Electrochemical capacitors based on pseudocapacitance
A different kind of capacitance can arise at electrodes of certain kinds, for example ruthenium dioxide, when the extent of faradaically admitted charge depends linearly, or approximately linearly, on the applied voltage. For such a situation, the electrode behavior is equivalent to, and measurable as, a capacitance. This capacitance can be large but it is faradaic and not electrostatic (that is non-faradaic) in origin. This is hence an important difference from the nature of double-layer capacitance, so it is called "pseudocapacitance". This kind of pseudocapacitance can originate when an electrochemical charge-transfer process takes place to an extent limited by a finite quantity of reagent or of available surface. Several examples of pseudocapacitance can arise, but the capacitance function is usually not constant and, in fact, is usually appreciably dependent on potential or state of charge.
However, when the process is surface limited, and is proceeding in several one-electron stages, a broad range of significant capacitance values arises as is found with ruthenium dioxide electrodes where the pseudocapacitance is almost constant (within 5%) over the full operating voltage range. Some other metal oxides behave similarly but only over smaller operating voltage ranges. The ruthenium dioxide pseudocapacitance provides one of the best examples of electrochemical (pseudo)capacitance as, in addition to the almost constant capacitance over a wide voltage range, its reversibility is excellent, with a cycle life over several hundred-thousand cycles. Furthermore, the pseudocapacitance can increase the capacitance of an electrochemical capacitor by as much as an order of magnitude over that of the double-layer capacitance. However, its cost prevents its large-scale use so that it has been employed mainly in military applications.
Another type of material exhibiting pseudocapacitive behavior that is highly reversible is the family of conducting polymers such as polyaniline or derivatives of polythiophene. These are cheaper than ruthenium dioxide but are less stable, giving only thousands of cycles (still quite attractive) over a wide voltage range. (See the Appendix for a more detailed discussion of the pseudocapacitance.)
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