Unveiling Pseudocapacitive Charge Storage Behavior in FeWO4 Electrode Material by Operando X-ray Absorption Spectroscopy

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 materials.