Enhanced Catalysis Of Electrochemical Hydrogen Evolution Reaction By P Doping In Ni₃S₂ Nanosheet Arrays

Due to the energy depletion and environmental problems,the economy dominated by the traditional energy（oil and coal）has been converted to new energy sources（solar energy,hydrogen energy and tidal energy）.As a renewable and clean energy,hydrogen with high-energy density and wide sources has become a promising alternative to the traditional energy.Electrolysis is one of the main ways to produce hydrogen from water in large-scale because of its simplicity,high efficiency and low cost.Therefore it is urgent to design high catalytic activity catalyst with low overpotential and high charge transfer rate in hydrogen evolution reaction（HER）to reduce the energy consumption.Generally,the HER rate in electrolytic water under alkaline conditions is 2～3 orders of magnitude slower than that under alkaline conditions.Consequently,the development of catalysts with high catalytic activity and stability under acidic conditions has far-reaching significance and value.As a transition metal chalcogenide,Ni₃S₂ has become a hotspot in the field of hydrogen evolution catalysis in recent years due to its low cost,abundant reserves and good catalytic properties.However,due to the slow hydrolysis kinetics,the application of Ni₃S₂ under alkaline conditions was limited.Meanwhile,as two important microscopic processes in alkaline HER,the hydrolysis kinetics and hydrogen adsorption associated with the electronic structure are still an open question.In this paper,the characteristics of Ni₃S₂ and its existing problems were studied.The phosphorus-doped Ni₃S₂ nano-sheets with controllable intrinsic electronic structure were fabricated.The electrocatalytic hydrogen evolution performance of P-doped Ni₃S₂was studied as well.The specific contents are demonstrated as follows:1.With nickel foam being as the nickel source and self-supporting current collector,thiourea as the sulfur source,3D self-supported P-doped Ni₃S₂ nanosheet arrays were constructed on the conductive Ni foam by hydrothermal method and sequential phosphorization treatment without phase separation.By controlling the phosphorus quality,the effect of different phosphorus doping content on the catalytic performance of HER was investigated.Finally,when the ratio of doped sulfur to phosphorus was1:1,the overpotential was 139 m V at the exchange current density of 10 m A/cm².With a tafel slope of 120 m V/dec,it has the most efficient HER performance.2.Futhermore,the Gibbs adsorption free energy of hydrogen protons and the water molecules in the low-index surface of Ni₃S₂ and phosphorus-doped Ni₃S₂ were simulated by first-principles calculation.The water was calculated by CLNEB method.Both in experiments and DFT calculations,the heteroatom doping can effectively modulate the electronic properties of Ni₃S₂ nanosheets and promote the intrinsic catalytic activity for alkaline HER.DFT calculations consistently elucidate that the introduction of P atoms can modify the electronic structure of Ni₃S₂,enhance the electrical conductivity,optimize the HER Gibbs free-energy(ΔG_Had),improve the water adsorption energy(ΔG_H2O)and reduce the barrier of water dissociation.Benefiting from the above disscusion,the as-synthesized P-doped Ni₃S₂/Ni foam exhibits outstanding HER performance than pristine Ni₃S₂/Ni foam.Remarkably,ICOOP and ICOHP analysis reveal that P doping makes electrons accumulate on the bonding state of Ni-O and H-OH bond of water in the transition state unfavorably bonded,which help us understand the essence of HER process more deeply.The capability to promote the dissociation kinetics of water from bonding analysis could offer a powerful platform to rationally design HER catalysts and beyond.