Theoretical Study Of Electrochemical CO₂ Reduction And O₂ Reduction Reaction

Due to the increasing emissions of Carbon dioxide(CO2)and the consumption of fossil fuels,the problems of global warming and energy crisis are becoming a great threat for human society.Electrochemical reduction of CO2 coupled with carbon capture and storage technology and powered by renewable energy(e.g.,wind and solar)to convert CO2 into chemical feedstocks represents a promising solution to mitigate our energy and environmental problems.The main obstacles for CO2 reduction reaction are high electric energy required to obtain useful chemicals and low selectivity.In addiation,the polymer electrolyte membrance full cells(PEMFCs)are an efficient device to convert hydrogen energy into electric energy.The sluggish kinetics of oxygen reduction reaction(ORR)on the cathode requires noble metal platinum(Pt)as ORR catalysts,which limits the commercialization of PEMFCs.For the electrochemical reduction of CO2,our research is mainly focused on Cu-based materials.We consider the effects of structure engineering,size changing and interface engineering of Cu-based materials for CO2 reduction based on the density functional theory.For oxygen reduction reaction,our research is mainly focused on Pt-Nx/C.The effects of defect types in graphene and the change of N content on oxygen reduction activity were studied based on the density functional theory.The main work and results can be summarized as:1.Structure engineering represents a powerful strategy for fine tuning the catalytic activity of catalysts.However,correlating the structural properties with catalytic performance is challenging because it is difficult to characterize the surface structure of catalysts.Herein,we demonstrate the effects of structural properties(e.g.,the coordination number and strain)in CO2 reduction reaction(CRR)by focusing on icosahedral,octahedral and cuboctahedral Cu nanoparticles with the diameters from 1.5 to 2.5 nm.A series of linear relations between the binding energy of CRR intermediates and generalized coordination number(GCN)or surface strain are established.We proposed that GCN and surface strain can be used as the predictive descriptors correlating the structural properties of Cu-based catalysts to its catalytic performance in CRR.By coupling of GCN and strain effects,we predicted that the octahedral Au@Cu core shell nanoparticle could lower the overpotentials to convert CO2 into CH4.2.The size of Cu NPs determines the activity and the selectivity for CO2 reduction.We study size-dependent changes on the adsorption of reaction intermediates and reaction free energy for CO2 reduction on Cu nanoparticles ranging from 0.5 to 2.5 nm.In general,the binding strengths of CO and O becomes weaken with increase of Cu NPs size and the binding energy of CO and O appears to converge to the crystal Cu(111)as the Cu nanoparticles size increases.We found that the adsorbate-induced surface charge perturbation on Cu nanoparticles becomes more local as the Cu nanoparticles size increases.For two-electron products(CO,HCOOH and H2),Cu147 appears near the top in the volcano plots for CO and H2,and Cu55 appears near the top in the volcano plots for HCOOH.To identify the active sites for the CO2 reduction,we compared the activity of different reaction sites,namely,facet,edge and corner sites of Cu nanoparticles,Cu(111),Cu(100),Cu(211)and hexagonal Cu nanowire.We find that the under-coordinated sites are more active than the low-index surfaces for CO2 reduction to CH4.3.Interface engineering represents a powerful strategy for fine tuning the catalytic activity of catalysts.The electrochemical reduction of CO2 to formic acid(HCOOH)and the competing hydrogen evolution reaction(HER),on eight different metal(111)and(211)surfaces(Tm(111)and Tm(211),(Tm=Ni,Cu,Rh,Pd,Ag,Os,Ir and Pt)ZnO bilayers and ZnO bilayers supported on X(111)(X=Cu,Ag and Au)surfaces,have been investigated using density functional theory calculations.There are two competing reaction pathways for CO2 reduction to HCOOH.In the first pathway,*COOH(where*indicates adsorbed species)is the intermediate and in the second pathway,*HCOO is the intermediate.It is impossible that the electroreduction of CO2 to HCOOH via the*COOH pathway because*COOH prefers reduction to*CO on metal surfaces.While the electroreduction of CO2 to HCOOH is possible via the*HCOO pathway.ZnO/X(111)surfaces are found to be the promising catalysts showing high selectivity for the electroreduction of CO2 to HCOOH.The origin of an extraordinary catalytic activity of ZnO/X(111)is also explained.The X(111)supported ZnO becomes metallic,in contrast with the wide gap in the unsupported ZnO bilayer.We attribute this behavior mainly to the strengthened binding of CO2 reduction intermediates.The occurrence of a net electron transfer from the support to*HCOO about-0.7 e,resulting in the formation of negatively charged*HCOO-,and this behavior resulting in the strengthened binding of*HCOO compared to*COOH.This is the reason why ZnO/X(111)showed the higher catalytic selectivity for HCOOH.4.M-Nx/C(M=Pt,Fe,Co)catalyst is the promising catalysts for ORR.Our research is focused on Pt-Nx/C.It is found that the graphene by introducing pyridinic N can make supported Pt single atom and Pt4 nanocluster accumulate more positive polarized charges,which upshift the d-band centers of the supported Pt single atom and Pt4 nanocluster toward the Fermi level.It is also observed that the more pyridinic N in grapheme is,the more positive polarized charges is accumulated by supported Pt single atom and Pt4 nanocluster.The positive charged sites is favour the dissociation of O2.We find that the energy barrier of O2 dissociation almost decrease linearly with the increase Pt positive polarized charges.The Pt4 nanocluster supported on N4 graphene(Pt4-N4/C)exhibit extremely low dissociation barriers with only 0.071 eV for O2.The improvement of oxygen decomposition activity depends on more positive polarization charge accumulating on Pt4 nanoparticle(0.49 e).Our work reveals that the Pt positive polarized charges can act as microscopic driving force for O2 dissociation.