Electrochemical double layer capacitors (EDLC) are widely used and known for their high power density. Today’s EDLC devices work in the voltage range of 2.5 V without noticeable loss of power density after 100’000 charge/discharge cycles. One measure to increase power and energy density of EDLCs is t extend the voltage window. At elevated electrode potentials, however, other processes than pure double layer charging start to contribute to charge storage.
For the first time the intercalation of the cation of a non-aqueous electrolyte ((C₂H₅)₄NBF₄ in propylene carbonate) into pyrolytic graphite (HOPG) serving as a model system could be monitored by in-situ atomic force microscopy (AFM). The image shows a line scan across a given step on the HOPG surface while changing its electrode potential. The alternating swelling (a)) / shrinking (b)) of the graphite crystal during cation intercalation and de-intercalation is clearly revealed.

The image stems from the Ph.D. thesis of F. P. Campana and was carried out in collaboration between the Electrochemistry Laboratory of the Paul Scherrer Institute (PSI, R. Kötz) and the Electrochemistry group of our Department (Prof. Hans Siegenthaler).

References:

• F. P. Campana;
  "Investigation of dimensional changes and film morphology at graphite electrodes in aprotic solutions by in situ atomic force microscopy"
  Ph.D. thesis, 2005.