Manganese oxides with rich redox chemistry have been widely used in (electro)­catalysis in applications of energy and environmental consequence. While they are ubiquitous in catalyzing the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), redox processes occurring on the surface of manganese oxides are poorly understood. We report valence changes at OER- and ORR-relevant voltages of a layered manganese oxide film prepared by electrodeposition. X-ray absorption spectra were collected in situ in O2-saturated 0.1 M KOH using inverse partial fluorescence yield (IPFY) at the Mn L3,2-edges and partial fluorescence yield (PFY) at the O K-edge. Overall, we found reversible yet hysteretic Mn redox and qualitatively reproducible spectral changes by Mn L3,2 IPFY XAS. Oxidation to a mixed Mn3+/4+ valence preceded the oxygen evolution at 1.65 V vs RHE, while manganese reduced below Mn3+ and contained tetrahedral Mn2+ during oxygen reduction at 0.5 V vs RHE. Analysis of the pre-edge in O K-edge XAS provided the Mn–O hybridization, which was highest for Mn3+ (eg 1). Our study demonstrates that combined in situ experiments at the metal L- and oxygen K-edges are indispensable to identify both the active valence during catalysis and the hybridization with oxygen adsorbates, critical to the rational design of active catalysts for oxygen electrocatalysis.

Electrochemical processes in lithium–oxygen (Li–O2 or Li–air) batteries are complex, with chemistry depending on cycling conditions, electrode materials and electrolytes. In non‐aqueous Li–O2 cells, reversible lithium peroxide (Li2O2) and irreversible parasitic products (i.e., LiOH, Li2CO3, Li2O) are common. Superoxide intermediates (O2−, LiO2) contribute to the formation of these species and are transiently stable in their own right. While characterization techniques like XRD, XPS and FTIR have been used to observe many Li–O2 species, these methods are poorly suited to superoxide detection. Raman spectroscopy, however, may uniquely identify superoxides from O−O vibrations. The ability to fingerprint Li–O2 products in situ or ex situ, even at very low concentrations, makes Raman an essential tool for the physicochemical characterization of these systems. This review contextualizes the application of Raman spectroscopy and advocates for its wider adoption in the study of Li–O2 batteries.

Redox flow batteries (RFBs) are promising energy storage candidates for grid deployment of intermittent renewable energy sources such as wind power and solar energy. Various new redox-active materials have been introduced to develop cost-effective and high-power-density next-generation RFBs. Electrochemical kinetics play critical roles in influencing RFB performance, notably the overpotential and cell power density. Thus, determining the kinetic parameters for the employed redox-active species is essential. In this Perspective, we provide the background, guidelines, and limitations for a proposed electrochemical protocol to define the kinetics of redox-active species in RFBs.

As one of the most theoretically promising next-generation chemistries, Li-O 2 batteries are the subject of intense research to address their stability, cycling, and efficiency issues. The recharge kinetics of Li-O 2 are especially sluggish, prompting the use of metal nanoparticles as reaction promoters. In this work, we probe the underlying pathway of kinetics enhancement by transition metal and oxide particles using a combination of electrochemistry, X-ray absorption spectroscopy, and thermochemical analysis in carbon-free and carbon-containing electrodes. We highlight the high activity of the group VI transition metals Mo and Cr, which are comparable to noble metal Ru and coincide with XAS measured changes in surface oxidation state matched to the formation of Li 2 MoO 4 and Li 2 CrO 4 . A strong correlation between conversion enthalpies of Li 2 O 2 with the promoter surface (Li 2 O 2 + M a O b ± O 2 → Li x M y O z ) and electrochemical activity is found that unifies the behaviour of solidstate promoters. In the absence of soluble species on charge and the decomposition of Li 2 O 2 proceeding through solid solution, enhancement of Li 2 O 2 oxidation is mediated by chemical conversion of Li 2 O 2 with slow oxidation kinetics to a lithium metal oxide. Our mechanistic findings provide new insights into the selection and/or employment of electrode chemistry in Li-O 2 batteries. Li 2 CO 3 , which can form from electrolyte decomposition and/or Electronic Supplementary Information (ESI) available: XRD of synthesized α-MnO2 nanowires, Electrochemical profile of Cr and Mo, Elucidation of surface oxides on metal nanoparticle nanoparticles, Identification of Li2MoO4 in pristine Mo:Li2O2 electrodes by XRD, Supplementary XAS for oxide electrodes, Probing of impurities effect on Li2O2 oxidation, Schematic and derivations for proposed conversion/delithiation mechanism, Tabulated values of reaction enthalpies, Table of theoretical estimation of enhancement trend based on proposed mechanism. SeeLithium-oxygen batteries have been referred to as the "holy grail" of battery chemistries for its potential to provide three times the gravimetric energy density of Li-Ion batteries and as such enable similar ranges as current internal combustion engines at comparable system weights. Thus far however, the Li-O 2 electrochemistry is confronted with severe instabilities of electrolyte and carbon-based cathodes which results in poor cycle life and efficiencies. More fundamentally, recharge requires large voltages for oxidation of the insulating Li 2 O 2 deposited on discharge resulting in low round trip efficiencies. In tackling the slow fundamental kinetics of recharge, researchers have resorted to adding transition metals and oxides (often called catalysts) to enhance the oxidation kinetics of Li 2 O 2 . The effort of identifying the best materials has yet to probe the mechanism of enhancement and thereby obtain predictive capability. In the present work, we examine the mechanistic origin of the influence of transition metals and oxides ...

Charging kinetics and reversibility of Na-O2 batteries can be influenced greatly by the particle size of NaO2 formed upon discharge, and exposure time (reactivity) of NaO2 to the electrolyte. Micrometer-sized NaO2 cubes formed at high discharge rates were charged at smaller overpotentials compared to nanometer-sized counterparts formed at low rates.

Bilayer molecular junctions were fabricated by using the alkyne/azide "click" reaction on a carbon substrate, followed by deposition of a carbon top contact in a crossbar configuration. The click reaction on an alkyne layer formed by diazonium reduction permitted incorporation of a range of molecules into the resulting bilayer, including alkane, aromatic, and redox-active molecules, with high yield (>90%) and good reproducibility. Detailed characterization of the current-voltage behavior of bilayer molecular junctions indicated that charge transport is consistent with tunneling, but that the effective barrier does not strongly vary with molecular structure for the series of molecules studied.

Silicon-based anodes for Li-ion batteries have been gaining a great deal of attention due to their high theoretical gravimetric energy density. Approaches for overcoming the challenge of pulverization associated with Sibased electrodes are required for efficient, reversible, and stable operation of such high energy batteries. This study focuses on addressing the source of pulverization of amorphous silicon films upon cycling, which is typically attributed to the formation of the c-Li 3.75 Si phase. Cross-sectional samples prepared by focused-ion beam milling revealed fractured sponge-like silicon structures after 150 cycles at a lithiation cutoff voltage of 5 mV Li , at which the c-Li 3.75 Si phase forms. Cycling at a higher lithiation cutoff voltage, 50 mV Li , however, resulted in a film with a higher degree of integrity, along with the absence of the c-Li 3.75 Si phase. These results clearly verify and underscore the deleterious effects of the c-Li 3.75 Si phase. Alternating carbon and silicon layers results in suppression of the formation of the c-Li 3.75 Si phase to a degree dependent upon the relative thicknesses of both the silicon and carbon layers. Best results were observed for multilayers of 8 nm Si/4 nm C, with which no evidence for the c-Li 3.75 Si phase up to 149 cycles was observed. Carbon interlayers were also found to beneficially lower the relative irreversible capacity loss due to solidelectrolyte interphase formation and associated electrical disconnection.

Interactions between carbohydrates (glycans) and glycan-binding proteins (GBPs) regulate a wide variety of important biological processes. However, the affinities of most monovalent glycan−GBP complexes are typically weak (dissociation constant (K d ) > μM) and difficult to reliably measure with conventional assays; consequently, the glycan specificities of most GBPs are not well established. Here, we demonstrate how electrospray ionization mass spectrometry (ESI-MS), implemented with nanoflow ESI emitters with inner diameters of ∼50 nm, allows for the facile quantification of low-affinity glycan−GBP interactions. The small size of the droplets produced from these submicron emitters effectively eliminates the formation of nonspecific glycan− GBP binding (false positives) during the ESI process up to ∼mM glycan concentrations. Thus, interactions with affinities as low as ∼5 mM can be measured directly from the mass spectrum. The general suppression of nonspecific adducts (including nonvolatile buffers and salts) achieved with these tips enables ESI-MS glycan affinity measurements to be performed on C-type lectins, a class of GBPs that bind glycans in a calcium-dependent manner and are important regulators of immune response. At physiologically relevant calcium ion concentrations (2−3 mM), the extent of Ca 2+ nonspecific adduct formation observed using the submicron emitters is dramatically suppressed, allowing glycan affinities, and the influence of Ca 2+ thereon, to be measured. Finally, we show how the use of submicron emitters and suppression of nonspecific binding enable the quantification of labile (prone to in-source dissociation) glycan−GBP interactions.