High-temperature PEM Fuel Cell Catalysts and Catalyst Layers

High-temperature PEM Fuel Cell Catalysts and Catalyst Layers 

1 Opportunities and Challenges for High-temperature PEM Fuel Cells 

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Figure 3.51. Single PEM fuel cell performance. Current-voltage relationship (open symbols) and implied power density (filled symbols), are shown for different operational temperatures in the range of 45-90°C, for a cell with a catalyst layer with incorporated PTFE (polytetrafluoroethene) to reduce water flooding, a low loading of a carbon-supported Pt catalyst layer (120 pg Pt cm ) and finally Nation intrusions. (From Z. Qi and A. Kaufman (2003). Low Pt loading high performance cathodes for PEM fuel cells. /. Power Sources 113,37-43. Used with permission from Elsevier.)...

In addition to loss of the platinum, the carlxm support that anchors the platinum crystallites and provides electrical coimectivity to the gas-diffusion media and bipolar plates is also subject to degradation. In phosphoric acid fuel cell, graphitized carbons are the standard because of the need for corrosion resistance in high-temperature acid environments [129], but PEM fuel cells have not employed fully graphitized carbons in the catalyst layers, due in large part to the belief that the extra cost could be avoided. Electrochemical corrosion of carbon materials as catalyst supports will cause electrical isolation of the catalyst particles as they are separated from the support or lead to aggregation of catalyst particles, both of which result in a decrease in the electrochemical active surface area of the catalyst and an increase in the hydrophUicity of the surface, which can, in turn, result in a decrease in gas permeability as the pores become more likely to be filled with liquid water films that can hinder gas transport. 

Fuel cells are power generation devices converting chemical energy into electric energy by electrochemical reactions. A typical fuel cell is comprised of two electrodes separated by an electrolyte, with a provision of reactant supply and product removal. Among various types of fuel cells, Ha-O -based polymer electrolyte membrane (PEM) fuel cells (PEMFC) have attracted special attention due to their high efficiency, low temperature operation and suitability for low to medium power generation. Basic components of a PEMFC are PEM, catalyst layer, gas diffusion layer and... 

Mazur P, Soukup J, Paidar M et al (2011) Gas diffusion electrodes for high temperature PEM-type fuel cells role of a polymer binder and method of the catalyst layer deposition. J Appl Electrochem 41 1013-1019... 

When residual water produced during fuel cell operation remains in the electrodes after the stack is shut down, problems can arise, particularly when the environmental temperature is <0 °C. When the stack is exposed to subzero conditions, the residual water will freeze, so the volume of the electrodes (in particular, the catalysts layers) will expand due to ice formation, which will lead to structural damage and decreased electrochemical active surface area. This has been reported as an additional degradation mechanism in PEM fuel cells [21]. However, if the PEM fuel cell is operated at high temperatures, less liquid water will remain in the electrode and thus decrease the impact of fuel cell structure failure caused by frozen water. 

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