Test procedure

The EIS measurements are carried out with the Energy-Lab XM System Energy-Lab XM System provided by AMETEKSI with Solartron Analytical brand coupled with the software XM-studio ECS. The device consists of a PGSTAT potentiostat/galvanostat, an internal 2A booster and the frequency response analyser, FRA. The system is controlled through the software XM-studio ECS. The Potentiostat/galvanostat enables to perform experiment on the cell. When used on its own it is able to run many types of DC tests, while combined with the FRA it can also measure impedance. It records data with maximum rate of 1M samples/second and it is able to work in a voltage range of ±8𝑉 up to ±300𝑚𝐴 and 1MHz bandwidth. The internal power booster is used to enhance the capability for testing higher power electrochemical systems and it is characterized by a maximum output of ±8𝑉/±2𝐴. The FRA allows impedance measurements, it can generate and analyse AC waveform with good accuracy over its range of operation over its full frequency range 10𝜇𝐻𝑧 − 1𝑀𝐻𝑧.

Figure 37 Energy-Lab XM System.

71 The test procedure foresees in order:

- the wire connection between the cell and the ModuLab XM,

- the creation of the project in the XM-studio software including the setting of the test input parameters, and finally

- the experiment running.

A proper electrode connection is required. A schematic configuration is reported in Fig.38.

The potentiostat is connected to the internal booster and the booster is connected to the cell.

The CE on the booster is connected to the positive terminal and the WE is connected to the negative terminal. The working and counter wires of the ModuLab XM are connected to the anode (positive electrode) and the cathode (negative electrode) respectively, while the reference wire is connected to both cathode and anode electrodes.

Figure 38 Schematic configuration of the connection between the cell and the device.

Prior to the running of the experiment, is the creation of the project in the XM-studio ECS software. This enables to run different experiments named steps: DC voltage control, DC current control, impedance voltage control and impedance current control. The focus of this work is the characterization of the electrolytic cell with the impedance spectroscopy technique thus the steps chosen are the last two: Impedance Voltage Control and Impedance Current Control.

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Figure 39 Types of experiments in the XM-studio ECS software.

For each step category there is a list of step types among which making the choice. For the impedance voltage control Potentiostatic impedance is selected while for the impedance current control the Galvanostatic impedance. These represent the two EIS measurement modes performed in this work.

Figure 40 Possible step types for Impedance Voltage Control and Impedance Current Control.

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For each EIS measurement mode the first step is the definition of the experiment Setup item consisting of different fields, in particular Hardware Requirement and Cell Setup.

Figure 41

The Hardware Requirement specifies the hardware and connection configuration for the experiment while the Cell Setup some specification about electrochemical cell such as the electrode active area, the type of reference electrode, density and equivalent weight.

Figure 42 Hardware requirements and cell setup.

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Thereafter, there is the definition of the test inputs for both the impedance experiments.

These are defined into two fundamental sub-screens:

- Scan Setup: this sub-screen specifies values associated with the excitation signal that is applied to the cell.

- Impedance Setup: it specifies the frequency range to be tested since the system superimposes a frequency, or a range of frequencies, on top of the DC scan that has been specified from the Scan Setup.

Figure 43

The Impedance Setup contains several fields to specify as shown in the Fig.44. In the Technique the frequency sweep- consisting of a single sinewave swept from the start frequency to the end frequency- is chosen as the type of the AC stimulus. Other possibilities are the single frequency where a single sinewave is applied to the cell at a fixed frequency or the multi-sine where multiple sinewaves, according to a Fast Fourier Transform, are swept from the start frequency to the end frequency.

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Figure 44 Impedance setup.

The amplitude defines the RMS amplitude of the AC signal, maintained at a fixed level during the step. This value is specified in absolute terms without reference to any other values, therefore in both potentiostatic and galvanostatic tests the absolute field is selected. As discussed in the previous section, the amplitude of the stimulus must be carefully chosen to ensure the linearity of the system response but also to provide a reasonable signal-to-noise ratio. Since the Frequency sweep is selected in the frequency field there are two fields representing the Start Frequency and End Frequency of the sweep. Generally, the frequency range used is between 10⁻³− 10⁶.

The signal applied to the FRA can be integrated to reject noise. The effect is to narrow the measurement bandwidth and thus increase the signal-to-noise ratio. Integration increases the measurement time, so there is a trade-off between the accuracy you require and the measurement speed. Auto-integration can be used when there is uncertainty about the degree of interference generated by the cell, so that the integration time is adjusted automatically to obtain a specified statistical accuracy in the measurement result. When auto-integration is off, this field can be used to specify a fixed auto-integration time. The units can be seconds or cycles, and a value in seconds is rounded up to cover the nearest number of whole cycles.

The Sweep type defines the method of variation of frequency during the sweep. Among linear and logarithmic the latter one is chosen. Also, the number of measurement points per decade during a logarithmic sweep should be at least 10.

The other field is the Scan Setup where the DC value of the stimulus is defined. Voltage, or potential as it is otherwise called, is always defined as a difference between two values. For the potentiostatic the potential has to be specified against no obvious reference value; there

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are many options but the most suitable is the vs. Reference where the value is specified relative to the reference voltage in the Cell Setup.

The galvanostatic impedance do not require a referencing method, thus the Scan Setup appears as follows

Finally, once that the wires are connected and the project has been created and set, the experiment is run.