Unveiling Pseudocapacitive Charge Storage Behavior in FeWO₄ Electrode Material by Operando X‐Ray Absorption Spectroscopy

Abstract

In nanosized 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. 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 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 materials.

Investigating the Cycling Stability of Fe2WO6 Pseudocapacitive Electrode Materials

“…On the other hand, the sample FWO-600 exhibits a quasi-rectangular shape CV with a specific capacitance of 23 F•g −1 at 20 mV•s −1 . Such a behavior is very similar to that observed for FeWO 4 material cycled under the same condition, [15,16], which has been attributed to the Fe 3+ /Fe 2+ redox couple [16].…”

“…The family of Fe-W-O has been studied in our team in the wolframite phase FeWO 4 , showing interesting performance as an electrode material for EC operated in a neutral aqueous electrolyte [14,15]. The role of the Fe 3+ /Fe 2+ redox couple has been recently proven to be responsible of the pseudocapacitive behavior [16]. Apart from its pseudocapacitive behavior, this compound is interesting to prepare high volumetric capacitance electrode due to its high density compared to carbon compounds.…”

Electrochemical Energy Storage: Questioning the Popular v/v1/2 Scan Rate Diagnosis in Cyclic Voltammetry

“…A series of successive experimental and modeling studies have shown that pseudocapacitance is not a viable concept but that the concerned materials behave as true capacitances once their electrochemical oxidation has made them become electronic conductors. These conclusions have not been challenged since, − but rather have been mentioned as a contribution to an open debate. , …”

“…These conclusions have not been challenged since, 8−10 but rather have been mentioned as a contribution to an open debate. 6,11 The v/v 1/2 Scan Rate Diagnosis Rid of the Notion of Pseudocapacitance. Once the misleading notion of pseudocapacitance is discarded and replaced by the standard true capacitance notion within the potential domain of interest, the discussion becomes actually simpler because the variation of the CV response with the scan rate is representative of two phenomena, namely, double-layer charging (referred to as the capacitive current) and faradaic current involving redox sites in the material (including its surface) and therefore associated with ion insertion.…”

Iron Tungsten Nanorods Electrode with High Capacitance: An Extraordinary Cycling Stability for Durable Aqueous Supercapacitors

“…However, this device was capable of withstanding 50 000 cycles with only a 7% capacitance downturn between the 100th and 50 000th cycles, demonstrating the FeWO 4 /FeWO 4 device’s exceptional stability. The excellent cycling ability of the assembled device could be attributed primarily to the use of FeWO 4 NRs as an electrode, where the ions of the electroactive iron are sustained in a specific crystallographic position . Also, its nanorod structure, which adapts to volume contraction and expansion during the cycle, demonstrates the supercapacitor’s highly stable nature .…”