The Key Role of Available Active Protons in Proton Conduction in Polyhydroxy Polymers

Proton exchange membrane fuel cells (PEMFCs) are regarded as one of the most promising clean and highly efficient power generation technologies. However, obtaining proton exchange membranes with superior performance remains a challenge for the development of PEMFCs. In this work, we synthesized a porous polyhydroxyl polymer, known as BTA, and successfully introduced almost equimolar phosphate and sulfonic acid groups (−SO3H and −PO4H2) into its framework through the simple postmodification process, resulting in BTA-P and BTA-S, respectively. The studies showed that the −SO3H group in BTA-S can ionize a large number of available protons, and its ion exchange capacity (IEC) reaches 1.65 mmol g–1, which is twice higher than that of BTA-P. The available protons can greatly enhance the proton transport efficiency, resulting in a 3 orders of magnitude increase in proton conductivity of BTA-S compared to BTA-P, reaching up to 3.97 × 10–1 S cm–1 at 80 °C and 95% relative humidity (RH). When a 5% BTA-S/Nafion composite is prepared as a proton exchange membrane for an H2/O2 fuel cell, the maximum power density reaches 952.6 mW cm–2, which is 61.9% higher than that of the recast Nafion membrane at 80 °C and 100% RH. This study reveals that the available proton density plays the most important role in proton conducting polymers.