Deep eutectic solvents (DESs) are an emerging class of mixtures characterized by significant depressions in melting points compared to those of the neat constituent components. These materials are promising for applications as inexpensive "designer" solvents exhibiting a host of tunable physicochemical properties. A detailed review of the current literature reveals the lack of predictive understanding of the microscopic mechanisms that govern the structure−property relationships in this class of solvents. Complex hydrogen bonding is postulated as the root cause of their melting point depressions and physicochemical properties; to understand these hydrogen bonded networks, it is imperative to study these systems as dynamic entities using both simulations and experiments. This review emphasizes recent research efforts in order to elucidate the next steps needed to develop a fundamental framework needed for a deeper understanding of DESs. It covers recent developments in DES research, frames outstanding scientific questions, and identifies promising research thrusts aligned with the advancement of the field toward predictive models and fundamental understanding of these solvents.

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.

Abstract:We propose a new method for simulating electron dynamics in open quantum systems out of equilibrium, using a finite atomistic model. The proposed method is motivated by the intuitive and practical nature of the driven Liouville vonNeumann equation approach of Sánchez et al. [J. Chem. Phys., 124, 214708 (2006)]. A key ingredient of our approach is a transformation of the Hamiltonian matrix from an atomistic to a state representation of the molecular junction. This allows us to uniquely define the bias voltage across the system while maintaining a proper thermal electronic distribution within the finite lead models. Furthermore, it allows us to investigate complex molecular junctions, including non-linear setups and multi-lead configurations. A heuristic derivation of our working equation leads to explicit expressions for the damping and driving terms, which serve as appropriate electron sources and sinks that effectively "open" the finite model system.Although the method does not forbid it, in practice we find neither violation of Pauli's exclusion principles nor deviation from density matrix positivity throughout our numerical simulations of various tight-binding model systems. We believe that the new approach offers a practical and physically sound route for performing atomistic time-dependent transport calculations in realistic molecular junctions.2

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.

Abstract:A parameter free version of the recently developed driven Liouville-von Neumann equation [J. Chem. Theo. Comp. 10, 2927-2941] for electronic transport calculations in molecular junctions is presented. A single driving rate, appearing as a fitting parameter in the original methodology, is replaced by a set of state-dependent broadening factors applied to the different single-particle lead levels. These broadening factors are extracted explicitly from the self-energy of the corresponding electronic reservoir and are fully transferable to any junction incorporating the same lead model. 2

We present insights into the lead-molecule coupling scheme in molecular electronics junctions. Using a "site-to-state" transformation that provides direct access to the coupling matrix elements between the molecular states and the eigenstate manifold of each lead, we find coupling bands whose character depends on the geometry and dimensionality of the lead. We use a standard tight-binding model to elucidate the origin of the coupling bands and explain their nature via simple "particle-in-a-box" type considerations. We further show that these coupling bands can shed light on the charge transport behavior of the junction. The picture presented in this study is not limited to the case of molecular electronics junctions and is relevant to any scenario where a finite molecular entity is coupled to a (semi)infinite system.

A non-orthogonal localized basis-set implementation of the driven Liouville von Neumann (DLvN) approach is presented. The method is based on block-orthogonalization of the Hamiltonian and overlap matrix representations, yielding non-overlapping blocks that correspond to the various system sections.An extended Hückel description of the Gold/benzene-dithiol/Gold junction is used to demonstrate the performance of the method. The presented generalization is an important milestone towards using the DLvN approach for performing accurate dynamic electronic transport calculations in realistic model systems, based on density functional theory packages that rely on atom-centered basis set representations.

Sequence of steps in the reaction of hydronium (H3O+) with sulfonate (SO3−) contributing to the proton structural diffusion mechanism in a low-hydration environment of a proton-exchange-membrane (PEM) mimic.