The Synthesis And Structural Modification Of Nickel/Iron Based Two-Dimensional Metal-Organic Frameworks Towards Electrocatalytic Oxygen Evolution

Two-dimensional metal-organic frameworks（MOFs）have emerged as homogeneous or heterogeneous oxygen evolution reaction（OER）catalysts due to their high specific surface area,abundant pore structures,fully exposed metal centers,and tunable metal composition and proportion.In recent years,the synthesis of Ni Fe-based two-dimensional MOFs and their application in the field of electrocatalytic OER have been reported a lot.However,MOFs catalysts are still not satisfied compared with commercial catalysts due to their poor conductivity,catalytic activity and stability.Based on the above problems,in this dissertation,we have prepared a series of two-dimensional Ni Fe-based MOFs through coupounding,modification and doping,and their electrocatalytic OER activities were studied.Besides,the catalytic mechanism of Ni and Fe sites in two-dimensional MOFs was investigated by electrochemical analysis,XPS and in-situ Raman techniques.The specific research results are as follows:（1）Nickel-iron layered double hydroxide-nickel foam（LDH-NF）nanosheet array composites was adopted as precursors,ion exchange method was achieved by using 2,6-naphthalenedicarboxylic acid（NDC）and 1,4-terephthalic acid（BDC）to prepare Ni Fe-naphthalenedicarboxylate MOF-nickel foam composite（NDC-NF）and Ni Fe-terephthalate MOF-nickel foam composite（BDC-NF）with the same morphology and Ni/Fe ratio as LDH-NF,to study the catalytic performance and mechanism of different catalysts in OER.In alkaline media,NDC-NF and BDC-NF have better catalytic performance compared with LDH-NF.All three catalysts undergo surface structural transformation during the catalytic process.The Ni²⁺ions are continuously oxidized to Ni³⁺and converted into oxyhydroxides as real active materials.During OER process,the MOF-NF can inhibit the dissolution of Ni ions.The metal active sites in MOFs are modified with more hydroxyl groups,which makes the metal active sites have a higher valence state and further improves the catalytic performance.（2）Through the competitive coordination mechanism between acetate ions and BDC ions,we prepared hierarchical Ni Fe-MOF microspheres（Ni Fe-MOF NSs）composed of two-dimensional ultrathin nanosheets with more（001）facets exposure.By exploring its growth process,we found that high concentration of acetate ions played an important role in the self-assemble of Ni Fe-MOF nanosheets.In the test system with glassy carbon electrode as work electrode,the catalytic performance of Ni Fe-MOF NSs is excellent,with an ultralow overpotential of 240 m V at a current density of 10 m A cm^-2 and a Tafel slope of 75.54 m V dec^-1.During the oxygen evolution reaction,the Ni Fe-MOF NSs will transform into two-dimensional Ni Fe oxides decorated with Ni OOH nanoparticles.In addition,more（001）crystal facet exposure induces more Ni²⁺ions to convert into Ni OOH,which further improves the OER performance.（3）As the NDC and BDC ligands have the same functional group（–COOH）,we constructed a dual-ligand two-dimensional MOF(Ni Fe-NDC_0.9BDC_0.1 MOF)with hierarchical microsphere morphology through co-cordination.Compared with Ni Fe-NDC,the introduced BDC ligands would induce enhancement of the metal-carboxylate bond,resulting in better catalytic activity for the OER.In the test system with glassy carbon electrode as work electrode,the optimized Ni Fe-NDC_0.9BDC_0.1 exhibits a low potential of 295 m V at a current density of10 m A cm^-2,a small Tafel slope of 69.4 m V dec^-1,and excellent stability in alkaline media.The introduction of BDC ligands into the Ni Fe-NDC framework not only strengthens the interaction between Ni and Fe atoms,but also facilitates the deeper conversion of MOFs into a mixed phase ofβ-Ni OOH,which promotes the catalytic activity.（4）Taking the two-dimensional Zn-BDC MOF as substrate,gradient doping of Ni and Fe in two-dimensional Zn-BDC MOF（Zn Nix%-BDC MOF and Zn Fex%-BDC MOF）by one-step solvothermal method and ion exchange method was achieved.With the increasing doping amount of Ni and Fe,the catalytic performance of both Zn Nix%-BDC MOF and Zn Fex%-BDC MOF is improved.This is because the increasing doping amount of Ni and Fe will provide more active sites,higher electrochemical active area,and faster charge/mass transfer rate.Through in-situ Raman analysis,we found that the main factor inducing the surface structural transformation of MOF materials during OER is that the metal ions are oxidized to higher valence state under the applied voltage,and the high valence metal ions will dissociate from the ligands.The dissociated ligand ions will dissolve in the reaction system,and high-valence metal ions will form metal（oxy）hydroxides as active materials to participate in the catalytic reaction.