(558bv) Understanding Fe-Based Catalysts for Electrochemical Ammonia Synthesis at Ambient Conditions

Electrochemical reduction of N₂ to NH₃ at ambient conditions may provide an alternative to the Haber−Bosch process for sustainable NH₃ production when powered by solar- or wind-derived electricity, as the Haber-Bosch process relies on the consumption of fossil fuels and is energy-intensive (consuming 1−2% of the global energy supply). However, the development of this process has been hindered by the lack of efficient catalysts for the N₂ reduction reaction (N₂RR) at ambient conditions, due to the barrier for N₂ activation and the competing hydrogen evolution reaction (HER). Fe-based catalysts have received a lot of attention as a promising candidate among the non-precious metal catalysts. Here, we present a study of the effect of the chemical states of Fe-based catalysts for N₂ electroreduction to NH₃. In contrast to pure metallic Fe catalyst, we developed an Fe/Fe oxide catalyst by in situ electroreduction of a pre-oxidized Fe foil in a neutral electrolyte. The resulting Fe/Fe oxide catalyst was composed of Fe and Fe₃O₄ nanocrystallites, as revealed by X-ray photoelectron spectroscopy and grazing incidence X-ray diffraction. The Fe/Fe₃O₄ catalyst shows greatly enhanced activity and selectivity for N₂RR at ambient conditions than the original Fe foil, achieving a Faradaic efficiency of 8.29% for NH₃ production at −0.3 V vs the reversible hydrogen electrode in a neutral electrolyte, which is around 120 times higher than that of the Fe foil. The high selectivity is enabled by an enhancement of the intrinsic N₂RR activity as well as an effective suppression of the HER activity.^[1] Comparative studies with Fe/Fe oxide samples derived at different pre-oxidation temperatures indicate that the N₂RR activity is affected by Fe/Fe oxide ratio. Furthermore, the N₂RR selectivity of the Fe/Fe₃O₄ catalyst is also superior to that of Fe, Fe₃O₄, and Fe₂O₃ nanoparticles. We are currently using in operando X-ray absorption spectroscopy to probe the chemical states of Fe-based nanocatalysts during N₂RR, to establish a correlation between the chemical states and NH₃ electrosynthesis activity on Fe-based catalysts.

References:

[1] Hu, L.; Khaniya, A.; Wang, J.; Chen, G.; Kaden, W. E.; Feng, X. Ambient electrochemical ammonia synthesis with high selectivity on Fe/Fe oxide catalyst. ACS Catal. 2018, 8, 9312−9319.