Fundamentals of Electrochemical Experiments - Part 2:Construction and Testing of Three-Electrode and Two-Electrode Systems

Differences Between Three-Electrode System and Two-Electrode System

The electrochemical workstation serves both as a power source capable of providing voltage and current and as an instrument for accurately measuring voltage and current. From a structural perspective, one can consider that a current meter is connected between the working electrode clamp and the counter electrode clamp, while a voltmeter is connected between the working electrode clamp and the reference electrode clamp. The three-electrode system refers to a configuration where the working electrode, reference electrode, and counter electrode are all connected to the electrolyte cell. The three electrodes form two circuits, with the circuit formed by the working electrode and reference electrode having a very small current, used to easure the electrode potential. The other circuit, formed by the working electrode and counter electrode, is used to measure the current, constituting the so-called "three-electrode, two-circuit" system commonly used in testing. Due to the substantial current passing through the system, resulting in solution voltage drop and polarization of the counter electrode, the potential of the working electrode is challenging to directly and accurately determine. This leads to the introduction of a reference electrode. The reference electrode has a highly stable potential, and its potential is known. The current in the reference electrode circuit is minimal, and the polarization and other voltage drops can be largely ignored. Consequently, the potential of the working electrode can be obtained from the reference electrode, while the current is directly measured through the working electrode-to-counter electrode circuit. In a two-electrode system, the reference electrode port is connected together with the counter electrode port, eliminating the reference electrode. The reference circuit and working circuit are combined, and the measured current remains the current in the working electrode circuit, while the voltage is the potential difference between the working electrode and the counter electrode.

Hardware Preparation Before Testing

1. Reactor (Electrolytic Cell): Can be purchased from manufacturers or designed independently. Typically made of glassware, commonly a five-neck electrolytic cell with three inserted electrodes, one gas inlet, and one gas outlet.
2. Electrodes: Reference electrode (commonly used Ag/AgCl, SCE, Hg/HgO, etc.), counter electrode (commonly used platinum wire, platinum mesh, graphite rod, etc.), working electrode (commonly used carbon paper, foam metal, glassy carbon electrode, rotating disk, rotating ring, etc.).
3. Electrochemical Workstation: Brands like Chenhua, Ivium, etc.
4. Others: Prepare electrolyte, make ink, install gas pipelines, etc.

Three-Electrode System Testing for Hydrogen Evolution Reaction (HER)

1. Working Electrode Preparation:

(1) Self-supporting electrodes can be directly used for testing.

(2) Prepare ink by dropping it onto the metal or glassy carbon electrode:

• Take a specific amount of catalyst, determine whether to add conductive carbon black based on the catalyst's conductivity. Add a certain amount of Nafion solution and a specific amount of solvent. Disperse with ultrasonication, choosing a solvent that can disperse the sample well and has sufficient surface tension to prevent ink overflow during coating. Typically, solvents include isopropanol or a mixture of ethanol and water.
• Use a pipette to take an appropriate amount of ink and drop it onto the prepared electrode surface. Allow it to dry. Note that if the loading amount is too large, apply the ink in several steps to avoid overflow.

2. Counter Electrode Selection:

In previous studies, researchers commonly used platinum wire or platinum mesh as the counter electrode. However, research indicates that in prolonged tests, electrochemical deposition occurs, leading to an artificially high activity of the working electrode due to Pt deposition on its surface. Therefore, when working and counter electrodes are in the same environment, as is the case in a one-compartment cell, it is recommended to avoid using platinum wire (mesh) as the counter electrode. Carbon rods or graphite electrodes are preferable.

3. Reference Electrode Selection:

The choice of reference electrode depends on the pH of the electrolyte:

• In acidic solutions, the saturated calomel electrode (SCE) is commonly used.
• In neutral solutions, Ag/AgCl is used.
• In alkaline solutions, Hg/HgO is employed.

4. Gas Inlet:

In theory, it is advisable to saturate with H2 for at least 30 minutes before HER testing. However, due to the high risk associated with H2, N2 or Ar can be used as alternatives. Note that their reactivity might be slightly compromised. Many research groups skip the gas saturation step in practical operations.

5. IR Compensation:

iR compensation comes in two main types – manual and automatic:

• Manual compensation: In EIS testing, compensate 95%-80% based on the measured solution resistance.
• Automatic compensation: Use the built-in compensation function of the electrochemical workstation. For example, in IVIUMSTAT, refer to the provided graph for illustration.

6. Linear Sweep Voltammetry (LSV) Testing:

Using IVIUMSTAT workstation as an example:

• Choose Linear Sweep, usually opting for Standard.
• Set Estart as the initial voltage, considering the electrolyte and reference electrode setup. If conversion to RHE is needed, Estart should be greater than 0 mV. Set Eend as the final voltage according to the sample, reference electrode, and testing requirements. Estep is the testing step size, typically testing every 5 mV. Scanrate is the scanning speed, usually below 5 mV/s. CurrentRange is the current testing range, selected based on testing requirements.

7. Cyclic Voltammetry (CV) Testing:

• Choose Cyclic Voltammetry, usually opting for Standard.
• Set Estart as the initial voltage based on the electrolyte and reference electrode. Vertex1 is the first vertex voltage, chosen based on the sample, reference electrode, and testing requirements. The sample starts scanning from Estart, and when reaching Vertex1, it begins reverse scanning. Vertex2 is the second vertex voltage, usually the same as Estart. The sample completes one scan from Vertex1 to Vertex2. Nscans is the number of scans, Scanrate is the scanning speed, and CurrentRange is the current testing range, all selected based on testing requirements.

Three-Electrode System Testing for Oxygen Evolution Reaction (OER)

1. Preparation of Working Electrode Similar to HER:

2. Selection of Reference Electrode:

The choice of the reference electrode depends on the pH of the electrolyte. Generally:In acidic solutions, the reversible hydrogen electrode (RHE) is commonly used.In neutral solutions, Ag/AgCl is used as the reference electrode.In alkaline solutions, Hg/HgO is employed as the reference electrode. However, it is essential to note that if a reference electrode encapsulated in polytetrafluoroethylene (PTFE) is used, it cannot be a porous ceramic one, as porous ceramics are unstable in alkaline environments. 3. Gas Inlet:

Before OER testing, it is necessary to saturate with O2. 4. iR Compensation, LSV, and CV Similar to HER:

Pay attention to modifying the testing range to the OER reaction range.

Two-Electrode System Testing for Overall Water Splitting

Compared to the three-electrode system, the two-electrode system has a simpler setup but often yields more complex results and corresponding analyses. In the case of overall water electrolysis testing, the two-electrode system involves connecting the working electrode (WE) to the anode catalyst, while the reference electrode (RE) and counter electrode (CE) are jointly connected to the cathode catalyst.

Using the IVIUMSTAT workstation as an example:

• Select Linear Sweep, usually opting for Standard. Set Estart as the initial voltage. Since the two-electrode system tests the potential difference between the anode and cathode, the range is typically between 1-1.2V. Set Eend as the final voltage based on the sample, reference electrode, and testing requirements. Adjust Estep for the testing step size.
• Choose Scanrate for the scanning speed, typically within the range of 5mV/s or lower. Set CurrentRange according to the testing requirements.