Thierry Brousse1 Nicolas Goubard-Bretesché1 Olivier Crosnier1 Camille Douard1 Christophe Payen1 Frédéric Favier2 Antonella Iadecola3 Richard Retoux4 Kazuaki Kisu5 Katsuhiko Naoi5 Etsuro Iwama5

1, University of Nantes/CNRS, Nantes Cedex 3, , France
2, ICG/University Montpellier 2, Montpellier, , France
3, Synchrotron SOLEIL, Gif-sur-Yvette, , France
4, CRISMAT, Caen, , France
5, TUAT, Tokyo, , Japan

Unlike carbon electrodes which enable charge storage through capacitive process, pseudocapacitive materials store charges through fast and reversible surface redox reactions that confers them a "capacitive like" behavior. In such peculiar materials multiple valency cations are involved in the charge storage mechanism as it was shown for MnO2 or RuO2 electrode. In order to understand the electrochemical behavior of pseudocapacitive materials, a deeper understanding of the role of electroactive cations is required. However, this is seldom reported in the literature. One probable reason for that is the high kinetic of charge storage mechanism, i.e. few seconds to charge or discharge an electrode, which does not ease the operando analysis of pseudocapacitive materials. Up to now, this issue has mainly been addressed by the use of in situ experiments which require long polarization steps at constant potentials that are not compatible with the time frame of electrochemical capacitor operation.
Metal tungstates (M2+WO4) represent an important group of inorganic high-density oxides that were recently proposed as potential electrode for electrochemical capacitors and Li-ion batteries. Nanosized iron tungstate (FeWO4) for example has been synthesized using various methods (polyol-mediated synthesis, hydrothermal synthesis, microwaves synthesis, ultra centrifugation synthesis). The different powders of FeWO4 have been electrochemically tested in aqueous electrolytes, exhibiting a pseudocapacitive behavior over a limited potential window. Subsequently, operando X-ray absorption spectroscopy was used for the first time on such pseudocapacitive electrode at SOLEIL Synchrotron on the ROCK beamline (Rocking Optics for Chemical Kinetics) in order to probe the charge storage mechanism while operating the electrode using potentiodynamic conditions. XAS measurements were performed on FeWO4 electrode at both Fe K-edge and W L3-edge, allowing very accurate determination of the oxidation state evolution of both Fe and W during cycling. Operando XAS enables to evidence the role of Fe2+/Fe3+ in charge storage while W6+ seem to act as a spectator cation. In this communication, the main results will be presented and the relationship between capacitance and the characteristics of the different FeWO4 powders will be detailed.