Various studies have been performed to combine 1D and 2D carbon nanostructures. Among various 1D carbon allotropes, CNTs (1D) have been extensively employed for incorporation into 2D nanocarbons. To date, CNTs have been extensively studied as the electrodes of EDLCs. CNTs have a high mechanical resilience and open tubular structure, making them promising as electrodes of high power energy devices. In addition, their tubular geometries provide good scaffolds or supports for other active materials. One of the main drawbacks of CNTs is their relatively small specific surface area (smaller than 500 m² g⁻¹ [2]) compared to those of other large-surface-area carbons (e.g. activated carbons), which hinders their applications in high-energy EDLCs.

Graphene was often chosen as a 2D nanocarbon for the electrodes of electrochemical energy storage devices due to its excellent properties including very high electrical and thermal conductivities, tunable bandgap, large surface area, and very high mechanical strength [13]. CNTs were combined with graphene for the electrode materials of rechargeable lithium-ion secondary batteries [14] and transparent electrodes [1]. The first graphene-based EDLCs were reported by Ruoff et al. [15]; the specific capacitance of the graphene-based supercapacitor was up to 135 F g⁻¹ in an aqueous electrolyte.

The main advantage of the incorporation of CNTs into graphene can be explained as follows. The performance of the graphene-based supercapacitor is limited owing to restacking of graphene during the processing. Therefore, the actual surface area of the graphene electrode is usually significantly smaller than the theoretical surface area (larger than 2600 m² g⁻¹) [16]. The addition of CNTs between graphene sheets provides spacers: thus, minimizing the restacking.

There have been numerous reports on explicitly employing CNTs as spacers to prevent the restacking of graphene. Cheng et al. [17] reported a graphene and single-walled CNT (SWCNT) composite film prepared by a blending process. The supercapacitors using graphene/SWCNT as electrodes exhibited very high specific capacitances of 290.6 F g⁻¹ and 201.0 F g⁻¹ in aqueous and organic electrolytes, respectively. It is reported that the addition of SWCNTs, which acted as a conductive additive, spacer, and binder, increased the energy density by 23% and power density by 31% compared to those of the graphene electrodes. Yang et al. [18] reported that the stacking of individual graphene sheets was effectively inhibited by introducing CNTs to form a 3D hierarchical structure. The specific capacitance of CNT/graphene was significantly higher than that of graphene (326.5 F g⁻¹ vs. 83 F g⁻¹). Preventing graphene sheets from restacking by use of hydrothermally introduced CNTS as spacers showed a high specific capacitance of 318 F g⁻¹[19]. Recent studies have been performed on the applications of CNTs as spacers including nitrogen-doped graphene/CNTs [20], chemical-free synthesis of graphene/CNTs (for lithium-ion batteries), graphene layers pillared with SWCNTs [21], flexible nitrogen-doped graphene/ CNT papers [22], and NiOH₂-graphene/CNTs [23].

Hybrid films of graphene and CNT were reported before the recent development of multi-dimensional nanocarbons. In 2008, Cai et al. reported the fabrication of conductive CNT films hybridized with graphitic oxide nanoparticles used as a carrier [24]. Tung et al. demonstrated nanocomposite comprised of chemically converted graphene and CNTs without surfactant to preserve the intrinsic electronic and mechanical properties of both components [1]. In a more recent report, an in situ reduction of exfoliated graphite oxides (GOs) in the presence of cationic poly(ethyleneimine) (PEI) was employed to perform self-assembly of multi-walled (MW) CNTs forming the hybrid films [25]. The sequential self-assembly of functionalized 2D graphene sheets and 1D CNTs through electrostatic interactions were reported to be suitable for the electrochemical measurements. In another recent study, a novel graphene foam was employed to make hybrid film of graphene foam/CNTs for flexible asymmetric supercapacitors [26].

Many approaches have been performed to prepare the desired graphene/CNT multi-dimensional nanocarbons, including microwave irradiation, solution based process, and CVD method. Each preparation method has advantages and disadvantages. In applications as electrodes of supercapacitors and batteries, a strong bonding of CNTs on graphene is important. In order to address the bonding issue, reports on direct growth of CNTs typically through the CVD method are increasing. Yu et al. reported a direct CNT growth on the graphene nanoplatelets (GNPs) via a thermal CVD process [27]. Vertical growths of CNTs on patterned grahenes were performed, yielding good flexibility and electrical conductivity [28]. Similar direct growths on graphenes or graphene-related nanocarbons were carried out by CVD method as the preparation method of CNT/graphenes.

Beidaghi et al. reported graphene/CNT composite micro-supercapacitors through electrostatic spray deposition. Their supercapacitors exhibited a high volumetric capacitance with a low RC time constant (4.8ms) [16].

CNTs and graphenes were combined to prepare the transparent conductors and electrode materials for lithium ion batteries [1]. CNTs and graphene sandwich structures with CNT pillars were fabricated by CVD for the electrodes in supercapacitors [1]. Graphene nanoplatelets were combined with CNTs without any buffer layers to improve both in- and through-plane thermal conductivities [1].

As graphenes are very thin and highly conductive, they are increasing employed as electrodes of flexible supercapacitors, which also formed another emerging field. The addition of CNTs into graphenes is beneficial as CNTs are highly conductive and porous, thus enhancing electronic and ionic tranports [29].

Another class of multi-dimensional nanocarbons are CNTs on various carbon films. Early demonstrations of CNTs on carbon films include an aligned growth of CNTs on graphitic substrates through plasma-enhanced CVD [30-32], supergrowth of aligned CNTs directly on carbon papers [33], and vertically aligned CNT growth on glassy carbon films by thermal CVD for cyclic voltammetry study [34]. The carbon films in these studies have several merits and advantages as the electrodes. For example, graphitic carbons are inexpensive and simple to handle. An early growth of CNTs showed low degree of alignment, while more recent works provided highly aligned or even vertically aligned CNTs on carbon films. In a study by Liu et al., vertically aligned CNTs were successfully formed on the substrates and their electrochemical properties as voltammetric electrodes were evaluated by obtaining cyclic voltammo-grams.

Fullerenes and their derivatives are considered promising as electrodes of supercapacitors because of their reversible redox charge storage and large surface areas (1100-1400 m²g⁻¹ [35]. However, reports on supercapacitors based on fullerenes or fullerenes combined with other carbon allotropes are limited in the literature mainly owing to the low specific capacitances [13].

However, numerous studies have been performed on multi-dimensional carbons combining fullerenes and other nanocarbons. For example, covalent bonding of 0D fullerenes onto 2D graphene oxide surface was achieved by Zhang et al. [36]. In their study, the functionalization of the graphite oxide was realized by fullerene insertion and chemical reaction through Fisher esterification, in which the esterification took place between the hydroxyl groups of the graphite oxide and the carboxyl groups of the fullerenoacetic acid.

Various electrochemical sensors can be fabricated on composites of fullerenes and various carbonaceous materials. A fullerene-C₆₀-modified edge plane pyrolytic graphite electrode was demonstrated to electrochemically investigate prednisone [37]. Dopamine sensors were fabricated using C₆₀-functionalized MWCNT films [38]. The C₆₀-MWCNTs film showed special selectivity and sensitivity for detection of dopamine than those of a bare MWCNT film. In another study, fullerenes were supported on graphene oxide through noncovalent conjunction through facile grinding method [39]. A phosphotungstic acid-graphene oxide-fullerene film was prepared to fabricate an electrode, which exhibited showed catalytic activity for redox of biomolecules.

The successful demonstrations of fullerenes-based nanocarbons, particularly for applications in the electrochemical sensors, suggest that supercapacitor electrodes made of fullerene nanocarbons could be feasible.

Despite their excellent chemical, physical, and electrochemical properties, graphenes naturally aggregate and stack to multilayers during the drying process in the electrode fabrication. Multilayered graphenes exhibit worse physical and chemical properties than those of the monolayer state [40,41]. In order to avoid the agglomeration, it was proposed that metal oxides such as SnO₂ or MnO₂ were inserted into the interlayers of graphene. The metal oxide “spacers” demonstrate another advantage of pesudocapacitances originating from metal oxides leading to the increase of the total capacitances. SnO₂ -loaded CNTs were used as spacers in graphenes [42]. γ-MnO₂ nanoflowers were anchored onto CNTs combined with graphene nanosheets formed the flexible electrodes [40]. The ternary components (γ-MnO₂-CNTs-graphenes) are reported to double the specific capacitance compared to that of the graphene-only electrodes, and the capacitance is the highest reported for symmetric supercapacitors containing MnO₂ and graphenes [40]. Similar structure, multilayer nanoflake structure composed of MnO₂/grapehene oxide with 3D hierarchical porous structure was reported as supercapacitor electrodes with enhanced stability [43]. In more recent applications, nitrogen-doped graphene foam/carbon nanotube/manganese dioxide were fabricated via chemical vapor deposition and hydrothermal method [44]. While MnO₂ has been extensively employed as the spacers, Rakhi et al. reported recently crystalline RuO₂ nanoparticles loaded to carbon nanocoils as spacers in reduced graphene oxide [45]. The RuO₂ -based ternary electrodeexhibited a high specific capacitance of 725 F g⁻¹ at a scan rate of 20 mV s⁻¹. The ternary electrodes were also employed as electrodes of lithium ion batteries. Si is one of the highest specific capacity anode materials but generally, it has poor cycle performance due to its significant volume expansion during the charging and discharging. In order to overcome this problem, ternary Si-based composite electrodes were demonstrated, in which Si nanoparticles were coated on a thin carbon layer and encapsulated in a graphene framework [46].