Anion Exchange

Article pubs.acs.org/JPCC

Mass-Transport Characteristics of Oxygen at Pt/Anion Exchange Ionomer Interface Prashant Subhas Khadke†,‡ and Ulrike Krewer*,‡ †

Max Planck Institute for Dynamics of Complex Technical Systems, Sandtorstrasse 1, 39106 Magdeburg, Germany Technische Universität Braunschweig, Institute of Energy and Process Systems Engineering, Franz-Liszt-Strasse 35, D-38106 Braunschweig, Germany

‡

ABSTRACT: This work quantifies the mass-transport characteristics of O2 at Pt/anion exchange ionomer by determining the solubility and diffusion coefficient of O2 in Tokuyama AS-4 anion exchange ionomer film. Electrochemical methods such as linear sweep voltammetry and chronoamperometry are performed on rotating disk electrode to determine these parameters. The diffusion process in AS-4 ionomer film is found to be Fickian in nature; that is, it is shown that the diffusion coefficient is independent of concentration and film thickness by calculating the diffusion coefficient at various film thicknesses and rotation speed of electrode. In comparison with Nafion ionomer, the magnitude of diffusion coefficient in anion exchange ionomer is found to be one order higher, and solubility is found to be one order lower. However, the overall permeability, defined by product of diffusion coefficient and solubility is similar to Nafion ionomer film. In addition, durability studies of anion exchange ionomer-coated Pt disk shows electrochemical stability of anion ionomer in alkaline media. With these studies, it is shown that O2 transport at the Pt/anion exchange ionomer interface is not expected to limit alkaline membrane fuel-cell cathode performance.

1. INTRODUCTION Alkaline membrane fuel cells (AMFCs) are increasingly becoming attractive due to the promising catalytic activity of nonprecious metals catalysts for oxygen reduction reaction (ORR).1−3 Recent material developments4−7 and availability of commercial membranes and ionomer (such as from Tokuyama, Japan) in the past decade have accelerated the research in this field, yet the performance of H2/O2 AMFC employing commercial anion exchange membrane (AEM) and anion exchange ionomer (AEI) is low in relation to proton exchange membrane fuel cell (PEMFC) in kinetic region, even with Pt loadings of ≥0.5 mg·cm−2 at anode and cathode,8−10 and the reason for this is still unclear. The most straightforward method to determine the performance-limiting factor is to measure the single electrode polarization of cathode/anode with respect to a fast reversible reaction such as hydrogen oxidation reaction (HOR)/hydrogen evolution reaction (HER). For the case where kinetically fast reaction is not feasible, an additional reversible hydrogen reference electrode is attached to membrane-electrolyte, which is positioned away from the direct current path between anode and cathode. This method of single electrode polarization measurement has been studied by several groups 11−14 in PEMFC employing Nafion membrane and ionomer by having one or more additional reference electrodes. Using this method, Zeng et al.15 concluded that anode limits the AMFC performance for catalyst layer (CL) with AEI. However, this method suffers from uncertainty in the measured potential mainly because the © 2014 American Chemical Society

measured potential is highly sensitive to the position of the reference electrode with respect to anode/cathode.13,14 Therefore, more studies based on other approaches are required to resolve the anode and cathode performance. Study of HOR/ HER kinetics in alkaline media using rotating disk electrode technique by Sheng et al.16 showed anode potential losses of ∼150 mV at 1.5 A/cm2 at a Pt loading of 0.05 mg·cm−2. Although this potential loss is significant and two orders of magnitude higher than in acidic media, it is expected to be much lower than 150 mV with Pt loading >0.5 mg·cm−2. Even though the HOR/HER kinetics is lower in alkaline media in relation to acidic media, it is still sufficiently higher than ORR kinetics in alkaline media and hence does not explain the low performance obtained in AMFC. Furthermore, it has been shown by several other groups16−18 using rotating disk electrode (RDE) technique that the ORR kinetics in alkaline media is as facile as in acidic media. Although studies at RDE setup show the kinetics of HOR and ORR to be sufficiently fast, the performance of AMFC, however, can be limited due to additional problems such as water flooding at anode, water scarcity at cathode, low ionic conductivity of CL, and low mass transport of reactants. Filpi et al.9 evaluated the performance of H2/O2 AMFC and proposed that the difference between estimated performance after kinetic and ohmic correction and Received: February 1, 2014 Revised: April 28, 2014 Published: May 1, 2014 11215

dx.doi.org/10.1021/jp5011549 | J. Phys. Chem. C 2014, 118, 11215−11223

The Journal of Physical Chemistry C

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Figure 1. Microscopic pictures of catalyst distribution on GC surface: water as solvent for loading of (a) 8 μgPt cm−2, (b) 16 μgPt cm−2, and (c) 24 μgPt cm−2; IPA as solvent for loading of (d) 8 μgPt cm−2, (e) 16 μgPt cm−2, and (f) 24 μgPt cm−2.

in designing and optimizing the fuel-cell electrode. The electrochemical stability and short-term durability of AEI is also evaluated. To our best knowledge the diffusion coefficient and solubility of oxygen in commercial Tokuyama AS-4 AEI have not yet been reported in the literature.

measured performance could be due to transport losses such as reactant transport and OH− transport. For a facile mass transport of reactant through the CL, permeability and diffusion coefficient of reactant in ionomer should be high. Recently, mass-transport parameters of O2 at Pt microelectrode/Tokuyama A-201 membrane interface have been reported by Gunasekara et al.20 The authors report about sixfold higher diffusion coefficient and seven-fold lower solubility of O2 in A201 AEM in relation to Nafion 112 membrane. Effectively, the permeability of O2 in both OH− exchanged AEM and Nafion 112 membrane was found to be of similar magnitude, whereas the permeability of O2 in AEM exchanged with carbonate ion was one magnitude lower. In this study, O2 transport parameters such as diffusion coefficient and solubility of O2 are determined at Pt/C−AEI interface using RDE technique. The RDE technique is chosen due to its well-defined regime in which the disk surface is uniformly accessible to reactant, and the diffusion boundary layer is precisely determined. Since the CL of AMFC comprises AEI and catalyst, in our study AEI is chosen instead of AEM. The transport parameters are important for modeling of AMFC and

2. EXPERIMENTAL SECTION 2.1. Catalyst Ink. The catalyst ink was prepared by adding 51.7 mg of 60 wt % Pt/C (Alfa Aesar, United Kingdom) to 40 mL of deionized (DI) water (Millipore, 18.2 MΩ·cm), which was ultrasonicated for 20 min in an ice bath and was further diluted so as to obtain 1.96 μgPt in 20 μL of pipetting volume. This suspension was ultrasonicated each time in an ice bath for 10 min before dropping on glassy carbon (GC) surface (0.247 cm2) to obtain a loading of ∼8 μgPt cm−2disk. The catalyst was dried under ambient conditions. For higher loadings, an additional 20 μL was dropped consecutively after the previous coating was dried. Another set of catalyst ink was prepared in iso-propanol (IPA) (Sigma-Aldrich, Germany)−water solution (IPA/water 99:1 vol %) using the same procedure previously described. A 10 μL aliquot of this solution was dropped on GC 11216



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surface to obtain a loading of ∼8 μgPt cm−2 disk. For higher loadings, an additional 10 μL was dropped consecutively after the previous coating was dried. The microscopic images of catalyst coated GC were taken by optical microscope (model DFC450C) supplied by Leica Microsystems, Switzerland. 2.2. Ionomer Film. Ionomer films of thickness 1, 2, and 5 μm on catalyst coated GC were obtained by dropping 10 μL of ionomer solution, diluted accordingly from commercially obtained AS-4 ionomer solution (5 wt % solids in IPA) supplied by Tokuyama, Japan. The density of recast ionomer film was assumed to be the same as A201 membrane (ρ = 1.06 g cm−3) supplied by Tokuyama, Japan. 2.3. Electrochemical Measurements. A standard threeelectrode RDE setup (Pine Instruments, model 636) was used for all electrochemical measurements. Mercury-mercurous oxide (MMO) (Belltec, Germany) was used as reference electrode, Pt mesh as counter electrode, and catalyst-coated GC as working electrode. All potentials referred herewith are converted with regard to RHE for easy comparison. The electrochemical measurements were performed in 0.1 M KOH under ambient conditions of temperature 22 ± 1 °C and atmospheric pressure. ORR voltammograms, impedance, cyclic voltammetry (CV), and chronoamperometry were measured by using an IM6e potentiostat procured from Zahner, Germany. Prior to each experiment, fresh 0.1 M KOH was used and GC was polished with 0.05 μm alumina powder (Bueler, Germany) followed by sonication and copious washing with DI water. High-frequency impedance measurements at 10 kHz were also carried out prior to each electrochemical measurement to determine uncompensated electrolyte resistance, it is found to be in range of 31 ± 1 Ω. CV measurements were performed at a scan rate of 50 mV s−1 in N2 (grade 5.0 Westfalen AG, Germany)-saturated electrolyte. All CVs are base-corrected, and base CV was performed on bare GC under similar operating conditions. The ORR voltammograms were measured at a scan rate of 10 mV s−1 with 300, 600, 1000, 1600, and 2500 rpm in O2-saturated electrolyte. The chronoamperometry was performed in O2-saturated electrolyte, and i versus t plot is obtained by changing the potential stepwise from 1.2 V, where no O2 reduction occurs to 0.5 V, where O2 reduction is diffusion-controlled. To account for contribution from doublelayer charging current, chronoamperometry is also performed in N2-saturated electrolyte and subsequently subtracted from the O2-saturated chronoamperometry curve. Short-term durability studies of AS-4 ionomer film were carried out by measuring ORR voltammograms for 1000 cycles. For this study, 5 μm film of AS-4 ionomer was coated on Pt disk instead of Pt/C particles. This allows us to exclude degradation or agglomeration of CL while cycling. Also, CVs under a N2 atmosphere were conducted before and after durability.

IPA, whereas in water it is much slower. Faster moving particles accompanied by higher collision force were usually found to either dislodge the smaller agglomerates stuck previously on GC or cling only to bigger agglomerates. As a result, the catalyst distribution after drying differs for IPA and water as solvent. Figure 1 shows the microscopic images of catalyst distribution on GC surface for IPA and water as solvent. The white background in the images is GC surface, and black specks are Pt/C particles. The images were taken after drying of each consecutive coating of catalyst in steps of 8 μgPt cm−2 and at the same location to trace the growth of agglomerates. For water as the solvent the catalyst is uniformly distributed with very few agglomerates, and the best coating is reached when the loading was kept at or below ∼16μgPt cm−2 (Figure 1a,b), with most of the particle sizes below 1 μm. For higher loadings, the particles start agglomerating, and occurrence of agglomerates >5 μm increases rapidly (Figure 1 c). For IPA, the distribution is significantly nonuniform, and bigger agglomerates are found even with 8 μgPt cm−2 (Figure 1d−f). In the case of IPA, a significant number of agglomerates grow in size at the same location during drying and when the loading is increased from 8 to 24 μgPt cm−2 (indicated as red circles in images d−f). In most of the literature, the loading of catalyst particles when using water as solvent varies between 10 and 50 μgPt cm−2. Although the diffusion-limited current is only marginally affected for loading of 10−50 μgPt cm−2, the best mass activity values at low overpotential are obtained when loading is kept below 20 μgPt cm−2.21−23 This is justified in this work by the microscopic images. For all studies discussed further in this paper, GC was coated with catalyst ink prepared in water as a solvent, with a loading of 8 μgPt cm−2. 3.2. Electrochemical Surface Area. Figure 2 shows the typical CV of Pt/C-coated disk with 1 μm ionomer film. The

3. RESULTS AND DISCUSSION 3.1. Catalyst Distribution on Glassy Carbon. It is of utmost importance that the catalyst distribution on GC should be uniform and reproducible while evaluating the catalyst activity.21 Preferably, the catalyst agglomerate size should be as low as possible for better catalyst utilization so that accurate and reproducible mass activity values are obtained. Monitoring the catalyst drying on GC surface with microscope from the time of ink dropping until drying gives insight into the factors leading to nonuniform catalyst distribution. As soon as the ink is dropped on GC, the catalyst particles move randomly inside the droplet. This Brownian-like motion was seen to be faster in

Figure 2. Cyclic voltammogram for Pt/C coated on GC with 1 μm ionomer film measured at a scan rate of 50 mV s−1 in N2-saturated 0.1 M KOH.

obtained CV is characteristic CV of Pt/C catalyst suggesting that ionomer is electrochemically inactive in the potential window of CV measurement. It shows the typical Pt−H adsorption/desorption region 0.1 to 0.5 V, double-layer region 0.5 to 0.6 V, and Pt-oxide region above 0.6 V similar to that reported previously.16 The electrochemical surface area (ECSA) was obtained by integrating the hydrogen adsorption/desorption region, subtracting the double-layer charging 11217



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However, it remains the same within experimental error for 1 and 0.5 μm thick film, suggesting that film resistance is negligible below 1 μm. 3.3.1. Mass Activity. As previously mentioned, the ionomer film resistance is found to be negligible for films ≤1 μm. Therefore, the kinetic data for 60 wt % Pt/C are extracted from catalyst coated with 1 μm ionomer film. The ORR polarization at 1600 rpm on Pt/C catalyst with varying ionomer film thickness is already shown in Figure 3. The onset reduction potential for ORR at Pt/C in alkaline media is unaffected by the presence of ionomer, which is 1.091 V and is comparable to previous reports.27 The mass-transfer-corrected Tafel plots derived from ORR voltammogram of Figure 3 is shown in Figure 4. The mass-transfer correction is performed using eq 1,

currents and then dividing the resulting coulombic charge by 2 to get the mean charge. This was further divided by 210 μC cm−2Pt,23 a charge associated with monolayer adsorption of hydrogen on polycrystalline Pt.24 The ECSA calculated from three independent experiments was found to be 14.58 ± 0.03 m2 g−1; consequently, the AreaPt is calculated as 0.294 cm2. In the following, the current densities obtained in electrochemical measurements are normalized to AreaPt. 3.3. Parameters Obtained from ORR Voltammograms. Figure 3 shows the negative going ORR voltammograms for

Figure 3. Polarization curves for negative going ORR in O2-saturated 0.1 M KOH on Pt/C coated with 5 μm (green square), 2 μm (blue triangle), 1 μm (red circle), and 0.5 μm (black inverted triangle) ionomer film thickness; scan rate: 10 mV s−1; rotation rate: 1600 rpm. The error bars represent deviation for three independent experiments performed for each film.

Figure 4. Mass activity for Pt/C in kinetic region of the ORR polarization. IR-corrected potential with mass-transfer correction (square). IR-corrected potential without mass-transfer correction (circle).

various film thicknesses at 1600 rpm. At any fixed potential in the limiting current region, the disk current is described by Koutecky−Levich (K−L) equation:25 1 1 1 = + i ik iL (1)

where the limiting current density is taken at 0.5 V and overall current density is iR-corrected. Mass activity of catalysts are determined by normalising ik values to mass of Pt in the electrode and are plotted against the potential, which is characteristic of kinetic control region. The mass activity value in A mgPt−1 at 0.9 V versus RHE is tabulated in Table 1 and compared with literature. The mass activity values obtained in the present work are comparable to mass activities reported in alkaline media. This value is also comparable to those studied in acidic media. It is to be noted that GC and carbon have appreciable ORR kinetics especially at high overpotential in alkaline media28 unlike acidic media. However, we have found (not shown here) that catalytic activity of bare GC at 0.9 V is negligible and a reasonable value of mass activity can be obtained for the catalyst at this potential. 3.3.2. Number of Electrons. The number of electrons is calculated from the slope of the K−L plot. The purpose of calculating n at particular potential is to substitute these values in appropriate equation (eq 1 in Section 3.3.3) so that the diffusion coefficient can be determined more precisely. Figure 5 shows the K−L plot derived from Figure 3 for various film thicknesses at different rotation rates, varying from 300 to 2500 rpm. The slope of K−L plots of 1, 2, and 5 μm film is given by 0.62nFD2/3υ−1/6CB. Assuming D = 1.9 × 10−5 cm2 s−1, υ = 0.01 cm2 s−1, and CB = 1.2 × 10−6 mol cm−3,29 the number of electrons, n, is calculated to be 2.76 ± 0.14. The ORR on Pt in

where i is the iR-corrected current density from disk, ik is voltage-dependent kinetic current density, and iL is the RPMdependent diffusion-limited current density and can be expressed as iL = (0.62nFD2/3υ−1/6C B)ω1/2

(2)

where n is number of electrons transferred, F is Faraday’s constant, D is the diffusion coefficient of the reactant in the electrolyte, υ is the viscosity of the electrolyte, CB is the bulk concentration or solubility of the reactant in the electrolyte, and ω is the rotation speed of the RDE. For the film-coated CL, there is an additional resistance offered by the film, and eq 2 is modified as follows.26 1 1 1 1 = + + i ik iL if (3) where if is film-diffusion limited current density dependent on film-thickness and can be expressed as nFCBDfL−1, where Df is the diffusion coefficient of the reactant in film and L is the film thickness. From Figure 3, it can be seen that i in the limiting current region decreases with increase in the film thickness. 11218



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Table 1. Comparative Mass-Activity Values of Pt/C with Various Metal Loading Pt/C catalyst

Pt loading (μg cm−2)

scan rate (mV s−1)

rotation speed (rpm)

60 wt % (Alfa Aesar) 10 wt % 10 wt % 20 wt % 40 wt % 60 wt % 80 wt % 20 wt % 40 wt % 40 wt % (ETEK) 40 wt % (ETEK) 20 wt % (ETEK) 20 wt % (ETEK) 45.9 wt % (TTK) 45.9 wt % (TTK)

10 33 14 28 57 85 113 14.3 14.3 13.7 13.7 14.3 14.3 12.7 12.7

10 5 1 1 1 1 1 5 20 5 20 5 20 5 20

1600 1600

a

1600 1600 1600 1600 1600 1600 1600 1600

electrolyte

mass-activity (A mg−1Pt) at 0.9 V vs RHE

ref

0.1 M KOH 0.1 M KOH 1 M NaOH 1 M NaOH 1 M NaOH 1 M NaOH 1 M NaOH 0.1 M HClO4 0.1 M HClO4 0.1 M HClO4 0.1 M HClO4 0.1 M HClO4 0.1 M HClO4 0.1 M HClO4 0.1 M HClO4

0.1 0.036 0.0896a 0.0804a 0.0582a 0.0451a 0.0335a 0.16 0.21 0.069 0.11 0.16 0.19 0.16 0.22

this work 39 18 18 18 18 18 21 21 22 22 22 22 22 22

Mass activity reported at 0.824 V vs RHE.

Figure 6. Schematic representation of concentration profile near electrode surface for a rotating electrode, i = 0 (solid line); i = limiting current (dashed line).

Figure 5. Koutecky−Levich plots for ORR at 0.5 V versus RHE in 0.1 M KOH on Pt/C with 5 μm (circle), 2 μm (triangle), and 1 μm (square) ionomer film thickness and theoretical curve for n = 4 (solid line).

i = (nF(Ci − 0)Df L−1)film = (nF(C B − Ci)Dδ −1)diffusion layer (4)

where Ci is concentration of reactant at film−electrolyte interface and can be rewritten as

alkaline media is commonly reported as a 4e transfer reaction19,30 and 2e transfer reaction on carbon and GC.31,32 The lower value of n obtained is attributed to the presence of both 4e and 2e transfer reactions, which compete with each other at high overpotential. 3.3.3. Film−Electrolyte Interface Concentration and Film Diffusion Coefficient. Schematic representation of concentration profile near electrode surface for a rotating electrode is shown in Figure 6 for the cases when the current is zero and when the current is diffusion-limited. The rotation of electrode in RDE creates a diffusion layer of thickness defined mathematically as δ = 1.61D1/3υ1/6ω−1/2. Because of rotation of electrode, the convection brings O2 outside this layer, where the concentration of O 2 is essentially equal to bulk concentration. In this case, the current is measured under steady-state conditions, and diffusion length is fixed and independent of time. It is equal to combined thickness of film thickness and δ. At steady state, the O2 flux through film and diffusion layer must be the same, and the respective limiting current density at high overpotential can be given as

Ci = C B −

iδ nFD

(5)

because i, n, F, and L are known and δ = 1.61D1/3υ1/6ω−1/2, Ci and subsequently Df can be calculated for each rotation speed. Figure 7 shows Ci value and corresponding Df value calculated for various RPM. It can be seen that Ci values increase with increase in RPM and are expected to be different for 2 and 5 μm film at a given RPM. The corresponding Df values are nearly constant at 6.57 × 10−6 cm2 s−1 for all RPMs. This shows that the diffusion coefficient is independent of concentration and film thickness. 3.4. Solubility of O2 in Film. The inset of Figure 8 shows a chronoamperometric curve for the 5 μm film measured at zero RPM at 0.5 V versus RHE. The current density for short times can be represented by Cottrell equation.25 11219



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Df and Cf are tabulated in Table 2 together with values reported for other electrolyte in literature. The Df values for Nafion membrane fall in same order, whereas the Cf data are a little scattered. More insight can be obtained by observing the variation of Df and Cf values in a single study. For example, Zhang et al.33 showed for Nafion membrane an increase in Df and permeability (given by CfDf) when the temperature was increased from 30 to 60 °C. On the contrary, the Cf was found to decrease with temperature increase; the rate of Df increase was sharp in relation to CfDf increase. In addition, they also found that the volume percentage of water content in membrane increased from 30.56 to 40.55% when the temperature was increased from 30 to 60 °C. Similar observations were made in another study by Takamura et al.,34 where they showed increase in Df and CfDf with relative humidity at constant temperature. Both groups33,34 also showed the existence of similar trends in other membranes. This shows the existence of the correlation between water content of membrane and transport parameters such as diffusion coefficient, solubility, and permeability. In particular, the diffusion coefficient is strongly correlated to water content. It is not clear at present whether the diffusion coefficient and permeability will increase with temperature if the water content remains same; however, with great certainty it can be said that diffusion coefficient and permeability in Nafion increase with increase in water content. This suggests that significant transport of O2 through membrane is more likely to happen through water channels formed in the pores of membranes and water content helps in O2 transport. It is previously reported by Li et al.35 and Duan et al.36 that water content in A201 membrane is higher in relation to Nafion membranes, and higher diffusion coefficient obtained in our studies is attributed to this factor. Interestingly, the values of Df and Cf obtained in this work are very similar to the value obtained for A201 AEM reported by Gunasekara et al.,20 even though their measurements were made by supplying humidified O2 at 1 atm pressure. Since both the A201 membrane and AS-4 ionomer are made up of the same polymer backbone with quaternary ammonium ion, the O2 transport mechanism can be assumed to be similar. If this is true, then it suggests that Cf is independent of O 2 concentration outside the film or membrane. This is contrary to what is reported with Nafion membrane,37 where Cf depends on the partial pressure of O2

Figure 7. Concentrations at film−electrolyte interface with 2 μm (square) and 5 μm (circle) film and corresponding diffusion coefficients at 2 μm (triangle) and 5 μm (star) versus RPM.

i(t ) =

nFDf 1/2Cf π 1/2t 1/2

(6)

where Cf is solubility of O2 in film that is identical to O2 concentration in film before the step potential is imposed and t is the measurement time. A plot of i versus t−1/2, as shown in 1/2 Figure 8, is a straight line with slope equal to ((nFD1/2 f Cf)/π ). At stationary electrode, convection is absent; under this condition the diffusion layer thickness is essentially the entire liquid electrolyte until the boundary of the vessel. In this case, the current is measured under unsteady-state conditions, and diffusion length depends on time scale of experiment. When the step potential is imposed, the concentration change near the electrode is a function of time, and at short times the concentration gradient is completely confined within film. The concentration profile near the electrode is schematically shown in Figure 8b, where the solid line represents initial concentration and the dotted line represents the change in concentration with respect to time once step potential is imposed. The diffusion length, given as L = (Dft)1/2, is the distance from the electrode where the concentration gradient at time t ceases to zero. For Df = 6.57 × 10−6 cm2 s−1 and L = 5 μm, t = 0.038 s, below 0.038 s the diffusion length is less than 5 μm. Figure 8 shows the Cottrell plot where t < 0.038 s, and from the slope, Cf can be determined.

Figure 8. (a) Cottrell plot for Pt/C−ionomer interface for 5 μm ionomer film thickness. Inset: chronoamperometry for ORR of a stationary electrode at the Pt/C−ionomer interface with an ionomer film thickness of 5 μm measured at 0.5 V vs RHE. (b) Schematic representation of concentration profile near electrode surface for a stationary electrode before step potential at t = 0 (solid line); concentration profile after imposing step potential at t > 0 (dotted line). 11220



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Table 2. Mass-Transport Parameters of O2 in Various Electrode Interfaces Df × 105 (cm2 s−1)a

medium

AS-4 ionomer film in 0.1 M KOH 0.657 A201 anion exchange membrane 0.429 Nafion membrane in 0.5 M K2SO4 0.024 Nafion membrane in 0.5 M H2SO4 0.070 Nafion membrane 100% RH 0.074 ± 0.0003 Nafion membrane in 1 N H2SO4 0.062 Nafion membrane in water 0.095 Nafion membrane 100% RH 0.075 Nafion membrane 100% RH 0.217 Nafion membrane in water 0.245 Nafion membrane 100% RH 0.6 Nafion ionomer film in 0.7 M H3PO4 0.2 ± 0.1 medium D × 105 (cm2 s−1)c 0.1 M KOH 1 M KOH 0.7 M H3PO4 0.5 M H2SO4 1 M H2SO4 H2O

Cf × 106 (mol cm−3)b 0.761 0.93 7.2 13 26 ± 1 18.7 9.34 10.65 6.68 8.56 0.4 3.7 ± 0.2 C × 106 (mol cm−3)d

1.9 1.65 1.94 ± 0.04

2.95

1.2 1.0 1.09 ± 0.01 1.13 0.9 1.3

temperature (°C)

ref

22 ± 1 25 20 20 25 25 30 30 50 50 50

this work 20 40 41 42 43 44 33 33 37 34 46 ref

temperature (°C) 20 25 25 25 25

29 45 46 40 45 40

Diffusion coefficient of O2 in ionomer film/membrane. bSolubility of O2 in ionomer film/membrane. cDiffusion coefficient of O2 in liquid electrolyte. dSolubility of O2 in ionomer in liquid electrolyte.

a

before 1000 ORR cycles, (2) Pt disk after 1000 ORR cycles, and (3) Pt disk after removing of ionomer film. It is clear that the CVs of cases 2 and 3 are similar and are very different from case 1. The CV in case 1 resembles more typical to CV of polycrystalline Pt, but CVs of cases 2 and 3 resemble to CV of Pt(111),30 suggesting that after 1000 ORR cycles, Pt(111) is predominantly exposed more than other planes. Since Pt(111) plane is more active for ORR30 in relation to other planes, we see an increase in the current in the activation region during ORR cycling. The CV results support the previous findings38 that the active sites of Pt disk undergo reorientation while cycling. The decrease in current in the limiting region is ascribed to delamination of film from Pt disk during cycling, which can increase the diffusion thickness and hence decrease the limiting current. This shows that AS-4 AEI is electrochemically stable during ORR cycling in alkaline media, and deviations in ORR voltammogram are caused only by changes in Pt disk morphology and ionomer delamination.

outside the membrane. Furthermore, this also suggests that the maximum current obtained in cathode of alkaline fuel cell will be limited by Cf. However, to confirm this, further studies are required. 3.5. Durability of AS-4 Ionomer Film. Figure 9 shows the ORR voltammograms for various cycles for ionomer coated on

4. CONCLUSIONS The mass-transport parameters such as diffusion coefficient and solubility of O2 are determined to identify the possible masstransport limitation of O2 at the cathode of AMFC. These parameters are determined by conducting linear sweep voltammetry and chronoamperometry at the Pt/AEI interface using RDE technique. We have found higher diffusion coefficient and lower solubility values for AEI in relation to values reported for proton exchange Nafion ionomer. This deviation is explained by higher water content of AEI. However, the effective permeability of O2 in AEI is found to be of similar magnitude as in Nafion ionomer, ruling out low availability of O2 as a limiting factor for AMFC cathode. In durability studies, the AEI is found to be electrochemically stable during potential cycling under O2-saturated alkaline electrolyte for up to 1000 cycles. With these studies, it is predicted that the optimum ionomer content at that cathode of AMFC is similar to the cathode of proton exchange fuel cell. These results show that

Figure 9. ORR voltammogram and CV of Pt disk coated with 5 μm ionomer film before and after aging by cycling. ORR after 10th (star), 300th (circle), 600th (triangle), and 1000th (square) cycles. Inset: CV under N2-saturated electrolyte before 1000 cycles (dashed line), after 1000 cycles (dotted line), and after removing ionomer (dashed-dotted line).

polycrystalline Pt disk electrode consisting of Pt(111), Pt(100), and Pt(110) crystalline planes. It can be seen that the current improves in the activation region, whereas it decreases in the limiting region. An explanation for improved activity is provided by CVs under a N2 atmosphere. Inset of Figure 9 shows CV measurements for Pt disk for three cases (1) Pt disk 11221



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O2 transport at Pt/AEI interface is facile and stable and is not expected to limit the cathode performance of AMFC.

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AUTHOR INFORMATION

Corresponding Author

*Tel:+49-531-3913030. Fax.: +49-531-3915932. E-mail: u. [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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