Design Of Highly Efficient Ni-based Electrocatalysts And The Study Of Performances Toward Water Oxidation

Utilizing renewable electricity to drive water splitting for hydrogen production is an important way to realize the storage and conversion of renewable energy.However,the oxygen evolution reaction（OER）half-reaction at the anode is a multi-electron-multi-proton,multi-step reaction process with high thermodynamic energy barriers and slow kinetics which hinders the large-scale applications.At present,precious metal catalysts such as Ir O₂ and Ru O₂ are mainly used in industrial electrocatalytic water oxidation.However,the reserves of precious metals are limited and it is difficult to apply for large-scale applications.Therefore,it is very necessary to design and synthesize non-precious metal water oxidation electrocatalysts to replace precious metal catalysts.The design and synthesis of non-noble based electrocatalysts is still constrained by issues,such as interfacial charge transfer and low surface catalytic conversion efficiency,and key scientific issues such as its structure-activity relationship and reaction mechanism are poorly understood.Nickel（Ni）is a metal with high earth-abundance,which has high activity for the hydrogen evolution half-reaction,and has the characteristics of easy control of structure,but it still has high OER overpotential and low intrinsic activity.Based on this,this thesis intends to take Ni-based catalysts as the main research object,and design electrocatalysts for water oxidation with excellent performance from the two key points of interface charge transfer improvement and catalytic site electronic structure regulation,and focus on the above two key scientific issues On this basis,the following research results were obtained:（1）Interface modification is used to regulate the interface properties between catalyst and electrode,as well as the interface interaction between catalyst particles.A new strategy of combing carbon nanotubes（CNTs）and doped carbon has been developed to synergistically promote the electrochemical performances of Ni Fe-based water oxidation catalyst.CNTs can effectively improve the interface between catalyst and carbon fiber paper（CFP）substrate,and the doped carbon that results from the precursor of glucose(C₆H₁₂O₆)can enhance the charge transfer and dispersion of Ni Fe-based catalyst particles,thus decreasing the charge transfer resistence between catalysts and electrolyte.In addition,the doped carbon will facilitate the formation of Fe-O-C bond,thereby enhancing its surface catalytic activity.Based on these improvements,highly efficient water oxidation catalyst（Ni Fe-C/CNT/CFP）is designed and synthesized which shows an overpotential of 202 m V at 10 m A cm^-2,Tafel slope of 38.2 m V dec^-1 and stability of 72 h in 1M KOH.Furthermore,the TOF value can reach 0.10 s^-1 at 1.48V（vs RHE）for this catalyst.（2）The interface modification with CNTs and doped carbon mentioned above are utilized to regulate the electron structure and interface charge transfer of Ni Fe-based bimetal catalyst,together with the coordination properties of polyoxometalates.The influence of PMA coordination and carbon doping on the electronic structure and catalytic performance of catalyst is further investigated systematically.The results indicate that carbon doping prompt the formation of Ni-O-Fe bond and optimize the electrotic structure of catalyst,as well as improve the interface charge transfer between catalyst and electrolyte.Highly efficient water oxidation electrocatalyst（Ni Fe-C-PMA/CNTs/CFP）is developed with the TOF value of 2.34s^-1 at 1.53 V（vs.RHE）in 1M KOH,which is superior to Ir O₂/CNTs/CFP(TOF=0.026 s^-1).Furthermore,it can operates stably at 10 m A cm^-2 for 100 hours,exhibiting excellent stability in alkaline electrolyte.Both the activity and stability of catalyst are improved simultaneously.（3）In response to the requirements of water oxidation electrocatalyst in different p H environments,such strategies as CNTs modifications and bimetallic composites are used to regulate the interface properties and electronic structure of catalyst.The synthesized bimetallic composite electrocatalyst（Ni Ir_x/CNTs/CFP）shows better OER performance than commercial Ir O₂ in a wide p H range（p H=0-14）.It has an overpotential of 196 m V at 10 m A cm^-2 and the mass activity of 2.54 A mg_Ir^-1 at 1.53 V（vs.RHE）in 0.5M H₂SO₄,showing a highly efficient operation of Ni Ir composite electrocatalysts in acidic electrolyte.Furthermore,It can run stably for 12 hours in different electrolytes.Experiments and DFT calculations show that the combination of Ni and Ir can significantly regulate their electronic structure,enhance the catalyst’s adsorption and activation toward water molecules。In addition,the strong electronic coupling interaction between Ni Ir bimetals and CNTs enhances the activation ability of catalyst toward intermediate species,improve the properties of water oxidation of catalysts,and greatly reduce the dosage of precious metals.In addition,the introduction of CNTs effectively improves the interface charge transport between the catalyst and CFP,as well as the micro-electronic structure of catalyst,thus commonly promoting the performances of water oxidation.Therefore,the catalytic performances of Ni-based electrocatalyst are improved in wide p H range.In this paper,Ni Fe-C/CNTs/CFP,Ni Fe-C-PMA/CNTs/CFP and Ni Ir_x/CNTs/CFP catalysts were synthesized by the strategy of interfacial modification,and the catalytic activity and stability were improved,respectively.Among them,Ni Fe-C/CNTs/CFP and Ni Fe-C-PMA/CNTs/CFP have excellent catalytic performances in alkaline system,while the catalyst Ni Ir_x/CNTs/CFP has good catalytic performance in a wide p H range.The experimental results show that the performance of the catalyst is closely related with the structure,morphology,interface properties and changes of electronic structure for the catalysts.Further theoretical studies showed that the rate-determining steps of the OER reaction for the catalysts Ni Fe-C/CNTs/CFP and Ni Fe-C-PMA/CNTs/CFP are related with the chemical process after electron-transfer step.While for the catalyst Ni Ir_x/CNTs/CFP in acidic electrolyte,the rate-determining step is the chemisorption reaction about the intermediate species from*O to*OOH.