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Developing stable and efficient electrocatalysts is vital for boosting oxygen evolution reaction (OER) rates in sustainable hydrogen production. High-entropy oxides (HEOs) consist of five or more metal cations, providing opportunities to tune their catalytic properties toward high OER efficiency. This work combines theoretical and experimental studies to scrutinize the OER activity and stability for spinel-type HEOs. Density functional theory confirms that randomly mixed metal sites show thermodynamic stability, with intermediate adsorption energies displaying wider distributions due to mixing-induced equatorial strain in active metal-oxygen bonds. The rapid sol-flame method is employed to synthesize HEO, comprising five 3d-transition metal cations, which exhibits superior OER activity and durability under alkaline conditions, outperforming lower-entropy oxides, even with partial surface oxidations. The study highlights that the enhanced activity of HEO is primarily attributed to the mixing of multiple elements, leading to strain effects near the active site, as well as surface composition and coverage.

 

Abstract When a three-dimensional material is constructed by stacking different two-dimensional layers into an ordered structure, new and unique physical properties can emerge. An example is the delafossite PdCoO 2 , which consists of alternating layers of metallic Pd and Mott-insulating CoO 2 sheets. To understand the nature of the electronic coupling between the layers that gives rise to the unique properties of PdCoO 2 , we revealed its layer-resolved electronic structure combining standing-wave X-ray photoemission spectroscopy and ab initio many-body calculations. Experimentally, we have decomposed the measured VB spectrum into contributions from Pd and CoO 2 layers. Computationally, we find that many-body interactions in Pd and CoO 2 layers are highly different. Holes in the CoO 2 layer interact strongly with charge-transfer excitons in the same layer, whereas holes in the Pd layer couple to plasmons in the Pd layer. Interestingly, we find that holes in states hybridized across both layers couple to both types of excitations (charge-transfer excitons or plasmons), with the intensity of photoemission satellites being proportional to the projection of the state onto a given layer. This establishes satellites as a sensitive probe for inter-layer hybridization. These findings pave the way towards a better understanding of complex many-electron interactions in layered quantum materials. 

Understanding the pathways and time scales underlying electrically driven insulator-metal transitions is crucial for uncovering the fundamental limits of device operation. Using stroboscopic electron diffraction, we perform synchronized time-resolved measurements of atomic motions and electronic transport in operating vanadium dioxide (VO₂) switches. We discover an electrically triggered, isostructural state that forms transiently on microsecond time scales, which is shown by phase-field simulations to be stabilized by local heterogeneities and interfacial interactions between the equilibrium phases. This metastable phase is similar to that formed under photoexcitation within picoseconds, suggesting a universal transformation pathway. Our results establish electrical excitation as a route for uncovering nonequilibrium and metastable phases in correlated materials, opening avenues for engineering dynamical behavior in nanoelectronics.

 

We investigate high‐valent oxygen redox in the positive Na‐ion electrode P2‐Na_0.67−x[Fe_0.5Mn_0.5]O₂(NMF) where Fe is partially substituted with Cu (P2‐Na_0.67−x[Mn_0.66Fe_0.20Cu_0.14]O₂, NMFC) or Ni (P2‐Na_0.67−x[Mn_0.65Fe_0.20Ni_0.15]O₂, NMFN). From combined analysis of resonant inelastic X‐ray scattering and X‐ray near‐edge structure with electrochemical voltage hysteresis and X‐ray pair distribution function profiles, we correlate structural disorder with high‐valent oxygen redox and its improvement by Ni or Cu substitution. Density of states calculations elaborate considerable anionic redox in NMF and NMFC without the widely accepted requirement of an A‐O‐A′ local configuration in the pristine materials (where A=Na and A′=Li, Mg, vacancy, etc.). We also show that the Jahn–Teller nature of Fe⁴⁺and the stabilization mechanism of anionic redox could determine the extent of structural disorder in the materials. These findings shed light on the design principles in TM and anion redox for positive electrodes to improve the performance of Na‐ion batteries.

 

We investigate high‐valent oxygen redox in the positive Na‐ion electrode P2‐Na_0.67−x[Fe_0.5Mn_0.5]O₂(NMF) where Fe is partially substituted with Cu (P2‐Na_0.67−x[Mn_0.66Fe_0.20Cu_0.14]O₂, NMFC) or Ni (P2‐Na_0.67−x[Mn_0.65Fe_0.20Ni_0.15]O₂, NMFN). From combined analysis of resonant inelastic X‐ray scattering and X‐ray near‐edge structure with electrochemical voltage hysteresis and X‐ray pair distribution function profiles, we correlate structural disorder with high‐valent oxygen redox and its improvement by Ni or Cu substitution. Density of states calculations elaborate considerable anionic redox in NMF and NMFC without the widely accepted requirement of an A‐O‐A′ local configuration in the pristine materials (where A=Na and A′=Li, Mg, vacancy, etc.). We also show that the Jahn–Teller nature of Fe⁴⁺and the stabilization mechanism of anionic redox could determine the extent of structural disorder in the materials. These findings shed light on the design principles in TM and anion redox for positive electrodes to improve the performance of Na‐ion batteries.

 

On‐chip dynamic strain engineering requires efficient micro‐actuators that can generate large in‐plane strains. Inorganic electrochemical actuators are unique in that they are driven by low voltages (≈1 V) and produce considerable strains (≈1%). However, actuation speed and efficiency are limited by mass transport of ions. Minimizing the number of ions required to actuate is thus key to enabling useful “straintronic” devices. Here, it is shown that the electrochemical intercalation of exceptionally few lithium ions into WTe₂causes large anisotropic in‐plane strain: 5% in one in‐plane direction and 0.1% in the other. This efficient stretching of the 2D WTe₂layers contrasts to intercalation‐induced strains in related materials which are predominantly in the out‐of‐plane direction. The unusual actuation of Li_xWTe₂is linked to the formation of a newly discovered crystallographic phase, referred to as Td', with an exotic atomic arrangement. On‐chip low‐voltage (<0.2 V) control is demonstrated over the transition to the novel phase and its composition. Within the Td'‐Li_0.5−δWTe₂phase, a uniaxial in‐plane strain of 1.4% is achieved with a change of δ of only 0.075. This makes the in‐plane chemical expansion coefficient of Td'‐Li_0.5−δWTe₂far greater than of any other single‐phase material, enabling fast and efficient planar electrochemical actuation.