Invited: Probing Pseudocapacitance Mechanisms in Metal-Oxide–Functionalized Carbon Nanoarchitectures with an Electrochemical Quartz Microbalance
Authors: Long Jeffrey W. , Sassin Megan B. , Rolison Debra R. , Beasley Christopher A. 
Published: February 2020

Transition-metal oxides that exhibit “pseudo-capacitance” are challenging high-surface-area carbons as the active material of choice for next-generation electrochemical capacitors (ECs).1 The electrochemical performance of such EC-relevant metal oxides as MnOx has been extensively reported,2 yet the underlying mechanisms responsible for pseudocapacitance are still a subject of debate in the scientific literature. Spectroscopic techniques have been used to verify reversible oxidation-state cycling of metal sites in these oxides (Mn(III/IV) in MnOx; Fe(II/III) in FeOx),3,4 but less is known about the participation of charge-compensating cations from the contacting electrolyte. In the case of mild aqueous electrolytes, redox reactions that give rise to pseudocapacitance may be coupled with insertion/ deinsertion of cations from the electrolyte salt (e.g., Li+, Na+, K+), protons that are available from the H2O solvent, or combinations thereof. Electrochemical quartz crystal microbalance (EQCM) techniques provide information on changes in electrode mass during electrochemical cycling that can address questions regarding ion insertion.5 Until recently the use of EQCM has been limited to well-defined thin-film electrodes, but advancements in EQCM methods now permit analysis of a wider range of substrates, including powder-composite electrodes of EC relevance.6,7

In this paper, we will describe the application of EQCM analysis to a class of electrode architectures comprising carbon nanofoam papers8 whose exterior and interior surfaces are exhaustively coated with nanometers-thick metal oxides that exhibit pseudocapacitance.4
,9,10,11 Carbon nanofoam papers are freestanding, device-ready electrode architectures of controllable macroscale dimensions (1–102 cm2 in area; 100–500 mm thickness) in which nanoscale dimensions of pore and solid can also be readily adjusted over a wide range (10 nm to several mm). The addition of a nanoscale metal-oxide coating to nanofoam substrates amplifies charge-storage capacity via pseudocapacitance, while the high-frequency response inherent to the nanofoam architecture is retained by ensuring that the metal oxide coating is conformal and self-limiting and that the pore network stays open in three dimensions.
For EQCM analysis of these nanofoam-based electrode architectures, we attach disk-shaped pieces of native or oxide-coated carbon nanofoam paper to Au-coated quartz crystals using a carbon-based conductive epoxy. Voltammetric cycling of the resulting electrodes in an EQCM cell yields the expected pseudocapacitance response in terms of both shape and magnitude, demonstrating effective electrical contact to the nanofoam paper. Coupled to the electrochemical response, we monitor mass changes during electrochemical cycling while also varying such parameters as aqueous electrolyte composition (Li+ vs. Na+ vs. K+), average pore size of the nanofoam (10–200 nm), and the type of metal-oxide coating (MnOx vs. FeOx). Preliminary EQCM experiments show that “depletion” effects6 are observed in the presence of Li+-containing electrolytes and for MnOx-coated nanofoams with pores <40 nm, in which the pseudocapacitance mechanism switches from Li+ to NO3
– insertion/deinsertion over the most positive regions of the active potential window of MnOx. The EQCM-observed depletion effect corresponds with the slowest time response of this series of MnOx–carbon nanofoams, as previously reported.12 We compare EQCM results obtained from nanofoam-based electrode architectures with those for thin-film versions of the respective metal oxides. Monitoring mass changes during electrochemical cycling will also allow us determine the conditions under which deleterious dissolution reactions may occur.References

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