Design Of Black Phosphorene Based Nitrogen Reduction Electrocatalyst And Optimization Of Electrolyte System

Ammonia is an important raw material for industrial and agricultural production.It is also considered as an ideal hydrogen carrier because of its hydrogen content up to 17.6%and its easy liquefaction.For a long time,the industrial synthesis of ammonia has been dependent on the Haber-Bosch process,but as we all know,although this technology has been developed for a hundred years,it still needs to be carried out under high temperature and high pressure,which not only consumes high energy,but also brings a lot of CO₂ emissions.Therefore,under the environment of energy conservation and emission reduction,it is urgent to develop a sustainable ammonia synthesis technology that can replace Haber-Bosch method.Among many potential alternatives for ammonia synthesis,electrochemical synthesis of ammonia,especially electrocatalytic nitrogen direct reduction of synthetic ammonia,is considered to be the most ideal replacement technology for synthetic ammonia because it can be carried out under mild conditions and can directly use water as the hydrogen source.However,although the technology has been proposed since the middle of the last century,it has not achieved the expected development,in the final analysis,because the technology faces several scientific challenges:first,because the bond energy of N≡N bond is too high(940.95 k J mol^-1),it is difficult to be efficiently activated in mild conditions;Secondly,the reduction potential of nitrogen reduction reaction is very close to hydrogen evolution potential in acidic electrolyte or alkaline electrolyte.Therefore,it is inevitable to face strong hydrogen evolution competitive reaction,coupled with the low solubility of nitrogen in many electrolytes,which restricts the yield and current efficiency of electrocatalytic nitrogen reduction synthesis of ammonia.In view of the above challenges faced by electrocatalytic nitrogen reduction synthesis of ammonia,this paper takes black phosphorene with high chemical activity as the model catalyst for nitrogen reduction,respectively from the design of high catalytic reaction active sites,the inhibition of hydrogen evolution competitive reaction,and the optimization of electrolyte reaction system.Research work has been carried out to simultaneously improve the ammonia production rate and current efficiency of electrocatalytic nitrogen reduction synthesis ammonia.The main research content and progress of this paper are summarized as follows:（1）First of all,in view of the difficult activation of nitrogen and nitrogen triple bond and the competitive reaction of hydrogen development,in this paper,on the basis of the successful preparation of black phosphorene,further by introducing metal tin,the preparation of tin doped black phosphorene nitrogen reduction catalyst with high activity and high selectivity for nitrogen reduction reaction.The innovative ideas of this catalyst design are as follows:i)Phosphene formed by in-situ spallation of black phosphorus nanosheets has potential high chemical reactivity,which is expected to realize efficient activation of nitrogen;ii)Introducing tin metal as the"sacrificial"site for hydrogen evolution to share the hydrogen evolution reaction on the catalyst can provide more opportunities for nitrogen to participate in adsorption activation on the active site of black phosphorus.Our results show that the Faradaic current efficiency of 31.58%can be achieved by using this catalyst,which is 11.87 times higher than that of undoped black phosphorene,and the maximum ammonia yield rate can reach 26.98μg h^-1 mg_cat.^-1.The results of theoretical calculation show that the adsorption energy of water at the active site of black phosphorene can be significantly reduced by Sn doping（Gibbs free energy decreases from 0.057 e V to-0.163 e V）.This result confirms that the hydrogen evolution competition at the active site of nitrogen reduction can be effectively weakened by the purposeful introduction of the hydrogen evolution"sacrifice"site into the nitrogen reduction catalyst.Thus,the selectivity of nitrogen reduction reaction can be significantly improved.（2）Inspired by the fact that metal lithium can spontaneously react with nitrogen,the design idea of constructing lithium-like nitrogen reduction catalyst is innovatively proposed in this paper.Based on the design idea of the catalyst,we used the defective black phosphorene with high chemical activity of lithium as the model catalyst to confirm the feasibility and advancement of this idea.At the same time,considering that defective black phosphorene is easy to be oxidized and difficult to be prepared in practice,we also proposed a strategy to achieve stable preparation of defective black phosphorene by using fluoridation protection.The results show that the stable defective black phosphorene(F-D-BP_ene)can be obtained in the reaction system with（CH₃CH₂CH₂）₄N（BF₄）as electrolyte solution by electrochemical means by simultaneous construction of defects and fluorination protection using bulk black phosphorus as raw material.The theoretical calculation results based on Bard charge analysis show that F-D-BP_ene has high chemical activity of transferring 0.88 electrons to nitrogen.Further nitrogen reduction experiments showed that the NH₃ yield of up to 70μg h^-1 mg_cat.^-1and the Faradaic efficiency of about 26%could be achieved synchronously by using F-D-BP_eneas the nitrogen reduction catalyst,which was about 2.5 times higher than that of defect-free black phosphorene.And almost superior to all other reported nonmetallic nitrogen reduction catalysts under similar test conditions.（3）Aiming at the difficult problem of competitive reaction of hydrogen evolution and low solubility of nitrogen in water electrolyte by electrocatalytic nitrogen reduction synthesis ammonia.In this paper,a methanol-mediated low proton electrochemical synthesis of ammonia reaction system was developed.By using methanol solution on one side of the H-cell（the side where the nitrogen reduction reaction occurred）and conventional water electrolyte on the other side,we effectively overcome the problem of hydrogen evolution competition reaction,and also effectively overcome the problem of proton source by using proton membrane between the two cells to conduct protons from the opposite side.Based on this reaction system,we use black phosphorus quantum dots（p-Ti₃C₂-BPQDs）anchored on porous Ti₃C₂ as catalyst to verify the superiority of the system.Reliable electrochemical performance evaluation results,including quantitative analysis of ¹⁵N₂ isotope,showed that p-Ti₃C₂-BPQDs achieved ammonia yield up to 376.70μg h^-1 mg_cat.^-1at the potential of-0.7 V vs Ag/Ag Cl.Faradaic efficiency up to 86.28%is achieved at the potential of-0.6V vsAg/AgCl.