Unveiling Pseudocapacitive Charge Storage Behavior in FeWO4Electrode Material by Operando X-Ray Absorption Spectroscopy

In nanosized FeWO(4)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. In order 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 is 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 are obtained in real time upon cycling the electrode, which allows 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 L-3-edge measurements point out the involvement of the Fe3+/Fe(2+)redox couple in the charge storage while W(6+)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 materials.