Battery innovation – supercapacitors
Laptop slow to charge? I-phone flat by lunchtime?
Enter the supercapacitor
Supercapacitors don’t act like normal batteries. Theoretically – and it is only a theory – they could act as a power supply that not only charges your devices in minutes, but also gives those devices a charge that lasts for a longer period of time.
A supercapacitor is a double-layer capacitor that has very high capacitance but low voltage limits. Supercapacitors store more energy than electrolytic capacitors and they are rated in farads (F). Supercapacitors store electrical energy at an electrode–electrolyte interface. They consist of two metal plates, which only are coated with a porous material known as activated carbon. As a result, they have a bigger area for storing much more charge.
The plates are immersed in an electrolyte made of positive and negative ions dissolved in a solvent. When a voltage is applied, two separate charged layers are produced on the surface with a small separation distance. This is why supercapacitors are often referred to as electric double-layer capacitors or EDLCs (Fig. 1).
1. The separation of charge distance in a double layer is on the order of a few angstroms (0.3-0.8 nm). (Courtesy of InTech)
Supercapacitors have many advantages. For instance, they maintain a long cycle lifetime—they can be cycled hundreds of thousands times with minimal change in performance. A supercapacitor’s lifetime spans 10 to 20 years, and the capacity might reduce from 100% to 80% after 10 or so years. Thanks to their low equivalent series resistance (ESR), supercapacitors provide high power density and high load currents to achieve almost instant charge in seconds. Temperature performance is also strong, delivering energy in temperatures as low as –40°C.
On the other hand, supercapacitors are offset by their low energy density. Thus, they can’t be used as a continuous power source. One cell has a typical voltage of 2.7 V; if higher voltage is needed, the cells must be connected in series.
Supercapacitors are used in many power-management applications requiring many rapid charge/discharge cycles for short-term power needs. Some of these applications include:
• Voltage stabilization in start/stop systems
• Electronic door locks in the event of power failures
• Regenerative braking systems
• Distribution microgrid
• Medical devices
• Energy harvesting
• Consumer electronics
• Kitchen appliances
• Real-time clock backup
• Utility meters
• Backup power
• Grid stability
• Wind energy
• Energy efficiency and frequency regulation
• Remote power for sensors, LEDs, switches
• Memory-backup SRAM
• Burst-mode power delivery
• Forklift hybridization