What is a super capacitor?
Supercapacitors, also known as ultracapacitors or electrochemical capacitors, are energy storage devices that store and release energy through the electrostatic separation of charges. Unlike traditional capacitors, which use dielectric material to store energy, supercapacitors store energy through the electrochemical double-layer effect and, in some cases, through a reversible faradaic redox reaction.
Construction of a super capacitor
The most common type is the electrochemical double-layer capacitor (EDLC). Super-capacitors are constructed from two electrodes, an electrolyte and a electrolyte separator that allows the transfer of ions, while providing insulation between the electrodes.
- Electrodes: Super-capacitors consist of a pair of electrodes, typically constructed from highly porous materials to obtain large surface area. Typical choices for electrode materials include activated carbon, graphene, carbon nano-tubes, and conductive polymers. These materials play a crucial role in facilitating the formation of an extensive electrochemical double-layer on their large surface area.
- Separator: A separator is positioned between the two electrodes to serve as a barrier, ensuring that direct contact between them is avoided, thus mitigating the risk of a short circuit. Generally, this separator comprises a porous membrane fabricated from materials such as cellulose or polymers. This membrane facilitates the passage of ions while simultaneously maintaining a physical separation between the electrodes.
- Electrolyte: Super-capacitors employ an electrolytic medium to facilitate the movement of ions between the pair of electrodes. The choice of electrolyte depends on the specific type of super-capacitor. Common electrolyte types include aqueous electrolytes (e.g., sulphuric acid, potassium hydroxide) for lower voltage super-capacitors and organic or ionic liquid electrolytes for higher voltage super-capacitors.
- Current Collectors: Current collectors typically consist of materials with high conductivity, such as foils composed of aluminium or copper. These collectors are affixed to the electrodes and serve the critical function of establishing electrical links to the external circuit.
- Packaging: The aforementioned components are arranged into a cylindrical configuration, which varies according to the super-capacitor’s specific design. This assembly is then enclosed within a container constructed from materials that exhibit chemical inertness towards the electrolyte. This container is employed to prevent any potential leakage and to uphold the super-capacitor’s overall stability.
Principle of operation of a EDLC (Electrochemical Double-Layer Capacitor)
- Charging: When a voltage is applied across the supercapacitor terminals as shown in Fig 2, ions from the electrolyte are attracted to the surface of the porous electrodes. Positive ions are attracted to the negative electrode, and negative ions are attracted to the positive electrode. This creates an electrochemical double layer at the electrode-electrolyte interface (also called Helmholtz layer).
- Energy Storage: The energy is stored in the form of an electric double-layer at the interface between the electrode and the electrolyte. This double layer can store a significant amount of electrical energy due to the large surface area of the porous electrodes and the high mobility of ions in the electrolyte.
- Discharging: When the supercapacitor is discharged, the stored energy is released as the ions return to the electrolyte, and the potential difference between the electrodes is reduced. This discharge process can happen rapidly, providing a burst of electrical power.
What is the difference betweeen conventional capacitor and a supercapacitor?
In conventional capacitors, energy is stored by storing electric charges between two physically separated conductive plates upon application of an electric field. No chemical reaction is used in conventional capacitors to store charge.
In supercapacitors (ELDC type), using chemical reaction of the special electrode and electrolyte, huge amount of charge is stored in volume similar to a conventional capacitor.
What is the difference between supercapacitor and a Li-ion battery?
| Property | Supercapacitor | Li-ion battery |
|---|---|---|
| Charge/Discharge time | 1 to 10 seconds | 10 to 600 minutes |
| Charge/Discharge efficiency | 85% to 98% | 70% to 85% |
| Charge/Discharge cycle | 100,000+ | 500+ |
| Min to Max Voltage | 0 to 2.3V | 3 to 4.2V |
| Specific energy | 1 to 5 | 100 to 240 |
| Specific power | 10,000+ | 1000 to 3000 |
| Operating temperature | -40C to +45C | 0C to +45C |
| Self discharge rate | High | Low |
| Safety | High | Low |
Supercapacitor power density and energy density
Supercapacitors have emerged as highly effective energy storage devices that serve as a vital link between larger, heavier battery-based systems and conventional bulk capacitors (refer Fig 3). Supercaps exhibit remarkable attributes, including the ability to endure rapid charge and discharge cycles far beyond the capabilities of rechargeable batteries. This unique feature positions supercapacitors as superior alternatives for various applications requiring short-term energy storage in relatively low-energy backup power systems, short-duration charging scenarios, peak load current buffering, and energy recovery systems.
Additionally, hybrid systems that combine batteries with supercapacitors are gaining traction, capitalizing on the high-current, short-duration power capabilities of supercapacitors while harnessing the long-term, compact energy storage advantages of batteries.
Supercapacitor advantages and applications
- High power density: Supercapacitors can deliver and absorb electrical energy at a much higher rate than traditional batteries. This high power density makes them well-suited for applications that require rapid energy bursts or quick charging and discharging.
- Rapid charging and discharging: Supercapacitors can be charged and discharged rapidly, often within seconds or milliseconds, without significant degradation in performance. This feature is valuable in applications where quick energy transfers are essential, such as regenerative braking in electric vehicles, to balance loads on power grids, standby/backup power supplies, or peak-load enhancers for hybrid vehicles.
- Long cycle life: Supercapacitors have a longer cycle life compared to many rechargeable batteries. They can withstand hundreds of thousands to millions of charge/discharge cycles, which makes them a reliable and durable energy storage solution.
- High efficiency: Supercapacitors have low internal resistance, which means they can efficiently convert stored energy into electrical power with minimal energy losses. This efficiency is particularly advantageous in applications where energy conservation is crucial.
- Wide operating temperature range: Supercapacitors can operate in a broad range of temperatures, from very cold to very hot conditions, without a significant impact on performance. This versatility makes them suitable for applications in extreme environments.
- Low maintenance: Supercapacitors require minimal maintenance compared to some other energy storage systems, like lead-acid batteries. They do not require regular deep cycling or maintenance charging, reducing overall operational costs.
- Durability: Supercapacitors are robust and resistant to mechanical vibrations and shocks, making them suitable for rugged environments and applications where reliability is critical.
- Safety: Supercapacitors are generally safer than some types of batteries, as they do not contain toxic chemicals, and they are less prone to thermal runaway events.
- Scalability: Supercapacitors can be easily scaled by connecting multiple cells in series or parallel to meet the specific energy and power requirements of a given application.
Supercapacitor disadvantages
- Low energy density: Supercapacitors typically store less energy per unit volume or mass compared to conventional chemical batteries. This makes them less suitable for applications that require high energy storage capacities.
- Voltage limitations: Supercapacitors have lower voltage ratings compared to batteries. They are often used in combination with other components to achieve higher voltage levels, which can increase system complexity and cost. Exceeding this voltage range can lead to irreversible damage or failure.
- Self-discharge rate: Supercapacitors have a higher self-discharge rate than batteries. This means they can lose their stored energy relatively quickly when not in use, making them less suitable for applications requiring long-term energy storage.
- Temperature sensitivity: Supercapacitor performance can be sensitive to temperature fluctuations. Extreme temperatures can impact their capacitance and internal resistance, affecting overall performance.
- Cost: Supercapacitors are typically more expensive per unit of stored energy than traditional batteries, which can make them less cost-effective for certain applications, especially those that require large energy storage capacities.
- Environmental concerns: Like batteries, supercapacitors may contain materials that can be harmful to the environment if not properly disposed of or recycled. Efforts are being made to develop more environmentally friendly supercapacitor materials, but this remains a concern.
- Safety considerations: While supercapacitors are generally considered safer than Li-ion batteries, they can release a significant amount of energy if mishandled or damaged. Proper safety measures and protection circuits are often necessary in high-power applications.