μblog: engineering from the trenches

I spent most of today working out how to perform basic cyclic voltammetry and control it using a NI USB-6009 acquisition board with LabView. The basic idea is that you place two electrodes, working electrode and counter electrode, into an electrolyte and sweep the potential between them at a constant rate, typically 50mV/sec. You then track the current going into the working electrode and plot that versus the applied voltage to determine electrode/electrolyte characteristics such as the electrode’s capacity to pass charge into the solution, mass transfer effects and redox potentials. For my purposes, I just want to create a method to have an untrained individual perform a simple test on electrochemically deposited iridium-oxide electrodes to see if they are fit to be used or should be trashed.

Since the USB device only outputs 0 and 5V power and has a 2.5V reference voltage, I designed the 2.5V to be effectively ground. Triangular waveform around 2.5V is written out on an analog-output channel and is then buffered by a voltage follower and connected to the working electrode. The counter electrode is held at 2.5V by way of a current follower, where the feedback resistor is either 1kOhm or 10kOhm resulting in the two gain settings (see schematic). Both of the op-amps used have a shutdown capacity, so the electrodes are not held at any fixed potential when the VI is not running providing that the digital IO output was in a low state before. The current signal is passed through a five-pole lowpass filter at about 10Hz. The resultant current is split between positive and negative phases and is integrated to give the anodic and cathodic charge transfer capacity (relative to the working electrode) respectively. Finally, the VI displays the graphical results and has the option to write ASCII log files.

This is the first version of this VI that was coded in a day so there may be errors and this should not be used for any important research. No quality or warantee is guaranteed or implied. The source code and the executable (LabView 8.2.1 runtime engine download required for executable to run on its own) are included in the ZIP file, so please enjoy them for non-commercial use. One known bug is that the executable runs automatically when opened which is undesired behavior, the executable should be stopped and then configured and then finally started again. Another wierdness is that the CV has a dip at the very begining, at the origin, probably due to the lowpass filter. Maybe there will be a version 2 sometime soon.

( elec-cv1.pdf ) ( elec-cv1.zip )

Update: I changed two small things in the original VI, mainly the CV now runs for two cycles and it records measurements from the second one only and the two feedback resistor values have been changed to 5.6kOhm and 56kOhm (from 1kOhm and 10kOhm).

( elec-cv101.zip )

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