Sequential doping of nitrogen and oxygen in single-walled carbon nanohorns for use as supercapacitor electrodes

Highlights

• N/O co-doped SWCNHs are prepared using a straightforward method.

• N/O co-doped SWCNHs can be used as active materials in supercapacitor electrodes.

• Electrochemical performance of SWCNHs is enhanced by N/O co-doping.

• N/O co-doping increases pseudocapacitive contribution and electrode bulk density.

Abstract

To store more energy in limited spaces, the volumetric performance of energy storage devices used in electric vehicles and portable electronics has attracted more research attention than their gravimetric performance. Herein, we describe the preparation of N/O co-doped single-walled carbon nanohorns (SWCNH) using chemical vapor deposition with pyridine followed by acid treatment to obtain a supercapacitor electrode material with high specific volumetric capacitance. The synthesized N/O co-doped SWCNH (N and O contents of 6.1 and 9.1 at%, respectively) electrode had a higher bulk density (1.05 g cm⁻³) than that of the pristine SWCNH electrode (0.86 g cm⁻³). Moreover, the N/O co-doped SWCNH supercapacitor electrode exhibited drastically increased specific volumetric, gravimetric, and areal capacitances (123 F cm⁻³, 117 F g⁻¹, and 91.4 μF cm⁻², respectively) in 1 M H₂SO₄ electrolyte, compared with those of a pristine SWCNH electrode (36 F cm⁻³, 42 F g⁻¹, and 11.4 μF cm⁻², respectively). The superior electrochemical performances are associated with enhanced pseudocapacitive contribution and high bulk density of electrode upon N/O co-doping. The simple method described herein for producing SWCNH electrodes with high bulk density and high specific volumetric capacitance should contribute to the development of supercapacitors with high volumetric performance.

Introduction

In recent years, the increased demand for electric vehicles and integrated portable electronics has prompted intense research in high-power energy storage systems. Supercapacitors have attracted considerable attention due to their advantageous properties, such as fast charge/discharge, high power density, and outstanding stability in cycle life (compared with rechargeable batteries) [[1], [2], [3], [4]]. In general, supercapacitors can be divided into two types based on their energy storage mechanism: electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. EDLCs store energy in pores present in the electrode materials (e.g., porous carbon) via physical adsorption and desorption of electrolyte ions [3,5,6]. In contrast, pseudocapacitors mainly depend on a fast and reversible Faradic reaction of electrolyte ions with specific electrode materials, such as metal oxides and conducting polymers [[7], [8], [9]]. Nowadays, porous carbon nanomaterials, such as single-walled carbon nanohorns (SWCNHs), carbon nanotubes (CNTs), and graphene, are extensively studied to replace the activated carbon materials commercially used for supercapacitor electrode fabrication.

SWCNHs have a set of exquisite physicochemical properties that originate from their unique structure. SWCNHs with a conical tubular structure exhibit diameters of 2–3 nm and lengths of 30–50 nm, which are similar to those of closed CNTs with a single-wall structure. Additionally, thousands of SWCNHs can combine to form spherical agglomerates of a dahlia-like morphology with diameters of 80–100 nm. In contrast, pristine SWCNHs have intrinsic micropores that are formed by interstitial spaces between adjacent SWCNHs [[10], [11], [12]]. Furthermore, due to higher reactivity at defect sites of the SWCNHs (e.g., pentagons and heptagons), the tips and sidewalls of closed SWCNHs can be opened by a simple hole-opening process, such as oxidation treatments in acid solution or O₂ gas atmosphere, resulting in high specific surface area (SSA) [[13], [14], [15], [16]]. Finally, SWCNHs have relatively high electrical conductivity due to their single graphene sheet structure. Such porous and electrically-conductive SWCNHs have been proposed as a promising material for EDLC applications.

Most recently, volumetric performance has been the focus of attention as a criterion for evaluating the energy storage ability of devices, which reflects how much energy can be stored in a restricted space. On the contrary, gravimetric performance does not directly reflect the ability to store energy in limited spaces in electric vehicles and portable electronics [[17], [18], [19]]. Most porous carbon nanomaterials with high SSAs exhibit high specific gravimetric capacitances, leading to a significant reduction in specific volumetric capacitance due to extremely low electrode density [20]. In this regard, to achieve high specific volumetric capacitance at a limited SSA, heteroatom-doping into the carbon structure has been developed as an effective strategy for tuning the intrinsic surface properties of carbon-based electrodes. The introduction of the heteroatoms into the carbon structure can improve the volumetric capacitance, as well as the gravimetric capacitance, by enhancing the pseudocapacitive contribution through a redox reaction between the electrolyte ions and heteroatom-doped electrode surface. Among the different types of heteroatoms, N-doping has been proposed as an approach to improve the specific capacitance of carbon electrodes while maintaining cycling stability. N can be partially or completely incorporated in the carbon structure, generating electrochemically active sites with increased electron density and electron-donating properties [21,22]. Unique electronic properties, originating from the conjugation between the lone-pair electrons of N and graphitic bonds, can enhance specific capacitances per unit SSA. N-doped carbon materials are generally prepared by direct annealing with N-containing precursors at high temperatures [[23], [24], [25]], plasma treatment using N₂ gas [26], and chemical vapor deposition (CVD) using an N-containing hydrocarbon source [21]. In addition, it is well known that O-based functional groups (e.g., quinones) that are present on the surface of a carbon material react with electrolyte ions, increasing the specific capacitance per unit SSA [[27], [28], [29]]. In this context, the synergistic effects of the functional groups based on O and other heteroatoms on the carbon structure surface are expected to improve the volumetric performance of supercapacitors.

In this study, N-doped SWCNHs were synthesized through CVD using a pyridine (C₅H₅N) source. The surface of these N-doped SWCNHs was additionally oxidized by nitric acid (NA, HNO₃) to prepare N/O co-doped SWCNHs. The electrochemical behaviors of these N/O co-doped SWCNH supercapacitor electrodes with high bulk density were investigated with special focus on their specific volumetric capacitance and their specific areal capacitance.