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Author ORCID ID is 0000000219216683
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  1. Spinel transition metal oxides (TMOs) have emerged as promising anode materials for lithium-ion batteries. It has been shown that reducing their particle size to nanoscale dimensions benefits overall electrochemical performance. Here, we use in situ transmission electron microscopy to probe the lithiation behavior of spinel ZnFe 2O 4 as a function of particle size. We have found that ZnFe 2O 4 undergoes an intercalation-to-conversion reaction sequence, with the initial intercalation process being size dependent. Larger ZnFe 2O 4 particles (40 nm) follow a two-phase intercalation reaction. In contrast, a solid-solution transformation dominates the early stages of discharge when the particlemore » size is about 6–9 nm. Using a thermodynamic analysis, we find that the size-dependent kinetics originate from the interfacial energy between the two phases. Furthermore, the conversion reaction in both large and small particles favors {111} planes and follows a core-shell reaction mode. Finally, these results elucidate the intrinsic mechanism that permits fast reaction kinetics in smaller nanoparticles.« less
  2. Pt alloy nanoparticles supported on Vulcan XC-72 (Pt/C) are the most effective catalysts for kinetically sluggish oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. However, significant performance degradation has been observed with the Pt/C catalysts due to agglomeration and Ostwald ripening of Pt nanoparticles largely resulting from the corrosion of carbon supports. In this paper, we developed a Pt alloy catalyst through annealing Pt nanoparticles deposited on nitrogen/metal co-doped large-size graphene tubes (NGTs). The in-situ formation of PtM (M: Co and Ni) alloy during the annealing process contributes to the improvement of the catalytic activity and stability. Duringmore » the accelerated stress tests (AST), after 20 000 potential cycles (0.6–1.0 V vs. RHE), the retained electrochemical surface area (ECSA) of the PtM/NGT catalyst is more than 2 times larger than that of the Pt/C catalyst. As for the AST tests of carbon corrosion, after 30 000 potential cycles (1.0–1.5 V vs. RHE) at room temperature, the NGT morphologies are well maintained and no ECSA loss of this PtM catalyst is observed, indicating excellent corrosion-resistance. Even at harsher 60 °C, the PtM/NGT catalyst exhibits only insignificant loss (6 mV) of E 1/2 while the Pt/C catalyst shows significant degradation (47 mV loss in E 1/2). Finally, the improved stability of PtM/NGT catalyst is attributed to the highly graphitized NGTs and possible synergistic effects between the NGT carbon support and the PtM alloy nanoparticles.« less
  3. Platinum group metal (PGM)-free catalysts that are also Fe-free are highly desirable for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells, as they avoid possible Fenton reactions. Here we report an efficient ORR catalyst that consists of atomically dispersed nitrogen-coordinated single Mn sites on partially graphitic carbon (Mn-N-C). Evidence for the embedding of the atomically dispersed MnN4 moieties within the carbon surface-exposed basal planes was established by X-ray absorption spectroscopy and their dispersion was confirmed by aberration-corrected electron microscopy with atomic resolution. The Mn-N-C catalyst exhibited a half-wave potential of 0.80 V vs. RHE, approaching that ofmore » Fe-N-C catalysts, along with significantly enhanced stability in acidic media. The encouraging performance of the Mn-N-C catalyst as a PGM-free cathode was demonstrated in fuel cell tests. First-principles calculations further support the MnN4 sites as the origin of the ORR activity via a 4e- pathway in acidic media.« less
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  4. Bimetallic and multi-component catalysts often exhibit superior activity and selectivity compared with their single-component counterparts. To investigate the origin of the composition dependence observed in the catalytic activities of CoPd bimetallic catalysts, the compositional and structural evolution of monodisperse CoPd alloy nanoparticles (NPs) were followed under catalytic CO oxidation conditions using ambient pressure X-ray spectroscopy (AP-XPS) and transmission electron microscopy (TEM). It was found that the catalysis process induced a reconstruction of the catalysts, leaving CoOx on the NP surface. The synergy between Pd and CoOx coexisting on the surface promotes the catalytic activity of the bimetallic catalysts. Such synergisticmore » effects can be optimized by tuning the Co/Pd ratios in the NP synthesis and reach a maximum at compositions near Co 0.26Pd 0.74, which exhibits the lowest temperature for complete CO conversion. Our combined AP-XPS and TEM studies provide a direct observation of the bimetallic NPs surface evolution under catalytic conditions and its correlation to catalytic properties.« less
  5. Surface segregation—the enrichment of one element at the surface, relative to the bulk—is ubiquitous to multi-component materials. Using the example of a Cu–Au solid solution, we demonstrate that compositional variations induced by surface segregation are accompanied by misfit strain and the formation of dislocations in the subsurface region via a surface di˙usion and trapping process. The resulting chemically ordered surface regions acts as an e˙ective barrier that inhibits subsequent dislocation annihilation at free surfaces. Using dynamic, atomic-scale resolution electron microscopy observations and theory modelling, we show that the dislocations are highly active, and we delineate the specific atomic-scale mechanisms associatedmore » with their nucleation, glide, climb, and annihilation at elevated temperatures. As a result, these observations provide mechanistic detail of how dislocations nucleate and migrate at heterointerfaces in dissimilar-material systems.« less
  6. Here, we present electrical and structural characterization of epitaxial LaTiO 3/SrTiO 3 heterostructures integrated directly on Si(100). By reducing the thicknesses of the heterostructures, an enhancement in carrier-carrier scattering is observed in the Fermi liquid behavior, followed by a metal to insulator transition in the electrical transport. The insulating behavior is described by activated transport, and its onset occurs near 1 electron per Ti occupation within the SrTiO 3 well, providing evidence for a Mott driven transition. We also discuss the role that structure and gradients in strain could play in enhancing the carrier density. The manipulation of Mott metal-insulatormore » behavior in oxides grown directly on Si opens the pathway to harnessing strongly correlated phenomena in device technologies.« less
  7. Recent research has explored combining conventional transition metal redox with anionic lattice oxygen redox as a new and exciting direction to search for high-capacity lithium-ion cathodes. For this study, we probe the poorly understood electrochemical activity of anionic oxygen from a material perspective by elucidating the effect of the transition metal on oxygen redox activity. We study two lithium-rich layered oxides, specifically lithium nickel metal oxides where metal is either manganese or ruthenium, which possess similar structure and discharge characteristics, but exhibit distinctly different charge profiles. By combining X-ray spectroscopy with operando differential electrochemical mass spectrometry, we reveal completely differentmore » oxygen redox activity in each material, likely resulting from the different interaction between the lattice oxygen and transition metals. This work provides additional insights into the complex mechanism of oxygen redox and development of advanced high-capacity lithium-ion cathodes.« less

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