Preparation Of NiCoFeO_x/(OH)_x Core-shell Structure And Research On Their Electrochemical Performance

In order to solve the energy crisis and environmental problems,the most effective way is to replace traditional fossil fuels with clean hydrogen energy."Green"hydrogen is produced by the electrolysis of water,and during the reaction,only water is formed as a by-product.The low reaction rate of oxygen evolution in electrolyzed water is the key to limiting electrochemical hydrogen production.Ru-based and Ir-based compounds are common materials for commercial high-performance OER catalysts,however,high cost and scarcity of resources limit their wide application.Therefore,among low-cost transition metals,oxides and hydroxides formed by elements such as Ni,Co,Fe,and Mn show good kinetic activity in the catalytic reaction of oxygen evolution.The activity and stability of the catalyst still need to be improved,and it has been found that the preparation of nanocomposite structures can improve this shortcoming.In view of the above problems,the following work is carried out in this paper to obtain efficient and stable transition metal-based oxygen evolution catalysts:（1）NiCo₂O₄was prepared on nickel foam by a combination of hydrothermal method and calcination method.The preparation conditions of NiCo₂O₄with the best oxygen evolution performance were explored,and the amount of NH₄F added was adjusted.With the increase of the amount,different shapes were obtained.The appearance of NiCo₂O₄.It was found that the nanowire-like NiCo₂O₄prepared at120℃hydrothermal temperature for 6 h had the best catalytic performance.In alkaline solution,the Tafel slope of the NiCo₂O₄/NF electrode is 149.40 m V·dec^-1,and it can reach a current density of 10 m A·cm^-2at an overpotential of 353 m V.（2）Using NiCo₂O₄nanowires as carriers,LDH nanosheets were supported on their surfaces by a hydrothermal method.In the LDH composed of Ni,Co,Mn and Fe,it is found that the core-shell structure NiFe-LDH/NiCo₂O₄composed of NiFe elements exhibits good oxygen evolution activity.The synthesized NiFe-LDH/NiCo₂O₄/NF（NiFe-LDH/NiCo₂O₄/Ni Foam）has only a Tafel slope of53.03 m V·dec^-1,and can reach a current density of 50 m A·cm^-2at an overpotential of363 m V.The main reason for the improved performance is that the formed core-shell structure can effectively exhibit the synergistic effect between metals,and the interaction between Ni,Co and Fe metals promotes each other to form optimal valence states.（3）By adjusting the NiFe-LDH loading of the shell and the ratio of Ni and Fe elements,the change of element valence affects the change of element valence inside,and the oxygen evolution performance of the catalyst is further improved.The electrode Tafel slope was only 50.20 m V·dec^-1when the total amount of input nitrate was 1.8 mmol,the loading amount was 0.8 mg·cm^-2,and Ni:Fe=1:1,and only 314 m V could reach 50 m A·cm^-2current density.