Modification Of Cobalt-based Nanomaterials And Their Electrochemical Ammonia Synthesis

The rapid development of electronic information technology is closely related to the global energy strategy.The technological innovation in high-energy-consuming industries and the development of emerging green energy can effectively alleviate the global energy crisis.The current industrial ammonia synthesis depends on the energy-intensive Haber-Bosch process,resulting in a large amount of energy waste.The electrocatalytic ammonia synthesis with mild reaction conditions is limited by the fact that N₂ is stable and difficult to activate.How to improve the efficiency of electrochemical ammonia synthesis has become a current problem.Therefore,this research work takes the efficient electrochemical ammonia synthesis as the basic orientation,takes the modification of catalysts and the use of new nitrogen sources as the basic means,and combines theoretical calculations and advanced nanotechnology to achieve efficient ammonia synthesis.The main research contents are as follows:(1)To solve the problems of easy agglomeration and insufficient activity of CoS₂nanoparticles,nitrogen-doped carbon nanobox with uniformly loaded CoS₂ nanoparticles(CoS₂@NC)is prepared to efficiently electrocatalyze N₂ reduction reaction(NRR)to synthesize ammonia.The in-situ recombination with carbon materials not only effectively increases the conductivity of the overall catalyst and accelerates electron transfer,but also facilitates the uniform distribution of CoS₂ nanoparticles and prevents agglomeration and deactivation.The resultant CoS₂@NC nanobox effectively increases the density of active sites,which is beneficial to material conversion and further improves the ammonia synthesis activity of NRR.Hence,it achieves a high NH₃ yield rate with 17.45?g mg^-1_cat.h^-1 and Faradaic efficiency with 4.6%at-0.15 V vs.reversible hydrogen electrode(RHE)in 0.1 M HCl electrolyte.In addition,the CoS₂@NC nanobox exhibits good chemical stability and wear resistance.(2)Owing to N₂ being stable and difficult to activate,NO₃^-with relatively weak activation energy is selected as the source of nitrogen.Fe element doping contributes to optimizing the electronic structure of spinel Co₃O₄ and adjusting the adsorption energy of reactants with catalyst surface to achieve a high-efficiency NO₃^-reduction reaction(NO₃RR)for ammonia production.Density functional theory(DFT)calculations demonstrate that Fe doping increases the projected density of states(PDOS)at the Fermi level of Co₃O₄,enhancing metallicity.In addition,the d-band center of Co₃O₄ is raised,making it more inclined to the Fermi energy level and optimizing the adsorption energy of the Co₃O₄ surface with reactants.All of these are beneficial to increasing the NH₃ yield rate of NO₃RR.In terms of experiments,Fe-doped Co₃O₄ nanoarray(Fe-Co₃O₄ NA/TM)can effectively improve the conductivity and optimize the reaction kinetics process,which is consistent with the theoretical results.It achieves efficient NO₃RR to produce NH₃ with a yield rate of 0.624 mg mg^-1_cat.h^-1 and Faradaic efficiency of 95.5%at-0.7 V vs.RHE in 0.1 M phosphate-buffered saline(PBS)electrolyte containing 50 mM NO₃^-.In addition,Fe-Co₃O₄ NA/TM exhibits excellent electrochemical stability and durability.The Zn-nitrate battery assembled with Fe-Co₃O₄ NA/TM as the cathode exhibits excellent performance.The power density of 0.75 mW cm^-2 is achieved when applied potential of0.3 V vs.Zn.The NH₃ yield rate of 70?g mg^-1_cat.h^-1 is realized when applied current density of 2 mA cm^-2.(3)Based on spinel Co₃O₄,Co₃O₄-NiFeLDH nanosheet array(Co₃O₄-NiFeLDH NA/TM)heterojunction is constructed to achieve efficient NO₃RR for NH₃ production.Such heterostructure effectively increases the density of active sites and accelerates material transfer,which is prepared through electrodeposition-air annealing-re-electrodeposition steps.The Co₃O₄-NiFeLDH NA/TM heterostructure has strong electronic interactions and charges redistribution,resulting in the formation of a built-in electric field(IEF)and further accelerating charge transfer.Therefore,it achieves a high NH₃ yield rate of 1.85 mg mg^-1_cat.h^-1 and a Faradaic efficiency of 97%at-0.9 V vs.RHE in 0.1 M PBS containing 50 mM NO₃^-electrolyte.Meanwhile,the Co₃O₄-NiFeLDH NA/TM heterojunction electrode exhibits excellent electrochemical stability and wear resistance.The Zn-nitrate battery is assembled with Co₃O₄-NiFeLDH NA/TM as the cathode,realizing synchronous power supply and NH₃ production.Specifically,the power density of 0.7 mW cm^-2 is achieved when applied potential of 0.28 V vs.Zn.The NH₃ yield rate of 71?g mg^-1_cat.h^-1 is realized when applied current density of 1.5 mA cm^-2.(4)CoP nanoarrays with different morphologies are prepared by a hydrothermal method-low temperature phosphating method using morphology control.In contrast,CoP nanowire arrays(CoP NWs/TM)possess excellent NO₃RR activity for NH₃ production,due to the abundant active sites and excellent electrical conductivity accelerating the reaction kinetics.DFT calculations explore the possible route of CoP for NO₃RR to synthesize NH₃ and confirm that the rate-determining step of NO₃RR is the process of the first H addition to^*NO₃ to form^*NO₃H.CoP NWs/TM with optimal morphology achieve a high NH₃ yield rate of 1.56±0.42 mg cm^-2 h^-1 and Faraday efficiency of 90.1±2.7%at-0.8 V vs.RHE in 0.1 M Na₂SO₄ containing 50 mM Na NO₃ electrolyte.Furthermore,the CoP NWs/TM with larger specific surface areas exhibit excellent electrochemical stability and wear resistance.