Cobalt-Doped Black TiO ₂ Nanotube Array as a Stable Anode for Oxygen Evolution and Electrochemical Wastewater Treatment

Abstract

TiO ₂ has long been recognized as a stable and reusable photocatalyst for water splitting and pollution control. However, it is an inefficient anode material in the absence of photoactivation due to its low electron conductivity. To overcome this limitation, a series of conductive TiO ₂ nanotube array electrodes have been developed. Even though nanotube arrays are effective for electrochemical oxidation initially, deactivation is often observed within a few hours. To overcome the problem of deactivation, we have synthesized cobalt-doped Black-TiO ₂ nanotube array (Co-Black NTA) electrodes that are stable for more than 200 h of continuous operation in a NaClO ₄ electrolyte at 10 mA cm ^–2. Using X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, electron paramagnetic resonance spectroscopy, and DFT simulations, we are able to show that bulk oxygen vacancies (O _v) are the primary source of the enhanced conductivity of Co-Black. Cobalt doping both creates and stabilizes surficial oxygen vacancies, O _v, and thus prevents surface passivation. The Co-Black electrodes outperform dimensionally stable IrO ₂ anodes (DSA) in the electrolytic oxidation of organic-rich wastewater. Increasing the loading of Co leads to the formation of a CoO _x film on top of Co-Black electrode. The CoO _x /Co-Black composite electrode was found to have a lower OER overpotential (352 mV) in comparison to a DSA IrO ₂ (434 mV) electrode and a stability that is greater than 200 h in a 1.0 M KOH electrolyte at a current density of 10 mA cm ^–2.