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Nonlinearity effects of electrochemical systems kinetics on impedance measurements are re-examined in the low-frequency (LF) domain through symbolic manipulation and numerical computation performed with Mathematica. It is a new approach to some problems not yet completely resolved in electrochemical impedance spectroscopy despite a number of publications on this subject in the specialized literature. This article is focused on electrochemical systems governed by Tafel kinetics under steady-state conditions, so-called Tafelian systems below, and perturbed by a sinusoidal variation of electrode potential vs. time, at low frequency. First, harmonic analysis of the system response to sinusoidal perturbation of potential with negligible Ohmic drop effects is dealt with. Next, the combined effects of Tafel kinetics and Ohmic drop are thoroughly examined. The current-potential response at low frequency is modelled using the Lambert W-function. New formulations are derived for the nonlinear polarization resistance of Tafelian systems in the presence of Ohmic drop. Closed-form or infinite-series formulations are derived for the amplitudes of fundamental and harmonic components of periodic current. Finally, the validity condition for impedance measurements carried out in the LF domain is derived for Tafelian systems. © 2012 Elsevier B.V. All rights reserved.

Montella C.,CNRS Physical Eletrochemistry Materials and Interfaces Lab
Journal of Electroanalytical Chemistry | Year: 2012

This article deals with the influence of interfacial CPE (constant phase element) behaviour on linear scan voltammograms (LSV) and cyclic voltammograms (CV). Theoretical investigation is carried out using the generalised Mittag-Leffler function which arises naturally in the solution of fractional order integral or differential equations. First, ohmic drop and CPE effects are analysed in the absence of electrochemical reaction through the closed-form expression of current vs. time. Next, the combined effects of ohmic drop, CPE and faradaic processes are modelled using the integral equation approach. Finally, the example of one-step electrochemical reaction, investigated under semi-infinite linear diffusion conditions, is dealt with for illustration of the calculation procedure. © 2011 Elsevier B.V. All rights reserved.

Martinelli A.,Chalmers University of Technology | Marechal M.,CNRS Physical Eletrochemistry Materials and Interfaces Lab | Ostlund A.,Chalmers University of Technology | Cambedouzou J.,CNRS Marcoule Institute for Separative Chemistry
Physical Chemistry Chemical Physics | Year: 2013

We report on how the local structure and the diffusional motion change upon increasing the alkyl chain length in 1-alkyl-3-methylimidazolium cation ionic liquids. This study has been performed by combining pulse field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy and small angle X-ray scattering (SAXS) experiments. The cationic side chain length varies from ethyl (n = 2) to hexadodecyl (n = 16), while the anion is always bis(trifluoromethanesulfonyl) imide (TFSI). We find that the self-diffusivity of the individual ionic species is correlated to the local structure in the corresponding ionic liquid, namely the nano-segregation into polar and non-polar domains. In agreement with previous results, we observe that for relatively short alkyl chains the cations diffuse faster than the anions; however we also note that this difference becomes less evident for longer alkyl chains and a cross-over is identified at n ≈ 8 with the anions diffusing faster than the cations. Our results indicate that this controversial behavior can be rationalized in terms of different types of cation-cation and anion-anion orderings, as revealed by a detailed analysis of the correlation lengths and their dispersion curves obtained from SAXS data. We also discuss the validity of the Stokes-Einstein relation for these ionic liquids and the evolution of the extrapolated cationic radius that was found to depend non-strictly linearly on n, in agreement with the cation-cation correlation lengths. © 2013 the Owner Societies.

Oury A.,French National Solar Energy Institute | Kirchev A.,French National Solar Energy Institute | Bultel Y.,CNRS Physical Eletrochemistry Materials and Interfaces Lab
Electrochimica Acta | Year: 2012

This work examines the oxygen evolution reaction (OER) taking place on α-PbO 2 electrode in methanesulfonic acid (MSA) medium and in sulphuric acid as a comparison, by means of cyclic voltammetry (CVA) and electrochemical impedance spectroscopy (EIS), for soluble lead acid flow battery applications. The influence of MSA concentration on OER is studied. EIS measurements highlighted the impact of the hydrated lead dioxide layer upon decreasing MSA or sulphuric acid concentration. The evolution of the Tafel curves plotted from EIS measurements and quasi-stationary currents while varying acid concentration was interpreted in the light of this hydrated layer which could enhance the electrocatalytic activity when it is thin, and on the contrary act as an electronic barrier when growing for low acid concentration. Both EIS and CVA revealed that OER on lead dioxide is less favoured in MSA than in sulphuric acid. It is finally concluded that a high-concentrated MSA electrolyte is better for lead acid flow battery application in terms of oxygen evolution. © 2011 Elsevier Ltd. All rights reserved.

Carral C.,CNRS Physical Eletrochemistry Materials and Interfaces Lab | Mele P.,CNRS Physical Eletrochemistry Materials and Interfaces Lab
International Journal of Hydrogen Energy | Year: 2014

A finite element model is developed to investigate the influence of the assembly phase of proton exchange membrane fuel cell (PEMFC) stacks on the mechanical state of the active layer (MEAs). Validated by experimental measurements, this model offers the possibility to analyze the influence of different parameters through the use of a complete parametric set, such as the number of cells and their position in the stack. The simulations show that a better uniformity of the MEA compression is obtained with the greatest number of cells, and at the center of the stack. The finite element analysis (FEA) is finally found to be an effective tool to show the influence of the assembly phase on the performance of PEMFCs, and will help the designer to adapt the future generations of stack to ensure the uniformity of the MEA mechanical strain. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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