Sub-2-nm Channels within Covalent Triazine Framework Enable Fast Proton-Selective Transport in Flow Battery Membrane

Ion conductive membranes (ICMs) with robust sub-2-nm channels show high proton transport rate in flow battery, but it remains a great challenge to precisely control the ion sieving of the membranes. Herein, as a promising proton-selective carrier, sulfonated piperazine covalent triazine framework (s-pCTF) with the channel size of ≈1.5 nm and abundant fast proton hopping sites is introduced into sulfonated poly(ether ether ketone) (SPEEK) to fabricate advanced ICM for vanadium flow battery (VFB) application. The interior protoplasmic channels of s-pCTF demonstrate significant Donnan exclusion effect, resulting in a high proton/vanadium ion selectivity in theory (6.22 × 10⁵). Meanwhile, the nitrogen-rich sub-2-nm channels yield fast proton highway, and exterior-grafted sulfonic acid groups further facilitate the proton transfer. By regulating the ion sieving and proton conductivity, the optimal hybrid membrane exhibits synchronously improved battery performance with an enhanced energy efficiency (92.41% to 78.53% at 40–200 mA cm⁻²) and long-term stability for 900 cycles over 400 h (EE: 87.2–85% at 120 mA cm⁻²), outperforming pure SPEEK and Nafion212 membranes. This study validates the applicability of organic porous CTF with sub-2-nm channels and desired functionality in ICMs for high-performance VFB application.