Formation and impact of nanoscopic oriented phase domains in electrochemical crystalline electrodes.

Electrochemical phase transformation in ion-insertion crystalline electrodes is accompanied by compositional and structural changes, including the microstructural development of oriented phase domains. Previous studies have identified prevailingly transformation heterogeneities associated with diffusion- or reaction-limited mechanisms. In comparison, transformation-induced domains and their microstructure resulting from the loss of symmetry elements remain unexplored, despite their general importance in alloys and ceramics. Here, we map the formation of oriented phase domains and the development of strain gradient quantitatively during the electrochemical ion-insertion process. A collocated four-dimensional scanning transmission electron microscopy and electron energy loss spectroscopy approach, coupled with data mining, enables the study. Results show that in our model system of cubic spinel MnO2 nanoparticles their phase transformation upon Mg2+ insertion leads to the formation of domains of similar chemical identity but different orientations at nanometre length scale, following the nucleation, growth and coalescence process. Electrolytes have a substantial impact on the transformation microstructure ('island' versus 'archipelago'). Further, large strain gradients build up from the development of phase domains across their boundaries with high impact on the chemical diffusion coefficient by a factor of ten or more. Our findings thus provide critical insights into the microstructure formation mechanism and its impact on the ion-insertion process, suggesting new rules of transformation structure control for energy storage materials.

Tertiary Hierarchical Complexity in Assemblies of Sulfur-Bridged Metal Chiral Clusters.

Self-assembly of three-dimensional structures with order across multiple length scales-hierarchical assembly-is of great importance for biomolecules for the functions of life. Creation of similar complex architectures from inorganic building blocks has been pursued toward artificial biomaterials and advanced functional materials. Current research, however, primarily employs only large, nonreactive building blocks such as Au colloids. By contrast, sulfur-bridged transition metal clusters (<2 nm) are able to offer more functionality in catalytic and biochemical reactions. Hierarchical assembly of these systems has not been well researched because of the difficulty in obtaining single-phase clusters and the lack of suitable ligands to direct structure construction. To overcome these challenges, we employ a rigid planar ligand with an aromatic ring and bifunctional bond sites. We demonstrate the synthesis and assembly of 1.2 nm sulfur-bridged copper (SB-Cu) clusters with tertiary hierarchical complexity. The primary structure is clockwise/counterclockwise chiral cap and core molecules. They combine to form clusters, and due to the cap-core interaction (C-H···π), only two enantiomeric isomers are formed (secondary structure). A tertiary hierarchical architecture is achieved through the self-assembly of alternating enantiomers with hydrogen bonds as the intermolecular driving force. The SB-Cu clusters are air stable and have a distribution of oxidation states ranging from Cu(0) to Cu(I), making them interesting for redox and catalytic activities. This study shows that structural complexity at different length scales, mimicking biomolecules, can occur in active-metal clusters and provides a new platform for investigation of those systems and for the design of advanced functional materials.

Cu(I) Reducibility Controls Ethylene vs Ethanol Selectivity on (100)-Textured Copper during Pulsed CO2 Reduction.

The electrochemical CO2 reduction reaction (CO2RR) can convert widely available CO2 into value-added C2 products, such as ethylene and ethanol. However, low selectivity toward either compound limits the effectiveness of current CO2RR electrocatalysts. Here, we report the use of pulsed overpotentials to improve the ethylene selectivity to 67% with >75% overall C2 selectivity on (100)-textured polycrystalline Cu foil. The pulsed CO2RR can be made selective to either ethylene or ethanol by controlling the reaction temperature. We attribute the enhanced C2 selectivity to the improved CO dimerization kinetics on the active Cu surface on predominately (100)-textured Cu grains with the reduced hydrogen adsorption coverage during the pulsed CO2RR. The ethylene vs ethanol selectivity can be explained by the reducibility of the Cu(I) species during the cathodic potential cycle. Our work demonstrates a simple route to improve the ethylene vs ethanol selectivity and identifies Cu(I) as the species responsible for ethanol production.

Unexpected Low-Temperature Performance of Li-O2 Cells with Inhibited Side Reactions.

In this work, we found that the side reactions of both the Li anode and cathode with the electrolyte can be obviously alleviated at low temperature. This favorable merit enables long cycle life of the Li-O2 cells at low temperature. At 0 °C, the cells can sustain stable cycling of 279 and 1025 cycles at 400 mA g-1 with limited capacities of 1000 and 500 mA h g-1, respectively. Even at -20 °C, the cell can be stably cycled for 83 cycles at 200 mA g-1 with a limited capacity of 500 mA h g-1.