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

摘要

New concepts have arisen in recent years concerning configurations of 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, 2-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 behavior. 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.