Stability in Potentiostat Mode – Capacitive Cells and Stability
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Chapter 9:
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 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 k
. With reference-electrode impedances greater than 20 k
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.
Improving Potentiostat Stability
There are a number of things that you can do to improve an unstable or marginally stable Reference 3000
potentiostat/cell system. This list is not in any particular order. Any or all of these steps may help.
Slow down the potentiostat. The Reference 3000 has five control-amplifier speed settings, which you
select in the software. Slower settings are generally more stable.
Increase the Reference 3000’s I/E stability setting. The Reference 3000 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 impedance. Make sure that you don’t 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 9-1. The capacitor insures that DC potential comes from the SCE, and AC potential
from the platinum wire. The capacitor value is generally determined by trial and error.
