Nanoporous Copper-based Catalysts For Highly-efficient Electrochemical Ammonia Synthesis

Ammonia（NH₃）plays a crucial role in the fields of fertilizers,pharmaceuticals,chemicals,fuel cells and hydrogen energy carriers.Currently,the conventional Haber-Bosch process produces more than 180 million tons of NH₃ annually,accounting for more than 90%of the total production.However,Haber-Bosch process operates under high temperatures and pressures,consumes fossil fuels and meanwhile emits large amounts of greenhouse gases.Therefore,it is imperative to develop low energy consumption,pollution-free and sustainable NH₃ production methods.In recent years,the rapid development of electrochemical technology has provided a new way for NH₃ production.With the aid of electrocatalysts,electrochemical reduction to NH₃ can be realized using renewable energy such as wind and solar energy to provide electric energy.At present,the commonly used electrochemical NH₃synthesis mainly includes nitrogen reduction reaction（NRR）and nitrate reduction reaction（NO₃RR）.Therein,NRR has important research significance because it can reduce abundant nitrogen（N₂）in nature into NH₃.However,N₂ molecule is difficult to be adsorbed and activated,and its solubility in electrolyte is very low,which reduces the NH₃ yield.In addition,NRR faces competition from hydrogen evolution reactions（HER）when conducted in aqueous electrolytes,resulting in a decrease in the Faraday efficiency（FE）for NH₃ synthesis.NO₃RR process can use the pollutant nitrate（NO₃^-）in wastewater as the reactant.This strategy could kill two birds with one stone by producing NH₃ and alleviating water pollution at the same time.However,this reduction process involves 8 electron transfer and faces difficulties such as slow reaction kinetics,multiple by-products and competition from HER,which lead to a decrease in NH₃ yield and selectivity during NO₃RR.To solve the above problems,applying efficient electrocatalysts for NRR and NO₃RR is one of the keys to improve the yield and efficiency for NH₃.As for the catalyst design,copper（Cu）-based materials own the advantages of low price,abundant reserves and weak HER activity.The structure of Cu can be optimized through various modification strategies to improve the intrinsic activity of catalyst.On this basis,the active sites of catalysts can be increased and the stability of catalysts can be improved by constructing nanoporous structure.Based on the above considerations,we designed nanoporous Cu-based catalysts in different ways for NRR and NO₃RR,respectively.The main research contents are as follows:1.Nanoporous Cu Mn alloy for highly efficient electrocatalytic nitrogen reduction to ammoniaCu₁₅Mn₈₅ alloy precursor was prepared using manganese（Mn）as the alloying material,and then nanoporous Cu Mn（np-Cu Mn）alloy was obtained as an efficient NRR catalyst by dealloying method at room temperature.This 3D nanoporous structure can increase the specific surface area,expose more active sites,and accelerate the electron transfer between catalyst and reactant.Compared with pure Cu and Mn,Cu Mn alloy exhibits enhanced NRR performances.When Cu:Mn=1:1,np-Cu Mn catalyst affords the NH₃ yield rate of 28.9μg h^-1 cm^-2 with the FE of 9.83%at-0.3 V（vs.RHE）.In addition,density functional theory（DFT）was used to analyze the N₂ adsorption energy and the reaction free energy during the NRR pathway on Cu Mn with different proportions.Alloyed Cu Mn achieved enhanced the adsorption energy for N₂,and Cu₅₀Mn₅₀ reduces the reaction energy of the rate determining step from 1.38 e V to 0.58 e V compared with Cu,which can promote the adsorption and activation of N₂.2.Interfacially engineered nanoporous Cu/MnO_x for enhanced electrocatalytic nitrate reduction to ammoniaCu-based materials are one of the most commonly used catalysts for NO₃RR because of their high activity.However,Cu exhibits relatively weak adsorption for NO₃^-and poor water molecules（H₂O）dissociation ability,which is difficult to meet the demand of NO₃RR hydrogenation process.Thus,manganese oxide（MnO_x）was introduced on the surface of nanoporous Cu,and the catalytic properities of Cu was optimized through interface regulation.X-ray photoelectron spectroscopy shows that there exists electron transfer from MnO_x to Cu at the interface between Cu and MnO_x,which effectively regulates the electronic structure of Cu.Represented by MnO,the DFT results show that there is obvious electron accumulation at the Cu/MnO interface.The presence of MnO facilitates the shift of Cu d band center towards the Fermi level,which is beneficial to enhance the interaction between catalyst and reaction intermediates and promote NO₃RR processes.According to the comparison of the reaction free energy of Cu/MnO and Cu catalyst in the NO₃RR and HER processes,the presence of MnO improves the NO₃RR activity of Cu and inhibits HER simultaneously,thus improves the selectivity of Cu/MnO.In 0.1 M Na₂SO₄ electrolyte containing 10 m M NO₃^-,nanoporous Cu/MnO_x（np-Cu/MnO_x）catalyst can obtain a high NH₃ yield rate of 5.53 mg h^-1 mg_cat.^-1with the FE of 98.2%at-0.6 V（vs.RHE）.This 3D nanoporous structure enables the np-Cu/MnO_x catalyst to maintain excellent catalytic performance and structural stability after 12 cycling tests.3.Nanoporous Ru-doped Cu for highly efficient electrocatalytic nitrate reduction to ammoniaIt is known that adjusting the electronic structure of Cu could improve the NO₃RR activity of the catalyst,but the performances of catalysts at low overpotentials need to be further improved.To solve this problem,Ru,which can promote H₂O molecular dissociation,was selected as the doping element,and nanoporous Ru-doped Cu（np Ru-Cu）was prepared as NO₃RR catalyst by alloy-dealloying method.The large pores in the structure are in favour of the diffusion of electrolyte and can accelerate the protons and mass transfer during the reaction.The small pores could help to increase the specific surface area and expose more active sites.Thus the activity and stability of the catalyst can be improved through this structure.Theoretical studies show that Ru-Cu regulates the d band center and electronic structure of Cu,improves the adsorption energy for NO₃^-and reduces the free energy of the rate determining step during NO₃RR processes.Ru doping successfully reduces the free energy required for H₂O dissociation,provides enough*H and inhibits HER,thus promotes the hydrogenation process and improves the selectivity for NO₃RR.np Ru-Cu catalyst can achieve a high NH₃ yield rate of 29.51 mg h^-1 mg_cat.^-1with the FE of 97.0%at-0.2 V（vs.RHE）under 0.1 M KOH containing 50 m M KNO₃ electolyte,manifesting the NO₃RR performances are improved at low potentials,which performs better than most reported catalysts.