Cobalt (II) Coordination-Tailored Composite Proton Exchange Membrane with Enhanced Proton Conductivity and Suppressed Hydrogen Permeation.

Balancing high proton conductivity with low hydrogen permeability remains a central challenge in the development of advanced proton exchange membranes (PEMs) for fuel cell applications. Herein, we report a Co(II)-coordination-engineered composite membrane based on Aquivion, in which lattice H2O and NO3- anions coordinated to Co(II) centers of a superprotonic coordination polymer (PCM-2) dynamically interact with humidified water and sulfonic acid (-SO3H) groups of Aquivion to establish an extended hydrogen-bonding network. This coordination-induced architecture facilitates efficient proton transport while simultaneously reinforcing the polymer matrix, thereby suppressing hydrogen crossover. The optimized membrane (AQV-2, 2 wt % PCM-2 in Aquivion) exhibits a high proton conductivity of 18.6 mS cm-1, representing a 61.04% enhancement over the pristine membrane (AQV-0, 11.55 mS cm-1), along with significantly reduced gas permeability. Structural integration of the one-dimensional PCM-2 framework further improves microphase-separated water domains and mechanical robustness. When evaluated in a hydrogen-oxygen fuel cell, AQV-2 delivers a superior peak power density of 643.2 mW cm-2 and a current density of 880.8 mA cm-2 at 0.6 V, outperforming the benchmark Nafion membrane with enhanced operational stability. These results highlight that Co(II)-mediated coordination within short-side-chain ionomers is an effective strategy to concurrently enhance proton transport and gas barrier properties, offering a promising pathway for next-generation high-performance PEMs.