1 methyl 2 pyrrolidone

Boron-doped Si (100) wafers were cleaned first in Milli-Q water and then in acetone/ethanol 1:1 for 10 min in an ultrasonic bath. Afterwards, they were treated for 10 min in an O₂-plasma with 30 W, followed by another cleaning in Milli-Q water in ultrasound (10 min). In-between the different steps, the wafers were dried with N₂.
Carboxylate-SAMs were prepared according to Hoffmann et al. [50 (link)]. After the cleaning procedure, a 3-aminopropyltriethoxysilane-SAM (APTES, Acros Organics, 99%) was prepared and functionalized with a 143 mm solution of succinic anhydride (Sigma-Aldrich, ≥99%) in 1-methyl-2-pyrrolidone (Sigma-Aldrich, ≥98.0%).
Polyelectrolyte layers were deposited according to Lipowsky et al. with a dipping robot DR 3 from Riegler & Kirstein, Germany [26 (link)]. Solutions of poly(styrene sulfonate) (PSS, Sigma-Aldrich, M ≈ 70,000 g mol⁻¹), poly-L-glutamic acid (PLGA, Sigma-Aldrich, M = 15,000–50,000 g mol⁻¹) and poly-L-lysine hydrobromide (PLL, Sigma-Aldrich, M = 15,000–30,000 g mol⁻¹) in Milli-Q water with a concentration of 1 mg mL⁻¹ were prepared. The pH of the PLL solution was adjusted to 9 with 0.3 m KOH. The sequence of the layer-by-layer deposition was (PLL + PLGA)₅ + PLL + PSS. The substrates were dipped into the polyelectrolyte solutions for 20 min, followed by several washing steps in Milli-Q water.

All raw materials were commercially available and were used without additional purification or modification. Activated carbon for supercapacitor applications (YP50F) was purchased from Kuraray (bulk density of 0.3 g mL⁻¹ and surface area of 1692 m² g⁻¹) and Carbon black Super P (≥99%) was purchased from Alfa Aesar (density of 160 ± 20 kg m⁻³). The remaining materials, Multiwalled Carbon Nanotubes (CNTs, 6–9 nm × 5 µm), 1-Methyl-2-pyrrolidone (NMP), Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and Carboxymethylcellulose (CMC) 90,000 Mw, were purchased from Sigma Aldrich, Madrid (Spain). Aluminium foil of 0.05 mm thickness was used as current collector (Goodfellow). The fabricated electrodes were assembled in CR2032 coin cells from MTI by using Whatman grade 4 cellulose paper separator and 1M tetraethylammonium tetrafluoroborate (TEABF₄) in acetonitrile (all purchased from Sigma Aldrich) as electrolyte.

For the preparation of activated carbon electrodes, 75 wt.% activated carbon, 15 wt.% binders (polyvinylidene fluoride (PVDF), EQ-Lib-PVDF, MTI Corporation, Richmond, CA, USA), 10 wt.% carbon black (EQ-Lib-SuperC45, MTI Corporation), and 1–2 mL of 1-methyl-2-pyrrolidone (≥99.0%, Sigma Aldrich, St. Louis, MI, USA) were mixed for 20 min. The prepared suspension was coated onto a titanium foil (MF-Ti-Foil-700L-105, MTI Corporation) current collector of 1 cm × 2 cm. Then, the prepared electrodes were dried at 130 °C for 12 h.