Anomalous capacitance increase in carbon nanopores

Article 1 : "Anomalous increase in carbon capacitance at pores below 1 nm ; Chmiola et al, Science 2006
_Electrochemical Double layer Capacitors, also called supercapacitors, store the charge by adsorption of ions from an electrolyte into high surface area porous carbons (1,000 to 2,000 m² per g). These porous carbons are obtained from activation process, which consists in controlled oxidation of carbon. There was a traditional belief stating that micropores (pores less than 1 nm that is less than the size of the solvated ions) were useless for capacitive storage since the ions could not access these confined pores. However, such a traditional view was not supported by experimental results since it was very difficult to prepare a series of microporous carbons with controlled pore size and narrow pore size distribution in the 0.6 – 1.5 nm range. In collaboration with Prof Gogotsi (Drexel University, Philadelphia), we prepared a series of carbon samples called Carbide-Derived carbons (CDCs) using an alternative method. CDCs are porous carbons obtained by extraction of metals from carbides (TiC, SiC and other) by etching in halogens at temperatures from 400°C to 1000°C :
MC + nCl2 —> MCl2 + C (1) where M is a metal and C the CDC
In this reaction, Ti is leached out from TiC and carbon atoms self-organize into an amorphous or disordered graphitic structure with the pore size that can be fine-tuned by controlling the chlorination temperature and other process parameters. Accordingly, a narrow uni-modal pore size distribution can be achieved in the 0.6 – 1.1 nm range and the mean pore size can be controlled with sub-Angstrom accuracy. These materials were used to understand the charge storage in micropores using 1M solution of NEt4BF4 in acetonitrile electrolyte. The normalized capacitance (F/m²) decreased with decreasing pore size until a critical value of 1 nm was reached (see Figure below), and then dramatically increased when the pore size approached the ion size. Since CDC samples were exclusively microporous, the capacitance increase for sub-nanometer pores clearly shows the role of micropores. Moreover, the gravimetric and volumetric capacitances achieved by CDC were, respectively, 50% and 80% higher compared to conventional activated carbons. It was proposed at that time that ions could access the micropores due to the partial or complete removal of their solvation shell.

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