A Study of the Catalytic Interface for O2 Electroreduction on Pt: The

Dec 11, 2008 - ... Engineering, The UniVersity of British Columbia, 2360 East Mall, ... V6T 1Z3, Canada, and Ballard Power Systems, 9000 Glenlyon Park...
0 downloads 0 Views 575KB Size
Download PDF
298

J. Phys. Chem. C 2009, 113, 298–307

A Study of the Catalytic Interface for O2 Electroreduction on Pt: The Interaction between Carbon Support Meso/Microstructure and Ionomer (Nafion) Distribution Anna Ignaszak,*,† Siyu Ye,‡ and Elo˜d Gyenge*,† Department of Chemical and Biological Engineering, The UniVersity of British Columbia, 2360 East Mall, VancouVer, Bristish Columbia, V6T 1Z3, Canada, and Ballard Power Systems, 9000 Glenlyon Parkway, Burnaby, Bristish Columbia, V5J 5J8, Canada ReceiVed: July 9, 2008; ReVised Manuscript ReceiVed: October 17, 2008

The catalytic activity of PEM fuel cell electrodes is determined by the complex physicochemical interactions among the components of the electrocatalytic interface: precious metal catalyst, support, and ionomer. In the present study, the effect of the carbon support meso- and microporosity was investigated in relation with both the Nafion content and distribution in the catalyst layer. The ionomer load was between 0.09 and 1.1 mg cm-2 in the catalyst layers prepared either by the Nafion-coated or Nafion-mixed techniques, while the Pt load was kept constant at 0.1 mg cm-2. Three supports were investigated: Vulcan XC-72R, Denka, and graphitized carbon (GC). Employing both the BET (Brunauer-Emmett-Teller) and the BJH (Barrett-JoynerHalenda) surface measurement techniques, a complete characterization of the support and supported catalyst (Pt/C) pore volume distribution and surface area in the micro- and mesopore size ranges was carried out. It was found that Pt nanoparticles (mean diameter between 4.1-4.9 nm by XRD) reduced the micropore volume of the carbon supports. Therefore, the supports with high BJH mesoporous area (Vulcan XC-72R and GC) yielded Pt/C catalysts with the highest electrochemically active Pt area as well. For the Denka support, characterized by the lowest BJH area, the Nafion-coated procedure gave about 7% larger electrochemically active area compared to the Nafion-mixed method. Regarding the oxygen electroreduction, the effective oxygen permeability in the catalyst layer, the intrinsic kinetic current density, and the area and mass-specific activities at 0.9 VRHE were determined as a function of support type, Nafion incorporation method, and load. Introduction The sluggish oxygen reduction reaction (ORR) kinetics continues to be a major challenge in the effort to develop commercially feasible PEM fuel cells. Some of the major research goals in this area are (i) reduction of the Pt load to (or below) 0.1 mg cm-2 by enhancing the intrinsic kinetic activity, (ii) improvement of the catalyst layer durability, and (iii) minimization of the catalyst surface area changes during extended periods of fuel cell operation. To accomplish these objectives a thorough fundamental understanding of the catalytic interface is required with an emphasis on the synergistic aspects involving the intrinsic activity of the Pt surface, electrochemically active area, support physicochemical and morphological properties, effective interfacial oxygen permeability and the distribution and spreading of the polymer electrolyte (e.g., Nafion). Regarding carbon-supported Pt catalysts, numerous studies have been reported on the kinetics of the oxygen reduction reaction (ORR) in acid media without the presence of the polymer-electrolyte.1-9 However, in operating fuel cells the polymer electrolyte provides the interfacial environment for oxygen reduction, it affects the surface concentration of O2 at the active sites and serves as the ion-conducting network for hydronium ions across the catalyst layer.10-15 Recently, much interest has been also devoted to the role of the electrocatalyst support.16-22 However, due mainly to cost * To whom correspondence should be addressed. E-mail: aignaszak@ chml.ubc.ca (A.I.); [email protected] (E.G.). † The University of British Columbia. ‡ Ballard Power Systems.

effectiveness, ubiquity, high surface area, and acceptable electronic conductivity, various carbon blacks remain the preferred support for PEM fuel cell electrodes. The activity of the carbon black supported catalyst depends not only on the primary macropore structure (diameter > 50 nm) formed by the aggregation and agglomeration of carbon grains, but also on the secondary meso- and micropore structure (diameter e 50 nm). Furthermore, the porous carbon structure plays an important role when the polymer electrolyte is incorporated as the conductive network for protons.23-26 Another important aspect of the carbon support pore size distribution relates to the capillary effects and the hydrophobic-hydrophilic balance. The micropores of the carbon support could be preferentially flooded due to capillary condensation as a function of the water-carbon contact angle. Therefore, micropores could constitute the channels required for liquid water transport away from the catalytically active sites. Furthermore, treatment with PTFE is commonly applied, in somewhat empirical fashion at present, to control the hydrophobic-hydrophilic balance of the porous structures in PEM fuel cells.27-29 Thick-film Pt/C electrodes made by deposition of a catalystNafion mixture onto a glassy carbon electrode showed that the film diffusion resistance plays an important role and extensive mathematical modeling was necessary to extract kinetic parameters from rotating disk electrode (RDE) data.30-32 Watanabe et al. applied the RDE method for studies of oxygen transport properties through the Nafion electrolyte.10 The critical thickness of the Nafion film covering the Pt electrode below which the O2 diffusion process was not the rate-determining step, was found to be 0.2 µm. This critical thickness was also corroborated by Schmidt et al. who developed a method for thin-film electrode

10.1021/jp8060398 CCC: $40.75  2009 American Chemical Society Published on Web 12/11/2008

Catalytic Interface for O2 Electroreduction on Pt preparation where a small amount of catalyst suspension is attached to the RDE disk and it is subsequently covered by a Nafion layer.31 Generally, the macrohomogeneous porous electrode model was employed in order to obtain the O2 permeability in the Nafion film (defined as the product of film oxygen concentration and diffusion coefficient).33-36 The aim of this work was to carry out a systematic investigation on the ORR activity with respect to the support meso/microstructure in conjunction with the Nafion loading and its application procedure to form the catalyst layer. Three different carbon blacks (Vulcan XC-72R, Denka, and graphitized carbon) were compared as supports for the Pt nanoparticles. Electrodes with a wide range of Nafion loading (0.09-1.1 mg cm-2) and two different preparation procedures (coated and mixed) were investigated. The employed characterization techniques included TEM, XRD, N2 adsorption by BET (BrunauerEmett-Teller method) and BJH (Barrett-Joyner-Halenda method), and RDE experiments. This is the first study the authors are aware of where a comprehensive correlation of the support micro- and mesopore size distribution (the latter obtained by BJH technique) is carried out in relation to its effect on the Nafion film distribution and electrocatalytic activity of the ORR. Experimental Section Materials. Pt/Vulcan XC-72R (HiSPEC4000 from Johnson Matthey, U.K.), Pt/Denka (Johnson Matthey, U.K.), and Pt/ graphitized carbon (GC; Tanaka Kikinzoku Kogyo K.K., Japan) catalyst were investigated for their crystallographic, morphological, and electrocatalytic properties. The metal content on the support was 40 wt % for Pt/Vulcan XC-72R and Pt/Denka and 50 wt % for Pt/GC. The three catalysts were prepared by their respective proprietary manufacturing methods. A typical preparation method patented by Johnson Matthey involves chemical reduction of the precursor Pt compound in an aqueous slurry containing the carbon support.37a The structures of the carbon black supports, Vulcan XC-72R (from Cabot Corporation, U.S.), Denka black (from Denka Kagaku Kogyo K.K., Japan), and graphitized carbon (from Polysciences, Inc. US), were separately studied. Ultrapure water (18 MΩ, Cole-Parmer Instrument Company, U.S.) was used to prepare all solutions and throughout the preparatory work. The following chemical stock solutions were used: sulfuric acid 95-98% reagent A.C.S grade (Fisher Scientific), glacial acetic acid (Fisher Scientific), isopropanol (Fisher Scientific), acetone (CHROMASOLV, Sigma-Aldrich), Nafion (5% solution in mixture of lower aliphatic alcohols and water, Sigma-Aldrich). High purity N2 and O2 (Praxair, Canada) gases were employed. Electrode Preparation. Prior to preparing the Pt/C electrodes, the glassy carbon disk substrate (PINE Instrument Company, U.S.A.) was mechanically polished with 0.05 µm Al2O3 slurry (Cypress Systems, Inc., U.S.A.), then rinsed in deionized water, cleaned ultrasonically in ultrapure water for 5 min, followed by rinsing in ultrapure isopropanol and acetone. To investigate the Nafion load and dispersion effect, two types of electrodes were prepared. I. Nafion-Coated Electrode. A total of 0.02 g of the supported catalyst powder was mixed with 2 mL of glacial acetic acid and placed in an ultrasonic bath for at least 15 min. From the catalyst ink suspension, 5 µL was carefully placed onto a glassy carbon disk (area 0.1963 cm2) and allowed to dry in air at room temperature, giving a catalyst loading of 0.1 mg cm-2 (for Pt/Vulcan XC-72R and Pt/Denka) and 0.125 mg cm-2 (for Pt/GC). To coat the supported catalyst with ionomer, a Nafion solution (5% wt) dissolved in ultrapure water was pipetted onto

J. Phys. Chem. C, Vol. 113, No. 1, 2009 299 the dry electrode to obtain Nafion loads in the range of 0.09-1.11 mg cm-2. The coated electrodes were dried in air at room temperature under cover. The chosen Nafion loads were sufficient to attach the carbon particles to the glassy carbon RDE. The dry thickness of the polymer electrolyte was calculated assuming a Nafion density of 1.98 g cm-3. II. Nafion-Mixed Electrode. A total of 0.02 g of the supported catalyst powder and 2 mL of acetic acid-Nafion mixture was placed in the ultrasonic bath for at least 15 min. The Nafion solution volume was chosen such that the same Nafion loading range was obtained as with method I, between 0.09 and 1.11 mg cm-2. The catalyst loading was the same 0.1 mg cm-2 (for Pt/Vulcan XC-72R and Pt/Denka) and 0.125 mg cm-2 (for Pt/GC). An aliquot of 5 µL of the suspension was carefully placed onto a glassy carbon disk and allowed to dry in air at room temperature. The dry thickness of the polymer electrolyte mixed into the supported catalyst, was calculated according to the following formula:

σmix )

VNafion VNafion ) Aeff (1 - γ) · ABET · 104 · mcat

(1)

where σmix (cm) is the Nafion thickness in the Naxion-mixed catalyst film, VNafion is the Nafion solution volume (cm3), Aeff is the effective area covered by Nafion (cm2), γ is the ratio of micropore volume to the total pore volume, ABET is the total area of the supported catalyst estimated by BET method (m2 g-1), and mcat is the catalyst mass (g). Note in eq 1 it is assumed that the micropores of the carbon support are inaccessible to Nafion micelles due to size exclusion. Electrochemical Characterization. A three-electrode electrochemical cell, thermostatted at 22 °C, was employed for all electrochemical measurements. A platinum mesh and mercury/ mercurous sulfate electrode (Hg/Hg2SO4, K2SO4 std.) were used as counter and reference electrodes, respectively. All potentials in this work are referred to a reversible hydrogen electrode (RHE). The potentiostat model 1100A Series Electrochemical Analyzer (CH Instruments, U.S.A.) was used for all electrochemical experiments. Pt ElectroactiWe Area. The hydrogen underpotential deposition method using N2 purged 0.5 M H2SO4 was used. Cyclic voltammetry was performed in the potential range of between 0 and 1.2 VRHE with a scan rate of 0.05 V sec-1 and sensitivity of 2 × 10-3 A V-1. Prior to the electrochemical area measurement, the 0.5 M H2SO4 solution was N2 purged for 1 h. The Pt/C electrode was conditioned by cycling between 0 and 1.2 V with scan rates of 0.05 and 0.1 V sec-1 until reproducible voltammograms were obtained. The Pt electroactive area was calculated as the average of both hydrogen adsorption and desorption regions after subtraction of the double layer capacitance. Catalytic ActiWity toward O2 Reduction. Rotating disk electrode voltammograms were recorded in O2 saturated 0.5 M H2SO4 solution by cathodic sweeping between 1.0 VRHE to 0.5 VRHE at a scan rate of 0.005 V sec-1 and sensitivity of 2 × 10-4 A V-1. During the measurements, the rotation rate (400-3000 RPM) was controlled by the electrode rotator model AFM SRX (PINE Instrument Company, U.S.A.). Prior to the experiments, the freshly prepared electrode was conditioned by cycling in the potential range of 0.4-1 V until reproducible voltammograms were obtained. X-ray Diffraction (XRD). A D8 Advance Bruker diffractometer with Cu KR1, radiation (1.54058 Å) was used to identify the crystallographic structure and phase composition of the catalysts. Measurements were performed at room temperature

300 J. Phys. Chem. C, Vol. 113, No. 1, 2009

Ignaszak et al.

TABLE 1: Pt Particle Characteristics as a Function of Support Pt /Vulcan XC-72R XRD TEM

Pt /Denka

Pt /GC

mean Pt particle dimension d (nm) 4.93 4.77 3.41 3.66

4.09 3.40

mean intercrystallite distance xi (nm) 4.14 2.46

1.96

in the angle range of 10-85° with a step of 0.04° and a step time of 1.5 s. The Pt particle size was calculated using the Scherrer equation applied to the peak width at half-maximum for Pt 111, Pt 200, and Pt 220 peaks after being normalized to the measured intensity of the Pt 111 peak. Morphology Observation. Transmission electron microscopy (TEM) micrographs of catalyst at 600000 times magnification were obtained using Hitachi H7600 microscope with an accelerating voltage of 120 kV. Surface Area and Pore Size Distribution. The surface area, pore size, and pore volume were determined with an ASAP 2020 analyzer (Micrometrics Instrument, U.S.A.). The samples were weighted and dried at 110 °C for 3 h under vacuum before they were secured to the analysis port. Adsorption-desorption isotherms were recorded at 77 K using N2 as the adsorbate. The adsorption data was analyzed using the ASAP 2020 software based on the BET isotherm and the BJH method. The BET analysis gives the surface area of the sample based on N2 monolayer physiosorption. The BJH method takes into account the capillary condensation using the Kelvin equation and it is useful for the determination of the pore size distribution generally for mesopores. Note in this work according to IUPAC nomenclature the micropores are defined as d < 2 nm; mesopores d ) 2-50 nm and macropores d > 50 nm. From the pore size distribution, a cumulative pore volume can be calculated. The employed technique for adsorption data analysis was valid exclusively for the micro- and mesopore range. Results and Discussion XRD and TEM Characterization of Pt/Vulcan XC-72R; Pt/Denka, and Pt/GC. All three catalysts showed characteristic diffraction peaks for the face-centered cubic Pt (space group: Fm3m) with crystal faces [111], [200], [220], and [311]. The average particle sizes calculated by the Scherrer equation20 were between 4.1 and 4.9 nm, which were slightly higher than the values obtained from TEM image processing (Table 1). A similar overestimation of Pt particle size from XRD measurement compared to TEM was found by Wikander et al. for Pt deposited on Vulcan XC-72R.37b The particle sizes reported in Table 1 agreed with the information supplied by the respective catalyst manufacturers. Figure 1 presents the TEM images for the three supported catalysts under study, together with the particle size distribution histograms. The Pt particle size distribution was estimated by measuring the size of sixty-five randomly chosen particles in the magnified TEM images. The narrowest particle size distribution was obtained for the Denka supported catalyst. Pt/Vulcan XC-72R and Pt/GC on the other hand, had a similar particle size distribution range between 2.25 and 4.7 nm. Furthermore, the type of carbon support had an effect on the Pt dispersion. A uniform and high degree of Pt dispersion was obtained on Vulcan XC-72R, while agglomerated Pt particles were observed on both Denka and graphitized carbon (GC). Interestingly, for the latter part of the Pt agglomeration was tubular-like (Figure

1). The intercrystallite distances38,39 calculated with eq 2 and reported in Table 1, support the visually observed agglomeration tendencies.

xi )

1 1-y · 30.5 · π · F · d3 · ABET,C · 3 y

(

0.5

)

(2)

where xi is the mean intercrystallite distance, F and d are the density and particle size of the metal, ABET,C is the surface area of carbon (by BET method), and y is the metal content of Pt/C. Carbon Support Meso- and Microstructure: Without and with Pt Nanoparticle Deposition. To understand the interaction of the support meso- and microsructure with the Pt nanoparticles, Figure 2a-d show the histograms of the support pore size distribution without Pt catalyst deposition. The micropore distribution curves (Figure 2d) were obtained from the analysis of the adsorption isotherm by the Dubinin-Astakhov method using the software Autosorb 1. Denka carbon exhibited the most uniform pore size distribution range, in contrast to graphitized carbon (exponential-like) and Vulcan XC-72R (bimodal; Figure 2a-c). Furthermore, Vulcan XC-72R had the highest fraction of micropores (with diameter 50 nm) formed primarily between the carbon particle agglomerates was not investigated in the present work. Table 2 summarizes the morphological differences among the three carbon supports with and without catalyst deposited. In the case of Vulcan XC-72R the commonly used BET method gave surface area values larger than the BJH approach (223.2 m2 g-1 vs 134 m2 g-1), which is expected since BET refers to the total surface area (including micropores), while BJH is typically valid for mesopores.40 Furthermore, the pore geometry (e.g., cylindrical, hexagonal) and tortuosity can affect the area measured by the BJH method via the statistical film thickness and meniscus correction for the Kelvin equation.41 Interestingly, in the case of both Denka and GC supports, the BET and BJH areas were approximately equal (64 and 75 m2 g-1, respectively; Table 2). When Pt nanoparticles were deposited on the three supports under investigation (Pt/C), the BET surface areas were smaller compared to the original values of the respective supports (Table 2). Moreover, the micropore volume was also reduced by the presence of the catalyst (Table 2). These measurements indicate unequivocally that Pt nanoparticles block some of the micropores in the support and the extra area due to N2 physiosorption on Pt in addition to carbon in the BET method does not make up the difference. This effect is especially pronounced in the case of Pt/Vulcan XC-72R since Vulcan XC-72R contains the highest fraction of micropores among the investigated carbon black supports (Table 2). Moreover, the average measured pore width increased when Pt is present, constituting yet another indication that the micropores are obstructed by catalyst nanoparticles and only the larger mesopores are open, which is also corroborated by the BJH area. For example, for the Pt/ Vulcan XC-72R and Pt/Denka catalysts, the BJH areas are only slightly lower than for the support itself since the mesopores are less obstructed by the catalyst particles. The case of the BJH area for Pt/GC on the other hand is somewhat surprising (Table 2). In this case, an increase of BJH area was observed compared to the pure support. The BJH area of Pt/GC can be explained by the large degree of agglomeration of Pt on GC forming tubular-like aggregates (Figure 1 TEM). These aggregates change considerably the pore geometry and tortuosity factors, hence, they modify the BJH area. Generally, the entire pore structure of the catalyst layer in a PEM fuel cell is also a strong function of the catalyst layer

Catalytic Interface for O2 Electroreduction on Pt

J. Phys. Chem. C, Vol. 113, No. 1, 2009 301

Figure 1. TEM images of Pt/C catalysts with histograms of the Pt size distribution: Pt/Vulcan XC-72R (a), Pt/Denka (b), and Pt/graphitized carbon (c).

manufacturing method and the conditions applied to produce the membrane-electrode assembly (e.g., compression and temperature). Therefore, performing ex-situ experiments on the supports and supported catalyst (i.e., not under fuel cell assembly and operating conditions) while it provides valuable insights,

one must take into account that the pore structure in operating fuel cells can be different. Electrochemical Pt Area. The electroactive surface area of Pt/C catalysts was estimated from the integrated charge (QH) in the hydrogen underpotential adsorption/desorption region of

302 J. Phys. Chem. C, Vol. 113, No. 1, 2009

Ignaszak et al. TABLE 2: Morphological Characteristics of the Carbon Black Supports and the Supported Pt/C Vulcan XC-72R 2

Denka

GC

-1

surface area (m g ) BET surface area 223.2 BJH cumulative surface area 134.0 pore volume (cm3 g-1) micropore volume 0.136 BJH cumulative pore volume 1.345 pore size (nm) average pore width 5.0

64.0 64.7 0.046 0.366 7.5

76.0 74.5 0.055 1.040 10.0

Pt/Vulcan XC-72R Pt/Denka Pt/GC surface area (m2g -1) BET surface area 146.1 BJH cumulative surface area 94.0 pore volume (cm3 g-1) micropore volume 0.022 BJH cumulative pore volume 0.316 pore size (nm) average pore width 6.0

Figure 2. Pore diameter and pore volume distribution for carbons (a-c): Vulcan XC-72R (a), Denka (b), graphitized carbon (c), and micropore distribution (d) obtained by deconvolution of the data from N2 adsorption at 77 K.

the cyclic voltammogram in the potential range of 0.05 to 0.4 V using the Pt loading (mcat) and assuming an anodic stripping charge of 0.21 mC cm-2 (a value generally accepted for the polycrystalline Pt electrode 42). The double layer charging current was subtracted from the total hydrogen desorption current

EAPt )

QH 0.21 · 10-3 · mcat

56.7 56.5

92.1 90.6

0.003 0.183

0.005 0.403

8.2

9.9

thermal treatment, humidity, support physicochemical properties, and possibly even the electric field could affect the aggregation of the Nafion macromolecules. In water-alcohol precursor solutions (1:1 vol. ratio of Nafion 5%-water) the size of micelles is 200 nm on average.11a However, in the Pt/C catalyst layer Uchida et al., employing electron tomography, observed Nafion agglomerates of 10-20 nm diameter,11b while Hiesgen et al. using in situ STM reported Nafion spherical globules (ionic cluster model) of about 6-9 nm diameter on graphite.11c Therefore, it is reasonable to assume that the microporosity of the support is not useful in the creation of an effective catalytic interface. There were interesting differences in the electrochemically active area as a function of support, ionomer distribution and loading (Figure 3). The catalyst layer preparation procedure (i.e., Nafion-coated or mixed) was least significant in the case of Vulcan XC-72R, both methods yielding electrodes with very similar electroactive area for all the investigated loadings. On the other hand, in the case of both graphitized carbon and Denka supports, whether the Nafion was mixed into the catalyst ink

(3)

Figure 3 shows the EAPt values of Pt/C electrodes prepared by Nafion-coated and Nafion-mixed methods with different ionomer loading. Regardless of the Nafion application procedure, the highest EAPt was obtained for Pt supported on Vulcan XC72R, with the maximum at 57 m2 g-1, which is close to the value reported by Paulus et al. (65 m2 g-1 for 20% Pt/Vulcan XC-72R electrode coated by Nafion film).7 The Pt/Denka catalyst had the lowest electroactive area (34-37 m2 g-1), while the Pt/GC electrochemical area was situated between those on Vulcan XC-72R and Denka. These trends correlate well with the support morphology characteristics given by Table 2, since Pt/Denka had the lowest BJH area as well as the lowest cumulative pore volume. In the catalyst layer, from the point of view of Nafion penetration and distribution, the macro- and mesopores (reflected by the BJH area) are the most important since the Nafion micelles are inaccessible to the micropores. The size of the Nafion micelles in the catalyst layer is not known precisely at this point in the literature and it depends on a number of factors such as: Nafion concentration, temperature, catalyst layer

Figure 3. Electrochemically active specific surface area of Pt/C catalysts as a function of Nafion loading and incorporation procedure. Legend: Pt/Vulcan XC-72R Nafion-coated (b) and Nafion-mixed (O) electrodes; Pt/Denka Nafion-coated (f) and Nafion-mixed (g) electrodes; Pt/graphitized carbon Nafion-coated (2) and Nafion-mixed (4) electrodes. Electrolyte: N2-purged 0.5 M H2SO4; 22 °C.

Catalytic Interface for O2 Electroreduction on Pt or applied as a coating had an effect on the measured electroactive area (Figure 3). For Pt/Denka the active Pt area estimated for Nafion-coated electrodes was about 7% higher than for the Nafion-mixed electrodes for the same ionomer content. This suggests that the spreading and adhesion of the Nafion in the Denka carbon might not be uniform, and nonuniform ionomer agglomeration can occur when mixed with Pt/C, blocking catalytically active areas. The same phenomenon was apparent in the case of Pt/GC as well, where for the Nafionmixed electrode a parabolic dependence of the electroactive area on Nafion loading was obtained with the maximum active area at 0.45 mg cm-2 Nafion loading (Figure 3). The spreading of the Nafion film on various carbon supports poses interesting questions that need to be addressed in future studies such as determination of the work of adhesion, interfacial energy, contact angle and spreading coefficient as a function of the carbon surface properties (e.g., surface functional groups and surface roughness). From our electrochemical area measurements (Figure 3) it is obvious that the three investigated supports behaved differently with respect to Nafion especially when the ionomer is mixed into the catalyst ink (the common way of preparing the fuel cell catalyst layers). The support physicochemical properties can affect the micelle-like microstructure of the polymer and the access of the liquid electrolyte and protons to the active metal centers. The liquid electrolyte could penetrate to the metal surface through the polymer film in two different ways. One way is through more or less straight channels of a pinhole-type formed in the Nafion film during preparation. Another way is penetration through the network of interconnected tortuous hydrophilic channels that are part of the morphology of the polymer electrolyte.43 Both transport pathways are dependent on the actual morphology of the polymer film on the support that in turn depends on the polymer/carbon interfacial properties as discussed before. McGovern et al. studied the effect of the Nafion impregnation method on the Pt electroactive surface area for the simpler case of the unsupported catalyst layer.15 It was found that Nafion, whether applied on the top or mixed with catalyst, reduces the electrochemically active surface area. The Pt specific area in Nafion-coated electrodes was almost two times higher compared to the Nafion-mixed electrode. Moreover, depositing Nafion on the top of unsupported Pt black changed the hydrogen peak shape and the peak position compared to the Nafion-free Pt electrode.15 In our studies, we also found that the bulk hydrogen oxidation peak was strongly influenced by the Nafion concentration and incorporation method (Figure 4 exemplifies the case of Pt/GC). The peaks in the hydrogen underpotential deposition region (Figure 4) showed a subtle dependence on Nafion load and incorporation forming the basis of the electrochemically active surface area calculation depicted by Figure 3. Oxygen Electroreduction Studies. RDE voltammetry was employed to obtain intrinsic kinetic and transport parameters for the ORR as a function of support morphology, Nafion impregnation method, and loading (coated and mixed electrodes, respectively). Figure 5 shows selected voltammograms for the ORR on Pt/Vulcan XC-72R, Pt/Denka, and Pt/GC, with a Nafion load of 0.45 mg cm-2. The extraction of intrinsic kinetic parameters for catalyst layers usually requires a combination of experimental data with porous electrode modeling.44-50 The RDE results in the mixedcontrolled domain (e.g., E ) 0.55 VRHE) were analyzed according to Lawson et al.3 (eq 4) in terms of the reciprocal

J. Phys. Chem. C, Vol. 113, No. 1, 2009 303

Figure 4. Cyclic voltammogram of Pt/GC as a function of Nafion load and incorporation method: (a) mixed and (b) coated. Electrolyte: N2-purged 0.5 M H2SO4; 22 °C.

Figure 5. O2 reduction current densities on Pt/C catalyst layer rotating disk electrodes prepared by Nafion-coated (a) and Nafion-mixed (b) procedures in O2-saturated 0.5 M H2SO4, 22 °C, at 1600 rpm; Nafion loading, 0.45 mg cm2.

relation considering the following current densities: (a) im external convective mass transfer limited current density given by the Levich equation (eq 5), (b) if film diffusion current density 32,44 expressed by eq 6, and (c) ik the intrinsic electrode kinetic current density which is a function of the overpotential according to the Tafel equation.

1 1 1 1 1 1 ) + + ) + i ik if im ik ilim

(4)

where i is the total current density,

im ) 0.62nFD2⁄3ν-1⁄6C0ω0 ) BC0ω1⁄2

(5)

where D and C0 are the diffusion coefficient and concentration of O2 in the bulk solution, respectively, ν is the kinematic viscosity of the electrolyte, and ω is the rotation rate in radians;

if ) nFCfDfL-1

(6)

where Cf and Df represent the effective O2 concentration and diffusion coefficient in the Nafion film of the catalyst layer and

304 J. Phys. Chem. C, Vol. 113, No. 1, 2009

Ignaszak et al.

L is the Nafion film thickness. The product Cf × Df is referred to as the effective O2 permeability in the Nafion. To determine the values of if and ik, 1/i was plotted versus 1/ω1/2 at a constant Nafion film thickness (Levich-Koutecky plot). According to eqs 4-6, the slope of these lines corresponds to 1/BC0, and the intercept (at ωf∞) is

Y)

1 L + ik nFCfDf

(7)

Thus, based on eq 7, a plot of Y versus L gives the intercept (Lf0) equal to 1/ik and the slope of (nFCfDf)-1. Figures 6a, 7a, and 8a show the plots of 1/i versus ω-1/2 (at E ) 0.55 VRHE) for the three supported catalysts layers, obtained by both Nafion-coated and Nafion-mixed methods. The 1/i versus ω-1/2 dependence was linear, with an intercept Y that was normalized to the electrochemically active Pt area and replotted against Nafion thickness (L) according to eq 7 (Figures 6b, 7b, and 8b). The term Y increases linearly with increasing Nafion film thickness L (i.e., the current is controlled by O2 diffusion in the film), and its intercept corresponding to Lf0 yields the current purely controlled by electrode kinetics (1/ik). The order of the intrinsic kinetic current densities ik obtained by this method for either Nafion-coated or mixed electrodes were as follows: Pt/Denka (2.1 mA cm-2Pt) > Pt/GC (1.1 mA cm-2Pt) > Pt/Vulcan XC-72R (0.82 mA cm-2Pt). Note the current density values in parenthesis refer to the mixed electrode case. Furthermore, the 1/i versus ω -1/2 dependence (Figures 6a, 7a, and 8a) for the electrodes coated by the ionomer was a strong function of the ionomer thickness (and load), while the ionomermixed electrode response was much less dependent on thickness. This is obviously due to the mass transfer influence imposed by the Nafion film at the Pt surface (eq 6) in the case of the thick-film coated electrode. The effective film thickness for

Figure 7. Koutecky-Levich plots (a) and inverse of current density (Koutecky-Levich intercept) vs Nafion film thickness (b) for Pt/Denka film prepared by Nafion-coated (f) and Nafion-mixed (g) procedures.

Figure 8. Koutecky-Levich plots (a) and inverse of current density (Koutecky-Levich intercept) vs Nafion film thickness (b) for Pt/ graphitized carbon film prepared by Nafion-coated (2) and Nafionmixed (4) procedures. Figure 6. Koutecky-Levich plots (a) and inverse of current density (Koutecky-Levich intercept) vs Nafion film thickness (b) for Pt/Vulcan XC-72R film prepared by Nafion-coated (b) and Nafion-mixed (O) procedures.

Nafion-mixed electrodes (calculated by eq 1) was almost up to 2 orders of magnitude smaller than the film thickness for coated electrodes.

Catalytic Interface for O2 Electroreduction on Pt

J. Phys. Chem. C, Vol. 113, No. 1, 2009 305

TABLE 3: O2 Permeability in Nafion Containing Supported Catalyst Layers Determined by the RDE Technique Using 0.5 M H2SO4 at 22 °Ca O2 permeability CfDf ( × 10-12 mol cm-1 s-1) Nafion-coated Nafion-mixed Nafion-coated Nafion-mixed Nafion-coated Nafion-mixed

Pt/Vulcan XC-72R 2.2 0.8 Pt/Denka 4.5 2.1 Pt/GC 4.1 0.6

bulk value in 0.5 M H2SO4 recast Nafion film on Pt disk bulk Nafion (membrane) film-catalyst electroded

20.34 6.3b 12.4c 3.6-6.4 3

references this work

idem 35, 36 49 35, 36 33, 34

a Comparative literature results are also given. b Assuming no change in dry film thickness following immersion in the electrolyte. c Assuming 65% increase in film thickness following immersion. d According to the macrohomogeneous model.

It is interesting to examine the oxygen transport for the two Nafion incorporation techniques in the catalyst layer. As shown by Table 3, the oxygen permeability (Df × Cf) obtained from eq 7 was consistently higher for the thick-film Nafion-coated electrodes regardless of the support type, which can be attributed to both the higher interfacial O2 concentration in the thicker Nafion film and the less tortuous O2 diffusion pathway in case of the coated versus mixed electrodes. Moreover, the calculated values for the Nafion-coated electrodes matched fairly well the O2 permeability range reported in the literature for either bulk Nafion or recast Nafion film on Pt disk (Table 3). Thus, the permeability data is consistent with the physical behavior of a thick film on top of the catalytically active surface regardless of the support type and morphology. The film oxygen permeability for the Nafion-mixed electrodes, on the other hand, was a strong function of the support and decreased in the order of Pt/Denka > Pt/GC > Pt/Vulcan XC-72R (Table 3). This trend is in reverse order with the corresponding cumulative pore volume of the supports determined by the BJH method (Table 2). The more tortuous the support pore structure, such as the case of the highly porous Vulcan XC-72R, the lower is the effective O2 permeability, which can be also expected based on the Bruggeman type of equation for effective diffusivity. The measured O2 permeability for the Pt/Denka catalyst was virtually identical to the value calculated by applying the macrohomogeneous model for porous electrodes prepared with ionomer mixed in the catalyst layer (3 × 10-12 mol cm-1 s-1).33,34 Turning our attention to the oxygen electroreduction kinetics, Figure 9, compares the specific current densities on electrochemically active area basis, obtained at 0.9 VRHE (kinetically controlled region). Generally, the Nafion-coated electrodes gave higher ORR specific activity regardless of the Nafion load or support type. Furthermore, the specific activities of the Nafionmixed electrodes decreased with Nafion load while for the coated electrode the highest Nafion load gave the best activity (Figure 9). These results can be explained by the enhanced O2 permeability of the coated electrode compared to the mixed type, as discussed previously (Table 3). For the coated electrodes the interfacial O2 concentration is higher, which in turn increases the specific activity at 0.9 VRHE by its effect on the exchange

Figure 9. Specific current density at 0.9 VRHE versus Nafion loading in O2-saturated 0.5 M H2SO4, 1600 rpm, 22 °C. Legend: Pt/Vulcan XC-72R in Nafion-coated (b) and Nafion-mixed (O); Pt/Denka in Nafion-coated (f) and Nafion-mixed (g); Pt/graphitized carbon in Nafion-coated (2) and Nafion-mixed (4) electrode.

current density. To confirm this hypothesis the exchange current densities were calculated from the kinetic region of the polarization curves (0.95 V > E > 0.83 V). The exchange current density increased with Nafion load for the coated electrode. Overall, the highest area-specific activities were obtained for the Nafion-coated Pt/Denka, 13.5 µA cm-2Pt at a Nafion load of 1.1 mg cm-2 (Figure 9). In the case of Nafion-mixed electrodes, the highest specific activity was observed for Pt/ Vulcan XC-72R, at the lowest explored Nafion load (0.09 mg cm-2), where the specific activity at 0.9 VRHE was about 11 µA cm-2Pt (Figure 9). One also has to consider that the experiments were performed with liquid electrolyte (0.5 M H2SO4) forming thin liquid layers surrounding the catalyst particles.51 The penetration of the 0.5 M H2SO4 solution into the polymer network is an essential component in facilitating the transport of both hydronium ions and oxygen.52-54 Furthermore, with regards to Figure 9, it is important to note that the specific activities at 0.9 VRHE obtained for the supported catalysts are about 20-fold lower compared to either bare Pt or Nafion-coated Pt disk (250 and 300 µA cm-2, respectively) reported by Gottesfeld et al.51 The reason for such a difference in activities between polycrystalline flat Pt disk and Pt nanoparticles supported on carbon comes from the structure of nanoparticles. The surface stresses, crystal facet distribution, atomic coordination number, and both particle-particle and particle-support interactions could affect the intrinsic kinetic currents for ORR.55-59 Employing the experimental data from the kinetic domain (0.95 V > E > 0.83 V) of the RDE polarization curves, Tafel slopes in the range of 100-120 mV dec-1 were calculated for all the electrodes used in the present work. These values are higher than the Tafel slopes typically reported for bare Pt in H2SO4 solutions (∼60-70 mV dec-1)59 and similar to those obtained on a Pt disk covered by Nafion film.51 It is expected that the presence of the Nafion film to increase the apparent Tafel slope compared to the expected intrinsic kinetic value, due to both ohmic voltage drop and mass transfer limitation effects.60,61 Figure 10 compares the mass specific activities at 0.9 VRHE for the coated and mixed electrodes as a function of Nafion load and support type. The mass specific activity was obtained as the product of the area specific activity (Figure 9) and the electrochemically active area (Figure 3). It must be noted that

306 J. Phys. Chem. C, Vol. 113, No. 1, 2009

Figure 10. Mass specific activity at 0.9 VRHE as a function of Nafion loading in O2-saturated 0.5 M H2SO4, 1600 rpm, 22 °C. Legend: Pt/ Vulcan XC-72R in Nafion-coated (•) and Nafion-mixed (O); Pt/Denka in Nafion-coated (f) and Nafion-mixed (g); Pt/graphitized carbon in Nafion-coated (2) and Nafion-mixed (4) electrode.

the data presented in Figures 9 and 10 was intentionally not corrected for the ohmic voltage drop across the catalyst layer because it was of interest to evaluate the true impact of the Nafion load and distribution. Based on Figure 10 it can be observed that generally, regardless of the support, the highest mass specific activity was obtained in the case of the Nafioncoated electrodes at a Nafion load of 1.1 mg cm-2. For Nafionmixed electrodes, however, the performance typically declined with an increase in Nafion load, for reasons discussed previously. The highest mass specific activity at 0.9 VRHE (5.8 A g-1 at 22 °C) was obtained for Nafion-coated Pt/Vulcan XC-72, followed by Pt/Denka (5.1 A g-1) and Pt/GC (4.8 A g-1). These mass specific activities compare favorably with literature data obtained in PEM single-cell fuel cells at 0.9 VRHE considering the different experimental conditions (temperature, O2 pressure, polymer electrolyte type, and catalyst dispersion). Gasteiger et al. summarized a number of ORR mass activity data for Pt/ Vulcan obtained in PEM single-cell fuel cells at 0.9 VRHE and 70-80 °C, giving a wide range from 10 A g-1 to 130 A g-1.30 Assuming the rule of thumb that an increase of temperature by 10 °C could double the exchange current density, and hence, the mass specific activity in the purely kinetic region (at 0.9 VRHE), the data shown by Figure 10 and tentatively extrapolated to 70-80 °C fits in the range indicated by Gasteiger et al.30 Conclusions The effect of carbon support meso- and microstructure (referred to also as secondary porosity11b) in conjunction with the Nafion load (between 0.09 and 1.1 mg cm-2) was investigated with regard to the ORR electrocatalytic activity of Pt/C for three supports (Vulcan XC-72R, Denka, and graphitized carbon). Two catalyst layer preparation procedures were employed, Nafion-coated and mixed, respectively. The catalyst load in all experiments was constant at 0.1 mg cm-2 (for Pt/Vulcan XC-72R and Pt/Denka) and 0.125 mg cm-2 (for Pt/GC). The employed load is considered to be commercially viable and represents a major research target for PEM fuel cell development. A thorough characterization of the supported catalyst meso- and microporous pore volume distribution and surface area was performed and correlated with the Pt electrochemically

Ignaszak et al. active surface area, oxygen permeability and intrinsic electrocatalytic activity. The optimal Nafion loading at which the highest Pt electrochemical surface area was obtained for all catalyst was 0.45 mg cm-2. The oxygen permeability in the catalyst layer Nafion film (CfDf), estimated from the analysis of Levich-Koutecky plots, was lower for the Nafion mixed electrodes compared to the coated electrodes and, furthermore, a strong influence of the support morphology was observed in the case of Nafion-mixed electrodes. Overall, the highest electrochemically active area specific current density at 0.9 VRHE (i.e., 13.5 µA cm-2 Pt at 22 °C) was obtained for the Pt/Denka catalyst prepared with the Nafion-coated procedure at a Nafion load of 1.1 mg cm-2. In contrast, when Nafion was mixed with the supported catalyst, Pt/Vulcan XC-72R generated the highest specific current density (i.e., 10.9 µA cm-2 Pt at 22 °C) for a low Nafion load of 0.09 mg cm-2. These results were explained based on the different interfacial characteristics affecting the effective oxygen permeability and concentration at the reaction sites. The present work underlined the importance of the synergy among the components of the electrocatalytic system (ionomer-support-catalyst) to improve the performance of PEM fuel cell cathodes. Acknowledgment. The authors gratefully acknowledge the generous financial support by Ballard Power Systems, Inc. (Burnaby, Vancouver). References and Notes (1) Gamez, A.; Richard, D.; Gallezot, P. Electrochim. Acta 1996, 41, 307. (2) Parthasarathy, A.; Martin, C. R.; Srinivasan, S. J. Electrochem. Soc. 1991, 138, 916. (3) Lawson, D. R.; Whiteley, L. D.; Martin, C. R.; Szentirmay, M. N.; Song, J. I. J. Electrochem. Soc. 1988, 135, 2247. (4) Paulus, U. A.; Wokaun, A.; Scherer, G. G.; Schmidt, T. J.; Stamenkovic, V.; Markovic, N. M.; Ross, P. N. Electrochim. Acta 2002, 47, 3787. (5) Wlodarczyk, R.; Kolary-Zurowska, A.; Marassi, R.; Chojak, M.; Kulesza, P. Electrochim. Acta 2007, 52, 3958. (6) Antolini, E.; Giorgi, L.; Pozio, A.; Passalacqua, E. J. Power Sources 1999, 77, 136. (7) Paulus, U. A.; Schmidt, T. J.; Gasteiger, H. A.; Behm, R. J. J. Electroanal. Chem. 2001, 495, 134. (8) Lee, S.-K.; Pyun, S.-I.; Lee, S.-J.; Jung, K.-N. Electrochim. Acta 2007, 53, 740. (9) Schmidt, T. J.; Gasteiger, H. A.; Sta¨b, G. D.; Urban, P. M.; Kolb, D. M.; Behm, R. J. J. Electrochem. Soc. 1998, 145, 2354. (10) Watanabe, M.; Igarashi, H.; Yosioka, K. Electrochim. Acta 1995, 40, 329. (11) (a) Arico, A. S.; Creti, P. L.; Antonucci, P. L.; Cho, J.; Kim, H.; Antonucci, V. Electrochim. Acta 1998, 43, 3719. (b) Uchida, H.; Song, J. M.; Suzuki, S.; Nakazawa, E.; Baba, N.; Watanabe, M. J. Phys. Chem. B 2006, 110, 13319. (c) Hiesgen, R.; Eberhardt, D.; Aleksandrova, E.; Friedrich, K. A. Fuel Cells 2006, 6, 425. (12) Cheng, X.; Yi, B.; Han, M.; Zhang, J.; Qiao, Y.; Yu, J. J. Power Sources 1999, 79, 75. (13) Thompson, S. D.; Jordan, L. R.; Forsyth, M. Electrochim. Acta 2001, 46, 1657. (14) Passalacqua, E.; Lufrano, F.; Squadrito, G.; Patti, A.; Giorgi, L. Electrochim. Acta 2001, 46, 799. (15) McGovern, M. S.; Garnett, E. C.; Rice, C.; Masel, R. I.; Wieckowski, A. J. Power Sources 2003, 115, 35. (16) Yamada, H.; Hirai, T.; Moriguchi, I.; Kudo, T. J. Power Sources 2007, 164, 538. (17) S¸en, F.; Go¨kag˘ac, G. J. Phys. Chem. C 2007, 111, 5715. (18) Auer, E.; Pietsch, J.; Tacke, T. Appl. Catal., A 1998, 173, 259. (19) Lobyntseva, E.; Kallio, T.; Alexeyeva, N.; Tammeveski, K.; Kontturi, K. Electrochim. Acta 2007, 52, 7262. (20) Liu, Z.; Gan, L. M. G.; Hong, L.; Chen, W.; Lee, J. Y. J. Power Sources 2005, 139, 73. (21) Hou, Z.; Yi, B.; Zhang, H. Electrochem. Solid-State Lett. 2003, 6, A232. (22) Kaiser, J.; Simonov, P. A.; Zaikovkii, V. I. J. Appl. Electrochem. 2007, 37, 1429.

Catalytic Interface for O2 Electroreduction on Pt (23) Uchida, M.; Aoyama, Y.; Eda, N.; Ohta, A. J. Electrochem. Soc. 1995, 142, 4143. (24) Uchida, M.; Fukuoka, Y.; Sugawara, Y.; Eda, N.; Ohta, A. J. Electrochem. Soc. 1996, 143, 2245. (25) Uchida, M.; Fukuoka, Y.; Sugawara,Ohara, H.; Ohta, A. J. Electrochem. Soc. 1998, 145, 3708. (26) Higuchi, E.; Uchida, H.; Watanabe, M. J. Electroanal. Chem. 2005, 583, 69. (27) Jordan, L. R.; Shukla, A. K.; Behrsing, T.; Avery, N. R.; Muddle, B. C.; Forsyth, M. J. Appl. Electrochem. 2000, 30, 641. (28) Kong, C. S.; Kim, D.-Y.; Lee, H.-K.; Shul, Y.-G.; Lee, T.-H. J. Power Sources 2002, 108, 185. (29) Yu, J.; Yoshikawa, Y.; Matsuura, T.; Islam, M. N. Electrochem. Solid-State Lett. 2005, 8, A152. (30) Gasteiger, H.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal., B 2005, 56, 9. (31) Schmidt, T. J.; Gasteiger, H. A.; Behm, R. J. J. Electrochem. Soc. 1999, 146, 1296. (32) Gough, D. A.; Leypoldt, J. K. Anal. Chem. 1980, 52, 1126. (33) Gloaguen, F.; Convert, P.; Gamburzev, S.; Velev, O. A.; Srinivasan, S. Electrochim. Acta 1998, 43, 3767. (34) Gloaguen, F.; Andolfatto, F.; Durand, R.; Ozil, P. J. Appl. Electrochem. 1994, 24, 863. (35) Ogumi, Z.; Takehara, Z.; Yoshizawa, S. J. Electrochem. Soc. 1984, 131, 769. (36) Ogumi, Z.; Kuroe, T.; Takehara, Z. J. Electrochem. Soc. 1985, 132, 2601. (37) (a) Keck, L., Buchanan, J. S. and Hards, G. A. U.S. Patent 5,068,161, November 26, 1991. (b) Wikander, K.; Ekstro¨m, H.; Palmqvist, A. E. C.; Lindbergh, G. Electrochim. Acta 2007, 52, 6848. (38) Watanabe, M.; Sei, H.; Stonehart, P. J. Electroanal. Chem. 1989, 261, 375. (39) Antolini, E.; Giorgi, L.; Cardellini, F.; Passalacqua, E. J. Solid State Electrochem. 2001, 5, 131. (40) Mushrif, S. H.; Rey, A. D.; Tekinalp, H. Ind. Eng. Chem. Res. 2008, 47, 3883–3890. (41) Kruk, M.; Jaroniec, M.; Sayari, A. Langmuir 1997, 13, 6267–6273.

J. Phys. Chem. C, Vol. 113, No. 1, 2009 307 (42) Nart, F. C.; Vielstich, W. Electrocatalysis. Handbook of Fuel Cell; John Wiley & Sons, Ltd.: New York, 2003; Volume 2, Chapter 21. (43) Zecevic, S. K.; Wainright, J. S.; Litt, M. H.; Gojkovic, S. L.; Savinell, R. F. J. Electrochem. Soc. 1997, 144, 2973. (44) Gough, D.; Leypoldt, K. Anal. Chem. 1979, 51, 439. (45) Perez, J.; Tanaka, A. A.; Gonzales, E. R.; Ticianelli, E. A. J. Electrochem. Soc. 1994, 141, 431. (46) Perez, J.; Gonzales, E. R.; Ticianelli, E. A. Electrochem. Acta 1998, 44, 1329. (47) Tamizhmani, G.; Dodelet, J. P.; Guay, D.; Lalande, G.; Capuano, G. A. J. Electrochem. Soc. 1994, 141, 41. (48) Gojkovic, S. L.; Zecevic, S. K.; Savinell, R. F. J. Electrochem. Soc. 1998, 145, 3713. (49) Claude, E.; Addou, T.; Latour, J.-M.; Aldebert, P. J. Appl. Electrochem. 1999, 28, 57. (50) Tanaka, A. A.; Fierro, C.; Scherson, D.; Yeager, E. A. J. Phys. Chem. 1994, 91, 3799. (51) Gottesfeld, S.; Raistrick, I. D.; Srinivasan, S. J. Electrochem. Soc. 1987, 134, 1455. (52) Yeager, H. L.; Kipling, B. J. Phys. Chem. 1979, 83, 1836. (53) Yeager, H. L.; O’Dell, B.; Twardowski, Z. J. Electrochem. Soc. 1982, 129, 85. (54) Yeager, H. L.; Steck, A. J. Electrochem. Soc. 1982, 128, 1880. (55) Kinoshita, K. J. Electrochem. Soc. 1990, 137, 845. (56) Peuckert, M.; Yoneda, T.; Dalla Betta, R. A.; Boudart, M. J. Electrochem. Soc. 1986, 113, 944. (57) Mukerjee, S. J. Appl. Electrochem. 1990, 20, 537. (58) Jalan, V. M.; Taylor, E. J. J. Electrochem. Soc. 1983, 130, 2299. (59) Kinoshita, K. Electrochemical Oxygen Technology; John Wiley & Sons Inc.: Hoboken, NJ, 1992; pp 37-40. (60) Newman, J.; Thomas-Alyea, K. E. Electrochemical Systems, 3rd ed.; John Wiley & Sons Inc.: Hoboken, NJ, 2004; pp 523-524, and 530532. (61) Gyenge, E. L. J. Power Sources 2005, 152, 105.

JP8060398