Carbon Aerogels Synthesized via Ambient Drying as Supercapacitor Electrode Materials with High Specific Capacitance

Carbon Aerogels (CAs) are potential candidates for electrochemical double layer electrode materials due to their unique properties such as high Specific Surface Area (SSA) up to ~1000 m²/g, controllable interconnected 3D-mesoporosity, good electrical conductivity and longer cycle stability. CAs are commonly prepared through a sol-gel polycondensation of resorcinol (R) with formaldehyde (F) and water by using Na₂CO₃ catalyst. The synthesis involves four steps: sol-gel formation, solvent exchange, supercritical drying, and pyrolysis. Supercritical drying is vital in creating mesoporosity while relative composition of R-F to water dictates pore-size distribution [1]. Recent studies [2] suggest that micropores of size comparable to that of electrolyte ions are crucial in obtaining high specific capacitance due to partial/complete de-solvation of ions into micropores.  Therefore, our study is focused on synthesis of microporous CAs by R-F polymerization using (NH₄)₂CO₃catalyst that allows drying aqua gel precursors at ambient conditions. Ambient drying has an additional advantage of producing high microporosity while eliminating two synthesis steps, e.g. solvent exchange and supercritical drying, which would significantly reduce the cost of the materials.

Organic gels (OGs) were formed by polymerization of resorcinol and formaldehyde with varying amounts of distilled water (0.883g in Gel1, 2.894g in Gel2 and 4.904g in Gel3 respectively) using (NH₄)₂CO₃ catalyst. The organic gels formed after polymerization were dried at ambient conditions for two days followed by pyrolysis at 900^oC in inert (N₂) atmosphere. Nitrogen adsorption-desorption measurements indicate that all three CA samples dried at ambient conditions have high microporosity and exhibit Langmuir type of monolayer adsorption isotherms. However, SSA of the materials decreases with amount of water content. BET analysis, SEM and TEM data will be presented for understanding the differences implied by the relative amount of R-F to water content. Supercapacitor electrodes were  made from slurry prepared by mixing CA materials in 1wt.% PVDF in NMP and  deposited onto etched copper foil (from MTI^®) using doctor-blade followed by drying in vacuum  and hot pressing at 2500 PSI. The electrochemical properties of the cells assembled in symmetric CR 2325 coin cell supercapacitor geometry using Celgard^® polypropylene (PP) membrane separator will be discussed in terms of cyclic voltammetry, galvanostatic charging-discharging, electrochemical impedance and durability tests.  The structure dependent electrochemical properties such as specific capacitance, energy density, power density, and cycle stability in aqueous (6M KOH) and organic electrolyte (NEt₄BF₄/1M Acetonitrile) will also be discussed in comparison to graphene platelets from STEM Chemicals as a baseline.

References:

[1] Horikawa T, Hayashi Ji, Muroyama K. Controllability of pore characteristics of resorcinol–formaldehyde carbon aerogel. Carbon. 2004;42:1625-33.

[2] Simon P, Gogotsi Y. Materials for electrochemical capacitors. Nat Mater. 2008;7:845-54.