Comparison of Forward and Reverse Reactions in Hydrogen Generation between GaN, InGaN, Nanocrystalline TiO2 and Pt Electrodes during Water Electrolysis

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The electron transport between n-, p-GaN, and p-In0.12Ga0.88N electrodes and electrolyte solution is studied by cyclic voltammetry under dark conditions, and compared with that between Pt and nanocrystalline TiO2/Ti electrodes. Both forward and reverse reactions of $\text{2H$^{+}$} + \text{2e$^{-}$}\rightleftharpoons \text{H$_{2}$}$ occur for Pt and TiO2/Ti electrodes. In contrast, in n-, p-GaN and p-In0.12Ga0.88N electrodes, only the forward reaction of $\text{2H$^{+}$} + \text{2e$^{-}$}\rightarrow \text{H$_{2}$}$ occurs and no reverse reaction is observed. These results are consistent with the finding that the conduction band-edge potentials of n-, p-GaN, and p-In0.12Ga0.88N are higher than the reduction potential of H+(aq) in electrolyte, determined by flat-band potential measurement. These differences in potential work as an energy barrier for the reverse electron transfer from H2 molecules to electrodes. These results also support the experimental results of spontaneous H2 generation caused by the band-gap excitation of p-GaN and p-In0.12Ga0.88N electrodes without applying bias voltage.
2008-12-25