Electrode parameters and operating conditions influencing the performance of anion exchange membrane fuel cells

Electrochimica Acta 277 (2018) 151e160

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Electrode parameters and operating conditions influencing the performance of anion exchange membrane fuel cells € rn Eriksson, Go € ran Lindbergh, Carina Lagergren, Annika Carlson*, Pavel Shapturenka, Bjo €m Rakel Wreland Lindstro Applied Electrochemistry, Department of Chemical Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 November 2017 Received in revised form 17 April 2018 Accepted 19 April 2018 Available online 27 April 2018

A deeper understanding of porous electrode preparation and performance losses is necessary to advance the anion exchange membrane fuel cell (AEMFC) technology. This study has investigated the performance losses at 50
C for varied: Tokuyama AS-4 ionomer content in the catalyst layer, Pt/C loading and catalyst layer thickness at the anode and cathode, relative humidity, and anode catalyst. The prepared gas diffusion electrodes in the interval of ionomer-to-Pt/C weight ratio of 0.4e0.8 or 29e44 wt% ionomer content show the highest performance. Varying the loading and catalyst layer thickness simultaneously shows that both the cathode and the anode influence the cell performance. The effects of the two electrodes are shown to vary with current density and this is assumed to be due to non-uniform current distribution throughout the electrodes. Further, lowering the relative humidity at the anode and cathode separately shows small performance losses for both electrodes that could be related to lowered ionomer conductivity. Continued studies are needed to optimize, and understand limitations of, each of the two electrodes to obtain improved cell performance. © 2018 Elsevier Ltd. All rights reserved.

Keywords: AEMFC Electrode performance Electrode morphology Ionomer content Pt/C catalyst

1. Introduction Anion exchange membrane fuel cells (AEMFCs) are a potential low-cost alternative to proton exchange membrane fuel cells (PEMFCs), as the alkaline environment enables the use of costeffective materials and non-precious metal catalysts [1e5]. Recent improvements have led to conductivity and cell performance approaching that of PEMFC [6,7]. However, AEMFCs still have insufficient polymer stability and therefore limited cell lifetime [1,3,4,8e10]. To solve the stability issues innovative polymer electrolyte materials with improved performance are being developed [1,8,11,12]. Another area of improvement is the catalyst which focuses on higher activity and lower cost for AEMFCs [13e17]. High cell performance and improved lifetime can also be achieved by improving the electrode structure and operating conditions. A wellperforming porous electrode needs a balance between ionomer for optimized ion-conduction, supported catalyst for high activity and pores for a continuous supply of reacting gases. The electrode structure is also of importance for water management, which in

* Corresponding author. E-mail address: [email protected] (A. Carlson). https://doi.org/10.1016/j.electacta.2018.04.137 0013-4686/© 2018 Elsevier Ltd. All rights reserved.

AEMFCs means sufficient water at the cathode for the oxygen reduction reaction while preventing flooding of the electrodes, primarily at the anode, at high current densities and excessive water production. An efficient morphology can be obtained by improving the electrode composition and preparation, which is a field less studied for AEMFCs. Further, poor water management is believed to be one of the reasons behind low stability [18] and can also be improved by cell design and operating conditions. For PEMFCs both ionomer content and preparation method have been shown to influence the electrode morphology and thereby the cell performance [19e22]. One parameter that needs to be considered is the ink solvent used during electrode preparation as it has been shown to affect the electrode morphology, especially when mixed solvents are used [20e22]. In studies on AEMFCs a mixture of iso-propanol and water as ink solvent is most commonly used during electrode preparation, but few solvent compositions are given in references [4,9,10,23,24]. When the compositions are given, mostly pure water [25,26] or 50/50 iso-propanol and water [24,27] are used, but no systematic study of different compositions has been reported. Therefore, an investigation is needed to clarify the effect of solvent composition during electrode preparation. Cell performance can also be related to the ratio of ionomer to supported catalyst. Gode et al. [19] showed that high amounts of

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ionomer will fill the porous structure and hinder the gas from reaching the catalyst in a study on PEMFCs. Other studies on PEMFCs [28e30] confirm that the ratio of ionomer to supported catalyst greatly affects the cell performance, but the optimal ratio varies. In AEMFC, Fukuta [10] found the highest performance for an ionomer content of 30 wt% when the electrode was manufactured on the membrane. Carmo et al. [31] instead found the best performing composition having 25 wt% ionomer content when manufacturing the electrode on the GDL. Using a different preparation method with PTFE as a binder Yang et al. [23] varied the amount of catalyst, while maintaining a constant ionomer loading, and showed yet another composition optimum at 20 wt% ionomer content. How the ionomer content affects the performance has not been fully clarified and therefore the material and preparation related issues cannot be separated. To better optimize AEMFC performance the contributions of the two separate electrodes needs to be considered. Today, there are few studies on the influence of rate-limiting electrode processes on AEMFC, all using different ionomers, making it hard to separate material properties from system related performance losses. In two studies introducing a reference electrode into the cell it was shown that the anode contributes the most to performance losses [32,33]. Others [34,35] have found that, as in PEMFC, it is the cathode that controls the cell performance, while still others indicate a mixed contribution [16]. The studies that claim high anode influence or mixed contribution were performed at relatively high current densities [16,32,33] and the ones claiming the cathode as most influencing compared the activity at lower current densities [34,35]. This could indicate a change in rate-limiting processes depending on operating potential, which has not been well studied. There are also several studies on the type of catalyst used at the cathode [13e16,25,26,36] and at least one study at the anode [37]. Change in type of catalyst can also help to identify the limitations of an electrode as altered cell performance. Again the influence on cell performance of the electrodes differs between the studies. Only a few of the studies combine the electrode variations with other operating conditions to identify if changes in performance are really related to altered electrode kinetics or other cell processes. From investigations of performance losses in AEMFC, it was found that water management or relative humidity of inlet gases greatly influence the performance [6,10,35,38e42]. However, some of these publications only present physical models [39e41], with little basis in experimental results. In one experimental study [35] it was found that cathode dry-out is a primary source of performance loss, but another study showed that inlet humidity at the cathode has little effect [10]. Further, it has been observed that a significant amount of water on the cathode side comes from back-diffusion through the membrane from the anode [42] and a final study showed flooding at both electrodes [6]. However, few of these studies related water balancing to other parameters such as ionomer content, electrode thickness or type of membrane. Therefore, it is necessary to gain a better understanding of the performance losses with regard to both electrode properties and relative humidity as a whole in an attempt to separate the different

phenomena. In order to fill the knowledge gaps stated above this study aims to investigate the effects of ink solvent composition and ionomer content on the final electrode structure and performance in AEMFCs. It also aims to gain a better understanding of how the cell performance losses are related to the anode and cathode separately, as well as how the relative humidity influences the two electrodes. 2. Experimental The influence on cell performance of solvent composition, ionomer content, Pt/C loading and catalyst layer (CL) thickness and relative humidity were investigated in this study. To have a stable and well known catalyst this study mostly uses platinum, as the main objective is to understand overall cell losses and avoid artifacts from less defined catalysts. In addition PtRu was studied as catalyst at the anode. The solvent composition was varied by changing the volume percentage of water in relation to total amount of ink solvent. The ionomer-to-Pt/C weight ratio was changed by varying the ionomer content in the ink with constant Pt/C loading. The loading and catalyst layer thickness were changed by applying different amounts of ink to the GDL. The effects of relative humidity were investigated by changing operating conditions. Finally, the catalyst powder was changed to PtRu at the anode to study its effect. The compositions and ratios for all tests are summarized in Table 1. 2.1. Electrode preparation Electrode inks were prepared from 36% Pt or 29.7% Pt and 23% Ru on carbon black from Tanaka Kikinzoku International K.K., AS-4 ionomer solution from Tokuyama Corp., iso-propanol (Analytical grade, Merck) and Milli-Q water (18.2 MUcm). The inks were alternately stirred and ultra-sonicated in 15 min periods for 1 h. Thereafter gas diffusion electrodes (GDEs) were prepared by pipetting 120, 60 or 15 mL of ink onto pre-cut 1 cm2 discs (11 mm diameter) of Sigracet 25BC gas diffusion layer (GDL), resulting in a loading of approximately 0.8, 0.4 or 0.1 mgPt cm2. Due to the surface tension, the 60 mL droplet forms a half-sphere that covers the whole surface area of the GDL, therefore the 120 mL electrodes were pipetted by applying 60 mL twice and the 15 mL were manually spread over the surface with the tip of the pipette. The electrodes were dried in two steps: firstly for 15 min in a fume hood at ambient conditions and secondly in vacuum for 30 min at room temperature. The catalyst loading was calculated from the weight difference of the GDLs before and after ink application with the assumption that the ionomer-to-Pt/C weight ratio was the same as in the ink. For statistical accuracy the performance was measured on a minimum of three cells. In addition, when the ink composition was varied at least three separate batches (approximately 400 mL) were prepared. In order to vary only one parameter at the time the weight fraction of Pt/C and ionomer in the ink remained constant when solvent composition was changed, and the solvent composition and Pt/C content remained constant when the ionomer

Table 1 Electrode layer/ink composition for different studies. Study

vol% H2 O (ink-solvent)

Ionomer:Pt/C

Loading [mg cm2]

Ionomer content wt%

Solvent composition Ionomer content Catalyst loading R.H. PtRu anode

30e70 40 40 40 40

0.6 0.2e1.5 0.6 0.6 0.6

0.4 0.4 0.1, 0.4, 0.8 0.4 0.33/0.25 Pt/Ru

37 17e60 37 37 37

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content in the ink was changed, as described in Table 1. 2.2. Fuel cell preparation and set-up Before mounting the cell, the membrane, A201 from Tokuyama Corp., was ion exchanged from carbonate to hydroxide form. The ion exchange was performed by submersing the membrane for 3  20 min in 1 M KOH (Merck pellets, and Milli-Q water) and then rinsing it for 3  20 min in Milli-Q water, to ensure that excess hydroxide was removed. The ion exchange took place just before mounting in the cell house in order to hinder the membrane from converting back to its carbonate form. The membrane was mounted wet, with dry, and non-ion-exchanged GDEs, on both the anode and cathode side. The membrane electrode assembly (MEA) was mounted in a cell house (Fuel Cell Technologies Inc.) using graphite current collectors with spiral shaped gas flow channels and a 4 N m torque. The cell was heated to 50
C under nitrogen purging prior to the electrochemical measurements. The volumetric gas flow of all carrier gases was maintained at a constant 7.4 Ln h1 (2.06 mLn h1 ) for all measurements. The inlet relative humidity (R.H.) was 95%, except in the study varying the relative humidity in the gases. 2.3. Electrode characterization All electrochemical measurements were performed with a PAR 273 A potentiostat except for the electrochemical impedance spectroscopy (EIS), which was performed with a Zahner IM6. To activate the catalyst and remove the remaining carbonates the cell was cycled between open circuit potential (OCP) of around 1 V and 0.1 V in oxygen/hydrogen gas with a scan rate of 10 mVs1 until no performance improvement was observed (approximately 25 cycles or 2 h). After activation, electrochemical characterization measurements were performed in the following order: polarization curve, EIS, potentiostatic hold, polarization curve, and cyclic voltammetry. Polarization curves were registered by a potentiodynamic sweep from OCP to 0.1 V at 1 mVs1, and IR-corrected using the built-in current interrupt method of the potentiostat. EIS was performed galvanostatically at 16 mA cm2 with oxygen/hydrogen gas flows. The impedance spectra were measured from 100 kHz to 20 mHz with an amplitude of 1 mA and with 8 points per decade. Another polarization curve was registered, as described previously, after a potentiostatic hold at 0.5 V for 30 min to establish the stability of performance. In the study of different relative humidity the cell was kept at 0.7 V for 1.5 h to stabilize, before additional polarization curves and EIS measurements. Finally, cyclic voltammetry (CV) was recorded, with nitrogen or argon at the working electrode and hydrogen and 5% hydrogen in argon at the counter electrode, between 0.05 V and 1.25 V or 0.8 V with a scan rate of 20 mVs1. The morphology of the GDEs was investigated by scanning electron microscopy (SEM) using a Hitachi S-4800 microscope at 1 or 10 keV from above and as cross-sections. The cross-sections were prepared by making an incision halfway through the back of the GDE with a scalpel and then breaking them after submersion in liquid nitrogen. From the cross-sections the catalyst layer thickness was measured and used to calculate the electrode porosity through equation (1). Here LCL is the thickness of the catalyst layer, Area is the geometrical electrode area, x is the mass fractions of ionomer and catalyst powder, r are the densities of ionomer (1.15 g cm3) and Pt/C powder (8.9 g cm3) and m is the mass of the catalyst layer.


Porosity ¼

.  Area,LCL  xiono ,mCL =riono  xPt=C ,mCL rPt=C Area,LCL (1)

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The measured thicknesses were also used to calculate the volumetric current densities for various tests, according to equation (2). Here i is the geometric current density, LCL;An is the thickness of the anode, LCL;Cat of the cathode and ivol the volumetric current density. The total volume of both the anode and the cathode was used as both electrodes affect the cell performance.

i  ivol ¼  LCL;An þ LCL;Cat

(2)

3. Results 3.1. Electrode structure optimization The partial volume of water in the ink was optimized in order to obtain a well performing porous electrode and a stable ink. The general trend for the range 30e70 vol% water, shown in Fig. 1, is higher performance with lower water content, with a small optimum for 40 vol% water in the ink solvent. At optimized conditions a maximum current density of 725 mA cm2 at 0.1 V and maximum power density of 180 mW cm2 at 0.5 V was obtained. SEM analysis of these electrodes, not shown here, shows that the ionomer is evenly distributed for 40 vol% water in the ink solvent. At high water content the ionomer is mainly found in a ring around the edge of the GDL, while the central part of the electrode has a porous structure similar to pure catalyst powder. The uneven distribution is probably due to phase separation during drying of the ink droplet and explains the poor performance observed. As a second step the amount of ionomer in the catalytic layer was varied for a fixed amount of supported catalyst to study how the ionomer content influenced the performance. Selected SEM images of ionomer-to-Pt/C weight ratios 0.2, 0.6 and 1.0 are shown in Fig. 2. The low magnification images show clear differences between the electrodes. Ionomer-to-Pt/C weight ratio 0.2 (Fig. 2a) has a homogeneous structure, ratio 0.6 (Fig. 2b) has large cracks and an uneven surface, and ratio 1.0 (Fig. 2c) has a smooth surface without cracks. At high magnification of the central regions, shown in the insets, the morphology is very porous for the ratio 0.2 (Fig. 2a) and is apparently deficient of ionomer, while for the 0.6 ratio catalyst agglomerates are observed with evenly distributed ionomer in the

Fig. 1. Influence of water content in the electrode ink solvent on IR-corrected polarization curves (solid lines) and power density curves (dashed lines). The water fraction in the solvent mixture with water, iso-propanol and ionomer solution is given in vol%. The curves were obtained in O2 =H2 AEMFCs at 50
C, 95%:R.H., from OCP to 0.1 V at 1 mVs1 with 0.4 mgPt cm2 electrodes.

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Fig. 3. Porosity (left axis) and catalyst layer thickness(right axis) for electrodes with varying ionomer content.

Fig. 2. SEM images, overviews and micro-structures (insets) for ionomer-to-Pt/C weight ratios a) 0.2, b) 0.6, and c) 1. Ratio 1 has two insets, the left from the top of the electrode and the right a cross-section, to better show the ionomer film formed at the electrode surface.

porous electrode (Fig. 2b). For ratio 1.0 the supported catalyst particles are totally covered by a thick ionomer-film, top left inset (Fig. 2c). This thick ionomer film is even better visible in the top right inset of Fig. 2c showing the cross-section of the electrode and this film was not found for samples with lower ionomer content. The samples not shown here (ionomer-to-Pt/C weight ratio 0.4, 0.8 and 1.5) follow the same trend of increasing ionomer content as expected. From cross-sectional SEM images the catalyst layer thicknesses were measured and used in equation (1) to determine the porosity, see Fig. 3. A decrease in porosity from approximately 75%e20% with increasing ionomer content was obtained, see Fig. 3.

The ionomer films formed on top of the electrodes when very high ionomer content (ratio 1 and 1.5) is used have been excluded from the porosity calculations. The cell voltage versus logarithmic respectively linear current density for varying ionomer-to-Pt/C weight ratios are presented in Fig. 4. The highest performance is obtained for ionomer-to-Pt/C weight ratios 0.4e0.8 (approx. 29e44 wt% ionomer content) being equal at low current densities. At higher current densities the 0.8 ratio shows a slightly better performance. For the two high ionomer ratios, 1.0 and 1.5, the electrode performance drops more sharply at higher current densities, shown in Fig. 4a. A small decrease is also observed below 60 mA cm2, more clearly resolved in the logarithmic curve in Fig. 4b. The ionomer-to-Pt/C weight ratio 0.2 has the lowest performance, both at low and high current densities. Further, the polarization curve for 0.2 ionomer-to-Pt/C weight ratio has no bend in the curve between 0.6 and 0.4 V that is found on the other curves and is often observed for AEMFC. To further explore the low current density properties for the different ionomer-to-Pt/C weight ratios, impedance spectra were recorded at 16 mA cm2. At this current density the effect of hydrogen crossover, mass transport and ohmic losses are expected to be minimal. The Nyquist plots in Fig. 4c show two semicircles for all compositions, as previously reported for AEMFCs by Zeng et al. [32,33] where they also showed that the frequency response of the anode and the cathode overlap and cannot be separated without the use of a reference electrode in the cell. Here the impedance is therefore discussed on a full cell basis. The right semicircle at medium to low frequency, dominates the spectra for ratios 0.4e1.5 and the size increases with ionomer content. The left semicircle, at higher frequencies, is smaller and similar for all ratios. However, for ratio 0.2 this semicircle is much larger and the maximum is around a frequency of 600 Hz, placing it in the high to medium frequency region. The right semicircle has a maximum at 15 Hz, which is a large shift compared to the other compositions' at 1 Hz. The lowest recorded high frequency resistance (HFR) of 131 mUcm2, is obtained for ratio 0.4 and the resistance increases linearly with higher ionomer content as seen more clearly in the inset of Fig. 4c. The differences in activity at low current density observed for 0.2 and to some degree 1.0e1.5 ratio in Fig. 4 suggests that the catalyst utilization is influenced by the ionomer content. Therefore, cyclic voltammetry (CV) was used to observe the changes in electrochemically active surface area (ECSA). In Fig. 5 it can be seen that the peak areas in the hydrogen adsorption and desorption region (0e0.3 V) and the corresponding surface oxide region (0.5e1.25 V), clearly vary for different ionomer contents. The 0.6 wt ratio has the most pronounced peaks and thereby a larger ECSA. For higher and

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Fig. 5. Cyclic voltammograms of electrodes with varying ionomer-to-Pt/ratio. The CV curves were obtained in N2 / 5% H2 in Ar from 0.05 to 1.2 V using a 20 mVs1 sweep rate with 0.4 mgPt cm2 and have been corrected for H2 partial pressure.

increases with ionomer content up to 0.6 wt ratio after which it seems to reach a steady value. Notably the 0.2 ionomer-to-Pt/C weight ratio CV has very small peaks and double layer capacitance, showing poor ionomer coverage and/or that the ionic conductivity is insufficient, and that most of the catalyst in the catalytic layer (CL) therefore is inaccessible for reactions.

3.2. Effect of Pt loading in the catalyst layer It is well known that in PEMFCs the cathode contributes the most to the polarization losses, due to sluggish kinetics of the oxygen reduction reaction compared to the hydrogen oxidation reaction at the anode. In AEMFC it is less clear which electrode contributes more, as discussed in the introduction. In order to obtain a better understanding to what extent each electrode influences the cell performance, the catalyst loading was varied between 0.1, 0.4, and 0.8 mgPt cm2 at the anode and cathode, respectively. For all experiments the opposite electrode had a loading of 0.4 mgPt cm2. To obtain the different loading, without changing the ink composition (as this also would have affected the electrode performance, see Fig. 1), the electrodes were made thicker or thinner. The SEM micrograph, Fig. 6, shows a crosssection of an electrode with 0.8 mgPt cm2 where three distinct layers can be observed; from the left is the carbon paper backing, followed by the microporous layer (MPL) and to the right the catalyst layer. The layer distinction was made based on EDS line scans, not shown here, that also showed that Pt was located solely in the catalyst layer and had not passed into the MPL. From the

Fig. 4. Results for different ionomer-to-Pt/C weight ratio, a) IR-corrected polarization curves, b) logarithmic plots and c) EIS Nyquist plot from 100 kHz to 20 mHz at 16 mA cm2 (Inset: High frequency intercepts). The curves were obtained in O2/H2 AEMFCs at 50
C, 95%:R.H., from OCP to 0.1 V at 1 mVs1 with 0.4 mgPt cm2.

lower ionomer contents both the adsorption and desorption peaks in the hydrogen region are smaller indicating that the catalytic sites are only partially accessible for the reaction, as the loading is the same for all electrodes. Further, the double layer capacitance

Fig. 6. Cross-section of an as-prepared GDE with 0.8 mgPt cm2 loading. Backing refers to carbon paper backing, MPL stands for microporous layer and CL for catalyst layer.

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images of the as-prepared electrodes the carbon paper backing thickness was determined to 160 ± 2 mm and the MPL to 77 ± 2 mm for all electrodes. The thicknesses of the catalyst layers were approximately 7, 35, and 67 mm for a loading of 0.1, 0.4, and 0.8 mgPt cm2, respectively, showing that the increased loading resulted in an approximately proportional thickness increase. The corresponding CVs for the different loading, shown in Fig. 7, confirm that also the ECSA based on the peaks for Pt oxide reduction, double layer capacitance, and the peaks from the hydrogen adsorption/ desorption region varies in accordance with loading. The calculated ECSA, determined by integrating the hydrogen adsorption/ desorption peaks, were approximately 12, 80, and 200 cm2 for the three electrodes and the double layer capacitance current densities were approximately 0.31, 1.4 and 3.4 mA cm2. The values are roughly proportional to the observed thicknesses. In Fig. 8 polarization curves on a normal and logarithmic scale for asymmetric cells with different loading at the anode (Fig. 8a,c) and the cathode (Fig. 8b,d) are presented. In the low current density region below 60 mA cm2, no significant differences are observed when varying the electrode loading at the anode, Fig. 8a). In contrast, if the cathode loading is decreased to 0.1 mgPt cm2 a significant loss in activity is observed in the same current density range, Fig. 8b). The activity at 0.95 V for 0.1 mgPt cm2 at the cathode is approximately one forth compared to that for 0.4 mgPt cm2 (see Table 2), which corresponds well with the decrease seen in ECSA and catalyst layer thickness. The same decrease is seen when a low loading of 0.1 mgPt cm2 is used on both electrodes, again showing little influence of the anode on the current density in this region. However, for the 0.8 mgPt cm2 cathode loading, only a small activity increase close to the OCP is observed, otherwise the activity is quite similar to that of the 0.4 loading electrode. As seen in the polarization curves for the anode (Fig. 8c) and the cathode (Fig. 8d), the influence of catalyst loading of the two electrodes on the performance differs for higher current densities. Best performance is observed for 0.4 mgPt cm2 on both electrodes, which is also clear from the peak power densities presented in Table 2. Decreasing the loading either at the cathode or the anode lowers the performance, but most significant is the loss for the low anode loading, see values in Table 2. In fact, low loading at the anode and high at the cathode is worse than low loading at both electrodes. Furthermore, increasing the loading at either electrode from 0.4

mgPt cm2 to 0.8 mgPt cm2 does not result in an increase as expected, instead it lowers the performance. On the anode side the performance is unaffected at low current densities, however, the performance losses increase with increased current density. Calculated volumetric current densities are presented in Table 2. They have been calculated based on the volume of both electrodes, equation (2), as the results above indicate that none of the electrodes contribution to performance are negligible. For 0.8 mgPt cm2 loading at either electrode the volumetric current density is 30% lower compared to that of 0.4 mgPt cm2 at 0.7 V, indicating that the added thickness does not contribute to the electrochemical activity. The cell with 0.1 mgPt cm2 loading at both electrodes instead shows a higher value, indicating that these electrodes are utilized to a larger degree. 3.3. Effect of relative humidity in inlet gases The effect of humidity in the cell was investigated by decreasing the relative humidity of the inlet gases to 70% either at the anode or the cathode. As seen in the logarithmic curves in Fig. 9a, the changes in relative humidity do not affect the activity in the low current density region. However, at higher current densities some performance losses are observed, see Fig. 9b. The strongest effect of lower R.H. in the inlet gas is seen at the cathode, as expected, since water is a reactant at this electrode. However, at the anode lowered humidity also results in a performance loss, although, at very high current densities a small performance increase is observed. Note that the loss in performance for intermediate potentials, 0.85e0.4 V, in Fig. 9b, is not related to an IR-drop across the cell as the polarization curves are IR-corrected. The Nyquist plots taken at 16 mA cm2, presented in Fig. 9c, show that independently of which electrode, lowered humidity results in an enlargement of the high frequency semicircle (between 100 kHz and 100 Hz) shifting the right semicircle further to the right. The size of the semicircle at lower frequencies is apparently not affected by the relative humidity. The minor effect of humidity on the high frequency intercept shows that the membrane resistance is not significantly increased at least not at this current density. 3.4. Effect of PtRu catalyst at the anode Finally, the effect of PtRu catalyst at the anode was investigated. As seen in Fig. 10, exchanging the Pt catalyst with PtRu at the anode (with a slightly different loading see Table 1) does not lead to significant improvements of the performance, however, the mass activity is improved as a lower amount of Pt was used. This is most noticeable at higher current densities where previously a lowering of the anode loading showed a significant loss in cell performance (Fig. 8). 4. Discussion

Fig. 7. Cyclic voltammograms of electrodes with different thicknesses. The CV curves were obtained in Ar/ 5% H2 in Ar from 0.05 to 1.2 V using a 20 mVs1 sweep rate and have been corrected for H2 partial pressure.

The results of this study show that the structure and composition of the electrodes are very important for the performance of AEMFC. Similar to PEMFC [20e22], the electrode porosity and distribution of ionomer must be optimized in order to achieve high catalyst utilization. This study shows that the morphology can be partly controlled by the solvent composition, i.e. the iso-propanol and water ratio in the ink. Having 40 vol% water in the ink solvent promotes an even distribution of ionomer and catalyst and gave the best performance and highest power density in this study. However, it is likely that the optimal solvent ratio will vary with the ink application method and the specific properties of the ionomer. A large variation of solvents are used in the literature [24e27,38] and as application method and operating conditions also vary,

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Fig. 8. Effect of catalyst loading a), c) at the anode, and b), d) at the cathode shown as polarization curves on linear and logarithmic scale. The polarization curves are IR-corrected and were obtained in O2/H2 AEMFCs at 50
C, 95%:R.H., from OCP to 0.1 V at 1 mVs1 with 0.1, 0.4 and 0.8 mgPt cm2 electrodes.

Table 2 Current density values at specific potentials, volumetric current densities calculated based on the total volume of both anode and cathode and the peak power densities for the curves shown in Fig. 8 Loading An/Cat mgPt cm2

i at 0.95 V mA cm2

i at 0.7 V mA cm2

ivol at 0.95 V mA cm3

ivol at 0.7 V mA cm3

Peak power density mW cm2

0.4/0.4 0.4/0.8 0.4/0.1 0.1/0.1 0.1/0.4 0.8/0.4

8.2 9.7 2.6 2.8 7.2 5.9

214.9 199.4 146.2 120.2 119.0 207.7

1.2 1.0 0.6 2.0 1.7 0.6

30.7 19.5 34.8 85.9 28.3 20.4

177 158 140 114 98 160

comparison between studies is difficult. However, in other studies using Tokuyama materials, inks using only water as solvent [25,26] seem to show lower performance than those with 50% iso-propanol [24,27], which is in line with our results, see Fig. 1. The method of pipetting on the GDL may cause additional effects due to uneven drying, but for a stable ink these effects should not significantly influence the results. Here the method was chosen for the high control of loading, and so the active catalyst area, resulting in good reproducibility of electrochemical performance. The optimal ionomer-to-Pt/C ratio was shown to be between 0.4 and 0.8 when the catalyst ink is pipetted on the GDL. The performance obtained is similar to those presented by Fukuta [10], Reshetenko et al. [43] and Yang et al. [23] who used the same components in the MEA. If the 0.8 ionomer-to-Pt/C weight ratio is exceeded, the low current density activity gradually decreases, as shown in the polarization curves in Fig. 4b and in the increased diameter of the low frequency semicircle shown in the Nyquist plot

in Fig. 4c. The increase of the low frequency semicircle can be due to local diffusion limitations in the ionomer covering the catalyst, insufficient gas phase diffusion across the electrode thickness due to low porosity and pores being blocked by ionomer, uneven current distribution due to insufficient ionic conductivity, or a combination of these [19]. As the structure of our electrodes with high ionomer content shows very low porosity, as seen from the SEM images and porosity estimation shown in Figs. 2 and 3, the limited performance in this case is probably related to concentration gradients in the gas phase already at low current densities. This suggests that the high ionomer content hinders oxygen and hydrogen transport through the electrodes. On the other hand, lowering the ionomer-to-Pt/C ratio from 0.4 to 0.2 leads to low catalyst utilization due to poor ionomer coverage, possibly combined with low hydroxide conductivity. This is evident when comparing the results for low ionomer content, Figs. 5 and 4b, to the 0.1 mgPt cm2 symmetrically low loading cell,

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Fig. 9. Effect of varying relative humidity on the anode and cathode respectively, a) IRcorrected logarithmic plots, b) IR-corrected polarization curves and power density curves and c) EIS Nyquist plot from 100 kHz to 20 mHz at 16 mA cm2. The curves were obtained in O2/H2 AEMFCs at 50
C, 70 and 95%:R.H., from OCP to 0.1 V at 1 mVs1 with 0.4 mgPt cm2 electrodes.

Figs. 7 and 8b, as they show similar activity at low current density in the logarithmic curves and roughly the same ECSA in the hydrogen adsorption region. Further, the calculated porosity indicate a highly porous catalyst layer, see Fig. 3. Regarding the catalyst utilization discussed above, the hydrogen adsorption and desorption peaks in all the cyclic voltammograms, Figs. 5 and 7, are very small compared to similar loading in PEMFC electrodes and could be interpreted as low ionomer coverage. However, comparing the current densities in double layer capacitance region at around 0.4 V, with corresponding PEMFC MEAs [44,45], they are of similar magnitude. This similarity suggests that

the ionomer covers the supported catalyst to the same extent in both types of cells, but the hydrogen peaks are still smaller in AEMFC. That the ionomer coverage of the catalyst is sufficient is further confirmed by the fact that there is no increase of the double layer capacitance above 0.6 ionomer-to-Pt/C weight ratio, Fig. 5. Further, hydrogen peaks have been observed to change in size after extensive electrochemical cycling on a rotating disk electrode with an ionomer film in KOH solution [46]. This indicates that the hydrogen peaks are not a valid measurement of real Pt-ECSA in AEMFC, at least not after activation and electrochemical measurements. The reasons for this could be that sites are not accessible for hydrogen adsorption or that hydrogen binds differently in AEMFC, which needs further investigation. However, looking at relative changes within a group of measurements still seems to give a qualitative idea of how the ECSA will vary. To gain a better understanding of cell performance losses related to the anode and cathode, the results from electrodes with different loading are of interest, Fig. 8. Lowering the loading at the cathode from 0.4 mgPt cm2 to 0.1 mgPt cm2 resulted in a proportional decrease of the current density around 0.95 V, Table 2, while no change was observed with similar lowered anode loading at the same voltage. This has in a previous study [35] been interpreted as cell performance limitations being dependent on the cathode. However, looking at the experimental results at higher current densities, above 60 mA cm2, our results do not support that cell performance is controlled purely by limitations at the cathode. As was shown by SEM analysis the electrode thickness is proportional to the loading. Thus, the effects of increased loading need to be considered in combination with the possible effects of limitations in gas and hydroxide ion transport across progressively thicker electrodes, as it could result in a non-uniform current distribution. This would explain why higher loading at either electrode does not increase the cell performance, as the additional amount of catalyst cannot be utilized. Non-uniform current distribution was also recently identified as a source for performance limitations in AEMFC through modeling work [47]. In fact, especially with high loading at the cathode, the longer transport distances in the electrode could result in transport limitations in the pores and explain the lower performance. Further, the increased performance when low loading is used at both electrodes, compared to only at the anode, suggests that thinner electrodes lead to improved mass transport. A higher utilization of the low loading electrodes can also be seen by the higher volumetric current densities for these electrodes, see Table 2. The influence of the anode on cell performance is less clear. However, the difference in impact of the anode loading with current density could also be explained by a non-uniform current distribution, as the whole electrode is not utilized at low currents. Even at 0.1 mgPt cm2 loading the current distribution could be so non-uniform that further increase of the loading has no effect on the cell performance at current densities below 60 mA cm2. The importance of higher loading at higher current densities would then suggest that a larger portion of the anode is utilized at these operating conditions. The increased utilization could possibly be related to local transport limitations. Investigating the effect of inlet gas humidity lowered to 70%:R.H. at either electrode showed a surprisingly low influence with only a slight decrease in performance, and mostly at intermediate current densities, Fig. 9. The observed behavior does not clearly correspond to either cathode dry-out or anode flooding, which has been seen previously in AEMFC [6,35]. Instead it should be considered that changes in humidity affects the ionomer conductivity in the electrodes, which can explain why no effect is seen at higher water production rate, Fig. 9b, as the ionomer is fully saturated. Further, the similarity between the Nyquist representations of the EIS data

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159

Fig. 10. Effect of PtRu catalyst at the anode, a) IR-corrected polarization curves and b) logarithmic plots were obtained in O2/H2 AEMFCs at 50
C, 95%:R.H., from OCP to 0.1 V at 1 mVs1 with 0.4 mgPt cm2 electrodes or 0:33=0:25 mgPt=Ru cm2.

when the humidity was decreased at the anode or the cathode, indicates that lowered humidity at either electrode influences the performance in the same way. This suggests relatively rapid water transport across the cell and that the increase in the low frequency intercept is related to slower hydroxide transport, which would affect the current distribution in the electrodes. The above results indicate that both electrodes influence the cell performance significantly. This is supported by Sheng et al. [34] who showed that, compared to acidic environment, the activation over-potential for the hydrogen oxidation reaction is no longer negligible. They also showed that depending on loading and current density it can approach the oxygen reduction reaction in magnitude [34]. The fact that both electrodes are considered to influence the cell performance is strengthened by previous studies showing mixed results between anode and cathode limitations [16,32,33,37,46]. The performance of the electrodes is affected by the electrode morphology. In our study the electrodes are quite porous, as they are thick with relatively low amounts of solid material. This means that the volume fraction of ionomer in the electrodes is low. A low amount of ionomer can lead to limited hydroxide conductivity and poor utilization of the electrodes. The high porosity can also explain the need for the slightly higher ionomer fraction in our study compared to others [10,23,31]. From the above discussed results it is clear that the morphology of the electrodes plays an important role in AEMFC and may cause non-uniform current distribution. It is therefore essential to obtain a balance between porosity and a homogeneous distribution of ionomer, ensuring high catalyst utilization and gas permeability, as well as, good hydroxide ion and electrical conductivity. Ionomer properties, such as water uptake, gas diffusion and interaction with the catalyst materials have been shown to influence the results [38,48], and should be considered. The preparation and material effects still need further clarification to optimize each of the two electrodes and improve the catalyst utilization. Studies are also needed for a deeper understanding of the influence of the separate electrodes on the cell performance.

shown that the highest performance is obtained with 40 vol% water content in the ink solvent when using Tokuyama AS-4 ionomer. Further, when comparing the influence of ionomer content in the catalyst layer, i.e. the ionomer-to-Pt/C weight ratio, a stable performance is observed for an interval between 0.4 and 0.8 with a slight optimum at 0.6 based on CV, polarization curves and EIS measurements. Higher ionomer content results in larger cell resistance and mass transport limitations due to reduced access to the catalytic sites. Lower ionomer content restricts the hydroxide ion conduction as the contact between the polymer electrolyte and the catalyst is reduced. It is also shown that the ionomer content to a large degree affects the electrode morphology. The above conclusions are limited to the Tokuyama AS-4 ionomer and might change when other materials are used. The catalyst loading and layer thickness were varied simultaneously with 0.1, 0.4 and 0.8 mgPt cm2 at the anode or cathode while the opposite electrode was kept constant. The results show that both electrodes influence the overall cell performance. At low current density, below 60 mA cm2, the cathode side of the cell has the highest influence on performance. However, changes in loading at the anode show an increasing effect with higher current densities. As the thickness of the electrodes varies in accordance with the loading, it influences the results and the effects seen cannot be directly related to amount of catalytic sites. The authors suggest that non-uniform current distribution can explain the difference observed for the two electrodes at low and high current densities. The non-uniform current distribution can be caused by limitations in hydroxide ion conductivity and concentration gradients of water and gases throughout the electrodes, and will change with thickness. Further, the effect of humidity on performance, investigated by decreasing the relative humidity from 95 to 70%, is comparably small, and is assumed to be related to lowered ionomer conductivity. Further studies, should be performed to clarify the influence of electrode kinetics and transport gradients in the cell on the performance losses and also use this to improve electrode morphology.

5. Conclusions This study has investigated cell performance losses with regard to electrode preparation, ionomer content, loading and catalyst layer thickness, relative humidity and PtRu as a catalyst at the anode. The results show that the structure and performance of GDEs are dependent on solvent composition in the ink used during preparation. By varying the water content in the ink consisting of water, iso-propanol, ionomer solution and supported catalyst it is

Acknowledgments This work was funded by the Swedish Vehicle Research and Innovation program supported by the Swedish Energy Agency, the governmental initiative StandUp for Energy and the IRES program through funding from the National Science Foundation under Award NSF-OISE-1358179.

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