Electrochemical impedance characteristics and electroreduction of oxygen at tungsten carbide derived micromesoporous carbon electrodes

Journal of Electroanalytical Chemistry 689 (2013) 176–184

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Electrochemical impedance characteristics and electroreduction of oxygen at tungsten carbide derived micromesoporous carbon electrodes E. Härk a, J. Nerut a, K. Vaarmets a, I. Tallo a, H. Kurig a, J. Eskusson a, K. Kontturi b, E. Lust a,⇑ a b

Institute of Chemistry, University of Tartu, 14A Ravila Str., 50411 Tartu, Estonia Department of Chemistry, Aalto University, P.O. Box 16100, FIN-00076 Aalto, Finland

a r t i c l e

i n f o

Article history: Received 16 March 2012 Received in revised form 10 September 2012 Accepted 25 September 2012 Available online 30 October 2012 Keywords: Tungsten carbide Oxygen electroreduction Carbide derived carbon Polymer electrolyte membrane fuel cell

a b s t r a c t The electrical double layer characteristics and oxygen electroreduction kinetics in 0.5 M H2SO4 aqueous solution has been studied at micromesoporous tungsten carbide derived carbon C(WC) electrodes. Carbon powders with various specific surface areas (1280–2116 m2 g1) have been prepared from WC at chlorination temperatures 900 °C, 1000 °C and 1100 °C. The porous structure of carbon substrate was characterised using nitrogen sorption, X-ray diffraction, high resolution TEM, electron energy loss spectroscopy, selected area electron diffraction and scanning electron microscopy with energy-dispersive Xray spectroscopy methods. Cyclic voltammograms at various potential scan rates from 2 to 70 mV s1, and rotating disc electrode data at rotation velocities from 0 to 3000 rev min1, were measured within the region of potentials from +0.4 V to 0.6 V vs. Hg|Hg2SO4|sat.K2SO4 in H2O (MSE). At E > 0.2 V, the electroreduction of oxygen is mainly limited by the charge transfer step, and at 0.6 V < E < 0.2 V, by the mixed kinetics. The oxygen electroreduction mainly proceeds through the peroxide formation intermediate step on all electrodes studied. Despite of that the electrodes tested were very stable during the electrochemical experiment, indicating that the C(WC) is a suitable catalyst support material for polymer electrolyte membrane fuel cell. The electroreduction rate of oxygen depends strongly on the structure (graphitisation level) of carbide derived carbon used for preparation of an electrode and the oxygen reduction overvoltage decreases in the order C(WC) 1100 °C > C(WC) 1000 °C > C(WC) 900 °C. Very high low-frequency capacitance values, independent of alternative current (ac) frequency at f < 0.1 Hz, have been established for C(WC) 1100 °C, demonstrating that at ac f ? 0, mainly pseudocapacitive behaviour with adsorption limited step of reaction intermediates has been observed. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Polymer electrolyte membrane fuel cells are promising energy sources for mobile and portable applications. Over the last several decades, an intensive research has focused on the development of electrocatalyst for the oxygen electroreduction reaction (ORR) because in a polymer electrolyte membrane fuel cell (PEMFC), the major limit on performance is the slow kinetics of the cathodic oxygen electroreduction [1–11]. During long lasting catalyst optimization studies, various carbon supports have been used [1,4,8,10,11], but influence of the structure of micromesopourous materials on the oxygen electroreduction kinetics has not fully understood. It has been recognized that, depending on the heattreatment temperatures, different carbon materials exhibit different physical properties, such as specific surface area, porosity, pore size distribution, electrical conductivity [12–16] and oxygen electroreduction activity [4,8,17]. ⇑ Corresponding author. Tel.: +372 737 5165. E-mail address: [email protected] (E. Lust). 1572-6657/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2012.09.039

ORR has also been studied on materials composed of WC [18] and W2C [19], prepared from Vulcan X-72 carbon and tungsten powder, using intermittent microwave heating. Pure WC and W2C were inactive within the potential region studied (E from 0.04 V to 0.49 V vs. MSE, and E from 0.01 V to 0.49 V vs. MSE, respectively) comparable with the potentials applied in this work [18,19]. However, if these materials were combined with platinum nanoparticles, they demonstrated higher activity towards ORR in 0.5 M H2SO4 solution compared with platinum based materials. Recent studies have revealed that the specific surface area, pore size distribution and crystallinity (i.e. graphitisation level and linear dimensions of graphitic areas) of various carbon supports have an important impact on the rate of electrochemical processes, influencing even fuel oxidation reaction mechanism and balance between rate limiting kinetics steps. Thus, the rate of charge transfer, adsorption or mass transfer steps depends on the carbon material support used [1–9,20]. For better performance of PEMFC catalysts the carbon supports with optimized micromesoporosity, pore size distribution and good electrical conductivity as well as optimal specific surface area would be designed. Our systematic

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studies in the field of electrical double layer capacitors [12–16] show that binary metal carbide derived carbon (CDC) materials have been characterised with very high specific surface area, low carbon degradation rate and chemical stability at extreme potentials applied (very low so-called ‘‘carbon corrosion rate’’), as well as increased electrical conductivity. It was demonstrated that carbon powders with well determined mircomesoporosity and pore size distribution, ratio of micro- and mesopore areas and volumes, can be synthesised using selective chemical reaction of molecular chlorine with various binary carbides [12–16]. Based on mentioned synthesis method, microporous C(TiC) and C(a-SiC), as well as partially mesoporous carbons C(VC) and C(WC), and mainly mesoporous C(Mo2C) have been synthesised using TiC, a-SiC, VC, WC and Mo2C as raw materials, respectively [12–16]. The main aim of this work was to study and compare the electrochemical properties of various micromesoporous C(WC) electrodes, prepared from WC powder at following synthesis temperatures Tsynt: 900 °C, 1000 °C and 1100 °C (referred to as C(WC) 900 °C, C(WC) 1000 °C and C(WC) 1100 °C, respectively [15]) and analyse an influence of materials porosity on the ORR in 0.5 M H2SO4 aqueous solution. Influence of the graphitisation level of C(WC) (i.e. ratio of the amorphous and graphitic areas), prepared at different Tsynt, on the rate of electrochemical processes has been demonstrated. In addition, the electrical double layer characteristics for systems under study have been calculated using impedance spectroscopy and cyclic voltammetry (CV) methods. Main information of this work will be used for the development of improved carbon supported Pt and Pt–Ru catalysts for oxygen reduction and methanol oxidation processes in PEMFC, respectively. 2. Experimental 2.1. Electrode preparation Glassy carbon disk electrode (GCDE), (5 mm diameter, 0.196 cm2) (Pine Instrument Company), pressed into a Teflon holder, served as the substrate for the catalyst, was polished to a mirror finish (0.05 lm alumina slurry, Buehler). After polishing GCDE was washed with Milli-Q+ water and sonicated in Milli-Q+ water for a few minutes. Thereafter GCDE was covered with a C(WC) 900 °C, C(WC) 1000 °C or C(WC) 1100 °C catalyst ink [4,11,21], prepared by suspending the C(WC) powder in Milli-Q+ water and agitated in an ultrasonic bath for 15 min for thorough wetting as well as to disperse the carbon powder homogeneously. Thereafter NafionÒ dispersion solution (Aldrich) was added to the catalyst mixture (to give a dry ink with fixed composition of 1 wt% of NafionÒ ionomer in the CDC electrode) and sonicated for next 30 min at room temperature to prepare uniformly dispersed ink. The catalyst ink was pipetted onto the GCDE surface and dried at room temperature. The mass of WC-CDC loading 1.6 ± 0.2 mg cm2 was found to be reproducible within ±5% (based on a series of experiments). The electrode was pretreated in MilliQ+ water during 30 min before assembling the test cell.

Table 1 Results of sorption measurements of electrode material prepared from WC-CDC synthesised at different fixed temperatures. Parameter/material chlorination temperature (°C)

SBET (m2 g1)

Smicro (m2 g1)

Vmicro (cm3 g1)

Vtot (cm3 g1)

C(WC) 900 C(WC) 1000 C(WC) 1100

1290 1280 2116

1280 1260 1980

0.60 0.61 1.26

0.64 0.66 1.53

SBET – BET surface area, Smicro – micropore area, calculated using t-plot method, Vtot – total pore volume, Vmicro – micropore volume.

Based on the data given in Table 1 and in Fig. 1, all materials under study have a hierarchical porous structure and complicated pore size distribution, i.e. they are micromesoporous materials, especially materials prepared at Tsynt = 1100 °C, where hysteresis in adsorption is very well detectable. The BET data obtained in this work are in a good agreement with corresponding data published in [15]. For additional characterisation, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) method was applied. The data obtained (Fig. 2) show that electrodes prepared have hierarchical structure and open micromesoporosity. EDX data (Table in Fig. 2) show that there is no chemical contamination of C(WC) electrode with residual chlorine, WCl3, or raw WC. In some areas the binding material NafionÒ filaments (as element F in surface chemical analyses Table) and small oxygen concentration (probably existing as functional groups at the surface) have been observed. For morphological studies, C(WC) was examined using high resolution transmission electron microscopy (HRTEM, Tecnai 12 instrument operating at the 120 kV accelerating voltage), and complementary techniques, such as electron energy loss spectroscopy (EELS) and selected area electron diffraction (SAED) methods (Fig. 3) [15]. Similarly to the results given in [15], the HRTEM, EELS and SAED data show that C(WC) 900 °C has mainly amorphous structure (not shown for shortness), but in some areas of C(WC) 1100 °C, the existence of a somewhat ordered graphitic particles in the sample surface layer (Fig. 3) has been observed [15], increasing noticeably the electronic conductivity of C(WC) 1100 °C in comparison with C(WC) 900 °C (demonstrated later by impedance data). The SAED pattern displays hk0-type diffraction rings of graphite (Fig. 3b), which are consistent with a nanostructure consisting of poorly-stacked graphene layers. EELS data i.e. more well developed and narrow p- and d-peaks for C(WC) 1100 °C compared with C(WC) 900 °C, (not shown for shortness) show that the carbon-K ionization edge gives important information about the bonding nature and the crystallinity of the material. Most importantly, in agreement with EDX data, the TEM-EELS data show that there is no WC, WCl3 or chlorine residuals in the electrode materials under study. The first-order and second order Raman spectra are in a good agreement with EELS and SAED data, indicating the partial graphitisation of C(WC) 1100 °C material, similarly to our previous data [15]. 2.3. Electrochemical measurements

2.2. Structural characterisation of electrodes The porous structure of C(WC) substrate was characterised using nitrogen sorption method (BET). The low-temperature N2 sorption experiments were performed at the boiling temperature of liquid nitrogen (195.8 °C) using the ASAP 2020 system (Micromeritics). Non-Local Density Functional Theory (NLDFT) combined with slit shape pore model and t-plot method was used for calculation of gas adsorption parameters [12–16], given in Table 1.

The electrochemical measurements were carried out in a threeelectrode electrochemical cell using a rotating disk electrode (RDE) system [21–25]. For ac impedance measurements [26], the electrode potential E was applied with a potentiostat/galvanostat (Autolab PGSTAT 100 with FRA, Eco Chemie B.V.), and the experiments were controlled with General Purpose Electrochemical System and Frequency Response Analyser software. Working electrode potentials were determined using a Hg|Hg2SO4, saturated K2SO4 reference electrode (MSE), separated from the cell by a Lug-

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Fig. 1. Low-temperature nitrogen adsorption/desorption isotherms for various C(WC) prepared at chlorination temperatures 900 °C, 1000 °C and 1100 °C (noted in figure) and differential volume vs. pore width (inset) for C(WC) prepared at chlorination temperature 1100 °C.

(a)

(c) Spectrum 2

Spectrum 1 Spectrum 3

(b)

Processing option : All elements analysed (Normalised) Spectrum C O F Spectrum 1 Spectrum 2 Spectrum 3

98.94 96.45 96.69

1.06 1.41

3.55 1.90

All results in weight% . Raw material C(WC) synthesised at chlorination temperature 1000 oC , 1% nafion suspension was added to prepare the micromesoporous carbon electrodes onto GCDE support.

Fig. 2. SEM images at different magnifications (with the magnification bar of 10 lm (a) and 50 lm (b)), and SEM–EDX data (c) for C(WC) 1000 °C powder based electrodes deposited onto GCDE (with addition of 1% NafionÒ binder).

gin capillary in order to avoid contamination and the distance between Luggin tip and working electrode surface was more than 1 cm (to avoid distortion effect for rotating disk electrode data). The counter electrode was a very large Pt wire mesh, separated from the main solution by a fritted glass membrane. The cyclic voltammetry (CV) curves were obtained on standing electrode at potential scan rates from 2 to 70 mV s1 and RDE data were measured at rotation velocities from 0 to 3000 rev min1

(v = 10 mV s1) and in the region of potentials from +0.4 V to 0.6 V vs. MSE. CV curves were corrected for IR-drop, where solution resistance Rs was obtained from impedance data at ac frequency f P 1  104 Hz, discussed later. First group of CV measurements was conducted in 0.5 M H2SO4 (Fluka, TraceSelectÒUltra) electrolyte solution saturated with Ar (99.9999%, AGA) to measure the so-called reference j, E-curve, where j is the current density, and the second group of j, E-curves was obtained

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Fig. 3. HRTEM micrograph (a) SAED pattern (b) and corresponding carbon-K ELNES (c) for partially graphitized C(WC), prepared at 1100 °C.

in the same solution, but saturated with the molecular oxygen (99.999%, AGA) for 30 min before the measurements to study the ORR kinetics (Figs. 4–7). The standby potential was fixed at + 0.40 V vs. MSE. Electrochemical impedance data were obtained at fixed ac frequencies f from 0.01 to 105 Hz (5 mV modulation) at fixed E,V: 0.5, 0.35, 0, +0.15, +0.25 and +0.4 vs. MSE. All measurements were carried out at temperature 22 ± 1 °C.

3. Results and discussion

Fig. 4. RDE data for C(WC) 900 °C powder prepared electrode (corrected for current densities in 0.5 M H2SO4 solution saturated with Ar) (a) and for various C(WC) powder based electrodes (b) deposited onto GCDE (m = 10 mV sec1) (corrected for current densities in 0.5 M H2SO4 solution saturated with Ar) for ORR on C(WC)|GCDE in 0.5 M H2SO4 solution saturated with O2, at 500, 1000, 2000 and 3000 rev min1, noted in figure.

3.1. Rotating disc electrode data The RDE data [21–25], given in Fig. 4 (current densities measured in Ar-saturated, i.e. deareated 0.5 M H2SO4 supporting electrolyte solution, given as solid line in Fig. 4a) show that high current density values (|j| > 25 A m2) have been achieved for the electrode, prepared from C(WC) 900 °C in 0.5 M H2SO4 solution saturated with O2. The current densities for ORR (Fig. 4) were minimal at the standby potential (+0.40 V) selected. After correction for the supporting electrolyte current densities in Ar saturated conditions, the RDE curves with shape (within the potential range from 0.59 V to 0.31 V vs. MSE) (Fig. 4b) characteristic for the mixed kinetics processes, have been observed. The RDE data (Fig. 4b) show that current densities noticeably depend on the C(WC) synthesis temperature Tsynt, increasing with the decrease of Tsynt. Thus, the mixed kinetic current densities depend strongly on the amorphous structure and on the graphitisation level of the carbon powder used for preparation of electrodes. The half-wave potential shifts toward more negative potentials, i.e. the electroreduction activity of O2 decreases with the increase of synthesis temperature of carbon powder. For the electrode, deposited using C(WC) 900 °C powder based ink, the comparatively high current densities have been measured within the mixed kinetics region. The higher current densities for more amorphous C(WC) 900 °C powder based electrode can be explained by the higher concentration of surface defects expressed at amorphous C(WC) 900 °C, (probably by the exsistance of edge planes), compared with that for partially graphitized C(WC) 1100 °C. CV and RDE data show that the current densities (within potential range

Fig. 5. Koutecky–Levich plots for C(WC) 1100 °C|GCDE, calculated from RDE data (corrected for current densities in 0.5 M H2SO4 solution, saturated with Ar) in 0.5 M H2SO4 solution at different E (vs. MSE), noted in figure.

from 0.6 V to 0 V vs. MSE) noticeably decrease with the increase of BET surface area (SBET) and total pore volume (Vtot) values (Table 1). To analyse the RDE data, the measured current density, j, was described by the following relation [22]

1 1 1 1 1 1 L þ ¼ þ þ ¼ þ j jk jD jf jk Bc0 x1=2 nFDf cf

ð1Þ

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Fig. 6. log jk, E-curves for various C(WC) powder based electrodes (noted in figure), calculated from RDE data for C(WC)|GCDE in 0.5 M H2SO4 solution, saturated with O2 (inset – the number of electrons transferred per O2 molecule calculated for C(WC) 900 °C|GCDE).

where jk and jD are the kinetic and diffusion limited current densities, respectively and the NafionÒ film diffusion limited current density, jf. The other parameters are the Levich constant B, the reactant concentration in the solution c0, the film thickness L, the reactant concentration in the NafionÒ film cf, and the diffusion constant in the NafionÒ film Df, the number of electrons transferred per one reduced O2 molecule n, and the Faraday constant F (96485 C mol1). For smooth electrodes, the jf term does not exist in Eq. (1) and j depends only on jk and jD, as shown in Eq. (2). Analysis of data presented in [22] demonstrates that for thin film electrode, i.e. at low catalyst loading, the role of jf is unimportant as the jf values are noticeably higher than jk, and jD values, and to the first approximation the simplified Eq. (2), known as Koutecky–Levich (K–L) equation, can be used for calculation of kinetic current densities. Taking into account the small thickness of catalyst layer and the low catalyst loading, as well as the fact, that the active catalyst layer thickness has been kept constant for all C(WC) electrodes prepared in this work, the kinetic current densities were calculated from the

Fig. 7. CCV, E-plots for C(WC) 1100 °C|GCDE in 0.5 M H2SO4 solution saturated with Ar (a) and O2 (b) at different voltammograms (inset in Fig. b) in the case of the 0.5 M H2SO4 solution saturated with oxygen (m = 10 mV sec1).

v (mV s1, noted in figure) and time stability of cyclic

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practically linear j1, x1/2 plots (at E 6 0.31 V) and were analysed using the K–L equation [1,4–6,23–25]

1 1 1 1 1  ; ¼ þ ¼ 1=6 x1=2 c b j jk jD nFkhet cbO2 0:62nFD2=3 O2 m O2

ð2Þ

where cbO2 is the concentration of O2 in the bulk (1.3  106 mol cm3 in 0.5 M H2SO4 at T = 298 K [5]), khet is the electrochemical rate constant for O2 reduction, DO2 is the diffusion coefficient of oxygen (2.2  105 ± 0.2 cm2 s1) [4,23–25], m is the kinematic viscosity of the solution (0.01 cm2 s1) [23–25] and x is the rotation velocity of the electrode. The K–L plots were linear (correlation coefficient R2 > 0.846) within the region of electrode potentials from 0.31 V to 0.59 V vs. MSE (Fig. 5) and the intercept of the extrapolated K–L plot (giving the kinetic current density jk value) depends on the potential applied. However, the slope for j1, x1/2 plots for electrode potentials from E = 0.31 V to 0.43 V is close to zero, demonstrating that the electrochemical reaction rate is controlled by processes independent of the electrode rotation rate, i.e. controlled by the faradaic charge transfer step and/or mass transfer process through the thin surface layer of catalyst that rotates together with the electrode. Fig. 6 demonstrates that the jk values noticeably depend on C(WC) material studied, being highest for material with lowest synthesis temperature, if E P 0.5 V. Thus, at less negative C(WC) electrode potentials, the crystallinity of C(WC) electrodes and graphitisation level has noticeable influence on the electrocatalytic activity of electrodes for oxygen electroreduction process under study. As summarised by Compton and Gara [27], the oxygen reduction on carbon electrode in acidic solutions usually proceeds through the more common two electron transfer mechanism, i.e. through the peroxide formation pathway. However, the number of electrons transferred during electroreduction process to O2 molecule, somewhat higher than 2, established in this work (inset in Fig. 6), is in a good agreement with literature data [1–3,5], and indicate that the peroxide formed partially reduces further at E < 0.3 V [27]. The number of electrons transferred depends somewhat on Tsynt, being highest for C(WC) 900 °C. More detailed analysis of CV data, given in Fig. 7, shows that nearly pseudocapacitive behaviour for Ar saturated (Fig. 7a), as well for O2 saturated C(WC)|GCDE systems, has been established (Fig. 7b). The values of capacitance, CCV, have been calculated using

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the relation CCV = j/v. CCV values, calculated for Ar and O2 saturated systems (Fig. 7a and b), only weakly depend on v applied, similarly to the micromesoporous carbon electrodes, tested by us before [13,14]. Very weak dependence of capacitance on v indicates the dominating pseudocapacitive behaviour of studied systems [26,28–31]. Comparison of data for Ar and O2 saturated solutions (Fig. 7a and b) show, that for O2 saturated system only weakly higher j and CCV values (Fig. 7b) have been obtained within the potential region from 0.2 V to 0.6 V vs. MSE, where ORR has been established using RDE data, indicating the slow mass transfer processes of O2 molecules inside micromesoporous carbon electrodes at normal pressure conditions applied in our CV experiments. In addition, the wide oxidation (with peak potential Eox) and reduction (with Ered) maxima in Fig. 7a and b, practically independent of O2 concentration in solution, have been established. Difference between Eox and Ered (DE) is lower than 50 mV and DE is nearly independent of the molecular oxygen concentration in solution indicating that the population density of the oxygen containing surface active groups is practically independent of O2 concentration in solution. Thus, at O2 diffusion limited conditions, the micromesoporous area is not active at atmospheric (normal) pressure conditions due to the absence of quick oxygen transfer into/inside of the microporous C(WC) structure. Based on the CV data (inset in Fig 7.b), the time stability of cyclic voltammograms in the case of 0.5 M H2SO4 solution, saturated with oxygen is good, despite of the fact that the oxygen reduction on these carbon materials proceeds mainly by the two electron mechanism, i.e. through the peroxide formation pathway. The stability of electrodes has been tested additionally using repetitive cyclation of electrode potential within the region from 0.6 V to 0.4 V vs. MSE. The data presented in Fig. 7b inset indicate, that the current densities are the same at the beginning and at the end of experiment thus after more than 100 potential cycles, within the E region mentioned, has been made. 3.2. Electrochemical impedance results Electrochemical impedance (EIS) data given as Nyquist (Z00 , Z0 ), phase angle h vs. log f and log (Z00 ) vs. log f plots [23,28–31], given in Figs. 8–10, show that in the region of low ac frequency (f < 10 Hz) the electroreduction of oxygen is mainly limited by the adsorption step limited process at the porous surface areas of

Fig. 8. Nyquist plots for C(WC)|GCDE system in 0.5 M H2SO4 solution, saturated with O2 (at different E, noted in figure).

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Fig. 9. Phase angle h vs. log f plots for various C(WC)|GCDE electrodes in 0.5 M H2SO4 solution, saturated with O2 (at different E, noted in figure).

the C(WC) electrode. At f > 102 Hz, the phase angle values (Fig. 9) are close to zero and in this frequency area [26] the impedance spectra are practically independent of oxygen concentration in 0.5 M H2SO4 solution. Only at f > 103 Hz, some differences in Nyquist plots have been established for O2 and Ar saturated solutions. Thus, the oxygen electroreduction reaction at open surface areas of C(WC) seems to be very quick (Figs. 8–10) and the kinetics of charge transfer step at open surface areas cannot be analysed using impedance method [26,30]. At lower ac frequency, f < 10 Hz, the ‘‘knees’’ in –Z00 , Z0 plots can be seen, indicating that the so called nearly ‘‘blocking adsorption’’ pseudocapacitive behaviour has been established, similarly to Pt–Ru–C(Mo2C) electrodes [21] and Ru|RuO2–C electrodes [30–36] as well as for C(WC) in acetonitrile or in aqueous Na2SO4 surface inactive electrolyte solutions [37,38]. Fig. 10b shows that the high frequency series resistance (Z0 (f ? 1) = Rs) is independent of potential applied and thus, the electrolyte series resistance Rs can be calculated at f P 10 kHz. These Rs values have been used for IR-compensation of j, E-curves, given in Figs. 4–6 [4–7]. Weak decrease of Z0 or Z, in comparison with C(WC) 900 °C and C(WC) 1000 °C, can be seen only for the

Fig. 10. Dependencies of log (Z0 0 ) (a) and log Z0 vs. log f plots (b) for various C(WC)|GCDE systems (noted in figure) in 0.5 M H2SO4 solution, saturated with O2 (at different E, noted in figure).

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Fig. 11. Dependencies of Cs and Cp on log f plots for various C(WC)|GCDE systems (noted in figure) in 0.5 M H2SO4 solution, saturated with O2 (at different E, noted in figure).

partially graphitized C(WC) 1100 °C electrode due to better solid phase conductivity. At f < 1 Hz, log Z0 (Fig. 10b) and log Z values increase with the decrease of negative electrode potential indicating the deviation of the system from the mixed kinetic mechanism at E P 0 V toward the adsorption limited mechanism at E 6 0.35 V. The phase angle values h  87° at f < 1 Hz, can be explained by the adsorption step limited processes at micromesoporous C(WC) electrodes in H2SO4 aqueous solution [17,24–26,30–35,37–41]. As it was demonstrated by Orazem et al. [28,39–41], the absolute value of the slope of the log (Z00 ) vs. log f plot is equal to the constant phase element (CPE) fractional exponent, where CPE impedance is given as ZCPE = Q(jx)aCPE (aCPE is CPE fractional pffiffiffiffiffiffiffiexponent and Q is the constant phase element coefficient, j ¼ 1 and x = 2pf). Fig 10a shows that a plot of the log (Z00 ), log f yields a straight line with a slope of 0.98 at frequencies lower than 10 Hz, which indicates CPE behaviour with a CPE exponent of 0.98. For more microporous C(WC) 900 °C|0.5 M H2SO4 + H2O interface, the aCPE values are somewhat lower than 0.96, demonstrating the more pronounced influence of mass transfer step on the mainly adsorption step limited processes. To the first approximation the impedance response for micromesoporous C(WC) based electrochemical systems reflects a distribution of reactivity that could represented with an equivalent electrical circuit as a CPE in parallel with charge transfer resistance [28]. Huang et al. [29] have shown that current and potential distributions induce a high frequency pseudo constant phase element behaviour in the global impedance response of an ideally polarized blocking electrode with a local ideally capacitive behaviour. It is important to mention, that for C(WC) 1100 °C, having highest SBET and more mesoporous structure, the log (Z00 ), log f plots are already linear at f 6 10 Hz, but for the more amorphous microporous C(WC) 900 °C, these plots are linear only at very low frequencies f 6 0.5 Hz. Thus, the time constants of mixed kinetic and adsorption limited processes depend noticeably on the porosity and crystallographic structure of the carbon material used. The adsorption limited processes prevail in the low frequency region and for the more efficient PEMFC systems, more mesomacroporous electrodes with higher defect concentration must be developed. Data in Figs. 8–10 show, that with the moderate increase of SBET (30%) and Vtot (40%) (Table 1), the limiting capacitance (Fig. 11) noticeably increases (3 times) with the rise of C(WC) powder synthesis temperature. The series capacitance Cs can be calculated from the imaginary part of impedance, given as Z00 = (2pfCsj)1,

pffiffiffiffiffiffiffi where j ¼ 1. High values of parallel capacitance Cp and parallel resistance Rp calculated from Nyguist plots using the well-known mathematical relationships [11–15,17,23] (Rp = |Z|2/Z0 and Cp = Z00 / (|Z|2x)), have been established for C(WC) 1100 °C (Fig. 11) at low frequency f < 1 Hz. At f ? 0, Cs, log f- and Cp, log f-plots overlap, indicating the nearly pseudocapacitive (so-called ‘‘blocking’’ adsorption) electrode behaviour [17,20,21,23] for C(WC)|0.5 M H2SO4 + O2 interface. Cs and Cp values depend noticeably on the electrode potential applied, and Cs and Cp are minimal at 0 V, i.e. in the region of zero charge potential [13,14,37,38]. Cs and Cp values increase with the rise of negative electrode potential applied, i.e. with the rate of ORR at electrodes limited by the adsorption rate of intermediate or reaction products at/inside micromesoporous C(WC) material.

4. Conclusions Micromesoporous carbon electrodes from C(WC) powders (with various specific surface areas from 1280 to 2116 m2 g1) were synthesised from WC at different fixed temperatures from 900 °C to 1100 °C, using chlorination method [15]. Analysis of XRD, SEMEDX, Raman, HRTEM, EELS, SAED, CV, RDE and EIS data show that highly porous electrodes have been prepared for which the moderate cathodic O2 electroreduction current densities (25 A m-2) in 0.5 M H2SO4, saturated with O2, were obtained. Hence, the C(WC) can be used as supports for various catalysts because of its moderate activity for O2 electroreduction in a wide potential region in acid media. Using the RDE method, at more negative potentials than 0.23 V vs. MSE, the number of electrons n transferred per one O2 molecule electroreduction calculated was (P2.6) being in a good agreement with literature data. However, analysis of impedance spectra shows nearly pseudocapacitive behaviour for C(WC) electrodes at low ac freguency region (f < 1 Hz), where probably the O2 electroreduction reaction rate is limited by the rate of intermediate (H2O2) or final product adsorption step. In the electrode potential region from 0 V to 0.4 V (vs. MSE), the very high series and parallel capacitance values 70–90 F g1 (calculated per active material weight) have been established for C(WC) 1100 °C. At all electrode potentials studied, series and parallel capacitance values coincide at frequency f 6 0.05 Hz, demonstrating the nearly socalled pseudocapacitive behaviour. Thus, C(WC) catalysts are inter-

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