Nafion, as a state-of-the-art solid electrolyte for proton exchange membrane fuel cells (PEMFCs), suffers from drastic decline in proton conductivity with decreasing humidity, which significantly restricts the efficient and stable operation of the fuel cell system. In this study, the proton conductivity of Nafion at low relative humidity (RH) was remarkably enhanced by incorporating multifunctional graphene oxide (GO) nanosheets as multifunctional fillers. Through surface-initiated atom transfer radical polymerization of sulfopropyl methacrylate (SPM) and poly(ethylene glycol) methyl ether methacrylate, the copolymer-grafted GO was synthesized and incorporated into the Nafion matrix, generating efficient paths at the Nafion-GO interface for proton conduction. The Lewis basic oxygen atoms of ethylene oxide (EO) units and sulfonated acid groups of SPM monomers served as additional proton binding and release sites to facilitate the proton hopping through the membrane. Meanwhile, the hygroscopic EO units enhanced the water retention property of the composite membrane, conferring a dramatic increase in proton conductivity under low humidity. With 1 wt % filler loading, the composite membrane displayed the highest proton conductivity of 2.98 × 10 S cm at 80 °C and 40% RH, which was 10 times higher than that of recast Nafion. Meanwhile, the Nafion composite exhibited a 135.5% increase in peak power density at 60 °C and 50% RH, indicating its great application potential in PEMFCs.
Graphene
oxide (GO) membrane, bearing well-aligned interlayer nanochannels
and well-defined physicochemical properties, promises fast proton
transport. However, the deficiency of proton donor groups on the basal
plane of GO and weak interlamellar interactions between the adjacent
nanosheets often cause low proton conduction capability and poor water
stability. Herein, we incorporate sulfonated graphene quantum dots
(SGQD) into GO membrane to solve the above dilemma via synergistically controlling the edge electrostatic interaction and
in-plane π–π interaction of SGQD with GO nanosheets.
SGQD with three different kinds of electron-withdrawing groups are
employed to modulate the edge electrostatic interactions and improve
the water swelling resistant property of GO membranes. Meanwhile,
SGQD with abundant proton donor groups assemble on the sp2 domain of GO via in-plane π–π
interaction and confer the GO membranes with low-energy-barrier proton
transport channels. As a result, the GO membrane achieves an enhanced
proton conductivity of 324 mS cm–1, maximum power
density of 161.6 mW cm–2, and superior water stability
when immersed into water for one month. This study demonstrates a
strategy for independent manipulation of conductive function and nonconductive
function to fabricate high-performance proton exchange membranes.
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