Pseudocapacitive FeWO4 Electrode: From Charge Storage Mechanism to Practical Use in Asymmetric Cell
Authors: Crosnier Olivier , Goubard-Bretesché Nicolas , Buvat Gaëtan , Douard Camille , Iadecola Antonella , Belin Stéphanie , Retoux Richard , Payen Christophe , Favier Frederic , Kisu Kazuaki , Iwama Etsuro , Naoi Katsuhiko , Doublet Marie-Liesse , Brousse Thierry 
Published: February 2020

Pseudocapacitive electrodes can be of interest in order to improve the volumetric energy density of electrochemical capacitors, so called supercapacitors, owing to the high density of metal oxides or nitrides as compared to activated carbon [1]. Among all the materials that have been proposed up to now, iron tungstate (FeWO4) represents an interesting candidate both from a fundamental and application point of views. It crystallizes in the wolframite-type structure, characterized by distorted [MO6] and [WO6] octahedra. Different low-temperature methods including polyol-mediated synthesis [2], hydrothermal synthesis [3], ultracentrifugation method [4] have been used in order to obtain high specific surface area nanoparticles which have been characterized and electrochemically tested as supercapacitor electrode materials in aqueous electrolytes [5]. FeWO4 has demonstrated a pseudocapacitive behavior that can be attributed to Fe2+/Fe3+ surface redox reactions and thus is closely related to the specific surface area.
To further investigate the charge-storage mechanism of FeWO4, in situ and operando X-ray absorption spectroscopy (XAS) experiments were performed at both Fe K- (7112 eV) and W L-edges (10204 and 12100 eV) on the new ROCK beamline at the SOLEIL synchrotron (France), using a specially designed 3-electrode-cell assembly. The composite working electrodes were composed of 60% FeWO4, 30% carbon black and 10% PTFE with a loading of about 10mg of FeWO4 per cm2. Cyclic voltammograms were obtained between -0.6 and 0 V vs. Ag/AgCl in neutral 5M LiNO3 at room temperature. An acquisition time of 100 ms per spectrum is possible on the ROCK beamline, thanks to its quick-EXAFS monochromator, thus allowing following very accurately the oxidation state evolution of both Fe and W in the electrodes during cycling. The XAS experiments show an evolution of the iron oxidation state (Fe2+/Fe3+) while cycling, thus confirming the Fe-related pseudocapacitive behavior of FeWO4, while no evolution was observed on the tungsten oxidation state. The characterizations and XAS results will be detailed in the presentation. Poor alteration of the crystal structure was also observed which open the way for the use of FeWO4 as possible long term cycling electrode in a supercapacitor.
Thus, FeWO4 was tested as negative electrode in an asymmetric electrochemical capacitor using cryptomelane-type MnO2 as the positive in a neutral aqueous electrolyte. Prior to assembling the cell, the electrodes have been individually tested in a 5M LiNO3 electrolyte solution to define both the adequate balance of active material in the supercapacitor and the proper working voltage window. The full asymmetric device has been cycled between 0 and 1.4 V for over 40 000 cycles and subjected to accelerated ageing tests under floating conditions at different voltages, without any significant change on its electrochemical behavior. This remarkable stability shows the interest of developing full oxide-based asymmetric supercapacitors operating in non-toxic aqueous electrolytes. Cell performances will be reported in this communication.
References
1) Nicolas Goubard-Bretesché, Olivier Crosnier, Gaëtan Buvat, Frédéric Favier, and Thierry Brousse, submitted to JPS, April 2016.
2) J. Ungelenk, M. Speldrich, R. Dronskowski, C. Feldmann, Solid State Sci. 31 (2014) 62.
3) S.H. Yu, B. Liu, M.S. Mo, J.H. Huang, X.M. Liu, Y.T. Qian, Adv. Funct. Mater. 13 (2003) 639.
4) K. Kisu, M. Iijima, E. Iwama, M. Saito, Y. Orikasa, W. Naoi and K. Naoi, J. Mater. Chem. A, 2014, 2, 13058-13068.
5) N. Goubard-Bretesché, O. Crosnier, C. Payen, F. Favier, T. Brousse, Electrochem. Commun. 57 (2015) 61.