Preparation Of Self-Supporting Nickel-Based Electrode And Efficiency Enhancement Strategy Of Electrocatalytic Nitrate For Ammonia Production

Under the background of energy shortage crisis and environmental pollution,the research on sustainable energy and resource utilization of pollutants is more urgent.Ammonia（NH₃）,as a low-cost chemical raw material,has the advantages of high energy density,no carbon dioxide generated by combustion,and easy compression,storage and transportation.It is one of the energy sources that is expected to meet the future energy demand of the world.At present,ammonia is mainly produced by the Haber-Bosch process,which consumes fossil energy and releases CO₂,which is not in line with the concept of“carbon neutrality”.Among the methods of preparing ammonia,electrochemical technology can be driven by electricity generated by clean and renewable energy（such as solar）,and utilize the interconversion of H in H₂O to store and transport energy,basically without carbon emissions.The above advantages make the electrochemical technology for production of ammonia become one of the research hotspots in the field of N resource utilization.At the same time,the accumulation of nitrate（NO₃^-）in groundwater and surface water is a serious threat to human health.Therefore,technology of electrocatalytic NO₃^-for its reductive conversion to recyclable NH₃under mild conditions is of great significance.However,the complex eight-electron transfer pathway and the competing hydrogen evolution reaction reduce the selectivity and efficiency of this reaction.Meanwhile,high operating costs,such as the bottleneck of EC,have been hindering the practical development of this technology.Improving NH₃yield and Faradaic efficiency as well as reducing cell voltage are the most critical elements to achieve energy savings.In view of the needs of improving the efficiency of NH₃synthesis and reducing EC via ERN,starting from the improvement of the most critical performance factors,this paper rationally designs the cathode materials and anode materials for NH₃synthesis via ERN,and the strategy of replacing OER with thermodynamically advantageous anodic half-reactions such as UOR and S（Ⅳ）OR to reduce the EC of NH₃synthesis.Therefore,this work may open up a new avenue for constructing efficient electrocatalysts to selectively convert nitrate to high value-added ammonia,providing a theoretical basis for clean carbon-free energy and nitrogen recovery.Based on the above results,the conclusions are drawn:（1）NFP nanoblocks were in situ constructed on a NF substrate by one-step phosphating.The NFP self-supporting cathode exhibits good ERN performance for NH₃production at-1.2 V,with NH₃selectivity（SE%）of 89.1%,Faradaic efficiency（FE%）of 95.4%,NH₃yield rate（Y）of 0.056 mmol h^-1mg^-1,and EC of 11.098k Wh/kg.Through the masking experiment and the test of Electron spin resonance（ESR）technology,it is proved that the atomic hydrogen（*H）generated in situ in the system effectively improved the removal of NO₃^-and the selectivity to NH₃.DFT calculations revealed that the（111）plane in Ni₂P crystals is the active site in ERN for NO₃^-adsorption and subsequent hydrogenation reduction reactions.Cycling experiments in real wastewater demonstrate the good stability of NFP as an ERN electrocatalyst.（2）In order to further improve the catalytic activity of ERN,the microstructure of cobalt and nickel-based catalysts was designed to increase the number of catalytic reaction active sites and enhance the synergy between multi-element atoms to improve their electrocatalytic activity.The Mn-CoNiLDH was formed on the NF substrate by a simple hydrothermal method,and after low-temperature phosphating,the Mn-CoNiP with three-dimensional micro-nano flower-like structure was finally formed.The interconnected flower-like structures provide fast channels for charge transfer on the NF substrate and also fully expose the active sites,thereby promoting the electrocatalytic activity.The Mn-CoNiP/NF cathode achieved a maximum R%of91.5%,a SE%of 92.1%,a FE%of 96.5%,a Y of 0.0589 mmol h^-1mg^-1,and EC of6.0364 k Wh/kg at-1.1 V.*H quenching experiments and the results of ESR indicated the direct electron transfer and active hydrogen reduction mechanism of nitrate.Mn-CoNiP/NF showed unique surface superhydrophilicity,low impedance,abundant active sites,metallicity,and unique d-electron structure,which reduces the overpotential for ammonia production via ERN and improves the FE as well as NH₃production.yield,thereby enhancing the ammonia production efficiency of ERN.The results of isotope tracer experiments confirmed that the N in the ammonia synthesized via ERN all came from the N in NO₃^-.Through the on-line detection of the gaseous products produced by ERN,it was found that only NH₃,NO and H₂were produced.Mn-CoNiP/NF exhibited excellent cycling stability.（3）In order to accelerate the sluggish anodic half-reaction OER kinetics of the Mn-CoNiP/NF cathodic ammonia production system,it is crucial to develop highly active and stable OER catalysts and strategies to replace OER with UOR.Co-Ni₃S₂/NF self-supporting anode material with three-dimensional nanosheet network structure was successfully prepared.The electrochemical characterization analysis showed that the Co-Ni₃S₂/NF anode had a high electrochemical active area（119.54cm²）and a small charge transfer resistance The Co-Ni₃S₂/NF anode exhibited good OER and UOR catalytic activities.And compared with OER,the voltage of UOR decreased significantly by 149 m V.The ERN+OER and ERN+UOR systems with Mn-CoNiP/NF cathode and Co-Ni₃S₂/NF anode both showed excellent ERN for NH₃production performance.Compared with the ERN+OER system with Mn-CoNiP/NF cathode and Pt anode,the EC of NH₃production decreased by 13.4%and21.7%respectively.By studying the effect of OER and UOR properties of Co-Ni₃S₂/NF anode on ERN for NH₃production via ERN with Mn-CoNiP/NF cathode,it was confirmed that the anode half-reaction can reduce the oxidation equilibrium potential and overpotential by improving its reaction kinetics,thereby reducing the cell as well as improving the efficiency of ERN.（4）To further enhance the efficiency of ERN for NH₃production,a variety of thermodynamically prone anodic reactions（UOR,S（Ⅳ）OR,S（Ⅳ）/UOR）were used to replace OER,respectively.The amorphous ANP self-supporting material,as a multifunctional catalyst,was prepared by one-step phosphating method.ANP showed excellent catalytic activity in ERN,OER,UOR and S（Ⅳ）OR.In order to simplify the preparation process of different electrodes and decrease the cost.multi-functional ANP electrode was used for multiple purposes,and it was used as both cathode and anode for ERN experiment.The results showed that OUR saved 17.5%energy compared to OER system;S（Ⅳ）OR saved 27.6%energy compared to OER system.Compared with the OER system,the yield rate of NH₃increased by 7.1%;compared with the OER system,S（Ⅳ）/UOR saved 32.1%energy,and the yield rate of NH₃increased by 12.6%compared with the OER system.The difference in EC for the yield rate of NH₃production in these systems were also mainly due to the difference in electrolytic cell voltage and yield rate of NH₃.This proved that the addition of urea and sodium sulfite can reduce the voltage required to drive the electrolytic cell,thereby reducing EC.