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submitted on 07.12.2019 and posted on 16.12.2019by Nicolas Goubard-Bretesché, Olivier Crosnier, Camille Douard, Antonella Iadecola, Richard Retoux, Christophe Payen, Marie-Liesse Doublet, Kazuaki Kisu, Etsuro Iwama, Katsuhiko Naoi, Frédéric Favier, Thierry Brousse
In nano-sized FeWO4 electrode material, both Fe and W metal cations are
suspected to be involved in the fast and reversible Faradaic surface
reactions giving rise to its pseudocapacitive signature. As for any
other pseudocapacitive materials, to fully understand the charge storage
mechanism, a deeper insight into the involvement of the electroactive
cations still has to be provided. The present paper illustrates how
operando X-ray absorption spectroscopy (XAS) has been successfully used
to collect data of unprecedented quality allowing to elucidate the
complex electrochemical behavior of this multicationic pseudocapacitive
material. Moreover, these in-depth experiments were obtained in real
time upon cycling the electrode, which allowed investigating the
reactions occurring in the material within a realistic timescale, which
is compatible with electrochemical capacitors practical operation. Both
Fe K-edge and W L3-edge measurements point out the involvement of the
Fe3+/Fe2+ redox couple in the charge storage while W6+ acts as a
spectator cation. The result of this study enables to unambiguously
discriminate between the Faradaic and capacitive behavior of FeWO4.
Beside these valuable insights toward the full description of the charge
storage mechanism in FeWO4, this paper demonstrates the potential of
operando X-ray absorption spectroscopy to enable a better material
engineering for new high capacitance pseudocapacitive electrode