HOW GRAPHENE IS REVOLUTIONIZING ENERGY STORAGE

Graphene ultracapacitors are energy-storage devices with graphene-based electrodes to combine very fast charge–discharge behaviour with higher capacitance than conventional activated-carbon-based supercapacitors. Graphene is unique material with atom-thin structure, high electrical conductivity, very large theoretical surface area, and mechanical flexibility- all useful for electrochemical double-layer storage.

Graphene for ultracapacitors

Ultracapacitor stores energy mainly by ion adsorption at electrode surface. Graphene has a theoretical surface area of about 2630 m²/g, and an ideal single-layer graphene has maximum theoretical capacitance about 550 F/g. Its high conductivity helps reduce internal resistance, so graphene devices can deliver large power with extremely fast response. Graphene gives much more space and easy access for electron movement and storage space in electrodes compared with activated carbon, thereby increasing capacitance and energy storage. Larger electron mobility permits much higher power delivery and faster charging capability.

Common application areas include regenerative braking, backup power, pulse power systems, wearable electronics, flexible devices, and micro-energy storage on chips and sensors. Graphene usage and manufacturing is not yet fully commercialized due to technical issues. Graphene is often used as CNTs, in composites with polymers, or metal oxides. Commercial production remains low-volume, which limits economies of scale and keeps prices high.

Graphene for Asymmetric Capacitors and Pseudocapacitors

Asymmetric capacitors have two electrodes of different types. Graphene is useful for asymmetric supercapacitors, where one electrode is graphene and the other is of different materials like activated carbon like metal oxides. This construction widens the voltage window and increases total energy storage, which can make supercapacitors competitive with batteries.

Graphene pseudocapacitors are made by mixing graphene with other active materials such as metal oxides, conducting polymers, nitrides, phosphides, selenides etc. Graphene mainly provides fast electron transport, mechanical support, and a framework that keeps active nanoparticles or polymer chains separated.

Graphene-carbon combinations use graphene plus carbon nanotubes (CNT) or porous carbon to prevent restacking and improve ion pathways. These electrodes improve specific capacitance, energy density, rate capability, and mechanical robustness.

Energy density and power of graphene hybrid ultracapacitors

Hybrid designs use pseudocapacitive materials such as metal oxides or conducting polymers with graphene. These mostly have one graphene electrode and one battery-type electrode construction. This allows extremely high energy storage by adding fast surface redox reactions. Energy density of graphene being much higher than activated carbon-based designs, hybrids reach levels of the order of batteries, and power densities several multiples of battery.

Graphene hybrid ultracapacitors can reach much higher energy density than plain EDLC supercapacitors, while still keeping very high power density and fast charging characteristics.

Graphene hybrid ultracapacitors are promising for regenerative braking, high-cycle industrial buffering, flexible electronics, and pulsed-power systems. They are not yet a full or universal replacement for lithium-ion batteries in long-range energy storage, but they approach battery-like energy levels in some asymmetric hybrid formats while preserving capacitor-like power delivery. For graphene hybrid ultracapacitors, energy densities in labs have reached around 150 Wh/kg, with very high power density. Actual energy density depends on design and construction.

Major Manufacturers of graphene capacitors

Leading manufacturers of graphene-based supercapacitors today are

1. Skeleton, Germany uses curved graphene, which looks like crumpled sheet of paper with large exposed surfaces and edges, allowing increased energy storage. Their supercapacitor modules reach extremely high peak power of 160 KW. Cell power densities of 48 KW/Kg are available, with energy densities double that of normal EDLC, a life of one million charge-discharge cycles.

2. NAWA, France grows Vertically Aligned Carbon Nanotubes (VACNT) graphene directly on aluminium or copper foils, acting as ‘nano-velcro’ structures. Their supercapacitors deliver 10 times the power and five time more energy (25-30 Wh/Kg) of normal EDLC, with ultra-fast recharge in seconds, and have a life of 1000,000 cycles. Power density is up to 100 times that of Lithium-ion batteries. Their UC-plus-battery hybrid systems are used in racing motorcycles.

3. Kilowatt Labs, New York manufactures graphene based ‘supercapacitor battery’ modular systems suitable for telecom, e-vehicles, forklifts, UPS, renewable energy farms etc. These systems feature energy density over 115 Wh/kg, charging times under 30 seconds, 1 million cycle life, and round-trip efficiency of 99%.  These products are called ‘Battery’ and since their energy levels enable them to directly replace Li-ion and other batteries.

4. GT Cap, China makes graphene-based supercapacitor batteries with energy densities up to 150 Wh/Kg. There are few others in China and outside who produce energy densities beyond 100 Wh/Kg using graphene materials.

5. Graphene Manufacturing Group, Australia makes piezo-supercapacitors for biomedical devices combining piezoelectric materials and supercapacitors for medical implants. These are self-charging under piezoelectric effect.  They also   produce Graphene Aluminium-ion battery pouch cells for fast charging (under 6 minutes) lithium-free alternatives.

Apart from ultracapacitors, graphene and CNT are also reshaping the electrodes of batteries to extend their energy density far beyond what has been possible up to now. Graphene creates additional spaces inside electrode volume for electron movement, thereby increasing the charge storage, which means additional energy storage. Batteries thus made also have better power density due to faster electron transfer between electrode and electrolyte.

Structure of graphene makes it useful in batteries to increase power density as also energy density. These batteries store higher energy beyond expectations, combined with high power delivery, thereby facilitating spread of electric vehicles with long driving range.

In summary, graphene is proving the new wonder material for supercapacitors as also batteries for increased energy storage in smaller volumes, leading to lighter devices. They also allow higher energy levels in miniature devices, leading to newer applications in many fields.