Electrocatalytic NO₃^- Reduction Performance Of Co/Ni-based Materials Enhanced By Vacancy/Interface Effect

Fossil energy is like a double-edged sword.While promoting the rapid development of human society,it also causes immeasurable harm to the environment.The disadvantages and non-renewability of fossil energy make it an irresistible trend to change the energy structure to renewable energy.Ammonia has a higher volume energy density and more mature storage and transportation technology than the ultimate clean energy（H₂）,which is expected to alleviate the increasingly prominent environmental and energy crisis.Additionally,ammonia is also an important chemical used in many areas of life.At present,people are actively exploring various green ammonia synthesis technologies to replace the dominant Haber-Bosch process with high energy consumption and heavy pollution.Electrocatalytic nitrate reduction（NITRR）into ammonia is one of them,which can realize the conversion of nitrate contaminant into high value-added ammonia,helping to alleviate environmental problems and energy crisis.In addition,NITRR possesses a broader application prospect owing to the lower energy barrier,higher efficiency and ammonia yield rate compared with electrocatalytic N₂reduction（NRR）.However,the complexity of the nitrate reduction process and the intense HER competition reaction seriously hinder the further raising of ammonia selectivity and Faraday efficiency（FE）.Therefore,it is urgent to develop highly active NITRR electrocatalysts and further explore their reaction mechanism.Transition metals with abundant unpaired d-orbital electrons are the focus of research in nitrate reduction.Particularly,the prominent NITRR activity and stability of precious metal-based transition metal catalysts,such as Pt,Pd and Ru,have attracted extensive attention from researchers.However,the scarcity and high cost of precious metals limit their large-scale application in industry,while the abundant reserves and low cost of non-precious metals exhibit great advantages.Among them,the Co-and Ni-based catalysts present outstanding catalytic activity.Although numerous measures have been considered to optimize their intrinsic activity,shortcomings such as inadequate exposure of active sites and poor conductivity still exist,resulting in considerable scope for enhancement of catalytic activity.Therefore,the objective of this work is to tailor the electronic structure of transition metal Co-based and Ni-based catalysts by vacancy engineering and interface engineering strategies to further enhance their NITRR activity.Specific research contents are as follows:（1）A Co₄N（Co₄N/NF）electrocatalyst supported by nickel foam（NF）with high conductivity and high porosity was designed for nitrate reduction based on the advantages of high conductivity,corrosion resistance,diversity of valence states and noble-metal-like behavior of non-noble metal-based transition metal nitrites.The experimental data illustrate that the surface morphology of Co₄N/NF catalyst changes obviously after NITRR,and it is transformed into nanosheet with smaller size and larger specific surface area.More importantly,a large amount of N is dissolved in the electrolyte during the electrochemical reduction process,and this irreversible dissolution induces enriched nitrogen vacancies in the crystal structure of Co₄N.The exposure of more reactive sites caused by morphological changes and the modulation of electronic structure by nonmetallic vacancies induced by the dissolution of nonmetallic species significantly enhance the NITRR activity of the catalyst in neutral environment.The optimized Co₄N/NF delivers an optimal FE of 95.4%and an ammonia selectivity of 99.4%at-0.44 V vs.RHE in 0.5 M K₂SO₄electrolyte with200ppm KNO₃-N,much higher than most catalysts reported to date.A series of comparison samples including Co P/NF,Co₉S₈/NF and V_s-Co₉S₈/NF were also designed to further verify the rationality of in situ nonmetal leaching-induced strategy to improve the activity of NITRR.The theoretical calculations indicate that the nitrogen vacancy can adjust the adsorption strength of intermediates and strengthen the adsorption of atomic hydrogen by optimizing the electronic structure of the catalyst,thus promoting the selective reduction of NO₃-to NH₃.（2）Since the heterogeneous catalytic reaction occurs on the surface of the catalyst,the catalytic activity and stability can be optimized by adjusting the surface properties of the catalyst,and the interface engineering strategy is an effective method.Due to the diversity of valence states and half-full d-orbitals of Mo,high conductivity and excellent durability of MoO₂,high NITRR activity of Ni-based catalyst and poor HER activity of MoO₂,coupling MoO₂with Ni may result in excellent NITRR activity.On the basis of these characteristics,MoO₂/Ni（MoO₂/Ni@NF）heterojunction micron rod arrays loaded on porous NF were synthesized by a two-step hydrothermal-hydrogen reduction method.The developed MoO₂/Ni@NF can be used as an efficient NITRR electrocatalyst,achieving high FE of 95.9%and ammonia selectivity of 96.9%at-0.3V vs.RHE in neutral electrolyte,much higher than its comparison samples and better than most reported NITRR catalysts.The experimental results demonstrate that MoO₂combined with Ni can induce electron redistribution and overcome the defect of excessive hydrogen evolution in pure Ni electrode.Additionally,the richer active sites and faster charge transfer kinetics of MoO₂/Ni@NF contribute to the further improvement of nitrate reduction activity.Theoretical calculations demonstrate that the heterostructure can optimize the desorption energy of ammonia and effectively inhibit HER.Therefore,the construction of electronically regulated MoO₂/Ni through interface engineering is an effective strategy to enhance the activity of NITRR.