New Insights into Pseudocapacitive Charge-Storage Mechanisms in Li-Birnessite Type MnO2 Monitored by Fast Quartz Crystal Microbalance Methods

Fast ionic transfer and transport properties continue to be one of the main pressing research concerns regarding energy storage materials (batteries, supercapacitors). Accompanying this search for optimal materials, appropriate characterization tools to assess key parameters of newly developed materials are required. In spite of its great relevance, fast electrogravimetric methods, i.e., coupling fast quartz crystal microbalance (QCM) and electrochemical impedance spectroscopy (EIS) (ac-electrogravimetry), have not yet been used for studying transfer and transport phenomena in materials for charge storage (except for the use of QCM along with cyclic voltammetry experiments (EQCM)). This coupled method, socalled ac-electrogravimetry, differs from classical EQCM and measures the usual electrochemical impedance, ΔE/ΔI (ω), and the mass variations of the film under a sinusoidal potential perturbation, Δm/ΔE (ω), simultaneously. This coupling has the ability to detect the contribution of the charged or uncharged species and to separate the anionic, cationic, and free solvent contributions during the various (pseudo)capacitive processes. The Li-birnessite type MnO2 thin films were studied in two different media, LiClO4 and NaClO4, by ac-electrogravimetry. The Li+ ions, Na+ ions, and their respective hydrated ionic species are detected to be involved in the pseudocapacitive charge storage of Li-birnessite type MnO2 thin films. The kinetics (fci) and resistance (Rti) of charged and noncharged species transferred at the electrode/electrolyte interfaces and the number of water molecules in the hydration shell of the ions are estimated considering integer values. The opposite flux direction of free water molecules was also detected by ac-electrogravimetry. This indicates that there is a population of hydrated Li+ or hydrated Na+ ions losing their hydration shell before being transferred at the electrode/electrolyte interfaces. Therefore, the effect of desolvation is clearly and experimentally demonstrated. The ac-electrogravimetry responses of the electrodeposited Li-birnessite type MnO2 thin films can serve as a gravimetric probe for studying the charge-storage mechanisms and extracting subtleties unreachable with classical tools. (Figure Presented). © 2014 American Chemical Society.