Venkateshkumar Prabhakaran

Rational design of efficient electrode-electrolyte interfaces for solid-state energy storage using ion soft landing.

The rational design of improved electrode–electrolyte interfaces (EEI) for energy storage is critically dependent on a molecular-level understanding of ionic interactions and nanoscale phenomena. The presence of non-redox active species at EEI has been shown to strongly influence Faradaic efficiency and long-term operational stability during energy storage processes. Herein, we achieve substantially higher performance and long-term stability of EEI prepared with highly dispersed discrete redox-active cluster anions (50 ng of pure ∼0.75 nm size molybdenum polyoxometalate (POM) anions on 25 μg (∼0.2 wt%) carbon nanotube (CNT) electrodes) by complete elimination of strongly coordinating non-redox species through ion soft landing (SL). Electron microscopy provides atomically resolved images of a uniform distribution of individual POM species soft landed directly on complex technologically relevant CNT electrodes. In this context, SL is established as a versatile approach for the controlled design of novel surfaces for both fundamental and applied research in energy storage.

Gas-Phase Fragmentation Pathways of Mixed Addenda Keggin Anions: PMo12-nWnO403– (n = 0–12)

AbstractWe report a collision-induced dissociation (CID) investigation of the mixed addenda polyoxometalate (POM) anions, PMo12-nWnO403– (n = 0–12). The anions were generated in solution using a straightforward single-step synthesis approach and introduced into the gas phase by electrospray ionization (ESI). Distinct differences in fragmentation patterns were observed for the range of mixed addenda POMs examined in this study. CID of molybdenum-rich anions, PMo12-nWnO403– (n = 0–2), generates an abundant doubly charged fragment containing seven metal atoms (M) and 22 oxygen atoms (M7O222–) and its complementary singly charged PM5O18– ion. In comparison, the doubly charged Lindqvist anion, (M6O192–) and its complementary singly charged PM6O21– ion are the dominant fragments of Keggin POMs containing more than two tungsten atoms, PMo12-nWnO403– (n = 3–12). The observed transition in the dissociation pathways with an increase in the number of W atoms in the POM may be attributed to the higher barrier of tungsten-rich anions towards isomerization. We present evidence that the observed distribution of Mo and W atoms in the major M6O192– and M7O222– fragment ions is different from that predicted by a random distribution, indicating substantial segregation of the addenda metal atoms in the POMs. Charge reduction of the triply charged precursor anion resulting in formation of doubly charged anions is also observed. This is a dominant pathway for mixed POMs having a majority (8–11) of W atoms and a minor channel for other precursors indicating a close competition between fragmentation and charge loss pathways in CID of POM anions. Graphical Abstractᅟ

In situ solid-state electrochemistry of mass-selected ions at well-defined electrode–electrolyte interfaces

Significance In situ characterization of kinetics at electrode–electrolyte interfaces (EEIs) is crucial to the rational design of efficient and sustainable solid-state electrochemical technologies. A significant advancement has been made to rationally understand processes at EEIs using solid-state in situ thin-film electrochemical cells fabricated using specially designed ionic liquid membranes with excellent mass-transfer properties. The in situ cells are used to characterize well-defined EEIs generated using ion soft-landing (SL) in controlled environments, both in vacuum and in the presence of reactant gases. Populating EEIs with precisely defined electroactive species using SL facilitates molecular-level understanding of electron transfer processes within deposited species and between species and electrodes, thus providing a powerful methodology to characterize technologically relevant EEIs during operation. Molecular-level understanding of electrochemical processes occurring at electrode–electrolyte interfaces (EEIs) is key to the rational development of high-performance and sustainable electrochemical technologies. This article reports the development and application of solid-state in situ thin-film electrochemical cells to explore redox and catalytic processes occurring at well-defined EEIs generated using soft-landing (SL) of mass- and charge-selected cluster ions. In situ cells with excellent mass-transfer properties are fabricated using carefully designed nanoporous ionic liquid membranes. SL enables deposition of pure active species that are not obtainable with other techniques onto electrode surfaces with precise control over charge state, composition, and kinetic energy. SL is, therefore, demonstrated to be a unique tool for studying fundamental processes occurring at EEIs. Using an aprotic cell, the effect of charge state (PMo12O403-/2-) and the contribution of building blocks of Keggin polyoxometalate (POM) clusters to redox processes are characterized by populating EEIs with POM anions generated by electrospray ionization and gas-phase dissociation. Additionally, a proton-conducting cell has been developed to characterize the oxygen reduction activity of bare Pt clusters (Pt30 ∼1 nm diameter), thus demonstrating the capability of the cell for probing catalytic reactions in controlled gaseous environments. By combining the developed in situ electrochemical cell with ion SL we established a versatile method to characterize the EEI in solid-state redox systems and reactive electrochemistry at precisely defined conditions. This capability will advance the molecular-level understanding of processes occurring at EEIs that are critical to many energy-related technologies.

In Situ Infrared Spectroelectrochemistry for Understanding Structural Transformations of Precisely Defined Ions at Electrochemical Interfaces

Understanding the intrinsic properties of electroactive species at electrode-electrolyte interfaces (EEIs) is essential to the rational design of high-performance solid-state energy conversion and storage systems. In situ spectroscopy combined with cyclic voltammetry (CV) provides insights into structural changes of electroactive species at functioning EEIs. Ion soft landing enables precisely controlled deposition of mass- and charge-selected ions onto electrode surfaces thereby avoiding the contamination inherent with conventional electrode preparation techniques. In this contribution, we describe a new approach for the simultaneous electrochemical and spectroscopic characterization of soft-landed ions at operating solid-state EEIs. The technique exploits a specially fabricated three-electrode cell that is compatible with in situ infrared reflection absorption spectroscopy (IRRAS) characterization of the soft-landed ions. Keggin polyoxometalate (POM) anions, PW12O403-, were selected as a model system for these experiments due to their multielectron redox activity, structural stability, and well-characterized IRRAS spectrum. In situ CV measurements indicated continuous multielectron transfer processes of the soft-landed PW12O403- anions over a large potential range of -2.1 to -0.3 V. A distinct shift in the wavenumber of the terminal W═Ot stretching vibration in the IRRAS spectra was observed during the multielectron reduction process. The results demonstrate the capabilities of the in situ spectroelectrochemical approach for examining structural changes of well-defined electroactive species during electron-transfer processes at operating solid-state EEIs.

Quantitative Extraction and Mass Spectrometry Analysis at a Single-Cell Level

Quantitative live cell mass spectrometry analysis at a subcellular level requires the precisely controlled extraction of subpicoliter volumes of material from the cell, sensitive analysis of the extracted analytes, and their accurate quantification without prior separation. In this study, we demonstrate that localized electroosmotic extraction provides a direct path to addressing this challenge. Specifically, we demonstrate quantitative mass spectrometry analysis of biomolecules in picoliter volumes extracted from live cells. Electroosmotic extraction was performed using two electrodes and a finely pulled nanopipette with tip diameter of <1 μm containing a hydrophobic electrolyte compatible with mass spectrometry analysis. The electroosmotic drag was used to drive analytes out of the cell into the nanopipette. Analyte molecules extracted both from solutions and cell samples were analyzed using nanoelectrospray ionization (nanoESI) directly from the nanopipette into a mass spectrometer. More than 50 metabolites including sugars and flavonoids were detected in positive mode in 2-5 pL volumes of the cytoplasmic material extracted from Allium cepa. Quantification of the extracted glucose was performed using sequential extraction of a known volume of the aqueous solution containing glucose- d2 standard of known concentration. We found that the ratio of the signal of glucose to glucose- d2 increased linearly with glucose concentration. This observation indicates that the approach developed in this study enables quantitative analysis of small volumes of metabolites extracted from cells. Furthermore, we observed efficient separation of hydrophilic and hydrophobic analytes through partitioning into the aqueous and hydrophobic electrolyte phase, respectively, which provides additional important information on the molecular properties of extracted metabolites.