Design,Synthesis And Electrocatalytic Water Splitting Performance Of Co-MOF-based Catalysts

With the increasing demand for energy in modern society,hydrogen is considered to be one of the most promising energy carriers for future fuel applications.Electrochemical water splitting is an efficient and clean hydrogen production technology that produces high-purity hydrogen.Water splitting consists of two half reactions:the hydrogen evolution reaction(HER)on the cathode and the oxygen evolution reaction(OER)on the anode.However,the actual reaction requires a large overpotential,especially the oxygen evolution reaction involves a complex multi-electron transfer steps,and the required voltage is much higher than the minimum theoretical value of 1.23 V.The electrocatalyst with excellent performance can reduce the energy cost required to achieve water splitting by reducing the overpotential.Therefore,it is of great significance to develop efficient and inexpensive HER and OER electrocatalysts.Transition metals have unique and tunable electronic structural characteristics,while metal-organic framework(MOF)materials have these unique advantages such as high specific surface area,porous crystalline structure,and adjustable chemical composition.Therefore,cobalt-based MOF are selected as the research object in this dissertation,and several synthetic strategies are designed to construct highly active and stable electrocatalyst,by exposing more active sites,improving electron transport capabilities,and establishing stable active structures.The details are shown as follows:Co-BPDC/Co-BDC two-dimensional(2D)heterojunction nanostructures with improved electrocatalytic activity were successfully constructed via a mild two-step solution route,employing Co2+ions as the center atoms,1,4-benzenedicarboxylate(BDC)and 4,4’-biphenyldicarboxylate(BPDC)as ligands.The BET area and ECSA of the 2D MOF-on-MOF material were larger than those of single Co-BPDC and Co-BDC,thus more active sites could be exposed.It was found that after the formation of the heterojunction,the thermal stability of the materials increased,and the electrocatalytic stability in the OER test also increased.The study found that these Co-MOFs were transformed into CoOOH after the CV activation before the OER test,and their morphology also changed.Co-BPDC and Co-BDC converted to CoOOH at different rates,which resulted in the formation of many oxygen vacancies in the Co-BPDC/Co-BDC heterojunction during CV activation.Since oxygen vacancies could significantly enhance the catalytic activity of the materials,the as-prepared Co-BPDC/Co-BDC heterojunction electrocatalyst showed significantly higher catalytic activity than the single Co-BPDC and Co-BDC,and was also higher than that of commercial IrO2.In the self-supporting electrode,no binder is needed.Thus,the electron transporting ability of the electrode can be improved.Carbon cloth(CC)is an excellent flexible conductive substrate material and can be used as a substrate for self-supporting catalysts.Cobalt nanosheet arrays were first grown on the CC by electrodeposition,and then the CC-supporting cobalt nanosheets(Co/CC)reacted with the ligand 4,4’-biphthalic acid in the presence of promper amounts of HCl.Co-BPDC nanoarrays were grown in situ on Co/CC to form a MOF-metal-conductive substrate three-layer heterojunction structure.The electrodeposited metal cobalt not only provided a metal source of the complex,but also effectively improved the conductivity of the electrode.The test results in alkaline solution indicated that the self-supporting Co-BPDC/Co/CC hybrid electrode by in-situ growth displayed not only excellent electrocatalytic performance for OER and catalytic stability,but also a HER catalytic performance.The introduction of metal heteroatoms can effectively improve the electrocatalytic activity.Co nanosheet arrays were first deposited on flexible CC substrate,and then Co/CC was reacted with 2,5-dihydroxyterephthalic acid in the presence of certain amounts of FeCl3 by the solvothermal technology,and well-aligned Co,Fe-MOF-74 nanospindles were in-situ grown on Co nanosheet arrays.Electrochemical performance studies showed that the synthesized Co,Fe-MOF-74/Co/CC three-layer hybrid self-supporting electrode performed very well on electrocatalytic OER,HER,and overall water splitting,it is a kind of efficiently bifunctional electrocatalyst for water splitting.The excellent catalytic activity should be attributed to the synergistic effect of Co and Fe,the excellent conductivity of the hybird electrode,and the superhydrophilic surface of the material.In addition,the veritable active species for OER and HER were elucidated.A self-sacrificing template strategy was designed for synthesis of α-Co(OH)2 hollow nano-dodecahedrons,employing ZIF-67 dodecahedrons and hexamethylenetetramine(HMT)as the reactants under a mild solvothermal pathway at 40℃.When NaCl was added to the system,Cl-doped α-Co(OH)2 hollow nano-dodecahedrons was successfully obtained.The hollow micro/nanostructures could reduce internal resistance,improve power capability due to large surface areas and short diffusion paths for ions and electrons,and prevent catalyst agglomeration.Experiments showed that the doped Cl would be etched away due to the oxidation after the CV activation,resulting in catalyst structure defects,so the electrocatalytic activity of the materials could be effectively improved.The Cl-doped α-Co(OH)2 had an overpotential of only 298 mV@10 mA cm-2 for OER,and long-term catalytic stability.The as-obtained Cl-doped α-Co(OH)2 hollow nano-dodecahedrons exhibited better OER activities than α-Co(OH)2 ones.