(119c) Membrane Degradation of Proton Exchange Membrane Devices Operating in an Oxygen Deprived Environment

Cation exchange membranes based on perfluoroionomers have been used extensively in fuel cells, electrolyzers and other devices for over 30 years. Frequently, these membranes are called upon to serve two important functions within an electrochemical cell. First they provide a barrier that effects a positive separation of reactants and/or products. Second, these membranes provide selective ionic conduction between the anode and cathode chambers of the cell. In this manner they help to complete the electrochemical circuit between the two half-reactions. In some cell designs, these membranes are also utilized as the central element in a gasket seal design, thereby preventing fluids leakage from the electrochemical cell to the external environment. Compromise of any of these important functions can lead to premature cell failure; limiting the overall reliability of the PEM system.

Today's service requirements for PEM electrochemical devices for stationary and transportation applications require the demonstration of thousands of hours of invariant performance under the challenging operating conditions. Extensive accelerated laboratory and in-service testing of PEM fuel cells designed for these applications has shown some form of degradation, manifesting itself in two modes. The first is a gradual unrecoverable performance loss often characterized by continual voltage decay at a specified current density. The second is a sudden failure of cell or stack associated with gas crossover through pinholes and tears or fissures formed in the polymer electrolyte membrane. Some work has been done to study each of these modes, however a complete picture regarding the mechanism of each of these degradation modes is not available.

While many researchers have studied the durability of PEM fuel cells, much is still not known about degradation modes and the effects of damage accumulation on operating cells. Extensive research has, however, been conducted to elucidate the chemical pathways for membrane decomposition. The detection of the decomposition products, such as hydrogen fluoride and other polymer chain fragments in the condensed effluent water provides a strong indicator of the health of perfluoroionomer membranes. Through these and other studies, researchers have indicated that most degradation mechanisms rely on the presence and participation of oxygen to either initiate or to propagate the failure mode.

But what is not well understood is what happens when the cell is removed completely from the oxygen environment. This paper explores the degradation characteristics of electrochemical cells based on PEM technology that are operated in an oxygen deficient environment. In this regard, degradation of PEM cells configured to operate in the hydrogen pump mode was characterized. Stack lifetime performance data are reported along with diagnostic studies of the condensate effluent and product gas quality from a 25-cell prototype stack. Performance results are compared to PEM fuel cells that use air as the oxidant.