Electrochemical and analytical techniques were utilized to study Ca electrodeposition in nonaqueous electrolytes. Linear sweep voltammograms obtained at Au and Pt ultramicroelectrodes (UMEs) exhibit an inverse dependence between current density and scan rate, indicative of the presence of a chemical reaction step in a chemical–electrochemical (CE) deposition process. However, the magnitude of change in current density as a function of scan rate is larger at the Au UME than at the Pt UME. COMSOL simulation reveals that the chemical reaction step rate (k c) obtained at the Pt UME is ∼10 times faster than that at the Au UME. Field desorption ionization mass spectrometry (MS) suggests that dehydrogenation of the borohydride anions by the metal substrate is the chemical reaction step. Pt is more efficient at abstracting hydride from borohydride ions than Au, leading to larger k c. Raman spectroscopy and electrospray ionization MS data show that Ca2+ ions are strongly coordinated with tetrahydrofuran and weakly interacting with BH4 – anions. Electron microscopy shows that the surface morphology of Ca electrodeposition is different between Au and Pt, with Au exhibiting a smooth deposit, while a patchier deposit is seen on Pt.
The rechargeable K-O battery is recognized as a promising energy storage solution owing to its large energy density, low overpotential, and high coulombic efficiency based on the single-electron redox chemistry of potassium superoxide. However, the reactivity and long-term stability of potassium superoxide remains ambiguous in K-O batteries. Parasitic reactions are explored and the use of ion chromatography to quantify trace amounts of side products is demonstrated. Both quantitative titrations and differential electrochemical mass spectrometry confirm the highly reversible single-electron transfer process, with 98 % capacity attributed to the formation and decomposition of KO . In contrast to the Na-O counterparts, remarkable shelf-life is demonstrated for K-O batteries owing to the thermodynamic and kinetic stability of KO , which prevents the spontaneous disproportionation to peroxide. This work sheds light on the reversible electrochemical process of K +e +O ↔KO .
In this work, the preparation and characterization of modified LiMn2O4 (LMO) cathodes utilizing chemisorbed alkylphosphonic acids to chemically modify their surfaces are reported. Electrochemical methods to study ionic and molecular mobility through the alkylphosphonate self‐assembled monolayers (SAMs) for different alkyl chain compositions, in order to better understand their impact on the lithium‐ion electrochemistry, are utilized. Electrochemical trends for different chains correlate to trends observed in contact angle measurements and solvation energies obtained from computational methods, indicating that attributes of the microscopic wettability of these interfaces with the battery electrolyte have an important impact on ionic mobility. The effects of surface modification on Mn dissolution are also reported. The alkylphosphonate layer provides an important mode of chemical stabilization to the LMO, suppressing Mn dissolution by 90% during extended immersion in electrolytes. A more modest reduction in dissolution is found upon galvanostatic cycling, in comparison to pristine LMO cathodes. Taken together, the data suggest that alkylphosphonates provide a versatile means for the surface modification of lithium‐ion battery cathode materials allowing the design of specific interfaces through modification of organic chain functionalities.
We investigate the behavior of Cu plating bath suppressor additives poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) using normal Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS), and electrochemical quartz crystal microbalance (QCM) measurements. Raman and SERS show a clear spectroscopic trend of increased intensity in higher wavenumber modes in the CH stretching region as the environment is changed from pure material to solution to surface for both PEG and PPG. The spectral changes associated with PEG are larger than those associated with PPG, suggesting that the relatively more hydrophilic PEG undergoes more conformational changes upon surface association relative to the more hydrophobic PPG. Calculations show that the observed spectroscopic trend is associated with increased gauche character in the polymer backbone. QCM measurements show PEG adsorbs to the surface only in the presence of Cl − , while PPG adsorbs to the surface both with and without Cl − present. In the presence of Cl − , PPG forms a denser surface layer (0.598 μg/cm 2 ) compared to PEG (0.336 μg/cm 2 ) on a Cu underpotential deposition (UPD) layer on Au. These differences are consistent with the increased hydrophobicity of PPG relative to PEG.
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