Ion intercalation dynamics of electrosynthesized mesoporous WO3 thin films studied by multi-scale coupled electrogravimetric methods

Abstract

Mesoporous WO₃ thin films were prepared electrochemically by using an ionic surfactant during the synthesis, and the electrochemical properties are investigated in comparison with their dense analogues. This report specifically highlights the suitability of a time resolved coupled electrogravimetric method to follow meticulously the ion intercalation/extraction phenomena which revealed the enhanced ion intercalation/extraction behavior of electrodeposited mesoporous WO₃ thin films for diverse applications in energy storage and electrochromism. This methodology (electrochemical impedance spectroscopy (EIS) and its coupling with a fast quartz crystal microbalance (QCM)) has the ability to detect the contribution of the charged or uncharged species during the electrochemical processes, and to deconvolute the global EQCM responses into the anionic, cationic, and the free solvent contributions. Our study identifies the involvement of several charged species (Li⁺, Li⁺·H₂O) in the compensation of charge, and H₂O molecules indirectly contribute to the process in both dense and mesoporous WO₃ thin films. Even a slight contribution of ClO₄⁻ ions was detected in the case of mesoporous analogues. The results of the study indicate that the transfer resistances of Li⁺ and Li⁺·H₂O are decreased when the WO₃ films are mesoporous. A more significant difference is observed for the larger and partially dehydrated Li⁺·H₂O ions, suggesting that increased surface area and pore volume created by mesoporous morphology facilitate the transfer of larger charged species. The relative concentration changes of cations are also magnified in the mesoporous films. The final concentration variations are higher in mesoporous films than that in the dense analogues; ∼4 times and ∼10 times higher for Li⁺ and for Li⁺·H₂O, respectively. To the best of our knowledge, an unambiguous identification of species other than desolvated cations (e.g. Li⁺ ions), the information on their transfer dynamics and quantification of the transferred species have never been reported in the literature to describe the charge compensation process in WO₃ based electrodes.