Crystal-Phase and Surface-Structure Engineering of Bi₂O₃ for Enhanced Electrochemical N₂ Fixation to NH₃

The nitrogen reduction reaction (NRR) for ammonia synthesis is hindered by weak N₂ adsorption/activation abilities and the hydrogen evolution reaction (HER). In this study, αBi₂O₃ (monoclinic) and βBi₂O₃ (tetragonal) were first synthesized by calcination at different temperatures. Experiments and calculations revealed the effects of Bi₂O₃ with different crystal phases on N₂ adsorption/activation abilities and HER. Then, αBi₂O₃-x and βBi₂O₃-x series catalysts with surface oxygen vacancies (OVs) and Bi⁰ active sites were synthesized through the partial in situ reduction method. The results demonstrate the following: (I) Tetragonal βBi₂O₃ can better adsorb N₂ and cleave the N≡N bond, thereby obtaining a lower NRR rate-limiting energy barrier (*N≡N → *N≡N-H, 0.51 eV). Meanwhile, βBi₂O₃ can effectively suppress HER by limiting proton adsorption (H⁺ + e^- → *H, 0.54 eV). Therefore, βBi₂O₃-x series catalysts exhibit higher NH₃ yield and FE than αBi₂O₃-x. Meanwhile, in situ FTIR further confirms that βBi₂O₃ could better adsorb/activate N₂, and the NRR distal mechanism occurs on the Bi₂O₃ surface. (II) The introduction of NaBH₄ promotes the conversion of part of Bi³⁺ on the Bi₂O₃ surface into Bi⁰ and releases OVs. The additional active sites (OVs and Bi⁰) enhance the overall catalyst's adsorption/activation capacity for N₂, further increasing the NH₃ yield and FE. Meanwhile, semimetal Bi⁰ can effectively limit electron accessibility, thereby inhibiting the combination of charges and adsorbed protons, reducing the HER reaction and improving the FE of NRR. Therefore, the introduction of NaBH₄ effectively improved the NH₃ yield and FE of the αBi₂O₃-x and βBi₂O₃-x series catalysts. After optimization, the βBi₂O₃-0.6 catalyst has the best NRR performance (NH₃ yield: 51.36 μg h^-1 mg^-1_cat.; FE: 38.67%), which is superior to the majority of bismuth-based NRR catalysts. This work not only studies the effects of Bi₂O₃ with different crystal phases on N₂ and HER reaction but also effectively regulates the active components of Bi₂O₃ surface, thereby realizing efficient NRR to NH₃ reaction, which provide valuable insights for the rational design of Bi-based NRR electrocatalysts.