Table 1. The ratio of nitrogen species with respect to the copper species of the modified electrodes and the reference compound Cu(DAT) as determined by XPS.

Sample	Elemental ratio N : Cu a
Cu(DAT)	5.6
CVAu|Cu(DAT)	3.0
CAAu|Cu(DAT)	2.6
PG|Cu(DAT)	4.2
CatAu|Cu(DAT)	2.1
Vulcan|Cu(DAT)	3.9
CatGC|Vulcan|Cu(DAT)	3.7

^aThe full areas of all species in the N 1s region and in the Cu 2p_3/2 region are used to determine the ratio.