Two-Dimensional\nAcid–Base Hydrogen-Bonded Organic\nFramework toward a Highly Conductive Proton Exchange Membrane

Hydrogen-bonded organic frameworks (HOFs) hold great\npotential\nfor realizing high-efficiency proton conduction because of their well-developed\nand ordered internal hydrogen bond networks. However, highly proton-conductive\nHOF aggregation (e.g., proton exchange membranes) has been seldom\nreported since the simultaneous regulation of intra-/interstructure\nof HOF remains a huge challenge. Herein, for the first time, two-dimensional\nHOF nanosheets with superior proton conductors (acid–base-type\nhydrogen bond) are prepared, followed by assembly into a regular free-standing\nlamellar membrane. We demonstrate the hydrogen bond-type transformation\nof HOF nanosheets from –COOH···–COOH\nto –COOH···–NH<sub>2</sub> with a precise\ncarboxyl ligand/amino ligand ratio of ∼1:1, which elevates\nthe horizontal conductivity as high as 323.2 mS cm<sup>–1</sup>. Meanwhile, the acidic and basic functional groups in adjacent nanosheets\ngenerate rich acid–base-type hydrogen bonds in the interlayer\nchannel, inducing misaligned stacking of HOF nanosheets. This optimizes\nthe distribution of functional groups and water molecules in the interlayer\nchannel and promotes the construction of highly developed water hydrogen\nbond networks in vertical direction. At 85 °C and 100% RH, the\nvertical conductivity of the HOF lamellar membrane reaches 156.2 mS\ncm<sup>–1</sup>, which is much higher than reported HOF aggregations\n(<20 mS cm<sup>–1</sup>) and comparable to the commercial\nNafion membrane. These afford satisfactory hydrogen fuel cell performances.