Study On Preparation And Electrocatalytic Properties Of Nickel-oxynitride Composite

Electrochemical water splitting is widely considered as a promising and attractive strategy to produce clean and renewable energy.Meanwhile,one of the feasible approaches for electrochemical application is to directly reduce nitrogen to ammonia by using electrocatalytic technology to simulate biological nitrogen fixation.Therefore,electrocatalytic technology has great potential in energy conversion.Transition metal oxides(TMOs)are widely used in the field of electrocatalysis because of their varied oxidation states,low cost and easy synthesis.At present,the problems of TMOs-catalyzed reactions are the slow reaction kinetics and low efficiency caused by electron conduction and ion adsorption transfer.In view of the above problems,in this thesis,a series of transition metal oxynitride composites were synthesized and improved the activity.The active sites of nickel-vanadium bimetallic oxide materials increased and electron transmission rate improved by introducing photoelectron and controlling core-shell morphology.The noble metal-like(platinum)hydrogen production activity was realized by photoelectric synergistic effect.The stable electrocatalytic overall water splitting on nickel-based nitrides was realized under the low voltage by reactive facets tuning and heterojunctions constructing.The adsorption competition of nitrogen with H*and H2O was effectively disposed by introducing oxynitride layer structure.The electrocatalytic nitrogen reduction to ammonia was further accomplished.Furthermore,the influence of material construction on electrocatalytic performance was systematically studied by means of the first principles theoretical calculation.The main works are as follows:1.Nickel-vanadium bimetallic oxides with seaurchin core-shell structure were prepared by solvent-thermal method.This structure had a large number of active sites,which could speed up the electron transport rate,thus making the material exhibit excellent photoenhanced electrocatalytic hydrogen evolution activity.The seaurchin structure could effectively extract photogenetated carriers and accelerate the carrier separation and transport rate in the reaction process.The elastic core-shell structure effectively alleviated the problems of oxidation corrosion and expansion of metal oxides and improved the stability of electrocatalytic hydrogen evolution.Under visible light irradiation,the as-synthesized material demonstrated improved HER activity,featured by a small overpotential of-90 mV at 10 mA·cm2,a small Tafel slope of 50 mV dec-1 and good stability.Drawing on the opto-electric synergy,photoelectrons were introduced in this system to effeiciently promote the electrocatalytic hydrogen generation.2.Nickel-molybdenum bimetallic nitride nanowire with the preferentially exposed(100)facet was designed and prepared by hydrothermal reaction and in situ N/O exchange of ammonia annealing.Phase characterization of the structure showed that the structure exposed more dominant reactive facets,and good electrocatalytic activity of water was realized.Meanwhile,the electrocatalytic mechanism was further investigated by density functional theory calculation.The one-dimensional nanowires anchored on nickel foam effectively improved electron conductivity and exposed much more dominant reactive facets.The facet-tuned NiMoN nanowire exhibited noble-metal like electrocatalytic HER/OER activity and excellent durable activity for overall water splitting,with a cell potential as low as 1.498 V at 20 mA·cm-2.DFT calculations further revealed that the catalytic activity of NiMoN(100)reactive facet was significantly greater than that on the(001)or(101)facets,owing to the low adsorption free energy of H*(ΔGH*)on the(100)facet.The results showed that the reactivity facet tuning could improve the adsorption performance of the reactivity group,thus promoting the electrocatalytic activity and stability of nickel-based nitrides.3.A series of nickel-based nitride heterostructures(MxN@NiMoN,M:Ni、Co、Cu)were designed and prepared by electroplate auxiliary and in situ N/O exchange.Physicochemical characterization and electrochemical analysis proved that this kind of structure could effectively enrich the active sites and improve the hydrogen/oxygen evolution activity.Boosted electrocatalytic overall water splitting was also achievedwith only a low cell voltage of 1.481 V to drive 10 mA·cm-2 and excellent durability of 48 h.The results showed that the design strategy of metal nitride heterostructure can be applied to other electrocatalytic materials.4.Nickel-vanadium oxynitride layer was designed and synthesised on corresponding oxide by hydrothermal reaction and in situ N/O exchange of ammonia annealing Physicochemical characterization revealed that nickel-vanadium bimetallic oxide is the core and corresponding oxynitride is the shell,and the structural advantages for nitrogen fixation were further analyzed by density functional theory.This core-shell structure effectly enhanced the electron conductivity,and the delocalized electron environment of oxynitride enhanced π backdonation,which is conducive to nitrogen absorption and activation.The elastic core-shell structure effectively alleviated the problems of metal oxides expansion and improved the stability of electrocatalytic nitrogen fixation.Experimentally,both ammonia production rate(～6.78 μg·h-1·cm2 cat.)and Faradic efficiency(～5.57%)were enhanced by 2-fold relative to that of its corresponding oxide under neutral condition.Results demonstrated that nickel-vanadium oxynitride layer has attractive applications in the fields of energy conversion or storage.