Functionalized carbons like N-doped Graphene or CNT have beneficial effect in super capacitor applications.. some reports as much as 300 F/g.. see this paper:

http://pubs.acs.org/doi/abs/10.1021/nl2009058

or this by paper on B & N co-doped graphene by Mullen group DOI: 10.1002/adma.201201948

Perhaps, this paper might help.

What is the choice for supercapacitors: graphene or graphene oxide?

Energy Environ. Sci., 2011, 4, 2826

In my investigation, graphene is one of the materials I used. Activated carbon is the first one I'm interested in. However, I haven't seen papers or breakthrough study on this kind of carbon material. How about that? I mean, OFGs have different impact on different carbon, don't they?

Surface functional groups on carbons can have beneficial effects on the specific capacitance because they can be supplier additional pseudo capacitance by redox reactions. But, they have adverse effects on the cycling stability of supercapacitors, self-discharge and leakage currents.

I did make an acid-treated carbon and found it has a large amount of oxygen containing functional groups. However, it had no capacitance in organic system.

no, generally in organic electrolyte it has not effect but in KOH and H2SO4 could be enough.

In my opinion, I guess that pseudocapacitance mechanism of oxygen-containing functional groups is different from those metal oxide. That might be the reason why there is no beneficial effect of functional groups in organic electrolyte. How do you think?

In addition, OCFGs had "negative" impact on the whole system.

Surface functional groups do not contribute in an orgsnic electrolyte, because there are no free protons. A quinone - hydroquiniône couple only works with protons.

For details see:

Correlation between capacitances of porous carbons in acidic and aprotic EDLC electrolytes

T. A. Centeno, M. Hahn, J.A. Fernández, R. Kötz and F. Stoeckli

Electrochemistry Communications 9 (2007) 1242-1246.

DOI: 10.1016/j.elecom.2007.01.031