TiO2/MnO2 composite electrode enabling photoelectric conversion and energy storage as photoelectrochemical capacitor

We prepared composite electrodes by using rutile TiO2 particles and γ-MnO2 particles, and evaluated their photoelectrochemical capacitor properties based on Na+ adsorption by light irradiation in aqueous electrolytes. By employing different synthesis method for TiO2 particles, we synthesized TiO2 particles with various particle sizes and crystallite sizes. An electrode of sol-gel-synthesized TiO2 showed higher photovoltages compared with an electrode of commercial TiO2. This probably originates from a larger contact area between electrode surface and electrolyte because of its smaller particle size than commercial TiO2's size. A further enhancement in photovoltage was attained for an electrode of a hydrothermally-synthesized TiO2 with good crystallinity. We consider that electron-hole recombination was suppressed because hydrothermal TiO2 has a lower density of lattice defect trapping the photoexcited carriers. As photoelectrochemical capacitor, a composite electrode consisting of hydrothermal TiO2 and MnO2 exhibited a 2.4 times larger discharge capacity compared with that of commercial TiO2 and MnO2. This result is attributed to an increased amount of Na+ adsorption induced by the enhanced photovoltage of TiO2.