The Design Of Electrocatalysts From Deep Eutectic Solvent For Electrocatalytic Nitrogen Fixation

Converting aerial nitrogen（N₂） to ammonia（NH₃）is primarily important to meet the growing demand of human life in general.As matters stand,the industrial NH₃ synthesis is undisputedly predominated by the traditional Haber-Bosch nitrogen fixation process under harsh reaction conditions,causing large amounts of global energy depletion and environmental crisis.Recently,the electrocatalytic N₂ reduction reaction（NRR）provides a promising way for renewable NH₃ synthesis,because it uses water as the hydrogen source and can be powered by electricity from clearer energy sources such as wind and solar.Although intensive research has been dedicated to electrocatalytic NRR,its NH₃ yield rate and faradaic efficiency are still unsatisfactory due to the high activation energy of extremely strong N=N bond and the occurrence of inevitable competitive hydrogen evolution reaction（HER）.Therefore,it is extremely urgent to design and exploit advanced catalysts for achieving admirable NRR performance.Due to the unique electronic structures,transition metal and main group metal elements can interact with N₂ molecular,which are beneficial to the adsorption and activation of N₂ molecules,so they are often used in the design of advanced NRR electrocatalysts.Deep eutectic solvent（DES）is a novel green solvent formed by the mixtures of two or three compounds which are capable of self-association via hydrogen bond interactions.Since DES has the multifunctional roles（solvents,structure-directing agents,templates）and plenty of advantages（biodegradability,low cost,high tenability,simple preparation）,it has triggered enormous research interest in the construction of advanced electrocatalysts with inimitable structures and morphologies.Therefore,in this paper,a novel preparation strategy using DES was applied to design and synthesize high efficient NRR electrocatalysts,and the following two research works were carried out:（1）Inspired by the excellent solubility of Fe₃O₄ in choline chloride/oxalic acid deep eutectic solvent,here,we reported an innovative DES-based regeneration strategy to fabricate porous Fe₃O₄ nanosheets utilizing commercial Fe₃O₄ powder as raw materials.After microwave heating of the DES-dissolved commercial Fe₃O₄ powder,nanosheet precursors were obtained.When these precursors were annealed under nitrogen atmosphere,porous Fe₃O₄ nanosheets with enriched oxygen vacancies were successfully fabricated.The as-prepared porous Fe₃O₄ nanosheets demonstrated a high NH₃ yield rate of 12.09 μg h^-1 mg^-1 cat.and an excellent faradaic efficiency of 34.38%at a low potential of-0.1 V versus RHE in the 0.1 M Na₂SO₄ electrolyte.Such high-performance electroactiviy towards NRR surpassed the previously reported Fe₃O₄ catalysts,holding great promise in NRR.（2）Bi-based materials especially Bi₂O₃ have been utilized for NRR due to its intrinsic suppression of HER,but the inferior conductivity and low faradaic efficiency hamper its popularity in NRR.Herein,we,for the first time,designed Mn-doped Bi₂O₃ nanosheets from choline chloride/oxalic acid deep eutectic solvent for electrochemical NRR.The Mn-doped Bi₂O₃ nanosheets demonstrated a high NH₃ yield rate of 23.54 μg h^-1 mg^-1cat.and an enhanced faradaic efficiency of 21.63%in 0.1 M Na₂SO₄ electrolyte at a lower potential of-0.1 V versus reversible hydrogen electrode,superior to the previous Bi₂O₃ catalysts.The result indicated that introducing Mn into Bi₂O₃ elevates the faradaic efficiency.for NRR by suppressing the adverse HER.