Black TiO₂ nanotube arrays: Bifunctional electrocatalytic performance for alkaline water splitting

Black TiO₂, with its defect-rich structure, outperforms conventional white TiO₂ in hydrogen and oxygen evolution reactions due to its superior light absorption and charge separation. This study focuses on the fabrication of black TiO₂ nanotube arrays (BTNTs) via electrochemical anodization, with the aim of evaluating their dual functionality as photoelectrodes for water splitting in alkaline conditions. This investigation addresses a significant gap in the literature regarding the performance of BTNTs in photoelectrochemical water splitting under alkaline conditions and visible light. Among the samples, BTNTs prepared with a 2-h anodization time (BTNTs-2h) displayed outstanding photoelectrochemical properties, achieving the highest performance metrics. Notably, the BTNTs-2h sample featured a high concentration of oxygen vacancies (OVs), which significantly enhanced electrical conductivity and accelerated charge transfer processes. The optimized BTNTs-2h electrode exhibited remarkably high photocurrent density of 2.6 mA cm⁻² at 1.23 V vs RHE for oxygen evolution and −50 mV for hydrogen evolution at a current density of −10 mA cm⁻². Furthermore, the BTNTs-2h exhibited minimal resistance in both reactions. Most impressively, when employed as both the cathode and anode in a two-electrode water electrolysis system, the BTNTs-2h achieved full water splitting at an exceptionally low cell voltage of 1.46 V at 10 mA cm⁻². These results underscore the potential of BTNTs, especially BTNTs-2h, as cost-effective and efficient materials for sustainable water splitting applications, paving the way for advancements in renewable energy conversion and storage systems essential for the future energy landscape.