Extending Electrochemical Quartz Crystal Microbalance Techniques to Macroscale Electrodes: Insights on Pseudocapacitance Mechanisms in MnOx-Coated Carbon Nanofoams

Electrochemical quartz crystal microbalance studies of MnO x -coated carbon nanofoams reveal that charge-compensation mechanisms associated with MnO x pseudocapacitance in mild aqueous electrolytes are dominated by anion insertion rather than more commonly reported cation ejection. Specific charge-compensation behavior depends on such factors as electrolyte composition, nanofoam pore size, and polarization amplitude. For example, MnO x –carbon nanofoams with average pore sizes of 5–20 nm, cycled in 2.5 M LiNO 3 , reveal a kinetically-hindered, mixed anion-cation charge-compensation mechanism, whereas the same nanofoam cycled in 2.5 M NaNO 3 shows only anion association. Nanofoams with larger pores (10–200 nm) that are cycled in 2.5 M LiNO 3 , reveal anion-only charge compensation. Our results demonstrate that critical new insights on charge-storage mechanisms are achieved using EQCM methods, even when analyzing practical, macroscale electrodes such as carbon nanofoams.