Supercapacitors are my kryptonite

Supercapacitors are energy storage devices which lie in a sweet spot of performance between capacitors and batteries. Supercapacitors typically exhibit higher power density than batteries, but also have higher energy density than capacitors as illustrated in the Ragone plot in Fig.1. This makes them an exciting prospect for applications such as grid stabilization and transportation. Recently, Airbus reported using supercapacitors to operate their emergency doors. Cars such as Toyota Yaris (Hybrid-R) and PSA Peugeot Citroen have employed supercapacitors for their start-stop fuel-saving systems [1]. There are two main types of supercapacitors: electric double layer capacitors (EDLCs) and pseudocapacitors. The primary difference between them, and also between batteries and capacitors, is their energy storage mechanism which results in varied performance and applicability for different functions.

Types of Supercapacitors

1. Electric Double-Layer Capacitors (EDLC)

EDLCs are the closest to traditional capacitors which typically consist of two conductive plates separated by a dielectric. These both employ a non-faradaic energy storage mechanism to store charge electrostatically. Meaning, there is no charge transfer on the electrode surface, and no change in the oxidation state of the electrode materials. However, EDLCs differ from capacitors in the nature of dielectric used. While traditional capacitors typically use dielectrics such as ceramics, polymer films, paper, mica, air, and oxide layers [3], EDLCs contain electrolyte in the gap between the metal plates. As such, they are able to reach higher energy density by storing energy in the electrode-electrolyte interface rather than in a conventional dielectric.

How does this work?

When a voltage is applied, charge accumulates at the electrode-electrolyte interface and forms a thin compact layer of counterions against the electrode surface known as the Helmholtz layer. This layer with separated charges acts as the dielectric in EDLCs. Energy is stored in the field between the separated charges, and the cell is charged and discharged by movement of ions in the Helmholtz layer. Due to the kinetically favorable nature of this electrostatic energy storage mechanism, EDLCs have very high power density which can reach up to 10 kW/kg [4]. Although they have rapid charge and discharge rates, their biggest shortcoming in comparison to batteries is their low energy density which averages at about 30 Wh/kg [4].

2. Pseudocapacitors

Pseudocapacitors exhibit higher energy density than EDLCs while still maintaining higher power density than batteries. This is made possible through a faradaic energy storage mechanism which involves redox reactions. This mechanism is similar to what occurs in batteries, but is drastically different in the electrode material response. Battery intercalation involves solid-state phase transitions within the electrode materials, and as such, is typically limited by diffusion processes. In pseudocapacitors, electrode materials such as conducting polymers and transition metal-based hydroxides and oxides specifically designed with open crystal frameworks are used. Short diffusion lengths and facile ion migration channels which do not undergo phase transitions from intercalation also play a part [5]. This offers fast and reversible ion insertion into the bulk electrode without phase transitions at a rate similar to surface redox reactions. Not only does this energy storage mechanism impact power and energy density, it also improves the cycle life compared to batteries as the electrode material is more stable during intercalation.  

Improving Supercapacitor Performance

Despite having superior energy density to capacitors and power density to batteries, there is room for further performance improvement in supercapacitors. Specific energy density is defined by: E = ½ CV². Increasing energy density can thus be achieved by increasing capacitance or voltage. 

1. Capacitance: Specific capacity is increased by increasing the amount of charge stored on the electrode surface which is based on the sites available. As such, improvements to capacitance mostly lie in electrode development. The preferred electrode material for ELDCs is carbon, which not only has high surface area and porosity, but additionally, chemical stability and good electronic conductivity [6]. Advances in carbon materials include hierarchical porous carbon materials (HPCMs) and doping with heteroatoms (e.g., N, O) to improve capacitance [7].

2. Voltage: Because energy scales quadratically with voltage, increasing the voltage has a larger impact on energy. However, the maximum voltage of a supercapacitor is limited by the electrolyte’s electrochemical stability window. More electrochemically stable electrolytes would enable higher voltages, and as such, higher energy density [8]. Aqueous electrolytes have a narrow voltage range around 1.23V; nevertheless, ionic liquids or organic electrolytes can be employed to expand the voltage window. It has been shown that supercapacitors using organic electrolytes can achieve a stable voltage in the range of 2.5–4.0 V [1]. However, problems associated with using organic electrolytes include relatively high equivalent series resistance, low ion diffusion rate, toxicity, and fire hazards [1]. Ionic liquids also enable higher voltages, but tend to be more viscous and less conductive which can result in poor rate performance [9]. 

3. Hybrids: Lots of development is being done on supercapacitors which combine the energy storage mechanisms of both EDLCs and pseudocapacitance in hybrid or asymmetric configurations. These use a carbon electrode to supply high power density and a metal oxide electrode for high energy density.  These hybrids are thus able to leverage the benefits from both types of supercapacitors.

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Sources :

[1] D. Pandey, K. S. Kumar, and J. Thomas, “Supercapacitor electrode energetics and mechanism of operation: Uncovering the voltage window,” Prog. Mater. Sci., vol. 141, p. 101219, Mar. 2024, doi: 10.1016/j.pmatsci.2023.101219.

[2] J. Castro-Gutiérrez, A. Celzard, and V. Fierro, “Energy Storage in Supercapacitors: Focus on Tannin-Derived Carbon Electrodes,” Front. Mater., vol. 7, Jul. 2020, doi: 10.3389/fmats.2020.00217.

[3] J. Cook, “Dielectric Capacitors: Use of Dielectric in Capacitors,” May 20, 2019. [Online]. Available: https://www.arrow.com/en/research-and-events/articles/dielectric-capacitors-explained-capacitor-dielectric-types

[4] D. Meena, R. Kumar, S. Gupta, O. Khan, D. Gupta, and M. Singh, “Energy storage in the 21st century: A comprehensive review on factors enhancing the next-generation supercapacitor mechanisms,” J. Energy Storage, vol. 72, p. 109323, Nov. 2023, doi: 10.1016/j.est.2023.109323.

[5] Y.-M. Wei, K. D. Kumar, L. Zhang, and J.-F. Li, “Pseudocapacitive materials for energy storage: properties, mechanisms, and applications in supercapacitors and batteries,” Front. Chem., vol. 13, Jun. 2025, doi: 10.3389/fchem.2025.1636683.

[6] Q. Zhou and H. Yao, “Recent development of carbon electrode materials for electrochemical supercapacitors,” Energy Rep., vol. 8, pp. 656–661, Nov. 2022, doi: 10.1016/j.egyr.2022.09.167.

[7] A. Hayat et al., “Recent advances in heteroatom-doped/hierarchical porous carbon materials: Synthesis, design and potential applications,” Prog. Mater. Sci., vol. 150, p. 101408, Apr. 2025, doi: 10.1016/j.pmatsci.2024.101408.

[8] P. Bhojane, “Recent advances and fundamentals of Pseudocapacitors: Materials, mechanism, and its understanding,” J. Energy Storage, vol. 45, p. 103654, Jan. 2022, doi: 10.1016/j.est.2021.103654.

[9] M. S. Arsha and B. V, “A 3.5 V Supercapacitor with Ultrahigh Energy and Power Capabilities using Thermally Deoxygenated Graphite Oxide Electrodes and Water-in-Salt Electrolyte,” Energy Fuels, vol. 38, no. 19, pp. 19076–19087, Oct. 2024, doi: 10.1021/acs.energyfuels.4c02984.