Controllable Metal-Organic Framework Reconstruction For Electrocatalytic Water Splitting And CO₂ Reduction

In the process of developing industrialization,energy supply is still highly dependent on fossil fuels,resulting in serious resource shortages and environmental pollution problems.The development of renewable green energy is an effective solution to solve the above problems,but its intermittence and unstability bring difficulties in the consumption of the converted electric energy.Converting the clean electrical energy into storable chemical energy through electrochemical methods is one of the ideal ways to achieve sustainable energy development and efficient synthesis of green chemicals.Among them,electrocatalytic water splitting to produce hydrogen is a potential means,because water is abundant and renewable,and the hydrogen as energy storage medium is the molecule with the highest energy density per unit mass.In addition,electrocatalysis reduction of CO2 into valuable fuels or chemicals,which can effectively reduce CO2 emissions and alleviate the greenhouse effect while storing energy,has also received widespread attention.The key to develop advanced electrocatalytic water splitting and CO2 reduction technologies is the rational design and controllable preparation of the electrocatalysts applied in the hydrogen evolution reaction(HER),oxygen evolution reaction(OER)and CO2 reduction reaction(CO2RR),aimed at reducing catalyst cost,accelerating the reaction Kinetic and achieving high-efficiency conversion of electrical energy into chemical energy.Metal-organic frameworks(MOFs)are ideal catalyst materials due to the large specific surface area,rich dispersed atomic-scale metal centers,and excellent chemical structure designability.However,most MOFs are composed of weak coordination bonds,which are prone to uncontrollable structural evolution during electrochemical processes.Therefore,starting from the practical application of electrocatalysis,this thesis utilizes the structural and compositional advantages of MOFs to construct a series of electrocatalytic water splitting and CO2RR catalysts based on the control of the topological structure transition through the study of its thermal,chemical and electrochemical reconstruction.Meanwhile,we track the reconstruction process by means of various in-situ characterizations to deeply clarify the structure-activity relationship and explore the reaction mechanism in depth.The main research results obtained are as follows:(1)We report a hierarchical nickel-carbon composite,fabricated by directly growing sheet-like Ni(BDC)on nickel foam prior to carefully regulated high-temperature annealing,as a highly efficient water splitting bifunctional catalyst.The hierarchical structure facilitates sufficient exposure of active sites and promotes the transport and diffusion of reactants and products.On this basis,taking advantage of the atomic-level dispersion of metal sites in MOFs,uniform Fe-doped Ni2P is fabricated using Fe-doped Ni(BDC)arrays as the structural template.The dopant atoms increase the electrochemical specific surface area of the MOFderived catalyst by regulating the morphology of the precursor,enhance the adsorption of HER intermediates by modulating the electronic structure of Ni2P,and finally improve the water splitting performance at high current densities.(2)A new strategy for the preparation of defect-rich layered double hydroxides(LDHs)via chemical reconstruction of MOFs has been developed,achieving a breakthrough in the activity of non noble metal doped LDHs for water splitting.The holey ternary CoFeNi-LDH synthesized via MOF-mediated topological transformation shows a unique ruffled sheet morphology with prevailing mesoscopic pores.The pores are mostly polygons with welldefined edges of high crystallinity,giving rise to atomistic lattice steps and defects enriched with under-coordinated metal centers.These under-coordinated metal centers help to achieve the synergistic effect among different metals which can stabilize the key HER intermediate*MH-OH by co-adsorption to facilitate the dissociation of water.On the other hand,they can also facilitate the conversion of LDHs into real OER active sites of metal oxyhydroxide,thereby improving the activity of water splitting.(3)For a more complex CO2RR system,we first investigate the effect of conducting support on the electrochemical reconstruction of Cu-based MOFs,clarify the structureactivity relationship between reconstructed metal catalysts and catalytic activity,and finally achieve the controllable construction of catalysts with high C2H4 and CH4 selectivity.The conducting support is beneficial to improve the current density and surface charge delocalization,thereby accelerating the reconstruction of MOFs to tiny Cu crystallites and helping stabilize them thereafter.The Cu crystallites comprise multifacets and grain boundaries,which enhance the*CO adsorption,promote the C-C coupling,and help to suppress the competitive reaction HER,leading to high C2H4 selectivity.Subsequently,uniform CoCu alloy catalysts are directly prepared on gas diffusion electrodes by electrochemical reconstruction of Co-doped Cu-based MOFs.The Co doping reduces the*CO concentration by enhancing surface water activation,suppressing the C-C coupling.On the other hand,the adsorbed H*on the Co sites is favorable for the hydrogenation of*CO to*COH,which ultimately improves the CH4 selectivity.