Peculiarities and requirements of asymmetric capacitor devices based on combination of capacitor and battery-type electrodes

New concepts have arisen in recent years concerning configurations of the so-called hybrid charge-storage devices in which a faradaic, rechargeable battery-type electrode is combined with a non-faradaic, electrochemical, double-layer type of electrochemical capacitor electrode, the combination now being referred to as an “asymmetric” device. Other hybrid systems combine a double-layer electrochemical capacitor device with a fuel-cell or a separate rechargeable battery. Following a discussion of dependence of cell voltage of regular, two-electrode, non-faradaic electrochemical capacitors on state-of-charge (SOC), comparisons are made with recently conceived hybrid combinations of a battery-type rechargeable electrode with a double-layer capacitor-type electrode. Advantages, disadvantages and requirements of such hybrid systems over regular electrochemical supercapacitors or batteries are discussed and critically examined in regard to available energy- and power-densities, energy and coulombic efficiency, cycle-life, voltage profiles on discharge, and self-discharge behaviour. Emphasis is given to importance of using a third electrode, as reference, especially in evaluations of discharge/recharge behaviour of each electrode of an hybrid pair having very different electrochemical characteristics, one faradaic, the other non-faradaic. Use of a reference electrode also provides the opportunity of following self-discharge behaviour of the individual electrodes independently.