Flash Joule Heating Equipment: Fast Joule Heating for Scalable and Green High Surface Area Graphene Production

Background

Since its discovery, graphene has been in the spotlight for its excellent properties, especially the high electrical conductivity of monolayer graphene of up to 5 × 106 s-m-1. Nowadays, graphene powders have been used in a variety of applications, including electrodes for energy storage devices and conductive fillers for polymer nanocomposites. However, in practical applications, it is usually difficult to utilize the excellent conductive properties of graphene-polymer composites. The main reason for this is that the interfacial contact between graphene sheets hinders the transmission of electrical energy, and thus the conductivity of graphene materials is usually much lower than the inherent conductivity of individual graphene sheets. To face this problem, some researchers have attempted to introduce similar sp2-hybridized covalently bonded carbon atom structures to connect graphene flakes while retaining the original excellent in-plane conductivity of graphene. For example, M. M. Slepchenkov and colleagues used single-walled carbon nanotubes to form single-walled carbon nanotube-graphene junctions of varying diameters in graphene nanopores to control the material's conductivity. Another group of researchers has constructed conductive networks using polymer materials as substrates. For example, J. Y. Suh and colleagues constructed conductive networks using a hot-pressing method in which 30 percent graphene was encapsulated in a polytetrafluoroethylene powder.

Among these attempted polymers, polyimide has been used by many researchers as a polymer matrix for graphene composites due to its promising applications in areas such as thin-film electrodes, integrated-circuit heat sinks, high-temperature structural materials, and lithium-ion batteries. In addition, it is widely used as one of the best pyrolytic precursors for the preparation of carbon thin films by carbonization and graphitization.

The main point of this paper

1.Characterization of graphene oxide (GO):

GO is the oxidized form of graphene, which contains a variety of functional groups and is itself an electrical insulator.

The electrical conductivity of GO can be improved by reduction.

2.GO reduction methods:

Including thermal annealing, laser annealing, microwave irradiation, etc.

Chemical methods (e.g., using hydrazine and sodium borohydride) are widely recognized, but suffer from long processing times, contamination, and the use of toxic agents.

3.Synthesis of mild graphene oxide (MOG):

Electrochemical stripping method is of great interest to obtain GO with low oxygen content.

The electrochemical exfoliation method has a short synthesis time, is scalable and uses non-toxic materials.

4.Advantages of electrochemical stripping method:

Use of inorganic salts containing SO4-anions, especially Na2SO4, low cost and low toxicity.

5.Disadvantages of the electrochemical stripping method:

Large graphite particles are stripped during synthesis, reducing yield.

Uneven distribution of electric field within the electrode volume.

Graphite electrode molding may introduce impurities.

6.Improved method:

Achee et al. proposed electrochemical stripping within the volume, using dialysis bags for electrodes and graphite powder.

Increased yield to 65% and improved quality of the resulting MOGs

Preparation of high specific surface area graphene materials by green electrochemical synthesis and Joule heating activation

Research Background and Interests:

The rapid development of electric vehicles, unmanned aerial vehicles, and wearable electronic devices has driven significant interest in the study of high specific surface area graphene for energy applications

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Graphene Synthesis Challenges:

The main challenges in graphene synthesis include scalability issues and long time and high energy consumption during carbon material activation.

Novel reactor development:

In this study, a novel reactor was developed for the green, simple and scalable electrochemical synthesis of graphene oxide with oxygen content as low as 14.1%.Graphene oxide

Activation by fast Joule heating:

The resulting materials were activated by fast Joule heating, using high-energy short electrical pulses to treat lightly oxidized graphene to obtain graphene-based porous carbon materials with high specific surface area.The graphene-based porous carbon material with high specific surface area was obtained.

Reason for increase in specific surface area:

The increase in specific surface area is attributed to the breakdown of pristine graphene flakes into smaller particles due to the explosive release of gaseous products.

Reduction effect of Joule heating:

Joule heating was able to instantaneously reduce graphene oxide with a decrease in resistance from >10 MΩ/sq to 20 Ω/sq. The regeneration of the sp2 carbon structure was confirmed by Raman spectroscopy

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Advantages of the method:

The method has the advantages of low energy intensity, simple operation, and environmental friendliness, and the prepared graphene materials have high specific surface area and electrical conductivity, which are suitable for various energy applications

Summarize

The results of the data indicate that the reactor developed in this study can produce multilayered lamellar MOG with oxygen content up to 14.1%.The functional group composition of the initial MOG correlates with the data obtained in other studies [16]. Also, the reactor design is remarkably scalable. The use of fast Joule heating increased the conductivity of aMOG by four orders of magnitude, which may be related to the reduction of the initial MOG to 4.2% oxygen by heat. Raman spectroscopy analysis shows that the sp2 grain size La increases with increasing fast Joule heating voltage, indicating that MOG has been reduced and the sp2 carbon lattice has been regenerated. At the same time, during fast Joule heating (32 ms), oxygen groups are explosively released as gaseous products, leading to the rupture of the initial MOG monoliths [23]. Atomic force microscopy studies have shown that the transverse size of aMOG decreases from 0.8 μm to 100 nm and the thickness from 6-20 nm to 0.8-1.2 nm compared to the initial MOG. aMOG has a hysteresis loop of the adsorption isotherm of the H1-type, which is characteristic of similarly sized and homogeneously distributed spherical particle agglomerates.