Fuel cells stability requirements

The long-term stability of Pd-based electrocatalysts is one of the unavoidable issues for PEM fuel cell applications. Pd-Pt-based ORR catalysts are more stable than Pd-transition metal alloys under harsh fuel cell conditions, but may still not meet the long-term fuel cell operation requirement due to the Pd leaching out. Future research may focus on improving the durability of Pd-based catalysts by surface modification and composition optimization. Core-shell type of catalyst with Pd-based materials as the core and Pt as the shell may be one of the most promising candidates to be used in the automotive fuel cell due to its low Pt content and high activity and stability. 

In summary, the improvement of AAEMs is an ongoing area of research and development There is no one membrane of choice currently and the conductivity and stability of some membranes are still an issue. However, there are signs that good AAEM performance is close and so catalyst development for the AAEM fuel cell is required to match performance and cost of the current PEM fuel cell technology. 

The choice of immobilization strategy obviously depends on the enzyme, electrode surface, and fuel properties, and on whether a mediator is required, and a wide range of strategies have been employed. Some general examples are represented in Fig. 17.4. Key goals are to stabilize the enzyme under fuel cell operating conditions and to optimize both electron transfer and the efficiency of fuel/oxidant mass transport. Here, we highlight a few approaches that have been particularly useful in electrocatalysis directed towards fuel cell applications. 

The general requirements for an SOFC anode material include [1-3] good chemical and thermal stability during fuel cell fabrication and operation, high electronic conductivity under fuel cell operating conditions, excellent catalytic activity toward the oxidation of fuels, manageable mismatch in coefficient of thermal expansion (CTE) with adjacent cell components, sufficient mechanical strength and flexibility, ease of fabrication into desired microstructures (e.g., sufficient porosity and surface area), and low cost. Further, ionic conductivity would be beneficial to the extension of... 

The next two chapters discuss two supporting components of the fuel cell stack —specifically, interconnects and sealants. The interconnect conducts the electrical current between the two electrodes through the external circuit and is thus simultaneously exposed to both high oxygen partial pressure (air) and low oxygen partial pressure (fuel), which places stringent requirements on the materials stability. Ceramic interconnects have been used, but metallic interconnects offer promise... 

A final issue that faces this class of catalysts is stability in the fuel cell environment. Deactivation of materials in a fuel cell environment has been shown to be minimal in some studies,31,137 and severe in others.128,142 More active catalysts seem more susceptible to deactivation. Deactivation has been linked to the formation of peroxide and the loss of metal from the catalyst.128 On the other hand, demetallization has also been observed in pyrolyzed samples that did not lose activity with time.84 Another possible mode of deactivation could be due to the oxidation of the carbon surface. However, it seems reasonable that a complete understanding of the deactivation mechanism would first require a well-developed understanding of the active site. 

The reformate gas contains up to 12% CO for SR and 6 to 8% CO for ATR, which can be converted to H2 through the WGS reaction. The shift reactions are thermodynamically favored at low temperatures. The equilibrium CO conversion is 100% at temperatures below 200°C. However, the kinetics is very slow, requiring space velocities less than 2000 hr1. The commercial Fe-Cr high-temperature shift (HTS) and Cu-Zn low-temperature shift (LTS) catalysts are pyrophoric and therefore impractical and dangerous for fuel cell applications. A Cu/CeOz catalyst was demonstrated to have better thermal stability than the commercial Cu-Zn LTS catalyst [37], However, it had lower activity and had to be operated at higher temperature. New catalysts are needed that will have higher activity and tolerance to flooding and sulfur. 

The latter devices are fuel cells that consist of ceramic components which have to fulfill extremely demanding criteria with regard to thermal, mechanical, chemical, and electrical properties. Just consider the electrolyte It does not only have to be thermally stable but also has to be mechanically and chemically compatible with the electrodes. It does not only have to be chemically stable over a very wide redox window but also has to maintain electrolyte properties within that window (redox stability). Owing to the high mobilities of the electronic carriers and the comparatively steep power law dependencies of their concentrations (see Part I), this requires an extremely high ratio of ionic versus electronic disorder at the reference point of p-n minimum (cf. Part I).2... 

Although the operating concept of SOFCs is rather simple, the selection of materials for the individual components presents enormous challenges. Each material must have the electrical properties required to perform its function in the cell. There must be enough chemical and structural stability to endure fabrication and operation at high temperatures. The fuel cell needs to run at high temperatures in order to achieve sufficiently high current densities and power output ... 

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