The Controllable Synthesis Of Nitrogen-Doped Carbon Based Nanofibers And Its Application For Electrocatalytic Ammonia Synthesis

Ammonia（NH₃）is an important industrial feedstock chemical,as a fertilizer in agriculture,a carbon-free energy carrier with high energy density.At present,ammonia synthesis is an important chemical technology with great significance,but it still relies on the traditional HaberBosch process,whose annual consumption accounts for 1～2%of the world’s energy consumption and releases a large amount of greenhouse gas-CO₂.Given the shortage of fossil fuels and global climate change,a clean,sustainable and efficient route to nitrogen fixation is in urgent.Electrochemical nitrogen reduction reaction（NRR）is a very attractive sustainable strategy,as it utilizes renewable electricity to convert water and N₂ to NH₃ under ambient conditions.In particular,utilizing H₂O as the hydrogen source,rather than raw hydrogen,can greatly reduce energy-intensive hydrogen production processes through methane or natural gas combustion,as well as fossil fuel and greenhouse gas emissions.However,to date,the NH₃ yield via NRR remains unsatisfactory.Therefore,it is necessary to develop high-efficiency and stable catalysts to improve the overall NRR catalytic performance.In this paper,the catalytic active sites of nitrogen doped carbon nanofiber catalysts were rationally regulated,which strengthen the adsorption and further hydrogenation process of N₂ molecules,thereby improving the conversion of N₂ to NH₃.A metal-free B,N co-doped porous carbon nanofiber（B/N-CNF）was developed by an electrospinning-calcination strategy.The contents of B and N dopants for the B/N-CNF were 29.9%and 27.6%,respectively.Beisdes,benefitting from a high surface area of 748 m² g^-1,B/N-CNF exhibits excellent electrocatalytic performance for NRR in alkaline electrolyte,in which the maximum Faradaic efficiency（FE）of 13.2%at-0.5 V and the NH₃ yield of 32.5 μg h^-1 mgcat^-1 at-0.7 V are achieved.Among them,the B-N bonds play a critical role in the NRR process,in which N atoms promote the electron conductivity and B atoms efficiently enhance the N₂ adsorption and charge transfer,thereby improving the whole NRR activity.An unusual NRR electrocatalyst with a single Zn（Ⅰ）site supported on hollow porous N-doped carbon nanofibers（Zn¹N-C）was synthesized by hydrothermal treatment of pre-synthesized ZIF8 supported on nitrogen-doped carbon nanofibers.The Zn¹N-C nanofibers exhibit an outstanding NRR activity with a high NH₃ yield rate of～16.1 μg NH₃ h^-1 mgcat^-1 at-0.3 V and Faradaic efficiency（FE）of 11.8%in alkaline media.Atomically dispersed Zn（Ⅰ）sites with Zn-N4 moieties serve as real active sites,and nearby graphitic N site significantly lowers the energy barrier for the formation of initial*NNH intermediate to accelerate hydrogenation kinetics,synergistically facilitates the NRR activity.A novel NRR catalyst consisting of atomically dispersed iron single-site embedded in porous N doped carbon nanofibers with abundant carbon defects（D-FeN/C）was synthesized via the introduction of iron species.The D-FeN/C catalyst achieves a remarkably high NH₃ yield rate of～24.8 μg h^-1 mgcat^-1 and Faradaic efficiency（FE）of 15.8%at-0.4 V in alkaline electrolyte.Intrinsic carbon defects dramatically enhance the water dissociation process and accelerate the protonation kinetics of D-FeN/C for NRR.Atomic Fe-N4 catalytic sites together with intrinsic carbon defects synergistically reduce the energy barrier of the protonation process and promote the proton-coupled reaction kinetics,thus boosting the whole NRR performance.A Fe single-atom anchored on nitrogen-doped carbon nanofiber catalyst containing molybdenum carbide particles（MoC-FeN/C）was synthesized by effectively controlling the electronic structure of Fe single-atom catalyst by molybdenum carbide loading.The MoC-FeN/C catalyst achieves the highest FE of 20.5%and NH₃ yield of 25.1 μg h^-1 mgcat^-1 at-0.3 V.The excellent NRR activity is mainly derived from the synergistic effect of the single-atom dispersed Fe-N4 active sites and MoC particle,which effectively accelerates the water dissociation process for proton spillover effect,thus N₂ protonation process wassignificantly boosted and NRR performance was effectively improved.