GRAPHENE

GRAPHENE

Graphene had been a matter of wonder since 2004 and research studies on its applications and potentials been vigorous since. Graphene claimed unbeatable potentials and capacities. But, now, a fellow competent has arose. With Boron much similar to Carbon, the new attention is towards Borophene, which is the two-dimensional monoatomic layer of Boron, just like Graphene for Carbon. So, before exploring Borophene, we shall now see for Graphene and its applications for energy.

Overview

Graphene is a two- dimensional single atomic layer of Carbon. It is a good conductor of electricity and very strong that it is planned to be used for space elevators. And the strength of Graphene is that, it is the strongest material in the world! The strength results from the power of the carbon-carbon tetravalent bonds. A good semiconductor, Graphene also stands a candidate for superconductors.

Graphene for Energy

An Eternal Source of Energy from Graphene

Under suitable circumstances, the temperature changes due to the surrounding heat causes the graphene sheet to ripple and buckle. The rippling is also said to an intrinsic property of graphene [a]. A mechanism of spontaneous mirror buckling, which occurs without any temperature gradient have also been proposed [g]. However, the freely suspended graphene membrane shakes and vibrates, curving convexly upward once, then concavely downwards, then again upwards and so. It periodically flips the curvature. We can generate energy out of this, placing a charge on the membrane, and keeping electrodes near it, such that when the membrane curves upward, it touches one electrode, and the other when downward, thereby making a flow of charge. This is a battery alternative, where we do not want to replace the batteries. Evidently, life becomes more easier, powering wristwatches to pacemakers; graphene being biocompatible.

All-Graphene-Battery

Lithium ion batteries (LIBs) have high energy density and give great promises for the future. But, their power density available is not suitable for large-scale applications, and it is expensive as well.[b] They take long hours to charge and eventually lose effectiveness as well. The low power density and high expense are closely related to the fundamental electrode reaction characteristic of those batteries, which is related to the slow solid-state diffusion of lithium and low electronic conductivity. Now, as for the lithium ion capacitors (LICs), which came with many recent advancements, there’s an imbalance of kinetics between the electrodes. The intercalation reaction in one electrode is far slower than the surface reaction in the other. [b]


This can be overcome if we could develop fast surface electrode reactions in both the electrodes, maintaining high energy density at the same time. Functionalization of graphene to enable fast surface reaction could lead to exceptionally high Li storage capability [c]. The functional groups on the graphene cathode acted as radical centres to store Li ions at high potential. [c,d]


The all-graphene-battery was introduced by utilising the fast surface electrode reactions of functionalized graphene cathodes by matching them with reduced GO anodes. Also, the fabrication of graphene was not mass-scalable. To realize, herein, all-graphene-battery, mass-scalable functionalized GO are used in cathode and anode, respectively, without utilizing lithium metals. All-graphene-battery delivers exceptionally high power density because both the anode and cathode exhibit fast surface reactions combined with porous morphology and high electrical conductivity. [b] Their similar chemistry and microstructure takes off the power imbalance drawback of LICs. [e] It retains its high energy density as well.


Thus, graphene can be integrated into the batteries, resulting in very high performances, as discussed with the all-graphene-battery. These supercapacitors charge immediately, run forever and never wear down.

Enhancing Direct Methanol Fuel Cell Performance

Fuel Cells, which directly convert chemical energy to electrical energy, that too with high efficiency are a promise for the future energy solutions.Among various types of fuel cells, one with greater attraction because of its liquid state and high energy density (compared to hydrogen gas), thereby offering advantages in transportation and storage, is Direct Methanol Fuel Cells (DMFCs). The addition of single layer graphene to an operational DMFC significantly enhances the performance of the cell once the temperature is raised above 60°C. The performance of the fuel cell is shown to increase linearly above this temperature. Results show that the maximum power density is increased at 75°C by 45% in comparison to the standard membrane electrode assembly without graphene. [k] Thus, the output of DMFC can be increased many folds by the application of graphene.

Environment-friendly Hydrogen Fuel Cell

The basic principle of a hydrogen fuel cell is that hydrogen fuel is combusted, and electrical energy is produced [f].


2H2 → 4H+ + 4e-


O2 + 4e- + 4H+ → 2H2O


A very great advantage of Hydrogen fuel cells - apart from their high efficiency and all, is that it causes no environmental pollution, as seen in the aforementioned electrode reactions; for the product is water and nothing harmful. But, one of the major drawback is the transportational difficulties owing to hydrogen’s weight. Birch reduction of few-layer graphene samples gives rise to hydrogenated samples containing 5 wt% of hydrogen [i]. The reduced material gets dehydrogenated on photothermal heating [j]. Thus, hydrogen integrated to graphene can be used to overcome the transportation issues of hydrogen, to an extend, and thus hydrogen fuel cells can be made more practical.

Nitrogen doped Graphene

As the surface defects increase, the gas molecules of graphene show more adsorption [l] When graphene is doped with Nitrogen, the reversible discharge capacity is double as compared to pristine graphene’s, since the surface defects got increased by doping with nitrogen. This doped graphene layer can be applied to Lithium batteries and even supercapacitors, as an electrode. The n-doped graphene electrodes could find applications in flexible film batteries as well. [m]

Lithium-Air Batteries

Specific capacities of li-air battery electrodes are more than five folds that of Li-ion batteries [n]. Thus, lithium-air batteries with more than ten times improved performance than Li-ion batteries are promising as energy sources. But, li-air batteries are seen to have many limitations, which we could overcome by the application of Graphene. As we did in Li-ion batteries, it’s not 2D sheets of graphenes here but the porous 3D graphene. [pu_11] The bimodal pores of these air electrodes account for their high capacities.The precipitation of discharge products (li2O2, liO2) has been a major drawback which limited the life of expected Li-air batteries. This can be overcome by the application of graphenes. The lattice defect sites on the functionalized graphene helps the discharge products be nano-sized.

REFERENCES

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