The Study On Design And Modulation Of Transition Metal Nitride And Phosphide Compound Catalysts

Developing new type and clean energy sources are the essential condition for sustainable development.Among all the promising energy candidates,hydrogen has been regarded as one of the most promising candidates.Water electrolysis is considered to be an eye-catching way for hydrogen production due to the high purity of the product and low environmental pollution,which makes it more desirable compared with other hydrogen production methods.Up to now,Pt,Ir and other novel metal based materials are still the most efficient electrocatalysts for water splitting,which results in a much higher cost in practical application.To solve this problem,high efficient electrocatalysts are the urgent need for the future development and large-scale application of water electrolysis.In this thesis,the controllable preparation of transition metal nitrides/phosphides(TMNs/TMPs)are investigated,and some useful strategies are applied to further improve their water electrolysis activities.At the same time,the possible mechanisms of the promoted catalytic performance are also investigated.Nanostructure constructing and elemental doping was applied to improve the water splitting performance of skutterudite-type Co P₃,which has the highest phosphorus content in all cobalt based phosphides.The Co P₃ nanoneedles were prepared on the carbon cloth by a two-step procedure:hydrothermal process and the following high-temperature vacuum phosphidation.To further modulating the electronic structure of Co P₃,different amount nickel was doped in the Co P₃ and the obtained electrocatalysts showed improved HER performances.It demonstrated that when the doping content of Ni reaches 7%(Co_0.93Ni_0.07P₃),the electrocatalyst demonstrated the highest activity with an overpotential of 87 m V vs RHE and a Tafel slope of 60.7 m V dec^-1.Moreover,this self-standing electrode also demonstrated an enhanced OER performance,which showed an overpotential of 212 m V and a Tafel slope of 60.7 m V dec^-1.Also,it has been proved that the oxidation happened during the OER process,and the electrocatalysts was finally turned to the corresponding metal oxides,which served as the real OER catalysts.Constructing heterostructure is another commonly used methods to improve the performance of electrocatalysts.Most heterostructure composites are composed with different materials,which may induce the doping effect at the interfaces and disturb the investigation of the function for heterostructure itself.In this section,proper amount of monoclinic Ni P₂(m-Ni P₂)was introduced into the commonly reported cubic Ni P₂(c-Ni P₂),and a newly kind of phase-junction electrocatalyst was proposed,which could exclude the doping effect at the interfaces and offer a platform to just investigate the function of heterostructure.By elevating the synthesis temperature,c-Ni P₂ could be partially transformed into m-Ni P₂,which resulted in the c/m-Ni P₂phase-junction.Compared with the most reported c-Ni P₂,the existence of m-Ni P₂could effectively promote the Volmer reaction,which is regarded as the rate degerming step for alkaline HER process.Moreover,the DFT calculation results demonstrated that,the c/m-Ni P₂ electrocatalyst had a more appropriate?G_H*,which further enhanced the HER performance.Experimental results showed that,when the content of m-Ni P₂ reaches 6.9%,the c/m-Ni P₂ phase-junction would demonstrate the best HER activity in alkaline media,with an overpotential of 134 m V,which is 26%and 96%lower than the values of c-Ni P₂ and m-Ni P₂,respectively.In the third part of this thesis,Ni₃N is chosen as the substrate and minute amount of Ru was loaded on it to serve as an efficient electrocatalyst for water splitting.It was found that Ru presented with the form of partial single atom and partial nanoparticle,which would greatly improve its catalytic sites,as well as the water dissociation step.Moreover,we introduced proper amount of F element in the Ni₃N substrate to modify its properties.Due to the strong electronegativity of F,the strong metal-support-interaction(SMSI)was enhanced,and the HER and OER activities was thus improved.This study also revealed doping with elements of strong electronegativity was a promising way to enhance SMSI,which was an efficient way to enhance the catalytic activities.Also,this study paves a new way for designing practical electrocatalysts with high catalytic performance,but low noble metal loadings.