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Chapter 4 -- Stability in Potentiostat Mode -- Capacitive Cells and Stability
Chapter 4 -- Stability in Potentiostat Mode
Capacitive Cells and Stability
All potentiostats can become unstable when connected to capacitive cells. The capacitive cell adds phase shift
to the potentiostat's feedback signal (which is already phase shifted). The additional phase shift can convert the
potentiostat's power amplifier into a power oscillator.
To make matters worse, almost all electrochemical cells are capacitive because an electrical double layer forms
next to a conductor immersed in a solution.
Potentiostat oscillation is an AC phenomenon. However, it can affect both AC and DC measurements.
Oscillation often causes excessive noise or sharp DC shifts in the system's graphical output. The PCI4
Potentiostat is often stable on less sensitive current ranges and unstable on more sensitive current ranges.
Whenever you see sharp breaks in the current recorded on the system, you should suspect oscillation.
The PCI4 has been tested for stability with cell capacitors between 10 pF and 0.1 F. In all but its fastest control
amp speed setting, it is stable on any capacitor in this range -- as long as the impedance in the reference
electrode lead does not exceed 20 kΩ. With reference electrode impedances greater than 20 kΩ, the PCI4
may oscillate. The RC filter formed by the reference electrode impedance and the reference terminal's input
capacitance filters out the high frequency feedback needed for potentiostat stability.
Longer cell cables make the problem worse by increasing the reference terminal's effective input capacitance.
Even when the system is stable (not oscillating), it may exhibit ringing whenever there is a voltage step applied
to the cell. The PCI4's D/A converters routinely apply steps, even when making a pseudo-linear ramp. While
this ringing is not a problem with slow DC measurements, it can interfere with faster measurements. The steps
taken to eliminate potentiostat oscillation also help to minimize ringing.
4-1
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