Preparation and characterization of novel zwitterionic lignin/Nafion composite membranes

In this study, a novel zwitterion-decorated lignin (ZL) containing amine and sulfonic acid groups was synthesized and ZL/Nafion composite membranes were fabricated as proton exchange membranes. Kraft lignin was modified using amino-silane and 1,3-propane sultone via a continuous grafting reaction to provide zwitterionic moieties. Chemical structural analyses confirmed the successful introduction of the zwitterion moiety into lignin. In particular, the surface charge of ZL was positive in an acidic medium and negative in a basic medium, suggesting that ZL is a zwitterionic material. ZL was incorporated into Nafion membrane to enhance its ion exchange capacity, thermal stability, and hydrophilicity. ZL/Nafion 0.5% with a proton conductivity of 151.0 mS/cm was found to be 55.3% higher than unmodified ML (methanol soluble lignin)/Nafion 0.5% (97.2 mS/cm), indicating that the zwitterion moiety of ZL can enhance the proton transport ability. In addition, the evaluation of oxidative stability confirmed that ZL/Nafion 2% was chemically more durable than pure Nafion. This confirmed that the use of lignin as an additive in the membrane can yielded positive results in terms of chemical durability and oxidation stability in Nafion. Therefore, ZL is expected to be utilized as a multifunctional additive and has the potential for fuel cell applications.