2.2. Experimental Procedure

GO was synthesized using Marcano’s method [20]. A total of 5 g of graphite powder and 18 g of potassium permanganate was slowly added to a mixture of 40 mL of phosphoric acid and 360 mL of sulfuric acid. The solution was stirred for 18 h at room temperature, then 3 mL of H₂O₂ was added. The solution was then filtered and centrifuged (4000 rpm for 10 min) so the supernatant could be decanted away. The mixture was then washed multiple times with 30% HCl and distilled water, following the procedure described elsewhere [13]. The obtained GO was placed in an oven for 4 h at 80 °C.

Activated carbon electrode was prepared using commercial activated carbon powder of high specific surface area as determined by N₂ adsorption (1034 m²/g). Carbon slurry was prepared as a suspension of activated carbon powder (3.2 g), carbon black (0.4 g) and poly(vinylidene fluoride PVDF, 0.4 g) in 25 mL N-Methyl-2-pyrrolidone (NMP). The mixture was stirred for 2 h and sonicated for 40 min to ensure homogeneity. The slurry was then deposited on a graphite sheet by using spray coating (Air brush Iwata, Fukushima, Japan). It was spray coated onto a graphite sheet with a thickness of 0.357 mm. The coated electrode was dried at 80 °C in an oven for 3–5 h. For the composite synthesis, the slurry was prepared by adding the as-prepared GO (5, 10 15 and 20 wt.%) to AC and the aforementioned procedure (for carbon slurry preparation) was followed.