Impacts of MnO2 Crystal Structures and Fe Doping in Those on Photoelectrochemical Charge-Discharge Properties of TiO2/MnO2 Composite Electrodes

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 (alpha-, beta-, gamma-, and delta-phases) in aqueous Na2SO4 solution. In a conventional electrochemical capacitor, the a-Mn0 2 electrode delivered the highest specific capacitance among the undoped MnO2 polymorphs because of its larger tunnel structure compared with beta-and gamma-MnO2. Since the electronic conductivity of the delta-MnO2 electrode was very low, its performance was poor despite its large interlayer spacing. Fe doping into delta-MnO2 improved its conductivity, leading to a remarkable enhancement in capacitance. The photoelectrochemical capacitor properties of the TiO2/alpha-MnO2 and TiO2/delta-MnO2 composite electrodes were improved by Fe doping into MnO2. In particular, the TiO2/Fe-doped delta-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.