A three-dimensional structure of ternary carbon for high performance supercapacitor

Highlights

• 3D-structured ternary carbon was synthesized by a one-step hydrothermal method.

• The 3D structure contains holey graphene, carbon nanotubes, and carbon nano-onions.

• The advantages of different-dimensional carbons were combined into one structure.

• The synergistic effect leads to high capacitance and ultrahigh rate capability.

• High energy and power densities in organic electrolyte were obtained.

Abstract

In supercapacitor applications, the energy storage capacitance and rate performance of graphene are severely weakened by the restacking of graphene sheets, which is a great challenge to overcome for fully exploring its supercapacitive properties. If one of the current strategies for promoting the ion diffusion of graphene is employed alone, only a limited improvement of supercapacitive performance of graphene can be achieved. Here, we combine three carbon allotropes into an all-carbon structure, and improve the ion diffusion and mitigate graphene restacking using combination of three strategies, including creating in-plane holes, self-assembling into three-dimensional (3D) structure, and adding spacers between graphene sheets. Thus a 3D hybrid-structured ternary-carbon (holey graphene/carbon nanotube/hollow carbon nano-onion, denoted as HG-CNT-HCNO) is synthesized as supercapacitor electrode material through a facile and effective one-step hydrothermal method. The synergistic effect of strategies and the different dimensional carbon allotropes endows the 3D structure with hierarchical porous structure, improved electron/ion transport, and increased energy storage sites. Consequently, the HG-CNT-HCNO exhibits high specific capacitance of 236.5 F g⁻¹, ultrahigh rate capability (capacitance retention of 97.9% as the current density increases from 0.5 to 40 A g⁻¹), and high electrochemical stability. Furthermore, the device with organic electrolyte shows high energy and power densities of 71.3 Wh kg⁻¹ and 7.5 kW kg⁻¹, respectively, demonstrating a high energy storage performance. The ternary-carbon structure may open up a new avenue for electrode performance optimization in the future energy storage systems by involving different dimensional carbon nanomaterials.

Introduction

Development of electrochemical energy storage is extremely important because of the rising depletion of the non-renewable sources and the growing demand of clean energy sources [1]. As one important class of energy storage devices, supercapacitor has attracted wide attention due to its advantages of high power capability, long cycle life, and high safety [2]. Different electrode materials have been employed to fabricate supercapacitor devices with electrochemical double-layer and pseudocapacitive energy storage behaviors, mainly including carbon materials, metal oxides, and conducting polymers [3]. Among them, carbon materials exhibit the most stable energy storage performance and are widely used in industrial supercapacitor devices, owing to the excellent conductivity, large surface areas, and high chemical/thermal stability [[4], [5], [6]]. Although pseudocapacitive materials can provide higher energy storage capacities, the degradation of stability and rate capability during cycling still remains a challenge to overcome. Therefore, developing supercapacitor through using all-carbon materials as electrode and enhancing their energy density is meaningful for achieving high and stable performance.

Activated carbon has been widely employed as electrode material on account of large surface area and highly porous structure. However, the enrichment of micropores, low conductivity and hydrophobicity nature limited the ion transportation and accessible surface area [7]. Carbon nanotube (CNT) has also been considered as high-performance electrode material owing to its robust mechanical strength and high conductivity. However, its hydrophobicity nature and high-cost chemical vapor deposition synthesis become obstacles for its large-scale application in supercapacitor [8]. Graphene, with a typical one-atom thick, two-dimensional honeycomb carbon nanostructure, is well known as a promising candidate for supercapacitors due to its low mass density, high specific surface area, and excellent intrinsic conductivity [9,10]. Compared to other carbon materials, graphene can be obtained by chemical or electrochemical reduction of graphene oxide (GO), exhibiting its advantage in facile and large-scale preparation [5]. Moreover, the incomplete removal of functional groups during the reduction can incorporate hydrophilicity and new functions into graphene, providing opportunities for constructing novel structures, tuning pore configuration and improving energy storage performance. However, graphene sheets are prone to restack with each other via π-π stacking interaction and van der Waals force during the synthesis and the subsequent electrode fabrication processes [11,12]. Consequently, the active surface area and pore volume for charge storage are greatly reduced, leading to the significant deterioration of its electrochemical capacitance (typically <200 F g⁻¹ in aqueous electrolytes), low mass diffusion rates and low rate capability [13,14]. Various attempts have been tried to increase the pore volume, improve the ion diffusion, and avoid the restacking of the graphene. The first strategy is to synthesize holey graphene (HG) with in-plane pores on the surface [15,16], which allow for a large ion-accessible surface area, efficient ion transport pathways and high packing density. The second strategy is to self-assemble 2D graphene sheets into 3D graphene hydrogel structures through a one-step hydrothermal method [17,18]. In such a hydrogel, the graphene sheets partially overlap in the 3D space to form interconnected porous microstructure, preventing the restacking of the graphene sheets and providing open channels for electrolyte ion diffusion. The third strategy is to add spacers between the graphene sheets [13,19]. CNTs, carbon black nanoparticles and MgAl-layered double oxide [20] have been demonstrated to be efficient spacers for mitigating the self-restacking of graphene sheets. Besides, the chemically reduced graphene usually has an electrical conductivity of about 100–200 S m⁻¹, two orders of magnitude lower than single-walled CNTs, which limits their performance in the high-rate supercapacitor applications. Therefore, CNTs are not only spacers for mitigating restacking, but also serve as the conductivity enhancers to efficiently increase the conductivity of the hybrid structure [21].

Carbon nano-onion (CNO), with a novel structure consisting of spherical and concentric closed graphitic shells, is a quasi-zero-dimensional member in the nanocarbon family [22]. Owing to the nanosized highly curved surface, CNOs present high power densities and rate capability when used as supercapacitor electrode materials [[23], [24], [25]]. Moreover, adding CNOs in pseudocapacitive materials can greatly increase the power density [26]. Our previous studies have demonstrated that the hollow CNOs (HCNOs) are good candidates for electrochemical energy storage [27,28]. Therefore, it is reasonable to assume that the CNOs can function as triple roles if hybridizing with graphene, including a spacer for mitigating restacking, a retainer for rate capability, and an enhancer for energy storage capacitance.

Holey graphene is dominated by micropores [15], while the 3D graphene hydrogel structure is enriched of macropores [17]. Adding spacer between the graphene sheets can only create more mesopores [20,21]. Therefore, employing only one strategy can not form hierarchical porous graphene structure, which is quite beneficial for electrolyte ion transport/storage [29,30]. As a result, if one of the above strategies for promoting ion diffusion is used alone, only a limited improvement of the energy storage performance of graphene can be achieved. Based on these, rational design of the graphene structure by utilizing the synergistic effect of the above strategies is desired to bestow the graphene with high supercapacitive performance.

In this work, we combined three carbon allotropes into a 3D hybrid-structured ternary-carbon as a supercapacitor electrode material. The hybrid structure was prepared via a one-step hydrothermal process using HG, CNTs and hollow CNOs (HCNOs). By involving the above three strategies, hierarchical porous structure can be obtained in this ternary-carbon along with high capacitance, ultrahigh rate capability, and high electrochemical stability, which are superior to the 3D-structured graphene, HG, and HG-CNT alone. The supercapacitor device with organic electrolyte also delivered high energy and power densities, and its real application as the power source was also demonstrated. The mechanism of the high energy storage and rate performance achieved by the 3D hybrid-structured ternary-carbon was proposed. The in-plane micropores and the 3D self-assembled structures form a hierarchical porous structure, allowing the most of the graphene sheets to be exposed for the diffusion/storage of ions. The incorporation of CNTs and HCNOs is capable to improve the rate and power capability, as well as the capacitance of the 3D hybrid structure. Our work demonstrates a way of combining different strategies and utilizing the synergistic effect of different dimensional carbon nanostructures for maximizing the performance of carbon electrodes for energy storage.