Anion-exchange synthesis of an MnCo2S4 electrocatalyst towards facilitated ultralong hydrogen evolution reaction in acidic and alkaline media

Abstract

Hydrogen (H₂) is deemed one of the prominent energy solutions of the 21st century due to the significant scientific and technological efforts devoted to the realization of a renewable carbon-free H₂-based economy. The generation of highly efficient clean H₂ fuel from the electrocatalysis of earth-abundant water is one of the key aspects of sustainable clean energy technologies. Traditionally, Pt-based catalysts have demonstrated an efficient hydrogen evolution reaction (HER), but their scarcity and high cost hinder their commercial applications. Herein, we demonstrate a facile synthesis of the MnCo₂S₄ nanosheet catalyst that was successfully transformed from MnCo₂O₄ through an anion-exchange reaction. The electrochemical performance of MnCo₂S₄ is considerably superior compared to that of MnCo₂O₄, demonstrating low overpotentials of −111 and −233 mV (1 M KOH) and −124 and −323 mV (0.5 M H₂SO₄) at current densities of 10 and 500 mA cm⁻², respectively, with a moderate Tafel slope of 63 mV dec⁻¹. The optimized catalyst showed excellent sustainability for an ultralong H₂ evolution up to 50 h at 10 and 100 mA cm⁻² in both media and an admirable endurance in a voltage-step response at various current rates (10 to 100 mA cm⁻²). The post-stability XPS reveals the partial alteration of cobalt into a metallic state on the catalyst surface without a significant change in the surface topography. The outstanding HER performance is credited to the increased number of electrochemically active sites and enhanced electronic conductivity resulting from the conversion of the oxide phase into a sulfide phase through anion exchange.