Ion Conducting Polymers (Ionomers) from Waste 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 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. Here, 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 x 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 sustained during humidification. On the other hand, expensive and not environment friendly, Nafion stiffened significantly upon humidification only. These results show the potentiality of LS x as an ideal candidate as ionomer binder for low-temperature, water-mediated ion conduction in energy conversion and storage devices.