Preparation And Performance Of Nickel-based Electrocatalysts For Urea Electrooxidation

Hydrogen energy is considered as the most promising energy carrier.Electrocatalytic water decomposition is an ideal way to produce hydrogen.However,the slow kinetics of the anodic oxygen evolution(OER)seriously restricts the hydrogen yield and energy conversion efficiency.Other anode reactions with low thermodynamic potential are expected to replace OER.Among them,urea oxidation reaction(UOR)has a lower thermodynamic potential of 0.37 V(vs.RHE),and its products are only CO2 and N2.Therefore,UOR becomes one of the ideal reactions for OER.However,the slow 6e-transfer process of UOR greatly limits its development in the field of electrolytic hydrogen production.Although some precious metals have excellent catalytic activity,their scarcity and high price severely limit their application in the field of electrocatalytic hydrogen production.Therefore,it is necessary to explore an efficient and abundant non-noble metal catalyst to improve the hydrogen production efficiency of urea electrooxidation.As a kind of high efficiency,low cost and abundant surface non-precious metal catalyst,nickel-based catalyst has received great attention.However,nickel-based catalysts still have some problems,such as insufficient exposure of active sites and poor stability.Therefore,in this paper,the exposure of active sites and the improvement of stability of nickel-based catalysts were achieved through the strategies of morphology regulation and electronic structure regulation,and two kinds of efficient urea electrooxidation catalysts were prepared and their performance was evaluated.Specific research contents are as follows:(1)Ni(OH)2 porous nanowire arrays doped with manganese(Mn-Ni(OH)2 PNAs)were prepared on the surface of nickel foam by hydrothermal method and electrochemical transformation.It was found that the precursor NiMn-BTC was completely converted to MnNi(OH)2 by electrochemical conversion method,in which the non-conductive organic ligands in the precursor MOF were destroyed,while the metal framework was intact.At the same time,the theoretical calculation shows that the doped Mn can adjust the electronic structure of Ni(OH)2,so as to significantly improve the electron density,optimize the reaction energy barrier of CO*/NH2*intermediates in UOR reaction,and effectively improve the intrinsic activity of the catalyst.Thus,Mn-Ni(OH)2 PNAs exhibit superior UOR electrochemical performance,requiring a potential of only 1.37 V(vs.RHE)at a current density of 50 mA/cm2,a Tafel slope of only 31 mV/dec,and excellent stability.In addition,the catalyst was coupled with a commercial Pt/C electrode to form a urea electrolytic system,showing excellent activity(requiring only 1.40 V battery voltage at a current density of 20 mA/cm2)and excellent durability(only 1.40%attenuation after 48 hours).(2)A novel hollow nanorod array composed of NiCoP nanosheets(NiCoP NSs@HNRAs)has been prepared by environmentally friendly and simple template-assisted electrodeposition and subsequent phosphating process.Hollow nanorods composed of nanosheets have a unique structural effect,which increases the surface area of the catalyst,exposes more electrochemical active sites,and improve the efficiency of electrolyte transfer,thus improving the catalytic activity of the catalyst.In addition,by introducing non-precious metal cobalt to form nickel-cobalt bimetallic phosphide,the synergistic effect between nickel-cobalt bimetals is generated,which promotes the reconstruction of the surface electronic structure of the catalyst,and significantly improves the electrocatalytic performance of nickel-cobalt bimetallic phosphide.Thus,NiCoP NSs@HNRAs exhibit superior UOR electrochemical performance,requiring a potential of only 1.35 V(vs.RHE)to achieve a current density of 50 mA/cm2 and has the smallest Tafel slope of 47 mV/dec.In addition,a urea-assisted electrolytic hydrogen production system was constructed with target catalyst NiCoP NSs@HNRAs as anode and commercial Pt/C catalyst as cathode,which showed high catalytic activity(1.43 V)and good stability at current density of 50 mA/cm2.