Characterizing heterogeneous dynamics at hydrated electrode surfaces

Abstract

In models of Pt 111 and Pt 100 surfaces in water, motions of molecules in the first hydration layer are spatially and temporally correlated. To interpret these collective motions, we apply quantitative measures of dynamic heterogeneity that are standard tools for considering glassy systems. Specifically, we carry out an analysis in terms of mobility fields and distributions of persistence times and exchange times. In so doing, we show that dynamics in these systems is facilitated by transient disorder in frustrated two-dimensional hydrogen bonding networks. The frustration is the result of unfavorable geometry imposed by strong metal-water bonding. The geometry depends upon the structure of the underlying metal surface. Dynamic heterogeneity of water on the Pt 111 surface is therefore qualitatively different than that for water on the Pt 100 surface. In both cases, statistics of this adlayer dynamic heterogeneity responds asymmetrically to applied voltage.The behavior of water adjacent to hydrated metal surfaces is central to the physics underlying electrochemistry, and as such there is much theoretical work devoted to this topic. References 1-14 are examples. Our recent work in this area 14 highlighted the role of interfacial properties occurring over nanosecond timescales. In this paper, we examine this dynamics in further detail, considering models of Pt-water interfaces and showing structural relaxations in the water ad-layers proceed exceedingly slowly via mechanisms that are spatially heterogeneous.Strong metal-water bonding forces ad-layer waters into structures that are antithetical to favorable hydrogen bonding. Accordingly, the ad-layers contain defects. Motions are one-to two-orders of magnitude faster in the proximity of these defects than elsewhere on the surface. Further, rearrangements of these defects require coordinated motions of several water molecules. This type of heterogeneous dynamics is common in glass-forming materials. 15,16 Some of the quantitative methods used to successfully interpret such behavior in molecular dynamics models of glass formers 16,17 are used here to elucidate the nature of water dynamics at metal surfaces. ModelAs in our earlier work, 13,14 we carry out molecular dynamics simulations with a surface model of Siepman and Sprik. 12 The model provides an empirical water-metal chemisorption potential. It includes effects of the metal's electronic polarizability and its response to applied voltage across an electrochemical cell. For the model parameters we consider, the electrodes most resemble platinum surfaces 18-21 with water-metal binding energies around 0.4eV and a metal lattice constant of 4.0Å. Consequently when the electrode is exposed to a bulk liquid reservoir, water molecules adsorb to the metal atoms and adopt a) Corresponding author. E-mail: chandler@berkeley.edu orientations that allow for hydrogen bond formation with molecules adsorbed to adjacent lattice sites. Snapshots of the ad-layers formed spontaneously from a slab of liquid water are shown in Panels (...

Hydrated Proton Structure and Diffusion at Platinum Surfaces

“…In Figure 2(a), this order parameter has been used to characterize the adsorbed water layer for the Pt(111) surface under an applied voltage of 1 V. The order parameter has been integrated over the simulation time to yield a result as a function of a⃗ ; the red regions represent areas of higher water mobility in which the adsorbed water layer reorients quickly and the blue regions of lower mobility. The result is consistent with the previous simulations 26,27 in two regards. First, the adsorbed water molecules reorient very slowly.…”

“…For Pt(100), boundary areas between different domains have a more narrow structure, consistent with previous MD simulations. 26,27 The different water structure gives rise to different diffusion behavior for the excess proton CEC, as calculated using eq 3.2 and presented in Figure 4(b). With a wall potential that confines the proton within the adsorbed water layer, the proton is even less mobile than for Pt(111).…”

“…26 Meanwhile, the adsorbed water molecules are entropically driven to form different domains on the platinum surface, giving rise to heterogeneity on larger time and length scale than can be observed using DFT calculations. 27 When considering proton transport at these platinum−water interfaces, one might imagine that the large-scale hydrogenbonded network will provide a good pathway for proton diffusion. However, simulations indicate that these surface water molecules reorient very slowly, which can inhibit proton diffusion.…”

Local order of liquid water at metallic electrode surfaces

“…This is a critical first step to validate current models of the electrochemical, i.e., ionic aqueous solution/metal, interface. [20][21][22][23] In this work, we overcome the limitation of classical potentials and analyze in detail, the structure and interfacial charge redistribution of liquid-water interacting with Pd and a) Author to whom correspondence should be addressed. Electronic mail: maria.fernandez-serra@stonybrook.edu Au (111) surfaces at ambient temperature, using first principles molecular dynamics.…”

Structural characterization of water-metal interfaces

“…Due to the efficiency, rarity, and cost of Pt, there is an active area of research focused on finding an alternative, abundant metal/semi-metal with similar properties [21,[23][24][25][26]. Many theoretical and experimental studies have been done to further understand the water-Pt interface [27][28][29][30][31][32][33][34]. In particular, Osawa et al [29] performed infrared absorption spectroscopy experiments to examine the structure of water next to an electrified, pristine Pt electrode.…”