Sulfonated Cellulose Membranes for Energy Storage Applications

Sammanfattning: In the ongoing efforts to reduce the dependency of mankind on fossil fuels for the supply of energy, renewable energy sources such as solar cells and wind turbines are employed to an increasing extent. Transitioning a large portion of electrical grids to intermittent power sources come with several problems that need to be taken into account and handled, such as ensuring supply at peak power demand and considering frequency regulation and other issues related to the stability of the grid. One possible way to increase the amount of intermittent energy sources while maintaining a stable grid and power supply is to use large scale energy storage systems to store energy that can then be used as needed.One of the most promising energy storage systems for this purpose is the redox flow battery, an electrochemical energy storage system in which the power output and total energy storage capacity are decoupled, the former relating to the area of the electrochemical cell and the latter to the amount of electrolyte. This decoupling is a great advantage since large electrolyte tanks can be used to store huge amounts of energy in a stationary manner.Redox flow batteries and other devices such as fuel cells and certain types of batteries are dependent on a selective membrane for their function. The membrane needs to efficiently transport certain species while blocking others, and the function of the membrane is often greatly influencing the performance of the devices that employ them. Current state-of-the-art ion selective membranes are often produced from PFSA-based materials, which are problematic in terms of sustainability and cost. Finding ways to replace such membranes with equally functional components produced from bio-based materials would be a large step forward in terms of improving the sustainability and cost-efficiency of large scale electrochemical energy storage.In this work, functionalized cellulose nanofibrils are used as starting material to produce novel bio-based selective membranes aimed to be employed in electrochemical energy storage systems, in particular redox flow batteries. The possibility to precisely tune the properties of membranes via the degree of modification of the starting material is investigated, as well as some strategies to further improve the performance of membranes via additives and post-fabrication modifications.