Electrode and cell preparation. Freestanding composite electrode films were prepared by modifying a previously reported method. Functionalized carbon material (f-CMK-3, f-YP-80F or f-BP) (50 mg) was mixed with ethanol (ca. 1.5 mL) to produce a slurry. This was sonicated for 15 min before being added to a PTFE dispersion in a few drops of ethanol in a watch glass.