An Investigation on the Degradation Behavior of an Asymmetric Supercapacitor of α-MnO₂//AC

Abstract

We have investigated the degradation behavior of a typical asymmetric supercapacitor cell operating in an aqueous solution of Na 2 SO 4 with a voltage window of 2 V, where α-MnO 2 and activated carbon (AC) were chosen as the active materials for positive and negative electrodes, respectively. In this study, potentials of α-MnO 2 and AC electrodes of the asymmetric cell were monitored with respect to an Ag/AgCl reference electrode during repetitive charge/discharge cycling. Since the charges passed at positive and negative electrodes are the same, a change in a certain parameter should be compensated by changes in the other parameters. An irreversible current due to reduction of water on AC electrode triggered capacitance fade in the initial charge/discharge cycling. This phenomenon made the lowest potential more positive to be deviated from the potential region where water reduction can occur, resulting in the following stable operation. Then, α-MnO 2 facilitated the oxidation of water, which deactivated α-MnO 2 itself probably due to an irreversible increase in reduced Mn sites. The deactivation of α-MnO 2 was compensated by expansion of its potential window in the cathodic scan, leading to further capacitance fade due to dissolution.Supercapacitor, also called electrochemical capacitor, can be categorized into two classes according to their charge-storage mechanisms; i.e., (i) electrochemical double-layer capacitor (EDLC) and (ii) pseudocapacitor. EDLCs store charge electrostatically via reversible ion adsorption/desorption at an electrode/electrolyte interface. Here, carbon-based materials with high conductivity and large surface area are commonly used as the electrode. The charge stored in EDLCs is limited in a range of 3-8 Wh kg -1 , which is much smaller than the energy density (>100 Wh kg -1 ) of batteries. 1 Extensive research has recently been devoted to pseudocapacitors which utilize faradaic reactions at the surface (and the bulk) of electroactive materials. The materials providing pseudocapacitance involve electrically conducting polymers (polyanilines, polythiophenes, polypyrroles, etc.) and transition metal oxides (ruthenium oxide, manganese oxide, iron oxide, nickel oxide, etc.). Among them, manganese oxides (mainly MnO 2 ) are often considered the most promising materials because of their high theoretical specific capacitance (1370 F g -1 ), low cost, and environmental benignity. 2,3 Pseudocapacitance of MnO 2 arises from the following redox reactions:where C + denotes electrolyte cations (Na + , K + , Li + , etc.). According to a comparative study performed by Devaraji et al., α-type MnO 2 has the highest specific capacitance (297 F g -1 at 0.5 mA cm -2 ) among various crystalline forms. 4 An expansion of the potential window of MnO 2 electrode is also an important subject with maintaining both electrochemical reversibility and rate capability because the energy density (E, Wh kg -1 ) is proportional to the square of the operating voltage (V, V).where C (F g -1 ) is the specific capacitance. Aque...

Macro‐ and Nano‐Porous 3D‐Hierarchical Carbon Lattices for Extraordinarily High Capacitance Supercapacitors

“…The degradation of the asymmetric MnO 2 /carbon supercapacitor can be attributable to the reduction of water on the carbon electrodes, and the deactivation and dissolution of MnO 2 , which is described in more detail in the literature. [ 62 ]…”

“…The degradation mechanisms of MnO 2 electrodes have been discussed in the literature, which can be mainly attributable to the dissolution of Mn ions into the electrolyte or the deactivation of MnO 2 by water oxidation. [60][61][62]…”

Sustainable eco‐friendly sub‐micron NaCl crystal powder‐assisted method to synthesize SiOx/C as anode materials originated from rice husk for lithium‐ion batteries

“…46‐1045) 27 . These characteristic peaks indicate that SiO 2 was present in the rice husk precursor, and it was partially reduced during the second carbonization to SiO x containing low‐valence Si 28 . Therefore, the B‐MRH_2nd C sample consisted of crystalline SiO 2 and amorphous C composites (SiO 2 /C).…”

“…27 These characteristic peaks indicate that SiO 2 was present in the rice husk precursor, and it was partially reduced during the second carbonization to SiO x containing low-valence Si. 28 Therefore, the B-MRH_2nd C sample consisted of crystalline SiO 2 and amorphous C composites (SiO 2 /C). The XRD patterns of S-MRH_1st C and S-MRH_2nd C resemble those of typical amorphous SiO x /C composites derived from rice husks (Figure 3C, D).…”

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