Anodic TiO2 nanotube arrays for photoconversion based hydrogen production

Abstract

Solar energy conversion and storage are potential technologies to improve energy security and achieve carbon neutrality. Among these technologies, the solar-to-chemicals conversion using photoelectrochemical cells to produce hydrogen fuel is a viable pathway. This work focuses on using highly active, stable, and flexible anodic TiO2 nanotube (TNT) electrodes in photoconversion cells for hydrogen production, such as the application of anode electrodes and cathode electrodes in water splitting (WS) cells. To improve solar energy capture, the two-anode reduction method and the stepwise cathodic reduction method were designed and verified as effective ways to modify pure anodic TNT. The performance of the reduced anodic TNT as a cathode in WS cell was -221.1 mA, which was 17,000-fold higher than that of TNT, and 5-fold that of commercial Ti. A stability test at CP@-100 mA for 24 h showed a decay of 1.3%. In the photoconversion cell, the onset potential of the reduced anodic TNT (TNT-C) has an anodic shift from -0.79 to 0.19 VRHE, corresponding to a performance increase from -110.06 to -210.66 mA at -1.0 VRHE. Self-improving performance was also found during the long-term tests, which were deduced to the self-formed composite structures, such as an n-p-n junction. When used as the anode, the reduced anodic TNT (TAR-2) gave a 3-fold enhancement over the pure one at 1.23 VRHE, 2.05 mA/cm2. The multi-size effects of the anodic TNT were investigated in a range of six orders of magnitude length scales ranging from 10-8 to 10-2 m. Increasing the nm-scale and cm-scale length size, the current density at 1.23 VRHE decreased. However, increasing the μm-scale length size, the performance at 1.23 VRHE increased. The light absorption intensity for reduced anodic TNT was enhanced, the light absorption range was broadened and the absorption edge had a redshift. The TNT-C showed six absorption peaks in the incident photon-to-electron conversion efficiency in the range of 365-1020 nm, while TNT showed an absorption edge at 430 nm. The electrical conductivity of the reduced samples has two arcs representing two electroactive interfaces with different kinetics in the Nyquist plot. The semiconductive arc was ten times smaller than that of pure anodic TNT, and the metallic arc was newly introduced indicating high electrical conductivity. The growth order of anatase TiO2 peaks was firstly reported and the growth rate of the anodic film was confirmed. The thesis shows that cathodic reduction methods, such as stepwise cathodic reduction, are promising for reducing the manufacturing costs of TNT-based electrodes.