Application Of LDH Materials In Electrochemical Ammonia Synthesis

With the over-exploitation and utilization of fossil energy,the dependence on these non-renewable energy sources and the environmental pollution caused by energy utilization have been widely concerned by scholars at home and abroad.Since entering the new era,the demand for material and energy in People’s Daily life is increasing day by day.It is urgent to solve the problem of resource and energy supply and greenhouse gas emission.Ammonia,as an indispensable energy carrier in industry,has been widely used in the production of"Haber method"in the past 100 years.However,due to its high energy consumption and the emission of a large number of greenhouse gases and many other drawbacks,finding a green and pollution-free way to replace ammonia has been brought into one of the leading topics in the energy industry today.In this paper,Ni Co-LDH and Co Mn-LDH are designed and synthesized as catalysts for electrochemical ammonia synthesis based on the polymetallic properties of LDH materials.The preparation of LDH materials is reflected through various characterization,the catalytic effect is evaluated by means of electrochemical testing,and the reaction mechanism at the atomic level is explored deeply through DFT simulation calculation.The main results are as follows:1.NiCo-LDH with hollow structure was prepared by using normal dodecahedral ZIF-67 material as sacrificial template under hydrothermal condition for electrochemical ammonia synthesis.Using ZIF-67 as template,Ni Co-LDH material was generated in ethanol by Co source and Ni²⁺ions,and the hollow structure LDH sample was obtained by sacrificing the template in hydrothermal condition.By macroscopically adjusting the stoichiometric ratio of the two materials in the preparation process,the precise control on the level of element content can be realized,and the preparation ratio with the strongest promoting ability to nitrogen reduction reaction can be found.The Ni Co-LDH sample obtained under the preparation ratio of Co:Ni=1:3 can achieve an ammonia gas yield of 52.8μg_NH3 h^-1mg^-1_cat and a Faradaic efficiency of 11.5%at-0.7 V vs.RHE potential,which is higher than the catalytic activity of most reported transition metal compounds as nitrogen reduction electrocatalysts.At the same time,in order to avoid the influence of nitrate in the raw material on nitrogen source,we have carried out argon atmosphere test and open circuit potential test successively,and no nitrogen and hydrogen compounds were produced.In order to ensure that the nitrogen source is completely from external nitrogen,we carried out isotope calibration method to compare the experimental results,and the conclusion is highly consistent.In this paper,the effect mechanism of electronegativity difference on polymetallic catalyst in nitrogen reduction reaction was investigated and confirmed by DFT theoretical calculation.The results show that there are Co-Ni-Ni,Co-Co-Ni,Co-Co-Co and Ni-Ni-Ni catalytic units on the surface of Ni Co-LDH.By calculating the reaction energy barrier at different stages of nitrogen reduction reaction,the promotion or inhibition effect of the presence of different catalytic units on nitrogen reduction reaction was compared.Projection density analysis shows that the co-rich site has a higher vacant orbital density distribution near the Fermi level,which greatly promotes the adsorption of nitrogen and enhances the nitrogen reduction reaction.The calculation of HER reaction showed that the introduction of Ni inhibited the occurrence of HER reaction to some extent and promoted the nitrogen reduction process.2.Two-dimensional lamellar CoMn-LDH material was prepared by one-step co-precipitation centrifugation for electrocatalytic nitrogen reduction reaction.According to previous studies on Ni Co-LDH materials,LDH materials with bimetals as the active sites of nitrogen reduction reaction have great potential in the field of electrochemical ammonia production.Therefore,we synthesized Co Mn-LDH materials for nitrogen reduction reaction with a more convenient method.Originally,the preparation cycle of the sacrificial precursor template by the hydrothermal method was too long.Now we directly use the co-precipitation method to make Co Mn-LDH materials with a typical lamellar structure,and replace the nitrates in the raw materials with chloride and sulfate.The catalytic ability of the samples for the nitrogen reduction process with different interlayer anions was observed.The yield of 23.5μg_NH3 h^-1 mg^-1_cat and Faraday efficiency of 15.6%were obtained at-0.6 V when the interlayer anion was sulfate ion.Different from previous studies,we replaced the variable of the ratio of metal ions on the material surface with that of interlayer ions.The difference of interlayer ions changed the spacing between LDH layers,which may be one of the properties that affect the nitrogen reduction performance of LDH materials.It is worth noting that Co may still be the reaction site of nitrogen reduction in the sample when conducting in situ Raman test,which further proves the determining role of electronegativity on the reaction site in LDH materials.