Synthesis Of Cobalt-based Metal-organic Frameworks (MOFs) For Electrochemical Catalysis

Hydrogen,as a high efficiency and zero pollution sustainable energy,has continuously stimulated broad interest in energy conversion and storage systems.Electrochemical water splitting is considered to be an effective and simple method to obtain H₂.However,the efficiency of water splitting is severely hampered by a sluggish half process,that is,the oxygen evolution reaction?OER?[4OH-?2H₂O + O2 + 4e-].As a four-electron transfer process associated with O-H bond breaking and O-O bond formation,OER requires a large overpotential and thus results in inherent sluggish kinetics.Generally,Ir O2 and Ru O2 are the representative and effective OER catalysts.However,their practical applications are widely impeded by the high cost and low abundance in the earth.Undoubtedly,it is an urgent and deserved significance to design and develop high activity alternatives based on nonprecious elements.Very recently,some metal-organic frameworks?MOFs?electrocatalysts with good chemical and physical stability,large specific surface area,high C content of coordinate organic ligands and controllable pore texture have been reported in many fields.Given that the overpotentials of many electrocatalysts towards OER are still large?ca.300-500 m V?,the MOFs-based or MOFs-derived electrocatalysts for OER have been prepared in this article in order to improve the overpotential and Tafel slope and analyze the mechanism during OER.The main details are as follows:1.Efficient charge transport is a key to the successful design of electrocatalyst.In this work,we report the in-situ growth of ultrathin MOF material?ZIF-67?nanosheets on conductive Ti@TiO₂/Cd S substrate for high-efficient electrochemical catalysis due to optimized charge transport.The ultrathin ZIF-67 nanosheets were grown on Ti@TiO₂/Cd S nanowire array substrates resulting unique 3D hierarchical structures,which were investigated as electrocatalyst for the oxygen-evolution reaction?OER?.The nanowire-supported ZIF-67 nanosheet electrode shows remarkable electrocatalytic activity and excellent stability toward OER,which can be explained by the rapid electron transport along the 1D Ti@TiO₂/Cd S nanowire to Ti substrates,large electrochemical active surface area and the rising valence state of Co ions induced by electronegative N atom in imidazole ligands.Compared with other MOFs catalysts,we obtained a very small Tafel slope?42 m V/dec?and a small overpotential?0.41 V?at 10 m A cm-2 for OER.2.The electrochemical splitting of water,as an efficient and large-scale method to produce H₂,is still hindered by the sluggish kinetics of the oxygen evolution reaction?OER?at the anode.Considering the synergetic effect of the different metal sites with coordination on the surface of electrocatalysts,the hybrids of Co/Fe phosphides?denoted as Co-Fe-P?is prepared by one-step phosphorization of Co Fe-MOFs for the first time as high-efficient electrocatalysts for OER.Benefiting from the synergistic effect of Co and Fe,the high valence of Co ions induced by strongly electronegative P and N and the large electrochemical active surface area?ECSA?originated from exposed nanowires on the surface of Co/Fe phosphides,the resultant Co-Fe-P-1.7 exhibits remarkable electrocatalytic performances for OER in 1.0 M KOH,affording an overpotential as low as 244 m V at a current density of 10 m A/cm²,a small Tafel slope of 58 m V/dec and good stability,which is superior to that of the state-of-the-art OER electrocatalysts.In addition,the two-electrode cell with Co-Fe-P-1.7 modified Ni Foam as anode and cathode in an alkaline electrolyte,respectively,exhibits the decomposition potential of ca.1.60 V at the current density of 10 m A/cm² and excellent stability.3.Electrochemical splitting of water for producing hydrogen?H₂?is still impeded by the large energy barriers of the oxygen evolution reaction?OER?at the anode.Therefore,designing of rational and effective electrocatalysts for OER is vital to sustainable energy technologies.In order to achieve this goal,we synthesized a bimetal-organic frameworks-74?denoted as Co Fe-MOF-74?through a facile hydrothermal method.Based on the excellent deprotonation of HO*,effective active sites of O atoms,partial-charge-transfer-activation?PCTA?effect on Co ions originated from Fe dopant and the large electrochemical active surface area?ECSA?,the resultant Co Fe-MOF-74 exhibits remarkable electrocatalytic performances towards OER in 1.0 M KOH,which all prove that Co Fe-MOF-74 is a promising electrocatalysts for OER.