Polymer-Grafted Graphene Oxide/Polybenzimidazole Nanocomposites for Efficient Proton-Conducting Membranes

In this study, we have functionalized graphene oxide (GO) by growing polymer chains on its surface and then utilized the polymer-g-GO as a nanofiller with oxypolybenzimidazole (OPBI) to make a highly efficient nanocomposite-based proton exchange membrane (PEM). Three different monomers, namely, acrylamide (AAM), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), and 3-sulfopropyl acrylate potassium salt (SPAK), were polymerized on the activated GO surface via surface-initiated reversible addition fragmentation chain-transfer polymerization to obtain three different types of polymer-g-GO, namely, pAAM-g-GO, pAMPS-g-GO, and pSPAK-g-GO. Furthermore, the chain length of grafted polymers in each case was altered in order to study the effects of the grafted polymer structure and chain length on the properties of nanocomposite PEMs. The exfoliation of GO nanosheets after polymer grafting was confirmed by studying the surface morphology using various microscopic techniques. Gel permeation chromatography and thermogravimetric analysis helped in measuring the chain length of grafted polymers and grafting density on the GO surface. Furthermore, we have impregnated polymer-g-GO as nanofillers by varying loading wt % into the OPBI to fabricate a mixed matrix membrane which upon doping with phosphoric acid (PA) converted into a mixed matrix PEM. The prepared nanocomposite PEM displayed exceptionally good thermal stability, significantly improved tensile properties, improved PA loading followed by superior proton conductivity, and remarkable PA retention when exposed to saturated water vapor. When the 2.5 wt % pSPAK-g-GO (where the pSPAK chain length is 19.6 kDa) mixed with OPBI, the resulting PEM showed a remarkably high proton conductivity value of 0.327 S cm^–1 at 160 °C, a significant 5-fold increment compared to the pristine OPBI membrane (0.067 S cm^–1 at 160 °C). To the best of our knowledge, this will be the first report on utilization of polymer-g-GO in polybenzimidazole membranes for high-temperature PEM application.