Two-Dimensional Acid–Base Hydrogen-Bonded Organic Framework toward a Highly Conductive Proton Exchange Membrane

Hydrogen-bonded organic frameworks (HOFs) hold great potential for realizing high-efficiency proton conduction because of their well-developed and ordered internal hydrogen bond networks. However, highly proton-conductive HOF aggregation (e.g., proton exchange membranes) has been seldom reported since the simultaneous regulation of intra-/interstructure of HOF remains a huge challenge. Herein, for the first time, two-dimensional HOF nanosheets with superior proton conductors (acid–base-type hydrogen bond) are prepared, followed by assembly into a regular free-standing lamellar membrane. We demonstrate the hydrogen bond-type transformation of HOF nanosheets from –COOH···–COOH to –COOH···–NH2 with a precise carboxyl ligand/amino ligand ratio of ∼1:1, which elevates the horizontal conductivity as high as 323.2 mS cm–1. Meanwhile, the acidic and basic functional groups in adjacent nanosheets generate rich acid–base-type hydrogen bonds in the interlayer channel, inducing misaligned stacking of HOF nanosheets. This optimizes the distribution of functional groups and water molecules in the interlayer channel and promotes the construction of highly developed water hydrogen bond networks in vertical direction. At 85 °C and 100% RH, the vertical conductivity of the HOF lamellar membrane reaches 156.2 mS cm–1, which is much higher than reported HOF aggregations (<20 mS cm–1) and comparable to the commercial Nafion membrane. These afford satisfactory hydrogen fuel cell performances.