In situ NMR and electrochemical quartz crystal microbalance techniques reveal the structure of the electrical double layer in supercapacitors

In situ NMR and electrochemical quartz crystal microbalance techniques reveal the structure of the electrical double layer in supercapacitors ; Griffin et al., Nature Materials 2015
_EQCM measurements were coupled to in-situ NMR experiments to directly quantify ion fluxes within working microporous carbon supercapacitor electrodes. Our results showed that charge storage mechanisms are different for positively and negatively polarized electrodes. At the negative electrode, the change of the ion population plotted versus the potential clearly shows that only counter ion adsorption is visible in a -1.5 V – 0V potential range, thus confirming previous results using different electrolyte (see paper 3. above). Anions are stabilized inside the electrode despite negative polarization. On the opposite, during positive polarizations within a 0 V – 1.5 V potential range, an ion exchange mechanism was evidenced where anions go inside the pores while cations leave the porous carbon electrode (see figure). EQCM measurement also conducted at constant potentials have confirmed these NMR results, and a partial ion desolvation has been measured for the cation during negative polarization. On positive side, the ion exchange mechanism well fitted the experimental plot meaning that acetonitrile molecules are not exchanged during the adsorption process ; they just re-organize inside the porous carbon structure.

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These results provide new molecular-level insight, with the methodology offering exciting possibilities for the study of pore/ion size, desolvation and other effects on charge storage in supercapacitors. It also highlights a lack of understanding of the charge storage mechanism. Understanding such mechanisms is of high importance since it would provide guidelines for preparing porous carbon with optimized structure for designing high energy density supercapacitors.


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