Active Materials Used in Supercapacitor Device Manufacturing

Supercapacitors, also known as ultracapacitors or electrochemical double-layer capacitors, are of considerable importance due to their ability to reach capacitance values in the thousands of Farads in terms of charge storage. Based on energy storage mechanisms, supercapacitors can be classified into two categories [1]. Electric double-layer capacitance (EDLC) and pseudocapacitance.

Introduction

The capacitance resulting from charge accumulation at the electrode/electrolyte interface depends strongly on the surface area of the electrode. While commercial supercapacitor cell samples have a maximum specific capacitance of 25–30 F/g, experimentally obtained cells typically achieve values exceeding 100 F/g [2]. For commercial supercapacitors, a maximum specific energy of E = 5–6 Wh/kg is achieved, whereas for lead-acid batteries this value is E = 35–40 Wh/kg. Lithium-ion batteries reach a maximum specific energy of E = 150–200 Wh/kg. To increase specific energy values, parameters such as the electrolyte solution used in the supercapacitor cell (organic or ionic), the specific capacitance of the cell, and the operating voltage are important. When carbon-based porous structures are used as active materials, charge accumulation in the thousands of Farads can be achieved depending on the proper design of the supercapacitor. Due to their high energy and power densities, supercapacitors are used in electronics, military applications, and hybrid electric vehicles [3, 4]. Fuel cells are electrochemical devices that convert chemical energy stored in fuel into electrical energy through electrochemical reactions without combustion. The U.S. Department of Energy allocates millions of dollars annually for these research efforts [5]. Automotive manufacturers such as General Motors, Ford, DaimlerChrysler, and Toyota have developed several types of prototypes powered by fuel cells [6]. In summary, the active materials used in supercapacitor device manufacturing closely affect the performance of the device. In this review article, you will find current active materials used in supercapacitor device production reported in the literature.

Metal Oxides

Cobalt oxide thin film electrodes were obtained on copper surfaces from cobalt chloride as the cobalt source in an ionic basic medium. The obtained method is a simple and inexpensive chemical coating technique. Supercapacitance and electrode stability tests were conducted in a KOH ionic electrolyte solution. The highest supercapacitance value was Csp = 118 F/g with energy (E = 5.8 Wh/kg) and power density (P = 0.33 kW/kg) [7]. Poly(3,4-ethylenedioxythiophene) (PEDOT)–NiFe2O4 conductive nanocomposite synthesis and electrochemical properties were studied to determine suitability as a supercapacitor electrode material [8]. Nanocrystalline nickel ferrites (5–20 nm) were synthesized by the sol-gel method. The nanocomposite electrode material achieved a specific capacitance of (Csp = 251 F/g), while NiFe2O4 (Csp = 127 F/g) and PEDOT (Csp = 156 F/g) were obtained. Ryu and colleagues investigated PANI–LiPF6 active electrode material as both symmetric and asymmetric supercapacitors. The activated carbon electrode increased the potential window of the supercapacitor and enabled long-term redox reactions [9]. Nanoscale manganese oxide (MnO2) was homogeneously incorporated into a carbon mesoporous structure for use as a supercapacitor electrode. A specific capacitance of Csp = 600 F/g was achieved. After 800 charge/discharge cycles, capacitive retention was achieved at the remarkably high value of 85% [10]. Faradic pseudocapacitor materials such as ruthenium oxide [11], cobalt-nickel oxide [12], nickel oxide [13], manganese oxide [14], and vanadium oxide [15] were studied within suitable potential windows. Metal nanoparticles, particularly gold and silver nanoparticles, have been researched over the past 10 years for their electronic, optical, antimicrobial, and catalytic properties [16]. NiO nanoparticles were incorporated into carbon spheres by air oxidation. The use of these spheres in the electrode material was studied for electric double-layer capacitance. Electrochemical measurements showed that the specific capacitance of ordered carbon spheres increased by 40%, rising to Csp = 205.3 F/g [17]. WO3 films were prepared on Ti surfaces coated with IrO2 using the electrodeposition method. At a scan rate of 50 mV/s, the specific capacitance was determined as Csp = 46 F/g [18].

Conducting Polymers

Polyaniline and its derivatives are used as electroactive materials in electrochemical capacitors due to high Faradaic pseudocapacitance [21] because of the presence of multiple oxidation states [20] in energy storage device applications [19]. Polypyrrole is used extensively for redox supercapacitor electrodes due to fast charge/discharge kinetics, low cost, easy synthesis, and high energy densities [22-24]. The synthesis and characterization of new electron-acceptor-donor-type bis(3,4-ethylenedioxythiophene)-(4,4'-dinonyl-2,2'-bithiazole) comonomers and their electrochemical polymerization on carbon fiber, Pt electrodes, and indium tin oxide (ITO)-coated glass were studied [25]. The polymer is electrochromic. It can be used as an active material. The band gap (Eg) value was calculated as 1.75 V from the onset of π–π* transitions. The λmax value is 2.15 eV. Electrochemical impedance spectroscopy (EIS) is one of the sensitive and reliable methods used to determine parameters such as double-layer capacitance, diffusion impedance, charge transfer, and solution resistance of a system [26]. Polyaniline doped with Zn2+ and H+ was synthesized in HCl solution and supercapacitor behavior was studied using 3- and 2-electrode systems in 1 M H2SO4 electrolyte by cyclic voltammetry (CV), charge/discharge, and electrochemical impedance spectroscopy (EIS) methods. A specific capacitance of Csp = 369 F/g and capacitance retention of 90% after 1000 cycles were obtained [27]. Poly(ethylene terephthalate)-based carbon electrode materials showed low specific capacitance at low current densities of Csp = 197 F/g reported in 2 M H2SO4 electrolyte solution [28]. Polyaniline redox supercapacitor devices in dimethyl sulfoxide electrolyte solution show lower bulk resistance than lithium secondary cells [29]. Polypyrrole film-coated stainless steel electrode was synthesized using 0.5 M p-toluenesulfonic acid and 0.1 M pyrrole by pulse galvanostatic method. A specific capacitance value of Csp = 403 F/g was obtained in 1 M H2SO4 solution [30].

Composite Materials

Polyaniline-carbon nanotube composite was synthesized by Qin using in situ polymerization [31]. The highest specific capacitance of the obtained composite active material was Csp = 560 F/g at 66% polyaniline composite ratio. After 700 cycles, capacitive loss was reported as 29.1%. PANI/graphene oxide (GO) layers synthesized by in-situ polymerization method at a ratio of [ANI]0/[GO]0 = 100:1 achieved the highest specific capacitance of Csp = 531 F/g in the 0–0.45 V potential window and at 200 mA/g constant current in charge/discharge analysis compared to PANI (Csp = 216 F/g) [32]. In the literature, the specific capacitance of activated carbon (AC)–MnOx electrode was determined as Csp = 93.8 F/g. AC was used at a 140% ratio in the electrode [33]. Carbon nanobeads were first developed by Lee and colleagues [34] using a simple pyrolysis technique for use in supercapacitor applications. The supercapacitor device operated with minimal IR drop after 100,000 cycles. As active material, poly(3,4-ethylenedioxythiophene)/polypyrrole composite electrodes were prepared by electropolymerization of EDOT on modified polypyrrole tantalum electrode surfaces. A specific capacitance of Csp = 230 F/g was obtained in 1 M LiClO4 solution and Csp = 290 F/g in 1 M KCl solution [35]. Hybrid asymmetric supercapacitor was coated with p-doped poly(aniline-co-manilic acid) and activated carbon on stainless steel electrode.

The supercapacitor achieved maximum specific capacitance (Csp = 102 F/g) at a scan rate of 10 mV/s [36]. Doped PANI nanofibers for supercapacitors were produced using FeCl3 and ammonium persulfate (APS) as oxidant.

A specific capacitance of Csp = 428 F/g was obtained in the potential range of −0.2 V to 0.8 V in 1 M H2SO4 solution. PANI electrode showed 83% capacitive retention [37]. Polyaniline/copper oxide (CuO), poly(3,4-ethylenedioxythiophene)/CuO, and polypyrrole/CuO nanocomposites were synthesized as supercapacitor electrode materials. The highest specific capacitance at a scan rate of 20 mV/s was Csp = 286.35 F/g for PANI/CuO nanocomposite obtained by CV method [38]. Polypyrrole/carbon aerogel composite material at different polypyrrole ratios was used as active electrode material for supercapacitors by chemical oxidation polymerization [39]. A specific capacitance of Csp = 433 F/g was obtained. Calcium carbide (CaC2) polyaniline composite material was synthesized by in-situ chemical oxidation polymerization [40]. The capacitance of the electrode from the composite material was obtained as Csp = 713.4 F/g at a scan rate of 1 mV/s from CV measurements. Capacitive retention was determined as 80.1% after 1000 cycles. Three-dimensional polypyrrole electrode was designed as a symmetric redox supercapacitor using microelectromechanical systems (MEMS) technology [41]. Graphene/CNT/PANI composite was synthesized by in-situ polymerization. The specific capacitance of the composite material was Csp = 1035 F/g at a scan rate of 1 mV/s in 6 M KOH solution [42]. Ultra-small silicon nanoparticles were used within polyaniline as supercapacitor electrode material. The supercapacitor's specific power was P = 220 kW/kg and specific energy was E = 30 Wh/kg [43].

Conclusions

For active materials used in supercapacitor devices, graphene, graphene hydrogel, activated carbon, metal oxides, conducting polymers, and nanocomposites can provide high capacitance values as well as high energy and power densities. It is inevitable that with good engineering of these materials, supercapacitors will find application in supercapacitor devices. Supercapacitors store charge for electricity faster than batteries. Their applications range from hybrid vehicles to mobile phones across many fields. In the near future, with the development of inexpensive and simple methods, supercapacitor devices along with fuel cells will find wider applications in daily life. Prof. Dr. Murat Ateş / Department of Chemistry – Faculty of Arts and Sciences – Namık Kemal University

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