Boosting electroreduction of nitrate to ammonia by modulating the crystalline phase of Fe2O3

Electrocatalytic nitrate reduction reaction (NO₃RR) provides an alternative to the conventional Haber-Bosch process for ammonia synthesis and is an effective method for removal of nitrate ions from polluted waters, which is highly significant from both energy and environmental perspectives. However, NO₃RR involves the complex eight-electron process alongside various nitrogen-containing intermediates and is also in competition with hydrogen evolution reaction, thus demanding highly active and selective electrocatalysts. In this work we prepare a Ni-doped Fe₂O₃ electrocatalyst via a solvent-free route. It is found that the addition of Ni induces the crystalline-phase transformation of Fe₂O₃ from γ-Fe₂O₃ to α-Fe₂O₃. The density functional theory (DFT) results reveal that compared to γ-Fe₂O₃, α-Fe₂O₃ gives rise to a lower potential-determining step (PDS) energy barrier, leading to the more thermodynamically favourable reaction. By modulating the crystalline phase, the optimal catalyst achieves high ammonia yield rates of > 5000 μg h⁻¹ cm⁻² and faradaic efficiencies of > 90 %, showcasing its high electrocatalytic activity and selectivity. From this perspective, this paper provides new insights and strategies for the green nitrate-to-ammonia conversion.