Preparation And Electrochemical Catalytic Reduction Performance Of High-efficiency Iron-based Electrodes

Given the escalating concerns surrounding environmental pollution and energy scarcity,there is a pressing need for the advancement of efficient technologies that promote energy conservation and emission reduction.Since conversion capacity and reaction rate,electrochemical technology has potential applications in environmental treatment and clean energy development.Electrode material play a crucial role in the application of electrochemical technology.Iron（Fe）-based materials have received particular attention in nitrate（NO₃^-）wastewater treatment and oxygen reduction reaction due to the properties of cheapness,environmentally-friendly and excellent electrochemical reduction activity.However,the prospects of Fe-based materials have been limited due to factors such as difficulties in large-scale synthesis,unsatisfactory catalytic activity and challenging product recovery.Therefore,in this thesis,natural Fe ores were used as raw materials to prepare electrodes and to construct an efficient electrochemical NO₃^-reduction（NO₃^-RR）system for ammonia（NH₄⁺/NH₃）production and in-situ recovery,thus realizing resourceful disposal of high concentration NO₃^-wastewater.Meanwhile,a new synthesis method of layered Fe single-atom catalyst（Fe SAC）with high loading was developed in this thesis.The strategy of interlayer iron-nitrogen structure（Fe N₄）enhanced electrochemical activity was first announced,which significantly improved the performance of NH₃production via NO₃^-RR and oxygen reduction reaction（ORR）,providing a new theoretical basis and technical method for resourceful disposal of NO₃^-and efficient conversion of clean energy.The main study contents and findings are as follows:（1）Based on the principle of NH₄⁺/NH₃ morphological transformation at the electrode interface,the electrochemically coupled membrane system with hematite（α-Fe₂O₃）as the electrode was constructed to solve the problems of low NO₃^-RR activity and difficult product NH₃ separation and recovery in the application of traditional iron-based electrodes,and realize the resourceful treatment of NO₃^-wastewater.The system showed high NO₃^--N conversion,NH₄⁺-N selectivity and recovery of 97.3,84.6%and81.7%,respectively,for the treatment of NO₃^--N at 1000 mg L^-1.Meanwhile,the energy consumption per kg of NH₃ synthesized was 62.2k Wh,representing one of the lowest values reported thus far.Importantly,electrochemical analysis and theoretical calculations indicate that the conversion of NO₃^-to NO₂^-is the rate-limiting step of NO₃^-RR.In addition,Fe²⁺/Fe³⁺cycle-mediated NO₂^-reduction is the key to selective NH₄⁺production.Notably,this study also constructed a NO₃^-RR system using natural hematite as the electrode,and the NO₃^--N conversion,NH₄⁺-N selectivity and recovery were as high as 93.7%,70.6%and 81.4%,respectively.In summary,this study provides an important reference for the resource-based treatment of NO₃^-wastewater.（2）Based on the high electronic conductivity and chemical stability of spinel structure,a high-temperature modulated ilmenite（Fe Ti O₃）crystal phase transformation strategy is proposed to solve the poor stability of natural hematite.After treatment at 900°C,the main phase of natural Fe Ti O₃ was transformed into Fe₂Ti O₅ and formed a heterojunction withα-Fe₂O₃ and Ti O₂,which was subsequently used as an electrode to construct an electrochemically coupled membrane system.The conversion of NO₃^--N,selectivity and recovery of NH₄⁺-N at 1000 mg L^-1of NO₃^--N were98.4%,82.3%and 83.7%,respectively.Meanwhile,the system has also been proven to operate continuously and stably for 400h（50 cycles）.Clearly,the results of NMR and electrochemical analysis not only confirm that NH₃ originates from NO₃^-,but also indicate that the synergistic interaction between heterojunctions and the redox of Fe²⁺/NO₂^-is the key to highly selective NH₃ production.In addition,the system was demonstrated to be applied to efficient NO₃^-reduction（94.7%）production of NH₃ and recovery（81.2%）of industrial wastewater with high NO₃^--N concentration(5527 mg L^-1),laying the foundation for the practical application of electrochemical resource-based treatment of NO₃^-wastewater.（3）Based on the ultra-high atomic utilization of SAC,an in-situ anchoring strategy was proposed to develop a novel layered Fe SAC with high-loading preparation method,which not only achieved a significant improvement in the efficiency of NH₃ production via NO₃^-RR,but also effectively solved the low electrical energy utilization of Fe oxide electrodes.Fe SAC with layered Fe N₄ structure were synthesized by using C₃N₄ for effective anchoring of Fe single atoms generated during thermal transport,with a maximum Fe loading of 4.8 wt.%.Combined with the physicochemical property analysis,the Fe SAC forming process was elucidated.The NO₃^-RR synthesis of NH₃ at optimal potential with Fe SAC as the electrode demonstrated a NH₃ yield of 7342.7μg h^-1 mg_cat.^-1and a Faraday efficiency of 95.7%.The electrochemical results clearly show that Fe²⁺is the active site for the reduction of NO₃^-to NH₃ catalyzed by Fe SAC,and the Fe²⁺site density of the bilayer Fe N₄ structure is 1.13 times higher than that of the monolayer,which provides a new direction for the preparation of NO₃^-RR selective synthesis of NH₃ electrode and resourceful treatment of NO₃^-wastewater.（4）Based on the excellent electrochemical activity and stability of SAC,an interlayer Fe N₄ enhanced ORR performance mechanism is proposed to realize the efficient application of Fe SAC in ORR,which effectively solves the difficulties of low ORR activity and device application of Fe-based catalysts.The bottom Fe N₄ site forms a coordination with the upper Fe N₄ site,which can adjust the electronic structure to be more suitable for the binding of oxygen intermediates,effectively reducing the ORR reaction overpotential and improving the turnover frequency.The best half-wave potentials of Fe SAC under alkaline and acidic conditions achieves 0.901 V and 0.74 V,respectively.The aqueous zinc-air cell assembled by Fe SAC has a maximum power of211.4 m W cm^-2 and can operate stably for 450h.Meanwhile,the maximum power of high temperature proton exchange membrane fuel cell assembled by Fe SAC is also up to 351.1 m W cm^-2.In summary,this study provides an important reference for the application of clean energy devices.83 Figures,22 Tables,271 References.