Al and Al-based composites with LiBH4 are of interest. Founded on first-principles calculation, Siegel et al. (2007) predicted that LiBH4 would react with Al to generate AlB2, LiH, and H2 at 280◦C under a hydrogen pressure of 1 bar. For the LiAlH4-LiBH4 system, Mao et al. (2009) found that the addition of TiF3 decreased the onset temperatures of H2 release by 64 and 150◦C compared with the undoped system. The decomposition enthalpy values of LiBH4 also reduced to 60.4 kJ/mol. He et al. (2019) studied the dehydrogenation performance of LiBH4/LiAlH4 composite, found that 8.7 wt.% of hydrogen was released at 500◦C, and defined a "Li-Al-B-H" compound. Soru et al. (2014) focused on the phase structural transformation of the LiAlH4 + LiBH4 system, which can produce 6.8 wt.% of hydrogen. According to a study by Carrillo-Bucio et al. (2017), a surface-oxidized 2LiBH4 + Al composite did not release hydrogen until heated to 400◦C under a 3 bar initial backpressure. With the catalytic effect of TiF3, the mixture obtained 9.3 wt.% of hydrogen release as compared to the 5.8 wt.% of the undoped mixture. In research by Zhang et al. (2018), a combined mixture of MgH2, LiBH4, and LiAlH4 showed superior performance, starting to release hydrogen at 280◦C and maintaining reversibility. In our previous studies (Li et al., 2012), the dehydrogenation temperatures of 2LiBH4 + Li3AlH6 doped with titanium were decreased by 80 and 50◦C, respectively, vs. the undoped system. From the previous studies, it can be summarized that Al can enhance the hydrogen storage performance of LiBH4 to some extent. However, the dehydrogenation behaviors of different Al source composite systems have not been systematically studied. In this work, three Al-based LiBH4 composite systems, 2LiBH4-Al, 2LiBH4-LiAlH4, and 2LiBH4-Li3AlH6 were prepared, and the hydrogen storage performance was investigated.

## MATERIALS AND METHODS

**Table 1** shows the raw materials used in this study. All powders were carefully stored in a glove box (MIKROUNA), in which

TABLE 1 | Raw materials used in the study.

noble gas was loaded and the oxygen content and water vapor content were kept below 1 ppm. A planetary mill (QM-3SP2) was utilized to prepare the composites.

The dehydrogenation properties of the materials were tested with a Sievert-type device. The changes in temperature and pressure over time were recorded. The ideal gas state equation was utilized to calculate the dehydrogenation capacity. Thermal analysis was performed using differential scanning calorimetry (DSC, Netzsch 449C Jupiter) combined with thermogravimetric analysis (TG, QMS 403C). To protect the samples from oxidation, high purity argon gas was added at a flow rate of 50 mL/min. An empty aluminum crucible was used as a reference during analysis. The phase compositions of the samples were determined by X-ray powder diffraction (X'Pert-PRO, PANalytical); its scanning range (2θ) was 15◦ -80◦ . A specific sealed device was used to protect samples from being oxidized or damped during analysis. Fourier transform infrared spectroscopy (FTIR, Tensor 27) was employed to detect some amorphous substances with a scanning rate of 30 cm−1 /min and a resolution ratio of 0.5 cm−1 .

## RESULTS AND DISCUSSION

## Preparation of Li3AlH6 Powder

In this work, Li3AlH6 powder was synthesized in-situ by ball milling LiH and LiAlH4. Approximately 15 g mixed powder of LiH and LiAlH4 was milled each time with a molar ratio of 2:1, a ball to powder ratio of 25:1, and a milling speed of 300 rpm. To suppress temperature rise in the ball milling, every 30 min was set as an operation cycle, which contained a stop time of 6 min, periodically. After ball milling for 20 h, XRD patterns were obtained and examined to characterize the mixed powder. **Figure 1** shows the XRD results of pure LiH powder, LiAlH4 powder, and the LiH and LiAlH4 powder after ball milling for 20 h. No impurity phases were observed in any of the samples, which indicates that the samples were highly pure and did not oxidize. As shown in **Figure 1C**, a few Li3AlH6 diffraction peaks are present, while the rest are of LiAlH4 and LiH. This shows that the synthesis reaction was not completed. Due to long milling time, LiH peaks were much lower than before. The LiH content was lower, and an amorphous structure was formed.

Further ball milling was carried out since the synthesis reaction to Li3AlH6 was not completed. The milling samples were taken for XRD analysis after every 10 h. **Figure 2** shows the XRD results of the milled powders after 20, 30, 40, and 50 h. Strong double peaks at 22◦ and 23◦ are detected, which are the characteristic peaks of Li3AlH6. After ball milling for 30 h, most of the diffraction peaks are Li3AlH6; however, some LiAlH4 and

powder of LiH and LiAlH4 after ball milling for 20 h.

LiH peaks can still be found at 27◦ , 43◦ , and 46◦ . As the ball milling time increased, the LiH and LiAlH4 content decreased, which shows that the synthesis reaction proceeds as the milling time increases. Nevertheless, the diffraction intensity of Li3AlH6 decreased, which shows that the longer milling time, the more likely the production of an amorphous phase becomes.

FTIR analysis was used to investigate the synthesized powder. In pure LiAlH4 (**Figure 3A**), the bending modes around 1,780 and 1,610 cm−1 correspond to the A-H stretching band, as was previously shown by Chen et al. (2001). After 50 h ball milling, this A-H stretching band disappeared in the 2LilH-LiAlH4 mixtures (**Figure 3B**). A new A-H stretching band was generated around 1,380 cm−1 , which belongs to Li3AlH6. This indicates the completion of the synthesis reaction.

## Characterization of the Composite Samples

The 2LiBH4 + M (M = Al, LiAlH4, Li3AlH6) composite systems were prepared by ball milling, respectively. The ball-milling time was 1 h, with a mole ratio of 2:1. The as-milled mixtures are presented in **Figure 4**. In the 2LiBH4-Al composite, the narrow and sharp diffraction peaks are of Al, while the other diffraction peaks are of LiBH4. The intensity of the LiAlH4 peaks in the 2LiBH4-LiAlH4 composite are also strong, but both LiBH4 and Li3AlH6 have attenuated diffraction peaks in 2LiBH4-Li3AlH6. The absence of other peaks indicated that there were no side reactions that could generate impurities or by-products during ball milling.

FIGURE 5 | DSC/TG curves of (A) pure LiBH4, (B) 2LiBH4-Al, (C) 2LiBH4-LiAlH4, and (D) 2LiBH4-Li3AlH6 samples after ball milling for 1 h.

## Dehydrogenation Performances

The dehydrogenation performances of the samples were investigated. The four samples were heated to 550◦C at a heating rate of 5◦C/min in an argon atmosphere. The DSC/TG results are shown in **Figure 5**. In the pure LiBH4 sample (**Figure 5A**), the endothermic peak at 120◦C denotes that LiBH4 transformed from an orthorhombic to a hexagonal crystal system. The endothermic peak at 286◦C is attributed to the melting of LiBH4. According to the TG curve, LiBH4 began to release hydrogen and lose weight at 400◦C. There is a dehydrogenation peak at around 469◦C, and the rate of dehydrogenation slowed down after that. Both the transformation and the melting peak of LiBH4 for each composite sample are shown in **Figures 5B–D**. All samples have lower transformation and melting temperatures than pure LiBH4 (transformation temperature: Td < Tc < Tb < Ta; melting temperature: Tc < Td < Tb < Ta). In the as-milled 2LiBH4-Al sample (**Figure 5B**), the wide endothermic range around 445◦C represents the decomposition of LiBH4. The last peak at 529◦C is the decomposition of LiH. As shown in **Figure 5C**, peaks at 190 and 198◦C indicated the dehydrogenation of LiAlH4, which were verified in our previous study (Li et al., 2012). LiAlH4 decomposed into Li3AlH6 at 190◦C, and Li3AlH6 started to generate LiH, Al, and release H2 at 198◦C. LiBH4 and LiH started to decompose at 435 and 472◦C, respectively. **Figure 5D** shows that the decomposition peak of Li3AlH6 is at 198◦C, which is consistent with the 2LiBH4-LiAlH4 sample. The decomposition peak of LiBH4 (416◦C) is lower than that of the 2LiBH4- LiAlH4 (435◦C) or 2LiBH4-Al samples (445◦C). In addition, the decomposition peak of LiH, which is around 452◦C, is also lower than that of the other composite systems. These results showed that the LiBH4 became more unstable due to the addition of Li3AlH6 and that hydrogen was released at a lower temperature. The main reason for this is that the Al present in Li3AlH6 was active enough to stimulate the dehydrogenation reaction.

The temperature-programmed desorption (TPD) method was implemented to study the dehydrogenation performance of pure LiBH4, 2LiBH4-Al, 2LiBH4-LiAlH4, and 2LiBH4-Li3AlH6; the results are shown in **Figure 6**. The samples were heated to 400◦C and held for 5 h. The pure LiBH4 began to release H2 at around 400◦C. Its desorption rate slowed down when 5 wt.% of H2 had been released. The 2LiBH4-Li3AlH6 sample released 9.1 wt.% H2 within 150 min, which is the fastest reaction kinetics in this work. Its decomposition process includes two main steps, with the release of 3 and 6.1 wt.% H2 as the first and second steps, respectively. In the 2LiBH4-LiAlH4 sample, LiAlH4 began to decompose at ∼130◦C and released 3.9 wt.% H2. Subsequently,

2LiBH4-Li3AlH6 samples.

its desorption rate was retarded. This composite released a total of 8 wt.% H2. The 2LiBH4-Al composite had a slower desorption rate. Only 5.7 wt.% H2 was released after 6 h, which is much lower than the 8.6 wt.% predicted by theoretical capacity. Previous research (Friedrichs et al., 2009) has reported that Al is an effective catalyst to activate LiBH4, and this phenomenon can also be observed in this study. However, the Al element generated from 2LiBH4-Li3AlH6 is much more effective than for the other samples. Since aluminum can be easily oxidized, an oxide film can easily form on the surface of the Al powder, which can slow down the dehydrogenation kinetics. In contrast, the Al in 2LiBH4-Li3AlH6, decomposed from Li3AlH6 exhibits high purity and is non-oxidized, resulting in the superior reaction kinetics of LiBH4.

## Characterization of the Dehydrogenation Materials

The XRD patterns of the dehydrogenated samples are shown in **Figure 7**. In **Figure 7A**, prominent diffraction peaks of LiH and Al are present. However, the LiBH4 phase was not observed, as its diffraction intensity is relatively weaker. In **Figure 7B**, residual peaks of LiBH4 are observed, which demonstrates incomplete dehydrogenation. In **Figure 7C**, besides LiH and Al, the dehydrogenation products contain AlB2, which was reported as the reversible phase (Friedrichs et al., 2009). AlB2 can accelerate the formation of LiBH4 in the reverse reaction. The AlB2 content was higher in 2LiBH4-Li3AlH6 than in the other samples, which signifies higher reversibility of 2LiBH4-Li3AlH6. There is an unexpected peak near 50◦ both in the 2LiBH4-LiAlH4 and 2LiBH4-Li3AlH6 samples, which was not identified but was also detected in previous reports (Yang et al., 2007).

## CONCLUSIONS

In this work, the dehydrogenation performance of three different Al source composite systems, 2LiBH4-Al, 2LiBH4-LiAlH4, and 2LiBH4-Li3AlH6, was analyzed. Elemental Al was the raw material and was used without further purification in the 2LiBH4-Al composite, in contrast to the other two samples, where it was decomposed from LiAlH4 or Li3AlH6. Li3AlH6 powder was prepared from LiH and LiAlH4. It can be concluded that the optimum synthetic conditions for ball milling are 50 h with a 25:1 ball to powder ratio at 300 rpm. The results demonstrate that Al, LiAlH4, and Li3AlH6 had a stimulative effect on LiBH4, allowing dehydrogenation at a relatively lower temperature. Additionally, 2LiBH4-Li3AlH6 was shown to have the best performance, with the endothermic peak of LiBH4 at 416◦C, 53◦C lower than that of the pure LiBH4 sample (469◦C). The TPD results also verified the superior results of 2LiBH4- Li3AlH6, which showed the best kinetics performance among the composite samples. 2LiBH4-Li3AlH6 released 9.1 wt.% H2 in only 150 min, which is over 95% of its theoretical hydrogen storage capacity. Its dehydrogenation product, AlB2, was reported as a reversible phase by researchers (Friedrichs et al., 2009), which could promote the reverse reaction of producing LiBH4. Further studies are needed to research the reversibility of the 2LiBH4- Li3AlH6 composite.

## DATA AVAILABILITY STATEMENT

All datasets generated for this study are included in the article/supplementary material.

## AUTHOR CONTRIBUTIONS

LC and YL: conception and design of study. YL and JH: acquisition of data. YL and XX: analysis and/or interpretation of data. YL: drafting the manuscript. DZ and SW: revising the manuscript critically for important intellectual content. YL, SW, DZ, JH, XX, and LC: approval of the version of the manuscript to be published.

## FUNDING

This work was supported by the National Natural Science Foundation of China (51671173) and the Curriculum Reform Project (KCSZ 201809 and JGXM201912).

## REFERENCES

**Conflict of Interest:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2020 Li, Wu, Zhu, He, Xiao and Chen. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Two-Dimensional Transition Metal Oxide and Hydroxide-Based Hierarchical Architectures for Advanced Supercapacitor Materials

Meili Guan1 , Qiuwan Wang1 , Xuan Zhang1 , Jian Bao1 , Xuezhong Gong2 \* and Youwen Liu3 \*

*1 Institute for Energy Research, School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, China, 2 National Center of International Joint Research for Hybrid Materials Technology, National Base of International Science and Technology Cooperation on Hybrid Materials, College of Materials Science and Engineering, Institute of Hybrid Materials, Qingdao University, Qingdao, China, 3 State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, China*

#### Edited by:

*Tengfei Zhou, University of Wollongong, Australia*

### Reviewed by:

*Ye Liu, South-Central University for Nationalities, China Li Shuang, China Three Gorges University, China*

#### \*Correspondence:

*Xuezhong Gong<EMAIL_ADDRESS>Youwen Liu<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Nanoscience, a section of the journal Frontiers in Chemistry*

Received: *04 March 2020* Accepted: *14 April 2020* Published: *15 May 2020*

#### Citation:

*Guan M, Wang Q, Zhang X, Bao J, Gong X and Liu Y (2020) Two-Dimensional Transition Metal Oxide and Hydroxide-Based Hierarchical Architectures for Advanced Supercapacitor Materials. Front. Chem. 8:390. doi: 10.3389/fchem.2020.00390* The supercapacitor has been widely seen as one of the most promising emerging energy storage devices, by which electricity is converted from chemical energy and stored. Two-dimensional (2D) metal oxides/hydroxides (TMOs/TMHs) are revolutionizing the design of high-performance supercapacitors because of their high theoretical specific capacitance, abundance of electrochemically active sites, and feasibility for assembly in hierarchical structures by integrating with graphitic carbon, conductive polymers, and so on. The hierarchical structures achieved can not only overcome the limitations of using a single material but also bring new breakthroughs in performance. In this article, the research progress on 2D TMOs/TMHs and their use in hierarchical structures as supercapacitor materials are reviewed, including the evolution of supercapacitor materials, the configurations of hierarchical structures, the electrical properties regulated, and the existence of advantages and drawbacks. Finally, a perspective covering directions and challenges related to the development of supercapacitor materials is provided.

Keywords: two-dimensional nanosheets, transition metal oxide and hydroxide, hierarchical structure, integration, supercapacitor

## INTRODUCTION

Global warming and the rapid depletion of fossil fuels are driving the development of sustainable and renewable energy. At present, the renewable energy sources of wind, tidal, geothermal, and solar energy have been researched and utilized (Yuksel and Ozturk, 2017; Cranmer et al., 2018; Mo et al., 2018; Song et al., 2018). However, these energy sources are highly dependent on nature, which is variable and unpredictable. Hence, an energy storage system is essential for efficiently collecting this natural energy, and, meanwhile, allows the energy to be rapidly exported in the form of stable electric energy. In recent years, the supercapacitor is becoming popular as a kind of energy storage device that is not only efficient and practical but also convenient and environmental friendly (Lukatskaya et al., 2016; Salanne et al., 2016; Wang et al., 2017).

The supercapacitor is also known as a Faraday quasi capacitor or electrochemical capacitor. It stores charges through a reversible redox reaction at the interface between electrode materials and electrolyte, unlike a traditional capacitor, it offers higher specific capacity and energy density (Simon and Gogotsi, 2010; Kate et al., 2018). Compared with a secondary battery, the supercapacitor possesses overwhelming advantages in terms of a shorter charging time and a long cycle life and, higher power density (Staaf et al., 2014; Zhao and Sun, 2018). Therefore, the supercapacitor is a kind of energy storage device that is intermediate between a traditional capacitor and a secondary battery that can be regarded as a complementary device between a battery and a traditional capacitor and can meet the needs of human beings for new energy.

The structural configuration of a supercapacitor mainly consists of electrode material, a diaphragm, electrolyte, and collecting fluid (Muzaffar et al., 2019). The energy storage is mainly from the charge transfer process at the interface of electrode and electrolyte, so the electrode material is the key factor in the performance of the supercapacitor.  In particular, carbon material is the most widely used electrode material due to its diversity and abundance as a resource. The capacitance over a carbon-based supercapacitor electrode is achieved through pure electrosorption of the electrical double-layer; thus, the structure of the electrode material will not collapse with the insertion and withdrawn of electrolyte ions during the charging and discharging processes. This leads to superior cycle stability, extending to tens of thousands or even hundreds of thousands of cycles (Frackowiak and Beguin, 2001; Liu et al., 2014). However, the energy density of carbon-based supercapacitors is low due to the storage mechanism, which rarely meets the needs of practical application. In contrast, conducting polymer electrode materials achieve high charge density by a reversible redox reaction of elements doping/dedoping, thus realizing large electric energy storage (Snook et al., 2011). However, the conducting polymer skeleton is prone to expanding and shrinking in the charging– discharging process, leading to low cycle stability (González et al., 2016). In addition, most of the conducting polymer materials are dense structures with limited interfaces, which restricts the amount of electrode material that is able to fully contact with the electrolyte and results in relatively low power density.

It is noteworthy that TMOs/TMHs exhibit potential for the fabrication of supercapacitor electrodes due to their high theoretic capacity and ultrahigh power density (Yuan et al., 2014; Wang et al., 2015; Zhang H. et al., 2015; Nguyen and Montemor, 2019). The energy storage can be achieved by either electrosorption or reversible redox reactions (Lee et al., 2010). Normally the valance state would change accompany with the charging–discharging process. Hence, for comparison, TMOs/TMHs have exhibited higher powder density and stability than traditional carbon and conducting polymer materials. Although problems arise from their narrow working voltage window, low conductivity, and small reaction area, these can be solved by structural engineering and composition, such as by controlling dimensionality and morphology (Zhu et al., 2014; Najafpour et al., 2015; Xu et al., 2015; Yue and Liang, 2015; Yu et al., 2015).

In this review, we summarize the recent progress in 2D TMOs/TMHs as supercapacitor electrode material. Further, 2D transition metal oxide/hydroxide-based hierarchical structures are covered. The different technical strategies in design and synthesis for optimizing the conductivity, structural stability, surface physical, and chemical properties, and structural morphology of the hierarchical composites are also detailed. A supercapacitor electrode with high specific capacitance, good rate capability, and excellent cycle stability can be obtained by tuning the electrochemical properties of the electrode materials. Finally, a perspective covering directions and challenges related to the development of supercapacitor materials is provided.

## TWO-DIMENSIONAL TRANSITION METAL OXIDES/HYDROXIDES

Transition metal oxides/hydroxides (TMOs/TMHs) are the most representative active electrochemical pseudocapacitor materials and are widely known for their high theoretical capacitance, abundance in nature, and high energy density (Jiang et al., 2012). However, poor intrinsic conductivity and other shortcomings have limited their performance as supercapacitor materials. Hence, 2D ultrathin TMOs/TMHs have been widely studied and applied in supercapacitors due to their advantages of high specific surface area and improved planar electronic conductivity (Gao et al., 2014; Liu W. et al., 2017; Zhu et al., 2018). Most importantly, the atomic thickness shortens the ion diffusion path and reduces ion diffusion resistance.

So far, there are two main strategies, that is, "top-down" and "bottom-up" methods, for obtaining 2D TMOs/TMHs (Sun et al., 2017; Mei et al., 2018), as shown in **Table 1**. "Top-down" mainly refers to the chemical or mechanical exfoliation of layered bulk materials, such as by mechanical exfoliation and liquid-phase exfoliation (Coleman et al., 2011; Rui et al., 2013; Zhang Y. Z. et al., 2015; Peng et al., 2017; Zavabeti et al., 2017; Tao et al., 2019). It is reported that MoO3, MnO2, and RuO2 nanosheets, etc., can be made at a large scale by ultrasonic exfoliation in ethanol/water and can be applied to solid-state symmetrical supercapacitors (Dutta et al., 2019). A liquid-phase exfoliation method with the aid of lithium (Li) intercalation and deintercalation was developed for the preparation of quasi-layered VO2 ultrathin nanosheet (Liu et al., 2012). The underlying mechanism is the insertion of Li ions to break the chemical bonds of the layers. Subsequent replacement of Li ions by larger molecules and ultrasonic treatment can result in a dispersed VO2 ultrathin nanosheet. The "bottom-up" synthetic protocol

TABLE 1 | Summary and comparison between top-down and bottom-up strategies for the fabrication of 2D TMOs/TMHs and their pros and cons.

starts from appropriate design at the atomic or molecular level by the aid of technologies, such as self-assembly (Sun J. et al., 2014; Hu et al., 2017; Sun et al., 2017), chemical vapor deposition (Lee and Sung, 2012; Liu et al., 2015), directional connection, and topological chemical transformation of layered intermediates (Liu et al., 2016; Wen et al., 2017; Bao et al., 2018), to synthesize a 2D nanostructure. For instance, five atomic layer thickness Co(OH)2 ultrathin nanosheet was synthesized by the orientation connection method and was assembled into an asymmetric vertical all-solid-state flexible supercapacitor with excellent specific capacitance and cycle stability (Gao et al., 2014). For non-layered 2D materials, Qiu and Zheng, for the first time, successfully observed the in-situ transformation of metal oxides from 3D nanoparticles as intermediate products to 2D oxide nanoflakes by LBNL's in-situ liquid-phase transmission electron microscopy technology, revealing the new evolution strategy of 3D to 2D materials at the atomic level, and also paving the way to obtaining ultrathin nanostructures from non-layered materials (Yang et al., 2019).

## 2D TMOs/TMHs-BASED HIERARCHICAL ENGINEERING FOR SUPERCAPACITOR MATERIALS

Although great efforts have been put into the development of 2D TMOs/TMHs for supercapacitor electrodes, challenges still exist when using a single electrode material. The drawback lies in low actual capacitance, limited improvement of energy density, and the poor rate capability caused by low conductivity, which have restricted the performance of such supercapacitors in practical applications (Jiang et al., 2012; Mahmood et al., 2019). In order to overcome the limitations of single electrode materials, it is necessary to combine 2D TMOs/TMHs with other low-dimensional nanomaterials to construct hierarchical nanostructures, which can not only optimize the configuration to overcome the agglomeration of 2D nanosheets but also complement and enhance the performance of different electrode materials so as to realize effective improvement of the performance for the supercapacitor device (**Figure 1**).

## TMOs/TMHs HIERARCHICAL STRUCTURES

The engineering of hierarchical structures by hybridizing different TMOs/TMHs is regarded as an efficient strategy to combine the advantages of different materials (Wu et al., 2013; Dinh et al., 2014; Zheng et al., 2016, 2018; Ouyang et al., 2019). The function of this kind of hierarchical structure can be generally summarized as overcoming the drawbacks of the individual components and preventing agglomeration of nanosheets. Thus, capacitance performance is predominantly enhanced because of the expected synergistic effect. Hierarchical structures with different morphologies can be formed through self-assembly (Huang et al., 2014; Feng et al., 2018), layer stacking (Zhu et al., 2012), and heterostructure core-shell engineering (Li et al., 2014; Sun Z. et al., 2014; Ho and Lin, 2019), which also have significant effects on the energy density and stability of the supercapacitors.

A 3D V2O5 architecture was constructed by using ultrathin V2O5 nanosheets as building blocks with a thickness of 4 nm via a freeze-drying process (Zhu et al., 2013). Due to the benefits of

its porous and ultrathin nature, as well as the 3D interpenetrating structure, a high surface area and shortened diffusion length were expected, thus enabling such a supercapacitor electrode to exhibit high capacitance, high energy density, and excellent stability (**Figures 2A–C**). A flexible film-like supercapacitor was fabricated via a vacuum filtration method by integrating MnO2 and Ti3C2 nanoflakes as the electrode, as shown in **Figure 2D** (Liu Y. et al., 2017). With the aid of good solubility, similar twodimensional geometry, the high theoretical capacity of MnO2, and the good conductivity of Ti3C2, the composite electrode exhibited good capacitance performance with a high massspecific capacitance of 305 F/g at a current density of 1 A/g. The fabricated symmetrical flexible supercapacitor device exhibited a maximum energy density of 8.3 Wh/kg, and the power density could reach 2,376 W/kg (**Figures 2E,F**). Characterizations and deeper understanding of the interaction between the stacked hierarchical structure and electrochemical performance revealed that the layered stacking structure is more conducive to improve the electrochemical performance of the electrode compared with traditional capacitors.

Forming a heterostructure or core-shell structures has been adopted as an efficient strategy for offering a large surface area for more Faradaic reaction sites and high conductivity to accelerate the charge transfer and therefore to improve the electrochemical performances of nanocomposites. For instance, a ZnCo2O4-MnO2 heterostructure on Ni foam was shown to be an active electrode material with a porous nanostructure, providing a considerably large electroactive area, and exhibited ideal capacitive behavior, with a maximum Cs of 2,057 F/g at a current density of 1 A/g and cycling stability of 96.5% after 5,000 cycles (Kumbhar and Kim, 2018) (**Figures 2G–I**). Unique core-shell arrays of CoFe2O4@MnO2 on nickel foam were also studied as an electrode material for a supercapacitor. Compared to the individual CoFe2O4 and MnO2 nanosheets, the composite electrode exhibited much higher specific area capacitance of 3.59 F/cm2 (≈1,995 F/g) at a current density of 2 mA/cm2 and a smaller semicircle in EIS, indicating faster ion insertion/extraction during electrochemical reactions, which can all be attributed to the hierarchical core-shell nanostructure. The asymmetric supercapacitor assembled using a CoFe2O4@MnO2

electrode had maximum energy density and maximum power density of 37 Wh/kg and 4,800 W/kg, respectively, with long-term cycling stability (91.4% retention after 2,250 cycles) (Gao et al., 2017).

Besides, mixed dimensional hierarchical structures have also been reported. For instance, a hybrid of NiCo2O4-MoS2 consisting of 2D NiCo2O4 nanosheets and 1D MoS2 nanowires were grown on a 3D Ni foam network, forming mixeddimensional hierarchical structures, as shown in **Figures 3A–C**. During the growth process, NiCo2O4 nanosheets intertwined on the Ni surface, providing sites for MoS2 nanowires, while a higher concentration of S2− lead to thinner nanowires. These features enable a high specific surface area and a shortened diffusion path for ions and electrons, which lead to the enhancement of the electron/ion transfer rate in the electrodes (Wen et al., 2018). The assembled supercapacitor device delivered a maximum energy density of 18.4 Wh/kg and a power density of 1200.2 W/kg with excellent stability (specific capacitance retention of 98.2% after 8,000 cycles) (**Figures 3D,E**).

## TMOs/TMHs/CARBON-BASED HIERARCHICAL STRUCTURES

Graphene, described as the "famous star" among carbon materials (Gong et al., 2016), it is an excellent two-dimensional scaffold upon which to construct composite materials with 2D TMOs/TMHs because its large surface area and the huge amount of cross-linking of the conjugated π-bond structure endow it with superior conductivity (in-plane carrier mobility up to 200,000 cm2 v −1 s −1 ) and physical structure stability (high mechanical strength and flexibility) (Novoselov et al., 2012). The electronic conduction and mechanical stability of the hierarchical structure formed are improved by the synergistic effect of the two components (Stankovich et al., 2006). Hence, a supercapacitor with excellent performance is expected due to the improved electrochemical reaction rate and cycle stability.

At present, a large amount of progress has been made in the preparation of high-performance energy storage materials by combining TMOs/TMHs with graphene. The application

molecular sheets stacked in a randomly interstratified manner. (E) Rate capability of the hybrid MnO2/Ti3C2 electrode compared with neat MnO2 and Ti3C2 electrodes. (F) Performance comparison of the MnO2/Ti3C2 hybrid with other reported materials on a Ragone plot (Liu W. et al., 2017). (G) Schematic of the coating of ZnCo2O4 nanoflakes with MnO2 nanosheets. (H) FESEM images and (inset) TEM images of ZnCo2O4 nanoflakes coated with MnO2 nanosheets. (I) Variation in Cs with respect to current density and cycling performance at 15 A/g for electrodes (Gao et al., 2017).

of pseudocapacitive V2O5 nanosheets and graphene in allsolid-state flexible thin-film supercapacitors (ASSTFSs) has been reported (Bao et al., 2014). By rationally integrating the two components, electron transfer was accelerated, and diffusion paths were shortened, leading to strong electrochemical performance, with a capacitance of 11,718 µF/cm2 at 0.2 A/m2 , an energy density of 1.13 µW h/cm2 at a power density of 10.0 µW/cm2 , and excellent long-term cycling stability for 2,000 charge–discharge cycles (**Figure 4**). The superior electrochemical performance thoroughly demonstrated the merit of a hierarchical architecture combining pseudocapacitive V2O5 nanosheets and graphene.

The Xie, group for the first time, fabricated a flexible allsolid-state thin-film pseudocapacitor using β-Ni(OH)2/graphene hybrid nanosheets as electrode materials via a layer-by-layer method (Xie et al., 2013). The assembly process of the β-Ni(OH)2/graphene hybrid includes electrostatic interaction between GO and Ni2+, interlayer Ni2+ diffusion, and confined (NiOH)2 crystallization, as well as simultaneous reduction of graphene (**Figures 5A–E**). The combination of highly conductive graphene and pseudocapacitive β-Ni(OH)2 guarantees high specific capacitance and excellent stability for this novel energy storage material. The fabricated all-solid-state thin-film pseudocapacitor exhibits a high volumetric specific capacitance of 660.8 F/cm and good cycling ability, as well as excellent flexibility with negligible degradation after 200 bending cycles (**Figures 5F,G**). Meanwhile, the ultrathin configuration of the hybrid nanosheets endows superior mechanical properties for the as-fabricated device, which can be regarded as a feasible energy supply for the exploitation of flexible electronics.

Following the all-solid-state planar configuration, the Xie group developed a quasi-2D ultrathin MnO2/graphene hybrid for

of a galvanostatic charge–discharge curve of the as-fabricated ASSTFS at a current density of 2.5 A/m2 . (D) Long-term cycling stability investigation of the ASSTFS after repeated bending/extension for 500 cycles (Bao et al., 2014).

fabricating a novel and high-performance planar supercapacitor (Peng et al., 2013). To make the best of the designed planar structure, a vacuum filtration method is adopted to produce films with controllable thickness and transferability. These hybrid 2D δ-MnO2/graphene thin films can be transferred onto a range of substrates such as PET, quartz, glass, and silicon wafer. By filling with a gel electrolyte of PVA/H3PO4, an all-solid-state planar supercapacitor is fabricated (**Figures 6A,B**). Owing to the planar peculiarity of both graphene and δ-MnO2 nanosheets, more electrochemically active surfaces for absorption/desorption of electrolyte ions were introduced, and charge transport was accelerated at the hybridized interlayer during the charging and discharging processes (**Figures 6C–E**). The device exhibited high specific capacitances of 267 F/g at a current density of 0.2 A/g and 208 F/g at 10 A/g and excellent rate capability and cycling stability (capacitance retention of 92% after 7,000 cycles) (**Figures 6F,G**).

after repeated bending/extension deformation (Xie et al., 2013).

In addition to these pioneering and representative research studies, numerous reports on the integration of TMOs/TMHs with graphene to prepare high-performance electrode materials for supercapacitors have revealed the general functions of the hierarchical structures (Wang G. et al., 2012; Mahmood et al., 2019; Nguyen and Montemor, 2019). The incorporation of graphene improves the conductivity of the pseudocapacitor electrode material and the constraints of the low specific capacitance of the graphene as a double-layer capacitor electrode material, and the advantages of the two are superimposed to achieve a substantial improvement in the capacitance performance, which is a feasible strategy for improving the electrochemical performance of the TMO/TMH electrode material.

## TMOs/TMHs/CONDUCTING POLYMER HIERARCHICAL STRUCTURES

Conducting polymer has become an important electrode material for pseudocapacitors due to the advantages of large capacity, good conductivity, facile synthesis, and low cost

working electrodes. (C) The hybrid thin film was formed by stacking layers of chemically integrated quasi-2D δ-MnO2 nanosheets and graphene sheets. (D) TEM image of the 2D hybrid structure with δ-MnO2 nanosheets integrated on graphene surfaces. (E) Schematic description of the 2D planar ion transport favored within the 2D δ-MnO2/graphene hybrid structures. (F) Comparison of specific capacitance values for the supercapacitors based on hybrids and based on graphene. (G) Capacitance retention (blue) and Coulombic efficiency (red) of the planar supercapacitor device based on hybrids over 7,000 charge/discharge cycles (Peng et al., 2013).

(Kalaji et al., 1999). In the past few years, conducting polymers have attracted increasing attention due to their great potential in supercapacitors. However, the conductive polymer-based electrode still suffers the drawbacks of low stability and poor mechanical properties, which restrict its application in fabricating supercapacitor devices (Liu et al., 2019). In terms of countermeasures, an inorganic-organic composite combination can improve the mechanical properties, electrochemical properties, and stability. Organic-inorganic composite electrode materials with excellent performance can be prepared to improve the specific capacitance and cycle stability of the electrode materials of supercapacitors.

As a representative conducting polymer, polyaniline (PANI) has frequently been used to combine with TMOs/TMHs to fabricate hierarchical structural hybrids for supercapacitor materials. As reported earlier, MoO3/PANI nanocomposites have been synthesized by a simple in-situ synthesis method using molybdenum oxide precursor precipitation and aniline monomer as raw materials, as shown in **Figures 7A–C** (Zheng et al., 2011). The PANI polymer chains are assembled between oxide layers, which cause the nanocomposite to exhibit a flexible layered hierarchical structure. The electrical properties of metal oxides as active supercapacitor materials are greatly improved, which gives them excellent conductivity (**Figure 7D**).

prepared hybrid PANI/MoO3 nanosheets. (D) I–V characteristics of the hybrid nanosheet (Zheng et al., 2011). (E) SEM images of the PANI/NiO electrode. (F) Specific capacity of the PANI/NiO electrode in different current densities and (inset) EIS curves. (G) Capacitance retention and energy efficiency curves of the PANI/NiO electrode produced in the best condition (Sun et al., 2016).

The outstanding performance was inferred to derive from the robust bonding through in-situ polymerization. Hence, nanocomposites prepared by in-situ reaction at the molecular level have significantly improved specific capacity and cycle stability, making them very suitable for the electrochemical energy storage application.

A PANI-NiO composite on nickel foam that was used as a supercapacitor electrode and was fabricated via a binderfree in-situ approach showed high specific capacitance (2,565 F/g at a current density of 1 A/g) and excellent cycling stability (with a high retention of 100% for almost 5,000 cycles) (Sun et al., 2016). The flower-like hierarchical structures offer a structural benefit enabling high levels of redox (**Figures 7E–G**). Detailed studies revealed that the Ni foam was the current collector and Ni source for in-situ deposition of NiO, establishing a steadier bonding between collector and active material. PANI that was deposited directly onto the electrode supported the structure and prevented the functional space structure from being destroyed and collapsing during the charging–discharging process. The resultant impressive cycling stability shows considerable potential for commonly used energy storage devices with long service lives.

Post-functionalization of TMOs/TMHs with PANI through polymerization provides the hybrid core-shell or sandwich hierarchical configurations. For example, adsorption of benzene amine on MoO3 nanobelts and further polymerization lead to the formation of coaxial heterostructure nanobelts of MoO3/PANI, which exhibited lower electric resistance than MoO3 nanobelts and pure PANI (**Figures 8A–F**). The supercapacitors achieved high specific capacitances of 714 F/g at a scan rate of I mV/s and 632 F/g at a current density of 1 A/g (Jiang et al., 2014).

curves of the two comparative materials at a current density of 2 A/g. (E) EIS spectra comparison and (F) cycling performance at a scan rate of 50 mV/s of the two comparative materials (Jiang et al., 2014). (G) Schematic illustration for the synthesis process of 3D MoO3/PANI hybrid nanosheet network film. (H) CV curves of MoO3, PANI, and 3D MoO3/PANI hybrid nanosheet network films in the potential range from −0.6 to 1 V at a scanning rate of 50 mV/s. (I) Nyquist plots of the MoO3, PANI, and 3D MoO3/PANI hybrid nanosheet network films (Zhang et al., 2017).

In order to overcome the low stability of conducting polymers as electrode material, a porous structure is usually introduced to guard against swelling and shrinking. In the case of a MoO3/PANI hybrid (Zhang et al., 2017), 3D MoO3 networks are formed through a freeze-drying process, in which the ice acts as a self-sacrificial template, and the different nanosheets are connected by van der Waals and hydrogen bonds. The final 3D MoO3/PANI hybrid networks were achieved by further electropolymerization of PANI onto the MoO3 surface (**Figure 8G**). Electrochemical impedance characteristics (EIS) measurements demonstrated good electrical conductivity and ion diffusion behavior (**Figures 8H,I**). The intrinsic structural advantages and the synergetic interaction of the 3D networks make such a hierarchical configuration a promising structure for supercapacitor materials.

It can be seen from the above work that preparing inorganicorganic composite electrode materials by combining 2D TMOs/TMHs with conductive polymers can effectively improve the cycling, mechanical properties, and specific capacitance of the electrode, which can be used as an effective and feasible scheme for improving the electrochemical performance of the electrode of a supercapacitor.

## SUMMARY AND PERSPECTIVES

A 2D TMOs/TMHs-based hierarchical structure combines the advantages of planar conductivity and a large specific surface area, making it an ideal candidate for the assembly of high-performance supercapacitors. To this end, finding new configurations with a hierarchical structure will guide the fabrication of supercapacitor materials with furtherimproved electrochemical performances. Besides TMOs/TMHs hierarchical structures, the integration of TMOs/TMHs with graphene or conductive polymer to form hierarchical structures has directed the recent steps forward in supercapacitor materials. In TMOs/TMHs-based hierarchical structures, graphene and conductive polymer are the key components for constructing flexible supercapacitors. By optimizing the TMOs/TMHs nanosheet orientation and stacking, electrons/ion transport channels were created, and performance was enhanced in the hierarchical energy storage devices. In summary, hierarchical structures form the prospective blueprint for fabricating highperformance supercapacitors by regulating morphologies and electronic properties.

Although great efforts have been put into supercapacitor materials in terms of the development of novel materials, synthetic technology, and structure engineering, a few problems remain at present. For instance, in TMOs/TMHs/graphene

## REFERENCES

hierarchical structures, the weak interaction forces between capacitive TMOs/TMHs and graphene at the heterostructure interface have been an impediment to improving electrochemical performance. However, very recently, graphene was demonstrated to eliminate the boundary effect and enable electron behavior of the hybrid comparable with that of a single crystal without grain boundary. It is believed that this discovery may inspire fresh research into hierarchical structures using graphene as a component. Given the advantages of conductive polymer, rational design and synthesis of new types of conductive polymers with higher conductivity and stability are highly desirable, as this would pave the way for next-generation supercapacitors with superior flexibility and performance.

Besides assembling hierarchical structures for improvement of supercapacitor performance, attention could be paid to developing new 2D electroactive materials, such as the newly emerging 2D transition metal carbides/nitrides (MXenes), which possess extremely high intrinsic electronic/ionic conductivity. High intercalation capacitance (∼1,000 F cm−3 ) in aqueous electrolytes make these promising electrode materials for the future. Meanwhile, supercapacitor performance is also dependent on the voltage window, which is strictly limited by the electrolyte. Thus, a possible way forward is to increase the working voltage by using green electrolytes with a high electronic conductivity and voltage window, such as ionic liquid electrolytes.

## AUTHOR CONTRIBUTIONS

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

## FUNDING

This work was financially supported by the National Natural Science Foundation of China for Youths (Grant Nos. 21701057, 21601067, 21905147), the Jiangsu Postdoctoral Research Foundation (Grant No. 1701109B), the China Postdoctoral Science Foundation (Grant No. 2017M611708), and the Scientific Research Startup Foundation of Jiangsu University (Grant No. 16JDG063).

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**Conflict of Interest:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2020 Guan, Wang, Zhang, Bao, Gong and Liu. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Hierarchical Porous Molybdenum Carbide Based Nanomaterials for Electrocatalytic Hydrogen Production

Yan Liu1 , Juanjuan Huo1 , Jiaojiao Guo1 , Li Lu1 , Ziyan Shen1 , Weihua Chen2 , Chuntai Liu2 and Hao Liu1,3 \*

*1 Joint International Laboratory on Environmental and Energy Frontier Materials, School of Environmental and Chemical Engineering, Shanghai University, Shanghai, China, 2 Key Laboratory of Materials Processing and Mold (Zhengzhou University), Ministry of Education, Zhengzhou, China, 3 Centre for Clean Energy Technology, School of Mathematical and Physical Sciences, Faculty of Science, University of Technology Sydney, Sydney, NSW, Australia*

#### Edited by:

*Tengfei Zhou, University of Wollongong, Australia*

#### Reviewed by:

*Yao Zheng, University of Adelaide, Australia Jintao Zhang, Shandong University, China Bin Wang, China Academy of Engineering Physics, China*

#### \*Correspondence:

*Hao Liu<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Nanoscience, a section of the journal Frontiers in Chemistry*

Received: *21 March 2020* Accepted: *23 April 2020* Published: *19 May 2020*

#### Citation:

*Liu Y, Huo J, Guo J, Lu L, Shen Z, Chen W, Liu C and Liu H (2020) Hierarchical Porous Molybdenum Carbide Based Nanomaterials for Electrocatalytic Hydrogen Production. Front. Chem. 8:426. doi: 10.3389/fchem.2020.00426* The electrocatalytic hydrogen evolution reaction (HER) for the preparation of hydrogen fuel is a very promising technology to solve the shortage of hydrogen storage. However, in practical applications, HER catalysts with excellent performance and moderate price are very rare. Molybdenum carbide (MoxC) has attracted extensive attention due to its electronic structure and natural abundance. Here, a comprehensive review of the preparation and performance control of hierarchical porous molybdenum carbide (HP-MoxC) based catalysts is summarized. The methods for preparing hierarchical porous materials and the regulation of their HER performance are mainly described. Briefly, the HP-MoxC based catalysts were prepared by template method, morphology-conserved transformations method, and secondary conversion method of an organic-inorganic hybrid material. The intrinsic HER kinetics are enhanced by the introduction of a carbon-based support, heteroatom doping, and the construction of a heterostructure. Finally, the future development of HP-MoxC based catalysts is prospected in this review.

Keywords: hierarchical structure, electrocatalytic reaction, hydrogen production, porous structure, molybdenum carbide

## INTRODUCTION

Hydrogen is a green energy with high energy density and excellent combustion performance (Martinez et al., 2019; Yang et al., 2019). HER is a key reaction for the renewable production of hydrogen. However, the actual reaction process is inefficient. In order to increase the conversion efficiency of the reaction process and reduce the reaction overpotential, a certain amount of catalyst is usually used (Chen et al., 2018; Huo et al., 2019; Ling et al., 2019). The ideal electrocatalyst for the HER is platinum (Pt) or other precious metals, but its application is severely limited by low richness and high cost (Khaselev and Turner, 1998; Nong et al., 2018).

MoxC has a wide range of applications in the fields of energy storage and conversion, for example, hydrodesulf'urization, denitrification (Wang et al., 2007; Ma et al., 2018), methanol reforming, electrolyte, etc. (Gao et al., 2010; Lin et al., 2017b; Yang et al., 2020). Density functional theory (DFT) calculations of carbides show that the hybridization of metal d orbitals with carbon s and p orbitals causes wider d-band structure, showing a d-band structure similar to Pt (Zhao et al., 2019b). This makes MoxC a promising alternative to precious metal catalysts. Conventional MoxC based catalysts generally have no voids or low porosity, resulting in low active surface area and poor wettability. Designing hierarchical porous micro/nanostructures can solve these problems. The hierarchical porous material has multi-stage pore structure, which is micropores (<2 nm), mesopores (2–50 nm) and macropores (>50 nm) (Li et al., 2019b). The properties and functions of a material depend on the characteristics of its structure, such as pore size, shape, porosity, etc. (Ryoo, 2019). In general, the presence of micropores provides a large surface area, mesoporous, and macroporous structures are effective in improving electrolyte penetration and promoting ion diffusion. The structure of hierarchical porous materials is usually assembled from nanoscale units by van der Waals forces, ionic bonds, covalent bonds and hydrogen bonds. The preparation of HP-MoxC based catalysts prevents the agglomeration of the nanoparticles, greatly increasing the specific surface area of the material and exposing more active sites (Kim et al., 2020). Compared with other non-precious metal catalysts, HP-MoxC has a hierarchical porous structure on the macro scale and a

d-band structure similar to Pt on the micro scale, which makes it exhibiting unique advantages.

This review focuses on the preparation and performance of HP-MoxC based catalysts, including soft-hard template method, morphology-conserved transformations, secondary conversion of organic-inorganic hybrid materials to construct catalysts with specific morphology. By introducing other conductive carriers, heterogeneous doping and construct heterostructured hybrids to optimize the HER performance of HP-MoxC based catalysts. **Table 1** shows the performance parameters of each catalyst mentioned in this article. Finally, an overview of the future development of HP-MoxC based electrocatalysts is outlined.

## CONSTRUCTION OF HP-MoXC WITH SPECIAL MORPHOLOGY

HP-MoxC based catalysts with a special morphology have many excellent properties such as rapid mass transfer, ultra-high surface area, controlled pore size and nano-effects (Niu et al., 2019; Wang et al., 2019b). Therefore, it is becoming more and

η*10, overpotentials to drive the current densities of 10 mA cm*−2 *.*

η*onset, oneset overpotential.*

more important to construct various forms of nano-catalytic materials. The use of nanotechnology makes it possible to expose as many active sites as possible during electrocatalysis, thereby improving HER performance (Hou et al., 2019). However, how to control the structural size and shape of materials still poses great challenges in the current research process.

## Template Method to Construct HP-MoxC Based Catalysts

The template method is one of the effective methods for preparing hierarchical porous materials, and can effectively control the morphology, particle size, and structure during the preparation process (Huang et al., 2019b; Zhao et al., 2019a). It is mainly divided into hard template method and soft template method. The hard template has rigid structure and specific morphology, and its morphology is copied into the target material by nano-replication technology. The obtained product has good dispersibility, controllable pore size and has been widely used (Chen et al., 2019; Feng et al., 2019).

Due to the stability of the hard template structure, the precursors are often used as "microreactor" in the synthesis process (Liu et al., 2019). The colloidal crystal (Thompson et al., 2019) contains a large amount of monodisperse colloidal particles, which are uniformly arranged in three dimensions. Using colloidal crystals as sacrificial hard templates, ordered and monodisperse pores can be introduced into the material. Kou et al. (2018) prepared hierarchical porous molybdenum carbide nanocrystals (uf-Mo2C/CF) with efficient HER performance by using uniformly-sized SiO2 microspheres as confined template (**Figure 1A**). Average size of nanocrystals is <2 nm. This

FIGURE 1 | Schematic illustration of synthesis and structures of various HP-MoxC based catalysts: (A) Schematic synthesis route and TEM image of the uf-Mo2C/C. Reprinted with permission from Kou et al. (2018) with permission from WILEY-VCH. (B) Schematic synthesis route and SEM/TEM images of the MoC-Mo2C/PNCDs. Reprinted with permission from Lu et al. (2019) with permission from WILEY-VCH. (C) Schematic synthesis route and SEM image of the P-Mo2C@C NWs. Reprinted with permission from Shi et al. (2017) with permission from Royal Society of Chemistry. (D) Schematic synthesis route and HR-TEM image of the Mo2C/G. Reprinted with permission from Huang et al. (2019c) with permission from WILEY-VCH. (E) Schematic synthesis route and TEM image of the Ni/Mo2C-NCNFs. Reprinted with permission from Li et al. (2019b) with permission from WILEY-VCH. (F) Schematic synthesis route and TEM image of the Mo-Mo2C. Reprinted with permission from Dong et al. (2018) with permission from Royal Society of Chemistry.

3D hierarchical porous structure enables a large number of mass transfer channels, high density of active sites, and high electrical conductivity, thereby providing a highly efficient and stable catalytic performance. Soft template method has no fixed structure and morphology. Soft templating agent mainly forms an organic phase with a certain morphology through intermolecular or intramolecular interaction forces (Xue et al., 2019; Zhang et al., 2019). In the process of synthesis, soft templating agent interacts with the inorganic phase to form an organic-inorganic phase with a certain morphology, thereby achieving the purpose of directional synthesis of nanomaterials. Yuan et al. (2019) used the precursor spheres formed by F127 and resoled phenolic resin to limit the growth of molybdenum carbide, and obtained ultra-small Mo2C particles encapsulated in situ in mesoporous carbon spheres (Mo2C@MCS). The ultrasmall particle size exposes more active sites, and the presence of a carbon substrate greatly reduces the resistance of the catalyst, thereby exhibiting excellent electrocatalytic performance in an alkaline medium (Liu et al., 2018; Huang et al., 2019d). This study provides an effective strategy for the synthesis of a MoxC@C catalyst.

## Morphology-Conserved Transformations

Although the template method can effectively control the morphology and pore size of the catalyst, the synthesis and the removal process of the template cause a lot of waste of resources and increase in cost, which hinders its application. The morphology-conserved transformations method has been developed as a simple and effective synthesis route (Tan et al., 2012). This method usually consists of two steps: first, a hierarchical porous metal intermediate compound is constructed and then converted into a carbon material by specific method. Metal organic frame materials (MOF) have become one of the most promising candidate precursors for the preparation of hierarchical porous materials due to their advantages such as uniform and controllable pore structure and large specific surface area (Dhakshinamoorthy et al., 2019; Garzón-Tovar et al., 2019). Shi et al. (2016) synthesized a highly active and stable MoC encapsulated by graphitized carbon shell (nanoMoC@GS) electrocatalyst by in-situ carburization of Mobased MOF, achieving "atomic-level contact" between Mo and organic species. The rich organic matter in the MOF generates a porous conductive carbon shell layer, which improves the ion transmission to the medium. Wu et al. (2015) adopted the "MOFs-assisted synthesis strategy" synthesized Cu-based MOFs (NENU-5) with MoCx nano-octahedral hydrogen evolution electrocatalysts with excellent HER activity. Recently, they used an ion exchange method to convert MO2− 4 groups by employing an exchange reaction between a zinc-based imidazole MOF (ZIF-8) and a metal salt (Na2MO4, M=Mo or W) in an organic solvent (Lu et al., 2019) (**Figure 1B**). The MO2− 4 group replaces the Zn (imidazolate)2− 4 group in ZIF-8, thereby effectively dispersing and fixing the metal source uniformly in the ZIF-8 framework (MoxC/PNCDs). Due to the structural advantages of ZIF-8, the low boiling point of Zn and the controlled exchange of MO2− 4 , ultrafine carbide nanocrystals with a porous nitrogen-doped carbon dodecahedron were successfully obtained. And achieve effective control of the carbide phase and composition, which not only provides a more stable active site, but also promotes electron transport during the HER process.

## Secondary Conversion of Organic-Inorganic Hybrid Materials (SC-OI-M)

Currently, there are few simple and diverse synthesis methods for new MOF materials, which greatly limits their development and application. In addition, conventional MOFs only have micropores and lack transmission channels such as mesopores and macropores, which will greatly reduce their transmission efficiency in the catalytic process. SC-OI-M refers to the integration of two counterparts into a single structure at the nanoscale (Wang et al., 2018b; Li et al., 2019a). This nano-scale single structure provides a periodic organic-inorganic structure, which has a "barrier effect" between the Mo sources during the high-temperature carbonization process, which can effectively prevent and promote the formation of nanostructures during high-temperature sintering. The hybrid materials generate a large amount of reducing gases such as CHx, CO, and H2 under high temperature decomposition to achieve the in-situ conversion of molybdenum carbide. The structure, morphology and composition of the synthesized molybdenum carbide have excellent controllability. Liao et al. (2014) synthesized nanoporous Mo2C Nanowires (np-Mo2C NWs) by pyrolyzing MoOx/amine hybrid precursors with sub-nanometer periodic structure. This became the beginning of the "SC-OI-M" for the development of highly active hydrogen evolution catalysts. β-Mo2C has outstanding HER performance, but it often exhibits excessive hydrogen absorption capacity, which restricts the desorption step of hydrogen atoms in HER (i.e., Heyrovsky/Tafel) (Wan et al., 2014). Shi et al. (2017) used electrostatic assembly to prepare a three-component hybrid precursor of MoOxphytic acid-polyaniline. P-doped β-Mo2C composite nanowire electrocatalyst (P-Mo2C@C NWs) was obtained after high temperature carbonization (**Figure 1C**). On the nanometer scale, the uniform ultra-small β-Mo2C particles provide abundant exposed catalytically active sites, and the structure of the one-dimensional nanowire facilitates the radial conduction of electrons. The resulting graphitized carbon greatly improves the overall conductivity and stability of the catalyst. At the atomic scale, the introduction of P atoms effectively increases the electron cloud density of the β-Mo2C Fermi level, and introduces steric hindrance effects, effectively weakens the Mo-H bond, reduces the 1GH∗ of β-Mo2C. Experimental and theoretical calculations show that suitable P doping (2.9%) can effectively balance the Volmer and Heyrovsky/Tafel processes in the HER and optimize the intrinsic activity of β-Mo2C for hydrogen evolution. Based on the controllability of this method, the researcher also proposed a series of electronic and spatial structure control methods. The formation of heterostructures such as by Co atom doping (Lin et al., 2016a), MoC-Mo2C (Lin et al., 2016b), and Fe3C-Mo2C (Lin et al., 2017a), the fine control of the surface and structure of β-Mo2C was realized, and the electrocatalytic performance was optimized.

## REGULATE THE CATALYTIC PERFORMANCE OF HP-MoXC BASED CATALYSTS

The key to construct a high-activity catalyst is using the advantages of HP-MoxC catalyst with high electronic conductivity and large specific surface area, combined with the regulation of active sites. There are two basic principles in the regulation of catalyst performance namely increasing the active site and enhancing the intrinsic activity of the material. For the former, it can be achieved by porous structure; for the latter, it can be achieved by heteroatom doping and constructing heterostructure.

## Introducing Other Conductive Carriers

Enriched active sites of sufficient unsaturated Mo and C atoms promote intimate contact between the electrolyte and the electrode material, thereby enhancing catalytic performance (Wang et al., 2019a; Cui et al., 2020). The introduction of a suitable matrix to form a strong coupling toward the catalyst can improve the intrinsic catalytic activity of MoxC, and also increase the conductivity of the catalyst due to the synergistic effect in the hybrid nanostructure, and hance, finally harvest desired electrochemical performance (Amrute1 et al., 2019; Xiong et al., 2019; Zhao et al., 2019c).

Up to now, a series of carbon materials such as reduced graphene oxide and carbon nanotubes have been used for supporting molybdenum carbide particles because of their large surface area and excellent electronic conductivity (Huang et al., 2019c; Li et al., 2019d; Wang et al., 2019e; Yu et al., 2019). Pan et al. (2014) used glucose as GO stabilizer, carbon source and reducing agent to prepare Mo2C nanoparticles grown on reduced graphene oxide (Mo2C-RGO), which showed excellent HER electrocatalytic activity. Huang et al. (2019c) developed a new technology for microwave-assisted ultrafast preparation of high performance carbon supported molybdenum carbide catalysts (Mo2C/G) (**Figure 1D**). They impregnated the carbon support with ammonium molybdate solution and irradiated it with microwave to obtain a carbon-supported molybdenum carbide catalyst. The carbon carrier material acts as an absorbing medium to promote in-situ rapid heating of the material, and a carbon source for forming Mo2C. This method can be applied to a variety of large-scale production of carbon carriers, including graphene, carbon nanotubes, commercial carbon black, and carbon fiber, etc. The in situ formation of the carbon support ensures a tight interfacial contact between the active material and the conductive substrate, thereby promoting rapid charge transfer between the substrate and the active material (Zhang et al., 2018; Han et al., 2019; Hou et al., 2019). Finding a way to synthesize molybdenum carbide/carbon support composites in one step is very valuable (Hsieha et al., 2019; Wu et al., 2019). Cheng et al. (2018) synthesized a 1 nm-sized molybdenum carbide nanoparticle in a carbon (Mo2C@NC) nanomesh through hydrothermal treatment, using dicyandiamide as a carbon and nitrogen source, ammonium molybdate as a molybdenum source. During the hydrothermal process, the intermolecular hydrogen bonding is used to self-assemble into band structure to limit the growth of particles. This mesoporous ribbon nanoweb structure provides a high specific surface area and a rich active site, greatly reduced the energy barrier. This material has excellent HER/ORR performance compared to other materials. This work demonstrates a simple template-free strategy for the synthesis of highly efficient non-precious metal catalysts with large specific surface areas. It also shows the possibility of replacing platinum-based catalysts with molybdenum carbide materials from theoretical to experimental evidence.

## Doping

The performance of the catalyst can also be controlled by doping. Heteroatom doping can be combined with addition of conductive support to enhance the intrinsic activity of the materials besides increasing the active site of the catalysts. The doping of heteroatoms into the lattice of the catalyst can adjust the electron and surface structure of the material, thereby affecting the adsorption free energy of the reaction intermediate on the surface, and improving the catalytic efficiency (Jia et al., 2017; Guo et al., 2019; Li et al., 2019a). At present, non-metal atoms (N, S, P, B, etc.), transition metals (Fe, Co, Ni, Zn, etc.) are introduced to replace Mo/C atoms in MoxC. It can adjust the intrinsic electron configuration of molybdenum carbide and improve the conductivity (Li et al., 2019c; Wang et al., 2019c,f; Zhong et al., 2019).

Nitrogen-doped nano-carbon support plays an important role in improving electrocatalytic activity (Wu et al., 2018a; Lyu et al., 2019). Wu et al. (2016) synthesized ultrafine Mo2C nanoparticles embedded within bacterial cellulose-derived 3D Ndoped carbon nanofiber networks (Mo2C@N-CNFs). Theoretical calculations demonstrate that excellent HER activity results from a strong synergistic effect between Mo2C nanocatalyst and N-CNF. Transition metal dopants (Ni, Co, Fe, etc.) can improve catalytic performance by adjusting the electronic configuration, creating new active sites and activating surrounding sites (Wu et al., 2018b; Cao et al., 2019). Li et al. (2019b) synthesized Ni/Mo2C nitrogen-doped carbon nanofibers (Ni/Mo2C-NCNFs) by using the electrospinning method (**Figure 1E**). Synergistic effect between Ni and Mo2C nanoparticles, high conductivity, large electrochemical active surface area and effective N doping significantly promote HER and OER due to strong hydrogen binding energy on Mo2C and high conductivity of Ni. This work provides a facile and effective way to produce low cost and high performance dual functional electrocatalysts for efficient overall water splitting. Since a single heteroatom doping has been shown to improve the catalytic performance of carbon materials, researchers developed binary or multi-heteroatom doped carbon materials to adjust the d-orbitals and optimize the electronic structure (Du et al., 2019; Ling et al., 2019; Kou et al., 2020). Ang et al. (2016) reported the formation of layered Mo2C by carburizing of molybdenum oxide/phenol/thioacetamide hybrids, followed by solvent stripping of layered Mo2C to further form N/S co-doped molybdenum carbide nanosheets. The synthesized nanosheet has an ultrathin thickness (1 nm) and a large specific surface area (139 m2 g −1 ). The incorporated N and S effectively improve the wettability of the material. Wang et al. (2018a) synthesized NP-Mo2C by direct carbonization. Theoretical calculations show that the doping of heteroatoms into carbon promotes the transfer of electrons in the catalyst, and the heteroatoms may also act as catalytic active sites (Gao et al., 2019; Singh et al., 2019). Owing to the synergistic coupling, the double doping of the N, P heteroatoms in Mo2C can remarkably improve the intrinsic activity of each active site.

## Hierarchical Porous Molybdenum Carbide-Based Heterostructure

Heterostructured hybrids have shown superior electrochemical performance compared to the corresponding single components (Liang et al., 2019; Yao et al., 2019). Through the interface electron transfer in a heterostructure, a large number of interfaces between multiple components can induce optimization of the electronic configuration (Wang et al., 2019d; He et al., 2020). Therefore, constructing heterojunction is an effective way to optimize the free energy of hydrogen adsorption.

The engineering design of molybdenum carbide-based heterostructures provides a new perspective for electrocatalysis. For example, heterostructures of Mo-Mo2C (Dong et al., 2018), Mo2C-Mo2N (Chen et al., 2013), MoP@Mo2C (Huang et al., 2018), and (Mo2C)x-(WC)1−x-QDs (Huo et al., 2017). The synergistic effect of the interface portion exhibits superior HER performance over the single component molybdenum carbide. Dong et al. (2018) obtained a new Mo-rich molybdenum carbide-based electrocatalyst (Mo-Mo2C) by calcining Mo3O10(C6H8N)2•2H2O precursor (**Figure 1F**). The overpotential of the Mo-Mo2C hydrogen evolution reaction is only 67 mV, and the Tafel slope is as low as 55 mV/dec. Its excellent HER performance can be attributed to the improvement of the internal charge transfer ability of the catalyst.

Recently, Huang et al. (2019a) constructed a Mo2C/VC heterostructure. The CO2 decomposed from Mg and NaHCO3 reacts at high temperature to generate a three-dimensional carbon network. At the same time, the micro-scale precursor V2MoO8 is broken into two-phase materials Mo2C and VC that are not completely separated and embedded in the threedimensional carbon network. The three-dimensional conductive carbon network and the cross-linked structure fully provide electron transportability and structural stability. Mo2C has strong

## REFERENCES

hydrogen adsorption capacity, while VC has strong hydrogen desorption capacity, so single component Mo2C@C and VC@C exhibit poor hydrogen evolution reactivity, while Mo2C/VC@C shows rapid adsorption capacity and rapid desorption kinetics. Based on the combination of experiment and theory, this experiment proposes that the method of preparing a rich interface structure by phase separation is a way to efficiently prepare high performance catalyst. This approach can be extended to other highly efficient heterogeneous catalysts and different energy sources in the future.

## DISCUSSION

In summary, molybdenum carbide-based materials are an ideal HER material. In this review, recent developments in the structural design and electronic regulation of molybdenum-based catalysts are illustrated. Template method, morphology-conserved transformations method, and secondary conversion of organic-inorganic hybrid materials method are effective strategies for synthesizing various molybdenum carbide-based materials. By compounding with a conductive carrier, element doping and designing a heterojunction can achieve electronic optimization of HER kinetics, greatly improving catalyst activity and stability. In practical applications, material combinations and properties, flexible selection of synthesis methods and performance control methods can synergistically achieve highly efficient catalysts. However, the large-scale application of electrolyzed water for hydrogen evolution still has a long way to go. In combination with the rapid development of molybdenum carbide in electrocatalytic hydrogen evolution in recent years. Future research on molybdenum carbide catalysts may focus on the development of efficient new synthetic methods, the development of molybdenum carbide hydrogen evolution devices, mechanism research, standardized test and the mining and understanding of structure-activity relationships.

## AUTHOR CONTRIBUTIONS

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

macrofilm for foldable lithium-ion battery withstanding variable weather. Adv. Energy Mater. 8:1802349. doi: 10.1002/aenm.201802349

enabled by nitrogen-doped carbon from imidazolium-based ionic polymers. Energy Environ. Sci. 12, 418–426. doi: 10.1039/C8EE02727G

**Conflict of Interest:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2020 Liu, Huo, Guo, Lu, Shen, Chen, Liu and Liu. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## Hierarchical 0D−2D Co/Mo Selenides as Superior Bifunctional Electrocatalysts for Overall Water Splitting

Lu Xia1,2, Hao Song1 , Xingxing Li 1 , Xuming Zhang1 \*, Biao Gao1,3, Yang Zheng1 \*, Kaifu Huo1 and Paul K. Chu3

*1 The State Key Laboratory of Refractories and Metallurgy and Institute of Advanced Materials and Nanotechnology, Wuhan University of Science and Technology, Wuhan, China, 2 The College of Resources and Environment Engineering, Wuhan University of Science and Technology, Wuhan, China, 3 Departments of Physics, Materials Science and Engineering, and Biomedical Engineering, City University of Hong Kong, Hong Kong, China*

#### Edited by:

*Guanglin Xia, Fudan University, China*

#### Reviewed by:

*Shichun Mu, Wuhan University of Technology, China Fei Ma, Xi'an Jiaotong University, China*

\*Correspondence:

*Xuming Zhang<EMAIL_ADDRESS>Yang Zheng<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Nanoscience, a section of the journal Frontiers in Chemistry*

Received: *02 March 2020* Accepted: *14 April 2020* Published: *19 May 2020*

#### Citation:

*Xia L, Song H, Li X, Zhang X, Gao B, Zheng Y, Huo K and Chu PK (2020) Hierarchical 0D*−*2D Co/Mo Selenides as Superior Bifunctional Electrocatalysts for Overall Water Splitting. Front. Chem. 8:382. doi: 10.3389/fchem.2020.00382* Development of efficient electrocatalysts combining the features of low cost and high performance for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) still remains a critical challenge. Here, we proposed a facile strategy to construct *in situ* a novel hierarchical heterostructure composed of 0D−2D CoSe2/MoSe2 by the selenization of CoMoO4 nanosheets grafted on a carbon cloth (CC). In such integrated structure, CoSe2 nanoparticles dispersed well and tightly bonded with MoSe2 nanosheets, which can not only enhance kinetics due to the synergetic effects, thus promoting the electrocatalytic activity, but also effectively improve the structural stability. Benefiting from its unique architecture, the designed CoSe2/MoSe2 catalyst exhibits superior OER and HER performance. Specifically, a small overpotential of 280 mV is acquired at a current density of 10 mA·cm−2 for OER with a small Tafel slope of 86.8 mV·dec−1 , and the overpotential is 90 mV at a current density of 10 mA·cm−2 for HER with a Tafel slope of 84.8 mV·dec−1 in 1 M KOH. Furthermore, the symmetrical electrolyzer assembled with the CoSe2/MoSe2 catalysts depicts a small cell voltage of 1.63 V at 10 mA·cm−2 toward overall water splitting.

Keywords: cobalt selenide, molybdenum selenide, hierarchical heterostructure, bifunctional electrocatalysts, overall water splitting

## INTRODUCTION

Hydrogen is a promising energy source that boasts a high power density and environmental friendliness; therefore, electrolysis of water is hotly pursued as a renewable, efficient, and pollution-free technique (Amiinu et al., 2017; Luo et al., 2018; Zhu et al., 2018). Electrocatalytic water splitting consists of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), and electrocatalysts as the chemical reaction centers play a critical role in the water splitting electrolyzer. Although some noble metal oxide catalysts (RuO2 and IrO2) have high electrocatalytic performance for the OER and some noble metal catalysts (Pt and Ir) deliver good electrochemical property in the HER, the high cost and scarcity restrict their wide industrial application (Trasatti, 1972, 1984). Therefore, noble-metal-free catalysts with high stability and efficiency are crucial to large-scale hydrogen production from water splitting. Currently, the OER activity in alkaline solution is the bottleneck in overall water splitting due to the sluggish kinetics arising from the multiproton-coupled electron transfer steps (Jamesh and Sun, 2018). In practice, the HER catalyst in the electrolyzer should be compatible with the OER catalyst and functions in the same medium. Hence, development of suitable bifunctional noble-metal-free electrocatalyts with both high HER and OER performance in alkaline media is of great significance.

In recent years, transition metal dichalcogenides (TMDs) have attracted significant research interests owing to their earthabundant reserves and acceptable activity for electrocatalytic HER (Xie et al., 2013; Zhang et al., 2017a; Xue et al., 2018; Wang et al., 2019). Particularly, layered MoSe2 has been considered as a promising HER electrocatalyst because of its unique structure features and high electrochemical activity (Shi et al., 2015; Chen et al., 2018a; Zhang et al., 2018). Theoretical research has demonstrated that the Gibbs free energy for H atom absorption on the edge of MoSe2 is lower than that of MoS2 due to the more metallic nature of MoSe2, revealing the higher HER performance (Tang et al., 2014; Lai et al., 2017; Yang et al., 2018). In addition, it also has been experimentally confirmed that the unsaturated Se edges in MoSe2 nanosheets are extremely active as the S edges in MoS2, which is responsible for the high HER activity (Jaramillo et al., 2007; Tang and Jiang, 2016). However, similar to MoS2, the HER activity of layered MoSe2 is largely limited by its poor conductivity and serious aggregation or restacking during the synthesis procedure (Mao et al., 2015; Qu et al., 2015), inhibiting the practical application of MoSe2 catalyst. Therefore, it is significant to improve the electrochemical activity of MoSe2-based catalyst. Recent works have shown that coupling MoSe2 with other transition metal selenides and constructing heterostrucurted materials could be an effective approach to further enhance the electrochemical performance of MoSe2. For instance, Wang et al. found that the<EMAIL_ADDRESS>nanowire delivered enhanced kinetics and performance for HER in alkaline conditions due to the high density of active edges of MoSe2 and the good conductivity of Ni0.85Se (Wang et al., 2017a). Zhang et al. synthesized 3D MoSe2/NiSe2 nanowires, which significantly enhanced HER activity with a low Tafel slope and overpotential in 0.5 M H2SO4, because the 3D structure affords more active sites (Zhang et al., 2017a). Liu et al. fabricated MoSe2-NiSe@carbon heteronanostructures and achieved glorious HER catalytic properties and excellent durability in both acidic and base conditions (Liu et al., 2018). In addition, the hierarchical mesoporous MoSe2@CoSe/N–C composite also exhibits outstanding HER activity (Chen et al., 2019b). Despite significant success, most of previous reports mainly focused on the improvement of HER performance, while the OER activity of MoSe2 catalyst in alkaline media has been ignored. Hence, the rational design and fabrication of MoSe2 based bifunctional electrocatalysts with satisfactory activity and stability toward overall water splitting in alkaline solution still remain a big challenge.

In this work, we developed a facile in situ phase separation strategy to construct a novel hierarchical heterostructure consisting of 0D−2D CoSe2/MoSe2 via the selenization of CoMoO4 nanosheets supported on a carbon cloth (CC) (**Figure 1**). Due to the in situ phase transformation, CoSe2

nanoparticles are uniformly anchored on MoSe2 nanosheets in the integrated structure, which can not only enhance reaction kinetics because of the synergetic effects, thus boosting the electrocatalytic activity, but also effectively suppress the aggregation/restacking of MoSe2 nanosheets, thereby improving the structural stability. Moreover, the hierarchical structure assembled by 0D−2D CoSe2/MoSe2 could provide abundant active sites for the electrochemical reactions. As a result, the designed CoSe2/MoSe2 architecture exhibits outstanding OER and HER performance in alkaline media. More specifically, a small overpotential of 280 mV is achieved at a current density of 10 mA·cm−2 for OER with a small Tafel slope of 86.8 mV·dec−1 , and the overpotential is 90 mV at a current density of 10 mA·cm−2 for HER with a Tafel slope of 84.8 mV·dec−1 in 1 M KOH. Moreover, the symmetrical electrolyzer assembled with the CoSe2/MoSe2 catalysts delivers a small cell voltage of 1.63 V at 10 mA·cm−2 toward overall water splitting.

## EXPERIMENTAL SECTION

## Synthesis of CoMoO4 Nanosheet

Firstly, a pristine carbon cloth (CC) was treated with nitric acid solution overnight, subsequently ultrasonicated in deionized (DI) water and dried in an oven at 80◦C for 2 h. After that, 1 mmol cobalt acetate, 1 mmol ammonium molybdate, 2 mmol urea, and 5 mmol ammonium fluoride were dissolved in 30 mL DI water, followed by ultrasonication for 30 min. Then, the homogeneous solution was poured into a 50-mL Teflon-lined stainless autoclave with the CC kept at 150◦C for 6 h. After cooling to room temperature, the CC was washed with DI water for several times and dried in a vacuum freeze-dryer overnight. Finally, the obtained sample was treated at 400◦C for 2 h with a ramp rate of 5◦C min−1 in an argon atmosphere.

## Synthesis of CoSe2/MoSe2

The as-prepared CoMoO4 precursor was reacted with 0.5 g selenium powder at 450◦C for 1 h under an Ar/H2 (90%/10%) atmosphere to form the CoSe2/MoSe2.

## Synthesis of CoSe2

The CoSe2 was prepared through two steps. Firstly, the treated CC was immersed in a 0.1 M Co(NO3)2 solution for the electrodeposition of Co (Yang et al., 2015). Then, the collected sample was reacted with 0.5 g selenium powder under an Ar/H2 (90%/10%) atmosphere at 450◦C for 1 h.

## Synthesis of MoSe2

Firstly, MoS2 was prepared via hydrothermal reaction with the CC at 200◦C for 12 h, followed by heating at 400◦C for 2 h to form MoO3 (Wu et al., 2018). Then, the obtained MoO3 was reacted with 0.5 g selenium powder at 450◦C for 1 h under an Ar/H2 (90%/10%) atmosphere.

## Preparation of Pt/C

Four milligrams of 20% Pt/C and 20 µL 5% Nafion solution were added into 1 mL solution of isopropanol and DI water (9:1) and then sonicated to form a uniform solution. Finally, the 1∗ 1 cm2

CC was soaked in the homogeneous solution and dried in air at atmospheric temperature.

## Preparation of RuO2

Four milligrams of RuO2 and 20 µL 5% Nafion solution were added into 1 mL solution of isopropanol and DI water (9:1), and then the sample was sonicated to form a uniform solution. Finally, the 1∗ 1 cm2 CC was soaked in the homogeneous solution and dried in air at atmospheric temperature.

## CHARACTERIZATION

The phase composition of the samples were characterized by X-ray diffraction (XRD, Bruker D8A A25), and the chemical states were determined through X-ray photoelectron spectroscopy (XPS, ESCALB 250Xi). The morphology and microstructure were recorded via field emission scanning electron microscopy (FE-SEM, FEI Nova NANOSEM 400) and high-resolution transmission electron microscopy (HR-TEM, JEM-2100 UHR STEM).

## ELECTROCHEMICAL MEASUREMENTS

All samples made use of a three-electrode system performed by a biologic VSP300 type electrochemical workstation (Biologic Science Instruments, France). The sample of CoSe2/MoSe2 was put on the electrode holder as the working electrode with a mass loading of 4 mg/cm2 , the saturated calomel electrode (SCE) was the reference electrode, and a carbon rod served as the counter electrode. The electrolyte was 1 M KOH solution with saturated N2. Linear sweep voltammetry (LSV) was characterized by polarization curves of OER with a scanning rate of 5 mV s −1 from 0 to 0.8 V vs. SCE. Similarly, the polarization curves of HER were determined under the same condition from 0 to −0.8 V vs. SCE. The potentials were standardized by a reversible hydrogen electrode (RHE) as shown in the following: E (RHE) = E (SCE) + 0.059 × pH with instrument automatic 85% iR compensation. The electrochemically active surface area (ECSA) was calculated by cyclic voltammetry (CV) performed from −0.3 to −0.2 V vs. SCE with different scanning rates of 40, 60, 80, 100, and 120 mV s−1 . Electrochemical impedance spectroscopy (EIS) measurements were conducted by biologic VMP3 (Biologic Science Instruments, France) from 100 KHz to 0.1 Hz. The overall water-splitting electrolyzer was performed with CoSe2/MoSe2 as electrodes and 1 M KOH as the electrolyte.

## RESULTS AND DISCUSSION

**Figure 2A** presents the FE-SEM image of the as-prepared CoMoO4 precursor, which presents uniform nanosheets (with a lateral size of 2µm) perpendicularly grown on the CC substrate with high coverage. After a selenization process, the obtained CoSe2/MoSe2 sample well maintains the pristine morphology of the CoMoO4 precursor (**Figure 2B**). Moreover, the high-magnification SEM image further reveals that lots of nanoparticles are well dispersed on the surface of the nanosheet (**Figure 2C**), implying the structure and phase evolution during the selenization treatment. The elemental maps in (**Figure 2D**) show that Mo, Co, and Se are uniformly distributed throughout the nanosheets. In addition, the low-resolution TEM images in (**Figures 2E,F**) display that nanoparticles are uniformly distributed on the nanosheet during the thermal reduction procedure, forming the 0D/2D structure. Furthermore, the highresolution TEM (**Figure 2G**) shows the lattice fringes of 0.26 nm and 0.65 nm corresponding to the (111) and (002) planes of CoSe2 and MoSe2, respectively (Qu et al., 2016; Liu et al., 2017), demonstrating the successful formation of the CoSe2/MoSe2 after the selenization reaction.

To investigate phase evolution during the selenization process, the crystal structure and phase composition of the obtained samples were characterized by X-ray diffraction (XRD) analysis (**Figure 3A**). The diffraction peaks of CoMoO4 precursor (in the black line) can be well indexed to the CoMoO4 phase (JCPDS No: 21-0868) (Wang et al., 2016). After the thermal reduction, some

FIGURE 2 | FE-SEM images of (A) CoMoO4 and (B,C) CoSe2/MoSe2; (D) elemental maps of CoSe2/MoSe2; (E,F) TEM of CoSe2/MoSe2; (G) HR-TEM images of CoSe2/MoSe2.

new diffraction peaks can be observed. The diffraction peaks at around 13.7 ◦ , 27.6 ◦ , 31.4 ◦ , and 37.8 ◦ can be assigned to the MoSe2 phase (JCPDS No: 77-1715) (Qu et al., 2016), while the other peaks could be attributed to the phase of CoSe2 (JCPDS No:53-0449) (Liu et al., 2017). The XRD result clearly manifests the successful phase separation of the CoSe2 and MoSe2 from the CoMoO4 precursor via the selenization process.

X-ray photoelectron spectroscopy (XPS) measurement was carried out to analyze the composition and chemical state of as-prepared samples. (**Figure 3B**) illustrates the high-resolution Co 2p peaks at 778.8 eV (Co 2p3/2), 793.7 eV (Co 2p1/2), 780 eV (Co 2p3/2), and 796 eV (Co 2p1/2), corresponding to CoSe2 and cobalt–oxide bond, while those peaks at 784.1 eV and 801.5 eV are the satellite peaks (Mu et al., 2016; Wang et al., 2017b; Gao et al., 2018). Furthermore, the fine Mo 3d XPS spectrum (**Figure 3C**) shows the main peaks at 228.8 eV and 231.9 eV, which represent the Mo 3d5/2 and Mo 3d3/2 of MoSe2 (Wang et al., 2018a,b). Additionally, the peak located at 230 eV can be ascribed to the Se 3s of MoSe2 (Zhao et al., 2018). The Se 3d XPS spectrum (**Figure 3D**) displays the characteristic of CoSe2 and MoSe2 at 54.5 eV and 55.4 eV in agreement with the Se 3d5/2 and Se 3d3/2, respectively (Gao et al., 2018). Moreover, the peak at around 59.8 eV is confirmed to correspond to the selenium–oxygen bond (Kong et al., 2014). According to these results, the selenization process induced the phase separation from CoMoO4 into the nanoscale CoSe2 and MoSe2.

It is generally recognized that highly efficient electrocatalysts worked in alkaline solution is the bottleneck for large-scale application of overall water splitting. Linear sweep voltammetry (LSV) at a scanning rate of 5 mV s−1 was characterized by the electrocatalytic HER and OER capacities of the samples by a three-electrode system in 1 M KOH solution with saturated N2. By contrast, CoSe2, MoSe2 (**Figure S1**), RuO2, and Pt/C catalysts were performed in the same condition. The HER polarization curves and corresponding Tafel slops

are depicted in (**Figures 4A,B**). The overpotential (η10) and Tafel slope of CoSe2/MoSe2 are 90 mV and 84.8 mV·dec−1 , which are better than those of CoMoO4 (277 mV, 123.6 mV·dec−1 ), CoSe2 (205 mV, 195.2 mV·dec−1 ), and MoSe2 (199 mV, 152.4 mV·dec−1 ). CoSe2 has a metallic character, which can promote the dissociation of water and provide protons under alkaline conditions, thus improving the HER performance of MoSe2 (Kwak et al., 2016). In addition, the hierarchical nanosheet array assembled by the CoSe2/MoSe2 provides abundant active sites for the electrochemical reaction at the phase interface, which can further enhance the HER performance (Zhang et al., 2017a). Therefore, the CoSe2/MoSe2 catalyst exhibits improved HER performance benefiting from the synergistic effect. The catalyst of Pt/C illustrates an overpotential (η10) (59 mV) and Tafel slope (36.9 mV·dec−1 ) in 1 M KOH that are similar to those in other literatures (Chen et al., 2018b; Wan et al., 2018). Moreover, the overpotential of CoSe2/MoSe2 is superior to those of recently reported selenide catalysts such as NiSe NWs/Ni Foam (96 mV) (Tang et al., 2015), EG/cobalt selenide/NiFe–LDH (260 mV) (Hou et al., 2016), o-CoSe2/P (104 mV) (Zheng et al., 2018), CoSe2 NCs (520 mV) (Kwak et al., 2016), Co0.75Ni0.25Se/NF (106 mV) (Liu et al., 2019), 1T MoSe2/NiSe (120 mV) (Zhang et al., 2019), and SWCNTs/MoSe2 (219 mV) (Najafi et al., 2019) (**Table S1**).

The electrocatalytic OER properties are determined by LSV and polarization measurements as shown in (**Figures 4D,E**). The CoSe2/MoSe2 catalyst shows a lower overpotential (η10 of 280 mV) than those of the CoMoO4 (352 mV), CoSe2 (322 mV), MoSe2 (404 mV), and RuO2 (318 mV), respectively. More importantly, the OER performance of the designed CoSe2/MoSe2 sample exceeds those of recently reported selenide catalysts in OER, for instance, the Ag–CoSe2 (320 mV) (Zhao et al., 2017), CoSe2 NCs (430 mV) (Kwak et al., 2016), CoSe2/DETA (392 mV) (Guo et al., 2017), NiCo2Se4 holey nanosheets (295 mV) (Fang et al., 2017), NiSe–Ni0.85Se/CP (300 mV) (Chen et al., 2018a), SWCNTs/MoSe2 (295 mV) (Najafi et al., 2019), 1T/2H MoSe2 (397 mV) (Li et al., 2019), and CoSe2@MoSe2 (309 mV) (Chen et al., 2019c) (**Table S2**). Furthermore, the corresponding Tafel slope of CoSe2/MoSe2 is 86.8 mV·dec−1 , which is smaller than those of the CoMoO4 (101.8 mV·dec−1 ), CoSe2 (124 mV·dec−1 ), MoSe2 (130 mV·dec−1 ), and RuO2 (93.4 mV·dec−1 ). The CoSe2/MoSe2 has lower overpotential and smaller Tafel, which can be attributed to its unique hierarchical heterostructure, facilitating electron transfer and accelerating OER kinetics. In this heterostructure, the transfer of electrons from CoSe2 phase to

MoSe2 phase in the CoSe2/MoSe2 interface can result in electronpoor Co species and electron-rich Mo species (Liu et al., 2018). It is believed that the Se anion can affect the electron transfer between Co and Mo species, which is important for boosting catalytic ability (Yan et al., 2019). Besides, the formation of CoOOH is the primary cause to promote OER activity (Liu et al., 2015), and the increased 3d−4p repulsion between the center of the metal d band and the center of the p band of the Se site further promotes the rapid transfer of dioxygen molecules, thus improving OER performance (Li et al., 2017).

To understand the effects of the structure and composition of prepared catalyst on the electrochemical performance, several CoSe2/MoSe2 catalysts were collected at different selenization temperatures and the HER and OER performance were evaluated

by LSV analysis (**Figure S2**). It can be seen that the CoSe2/MoSe2 sample obtained at 450◦C (CoSe2/MoSe2-450) possesses better electrocatalytic properties than other counterparts, which can be ascribed to its superior structure. As shown in (**Figure S3**), with the selenization temperature increasing, the size of nanoparticles on the surface of nanosheets increased as well, indicating higher crystallinity. Generally, larger particle size will reduce the active surface of catalyst (Zhang et al., 2017b; Chen et al., 2019a). Therefore, when the selenization temperature elevated to 500◦C (CoSe2/MoSe2-500), the catalytic performance slightly declined owing to its larger particle size and lower active surface. In addition, (**Figure S4**) displays the composition of the CoSe2/MoSe2 catalysts achieved at a different selenization temperature. As can be seen, when the selenization process proceeded at low temperature, the obtained CoSe2/MoSe2 catalyst has poor MoSe2 phase and low crystallinity, which are responsible for the poor electrochemical catalytic performance of the catalysts (CoSe2/MoSe2-350 and CoSe2/MoSe2-400). Therefore, the catalyst synthesized at 450◦C shows the best performance, benefiting from the appropriate crystal structure and phase composition.

The electrochemically active surface area (ECSA) of asprepared catalyst was evaluated by the double-layer capacitance (Cdl), which was measured by CV in a non-Faradaic reaction potential range (Deng et al., 2015). The Cdl values of the CoSe2/MoSe2 (1.6 mF cm−2 ) is higher than those of CoSe2 (0.63 mF cm−2 ) and MoSe2 (0.8 mF cm−2 ), as shown in (**Figure 5**), suggesting more active sites of the CoSe2/MoSe2 catalyst. Furthermore, the smaller Rct value for the CoSe2/MoSe2 catalyst in the EIS measurement (**Figure S5**) implies the promoted charge transfer and boosted kinetics, which can be ascribed to the abundant interfaces and synergetic effect between the CoSe2 and MoSe2.

The structural stability is another significant parameter for catalysts in HER and OER. (**Figures 4C,F**) show a galvanostatic for CoSe2/MoSe2 catalyst in both the HER and OER processes. The morphology and composition of the catalyst after galvanostatic cycling are characterized by SEM and XPS. The

CoSe2/MoSe2 could well inherit the pristine sheet-like structure, demonstrating good structural stability. In addition, the fine XPS spectra of the Co 2p, Mo 3d, Se 3d acquired from the sample of CoSe2/MoSe2 after galvanostatic measurement confirm the reservation of CoSe2 and MoSe2 (**Figure S6**), indicating phase stability during the electrochemical reactions.

To investigate its practical application of the obtained catalyst, an overall water splitting electrolyzer is assembled with CoSe2/MoSe2 as electrodes in 1 M KOH. It can decompose water at a low cell voltage of 1.63 V (current density at 10 mA·cm−2 ) (**Figure 6A**), and the efficiency is similar to those constituting of the noble-metal-based cathode and anode (RuO2 vs. Pt/C). Moreover, the overall water splitting performance of the CoSe2/MoSe2 is better than those of other recently reported non-noble metals at the same current density, such as (Ni,Co)0.85Se NSAs (1.65 V) (Xiao et al., 2018), a-CoSe/Ti mesh (1.65 V) (Liu et al., 2015), CoOx-CoSe (1.64 V) (Xu et al., 2016), Co0.85Se@NC (1.76 V) (Meng et al., 2017), CoB2/CoSe2 (1.73 V) (Guo et al., 2017), NiSe2/Ni (1.64 V) (Zhang et al., 2018), 1T/2H MoSe2/MXene (1.64 V) (Li et al., 2019), and Ni3Se2/CF (1.65 V) (Shi et al., 2015) (**Table S3**). Additionally, CoSe2/MoSe2 electrolyzer exhibits a slight increase in the potential after being cycled for 12 h in alkaline solution (**Figure 6B**).

## CONCLUSION

In summary, a novel hierarchical 0D−2D Co/Mo selenide was developed by a facile in situ phase separation strategy. Benefiting from its unique structure and composition, the constructed CoSe2/MoSe2 catalyst exhibits small η10 of 280 mV and 90 mV

## REFERENCES

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and Tafel slopes of 86.8 mV·dec−1 and 84.8 mV·dec−1 for OER and HER, respectively. Furthermore, the electrolyzer comprising CoSe2/MoSe2 as the bifunctional catalyst shows a small water splitting cell voltage of 1.63 V at a current density of 10 mA·cm−2 . This work provides insights into rational design and development of economical and valid bifunctional catalysts for overall water splitting.

## DATA AVAILABILITY STATEMENT

All datasets generated for this study are included in the article/**Supplementary Material**.

## AUTHOR CONTRIBUTIONS

LX implemented the experiment, analyzed the data and wrote the article. HS, XL, XZ, and BG participated in the formulation of the experimental scheme. YZ, KH, and PC revised the article.

## FUNDING

This work was financially supported by the National Natural Science Foundation of China (Nos. 51572100, 61434001, and 31500783).

## SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2020.00382/full#supplementary-material

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**Conflict of Interest:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2020 Xia, Song, Li, Zhang, Gao, Zheng, Huo and Chu. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Influence of K2NbF7 Catalyst on the Desorption Behavior of LiAlH4

Nurul Amirah Ali, Noratiqah Sazelee, Muhammad Syarifuddin Yahya and Mohammad Ismail\*

*Energy Storage Research Group, Faculty of Ocean Engineering Technology and Informatics, Universiti Malaysia Terengganu, Terengganu, Malaysia*

In this study, the modification of the desorption behavior of LiAlH4 by the addition of K2NbF7 was explored for the first time. The addition of K2NbF7 causes a notable improvement in the desorption behavior of LiAlH4. Upon the addition of 10 wt.% of K2NbF7, the desorption temperature of LiAlH4 was significantly lowered. The desorption temperature of the LiAlH4 + 10 wt.% K2NbF7 sample was lowered to 90◦C (first-stage reaction) and 149◦C (second-stage reaction). Enhancement of the desorption kinetics performance with the LiAlH4 + 10 wt.% K2NbF7 sample was substantiated, with the composite sample being able to desorb hydrogen 30 times faster than did pure LiAlH4. Furthermore, with the presence of 10 wt.% K2NbF7, the calculated activation energy values for the first two desorption stages were significantly reduced to 80 and 86 kJ/mol; 24 and 26 kJ/mol lower than the as-milled LiAlH4. After analysis of the X-ray diffraction result, it is believed that the *in situ* formation of NbF4, LiF, and K or K-containing phases that appeared during the heating process promoted the amelioration of the desorption behavior of LiAlH4 with the addition of K2NbF7.

Edited by: *Guanglin Xia, Fudan University, China*

#### Reviewed by:

*Xuezhang Xiao, Zhejiang University, China Chu Liang, Zhejiang University of Technology, China*

\*Correspondence: *Mohammad Ismail<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Inorganic Chemistry, a section of the journal Frontiers in Chemistry*

Received: *16 February 2020* Accepted: *01 May 2020* Published: *12 June 2020*

#### Citation:

*Ali NA, Sazelee N, Yahya MS and Ismail M (2020) Influence of K2NbF7 Catalyst on the Desorption Behavior of LiAlH4. Front. Chem. 8:457. doi: 10.3389/fchem.2020.00457* Keywords: hydrogen storage, lithium aluminum hydride, desorption, catalyst, metal halide

## INTRODUCTION

The excessive consumption of fossil fuels and the emission of carbon dioxide are the roots of environmental pollution. As a resolution to this global issue, the utilization of clean, and sustainable energy resources such as hydrogen, wind, and solar has become an inescapable need. Recently, hydrogen has received a large amount of attention as a future energy carrier. Hydrogen promises to be a clean and renewable energy carrier. Moreover, the production of hydrogen can be achieved from various resources, both renewable (e.g., solar, wind, and hydro) and non-renewable (e.g., natural gas and coal; Winter, 2009; Parra et al., 2019). Furthermore, energy production via hydrogen-oxygen reaction will only produce water as a by-product (Crabtree et al., 2004).

In pursuit of the success of hydrogen as a future energy carrier, the need for an efficient and reliable storage method has become the top priority. In general, there are three forms of hydrogen storage which are: (i) compressed hydrogen gas, which requires high pressure, (ii) liquefaction, and (iii) solid-state hydrogen storage via hydrides (Dalebrook et al., 2013; Zhang et al., 2016; Barthelemy et al., 2017). Solid-state hydrogen storage has been perceived to be an efficient and favorable method because of its safety, storage requirements, and storage capacity.

Lithium aluminum hydride (LiAlH4) has major benefits and is the preferable solid-state material. LiAlH4 is attractive due to its low temperature of hydrogen release and high storage capacity (10.6 wt.%; Andrei et al., 2005; Ares et al., 2008). The desorption process of LiAlH4 occurs in three stages, as follows:

$$\text{3LiAlH}\_4 \rightarrow \text{Li}\_3\text{AlH}\_6 + \text{2Al} + \text{3H}\_2 \tag{1}$$

$$\text{Li}\_3\text{AlH}\_6 + 2\text{Al} \rightarrow \text{ 3LiH} + \text{ 3Al} + \text{ 3/2H}\_2 \tag{2}$$

$$\text{3LiH} + \text{ 3Al} \rightarrow \text{ 3LiAl} + \text{ 3/2 H}\_2 \tag{3}$$

The first reaction (1) occurs in a temperature range of 150–175◦C and desorbs 5.2 wt.% of the hydrogen. The second reaction (2) takes places at 180–220◦C and desorbs 2.6 wt.% of the hydrogen, while the third reaction (3) happens at temperatures > 400◦C, with 2.6 wt.% of the hydrogen desorbed.

In spite of its advantages, LiAlH4 has some shortcomings, such as irreversible and slow desorption kinetics (Pukazhselvan et al., 2012). Moreover, the thermal decomposition in reaction 3 is considered incompatible with applied applications due to its high requirement for temperature (>400◦C) to release hydrogen.

Among these catalysts, previous studies have revealed that metal halides provide essential catalytic effects on the performance of LiAlH4. Cao et al. (2018) reported that the addition of ScCl3 had a superior effect on the performance of lithium alanates. The desorption process of the LiAlH4-10 mol% ScCl3 sample began at a lower temperature (∼120◦C), while the undoped LiAlH4 released hydrogen from around 150◦C. Besides, the time needed to complete the dehydrogenation process was shortened with the addition of 1–10 mol% ScCl3. Meanwhile, Sun et al. (2008) found that NiCl2 significantly boosted the desorption behavior of LiAlH4. A composite sample of LiAlH4-NiCl2 demonstrated three times the desorption rate of pure LiAlH4, which was not able to desorb any hydrogen at 100◦C. It was believed that the LiAlH4-NiCl2 sample presented this notable improvement due to the formation of Ni, which plays a vital role in accelerating the LiAlH4-NiCl2 system. Another investigation of the catalytic effect of metal halides was carried out by Liu et al. (2012). They proved that a LiAlH4-TiCl3 sample could release hydrogen at a lower temperature (80◦C) than the pure LiAlH4. Furthermore, the dehydrogenated sample had good cyclability, with the composite sample able to retain a high capacity for hydrogen (6.4 wt.%) even after completing the 3rd cycle. Moreover, Ismail et al. (2010) observed that the composite sample of LiAlH4-1 mol NbF5 showed a 5–6 times faster dehydrogenation rate than the milled LiAlH4.

Additionally, the LiAlH4-NbF5 composite sample had lower activation energy; 67 kJ/mol (first-stage reaction) and 77 kJ/mol (second-stage reaction), respectively. However, the improvement of LiAlH4 through the addition of a catalyst is still lacking, and further enhancements still need to be carried out. Moreover, different catalysts will enable different effects and performances. Therefore, it is interesting to enhance the desorption performance of LiAlH4 by the addition of other metal halides.

Metal halides, especially fluorides, are known to be highly effective catalysts for solid-state materials (Sulaiman et al., 2016; Yap et al., 2017; Youn et al., 2017). A number of researchers have reported that niobium fluoride exhibits a notable effect on the hydrogenation behavior of metal hydrides and the complex hydrides. A study conducted by Luo et al. (2008) revealed that the addition of 2 mol of NbF5 led to faster desorption kinetics for the MgH2-NbF5 sample as compared to pure MgH2. At 573 K, the MgH2-NbF5 sample could desorb 4.7 wt.% of hydrogen, while the pristine MgH2 desorbed almost no hydrogen. Other than that, Kou et al. (2014) added NbF5 to LiBH4 and demonstrated notable improvement on the desorption performance of LiBH4. In comparison to the milled LiBH4, which started to desorbed hydrogen at >400◦C, the composite sample of LiBH4-NbF5 had a lower desorption temperature, 60◦C. Moreover, Wang and colleagues (Wang et al., 2020) showed that a composite of Mg(BH4)2-doped NbF5 possessed the best dehydrogenation performance, with the ability to release hydrogen at low temperature (120◦C), as compared to amorphous Mg(BH4)2 (126.9◦C), and pristine Mg(BH4)2 (282.7◦C). Meanwhile, Cheng et al. (2018) demonstrated that upon the addition of NbF5, the composite of 4LiBH4-MgH2- Al exhibited excellent kinetics and reversibility performance. It took <4 h to achieve 90% of the total amount of hydrogen desorption. Other than that, Xiao et al. (2012) proved that the performance of LiBH4/MgH2 was significantly improved with the addition of NbF5. Here, the addition of NbF5 not only had reduced the onset decomposition temperature but also improved the dehydrogenation and absorption rates.

On the other hand, potassium (K) is another well-known additive for hydrogen storage systems. Wang et al. (2009) demonstrated that the addition of K significantly boosted the desorption process of Mg(NH2)2/LiH by reducing the overall reaction temperature. Furthermore, Dong et al. (2014) revealed that superior results for the hydrogenation performance of the LiH-NH3 system were obtained by the addition of various potassium compounds.

In respect to this matter, it is interesting to mix niobium fluoride with potassium as a ternary compound in the form of K2NbF7 and to study its potential catalytic effect. To date, no studies have been conducted using doped K2NbF7 as a catalyst for LiAlH4. Moreover, previous studies reported that K2NbF7 enables a remarkable improvement in the hydrogen storage performance of MgH2 (Yahya et al., 2018; Yahya M. S. and Ismail M., 2018). Thus, it is of great interest to explore the influence of K2NbF7 on the desorption performances of LiAlH4. It is anticipated that the addition of K2NbF7 will have notable effects on the desorption and kinetic performances of LiAlH4.

## EXPERIMENTAL DETAILS

Commercial powders of LiAlH4 (purity 95%) and K2NbF7 (purity 98%) were obtained from Sigma Aldrich and were used without any modification. To minimize exposure to oxygen and water moisture, the samples were prepared and handled in the Ar-filled Mbraun Unilab glove box. In this study, 10 wt.% of K2NbF7 was mechanically milled together with LiAlH4 to explore its effect on the desorption behavior of LiAlH4. The milling process was done in a planetary ball mill (NQM-0.4) for 1 h, starting with 0.5 h of milling, followed by 6 min of rest time, and then another 0.5 h of milling in a different rotation direction at a speed of 400 rpm. The samples were placed in a hardened stainless-steel jar with four stainless balls, each 1 cm in size. The ratio of the balls to the weight of the powder was 40:1. For comparison purposes, the as-received LiAlH4 was treated under the same conditions.

The hydrogenation performances of LiAlH4 + 10 wt.% K2NbF7 were studied with temperature-programmed desorption (TPD) using Sievert-type pressure-composition-temperature (PCT) equipment (Advanced Materials Corporation). In order to determine the initial decomposition temperature, the sample was heated from room temperature to 250◦C (heating rate: 5 ◦C/min). Other than that, the desorption kinetics performances were evaluated at 90◦C under 1.0 atm of pressure. The apparent activation energy, EA, was determined using differential scanning calorimetry (DSC, Mettler Toledo, DSC/TGA 1), loading 5– 7 mg of the samples into a crucible and heating from 25 to 300◦C at heating ramps of 15, 20, 25, and 30◦C/min under an argon flow (50 ml/min). In terms of the morphology and phase structure characterizations, the samples were analyzed using scanning electron microscopy (SEM: JEOL JSM 6350LA), X-ray diffractometry (XRD, Rigaku Miniflex), and Fourier transform infrared (IR Shimadzu Tracer-100).

## RESULTS AND DISCUSSION

**Figure 1** demonstrates the TPD results of the LiAlH4 and modified LiAlH4 system. The results show that the as-received and as-milled LiAlH4 have similar desorption processes that occur in two stages of desorption, as in Equations (1, 2), with 7.4 wt.% hydrogen capacity. Before the ball milling process, the first stage of desorption occurred at 147◦C, with 5 wt.% of hydrogen released. Meanwhile, the desorption process for the second stage was recorded to happen at around 175◦C, with a capacity of 2.4 wt.% of the hydrogen. After the milling process, the initial desorption temperature of the sample was similar to that of pure LiAlH4 but with slight temperature reductions to 144◦C (first stage) and 174◦C (second stage). This phenomenon showed that the 1-h milling process had an insignificant effect on the desorption behavior of LiAlH4. In contrast, the addition of 10 wt.% of K2NbF7 significantly decreased the decomposition temperature for both stages, to 90 and 149◦C. However, the amount of hydrogen released from the LiAlH4 + 10 wt.% K2NbF7 sample was decreased to 6.3 wt.%. This is expected due to the dead weight of K2NbF7, which does not hold any hydrogen.

Further study on the catalytic activity of K2NbF7 was performed based on the desorption kinetics experiment. **Figure 2** depicts a comparison of the hydrogen desorption at 90◦C for LiAlH4 and LiAlH4 modified by the addition of 10 wt.% K2NbF7. It is noticeable that within 120 min, the undoped LiAlH4 was only able to desorb a small amount of hydrogen; 0.1 wt.% for the as-received LiAlH4 and 0.4 wt.% for the as-milled LiAlH4. Surprisingly, with the addition of 10 wt.% K2NbF7, the doped sample desorbed ∼3.2 wt.% H2 within the same duration. This desorption rate was 30 times faster than that of the as-received LiAlH4. This enhancement may be correlated to the formation of surface defects and active materials through the reaction of the LiAlH4 + 10 wt.% K2NbF7 composite (Cai et al., 2016).

In terms of thermal behavior, DSC experiments were conducted for the doped and un-doped LiAlH4 samples. **Figure 3** displays the DSC curves of the samples at a heating ramp of 15◦C/min. Both the doped and un-doped LiAlH4 have two endothermic and exothermic peaks. The first exothermic peak corresponds to the reaction of LiAlH4 with surface hydroxyl groups, while the first endothermic peak is ascribed as its melting process. The second exothermic peak is attributed to the decomposition of LiAlH4, as described in Equation (1), and the second endothermic peak correlates with the decomposition of Li3AlH6, as described by Equation (2). Both samples exhibit similar thermal behavior, but the peaks of the LiAlH4 + 10 wt.% K2NbF7 sample occur at a lower temperature as compared to as-milled LiAlH4.

Fundamentally, the enhancement of the initial temperature to release hydrogen and the faster desorption kinetics rates are correlated with the energy barrier of LiAlH4. In this study, the decomposition activation energy (EA) is the least possible amount of energy needed by LiAlH4 to begin the hydrogen desorption

process. **Figure 4** shows DSC traces for several heating ramps (15, 20, 25, and 25◦C/min). By referring to the plots, the activation energies for both decomposition stages of the as-milled LiAlH4 and LiAlH4 + 10 wt.% K2NbF7 samples were determined using the Kissinger analysis, as in equation (4):

$$\ln\left[\beta/T\_{\mathcal{P}}\right.^{2}\right] = -E\_{A}/\mathcal{R}T\_{\mathcal{P}} + A\tag{4}$$

where β, Tp, R, and A are the heating rate, peak temperature in the DSC curve, gas constant, and linear constant, respectively. The apparent activation energy was determined from the slope of ln [β / Tp 2 ] versus 1000/Tp, as shown in **Figure 5**.

The activation energy was calculated based on the second exothermic (decomposition of LiAlH4) and second endothermic (decomposition of Li3AlH6) reactions. For the as-milled LiAlH4, the activation energy values were 104 and 112 kJ/mol for the first two stages of reaction, respectively. After the addition of 10 wt.% K2NbF7, the activation energy values dropped to 80 kJ/mol (first stage) and 86 kJ/mol (second stage), 23% lower than those of the un-doped LiAlH4. These results are in good agreement with other studies that prove the addition of a catalyst is able to reduce the activation energy of LiAlH4. **Table 1** lists the activation energy from previous studies for comparison purposes. The reduction in these activation energies verifies that K2NbF7 plays a major role in enhancing the desorption kinetics performance of LiAlH4.

The morphological structures of the doped and un-doped LiAlH4 were examined using SEM equipment. **Figure 6** shows SEM images of the un-doped and doped-LiAlH4 samples. As shown in **Figure 6**, the pure LiAlH4 exhibits larger particle sizes than the milled sample. The as-received LiAlH4 (**Figure 6A**) has larger (15–40µm), non-uniform rod-shaped particles. Furthermore, the as-received LiAlH4 shows a uniform size distribution and consists of "blocky" particles, consistent with the report by Varin and Zbroniec (2010). Meanwhile, after 1 hour of milling, the milled LiAlH4 (**Figure 6B**) displays a reduction in particle sizes but with some agglomeration and inconsistency in particle size. Then, with the addition of 10 wt.% of K2NbF7 (**Figure 6C**), the morphological structure of the sample was notably enhanced. The doped sample has smaller

TABLE 1 | Activation energy of catalyst-doped LiAlH4 from previous studies.

particle sizes and is less agglomerated. This observation is in line with numerous research results that have shown a reduction of particle sizes with the addition of a catalyst (Aguey-Zinsou et al., 2007; Ali et al., 2018; Yahya M. and and Ismail M., 2018). In this study, K2NbF7 functioned as a dispersing agent that impeded the sample from agglomerating. The particle size is important because smaller particles provide more area for surface defects and additional grain boundaries (Schulz et al., 1999; Sakintuna et al., 2007; Ranjbar et al., 2009). As a consequence, the desorption kinetics of LiAlH4 will be improved.

**Figure 7** presents the XRD profiles of the as-received LiAlH4, as-milled LiAlH4, and LiAlH4-K2NbF7 sample. The XRD characterization was performed to explore the reaction process and the mechanism that operated during the milling process. **Figure 7A** displays the XRD pattern of the as-received LiAlH4 and shows that only the LiAlH4 phase was detected, which confirms the purity of the LiAlH4. The XRD pattern of the milled LiAlH4 **Figure 7B** shows similar peaks to the as-received LiAlH4. This result shows that LiAlH4 has high stability during the milling process and agrees well with a previous study (Ismail et al., 2010). Meanwhile, with the addition of 10 wt.% of K2NbF7 (**Figure 7C**), only LiAlH4 and Al peaks are visible and no peak of K2NbF7 was detected, suggesting that the amount of catalyst was too small to be picked up by the XRD. The appearance of Al peaks indicates that a part of the LiAlH4 had decomposed to Li3AlH6 and Al (reaction 1) during the milling process in the presence of 10 wt.% K2NbF7. Surprisingly, the XRD result for the 10 wt.% K2NbF7-doped LiAlH4 sample does not show any peaks of Li3AlH6. Additional characterization was carried out for a doped sample with 30 wt.% K2NbF7 (**Figure 7D**). A K2NbF7 peak was against not detected by the XRD for this sample. This may be because the K2NbF7 is in an amorphous state. Similar phenomena were reported by previous studies, where several catalysts like TiO2 and TiF3 were not detected by the XRD after the milling process (Ismail et al., 2011; Zang et al., 2015). However, for the LiAlH4 + 30 wt.% K2NbF7 sample, diffraction peaks corresponding to the decomposition product, Al and Li3AlH6, were detected. Meanwhile, unlike for the LiAlH4 + 10 wt.% K2NbF7 sample, for which only peaks of Al were detected while peaks of Li3AlH6 could not be discovered by XRD.

**Figure 8** shows the IR spectra of the as-received LiAlH4, asmilled LiAlH4, and LiAlH4 + 10 wt.% K2NbF7 in the range of 800 to 2,000 cm−1 . The FTIR characterizations were conducted to identify the presence of Li3AlH6 in the 10 wt.% K2NbF7 doped LiAlH4 sample. For all samples, two distinct regions of Al-H modes were detected at around 800–900 cm−1 ([AlH4] − stretching modes) and 1,600–1,800 cm−1 ([AlH4] − bending modes), respectively. Furthermore, with the addition of 10 wt.% K2NbF7, a weak IR absorption peak at 1,398 cm−1 was detected, which indicates the presence of Li3AlH6. This result suggests that with the addition of 10 wt.% K2NbF7, LiAlH4 was partially decomposed to Li3AlH6 and Al (reaction 1) during the milling process, consistent with the XRD results (**Figure 7C**).

To investigate the specific mechanism that is related to the enhanced desorption performance of LiAlH4, the dehydrogenated sample was examined using XRD. The XRD pattern for the dehydrogenated sample is depicted in **Figure 9**. After the dehydrogenation process at 250◦C, the main peaks observed are the LiAlH4 dehydrogenation products, LiH and Al, which indicates complete dehydrogenation of LiAlH4. In addition, peaks for LiF and NbF4 were detected after the dehydrogenation process. However, the peak of the K-containing

phase was not detected after the dehydrogenation process, potentially due to the low amount of catalyst.

Niobium fluoride is well established as a promising catalyst that plays a vital role in enhancing the hydrogenation performance of solid-state material (Luo et al., 2007; Malka et al., 2011; Mao et al., 2013). It is reasonable to state that the NbF4 that formed in situ after the desorption process contributes to a remarkable amelioration of the desorption behavior of LiAlH4. This result well-agreed with previous research that demonstrates the outstanding dehydrogenation performance of LiAlH4-NbF5