Electronic structure tuning for enhanced oxygen evolution performance of a NiMnFeCr medium entropy alloy

Abstract

Electronic structure tuning of an electrocatalyst is an effective strategy for improving the efficiency of water electrolysis process and multi metallic electrocatalysts provide for better opportunity for tuning the electronic structure compared to monometallic electrocatalyst. Medium entropy alloys (MEAs) are multicomponent alloys and can be potentially used for electrocatalysis to produce hydrogen as it can be amenable to easy electronic structure tuning. Herein we report, the synthesis of MEA NiMnFeCr and demonstration of its utility in being a good platform for the electrocatalytic oxygen evolution reaction. X ray diffraction (XRD) was used to study the phase constituents of the MEAs. Differential scanning calorimetry (DSC) was employed to study the phase transformation temperature of the as-milled MEA powder while optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) allowed investigation of the microstructure of the synthesized alloy. Electrocatalytic properties were studied by an electrochemical workstation which incorporated linear sweep voltammetry (LSV), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The electronic structure of the as-sintered MEA was studied using ultraviolet photon spectroscopy (UPS). The experimental results showed that the MEA synthesized by sintering at 1200 °C and holding for 1 h (referred to as T1) exhibited an overpotential of 300 mV under 100 cycles of CV activation. The measured overpotential was lower than the state-of-the-art, expensive electrocatalysts such as Iridium oxide and Ruthenium oxide. This superior electrocatalytic activity could be attributed to the presence of porosity and in situ formed Mn3O4 oxide layers during cyclic voltammetry (CV) activation. Furthermore, the multi metallic nature of MEA was more amenable to better tuning of the electronic structure in the MEA by lowering the work function thereby improving the catalytic activity.