Supercapacitors

Supercapacitors (SC), also known as ultracapacitors, electrochemical capacitors or electrochemical double layer capacitors (EDLCs), are electrochemical devices that are used to store and deliver electrical energy. They are characterized by their high power density, long life, and fast charging and discharging capabilities, but they lack high energy density.

SC are similar to capacitors in that they store electrical energy in an electric field between two conductive plates. However, unlike capacitors, which store energy using a physical separation of charges, SC store energy by electrostatically adsorbing ions onto the surface of the conductive plates. This allows them to store much more energy than traditional capacitors and to charge and discharge much faster.

SC have a number of advantages over other energy storage technologies, such as batteries. They have a much longer life, typically lasting for millions of charge/discharge cycles, compared to hundreds or thousands of cycles for batteries. They can also be charged and discharged much faster than batteries, making them well-suited for applications that require quick bursts of power.

These are used in a wide range of applications, including power backup, energy harvesting, electric vehicles, and portable electronic devices. They are also used in hybrid systems in combination with batteries or fuel cells to improve overall performance and extend the life of the system. 

Charge-Discharge Profile

They have quasi-triangular GCD profile and near rectangular-shaped CV which are characteristic of SC that use EDLC as the mechanism for energy storage, while, different types of SC, such as those that use pseudocapacitance, may exhibit different charge/discharge profiles and CV shapes.

• The quasi-triangular galvanostatic (GCD) profile refers to the shape of the current vs. time graph during the charging and discharging process of a SC. During charging, the current starts high and gradually decreases as the capacitor becomes more charged, resulting in a triangular-like shape. During discharging, the current remains relatively constant until the capacitor is nearly depleted, resulting in a similar triangular-like shape. The term "quasi" is used because the shape is not a perfect triangle due to factors such as internal resistance and voltage drop.
• The near rectangular-shaped cyclic voltammogram (CV) refers to the shape of the current vs. voltage graph during the cyclic voltammetry technique used to measure the capacitance of a supercapacitor. The graph shows a sharp increase in current as the voltage is ramped up, followed by a plateau where the current remains relatively constant as the voltage continues to increase, and then a sharp decrease in current as the voltage is ramped back down. The shape of the CV curve is nearly rectangular due to the rapid and reversible charge/discharge of the electrochemical double-layer.

Metrics & Benchmark

The performance of supercapacitors can be evaluated using several metrics and benchmarks, while the ideal benchmarks can vary depending on the specific application and operating conditions, and that there are trade-offs between different metrics that must be considered when designing supercapacitors for different applications. The metrics with benchmarks are as follows:

• Energy density: The amount of energy that can be stored per unit of volume or mass. The higher the energy density, the more energy can be stored in a smaller device. (>5 Wh/kg)
  
• Power density: The rate at which energy can be delivered per unit of volume or mass. The higher the power density, the faster energy can be delivered. (>5 kW/kg)
  
• Cycle life: The number of charge/discharge cycles a supercapacitor can endure before its performance degrades significantly. A high cycle life is desirable as it ensures a longer lifespan for the device. (>10,000 cycles)
  
• Efficiency: The ratio of the amount of energy output to the amount of energy input. A high efficiency means that less energy is wasted during charge/discharge cycles. (>95%)
  
•  Equivalent series resistance (ESR): The resistance of the supercapacitor's internal components that can limit its ability to deliver high power quickly. A low ESR is desirable as it enables the device to deliver energy more efficiently. (<10mΩ)
  

Supercapacitors  Vs LIBs

They both are two different types of energy storage devices that have their own unique characteristics and are used in a variety of applications depending on the specific energy and power requirements of the system.

• One of the main differences between SC and lithium-ion batteries is their energy density. LIB have a much higher energy density than SC, meaning they can store more energy per unit of mass or volume. This makes them well-suited for applications that require a long-lasting power source, such as laptops, cell phones, and electric vehicles.
• On the other hand, SC have a much higher power density than LIB, meaning they can deliver or receive energy much more quickly. This makes them well-suited for applications that require quick bursts of power, such as regenerative braking in hybrid electric vehicles, power backup, and energy harvesting.
• Another difference between the two technologies is their charging and discharging rate. SC can be charged and discharged much faster than LIB, making them well-suited for applications that require frequent charge/discharge cycles.
• Finally, SC have a much longer lifespan than lithium-ion batteries, typically lasting for millions of charge/discharge cycles compared to hundreds or thousands of cycles for batteries. However, LIB have a higher energy density, which allows them to store more energy and therefore last longer in some applications.

Lifetime

Even though SC have ten thousands or millions of cycles, depending on the application requirements they can be used till they last their required increase in SR or decrease in capacitance (energy). Prolonged exposure to elevated temperatures, high voltage or/and current will lead to electrochemical degradation and therefore decrease in lifetime.