Crystal-Phase and Surface-Structure Engineering of Bi 2 O 3 for Enhanced Electrochemical N 2 Fixation to NH 3 .

The nitrogen reduction reaction (NRR) for ammonia synthesis is hindered by weak N 2 adsorption/activation abilities and the hydrogen evolution reaction (HER). In this study, αBi 2 O 3 (monoclinic) and βBi 2 O 3 (tetragonal) were first synthesized by calcination at different temperatures. Experiments and calculations revealed the effects of Bi 2 O 3 with different crystal phases on N 2 adsorption/activation abilities and HER. Then, αBi 2 O 3 - x and βBi 2 O 3 - x series catalysts with surface oxygen vacancies (OVs) and Bi 0 active sites were synthesized through the partial in situ reduction method. The results demonstrate the following: (I) Tetragonal βBi 2 O 3 can better adsorb N 2 and cleave the N≡N bond, thereby obtaining a lower NRR rate-limiting energy barrier (*N≡N → *N≡N-H, 0.51 eV). Meanwhile, βBi 2 O 3 can effectively suppress HER by limiting proton adsorption (H + + e - → *H, 0.54 eV). Therefore, βBi 2 O 3 - x series catalysts exhibit higher NH 3 yield and FE than αBi 2 O 3 - x . Meanwhile, in situ FTIR further confirms that βBi 2 O 3 could better adsorb/activate N 2 , and the NRR distal mechanism occurs on the Bi 2 O 3 surface. (II) The introduction of NaBH 4 promotes the conversion of part of Bi 3+ on the Bi 2 O 3 surface into Bi 0 and releases OVs. The additional active sites (OVs and Bi 0 ) enhance the overall catalyst's adsorption/activation capacity for N 2 , further increasing the NH 3 yield and FE. Meanwhile, semimetal Bi 0 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 NaBH 4 effectively improved the NH 3 yield and FE of the αBi 2 O 3 - x and βBi 2 O 3 - x series catalysts. After optimization, the βBi 2 O 3 -0.6 catalyst has the best NRR performance (NH 3 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 Bi 2 O 3 with different crystal phases on N 2 and HER reaction but also effectively regulates the active components of Bi 2 O 3 surface, thereby realizing efficient NRR to NH 3 reaction, which provide valuable insights for the rational design of Bi-based NRR electrocatalysts.