Interaction of phosphoric acid with cell components in high temperature polymer electrolyte fuel cells

A high-temperature polymer electrolyte fuel cell (HT-PEFC) is an efficient and clean energy converting device. The protonic conductivity of the electrodes and the polybenzimidazole-type membrane is assured by phosphoric acid. The electrochemical reactions in HT-PEFCs of hydrogen and oxygen to water take place in the electrodes of the membrane electrode assembly (MEA) which are partly soaked with phosphoric acid. The performance of a HT-PEFC depends mainly on the interactions between all cell components as well as on the amount, the concentration and distribution of phosphoric acid inside these components. Whereas, the concentration and distribution is a function of current density and water vapour distribution in the MEA. The reaction between phosphoric acid and poly[2,5-benzimidazole] (ABPBI) membrane was investigated with the focus on kinetics and thermodynamics. The maximum acid doping levels were estimated experimentally at different doping times, doping temperatures and concentrations of the phosphoric acid bath. Afterwards, the reaction mechanisms were examined. The studies involved the amount and type of the coordinating sites of the polymer, the activation energy, the rate-limiting step, the equilibrium constant and the dissociation constant. In the MEA, the phosphoric acid is uneven distributed in the ABPBI membrane and the catalyst layers of gas diffusion electrode (GDE). This distribution changes with total acid uptake and the GDE structure. When the cells are operated under constant current, equilibrium between water production and water removal in the electrodes is established. The water removal is associated with the transport properties of the MEA components and the flow field geometry of the cell. As a result, the phosphoric acid concentration is influenced in the catalyst layer. In addition, the oxygen distribution in the catalyst layer is sensitive to the structure of GDEs, the flow properties of the flow field, the concentration and distribution of phosphoric acid in the catalyst layer. A series of experiments revealed that the optimum cell performance was achieved at high phosphoric acid doping level, thick cathode catalyst layer with the high Pt loading and a medium anode thickness with about half the Pt loading compared to the cathode. It was further demonstrated that a spiral flow field which allows gas cross over between adjacent flow channels resulted in better cell performances and lifetime compared to a conventional serpentine flow field.