Sat 11 Nov 2006
The basic function of analytical electrochemistry is to look at the response of a cell (set of electrodes in electrolyte). This often accomplished with a potentiostat, an instrument that can set the potential between a pair of electrodes and track the current or vise versa. A commercial, and often expensive, potentiostat generally has six to nine orders of magnitude of dynamic range in both current and voltage mode (10pA to 10A and 100uV to 100V for the best). However, if you can narrow down the range that you will be working with, you can make yourself a high-performance and inexpensive potentiostat that works over the range of interest. Since we primarily are interested in characterizing various types of iridium-oxide electrodes, the current range will be 1uA to 1mA and the voltage will be 10mV to 10V, so only three orders of magnitude are required. This can easily be realized with a pair of op amps, one working as a current follower (program voltage and track current) and the other as a current source with shutdown functions and necessary logic to ensure that only one driving op amp is running at a time.
The cell is pretty simple: a working electrode, a reference electrode and a counter electrode. The working electrode is the unit under test where you program the voltage between it and the reference electrode or the current between it and the counter electrode. The voltage is measured between the working and reference electrodes in current program mode and the current is measured between the working and counter electrode in voltage program mode. The working electrode is then subjected to various regimes where the relationship between voltage and current is tracked and is characteristic of surface chemistry. Finally, the open circuit potential can be measured between the reference and working electrode to get the cell’s galvanic potential. This potential is generally the difference of the working and reference electrodes’ standard electrode potentials (notice that Lithium’s standard potential is around -3V, that is why most Lithium-ion cells are 3.7V at full charge). Here is a potentiostat design guide as well as an application note regarding feedback stability. Those who wish to learn more should look at Electroanalytical Chemistry by Bard and Faulkner (ISBN 0824790928).
( std_pot.pdf ) ( potstae2.pdf ) ( an4.pdf )
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