The Mechanism of Hydrogen Oxidation at Gold and Nickel Flag Electrodes in Molten Li/K Carbonate

Abstract

This research was performed to study the anode reaction in a molten carbonate fuel cell. Chronoamperometry experiments and impedance measurements were carried out at gold and nickel flag‐type electrodes in a 62 mole percent eutectic carbonate mixture at 923 K as a function of gas composition , and as a function of temperature between 823 and 1073 K. The extrapolation technique, generally applied to evaluate chronoamperometry results, is shown to yield systematic errors for the hydrogen oxidation studies in molten carbonates. A direct fit of the Gerischer‐Vielstich relation yields more reliable results for gold. The current responses on Ni cannot be described by the Gerischer‐Vielstich relation. The high reaction orders for hydrogen (0.7 to 1.0), medium for carbon dioxide (order 0.5), and the low or even negative reaction order for water (0 to −0.25), indicate the hydrogen oxidation follows a Volmer‐Heyrovsky‐type mechanism on gold flag electrodes. The reaction rate at nickel electrodes was found to be too high to be studied accurately using impedance measurements and chronoamperometry on flag electrodes. In spite of the poor accuracy, reaction orders indicate that a final chemical reaction is the rate‐determining step in the reaction mechanism on nickel. The activation energy for the exchange current density was found to be in the order of with a tendency to be higher for the gases richer in hydrogen. The activation energy for the Warburg coefficient (diffusion) was found to be about.