Membranes and separators are crucial components in many processes and devices. The state-of-the-art fossil-based membranes have a high carbon footprint, and polyfluorinated membranes are increasingly phased out. These limitations lead to an inevitable transition that calls for carbon-neutral membranes with the same or even better performance that can be produced at scale and low cost. Cellulose membranes have the potential to fulfill these criteria if they can be tuned for different purposes. A way to tailor cellulose membranes by preparing them from cellulose fibrils of different refining degrees is presented. The membranes’ effective pore size and permeability to PEG, Fluorescein, and different ions were characterized. The membranes were efficiently used as separators in aqueous-based Zn-ion batteries and PEDOT supercapacitors. This work demonstrates a route toward high-performing and versatile cellulose membranes that can be produced at scale in a more sustainable membrane industry.