**Nanoalloy Electrocatalysts for Electrochemical Devices**

Editor

**C ´esar Augusto Correia de Sequeira**

MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade • Manchester • Tokyo • Cluj • Tianjin

*Editor* Cesar Augusto Correia de ´ Sequeira University of Lisbon Lisboa Portugal

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This is a reprint of articles from the Special Issue published online in the open access journal *Nanomaterials* (ISSN 2079-4991) (available at: https://www.mdpi.com/journal/nanomaterials/ special issues/nanoalloy electrocatalysts).

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## **Contents**

## **About the Editor**

#### **C ´esar Augusto Correia de Sequeira**

Cesar AC Sequeira, PhD, has been a member of the faculty staff of Instituto Superior T ´ ecnico ´ (Univ. of Lisbon), maintaining his academic career in fundamental and technological electrochemistry for more than 50 years. He is author/co-author of over 280 professional papers, 330 scientific communications, and 42 books and book chapters in the areas of corrosion science and technology, electrochemistry, and materials science. He has directed numerous workshops, research projects, and advanced short courses. He is a Fellow of The Royal Society of Chemistry (UK) and of The Institute of Materials (UK), and is an Emeritus Member of The Electrochemical Society (USA).

## *Editorial* **Editorial for the Special Issue on "Nanoalloy Electrocatalysts for Electrochemical Devices"**

**César A. C. Sequeira**

Materials Electrochemistry Group, CeFEMA, Instituto Superior Técnico, University of Lisbon, 1049-001 Lisboa, Portugal<EMAIL_ADDRESS>
Nanoscale science and technology dealing with materials synthesis, nanofabrication, nanoprobes, nanostructures, nanoelectronics, nano-optics, nanomechanics, nanodevices, nanobiotechnology, and nanomedicine is an exciting field of research and development in Europe, the United States, and other countries around the world [1–4]. Over the past two decades, considerable progress has been made with countless discoveries advancing the understanding of the nanoscale and enabling applications from advanced electronic, magnetic, and electrochemical devices, energy, and defense to life-saving medicines [5–10]. At the heart of these fields are specific complexes of solid units that possess sizes between one and hundred nanometers (1–100 nm), which we call "nanomaterials" [11,12]. These materials have a very large surface area to volume ratio when compared with bulk material and can be produced using chemical or physical methods. They are small enough to confine their electrons and produce quantum effects, thus leading to unexpected chemical, mechanical, magnetic, electronic, electrochemical, and optical properties. They are classified into three major categories—namely, nanoparticles, nanoclays, and nanoemulsions. Nanoparticles can exist as nanostructures or as composites. All nanostructures can be built from elementary units or blocks having low dimensionality. It could either be zero dimension, one dimension, two dimensions, or three dimensions. If only one dimension of a three-dimensional nanostructure is of nanoscale, the structure is referred to as a quantum well; if two dimensions are of nanometer scale, the structure is referred to as a quantum wire; and if all three dimensions are of nanometer scale, the structure is referred to as a quantum dot. Hence, a quantum dot has all three dimensions in the nanorange and is the ultimate example of nanomaterials. The word quantum is associated with these three types of nanostructures because changes in properties arise from the physics of quantum mechanics. Nanoparticles are either organic or inorganic. Inorganic nanoparticles are more stable than organic nanoparticles but are limited by their stability in terms of either mechanical or chemical properties. Nanoclays are prepared using ion-exchange reactions, and usually, their chemical composition is characterized by FTIR. Nanoemulsions can be synthesized by high (e.g., ultrasonification, microfluidization) or low energy emulsification (e.g., solvent displacement, phase inversion composition). In parallel to the development of applications, considerable worldwide attention has focused on the potential implications of nanomaterials and vastly improved our understanding of nanosafety and other emerging nanosciences. Early focus areas of nanoscience, such as nanophotonics, have grown into their own fields. New suites of tools are now available to fabricate and characterize nanomaterials and enable their integration into devices and systems. Additionally, a vibrant, interdisciplinary, and international nanotechnology community has developed [13–23].

With this in mind, and considering that, as a chemical engineer with a doctoral thesis in material science, I have been working for many years in applied domains of electrochemistry in which metallic, bimetallic, trimetallic and other nanoalloys find several applications in the areas of partial oxidation/reduction of fuel cells, catalysis of methane and synthetic ammonia, energy storage, water electrolyzers, electrochemical detectors,

**Citation:** Sequeira, C.A.C. Editorial for the Special Issue on "Nanoalloy Electrocatalysts for Electrochemical Devices". *Nanomaterials* **2022**, *12*, 132. https://doi.org/10.3390/nano12010132

Received: 8 December 2021 Accepted: 29 December 2021 Published: 31 December 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

contaminant degradation in water and wastewater, electronic devices, nanomedicine, cellphones, biosensing, drug delivery, electric cars, electrochemical sensors, electrodegradation devices, hydrogen peroxide producers, etc., I was encouraged by the Nanomaterials Coordination Office to advance a Special Issue providing examples of ongoing nanotechnology research, current applications, and an overview of new elements of nanoalloy electrocatalysts, reporting their synthesis, characterization, modeling, unique properties and low cost for electrochemical devices.

Many techniques have been developed to synthesize and fabricate nanomaterials and, in particular, metallic nanoparticles, with controlled shape, size, dimensionality, and structure. The performance of these materials depends on their properties. The properties, in turn, depend on the atomic structure, composition, microstructure, defects, and interfaces, which are controlled by thermodynamics and kinetics of the synthesis. There are two technical approaches for the synthesis of metallic nanoparticles—namely, top-down and bottom-up processes. The former involves the breaking down of the bulk material into nanosized structures or particles. It is inherently simpler and depends either on the removal or division of bulk material or on miniaturization of bulk fabrication processes to produce the desired structure with appropriate properties. Usually, the prepared surface structure contains many impurities and structural defects. Examples of top-down techniques are high-energy wet ball milling, electron beam lithography, atomic force manipulation, gasphase condensation, aerosol spray, etc. The latter approach refers to the build-up of material from the bottom: atom by atom, molecule by molecule, or cluster by cluster. The organometallic chemical route, reversed micellar route, sol–gel synthesis, colloidal precipitation, hydrothermal synthesis, template-assisted sol–gel, electrodeposition, etc., are some of the well-known bottom-up techniques reported for the preparation of diverse nanoparticles. It has been found that the Gibbs free energy, the thermodynamic equilibrium, and the kinetic methods are the main strategies for the synthesis of nanoparticles by the bottom-up approach.

Many of the bottom-up techniques (or molecular nanotechnology) are still under development or are just beginning to be used for commercial production, belonging to the process intensification philosophy, whose peculiar operative conditions intend to maximize the mass/heat/momentum transfer inside the processing unit itself [2,24,25]. The objective of process intensification is not so as to "increase the efficiency and reduce the plant size", but also to enhance the production efficiency and process safety, to lower the cost, and to minimize the waste at source for reducing the environmental pollution [2].

The spinning disk reactor is a promising process intensification technology for heat and mass transfer rate enhancement. The hydrothermal route is another relevant technique that, according to Chen et al. [25] allowed the synthesis of well-dispersed zinc oxide and carbon co-modified LiFePO4 spherical particles (promising cathode materials for lithium-ion batteries). Green and bio-based technologies permit also to achieve reproducible nanomaterials applied in biomedical, medical, and electrochemical fields. In this context, electrochemical devices that address rising energy demands with the corresponding disadvantages are experiencing a change in this situation by process intensification. For example, Carmo et al. [5] reported Pt57.5 Cu14.7 Ni5.3 P22.5 bulk metallic glass (Pt-BMG) nanowires, which circumvent the performance problems of electrochemical devices. Pt-BMG nanowires catalysts were fabricated by a nanoimprinting approach, with high conductivity and activity for methanol and ethanol oxidation. After 1000 cycles, these nanowires, maintain 96% of their performance—2.4 times as much as conventional Pt/C catalysts. Their properties make them ideal candidates for energy conversion/storage and sensors. Using a contemporary PtCu-alloy electrocatalyst as a model system, a unique approach caused by the process of dealloying was presented by Ruiz-Zepeda et al. [11]. Observing detailed structures of the same nanoparticle at different stages of electrochemical treatment, they found new insights on properties such as size, faceting, strain, and porosity development. Furthermore, based on microscopy data and Kinetic Monte Carlo (KMC) simulations, they provided further feedback into the physical parameters governing electrochemically induced structural dynamics. This unique approach paves the way for an understanding of the structure–stability relationship.

In the area of alkaline polymer electrolyte fuel cells (APEFCs), a series of Ni-Cu binary alloy films were prepared using a combinatorial magnetron computing method, and their corresponding electrocatalytic performances toward HOR in alkaline media were systematically studied [19]. A volcano-type correlation between HOR activity and Cu doping content was revealed. The impart of Cu addition on the HOR catalytic activity (and antioxidation property) of Ni was deeply understood.

Allowing two kinds of metal, such as platinum group metals with 3d transition metals, has a great interest in advanced technological applications due to enhanced chemical stability and catalytic activities via increased active surface area [20,22]. In particular, loading Pt-based nanoparticles with ferromagnetic Fe, Co, and Ni atoms result in a reduction in the production cost, while the catalytic activities increase, such as hydrogen evolution and oxygen reduction reactions, the magnetic properties controlled, such as coercivity and saturation magnetization, for practical applications [6]. While Pt Fe alloy nanoparticles are widely used for permanent magnets (uniaxial magnetocrystalline anisotropy) and good chemical stability, PtNi nanoparticles is a promising material for magnetoresistive sensors, ultra-high density data recording technologies, and magnetic resonance imaging (MRI) applications, and exhibits better performance and durability for catalytic activities than commercially available Pt/C [26].

Electrodeposition is one of the practical methods to make nanocrystalline (NC) materials. There are two classes of deposition methods—electrodeposition and electroless deposition; both are inexpensive methods to manufacture relatively ductile metallic bulk materials or films/coatings. Electrodeposition is usually used for making thicker films (with higher deposition rates), while electroless deposition is typically employed for making high-quality thin coatings. The grain size and microstructure of electrodeposited films are controlled by tuning deposition parameters. Furthermore, these deposition parameters can be changed continuously or discontinuously to make functionally graded materials. The density, thickness, grain orientation of the deposited films can also be affected by the substrate conditions. To date, electrodeposition methods have been used to make various NC-materials (<100 nm in grain size) including Ni, Co, Pd, Cu, Zn, Ni-P, Ni-Fe, Ni-W, Zn-Ni, Ni-Fe-Cr, Ni-SiC, etc. The hardness, wear resistance, and electrical resistivity of most of these NCs are strongly dependent on grain size, surface morphology, and preferred orientation, and these effects are better exhibited by pulse than by direct current electrodeposition.

The corrosion behavior of NC alloys has been assessed by several techniques in various environments [27–31]. First results on the corrosion of crystallized melt-spun materials were obtained in Fe-Ni-Cr-P-B alloys in chloride–sulfate and sulfuric acid solutions [27,28]. It was determined that the corrosion resistance of the NC made of Fe32-Ni36-Cr14-P12-B6 was significantly greater than that of the amorphous counterpart. This improved corrosion resistance was attributed to the observed enrichment of Cr in the passive layer due to rapid interface boundary diffusion. Zeiger et al. [29] studied NCs made of Fe8Al in sodium sulfate solution. At the earlier stage of the formation of oxide film, this region becomes impoverished in Al. Diffusion of Al occurs due to a gradient in the Al content between the region beneath the Al oxide and the bulk alloy. The diffusion preferentially occurs through the grain boundaries and is enhanced in NC alloy due to its higher density of intergranular paths. As in NC alloy, the protective Al oxide is formed rapidly, and the uniformity and the compactness of passive film are enhanced. This film constitutes an effective barrier and protects the alloy against corrosion. Thus, it is possible to apply Fe8Al alloy films on carbon steel surfaces as protective measures. Barbucci et al. [30] investigated the corrosion behavior of Cu90Ni10 alloy in neutral media containing chlorides. They reported a decrease in the protective properties of the passive layer in the nanostructured alloy that was found to depend on the oxygen concentration. The increased amount of grain boundary in the nanostructured alloy could justify the loss of oxide compactness as a result of its irregular

growth on the surface. Alves et al. [31] found that NC alloys made of (Ni70 Mo30)90B10 are less sensitive to corrosion in alkaline solutions than the coarse-grained material.

Engineering alloys for high-temperature applications rely on the formation of protective oxide films such as Al2O3 and Cr2O3 to resist high temperature and corrosive environments. The high density of grain boundaries in an NC material provides fast diffusion paths, promoting selective oxidation of protective oxide scales with better adhesion to the substrate. Consequently, the percentage of passivating elements (such as Al and Cr) in the alloy/composite that is required to form a complete protective oxide scale can be substantially reduced. Experimental results indicate that when the grain size of the NC comprising Ni-20Cr-Al coatings was ∼ 60 nm, alloys containing ∼ 2 wt% Al could form a complete *α*-Al2O3 scale at 1000 ◦C in air. This concentration is only one-third of the required Al% for Ni-20Cr alloys with normal grain size [8].

The interest in Ti alloys and TiAl intermetallic alloys is growing in recent years because of their excellent ratio between mechanical properties and density [32]. However, for the last 15 years, the application was restricted at temperatures up to 873–973 K by the resistance to oxidation. For this reason, it was of great importance to study protective coatings able to raise its temperature of performance above 1073 K. A TiAl3 layer was obtained on the surface of TiAl intermetallic samples, depositing an Al coating by arc physical vapor deposition, followed by a thermal treatment. The mechanical behavior of the thermally treated layer was investigated by means of scratch tests and nanoindentation. Oxidations at 1123 K were carried out on coated and uncoated samples, in order to study the effect of the coating on the oxidation resistance of the samples. The aluminide coating provides protection against oxidation in air at the considered temperatures due to its ability to form a continuous Al2 O3 scale on the surface. Thus, these nano- or submicro-alloy coatings provide powerful tools for TiAl intermetallics to be used as high-temperature structural materials [8].

Many of the latest contributions on the possibilities and challenges to overcome the drawbacks that nanotechnology still exhibits and that prevent its large-scale development have been collected above, emphasizing nanoalloys for electrochemical devices. This Special Issue deals with these multimetallic nanoparticles and their theoretical and computational studies, as well as their synthesis, characterization, chemical and physical properties, and recent electrochemical application-driven research. The Guest Editor is very satisfied with the contributions received and finally published.

**Funding:** This research received no extra funding.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Article* **Synthesis and Characterization of PdAgNi/C Trimetallic Nanoparticles for Ethanol Electrooxidation**

**Ahmed Elsheikh 1,2,\* and James McGregor 2,\***

**Abstract:** The direct use of ethanol in fuel cells presents unprecedented economic, technical, and environmental opportunities in energy conversion. However, complex challenges need to be resolved. For instance, ethanol oxidation reaction (EOR) requires breaking the rigid C–C bond and results in the generation of poisoning carbonaceous species. Therefore, new designs of the catalyst electrode are necessary. In this work, two trimetallic PdxAgyNiz/C samples are prepared using a facile borohydride reduction route. The catalysts are characterized by X-ray diffraction (XRD), Energy-Dispersive X-ray spectroscopy (EDX), X-ray photoelectron Spectroscopy (XPS), and Transmission Electron Microscopy (TEM) and evaluated for EOR through cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). The XRD patterns have shown a weak alloying potential between Pd, and Ag prepared through co-reduction technique. The catalysts prepared have generally shown enhanced performance compared to previously reported ones, suggesting that the applied synthesis may be suitable for catalyst mass production. Moreover, the addition of Ag and Ni has improved the Pd physiochemical properties and electrocatalytic performance towards EOR in addition to reducing cell fabrication costs. In addition to containing less Pd, The PdAgNi/C is the higher performing of the two trimetallic samples presenting a 2.7 A/mgPd oxidation current peak. The Pd4Ag2Ni1/C is higher performing in terms of its steady-state current density and electrochemical active surface area.

**Keywords:** intermetallic XRD patterns; alloy formation; ethanol electrooxidation; metal segregation; borohydride reduction; fuel cells

#### **1. Introduction**

Fuel cells are electrochemical energy conversion devices capable of converting the fuel chemical energy into electricity [1,2]. They possess multiple technical and environmental advantages considering the decaying fossil fuel resources and growing energy needs. Their operation is theoretically 100% efficient because they are not restricted by the Carnot thermal/mechanical energy conversion limitation of conventional heat engines [3]. Direct ethanol fuel cells (DEFCs) are fed with liquid ethanol instead of hydrogen. DEFCs are friendly to the environment and present high flexibility to supply diverse energy needs. As a liquid, ethanol is easy to store and transport and presents a high energy density (8 kWh/kg) [4–7]. Furthermore, it can be produced from biomass resources such as sugar cane, corn, and, more recently, from diverse agricultural and biomass wastes and residues which means it is sustainable and carbon-neutral [5]. Despite these advantages, the full commercialization of DEFCs is far from being realized. There are difficult-to-overcome challenges mainly in the form of cell fabrication costs. All low-temperature fuel cells need active noble metal (Pt) catalysts to push the fuel oxidation and oxidant reduction reactions forward. Another challenge is that most C-containing fuels generate CO–species during their oxidation. These poison catalytic sites and prevent further reaction, even if present only in low concentrations, e.g., 50 ppm [8,9]. Additionally, ethanol contains a strong C–C

**Citation:** Elsheikh, A.; McGregor, J. Synthesis and Characterization of PdAgNi/C Trimetallic Nanoparticles for Ethanol Electrooxidation. *Nanomaterials* **2021**, *11*, 2244. https:// doi.org/10.3390/nano11092244

Academic Editor: Eva Pellicer

Received: 28 July 2021 Accepted: 27 August 2021 Published: 30 August 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

bond thereby favoring its incomplete oxidation to acetate instead of CO2 and releasing only 4 electrons instead of 12 [10,11]. Pt is very active for fuel cell electrocatalysis but is very expensive and scarce at the same time. Its expense can account for half of the cell fabrication [12]. There is, therefore, a need to find a Pt-alternative that presents comparable activity but also is more abundant to decrease the cell fabrication cost. Pd is more abundant than Pt and has shown a comparable performance for fuel cell electrocatalysis in alkaline electrolyte [13–15]. The fluctuating Pd and Pt prices are a complex phenomenon that involves technological, economic, and political contributions [16,17]. South Africa, Russia, Zimbabwe, Canada, and USA have Pd and Pt deposits. Prior to 1990, Pd was generally cheaper than Pt to mine and produce. However, with the advancement of Pd-based autocatalytic systems by the mid-90s, the demand and cost of Pd soared. By 2000, the price of Pd was much higher than that of Pt [18]. However, in 2007, the prices of Pt and Pd were reported \$1450/ounce and \$450/ounce, respectively [19]. Still, however, Pd is more attractive for automakers because it is not easy to switch back to Pt-catalytic converters. Furthermore, the diversity of Pd production by country and region is higher than that of Pt. South Africa, solely, controls Pt production while Pd mining capabilities are shared by Russia, South Africa, and North America [18]. This is an important factor due to the potential of a future shortage of Pt which is less likely in the case of Pd [20]. Another advantage of Pd over Pt in this particular EOR application is that it is more CO tolerant than Pt, however, Pt is more CO2 selective than Pd [21]. To achieve a high-surface-area, Pd nanoparticles are dispersed on an inert, conducting, and porous material such as carbon black.

To further enhance the performance pf Pd and to decrease cell fabrication expenses, it is recommended to add another metal (or two) during the synthesis. The cocatalyst metal is expected to activate water and generate OH species to facilitate the oxidation of adsorbed ethoxy species. Moreover, it would modify the Pd geometry and electronic configuration and tune in its adsorption characteristics. Ni has been proven a beneficial cocatalyst when added to Pd because it can generate OH at a lower applied potential and modify the electronic structure of Pd [22–27]. The coexistence of Ni and Ni(OH)2 can enhance alcohol oxidation on transition metals in acidic and basic electrolytes [6,28]. Feng et al. [29] prepared unsupported porous Pd and PdNi catalysts for ethanol electrooxidation sowing that PdNi exhibits enhanced electrocatalytic performance when compared to monometallic Pd. This enhancement is ascribed to the electronic and bifunctional effects of Ni. Zhang et al. [27] prepared various-proportion PdxNiy/C catalysts for ethanol oxidation reaction. They found that Ni can generate oxygen species at lower applied potential, recovering Pd active sites and thus promoting ethanol oxidation, while through the microemulsion syntheis method it was possible to control PdNi particle size and make efficient contact between Pd and Ni. You et al. [30] prepared bimetallic PdAg dendrites with various composition and porous structure and evaluated them towards ethanol oxidation. Alloying between Pd and Ag shifts up the Pd d-band center leading to more tolerance for intermediates and poisons during EOR. Elsewhere, Li et al. [31] prepared PdAg nanoparticles supported on reduced graphene oxide (RGO), noting enhanced ethanol and methanol oxidation on PdAg when compared to Pd only. Oliveira et al. [32] prepared PdAg alloys and tested their efficacy for oxygen reduction reaction (ORR) and EOR. The kinetics of both reactions are promoted on PdAg as compared to Pd only. Additionally, the alloys maintained a higher selectivity for ORR in presence of ethanol. Similar conclusions on the benefits of adding Ag to Pd have been reported elsewhere [33,34].

In this work, trimetallic samples of C-supported PdAgNi nanoparticles are prepared, characterized, and evaluated for ethanol oxidation reaction (EOR) for the first time. Pd-based bimetallic nanocatalysts have been extensively investigated and reported for fuel cell electrocatalysis and EOR. Yet, a few groups have pursued the synthesis and application of C-supported trimetallic samples for similar purposes [35–43]. It is postulated that adding two metals is likely to provide benefits towards ethanol oxidation through altering Pd geometry, surface configuration, adsorption capacity, and coordination [35,37–39,44,45]. However, several factors such as the metal chemistry, synthesis method, reducing agent, and support structure may adversely affect the outcome. The ultimate objective is to design new electrodes that are cost-effective, EOR-active, CO-tolerant, and CO2-selective.

#### **2. Materials and Methods**

The applied borohydride reduction synthesis method follows [46–48]. Table 1 shows the added precursor quantities of Vulcan carbon (XC72R), PdCl2, NiCl2, and AgNO3 to prepare the monometallic and trimetallic samples. The theoretical metal loading was fixed at 12 Wt. %. The metal and carbon precursors were sonicated in a mixture of 2-propanol and water (50/50 *v*/*v*) for a few minutes. The theoretical Pd:Ag:Ni molar ratio is 57:28:15 and 34:33:33 for Pd4Ag2Ni1/C and PdAgNi/C, respectively. The metallic salts were purchased from Sigma-Aldrich (Gillingham, United Kingdom) and the vulcan carbon precursor from Cabot Corp (Boston, MA, USA).

**Table 1.** Stochiometric added quantities of C, PdCl2, AgNO3, NiCl2 to synthesize Pd/C. PdAgNi/C, and Pd4Ag2Ni1/C.

KBr was added as a capping agent following the anion exchange method; the larger Br− ion is capable of replacing the smaller Cl− ion in the vicinity of Pd [49,50]. Consequently, Br− ions can surround the recently reduced metal hindering its coalescence. The KBr/Metal atomic ratio was adjusted to 1.5. After that, the mixture was stirred for 10 min followed by adding the NaBH4 solution (0.5 M, 15 mL) in one portion. Subsequently, the whole mixture was vigorously stirred for 30 min. Finally, the wet powder was dried at 80 ◦C in vacuum oven overnight.

X-ray diffraction (XRD) was undertaken to analyze the catalyst structure. The equipment is a Bruker D2 Phaser (Billerica, MA, USA) using Cu Kα radiation at 30 kV, 10 mA, and 12◦/min scan-rate. The chemical composition of the catalysts was examined through energy-dispersive X-ray (EDX) spectroscopy conducted on a JEOL JSM 6010LA scanning electron microscope (SEM) (Akishima, Tokyo, Japan). Each catalyst surface was examined twice applying two different accelerating voltages: 10 kV and 20 kV. The different accelerating voltages change the interaction volume resulting from the electron beam-sample surface interaction. Thus, applying two different voltages enables the composition at two different depths to be studied [51]. Transmission electron microscopy (TEM) was undertaken to examine the catalyst surface morphology using a Phillips C100 microscope (Hillsboro, OR, USA) operating at 100 kV and equipped with a LaB6 filament. X-ray photoelectron spectroscopy (XPS) was applied to investigate the metal oxidation state and surface composition of the prepared electrocatalysts. A Thermo Scientific K-Alpha+ spectrometer (Waltham, MA, USA) equipped with an Al-X ray source (72 W) was used. The pass energy to record the data was 150 eV for survey scans and 40 eV for high-resolution scans. The survey scan step size is 1 eV, and that of the high-resolution scans is 0.1 eV. Low-energy electrons and argon ions were used to neutralize the charge. CasaXPS (Teignmouth, UK) was used to analyze the data implementing a Shirley-type background and Scofield crosssections with an energy dependence of −0.6.

The catalysts were evaluated for EOR via cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). For this, a 3-electrode half-cell was used in which the working electrode is a glassy carbon electrode (GCE, Ø 3mm) coated with each catalyst ink. The reference and counter electrodes were Ag/AgCl (sat' KCL) and Pt wire, respectively. The applied potential is converted to normal hydrogen electrode

(NHE). A Gamry 600 portable Station from Gamry Instruments Inc. (Warminster, PA, USA) was used to perform the electrochemical evaluation. The catalyst ink was prepared by dispersing 5 mg of its powder in 25 μL (Nafion® 117, 5%) and 2000 μL of ethanol (100%). The mixture was then sonicated for 1 h. Before drop-casting the working electrode with ink, it was polished with alumina powder (1 and 0.05 μm, respectively) to produce a mirror-like surface. Then, 25 μL of the ink was painted on GCE at 5 μL intervals.

#### **3. Results**

#### *3.1. Physical Characterization* X-ray Diffraction (XRD)

Figure 1 shows the XRD patterns of Pd/C, Ni/C, Ag/C, PdAgNi/C, and Pd4Ag2Ni1/C. The 25◦ peak present in all patterns is attributed to the semi-crystalline graphitic nature of Vulcan carbon. For the Ni pattern, the metal peaks are of lower intensity than the carbon peaks as Ni is present in the form of hydroxide and oxide, indicated by peaks at 34◦, 60◦, and 42.5◦, respectively. The Ag pattern, on the other hand, shows the (111), (200), (231), (220), and (311) reflections at 38◦, 44◦, 46◦, 65◦, and 77◦, respectively resembling a face-centered cubic Ag (JCPDS card, File No. 04-0783) [52]. A similar Ag pattern has been obtained previously [31]. The Pd pattern presents (111), (200), (220), and (311) reflections located at 39.8◦, 46◦, 76.7◦, and 82.2◦ indicative of Pd (JCPDS card, File No. 46-1043). Considering the XRD patterns of both trimetallic samples, it can be noted that a significant peak overlapping occurs. The high intensity peaks present at lower-angles align well with the pure Ag peak positions, whereas the lower-intensity, broader, peaks at higher-angles align with the peak positions of Pd. Monometallic Ni/C shows Ni(OH)2 and Ni (111) peaks at 34.5◦, 60.1◦, and 42.7◦, respectively.

**Figure 1.** X-ray diffraction patterns of Pd/C, Ag/C, Ni/C, PdAgNi/C, and Pd4Ag2Ni1/C.

#### *3.2. Energy Dispersive X-ray Spectroscopy (EDX)*

Table 2 lists the elemental metal molar and weight compositional ratios of PdAgNi/C and Pd4Ag2Ni1/C. While the average total metal loading of Pd4Ag2Ni1/C as measured by EDX is close to the theoretical loading at ~12 wt. %, it is slightly higher (16 wt. %) in the case of PdAgNi/C. As EDX is a surface-weighted technique, this indicates the preferential distribution of metal in the surface region of the catalyst. Furthermore, higher Ni atomic concentrations are observed at 10 kV than at 20 kV indicating the preference of Ni to be distributed in the near-surface region.

**Table 2.** EDX elemental metal composition of PdAgNi/C and Pd4Ag2Ni1/C detected at 20 kV and 10 kV voltages.

\* wt. %: weight percentage %; \*\* at. %: atomic percentage %.

The PdAgNi/C surface can therefore be considered as relatively rich in Ni, while for Pd4Ag2Ni1/C the Ni content in the surface region is comparable to that of Ag even though the theoretical Ni quantity in the bulk sample is half of that of Ag. Figure 2 shows the elemental EDX maps of Pd, Ni, Ag, and C in both PdAgNi/C and Pd4Ag2Ni1/C. Examining the maps of Pd4Ag2Ni1/C (A–D) shows that the carbon support and metal species are evenly dispersed across the catalyst surface.

**Figure 2.** EDS elemental maps of Pd4Ag2Ni1/C (**A**–**D**) and PdAgNi/C (**E**–**H**) taken at 10 kV.

*3.3. Transmission Electron Microscopy (TEM)*

Figure 3 shows TEM micrographs (A, C, D) of Pd/C, Pd4Ag2Ni1/C, and PdAgNi/C. The respective particle size distribution is, also, shown (B, D, F). From inspection of the

Pd/C micrograph, it can be noted that Pd nanoparticles (average size 6.7 nm) are dispersed over larger carbon aggregates (40–60 nm). Additionally, some particle agglomeration can be noted. The particle size distribution was measured by manually selecting the metal particles through use of ImageJ software. The average particle size and standard deviation of Pd/C are 6.7 and 5 nm, respectively. The high standard deviation value is suggestive of the high variation in particle size as a consequence of high particle agglomeration. In contrast, the average particle size of Pd4Ag2Ni1/C is 5.6 nm and the standard deviation to 2.7 nm. This is possibly due to the contradicting forces that affect the particle aggregation during both nucleation and growth. It is extensively reported that preparing multi-metallic samples results in different attraction and repulsion forces and, therefore, smaller particle sizes [22,24,41,53–56]. The same trend is noted in PdAgNi, which has a smaller total quantity of Pd; in this case the average particle size and standard deviations are 4.4 nm and 3 nm, respectively.

**Figure 3.** TEM Micrographs and the respective particle size distribution of Pd/C (**A**,**B**), Pd4Ag2Ni1/C (**C**,**D**), and PdAgNi/C (**E**,**F**).

#### *3.4. X-ray Photoelectron Spectroscopy (XPS)*

XPS analysis was undertaken to investigate the metal oxidation state and surface composition of the samples. Figure 4 shows the XPS full survey scans of Pd/C, PdAgNi/C, and Pd4Ag2Ni1/C. The C1s, Pd 3d, Ag 3d, Pd 3p (overlapping with O 1s), and Ni 2p are located approximately at 284, 335, 370, 532, and 856 eV, respectively.

**Figure 4.** XPS full surveys of Pd/C, Pd4Ag2Ni1/C, and PdAgNi/C.

Figure 5 shows the detailed elemental peaks of Pd 3d, Ag 3d, and Ni 2p in Pd/C, Pd4Ag2Ni1/C, and PdAgNi/C. The Pd 3d of Pd/C (A) is deconvoluted into a high-energy band (340.4 eV) of Pd 3d3/2and a low-energy band (335.48 eV) of Pd 3d5/2. Additionally, the PdO peaks are visible around 342 eV and 337 eV. Table 3 lists the XPS surface concentrations of C, O, Pd, Ag, and Ni for the three samples. The XPS surface concentration Pd of Pd/C is 1.63 At. % and 0.45% for Pd0 and Pd2+, respectively. The presence of Pd oxide is indicative of Pd air instability which could be improved by adding another metal. PdAgNi/C also presents a significant fraction of Pd2+—16% of the total Pd present. Pd4Ag2Ni1/C however, with a lower overall Pd content, presents only metallic Pd0, as shown in Figure 5D. The Ag 3d double-peak is located at 374 and 368 eV for 3d3/2 and 3d5/2 (Figure 5E), respectively. The Ni 2p peaks of Ni<sup>0</sup> and Ni+2 of Pd4Ag2Ni1/C are present at 852, 856, and 862 eV (Figure 5D), respectively, with no visible satellite peaks. The Ni XPS surface concentration of that sample is 0.05 at. % (for metallic Ni) and 0.73 at. % (for Ni(OH)2).

**Table 3.** XPS surface atomic concentration of C, O, Pd, Ag, and Ni in Pd/C, Pd4Ag2Ni1/C, and PdAgNi/C.

**Figure 5.** XPS elemental peaks of Pd 3d in in Pd/C (**A**), valence band spectra (**B**) and Pd 3d5/2 (**C**) of Pd/C, Pd4Ag2Ni1/C, and PdAgNi/C, Pd 3d, Ag 3d, Ni 2p, of Pd4Ag2Ni1/C (**D**) and PdAgNi/C (**E**).

#### *3.5. Electrochemical Evaluation*

Figure 6 shows the cyclic voltammograms of Pd/C, PdAgNi/C, and Pd4Ag2Ni1/C performed in 1M KOH at 50 mV/s. It is noteworthy that the thinnest profile is observed for Pd/C. This is indicative of a thinner double-layer, and consequently, faster charging/discharging behavior for Pd/C [42]. As Pd is known to absorb hydrogen within its bulk, the Habs/ads peak in the forward scan at ~−600 mV is not present. This peak is, however, more pronounced on both trimetallic samples, which may be ascribed to the presence of Ag. Upon increasing the potential, the OH adsorption peak between −400 and −200 mV is more apparent for monometallic Pd/C than for the trimetallic samples. OH adsorption can be considered as the onset of surface oxidation, however oxidation of Pd commences at approximately 0.0 mV vs. NHE. Ag surface oxidation is noted at ~470 mV followed by Ni oxidation up to the scan end at 600 mV. Both Ni and Ag oxidation peaks are more pronounced for PdAgNi/C than for Pd4Ag2Ni1/C as the former contains higher quantities of Ni and Ag. In the reverse scan, distinctive reduction peaks for Ni, Ag, and Pd appear at 450, 220, and −200 mV, respectively. A small shoulder on the Pd reduction peak is noted for Pd4Ag2Ni1/C and Pd/C which is not clear for PdAgNi/C. This is likely to be reflective of the reduction of different Pd oxides.

**Figure 6.** CV voltammograms of Pd/C, Pd4Ag2Ni1/C, and PdAgNi/C in 1M KOH at 50 mV/s.

Figure 7 shows the cyclic voltammograms of the three catalysts in 1M KOH+EtOH at 50 mV. The addition of ethanol suppresses the Hands/abs peak as reported elsewhere [55,57]. With the start of OH adsorption, adsorbed ethoxy species undergo oxidation and are removed from Pd sites, thereby making these sites available for further fuel oxidation. As the adsorbed OH increases with increasing potential, the free Pd sites increase, and an increasing current is drawn. While the onset oxidation potential in case of Pd/C is −390 mV, it is shifted to −500 mV in case of PdAgNi/C and Pd4Ag2Ni1/C which suggests a reduction in the activation barrier against ethanol oxidation on the surface of both trimetallic samples. A similar shift in potential is noted for the oxidation peak on PdAgNi sample compared to Pd. The peak current density in case of PdAgNi/C (2700 mA/mgPd) is higher than Pd4Ag2Ni1/C (2300 mA/mgPd) and the smallest peak current density is of Pd/C (1.8 A/mgPd).

**Figure 7.** CV Voltammograms of Pd/C, Pd4Ag2Ni1/C, and Pd/C in 1 M KOH+EtOH at 50 mV/s.

When the catalyst surface can no longer adsorb OH, the surface Pd oxides thereby decreasing the number of Pd active sites as the potential increases. This ultimately decreases the drawn current up to the end of the forward scan. It is noteworthy that around the forward current peak, the reactants are depleted much faster than at the beginning of the scan [8]. A shoulder peak is noted on PdAgNi/C at 200 mV which may be suggestive of the oxidation of ethoxy intermediates on Pd. At the end of the PdAgNi forward scan, a sharp current rise is noted due to the Ni oxidation that also appears in the voltammogram conducted in the absence of ethanol. The sharp peak in the inverse scan is due to the removal of the incompletely oxidized intermediates from EOR and recovery of Pd active sites. It is noteworthy that the forward sweep current peak is higher than the reverse one on all catalysts which is a positive indication of the ability of the catalyst to tolerate poisoning species [58,59]. Therefore, CO-like species are less likely to block further EOR on the catalysts developed in the present work.

Figure 8 shows the chronoamperometric scans of Pd/C, Pd4Ag2Ni1/C, and PdAgNi/C performed at fixed potentials of −300 mV and +100 mV vs. NHE. The former was chosen because it is in the middle of the window in which OH was adsorbed causing the oxidation and removal of ethoxy species from Pd surface. The latter was chosen because it is greater than the potential at which the Pd surface oxidation starts. These data show that the CA current density of PdAgNi/C sample—though higher than that of Pd4Ag2Ni1/C at the two CA steps—decays much faster at the higher-potential (+0.1 V) to the extent that it drops below Pd4Ag2Ni1/C towards the end of the CA duration. According to EDX and XPS, the surface of PdAgNi/C is Ni-rich and correspondingly deficient in Pd and Ag. PdAgNi/C, therefore, presents fewer accessible Pd surface sites for EOR than Pd/C or Pd4Ag2Ni1/C. During the high-potential CA scan, the number of available Pd sites further decreases through site poisoning via the strong adsorption of EOR intermediates. The PdAgNi voltammograms (Figures 6 and 7) show that OH adsorption does not continue at +0.1 V. Therefore, at high potential, PdAgNi/C is susceptible to poisoning by carbonaceous species from EOR. In contrast, Pd4Ag2Ni1/C generates a steadier CA current density than PdAgNi/C at +0.1 V; which is ascribed to the higher concentration of Pd in the surface region for the former, providing a greater number of available Pd sites and hence greater resistance to poisoning. Pd/C achieves an intermediate current decay between PdAgNi/C and Pd4Ag2Ni1/C which could be explained by two factors: the strong adsorption of CO-like species and the abundance of Pd surface sites.

**Figure 8.** Chronoamperometric (CA) scans of Pd/C, Pd4Ag2Ni1/C, PdAgNi/C at −0.3 V and +0.1 V vs. NHE.

Figure 9 shows the EIS spectra of Pd4Ag2Ni1/C, and PdAgNi/C recorded at −0.3 V, 0.0 V, and +0.3 V vs. NHE between 10,000 Hz and 0.1 Hz while the voltage amplitude is 5 mV. Each experiment was preceded with a 10-min potentiostatic scan to compensate for the current perpetuation. EIS spectra at −0.3 V and 0.0 V represent, to some extent, a semi-circle due to the interchangeable interaction of the double-layer capacitance through the electrolyte-electrode interface and the charge-transfer resistance. However, at +0.3 V, the effect of charge-transfer resistance is such that with further decreasing frequency, the

arc continues to rise vertically. This is a consequence of more than one factor. For instance, increasing the applied potential affects the charge-transfer resistance more significantly than the double-layer capacitance. Moreover, at +0.3 V no further EOR occurs due to the Pd surface oxidation and current decrease (Figure 7). It is noteworthy the arc size at −0.3 V and 0.0 V of Pd4Ag2Ni1/C is slightly smaller than their counterparts of PdAgNi/C.

**Figure 9.** Electrochemical impedance spectroscopy (EIS) of Pd4Ag2Ni1/C (**A**) and PdAgNi/C (**B**) at −0.3 V, 0.0 V, and 0.3 V vs. NHE.

Gamry Echem Analyst software was used to construct an electrical model (Figure 10) that represents the physical phenomena contributing to the impedance. Rs represents the solution resistance measured at the intersection point with the *x*-axis (−Z"= 0, frequency = 10 kHz) and for both catalysts, it is approximately 14 Ω and does not change with the applied potential. The constant-phase element (Ø) represents the double-layer capacitance effect due to opposite-charge accumulation at the electrode-electrolyte interface. Rct represents the charge-transfer resistance that reflects on the specific activity of each catalyst. Rct values of Pd4Ag2Ni1/C are estimated to be 150 Ω and 183 Ω at −0.3 V and 0.0 V vs. NHE, respectively. Larger values of 285 Ω and 290 Ω—at the same respective potentials—are obtained with PdAgNi/C. This further demonstrates the improved performance of Pd4Ag2Ni1/C towards EOR when compared to PdAgNi/C. It is difficult to estimate the Rct value at +0.3 V due to the complex phenomena encountered and missing EOR at such high potential and several EIS models were inaccurate enough to estimate reasonable Rct values at that potential.

**Figure 10.** EIS electrical model representative of the physical quantities in a half-cell 3-electrode.

#### **4. Discussion**

The XRD peak separation of Pd and Ag (Figure 1) implies that there is substantial separation of Pd and Ag in the nanoparticles and therefore that it is less likely a nanoalloy has been formed. Separate Ag and Pd peaks were also reported by other researchers [31,60], while other groups have found singular peaks that resemble both Pd and Ag [30,39,61].

Olivera et al. [32] annealed electrolessly deposited Pd and Ag films on steel discs to produce Pd-Ag alloy films. Before annealing, distinctive Pd and Ag phases were detected by XRD, with a single peak observed after annealing. The Ni(OH)2 individual peak disappearance in the XRD patterns of both trimetallic samples is suggestive of a high degree of mixing between Pd and Ni whose atoms could be assumed incorporated into the Pd lattice. The separate peaks of Ni(OH)2 are commonly seen when synthesizing bimetallic and trimetallic Ni-containing catalysts [22,29,53–55,62,63]. The surface metal loading of PdAgNi/C (16 wt. %), as measured by ESX, is 4 wt. % higher than the nominal bulk loading (Table 1), with Ni segregating to the surface. The opposite trend is noted for Ag which segregates into the catalyst core. The Pd: Ag: Ni molar ratio at 10 kV is 4:1:2 while it is 4:1.6:1 at 20 kV which shows the Pd concentration does not change significantly with analysis depth, but the Ag and Ni ones do. A similar observation is noted for PdAgNi/C whose Pd:Ag:Ni at 10 kV is 2:1:4 while it is 1.75:1:2.25 at 20 kV.

According to the XPS measurements (Table 3), Ag is present exclusively in the metallic state with a concentration of 0.4 at. % and 0.32 at. % for Pd4Ag2Ni1/C and PdAgNi/C, respectively. This is despite the theoretical Ag loading being lower in Pd4Ag2Ni1/C (Table 1). It is noteworthy that XPS atomic Pd:Ag:Ni ratio is 4:1:2 while the ratio from surface-weighted EDX spectroscopy measurements (Table 2) is 4.4:1.7:1. The higher Ni XPS proportion signifies its segregation into the top surface unlike Ag. In PdAgNi/C, which has a lower fraction of Pd than Pd4Ag2Ni1/C, the Pd air stability is lost and 15% of Pd exists in an oxidized form (Figure 5E) Figure 5G shows the Ni 2p peak of PdAgNi/C and two satellite peaks which are potentially due to the multiple Ni excitations. No metallic Ni was detected for this sample and the Ni surface concentration equals 2.01 at.%. Therefore, the XPS-derived Pd:Ag:Ni molar ratio is 2.7:1:6.7 while the EDX spectroscopy derived ratio (Table 2) is 1.7:2.3:1. Once again this is probably due to the Ni tendency to segregate into the surface and Ag and little Pd tendency to segregate into the core. In case of trimetallic samples, the Pd 3d peak is shifted 0.05 eV (Figure 5C) to higher binding energy than Pd/C, which may be suggestive of an electron loss from Pd to either Ag or Ni. Figure 5C shows the valence band of Pd/C, Pd4Ag2Ni1/C, and PdAgNi/C which indicates the *d*-band center of the two trimetallic samples are shifted approximately 1 eV from the Fermi level compared to their monometallic counterpart. Note that the impact of particle size on *d*-band center can be neglected as the particle size is ~5 nm and instead shifts in the *d*-band center are the consequence of charge transfer from Pd to other metals [35,64].

As Habs/des peak is missing in Figure 6, it is more appropriate to estimate electrochemical active surface area (ECSA) using the PdO reduction peak in the reverse scan. The PdO reduction area in the reverse scan is the smallest in case of Pd/C while higher reduction currents are obtained with PdAgNi/C and Pd4Ag2Ni1/C. The PdO reduction peak is used as a measure of the Pd fraction exposed on the surface, and therefore, available to undertake redox reactions. The electrochemical active surface area (ECSA, cm2/mg) is calculated according to [26]:

$$ECSA = \frac{Q}{0.43 \times [Pd]}$$

where *Q* is the charge extracted from PdO reduction peaks in mC, 0.43 mC/cm<sup>2</sup> is the monolayer reduction charge of Pd, and [Pd] is the weight of Pd on GCE in mg. This indicates ECSA is highest on Pd4Ag2Ni1/C followed by PdAgNi/C and finally Pd/C (Table 4). This could be explained by the smaller particle size of both trimetallic samples compared to the monometallic one according to TEM (Figure 3). The three catalysts have very significantly larger ECSAs compared to the other materials listed in Table 4. This is indicative of the high Pd surface fraction; although Ni has a higher tendency to segregate into the surface as shown by EDX and XPS analyses. The low overall metal loading (12 wt. %) decreases the overall metal surface energies and aggregation potential leading to a thicker and rougher catalyst layer, while the KBr capping works against agglomeration by surrounding the individual nanoparticles during synthesis with Br−<sup>1</sup> ions. The ethanol-enhanced CV performance on both trimetallic samples is a reflection of

the Ag and Ni effects. This indicates that more EOR Pd active sites exist on the trimetallic sample surfaces than on Pd/C. A similar finding was previously reported regarding the activity of PdAgNi towards formic acid oxidation [39]. The lower oxidation onset potentials and higher forward current peak are advantageous for the three samples in this work compared to many other previously published catalysts [58,65–67]. Table 4 compares the EOR onset oxidation current density and current peak obtained with this work catalysts and previously published ones. The reverse current peak of Pd/C, Pd4Ag2Ni1/C, and PdAgNi/C are smaller than their forward counterparts. This is indicative of a good Pd tolerance towards the poisoning species. When of/jb is >1, it implies less incompletely oxidized ethoxy intermediates on Pd surface which suggests the catalyst is more likely to recover its surface-active sites and proceed with further EOR. When it is <1, it suggests there are more incompletely-oxidized carbonaceous species on the catalyst surface that are strongly bound to active sites. Adding Ni to the Pd-based catalyst has, in some previous works, resulted in jf/jb decreasing below 1 [29,43], although other works observe the opposite trend [42,54,68].

**Table 4.** Comparison of the ECSA (cm2/mg), Eonset (mV) and *j*<sup>p</sup> (A/mgPd) in 1M EtOH of this work and previously published catalysts.

It should be noted the chronoamperometric currents drawn on trimetallic catalysts (Figure 8) are substantially higher than that on Pd/C which suggests a higher catalyst capability to recover the Pd active sites by generating OH species which indicates the addition of Ag and Ni is beneficial for EOR catalytic performance. The catalyst containing the least Pd (PdAgNi/C) draws the highest current density (−0.3 V). This implies that this surface is highly active for the generation of oxygen species, facilitating the removal of adsorbed ethoxy species. Ni surface segregation and Ag core segregation, as determined through EDX spectroscopy and XPS, may in part explain the enhanced V-*j* behavior of PdAgNi/C. A somewhat similar behavior is noted for the other trimetallic sample. Note, however, that a very high level of OH adsorption on the catalyst surface would negatively impact the overall catalyst performance as many of the EOR-active surface sites would be occupied by OH.

The enhanced the performance of the trimetallic catalysts in the present work is likely due to Ni and Ag generating OH species resulting in the larger ECSA for both samples as shown in Figure 6. The current density drawn on three catalysts at +0.1 V is higher than at −0.3 V due to the increased activation voltage and OH adsorption. It is also noteworthy that at −0.3 V vs. NHE, the three catalysts exhibit similar behavior in terms of the current decays as the scan proceeds. However, at −0.1 V vs. NHE, Pd4Ag2Ni1/C shows better stability (stable current decay) than PdAgNi/C even though the former draws a higher current for the majority of the 30-min scan. Although the initial current decay at −0.3 V is similar on both Pd4Ag2Ni1/C and PdAgNi/C, at +0.1 V the current decay rate is higher for PdAgNi/C than for Pd4Ag2Ni1/C and the former draws a higher current at the scan end. Several factors likely contribute to this enhanced performance upon adding both Ag and Ni. For instance, the activation barrier of EOR on both trimetallic samples less than that on Pd/C due to the reduction of the bandgap energy, indicated by the smaller activation overvoltage [8]. Also, the large number of active sites verified by the smaller TEM particle size in the case of both trimetallic samples, will play a positive role. Another factor that may contribute to the observed higher ESCA values as compared to previous studies is that the experiment was performed with the electrolyte being stirred. This may help to ensure the uniform distribution of reactants/products and reduce the effects of mass transport and diffusion.

Compared to many reported synthesis methods, the currently applied method is quicker and easier because it is a room-temperature one-pot synthesis. Furthermore, it was concluded in a short time (30 min). Many of the previous synthesis protocols include use of high temperature and/or longer synthesis times [29,42,43,54,68]. Also, the smaller metal loading in the present work is potentially beneficial for the physical and electrochemical characteristics of the supported catalyst. Additionally, it is noted that Ni tends to segregate into the surface while Ag tends to segregate into the bulk of trimetallic particles. Furthermore, the trimetallic catalysts are cost-effective than those consisting only of platinum-group metals. The collective characterization and evaluation results suggest that Pd4Ag2Ni1/C is the best performing catalyst and that adding a significant Ni quantity (>35 At. %) would adversely impact the catalyst physical and electrocatalytic performance

#### **5. Conclusions**

C-supported PdAgNi intermetallic catalysts have been successfully prepared through a straight-forward chemical synthesis method suitable for up-scaling for the first time. Adding small quantity of Ni in trimetallic catalysts has a beneficial impact but increasing the Ni content can harm the EOR performance. A relatively low metal loading (12 Wt. %) and adding KBr as a capping agent are recommended to produce a well-dispersed catalyst with a high electrochemical surface area (ECSA). However, the followed protocol does not produce a highly order alloy structure of Pd and Ag as indicated by XRD. Although the Pd4Ag2Ni1/C surpassed the PdAgNi/C in terms of ECSA and ethanol oxidation potential, the latter outperformed it in terms of the oxidation current peak and steady chronoamperometric current. Adding Ag and Ni to Pd not only improves its catalytic performance towards EOR, but considerably decreases the catalyst preparation cost. While Ag tends to segregate to the bulk of the PdAgNi nanoparticle, Ni tends to segregate to its outer surface layers. Pd4Ag2Ni1/C combines both high activity and stability towards EOR.

**Author Contributions:** Conceptualization, A.E. and J.M.; methodology, A.E.; software, A.E.; validation, J.M. and A.E.; investigation, A.E.; resources, J.M.; data curation, A.E.; writing—original draft preparation, A.E.; writing—review and editing, J.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** Ahmed Elsheikh is grateful for the financial support of Newton-Mosharafa Fund (Reference No. NMJ8/15) of his Ph.D.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** XPS data collection was performed at the EPSRC National Facility for XPS ('HarwellXPS'), operated by Cardiff University and UCL, under contract No. PR16195.

**Conflicts of Interest:** The authors declare there is no conflict of interest related to this work.

#### **References**

## *Article* **Carbon-Supported Trimetallic Catalysts (PdAuNi/C) for Borohydride Oxidation Reaction**

**Ahmed M. A. ElSheikh 1,2,†, Gordana Backovi´c 3,†, Raisa C. P. Oliveira 3, César A. C. Sequeira 3, James McGregor 2, Biljana Šljuki´c <sup>3</sup> and Diogo M. F. Santos 3,\***

**Abstract:** The synthesis of palladium-based trimetallic catalysts via a facile and scalable synthesis procedure was shown to yield highly promising materials for borohydride-based fuel cells, which are attractive for use in compact environments. This, thereby, provides a route to more environmentally friendly energy storage and generation systems. Carbon-supported trimetallic catalysts were herein prepared by three different routes: using a NaBH4-ethylene glycol complex (PdAuNi/CSBEG), a NaBH4-2-propanol complex (PdAuNi/CSBIPA), and a three-step route (PdAuNi/C3-step). Notably, PdAuNi/CSBIPA yielded highly dispersed trimetallic alloy particles, as determined by XRD, EDX, ICP-OES, XPS, and TEM. The activity of the catalysts for borohydride oxidation reaction was assessed by cyclic voltammetry and RDE-based procedures, with results referenced to a Pd/C catalyst. A number of exchanged electrons close to eight was obtained for PdAuNi/C3-step and PdAuNi/CSBIPA (7.4 and 7.1, respectively), while the others, PdAuNi/CSBEG and Pd/CSBIPA, presented lower values, 2.8 and 1.2, respectively. A direct borohydride-peroxide fuel cell employing PdAuNi/CSBIPA catalyst in the anode attained a power density of 47.5 mW cm−<sup>2</sup> at room temperature, while the elevation of temperature to 75 ◦C led to an approximately four-fold increase in power density to 175 mW cm−2. Trimetallic catalysts prepared via this synthesis route have significant potential for future development.

**Keywords:** palladium; trimetallic catalysts; nanoparticle; borohydride oxidation; direct borohydride peroxide fuel cell; kinetic parameters

#### **1. Introduction**

Fuel cells (FCs) are electrochemical energy conversion devices with superior performance compared to conventional internal combustion engines, as they are not limited by the Carnot cycle [1]. Additionally, FCs are more environmentally friendly than conventional energy generation processes since they generally do not generate carbon dioxide (with few exceptions, such as direct alcohol/formic acid FCs) and do not rely on the consumption of fossil resources. Consequently, FC research and development has grown linearly with the worldwide energy demand and is projected to increase as FCs replace conventional heat engines in various applications [2]. However, many challenges need to be overcome if mass uptake of fuel cells is to be realized, including issues of safety, fuel source, and ensuring high efficiencies at low operating temperatures.

Low-temperature FCs targeted for use in portable applications and, particularly, for use in the transportation area have recently undergone rapid development [3]. They are

**Citation:** ElSheikh, A.M.A.; Backovi´c, G.; Oliveira, R.C.P.; Sequeira, C.A.C.; McGregor, J.; Šljuki´c, B.; Santos, D.M.F. Carbon-Supported Trimetallic Catalysts (PdAuNi/C) for Borohydride Oxidation Reaction. *Nanomaterials* **2021**, *11*, 1441. https:// doi.org/10.3390/nano11061441

Academic Editor: Stefano Agnoli

Received: 4 May 2021 Accepted: 24 May 2021 Published: 29 May 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

compact, lightweight, and can attain high power densities. Polymer electrolyte membrane fuel cell (PEMFC), the most developed FC, most commonly comprises an H2-supplied anode and an O2-supplied cathode separated by a polymeric membrane. Nevertheless, employing H2 as fuel presents several challenges to widespread adoption, including safety, storage, and supply. Furthermore, H2-stream pretreatment is usually required in order to remove trace carbon monoxide, which poisons the electrode. To address these issues, H2/O2 PEMFCs can be replaced with direct liquid fuel cells (DLFCs). DLFCs can use organic and inorganic liquid fuels, which usually incorporate a supporting electrolyte in their composition [4].

Solutions of complex hydrides (e.g., LiBH4, NaBH4, or KBH4) are examples of hydrogencontaining compounds that can be used as fuels in DLFCs. Sodium borohydride (NaBH4) presents many advantages over other fuels. The open-circuit voltage (OCV) for the direct borohydride fuel cell (DBFC, 1.64 V) is higher than that of the direct methanol fuel cell (DMFC, 1.21 V) and the H2/O2 PEMFC (1.23 V). While the oxidation of H2 and methanol generates two and six electrons, respectively, per molecule, BH4 − direct oxidation generates eight electrons. Consequently, DBFC possesses specific energy over 50% higher than that of the DMFC [5]. Indig and Snyder [6] demonstrated in the 1960s that electricity could be produced via anodic oxidation of borohydride and O2 cathodic reduction, as shown in Equation (1) and Equation (2), respectively, to yield the overall fuel cell reaction (Equation (3)), where SHE stands for standard hydrogen electrode.

$$\text{BOR} \quad \text{NaBH}\_4 + 8\,\text{OH}^- \rightarrow \text{NaBO}\_2 + 6\,\text{H}\_2\text{O} + 8\,\text{e}^- \qquad \text{E}^0 = -1.24 \,\text{vs. SHE} \tag{1}$$

ORR 2 O2 +4H2O+8e<sup>−</sup> <sup>→</sup> 8 OH<sup>−</sup> E0 = 0.40 vs. SHE (2)

$$\text{Overall} \quad \text{NaBH}\_4 + 2\text{ O}\_2 \rightarrow \text{NaBO}\_2 + 2\text{ H}\_2\text{O} \qquad \quad \text{E}\_0 = 1.64\text{ V} \tag{3}$$

Many authors have discussed the advantages, disadvantages, and state-of-the-art in DBFCs, including postulating the replacement of O2 by hydrogen peroxide (H2O2) [7–10]. The use of H2O2 as an oxidant has become more popular in recent years, in particular as this results in an entirely liquid FC, thereby facilitating its use in applications where a compact design is required. When H2O2 is used as the oxidant, the catholyte consists of an acidic solution of H2O2 in order to avoid its decomposition into H2O and O2. Consequently, the DBFC in which the hydrogen peroxide reduction reaction (HPRR) takes place at the cathode and the BOR at the anode is known as the direct borohydride-peroxide fuel cell (DBPFC). It can generate electricity through Equations (1) and (4), with Equation (5) being the overall FC reaction.

$$\text{HPRR} \quad \text{H}\_2\text{O}\_2 + 2\text{e}^- + 2\text{H}^+ \rightarrow 2\text{ H}\_2\text{O} \qquad \quad \text{E}^0 = 1.78 \text{ V vs. SHE} \tag{4}$$

$$\text{Overall} \quad \text{NaBH}\_4 + 4\text{H}\_2\text{O}\_2 \rightarrow \text{NaBO}\_2 + 6\text{H}\_2\text{O} \qquad \quad \text{E}^0 = 3.01 \text{ V vs. SHE} \tag{5}$$

The majority of published research on DBFCs discusses the need for effective and low-cost anode and cathode materials in order to facilitate the commercialization of DBFCs. This is a result of the high operating costs of the DBFC, which is correlated with the performance, efficiency, and cost of the electrocatalysts [5,8–11]. In fact, anodic catalysts for DBFCs must have high catalytic activity for BOR and be inert towards BH4 − hydrolysis, a chemical side reaction where BH4 − is consumed, generating H2 and BO2 −. In addition, DBFC anodes need to exhibit chemical stability in a basic solution. Common anodic catalysts for DBFCs include noble metal catalysts such as Au [12–16], Pt [17–20], and their corresponding alloy materials [21–24], as well as some nonprecious metal catalysts, e.g., Ni [25,26], Zn [27], and Co [28–30]. In particular, Au has been extensively studied, as it was originally thought to be capable of inhibiting borohydride hydrolysis; however, it exhibits rather slow kinetics for borohydride oxidation [31,32]. Studies involving Au, Pd, and Pt bulk electrodes for BOR revealed that surface poisoning by reaction intermediates, including BHads or BH3, was significantly different for the three noble metals. Results have shown that this effect was more severe for Pt and also that the Au surface was the

least affected by poisoning [33]. Despite this observation, Pt-based catalysts typically demonstrate faster electrode kinetics in BOR [24,34,35]; however, limited resources hinder their application as DBFC anodic catalysts. Pd is a promising alternative to Pt, presenting similar chemical properties and reactivity. Pd is considerably more abundant in the Earth's crust than Pt (ca. fifty times) and hence presents fewer challenges regarding resource scarcity [36–38]. Therefore, Pd has been tested in FCs both as a cocatalyst with Pt and as a Pt-free catalyst [37,39–41]. Decreasing the required quantity of Pd by mixing it with non-noble metals in a well-designed catalytic structure capable of efficient borohydride oxidation can decrease the cost of membrane electrode assemblies (MEAs) and boost the commercialization of DBFCs. It was previously found that co-doping can improve the catalytic performance of noble metal catalysts [42–44]. For example, a Pd-Co/C nanocatalyst has been demonstrated as an electrocatalyst for formic acid oxidation and methanol oxidation [45,46].

Previous studies reported Pd nanocatalysts and Pd nanoalloys as anodic catalysts for BOR in DBFCs [47,48]. Furthermore, Simões et al. [49] analyzed how Pd, Au, and Pd-Au carbon-supported nanoparticles with different compositions performed during BOR in alkaline media. PdxAu1-x/C electrocatalyst exhibited an open circuit potential (OCP) lower than Pd and Au did. Merino-Jimenez et al. [50] evaluated a titanium-supported and carbon nanofiber-covered Pd-Ir alloy for BOR. The authors concluded that alloying Pd with Ir improves Pd electroactivity towards BOR. This enhancement was associated with the favoring of BOR (direct oxidation) and suppression of hydrogen evolution reaction (HER). Furthermore, Grimmer et al. [51] studied BOR at Pd/C by cyclic voltammetry, rotating disc electrode (RDE) measurements, chronoamperometry coupled to NMR spectrometry, and in situ Fourier transform infrared spectroscopy (FTIR). RDE results yielded n = 4 (at 0.4 V vs. reversible hydrogen electrode (RHE)) and n = 8 (at 0.8 V vs. RHE). Thus, at 0.8 V vs. RHE, BOR is 100% direct, while at 0.4 V, two different pathways are proposed to operate: (i) the total oxidation of BH4 − to BO2 − and two H2 molecules and (ii) a four-electron electrooxidation with the production of BH2 − species. The authors also suggested that at low potentials, Pd-BH2OH species block surface sites, while at higher potentials, this intermediate is fully oxidized (n = 8).  Recently, by screening selected metal combinations (e.g., Pd with Co, Au, Ti) over a wide range of compositions, bimetallic and trimetallic Pd-based materials with optimum compositions that exhibited good activity for BOR were identified [51–56]. According to multiple published reports, Ni presents several benefits when added as a cocatalyst to C-supported Pd. First, it is an Earth-abundant metal, and therefore its use tackles issues of resource scarcity and reduces cost. Secondly, it is oxyphilic and capable of generating the oxygen species necessary for the redox reaction. Finally, the presence of Ni can exert an electronic effect over Pd, altering and potentially enhancing the catalytic properties of the latter [57–59].

In this work, investigations on the oxidation of BH4 − in alkaline media at different working conditions are presented for a selection of trimetallic carbon-supported catalysts based on Pd (PdAuNi/C), prepared by a facile and scalable synthetic route. There is a consensus that adding a second metal as a cocatalyst to Pd is beneficial for its electrocatalytic performance. This is due to the electronic, geometric, and bifunctional effects that the second metal can provide. Previous works have considered multiplying the catalytic benefits by adding two metals to Pd instead of one [56,60–66]. It is hypothesized that adding two metals, such as Au and Ni, each of which is known to be a beneficial addition to Pd individually [67–69], would significantly enhance Pd electrocatalytic performance. A small number of previous studies have previously investigated PdAuNi electrocatalysts [67–72]. Au [12,73–77] and Ni [25,78–80], like Pd, have been proven to possess good catalytic activity towards BOR, which makes their incorporation in a catalyst additionally attractive. Specifically, in the case of Ni electrodes, the formation of β-NiOOH has been pointed out as essential for the oxidation of BH4 − [26]. Furthermore, to maximize the potential for

BOR, a high-surface inert, mesoporous, and conductive support material is necessary to anchor the trimetallic nanoparticles and prevent their coalescence. Vulcan carbon (XC-72R) is commonly applied for this application due to its high surface area, mesoporous structure, and affordable cost. It was therefore chosen to support the PdAuNi and Pd nanoparticles. There are various chemical and physical synthetic routes in the literature to prepare dispersed metal nanoparticles [81]. The synthesis protocol is thought to play a crucial role and contribute to the catalyst physicochemical and catalytic characteristics. Any chosen method should balance between fine-tuned catalytic properties and ease of preparation, leading potentially to facile mass production [82]. NaBH4 metal reduction is a very common means to prepare C-supported catalysts due to its easy procedure and quality-produced catalysts. Moreover, there is more than one NaBH4 protocol to prepare Pd-supported nanoparticles. This work aims to synthesize three carbon-based trimetallic catalysts based on different borohydride reduction protocols. The physicochemical and morphological properties of those materials were determined and compared. Their performance as electrocatalysts for BOR was analyzed and discussed considering the different synthesis routes employed and the final properties obtained for each material.

#### **2. Materials and Methods**

#### *2.1. Chemicals*

Metallic precursors (PdCl2, NiCl2, and AuCl3) to synthesize PdAuNi/C electrocatalysts were purchased from Sigma-Aldrich (St. Louis, MO, USA). Vulcan carbon (XC-72R), purchased from Cabot Corp (Boston, MA, USA), was used to support the synthesized metal nanoparticles. NaBH4 (SB, 96 wt.%), ethylene glycol (EG, 99.5 wt.%), 2-propanol (IPA, 99 wt.%), NaOH (85 wt.%), and ethanol (100%) were purchased from Sigma-Aldrich. For the electrochemical measurements, NaBH4 (98 wt.%, Scharlau, Barcelona, Spain) and H2O2 (30 vol.%, Carlo Erba, Barcelona, Spain) were used. All chemicals were used as received without further purification.

#### *2.2. Catalyst Preparation*

Three borohydride reduction protocols using (i) a NaBH4-ethylene glycol complex, (ii) a NaBH4-2-propanol complex, and (iii) a three-step route were deployed to synthesize carbon-supported PdAuNi catalysts denoted herein as PdAuNi/CSBEG, PdAuNi/CSBIPA, and PdAuNi/C3-step, respectively. The theoretical metal loading based on the quantity of metal salts used in the synthesis was 12 wt.% for PdAuNi/CSBIPA and 20 wt.% for both PdAuNi/CSBEG and PdAuNi/C3-step. Although electrocatalysts for laboratory studies usually load the carbon support with 20 to 40 wt.% of metal, the use of lower loadings is more representative of industrial catalyst loadings which are typically ~5 wt.% [83].

The SBEG synthesis protocol uses a NaBH4-ethylene glycol complex, following previous work [36,84,85]. Vulcan carbon (202 mg) was first dispersed in ethylene glycol (10 mL). The metallic precursor solution containing PdCl2 (28 mg mL−1), AuCl3 (55 mg mL−1), and NiCl2 (36.7 mg mL−1) was then suspended in ethylene glycol (10 mL). The two suspensions were subsequently mixed. Next, a solution of NaBH4 (20 mL) was slowly added to the metal and carbon mixture under magnetic stirring. The mixture was heated to 35 ◦C and kept at that temperature for 3.5 h. The mixture was then left to cool and washed copiously with deionized water until neutralization. Following this, it was dried in a vacuum oven at 80 ◦C for 2 h.

The second reduction protocol (SBIPA) applied NaBH4 following methods described in previous reports [86–89]. The Vulcan carbon and metal precursors were sonicated in a mixture of 2-propanol/water (50 vol.%). KBr was added to stabilize the metal nanoparticles. The mechanism of this is that the larger Br− ions substitute for Cl− ions in solution, stabilizing Pd particles and preventing further particle agglomeration [90]. Unlike an organic surfactant such as PVP, KBr can be washed off easily after synthesis from the particle surface. The KBr/metal atomic ratio was 1.5. The mixture was stirred for 10 min, followed by the addition of NaBH4 solution (0.5 M, 15 mL). Stirring then continued for

another 30 min. Following this, the sample was washed under vacuum filtration. Finally, the wet powder was dried at 80 ◦C in a vacuum oven overnight.

The third protocol to prepare PdAuNi/C (three-step) aimed to compare the stepwise reduction of metals on the carbon surface to the co-reduction procedure practiced in the first two protocols. Consecutive reduction of different metals can lead to the formation of core@shell structures, such as Ru@Pd/C [91], Au@Pd [92,93], and Cu@Pd [94]. The NaBH4- 2-propanol reduction mixture was once more used to prepare this catalyst. The procedure started with sonicating Vulcan carbon (202.4 mg) in a mixture of 2-propanol/water (50 vol.%). Then, NiCl2 was added and kept in the solution under stirring for 1 h. This was longer than the reduction time required for Au and Pd reduction time, as Ni is more oxyphilic and less reducible [95,96]. Subsequently, AuCl3 was added with stirring for 30 min. Finally, PdCl2 was added with stirring for 30 min. To illustrate the impact of adding both Au and Ni onto Pd, a monometallic C-supported Pd catalyst (Pd/CSBIPA) was synthesized using the SBIPA procedure.

#### *2.3. Physical Characterization*

The crystal structure of the prepared catalysts was investigated using X-ray diffraction (XRD) with a Bruker D2 Phaser operating using Cu Kα radiation at 30 kV, 10 mA, and 12◦ min−<sup>1</sup> scan rate. To evaluate the crystallite size, the Scherrer equation was applied using the Pd (111) peak details. To quantify the metallic species in each catalyst powder, inductively coupled plasma optical emission spectrometry (ICP-OES) was applied using a Spectro Ciros Vision spectrometer by Spectro Analytical Instruments Inc. (Kleve, Germany). The samples were first digested in HNO3 (2%), followed by 10-fold dilution in aquaregia (8%). The surface-weighted chemical composition of the catalysts was examined through energy-dispersive X-ray spectroscopy (EDX) attached to a JEOL JSM 6010LA scanning electron microscope (SEM) by JEOL Ltd. (Tokyo, Japan). Each catalyst surface was examined twice applying two different accelerating voltages: 10 kV and 20 kV [97] enabling studying the composition at two different depths. The catalyst morphology was examined by transmission electron microscopy (TEM) employing a Phillips C100 microscope (Hillsboro, OR, USA) operating at 100 kV applying LaB6 filament. The TEM samples were prepared by suspending 5 mg of each sample powder in 2 mL of ethanol. Then, the mixture was sonicated for 1 h. A spray atomizer was then used to take 10 μL of the suspension to paint the holey carbon-coated Cu grid. The grid was left to dry for 24 h. The surface chemistry of the prepared electrocatalysts was studied by X-ray photoelectron spectroscopy (XPS) employing a Thermo Scientific K-Alpha+ spectrometer (Waltham, MA, USA). An Al-X ray source (72 W) and 400-μm2 were applied. The pass energy to record the data was 150 eV for survey scans and 40 eV for high-resolution ones. The survey scan step size was 1 eV, but that of the high-resolution was 0.1 eV. Low-energy electrons and argon ions were used to neutralize the charge. CasaXPS from Casa Software Ltd. (Teignmouth, UK) was used to analyze the data and employed a Shirley-type background and Scofield cross-sections with an energy dependence of −0.6.

#### *2.4. Working Electrode Preparation*

Catalytic inks were prepared by ultrasonically dispersing 5 mg of each catalyst in 2 mL of ethanol and 25 μL of 5 wt.% Nafion suspension for 30 min. Subsequently, 5 μL of the prepared ink was deposited onto a glassy carbon electrode (GC, A = 0.0707 cm2, inactive for BOR [98]) and left to dry at room temperature for 1 h. Catalyst loading was 0.175 mg cm−<sup>2</sup> for all catalysts. The total metal loadings (calculated based on the ICP-OES results) were 0.021, 0.027, 0.039, and 0.015 mg cm−<sup>2</sup> for Pd/CSBIPA, PdAuNi/CSBEG, PdAuNi/C3-step and PdAuNi/CSBIPA, respectively. Pd loadings were 2.51, 2.41, 4.77, and 5.77 μg cm−<sup>2</sup> for Pd/CSBIPA, PdAuNi/CSBEG, PdAuNi/C3-step, and PdAuNi/CSBIPA, respectively.

#### *2.5. Electrochemical Evaluation*

Electrochemical characterization was carried out in a conventional three-electrode setup of 125 mL. An ALS 2325 bipotentiostat (ALS Co., Ltd, Japan) combined with an RRDE-3A apparatus was used for fundamental experiments. Pt served as a counter electrode, and a saturated calomel electrode (SCE) as a reference electrode. All potential values within this paper were converted and are presented relative to RHE.

BOR measurements were carried out using cyclic and linear scan voltammetry. A 2 M NaOH solution was employed as the supporting electrolyte for all fundamental experiments. Cyclic voltammetry (CV) measurements were carried out in the potential range from OCP to 1.2 V applying scan rates from 5 to 1000 mV s−1. Furthermore, the temperature effect was analyzed in the 25–65 ◦C temperature range. The BH4 − concentration influence was studied in the 0.01–0.12 M range. Studies on the effect of temperature and concentration were conducted at a scan rate of 50 mV s−1. RDE studies were carried out by linear scan voltammetry (LSV) at 10 mV s−<sup>1</sup> by applying different rotation rates (0–2400 rpm). Apart from the concentration study, all experiments were conducted in a 0.03 M NaBH4 in 2 M NaOH solution. Experiments were run at 25 ◦C and 0 rpm unless otherwise stated.

#### *2.6. Fuel Cell Testing*

A laboratory-scale direct borohydride peroxide fuel cell (DBPFC, V = 100 mL for both anodic and cathodic compartments) was assembled and tested. The two compartments were separated by a Nafion®117 (DuPont, Wilmington, DE, USA) cation-exchange membrane. A PAR 273A (Princeton Applied Research, Inc., Oak Ridge, TN, USA) potentiostat using PowerSuite software (PowerCV+ PowerSTEP) was employed for control of the experiments. The anolyte and catholyte were 1 M NaBH4 in 4 M NaOH and 5 M H2O2 in 1.5 M HCl solutions, respectively. The chosen compositions are proven to be optimal in previous studies [99,100]. PdAuNi/CSBIPA was employed as the anodic catalyst (1 cm2, 0.100 mg cm−<sup>2</sup> catalyst loading, 0.012 mg cm−<sup>2</sup> metal loading). The catalytic ink used for the anode was prepared as described in Section 2.4, with a GC tip used as substrate electrode. The cathode was a Pt mesh with a high surface area (Johnson Matthey, A = 50 cm2) to ensure that fuel cell performance was not limited by the cathodic process. Interelectrode distance was the lowest possible to minimize ohmic resistance. DBPFC performance was analyzed in the 25–65 ◦C temperature range. Cell polarization curves were recorded, and the corresponding power density curves were plotted.

#### **3. Results and Discussion**

#### *3.1. Physical Characterization Results*

Figure 1 shows the XRD patterns of Pd/C and the three trimetallic catalysts. All catalysts showed one broad peak at 25◦, typical of graphitic Vulcan carbon. For the monometallic Pd/C, the pattern showed four peaks that are characteristic of Pd (111), (200), (220), and (311) faces at 39.8◦, 46.5◦, 68.3◦, and 82.1◦, respectively (PDF#46-1043). However, for both trimetallic catalysts prepared by co-reduction (SBEG and SBIPA), there was a small shift in the peak positions to lower diffraction angles. This shift is ascribed to alloying among the three metals. Pd and Au, in particular, are well-known to exhibit remarkable alloying behavior [95,101–108]. Although Pd and Ni are known to form a solid solution for their bulk alloys, their potential for formulating a nanoalloy is lower [96,107–109]. It is not uncommon to see distinctive Ni(OH)2 peaks at 60.1◦, which is indicative of bulk Ni species segregation. In the present study, the shift in the two co-reduced trimetallic samples was suggestive of a nanoalloy formation among the metals. However, the appearance of Ni(OH)2 peaks at 33.4◦ and 60.1◦ in the case of PdAuNi/CSBEG implied there was some Ni segregation in the bulk of that catalyst. Additionally, these Ni(OH)2 peaks were also present in the three-step catalyst, although smaller than that of PdAuNi/CSBEG.

**Figure 1.** (**A**) XRD patterns of PdAuNi/CSBEG, PdAuNi/CSBIPA, Pd/CSBIPA, and PdAuNi/C3-step and (**B**) the enlarged (111) facet for the four catalysts.

In contrast to PdAuN/C3-step and PdAuNi/CSBEG, the catalyst synthesized via the SBIPA protocol did not show any individual Ni peaks. This indicated a higher potential of mixing of the three metals when the SBIPA protocol was used. KBr addition may also have contributed due to the replacement of the smaller Cl− anion with Br− during synthesis [110].

Unlike the SBEG and SBIPA catalysts, the XRD peaks of PdAuNi/C3-step were closer to pure Au (PDF#04-0784) than pure Pd (PDF#46-1043). This suggested the formation of Au@Pd (core@shell) structure as a consequence of the stepwise reduction process as previously reported [92,93]. Another important observation is that the XRD peak shift to lower diffraction angles was suggestive of the expansion of the Pd crystal lattice by the incorporation of the larger Au atoms into the Pd lattice. Ni is, theoretically, expected to contract the lattice, not to expand it. However, previous studies have found that adding Ni to Pd does not seem to displace the Pd diffraction peaks and therefore Ni does not necessarily contract the Pd lattice [59,111,112]. The peak shift in the case of PdAuNi/CSBEG and PdAuNi/CSBIPA is therefore consistent with lattice expansion and tensile strain that could upshift the *d*-band center and decrease the adsorbate-Pd bond strength [105].

The XRD Pd(111) peak of the four catalysts was analyzed to extract the peak broadening and diffraction angle values of each sample, and subsequently to determine their structural parameters: particle size, interplanar distance, and lattice constants (Table 1). The detailed analysis is explained in section "1. XRD calculation" of supplementary information (SI). While the interplanar distance and lattice constant were the same for Pd/CSBIPA, PdAuNi/CSBIPA, PdAuNi/CSBEG (2.26 and 3.92 Å, respectively), it has increased in the case of PdAuNi/C3-step to 2.33 and 4.03 Å, respectively. It is noteworthy that the smallest particle size and highest broadening (FWHM) were those of PdAuNi/CSBIPA, followed by the monometallic Pd/CSBIPA, PdAuNi/CSBEG, and, finally, PdAuNi/C3-step. Factors other than crystallite size can also contribute to peak broadening, e.g., the instrument employed and the lattice strain. However, the Scherrer equation (Equation (S3)) is known to provide a reliable approximation of the crystallite size. The large particle size of PdAuNi/C3-step could be attributed to its Au-like structure, which presents larger lattice and interplanar distances.

**Table 1.** Structural and morphological parameters of Pd/CSBIPA, PdAuNi/CSBIPA, PdAuNi/CSBEG, and PdAuNi/C3-step.

The bulk elemental composition of the synthesized catalysts was investigated by ICP-OES, as illustrated in Table 2. For Pd/CSBIPA, the identified Pd loading was 12 wt.%, equivalent to the theoretical value. For PdAuNi/CSBIPA, PdAuNi/CSBEG, and PdAuNi/C3-step, Pd loadings of 9 wt.%, 16 wt.%, and 22 wt.% were identified, respectively.

**Table 2.** ICP-OES metal composition of Pd/CSBIPA, PdAuNi/CSBIPA, PdAuNi/CSBEG, and PdAuNi/C3-step.

While ICP-OES determines the whole composition of the catalysts, the catalytically active sites are not dispersed throughout the bulk of the material. Instead, interaction with reactants takes place at surface sites. Therefore, the surface-weighted technique of EDX was used to gain insights into the composition of the catalysts in the catalytically relevant surface region. Figure 2 shows EDX metal elemental maps of PdAuNi/CSBEG.

**Figure 2.** Elemental maps of (**A**) Pd, (**B**) Au, (**C**) Ni, (**D**) C, and (**E**) O obtained by EDX spectroscopy of PdAuNi/CSBEG at 10 kV.

While the Pd (A) and Au (B) maps show similar and relatively even distribution of both metals, the Ni map (C) shows three areas that were more densely populated with Ni species, indicating that Ni was not homogeneously distributed at the surface. The regions of high Ni density correspond to regions of low density in the C map (D) and in the O map (E). This may suggest that some bulk segregation of Ni(OH)2 had occurred, as supported by XRD data where a Ni(OH)2 peak was observed, Figure 1.

Table 3 shows the quantitative composition data extracted from EDX in both weight and atomic percent for the three trimetallic catalysts investigated. This aimed to highlight the different surface metal segregation by applying two accelerating voltages: 10 kV and 20 kV, the latter providing greater surface penetration. Unlike Pd and Au, the atomic concentration of Ni at 10 kV was larger than that at 20 kV. This further confirms that Ni segregation to the catalyst surface occurred. EDX analyses revealed higher metal loading than either ICP-OES or the theoretical compositions, as the synthesis resulted in a heterogeneous composition throughout the carbon support, with metal preferentially located towards the surface, beneficial for catalysis.

**Table 3.** Quantitative EDX analysis of PdAuNi/CSBEG, PdAuNi/C3-step, and PdAuNi/CSBIPA at 10 kV and 20 kV.

Figure 3 shows the EDX maps of PdAuNi/CSBIPA. Unlike PdAuNi/CSBEG, the Au (A), Pd (B), and Ni (C) maps show that the three metals were co-located with high-density regions of one metal corresponding to high-density regions of the others. Areas of low metal density corresponded to density regions in the C map (D). This is anticipated, as at regions of low metal density, the bare C support was exposed at the surface. Although to a lesser extent than PdAuNi/CSBEG, the quantitative data for PdAuNi/CSBIPA (Table 3) indicated a slightly higher concentration of Ni at 10 kV (2.90 at.%) as compared to that at 20 kV (2.52 at.%), perhaps indicating some surface segregation of Ni for this material as well. Overall, PdAuNi/CSBIPA showed relatively homogeneous metal distribution throughout the surface region, indicative of good mixing characteristics of the three metals in this case. This was further supported by the absence of any distinctive Ni peaks in this catalyst XRD pattern. Similar to PdAuNi/CSBIPA, the EDX analysis of PdAuNi/C3-step revealed that Pd, Au, Ni, and C were co-located with each other (Figure S1), suggesting good mixing and little Ni segregation in this material.

Figure 4 shows representative TEM micrographs obtained for the Pd/CSBIPA, PdAuNi/CSBIPA, PdAuNi/CSBEG, and PdAuNi/C3-step electrocatalysts, with additional TEM images being included in SI (Figures S2–S5). Examining the micrographs, the presence of highly dispersed metal nanoparticles on the carbon surface could be inferred. The very small particles indicated metal nanoparticles, while the large grey particles (40–60 nm) indicated the carbon aggregates. However, some particle aggregation and agglomeration could also be seen. This could be due to the synthesis method and/or being a monometallic system, which can promote particle growth and agglomeration more than a bimetallic or trimetallic one [56]. Comparing the trimetallic PdAuNi/CSBIPA catalyst with the monometallic one prepared by applying the same protocol suggests that the latter explanation may apply in this case.

**Figure 3.** Elemental maps of (**A**) Pd, (**B**) Au, (**C**) Ni, and (**D**) C of PdAuNi/CSBIPA obtained by EDX spectroscopy at 10 kV.

**Figure 4.** TEM micrographs and corresponding particle size distribution histograms of (**A**) Pd/CSBIPA, (**B**) PdAuNi/CSBIPA, (**C**) PdAuNi/CSBEG, and (**D**) PdAuNi/C3-step.

PdAuNi/CSBIPA particles (1–6 nm, consistent with XRD analysis, Table 1) were very well dispersed, and less particle aggregation could be seen as compared to Pd/C. The reverse situation was noted for the PdAuNi/CSBEG, which showed significant particle aggregation. Above those few carbon particles, high metal particle aggregation could be seen. This could be promoted by the NaBH4-ethylene glycol reduction complex at 40 ◦C. Additionally, as discussed above with reference to the EDX map (Figure 3C), the high Ni content in this catalyst might contribute to this agglomeration, noting that Ni exists only in the form of Ni(OH)2 (Figure 5).

**Figure 5.** XPS spectra of Pd 3d in PdAuNi/CSBIPA, Pd/CSBIPA, PdAuNi/CSBEG, and PdAuNi/C3-step, showing Pd 3d3/2 and Pd 3d5/2 peaks.

The preparation method may also play a role in the promotion of agglomeration. For PdAuNi/CSBEG, the metal precursors were initially mixed in ethylene glycol, followed by the slow addition of NaBH4. Once the NaBH4 addition commenced, the orange/brown solution turned black instantly, alongside the formation of bubbles. PdAuNi particles had most likely been reduced and segregated before the carbon support was added. For PdAuNi/C3-step, on the other hand, Ni was initially reduced on the carbon surface (for 30 min), followed by Au (for 30 min), and finally Pd (for 30 min). As is clear from the XRD and EDX measurements, Ni segregation occurred once again, further verified by the observation of Ni(OH)2 peaks, albeit in lower intensity than for PdAuNi/CSBEG. Additionally, Au had most likely formed a core@shell structure with Pd, since the XRD pattern was more representative of Au than of Pd. Although to a lesser degree than PdAuNi/CSBEG, significant particle aggregation was observed, and particle size was the largest in comparison to the other catalysts. Following PdAuNi/C3-step, PdAuNi/CSBEG particle size was larger than that of both Pd/CSBIPA and PdAuNi/CSBIPA. The PdAuNi particle sizes obtained were slightly smaller than previously reported for trimetallic systems [68,69].

Figure 5 shows the XPS measured data for the Pd 3d peak positions of the four catalysts. The molar Pd:Au:Ni ratio of PdAuNi/CSBIPA equaled 64:17:19, while it was 5:1:94 and 58:6:36 for PdAuNi/CSBEG and PdAuNi/C3-step, respectively. This showed the abundant Pd presence on the surface of PdAuNi/C3-step and PdAuNi/CSBIPA samples, while PdAuNi/CSBEG contained mainly Ni (94 at.%) and only 6 at.% Pd and Au combined. Furthermore, it is noteworthy that the highest oxygen content (5.19 at.%) existed in PdAuNi/CSBEG, likely a consequence of the presence of significant Ni(OH)2 (2.19 at.%). The Pd 3d peak of Pd/CSBIPA (Table S1 in SI) could be deconvoluted into a high-energy band corresponding to Pd 3d3/2 (340.73 eV) and a low-energy band corresponding to Pd 3d5/2 (335.43 eV). Those bands were shifted approximately 0.40 eV to higher binding energy compared to pure Pd, due to the interaction with the C support. Moreover, the Pd 3d binding energies of PdAuNi/CSBEG and PdAuNi/CSBIPA samples were shifted ~0.5 eV and 0.15 eV, respectively, to lower binding energies. On the contrary, Pd 3d of PdAuNi/C3-step

was shifted 0.05 eV to binding energy higher than that of the Pd/CSBIPA sample. Pd 3d peak positions are shown in Figure 5. Ni 2p and Au 4f were also shifted in the three trimetallic samples compared to pure Au and Ni, the binding energies of which were 84 eV and 855.6 eV (Ni(OH)2), respectively. While the Ni peaks shifted towards higher binding energy compared to pure Ni, the reverse trend was observed for Au. The Ni 2p shift was estimated to be +0.3 eV, +0.7 eV, and +0.8 eV for PdAuNi/CSBIPA, PdAuNi/CSBEG, and PdAuNi/C3-step, respectively. The Au 4f shift was −0.3 eV, −0.4 eV, and −0.2 eV for PdAuNi/CSBIPA, PdAuNi/CSBEG, and PdAuNi/C3-step, respectively. The shift in the binding energy was indicative of a change in the adsorption behavior of the respective catalysts. The presence of Au and Ni potentially explains why PdO was not detected in the trimetallic sample, unlike in the monometallic one, which contained 20 wt.% of Pd as PdO. Adding both Au and Ni enhanced Pd air stability. The full XPS surveys are illustrated in Figure S6. The individual elemental (Pd 3d, Au 4f, and Ni 2p) XPS peaks are available in Table S1, which confirms that Pd and Au existed in the metallic state in the trimetallic samples, Pd existed in metallic and oxide form in the monometallic sample, and Ni predominantly existed as Ni(OH)2. The Ni 2p peak in the PdAuNi/CSBIPA sample contained some Ni metal alongside Ni(OH)2, unlike PdAuNi/CSBEG and PdAuNi/C3-step. This further exemplifies the good mixing of the three metals obtained by the SBIPA protocol, which was corroborated by the XRD and EDX data.

PdAuNi/CSBEG has furthermore achieved the highest shift of Pd 3d to the left, which was also the broadest from the four-sample Pd 3d peaks as reported for the addition of Ni to Pd [108]. Examining the Ni 2p peaks of the three trimetallic samples, two highbinding-energy satellite peaks were adjoined to the main peaks of Ni, which indicates Ni multi-electron excitations [59,96,111,112]. Additionally, in the case of PdAuNi/CSBEG and PdAuNi/C3-step samples, the single pattern of two Ni 2p peaks suggests the presence of only one oxide species, Ni(OH)2, in contrast to the findings of Ramulifho et al. [113]. In terms of co-reduction, Au is the easiest of the three metals to reduce, followed by Pd. On the contrary, Ni is the slowest to reduce [95,114]. Therefore, it may not be incorrect to assume that the surface of the trimetallic catalysts (especially PdAuNi/CSBEG) was richest in Ni and poor in Au, which was evident from the XPS and EDX measurements. On the other hand, the core would be highly populated with Au and lacking a substantial Ni proportion.

#### *3.2. Borohydride Oxidation Reaction Studies*

The catalytic performance of the three PdAuNi/C electrocatalysts towards the BOR was initially investigated by scanning comparative CVs in 2 M NaOH with 0.03 M NaBH4 and in pure 2 M NaOH. Along with evaluating the activity of the synthesized trimetallic catalysts, the response of monometallic Pd/C was also evaluated in order to better understand the role played by Au and Ni. The CVs of three catalysts as well as of monometallic Pd/C depicted in Figure 6A–C show the absence of any relevant anodic peaks in the BOR potential region in NaOH solution, implying that the current generated during CVs recorded in the presence of BH4 − originated from its oxidation. Based on the shape of the CVs and similar OCP values of ca. −0.12 V, it could be assumed that the studied catalysts promoted similar reaction mechanisms. Similar OCP values were reported in other recent studies on BOR at Pd-containing materials [53,54]. Within the scanned potential range, CVs of three different catalysts showed two oxidation peaks in the positive scan, while an additional sharp oxidation peak was evident in the reverse scan. The first oxidation peak appeared as a wide oxidation hump at ca. 0.29 V for the PdAuNi/CSBEG and PdAuNi/CSBIPA catalysts, while in the case of PdAuNi/C3-step, this peak was more pronounced and positioned at more positive potential (0.38 V). It is believed that this peak can be attributed to H2 oxidation, which originates from the borohydride hydrolysis reaction, and to the initial oxidation of adsorbed BH3OH− [54]. The second anodic peak at ~0.8 V for all examined samples can be considered to originate from electrooxidation of BH4 − as well as BH3OH− [48,54]. PdAuNi/CSBIPA gave the best catalytic response, reaching the peak current density of 46 mA cm−<sup>2</sup> at 50 mV s−1, a value considerably higher than those reached employing carbon-supported Pd-Cu [115], Pd-Au [116], and Au-Ni [117] alloy nanoparticles. Additionally, the oxidation peak on the cathodic scan in the potential window from 0.5 to 0.8 V for all three studied samples was most likely related to the oxidation of BH3OH− [118], which was formed during the anodic scan and stayed adsorbed on the oxidized electrode surface until it was reactivated by reduction of the surface oxides.

**Figure 6.** CVs (third cycle) in the absence (2 M NaOH) and presence (2 M NaOH + 0.03 M NaBH4) of BH4 − for (**A**) PdAuNi/CSBEG, (**B**) PdAuNi/CSBIPA, and (**C**) PdAuNi/C3-step electrocatalysts, and (**D**) comparative CVs of trimetallic and Pd/CSBIPA monometallic catalysts in the presence of BH4 −. CVs ran at 50 mV s−<sup>1</sup> and 25 ◦C.

The catalytic performance of the synthesized materials was further compared with the response obtained for monometallic Pd/CSBIPA. Figure 6D reveals the inferior catalytic activity of Pd/CSBIPA. Introducing Au and Ni into Pd provided more active sites where BH4 − adsorption could take place and as such, trimetallic catalysts were expected to exhibit higher activity towards BOR. Improved catalytic properties can be explained by the produced synergistic effect of combining Pd with Au, a metal that is relatively inert towards hydrolysis, and Ni, a 3d transition metal, and the resultant electronic effect. In terms of electronic interaction, as XPS results suggest, the addition of Ni may have influenced the atomic band structure of the other two metals. This can further lead to the reduced adsorption energy of BOR intermediates, thus preserving those active sites for BH4 − adsorption and further encouraging BOR activity [119].

The PdAuNi/CSBIPA highest catalytic activity most likely resulted from the absence of segregation of Ni. This enabled an improved degree of alloying in comparison with the other two studied catalysts, as confirmed by XRD results. Additionally, the enhanced catalytic activity of PdAuNi/CSBIPA could also be associated with the better dispersed multi-metallic particles of smaller average size, confirmed by TEM analysis and the respective EDX elemental maps. Smaller particle size and interparticle distance produced larger surface area and, therefore, were deemed beneficial for catalytic reactions. However, caution must be practiced as Antolini et al. [120] reviewed that decreasing the particle size below 5 nm and interparticle distance below 20 nm adversely impacts the reduction of oxygen, as well as reactions of ethanol and methanol oxidation on Pt surfaces. The situation would be worse for a highly aggregated particle system due to exceedingly smaller interparticle distance. That is a likely reason for the lower PdAuNi/CSBEG performance in the present study. The CVs shown in Figure 6 are also available in specific current density (mA mgmetal<sup>−</sup>1) in SI (Figure S7).

BOR kinetic parameters are summarized in Table 4 for each catalyst.

**Table 4.** Kinetic parameters for BOR at the four studied catalysts.

For all samples, an upward trend of current density with increasing polarization rate, as well as a small shift in the position of the characteristic BH4 − oxidation peak to more positive potentials were observed. Figure 7A illustrates this behavior for PdAuNi/CSBIPA as the best material. This allowed for the determination of the charge transfer coefficient (α) values using the expression describing the peak potential dependence on the logarithm of polarization rate (Ep vs. ln ν) for irreversible processes [12,121]. The values were found to be relatively similar for all studied catalysts (Table 4), with PdAuNi/CSBIPA showing the highest α value. High values indicate that BOR at examined electrocatalysts was an irreversible and diffusion-controlled process. The obtained values are comparable to the ones reported for catalysts containing Pd, Au, and Ni [12,25,47,79,100,121].

In the case of PdAuNi/CSBIPA (Figure 7B), a significant increase in anodic current densities when recording the CVs starting from 0.01 M to 0.12 M NaBH4 concentration was evident. The CVs revealed that the anodic current density value amounted to 8.5 mA cm−<sup>2</sup> for the lowest concentration (0.01 M), while a value nine times higher was achieved for the highest NaBH4 concentration. The observed trend corresponded to enhanced mass transfer, and this behavior agrees with previous reports employing Au-, Pd-, and/or Ni-based electrocatalysts [48,78,79,122,123]. In addition, the oxidation process started at lower potentials but reached its maximum at more positive potential values. The order of reaction (β) values was obtained using ln j vs. ln c plot slope and ranged from 0.91 for PdAuNi/CSBEG to 1.12 for PdAuNi/C3-step.

CVs obtained at elevated temperatures also exhibited higher current densities for all studied electrocatalysts. This behavior was expected, since higher temperatures lead simultaneously to a decrease in solution viscosity, resulting in a higher BH4 − diffusion coefficient, and faster electron transfer kinetics. By constructing the Arrhenius plots [121], depicted in the inset of Figure 7C, and applying the Arrhenius equation, the values of the apparent activation energy, Ea app, were estimated. PdAuNi/CSBIPA showed the lowest value of 16.7 kJ mol<sup>−</sup>1, while the highest value of this parameter was observed for PdAuNi/CSBEG (34 kJ mol−1). Similar values were reported for mono- and bimetallic noble metal or/and transition metal-containing materials. For instance, BOR at carbon-supported Pd catalyst (prepared using different synthesis methods and types of carbon support) [53,124] has been shown to proceed with activation energy ranging from as low as 10 kJ mol−<sup>1</sup> to 26 kJ mol<sup>−</sup>1. Moreover, Ea app of BOR at a commercially available Pt/C [22] catalyst was reported to have a value of 34 kJ mol<sup>−</sup>1, while alloying with transition metals (Co and Ni) resulted in lower values (25 and 20 kJ mol<sup>−</sup>1, respectively).

**Figure 7.** CVs of PdAuNi/CSBIPA at (**A**) different scan rates, ν, and the derived Ep vs. ln ν plots (inset), (**B**) different NaBH4 concentrations and the derived ln j vs. ln c plots (inset), (**C**) different temperatures and the derived Arrhenius plots (inset), and (**D**) LSVs of PdAuNi/CSBIPA at 10 mV s−<sup>1</sup> and different rotation rates and the derived Koutecky–Levich plot (inset). The electrolyte solution used was 0.03 M NaBH4 + 2 M NaOH, polarization rate was 50 mV s−1, with room temperature (25 ◦C), unless otherwise noted.

The reaction kinetics were additionally examined using RDE LSVs, and polarization curves were obtained; the corresponding j−<sup>1</sup> vs. ω−1/2 plot for the currents taken at 0.9 V (inset) is shown in Figure 7D. From the slope of j−<sup>1</sup> vs. ω−1/2 plots and using the Koutecky– Levich equation, n values were obtained [125]. The shift of half-wave potential with an increase of the electrode rotation speed confirmed the irreversibility of the reaction.

The n values for PdAuNi/C3-step and PdAuNi/CSBIPA electrocatalysts (7.4 and 7.1, respectively) were determined to nearly match the theoretical value of 8, while significantly lower values were obtained for the other two materials (2.8 and 1.2 for PdAuNi/CSBEG and Pd/CSBIPA, respectively). Although in terms of current densities, PdAuNi/C3-step showed significantly lower values than PdAuNi/CSBIPA, the highest n value could be explained by Au@Pd (core@shell) structure formation previously identified in the XRD analysis. Such a structure makes the behavior of the catalyst more Au-like due to the core position of those nanoparticles and as such, less active toward the BH4 − hydrolysis reaction [12,100,126]. The lower faradaic efficiency of PdAuNi/CSBEG and Pd/CSBIPA was most likely a consequence of the higher hydrolysis rates at those two electrocatalysts. Obtained n values for PdAuNi/CSBEG and Pd/CSBIPA are similar to the ones obtained for other Pd-containing materials reported in previous studies. For instance, Martins et al. [54] reported n values of 2 and 5.6 during BOR at Pd nanoparticles with two different types of biobased carbon supports. For other Pd-based catalysts, depending on NaBH4 concentration, n values between 2 and 6 [53,118] were obtained, indicating hydrolysis occurrence in parallel with the oxidation of BH4 −. Song et al. [127] recently reported high BOR current densities for a ternary CuPdNi electrode, but at high overpotentials and an n value of 4.9, far from those obtained for the two best catalysts in this work. Alloying Pd

with Au resulted in n ~6, as reported by Simões et al. [57], also lower than values than obtained herein for PdAuNi/CSBEG and Pd/CSBIPA. Figure 7 is also available in specific current density (Figure S8 of SI).

#### *3.3. Fuel Cell Testing*

Figure 8 (available in SI in specific current density, Figure S9) depicts the polarization and power density curves of a single cell with PdAuNi/CSBIPA anode operating at two different temperatures, 25 and 75 ◦C, along with anode and cathode potentials measured during the cell operation and experimentally measured cell voltage. The cathode potential measured in zero current regime (open-circuit conditions) at 25 ◦C was almost identical to the equilibrium potential of H2O2 reduction given in Equation (4) (E0 = 1.78 V), suggesting direct H2O2 reduction. As for the cell operating at elevated temperature (75 ◦C), this value was slightly less positive, as expected, implying H2O2 disproportionation was followed by O2 reduction to a certain extent [18,21]. On the other hand, the anode potential measured during the experiment was significantly higher than the equilibrium potential for BOR described by Equation (1), implying that the anodic process also involved side reactions, such as oxidation of H and BH3OH− generated during spontaneous hydrolysis of BH4 <sup>−</sup> [128]. Moreover, from Figure 8B,C, a small difference between cell voltage, Ecell, and the calculated potential difference of the half-reactions, Ec–Ea, could be observed, which revealed minor resistance of system components.

**Figure 8.** (**A**) Polarization and power density curves at 25 and 75 ◦C for a DBPFC using a PdAuNi/CSBIPA anode and a Pt cathode. The cathode and anode potentials, their potential difference, and Ecell are also represented at (**B**) 25 ◦C and (**C**) 75 ◦C.

Considering the pH values of electrolytes, the theoretical voltage value of such fuel cells was expected to be close to 3 V. However, the cell polarization curves show an OCV value of ~1.9 V at both temperatures. This fluctuation in OCV value was mainly triggered by the mixed potential occurrence on the anodic side and it agrees with the reports from the literature for DBPFCs studies performed under conditions similar to the ones reported herein [18,125,128]. After the initial voltage decay due to activation losses, cell voltage continued to decrease linearly with current density, while mass transport effects were not evident. The power density value reached its maximum of 47.5 mW cm−<sup>2</sup> at 0.6 V and 78 mA cm<sup>−</sup>2. As expected, at an elevated temperature of 75 ◦C, the kinetics of both fuel cell reactions (BOR and HPRR) was faster, and a significant improvement in cell performance was displayed. The maximum value of power density that was obtained at this temperature was almost four times higher than that at 25 ◦C, amounting to 175 mW cm−2. This value was exhibited at a cell voltage value of 0.7 V, while current density reached 247 mA cm<sup>−</sup>2. The comparison of the DBPFC results obtained herein with the ones from the literature is shown in Table 5.

*Nanomaterials* **2021** , *11*, 1441

 Ni@Pd (core@shell structure) supported on polyaniline (PANI).

 Pt supported on multiwalled carbon nanotubes (MWCNTs).

PdAuNi/CSBIPA exhibited the best performance among the presented electrocatalysts, even when compared with the typical (and expensive) electrocatalysts exclusively composed of noble metals (e.g., Pd [128], Au/C [129]). It shows that the alloying effect, especially between noble and low-cost metals, is one of the best ways to decrease the price of electrocatalysts while increasing their catalytic activity. In fact, the highest activity of PdAuNi/CSBIPA towards BOR, i.e., the most pronounced synergistic effect, might be due to the complete mixing of three metals by the SBIPA procedure, as indicated by XRD analysis. Furthermore, XRD and XPS analysis indicated the decrease of adsorbate-Pd bond strength that could lead to faster release and subsequent oxidation of the reaction intermediates. Thus, PdAuNi/CSBIPA was demonstrated to be 1.5 and 1.6 times more active (in terms of current density) for BOR than Ni@Pd/PANI [130] and PtNi/C [131] electrocatalysts, which reflects the advantage of using a trimetallic alloy, probably due to the mentioned synergic effect. Carbon-supported AuxNiyCuz trimetallic electrocatalysts [119] exhibited better results than PdAuNi/CSBIPA at high Au amounts (Au2NiCu). For the other compositions, PdAuNi/CSBIPA was demonstrated to be a more effective anode, showing that the affinity between the metals is crucial to promote a notable alloying effect.

#### **4. Conclusions**

PdAuNi carbon-supported trimetallic catalysts were successfully synthesized by three different routes. A PdAuNi/CSBIPA catalyst, synthesized via reduction using a NaBH4- 2-propanol complex, presented the best alloying effect, while the other two materials showed the presence of segregated Ni species, as confirmed by XRD, TEM, and XPS analyses. The results obtained from CV and LSV measurements revealed that the catalysts performed well towards BOR. The PdAuNi/CSBIPA catalyst exhibited the highest activity, evidenced by the high number of exchanged electrons amounting to 7.1, and the lowest apparent activation energy (ca. 16.7 kJ mol−1). Furthermore, DBPFC operating at two different temperatures (25 and 75 ◦C) employing a PdAuNi/CSBIPA anode showed a maximum power density of 47.5 mW cm−<sup>2</sup> at 25 ◦C, with a significant improvement with a temperature increase and peak power density of 175 mW cm−<sup>2</sup> at 75 ◦C. Considering the economic benefits of alloying and using support materials, and the results obtained herein, it can be concluded that PdAuNi/CSBIPA is an interesting candidate for the application in DBPFC as anode material.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/nano11061441/s1, Figure S1: (A) Pd, (B) Au, and (C) Ni elemental maps of PdAuNi/C3-step obtained by EDX spectroscopy at 20 kV, Figure S2: TEM images of Pd/CSBIPA electrocatalyst, Figure S3: TEM image of PdAuNi/CSBIPA electrocatalyst, Figure S4: TEM image of PdAuNi/CSBEG electrocatalyst, Figure S5: TEM images of PdAuNi/C3-step electrocatalyst, Figure S6: XPS survey spectra of Pd/CSBIPA, PdAuNi/CSBIPA, PdAuNi/CSBEG, and PdAuNi/C3-step, Figure S7: CVs (third cycle) in 2 M NaOH + 0.03 M NaBH4 for (A) PdAuNi/CSBEG, (B) PdAuNi/CSBIPA, and (C) PdAuNi/C3-step electrocatalysts and (D) comparative CVs of trimetallic and Pd/CSBIPA monometallic catalysts. CVs run at 50 mV s−<sup>1</sup> and 25 ◦C, Figure S8: CVs of PdAuNi/CSBIPA at (A) different scan rates, ν, and the derived Ep vs. ln ν plots (inset), (B) different NaBH4 concentrations and the derived ln j vs. ln c plots (inset), (C) different temperatures and the derived Arrhenius plots (inset), and (D) LSVs of PdAuNi/CSBIPA at 10 mV s−<sup>1</sup> and different rotation rates and the derived Koutecky–Levich plot (inset). The electrolyte solution used was 0.03 M NaBH4 + 2 M NaOH, polarization rate was 50 mV s<sup>−</sup>1, and room temperature (25 ◦C), unless otherwise noted, Figure S9: (A) Polarization and power density curves at 25 and 75 ◦C for a DBPFC using a PdAuNi/CSBIPA anode and a Pt cathode. The cathode and anode potentials, their potential difference, and Ecell are also represented at (B) 25 ◦C and (C) 75 ◦C., Table S1: XPS elemental peaks of Pd 3d, Au 4f, and Ni 2p in Pd/CSBIPA, PdAuNi/CSBIPA, PdAuNi/CSBEG, and PdAuNi/C3-step.

**Author Contributions:** Conceptualization, C.A.C.S., B.Š., J.M., and D.M.F.S.; formal analysis, A.M.A.E., G.B., and R.C.P.O.; investigation, A.M.A.E. and G.B.; writing—original draft preparation, A.M.A.E., G.B., R.C.P.O., and C.A.C.S.; writing—review and editing, J.M., B.Š., and D.M.F.S., visualization, A.M.A.E., R.C.P.O. and D.M.F.S.; supervision, J.M., B.Š., and D.M.F.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by a Newton-Mosharafa Scholarship NMJ8/15 (A. Elsheikh) and by the Portuguese Foundation for Science and Technology (FCT, Portugal) through a research grant within project UID/CTM/04540/2013 (G. Backovi´c), a research contract in the scope of programmatic funding UIDP/04540/2020 (D.M.F. Santos), a Ph.D. grant SFRH/BD/137470/2018 (R.C.P. Oliveira), and contract no. IST-ID/156/2018 (B. Šljuki´c).

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interests. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**

## *Article* **Hydrogen Nanometrology in Advanced Carbon Nanomaterial Electrodes**

**Rui Lobo 1,2,\*, Noe Alvarez <sup>3</sup> and Vesselin Shanov <sup>4</sup>**

**Abstract:** A comparative experimental study between advanced carbon nanostructured electrodes, in similar hydrogen uptake/desorption conditions, is investigated making use of the recent molecular beam-thermal desorption spectrometry. This technique is used for monitoring hydrogen uptake and release from different carbon electrocatalysts: 3D-graphene, single-walled carbon nanotube networks, multi-walled carbon nanotube networks, and carbon nanotube thread. It allows an accurate determination of the hydrogen mass absorbed in electrodes made from these materials, with significant enhancement in the signal-to-noise ratio for trace hydrogen avoiding recourse to ultra-high vacuum procedures. The hydrogen mass spectra account for the enhanced surface capability for hydrogen adsorption in the different types of electrode in similar uptake conditions, and confirm their enhanced hydrogen storage capacity, pointing to a great potential of carbon nanotube threads in replacing the heavier metals or metal alloys as hydrogen storage media.

**Keywords:** hydrogen storage; nanometrology; carbon nanotubes; graphene; nanotubes thread; desorption spectrometry

#### **1. Introduction**

The development of a sustainable energy system is one of the most important challenges today and electrocatalytic hydrogen evolution is one of the most important ways to develop a sustainable energy system based on hydrogen technologies [1]. Together with suitable safe storage methods, this will pave the way for making this light fuel gas a sustainable energy source. Among electrochemical energy conversion advances, hydrogen energy is considered the most powerful candidate to alternate fossil energy due to its clean, renewable, and environmentally friendly properties and high energy density [2–4]. Despite the fact that in electrocatalytic water splitting the Pt cathode with near-zero overpotential is considered the most effective catalytic cathode, it has a high cost and platinum is a scarce resource [5]. With the purpose of seeking a low-cost and efficient catalytic cathode, many materials have been extensively explored by several authors.

Currently, many studies have been devoted to the use of hydrogen storage solids as active materials for application in fuel cells, and as a means of intermittency mitigation in energy supplies, with the aim of carbonic neutrality and renewable clean energy as urgent objectives. Activated carbons are ineffective in hydrogen storage systems because only a small fraction of pores in their pore-size distributions are small enough to interact strongly with hydrogen molecules at the gas phase. For small-sized systems, surface- or interface-related sites assume crucial importance and can change the overall solubility of hydrogen. Thus, for improving the kinetic characteristics of hydrogen storage in materials,

**Citation:** Lobo, R.; Alvarez, N.; Shanov, V. Hydrogen Nanometrology in Advanced Carbon Nanomaterial Electrodes. *Nanomaterials* **2021**, *11*, 1079. https://doi.org/10.3390/ nano11051079

Academic Editor: Diego Cazorla-Amorós

Received: 13 March 2021 Accepted: 20 April 2021 Published: 22 April 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

great attention has been paid to thermal desorption studies and calibration [6–8]. Thermal desorption spectrometry (TDS) assures a good accuracy in monitoring hydrogen absorbed in solid materials [9], and it is suitable for monitoring hydrogen evolution following the application of a thermal ramp, in contrast to the determination of static sorption isotherms. The use of a mass spectrometer allows evolved gas species to be identified, whereas the other methods are non-selective. The binding energy of adsorbed molecules varies with the nature of the adsorbate/surface materials, and desorption temperature gives information on the binding energy [10,11].

An additional goal consists of looking for low weight and high performance hydrogen active storage media made from carbon nanomaterial assemblies. Several carbon nanomaterial assembly-based electrodes are used in this work for comparing their relative performance. With this purpose in mind, the recent molecular-beam thermal desorption spectrometry (MB-TDS) is used in this work [12]. This technique is a variant of thermal desorption spectrometry which has been developed to detect the hydrogen release by the lowest amounts of solid samples, with several advantages [12–16].

#### **2. Materials and Methods**

A tuned desorption spectrum for hydrogen typically displays peaks caused by desorption with an origin on different adsorbed phases, and the total desorbed quantity of hydrogen is determined from the integral of the mass spectrometer signal, providing it can be adequately calibrated [12]:

$$\mathfrak{m}\_{\Gamma} = \int\_{\mathfrak{t}\_{0}}^{\mathfrak{t}\_{1}} \mathfrak{n}(\mathfrak{t}) \mathfrak{d}\mathfrak{t} \tag{1}$$

where nT represents the number of moles of desorbed hydrogen and n(t) the molar hydrogen desorption rate as a function of time.

Within the range of temperatures used in this work (below 400 K) hydrogen desorption from chemisorbed sites in the material is not detected and so one can simplify the nonequilibrium kinetics of the global desorption process by the following three elementary steps:

$$\ln\left(\frac{T\_{\rm m}^2}{\beta}\right) \approx \frac{E\_a^{\rm dif}}{k\_B T \left(T\_{\rm m}\right)}\tag{2}$$

where *Ea dif* is the diffusion activation energy, *kB* the Boltzmann constant, and *β* the heating rate.

The activation energy of desorption *Ed* can then be determined from the linear dependence of ln(*β*/*Tm*2) on 1/*Tm* and from these considerations, it is then possible to confirm or not, if desorption is a diffusion-controlled process [17,18].

Pumping out the heated oven (with the sample inside), and considering that desorption activation energy, *Ed* it is possible to show that desorption activation energy can be obtained from the experimental MB-TDS desorption curves making use of the equation [19]:

$$\ln\left(\frac{\beta}{T\_m^2}\right) = -\frac{E\_d}{RT\_m} + \ln\frac{RB}{E\_d} \tag{3}$$

where *B* stands for the Arrhenius pre-exponential factor, and *R* is the universal gas constant.

Molecular-beam thermal desorption mass spectrometry is applied to the determination of electrochemical hydrogen uptake and release by the samples. The MB-TDS apparatus has already been described elsewhere [12] and is merely schematically displayed in Figure 1, where a composite molecular beam of known intensity is produced from the degassing solid sample at a certain temperature inside the oven.

**Figure 1.** Top view of the molecular-beam thermal desorption spectrometry (MB-TDS) configuration.

By tuning the mass spectrometer to the hydrogen gas, one can monitor its evolution in time, thus measuring the amount of hydrogen desorbed from the sample. The effusion beam can be geometrically defined as well as its fraction detected by the quadrupole mass spectrometer (QMS) located in the forward direction (Figure 1).

Values of the order of 1 ◦C min−<sup>1</sup> have typically been used for the heating rate of each sample. The hydrogen partial pressure background variation can be described by a temporal decay [12,13].

By subtracting the residual hydrogen gas background (measured without the beam) from the total amount of hydrogen impinging in the quadrupole spectrometer, one can compute the real amount of hydrogen coming from the sample [12,13]. The ratio of the hydrogen background partial pressure variations, for the situations of heating-on and heating-off, lead to the simple following expression [12]:

$$\frac{N\_0 + N\_b}{N\_0} = 1 + \frac{A\_b}{A\_0} \tag{4}$$

*N* and *A* stand, respectively, for the number of hydrogen molecules and the area under the desorption curve, with the subscripts "0" and "*b*" assigned respectively to the situations "without the beam" and "with the beam".

Before introducing a sample in the oven of the MB-TDS apparatus, it needs to be submitted to hydrogen uptake, and this is done by electrochemical hydrogen charging. This procedure requires a previously experimental preparation of cathodes, which is summarized here for different carbon nanomaterials.

(i) Graphene

Graphene structures have received considerable interest due to their outstanding properties such as interconnected porous structures, enormous specific surface area, good mechanical stability, and flexibility to tailor their surface chemistry [20–22].

A 3D-graphene in the form of a pellet, possesses a monolithic structure with high electrical conductivity (148 S cm−1), well-controlled mesopore size (~2 nm), and good electromechanical properties. Details about the synthesis and characterization of 3D graphene have been published by our group elsewhere [23–26]. In brief, 3D graphene pellet is synthesized by chemical vapor deposition (CVD), using nickel powder as a catalyst, and it can be further processed into a graphene paper by pressing. It possesses electrical conductivity of above 1100 S cm−<sup>1</sup> and exhibits breaking stress of about 20 MPa.

Since Ni is catalytic, the etching with HCl was active for one hour, which is believed to be time enough to assure a complete depletion of Ni contamination. There was no determination of the Ni that remained.

The morphology of graphene with high surface roughness could contribute to a good adhesion with any substrate of practical interest due to the existence of van der Waals forces. Figure 2a displays a scanning electron microscopy (SEM) image of the 3D graphene pellet and Figure 2b shows Raman spectra of the same sample conducted at different spots.

**Figure 2.** (**a**) Scanning electron microscope (SEM) of 3D graphene pellet; (**b**) Raman spectroscopy of 3D graphene pellet conducted at altered spots on the sample, revealing a different number of graphene layers within the probed flakes.

After etching the Ni catalyst with HCl acid, the graphene pellet obtained was transferred onto a Kapton (polyimide) film and dried at 40 ◦C. A good adhesion between graphene and Kapton substrate was created due to van der Waals forces and the unique morphology of the 3D graphene. The graphene pellet with the dimension 5 mm × 5 mm was connected to a silver wire using silver conductive epoxy, which was then coated with non-conductive epoxy. After drying overnight at room temperature, the working electrode was ready for use.

#### (ii) Single-walled carbon nanotube networks