Interfacial Redox Phenomena for Enhanced Aqueous Supercapacitors

Abstract

The paper reports on electrochemical characterization of several supercapacitors operating in aqueous electrolytes and discusses a strategy for enhancing their performance. Various electrolytes such as alkaline (6 mol L −1 KOH), acidic (1 mol L −1 H 2 SO 4 ) and neutral (1 mol L −1 KI and 1 mol L −1 Na 2 S 2 O 3 ) were investigated in terms of electrochemical stability and redox activity. A new concept of capacitor with two electrodes operating in different electrolytes is demonstrated. The positive electrode worked in KI solution whereas KOH solution served as an electrolytic medium for the negative electrode. As a result, improved capacitance values and energy density for a capacitor with a combined electrolyte have been obtained because of the beneficial redox phenomena on both electrodes. Electrochemistry with its fundamental and applied branches gives an excellent feedback for a dramatic increase of energy demand and offers several solutions for various energy conversion and storage systems. At the interface between electrode and electrolyte, several mechanisms of energy conversion and storage might be exploited in batteries, fuel cells and electrochemical capacitors. Electrochemical capacitors (quite often named as supercapacitors, ultracapacitors, etc.) are excellent devices for power demand but they suffer from moderate energy density.1-3 This feature greatly distinguishes them from batteries which store energy in a faradaic manner and provide high energy but moderate power. Therefore, their application and final target is different. Moreover, one should conclude that the competition between supercapacitors and batteries is pointless and it is more reasonable to consider them rather as complementary to each other. 4 On the other hand, a remarkable interest of scientific community is observed in the development of novel materials based on transition metal oxides and conducting polymers providing good capacitive behavior but involving slow faradaic processes in order to enhance the energy performance and introducing pseudocapacitance. 5-11Another trend in recent research in supercapacitor development is to exploit redox processes originating from electrolyte solution. Contrary to redox active electrode materials, such a concept offers fast charge transfer reactions on the electrode surface, limited either by diffusion of a redox specimen from the electrolyte bulk to the interface or specific adsorption of ions within the pores of the electrode. To date, redox active electrolytes are based on iodide/iodine, bromide/bromine or hydroquinone/quinone redox couples.12-19 A variety of halide oxidation states allows reaching a remarkable contribution of pseudocapacitance; however, only iodide-and bromide-based electrolytic solutions seem to be promising for supercapacitor application. Fluorine and chlorine have too high formal oxidation potentials (i.e. above oxygen evolution potential) to be successfully and reversibly reached in aqueous solutions. Additionally, the reversibility of the redox reaction strongly de...

Appropriate methods for evaluating the efficiency and capacitive behavior of different types of supercapacitors

“…According to Ref. [16], the low η E ≈ 50% at 1.6 V for the KI system could be due to a blockage of the hydrogen adsorption sites by the iodide reaction products (iodine and iodates), leading to irreversible di-hydrogen evolution. Misleading coulombic efficiency values up to 100%, without evaluation of energy efficiency, have also been reported for other faradaic systems (conducting polymer, functionalized carbon with redox electrolyte, etc.)…”

A High‐Voltage Electrochemical Cell Operating with Two Aqueous Electrolytes of Different pH Values

“…The voltage limit for cells in aqueous electrolytes is the positive electrode, due to oxidation of the carbon electrode material and/or corrosion of current collectors when the thermodynamic potential of water oxidation is exceeded ,. To solve such issue, Frąckowiak et al suggested an asymmetric cell with KOH and H 2 SO 4 (or LiNO 3 ) electrolytic media for the negative and positive electrodes, respectively, both separated by two porous glassy fiber membranes ,. Unfortunately, a sharp capacitance decrease of the (−) KOH/LiNO 3 (+) system occurred in the first 500 cycles, which suggests possible mixing of electrolytes due to the use of a porous separator …”

Redox‐Mediator‐Enhanced Electrochemical Capacitors: Recent Advances and Future Perspectives

“…Many efforts have been devoted to the redox‐mediator‐enhanced supercapacitors. As a result, a variety of redox mediators and electrolyte systems have been developed …”

“…Therefore, the CV curve of the positive electrode now shows a pair of redox peaks and the negative one a rectangular shape (Figure g) and the GCD curve of the positive electrode exhibits the sloping potential plateaus whereas the potential of the negative electrode varies linearly with time (Figure h). After the first successful attempt of enhancing the capacitance by introducing Ag + as mediator, a large number of redox mediators were developed for supercapacitors . Based on the used redox mediators, the single‐redox‐mediator‐enhanced supercapacitors can be divided into three categories: metal‐mediator‐based supercapacitors, inorganic‐mediator‐based supercapacitors, and organic‐mediator‐based supercapacitors.…”

“…[54] To extend the voltage window and increase the capacitance contribution of the redox mediators, ap H-combined capacitorw as assembled by using KOH and KI as negative and positive electrolytes, respectively,a nd glassy fiber as the separator.T he voltage could be increased up to 1.5 V, exceeding the stability window of aqueous electrolytes ( Figure 6). [55] Different from the separated electrolyte system,amixed electrolytes ystem with KI and KOH, reported by Ji and co-workers, was proposed for carbon-based supercapacitors. [56] The capacitor could also be charged to 1.6 Vr esulting from the adsorption/desorption of hydrogen at the negative electrode and the IO 3 À /I À redox reaction at the positive electrode.…”