I. INTRODUCTION
This issue of the Journal of Chemical Physics highlights recent developments in the study of electrode-electrolyte interfaces. At such an interface, the electrolyte phase is anisotropic and often subject to large electric fields originating from the electrodes and modulated by the screening response of the electrolyte. These fields serve, for better or worse, to drive the transfer of charge (e.g., electrons and protons), the migration of ionic species, and the reorientation of polar solvent molecules. Due to these phenomena, and others such as specific adsorption, electrode-electrolyte interfaces support many chemical and physical processes that are not favorable in bulk electrolyte environments. A wide range of scientific and industrial technologies have long leveraged these interfaces for applications including catalysis, chemical synthesis, energy storage and conversion, corrosion prevention, and electrical and chemical sensing.
Despite their technological importance, much remains to be understood about the chemical and physical properties of electrode-electrolyte interfaces. For example, electrode nanostructure can lead to changes in interfacial reactivity that are not easily explained with standard theory. Coupling between electron and ion (e.g., proton) transport can complicate reaction mechanisms in ways that are difficult to uncover based on typical electrochemical experiments. Furthermore, the molecular scale structure and dynamics of these interfaces remain cloudy due to limitations in simulation and spectroscopic characterization. All of these problems, and many more, require continued advances in both experiment and theory.
This issue includes 31 manuscripts addressing various aspects of electrode-electrolyte interfaces and the systems they comprise. Some manuscripts focus on the chemical reactivity of these systems. Others study nonreactive effects, such as the influence of the electrode on the properties of the electrolyte solution. Many manuscripts address the role of these interfaces in applications of energy conversion and storage, such as in batteries and supercapacitors, while some aim to advance conceptual frameworks, theoretical models, and experimental methodologies. Taken together, this body of literature provides an overview of some of the most pressing scientific problems surrounding electrode-electrolyte interfaces and highlights the exciting studies that are being performed to solve these problems.
II. SUMMARY OF AREAS COVERED
As part of this commentary, we summarize the contents of the issue categorically. The accepted manuscripts could be roughly sorted into four different categories, as described in the subsections below. Of course, many of the manuscripts described below could easily fall under multiple categories, including those beyond the four we highlight below.
A. Electrochemical reactivity
Electrode-electrolyte interfaces enable modes of chemical reactivity and chemical control that are not available in standard bulk-phase environments. In this issue, Spencer et al.1 and Wei et al.2 explore the roles of proton insertion in enhancing the activity of electrodes for the hydrogen evolution reaction. Goyal and Koper consider the influence of mass transport conditions on the kinetics of the same reaction.3 Botello et al. study the thermodynamics of hydrogen adsorption onto Pt electrodes, an important step in a variety of electrochemical reactions.4 Wang and Abruna derive new insight into the mechanism of methanol and formic acid oxidation.5 Wu et al. explore the influence of hydrogen bond donor molecules on the stability of Au surfaces in deep eutectic solvents.6 Gibson et al.7 use molecular dynamics (MD) simulations to study the influence of ion solvation effects on electrokinetic properties and Qin et al.8 us ab initio simulation to study CO activation.
B. Properties of the adlayer and the electric double-layer
The region of an electrolyte solution in close proximity to the electrode is often referred to as the electric double-layer (EDL). Within the double layer, the properties of the electrolyte solution can differ significantly from those the bulk. Many contributions to this issue focus on improving our understanding of the EDL. Kim et al. use spectroscopy to study double-layer structure.9 Li et al. use ab initio MD simulation to examine the influence of stepped electrode surfaces EDL properties.10 Xu and Jiang use ab initio simulation to study proton dynamics in water confined within electrode heterostructures.11 Shandilya et al. explore the effects of molecular asymmetry on the interfacial capacitance of water-electrode interfaces.12 Mikkelsen et al. and Davile López et al. use MD simulation to analyze the properties of water adlayers on Pt electrodes.13,14 Serva et al. consider how hydrophobic effects influence the thermodynamics of electrochemical reactions.15
C. Electrode-electrolyte interfaces for energy storage applications
Many studies focus on advancing the field of energy storage. Halim et al. study interfacial charge storage via ion exchange.16 Morey et al. investigate how electrode patterning can help guide the deposition of Li+ in batteries.17 Wang et al. explore novel electrode materials for Li-ion batteries.18 Verkholyak et al. and Lahrar et al. use theory to study the effects of dimensional confinement on capacitive energy storage.19,20 A few contributions focus on aqueous electrolytes at very high ion concentration (so-called water-in-salt solutions). Ueno et al. describe how high concentrations of electrolytes can increase the chemical stability of water molecules,21 and Degoulange et al. consider ion activities and liquid junction potentials.22
D. Developing concepts, models, and methods
Several authors contributed advances in theoretical or experimental methodology. Isogai et al. develop spectroscopic probes of interfacial vibrations.23 Ahrens-Iwers and Meiβner introduce a new package for simulating constant potential electrodes in MD simulation.24 Maxian et al. present a method for computing electrostatic energies in periodic double-slit systems.25 Steinrück models cyclic voltammetry during the formation of solid-electrolyte interface. Some papers present new conceptual advances. Montes-Campos et al. study the role of solvation in the approach of alkali ions to electrodes in ionic liquid solvents.26 Pireddu et al. present molecular insight into the induction of charge on electrodes.27 Rustam et al. use simulation to explore the interplay be-tween surface functionalization and electrolyte structure and dynamics.28 Cats and van Roij draw connections between differential capacitance and the EDL structure.29
III. CONCLUSIONS
This special issue collects a wide range of exciting studies aimed at furthering our understanding of the electrode-electrolyte interface. The breadth of approaches highlights the synergy that has emerged in the community that studies these systems. While much remains to be understood about these systems, the outstanding efforts of this scientific community will continue to advance our understanding, drive technological advances, and improve device safety, performance, and efficiency. The manuscripts that follow highlight these efforts.
ACKNOWLEDGMENTS
The guest editors would like to thank all the authors who contributed to this issue. We are also grateful for the work of the journal editors and the reviewers who strive to maintain the high quality of this Journal. Finally, we thank the editorial staff for all their help in organizing this special issue.