In situ fragmented bismuth nanoparticles for electrocatalytic nitrogen reduction

Abstract

The electrochemical nitrogen reduction reaction (NRR) is a promising alternative to the energy-intensive Haber–Bosch process for ammonia synthesis. Among the possible electrocatalysts, bismuth-based materials have shown unique NRR properties due to their electronic structures and poor hydrogen evolution activity. However, identification of the active sites and reaction mechanism is still difficult due to structural and chemical changes under reaction potentials. Herein, in situ Raman spectroscopy, complemented by electron microscopy, is employed to investigate the structural and chemical transformation of the Bi species during the NRR. Nanorod-like bismuth-based metal–organic frameworks are reduced in situ and fragment into densely contacted Bi⁰ nanoparticles under the applied potentials. The fragmented Bi⁰ nanoparticles exhibit excellent NRR performance in both neutral and acidic electrolytes, with an ammonia yield of 3.25 ± 0 .08 µg cm⁻² h⁻¹ at −0.7 V versus reversible hydrogen electrode and a Faradaic efficiency of 12.11 ± 0.84% at −0.6 V in 0.10 m Na₂SO₄. Online differential electrochemical mass spectrometry detects the production of NH₃ and N₂H₂ during NRR, suggesting a possible pathway through two-step reduction and decomposition. This work highlights the importance of monitoring and optimizing the electronic and geometric structures of the electrocatalysts under NRR conditions.