Elevated Temperature Proton Exchange Membrane Water Electrolysis: Electrochemical and Product Gas Analysis

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Operating proton exchange membrane (PEM) water electrolyzers at elevated temperatures is a potential pathway to increase operational efficiency and reduce hydrogen production costs. In this study, an in-house developed elevated temperature PEM electrolysis test station operated with liquid water is coupled with online mass spectrometry to investigate the true H2 production efficiency of perfluorosulfonic acid-based 5 cm2 single cells in the temperature window of 60 to 120 °C via electrochemical characterization techniques and a product gas analysis. Thermodynamic and kinetic improvements responsible for reducing the required cell voltage for water splitting by 103 ± 4 mV at low current densities when increasing the operating temperature by 60 K are analyzed. A temperature-induced reduction in ohmic and anode catalyst layer resistances is observed and possible reasons considering intrinsic and interfacial property changes are postulated. The H2 production rate increases by approximately 80% when elevating the operating temperature from 60 to 120 °C at a given cell potential of 1.8 V. Furthermore, hydrogen crossover and true Faradaic efficiencies are determined by closing the hydrogen and oxygen mass balance, revealing that operation at 120 °C with the given cell configuration is neither efficient (the Faradaic efficiency decreases on average by about 4%) nor safe, presumably due to microstructural changes resulting from enhanced creep operating in the vicinity of the ionomer’s glass transition temperature.