(Invited) Discussion on the Way(s) to Analyze the Electrochemical Impedance Spectroscopy Spectra for the Reduction of Nitric Acid on Passive Metals
Authors: Benoit Marie , Bataillon Christian , Gwinner Benoit , Miserque Frédéric , Sanchez-Sanchez Carlos , Tribollet Bernard , Vivier Vincent 
Published: February 2020

This work was performed in the context of the reprocessing of the spent nuclear fuel, where nitric acid is used for the dissolution of the fuel. The devices are made of passivating metals like stainless steel or zirconium that naturally form a protective oxide film in contact with nitric acid. This passive film limits the oxidation kinetics of the metal but also the reduction of nitric acid. To clarify the way by which the passive film influences the kinetics of the nitric acid reduction, electrochemical impedance spectroscopy (EIS) measurements were performed at different potentials in the cathodic domain. The objective of this presentation is to discuss the different ways that can be proposed to analyze such EIS spectra. The main difficulty (but also the main interest) is that both information relative to the material and the nitric acid reduction are combined in the EIS spectra. It seems that the high frequency range concerns mainly the metal properties, whereas the low frequency range contains mainly the information about the reduction reaction.

Having corrected the spectra by the electrolyte resistance, the extrapolation at infinite frequency was first considered. A Cole and Cole representation allowed us extrapolating a pure capacitive behavior at infinite frequency [1]. This was mainly attributed to the capacitive behavior of the oxide film with a minor contribution of the electrical double layer. By this way, the thickness of the oxide was estimated and the result was successfully compared with the estimations from ex situ XPS measurements [2]. In the high frequency range, a non-ideal capacitive behavior was observed corresponding to the non-ideal structure of the oxide film. The coherence with the description of the oxide film by Jonscher (dielectric losses) [1, 3] or in terms of the power law model (distribution of resistivity in the oxide) [4-6] was successively studied. In the low frequency range, the charge transfer resistance was estimated. The dependency of this charge transfer resistance with the potential, allowed us estimating the kinetic constant of the reduction reaction. Moreover a capacitive behavior was also observed, which could be attributed (as a function of the thickness of the oxide film) to the electrical double layer or the space charge capacitance (Mott-Schottky).

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