Aqueous Zn/MnO2 batteries have garnered significant interests owing to their abundance, high theoretical specific capacity, safety, and low cost. However, large-scale application of these systems is limited by the incomplete understanding of the MnO2 reaction chemistry. The different crystal lattice structures among MnO2 polymorphs contribute to the variations in reported reaction mechanisms. Among them, ε-MnO2 polymorph, the dominant phase in electrolytic manganese dioxide (EMD), is notably observed during the charge cycles of aqueous Zn/MnO2 batteries. In this work, we investigate the electrochemical behavior of an ε-MnO2 cathode synthesized via electrodeposition from a ZnSO4 and MnSO4 electrolyte, onto a 3-dimensional carbon cloth substrate. Proton intercalation emerges as the dominant charge storage mechanism, critically enabling the reversibility of ε-MnO2 during cycling, as revealed by operando synchrotron X-ray diffraction and X-ray absorption spectroscopy. Additionally, a proton-coupled dissolution/redeposition pathway operates alongside minor Zn2+ intercalation, as quantified by Rietveld refinement. Morphological and chemical heterogeneities are studied by transmission X-ray microscopy further validates this reaction mechanism. These mechanistic insights provide the foundation for rationally designing Zn/MnO2 batteries with optimized proton dynamics and charge transfer, advancing these systems as a viable solution for safe, cost-effective grid-scale energy storage.

Copper-based electrocatalysts are widely explored for electrochemical nitrate remediation, yet their stability under operating conditions remains poorly understood. While nanoscale and subnanoscale Cu motifs are known to restructure during the nitrate reduction reaction (NO3RR), how these transformations translate into irreversible material loss remains unclear. Here, we quantify Cu dissolution during NO3RR as a function of catalyst architecture and electrolyte chemistry using two model systems: single-atom Cu (Cu1) and Cu nanoparticles (CuNP). Time-resolved leaching measurements, integrated with in situ X-ray absorption spectroscopy (XAS), reveal measurable Cu loss for both catalysts during NO3RR. Dissolution is concentrated at the initiation of electrolysis, coinciding with rapid restructuring, and depends strongly on morphology, with CuNP consistently exhibiting greater Cu loss than Cu1. In situ XAS reveals that Cu1 forms transient metallic clusters under reduction that largely redisperse upon returning to open-circuit voltage, whereas CuNP undergoes reduction of an oxidized surface layer accompanied by sustained Cu loss during electrolysis. Notably, the presence of nitrate significantly intensifies restructuring and Cu loss, highlighting the critical role of electrolyte composition. These findings establish a direct link between electrochemical restructuring and Cu dissolution, unveiling electrolyte-dependent interactions as key determinants of catalyst durability in electrochemical nitrate conversion.

Gaining a comprehensive understanding of how materials behave at the interface with molten salt is of utmost importance for the advancement of sustainable energy solutions in the future. Prior work on the impact of molten salts on the initial stages of reactions involving metal microparticles showed that due to the large size of these particles, the analysis yielded a combination of effects occurring on the particle's surface and within its bulk. Here, we use metallic nanoparticles specifically in the range of approximately 30-50 nm to understand the chemical transformations taking place at the interface between the particles and salt. By employing this approach, we aim to fully characterize the structure and kinetics of transformation for all species involved. Our synchrotron X-ray multimodal approach combines various techniques such as X-ray nano-imaging, absorption spectroscopy, diffraction/scattering and electron microscopy (STEM/EDS) to study the various corrosion mechanism and changes in the structure of the nanoparticles during the interaction with the salt. Keywords -Nanoparticles, corrosion, molten salts, X-ray tomography, XAS, XRD, PDF

Nitrogenases employ Fe-S co-factors to reduce the weakly π-acidic ligand N2 to NH3. The Mo nitrogenase is more efficient than the V and Fe-only nitrogenases, but how the Mo ion enhances N2 reduction is not fully understood. In this study, we undertake a comparative structural and spectroscopic analysis of synthetic [MoFe3S4] and [Fe4S4] clusters that ultimately reveals how the presence of Mo impacts the ability of Fe-S clusters to interact with π-acids. Specifically, we find that compared with the Fe centers in the [Fe4S4] clusters, those in the [MoFe3S4] clusters are more adept at backbonding to their weakly π-acidic N-heterocyclic carbene ligands. Such differences can be attributed to enhanced metal-sulfide mixing and Mo–Fe delocalization in the [MoFe3S4] clusters, whereby the diffuse Mo 4d orbitals more efficiently transfer electron density from the sulfides to the Fe ions and, ultimately, to Fe-bound π-acids.

Sodium-ion batteries are a promising alternative to lithium-ion batteries for select applications, offering comparable performance at lower cost and reduced reliance on critical minerals. Among the cathode candidates, Fe-and Mn-rich layered oxides free of Co and Ni are particularly attractive but suffer from poor electrochemical performance due to Fe migration and irreversible structural phase transformations. Biphasic O3/P2 cathodes attempt to improve the performance by leveraging the synergistic advantages of each phase. However, they usually do not have sufficient Na content. Further, studies typically lack a design strategy to tune the O3:P2 phase fraction to optimize performance. Here, we introduce design principles to control O3:P2 phase fractions in high-Na-content NaTMO2 materials without altering the overall composition. Using Na0.78(Li0.04Mg0.02-Fe0.38Mn0.5)O2 (NLMFM) as a model system, we demonstrate that tuning the calcination temperature from 600 degrees C to 1000 degrees C systematically increases the O3:P2 fraction by modifying the chemical potential of O2 (& micro;O2) in the calcination environment, which in turn influences the O stoichiometry in the materials and changes the TM oxidation states. We show that higher temperature (lower & micro;O2) synthesis results in an O3-rich biphasic material whose minority P2 fraction has increased Na content compared to the P2 fraction of the lower temperature (higher & micro;O2), P2-rich material. This delays detrimental P-O transitions and suppresses Fe migration in the P2 fraction, resulting in the higher temperature (lower & micro;O2), O3-rich samples exhibiting superior structural stability and electrochemical performance compared to the samples synthesized at lower temperatures (higher & micro;O2). Our findings establish a framework for phase-engineering Fe-and Mn-rich layered NaTMO2 cathodes via processing conditions alone, providing a new pathway toward high-performance, sustainable sodium-ion batteries.

The lack of mechanistic understanding and catalyst design principles for alkaline electrolytes, especially for the sluggish oxygen reduction reaction, has impeded the advancement of alkaline fuel cells. Here we propose a modified volcano plot and apply this rationale to strategically design Pt nanosheets with PdHx nanosheets substrates. This catalyst exhibited high stability with a specific activity of 1.71 mA cm−2 at 0.95 V versus the reversible hydrogen electrode, surpassing the benchmark of Pt/C by 49-fold. Spectroscopic, electrochemical and electron microscopic characterizations revealed that such performance enhancement originated from tensile-strained Pt{111} facets, improving oxidative stability and suppressing carbon corrosion. In fuel cell testing, the catalyst enabled a peak power density of 1.67 W cm−2 with a loading of 10 µgPGM Cathode cm−2. Further optimization delivered a peak power density of 21.7 W mg−1PGM Cathode+Anode with a total specific catalyst cost US$1.27 kW−1, surpassing the US Department of Energy’s Pt group metal loading and cost targets. This study provides valuable insights into catalyst design for the alkaline oxygen reduction reaction. Volcano plots are useful catalyst design tools for acidic oxygen reduction reaction, but have proved less successful for alkaline oxygen reduction reaction. Here, a modified volcano plot for this environment is developed and used to design PdHx@Pt nanosheets with high oxygen reduction reaction performance.

Metal-oxide-supported Pt-group metal catalysts are essential for industrial reactions, including CO oxidation in automotive emission control. Maximizing the utilization of these metals is crucial for designing efficient catalysts. Therefore, understanding how the metal electronic properties and metal–support interactions change with metal nuclearity, especially in the subnanometer regime, is essential for advancing the catalytic performance. This study investigates the reactivity of CO oxidation on Pt supported on anatase TiO2 with varying Pt nuclearity from single-atoms, 0.9 nm clusters to 1.8 nm nanoparticles. Kinetic measurements and in-situ infrared spectroscopy reveal that CO oxidation activity and the Pt resistance to oxidation increase with nuclearity. Additionally, in-situ infrared and X-ray absorption spectroscopy results show that the electron density on Pt under CO oxidation increases with Pt nuclearity. The reaction mechanism is shown to change as a function of nuclearity. For 0.9 nm clusters and 1.8 nm nanoparticles, TiO2 lattice oxygen is involved in the reaction, likely via the Mars–van Krevelen mechanism. In contrast, on single-atoms, oxygen from the lattice is not labile, and extra oxygen adatoms adsorbed on the Pt-Ti interface participate in CO2 formation, leading to a significantly lower activity than 0.9 nm and 1.8 nm Pt. Kinetic analysis coupled with temperature-programmed reduction by CO reveals that the higher activity is due to lower oxygen vacancy formation energy induced by larger Pt nuclearity. Overall, our study demonstrates that Pt nuclearity influences the reducibility of TiO2 through electronic interaction, which gives rise to the structure-sensitivity of CO oxidation on Pt/TiO2 catalyst.

Sodium-ion batteries are a promising lower-cost alternative to lithium-ion batteries, but further improvements in electrochemical performance are required. One strategy to increase capacity is to enable reversible high-valent cationic and anionic redox in layered cathode materials; however, this is typically accompanied by structural degradation. Here, we elucidate the mechanism by which Fe-doped Na2Mn3O7, featuring ordered transition metal-vacancies, achieves reversible high-valent redox. Using Mossbauer spectroscopy, soft X-ray absorption spectroscopy (XAS), and in-situ hard XAS, we demonstrate reversible high-valent cationic redox involving both Fe and Mn while in-situ Raman confirms the absence of local structural degradation associated with oxygen redox. Combining in-situ X-ray diffraction with theoretical calculations, we further identify a previously unreported global phase transition from the P1 to the P21/c space group during electrochemical cycling and develop a physical model describing this structural evolution. These results provide insights for structurally stable layered sodium transition metal oxide cathodes with reversible high-valent redox.

A major hurdle to the implementation of single-atom catalysts (SACs) in real-world systems is a poor understanding of their stability under operating conditions, which is particularly relevant due to the high surface free energy of SACs. Here, we evaluated the aggregation behavior of a suite of SACs varied by metal identity (Fe, Co, Ni, and Cu) during electrocatalytic nitrate reduction using in situ X-ray absorption spectroscopy. The metal center had significant influence on reconstruction, where under identical applied reductive potentials, SACs underwent varying levels of reconstruction, ranging from no discernible change to complete reduction into metallic nanoparticles. Such in situ experiments revealed Cu SACs to be the most susceptible to aggregation, prompting a deeper investigation into how coordination environment (O-, B-, and N-graphene) affected Cu SAC aggregation. We further conducted density functional theory calculations to elucidate the relationship between Cu SAC structure and stability. This work deconvolutes the relationship between SAC architecture and stability, which is essential to evaluate and explain for the realization of SACs for electrocatalysis.

Directly converting methane (CH4) into liquid oxygenates (e.g., methanol) can circumvent the cost and engineering limits of natural gas transportation and storage. However, oxygenate yields from CH4 remain low, and sulfur present in natural gas hinders activity in most catalysts. To overcome these barriers, we employ bulk molybdenum disulfide (MoS2), a low-cost, robust catalyst which selectively produces large quantities of liquid oxygenates (>900 & micro;mol/g(cat)center dot hr) from methane in the presence of hydroxyl (OH center dot) radicals produced from dilute hydrogen peroxide (H2O2) at 75 degrees C. Under realistic reaction conditions, MoS2 partially and reversibly adopts a metastable, more electrically conductive phase (1T') that can only be observed through in situ structural probes. Herein, we elucidate that redox synergy between H2O2 and MoS2 produces active OH center dot radical species that selectively transform CH4 to surface methoxy species at the gas-solid liquid interface, leading to the unitary production of liquid oxygenate at a rate competitive with more costly precious metal catalysts, without additional catalyst preparation steps.

Resolving the local structure motifs that characterize phase evolution as a function of composition is a key challenge in structure characterization of complex materials. In this study, we combine first-principles simulations and x-ray absorption near-edge structures (XANES) analysis to gain insights into the structure evolution revealed by measurements across a combinatorial zinc titanate thin film, which was grown with smoothly varying composition over a wide range of the Ti:Zn ratio. Specifically, we propose a cluster blind-signal-separation (cBSS) method for XANES spectral analysis based on a library of the structures and spectra of representative local motifs. In addition to motifs from zinc titanate crystals, two types of Ti-defect models constructed in this study are key to the understanding of the structure characteristics in the Zn-rich region. The cBSS method makes use of both spectral clustering of the simulated site-XANES spectra library and the BSS procedure to construct high-fidelity spectral basis functions from an experimental spectral sequence. The method provides a rigorous measure of the spectral sensitivity and basis completeness. The results of the XANES analysis are corroborated with other experimental modalities, including x-ray diffraction and spectroscopic ellipsometry, to validate the cBSS method. The calculated motif weights resulting from fitting the XANES spectra with the cBSS basis probe the atomic structure characteristics of both crystalline and amorphous phases as a function of the Ti/Zn composition. The insights of the local structure motif evolution are pivotal to the understanding of the nonmonotonic trend in the optical gap, which may lead to potential applications through tuning the optical properties of zinc titanate. The workflow of the XANES spectral analysis developed in this work can be generalized to construct the structure-property relationship in a broad material space.

While current methods use oxidizable metals as electron donors to effectively reduce Fe3+, they suffer from the irreversible oxidation of these metals, ultimately compromising the catalyst's longevity. To address this challenge, we engineered the second coordination shell of a single-atom Fe center by doping boron (B) onto a graphene-based support (Fe1/B-graphene) and utilized H2O2 as the electron source for efficient Fe2+ regeneration. Experimental results, supported by theoretical calculations, revealed that the Fe-O-B motif functions like a micro galvanic cell, with intermediary O atoms facilitating electron transfer between electrodes. Specifically, electrons consumed during H2O2 activation at Fe1 sites (positive electrode) are replenished by electrons extracted from H2O2 at B atoms (negative electrode), where the activation energy for H2O2 oxidation is significantly lower than that at Fe1 sites. This study offers inspirational insights into the design of Fenton catalysts through precise regulation of the second coordination shell, demonstrating the potential of tailoring the outer coordination environment of single-atom catalysts to enhance catalytic performance across various reactions.

The scalable and facile preparation of single-atom catalysts remains a critical challenge. Here, we introduce diluted atomic layer deposition (DALD), a unique approach for synthesizing supported metal catalysts with precisely tunable loadings. Unlike conventional metal deposition by ALD which uses pure metal precursors, DALD employs a diluted precursor mixture, combining organometallic precursors with the corresponding free ligand in controlled ratios. The method enables precise control over metal loadings, allowing the synthesis of structures ranging from nanoparticles to isolated single atoms, as exemplified by Ir, Rh, and Pt on high-surface-area γ-Al2O3. With its inherent simplicity and exceptional efficiency in metal precursor utilization, DALD represents a highly scalable strategy, unlocking opportunities for integrating single-atom catalysts into industrial processes.

While single-atom catalysts (SACs) are often touted for their maximal atomic efficiency and enhanced activity compared to nanoparticle counterparts, our understanding of SAC stability is severely lacking. The atomic dispersion of SACs leads to a high surface free energy, which can result in single-atom aggregation. Such instability can be further exacerbated by reaction conditions, such as temperature, gaseous atmosphere, or applied potential. Here, we investigated the stability of a series of SACs of various metal identities during electrocatalytic nitrate reduction by potential-resolved in situ X-ray absorption spectroscopy (XAS). We demonstrate the metal center to have significant influence on stability, where under an identical applied potential pattern, SACs underwent varying levels of reconstruction ranging from minimal change to complete reduction into metallic particles. The in situ XAS experiments revealed Cu SACs to be the most vulnerable to aggregation, prompting a deeper investigation into how the coordination environment surrounding Cu affected aggregation. We lastly conducted density functional theory calculations to determine the relationship between Cu SAC structure and stability. Determining the relationship between SAC structure and stability is essential to evaluate for the implementation of SACs to real-world electrocatalytic systems, as the line between stability and activity is not always clear.

Tailoring and stabilizing the active sites of supported noble-metal catalysts to a semioxidized state with unsaturated coordination remain a long-standing challenge in heterogeneous catalysis. Herein, we develop a reaction-atmosphere-driven evolution approach for dynamic structural tuning of semioxidized metal sites in supported Pt catalysts. N2O as an alternative oxidant is used over Pt/TiO2 in CH4 combustion to dynamically prompt the transformation of Pt0 nanoclusters into Ptδ+ (0 < δ < 2) nanoclusters. Compared to CH4 combustion with O2 that inclines to overoxidize Pt0, the catalytic activity of CH4-N2O combustion is distinctly boosted, achieving complete CH4 combustion at only 200 °C, outperforming the state-of-the-art catalysts using O2 as the oxidant. Computational and experimental studies validate that N2O triggers less electron transfer from Pt than from O2, thereby facilitating the formation and preservation of Ptδ+ species during CH4 combustion. The newly emerged semioxidized Ptδ+ species with oxygen-deficient coordination structures simultaneously enhance lattice oxygen activation and the first C-H bond dissociation of CH4, contributing to ultralow temperature activity. Our work demonstrates that modulating the reaction atmosphere to achieve the structural dynamic evolution of semioxidized metal sites can provide new strategies for designing highly efficient catalysts for low-temperature CH4 combustion.

Understanding and controlling the physical and chemical processes at molten salt‐alloy interfaces is vital for molten‐salt nuclear reactors. Corrosion processes in molten salts are highly dependent on the redox potential of the solution that changes with the addition of fission and corrosion products. Therefore, reactor designers develop online electrochemical methods of salt monitoring. But electrochemical spectroscopy relies on the deconvolution of broad peaks, a process that may be imprecise in the presence of multiple species in the solution. Here, we describe our developments towards monitoring the concentration and the chemical state of corrosion products in the melt by a combination of electrochemistry and X-ray absorption spectroscopy. We placed NiCr foil in molten FLiNaK and found the presence of both Ni2+ ions and metallic Ni in the melt, which we attribute to the disintegration of the corroding foil due to Cr dealloying. Although extremely challenging, spectroelectrochemical measurements add a promising rich new data stream for online salt monitoring.

Aqueous Zn/MnO 2 batteries have garnered significant interests owing to their abundance, high theoretical specific capacity, low cost, and safety. However, the large-scale application of these systems is limited by the lack of understanding of the reaction mechanism of the MnO 2 cathode. An electrodeposited polymorph of MnO₂ has been identified as the dominant phase that forms regardless of the initial polymorph in the charge cycles (1.75 V vs Zn/Zn +2 ). This work investigates the reaction mechanism of the electrodeposited polymorph of MnO₂. A multimodal approach combining synchrotron X-ray diffraction, absorption spectroscopy, and nano-tomography was used to study the structural evolution, reaction chemistry, and the morphological and chemical changes of the electrodeposited cathode. Full-field nano-tomography visualizes these changes in the three-dimensional structure of the electrodeposited cathode after discharge/charge cycle. X-ray absorption near-edge structure (XANES) spectroscopy showed the chemical heterogeneity of the cathode material. Overall, these findings highlight the factors limiting the specific capacity, paving the way for practical applications in grid-scale energy storage systems.

PdAu/SiO2 catalysts were synthesized by strong electrostatic adsorption (SEA) and characterized by TEM, DRIFTS, XRD, XAS, and O-2-TPD. The use of group 1 alkali salt solutions to control pH during SEA syntheses led to uptake of alkali metals observed reductions in the densities of terminal silanol groups of the SiO2 support. In the absence of alkali metals, PdAu/SiO2 catalyzes oxidative C-N bond formation between methanol and dimethylamine (DMA), yielding dimethylformamide (DMF) with similar to 95 % carbon selectivity (CO2 similar to 5 %) at temperatures below 413 K. When Na, K, and Cs were present on the catalyst, methyl formate (MF) and tetramethylurea (TMU) were observed as additional products (combined similar to 30 % carbon selectivity) while only TMU was detected for the Li-promoted catalyst. Total coupling product rate increased for promoted samples in the order Li < Na < Cs < K, and the apparent kinetics over the Cs-promoted catalyst were distinct from those over the alkali-free catalyst as the apparent reaction order with respect to DMA decreased and the apparent activation energy increased. This work demonstrates the sensitivity of oxidative coupling reactions to alkali metal promoters and the opportunity to achieve alkali promotion of metal catalysts during SEA synthesis.

Lithium (Li) chloride and iron oxychloride (FeOCl), typically nonconductive, were combined to form a [Li1+dCl]d+/[FeOCl]d- heterointerface composite material (LFH), achieving ionic conductivities of >1 mS cm-1. Analysis techniques (scanning transmission electron microscopy [STEM] and electron energy-loss spectroscopy [EELS]) indicated that the microstructure of LFH consisted of an amorphous LiCl-based shell surrounding a crystalline FeOCl-based core. Electrochemical measurements alongside solid-state 6,7Li nuclear magnetic resonance (NMR) and molecular dynamic simulations revealed Li+ as the sole conductive species, with a diffusion barrier of '0.25 eV. X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) results further supported interstitial Li+ diffusion at the heterointerface and within the LiCl phase, made possible by the heterointerface. Despite susceptibility to electronic conductivity, iron's defects and multivalency (Fe3', Fe2') enable the Fe-O-Cl framework to accept Cl-, facilitating Li'ionic conduction. A prototype solid-state cell (showing 97% Coulombic efficiency) demonstrated the viability of this heterointerface design for applications in energy storage.