Ti_0.87O₂ Nanosheet-Associated Aramid Nanofiber Ordering in Ultrathin Membranes for Proton Gradient Energy Conversion

In membrane-based reverse electrodialysis (RED), ion-selective membranes convert ionic concentration gradients directly into electrical output. Protons are often treated within the general framework of cation selectivity, while how membrane structure specifically regulates proton transport beyond conventional cation transport remains insufficiently understood. Here, freestanding aramid nanofiber (ANF)/Ti_0.87O₂ membranes approximately 3 μm thick are prepared by co-dispersion in trifluoromethanesulfonic acid (TfOH), spin coating, aqueous regeneration, and confined drying. X-ray diffraction and two-dimensional wide-angle X-ray scattering show that Ti_0.87O₂ incorporation is accompanied by enhanced ANF ordering and preferential orientation. After normalization to the bulk HCl/KCl conductivity ratio, the proton preference factor increases from approximately 1.0 for ANF to 1.67 at a Ti_0.87O₂ mass fraction of 0.5, while the apparent activation energy is lower for HCl than for KCl transport (0.18 versus 0.24 eV). Under a 1000-fold HCl concentration gradient (1 M/0.001 M), the membrane delivers 51.52 W m^-2 and retains 88.1% of its short-circuit current over approximately 33 h. Further integration into a 10-unit stack yields an open-circuit voltage of 1.77 V, demonstrating modular voltage scaling. Together, these results suggest that Ti_0.87O₂ nanosheets promote ANF ordering and contribute to proton-favorable transport behavior.