Hierarchical graphene network sandwiched by a thin carbon layer for capacitive energy storage

Abstract

A simple yet efficient one-step route is reported to prepare a three-dimensional interconnected thin-layer carbon network consisting of graphene sheets sandwiched by a porous carbon layer (PCG). Our route relies on the direct pyrolysis of ethylene diamine tetraacetic acid tripotassium salt (EDTA-3K) in the presence of polyelectrolyte-functionalized graphene oxide (GO) at 750 °C. The decomposition of EDTA-3K at elevated temperature yields carbon and potassium species, which in turn serves as an activation agent to generate highly porous carbon layer on graphene sheets. Due to the reduced ion diffusion length, improved wettability and electrical conductivity, PCG electrode exhibits a higher specific capacitance, better rate capability together with smaller ion transport resistance in both aqueous KOH and organic electrolyte as compared with the counterpart carbon electrode prepared by direct pyrolysis of EDTA-3K in the absent of GO. Moreover, PCG electrode shows excellent cycling stability with 97% and 94.3% capacitance retention after 20,000 and 5000 consecutive charge-discharge cycles in aqueous and non-aqueous electrolytes, respectively. Further constant voltage floating experiment by holding the cell voltage at 2.5 V for 200 h confirms the high stability of PCG electrode in TEABF₄/AN electrolyte. These performance demonstrate that PCG hybrid could be one of promising candidates for electrochemical energy storage.

Introduction

Carbon materials with various dimensions hold great promises as electrode materials for electrochemical energy storage in view of their large specific surface area (SSA), good electrical conductivity and excellent chemical and thermal stability. Carbon-based supercapacitors store energy through fast and reversible electrostatic ion adsorption at the electrode/electrolyte interfaces [1], [2]. The very quick energy storage and delivery enable supercapacitor to show high power density, fast charge-discharge rate and outstanding cycling stability [3], [4]. These intrinsic characters are extremely important for extensive applications of supercapacitors in various hybrid electric vehicles, uninterrupted power supply and portable equipments and electronic devices.

Graphene, as an representative 2D carbon materials, has aroused tremendous attention over the past decades due to its large theoretic surface area (2630 m² g⁻¹), superior electrical conductivity, excellent mechanical property and atom-thick planar structure [5], [6], [7], [8], [9]. The intriguing properties of graphene are very attractive for applications as supercapacitor electrode. However, due to the strong sheet-to-sheet van der Waals and π-π stacking, chemically derived graphene tends to agglomerate and restack, resulting in dramatic decrease in surface area for efficient energy storage. To prevent severe stacking of graphene sheets, a promising way is to build graphene sheets into various 3D architectures, including graphene foams [10], sponges [11], hydrogels [12], and aerogels [13], [14], [15], [16]. The interconnected micrometer-scaled macropores can not only store large amount of electrolyte ions but also serve as ion channels for rapid ion motion at high charging/discharging rates. It is well accepted that the charge storage capacity of graphene electrode strongly depends on its ion-accessible surface area. Unfortunately, in building graphene architectures, the physical cross-linking of individual GO/rGO sheets usually causes the local overlapping or coalescing of GO/rGO sheets [17], lowering the ion-accessible SSA and resulting in a poor capacitance performance of graphene electrode.

Chemical activation of carbonaceous precursors with KOH is considered to be a representative route to create a large number of micropores on carbon product [18], [19], [20], [21], The typical preparation process involves mixing the starting carbonaceous precursors with KOH, followed by subsequent chemical activation at high temperature. The reaction of carbon atoms with KOH could generate vacancies, which progressively extend into nanopores on the carbon products [22], [23]. However, homogeneous mixing of the carbon precursors with activation agents at an atomic level remains a challenge. Alternatively, pyrolysis of organic salts represents a facile method to prepare hierarchical porous carbon with a large SSA [24], [25], [26]. In this process, the K- or Na-containing precursor converts into carbon with concomitant formation of the corresponding carbonate salts, which serve as the activation agents to react with carbon atoms to generate nanopores [25]. This strategy allows easy control over the porosity of the carbon products by selecting appropriate precursors and changing the pyrolysis temperature.

Herein, we present a facile one-step procedure for preparing hierarchically porous graphene network by directly pyrolyzing the mixture of ethylene diamine tetraacetic acid tripotassium salt (EDTA-3K) and graphene oxide (GO) (Fig. 1). To favor uniform adsorption of EDTA anions on GO sheets, the GO sheets were first functionalized with poly(diallyldimethylammonium chloride) (PDDA) to endow a positive surface charge. Afterward, by adding EDTA-3K solution into PDDA-functionalized GO suspension, EDTA anions could uniformly adsorb on GO sheets. Finally, pyrolysis of the EDTA-3K/GO composite at temperature of 750 °C yields a hierarchical porous carbon (PCG), which is composed of interconnected graphene sheets sandwiched by a thin-layer of porous carbon with thickness of 15–30 nm. As supercapacitor electrode, the sandwich-like PCG exhibits significantly improved capacitive performance in both aqueous and non-aqueous electrolyte. These performances are better than the counterpart carbon electrode prepared by direct pyrolysis of EDTA-3K (AC), making the reported method one of promising route for preparing high surface area graphene-based hybrid electrodes for electrochemical energy storage.