Operando UV Vis Spectroscopy Research Articles

Phase transitions in NiO during the oxygen evolution reaction assessed via electrochromic phenomena through operando UV–Vis spectroscopy

Due to its high activity and stability, nickel oxide (NiO) has shown significant promise as an electrocatalyst for the alkaline oxygen evolution reaction (OER). In parallel, NiO exhibits a well-known electrochromic phenomenon, changing its optical properties in response to an applied electric potential. This study investigates the relationship between NiO phase changes that occur during the OER and the connected optical modulation using operando UV–visible reflectance spectroscopy. The correlation between the OER activity and the electrochromic behavior of NiO is explored, providing insights into the underlying physicochemical mechanisms governing both phenomena. Strong reduction of the reflected light at higher applied potentials cannot be attributed solely to the change in optical bandgap due to the phase change or the change in the material's refractive index when different phases form. Therefore, in-gap states responsible for increasing the absorption at higher applied potentials were experimentally characterized by ultraviolet photoelectron spectroscopy (UPS) and X-ray absorption spectroscopy (XAS). The results show that phase changes in NiO during OER influence its optical absorption characteristics, which in turn give access to following active phase changes of the electrocatalyst material through a non-invasive, optical, operando probe in real time allowing for kinetic studies of the involved solid-state conversion reactions.

Reaction kinetics of lithium-sulfur batteries with a polar Li-ion electrolyte: modeling of liquid phase and solid phase processes.

The present investigation fits the reaction kinetics of a lithium-sulfur (Li-S) battery with polar electrolyte employing a novel two-phase continuum multipore model. The continuum two-phase model considers processes in both the liquid electrolyte phase and the solid precipitates phase, where the diffusion coefficients of the Li+ ions in a solvent-softened solid state are determined from molecular dynamics simulations. Solubility experiments yield the saturation concentration of sulfur and lithium sulfides in the polar electrolyte employed in this study. The model describes the transport of dissolved molecular and ion species in pores of different size in solvated or desolvated form, depending on pore size. The Li-S reaction model in this study is validated for electrolyte 1 M LiPF6 in EC/DMC. It includes seven redox reactions and two cyclic non-electrochemical reactions in the cathode, and the lithium redox reaction at the anode. Electrochemical reactions are assumed to take place in the electrolyte solution or the solid state and cyclic reactions are assumed to take place in the liquid electrolyte phase only. The determination of the reaction kinetics parameters takes place via fitting the model predictions with experimental data of a cyclic voltammetry cycle with in operando UV-vis spectroscopy.

Operando UV-vis spectroscopy for real-time monitoring of nanoparticle size in reaction conditions: a case study on rWGS over Au nanoparticles.

We propose the use of surface plasmon resonance (SPR) as a distinctive marker for real-time monitoring in reaction conditions of gold nanoparticles supported on α-Al2O3. The study leverages the SPR shape-and-size dependency to monitor metal nanoparticles in reaction conditions, evidencing an influence of both dimensions and agglomerations on the SPR peak position. Operando measurements, coupling UV-vis spectroscopy and catalytic testing, allows to follow the dynamics during nanoparticle formation (Au3+ to Au0 reduction) and during the reverse water gas shift reaction (CO2 + H2 → CO + H2O). The catalyst structure and stability in reaction conditions was further confirmed by operando X-ray spectroscopy and PXRD data. Overall, this approach enables the direct acquisition of information on the structure-activity relationship of metal-based supported catalysts under actual reaction conditions.

Selectivity descriptors of the catalytic n-hexane cracking process over 10-membered ring zeolites.

Zeolite-mediated catalytic cracking of alkanes is pivotal in the petrochemical and refining industry, breaking down heavier hydrocarbon feedstocks into fuels and chemicals. Its relevance also extends to emerging technologies such as biomass and plastic valorization. Zeolite catalysts, with shape selectivity and selective adsorption capabilities, enhance efficiency and sustainability due to their well-defined network of pores, dimensionality, cages/cavities, and channels. This study focuses on the alkane cracking over 10-membered ring (10-MR) zeolites under industrially relevant conditions. Through a series of characterizations, including operando UV-vis spectroscopy and solid-state NMR spectroscopy, we intend to address mechanistic debates about the alkane cracking mechanism, aiming to understand the dependence of product selectivity on zeolite topologies. The findings highlight topology-dependent mechanisms, particularly the role of intersectional void spaces in zeolite ZSM-5, influencing aromatic-based product selectivity. This work provides a unique understanding of zeolite-catalyzed hydrocarbon conversion, linking alkane activation steps to the traditional hydrocarbon pool mechanism, contributing to the fundamental knowledge of this crucial industrial process.

Multiple active sites for glucose conversion identified in Sn-Beta catalysts by CPMG MAS NMR and operando UV-Vis spectroscopy

Sn-Beta is an emerging heterogeneous zeolite catalyst that displays high activity for the catalytic conversion of glucose to desirable bio-based chemicals. Due to its promising catalytic properties, the synthesis and characterisation of Sn-Beta catalysts of different composition and preparation method have been the subject of a great number of studies. However, development of structure-activity-lifetime relationships for glucose conversion chemistry has been obstructed by several challenges, making the tailored preparation of high performance Sn-Beta catalysts harder to achieve. In this manuscript, we undertake continuous kinetic studies of four different Sn-Beta catalysts for glucose conversion, and pair the kinetic insights obtained with operando UV-Vis spectroscopy and quantitative 119Sn CPMG MAS NMR. In doing so, we reveal that at least two types of Sn species that are active for glucose conversion can exist in Sn-Beta catalysts, but that the two Sn species exhibit different levels of activity, selectivity and stability. In particular, we find that a Sn species resonating at − 690 ppm is characterised by high activity and is primarily selective towards the isomerisation of glucose to fructose, whereas a second Sn species resonating at − 658 ppm is characterised by lower activity for glucose conversion, and preferential selectivity towards alkylation and β-dehydration. Our studies also demonstrate that both Sn sites also possess different levels of stability during continuous operation. Consequently, we reveal that it is the ratio amongst these species that governs the overall catalytic performances of Sn-Beta catalysts for biomass conversion processes.

Internal Magnetic Fields for Improved Cyclability in Aqueous Zn-Br2 Static Batteries

Owing to their high safety and low-cost, aqueous zince-bromine batteries (AZBBs) are envisioned as being highly applicable in both portable and stationary energy storage. The highly reversible Zn/Zn2+ (-0.76 V vs. RHE) and Br-/Br2 (1.08 V vs. RHE) redox couples have been employed in Zn-Br2 flow batteries (1.84 V vs. RHE), although widespread implementation has not been realized. This is largely because parasitic losses in overall battery performance rapidly occur due to i) dendritic growth of Zn deposits during the repetitive discharge process, leading to internal short circuits and ii) the cross-diffusion of highly soluble Br- (Br3 -) ions causing severe self-discharge and reduced cycle life. Common approaches to mitigate these problems include electrode engineering and incorporating additives and ion-selective membranes into the system for improving cycle life and coulombic efficiency. This talk will highlight the benefits of fabricating an aqueous Zn-Br2 static battery with internally contained magnetic fields, (~ 30, 40, 50 and 60 mT) at the anode and cathode by incorporating 1 mm thick Nd permanent magnets. Here we show that moderate internal magnetic fields are capable of simultaneously mitigating dendrite growth while drastically reducing cross-diffusion, leading to enhanced cycle life and improved energy and voltage efficiency. Operando UV-Vis spectroscopy, scanning electro microscopy (SEM), and electrochemical methods (electrochemical impedance spectroscopy and cyclic voltammetry) were used to to rationalize the effect of magnetic fields on the performance and cyclability of AZBBs.

Investigation and Determination of Electrochemical Reaction Kinetics in Lithium-Sulfur Batteries with Electrolyte LiTFSI in DOL/DME

A major challenge in the simulation of Li-S batteries is that the electrochemical reaction parameters supplied from the fitting of 0-d or 1-d models depend on the cathode and separator microstructure, so these parameters cannot be used in the design and optimization of material microstructure. The present investigation fits the electrochemical reaction kinetics employing a continuum model taking into account the pore size distribution and tortuosity of cathode and separator in the transport of sulfur molecules and ion species (Li+ ion, electrolyte and sulfide anions) in solvated or desolvated form depending on pore size. Hence, the specified reaction kinetics parameters are independent from the material microstructure. The Li-S redox reaction model includes six redox reactions in the cathode and the lithium redox reaction at the anode. Reactions are assumed to take place in the electrolyte solution rather than in the solid phase of sulfur or lithium sulfide precipitates, where dissolution/precipitation kinetics is modeled especially for the low solubility compounds: sulfur, Li2S and Li2S2. The fitting exercise is conducted based on experimental data of a cyclic voltammetry cycle accompanied by in operando UV–vis spectroscopy. The investigated battery has electrolyte LiTFSI in DOL/DME and no electrocatalysts in any part of the cell.

The Role of Adsorbed and Lattice Oxygen Species in Product Formation in the Oxidative Coupling of Methane over M2WO4/SiO2 (M = Na, K, Rb, Cs)

MnOx–Na2WO4/SiO2 is one of the best-performing catalysts in the oxidative coupling of methane (OCM) to C2 hydrocarbons (C2H6 and C2H4). The current mechanistic concepts related to the selectivity to the desired products are based on the involvement of crystalline Mn-containing phases, the molten Na2WO4 phase, surface Na–WOx species, and the associated lattice oxygen. Using in situ X-ray diffraction, operando UV–vis spectroscopy, spatially resolved kinetic analysis of product formation in steady-state OCM tests, and temporal analysis of products with isotopic tracers, we show that these phases/species are not categorically required to ensure high selectivity to the desired products. M2WO4/SiO2 (M = Na, K, Rb, Cs) materials were established to perform similarly to MnOx–Na2WO4/SiO2 in terms of selectivity–conversion relationships. The unique role of the molten Na2WO4 phase could not be confirmed in this regard. Our alternative concept is that the activity of M2WO4/SiO2 and product selectivity are determined by the interplay between the lattice oxygen of M2WO4 and adsorbed oxygen species formed from gas-phase O2. This lattice oxygen cannot convert CH4 to C2H6 but oxidizes CH4 exclusively to CO and CO2. Adsorbed monoatomic oxygen species reveal significantly higher reactivity toward overall CH4 conversion and efficiently generate CH3 radicals from CH4. These reactive intermediates couple to C2H6 in the gas phase and are oxidized, to a lesser extent, by the lattice oxygen of M2WO4 to CO and CO2. Adsorbed diatomic oxygen is involved in the direct CH4 oxidation to CO2. The electronegativity of alkali metal in M2WO4 was established to affect the catalyst ability to generate adsorbed oxygen species from O2. This knowledge opens the possibility to influence product selectivity by controlling the coverage by adsorbed and lattice oxygen via reaction conditions or catalyst design.

Detecting Cage Crossing and Filling Clusters of Magnesium and Carbon Atoms in Zeolite SSZ-13 with Atom Probe Tomography.

The conversion of methanol to valuable hydrocarbon molecules is of great commercial interest, as the process serves as a sustainable alternative for the production of, for instance, the base chemicals for plastics. The reaction is catalyzed by zeolite materials. By the introduction of magnesium as a cationic metal, the properties of the zeolite, and thereby the catalytic performance, are changed. With atom probe tomography (APT), nanoscale relations within zeolite materials can be revealed: i.e., crucial information for a fundamental mechanistic understanding. We show that magnesium forms clusters within the cages of zeolite SSZ-13, while the framework elements are homogeneously distributed. These clusters of just a few nanometers were analyzed and visualized in 3-D. Magnesium atoms seem to initially be directed to the aluminum sites, after which they aggregate and fill one or two cages in the zeolite SSZ-13 structure. The presence of magnesium in zeolite SSZ-13 increases the lifetime as well as the propylene selectivity. By using operando UV-vis spectroscopy and X-ray diffraction techniques, we are able to show that these findings are related to the suppression of aromatic intermediate products, while maintaining the formation of polyaromatic compounds. Further nanoscale analysis of the spent catalysts showed indications of magnesium redistribution after catalysis. Unlike zeolite H-SSZ-13, for which only a homogeneous distribution of carbon was found, carbon can be either homogeneously or heterogeneously distributed within zeolite Mg-SSZ-13 crystals as the magnesium decreases the coking rate. Carbon clusters were isolated, visualized, and analyzed and were assumed to be polyaromatic compounds. Small one-cage-filling polyaromatic compounds were identified; furthermore, large-cage-crossing aromatic molecules were found by isolating large coke clusters, demonstrating the unique coking mechanism in zeolite SSZ-13. Short-length-scale evidence for the formation of polyaromatic compounds at acid sites is discovered, as clear nanoscale relations between aluminum and carbon atoms exist.

Spectroelectrochemical Analysis of the Water Oxidation Mechanism on Doped Nickel Oxides.

Metal oxides and oxyhydroxides exhibit state-of-the-art activity for the oxygen evolution reaction (OER); however, their reaction mechanism, particularly the relationship between charging of the oxide and OER kinetics, remains elusive. Here, we investigate a series of Mn-, Co-, Fe-, and Zn-doped nickel oxides using operando UV–vis spectroscopy coupled with time-resolved stepped potential spectroelectrochemistry. The Ni2+/Ni3+ redox peak potential is found to shift anodically from Mn- < Co- < Fe- < Zn-doped samples, suggesting a decrease in oxygen binding energetics from Mn- to Zn-doped samples. At OER-relevant potentials, using optical absorption spectroscopy, we quantitatively detect the subsequent oxidation of these redox centers. The OER kinetics was found to have a second-order dependence on the density of these oxidized species, suggesting a chemical rate-determining step involving coupling of two oxo species. The intrinsic turnover frequency per oxidized species exhibits a volcano trend with the binding energy of oxygen on the Ni site, having a maximum activity of ∼0.05 s–1 at 300 mV overpotential for the Fe-doped sample. Consequently, we propose that for Ni centers that bind oxygen too strongly (Mn- and Co-doped oxides), OER kinetics is limited by O–O coupling and oxygen desorption, while for Ni centers that bind oxygen too weakly (Zn-doped oxides), OER kinetics is limited by the formation of oxo groups. This study not only experimentally demonstrates the relation between electroadsorption free energy and intrinsic kinetics for OER on this class of materials but also highlights the critical role of oxidized species in facilitating OER kinetics.

Substrate specificity in the biomimetic catalytic aerobic oxidation of styrene and cyclohexanone by metalloporphyrins: kinetics and mechanistic study

Substrate specificity is a hallmark of enzymatic catalysis. In this work, the biomimetic catalytic oxidation of styrene and cyclohexanone by iron (III) porphyrins and molecular oxygen was carried out, and remarkable differences in efficiency were observed. The specificity of the substrates for biomimetic catalytic oxidation was investigated by kinetics and mechanistic studies. Kinetics studies revealed that the oxidation of styrene followed Michaelis–Menten kinetics with KM ​= ​8.99 ​mol L-1, but the oxidation of cyclohexanone followed first-order kinetics with kobs ​= ​1.46 ​× ​10−4 ​s−1, indicating that the styrene epoxidation by metalloporphyrins exhibited characteristics of enzyme-like catalysis, while the oxidation of cyclohexanone was in agreement with the general rules of chemical catalysis. Different catalytic mechanisms for the two substrates were discussed by operando electron paramagnetic resonance spectroscopy, operando UV–vis spectroscopy, and KI/starch experiments. Substrate specificity was concluded to be attributed to the stability of high-valence species and oxygen transfer rate.

Elucidating the Nature of Active Sites and Fundamentals for their Creation in Zn-Containing ZrO2–Based Catalysts for Nonoxidative Propane Dehydrogenation

Environmentally friendly and low-cost catalysts are required for large-scale nonoxidative dehydrogenation of propane to propene (PDH) to replace currently used CrOx- or Pt-based catalysts. This work introduces ZnO-containing ZrO2- or MZrOx-supported (M = Ce, La, Ti or Y) catalysts. The most active materials outperformed the state-of-the-art catalysts with supported CrOx, GaOx, ZnOx, or VOx species as well as bulk ZrO2-based catalysts without ZnO. The space–time yield of propene of 1.25 kgC3H6·kg–1cat·h–1 at a propane conversion of about 30% with a propene selectivity of 95% was obtained over Zn­(4 wt %)/TiZrOx at 550 °C. For deriving key insights into the structure of active sites, reactivity, selectivity, and on-stream stability, the catalysts were characterized by XRD, HRTEM, EDX mapping, XPS, X-ray absorption, CO-TPR, CO2-TPD, NH3-TPD, pyridine-FTIR, operando UV–vis spectroscopy, Raman spectroscopy, TPO, and temporal analysis of products. In contrast with previous reports that used bulk ZrO2-based catalysts without ZnO, coordinatively unsaturated Zr cations are not the main active sites in the ZnO-containing catalysts. Supported ZnOx species were concluded to participate in the PDH reaction. The current X-ray absorption analysis proved that their structure is affected by the type of metal oxide used as a dopant for ZrO2 and by the crystallinity of ZrO2. Isolated tricoordinated Zn2+ species were concluded to show high activity and on-stream stability. Their intrinsic activity is enhanced when TiO2 and ZrO2 coexist in the support or when ZrO2 is promoted by TiO2. This is probably due to accelerating hydrogen formation in the course of the PDH reaction as concluded from temporal analysis of products with sub-millisecond resolution. The results of temperature-programmed oxidation of spent catalysts as well as ex situ Raman and operando UV–vis studies enabled us to conclude that the high on-stream stability of isolated tricoordinated Zn2+ species in the PDH reaction is related to their low ability to form coke. In general, the tendency for coke formation seems to increase with an increase in the degree of ZnOx agglomeration.

Operando UV–Vis spectroscopy as potential in-line PAT system for size determination of functioning metal nanocatalysts

Metal nanoparticle-catalyzed reactions such as hydrogenation, cross-coupling, carbonylation, and hydroformylation reactions are the most widely used reactions in the pharmaceutical and fine chemical industries. However, there is no operando spectroscopic technique that exists to monitor the size of functioning nanocatalyst. By exploiting localized surface plasmon resonance of catalytically relevant nanostructures, such as monometallic (e.g., Pd, Pt, Ni, Rh, Au, and Cu) nanoparticles and bimetallic core–shell (e.g., Ag-Pd) nanoparticles, we show UV–Vis spectroscopy can be used to determine the size of functioning nanocatalyst. Based on our finite-difference time-domain simulations, it is possible to detect leaching of even a monolayer of atoms from the surface of widely used metal nanocatalysts with a conventional UV–Vis spectrometer. This sensitive, inexpensive and robust spectroscopic approach can be potentially used as in-line process analytical technology (PAT) in pharmaceutical development and manufacturing.

Unraveling the Origins of the Synergy Effect between ZrO2 and CrOx in Supported CrZrOx for Propene Formation in Nonoxidative Propane Dehydrogenation

In this work, steady-state tests of propane dehydrogenation, density functional theory calculations, operando UV–vis spectroscopy, ex situ and in situ electron paramagnetic resonance spectroscopy, ...

Form of the catalytically active Pd species during the direct synthesis of hydrogen peroxide

AbstractDirect synthesis of H2O2 could produce H2O2 at lower cost than the Riedl–Pfleiderer process, which would enable the broader use of H2O2 for industrial oxidations. The addition of inorganic acids and halides to the solvent increase H2O2 selectivities on Pd nanoparticles but also dissolve Pd and produce dynamic mixtures of complexes including heterogeneous (Pd0) and homogeneous (Pd2+) species, any of which may contribute to H2O2 formation. We combine kinetic measurements and in operando UV–vis spectroscopy to determine how H2O2 rates and selectivities depend on concentrations of these forms of Pd. Introducing HCl to the solvent increases the concentration of Pd2+ complexes by oxidizing Pd0 nanoparticles. The rates of primary H2O2 formation and H2O2 selectivities do not depend directly on the population of the Pd2+ species for catalysts tested in water or methanol. These results suggest that Pd nanoparticles form H2O2 and detectable homogeneous complexes do not act as catalysts.

Hematite Photoanodes for Solar Water Splitting: A Detailed Spectroelectrochemical Analysis on the pH-Dependent Performance

The effect of the electrolyte pH on the performance of metal oxide photoanodes for solar water oxidation has not been fully resolved, and contrasting views have been presented in recent reports. Herein, a comprehensive set of spectroelectrochemical techniques (impedance spectroscopy, intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS), and operando UV–vis spectroscopy) are deployed to clearly uncover the role of pH on the performance of hematite photoanodes. Our results reveal that, despite the presence of high-valent iron-oxo active sites over a wide pH range (7–13.6), the observed performance improvement with increasing pH is mainly driven by the reduction of surface accumulated charges, in the form of reactive intermediate species, that alleviates Fermi level pinning (FLP). Interestingly, IMPS data provides compelling evidence that the mitigation of FLP originates from changes in the reaction mechanism which boost the rate of charge transfer reducing, in turn, the surface charging. A...

H-ZSM-5 Materials Embedded in an Amorphous Silica Matrix: Highly Selective Catalysts for Propylene in Methanol-to-Olefin Process

H-ZSM-5 materials embedded in an amorphous silica were successfully synthesized with three different Si/Al ratios (i.e., 40, 45, and 50). The presence of the MFI structure in the synthesized samples was confirmed by X-ray diffraction (XRD), Fourier transform infra-red (FT-IR), and solid state-nuclear magnetic resonance (SSNMR) techniques. The morphology and textural properties of the samples were investigated by scanning electron microscopy (SEM), TEM, and N2-physisorption measurements. Furthermore, acidic properties of the synthesized catalysts have been studied by NH3-TPD and FT-IR spectroscopy of CO adsorption studies. Variation of the Si/Al ratio affected the crystal morphology, porosity, and particle size, as well as the strength and distribution of acid sites. The synthesized zeolite materials possessed low acid-site density and exhibited high catalytic activity in the methanol-to-olefin (MTO) reaction. To study the intermediate species responsible for catalyst deactivation, the MTO reaction was carried out at high temperature (500 °C) to accelerate catalyst deactivation. Interestingly, the synthesized catalysts offered high selectivity towards the formation of propylene (C3=), in comparison to a commercial microporous crystalline H-ZSM-5 with Si/Al = 40, under the same reaction conditions. The synthesized H-ZSM-5 materials offered a selectivity ratio of C3=/C2= 12, while it is around 2 for the commercial H-ZSM-5 sample. The formation of hydrocarbon species during MTO reaction over zeolite samples has been systematically studied with operando UV-vis spectroscopy and online gas chromatography. It is proposed that the strength and type of acid sites of catalyst play a role in propylene selectivity as well as the fast growing of active intermediate species. The effective conversion of methanol into propylene in the case of synthesized H-ZSM-5 materials was observed due to possession of weak acid sites. This effect is more pronounced in H-ZSM-5 sample with a Si/Al ratio of 45.

The effect of phase composition and crystallite size on activity and selectivity of ZrO2 in non-oxidative propane dehydrogenation

A series of bare ZrO2 materials composed either of monoclinic or tetragonal phase with the size of crystallites varying between 3.7 and 43.4 nm were prepared by precipitation, hydrothermal or thermal calcination methods. The samples were characterized by XRD, BET, CO-TPR, NH3-TPD, electrical conductivity measurements and operando UV–vis spectroscopy. Density functional theory calculations provided molecular insights into the kind of active site and individual steps of propane dehydrogenation to propene and hydrogen. Regardless of the phase composition, two Zr cations located at an oxygen vacancy, i.e. coordinatively unsaturated Zr cations (Zrcus sites) having, nevertheless, different chemical environment in tetragonal and monoclinic ZrO2, were concluded to be responsible for homolytic breaking of CH bonds in propane. Monoclinic ZrO2 showed, however, higher rate of propene formation and higher propene selectivity than ZrO2 stabilized in the tetragonal phase. Both the activity and the selectivity to propene increased with a decrease in the size of crystallites. The effects of phase composition and crystallite size on the PDH performance were related to the ability of ZrO2 to release lattice oxygen upon reductive catalyst treatment and thus to create Zrcus sites. The knowledge derived can be used for further optimizing PDH performance of ZrO2-based catalysts and also extended to other non-reducible metal oxides.

(Invited) Electrolyte Design and Solution Mediation Processes in Lithium-Oxygen and Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) and lithium-Oxygen (Li-O2) batteries have attracted intensive research attentions owing to their potential to provide higher gravimetric energy density than conventional Li-ion batteries. However, Li-S and Li-O2 batteries have been suffering from poor cycle life, low round-trip efficiency and low practical energy density compared to conventional Li-ion batteries. Different from intercalation-based charge storage, Li-S and Li-O2 both operate based on solution-mediated redox processes. The development of stable and reversible Li-S and Li-O2 chemistries requires fundamental understanding on how solvent and salt affect the reaction mechanism and mediation processes. In this presentation, we apply in situ/operando spectroscopy/analytical techniques to study electrolyte structure-function relationship in Li-S and Li-O2 batteries. We exploit online electrochemical mass spectrometry, operando UV-Vis spectroscopy, rotating rink disk electrode and synchrotron-based X-ray absorption near edge structure spectroscopy to investigate the role of solvent and salt (e.g., donicity, electron-accepting ability, dielectric constant etc) on the reaction mechanism and solution mediation processes in sulfur and oxygen redox chemistries. We will discuss the interplay between solution mediation and solid phase precipitation and the synergistic effect of employing redox mediators and novel cell design in improving the rate and cycle life of Li-S and Li-O2 batteries.

A Solvent-Controlled Oxidation Mechanism of Li2O2 in Lithium-Oxygen Batteries

Summary Long-unresolved discrepancies in the oxidation mechanism of lithium peroxide (Li2O2) impede electrode/electrolyte development for Li-O2 batteries. In this study, we report direct evidence of the formation of soluble LiO2 upon the oxidation of Li2O2 and reveal a strong solvent-controlled Li2O2-oxidation reaction mechanism. We exploit a thin-film rotating ring-disk electrode to show that soluble LiO2 is generated when oxidizing Li2O2 in high-donicity solvent but is absent in low-donicity glyme-based solvent. Synchrotron-based X-ray absorption near-edge structure spectroscopy further proved the presence of LiO2 upon Li2O2 oxidation in high-donicity solvent but not in low-donicity solvent. We show that preferential formation of soluble LiO2 in the high-donicity solvent could account for poor cycling stability, which suggests that strategies that bypass the formation of soluble LiO2 upon Li2O2 oxidation are critical. Our work offers new insights in resolving the discrepancy in Li-O2 charging mechanism and design strategies to achieve efficient and long-life Li-O2 batteries.