Fabrication Of High Efficient Transition Metal Electrocatalysts And Their Applications In Electrosyntheis Of Ammonia

Ammonia（NH₃）is one of the most widely produced chemicals worldwide with a key role in the growth of global economy.Traditional ammonia synthesis by the Haber-Bosch process operates at high temperature and pressure with low hydrogen conversion efficiency,high energy consumption and severe environmental pollution.With the development of artificially synthetic ammonia technology,it has aroused great concern and attention that the electrochemical ammonia synthesis at ambient conditions embraces the advantages of lower energy consumption,utilization of water as hydrogen source and no emission of CO₂ and so on.The key of electrochemical reduction of nitrogen to ammonia lies in the development of low-cost and high-efficiency electrocatalysts.At present,the reported electrocatalysts for the nitrogen reduction reaction（NRR）can be mainly divided into three categories:nonmetallic carbon catalysts,transition metal catalysts and noble metal catalysts.However,the reported works have demonstrated that nonmetallic carbon catalysts possess low NRR activity and efficiency,and noble metal catalysts with high NRR activity are expensive and resource-poor,limiting their large-scale application.Because of its unique d-orbital structure and abundant electron cloud density,transition metal catalysts can efficiently adsorb N2 molecule and weaken its stable N≡N triple bond to achieve high-efficiency activation of N2 molecules,but the transition metal d-orbital electron is also conducive to the formation of metal-H bond,which leads to the competitive electrocatalytic hydrogen evolution reaction（HER）,thus resulting in poor NRR selectivity.On the basis of the above mentioned issues,this thesis work is mainly focused on designing and constructing of high-efficiency transition metal NRR electrocatalyst with precise regulation of defects,compositions,coordination structures etc.for electrocatalytic NRR to NH3.The NRR mechanism and pathway were revealed by the combined method of theoretical and experimental analysis.The main research contents and results are summarized as follows:1.Based on defect regulating strategy,Cu doped CeO₂ nanorods catalyst was synthesized by a facile hydrothermal method.Cu doping amount in catalyst can be precisely regulated.When the Cu doping content is 3.9 wt.%,the obtained catalyst（Cu-CeO₂-3.9%）exhibited the highest NH3 yield rate（5.3×10-10 mol s-1 cm-2）and faradaic efficiency（FE）（19.1%）at-0.45 V（vs.RHE）.The superior NRR activity of Cu-doped CeO₂ nanorods catalyst is mainly ascribed to Cu doping in CeO₂ to efficiently regulate the oxygen vacancies concentration,which are the catalytic active sites toward the NRR to NH3.2.Based on composition regulating strategy,cobalt phosphide embedded into carbon nanotubes（CoP/CNs）were fabricated by pyrolysis of cobalt based metal organic framework（MOF）precursor combined with phosphorization treatment at high temperature.The largest NH3 yield rate of CoP/CNs can be achieved to be 48.9μg h-1 mgcat.-1 at-0.4 V（vs.RHE）with a faradaic efficiency（FE）of 8.7%.Based on the theoretical calculations results the dissolved N2 in alkaline electrolyte can be adsorbed and activated on the positively charged cobalt sites of CoP and the negatively charged neighboring phosphorous sites become the anchoring points for protons to efficiently inhibit hydrogen evolution reaction（HER）activity,thus improving the performance of electrocatalytic NRR to NH3.3.Based on coordination regulating strategy,Co single atoms anchored on N-doped carbon（Co-SAs/NC）was synthesized by carbonizing a mixture of cobalt acetate,tartaric acid and polyvinylpyrrolidone（PVP）.The resultant Co-SAs/NC can afford an NH3 yield rate of 9.7 μg h-1 mgcat.-1（951 pg h-1 mgCo-1）with a faradaic efficiency（FE）of 7.5%at-0.45 V（vs.RHE）.After the laser-irradiation treatment,the Co-SAs/NC exhibits significantly enhanced NRR activity in the same electrolyte.This significantly enhanced NRR performance is because the laser-irradiation treatment liberates more Co-N3 active sites on Co-SAs/NC.The theoretical calculations results identify the Co-N3 site being the NRR active center,the possible mechanism of NRR on Co-N3 site is the enzymatic hydrogenation mechanism.4.Based on the above-mentioned work,O-coordinated Fe single atoms（Fe-SAs）anchored on lignocellulose（LC）converted graphitic carbon substrate（Fe-SAs/LCC）was developed for the first time using N-free and O-containing LC as adsorption regulator by high-temperature pyrolysis approach.The Fe-SAs/LCC loaded on carbon cloth electrode can afford a high NH3 yield rate and faradaic efficiency（FE）of 32.1μg h-1 mgcat.-1（5350 μg h-1 mgFe-1）and 29.3%,respectively.An exceptional NH3 yield rate of 307.7 μg h-1 mgcat.-1（51,283 μg h-1 mgFe-1）with a near record FE of 51.0%can be achieved from the electrocatalyst immobilized on a glassy carbon electrode.Based on the theoretical calculations results,the NRR process catalyzed by Fe-SAs/LCC is likely to occur through an alternating hydrogenation pathway.5.To enrich the kinds of single-atom catalyst family,O/N-coordinated Cu single-atom catalysts（Cu-SAs/BCC）were fabricated using bacterial cellulose（BC）with rich O-and N-containing functional groups as adsorption regulator by high-temperature pyrolysis method.The X-ray absorption spectra analysis of Cu-SAs/BCC confirms the stable CuO4 and CuO3N configuration structures in the catalyst.When the Cu load content is 0.64wt.%,the obtained catalyst（Cu-SAs/BCC-63/0.64wt.%）exhibited the highest NH3 yield rate（30.7 μg h-1 mgcat.-1）and faradaic efficiency（FE）（33.4%）at-0.20 V（vs.RHE）.The electron-spin influence under high density of single atomic Cu may result in the decrease of NRR activity of an individual single atomic Cu active site.The theoretical calculations results suggest that the formed CuO4/CuO3N single atomic sites on BC converted graphitic carbon possess high NRR activity.The possible mechanism mainly follows the enzymatic hydrogenation mechanism.