Comparative study on proton conductivity of a polypyridinyl multicarboxylate-based hydrogen-bonded organic framework and related chitosan composite membrane

Abstract

Although hydrogen-bonded organic frameworks (HOFs) have received particular attention in the field of proton conduction owing to their richly H-bonded networks and porous structures, there are few related reports, especially on the study of the related proton exchange membranes (PEMs). In this paper, a multi-pyridine and multi-carboxylate-based HOF (H₃tpt = 4-(2,4,6-tricarboxyphenyl)-2,2′,6′,2′′-terpyridine, namely HOF 1) was successfully prepared and introduced in a chitosan (CS) matrix to fabricate proton-conductive composite membranes. Firstly, the excellent thermal-, water-, and acid-stability of HOF 1 were confirmed via various measurements. Secondly, an AC impedance test demonstrated its temperature- and humidity-dependent proton-conductive properties. Based on the above preliminary research, we tried to dope it into CS to prepare a composite membrane (denoted as CS/HOF-x, where x is the doping amount of 1 in the hybrid film). Fortunately, one composite membrane (CS/HOF-4) with ultra-high proton conductivity (σ) (2.61 × 10⁻² S cm⁻¹ under 100 °C and 98% RH) was fabricated using the casting method; its best σ value is 54 and 2 times greater than that of HOF 1 (4.83 × 10⁻⁴ S cm⁻¹) and the pure CS membrane, respectively, and is comparable to that of Nafion under similar conditions. Further, the proton conduction mechanism of the HOF 1 composite membrane was proposed. This research represents the first time that HOF has been doped into CS to make a high-efficiency proton-conducting composite membrane, which will lay a good foundation for its application in fuel cells and other related fields.