Electrophoretic deposition of a supercapacitor electrode of activated carbon onto an indium-tin-oxide substrate using ethyl cellulose as a binder

Abstract

A transparent energy storage device is an essential component for transparent electronics. The increasing demand for high-power devices stimulates the development of transparent supercapacitors with high power density. A transparent electrode for such supercapacitors can be assembled via the electrophoretic deposition of an active material powder with a binder onto a transparent substrate. The properties of the binder critically influence the electrochemical behavior and performance of the resulting electrode. Ethyl cellulose (EC) is known as an eco-friendly, transparent, flexible, and inexpensive material. Here, we fabricated an electrode film with EC binder via electrophoretic deposition on an indium tin oxide (ITO) substrate instead of using the conventional polytetrafluoroethylene (PTFE) binder. The assembled electrodes with EC and PTFE were compared to investigate the feasibility of EC as a binder from different perspectives, including homogeneity, wettability, electrochemical behavior, and mechanical stability. The EC enabled the formation of a homogeneous film composed of smaller particles and with a higher specific capacitance compared with films prepared with PTFE. The annealing improved the adhesion strength of the EC because of its glass transition; however, its hydrophobic nature limited utilization of the active material for charge storage. Subsequent electrochemical activation improved the wettability of the electrode, resulting in an increased capacitance of 60 F g⁻¹. Furthermore, even with the lower wettability of EC compared with that of PTFE, better rate performance was possible with the EC electrode. The increased mechanical stability after the annealing process ensured an excellent cycle life of 95 % capacitance retention for 15,000 cycles.

Introduction

Increasing demand for transparent electronics has led to the design of transparent energy storage devices because the energy system is the main component for powering the electronics [1], [2]. Recent smart electronics increasingly require greater current delivery to perform multiple tasks, requiring an energy device with high power performance [3], [4]. Unlike conventional batteries with poor power density, supercapacitors have a high power density as a consequence of the fast ion-adsorption mechanism responsible for storing electrical charge. Thus, supercapacitors should be developed in a transparent form factor for incorporation into transparent electronic devices.

Supercapacitors are electrochemical energy devices that principally comprise two electrodes that emit or accept the electrons and an electrolyte through which ions transport between the two electrodes. As an electrolyte for supercapacitors, aqueous-based solutions are preferred because of their safety, environmentally friendliness, low cost, easy manufacture, and high conductivity [5], [6], [7], [8]. Moreover, an aqueous solvent ensures transparency of electrolytes prepared by dissolving a salt in water as long as the ions composing the salt do not impart a dark tint to the solution.

A transparent electrode needs to be developed as an essential component for a transparent energy device. As a current collector to deliver the electrical current to an electrode material, a transparent conducting material should be used instead of the traditional opaque metals. Among various transparent polymers and oxide films, the widely used transparent conductive material indium tin oxide (ITO) is suitable because of its high transmittance and reasonable electrical conductivity [9], [10], [11]. Although a colored active material is placed on the transparent current collector, a transparent electrode can be attained by aligning the electrode material with the opening on the transparent substrate [12], [13], [14]. Thus, any existing active material can be used to prepare a transparent electrode. We adopted this strategy of patterning an active material on a transparent substrate. Activated carbon is a typical supercapacitor active material because of its high natural abundance, cost effectiveness, commercial availability, and high specific surface area [15], [16], [17], [18]. For the scalable production of an electrode, powdered activated carbon can be deposited with a conducting additive and binder onto an ITO substrate. Electrophoretic deposition (EPD) is economical, convenient, and more rapid than other deposition techniques because it enables a material dispersed in bulk solution to be deposited onto a conducting substrate under an electric field [19], [20], [21], [22], [23]. Furthermore, an electrophoretic-deposited material shows good adhesion to their substrate and is homogeneously deposited onto it [24], [25]. Therefore, the combination of the EPD method and a powdered-type active material enable the fabrication of a transparent electrode suitable for commercial mass production.

An active material film deposited by EPD onto an ITO substrate should exhibit a substantial specific capacitance and reasonable mechanical stability when used as an electrode. Electronic connectivity between the active material powders should be assured to exploit their intrinsic specific capacitance. A binder used to deposit the active material powders onto an ITO substrate for fabricating an electrode should (1) be inexpensive and environmentally friendly, (2) exhibit high film formability to enable homogeneous deposition of the active material onto the surface, (3) exhibit high flexibility and transparency for use with a flexible, transparent substrate, and (4) bind the active material powders through high adhesive strength. To satisfy these requirements, a binder material should be critically selected on the basis of the aforementioned properties.

In previous studies, conventional binders have been adopted to form electrodes without careful consideration of the deposition technique and substrate. Conventional binders used to fabricate electrodes include polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) [26], [27], [28]. Both of these materials exhibit high binding strength, good electrochemical stability, and superior chemical and thermal resistance when used as a binder [29]. However, PVDF requires a toxic and flammable organic solvent when used as a binder in the fabrication of an electrode [30]. In addition, fluorinated polymers can pollute water and soil and are expensive. An electrode with either of these binders cannot be recycled and is environmentally hazardous [31], [32].

To compensate for the shortcomings of traditional binders, researchers have developed low-cost and eco-friendly binder materials. Carboxymethyl cellulose (CMC) is a good prospective binder because of its ecological compatibility and low cost [33]. However, the stiffness and brittleness of CMC result in cracking and failure of the assembled electrode during operation [33], [34]. In addition, CMC cannot be universally used because its high water solubility adversely affects the performance of assembled electrodes used in aqueous-based energy systems [31], [35]. Meanwhile, Na-alginate was demonstrated as an environmentally benign, low-cost, and electrochemically stable binder for a battery electrode [36], [37]. Moreover, due to its higher viscosity, Na-alginate is beneficial to prepare the stable slurry for an electrode than Na-CMC [38], [39]. Natural cellulose has been also studied recently as an alternative binder based on its lower cost than the traditional fluorinated polymer or even CMC binder [30], [40]. The insolubility of this water and almost all organic solvents allows natural cellulose to be used as a binder basically in all kinds of electrolyte [30], [41]. Only ionic liquids can dissolve natural cellulose, so that the expensive ionic liquid should be employed to fabricate the electrode slurry [42].

Recently, ethyl cellulose (EC) has attracted attention as a binder because of its good electronic properties and mechanical stability [43], [44], [45], [46], [47]. Moreover, the transparency and flexibility of EC are beneficial in fabricating an electrode for transparent devices [48], [49]. In the present work, we fabricated an electrode with powdered activated carbon for use in an aqueous-based transparent supercapacitor. To this end, the powdered active material was deposited onto an ITO substrate by the EPD method as a convenient and rapid deposition technique. As a binder material, the feasibility of EC was explored from the perspective of various essential characteristics, including film formability, mechanical strength, and electrochemical performance. An electrode assembled with EC was analyzed and compared with an electrode assembled with the conventional PTFE binder. The superiority of EC over PTFE as a binder is discussed on the basis of their analyzed properties.