Woven graphene is an intriguing contender for applications in next-generation energy storage and conversion devices due to its intrinsic physical qualities and the high degree of tunability of its electronic properties.
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What is Entangled Graphene?
Twisted graphene, also known as twisted bilayer graphene, is a unique structure that can be created by stacking two layers of graphene at a small angle, often between one and two degrees. Since the hexagonal lattices of the two layers of graphene are not fully aligned, a new unit cell with larger dimensions and a moiré pattern is produced.
The resulting structure of twisted graphene is particularly fascinating from a scientific point of view as it possesses a range of unusual physical and electrical properties not found in either single-layer graphene or graphite in its bulk form. These characteristics are a result of the interlayer coupling that occurs between the two graphene layers, which is mainly determined by the torsion angle of the graphene layers.
Twisted graphene has a flat electron band structure at particular “magic” twist angles, such as 1.1 degrees and 1.8 degrees, where the energy levels of the electrons are spread over a wide range of momentum space , resulting in an almost zero density of states. This results in a variety of intriguing phenomena, such as Mott insulator behavior, superconductivity, and topological states.
Batteries and supercapacitors
Energy is stored in batteries as chemical potential energy, which is transformed into electrical energy when needed. To increase the amount of energy that can be stored, battery electrodes are often made of materials with a high surface area, such as activated carbon or metal oxides. Due to the moiré pattern generated by the two layers, twisted graphene has a huge surface area, which enhances its ability to store energy. Furthermore, the flat electric band structure at the magic angles could be used to increase the performance of the battery and supercapacitor.
Supercapacitors, on the other hand, store energy in the form of electric charges that can be released rapidly. To maximize capacitance, supercapacitor electrodes are often composed of materials with high electrical conductivity and surface area. Due to its unique electronic properties, woven graphene has high electrical conductivity, and its huge surface area can also increase the capacity of the material.
For example, researchers at India’s Jawaharlal Nehru Center for Advanced Scientific Research have reported using twisted multilayer graphene to create an ultrafast supercapacitor with an ultra-high frequency response in the order of 10,000 Hz, the highest ever reported for any supercapacitor. until today.
In solar cells, sunlight is transformed into electrical energy by absorbing photons in a material having a bandgap that corresponds to the energy of the photons. The absorbed energy excites the material, which can then be collected as an electric current. The performance of a solar cell is determined by various parameters, including the material’s bandgap, electrical conductivity and light absorption.
The moiré pattern of twisted graphene can behave like a photonic crystal, increasing light absorption and improving solar cell efficiency. Furthermore, the flat electronic band structure at the magic angles may result in improved charge carrier mobility, which is critical for effective charge separation and collection in solar cells.
Photovoltage from a 10° twisted bilayer graphene photodetector demonstrates a seven-fold (700%) photovoltage enhancement at the best angle of incidence, according to researchers at Nankai University in Tianjin.
Fuel cells are electrochemical devices that convert chemical energy directly into electrical energy. They consist of an electrolyte and two electrodes: an anode and a cathode. The fuel is oxidized in the anode to produce electrons and protons. Protons move through the electrolyte towards the cathode while electrons travel through an external circuit, providing an electric current. The protons react with the oxygen in the cathode to generate water, completing the electrochemical reaction.
Due to its unique electronic properties, woven graphene has high electrical conductivity, which can improve the performance of fuel cells. Furthermore, the huge surface area of twisted graphene can increase the surface area of the electrodes and allow for adsorption of reactants, improving fuel cell efficiency.
Woven graphene could be used in water splitting applications due to its large surface area. The twisted graphene moiré pattern may also serve as a template for the formation of catalytic nanoparticles, increasing the efficiency of the water splitting process.
Due to its unique electrical characteristics, woven graphene could be a viable material for thermoelectric applications. The flat electronic band structure at the magic angles can result in a high Seebeck coefficient, which is required to convert heat into electricity.
Challenges and opportunities
The longevity of energy storage can be improved by using twisted graphene due to its mechanical and chemical stability. Compared to other high-performance energy storage materials, the production cost of woven graphene is significantly lower, making it a promising candidate for widespread use.
However, before woven graphene can be used in a practical energy storage system, several hurdles have to be overcome. For example, there is still a technical challenge in mass producing high quality twisted graphene. The complexity of the electrical environment generated by the moiré pattern of twisted graphene adds another level of difficulty to the challenge of developing an efficient energy storage device based on this material.
Overall, research into solar cells using woven graphene is exciting and growing rapidly. To fully understand the potential of this material and optimize its use in practical applications, further research and development is needed.
Read on: What is Twisted Graphene?
References and further reading
Gupta, N, Mogera, et al. (2022) Ultrafast planar microsupercapacitor based on defect-free twisted multilayer graphene, Materials Research Bulletin. P. 111841 https://www.sciencedirect.com/science/article/abs/pii/S0025540822001143?via%3Dihub
Xi, Wei, et al. (2016) “Photovoltage enhancement in twisted bilayer graphene using surface plasmon resonance”. Advanced optical materials. pp. 1703-1710. https://onlinelibrary.wiley.com/doi/10.1002/adom.201600278
Mogera, Umesha and Giridhar U. Kulkarni. (2020) “A New Breakthrough in Graphene Research: Twisted Graphene”. Carbon. pp. 470-487. https://www.sciencedirect.com/science/article/abs/pii/S0008622319309625?via%3Dihub
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