Potential and Current Control

Control of Potential The lowest cost method of potential control is to connect the sample to a low resistance electrode with a stable potential

Control of Potential For a variable potential, a voltage source can be inserted between the two electrodes

Control of Potential If the potential is monitored, it can be manually adjusted to the desired value

Control of Potential However, the above techniques suffer from difficulties when trying to control potential in some systems (e.g. active-passive transitions) For these situation the potentiostat provides near-ideal control characteristics

The Potentiostat - Practical

The Potentiostat - Practical The Working Electrode is connected to 0 V of the power supply. It may be internally connected to mains earth, or, as here, a separate earth (or ground) terminal may be provided - this should be connected to the Working Electrode terminal if possible

The Potentiostat - Practical The Reference Electrode input is connected to the inverting input of the amplifier. It is a high resistance input.

The Potentiostat - Practical There may be a potential output that is derived from the difference between the reference electrode and working electrode terminals. It will have a low output impedance, and the reference electrode input will not be affected by loads connected to this output (it is said to be buffered)

The Potentiostat - Practical The Output from the amplifier and the connection to the Counter Electrode may be separated on some potentiostats. This allows a resistor to be connected between the two terminals, with an internal buffer amplifier providing the same voltage at the current output terminals

The Potentiostat - Practical An internal source of controlled voltage allows the control potential to be set. External input terminals allow an external control voltage (e.g. from a sweep generator) to be used

The Potentiostat - Practical Some potentiostats have an internal meter to monitor cell potential or current - beware of the current ranges, as the resistors are often damaged by excessive currents

The Potentiostat - Practical Various forms of IR compensation allow for automatic or semi-automatic correction for the resistive potential difference between the tip of the Luggin probe and the specimen surface

The Potentiostat - Practical The overload indicator (where fitted) provides a warning that the potentiostat is no longer able to control the potential, because too much current or too large a potential is required

Problems with Potentiostats - 1 Oscillation The potentiostat relies on negative feedback to control the potential However, delays in the feedback loop due to the charging of capacitances can shift the phase of an ac signal such that the feedback becomes positive This causes oscillation, typically in the kiloherz region

Problems with Potentiostats - 1 Oscillation - a solution A cure for this problem (and a method of diagnosis) is to fit a 1 F capacitor between the counter and reference electrode terminals This produces a strong negative feedback at high frequencies that swamps the oscillation Unfortunately, it also slows down the response of the potentiostat

Problems with Potentiostats - 2 Noise Pickup The reference electrode input is a high impedance point, and is very sensitive to noise pickup (most commonly at mains frequency) Mains frequency noise is rejected by standard digital multimeters, and is often overlooked But what does it do to the electrochemistry?

Problems with Potentiostats - 2 Noise Pickup - solutions Check for noise (or oscillation) with an oscilloscope connected between the counter electrode and the working electrode (not the reference electrode, as the connection of the oscilloscope may affect the behaviour, and the observed potential at the reference electrode will be held constant by the action of the potentiostat)

Problems with Potentiostats - 2 Noise Pickup - solutions Screen the reference electrode with a conductor connected to ground (or the working electrode) Screen the whole system by mounting it in a Faraday Cage (e.g. a metal box)

Problems with Potentiostats - 3 Electronic Noise Some instruments may produce a significant level of noise - check by performing an experiment on a dummy cell made of electronic components, and with similar characteristics to the real cell

Problems with Potentiostats - 4 Overloading If the control of the cell potential requires either a higher voltage or a higher current than the potentiostat can deliver, then the potential will not be controlled Watch out for very constant current traces, as these are not usual for real systems Check the overload indicator if one is fitted to the potentiostat

Problems with Potentiostats - 4 Overloading - solutions If the current is limiting use a smaller electrode If the cell voltage is limiting use a larger counter electrode reduce the spacing between the counter and working electrodes increase the conductivity of the solution (if possible)