Voltage Noise And Dc Measurements; Shunt Resistance And Capacitance; Hints For System And Cell Design; Faraday Shield - Gamry Instruments Interface 1010 Operator's Manual

Measurement of Small Signals – Hints for System and Cell Design

Voltage Noise and DC Measurements

Often the current signal measured by a potentiostat shows noise that is not the fault of the current-
measurement circuits. This is especially true when you are making DC measurements. The cause of the current
noise is noise in the voltage applied to the cell.
Assume that you have a working electrode with a capacitance of 40 µF. This could represent a 1 cm
bare metal immersed in an electrolyte solution. You can roughly estimate the capacitance of the electrical
double-layer formed by a metal/electrolyte interface as 20 µF/cm
surface, which is larger than the geometric area, because even a polished surface is rough. The impedance of
this 40 µF electrode, assuming ideal capacitive behavior, is given by:

Z = 1/j C
At 60 Hz, the magnitude of the impedance is about 66 Ω.
Apply an ideal DC potential across this ideal capacitor and you get no DC current.
Unfortunately, all potentiostats have noise in the applied voltage. This noise comes from the instrument itself
and from external sources. In many cases, the predominant noise frequency is the AC power-line frequency.
Assume a realistic noise voltage, V
potentiostats). Further, assume that this noise voltage is at the US power-line frequency of 60 Hz. It will create
a current across the cell capacitance:
/Z  10 × 10
I = V
n
This rather large noise current prevents accurate DC-current measurement in the low nA or pA ranges.
In an EIS measurement, you apply an AC excitation voltage that is much bigger than the typical noise voltage,
so this is not a factor.

Shunt Resistance and Capacitance

Non-ideal shunt resistance and capacitance arise in both the cell and the potentiostat. Both can cause
significant measurement errors.
Parallel metal surfaces form a capacitor. The capacitance rises as the area of the metal increases and as the
separation between the metal surfaces decreases.
Placement of the wires and electrodes has a large effect on shunt capacitance. If the clip leads connecting to
the working and reference electrodes are close together, they can form a significant shunt capacitor: values of
1 to 10 pF are common. This shunt capacitance cannot be distinguished from "real" capacitance in the cell. If
you are measuring a paint film with a 100 pF capacitance, 5 pF of shunt capacitance is a very significant error.
Shunt resistance in the cell arises because of imperfect insulators. No material is a perfect insulator (one with
infinite resistance). Even PTFE, which is one of the best insulators known, has a bulk resistivity of about
12
10 Ω m. Worse yet, surface contamination often lowers the effective resistivity of good insulators. Residual
∙
water films can be a real problem, especially on glass.
Shunt capacitance and resistance also occur in the potentiostat itself. Specifications for the Interface 1010 in
Potentiostat Mode, in Appendix A, contain equivalent values for the potentiostat's R
can be measured by an impedance measurement with no cell.
In most cases, the cell's shunt resistance and capacitance errors are larger than those from the potentiostat.

Hints for System and Cell Design

Faraday Shield

A Faraday shield surrounding your cell is mandatory for very low-level measurements. Such a shield reduces
both current noise picked up directly on the working electrode and voltage noise picked up by the reference
electrode.
, of 10 µV (this is lower than the noise level of most commercial
n
/66  150 nA
−6
2 . The area is the microscopic area of the
62
polished
2
and C . These values
shunt shunt