Abstract
Fuel cell-based anion-exchange membranes (AEMs) and proton exchange membranes (PEMs) are considered to have great potential as cost-effective, clean energy conversion devices. However, a fundamental atomistic understanding of the hydroxide and hydronium diffusion mechanisms in the AEM and PEM environment is an ongoing challenge. In this work, we aim to identify the fundamental atomistic steps governing hydroxide and hydronium transport phenomena. The motivation of this work lies in the fact that elucidating the key design differences between the hydroxide and hydronium diffusion mechanisms will play an important role in the discovery and determination of key design principles for the synthesis of new membrane materials with high ion conductivity for use in emerging fuel cell technologies. To this end, ab initio molecular dynamics simulations are presented to explore hydroxide and hydronium ion solvation complexes and diffusion mechanisms in the model AEM and PEM systems at low hydration in confined environments. We find that hydroxide diffusion in AEMs is mostly vehicular, while hydronium diffusion in model PEMs is structural. Furthermore, we find that the region between each pair of cations in AEMs creates a bottleneck for hydroxide diffusion, leading to a suppression of diffusivity, while the anions in PEMs become active participants in the hydronium diffusion, suggesting that the presence of the anions in model PEMs could potentially promote hydronium diffusion.
Highlights
Fuel cell-based anion exchange membranes (AEMs) constitute some of the cleanest low-cost electrochemical devices [1,2,3,4,5]
Combining the results presented in Section 4.3.1, we conclude that the factors required to increase the hydronium reactivity in the proton exchange membranes (PEMs) model relate to the non-uniformity of the water distribution along the membrane, which gives rise to a high probability of obtaining a coordination numbers (CNs) value of ~1 for Onext, an incomplete second solvation shell for the hydronium ions, and fewer water molecules in the vicinity of the anions oxygens (i.e., SO3− )
For the PEM model, hydronium ion diffusion is structural rather than vehicular, with the participations of the anions according to the reaction: SO3− +H3 O+ ↔ SO3 H + H2 O [55]
Summary
Fuel cell-based anion exchange membranes (AEMs) constitute some of the cleanest low-cost electrochemical devices [1,2,3,4,5]. This is largely due to the use of an alkaline environment, which eliminates the need for precious metal catalysts [2,3,5,6,7,8,9,10,11]. Enhancing hydroxide ion conductivity and membrane stability remains a key hurdle to realizing the potential of AEM fuel cells [2,3,5]. The sulfonate anionic functional end group (SO3− ) is one of the most widely used groups in PEM fuel cell devices [14,19,20,21,22,23,24,25,26,27,28]
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