Impacts of MnO<sub>2</sub> Crystal Structures and Fe Doping in Those on Photoelectrochemical Charge–Discharge Properties of TiO<sub>2</sub>/MnO<sub>2</sub> Composite Electrodes
Hiroyuki Usui, Shin Suzuki, Yasuhiro Domi, Hiroki Sakaguchi
Abstract
We investigated the impacts of MnO2 crystal structures and Fe doping into the MnO2 crystal structures on photoelectrochemical charge–discharge properties of composite electrodes composed of TiO2 and MnO2 polymorphs (α-, β-, γ-, and δ-phases) in aqueous Na2SO4 solution. In a conventional electrochemical capacitor, the α-MnO2 electrode delivered the highest specific capacitance among the undoped MnO2 polymorphs because of its larger tunnel structure compared with β- and γ-MnO2. Since the electronic conductivity of the δ-MnO2 electrode was very low, its performance was poor despite its large interlayer spacing. Fe doping into δ-MnO2 improved its conductivity, leading to a remarkable enhancement in capacitance. The photoelectrochemical capacitor properties of the TiO2/α-MnO2 and TiO2/δ-MnO2 composite electrodes were improved by Fe doping into MnO2. In particular, the TiO2/Fe-doped δ-MnO2 electrode presented a significant improvement. This was because the photoinduced electrons could move easily in the MnO2 layer due to its improved conductivity, thereby promoting the Na+ storage reaction.