A novel method involving the fabrication of Mo–W mixed oxide (MoxW1–xO3) is proposed to modify the modest reaction kinetics and poor cycling stability of MoO3 material. By a simple coelectrodeposition method, a series of MoxW1–xO3 oxides is deposited on a TiO2 nanotube array substrate. Because of the differences between Mo6+ and W6+ in nature, there is significant distortion existing in the mixed oxides, leading to their decreased crystallite size and enlarged lattice space, which facilitates ion diffusion in the solid. As results, the mixed oxides show much better balance between specific capacitance and cycling stability than the bare MoO3 or WO3 sample, which suffers from either poor cycling stability or low electrochemical activity. Impressively, the optimal Mo–W mixed oxide exhibits a high specific capacitance of 517.4 F g–1 at 1 A g–1, and, moreover, it retains 89.3% of the capacitance even at a high current density of 10 A g–1, demonstrating ultrahigh rate capability. These findings reveal the potential of the Mo–W mixed oxide for constructing advanced ultrahigh power supercapacitors.Keywords: energy storage; mixed oxide; molybdenum oxide; titania nanotube arrays; tungsten oxide; ultrahigh rate supercapacitor;

•Successful fabrication of TiO2@MnO2 nanotube arrays by pulsed electrodeposition.•A high current pulse is used to create sufficient nucleation trend on tube walls.•TiO2@MnO2 nanotube arrays structure exhibits enhanced electrochemical performance.•The application of TiO2@MnO2 nanotube arrays for high-performance supercapacitors.The fabrication of TiO2@MnO2 nanotube arrays (NTAs) structure is accomplished via a pulsed electrodeposition technique. The successful preparation of this structure is owing to the high current pulse used, which creates a sufficient nucleation trend on the wall of the entire TiO2 nanotube layer, that results in the continuous deposition of MnO2 along the nanotube walls. The synthesized TiO2@MnO2 NTAs sample is found to exhibit excellent electrochemical behavior with high specific capacitance of 425.0 F g−1 and areal capacitance of 150.9 mF cm−2, ranking in the top level ever achieved by a MnO2-TiO2 composite. Furthermore, a symmetric supercapacitor device is assembled using two strips of the TiO2@MnO2 NTAs electrode that yielded energy density as high as 7.4 Wh kg−1, a much higher value than most of the values reported for symmetric MnO2//MnO2 supercapacitors. The remarkable electrochemical properties of the TiO2@MnO2 NTAs sample make it an attractive candidate for the application of constructing high-performance supercapacitors.

In this study, a photocatalytic material g-C3N4–Ti3+/TiO2 nanotube arrays was prepared by a facile and viable approach involving a heat treatment followed by an electrochemical reduction step, and it was characterized using instrumental techniques such as X-ray diffraction pattern, Fourier transform-infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and UV-vis diffuse reflectance spectra. The photocatalytic efficiency of the as-prepared samples towards treating aqueous solution contaminated with phenol was systematically evaluated by a photoelectrocatalytic method and found to be highly dependent on the content of the g-C3N4. At the optimal content of g-C3N4, the apparent photocurrent density of g-C3N4–Ti3+/TiO2 was four times higher than that of the pristine TiO2 under visible-light illumination. The enhanced photoelectrocatalytic behavior observed for g-C3N4–Ti3+/TiO2 was ascribed to a cumulative impact of both g-C3N4 and Ti3+, which enhances the photoresponsive behavior of the material into the visible region and facilitates the effective charge separation of photoinduced charge carriers.

•Self-doped TNTs photocatalyst is prepared by electrochemical reduction.•Self-doped TNTs photoelectrodes show an enhanced photoelectrochemical activity.•Self-doping can enhance both the absorption in visible region and electrical conductivity of the TNTs.•Self-doping can decrease the Rct between the solid-electrolyte interface.In this study, a simple electrochemical reduction approach is reported to enhance the photoelectrochemical performance of TiO2 nanotube arrays towards degrading water contaminants such as Rhodamine B, phenol and E. coli K-12. The results obtained from X-ray diffraction and X-ray photoelectron spectroscopy demonstrate that oxygen vacancies i.e., Ti3+ self-doping, were formed in the lattices of TiO2 nanotube arrays during the electrochemical reduction process of pristine TiO2 nanotube arrays at different negative potentials ranging from −1.2 to −1.5 V. Comparing the pristine TiO2 nanotube arrays, the treatment TiO2 nanotube arrays samples by electroreduction process were found to be showing enhanced photoelectrocatalytic activity in the UV and visible regions in the entire potential window tested. Further, the photocurrent density of self-doped TiO2 nanotube arrays sample prepared at −1.3 V was 250% higher than that of the pristine TiO2 under visible-light illumination. Impedance analysis revealed that the electrical conductivity of the nanotubes significantly enhanced after self-doping. The photoelectrocatalytic activity of the self-doped TiO2 nanotube increased dramatically due to the enhanced electrical conductivity and absorption in the visible light region, as well as an accelerated charge transfer rate between the interface of solid and electrolyte.

•The PFC system of WO3/TiO2-CuO/TiO2 provides an interior bias potential (0.18 V).•The photoelectrodes of the PFC system can be stimulated by visible light.•The PFC is effective for photocatalytic degradation of Rhodamine B.A visible-light driven Photocatalytic Fuel Cell (PFC) system comprising WO3/TiO2 and CuO/TiO2 nanotube array materials as photoanode and photocathode, was established with a dual objective of degrading an organic water pollutant and generating electric power as well. Under illumination, the Fermi level of WO3/TiO2 nanotube arrays photoanode was higher than that of CuO/TiO2 nanotube arrays photocathode. Arising an interior bias (0.18 V) induced the transfer of electrons from the photoanode across the external circuit to the photocathode and combination with the holes produced therein with electric power generation. In this manner, the separation of electron/hole pair was achieved in the photoelectrodes by releasing the holes of WO3/TiO2 nanotube arrays photoanode and electrons of CuO/TiO2 nanotube arrays photocathode. Using this PFC based system, the degradation of an organic water pollutant, Rhodamine B, was successfully accomplished with determining its decolorization and the variation in total organic carbon (TOC) content. The decolorization and TOC removal were obtained to the extent of 100% and 57%, respectively at 4 h reaction time. The stability of the photoelectrodes for scaling up applications could also be confirmed by the repeated several experimental runs using the electrodes. The proposed photoelectrocatalytic system provides a self-sustained and energy-saving methodology for wastewater treatment with a parallel energy production.

The application of highly ordered TiO2 nanotube arrays (NTAs) for energy storage devices such as supercapacitors has been attractive and of great interest owing to their large surface area and greatly improved charge-transfer pathways compared to those of nonoriented structures. Modification of the semiconductor nature of TiO2 is important for its application in constructing high-performance supercapacitors. Hence, the present study demonstrates a novel method involving fabrication of self-doped TiO2 NTAs by a simple cathodic polarization treatment on the pristine TiO2 NTAs to achieve improved conductivity and capacitive properties of TiO2. The self-doped TiO2 NTAs at −1.4 V (vs SCE) exhibited 5 orders of magnitude improvement on carrier density and 39 times enhancement in capacitance compared to those of the pristine TiO2 NTAs. Impedance analysis based on a proposed simplified transmission line model proved that the enhanced capacitive behavior of the self-doped TiO2 NTAs was due to a decrease of charge-transport resistance through the solid material. Moreover, the MnO2 species was introduced onto the TiO2 NTAs by an impregnation–electrodeposition method, and the optimal specific capacitance achieved (1232 F g–1) clearly confirmed the suitability of self-doped TiO2 NTAs as effective current collector materials for supercapacitors.

•TiO2 nanotube arrays were modified by a facile cathodic reduction process.•Oxygen vacancies were formed and hydroxyls were introduced on the surface of TiO2.•The electrochemical activity and conductivity of modified sample were improved.•The modified sample showed much enhanced capacitance over the pristine one.TiO2 nanotube arrays are modified by a facile cathodic reduction process treatment and the results are discussed in terms of their electrochemical activity and conductivity. The instrumental characterizations such as X-ray photoelectron spectroscopy and Raman spectroscopy indicate that the formation of oxygen vacancies in the lattice and introduction of hydroxyl groups on the surface of TiO2 take place. The capacitance of the modified sample is found to be 13 times larger than the pristine TiO2 nanotube arrays. This work reveals a feasible and simple method to improve electrochemical activity and conductivity of TiO2 for supercapacitors application.

In this study, a photocatalytic material g-C3N4–Ti3+/TiO2 nanotube arrays was prepared by a facile and viable approach involving a heat treatment followed by an electrochemical reduction step, and it was characterized using instrumental techniques such as X-ray diffraction pattern, Fourier transform-infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and UV-vis diffuse reflectance spectra. The photocatalytic efficiency of the as-prepared samples towards treating aqueous solution contaminated with phenol was systematically evaluated by a photoelectrocatalytic method and found to be highly dependent on the content of the g-C3N4. At the optimal content of g-C3N4, the apparent photocurrent density of g-C3N4–Ti3+/TiO2 was four times higher than that of the pristine TiO2 under visible-light illumination. The enhanced photoelectrocatalytic behavior observed for g-C3N4–Ti3+/TiO2 was ascribed to a cumulative impact of both g-C3N4 and Ti3+, which enhances the photoresponsive behavior of the material into the visible region and facilitates the effective charge separation of photoinduced charge carriers.