Increasing Cation Size Research Articles

Impact of mobile and stationary phases on siRNA duplex stability in liquid chromatography

Nucleic acid duplexes are typically analyzed in non-denaturing conditions. Melting temperature (Tm) is the property used to measure duplex stability; however, it is not known how the chromatographic conditions and mobile phase composition affect the duplex stability. We employed differential scanning calorimetry (DSC) method to measure the melting temperature of chemically modified silencing RNA duplex (21 base pairs, 0.15 mM duplex concentration) in mobile phases commonly used in reversed-phase, ion-pair reversed-phase, size exclusion and hydrophilic interaction chromatography. We investigated mobile phases consisting of ammonium acetate, alkylammonium ion-pairing reagents, alkali-ion chlorides, magnesium chloride, and additives including methanol, ethanol, acetonitrile and hexafluoroisopropanol. Increasing buffer concentration enhanced the duplex stability (Tm was 67.1 – 78.2 °C for 10–100 mM [Na+] concentration). The melting temperature decreases with the increase in cation size (70.2 °C in 10 mM [Li+], 68.1 °C in 10 mM [NH4+], 65.6 °C in 10 mM [Cs+], and 56.6 °C in 10 mM [triethylammonium+] solutions). Inclusion of 20 % of organic solvent in buffer reduced the melting temperature by 1–3 °C, and denaturation power increases in the order MeOH<EtOH<MeCN. Next, we investigated the RNA duplex melting using selected chromatographic techniques and within the 10–90 °C column temperature range. Melting temperature obtained with size exclusion chromatography in 25 mM sodium phosphate buffer was ∼ 70 °C, about 5 °C lower compared to DSC values. The apparent melting temperature for a reversed-phase chromatography experiment was < 10 °C for 10 mM ammonium acetate mobile phase with acetonitrile eluent. Ion-pair reversed-phase with 10 mM triethylamine, 100 mM hexafluoroisopropanol and MeCN as eluent yielded Tm 49.3–54.9 °C. Hydrophilic interaction chromatography experiments with 10 mM ammonium acetate and ∼ 50 % acetonitrile mobile phase showed high duplex stability (Tm ∼ 80 °C). The observed chromatographic Tm values suggest that the formation of an organic/aqueous solvent layer on the chromatographic sorbent surface affects the duplex stability more significantly than the composition of the mobile phase alone.

Diffusion and structure of propylene carbonate-metal salt electrolyte solutions for post-lithium-ion batteries: From experiment to simulation.

The diffusion of cations in organic solvent solutions is important for the performance of metal-ion batteries. In this article, pulsed field gradient nuclear magnetic resonance experiments and fully atomistic molecular dynamic simulations were employed to study the temperature-dependent diffusive behavior of various liquid electrolytes representing 1M propylene carbonate solutions of metal salts with bis(trifluoromethylsulfonyl)imide (TFSI-) or hexafluorophosphate (PF6-) anions commonly used in lithium-ion batteries and beyond. The experimental studies revealed the temperature dependence of the diffusion coefficients for the propylene carbonate (PC) solvent and for the anions following an Arrhenius type of behavior. It was observed that the PC molecules are the faster species. For the monovalent cations (Li+, Na+, K+), the PC solvent diffusion was enhanced as the cation size increased, while for the divalent cations (Mg2+, Ca2+, Sr2+, Ba2+), the opposite trend was observed, i.e., the diffusion coefficients decreased as the cation size increased. The anion diffusion in LiTFSI and NaTFSI solutions was found to be similar, while in electrolytes with divalent cations, a decrease in anion diffusion with increasing cation size was observed. It was shown that non-polarizable charge-scaled force fields could correspond perfectly to the experimental values of the anion and PC solvent diffusion coefficients in salt solutions of both monovalent (Li+, Na+, K+) and divalent (Mg2+, Ca2+, Sr2+, Ba2+) cations at a range of operational temperatures. Finally, after calculating the radial distribution functions between cations, anions, and solvent molecules, the increase in the PC diffusion coefficient established with the increase in cation size for monovalent cations was clearly explained by the large hydration shell of small Li+ cations, due to their strong interaction with the PC solvent. In solutions with larger monovalent cations, such as Na+, and with a smaller solvation shell of PC, the PC diffusion is faster due to more liberated solvent molecules. In the salt solutions with divalent cations, both the anion and the PC diffusion coefficients decreased as the cation size increased due to an enhanced cation-anion coordination, which was accompanied by an increase in the amount of PC in the cation solvation shell due to the presence of anions.

Effect of alkali metal cations in alkaline iron battery electrodes

Abstract Rechargeable alkaline iron batteries (e.g. Ni-Fe and Fe-air) have been extensively studied recently as viable energy storage systems for renewable energy sources. However, inherent issues such as passivation of the iron and parasitic hydrogen evolution reaction (HER) on the electrode surface limit their full capability. Multiple approaches to improving iron electrode performance have been conducted, few of which focused on electrolyte composition. While alkali metal (AM) cations on the electrolyte do not directly participate in the electrochemical reactions, their intrinsic characteristics can dictate the performance of the electrode. Investigating the interface interactions and electrical double layer (EDL) structure can provide a deeper insight into the operation of iron electrodes in an alkaline solution. In this work, we investigated the effect of alkali metal cations (Li+, Na+, K+, Cs+) in the electrolyte solution in inhibiting passivation and HER on electrodeposited iron on carbon paper (Fe/CP) electrodes. The electrochemical measurements show that the iron redox and HER activities of the electrode increased with increasing cation size in the electrolyte. The non-covalent interactions between hydrated alkali metal cations and adsorbed OH species resulted to the formation of quasi-adsorbed clusters which can block active sites on the electrode surface. Furthermore, the concentration of these clusters decreases with increasing cation size which resulted to higher EDL capacitance and ECSA values of the electrode. The results of this work provide a better understanding of the surface reactions on iron electrodes and can help in developing novel techniques for suppressing passivation and parasitic HER on rechargeable alkaline iron batteries.

Cation induced changes to the structure of cryptophane cages.

Here the monocation complexes of seven anti-cryptophanes are examined with high-resolution ion-mobility mass spectrometry. The relative size of the [cation + cryptophane]+ complexes were compared based on their measured mobilities and derived collisional cross sections. A paradoxical trend of structural contraction was observed for complexes of increasing cation size. Density functional theory confirmed encapsulation occurs for cation = Na+, K+, Rb+, Cs+ and NH4+. However, cation = Li+ preferred oxygen coordination at a linker over encapsulation within the cavity, leading to a slightly larger gas phase structure overall. Protonated cryptophanes yielded much larger collision cross sections via imploded cryptophane structures. Thus, competing physical effects led to the observed non-periodic size trend of the complexes. Trends in complexation from isothermal titration calorimetry and other condensed phase techniques were borne out by the gas phase studies. Further, predicted cavity sizes compared with the gas phase experimental findings reveal more about the encapsulation mechanisms themselves.

X-ray and molecular dynamics study of the temperature-dependent structure of FLiNaK

The atomic structure of FLiNaK and its evolution with temperature are examined with x-ray scattering and molecular dynamics (MD) simulations in the temperature range 460–636 °C. In accord with previous studies, it’s observed that the average nearest-neighbor (NN) cation-anion coordination number increases with increasing cation size, going from ∼4 for Li-F to ∼6.4 for K-F. In addition, we find that there is a coupled change in local coordination geometry – going from tetrahedral for Li-F to octahedral for Na to very disordered quasi-cuboidal for K. The varying geometry and coordination distances for the cation-anion pairs cause a relatively constant F-F next-nearest neighbor (NNN) distance of approximately 3.1 Å. This relatively fixed distance allows the F anions to assume an overall correlated structure very similar to that of a hard-sphere liquid with an extended radius which is beyond the normal F ion size but reflects the cation-anion coordination requirements. Careful consideration of the evolution of the experimental atomic distribution functions with increasing temperature shows that the changes in correlation at each distance can be understood within the context of broadening asymmetric neighbor distributions. Within the temperature range studied, the evolution of F-F correlations with increasing temperature is consistent with changes expected in a hard-sphere liquid simply due to decreasing density.

Ab-initio investigation of the fundamental properties of metals (X = Be, Mg, and Ca) encapsulated CsXO3 tin-based perovskite materials

Perovskite material are widely used in Solar cell device due to it promising properties and power conversion efficiencies (PCE). Most importantly, properties of the absorber layer such as the band gap, electronic, optical, chemical composition and the grain size of the perovskite absorbed layer are very sacrosanct when determining efficacy of the material for such application. Thus, in this work, the optical, structural stability, electronic, and mechanical properties of non-halide cubic perovskite CsXO3 (X = Mg, Be and Ca) was extensively studied using the first principles Density functional theory (DFT) approach. The structural, electronic and optoelectronic properties was evaluated using Quantum Espresso Simulation Package (QESP) with the plane wave self-consistent field (PWscf) code. The structural computation reveals that the Lattice constant as well as total energy of CsXO3 (X = Be, Mg and Ca) increases as the size of cation X (X = Be, Mg and Ca) increases such that CsCaO3 possess the highest energy. Contrary, the fermi energy decreases with increase in cationic size, however, CsCaO3 has the highest lattice constant value 4.3743 eV. Electronic properties investigation shows that the studied perovskite has a metallic property with no forbidden band. zero band gap materials can adsorb photon across the entire solar spectrum, including lower-energy photons which is an advantage over a non-zero band gap which can only absorb photon with energies higher than the band gap energy can be absorbed and converted into electricity. This enables more efficient utilization of sunlight and increased overall energy conversion. To ascertain the effect of atomic exchange and orbital contribution in the formation of electronic properties, partial density of state (PDOS) was calculated and the result reveal that electron transition occurs mostly from p-orbital oxygen atom having major contribution on the valence band to the d-orbital of Caesium atom having major contribution on the conduction band and X-cation having minor contribution. CsXO3 (X = Be, Mg and Ca) also shows fascinating optical properties and thus can be used to improve solar cell device.

Noncovalent Host-Guest Complexes of Artemisinin with α-, β-, and γ- Cyclodextrin Examined by Structural Mass Spectrometry Strategies.

Cyclodextrins (CDs) are a family of macrocyclic oligosaccharides with amphiphilic properties, which can improve the stability, solubility, and bioavailability of therapeutic compounds. There has been growing interest in the advancement of efficient and reliable analytical methods that assist with elucidating CD host-guest drug complexation. In this study, we investigate the noncovalent ion complexes formed between naturally occurring dextrins (αCD, βCD, γCD, and maltohexaose) with the poorly water-soluble antimalarial drug, artemisinin, using a combination of ion mobility-mass spectrometry (IM-MS), tandem MS/MS, and theoretical modeling approaches. This study aims to determine if the drug can complex within the core dextrin cavity forming an inclusion complex or nonspecifically bind to the periphery of the dextrins. We explore the use of group I alkali earth metal additives to promote the formation of various noncovalent gas-phase ion complexes with different drug/dextrin stoichiometries (1:1, 1:2, 1:3, 1:4, and 2:1). Broad IM-MS collision cross section (CCS) mapping (n > 300) and power-law regression analysis were used to confirm the stoichiometric assignments. The 1:1 drug:αCD and drug:βCD complexes exhibited strong preferences for Li+ and Na+ charge carriers, whereas drug:γCD complexes preferred forming adducts with the larger alkali metals, K+, Rb+, and Cs+. Although the ion-measured CCS increased with cation size for the unbound artemisinin and CDs, the 1:1 drug:dextrin complexes exhibit near-identical CCS values regardless of the cation, suggesting these are inclusion complexes. Tandem MS/MS survival yield curves of the [artemisinin:βCD + X]+ ion (X = H, Li, Na, K) showed a decreased stability of the ion complex with increasing cation size. Empirical CCS measurements of the [artemisinin:βCD + Li]+ ion correlated with predicted CCS values from the low-energy theoretical structures of the drug incorporated within the βCD cavity, providing further evidence that gas-phase inclusion complexes are formed in these experiments. Taken together, this work demonstrates the utility of combining analytical information from IM-MS, MS/MS, and computational approaches in interpreting the presence of gas-phase inclusion phenomena.

Understanding Cation Effects on the Hydrogen Evolution Reaction

The hydrogen evolution reaction (HER) is known to be influenced by the identity of alkali metal cations in the electrolyte. But a clear understanding of this behavior has not been developed. Here, we present the results of experimental and theoretical studies that describe how alkali metal cations influence the HER in acidic and basic electrolytes. Alkali metal cations are shown to have no systematic effect on HER rates in acid. In alkaline media, rates decrease with increasing cation size over Ir, Pd, and Pt (Li+ > Na+ > K+ > Cs+) and increase with cation size over Cu, Ag, and Au (Li+ < Na+ < K+ < Cs+). We argue that interfacial cations lower the activation barrier for water dissociation, an elementary step unique to the HER in alkaline media. HER rates are limited by this step on Cu, Ag, and Au but are not over Ir, Pd, and Pt, explaining the inverted activity trend. Based on ab initio molecular dynamics simulations, we suggest that trends with cation size are attributable to the greater willingness of large, weakly solvated cations to approach the electrode surface.

Cation assisted binding and cleavage of dinitrogen by uranium complexes.

The role of alkali promoters in N2 cleavage by metal complexes remains poorly understood despite its relevance to the industrial production of ammonia from N2. Here we report a series of alkali bound-oxo-bridged diuranium(iii) complexes that provide a unique example of decreasing N2 binding affinity with increasing cation size (from K to Cs). N2 binding was found to be irreversible in the presence of K. A N2 complex could be isolated in the solid state in the presence of the Rb cation and crystallographically characterized, but N2 binding was found to be reversible under vacuum. In the presence of the Cs cation N2 binding could not be detected at 1 atm. Electrochemical and Computational studies suggest that the decrease in N2 binding affinity is due to steric rather than electronic effects. We also find that weak N2 binding in ambient conditions does not prevent alkali assisted N2 cleavage to nitride from occurring. More importantly, we present the first example of cesium assisted N2 cleavage leading to the isolation of a N2 derived multimetallic U/Cs bis-nitride. The nitrides readily react with protons and CO to yield ammonia, cyanate and cyanide.

Role of metal and anions in organo-metal halide perovskites CH3NH3MX3 (M: Cu, Zn, Ga, Ge, Sn, Pb; X: Cl, Br, I) on structural and optoelectronic properties for photovoltaic applications.

Methylammonium metal halide perovskites have recently been explored for new uses due to their unique and exciting optoelectronic properties. Their exceptional electronic properties have often been attributed to the overlap between the metal cation s and halogen p states. In this study, density functional theory calculations have been carried out based on the orthorhombic phase of the organometal trihalide perovskite CH3NH3MX3 (M: Cu, Zn, Ga, Ge, Sn, Pb; X: Cl, Br, I) to systematically investigate the effects of the metal cation and halogen anion on the structural, electronic, and optical properties for solar cell applications. The calculated lattice parameters agree well with previously obtained experimental and theoretical results. All of these perovskites are direct band gap compounds at the G symmetry point, except CH3NH3GaX3. The band gap increases from iodide to chloride and also with the metal cation size, from Ge to Pb or Cu to Zn. Furthermore, metal halide perovskites show blue shifts in their optical absorption spectra with an increase in metal cation size. Among the studied examples, CH3NH3GaBr3 and CH3NH3CuCl3 absorb a wide range of light, from UV to the visible region, and possess very unusual high dielectric constants and refractive indices. Our calculations reveal that CH3NH3SnI3, CH3NH3GeI3, and CH3NH3ZnI3 are favorable candidates for lead-free photovoltaic applications.

Whiteite-(MnMnMn), a new jahnsite-group mineral species from the Foote mine, North Carolina, USA, and chemical pressure effects in jahnsite-group minerals.

AbstractWhiteite-(MnMnMn), Mn2+Mn2+Mn2+2Al2(PO4)4(OH)2⋅8H2O, is a new whiteite-subgroup member of the jahnsite group from the Foote Lithium Company mine, Kings Mountain district, Cleveland County, North Carolina, USA. It was found in small vugs of partially oxidised pegmatite minerals on the East dump of the mine, in association with eosphorite, hureaulite, fairfieldite, mangangordonite, whiteite-(CaMnMn) and jasonsmithite. It occurs as sugary aggregates of blade-like crystals up to 0.1 mm long and as epitaxial overgrowths on whiteite-(CaMnMn). The crystals are colourless to very pale brown, with a vitreous lustre and a white streak. The blades are flattened on {001} and elongated along [010], with poor cleavage on {001}. The calculated density is 2.82 g⋅cm–3. Optically it is biaxial (–) with α = 1.599(2), β = 1.605(2), γ = 1.609(2) (white light); 2V (calc.) = 78.2°, having no observable dispersion or pleochroism, and with orientation X = b. Electron microprobe analyses and structure refinement gave the empirical formula (Mn2+0.59Ca0.38Na0.03)Σ1.00Mn1.00(Mn2+1.04Fe3+0.58Fe2+0.23Zn0.16Mg0.08)Σ2.09Al2.04(PO4)3.89(OH)3.18(H2O)7.26. Whiteite-(MnMnMn) is monoclinic, P2/a, a = 15.024(3) Å, b = 6.9470(14) Å, c = 9.999(2) Å, β = 110.71(3)°, V = 976.2(4) Å3 and Z = 2. The crystal structure was refined using synchrotron single-crystal data to wRobs = 0.057 for 2014 reflections with I &gt; 3σ(I). Site occupancy refinements confirm the ordering of dominant Mn in the X, M1 and M2 sites of the general jahnsite-group formula XM1(M2)2(M3)2(H2O)8(OH)2(PO4)4. A review of published crystallochemical data for jahnsite-group minerals shows a consistent chemical pressure effect in these minerals, manifested as a contraction of the unit-cell parameter, a, as the mean size of the X and M1 site cations increases. This is analogous to negative thermal expansion, but with increasing cation size, rather than heating, inducing octahedral rotations that result in an anisotropic contraction of the unit cell.

Influence of the Cubic Sublattice on Magnetic Coupling between the Tetrahedral Sites of Garnet.

We present a study on the nuclear and magnetic structures of two iron-based garnets with magnetic cations isolated on tetrahedral sites. Ca2YZr2Fe3O12 and Ca2LaZr2Fe3O12 offer an interesting comparison for examining the effect of increasing cation size within the diamagnetic backbone of the garnet crystal structure, and how such changes affect the magnetic order. Despite both systems exhibiting well-pronounced magnetic transitions at low temperatures, we also find evidence for diffuse magnetic scattering due to a competition between the nearest-neighbor, next nearest-neighbor, and so on, within the tetrahedral sites. This competition results in a complex noncollinear magnetic structure on the tetrahedral sublattice creating a mixture of ferro- and antiferromagnetic interactions above the long-range ordering temperature near 20 K and suggests that the cubic site of the garnet plays a significant role in mediating the superexchange interactions between tetrahedral cations.

Comparison of Ionic Transport Properties of Non-Aqueous Lithium and Sodium Hexafluorophosphate Electrolytes

To bridge the gap between current lithium-ion battery technology and alternative cell chemistries such as, e.g., sodium-ion batteries, the majority of the research in this field focuses on the improvement of the cell’s energy density by the development of new active materials for reversible storage of sodium ions. On the other hand, the power density, which is determined by the ionic transport and thermodynamic parameters in the electrolyte, namely the conductivity, the thermodynamic factor, the transference number, and the diffusion coefficient, is attracting little attention. In this contribution, we determine these electrolyte properties for 0.1 M to 2 M LiPF6 and NaPF6 in a mixture of ethylene carbonate and diethyl carbonate (EC:DEC (1:1 v:v)) and use them in 1D simulations to show their impact on the theoretical discharge rate performance of the lithium and sodium cell chemistry. We show that the increased cation size of sodium and its corresponding weaker solvent interactions are beneficial for high power applications and that the improved ionic transport properties would allow for a substantial increase of either the (dis)charge currents or the electrode areal loading, compared to the well-established lithium system.

Study and optimization of Diels-Alder reaction of piperine in aqueous ionic solutions using Gn.HCl as a catalyst

Study and optimization of Diels-Alder reaction of piperine in aqueous ionic solutions using Gn.HCl as a catalyst

Direct observation of ion emission from charged aqueous nanodrops: effects on gaseous macromolecular charging.

Mechanistic information about how gaseous ions are formed from charged droplets has been difficult to establish because direct observation of nanodrops in a size range relevant to gaseous macromolecular ion formation by optical or traditional mass spectrometry methods is challenging owing to their small size and heterogeneity. Here, the mass and charge of individual aqueous nanodrops between 1–10 MDa (15–32 nm diameter) with ∼50–300 charges are dynamically monitored for 1 s using charge detection mass spectrometry. Discrete losses of minimally solvated singly charged ions occur, marking the first direct observation of ion emission from aqueous nanodrops in late stages of droplet evaporation relevant to macromolecular ion formation in native mass spectrometry. Nanodrop charge depends on the identity of constituent ions, with pure water nanodrops charged slightly above the Rayleigh limit and aqueous solutions containing alkali metal ions charged progressively below the Rayleigh limit with increasing cation size. MS2 capsid ions (∼3.5 MDa; ∼27 nm diameter) are more highly charged from aqueous ammonium acetate than from its biochemically preferred, 100 mM NaCl/10 mM Na phosphate solution, consistent with ion emission reducing the nanodrop and resulting capsid charge. The extent of charging indicates that the capsid partially collapses inside the nanodrops prior to the charging and formation of the dehydrated gaseous ions. These results demonstrate that ion emission can affect macromolecular charging and that conformational changes to macromolecular structure can occur in nanodrops prior to the formation of naked gaseous ions.

Polyacrylamide adsorption on (1 0 1) quartz surfaces in saltwater for a range of pH values by molecular dynamics simulations

The adsorption of partially hydrolyzed polyacrylamide (HPAM) on quartz plays a key role in solid-liquid separation processes that seek to recover water for processing. In this work, computational molecular dynamics is used to study the water-quartz interface in the presence of HPAM under conditions of high salt and wide pH range. The salts used are mostly those of seawater. Small water-structure maker cations are preferentially adsorbed onto the polymer rather than onto the quartz surfaces. In freshwater, the adsorption of HPAM on quartz is reduced to very few contacts, however in saltwater the adsorption contacts of HPAM increase although decrease with surface charge, mainly due to electrostatic repulsion. In the presence of monovalent cations and for any surface charge on the quartz, above the isoelectric point, the adsorption contacts of HPAM decreases as the cation size increases, which is expected considering that smaller cations are more effective in neutralizing both quartz and HPAM. The results show that the adsorption of HPAM occurs exclusively via salt bridges. One way of describing the conformation of an HPAM chain adsorbed onto quartz is to reduce it to the skeleton of carbon atoms of the polymer chain and then determine the fraction of these atoms in loops, trains, and tails. In general, the adsorption contacts of HPAM are few, the carbon fractions in trains and loops are tiny, and therefore most of the carbon atoms in HPAM remain in long tails ready to get entangled with other tails thus favoring the flocculation process. The results are expected to contribute to sustainable water management through the right choice of flocculant based on molecular aspects.

Molecular dynamics study on TOTO-based ionic liquids with different cations

Ionic liquids (ILs) have good designability and promising prospect of application. By changing different cations or anions, ILs possessing better and specific properties can be synthesized. In this paper, molecular dynamics (MD) simulation is applied to study ILs with the anion of 2,5,8,11-tetraoxatridecan-13-oate (TOTO), which are named as TOTO-based ILs. Attention is mainly paid to the effect of five different types of cations, including organic choline (Ch), tetraethylammonium (TEA), tetrapropylammonium(TPA), and tetrabutylammonium (TBA) and inorganic sodium (Na) ions, on the structure and properties of TOTO-based ILs. Results about conformation and length distribution of TOTO chain, distance between cations and TOTO anions, distribution of coordination number for cation-anion, and dynamic coordination are analysed, showing that changing cations makes no considerable influence on gauche probability along TOTO chain. All organic cation-TOTO ILs have similar TOTO chain length distribution, and similar radical function distributions between cations and anions. No matter inorganic cations and organic cations, these cations prefer to interact with carboxyl oxygen atoms. For organic cation-TOTO systems, interionic coordination interactions are weakened with the increase of cation size. Due to the hydroxyl group in one Ch alkyl chain, Ch-TOTO system exhibits some particular properties. The introduce of functional group can tune ILs properties and promote the design of task-specific ILs.

Impact of Solvation Energetics on the Oxygen Reduction Reaction in Alkali-Metal/Oxygen Batteries

Alkali-metal/Oxygen batteries are a potential high-energy alternative to Li-ion batteries1. In these systems, the critical performance bottleneck is the oxygen reduction reaction (ORR) which in turn depends on the non-aqueous electrolyte that mediates ion and oxygen transport2. Herein, we show that a multifold increase of the practical discharge power of these batteries can be achieved by tuning the tripartite interactions between the salt cation, anion and the solvent in the electrolyte3. A single descriptor of the solvation energetics, the solvent reorganization energy (λ), is shown to capture the influence of electrolyte composition on both reactant transport (DO2 ) and reaction kinetics (k) at the electrode and found to be broadly applicable to alkali-metal/oxygen (i.e. Li-, Na- and K-O2) batteries. Increasing cation size (from Li+ to K+) doubled k/λ, indicating an increased sensitivity of k to the choice of anion, while variations in DO2 were minimal over this cation size range. These multifarious effects were captured in the Electrochemical Thiele Modulus incorporating Marcus-Hush kinetics and guidelines are presented for the choice of electrolyte solvent and salt when designing systems for high discharge rates. In a model symmetric K-O2 cell4, both the formation of solvent-separated ion-pairs and the existence of unsolvated anions reduced overpotential losses by 200mV compared to electrolytes with partial anion solvation, successfully demonstrating the rational, ab-initio design of electrolytes for high-power alkali-metal/Oxygen batteries.

Fluoride Ion Interactions in Alkali-Metal Fluoride-Diol Complexes.

The activity of F- is an important factor in the design of both inorganic and organic reactions involving fluorine compounds. The present study investigates interactions of F- with diols in alkali-metal fluoride-diol complexes. Increases in the reactivities of alkali-metal fluorides and their solubilities in alcohols is observed with increasing cation size. The difference in alkali-metal ion size produces different structural motifs for F--diol complex salts. The CsF complex salt with ethylene glycol (EG), CsF-EG, has a layered structure, whereas the Rb and K complex salts, (RbF)5-(EG)4 and (KF)5-(EG)4, form columnar structures. Comparison of the CsF complex salts with three different diols- EG, 1,3-propylene glycol (PG13), and 1,4-butylene glycol (PG14)-revealed that the diol chain length affects the bridging mode in their layered structures. EG bridges two OH oxygen atoms within the same CsF layer in CsF-EG, whereas PG13 and BG14 bridge two OH oxygen atoms in different CsF layers in (CsF)2-PG13 and CsF-BG14, respectively. The F- ion coordination environment involves interactions between alkali-metal ions and H atom(s) in the diol OH groups, where the F-···H interactions are more dominant than the F-···M+ interaction, based on Hirshfeld surface analyses. The O-H bond weakening observed by infrared spectroscopy also reflects the strengths of the F-···H interactions in these complex salts.

Electrochemical Polarization Dependence of the Elastic and Electrostatic Driving Forces to Aliovalent Dopant Segregation on LaMnO3.

Segregation of aliovalent dopant cations is a common degradation pathway on perovskite oxide surfaces in energy conversion and catalysis applications. Here we focus on resolving quantitatively how dopant segregation is affected by oxygen chemical potential, which varies over a wide range in electrochemical and thermochemical energy conversion reactions. We employ electrochemical polarization to tune the oxygen chemical potential over many orders of magnitude. Altering the effective oxygen chemical potential causes the oxygen nonstoichiometry to change in the electrode. This then influences the mechanisms underlying the segregation of aliovalent dopants. These mechanisms are (i) the formation of oxygen vacancies that couples to the electrostatic energy of the dopant in the perovskite lattice and (ii) the elastic energy of the dopant due to cation size mismatch, which also promotes the reaction of the dopant with O2 from the gas phase. The present study resolves these two contributions over a wide range of effective oxygen pressures. Ca-, Sr-, and Ba-doped LaMnO3 are selected as model systems, where the dopants have the same charge but different ionic sizes. We found that there is a transition between the electrostatically and elastically dominated segregation regimes, and the transition shifted to a lower oxygen pressure with increasing cation size. This behavior is consistent with the results of our ab initio thermodynamics calculations. The present study provides quantitative insights into how the elastic energy and the electrostatic energy determine the extent of segregation for a given overpotential and atmosphere relevant to the operating conditions of perovskite oxides in energy conversion applications.