Graphene Based Materials

#### **Chapter 4**

## Graphene-Based Materials for Supercapacitor

*Abu Jahid Akhtar*

#### **Abstract**

Graphene, a one-atomic-thick film of two-dimensional nanostructure, has piqued the attention of researchers due to its superior electrical conductivity, large surface area, good chemical stability, and excellent mechanical behaviour. These extraordinary properties make graphene an appropriate contender for energy storage applications. However, the agglomeration and re-stacking of graphene layers due to the enormous interlayer van der Waals attractions have severel*y* hampered the performance of supercapacitors. Several strategies have been introduced to overcome the limitations and established graphene as an ideal candidate for supercapacitor. The combination of conducting polymer (CP) or metal oxide (MO) with graphene as electrode material is expected to boost the performance of supercapacitors. Recent reports on various CP/graphene composites and MO/graphene composites as supercapacitor electrode materials are summarised in this chapter, with a focus on the two basic supercapacitor mechanisms (EDLCs and pseudocapacitors).

**Keywords:** Supercapacitor, EDLC, Pseudo-capacitance, Metal oxides, Conducting polymers

#### **1. Introduction**

Energy storage devices are important in today's world to meet the increasing demand for reliable and portable power sources [1–3]. Supercapacitors, also known as ultracapacitors, are electrochemical energy storage devices that are lightweight, can operate at a wide range of temperatures, have a long life cycle, and are shielded to make their work easier [4, 5]. With a number of such advantages, the supercapacitors emerged in a variety of applications in hybrid or electric vehicles, electronics and aircrafts [4, 5]. Today, supercapacitor manufacturers mostly use coconut activated carbon as an electrode material due to its high specific surface area, low price and mass production capability. However, with increased energy demand, significant research efforts have been made to find ideal electrode materials for the production of advanced energy storage systems. Energy is stored in supercapacitors via two energy storage mechanism, namely electrochemical double layer capacitance (EDLC) and pseudocapacitance. So, in order to improve supercapacitor efficiency, both of these mechanisms must be incorporated on a single electrode material.

Graphene, a one-atom-thick 2D single layer of sp2 -bonded carbon atoms with the hexagonal lattice structure, is considered as the basic building block material for all carbon materials [6, 7]. Graphene has emerged as an appropriate candidate for energy storage applications due to its high electrical (108 S/m) and thermal

conductivity (5000 W/m/K), large surface area (2.63X106 m<sup>2</sup> /kg), high transparency (absorbance of 2.3%), good chemical stability, and excellent mechanical behaviour (breaking strength of 42 N/m and Young's modulus of 1.0 TPa) [8, 9]. Various methods such as chemical vapour deposition (CVD) of hydrocarbons, epitaxial growth on electrically insulating surfaces such as SiC, micromechanical exfoliation of graphite (Scotch tape method), oxidation–exfoliation–reduction of graphite powder may be used to synthesise graphene sheets of various sizes and defect contents [10]. Among these methods, graphene sheet which is grown by chemical vapour deposition (CVD) of hydrocarbon [11] has the superior quality with minimal defects. However, CVD prepared graphene would not be an ideal contender for EDLC electrode material, as it is too costly to produce and is hardly scalable. On the other hand, graphene produced by a chemical or thermal exfoliation process [12] of graphite is relatively inexpensive but has more surface defects which prevent graphene from being used in high-speed electronic, photonic/ optoelectronic devices. But these defects play an important role in supercapacitor applications. So graphene with defects is used as an acceptable supercapacitor material. However, due to the large interlayer van der Waals attractions, re-stacking of graphene layers can severely reduce the available electrochemical surfaces, obstructing ion diffusion and ultimately limiting electrochemical efficiency, and the lack of fast Faradic pseudocapacitive behaviour have severely hampered supercapacitor performance. In order to address the issue graphene is often combined with other materials such as different metal oxides (MP) and conducting polymers (CP) to further increase its electrochemical performance. Coupling MPs or CPs to graphene has been shown to be an effective approach to improving the cycling stability, energy and power density of the supercapacitor device by introducing pseudocapacitance [13–37]. This chapter summarises recent studies on various CP/graphene composites and MO/graphene composites as supercapacitor electrode materials, with an emphasis on the two basic supercapacitor mechanisms (EDLCs and pseudocapacitors).

#### **2. Graphene-metal oxide nanocomposites for supercapacitor applications**

Metal oxide supercapacitors have gotten a lot of attention in recent years because of their high theoretical basic capacitance, low cost, environmental friendliness, and natural abundance [38–45]. Metal oxides also allow rapid, reversible faradic reactions to the electrode-electrolyte interface [46] resulting in large specific capacitances. However, the power density and cycling stability of the metal oxide based supercapacitor device are limited by poor electronic and ionic conductivity of metal oxides. So metal oxides are often combined with graphene to address these drawbacks, and it is expected that hybrid metal oxide/graphene nanostructures can increase supercapacitor performance for large-scale energy storage systems.

#### **2.1 Graphene-manganese oxide nanocomposites**

Manganese oxide-graphene composite is the most studied electrode material for supercapacitor devices among metal oxides [13–15]. The charge storage mechanism of a MnO2 electrode involves a transition in manganese (Mn) oxidation state from III to IV. The reversible insertion/extraction of electrolyte cations to balance the charge during reduction/oxidation of Mn+3/Mn+4 gives MnO2 its pseudocapacitive properties [47, 48].

#### *Graphene-Based Materials for Supercapacitor DOI: http://dx.doi.org/10.5772/intechopen.98011*

He et al. [13] used electrodeposition to build freestanding, lightweight (0.75 mg/cm<sup>2</sup> ), ultrathin (<200 μm), highly conductive (55 S/cm), and flexible three-dimensional (3D) graphene networks filled with MnO2 as the flexible supercapacitor electrode material. The composite with 9.8 mg/cm<sup>2</sup> MnO2mass loading (92.9% of the total electrode mass) had a capacitance of 1.42 F/cm<sup>2</sup> in a scan rate of 2 mV/s. He et al. further optimised the MnO2 content in the composite material for realistic applications and achieved a maximum specific capacitance of 130 F/g.

Another research [14] rendered graphene/ MnO2 composites by chemically reducing GO/ MnO2 with both hydrazine hydrate (H-RGO/MnO2) and sodium borohydride (S-RGO/ MnO2) as reducing agents. The H-RGO/MnO2 showed a specific capacitance of 327.5 F/g, which is higher than that of the S-RGO/MnO2 (278.6 F/g). Kim et al. proposed that using the hydrazine reduction process to fabricate MnO2 on graphene oxide surfaces is a promising fabrication method for supercapacitor electrodes.

For producing highly efficient graphene/metal oxide-based hybrid supercapacitors, Wang et al. [15] described in situ synthesis of 3D-graphene/MnO2 foam composite using a combination of chemical vapour deposition and hydrothermal process. High crystallinity and low contact resistance were observed during in situ conformal growth of 3D-graphene/MnO2 composites. In the supercapacitor, the 3D-graphene/MnO2 composite electrode demonstrated high specific capacitance (333.4 F/g at 0.2 A/g) and excellent cycling stability (92.2% retention at 0.2 A/g after 2000 cycles).

Thus these methods for fabricating graphene/MnO2 composites offers a promising means of producing energy storage electrode materials for supercapacitor devices with high efficiency.

#### **2.2 Graphene-iron oxide nanocomposites**

Iron oxides drew interest as a potential electrode material due to their natural abundance, high thermal stability, and low toxicity [42, 43]. However, in terms of power density and cyclic stability, iron oxide struggles as an electrode material and must be combined with graphene to overcome the problem.

Reduced graphene oxide- Fe3O4 (RGO-Fe3O4) nanocomposite was synthesised by Ghasemi and co-worker [16] using a simple electrophoretic deposition (EPD) method followed by an electrochemical reduction procedure. On RGO, Fe3O4 nanoparticles with a diameter of 20–50 nm are uniformly assembled. At a current density of 1 A/g, RGO- Fe3O4 had a specific capacitance of 154 F/g, which is greater than RGO (81 F/g) in Na2SO4 electrolyte. The electrochemical behaviours of this study also revealed that adding surfactant to aqueous Na2SO4 solution would boost the capacitance of RGO- Fe3O4 electrodes. RGO- Fe3O4 electrode in Na2SO4 electrolyte containing t-octyl phenoxy polyethoxyethanol (Triton X-100) showed capacitance of 236 F/g at 1 A/g, with 97% of the initial capacitance retained after 500 cycles.

Qu et al. [17] have shown that the increase in electrochemical capacitive performance of 2D Fe3O4 -graphene nanocomposites was mainly due to the optimization of electrochemical surfaces by avoiding graphene re-stacking due to the uniform Fe3O4 surface deposition and synergistic effect of Fe3O4 and graphene. The hybrid capacitor shows a capacitance value of 304 F/g. In addition, Fe3O4 -graphene nanocomposites have achieved higher power density.

In another work Tang et al. [18] synthesised three-dimensional (3D) iron oxide/graphene aerogel hybrid using an innovative in situ hydrothermal process for supercapacitor applications. This material Fe2O3/GA hybrid electrode were used to make a highly flexible all-solid-state symmetric supercapacitor system.

The device provided a high specific capacity of 440 F/g and was suitable for different bending angles. 90% of the capacitance was also preserved after 2200 cycles, indicating strong cycling stability. These excellent electrochemical performances suggest that graphene-iron oxide nanocomposites have huge potential in energy application.

#### **2.3 Graphene-nickel oxide nanocomposites**

Nickel oxide (NiO) has been shown to be one of the most promising electrode materials for supercapacitors [38, 39]. However, the efficacy of NiO has been found to be reduced due to its low electrical conductivity, resulting in poor performance in electrochemical devices. Researchers are trying to boost its efficiency by linking it to graphene.

A simple solvothermal-induced self-assembly method was used by Gui and co-worker [19] to make three-dimensional nickel oxide/graphene aerogel nanocomposites (NiO/GA). With an extremely large working potential window, the NiO/GA electrodes attained a specific capacitance of 587.3 F/g at 1A/g. The NiO/GA had excellent cycling reliability, with only a minor decrease in capacitance after 1000 cycles.

Zhao et al. [20] demonstrated NiO's electrochemical properties by growing NiO mesoporous nanowalls on rGO nanosheets on 3D nickel foams, referring to the process as binder-free electrode preparation. The NiO-graphene nanocomposite 3D porous foam composite substrate offers an appropriate structure for electron collection and electrolyte/ion diffusion via the active materials, resulting in a high specific capacitance of 950 F/g at a current density of 5 A/g with excellent cycling stability.

In another work Choi et al. [21] demonstrated the synthesis of 3D porous graphene/NiO nanoparticle composites (3D-RGNi) by a facile method. The prepared 3D-RGNi had a large electrochemically active surface region. The as-synthesised 3D-RGNi electrode had a large specific capacitance of 1328 F/g at 1A/g and superb rate capability, with 87% of the capacitance retained after 2000 cycles. The synergistic effects of the rGO network and NiO nanoparticles, as well as the highly porous structure of 3D-RGNi, are attributed with these high capacitance results.

Hence, it is expected that graphene-Nickel oxide nanocomposites might serve as a favourable materials for energy storage applications.

#### **2.4 Graphene-cobalt oxide nanocomposites**

Apart from having a high theoretical capacitance (3560 F/g) and being abundant in nature, cobalt-oxide based nanomaterials are also said to be environmentally friendly [49, 50]. The faradaic redox transitions of interfacial oxy-cation species trigger the pseudo capacitance of hydrous Cobalt oxides [51]. The formation of cobalt oxide phases with a transition between Co(II), Co(III), and Co(IV) oxidation states explains the charge–discharge process of cobalt oxide in alkaline electrolyte.

Akhtar et al. [22] presented the preparation of nanostructured cobalt oxide/reduced graphene oxide (Co3O4/rGO) composites for potential materials in supercapacitor applications using a basic one-step cost-effective hydrothermal technique. The v nanoparticles in the Co3O4/rGO nanocomposites were layered over the surface of the rGO sheets. In a three electrode cell system, Co3O4/rGO nanocomposite based electrode had a specific capacitance of 754 F/g and after 1000 continuous cycles, the material abled to maintained 96% of its initial capability.

Chen et al. [23] used a simple hydrothermal technique to manufacture cobalt oxide (Co3O4) nanowires on three-dimensional (3D) graphene foam. The free-standing electrode for supercapacitor application was prepared from the

synthesised 3D graphene/Co3O4 composite. It showed a high specific capacitance of 1100 F/g and excellent cycling stability at a current density of 10 A/g.

#### **3. Graphene-conducting polymers for supercapacitor applications**

Conducting polymers (CPs) attains lots of attention in academia and industry as electrode materials for supercapacitor. Polyaniline (PANI), polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) are type of conducting polymers [34, 52–56] that are extensively studied for supercapacitor device by offering very fast redox reaction with an electrolyte which can lead to pseudo-capacitance. However, CPs have a disadvantage of long-term stability due to the mechanical degradation of CPs. Thus graphene is often used to overcome the issue with CPs and guide to long term cycling stability, which is vital for supercapacitor devices.

#### **3.1 Graphene- polyaniline (PANI) composite**

PANI exhibit excellent conductivity and stability and have been widely used in energy storage devices [52, 53]. During the last few years, graphene/PANI composite nanocomposites have been used as electrode materials for supercapacitors [24–28]. Wu et al. [24] demonstrated synthesis of polyaniline nanofiber on graphene by in situ polymerisation of aniline monomer in the presence of graphene oxide under acid condition. The supercapacitor devices shows a specific capacitance of 480 F/g at a current density of 0.1 A/g and the material retained 70% of the original capacitance value after 1000 cycles. Gomes et al. [25] aimed to improve the cyclic stability by preparing hierarchical assembly of graphene/polyaniline nanostructures by microemulsion polymerisation, followed by the incorporation of graphene oxide nanosheets by hierarchical organisation. Hierarchical nanostructures showed a specific capacitance of 448F/g which is almost double of that of PANI due to the synergistic combination of graphene and PANI nanostructures. At the same time almost 81% capacity retention was achieved for the material compared to 38% for PANI after 5000 cyclic operations. Cheng et al. [26] prepared graphene–PANI composite paper as a flexible electrode, by combining the advantages of high conductivity, mechanical strength, and flexibility of graphene paper and large capacitance of the PANI. Based on these properties, this flexible graphene–PANI electrode material displayed a good tensile strength of 12.6 MPa and a stable large electrochemical capacitance of 233 F/g and 135 F/cm3 for gravimetric and volumetric capacitances, respectively.

Zhang et al. [27] reported a novel method to prepare flexible graphene/polyaniline paper (GPp) as supercapacitor electrodes through controlled in-situ polymerisation followed by roll coating in order to increase the electrochemical properties. This GPp deliver a high specific capacitance of 838 F/g at a current density of 1 A/g and high retention of 93.7% at 10 A/g over 5000 cycles. Kinetics analysis of the material shows that the GPp stores both surface capacitance and diffusion capacitance. The asprepared GPp also showed a specific energy density as high as 40 Wh/kg and a power density of 10 kW/kg. The authors also succeeded to light the light emitting diode (LED) connected with the fabricated GPp device.

Another method which is commonly used for the preparation of graphene polymer composite is physical mixing in a given solvent. Flexible Graphene/Polyaniline Nanofiber Composite films were prepared by vacuum filtration of the mixed dispersions of both rGO and PANI nanofibers [28]. The film was mechanically stable and showed a good flexibility. The supercapacitor devices showed large electrochemical capacitance of 210 F/g at a low discharge rate of 0.3 A/g.

#### **3.2 Graphene-Polypyrrole (PPy)**

PPy is emerged as attracting material for supercapacitor [54, 55] because of its simple synthesis procedure and it has good thermal and electrical conductivity. However, the thin PPy film forms aggregated a cauliflower-like structure which is not favourable for supercapacitor applications. In order to address the issue PPy is often combined with graphene to improve its electrochemical performance. Zhang et al. [29] synthesised graphene and polypyrrole composite via in situ polymerisation of pyrrole monomer in the presence of graphene under acid conditions An even composite is formed with polypyrrole being uniformly bounded by graphene sheets. Electrochemical performance of the composite material are higher than pure samples with the maximum capacitance of 482 F/g and excellent cycling performance (95% retention after 1000 cycles). Liu et al. [30] demonstrated preparation of hierarchical graphene/polypyrrole nanocomposites via in-situ polymerisation of self-assembled pyrrole. The nanomaterial attained outstanding conductivity of ∼1980 S/cm and demonstrated promising potential in supercapacitor, with a specific capacitance value 650 F/g at 0.45 A/g current density. Furthermore, the device showed energy density of 54.0 W h/kg at 1 mA current, and power density of 778.1 W/kg at 5 mA current. In another work Akhtar et al. [31] studied the electrochemical performance of graphene/polypyrrole layered type structure. Charge transport was investigated in this study to determine the relative contributions of graphene and polypyrrole in charge transport and storage mechanism, with the aim of improving device properties. Electrochemical supercapacitor fabricated using this layered composite exhibited a large value (~931 F/g) of specific capacitance.

#### **3.3 Graphene/(PEDOT:PSS) composite**

PEDOT: PSS attract as one of the potential electrode materials due to its good electrical conductivity, transparency, ductility, and stability [34, 56]. Wu et al. [32] demonstrated ultrathin printable graphene supercapacitors based on solutionprocessed electrochemically exfoliated graphene hybrid films on an ultrathin poly(ethylene terephthalate) substrate. The device exhibited an unprecedented volumetric capacitance of 348 F/cm3 at an ultrahigh scan rate of 2000 V/s, and AC line-filtering performance. This method can be possibly used for large-scale production of printable, thin and lightweight supercapacitor devices.

Fibre-shaped supercapacitors [33] with high mechanical and electronic properties based on hollow rGO/PEDOT: PSS (HCF) have gained tremendous attention because of their tiny volume, low weight, high flexibility, and good wearability. This novel fibre-shaped supercapacitor showed a high specific capacitance of 304.5 mF/cm2 at 0.08 mA/cm2 and an energy density of 27.1 mWh/cm2. In another work highly flexible, bendable and conductive rGO-PEDOT/PSS films were prepared by Chen et al. [34] The assembled device could be bent and twisted without harming the electrochemical performance of the device. A high areal capacitance of 448mF/cm2 was achieved at a scan rate of 10mV/s and when the device was fully charged the device was powerful enough to power a LED for 20seconds.

#### **3.4 Graphene/other polymers composite**

Besides the above-mentioned conducting polymers there are others polymers that have been combined with graphene for supercapacitor devices. Gupta and co-workers [35] synthesised Poly (3-hexylthiophene)/graphene composites via both in-situ and ex-situ growth technique to investigate supercapacitive behaviour. They observed that in-situ growth of P3HT forms better composites with graphene than

*Graphene-Based Materials for Supercapacitor DOI: http://dx.doi.org/10.5772/intechopen.98011*

ex-situ growth. The values of specific capacitance for ex-situ and in-situ samples were found to be 244 F/g and 323 F/g respectively at a current density of 200 mA/g. Thus in-situ P3HT/graphene composite showed superior storage capacity in comparision to ex-situ sample. Electrochemical performance was also studied for graphene (G)–polyethylenedioxythiophene (PEDOT) nanocomposites as electrode material [36]. This manuscript presented the capacitance studies on supercapacitor G-PEDOT electrode with respect to stability of material, specific capacitance and electrical conductivity. Specific capacitance value for G-PEDOT sample was estimated to be 374 F/g. Wu et al. [37] reported conjugated polyfluorene imidazolium ionic liquids (coPIL) intercalated reduced graphene oxide (coPIL-RGO) for high performance supercapacitor. coPIL-RGO based device showed a specific capacitance of 222 F/g at a low current density of 0.2 A/g in 6 M KOH and 132 F/g at a current density of 0.5 A/g in ionic liquid electrolyte 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4), respectively.

#### **4. Conclusion**

Metal oxide and Conducting polymers have gotten a lot of attention in nextgeneration supercapacitor electrode research because of their simple synthesis procedure, low cost and high pseudocapacitance. Simultaneously, pure metal oxide and conducting polymers have a number of flaws, including low electrical conductivity, weak cyclic stability, and low energy and power density. Graphene/conducting polymer composites and graphene/metal oxide composites outperform conducting polymers and metal oxides in terms of cyclic durability, energy density, and power density. Simultaneously, in recent years, considerable focus has been placed on structural architecture, material fabrication, and device performance evaluation. To accomplish the expected full-scale realistic application, both the efficiency and reproducible quantity of the electrode materials must be improved in the immediate future.

#### **Author details**

Abu Jahid Akhtar Department of Physics, Diamond Harbour Women's University, West Bengal, India

\*Address all correspondence to: jahid.dhwu@gmail.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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[47] Kuo S-L, Wu N-L. Investigation of pseudocapacitive charge-storage reaction of MnO2∙ nH2O supercapacitors in aqueous electrolytes. Journal of The Electrochemical Society. 2006; 153: A1317.

*Graphene-Based Materials for Supercapacitor DOI: http://dx.doi.org/10.5772/intechopen.98011*

[48] Kozawa A, Powers R. The Manganese Dioxide Electrode in Alkaline Electrolyte; The Electron-Proton Mechanism for the Discharge Process from MnO2 to MnO1. 5. Journal of The Electrochemical Society. 1966; 113: 870.

[49] Lokhande V, Lokhande A, Lokhande C, Kim JH, Ji T. Super capacitive composite metal oxide electrodes formed with carbon, metal oxides and conducting polymers. J Journal of Alloys Compounds. 2016; 682: 381-403.

[50] Liao Q, Li N, Jin S, Yang G, Wang C. All-solid-state symmetric supercapacitor based on Co3O4 nanoparticles on vertically aligned graphene. ACS nano. 2015; 9: 5310-5317.

[51] Wang X-f, Ruan D-b, You Z. Pseudocapacitive behavior of cobalt hydroxide/ carbon nanotubes composite prepared by cathodic deposition. Chinese journal of chemical physics. 2006; 19: 499.

[52] Ryu KS, Kim KM, Park N-G, Park YJ, Chang SH. Symmetric redox supercapacitor with conducting polyaniline electrodes. Journal of Power Sources. 2002; 103: 305-309.

[53] Gupta V, Miura N. High performance electrochemical supercapacitor from electrochemically synthesized nanostructured polyaniline. Materials Letters. 2006; 60: 1466-1469.

[54] Huang Y, Li H, Wang Z, Zhu M, Pei Z, Xue Q, Huang Y, Zhi C. Nanostructured polypyrrole as a flexible electrode material of supercapacitor. Nano Energy. 2016; 22: 422-438.

[55] Sharma R, Rastogi A, Desu S. Pulse polymerized polypyrrole electrodes for high energy density electrochemical supercapacitor. Electrochemistry Communications. 2008; 10: 268-272.

[56] Manjakkal L, Pullanchiyodan A, Yogeswaran N, Hosseini ES, Dahiya R. A wearable supercapacitor based on conductive PEDOT: PSS-coated cloth and a sweat electrolyte. Advanced Materials. 2020; 32: 1907254.

#### **Chapter 5**

## Graphene Functionalization towards Developing Superior Supercapacitors Performance

*Abd Elhamid M. Abd Elhamid, Heba Shawkey, Ahmed A.I. Khalil and Iftitan M. Azzouz*

#### **Abstract**

Graphene is known as the miracle material of the 21st century for the wide band of participating applications and epic properties. Unlike the CVD monolayer graphene, Reduced graphene oxide (RGO) is a commercial form with mass production accessibility via numerous numbers of methods in preparation and reduction terms. Such RGO form showed exceptional combability in supercapacitors (SCs) where RGO is participated to promote flexibility, lifetime and performance. The chapter will illustrate 4 critical milestones of using graphene derivatives for achieving SC's superior performance. The first is using oxidized graphene (GO) blind with polymer for super dielectric spacer. The other three types are dealing with electrolytic SCs based on RGO. Polyaniline (PANI) was grown on GO for exceptionally stable SCs of 100% retention. Silver decoration of RGO was used for all-solid-state printable device. The solid-state gel electrolyte was developed by adding GO to promote current rating. Finally, laser reduced graphene is presented as a one-step and versatile technique for micropatterning processing. The RGO reduction was demonstrated from a laser GO interaction perspective according to two selected key parameters; wavelength and pulse duration.

**Keywords:** Polyaniline/RGO, Solid-state, laser-induced graphene, super dielectric spacer, flexible supercapacitor

#### **1. Introduction**

Supercapacitors (SCs) are a key block elements in our energy storage perspective that could stand alone or be combined with various types of batteries [1]. Unlike batteries, SCs possess unique features of high power and millions of cycling [2]. Yet, SCs energy density could not match the nowadays batteries [3], the development is focused on extending energy density and engineering flexible devices [2]. The storage mechanism is conducted to the SCs type as following; electric double-layer (EDL), pseudocapacitors and hybrid structure [4]. EDL is the simplest form of storing energy electrostatically due to electrolyte ions, whereas, pseudocapacitors is based on reversible redox reactions through active material's surface resulting in more than 10 times capacitance value than EDL. The hybrid type is combining faradic redox and non-faradic EDL reactions that showing near battery like performance.

Graphene is one of the most discussed electrodes material in energy storage due to outstanding electrical, mechanical and electrochemical performance [4]. Graphene functionalization in SCs is expected to lead the next SCs's generation processing as a result of the following; (i) high surface area of 2630 m<sup>2</sup> /g correlated with low theoretical density of 2.28 g/cm3 , (ii) high carrier mobility and electrical conductivity that promote using of graphene as a compact active material/current collector, and (iii) Young's modulus of 1 TPa indicating excellent mechanical strength enabling perfect impeding in flexible and wearable electronics. The easyto-get graphene form known as reduced graphene oxide (RGO) that is obtained from the reduction of highly oxidized exfoliated graphene oxide (GO). Crude RGO active material suffers from low specific capacitance and restacking over time that minimizes accessible surface area [5]. Therefore, many attempts of adding another material such as conducting polymers, metal oxides and metal NPS were reported [3, 6].

Despite a large number of graphene-based SCs reports, this chapter is focusing on selected milestones on using graphene in SCs according to extensive research work as well as others' reports. Four main points considered graphene derivatives, the first is using graphene oxide/polymer blind as a super dielectric spacer for double layer AC supercapacitor. The other technique is using RGO as efficient nucleation sites for polyaniline (PANI). After that, silver metal NPs decoration/ RGO was used to fabricate flexible all-solid-state SCs of high power. Finally, laser-induced graphene is a one-step technique to obtain miniaturized 3D RGO based SCs.

#### **2. Dielectric supercapacitor of pseudo 2D GO**

Materials of high dielectric constant are directed for large capacitance circuit element, which enables minimizing dimensions in integrated circuits and basic storage elements, etc. [7–10]. Despite well-known ceramic materials such as Barium titanate-based composites, polymers are low cost and scalable but hold significantly low dielectric response. Thus, nano-composites doped inside a polymer matrix is a promising candidate for promoting dielectric characteristics for wearable and flexible electronics [5]. Among enormous compositions types, RGO and GO are blinded in polymer to expand the dielectric response via two different technique, the first is by forming multiple micro-capacitors inside the spacer while the other is using the oxide function group strong polarization [11]. The present section is revealing the potential of using graphene-based polymer composite potential as a super dielectric spacer for RF SC applications [12].

#### **2.1 Double plate SC of PVA/GO spacer fabrication process**

GO suspension of 1 gram in 0.1 L DI water and 8% PVA were used for preparing the mixture. The weight ratio blind was as following: 10%, 20% and 50% of GO to PVA using facile colloidal mixing method at 70°C. The resulted paste was coated on cleaned Al foils strips 0.08 × 40x50 mm3 to form the compacted spacer. After mild evaporation, the top Al foil was attached to the three different ratios GO/PVA. The double-layer capacitor's spacer thickness was adjected to 475 μm after multiple drying processing using a hot press. The fabricated SCs electric characterization was conducted using LRC meter and multi-channels Potentiostat/Galvanostat. **Figure 1** illustrates a schematic for the whole study steps [12].

*Graphene Functionalization towards Developing Superior Supercapacitors Performance DOI: http://dx.doi.org/10.5772/intechopen.98354*

**Figure 1.** *Schematic of the GO/PVA and GO dielectric spacer study.*

#### **2.2 The mega dielectric value of double**

The three different double plate capacitors of (10%, 20% and 50%) GO/PVA weight ratio are measured from 20 Hz to 1 MHz, the associated dielectric constants were estimated by knowing area and thickness of the spacers. The whole values are located in the range of mega value (106 ) as showed in **Figure 2(a)**. The dielectric decay over frequency is expected due to weak dipoles response. At low oscillating frequency, the corresponding oxide functionalized groups have the necessary time for alignment correlated with the applied electric field, which will promote the dielectric constant value due to strong polarization. Whereas, the increase in oscillating frequency will eliminate contributed ionic and space charge as well as cause a systematic drop in the dielectric response [13]. Thus, the GO filling inside the PVA polymer matrix achieved high dielectric SC caused by Maxwell- Wagner-Sillar theory [11]. The phase angle in **Figure 2(b)** is an indication of the leakage current through GO/PVA spacers. The higher phase angle than −90 degree resulted from free and bounded charges within the GO/PVA interface. Accordingly, SC's quality factor is linked to the measured phase angle. While the 10% gave the best spacer performance due to low water contents and residuals ions, the 50% ratio showed the lowest dielectric constant as well as quality factor. Low phase angle could be attributed to conductive graphitic defects generated during GO preparation.

The imaginary part (ε՛՛) represents electric field dissipation into heat. The obtained losses could be attributed to GO lattice defects, conductive defects, water contents and ions residuals [13–15]. **Figure 2(c)** present the complex dielectric susceptibility curve (Cole-Cole), where ε is the complex permittivity, ε՛ real value and ε՛՛ represent the imaginary part. Several GO oxide functional groups, defects, polymeric chain/GO interface and water molecules will result direct complex time response confirmed by Cole-Cole complex shapes. The 20% GO/PVA based SC cyclic voltammetry is presented in **Figure 2(d)**, which matched the previously measured mega dielectric constant value and performance. Those results indicate balanced filler loading within the polymer matrix of such 20% GO weight value [16]. Finally, the following **Table 1** reports the high dielectric value of using either crude GO or as a polymer matrix filler.

#### **Figure 2.**

*The three GO/PVA ratio (a) dielectric constants, (b) phase angle, (c) Cole-Cole plot. And (d) the 20% GO/ PVA ratio CV at different scan rates. Reproduced with permission from Ref. [12]. Copyright 2021, Elsevier Ltd.*


#### **Table 1.**

*Dielectric constant of GO-based spacers. Reproduced with permission from Ref. [12]. Copyright 2021, Elsevier Ltd.*

#### **3. Conductive polymer graphene composite**

#### **3.1 Superior stable PANI reinforced RGO SC**

An effective technique to not only prevent RGO restacking but also extend the electrochemical performance is surface composition via polymeric material [21]. PANI, polythiophene and polypyrrole are the most studied conductive

#### *Graphene Functionalization towards Developing Superior Supercapacitors Performance DOI: http://dx.doi.org/10.5772/intechopen.98354*

polymers that participated in various application like SC electrodes, sensors, etc. [22]. Among those, PANI is preferable in SC applications due to low cost, controlled conductivity, easy processing and high electrochemical performance [23]. Unfortunately, poor stability is the main drawback. Hence, covalent grafting of PANI with a graphitic based material could promote lifetime, porosity and conductivity [24]. Oxidized graphene function groups are excellent nucleation sites for efficient covalent bonded PANI polymerization. Another advantage of using RGO/ PANI composite in SC application is enabling the EDL behavior as well as pseudocapacitance [25].

This section discusses PANI/RGO synthesis via two steps (i) *in situ* polymerization of distilled aniline on GO surface via initially adsorbed Fe2+ and (ii) reduction of the composite using hydrazine hydrate. The fabricated symmetric SCs was tested using four different electrolytes namely; sulfuric Acid, phosphoric acid. Potassium hydroxide and sodium sulfate to study the performance over a wide range of transported ions [26].

#### **3.2 Preparation of PANI-RGO SC**

2.25 g of Fe2SO4 was dissolved in 0.05 L DI water and was drop wised to 0.475 L of GO suspension (10 g/L) while stirred for 2 h. Aniline monomer was double distilled under vacuum for fresh using, and dissolved in 1 M (0.25 L HCl). The Fe2SO4 was rabidly added to GO suspension followed by adding Ammonium peroxydisulfate (APS) (4.5 g in 0.25 L 1 M of HCl and was kept stirring overnight at ambient conditions. After collecting and drying, the mixture was dissolved in 1 L DI water followed by sonication for reduction step. Hydrazine hydrate 1:1 to GO weight ratio was added in a boiling water bath. The electrodes PANI/RGO active material was conducted to another polymerization step before collected and drying in vacuum overnight. The paste was prepared by adding 8% PVDF to 92% PANI/RGO then coated 304 Stainless steel foil coated by sputtered 500 nm Pt current collector (sheet resistance is 2 Ω/□). **Figure 3** illustrates SC fabrication and characterization steps schematic [26].

**Figure 3.** *Schematic of the PANI/RGO based SCs fabrication and measurements.*

#### **3.3 Plausible growth mechanism of PANI on RGO**

The analysis of PANI polymerization, as well as GO reduction, was confirmed via standard characterization like XRD, Raman & EDX. However, the microscopic imaging is defining a more detailed view of folded RGO and PANI structure. SEM images are presented in **Figure 4(a)** and **(b)** that are showing the high load of PANI completely cover the RGO flakes in thick wood like shape and PANI/RGO flakes, respectively. TEM images illustrate RGO flake folded within dark region **Figure 4(c)**. Whereas, higher magnification TEM image present PANI growth on the RGO surface in tiny islands, which could be attributed to Fe2+ sites and GOs' function groups.

Dissociated FeSO4 ions will be functionalized on the dispersed GO flakes. The distilled aniline monomer started to adsorbed on the GO surface [26]. By adding the APS/HCl, Fe (II) will be oxidized to Fe (III) and forming the major oxidation centers and the seed for PANI chains. The applied 1:1 weight ratio between aniline and GO will produce a high load of PANI correlated of high polymerization degree [27]. Fe (II)/APS is promoting effective and rapid polymerization due to direct bonding of the Fe (II) on pseudo-2D GO surface [27]. Thus, APS will react with Fe (II) ions not aniline due to low oxidation potential and produce sulfate radical anions (Eq. (1)) [28]:

$$\text{Fe}^{2+} + \text{S}\_{\text{z}}\text{O}\_{\text{s}}^{\text{z}-} \rightarrow \text{Fe}^{3+} + \text{SO}\_{\text{4}}^{\text{z}-} + \text{SO}\_{\text{4}}^{-\text{.}} \tag{1}$$

#### **Figure 4.**

*PANI-RGO at different magnification scale; (a) & (b) SEM images. (c) & (d) TEM images. Reproduced with permission from Ref. [26]. Copyright 2021, Elsevier Ltd.*

*Graphene Functionalization towards Developing Superior Supercapacitors Performance DOI: http://dx.doi.org/10.5772/intechopen.98354*

#### **3.4 Exceptionally stable PANI/RGO SC**

The PANI/RGO based SCs were tested in various electrolytes to study the performance of the composite in strong alkaline, strong acid, weak acid and natural medium. Being a time-domain process, galvanostatic charge/discharge test is a

#### **Figure 5.**

*Different electrolytes, (a) Galvanostatic charge/discharge curves cycle stabilities of different electrolytes. (b) Retention test for 460 cycles at 1A and (c) selected retention for 5000 cycles at 3A. Reproduced with permission from Ref. [26]. Copyright 2021, Elsevier Ltd.*

versatile technique of defining specific capacitance, power, energy and lifetime. **Figure 5(a)** presents different electrolyte-based SC at different current. The estimated specific capacitance (0.1A) using H2SO4, H3PO4, Na2SO4 and KOH were 288, 126, 84 and 480 F/g, respectively. The capacitance of the composite could be expanded by using a highly conducting current collector, hot pressing of the active material, using standard cell configuration and a proper separator.

The remarkable result of the prepared PANI/RGO composite is the superior stability in various electrolytes as PANI is degradable at high temperature, high current and charging/discharging process. The first applied retention test at 1A was for nearly 500 cycles **Figure 5(b)**.

The pure EDL performance in alkaline KOH and natural Na2SO4 electrolytes present zero decay, whereas, sulfuric and phosphoric show ~75% retention due to aniline doping/degradation during charging. The 5000 cycles (**Figure 5(c)**) are considered to be relatively long cycling for a conducting polymer. The highest obtained capacitive results using H2SO4 and KOH were conducted to a 3A/g of current value for more aggressive cycling. Remarkably, KOH based SC maintained the 100% value correlated with the highest achieved capacitance. On the other hand, the sulfuric doping of PANI leads to an increase in the retention value behind 100% [29].

**Table 2** presents the obtained results of PANI/RGO concerning others' reports comparatively to clear the role of RGO in enhancing the PANI super capacitive performance. It's worth mentioning that, 3-electrode setup multiplies the actual


#### **Table 2.**

*The PANI-graphene (G) based SCs report. Reproduced with permission from Ref. [26]. Copyright 2021, Elsevier Ltd.*

#### *Graphene Functionalization towards Developing Superior Supercapacitors Performance DOI: http://dx.doi.org/10.5772/intechopen.98354*

applied voltage that causes a massive increase in measured capacitance of about triple times when compared to the 2-electrode [38]. However, the 2-electrode cell configuration is preferable for practical SCs analysis. The electrochemical stability could be attributed to Fe(II) surface adsorbed on the GO and assisted APS strong locating oxidation.

#### **4. Metal/RGO for solid-state and flexible structure**

#### **4.1 Printed devices based on doped graphene**

Modern applications of wearable and flexible electronics require a convenient storage device to meet the rapid demands of energy consumption of devices such as ITO sensors and implantable bio-devices [39]. Long lifetime, high power profile, easy manufacturing and eco-friendly structure make the SCs is the best up-to-date candidate for powering such devices [40]. Nevertheless, solid and flexible SCs possess much lower power and energy densities than liquid-based devices that limit the practical coupling with systems. Accordingly, intense research work is applied for promoting the corresponding electrochemical characteristics. The electrodes active materials are already solid but the liquid electrolytes need sophisticated packaging that prevents planer designing. Ionic liquid salts, solid and electrolyte intercalated polymer are different types of the used solid electrolyte. In particular, gel polymer is dissociated ions from acid or base blinded in a polymer matrix. Solid-state gel polymer electrolytes present relatively good ionic conductivity, low cost, high stability, simple principle, reliable, environmentally friendly and safe to handle [41].

This part is using graphene to fabricated flexible, half printed and solid-state SC, the Ag decorated the RGO flakes to prevent restacking. The other functionalization of graphene oxide was for developing the gel polymer to promote high current and scan rate performance [42]. Despite the weak acidic nature of phosphoric acid, it was used as the main ions source inside the GP due to compatibility with RGO and stability over time other than strong acids like sulfuric.

#### **4.2 SCs fabrication procedures**

GO suspension was the start material (g/200 mL). Silver nitrate (1 g/50 mL) was added with two different volume ratios 4 ml and 16 ml to a 100 mL of GO suspension, respectively. The two ratios mixture were conducted to Hydrazine hydrate as a strong reduction agent and will be known as sample 1 (4 ml AgNO3) while sample 2 (16 ml AgNO3). The developed gel polymer was performed by dissolving 10% PVA at first and adding an equal weight of phosphoric acid. Finally, GO and PANI of weight 0.03 and 0.01 g were added to the GP, respectively.

A handmade plastic mask was used to make Ag/RGO electrode of dimensions 1x4 cm2 (0.1 g) using doctor blade. After drying, the GP was coated followed by the second electrode. Lastly, the two devices were peeled off and a metal contact was sputtered on both sides. The schematic in **Figure 6**. illustrates devices fabrication procedures and applied electrochemical measurements [42].

#### **4.3 The remarkable obtained printed SCs performance**

TEM image in **Figure 7**. illustrates GO, RGO and Ag-doped RGO, respectively. The folded GO single layer is a clear sheet and the reduction of RGO make defects

#### **Figure 6.**

*Schematic of the Ag/RGO flexible SCs fabrication and measurements.*

and multilayer represented by the dark regions. However, the silver nanoparticles incorporated RGO could be observed in the dark spots, the RGO do not present the dark defects and multilayered restacked shape due to the intercalated Ag particles. Thus, the as-prepared active material of Ag/RGO will show higher surface area, stability and conductivity. **Figure 7(d)** presents a captured image of the multilayers printed symmetric SC on the support plastic film. And **Figure 7(e)** represents SC after peeling-off and current collectors metalization. Silver was used for metalization to reduce mismatch and reduce sheet resistance. The fabricated SCs were stretchable and flexible.

The Ag/RGO SCs was tested at a relatively high current value of 2A/g for a solidstate device by applying galvanostatic cycling. **Figure 8** shows the first three cycles where the time scale of sample 2 is nearly four-fold than sample 1 based SC. Sample 1 is located within the millisecond range, which confirms Ag doping ratio impact on such solid-state SC capacitive value. **Table 3** incorporates the estimated specific capacitance results of sample 1 and 2 via measured cycling voltammetry which is in good agreement with galvanostatic cycling results.

The electrochemical impedance spectroscopy is playing a key role in defining the solid electrolyte performance, the measurement was performed at a wide range from 0.1 Hz to 2 MHz **Figure 8(b)**. Nyquist plot and equivalent circuit measurements were obtained using VSP300 EC-Lab., the matched equivalent circuit is [R1 + C1/(R2 + Q2/(R3 + W3))], Q2 a constant phase element as a timeconstant distribution function. R1 charge transfer resistance within electrodes. The low resistance region slop behavior is due to phosphoric weak acid natural [43]. Electrode/electrolyte interface is some sort of a junction represented by a small signal response equivalent circuit. The resistance originated from the charge transfer across such interfacial potential barrier besides electrolyte internal ohmic resistance. At extremely low frequency, the charge transfer resistance is polarization resistance. Whereas, the high frequency will reduce the resistance till reaching R1 value. R2 and R3 are the resistance of (silver current collector+ Ag/RGO active material+ metal/active material Interface contact). They are not equal as the peeling off cause roughness of one surface over the other that modified contact points and

*Graphene Functionalization towards Developing Superior Supercapacitors Performance DOI: http://dx.doi.org/10.5772/intechopen.98354*

#### **Figure 7.**

*TEM images of GO, RGO and Ag/RGO. Digital images of the flexible SCs. (d) on a plastic substrate. (e) after peeling and silver coating. (f) Final SC structure. Reproduced with permission from Ref. [42]. Copyright 2021, IEEE.*

induced structural defects. During SC charging/discharging, Warburg impedance (W3) represnts semi-infinite one direction liner diffusion of electrolytic ions. The C1 is the magnitude of the two electrodes capacitance. The equivalent series resistance of sample 1 and sample 2 were 2.9 and 0.5317 ohms, respectively. Despite the obtained relatively small ESR for a solid-state SC, it could be further enhanced by the following; (i) using a high mobility ions based electrolyte, (ii) using a strong electrolyte, (iii) full printed structure using metallic ink as plasma deposition induce defects and (iv) applying layer by layer systematic hot compression and vaccum drying.

#### **Figure 8.**

*(a) Charging/discharging test at 2A/g. (b) EIS test. Reproduced with permission from Ref. [42]. Copyright 2021, IEEE.*


#### **Table 3.**

*Estimated specific capacitance at different scan rates. Reproduced with permission from Ref. [42]. Copyright 2021, IEEE.*

*Graphene Functionalization towards Developing Superior Supercapacitors Performance DOI: http://dx.doi.org/10.5772/intechopen.98354*

## **5. Laser-assisted RGO SC fabrication**

#### **5.1 3D RGO SC with laser irradiation**

Conventional SCs are obeying two main configurations, the first is parallel plates and in-between a dielectric separator, the other is three parallel plates and in-between two dielectric separators to form two parallel-connected identical SCs. Those two designs are using a 3D structure and not convenient for planer, compact and miniaturized applications [44]. The developed IDE designs into interdigitated pattering of controlled dimensions will show diverse features over the conventional structure such as [45]; (i) fabrication on-plane macro/micro/nano in-plan SC, (ii) fast sidelong ions transportation, (iii) terminate separator usage and (iv) high power/energy due to confinement of electric field. For creating such fingers patterns SC, lithography techniques are used as it's a matter of certain dimensions etching processing. Photolithography, screen printing, laser dry etching and lift-off processing are used to fabricate complicated SCs designs [46]. However, most of SCs active material will

**Figure 9.**

*(a) LDWT setup for SC patterning process. (b) and (c) microscopic images of ongoing using a nanosecond laser in micro SCs processing.*

be affected by exposure to chemicals and multi-processing steps which will not match the easy-to-fabricated principle. Thus, laser processing can apply etching as well as controlled structure modification via laser material interaction.

GO is perfect a starting material for laser processing. Dispersion could be functionalized with various nanoparticles and deposited directly on substrates via liquid-phase processing [47]. The concept of using RGO as a commercial source of graphene especially in electronics applications is to reduce the oxygen groups to a highly conductive 2D layered structure. The previously mentioned flexible and PANI/RGO SC are conducted to chemical reduction techniques. However, there are various reduction techniques such as; thermal annealing, microwave flash reduction, laser-assisted processing and high-density UV reduction that affect the resulted RGO characteristics [48]. One of the effective techniques to reduce the dielectric GO into 3D network RGO for SC applications is laser direct writing


#### **Table 4.** *Different laser type for SCs fabrication process.*

*Graphene Functionalization towards Developing Superior Supercapacitors Performance DOI: http://dx.doi.org/10.5772/intechopen.98354*

technique (LDWT) that enables fast, low cost, easy-processing and maskless patterning [46]. Laser processing of SCs is accessible for most of available rigid and flexible substrates.

#### **5.2 LDWT of RGO based miniaturized SC**

**Figure 9** presents the LDWT processing main parameters that affect the resulted SC performance. Many reports discuss the corresponding laser parameters, interactions and resulted RGO characteristics [49, 50]. During laser processing, GO reduction is governed by photothermal oxide group removal and photochemical bond breaking. Laser wavelength and pulse duration are the two main key parameters of SC processing. The laser wavelength controls the major photoreduction process of being photochemical (≤ 400 nm) or photothermal (≥ 400 nm).

photochemical process is directly conducted to photon energy that can break oxygen functional groups. The photothermal process is focused on near-infrared region and infrared bands. The deposited thermal energy on GO is maximized upon increasing the beam density to induce local heating within a certain heataffected zone. This sort of heat can break oxygen radicals as well. In addition, long wavelengths of fast/ultrafast pulse duration could apply photochemical reduction through nonlinear processes and multiphoton absorption [51]. **Table 4** is concluding the GO reduction process using most common laser sources [52–54].

#### **6. Conclusions**

Finally, graphene not only proved effective participation in different types of SCs but showing superior electrochemical and electromagnetic performance. PVA/GO composite could work as an efficient super dielectric spacer of mega value 106 thanks to the intercalated water dipoles and oxide functional groups. Maxwell- Wagner- Sillar effect expanded the PVA blind dielectric response. By using Fe(II) as a growth mediated oxidizer for PANI growth on GO surface, 100% retention at high current after 5000 cycles at 3A/g was obtained. The PANI/RGO showed EDL performance for H3PO4, Na2SO4 and KOH. For a flexible and printed SC fabrication, silver doped RGO has 29.5 Wh/Kg energy at 2A. The high current rating was promoted via GO impeding inside the electrolytic solid polymer matrix. The miniaturization of one-step fabricated SC is using laser-assisted reduction technique. The CO2 is a cost-effective direct heating source that induces thermal reduction, whereas, a femtosecond laser is using cold processing that reduces the oxide groups but with a low degree of graphenezation. Nanosecond laser sources combine thermal and photochemical reduction through the GO film.

#### **Conflict of interest**

We declare no conflict of interest.

*Supercapacitors for the Next Generation*

#### **Author details**

Abd Elhamid M. Abd Elhamid1,2\*, Heba Shawkey3 , Ahmed A.I. Khalil1 and Iftitan M. Azzouz1

1 Laser Sciences and Interactions Department, National Institute of Laser Enhanced Sciences (NILES), Cairo University, Giza, Egypt

2 Nanotechnology Laboratory, Electronics Research Institute, El Nozha, Egypt

3 Microelectronics Department, Electronics Research Institute, El Nozha, Egypt

\*Address all correspondence to: a.m.abdelhamid@eri.sci.eg

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Graphene Functionalization towards Developing Superior Supercapacitors Performance DOI: http://dx.doi.org/10.5772/intechopen.98354*

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Section 4
