A Study On Structure-activity Relationship Of Water Oxidation On Mononuclear Catalysts

Hydrogen production via water splitting utilizing renewable energy is recognized as one of the most intriguing routes to address energy and environmental issues.Water oxidation is considered as the bottleneck-type reaction in water splitting due to its sluggish kinetics and four electron transfer.It is imperative to develop cheap,highly active and robust water oxidation catalysts.The catalytic performance of catalysts is highly dependent on the electronic structure of catalysts,which is relevant to the metal site(d electronic structure)and its coordination atom(coordination environment).As a consequence,the study on the structure-performance relationship of catalysts will guide the rational design of highly active water oxidation catalysts,which still remains a challenge.Taking the advantages of uniformity of active sites in mononuclear catalysts,the mononuclear catalysts are selected as models for studying the structureperformance relationship.In this thesis,focusing on the C,N coordinated mononuclear catalysts,we have studied the effects of the metal sites(d electronic structure)and the coordination environment of the metal sites in mononuclear catalysts on oxygen evolution reaction(OER)performance.Furthermore,the impacts of electronic structure of catalyst on OER pathway and OER performance have also been investigated.The main results are summarized as follows:1.Three solid mononuclear Co catalysts were synthesized by embedding CoN4Cl,CoCN3Cl and CoC4Cl structures in graphene.We found that the CoCN3Cl structure shows a higher water oxidation performance than CoC4Cl and CoN4Cl.And the DFT calculation further disclosed the change rule of OER performance and RDS(Ratedetermining step)for the structure of CoC(4-x)NxCl(x=0,1,2,3,4).When C in the first coordination sphere is replaced by N atoms,a)the binding strength of Co=O*in these structure is getting weak;b)the reaction barrier for Co-OH*→Co=O*is increased,while the reaction barrier for Co=O*→Co-OOH*is decreased.Therefore,the RDS of water oxidation over these catalysts will change from Co=O*→Co-OOH*(CoC4Cl)to Co-OH→Co=O*(CoCN3Cl and CoN4Cl);c)The CoCN3Cl structure with the most suitable binding strength to*O shows a higher water oxidation performance than CoC4Cl(CoN4Cl)with a strong(weak)binding strength to*O.This work inspires us the importance of coordination environment of the metal nucleus in catalysts for RDS manipulation and activity optimization in OER through modulating the binding strength between active site and reaction intermediates.2.Mononuclear catalysts with M(M=Mn,Fe,Co,Ni)and N atoms embedded into graphene have been prepared and selected as a model catalyst to study the impacts of metal sites(Mn,Fe,Co,Ni)on catalytic OER performance.It is revealed that these mononuclear catalysts show an OER activity trend of Mn>Fe>Co≈Ni for chemical water oxidation system(Ce4+,pH≈1),while the activity trend remains Mn>Fe>Co≈Ni for the electrocatalytic water oxidation system(1 M KOH).And the experimental observed trend is consistent with the DFT-predicted one.3.Taking mono-CoPc and bi-CoPc as examples,we found that the bi-CoPc catalyzes water oxidation more efficiently than mono-CoPc.Kinetic analysis reveals that the ratedetermining step(RDS)of oxygen evolution reaction(OER)over both two catalysts is a nucleophilic attack process involved with a hydroxy anion(OH’).The difference is that the substrate nucleophilically attacked by OH-for bi-CoPc is the phthalocyanine cation-radical species(CoⅡ-Pc-Pc·+-CoⅡ-OH)that is formed from the oxidation of phthalocyanine ring,while cobalt oxidized species(Pc-CoⅢ-OH)is involved for monoCoPc as evidenced by UV-vis spectroelectrochemistry technique.The DFT simulated OER pathway is consistent with experimental results,which confirm that the bi-CoPc can effectively stabilize the accumulated oxidative charges in phthalocyanine ring,and thus bestowing a higher OER performance on bi-CoPc.This work discloses the role of electronic structure of active sites in water oxidation route and catalytic performance,which may guide the rational design of highly active catalysts.