New intermediate temperature proton conductors: Hydrated heavy alkali thio-hydroxogermanates

Fuel cells are emerging as popular alternative power sources due to their high efficiencies, low green house gas emissions, and the possibility of using renewable hydrogen fuels. At the heart of the proton-based fuel cell is the proton exchange membrane (PEM) transporting protons from the anode to the cathode while providing electronic insulation. Hydrated polymers such as Nafion® are the most popular PEM for many applications, but they are restricted by strong humidification requirements to operating temperatures below 100 °C. Alternative membrane materials are critically needed for the intermediate temperature range of 100 to 300 °C where higher efficiencies and easier water (steam) management are possible. Here we report a new class of solid inorganic hydroxide proton conductors with mixed anions, specifically hydrated alkali thio-hydroxogermanates. As an example, proton conductivities for the amorphous cesium thio-hydroxogermanate Cs2GeS2(OH)2·xH2O (0 < x < 5) range from 10− 3 to 10− 2 (Ω cm)− 1 for temperatures of 100 to 275 °C under low relative humidities (R.H. < 12%). And unlike typical hydrated PEMs which rapidly dehydrate above 100 °C, the conductivity of these thio-hydroxogermanates decays slowly (∼days) under low R.H.