PEDOT-modified laser-scribed graphene films as bginder– and metallic current collector–free electrodes for large-sized supercapacitors

Highlights

• The laser writing technique has been applied to prepare porous graphene.

• The surface of laser-scribing graphene films has been modified with PEDOT.

• The compositecan be directly applied for supercapacitors without metallic current collector.

• The process does not require an additive or solvent and it can be performed at low temperature.

Abstract

The rapid development of wearable electronic devices and energy storage devices has increased the demand for flexible, lightweight, and durable supercapacitors. Nevertheless, the cost-effective synthesis of suitable active materials and the facile fabrication of electrodes for energy storage systems remain a challenge for practical applications. In this study, we developed a scalable method for the fabrication of graphene-based supercapacitors, by using a CO₂ infrared laser to transform polyimide (PI) films into porous graphene. Furthermore, when modified with oxidatively polymerized poly (3,4-ethylenedioxythiophene) (PEDOT), the conductivity of the graphene films was enhanced significantly. The resultant films could be fabricated directly for use in supercapacitors without employing metallic current collectors. The current collector–free supercapacitors exhibited excellent electrochemical properties, rivaling those obtained from corresponding devices featuring metallic current electrodes. An assembled device having a large working area (4 × 4 cm²) displayed reversible capacities of 115.2, 97.0, and 78.4 F/g at rates of 0.5, 2, and 6 A/g, respectively. Moreover, only slight losses in capacitance occurred after 4000 charge/discharge cycles and 2000 bending cycles, indicative of remarkable cycling life and mechanical stability.

Introduction

Supercapacitors combine the energy storage properties of batteries with the power discharge characteristics of capacitors. Unlike conventional batteries, which use chemical reactions to store energy, supercapacitors store energy by forming an electric double layer at the interface between an electrolyte and an electrode. Such an arrangement implies that supercapacitors can be charged and discharged much faster, and with longer cycling life, than batteries [1], [2]. Through this mechanism of energy storage, a higher surface area and conductivity will result in a greater amount of charge being stored within a supercapacitor. Because the two-dimensional material graphene has an extremely high surface area and remarkable electric conductivity [3], [4], [5], [6], it is a suitable electrode material for use in supercapacitors.

Graphene can be prepared using a variety of techniques that can be grouped mainly into top-down and bottom-up approaches [7], [8], [9]. The top-down approaches focus on the exfoliation of bulk graphite through, for example, mechanical cleavage, liquid phase exfoliation, and the oxidation of graphite to graphite oxide. In bottom-up approaches, graphene can be prepared through molecular growth from small molecular carbon precursors using chemical vapor deposition or epitaxial growth on a substrate, where the thickness can be controlled through the use of various substrate catalysts and growing parameters. In general, top-down approaches offer higher yields at lower cost; bottom-up methodologies can produce higher quality graphene, but they can be tedious to perform. Therefore, the remains room for the development of facile, non-toxic, and high-throughput routes for the synthesis of graphene suitable for application in supercapacitors.

In general, the standard procedure for the fabrication of electrodes for supercapacitors includes (i) removal of moisture from the active material, (ii) preparation of a slurry, and (iii) a wet coating process [10], [11]. A higher degree of dehydration of the active material can minimize any side effects caused by water molecules during the charge/discharge process, leading to better durability. After drying, the active material should be blended uniformly with a conductive carbon and a binder in a solvent (usually at a weight ratio of 85:10:5) to form a stable, well-mixed slurry. The binder and solvent that have been used most commonly in commercial manufacturing are poly (vinylidene fluoride) (PVDF) and N-methyl-2-pyrrolidone (NMP), respectively. The as-prepared slurry is then cast on a metallic current collector, followed by heating treatment to evaporate the NMP. In this standard process, the use of additives (binder and conductive carbon) increases the weight of the electrode, resulting in a lower energy density. Moreover, evaporation of the solvent after wet coating is a time- and energy-consuming process (usually taking 12–24 h at 120 °C). Furthermore, the high cost and potential pollution of NMP are unfavorable aspects for mass production.

The laser writing technique has been developed recently as an effective method for fabricating supercapacitive electrodes by directly transforming graphene oxide [12], [13], [14] or polyimide (PI) [15], [16], [17], [18], [19] into porous graphene films. This approach has some advantages over the conventional process; for example, it does not require an additive or solvent and it can be performed at low temperature. Moreover, the PI polymer serves not only as a carbon source for porous graphene growth but also as a substrate upon which to directly form a graphene electrode. Therefore, the synthesis of graphene and the coating of the electrode can be integrated into a single process, without the need for metallic current collectors. These features make laser writing a perfect process for the inexpensive production of graphene-based supercapacitors displaying high energy density.

The laser scribing technique has been used previously, however, only to demonstrate the fabrication of microsupercapacitors. For real applications, supercapacitors of large size will be required to satisfy the need for large amounts of stored charge. Unfortunately, the sheet resistance of porous PI-derived graphene (PIDG) remains too high at present, and its electrochemical properties would be very poor when prepared at large size. To address these issues, in this study we developed an approach to lower the sheet resistance of porous PIDG by modifying it interfacially with highly conductive poly (3,4-ethylenedioxythiophene) (PEDOT), which enhanced the number of pathways for charge transport and, thereby, lowered the resistance. We have fabricated symmetric supercapacitors based on PEDOT-PIDG composite and having an active area of 4 cm × 4 cm. These large-sized supercapacitors delivered a specific capacity of 115.4 F/g at 0.5 A, with excellent rate performance and superior cycling life.