Experimental And Theoretical Combined Study On Electrocatalytic Ammonia Synthesis Under Mild Conditions

Ammonia(NH3)plays an important role in modern society.Owing to the wide use of ammonia-based fertilizers,global grain production greatly increased in the past century.Beyond its industrial use in pharmaceuticals,explosives,and plastic,ammonia is also a promising clean,hydrogen-based,and non-carbon fuel.Typically,ammonia is artificially synthesized from N2 and H2 through the Haber-Bosch process,in which high temperature(300-500?)and extremely high pressure(150-300 atm)are required.As a result,it is estimated that this process consumes more than 5%of the worldwide natural gas supple and emits?1.5%of the global greenhouse gas.Obviously,it is of significance to develop new ammonia synthesis technologies to tackle the fossil energy crisis and global warming.In recent years,electrocatalytic ammonia synthesis(EAS)from N2 and H2O under mild conditions utilizing renewable electricity has drawn much attention from the scientific community owing to its potential in producing ammonia through a clean and sustainable route.In this thesis,the main research goal is to design and prepare efficient EAS catalyst that could achieve a high NH3 generation rate(RNH3)and Faradaic efficiency(FE).Because of the wide application of Fe-based materials in the field of ammonia synthesis,three materials with different chemical composition,microstructure,and morphology-S31608 stainless steel,CoFe-LDH,and perovskite LaFeO3-were respectively used as catalysts to conduct the EAS study.According to the characteristics of different materials,three EAS microkinetic models were summarized,and the competition between EAS and hydrogen evolution reaction(HER)was explored from the kinetic point of view.The intrinsic mechanism of the three materials was studied by combined experimental study and theoretical calculation to reveal the "structure-performance" relationship between the catalytic performance and chemical composition,microstructure,and morphology.The major contents and conclusions are as follows:(1)IC was established in different matrixes with low NH4+concentration(<2 mg L-1)to quantify the produced ammonia in EAS experiment.The advantages of high reproducibility,low detection limit,and high precision of IC were confirmed.Strong robustness of IC was demonstrated by studying the influences of pH,supporting electrolyte,co-existing transition metal cations,and co-existing organic compounds on NH4+quantification.By using IC,background values of NH3 in reagent water,inorganic reagents,and organic reagents and NH3 contamination from surroundings were identified.The ammonization effect of Nafion membranes and the operations that could prevent the underestimation in quantification were systematically investigated.Further,a reliable level of synthesized ammonia(>240 ?g L-1)was identified based on the determined background values of materials and contaminations from the surroundings.Finally,a comprehensive and reasonable EAS research protocol regarding how to design and run the experiments were given,which not only lays a foundation for the following EAS studies,but also gives some valuable suggestions in this field.(2)S31608 stainless steel electrode was used to realize EAS.In a single-chamber system,a RNH3 of 4.70×10-11 mol s-1 cm-2 was achieved in 25 mol kg-1 NaOH at 170?.However,the applied cell voltage has little effect on RNH3,implying the EAS process on S31608 stainless steel is spontaneous.Two possible spontaneous electrochemical mechanisms were proposed based on thermodynamics,these are dissociative mechanism on the fresh exposed surface of pure metals and associative mechanism on the surface of oxides in the passive film.It is demonstrated that the dissociative mechanism mainly occurs on the surface of Cr and Fe metals,and the micro-anodic dissolution of Fe is the driving force of this spontaneous process.Further,the applied cell voltage could reduce the dissolved Fe and make the whole process closed-loop;however,it can not promote the EAS catalytic performance.The associative mechanism was studied by density functional theory(DFT)calculations.Fe3O4 in the passive film is demonstrated to be able to effectively adsorb and activate the N2 molecule since it has large adsorption energy of the N2 molecule.By calculating the Gibbs free energy change(AG)of elementary reactions in EAS,associative enzymatic pathway on Feoct site is demonstrated to has a high catalytic activity,which means the associative mechanism is confirmed theoretically.(3)CoFe-LDH modified electrode was used to realize EAS.In a single-chamber system,a hig RNH3 of 1.10×10-9 mol s-1 cm-2 was achieved at the cell voltage of 2.10 V in 25 mol kg-1 KOH at 80?.However,the FE is low(0.13%),which should be attributed to the violent HER.To inhibit HER,EAS experiments were carried out in a double-chamber system.A much higher FE of 14.18%was obtained at the cathodic potential of-0.75 V(vs.Hg/HgO electrode)in 0.01 M KOH at 25 ?.However,the RNH3 decreased to 2.20×10-11 mol s-1 cm-2,highlighting the necessity to keep a good balance between the RNH3 and FE.The influences of different factors on EAS performance were consistent with the EAS microkinetic model.It is demonstrated that the EAS catalytic activity of the cathode was originated from CoFe-LDH,and the synergistic effect of Co and Fe is important.DFT calculations indicate both the Co site and Fe site on the surface fo CoFe-LDH could adsorb and activate N2 molecules.The alternating association pathway on the Co site is demonstrated to has higher activity by calculating the ?G of elementary reactions in EAS.The density of states(DOS)analysis discloses the nature of the synergistic effect of Co and Fe,that is,the shift of the d-band center of Co site and the improvement of conductivity.(4)Cs-and Ni-doped perovskite LaFeO3 modified electrode was used to realize EAS.The results show doping could improve the EAS catalytic activity of perovskite LaFeO3,and the best EAS performance was achieved with the doping concentration of 20%(LCFN82).In a single-chamber system,a high RNH3 of 3.34×10-10 mol s-1 cm-2 was obtained with a high FE of 1.27%at the cell voltage of 2.00 V in 2 M KOH at 80? with LCFN82 as the catalyst.The highest FE of 1.99%with an RNH3 of 2.35×10-10 mol s-1 cm-2 was achieved at the cell voltage of 1.80 V,which means high RNH3 and FE were achieved simultaneously.The influences of different factors on EAS performance were consistent with the EAS microkinetic model.It is demonstrated that the oxygen vacancies(OVs)provided by perovskite structure is the precondition of high EAS catalytic activity,and LCFN82 has the highest number of OVs on the surface.The formation mechanism of OVs and the effect of dopant were studied by DFT calculations.It is indicated that the OV site could strongly adsorb and activate the N2 molecule.The associative enzymatic pathway on the OV site is demonstrated to has a very high activity by calculating the ?G of elementary reactions in EAS.