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Supercapacitor Designs and Materials


High power density bi-polar designs coupled with improved electrolytes and proprietary technology to substantially reduce internal resistance result in electric double layer capacitors (EDLC or supercapacitors) that achieve new levels of reliability and performance in power density and on-demand power delivery.

Electrochemical double layer capacitors (EDLC), or supercapacitors, are finding increased application as rugged, reliable and long-lived energy storage devices. EDLC’s can be used in everything from small portable DC power supplies to large installations ensuring power quality on AC electrical grids.

These supercapacitors have several advantages as energy storage devices compared to both batteries and conventional capacitors. They are capable of delivering more power than similarly sized batteries and store more energy than conventional capacitors. In addition, EDLC’s can be charged much faster than batteries and are capable of hundreds of thousands of charge - discharge cycles, as compared to hundreds of cycles for most batteries. 

As shown in the diagram to the left, EDLC devices store energy in the form of separated charge, specifically as an electric field (double layer) created between electrodes with a porous material such as activated carbon. 

EDLC power density is a function of the equivalent internal resistance (ESR). Lower ESR means a smaller RC time constant and more power (energy delivered per unit time). Using designs and materials that reduce internal resistance can greatly increase EDLC power and efficiency.

Electrolyte systems also play an important role in determining EDLC performance. In carbon-based EDLC’s, electrolytes can be either aprotic organic systems with quaternary salts as charge carriers, or aqueous systems such as 30% sulfuric acid. Organic systems are characterized by higher resistance (ca. 40 cm at 25˚C (77F)) and higher working voltage (ca 2.5 V), while aqueous systems operate at lower voltage (ca. 1.2V) and lower resistance (ca 1.5 cm at 25˚C (77F)).

Higher voltages are desirable because maximum power of an EDLC increases as the square of the output voltage. However, the higher conductivity of aqueous electrolytes means that aqueous systems can deliver more power for comparable sized units, especially if bipolar electrode designs are employed.

In both conventional EDLC and bipolar EDLC’s electrode design is very important. The total internal surface area of the electrode material determines total capacitance of the unit.  The thicknesses of the electrode, as well as the electrode-to-current collector contact resistance are important factors in determining ESR.

Enerize has proprietary technology to reduce interface resistance between the current collector and electrode material, as well as proprietary treatments for carbon materials used in electrodes to reduce ESR and increase the power of EDLC’s.

Bipolar Electrode EDLC Designs

A recent advance in EDLC design is the use of bipolar electrodes. In bipolar designs, there are no separate cells as individual assemblies within a unit or module. As shown in the diagram below, the electrode of one cell is connected with the electrode of opposite polarity of the next cell by a means that is impermeable for electrolytes, but is an electrical conductor. The electrode and separator pairs must be sealed to prevent leakage current.

Thus, a set of cells in series does not require external circuits to connect one to the next. The cells are connected internally. In such a design, the effective cross section of the conducting surface is equal to the square of that for an individual electrode. This enables the bipolar module to deliver thousands amperes of current without overheating or significant voltage drop.

 Bipolar designs allow higher voltage modules to be constructed within a single case rather than wiring together discrete EDLC cells in a series-parallel configuration to achieve the required voltage and current ratings. As shown in the Table below, bipolar EDLC’s modules have higher power densities than conventional EDLC multi-cell circuit.

In addition to research aimed at increasing supercapacitor performance, Inter-Intel has partnered with another leading design and manufacturing house to develop and market advanced bipolar electrode supercapacitors that are engineered for specific applications such as wind turbine pitch control, power quality, etc.

One such device, providing 18 kJoule at 200V, is shown here. Bi-polar electrode devices require fewer components than conventional EDLC devices to store a given amount of energy. Bipolar electrode devices therefore have high power density and do not require as much external circuitry to control voltage leveling.

Comparative Performance of Supercapacitors Having Conventional and Bipolar Designs

The table below shows a comparison of high-voltage bipolar supercapacitor system with aqueous electrolyte and a conventional high-voltage system with non-aqueous electrolyte. The two units have approximately the same weight. The conventional system is comprised of many separate cells that are configured to provide the required voltage (350 Volts) and initial discharge current (157 amps) for a power rating of 55kW. 

In the bipolar design, the system is comprised of just a few high voltage modules and not cells per se.   The bipolar module was selected to provide the same voltage and initial discharge current. As shown in the table however, the bipolar module has a higher power density, greater delivered energy, and is substantially more efficient than the conventional system.

Table 1: Comparative Performance Data for Conventional and Bipolar Electrode Supercapacitors

PARAMETER

Conventional system with non
aqueous electrolyte
159 caps in series, 4 strings in parallel

Bipolar system with aqueous electrolyte
 modules in series

1. Operating voltage

                      320 V

2. End of discharge voltage

                     175 V

3. Capacity

2.52 F

1.03 F

4. ESR

516 mW

310 mW

5. Stored energy

154 kJ

63 kJ

*6. Delivered energy (55 kW discharge)

5.6 kJ

19 kJ

*7. Discharge time (55 kW) to ½ V

0.1 sec

0.35 sec

8. Initial discharge current

                       157 A

9. End of discharge current (0.5 U nom)

                        314 A

*10. Weight

25.4 kg

26.3 kg

*11. Energy density (delivered)

0.224 kJ/kg

0.722 kJ/kg

*12. Power density max (V2/4 ESR)

2.33 kW/kg

3.75 kW/kg

*13. Energy efficiency, 55 kW (delivered/stored *100%)

3.64 %

30.2 %

Note: The weight of casing for non-aqueous system was not taken into account for these calculations.

From the above table, it is evident that bipolar systems have advantages in specific power and “delivered” specific energy as compared to the conventional systems with non-aqueous electrolyte and circuit of separate cells at up to ~ 1 sec of discharge.

This comparison confirms that EDLC designs featuring bipolar electrodes and non-aqueous electrolyte can have dramatically increased efficiency compared to the current EDLC’s with non-aqueous electrolyte. 

Enerize stands ready to develop and deliver ECLD solutions for your specific applications. As shown above our bipolar electrode units have higher power densities than conventional EDLC units and can be adapted to form factors that will meet most application requirements.

 

 

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