Carbon corrosion durability

Similar ECA loss phenomena have been observed in PEMECs. Understanding ECA loss and carbon corrosion mechanisms may help with designing more durable... 

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. 

In a PEFC, oxygen and hydrogen crossover is important because of the obvious performance loss, the development of a mixed potential, and even durability issues due to hydrogen peroxide generation platinum migration, and possible carbon corrosion [69]. Furthermore, crossover becomes increasingly important as the membranes used become thinner. Presented in this section are the parameters and governing equations to model this phenomenon. 

Perry ML, Patterson TW, Reiser C (2006) Systems strategies to mitigate carbon corrosion in fuel cells durability— fuel starvation and start/stop degradation. ECS Trans 3 783-795... 

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]. 

The last three chapters are dedicated to improving the durability of the catalyst/ electrode. Chapter 22 reports the development and evaluation of bimetallic Pt-Ru (Ir) oxygen evolution catalysts on 3M s nanostructured thin film (NSTF). This type of catalyst may significantly reduce carbon corrosion and Pt dissolution during transient conditions of fuel cells. Chapter 23 discusses the unique properties of carbide-modified carbon as the support for fuel cell catalysts. The final chapter gives a comprehensive review of novel materials other than carbon black as catalyst support. The interactions between the supports and catalysts are intensively discussed in the last two chapters. 

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... 

Non-supported catalysts seem a way not only to avoid durability issues related to the support (such as carbon corrosion), but also introduce extended surfaces and therefore the possibility of higher activity. The NSTF is an example, but also... 

We also discussed the durability issues of PEM fuel cell catalysts which have been recently recognized as one of the major barriers to the commercialization of fuel cells [10]. To use carbon with a higher graphitization degree as support materials and to alloy Pt with other metals can improve the catalyst durability. Compared with carbon black, several research groups have reported that the use of CNTs can be promising in effectively reducing the carbon corrosion problem [213]. 

The membrane is typically negatively impacted by low humidity conditions, with the acceleration of mechanical failure observed. On the other hand, the catalyst layer durability is generally higher under drier conditions. The dominant degradation mechanisms of Ft dissolution and carbon corrosion are both accelerated under wetter conditions. 

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]. 

Durability is the capability of a PEM fuel cell stack to resist permanent change in performance over time. A durability failure may not cause catastrophic failure in the fuel cell. However, this mode of failure will decrease the performance of the fuel cell. It also can involve irreversible failures, such as electrochemical surface area reduction, carbon corrosion, etc., and losses mainly related to ageing [12]. 

This chapter is devoted to a review of material issues for HT-PEMFC components and is built on relevant evaluation of data from literature on membrane oxidation, acid loss, platinum dissolution, and carbon corrosion. Finally, the state-of-the-art durability of PBI-based fuel cells is summarized. For certain applications, like in mobile auxiliary power units exposed to severe vibrations and road dust or to maritime saline mists, the picture becomes more complicated and knowledge today is rather limited. 

A suitable strategy to minimize the carbon corrosion issue is to use carbon materials which have higher graphite contents and therefore fewer structural defects at which the oxidation initiates [81]. Heat-treatment of carbon blacks at elevated temperature is known to be able to impart the graphitic character to the carbon black [77, 82, 83]. As seen from Fig. 22.5, graphitization of the carbon blacks improved the stability and catalyst durability, though at the expense of a significant decrease in the specific surface area of the support material. The specific surface area loss was found to be primarily due to the elimination of pores less than... 

Figure 11.30 displays the calculated cell potential evolntion at fixed current, for a model acconnting for the cathode carbon corrosion alone (and induced Pt coarsening) and for a model acconnting for both PEM side-chain degradation -i-carbon corrosion, for two PEM thicknesses (25 pm and 50 pm). The potential behavionr overlaps for the two models applied to the PEM with an initial thickness of 50 pm, which indicates that carbon corrosion is the dominant aging mechanism. In contrast to this, for the 25 pm PEM case, the coupled model provides a lower durability compared to the model accounting for the carbon corrosion alone. 

Carbon corrosion can also arise from a nonuniform distribution of fuel on the anode side (partial hydrogen coverage) and from crossover of reactant gas through the membrane. Local fuel starvations can cause this type of carbon corrosion. Because of its complexity and consequence to the durability of the fuel cell catalyst layer, local fuel starvation is both a widely studied and researched phenomena. 

Although it can be concluded from the above evidence that greater edge plane exposure and number of defected sites usually have a positive impact on catalytic activity of the carbon nanostructured supports, it is also known that the corrosion of carbon materials is initiated at these edge planes. This relationship between catalyst support enhancement and carbon corrosion needs to be kept in mind when developing novel nanostructured carbon catalysts. Unfortunately, the influence of defects in CNTs and CNFs on the durability of these structures is stiU not quite known and little research is published in this area. 

Although the use of these OMCs have been utilized in recent years with great success as supports for PEMFC catalysts, not much work has been done on assessing the durability of these supports for longterm applications. And although these novel nanostructured carbons may display increased tolerance to the harsh operating conditions in fuel cells with respect to carbon blacks, carbon corrosion is still thermodynamically possible and not completely eliminated. 

---