Synergistic Doping in Proton‐Conducting Oxides Unlocks High Conductivity and Low Proton Trapping

ABSTRACT The development of proton‐conducting oxides is pivotal for advancing solid oxide fuel cells (SOFCs), yet it is perpetually hampered by a critical trade‐off between high proton conductivity and good sinterability. Conventional doping strategies, typically employing single elements, often improve one property at the expense of the other. Here, we break this long‐standing compromise by introducing a multi‐element doping strategy, creating a novel proton conductor, BaCe _0.7 Zr _0.1 Gd _0.04 Nd _0.04 Sm _0.04 Y _0.04 Yb _0.04 O ₃ (GNSYYb), which simultaneously exhibits superior sinterability, the highest proton defect concentration, and the largest grain size among all singly doped analogues. As a result, GNSYYb possesses the highest bulk conductivity. More profoundly, comparative H ₂ O/D ₂ O isotope studies indicate a significantly mitigated proton trapping effect in GNSYYb, as evidenced by its lower activation energy shift and a conductivity ratio ( σ _H / σ _D ) closer to the theoretical value. This reduction in trapping energy barriers is identified as a key factor for its enhanced proton mobility. Consequently, a proton‐conducting SOFC (H‐SOFC) with a thin GNSYYb electrolyte demonstrates an exceptional peak power density of 2399 mW cm ⁻² at 700°C, outperforming most reported BaCeO _3– BaZrO ₃ ‐based cells. This work not only presents a superior electrolyte candidate for H‐SOFCs but also establishes multi‐element doping as a transformative paradigm for designing high‐performance proton‐conducting oxides.