Electrochemical behavior of a Ni3N OER precatalyst in Fe-purified alkaline media: the impact of self-oxidation and Fe incorporation

Abstract

Nickel nitride (Ni₃N) is known as one of the promising precatalysts for the electrochemical oxygen evolution reaction (OER) under alkaline conditions. Due to its relatively low oxidation resistance, Ni₃N is electrochemically self-oxidized into nickel oxides/oxyhydroxides (electroactive sites) during the OER. However, we lack a full understanding of the effects of Ni₃N self-oxidation and Fe impurity incorporation into Ni₃N from electrolyte towards OER activity. Here, we report on our examination of the compositional and structural transformation of Ni₃N precatalyst layers on Ni foams (Ni₃N/Ni foam) during extended periods of OER testing in Fe-purified and unpurified KOH media using both a standard three-electrode cell and a flow cell, and discuss their electrocatalytic properties. After the OER tests in both KOH media, the Ni₃N surfaces were converted into amorphous, nano-porous nickel oxide/(oxy)hydroxide surfaces. In the Fe-purified electrolyte, a decrease in OER activity was confirmed after the OER test because of the formation of pure NiOOH with low OER activity and electrical conductivity. Conversely, in the unpurified electrolyte, a continuous increase in OER activity was observed over the OER testing, which may have resulted from the Fe incorporation into the self-oxidation-formed NiOOH. Our experimental findings revealed that Fe impurities play an essential role in obtaining notable OER activity using the Ni₃N precatalyst. Additionally, our Ni₃N/Ni foam electrode exhibited a low OER overpotential of 262 mV to reach a geometric current density of 10 mA cm_geo⁻² in a flow cell with unpurified electrolyte.