Modulating The Electronic Structure Of Transition Metal Based Catalysts For Electrocatalytic Nitrogren Reduction

Industrial ammonia production is still dominated by the traditional Haber-Bosch process reacting at high pressures and temperatures,while electrocatalytic nitrogen reduction reaction（ENRR）has been emerging as an energy-saving and on-demand technique for ammonia production under ambient conditions by utilizing renewable energy sources.However,the low solubility of N₂ and high activation energy of N≡N bonds lead to unsatisfactory ammonia（NH₃）yield and poor Faraday efficiency（FE）of ENRR.Therefore,it is crucial to fabricate efficient electrocatalysts to boost the ENRR performance.Transition metal（TM）elements possess unique d orbitals electron structure,and TM based materials have been widely investigated as electrocatalysts for ENRR.In this thesis,TM based catalysts,i.e,VS_x and Co₃O₄,were prepared as high performance ENRR catalysts.By developing effective strategies of regulating the electronic structure of transition metals,this thesis devotes to unraveling the relationship between the electronic structure of catalysts and the corresponding ENRR performance,as well as to clarifying the catalytic process and mechanism of ENRR at TM based catalyst:（1）We designed deliberately and prepared VS_x and VS₂ catalysts by using microwave hydrothermal method.VS_x has hybrid valences(V²⁺and V⁴⁺),and VS₂ has only V⁴⁺.The results showed that VS₂ delivers a high NH₃yield of 41.21μg h^-1 mg_cat^-1and FE of 35.52%at-0.3 V vs.RHE,which are much higher than that of VS_x.In-situ Raman results demonstrated that V⁴⁺of VS₂ were exhausted gradually under the harsh reducing conditions of ENRR,leading to the deactivation of catalyst.The first-principles calculations（DFT）confirmed that the unique electron configuration（3d¹）of V⁴⁺offers an ideal electronic platform for the“donation and back-donation”process of ENRR on TMs.Moreover,V⁴⁺can accept excessive electrons under cathodic potential to maintain the stability of catalyst.Therefore,we deduced that V⁴⁺can preferably act as a highly catalytic active center in VS_x for ENRR.（2）We prepared C⁴⁺ions doped Co₃O₄（C-Co₃O₄）by using a hydrothermal and sinter method to further explore the effect of TM electronic structure on the ENRR performance.Comparing to undoped Co₃O₄,C-Co₃O₄exhibited higher NH₃ yield of 38.49μg h^-1 mg^-1_cat and FE of 15.10%.The experiments and DFT results showed that the introduction of C⁴⁺redistributes successfully the charges density of Co₃O₄,resulting in a strong built-in electric field（IEF）inside the catalyst.Mechanism studies showed that the modulated electronic structure of C-Co₃O₄ is essentially responsible for the enhanced ENRR performance.In summary,VS_x and Co₃O₄ based materials were prepared as high performance ENRR electrocatalysts.Combining with the experimental results and DFT calculations,we identified the active sites of VS_x and unraveled the relationship between ENRR activity and electronic structure of VS_x.By introducing C⁴⁺ions to Co₃O₄,we obtained C-Co₃O₄ catalyst with high ENRR performance.Mechanism studies showed that the C⁴⁺doping induced IEF could modulate the electronic structure of Co₃O₄,thus boosting the catalytic performance.These in-depth studies elaborated that optimizing the electron structure of TM based materials is a reasonable pathway to improve their ENRR performance,which offers a valuable information for designing and preparation of advanced catalysts.