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1. Tailored (La0.2Pr0.2Nd0.2Tb0.2D0.2)2Ce2O7 as a Highly Active and Stable Nanocatalyst for the Oxygen Evolution Reaction

   https://doi.org/10.1002/smll.202305789  

   Paladugu, Sreya ; Abdullahi, Ibrahim Munkaila ; Jothi, Palani Raja ; Jiang, Bo ; Nath, Manashi ; Page, Katharine ( March 2024 , Small)

   Designing highly active and robust catalysts for the oxygen evolution reaction is key to improving the overall efficiency of the water splitting reaction. It has been previously demonstrated that evaporation induced self‐assembly (EISA) can be used to synthesize highly porous and high surface area cerate‐based fluorite nanocatalysts, and that substitution of Ce with 50% rare earth (RE) cations significantly improves electrocatalyst activity. Herein, the defect structure of the best performing nanocatalyst in the series are further explored, Nd₂Ce₂O₇, with a combination of neutron diffraction and neutron pair distribution function analysis. It is found that Nd^{3 +}cation substitution for Ce in the CeO₂fluorite lattice introduces higher levels of oxygen Frenkel defects and induces a partially reduced RE_1.5Ce_1.5O_{5 +x}phase with oxygen vacancy ordering. Significantly, it is demonstrated that the concentration of oxygen Frenkel defects and improved electrocatalytic activity can be further enhanced by increasing the compositional complexity (number of RE cations involved) in the substitution. The resulting novel compositionally‐complex fluorite– (La_0.2Pr_0.2Nd_0.2Tb_0.2Dy_0.2)₂Ce₂O₇is shown to display a low OER overpotential of 210 mV at a current density of 10 mAcm⁻²in 1M KOH, and excellent cycling stability. It is suggested that increasing the compositional complexity of fluorite nanocatalysts expands the ability to tailor catalyst design.

    

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2. On the thermal and mechanical properties of Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 O across the high-entropy to entropy-stabilized transition

   https://doi.org/10.1063/5.0122775  

   Rost, Christina M. ; Schmuckler, Daniel L. ; Bumgardner, Clifton ; Bin Hoque, Md Shafkat ; Diercks, David R. ; Gaskins, John T. ; Maria, Jon-Paul ; Brennecka, Geoffrey L. ; Li, Xiadong ; Hopkins, Patrick E. ( December 2022 , APL Materials)

   As various property studies continue to emerge on high entropy and entropy-stabilized ceramics, we seek a further understanding of the property changes across the phase boundary between “high-entropy” and “entropy-stabilized” phases. The thermal and mechanical properties of bulk ceramic entropy stabilized oxide composition Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 O are investigated across this critical transition temperature via the transient plane-source method, temperature-dependent x-ray diffraction, and nano-indentation. The thermal conductivity remains constant within uncertainty across the multi-to-single phase transition at a value of ≈2.5 W/mK, while the linear coefficient of thermal expansion increases nearly 24% from 10.8 to 14.1 × 10 −6 K −1 . Mechanical softening is also observed across the transition. 

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3. (Hf 0.2 Zr 0.2 Ta 0.2 Nb 0.2 Ti 0.2 )C high‐entropy ceramics with low thermal conductivity

   https://doi.org/10.1111/jace.15779  

   Yan, Xueliang ; Constantin, Loic ; Lu, Yongfeng ; Silvain, Jean‐François ; Nastasi, Michael ; Cui, Bai ( May 2018 , Journal of the American Ceramic Society)
4. The role of Co valence in charge transport in the entropy‐stabilized oxide (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 )O

   https://doi.org/10.1111/jace.18820  

   Jacobson, V. ; Huang, J. ; Titus, C. J. ; Smaha, R. W. ; Papac, M. ; Lee, S. J. ; Zakutayev, A. ; Brennecka, G. L. ( October 2022 , Journal of the American Ceramic Society)

   Many of the studies on the entropy‐stabilized oxide (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O have been heavily application‐based. Previous works have studied effects of cation stoichiometry on the entropy‐driven reaction to form a single phase, but a fundamental exploration of the effects of anion stoichiometry and/or redox chemistry on electrical properties is lacking. Using near‐edge X‐ray absorption fine structure (NEXAFS) and electrical measurements, we show that oxidizing thin film samples of (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O affects primarily the valence of Co, leaving the other cations in this high‐entropy system unchanged. This oxidation increases electrical conduction in these thin films, which occurs via small polaron hopping mediated by the Co valence shift from 2+ to a mixed 2+/3+ state. In parallel, we show that bulk samples sintered in an oxygen‐rich atmosphere have a lower activation energy for electrical conduction than those equilibrated in a nitrogen (reducing) atmosphere. Combining feasible defect compensation scenarios with electrical impedance measurements and NEXAFS data, we propose a self‐consistent interpretation of Co redox‐mediated small polaron conduction as the dominant method of charge transfer in this system.

    

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5. Thermally driven mesoscale chemomechanical interplay in Li 0.5 Ni 0.6 Mn 0.2 Co 0.2 O 2 cathode materials

   https://doi.org/10.1039/C8TA08973F  

   Wei, Chenxi ; Zhang, Yan ; Lee, Sang-Jun ; Mu, Linqin ; Liu, Jin ; Wang, Chenxu ; Yang, Yang ; Doeff, Marca ; Pianetta, Piero ; Nordlund, Dennis ; et al ( November 2018 , Journal of Materials Chemistry A)

   While Li ion batteries are intended to be operated within a mild temperature window, their structural and chemical complexity could lead to unanticipated local electrochemical events that could cause extreme temperature spikes, which, in turn, could trigger more undesired and sophisticated reactions in the system. Visualizing and understanding the response of battery electrode materials to thermal abuse conditions could potentially offer a knowledge basis for the prevention and mitigation of the safety hazards. Here we show a comprehensive investigation of thermally driven chemomechanical interplay in a Li 0.5 Ni 0.6 Mn 0.2 Co 0.2 O 2 (charged NMC622) cathode material. We report that, at the early stage of the thermal abuse, oxygen release and internal Li migration occur concurrently, and are accompanied by mechanical disintegration at the mesoscale. At the later stage, Li protrusions are observed on the secondary particle surface due to the limited lithium solubility in non-layered lattices. The extraction of both oxygen and lithium from the host material at elevated temperature could influence the chemistry and safety at the cell level via rearrangement of the electron and ion diffusion pathways, reduction of the coulombic efficiency, and/or causing an internal short circuit that could provoke a thermal runaway. 

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