Pyrroloquinoline quinone (PQQ)-dependent glucose dehydrogenase (GDH) is of interest because of its high activity and independence of dissolved oxygen in catalyzing the transfer of electrons from glucose to an electron mediator. It can be studied by SECM using FB mode or G/C mode. Because galactosidase (Gal) which is not an oxidoreductase cannot be imaged in the conventional FB mode, it was studied in the G/C mode. Zhao and Wittstock (G. Wittstock et al, 2004) investigated a multienzyme system composed of immobilized Gal and PQQ-dependent GDH with a new detection method which combined the advantages of the FB and G/C modes and allowed high sensitivity and lateral resolution in SECM imaging. Gal and GDH were immobilized on separate batches of paramagnetic microbeads.In conventional GC imaging of Gal, p-aminophenyl- -D-galactopyranoside (PAPG) is added to the solution, generating p-aminophenol (PAP) which can be oxidized to p-quinoneimine (PQI) under diffusion controlled conditions at the tip. The electrochemically produced PQI diffuses to the microspot, where it is reconverted into PAP by immobilized GDH in the presence of d-glucose. Therefore, SECM feedback loop was formed when PAP diffused back to the UME. The UME current, iT, is caused by the G/C contribution from Gal and the feedback amplication by GDH. After the addition of glucose to the assay solution, a clear increase in the current strength was observed above the spot. Moreover, the peak current observed in the combined mode is about 1.8 times stronger than that observed in the conventional GC mode. Yamada et al. (Yamada, et al. 2005) have used a share force-based tip–substrate position SECM system to image a platinum-patterned array electrode and a diaphorase/albumin coimmobilized glass surface. A standing approach mode to avoid contact between the tip and the substrate was used, which repeated the approach and retraction at each data point of the surface, to obtain simultaneously both a current image and a topographic image.

## **8.1.3 Electrocatalytic activity**

Catalyzed heterogenous reaction has been extensively investigated by SECM. Different electrode materials and pH of the solution have different catalytic activities. Hydrogen peroxide decomposition catalyzed by different solid surfaces which were catalase-modied nylon surface and Pt nanoparticles immobilized glass was performed by using an amalgam Au–Hg disk tip. Rate constants for decomposition reaction on immobilized catalase and Pt nanoparticles were measured at different pH values. Catalase-modied nylon surface exhibited obvious activity in the pH range 5 < pH < 10, with a maximum at around pH 7. The value of ks is between 1 × 10−3 and 3 × 10−3 cm s−1. The activity of immobilized Pt particles (5–10 nm average diameter) showed a maximum at pH 11.9 in the pH range 10 < pH < 13 with an apparent rate constant (ks = 5 × 10−3 cm s-1). Zhou et al. (A.J. Bard et al., 2000) reported the SECM feedback behavior of the H+/H2 mediator system at 10 mM concentrations of strong acid at different substrates and demonstrated that the feedback of the redox couple depended on the catalytic activity of the substrate surface for hydrogen oxidation. The prohibitive effect of adsorbed anions (Br− and I−) and reduction products of NO3− for catalytic H2 oxidation on the Pt surface was also studied. The kinetics of hydrogen oxidation on Pt, Ir, and Ru substrates in acidic media was investigated. The rate constants for hydrogen oxidation on Pt (k◦ = 0.22 cm s−1), Ir (k◦ = 0.25 cm s−1) and Rh (k◦ = 0.010 cm s−1) were obtained by tting approach curves. The SECM can be used to quantitatively map the rate constant for the hydrogen oxidation reaction. Recently, a screening method was presented that quantitatively detects protons at a surface using the SECM. The variation in reactivity of the platinum catalyst gradient toward the hydrogen oxidation reaction was measured directly as a function of spatial position. The local rate constant value was proportional to the local platinum surface coverage. A tip reaction was employed to measure the activity of PtxRuy and PtxRuyMoz electrodeposited catalyst toward the hydrogen oxidation reaction in the absence and presence of a monolayer of carbon monoxide. The activity and the onset potential of various catalysts toward the hydrogen oxidation reaction determined by performing screening studies as a function of composition electrode potential. The use of SECM can effectively weaken the hydrogen oxidation activity and poison (CO) tolerance of the catalysts. Bard (A.J. Bard et al, 2003) detected and imaged electrocatalytic activity for the oxygen reduction reaction (ORR) in acidic medium of different electrode materials by operating in a modied TG/SC mode. When the tip placed at 30 m from the substrate was moved in the x–y plane, activity-sensitive images of heterogeneous surfaces, e.g., with Pt and Au electrodes, were obtained from the substrate current. Moreover, the images were obtained by scanning an array of Pt and Ru spots supported on glassy carbon (Fig. 5). It showed Ru metal had weaker ORR activity than Pt. The electroactive of trodes can be sufciently discriminated by SECM-image. Later, the heterogeneous kinetics for the ORR has been studied by TG/SC mode. The electrocatalysis of O2 reduction at copper (II)–poly-l-histidine complex was studied and the activity of arrays of Cu2+–poly-his complex spots of various compositions were imaged. After the electrocatalytic activity for the ORR of different electrode materials were studied by TG/SC mode, Fernandez et al. (A.J. Bard et al, 2005) reported the electrocatalytic activity of bimetallic (and trimetallic) materials for the ORR in acidic medium by using SECM in a new rapid-imaging mode.

### **8.2 Liquid / liquid interface**

### **8.2.1 ET**

146 Recent Trend in Electrochemical Science and Technology

where kcat is the minimum catalytic reaction rate at substrate saturation, *enz* mol cm−2 is the surface coverage of enzyme, rs is the radius of the enzyme modied spot, c` and D are the detection limit for the species observed at the tip and the diffusion coefcient of the detected component, respectively. This relation indicates that the sensitivity of the experiment to kinetics is increased for large sample radius rs, low c`, low D, and high surface coverage of

SECM was used to study biocatalytic reactions inside the enzyme layer of a biosensor. Concentration changes of substrate, products and pH changes in the reaction layer were probed by amperometric and potentionmetric tip. Many enzyme reactions and metabolic processes of microorganisms can cause pH changes. The pH sensitive antimony (Sb) tip was selected to check pH changes. The current–distance curves can be useful to give information about an optimal reaction layer thickness. These curves for both H2O2 and O2 concentration proles showed the products have local maximum inside reaction layer approximately

Pyrroloquinoline quinone (PQQ)-dependent glucose dehydrogenase (GDH) is of interest because of its high activity and independence of dissolved oxygen in catalyzing the transfer of electrons from glucose to an electron mediator. It can be studied by SECM using FB mode or G/C mode. Because galactosidase (Gal) which is not an oxidoreductase cannot be imaged in the conventional FB mode, it was studied in the G/C mode. Zhao and Wittstock (G. Wittstock et al, 2004) investigated a multienzyme system composed of immobilized Gal and PQQ-dependent GDH with a new detection method which combined the advantages of the FB and G/C modes and allowed high sensitivity and lateral resolution in SECM imaging. Gal and GDH were immobilized on separate batches of paramagnetic microbeads.In

to the solution, generating p-aminophenol (PAP) which can be oxidized to p-quinoneimine (PQI) under diffusion controlled conditions at the tip. The electrochemically produced PQI diffuses to the microspot, where it is reconverted into PAP by immobilized GDH in the presence of d-glucose. Therefore, SECM feedback loop was formed when PAP diffused back to the UME. The UME current, iT, is caused by the G/C contribution from Gal and the feedback amplication by GDH. After the addition of glucose to the assay solution, a clear increase in the current strength was observed above the spot. Moreover, the peak current observed in the combined mode is about 1.8 times stronger than that observed in the conventional GC mode. Yamada et al. (Yamada, et al. 2005) have used a share force-based tip–substrate position SECM system to image a platinum-patterned array electrode and a diaphorase/albumin coimmobilized glass surface. A standing approach mode to avoid contact between the tip and the substrate was used, which repeated the approach and retraction at each data point of the surface, to obtain simultaneously both a current image

Catalyzed heterogenous reaction has been extensively investigated by SECM. Different electrode materials and pH of the solution have different catalytic activities. Hydrogen peroxide decomposition catalyzed by different solid surfaces which were catalase-modied nylon surface and Pt nanoparticles immobilized glass was performed by using an amalgam

200µm far from the substrate/enzyme layer boundary.

conventional GC imaging of Gal, p-aminophenyl-

and a topographic image.

**8.1.3 Electrocatalytic activity**

enzyme.

The kinetics of heterogeneous electron transfer depends on the driving force at the ITIES. At low overpotential, the kinetics of ET obeys the Bulter–Volmer (B–V) theory. As the driving force is increased, the Marcus inverted region appears. The existence of a Marcus inverted

Electron Transfer Kinetics at Interfaces Using SECM (Scanning Electrochemical Microscopy) 149

effect of aqueous ionic strength was studied by the addition of salt to the aqueous by using

Recently, Sun et al. (Y.H. Shao et al, 2003) demonstrated the existence of an inverted region by the use of SECM with a three-electrode setup and variable concentration ratio of redox species in both phases. For the ET reaction between ZnPor in DCE and Fe(CN)63− in aqueous phase, at lower applied potential (0.54- *WEO* <440 mV, Ewo is the externally applied potential), the increasable trend was obvious. When the applied potential ( *WEO* ) is given at >440 mV, the values of log k12 were increasedslowly and reached a maximum at about 500 mV. When the driving force exceeded 600 mV, the rate constant of an ET reaction

best-tting of the experimental curve. The ET reaction between TCNQ in 1, 2-dichloroethane (DCE) and Fe(CN)63− in the aqueous phase was also studied (Fig. 6b). The reorganization energy is 28.9 kJ mol−1. Compared with the studies of the forward reaction between TCNQ

An interface between two immiscible electrolyte solutions is considered as the simplest model for biomembranes. There is a great deal of interest in understanding charge-transfer at modied interfaces. Amphiphilie lipid molecules dissolved in an organic phase spontaneously form a monolayer lm at the water/organic interface. Although the lipids make the CT at the L/L interface more complicated, the lipid-modied interface may be a great help to study the electrical properties of membrane-bound components that control ion-transfer and electron-transfer across biological membranes. The ET reactions at lipid modied L/L interfaces have been investigated by SECM. The ET rate depends on the lipid concentration and the chain length. On the one hand, the ET rate through the ITIES decreases when the lipid concentration increases, but the ET rate reaches a limiting value and doesn't change for lipid concentrations higher than 50 Mm because of a complete lipid monolayer formed at the ITIES. On the other hand, the ET rate increases as the lipid chain length increases. The blocking effect of the lipid monolayerm makes the ET rate signicantly decreased. In addition, the extent of the blocking effect apparently depends on the driving force for ET process. The dependence of log kf on driving force for ET between Fe(CN)64−

with a xed concentration of THAP (tetra-n-hexylammonium perchlorate) in DCE.

and ZnPor+ was linear with aslope of=0.59 at lower overpotential ( 630 *o W E O*

a larger overpotential, the ET rate for Co (II) sepolchrate and V2+ decreased with the increasing driving force, which was consistent with Marcus theory inverted region behavior. To investigate the bionic ET between dopamine (DA) and ferrocene (Fc) at the water/1, 2 dichloroethane (w/DCE) interface, the recently proposed three-electrode setup is adopted based on a glassy carbon electrode and it can be used to study the CT at a L/L interface with

For an ET process, the solvent effects have recently aroused much attention, such as the effects of dielectric constants and viscosities, which mainly inuence on its activation Gibbs energy and pre-exponential factor. The dependence of rate constant of the ET reaction at the interface upon the solution viscosity is evaluated using SECM. Liu and Mirkin (M.V. Mirkin et al, 1999) have investigated the effect of solvent dynamics on the rate of ET from ZnPor to Ru(CN)63− using various organic solvents. The ET rate was found to be essentially independent of the potential drop across the interfacial boundary when the organic redox

for the oxidation of TCNQ− in DCE by aqueous Fe(CN)63− was studied using SECM

4− in the aqueous phase, the dependence of the back ET rate constants

= 47.2 kJ mol− 1) was obtained from th

*nm* ). At

approach curves of SECM .

in DCE and Fe(CN)6

different phase ratios.

on *OW*

diminished (Fig. 6a). The reorganization energy (

Fig. 3. ORR images obtained by the TG–SC mode of an array of Pt (left spot and right row) and Ru (middle row) spots supported on glassy carbon: scan rate 600μms−1; d = 30nA; iT = 210 nA; ES = 0.1 V. (A.J. Bard et al, 2003)

region at a lipid modied ITIES or unmodied ITIES was demonstrated by SECM. The previous SECM experiments were often performed with a high concentration of reactant in phase 2 compared with that of the mediator in phase 1. Barker et al. (A.L. Barker et al, 1999) developed a theoretical model that allowed the use of a relatively low concentration of the reactant in the second phase. When the concentration ratio Kr of aqueous to organic reductant ( 2 1 \*, \*, Re Re / *W O KC C r d d* ) is decreased, the interfacial ET rate decreases. This approach is benecial in studying the fast kinetic of the ET reaction. Recently, Li et al. (P.R. Unwin et al, 2007) illustrated how the use of small values of Kr is optional for fast kinetics studied and the effect of Galvani potential on the ET rate constants of the reaction between TCNQ− and Ru(bipy)3+3 at the ITIES. The ET reaction between electrogenerated ZnPor+ (oxidized form of zinc-21H, 23H-tetraphenylporphine) and different redox species at different water/organic solvent interfaces were studied. The redox reactions at the tip and ITIES are as follows:

$$ZnPor(o) - e^- \rightarrow ZnPor^+(o) \quad \text{(at the tip)}\tag{27}$$

$$ZnPor^{+}(o) + \text{Re}\, d(\mathcal{W}) \to ZnPor(o) + \text{Ox(\mathcal{W})} \quad \text{(at the TITES)}\tag{28}$$

where the redox couple represents Ru(CN)63−/4−,Mo(CN)8 3−/ 4−, Fe(CN)63−/ 4−, W(CN)83−/ 4−, Fe(EDTA)−/2−,Ru(NH3)63+/2+, Co(Sep)3+/2+, and V3+/2+.

Although the solvent used for the Fe(CN)64− study was different from that of the other three reactants, the overall trend is consistent with the prediction of Marcus theory. Moreover, the

Fig. 3. ORR images obtained by the TG–SC mode of an array of Pt (left spot and right row) and Ru (middle row) spots supported on glassy carbon: scan rate 600μms−1; d = 30nA; iT =

region at a lipid modied ITIES or unmodied ITIES was demonstrated by SECM. The previous SECM experiments were often performed with a high concentration of reactant in phase 2 compared with that of the mediator in phase 1. Barker et al. (A.L. Barker et al, 1999) developed a theoretical model that allowed the use of a relatively low concentration of the reactant in the second phase. When the concentration ratio Kr of aqueous to organic

is benecial in studying the fast kinetic of the ET reaction. Recently, Li et al. (P.R. Unwin et al, 2007) illustrated how the use of small values of Kr is optional for fast kinetics studied and the effect of Galvani potential on the ET rate constants of the reaction between TCNQ− and Ru(bipy)3+3 at the ITIES. The ET reaction between electrogenerated ZnPor+ (oxidized form of zinc-21H, 23H-tetraphenylporphine) and different redox species at different water/organic solvent interfaces were studied. The redox reactions at the tip and ITIES are as follows:

( ) Re ( ) ( ) ( ) *ZnPor o d W ZnPor o Ox W* (at the ITIES) (28)

where the redox couple represents Ru(CN)63−/4−,Mo(CN)83−/ 4−, Fe(CN)63−/ 4−, W(CN)83−/ 4−,

Although the solvent used for the Fe(CN)64− study was different from that of the other three reactants, the overall trend is consistent with the prediction of Marcus theory. Moreover, the

Re Re / *W O KC C r d d* ) is decreased, the interfacial ET rate decreases. This approach

*ZnPor o e ZnPor o* ( ) ( ) (at the tip) (27)

210 nA; ES = 0.1 V. (A.J. Bard et al, 2003)

\*, \*,

Fe(EDTA)−/2−,Ru(NH3)63+/2+, Co(Sep)3+/2+, and V3+/2+.

reductant ( 2 1

effect of aqueous ionic strength was studied by the addition of salt to the aqueous by using approach curves of SECM .

Recently, Sun et al. (Y.H. Shao et al, 2003) demonstrated the existence of an inverted region by the use of SECM with a three-electrode setup and variable concentration ratio of redox species in both phases. For the ET reaction between ZnPor in DCE and Fe(CN)6 3− in aqueous phase, at lower applied potential (0.54- *WEO* <440 mV, Ewo is the externally applied potential), the increasable trend was obvious. When the applied potential ( *WEO* ) is given at >440 mV, the values of log k12 were increasedslowly and reached a maximum at about 500 mV. When the driving force exceeded 600 mV, the rate constant of an ET reaction diminished (Fig. 6a). The reorganization energy ( = 47.2 kJ mol− 1) was obtained from th best-tting of the experimental curve. The ET reaction between TCNQ in 1, 2-dichloroethane (DCE) and Fe(CN)63− in the aqueous phase was also studied (Fig. 6b). The reorganization energy is 28.9 kJ mol−1. Compared with the studies of the forward reaction between TCNQ in DCE and Fe(CN)64− in the aqueous phase, the dependence of the back ET rate constants on *OW* for the oxidation of TCNQ− in DCE by aqueous Fe(CN)63− was studied using SECM with a xed concentration of THAP (tetra-n-hexylammonium perchlorate) in DCE.

An interface between two immiscible electrolyte solutions is considered as the simplest model for biomembranes. There is a great deal of interest in understanding charge-transfer at modied interfaces. Amphiphilie lipid molecules dissolved in an organic phase spontaneously form a monolayer lm at the water/organic interface. Although the lipids make the CT at the L/L interface more complicated, the lipid-modied interface may be a great help to study the electrical properties of membrane-bound components that control ion-transfer and electron-transfer across biological membranes. The ET reactions at lipid modied L/L interfaces have been investigated by SECM. The ET rate depends on the lipid concentration and the chain length. On the one hand, the ET rate through the ITIES decreases when the lipid concentration increases, but the ET rate reaches a limiting value and doesn't change for lipid concentrations higher than 50 Mm because of a complete lipid monolayer formed at the ITIES. On the other hand, the ET rate increases as the lipid chain length increases. The blocking effect of the lipid monolayerm makes the ET rate signicantly decreased. In addition, the extent of the blocking effect apparently depends on the driving force for ET process. The dependence of log kf on driving force for ET between Fe(CN)64− and ZnPor+ was linear with aslope of=0.59 at lower overpotential ( 630 *o W E O nm* ). At a larger overpotential, the ET rate for Co (II) sepolchrate and V2+ decreased with the increasing driving force, which was consistent with Marcus theory inverted region behavior.

To investigate the bionic ET between dopamine (DA) and ferrocene (Fc) at the water/1, 2 dichloroethane (w/DCE) interface, the recently proposed three-electrode setup is adopted based on a glassy carbon electrode and it can be used to study the CT at a L/L interface with different phase ratios.

For an ET process, the solvent effects have recently aroused much attention, such as the effects of dielectric constants and viscosities, which mainly inuence on its activation Gibbs energy and pre-exponential factor. The dependence of rate constant of the ET reaction at the interface upon the solution viscosity is evaluated using SECM. Liu and Mirkin (M.V. Mirkin et al, 1999) have investigated the effect of solvent dynamics on the rate of ET from ZnPor to Ru(CN)63− using various organic solvents. The ET rate was found to be essentially independent of the potential drop across the interfacial boundary when the organic redox

Electron Transfer Kinetics at Interfaces Using SECM (Scanning Electrochemical Microscopy) 151

Fig. 4. Voltammetric observation of electron transfer between ZnTPP and K4Fe(CN)6. (A) Cyclic voltammogram for 10 mM K4Fe(CN)6 at an uncoated EPG electrode: (C) Cyclic voltammogram with the electrode covered with 1.5 µL of NB containing 1 mM ZnTPP and

some advantages, including the availability of nanometer-sized pipet tips and the possibility to work without redox mediator in solution. Most IT reactions at the ITIES are often very fast and hard to measure. A mass-transfer rate sufciently high for measurements of rapid IT kinetics under steady-state conditions can be obtained using nanometer-sized pipets based ITIES. Cai et al. measured the kinetic parameters for two rapid simple ion transfer (IT) reactions—the transfers of tetraethylammonium (TEA+) and tetramethylammonium (TMA+) ions between DCE and water. An excellent agreement of the k◦ values was obtained for the forward (from DCE to water) and reverse (from water to DCE) transfer of TEA+. Ion transfer across L/L interface facilitated by various types of ionophores has been extensively investigated for different cations. The process of facilitated K+ transfer by dibenzo-18-crown-6 (DB18C6) from water into DCE has been studied extensively in the past. The reactions

*K W DB C DCE K DB C DCE* ( ) 18 6( ) 18 6 ( ) (at the pipet tip) (29)

*K DB C DCE K W DB C DCE* 18 6 ( ) ( ) 18 6( ) (at the ITIES) (30)

The tip current was produced by the facilitated transfer of K+ from the aqueous solution inside the pipette into DCE assisted by DB18C6. With the concentration of K+ inside apipette is much higher than that of DB18C6 in DCE phase, the tip current is limited by diffusion of DB18C6 to the pipet orice. When the tip approaches the aqueous layer, DB18C6 is regenerated to its neutral form by interfacial dissociation and diffuses back to the tip to establish the feedback effect. The kinetics of these processes has been determined using

supporting electrolyte (0.01 M TBAClO4).

studied in the system can be described as follows:

reactant is a neutral species. Miao et al. (A.J. Bard et al, 2002) studied the heterogeneous electron-transfer kinetics for the oxidation of ferrocenemethanol (FcCH2OH) over the whole composition range of dimethyl sulfoxide (DMSO)–water solutions of different viscosities ( ) containing 50.0 mM (CH3)4NClO4 (TMAP) at a Pt microelectrode by the SECM technique. Bai et al. (Y.H. Shao et al, 2003) reported the effect of solution viscosity on heterogeneous ET reaction kinetics between TCNQ and ferrocyanide. The k12 dependence on viscosity was explained. The ET rate constant was shown to be inversely proportional to the aqueous solution viscosity and the solvent longitudinal relaxation time, and directly proportional to the diffusion coefcient of the electroactive species. TLCV is demonstrated to be a useful means for investigating the kinetics of heterogeneous consecutive ET. (revised Scheme 3 and Fig.4) X.Q. Lu. et al. (X.Q. Lu et al., 2010) study two-step electron transfer of ZnTPP/[Fe(CN)6]4-, indicating that the Bulter-Volmer (B-V) theory is suitable for the consecutive electron transfer. This system provides an interesting example of how the relatively new theory of TLCV is used to probe more complex biological redox chemistry.

## **8.2.2 Ion transfer (IT) at the ITIES**

Heterogeneous IT reactions are essential for many biological and technological systems. Especially, simple IT reactions are numerous and of great importance for biological systems. Wei et al. were the rst to apply the SECM for investigating the IT process at the ITIES induced by ET. They demonstrated IT limitation on the overall kinetics in two cases: the organic phase without electrolyte and the effect of the concentration of the potential determining ion (TEA+) on the shape of feedback curves. SECM is possible to probe simple (unassisted) IT reactions and map ion-transfer reactivity of the interfacial boundary with ion-transfer feedback mode of the SECM and basing some ions (mainly Na+ and K+), play a fundamental role in the excitability of the nerves and muscles because they are transported through the ionic channels of their cells, the ion transport traversing the "Ion Channels" was investigated by Scanning Electrochemical Microscopy (SECM). The ion-transfer feedback mode of the SECM that is used for imaging of solid/liquid and liquid/liquid interfaces has

Scheme 3. Simplied Two-Step Electron Transfer Process of Respiratory Chain in the Mitochondria and the Corresponding Reaction at the Electrode

reactant is a neutral species. Miao et al. (A.J. Bard et al, 2002) studied the heterogeneous electron-transfer kinetics for the oxidation of ferrocenemethanol (FcCH2OH) over the whole composition range of dimethyl sulfoxide (DMSO)–water solutions of different viscosities

 ) containing 50.0 mM (CH3)4NClO4 (TMAP) at a Pt microelectrode by the SECM technique. Bai et al. (Y.H. Shao et al, 2003) reported the effect of solution viscosity on heterogeneous ET reaction kinetics between TCNQ and ferrocyanide. The k12 dependence on viscosity was explained. The ET rate constant was shown to be inversely proportional to the aqueous solution viscosity and the solvent longitudinal relaxation time, and directly proportional to the diffusion coefcient of the electroactive species. TLCV is demonstrated to be a useful means for investigating the kinetics of heterogeneous consecutive ET. (revised Scheme 3 and Fig.4) X.Q. Lu. et al. (X.Q. Lu et al., 2010) study two-step electron transfer of ZnTPP/[Fe(CN)6]4-, indicating that the Bulter-Volmer (B-V) theory is suitable for the consecutive electron transfer. This system provides an interesting example of how the relatively new theory of TLCV is used to probe more complex biological redox chemistry.

Heterogeneous IT reactions are essential for many biological and technological systems. Especially, simple IT reactions are numerous and of great importance for biological systems. Wei et al. were the rst to apply the SECM for investigating the IT process at the ITIES induced by ET. They demonstrated IT limitation on the overall kinetics in two cases: the organic phase without electrolyte and the effect of the concentration of the potential determining ion (TEA+) on the shape of feedback curves. SECM is possible to probe simple (unassisted) IT reactions and map ion-transfer reactivity of the interfacial boundary with ion-transfer feedback mode of the SECM and basing some ions (mainly Na+ and K+), play a fundamental role in the excitability of the nerves and muscles because they are transported through the ionic channels of their cells, the ion transport traversing the "Ion Channels" was investigated by Scanning Electrochemical Microscopy (SECM). The ion-transfer feedback mode of the SECM that is used for imaging of solid/liquid and liquid/liquid interfaces has

Scheme 3. Simplied Two-Step Electron Transfer Process of Respiratory Chain in the

Mitochondria and the Corresponding Reaction at the Electrode

(

**8.2.2 Ion transfer (IT) at the ITIES**

Fig. 4. Voltammetric observation of electron transfer between ZnTPP and K4Fe(CN)6. (A) Cyclic voltammogram for 10 mM K4Fe(CN)6 at an uncoated EPG electrode: (C) Cyclic voltammogram with the electrode covered with 1.5 µL of NB containing 1 mM ZnTPP and supporting electrolyte (0.01 M TBAClO4).

some advantages, including the availability of nanometer-sized pipet tips and the possibility to work without redox mediator in solution. Most IT reactions at the ITIES are often very fast and hard to measure. A mass-transfer rate sufciently high for measurements of rapid IT kinetics under steady-state conditions can be obtained using nanometer-sized pipets based ITIES. Cai et al. measured the kinetic parameters for two rapid simple ion transfer (IT) reactions—the transfers of tetraethylammonium (TEA+) and tetramethylammonium (TMA+) ions between DCE and water. An excellent agreement of the k◦ values was obtained for the forward (from DCE to water) and reverse (from water to DCE) transfer of TEA+. Ion transfer across L/L interface facilitated by various types of ionophores has been extensively investigated for different cations. The process of facilitated K+ transfer by dibenzo-18-crown-6 (DB18C6) from water into DCE has been studied extensively in the past. The reactions studied in the system can be described as follows:

$$K^{+}\text{(IV)} + DB18C6\text{(DCE)} = \left[K^{+}DB18C6\right]\text{(DCE)}\quad\text{(at the pipe tip)}\tag{29}$$

$$\left[K^{+}DB18C6\right] \text{(DCE)} = K^{+} \text{(W)} + DB18C6 \text{(DCE)} \text{ (at the ITIES)} \tag{50}$$

The tip current was produced by the facilitated transfer of K+ from the aqueous solution inside the pipette into DCE assisted by DB18C6. With the concentration of K+ inside apipette is much higher than that of DB18C6 in DCE phase, the tip current is limited by diffusion of DB18C6 to the pipet orice. When the tip approaches the aqueous layer, DB18C6 is regenerated to its neutral form by interfacial dissociation and diffuses back to the tip to establish the feedback effect. The kinetics of these processes has been determined using

Electron Transfer Kinetics at Interfaces Using SECM (Scanning Electrochemical Microscopy) 153

interfacial rate constant decreased with increasing concentration of C18:0. This inhibited behaviour was explained in terms of a simple energy barrier mode. Cannan et al. (S. Cannan et al., 2004) have reported the kinetics of oxygen transfer across an L-R-phosphatidylethanolamine, distearoyl (DSPE) monolayer spread at three different interfaces by using SECM combined with a Langmuir trough. At all three interfaces, oxygen transfer was diffusion controlled when the monolayer was in the liquid expanded state, but the rate of transfer decreased with increasing surface pressure in the liquid condensed state. For the decane/water interfaces, oxygen transfer was diffusion-controlled up to surface pressures of approximately 20 mN m−1 for the thin layer and 40 mN m−1 for the thicker layer. At higher pressures, the rate of oxygen transfer decreased rapidly, especially at the thick decane interface. Moreover, the rate constant for oxygen transfer at thick oil/water was higher than that at thin oil/water or air/water for any given surface pressure. The phenomenon showed oxygen had an easier permeation at a thick oil/water interface where the hydrocarbon tail

In recent years, the applications of the SECM have greatly increased in numerous elds due to the fabrication of miniaturized tips and new combinations with other techniques, for instance, with atomic force microscopy, optical microscopy (OM), single-molecule uorescence spectroscopy (SMFS), electrochemical quartz crystal microbalance (EQCM), Langmuir trough technique, electrochemical scanning tunneling microscopy (ECSTM), scanning force microscopy (SFM) and surface plasmon resonance imaging (SPR-i) These combinations with SECM make the spatial resolution, accuracy, sensitivity further improved. In addition, more information about the topographic, optical, kinetic, or photoelectrochemical properties in situ

at various interfaces can be obtained. Obviously, these combinations are to be a trend.

for the transfer of oxygen across a DSPE monolayer spread at the

interface between (a) air/water, (b) thin decane lm/water, and (c) thick decane/water (S.

The use of mesoporous oxide films as a substrate to anchor the dye molecules allows sunlight to be harvested over a broad spectral range in the visible region. Similarly to chlorophyll in the green leaf, the dye acts as an electron transfer sensitizer. Upon excitation by light, it injects an electron into the conduction band of the oxide, resulting in the separation of positive and negative charges. Charge transfer from photoexcited dyes into

**8.3 Photoinduced charge transfer at the solid/electrolyte interface**

region of the phospholipid was less ordered (Fig. 6).

Fig. 5. Plot of ln k`vs.

Cannan et al., 2004).

Ⅱ

SECM combined with nanopipet and a three-electrode arrangement. A new method in which an SECM can be combined with a polarized L/L interface was introduced. Recently, the group of Shao (Y.H. Shao et al, 2006) has investigated the dependence of rate constants (kf) on the potential drop woof facilitated ion-transfer reaction at the W/DCE interface. The FIT rate constants kf were found to be dependent upon the driving force. When the driving force was low, the dependence of ln kf on the driving force was linear with a transfer coefcient of about 0.3 in the case of a facilitated Na+-transfer reaction, which followed theclassical Butler–Volmer theory. At higher driving, the Marcus inverted region was observed in the facilitated Li+-transfer system In addition, it was reported that the ion transfer across the micro-water/nitrobenzene interface supported at the tip of a micropipet could be facilitated by a functionalized fullerene derivative using cyclic voltammetry (CV) and osteryoung square wave voltammetry (OSWV).

The kinetic information about IT reactions at planar bilayer lipid membranes (BLMs) was extracted by SECM with a Pt microelectrode as the tip, where membrane permeability to conventional redox mediators was studied. The voltammetric K+-selective micropipet electrodes were used in the SECM feedback and generation–collection experiments to probe K+ transfer through gramicidin channels in BLMs. Afterwards, Ti(I) can be used as a surrogate for K(I), so that ion transport across gramicidin channels can be monitored using an amperometric UME instead of an ion-selective probe. An apparent heterogeneous rate constant (khet = 2.8 ( ± 0.1) × 10−4 cm s−1) for the transport of Tl(I) through the gramicidin to the tip was obtained . The charge transfer coupling (CTC) processes at L/L interface are common phenomena, which play an important role in many chemical and biological systems.Moreover, the processes of coupling ET–IT and IT–IT have been probed by SECM.

## **8.2.3 Molecular transfer**

Studies of molecular transfer processes of non-charged species at the L/L interfaces can be performed by SECM. The effect of fatty alcohol monolayers on the rate constant for Br2 transfer across the water/air (w/A) interface has been investigated using SECM-DPSC. The intracellular transfer of oxygen is of importance in bioenergetics and the diffusion of O2 through membranes is an active area of research .The adequate supply of oxygen to cells is necessary for the growth of multicellular systems; lack of oxygen can cause cell death or necrosis. Scanning electrochemical microscopy induced transfer (SECMIT) was introduced by the Unwin's group. SECMIT is a development of the SECM equilibrium perturbation method. It can be used to characterize reversible phase transfer processes at a wide variety of interface. In other words, the SECM tip can be used to locally deplete the concentration of an electroactive species near the ITIES and induce the transfer of this species from the second liquid phase. It was shown to be a valuable method for probing the dynamics of partitioning of electroactive solutes between two immisciblen phases. For the special case of slow interfacial transfer, the theory is conceptually similar to simple (unassisted) IT. Recently, SECMIT was applied to investigate the transport of molecular oxygen across phosphatidylcholine monolayers adsorbed at the interface between a buffered aqueous phase and DCE. The inuence of different lipids, a series of 1, 2-diacyl-sn-glycero-3 phosphocholines (C14:0, C16:0, C18:0), on the dioxygen transfer process acrossn the interface was studied, which demonstrated that a monolayer of C18:0 formed a barrier for the transport of a small molecule (O2) from DCE to water but there is relatively small effect on dioxygen transfer across C14:0 and C16:0 monolayers at ambient temperature. The

SECM combined with nanopipet and a three-electrode arrangement. A new method in which an SECM can be combined with a polarized L/L interface was introduced. Recently, the group of Shao (Y.H. Shao et al, 2006) has investigated the dependence of rate constants (kf) on the potential drop woof facilitated ion-transfer reaction at the W/DCE interface. The FIT rate constants kf were found to be dependent upon the driving force. When the driving force was low, the dependence of ln kf on the driving force was linear with a transfer coefcient of about 0.3 in the case of a facilitated Na+-transfer reaction, which followed theclassical Butler–Volmer theory. At higher driving, the Marcus inverted region was observed in the facilitated Li+-transfer system In addition, it was reported that the ion transfer across the micro-water/nitrobenzene interface supported at the tip of a micropipet could be facilitated by a functionalized fullerene derivative using cyclic voltammetry (CV)

The kinetic information about IT reactions at planar bilayer lipid membranes (BLMs) was extracted by SECM with a Pt microelectrode as the tip, where membrane permeability to conventional redox mediators was studied. The voltammetric K+-selective micropipet electrodes were used in the SECM feedback and generation–collection experiments to probe K+ transfer through gramicidin channels in BLMs. Afterwards, Ti(I) can be used as a surrogate for K(I), so that ion transport across gramicidin channels can be monitored using an amperometric UME instead of an ion-selective probe. An apparent heterogeneous rate constant (khet = 2.8 ( ± 0.1) × 10−4 cm s−1) for the transport of Tl(I) through the gramicidin to the tip was obtained . The charge transfer coupling (CTC) processes at L/L interface are common phenomena, which play an important role in many chemical and biological systems.Moreover, the processes of coupling ET–IT and IT–IT have been probed by SECM.

Studies of molecular transfer processes of non-charged species at the L/L interfaces can be performed by SECM. The effect of fatty alcohol monolayers on the rate constant for Br2 transfer across the water/air (w/A) interface has been investigated using SECM-DPSC. The intracellular transfer of oxygen is of importance in bioenergetics and the diffusion of O2 through membranes is an active area of research .The adequate supply of oxygen to cells is necessary for the growth of multicellular systems; lack of oxygen can cause cell death or necrosis. Scanning electrochemical microscopy induced transfer (SECMIT) was introduced by the Unwin's group. SECMIT is a development of the SECM equilibrium perturbation method. It can be used to characterize reversible phase transfer processes at a wide variety of interface. In other words, the SECM tip can be used to locally deplete the concentration of an electroactive species near the ITIES and induce the transfer of this species from the second liquid phase. It was shown to be a valuable method for probing the dynamics of partitioning of electroactive solutes between two immisciblen phases. For the special case of slow interfacial transfer, the theory is conceptually similar to simple (unassisted) IT. Recently, SECMIT was applied to investigate the transport of molecular oxygen across phosphatidylcholine monolayers adsorbed at the interface between a buffered aqueous phase and DCE. The inuence of different lipids, a series of 1, 2-diacyl-sn-glycero-3 phosphocholines (C14:0, C16:0, C18:0), on the dioxygen transfer process acrossn the interface was studied, which demonstrated that a monolayer of C18:0 formed a barrier for the transport of a small molecule (O2) from DCE to water but there is relatively small effect on dioxygen transfer across C14:0 and C16:0 monolayers at ambient temperature. The

and osteryoung square wave voltammetry (OSWV).

**8.2.3 Molecular transfer**

interfacial rate constant decreased with increasing concentration of C18:0. This inhibited behaviour was explained in terms of a simple energy barrier mode. Cannan et al. (S. Cannan et al., 2004) have reported the kinetics of oxygen transfer across an L-R-phosphatidylethanolamine, distearoyl (DSPE) monolayer spread at three different interfaces by using SECM combined with a Langmuir trough. At all three interfaces, oxygen transfer was diffusion controlled when the monolayer was in the liquid expanded state, but the rate of transfer decreased with increasing surface pressure in the liquid condensed state. For the decane/water interfaces, oxygen transfer was diffusion-controlled up to surface pressures of approximately 20 mN m−1 for the thin layer and 40 mN m−1 for the thicker layer. At higher pressures, the rate of oxygen transfer decreased rapidly, especially at the thick decane interface. Moreover, the rate constant for oxygen transfer at thick oil/water was higher than that at thin oil/water or air/water for any given surface pressure. The phenomenon showed oxygen had an easier permeation at a thick oil/water interface where the hydrocarbon tail region of the phospholipid was less ordered (Fig. 6).

In recent years, the applications of the SECM have greatly increased in numerous elds due to the fabrication of miniaturized tips and new combinations with other techniques, for instance, with atomic force microscopy, optical microscopy (OM), single-molecule uorescence spectroscopy (SMFS), electrochemical quartz crystal microbalance (EQCM), Langmuir trough technique, electrochemical scanning tunneling microscopy (ECSTM), scanning force microscopy (SFM) and surface plasmon resonance imaging (SPR-i) These combinations with SECM make the spatial resolution, accuracy, sensitivity further improved. In addition, more information about the topographic, optical, kinetic, or photoelectrochemical properties in situ at various interfaces can be obtained. Obviously, these combinations are to be a trend.

Fig. 5. Plot of ln k`vs.Ⅱ for the transfer of oxygen across a DSPE monolayer spread at the interface between (a) air/water, (b) thin decane lm/water, and (c) thick decane/water (S. Cannan et al., 2004).

### **8.3 Photoinduced charge transfer at the solid/electrolyte interface**

The use of mesoporous oxide films as a substrate to anchor the dye molecules allows sunlight to be harvested over a broad spectral range in the visible region. Similarly to chlorophyll in the green leaf, the dye acts as an electron transfer sensitizer. Upon excitation by light, it injects an electron into the conduction band of the oxide, resulting in the separation of positive and negative charges. Charge transfer from photoexcited dyes into

Electron Transfer Kinetics at Interfaces Using SECM (Scanning Electrochemical Microscopy) 155

separation of electron transfer and ion transfer processes. These advantages of SECM make it become a ver- satile technique for determining electron-transfer kinetics, micropatterning and imaging of cellular activities of single cells. The development of UME and the combination of SECM and other techniques show much higher spatial resolution and precision. With the improvement of lateral resolution and sensitive detection, SECM will extend a wide variety of applications and show a great potential as analytical and microfabrication tool, especially in

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**10. References**

semiconductors was discovered more than a century ago in a famous experiment. He observed that the photoelectric effect reported earlier by Becquerel on silver plates was enhanced in the presence of erythrosin dye. A few years before, Vogel in Berlin had associated dyes with the halide semiconductor grains to make them sensitive to visible light. This led to the first panchromatic film, able to render the image of a scene realistically in black and white. However, the clear recognition of the parallelism between the two procedures, a realization that the same dyes in principle can function in both systems, and a verification that their operating mechanism is by injection of electrons from photoexcited dye molecules into the conduction band of the n-type semiconductor substrates date to the 1960s. In subsequent years the idea developed that the dye could function most efficiently if chemisorbed on the surface of the semiconductor. The concept emerged of using dispersed particles to provide a sufficient interface, then photo-electrodes were employed. Finally, the use of nanocrystalline Ti02 films sensitized by a suitable molecular dye provided an important technological breakthrough. These mesoporous membranes have allowed in effect for the first time the development of a regenerative photoelectrochemical cell based on a simple molecular light absorber, which attains a conversion efficiency commensurate with that of silicon-based photovoltaic devices, but at a much lower cost.

The photoinduced electron-transfer (PET) processes are of vital importance in the versatility of biological and chemical systems, which exhibited extensive research prospects, such as photocatalysis, photo-to-electric conversion, photosynthesis, and photo-induced supramolecular electron-transfer (ET). Photoelectrochemical properties of porphyrin compounds, including porphyrin-fullerene dyad and ferrocene-porphyrin-fullerene triad, were investigated by self-assembled monolayers (SAMs) on nanostructured substance and electrode surfaces. With the development of SECM techenology, it has been demonstrated to be an effective means of determining electron transfer (ET) kinetics on polymer films at electrodes, Langmuir monolayers at air-water interfaces, liquid-liquid interfaces, phospholipid bilayers, the traditional metal electrode-electrolyte solution interfaces, thiolporphyrin self-assembled monolayers, as well as the semiconductor/electrolyte interface (SEI). As well all known, almost each-step ET occurring in nature can not proceed without light, indicating the studying on PET has important practical implications. Because porphyrin molecules were an important class of conjugated organic molecules for light harvesting in photosynthesis, and exhibited ultrafast electron injection, slow charge-recombination kinetics, high absorption coefficients and good chemical stability, the carboxylic groups on some porphyrins can spontaneously bind to TiO2 nanoparticles to obtain the excellent photosensitized material (H.X. Ju et al., 2011). By fitting experimental feedback curves to theory ones, the heterogeneous rate constant (keff) is estimated, meanwhile, the dependence of keff with light source wavelength and intensity is determined (X.Q. Lu et al., 2011).

## **9. Scope**

The scanning electrochemical microscopy has been proven to be a powerful instrument for the quantitative investigation and surface analysis of a wide range of processes that occur at interfaces. SECM has the advantage that measurements are carried out under steady-state conditions, eliminating the resistive potential drop in solution, double layer charging current and some difculties that are frequently associated with the other traditionally electrochemical techniques. In addition, the SECM technique has provided a great advantage of easy separation of electron transfer and ion transfer processes. These advantages of SECM make it become a ver- satile technique for determining electron-transfer kinetics, micropatterning and imaging of cellular activities of single cells. The development of UME and the combination of SECM and other techniques show much higher spatial resolution and precision. With the improvement of lateral resolution and sensitive detection, SECM will extend a wide variety of applications and show a great potential as analytical and microfabrication tool, especially in life sciences and in the material.

## **10. References**

154 Recent Trend in Electrochemical Science and Technology

semiconductors was discovered more than a century ago in a famous experiment. He observed that the photoelectric effect reported earlier by Becquerel on silver plates was enhanced in the presence of erythrosin dye. A few years before, Vogel in Berlin had associated dyes with the halide semiconductor grains to make them sensitive to visible light. This led to the first panchromatic film, able to render the image of a scene realistically in black and white. However, the clear recognition of the parallelism between the two procedures, a realization that the same dyes in principle can function in both systems, and a verification that their operating mechanism is by injection of electrons from photoexcited dye molecules into the conduction band of the n-type semiconductor substrates date to the 1960s. In subsequent years the idea developed that the dye could function most efficiently if chemisorbed on the surface of the semiconductor. The concept emerged of using dispersed particles to provide a sufficient interface, then photo-electrodes were employed. Finally, the use of nanocrystalline Ti02 films sensitized by a suitable molecular dye provided an important technological breakthrough. These mesoporous membranes have allowed in effect for the first time the development of a regenerative photoelectrochemical cell based on a simple molecular light absorber, which attains a conversion efficiency commensurate with

The photoinduced electron-transfer (PET) processes are of vital importance in the versatility of biological and chemical systems, which exhibited extensive research prospects, such as photocatalysis, photo-to-electric conversion, photosynthesis, and photo-induced supramolecular electron-transfer (ET). Photoelectrochemical properties of porphyrin compounds, including porphyrin-fullerene dyad and ferrocene-porphyrin-fullerene triad, were investigated by self-assembled monolayers (SAMs) on nanostructured substance and electrode surfaces. With the development of SECM techenology, it has been demonstrated to be an effective means of determining electron transfer (ET) kinetics on polymer films at electrodes, Langmuir monolayers at air-water interfaces, liquid-liquid interfaces, phospholipid bilayers, the traditional metal electrode-electrolyte solution interfaces, thiolporphyrin self-assembled monolayers, as well as the semiconductor/electrolyte interface (SEI). As well all known, almost each-step ET occurring in nature can not proceed without light, indicating the studying on PET has important practical implications. Because porphyrin molecules were an important class of conjugated organic molecules for light harvesting in photosynthesis, and exhibited ultrafast electron injection, slow charge-recombination kinetics, high absorption coefficients and good chemical stability, the carboxylic groups on some porphyrins can spontaneously bind to TiO2 nanoparticles to obtain the excellent photosensitized material (H.X. Ju et al., 2011). By fitting experimental feedback curves to theory ones, the heterogeneous rate constant (keff) is estimated, meanwhile, the dependence of

keff with light source wavelength and intensity is determined (X.Q. Lu et al., 2011).

The scanning electrochemical microscopy has been proven to be a powerful instrument for the quantitative investigation and surface analysis of a wide range of processes that occur at interfaces. SECM has the advantage that measurements are carried out under steady-state conditions, eliminating the resistive potential drop in solution, double layer charging current and some difculties that are frequently associated with the other traditionally electrochemical techniques. In addition, the SECM technique has provided a great advantage of easy

**9. Scope**

that of silicon-based photovoltaic devices, but at a much lower cost.

**Part 3**

**Electrochemical Energy Storage Devices**

## **Part 3**

## **Electrochemical Energy Storage Devices**

156 Recent Trend in Electrochemical Science and Technology

[36] C. Zhao, G. Wittstock. (2004). *Angew. Chem*. Int. Ed. 43, 4170. [37] T. Koike., H. Yamada, et al. (2005). *Anal. Chem*. 77,1785.

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[46] W.J. Miao, Z.F. Ding, A.J. Bard. (2002). *J. Phys. Chem. B.* 106, 1392. [47] X. Q. Lu., Sun P., et al., (2010) Anal. Chem., 82, 8598–8603. [48] Y.H. Shao., Y.M. Bai, et al. (2003). *Electrochim. Acta.* 48, 3447. [49] Y.H. Shao., P. Sun., et al. (2002). *Angew. Chem*. Int. Ed. 41, 3445.

[50] S. Cannan, J. Zhang, F. Grunfeld, P.R. (2004). *Unwin, Langmuir*. 20, 701.

[44] Y.H. Shao., P. Sun, et al. (2003). *J. Am. Chem. Soc*. 125, 9600. [45] B. Liu, M.V. Mirkin. (1999). *J. Am. Chem. Soc*. 121, 8352.

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**7**

*Russia*

**Studies of Supercapacitor Carbon**

D.A. Bograchev, V.E. Sosenkin and V.S. Bagotsky

*A. N. Frumkin Institute of Physical Chemistry and Electrochemistry,*

Yu.M. Volfkovich, A.A. Mikhailin,

*Russian Academy of Sciences, Moscow,*

 **Electrodes with High Pseudocapacitance**

During the last decades different new capacitor types were developed based on electrochemical processes. According to Conway [1] an electrochemical capacitor is a device in which different quasi-reversible electrochemical charging/discharging processes take place and for which the shape of the charging and discharging curves is almost linear, similarily to those in common electrostatic capacitors [1-13]. Electrochemical capacitors can

**Electrolytic capacitors** based on aluminium foils and liquid electrolytes are well-known for many decades. In them a thin film (thickness in the order of micrometers) of aluminum oxide prepared by electrochemically oxidizing the Al foils serves as dielectric film. Their

**Electrochemical supercapacitors (ESCs)** can be subdivided into electrical double-layer capacitors (EDLCs), pseudo-capacitors, and hybride-type capacitors. Historically the first ESCs which were developed were EDLCs. Up to now they remain the most important ESC version. The first prototypes of EDLCs were developed in the 1970s in Russia by N. Lidorenko and A. Ivanov [14] and also in Japan under the names "molecular energy

**The double-layer capacitor (EDLC)** comprise two porous polarizable electrodes. The accumulation of energy in them proceeds through dividing positive and negative electrical charges between the two electrodes while maintaining a potential difference *U* between them. The electrical charge on each electrode depends on the electrical double-layer (EDL) capacity. Due to the very low thickness of this layer (tenths of a nanometer) the capacity value referred to unit of electrode's surface area is much higher than for electrolytic

In order to achieve high capacity values in EDLCs highly dispersed carbonaceous electrodes with a high specific surface area of 1000-3000 m2/g are used, such as activated carbon (AC) or activated carbon cloths (ACC), nanofibers, nanotubes graphene sheets. The specific

be classified as film-type (dielectric), electrolytic and supercapacitors.

specific energy is of the order of some hundredths Wh/L.

capacitors. Hence the term "*supercapacitor*" was introduced.

energy density of such capacitors reaches values of 1-20 Wh/L.

**1. Introduction**

accumulators" and "*Ionistors*".

## **Studies of Supercapacitor Carbon Electrodes with High Pseudocapacitance**

Yu.M. Volfkovich, A.A. Mikhailin, D.A. Bograchev, V.E. Sosenkin and V.S. Bagotsky *A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia*

## **1. Introduction**

During the last decades different new capacitor types were developed based on electrochemical processes. According to Conway [1] an electrochemical capacitor is a device in which different quasi-reversible electrochemical charging/discharging processes take place and for which the shape of the charging and discharging curves is almost linear, similarily to those in common electrostatic capacitors [1-13]. Electrochemical capacitors can be classified as film-type (dielectric), electrolytic and supercapacitors.

**Electrolytic capacitors** based on aluminium foils and liquid electrolytes are well-known for many decades. In them a thin film (thickness in the order of micrometers) of aluminum oxide prepared by electrochemically oxidizing the Al foils serves as dielectric film. Their specific energy is of the order of some hundredths Wh/L.

**Electrochemical supercapacitors (ESCs)** can be subdivided into electrical double-layer capacitors (EDLCs), pseudo-capacitors, and hybride-type capacitors. Historically the first ESCs which were developed were EDLCs. Up to now they remain the most important ESC version. The first prototypes of EDLCs were developed in the 1970s in Russia by N. Lidorenko and A. Ivanov [14] and also in Japan under the names "molecular energy accumulators" and "*Ionistors*".

**The double-layer capacitor (EDLC)** comprise two porous polarizable electrodes. The accumulation of energy in them proceeds through dividing positive and negative electrical charges between the two electrodes while maintaining a potential difference *U* between them. The electrical charge on each electrode depends on the electrical double-layer (EDL) capacity. Due to the very low thickness of this layer (tenths of a nanometer) the capacity value referred to unit of electrode's surface area is much higher than for electrolytic capacitors. Hence the term "*supercapacitor*" was introduced.

In order to achieve high capacity values in EDLCs highly dispersed carbonaceous electrodes with a high specific surface area of 1000-3000 m2/g are used, such as activated carbon (AC) or activated carbon cloths (ACC), nanofibers, nanotubes graphene sheets. The specific energy density of such capacitors reaches values of 1-20 Wh/L.

Studies of Supercapacitor Carbon Electrodes with High Pseudocapacitance 161

higher specific energy (up to 10-20 Wh/kg). The lower Umax value for symmetric AC systems in comparison with the hybride versions is due to the fact that at not very high anodic potentials (0.9-1.0 V) an oxidative carbon corrosion is observed while hydrogen evolution potential on the the hybride's negative AC electrode remains at very negative values up to -0.8 V). An advantage of hybride supercapacitors over the corresponding storage batteries is the much higher cycling capability, and the possibilities of faster charging and of easier hermetically sealing. Hybride PbO2/H2SO4/AC capacitors are used

In [19] a hybride supercapacitor with a RuO2 positive electrode and an AC negative

An important feature of supercapacitors in comparison with storage batteries is the possibility of rapid charging and discharging in very broad time intervals ranging from less than one second to several hours. Correspondingly supercapacitors can be subdivided into power units with high values of specific power and into energy units with high specific

Power supercapcitors allow carrying out the charging and discharging processes in very short time periods (from fractions of a second to minutes) and obtaining herewith high power characteristics from 1 to 5 kW/kg in concentrated aqueous solutions with high specific conductivity. Measurements for highly dispersed carbon electrodes in the energy capacitor operation modes usually yield the specific charge values in the range of 40 to 200 C/g [20, 21]. In the case of carbon materials, the limiting capacity obtained in [22] was 320 F/g due to a considerable contribution of pseudocapacitance of reversible surface group redox reactions (thus, they are not pure EDLCs anymore). In [23] high power characteristics (above 20 kW/kg) were obtained for electrodes based on single-wall carbon nanotubes (SWCNT). Such high power values can be explained by the regularity of the SWCNT's pore structure. As can be seen from Fig. 1 representing a SEM picture of this material, the pores are neither corrugated nor curving, thus providing for a high conductivity and, therefore,

The electrodes used in energy–type ECSCs are often electrodes on which rather reversible faradaic processes occur. Such electrodes include electrodes based on electron–conducting polymers (polyaniline, polythiophene, polypyrrole etc.) and also electrodes based on some oxides of variable–valency metals (oxides of ruthenium, iridium, tungsten, molybdenum, zirconium etc.) [1, 2]. These electrodes feature different limitations for practical application, such as expensiveness, insufficient cyclability due to degradation processes etc. Using nonaqueous electrolytes in ECSC with electrodes based on highly dispersed carbon materials allows obtaining high (up to 3–3.5 V) charging voltage values, which significantly enhances the energy but limits the power of capacitors due to low conductivity of these electrolytes [1, 2, 24]. Aqueous alkali solutions allow obtaining rather high power values, but the low operating voltage range (about 0.8 V) decreases the energy characteristics of ECSC. Aqueous electrolytes with the highest conductivity are sulfuric acid solutions with concentrations from 30 to 40 wt. %. Besides, the working voltage range in the region of reversible processes proves to be above 1 V due to the relatively low corrosion activity

towards carbon as compared to other aqueous electrolytes.

electrode was described. A high specific energy of 26.7 Wh/kg was reported.

in wheelchairs and in electrical motor-buses.

energy values.

for a high power.

For a EDLSC with ideal polarizable electrodes the energy A delivered through a single discharge can be represented as:

$$\mathbf{A} = \begin{pmatrix} \mathbf{1}/\mathbf{2} \end{pmatrix} \mathbf{C} \begin{bmatrix} \begin{pmatrix} \mathbf{U}\_{\max} \end{pmatrix} \, \mathbf{2} \text{ (} \mathbf{U}\_{\min} \text{)} \, \mathbf{2} \end{bmatrix} , \tag{1}$$

where С is the average electrode's capacity, and Umax and Umin are the initial, resp. the final values of discharge voltage. For full discharge until Umin = 0 the maximal discharge energy will be: A = Аmax = (1/2)C[(Umax ) 2].

The development of EDLCs was induced by a necessity for rechargeable power sources with higher enegy values, and power capabilities, and much better cyclability properties than those for existing storage batteries. Among the most remarkable features of EDLCs are excellent cyclability (hundred of thousands charge/discharge cycles as compared with hundreds of cycles for storage batteries) and the possibility to deliver for short periods high power and current densities, and also the possibility to be used at high and low temperatures (up to +60oC and down to -500C). The cycling efficiency (ratio of energy consumed during charging and delivered during discharge) is about 92-95 %. Taking into account these properties, very promising are the following fields of applications for EDLCs: in ICE vehicles (in parallel with storage batteries) for starting purposes, delivering the necessary initial peak power (especially at low temperatures) and thus increasing the battery life time; and also in all-electric and in hybride vehicles for energy recuperation during slow-down and braking.

In *pseudocapacitors* electrical charges are accumulated mainly as the result of fairly reversible redox reactions (faradaic pseudo-capacity). Many such reactions are known in which oxides and sulfides of transition metals RuO2, IrO2, TiS2 or their combinations take part.

One of the important achievements of modern electrochemistry is the development of *electron-conducting polymers*. Electrochemical reactions in systems with conjugated double bonds such as polyanilin, polythiophene, polypyrolle, polyacethylene and others, are reversible and can be used in supercapacitors. Such processes are called electrochemical doping or dedoping the polymers with anions and cations. The electron conductivity during doping is due to the formation of delocalized electrons or electron-holes and their migration in the system of polyconjugated double-bonds under the influence of an applied electrical field. The use of some electron-conducting polymers as supercapacitor electrodes is based on the high reversibility of doping and dedoping reactions and the high conductance values of such polymers. The specific energy values of pseudocapacitors are fairly high 10-50 Wh/L and their cyclability reaches hundred thousands cycles. A disadvantage of supercapacitors based on transition metal oxides and sulfides is their high price. A disadvantage of those based on electron-conducting polymers is their insufficient stability.

Recently some *hybride-type supercapacitors* were developed in which different types of electrodes are used. In [15-18] capacitors were investigated in which the positive electrodes were based on metal oxides, and the negative electrodes on activated carbon ( e.g. the system NiOOH/KOH/AC or the system PbO2/H2SO4/AC. In both of these two systems as positive electrodes conventional electrodes from alkaline, resp. lead-acid storage batteries are used. An advantage of the hybride-type supercapacitors as compared with their analogues, with symmetrical AC, is their higher Umax value and correspondingly their

For a EDLSC with ideal polarizable electrodes the energy A delivered through a single

where С is the average electrode's capacity, and Umax and Umin are the initial, resp. the final values of discharge voltage. For full discharge until Umin = 0 the maximal discharge energy

The development of EDLCs was induced by a necessity for rechargeable power sources with higher enegy values, and power capabilities, and much better cyclability properties than those for existing storage batteries. Among the most remarkable features of EDLCs are excellent cyclability (hundred of thousands charge/discharge cycles as compared with hundreds of cycles for storage batteries) and the possibility to deliver for short periods high power and current densities, and also the possibility to be used at high and low temperatures (up to +60oC and down to -500C). The cycling efficiency (ratio of energy consumed during charging and delivered during discharge) is about 92-95 %. Taking into account these properties, very promising are the following fields of applications for EDLCs: in ICE vehicles (in parallel with storage batteries) for starting purposes, delivering the necessary initial peak power (especially at low temperatures) and thus increasing the battery life time; and also in all-electric and in hybride vehicles for energy recuperation

In *pseudocapacitors* electrical charges are accumulated mainly as the result of fairly reversible redox reactions (faradaic pseudo-capacity). Many such reactions are known in which oxides

One of the important achievements of modern electrochemistry is the development of *electron-conducting polymers*. Electrochemical reactions in systems with conjugated double bonds such as polyanilin, polythiophene, polypyrolle, polyacethylene and others, are reversible and can be used in supercapacitors. Such processes are called electrochemical doping or dedoping the polymers with anions and cations. The electron conductivity during doping is due to the formation of delocalized electrons or electron-holes and their migration in the system of polyconjugated double-bonds under the influence of an applied electrical field. The use of some electron-conducting polymers as supercapacitor electrodes is based on the high reversibility of doping and dedoping reactions and the high conductance values of such polymers. The specific energy values of pseudocapacitors are fairly high 10-50 Wh/L and their cyclability reaches hundred thousands cycles. A disadvantage of supercapacitors based on transition metal oxides and sulfides is their high price. A disadvantage of those based on electron-conducting polymers is their insufficient stability. Recently some *hybride-type supercapacitors* were developed in which different types of electrodes are used. In [15-18] capacitors were investigated in which the positive electrodes were based on metal oxides, and the negative electrodes on activated carbon ( e.g. the system NiOOH/KOH/AC or the system PbO2/H2SO4/AC. In both of these two systems as positive electrodes conventional electrodes from alkaline, resp. lead-acid storage batteries are used. An advantage of the hybride-type supercapacitors as compared with their analogues, with symmetrical AC, is their higher Umax value and correspondingly their

and sulfides of transition metals RuO2, IrO2, TiS2 or their combinations take part.

A = (1/2) C [(Umax ) 2 - (Umin) 2 ] , (1)

discharge can be represented as:

will be: A = Аmax = (1/2)C[(Umax ) 2].

during slow-down and braking.

higher specific energy (up to 10-20 Wh/kg). The lower Umax value for symmetric AC systems in comparison with the hybride versions is due to the fact that at not very high anodic potentials (0.9-1.0 V) an oxidative carbon corrosion is observed while hydrogen evolution potential on the the hybride's negative AC electrode remains at very negative values up to -0.8 V). An advantage of hybride supercapacitors over the corresponding storage batteries is the much higher cycling capability, and the possibilities of faster charging and of easier hermetically sealing. Hybride PbO2/H2SO4/AC capacitors are used in wheelchairs and in electrical motor-buses.

In [19] a hybride supercapacitor with a RuO2 positive electrode and an AC negative electrode was described. A high specific energy of 26.7 Wh/kg was reported.

An important feature of supercapacitors in comparison with storage batteries is the possibility of rapid charging and discharging in very broad time intervals ranging from less than one second to several hours. Correspondingly supercapacitors can be subdivided into power units with high values of specific power and into energy units with high specific energy values.

Power supercapcitors allow carrying out the charging and discharging processes in very short time periods (from fractions of a second to minutes) and obtaining herewith high power characteristics from 1 to 5 kW/kg in concentrated aqueous solutions with high specific conductivity. Measurements for highly dispersed carbon electrodes in the energy capacitor operation modes usually yield the specific charge values in the range of 40 to 200 C/g [20, 21]. In the case of carbon materials, the limiting capacity obtained in [22] was 320 F/g due to a considerable contribution of pseudocapacitance of reversible surface group redox reactions (thus, they are not pure EDLCs anymore). In [23] high power characteristics (above 20 kW/kg) were obtained for electrodes based on single-wall carbon nanotubes (SWCNT). Such high power values can be explained by the regularity of the SWCNT's pore structure. As can be seen from Fig. 1 representing a SEM picture of this material, the pores are neither corrugated nor curving, thus providing for a high conductivity and, therefore, for a high power.

The electrodes used in energy–type ECSCs are often electrodes on which rather reversible faradaic processes occur. Such electrodes include electrodes based on electron–conducting polymers (polyaniline, polythiophene, polypyrrole etc.) and also electrodes based on some oxides of variable–valency metals (oxides of ruthenium, iridium, tungsten, molybdenum, zirconium etc.) [1, 2]. These electrodes feature different limitations for practical application, such as expensiveness, insufficient cyclability due to degradation processes etc. Using nonaqueous electrolytes in ECSC with electrodes based on highly dispersed carbon materials allows obtaining high (up to 3–3.5 V) charging voltage values, which significantly enhances the energy but limits the power of capacitors due to low conductivity of these electrolytes [1, 2, 24]. Aqueous alkali solutions allow obtaining rather high power values, but the low operating voltage range (about 0.8 V) decreases the energy characteristics of ECSC. Aqueous electrolytes with the highest conductivity are sulfuric acid solutions with concentrations from 30 to 40 wt. %. Besides, the working voltage range in the region of reversible processes proves to be above 1 V due to the relatively low corrosion activity towards carbon as compared to other aqueous electrolytes.

Studies of Supercapacitor Carbon Electrodes with High Pseudocapacitance 163

There are only few papers on mathematic modeling of processes in ECSC. In [26], an EDLC model was developed taking into account EDL charging, potential distribution in a porous electrode due to ohmic energy losses and its porous structure. The calculation results agreed well with experimental galvanostatic charging–discharge curves. In [27, 28] an operation theory was developed for electrodes based on electron–conducting polymers used in supercapacitors. The theory takes into account EDL charging, potential distribution in a porous electrode, electrochemical kinetics, intercalation of counterions in the polymer phase, and a nonsteady–state solid–phase diffusion of counterions in this phase. Simulation of the experimental discharge curves allowed obtaining the values of the parameters of processes occurring in the electrode: the solid–phase diffusion coefficient, EDL specific capacitance, and exchange current density of the electrochemical reaction. In [29], a model of a SWCNT electrode was developed taking into account EDL charging, hydrogen electrosorption–desorption, and kinetics of hydrogen electrooxidation–electroreduction according to the Volmer theory. In [30] an AC–based electrode impedance model accounted for EDL charging and intercalation processes, using statistical thermodynamics. However, in [29, 30] there was no comparison between

The following electrochemical methods were used in this work: cyclic voltammetry, galvanostatic, and impedance techniques. The electrochemical impedance spectra were obtained using an electrochemical measurement system consisting of a Solartron 1255 frequency analyzer, Solartron 1286 potentiostat, and a computer, and also using a FRA impedance meter. Measurement of CVs and galvanostatic curves was carried out using Solartron 1286 and PI-50 potentiostats. Prolonged cell cycling was achieved using a Zaryad 8k cycling device. Apart from the impedance technique, the dc charged electrode resistance was measured as follows. After deep anodic charging, the cell for charging was disassembled and the working electrode was removed. Further, in order to remove the acid residue, the electrode was dried in contact with microporous acidproof filter paper. Then it was placed into a special four–electrode measurement cell and pressed between two liners of foil of thermally expanded graphite (TEG); copper disks with copper current leads were placed on the rear sides of this set. Further, the cell was sealed and the dependence of voltage on current was measured, from which the electrode resistance was calculated. An important feature of the method is that the cell with copper current leads must be assembled in a predetermined period of time (in our case, 3 min), as a deeply charged electrode undergoes gradual oxidation in air. An advantage of this method over impedance analysis is that it allows eliminating the polarization contribution into resistance as in the case of the

impedance technique. Due to these limitations, both of these methods were used.

The main electrochemical measurements were carried out in a specially designed teflon filter–press cell with carbon current leads that allowed performing studies in a wide range of potentials. Its scheme is shown in Fig. 2,a and its photograph is presented in Fig. 2,b. Fig. 3 shows schematically the electrochemical group of this cell representing a matrix system, in which the electrolyte is in the pores of the electrode and separator. The separator is clamped between two ring–shaped acidproof rubber gaskets that are chosen depending on the

the theory and experiment

**2. Experimental methods**

Fig. 1. SEM image of single-wall carbon nanotubes (SWCNT)

In [25], a very high maximum amount of electricity of 1150 C/g was obtained with electrodes based on ADG type activated carbon with the specific surface area of 1500 m2/g. This value was reached after deep cathodic charging to the potentials of –0.3 to –0.8 V RHE. On the basis of these data and on the basis of other various experiments it was assumed that such a high amount of electricity is obtained as a result of hydrogen intercalation into AC carbon limited by solid–phase diffusion. These experiments are as follows: a very slow (20 h and more) deep charging process, memory effect under potential scanning in the positive direction, absence of correlation between the specific surface area and specific capacitance values, linear dependence of current maximums on the square root of the potential sweep rate, and some other experimental data. It was concluded in this paper that the most probable limiting amount of electricity is that corresponding to the formation of a compound С6Н similar to the compound of С6Li for negative carbon electrodes in rechargeable lithium–ion cells. However that formation of the C6H compound according to Faraday's law requires consumption of 1320 C/g that was not obtained in [25].

Therefore, the aim of the present investigation was to reach the maximum possible capacitance values on AC and also a more detailed study of mechanisms of the processes occurring under deep cathodic charging. Besides that, the aim of the investigation was to develop a mathematical model of these processes and to compare it with experimental data.

There are only few papers on mathematic modeling of processes in ECSC. In [26], an EDLC model was developed taking into account EDL charging, potential distribution in a porous electrode due to ohmic energy losses and its porous structure. The calculation results agreed well with experimental galvanostatic charging–discharge curves. In [27, 28] an operation theory was developed for electrodes based on electron–conducting polymers used in supercapacitors. The theory takes into account EDL charging, potential distribution in a porous electrode, electrochemical kinetics, intercalation of counterions in the polymer phase, and a nonsteady–state solid–phase diffusion of counterions in this phase. Simulation of the experimental discharge curves allowed obtaining the values of the parameters of processes occurring in the electrode: the solid–phase diffusion coefficient, EDL specific capacitance, and exchange current density of the electrochemical reaction. In [29], a model of a SWCNT electrode was developed taking into account EDL charging, hydrogen electrosorption–desorption, and kinetics of hydrogen electrooxidation–electroreduction according to the Volmer theory. In [30] an AC–based electrode impedance model accounted for EDL charging and intercalation processes, using statistical thermodynamics. However, in [29, 30] there was no comparison between the theory and experiment

## **2. Experimental methods**

162 Recent Trend in Electrochemical Science and Technology

In [25], a very high maximum amount of electricity of 1150 C/g was obtained with electrodes based on ADG type activated carbon with the specific surface area of 1500 m2/g. This value was reached after deep cathodic charging to the potentials of –0.3 to –0.8 V RHE. On the basis of these data and on the basis of other various experiments it was assumed that such a high amount of electricity is obtained as a result of hydrogen intercalation into AC carbon limited by solid–phase diffusion. These experiments are as follows: a very slow (20 h and more) deep charging process, memory effect under potential scanning in the positive direction, absence of correlation between the specific surface area and specific capacitance values, linear dependence of current maximums on the square root of the potential sweep rate, and some other experimental data. It was concluded in this paper that the most probable limiting amount of electricity is that corresponding to the formation of a compound С6Н similar to the compound of С6Li for negative carbon electrodes in rechargeable lithium–ion cells. However that formation of the C6H compound according to Faraday's law requires consumption of 1320 C/g that

Therefore, the aim of the present investigation was to reach the maximum possible capacitance values on AC and also a more detailed study of mechanisms of the processes occurring under deep cathodic charging. Besides that, the aim of the investigation was to develop a mathematical model of these processes and to compare it with experimental

Fig. 1. SEM image of single-wall carbon nanotubes (SWCNT)

was not obtained in [25].

data.

The following electrochemical methods were used in this work: cyclic voltammetry, galvanostatic, and impedance techniques. The electrochemical impedance spectra were obtained using an electrochemical measurement system consisting of a Solartron 1255 frequency analyzer, Solartron 1286 potentiostat, and a computer, and also using a FRA impedance meter. Measurement of CVs and galvanostatic curves was carried out using Solartron 1286 and PI-50 potentiostats. Prolonged cell cycling was achieved using a Zaryad 8k cycling device. Apart from the impedance technique, the dc charged electrode resistance was measured as follows. After deep anodic charging, the cell for charging was disassembled and the working electrode was removed. Further, in order to remove the acid residue, the electrode was dried in contact with microporous acidproof filter paper. Then it was placed into a special four–electrode measurement cell and pressed between two liners of foil of thermally expanded graphite (TEG); copper disks with copper current leads were placed on the rear sides of this set. Further, the cell was sealed and the dependence of voltage on current was measured, from which the electrode resistance was calculated. An important feature of the method is that the cell with copper current leads must be assembled in a predetermined period of time (in our case, 3 min), as a deeply charged electrode undergoes gradual oxidation in air. An advantage of this method over impedance analysis is that it allows eliminating the polarization contribution into resistance as in the case of the impedance technique. Due to these limitations, both of these methods were used.

The main electrochemical measurements were carried out in a specially designed teflon filter–press cell with carbon current leads that allowed performing studies in a wide range of potentials. Its scheme is shown in Fig. 2,a and its photograph is presented in Fig. 2,b. Fig. 3 shows schematically the electrochemical group of this cell representing a matrix system, in which the electrolyte is in the pores of the electrode and separator. The separator is clamped between two ring–shaped acidproof rubber gaskets that are chosen depending on the

Studies of Supercapacitor Carbon Electrodes with High Pseudocapacitance 165

The electrolyte used in this study was mainly concentrated sulfuric acid with concentrations from 30 to 60%. The electrode used was a СН900-20 activated carbon cloth (ACC) (Japan). The electrodes had a surface area of 3 cm2 , a thickness (in the compact state under the

Studies of the porous structure and hydrophilic–hydrophobic AC properties were carried out using the method of standard contact porosimetry (MSCP) technique [32] with evaporation of octane and water. The method is based on the laws of capillary equilibrium. If two (or more) porous bodies partially filled with a wetting liquid are in capillary equilibrium, the values of the liquid's capillary pressure in these bodies are equal. The value of the capillary pressure according to the Laplace equation is *p*c = 2 cos/r, where r – is the pore radius, - the surface tension, - the wetting angle. In this method the amount of a wetting liquid in the test sample (*V*t ) is measured; Simultaneously, the amount of the same wetting liquid (*V*s) is measured in a standard specimen of known porous structure. The liquids in both porous samples are kept in contact. After some time a thermodynamic equilibrium is reached. The measurements are performed for different overall amounts of the liquid *V*0=*V*s+*V*t. During the experiment this overall amount is changed by gradual evaporation of the liquid. The MSCP with appropriate standard samples can be used to measure pore sizes in the range from 1 to 3·105 nm. The MSCP has several substantial advantages over mercury porosimetry and other porosimetric methods. It has the

conditions in which they are commonly used in different devices;

Fig. 3. Electrochemical group of the Teflon cell.

possibility:

materials and even powders;

pressure of 0.5 MPa) of about 0.4 mm, and a mass of about 0.06 g.

electrode thickness to provide reliable separation of the working and auxiliary electrodes. A fine–texture Grace–type separator (polyethylene with silica gel) was used in the cell. The capacitance of the AC auxiliary electrode was much higher than that of the working electrode, which allowed eliminating interaction of electrodes through the evolved gas. Application of graphite current leads instead of the usual metallic (most often, platinum) ones allowed reaching high negative potentials (up to –1 V RHE), which in its turn allowed obtaining ultrahigh capacitance values (see below).

Fig. 2.a Electrochemical cell. The principal details: 4 – a graphite current lead, 12 – current distribution layer of thermal expanded graphite, 13 – electrodes, 14 – separator.

Fig. 2.b Photograph of the electrochemical cell

electrode thickness to provide reliable separation of the working and auxiliary electrodes. A fine–texture Grace–type separator (polyethylene with silica gel) was used in the cell. The capacitance of the AC auxiliary electrode was much higher than that of the working electrode, which allowed eliminating interaction of electrodes through the evolved gas. Application of graphite current leads instead of the usual metallic (most often, platinum) ones allowed reaching high negative potentials (up to –1 V RHE), which in its turn allowed

Fig. 2.a Electrochemical cell. The principal details: 4 – a graphite current lead, 12 – current

distribution layer of thermal expanded graphite, 13 – electrodes, 14 – separator.

obtaining ultrahigh capacitance values (see below).

Fig. 2.b Photograph of the electrochemical cell

Fig. 3. Electrochemical group of the Teflon cell.

The electrolyte used in this study was mainly concentrated sulfuric acid with concentrations from 30 to 60%. The electrode used was a СН900-20 activated carbon cloth (ACC) (Japan). The electrodes had a surface area of 3 cm2 , a thickness (in the compact state under the pressure of 0.5 MPa) of about 0.4 mm, and a mass of about 0.06 g.

Studies of the porous structure and hydrophilic–hydrophobic AC properties were carried out using the method of standard contact porosimetry (MSCP) technique [32] with evaporation of octane and water. The method is based on the laws of capillary equilibrium. If two (or more) porous bodies partially filled with a wetting liquid are in capillary equilibrium, the values of the liquid's capillary pressure in these bodies are equal. The value of the capillary pressure according to the Laplace equation is *p*c = 2 cos/r, where r – is the pore radius, - the surface tension, - the wetting angle. In this method the amount of a wetting liquid in the test sample (*V*t ) is measured; Simultaneously, the amount of the same wetting liquid (*V*s) is measured in a standard specimen of known porous structure. The liquids in both porous samples are kept in contact. After some time a thermodynamic equilibrium is reached. The measurements are performed for different overall amounts of the liquid *V*0=*V*s+*V*t. During the experiment this overall amount is changed by gradual evaporation of the liquid. The MSCP with appropriate standard samples can be used to measure pore sizes in the range from 1 to 3·105 nm. The MSCP has several substantial advantages over mercury porosimetry and other porosimetric methods. It has the possibility:

Studies of Supercapacitor Carbon Electrodes with High Pseudocapacitance 167

Fig. 5 presents the dependence of the wetting angle on the pore radius calculated from Fig. 5 according to [32]. As may be seen, the values for the whole pore radius range are close to 90о. Therefore, though most of the pores are hydrophilic with < 90о, they are still badly wetted. This and also the presence of fully hydrophobic pores may point to high amounts of graphite (graphene) impurities that are practically hydrophobic. The complex curve character in Fig. 5 is largely due to a nonuniform distribution of surface groups in the pores

01234

In this work, studies of AC were carried out in a large range of potentials from –0.8 to 1.0 V RHE. It is convenient to present the CV (especially those measured at different potential sweep rates w), graphically in the form of capacitance–voltage curves constructed in the coordinates of differential capacitance (С) vs. potential (E), where С = dQ/d = I d /dE = I/

Fig. 6 compares the cyclic capacitance–voltage curves measured in 48.5% H2SO4 at different potential sweep rates in two ranges of potentials: in the reversibility range (from 0.1 to 0.9 V) and in the deep charging range (from –0.8 to 1 V). As follows from curve 4 measured in the reversibility range, only electric double layer (EDL) charging occurs here, while pseudocapacitance of redox reactions of surface groups is very low in this case. This distinguishes the СН900-20 ACC from ADG AC in which a considerable contribution is introduced by the pseudocapacitance of fast redox reactions of surface groups [25]. As follows from curve 4, the value of the EDL capacitance is approximately 160 F/g. Taking into account that Sphi = 870 m2/g, we obtain СDEL = 18.4 F/cm2 for unit of the true hydrophilic carbon surface ara. This value is close to the classical EDL capacitance value of platinum [33]. Much lower СEDL values presented in [34], in our opinion, are explained by the fact that the values of the full specific surface area measured using the BET

log r, (nm) Fig. 5. Dependence of wetting angle on the pore radius for the СН900-20 AC cloth.

w, I is the current, Q is the amount of electricity, w = dE/d, is the time.

of different radii.

0

10

20

30

40

50

60

70

80

90 o

One of the most pronounced advantages of the MSCP is the possibility to investigate the wetting (hydrophilic/hydrophobic or liophilic/liophobic) properties of porous materials. Primarily, the MSCP measures the distribution of pore volume vs. the capillary pressure *p*c, i.e. vs. the parameter *r*\*=*r*/cos (henceforth, this parameter is called effective pore radius). For partially hydrophobic materials (for which >0) the porosimetric curves measured with water are shifted towards higher values of *r*\* with respect to the curves measured with octane that wets most materials almost ideally (=00). The value of this shift for a certain value of pore volume *V*n and of the corresponding pore radius *r*n allows to determine the wetting angle of water for pores with the radius *r*n:

$$\cos \theta = r\_{\mathbf{n}} / r^\*. \tag{2}$$

For porous materials the wetting angles (*r*) for pores of different size can be different.

## **3. Experimental results and discussion**

Fig. 4 presents integral pore radius distribution curves measured using MSCP with octane and water for the СН900-20 AC cloth. As follows from this figure, this cloth has a very wide pore spectrum: from micropores with radii r ≤ 1 nm to macropores with r > 100 m, i.e. in the range of more than 5 orders of magnitude. It is of interest that this cloth contains micropores and also macropores with r > 1 m, but there are practically no mesopores with 1 nm < r < 100 nm. Micropores provide high full specific surface with Sf = 1520 m2/g and hydrophilic specific surface Sphi = 870 m2/g. According to [32], porometric curves are measured using octane for all pores and using water only for hydrophilic pores. As follows from these curves, the full porosity (by octane) was 86%, the hydrophilic porosity was 78.5%, and the hydrophobic porosity iwas 7.5 %.

Fig. 4. Integral pore radius distribution curves measured in octane (1) and water (2) for a СН900-20 AC cloth.

One of the most pronounced advantages of the MSCP is the possibility to investigate the wetting (hydrophilic/hydrophobic or liophilic/liophobic) properties of porous materials. Primarily, the MSCP measures the distribution of pore volume vs. the capillary pressure *p*c, i.e. vs. the parameter *r*\*=*r*/cos (henceforth, this parameter is called effective pore radius). For partially hydrophobic materials (for which >0) the porosimetric curves measured with water are shifted towards higher values of *r*\* with respect to the curves measured with octane that wets most materials almost ideally (=00). The value of this shift for a certain value of pore volume *V*n and of the corresponding pore radius *r*n allows to determine the

For porous materials the wetting angles (*r*) for pores of different size can be different.

Fig. 4 presents integral pore radius distribution curves measured using MSCP with octane and water for the СН900-20 AC cloth. As follows from this figure, this cloth has a very wide pore spectrum: from micropores with radii r ≤ 1 nm to macropores with r > 100 m, i.e. in the range of more than 5 orders of magnitude. It is of interest that this cloth contains micropores and also macropores with r > 1 m, but there are practically no mesopores with 1 nm < r < 100 nm. Micropores provide high full specific surface with Sf = 1520 m2/g and hydrophilic specific surface Sphi = 870 m2/g. According to [32], porometric curves are measured using octane for all pores and using water only for hydrophilic pores. As follows from these curves, the full porosity (by octane) was 86%, the hydrophilic porosity was

012345

Fig. 4. Integral pore radius distribution curves measured in octane (1) and water (2) for a

log r ( nm)

cos = *r*n /*r*\*. (2)

2

1

wetting angle of water for pores with the radius *r*n:

**3. Experimental results and discussion**

78.5%, and the hydrophobic porosity iwas 7.5 %.

0.0

0.5

1.0

1.5

V,

СН900-20 AC cloth.

с

m3/g

2.0

2.5

3.0

3.5

4.0

Fig. 5 presents the dependence of the wetting angle on the pore radius calculated from Fig. 5 according to [32]. As may be seen, the values for the whole pore radius range are close to 90о. Therefore, though most of the pores are hydrophilic with < 90о, they are still badly wetted. This and also the presence of fully hydrophobic pores may point to high amounts of graphite (graphene) impurities that are practically hydrophobic. The complex curve character in Fig. 5 is largely due to a nonuniform distribution of surface groups in the pores of different radii.

log r, (nm) Fig. 5. Dependence of wetting angle on the pore radius for the СН900-20 AC cloth.

In this work, studies of AC were carried out in a large range of potentials from –0.8 to 1.0 V RHE. It is convenient to present the CV (especially those measured at different potential sweep rates w), graphically in the form of capacitance–voltage curves constructed in the coordinates of differential capacitance (С) vs. potential (E), where С = dQ/d = I d /dE = I/ w, I is the current, Q is the amount of electricity, w = dE/d, is the time.

Fig. 6 compares the cyclic capacitance–voltage curves measured in 48.5% H2SO4 at different potential sweep rates in two ranges of potentials: in the reversibility range (from 0.1 to 0.9 V) and in the deep charging range (from –0.8 to 1 V). As follows from curve 4 measured in the reversibility range, only electric double layer (EDL) charging occurs here, while pseudocapacitance of redox reactions of surface groups is very low in this case. This distinguishes the СН900-20 ACC from ADG AC in which a considerable contribution is introduced by the pseudocapacitance of fast redox reactions of surface groups [25]. As follows from curve 4, the value of the EDL capacitance is approximately 160 F/g. Taking into account that Sphi = 870 m2/g, we obtain СDEL = 18.4 F/cm2 for unit of the true hydrophilic carbon surface ara. This value is close to the classical EDL capacitance value of platinum [33]. Much lower СEDL values presented in [34], in our opinion, are explained by the fact that the values of the full specific surface area measured using the BET

Studies of Supercapacitor Carbon Electrodes with High Pseudocapacitance 169

assumed in [25] that during a deep cathodic charging of AC, as a result of hydrogen intercalation a compound CxH and in he limiting case C6H is formed. On the basis of the shape of the potentiodynamic curves it is also possible to assume that two processes take place: a fast hydrogen chemosorption at the interface of carbon and electrolyte in the pores and a process of hydrogen intercalation into AC with a slow hydrogen solid–phase

0,1 1 10 100 t, h

Fig. 7. Dependence of specific amount of electricity under discharge of the СН900-20 electrode in 40.3% sulfuric acid on the charging time under the charging potential of - 250

Fig. 8 presents the dependence of the active resistance of the electrode on charging time at Е = –400 mV measured using the impedance technique in 40% sulfuric acid. As can be seen, the active resistance of the electrode grows in the course of the charging process. Both the solid phase and electrolyte in pores contribute to the resistance. The dependence of electric resistance on the charging time measured under constant current according to the above mentioned technique is more significant. Fig. 9 presents such a dependence of the dc resistance of a СН900-20 electrode on the charging time in 40.3 % sulfuric acid at Е = –0.34 V. As seen from this figure, the electrode resistance grows significantly with an increase in the AC charging time. One should keep in mind that the above parameters were obtained on activated carbon cloth (СН900-20), which practically eliminates the effect of contact resistance between its separate fibers, as the fibers represent practically parallel transport paths for electrons. Therefore, the above data may be explained by a change in the bulk phase chemical composition in the course of charging. Thus, both resistance measurement techniques point to its increase as dependent on the charging time. This can also be explained by a change in the solid phase composition: from С to СхН and, in the limit, to

The maximum specific charge of 1560 C/g was obtained after charging for 22 h at the potential of E = –0.31 V RHE in 56.4% H2SO4. No close value was described in the literature

diffusion.

Q C/g

mV.

С6Н.

technique were used, while it is known that carbon materials have both hydrophilic and hydrophobic pores. The MSCP used in this work allows to obtain the values of the hydrophilic surface area Sphi and thus the СDEL values per unit of the real working surface area.

Fig. 6. Capacitance-voltage cyclic curves for СН900-20.

In the range of negative potentials (< –0.1 V) (curves 1, 2, 3), faradaic processes with a very high pseudocapacitance are observed. As may be seen, the EDL capacitance decreases in the range of negative potentials. This is probably due to the partial surface blocking by adsorbed particles. There are two pronounced maximums in the anodic branches of curves 2, 3 measured at low w values, which points to the probable occurrence of two slow processes. There is a single deep maximum corresponding to a very high amount of electricity under deep cathodic charging of AC in catodic branches of curves 1, 2, 3.

The method of galvanostatic curves was used to measure the amounts of electricity after long–term charging at negative potentials. Fig. 7 presents the dependence of the amount of electricity Q under discharge on the charging time at potential Е = –250 mV in 40.3% H2SO4. As may be seen, the Q value grows very fast at very low charging times of seconds and minutes and continues increasing further for many tens of hours. Such very slow growth can be explained by a very slow diffusion in a solid phase. Therefore it is possible to assume the existence of a hydrogen intercalation into AC carbon controlled by slow solid–phase hydrogen diffusion. This is also evidenced by a proportionality of the limiting current to the square root of the potential sweep rate and also by a number of other experimental data obtained in [25] (memory effect under potential sweep in the positive direction, absence of correlation between the specific surface area value and specific capacitance etc.) It was

technique were used, while it is known that carbon materials have both hydrophilic and hydrophobic pores. The MSCP used in this work allows to obtain the values of the hydrophilic surface area Sphi and thus the СDEL values per unit of the real working surface

4

In the range of negative potentials (< –0.1 V) (curves 1, 2, 3), faradaic processes with a very high pseudocapacitance are observed. As may be seen, the EDL capacitance decreases in the range of negative potentials. This is probably due to the partial surface blocking by adsorbed particles. There are two pronounced maximums in the anodic branches of curves 2, 3 measured at low w values, which points to the probable occurrence of two slow processes. There is a single deep maximum corresponding to a very high amount of electricity under deep cathodic charging of AC in catodic branches of

The method of galvanostatic curves was used to measure the amounts of electricity after long–term charging at negative potentials. Fig. 7 presents the dependence of the amount of electricity Q under discharge on the charging time at potential Е = –250 mV in 40.3% H2SO4. As may be seen, the Q value grows very fast at very low charging times of seconds and minutes and continues increasing further for many tens of hours. Such very slow growth can be explained by a very slow diffusion in a solid phase. Therefore it is possible to assume the existence of a hydrogen intercalation into AC carbon controlled by slow solid–phase hydrogen diffusion. This is also evidenced by a proportionality of the limiting current to the square root of the potential sweep rate and also by a number of other experimental data obtained in [25] (memory effect under potential sweep in the positive direction, absence of correlation between the specific surface area value and specific capacitance etc.) It was

area.

curves 1, 2, 3.

2 1

Fig. 6. Capacitance-voltage cyclic curves for СН900-20.

3

С, F/g

0

500

1000

assumed in [25] that during a deep cathodic charging of AC, as a result of hydrogen intercalation a compound CxH and in he limiting case C6H is formed. On the basis of the shape of the potentiodynamic curves it is also possible to assume that two processes take place: a fast hydrogen chemosorption at the interface of carbon and electrolyte in the pores and a process of hydrogen intercalation into AC with a slow hydrogen solid–phase diffusion.

Fig. 7. Dependence of specific amount of electricity under discharge of the СН900-20 electrode in 40.3% sulfuric acid on the charging time under the charging potential of - 250 mV.

Fig. 8 presents the dependence of the active resistance of the electrode on charging time at Е = –400 mV measured using the impedance technique in 40% sulfuric acid. As can be seen, the active resistance of the electrode grows in the course of the charging process. Both the solid phase and electrolyte in pores contribute to the resistance. The dependence of electric resistance on the charging time measured under constant current according to the above mentioned technique is more significant. Fig. 9 presents such a dependence of the dc resistance of a СН900-20 electrode on the charging time in 40.3 % sulfuric acid at Е = –0.34 V. As seen from this figure, the electrode resistance grows significantly with an increase in the AC charging time. One should keep in mind that the above parameters were obtained on activated carbon cloth (СН900-20), which practically eliminates the effect of contact resistance between its separate fibers, as the fibers represent practically parallel transport paths for electrons. Therefore, the above data may be explained by a change in the bulk phase chemical composition in the course of charging. Thus, both resistance measurement techniques point to its increase as dependent on the charging time. This can also be explained by a change in the solid phase composition: from С to СхН and, in the limit, to С6Н.

The maximum specific charge of 1560 C/g was obtained after charging for 22 h at the potential of E = –0.31 V RHE in 56.4% H2SO4. No close value was described in the literature

Studies of Supercapacitor Carbon Electrodes with High Pseudocapacitance 171

Fig. 10 presents the dependence of the maximum specific charge of СН900-20 on the sulfuric acid concentration (in the range from 34 to 56%) for the discharge time of 18 h. As follows from this figure, the specific charge grows with an increase in the sulfuric acid concentration. The following explanation is possible: It is known from the literature that: 1) in concentrated sulfuric acid solutions a sulfuric acid intercalation occurs into graphite and graphite-like materials, that is enhanced at an increase in the concentration [35–37]. During this intercalation the gap between graphene layers where the acid penetrates grows. 2) Activated carbons to a certain degree are swelling during adsorption of different adsorptives [38]. As graphite-like impurities are contained in AC [39, 40]. Taking intp account these facts, it is possible to assume that in the described conditions a **double intercalation** occurs. Sulfuric acid is intercalated into AC expanding the interlayer (intergraphene) space. Hydrogen atoms are then directed into this space under deep cathodic charging of AC. This interlayer space serves as a transport route for hydrogen. Then hydrogen interacts with the graphene layers with the ultimate

formation of compound **С6Н (carbon hydride or hydrogen carbide)** (see Fig. 11).

20 40 60 C, %

Fig. 10. Dependence of the maximum specific charge of СН900-20 on the sulfuric acid

According to Faraday's law, formation of compound С6Н requires 1320 C/g. The maximum value of Qmax = 1560 C/g was obtained in our investigation. Therefore, other rechargeable processes require 240 C/g. This value includes in the first instance the value of QEDL = СEDL х ∆Е, where ∆Е is the range of potentials. In this case, ∆Е = 1.4 V, so to the first approximation, СEDL = 240/1.4 = 170 F/g. This is approximately the same value as the one obtained from curve 4 in Fig. 3. It must be noted that , this is an approximate estimate, as the СEDL value somewhat depends on the potential and also as a small contribution into the Qmax value is made by the pseudocapacitance of redox reactions of surface groups. Nevertheless, this approximate estimate shows the correctness of the assumed mechanism of deep AC charging. Though the process of electrochemical chemosorption on the carbon/electrolyte

600

concentration for the charging time of 18 h.

700

800

900

1000 1100

Q, C/g

1200

1300

1400

1500

Fig. 8. Dependence of active resistance of the electrode on the charging time at Е = –400 mV measured using the impedance technique in 40% sulfuric acid.

Fig. 9. Dependence of electron resistance of the СН900-20 electrode on the charging time in 40.3 % sulfuric acid at Е = –0.34 V measured under constant current.

0 2000 4000 6000 8000 10000 12000 14000 16000

t, s Fig. 8. Dependence of active resistance of the electrode on the charging time at Е = –400 mV

> 0 0,5 1 1,5 2 2,5 t, h

Fig. 9. Dependence of electron resistance of the СН900-20 electrode on the charging time in

40.3 % sulfuric acid at Е = –0.34 V measured under constant current.

measured using the impedance technique in 40% sulfuric acid.

0.4

0

2

4

6

R, Oh

m

8

10

12

0.5

0.6

R, Ohm

0.7

0.8

Fig. 10 presents the dependence of the maximum specific charge of СН900-20 on the sulfuric acid concentration (in the range from 34 to 56%) for the discharge time of 18 h. As follows from this figure, the specific charge grows with an increase in the sulfuric acid concentration. The following explanation is possible: It is known from the literature that: 1) in concentrated sulfuric acid solutions a sulfuric acid intercalation occurs into graphite and graphite-like materials, that is enhanced at an increase in the concentration [35–37]. During this intercalation the gap between graphene layers where the acid penetrates grows. 2) Activated carbons to a certain degree are swelling during adsorption of different adsorptives [38]. As graphite-like impurities are contained in AC [39, 40]. Taking intp account these facts, it is possible to assume that in the described conditions a **double intercalation** occurs. Sulfuric acid is intercalated into AC expanding the interlayer (intergraphene) space. Hydrogen atoms are then directed into this space under deep cathodic charging of AC. This interlayer space serves as a transport route for hydrogen. Then hydrogen interacts with the graphene layers with the ultimate formation of compound **С6Н (carbon hydride or hydrogen carbide)** (see Fig. 11).

Fig. 10. Dependence of the maximum specific charge of СН900-20 on the sulfuric acid concentration for the charging time of 18 h.

According to Faraday's law, formation of compound С6Н requires 1320 C/g. The maximum value of Qmax = 1560 C/g was obtained in our investigation. Therefore, other rechargeable processes require 240 C/g. This value includes in the first instance the value of QEDL = СEDL х ∆Е, where ∆Е is the range of potentials. In this case, ∆Е = 1.4 V, so to the first approximation, СEDL = 240/1.4 = 170 F/g. This is approximately the same value as the one obtained from curve 4 in Fig. 3. It must be noted that , this is an approximate estimate, as the СEDL value somewhat depends on the potential and also as a small contribution into the Qmax value is made by the pseudocapacitance of redox reactions of surface groups. Nevertheless, this approximate estimate shows the correctness of the assumed mechanism of deep AC charging. Though the process of electrochemical chemosorption on the carbon/electrolyte

Studies of Supercapacitor Carbon Electrodes with High Pseudocapacitance 173

the contribution into this DEL capacitance value is 240 C/g, so the dominating contribution to the Qmax value is made by the specific charge Q = 1320 C/g corresponding to formation of the С6Н compound (pseudocapacitance charge). In principle, if one uses other activated carbons and cloths, the DEL capacitance value may somewhat increase due to an increase in the AC specific surface area that in our case was 1520 m2/g. However, this must not result

Fig. 13 shows an impedance Cole-Cole plot (capacitive impedance component vs. resistitive component) for an activated carbon electrode in a 30% sulfuric acid solution at E=0.430 V measured in the frequency range from 100 kHz to 0.01 GHz. In this figure also shown is a fitting plot calculated from the staircase-type equivalent circuit shown in Fig. 14. Such a circuit-type is often used for EDLC electrodes [1]. We interprete this circuit by the simultaneous proceeding of different events: DL-charging, redox-reactions of different surface groups, hydrogen chemosorption and hydrogen intercalation. As the size of micropores is comparable to the DL- thickness, the DL capacity and the kinetic parameters of surface-group reactions can vary for pores of different size. Each of these processes has its own "stairstep" in the equivalent circuit. The absence in this circuit of a Warburg impedance, corresponding to a hydrogen diffusion in the solid phase, can be explained by the time constants of such a diffussion being much higher than the lowest frequencies (0.01

Fig. 13. Impedance plot for an activated carbon electrode in a 30% sulfuric acid solution at

in any significant increase in the Qmax value.

Hz) which can be used for such measurements.

E=0.430 V

Fig. 11. Structural formula of compound С6Н.

interface and the bulk process of hydrogen intercalation occur at different rates, both of them eventually lead to formation of the single bulk compound С6Н.

Fig. 12 presents the obtained dependence of Q on the charging potential. It can be seen that the curve reaches a plateau at Е < –250 mV. The very fact of reaching a plateau points to a saturation of the carbon bulk and surface by hydrogen atoms, i.e., evidences indirectly the saturation of the Сх–Н chemical bonds at the given potential and H2SO4 concentration.

Fig. 12. Dependence of specific capacitance on the potential of charging for 18 h in 40.3% H2SO4.

Thus, neither the further increase in the concentration of H2SO4 above 56.4%, nor the further decrease in the charging potential below –250 mV, nor any increase in the charging time result in an increase in the maximum specific charge Qmax = 1560 C/g. As pointed out above,

H H H

interface and the bulk process of hydrogen intercalation occur at different rates, both of

H H H

Fig. 12 presents the obtained dependence of Q on the charging potential. It can be seen that the curve reaches a plateau at Е < –250 mV. The very fact of reaching a plateau points to a saturation of the carbon bulk and surface by hydrogen atoms, i.e., evidences indirectly the saturation of the Сх–Н chemical bonds at the given potential and H2SO4 concentration.

> -550 -500 -450 -400 -350 -300 -250 -200 -150 -100 E, mV

Fig. 12. Dependence of specific capacitance on the potential of charging for 18 h in 40.3%

Thus, neither the further increase in the concentration of H2SO4 above 56.4%, nor the further decrease in the charging potential below –250 mV, nor any increase in the charging time result in an increase in the maximum specific charge Qmax = 1560 C/g. As pointed out above,

them eventually lead to formation of the single bulk compound С6Н.

Fig. 11. Structural formula of compound С6Н.

400

500

600

700

Q, C/g

H2SO4.

800

900

1000

the contribution into this DEL capacitance value is 240 C/g, so the dominating contribution to the Qmax value is made by the specific charge Q = 1320 C/g corresponding to formation of the С6Н compound (pseudocapacitance charge). In principle, if one uses other activated carbons and cloths, the DEL capacitance value may somewhat increase due to an increase in the AC specific surface area that in our case was 1520 m2/g. However, this must not result in any significant increase in the Qmax value.

Fig. 13 shows an impedance Cole-Cole plot (capacitive impedance component vs. resistitive component) for an activated carbon electrode in a 30% sulfuric acid solution at E=0.430 V measured in the frequency range from 100 kHz to 0.01 GHz. In this figure also shown is a fitting plot calculated from the staircase-type equivalent circuit shown in Fig. 14. Such a circuit-type is often used for EDLC electrodes [1]. We interprete this circuit by the simultaneous proceeding of different events: DL-charging, redox-reactions of different surface groups, hydrogen chemosorption and hydrogen intercalation. As the size of micropores is comparable to the DL- thickness, the DL capacity and the kinetic parameters of surface-group reactions can vary for pores of different size. Each of these processes has its own "stairstep" in the equivalent circuit. The absence in this circuit of a Warburg impedance, corresponding to a hydrogen diffusion in the solid phase, can be explained by the time constants of such a diffussion being much higher than the lowest frequencies (0.01 Hz) which can be used for such measurements.

Fig. 13. Impedance plot for an activated carbon electrode in a 30% sulfuric acid solution at E=0.430 V

Studies of Supercapacitor Carbon Electrodes with High Pseudocapacitance 175

above mechanism of double intercalation of H3PO4 and hydrogen into AC carbon during

0 10 20 30 40 50

t, h

The above data point to high prospects of using ACs under the conditions of deep cathodic

We developed a two–dimensional mathematical model of charging–discharge for highly

Hydrogen solid–phase diffusion may be assumed to be one–dimensional due to strong anisotropy of carbon crystallites forming the porous structure, as there is practically no diffusion in transverse direction to graphene layers [40]. Then the equation for hydrogen

where с is the hydrogen concentration in the AC carbon, t is the time, x is the shortest axial coordinate of the crystallite, D is the longitudinal solid–phase diffusion coefficient of hydrogen in the crystallite. The boundary condition at the interface of carbon/electrolyte in pores is:

*p*

*c i nFD*

where n is the charge transferred, F is Faraday's number, ir is the current density determined by the kinetic dependence of the reaction. Let us also assume that there is a certain length H corresponding to half the crystallite length, where there is no hydrogen diffusion flux due to

2 2 *c c D t x*

0

*x*

*x*

, (3)

, (4)

**4. Mathematic charging–discharge model of electrode based on activated**

deep cathodic charging.

1,2

Fig. 16. Cyclic charging–discharge curves for 8 cycles.

dispersed carbon accounting for the obtained experimental results.

diffusion for crystallites may be written in a one–dimensional form:

charging for energy type supercapacitors.

0

0,2 0,4

0,6 0,8 1

E, V

**carbon**

the symmetry:

Fig. 14. Equivalent circuit fitting the plot on Fig. 13, the values of the circuit's components are presented in the table below the figure

Galvanostatic cycling of the СН900-20 electrode was carried out at the current density of 1.2 mA/cm2 in the range of potentials from –0.32 V to 1.03 V. Fig. 15 presents the dependence of specific discharge capacitance Q on number of cycles N. One may see that the Q value changed little in 100 cycles. Then the cycling was stopped, as it had already taken a lot of time: 588 h. The mean Q value was 940 C/g. At N = 100, the obtained overall discharge capacitance was 26.3 A h/g. Fig. 16 presents cyclic charge–discharge curves for 8 cycles. The difference between these curves and the ideal "sawtooth" is related to occurrence of the above pseudocapacitance processes.

Fig. 15. Dependence of specific discharge capacitance Q on the number of cycles.

Besides the masurements of deep cathodic charging of СН900-20 ACC in H2SO4 solutions, similar measurements were carried out in a 90% aqueous H3PO4 solution. The specific charge value calculated using this plot was 1200 C/g. As according to [37], phosphoric acid is also intercalated into graphite and graphite-like compounds, these results agree with the

N 1 2 3 4 5 6 R, Ohm 0.29 1.99 1.2 0.57 2.22 2.86 С, F 3.210-5 1.41 3.43 8.210-2 4.010-3 4.410-5 Fig. 14. Equivalent circuit fitting the plot on Fig. 13, the values of the circuit's components

Galvanostatic cycling of the СН900-20 electrode was carried out at the current density of 1.2 mA/cm2 in the range of potentials from –0.32 V to 1.03 V. Fig. 15 presents the dependence of specific discharge capacitance Q on number of cycles N. One may see that the Q value changed little in 100 cycles. Then the cycling was stopped, as it had already taken a lot of time: 588 h. The mean Q value was 940 C/g. At N = 100, the obtained overall discharge capacitance was 26.3 A h/g. Fig. 16 presents cyclic charge–discharge curves for 8 cycles. The difference between these curves and the ideal "sawtooth" is related to occurrence of the

0 10 20 30 40 50 60 70 80 90 100 N

Besides the masurements of deep cathodic charging of СН900-20 ACC in H2SO4 solutions, similar measurements were carried out in a 90% aqueous H3PO4 solution. The specific charge value calculated using this plot was 1200 C/g. As according to [37], phosphoric acid is also intercalated into graphite and graphite-like compounds, these results agree with the

Fig. 15. Dependence of specific discharge capacitance Q on the number of cycles.

are presented in the table below the figure

above pseudocapacitance processes.

0

200

400

600

Q,

С/g

800

1000

1200

above mechanism of double intercalation of H3PO4 and hydrogen into AC carbon during deep cathodic charging.

Fig. 16. Cyclic charging–discharge curves for 8 cycles.

The above data point to high prospects of using ACs under the conditions of deep cathodic charging for energy type supercapacitors.

## **4. Mathematic charging–discharge model of electrode based on activated carbon**

We developed a two–dimensional mathematical model of charging–discharge for highly dispersed carbon accounting for the obtained experimental results.

Hydrogen solid–phase diffusion may be assumed to be one–dimensional due to strong anisotropy of carbon crystallites forming the porous structure, as there is practically no diffusion in transverse direction to graphene layers [40]. Then the equation for hydrogen diffusion for crystallites may be written in a one–dimensional form:

$$\frac{\partial \mathcal{C}}{\partial t} = D \frac{\partial^2 \mathcal{C}}{\partial \mathbf{x}^2} \,, \tag{3}$$

where с is the hydrogen concentration in the AC carbon, t is the time, x is the shortest axial coordinate of the crystallite, D is the longitudinal solid–phase diffusion coefficient of hydrogen in the crystallite. The boundary condition at the interface of carbon/electrolyte in pores is:

$$\dot{\mathbf{u}}\_p = nFD \frac{\partial \mathbf{c}}{\partial \mathbf{x}} \bigg|\_{\mathbf{x}=\mathbf{0}} \tag{4}$$

where n is the charge transferred, F is Faraday's number, ir is the current density determined by the kinetic dependence of the reaction. Let us also assume that there is a certain length H corresponding to half the crystallite length, where there is no hydrogen diffusion flux due to the symmetry:

Studies of Supercapacitor Carbon Electrodes with High Pseudocapacitance 177

[38]: *l* ~ 1/ s , where is the carbon density in activated carbons. ~ 2 g/cm3. Assuming that s

If the adsorption rate is much higher than the rate of hydrogen solid–phase diffusion (as in

where СА is the hydrogen concentration at the interface. In the case of the galvanostatic

0, *y y L*

where L is the porous electrode thickness. I is the overall current density per electrode unit

The system of equations (3)–(13) is the system with parameters twice distributed along the x and y axes; it describes hydrogen intercalation and adsorption in the porous carbon

This system of equations was solved numerically using the COMSOL Multiphysics

The fitting yielded better convergence of these curves for the set of system parameters presented in Table 1 as followed from the comparison of calculated and experimental