ZIF-67 Derived Co₃O₄/CoFe-LDH with Strongly Coupled Interfacial Junction as an Enhanced Electrocatalyst for Oxygen Evolution Reaction

Development of renewable and clean energy production has become an urgent issue owing to the increasing concerns about depletion of fossil fuels and environmental crisis. Producing hydrogen using water electrolysis is regarded as promising strategy for solving the problem owing to its environmental friendliness and high energy density of H₂. However, the sluggish kinetics and large overpotential of oxygen evolution reaction (OER) at the anode remains a bottleneck in water electrolysis. Until now, noble metal-based oxides, such as Ir/Ru based oxides, are generally used as state-of-the-art electrocatalysts for OER. Unfortunately, their scarcity, high cost and low durability during long-term operation have seriously hindered the large-scale applications of water electrolysis system. Accordingly, development of earth-abundant electrocatalysts with high activity and stability is essential to substitute the expensive electrocatalysts for water electrolysis. Spinel-structured cobalt oxide (Co₃O₄), which is constructed by combination of Co²⁺ and Co³⁺, is one of the promising electrocatalysts for OER due to rich redox properties and high corrosion resistance. Also, the bimetallic layered double hydroxide (LDH) structure is generally known as highly efficient electrocatalyst owing to its large surface area and flexible open structure.<br/>Based on the above considerations, we synthesized the core-shell structured Co₃O₄/CoFe-LDH composite with abundant oxygen vacancies via co-precipitation, heat treatment, and subsequent sonochemistry method. First, ZIF-67 was prepared by the co-precipitation reaction of Co²⁺ with 2-methylimidazole in methanol solution. Second, the ZIF-67 was annealed and oxidized at high temperature to prepare dodecahedral-shaped Co₃O₄ (Co₃O₄@C). After that, the Co₃O₄ was partially reduced under H₂/Ar flow to induce oxygen vacancies into Co₃O₄ (V_o-Co₃O₄@C). Finally, the Co²⁺ and Fe³⁺ ions (molar ratio of 2:1) dissolved in DI water, and as-prepared V_o-Co₃O₄@C was added into the solution. After stirred for about 30 min to promote adsorption of metal ions on the surface of V_o-Co₃O₄@C, 1M KOH was injected into the above mixture, and ultrasonicated for 30 min. In this step, the adsorbed metal ions react with OH^- ions, resulting in formation of CoFe LDH on the surface of V_o-Co₃O₄(CoFe-LDH/V_o-Co₃O₄@C). Owing to the electrostatic interaction between oxygen defect sites and metal ions, the CoFe-LDH/V_o-Co₃O₄@C has strongly coupled interfacial junction. Construction of interfacial junction of different phase is effective way to boost charge transfer efficiency. Additionally, it also can prevent the detachment of CoFe-LDH, thereby enhancing the long-term stability of electrocatalyst during OER process. As a result, the CoFe-LDH/V_o-Co₃O₄@C exhibited significantly improved electrocatalytic OER activity with low overpotential and Tafel slope, which are even superior to state-of-the-art RuO₂ electrocatalyst. Moreover, the prepared electrocatalyst showed stable operation over 100 h at a current density of 10 mA cm^-2 in alkaline solution. We believe that this research will provide a new insight into the preparation of transition metal-based composite materials for energy storage and conversion system.