Synthesis And Electrocatalytic Properties Of Nickel/Cobalt-based Compounds Nanostructures Modified With Transition Metal Heteroatoms

The production of hydrogen by electrocatalytic water splitting is a promising and competitive technology for the production of renewable energy.But until now,the main challenge for commercial applications of electrocatalytic water splitting toward mass production of hydrogen has been the sluggish 4 e^-reaction kinetics of the anodic oxygen evolution reaction?OER?,which requires a large overpotential,resulting in an overall low efficiency of electrolysis.There are two effective strategies to address this challenge:?1?Design high-activity and low-cost OER and HER bifunctional catalysts to accelerate OER dynamics,thereby reducing the energy requirements for hydrogen production through overall water splitting;?2?Replacing OER with other small molecules oxidation reactions those are much easier to occur?such as hydrazine,urea,methanol,dihydroisoquinoline,etc.?.Among them,urea oxidation reaction?UOR?is an ideal candidate for hydrogen production with low energy consumption due to its low price and extremely low thermodynamic electrolytic potential?0.37 V?as well as non-toxic oxidized products?CO₂ and N₂?.Based on these two strategies,this thesis designed and synthesized Ni/Co-based compounds nanostructures modified with transition metal heteroatom for effective hydrogen production,including Ag nanoparticles@Ni₃S₂ and Fe-doped Ni₃S₂ nanowires for hydrogen production through overall water splitting as well as Cu-doped Co?OH?₂ nanosheets-array material for urea-assisted hydrogen production.The relationship between composition,structure and electrocatalytic performance is deeply discussed.The main contents of this thesis are summarized as follows:1.Ag@Ni₃S₂ loaded on nickel foam was synthesized by solvothermal method and room-temperature reduction method.Loading Ag nanoparticles onto Ni₃S₂ nanosheets film can improve the conductivity and provide more conductive paths,resulting in faster charge transfer.The interaction between Ag and Ni₃S₂ optimizes the adsorption and desorption characteristics of oxygen/hydrogen-containing intermediates and lower the free energy of hydrogen adsorption,and thus improved electrocatalytic performance.The optimal 5.1%Ag@Ni₃S₂ merely demands a small overpotential of 237 mV to reach 30 mA cm^-2 toward OER.Moreover,it also showed a decent HER catalytic performance,which can drive 10 mA cm^-2 at an overpotential of 146 mV for HER.A water electrolyzer assembled with 5.1%Ag@Ni₃S₂ electrodes only requires a low cell voltage of 1.57 V to achieve 10 mA cm^-2.2.Fe-doped Ni₃S₂ nanowires with diameters of ca.17 nm and lengths of 1.4?2?m aresynthesized on Ni foam through a one-step solvothermal route for alkaline water splitting,which display notable active and excellent durability for both OER and HER under high current densities.The optimal Fe_13.7%-Ni₃S₂ nanowires electrode can attain 200 mA cm^-2 at a fairly low overpotential of 223 mV,and 500 mA cm^-2 at 245 mV toward OER.Furthermore,it yields a considerable low overpotential of 109 mV to garner 10 mA cm^-2,and 246 mV for500 mA cm^-2 toward HER.The incorporation of iron simultaneously modifies the electronic structure and morphology of Ni₃S₂,which not only enhances the conductivity but also generates abundant active edge sites.The slight surface-restricted oxidation of nanowires in a strongly basic electrolyte in situ generates a large number of interfaces,which enables the reactivity and durability for both OER and HER.Accordingly,an alkaline water electrolyzer with two Fe_13.7%-Ni₃S₂ electrodes only requires a low cell voltage of 1.53 V to achieve 10mA cm^-2,and 1.95 V to 500 mA cm^-2 with striking stability.In addition,this Fe_13.7%-Ni₃S₂couple can be consistently driven by a 1.5 V single-cell battery.3.Cu doped Co?OH?₂ nanosheets array on Ni foam was synthesized by a one-step liquid-phase approach for efficient production of hydrogen via urea-assisted.Cu²⁺integration significantly promotes the electron transport and increases the exposed active sites,along with enhanced hydrophilicity,greatly improves UOR and HER behaviors.The defects caused by Cu doping promoted the adsorption of the active site toward water and further enhanced the hydrophilicity and compatibility of the catalyst surface.As a result,the optimal Cu_6.2%-Co?OH?₂ exhibits excellent UOR activity,which 10 and 500 mA cm^-2 can be achieved at a considerable low potential of 1.31 and 1.418 V,respectively.Furthermore,it provides a considerable low overpotential of 76 mV to attained 10 mA cm^-2,and 234 mV for 500 mA cm^-2 toward HER.A urea-alkali electrolyzer with optimal Cu_6.2%-Co?OH?₂electrodes can provide 10 mA cm^-2 at a fairly low voltage of 1.389 V,and 500 mA cm^-2 at1.781 V with remarkable operational stability.The design of the catalyst shed light on a structural optimization strategy of Co-based materials by suitable element doping for high-performance urea-assisted hydrogen generation.