Spinel NiCo₂O₄-based Nanomaterials:Preparation And Electrocatalytic Properties Investigation

To solve the environmental pollution and energy crisis,electrochemical energy storage and conversion devices have attracted great attention,especially for electrocatalytic oxygen evolution reaction?OER?.Although OER is one of the effective ways to generate clean energy,its slow kinetics severely limits its application.Choosing an efficient electrocatalyst could promote the four-electron oxidation process and make the OER process go smoothly at low overpotential or onset potential to reduce its activation barrier.It is well known that RuO₂ and IrO₂ are the most active catalysts for OER.However,the high cost and scarcity of noble metals hinder their wide application.Therefore,searching for cheap and efficient OER electrocatalysts has become a key issue for improving the efficiency of energy production technologies.Recently,spinel-type NiCo₂O₄,as a bimetallic transition metal oxide electrode material,has become a hot material in the field of electrocatalysis research due to its low cost,simple preparation,complex composition,rich oxidation state and high electroactivity and stability.However,as an OER electrocatalyst,it still has some problems,such as small specific surface area,less active sites and weak conductivity,which greatly limit its practical application.In order to improve the electrocatalytic OER activity of NiCo₂O₄,elemental doping,compounding with Ni foam and transition metal oxides and morphology control have been developed to overcome these drawbacks.The main research contents are listed as follows:?1?A series of three-dimensional porous flower-like zinc doped NiCo₂O₄ electrocatalysts have been successfully prepared by a facile solvothermal process.The obtained products are characterized by various techniques,and their electrocatalytic performances have also been assessed by OER test.According to the electrochemical characterization,it is demonstrated that Zn_0.15Ni_0.85Co₂O₄?i.e.,ZNCO-0.15?nanoflowers show a lower overpotential,smaller Tafel slope,higher electrochemical active surface area,as well as a larger turnover frequency value than those of Co₃O₄,NiCo₂O₄,and ZnCo₂O₄ nanoflowers,indicating that introduction of an optimal content of zinc ions plays an important role in enhancing electrocatalytic performances for OER.The enhanced OER performance of ZNCO-0.15 could be attributed to the increased number of active sites(Co³⁺),accelerated kinetic rate,and promoted conductivity of the catalyst with the incorporation of zinc ions.?2?NiCo₂O₄/ZnFe₂O₄?NCO/ZFO?nanocomposite with hierarchical structure has been successfully prepared by electrospinning and solvothermal methods.The uniform growth of NiCo₂O₄ nanosheets?NCO NSs?on ZnFe₂O₄ nanofibers?ZFO NFs?provides larger specific surface area,increases the exposure of active sites and enhances the structural stability;moreover,the synergistic effect between NCO NSs and ZFO NFs improves the conductivity and accelerates the electron transfer rate,thereby improving the catalytic activity and stability of NCO/ZFO nanocomposites in OER.The NCO/ZFO nanocomposites shows a low overpotential of 340 mV at 10 mA cm^-2 and a residual current density of 81%after 50 h continuous reaction,also its catalytic activity is superior to that of NCO NSs and ZFO NFs,respectively.?3?ZIF-67 is self-grown on the NiCo-LDH nanosheets-modified Ni foam?NF?surface.Its derivative,the hierarchically structured Co3O4/NiCo2O4/Ni foam?CO/NCO/NF?composite is successfully fabricated by the oxidation of ZIF-67/NiCo-LDH/NF,which exhibits excellent electrocatalytic performance.When assessed as an electrode for OER,the CO/NCO/NF electrode shows low overpotentials of 320 and 407 mV at 10 and 50 mA cm^-2,respectively,in 0.1 M KOH aqueous electrolyte and an excellent long-term electrochemical durability with 88%current preservation during a 60 h test.It is also found that the OER activities of CO/NCO/NF electrode are superior to those of NCO/NF and CO/NF electrodes,demonstrating that the strong interaction of the interconnected interface of Co3O4 nanocages and NiCo₂O₄ nanosheets is advantageous to increase the number of Co³⁺,improve conductivity,enhance structural stability,facilitate electronic transfer and reduce reversible ion accumulation at the interface.In addition,the hierarchical architecture of the composite is binder-free and good for enhancing mass transport,boosting the dissipation of gases,as well as exposing catalytic active sites in the process of OER.?4?NiCo-MOF/LDH/NF nanocomposite has been synthesized by an in-situ growth of bimetallic MOFs on NiCo-LDH nanowire-modified Ni foam?NF?surface via the hydrothermal method.Subsequently,the NiCo2O4/NiCo2O4/Ni foam?NCO/NCO/NF?composite with a wire-penetrated-cage hierarchical structure are prepared by oxidative calcination the NiCo-MOF/LDH/NF.After a series of characterization and OER performance evaluations,NCO/NCO/NF composite exhibits a low overpotential of 320 mV at 10 mA cm^-2and a current density retention of 91%after 100 h oxidation reaction,which indicates that it has excellent catalytic activity and durability in electrocatalytic OER.Moreover,the wire-penetrated-cage hierarchical structure of NCO/NCO/NF and the synergistic effect of NiCo2O4 hollow nanocages,NiCo2O4 nanowires and Ni foam increase the specific surface area and the number of active sites,provide a fast electron transfer path,improve the conductivity,accelerate kinetic reaction rate,as well as enhance structural stability,thus improving the OER performance of NCO/NCO/NF composite.