Solar energy storage by a microfluidic all-vanadium photoelectrochemical flow cell with self-doped TiO2 photoanode

All-vanadium photoelectrochemical flow cell, which combines the vanadium redox flow battery and the photoelectrochemical flow cell, is a promising technology to store solar energy in reversible redox pairs. The development of a high-performance photoanode is vital to promote the storage of solar energy. In this work, we developed a self-doped TiO2 photoanode and applied it to a microfluidic all-vanadium photoelectrochemical flow cell (μVPFC). The self-doped TiO2 photoanode was simply prepared by annealing the TiO2 photoanode with NaBH4 in the nitrogen atmosphere, by which the self-dopant led to the formation of a disordered layer on the surface and created a mid-band. As a result, the light absorption region was extended and the electron-hole pair separation efficiency was enhanced, promoting the capability of the μVPFC with the self-doped TiO2 photoanode. The superiority of the self-doped TiO2 photoanode was confirmed by its excellent photoelectrochemical performance and vanadium ion conversion rate. The μVPFC with the self-doped TiO2 photoanode yielded an average photocurrent density as high as 0.064 mA·cm−2 in 6-h operation, which was much higher than the reported TiO2 and Ti2O3 photoanodes and presented the improvements by approximately 167% and 60%, respectively. In addition, intensifying the intensity of light and concentration of vanadium ion enabled the performance of the μVPFC with the self-doped TiO2 photoanode to be promoted.