Towards enhanced energy density of graphene-based supercapacitors: Current status, approaches, and future directions

Journal of Power Sources 396 (2018) 182–206

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Review article

Towards enhanced energy density of graphene-based supercapacitors: Current status, approaches, and future directions

T

Shao Ing Wonga, Jaka Sunarsoa,∗, Basil T. Wonga, Han Linb, Aimin Yuc, Baohua Jiab,∗∗ a

Research Centre for Sustainable Technologies, Faculty of Engineering, Computing and Science, Swinburne University of Technology, Jalan Simpang Tiga, 93350, Kuching, Sarawak, Malaysia b Centre for Micro-Photonics, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, Victoria, 3122, Australia c Department of Chemistry and Biotechnology, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, Victoria, 3122, Australia

H I GH L IG H T S

current status of graphene-based supercapacitors in energy storage market. • Discuss approaches to high energy density graphene-based SC. • Review • Provide potential strategies to develop high energy density graphene-based SC.

A R T I C LE I N FO

A B S T R A C T

Keywords: Graphene Supercapacitors Energy density Approaches

Despite high power density, fast charging/discharging rate, and long operational lifetime, large-scale application of supercapacitor (SC) is limited by its intrinsically low energy densities (of 5–8 Wh kg−1 (gravimetric) and 5–8 Wh L−1 (volumetric)), which are at least 10-fold lower than battery. Since the invention of graphene in 2004, graphenebased SCs have set the upper performance limit of the symmetric carbon-based SCs due to superior electrical conductivity and very high accessible surface area of graphene. Still, only two companies have commercialised graphene-based supercapacitors thus far. Their maximum achievable energy density (i.e., 11.65 Wh kg−1) is too low to make them competitive against batteries in high-energy applications. To this end, this comprehensive review focuses on the material- and device-level approaches to high energy density graphene-based conventional macroscale SCs (≥11.65 Wh kg−1) and flexible SCs and microsupercapacitors (≈0.3–10 mWh cm−3; ≈300–16000 μWh cm−2). It includes a description on how each approach is implemented and an explanation of how each can provide effective research results. This review also meticulously discusses the underlying challenges and possible solutions to achieve high energy density graphene-based SCs in practice.

1. Introduction In the past two decades, supercapacitors (SCs) have been developed intensely to complement batteries or replace them in high power applications such as electronic devices and hybrid electric vehicles due to their fast charge/discharge rate and large power density [1–6]. The configuration of a typical SC is somewhat similar to a battery in that it contains a pair of polarisable electrodes, a separator, and an electrolyte [7]. However, charge storage mechanisms of SCs are different from that of batteries, which mainly operates Faradaically via intercalation and de-intercalation of electrolyte ions into and from the bulk of the active material [8,9]. The charge storage mechanisms of SCs can be classified

into two types i.e., electric double layer (EDL) capacitive mechanism that stores charges electrostatically through electrolyte ion adsorption at the electrode/electrolyte interface [10,11] (Fig. 1(a) and (b)), and the pseudocapacitive mechanism, which stores charges Faradaically mainly through charge transfer on or near the electrode surface (Fig. 1(c)), or intercalation and de-intercalation of electrolyte ions into or out of voids of the electrode materials [7,12,13] (Fig. 1(d)). Although pseudocapacitive materials undergoes fast redox reactions, they exhibit EDLC-like electrochemical behaviours over large ranges of potential, i.e., nearly rectangular cyclic voltammetry (CV) curve and linear galvanostatic charge-discharge (GCD) curve. This characteristic highlights the difference between pseudocapacitive and battery materials [14,15].

∗ Corresponding author. Research Centre for Sustainable Technologies, Faculty of Engineering, Computing and Science, Swinburne University of Technology, Jalan Simpang Tiga, 93350, Kuching, Sarawak, Malaysia. ∗∗ Corresponding author. E-mail addresses: [email protected], [email protected] (J. Sunarso), [email protected] (B. Jia).

https://doi.org/10.1016/j.jpowsour.2018.06.004 Received 28 January 2018; Received in revised form 25 April 2018; Accepted 1 June 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.

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1.1. Graphene and its derivatives Graphene is a one-atom thick, two-dimensional graphite layer arranged in a honeycomb structure [29]. Along with its large theoretical surface area of 2630 m2 g−1 [30,31], graphene was found to have an intrinsic areal capacitance of 21 μF cm−2, which sets the upper capacitance limit for all carbon materials [32]. Graphene can provide an EDL capacitance value of up to 550 F g−1, provided the entire surface area of graphene is fully utilised [32]. Besides large surface area, graphene, in its pristine form, exhibits remarkable electrical conductivity (> 103 S m−1) [33–35] and outstanding electron mobility (> 104 cm2 V-1 s-1) [36,37]. These intriguing properties make graphenebased materials promising for SC applications [38–40]. Monolayer and few-layer graphene can be obtained from chemical reactions involving molecular precursors via a bottom-up approach whereas its derivative—graphene oxide (GO) is formed via graphite exfoliation using a top-down approach [41,42]. In bottom-up approach, graphene is deposited on a substrate after directly synthesised from organic sources such as methane and other hydrocarbon precursors. This approach encompasses chemical vapour deposition (CVD) [43–45] and epitaxial growth [46–48]. Top-down approach, on the other hand, involves exfoliation of graphene from graphite. The techniques include mechanical exfoliation of graphite [49–51], and chemical exfoliation of graphite [52–54]. In contrast to bottom-up approach, these top-down techniques require subsequent GO reduction due to the presence of oxygen-containing functional groups on the GO resulting from oxidation of graphite. Defect-free pristine graphene is generally difficult and expensive to synthesise, especially on industrial scale [55,56]. Chemically-reduced GO is the most commonly prepared alternative to graphene for most applications [34,57,58]. Generally, graphite is first oxidised to form graphene oxide using Hummers' method [59] or modified Hummers' method [60,61], in which concentrated sulphuric acid (H2SO4) intercalates graphite and potassium permanganate (KMnO4) oxidises the acid-intercalated graphite. The oxidation reaction introduces functional groups, for example hydroxyl (–OH) and epoxy (C–O–C) groups to the basal planes, and carbonyl (–C=O) and carboxyl (–COOH) groups to the sheet edges [34,62,63]. The presence of these functional groups in GO contributes to the distinct physical and chemical properties of GO relative to graphene. On the upside, the oxygen-containing functional groups in GO create electrostatic repulsion and weaken the Van der Waals forces between graphene layers. The increase of the interlayer spacing is beneficial for water penetration during subsequent GO exfoliation through sonication [45,64] as well as electrolyte ion intercalation. In addition, those functional groups with anionic or polar character have high affinity to water molecules, which makes GO hydrophilic and is readily dispersed in water and some organic solvents (e.g., ethylene glycol) [34,65–67]. Such hydrophilicity leads to the increased wettability between GO and electrolytes and hence, improved ion accessibility to the electrode [68]. On the down side, excessively high amount oxygen functionalities in GO cause substantial decrease in electrical conductivity as they disrupt the conductive sp2 domains of graphene

Fig. 1. Schematic diagram of different mechanisms of capacitive energy storage. Arising from adsorption of negative electrolyte ions on the positive electrodes, EDL capacitance develops at electrodes consisting of (a) carbon particles or (b) porous carbon. Pseudocapacitive mechanisms include (c) redox pseudocapacitance, as induced by hydrous RuO2, and (d) intercalation pseudocapacitance, where Li+ ions from electrolyte are inserted into the active materials. Reproduced with permission [267].

Electric double layer capacitors (EDLCs), the most common rechargeable power devices at present, comprise of two similar EDL electrodes. Electrode materials with large surface area and porous structure are thus desirable for charge accumulation and rapid ionic motion in EDLCs. To date, porous carbonaceous materials, especially activated carbon (AC), have been widely used as EDL electrodes in commercially available SCs [16–18]. AC offers large surface area (i.e., 1000–2000 m2 g−1) [19,20] to trap a large amount of ions in the electrode/electrolyte interface for maximum capacitance (i.e., how much electric charge can be stored in a SC) [21]. The large surface area also endows AC with a theoretical area-normalised EDL capacitance (i.e., how much charge can be stored in a given area of active materials) of 15–25 μF cm−2 [13]. Still, ACs used in commercial EDLCs suffer from several limitations, for example, the presence of bottle-necked shaped pores and relatively low electrical conductivity (10–100 S m−1) [22–24]. This limits ion transport and the resultant specific capacitance to below 200 F g−1, which in turn translates to a low maximum energy density (i.e., how much energy is available in a SC) of only 5 Wh kg−1 in the resultant SCs [21,25]. Such an energy density is at least an order of magnitude lower than that of the commercial batteries (see Table 1). Recent research efforts therefore attempt to develop novel carbonbased electrode materials including graphene for use of SCs with high power and energy densities.

Table 1 Comparison of performances between lithium-ion batteries and commercial SCs. Reproduced with permission [26–28]. Characteristic

Standard lithium-ion batteries

Commercial SCs

Charge/discharge time (s) Cell voltage (V) Cycle life Energy density

3600–14400 (1–4 h) 3.2–3.7 500–2000 100–265 220–400 100–1000 220–400

1–10 2.3–3.0 > 100000 2.3–8 5 (typical) 3000–40000 Up to 10000

Power density

Gravimetric (Wh kg−1) Volumetric (Wh L−1) Gravimetric (W kg−1) Volumetric (W L−1)

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not only graphene-based conventional SCs as with Skelcap and CSRCAP, but also flexible thin-film and fibre SCs as well as interdigitated microsupercapacitors (MSCs), which have been increasingly developed at laboratory level. To define the quoted high energy density in this review, we have set the baseline (see Table 2). It is worth to note that there are distinction between conventional, flexible SCs and MSCs when expressing particular performance metrics (e.g., gravimetric, volumetric, or areal parameters) [94,95]. However, the energy densities of the graphene-based flexible SC and MSC prototypes are expected to be far less than that provided by those commercial thin-film batteries since no commercial conventional SC has outperformed battery in terms of energy density to date.

[69–71]. Therefore, in contrast to graphene, GO is an electronic insulator [69]. Reduction of GO is thus necessary to recover its lost electrical conductivity for practical applications. Reduced graphene oxide (RGO) is normally obtained through thermal reduction [72–74], chemical reduction [75–77], or electrochemical reduction [78] to moderately remove surface functional groups and then restore its originally high electrical conductivity. Porous graphene is a modified graphene-related materials with nanopores in the planes. The distinctive pore structure in porous graphene sets it apart from graphene in term of properties [79,80]. Pores in porous graphene can be categorised into two types: in-plane pores that exist in a monolayer graphene, and interlayer spacing (interlayered pores) found in 2D laminar (i.e., multi-layered) materials [81]. The concurrent presence of both pores in hierarchical structure can be found in 3D graphene framework [82–84]. An optimum in-plane porosity can provide more accessible interior volumes and surface area [85,86] while the well-controlled interlayer spacing can suppress graphene sheets re-stacking and allows efficient ion diffusion across neighbouring layers [87,88]. The synergistic effect of both pores in graphene-based electrodes can greatly enhance charge storage capability of SCs [43,89,90].

2. Useful approaches and performances of high energy density graphene-based supercapacitors 2.1. Material-level approaches Since energy density is a product of cell capacitance and squared cell voltage, graphene-based electrode materials and electrolytes are two most important determinants of graphene-based SC energy density. The former greatly governs the cell capacitance while the latter determines the cell working voltage. In this regard, graphene-based electrode materials and electrolytes have been widely explored for graphene-based SC with high energy density.

1.2. Current status of graphene-based supercapacitors The commercialisation of graphene-based SCs is still uncommon—only two companies have commercialised graphene-based conventional macroscale SCs i.e., consist of two graphene-based electrodes with electrolyte-saturated separators sandwiched between them in sealed casings. Since 2012, Skeleton Technologies from Europe has commercialised a series of high performance symmetric EDLCs called SkelCap that are based on curved graphene. The 2.85 V/4500 F graphene-based SCs were reported to have achieved an energy density of 9.6 Wh kg−1 without compromising its power density (a 95%-efficient power capability of 1730 W kg−1) [91]. Besides that, a Chinese-based company known as Ningbo CRRC New Energy Technology has launched CSRCAP 3 V/12000F SCs with electrodes in composite of graphene and activated carbon, which can achieve energy density as high as 11.65 Wh kg−1 [92]. The energy performance of these graphene-based SCs surpasses those (3–5 Wh kg−1) of commercial carbon/carbon symmetric EDLCs from Maxwell, Panasonic, EPCOS, and Nesscap [93]. The SkelCap and CSRCAP SC performances are almost even on a par with some asymmetric carbon devices carrying an energy density of 10–12 Wh kg−1, for instance those manufactured by Fuji Heavy Industry and JSR Micro [93]. This suggests the excellent energy performance of graphene electrodes compared to activated carbon ones and the high potential of graphene-based SCs to approach energy performance of batteries. Therefore, in this review, we discuss deeply some useful material– and device–level approaches to high energy density of graphene-based SCs, as well as the rationale behind each approache. This paper covers

2.1.1. Graphene production methods One of the common approaches to energy density improvement in graphene-based SCs is altering the manufacturing conditions, e.g., type and amount of chemical used, temperature, and exposure time to chemical, heat or electric current, during graphene production. These manufacturing conditions have significant effect on porous structure of graphene, which in turn determines the electrochemical performance of SC. 2.1.1.1. Chemical reduction of graphene oxide. To achieve a high cell specific capacitance of 154.1 F g−1 (at a current density of 1 A g−1) and an impressive practical energy density of 21.4–42.8 Wh kg−1, Liu et al. [98] assembled two identical electrodes made of curved graphene sheets in a coin cell filled with IL electrolyte, i.e., [EMIM][BF4] that works at a voltage up to 4.5 V. They first injected the GO suspension into a forced convection in which compressed air was introduced to keep the solid GO particles in motion. Upon removal of the solvent, the GO sheets shrunk and curled up. The curved morphology prevented face-to-face aggregation of GO sheets during packing and compression into an electrode structure, thus maintaining a mesoporous structure with pore sizes in the range of 2–25 nm. The formation of mesopores was essential for good accessibility of IL-based electrolytes, which are significantly larger in molecular size (0.9–1.3 nm) [99] compared to those of conventionally used aqueous electrolytes (around 0.7 nm) [100]. The dried curved GO was later reduced to graphene in the presence of hydrazine. During the chemical reduction, hydrazine reacted with epoxy groups (C–O–C) on the plane of GO sheets to produce CO2 and steam [101]. The decomposition of the functional groups restored the conjugated areas of GO sheets; providing more pathways for carrier transport within the graphene sheets. Despite strong reducing capabilities, hydrazine is a very toxic compound [102]. Therefore, Perera et al. [103] introduced a green reducing agent—sodium hydroxide (NaOH)—to reduce GO via hydrothermal (i.e., an aqueous-based reaction at elevated temperature and pressure) deoxygenation process at 120 °C. While detaching large oxidised debris bonded to the GO during hydrothermal treatment, the strong alkali, unlike hydrazine, did not disrupt the graphene structure. The unchanged large sp2 domain in alkali-deoxygenated GO (hydrothermalised GO or hGO) resulted in lower charge resistance due to the more efficient charge transfer kinetics and shorter diffusion path for electrons. This facilitated ion adsorption that created double layers at

Table 2 Baseline for high energy density graphene-based SCs discussed in this review. Configuration of SC Conventional SC Flexible SC and MSC

Energy density

Baseline −1

Gravimetric (Wh kg ) Volumetric (Wh L−1) Volumetric (mWh cm−3) Areal (μWh cm−2)

11.65a –b 0.3–10c 300–16000c

a Value based on the maximum energy performance of commercial graphene-based EDLC. b No information available for commercial graphene-based EDLC. c Information taken with reference to commercial thin-film batteries and microbatteries e.g., Thinergy from Infinite Power Solutions, EnFilm from STMicroelectronics, MS Li from Seiko, and 4 V/500 μAh lithium thin-film battery [96,97].

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Fig. 2. (a) Schematic illustration of the synthesis of a-FG; SEM images of (b) pure GO (without washing with H2SO4 solution) and (c) a-FG, respectively; and (d) Photograph showing the coin-cell SC using ionic-liquid as electrolyte that could power the commercial white LED (denoted with red circle). Reproduced with permission [106]. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

the electrode/electrolyte interface. At a 0.5 A g−1 current density, the device fabricated with hGO (with specific capacitance of 145 F g−1) in [Li][TFSI]/AN organic electrolyte was reported to have an energy density and a power density of 30 Wh kg−1 and 1.2 kW kg−1, respectively (without considering the weight of the coin cell packaging). The high device performance was attributed to Faradaic reaction (Li+ + 6C + e– ⇆ LiC6) [104] arising from intercalation of Li+ ions into the layers of graphene electrode.

had demonstrated its practical capability by lighting a commercial white LED for more than 20 h (Fig. 2(d)). Amir et al. [107] suggested multi-step reduction involving GO reduction in NaOH and low-temperature thermal annealing at 150 °C to obtain facile reduction of GO-RuO2. Besides cleaving oxygen functionalities of GO, the reduction by NaOH prevented the growth and recrystallization of RuO2 nanoparticles (NPs), which might hinder the ion transport across microscopic surface. Such chemical reduction effectively decreased the temperature required for subsequent annealing during which chemically bound water and labile functional groups were further removed, respectively, from RuO2 NPS and RGO. The higher deoxygenation efficiency of dual-stage reduction compared to single reduction process [101], along with quick ion transport through nanopores originated from the ultra-small RuO2 NPS with sizes ranging from 1 to 2 nm, provided very high specific capacitance (509.4 F g-1 at a current density of 1.0 A g−1) for the annealed RGO-RuO2 (a-RGO-RuO2) electrode. In addition to the redox reactions of RuO2, the aforementioned reasons explained why the energy density of the symmetric SC could reach 16.7 Wh kg−1 even in an H2SO4-based aqueous electrolyte that provides a narrow potential window of about 1.0 V.

2.1.1.2. Thermal reduction of graphene oxide. Since chemical reduction of GO suspension can cause defects in graphene structure, which deteriorates its high carrier mobility and chemical stability [105], thermal reduction alone or a subsequent thermal reduction on chemically-treated RGO have been massively adopted to produce highquality RGO or graphene. Fang et al. [106] constructed a highperformance symmetric SC from functionalised graphene (a-FG) obtained via acid-assisted ultrarapid thermal-processing technique (Fig. 2(a)). GO prepared by the modified Hummers' method was first washed with H2SO4 solution to intercalate the acid molecules between GO sheets. Then, the acid-incorporated GO (a-GO) experienced a thermal treatment at 900 °C that only lasted a few seconds. The removal of acid molecules during the thermal process facilitated the exfoliation of aggregated a-GO (Fig. 2(b) and (c)); endowing a-FG with a large surface area that enabled high ion storage and hence EDL capacitance. The ultrarapid thermal reduction allowed the preservation of hydrophilic functional groups, which not only improved the wetting between a-FG surface and the polar electrolyte, but also added pseudocapacitance. At the same time, the high-temperature (900 °C) treatment could ensure the recovery of sp2 domains of the GO sheets that led to the formation of electrically conductive paths for electrolyte ions. These synergistic effects were validated by the high specific capacitance (417 F g−1 for a current density of 0.05 A g−1) of the a-FG electrodes in KOH aqueous electrolyte and the outstanding energy density (above 150 Wh kg−1) of the coin-type SC with IL (based on electrode weight only). This single IL-based SC cell

2.1.1.3. Electrochemical reduction of graphene oxide. In comparison to chemical and thermal reductions, electrochemical reduction of GO is ‘green’ and fast, and avoids the use of toxic solvents that contaminates and affects the performance of the SC [78]. Pham and Dickerson [108] assembled a SC with an energy density of 6.02 Wh L−1, which was based on highly-packed electrochemically reduced GO (ERGO) that had a high volumetric capacitance of 176.5 F cm−3. The authors discovered that during pyrolysis between 150 °C and 250 °C, the mass loss in ERGO was negligible while pristine GO lost 54% of its mass due to the release of the residual water between adjacent pristine GO sheets and CO2 and steam from the functional groups [109]. This indicated the decomposition of most oxygen functional groups and the restoration of thermally stable conjugated C=C bonds in ERGO upon the 185

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performance. Upon NaOH treatment, zinc oxide (ZnO) NPs template was removed from the mixture with RGO to produce porous RGO (PRGO) with homogenous pore distribution. The surface functional groups of PRGO reacted with the hydroxyl groups from NaOH to form abundant oxygen functionalities for Faradaic reactions. With 46.1% of the total capacitance contributed by pseudocapacitance, the specific capacitance of oxygen-functionalised PRGO (PRGO-O)-based symmetric SC with H2SO4 aqueous electrolyte was 322.1 F g-1 at a 5 mV s−1 scan rate. This translates to the SC energy density of 38.8 Wh kg−1. Such high energy density was attributed to the enhance specific surface area (SSA) in the interconnected pore structure (i.e., the mesopores resulting from the ZnO templates that bridged the pore space between isolated macropores derived from the original RGO powder). The use of alkali treatment for pore creation is also beneficial towards SC performance. Zhu et al. [118] presented a simple KOH activation of microwave-exfoliated GO (MEGO) that enabled an exceptionally high SSA of 3100 m2 g−1, which exceeded the theoretical limit of graphene's SSA. The powders composed of GO sheets were first irradiated in a microwave oven to exfoliate the overlapped sheets by eliminating a large amount of oxygen functional groups on the edge of the particles. The as-made MEGO powders were then dispersed in KOH solution for activation. Contrary to the work previously described, Zhu et al. [118] found out that the activation process suppressed the oxygen functionalities of MEGO and allowed enhanced exposure of graphene sheets to the electrolyte. The KOH activation also yielded a continuous three-dimensional (3D) network of nanoscale pores ranging from 1 to 10 nm in electrode. The improved porosity was ascribed to two probable mechanisms during carbon activation with KOH between 600 °C and 1200 °C, i.e., (i) the chemical etching of carbon framework resulting from the redox reactions between potassium compounds; and (ii) the physical activation where formation of gas bubbles takes places during gasification of carbon [119–121]. The very large increase in pore volume and SSA of the a-MEGO relative to MEGO, which translated to enhanced density and conductivity of the material, was also observed in this work. This was the result of the intercalation of metallic potassium into the carbon matrix during activation; causing irreversible expansion of carbon lattices [122]. Therefore, at a 5.7 A g−1 current density, the a-MEGO electrode exhibited a satisfactory electrical conductivity of 500 S m−1 and specific and volumetric capacitances up to 166 F g−1 and 60 F cm−3, respectively in [BMIM][BF4]/AN organic electrolyte that allowed a maximum working voltage of 3.5 V. The symmetrical SC based on a-MEGO was expected to deliver a remarkable energy density of above 20 Wh kg−1 with a power density of up to 75000 W kg−1 in practice. Using the similar preparation method as previously described (Fig. 3(a)), Kim et al. [123] fabricated a comparable SC having a maximum practical energy density of 16.5 Wh kg−1 and a power density of 7 kW kg−1. They introduced an additional step, i.e., the drying of GO in aerosol spray prior to microwave irradiation and KOH activation. This process served to transform the GO sheets into crumpled and hollow GO spherical particles that contributed to macropores in the carbon (Fig. 3(b) and (c)). Interspersed with small mesopores that derived from the chemical activation with KOH, the unique macroporous structure of the activated GO provided high permeability to electrolyte ions; leading to both high gravimetric (129 F g−1) and volumetric (58 F g−1) capacitances in [EMIM][TFSI]/AN organic electrolyte for a current density of 1.1 A g−1 (Fig. 3(d)).

electrochemical reduction. Besides electrochemical reduction, hydraulic compression of ERGO contributed to the high volumetric capacitance and energy density as well. When compressed up to 156 MPa, the ERGO deposited on the nickel foam (NF) was pushed into interstitial micropores of NF; thus occupying the pores completely. The thickness of ERGO electrodes decreased afterwards by 137 times, which led to a corresponding increase in graphene packing density, from 0.0097 to 1.32 g cm−3. The formation of compact pore structure lowered pore volume, which in turn profoundly improved volumetric performances of ERGO by a factor of 112 in this work. Li et al. [110] proposed the use of simultaneous electrochemical deposition and reduction of GO on PANI arrays to manufacture a highenergy-density (up to 137.3 Wh kg−1) SC. During the growth of alternate PANI layers, the previously deposited GO on these layers was reduced in situ. The removal of surface functional groups reduced the band gap (i.e., the minimum energy required to excite an electron to an energy level above its ground state where it can participate in conduction) [111] of GO and composite, and thereby elevating charge carriers density. Faster electron transfer could be expected when more layers of hierarchical nanostructures (especially graphene layers) were introduced. Since specific capacitance is directly proportional to the number of electrons transferred (or charge) [112], the specific capacitance of the electrode in [TBA][PF6]/AN organic electrolyte increased from 79 to 224 F g−1 (at a 1.0 A g−1 current density) when the number of arrays was extended from one to three. 2.1.1.4. Alternative reduction methods of graphene oxide. While many researchers have reported high deoxygenation efficiency using conventional reduction techniques described above, Mohanapriya et al. [113] proved that reduction of GO by solar irradiation alone could promote the electrode specific capacitance to 187.1 F g−1 and hence high SC energy density (58.5 Wh kg−1) for a current density of 0.5 A g−1 in [EMIM][BF4]. Irradiated by a focused solar light, the oxygen functional groups attached on GO surfaces were immediately decomposed due to the thermal heating effect, and the graphitic layers of GO was exfoliated simultaneously due to the developed pressure that could easily overcome Van der Waals force between the layers. The rapid reduction of GO, which took only 2 minutes to complete, led to the folded sheets and the wrinkled structure; thus maintaining the high surface area of the electrode by minimising the re-stacking of solar reduced GO (SRGO) sheets. The restoration of sp2 network during solar irradiation increased the electrical conductivity of SRGO to 1986 S m−1; facilitating the charge transport through the active materials. In another work, Sun et al. [114] developed SC electrode based on graphene-poly (3,4-ethylenedioxythiophene) composite (G-PEDOT) via an in situ microwave heating. Under 20-minute microwave irradiation, EDOT monomers that initially adsorbed on the GO surface were oxidised by GO to become EDOT+ that later joined together to form PEDOT. At the same time, the electrons released by EDOT were transferred to GO to convert defect sp3-hybridised carbon in GO to sp2 bonded network in graphene; indicating the diminished oxygen-containing functional groups in GO-PEDOT. This phenomenon combined with the anchoring of the PEDOT on the graphene could mitigate sheets agglomeration and expand the contact area with the electrolyte solution, which promoted ion transport in the electrode and led to a remarkable specific capacitance of 280 F g-1 at a 0.5 A g−1 current density in an H2SO4-based aqueous solution. The maximum achievable energy density of the SC was 38.9 Wh kg−1 with the corresponding power density of 250 W kg−1. Notably, the energy density was well maintained at 34 Wh kg−1 when power density was increased to 25000 W kg−1; highlighting the electrode potential for high-energy and high-power applications.

2.1.1.6. Doping of graphene oxide. Both GO synthesised via top-down approach and graphene produced via bottom-up approach can be further functionalised to achieve desired surface characteristics, physicochemical properties, and processability for energy storage applications. Heteroatom-doping and chemical functionalisation are the common methods to fabricate multifunctional graphene-based materials. Heteroatom dopants such as nitrogen, boron, and sulphur can alter the chemical and electronic properties of GO or graphene [124,125].

2.1.1.5. Alkaline hydroxide treatment on graphene oxide. Alkali hydroxide has often been used solely as a chemical reducing agent in numerous studies [103,115,116] but Hwang et al. [117] proved that NaOH can also be used to functionalise RGO to improve device 186

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Fig. 3. (a) Schematic illustration of the preparation process of asMEGO. The crumpled GO was first formed when GO fine droplets in aerosol were dried to evaporate water inside them; and SEM micrographs of (b) crumpled GO particles, (c) hollow spheres of sMEGO, and (d) porous asMEGO, respectively. Reproduced with permission [123].

prepared the BNG film via layer-by-layer assembly of anionic GO NSs and cationic poly-L-Lysine (PLL) which acted as nitrogen-containing precursor, followed by intercalation of H3BO3 (as boron precursor) within the layers, and annealing treatment. Such modified approach is effective because the BNG film showed a high volumetric capacitance (i.e., 488 F cm-3 at a 10 mV s−1 scan rate). The BNG-MSCs enclosed within H2SO4/PVA gel electrolyte demonstrated a maximum practical volumetric energy density (3.4 mWh cm−3), which was comparable to the 4 V/500 μAh commercial lithium thin-film battery and significantly outperformed the pure graphene-based MSC (MPG-MSCs-PET) they previously tested. In addition to the creation of micropore fillers from the decomposition of PLL and H3BO3 during thermal treatment, the enhanced performance of the BNG-MSC was mainly due to two factors, i.e., (i) the addition of pseudocapacitance from the presence of the newly created electrochemically active moieties (i.e., B-N-C) by codoping with dual heteroatoms; (ii) the incorporation of N and B atoms into the graphene lattice that improved the interface wettability of the electrode with the electrolyte and led to thickened EDL.

Hao et al. [126] reported nitrogen-doped graphene-based hierarchical porous carbon aerogel (that is created by replacing the liquid content in the gel with air via freeze-drying) with a high specific capacitance of about 197 F g-1 at a 0.2 A g−1 current density for use of a SC that could exhibit an energy density of 27.4 Wh kg−1 at a 400 W kg−1 power density. Prior to high-temperature activation with KOH, graphene nanosheets (GNSs) and amorphous carbon derived from carbonisation of chitosan (i.e., natural polymer extracted from the shell of crustaceans such as crabs and shrimps) aerogel were homogenously doped with N atoms from amino acid (–NH2 groups) in chitosan. In addition to accommodating more ions on the electrode surface given the large binding energy of pyridine N and pyrrolic N with potassium ions in electrolyte (KOH/PVA gel), the pseudocapacitive interactions between N species in the carbon aerogel and the electrolyte ions led to an increase in capacitance. Stemmed from the chemical activation, the optimised mesopore and micropore volumes in the carbon aerogel that enhanced the ion transport and the charge storage, respectively, also ensured the outstanding capacitive behaviour of the SC. Wang et al. [127] co-doped graphene with nitrogen and sulphur (NS-G) via a hydrothermal route using amino acid and GO as precursors. They found substantially higher N or S concentration in codoped graphene aerogel than in a single doped sample. This was probably due to the presence of both dopants that could promote the embedding of one another in neighbouring rings. While graphitic N raised the electrical conductivity of graphene, the high contents of pyrrolic N and thiophenic-like sulphur and sulphone in NS-G contributed to pseudocapacitance of the electrode. Moreover, N and S dopants partially replaced oxygen attached to GO and then introduced a structural irregularity into the hexagonal crystal lattice of graphene; causing the crumpling of graphene. This induced the formation of 3D porous network that allowed the effective adsorption and diffusion of electrolyte ions. The combined effects of N and S in graphene manifested into a SC specific capacitance of 212 F g-1 at a 10 A g−1 current density and an energy density of 29.4 Wh kg−1 at a 10000 W kg−1 maximum power density for SC in an aqueous electrolyte. Wu et al. [128] co-doped graphene film with boron and nitrogen (BNG film) so that higher MSC performance could be achieved. They

2.1.1.7. Covalent and non-covalent functionalisation of graphene oxide. Covalent functionalisation can be used to introduce new functional groups for the preparation of special materials with tunable functionalities. It is generally difficult to realise the covalent modification in pristine graphene given its perfect lattice structure and the high energy barrier required to disrupt the conjugation structure. To this end, an alternative—GO with interrupted conjugated domains and rich oxygen functionalities—can be used for further modification that allows permanent bonding between the graphene and modifier [129]. Bag et al. [130] recommended the covalent functionalisation of RGO with hydrophilic imidazolium-based IL (RGO-Im-IL). Their resultant symmetric aqueous device delivered high energy density of 36.67 Wh kg−1 at a 2000 W kg−1 power density in a potential window of 0–2 V. When mixture of amine-terminated ionic-liquid (IL-NH2) was refluxed at 170 °C, the removal of oxygen functionalities in GO and covalent attachment of Im-IL onto the carbon framework occurred. Specifically, the reaction of –NH2 with the carboxyl (–COOH), epoxide (C–O–C), and carbonyl (C=O) groups of GO resulted in covalent 187

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336.7 F g-1 at a 0.1 A g−1 current density and delivered an energy density up to 108 Wh kg−1. Yan et al. [143] specially synthesised functionalised graphene sheets (FGN-300) via low-temperature (300 °C) thermal reduction of GO. Magnesium hydroxide (Mg(OH)2) was employed as template skeleton in this work. The obtained SC electrode delivered ultrahigh gravimetric and volumetric capacitances of 456 F g−1 and 470 F cm−3, respectively, almost 3.7 times and 3.3 times higher than the hydrazine reduced graphene reported in their previous work [144]. The electrostatic attraction between Mg(OH)2 NSs and GO sheets prevented the latter from re-stacking and thus ensured high surface area for charge accumulation on the electrode surface. Since Mg(OH)2 was intercalated between the GO layers, the subsequent reduction of GO sheets was performed at low temperature (300 °C) so that large amount of oxygencontaining functional groups could be retained on the RGO layers to provide more pseudocapacitance while improving the surface wettability of electrode in electrolyte. During heat treatment of GO, a slow heating rate was used (3 °C min−1) to limit the sheets expansion so that low pore volume and high powder density were attained, which in turn translate to enhanced volumetric performance. After dissolving Mg (OH)2 in HCl, the footprints (dented surface of RGO sheets) the templates had left behind served as ion-buffering reservoirs that could shorten ion diffusion distance and facilitate the rapid ion transport to the interior of RGO layers. The constructed symmetric SC exhibited a specific capacitance of 58.6 F g-1 at a 2 mV s−1 scan rate in Na2SO4 electrolyte. The SC also displayed high energy densities of 26.4 Wh kg−1 and 27.2 Wh L−1, which were higher than the previously reported carbon-based symmetric SC in aqueous electrolyte [145–148].

attachment of Im-IL at the periphery and basal plane of the RGO. The bulky hydrophilic imidazolium moiety not only increased the interlayer distance between RGO sheets, but also enhanced the hydrophilicity of RGO and hence the wettability of RGO-Im-IL. These features facilitated easy permeation of electrolyte to the electrode. Notably, the hydrophilisation of RGO did not affect the electrical conductivity despite the prediction that the functionalisation on the RGO surface should disrupt the conjugation in the carbon network. In their work, the electrical conductivity of RGO-Im-IL was 156400 S m−1, which was 20% higher than that of non-functionalised RGO. They attributed these to two factors, i.e., (i) partial restoration of extended conjugation upon the removal of oxygen functional groups; and (ii) covalent functionalisation that preferably took place at the edge of RGO without substantially altering its conjugation structure. Therefore, a one-fold increase in specific capacitance (i.e., up to 66 F g-1 at a 2 A g−1 current density) was observed in the RGO-Im-IL-based SC. Non-covalent functionalisation that is used for the surface modification of the sp2 network, can be performed onto graphene-based materials via π-π attraction [131,132], hydrogen bonding [133,134], and hydrophobic interaction [135,136] between the absorbed molecules and graphene surface. For example, Jana et al. [137] embedded high-conductivity sulfanilic acid azocromotrop (SAC) on GO surface through π-π interaction (interaction between π-orbitals of a molecular system) for enhanced SC energy storage performance. The hydrophilic functionalities (i.e., –SO3H) of SAC in the surface modified graphene minimised the re-stacking of GO sheets; leading to a floppy and porous structure for better penetration of electrolyte into the SAC-RGO electrode (reduced with hydrazine). The interaction between the aromatic moiety of SAC and the basal plane of graphene without disturbing the conjugated sp2 network of the honeycomb lattice also helped to increase the electrical conductivity from 318 S m−1 in pure RGO to 551 S m−1 in SAC-RGO. The redox reactions between –SO3H/OH functionalities of SAC-RGO and the H+ ions of H2SO4 contributed its pseudocapacitance to the electrode; leading to a high specific capacitance of 366 F g-1 at a 1.2 A g−1 current density. Operated at the potential range of 0–1.4 V, the asymmetric SC with SAC-RGO as the positive electrode and RGO as the negative electrode exhibited a specific capacitance of 95 F g-1 at a 1.4 A g−1 current density and a maximum energy density of 25.86 Wh kg−1; corresponding to a power density of 980 W kg−1.

2.1.1.9. Composite electrodes (Transition metal oxide and conducting polymer). Since electrodes based on pure RGO or graphene only exhibit EDL capacitances that limit energy density achievement in SC, pseudocapacitive materials especially transition metal oxides and conducting polymers undergoing fast redox reaction can be incorporated into RGO or graphene to form nanocomposites with enhanced electrochemical properties such as higher normalised capacitances. Wu et al. [149] synthesised a 3D vanadium pentoxide (V2O5)/graphene hybrid aerogel (V2O5/GN-Ae) via in situ self-assembly of V2O5 nanofibres and GO sheets followed by annealing to reduce GO. During thermal treatment, the V4+ ions were oxidised to V5+, and the V2O5 nanofibres were partly crystallised (i.e., became shorter and smaller [150]). The size reduction of nanofibers led to abundant electron transport sites and hence quick movement of electrons [151]. The hybrid aerogel supported by the randomly-oriented V2O5 nanofibre scaffolds exhibited hierarchical porous structure in which macropores acts as ion-buffering reservoir, mesopores facilitates ion transport, and micropores enhances charge storage [152,153]. The V2O5/GN-Ae delivered a high specific capacitance of 486 F g-1 at a 0.5 A g−1 current density and an energy density of 68 Wh kg−1 that corresponds to a power density of 250 W kg−1. Zhang et al. [154] successfully demonstrated the role of conducting polymer in attaining high performance of SC. At a current density of 1 A g−1, the RGO/PANI film delivered a specific capacitance of 1182 F g−1 (Fig. 4(d)), which was about three times that of RGO (∼360 F g−1) prepared using the same protocol but without embedment of PANI. Zhang et al. [154] introduced steam water technique where the equilibrium of steam pressures and water evaporation forces exerted on the RGO networks helped to generate porous graphene film with which PANI fibres were wrapped (Fig. 4(a) and (b)). The resultant 3D hierarchical structure promoted electrolyte infiltration into the layers of graphene sheets (Fig. 4(c)). The incorporation of PANI fibres not only contributed to Faradaic capacitance, but also impeded the re-stacking of RGO NSs. This led to high SSA that improved the capacitance relative to the case without PANI fibres. These behaviours dramatically increased the maximum energy density by three times, i.e., from 9.76 Wh kg−1 for RGO-based SC to 28.06 Wh kg−1 for RGO/PANI-based SC in H2SO4.

2.1.1.8. Template synthesis of porous graphene oxide. Template method has been developed to synthesis porous GO with different pore structures and sizes. This method involves the introduction of suitable solid nanostructure solid or supramolecular aggregates into GO sheets, followed by the removal these template skeletons from GO [138–141]. Lee et al. [142] utilised the ice-templated self-assembly technique to make vertically porous 3D vanadium phosphate (VOPO4)-graphene composite (VP3D-VOPO4-RGO) that showed an electrode specific capacitance of 527.9 F g-1 at a 0.5 A g−1 discharge current density. The electrode was simply prepared by freeze-drying the frozen nickel foam filled with the mixture of GO and VOPO4, followed by reduction using hydrazine. As liquid suspension froze in a liquid nitrogen, the dispersed GO and VOPO4 nanosheets (NSs) in nickel foam were expelled from the perpendicularly growing ice crystals and accumulated radially among themselves; generating interstacked 3D VOPO4-GO upon removal of ice template. The unique structure held the VOPO4 NSs firmly and allowed most of them to take part in the pseudocapacitive reaction during pseudocapacitive reaction. The maximised plane-to-plane contact between VOPO4 and RGO also enabled facile electron transfer from VOPO4 to the highly conductive RGO framework. Furthermore, the vertically-aligned microchannels increased the accessible surface area and shortened the ion diffusion length, which enhanced the device specific capacitance. By operating in KOH electrolyte at 1.6 V potential, the coin-shaped asymmetric SC consisting of VP3D-VOPO4-RGO as the positive electrode and VP3DRGO as the negative electrode showed a specific capacitance of 188

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Fig. 4. (a) Schematic diagram of the preparation procedure of RGO/PANI film through steam water technique; (b) TEM image of RGO/PANI, which shows that PANI fibres (denoted with white arrows) were wrapped with RGO NSs; (c) FESEM images displaying the cross-section of RGO/PANI; suggesting the porous structure in the hybrid film; and (d) The electrochemical performances of RGO/PANI, pure RGO, and PANI fibres in three-electrode tests at different current densities. Reproduced with permission [154].

increased the conductivity from 47.5 S m−1 for pure RGO paper to 52.9 S m−1 for RGO/PPy paper, but also led to the formation of hierarchical framework that eased the infiltration of gel electrolyte and induced pseudocapacitance.

Apart from these, Xia et al. [155] also demonstrated that RGO/ MoO3/PANI ternary composite electrode (RGO (MP)8) could exhibit higher electrochemical performance than either RGO/PANI or MoO3/ PANI binary composite electrode. In the ternary composite, graphene sheets acted as the supporting material to anchor uniform MoO3 and PANI NPs whereas MoO3 served as a doping agent that combined RGO and PANI with hydrogen bonding to form interconnected structure that ensures facile ion movement into the electrode. The reversible redox reactions of MoO3 and PANI, and the residual oxygen functionalities of RGO were the contributing factors of the additional pseudocapacitance that in turn increased total capacitance. The RGO/MoO3/PANI electrode achieved a specific capacitance of 553 F g-1 at a 1 mV s−1 scan rate in an acidic H2SO4 electrolyte, which was significantly higher than MoO3/PANI (261 F g−1) and RGO/PANI (295 F g−1) in the same electrolyte. This proved the synergistic effects of the components in the ternary structure on the electrochemical performance. The energy density of the RGO/MoO3/PANI-based SC reached 76.8 Wh kg−1 at a power density of 276.3 W kg−1. In a neutral Na2SO4 electrolyte, however, the RGO/MoO3/PANI electrode showed decreased specific capacitance (363 F g−1), and the SC exhibited a lower energy density (72.6 Wh kg−1 that corresponds to a power density of 217.7 W kg−1) due to the absence of H+ ions, which provided better ionic diffusion into the pores as mobile species [156], and the missing redox transition of PANI (i.e., between a semi-conducting state and a conductive form). Still, this RGO/MoO3/PANI electrode was considered promising for high-performance SCs in both acidic and neutral electrolytes. Yang et al. [157] reported the synergistic capacitive ability of RGO/ PPy hybrid paper (obtained through vacuum filtration of suspension) in all-solid-state flexible SC with H2SO4/PVA gel as electrolyte. The symmetric SC could deliver an areal capacitance as high as 512 mF cm2 at a 1 mA cm−2 current density and a volumetric capacitance as large as 59.9 F cm-3 at a 100 mA cm−3 current density. It also showed maximum areal density and volumetric density of 71.2 μWh cm−2 and 8.32 mWh cm−3, respectively. The introduction of highly conductive PPy nanotubes (NTs) into the electrode was the main reason behind the exceptional performance of the flexible SC. PPy NTs presence not only

2.1.1.10. Composite electrodes (Carbonaceous materials). The hybrid of graphene with transition metal oxides and/or with conducting polymers generally show high energy density. Nevertheless, the pseudocapacitive materials' poor conductivity, less reversible redox reactions, and intrinsic structural degradation during the redox process may reduce the power density and shorten the life cycle of SC; thus restricting their practical application [158–161]. Alternatively, carbonaceous materials, for example, carbon nanotubes and carbon nanofibers with large SSA have been incorporated into graphene via electrostatic assembly. More importantly, IL electrolytes, which can extend the operating voltage window of the SC, are normally used in the SCs assembled with graphene/carbon composite electrodes to achieve comparable device energy density without sacrificing other electrochemical properties of SC. Zhang et al. [162] made two identical composite electrodes from carbon spheres and graphene (CSG), which were then assembled into a high-performance symmetric SC. During hydrothermal process, the GO sheets were functionalised with positively charged polymer that was produced by ionisation of PDDA. This action aimed to create surface positive charges on GO so that negatively charged carbon spheres could sandwich themselves in between the positively charged GO sheets to form 3D porous structure with enhanced SSA. The advantage of using carbon spheres as the spacers was reflected in the specific capacitances and energy densities of CSG//CSG SC. In KOH aqueous electrolyte, the symmetric device could attain a specific capacitance of 77.2 F g−1 for a discharge current density of 1 A g−1 and a maximum energy density of 18.1 Wh kg−1 at a power density of 650 W kg−1. In [EMIM][BF4] IL, the device showed a slightly reduced specific capacitance (70 F g−1) but a significantly higher energy density (87.5 Wh kg−1 that corresponds to a power density of 1500 W kg−1). Despite the high viscosity of IL that limited ions diffusion and device specific capacitance in [EMIM][BF4] 189

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2.1.1.11. Composite electrodes (2D materials). Similar to graphene, other 2D materials such as transition metal carbides or carbon nitrides (MXenes) [169,170], transition metal dichalcogenides (TMDs) [171,172], and layered double hydroxides (LDHs) [173,174] have excellent electronic properties and exceptional mechanical robustness, which provide advantages to achieve both high capacitive performance and high flexibility for energy storage applications. Semiconductor TMDs is usually represented by MX2, where a transition metal atom of groups 4 to 10 (e.g., M = Mo, W, and We) is sandwiched between two chalcogen atoms (e.g., X = S, Se, and Te) [175]. Ratha and Rout [176] attempted to improve the electrochemical performance of SC electrode by coating tungsten disulphide (WS2) on GO, which was simultaneously reduced to RGO via a simple hydrothermal method. Given the presence of functional groups at its edges and basal planes, GO acted as novel substrate for the nucleation and subsequent growth of WS2. Following such nucleation and growth, these oxygen functionalities were removed, and the electrical conductivity was increased. Since the covalently bonded S–W–S layers was separated by a weak Van der Waals gap, Na+ ions from Na2SO4 electrolyte could intercalate into the gap between the WS2 layers easily; triggering redox reaction (WS2 + Na+ + e– ⇆ WS–Na+) and providing Faradaic contribution to the overall performance of electrode. Combined with the increasing utilisation of electrochemically active WS2, the enhanced properties endowed the electrode with a high specific capacitance of 350 F g−1 for a 0.5 A g−1 discharge current density. For the WS2-RGO//WS2-RGO SC, the maximum energy density was 49 Wh kg−1, which was higher those of WS2//WS2 (10 Wh kg−1) and RGO//RGO (20 Wh kg−1) SCs. Thangappan et al. [177] also adopted the synthesis method as previously described to fabricate molybdenum disulphide/graphene composite (MoS2/G) for high-performance SC electrode. Mo2+ ions were adsorbed onto the negatively-charged GO via electrostatic interaction; inducing in situ reduction of GO sheets during hydrothermal process. Despite being less conductive, the MoS2 NSs with abundant open space provided the composite with high surface area that increased the contact area between the electrolyte and electrode; creating efficient transport pathways for charged ions. The synergistic effects of MoS2 and graphene was clearly verified by a high electrode specific capacitance of 270 F g-1 at a 0.1 A g−1 current density and a maximum device energy density of 37.5 Wh kg−1 that corresponded to a power density of 250 W kg−1 in Na2SO4 aqueous solution. LDHs, also called anionic clays, are layered compounds that consist of positively charged metal hydroxides and interlayer anions accompanied with hydroxyl groups to compensate the net positive charge [178]. Their exciting properties, for example, large surface area, excellent anion-exchange capabilities, and tuneable composition, render them attractive for SC electrodes [179–181]. Instead of using in situ growth of nickel-aluminium (Ni-Al) LDH on graphene NSs via hydrothermal process as describe above, Wimalasiri et al. [182] prepared hierarchical layered Ni-Al LDH/graphene (Ni-Al LDH/G) composite by mixing Ni-Al LDH suspension with GO solution, followed by chemical reduction of GO with hydrazine monohydrate. The composite electrode displayed an electrode specific capacitance of 915 F g−1 (which was higher than Ni-Al LDH/G produced in situ via a direct hydrothermal synthesis [183]) at a 2.0 A g−1 current density and a maximum SC energy density of 27.6 Wh kg−1 at a power density of 500 W kg−1 in a narrow potential of 0.48 V. This impressive result was mainly attributed to pseudocapacitance brought on by the oxidation and reduction of Ni atoms in the composite electrode. The mutual electrostatic attractions between both NSs drove face-to-face assembly of graphene and Ni-Al LDHs, which exposed Ni atoms in close contact to graphene. This phenomenon led to faster electron transfer through graphene during redox reaction. A higher electrochemically active surface area was also realised for energy storage after Ni-Al NSs were sandwiched between graphene to hinder the aggregation of graphene. Li et al. [184] developed the solution of hybrid inks based on electrochemically exfoliated graphene (EG) and MXene NSs (Ti3C2Tx

[5,163], the wider potential window offered by IL (0–3.0 V) that largely exceeded that accessible in aqueous electrolyte (0–1.3 V), enhanced the energy density of the as-prepared symmetric SC, since potential window is a dominant factor for energy density. Pham et al. [164] used the similar concept, i.e., electrostatic selfassembly to prepare activated graphene/single-walled carbon nanotubes (ac-G/SWCNTs) films. After grafted with the cationic surfactant, i.e., CTAB, positively charged SWCNTs intercalated into the negatively charged GO layers to obstruct the agglomeration of GO sheets to obtain enhanced specific capacitance. Activation of G/SWCNTs with KOH at high temperature (800 °C) not only partially removed the functional groups attached to GO to improve electrical conductivity up to 39400 S m−1, but also developed micropores and mesopores for efficient diffusion of larger IL ions. With a moderate packing density of 1.06 g cm−3, the optimised ac-G/SWCNTs film achieved both high gravimetric and volumetric capacitances (199 F g−1 and 211 F cm−3, respectively) at a 0.5 A g−1 current density. A medium packing density of graphene electrode is preferable because a too-high density improves volumetric performances but diminishes gravimetric performance and vice versa [165]. The symmetric coin cell could deliver the highest gravimetric energy density of 110.6 Wh kg−1 and a maximum volumetric energy density of 117.2 Wh L−1, which corresponds to power densities of 500 W kg−1 and 550 W L−1, respectively, in [EMIM][BF4]. Lei et al. [166] reported a high-performance SC electrode (i.e., specific capacitance of 144.4 F g-1 at a 0.2 A g−1 current density) consisting of ordered mesoporous carbon CMK-5 platelets (functionalised with PDDA for surface modification) and RGO. The CMK-5 was chosen to be incorporated in RGO because of its small particle size (0.8–2.6 nm) [167], which could greatly enlarge accessible surface area of the electrode and shorten the diffusion path of the electrolyte ions after the positively-charged CMK-5 platelets intercalated between the negatively-charged RGO sheets. The symmetric RGO/CMK-5 cell with [EMIM][BF4] electrolyte exhibited an energy density as high as 60.7 Wh kg−1 at a power density of 174 W kg−1, which was tremendously higher than that measured in a KOH aqueous electrolyte (5.2 Wh kg−1) and an LiPF6 organic electrolyte (23.1 Wh kg−1). This suggested the crucial role of IL in boosting the energy density of SC, particularly in carbon-based device. Tamailarasan and Ramaprabhu [168] proposed the use of functionalised multi-walled carbon nanotubes/hydrogen-exfoliated graphene/ IL ternary composite electrode (f-MWCNTs/HEG/[BMIM][TFSI]) in SC to achieve extremely high energy density (170.66 Wh kg−1 corresponding to a power density of 5710 W kg−1). Besides serving as a bridge for electron transfer between graphene layers, the intrusion of fMWCNTs into the graphene layers also increased the amount of mesopores in the nanocomposite; thus enabling the better accommodation of large-sized IL ions inside the pores. The wrinkles appeared on the surface of HEG was also a contributing factor for the mesoporosity increase and hence, specific capacitance. The presence of solid-likelayered [BMIM][TFSI] in the ternary electrode appeared to be vital since the specific capacitance was improved by 32.66% relative to that of f-MWCNTs/HEG electrode. Such large discrepancy in performance was attributed to the adsorption of IL on the surface of HEG, which inhibited the aggregation of HEG upon drying, thereby increasing the number of ion diffusion path. The specific capacitance of the composite electrode (201 F g-1 at a 2 A g−1 current density) highlighted the synergistic effects of the three components on electrode performance. Another important highlight in this research is the use of hydrophobic [BMIM][TFSI] IL as electrolyte. Although ILs are well-known for their wide potential windows, the presence of moisture may reduce the potential windows of hydrophilic ILs such as [BMIM][BF4], which had its window reduced by 2.0 V in this work. On the other hand, [BMIM] [TFSI] maintains its wide potential window of 3.5 V in the course of this study so that the energy density was retained even at high current discharge rate.

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such SC was demonstrated when three units of the flexible device connected in series could light up a red LED at 2.5 V despite 30% decrease in an areal capacitance (17 mF cm−2) (Fig. 5(d)). This was attributed to the three-fold increase in an areal energy density (15 μWh cm−2) resulting from the widened operating voltage. Instead of using GO dispersion, Xiong et al. [194] prepared binderfree, large-area (1316 m2 g−1) porous RGO film from GO hydrogel. Such innovative method was viable and reliable because the SC made of the RGO film and H2SO4-soaked filter paper presented a large areal capacitance of 71 mF cm-2 at a 1 mA cm−2 current density and an areal energy density of 9.8 μWh cm−2 that corresponded to an areal power density of 500 μW cm−2. The viscous GO hydrogel obtained beforehand was blade-casted on the glass substrates to form thin GO films, which were then reduced by hydroiodic acid/acetic acid solution. While macropores in the graphene network could serve as an ion reservoir to provide more diffusion paths for electrolyte ions, the absence of binder or conductive additives here was the additional contributing factor of the high performance of the device. In this regard, larger mass loading of active material was allowed, which enhanced charge storage and hence EDL capacitance. Tao et al. [145] used evaporation-induced drying technique to produce binderless, highly compact yet porous graphene macroform (HPGM) with high volumetric capacitance of 376 F cm−3 and 171 F cm−3 in KOH aqueous solution and [TEA][BF4]/AN organic electrolyte, respectively, for high-performance SC. The hydrogel resulted from hydrothermal treatment was vacuum dried at room temperature to remove the trapped water, which caused the interlinking of graphene NSs and about 98.75% of volume shrinkage. Such interlinkage provided the monolithic HPGM with mouldability, which made the electrode additive- or binder-free. Tao et al. [145] ascribed the much improved device-level energy density (of 37.1 Wh L−1 in [TEA] [BF4]/AN organic electrolyte and of 13.1 Wh L−1 in KOH aqueous solution) to this feature, in addition to the electrode's dense microstructure and interwined canal-like network for fast ion transport.

where T are terminal groups, for example, O, OH, or F, and x is the number of terminal groups) for the construction of high-volumetricenergy-density (3.4 mWh cm−3 at a volumetric power density of 200 mW cm−3) flexible SC. MXenes are generally produced by etching out the A element from MAX phase family (where M represents an early transition metal, A denotes element from groups 13 and 14, and X can be carbon and/or nitrogen) such as Ti3AlC2, Ti2AlC, and Ta4AlC3 through hydrofluoric acid treatment accompanied with ultrasonication [185,186]. In the EG/MXene electrode obtained through filtration of the solution processable ink, the small sized MXene (about 200 nm) served as conducting spacers to hinder the irreversible π-π stacking between the graphene sheets. A large accessible area was thus available for efficient ion adsorption. Previous studies have demonstrated that MXene shows good electrical conductivity and hydrophilic behaviour [185,187]. Therefore, Ti3C2Tx between graphene layers also served as active material and ideal buffer for enhanced electrolyte shuttling. The combined effect between EG and MXene enabled high volumetric capacitance of the EG/MXene-based flexible SC (184 F cm−3 for a current density of200 mA cm−3), which was superior to those of SC based on EG (22 F cm−3) and MXene (4 F cm−3). Three EG/MXene-based devices in series could power a red LED for 8 minutes; indicating high energy storage capacity of the flexible SC with H3PO4/PVA electrolyte sandwiched between two identical electrodes. 2.1.1.12. Binderless electrodes. Self-supporting graphene-based electrodes can be formed without the need for conducting additives or polymer binder components. While the conductive additive improves the electrical connectivity of the electrode materials, it alters the porosity of the carbon electrode and affects electrolyte diffusion, thereby hardly contributing to capacitance [188]. The insulating binder decreases the electrode conductivity and suppresses electron transport; thus leading to poor electrochemical performance of SC [189,190]. Therefore, the absence of these inactive materials likely leads to high specific capacitances of the electrodes. Sun et al. [191] composed a SC based on self-assembled graphene organogel (i.e., gel with organic solvents as liquid component) that delivered a maximum energy density as high as 43.5 Wh kg−1 at a power density of 780 W kg−1. After solvothermal reduction of GO dispersion in propylene carbonate (PC), the flexible RGO NSs regionally overlapped and coalesced through π-π interactions; forming macroscopic self-assembled graphene organogel (SGO). The higher electrical conductivity of SGO (4 S m−1) than that (0.5 S m−1) of hydrogel (SGH) indicated that the solvothermal reduction of GO in PC was more effective than that in aqueous medium. The SGO had a high specific capacitance of 140 F g−1 for a 1 A g−1 discharge current density. The notable specific capacitance was partly attributed to the absence of polymer binder or conducting additives when fabricating electrode using SGO. The use of polymer binder has a negative impact on the properties of active materials because of the reduction in the effective surface area [192] and the increase in the electrical resistivity [190]; thereby decreasing specific capacitance. In [TEA][BF4]/PC organic electrolyte, the maximum energy density of SGO-based symmetric SC was slightly higher than that based on SGH (i.e., 43.5 Wh kg−1 for the former and 32 Wh kg−1 for the latter). Weng et al. [193] engineered a flexible polymer SC based on binderfree graphene-cellulose paper (GCP) electrode that had a cell areal capacitance of 46 mF cm−2 and areal energy density of 4 μWh cm−2 (Fig. 5(b)). The binder was not needed as graphene NSs (GNSs) were anchored on the surface of the cellulose fibres (filter paper) by electrostatic interaction between the functional groups on the fibres and the negatively charged GNSs (Fig. 5(a) and (c)). While GNSs filled the voids between the fibres to form a conductive interwoven network, the cellulose fibres acted as an electrolyte reservoir by substantially absorbing electrolyte to facilitate ion transport for increased charge storage. The H2SO4/PVA gel functioned not only as an electrolyte, but also as a separator; thus minimising the device thickness. The high performance of

2.1.1.13. Other approaches. Apart from the aforementioned approaches, other novel graphene production methods have also been adopted to boost the energy density of SC. For example, Lin et al. [195] employed conventional photolithography and CVD techniques to manufacture graphene/carbon nanotube carpets (G/CNTCs)-based MSC on nickel electrodes (which also served as current collectors). They discovered that high-temperature water etching where hydrogen and water vapour were fed to the post-CNTCs-growth G/CNTCs-MSC could substantially boost the capacitance of the micro-device. For example, the areal capacitance of the assembled MSC increased by 103% from 0.7 mF cm−2 to 1.42 mF cm−2 after undergoing wateretching process. This enhancement was probably due to two factors, i.e., (i) water could etch amorphous carbon deposited on the surface of CNTs, thereby lowering the effective series resistance between the electrolyte and CNTs; and (ii) water could attack the defect sites of the CNTs to produce oxygen or hydroxyl functional groups, which increased the wettability of the CNTs and decreased the interfacial resistance. Benefitting from the presence of CNTCs and the water etching process, the G/CNTCs-MSC displayed a specific capacitance of 1.96 F cm−3 and a maximum volumetric energy density of 2.42 mWh cm−3 in [BMIM][BF4]; making it comparable to commercial lithium thin-film batteries. Xie et al. [196] applied simple laser technology to produce pure laser-processed graphene-based micro-planar SC (LPG-MPS) that displayed high volumetric energy performances. They used low and high power outputs to successively reduce and ablate GO thin layer, which was previously deposited on the nickel film via electrostatic spray. The precise control over reduction degree by tuning the laser power makes GO photoreduction contributed positively to the MSC performance. The excessive decomposition of these oxygen groups, however, may cleave carbon atoms. This behaviour causes the splitting of graphene sheets 191

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Fig. 5. (a) Schematic illustration of the preparation process of GCP membrane; (b) Photograph of a GCP membrane displaying its flexibility; (c) TEM image of GCP membrane that demonstrates GNSs anchored on the cellulose fibre surface; and (d) Photograph of a red LED powered by the tandem SCs. Reproduced with permission [193].

conductive bridges between RGO sheets. Such hierarchical structure with large SSA (254.6 m2 g−1) and large amount of mesopores provided favourable channels for electrolyte penetration and exposed more inner area for ion adsorption; thereby facilitating the rapid charge-discharge and increasing the capacitance of the fibre electrode. The potential application of the flexible SC as an energy storage component in real life was validated when three SCs connected in series successfully powered a red LED after the cell operating voltage window was expanded from 1 V to 3 V. For instance, Xu et al. [206] reported a free-standing holey graphene framework (HGF), which had high packing density and efficient ion transport pathway to enable high-performance SC electrodes with a maximum specific capacitance of 298 F g−1 and a maximum volumetric capacitance of 212 F cm−3. During hydrothermal process, GO dispersion was self-assembled to form hydrogel, and the initially added hydrogen peroxide (H2O2) was partially oxidised and etched the carbon atoms around the active defective sites of GO; leaving behind carbon vacancies. These vacancies gradually extended into nanopores for efficient ion diffusion. The RGO sheets were later mechanically compressed to form freestanding porous graphene film with about 60-fold increase in packing density (from 0.012 g cm−3 to 0.71 g cm−3) without significantly altering the stacking characteristics of RGO. In other words, the volumetric performance of the electrode that is controlled by the pore volume was substantially enhanced without compromising gravimetric performance which depends on the accessible surface area. Accordingly, with remarkable electrical conductivity of 1000 S m−1, the fully packaged HGF//HGF device could deliver a high gravimetric and volumetric stack energy densities of 35.1 Wh kg−1 and 49.2 Wh L−1, respectively in [EMIM][BF4]/AN. These values are comparable to those (30–50 Wh kg−1 and 50–90 Wh L−1) of lead-acid batteries [207]; making it an ideal electrode material for practical applications. By conducting pulse electrochemical deposition technique, Xie et al. [208] coated CVD-grown graphene with PANI to form wavy-shaped composite electrode film, which were later assembled into a stretchable SC exhibiting an energy density of 23.2 Wh kg−1 that corresponds to a power density of 399 W kg−1 for a 0.8 V cell voltage. Such outstanding result was closely related to the pulse electrodeposition method used (pulse length of 4 s for 60 cycles with a peak current density of 2 mA cm−2) in addition to the high-quality graphene formed by CVD. Compared to direct-current electrodeposition that generated dense PANI coating on the outermost graphene surface only, the pulse electrodeposition coated PANI thin film on the inside part of the porous graphene. In contrast to the dense coating, the thin film of PANI could retain both high surface area and porous structure of the electrode; leading to efficient ion diffusion and electron propagation into the inner region of the composite electrode. This fact, along with the pseudocapacitance contribution from PANI, resulted in high cell specific capacitance of 261.24 F g-1 at a 0.38 A g−1 current density. Li et al. [209] confirmed that amine-functionalised graphene quantum dots (GQDs) deposited electrophoretically within vertically

into small fragments that distorts their carbon planes [34,197], which significantly reduces the electrical conductivity [198]. Therefore, to further improve the electrical conductivity and the electrode wettability, the LPG electrode array was exposed to nitrogen plasma atmosphere prior to applying the gel electrolyte on it so that some carbon atoms were replaced by nitrogen atoms that introduced pseudocapacitance [199,200]. Benefitting from the laser irradiation and nitrogen plasma treatment, the LPG-MPS casted with [EMIM][BF4] showed an areal capacitance of 2.97 mF cm-2 at a 0.3 mA cm−2 current density and a high volumetric energy density of 5.7 mWh cm−3 that corresponds to a power density of 830 mW cm−3, which was comparable to commercial lithium thin-film batteries but with more than 100 times higher power density. Li et al. [201] recommended the use of prevalent additive manufacturing technology, i.e., inkjet printing to form 12 in-plane interdigitated graphene electrodes of the MSC built on a flexible PI substrate. The graphene/ethyl cellulose (G/EC) ink, which served as the electrode material, was inkjet-printed on the PI, and annealed at 350 °C before H3PO4/PVA electrolyte deposition. The printable G/EC-based MSC showed a high volumetric capacitance of 9.3 F cm-3 at a 250 mA cm−3 current density and a high volumetric energy density of 1.29 mWh cm−3 that corresponds to a volumetric power density of 0.278 mWh cm−3. The use of pristine graphene instead of oxidised graphene was the primary reason behind the satisfactory performance of the micro-device. The pristine graphene was obtained through direct liquid-phase exfoliation of graphite without pre-oxidation process. A low oxygen content and minimal oxygen functionalities were thus observed in the graphene sheets. This resulted in lower resistance to the electrolyte ion movement into and out of the graphene layers and larger electrode surface area for ion adsorption. Annealing of G/EC inks at 350 °C was beneficial to the micro-device performance. This was because the cellulose derivatives (EC in this work) could thermally decompose into aromatic species [202,203], and the resulting π-π stacking between EC residues and graphene sheets allowed relatively efficient charge transport through the electrode network [204]. Besides that, Ma et al. [205] engineered a well-performing flexible symmetric SC using porous carbon black/RGO hybrid fibres (CB/RGO) prepared by wet spinning method (i.e., homogeneous dispersion of carbon black/GO was injected from syringes into a rotating coagulation bath) followed by chemical reduction using hydroiodic (HI) solution. The assembled SC consisted of two parallel electrolyte-coated fibres placed on scotch tape which exhibited a volumetric capacitance of 79 F cm−3 (equivalent to 125.8 F g−1) at a 320 mA cm−3 current density. The SC could deliver a volumetric energy density up to 2.8 mWh cm−3 that corresponded to a volumetric power density of 80 mW cm−3. The maximum energy density of CB/RGO//CB/RGO cell was comparable to that of the commercial 4 V/500 μAh lithium thin-film battery. This was primarily due to the formation of hierarchically interconnected porous structure in the composite electrode after introducing CB that effectively hindered the re-stacking of RGO sheets and served as 192

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2.1.2. Electrolytes Zhang et al. [218] explored the energy performance of phenolic resin/GO film (PF16G-HA) in [EMIM][BF4] IL electrolyte (33 Wh kg−1) and [TEA][BF4]/AN organic electrolyte (17 Wh kg−1). In addition to the highly wrinkled single-layer graphene sheets (4–6 nm) with some few-layer sheets and high electrical conductivity, the mesoporous structure was also present in PF16G-HA, in which the majority of the pores had size of less than 10 nm. Such mesopores provided high accessibility for electrolyte ions, resulting in a high gravimetric capacitance of 231 F g−1 and volumetric capacitance of 92 F cm−3 for PF16GHA in [EMIM][BF4]. Surprisingly, the normalised capacitances of electrode in the IL was higher than those (202 F g−1 and 80 F cm−3) in the organic electrolyte, i.e., [TEA][BF4]/AN. This is counterintuitive to the traditional view that IL-based SCs exhibit lower specific capacitance than those based on organic electrolytes [103,219], given the higher viscosity of IL. Such deviation can be rationalised in terms of the low accessibility of [TEA][BF4]/AN to micropores in PF16G-HA. Although 73% of the electrode total pore volume came from the mesopore size range (2–5 nm), the micropores (peak values at 0.5, 0.8, and 1 nm) made up the rest of the pores. These micropores could be easily accessible for the small sizes of [EMIM][BF4] ions (∼0.76 nm for EMIM+ and ∼0.33 nm for Et4¯) but not for large sizes of [TEA][BF4]/AN (∼1.3 nm for Et4N+·7AN and ∼1.1 nm for BF4¯·9AN) [220–222]. Therefore, despite the lower viscosity of [TEA][BF4]/AN relative to [EMIM][BF4], the E-SSA of PF16G-HA and its normalised capacitance were lower in [TEA][BF4]/AN. El-Kady et al. [223] showed that IL-based laser-scribed graphene (LSG) SCs (Fig. 6(a)) had the highest energy density relative to those based on aqueous and organic electrolyte as well as solid-state electrolyte. The volumetric stack energy density of the flexible LSG-based SC could reach as high as 1.36 mWh cm−3 for a volumetric power density of 800 mW cm−3 in [EMIM][BF4], which was about 17 and 18.6 times higher than those in a H2SO4 aqueous solution and a H3PO4/PVA aqueous-based gel electrolyte, respectively (Fig. 6(c)). Such higher volumetric energy density was attributed to wider potential window of 4 V provided by [EMIM][BF4] (relative to 1 V for the aqueous-based gel electrolyte). On the other hand, the all-solid-state SC with H3PO4/PVA gel electrolyte (Fig. 6(b)) exhibited a volumetric energy density of 0.073 mWh cm−3 (that corresponded to a volumetric power density of 200 mW cm−3), which was lower than all its liquid electrolyte based LSG counterparts. This could be due to the decrease in the ion conductivity as solution viscosity increased in the gel, which negatively influence the ion accommodation in the pores [224,225]. However, such flexible LSC SC still holds promise for commercial applications if an IL-polymer electrolyte is preferable over an aqueous gelled electrolyte, given that the ionic-liquid based LSG-SC prototype could even light up a red LED for 24 minutes after being charged at a constant potential of 3.5 V (Fig. 6(d)). To attain large areal mass of electrode that maximise areal capacitance of electrode as well as of whole cell, Liu et al. [226] studied the effect of different aqueous electrolytes on the electrochemical performance of a flexible SC built with N, P-co-doped carbon nanofibres/ graphene composite (N, P-CNFs/GN) coated on bacterial cellulose (BC). The highest areal capacitances of the composite paper were 1990 mF cm−2 and 2588 mF cm−2 in a KOH and an H2SO4 aqueous solutions, respectively, at a 2 mA cm−2 current density. The symmetric device based on N, P-CNFs/GN/BC achieved the largest areal capacitance of 690 mF cm−2 and the largest areal energy density of 96 μWh cm−2 in a KOH aqueous solution. In an H2SO4 aqueous solution, this SC exhibited even higher areal capacitance of 898 mF cm−2 and areal energy density (244 μWh cm−2). This can be explained by the lack of H+ ions in KOH alkali solution [227]. H+ ions in the acidic H2SO4 electrolyte were adsorbed on the oxygen functional groups of graphene sheets, and then involved in the electron transfer [228,229]. This phenomenon suggested the addition of both EDL capacitance and pseudocapacitance to the electrode immersed in H2SO4. In an alkaline

ordered TiO2 nanotube arrays (TNAs) could deliver very high specific capacitance of 595 F g−1 and areal capacitance of 194 mF cm−2, which were advantageous to aqueous-based SC in terms of energy density. In contrast to pristine graphene, GQDs exhibit non-zero, tuneable band gap [210] that helps to control current leakage and power dissipation [211]; making it more promising for practical applications. In the study done by Li et al. [209], the zero-dimensional GQDs (single- or few-layer graphene with diameters below 20 nm) [212–214] were co-functionalised by amine and hydroxyl moieties at edge sites during hydrothermal molecular fusion process. The ultrahigh normalised capacitances of the amine-enriched GQDs mainly stemmed from the reversible Faradaic reaction of amine with H+ ions from aqueous solution (C–NH–NH2 ⇆ C=N–NH2 + H+ + e¯). The TNAs undergoing vacuum annealing prior to electrodeposition process had high concentration of oxygen vacancies that induced diffusion current, thereby enhancing their electrical conductivity [215] and electrical transport between the Ti collector and the carbon layers separated by them. These explain why the symmetrical GQDs-based SC could show a maximum energy density of 21.8 Wh kg−1 and a power density of 250 W kg−1. Three and four tandem GQD-based SC charged at 1 mA cm−2 could power red and green LED, respectively, for more than 2 minutes, demonstrating its practicality in real energy storage application. Jiang et al. [216] established a new synthesis approach, i.e., ammonium-assisted chemical blowing by foaming sucrose into the bubble networks of sucrose-derived polymers, to effectively create 3D strutted graphene (SG). During the synthesis, the polymers derived from the heating of sucrose or household sugars were blown into crowded bubble networks due to the released gases from the decomposition of ammonium salts (NH4Cl and (NH4)2CO3). After annealing, the polymer bubbles were transformed into polyhedron (i.e., 3D geometric objects constructed from polygonal faces) enclosed by graphene membranes and supported by graphite struts. The struts connecting three or four bubbles served as scaffolds to support large-surface graphene membrane and to obstruct re-stacking of graphene that might affect its properties. The ripples present on the surface of graphene membranes also prohibited the membranes from agglomerating. The stomata in the struts helped to reduce the fraction of struts in SG; thus allowing high surface area of graphene membrane (710 m2 g−1) for electron propagation. The SG also exhibited high porosity where the large amount of mesopores and macropores respectively contributed largely to SSA and high pore volume of the structure. Despite the poor electrical conductivity of SG (0.5–2 S m−1), these unique characteristics rendered the high electrode specific capacitance of 190 F g-1 at a 1 A g−1 current density and remarkable SG-based symmetric SC energy density as high as 50 Wh kg−1 in [TEA][BF4]/AN organic electrolyte that allowed operation voltage of 2.7 V. Chen et al. [217] found that Cu NPS/RGO composite film could exhibit high specific capacitance of 344 F g-1 at a 1 A g−1 current density. The electrode was prepared by adding Cu(NO3)2 solution into GO suspension and then annealed in air for reduction of GO and synthesis of Cu NPs. Redox reactions of both the remaining oxygen-containing functional groups (especially hydroxyl groups) on the RGO flakes and the Cu NPs contributed to pseudocapacitance, which made up 56.4% of the total capacitance. Cu was chosen as the dopant given its very high electrical conductivity (about 6 × 107 S m−1). The redox activity of Cu NPs took place at the surface layer only; leaving behind the metallic Cu core region. Thus, the electrodes could maintain high conductivity for rapid and effective ion diffusion into the interior electrode surfaces. The Cu NPs also separated stacked RGO flakes so that more hydrated ions could be adsorbed on the RGO surface; translating to high specific capacitance. The electrochemical test in Na2SO4 displayed that the SC based on Cu-decorated RGO exhibited a maximum energy density of 47 Wh kg−1 that corresponded to a power density of 804 W kg−1. This indicated the potential application of Cu/RGO electrode in flexible SC with aqueous polymer gel as electrolyte.

193

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Fig. 6. (a) Schematic illustration of the fabrication process of LSG-SC. Inset shows the cross-sectional SEM images of the GO and LSG. It is observed that the lowpower infrared laser convert the stacked GO sheets into well-exfoliated few-layered LSG film accompanied with a substantial film expansion. (b) Illustration and digital image (Inset) of the flexible all-solid-state LSG-SC design. (c) Ragone plot that compares the performances of LSG-SCs in different electrolytes (denoted with black circles) and those of the commercially available energy storage devices. (d) Photograph showing LSG-SC in [EMIM][BF4] lighting a red LED. Reproduced with permission [223].

ones [231,232]. In the second case of asymmetric configuration as previously mentioned, the Faradaic capacitance will be introduced due to the use of pseudocapacitive electrode. Choi et al. [233] constructed an asymmetric SC in such a way that MnO2-embedded embossed chemically modified graphene (MnO2/eCMG) and embossed chemically modified graphene (e-CMG) functioned as positive electrode and negative electrode, respectively, in an Na2SO4 aqueous electrolyte. The MnO2 deposition on e-CMG indeed doubled the electrode specific capacitance from 202 F g−1 to 389 F g-1 at a current density of 1 A g−1, which could enhance energy density. The asymmetric configuration was also crucial as it allowed the device to operate up to 2.0 V, which was at least 0.8 V higher than symmetric eCMG//e-CMG and MnO2/e-CMG//MnO2/e-CMG SCs. The asymmetric MnO2/e-CMG//e-CMG device could deliver a maximum energy density of 44 Wh kg−1, about 2.5 times higher than the symmetric MnO2/eCMG//MnO2/e-CMG SCs. Yan et al. [234] also compared the energy performance of symmetric and asymmetric sandwiched porous carbon layer/graphene hybrids (SGC) based SCs. With symmetric configuration, SGC//SGC SC could operate within the cell voltage range of 0–1.8 V in a Na2SO4 neutral electrolyte due to the presence of oxygenated surface functionalities that could extend the anodic limit [156]. Benefitting from its specific capacitance (57 F g−1) and a wide operating range (1.8 V), the symmetric device could achieve a maximum gravimetric energy density of 25.7 Wh kg−1

aqueous solution, however, the redox reaction was mostly caused by the insertion/de-insertion of large-sized hydrated K+ ions in the pores; resulting in a relatively low pseudocapacitance [230]. Still, the high performance of SC in both aqueous solutions highlighted the potential of the co-doped graphene-based paper electrode for use in the practical SC (see Table 3). 2.2. Device-level approaches Useful strategies to achieve high energy density in graphene-based SC can be extended to device level. SC configurations, ranging from the use of two dissimilar electrodes to the arrangement of electrodes in the cell, play a crucial role in this regard. 2.2.1. Asymmetric supercapacitors Similar to conventional EDLCs, an asymmetric SC is composed of two capacitive electrodes. The asymmetric configuration in SC can be obtained with: (i) same materials on two electrodes (e.g., carbon) but with different loadings; or (ii) different materials on two electrodes—one is derived from carbon-based materials while the other one is made of pseudocapacitive materials. The purpose of the asymmetric configurations is to push energy density to a higher level because it combines complementary potential windows of the positive and negative electrodes that lead to wider cell voltage compared with symmetric 194

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Table 3 Summary of electrochemical performances of graphene-based SCs. Electrode materials

Electrolyte

Normalised capacitance (rate)

Electrical conductivity

Energy density

Power density

Capacitance retention (no. of cycles)

Ref.

Curved G hGO

[EMIM][BF4] [Li][TFSI]/AN

154.1 F g−1 (1 A g−1) 145 F g−1 (1 A g−1)

– –

21.4–42.8 Wh kg −1b 30 Wh kg−1 (max)

> 90% (500) 80% (100)

[98] [103]

a-FG

Ionic liquid

–

[106]

H2SO4

∼150 Wh kg−1 (max) 16.7 Wh kg−1 (max)

88% (10000)

a-RGO-RuO2

417 F g−1 (0.05 A g−1) in KOH 509.4 F g−1 (1 A g−1)

9.838 kW kg−1 (max) 1.2 kW kg−1 12.5 kW kg−1 (max) – 1 kW kg−1 10 kW kg−1 (max) 0.35 kW L−1 5.4 kW L−1 (max) 1.98 kW kg−1

88.6% (2000)

[107]

79% (1000)

[108]

60% (1000)

[110]

1.58 kW kg−1 10.5 kW kg−1 (max) 0.25 kW kg−1 25 kW kg−1 (max) 0.402 kW kg−1 41.5 kW kg−1 (max) ∼75 kW kg−1b

108% (1000)

[113]

95% (10000)

[114]

110% (10000)

[117]

97% (10000)

[118]

∼20 kW kg−1b

94% (1000) in neat [EMIM][TFSI] 92.1% (10000)

[123]

93% (2000)

[127]

–

[128]

97% (5000)

[130]

72% (5000)

[137]

∼100% (1000)

[142]

134% (10000)

[143]

90% (20000) 108% (10000)

[149] [154]

86.6% (200)

[155]

224 F g−1 (1 A g−1)a

–

187.1 F g−1 (0.5 A g−1)

1986 S m−1

137.3 Wh kg−1 (max)a 58.5 Wh kg−1 (max)

176.5 F cm

PANI/RGO

[TBA][PF6]/ AN [EMIM][BF4] H2SO4

280 F g

−1

−1

(max)

KOH

212 F g−1 (10 A g−1)c

–

29.4 Wh kg−1 (max)

H2SO4/PVA gel

488 F cm−3 (10 mV s−1)

23000 S m−1

3.4 mWh cm−3 (max)d 36.67 Wh kg−1 (max) 25.86 Wh kg−1 (max)

Na2SO4 H2SO4

VOPO4-RGO (+)//RGO (−)

KOH

FGN-300

KOH

V2O5/GN-Ae RGO/PANI

Na2SO4 Na2SO4 H2SO4

RGO/MoO3/PANI

H2SO4 (Acidic)

ac-G/SWCNT

38.9 Wh kg

–

SAC/RGO (+)//RGO (−)

Carbon spheres/graphene

–

)

−1

a

∼21 Wh kg−1 (max)b ∼16.5 Wh kg−1 (max)b 27.4 Wh kg−1 (max)

[BMIM][BF4]/ AN [EMIM][TFSI]/ AN KOH/PVA gel

RGO/PPy

−1

500 S m−1

a-MEGO

RGO-Im-IL

−1 a

(max)

166 F g−1 (5.7 A g−1) 60 F cm−3 (5.7 A g−1) 129 F g−1 (1.1 A g−1) 58 F cm−3 (1.1 A g−1) 197 F g−1 (0.2 A g−1)

322.1 F g

BNG

(0.5 A g

−1

)

38.8 Wh kg

H2SO4

N-self doped graphenebased carbon aerogel NS-G aerogel

(1 A g

–

PRGO-O

asMEGO

−1

6.02 Wh L

KOH

G/PEDOT

−1

–

ERGO

SRGO

−3

–

Na2SO4 (Neutral) H2PO4/PVA gel

66 F g

−1

(5 mV s

(2 A g

)

–

−1 c

−1

)

−1

156400 S m

−1

366 F g (1.2 A g ) (+)a 95 F g−1 (1.4 A g−1)c 527.9 F g−1 (0.5 A g−1) (+)a 336.7 F g−1 (0.1 A g−1)c 456 F g−1 (0.5 A g−1)a 470 F cm−3 (0.5 A g−1)a 58.6 F g−1 (2 mV s−1)c 486 F g−1 (0.5 A g−1) 1182 F g−1 (1 A g−1)a 808 F g−1 (1 A g−1) 553 F g−1 (1 mV s−1)a 363 F g

−1

(1 mV s

−1 a

)

−1

551 S m

(max)

2 kW kg 20 kW kg−1 (max) 0.98 kW kg−1 3.78 kW kg−1 (max) 90 W kg−1

2.9 S m−1

26.4 Wh kg−1 (max) 27.2 Wh L−1 (max)

– –

68 Wh kg−1 (max) 28.06 Wh kg−1 (max) 76.8 Wh kg−1 (max)a

–

−1

–

72.6 Wh kg

52.9 S m−1

71.2 μWh cm−2 (max) 8.32 mWh cm−3 (max)

(max)

a

[EMIM][BF4]

–

87.5 Wh kg−1 (max)

KOH

77.2 F g−1 (1 A g−1)c

–

18.1 Wh kg−1 (max)

[EMIM][BF4]

199 F g−1 (0.5 A g−1)

39400 S m−1

110.6 Wh kg−1 (max) 117.2 Wh L−1 (max)

211 F cm

(0.5 A g

−1

)

RGO/CMK-5

[EMIM][BF4]

144.4 F g−1 (0.2 A g−1)

–

60.7 Wh kg−1 (max)

f-MWNT/HEG/[BMIM] [TFSI] WS2/RGO MoS2/RGO

[BMIM][TFSI]

201 F g−1 (2 A g−1)

–

Na2SO4 Na2SO4

−1

350 F g (0.5 A g ) 270 F g-1 (0.1 A g-1)a

– –

170.66 Wh kg−1 (max) 49 Wh kg−1 (max) 37.5 Wh kg−1 (max)a

Ni-Al LDH/G

KOH

915 F g−1 (2 A g−1)a

–

27.6 Wh kg−1 (max)a

EG/MXene

H3PO4/PVA gel

216 F cm−3 (100 mA cm−3) 140 F g−1 (1 A g−1)

–

3.4 mWh cm−3 (max) 43.5 Wh kg−1 (max)

SGO GCP

[TEA][BF4]/PC H2SO4/PVA gel

−1

46 mF cm

−2

(1 mV s

2Sm −1 c

)

[126]

−1

512 mF cm−2 (1 mA cm−2) 59.9 F cm−3 (100 mA cm−3) 197.6 F g−1 (0.5 A g−1) 70.0 F g−1 (1 A g−1)c

−3

0.4 kW kg 20 kW kg−1 (max) 0.5 kW kg−1 10 kW kg−1 (max) 910 W cm−3 (max)d −1

108 Wh kg−1 (max)

–

−1

−1

–

4 μWh cm

−2

(max)

10 kW kg (max) 130 W kg−1 −1 16.6 kW kg (max) 250 W kg−1 250 W kg−1 2.5 kW kg−1 (max) 276.3 W kg−1 10.294 kW kg−1 (max) 217.7 W kg−1 3.994 kW kg−1 (max) 1 mW cm−2

73.4% (200) 82.7% (10000)

[157]

90.1% (5000)

[162]

120 mW cm−3

1.5 kW kg−1 15 kW kg−1 (max) 650 W kg−1 5 kW kg−1 (max) 0.5 kW kg−1 400 kW kg−1 (max) 0.55 kW L−1 450 kW L−1 (max) 174 kW kg−1 10.1 kW kg−1 (max) 5.71 kW kg−1 148.43 kW kg−1 (max) – 250 W kg−1 2.5 kW kg−1 (max) 500 W kg−1 2.4 kW kg−1 (max) 200 mW cm−3 1600 mW cm−3 (max) 780 kW kg−1 16 kW L−1 (max) 0.035 mW cm−2

95.3% (5000) 98.2% (10000)

[165]

90% (2000)

[166]

98% (1000)

[168]

100% (1000) 89.6% (1000)

[176] [177]

95% (1500)

[182]

85.2% (2500)

[184]

83% (1000)

[191]

99.1% (5000)

[193]

(continued on next page)

195

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Table 3 (continued) Electrode materials

Electrolyte

Normalised capacitance (rate)

Electrical conductivity

Energy density

Power density

Capacitance retention (no. of cycles)

Ref.

RGO

H2SO4

112 S m−1

9.8 μWh cm−2 (max)

[194]

KOH

16 S m−1

8.35 Wh kg−1 (max)

5 mW cm−2 40 mW cm−2 (max) 27 W kg−1 17 kW kg−1 (max) 39.5 W L−1 5.9 kW L−1 (max) 62.5 W kg−1 98.8 W L−1 46 W cm−3 (max)

98.3% (5000)

HPGM

71 mF cm−2 (1 mA cm−2) 238 F g−1 (0.1 A g−1)

96% (4000)

[145]

98.4% (8000)

[195]

376 F cm

G/CNTC

[TEA][BF4]/ AN [BMIM][BF4]

LPG

[EMIM][BF4]

G/EC

H3PO4/PVA gel

CB/RGO

H3PO4/PVA gel

HGF

[EMIM][BF4]/ AN

−3

−1

(0.1 A g

−1

)

13.1 Wh L

−1

108 F g (0.1 A g ) 171 F cm−3 (0.1 A g−1) 3.93 mF cm−2 1.96 F cm−3 (50 mA cm−3)c 2.97 mF cm−2 (0.3 mA cm−2)d 0.694 F cm−3 (1000 mV s−1)d 9.3 F cm−3 (250 mA cm−3)d 79 F cm−3 (320 mA cm−3) 298 F g−1 (1 A g−1)

H3PO4/PVA gel

Amine-GQDs

H2SO4

SG Cu/RGO PF16G-HA

[TEA][BF4]/ AN Na2SO4 [EMIM][BF4]

LSG

[TEA][BF4]/ AN [EMIM][BF4] [TEA][BF4]/ AN H2SO4 H3PO4/PVA gel

N, P-CNFs/GN/BC

H2SO4 KOH

(max)

−1

23.5 Wh kg 37.1 Wh L−1 2.42 mWh cm−3 (max)

–

–

–

5.7 mWh cm−3 (max)d

830 mW cm−3

–

[196]

–

1.29 mWh cm−3 (max)d 2.8 mWh cm−3 (max) 35.1 Wh kg−1 (max)b

278 W cm−3 (max)

95% (10000)

[201]

80 mW cm−3

95.9% (2000)

[205]

290 W kg 7 kW kg−1 (max) 400 W L−1 10 kW L−1 (max) 399 W kg−1 799 W kg−1 (max) 0.25 kW kg−1 27 kW kg−1 (max)

91% (10000)

[206]

89% (1000)

[208]

90% (10000)

[209]

340 kW kg−1 400 kW kg−1 (max) 0.8 kW kg−1 137 kW kg−1 55 kW L−1 109 kW kg−1 44 kW L−1 800 mW cm−3

88% (5000)

[216]

125% (8000) 94% (5000)

[217] [218]

550 mW cm−3

–

1490 S m−1 1000 S m

−1

212 F cm−3 PANI/G

−1

49.2 Wh L−1 (max)b

261.24 F g−1 (0.38 A g−1)c 595 F g−1 (1 A g−1) 194 mF cm−2 (0.2 mA cm−2) 190 F g−1 (1 A g−1)

–

344 F g−1 (1 A g−1)a 231 F g−1 (1 A g−1) 92 F cm−3 (1 A g−1) 202 F g−1 (1 A g−1) 80 F cm−3 (1 A g−1) 5.20 mF cm−2 (5 A g−1)d 0.70 F cm−3 (1 A g−1)d 4.82 mF cm−2 (1 A g−1)d 0.67 F cm−3 (1 A g−1)d 4.04 mF cm−2 (1 A g−1)d 0.44 F cm−3 (1 A g−1)d 3 mF cm−2 (1 A g−1)d 0.46 F cm−3 (1 A g−1)d 898 mF cm−2 (2 mA cm−2)c 690 mF cm−2 (2 mA cm−2)c

23.2 Wh kg−1 (max) −1

–

21.8 Wh kg

0.5–2 S m−1

50 Wh kg−1 (max)

– 303 S m−1

47 Wh kg−1 (max)a 33 Wh kg−1 (max)b 39 Wh L−1 17 Wh kg−1 (max)b 20 Wh L−1 1.36 mWh cm−3 (max)d 0.7 mWh cm−3 (max)d 0.08 mWh cm−3 (max)d 0.073 mWh cm−3 (max)d 244 μWh cm−2 (max) 96 μWh cm−2 (max)

– – – – 2.45 S m−1

(max)

−1

200 mW cm 200 mW cm

−3

−3

1.399 mW cm−2 35.03 mW cm−2 (max) 1.015 mW cm−2 19.98 mW cm−2 (max)

> 99% (5000) –

[223]

96.5% (10000) > 97% (10000) –

[226]

99.6% (10000)

a

The value is derived from data obtained through three-electrode test. The value estimates the energy performance of packaged cell in real life since the calculation includes the packaging. c The value shows the normalised capacitance of two electrodes instead of a single electrode. d The value estimates the performance of whole device. The areal performance is evaluated with respect to the entire projected surface area of the device. The volumetric performance is calculated considering the volume of device stack including the volume of graphene-based electrodes, the interspaces between the electrodes, gel electrolyte, current collector, and electrolyte separator (if there is any), but without taking into account the packaging. b

with the corresponding volumetric energy density of 11.3 Wh L−1. The device performance could be further augmented to reach a specific capacitance of 159 F g−1 with the corresponding maximum energy density of 88 Wh kg−1 and practical energy density of 26.4 Wh kg−1 by fabricating an asymmetric SC using graphene/MnO2 and SGC as the positive and negative electrodes, respectively. Such high performance was ascribed not only to the large pseudocapacitance provided by the MnO2 NPs [235], but also to the extended cell voltage (0–2 V). Foo et al. [236] reported a high-areal-energy-density (89 μWh cm−2 for an areal power density of 83 μW cm−2) asymmetric flexible device with free standing V2O5-RGO and RGO as positive electrode and negative electrode, respectively, in a LiClO4/PC organic electrolyte. In addition to the use of pseudocapacitive transition metal oxide (V2O5), the authors attributed the outstanding performance, i.e., the cell areal capacitance of up to 95 mF cm−2, to the expansion of the potential

window from 1.6 V to 2.5 V as the result of asymmetric configuration. The assembled device was highly promising for application in flexible SC as eight LED bulbs, which typically requires 2 AA batteries (3 V) as power supply, could be lighted up by only one unit of the SC. Ma et al. [237] developed a novel fibre-shaped asymmetric linear SC where MnO2 nanorods/RGO hybrid fibres and MoO3 nanorods/RGO hybrid fibres served as positive electrode and negative electrode, respectively, to double the cell operating window (from 0.8 V to 1.6 V) for enhanced energy density. Both reduced composite fibres were coated with H3PO4/PVA gel electrolyte before twisted together to maximise the contact between electrode and electrolyte for enhanced electrolyte ion diffusion. Benefitting from the introduction of pseudocapacitive materials and the asymmetric configuration, the volumetric capacitance of the flexible SC was as high as 53.5 F cm-3 at a 100 mA cm−3 current density. The device maintained its capacitance at 26.3 F cm−3 even 196

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when the current density was increased to 4000 mA cm−3; implying its good rate capability. The asymmetric linear SC also displayed a high volumetric energy density of 18.2 mWh cm−3 at a power density of 76.4 mW cm−3, which made it promising for application in flexible electronic devices. Choi et al. [238] devised a solid-state flexible asymmetric SC based on an IL-functionalised chemically-modified graphene (IL-CMG) film (as the negative electrode) and hydrous RuO2-IL-CMG composite film (as the positive electrode), separated by H2SO4/PVA electrolyte. The asymmetric device could operate up to 1.8 V and deliver a high energy density of 19.7 Wh kg−1 at a power density of 0.5 kW kg−1, which is higher than a maximum energy density of 2.95 Wh kg−1 at a power density of 0.23 kW kg−1 of symmetric SC based on IL-CMG film. This comparison indicated the beneficial roles of functionalisation, composite electrode structure, and asymmetric configuration toward high energy density and power density attainment. For example, [BMIM][BF4] ILs promoted the stabilisation of CMG sheets in water solvents through electrostatic attractions between imidazolium cations in the ILs with the carboxylic acid groups at the edges of CMG sheets [239]. The IL functional groups on the surface of IL-CMG hybrids also served as the nucleation and growth sites for the deposition of RuO2 NPs. The introduction of RuO2 doubled the electrode specific capacitance, from 122 F g−1 for IL-CMG to 233 F g−1 for RuO2-IL-CMG. This is consistent with the fact that the asymmetric device showed a specific capacitance of 175 F g−1, which was larger than that (85 F g−1) of IL-CMG//IL-CMG SC.

Specifically, the reversible Faradaic redox reaction from NiCo2S4 to NiSOH and CoSOH as well as the conversion between CoSOH and CoSO added significant pseudocapacitance to the total capacitance. The metallic conductivity of NiCo2S4 was also a key factor towards the excellent performance of the SC [243]. It increased the conductivity of the electrode by 3.4 times (from 3900 S m−1 for GF to 13300 S m−1 for GF/ NiCo2S4), which facilitated the ion diffusion into the interior surface of positive electrode and thereby increased the charge storage. By connecting three GF/NiCo2S4//GF asymmetric SC having a cell voltage of 1.5 V each in series, a LED could be lighted up; confirming the great potential of GF/NiCo2S4 as an electrode in SC. 2.2.3. In-plane architecture Trigueiro et al. [244] constructed an ‘in-plane’ high-energy-density reduced graphene oxide/multi-walled carbon nanotube (RGO/ MWCNT) SC (41.3 Wh kg−1) with [EMIM][TFSI] as electrolyte. Different from conventional stacked design, the electric field applied within ‘in-plane’ device was in the same direction as the preferential arrangement of the electrode materials and the movement direction of ions; thus giving an increased ability of the electrolyte to percolate into the carbon materials to allow for full utilisation of the electrochemical surface area. This new strategy, in addition to the addition of MWCNT that hindered RGO NSs restacking, is a reason behind the high specific capacitance of the electrode (153.7 F g-1 at a 0.2 A g−1 current density) that is closely related to device energy density. Wu et al. [245] developed high performance (a volumetric stack capacitance of 17.5 F cm−3 and a volumetric energy density of 2.5 mWh cm−3) all-solid-state graphene-based in-plane MSC on flexible PET substrate (MPG-MSC-PET) via micro-patterning of graphene films (Fig. 8(d)). The lithography technique was applied to manufacture the interdigital micro-electrode pattern through deposition of gold current collectors on the MPG film (i.e., GO film after reduction), followed by oxidative etching of the unexposed areas in an oxygen plasma (Fig. 8(a)). Besides the high conductivity of the MPG film resulting from the addition of carbon source (e.g., C+ and CHn+, and CHn, where n = 1–3) that repairs the GO defects, the in-plane geometry of MPG film was of utmost importance to the SC performance enhancement because it shortened the diffusion length of ions, so the ions between the microelectrodes gaps could be rapidly transported along the planar graphene sheets (Fig. 8(b)). This factor contributed to the high capacitance at high rates. The MPG-MSC-PET could certainly offer a nano- or microscale power source sufficient to fulfill applications that require higher energy since its volumetric energy density was comparable to that of commercial lithium thin-film batteries (Fig. 8(c)).

2.2.2. Hybrid supercapacitors Similar to asymmetric SCs, hybrid SCs are fabricated to obtain higher energy density relative to EDLCs via: (i) integration of Faradaic and capacitive storage mechanisms in a device; and (ii) wider operating voltage. The main difference between asymmetric and hybrid SCs is that the former consists of two capacitive electrodes while the latter is built with one capacitive electrode and one battery-type electrode. Zhang et al. [240] designed a hybrid SC with Fe3O4/G as negative electrode and 3D porous graphene (3DGraphene) as positive electrode immersed within LiPF6-containing organic electrolyte (Fig. 7(a) and (c)) to improve the practical energy density from 10 Wh kg−1 (for 3DGraphene//3DGraphene SC device) to 30 Wh kg−1 (that corresponded to a volumetric energy density of 16 Wh L−1 and packaged power densities of 1000 W kg−1 and 540 W L−1) (Fig. 7(d)). This hybrid configuration increased the energy storage capacity from three aspects, i.e., (i) larger operating voltage; (ii) 3.5-fold higher specific capacity of the Fe3O4/G (negative electrode) relative to that of the graphene it replaced; and (iii) higher specific capacitance of the 3DGraphene (positive electrode) when used with Li-ion electrolytes [241]. During the charge/discharge process, PF6¯ ions from electrolyte were absorbed to/ desorbed from the 3DGraphene (positive electrode) while Li+ ions were intercalated into/de-intercalated from the layers of Fe3O4/G nanocomposite (negative electrode). At a 90 mA g−1 current density, the negative electrode exhibited a specific capacity as high as 1089 mA h g1 , which was higher than the theoretical capacity of Fe3O4 (924 mA h g1 ) and that of bare Fe3O4 (723 mA h g-1) (Fig. 7(b)). This feature along with operating voltage of 3 V largely contributed to the remarkable energy density hybrid SC, which was comparable to Ni-metal hydride and lead acid batteries (Fig. 7(e)). Cai et al. [242] constructed a flexible all-solid-state GF/NiCo2S4// GF hybrid SC with a stack volumetric capacitance of 9.5 F cm−3 and a stack volumetric energy density reaching up to 3.0 mWh cm−3 which was on a par with that of the 4 V/500 μAh lithium thin-film battery. The composite positive electrode (GF/NiCo2S4) showed both impressive areal capacitance of 568 mF cm−2 and volumetric capacitance of 300 F cm-3 at 0.5 mA cm−2 and 175.7 mA cm−3 current densities, respectively, in a KOH aqueous solution. Cai et al. [242] credited the remarkable normalised capacitances of the hybrid fibres and the device to the presence of transition metal sulphide (NiCo2S4) that is known to have rich redox activity of Co2+/Co3+/Co4+ and Ni2+/Ni3+.

2.2.4. Other approaches El-Kady and Kaner [246] developed an interdigitated flexible graphene MSC using laser scribing—an infrared laser was used to reduce GO to laser-scribed graphene (LSG). The authors observed that higher amount of interdigitated electrodes per unit area led to higher energy and power for the micro-device. Specifically, compared to LSG-MSC with 4 and 8 interdigitated electrodes, the MSC with 16 interdigital electrodes showed a higher volumetric stack capacitance of 3.05 F cm3 at a 16.8 mA cm−3 current density in H2SO4/PVA gel electrolyte. This was because both the pathway for ion diffusion from the electrolyte to the electrode and the mean ionic diffusion pathway between two microelectrodes were minimised. Such minimum pathways provided the maximum available electrochemical surface area, which translated to enhanced capacitance and the fast charge/discharge rates. The energy density of LSG-MSC was reported to reach up to 2.1 mWh cm−3 when encapsulated with ionogel (fumed silica/[BMIM][TFSI]). The LED could be illuminated by connecting it to a tandem device (i.e., four inseries H2SO4/PVA electrolyte-based LSG-MSCs); demonstrating fine performance of the micro-device for practical applications (see Table 4). 197

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Fig. 7. (a) Schematic illustration of the preparation procedure of Fe3O4/G nanocomposite and 3DGraphene; TEM images of (b) bare Fe3O4 and (c) Fe3O4/G; showing the larger particles (200–500 nm) for the former, which are about two times larger than those in the latter, due to severe agglomeration; (d) Ragone plot comparing the performances of 3DGraphene-based symmetric and asymmetric SCs; and (e) Ragone plot comparing the energy-power densities performances of Fe3O4/G// 3DGraphene asymmetric device and the commercial batteries. Reproduced with permission [240].

3. Challenges and outlook

challenge to the commercialisation of low-cost, high performance SCs based on graphene. 2. Low working cell voltage and its effect on energy density. The working voltage of SCs is closely associated to the potential window of electrolyte used. In this regard, ILs have been extensively used as electrolytes in laboratory-level graphene-based SCs due to their wider potential windows compared to organic electrolytes which are commonly utilised in most commercial SCs. However, practical applications of ILs in SCs remains unclear due to their high process complexity, high viscosities at room temperature, and high cost. For example, a recent study has demonstrated that the energy density of a SC based on an IL with an ESPW of 6 V and a viscosity of 481 mPa s is three times lower than that of an SC based on an IL with an ESPW of 3.9 V and a viscosity of 52.6 mPa s [249]. Such a trade-off between ESPW and viscosity of IL electrolytes poses major challenge to the development of high energy density graphene-based SCs. 3. Compatibility issue between the electrolyte and graphene-based electrode materials and its effect on capacitance. Major attempts have been devoted to improve the physicochemical properties of organic and IL electrolytes for SC applications given their relatively high ESPW but low ionic conductivities. Relatively low attention have been given to the microscopic behaviour of electrode/electrolyte interfaces and the effects of pore size on the structure. Understanding the compatibility between electrolyte and electrode materials is particularly essential because it significantly affects the ion transport between the two components, which in turn determines the cell performance. Such electrolyte-electrode compatibility in SC is primarily governed by the wetting of electrode with electrolyte. A lot of studies reported that a good wettability between graphene-based electrode and an electrolyte can be attained by retaining a small amount of oxygen-containing functional groups on the surface of graphene sheets. However, this is not always true. For example,

3.1. Challenges towards high energy density graphene-based supercapacitors The successful commercialisation of conventional graphene-based macroscale SCs, i.e., SkelCap and CSRCAP SCs, and the consistently good experimental results from the majority published scientific works have marked the significant advances in the energy performance of graphene-based SCs relative to the current available EDLCs. Nevertheless, the energy density of the currently available graphenebased SCs is only one-tenth that of the AA rechargeable batteries, which highlights its incapability to compete with the existing battery in practical applications. Although intensive efforts have been given to develop high energy density graphene-based SCs, there are some key challenges in this direction: 1. Low-scale graphene at high cost. Since graphene does not exist naturally, the key issue in producing graphene-based SCs lies in manufacturing large quantities of graphene at an affordable price. Currently, Hummers' method and CVD are the two most widely used synthesis routes to produce graphene-based materials (in the form of film, solution, or powder) in industry (i.e., Sigma Aldrich, Graphenea, and Graphene Supermarket). In the conventional Hummers process, the conversion of graphite to GO is often incomplete, leading to low product yield. Despite the high possibility to synthesise high-quality graphene in a large scale via CVD, its complicated process and expensive substrates result in expensive graphene. For instance, 1 g of reduced graphene oxide powder and a 60 mm × 40 mm monolayer graphene on Cu are currently sold at an extremely high cost, about USD 97 and USD 172, respectively [247,248]. Undoubtedly, the cost issue remains a profound 198

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Fig. 8. (a) Schematic illustration of the synthesis of MPG-MSC-PET. (b) The in-plane geometry of MPG film that reveals the shorter diffusion length of electrolyte ions along the graphene sheets compared to stacked geometry. (c) Ragone plot of MPG-MSC (before transferred to PET substrate) and commercial energy storage devices that confirms MPG-MSC comparable electrochemical performance with respect to commercial batteries and capacitors. (d) Photograph of MPG-MSC on flexible PET substrate. Reproduced with permission [245].

are interrelated. For example, increasing pore volume reduces electrode packing density, which is advantageous to gravimetric performance but detrimental to volumetric performance. Furthermore, the high device capacitance is always determined by the synergistic effect of several factors, not by a single factor. For instance, high electrical conductivity of a graphene-based electrode does not necessarily translate into high device capacitance because other variables such as surface chemistry (i.e., oxygen-containing functional groups) and pore size distribution may have an equally strong influence on the cell performance. 5. Undesirable effect of current collector on graphene-based SCs. As in other non-metal based SCs, metal foils such as aluminium, nickel, and stainless steel are mostly used as current collectors in graphenebased SCs to enhance the electron conduction within the device. The main technical challenge of using a metal current collector is the contact resistance at current collector/active materials interface, which can be a major contributor to cell internal resistance [251]. Interestingly, the internal resistance decreases not only device power density, but also energy density. This is because the internal resistance contributes to IR drop (i.e., reflected in the initial portion of discharge curve), which reduces the usable cell voltage window (and capacitance) and hence energy density [252]. Furthermore, these metallic current collectors are usually heavy and inactive. They increase the weight and volume of overall device without contributing to energy storage; thus reducing the gravimetric and volumetric energy densities of graphene-based SC [253]. 6. Inconsistencies in SC performance evaluation and characterisation. In current practices, such inconsistencies are brought on by common sources, which include testing methods, test instruments, and the evaluation methods (e.g., the use of two- and/or three-electrode configurations, the performance metrics derivation using cyclic voltammetry and/or galvanostatic charge-discharge curves, and the

hydrophobic IL electrolytes (due to the presence of hydrophobic cations) have high affinity toward hydrophobic electrode materials. Considering that the surface oxygen-containing functional groups on graphene derivatives are hydrophilic in nature, these two components are less likely to be compatible with each other [239]. Such poor compatibility prevents the electrolyte ions from fully accessing the inner surface area of electrode, thus limiting the capacitance and energy density. The in-depth fundamental understanding of EDL dynamics in complex electrode/electrolyte systems becomes crucial given the recent emergence of various new electrolytes such as ILmixtures, IL-organic solvent mixtures, and gel-polymer electrolytes. Likewise, a better understanding of the relationship between pore size on interfacial properties and ion transport is necessary. Although a number of reports on this subject is available, conflicting results appear with several studies showing that the maximum normalised capacitance of graphene-based SC can be achieved by matching the electrode pore size to the electrolyte ion size [218,250] while other studies reporting that a higher capacitance was obtained at pore size larger than the ion size (not at pores matching ion size). Such controversy highlights the need of further systematic and detailed study to evaluate the dependence of the capacitance of various electrolytes on EDL structure and electrode pore size from a microscopic view. 4. Lack of optimisation of graphene-based electrode parameters. Despite tremendous amount of experimental work on energy density enhancement of graphene-based SCs, fewer reports have demonstrated the optimisation of electrode parameters. In fact, such optimisation is essential to obtain better device performance, since the key physiochemical properties of graphene electrode materials such as pore volume, packing density, effective specific surface area, thickness, surface chemistry all have direct effect on energy and power densities. It is worth to point out that some aforementioned properties 199

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Table 4 Summary of electrochemical performances of graphene-based SCs. Electrode materials

Electrolyte

Normalised capacitance (rate)

Electrical conductivity

Energy density

Power density

Capacitance retention (no. of cycles)

Ref.

MnO2/e-CMG (+)//eCMG (−)

Na2SO4

–

44 Wh kg−1 (max)

11.2 W kg−1 25 kW kg−1 (max)

95% (1000)

[233]

SGC-2

Na2SO4

389 F g−1 (1 A g−1) (+)a 202 F g−1 (1 A g−1) (−)a ∼79 F g-1 (1 A g−1)c 57 F g−1c

–

25.7 Wh kg−1 (max) 11.3 Wh L−1

–

[234]

159 F g−1c

–

95 mF cm−2 (0.025 A g−1)c

–

26.4 Wh kg−1 (max)b 89 μWh cm−2 (max) 13.3 Wh kg−1 (max) 18.2 mWh cm−3 (max) 19.7 Wh kg−1 (max)b 30 Wh kg−1 (max)b 16 Wh L−1b

100 W kg−1 45 kW kg−1 (max) 45 W L−1 18.3 kW L−1 (max) 320 W kg−1 597 kW kg−1 (max) 0.083 mW cm−2

G/MnO2 (+)//SGC-2 (−) V2O5-RGO (+)//RGO (−)

MnO2/rGO (+)//MoO3/ rGO (−) RuO2-IL-CMG (+)//ILCMG (−) Fe3O4/G (+)//3DG (−)

LiClO4/PC

H3PO4/PVA gel H2SO4/PVA gel

53.5 F cm−3 (100 mA cm−3)c 175 F g−1 (0.5 A g−1)c

–

–

2000 S m−1 29700 S m−1

GF/NiCO2S4 (+)//GF (−) RGO/MWCNTs

[EMIM][TFSI]

1098 mAh g−1 (90 mA g−1)()̶ ; 187 F g−1 (0.05 A g−1) (+) 9.5 F cm−3 (175.5 mA cm−3)d 153.7 F g−1 (0.2 A g−1)

MPG

H2SO4/PVA gel

17.5 F cm−3 (10 mV s−1)d

LSG

LiPF6-containing organic electrolyte

KOH/PVA gel

H2SO4/PVA gel Ionogel (Fumed silica/[BMIM][TFSI])

-3

3.05 F cm (16.8 mA cm3)d 2.35 F cm−3 (16.8 mA cm−3)d

–

–

– –

3 mWh cm−3 (max)d 41.3 Wh kg−1 (max) 2.5 mWh cm−3 (max)d 0.5 mWh cm−3 (max)d 2.1 mWh cm−3 (max)d

– > 85% (8000)

[236]

96.8% (3000)

[237]

97% (660)

[238]

70% (1000)

[240]

0.3 mW cm−3 (max)

92% (2000)

[242]

3.5 kW kg−1 (max)

88% (2000)

[244]

–

∼99.1% (100000)

[245]

97% (2000)

[246]

12.5 W kg−1 76.4 mW cm−3 3269 mW cm−3 (max) 0.5 kW kg−1 6.8 kW kg−1 (max) 1 kW kg−1 540 W L−1

−3d

1.8 W cm 30 W cm−3 (max) 2.4 W cm−3 141 W cm−3 (max)

∼100% (30000)

a

The value is derived from data obtained through three-electrode test. The value estimates the energy performance of packaged cell in real life since the calculation includes the packaging. c The value shows the normalised capacitance of two electrodes instead of a single electrode. d The value estimates the performance of whole device. The areal performance is evaluated with respect to the entire projected surface area of the device. The volumetric performance is calculated considering the volume of device stack including the volume of graphene-based electrodes, the interspaces between the electrodes, gel electrolyte, current collector, and electrolyte separator (if there is any), but without taking into account the packaging. b

graphene 100 times faster than conventional CVD systems, cut costs by 99%, and produce material with enhanced electronic qualities. Recently in 2017, researchers from Australia have also developed a cost-effective modified CVD route (called ‘GraphAir’ technology) that produces graphene from cheap soybean cooking oil in ambient air [255]. Requiring neither high temperatures annealing nor expensive vacuum processing as in conventional CVD method, the ambient-air synthesis of graphene film is more simple, safer, and cheaper. Such unique technology can potentially produce graphene on a large scale if continuous production process is adopted. In another attempt on developing sustainable techniques for the largescale production of graphene from natural sources, a China-based graphene company called Shangdong Longju New Materials Technology recently announced that it has completed the installation and commissioning of a pilot biomass graphene production line and has put it into operation. The facility uses corncob waste to make few-layer biomass graphene. The production line's annual capacity is said to be five tons and is expected to increase to 300 tons. Future works along these directions are warranted to overcome the cost issue of graphene. It is expected that in several years, bulk graphene prices will drop below that of silicon, enabling graphene to enter all markets now dominated by silicon. 2. Expand the cell working voltage to increase the energy density. The operating voltage range of a graphene-based SC can be increased by expanding the ESPW of the electrolyte used. Given the ESPW/ionic conductivity trade-off, an optimum device energy density is most likely to be attainable if IL with moderately high ESPW (which is still higher than that of organic electrolyte) and low viscosity (and

performance metrics normalisation to the mass, volume, or area of electrode materials or whole devices). Such inconsistencies makes the comparison of the experimental data from a wide range of literature challenging and inaccurate. Notably, various groups around the world e.g., The United States Advanced Battery Consortium (USABC), International Electrotechnical Commission (IEC), the University of California-Davis, and European Council for Automotive R&D (EUCAR) have developed their own procedures to test SC. With the data interpreted being different for each testing procedure, there is a need to universalise the testing procedure (i.e., test conditions and experimental setups) and calculation methods that derive the performance metrics. 3.2. Future directions The aforementioned challenges should be resolved to realise high energy density graphene-based SC in the near future. Several most promising directions are as follows: 1. Develop alternative graphene synthesis methods that can produce high quality graphene with lower manufacturing costs. To reduce graphene cost, researchers from University of Glasgow proposed the replacement of rough copper foil currently used in CVD with ultra-smooth, commercially-available copper foil (costs around $1 per m2, which is 115 times cheaper than the former) [254]. A substantial reduction in graphene production cost is thus anticipated with the use of smooth copper foil. Besides, growing graphene in an industrial resistive-heating cold wall CVD system was claimed to produce 200

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multi-parameter optimisation of porous electrode has so far been developed [259]. A robust model for multi-parameter optimisation based on graphene-based electrode can thus be developed to predict the optimum electrode architecture. The physical, structural, and electrical properties of the electrode materials and components should be considered when modelling and estimating the electrode parameters so that a reliable guideline for the practical design of the high performance electrode can be obtained. Experimental test also plays an important role in the electrode design optimisation since experimental conditions (e.g., the degree of activation, the degree of GO reduction, etc.) should be manipulated to obtain the predicted optimum electrode properties. Most importantly, such optimisation should consider not only the gravimetric performance, but also the volumetric performance since they offset each other. 5. Minimise the undesirable effect of current collectors on SC performance. Although etching treatment on metal foils (i.e., creating porosity on the metallic surface) is not commonly practiced since its introduction in the past decade, it has been proven useful in reducing the contact resistance between the electrode and the current collectors due to the enhanced interfacial surface area [260,261]. The etching procedure can be chemically or electrochemically carried out. Besides that, etched metal current collector can be coated with a layer of carbon materials to further roughen the metallic surface for better adhesion of graphene electrode materials (to reduce interfacial resistance) and more paths for electron transfer (to increase capacitance) [262,263]. Several research works even reported the single use of conductive carbon materials (e.g., carbon nanotubes (CNT) film and graphite foil) as current collectors in carbon-based SCs [264,265]. While CNT film can provide stronger adhesion to the graphene electrodes, it is reported to be as efficient as a gold film to transport electrons [264]. More interestingly, graphene can function as both electrode materials and current collector, given its unique properties (e.g., large surface area and high electrical conductivity) [266]. The absence of the current collector not only minimises the interfacial resistance and hence charge transfer resistance, but also decreases the overall weight and volume of device for improved electrochemical performance. However, graphene-based materials used in current collector-free SC should have very high electrical conductivity to ensure efficient electron transport that enables the device to operate at maximum power. Further research and development efforts on the discussed subjects are required to investigate their practical feasibility in terms of energy and power performances. 6. Develop consistent SC performance evaluation and characterisation methods. In parallel with the growing interest of global research communities in graphene-based SCs, the development of a worldwide standard protocol is imperative for accurate performance measurement and comparison of various graphene-based SCs. Two key issues that should be addressed are: (i) consistency in electrochemical testing procedure; and (ii) consistency in performance evaluation of an electrode material and/or its associated SC. The allencompassing experimental procedures should be reproducible and universally applicable. For performance evaluation, there are two areas that should be taken note of. First, the representative normalisation parameter for conventional macroscale SC, flexible SC, and MSC should be standardised. In the case of conventional macroscale SC, researchers typically provide only single normalised value of capacitance (either gravimetric or volumetric capacitance), which makes it difficult to assess the true device performance given the trade-off between gravimetric and volumetric capacitances. In this light, it is recommended that the standardisation of the performance metrics can be based on at least two representative normalisations (to mass, volume, and/or area). On the other hand, due to the limited size (volume and area) available for electrochemical reaction, the representative normalised performance metrics for flexible SC and MSC should be meaningful and useful to reflect their

hence high ionic conductivity) is used as an electrolyte in graphenebased SC. In the case of organic electrolytes, some possible ways to improve ESPW include develop new organic solvents and/or modify the combination of organic electrolytes. For example, [SBF][BF4]/ 2,3-BC (i.e., spirobipyrrolidinium tetrafluoroborate with 2,3-butylene carbonate, an alkylated cyclic carbonate) showed a high ESPW up to 3.5 V, which far exceeds that (2.5–2.7 V) of its PC-based counterpart [256]. The high electrolyte stability at such high voltage is ascribed to its excellent degradation resistance following the alkyl groups substitution at the 4th and/or 5th position of 2,3-BC. Despite the ESPW/ionic conductivity trade-off in most electrolytes, the 2,3-BC has viscosity (of 2.8 mPa s at 25 °C) that is as low as that (of 2.5 mPa s at 25 °C) of PC, which is important to maintain the capacitance at mid-to-high current rates. Given its high ESPW and low viscosity, 2,3-BC becomes a very promising candidate as an alternative solvent for SCs. Still, further investigation on 2,3-BC is necessary to thoroughly explore the effects of its other physicochemical properties on the overall SCs performance. Likewise, [SBF] [BF4] with linear sulfone (i.e., ethyl isopropyl sulfone (EiPS)) as an organic solvent can exhibit a high ESPW of 3.3–3.7 V, which is around 0.6–1.2 V higher than that of PC-based electrolytes [257]. The higher ESPW in the EiPS-based electrolyte may be attributed to the low reactivity between EiPS and the released water, unlike the PC case. However, EiPS has high viscosity (of 5.5 mPa s) at room temperature (twice that of PC), which may offset its high ESPW. Such behaviour is consistent with the parallel increase of viscosity and ESPW for most electrolytes. A possible strategy to overcome this issue is the combination of a salt with high ionic conductivity (or solubility) with an organic solvent(s) with high ESPW. 3. Enhance compatibility between the electrolyte and graphene-based electrode materials to improve capacitance. Given the limited studies on the compatibility between graphene-based electrodes and various electrolytes as well as the controversy over the effect of pore size on capacitance, more theoretical and experimental work on these subjects are required. A possible method is to perform theoretical studies via density functional theory (DFT). DFT provides a unified theoretical basis for computational modelling of the microscopic behaviour at electrode/electrolyte interface with minimal computation cost and molecular details. The DFT approach can be used to simulate the physical processes of charge adsorption in SCs and probe different pore range scenarios from ion size to macroscopic scales [258]. On the other hand, experimental studies involving in situ characterisation of kinetics at the electrode/electrolyte interface via powder X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) are recommended. A comparison between the DFT predications and the experimental results can provide fundamental understandings on the ion dynamics in certain electrolytes, the solvation/ de-solvation mechanism, and the charge storage mechanism in the electrode structure. Such understanding can give guidelines towards the appropriate selection of electrolyte for particular SC or how electrode materials should be developed to suit a particular electrolyte. Molecular simulation becomes more important particularly to predict the effects of hard-to-measure electrolyte properties such as the size of solvent molecule (if there is any solvent), the ionic concentration, the ionic charge, and the pore shape (e.g., slit, cylindrical, spherical, and in-bottle shaped pores) on the capacitance. 4. Optimise graphene-based electrode properties. An optimisation of the electrode properties can be performed through the combination of model predictions and experimental verifications. The optimisation based solely on experimental approaches is challenging and timeconsuming given the requirements to optimise simultaneously multiple electrode parameters and ensure reproducibility. In this light, modelling can be used to obtain an optimum electrode architecture. The obtained result requires further experimental verification. While models for two-parameter optimisation have been established and widely utilised, only one model for simultaneous 201

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true performances. Second, it must be made clear that the capacitance value should be normalised with respect to the quantities of the active materials, the entire electrode, or the whole device. This issue has been brought up but it remains inapplicable widely. Moreover, to minimise wide variations in the reported results arisen from such inconsistency and to obtain an accurate comparison between electrode performances, the standard reporting format should be implemented.

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Nomenclature AC: Activated carbon CV: Cyclic voltammetry CVD: Chemical vapour deposition EDL: Electric double layer EDLC: Electric double layer capacitor ESPW: Electrochemical stability potential window G: Graphene GCD: Galvanostatic charge/discharge curve GF: Graphene fibre GO: Graphene oxide IR: Ohmic resistance drop IL: Ionic liquid LDH: Layered double hydroxide MSC: Micro-supercapacitor MWCNT: Multi-walled carbon nanotube MXene: Transition metal carbide or carbon nitride NP: Nanoparticle NS: Nanosheet PANI: Polyaniline PEDOT: poly(3,4-ethylenedioxythiophene) PPy: Polypyrrole RGO: Reduced graphene oxide SEM: Scanning electron microscopy SSA: Specific surface area TEM: Transmission electron microscopy [BMIM][BF4]/AN: 1-butyl-3-methylimidazolium tetrafluoroborate in acenotrile [EMIM][TFSI]: 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [EMIM][BF4]: 1-ethyl-3-methylimidazolium tetrafluoroborate [PYR14][TFS]/AN: N-butyl-n-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide [TBA][PF6]/AN: Tetrabutylammonium hexafluorophosphate in acetonitrile [TEA][BF4]/AN: Tetraethylammonium tetrafluoroborate in acetonitrile [TEA][BF4]/PC: Tetraethylammonium tetrafluoroborate in propylene carbonate

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