Interplaying Intrinsic and Extrinsic Proton Conductivities in Covalent Organic Frameworks

Abstract

A sulfonic-acid-based covalent organic framework (TpPa-SO₃H) has been synthesized that exhibits intrinsic proton conductivity under anhydrous conditions. The sulfonic acid groups are aligned on the two-dimensional (2D) layers at periodic intervals and promote the proton hopping inside the hexagonal one-dimensional channel. The intrinsic proton conductivity of TpPa-SO₃H was measured as 1.7 × 10^–5 S cm^–1 at 120 °C under anhydrous conditions. To enhance the proton conductivity, we have synthesized a hybrid COF TpPa-(SO₃H-Py) by a ligand-based solid-solution approach that contains sulfonic acid as the acidic site, as well as pyridine as the basic site, in order to immobilize acidic proton carrier molecules. Impregnation of phytic acid molecules inside the framework increases the anhydrous proton conductivity up to 5 × 10^–4 S cm^–1 at 120 °C. Such an approach highlights the advantage and first-time use of hybrid COF for interplaying intrinsic to extrinsic proton conductivity.