Chapter 8 -- Stability In Potentiostat Mode; Capacitive Cells And Stability - Gamry Reference 3000 Operator's Manual

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Chapter 8 -- Stability in Potentiostat Mode--Capacitive Cells and Stability

Chapter 8 -- Stability in Potentiostat Mode

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 Reference

3000 Potentiostat can be 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 Reference 3000 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 Reference 3000 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 Reference 3000'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.

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