Electrochemistry is central to many applications, ranging from biology to energy science. Studies now involve a wide range of techniques, both experimental and theoretical. Modelling and simulations methods, such as density functional theory or molecular dynamics, provide key information on the structural and dynamic properties of the systems. Of particular importance are polarization effects the electrode/electrolyte interface, which are difficult to simulate accurately. Here we show how these electrostatic interactions are taken into account in the framework of the Ewald summation method. We discuss, in particular, the formal set up for calculations that enforce periodic boundary conditions in two directions, a geometry that more closely reflects the characteristics of typical electrolyte/electrode systems and presents some differences with respect to the more common case of periodic boundary conditions in three dimensions. These formal developments are implemented and tested in MetalWalls, a molecular dynamics software which captures the polarization of the electrolyte and allows the simulation of electrodes maintained at a constant potential. We also discuss the technical aspects involved in the calculation of two sets of coupled degrees of freedom, namely the induced dipoles and the electrode charges. We validate the implementation, first on simple systems, then on the well-known interface between graphite electrodes and a room-temperature ionic liquid. We finally illustrate the capabilities of MetalWalls by studying the adsorption of a complex functionalized electrolyte on a graphite electrode.

Metastable photoacids (mPAH), such as protonated merocyanine, are organic molecules that release protons under illumination, providing spatiotemporal control of pH. While merocyanine-type photoacids enable fully reversible pH cycling in water, their solubility is limited and chemical stability <24 hours. In recent studies, structure modifications have been the main direction in improving photoacid properties. In this work we introduce a new pathway to increase stability and solubility of photoacids by tuning their solvation in binary solvent mixtures. We show that a modified solvation environment supports highly reversible, large and stable pH modulation. We demonstrate >350 hours of light-induced pH cycling with 10 times more protons released per cycle in water-DMSO mixtures compared to pure water. This solva-tion engineering approach can be applied to other metastable photoacids, serving as a steppingstone towards practical use in applications where long-term stability is critical.

Redox-active organic species play an important role in catalysis, energy storage, and biotechnology. One of the representatives is 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical, used as a mediator in organic synthesis and considered a safe alternative to heavy metals. In order to develop a TEMPO-based system with well-controlled electrochemical and catalytic properties, a reaction medium should be carefully chosen. Being highly conductive, stable, and low flammable fluids, ionic liquids (ILs) seem to be promising solvents with easily adjustable physical and solvation properties. In this work, we give an insight into the local structure of ILs around TEMPO and its oxidized form, TEMPO+, underlining striking differences in solvation of these two species. The analysis is coupled with a study of thermodynamics and kinetics of oxidation in the frame of Marcus theory. Our systematic investigation includes imidazolium, pyrrolydinium, and phosphonium families combined with anions of different size, polarity, and flexibility, opting to provide a clear and comprehensive picture of the impact of the nature of IL ions on the behavior of radical/cation redox pair. The obtained results will help to explain experimentally observed effects and to rationalize the design of TEMPO/IL systems.