Hierarchical Self-Assembly of Microphase-Separated Membranes Enables Contiguous Low-Resistance Channels for Exceptional Low-Humidity Proton Conduction in PEMFCs

Achieving high proton conduction in proton exchange membranes (PEMs) at low relative humidity (RH) remains challenging due to the suppression of the Grotthuss mechanism. Engineering microphase separation with aligned hopping sites is an effective strategy to drive the mechanism shift, yet simultaneously maintaining membrane durability remains difficult. Here, we propose a hierarchical self-assembly strategy that precisely introduces guest hopping sites into the microphase-separated region without damaging the membrane's durability. By regulating the polyimide (PI) doping level in sulfonated polyetheretherketone (SPEEK) membranes, an optimal balance of proton conduction and stability is achieved at a bridged ─N─…SO₃H─ acid-base pair ratio of 13%. Precise hybridization of acid-base hydrogen-bonded organic frameworks (ABHOFs) into microphase-separated regions constructs guest acid-base pair sites and promotes the ordered arrangement of host sites. Spatial alignment of guest-host hopping sites creates long-range and low-resistance channels. Both site and energy matching enable a transition from vehicle-mediated to Grotthuss mechanisms under low RH. The resulting SP/PI@ABHOF membranes exhibit high proton conductivities of 193.6/50.8 mS cm^{- 1} at 80°C under 100%/20% RH, with peak power densities reaching 1221.2/419.6 mW cm^{- 2}. Significant enhancements in mechanical strength, cyclic stability, and hydrogen-permeation current density were also achieved, surpassing the performance of benchmark PEMs, including Nafion 212 and GORE membranes.