Investigation Of Copper/cobalt-based Catalysts For Electrochemical Ammonia Synthesis

Ammonia（NH₃）is an important chemical feedstock in producing of fertilizers,chemicals,and pharmaceuticals,and has also been considered as a promising carbon-free energy carrier.At present,the industrial production of NH₃ is heavily relied on Haber–Bosch process,which operates at high temperature and high pressure.This method not only consumes significant amounts of fossil fuels but also results in the release of large quantities of greenhouse gases,contributing to environmental pollution.Therefore,it is essential to develop low-energy,pollution-free and sustainable NH₃ production methods.In recent years,researchers have focused a lot of emphasis on the electrochemical reduction method for NH₃production.This approach,which employs the electric energy transmitted from renewable energy sources like solar and wind power,can be carried out in ambient settings utilizing water as the hydrogen source.At present,the commonly methods of electrochemical NH₃synthesis mainly include nitrogen reduction reaction（NRR）and nitrate reduction reaction（NO₃RR）.Researchers have been interested in NRR because nitrogen（N₂）is plentiful and stable in nature,which makes it a great raw material for the synthesis of NH₃.However,the low solubility and stable N≡N bonds leading the NRR still suffers from low NH₃ yield.Thus,well-designed catalysts are needed to enhance the NH₃ yield and selectivity of NRR.Nitrate（NO₃^-）is a common water pollutant.The use of NO₃^-as a feedstock for the synthesis of NH₃is a two-for-one strategy that solves the problem of wastewater treatment while obtaining an economically valuable product NH₃.However,Unfortunately,electrochemical NO₃RR for NH₃ production often suffers from poor NH₃ selectivity because the NO₃^--to-NH₃ conversion involves a complex eight-electron/nine-proton process,and NO₃^-electroreduction can occur via multiple reaction pathways to generate a variety of by-products.It is also necessary to design a reasonable catalyst to optimize and enhance its reaction path and selectivity.Transition metals have excellent electrical conductivity and unique d-electronic structures,and the electrons of transition metals can be modified by various strategies to meet different catalytic demands and improve the electrochemical NH₃ synthesis activity of the catalysts.In this thesis,we choose copper（Cu）and cobalt（Co）among transition metals as the core of the research,employing various strategies such as heteroatom doping and construction of heterogeneous interfaces to improve catalytic activity,selectivity,and stability of Cu/Co-based catalysts in the electrochemical synthesis of NH₃.The main research contents are as follows:1.In order to break the stable N≡N bond,it requires a large overpotential to activate the N₂ molecule in aqueous electrolytes,which makes the applied potential used for electrochemical NRR moving to the zone of hydrogen evolution reaction（HER）.Consequently,the HER becomes the dominating reaction and severely inhibits the NRR in aqueous electrolyte during the electrochemical NRR,leading to low NH₃ yield and FE.One effective approach to improve the NH₃ yield and FE of electrochemical NRR is the use of HER-inactive materials as electrocatalysts.Based on the above views,Cu and silver（Ag）with poor HER activities were selected and synthesized Ag-doped Cu nanosheets grown on carbon paper(Ag-Cu _NS/CP)via a simple electrochemical deposition method.The Ag-Cu _NS/CP catalyst has a unique corrugated nanosheet structure,which can expose more active sites.The porous structure formed by cross-linking nanosheets with each other facilitates the electron transfer between the catalyst and the reactants in solution.Besides,X-ray photoelectron spectroscopy shows that the doping of Ag leads to the formation of electron-deficient Cu atoms in Ag-Cu _NS/CP,which can suppress hydrogen evolution reaction and enhance N₂ adsorption in neutral media,leading to enhanced NRR activity.In 0.1 M Na₂SO₄electrolyte,the Ag-Cu _NS/CP catalyst exhibits a high FE of 20.9%with a NH₃ yield rate of61.5μg h^-1 mg_cat.^-1at-0.4 V（vs.RHE）,which performs better than most reported catalysts.2.Cu-based materials have been one of the most commonly utilized catalysts for NO₃RR because of their high activity.However,the weak adsorption capacity of Cu for NO₃^-and the strong adsorption of intermediates make the Cu-based catalysts susceptible to the accumulation of intermediates in the NO₃RR reaction.Besides,the poor hydrolysis capacity of Cu is difficult to meet the demands of the NO₃RR hydrogenation process.For this purpose,we designed cuprous oxide/copper（Cu₂O/Cu）heterostructures to improve the adsorption capacity of NO₃^-on the catalysts and doped Co into Cu₂O structure to accelerate the hydrolysis and provide free hydrogen（*H）for the NO₃RR reaction in hydrogenation process.On this basis,we reported the preparation of heterostructured Co-doped Cu₂O/Cu nanoparticles embedded in a carbon framework（Co-Cu₂O/Cu@C）catalyst and applied in electrochemical reduction of NO₃RR for NH₃ synthesis.The Co-Cu₂O/Cu@C catalyst exhibits a remarkable NO₃RR activity with an NH₃ yield rate of 37.86 mg h^-1 mg_cat.^-1and a FE of 98.1%at-0.4 V（vs.RHE）in 0.1 M KOH electrolyte containing 0.05 M NO₃^-,which are higher than those obtained for most of the Cu-based catalysts under similar conditions.Density functional theory（DFT）calculations indicate that the formation of Co-Cu₂O/Cu interface decreases the reaction barrier of electrochemical NO₃^--to-NH₃ conversion and the introduction of Co promotes H₂O adsorption/dissociation to supply*H for the hydrogenation step,leading to an enhanced NO₃RR performance3.Due to the slow kinetics of the NO₃RR process and the variety of intermediate products,it is necessary to design catalysts with multifunctional active sites to accelerate the kinetics of the NO₃RR reaction and to promote the conversion of intermediate products to NH₃.Moreover,it is equally important to explore high-efficiency and stable electrocatalysts for NO₃RR.Addressing this problem,we selected Co-based catalysts with excellent NO₃RR activity and partially phosphorylated them to obtain Co/Co₂P catalysts with heterostructures for efficient electrocatalytic NO₃RR reaction for the NH₃synthesis.Co₂P has excellent hydrolysis ability and can provide sufficient*H for the hydrogenation progress of N-containing intermediates in the NO₃RR reaction.The synergistic effect of Co/Co₂P tandem catalysts optimized the kinetic rate of intermediate reaction of NO₃RR and achieved a high rate of NH₃ generation over the catalysts at low overpotential.In addition,Co/Co₂P nanoparticles are encapsulated in nitrogen-doped graphitized carbon nanotubes.This structure provides good protection for the internal Co/Co₂P nanoparticles while improving the catalyst conductivity,promoting the dispersion of Co/Co₂P nanoparticles,and increasing the active area of the catalyst,resulting in excellent stability.The Co/Co₂P@NG/CNT catalyst exhibits a remarkable NO₃RR activity with an NH₃ yield rate of 47.96 mg h^-1 mg_cat.^-1and a FE of 99.24%at-0.2 V（vs.RHE）in 0.1 M KOH electrolyte containing 0.05 M NO₃^-,and maintains excellent catalytic performance and structural stability after 30 cycling tests.