Ordered Lattice at Polycrystalline‐Single Crystal Island Interfaces for Efficient CO ₂ /H ₂ O Co‐Electrolysis in Solid Oxide Electrolysis Cells

ABSTRACT Solid oxide electrolysis cells (SOECs) hold promise for large‐scale co‐electrolysis of CO ₂ and H ₂ O to produce syngas (CO/H ₂ ), a key step toward sustainable fuel production. However, the efficiency of this process is often limited by sluggish electrode kinetics and inefficient interfacial charge transfer. Here, we construct a polycrystalline‐single crystal island homojunction via epitaxial growth to overcome these limitations. In this architecture, single‐crystal BaFeO ₃ and BaCo _x Fe _1‐x O ₃ islands are grown on a polycrystalline Ba _0.7 Gd _0.3 FeO ₃ (110) substrate. The resulting structure achieves two major functions: it spatially separates CO ₂ and H ₂ O adsorption sites, while the highly ordered polycrystalline‐single crystal interface, free from dangling bonds, significantly enhances charge transfer kinetics. The BaCo _0.1 Fe _0.9 O ₃ /Ba _0.7 Gd _0.3 FeO ₃ composite electrode delivers a faradaic efficiency of 95.9% for CO ₂ reduction and shows no notable degradation over 300 h of continuous operation. Moreover, directional doping of Co into the BaFeO ₃ single‐crystal islands precisely modulates the adsorption behavior toward CO ₂ and H ₂ O, allowing a broadly adjustable syngas ratio (CO:H ₂ = 1:1.1 to 1:5.8). This ordered ‘polycrystalline‐single crystal’ structure establishes a materials design strategy in which ordered interfaces and spatially separated active sites converge to overcome the kinetic and transport limitations that have long hindered high‐efficiency co‐electrolysis in SOECs.