Preparation And Electrocatalytic Water Splitting Properties Of Composite Bifunctional Catalysts Based On Nonmetallic Elements Doped Nb₄C₃T_x MXene

Electrochemical water decomposition technology is a green,clean and pollution-free new energy technology that can decompose water into hydrogen and oxygen,which is of great significance for alleviating global energy crisis and improving environmental pollution.The efficiency of electrochemical decomposition of water depends on the electrocatalyst used.But the high cost and scarcity of precious metal catalysts limit their large-scale commercial applications.Therefore,the development and preparation of low-cost non-noble metal electrocatalysts with high activity and stability will help to improve the efficiency of water decomposition.Two-dimensional transition metal carbon/nitride（MXene）has become one of the research hotspots in the field of electrocatalytic water decomposition due to its good lamellar hydrophilicity and conductivity.MXene as catalyst substrate can not only improve the HER performance of electrocatalyst,but also promote the OER kinetics which is considered to be lagging.The doping of non-metallic heteroatoms can effectively regulate the surface chemistry of MXene so as to accelerate electron/ion transport and increase the catalytic active site,thus promoting the electrocatalytic activity.In this paper,Nb₄C₃T_x MXene based catalyst doped with sulfur（S）,nitrogen（N）,and phosphorus（P）elements was prepared,and its surface morphology,composition,and electrochemical performance were investigated.The main research contents are as following:（1）Multilayer Nb₄C₃T_x MXene was prepared by etching Nb₄Al C₃ with Li F/HCl solutions,and then the multilayer Nb₄C₃T_x with less fluorine was obtained by alkali treatment.Using Na₂S·9H₂O as the S source,S-doped multilayer Nb₄C₃T_x catalyst was prepared by hydrothermal method.The composite catalyst was characterized by HRTEM,SEM,XRD,XPS,Raman and other testing methods.The as-prepared catalyst has showed excellent activity in HER,OER and electrolytic water process.The overpotential for HER and OER process is 118 and 320 m V to achieve the current density of 10 m A cm^-2,respectively.The as-prepared catalyst shows a long-term stability of 24 h.Moreover,only 1.60 V（vs.RHE）is need to drive the electrolytic water for the the fully hydrolyzed pool.（2）N-doped bifunctional catalyst（N-Nb₄C₃T_x）was prepared through hydrothermal synthesis method by using hexamethylenetetramine（HMTA）as the N source.The composite catalyst was characterized by HRTEM,SEM,XRD,XPS,Raman and other testing methods.In 1.0 M KOH electrolyte solution,the overpotential for HER and OER process is 96 and 291 m V to achieve the current density of 10 m A cm^-2,respectively.The stability for the as prepared catalyst is improved to 24 h.Moreover,only 1.58 V（vs.RHE）is need to drive the electrolytic water.（3）By using sodium hypophosphate（NaH₂PO₂）as the P source,P element is introduced into few-layered Nb₄C₃T_x MXene by phosphating at a low temperature to obtain a bifunctional catalyst denoted as P-Nb₄C₃T_x.The P-Nb₄C₃T_x composite catalyst is characterized by HRTEM,SEM,XRD,XPS,Raman and other testing methods.The overpotential to reach a current density of 10 m A cm^-2in 1.0 M KOH electrolyte for HER and OER process is 92 and 278 m V,respectively.Moreover,only 1.57 V（vs.RHE）is required to drive the electrolytic water.In addition,the stability of P-Nb₄C₃T_x can reach 36 h.