Hydrangea-like multi-scale carbon hollow submicron spheres with hierarchical pores for high performance supercapacitor electrodes

Highlights

• Uniform hydrangea-like multi-scale carbon hollow submicron spheres are fabricated.

• HCSSg contains carbon nanosheets (about 10 nm thickness) and mesoporous channels.

• HCSSg with hierarchical pores possesses high surface area and bulk density.

• HCSSg exhibits high gravimetric and outstanding volumetric capacitive performance.

• HCSSg demonstrates good rate capability and cycle stability.

Abstract

Uniform hydrangea-like multi-scale carbon hollow submicron spheres (HCSSg) are fabricated by a simple hydrothermal method using glucose as carbon source and fibrous silicon dioxides spheres as shape guide. Structure characterization suggests that petal-like partially graphitized carbon nanosheets with the thickness of about 10 nm arranged in three dimensions (3D) to form the hydrangea-like hollow spheres (size ranging from 250 to 500 nm) with mesoporous channels, which can be conducive to be a high specific surface area (934 m² g⁻¹) and bulk density (0.87 cm g⁻³), hierarchical pores structure with good conductivity. As a result, the HCSSg has been demonstrated to be a supercapacitor electrode material with high gravimetric (386 F g⁻¹ at 0.2 A g⁻¹) and outstanding volumetric (335 F cm⁻³) capacitance, good rate capability and cycling stability with 94% capacitance retention after 5000 cycles in aqueous electrolytes, thus suggesting its application potential.

Introduction

As energy storage devices, supercapacitors (SCs) with high power and energy density, excellent cycling stability, and superior reversibility are now attracting intensive attention for many portable systems and hybrid electric vehicles [1], [2], [3], [4]. In general, based on the stores electric energy mechanism, the supercapacitors can be often divided into two types: electrochemical double layer by the physical charge adsorption/desorption at the electrode/electrolyte interface, and pseudocapacitors depending on fast Faradaic redox reaction to harvest energy. The active electrode material with high capacitive performance is indispensable for developing an advanced supercapacitor device. The porous carbon are often used as the active electrode materials for commercial supercapacitors but they show relatively low energy density due to low specific capacitance [5]. For this, the transition metal oxides and conducting polymers have been widely explored as pseudocapacitors to improve their capacitance and energy density [6], [7], [8]. However, it usually suffers from poor cyclability and rate capability. So Research has thus been focused on increasing the specific capacitance (gravimetric and volumetric capacitance) and energy density of supercapacitors without sacrificing the cycle life or high power density.

In the recent years, carbonaceous materials including carbon nanotubes, graphene, carbons nanocages, hierarchical porous carbon, etc. have been investigated as good candidates for supercapacitors [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Among these, hydrothermal carbons (HTCs) with distinctive structure and chemistry, obtained easily through an environmentally friendly process using either pure carbohydrates (glucose, sucrose, starch, cellulose, etc.) or lignocellulosic biomass have received increasing attention over the last years [23], [24], [25], [26], [27]. Since HTCs usually exhibit relatively low energy storage performance due to their little or no porosity, many strategies including chemical activation with suitable agents (e.g., KOH) [28], [29], or hard template methods during the hydrothermal carbonization step have been adopted in order to introduce micro- and/or mesopores and enlarge surface areas and pore volume [30], [31]. For example, the carbon hollow nanospheres with relatively high surface area (658 m² g⁻¹,) and pore volume (1.1 cm³ g⁻¹) prepared through hard template method using silicon spheres as shape guides exhibit relatively high gravimetric capacitance and good cycle stability [31]. Regrettably, good volumetric capacitive performance has been not achieved or overlooked in most of these cases due to the relatively large pore volume of HTCs which may be conducive to be the low bulk density. In fact, the volumetric performance is a more reliable parameter than the gravimetric one for practical supercapacitor applications, especially for compact and portable energy-storage systems. In addition, the rate capability which is related to the conductivity or ion transport channels of the materials is another important parameter for supercapacitors. Therefore, carbon-based materials with desirable morphology, porous structure, relatively high surface area and bulk density are preferable for high gravimetric and volumetric performance supercapacitors.

In this study, novel uniform hydrangea-like architectures carbon hollow submicron spheres were prepared directly using fibrous silicon dioxides spheres as template by hydrothermal treatment with glucose, and followed by carbonizing and etching templates. The prepared carbon hollow submicron spheres possess relatively high specific surface area and bulk density with many mesoporous channels, and hence exhibited excellent energy storage performance (386 F g⁻¹ or 335 F cm⁻³ at 0.2 A g⁻¹), good rate capability (171 F g⁻¹ at 15 A g⁻¹) and high cycling stability (capacitance preserved at 95% after 1000 cycles). Such remarkable electrochemical performance makes it an attractive electrode material for various energy storage devices applications.