Thermo-mechanics of cathode is closely related to the durability of high-temperature solid oxide fuel cells (SOFCs), with two main mechanical failures during thermal cycling: interface delamination and bulk cracking of cathode. Bulk cracking, caused by insufficient fracture strength/stiffness is a big concern but often overlooked. Here, we introduce chemical hydration to offset the thermal expansion, enhancing the cathodic mechanical stiffness and fracture strength, thus promoting the thermo-mechanical durability of cathode in proton ceramic fuel cells (PCFCs). Such chemical-induced expansion offset is achieved by strengthening intergranular bonding inside the bulk cathode after the hydration, preventing granule detachment during thermal shrinkage. As a demonstration, the stiffness-enhanced air electrode (BaCo_0.7Ce_0.15Y_0.15O₃, noted as s-BCC-Y) exhibits 86% enhancement of fracture strength, thus thermal cycling stability with almost no degradation after 35 harsh thermal cycles between 600 and 300 °C, surpassing pristine BaCo_0.7Ce_0.3O₃ and many cobalt-free PCFC cathodes. Benefitted from the improved stiffness of cathode, full cell with the s-BCC-Y electrode demonstrates enhanced power output. This work highlights the importance of bulk cathode thermo-mechanics in developing robust SOFCs for high temperature energy applications.

Delamination and cracking of air electrodes are two mechanical causes to the degradation of high-temperature electrochemical ceramic cells. While compositing negative thermal expansion (NTE) materials can tackle delamination by lowering the thermal expansion coefficient (TEC) of air electrode, it can exacerbate cracking due to large thermal stress between particles of NTE and positive thermal expansion perovskites (PTE). Here, we introduce interfacial oxides to “wedge” the NTE-PTE interface, thereby resisting cracking inside the bulk of the air electrode through reactive calcination at near-melting temperatures. This concept is demonstrated by compositing negative thermal expansive HfW₂O₈ with Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3–δ (perovskite), forming Co₃O₄, Fe₃O₄, BaHfO₃ and Sr₃WO₆ as wedging phases. Enhanced bulk modulus (by 102%), hardness (by 138%), and mitigated TEC (reduced by 35%) are simultaneously achieved, which enhances the durability of the air electrode over 40 rigorous thermal cycles between 600 °C and 300 °C, and even with no decay after two years of exposure to ambient air. This method offers an effective strategy for developing mechanical-robust electrodes of high-temperature electrochemical cells.