E.g. graphite, graphene, nanotubes, diamonds with mass around 10 mg, two electrode cells, and current around 1A/g in non-aqueous solution (3V). We see so many different values in the literature that is hard to known which is the most appropriate. Can we say 5uF/cm^2 for a flat carbon substrate?
I often used to read that a clean graphite surface has a specific capacitance of 20 µF/cm2, and I indeed used to find this order of magnitude when testing carbon gels as EDLC electrodes. But I also observed 2 times this value in other carbons and even 3 times higher in doped carbons. A value of 5 µF/cm2 seems definitely too low to me.
Alain
Hi Alain Celzard,
Thank you for your answer.
I got that value (5uF/cm^2) from both HOPG and GC in potassium nitrate aqueous media. I have observed that 5uF/cm^2 in my CV and EIS, but I saw some works reporting up to 30 uF/cm^2.
Anyway, regarding the most appropriate value, should that value be approximately the same for all carbon materials?
I was wondering if I could extrapolated those values for CNTs, after define that using HOPG and GC electrodes. What do you think?
Your value is fully consistent with other works, see Fig. 4 of :
http://www.ifc.dicp.ac.cn/library/cailiao/pdf1/Materials%20for%20electrochemical%20capacitors.pdf
but I would not do any extrapolation as the specific capacitance may change dramatically with pore size and redox properties of the carbon surface.
Alain
Hi Hudson,
Just to add some personally measured values, something between 5-10 μF cm–2 is what I have observed the most. In particular, values like 7.5 μF cm–2 were obtained for activated carbon and CNTs, in dilute NaCl electrolyte. Also in your case I believe something similar could have been obtained given that have gone to slow enough rates (0.1-0.5 mV s–1 for CV and 1-5 mHz for EIS). I have never measured something above 10 μF cm–2, though I agree it depends on both the electrolyte and pore structure of the active material. Nevertheless, including those factors, a capacitance above 15 μF cm–2 is too much in my opinion. The only way I can explain that, is by the presence of some functional groups, totally changing the surface chemistry and charge density of carbon. In other words, it wouldn’t be carbon, in its narrow sense, any more.
Some more references:
R.H. Baughman, Carbon Nanotube Actuators, Science. 284 (1999) 1340–1344. doi:10.1126/science.284.5418.1340.
O. Barbieri, M. Hahn, A. Herzog, R. Kötz, Capacitance limits of high surface area activated carbons for double layer capacitors, Carbon. 43 (2005) 1303–1310. doi:10.1016/j.carbon.2005.01.001.
J. Chmiola, Anomalous Increase in Carbon Capacitance at Pore Sizes Less Than 1 Nanometer, Science. 313 (2006) 1760–1763. doi:10.1126/science.1132195.
C. Largeot, C. Portet, J. Chmiola, P.-L. Taberna, Y. Gogotsi, P. Simon, Relation between the Ion Size and Pore Size for an Electric Double-Layer Capacitor, J. Am. Chem. Soc. 130 (2008) 2730–2731. doi:10.1021/ja7106178.
H. Ji, X. Zhao, Z. Qiao, J. Jung, Y. Zhu, Y. Lu, L.L. Zhang, A.H. MacDonald, R.S. Ruoff, Capacitance of carbon-based electrical double-layer capacitors, Nat. Commun. 5 (2014). doi:10.1038/ncomms4317.
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