Publication date: 15th December 2014
Combination of a solar cell and an electrochemical cell for water splitting is expected high energy conversion efficiency from solar to hydrogen energy conversion [1]. The overpotential for anodic electrode is, however, one of the large energy loss processes. Although many electrochemical water oxidation catalysts like RuOx, CoOx, NiOx, and MnOx are proposed to reduce the overpotential, the mechanisms of overpotential reduction has been obscure. In this report, the carrier transfer mechanisms are tried to be clarified using the proposed mechanisms of water oxidation by n-type GaN.
The water oxidation reaction by n-type GaN photoelectrode is summarized that the process occurs via electron transfer from water reduction state to intermediate state of GaN. The process is similar to the Gericher model, which holds under single electron and outer sphere transfer, even under the condition of 4-electron transfer. The important characteristic is that the carrier lifetime of the intermediate state is extremely long compared from that for the usual trap states which can be waited the reactant formation change to the next electron transfer can occur. The origin of the long lifetime is still under discussion but one of the possibilities is the break of the crystal symmetry. The symmetry break supresses the radiative recombination of the exciton-like donor-acceptor carrier which acceptor locates at the intermediate state.
The interesting point for the electrochemical catalysts for water oxidation is that the materials are transition metal oxides. Some of the transition metal oxides have intermediate isolated d-band, which band is usually filled by electron, in-between the usual conduction and valence band. This isolated band has a possibility to have long-life hole due to the deviation of the stoichiometry of oxides and/or the element change from oxygen to hydroxide ion. This condition is similar to the n-type GaN photoanode.
[1] K. Fujii et al., Int. J. Hydrogen Energy 38 (2013) 14424.
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