WHAT ARE ASYMMETRIC ULTRACAPACITORS?

The two electrodes of ultracapacitors in EDLC and pseudocapacitors are of same composition. These are symmetrical capacitors, in that the two electrodes are identical in construction. It is possible to have the two electrodes from different materials or types of construction to get better energy storage capacity and higher working voltages. Ultracapacitors using such construction are called Asymmetric Capacitors, and are widely used in practice. They have the benefit of much higher voltages from 3.2 V to 4.2 V, with very high energy densities, up to 10 times of EDLC or even more.

The two electrodes can be from a number of combinations of materials. Possible combinations of asymmetric capacitors are – (1) carbon based electrode (EDLC) + pseudocapacitor electrode. (2) composite material combination and (3) one battery type electrode, while the other being ultracapacitor type electrode. Use of battery type electrode combines the characteristics of battery and ultracapacitor in one physical unit.

The electrochemical window of asymmetric supercapacitors (ASCs) generally exceeds that of symmetric supercapacitors (SSCs), providing significant advantages in energy storage capabilities. Traditional ultracapacitors have lower ESR and can be discharged fully for safety. Asymmetric capacitors cannot be fully discharged.

Types of Electrodes in Asymmetric capacitors

CNT and Graphene Electrodes

Carbon nanotubes (CNT) are tubular hollow nanostructures of carbon in different forms, and offer very high surface area. The hollow structure allows high penetration of electrolyte, thereby increasing capacitance and energy density several times over activated carbon. 

Graphene is one atom thick sheet of carbon, and is the best conductor known. It offers very high surface area, and allows thorough contact of electrolyte with full available surface, giving much higher capacitance. High conductivity of graphene allows fast electron transfer and therefore higher power density and rapid charge-discharge rates. Aligned structure of graphene also imparts greater mechanical stability. Graphene is becoming common nowadays for EDLC carbon electrodes.

Pseudocapacitor Electrode

Pseudocapacitor electrode uses redox materials like metal hydroxide (MOH) of transition metals oxides, metal hydroxides, metal nitrides, or conducting polymers. Barrier voltage of these being much higher, these give far greater overall voltage compared with EDLC, and hence higher energy density. Number of nanostructured materials have been developed to get much improved voltage level, higher electrolyte penetration and lower ESR.

Conductive Polymers

Conductive polymers are used for electrodes because of their long-term stability. These capacitors may utilize a combination of liquid electrolytes, which can include aqueous, organic, and ionic liquid types. These electrolytes are chosen for their high ionic conductivity and low viscosity, which facilitate rapid ion transport during charge and discharge cycles. Enhanced ionic conductivity reduces equivalent series resistance (ESR), leading to improved power density and efficiency. Conducting polymers are not yet common, but good research is on in this field.

Battery Type Electrode

Battery-type electrodes in ultracapacitors, particularly in hybrid configurations, enhance the performance of these energy storage devices by combining the benefits of both supercapacitors and batteries. This combines the advantages of ultracapacitor and battery. Energy storage and power delivery are between ultracapacitor and battery. Capacitance at graphite electrode is much lower than that of battery type, while is many times higher. Therefore, voltage at EDLC electrode is much higher than the other, while battery type electrode gives high capacitance. Working voltages are similar to Li-ion battery, varying from 3.2 V to 4 V.

Lithium ion capacitors (LIC) are the most common in hybrid ultracapacitors. The working mechanism involves a process called intercalation. This process at this electrode is similar to that in a battery electrode. Electrolyte ions directly enter empty spaces in electrode nanostructure under electric field, allowing deepest possible penetration of ions. This gives highest capacitance values and energy levels. Energy densities are in between a battery and ultracapacitor, and among the highest in ultracapacitor types. Cycle life is much lower than EDLC, but is several times higher than battery, and their high power delivery and energy density makes them ideal for several applications.

Effect of Electrolyte

Choice of electrolyte significantly impacts the voltage window of ultracapacitors, influencing their energy density and overall performance. Different electrolytes have varying electrochemical stability windows, which define the maximum voltage at which they can operate without decomposing. Organic and ionic electrolytes used in asymmetric capacitors enable much higher rated voltages up to 4 V. Electrolytes play a crucial part in their characteristics, and have to be judiciously selected for a particular type of capacitor and electrode configuration.

Characteristics of Asymmetric capacitors

• Working voltage is 25% higher than symmetric type.
• Capacitance 6 to 9 times higher than EDLC.
• These factors give much smaller size than EDLC capacitors.
• Energy densities are in between EDLC and batteries. Densities of 25 to 85 Wh/Kg, or even beyond, are available.
• Leakage current is about ten times lower.
• Power delivery capacity (though lower than EDLC) is much higher than battery.
• Charging times are slower than other types of ultracapacitors, but much higher than batteries.
• Self-discharge rates are much lower than traditional ultracapacitors.

Asymmetric capacitors with graphene electrodes are finding increasing applications where high energy density, coupled with higher power delivery and higher efficiency is desired.

Limitations of Asymmetric capacitors

• Asymmetric capacitors cannot be safely discharged to zero, and must be limited to 50% of design rated voltage. A 3.6 V capacitor can be discharged to lowest limit of 1.8 V only.
• This means 75% of overall stored energy can be utilized.
• Higher ESR and higher values means their time constant is higher, and charging times are much longer than conventional ultracapacitors.
• Asymmetric capacitors often need monitoring of voltages similar to BMS in batteries, when used in modules.
• Life of hybrid capacitor is much lower, at 500,000 cycles, while that of EDLC can go to million cycles.  
• They can carry dangerous residual voltages and stored energy even in discharged condition.

Utmost care and precautions are needed (as for hazardous substances) during transport since they cannot be fully discharged

Applications of Asymmetric capacitors

• Energy storage systems for renewable energy sources, like solar or wind energy.
• Grid energy storage systems for power quality and system stability.
• Electric vehicles use them in regenerative braking systems, and several other functions.
• Ultracapacitor powered buses and trams are getting popular because of their ultra-fast charging and superior performance for city services.
• Remote locations can use them in place of batteries due to almost no maintenance needs and ease of installations, coupled with their higher temperature capacity compared to batteries.
• Industrial energy storage
• Portable electronics like smartphones, laptops, and energy harvesting devices.