Sr_xTi_0.6Fe_0.4O_3−δ (x = 1.0, 0.9) catalysts for ammonia synthesis via proton-conducting solid oxide electrolysis cells (PCECs)

Ammonia is a promising carbon-free energy carrier. Ammonia is usually industrially synthesized via the Haber-Bosch method under high pressures and temperatures, which requires high energy consumption. In comparison, the electrocatalytic reduction of N₂ is a green, eco-friendly, and pollution-free method for ammonia synthesis if the electricity is generated using a renewable source. Therefore, the development of highly efficient electrocatalysts for the N₂ reduction reaction (NRR) would be significant. Herein, perovskites Sr_xTi_0.6Fe_0.4O_3−δ (S_xTF, x = 1 and 0.9) with tunable oxygen vacancies (OVs) were prepared and used as NRR electrodes for proton-conducting solid oxide electrolysis cells (PCECs). These PCECs were used to synthesize NH₃ from N₂ and H₂. STF and S_0.9TF showed maximum ammonia synthesis rates of 6.84 × 10⁻⁹ (±0.25 × 10⁻⁹) mol cm⁻² s⁻¹ and 4.09 × 10⁻⁹ (±0.80 × 10⁻⁹) mol cm⁻² s⁻¹, with corresponding Faraday efficiencies of 2.79% (±0.12%) and 2.01% (±0.09%) at 650 °C and 0.6 V. The enhanced NRR performance of S_0.9TF was mainly attributed to the improved adsorption and activation of N₂ by the abundant OVs, Ti³⁺ and the exsolved Fe active particles. This work offers a promising strategy for the design of materials for the electrochemical synthesis of NH₃via PCECs.