Electrochemical Quartz Microbalance (EQCM) study of ion dynamics in nanoporous carbons

Electrochemical Quartz Microbalance (EQCM) study of ion dynamics in nanoporous carbons ; Tsai et al, JACS 2014
Electrochemical quartz crystal microbalance (EQCM) experiments were achieved to characterize ion adsorption in porous carbide-derived carbons under polarization, in 2M EMI+,TFSI- in acetonitrile electrolyte. We experimentally measured solvation numbers for EMI+ cation, evidencing a partial desolvation when cations were adsorbed in confined carbon pores. The extent of desolvation was measured to increase when decreasing the carbon pore size. As a result, cations enter with 3.4 acetonitrile molecule in 1 nm pores while EMI+ cations are surrounded by about 8 solvent molecules in bulk electrolyte. EMI+ cations confined in 0.65 nm pore size carbon have only 1.2 solvent molecules. The results suggest that EMI+ owns higher mobility than TFSI- in these electrolytes.
More importantly, they show two different charge storage mechanism depending on the electrode polarity.
For negatively charged electrodes, the charge balancing in nanoporous carbon electrodes in achieved through counter-ion (cation) adsorption. For positive polarization, the smaller slope of the electrode weight change versus the charge in the electrode evidences an ion exchange mechanism, where anions enter the porous electrode while cations leave.

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