Hydroxide Ion Diffusion in Anion-Exchange Membranes at Low Hydration: Insights from Ab Initio Molecular Dynamics

Abstract

Operation of anion-exchange membrane (AEM) fuel cells (AEMFCs) results in gradients in the cell that can lead to low-hydration conditions within the cell. It is therefore important to investigate hydroxide ion diffusion in AEMs with low water-to-cation ratios (λ ≤ 4, λ≡n H 2 O /n cation ). In this work, ab initio molecular dynamics simulations are presented to explore hydroxide ion solvation complexes and diffusion mechanisms in model AEMs at low hydration. By changing the cation spacing within the AEM and the degree of hydration, six different idealized AEM models are created in which the water distribution is not uniform. It is shown that distinct water distributions impart unique OH − diffusion mechanisms that fall into three regimes. The observed mechanisms, nondiffusive, vehicular, and a mixture of structural and vehicular diffusion, depend on the presence or absence of a second solvation shell of the hydroxide ion and on the local water structure. The results suggest that the water distribution is a better descriptor than the value of λ for classifying AEMs under low-hydration conditions. These results enable us to posit idealized mechanisms for the three diffusion regimes and to define requirements for promoting OH − conductivity in high-performance AEMFC devices operating under lowhydration conditions.

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

“…While in bulk solution, the hydroxide ions typically share a similar solvation patterns, which results in a similar diffusion mechanism in this low-hydration AEM model, the heterogeneity of the environments of each hydroxide suggests that over the time scales of the simulations, each OHmight be governed by different diffusion mechanisms, which we discuss next. (a,c) taken with permission from Reference [53]. Copyright 2019 American Chemical Society.…”

“…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.…”

The alkali degradation of LDPE-based radiation-grafted anion-exchange membranes studied using different ex situ methods

“…Degradation is more rapid at RH ¼ 50% when the temperature is raised to 80 C, while operating these AEMs at 100 C at reduced RHs is not advised! As well as AEM water content and water diffusivity being critical for high OH À conductivities and high AEMFC performances, [47][48][49][50][51][52][53] hydration is also key to enhancing the alkali stabilities of AEMs. 45,[54][55][56][57][58][59][60] Note, a very recent result shows that LDPE-TMA-based RG-AEMs can be successfully employed in AEMFCs at 125 C for reasonable periods of time when suitably hydrated.…”