Phosphate ions-functionalized and wettability-tuned nickel ferrite for boosted oxygen evolution performance

Nickel ferrite (NiFe₂O₄) has been explored as a promising oxygen evolution reaction (OER) electrocatalyst for water splitting owning to its earth-abundant and considerable water oxidation catalytic activity. Nevertheless, its practical electrocatalytic performance towards OER is still undesirable due to the sluggish OER kinetics and high overpotential gap on the water oxidation anode side. In this work, in order to enhance the electrochemical water oxidation performance of NiFe₂O₄, the surface of NiFe₂O₄ is functionalized with phosphate ions (Pi) by using a facile incipient impregnation and following calcination process. Results demonstrate that the OER properties of NiFe₂O₄ under alkaline conditions can be dramatically boosted by the surface Pi functionalization. In 1.0 M KOH solution, the resulting NiFe₂O₄-Pi on glassy carbon (GC) electrode demonstrates quite lower overpotential of 332 mV (10 mA/cm²) and Tafel slope of 57 mV/dec compared to that of pristine NiFe₂O₄ (443 mV@10 mA/cm² and 96 mV/dec), which is also better than that of commercial RuO₂ electrocatalysts (348 mV@10 mA/cm² and 80 mV/dec). Moreover, such electrocatalyst on nickel foam electrode also realizes superior OER durability to afford a current density of 70 mA/cm² at overpotential of only 300 mV for at least 28 h. The excellent electrocatalytic water oxidation activities of NiFe₂O₄-Pi can be attributed to the tuning electronic property and surface wettability by Pi ions functionalization. This work provides us a novel and effective approach to modify the photo-/electrocatalytic activity for transition metal oxides.