Fabrication Of High Efficient Metal And Metal Oxide Electrocatalysts And Their Applications In Nitrogen Fixation

Nitrogen is an indispensable molecule in the biosynthesis of the basic building-block molecules and plays an vatal role in life on earth.Recently,the electrocatalytic nitrogen fixation technology to synthesize ammonia and nitrate(mainly focus on electrochemical nitrogen reduction reaction(NRR))which owns the advantages of green,economic friendly,lower energy consumption is regarded as a hopeful way to substitute the traditional Haber-Bosch process and Ostwald process with high energy consumption and high input.Regrettably,the biggest challenge for electrochemical nitrogen fixation is still the limited target production rate and selectivity.Therefore,it’s highly needed to explore high-efficient electrocatalysts and rational design of effective electrolysis system.Laser technology in liquid including laser ablation in liquid(LAL)and laser irradiation in liquid(LIL)technology is an emerging method to fabricate nanomaterials and nanostructures based on the interaction between laser and materials in liquid.Compared with traditional methods preparing nanomaterials,laser technology in liquid which is featured with high temperature,high pressure and quick cooling can transform the inert or low active bulk target and powder materials into highly active and reactive nanomaterials,and its unique photothermal effect can achieve effective construction and regulation of the surface defects,components,and structures of metal and metal oxide nanomaterials,and therefore directly or indirectly improving the electrocatalytic activity of electrocatalyst.Based on these,our work aim to using LAL and LIL technology to fabricate small-sized metal and metal oxide nanomaterials,regulating its surface defects,composition and size to enhance the intrinsic activity and increase the apparent activity.Furthermore,fluidized electrocatalysis system and electrocatalysis device are also designed and modified to take full advantages of the catalytic active sites of electrocatalyst and accelerate the electrochemical reaction kinetics,therefore achieving high-efficient electrochemical nitrogen fixation.The main research contents and results are summarized as follows:1.Nano-sized Au catalysts have already present great potential in electrochemical NRR,and their catalytic activity can be further improved by surface defects construction and structure regulation.In this work,core-shell structured Au@C composite were fabricated by laser ablation of Au target in acetone solvent,exhibiting obviously improved NRR activity.The experimental results demonstrate that ultrathin graphitic carbon shell layers can not only interact with Au nanoparticles to form Au-O-C bond,providing additional catalytic active sites for the NRR,but also can effectively prevent Au nanoparticles from aggregation during the NRR.Compared to the pure Au nanoparticles(Au-NS),the NRR activity of Au@C electrocatalyst was obviously increased,affording the largest NH3 yield rate(RNH3)of 241.9 μg h-1 mgcat.-1 and highest faradaic efficiency(FE)of 40.1%under optimal applied potential of-0.45 V(vs.RHE).In addition,the DFT calculation confirms the role of carbon vacancies in Au@C for the electrocatalytic NRR.2.The electrochemical NRR performance is highly depended on the development of high-efficiency NRR electrocatalysts and catalysis system.Ultrasmall-sized silver nanodots(AgNDs)were synthesized by laser ablation of Ag target in deionized water.Individual Ag nanodot can provide abundant catalytic active sites(e.g.,facet,edge,corner)during the electrocatalytic NRR.To utilize the catalytic active sites more effectively,a fluidized electrocatalysis system using Ti mesh as the current collector and aqueous AgNDs as the catalyst was constructed.The electrochemical experiment results indicate that utilizing the developed fluidized electrocatalysis system,a large NH3 yield rate of 600.4±23.0 μg h-1 mgAg-1 with a faradaic efficiency(FE)of 10.1±0.7%can be achieved at-0.25 V(vs.RHE),which was 7.5 times higher than that of using the conventional catalyst-loading electrocatalysis system.In order to further enhance the FE of the fluidized electrocatalysis system,a TiO2 layer with riched oxygen vacancies was modified on metallic Ti mesh(Ov-TiO2/Ti).The surface Ov-TiO2 layer can not only effectively reduce the cathodic current of metallic Ti mesh,but also can serve as the electrocatalyst providing catalytic active sites for the enhanced NRR performance.The experimental results demonstrate that using Ov-TiO2/Ti as current collector,the FE of the fluidized electrocatalysis system is 2 times higher than that of using Ti mesh.Moreover,a“S-type" Ti plate-based two-electrode configured flow-type electrochemical reactor was developed to relize the two-electrode flowing NRR reaction.3.In comparsion with noble metal,non-noble metal oxide catalysts own the merits of low cost,easy availability,highly tunable composition.Surface defect construction and size regulation can improve their intrinsic activity and apparent activity.Ultrafine Co3O4 nanoparticles with abundant oxygen vacancies(Ov)were prepared by laser ablation of cobalt target in deionized water.The extremely rapid quenching process after high-energy pulsed laser beaming leads to insufficient oxidation of cobalt and restricts a continuous growing of the oxidation product,bringing about rich oxygen vacancies in the formed ultrafine Co3O4 NPs.The surface Ov can work as the N2 adsorption and activation catalytic active sites,while the small size can bring about more exposed active site numbers.Thanks to the highly dispersed property of L-Co3O4-4.0,their NRR activity was evaluated employing the developed fluidized electrocatalysis system.The experimental results indicate that using the fluidized electrocatalysis system,ultrafine Co3O4 NPs with rich oxygen vacancies can afford an NH3 yield rate of 235.0 μg h-1 mgcat.-1 and a faradaic efficiency(FE)of 16.3%under optimal applied potential of-0.30 V(vs.RHE).Based on this,a series of small-sized transition metal oxides(ZnO,MoO3,TiO2,NiO)with rich oxygen vacancies fabricated by LAL were extended for efficiently electrocatalytic NRR in the fluidized electrocatalysis system,confirming the generic applicability of the fluidized electrocatalysis system for small-sized nanoparticle electrocatalysts.4.The developed fluidized electrocatalysis system can overcome the disadvantages of the conventional catalyst-loading electrocatalysis system,but the kinetic reason behind is still unknown.Ultrasmall-sized V2O5 nanodots with abundant oxygen vacancies(Ov)were fabricated by laser irradiation of commercial V2O5 powder in deionized water for efficiently electrocatalytic NRR and NOR in the fluidized electrocatalysis system.The electrochemical experiment results indicate that utilizing the developed fluidized electrocatalysis system,a large NH3 yield rate of 575.3 μg h-1 mgcat.-1 with a faradaic efficiency(FE)of 28.6%can be achieved at-0.20 V(vs.RHE),while a large NO3-yield rate of 1388.0 μg h-1 mgcat.-1 with a FE of 7.8%can be achieved at 2.4 V(vs.RHE).The small size of V2O5 nanodots can provide abundant catalytic active sites(e.g.,facet,edge,corner)for the electrocatalytic nitrogen fixation,while the surface Ov are conducive to promote N2 adsorption and activation.More importantly,Monte Carlo simulation and finite difference method were employed to simulate the kinetic models of the fluidized electrocatalysis system and the conventional catalyst-loading electrocatalysis system,respectively.The simulation results confirm that utilizing the fluidized electrocatalysis system can effectively accelerate the electrochemical reaction kinetics and greatly improve the production rate,providing theoretical support for optimizing experimental conditions and understanding the kinetic reasons behind the experimental results.