Meaning
Voltage difference between the reference electrode and the working electrode that is not accounted for by the potentiostat control loop. An uncompensated ir drop occurs because of the resistance of the electrolyte and the physical distance between the electrodes. This error causes the actual potential at the electrode surface to be different from the setpoint.
It marks the boundary where the instrument no longer has direct control over the electrochemical reaction.
Resistance Origin
Electrolyte conductivity and the geometry of the cell determine the magnitude of this loss. Even in highly conductive solutions, the uncompensated ir drop can be several millivolts during high-current pulses. This resistance is often called the solution resistance and is a fundamental part of the cell environment.
Placing the reference electrode closer to the working electrode reduces the volume of electrolyte contributing to the error.
Feedback Loop
Potentiostats use a feedback mechanism to maintain the desired voltage at the working electrode. If the uncompensated ir drop is high, the instrument thinks it has reached the target voltage while the surface potential is still lagging. Modern instruments include a compensation circuit that measures this resistance and adds an equivalent voltage to the output.
The resulting error can lead to an incorrect estimation of the exchange current density and other fundamental electrochemical parameters.
Measurement Techniques
Current interrupt or high-frequency impedance methods are used to determine the exact resistance value. Once the value is known, the uncompensated ir drop can be corrected in real-time or during post-processing. This correction is fundamental for determining the true overpotential of an electrocatalyst or the state of health of a battery.
The precision of the correction depends on the stability of the electrolyte resistance over time.
