Preparation Of Transition Metal Nanomaterials And Their Application In Electrocatalytic Nitrogen Fixation

As one of the largest industrial chemicals in the world,NH₃ plays an essential role in fertilizer,dyes,explosives,synthetic fibers,and resins.Nitrogen fertilizer produced with NH₃ as raw material can effectively increase the nitrogen content of soil and greatly promote the growth of plants,which can to a large extent solve the food shortage problem caused by explosive population growth.However,the industrial-scale production of NH₃ is dominated by the Haber-Bosch process,which operates under harsh reaction conditions and needs H₂ that is mostly obtained by the reforming of fossil hydrocarbons to serve as the reducing agent.Furthermore,the process consumes large amounts of energy with low energy efficiency and emits large amounts of CO₂,leading to serious climate problems.Therefore,a sustainable and environmentally friendly approach for artificial NH₃ synthesis at mild conditions is urgently to explore.Electrochemical N₂ reduction under ambient conditions is an eco-friendly and sustainable method for NH₃ synthesis.However,it is seriously inhibited by the N≡N bond with ultrahigh stability,especially the first dissociation energy is up to 410 k J mol^-1.Moreover,in aqueous system,NRR and hydrogen evolution reaction（HER）have similar reduction potentials,leading a compromise selectivity for NRR.Therefore,design and development of such catalyst with high active and selective for NRR plays a key role in this process.Although noble metal-based electrocatalysts（Ag,Au,Pd）can effectively activate the N₂,but their widespread use is restrained by scarcity and cost.An immediate outlook for large-scale industrial applications points toward using systems without expensive precious metals,which strongly encourages the development of noble-metal-free NRR electrocatalysts.Nanomaterials are extensively applied in electrocatalysis field,due to their have properties that are different from their atomic state,such as quantum size,small size,surface,dielectric limiting,and coulomb blocking effect.Moreover,most of transition metals recently are received widespread attention,because their are earth-abundant,environmentally friendly and have d-orbitals that are easy to form bonds.In this paper,through electrospinning,high-temperature annealing,displacement reaction and other methods,we successfully prepared transition metal nano-material catalysts based on Mn,Fe,Cu.Then,we use morphology control,introduce defects and other strategies to improve the performance of NRR.The main research contents are as follows:1)Mn are widely distributed in my country.Because of their abundant,cheap,and easy availability,Mn has good prospects for large-scale industrial applications.Hence,we use the electrospinning process to prepare spinel Li Mn₂O₄ one-dimensional nanofiber.When tested in 0.1 M HCl,at-0.50 V,this electrocatalyst affords an excellent faradaic efficiency（FE）of 7.44%and a large NH₃ yield rate of 15.83μg h^-1 mg^-1_cat..,outperforming reported NRR electrocatalysts.Futhermore,it also shows high electrochemical stability and durablely for N₂ fixation to NH₃.Sush excellent performance of Li Mn₂O₄ benefits from the presence of Li in the aqueous solution-based NRR system,which can inhibit HER,thereby increasing the selectivity of NRR.Morover,Li Mn₂O₄ with one-dimensional nanostructures has large specific area exposing more active sites,thus boosting the NRR.2)Porous La Fe O₃ nanofiber with oxygen vacancy defect（Vo-La Fe O₃）was prepared using electrospinning technique.When tested in 0.1 M HCl,this catalyst achieves a high FE of 8.77%and a large NH₃ yield rate of 18.59μg h^-1 mg^-1_cat.at–0.55 V.Notably,it also shows high electrochemical stability and durability.This excellent performance is greatly benefited from the introduction of oxygen vacancies.The existence of oxygen vacancies in the La Fe O₃ effectively regulates the electronic structure and promotes charge transfer.In addition,the one-dimensional porous nanostructure of La Fe O₃ exposes more active sites and facilitates contact with reactants,thus greatly promotes the NRR.Density functional theory calculations reveal that,by introducing oxygen vacancy on La Fe O₃,the subsurface metallic ions are exposed with newly localized electronic states near the Fermi level,which facilitates the adsorption and activation of N₂ molecules as well as the subsequent hydrogenation reactions.3)Cu are abundant in all over the world,because of the characteristics of small resistivity and low price,it has become an excellent electronic conductor material.The dendritic Cu nanostructure was prepared by the simple galvanic replacement method.When tested in 0.1 M HCl,such a catalyst achieves a high FE of 15.12%and a large NH₃yield rate of 25.63μg h^-1 mg^-1_cat.at–0.40 V,outperforming most aqueous-based NRR electrocatalysts.Remarkably,it shows a high electrochemical stability and an excellent selectivity for N₂ fixation to NH₃.Spikes on the surface of materials can markedly amplify the local electric field,which may increase the reagent concentration near the tips of the electrodes.Moreover,dendritic Cu nanostructure with abundant-oriented tips can drastically promote interactions between electrocatalysts and reactants,and thus boosting the performance of NRR.