Alteration of PTFE in AFC, DMFC and PEFC electrodes during start-up procedure and during operation

Kurzfassung

The life time of fuel cells is a critical topic for the commercialization of fuel cells. One of the aging components in low temperature fuel cells are the electrodes. The electrodes consist of the catalyst for the electrochemical reaction, carbon structures and additional materials to define the structure and physical properties. In the low temperature fuel cells a hydrophobic pore system is needed for the transport of gases and a hydrophilic pore system for the transport of water. Two different degradation processes are important especially in PEFC: the aging of the catalysts and alteration of transport processes induced by changes in the physical properties. The alteration of the catalyst is related to electrochemical processes. The change of the transport process due to the operation time is correlated with the alteration of the hydrophobicity of the electrodes. In all low temperature fuel cells polytetrafluroethylene (PTFE) is used to give the electrode their hydrophobicity. The PTFE is not complete stable under fuel cell operation, therefore the hydrophobicity decreases. The alteration of the PTFE in the low temperature fuel cell electrodes depends on the electrode and fuel cell type as well as on the operating conditions. Some electrodes have a polymer film on the surface, which covers the catalysts, e.g. AFC nickel anodes produced by a rolling process or PEFC and DMFC electrodes, which were prepared by an wet process including an tempering step; other electrodes have a homogeneous distribution of the components without an polymer film on the surface like the dry sprayed DLR electrodes. Electrodes with a polymer film on the surface needs a start-up procedure, which change this covering PTFE film whereby the PTFE film will be partially removed. The typical time for this procedure is few hours up to few tens of hours. Under normal fuel cell operation in all fuel cell types the PTFE is changed more dramatically the anode compared to the cathode and this process is much faster in the DMFC than in the PEFC or AFC.