Ultracapacitors with high frequency response
An electric double layer capacitor (EDLC) is disclosed including: a first electrode including a first current collector and first plurality of carbon nanotubes (CNTs) disposed substantially directly upon the first current collector; a second electrode comprising a second current collector and second plurality of CNTs disposed substantially directly upon the second current collector; and an electrolyte disposed between and in contact with (e.g., wetting) the first and second electrodes. In some embodiments, the EDLC is configured to have a capacitive frequency window comprising about 1 Hz to about 50 Hz.
1. An electric double layer capacitor (EDLC) comprising:
a first electrode comprising a first current collector and first plurality of aligned carbon nanotubes (CNTs) formed on the first current collector;
a second electrode comprising a second current collector and second plurality of carbon nanotubes (CNTs) formed on the second current collector; wherein for each of the first and the second current collectors, the first plurality of CNTs and the second plurality of CNT's are separated from the respective current collector by only thin regions of CNT growth catalyst material disposed directly on the respective current collector in a single layer and proximal to bases of the CNTs; wherein the catalyst material is configured to self assemble into a plurality of nanodots during CNT formation and regions of the catalyst material include localized islands each proximal to a base of one of the CNTs; wherein the first plurality of CNTs and the second plurality of CNTs each have an average CNT height of less than about 20 μm;
an electrolyte contacting the first and second electrodes and comprising cations and ions; wherein the electrolyte comprises a salt of the formula:
wherein R 1 and R 2 can be the same or different and are each a straight-chain or branched alkyl having 1 to 4 carbon atoms, and X— is an anion that comprises [B(CN)4] − or [BFn(CN)4-n] − , where n=0, 1, 2 or 3;
wherein the EDLC is configured to have a capacitive frequency window comprising a frequency range of about 1 Hz to an upper boundary of at least about 150 Hz, wherein the upper boundary is a cutoff frequency, f3db, that is defined by a 3 db reduction in capacitance.
2. The EDLC of claim 1 , comprising a separator porous to the electrolyte and configured to separate the first electrode from the second electrode.
3. The EDLC of claim 1 , wherein the first plurality of CNTs and the second plurality of CNTs are vertically aligned.
4. The EDLC of claim 1 , wherein first plurality of CNTs and the second plurality of CNTs comprise multi-walled CNTs.
5. The EDLC of claim 1 , wherein first plurality of CNTs and the second plurality of CNTs consist essentially of multi-walled CNTs.
6. The EDLC of claim 1 , wherein the first plurality of CNTs and the second plurality of CNTs each have an average CNT height of less than about 10 μm.
7. The EDLC of claim 1 , wherein the first plurality of CNTs and the second plurality of CNTs each have an average CNT height of less than about 5 μm.
8. The EDLC of claim 1 , wherein each of the first current collector and the second current each comprises a thin metallic layer having the respective plurality of CNTs disposed thereon.
9. The EDLC of claim 8 , wherein for each of the first and the second current collectors, the surface of the current collector is substantially free of oxide material other than the thin regions of CNT growth catalyst material.
10. The EDLC of claim 8 , wherein at least one of the current collectors has of roughened surface.
11. The EDLC of claim 10 , wherein the roughened surface comprises a wet etched or dry etched surface.
12. The EDLC of claim 11 , wherein:
the first electrode comprises a first plurality of stacked metallic current collector plates, each connected to a first common conductive lead, each metallic current collector plate having a respective plurality of CNTs disposed on at least one surface of the plate; and
the second electrode comprises a second plurality of stacked metallic current collector plates, each connected to a second common conductive lead, each metallic current collector plate having a respective plurality of CNTs disposed on at least one surface of the plate; and wherein the first plurality of stacked metallic current collector plates is interdigitated with the second plurality of stacked metallic current collector plates; and wherein a separator porous to the electrolyte is disposed in a serpentine fashion between the interdigitated stacked metallic current collector plates to separate the first electrode from the second electrode.
13. The EDLC of claim 12 , wherein the first plurality of stacked metallic current collector plates comprises at least 3 plates, and the second plurality of stacked metallic current collector plates comprises at least 2 plates.
14. The EDLC of claim 12 , wherein at least one of the metallic current collector plates has CNTs disposed on at least two opposing sided of the plate.
15. The EDLC of claim 1 , wherein the thin regions of CNT growth catalyst are less than 50 nm thick along a direction normal to a surface of the current collector.
16. The EDLC of claim 1 , wherein the thin regions of CNT growth catalyst are less than 20 nm thick along a direction normal to a surface of the current collector.
17. The EDLC of claim 1 , wherein the thin regions of CNT growth catalyst are less than 10 nm thick along a direction normal to a surface of the current collector.