Cathode corrosion

Carbon is used extensively in fuel cells due to the favorable properties of high electrical and thermal conductivity, the relatively low cost, the absence of damaging degradation products when compared to metals, and the ability to form various structures from high surface area particles for the carbon support and in the microporous layer, to carbon or graphitic fibers for the gas diffusion substrate, to graphite or carbon-polymer composite flow field plates. Dicks provides an overview of the role of carbon in fuel cells [45]. 

Although carbon is thermodynamically unstable in the fuel cell environment, the slow kinetics of the corrosion reaction (equation (1.34)) allow quite stable operation under controlled conditions, such as steady state operation. 

Combined with the presence of the Pt catalyst, which accelerates this reaction, measurable CO2 production occurs above 1.0 V. While this indicates that slow corrosion will occur at open circuit potential, the rate is still relatively low, but is enough to contribute to degradation over longer lifetimes. However, the key driver for corrosion of the carbon support is the increased potentials that typically occur during shutdown or startup, when the cathode potential can go up to 1.5 V or higher [461. 

Schematic of the fuel cell environment and electrochemical potentials during a partial fuel starvation. (From Lauritzen, M.V., et al. 2007. /. New Mat. Electrochem. Sys. 10 143-145. With permission. 

MEA cross section picture comparison of MEAs at 0 and 30 h of corrosion cycling. (Left) 0 h, prior to corrosion occurring (right) 30 h, after corrosion occurred cathode catalyst layer thinned by 65%. (From Young, A.P., et al. 2009. /. Electrochem. Soc. 156 (8) B913-B922. With permission.) 

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A process resulting in a decrease in touglmess or ductility of a metal due to absorjDtion of hydrogen. This atomic hydrogen can result, for instance, in the cathodic corrosion reaction or from cathodic protection. 

Baeckman W v, Schenk W and Prinz W 1997 Handbook of Cathodic Corrosion Protection Theory and Practice of Electrochemical Protection Processes (Flouston, TX Gulf)... 

Asahi also reports an undivided cell process employing a lead alloy cathode, a nickel—steel anode, and an electrolyte composed of an emulsion of 20 wt % of an oil phase and 80 wt % of an aqueous phase (125). The aqueous phase is 10 wt % K HPO, 3 wt % K B O, and 2 wt % (C2H (C4H )2N)2HP04. The oil phase is about 28 wt % acrylonitrile and 50 wt % adiponitrile. The balance of the oil phase consists of by-products and water. The cell operates at a current density of 20 A/dm at 50°C. Circulated across the cathode surface at a superficial velocity of 1.5 m/s is the electrolyte. A 91% selectivity to adiponitrile is claimed at a current efficiency of 90%. The respective anode and cathode corrosion rates are about mg/(Ah). Asahi s improved EHD process is reported to have been commercialized in 1987. 

The primary corrosion product PbHj is unstable and decomposes in a subsequent reaction into lead powder and hydrogen gas. Figures 2-11 and 2-12 are typical examples of cathodic corrosion of amphoteric and hydride-forming metals. 

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