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 ßBi2O3 (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 ßBi2O3-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 ßBi2O3 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, ßBi2O3 can effectively suppress HER by limiting proton adsorption (H+ + e- → *H, 0.54 eV). Therefore, ßBi2O3-x series catalysts exhibit higher NH3 yield and FE than αBi2O3-x. Meanwhile, in situ FTIR further confirms that ßBi2O3 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 ßBi2O3-x series catalysts. After optimization, the ßBi2O3-0.6 catalyst has the best NRR performance (NH3 yield: 51.36 µg h-1 mg-1cat.; 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.