Highly efficient bimetallic counter cations-based tungsten bronzes electrocatalysts developed for sustainable oxygen evolution in acidic solution

Abstract

This study involves the designing of monophosphate tungsten bronzes (MPTBs) with a combination of earth-abundant Al and Ni counter cations synthesized via the solution combustion as robust, highly efficient electrocatalysts for oxygen evolution reaction (OER) in 1.0 M H2SO4 solution. A series of mixed Al and Ni counter cations-based MPTBs was prepared at different temperatures and characterized with various sophisticated techniques concerning their compositions, morphologies, microstates, and electronic states. The catalyst with Al/Ni ratio of 1:3 of tetragonal cube-shaped MPTB calcined at 700 °C ((Al1/3,Ni)-MPTB7) exhibited an extraordinary OER performance with a larger electrochemically active surface area, almost zero onset overpotential (η), and a lower solar cell-equivalent η (i.e., at 10 mA cm−2) of 199 mV that is lower than that obtained with the state-of-the-art RuO2 catalyst. A progressive enhancement of the performance of the catalysts was noticed for a 12 h OER run, ensuring the robustness of the materials developed for a long-term application. The mixed valency states of tungsten (W), i.e., W5+/W6+ states, along with higher oxygen vacancies of (Al1/3,Ni)-MPTB7 are evaluated to tender suitably oriented active sites for effective adsorption of H2O during the OER. A substantial increase in oxygen vacancies and W5+ state, particularly the W5+4f7/2 state in the catalysts, resulted from a spin-exchange phenomenon during a long-term OER is associated with OER catalytic enhancement. The thus-developed acid-stable, robust MPTBs with mixed Al and Ni counter cations would be an alternative to the pricy noble metal counterparts used in clean energy water splitting technology.