  Both Faradaic and non-Faradaic processes can take place at an electrode. The
difference between the two processes is clearly discussed in several classical
sources, starting with Grahame (1952). However, later reference to charge
transfer across the metal-solution interface as a defining feature of a
Faradaic process, has led to ambiguities. Following Grahame, in a Faradaic
process, charged particles transfer across the electrode, from one bulk phase
to another. Thus, in a Faradaic process, after applying a constant current, the
electrode charge, voltage and composition go to constant values. Instead, in a
non-Faradaic (capacitive) process, charge is progressively stored. We
characterize the intercalation material nickel hexacyanoferrate by two
electrochemical methods and compare with theory. Data for the capacitance of
this material is well described by the extended Frumkin isotherm. This data,
and the correspondence with theory, demonstrates that this is a capacitive
material and ion and charge storage in this material a non-Faradaic electrode
process. Cyclic Voltammetry (CV) diagrams for this material have broad peaks
for certain potential windows, and rectangular shapes for other conditions,
both experimentally and in theoretical calculations based on a RC network model
that includes how capacitance is a function of charge. Measured and predicted
CV diagrams are in perfect agreement with one another. This shows that (broad)
peaks in CV diagrams do not establish whether an electrode material is Faradaic
or not.
