Tafel equation

The Tafel equation is an equation in electrochemical kinetics relating the rate of an electrochemical reaction to the overpotential. The Tafel equation was first deduced experimentally and was later shown to have a theoretical justification. The equation is named after Swiss chemist Julius Tafel." It describes how the electrical current through an electrode depends on the voltage difference between the electrode and the bulk electrolyte for a simple, unimolecular redox reaction ". Ox + n e^- \leftrightarrows Red Where an electrochemical reaction occurs in two half reactions on separate electrodes, the Tafel equation is applied to each electrode separately. On a single electrode the Tafel equation can be stated as: where

the plus sign under the exponent refers to an anodic reaction, and a minus sign to a cathodic reaction, *\eta : overpotential, V
A : "Tafel slope", V
i : current density, A/m2 *i_0 : "exchange

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Modeling and Optimal Control of a Redox Flow Battery

Véronique Amstutz, Alberto Battistel, Julien Léo Billeter, Dominique Bonvin, Timm Faulwasser, Hubert Girault, Heron Vrubel, Daniel Wrang

Vanadium Redox Flow Batteries (VRFB) can be used as energy storage device, for example to account for wind or solar power fluctuations. In VRFBs charge is stored in two tanks containing two different vanadium solutions. This approach decouples the storage capacity and the power supply which is dependent only on the number and size of the cells [1]. A control specific model of a VRFB is proposed, which captures the essential dynamic properties of the battery while ignoring all fluid mechanical elements. The model of the battery is a nonlinear DAE comprising a differential equation for the state of charge SoC and algebraic equations based on the Butler-Volmer equation for the current I and the voltage U, the power P = U∙I being set by the operator. The battery typically operates in constant power mode, but the control system limits its operating range by switching to a constant voltage when upper or lower voltage thresholds are reached. In addition, this VRFB contains a secondary flow circuit, where the electrolyte discharge reactions produce hydrogen and oxygen [2]. Model parameters are estimated by minimizing the difference between the measured and modeled SoC over time, the other states (I, U and P) being compared with measured data. The model is validated using independent measurements, which show good fits. Finally, the dynamic model will be used to formulate and numerically solve the problem of optimal battery operation in different scenarios. [1] C. Blanc, A. Rufer, Chapter 18, in Paths to Sustainable Energy, InTech, 2010 [2] V. Amstutz et al., Energy and Environmental Science 7, 2350-2358 (2014)

2016

IrOOH nanosheets as acid stable electrocatalysts for the oxygen evolution reaction

Filip Mateusz Podjaski

In solids, heterogeneous catalysis is inherently bound to reactions on the surface. Yet, atomically efficient preparation of specific surfaces and the characterization of their properties are impeding its applications towards a clean energy future. Here, we present the synthesis of single layered IrOOH nanosheets and investigations of their structure as well as their electrochemical properties towards oxygen evolution under aqueous acidic conditions. The nanosheets are synthesized by treating bulk IrOOH with a tetrabutylammonium hydroxide solution and subsequent washing. Electron diffraction shows that the triangular arrangement of the edge sharing Ir(O,OH)(6) octahedra found in the layers of bulk IrOOH is retained after exfoliation into single layers. When incorporated as an active component in Ti electrodes, the nanosheets exhibit a Tafel slope of 58(3) mV dec(-1) and an overpotential of eta(-2)(10 mA cm) = 344(7) mV in 0.1 M HClO4, while retaining the trivalent oxidation state of iridium. They outperform bulk rutile-IrO2 and bulk IrOOH as electrocatalytic water oxidation catalysts under the same conditions. The results of this study on the structure-property relationships of low valence IrOOH nanosheets offer new pathways for the development of atom efficient, robust and highly active oxygen evolution catalysts.

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Chlorine evolution at highly boron-doped diamond electrodes

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The Cl2 evolution reaction was studied at highly boron-doped diamond thin film electrodes. The comparison of this carbonaceous material with graphite and glassy carbon points out the similar behavior in terms of diagnostic parameters and related mechanisms, without the mech. fragility of these materials. Tafel slopes at different chloride concns. in soln. (from 0.1-4 M NaCl in 0.01 M HClO4) and at different pHs were detd., together with the reaction orders with respect to Cl- and H+. All measurements were carried out at a const. ionic strength. The electrode characterization was done by cyclic voltammetry, showing a detailed picture for the chloride oxidn. and the redn. of the evolved Cl2. Significant surface modifications occur when the electrode works as an anode for oxygen evolution, while Cl2 evolution does not seem to cause severe changes. As shown by tests carried out following a method suggested in the literature, the faradaic yield for Cl2 prodn. is expected to be very high. In dil. chloride media and at neutral-weakly alk. pH, a faradaic yield of ~65% was found; this makes use of highly doped diamond electrodes in, e.g., seawater electrolysis, quite promising. [on SciFinder (R)]

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