A mixed proton–electron-conducting cathode with a Ru nanoparticle catalyst for electrochemical ammonia synthesis based on a proton-conducting BZCYYb electrolyte

Abstract

Electrochemical ammonia synthesis (EAS) has emerged as a promising alternative to the traditional Haber–Bosch process. Indeed, N₂ activation in room-temperature EAS systems remains a formidable challenge due to the strong NN bond. Solid oxide proton conductor EAS (PCEAS) electrolysis cells operating at intermediate temperatures offer a promising solution by utilizing both temperature and potential. In this process, the design of the cathode is crucial, requiring abundant proton and electron conduction channels, along with highly active catalysts. Herein, we design a cathode composed of ruthenium–La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ–BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (Ru–LSCF–BZCYYb) to meet the aforementioned requirements for PCEAS. LSCF and BZCYYb form a porous skeleton at the cathode, with Ru nanoparticles dispersed on the surface of this structure. This configuration features numerous triple-phase boundaries (TPBs), facilitating the contact between activated N₂, H⁺, and e⁻, thereby promoting electrochemical ammonia synthesis. The impregnated Ru–LSCF–BZCYYb|BZCYYb|Ni–BZCYYb PCEAS electrolysis cell exhibited a maximum NH₃ formation rate of 5.14 × 10⁻¹¹ mol s⁻¹ cm⁻² and a maximum Faraday efficiency (FE) of 0.128% at 400 °C and −0.2 V with H₂ and N₂ as feedstock gases. Its yield surpassed those of the mixed Ru–LSCF–BZCYYb|BZCYYb|Ni–BZCYYb and the impregnated Ru–BZCYYb|BZCYYb|Ni–BZCYYb by a factor of 3.9 and 11.5, respectively. The authenticity of ammonia synthesis is confirmed using the ¹⁵N₂ isotope combined with NMR detection. This study also achieved EAS using water as the hydrogen source. This approach would better meet the future demand for EAS by directly using N₂ and H₂O.