ELECTROCHEMICAL CAPACITORS (ECS)

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Chapter 3 Synthesis and characterization techniques

This chapter consists of a brief description of the synthesis and characterization techniques exploited in this study, as well as accounting for the synthesis of all electrode materials utilized in this study and details of the parameters used for characterization techniques.

Synthesis techniques

Chemical vapor deposition technique (CVD)

CVD is a fascinating growth route which was established in the 1960s and 1970s for the preparation of carbon fibers and carbon nanofibers [224,225]. In the CVD method, substrates are inserted at the middle of the reactor chamber under a continuous flow of a mixture of gaseous precursors at a specific temperature. The CVD technique is an economical, reliable and extremely efficacious synthesis technique that is highly suited for the production of good quality, large-scale and uniform layer deposition materials [226]. In this work, the CVD method is mainly utilized for the production of graphene foam (GF) and it contains a reactor chamber (a 2-inch diameter quartz tube) connected through flow meters to various gas sources and enfolded in an electric furnace as shown in Figure 3.1. The gases utilized for the production of GF in this work are of good grade, which includes argon (Ar, grade 5 – 99.999%), hydrogen (H2, grade 5 – 99.999%) and methane (CH4, grade 4.5 – 99.995%).
The common substrates utilized for the production of CVD graphene are catalytic Ni and Cu transition metals [224,227,228] and CH4 is the carbon source, while H2 and Ar are the carrier gases utilized to improve the surface reaction and reaction rate, thereby enhancing the deposition of graphene on the substrate. Chen et al. reported a 3D structure of graphene termed graphene foam (GF) [229] by direct deposition of the carbon onto a nickel foam (NF) substrate using the CVD technique, which was followed by quenching once the deposition was completed. Several quenching rates such as fast, medium and slow determine the thickness and quality of the GF prepared because of the non-equilibrium characteristics of the precipitation mechanism as depicted in Figure 3.2 [230,231]. Medium quenching rate provides uniform carbon segregation and yields few layer graphene, whereas, slow and fast quenching rates produces graphene with patches when NF is utilized as a substrate [232]. In addition to the quenching rate, the microstructure of the Ni substrate is highly essential in determining the morphology of the graphene produced [232].

Synthesis of GF

Atmospheric pressure chemical vapor deposition (AP-CVD) growth system was employed to synthesize graphene foam (GF) on a nickel foam (NF) template (from Alantum, Munich, Germany with an areal density of 420 g m^-2 and 1.6 mm thickness). The synthesis was reported in our previous work [129]. Briefly, the NF was loaded at the center of a quartz reactor which was heated up to 1000 °C under Ar:H2 gas ratio of 300:200 sccm. Once the desired temperature is attained, i.e., 1000 °C, it is kept constant for 60 min in order to remove the oxide layers and impurities on the NF before introduction of the carbon source (CH4 gas) at the same temperature. 10 sccm of CH4 (carbon source) was added to the previous gas ratio for 5 min. The quartz reactor chamber was then pushed immediately to a lower temperature region to help rapidly cool the sample, which consists of graphene deposited on NF. Finally, GF was obtained by etching away the nickel template in a 3.0 M HCl solution at 80 °C, for several hours. The GF obtained generally float on the solution, meaning that there is a very low concentration of Ni in the GF, if any. Subsequently, the floating GF was continuously washed with deionized water and finally dried at 60 °C in an electrical oven.

Hummer’s method

Hummer’s method was established in 1958 [233], it is a reliable chemical process commonly utilized to produce a reasonable amount of graphite oxide by engineers and scientists. It is more efficient, and faster way of preparing graphite oxide than other methods (e.g. Staudenmeier–Hoffman–Hamdi method [234], etc.) formally used for the production of graphite oxide. This method involves addition of potassium permanganate to a solution containing graphite, sodium nitrate, and sulfuric acid. Recently, Hummer’s method was found to be modified to make an atom-thick version of a substance called graphene oxide (GO).

Synthesis of GO

GO was prepared by the modification of the established Hummer’s method from graphite powder. Briefly, 5.0 g of graphite powder was poured into a round bottom flask containing 15 mL of concentrated H2SO4, followed by stirring and the addition of fuming HNO3 and continuous stirring of the mixture at room temperature for 24 h. Subsequently, the mixture was centrifuged four times, washed with deionized water and dried in an electric oven for 24 h at 60 °C to obtain a graphite intercalation compound powder (GICP). Thermal expansion of the GICP to give expanded graphite (EG) was achieved after heating the GICP at 1050 °C for 15 s. Finally, 1.0 g of EG (a precursor for GO) was dissolved in a 200 mL of concentrated H2SO4 in a 500 mL three-necked flask, followed by slow addition of 10 g KMnO4 and the mixture was transferred into an ice bath, followed by slow addition of 200 mL of deionized water and 50 mL H2O2. The mixture was then stirred for 30 min, resulting in a light brown suspension of GO. The obtained sample of GO was washed with aqueous HCl in a ratio 9:1 of water:HCl, followed by continuous centrifugation and washing with deionized water until a pH of about 7 was achieved, then dried at 80 °C for 12 h [235,236].

Hydrothermal and solvothermal techniques

The hydrothermal technique was first reported by a German geologist Karl Emil von Schafhäutl in 1845 [237] and is the common technique for the growth of crystalline materials. This method takes into consideration different procedures of crystallizing materials from high-temperature aqueous solutions at high vapor pressures and the crystallization vessel normally utilized is an autoclave. The hydrothermal method is an economical and a direct method employed for the preparation of nanomaterials or composite nanomaterials of different architectures and distinctive chemical and physical characteristics, as well as several morphologies that include rods, sheets, flowers, spheres etc. [57,113,238–242]. Water is an essential solvent because of its amazing characteristics as a reaction medium at hydrothermal conditions in which it behaves in a different way from the water under normal conditions.

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