We present a 'computational microscopy' analysis (targeted molecular dynamics simulations) of the structure and performance of conductive metal organic framework (MOF) electrodes in supercapacitors with room temperature ionic liquids. The molecular modeling predicts the characteristic shapes of the potential dependence of electrode capacitance, relying on the structure of MOF electrodes and particularly how ions transport and reside in MOFs under polarization. Transmission line model was adopted to characterize the charging dynamics process and build up a bridge to evaluate the capacitive performance of practical supercapacitor devices at macroscale from the simulation-obtained data at nanoscale. Such nanoscale-to-macroscale analysis demonstrates the potential of MOF supercapacitors for achieving unprecedentedly high volumetric energy and power densities. The investigation gives molecular insights into the preferred structures of MOF for achieving these results, which could provide a blueprint for future experimental characterization of these new systems.

In concentrated electrolytes with asymmetric or irregular ions, such as ionic liquids and solvent-in-salt electrolytes, ion association is more complicated than simple ion-pairing. Large branched aggregates can form at significant concentrations at even moderate salt concentrations. When the extent of ion association reaches a certain threshold, a percolating ionic gel network can form spontaneously. Gelation is a phenomenon that is well known in polymer physics, but it is practically unstudied in concentrated electrolytes. However, despite this fact, the ion-pairing description is often applied to these systems for the sake of simplicity. In this work, drawing strongly from established theories in polymer physics, we develop a simple thermodynamic model of reversible ionic aggregation and gelation in concentrated electrolytes accounting for the competition between ion solvation and ion association. Our model describes, with the use of several phenomenological parameters, the populations of ionic clusters of different sizes as a function of salt concentration; it captures the onset of ionic gelation and also the post-gel partitioning of ions into the gel. We discuss the applicability of our model, as well as the implications of its predictions on thermodynamic, transport, and rheological properties.

Using molecular dynamics simulations and theoretical analysis of velocity-autocorrelation functions, we study ion transport mechanisms in typical room-temperature ionic liquids. We show that ions may reside in two states: free and bound with an interstate exchange. We investigate quantitatively the exchange process and reveal new qualitative features of this process. To this end, we propose a dynamic criterion for free and bound ions based on the ion trajectory density and demonstrate that this criterion is consistent with a static one based on interionic distances. Analyzing the trajectories of individual cations and anions, we estimate the time that ions spend in bound "clustered states" and when they move quasifreely. Using this method, we evaluate the average portion of "free" ions as approximately 15%-25%, increasing with temperature in the range of 300-600 K. The ion diffusion coefficients and conductivities as a function of the temperature calculated from the velocity and electrical-current autocorrelation functions reproduce the reported experimental data very well. The experimental data for the direct-current conductivity (constant ionic current) is in good agreement with theoretical predictions of the Nernst-Einstein equation based on the concentrations and diffusion coefficients of free ions obtained in our simulations. In analogy with electronic semiconductors, we scrutinize an "ionic semiconductor" model for ionic liquids, with valence and conduction "bands" for ions separated by an energy gap. The obtained band gap for the ionic liquid is small, around 26 meV, allowing for easy interchange between the two dynamic states. Moreover, we discuss the underscreening paradox in the context of the amount of free charge carriers, showing that the obtained results do not yet approve its simplistic resolution.

In supercapacitors based on ionic liquid electrolytes, small amounts of absorbed water could potentially reduce the electrochemical window of electrolytes and cause performance degradation. The same would take place if ionic liquids are used as solvents for electrocatalysis involving the dissolved molecular species. In this work, we carry out molecular dynamics simulations, with gold and carbon electrodes in typical ionic liquids, hydrophobic and hydrophilic, to study electrosorption of water. We investigate the effects of hydrophobicity/hydrophilicity of ionic liquids and electrodes on interfacial distribution of ions and electrosorbed water. Results reveal that using hydrophilic ionic liquids would help to keep water molecules away from the negatively charged electrodes, even at large electrode polarizations. This conclusion is supported by electrochemical cyclic voltammetry measurements on gold and carbon electrodes in contact with humid ionic liquids. Thereby, our findings suggest potential mechanisms for protection of electrodes from water electrosorption.

Humid hydrophobic ionic liquids—widely used as electrolytes—have narrowed electrochemical windows due to the involvement of water, absorbed on the electrode surface, in electrolysis. In this work, we performed molecular dynamics simulations to explore effects of adding Li salt in humid ionic liquids on the water adsorbed on the electrode surface. Results reveal that most of the water molecules are pushed away from both cathode and anode, by adding salt. The water remaining on the electrode is almost bound with Li+, having significantly lowered activity. The Li+-bonding and re-arrangement of the surface-adsorbed water both facilitate the inhibition of water electrolysis, and thus prevent the reduction of electrochemical windows of humid hydrophobic ionic liquids. This finding is testified by cyclic voltammetry measurements where salt-in-humid ionic liquids exhibit enlarged electrochemical windows. Our work provides the underlying mechanism and a simple but practical approach for protection of humid ionic liquids from electrochemical performance degradation.

As a class of porous materials with crystal lattices, metal–organic frameworks (MOFs), featuring outstanding specific surface area, tunable functionality, and versatile structures, have attracted huge attention in the past two decades. Since the first conductive MOF is successfully synthesized in 2009, considerable progress has been achieved for the development of conductive MOFs, allowing their use in diverse applications for electrochemical energy storage. Among those applications, supercapacitors have received great interest because of their high power density, fast charging ability, and excellent cycling stability. Here, the efforts hitherto devoted to the synthesis and design of conductive MOFs and their auspicious capacitive performance are summarized. Using conductive MOFs as a unique platform medium, the electronic and molecular aspects of the energy storage mechanism in supercapacitors with MOF electrodes are discussed, highlighting the advantages and limitations to inspire new ideas for the development of conductive MOFs for supercapacitors.

Water-in-salt electrolytes (WiSEs) are a class of super-concentrated electrolytes that have shown much promise in replacing organic electrolytes in lithium-ion batteries. At the extremely high salt concentrations of WiSEs, ionic association is more complicated than the simple ion pair description. In fact, large branched clusters can be present in WiSEs, and past a critical salt concentration, an infinite percolating ionic network can form spontaneously. In this work, we simplify our recently developed thermodynamic model of reversible ionic aggregation and gelation, tailoring it specifically for WiSEs. Our simplified theory only has a handful of parameters, all of which can be readily determined from simulations. Our model is able to quantitatively reproduce the populations of ionic clusters of different sizes as a function of salt concentration, the critical salt concentration for ionic gelation, and the fraction of ions incorporated into the ionic gel, as observed from molecular simulations of three different lithium-based WiSEs. The extent of ionic association and gelation greatly affects the effective ionic strength of solution, the coordination environment of active cations that is known to govern the chemistry of the solid-electrolyte interface, and the thermodynamic activity of all species in the electrolyte.

Using electrodes with subnanometer pores and ionic liquid electrolytes can improve the charge storage capacity at the expense of the charging rate. The fundamental understanding of the charging dynamics of nanoporous electrodes can help to avoid compromising the power density. In this work, we performed molecular dynamics simulations to reveal the charging mechanism of subnanometer pores in ionic liquids. Different from the traditional view that a smaller pore results in slower charging, a non-monotonic relation is found between the charging rate and pore size, in which the charging process is accelerated in some subnanometer pores. Our analysis uncovers that the mechanism of the charging enhancement can be attributed to the transition of in-pore ion structure.