Constructing Built-in Electric Field At The Surface/Interface Of Electrocatalyts For Enhanced Electrocatalytic Nitrogen Reduction

Ammonia（NH₃）serves as a sought-after clean energy carrier,experiencing substantial demand across diverse industrial sectors,including fertilizer manufacturing,pesticide production and energy conversion.Currently,the industrial synthesis of ammonia heavily depends on the conventional Haber-Bosch process,which entails the utilization of elevated temperatures and pressures.Unfortunately,this approach entails considerable costs and contributes to the release of substantial amounts of carbon dioxide,thereby exacerbating environmental apprehensions.Electrocatalytic nitrogen reduction（ENRR）has garnered significant attention among researchers owing to its remarkable attributes of favorable reaction parameters,minimal energy requirements and eco-friendliness.Presently,the field of NRR encounters several challenges:（1）the nitrogen（N₂）solubility in the electrolyte is limited,（2）the formidable task of cleaving the highly stable N≡N triple bond and（3）the presence of the competitive hydrogen evolution reaction（HER）.Consequently,it becomes imperative to explore reasonable approaches to enhance the performance of ENRR.This study primarily focuses on addressing the critical scientific issues in electrochemical nitrogen reduction and conducts catalyst design research to propose the mechanism of enhancing electrocatalytic performance through the induction of intrinsic electric fields.Through interfacial engineering,different types and strengths of intrinsic electric fields are introduced into the catalyst system to investigate the application of electric field-induced effects in ENRR.The primary research objectives encompass the following areas:（1）A unique urchin-inspired cobalt oxide（U-L Co₃O₄）electrocatalyst was synthesized via a hydrothermal method,which exhibits exceptional ENRR activity.In a 0.5 M Li Cl O₄ electrolyte,the catalyst demonstrates an impressive ammonia yield of 49.10μg h^-1 mg_cat^-1 and Faraday efficiency（FE）of 18.50%.These results surpass those achieved by the spherical Co₃O₄（S-L Co₃O₄）.Experimental findings and computational simulations substantiate that the U-L Co₃O₄ with high-curvature tip induces a local electric field,akin to the natural phenomenon of a"lightning rod effect."Consequently,the deliberately designed strong local electric field effectively promotes Li⁺accumulation on the surface of electrocatalyst,leading to a heightened local concentration of N₂on the electrocatalyst surface.This,in turn,enhances the NRR reaction activity while concurrently inhibiting the hydrogen evolution reaction.（2）We synthesized Co O-Co₃O₄ catalysts featuring interfacial electric fields to significantly enhance the performance of ENRR.The Co O-Co₃O₄catalyst exhibited an impressive NH₃ yield of 59.96μg h^-1 mg_cat^-1 and the Faraday efficiency（FE）of 22.37%in a 0.1 M Li₂SO₄ electrolyte.In situ measurements and theoretical calculations revealed that the interfacial electric field created within the Co O-Co₃O₄ electrocatalyst effectively captured inert N₂ molecules by forming Co-N bonds.Furthermore,this interfacial electric field facilitated enhanced hybridization of theσ-d orbitals between the N₂molecule and the Co site of Co O-Co₃O₄,thus enabling efficient activation of the N₂ molecule.（3）Building upon these findings,we further investigated the influence of interfacial electric field strength on the activity of ENRR.A strategy was proposed to modulate the oxygen vacancy concentration in WO₃ to enlarge the bandgap,which can induce more electron transfer from WO₃ to C₃N₄,thereby enhancing the interfacial electric field strength in the WO₃-C₃N₄ material.The WO₃-C₃N₄-R catalyst exhibited a NH₃ yield of 43.50μg h^-1 mg_cat^-1 and achieved up to 11.20%FE in a 0.1 M Li₂SO₄ electrolyte.Combined density functional theory（DFT）calculations with in situ Raman and FTIR spectra demonstrate that the engineered intensified interfacial electric field in WO₃-C₃N₄-R can enhance the adsorption of N₂ molecules by forming strong W-N bonds and the polarization of N≡N bond through an"acceptance-donation"mechanism,resulting in a promoted ENRR kinetics.（4）Based on the investigation of interfacial electric field in（2）and（3）,we constructed a WS₂-WO₃ heterojunction with an intrinsic interfacial electric field to further explore the underlying mechanism of how the interfacial electric field enhances the performance of ENRR.We fabricated a WS₂-WO₃heterojunction with an interfacial electric field to effectively elevate the d-band center of W,thereby strengthening the adsorption of intermediates.Experimental findings unequivocally demonstrate the remarkable efficacy of this approach in enhancing ENRR performance.Notably,the WS₂-WO₃heterojunction exhibited an improving NH₃ yield of 62.38μg h^-1 mg_cat^-1 and a commendable Faraday efficiency of 24.24%.Furthermore,in situ characterization and theoretical calculations elucidate that the strong interfacial electric field in WS₂-WO₃ induces an upward shift of the d-band center of W toward the Fermi level,resulting in augmented intermediate adsorption.Consequently,this enhanced adsorption facilitates improved kinetics of the ENRR reaction.（5）Based on the research conducted in（1）-（4）,we designed a WO₃-C₃N₄heterojunction where the interfacial electric field formed at the heterojunction interface couples with the atomic local electric field induced by W-N bonding,resulting in an interface-local coupled electric field.This interface-local coupled electric field endows the fabricated WO₃-C₃N₄ heterojunction with excellent ENRR activity,yielding 54.87μg h^-1 mg_cat^-1 with an impressive Faraday efficiency of 24.33%at-0.3 V vs.RHE.Experimental findings corroborated by density functional theory（DFT）calculations,underscore the efficacy of this interface-local coupling electric field in augmenting the d_z²occupancy at the active site of W.Consequently,this enhancement significantly improves N₂ adsorption and activation processes,thereby propelling the observed enhancement in ENRR performance.In summary,this dissertation investigates the application of electric field effects in ENRR by employing interfacial engineering to introduce different types and strengths of intrinsic electric fields into the catalyst system.Adopting an experimental characterization combined with theoretical simulation research approach,the formation mechanism of electric fields and the underlying nature of their induced enhancement of electrochemical reactions are elucidated.These studies provide insights into the mechanisms by which electric field effects influence ENRR activity and offer guidance for the rational design of catalysts.