Controlled Preparation Of Two-Dimensional Transition Metal-Based Electrocatalysts And Their Reaction Mechanism For Overall Water Splitting To Hydrogen

In the context of carbon neutrality,the development of high energy density,environmentally benign,and carbon-free hydrogen energy have become a priority.One of the most efficient methods for hydrogen production is to generate electricity using intermittent energy sources（such as tidal,wind,and solar）,followed by hydrogen generation by whole-water decomposition.Because of the drawbacks of traditional noble metal-based electrocatalysts,such as their high cost,poor stability,and limited natural abundance,researchers have concentrated on producing low-cost transition metal-based electrocatalysts with high natural abundance.Two-dimensional materials,such as two-dimensional transition metal-based electrocatalysts,have become the focus of research in transition metal-based electrocatalysts because they have excellent two-dimensional morphology,expose abundant active sites,and are capable of fast and directional electron transfer.Two types of two-dimensional transition metal-based electrocatalysts are investigated in this research,including two-dimensional conjugated metal-organic frameworks and two-dimensional molybdenum sulfide-based electrocatalysts,and the results are summarized as follows.（1）MOF-derived M-OOH with rich oxygen defects by in situ electro-oxidation reconstitution for a highly efficient oxygen evolution reactionThe effective,simple,and green preparation of low-cost oxygen evolution reaction（OER）electrocatalysts remains a problem.Herein,a novel in situ electro-oxidation reconstitution strategy is first adopted to derive metal-oxyhydroxides with rich oxygen-defects（M-OOHv）on the surface of Fe₂Co-MOF with nickel foam（NF）as the substrate,named Fe₂Co-MOF@M-OOHv-ER/NF,which exhibits an extremely low overpotential of 224 m V at a current density of 10 m A·cm^-2 in 1 M KOH for the OER,with a small Tafel slope of44.87 m V·dec^-1 and a high electrochemically active surface area of 50.5 cm².The in situ Raman reveals the transition of the MOF phase to the M-OOHv phase,which is the real active site for the OER.Furthermore,X-ray photoelectron spectroscopy,photoluminescence,and density functional theory calculations prove that the bimetal synergistic effect and the oxygen defects in the M-OOHv,regulating the electron density of states,are the real reasons for the higher catalytic activity for the OER.In short,the newly prepared Fe₂Co-MOF@M-OOHv-ER/NF not only can be prepared by an efficient,simple,and green strategy but also has excellent oxygen evolution performance.（2）Lattice distortion of crystalline-amorphous nickel molybdenum sulfide nanosheets for high-efficiency overall water splitting:libraries of lone pairs of electrons and in situ surface reconstitutionLattice distortion is an important way to improve the electrocatalytic performance and stability of two-dimensional transition metal materials（2d-TMSs）.Herein,a lattice distortion nickel-molybdenum sulfide electrocatalyst on foam nickel（Ni Mo S₄-12/NF）has been synthesized through a novel,simple,and effective crystalline-amorphous strategy.The electrocatalyst only requires1.47 V to obtain 10 m A·cm^-2 for overall water splitting（OWS）and can function stably for 100 h at a current density of 100 m A·cm^-2,demonstrating excellent electrocatalytic performance and stability.From the results of the transmission electron microscopy（TEM）and electron paramagnetic resonance spectroscopy（EPR）,it can be seen that the（104）crystal lattice of Ni Mo S₄-12 undergoes interface strain under the crystalline-amorphous state,resulting in rich sulfur defects caused by lattice distortion,which could improve the intrinsic catalytic activity of Ni Mo S₄-12.According to the differential charge density analysis,around the sulfur defects,the Mo and Ni atoms with abundant lone pairs of electrons acted as libraries of lone pairs of electrons to enable an efficient hydrogen evolution reaction（HER）.From the total density of states（TDOS）and the Gibbs free energy of hydrogen adsorption(ΔG_H*),the libraries of lone pairs of electrons not only effectively optimized the distribution of the surface electron density of states at the Fermi level,but also reduced theΔG_H*,thereby improving the intrinsic HER electrocatalytic performance.The in situ Raman test results demonstrate that during the oxygen evolution reaction（OER）,the surface of the nickel molybdenum sulfide was reconstructed,and highly active Ni-OOH was generated.From the calculated free energy diagrams,the Ni-OOH could optimize the reaction barrier of the rate-determining step（RDS）for the OER to enhance the slow oxygen evolution reaction kinetics.This work will contribute to the rational design of a 2d-TMSs electrocatalyst,as well as an investigation of the catalytic mechanism.