Research On Designing Non-noble Metal Organic Frameworks Based Electrocatalysts For Oxygen Evolution Reaction

Electrocatalytic water splitting has been well recognized as one of the most important strategy to generate hydrogen energy.However,during the process of this reaction,the efficiency of water splitting is limited due to the oxygen evolution reaction（OER）occurring at the anode,which involves a complex four-electron transfer process with sluggish kinetics.At present,although the noble metal-based catalysts can effectively improve the efficiency of oxygen evolution,their low reserves and high costs limit the commercial application of the noble metal based OER catalysts.Therefore,it is urgent to develop efficient non-noble metal based electrocatalysts for OER.Metal-organic frameworks（MOFs）have been widely aroused as a class of versatile porous materials in designing non-noble metal based OER catalysts due to their high specific surface area,abundant active metal sites and functional groups.However,the research in this area was still at its initial stage,and the design strategy of non-noble MOFs based materials for oxygen evolution was still confronted with two scientific problems:（1）design strategies of multi-component synergistic catalysts need to be developed.（2）the intrinsic catalytic activity of MOFs and its influencing factors need to be further explored.Based on the above backgrounds,this dissertation is focused on the strategy of designing non-noble MOFs based OER catalysts with four progressive aspects:MOF-derived strategy,conductive MOF’s design strategy,MOF’s valence engineering strategy and MOF’s ligand partial substitution strategy.The research contents are as follows:（1）A MOF derived interface reconstruction strategy was proposed by constructing 3D nanowall arrays（NWs）on nickel foam（NF）,which are composed of copper nitride/phosphide（Cu₃N-Cu₃P）heterostructure coated by ultrathin N,P,S-tri-doped carbon（NPSC）.The resultant MOF-derived material integrated the good charge transfer performance of Cu₃N and Cu₃P,which can effectively enhance its conductivity.While the as formed ultrathin NPSC layer can promote the mass transfer and protect the inside Cu₃N-Cu₃P heterostructure.The as prepared Cu₃N-Cu₃P/NPSCNWs@NF has showed excellent OER electrocatalytic activity with a low overpotential of 240 m V at 10 m A cm^-2 in 1.0 M KOH over 36 hours,which was better than that of pristine Cu-MOF（407 m V）.（2）As the catalyst prepared via above MOF-derived strategy may not reflect the intrinsic catalytic activity of MOF,basing on the above research,a conductive MOF’s design strategy was proposed to fabricate a 2D MOF containing both covalent and coordinate effect（2D covalent organometallic nanosheet,2D COMS-Co）.In the structure of 2D COMS-Co,the strong coordination structure Co N₆ can promote the charge transport and prevent the aggregation of Co during the catalytic reaction,while the 2D structure formed by covalent bond topology can expose the active site（i.e.,Co）in electrolyte,which can contribute to the enhancement of electrocatalytic activity and stability.The as-obtained 2D COMS-Co shows outstanding charge transfer properties,which demonstrated its promising application in the field of electrocatalysis.The OER test indicated that 2D COMS-Co shows excellent OER electrocatalytic activity with a low overpotential of 319 m V at 10 m A cm^-2 in 1.0 M KOH.Moreover,the electrocatalytic performance and 2D morphology could still be preserved after long time stability test.（3）For the complicated preparation process of the conductive MOF’s design strategy in the previous chapter,a MOF’s valence engineering strategy was proposed to directly prepare electrocatalyst with excellent OER performance.The theoretical results first predicted that the oxygen species adsorption ability of low valence Fe（II）-MOF-74 was better than that of Fe（III）-MOF-74,which was more favorable for OER electrocatalysis.Different valence Fe（II）and Fe（III）based pristine MOF-74 nanoarrrays（NAs）on NF were further synthesized via valence engineering strategy.The results of OER test and free energy calculation indicated that Fe（II）-MOF-74 NAs@NF possessed better OER catalytic performance than that of Fe（III）-MOF-74 NAs@NF with a low overpotential of 207 m V at10 m A cm^-2 in 1.0 M KOH without any distinct change after continuous 72 hours.（4）Based on the previous chapter,a MOF’s ligand partial substitution strategy was proposed to prepare electrocatalyst in order to further promote the OER catalytic activity of Fe²⁺metal site.The theoretical studies were first performed and the results revealed that when the 2,5-dihydroxyterephthalic acid（DHTA）in Fe（II）-MOF-74 was partial substituted by the asymmetry ligand 2-aminoterephthalic acid（ATA）,Fe²⁺can be changed to coordination unsaturated metal sites and the band gap value was decreased,indicating that conductivity of ATA substituted Fe（II）-MOF-74 has been improved,which is beneficial to the improvement of electrocatalytic performance.Taking the advantages of the proposed valence engineering method and ligand substitution strategy,a series of（ATA）_x（DHTA）_1-x-Fe（II）-MOF-74（0≤x<1）were fabricated with different substitution ratio.Benefited from the optimal number of coordination unsaturated Fe²⁺sites,the as-prepared（ATA）_0.6（DHTA）_0.4-Fe（II）-MOF-74 showed the highest OER electrocatalytic activity,just required a low overpotential of 189 m V to drive a current density of 10 m A cm^-2 in 1.0 M KOH,with a long time stability under different current densities.This performance was better than that of the Fe（II）-MOF-74（237 m V）,corresponding with the above theoretical prediction results.