By Aditya Singhal
Supercapacitors coupled with a regular battery form a specific kind of “super battery”, offering faster charging and more efficient energy storage. According to researchers at Washington State University, these hybrid batteries are one of the most condensed forms of energy storage. They can be constructed in any size, and have a low cost and are lightweight. Current uses include emergency radios and flashlights, and the technology could eventually find applications in cell phones and automobiles as well.
Advanced capacitor technology
Capacitors are charge storage devices having two conductive surfaces separated by a dielectric. Supercapacitors or ultracapacitors use porous conductive electrodes separated by a few nanometres,and charge is stored at the interface of the electrolyte and the electrode. The performance difference between a regular and supercapacitor is considerable.
A supercapacitor has been proved to have capacitance as high as 1700 Farads/gram. It has a very long life cycle of 1 million or 30,000h (cycle efficiency of about 95%) and is charged and discharged to the maximum limit in seconds. They have a high specific power of up to 10,000 W/Kg and low internal resistance, which aids in supporting high load currents. Although the power is high, the energy density of a supercapacitor is 0.33 – 3.89 W-h/L, which is about one tenth of a lithium ion battery.
A battery on the other hand is an electrochemical device, which has limited life, significant environmental impact and limitations in power delivery, attributes prevalent in all types of batteries. Batteries can nevertheless store more charge than supercapacitors. A lithium ion battery can store charge up to 730 W-h/L, however, a lithium ion battery has a low power density of about 0.4 kW/Kg. Hybrids combining the two technologies are expected to maximize the strengths of each form of energy storage with few disadvantages.
Graphene adds performance, capacity
In one form of a hybrid battery the positive electrode is an electrochemical double layer capacitor (EDLC) or supercapacitor and the negative electrode is a Li-ion battery, connected in series. Graphene is used as the core material for both these devices. Graphene nanocomposites are used in the negative electrode and highly porous three dimensional graphene with a large surface area is used in the positive electrode.
A Fe3O4/G//3DGraphene hybrid battery has been constructed by researchers in China, (Fan Zhang, Tenqfei Zhang et al,) which generates an ultrahigh energy density of 147 W-h/kg (power density of 150 W/kg-1). Their work has been published in the Journal of Energy and Environmental Science. This energy density is comparable to that of a lithium-ion battery and the power density to that of a symmetric supercapacitor.A hybrid or a super battery can store more than double the energy by volume of a regular supercapacitor. This is much less than a battery; however, a hybrid can undergo the charge-discharge process over 20,000 times as compared to the few 400 – 1200 life cycles of a lithium ion battery.
Redefining battery engineering
Applications with too much energy to store in a supercapacitor, but no time to charge a lithium ion battery, are ideal for superbatteries. Such hybrids have been used in wind turbines and for regenerative braking. Some other industries employing the use of this technology are telecommunication and the automotive industry. Currently lithium ion batteries are being used extensively to power our cell phones. Supercapacitors can also do so, however their major issue of size and low charge storage is quite a hindrance in this regard. Extensive research is being conducted in laboratories all over the world to invent new functional materials, which can help in greater energy storage with a smaller surface area. Highly porous carbon and mixed metal oxides are being researched for electrode materials in the National Chemical Laboratory, Pune, India, to get the desired results. The use of supercapacitors in vehicles is not new; they were used by the US military to start the engines of submarines and tanks and will are being used in hybrid electric vehicles like the Toyota Prius.
Graphene: high performance but expensive
Superbatteries are a promising technology, but at present has issues of stability and longevity. The results obtained in a laboratory may take time to implement on a mass-production scale. Advanced materials like graphene with high purity and good structural integrity are required, and are expensive: Graphene is an example, costs of as much as $109 for 25g of material.
Graphene has a very high theoretical area, which improves its gravimetric energy density (or specific energy measured by weight as W-h/Kg) but not the volumetric energy density (or energy density measured by size as W-h/L). It is very important to devise structural supercapacitors with high volumetric energy density to enhance the different applications of this device. Energy density improvement is the biggest concern, which researchers in Europe and Asia are striving hard to improve.