Mixed Mode (MM)

Description

Mixed Mode is a flexible technique that allows for switching between potentiostatic, galvanostatic, and open circuit measurements during a single run.

The Mixed Mode uses different stages similar to the levels during Multistep Amperometry or Potentiometry, but each stage can be galvanostatic or potentiostatic independent of the previous stage.

The available stage types are ‘ConstantE’, ‘ConstantI’, ‘SweepE’, ‘OpenCircuit’ and ‘Impedance’. ‘SweepE’ offers a potential linear sweep (ramp), so as a regular LSV step. During an Impedance stage the impedance is measured by applying a small AC potential superimposed with a DC potential. This corresponds to an EIS single frequency step (Scan type = Default, Frequency type = Fixed).

Each stage can use the previous stage’s potential as a reference point, for example, a constant current is applied for a fixed period and afterward, the reached potential is kept constant for a fixed period.

Furthermore, each stage can end, because a fixed period has elapsed, or certain criteria are met. Available criteria include reaching a maximum current, minimum current, maximum potential, and minimum potential.

Application

These modes are useful especially for energy conversion and storage research, i.e.: battery, solar cell, or supercapacitors research. A classic test for batteries involves charging and discharging them through multiple cycles. A constant current is applied, and the potential change is recorded. If a certain potential is reached, the next stage is triggered, which is usually applying the inverted constant current. This is repeated for multiple cycles. While this method could be performed with Multistep Potentiometry as soon as further steps are introduced the Mixed Mode is necessary. For example, to determine the capacity you would like to discharge a battery, but you need to take care that the terminal potential isn’t crossed. First the battery is charged. You can apply a constant current until a set potential is reached, e.g.: the termination potential. After that, the termination potential is kept constant until a minimum current limit is reached or the time has elapsed. Then this process is repeated with a negative current to discharge the battery until a minimum potential is reached. Impedance steps can be inserted between to determine the parameters called State of Charge (SoC) and State of Health (SoH) of the battery.