Dipobrato Sarbapalli

Nonaqueous redox flow batteries are a promising technology that utilize redox-active species (i.e., redoxmers) in solution to store energy via electron-transfer (ET) reactions with electrodes. However, electron transfer (ET) phenomena at the interface of graphitic electrodes and nonaqueous media are poorly understood, with several non-idealities in the use of conventional models such as the Butler–Volmer model reported. Possibilities for these non-idealities include the adsorption of redox species at the electrode, fundamental ET limitations related to the density of states at the electrode, and the presence of chemical and spatial heterogeneities at the surface of the electrode. To this point, we present a computational and experimental approach to comparatively investigate the ET behavior of two redoxmers, ferrocene (Fc) and 2,3-dimethyl-1,4-dialkoxybenzene (C7) on single layer graphene (SLG), hydrogen-functionalized SLG (H-SLG), and pristine and hydrogen-functionalized graphite electrodes. Scanning electrochemical microscopy (SECM) experiments revealed enhanced ET kinetics for both redoxmers on H-SLG electrodes compared to pristine SLG electrodes, with the degree of functionalization playing a key role in this enhancement. Electrodes such as boron-doped diamond and hydrogenated graphite mirrored these enhancements. Density functional theory (DFT) calculations indicate only small differences in the binding strengths for Fc and C7 redoxmers on SLG and H-SLG surfaces, but Marcus–Hush–Chidsey (MHC) kinetic theory analysis suggests that the density of states (DOS) of the carbon electrode likely plays a crucial role in the observed ET enhancement. These findings refine our initial assumption of binding energy (BE) as a dominant factor for interfacial behavior in the case of Fc and C7 redoxmers. Our findings create new opportunities to explore systems with varying degrees of surface modification to understand and design better redox flow batteries.

The fundamental process in non‐aqueous redox flow battery (NRFB) operation revolves around electron transfer (ET) between a current collector electrode and redox‐active organic molecules (redoxmers) in solution. Here, we present an approach utilizing scanning electrochemical microscopy (SECM) to evaluate interfacial ET kinetics between redoxmers and various electrode materials of interest at desired locations. This spot‐analysis method relies on the measurement of heterogeneous electron transfer rate constants (k_f or k_b) as a function of applied potential (E−E⁰′). As demonstrated by COMSOL simulations, this method enables the quantification of Butler‐Volmer kinetic parameters, the standard heterogeneous rate constant, k⁰, and the transfer coefficient, α. Our method enabled the identification of inherent asymmetries in the ET kinetics arising during the reduction of ferrocene‐based redoxmers, compared to their oxidation which displayed faster rate constants. Similar behavior was observed on a wide variety of carbon electrodes such as multi‐layer graphene, highly ordered pyrolytic graphite, glassy carbon, and chemical vapor deposition‐grown graphite films. However, aqueous systems and Pt do not exhibit such kinetic effects. Our analysis suggests that differential adsorption of the redoxmers is insufficient to account for our observations. Displaying a greater versatility than conventional electroanalytical methods, we demonstrate the operation of our spot analysis at concentrations up to 100 mM of redoxmer over graphite films. Looking forward, our method can be used to assess non‐idealities in a variety of redoxmer/electrode/solvent systems with quantitative evaluation of kinetics for applications in redox‐flow battery research.

Na-ion batteries (NIBs) are proposed as a promising candidate for beyond Li-ion chemistries, however, a key challenge associated with NIBs is the inability to achieve intercalation in graphite anodes. This phenomenon has been investigated and is believed to arise due to the thermodynamic instability of Na-intercalated graphite. We have recently demonstrated theoretical calculations showing it is possible to achieve thermodynamically stable Na-intercalated graphene structures with a fluorine surface modifier. Here, we present experimental evidence that Na ⁺ intercalation is indeed possible in fluorinated few-layer graphene (F-FLG) structures using cyclic voltammetry (CV), ion-sensitive scanning electrochemical microscopy (SECM) and in situ Raman spectroscopy. SECM and Raman spectroscopy confirmed Na ⁺ intercalation in F-FLG, while CV measurements allowed us to quantify Na-intercalated F-FLG stoichiometries around NaC _14–18 . These stoichiometries are higher than the previously reported values of NaC ₁₈₆ in graphite. Our experiments revealed that reversible Na ⁺ ion intercalation also requires a pre-formed Li-based SEI in addition to the surface fluorination, thereby highlighting the critical role of SEI in controlling ion-transfer kinetics in alkali-ion batteries. In summary, our findings highlight the use of surface modification and careful study of electrode-electrolyte interfaces and interphases as an enabling strategy for NIBs.

Lattice oxygen loss during cathode charging significantly limits the charge storage capacity of lithium-ion batteries (LiBs). Therefore, elucidating the oxygen loss and subsequent surface reconstruction phenomena remains an ongoing pursuit with practical implications. In this article, we report an in situ oxygen detection strategy using scanning electrochemical microscopy (SECM) that reveals an unprecedented two-stage oxygen evolution behavior from commercial cathodes. This highly sensitive SECM method captured an unreported transient oxygen release at less than 3.3 V vs Li ⁺ /Li during the first charge cycle of LiCoO ₂ , LiNi _0.33 Mn _0.33 Co _0.33 O ₂ and LiNi _0.8 Mn _0.1 Co _0.1 O ₂ . At the main oxygen loss process above 3.3 V vs Li ⁺ /Li, SECM mapping highlighted spatial and temporal heterogeneities. Finite element simulations were used to quantify the rate of instantaneous oxygen release, with rates of ∼30 pmol cm ⁻² s for the steady-state oxygen evolution. This SECM approach revealed incipient degradation processes and created new quantitative and spatially resolved opportunities for investigating degradation in operating LiB cathodes.

This study focuses on understanding how salicylic acid and methanol (SAM), commonly used to dissolve certain calcium silicates in portland cement, affect geopolymer gels and other sodium aluminosilicate hydrates. The technique has been used in the past to selectively dissolve hydrated calcium silicates formed through alkali activation of aluminosilicate mixtures containing calcium in the expectation that the extraction does not dissolve alkali aluminosilicate hydrates. However, the work presented here reveals that SAM solution does, in fact, dissolve aluminosilicate hydrates. Zeolite A, a three‐dimensional, low‐silica, crystalline sodium aluminosilicate hydrate, was fully dissolved upon SAM extraction, zeolite X was partially dissolved in the SAM extraction, and only zeolite Y was completely unaffected by SAM extraction. Similar behavior was observed with amorphous sodium hydroxide‐metakaolin geopolymer products—at an early age (1 day), SAM dissolved all the gel and left only unreacted metakaolin, and at a later age (7 days), SAM partially dissolved the geopolymer gel. Nuclear magnetic resonance spectroscopy indicated that dissolution requires the presence of silicon at the Q ⁴ (4Al) sites. Extraction was seen to involve some dealumination and complete removal of sodium. The mechanism of aluminosilicate dissolution in SAM appears to be acid attack followed by removal of aluminum via chelation with salicylic acid.

The solid electrolyte interphase (SEI) is a dynamic, electronically insulating film that forms on the negative electrode of Li⁺ batteries (LIBs) and enables ion movement to/from the interface while preventing electrolyte breakdown. However, there is limited comparative understanding of LIB SEIs with respect to those formed on Na⁺ and K⁺ electrolytes for emerging battery concepts. We used scanning electrochemical microscopy (SECM) for the in situ interfacial analysis of incipient SEIs in Li⁺, K⁺ and Na⁺ electrolytes formed on multi‐layer graphene. Feedback images using 300 nm SECM probes and ion‐sensitive measurements indicated a superior passivation and highest cation flux for a Li⁺‐SEI in contrast to Na⁺ and K⁺‐SEIs. Ex situ X‐ray photoelectron spectroscopy indicated significant fluoride formation for only Li⁺ and Na⁺‐SEIs, enabling correlation to in situ SECM measurements. While SEI chemistry remains complex, these electroanalytical methods reveal links between chemical variables and the interfacial properties of materials for energy storage.

Choosing reference electrodes for nonaqueous electrochemical measurements, especially in energy storage research, is challenging due to lengthy experiments (>1 day), the lack of alternatives to the commonly used Ag/Ag⁺ reference electrode (RE), the introduction of junction potentials, and the possibility of sample contamination. Often, quasi-reference electrodes (QREs) such as Ag wires and Li metal strips are used. However, small changes in electrolyte composition can cause large potential drifts, and their surfaces may be reactive to the solution. Here, we propose an alternative QRE based on polypyrrole electrodeposited on Pt wire (PPyQRE) encased in a glass tube with the open end sealed with commercial frits. While freestanding PPyQRE wires have been reported in the literature, simple encasing of the PPyQRE overcomes the above-mentioned drawbacks of QREs while providing a reliable reference potential that is closer to the performance of an RE. During cyclic voltammetric and bulk electrolysis testing of a redox mediator in solution, the encased PPyQRE exhibited stable reference potentials over multiple charge/discharge cycles with minimal drift (∼5 mV) after ∼2.25 days of operation. We also tested the reliability of our reference during the testing of multilayer graphene Li-ion anodes, which often involve cycling samples at highly reducing potentials (<−3 V vs Fc/Fc⁺) over long durations (>1 day). In the same testing conditions, the Ag/Ag⁺ electrode led to observable Ag deposits on the graphene and large potential drifts (∼50 mV), while the PPyQRE exhibited no measurable drift and revealed changes in voltammetric features that were obscured by reference drift when using Ag/Ag⁺. Minor reference drifts of ∼30 mV over long usage of the PPyQRE (∼2 months) can be addressed by calibration with a ferrocene couple at the end of experiments. These results highlight the advantages of using an encased PPyQRE as a simple and practical reference electrode for electrochemical measurements in the field of nonaqueous energy storage research.

Alkali ion intercalation is fundamental to battery technologies for a wide spectrum of potential applications that permeate our modern lifestyle, including portable electronics, electric vehicles, and the electric grid. In spite of its importance, the Nernstian nature of the charge transfer process describing lithiation of carbon has not been described previously. Here we use the ultrathin few-layer graphene (FLG) with micron-sized grains as a powerful platform for exploring intercalation and co-intercalation mechanisms of alkali ions with high versatility. Using voltammetric and chronoamperometric methods and bolstered by density functional theory (DFT) calculations, we show the kinetically facile co-intercalation of Li⁺ and K⁺ within an ultrathin FLG electrode. While changes in the solution concentration of Li⁺ lead to a displacement of the staging voltammetric signature with characteristic slopes ca. 54–58 mV per decade, modification of the K⁺/Li⁺ ratio in the electrolyte leads to distinct shifts in the voltammetric peaks for (de)intercalation, with a changing slope as low as ca. 30 mV per decade. Bulk ion diffusion coefficients in the carbon host, as measured using the potentiometric intermittent titration technique (PITT) were similarly sensitive to solution composition. DFT results showed that co-intercalation of Li⁺ and K⁺ within the same layer in FLG can form thermodynamically favorable systems. Calculated binding energies for co-intercalation systems increased with respect to the area of Li⁺-only domains and decreased with respect to the concentration of –K–Li– phases. While previous studies of co-intercalation on a graphitic anode typically focus on co-intercalation of solvents and one particular alkali ion, this is to the best of our knowledge the first study elucidating the intercalation behavior of two monovalent alkali ions. This study establishes ultrathin graphitic electrodes as an enabling electroanalytical platform to uncover thermodynamic and kinetic processes of ion intercalation with high versatility.

Every electrochemical energy storage and conversion device requires electrodes to exchange charge with the electrolyte and move electrons through an external circuit. We are used to describing electrode performance in terms of single-valued, averaged measurements: a peak current, a charge transfer resistance, a capacitance, and a charge capacity. However, these characterizations escape the complexity of most practical materials. Electrodes are not idealized current collectors but rather heterogeneous surfaces on which individual features strongly influence device performance by dictating local charge transfer. Thus, it would be desirable to understand reactivity at such locality, whatever its length scale.