Fundamentals of Electrochemical Experiments – Part 1:Basic Functions and Usage of Electrochemical Workstation

LinearSweepVoltammetry(LSV)

The potential changes linearly with time, measuring the process of current variation with voltage. Linear sweep voltammetry is generally divided into two categories: when the scan speed is slow enough, the electrode surface is considered to be in a steady state. At this time, the current response curve with respect to voltage is called the steady-state polarization curve, commonly known as the polarization curve. The current at this time is the Faradaic current. When the scan speed is relatively fast, the electrode surface is in a transient state, and we refer to it as a voltammetric curve. The current at this time includes both Faradaic and non-Faradaic currents.

Taking the IVIUMSTAT workstation as an example, the setup interface is shown in Figure 1:

1. Select Linear Sweep, usually using Standard.
2. Estart is the starting voltage, and Eend is the ending voltage. It is determined and set based on the electrolyte and reference electrode. For example, OER generally corresponds to 1.1-2.0V under RHE. Adjustments can be made according to requirements.
3. Estep is the testing step size, generally 1-10mV. In the example shown, it is set to test at every 5mV.
4. Scanrate is the scan speed. Glassy carbon electrodes generally choose 5 or 10mV/s, while carbon paper, foam nickel, etc., choose below 2mV/s.
5. CurrentRange is the current testing range, selected based on testing requirements. The smaller the range, the higher the current accuracy. It is generally adjusted to a current value slightly larger than the tested result.

Notes for LSV testing:

1. Before testing the LSV curve, the sample usually undergoes several cycles of CV curves to activate the sample surface.
2. Before testing, observe whether there are bubbles in the reference electrode to avoid a short circuit.
3. If the current is found to be too large during testing, immediately stop the scan and check if the device is short-circuited.
4. If the current is found to be too small during testing, immediately stop the scan and check if the device is open-circuited.

CyclicVoltammetry(CV)

Changing the input signal of the voltammetric curve to a cyclic triangular wave is referred to as a cyclic voltammogram. The obtained current-voltage curve includes two branches. If the potential scans towards the cathode direction in the first half, the electroactive substance is reduced on the electrode, generating a reduction wave. When the potential scans towards the anode in the second half, the reduction product will be oxidized on the electrode again, producing an oxidation wave. Therefore, after one cycle of the triangular wave scan, the electrode completes a cycle of the reduction and oxidation process. Hence, the potential range of the scan needs to allow alternating reduction and oxidation reactions to occur on the electrode, so this method is called cyclic voltammetry. Using the cyclic voltammetry method, on the one hand, it can quickly observe electrode processes occurring within a wide potential range, providing rich information about the electrode process. On the other hand, by analyzing the shape of the scan curve, it is possible to estimate electrode reaction parameters.

Taking the IVIUMSTAT workstation as an example, as shown in Figure 2:

1. Select Cyclic Voltammetry, usually using Standard.
2. Estart is the starting voltage, set according to the electrolyte and reference electrode: Vertex1 is the vertex voltage 1, chosen based on the sample, reference electrode, and testing requirements. The sample starts scanning from Estart, and when reaching Vertex1, it begins the reverse scan; Vertex2 is the vertex voltage 2, usually the same as the numerical value of Estart. When the sample scans from Vertex1 in the reverse direction to Vertex2, one scan cycle is completed. This range value can be referenced from LSV.

Nscans is the number of scan cycles, Scanrate is the scan speed, CurrentRange is the current testing range, all selected based on testing requirements.

ElectrochemicalImpedanceSpectroscopy(EIS)

There are two commonly used types of electrochemical impedance spectroscopy: Nyquist plots and Bode plots. In electrochemical research, Nyquist plots are more commonly encountered. In the Nyquist plot, Z' (real part) and Z'' (imaginary part) represent the charge transfer resistance (Rct) on the electrode surface, and its value is equivalent to the diameter of the semicircular part. It is used to describe the characteristics of the interface between the electrode and the electrolyte. The Nyquist plot consists of two parts: the semicircular part in the high-frequency region corresponds to the electron transfer-limiting process, and the linear part in the low-frequency region corresponds to the diffusion-limiting process.By calculating the charge transfer resistance Rct on the electrode surface, it is possible to interpret certain phenomena during experimental processes such as photocatalysis and electrocatalysis.

The test setup for EIS, as shown in Figure 3:

1. Use a three-electrode system.
2. Set the test voltage, usually choosing the voltage at a current density of 10 mA/cm² or open-circuit voltage.
3. Set the test frequency from high frequency (100,000 Hz) to low frequency (0.01 Hz), typically choosing a frequency of 36. Other parameters can use default values.

Stability Testing

Stability testing can be conducted using chronoamperometry, chronopotentiometry, and cyclic voltammetry (CV) to assess the stability of catalysts. In chronoamperometry stability testing, the voltage of the working electrode is controlled to remain constant, while the current flowing through the working electrode is measured over time. In contrast, in chronopotentiometry, the current of the working electrode is controlled to remain constant, and the voltage of the working electrode is measured over time. Cyclic voltammetry involves measuring the electrochemical stability of the catalyst by testing the linear sweep voltammetry (LSV) curves before and after CV scans within a specific potential range.Chronoamperometry and chronopotentiometry have a limitation when measuring the stability of electrocatalysts, as they can only measure at a constant current or voltage. On the other hand, multiple-step chronoamperometry, which controls different voltages simultaneously while measuring the current passing through the electrode over time, can assess the stability of electrocatalysts at different potentials.

Using Chronoamperometry as an Example, the Parameter Settings Interface is Illustrated in Figure 4:

1. Select "transients -> ChronoAmperometry."
2. Click on "levels" to set a single or multiple-step voltage, along with the corresponding time for each step.
3. After setting, click "close" to exit.
4. Set the current range to prevent over-ranging during the test.
5. After clicking "test," promptly check if the test is normal before leaving. It is advisable to conduct experiments in a fume hood to prevent the accumulation of hydrogen.

Note: During prolonged tests, the instrument may experience freezing or data loss due to the large volume of data. Therefore, regular checks and data saving are crucial.