The Research On Structure-control And Overall Water Splitting Performance Of Ni-Mo Based Sulfides

The increasing demand of human society promoted the over-exploitation of natural resources.People are forced to seek a new energy resource to solve the problem of energy exhaustion and environmental pollution caused by over-exploitation.In comparison to fossil,hydrogen energy has a superior efficiency and density.Meanwhile,zero carbon dioxide emission and good environmental compatibility assist hydrogen to be a preferred substitute for traditional resources.At present,overall water splitting is a promising way for hydrogen production.However,the inevitable overpotential generated by cathode and anode increases the energy consumption and reduces the efficiency.For this purpose,developing an electrocatalyst material with high activity can availably address this issue.In this paper,a transition metal sulfide was controlled through a series of methods such as dealloying,hydrothermal and chemical vapor deposition.Besides,the parameters of those methods were analyzed and optimized to obtain a high-efficient and stability electrocatalyst for overall water splitting.Transition metals are considered as potential materials benefit from earth-abundant and low-cost.And combining transition metals with anions is a futuristic strategy to exploit high-efficient composites.Transition metal sulfide has been extensively studied because of its suitable band gap and multiple valence states.In the first chapter,a heterostructure MoS₂/NiS nanoflakes were synthesized on nano-porous Ni by one-pot dealloying and two-step hydrothermal.The microstructures and phase compositions of materials were characterized by SEM,TEM,XRD and XPS,the electrocatalytic performances were represented by LSV,Tafel and EIS.The results show that MoS₂/NiS/Ni exhibits a better electrocatalytic performance than bare MoS₂,NiS and Ni.The overpotential to reach a current density of 10 m A·cm^-2 is 118 m V for HER and to reach current density of 50 m A·cm^-2 is 283 m V for OER.When assembled as bifunctional electrodes for overall water splitting,a low cell voltage of 1.51 V was required to attain the current density of 10 m A·cm^-2.After a long-term durability test,there is no obvious voltage attenuation reveals that an excellent stability MoS₂/NiS/Ni has.Such a performance is attributed to the synergy between MoS₂ and NiS,also the electron transport channel between nano-porous Ni.In the second chapter,based on the product fabricated in two-step hydrothermal,a core-shell MoS₂/Ni₃S₂ nanospheres was prepared by heat treatment combing chemical vapor deposition.The SEM and TEM results show that the nanosheet MoS₂are uniformly attached to the surface of nanospheres Ni₃S₂,which forms a core-shell structure.The electrochemical test results show that MoS₂/Ni₃S₂ exhibits a superior activity than bare MoS₂ and Ni₃S₂.The overpotential to reach current density of 10m A·cm^-2 is 109 m V for HER and 232 m V to reach 50 m A·cm^-2 for OER.When engaged as both cathode and anode,a low cell voltage of 1.49 V was required to reach the current density of 10 m A·cm^-2.The durability test result shows that an unobvious deviation of voltage after 12 h electrolysis.The outstanding performance of MoS₂/Ni₃S₂ is results from the unique core-shell structure provides more activity sites and the interface between MoS₂ and Ni₃S₂ accelerates the transport speed through two phases.Defect engineering is a kind of micro control method,which has significant influence on optical,electrical and thermal properties of materials,and has been widely researched in functional materials.Therefore,a S-defect core-shell MoS₂/Ni₃S₂nanosphere was synthesized on the basis of obtained core-shell MoS₂/Ni₃S₂ nanosphere via one-step heat treatment in H₂/Ar atmosphere.In comparison with MoS₂/Ni₃S₂,the import of S defects increases more electron vacancies in core-shell structure and makes it more convenient to adsorb H*in the process of HER.As expected,the S-defect MoS₂/Ni₃S₂ exhibits a better performance than MoS₂/Ni₃S₂.In the current density of10 m A·cm^-2,a quite low overpotential of 92 m V for HER was reflected by S-defect MoS₂/Ni₃S₂,which the value has a 16 m V decrease than MoS₂/Ni₃S₂.And the overpotential at current density of 50 m A·cm^-2 is 219 m V,11 m V smaller than MoS₂/Ni₃S₂.The cell voltage to reach the current density of 10 m A·cm^-2 is 1.41 V for overall water splitting.After a long-term durability test,there are 35 m V voltage deviation,which may due to the instability of core-shell caused by defects.