Coordinate Structure Design Of Single-atom Catalysts For CO₂ Electrochemical Reduction

Electrochemical reduction of CO2 to fuels with wind power/solar power has been an effective way to develop renewable energy to achieve the national goal of carbon neutrality.What’s more,electrocatalytic CO2 reduction to CO has become a research hotspot in the field of carbon transformation and utilization due to its high efficiency,mild reaction conditions and high product selectivity.However,there are many shortcomings such as low current density,severe HER and high cost in CO2 RR.Thus,it is urgently needed to develop good conductivity,high activity and low-cost catalysts to improve CO2 RR performance.Herein,single atom carbon-based catalysts were studied due to its large reaction surface,abudent active sites and fast charge transfer.The adsorption energy of intermediates was decreased through optimizing coordination environment and electronic structure of the active center,thus promoting CO2 activation,COOH* adsorption and CO* desorption,improving current density and selectivity for CO2 RR.Axial Ni-N coordination structure enhances charge-polarization effect of active-site.The phthalocyanine nickel molecule has a high CO selectivity in CO2 RR,while it is poor conductivity and easy to reunite.To enhance current density and stability in CO2 RR,Ni Pc/NC was proposed.XAFS spectroscopy confirmed that Ni-N5 structure composed of Ni-N4 site in Ni Pc and an axial N atom doped in carbon matrix.DFT calculations revealed that axial Ni-N coordination enhanced charge polarization effect of the active site and COOH* adsorption,thus reducing the energy barrier in the rate-limiting step.The FECO was ≥93% over Ni Pc/NC catalyst in a wide potential range of-0.5 to-0.8 V(vs.RHE),and the peak FECO reached 98% at-0.5 V,significantly improving the current density and stability of the reaction system.Bilayer coordination structure of Ni-Fe single atoms enhances valence state of Fe atom.To further improve current density,bilayer coordination structure of Ni-Fe single atom catalysts were constructed.It was worth noting that the valence state of Fe species in Fe Pc@Ni NC reached the maximum valence.DFT calculations revealed that the high valence state of Fe atoms in the Fe Pc@Ni NC catalyst boosted adsorption of COOH* and desorption of CO* intermediate.The current density of the Fe Pc@Ni NC catalyst was increased to 12.7 m A·cm-2 at-0.7 V,showing a stable current density with constant CO selectivity at-0.8 V during continuous electrolysis over 18 h.Hydroxyl group modifies Ni-N active site promoting charge-polarization of CO2.To further reduce the cost of catalysts such as phthalocyanine molecules,single Niatoms modified with hydroxyl in cheap honeycomb-like carbon matrix was proposed.XAFS and XPS measurements confirmed that the active-center structure consisted of single Ni atoms and the adjacent hydroxyl(H-Ni/NC).DFT calculations showed that an active region was formed between Ni atom and hydroxyl group,C atom in CO2 bound to Ni atom,and hydrogen bond was formed between H in hydroxyl with O in CO2,promoting polarization and bond activation of CO2.The barrier of rate-limiting reduced to +1.05 e V,and the FECO reached 97% over H-Ni/NC catalyst at-0.7 V.Constructing the structure of nitrogen and phosphorus co-coordination decreased the valence state of Ni single atom.To solve the excessive charge transfer from Ni to N caused by high electronegativity of N atoms,N-P co-coordinated Ni single atom catalyst was proposed by doping with low negative P atoms.Herein,XAFS measurements revealed that the active site of the Ni-NPC catalyst is Ni-N3 P.DFT calculations indicated that the P-doping decreased the valence state of Ni atom,reducing the energy barrier of limiting-step to +0.97 e V and thus promoting CO2 RR.Ni-NPC exhibited a large current density of 22.6 m A· cm-2 at-0.8 V vs RHE.Adjusting the nitrogen-carbon co-ordination ratio optimized the PDOS of Ni3 d orbit.To avoid environmental pollution caused by phosphorus doping,atomically dispersed Ni-based catalysts with various N/C coordination numbers(named Ni@Nx Cy)were fabricated via simply adjusting the carbonization temperature.EXAFS fitting analysis confirmed that N coordination number decreased from 4 to 1 in Ni@Nx Cy catalysts when pyrolysis temperature increased from 800 to 1100 ℃,whereas C coordination number showed an opposite trend.DFT calculation clarified that Ni-N2C2 site was favorable to generate more active Ni 3d orbitals,which can effectively promote the rapid transfer of electrons and the adsorption of reaction intermediates,thus effectively reducing the energy barrier of rate-determining step to+0.80 e V.The Ni@Nx Cy-1000 catalyst with the optimum coordination numbers of two N and two C atoms pyrolyzed at 1000 ℃ achi e Ved the highest FECO of 98.7% and high current density of 18.5 m A×cm-2 at the potential of-0.7 V vs RHE.