In electrochemical and photoelectrochemical reactions, an ideal catalyst can manifest higher current density at a smaller overpotential.
The Tafel slope provides an essential reference for exploring reaction mechanisms, especially in elucidating the rate-determining steps and reaction pathways. In electrochemical and photoelectrochemical experiments, the kinetic relationship is generally expressed using the Butler-Volmer equation[1]:
i: Current density
i0: Exchange current density
αa: Anodic electron transfer coefficient
αc: 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 originates from the anodic current, and the cathodic current can be neglected. Equation (1) can be simplified as:
Where η represents the overpotential. Equation (2) is also known as the Tafel equation. Taking the logarithm of both sides of the Tafel equation results in:
Where b represents the Tafel slope, which can be obtained from the LSV curve. The Tafel slope can also be expressed further as:
From this, it can be seen that a smaller Tafel slope value corresponds to a faster increase in current density, indicating a more rapid catalytic kinetics and better catalytic activity.
How can we infer the reaction mechanism based on the experimentally determined Tafel slope?
Firstly, Tafel slope can be used to deduce the rate-controlling step of the reaction. Generally, in photoelectrochemical reaction experiments, the performance enhancement effects of working electrodes such as HER, OER, or CO₂RR need to be tested.
During testing, it is necessary to monitor the open-circuit "potential-time curve." When the test system remains static for 15 minutes and the open-circuit potential stabilizes, you can start testing the Tafel curve. The lowest point of the Tafel curve will be lower than the open-circuit potential, and it is recommended to subtract 0.1 V from the open-circuit potential as a reference. A smaller scan rate and longer test duration will yield more accurate results.
It should be noted that Tafel curve testing is highly corrosive, and a sample can only be tested once. It is recommended to perform the Tafel curve test after other non-corrosive tests are completed. If the results are not ideal, the sample should be prepared again, and the electrolyte should be replaced for further testing.
![Figure 1. Application schematic of the classical Tafel method in non-redox buffer systems[2].jpg](/Uploads/UserFile/Image/1/20220715/62d17d111b17b.jpg "Figure 1. Application schematic of the classical Tafel method in non-redox buffer systems[2].jpg")
Figure 1. Application schematic of the classical Tafel method in non-redox buffer systems[2]
According to the reaction mechanism, in Figure 1, I1,a and I2,a represent the cathodic slope and anodic slope, respectively, obtained from the extrapolation method. There are mainly two fitting methods:
① Manual calculation:
Use Origin software to install the Tafel Extrapolation plugin for calculation. It should be noted that when fitting the data, log(i) should be used as the X-axis, and E as the Y-axis, otherwise, the obtained slope will be the reciprocal of the actual slope.
② Automatic calculation:
Use the software provided with the electrochemical workstation, which is the most convenient method.
Figure 2. Tafel plots[3-4]
By calculating the LSV, the Tafel curve can be obtained, which further reveals the catalytic kinetics information of HER. For HER, the theoretical Tafel slope is 120 mV/dec, 40 mV/dec, and 30 mV/dec, corresponding to the Volmer-Heyrovsky step, Heyrovsky step, and Tafel step, respectively[5].
For the HER reaction, the Volmer-Heyrovsky mechanism is as follows:
A smaller Tafel slope indicates a faster kinetic process, indicating that the catalyst can achieve the required current at a lower overpotential.
References
[1] Stephan Enthaler*, Jan von Langermann*, Thomas Schmidt*. Carbon Dioxide and Formic Acid-the Couple for Environmental-Friendly Hydrogen Storage? [J]. Energy Environmental Science, 2010, 3, 1207.
[2] 秦越强,左勇,申淼. FLiNaK-CrF₃/CrF₂氧化还原缓盐体系对316L不锈钢耐蚀性能的影响[J].中国腐蚀与防护学报, 2020, 40(02):182.
[3] Ya Zhang, Lang Hu, Yongcai Zhang*, et.al. NIR Photothermal-Enhanced Electrocatalytic and Photoelectrocatalytic Hydrogen Evolution by Polyaniline/SnS₂ Nanocomposites[J]. ACS Applied Nano Materials, 2022, 5: 391.
[4] Priti Sharma, Debdyuti Mukherjee, Yoel Sasson*, et. al. Pd doped carbon nitride (Pd-g-C₃N₄): an efficient photocatalyst for hydrogenation via an Al-H₂O system and an electrocatalyst towards overall water splitting[J]. Green Chemistry, 2022, DOI: 10.1039/d2gc00801g.
[5] Guoqiang Zhao, Kun Rui, Wenping Sun*, et. al. Heterostructures for electrochemical hydrogen evolution reaction: a review [J]. Advanced Functional Materials, 2018, 28(43): 1803291.
