Direct Visualization of Asymmetric Behavior in Supported Lipid Bilayers at the Gel-Fluid Phase Transition

AbstractWe utilize in situ, temperature-dependent atomic force microscopy to examine the gel-fluid phase transition behavior in supported phospholipid bilayers constructed from 1,2-dimyristoyl-sn-glycero-3-phosphocholine, 1,2-dipentadecanoyl-sn-glycero-3-phosphocholine, and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine. The primary gel-fluid phase transition at Tm occurs through development of anisotropic cracks in the gel phase, which develop into the fluid phase. At ∼5°C above Tm, atomic force microscopy studies reveal the presence of a secondary phase transition in all three bilayers studied. The secondary phase transition occurs as a consequence of decoupling between the two leaflets of the bilayer due to enhanced stabilization of the lower leaflet with either the support or the water entrained between the support and the bilayer. Addition of the transmembrane protein gramicidin A or construction of a highly defected gel phase results in elimination of this decoupling and removal of the secondary phase transition

Elucidating Zn and Mg Electrodeposition Mechanisms in Nonaqueous Electrolytes for Next-Generation Metal Batteries

Cyclic voltammetry and linear sweep voltammetry with an ultramicroelectrode (UME) were employed to study Zn and Mg electrodeposition and the corresponding mechanistic pathways. CVs obtained at a Pt UME for Zn electroreduction from a trifluoromethylsulfonyl imide (TFSI^–) and chloride-containing electrolyte in acetonitrile exhibit current densities that are scan rate independent, as expected for a simple electron transfer at a UME. However, CVs obtained from three different Mg-containing electrolytes in THF exhibit an inverse dependence between scan rate and current density. COMSOL-based simulation suggests that Zn electrodeposition proceeds via a simple one-step, two-electron transfer (E) mechanism. Alternatively, the Mg results are best described by invoking a chemical step prior to electron transfer: a chemical–electrochemical (CE) mechanism. The chemical step exhibits an activation energy of 51 kJ/mol. This chemical step is likely the disproportionation of the chloro-bridged dimer [Mg_2(μ–Cl)_3·6THF]^+ present in active electrodeposition solutions. Our work shows that Mg deposition kinetics can be improved by way of increased temperature

Modulus variation of composite graphite electrodes in lithium-ion batteries during electrochemical cycling

Graphite is currently the most common anode material used in commercial lithium-ion batteries. During battery charging and discharging processes, lithium ions intercalate into and deintercalate from graphite, forming several distinct stages of graphite-lithium intercalation compounds (G-LICs). Each stage of G-LIC has a unique spacing between graphene layers, with the spacing increasing for increasing lithium content. In graphite-based composite electrodes (graphite particles in a porous polymer matrix), the changing layer spacing leads to stress and strain evolution on the composite length scale. In two separate experiments, we use substrate-curvature measurements to monitor stress changes in a thin electrode constrained on an inert, rigid substrate, and we use digital image correlation to track strain changes in a free-standing, unconstrained electrode. Combining the in-situ stress and strain analyses enables us to extract the change in the apparent modulus of the composite graphite electrode as a function of electrode potential and lithium content. As expected, we found that constrained electrodes develop compressive stress during lithiation (~10 MPa) and that unconstrained electrodes undergo free expansion (~1.5% linear strain). Interestingly, the apparent modulus of the electrode increases the most significantly during the formation of the dilute stage I compound, increases slightly with the formation of the stage IV, dilute stage II, and stage II compounds, and then decreases with the formation of the stage I compound (LiC 6). During delithiation, unconstrained electrodes contract, recovering nearly their original size. In constrained electrodes during delithation, however, the compressive stress is first relaxed, and then a tensile stress develops and is subsequently relaxed. The tensile stress leads to an apparent softening of the composite electrode over a broad range of electrode potential and capacity. At the end of one complete lithiation/delithiation cycle, the apparent modulus returns to approximately its original value. The evolution of stress, strain, and modulus data provides quantitative information on the coupled electro-chemo-mechanical response of battery electrodes and insight on material strategies to increase battery reliability

Outer-Sphere Contributions to the Electronic Structure of Type Zero Copper Proteins

Bioinorganic canon states that active-site thiolate coordination promotes rapid electron transfer (ET) to and from type 1 copper proteins. In recent work, we have found that copper ET sites in proteins also can be constructed without thiolate ligation (called “type zero” sites). Here we report multifrequency electron paramagnetic resonance (EPR), magnetic circular dichroism (MCD), and nuclear magnetic resonance (NMR) spectroscopic data together with density functional theory (DFT) and spectroscopy-oriented configuration interaction (SORCI) calculations for type zero Pseudomonas aeruginosa azurin variants. Wild-type (type 1) and type zero copper centers experience virtually identical ligand fields. Moreover, O-donor covalency is enhanced in type zero centers relative that in the C112D (type 2) protein. At the same time, N-donor covalency is reduced in a similar fashion to type 1 centers. QM/MM and SORCI calculations show that the electronic structures of type zero and type 2 are intimately linked to the orientation and coordination mode of the carboxylate ligand, which in turn is influenced by outer-sphere hydrogen bonding

Effect of the Hydrofluoroether Cosolvent Structure in Acetonitrile-Based Solvate Electrolytes on the Li^+ Solvation Structure and Li–S Battery Performance

We evaluate hydrofluoroether (HFE) cosolvents with varying degrees of fluorination in the acetonitrile-based solvate electrolyte to determine the effect of the HFE structure on the electrochemical performance of the Li–S battery. Solvates or sparingly solvating electrolytes are an interesting electrolyte choice for the Li–S battery due to their low polysulfide solubility. The solvate electrolyte with a stoichiometric ratio of LiTFSI salt in acetonitrile, (MeCN)_2–LiTFSI, exhibits limited polysulfide solubility due to the high concentration of LiTFSI. We demonstrate that the addition of highly fluorinated HFEs to the solvate yields better capacity retention compared to that of less fluorinated HFE cosolvents. Raman and NMR spectroscopy coupled with ab initio molecular dynamics simulations show that HFEs exhibiting a higher degree of fluorination coordinate to Li+ at the expense of MeCN coordination, resulting in higher free MeCN content in solution. However, the polysulfide solubility remains low, and no crossover of polysulfides from the S cathode to the Li anode is observed

Effect of the Hydrofluoroether Cosolvent Structure in Acetonitrile-Based Solvate Electrolytes on the Li^+ Solvation Structure and Li–S Battery Performance

We evaluate hydrofluoroether (HFE) cosolvents with varying degrees of fluorination in the acetonitrile-based solvate electrolyte to determine the effect of the HFE structure on the electrochemical performance of the Li–S battery. Solvates or sparingly solvating electrolytes are an interesting electrolyte choice for the Li–S battery due to their low polysulfide solubility. The solvate electrolyte with a stoichiometric ratio of LiTFSI salt in acetonitrile, (MeCN)_2–LiTFSI, exhibits limited polysulfide solubility due to the high concentration of LiTFSI. We demonstrate that the addition of highly fluorinated HFEs to the solvate yields better capacity retention compared to that of less fluorinated HFE cosolvents. Raman and NMR spectroscopy coupled with ab initio molecular dynamics simulations show that HFEs exhibiting a higher degree of fluorination coordinate to Li+ at the expense of MeCN coordination, resulting in higher free MeCN content in solution. However, the polysulfide solubility remains low, and no crossover of polysulfides from the S cathode to the Li anode is observed