Design And Preparation Of Copper-cerium Bimetallic Electrocatalyst For Nitrogen Reduction Reaction

In recent years,electrocatalytic ammonia synthesis is a green and environmentally friendly ammonia synthesis method.The reaction rate can be adjusted by voltage when water is used as a hydrogen source.It is considered to be expected to replace the industrial ammonia Haber-Bosch method.At present,the electrocatalytic ammonia synthesis reaction can be carried out at room temperature and under normal pressure,but the currently used catalysts are very limited in the rate of ammonia production and the Faraday efficiency.The difficult-to-activate nitrogen molecules and the accompanying competitive hydrogen evolution reaction during the reaction have become the difficulties to overcome in the field of synthetic ammonia.Therefore,it is urgently requested to develop effective strategies to rationally design efficient catalyst systems,with optimized catalytic active sites on the catalyst surface so as to enhance the N₂ adsorption and activation for improving the nitrogen reduction performance while inhibiting the hydrogen evolution side reaction.This thesis developed three types of Cu-Ce bimetallic non-precious metal-based electrocatalysts including bimetallic Cu@Cu/Ce-MOFs,oxygen vacancy-riched Cu@Cu₂O-CeO₂-C,and fluoride ion-modified oxygen vacancy-riched Cu@Cu₃P-Ce PO₄:F growth on a Cu mesh substrate for catalyzing the nitrogen reduction reaction（NRR）.The main content of this thesis includes the following aspects:In the first part of the thesis,a simple room temperature in-situ synthesis method was used to prepare a self-supporting bimetallic copper and cerium MOF catalyst（Cu@Cu/Ce-MOFs）on a copper mesh.By adjusting the molar ratio of Cu/Ce,the optimum Cu@Cu/Ce-MOFs electrocatalyst used as a cathode electrode showed an efficient catalytic performance for NRR with an ammonia yield of 14.83μg h^-1 cm^-2and Faraday efficiency of 12.45%at-0.20 V（vs.RHE）in 0.10 M KOH solution.The self-supported monolithic Cu@Cu/Ce-MOFs electrocatalyst with a mesoporous MOF structure and a macroporous Cu mesh substrate can largely improve its conductivity and mass transfer efficiency with a large number of Cu/Ce catalytic active sites formed on the surface,thus greatly promoting the NRR catalytic performance.In the second part of this thesis,the Cu@Cu₂O-CeO₂-C electrocatalyst with an oxygen vacancy-riched porous hybrid structure on a Cu mesh substrate via the pyrolysis of the above synthesized Cu@Cu/Ce-MOFs.The Cu@Cu₂O-CeO₂-C electrocatalyst showed a superior electrocatalytic performance for NRR with an ammonia yield of 22.73μg h^-1cm^-2 and a Faraday efficiency（FE）is 18.21%at-0.30 V（vs.RHE）in 0.10 M KOH,which was higher than those similar electrocatalysts reported lately.The superior electrocatalytic performance of the Cu@Cu₂O-CeO₂-C electrocatalyst for NRR may be derived from the coupling of CeO₂ with Cu₂O to provide more active sites and simultaneously modify the electronic structure of Cu₂O for boosting the NRR electrocatalytic activity.Its catalytic mechanism combined with acid-base theory,the nitrogen with a lone pair of electrons is the Lewis base,which can chemically adsorb the Lewis acid or the hydrogen bond modified on the catalyst surface.The oxygen vacancy is generally positively charged and can be used as the Lewis acid.In the third part of this thesis,to further improve the NRR electrocatalytic performance,the fluoride ion was introduced into Cu@Cu-MOF-Ce PO₄ through a hydrothermal method following a pyrolytic phosphating process to obtain a fluoride ion-doped and oxygen vacancy-riched Cu@Cu₃P-Ce PO₄:F electrocatalyst.Compared with the Cu@Cu₃P-Ce PO₄ electrocatalyst,the Cu@Cu₃P-Ce PO₄:F electrocatalyst exhibited a higher ammonia yield of 28.64μg h^-1 cm^-2 and a higher Faraday efficiency of 48.26%at-0.25 V（vs.RHE）in 0.05 M H₂SO₄ solution.The introduction of F ions endowed more catalytic active sites to improve the catalytic selectivity and stability while inhibit the the competitive reaction.