Research On La_0.7Sr_0.3Fe_0.8Co_0.2O_3-δ Based Catalyst For Electrochemical Ammonia Synthesis

Compared with hydrogen,ammonia（NH₃）is a hydrogen carrier with the feature of easy storage and zero carbon emissions,which is of great significance for the construction of future hydrogen energy society.The current ammonia synthesis method based on the Bern-Haber method is highly energy-consuming and polluting,so it is urgent to find a green and pollution-free ammonia synthesis method.The Solid Oxide electrolysis cell（SOEC）offers an environmentally friendly solution for ammonia synthesis through the electrochemical catalytic reaction of nitrogen and hydrogen（or water vapor）at medium-high temperatures.However,the currently reported electrode catalysts have low activity and strong competitive hydrogen precipitation reaction（HER）,resulting in very low ammonia synthesis rate and Faraday efficiency（FE）.SOEC electrode catalysts with high catalytic activity of nitrogen reduction reaction（NRR）and inhibition of HER activity need to be developed to achieve efficient synthesis of NH₃.The first step in the electrochemical catalytic reduction of nitrogen is adsorption on the catalyst surface,followed by the activation of nitrogen.Due to the high bond energy of the nitrogen-nitrogen triple bond,which is not easily broken,a suitable catalyst needs to be designed for the activation of the N≡N bond.It has been reported that the perovskite oxide La_1-xSr_xFe_yCo_1-yO_3-δ（LSFC）has good electrochemical catalytic performance,and the element Ni has a catalytic effect on the NRR reaction and Bi has an inhibitory effect on the hydrogen precipitation reaction.Based on the catalytic effect of oxygen vacancy on nitrogen adsorption and activation,La_0.7Sr_0.3Fe_0.7Co_0.2Ni_0.1O_3-δ（LSFCNi）,La_0.7Sr_0.3Fe_0.7Co_0.2Bi_0.1O_3-δ（LSFCBi）as two kinds of perovskite oxide materials and SOEC cathode was designed and synthesized,with Ba Zr_0.1Ce_0.7Y_0.7Yb_0.1O_3-δ（BZCYYb）as electrolyte and Ni-BZCYYb as anode in this paper.the electrochemical performance of these two catalysts for ammonia synthesis was systematically analyzed by studying the NH₃ generation rate and Faraday efficiency（FE）of SOEC at different temperatures and different applied voltages.1.The performance of LSFCNi for ammonia synthesis when used as SOEC cathode was investigated.The XPS characterization indicated that a large number of oxygen vacancies were formed in LSFCNi,which would be conducive to the adsorption and activation of nitrogen.The electrochemical synthesis of ammonia using LSFCNi as SOEC cathode was studied.N₂ at a flow rate of 60 mL min^-1 to the SOEC cathode and H₂ at a flow rate of 30 mL min^-1 to the anode.The ammonia synthesis rates were measured at different temperatures（550℃～650℃）and applied voltages（0.2V～0.8V）.The results show that the rate of electrochemical ammonia synthesis is the highest at 600℃with0.2V voltage,and the Faraday efficiency is the highest with 0.4 V voltage,which are4.1×10^-9 mol s^-1 cm^-2 and 4.6%,respectively.LSFCNi single cell is stable and its microstructure is not damaged after 100 h constant voltage operation at 600℃and 0.2 V voltage.It is proved that the cathode material is a good performance catalyst for electrochemical ammonia synthesis.2.The performance of LSFCBi for ammonia synthesis when used as SOEC cathode was studied.The linear sweep voltammetry（LSV）test showed that the catalyst doped with Bi could inhibit HER to some extent.Electrochemical synthesis of ammonia using LSFCBi as SOEC cathode catalyst was studied.The results show that when 0.4 V voltage is applied at 600℃,the maximum ammonia synthesis rate and Faraday efficiency are6.8×10^-9 mol s^-1 cm^-2 and 6.7%,respectively.The results showed that LSFCBi catalyst increased the activity of NRR and had a certain inhibitory effect on HER.The catalytic performance of LSFCBi is better than that of LSFCNi,which improves the ammonia synthesis rate and Faraday efficiency to a certain extent.The stability of LSFCBi single battery was tested when 0.4 V voltage was applied at 600℃.The structure of the battery is intact after 100 h.In summary,two kinds of perovskite materials LSFCNi and LSFCBi as SOEC cathode catalysts can both catalyze the NRR reaction for the electrochemical synthesis of NH₃.LSFCBi has certain inhibitory effect on HER.This work provides some theoretical support for the design of NRR catalyst materials.