Supercapacitors are categorized into three distinct types: Contact online >>
Supercapacitors are categorized into three distinct types:
What is a supercapacitor and how does it work?
Figure 1: Illustration of double layer formation and electric potential distribution in one electrode of a charged supercapacitor. (Image is in the public domain)
The double-layer mechanism implies that the amount of charge stored per unit voltage (i.e., the capacitance) is proportional to the interfacial area of the electrodes. Thus, supercapacitor electrodes tend to be made from extremely porous materials whose surface areas can be in excess of 1000 m2 (about one-fifth the size of a football field) per gram of material.
*Some specially designed supercapacitors, called pseudocapacitors, may also store charge via rapid redox reactions that do not result in any phase transition of the electrode material. (See page on pseudocapacitance.) Pseudocapacitors offer energy densities around 30 Wh/kg, better than supercapacitors based only on double-layer capacitance but still much less than lithium-ion batteries.[3]Figure 2: Plot of gravimetric (mass-based) power density versus energy density for various capacitor and battery technologies. Source: Wikimedia Commons, user Shaddim (CC by-SA 3.0)
Because of their robustness and fast mechanism for storing and releasing charge, supercapacitors are ideally suited for applications that require high power (a large current delivered in a short time) over many cycles. As one example, they can recover energy from the repetitive braking of automobiles or trains, which would normally be wasted as heat.[4,5] Other examples are cold startup of large diesel engines, radar pulses, and energy storage for solar-powered LED streetlights.[6–8]
Supercapacitors can also act as a power buffer, mitigating the effects of high peaks and brief interruptions of current. They have been used as backup power supplies for computer memory modules and pitch control system in wind turbine actuators.[9,10] In addition, they have been proposed as buffers for the high pulse power that is drawn by some electric vehicles while charging, which would otherwise reduce the efficiency of the grid.[11]
[1] J. Bird, Electrical Circuit Theory and Technology, Routledge, 2014.
[2] A.K. Singh, D. Sarkar, G.G. Khan, K. Mandal, Unique hydrogenated Ni/NiO core/shell 1D nano-heterostructures with superior electrochemical performance as supercapacitors, Journal of Materials Chemistry A. 1 (2013) 12759. doi:10.1039/c3ta12736b.
[3] H.D. Abruna, Y. Kiya, J.C. Henderson, Batteries and electrochemical capacitors, Physics Today. 61 (2008) 43–47. doi:10.1063/1.3047681.
[4] P. Weissler, Mazda introduces supercapacitor-type regenerative braking – SAE International, (2013). (accessed June 15, 2016).
[6] H. Liu, Z. Wang, S. Qiao, Y. Liu, Improvement of engine cold start capability using supercapacitor and lead-acid battery hybrid, in: Twenty-Third Annual IEEE Applied Power Electronics Conference and Exposition, 2008. APEC 2008, 2008: pp. 668–675. doi:10.1109/APEC.2008.4522793.
[7] D. Terrones, Tecate Group – Markets & Applications. – Military/Aerospace – Radar, (n.d.). (accessed June 15, 2016).
[8] Nippon Chemi-Con Corporation, Nippon Chemi-Con, Stanley Electric and Tamura announce: Development of “Super CaLeCS”, an environment-friendly EDLC-powered LED Street Lamp, (2010). (accessed June 15, 2016).
[9] Cooper Bussmann, 5.5V Supercapacitors for Hold-Up & Memory Back-Up Applications, (2007). (accessed June 15, 2016).
[10] Maxwell Technologies | Wind Pitch Control Solutions, Maxwell Technologies. (n.d.). (accessed June 15, 2016).
[11] M. Farhadi, O.A. Mohammed, Real-Time Operation and Harmonic Analysis of Isolated and Non-Isolated Hybrid DC Microgrid, IEEE Transactions on Industry Applications. 50 (2014) 2900–2909. doi:10.1109/TIA.2014.2298556.
Pseudocapacitance
In certain electrode / electrolyte systems at certain applied potentials, some ions may actually participate in reversible redox reactions at or near the electrode surface. Unlike lithium intercalation in lithium-ion batteries, these redox processes are inherently fast and do not result in phase transformation of the electrode material. This represents a second mechanism for capacitive energy storage, known as pseudocapacitance. In many cases, pseudocapacitance offers much higher energy density compared to double-layer capacitance. Two major types of pseudocapacitance are used in supercapacitor technology:
Figure 1: Illustration of redox pseudocapacitance. Source: [1]
Experimentally, electrodes made of hydrous ruthenium oxide mixed with amorphous carbon and coated on graphite have achieved a specific capacitance as high as 1340 F/g at a voltage sweep rate of 25 mV/s, very close to the theoretical capacitance value.[3]
Figure 2: Illustration of intercalation pseudocapacitance. Source: [1]
One example is illustrated with the orthorhombic polymorphs of Nb2O5 (T-Nb2O5) and V2O5, both of which form layered sheet structures.[4–6] When a lithium-based electrolyte is used, Li+ ions can diffuse rapidly through the interlayer voids and bind reversibly to the oxide, losing or gaining an electron in the process.
In the case of T-Nb2O5, the maximum amount of lithium that can be accommodated is x = 2. This corresponds to a theoretical capacity of ~200 mAh/g; 130 mAh/g (65% of theoretical) has been attained experimentally at a rate of 10C (meaning that the current would discharge or charge the entire theoretical capacity in one-tenth of an hour or 6 minutes).[4] In comparison, most commercial lithium-ion batteries cannot be charged at a rate over 3C (discharging / charging the theoretical capacity in one-third of an hour) without significant reduction in capacity and service life.
[1] V. Augustyn, P. Simon, B. Dunn, Pseudocapacitive oxide materials for high-rate electrochemical energy storage, Energy & Environmental Science. 7 (2014) 1597. doi:10.1039/c3ee44164d.
[2] W. Dmowski, T. Egami, K.E. Swider-Lyons, C.T. Love, D.R. Rolison, Local Atomic Structure and Conduction Mechanism of Nanocrystalline Hydrous RuO2 from X-ray Scattering, J. Phys. Chem. B. 106 (2002) 12677–12683. doi:10.1021/jp026228l.
[3] C.-C. Hu, W.-C. Chen, K.-H. Chang, How to Achieve Maximum Utilization of Hydrous Ruthenium Oxide for Supercapacitors, J. Electrochem. Soc. 151 (2004) A281–A290. doi:10.1149/1.1639020.
[4] V. Augustyn, J. Come, M.A. Lowe, J.W. Kim, P.-L. Taberna, S.H. Tolbert, H.D. Abruña, P. Simon, B. Dunn, High-rate electrochemical energy storage through Li+ intercalation pseudocapacitance, Nat Mater. 12 (2013) 518–522. doi:10.1038/nmat3601.
[5] M. Liu, C. Yan, Y. Zhang, Fabrication of Nb2O5 Nanosheets for High-rate Lithium Ion Storage Applications, Scientific Reports. 5 (2015) 8326. doi:10.1038/srep08326.
[6] Z. Tong, H. Xu, G. Liu, J. Zhao, Y. Li, Pseudocapacitive effect and Li+ diffusion coefficient in three-dimensionally ordered macroporous vanadium oxide for energy storage, Electrochemistry Communications. 69 (2016) 46–49. doi:10.1016/j.elecom.2016.05.017.
Supercapacitor Materials and Construction
Other materials that have been investigated for double-layer capacitance are vertically aligned carbon nanotubes,[2] porous 3D assemblies of graphene sheets, and carbon aerogels derived from organic polymers. Like activated carbon, these forms of carbon have very high specific surface area and electrical conductivity to maximize charge storage and rate capability. Vertical alignment of the carbon nanotubes, in particular, is hypothesized to create better conduction pathways for electrons and diffusion pathways for ions in the electrolyte.
Supercapacitors relying on pseudocapacitance as the primary charge storage mechanism tend to be based on transition metal oxides, such as RuO2, MnO2, Nb2O5, V2O5, and Fe3O4. These can be synthesized by a variety of methods, including sol-gel, co-precipitation, and hydrothermal technique, and may be processed into composites with the forms of carbon described above to improve electron transport properties as well as add double-layer capacitance.
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