Conjugated acid-base pairs in Keggin-type polyoxometalate-based metal-organic frameworks enhance proton conduction

Abstract

To achieve high proton conductivities, it is pivotal to design and select appropriate functional organic ligands for constructing polyoxometalate-based metal–organic frameworks (POMOFs). In this work, the effects of functional groups upon the molecular electrostatic potentials and pKa of two organic ligands (4-abpt, theoretical pKa value of 25.13; Hdpt, theoretical pKa value of 9.24) are first investigated by theoretical calculations. And then, two new proton-conductive decorated POMOFs, {[Cu(4-abpt)2(H2O)][Cu(4-abpt)(H2O)][SiW12O40]}·6H2O (CUST-875) and {[Cu(Hdpt)(H2O)2] [Cu(Hdpt)][SiW12O40]}·2H2O (CUST-876), have been designed and obtained via hydrothermal method. Besides, their proton conduction performances and influencing factors also have been investigated in more details. It is found that more water molecules adsorption and the conjugated acid-base pairs played substantial roles in proton conduction. As a consequence, the three-dimensional framework CUST-875, composed of the higher theoretical pKa value organic ligand 4-abpt, exhibited a higher proton conductivity of 1.07 × 10−4 S cm−1 at 95 °C and 98 % relative humidity (RH), surpassing that of CUST-876 by three orders of magnitude. Additionally, CUST-875 possesses the ability to degrade H2O2, which readily generates during charging and discharging of battery and has adverse effects on the battery life. This work contributes new insights into the high efficiency preparation and structure–property relationships of new polyoxometalate (POM)-based crystalline proton conducting materials with relatively high intrinsic proton conductivities.