Improved catalytic activity on transitioning from inverse to normal spinel in Zn2−xGa2xSn1−xO4: a robust bifunctional OER and HER electrocatalyst

Abstract

Water splitting by electrolysis is considered as one of the best methods for the production of hydrogen which is a clean and green fuel that can replace the existing non-renewable sources. Herein, Zn_2−xGa_2xSn_1−xO₄ with oxygen vacancies has been synthesized via a solid state method. Furthermore, the influence of oxygen vacancies on electrocatalytic activity is investigated systematically. The presence of oxygen vacancies has been confirmed by X-ray photoelectron spectroscopy (XPS) and positron annihilation lifetime spectroscopy (PALS) and it was found that higher oxygen vacancy concentration is present in zinc gallate. As a result, ZnGa₂O₄ exhibits prominent hydrogen evolution reaction (HER) achieving a current density of 20 mA cm⁻² with a low overpotential of 360.0 mV and prominent oxygen evolution reaction (OER) achieving a current density of 10 mA cm⁻² at an overpotential of 370.0 mV, and the voltage required for overall water splitting is 2.0 V @ 10 mA cm⁻². The catalyst also exhibits good stability for up to 12 hours and it exhibited a faradaic efficiency of 92% for the OER and 95% for the HER. The role of oxygen vacancies adds multifunctionalities in these materials in terms of light emission. ZnGa₂O₄ has shown superior photoluminescence compared to Zn₂SnO₄ and interestingly there was also color tunability from orange to blue region going from zinc stannate to zinc gallate.