4.8 Cellulose membranes for energy storage devices and beyond

 Ionomer membranes are crucial for the function of a wide range of electrochemical applications within areas such as energy storage, hydrogen production and water purification. State-of-the-art membranes of today have drawbacks like environmentally problematic production, poor recyclability and a high cost limiting the economic viability of systems reliant on such membranes.  Bio-based nanomaterials have gained interest in recent years as starting material for the development of a novel generation of green and functional low-cost membranes. Cellulosic nanomaterials are of particular interest due to a combination of favourable properties: abundancy of cellulose as a renewable material in nature, low cost, excellent mechanical properties, ability to form porous networks at different scales, and high variability in possible functionalization routes.  In this work, a two-step chemical modification route has been employed to produce functionalized cellulose nanofibrils for use as starting material to develop a new type of membrane. The primary aim is to use these membranes in energy storage devices based on aqueous electrolytes, such as aqueous organic redox flow batteries, where a combination of ionic conductivity and selectivity towards certain molecules is required. The possibility to tune some of the most important properties of the membranes via the degree of chemical modification has additionally been studied.