Influence of Proton Activity in H2/H2 Cells: Implications for Fuel-Cell Operation with Low Relative Humidities

Abstract

The proton activity term is typically ignored in the Nernst equation because of the definition of the unit activity of protons within catalyst layers in proton-exchange membrane fuel cells although the relative humidities of an anode (RHA) and a cathode (RHC) can be different. Herein, we investigate the effect of proton activity on the open-circuit voltage (OCV) of a H2/H2 cell by individually controlling RHA and RHC at ≤30%. The OCV was thermodynamically estimated by applying the correlations of the RH and water uptake of a Nafion® membrane. The OCV experimentally increased with an increase in the humidity difference: the highest OCV of 77 mV was observed at RHA 30% and RHC 0%. The electro-osmotic coefficient (ξ) was calculated and found to be 0.73 at 5%–30% RHC and 30% RHA. The kinetic current (i k ) of the oxygen-reduction reaction was measured by the rotating disk electrode method to verify the influence of proton activity (a H+ ). i k was described as i k ∝ a H+ −β , with β values of 0.29 and 0.45 for H2SO4 and CF3SO3H, respectively, at 0.9 V. The results demonstrate that for the dry operation of fuel cells, especially for heavy-duty applications, proton activity effects within ionomers must be considered.