Regulation Of Electronic Structure In Cobalt-Based Catalysts And Their Electrochemical Nitrate Reduction Performance For Ammonia Synthesis

Ammonia（NH₃）is an essential chemical for human survival and ecosystems.Due to its high energy density and hydrogen storage capacity,it is considered a key carrier for green hydrogen and energy transition.Currently,NH₃ synthesis primarily relies on the energy-intensive and carbon-intensive Haber-Bosch（H-B）process.In the context of the global energy crisis and carbon neutrality,there is an urgent need to develop sustainable and environmentally friendly alternatives.Electrocatalytic nitrate（NO₃^-）reduction reaction（e-NO₃RR）can alleviate NO₃^-pollution while achieving large-scale NH₃production under ambient conditions,offering the dual benefits of"turning waste into treasure".However,e-NO₃RR involves a complex 8-electron transfer process and competes with the hydrogen evolution reaction（HER）,making the design of highly selective e-NO₃RR electrocatalysts a key challenge.Cobalt-based catalysts are highly promising e-NO₃RR electrocatalysts due to their natural abundance,low cost,and unique 3d electronic structure.However,the inherent conductivity and intrinsic activity of pure cobalt-based materials hinder their application.This thesis employs strategies such as atomic doping,defect introduction,and heterojunction interface construction,combined with density functional theory（DFT）calculations,to develop a series of highly selective e-NO₃RR electrocatalysts.The relationship between e-NO₃RR activity and the electronic structure of the catalysts was studied,revealing the reaction mechanisms of cobalt-based catalysts and providing new insights and theoretical basis for the design of e-NO₃RR catalysts.The main work and conclusions are as follows:（1）Firstly,the catalytic ability of Co₃O₄ towards e-NO₃RR is enhanced by introducing a highly active noble metal element,Pd,into the crystal lattice of Co₃O₄.Theoretical calculations showed that Pd doping shifts the d-band center of Co₃O₄ closer to the Fermi level,regulating the electronic structure of the catalyst,promoting NO₃^-adsorption,and lowering the reaction barrier for e-NO₃RR,resulting in excellent e-NO₃RR activity.Experimental results demonstrated that Pd-Co₃O₄/TM exhibits a lower e-NO₃RR onset potential,with NH₃ Faradaic efficiency（FE）remaining above 95%over a wide potential range,compared to Co₃O₄/TM,which has superior NH₃ synthesis performance.The maximum NH₃ yield of Pd-Co₃O₄/TM reached 745.6μmol h^-1 cm^-2,with an optimal FE of 98.7%,and it showed good chemical stability.Additionally,Zn-NO₃^-batteries assembled with Pd-Co₃O₄/TM as the cathode achieved a high power density of 3.9 m W cm^-2 and an excellent FE of 98.8%,providing multifunctional benefits of power output,NO₃^-removal,and NH₃ synthesis.（2）In addition to atomic doping strategies,introducing vacancy defects is also an effective method for regulating electronic structures.In this study,oxygen vacancy-rich Co Ti O_3-x nanomaterials were prepared through electrospinning,high-temperature annealing,and plasma treatment.Performance tests for e-NO₃RR showed that the electrocatalyst Co Ti O_3-x/CP,obtained by coating carbon paper（CP）with Co Ti O_3-xnanofibers,could efficiently reduce NO₃^-to NH₃.In a 0.1 M NO₃^-Na OH electrolyte,Co Ti O_3-x/CP achieved an NH₃ yield of 30.4 mg h^-1 mg_cat.^-1(858.8μmol h^-1 cm^-2)with an FE of 92.6%,superior to the pristine Co Ti O₃/CP,and exhibited good electrochemical stability.DFT calculations indicated that the introduction of oxygen vacancies enhanced its conductivity,facilitating NO₃^-reduction.Co Ti O_3-x demonstrated excellent e-NO₃RR activity,with the rate-determining step being^*NH₂→^*NH₃ and a low free energy barrier of approximately 0.41 e V,lower than the original Co Ti O₃（1.15 e V）.（3）Based on the research of the previous two works,further investigations were carried out to regulate the materials.Owing to the differences in electronic structures between different materials,heterojunction interfaces can form built-in electric fields,inducing charge redistribution,thereby effectively regulating the electronic structure and improving reaction kinetics.In this study,Co₃O₄ nanosheets modified Ti O₂ nanobelt arrays were loaded onto a titanium plate（TP）through hydrothermal reaction,electrodeposition,and calcination,forming Co₃O₄@Ti O₂/TP p-n heterojunction arrays as e-NO₃RR electrocatalysts for NH₃ synthesis.Benefiting from the interaction of the heterostructure,Co₃O₄@Ti O₂/TP achieved an NH₃ yield of 875μmol h^-1 cm^-2 with an FE of 93.1%,superior to Co₃O₄/TP and Ti O₂/TP.Theoretical calculations revealed that the construction of the p-n heterojunction induced charge redistribution between Co₃O₄and Ti O₂,reducing the bandgap and improving the catalyst’s conductivity.It also optimized the adsorption free energy of reaction intermediates,suppressing HER,and promoting selective NH₃ synthesis via e-NO₃RR.（4）Utilizing the semiconductor properties of Ti O₂,it was combined with metal Co to form a Schottky heterojunction,creating a built-in electric field to accelerate charge transfer during e-NO₃RR.Titanium salts generated via hydrothermal reaction on a TP underwent ion exchange with cobalt salts,followed by annealing to obtain Co@Ti O₂/TP Schottky heterojunction materials.Theoretical and experimental results indicated that the"point-face"contact between Co and Ti O₂ at the interface induced charge transfer,forming a built-in electric field that could regulate the electronic structure of Co@Ti O₂,increasing the electronic density on the Co surface and promoting NO₃^-adsorption,thereby enhancing the e-NO₃RR reaction.Additionally,Co nanoparticles were uniformly dispersed on Ti O₂ nanobelts,effectively preventing aggregation and ensuring the activity of the electrode material.The Co@Ti O₂/TP electrocatalyst exhibited a FE of 96.7%and an NH₃ yield of up to 800.0μmol h^-1 cm^-2 under neutral conditions,maintaining excellent stability in continuous 50-hour electrolysis tests.