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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 H 2 SO 4 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. Image 1 • 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 FROM AUTHOR]

The low density of porous carbon nanomaterials for supercapacitor electrodes has limited their widespread application, despite their ultra-high gravimetric capacitance. In this work, we successfully prepared highly densified composite electrodes of graphene and single-walled carbon nanohorns (SWCNHs) using a simple spray-drying method that is suitable for mass production. To prepare the high-density composite electrodes, water-based mixtures of oxidized SWCNHs (NHOs) and graphene oxides (GOs) were spray-dried in heated air; after spray-drying, GOs dispersed in water were agglomerated in spherical clusters containing NHO nanoparticles. The reduced spray-dried GO/NHO (rS-GO/NHO) composite electrodes exhibited an extremely high bulk density of 1.23 g·cm−3, which is almost double that of commercial activated carbon (AC) and reduced NHO (r-NHO) electrodes, and three times higher than that of rS-GO electrodes. Of the materials tested, the rS-GO/NHO composite electrode had the highest volumetric capacitance (80 F·cm−3 at 1 mA·cm−2) and a low sheet resistance (0.005 Ω· sq.−1), which are far superior to those of commercial AC (57 F·cm−3 at 1 mA·cm−2 and 0.293 Ω·sq.−1, respectively), without the need for a conductive material, such as carbon black. We expect that these high-density graphene/SWCNH composite electrodes with high volumetric capacitances can be substituted for commercial AC materials in energy storage devices, such as supercapacitors. Unlabelled Image • High-density graphene/single-walled carbon nanohorn composite was produced by facile spray-drying method. • The proposed method is suitable for mass production of this composite material. • The best composite electrode had a bulk density double that of commercial activated carbon electrodes. • The composite supercapacitor electrode exhibited high volumetric capacitance. [ABSTRACT FROM AUTHOR]

[Display omitted] • Porous S-doped graphene was prepared by the chemical vapor deposition of thiophene. • Effects of S-doping on supercapacitor electrode performance were investigated. • Electrochemical performance of porous S-doped graphene was enhanced by S-doping. • S-doping increased the electrical conductivity, wettability, and pseudocapacitance. In this study, we successfully synthesized porous S-doped graphene spheres (SGSs) with an S content of 1.1 at% by chemical vapor deposition on a spherical layered double hydroxide (LDH) template using thiophene (C 4 H 4 S) as the S-containing carbon source. The doped S atoms were removed from the SGSs by heat treatment under a H 2 atmosphere to give desulfurized graphene spheres (De-SGSs) that retained the original pore structure. The effects of the S-doping on the electrochemical behavior of the SGS supercapacitor electrode were investigated by comparing these behaviors with those of the De-SGS electrode. The results revealed that the SGS electrode exhibited electrical conductivity and surface wettability with the electrolyte that were superior to those of the De-SGS electrode. Furthermore, the SGS electrode showed an extremely high capacitance retention of approximately 100% at a current density of 20 mA cm−2 in an electrolyte of 1 M tetraethylammonium tetrafluoroborate in propylene carbonate (TEABF 4 /PC), while that of the De-SGS electrode under same conditions was approximately 40%. Thus, due to its excellent electrical conductivity, wettability, and the pseudocapacitive contribution of its S-dopants, the SGS electrode demonstrates an electrochemical performance that is superior to that of the De-SGS electrode. [ABSTRACT FROM AUTHOR]