Fig. 7: Experimental methods.

a Schematic illustration of carbon xerogel synthesis procedure. The precursor solution (sol) is allowed to undergo polycondensation by heat treatment to form a gel. The water medium is then replaced by ethanol and followed by supercritical drying with carbon dioxide to remove the liquid phase. The dried sample is then carbonized by pyrolysis. b Schematic of the experimental setup for the ultrasonic pulse transmission method. c A typical waveform measured for the carbon xerogel sample and the determination of the time of flight relative to the reference waveform. d Schematic of the adsorption-ultrasonic experimental setup.