Steady-State and EIS Investigations of Hydrogen Electrodes and Membranes in Polymer Electrolyte Fuel Cells

Electrochemical impedance spectroscopy (EIS) and steady-state models have been developed for the porous hydrogen electrode with water concentration dependence and water transport in a polymer electrolyte fuel cell membrane. Because the hydrogen electrode performance is influenced by its water content, the hydrogen electrode model was coupled to the membrane model. The EIS model for the hydrogen electrode gave three to four loops in the complex plane plots. The high-frequency semicircle was attributed to the Volmer reaction and the medium-frequency semicircle to the hydrogen adsorption. The additional low-frequency loops were connected to changes in the hydrogen electrode performance with water concentration, due to changes in kinetics or proton conductivity. Those loops appear in a frequency range depending on the water transport in the membrane, changing with D ∕ L m 2 , where D is the water diffusivity and L m is the membrane thickness. Modeling of the membrane alone showed that the membrane gives rise to a loop in EIS. The difference between the high- and low-frequency intercepts of the loop is i d R ∕ d i , where the high-frequency intercept is equal to the membrane resistance. The loop appears in the same frequency range as the hydrogen electrode low-frequency loops and thus overlaps.