Consider an electric vehicle that takes only a few seconds to recharge while braking and then using that energy to power up the vehicle until the next stop. Imagine a lightweight smartphone that, after being plugged in the wall for a minute, is fully charged. Although this may seem far-fetched, innovations in graphene supercapacitors are bringing these possibilities to life.
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Brief Introduction to Supercapacitors
Although batteries have a high energy density, they have a bad reputation for charging and discharging slowly. Capacitors charge quickly, but they can only store a limited number of charges for a short time. A supercapacitor combines the best characteristics of both a battery and a capacitor.
In a supercapacitor, energy is stored electrostatically on the surface of the material, unlike batteries, which involve chemical reactions. They are composed of two metal plates coated with a porous material called activated carbon, a separator in the middle, and an electrolyte. The electrolyte can be either a liquid or a solid-state, or quasi-solid-state material.
When a voltage is applied to the surface of a supercapacitor, two separate charged layers appear. Because of this, supercapacitors are also known as electric double-layer capacitors, or EDLCs. To further improve the energy and power densities of supercapacitors, scientists are looking for new materials that can store more energy. Graphene is one of these materials.
Graphene Supercapacitors: The Next Generation Energy Storage Technology
Graphene is often suggested as a replacement for activated carbon in supercapacitors, due to its high relative surface area of 2630 m2/g, which is better at storing electrostatic charge with almost no degradation over long-term cycling.
A graphene supercapacitor is capable of storing as much energy as a battery and can be fully recharged in one or two minutes. Moreover, graphene supercapacitor technology is both environmentally friendly and much safer than current battery technology because it can operate without exploding or overheating
Graphene’s exceptional electrical conductivity (up to ~ 20,000 S/cm), flexibility and high mechanical strength with a Young's modulus of 1TPa make it a promising candidate for wearable and flexible supercapacitor technology.
The most critical aspect to keep in mind is the efficiency of the supercapacitor. Recently, scientists from First Graphene Limited have produced graphene supercapacitors that can store 140 F/g. This is a watershed moment in supercapacitor materials when compared against the current technology of activated carbon, which typically has a specific capacitance of only 35 F/g.
Recent Development for Industrialization of Graphene Supercapacitors
Due to the increased difficulty in synthesizing mass-scale graphene, the cost of supercapacitor materials frequently exceeds the cost of battery materials. However, recent large-scale graphene supercapacitor synthesis breakthroughs may make them battery competitors.
Prof. Richard Kaner and his team from the University of California, for example, used a simple DVD writer on a home PC to produce scalable fabrication of more than 100 graphene micro-supercapacitors over a large surface in less than 30 minutes. The reported power density of these micro-supercapacitors is 200 W/cm3, which is among the highest ever recorded for any supercapacitor.
In another study, researchers created mass production of green flexible graphene microsupercapacitor by irradiating femtosecond laser pulses on fallen leaves. The synthesized graphene microelectrodes demonstrated an outstanding areal capacitance of 34.68 mF/cm2 and capacitance retention of ≈99% after 50 000 charge/discharge cycles.
Graphene Supercapacitor Scalability: From Lab to Industry
Companies like Maxwell Technology, Skeleton Technology and The Paper Battery Co are leading the charge to integrate graphene into supercapacitors technology. For example, Skeleton Technologies' graphene-powered ultracapacitor SkelCap SCX5000 is a 3.0V ultracapacitor with an energy density of 16.0 Wh/L, significantly higher than other ultracapacitors currently on the market.
CRRC has developed 3 Volt/12,000 F graphene-based supercapacitor, which is said to be able to power a tram for 6 km with only 30 seconds of charging and 2.8 Volt/30,000 F capacitor can power a bus for up to 10 km after one minute charge. In addition to improvements in performance, the new products are also more energy-saving and environmentally friendly.
Graphene Hybrid Supercapacitor: A New Rival to the Battery Technology
One issue with supercapacitors so far has been their low energy density. Batteries, on the other hand, have been widely used in consumer electronics. However, after a few charge/discharge cycles, they wear out and have safety issues, such as overheating and explosions.
Hence, scientists started working on coupling supercapacitors and batteries as hybrid energy storage systems. For example, Prof. Roland Fischer and a team of researchers from the Technical University Munich have recently developed a highly efficient graphene hybrid supercapacitor. It consists of graphene as the electrostatic electrode and metal-organic framework (MOF) as the electrochemical electrode.
The device can deliver a power density of up to 16 kW/kg and an energy density of up to 73 Wh/kg, comparable to several commercial devices such as Pb-acid batteries and nickel metal hydride batteries. Moreover, the standard batteries (such as lithium) have a useful life of around 5000 cycles. However, this new hybrid graphene supercapacitor retains 88% of its capacity even after 10,000 cycles.
The Future of Graphene Supercapacitors
Within the next five years, graphene supercapacitors are likely to be utilized in laptops, smartphones, electronics, public transportation, and many other applications due to increased development in terms of energy storage limits.
CRRC, for example, has announced the launch of an electric train that uses a graphene supercapacitor to store energy when braking and then convert that energy to power the train when accelerating.
We will see a flexible, lightweight smartphone powered by graphene supercapacitor in the next few years that charges at a much faster rate than current batteries and lasts for a very long time.
Continue reading: A Review of Graphene in Energy Storage Devices
References and Further Reading
Jayaramulu, K., et al (2020) Covalent Graphene‐MOF Hybrids for High‐Performance Asymmetric Supercapacitors. Advanced Materials, 33(4), p.2004560. https://doi.org/10.1002/adma.202004560
Le, T., et al (2021) Green Flexible Graphene–Inorganic‐Hybrid Micro‐Supercapacitors Made of Fallen Leaves Enabled by Ultrafast Laser Pulses. Advanced Functional Materials, p.2107768. https://doi.org/10.1002/adfm.202107768
El-Kady, M. and Kaner, R., (2013) Scalable fabrication of high-power graphene micro-supercapacitors for flexible and on-chip energy storage. Nature Communications, 4(1). https://doi.org/10.1038/ncomms2446
CRRC ZELC EUROPE. (2018) Graphene supercapacitor DMU fleet takes shape - CRRC ZELC EUROPE. [online] Available at: https://crrczelc-europe.com/2018/05/24/graphene-supercapacitor-dmu-fleet-takes-shape
Skeleton Technologies (2022) SkelCap SCX Series Ultracapacitor Cells. [online] Skeletontech.com. Available at: https://www.skeletontech.com/skelcap-scx-ultracapacitor-cells