Preparation Of NiFe₂O₄ And Their Electrocatalytic Activity For Water Oxidation

Efficient,economic,and robust electrocatalysts for water splitting are highly desired for future clean energy supply.However,due to the intrinsic sluggishness of the four-electron/four-proton process,the low-efficiency of the oxygen evolution reaction?OER?has not yet been resolved so far and is still regarded as a bottleneck in the integrated overall water splitting systems.Numerous low-cost first-row?3d?transition metal oxides have been widely explored because of their high natural activities in alkaline electrolyte for oxygen evolution reaction?OER?.However,the catalytic efficiency of pure NiFe₂O₄ is quite low due to its limited number of active sites and poor electrical conductivity.Therefore,this present study focuses on improving the OER activity of NiFe₂O₄ with deep understanding its catalytic mechanism.1.Defect engineering of mesoporous NiFe₂O₄ for highly enhanced water oxidationHighly ordered crystalline mesoporous NiFe₂O₄ with high surface area was prepared by a one-step-impregnation hard-template method.Using defect engineering,we have successfully overcome its disadvantages for the first time by increasing the number of oxygen vacancies in NiFe₂O₄.HAADF-STEM,XAS and XPS were employed to characterize the morphologies and electron configurations.The existence of oxygen vacancy not only shifts up the d-band center,strengthens the adsorption of H2O,and thus provides more active catalytic sites,but also tunes the electron configuration and creates massive number of defective donor states in the band gap to facilitate charge transfer processes.The optimal defective NiFe₂O₄ catalyst showed significantly enhanced catalytic efficiency for OER which is far better than its defect-free counterpart as well as any other reported NiFe₂O₄ systems.2.Carbon-encapsulated NiFe₂O₄ nanoparticles with extraordinary high mass activity and durability for electrocatalytic water oxidationCarbon-encapsulated NiFe₂O₄ nanoparticles were synthesized by treating Ni/Fe-oleate complex under vaccum at a high temperature,which demonstrated excellent catalytic activity for OER with an onset overpotential of 220 mV and overpotential of 300 mV at a current density of 10 mA cm-2.Notably,the mass activity reached up to 5000 A g-1 with a mass loading of 1 ?g,and the durability under an applied volatage of mA cm-2 can exceed 250 hours.The good OER performance is probably attributed from its ultrasmall nano size and the protective carbon layer.