Synthesis And Studies Of Nickel/Cobalt-Based Oxides And Chalcogenides For Electrocatalytic Water Splitting

The increasing consumption of non-renewable fossil fuels has lead to serious environmental problems and energy crisis.As a kind of renewable energy with a wide range of sources,high energy density and clean combustion products,hydrogen can be an effective supplement and substitute for fossil fuels.Hydrogen production from water electrolysis has attracted much attention as a clean and efficient industrial hydrogen production technology.Due to the large overpotential of the electrodes,the driving voltage of electrocatalytic hydrogen production is much higher than the theoretical value,resulting in high energy loss.Therefore,developing a simple method to synthesize cheap,highly active and stable electrocatalysts is the key to reducing the whole cost of water electrolysis.In this dissertation,I synthesized and studied the electrocatalytic performance of low-cost nickel/cobalt-based chalcogen compounds,I mainly focused on how to design,control and optimize the catalysts during different synthesis processes,and realized overall water splitting catalyzed by bifunctional catalysts at a low voltage.Aiming at the problem of low catalytic activity and low active sites for HER catalysts,the nickel disulfide nanosheet arrays with large surface area were prepared by a soft template method combining with chemical vapor conversion process.The pyrite-type nickel disulfide catalysts have high intrinsic catalytic activity,and its special nanosheet array structure can provide more active sites for electrocatalytic reactions,thereby increasing the hydrogen evolution reaction rate.The as-synthesized nickel disulfide nanosheet arrays have a good pH adaptability,they exhibit good electrocatalytic activity toward hydrogen evolution in both acidic and alkaline electrolytes with excellent durability,need the overpotentials of-240 mV and-190 mV to drive 10 mA/cm2,respectviely.In order to further improve the conductivity and intrinsic catalytic activity of HER electrocatalysts,I chose metal organic frameworks(MOFs)as both precursor and template.During the conversion,the nitrogen-doped carbon matrix formed through the carbonization of organic ligands,which provided a highly conductive support and additional catalytically active sites for the catalyst.Meanwhile,the modified transition metal ions were introduced into the CoSe2 crystals,activating the catalytic active site,so that the catalyst crystal plane had a better Gibbs free energy for hydrogen evolution.The obtained Fe-CoSe2@NC nanoparticles have excellent catalytic activity for hydrogen evolution,only need the overpotential of-143 mV to drive 10 mA/cm2.The Tafel slope is 40.9 mV/dec,which is quite close to the precious platinum and expected to become a substitute for precious metal electrodes.By adjusting and designing the chemical composition and crystal structure of the catalyst,I prepared hexagonal Ni0.85Se and inverse spinel NiCo2O4 with bifunctional electrocatalytic activity.In the alkaline electrolyte,the overpotentials of Nio.ssSe/GS electrode at 10 mA/cm2 were-200 mV and 302 mV for HER and OER,respectively,and for NiCo2O4 were-198 mV and 280 mV,respectively,which significantly reduce the drive voltage of single electrode material in water electrolysis system.The results show that both catalysts have excellent electrical conductivity and redox ability,and can be used as highly-efficient bifunctional electrodes in overall water splitting system for the replacement of precious metal electrodes.