Capacitive and faradic charge components in high-speed carbon nanotube actuator

When submitted to square-wave potentials of ±2 V, single-walled carbon nanotube films with an ionic liquid, taking part of an actuator, show high-frequency electromechanical responses (up to 100 Hz), with anodic current evolutions showing two maxima. Similar anodic chronoamperometric responses have been obtained from films of conducting polymers exchanging balancing anions with the electrolyte during reaction, being described by two stretched exponential functions deduced on physico-chemical bases: oxidation under nucleation–relaxation structural control and oxidation/swelling completion under diffusion control. Now by adding a third stretched function, describing a greater contribution now of the charge of the electrical double layer, a good mathematical fit of the chronoamperometric response of the carbon nanotube films was carried out. By recovering the original physico-chemical meaning of the electrochemical stretched functions, capacitive and structural faradic components of the chronoamperometric charge were quantified as a function of the applied frequency (1–10 Hz). The capacitive component of the charge ranges with frequency between 8.6% and 12.2% of the involved charge. The faradic (reactive) component ranges between 91.4% and 87.8%, being the main contributor to the volume variations in the actuating films. The order of magnitude of the diffusion coefficients was 10−6 cm2 s−1, in good agreement with the fast response of the reactive actuators constructed with those materials. A good separation of capacitive and faradic components will allow a better design of electrochemical applications of single-walled carbon nanotube films.

Anodic chronoamperometric response of single walled carbon nanotubes with an IL is fitted by three stretched exponential functions including structural faradic (oxidation–relaxation and oxidation–swelling–diffusion) and capacitive (EDL) components.Figure optionsDownload as PowerPoint slideHighlights
► Films of carbon nanotubes (SG-SWNTs) were studied by square-wave potentials in EMITFSI IL.
► Anodic chronoamperometric responses present two maxima: not fitted by capacitive currents.
► Stretched functions deduced for conducting polymers allow a good fitting of the responses.
► Charge components are quantified and their physico-chemical meaning describe structural changes.
► Capacitive and faradic charge percentages range with frequency between 8–12% and 92–88%, respectively.