6 Search Results

Perovskite oxides are an important class of oxygen evolution reaction (OER) catalysts in alkaline media, despite the elusive nature of their active sites. In this work, we demonstrate that the origin of the OER activity in a La_1-xSrxCoO₃ model perovskite arises from a thin surface layer of Co hydr(oxy)oxide (CoO_xH_y) that interacts with trace-level Fe species present in the electrolyte, creating dynamically stable active sites. Generation of the hydr(oxy)oxide layer is a consequence of a surface evolution process driven by the A-site dissolution and O-vacancy creation. In turn, this imparts a 10-fold improvement in stability against Co dissolution and amore » 3-fold increase in the activity-stability factor for CoO_xH_y/ LSCO when compared to nanoscale Co-hydr(oxy)oxides clusters. Our results suggest new design rules for active and stable perovskite oxide-based OER materials.« less

Versatile chemical transformations of surface functional groups in two-dimensional transition-metal carbides (MXenes) open up a previously unexplored design space for this broad class of functional materials. We introduce a general strategy to install and remove surface groups by performing substitution and elimination reactions in molten inorganic salts. Successful synthesis of MXenes with oxygen, imido, sulfur, chlorine, selenium, bromine, and tellurium surface terminations, as well as bare MXenes (no surface termination), was demonstrated. These MXenes show distinctive structural and electronic properties. For example, the surface groups control interatomic distances in the MXene lattice, and Ti _{n +1} C _n ( nmore » = 1, 2) MXenes terminated with telluride (Te ²⁻ ) ligands show a giant (>18%) in-plane lattice expansion compared with the unstrained titanium carbide lattice. The surface groups also control superconductivity of niobium carbide MXenes.« less

Chemical lithiation using lithium metal dissolved in liquid ammonia is introduced for the first time as a viable, potentially scalable method to over-lithiate cathode materials, in this case, the 5V spinel Li_1+xNi_0.5Mn_1.5O₄. In this formula the value of x represents the amount of extra lithium inserted into the spinel. Such over-lithiated cathodes can subsequently be used to pre-lithiate high-energy anodes in a lithium-ion battery configuration during the first charge step. Lithiated 5V spinel Li_1+xNi_0.5Mn_1.5O₄ cathode materials prepared by this technique show higher first delithiation capacities, confirming the chemically inserted lithium is electrochemically active. Full cells with a Si-graphite anode andmore » the Li_1+xNi_0.5Mn_1.5O₄ (x=0.62) cathode show a 23 % higher reversible capacity in the first cycle than LiNi_0.5Mn_1.5O₄ baseline cells, and improved capacity retention. The extra, chemically inserted lithium therefore sacrificially compensates for the loss of lithium at the anode allowing higher utilization of the cathode capacity in following cycles.« less

Zero-dimensional PbSe quantum dots are heterogeneously nucleated and grown onto two-dimensional zincblende CdSe nanoplatelets. Electron microscopy shows ad-grown dots predominantly decorate edges and corners of the nanoplatelets. Spectroscopic characterizations relate type I electronic alignment as demonstrated via photoluminescence excitation spectroscopy enhancement of near-infrared emission. Transient photoluminescence and absorption convey ultrafast transfer of excitons to the lower energy semiconductor dots. Furthermore these structures combine benefits of large absorption cross sections of nanoplatelets and efficient near-infrared emission of PbSe with quantum confinement tuning of energy gap.