Asymmetric and Hybrid Devices in Aqueous Electrolytes

Abstract

IntroductionCarbon-based symmetrical electrochemical double-layer capacitors (EDLCs) demonstrate higher energy density and power capability in organic-based electrolytes compared to aqueous-based electrolytes owing to their high operating voltage (2.5-2.7 V). Indeed, despite a lower capacitance of carbon-based electrodes in organic electrolytes (C), the maximum energy density (E max ) is expressed asand is proportional to the square of the maximum operating voltage (U max ), which is limited to the water electrochemical stability window that cannot theoretically exceed 1.23 V. Thus, even if the capacitance of a symmetrical carbon-based device in an aqueous electrolyte is twice that of an EDLC in an organic electrolyte (C aq = 2C org ), the maximum operating voltage of an organic-based device is more than twice that of an aqueous-based capacitor (U org = 2U aq ) resulting in:Obviously, this simple calculation does not take into account additional factors such as electrolyte concentration, ionic conductivity, packaging, and so on, but some more detailed theoretical calculation [1] points out the superiority of symmetrical carbon-based EDLC in an organic electrolyte (5.7 vs 1.7 Wh kg −1 for symmetrical carbon-based devices in aqueous electrolytes). Practically, the superiority of organicbased carbon/carbon devices has been demonstrated and most commercial-based devices use organic electrolytes. However, aqueous-based devices possess a number of advantages that can be emphasized in a commercial system, such as their high ionic conductivity, which can be useful to achieve high power density [2,3]. In addition, the electrothermal safety of aqueous-based devices will be greater in all cases than for organic-based electrolytes [4], which is one of the major points now addressed by electrochemical capacitor (EC) manufacturers as high currents and fast cycling rates are usually required, thus possibly leading to thermal rather than chemical runaway of the

Fluorine‐Doped Fe2O3 as High Energy Density Electroactive Material for Hybrid Supercapacitor Applications

“…This satisfactory kinetic reactivity even at such a high scan rate makes these binary oxides promising as electrode materials for high-performance HSC applications. [8] Moreover, the enclosed area under the CV curves differs in that the F-Fe 2 O 3 /AC cell has a stronger current response than the Fe 2 O 3 /AC cell, thus indicating its superior energy storage property compared to bare hematite phase. The higher current response of F-Fe 2 O 3 /AC cell might be attributed to well-developed particles with largely exposed contact areas as well as a good contact towards the current collector.…”

“…An example is the use of a pseudocapacitive electrode coupled with activated carbon (AC); however, in such a system the operating potential cannot be widened beyond 2 V in an aqueous medium. [7,8] As a result, the obtained energy density is not at the expected level. Therefore, we made an attempt to use organic solutes for increasing the energy density of an HSC.…”

Burgeoning Prospects of Spent Lithium‐Ion Batteries in Multifarious Applications

“…[120][121][122][123] Cobalt oxide (Co 3 O 4 ) has attracted great attention in the energy storage from the transition metal oxide group owing to its electrochemical reactivity, redox reversibility, and high theoretical capacity endowed with potential applications. [120][121][122][123] Cobalt oxide (Co 3 O 4 ) has attracted great attention in the energy storage from the transition metal oxide group owing to its electrochemical reactivity, redox reversibility, and high theoretical capacity endowed with potential applications.…”

Biomass‐Derived Carbon Materials as Prospective Electrodes for High‐Energy Lithium‐ and Sodium‐Ion Capacitors

“…In addition, the discovery of fast Faradaic reactions, that is, pseudocapacitance on the surface of nanostructured transition metal oxides( later extended to sulfides) has led to opportunities to construct asymmetrics upercapacitors (ASCs) with aqueous-based solutions at wider potentials. [7][8][9][10] Similar to AC, the Faradaic reaction is related to the specific surface area of the transition metal oxide.T ypically,atransition metal oxide that undergoes aF aradaic reaction is engaged as ap ositive terminal and AC remains as an egative electrode for aw ide range of alkali metal ions (Na + + ,L i + + ,K + + ,e tc.). MnO 2 is ap erfect example of ap ositive electrode for use in ASCs, regardless of structural properties, [9,11,12] which leads to an otable increase in energy density compared to symmetrical assemblies, as in EDLCs.…”

“…[7][8][9][10] Similar to AC, the Faradaic reaction is related to the specific surface area of the transition metal oxide.T ypically,atransition metal oxide that undergoes aF aradaic reaction is engaged as ap ositive terminal and AC remains as an egative electrode for aw ide range of alkali metal ions (Na + + ,L i + + ,K + + ,e tc.). MnO 2 is ap erfect example of ap ositive electrode for use in ASCs, regardless of structural properties, [9,11,12] which leads to an otable increase in energy density compared to symmetrical assemblies, as in EDLCs. Similar kinds of studies have also been conducted in organicbased solutions, in whicht he pseudocapacitance component is replaced with (mostly)i nsertion-type electrodes (a perfect Faradaic reaction in which the bulk is involved in the reaction, rathert han the surface;i ns ome cases, conversion anda lloying have been employed)a nd the AC acts as ac ounter electrode in the presence of aL i + + -containing organic solution.T his new configuration has been termed "lithium-ion capacitors" (LICs), or lithium-ion hybrid electrochemical capacitors;f or example, insertion-type spinel Li 4 Ti 5 O 12 has been used as anegative electrode, with AC as ap ositive electrode.…”