Lawrence Berkeley National Laboratory

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.