Stability in Potentiostat Mode – Capacitive Cells and Stability
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Chapter 8: 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 Interface 1010
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 Interface 1010 has been tested for stability with cell capacitors between 10 pF and 3000 F. In all but its
fastest control amplifier 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 k. With reference-electrode impedances greater than 20
k the Interface 1010 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 Interface 1010’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.
Improving Potentiostat Stability
There are a number of things that you can do to improve an unstable or marginally stable Interface 1010
potentiostat/cell system. This list is not in any particular order; any or all of these steps may help.
• Slow down the potentiostat. The Interface 1010 has five control-amplifier speed settings, which can be
selected in software. Slower settings are generally more stable.
• Increase the Interface 1010 I/E stability setting. The Interface 1010 includes three capacitors that can
be placed in parallel with its I/E converter resistors. These capacitors are connected to relays that are
under software control. Contact your local Gamry Instruments representative for more information
concerning changes in these settings.
• Lower the reference electrode’s impedance. Make sure that you do not have a clogged reference-
electrode junction. Avoid asbestos-fiber reference electrodes and double-junction electrodes. Avoid
small-diameter Luggin capillaries. If you do have a Luggin capillary, make sure that the capillary’s
contents are as conductive as possible.
• Add a capacitively-coupled low-impedance reference element in parallel with your existing reference
electrode. The classic fast combination reference electrode is a platinum wire and a junction-isolated
SCE, see Figure 8-1) The capacitor ensures that the DC potential comes from the SCE and AC potential
from the platinum wire. The capacitor value is generally determined by trial and error.
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