Synergistic effect of Bi and Fe for suppressing hydrogen evolution reaction (HER) and enhancing electrochemical nitrogen reduction reaction (eNRR) performance

The electrochemical nitrogen reduction reaction (eNRR) displays significant potential for the synthesis of ammonia. However, eNRR electrocatalysts with high catalytic activity and selectivity still need to be developed. In this study, a Bi-doped La_0.9Bi_0.1FeO_3-δ (LBiF) perovskite was fabricated as a potential cathode catalyst and incorporated into a protonic ceramic electrolysis cell (PCEC) for eNRR. Electrochemical tests demonstrated that the LBiF cathode reveals higher eNRR catalytic activity and enhanced hydrogen evolution reaction (HER) inhibition compared to undoped LaFeO₃ (LF). Characterization of the LBiF cathode following 90 h of NH₃ synthesis revealed surface reconstruction of the catalyst during eNRR. Particularly, Bi³⁺ is partially reduced to Bi²⁺ and metallic Bi, resulting in A-site defects and an increased concentration of oxygen vacancies (OVs) and Fe⁴⁺ in accordance with the charge neutrality principle. Both the OVs and Fe⁴⁺ can accept electron pairs provided by N₂ due to their unsaturated electronic structures. Furthermore, Bi doping inhibits the HER because Bi preferentially binds to N instead of H. Bi doping creates new Bi³⁺/Bi²⁺/Bi⁰ redox electron pairs, establishing an electron transfer path that improves perovskite conductivity and eNRR electrocatalytic activity. This study sets a foundation for designing eNRR catalysts aimed at suppressing the HER.