(655g) Ion Conducting Polymers from Kraft Lignin for Electrochemical Devices

An innovative lignin valorization effort, designing lignin-rich waste as efficient and cost-effective energy materials, can aid in both bio- and energy economy. One major technical limitation of electrochemical devices (e.g., fuel cells) is the weak ion conductivity within the 2-30nm thick catalyst binding, ion-conducting polymer (ionomer) layer over electrodes. Also low-cost ionomer formulations can lower down the production cost of fuel cells. In this work, we strategically sulfonated kraft lignin (a by-product of pulp and paper industries) to design ionomers with varied ion exchange capacities (IECs) (LS x; x=IEC) that is cheap, environment friendly and can potentially overcome this interfacial ion conduction limitation. Unlike commercial lignosulfonate, the water solubility of LS x was overcome by controlling the sulfomethylation and cross-linking reactions. The proton conductivity, water uptake, ionic domain characteristics, density, and water mobility were measured in sub-micron thick LS 1.6 and LS 3.1 films and compared with the current state-of-the-art ionomer, Nafion thin films. LS 1.6 showed much higher ion conductivity than Nafion and LS 3.1 in films with similar thickness despite of their water uptakes. Within the three-dimensional, less dense, branched architecture of LS 1.6 macromolecules, the –SO₃H and –OH groups are in close proximity facilitating the formation of larger ionic domains with highly mobile water molecules. As compared to LS 1.6, LS 3.1 showed a higher glass transition temperature and film stiffness at dry state, which did not change upon humidification. On the other hand, Nafion stiffened significantly upon humidification only. These results demonstrate the potential of LS x as an ideal candidate as ionomer binder for low-temperature, water-mediated ion conduction in energy conversion and storage devices.