The Synthesis Of Cofe-ldh Modified Electrode And Electrochemical Ammonia Synthesis Property Under Mild Conditions

Ammonia is an important chemical raw material and an ideal hydrogen energy carrier in the future.Although the reaction for NH3 synthesis from N2 and H2 is a thermodynamically spontaneous process under normal temperature and pressure conditions(K?=6.8×105),the N?N bond in the N2 molecule is extremely stable(bond energy 945 kJ mol-1).Consequently,the activation energy of reaction is high.Currently,energy-intensive and CO2-extensive Haber-Bosch ammonia synthesis is carried out on a tremendous scale.Therefore,researchers around the world are looking for ammonia synthesis technology under mild conditions,among them,electrochemical ammonias synthesis is considered to be one of the most promising pathway.The core scientific issue of electrochemical ammonia synthesis is to find highly efficient electrochemical catalysts.Researchers have tried noble metal-based(including Pd,Ru,Pt/C,Au)catalysts and iron-based catalysts,etc.in electrochemical ammonia synthesis systems.All of the catalysts above are single-metal materials.Based on the theory of "volcano plots",this paper proposes to prepare a catalyst by combining metal elements(such as Mo,Fe)which are favorable for N2 adsorption with metal elements(such as Co,Ni)which are beneficial to the desorption of NHx,and it is expected to improve electrochemical ammonia synthesis catalytic activity.To this end,this paper used hydrothermal method to grow CoFe-LDH modified film with three-dimensional nanostructures on the nickel foam substrate,and characterized the performance of electrochemical ammonia synthesis under mild conditions.The results are as follows:(1)The density functional theory(DFT)theoretical calculation of the N2 adsorption capacity of CoFe-LDH surface shows that N2 form a stable chemical adsorption to Co and Fe elements on the surface of CoFe-LDH,which effectively weakens the N?N bond strength.It is beneficial to the occurrence of hydrogenation reaction.Based on the above DFT calculation results,a microscopic kinetic model for the electrochemical ammonia synthesis from N2 molecules on CoFe-LDH was proposed.The reaction of adding the first hydrogen to the adsorbed N2 was a rate-determining-step.(2)CoFe-LDH modified film electrodes with different Co and Fe mole ratios were prepared on the foamed nickel substrate by hydrothermal method by adjusting the feed ratio of Co and Fe sources.The structure and morphology of the materials were characterized by SEM,HRTEM,XRD,XPS,FT-IR and BET.The results show that the modified films with different Co and Fe mole ratios have perfect LDH crystal structure,among which,the CoFe-LDH modified film with a Co and Fe mole ratio of 5:5 is about 40 nm thick,and forms a surface open pore structure with a pore diameter of 50-300 nm,and the specific surface area reaches 22.62 m2 g-1.The electrochemical properties of LDH with different Co and Fe mole ratios were investigated by electrochemical tests(ECSA,LSV,etc.).The results show that material with a Co and Fe mole ratio of of 5:5 has a large ECS A value(2 CdI=615.3 ?F cm-2).(3)In the single-chamber electrolysis cell without diaphragm,the above-mentioned CoFe-LDH modified membrane electrode was used as the cathode,and the results of ammonia synthesis using N2 and H2O as raw materials showed that the electrolysis conditions such as cathode potential,temperature and N2 flow rate had an effect on the performance of ammonia synthesis.Under optimized electrolysis conditions:the cell pressure is 2.1 V,the reaction temperature is 80?,the N2 flow rate is 60 mL min-1,the KOH concentration is 17.8 mol L-1,and the CoFe-LDH with Co and Fe ratio of 5:5 is a working electrode,and the synthesis rate and faradaic efficiency were 1.10×10-9 mol s-1 cm-2 and 0.095%,respectively.