Preparation And Electrocatalytic Performance Of Transition Metals And Compounds Based On ZIF-67 And Its Derivatives

Currently,Pt/C and RuO₂/Ir O₂ are known as commercial hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）catalyst.However,the high price and low reserve of noble metal-based catalysts is not conducive to large-scale electrolysis of water to produce hydrogen.Therefore,combining precious metals with transition metal materials or researching transition metal compounds to replace precious metal catalysts for large-scale industrial hydrolysis have significant meaning.The zeolitic imidazolate framework（ZIF）has been widely used in the field of catalysis due to its large specific surface area and pore volume,adjustable pore structure,diversified structure and function,unsaturated metal active sites and other advantages.In this paper,using ZIF-67 as a basis material,which transformed into a series of transition metal and their compounds:borides,sulfides and phosphate with specific reactants by high temperature carbonization or hydrothermal reaction to reduce the reaction barrier of overall water splitting and improve catalytic efficiency.The details are as follows:Using the Co,N co-doped porous carbon（Co-NC）derived from ZIF-67 as the precursor,RuCo alloy embedded in the doped-carbon（RuCo@Co-NC）was prepared through precious metal’s substitution to catalyze HER reaction.Due to the large specific surface area and abundant pore structure of Co-NC exposed more active sites,and good electrical conductivity result in accelerating charge transfer and transport,the prepared RuCo@Co-NC catalyst had excellent HER catalytic performance in both acidic and alkaline media.In 1 M KOH solution,the overpotential of RuCo@Co-NC-800 catalyst was only 23 m V at 10 m A cm^-2,exceeding the 20 wt%Pt/C.In 0.5 M H₂SO₄ solution,it needed 110 m V overpotential to reach 10 m A cm^-2.In addition,RuCo alloy embedded in doped-carbon could effectively reduce the agglomeration of the active site and the corrosion of electrolyte,thereby enhancing the HER stability of RuCo@Co-NC-800.The powder samples need to use binder in the testing process and cannot withstand high current density,so the research and development of self-supporting transition metal catalysts is of great significance.A self-supporting bimetallic boride nanorods（Co₄B-Ni_xB/NF-800）were prepared by using boron carbide as boron source and reacting with ZIF-67 nanorods grown on nickel foam（NF）at 800℃.Co₄B-Ni_xB/NF-800 catalyst has a large specific surface area,which is conducive to exposing more marginal active sites,and the connection between nanorods and NF matrix facilitates to effective charge transfer in the electrolysis process and release of bubbles on the catalyst surface.The synergistic effect of bimetallic borides is beneficial to reduce the reaction barrier of electrochemical process and facilitate the OER reaction.The Co₄B-Ni_xB/NF-800 required the overpotential of 247 and 296 m V to achieve 10 and 50 m A cm^-2,respectively,which is comparable to Ir O₂/C.In addition,Co₄B-Ni_xB/NF-800 also exhibited outstanding electrochemical stability due to its unique core-shell structure.Using ZIF-67,NF,Na₂Mo O₄ and thiourea as raw materials,the Co-doped bimetallic sulfide nanorods（Co-Ni S@Mo S₂）were prepared during one-step solvothermal reaction for overall water splitting.Bifunctional catalyst can simplify the device and save cost.The unique core-shell structure of Co-Ni S@Mo S₂ catalyst can not only promote charge transfer and bubble diffusion,but also conduce to expose more active sites.Co doping can also cause lattice distortion,resulting in a large number of defects and unsaturated active sites.Co-Ni S@Mo S₂ catalyst had excellent HER and OER catalytic activities,requiring only 140 and 171 m V overpotential to reach 50 m A cm^-2,respectively.And in a two-electrode device of overall water splitting,an external voltage of 1.54 V to achieve the current density of 50 m A cm^-2.Besides,Co-Ni S@Mo S₂ catalyst had fine endurance due to its unique core-shell structure.Using Co-NC derived from ZIF-67 as Co source and NF as Ni source,a self-supporting nickel-cobalt bimetallic phosphate（NiCoPO@NC/P-NF-e）was prepared by phosphating,in which Ni_xCo_1-x（PO₃）₂ particles were embedded in doped-carbon and loaded on partially phosphating NF.Doped-carbon can not only improve the conductivity of the sample,accelerating the charge transfer and transfer,but also reduce the agglomeration of Ni_xCo_1-x（PO₃）₂ nanoparticle and the corrosion of electrolyte.NiCoPO@NC/P-NF-e catalyst exhibited excellent HER catalytic performance in both acidic and alkaline electrolytes.In 0.5 M H₂SO₄ solution,the catalyst needed 138 m V to reach 10 m A cm^-2.In 1 M KOH solution,the HER and OER overpotential of NiCoPO@NC/P-NF-e catalyst at 10 m A cm^-2 were 73 and 221 m V,respectively.The overall electrolysis required an external voltage of 1.50 V to achieve 10 m A cm^-2.What’s more,NiCoPO@NC/P-NF-e catalyst exhibited excellent stability of HER,OER and overall water splitting due to the active site embedded in doped-carbon.