Doping Engineering Of Electrocatalysts Toward Efficient Water Splitting

Energy shortage and environmental pollution are two major problems in the development of human society today.To address these,it is particularly important to develop sustainable technologies for high efficient and green energy conversion and storage.Hydrogen production via water electrolysis is a key part of achieving this goal.Water electrolysis consists of two half reactions:hydrogen evolution reaction（HER）at the cathode and oxygen evolution reaction（OER）at the anode.However,both of them are kinetically sluggish and have high energy barriers,resulting in a low energy conversion efficiency of overall water splitting.Therefore efficient electrocatalysts are highly demanded.The precious metals Pt and Ru/Ir are considered as the best HER and OER electrocatalysts,respectively.Their wide application however is limited by their scarce reserves and high prices.Thus,reducing the loading of precious metals in catalysts or replacing it with non-precious ones is an effective strategy to solve this problem.Doping engineering can significantly adjust the microscopic structure and electronic property of the host materials and is widely employed to tailor their catalytic performance.It is one of exciting research areas in recent years to design and synthesize efficient electrocatalysts based on doping engineering and further to explore the mechanism behind doping enhanced activity.In this thesis,two efficient heteroatom-doped electrocatalysts were synthesized,and their formation mechanism,active sites,electrocatalytic performance and enhancing mechanism of heteroatoms were systematically investigated.The main research contents are as follows:（1）Ni Fe based catalysts exhibit promising electrocatalytic activity toward OER in alkaline media.The introduction of Fe leads to the formation of Fe-O-Ni sites,which are considered to be the true catalytic activity centres of this type of catalyst.However,the doping of Fe induces enhanced disorder level of the Ni OOH host and possible formation of insert Fe OOH phase,both of which are harmful to OER activity.Thus,maximize the density of surface Fe-O-Ni active sites while maintaining a high structural order level of the Ni OOH host is a feasible strategy to enhance the activity of Ni Fe catalysts.In the first work,a unique photochemical-electrochemical method is developed to fabricate Fe-doped Ni OOH nanoparticle electrocatalyst with uneven Fe distribution on the three-dimensional carbon cloth scaffold（Fe-Ni OOH@CC）.The characterization results show that the abundant Fe-doped Ni OOH active sites exist on the surface of Fe-Ni OOH@CC,while the inside is the highly crystalline Ni OOH matrix with a low structural disorder level.It exhibits a higher OER catalytic activity and excellent stability than Ru O₂ and other Ni Fe analogues in alkaline electrolyte.Moreover,this photochemical-electrochemical strategy is applicable to other transition metals and substrates.It is a general strategy for synthesis of OER electrocatalysts.（2）Heteroatom-doped carbon is considered as a promising support for active metal nanoparticles as it can provide abundant nucleation and growth sites for active metal nanoparticles,and strengthen the attachment of active metal nanoparticles for improved stability.More importantly,the doped carbon can regulate the electronic structure via strong support-metal electronic interaction,thus enhancing the intrinsic catalytic activity of metal nanoparticles.In the second work,graphene sheets are doped with N atoms by radio-frequency N₂ plasma treatment at near-room temperature,and then complexed with Ir³⁺ions,and finally Ir nanoparticles loaded on N-doped graphene（Ir@N-G-600）is synthesized by pyrolysis in inert atmosphere.Although with very low Ir loading,as-synthesized Ir@N-G-600 exhibits excellent activity and stability toward HER,OER and water splitting in both acidic and alkaline media.Further investigation results show that the enhanced catalytic activity of Ir@N-G-600 is due to the abundant coordination between Ir and N atoms,which enriches the electron density of Ir atoms to form a favorable electronic configuration for HER and OER processes.In summary,this thesis develops two effective doping engineering strategies and achieves heteroatom-doping of the host materials,one of which is the doping of the metal active phase,and the other is the doping of the carbon support materials with more universal significance.Two efficient electrocatalysts,Fe-doped Ni OOH nanoparticles with uneven Fe distribution and Ir nanoparticles supported on N-doped graphene nanosheets,are prepared respectively,and their formation mechanism,electrocatalytic performance and structure-activity relationship are deeply investigated.