Material design based on impurity element doping for photoelectrochemical capacitor composite electrodes using metal oxides

Abstract To evaluate the properties of TiO2/MnO2 composite electrodes for use as photoelectrochemical capacitors, Zn-doped TiO2 with a rutile-type structure hydrothermally synthesized as a photoelectric conversion material. The substitutional solid-solution of 2 at.% Zn in TiO2 successfully increased the oxygen vacancy amount and the electronic conductivity. A wider optical band gap was observed for the Zn-doped TiO2 compared with an undoped TiO2, indicating that Zn doping increased the carrier concentration due to the generation of oxygen vacancies. The photovoltage of the TiO2 electrode was drastically enhanced from 250 to 1600 mV by 2 at.% Zn doping. The charge-discharge properties of the Zn-doped TiO2/MnO2 composite electrodes were evaluated by photoirradiation of Zn-doped TiO2, where Na+ adsorption−desorption reactions occurred on the MnO2 surface. The Zn-doped TiO2/MnO2 composite electrodes afforded a larger discharge capacity of 13 mA h cm−2 compared to 7 mA h cm−2 for the TiO2/MnO2 composite. Plausibly, the higher carrier concentration increased the Fermi level above the conduction band of TiO2, enabling conducting electrons to overcome the energy barrier between the TiO2 particles. Doping Fe into MnO2 in the composite electrode further enhanced these properties, and Fe doping increased the capacity of the composite from 13 to 25 mA h cm−2. These results demonstrate that the commonly used approach of doping with impurity elements can improve the properties of various electrode materials for use as photoelectrochemical capacitors.