Influence of Proton Activity Gaps between Electrodes on Open-Circuit Potential of H2/H2 and H2/Air Cells

In polymer electrolyte fuel cells, proton activity thermodynamically correlates with the relative humidity (RH) as well as water uptake (λ) of the ionomer electrolyte. Proton activity differences between the two electrodes arise from RH control, and the activity terms of the Nernst equation could become significant. We investigated the effect of proton activity on open-circuit potentials (OCPs) and cell performance by controlling the RH of H2/H2 and H2/air cells, and on the kinetic current (ik) using a rotating disk electrode (RDE). In the H2/H2 cell, OCPs were caused by the RH difference between electrodes; thermodynamic analysis was performed to rationalize the experimental OCP by correlating the RH and λ. The H2/air cell performance was low owing to the high resistance and low exchange current density at RH < 30%. The RDE measurements showed that ik decreased with increasing concentrations (proton activities) of HClO4, H2SO4, and CF3SO3H (TFMSA).