2D Co‐Assembly of Solid Acids Enables Stable Superprotonic Conduction Above 260°C

ABSTRACT High‐temperature proton exchange membrane fuel cells (HT‐PEMFCs) offer enhanced catalytic activity, improved fuel tolerance, and simplified thermal and water management compared with conventional low‐temperature systems. However, their development is hindered by the lack of proton‐conducting membranes that maintain stability and conductivity under anhydrous conditions above 200°C. Here, we present a solid‐acid nanosheet membrane based on a nanosheet co‐assembly strategy, in which antimony phosphate (APA) and phosphotungstic acid (PWA) nanosheets are integrated into a confined lamellar architecture through direct acid–acid coupling to form the APA/PWA nanosheet‐assembled (APA/PWA‐NS) membrane. This design promotes dense stacking and confined acid layers within interlamellar nanochannels, overcoming the weak interparticle contact and poor mechanical integrity of conventional pressed solid‐acid pellets. Upon thermal treatment, the ordered lattice transforms into a disordered yet interconnected hydrogen‐bond network that preserves efficient Grotthuss‐type proton transport up to 280°C. The membranes exhibit proton conductivity above 0.1 S cm −1 at 260°C, with strong thermomechanical stability and suppressed hydrogen crossover. Fuel cell testing under anhydrous H 2 /O 2 delivers peak power densities of 0.767 W cm −2 at 260°C, with minimal degradation over extended operation. This work establishes nanosheet‐enabled acid–acid coupling as a robust strategy for superprotonic conduction in HT‐PEMFCs.