Theory of water management at the anode side of polymer electrolyte fuel cell membranes

The behavior of the water transport and the water concentration profile at the anode side of the membrane in a polymer electrolyte fuel cell is considered theoretically. A linear transport equation based on the diffusion of water and electroosmotic water drag was taken into account. Two kinds of boundary value problem were treated: (1) for semi-infinite boundary conditions, analytical solutions for water concentration at a given time and location were obtained, under either humidified or non-humidified conditions at the anode side of the membrane; (2) for finite boundary conditions, the steady-state water concentration profile was obtained with two kinds of boundary condition at the cathode side. The effect of various fuel cell operation and membrane state parameters on the water concentration was evaluated in a systematic way. Among those parameters tested, the current density and water penetration parameters are essential in determining membrane water content. Membrane thickness and the diffusion coefficient of water are important parameters in the case of finite boundary conditions. It turned out that contamination of the membrane by foreign impurities (e.g. NaCl), even on the surface, will cause a serious effect on the water depletion at the anode side. Water supply from the anode side of the membrane results in a considerable improvement of the membrane state with regard to water depletion. Although the method was fairly simplified in comparison with the most detailed analysis of membrane water content, it gave explicit and easy-to-handle equations for the purpose of quantitative or semi-quantitative evaluation of the membrane water management in polymer electrolyte fuel cell operations.