XPS and SEM investigation of the structural changes of electrodes in low temperature fuel cells during long-term operation

Abstract

Fuel cells allow an environmentally friendly and highly efficiently conversion of chemical energy to electricity. Therefore, fuel cells have a high potential to become important components of a energy-efficient and sustainable economy. The main challenges in the development of fuel cells are cost reduction and long-term durability. In this contribution, investigations of various types of low temperature fuel cells – the polymer electrolyte membrane fuel cell (PEFC), direct methanol fuel cell (DMFC) and the alkaline fuel cell (AFC) – are presented during long-term operation. For all types a decrease of the electrochemical performance with operation time is observed. The active parts in the fuel cells are the membrane-electrode assemblies in the case of PEFCs and DMFCs and the electrodes in contact with liquid electrolyte in the case of AFCs. The electrodes have the function to catalyse the reaction and to allow a facile transport of the reacts and educts. Two important and distinct structural degradation processes can be identified by surface analysis of the electrodes before and after fuel cell operation: first, the decomposition of PTFE which is used as an organic binder and as a hydrophobic agent in the electrodes and second, a change of the structure of the catalysts. The observed decomposition of the PTFE is associated with a decrease of the hydrophobicity of the electrode. A loss of hydrophobicity influences drastically the required operation conditions and leads to a more critical water management of the fuel cell. In contrast, the alteration of the catalysts structure in the electrodes causes an irreversible decrease of the electrochemical performance. The observed alteration of the catalysts depends on the fuel cell type and on the operation conditions: e.g. in AFC anodes a disintegration of the nickel catalyst particles is observed, whereas in PEFCs a particle agglomeration of the platinum catalysts at the cathodes is detected.