Cobalt Foam Load Iron Group Metal Phosphide/hydroxideself-supporting Electrodes And Their Electro-catalytic Hydrogen Evolution Reaction Properties

Metal phosphides not only have high physical and chemical stability,but also superior catalysts toward hydrogen evolution reaction reduction（HER）.Taking commercial nickel foam as the substrate to prepare self-supporting electrodes,not only significantly improves the catalytic efficiency toward HER,but also can be directly used as an electrode,and continue to work stably under high current density.However,the widely used commercial nickel foam currently has an overall pore size distribution between 450 and 3200μm,and has a limited specific surface area.Reducing the pore diameter of porous substrate and increasing the specific surface area is believed to benefit for improving the catalytic performance.On this base,we prepared cobalt foam with high porosity and small pore size via freeze-drying and high temperature sintering method,using cobalt powder and chitosan as raw materials.After that,the iron group metal hydroxide is hydrothermally grown on its surface followed by low-temperature phosphating.Finally,cobalt foam-supported phosphide/hydroxide self-supporting electrodes were prepared.The effects of phosphating temperature,metal salt species and single/double metal phosphide on the HER performance were studied.The results indicate that:（1）Via freeze-drying and high-temperature reduction sintering,cobalt foam with porosity of 90%was prepared.The pore size was only tens of micrometers,and the pore interconnected to form a three-dimensional structure.This structure not only provides a large surface area,but also facilitates the diffusion of electrolyte and gas during the HER process.（2）Taking the as-prepared cobalt foam as the substrate,single metal hydroxide was grown on the surface by hydrothermal method.After phosphating,Co foam load single iron,cobalt and nickel phosphide/hydroxide electrodes were prepared.The order of HER performance of the three electrodes is:Cobalt foam load iron phosphide electrode（Fe₂P/CF-350,350 means that the phosphating temperature is 350℃）>Cobalt foam load cobalt phosphide electrode（Co P/CF-350）>Cobalt foam load nickel phosphide electrode（Ni₂P/CF-350）.The prepared Fe₂P/CF-350 electrode has the best HER performance among the three types of electrodes,which can reach a current density of-10 mA/cm² under overpotential of-50 mV with Tafel slope of 60 m V/dec.（3）Cobalt foam load nickel-cobalt phosphide/hydroxide electrode was prepared by similar method.The electrode phosphatized at 350°C has the best HER performance,which can reach a current density of-10 m A/cm² at overpotential of-47mV with Tafel slope of 59 m V/dec,and can work stably more than 16 h under high current density over-500 mA/cm².The calculation results of density functional theory show that the adsorption and dissociation of water molecules on the NiCoP/NiOOH（0001）plane is easier than that on the Co₂P（0001）plane,and the formation of the Ni CoP/NiOOH heterojunction is conducive to electron transport,thus NiCoP/NiOOH has a high intrinsic HER catalytic activity.（4）Cobalt foam load iron-cobalt phosphide/hydroxide electrode and cobalt foam load iron-nickel phosphide/hydroxide electrode were prepared by similar method.The electrodes phosphatized at 350°C have the best HER performance,Fe CoP/CF-350 can reach a current density of-10 m A/cm² at overpotential of-44 mV with Tafel slope of92 mV/dec,and Fe NiP/CF-350 can reach a current density of-10 mA/cm² at overpotential of-39 mV with Tafel slope of 59 mV/dec.Both electrodes can work stably more than 16 h under high current density over-500 m A/cm².（5）The purchased Ni foam load nickel-cobalt,iron-cobalt and iron-nickel phosphide/hydroxide bimetal electrodes were prepared by similar method and compared with electrodes supported by cobalt foam.The results showed that the performance sequence of the electrodes was as follows:Co foam load bimetal electrodes>Co foam load single metal electrodes>Co foam directly phosphating>The purchased Ni foam load bimetal electrodes.The results showed that both the synergistic effect of bimetal active sites and the increase of specific surface area of the electrode could effectively improve the performance toward HER.