Electrochimica Acta 52 (2007) 4124–4131 The use of channel flow cells for electrochemical kinetic studies in high temperature aqueous solutions George R. Engelhardt a , Ritwik Biswas b , Zaki Ahmed c , Serguei N. Lvov d , Digby D. Macdonald e,∗ a OLI Systems Inc, 108 The American Road, Morris Plains, NJ 07950, USA b Siemens Power Generation, 4400 Alafaya Trail, Orlando, FL 32826, USA c Research Institute, King Faud University of Petroleum and Minerals, Dhahran, Saudi Arabia d Department of Energy and Geo-Environmental Engineering, The Pennsylvania State University, University Park, PA 16802, USA e Center for Electrochemical Science and Technology, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA Received 13 November 2006; received in revised form 22 November 2006; accepted 22 November 2006 Available online 17 January 2007 Abstract It has been shown that, for the case of one-step reactions of arbitrary order, the relationship between the average current density and the limiting current density on a working electrode mounted on the inner radius of an annular flow channel of arbitrary length obeys, with great accuracy, the same relations as does a reaction on a uniformly accessible surface. This allows us to combine the advantages of non-uniformly accessible surfaces (high sensitivity, and no need to use rotating contacts) with the advantages of uniformly accessible surface systems (simple treatment of experimental data). This feature can be very important when investigating systems at high temperatures and pressures, where RDEs are difficult to employ. Using this approach, and by employing previously measured polarization data, the kinetic parameters (exchange current density and anodic transfer coefficient) for the oxidation of hydrogen on platinized nickel in 0.1 M NaOH + 0.7 × 10 −3 mH 2 at temperatures between 25 ◦ C and 300 ◦ C have been derived. The anodic transfer coefficient is found to be almost temperature independent with a value of 0.43. The exchange current density displays Arrhenius behavior with temperature, increasing from 1.9 × 10 −4 A cm −2 at 25 ◦ C to 3.9 × 10 −3 A cm −2 at 300 ◦ C. © 2007 Elsevier Ltd. All rights reserved. Keywords: Annular flow channel; Uniformly accessible electrode; Hydrogen oxidation; Elevated temperature 1. Introduction The quantitative description of many corrosion and electro- chemical processes at elevated temperatures is often restricted by the lack of accurate experimental data for transport coeffi- cients and charge transfer kinetic parameters. Only a few data for reaction rate constants, exchange current densities, and transfer coefficients for temperatures higher than 100 ◦ C are available for even the most commonly studied reactions. For this reason, unknown parameters are often set equal to the same parame- ters for different electrochemical reactions (that are known), or values are simply assumed without explanation or justification. For example, it is sometimes assumed that the kinetic param- ∗ Corresponding author. E-mail address: ddm2@psu.edu (D.D. Macdonald). eters (rate constant and transfer coefficient) for hydrogen ion reduction coincide with those for the reduction of water [1]. Also, it should be noted that, even in cases where direct exper- imental data for the exchange currents and transfer coefficients are available, the accuracy of the data are often questionable, because they are not obtained from cells possessing well-defined hydrodynamic and mass transfer properties. To our knowledge, the sole laboratory system having well-controlled hydrodynamic characteristics that has been used for measuring kinetic param- eters of redox reactions at elevated temperatures (T > 200 ◦ C) is the annular flow channel [2,3]. For elevated temperature work, this system has some advantages over the rotating disk electrode (RDE), because of the difficulty of devising rotating electrical contacts for use at high temperatures and pressures. A second issue concerns the uniform accessibility of the reac- tion surface. While the RDE represents a uniformly accessible surface when operating under complete mass transfer control, 0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.11.027