Dielectric permittivity is central to many biological and physiochemical systems, as it affects the long-range electrostatic interactions. Similar to many fluid properties, confinement greatly alters the dielectric response of polar liquids. Many studies have focused on the reduction of the dielectric response of water under confinement. Here, using molecular dynamics simulations, statistical-mechanical theories, and multiscale methods, we study the out-of-plane (z-axis) dielectric response of protic and aprotic fluids confined inside slit-like graphene channels. We show that the reduction in perpendicular permittivity is universal for all the fluids and exhibits a Langevin-like behavior as a function of channel width. We show that this reduction is due to the favorable in-plane (x–y plane) dipole–dipole electrostatic interactions of the interfacial fluid layer. Furthermore, we observe an anomalously low dielectric response under an extreme confinement.

Electric double layers (EDLs), occurring ubiquitously at solid−liquid interfaces, are critical for electrochemical energy conversion and storage processes such as capacitive charging and redox reactions. However, to date the molecular-scale structure of EDLs remains elusive. Here we report an advanced technique, electrochemical three-dimensional atomic force microscopy (EC-3D-AFM), and use it to directly image the molecular-scale EDL structure of an ionic liquid under different electrode potentials. We observe not only multiple discrete ionic layers in the EDL on a graphite electrode but also a quasi-periodic molecular density distribution within each layer. Furthermore, we find pronounced 3D reconfiguration of the EDL at different voltages, especially in the first layer. Combining the experimental results with molecular dynamics simulations, we find potential-dependent molecular redistribution and reorientation in the innermost EDL layer, both of which are critical to EDL capacitive charging. We expect this mechanistic understanding to have profound impacts on the rational design of electrode− electrolyte interfaces for energy conversion and storage.

Conversion of CO2 to energy‐rich chemicals using renewable energy is of much interest to close the anthropogenic carbon cycle. However, the current photoelectrochemical systems are still far from being practically feasible. Here the successful demonstration of a continuous, energy efficient, and scalable solar‐driven CO2 reduction process based on earth‐abundant molybdenum disulfide (MoS2) catalyst, which works in synergy with an inexpensive hybrid electrolyte of choline chloride (a common food additive for livestock) and potassium hydroxide (KOH) is reported. The CO2 saturated hybrid electrolyte utilized in this study also acts as a buffer solution (pH ≈ 7.6) to adjust pH during the reactions. This study reveals that this system can efficiently convert CO2 to CO with solar‐to‐fuel and catalytic conversion efficiencies of 23% and 83%, respectively. Using density functional theory calculations, a new reaction mechanism in which the water molecules near the MoS2 cathode act as proton donors to facilitate the CO2 reduction process by MoS2 catalyst is proposed. This demonstration of a continuous, cost‐effective, and energy efficient solar driven CO2 conversion process is a key step toward the industrialization of this technology.

The electrical-double layer (EDL) model is fundamental to our understanding of interactions in ionic solutions, and is widely used in chemical, biological, and technological contexts, particularly in the description of aqueous electrolyte solutions. However, recent experiments have raised questions regarding the validity of this model in polar, aprotic solvents; some observations, such as a surface potential that changes sign with increasing salt concentration, are not consistent with the EDL picture. We demonstrate in a model system, acetonitrile at a silica interface, that solvent organization dictates the ionic distributions. Ion-transport measurements in nanopores, surface-selective spectroscopy, and molecular dynamics simulations reveal that the distribution of ions in acetonitrile at a silica interface is determined by the lipid-bilayer-like organization that the interface imposes upon the liquid, which accounts for the change in sign of the potential. Our findings emphasize the importance of including solvent molecules and ions explicitly in descriptions of solid/liquid interfaces.

We develop coarse-grained force fields (CGFFs) for computationally efficient and accurate molecular simulation of imidazolium-based ionic liquids. To obtain CGFF parameters, we employ a systematic coarse-graining approach based on the relative entropy (RE) method to reproduce not only the structure but also the thermodynamic properties of the reference all-atom molecular model. Our systematic coarse-graining approach adds a constraint to the RE minimization using the Lagrange multiplier method in order to reproduce thermodynamic properties such as pressure. The Boltzmann inversion technique is used to obtain the bonded interactions, and the non-bonded and long-range electrostatic interactions are obtained using the constrained relative entropy method. The structure and pressure obtained from the coarse-grained (CG) models for different alkyl chain lengths are in agreement with the all-atom molecular dynamics simulations at different thermodynamic states. We also find that the dynamical properties, such as diffusion, of the CG model preserve the faster dynamics of bulky cation compared to the anion. The methodology developed here for reproduction of thermodynamic properties and treatment of long-range Coulombic interactions is applicable to other soft-matter.