Conductivity and Hygroscopic Expansion of Ionomer Thin Films: Effect of Cobalt Exchange and Thermal/Aqueous Treatment

The replacement of pure Pt catalysts with Pt–Co and Pt–Ni alloy catalysts has helped reduce the cost of polymer electrolyte fuel cells but has also introduced long-term performance degradation due to leaching out of cobalt and nickel into the catalyst layer ionomer and polymer electrolyte membrane. This work reports the impact of exchange of protons with cobalt ions on the humidity-dependent (0–90% RH) hydration and conductivity of ∼30 nm thin ionomer films at a fuel cell-relevant temperature (80 °C) for seven different ionomers varying in equivalent weight from 620 to 1100 g/mol of sulfonic acid. A significant suppression (up to 2 orders of magnitude at low RH) in ionic conductivity was observed for all ionomers upon exchange of protons with cobalt ions, consistent with prior studies. From a corresponding measurement of film swelling characteristics of H+-form and Co2+-exchanged ionomers, not reported earlier, the present study provides a clear piece of evidence that the water content of the ionomer films decreases upon Co2+ exchange. This suppression in water uptake is the dominant cause of the reduction in ionic conductivity of ionomer films upon Co2+ exchange. The most interesting finding of the study is that a large variation in conductivity between the H+ form and Co2+ form of ionomer films at a given RH is significantly minimized when conductivity is correlated with the water content (λ = number of water molecules per protogenic group). Accordingly, the work introduces a new universal relationship for ionic conductivity and water content of the ionomer films.