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Introduction
Published in Ramanathan Srinivasan, Fathima Fasmin, An Introduction to Electrochemical Impedance Spectroscopy, 2021
Ramanathan Srinivasan, Fathima Fasmin
We want to develop the equations relating the current to the potential in an electrochemical reaction. In the previous section, we saw that the rate constant depends exponentially on the potential. For a simple electron transfer reaction, under certain conditions, the reaction rate is proportional to the rate constant. Independently, based on experimental observations, Julius Tafel proposed a relationship between the current and the potential in many electrochemical reactions as i=constant × ebE and this is also referred to as the Tafel equation. Here, b is a parameter that depends on the electrode material and the electrolytes.
Proton Exchange Membrane Water Electrolysis
Published in Lei Zhang, Hongbin Zhao, David P. Wilkinson, Xueliang Sun, Jiujun Zhang, Electrochemical Water Electrolysis, 2020
Zhao Jin, Shuai Hou, Zhaoyan Luo, Rongpeng Ma, Yang Li, Yibo Wang, Junjie Ge, Changpeng Liu, Wei Xing
From the extrapolation of the linear region of a plot of overpotential versus log J, we can obtain the Tafel slopes and current density of catalysts (J0), which are also very important electrode parameters. Without other critical information, the value of the electrode area-normalized J0 is not an ideal parameter for assessing catalytic performance because it is neither a relevant electrochemical parameter nor a fundamental material property, which depends on the turnover frequency per site and the total number of sites. When combined with the Tafel slope, the exchange current density can become an important parameter. When applied in the Tafel equation, these two parameters together can calculate the overpotential required to achieve any current density. Ultimately, the Tafel slope and onset potential can determine the overpotential required to reach any working current density on electrodes. Thus, low Tafel slopes are desirable, especially for high operating current applications such as water electrolyzers. More importantly, the Tafel slope can also reflect the HER mechanism. The Volmer step is a rate-determining step, the corresponding Tafel slope is 120 mV dec−1; the Heyrovsky step is a rate-determining step, the corresponding Tafel slope is 40 mV dec−1; and the Tafel step is rate-determining step, the corresponding Tafel slope is 30 mV dec−1.
Photoelectrocatalytic Carbon Dioxide Reduction to Value-Added Products
Published in Anirban Das, Gyandshwar Kumar Rao, Kasinath Ojha, Photoelectrochemical Generation of Fuels, 2023
Paras Kalra, Cini M. Suresh, Rashid, Pravin P. Ingole
According to the Tafel equation, Δ=blog(j)+a, which relates the overpotential and the logarithm of the current density (log j), a Tafel plot can be drawn and fitted. This equation corresponds to that of a straight line (y = mx + c), thus the Tafel slope “b” can be found from the slope of the Tafel curve. Here, overpotential η describes the difference between the electrode potential required to carry the reaction at a particular rate (also correspond to or given by current density) and the thermodynamic standard potential and “a” is the exchange current density which can be found from the intercept of the Tafel plot. As overpotential is related to the current density as a logarithmic function in the Tafel equation, it indicates that an increase in the overpotential is necessary for a ten-fold increment in the current density. The Tafel slope specifically quantifies the sensitivity of the current density of a particular material with the applied overpotential. The smaller the Tafel slope, the more desirable is the electrocatalyst because a small value of b shows that the current density increases steeply with the overpotential. Tafel slope provides more information on the possible reaction pathways and rate-determining steps (RDS). The experimentally determined value of the Tafel slope can be compared with the theoretical value found from microkinetic model to arrive at a conclusion of the RDS and reaction pathway. For example, the RDS of the electrochemical CO2 RR process is the formation of CO2[·]− anion radical, the theoretical value of b for this reaction is 118 mV per decade which can be correlated with the experimental reaction to get an insight.
Electrodeposition of amorphous Ni–P layers, thermal treatment and corrosion behaviour
Published in Transactions of the IMF, 2019
The Tafel equation provides the link between the applied overpotential, η/V to the current flowing through the external circuit of the electrochemical system, i/A.40 Equation (2) is actually the logarithmic expression of the well-known Buttler–Volmer kinetic equation of heterogeneous electrode kinetics. It is used for determination of the current when there is no mass transfer limitation.
Ultralow Pt0 loading on MIL-88A(Fe) derived polyoxometalate-Fe3O4@C micro-rods with highly-efficient electrocatalytic hydrogen evolution
Published in Journal of Coordination Chemistry, 2020
Ming-Liang Wang, Di Yin, Yun-Dong Cao, Guang-Gang Gao, Tao Pang, Lulu Ma, Hong Liu
To study the HER performance and catalytic mechanism of Pt-POMFe electrocatalyst, HER activities of MIL-88A(Fe), Pt-POMFe and Pt/C catalysts (Pt content is 20 wt%) have been explored in acidic medium (0.5 M H2SO4 solution). By the LSV polarization curves (Figures 4a and S5), Pt-PW12Fe-400 catalyst shows the optimal performance with overpotential of 28 mV in the current density of 10 mA·cm−2. It is much smaller than the MIL-88A (Fe) sample (160 mV) and comparable to the Pt/C sample (26 mV). The result indicates that the Pt-PW12Fe-400 catalyst has high Pt utilization as well as good electrocatalytic activity. With increase of calcination temperature, the catalytic activity of Pt-POMFe decreases after calcination (Figure S5). Excessive carbonization temperature may lead to skeleton collapse and aggregation of Pt-POMFe micro-rods, which reduces the number of exposed active sites and thus affects its electrocatalytic activity (Figure S6). In order to further study the catalytic mechanism of Pt-POMFe, Tafel plots derived from the corresponding LSV curves of the four samples are shown in Figure 4b. Tafel slope is usually used to analyze the kinetics of HER process, its linear region fits to the Tafel equation (η = b log j + a, b is the Tafel slope). Tafel slopes of Pt-PW12Fe-400, Pt-PMo12Fe-450, MIL-88A(Fe) and Pt/C catalyst are 30, 67, 47 and 29 mV·decade−1, separately. The small Tafel slope of Pt-PW12Fe-400 suggests that the hydrogen evolution process is achieved via combination of two adsorbed hydrogens on the electrode surface to form molecular hydrogen (Volmer-Tafel mechanism) [61]. In addition, the activity of the catalyst was further measured using EIS. The charge transfer resistance (Rct) in the Nyquist diagram was derived from the high frequency. EIS data (Figure 4c) show that the Rct of Pt-PW12Fe-400 sample is 40 Ω, which is smaller than Pt/C catalyst (120 Ω), and much lower than MIL-88A(Fe) (314 Ω) and other reference samples. This indicates that the Pt-PW12Fe-400 catalyst possesses excellent electron transfer ability.
Coordination chemistry of silver(I), gold(I) and nickel(II) with bis N-heterocyclic carbenes: applications in electrocatalytic hydrogen evolution reaction
Published in Journal of Coordination Chemistry, 2022
B. M. Geetha, Zhoveta Yhobu, V. Monica, Jan Grzegorz Małecki, D. H. Nagaraju, Mohammad Azam, Saud I. Al-Resayes, Srinivasa Budagumpi
The understanding of the mechanism in any catalytic process is crucial in designing more efficient catalysts and the same principle is also implemented in the design of an electrocatalyst. In HER, the mechanism consists of several processes at the surface of the electrode. The HER mechanism in acidic electrolyte is described in Equations (2)–(4), where M denotes the metal center. The first step, i.e., Equation (2) involves a proton adsorption to the electrode accompanied by an electron transfer on an empty active metal site yielding the adsorption of a hydrogen atom; this is termed the Volmer step. The Volmer step occurs in one of two different pathways, i.e., the Tafel or the Heyrovsky pathway. The Heyrovsky pathway involves the transfer of a second electron coupled with a second proton transfer from the electrolyte to evolve H2 gas as shown in Equation (3); this reaction is also called the ion + atom reaction. The other plausible pathway involves the combination of two adsorbed hydrogen atoms on the electrode surface to yield H2 as shown in Equation (4); this pathway is termed the Tafel reaction or the combination pathway [36]. To ascertain the mechanism of the electrocatalyst, Tafel plots are drawn by using the Tafel equation (η = a + b log j, where η is the overpotential, j is current density, and b=Tafel slope), which ultimately relays the Tafel slope value of the electrode. As highlighted from Equations (2)–(4), the equations are attributed to different Tafel slopes, i.e., 30, 40, and 120 mV dec−1 as the rate determining steps of Tafel, Heyrovsky, and Volmer, respectively. The Tafel slope values obtained for the prepared electrodes are exhibited in Figure 4c and d; the Tafel slope values are 172–255 mV dec−1. The high Tafel slope of the complexes and composites indicate that the discharge of the hydrogen gas is slow and sluggish, implying that the mechanism is combination of Volmer step and Heyrovsky step as the rate determining step [37]. This inference from the Tafel slope directly corroborates to the electrochemical activity displayed by the modified electrodes whereby 4-MWCNT, which shows the least Tafel slope, i.e., 172 mV dec−1, demonstrates the best HER activity. Comparison of the overpotentials and Tafel slopes is summarized in Table 1.