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Abstract: Oxygen electrocatalysis at transition metal oxides is one of the key challenges underpinning electrochemical energy conversion systems, involving a delicate interplay of the bulk electronic structure and surface coordination of the active sites. In this work, we investigate for the first time the structure–activity relationship of A2RuMnO7 (A = Dy3+, Ho3+, and Er3+) nanoparticles, demonstrating how orbital mixing of Ru, Mn, and O promotes high density of states at the appropriate energy range for oxygen electrocatalysis. The bulk structure and surface composition of these multicomponent pyrochlores are investigated by high-resolution transmission electron microscopy, X-ray diffraction, X-ray absorption spectroscopy, X-ray emission spectroscopy (XES), and X-ray photoemission spectroscopy (XPS). The materials exhibit high phase purity (cubic fcc with a space group Fd3̅m) in which variations in M–O bonds length are less than 1% upon replacing the A-site lanthanide. XES and XPS show that the mean oxidation state at the Mn-site as well as the nanoparticle surface composition was slightly affected by the lanthanide. The pyrochlore nanoparticles are significantly more active than the binary RuO2 and MnO2 toward the 4-electron oxygen reduction reaction in alkaline solutions. Interestingly, normalization of kinetic parameters by the number density of electroactive sites concludes that Dy2RuMnO7 shows twice higher activity than benchmark materials such as LaMnO3. Analysis of the electrochemical profiles supported by density functional theory calculations reveals that the origin of the enhanced catalytic activity is linked to the mixing of Ru and Mn d-orbitals and O p-orbitals at the conduction band which strongly overlap with the formal redox energy of O2 in solution. The activity enhancement strongly manifests in the case of Dy2RuMnO7 where the Ru/Mn ratio is closer to 1 in comparison with the Ho3+ and Er3+ analogs. These electronic effects are discussed in the context of the Gerischer formalism for electron transfer at the semiconductor/electrolyte junctions.

Open Access

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Abstract: Uranyl oxalate (UO2C2O4·xH2O) may exist at the back-end of the nuclear fuel cycle (NFC) as an intermediate in spent fuel reprocessing. The conditions used in aqueous reprocessing and thermal treatment can affect the physical and chemical properties of the material. Furthermore, trace impurities, such as Fe, may incorporate into the structure of these materials. In nuclear forensics, understanding relationships between processing variables aids in determination of provenance and processing history. In this study, the thermal decomposition of UO2C2O4·3H2O and phase analysis of its thermal products are examined. Their morphologies are discussed with respect to a matrix of solution processing conditions.

Open Access

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Abstract: In the search for high energy density cathodes for next-generation lithium-ion batteries, the disordered rocksalt oxyfluorides are receiving significant attention due to their high capacity and lower voltage hysteresis compared with ordered Li-rich layered compounds. However, a deep understanding of these phenomena and their redox chemistry remains incomplete. Using the archetypal oxyfluoride, Li2MnO2F, we show that the oxygen redox process in such materials involves the formation of molecular O2 trapped in the bulk structure of the charged cathode, which is reduced on discharge. The molecular O2 is trapped rigidly within vacancy clusters and exhibits minimal mobility unlike free gaseous O2, making it more characteristic of a solid-like environment. The Mn redox process occurs between octahedral Mn3+ and Mn4+ with no evidence of tetrahedral Mn5+ or Mn7+. We furthermore derive the relationship between local coordination environment and redox potential; this gives rise to the observed overlap in Mn and O redox couples and reveals that the onset potential of oxide ion oxidation is determined by the degree of ionicity around oxygen, which extends models based on linear Li–O–Li configurations. This study advances our fundamental understanding of redox mechanisms in disordered rocksalt oxyfluorides, highlighting their promise as high capacity cathodes.

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Abstract: The design of selective and efficient catalysts for electrochemical CO2 reduction is highly desirable yet still challenging, in particular, if the aim is to make them binder-free and self-standing. Here, we report a new and straightforward strategy to incorporate Ni single atoms into a commercially available carbon paper to prepare a self-standing electrode. This is accomplished by consecutive acid activation, adsorption of Ni2+ ions, and pyrolysis steps. Structural characterizations and calculations based on density functional theory consistently suggest that the Ni single atoms are coordinated with three N and one S atoms on the carbon paper. When used for CO2 electroreduction, the electrode exhibits an optimal selectivity (91%), activity (3.4 mA cm−2), and stability (at least 14 h) for CO production in water at an overpotential of 660 mV. This report may inspire the design and incorporation of single atoms of various metal types into carbon papers, or other kinds of carbon substrates, for a wide range of electrocatalytic processes.

Open Access

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Abstract: Cost and lifetime currently hinder widespread commercialization of polymer electrolyte membrane fuel cells (PEMFCs). Reduced electrode Pt loadings lower costs; however, the impact of metal loading (on the support) and its relation to degradation (lifetime) remain unclear. The limited research on these parameters stems from synthetic difficulties and lack of in situ analytics. This study addresses these challenges by synthesizing 2D and 3D Pt/C model catalyst systems via two precise routes and systematically varying the loading. Pt dissolution was monitored using on-line inductively coupled plasma mass spectrometry (on-line-ICP-MS), while X-ray spectroscopy techniques were applied to establish the oxidation states of Pt in correlation with metal loading. Dissolution trends emerge which can be explained by three particle proximity dependent mechanisms: (1) shifts in the Nernst dissolution potential, (2) redeposition, and (3) alteration of Pt oxidation states. These results identify engineering limitations, which should be considered by researchers in fuel cell development and related fields.