Effective electrode design and the reaction mechanism for electrochemical promotion of ammonia synthesis using Fe-based electrode catalysts

Abstract

The electrochemical promotion of ammonia formation on Fe-based electrode catalysts is investigated using proton-conducting-electrolyte-supported cells of H₂–Ar, Pt|BaCe_0.9Y_0.1O₃ (BCY)| Fe-based catalysts, H₂–N₂ at temperatures between 550 °C and 600 °C, and ambient pressure. To clarify the reaction mechanism, the ammonia formation rate is examined using two cathodes: (I) a porous pure Fe electrode with a shorter triple phase boundary (TPB) length and (II) a cermet electrode consisting of Fe–BCY (or W–Fe–BCY) with a longer TPB length. Using the different electrode structures, we investigate the effects of cathodic polarization, hydrogen partial pressure, and electrode materials. The porous pure Fe electrode shows better performance than the Fe–BCY cermet electrode, which suggests that the ammonia formation is accelerated by the electrochemical promotion of catalysis (EPOC) effect on the Fe surface rather than the charge-transfer reaction at the TPB. The electrochemical promotion is governed by a dissociative mechanism, i.e., acceleration of direct N₂ bond dissociation with cathodic polarization on the Fe surface, with a smaller contribution by a proton-assisted associative mechanism at the TPB. These findings indicate that the porous pure Fe electrode is more effective for ammonia formation than the (W–)Fe–BCY cermet electrode. Despite the relatively short TPB length, the porous pure Fe cathode achieves a very high ammonia formation rate of 1.4 × 10⁻⁸ mol cm⁻² s⁻¹ (450 μg h⁻¹ mg⁻¹) under appropriate conditions. This significant result suggests that the effective double layer spreads widely on the Fe electrode surface. Using the identified reaction mechanism, we discuss key processes for improving ammonia formation.