Proton Transport under External Applied Voltage Zhen Cao, † Revati Kumar, ‡ Yuxing Peng, † and Gregory A. Voth* ,† † Department of Chemistry, James Frank Institute, Computation Institute, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637, United States ‡ Department of Chemistry, Louisiana State University, 736 Choppin Hall, Baton Rouge, Louisiana 70803, United States ABSTRACT: Proton transport through an electrolyte layer between platinum electrodes under a range of applied voltages is explored using reactive molecular dynamics simulation. The proton transport process is decomposed into vehicular and Grotthuss hopping components, and the two mechanisms and their correlation are investigated as a function of applied voltage. At higher applied voltages, the effect of the hopping mechanism is much larger as compared with the vehicular mechanism. As the voltage is increased, the net correlation between the two mechanisms goes from negative to positive, and both the hopping frequency as well as the number of consecutive forward hops increases. This behavior results in a larger total diffusion constant at higher values of the voltage. The behavior of the hydrated excess proton is therefore substantially different under an applied external voltage than in the normal bulk water environment. I. INTRODUCTION Hydrated excess protons are the primary charge transport species in a number of systems ranging from energy storage to biological membranes. 1-4 In practice, this transport process often takes place under an electrostatic potential difference, and, in particular, in energy storage systems, the electrolyte is typically confined between electrodes with a voltage applied across the cell. Atomistic simulations can provide key insight, from both a dynamical as well as structural perspective, 5-16 into the charge transport processes in these systems. This, in turn, can be used to interpret experimental observations, 17-22 leading ultimately to the guided optimization of energy storage systems based on proton transport. However, comparatively few simulations have been carried out to study proton transport in electrochemical systems, and these concentrate on the electrochemical reduction of protons at the electrode sur- face 14,23,24 rather than the actual transport through the electrolyte. Moreover, recent work on the neat water platinum interface has revealed some interesting features including hindered dynamics and structuring of the water at these interfaces. 25,26 These dynamical effects are comparatively long- lived (tens of nanoseconds), and the structural correlations extend over large length scales (several nanometers). These studies suggest that simulations spanning long length and time scales will be necessary to study the proton transport mechanism near these types of metal/aqueous electrolyte interfaces. Computational investigations have been carried out to accurately model proton transport in the electrolyte region between electrodes in the presence and absence of an applied external voltage, resulting in fundamental insight into this complex charge transport process. In addition to the usual vehicular diffusion, the proton can hop between water molecules, giving rise to the “Grotthuss shuttling” process. 27-29 This makes the modeling of proton transport particularly challenging because one needs a reactive description, lacking in conventional empirical force fields, involving both bond dissociation and formation. Ideally, one would like to employ ab initio molecular dynamics (MD) to study these types of systems, but the prohibitive computational cost precludes such studies in systems with time scales greater than the order of tens of picoseconds. Reactive MD models have been developed 28,30-32 to study proton transport in a wide range of systems, from biological ion channels, 32,33 to fuel cells. 34 The reactive methodology is based on a multiconfigura- tional approach, wherein the system is described as a linear combination of states, each with a different bonding topology, which is propagated in time allowing for smooth transitions from reactants to products. Previous studies have demonstrated that proton transport in aqueous media is particularly sensitive to the hydrogen bond environment. 35-37 Interestingly, the vehicular transport involves breaking of the neighboring hydrogen-bond network to let the hydronium transfer as a whole, while the Grotthuss mechanism requires a more complex process. 28,38-40 Modeling proton transport in an aqueous electrolyte in the presence of an external voltage applied between the electrodes Special Issue: James L. Skinner Festschrift Received: January 31, 2014 Revised: April 9, 2014 Published: April 10, 2014 Article pubs.acs.org/JPCB © 2014 American Chemical Society 8090 dx.doi.org/10.1021/jp501130m | J. Phys. Chem. B 2014, 118, 8090-8098