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Chapter 7: 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
600+ 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 600+ has been tested for stability with cell capacitors between 10 pF and 0.1 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 kΩ. With reference-electrode impedances
greater than 20 kΩ the Reference 600+ 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 600+'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 600+
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 600+ has five control-amplifier speed settings, which can
be selected in software. Slower settings are generally more stable.
•
Increase the Reference 600+'s I/E stability setting. The Reference 600+ includes three capacitors that
can be connected in parallel with its I/E converter resistors. These capacitors are connected to relays
that are controlled by software. 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 7-1. The capacitor ensures that 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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