Study On Atomic-level Structure Regulation Of Electrochemical Nitrogen Fixation Catalysts And Their Reaction Mechanism

Ammonia,one of the most basic chemical raw materials,can not only be used in the production of fertilizers,but also considered as an ideal hydrogen energy carrier in the future.From the perspective of economic development and/or human scientific and technological progress,it is very important to develop green and efficient ammonia synthesis technology.Electrochemical nitrogen fixation technology has recently attracted widespread attentions in the world due to the mild synthesis conditions,rich sources of raw materials（N₂ and H₂O）,simple craft,environmental friendliness and so on.In addition,such technology drived by clean electric energy can be well compatible with the intermittent nature of renewable energy,which is expected to realize distributed and modular synthesis of ammonia.However,its development is seriously impeded by the robust N≡N triple bond with extremely high bond energy and the competitive hydrogen evolution reaction.The core problem is the lack of efficient and stable nitrogen fixation catalysts.Herein,the key role of"cationic Mo vacancy"in enhancing nitrogen fixation activity,the promotion of nitrogen fixation reaction efficiency by"low coordination step atom"and the optimization of nitrogen fixation reaction performance by fine"core-shell effect"have be studied in detail by the atomic-level structure regulation of catalysts.The details are as follows:（1）Taking advantage of the disparity in nitride formation conditions between silicon oxide and molybdenum oxide,and the difference in electronegativity of O and N elements,a simple and effective chemical etching-sacrificial hard template strategy is proposed to introduce cation Mo vacancies on the surface of Mo N nanocrystals（MV-Mo N@NC）,and employed it for the first time in the study of electrochemical nitrogen reduction reaction（e NRR）.Compared with the anionic vacancy e NRR catalyst reported in the previous literature,MV-Mo N@NC exhibits impressive e NRR activity,in which the maximum NH₃ yield rate is 76.9μg h^-1 mg^-1_cat..Meanwhile,this catalyst has also high Faraday efficiency（FE）of 6.9%and outstanding electrochemical stability.In addition,the Mars-van Krevelen pathway during NRR process over MV-Mo N@NC is identified by joint ¹⁵N₂ isotopic tracer experiments with nuclear magnetic resonance spectroscopy.First-principles calculations reveal the critical role of Mo vacancy in regulating the electronic properties of Mo N and shifting the rate-limiting step of NRR that significantly reduces the reaction barrier from 1.40 e V to 0.61 e V.（2）A unique strategy of plasma-assisted calcination and sacrificial hard-template is developed to synthesize iridium diphosphide nanocrystals with abundant surface step atoms anchored in P,N-codoped porous carbon nanofilms（Ir P₂@PNPC-NF）,where the edges of the Ir P₂ nanocrystals are extremely irregular,and the ultrathin PNPC-NF possesses a honeycomb-like macroporous structure.These characteristics ensure that Ir P₂@PNPC-NF delivers superior NRR performance with an NH₃ yield rate of 94.0μg h^-1 mg^-1_cat.at-0.2 V vs.RHE and a FE of 17.8%at-0.1 V vs.RHE.Density functional theory calculations reveal that compared with the platform surface（111）,the low-coordination step surface（313）is more conducive to the adsorption/activation of N₂ molecules.Such unique NRR performance originates from the low-coordinate step atoms on the edges of Ir P₂ nanocrystals,which can lower the reaction barrier to improve the NRR activity and simultaneously inhibit hydrogen evolution to achieve a high FE for NH₃ formation.In addition,such a plasma-assisted strategy is general and can be extended to the synthesis of other high-melting-point noble-metal phosphides with abundant step atoms.（3）Through precise-time nitridation control,a series of 2D porous core-shell V₂O₃/VN nanomeshs with gradient thickness of nitride shell are rationally constructed,and the influence of the core-shell effect on e NRR activity is studied for the first time.Interestingly,as the thickness of the nitride shell increases,the e NRR activity of the core-shell material first increases and then decreases.Among them,the sample V₂O₃/VN-2 with a nitriding time of 2 min has the best e NRR performance with an NH₃yield rate of 59.7μg h^-1 mg^-1_cat.at-0.4 V vs.RHE and a FE of 34.9%at-0.2 V vs.RHE.To achieve the intrinsic activity for each sample under different voltages,the concept of the roughness factors（RFs）is introduced by normalizing their NH₃ yield rates using electrochemical active surface area（ECSA）.Similarly,under the same voltage,their intrinsic activitys still show a trend of first increasing and then decreasing,suggesting that only the sample with appropriate thickness of nitride shell has a best intrinsic NRR activity.In addition,the consecutive eight cycle tests and the long-term constant-voltage test jointly confirm that V₂O₃/VN-2 has a good stability.Furthermore,the ¹⁵N isotope labeling experiment also confirm the possible Mv K reaction path in the catalytic process.In summary,based on the understanding of the electrochemical nitrogen fixation reaction,a reasonable design idea for fine-tuning high-efficiency catalysts is proposed.Nither NH₃ yield rate or FE,the e NRR performance has been significantly improved.The implementation of this subject provides a certain scientific basis and theoretical guidance for the development of novel high-performance e NRR catalysts and the research on the mechanism of nitrogen fixation reaction.