  Increasing energy demands of modern society requires deep understanding of
the properties of energy storage materials as well as their performance tuning.
We show that the capacitance of graphene oxide (GO) can be precisely tuned
using a simple electrochemical reduction route. In situ resistance
measurements, combined with cyclic voltammetry measurement and Raman
spectroscopy, have shown that upon the reduction GO is irreversibly
deoxygenated which is further accompanied with structural ordering and
increasing of electrical conductivity. The capacitance is maximized when the
concentration of oxygen functional groups is properly balanced with the
conductivity. Any further reduction and de-oxygenation leads to the gradual
loss of the capacitance. The observed trend is independent on the preparation
route and on the exact chemical and structural properties of GO. It is proposed
that an improvement of capacitive properties of any GO can be achieved by
optimization of its reduction conditions.
