Interface contribution to the electrode performance of proton exchange membrane fuel cells – Impact of the ionomer

Interface contribution to the electrode performance of proton exchange membrane fuel cells – Impact of the ionomer

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Interface contribution to the electrode performance of proton exchange membrane fuel cells e Impact of the ionomer Shuang Ma Andersen a,*, Laila Grahl-Madsen b a

Department of Chemical Engineering, Biotechnology and Environmental Technology, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark b IRD Fuel Cells A/S, Kullinggade 31, DK-5700 Svendborg, Denmark

article info

abstract

Article history:

The commercialization of proton exchange membrane fuel cells (PEMFCs) is closer to the

Received 28 August 2015

reality than ever before. Electrode interface development can bring a boost to the last few

Received in revised form

steps. Here, we explore electrode properties from its interface structure, especially the

9 November 2015

ionomer phase. Electrodes containing identical catalyst but various ionomer loading (0, 10,

Accepted 19 November 2015

20, 30, 40 and 50 wt.%) were prepared. An optimal value of electrode performance, stability

Available online 29 December 2015

and platinum dissolution was observed respectively for the electrode containing around 30 wt.% ionomer. The platinum particle increment monotonically decreased with the in-

Keywords:

crease of the ionomer. The electrode surface studies surprisingly reveal that the ionomer

Nafion ionomer

coverage increases linearly with the ionomer loading/content only up to a certain extent

Catalyst

(around 30 wt.% in this case); further increase of the ionomer content triggers ionomer

Interface

agglomeration, which leaves lower coverage of the catalyst. The electrode structure is

Durability

directly related to the electrode performance.

Optimization

Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Low temperature proton exchange membrane fuel cell (LTPEMFC) is one of the most promising future energy conversion devices for both stationary and portable energy consumers. Nonetheless, LT-PEMFC still suffers from high capital cost, slow oxygen reduction reaction and durability issues, which slowdown its progress towards the market. Massive efforts have been devoted into exploration of alternative materials, especially to replace noble metal catalyst [1e3], enhancement of reaction kinetics [4e6] and improvement of cell stability

[7e9]. Moreover, recent studies [10e13] have demonstrated that electrode structure and the resulting three-phase boundary microstructure have a great impact on the electrode performance. Though many advances have been achieved for nonprecious metal (NPM) catalyst for PEMFC, such as iron or cobalt -based nanostructures on nitrogen functionalised mesoporous carbons [14,15], platinum based catalysts are still under extensive investigation and showing state of the art performance. However, for PEMFC to reach ultimate commercialization focusing solely on the catalytic activity is not sufficient; therefore, a lot of work is devoted to improve

* Corresponding author. Tel.: þ45 6550 9186. E-mail address: [email protected] (S.M. Andersen). http://dx.doi.org/10.1016/j.ijhydene.2015.11.101 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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the catalyst stability and reduce costs. Based on independent studies from various research groups [16e18], the degradation mechanisms of the platinum nanoparticle utilized in PEMFCs can be generally categorized into the following 5 aspects (1) platinum dissolution: Pt is oxidized and converted into soluble specious; (2) Ostwald ripening: dissolved Pt species from smaller particles redeposit onto the surfaces of larger particles; (3) coalescence: Pt nanoparticles migrate on the surface of a support and coalesce; (4) detachment: Pt nanoparticles detach from the carbon support and (5) support corrosion: the catalyst support is oxidized and isolates from the catalyst. Some of the degradation mechanisms are more directly related to the intrinsic properties of the catalyst [19] (crystallinity, nano-architecture, degree of alloy, etc.), or support [7,20] (amorphous, nano-carbon or ceramic and their surface properties such as porosity, roughness, etc.) or the interaction between the catalyst and the support [21,22]. Nevertheless, all degradation mechanisms are either directly or indirectly associated with the surrounding transport media [10]. In a fuel cell electrode structure, the impregnated ionomer is a direct physical barrier for the catalyst coalescence and detachment. Pt dissolution, Ostwald ripening and support corrosion all involve chemical reaction and equilibrium. The reaction kinetic and equilibrium are largely influenced by the transport media e properties of the proton conductive ionomer, such as its distribution, hydration, crystallinity, interaction with other components etc. Therefore, systematic study of the ionomer phase in the electrode structure can be highly valuable, as experienced by others [23e26] as well. In state-of-the-art LT-PEMFCs, protonic conductive ionomer is a vital component in the electrode to maximize reaction region. A normal fuel cell electrode contents 20e50% ionomer by weight [27e29], which corresponds 30e60% by volume depending on hydration. This indicates that the ionomer phase contributes significantly in the electrode structure, and it is influential to the catalyst accessibility and performance. Study of the ionomer content in an electrode is not a new subject. Component optimization is almost a standard engineering procedure for any new catalyst to be utilized in a fuel cell. Typically an optimal content is selected based on the peak performance in a single cell [30,31]. Besides, the ionomer content is also an active topic within simulation studies [32,33]. Electrode structure directed analysis and understanding of the electrode performance are important subjects of the field. Though, one of the biggest challenges of fuel cell experiments is variation of several parameters at the same time. Degradation of the cell performance can be due to anode, cathode or various issues occurred in the membrane (if we only limit the discussion within membrane electrode assembly). In this sense, ex-situ accelerated stress test has the advantage of varying one parameter at a time for systematic studies, since the counter electrode, the reference electrode and the electrolyte in the system are stable elements. Besides, fuel cell testing is costly and time consuming. In this work, catalyst ionomer electrodes following a standard electrode preparation recipe with various ionomer contents were prepared. An electrochemical accelerated stress test (AST) as a simulated start and stop cycle [34], focusing on a combined degradation of both catalyst and

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support, was carried out on the electrodes. Such events in real life, when not well managed, are known to be the most harmful regarding the stability of the PEMFC components [35], since formation of an air/fuel boundary at the anode will significantly increase the local electrode potential at the cathode to a value as high as 1.6 V [34]. The electrode property investigation was carried out in an ex-situ three-electrode electrochemical cell, which has the advantage of studying one parameter at a time, rather than in-situ test, which combines several variations at the same time. Besides, fuel cell test is time consuming and costly. The platinum electrochemical surface area, stability, dissolution, catalyst nano particle enlargement and the electrode surface chemistry were systematically studied.

Experimental Catalyst powder from Johnson Matthey, Hispec 9100, was used as received. The catalyst is 57 wt.% Pt black with a diameter around 3e4 nm supported on high surface area carbon. The catalyst ionomer electrode is a composite of the catalyst powder and the Nafion ionomer (Ion Power, Inc) of different percent. They were homogeneously mixed in an aqueous solution of water and ethanol mixture (1:3) with an ultrasonic horn for 10e15 min. Then, the ink was spray printed on a gas diffusion layer (SIGRACET®) placed on a heating table of temperature 60  C. The printed electrodes were used as the working electrode in the electrochemical characterization and degradation. In this work, content of the ionomer in the electrode was studied. A detailed sample list is summarized in Table 1. The electrochemistry was performed in a traditional threeelectrode electrochemical cell. The electrodes were subjected to the potential cycling between 0.4 and 1.6 V vs. RHE for 2500 cycles with sweep rate of 1 V/s in 1 M H2SO4 at room temperature. A carbon rod was used as the counter electrode and a radiometer® Hg/Hg2SO4 was used as the reference electrode. Ar purging was maintained during the measurements with a constant flow of 0.2 mL/s. The experiments were carried out with an electrochemical workstation (Zahner® IM6e). The connection between the sample and the device was established with a 0.2 mm thick gold wire. The powder X-ray diffraction (XRD) pattern was collected by the use of a Panalytical X'Pert diffractometer. Data treatment was assisted by X'Pert HighScope Plus. Dissolved platinum in the aqueous electrolyte at the end of the AST was analysed with a Varian® Atomic Adsorption Spectrotrometer

Table 1 e Sample specification. Catalyst ID Hispec Hispec Hispec Hispec Hispec Hispec

9100 9100 9100 9100 9100 9100

Ionomer loading (wt.% to the catalyst loading)

Pt loading (mg/cm2)

0.0 9.4 19.2 28.7 38.5 48.4

0.663 0.486 0.460 0.464 0.512 0.400

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(AAS) equipped with a graphite furnace providing high sensitivity. X-ray photoelectron spectroscopy (XPS) analysis was performed using a SPECS® system with Mg Ka (1253.6 eV) as the X-ray source. The signal was collected according to an optimized condition developed earlier [36]. All binding energies were calibrated with respect to C 1s: CeC peak at 284.5 eV. The data was analysed using CasaXPS™ and presented with Origin® Pro 9.1. Contact angle was carried out by placing a 20 mL Milli-Q® (Millipore, MA USA) water droplet on the sample surface and a snapshot was taken with an Olympus E3 camera equipped with a 55 mm lens on a bellows unit. Images analysis was assisted by software MB-Ruler.

Results Electrochemical characterization Typical cyclic voltammograms with characteristic hydrogen adsorption/desorption, platinum oxidation and reduction were observed for all electrodes of different ionomer contents. A typical series of AST voltammograms is shown in supplementary information, Fig. S1. The specific electrochemical surface area (ECSA) was determined from the charge transfer of hydrogen adsorption and normalized to the platinum loading. A gradual increase of the ECSA within the first 100 cycles was typically observed, which is related to a selfcleaning process or surface structure optimization. Max specific ECSA is the maximum ECSA value observed in the initial performance. The electrode stability is evaluated as the percentage change of the ending ECSA (after 2500 cycles according to AST) towards the max ECSA. The values of the max specific ECSA, utilization (the catalyst particle is estimated with 4 nm in diameter) and durability of the various electrodes are summarized in Fig. 1(A). Volcano trends were observed for both initial performance and durability with increasing amount of ionomer content in the electrode structure. This indicates a certain optimized electrode structure for the electrode containing 30 wt.% ionomer, which leads to both high performance and good durability.

Platinum dissolution The acid solution as the electrolyte in the accelerated stress test was analysed using atomic adsorption spectroscopy (AAS). The platinum dissolution amount as percentage of the original loading is presented in Fig. 1(B). A clear inverse volcano shape shows that the electrode containing around 30 wt.% ionomer has the lowest Pt dissolution. Gradually higher dissolution amounts were observed for the electrodes with both lower and higher ionomer content. When the Pt dissolution amount is normalized to the electrochemical surface area, the same trend retains. Moreover, larger differences were observed: the highest dissolution value is close to 4 times that of the lowest value among the electrodes studied in this work. The inverse volcano shape corresponds well with the durability performance in the electrochemical characterization. This also indicates that physical detachment of the catalyst particle is not severe.

XRD The various catalyst ionomer electrodes were studied with Xray diffraction before and after the electrochemical AST. Characteristic Pt diffraction was detected at around 2q ¼ 39.6 , 45.9 , 67.6 , 81.70 and 86.8 , which correspond to Pt (111), Pt (200), Pt (220), Pt (311) and Pt (222). Typical XRD patterns are shown in supplementary information, Fig. S2. Pt crystallite size was evaluated from the peak broadening of Pt (111) based on the Scherrer formula. The particle size and the relative change are summarized in Fig. 1(C). In general, platinum nanoparticle size was found to be increased after the AST. Moreover, a fair trend shows that the particle size increment decreases with increasing ionomer content in the electrode. However, we need to point out that the fundamental observation mechanism and application range between XRD and electron microscopies (such as SEM or TEM) are different [37,38]. The Pt crystallite size as determined by using XRD may be influenced by aggregation of the minor particles, contour, porosity or micro-strain, which may require further justification by other studies; though a clear trend is documented in this work.

Contact angle measurements The electrode was briefly rinsed with distilled water and surface dried with fine paper napkins to remove excess liquid. Nice truncated water spheres are readily formed on the electrode surface. The contact angle was measured by fitting a tangent to the three-phase point, where liquid surface touches the solid surface. The results are summarized in Fig. 1(D). The pristine electrodes with various ionomer content show significantly different wetting property. A maximum contact angle of around 126 was observed for the electrode of around 30 wt.% ionomer, indicating a hydrophobic surface. The electrodes containing around 10, 20 and 40 wt.% ionomer appear slightly less hydrophobic, with contact angles above 105 . The electrode without ionomer shows hydrophilic nature with a contact angle of around 70 . The electrode of the highest ionomer content of around 50 wt.% has a contact angle just below 90 . Due to its natural oxide content, hydrophilic metal surface is well known. Besides, amorphous carbon has a rich content of carbon oxides. Those properties render the pure catalyst electrode a good hydrophilicity. With increasing content of the ionomer, due to a strong interaction between the ionomer and the catalyst [39,40], the ionomer starts to form a thin film over the catalyst. Though the polymer contains both hydrophobic backbones and hydrophilic side chains, the hydrophilic groups primarily interact/adsorb with the hydrophilic catalysts, and leave the hydrophobic backbones as an outer shell, which is observed by various groups [41e43]. This leads to a gradual increase of the contact angle with increasing amount of the ionomer up to around 30 wt.%. However, with further increase of the ionomer, the contact angle was found to drop, which will be discussed later. After the potential cycling, all electrodes were found to have lower contact angle, especially for the electrodes of low ionomer content (30 wt.%). This might indicate degradation,

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Fig. 1 e (A) Initial performance and stability of the electrodes in relation to the ionomer content as evaluated with cyclicvoltametry and accelerated stress test. (B) Percentage of platinum dissolution after the AST in relation to the electrode ionomer content in the electrode as evaluated by the atomic adsorption spectroscopy. (C) Platinum nanoparticle size before and after the AST in relation to the electrode ionomer content as evaluated with X-ray diffraction. (D) Water droplet contact angle dependence of the electrode ionomer content.

dissolution or restructuration of the ionomer phase. The contact angles of the electrodes with high ionomer content appear relatively less sensitive to the treatment. Understanding of the observation requires additional experiments. The explanation will be provided in combination with other measurements.

XPS surface analysis X-ray photoelectron spectroscopy (XPS) was directly conducted on the catalyst ionomer electrodes without further treatment. During the XPS experiment, most electrodes (both before and after the AST) experience no effect of charging; except the pristine electrode containing around 50 wt.% ionomer, which showed a shift of around 0.3 eV towards high binding energy (BE). This indicates slightly poorer electron conductivity of the electrode than the others. Surface element compositions based on the survey spectra of the electrodes before and after the AST are shown in Fig. 2. Examples of the survey spectra are available in supplementary information, Fig. S3. Major elements detected on the electrode surface are carbon, fluorine (for ionomer containing samples), oxygen and platinum, as expected. Before the AST, with increasing amount of the ionomer, the surface carbon decreases until

around 30 wt.%; surprisingly, further increase of the ionomer leads to a gradual increase of the surface carbon. Similar trend was also observed for the platinum. An opposite trend was observed for the fluorine. Surface oxygen content turns out to be relatively higher for ionomer containing electrode than the pure catalyst electrode. This is probably due to the rich oxygen content in the functional groups. After the AST, in general, surface carbon decreases with increase of the original loading of the ionomer. Moreover, there is less carbon detected on the surface than before the treatment, which is probably due to the carbon corrosion. This corresponds with higher oxygen detected on the electrode surface after the AST as well. For low ionomer contenting electrodes, higher amount platinum was detected on the surface after the AST, which is probably due to platinum migration and reduction of the soluble platinum species; for high ionomer containing electrodes, relatively little change was observed. Fluorine content was found to be rather stable for the low ionomer containing electrodes and to be increased for the high ionomer containing electrodes after the AST. Further studies were carried out on high resolution deconvolution spectra. Though XPS observes only the top a few nanometres of the sample, it represents the electrode interface structure. Slight stripping of the surface with a piece of Scottish tape was also carried out. Similar observation was

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Fig. 2 e XPS survey spectra analysis (A) surface element concentration in relation to the ionomer content in the electrode before and after the electrochemical AST.

retained, indicating comparable element content through the electrode.

C 1s Nice series of carbon 1s high resolution deconvolution spectra on the electrodes of different ionomer content before and after the electrochemical AST are presented in Fig. 3. The two dominating peaks appearing at binding energy (BE) around 284.5 and 291.2 eV are due to CeC (from the catalyst carbon

support) and CF2 (from the Nafion ionomer). Signal at BE around 285.6 eV is due to defective carbon bonding. Signals within BE region between 289.5 and 285.0 eV are due to carbon oxides or CO (e.g. CeO, C]O, OCO). Signals at BE around 289.0 and 293.4 are due to CF and CF3 respectively. Since the signal from CF2 indicates ionomer and the signal from CeC indicates catalyst surface appearance, the relative amount between CF2 and CeC may reflect surface composition and catalyst coverage by the ionomer. The ratio between

Fig. 3 e XPS C 1s spectra high resolution spectra (A) before and (B) after the AST in relation to the electrode ionomer content.

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Fig. 4 e The ratio between (A) CF2 and CeC and (B) CO and CeC in C 1s high resolution spectra before and after the AST in relation to the ionomer content.

CF2 and CeC before and after the electrochemical AST is summarized in Fig. 4(A). In the pristine electrodes, CF2/CeC is seen to increase with increasing content of the ionomer in the electrode until around 30 wt.%. Further increase of the ionomer does not contribute in more CF2 relatively to the CeC. A huge reduction of CF2/CeC was observed for the electrode containing 50 wt.% ionomer. After the stress test, the ratio between CF2 and CeC was found rather linearly related to the ionomer content. This might indicates that a more uniform distribution of the polymer chain on the surface after the treatment. The ratio between the carbon oxides (CO) and the graphitic carbon (CeC) is plotted in relation to the ionomer content, Fig. 4(B). In the pristine samples, a clear difference was observed for the pure catalyst electrode and ionomer containing electrodes. In the electrode without ionomer, the carbon oxides are due to the oxygen groups of the carbon surface. While, 2e3 times carbon oxides were observed in the catalyst ionomer electrodes, which are mainly due to the oxygen containing functional groups in the Nafion ionomer or possible interactions between the functional groups and the catalyst supports. In general, a gradual increase of the carbon oxides was observed with the increase of the ionomer content in the electrodes. After the stress test, the carbon oxides were found to be increased for the pure catalyst electrode (due to the carbon corrosion), but to be decrease for all ionomer containing electrodes (especially 50 wt.%). This might indicate degradation of the functional groups in the polymer structure due to the AST.

samples was found to increase up to 30 wt.% ionomer and then decrease with higher ionomer content; for the stressed samples, in general the ether was found to be increased, indicating higher presence on the surface. The surface carbon oxides concentration is shown in Fig. 6(B). In general, the oxides were found to decrease with the increase amount of the ionomer in the electrode. Larger amount of the carbon oxides was observed after the AST, which is probably related to the carbon corrosion. A higher degree of the oxides formation was observed for the electrode of no or low ionomer content, which might indicates that the ionomer prevents carbon from corrosion reaction during the AST. On the whole, the information from the XPS survey, C 1s and O1s, corresponds well with each other and provide deep insights to the surface structure of the electrodes.

Discussion Based on the ECSA, the stability, and the measured Pt dissolution, the results demonstrate that the electrode of around 30 wt.% ionomer is of the best performance among the electrodes of the identical catalyst. The various electrode performances and responds toward the accelerated stress test are largely affected by the electrode structure or the interface between the catalyst and the Nafion ionomer. Therefore, a close examination of the electrode structure is of great importance.

O 1s

Catalyst surface exposure (CSE)

Oxygen 1s high resolution deconvolution spectra on the electrodes are presented in Fig. 5. The contribution of the signal can be classified into following 3 categories: (1) platinum oxides (PtO) at around BE 531.2 eV, which is due to the platinum catalyst; (2) carbon oxides (C]O and CeO) at around BE 532.3 and 533.3 eV, respectively, which are due to simple bonding (single or double bond) between carbon and oxygen; (3) ether at around BE 534.6 eV, which is primarily due to the ether group in the polymer. The surface ether concentration is summarized in Fig. 6(A). Similar to CF2/CeC in C1s (Fig. 4(A)), the value for the pristine

Due to an interaction between the catalyst and the ionomer, an ionomer covered catalyst structure is formed in the electrode. Based on the surface concentration of the carbon and fluorine in the survey spectra, CF2/CeC in C 1s spectra and surface ether in O 1s spectra, the initial catalyst surface exposure (CSE) decreases with increasing amount of the ionomer content in the electrode until around 30 wt.%; further increase of the ionomer turned out to be less effective in covering the catalyst surface. This suggests that there might be segregation of catalyst and ionomer in the electrodes of

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Fig. 5 e Ether concentration in O 1s high resolution spectra (A) before and (B) after the AST in relation to the ionomer content.

Fig. 6 e Carbon oxides concentration in O 1s high resolution spectra before and after the AST in relation to the ionomer content.

high ionomer content. The reason for such segregation might be formation of a more thermodynamically stable interface, when ionomer aggregates or a certain micelle structures are established in the aqueous solution or during the drying

process. An over simplified sketch on the electrode structure in relation to the ionomer content is presented in Fig. 7. The catalyst surface exposure also corresponds well with the wetting property study (section Contact angle

Fig. 7 e Sketch on electrode interface structure evolution with increasing content of the ionomer.

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measurements). It explains the decline of the contact angle for the electrodes with high ionomer content (around 40 and 50 wt.%). Moreover, the ionomer aggregates may also the reason accounts for the charging issue in the XPS measurement. Therefore, the electrode with high ionomer content showing low performance is not only due to its poor electronic conductivity, as we earlier observed with Kelvin probe microscopy [44], but also likely be caused by a completely different interface structure and high surface presence of the catalyst. After the AST, the ionomer coverage on the catalyst was found to be more proportional to its initial loading, based on the XPS survey and the high resolution spectra, (though certain variation was still detected for the 50 wt.%). This implies that, the electrochemical stress treatment homogenizes the ionomer catalyst mixture by breaking down the ionomer aggregates; the catalysts in the electrode of high ionomer content turned out to be better covered with the ionomer after the AST.

The electrode performance as function as the electrode structure Based on the information from the ECSA, durability, platinum dissolution, there is a direct connection between the electrode performance and the electrode structure or the particular catalyst ionomer interface, as shown in Fig. 8. The surface carbon concentration determined with the XPS measurements is used as an estimation of the catalyst surface exposure (CSE). A fair relation is found between the CSE and max specific ECSA, Fig. 8(A). The enhanced active surface area is probably due to a better accessibility of the catalyst to the electrolyte via a surface modification of the catalyst from the ionomer catalyst interaction. Fair relations are also observed

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between the CSE and the electrochemical stability, Fig. 8(B). This can be attributed to a protection effect from the ionomer, since the ionomer surface coverage on the catalyst can effectively slow down the platinum catalyst from migration, coalescence, detachment, dissolution (see later) and carbon corrosion, as we observed before as well [11,45]. This leads to improved electrode durability. Depending on below or above the optimal ionomer content, the effectivity of the protection turns out to be different, which is probably closely related to the dissolution behaviour. In Fig. 8(C), a fine relation is observed between the CSE and the platinum dissolution below the optimal ionomer content. This clearly implies that the ionomer coverage prevents platinum from dissolution. Since identical catalyst was applied in the electrodes, their intrinsic ability to be dissolved should be comparable. However, due to the different interface structure, the local potential applied on the catalyst might be lower than the actual potential. Another possibility is that the reaction kinetic is limited by the transport media, which depends on the proton conductivity of the Nafion ionomer network in the electrode. This could be the bottleneck of the reaction. Furthermore, another reasonable relation is found between the CSE and the Pt dissolution for the electrodes of high ionomer content (around 40 and 50 wt.%). The slop of the linear fitting is significantly different from the previous dissolution fitting for electrodes of low ionomer content, which indicates different reaction mechanisms. This might be due to formation of the ionomer aggregates containing enhanced hydrophilic domains, which may promote transport of the soluble species and therefore enhance the platinum dissolution reactions.

Degradation of the electrode Substantial electrode degradations have happened during the electrochemical stress test. Based on the available evidences it can be categorized into platinum coalescence, migration, dissolution, detachment; carbon corrosion; ionomer degradation and electrode structure change. The focus of the electrode degradation is the ionomer, the interaction between the ionomer and the catalyst and the consequent overall electrode property change. Though the ionomer coverage on the catalyst of the electrode with high ionomer content (40 and 50 wt.%) seem to be improved (Figs. 2 and 4(A)) after the AST, due to the damage of the ionomer side chains and surface property change of the catalysts, the resulting electrode structure does not help in preserve the performance. The catalyst and the ionomer degradations will always affect and exacerbate each other in the electrodes. Platinum coalescence was found not affected by the ionomer interface structure (Fig. 1(C)). This might indicates that the ionomer aggregates are also effective in preventing the small Pt particles from merging.

Conclusions

Fig. 8 e The electrode performance in relation to the catalyst surface exposure (CSE).

Electrodes containing identical catalysts but various ionomer contents were prepared in the same manner and characterized with various measurements. The catalyst surface

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exposure (CSE) was found to be decreased with increasing amount of the ionomer up to around 30 wt.%, (due to a strong interaction between the ionomer and the catalyst forming an ionomer covered catalyst composite); further increase of the ionomer turns out to increase the CSE, which is probably due to formation of ionomer aggregates. The electrochemical performance and the component degradation are directly related to the electrode interface structure: the ECSA and the stability increase and the Pt dissolution decrease with the decrease of the CSE. A proper ionomer covered catalyst interface is an optimal electrode structure for the LT-PEMFCs. Interface study is a powerful tool for the electrode structure optimization.

Acknowledgements The authors appreciate the financial support from the Danish PSO through the project DuraPEM III (ForskEl J.No 2013-112064) and the project UpCat (ForskEl-projekt J.No 2015-112315), and from the Danish Council for Strategic Research, Innovationsfonden, through the 4M Centre (J.No 12-132710).

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.11.101.

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