Construction And Electrochemical Performance Of Self-supporting Nickel Phosphide-based Electrode

The ever-growing global demand for clean and sustainable energy sources has stimulated the enormous endeavors to develop renewable and zero-emission energy carriers.Hydrogen has been acknowledged as the most ideal energy carrier on account of its superb energy density,carbon-free emission and perfect recyclability.Among many hydrogen production technologies,electrochemical cathodic dissociation of water is a promising way to produce hydrogen,and the electrocatalyst of cathode hydrogen evolution reaction（HER）is the key to determine the conversion efficiency of hydrogen energy.Nickel phosphide（Ni₂P）electrode material is considered as one of the ideal electrocatalytic materials in the future due to its unique physical and chemical properties and potential catalytic activity.However,the electrochemical property of the single-component Ni₂P is severely limited,because of the low intrinsic activity and the limited availability of electrochemical reactive sites of this catalyst.In this paper,the control strategies such as the phase regulation,heterostructure construction and the design of electrode structure have been optimized and modified to obtain efficient HER electrocatalysts at a wide p H range.And the developed HER electrode material is applied to the nitrate reduction and graphene oxide reduction engineering fields.The mechanisms of hydrogen evolution reaction,nitrate reduction and graphene oxide reduction are described by the density functional theory（DFT）calculation.The specific studies are summarized as follows:（1）A self-supporting nickel phosphorus/nickel phosphide/carbon cloth（Ni-P/Ni₂P/CC）electrode material was prepared by a three-step coupling method of nickel phosphorus electrodeposition,hydrothermal synthesis and low temperature phosphorization.Characterization analysis shows that the amorphous Ni-P nanoparticles were successfully loaded on the remaining sites on CC substrate unoccupied by Ni₂P nanosheets and the surface of Ni₂P nanosheet arrays,realizing the construction of high-density active sites and synergically enhancing their HER catalytic activity.In the process of the electrodeposited Ni-P nanoparticles,the influence of deposition time on the morphology and catalytic performance of the catalytic electrode was investigated.The Ni-P/Ni₂P/CC-2 catalytic electrode with the deposition time of 120 s has a low overpotential of 95 m V to afford a current density of 10 m A cm^-2,exhibiting excellent alkaline HER performance;and this catalytic electrode behaves outstanding electrochemical stability.（2）The nanoheterostructure cobalt phosphide/nickel phosphide/CC（Co P/Ni₂P/CC）catalytic electrode with the coexistence of Co P nanoleaves and Ni₂P nanosheets was fabricated via a sequential three-step process（i.e.,hydrothermal synthesis,co-precipitation and phosphorization process）.According to the characterization analysis,the nanoleaf-like Co P arrays are densely and uniformly anchored on nanosheet-type Ni₂P skeleton,showing a unique nanoleaf/nanosheet-integrated hierarchical structure.In the co-precipitation process,the effect of aging time on the HER performance of Co P/Ni₂P/CC catalytic electrode was studied.The Co P/Ni₂P/CC-4 catalytic electrode with the aging time of 4 h shows excellent catalytic performance at a wide p H range(overpotentials of 67,71 and 95m V to afford 10 m A cm^-2in 0.5 M H₂SO₄,1 M KOH and 1 M PBS,respectively)and desirable durability（without noticeable decrease in activity after the test of 24 h）,showing a good application prospect.（3）The electrochemical HER process of the Co P/Ni₂P/CC-4 functional electrode can be innovatively applied to realize the disposal of nitrate and reduction of graphene oxide.The HER process of Co P/Ni₂P/CC-4 electrode can promote the formation of strongly reduced adsorption hydrogen（H*）,which is conducive to the conversion of NO₃^--N to NH₄⁺-N and N₂.The removal efficiency of NO₃^--N is 86.8%,and the selectivity to NH₄⁺-N is high（about 70.9%）.This will expand the application of nickel phosphide-based HER electrode material for controlling water eutrophication,especially in the disposal of nitrate.Meanwhile,the generated H*of the HER process has strong reducing ability,which can facilitate the reduction of oxygen-containing functional groups（C-O-C,C-OH and HO-C=O）on the surface of GO.Based on the analysis results,the r GO-HER sample shows a higher degree of reduction and the significant removal of oxygen-containing groups.Compared with C-OH and C-O-C groups,the HO-C=O group of r GO-HER sample is not easily reduced.The HER process of Co P/Ni₂P/CC electrode is a green and economic pathway to obtain graphene materials,extending the engineering application of HER technology.（4）The density functional theory（DFT）calculation is applied to investigate the adsorption energy of water(ΔE_H2O*)and the Gibbs free energy of hydrogen adsorption(ΔG_H*).The optimal adsorption site of water on Ni₂P（111）surface is determined as Ni₅₉,and the optimal adsorption site of hydrogen is determined as P₆₈.The transition state（TS）energy barrier calculation result shows that the water dissociation kinetics of Ni₂P（111）surface is slow,and the HER process is endothermic.Meanwhile,the optimal adsorption site and the reduction pathway of NO₃^-on Ni₂P（111）surface were investigated.In the reduction of NO₃^-by the H*,the DFT calculation confirms that the adsorption of NO₃^-is by means of electrostatic or covalent bond between O atom in NO₃^-and Ni₁₂active site of Ni₂P surface.TS calculation indicates that the formation of the N-H bond mediated by the H*is more kinetically favorable than the formation of the N-N bond.Therefore,Ni₂P catalytic electrode is more favorable for the conversion of nitrate to ammonia/ammonium,exhibiting a high selectivity for NH₄⁺.In addition,the reaction pathway for the reduction of oxygen-containing functional groups（C-O-C,C-OH and HO-C=O）by the H*on GO surface was calculated.DFT calculation shows that the oxygen-containing functional groups of GO surface follow the reduction order:C-O-C>C-OH>HO-C=O,which provides theoretical basis for experimental research.