Influence of activated carbon pore structure on oxygen reduction at catalyst layers supported on rotating disk electrodes

Carbon 42 (2004) 3115–3121 www.elsevier.com/locate/carbon

Influence of activated carbon pore structure on oxygen reduction at catalyst layers supported on rotating disk electrodes Jun Maruyama a

a,*

, Ken-ichi Sumino b, Masayuki Kawaguchi b, Ikuo Abe

a

Environmental Technology Department, Osaka Municipal Technical Research Institute, 1-6-50, Morinomiya, Joto-ku, Osaka 536-8553, Japan b Department of Materials Science, Osaka Electro-Communication University, 18-8, Hatsu-cho, Neyagawa, Osaka 572-8530, Japan Received 6 April 2004; accepted 24 July 2004 Available online 17 September 2004

Abstract Carbon materials are often used as catalyst supports, and for catalysts in electrodes of a polymer electrolyte fuel cell, carbon black has been used. Recently, it was found, however, that activated carbon could replace carbon black and besides, significantly improve the activity of the electrode catalyst layer for oxygen reduction. In the present study, to optimize the pore structure of activated carbon for further activity improvement, the influence of the pore structure on the activity was investigated using activated carbon of various specific surface areas and mean pore diameters. A catalyst layer was formed from activated carbon loaded with platinum and a polymer electrolyte. The activity of the layer was measured in an oxygen-saturated perchloric acid solution, supporting the layer on a rotating glassy carbon disk electrode. We found that increases in the specific surface area and mean pore diameter increased the activity and that the latter was more effective than the former mainly due to the enhanced mass-transfer in the pores; the catalyst layer formed from activated carbon with the largest mean pore diameter was the most active. Unless pores excessively develop and lose connections between particles, a large pore diameter is therefore desired for the fuel cell electrodes.
2004 Elsevier Ltd. All rights reserved. Keywords: A. Activated carbon; B. Catalyst support; C. BET surface area; D. Electrochemical properties, Transport properties

1. Introduction Catalysts in the electrodes of a polymer electrolyte fuel cell (PEFC) play a crucial role in generating electricity from chemical energy as the electrode reactions occur on them. The electrode reactions in the PEFC are H2 oxidation (Eq. (1)) and O2 reduction (Eq. (2)) occurring at the anode and the cathode, respectively [1–4]. H2 !2Hþ + 2e

ð1Þ

O2 + 4Hþ + 4e !2H2 O

ð2Þ

*

Corresponding author. Tel.: +81 6 6963 8041; fax: +81 6 6963 8049. E-mail address: [email protected] (J. Maruyama). 0008-6223/$ - see front matter
2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.07.023

Dispersed platinum loaded on electron-conductive carbon black (Pt/C) is currently used as the catalyst and mixed with a perfluorosulfonate ion-exchange resin to form a catalyst layer in the electrode. The diameter of the dispersed Pt particles is 1–5 nm, giving them a large specific surface area (50–200 m2 g1) greater than that of platinum black (2–30 m2 g1) [4–6]. It was reported that the PEFC performance was greatly improved by efficient utilization of the dispersed Pt supported on carbon black in the catalyst layer rather than platinum black [5–10], which suggests that carbon support is essential to achieve a high performance in the PEFC. However, its activity, especially for cathodic O2 reduction, is not sufficient yet, causing an energy loss in the energy conversion, which appears as a large cell-voltage drop; 0.4–0.5 V from the theoretical value of 1.2 V at a current density of 1 A cm2 [1,11].

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The specific surface area of the dispersed Pt particles would be further increased by using activated carbon due to its high surface area instead of carbon black [12]. In addition, the high surface area of activated carbon is favorable for Pt particles to function independently without interference from neighboring Pt particles. The variety of raw material sources is another possible advantage of using activated carbon, which can be produced by carbonization and activation of waste materials including waste wood and textiles [13–15]. In the recent studies, we revealed that partial replacement of carbon black by conventional activated carbon in the catalyst layer improved the activity of the layer for O2 reduction [16]. We further found that the adsorption of trifluoromethane sulfonic acid (CF3SO3H) in the pores of Pt/AC significantly enhanced the activity, approximately an 8-fold increase at most, compared to a conventional catalyst layer formed by Pt/C, and attained a 0.1–0.15 V improvement in electrode potential [17]. It was necessary to use an electron-conductive agent such as carbon black in the catalyst layer formed from Pt/AC, which would increase the volume of the layer, and consequently, the volume of the fuel cell. However, this could be minimized by using a slightly thinner gas-diffusion layer, since the typical thickness of a catalyst layer is 5–30 lm, much smaller than that of a gas-diffusion layer, 300–400 lm [18–21]. Activated carbon has various pore structures depending on the raw materials, carbonization conditions, and activation processes. Optimization of the pore structure would improve the activity for O2 reduction. In the present study, we investigated the influence of the Pt/AC pore structure on O2 reduction at the catalyst layer supported on a rotating disk electrode. We formed catalyst layers from activated carbon of various specific surface areas and mean pore diameters by mixing them with Nafion, which is the most widely used commercial perfluorosulfonate ion-exchange resin for the PEFC. We found that the activity of the catalyst layers was clearly dependent on the pore structure, and the catalyst layer formed by the activated carbon with the largest pore diameter among the activated carbons used in this study was the most active.

2. Experimental 2.1. Materials Four kinds of activated carbons were used in the present study. For convenience, they are hereafter called AC-1, AC-2, AC-3, and AC-4. The adsorption isotherms of nitrogen onto AC-n (n = 1–4) were measured using an automatic N2 adsorption apparatus (Belsorp 28, Nihon Bell) at 196 C. The specific surface areas determined by Brunauer–Emmet–Teller (BET) plot,

Table 1 Specific surface area, pore volume, and mean pore diameter of activated carbon used as the support of Pt Activated carbon

AC-1

AC-2

AC-3

AC-4

Specific surface area (m2 g1) Pore volume (mm3 g1) Mean pore diameter (nm)

1038 460 1.77

1831 839 1.83

1187 724 2.25

1467 1149 3.13

pore volumes, and mean pore diameters of AC-n are shown in Table 1, where the mean pore diameter was calculated assuming the pores to be cylindrical and using the equation d ¼ 4V p =S

ð3Þ

where d is the mean pore diameter, Vp is the pore volume, and S is the specific surface area. Differential pore-volume distributions were also obtained using the isotherm. Hydrogen hexachloroplatinate hexahydrate (H2PtCl6 Æ 6H2O, Nacalai tesque, 98.5%) and sodium tetrahydroborate (NaBH4, Kanto Chemical, 98%) were dissolved in ethanol (dehydrated, Kanto Chemical, reagent grade) to prepare 0.2 M and 1 M solutions, respectively, which were used for the platinum loading on the activated carbon. High-purity water was obtained by circulating ion-exchanged water through an Easypure water-purification system (Barnstead, D7403). Perchloric acid (70%, Tama Chemical, analytical grade) was diluted with the high-purity water to prepare 0.1 M HClO4. Nafion solution [equivalent weight (molar mass per mol of ion-exchange site) = 1100, 5 wt.% dissolved in a mixture of lower aliphatic alcohols and 15– 20% water] was purchased from Aldrich. Argon and oxygen gases were of ultra-high purity. 2.2. Platinum loading on activated carbon Platinum was loaded on the activated carbon according to the method reported by Brown and Brown [22]. A 70.2 ± 0.1 mg sample of activated carbon and 0.2 ml of 0.2 M H2PtCl6 ethanol solution were added to 4 ml of ethanol. After shaking for 2 days, 1 ml of 1 M NaBH4 ethanol solution was added to the mixture under an Ar atmosphere to produce 10 wt.% Pt/AC. When the activated carbon was AC-n, the obtained catalyst was represented as Pt/AC-n. The platinum dispersion was observed by a transmission electron micrograph (TEM) using JEM-1200EX, JEOL. 2.3. Fabrication of catalyst layer A GC RDE (BAS), which consisted of a GC rod sealed in a Kel-F holder, was polished with a 2000 grit emery paper (Sumitomo 3 M) and then ultrasonically cleaned in high-purity water for use as a support for the catalyst layer. The geometric surface area of the electrode was

J. Maruyama et al. / Carbon 42 (2004) 3115–3121

—1

1000

—1

nm )

1200

800

AC-2

3

dV p/dr p (mm g

0.0707 cm2 (diameter 3 mm). A 11.1 ± 0.1 mg sample of Pt/AC and 10.0 ± 0.1 mg of carbon black (Vulcan XC72R, Cabot) as an electron-conductive agent were added to 1 ml of the Nafion solution and the mixture was ultrasonically dispersed to produce a catalyst paste [23]. One microlitre of the paste was pipetted onto the GC surface, and, to shield it from the irregular air stream generated by a ventilator, the electrode was immediately placed under a glass cover until the layer was formed [24]. After removing the glass cover, the electrode was further dried overnight at room temperature.

600 AC-4

400 AC-3

3. Results and discussion 3.1. Characterization of Pt/AC The differential pore-volume distributions of AC-n are shown in Fig. 1. Comparing AC-1 and AC-2, the micropores highly developed in AC-2 that possessed the nearly equal mean pore diameter to AC-1, but had a larger specific surface area. The differential pore volume reached a maximum at rp = 0.8 nm. The distributions for AC-3 and AC-4 exhibited peaks at rp = 0.9 nm and at rp = 1.1 nm; the latter were in the mesopore region, to which a large part of the pore volume belonged. In this region, the differential pore volume increased in the order: AC-1 < AC-2 < AC-3 < AC-4. On the whole, the difference in the differential pore-volume

AC-1

200 0 0

1

2

3

4

r p/nm

2.4. Electrochemical measurements

60 50

—1

—1

nm )

70

3

dV p/dr p (mm g

An electrochemical analyzer (100B/W, BAS) and an RDE glass cell were used for the cyclic voltammetry and the measurement of current–potential relations. The glass cell was cleaned by soaking in a 1:1 mixture of concentrated HNO3 and H2SO4, followed by a thorough rinsing with high-purity water and finally steamcleaning [25]. The counter-electrode was a Pt wire and the reference electrode was a reversible hydrogen electrode (RHE). All potentials were referred to the RHE. Cyclic voltammograms for the catalyst layers were recorded in 0.1 M HClO4 at 25 C. The potential was scanned between 0.05 and 1.3 V at a scan rate of 50 mV s1. Before recording, the potential was repeatedly scanned between 0.05 and 1.4 V to remove the residual impurities. Current–potential relations were measured in O2-saturated 0.1 M HClO4 at 25 C at various rotation speeds. The scan rate of the potential was fixed at 10 mV s1. Prior to the measurements, the electrode was repeatedly polarized at 0.05 and 1.3 V alternately [26]. The potential was finally stepped to 0.2 V and then swept in the positive direction from 0.2 to 1.2 V to obtain the current–potential relationship. The background current was similarly measured in an Ar atmosphere without rotation.

3117

40 30

AC-4

20

AC-1

AC-2

AC-3

10 0 4

6

8

10

12

14

r p/nm

Fig. 1. Differential pore-volume distributions of AC-1 (very thick line), AC-2 (very thin line), AC-3 (thin line), and AC-4 (thick line). Vp: pore volume. rp: pore radius.

distributions between all the activated carbons was observed across all regions. Transmission electron micrographs of Pt/AC-n are shown in Fig. 2. The dispersed platinum was visualized as very small black particles and the carbon supports as gray areas. Although Pt aggregation was observed in several parts, Pt was loaded almost uniformly inside the particles of activated carbon. The particle size of the dispersed Pt was not homogeneous; however, most of them were approximately below 10 nm, except for the aggregated part. Although homogeneous Pt particles could have been obtained using a more complicated loading method, a simple method was used in the present study because its attention was focused on investigating the effect of the pore structure of activated carbon, not particularly on developing a Pt catalyst with a high surface area. According to the results in Fig. 1, the pore-volume distributions for AC-n were different over the entire pore size range, well exceeding the Pt particle size, which would generate the difference in the electrochemical characteristics of the catalyst layers formed from Pt/AC-n. 3.2. Electrochemically active surface area in Pt/AC The electrochemical surface property of Pt/AC-n in contact with Nafion was characterized by cyclic

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Fig. 2. Transmission electron micrographs of (a) Pt/AC-1, (b) Pt/AC2, (c) Pt/AC-3, and (d) Pt/AC-4. Acceleration voltage was 120.0 kV. Scale bars in all the micrographs correspond to 50 nm.

voltammetry. Fig. 3 shows the cyclic voltammograms for the catalyst layers formed from Pt/AC-n in Ar-saturated 0.1 M HClO4. The sign of the current due to oxidation reactions was taken as positive. The current was generated only from the surface that was in contact with Nafion, the electrochemically active surface, and 200 Pt/AC-1 Pt/AC-3

Current /µA

100

0

Pt/AC-4

Pt/AC-2

-100

-200 0

0.5

1

was mostly attributable to the double-layer charging of the activated carbon surface and the redox reaction of the quinone-like surface functional groups. The peaks caused by the Pt surface reactions were reduced by the overlapping of these currents or by the loss in the crystal faces due to dispersion [16,24,27,28]. It was impossible to determine the Pt surface area from the charge caused by the hydrogen adsorption and desorption. However, it could be assumed that the surface functional groups and Pt were almost uniformly distributed in the Pt/AC pores based on the transmission electron micrographs of Pt/ AC-n. The charge which corresponds to the enclosed area in the voltammogram is, therefore, nearly proportional to the electrochemically active surface area. If Nafion molecules fully penetrated into the pores and the entire surface in the Pt/AC pores was in contact with Nafion, the ideal state, the electrochemically active surface area would be exactly proportional to the specific surface area of the Pt/AC, although this state is not practically realized. The current at the catalyst layer formed from Pt/AC2 was smaller than at the Pt/AC-1 layer. Given the larger specific surface area of Pt/AC-2 vs. Pt/AC-1 and nearly the same mean pore diameter, penetration of the Nafion molecules into Pt/AC-2 was assumed to be superior or at least equivalent to that in Pt/AC-1, which led to a similar or larger active surface area [29]. A possible reason for this decrease was the decrease in the connections between the activated carbon and the electron-conductive agent in the catalyst layer to reduce the number of active particles, and consequently, the active surface area, since it could be assumed that the development of pores in the activated carbon caused the decrease in the connection on the outer surface of the activated carbon particles, although a detailed characterization of the catalyst layer, such as measurement of the electron conductivity, would be necessary. The current at the Pt/AC-3 layer was nearly the same as that at the Pt/AC-1 layer. Although the development of pores in AC-3 compared to AC-1 might also hinder the connection in the Pt/AC-3 layer, an increase in the active surface area by easier penetration of the Nafion molecules due to the larger mean pore diameter than in Pt/AC-1 could compensate for the decrease. When the mean pore diameter further increased, as shown in the Pt/AC-4 layer, the current exceeded that at the Pt/AC1 layer. These results indicate that, for achieving an increase in the active area, an increase in specific surface area is not necessarily required, but enlargement of the mean pore diameter is effective.

1.5

Potential/V vs. RHE

Fig. 3. Cyclic voltammograms for catalyst layers formed from Pt/AC1 (very thick line), Pt/AC-2 (very thin line), Pt/AC-3 (thin line), and Pt/ AC-4 (thick line) supported on GC RDE in Ar-saturated 0.1 M HClO4. Scan rate: 50 mV s1.

3.3. Cathodic oxygen reduction at catalyst layer formed from Pt/AC-n Oxygen reduction currents at the catalyst layers, I, were measured in O2-saturated 0.1 M HClO4 with the

J. Maruyama et al. / Carbon 42 (2004) 3115–3121

1 1 1  ¼ þ I I K 0:620nFAD2=3 cm1=6 x1=2

ð4Þ

where n is the number of electrons involved in O2 reduction per molecule, F is the Faraday constant, A is the geometric area of the GC electrode, D is the diffusion coefficient of O2 in solution, c is the concentration of O2 in solution, m is the kinematic viscosity of the solution, and x is the angular frequency of the rotation. Fig. 5 shows 1/I vs. x1/2 plots for O2 reduction at 0.2 V and n that were calculated using the slope of the plot and the following values [29,31,32]: F, 96 485 C mol1; A, 0.0707 cm2; D, 1.9 · 105 cm2 s1; c,

0.00

I /mA

-0.10 Pt/AC-1

-0.20 Pt/AC-3

-0.30 Pt/AC-2

-0.40 0.2

0.4

0.6

Pt/AC-4

0.8

1

1.2

Potential/V vs. RHE Fig. 4. Hydrodynamic voltammograms for O2 reduction at catalyst layer formed from Pt/AC-1 (very thick line), Pt/AC-2 (very thin line), Pt/AC-3 (thin line), and Pt/AC-4 (thick line) supported on GC RDE in O2-saturated 0.1 M HClO4. Scan rate: 10 mV s1. Electrode rotation speed: 2000 rpm.

8 n = 3.6 n = 3.5

—1 —1 /mA

6

n = 3.8

4

n = 3.6

I

electrodes rotated at various speeds. Fig. 4 shows the relationships between the electrode potential and the current at 2000 rpm. The sign of the current due to reduction reactions was taken as negative as shown in Fig. 3. The O2 reduction current shown in Fig. 4 was obtained by subtracting the background current from the measured current. The currents at the catalyst layers formed from Pt/AC-n (n = 2–4) were larger than that at the Pt/AC-1 layer; an increase in the specific surface area and mean pore diameter enhanced O2 reduction. On close inspection, the behaviors of the Pt/AC-2 layer and the Pt/AC-3 layer were different; at the former, the current increased across all potential regions, whereas at the latter, the increase was observed only below 0.7 V and, at below 0.6 V, was greater than that at the former one. The current shown in Fig. 4 arose from O2 reduction inside the catalyst layer, but included the influence of mass-transfer in 0.1 M HClO4 solution in which the catalyst layer was immersed. The activities of the catalyst layers for O2 reduction were evaluated using the reduction current free of the influence of mass-transfer in the solution, IK, determined by the equation shown below [16,17,23,30].

3119

2

0

0

0.04

0.08 0.12 ω—1/2/rad—1/2 s1/2

0.16

Fig. 5. 1/I vs. x1/2 plots for O2 reduction at catalyst layers formed from Pt/AC-1 (s), Pt/AC-2 (n), Pt/AC-3 (), and Pt/AC-4 (h) supported on GC RDE in 0.1 M HClO4 under O2 atmosphere. Electrode potential: 0.2 V.

1.18 · 106 mol cm3; and m, 9.87 · 103 cm2 s1. The values of n were almost the same for all the catalyst layers and nearly four, indicating that O2 was reduced to H2O (Eq. (2)) free of H2O2, which is generated through a 2-electron reduction (Eq. (5)), which damages the polymer electrolyte and lowers the reduction current [16,33,34], and that n was independent of the pore structure. O2 + 2Hþ + 2e !H2 O2

ð5Þ

3.4. Influence of pore structure on activity for oxygen reduction at Pt/AC The relationships between the electrode potential, E, and IK for O2 reduction at catalyst layers formed from Pt/AC-n are shown in Fig. 6. Enhancement of the specific surface area and mean pore diameter also increased IK. In order to examine in detail the influence of the pore structure of the activated carbon on the activity, the ratios of IK at the Pt/AC-n (n = 2–4) layer to that at the Pt/AC-1 layer were calculated as enhancement factors; Pt/AC-1 was used as the standard due to its smallest specific surface area and mean pore diameter. Table 2 lists IK and the enhancement factors in the low and high potential regions. The influence of the pore structure in relation to the potential region was different depending on the specific surface area and mean pore diameter. At the Pt/AC-2 layer, the enhancement factors were greater in the high potential region than in the low potential region, but the behavior was opposite at the Pt/AC-3 and Pt/AC-4 layers. The enhancement in the high potential region is mainly attributable to the increase in the activity of the Pt particle for O2 reduction [16] and that in the low potential region, to enhancement of the

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1.2

Potential/V vs. RHE

1 0.8 0.6 0.4 0.2 0 -6

-5

-4 log(—I K /A)

-3

-2

Fig. 6. Tafel plots of IK for O2 reduction at catalyst layers formed from Pt/AC-1 (s), Pt/AC-2 (n), Pt/AC-3 (), and Pt/AC-4 (h) supported on GC RDE in O2-saturated 0.1 M HClO4.

Table 2 IK and enhancement factors (E.F., shown in parentheses) in low and high potential regions E/V vs. RHE

Activated carbon AC-1

0.20 0.25 0.30 0.85 0.90 0.95

IK/mA (E.F.) IK/mA (E.F.) IK/mA (E.F.) IK/lA (E.F.) IK/lA (E.F.) IK/lA (E.F.)

0.87 0.88 0.89 150.5 55.1 12.6

AC-2

AC-3

AC-4

1.08 (1.23) 1.11 (1.26) 1.13 (1.26)

1.03 (1.17) 1.04 (1.18) 1.05 (1.17)

3.25 (3.72) 2.79 (3.17) 2.90 (3.24)

204.7 (1.36) 72.0 (1.31) 17.9 (1.42)

161.1 (1.07) 62.3 (1.13) 16.2 (1.28)

332.7 (2.21) 100.2 (1.82) 19.1 (1.52)

The enhancement factor was defined here as the ratio of IK at Pt/ACn (n = 2–4) layer to that at Pt/AC-1 layer.

mass-transfer in the Pt/AC pores; i.e. O2 diffusion and H+ migration [17]. At the Pt/AC-2 layer, in spite of the lower electrochemically active surface area, the activity of the layer was greater than that of the Pt/AC-1 layer. Enhancement of the specific surface area improved the separation of the Pt particles to avoid interference from neighboring Pt particles with oxygen reduction, leading to the higher activity of each particle. Watanabe et al. proposed that there is a potential reaction space around each Pt particle; dissolved O2 may diffuse to the Pt particle and be reduced [12]. With a decrease in the distance between the Pt particles, the space starts overlapping at some critical distance, then the overlapping increases, implying a decrease in the total potential reaction space. This phenomenon could be regarded as interference from the neighboring Pt particles. It was also attributable to the Pt surface area that might have been increased

due to the higher surface area of the activated carbon, although it could not confirmed in the present study by TEM observation due to the wide range of the Pt particle size distribution. Contrary to AC-2 that possessed a higher surface area than AC-1 and nearly the same mean pore diameter, AC-3 possessed nearly the same surface area and higher mean pore diameter, leading to improvement of the mass-transfer in the pores. Further enhancement of the mean pore diameter significantly improved the activity, as exhibited at the Pt/AC-4 layer. The improvement in the mass-transfer also enhanced oxygen reduction in the high potential region since oxygen reduction was partly controlled by the mass-transfer even in this region [35]. The higher electrochemically active surface area was another reason for the enhancement in the high potential region. Given the small effect of the higher specific surface area of the activated carbon, the enhancement observed at the Pt/AC-4 layer was, therefore, mostly attributable to the effect of the greater pore size. It should be noticed again that the mean pore diameter was calculated by assuming the pores to be cylindrical and consideration of the pore shape is not included. However, the effect of the surface area expansion would be independent of the shape. To the effect on the masstransfer, the cross section of the pore opening would be important, unless the distance between the nearest pore walls is smaller than the molecular size of O2, 0.28 · 0.39 nm [36]. It should also be noticed that there could be other factors that affect the activity, such as the surface properties of the activated carbon. Nevertheless, the results obtained in the present study were consistently explained by the effect of the pore structure.

4. Conclusions Activated carbon loaded with dispersed Pt was used as a catalyst for the electrode reactions in a polymer electrolyte fuel cell in the form of a layer that consisted of the catalyst, a polymer electrolyte, and an electronconductive agent. The activity of the layer for cathodic oxygen reduction was measured by varying the pore structure of the activated carbon characterized by its specific surface area and mean pore diameter in order to optimize the pore structure for the activity improvement. A higher specific surface area and mean pore diameter increased the activity, despite the possibility that the loss of connection between the activated carbon and the electron-conductive agent in the catalyst layer might be caused by the enhanced development of pores. The effect of the increased pore size was greater than that of the surface area expansion, which was mainly caused by increased O2 and H+ transfer in the pores and also by the increased electrochemically active surface due to easier penetration of the polymer molecules

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into the pores. We could, therefore, conclude that activated carbon that possesses wide pores free of excessive pore development is appropriate for the catalyst in PEFC electrodes.

References [1] Gottesfeld S, Zawodzinski TA. Polymer electrolyte fuel cells. In: Alkire RC, Gerischer H, Kolb DM, Tobias CW, editors. Advances in electrochemical science and engineering, vol. 5. Weinheim: Wiley-VCH; 1997. p. 195–301. [2] Dhar HP. On solid polymer fuel cells. J Electroanal Chem 1993;357(1–2):237–50. [3] Appleby AJ, Foulkes FR. Fuel cell handbook. New York: Van Nostrand Reinhold; 1989. [4] Costamagna P, Srinivasan S. Quantum jumps in the PEMFC science and technology from 1960s to the year 2000. Part 1. Fundamental scientific aspects. J Power Sources 2001;102(1–2): 242–52. [5] Feltham AM, Spiro M. Platinized platinum electrodes. Chem Rev 1971;71(2):177–93. [6] Bett J, Lundquist J, Washington E, Stonehart P. Platinum crystallite size consideration for electrocatalytic oxygen reduction––I. Electrochim Acta 1973;18(5):343–8. [7] Watanabe M, Uchida M, Motoo S. Preparation of highly dispersed Pt + Ru alloy clusters and the activity for the electrooxidation of methanol. J Electroanal Chem 1987;229(1–2): 395–406. [8] Kinoshita K. Electrochemical oxygen technology. New York: John Wiley & Sons; 1992. pp. 168–176. [9] Wilson MS, Gottesfeld S. Thin-film catalyst layers for polymer electrolyte fuel cell electrodes. J Appl Electrochem 1992;22(1):1–7. [10] Wilson MS, Gottesfeld S. High performance catalyzed membranes of ultra-low platinum loadings for polymer electrolyte fuel cells. J Electrochem Soc 1992;139(2):L28–30. [11] Bernardi DM, Verbrugge MW. A mathematical model of the solid-polymer-electrolyte fuel cell. J Electrochem Soc 1992;139(9):2477–91. [12] Watanabe M, Sei H, Stonehart P. The influence of platinum crystallite size on the electroreduction of oxygen. J Electroanal Chem 1989;261(2):375–87. [13] Iwasaki S, Fukuhara T, Abe I, Yanagi J, Mouri M, Iwashima Y, et al. Adsorption of alkylphenols onto microporous carbon prepared from coconut shell. Synth Met 2002;125(2):207–11. [14] Iwasaki S, Fukuhara T, Yoshimura Y, Sakaguchi R, Shibutani Y, Abe I. Removal of endocrine-disrupting chemicals in water using textile-related wastes. I. Removal of 4-nonylphenol by microporous carbons prepared from cotton waste. SenI Gakkaishi 2001;57(12):359–63. [15] Abe I, Fukuhara T, Maruyama J, Tatsumoto H, Iwasaki S. Preparation of carbonaceous adsorbents for removal of chloroform form drinking water. Carbon 2001;39(7):1069–73. [16] Maruyama J, Abe I. Application of conventional activated carbon loaded with dispersed Pt to PEFC catalyst layer. Electrochim Acta 2003;48(10):1443–50. [17] Maruyama J, Abe I. Effective utilization of nano-spaces in activated carbon for enhancing catalytic activity in fuel cell electrodes. J Electrochem Soc 2004;151(3):A447–51.

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[18] Gu¨lzow E, Schulze M, Wagner N, Kaz T, Reissner R, Steinhilber G, et al. Dry layer preparation and characterisation of polymer electrolyte fuel cell components. J Power Sources 2000;86(1–2): 352–62. [19] Jaouen F, Lindbergh G, Sundholm G. Investigation of masstransport limitations in the solid polymer fuel cell cathode. I. Mathematical model. J Electrochem Soc 2002;149(4):A437–47. [20] Li G, Pickup PG. Ionic conductivity of PEMFC electrodes. Effect of Nafion loading. J Electrochem Soc 2003;150(11):C745–52. [21] Guo Q, White RE. A steady-state impedance model for a PEMFC cathode. J Electrochem Soc 2004;151(4):E133–49. [22] Brown HC, Brown CA. Hydrogenation of nitroaromatics in the presence of the new platinum metal and carbon-supported platinum metal catalysts. J Am Chem Soc 1962;84(14):2828. [23] Gojkovic´ SL, Zecˇevic´ SK, Savinell RF. O2 reduction on an inktype rotating disk electrode using Pt supported on high-area carbons. J Electrochem Soc 1998;145(11):3713–20. [24] Maruyama J, Abe I. Influence of anodic oxidation of glassy carbon surface on voltammetric behavior of Nafion-coated glassy carbon electrodes. Electrochim Acta 2001;46(22):3381–6. [25] Chu D, Tryk D, Gervasio D, Yeager EB. Examination of the ionomer/electrode interface using the ferric/ferrous redox couple. J Electroanal Chem 1989;272(1–2):277–84. [26] Razaq M, Razaq A, Yeager E, DesMarteau DD, Singh S. Perfluorosulfonimide as an additive in phosphoric acid fuel cell. J Electrochem Soc 1989;136(2):385–90. [27] Maruyama J, Inaba M, Katakura K, Ogumi Z, Takehara Z. Influence of Nafion film on the kinetics of anodic hydrogen oxidation. J Electroanal Chem 1998;447(1–2):201–9. [28] Yoshitake H, Yamazaki O, Ota K. In situ X-ray absorption fine structure study on structure transformation and electronic state of various Pt particles on carbon electrode. J Electrochem Soc 1994;141(9):2516–21. [29] Maruyama J, Abe I. Cathodic oxygen reduction at the catalyst layer formed from Pt/carbon with adsorbed water. J Electroanal Chem 2003;545(1):109–15. [30] Paulus UA, Schmidt TJ, Gasteiger HA, Behm RJ. Oxygen reduction on a high-surface area Pt/Vulcan carbon catalyst: thinfilm rotating ring-disk electrode study. J Electroanal Chem 2001;495(2):134–45. [31] Zecˇevic´ SK, Wainright JS, Litt MH, Gojkovic´ SL, Savinell RF. Kinetics of O2 reduction on a Pt electrode covered with a thin film of solid polymer electrolyte. J Electrochem Soc 1997; 144(9):2973–82. [32] Mello RMQ, Ticianelli EA. Kinetic study of the hydrogen oxidation reaction on platinum and Nafion covered platinum electrodes. Electrochim Acta 1997;42(6):1031–9. [33] Tarasevich MR, Sadkowski A, Yeager E. Oxygen electrochemistry. In: Bockris JO, Conway BE, Yeager E, Khan SUM, White RE, editors. Comprehensive treatise of electrochemistry, vol. 7. New York: Plenum; 1983. [chapter 6]. [34] Maruyama J, Inaba M, Morita T, Ogumi Z. Effects of the molecular structure of fluorinated additives on the kinetics of cathodic oxygen reduction. J Electroanal Chem 2001;504(2): 208–16. [35] Perry ML, Newman J, Cairns EJ. Mass transport in gas-diffusion electrodes: a diagnostic tool for fuel-cell cathodes. J Electrochem Soc 1998;145(1):5–15. [36] Tamaru T. Gas separation by PSA method. In: Ikuo A, Hideki T, editors. Activated carbon application technology. Tokyo: Technosystem; 2000. p. 217–21.