Preparation Of Three-dimensional Nickel-based Phosphides And Their Catalytic Performance For The Electrooxidation Of Urea

Urea electrooxidation has broad and promising applications in direct urea fuel cells and urea electrolysis for hydrogen production.Direct urea fuel cell?DUFC?is a new type of fuel cell device can generate electricity by using industrial wastewater or human and animal urine as anodic fuel.Hydrogen production by urea electrolysis is achieved by applying electric energy to an aqueous solution containing urea,where urea oxidation reaction?UOR?occurs at the anode of the electrolytic cell,and H₂ is obtained at the cathode from hydrogen evolution reaction?HER?.Through this way,the dual purpose of purifying sewage and producing clean energy is thereby achieved.However,the problem of the high onset oxidation potential of UOR,the poor catalyst activity and the not clear reaction mechanism still limit their broader development.Therefore,this paper develops multifunctional catalysts suitable for high-activity DUFC anodes and high-efficiency urea electrolysis from the aspects of catalyst structure,modification and single-electrode reaction mechanism.The specific contents are:1.Porous Ni₂P nanosheet electrode supported on nickel foam is fabricated by a simple hydrothermal reaction with further low-temperature phosphating strategy.The synthesized Ni2P owns porous structure connected by a plurality of nanoparticles,the pore size of which is distributed between 30 and 90 nm.The rough and porous nanosheets increase the specific surface area of the Ni2P catalyst,thus exposing more active sites of electrooxidation reaction.And the open three-dimensional structure provides sufficient electrolyte transport channels and reaction sites to accelerate the interfacial reaction.Electrochemical results show that the porous Ni2P@Ni foam nanosheet electrode perform excellent catalytic property towards urea oxidation.In the 5 M KOH and 0.6 M CO?NH2?2 solution,the peak current density reaches750 mA cm^-2 at 0.6 V?vs.Ag/AgCl?,which is 300 mA cm^-2 higher than the Ni?OH?₂@Ni foam precursor.The reactant diffusion coefficient of urea electrooxidation process(D_urea)is6.23×10^-55 cm² s^-1,and the activation energy E_a is 11.93 kJ mol^-1.The urea oxidation behavior on Ni2P electrode surface is a typical E-C reaction mechanism.The onset oxidation potential of urea is the formation equel to the generation potential of NiOOH.At the higher potential,direct electrooxidation of urea may occur.2.Three-dimensional NiCoP nanowire electrode supported on nickel foam was obtained by adding cobalt salt during the hydrothermal process with further phosphating treatment.The synthesized NiCoP exhibits a synergistic effect of Ni-Co on the morphology features,that is,the overall structure maintains a nano-sheet array,while the details of the individual sheets are composed of lots of closely connected NiCoP nanowire cones.Besides,the phosphatized nanowires become more loose and rough.Each nanowire have a diameter of 40-60 nm and a length of about 500 nm.The electrochemical results show that the current density of the NiCoP@Ni foam electrode in the 5 M KOH and 0.6 M CO?NH₂?₂ solution is 594 mA cm^-2 at0.6 V(slightly lower than 654 mA cm^-2 of Ni₂P@Ni foam).While the onset oxidation potential of NiCoP is only 0.02 V?200 mV negatively compared to Ni₂P?.In the 1 M KOH solution,the Nyquist arc radius of the NiCoP electrode in the high frequency region is also significantly smaller than that of Ni₂P.The above results confirmed that Co has no catalytic activity towards urea oxidation,but participates in the form of driving Ni?OH?2/NiOOH conversion reaction.That is,the doping of Co can greatly reduces the formation potential of NiOOH,therefore the onset oxidation potential of urea is negatively shifted.