Fabrication Of Co-based Metal-organic Frameworks For Electrocatalytic Oxygen Evolution

Environmental pollution caused by the consumption of fossil fuels is major problems in a worldwide scale.As a clean and pollution-free renewable energy,hydrogen energy is considered to be an ideal alternative to fossil energy.The production of hydrogen via electrochemical water splitting realizes the transformation from electric energy to chemical energy,which arouses scientist's research interest.Oxygen evolution reaction?OER?regarded as the bottleneck of the overall water splitting,is kinetically sluggish due to four electrons and four protons transfer.Therefore,it is urgent to develop efficient catalysts towards OER.Metal-organic frameworks?MOFs?as a class of crystalline porous materials,are self-assembled by metal ions/clusters and organic ligands through coordination bonds.They are widely applied to gas storage,drug delivery,sensing,catalysis and so on due to the large specific surface area,adjustable pore size and diversity in metal centers and organic ligands.In OER reaction,MOFs-derived metal oxide/carbon composites fabricated under high temperature pyrolysis have been proved to be effective.However,the reduction of active sites and structural collapse are unavoidable.In this regards,direct use of MOFs as OER electrocatalysts without extra pyrolysis treatment is very attractive.Based on the above background,cobalt/iron bimetal-organic frameworks?Co_xFe-MOFs?with different metal ratios were synthesized by simple ultrasonic method with Co²⁺and Fe³⁺as metal sources and 1,3,5-benzenetricarboxylic acid as organic ligand.By optimizing the ratio of Co²⁺and Fe³⁺,Co₂Fe-MOF exhibits the optimized electrocatalytic OER performance.In 1M KOH solution,Co₂Fe-MOF requires an overpotential of 280 mV to reach a current density of 10 mA cm^-2 with a Tafel slope as low as 44.7 mV dec^-1 as well as long-term stability for12000 s.Besides,the highest turnover frequency?TOF?of 0.032 s^-1 was obtained at the overpotential of 280 mV.In addition,cobalt-based electrodes with self-supporting hierarchical structure?denoted as Co-MOF-74/CCH/CF?were synthesized by simple hydrothermal method with copper foam as the substrate and cobalt carbonate hydroxide as a template.The resulting electrode was directly used as the working electrode for electrocatalytic OER.In 1 M KOH solution,Co-MOF-74/CCH/CF produced a current density of 100 mA cm^-2 at a low overpotential of 292 mV with a small Tafel slope of 40.7 mV dec^-1.Moreover,it showed stability for at least 20 h in the constant current electrolysis experiment at a current density of10 mA cm^-2,and no significant morphology change was observed for the catalyst before and after long-term electrolysis.