Coherent Exsolution of Ultrafine Ru Nanoparticles on Double Perovskite Boosts CO ₂ and H ₂ O Electrolysis via Strong Metal‐Support Interactions

ABSTRACT Solid oxide electrolysis cells (SOECs) hold significant potential for efficient, high‑rate conversion of CO 2 and H 2 O into syngas driven by renewable electricity. However, SOEC performance is often constrained by the scarcity of fuel electrode materials that combine high activity with structural stability for CO 2 and H 2 O electrochemical reduction. Here, we develop a rationally designed fuel electrode, Sr 1.95 FeMg 0.3 Mo 0.6 Ru 0.1 O 6‐δ (SFMMRu), that exhibits coherent exsolution of ultrasmall Ru nanoparticles (1‐5 nm) with an exceptionally high surface number density (∼2.7 × 10 4 particles µm −2 ). The resulting high density of stable metal‐oxide interfaces markedly enhances CO 2 and H 2 O electroreduction kinetics. As a result, the SFMMRu exhibits low polarization resistances of 0.254 Ω cm 2 in 50% CO/CO 2 and 0.143 Ω cm 2 in 40% H 2 O/H 2 at 800°C. In SOEC operation at 1.5 V, high current densities of 2.24 A cm −2 and 2.88 A cm −2 are achieved for CO 2 and H 2 O electrolysis, respectively. Long‐term operation exceeding 200 h at 1 A cm −2 in pure CO 2 demonstrates the high durability of the fuel electrode. Advanced electron microscopy combined with density functional theory calculations indicate that the formation of ultrasmall, densely distributed Ru nanoparticles stems from strong metal‐support interactions between the Ru phase and the perovskite substrate.