Ab Initio Molecular Dynamics Study of Hydroxide Diffusion Mechanisms in Nanoconfined Structural Mimics of Anion Exchange Membranes

Abstract

The development of reliable, cost-effective polymer architectures for use as anion exchange membranes (AEMs) is an important challenge facing emerging electrochemical device technologies. Elucidation of key design principles underlying these electrolytes requires a fundamental understanding of the hydroxide ion transport mechanism in the aqueous region of an AEM. To this end, we have carried out a series of atomistic ab initio molecular dynamics calculations. To mimic the complex AEM nanoconfined environment, we employ graphane bilayers or carbon nanotubes to which selected cationic groups are attached and which are subsequently filled with water and hydroxide ions to achieve target waterto-cation ratios and overall electrical neutrality. The complex structure of water under nanoconfinement differs from the bulk and is controlled by the shape and size of the confining volume. Consequently, the local hydroxide ion diffusion mechanisms in different chemical and geometric environments is also seen to differ from that in bulk aqueous solution and depends on a number of design parameters, including hydration level, cation spacing, and cell geometry. An exploration of this large parameter space will be presented in a series of reports; in this first one, we introduce analysis tools to characterize the system, elucidate hydroxide transport mechanisms, and present our first set of case studies.

OH− and H3O+ Diffusion in Model AEMs and PEMs at Low Hydration: Insights from Ab Initio Molecular Dynamics

“…Dispersion forces were accounted for using the Dispersion-Corrected Atomic Core Pseudopotential (DCACP) scheme [ 61 , 62 ] within the Kohn–Sham formulation of Density Functional Theory using the B-LYP exchange-correlation functional [ 63 , 64 ]. The B-LYP+DCACP has been selected for this study, as it has previously been shown to produce satisfactory results for water-acene interactions [ 65 ], liquid water [ 66 ], and hydronium diffusion in bulk water [ 38 , 39 , 40 , 67 ] A detailed description of the construction of the initial structures and of the computational methodology can be found in the Supplementary Information (SI) and in References [ 52 , 53 , 54 , 55 ].…”

“…The CN values found suggest that before a PT occurs between the anion nascent water molecule, the water must have a first solvation shell of one water o and an incomplete second solvation shell. Values of δ > 0.5 are considered to be inactive complexes with respect to PT, while values of δ < 0.1 are considered to be "active" and are associated with PT events [41,42,52,69,70]. In Figure 7b, we present the O next O RDF and CNs for δ < 0.1 and δ > 0.5, where, O next represents the first neighbor oxygen to the SO 3 H oxygen, and O represents water and hydronium oxygens.…”

“…Hydroxide and hydronium ions serve as charge carriers, respectively in AEMs and PEMs, and these systems are then solvated with water molecules [ 57 ]. In previous studies, we explored different GB systems as mimics of different AEM and PEM environments [ 52 , 53 , 54 , 55 ] in which the nanoconfined structure in these systems model the layered arrangement recently reported in References [ 16 , 57 ]. Of the various systems studied, we ultimately chose two representative examples that best captured the hydroxide and hydronium ion diffusion mechanisms described above.…”

“…We found that the water molecules in these models exhibit intriguing and unusual structures that depend on the shape and size of the confining volume, the hydration level, and the cation spacing. Specifically, for systems in which two-dimensional graphane bilayers (GBs) are used to mimics the actual polymer architectures [ 52 , 53 , 54 , 55 ], under low hydration values (λ < 5, where λ is the number of water molecules per cation/anion), the water assumes a non-uniform distribution, which is characterized by the formation of void areas throughout the system [ 53 , 55 ]. Consequently, the local hydroxide and hydronium diffusion mechanisms were shown to differ from their respective bulk solution mechanisms [ 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 ] in ways that are strongly dependent on the water structure.…”

“…In this study, we aim to elucidate the differences in the atomistic details of the hydroxide and hydronium ion diffusion mechanisms in AEMs and PEMs in confined environments and under low hydration conditions. To this end, we apply a similar protocol from our previous studies [ 52 , 53 , 54 , 55 ] to explore hydroxide and hydronium diffusion in architecturally distinct AEMs and PEMs, employing nano-confined structures. We employ fully atomistic AIMD [ 51 ] to simulate the molecular behavior, solvation patterns, and ion diffusion mechanisms within the model AEMs and PEMs.…”