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The difference between Faradaic and non-Faradaic electrode processes

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.