Hierarchical heterostructure of NiFe2O4 nanoflakes grown on the tip of NiCo2O4 nanoneedles with enhanced interfacial polarization effect to achieve highly efficient electrocatalytic oxygen evolution

Abstract

In this work, a heterostructure oxygen evolution reaction (OER) electrocatalyst consisting of NiFe2O4 nanoflakes grown on the tip of NiCo2O4 nanoneedles arrays supported on nickel foam (NCO-NNs@NFO-NFs/NF) was obtained via simultaneous cation (Co3+ by Fe3+) and ligand exchange (-OH by 1,4-benzenedicarboxylic acid) and further annealing of the hydroxide/MOF precursor in air. HRTEM and Raman characterizations confirm the formation of NCO/NFO heterostructure. XPS characterization and charge density difference calculations clarify the electron transfer from Fe to Ni and Co at the NCO/NFO heterointerface. Electrochemical tests show that NCO-NNs@NFO-NFs/NF possesses excellent OER electrocatalytic activity (overpotential of 265 mV@50 mA cm−2) and durability (stable for 100 h@1 A cm−2). NCO-NNs@NFO-NFs/NF‖20 % Pt/C two-electrode system can achieve 10 mA cm−2 at 1.398 V for anion exchange membrane (AEM) water splitting in 1.0 M KOH at 85 °C, which exceeds the activity of using commercial RuO2 as an anode at 25 °C, combined with the stability of 100 h at 10 mA cm−2 at 25 °C, demonstrating the practical application in industrial AEM process. XPS and HRTEM characterizations of NCO-NNs@NFO-NFs/NF after the OER V-t test, along with ex-situ Raman characterization, confirm that Co and Fe sites undergo surface reconstruction to form specific CoOOH/FeOOH interfaces, as to exert the synergistic effect of Co and Fe dual active sites during OER process. DOS calculations reveal that the density of state at Fermi energy level increases after the introduction of Fe, thereby improving conductivity and facilitating the activation of adsorbed oxygen species. This study shows a significant methodology for simultaneous improvement of the electrocatalytic stability and activity of spinel oxides-based heterostructures, which is critical for practical application.