Stabilizing Hydrogen Adsorption through Theory-Guided Chalcogen Substitution in Chevrel-Phase Mo6X8 (X=S, Se, Te) Electrocatalysts

Nanoscale palladium-Mo6S8/carbon nanowires toward efficient electrochemical hydrogen evolution and hydrogen peroxide detection

Chevrel phase (CP) molybdenum sulfides (Mo6S8) have attracted extensive research attention in the field of energy conversion and storage due to their unique electronic structures and rich open channels. However, comprehensive understanding of intrinsic kinetic mechanisms governing the electrocatalytic bi-functional hydrogen evolution reaction (HER) and hydrogen peroxide (H2O2) sensing on CP-based composites is still lacking. Herein, nanosized palladium (Pd) and Mo6S8 particles were assembled in carbon nanowires (C NWs) via electrospinning followed by pyrolysis. The as-obtained novel Pd-Mo6S8/C NWs exhibited excellent performance in terms of a low overpotential of -194 mV at η10 for HER, and an ultrahigh sensitivity of 2231 μA mM-1 cm-2 with a limit of detection of 25 nM for H2O2 sensing. The experimental and theoretical findings demonstrated that Pd and Mo6S8 nanoparticles (NPs) exhibited exceptional catalytic activity and strong electronic interactions. The synergistic effects of these two components could effectively modulate the binding strength of reactants and intermediates on the catalyst surface, ultimately leading to improved electrochemical catalytic performance toward reduction of small molecules. Moreover, verification of the stable tolerance in various environments and good selectivity of the electrocatalyst promoted the further use of Pd-Mo6S8/C NWs-based electrochemical sensing system for sensing additional H2O2 in milk samples, proving the widespread potential of this material for practical applications. This study significantly advances the understanding of nanoscale and bi-functional CP-based composites.

Heterointercalation in Chevrel-Phase Sulfides: A Model Periodic Solid for the Investigation of Chain Electron Transfer

Modulation of electron density localization on periodic crystal solids through electron transfer from interstitial cations can directly influence the bonding configurations of small-molecule intermediates at the catalyst binding site. This study presents the microwave-assisted solid-state synthesis of four heterointercalant Chevrel-phase (CP) sulfides with varying metal cation intercalants with compositional and electronic structure investigations of the electron density redistribution as a result of intercalation. The heterointercalant CP sulfides, with the general formula CuxMyMo6S8 (where M = Cr, Mn, Fe, Ni; x, y = 1.5-2.5), are presented here for the probe reaction of electrochemical CO2 reduction. A change in product selectivity is observed toward the production of methanol at low overpotentials of -0.5 V vs reversible hydrogen electrode (RHE), as a result of the intercalant combination present within the CP interstitial cavity. Structural confirmation of all materials was examined through Rietveld refinement of the powder X-ray diffraction (PXRD) data, high-resolution transmission electron microscopy (HR-TEM), and selected-area electron diffraction (SAED). Electron transfer from the intercalated metal cations to the Mo6S8 cluster was investigated via X-ray photoelectron spectroscopy (XPS) of the intercalated metal cations and the chalcogenide cluster. Electron transfer was further confirmed through X-ray absorption analysis (XAS) of the K-edges of Mo and intercalants. Intermediate studies of electrochemical reduction of formaldehyde to methanol resulted in a faradaic efficiency of ∼78% methanol production on CuxNiyMo6S8. The results presented herein identify distinct principles for materials design that can be utilized in other compositional spaces within the broad families of periodic crystal solids.

Tailoring Synergistic Ion Environment for Copper Telluride toward High-Capacity and Ultrastable Acidic Multivalent-Ion Batteries

Acidic batteries permit a reliable energy supply at low temperatures with low cost and intrinsic safety, yet the development of stable acid-resistant electrodes with high capacity and a reliable lifespan is still challenging. Herein, nonstoichiometric copper telluride (Cu2-xTe) nanosheets are first explored as high-performance electrodes for acidic batteries to provide a stable capacity release of 409 mAh g-1 with a record-breaking lifespan of 40 000 cycles and excellent kinetics, enabling operation at a high current density of 20 A g-1. In contrast to the inherent perception of corrosive destruction of electrode materials by strongly acidic environments, the electrolyte environment enriched with copper ions and hydrogen ions synergistically stabilizes the Cu2-xTe electrode and drives reversible multielectron transfer asymmetric deep conversion, which is confirmed by in situ synchrotron X-ray diffraction, X-ray absorption spectroscopy, first-principal calculations, and composite electrochemical characterization. Therefore, Cu2-xTe provides an impressive accumulation capacity of over 4764 Ah g-1, exceeding that of most acidic batteries, and works well at -20 °C. High-performance Cu2-xTe electrodes also promote the establishment of Cu2-xTe//Mn2O3 and Cu2-xTe//Fe acidic full cells enabling stable operation at room temperature and low temperature, offering promising opportunities for electrode progress in advanced acidic batteries.

Extending the Chevrel-type superatoms to the nitrogen family

Chevrel-type superatoms refer to the ligated transition metal chalcogenide clusters M6E8L6, where the octahedral M6 is face-capped with cubic chalcogen E8 (E = S/Se/Te). Most transition metals can form such superatoms and many organic and inorganic ligands can be substituted in solution reactions, which allows these atomic precision species to be easily functionalized and their properties to be tunable. No test was reported for substituting the chalcogens with pnicogens in this class of materials. In this article, we try to discover if such substitutions are possible. Combining different transition metals and ligands, theoretical computations show that [M6Q8(CN)6]2- (Q = P, As, Sb) for M = Co, Rh, Ir, and Ni, Pd, Pt have closed electronic shells and possess enhanced thermal and chemical stabilities. Analyses of the electronic structures indicate high similarity between the M-M and M-Q/M-S interactions in these species.

CuS-NiTe2 embedded phosphorus-doped graphene oxide catalyst for evaluating hydrogen evolution reaction

The hydrogen evolution reaction (HER), a crucial half-reaction in the water-splitting process, is hindered by slow kinetics, necessitating efficient electrocatalysts to lower overpotential and enhance energy conversion efficiency. Transition-metal electrode materials, renowned for their robustness and effectiveness, have risen to prominence as primary contenders in the field of energy conversion and storage research. In this investigation, we delve into the capabilities of transition metals when employed as catalysts for the HER. Furthermore, we turn our attention to carbon nanomaterials like graphene, which have exhibited tremendous potential as top-performing electrocatalysts. Nevertheless, advancements are indispensable to expand their utility and versatility. One such enhancement involves the integration of phosphorus-doped graphene. Our research focuses on the synthesis of CuS-NiTe2/PrGO, a nanocomposite with a crystalline structure, through a straightforward method. This nanocomposite exhibits enhanced catalytic activity for the HER, boasting a Tafel slope of 57 mV dec-1 in an acidic environment. Consequently, our findings present a straightforward and efficient approach to developing high-performance electrocatalysts for HER.

A Low-Temperature Synthetic Route Toward a High-Entropy 2D Hexernary Transition Metal Dichalcogenide for Hydrogen Evolution Electrocatalysis

High-entropy (HE) metal chalcogenides are a class of materials that have great potential in applications such as thermoelectrics and electrocatalysis. Layered 2D transition-metal dichalcogenides (TMDCs) are a sub-class of high entropy metal chalcogenides that have received little attention to date as their preparation currently involves complicated, energy-intensive, or hazardous synthetic steps. To address this, a low-temperature (500 °C) and rapid (1 h) single source precursor approach is successfully adopted to synthesize the hexernary high-entropy metal disulfide (MoWReMnCr)S2 . (MoWReMnCr)S2 powders are characterized by powder X-ray diffraction (pXRD) and Raman spectroscopy, which confirmed that the material is comprised predominantly of a hexagonal phase. The surface oxidation states and elemental compositions are studied by X-ray photoelectron spectroscopy (XPS) whilst the bulk morphology and elemental stoichiometry with spatial distribution is determined by scanning electron microscopy (SEM) with elemental mapping information acquired from energy-dispersive X-ray (EDX) spectroscopy. The bulk, layered material is subsequently exfoliated to ultra-thin, several-layer 2D nanosheets by liquid-phase exfoliation (LPE). The resulting few-layer HE (MoWReMnCr)S2 nanosheets are found to contain a homogeneous elemental distribution of metals at the nanoscale by high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) with EDX mapping. Finally, (MoWReMnCr)S2 is demonstrated as a hydrogen evolution electrocatalyst and compared to 2H-MoS2 synthesized using the molecular precursor approach. (MoWReMnCr)S2 with 20% w/w of high-conductivity carbon black displays a low overpotential of 229 mV in 0.5 M  H2 SO4 to reach a current density of 10 mA cm-2 , which is much lower than the overpotential of 362 mV for MoS2 . From density functional theory calculations, it is hypothesised that the enhanced catalytic activity is due to activation of the basal plane upon incorporation of other elements into the 2H-MoS2 structure, in particular, the first row TMs Cr and Mn.

Optical and electronic properties enhancement via chalcogenides: promising materials for DSSC applications

CONTEXT

Comparatively, metal-free sensitizers featuring the chalcogen family receive less attention despite known electronic properties for metal-chalcogenide materials. This work reports an array of optoelectronic properties using quantum chemical methods. Observed red-shifted bands within the UV/Vis to NIR regions with absorption maxima > 500 nm were consistent with increasing chalcogenide size. There is a monotonic down-shift in the LUMO and ESOP energy consistent with O 2p, S 3p, Se 4p, to Te 5p atomic orbital energies. Excited-state lifetime and charge injection free energies follow the decreasing order of chalcogenide electronegativity. Adsorption energies of dyes on TiO₂ anatase (101) range between - 0.08 and - 0.77 eV. Based on evaluated properties, selenium- and tellurium-based materials show potential use in DSSCs and futuristic device applications. Therefore, this work motivates continued investigation of the chalcogenide sensitizers and their application.

METHODS

The geometry optimization was performed at B3LYP/6-31 + G(d,p) and B3LYP/LANL2DZ level of theory for lighter and heavier atoms, respectively, using Gaussian 09. The equilibrium geometries were confirmed by the absence of imaginary frequencies. Electronic spectra were obtained at CAM-B3LYP/6-31G + (d,p)/LANL2DZ level of theory. Adsorption energies for dyes on a 4 × 5 supercell TiO₂ anatase (101) were obtained using VASP. The dye-TiO₂ optimizations were employed using GGA and PBE with the PAW pseudo-potentials. The energy cutoff was set at 400 eV and convergence threshold for self-consistent iteration was set to 10^-4, and van der Waals were accounted using DFT-D3 model and on-site Coulomb repulsion potential set at 8.5 eV for Ti.

Recent Advances in Transition Metal Tellurides (TMTs) and Phosphides (TMPs) for Hydrogen Evolution Electrocatalysis

The hydrogen evolution reaction (HER) is a developing and promising technology to deliver clean energy using renewable sources. Presently, electrocatalytic water (H₂O) splitting is one of the low-cost, affordable, and reliable industrial-scale effective hydrogen (H₂) production methods. Nevertheless, the most active platinum (Pt) metal-based catalysts for the HER are subject to high cost and substandard stability. Therefore, a highly efficient, low-cost, and stable HER electrocatalyst is urgently desired to substitute Pt-based catalysts. Due to their low cost, outstanding stability, low overpotential, strong electronic interactions, excellent conductivity, more active sites, and abundance, transition metal tellurides (TMTs) and transition metal phosphides (TMPs) have emerged as promising electrocatalysts. This brief review focuses on the progress made over the past decade in the use of TMTs and TMPs for efficient green hydrogen production. Combining experimental and theoretical results, a detailed summary of their development is described. This review article aspires to provide the state-of-the-art guidelines and strategies for the design and development of new highly performing electrocatalysts for the upcoming energy conversion and storage electrochemical technologies.

Dual interfacial engineering of a Chevrel phase electrode material for stable hydrogen evolution at 2500 mA cm-2

Constructing stable electrodes which function over long timescales at large current density is essential for the industrial realization and implementation of water electrolysis. However, rapid gas bubble detachment at large current density usually results in peeling-off of electrocatalysts and performance degradation, especially for long term operations. Here we construct a mechanically-stable, all-metal, and highly active CuMo₆S₈/Cu electrode by in-situ reaction between MoS₂ and Cu. The Chevrel phase electrode exhibits strong binding at the electrocatalyst-support interface with weak adhesion at electrocatalyst-bubble interface, in addition to fast hydrogen evolution and charge transfer kinetics. These features facilitate the achievement of large current density of 2500 mA cm⁻² at a small overpotential of 334 mV which operate stably at 2500 mA cm⁻² for over 100 h. In-situ total internal reflection imaging at micrometer level and mechanical tests disclose the relationships of two interfacial forces and performance of electrocatalysts. This dual interfacial engineering strategy can be extended to construct stable and high-performance electrodes for other gas-involving reactions.

Stable electrodes which operate at large current density are essential for industrial water electrolysis. Here, a highly active Chevrel phase electrode is reported to achieve 2500 mA/cm⁻² current density for 300 hours at small overpotentials.

Nanocubes of Mo6S8 Chevrel phase as active electrode material for aqueous lithium-ion batteries

The development of intrinsically safe and environmentally sustainable energy storage devices is a significant challenge. Recent advances in aqueous rechargeable lithium-ion batteries (ARLIBs) have made considerable steps in this direction. In parallel to the ongoing progress in the design of aqueous electrolytes that expand the electrochemically stable potential window, the design of negative electrode materials exhibiting large capacity and low intercalation potential attracts great research interest. Herein, we report the synthesis of high purity nanoscale Chevrel Phase (CP) Mo₆S₈via a simple, efficient and controllable molecular precursor approach with significantly decreased energy consumption compared to the conventional approaches. Physical characterization of the obtained product confirms the successful formation of CP-Mo₆S₈ and reveals that it is crystalline nanostructured in nature. Due to their unique structural characteristics, the Mo₆S₈ nanocubes exhibit fast kinetics in a 21 m lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte as a result of the shorter Li⁺ ion diffusion distance. Full battery cells comprised of Mo₆S₈ and LiMn₂O₄ as negative and positive electrode materials, respectively, operate at 2.23 V delivering a high energy density of 85 W h kg^-1 (calculated on the total mass of active materials) under 0.2 C-rate. At 4 C, the coulombic efficiency (CE) is determined to be 99% increasing to near 100% at certain cycles. Post-mortem physical characterization demonstrates that the Mo₆S₈ anode maintained its crystallinity, thereby exhibiting outstanding cycling stability. The cell outperforms the commonly used vanadium-based (VO₂ (B), V₂O₅) or (NASICON)-type LiTi₂(PO₄)₃ anodes, highlighting the promising character of the nanoscale CP-Mo₆S₈ as a highly efficient anode material. In summary, the proposed synthetic strategy is expected to stimulate novel research towards the widespread application of CP-based materials in various aqueous and non-aqueous energy storage systems.

X-ray absorption spectroscopy insights on the structure anisotropy and charge transfer in Chevrel Phase chalcogenides

The electronic structure and local coordination of binary (Mo₆T₈) and ternary Chevrel Phases (M_xMo₆T₈) are investigated for a range of metal intercalant and chalcogen compositions. We evaluate differences in the Mo L₃-edge and K-edge X-ray absorption near edge structure across the suite of chalcogenides M_xMo₆T₈ (M = Cu, Ni, x = 1-2, T = S, Se, Te), quantifying the effect of compositional and structural modification on electronic structure. Furthermore, we highlight the expansion, contraction, and anisotropy of Mo₆ clusters within these Chevrel Phase frameworks through extended X-ray absorption fine structure analysis. Our results show that metal-to-cluster charge transfer upon intercalation is dominated by the chalcogen acceptors, evidenced by significant changes in their respective X-ray absorption spectra in comparison to relatively unaffected Mo cations. These results explain the effects of metal intercalation on the electronic and local structure of Chevrel Phases across various chalcogen compositions, and aid in rationalizing electron distribution within the structure.

How the Bioinspired Fe2Mo6S8 Chevrel Breaks Electrocatalytic Nitrogen Reduction Scaling Relations

The nitrogen reduction reaction (NRR) is a renewable alternative to the energy- and CO₂-intensive Haber-Bosch NH₃ synthesis process but is severely limited by the low activity and selectivity of studied electrocatalysts. The Chevrel phase Fe₂Mo₆S₈ has a surface Fe-S-Mo coordination environment that mimics the nitrogenase FeMo-cofactor and was recently shown to provide state-of-the-art activity and selectivity for NRR. Here, we elucidate the previously unknown NRR mechanism on Fe₂Mo₆S₈ via grand-canonical density functional theory (GC-DFT) that realistically models solvated and biased surfaces. Fe sites of Fe₂Mo₆S₈ selectively stabilize the key *NNH intermediate via a narrow band of free-atom-like surface d-states that selectively hybridize with p-states of *NNH, which results in Fe sites breaking NRR scaling relationships. These sharp d-states arise from an Fe-S bond dissociation during N₂ adsorption that mimics the mechanism of the nitrogenase FeMo-cofactor. Furthermore, we developed a new GC-DFT-based approach for calculating transition states as a function of bias (GC-NEB) and applied it to produce a microkinetic model for NRR at Fe₂Mo₆S₈ that predicts high activity and selectivity, in close agreement with experiments. Our results suggest new design principles that may identify effective NRR electrocatalysts that minimize the barriers for *N₂ protonation and *NH₃ desorption and that may be broadly applied to the rational discovery of stable, multinary electrocatalysts for other reactions where narrow bands of surface d-states can be tuned to selectively stabilize key reaction intermediates and guide selectivity toward a target product. Furthermore, our results highlight the importance of using GC-DFT and GC-NEB to accurately model electrocatalytic reactions.

Multi-dimensional designer catalysts for negative emissions science (NES): bridging the gap between synthesis, simulations, and analysis

Negative emissions technologies will play a critical role in limiting global warming to sustainable levels. Electrocatalytic and/or photocatalytic CO2 reduction will likely play an important role in this field moving forward, but efficient, selective catalyst materials are needed to enable the widespread adoption of these processes. The rational design of such materials is highly challenging, however, due to the complexity of the reactions involved as well as the large number of structural variables which can influence behavior at heterogeneous interfaces. Currently, there is a significant disconnect between the complexity of materials systems that can be handled experimentally and those that can be modeled theoretically with appropriate rigor and bridging these gaps would greatly accelerate advancements in the field of Negative Emissions Science (NES). Here, we present a perspective on how these gaps between materials design/synthesis, characterization, and theory can be resolved, enabling the rational development of improved materials for CO2 conversion and other NES applications.

Machine Learning Guided Synthesis of Multinary Chevrel Phase Chalcogenides

The Chevrel phase (CP) is a class of molybdenum chalcogenides that exhibit compelling properties for next-generation battery materials, electrocatalysts, and other energy applications. Despite their promise, CPs are underexplored, with only ∼100 compounds synthesized to date due to the challenge of identifying synthesizable phases. We present an interpretable machine-learned descriptor (H_δ) that rapidly and accurately estimates decomposition enthalpy (ΔH_d) to assess CP stability. To develop H_δ, we first used density functional theory to compute ΔH_d for 438 CP compositions. We then generated >560 000 descriptors with the new machine learning method SIFT, which provides an easy-to-use approach for developing accurate and interpretable chemical models. From a set of >200 000 compositions, we identified 48 501 CPs that H_δ predicts are synthesizable based on the criterion that ΔH_d < 65 meV/atom, which was obtained as a statistical boundary from 67 experimentally synthesized CPs. The set of candidate CPs includes 2307 CP tellurides, an underexplored CP subset with a predicted preference for channel site occupation by cation intercalants that is rare among CPs. We successfully synthesized five of five novel CP tellurides attempted from this set and confirmed their preference for channel site occupation. Our joint computational and experimental approach for developing and validating screening tools that enable the rapid identification of synthesizable materials within a sparse class is likely transferable to other materials families to accelerate their discovery.

Transition Metal Chalcogenides as a Versatile and Tunable Platform for Catalytic CO2 and N2 Electroreduction

Group VI transition metal chalcogenides are the subject of increasing research interest for various electrochemical applications such as low-temperature water electrolysis, batteries, and supercapacitors due to their high activity, chemical stability, and the strong correlation between structure and electrochemical properties. Particularly appealing is their utilization as electrocatalysts for the synthesis of energy vectors and value-added chemicals such as C-based chemicals from the CO₂ reduction reaction (CO₂R) or ammonia from the nitrogen fixation reaction (NRR). This review discusses the role of structural and electronic properties of transition metal chalcogenides in enhancing selectivity and activity toward these two key reduction reactions. First, we discuss the morphological and electronic structure of these compounds, outlining design strategies to control and fine-tune them. Then, we discuss the role of the active sites and the strategies developed to enhance the activity of transition metal chalcogenide-based catalysts in the framework of CO₂R and NRR against the parasitic hydrogen evolution reaction (HER); leveraging on the design rules applied for HER applications, we discuss their future perspective for the applications in CO₂R and NRR. For these two reactions, we comprehensively review recent progress in unveiling reaction mechanisms at different sites and the most effective strategies for fabricating catalysts that, by exploiting the structural and electronic peculiarities of transition metal chalcogenides, can outperform many metallic compounds. Transition metal chalcogenides outperform state-of-the-art catalysts for CO₂ to CO reduction in ionic liquids due to the favorable CO₂ adsorption on the metal edge sites, whereas the basal sites, due to their conformation, represent an appealing design space for reduction of CO₂ to complex carbon products. For the NRR instead, the resemblance of transition metal chalcogenides to the active centers of nitrogenase enzymes represents a powerful nature-mimicking approach for the design of catalysts with enhanced performance, although strategies to hinder the HER must be integrated in the catalytic architecture.