Constructing a Highly Cross-Linked Hydrogen Network by Integrating Poly(4,4′-diphenylether-5,5′-benzimidazole) with Phosphoric Acid-Modified COFs

It is critical for phosphoric acid (PA)-doped high-temperature proton exchange membranes (HT-PEMs) to retain functional durability in gas separation and proton conduction due to the plasticization effect of doped PA and its leaching. To solve the problem, we propose a novel type of HT-PEMs by incorporating PA-modified covalent organic frameworks (COFs) in poly[2,2 ′ -( p -oxidiphenylene)-5,5 ′ -benzimidazole] (OPBI). Two COFs with comparable pore sizes (1.2 and 1.3 nm) and the same number of total nitrogen atoms, but with or without ketone (C═O) groups, are designed to investigate the effect of the potential number of hydrogen bonds on the performance. The 12%Tp/PA@OPBI membrane (molar percent of ketone-rich Tp to OPBI is 12) possesses a highly cross-linked hydrogen-bond network, which endows the membrane with PA retention rates of 87.2% and 88.6% after exposing over 250 h under 40% RH at 50 °C and 0% RH at 160 °C. This membrane doped with 290 wt % PA delivers an anhydrous proton conductivity of 155 mS cm –1 at 180 °C and enables a peak power density of 707 mW cm –2 for a single H 2 /O 2 fuel cell without backpressure. Moreover, a fuel cell assembled with 12%Tp/PA@OPBI retains an initial performance of 88% after 30 start-up and shut-down cycles from room temperature to 160 °C.