Catalyst ageing and degradation in polymer electrolyte membrane fuel cells

7 Catalyst ageing and degradation in polymer electrolyte membrane fuel cells K. J. J. MAYRHOFER, MPI für Eisenforschung, Germany and M. ARENZ, University of Copenhagen, Denmark

Abstract: This chapter discusses performance degradation of low temperature fuel cell catalysts due to the influence of dynamic operation conditions. The aim is to highlight fundamental studies on the chemical degradation of the catalyst and summarize recent results. A short overview of catalyst ageing mechanisms is given, then techniques for their investigation are introduced. The chapter then focuses on the methodology and results of identical location transmission electron microscopy (IL-TEM), as originally developed in our laboratory. Key words: fuel cell catalyst degradation, identical location transmission electron microscopy (IL-TEM), high surface area carbon supported nanoparticles.

7.1

Introduction

A fuel cell directly converts chemical into electric energy. In contrast with combustion engines, the theoretical conversion efficiency of this process is not limited by the Carnot cycle and therefore much higher efficiency values can be obtained. Due to the high conversion efficiency, the low overall emission of pollutants, as well as the increasing constraints upon fossil fuels, the concept of electric powertrains utilizing a combination of fuel cells and batteries is considered a potential alternative to replace internal combustion engines in today’s vehicles. Because of its high power density the most suitable fuel cell for such an application is the polymer electrolyte membrane fuel cell (PEMFC), which uses hydrogen fuel and oxygen, e.g. from air. In the last decade substantial progress has been made on many technical aspects concerning a commercial large-scale deployment of PEMFCs; manufacture of fuel cell stacks, their implementation in vehicles, and even long-term field trials are nowadays standard practice. However, in order to render PEMFCs commercially viable for such demanding large-scale applications, two fundamental scientific problems persist: improvements of cathode electrocatalysts both in terms of activity and, perhaps more importantly, stability. The catalysts of choice for PEM fuel cells are Pt-based nanoparticles supported on high surface area carbons. In recent years the main research effort was in the improvement of catalytic activity for the oxygen reduction reaction (ORR) at the cathode. It is now demonstrated by a wealth of experimental and theoretical investigations, that the reaction rate for the ORR can be considerably increased by 178 © Woodhead Publishing Limited, 2012

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alloying platinum with a second transition metal, such as Co, Ni or Cu.1–10 With this success, the focus of future studies shifts towards the stability of these catalysts.9, 11 The gradual loss of performance of low temperature fuel cells is especially a critical issue for their commercialization in the very demanding application of powering electric vehicles, which need to sustain several thousand hours of operation. The reasons for performance degradation in fuel cells can be multiple, and range from impurities in the fuel to a degradation of the membrane electrolyte. Although many processes are interrelated to each other, three different categories are distinguished, i.e. mechanical, thermal and chemical degradation. As a complete coverage of all phenomena is beyond the scope of this chapter, we concentrate on performance degradation directly related to the influence of dynamic operation conditions on the chemical degradation of the catalyst. The aim is to highlight catalyst degradation from a fundamental point of view, which is one of the core research areas of the authors.

7.2

Catalyst ageing mechanism

It was shown in many studies concerning phosphoric acid (PA) and proton exchange membrane (PEM) fuel cells that catalyst degradation is one of the major reasons for gradual performance losses and that its cause can be mainly related to a loss in accessible surface area of the active catalyst components.12 In addition to this, in PEM fuel cells the degradation of the membrane electrolyte is a critical issue. Considering the influence of the catalyst, peroxide formed as a side product in the ORR and the dealloying of Pt-alloy catalysts due to the segregation and dissolution of non-noble components might have a continuous detrimental effect on the ion conductance of the membrane electrolyte.13–15 The mechanisms concerning the loss of electrochemical surface area (ECSA) for pure Pt nanoparticles supported on carbon are described below. The catalyst ageing of Pt-alloys by the dissolution of non-noble components is then discussed. The mechanisms responsible for the ECSA loss of Pt are, however, valid for Pt-alloy-based catalysts as well. For the loss in ECSA of Pt in PEM fuel cells there can be three fundamentally different mechanisms identified (see Fig. 7.1). These are Pt dissolution, the migration and concomitant coalescence of Pt nanoparticles on the support, and the detachment of Pt nanoparticles from the support.16, 17 Pt dissolution can be further sub-divided into two processes, resulting either in Pt re-deposition onto larger particles (electrochemical Ostwald ripening) or in the precipitation of Pt crystallites close to the membrane electrolyte due to reduction by hydrogen gas diffusing through the membrane. In addition to these three processes concerning Pt, the complete oxidation of the carbon support to carbon dioxide can occur under certain conditions, which results in the loss of electrical contact with the Pt nanoparticles. The stability of bulk platinum can be characterized by its so-called Pourbaix diagram, describing its thermodynamic behaviour as a function of electrode

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7.1 Different degradation mechanisms. (a) Pt dissolution leading either to re-deposition on larger Pt particle (electrochemical Ostwald ripening) or to the formation of Pt crystallites via the reduction with H2 from the anode side. (b) Particle migration and coalescence. (c) Particle detachment from the carbon support.18

potential and solution pH.19 The Pourbaix diagram indicates that, in the absence of any complexing agents, metallic platinum is a very noble metal, which has a domain of stability covering the majority of the stability region of water. Pt dissolution may occur via two processes, i.e. direct dissolution, and via Pt oxide formation which is then chemically dissolved. At 25 °C only a small corrosion region around 1 V for pH values below 0 exists. At higher pH values the formation of a protective oxide layer is indicated.19 Nevertheless, there are several reports of ample platinum dissolution under PEM fuel cell operating conditions.16, 20, 21 Recent investigations concerning the solubility of Pt indicate that it depends on various factors. While the temperature has an unambiguous effect on solubility, the influence of the solution pH, the nature of the anions in solution, the Pt particle size and the electrode potential are not straightforward. The influence of temperature was investigated in an acid solution, showing a clear increase in solubility with increasing temperature independent of electrode potential and electrolyte.22, 23 The temperature influence follows an Arrhenius type relationship, and the studies indicate that the dissolution reaction is an endothermic process. Mitsushima et al. determined the apparent enthalpy of the dissolution reaction of Pt to 23–25 kJ mol−1, for different concentrations of sulfuric, perchloric and trifluoromethanesulfonic acid, indicating that the anions in the electrolyte do not directly affect the dissolved species.22 However, it is noteworthy in this respect that despite the similar/identical apparent dissolution enthalpy, the absolute values determined for the Pt solubility in different acid solutions depend on the electrolyte. At room temperature, the reported value for air-saturated 1M HClO4 (4.7 × 10−6 mol dm−3) is about 50% higher than for 1M H2SO4 (3.0 × 10−6 mol dm−3).22 However, the influence of chloride anions was not investigated, which would be particularly interesting as it might enhance the solubility due to the formation of hexachloro-Pt complexes. Interestingly, it is reported that in contrast to oxygen- or air-saturated electrolyte, no Pt solubility in nitrogen-saturated solution was detectable. This finding was interpreted due to a

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change in the open circuit potential from about 1 VRHE (oxygen) to 0.4 VRHE (nitrogen), thus changing the Pt surface from an oxide covered to a bare surface. It is less clear if under controlled potentiostatic conditions oxygen saturation of the electrolyte has any influence on the Pt solubility. The difference in Pt solubility obtained for the different acids might be partially related to the influence of the solution pH. Although some literature considering the solution pH and the particle size exists, a general comparison of the different studies is difficult as they were not performed under identical conditions. Discussing the influence of the pH of aqueous solutions on the dissolution of Pt, one has to distinguish between acid and neutral/alkaline electrolytes. It is reported that in acid electrolyte Pt dissolution under thermodynamic conditions (no potential control) increases with decreasing solution pH,22 whereas in the pH range 4–10 the solubility increases with increasing pH.24 Comparing both studies it is evident that for acid electrolyte the Pt solubility is considerably higher than in neutral/alkaline electrolyte. While for Pt black powder at room temperature and in acid solution solubilities in the order of 3–6 × 10−6 mol dm−3 are reported, in neutral and alkaline solutions the solubility is in the range of 1–5 × 10−8 mol dm−3. In addition, the values for bulk Pt in neutral/alkaline solutions are reduced by an additional factor of 102 as compared to Pt powder under the same conditions. The influence of the electrode potential has been extensively studied in previous decades by several groups.25–30 In general, it is distinguished between fixed potential conditions and potential cycling, i.e. conditions that occur under varying power loads in a fuel cell. It has been shown in several studies that, when compared with fixed potential conditions, the catalyst degradation upon potential cycling is greatly enhanced. At fixed potential conditions the influence of the potential on the Pt solubility is in general reported to increase with increasing the applied potential. Furthermore, in some studies it is demonstrated that Pt solubility decreases when applying fairly high potentials. For example, Wang et al. report that the equilibrium concentration of dissolved Pt increases monotonically from 0.65 to 1.1 V vs standard hydrogen electrode (SHE) and decrease at potentials higher than 1.1 V.30 The reason for this effect seems to be related to the fact that at high potentials OH groups adsorb on Pt and oxide formation sets in, which protects the metal from dissolution. Potential cycling, by comparison, induces a constant change from an OH/oxide covered to a bare Pt surface, inducing accelerated dissolution via the chemical pathway. On supported catalysts alternative processes such as particle detachment and migration and coalescence of Pt nanoparticles might be responsible for the accelerated degradation as well (see below). Pt dissolution in a fuel cell can lead to two effects: either in electrochemical Ostwald ripening or in the precipitation of Pt crystallites due to reduction by hydrogen gas diffusing from the anode through the membrane. Both effects involve the transport of soluble platinum species through the electrolyte. The transport of Pt ions due to their diffusion on the carbon support seems unlikely,

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although such a scenario has been concluded by Bett et al. in earlier studies.31 However, it could not explain the detection of Pt in the membrane electrolyte of PEM fuel cells after degradation. As reported by different groups, after potential cycling of membrane electrode assemblies a platinum band is formed in the ionomer membrane, which was related to the precipitation of Pt species by hydrogen.16, 20 In electrochemical Ostwald ripening the soluble platinum species are formed predominantly from smaller Pt particles in the catalyst and the Pt re-deposits on larger ones, thus leading to a general growth in particle size. In theory, a distinctive feature of electrochemical Ostwald ripening is therefore that log-normal plots of the particle size distribution in aged catalysts should exhibit a tailing towards small particle sizes. The migration-coalescence degradation mechanism involves the migration of intact Pt nanoparticles on the carbon support and their coalescence upon impact with other Pt particles. In general this process is readily observed in gas-phase sintering studies of Pt/C catalysts. Its identification at the solid–liquid interface is more difficult and based on comparison with models. The basis of these comparisons is that in contrast to the electrochemical Ostwald ripening process, a migration-coalescence mechanism should result in a tailing towards larger particle sizes in log-normal plots of the particle size distribution of aged catalysts. So far, however, little is known about the mechanism of particle migration on a carbon support. In the gas phase, particle migration was only observed at temperatures above 500 °C.32 Very recent investigations of the authors on Pt nanocluster model systems indicate that, even at room temperature, at the solid–liquid interface Pt nanoclusters migrate on an amorphous carbon film upon potential cycling.33 The detachment of Pt nanoparticles from the carbon support is difficult to assess by conventional methodologies for the investigation of catalyst degradation. It involves the loss of the mechanical/electrical contact of the Pt nanoparticles to the support. In the literature this process is often associated with the complete oxidation of the carbon support to carbon dioxide. Recent investigations by the authors, however, showed a case where particle detachment occurred without any visible loss of carbon when using transmission electron microscopy (TEM) micrographs (see below).34 It can therefore be assumed that particle loss might occur due to the incomplete oxidation of the carbon support as well. Furthermore, it is noteworthy that in the study by the authors, the Pt particles that were detached from the carbon support were washed into the electrolyte where they could be detected by inductively-coupled plasma optical emission spectroscopy (ICPOES) analysis.34 In a MEA, however, much denser catalyst layers are used and detached particles may well be trapped in the layer after detachment from the support. Thus particle detachment might induce an agglomeration of particles. Although inducing the loss of electrical contact of the Pt particles, the complete oxidation of the carbon support to form carbon dioxide should be distinguished from the particle detachment process. As for the previously discussed degradation mechanisms, the complete oxidation of the carbon support is a complex function

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of temperature, electrochemical potential and water partial pressure in a fuel cell. Furthermore, the carbon morphology plays an important role for the corrosion rate. A complete coverage of all aspects of carbon corrosion and its influence on PEMFC degradation, however, is beyond the scope of this chapter. The interested reader is referred to the following references.35–37 For Pt-alloy based electrocatalysts, in addition to the discussed mechanisms the dealloying of the non-noble components occurs under applied conditions. In studies of Pt-alloy bulk electrodes,5 as well as at carbon supported catalysts13 it was demonstrated that alloys of Pt with other less-noble transition metals are not stable in an acidic environment but the non-noble component is leached into the electrolyte. In a fuel cell, the metal ions are not simply washed into the liquid electrolyte, as in half-cell measurements, but might partially replace the protons in the membrane electrolyte, thus diminishing its proton conductivity. This effect may be the main reason for a quite poor performance reported for different Pt-Co based catalysts in FC stack tests under drive cycle conditions.15 The detrimental effect of metal dissolution in alloy catalysts can be reduced by a proper pretreatment, such as leaching the alloy catalyst in concentrated acid. In such a treatment the alloy composition is considerably changed; nevertheless the beneficial effect on the catalytic activity of the oxygen reduction reaction is still (partially) preserved. A topic which has received little attention so far, however, is the fact that the reaction conditions in a fuel cell might induce segregation effects in alloy catalysts. Mayrhofer et al. demonstrated in simple half-cell measurements performed in alkaline electrolyte that, for a conventional, untreated PtCo/C catalyst, potential cycling leads to a segregation of Co to the surface of the catalyst.14 So far, little is known about the mechanism; however, a similar effect was also reported in gas phase studies.38 At the time of writing it is still not understood exactly how these degradation processes depend on the reaction conditions or even the catalyst support, and therefore to what extent the respective mechanisms account for the observed degradation in fuel cells16, 39–41 Detailed insights into FC catalyst corrosion, however, do lay the basis for strategies to synthesize improved, degradationresistant catalysts. The reasons behind observed discrepancies can in part be attributed to differences in the applied experimental conditions, e.g. dynamic load cycles vs constant load. In addition, by conventional methods it is difficult to distinguish between the different mechanisms. For example, the distinction between electrochemical Ostwald ripening and a particle migration mechanism is in general quite difficult. Both mechanisms lead to a growth of the catalyst particles and in most studies the distinction is based on comparison of the experimental data with predictions from theoretical models. Furthermore, most degradation experiments are performed using membrane electrode assemblies (MEAs). Although certainly needed for applications, studies using MEAs are not only rather time-consuming, difficult to analyze and expensive, but also prone to a large number of experimental variables in MEA

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preparation. The preparation of the MEA, however, is usually proprietary information and is consequently not discussed in the literature. Accelerated tests in the form of half-cell measurements are therefore a promising approach to perform more fundamental and systematic fuel cell catalyst degradation studies.

7.3

Characterization of catalyst degradation

In fuel cell applications the catalyst degradation is typically investigated by electrochemical means. Performance loss or the reduction of the active surface area is recorded over time, in order to quantify the degradation, depending upon operating conditions. Alterations of the composition and structure of the catalyst/ catalyst layer are investigated after the electrochemical testing, providing a snapshot of the final constitution of the electrode. The information gained from such post mortem analysis of the MEA can then be compared to the analysis of the initial state so that, ideally, alterations can be correlated with the application procedure. Scanning electron microscopy (SEM) on MEAs cut perpendicularly to their layers can provide information on the distribution of the catalyst on the anode and the cathode sides, losses of carbon support due to complete oxidation, as well as eventual deposition of metal in the membrane.16, 42 When the catalyst is scraped off the MEA, transmission electron microscopy can be employed to determine the particle size distribution and shape.18 In both cases energydispersive X-ray spectroscopy (EDX) additionally helps to elucidate the elemental distribution within the catalyst/catalyst layer, which is of relevance when alloys or mixed-metal catalysts are applied. The same could also be achieved with an aberration-corrected scanning transmission electron microscope (STEM) equipped with a high-angular annular dark-field detector (HAADF), with a spatial resolution below one nanometer.43 Moreover, powder X-ray diffraction (XRD) provides information on the crystalline structure of the catalyst, particle size via the Scherrer formula, and semi-quantitative composition of alloy systems. Due to the general inhomogeneity of industrial high surface area catalysts, various samples of one MEA are analysed by a combination of these techniques, and statistical methods are necessary for a convincing data interpretation. The investigation of catalysts utilizing these methodologies is not straightforward at all. In a fuel cell, various reaction parameters can influence the degradation of the catalyst (humidity, concentration gradients, local effects, temperature, pressure, etc.), but the MEA preparation and the catalyst support also play crucial roles. Moreover, an ex situ characterization before and after a performance test that can often last for hundreds of hours is scientifically rather unsatisfying, in particular with respect to establishing a fundamental understanding of the degradation mechanism. As a consequence a lot of effort has been invested over the last decade in investigating fuel cell catalysts under more defined conditions and designing in situ characterization techniques.

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As a consequence, electrochemical measurements in standard three-electrode electrochemical cells with small amounts of catalyst and liquid electrolytes attracted increasing attention, and had a positive impact on recent developments.44, 45 Half-cell investigations of this type are far less complex; however, they are still directly comparable to studies in fuel cells. Since the experimental conditions can be controlled more precisely and concentration gradients do not occur on the electrode surface, the impact of individual parameters can be investigated in a straightforward manner.46 Furthermore, the potential of the working electrode can be determined more accurately, since in general the influence of the counter electrode and solution/membrane resistance can be neglected. The thin-film technique developed by Schmidt et al.,47–49 for instance, extended the rotating disc electrode (RDE) measurements from ideally flat electrode surfaces to undefined, porous electrode structures. This has not only improved the analysis of the activity of various supported and unsupported electrocatalysts, but also the electrochemical investigation of their long-term stability. Other methods with forced convection have also been utilized to mimic controlled diffusion in a fuel cell on a much more defined and smaller level with similar results, such as electrochemical flow cells4, 50 and microelectrode setups.51, 52 Combined with high-throughput testing, these methods have become a very powerful extension for the fast screening of properties of materials, in particular with respect to their long-term behaviour.53–55 In addition, complimentary techniques can readily be combined with the halfcell measurements and provide additional information on degradation processes. In situ Fourier-transform-infrared spectroscopy (FTIR) can be utilized to determine adsorbed species on the catalyst that might block the active sites for reactions.56, 57 Moreover, in a flow cell setup, differential electrochemical mass spectrometry (DEMS) has been established as an excellent complimentary technique for the analysis of gaseous reaction products,58, 59 which aids, for instance, estimation of the potential dependent carbon support oxidation. It was recently demonstrated by the authors that it is feasible to investigate the identical location of an electrode after an electrochemical treatment utilizing IL-TEM34, 60 (Fig. 7.2). This technique enables the tracking of changes of the size and location of individual catalyst particles visually, even on the nanometer scale, which can then be directly related to the treatment procedure in between. Thus the fundamental degradation processes described in the previous chapter can be easily distinguished from one another, without counting large numbers of particles and comparing the statistical evaluation of different catalyst regions. Since this methodology is not destructive, as are the regular TEM approaches for fuel cell catalysts, continuous evaluation of the degradation in between various treatments can be made. Despite all the benefits of these more defined measurements for the investigation of fundamental processes, they cannot completely substitute MEA testing. In order to understand effects due to gradients within a fuel cell, cell reversal due to blocking

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7.2 IL-TEM micrographs recorded with a magnification of 100 k of the catalyst before (a, c) and after electrochemical treatment (b, d) in 0.1 mol l−1 HClO4. The electrochemical treatment between A and B was 3600 cycles with 1.0 V s−1 in the potential region of 0.05 and 1.05 VRHE, between (c) and (d) in the potential region of 0.40 and 1.4 VRHE. The according Pt particle size distributions are depicted next to the corresponding images (including data from several other micrographs).60 Source 60: reprinted with permission from Elsevier.

and similar applied issues, in situ methodologies become unavoidable. For instance, extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) offer the possibility to study the catalyst directly in a fuel cell during operation, without the necessity of ultra-high vacuum conditions.61–63 Whereas with EXAFS the amount, constitution and distance of neighbouring atoms of a certain chemical element in a catalyst/crystal can be determined, XANES provides information about adsorbed molecules on the solid surface and the oxidation state of surface elements.64 This direct observation of the catalyst properties, dependent upon potential and current, as well as the location in the MEA, elegantly complements the information from the previously described techniques.65

7.4

Identical-location transmission electron microscopy

Although still in its infancy, identical-location transmission electron microscopy (IL-TEM) has already proven to be a very valuable and efficient methodology for

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the investigation of fundamental catalyst degradation processes. Its great potential has so far only been discussed for fuel cell catalysts based on Pt nanoparticles supported on a high surface-area carbon, but the advantages also for other electrocatalytic systems are obvious. IL-TEM investigations are generally performed on a TEM finder grid made of gold, coated with a thin conducting film of carbon. Both materials are inert and stable under the regular operating conditions of fuel cells, making these grids a perfect working electrode support. In order to deposit the catalyst onto the grids, however, they first must develop hydrophobicity by an exposure to a glow discharge in argon plasma. Very low amounts of catalysts can then be loaded onto the grid by wet impregnation, leading to a uniform distribution of ideally well-separated and non-overlapping particles. These particles can be investigated in a regular TEM with different magnification in such a way that the quadrant and location on the finder grid will be remembered for the post-treatment analysis by means of the alphabetic index. Those quadrants with a complete carbon film should be preferentially chosen for the analysis, since defective quadrants tend to deteriorate or even break under the following treatment. After evaluating the initial state of the particles, the grid is transferred to an electrochemical cell and connected to a potentiostat as a working electrode. A wide range of electrochemical treatments can thus be imposed on the catalyst. Afterwards the grid must be washed with ultrapure water to remove electrolyte components, dried thoroughly and transferred back to the TEM to be positioned at the identical locations as before the treatment. Since the loading on the grid has to be kept as low as possible, the electrochemical response of the sample is usually governed by the supporting grid. As a consequence the impact of the treatment on catalyst composition and activity has to be determined in a separate experiment utilizing, for instance, the thin-film RDE technique. In order to assure the identical treatment procedure in this case, however, the RDE can be connected to the same lead as the TEM grid and placed in the electrochemical cell as a parallel working electrode. This guarantees the absolute comparability of the macroscopic information from electrochemical measurements with the microscopic information gained from IL-TEM (see Plate III in the colour section between pages 222 and 223). The transfer from the TEM in ultra-high vacuum to the electrochemical cell and back including a mild treatment procedure has been proven not to affect the catalyst structure. Thus IL-TEM analysis is indeed solely reflecting the impact of the electrochemical treatment, and only when the catalyst is subjected to a corrosive treatment various changes can be observed. Due to the major advantage of being capable to directly compare the location of individual particles, their absolute number and their size distribution, it has been demonstrated that whole particles can detach from the carbon support surface upon an accelerated degradation test (see Fig. 7.2). This fast potential cycling between a reduced and an oxidized state is a special experiment, which is often used in the literature to mimic the dynamics in a fuel cell. Considering these IL-TEM results, however, it

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should be scrutinized whether this approach truly reflects real operating conditions and the concomitant degradation processes, or if another treatment approach is more suitable. Under different circumstances other effects could occur such, as for instance, the movement/coalescence of whole particles or the dissolution and re-deposition of single atoms. In order to distinguish between exceptional events and a prevalent degradation mechanism, a significant statistical analysis is still unavoidable in these cases. Therefore micrographs of well-separated and unambiguous catalyst particles have to be analysed; however, any overlapping regions must be rejected. These are usually not interpretable, simply because coalesced particles cannot be distinguished from overlapping ones in this two-dimensional projection of the three-dimensional catalyst agglomerate. Moreover, the interpretation of the results can be further complicated when the catalyst flakes as a whole or when parts of it are not sufficiently attached to the carbon film of the TEM grid, as in the exceptional example shown in Plate III. The catalyst particle then disengages during the treatment and docks on in a different configuration and even location upon drying and transfer to ultra-high vacuum. Although there are still some challenges regarding this methodology and room for improvements, non-destructive IL-TEM has already brought new incentives to the characterization of electrocatalyst degradation and will continue to complement fundamental electrochemical studies.

7.5

Future trends

An open issue that has still not been resolved sufficiently and needs to be addressed to a greater extent is the transfer of know-how from fundamental investigations in half-cells to application in real fuel cells by a direct comparison of degradation processes under various similar conditions. This should aid to verify the expected identical behaviour despite the quite different experimental approach, and will additionally justify the reduction of the complexity of studies in order to gain an improved understanding of the underlying processes. Conclusions drawn from measurements utilizing complementary analysis techniques will then be put to a test, and their applicability directly proven. This is particularly important considering the trend in fuel cell research to high-throughput screening of catalyst stability using accelerated testing procedures in multiple cell setups, since it is not absolutely clear at this point whether the currently applied measurement protocols are of any relevance for the true long-term behaviour of the systems. In contrast, it might be that the degradation mechanisms during some fast screening procedures are quite different from processes in applications, and thus the simulation conditions have to be reconsidered. While other characterization techniques are regularly employed in degradation studies of low-temperature fuel cell catalysts, IL-TEM has only recently shown its potential and is not yet commonly applied. So far only a couple of reaction

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conditions on a single catalyst have been tested in our laboratory, and whole ranges of experiments and systems still have to be investigated. So the effects of the absolute potential, potential regions and cell dynamics have to be clarified, as has to be the impact of temperature, reaction gases and electrolyte pH, since all of these parameters might be decisive for the degradation mechanism. Moreover, stability studies should be extended to catalysts with different particle sizes, which are known to also have unequal behaviour with respect to their specific activity.66 Alloy catalysts of Pt and transition metals like Co, Fe, Ni or Cu and mixed metal catalysts such as PtAu and PtPdAu, for which an improved stability has been often reported in the literature, could also be very interesting systems for IL-TEM. As long as the support of the catalyst provides a distinctive feature of the particle location as well as remains relatively unaltered during the degradation treatment, there is no limit for this methodology. The final goal of such fundamental investigations of fuel cell catalysts is to create a map of physical and chemical degradation processes that is independent of the experimental conditions. Only a substantial understanding of the origin of Ostwald ripening, particle coalescence/agglomeration, particle detachment and carbon corrosion and their exact mechanisms will enable avoidance of their occurrence during applications in the future. It is conceivable that, based on this fundamental know-how, catalyst systems with defined structures and compositions will be designed systematically in order to achieve improved long-term stable properties, just as it is currently the case for the optimization of catalysts concerning their specific activity.

7.6

References

1. Ralph, T. R. and Hogarth, M. P. ‘Catalysis for low temperature fuel cells; Part I: The cathode challenges’. Platinum Metals Review 46, 3 (2002). 2. Thompsett, D., ‘Pt alloys as oxygen reduction catalysts’, in Handbook of fuel cells – Fundamentals, Technology and Application vol.3, eds. Vielstich, W., Gasteiger, H. and Lamm, A., 467 (Wiley, 2003). 3. Stamenkovic, V., Mun, B. S., Mayrhofer, K. J. J., Ross, P. N., Markovic, N. M., et al., ‘Changing the activity of electrocatalysts for oxygen reduction by tuning the surface electronic structure’. Angewandte Chemie–International Edition 45, 2897–2901 (2006). 4. Colmenares, L., Guerrini, E., Jusys, Z., Nagabhushana, K. S., Dinjus, E., et al., ‘Activity, selectivity, and methanol tolerance of novel carbon-supported Pt and Pt3Me (Me = Ni, Co) cathode catalysts’. Journal of Applied Electrochemistry 37, 1413–1427 (2007). 5. Stamenkovic, V. R., Mun, B. S., Arenz, M., Mayrhofer, K. J. J., Lucas, C. A., et al., ‘Trends in electrocatalysis on extended and nanoscale Pt-bimetallic alloy surfaces’. Nature Materials 6, 241–247 (2007). 6. Stamenkovic, V. R., Fowler, B., Mun, B. S., Wang, G., Ross, P. N., et al., ‘Improved oxygen reduction activity on Pt3Ni(111) via increased surface site availability’. Science 315, 493–497 (2007). 7. Koh, S., Leisch, J., Toney, M. F. and Strasser, P. ‘Structure-activity-stability relationships of Pt-Co alloy electrocatalysts in gas-diffusion electrode layers’. J. Phys. Chem. C 111, 3744–3752 (2007).

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Plate III IL-TEM image of a certain location on the catalyst (a) before and (b) after an electrochemical treatment consisting of potential cycling between 0.05 and 0.65 VRHE in CO saturated in 0.1 mol l−1 HClO4 and between 0.05–1.2 VRHE in Ar saturated electrolyte. The sweep rate was 20 mVs−1, the respective duration 1 h. The coloured edges and arrows aid visualization of the changes. (Source: Ref. 60, reprinted with permission from Elsevier.)

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