Photocatalysis Lecture 2 | Basics of Tafel Slope

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In electrochemical and photocatalytic reactions, an ideal catalyst can exhibit high current density at relatively low overpotentials. Tafel slope provides crucial insights for exploring reaction mechanisms, especially in elucidating the rate-determining steps and reaction pathways.

In electrochemical and photocatalytic experiments, kinetic relationships are generally represented by the Butler-Volmer equation^[1]:

i: Current density

i₀: Exchange current density

α_a: Anodic electron transfer coefficient

α_c: Cathodic electron transfer coefficient

n: Number of electrons transferred in the reaction

F: Faraday's constant

E: Applied voltage

R: Universal gas constant

T: Thermodynamic temperature

At high anodic potentials, the current mainly comes from the anodic current, and the cathodic current can be neglected, simplifying equation (1) to

Where η represents overpotential, and equation (2) is also known as the Tafel equation. Tafel slope can be obtained from the LSV curve. The Tafel slope can also be expressed as:

From this, it can be seen that the smaller the Tafel slope, the faster the increase in current density, indicating faster catalytic kinetics and better catalytic activity.

How can we infer the reaction mechanism based on the experimentally measured Tafel slope?

First, use the Tafel slope to infer the rate-controlling step of the reaction. Generally, photocatalytic reaction experiments require testing the performance improvement of the working electrode for processes like HER, OER, or CO₂RR.

During testing, check the open-circuit "potential-time curve." When the test system is still for 15 minutes and the open-circuit potential is stable, you can start testing the Tafel curve. The lowest point of the Tafel curve will be lower than the open-circuit potential. It is recommended to subtract 0.1 V from the open-circuit potential as a reference. A smaller scan speed and longer test time will result in more accurate results.

It should be noted that Tafel curve testing is highly corrosive, and each sample can only be tested once. It is recommended to test the Tafel curve last after other non-corrosive tests have been completed. If the results are not ideal, you need to prepare the sample again and replace the electrolyte for retesting.

Figure 1. Application schematic of the classic Tafel method in non-oxidation-reduction buffer systems^[2]

According to the reaction mechanism, I_1,a and I_2,a in Figure 1 represent the cathodic slope and anodic slope, respectively, obtained by extrapolation. There are mainly two methods for fitting:

① Manual calculation

Use the Origin software with the Tafel Extrapolation plugin installed for calculation. It is important to note that when fitting the data, use log(i) as the X-axis and E as the Y-axis; otherwise, the slope obtained will be the reciprocal of the actual slope.

② Automatic calculation

Use the software provided by the electrochemical workstation, which is the most convenient method.

Figure 2. Tafel plots^[3-4]

Through LSV, Tafel curves can be calculated to further reveal the catalytic kinetic information of HER. For HER, the theoretical Tafel slopes are 120 mV/dec, 40 mV/dec, and 30 mV/dec, corresponding to the Volmer-Heyrovsky mechanism, Heyrovsky mechanism, and Tafel mechanism^[5].

For the Volmer-Heyrovsky mechanism in the HER reaction:

A smaller Tafel slope means faster kinetic processes, indicating that the catalyst can achieve the required current at lower overpotentials.

References