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

Abstract

Electrochemical quartz crystal microbalance studies of MnOx-coated carbon nanofoams reveal that charge-compensation mechanisms associated with MnOx 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, MnOx-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) Transition metal oxides that exhibit "pseudocapacitance", capacitor-like behavior that arises from faradaic reactions, are challenging high-surface-area carbons, which rely primarily on doublelayer capacitance, as active materials in next-generation electrochemical capacitors (ECs).1-4 Because pseudocapacitance involves electrontransfer reactions, the quantity of charge-storage per mass or volume often surpasses that achieved with double-layer capacitance alone. The enhanced charge-storage capacity provided by pseudocapacitive metal oxides compensates for the voltage limitations of aqueous-electrolyte ECs, resulting in asymmetric aqueous EC designs 5-7 that provide competitive performance and safer operation compared to conventional symmetric carbon-carbon ECs that use organic electrolytes.Manganese oxides (MnOx) have emerged as one of the most important classes of pseudocapacitive materials due to their low cost, competitive specific capacitance, availability in a wide range of compositions and crystal habits, and adaptability to a variety of electrode architectures. [8][9][10][11] The electrochemical characteristics of MnOx have been extensively demonstrated, yet the underlying mechanisms responsible for pseudocapacitance are still a subject of debate. Spectroscopic methods have been used to confirm that pseudocapacitive charge storage is supported by reversible toggling of the Mn oxidation state (ranging between +3 and +4, but typically <1 e -per Mn site) during electrochemical cycling in aqueous electrolytes. [12][13][14] Changes in Mn oxidation state must be accompanied by insertion or adsorption of charge-balancing ions from the contacting electrolyte. In the case of mild aqueous electrolytes, charge compensation typically occurs via cation-insertion/association mechanisms where the cations are supplied by either the electrolyte salt (e.g., Li + , Na + , or K + ), the H 2 O solvent (in the form of H + or H 3 O + ), or combinations thereof.14-20 Because of the prospective participation of multiple types of ions (in varying degrees of solvation), deconvolution of MnOx pseudocapacitance mechanisms has proven difficult. The ability to draw broader conclusions regarding MnOx pseudocapacitance mechanisms has been further complicated by the wide range of materials that are broadly identified in the literature a...

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