A morphology controlled surface sulfurized CoMn2O4 microspike electrocatalyst for water splitting with excellent OER rate for binder-free electrocatalytic oxygen evolution

Abstract

Transition metal mixed oxides have drawn extensive interest as oxygen evolution electrocatalyst alternatives to noble metal-based materials but generally involve prolonged synthesis routes and limited electrocatalytic activity and stability. Herein we report surface sulfurized CoMn₂O₄ microspikes (S–CoMn₂O₄-MSs) grown directly at low temperature and ambient atmosphere using a two-step facile synthesis protocol, requiring only 310 s. S–CoMn₂O₄-MSs showed excellent OER activity with an overpotential of 300 mV at 10 mA cm⁻² while pristine Mn₃O₄ and Co₃O₄ required 580 and 410 mV to deliver the same current density value. Moreover, we rationally designed the study to gradually decrease the Tafel slope value from 307.5 to 26.2 mV dec⁻¹ for the best electrocatalyst, with excellent stability at 20 mA cm⁻² for 24 h without any binder. This overall performance compares well with very recently reported oxides and CoMn₂O₄ based systems and the as-synthesized material showed highly competitive overall performance. During this work, the stability of the S²⁻ layer remained the main concern and was confirmed by several characterization techniques. We investigated the surface blockage role of the surface S²⁻ layer and, therefore, optimized the degree of sulfurization for best performance. In the end, we attributed and supported the exciting performance of S–CoMn₂O₄ MSs to the synergistic effect of Co and Mn, their unique morphology, the easily oxidizable lower oxidation states of cobalt (due to surface sulfurization treatment), and the large interior structure for increased contact with the electrolyte.