An Overview of Supercapacitors

Catalog

I Introduction

In brief, a supercapacitor (also called an ultracapacitor) is a high-capacitance, polarized electrolytic device. “High” here refers to capacitance (farads), not necessarily physical size.

For ordinary electrolytic capacitors, larger capacitance and voltage ratings generally require larger packages. Electrolytic capacitors typically provide capacitance values from microfarads up to several thousand microfarads (and some specialized types approach 1F) with voltage ratings up to hundreds of volts. As voltage rating rises, available capacitance per volume usually falls.

Supercapacitors follow similar size trends, but they operate in a different regime. Typical single-cell electric double-layer capacitors (EDLCs) have capacitances ranging from a few farads to several thousand farads, with single-cell rated voltages commonly 2.3–3.0 V (organic electrolytes). Multiple cells are placed in series to reach higher system voltages (tens to hundreds of volts) with balancing circuitry. As capacitance and system voltage increase, volume also increases.

Early supercapacitors with tens of farads were mainly used in high-power supplies and industrial modules. Today, compact low-voltage EDLCs and lithium-ion capacitors (LICs) appear in consumer and IoT devices for short-term backup, peak-power assist, real-time clocks, and energy harvesting buffers.

Although supercapacitors and electrolytic capacitors look similar, their electrical behavior differs substantially. For example, a common 10 µF, 25 V electrolytic capacitor can be similar in size to a 1–10 F, 2.7 V EDLC. With recent advances, multi-cell supercapacitor modules can achieve 16–160 V (and higher) with integrated protection and balancing, but each individual EDLC cell remains limited to a few volts.

Inside Look Of A Supercapacitor vs Electrolytic Capacitor

II Structure of Supercapacitors

In principle, a supercapacitor stores charge electrostatically and (for some types) through fast, reversible surface redox reactions. The most common type, the electric double-layer capacitor (EDLC), relies on an electronic double layer at the electrode–electrolyte interface—this is the key distinction from conventional electrolytic capacitors and the reason for its much higher capacitance per volume.

Figure 1. Structure of a Standard Capacitor

A standard capacitor uses a dielectric layer between two electrodes attached to metal plates (Fig. 1). Depending on type, the dielectric can be alumina (Al₂O₃), tantalum pentoxide (Ta₂O₅), barium titanate (BaTiO₃), polypropylene, polyester, etc. Capacitance depends on dielectric permittivity, plate area, and plate separation.

Figure 2. Capacity and Voltage Characteristics of Different Materials

In a single-layer dielectric structure, you can increase capacitance mainly by increasing area or decreasing separation. All such methods increase volume for a given voltage rating—there are practical limits on how thin the dielectric can be before breakdown.

An electric double-layer capacitor (EDLC) addresses this by using highly porous carbon electrodes (activated carbon, carbon nanotubes, graphene-like carbons) and an electrolyte (aqueous or organic). The immense surface area (often >1000–2000 m²/g) and nanometer-scale ion separation at the interface yield capacitance increases of several orders of magnitude compared with conventional capacitors of similar size. Conductive polymers and metal oxides can also be used in pseudocapacitors (see Section IV).

Figure 3. Structure of Supercapacitors

Voltage capability remains the key limitation of EDLC cells. Because the effective “dielectric” is a nanometer-scale ion layer that is not a true solid insulator, single-cell rated voltages are low (generally 2.3–3.0 V for organic electrolytes; aqueous cells are typically ≤1.6 V unless using advanced “water-in-salt” systems). Higher system voltages are achieved by series-connecting cells with active or passive balancing.

III Working Principle of Supercapacitors

A supercapacitor stores energy primarily through an electric double layer formed at the electrode–electrolyte interface due to Coulombic attraction (Fig. 4). In pseudocapacitors and hybrids, fast surface redox adds Faradaic charge storage.

Figure 4. Coulomb Force

Applying a potential across two porous electrodes in electrolyte draws counter-ions to each surface, forming double layers that behave like capacitors in series (one at each electrode surface).

For an ideal parallel-plate capacitor, the governing relationships are:

C = ε_r ε₀ A / d

E = ½ C (ΔV)²

where A is plate area and d is separation. EDLCs effectively maximize A via porous carbons and minimize the effective d at the Helmholtz layer, resulting in very high capacitance with very low equivalent series resistance (ESR).

Typical activated carbon electrodes provide >1200 m²/g surface area, and the interfacial charge separation distance is on the order of nanometers, allowing specific capacitances >100 F/g (electrode basis) and low ESR suitable for high peak currents.

IV Classification of Supercapacitors

Supercapacitors have evolved in structure, materials, and performance. Broadly, they are classified by working principle and by electrolyte.

1. Classification by Principle

(1) Electric Double-Layer Capacitors (EDLCs)

EDLCs store charge at the electrode/solution interface via ion adsorption without bulk chemical transformation. When a field is applied, anions and cations migrate to opposite electrodes and form stable double layers (Fig. 5). Removing the field leaves a stored potential that can deliver current when connected to a load.

Figure 5. Operating Principle in Electric Double-Layer Capacitors

(2) Pseudocapacitors (Faradaic Quasi-capacitors)

Proposed by Conway, pseudocapacitors add fast, reversible surface redox (e.g., UPD—underpotential deposition) to double-layer storage, yielding higher specific capacitance than pure EDLCs. Electroactive materials (conductive polymers, transition-metal oxides) contribute surface-confined Faradaic charge while retaining high power capability.

Figure 6. The Mechanism of Underpotential Deposition (UPD)

During discharge, ions return to the electrolyte through the reverse redox reaction and the stored charge is delivered to the external circuit. Hybrids such as lithium-ion capacitors (LICs) pair a capacitor-type cathode with a battery-type anode to raise energy density at some cost to power density.

2. Classified by Electrolyte

Supercapacitor electrolytes are typically aqueous (acidic, alkaline, or neutral) or organic (e.g., acetonitrile (ACN) or propylene carbonate (PC) with salts like TEABF₄ or LiPF₆). Aqueous systems have high ionic conductivity but lower cell voltage (water splitting), while organic systems allow higher cell voltages with lower conductivity. Recent “water-in-salt” aqueous electrolytes extend aqueous cell voltages toward ~2.2–2.5 V in specialized designs.

Figure 7. (a) CVs comparing aqueous vs. organic electrolytes; (b) Ragone plots illustrating energy/power trade-offs.

V Main Parameters and Performance of Supercapacitors

1. Main Parameters

(1) Service Life

Supercapacitor life is defined by capacitance and ESR drift over time/temperature/voltage. Cycle life commonly exceeds 500,000 charge–discharge cycles (and can reach into the millions) because storage is largely interfacial. Calendar life is strongly temperature- and voltage-dependent. End-of-life is usually specified at 20–30% increase in ESR and/or 20–30% loss of capacitance (not a fixed “63.2%” capacity threshold).

(2) Voltage

Each EDLC cell has a strict maximum rated voltage (often 2.3–3.0 V for organic systems). Over-voltage is destructive: electrolyte decomposition generates gas, increases ESR, causes swelling/venting, and shortens life. Operating near the recommended voltage and minimizing time at high voltage extends life. Never exceed the rated voltage of any cell; limit to ≤100% of rating unless the manufacturer specifies otherwise.

(3) Temperature

Typical operating ranges are −40 °C to +65 °C (some parts to +85 °C). Life follows an Arrhenius relationship: a common rule of thumb is every +10 °C roughly halves life (exact values are part-specific). At low temperature, ESR rises; some makers allow slightly higher operating voltage at low temperatures within datasheet limits, but this must follow manufacturer guidance.

(4) Discharge

For pulse loads, low ESR and peak current capability dominate; for small continuous loads, total capacitance and leakage current matter. Designers should evaluate voltage droop (ΔV = I·Δt / C) and ESR-related instantaneous sag (I·ESR).

(5) Charging

Common methods include constant-current (CC), constant-voltage (CV), or CC/CV blends. A series resistor or current-limited source prevents inrush and reduces stress. Reverse voltage must be avoided—EDLCs are polarized. For series strings, use passive resistive balancing or active balance ICs to prevent cell over-voltage due to parameter mismatch.

Figure 8. Constant Current & Constant Voltage Charging

2. Performance Characteristics

(1) Advantages

1) High Power Density

Very low ESR enables kW/kg power levels and rapid charge/discharge—far beyond batteries of similar size.

2) Long Cycle Life

No bulk phase change; cycle life commonly >500k with minimal degradation when used within ratings.

3) Short Charging Time

Minutes (or seconds) to reach target voltage with proper current limiting—ideal for energy buffering and peak shaving.

4) High Power with Moderate Energy

EDLC energy density is typically ~3–10 Wh/kg (modules); LICs can reach ~10–20+ Wh/kg, still far below Li-ion batteries (100–250+ Wh/kg), but with much higher power.

Figure 9. Power vs. Energy Density

5) Long Storage Life

With appropriate storage (cool, dry, partial voltage), calendar life can exceed 10 years. Leakage current is higher than film/electrolytic capacitors, so system design should account for it.

6) Wide Operating Temperature Range

Many parts operate from −40 °C to +65 °C (some to +85 °C) with predictable ESR change.

(2) Disadvantages

Despite strengths, supercapacitors also have limitations:

1) Leakage and Packaging Considerations

Poor mechanical placement or thermal stress can induce electrolyte venting or seal failure. Follow mounting guidelines to avoid mechanical load on terminals/can.

2) AC Usage Caveats

EDLCs are polarized and must not be reverse-biased. They are intended for DC and low-frequency applications; they are not drop-in replacements for AC line-rated capacitors. With proper DC bias and ripple limits, they can handle significant ripple current in DC systems.

Figure 10. An Aluminum Electrolytic Capacitor

3) Cost vs. Batteries (Energy Storage)

Cost per Wh remains higher than lithium-ion batteries; however, cost per kW can be favorable due to superior power capability and cycle life. System-level hybrids often yield the best economics.

VI The Identification of Supercapacitors

1. Identification of Supercapacitors and Batteries

(1) EDLCs based on double-layer theory do not rely on bulk electrochemical reactions. They can be discharged to near 0 V without damage (observe any minimum storage voltage in the datasheet). Shorting the terminals for storage is not generally recommended; follow the manufacturer’s storage instructions.

Batteries must not be shorted and are not intended to be discharged to 0 V; doing so may cause damage or safety issues.

(2) Reverse voltage is not allowed for EDLCs; they are polarized. Batteries are also polarity-sensitive, but some chemistries include protection circuits. Always respect polarity for both devices.

(3) The voltage–charge relationship of an ideal capacitor is linear (V ∝ Q). Battery voltage typically changes nonlinearly with state of charge and depends on load profile.

Batteries and Supercapacitors

2. Identification of Electric Double-Layer Supercapacitor and Electrochemical (Pseudocapacitor/Hybrid) Supercapacitor

Electrochemical (pseudocapacitor or hybrid) devices add Faradaic contribution, so their energy density can exceed that of pure EDLCs. EDLCs typically have lower ESR and extremely high power capability. Many pseudocapacitor and LIC cells should not be discharged to 0 V—observe the chemistry-specific lower voltage limit in the datasheet.

3. Identification of Supercapacitors in Aqueous and Organic Solutions

Organic EDLCs: single-cell ratings ~2.3–3.0 V (2.7 V is common). Higher energy density than conventional aqueous EDLCs; lower ionic conductivity; flammability considerations.

Aqueous EDLCs: typical single-cell ratings ≤1.6 V; very low ESR and excellent low-temperature performance; larger physical size for the same energy. Advanced “water-in-salt” can raise cell voltage in specialized designs.

Figure 11. Button Batteries

In general, organic systems offer higher energy density; aqueous systems excel in power and safety. ESR of modern EDLCs is very low in both families, but aqueous typically has the edge on absolute ESR.

VII Precautions for Use

Not all aspects are superior to batteries for every use case. Manufacturing and application constraints require careful design. Key precautions:

1. Supercapacitors are polarized; confirm polarity before use and prevent reverse bias (use diodes or bidirectional protection where needed).

2. Do not exceed rated voltage. Over-voltage causes electrolyte decomposition, heating, gas generation, capacitance loss, and ESR rise, shortening life or causing failure.

3. Avoid high-frequency charge/discharge without proper ripple current analysis. While EDLCs handle large ripple with DC bias, they are not AC line capacitors.

4. Keep away from heat sources; manage ambient temperature to extend life (use heatsinking/airflow where appropriate).

5. For backup-power use, account for ESR-related instantaneous voltage droop at load step. Size C and ESR to meet minimum hold-up voltage (see Fig. 12).

Figure 12. A Backup Supply Circuit Diagram

6. Avoid environments with >85% RH or corrosive gases; corrosion can damage leads and can.

7. Store at −30 to +50 °C and RH <60%, partially charged if specified; follow maker guidance for long-term storage voltage.

8. On double-sided PCBs, ensure clearances so the can/case cannot short exposed conductors. Observe creepage/clearance around terminals.

Figure 13. A Double-sided PCB Board

9. Do not let the can contact the PCB during soldering; heat can wick into the vent and degrade seals.

10. Do not tilt or twist after soldering; avoid mechanical stress on leads/terminals.

11. Avoid overheating during soldering (observe time/temperature profiles); excessive heat reduces life.

12. Clean flux residues; ionic residues can create leakage paths or corrosion.

13. Series strings require cell voltage balancing (resistive or active). Without it, cell-to-cell variation can over-voltage one or more cells, causing premature failure.

14. For any atypical application conditions, consult the manufacturer’s datasheet and application notes.

VIII Main Application Fields of Supercapacitors

1. Application in Solar Energy System

Solar PV systems—stand-alone, grid-tied, and hybrid—benefit from energy storage. Batteries provide energy capacity; supercapacitors provide high-power buffering (MPPT dynamics, inverter ride-through, and transient smoothing). EDLC banks can handle brief irradiance changes (cloud transients), reduce battery stress, and improve system reliability and lifetime.

Figure 14. The Basic Operation of a PV Cell

2. Application in Wind Power System

Wind power is inherently variable; fluctuations in the 0.01–1 Hz band degrade power quality. Short-term storage using supercapacitors smooths these fluctuations, stabilizes bus voltage, supports pitch/ yaw actuators, and reduces cycling on batteries. EDLCs also provide black-start support and grid-code fault-ride-through in modern converters.

Figure 15. Traditional Wind Power Generation

3. Supercapacitors in the Development of New Energy Vehicles

In EVs/HEVs, supercapacitors complement batteries: they supply bursts of power for start/acceleration and capture regenerative-braking energy efficiently, while batteries provide sustained energy. This hybridization improves fuel economy and extends battery life. 48-V mild hybrids, start-stop systems, electric buses/trams, and fuel-cell vehicles widely employ EDLC modules and, increasingly, LICs where higher energy is needed.

Figure 16. New Energy Vehicles

During deceleration, downhill travel, and braking, supercapacitors quickly recover energy. During launches and sudden accelerations, the EDLC provides peak power, reducing battery stress and improving overall system responsiveness and life.

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