Capacitance of electrochemical double layer capacitors

The general aim of the present study was to conduct phenomenological investigation of EDLCs capacitance ‘stability’ under direct current conditions. Commercial EDLCs and laboratory test-devices were tested from the point of view of their ‘capacitance’ stability. Changes of potential with time during EDLCs galvanostatic charging/discharging (constant current I) had linear character, typical of capacitors. Values of I(ΔU/Δt)−1 should be the same at each time, indicating the device capacitance C. However, analysis of galvanostatic charging/discharging data showed that I(ΔU/Δt)−1 values were not constant during the process. Current flowing through the circuit during EDLC cyclic voltammetry experiments, may be almost constant, but I(ΔU/Δt)−1 (pointing to the capacitance value) changed significantly. When a capacitor was discharged through a resistor, the following function of time t, resistance R and potential U should be constant: t/R ln(U0/U) and equal device capacitance C. However, t/R ln(U0/U) values calculated from discharge curves were not constant. Changes were considerably high at the beginning of the discharge. The substantial instability of ‘capacitance’ value during the operation of EDLCs suggests that it can hardly be used as the proportionality constant between energy accumulated by an EDLC and the voltage in the second power.