Measurement of Supercapacitors
Supercapacitors, also known as ultracapacitors or electrochemical capacitors, are promising energy storage devices that bridge the gap between electrolytic capacitors and batteries. Thanks to their high power and long lifespan, supercapacitors are widely used in applications ranging from SRAM to high-speed trains. The key to such high performance is often a low equivalent series resistance (ESR), which reduces the energy wasted via Ohmic heating during charging and discharging cycles. Characterization of the supercapacitor impedance at different frequencies is thus important to determine the ESR and other critical device parameters. At low frequencies, such characterization sets a unique measurement challenge that requires an instrument capable of measuring current and voltage directly and of measuring impedance with high accuracy.
Traditionally, the galvanostatic charging/discharging method gives the ESR as a static value independent of frequency and test voltage, and the result can thus differ significantly depending on the operating conditions. Further, if the used current is high, irreversible electrochemical processes may greatly shorten the lifespan of the device.
With the Zurich Instruments MFIA Impedance Analyzer, it is possible to avoid the above drawbacks by measuring supercapacitors over a wide frequency range, from 1 mHz to 5 MHz, with a small AC test signal. In the four-terminal configuration shown in Figure 1, voltage and current on the device are monitored simultaneously and can be displayed together with calculated impedance parameters in the Sweeper module of the LabOne instrument control software.
Switching visualization between Bode and Nyquist plots (see Figure 2) helps to determine other useful parameters in addition to the device's capacitance: these include the ESR, the charge transfer resistance and the Warburg elements with their corresponding frequencies. The ESR is an indicator of the 'health status' of the device as a whole.
The charge transfer resistance and the Warburg elements relate to the interface between the electrode and the electrolyte, and are especially relevant to electrochemists who study the device's materials on the microscale. Thanks to the time-domain LabOne Plotter module (see Figure 3) and the configurable LabOne APIs, these parameters can be swiftly streamed to the computer and monitored over the device's lifespan.
The Benefits of Choosing Zurich Instruments
- Minimize the measurement time by probing two frequencies simultaneously, or by recording all relevant impedance parameters in a single sweep.
- Automate your measurements easily and record test parameters as a function of frequency, test signal amplitude and bias offset with the LabOne software and its APIs.