Preparation Of Nitrogen-Doped Nickel-Based Catalysts And Their Performance For Urea Electrolysis

Urea electrolysis（UE）is an environmentally friendly sustainable energy technology that can produce hydrogen from urea in wastewater.Additionally,urea itself can be used as a fuel in a direct urea fuel cell（DUFC）,converting chemical energy into electrical energy.However,both techniques are hindered by the slow kinetics of the urea oxidation reaction（UOR）on the anode.Therefore,it is necessary to develop high-efficiency UOR electrocatalysts.A large number of electrocatalysts have problems such as low oxidation current density,large overpotential,poor stability and conductivity,etc.Improving catalytic performance by reducing the reaction barrier and increasing the reaction rate is the key to designing nickel-based materials.In this paper,two Ni-based catalysts were prepared by nitrogen doping modification strategy.The introduction of N atoms adjusted the active center and promoted the interaction between the catalyst and urea molecules,aiming to develop efficient catalysts for accelerating the reaction kinetics,reducing the overpotential and energy consumption.Bifunctional catalysts suitable for efficient urea oxidation electrolysis and hydrogen evolution reaction were developed.（1）A series of nitrogen doped nickel nanoparticles（Ni-NCNT）supported on carbon nanotubes were prepared by calcination at 800℃after mixing nickel nitrate and dicyandiamide by a simple catalytic pyrolysis method.The oxidation mechanism of Ni-based catalysts and the effects of the introduction and adjustment of N atoms on the catalytic performance of Ni-NCNTs for UOR were investigated.Through electrochemical tests and in situ Raman spectroscopic characterizations,the active source of the catalysts was Ni^2+δO（OH）_ads,which were in situ generated active intermediate oxides,and the reaction kinetics were much faster than those on the Ni³⁺active sites.Nitrogen atoms infiltrated into the Ni atomic lattice during vapor deposition,which significantly modulated the electronic structure of metallic Ni and accelerated the pre-formation of Ni^2+δO（OH）_ads.At the same time,the introduction of nitrogen made the D-band center of nickel move up,which enhanced the adsorption of urea and corresponding intermediates,and improved the catalytic activity of UOR.Therefore,the Ni-NCNT electrocatalyst had good UOR and HER electrocatalytic performance and the ability of urea-assisted total water splitting.（2）The foamed nickel-supported porous Ni₂P nanosheet electrodes were obtained by hydrothermal reaction and further low-temperature phosphating treatment,and N-Ni₂P catalyst could be obtained by feeding NH₃ during the phosphating process,the amount of nitrogen incorporated into the nanosheets could be adjusted by controlling the ratio of NH₃ to Ar atmosphere.The nickel phosphide-based catalyst had a rough and porous surface and exhibited super-hydrophilicity,which facilitated the direct contact between reactants and active species and the release of gaseous products.Nitrogen modulated the surface charge distribution of nickel phosphide,activated the adsorbed urea molecules,and reduced the UOR reaction overpotential.The Tafel slope after LSV calculation showed that the introduction of N accelerated the UOR reaction kinetics.At the same time,the electrode had excellent electrochemical stability,and the N-doped N-Ni₂P catalyst material could drive the oxidation current up to 1 A,with a lower onset potential and higher activity than conventional nickel-based catalysts.It was a very promising UOR electrocatalyst.