Preparation Of Atomic-level Dispersed Nickel Catalysts For Electrochemical Carbon Dioxide Reduction Reaction

Electrocatalytic reduction of carbon dioxide（CO2RR）to value-added chemicals and fuels associated with renewable energy sources opens up a promising way for tackling global warming issues,alleviating energy crisis and realizing carbon neutrality.Specifically,the electrosynthesis of CO from CO2RR represents one immediate and available avenue for current CO2 valorization as it is an energy-rich precursor for further valued products（methane,methanol,and diesel fuel,etc.）synthesis via the established Fischer-Tropsch process.However,the current conversion efficiency for CO2RR cannot meet the requirement of the relevant industry due to the sluggish reaction kinetics and fast competing hydrogen evolution reaction（HER）.Therefore,the development of highly efficient electrocatalysts for fast CO2RR process is critical for high energy conversion efficiency.Upon decades of explorations,the state-of-art catalysts for electrochemical CO2 conversion to CO are still limited to the noble metal based materials（e.g.Au,Ag）.However,their scarcity,high cost,and questionable stability block their large-scale application.To address these challenges,atomically dispersed earth-abundant transition metal atoms（eg,Fe,Co,Ni,Zn and et al.）based catalyst has been developed as one of the most promising alternatives of noble metal catalysts due to their unique features of uniform active sites,maximum atom utlization and excellent catalytic performance.This thesis focuses on the controllable design and construction of atomically dispersed Ni-based catalysts including single-atom,dual-atom and fully exposed cluster catalystic systems,as well as to investigate their CO2RR process and the relevant mechanism studies in order to clarify the influence of different catalytic sites of the Ni-based catalysts toward boosting the CO2RR.The thesis consists the following aspects:1.Universal synthesis of various dual-atom catalysts（DACs）with uniform M2N6（M=Ni,Zn,Pd,Mn）motifs on nitrogen-doped carbon sheet（named as M2NC）on a large scale was developed.Importantly,this method enables a controllable dispersion of well-defined dual-atom and single-atom on NC via carefully manipulating the precursors.Taking Ni2NC as an example,we found Ni2N6 sites exhibited different CO2 activation ways,increased adsorption energies of key intermediates,and faster CO2RR compared to the single NiN4 site of SACs.Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy（HAADF-STEM）and X-ray absorption fine structure（XAFS）measurements identify the atomic dispersion of M2N6 on NC.Ni2NC achieved a record performance with CO partial current density up to～1 A cm～2 and turnover frequency（TOF）of 77,500 h-1 at～99%Faradaic efficiency（FE）,as well as 30 h stability at～290 mA cm-2.In-situ XAFS analyses with DFT calculations elucidate that the enhanced adsorption and activation of*COOH intermediates by Ni dual-atom sites and hydrogen bonding in water accelerate the kinetics of CO2RR.2.Various atomically dispersed N,S co-coordinated Ni based catalyst was synthsized by a controable pyrolysis temperature method,realizing controlable engneering of the fully exposed Ni cluster to Ni single atom sites.The fully exposed Ni cluster catalyst with fully exposed catalytic sites and multiple adjacent metal centers exhibits excellent CO current density（jco）of～347 mA cm-2 at-0.8 V vs.reversible hydrogen electrode（RHE）,which can be totally competent for the industrial electrolysis CO2RR process.Impressively,a high selectivity of CO with Faradaic efficiency（FEco）of 99%was reached at a low overpotential of 390 mV.In addition,the catalyst exhibited negligible current density and selective decays in a constant potential stability test conducted in 0.5 M potassium hydroxide（KOH）electrolyte for 20 hours.Density functional theory（DFT）calculations affirm that the charge arrangement of multiple Ni centers regulated by the introduced S atoms can enhance the adsorption of*COOH intermediates on Ni sites and the synergestic effect of multiple Ni atoms sites reduce the energy barrier of CO2RR,which together endow Ni FECC catalyst with excellent CO2RR performance.