arXiv subjects

Mario Burbano

Publications and source records attributed to Mario Burbano.

6 recordsLinked to original sources

arXiv2018

Ion-ion correlations across and between electrified graphene layers

When an ionic liquid adsorbs onto a porous electrode, its ionic arrangement is deeply modified due to a screening of the Coulombic interactions by the metallic surface and by the confinement imposed upon it by the electrode's morphology. In particular, ions of the same charge can approach at close contact, leading to the formation of a superionic state. The impact of an electrified surface placed between two liquid phases is much less understood. Here we simulate a full supercapacitor made of the 1-butyl-3-methylimidazolium hexafluorophosphate and nanoporous graphene electrodes, with varying distances between the graphene sheets. The electrodes are held at constant potential by allowing the carbon charges to fluctuate. Under strong confinement conditions, we show that ions of the same charge tend to adsorb in front of each other across the graphene plane. These correlations are allowed by the formation of a highly localized image charge on the carbon atoms between the ions. They are suppressed in larger pores, when the liquid adopts a bilayer structure between the graphene sheets. These effects are qualitatively similar to the recent templating effects which have been reported during the growth of nanocrystals on a graphene substrate.

cond-mat.mtrl-sci↗

arXiv2017

Confinement Effects on an Electron Transfer Reaction in Nanoporous Carbon Electrodes

Nanoconfinement generally leads to drastic effect on the physical and chemical properties of ionic liquids. Here we investigate how the electrochemical reactivity in such media may be impacted inside nanoporous carbon electrodes. To this end, we study a simple electron transfer reaction using molecular dynamics simulations. The electrodes are held at constant electric potential by allowing the atomic charges on the carbon atoms to fluctuate. We show that the $\mathrm{Fe^{3+}}/\mathrm{Fe^{2+}}$ couple dissolved in an ionic liquid exhibits a deviation with respect to Marcus theory. This behavior is rationalized by the stabilization of a solvation state of the Fe$^{3+}$ cation in the disordered nanoporous electrode that is not observed in the bulk. The simulation results are fitted with a recently proposed two solvation state model, which allows us to estimate the effect of such a deviation on the kinetics of electron transfer inside nanoporous electrodes.

cond-mat.mtrl-sci↗

arXiv2015

Sparse Cyclic Excitations Explain the Low Ionic Conductivity of Stoichiometric Li$_{7}$La$_{3}$Zr$_{2}$O$_{12}$

We have performed long time-scale molecular dynamics simulations of the cubic and tetragonal phases of the solid lithium-ion--electrolyte Li$_{7}$La$_{3}$Zr$_{2}$O$_{12}$ (LLZO), using a first-principles parameterised interatomic potential. Collective lithium transport was analysed by identifying dynamical excitations; persistent ion displacements over distances comparable to the separation between lithium sites, and string-like clusters of ions that undergo cooperative motion. We find that dynamical excitations in c-LLZO are frequent, with participating lithium numbers following an exponential distribution, mirroring the dynamics of fragile glasses. In contrast, excitations in t-LLZO are both temporally and spatially sparse, consisting preferentially of highly concerted lithium motion around closed loops. This qualitative difference is explained as a consequence of lithium ordering in t-LLZO, and provides a mechanistic basis for the much lower ionic conductivity of t-LLZO compared to c-LLZO.

cond-mat.mtrl-sci↗

arXiv2014

Ceria co-doping: Synergistic or average effect?

Ceria (CeO$_{2}$) co-doping has been suggested as a means to achieve ionic conductivities that are significantly higher than those in singly-doped systems. Rekindled interest in this topic over the last decade has given rise to claims of much improved performance. The present study makes use of computer simulations to investigate the bulk ionic conductivity of Rare Earth (RE) doped ceria, where RE = Sc, Gd, Sm, Nd and La. The results from the singly doped systems are compared to those from ceria co-doped with Nd/Sm and Sc/La. The pattern that emerges from the conductivity data is consistent with the dominance of local lattice strains from individual defects, rather than the synergistic co-doping effect reported recently and, as a result no enhancement in the conductivity of co-doped samples is observed.

cond-mat.mtrl-sci↗

arXiv2013

Strain effects on the ionic conductivity of Y-doped ceria: A simulation study

In this paper we report a computational study of the effects of strain on the conductivity of Y-doped ceria (YDC). This material was chosen as it is of technological interest in the field of Solid Oxide Fuel Cells (SOFCs). The simulations were performed under realistic operational temperatures and strain (ε) levels. For bulk and thin film YDC, the results show that tensile strain leads to conductivity enhancements of up to 3.5x and 1.44x, respectively. The magnitude of these enhancements is in agreement with recent experimental and computational evidence. In addition, the methods presented herein allowed us to identify enhanced ionic conductivity in the surface regions of YDC slabs and its anisotropic character.

cond-mat.mtrl-sci↗

arXiv2011

A Dipole Polarizable Potential for Reduced and Doped CeO$_2$ from First-Principles

In this paper we present the parameterization of a new interionic potential for stoichiometric, reduced and doped CeO$_2$. We use a dipole-polarizable potential (DIPPIM) and optimize its parameters by fitting them to a series of DFT calculations. The resulting potential was tested by calculating a series of fundamental properties for CeO$_2$ and by comparing them to experimental values. The agreement for all the calculated properties (thermal and chemical expansion coefficients, lattice parameters, oxygen migration energies, local crystalline structure and elastic constants) is within 10-15% of the experimental one, an accuracy comparable to that of ab initio calculations. This result suggests the use of this new potential for reliably predicting atomic-scale properties of CeO$_2$ in problems where ab initio calculations are not feasible due to their size-limitations.

physics.comp-ph↗