Plasma-assisted rhodium incorporation in nickel–iron sulfide nanosheets: enhanced catalytic activity and the Janus mechanism for overall water splitting

Abstract

NiFe catalysts show compelling oxygen evolution reaction (OER) activity in alkaline solution but their hydrogen evolution reaction (HER) activity is limited. In this work Rh was incorporated in Fe-doped Ni₃S₂ nanosheets with the assistance of hydrogen plasma to significantly enhance the HER/OER catalytic activity. The resultant catalyst, p-Rh/Fe-Ni₃S₂/NF, was composed of Rh/Rh₂S₃ heterostructured clusters embedded in Fe-Ni₃S₂ nanosheets; it needs overpotentials of only 108 and 237 mV to achieve HER and OER currents of 100 mA cm⁻², respectively, and drives overall water splitting at 1.79 V with a current of 100 mA cm⁻² as a bifunctional catalyst. Post-characterization and density functional theory (DFT) calculation reveal its operando evolution behavior and a unique Janus catalytic mechanism for the HER and OER. For the HER, a Rh/Rh₂S₃ heterointerface with strong H₂O adsorption energy and optimal H adsorption energy is the active site; in the OER, it experiences surface transformation under operando conditions; an in situ formed Rh/Fe-doped NiOOH layer with strengthened binding of *O facilitates the rate determining step of *O formation to accelerate the OER process. Hydrogen plasma treatment not only helps partially reduce Rh to form HER-active Rh/Rh₂S₃ heterostructures but also induces rich defects/vacancies to facilitate surface reconstruction/phase transformation to form an active co-doped NiOOH phase under OER conditions. This work may shed light on the promotive role of noble metals on NiFe-based catalysts for water splitting.