Preparation Of Copper-based Nanomaterials And Their Performance In Electrocatalytic Reduction Of Nitrate

Ammonia（NH₃）is an important agricultural feedstock and a promising clean energy source.Because ammonia has greater energy density and lower transportation cost than hydrogen,it is also considered a promising next-generation energy carrier.Currently,NH₃ is mainly synthesized by the Haber-Bosch method,in which N₂ and H₂ are used as raw materials,and iron is used as a catalyst under high pressure and high temperature conditions.Due to the high greenhouse gas emissions and high energy consumption during the synthesis process,researchers are looking for a more economical and environmentally friendly alternative method.The electrochemical synthesis of NH₃ is currently attracting attention because of its low energy consumption and zero pollution.In recent years,electrocatalytic nitrogen reduction reaction（NRR）is a new strategy for the synthesis of NH₃ that has received much attention.However,the high dissociation energy due to the stable N≡N bond,and the weak adsorption of N₂ on the catalyst with ultra-low water solubility lead to the poor efficiency of NRR.On the other hand,nitrate（NO₃^-）is not only a common source of nitrogen in nature but also widely present in industrial and domestic wastewater,which affects human health.In addition,the enrichment of NO₃^-has caused serious environmental problems,such as eutrophication and global acidification.Unlike N₂,NO₃^-has high water solubility and the dissociation energy of N=O bond(204 k J mol^-1)is 4.6 times lower than that of N≡N bond(941 k J mol^-1),which makes it more reactive.Therefore,the NO₃^-reduction reaction（NO₃RR）has greater potential than NRR as an alternative to the Haber-Bosch method.Transition metal-based electrocatalysts have gradually become a potential alternative to precious metals because of their low cost,high activity and abundant resources.Especially,copper-based materials with high activity and high nitrite selectivity in alkaline electrolytes,their low starting potential,high adsorption capacity for nitrate and relatively low HER activity have good prospects for the application of electrocatalytic NO₃RR.However,problems such as corrosion of equipment due to strong alkalinity limit its application in actual production.Therefore,this work aims to seek new copper-based electrocatalysts with excellent electrocatalytic performance in neutral media to promote the large-scale production application of electrocatalytic reduction of nitrates to ammonia.We focus on improving the intrinsic activity and selectivity of copper-based catalysts by adjusting the components.The main contents and conclusions are shown below:（1）First,Cu HN/NF electrocatalysts were prepared by rapidly growing copper nitrate on nickel foam by a simple molten salt method,and then R-Cu HN/NF nanorods containing oxygen vacancies were formed by in-situ electrochemical reduction.It was found that R-Cu HN/NF catalyst showed good electrocatalytic performance of NO₃RR in 0.1M PBS and 50 m M Na NO₃solution in a three-electrode H-type electrolytic cell.The NH₃ yield of the catalyst was 20.14mg h^-1 cm^-2 at-0.95 V（vs.RHE）,while the Faraday efficiency（FE）was 99.38%at-0.55 V（vs.RHE）.The produced NH₃ was verified to be derived from nitrate reduction using the ¹⁵N isotope ¹H NMR..The theoretical calculation（DFT）shows that the high selectivity of NH₃ is enhanced by electron redistribution,abundant oxygen vacancy and favorable charge/mass transfer during electrochemical reduction.（2）On the basis of R-Cu HN/NF catalyst,heterojunction electrocatalyst(Cu₂₊₁O@Co₃O₄/NF)containing oxygen vacancy nanoarray was further prepared by in situ chemical oxidation,electrodeposition and mild annealing in three steps,and the same synthesis conditions,and heterojunction electrocatalyst Cu₂₊₁O@Co₃O₄/CF on copper foam（CF）substrate was prepared,and the comparative study found that Cu₂₊₁O@Co₃O₄/NF was significantly better than Cu₂₊₁O@Co₃O₄/CF.There was high Faraday efficiency（FE）of 99.78%and high NH₃ yield of 21.37 mg h^-1 cm^-2 in solutions of 0.1 M PBS and 50 m M Na NO₃ with Cu₂₊₁O@Co₃O₄/NF at-0.85 V（vs.RHE）.The ¹⁵N isotope ¹H NMR also clarified that the NH₃produced originated from the reduction of nitrate.In combination with the characterization,it was found that its superior electrocatalytic performance should be attributed to the synergistic effect of electron transfer at the oxygen vacancy and heterojunction interfaces,which inhibits the occurrence of hydrogen precipitation reaction（HER）.（3）By means of hydrothermal reaction,Ni（OH）₂ was loaded on Cu₂₊₁O@Co₃O₄/NF nanoarrays,and 3D nanofloral heterojunction with interleaved nanowires（Cu O@Co₃O₄@Ni（OH）₂/NF）was obtained.The results showed that the catalyst exhibited good electrocatalytic performance from NO₃^-to ammonia under both neutral and alkaline conditions:in 0.1M PBS solution,the NH₃ yield reached the maximum 21.37 mg h^-1 cm^-2 at-0.95 V（vs.RHE）,and the Faraday efficiency（FE）was 94.19%.At-0.45 V（vs.RHE）,NH₃ yield and Faraday efficiency（FE）were 11.11 mg h^-1 cm^-2 and 96.91%,respectively.While the performance of 50 m M NO₃^-reduction with 1.0 M Na OH as electrolyte found a Faraday efficiency（FE）of 94.85%at-0.28 V（vs.RHE）and 24.49 mg h^-1 cm^-2 NH₃ yield at-0.58 V（vs.RHE）.These results indicate that the nanowire structure of Cu O@Co₃O₄@Ni（OH）₂/NF catalyst can expose more active sites and facilitate electron transport,thus improving the electrocatalytic performance of NO₃RR.