Durability carbon-support corrosion

Durable and reliable operation for several thousand hours are considered crucial for the successfully commercialization of DAFC. The factors that determine a PEM fuel cell s lifetime (as platinum-particle dissolution and sintering, carbon-support corrosion, and membrane thinning) is currently studied by many researchers in order of increase the lifetime without increasing cost or losing performance [83-85]. The relative contribution of each component s degradation to the degradation of the entire fuel cell is not completely understood yet. 

To reach the requirements of performance and durability for both automotive and stationary PEFC appUcations [20], catalyst durability has become an important issue both for of academic and industrial R D. Cathode degradation in operating PEFCs mainly occurs tmder transient conditions, leading both to Pt dissolu-tion/degradation and carbon-support corrosion. At typical operational pH conditions and above 0.9 V, two main mechanisms lead to Pt degradation (i) diffusion of dissolved Pt species... 

Gallagher KG, Darling RM, FuUct TF (2009) Carbon-support corrosion mechanisms and models. In Vielstich W, Gasteiger HA, Yokokawa H (eds) Handbook of fuel cells - advances in electrocatalysis, materials, diagnostics, and durability, vol 5. Wiley, Chichester, pp 819-828... 

In the first case it is observed a dramatic potential collapse, a clear signature of the electro-active area loss (drivenby the carbon support corrosion). Also observed are an optimal current with maximal durabihty (0.5 A), as already shown. " In the second case, the current offering the maximal durability corresponds to the OCC, and a bell-shaped potential collapse is observed, which provides a signature of... 

Other than ex situ testing based on half-cell systems, in situ tests in full cells are conducted to further understand the durability issues for Pt-alloy based catalysts. Comparison of the results from different test systems can yield much useful information. Investigating the influence of carbon support corrosion, Pt dissolution, and sintering, Arico et al. (2008) evaluated the stability of Pt/C and Pt-Co/C in both a gas-fed sulfuric acid electrolyte half-cell at 75°C and a PEM fuel cell at 130°C. Results in sulfuric acid revealed the Pt-Co alloy to be more stable than Pt after cycling and more sensitive to carbon support corrosion. In high-temperature PEM fuel cell testing, Pt-Co/C showed smaller sintering effects than Pt/C. 

Wang J, Yin G, Shao Y, Zhang S, Wang Z, Gao Y (2007) Effect of carbon black support corrosion on the durability of Pt/C catalyst. J Power Sources 171 331-339 Maass S, Finsterwalder F, Frank G, Hartmann R, Merten C (2008) Carbon support oxidation in PEM fuel cell cathodes. J Power Sources 176 444-4.51... 

The right choice of a carbon support greatly affects cell performance and durability. The purpose of this chapter is to analyze how structure and properties of carbon materials influence the performance of supported noble metal catalysts in the CLs of the PEMFCs. The review chapter is organized as follows. In Section 12.2 we give an overview of carbon materials utilized for the preparation of the catalytic layers of PEMFC. We describe traditional as well as novel carbon materials, in particular carbon nanotubes and nanofibers and mesoporous carbons. In Section 12.3 we analyze properties of carbon materials essential for fuel cell performance and how these are related to the structural and substructural characteristics of carbon materials. Sections 12.4 and 12.5 are devoted to the preparation and characterization of carbon-supported electrocatalysts and CLs. In Section 12.6 we analyze how carbon supports may influence fuel cell performance. Section 12.7 is devoted to the corrosion and stability of carbon materials and carbon-supported catalysts. In Section 12.8 we provide conclusions and an outlook. Due to obvious space constraints, it was not possible to give a comprehensive treatment of all published data, so rather, we present a selective review and provide references as to where an interested reader may find more detailed information. 

Finally, we addressed the complex problem of carbon corrosion, which is particularly relevant for PEMFC durability and thus commercialization of PEMFC technology. Carbon supports with an ordered crystalline structure, such as graphi-tized carbons, CNTs, and CNFs, as well as pyrolytic carbons of the Sibunit family hold out hope for the development of CLs with higher durability. More systematic studies are required to unveil the complex influence of the structure and morphology of carbon supports on the performance of the CLs and eventually, to develop a new generation of structurally ordered tailored materials for PEMFC applications with enhanced catalytic activities, low noble metal contents, and high dmabilities. 

Corrosion problems from the different issues related to the durability of the MEAs in the PEMFCs, corrosion of the carbon support is one of the most significant in terms of improving the stability of the electrodes. Carbon blacks are relatively stable with respect to other kind of carbons, but still their behaviour is not ideal for long-term use of FCs. 

As mentioned earlier, CB is prone to oxidation, the so-called carbon corrosion, which results in the loss of surface area, changes in the pore structure and finally also leads to sintering of the supported nanoparticles and eventually their loss from the support surface. This affects both the kinetics of the reaction and the electrode s mass transport behavior resulting in a significant loss of performance with operation time. Consequently, carbon support durability is considered to be a major barrier for the successful commercialization of fuel cell technology in the automotive sector. So much so, during the last decade, more than 60 publications dealt with carbon corrosion mechanisms in fuel cell apphcation [82]. 

Superior durability aspects of the NSTF electrode in comparison to carbon-support-dispersed catalysts are related to (i) its non-electron-conducting support that eliminates carbon corrosion that can cause significant increases in gas transport resistance and (ii) its bulk-like-Pt surface that is more resistant to Pt dissolution [67, 75, 76]. However, NSTF faces higher sensitivity to contaminants due to its lower Pt surface area [77] and other operational challenges related to its unique structure [78-81], notably its electrode thickness which is l/30th of conventional electrode. Figure 13.8... 

The issue of carbon corrosion has received considerable attention in recent years. There are several drivers for this (1) the cost drivers for commercialization require the use of high performance catalysts with less durable carbon catalyst supports, (2) the need for system simplification and low cost prevents additional control systems to be implemented to avoid the carbon corrosion conditions, and (3) the use of the fuel cells subjected to "real world" conditions as opposed to carefully controlled demonstration projects, with very dynamic duty cycles and many start-up/shutdown cycles. This increased attention has resulted in new or improved measurement techniques and several studies and reviews on the high cathode potential and associated carbon corrosion mechanism [39,40,48-51]. 

Wang J, Yin G, Shao Y, et al. (2007), Effect of carbon black support corrosion on the durability ofPt/C caia ysi Journal of Power Sources 171, 331-39. 

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