Electrochemical Double Layers in Electrochemical Energy Storage and Conversion: A Multiscale Simulation Study
Authors: Franco Alejandro A. , Nguyen Trong-Khoa , Xue Kan-Hao , Quiroga Matias , Huang He 
Published: February 2020

The nanometric liquid/solid electrochemical interface is the pivotal scale behind the operation principles of low-temperature batteries and fuel cells. It involves ionic transport by diffusion and electro-migration in the electrolyte, surface multi-step REDOX reactions, among other mechanisms. Developing an understanding of its functional role is thus crucial to help on determining the limiting factors of batteries and fuel cells performance and stability [1-3].
A very significant amount of work has been carried out since more than 100 years to understand the structure of such interfaces in electrochemical systems (also known as electrochemical double layers -EDL-) but under ideal conditions (equilibrium conditions, absence of REDOX reactions): this is not representative of the non-equilibrium conditions typically found in electrochemical power generators.
In this work a new non-equilibrium EDL multiscale model is presented, able to capture the following features of relevance on designing high performant and stable electrolytes in batteries and fuel cells [4]:
- the impact of solvent composition, additives and lithium salt concentrations onto the charge-discharge curves in lithium ion and lithium air batteries (LIBs and LABs), and thus discriminating between thermodynamic and kinetic effects;
- the impact of the platinum-based catalysts oxidation state onto the structure of the EDL, the ionomer morphology and the effectiveness of the Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells (PEMFCs).
The model is supported on an extension of the so-called "Poisson-Nernst-Planck approach" and accounts for solvent orientational polarization effects, finite ion size effects and extended Transition State Theory. REDOX reactions occuring on the surface of the active particles (LIBs and LABs) and catalysts (PEMFCs) are described through an elementary kinetic approach parametrized with DFT-calculated activation energies [5]. The influence of the coverage onto the effective activation barriers is evaluated through on lattice Kinetic Monte Carlo simulations within the Variable Step Size Method (VSSM) describing surface diffusion, reactions between adspecies and adsorption/desorption.
Some application cases of this model are discussed: for example, the model reproduces nicely well complex phenomena such as the irreversible Nafion reduction/oxidation peaks observed in cyclic voltammetry experiments in [6] (PEMFC), and it predicts how solvent composition impacts polarization and lithium ion transport properties (LIB/LAB). Finally, a cell device multiscale model integrating these EDL models is presented [7], and the EDL impact onto the overall cell performance dynamics (as function of the applied potentiodynamic and galvanodynamic conditions in PEMFCs, and cycling rate for batteries) is studied.

Acknowledgements. Prof. Dominiquer Larcher and Dr. Abdelouahab El Kharbachi (LRCS) are gratefully acknowledged for their experiments devoted to our models validation.

References

[1] A.A. Franco, Multiscale modeling of electrochemical devices for energy conversion and storage, book chapter in: Encyclopedia of Applied Electrochemistry, edited by R. Savinell, K.I. Ota, G. Kreysa (publisher: Springer, UK) (2013).
[2] A.A. Franco, Multiscale modeling methods for electrochemical energy conversion and storage, book chapter in: Multiscale Modeling Methods for Applications in Materials Science, edited by I. Kondov, G. Sutmann (publisher: CECAM & FZ Jülich, Germany), IAS Series, Volume 19, ISBN 978-3-89336-899-0 (2013).
[3] A.A. Franco, K.H. Xue, ECS J. Solid State Sc.
Tech., 2 (10) (2013) M3084.
[4] K.H. Xue, M. Quiroga, H. Huang, T.K. Nguyen, A.A. Franco, in preparation (2013).
[5] R. Ferreira de Morais, D. Loffreda, P. Sautet, A. A. Franco, Electrochim. Acta, 56(28) (2011) 10842.
[6] R. Subbaraman et al., Journal of Physical Chemistry C, 114 (18) 8414.
[7] www.modeling-electrochemistry.com

Figure. Schematics of an EDL formed at the vicinity of a growing lithium oxide layer during a LAB discharge (a), and calculated adlayer polarization (b) and solvents coverage (c) onto the lithium oxide layer as function of the surface charge density and for two solvent volume compositions.