Photocharged Water Splitting Employing a Nickel(II) Tellurium Oxide (Photo)anode in Alkaline Medium

Nickel-based oxides and (oxy)hydroxides are known to be electrocatalytically active for the oxygen evolution reaction (OER) in an alkaline medium. However, the aforementioned materials would be more auspicious, especially for photoelectrocatalysis, if their optical properties fell within the range of the solar spectrum. These properties could be attained by either doping or by creating a specific crystal structure. In this study nickel(II) tellurium oxide was synthesized by employing the Pechini sol-gel synthesis route and calcined at 400 °C (NTO-400), 600 °C (NTO-600), and 800 °C (NTO-800). It was physically characterized and (photo)electrochemically tested for the OER in alkaline medium. Under electrocatalytic (EC) conditions, NTO-600 displayed the lowest overpotential, 353 mV, to drive a current density of 10 mA cm^-2 and exhibited the lowest charge transfer resistance of 20.75 Ω. Under photoelectrocatalytic (PEC) conditions, NTO-600 (with a bandgap of 2.50 eV) realized a current density increase of 55% compared to pure EC conditions. Remarkably, this current density gain is (to a greater extent) maintained, subsequent to the termination of illumination, which points to photocharging of the material and the subsequent utilization of the stored charge to drive the OER (in the dark). Solar energy conversion, storage, and subsequent utilization is therefore realized through the use of nickel(II) tellurium oxide.