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Back-exchange: a novel approach to quantifying oxygen diffusion and surface exchange in ambient atmospheres

Cooper, Samuel J. and Niania, Mathew and Hoffmann, Franca and Kilner, John A. (2017) Back-exchange: a novel approach to quantifying oxygen diffusion and surface exchange in ambient atmospheres. Physical Chemistry Chemical Physics, 19 (19). pp. 12199-12205. ISSN 1463-9076. doi:10.1039/c7cp01317e.

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A novel two-step Isotopic Exchange (IE) technique has been developed to investigate the influence of oxygen containing components of ambient air (such as H₂O and CO₂) on the effective surface exchange coefficient (k*) of a common mixed ionic electronic conductor material. The two step ‘back-exchange’ technique was used to introduce a tracer diffusion profile, which was subsequently measured using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The isotopic fraction of oxygen in a dense sample as a function of distance from the surface, before and after the second exchange step, could then be used to determine the surface exchange coefficient in each atmosphere. A new analytical solution was found to the diffusion equation in a semi-infinite domain with a variable surface exchange boundary, for the special case where D* and k* are constant for all exchange steps. This solution validated the results of a numerical, Crank-Nicolson type finite-difference simulation, which was used to extract the parameters from the experimental data. When modelling electrodes, D* and k* are important input parameters, which significantly impact performance. In this study La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O_(3−δ) (LSCF6428) was investigated and it was found that the rate of exchange was increased by around 250% in ambient air compared to high purity oxygen at the same pO₂. The three experiments performed in this study were used to validate the back-exchange approach and show its utility.

Item Type:Article
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URLURL TypeDescription
Cooper, Samuel J.0000-0003-4055-6903
Hoffmann, Franca0000-0002-1182-5521
Additional Information:© 2017 the Owner Societies. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Submitted 28 Feb 2017; Accepted 16 Apr 2017; First published 27 Apr 2017. The authors would like to thank Dr Richard Chater and Dr Sarah Fearn of Imperial College London for the guidance with the ToF-SIMS analysis, as well as Dr Renaud Podor of the L'Institut de Chimie Séparative de Marcoule (ICSM) for his work on the HT-ESEM. The authors would also like to acknowledge the EPSRC grant EP/J003085/1 for supporting this work.
Funding AgencyGrant Number
Engineering and Physical Sciences Research Council (EPSRC)EP/J003085/1
Issue or Number:19
Record Number:CaltechAUTHORS:20200330-153150788
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:102177
Deposited By: Tony Diaz
Deposited On:30 Mar 2020 22:42
Last Modified:16 Nov 2021 18:09

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