Modulating Electronic Structures Of Ternary Sulfides By Nitrogen Incorporation For Efficient Electrocatalytic Water Splitting

The development of a bifunctional electrocatalyst with high activity and high stability that can be applied to both hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)is of great significance to the utilization of renewable energy.In this study,we propose an in-situ N-doping strategy to improve the activity and stability of ternary sulfide in the process of total water electrolysis by rationally adjusting the space electron density of atoms in the catalyst.The results show that this modified N-doped ternary sulfide has a good kinetic driving force and provides a rich catalytic active center for the HER and OER reactions.This self-supported N-NiMoS4 electrode can provide a current density of 10 mA cm-2 for total water electrolysis in a two-electrode system at a very low battery voltage of 1.54 V(only 310 mV higher than the theoretical potential).The value is 130 mV lower than the unmodified NiMoS4 electrode.In addition,the electrochemical stability tests show that the initial current density retention rate of the electrode after continuous operation for 100 h is 76.6%,which is better than that of the NiMoS4 electrode after 25 h.In addition,this paper also synthesized N-CoMoS4 electrode material by the same method and analyzed its electrochemical HER performance.The results show that N-CoMoS4 electrode material only needs 95 mV to provide a current density of 10 mA cm-2,which is 51 mV lower than that of CoMoS4 electrode material.At the same time,the electrochemical stability tests also shows that the HER catalytic stability of the N-CoMoS4 electrode material is much better than that of the CoMoS4 electrode material.Therefore,this in-situ N-doping strategy to effectively improve the activity and stability of the electrocatalyst can provide valuable insights in the development of high-efficiency electrocatalysts through electronic structure control.Specific details as follows:① Study on the overall water splitting electrochemical performance of in-situ N-doped NiMoS4 by hydrothermal method.NiMoO4,NiMoS4 and N-NiMoS4 catalyst materials were prepared by hydrothermal method using sodium molybdate dihydrate(Na2MoO4·2H2O)as molybdenum source,nickel nitrate hexahydrate(Ni(NO3)2·6H2O)as nickel source,sodium sulfide(Na2S)or thioacetamide(TAA)as sulfurization or nitride agent.A three-electrode system was constructed,and the electrochemical performance of the material was analyzed and compared using an electrochemical workstation to explore the effect of N-doping on the electrochemical performance of the ternary sulfide.The results show that the performance of optimized NiMoO4(N-NiMoS4)is significantly better than other materials,only 109 mV and 238 mV overpotential can drive the current density of 10 mA cm-2 for HER and OER,and the current density retention rate after continuous working for 100 h are 83.4%and 77.8%respectively.In addition,the two-electrode test also shows that this self-supporting N-NiMoS4 electrode only needs a potential of 310 mV higher than the theoretical value to provide a current density of 10 mA cm-2 for the overall water splitting in the system.The current density retention rate after continuous working for 100 h is 76.6%.This result has a certain degree of advantage over NiMoS4 electrode materials,confirming that N-doping can improve the electrocatalytic performance of ternary sulfide NiMoS4.② Study on electrochemical performance of hydrogen evolution of in-situ N-doped CoMoS4 by hydrothermal method.In.order to investigate whether N-doping has a wider application range for electrolysis of water electrolysis,we using sodium molybdate dihydrate(Na2MoO4·2H2O)as the molybdenum source and cobalt nitrate hexahydrate(Co(NO3)2·6H2O)as the cobalt source.Anhydrous sodium sulfide(Na2S)or thioacetamide(TAA)were used as sulfurization or nitride agent to prepare CoMoO4,CoMoS4,and N-CoMoS4 catalyst materials,respectively.And N-CoMoO4 material was prepared by heat treatment with ammonia gas.Analysis and comparison of HER performance for the four electrocatalysts show that the N-CoMoS4 electrode material only needs 95 mV to provide a current density of 10 mA cm-2,which is 51 mV decrease compared to the CoMoS4 electrode material.At the same time,the electrochemical stability of the HER reaction shows that the current density retention rate of the N-CoMoS4 electrode material after continuous operation for 120 h is 85.0%.This result shows that the stability is much superior than the CoMoS4 electrode material.