Supercapacitors: A Comparative Analysis

Transcription

1 Supercapacitors: A Comparative Analysis Authors: Sneha Lele, Ph.D., Ashish Arora, M.S.E.E., P.E. Introduction Batteries, fuel cells, capacitors and supercapacitors are all examples of energy storage devices. Unlike batteries and fuel cells that rely upon the conversion of chemical energy into electrical energy, capacitors rely upon the physical separation of electrical charge across a dielectric medium such as a polymer film or an oxide layer. Each type of energy storage device provides a different combination of power and energy density ( Figure 1). Figure 1: Specific power vs. specific energy for selected energy storage devices 1

2 At a high level, supercapacitors (also called as Electrochemical Double Layer Capacitors or EDLCs) are one of the highest-capacity capacitors. Table 1 provides a comparison of selected properties of various energy storage devices. Unlike conventional batteries, supercapacitors have a low energy density which makes them unsuitable for use as a continuous power source. However, supercapacitors have several advantages including the ability to handle high load currents, wide operating temperature ranges, long lifetimes and the ability to be charged almost instantly when compared to batteries. The combination of these advantages make supercapacitors suitable for use in power management applications requiring rapid charge/discharge cycles for short term power needs. Supercapacitors are often found in applications ranging from backup power systems, consumer electronics, medical devices, regenerative braking systems etc. [1]. Table 1: Comparison of selected properties of various energy storage devices 2 A common application of supercapacitors is their use in combination with batteries. The supercapacitors in these applications function to relieve the battery of the most severe load current demands of the application by meeting the peak power requirements and allowing the battery to supply the average load. This reduction in the peak load draw from the battery helps significantly extend battery life.

3 Principal of Operation Supercapacitors store charge in a similar way to conventional capacitors, but the charge does not accumulate in two conductors, but in the interface between the surface of a conductor and an electrolytic solution. They incorporate electrodes with much higher surface areas and much thinner dielectrics that decrease the distance between the electrodes thus increasing the capacity. Simply put, when connected to a power source, positive and negative electrolyte ions get stuck to the surface of the electrodes to charge the capacitor, and bounce off during discharge. Figure 2. Principle of operation of a supercapacitor 3 Figure 2 shows a basic capacitor construction with a separator between two metal electrodes. When charging, the negative electrode gains a negative charge, and the other end (the positive electrode) gains a positive charge. The permeable insulator in between physically separates the two electrodes, and is inert in nature to maintain the electrolyte s conductivity. When connected to a reverse voltage, the path of the electrons reverses to allow the capacitor to discharge [12]. A typical supercapacitor is constructed with two metal foils each coated with an electrode material such as activated carbon. These electrodes are separated by an ion-permeable insulator to protect the electrodes against short circuits. The assembly is then soaked with an electrolyte, which serves as the conductive connection between the electrodes across the separator. This construction is subsequently rolled or folded into a cylindrical shape and is stacked in an aluminum can. Figure 3 shows the construction of a 5 Farad off-the-shelf supercapacitor.

4 Figure 3. Disassembly of a supercapacitor Typical Characteristics Supercapacitors have advantages in applications where a large amount of power is needed for a relatively short time, or where a very high number of charge/discharge cycles or a longer lifetime is required. Some of the characteristics to consider when selecting a supercapacitor, based on the application needs, are as follows: 1. Operating voltage 2. Energy capacity 3. Specific energy and specific power 4. Lifetime 5. Capacitance 6. Operating temperature voltage 7. Self-discharge rate, etc.

5 Standards Some of the standards used to evaluate supercapacitors include [11]: UL 810A (Standard for Electrochemical Capacitors) IEC (Electric double-layer capacitors for use in hybrid electric vehicles - Test methods for electrical characteristics) IEC (Fixed electric double-layer capacitors for use in electronic equipment) BS/EN (Railway applications. Rolling stock equipment. Capacitors for power electronics. Electric double-layer capacitors) Cycle Life Experiments Performed In this section, we discuss the findings of our experiment which was focused on the effects of operating a supercapacitor outside its specifications. Two commercially produced supercapacitors were evaluated to study the effect of capacitor charge voltage on capacity degradation. The capacitors used for this experiment had a nominal capacitance rating of 5 F and were rated for a charge voltage of 2.7 V per the manufacturer s datasheet. Parameter Specification Capacitance 5 Farads Nominal Rated Voltage 2.7 V Operating Temperature Range -40ºC to 70ºC Table 2. Specifications of tested capacitor per datasheet During the experiments performed at room temperature, the two capacitor were subjected to 1000 charge-discharge cycles. While the discharge profile for the two capacitors was the same, the charge profile for one of the capacitors involved charging it its rated voltage (2.7 V) while the other capacitor was over-charged to a voltage exceeding its rating (3.5 V) during each charge cycle. Figure 4 and Figure 5 show the charge and discharge profile used for the two tested capacitors.

6 Figure 4. Charge-discharge cycles for rated capacitor Figure 5. Charge-discharge cycles for stressed capacitor

7 Figure 6 and Figure 7 show the loss of capacity over 500 charge-discharge cycles for the two capacitors. Figure 6 Capacity of rated supercap over 500 charge-discharge cycles Figure 7 Capacity of stressed supercap over 500 charge-discharge cycles

8 Operation Outside Specifications The supercapacitor s voltage is limited due to the decomposition voltage of the electrolyte. Charging the supercapacitor above its rated voltage results in an acceleration of electrochemical reactions because of the presence of impurities in the electrodes and in the electrolyte, moisture ingress etc. This accelerates the degradation of the electrolyte. The goal of the experiment was to demonstrate that operating a supercap with a voltage exceeding its rated voltage impacts its degradation and as a result its lifetime. After 1000 charge/discharge cycles, the capacity of the stressed (over-charged) supercapacitor had degraded by approximately 37% whereas the capacity of the supercapacitor cycled at its rated voltage had degraded by only approximately 6% ( Table 3). Initial capacity Post-cycling capacity Capacity loss (%) (mah) (mah) Rated capacitor % Stressed capacitor % Table 3. Capacity degradation over time A disassembly and visual inspection of the two capacitors demonstrated the effects of over-charging a supercapacitor. Figure 8. Stressed capacitor (left) vs capacitor operated within specifications (right)

9 Figure 9. Stressed capacitor (left) vs capacitor operated within specifications (right) Figure 10 Stressed capacitor (top) vs capacitor operated within specifications (bottom) Summary The many advantages of supercapacitors make them an attractive component for use in energy storage application. However, it is critical to ensure that the supercapacitors are operated within their specifications. Operating the supercapacitors outside their rated specifications can very quickly eliminate any advantages that the supercapacitors bring to an application. References: 1. Maria Guerra, Can Supercapacitors Surpass Batteries for Energy Storage? in Electronic Design. 2 R. Kötz, P.W. Ruch, D. Cericola, Aging and failure mode of electrochemical double layer capacitors during accelerated constant load tests, In Journal of Power Sources, Volume 195, Issue 3, 2010, Pages , ISSN Murata Supercapacitor Technical Note, No. C2M1CXS-053L, murata 4 Winter, Martin and Brodd, Ralph J., What Are Batteries, Fuel Cells, and Supercapacitors, Chemical Reviews, Volume 104, Number 10, Issue 2004, Pages

10 5 A. K. Shukla, S. Sampath and K. Vijayamohanan, Electrochemical supercapacitors: Energy storage beyond batteries, Current Science, Vol. 79, No. 12 (25 December 2000), pp S. Pay and Y. Baghzouz, "Effectiveness of battery-supercapacitor combination in electric vehicles," 2003 IEEE Bologna Power Tech Conference Proceedings, 2003, pp. 6 pp. Vol.3. 7 Vlasta Sedlakova, Josef Sikula, Juraj Valsa, Jiri Majzner and Petr Dvorak, Supercapacitor charge and self-discharge analysis, September 2013, ESA/ESTEC, Noordwijk, The Netherlands 8 Extending Battery Life in Transportation and Mobile Applications with Supercapacitor Technology, Eaton 9 A. Hammar, P. Venet, R. Lallemand, G. Coquery and G. Rojat, "Study of Accelerated Aging of Supercapacitors for Transport Applications," in IEEE Transactions on Industrial Electronics, vol. 57, no. 12, pp , Dec P. Kreczanik, P. Venet, A. Hijazi and G. Clerc, "Study of Supercapacitor Aging and Lifetime Estimation According to Voltage, Temperature, and RMS Current," in IEEE Transactions on Industrial Electronics, vol. 61, no. 9, pp , Sept Maria Guerra, Can Supercapacitors Surpass Batteries for Energy Storage? in Electronic Design. 12 Marin S. Halper, James C. Ellenbogen, Supercapacitors: A Brief Overview, Technical note no. MP 05W by MITRE Nanosystems Group. 1 xx.com/wp content/uploads/2015/04/ragone Plot All Energy Devices colour blocks.png 2 xx.com/resource/energy storage technologies/ %20Supercapacitors:%20A%20Brief%20Overview.pdf