Using first-principles simulations, we predict a high-performance solid electrolyte with composition Na 10 GeP 2 S 12 for use in sodium-sulfur (Na-S) batteries. The thermodynamic stability of its structure is established through determination of decomposition reaction energies and phonons, while Na-ionic conductivity is obtained using ab initio molecular dynamics at elevated temperatures. Our estimate of the room-temperature (RT) conductivity is 4.7 Â 10 À3 S cm À1 , which is slightly higher than those of other superionic solid electrolytes such as b 00 -alumina and Na 3 Zr 2 Si 2 PO 12 , currently used in practical hightemperature Na-S batteries. Activation energy obtained from the Arrhenius plot (in the range 800-1400 K) is 0.2 eV, which is slightly lower than the typical values exhibited by other ceramic conductors (0.25-1 V) (Hueso et al., Energy Environ. Sci., 2013, 6, 734). We show that soft Na-S phonon modes are responsible for its thermodynamic stability and the lower activation barrier for diffusion of Na-ions.Finally, the calculated electronic bandgap of 2.7 eV (a wide electrochemical window) augurs well for its safe use in sodium batteries. Opening up a possibility for realizing RT operation of Na-S batteries, our prediction of a new phase in the Na-Ge-P-S system will stimulate experimental studies of the material.

With a view towards optimizing gas storage and separation in crystalline and disordered nanoporous carbon-based materials, we use ab initio density functional theory calculations to explore the effect of chemical functionalization on gas binding to exposed edges within model carbon nanostructures. We test the geometry, energetics, and charge distribution of in-plane and out-of-plane binding of CO(2) and CH(4) to model zigzag graphene nanoribbons edge-functionalized with COOH, OH, NH(2), H(2)PO(3), NO(2), and CH(3). Although different choices for the exchange-correlation functional lead to a spread of values for the binding energy, trends across the functional groups are largely preserved for each choice, as are the final orientations of the adsorbed gas molecules. We find binding of CO(2) to exceed that of CH(4) by roughly a factor of two. However, the two gases follow very similar trends with changes in the attached functional group, despite different molecular symmetries. Our results indicate that the presence of NH(2), H(2)PO(3), NO(2), and COOH functional groups can significantly enhance gas binding, making the edges potentially viable binding sites in materials with high concentrations of edge carbons. To first order, in-plane binding strength correlates with the larger permanent and induced dipole moments on these groups. Implications for tailoring carbon structures for increased gas uptake and improved CO(2)/CH(4) selectivity are discussed.

A combination of ab initio and classical Monte Carlo simulations is used to investigate the effects of functional groups on methane binding. Using Møller–Plesset (MP2) calculations, we obtain the binding energies for benzene functionalized with NH2, OH, CH3, COOH, and H2PO3 and identify the methane binding sites. In all cases, the preferred binding sites are located above the benzene plane in the vicinity of the benzene carbon atom attached to the functional group. Functional groups enhance methane binding relative to benzene (−6.39 kJ/mol), with the largest enhancement observed for H2PO3 (−8.37 kJ/mol) followed by COOH and CH3 (−7.77 kJ/mol). Adsorption isotherms are obtained for edge-functionalized bilayer graphene nanoribbons using grand canonical Monte Carlo simulations with a five-site methane model. Adsorbed excess and heats of adsorption for pressures up to 40 bar and 298 K are obtained with functional group concentrations ranging from 3.125 to 6.25 mol % for graphene edges functionalized with OH, NH2, and COOH. The functional groups are found to act as preferred adsorption sites, and in the case of COOH the local methane density in the vicinity of the functional group is found to exceed that of bare graphene. The largest enhancement of 44.5% in the methane excess adsorbed is observed for COOH-functionalized nanoribbons when compared to H terminated ribbons. The corresponding enhancements for OH- and NH2-functionalized ribbons are 10.5% and 3.7%, respectively. The excess adsorption across functional groups reflects the trends observed in the binding energies from MP2 calculations. Our study reveals that specific site functionalization can have a significant effect on the local adsorption characteristics and can be used as a design strategy to tailor materials with enhanced methane storage capacity.

Organic ionic plastic crystals (OIPCs) are a unique class of materials that exhibit a short-range disorder on the molecular level but are ordered at higher length scales. Recent experiments in our group have shown that the OIPC methyl(diethyl)isobutylphosphonium hexafluorophosphate ([P][PF]) exhibits a high ideal selectivity of 30 with respect to CO and N at 35 °C. Here, we employ classical molecular dynamics simulations for studying gas uptake in the OIPC [P][PF] at different temperatures. Both adsorption and absorption of the gases CO, N, O, and CH were estimated using a gas/solid interface model. The observed trend in gas uptake was CO > CH > O > N. The CO uptake was found to be dependent on both the OIPC structure and temperature. Owing to phase transitions and intermolecular motions, the solid gets disordered with increasing temperatures. The study finds that such disordering effects can also be caused by interface effects, which can enhance gas absorption. The results qualitatively confirmed that the OIPC can be effective in separating CO not only from N, as per the previous experimental study, but also from O and CH. However, strategies to improve the free volume in the material become important.

An organic ionic plastic crystal (OIPC), methyl(diethyl)isobutylphosphonium hexafluorophosphate [P][PF], was investigated for CO and N absorption using molecular simulations. Ab initio calculations showed that both the cation and anion exhibit larger binding energy for CO compared with N. The CO absorption, as calculated from classical molecular dynamics simulations, increased by a factor of 7.5 from 275 K to 325 K, while that of N showed low absorption at both temperatures. The simulations suggest that the significant increase in CO absorption at 325 K is attributed to a higher degree of disorder and increase in the free volume due to the gas/solid interfaces. While the ab initio calculations were helpful in identifying specific interaction sites on the constituent ions, the classical MD simulations elucidated the importance of interfaces in gas absorption studies in this material. The results show that the OIPC can be a promising material for CO separations from CO/N mixture.

The free energy calculation shows the different free energy changes of the adsorption and absorption of gas molecules into an organic ionic plastic crystal, successfully predicting the gas selectivity of this new type of gas separation material.

Supercapacitors deliver higher power than batteries and find applications in grid integration and electric vehicles. Recent work by Chmiola et al. (2006) has revealed unexpected increase in the capacitance of porous carbon electrodes using ionic liquids as electrolytes. The work has generated curiosity among both experimentalists and theoreticians. Here, we have performed molecular simulations using a recently developed technique (Punnathanam, 2014) for simulating supercapacitor system. In this technique, the two electrodes (containing electrolyte in slit pore) are simulated in two different boxes using the Gibbs ensemble methodology. This reduces the number of particles required and interfacial interactions, which helps in reducing computational load. The method simulates an electric double-layer capacitor (EDLC) with macroscopic electrodes with much smaller system sizes. In addition, the charges on individual electrode atoms are allowed to vary in response to movement of electrolyte ions (i.e., electrode is polarizable) while ensuring these atoms are at the same electric potential. We also present the application of our technique on EDLCs with the electrodes modeled as slit pores and as complex three-dimensional pore networks for different electrolyte geometries. The smallest pore geometry showed an increase in capacitance toward the potential of 0 charge. This is in agreement with the new understanding of the electrical double layer in regions of dense ionic packing, as noted by Kornyshev's theoretical model (Kornyshev, 2007), which also showed a similar trend. This is not addressed by the classical Gouy-Chapman theory for the electric double layer. Furthermore, the electrode polarizability simulated in the model improved the accuracy of the calculated capacitance. However, its addition did not significantly alter the capacitance values in the voltage range considered.

T here is an error in Figure 8a that relates to the calculation of the radial distribution function for certain atoms. The mistake is in the range of the Y axis of the plot. In the corrected figure (shown here), the Y-axis values range from 0 to 3 compared with 0 to 7 in the original figure in the manuscript. However, the shape of the plot and the peak positions on the X axis remain unchanged. Whereas Figure 8b is unaffected, the range of its Y axis has also been readjusted for consistency with that of the corrected Figure 8a, and both are shown below.The change does not affect the conclusions or any part of the text, as the discussions involved the peak positions on the X axis of the plot. We apologize for any inconvenience caused due to the error. Figure 8. Radial distribution plots between the phosphorus atoms of the cations and anions in the bulk ordered (a) and disordered structure (b), both at 325 K.