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

By Christopher A. Beasley, Megan B. Sassin, Jeffrey W. Long
Published in Journal of The Electrochemical Society ECS Publications 2014

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 LiNO₃, reveal a kinetically-hindered, mixed anion-cation charge-compensation mechanism, whereas the same nanofoam cycled in 2.5 M NaNO₃ shows only anion association. Nanofoams with larger pores (10–200 nm) that are cycled in 2.5 M LiNO₃, 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.Transition metal oxides that exhibit “pseudocapacitance”, capacitor-like behavior that arises from faradaic reactions, are challenging high-surface-area carbons, which rely primarily on double-layer capacitance, as active materials in next-generation electrochemical capacitors (ECs) Because pseudocapacitance involves electron-transfer 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 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. 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.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 H₂O solvent (in the form of H⁺ or H₃O⁺), or combinations thereof. 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 as “MnO₂” or “MnOx”, but which have widely varying crystalline structures and compositions. The electrochemical behavior of manganese oxides in the aqueous electrolytes of interest for ECs depends highly on the specific crystal habit of the oxide (e.g., layered vs. tunnel structures).The electrochemical quartz crystal microbalance (EQCM) provides information on changes in electrode mass during electrochemical cycling, and thus can be used to address questions regarding electrochemical ion-insertion/association reactions for such materials as MnOx. In the past, EQCM analyses were limited to well-defined thin-films (i.e., acoustically thin), but recent advancements in EQCM methods now permit the analysis of a wider range of substrates, including EC-relevant powder-composite inksCarbon nanofoam papers are freestanding, device-ready electrode architectures of controllable macroscale dimensions (many cm2 in area; 70–500 μm thickness; see Figure 1a,1b) and nanoscale feature sizes (pore and solid domains from 10 nm to several μm). Self-limiting redox-deposition protocols can be used to generate conformal, nanoscale coatings of EC-relevant metal oxides (MnOx, FeOx, RuO₂) on the interior and exterior surfaces of the nanofoams such that the through-connected nature of the nanofoam pore structure is maintained, even after extensive oxide deposition. The incorporated metal oxides provide pseudocapacitance that significantly enhances the charge-storage capacity of the nanofoam beyond its inherent double-layer capacitance, while the nanoscale dimensions of the oxide coatings and readily available supply of ions within the electrolyte-filled pore structure of the nanofoam support high-rate charge–discharge response. This class of electrode materials has proved an effective platform with which to investigate the interplay of structure and electrochemical performance for pseudocapacitive oxides and also exhibits “plug-and-play” function for ready incorporation into EC devices. In this paper, we use MnOx-coated carbon nanofoams to demonstrate that EQCM techniques can be exploited to elucidate pseudocapacitance mechanisms in practical, macroscale electrode structures.

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