Tailoring MMn2O4 spinels anchored on nitrogen-doped reduced graphene oxide for high performance bifunctional water splitting

Abstract

Developing highly efficient, durable, and low-cost oxygen evolution reaction (OER) electrodes remains a key barrier that needs to be overcome for producing green H₂ through alkaline water electrolysis (AWE). A series of transition metal cation-substituted manganese oxides (M–Mn₂O₄, where M = Co, Cu, Fe, and Zn) over nitrogen-doped rGO (N–rGO) nanosheet (M–Mn₂O₄@N–rGO) heterostructures have been developed using a facile solvothermal strategy. Among the tested compositions, the FeMn₂O₄@N–rGO heterostructures exhibited superior OER catalytic activity, with an overpotential (η) of ∼330 mV at 10 mA cm⁻² along with stability over 96 h in 1 M KOH. In addition to OER activity, the FeMn₂O₄@N–rGO heterostructure exhibited better hydrogen evolution reaction (HER) performance, achieving an overpotential of 153 mV at a current density of 10 mA cm⁻². The coexistence of Fe³⁺/Fe²⁺ and Mn³⁺/Mn⁴⁺ redox couples provides multiple electron transfer pathways during water splitting. When assembled as a bifunctional electrode for overall water splitting (FeMn₂O₄@N–rGO as both the anode and cathode), the system delivers a cell voltage of 1.57 V at 10 mA cm⁻², with operational stability over 10 h. The robust spinel framework and strong metal–support interactions confirm that FeMn₂O₄@N–rGO is a viable electrocatalyst for AWE systems.