A Generalized Expression for the Tafel Slope and the Kinetics of Oxygen Reduction on Noble Metals and Alloys
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The usually employed expression for the Tafel slope is modified to take into account the potential which is effective in charge transfer within the double layer. The exchange currents and cathodic Tafel slopes are obtained on noble metal electrodes having different number of holes in the d‐band. Using the conventional and modified Tafel slopes, possible paths and rate‐determining steps are suggested for the cathodic reduction of oxygen.Keywords:
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Noble metal
The application of various electrochemical techniques to evaluate the activity of supported nano-size electrocatalysts for the oxidation of a specific fuel for fuel cell applications is examined. Cyclic voltammetry (CV) on both static and dynamic (rotating disc electrode, RDE) electrodes, and fuel cell station tests were the main electrochemical techniques used in this study. It was found from both static and dynamic CV and the fuel cell station tests that the most active catalyst is the one that shows the most negative oxidation peak potential. According to the Tafel equation, a lower anodic/cathodic overpotential is clear evidence of higher catalytic activity. This can be achieved for a specific current load by an electrocatalyst that exhibits as low a Tafel slope, and as high an exchange current density, as possible. RDE and fuel cell station tests show that the best performance was recorded for those electrocatalysts which have values of the Tafel slope (ba) and the exchange current density (io) that, on balance, give rise to the lowest overpotential. Therefore, CV and RDE are the recommended electrochemical techniques for a reliable assessment of electrocatalysts prior to performing a lrealr fuel cell test.
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CoWO 4 and NiWO 4 have been prepared by a co-precipitation method and investigated as electrocatalysts for the oxygen evolution reaction in 1 M KOH by electrochemical impedance spectroscopy. The electrode kinetic parameters such as the electrochemical active surface area, exchange current density, Tafel slope, and the reaction order are determined. Results have shown that the values of the Tafel slope and reaction order obtained from the EIS study excellently match with those determined by dc techniques.
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Tafel analysis is widely used to characterize electrochemical kinetics and assess the properties of electrocatalysts for use in fuel cells, electrolyzers, and other applications. This method is limited in part by the subjective determination of linearity, as the kinetic parameters obtained by the regression may vary significantly depending on the chosen linear region. In an effort to increase measurement quality and decrease subjectivity, an algorithm has been developed in Microsoft ® Excel ® that generates a Tafel plot from an LSV and determines the exchange current density j 0 , charge transfer coefficient α, and Tafel slope of closest fit. Comparisons of kinetic parameters between conventional and algorithmic Tafel analysis are made for the hydrogen evolution reaction (HER, 2H + + 2e - ⇌ H 2 ) for different electrodes. The algorithmic parameters correlate well with conventional methods and show increased measurement precision. Similar agreement is observed between literature and algorithmic fits of representative Tafel plots. The developed algorithm allows for straightforward, rapid, and user bias limited Tafel analysis and can be used to increase measurement quality.
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The development and characterization of active and selective catalysts is critical for the simulation and optimization of electrochemical synthesis of chemicals and fuels using renewable energy. The rate of electrochemical generation of a specific product as a function of electrode potential can be described by a Tafel equation, which depends on two parameters: the Tafel slope (or the related transfer coefficient) and the exchange current density. However, common methods for calculating Tafel slopes are subjective and limited by data insufficiency resulting from challenges associated with product quantification, and, as shown here, the effects of mass transport, bulk reaction occurring in the mass-transfer boundary layer, and the occurrence of competitive surface reactions. Errors in the Tafel slope extracted from experimental data can also lead to errors in the exchange current density estimation. To address these issues, we present a technique that leverages statistical learning methods informed by physics-based modeling to calculate kinetic parameters (the transfer coefficient and exchange current density) with quantified uncertainty. The method is applied to 21 sets of data for the electrochemical reduction of CO2 to CO and H2 on Ag catalysts acquired under similar experimental conditions. We find that fitted values for the transfer coefficient and exchange current density do not converge to a unique set of values, and that there is an apparent correlation of these parameters; however, the most probable value of the exchange coefficient for CO and H2 formation correspond reasonably well with the DFT-predicted values of this parameter. While the system explored is relatively simple, the techniques developed can be used to evaluate the transfer coefficient and exchange current density for many other electrochemical processes.
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The kinetics of the hydrogen evolution reaction at various Zr‐based laves‐phase hydrogen‐storage alloy electrodes was investigated by measuring an overpotential‐decay curve immediately after the interruption of an applied constant current. The measured overpotential was divided into two partial overpotentials corresponding to the elementary reactions, i.e., the Volmer (discharge) and Tafel (combination) reactions. The ratio of exchange current density for the Volmer (discharge) reaction to that for the Tafel (combination) reaction increased with increasing unit‐cell volume of the hydrogen‐storage alloys used, suggesting that the Tafel (combination) reaction is effectively suppressed.
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Simulation of the electrocatalytic activity of cobalt for hydrogen evolution reaction was performed in 1 M NaOH solution. The simulation of current – potential correlation was performed using the SigmaPlot program. For Tafel slope of -115 mV dec -1 and exchange current density of 5.6×10 -6 A cm -2 , a linear polarization response for cobalt was modelled. Simulation of cobalt impedance spectra in 1 M NaOH solution was also performed, using the ZView program. Electrochemical impedance spectra were modelled using Randles simple electrical equivalent circuit and presented in form of Nyquist and Bode plot. From these experimental data, electrode surface roughness, exchange current density and Tafel slope were derived.
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To obtain Tafel constant and exchange current density data for the U/U3+ couple, a uranium working electrode was examined potentiodynamically at ±15 mV overpotential at five temperatures in molten LiCl/KCl eutectic salt. This pre-Tafel region was used to avoid the presence of an observed buildup of material at uranium anodes at Tafel region overpotentials. Data were analyzed using the Oldham-Mansfeld method. The Tafel constant varied inversely with temperature, due to a direct relationship between temperature and transfer coefficient. Exchange current density followed the relation io = 20396e−4143/T. This temperature-exchange current density relationship yielded an activation energy for the U/U3+ reaction of 34.4 kJ/mol.
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