Tuning Electron Cloud Density in MOF Nanosheets for Enhanced Proton Conduction in PEMFCs

Precise construction of proton transfer pathways and modulation of proton carrier microenvironments are crucial for achieving high proton conduction in proton exchange membranes (PEMs). Despite extensive efforts to tailor these microenvironments, the underlying regulation of electron cloud density around proton carriers, intrinsically linked to proton conduction, remains largely overlooked. Herein, MIL‐53‐SO3H‐X (X = –F, –Br, –H, –CH3) nanosheets are synthesized via solvothermal coordination of Cr3+ ions with functionalized terephthalic acid (BDC‐X) ligands, followed by post‐synthetic sulfonation at unsaturated Cr3+ sites to anchor –SO3H groups. Density functional theory (DFT) calculations and experimental results reveal that electron‐withdrawing –F groups enhance the electron cloud density around –SO3H sites, shortening transfer distance, aligning water molecules, and lowering the proton transfer barrier. Conversely, electron‐donating –CH3 groups deplete the electron cloud density in the –SO3H region, disrupting proton transport pathways. The MIL‐53‐SO3H‐F nanosheet achieves an intrinsic proton conductivity of 298.3 mS cm−1 at 80 °C and 98% RH, a 4.5‐fold increase over MIL‐53‐SO3H‐CH3 nanosheets (65.5 mS cm−1), as measured by the comb electrode. The corresponding lamellar membrane delivers a high peak power density of 734.8 mW cm−2 and remains stable for over 100 h in H2/O2 PEM fuel cells (PEMFCs).