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Tafel Slope basics

Photoelectric catalytic series 2022-07-27 1 230

In electrochemical and photoelectrochemical reactions, ideal catalysts can exhibit high current density at a small overpotential. 

Tafel slope can provide important reference for exploring the reaction mechanism, especially in clarifying the reaction rate determining steps and reaction paths. 

In electrochemical and photoelectrochemical experiments, the dynamic relationship is generally expressed by Butler-Volmer formula[1].

光电催化第二讲|Tafel斜率基础知识.jpg(1)

i:Current density 

i0:Exchange current density 

αa:Anode electron transfer coefficient 

αc:Cathode electron transfer coefficient 

n:The number of electrons transferred in the reaction  

F:Faraday constant 

E:Applied voltage  

R:Universal gas constant  

T:Thermodynamic temperature

At high anode potential, the current mainly comes from anode current, and the cathode current can be ignored. Formula (1) can be simplified as

光电催化第二讲|Tafel斜率基础知识.jpg(2)

Where η is overpotential, formula (2) can also be called Tafel formula, which can be changed by taking logarithms of both sides of Tafel formula

光电催化第二讲|Tafel斜率基础知识.jpg(3)

Where b represents the Tafel slope, which can be obtained from LSV curve. Tafel slope can be further expressed as:

光电催化第二讲|Tafel斜率基础知识.jpg(4)

Thus, the smaller the Tafel slope value is, the faster the current density increases, indicating that the catalyst has faster kinetics and better catalytic activity.

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

Firstly, the Tafel slope is used to deduce the reaction speed control steps. Generally, the performance improvement effect of working electrode SUCH as HER, OER or CO2RR should be tested in photoelectric chemical reaction experiments. 

Test, to test for open circuit potential - time curve, when the test system static after 15 min, and the open circuit voltage stability, can start testing Tafel curves, Tafel curves nadir is smaller than the open circuit potential, proposed to the open circuit potential value as a reference, after subtracting 0.1 V scanning speed value is smaller, the longer the test of time, the result more accurate. 

It should be noted that Tafel curve test is highly corrosive and one sample can only be tested once. It is recommended to test Tafel curve at last after other non-corrosive tests are completed. If the results are not ideal, the sample needs to be prepared again and the electrolyte needs to be replaced before testing.

图1. 经典Tafel方法在非氧化还原缓冲体系中应用原理图[2].jpg

Figure 1. Schematic diagram of classical Tafel method applied in non-redox buffer system[2]

According to the reaction mechanism, in Figure 1I1,a、I2,aAre respectively cathode slope and anode slope, which are obtained by extrapolation. There are two fitting methods: 

① The manual calculation  

Use Origin software to install the Tafel Extrapolation plug-in for calculations. It should be noted that the data fitting should take log(I) as the X-axis and E as the Y-axis, otherwise the slope obtained is the reciprocal of the actual slope; 

② Automatic calculation 

The most convenient method is to use electrochemical workstation with its own software.

Tafel斜率.jpg

图2. Tafel plots[3-4]

The Tafel curve was obtained by LSV calculation, which could further reveal the catalytic kinetics information of HER. For HER, the theoretical Tafel slope is 120 mV/dec, 40 mV/dec, 30 mV/ Dec corresponding to volmer-Heyrovsky step, Heyrovsky step, Tafel step respectively[5]。 

The Volmer-Heyrovsky mechanism in HER reaction is as follows:

光电催化第二讲|Tafel斜率基础知识.jpg

A smaller Tafel slope means a faster kinetic process, indicating that the catalyst can achieve the desired current at a lower overpotential.

Reference

[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氧化还原缓冲熔盐体系对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.



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