Dramatically improved energy conversion and storage efficiencies by simultaneously enhancing charge transfer and creating active sites in MnO_x/TiO₂ nanotube composite electrodes

We report a general approach to achieve greatly boosted efficiency of electrochemical process in the photoelectrochemical (PEC) water splitting reaction and supercapacitive process by simultaneously enhancing charge transfer and creating active sites in TiO₂ nanotube arrays (TNAs) electrodes. Shallow donor defects self-doping and MnO_x deposition are adopted to fulfill the two desired roles, respectively. Our results clearly demonstrated that the enhanced charge transfer resulted from vacuum induced defect self-doping could dramatically improve the water splitting performance and areal capacitance of the pristine TNAs, whilst the deposited manganese oxide species, serving as electrochemically active sites, can further substantially boost their energy conversion and storage efficiencies. As a result, the MnO_x/TNAs anode exhibited a photoconversion efficiency of 0.56% in neutral Na₂SO₄, which is the highest value of TiO₂ anode in the same electrolyte under AM 1.5G illumination. Also, the composite electrode presented an areal capacitance of 12.51mFcm^-2 at 5mVs^-1, nearly three orders of magnitude lager than that of the pristine TNAs counter electrode. Moreover, a specific capacitance of 1117Fg^-1 (at 5mVs^-1) was achieved based on the mass of deposited MnO₂, which also ranks in the top performance level of MnO₂ based supercapacitor devices. It is hoped that the present work may shed new light on achieving enhanced performance in energy conversion and storage devices.