Preparation Of CoFe-based Nanoarray Composite Electrocatalysts And Their Electrocatalytic Water Splitting Activities

Nowadays,resource depletion and environmental pollution problems have severely hampered the global energy development.Exploring clean and sustainable energy with abundant resources,low prices,high efficiency and no pollution is of great important for the alleviation of the current energy crisis.Hydrogen is regarded as an ideal candidates because of its renewable,higher calorific value and high energy conversion efficiency.Electrochemical water splitting corresponds to the concept of eco-friendly energy.Among them,the electrochemical water splitting technology under alkaline electrolyte exhibits a wide range of industrial applications.It was found that cobalt-iron-based transition metal materials were extensive researched benefit from the advantages of resource-rich,cheap,and excellent electrocatalytic intrinsic performance.However,current reported show that traditional cobalt-iron-based transition metal materials with disadvantages such as large overpotential and few active sites exposed.Therefore,we synthesize cobalt-iron-based nanosheet array support on 3D and porous Ni foam by simple,green and mild condition,and a series of novel composite nanoarray electrocatalysts were constructed,which greatly improved the electrocatalytic activity.The relationship between the composition of the nanoarray structure and the performance toward water splitting was systematically studied.The specific research content was as follows:1.The precursor CoFe-LDH nanosheet arrays were successfully synthesized by facile hydrothermal method supported on high conductivity and three-dimensional porous Ni foam.Subsequently,the CoFe-LDH was modified by NiFe-LDH nanosheet to form CoFe@NiFe/NF through electrodeposition.And by controlling the electrodeposition time to adjust the loading capacity of NiFe-LDH on the surface,the composite ratio was confirmed.The characterization methods of XRD,XPS,SEM,TEM and HRTEM were used to analyze the composition and structure of CoFe@NiFe/NF.The results of the performance of electrocatalytic water splitting under alkaline conditions show that the optimal composite ratio CoFe@NiFe-200/NF catalyst has lower overpotential towards both hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?,the low overpotentials of 240 and 190 mV at a current density of 10 mA cm^-2 for HER and OER,respectively.In the two-electrode system,CoFe@NiFe-200/NF||CoFe@NiFe-200/NF exhibits excellent overall water splitting performance,and the potential as low as 1.59 V was needed to achieve the current density of 10 mA cm^-2.It was found that the combination of CoFe-LDH and NiFe-LDH increased the specific surface area and exposed more electrochemical active sites,which were the key factors to improve the electrocatalytic performance.2.In this work,holey cobalt-iron nitride nanosheet arrays which grown on 3D and porous Ni foam substrate?CoFeN_x HNAs/NF?were prepared via a facile hydrothermal and thermal nitridation method.Combined with XRD,XPS,SEM,TEM and HRTEM analysis,we analyse the samples of crystal phase structure and microstructure,and further investigate the interaction between cobalt-iron nitride.The results of the performance of electrocatalytic water splitting under alkaline conditions show that the CoFeN_x HNAs/NF exhibited high catalytic performance with overpotentials of 200 and 260 mV at the current density of 10 mA cm^-2 for HER and50 mA cm^-2 for OER,respectively.Furthermore,when using CoFeN_x-500 HNAs/NF as both anode and cathode,the alkaline electrolyzer performs excellent overall water splitting performance.The results of the study on the enhancement mechanism of electrocatalytic performance show that the unique HNAs architecture not only can enlarge the active surface areas to expose abundant active sites but also facilitate the charge/mass transfer.And the metallic characteristics of the Co₂N and Fe₄N favor the charge transfer between the catalyst and current collector,thus enhancing the HER and OER kinetics.3.The CoFe-LDH nanosheet array was supported on nickel foam,the CoFe-PBA nanocubes/CoFe-LDH were prepared by aging at room temperature,and then Co_xP-Fe₂P/NF heterostructure electrocatalyst was synthesized by phosphidation.By means of XRD,XPS,SEM,TEM and HRTEM,the crystal structure and microstructure of the as-prepared samples were analyzed,and the interfacial interaction between heterogeneous structures was investigated.The electrocatalytic performance of water splitting were measured in alkaline electrolyte,which performed excellent activities with a low overpotential of 75 mV and 265 mV to reach the current density of 10 mA cm^-2 toward HER and OER,respectively.Furthermore,Co_xP-Fe₂P/NF also exhibited preeminent overall water splitting activities and stability in two-electrode system.The results of the study on the enhancement mechanism of electrocatalytic performance show that the combination of CoFe-LDH and PBA could increase the electrochemical surface areas.And the original array structure could maintain after phosphidation.Meanwhile,the phosphide improved the conductivity and further enhanced the catalytic efficiency.