Svyatoslav Kondrat

Accelerating charging dynamics in subnanometre pores

Supercapacitors have exceptional power density and cyclability but smaller energy density than batteries. Their energy density can be increased using ionic liquids and electrodes with subnanometre pores, but this tends to reduce their power density and compromise the key advantage of supercapacitors. To help address this issue through material optimization, here we unravel the mechanisms of charging subnanometre pores with ionic liquids using molecular dynamics simulations, navigated by a phenomenological model. We show that charging of ionophilic pores is a diffusive process, often accompanied by overfilling followed by de-filling. In sharp contrast to conventional expectations, charging is fast because ion diffusion during charging can be an order of magnitude faster than in the bulk, and charging itself is accelerated by the onset of collective modes. Further acceleration can be achieved using ionophobic pores by eliminating overfilling/de-filling and thus leading to charging behaviour qualitatively different from that in conventional, ionophilic pores.

Dynamics of Ion Transport in Ionic Liquids.

A gap in understanding the link between continuum theories of ion transport in ionic liquids and the underlying microscopic dynamics has hindered the development of frameworks for transport phenomena in these concentrated electrolytes. Here, we construct a continuum theory for ion transport in ionic liquids by coarse graining a simple exclusion process of interacting particles on a lattice. The resulting dynamical equations can be written as a gradient flow with a mobility matrix that vanishes at high densities. This form of the mobility matrix gives rise to a charging behavior that is different to the one known for electrolytic solutions, but which agrees qualitatively with the phenomenology observed in experiments and simulations.

The effect of composition on Diffusion of macromolecules in a crowded environment

We study diffusion of macromolecules in a crowded cytoplasm-like environment, focusing on its dependence on composition and its crossover to the anomalous subdiffusion. The crossover and the diffusion itself depend on both the volume fraction and the relative concentration of macromolecules. In accordance with previous theoretical and experimental studies, diffusion slows down when the volume fraction increases. Contrary to expectations, however, the diffusion is also strongly dependent on the molecular composition. The crossover time decreases and diffusion slows down when the smaller macromolecules start to dominate. Interestingly, diffusion is faster in a cytoplasm-like (more polydisperse) system than it is in a two-component system, at comparable packing fractions, or even when the cytoplasm packing fraction is larger.

Does metabolite channeling accelerate enzyme-catalyzed cascade reactions?

Metabolite or substrate channeling is a direct transfer of metabolites from one enzyme to the next enzyme in a cascade. Among many potential advantages of substrate channeling, acceleration of the total reaction rate is considered as one of the most important and self-evident. However, using a simple model, supported by stochastic simulations, we show that it is not always the case; particularly at long times (i.e. in steady state) and high substrate concentrations, a channeled reaction cannot be faster, and can even be slower, than the original non-channeled cascade reaction. In addition we show that increasing the degree of channeling may lead to an increase of the metabolite pool size. We substantiate that the main advantage of channeling likely lies in protecting metabolites from degradation or competing side reactions.

Phase behaviour and structure of a superionic liquid in nonpolarized nanoconfinement

The ion-ion interactions become exponentially screened for ions confined in ultranarrow metallic pores. To study the phase behaviour of an assembly of such ions, called a superionic liquid, we develop a statistical theory formulated on bipartite lattices, which allows an analytical solution within the Bethe-lattice approach. Our solution predicts the existence of ordered and disordered phases in which ions form a crystal-like structure and a homogeneous mixture, respectively. The transition between these two phases can potentially be first or second order, depending on the ion diameter, degree of confinement and pore ionophobicity. We supplement our analytical results by three-dimensional off-lattice Monte Carlo simulations of an ionic liquid in slit nanopores. The simulations predict formation of ionic clusters and ordered snake-like patterns, leading to characteristic close-standing peaks in the cation-cation and anion-anion radial distribution functions.

Capacitance-Power-Hysteresis Trilemma in Nanoporous Supercapacitors

Nanoporous supercapacitors are an important player in the field of energy storage that fill the gap between dielectric capacitors and batteries. The key challenge in the development of supercapacitors is the perceived trade-off between capacitance and power delivery. Current efforts to boost the capacitance of nanoporous supercapacitors focus on reducing the pore size so that they can only accommodate a single layer of ions. However, this tight packing compromises the charging dynamics and hence power density. We show via an analytical theory and Monte Carlo simulations that charging is sensitively dependent on the affinity of ions to the pores, and that high capacitances can be obtained for ionophobic pores of widths significantly larger than the ion diameter. Our theory also predicts that charging can be hysteretic with a significant energy loss per cycle for intermediate ionophilicities. We use these observations to explore the parameter regimes in which a capacitance-power-hysteresis trilemma may be avoided.

Discrete-continuous reaction-diffusion model with mobile point-like sources and sinks

Abstract.In many applications in soft and biological physics, there are multiple time and length scales involved but often with a distinct separation between them. For instance, in enzyme kinetics, enzymes are relatively large, move slowly and their copy numbers are typically small, while the metabolites (being transformed by these enzymes) are often present in abundance, are small in size and diffuse fast. It seems thus natural to apply different techniques to different time and length levels and couple them. Here we explore this possibility by constructing a stochastic-deterministic discrete-continuous reaction-diffusion model with mobile sources and sinks. Such an approach allows in particular to separate different sources of stochasticity. We demonstrate its application by modelling enzyme-catalysed reactions with freely diffusing enzymes and a heterogeneous source of metabolites. Our calculations suggest that using a higher amount of less active enzymes, as compared to fewer more active enzymes, reduces the metabolite pool size and correspondingly the lag time, giving rise to a faster response to external stimuli. The methodology presented can be extended to more complex systems and offers exciting possibilities for studying problems where spatial heterogeneities, stochasticity or discreteness play a role.Graphical abstract

Two tributaries of the electrical double layer

This special issue of Journal of Physics: Condensed Matter focuses on physical aspects of capacitive energy storage and harvesting devices based on electrical double layers. An electrical double layer (EDL) is an interfacial structure in which charges are separated between two neighboring phases or layers. Originally introduced by Helmholtz in 1850, the concept of EDLs has found numerous applications in various areas. Perhaps two of the fastest growing areas are the capacitive energy storage and energy harvesting. This special issue covers a wide range of topics in these areas, ranging from solving fundamental problems to proposing specific working devices. Electrical double layer capacitors (i.e., capacitive energy storage systems based on EDLs) were first developed in the labs of General Electric (late 1950s) and Standard Oil of Ohio (1960s). However unusual it may sound, they were abandoned as unusable, but luckily reborn by the NEC Corporation who marketed them as ‘supercapacitors’ in 1978. Supercapacitors consist of porous electrodes immersed in an electrolyte medium and store energy in the EDLs at the electrode/electrolyte interfaces [1, 2]. As a result of this simple setup that typically involves no chemical reaction, supercapacitors are distinguished by high power densities and unusual cyclability (reaching more than 1000 000 cycles), but have only moderate energy densities. Largely for the latter reason, supercapacitors did not play an important role in the mainstream energy storage applications historically. However, recent breakthroughs [3, 4] in developing novel electrode materials and room-temperature ionic liquids (ILs) have given a new life to supercapacitors, which now complement and in some cases even compete with the conventional energy storage devices such as batteries. Indeed, the market of then-abandoned supercapacitors was worth about

Superionic state in double-layer capacitors with nanoporous electrodes

1.8 billion in 2014 and was estimated to show a year-on-year growth rate of about 9.2% according to some sources [5]. Works on supercapacitors appear now in a wide spectrum of journals covering chemical, material, energy, nanotechnology and applied physics sciences (for the latest review see e.g. [6]). In this special issue we attempted to collect papers focusing specifically on the physical aspects of the supercapacitor research, in all their complexity and depth, highlighting new challenges and yet unresolved problems. A distinct focus is however on electrolytes and electrolyte-electrode interactions. Why is this important? The energy stored in a supercapacitor is roughly CV2/2, where C is capacitance and V the applied voltage. Now, by increasing the voltage from 1 V (electrochemical window of a typical aqueous electrolyte) to 3 V or more (room-temperature ionic liquids), we increase the stored energy almost by an order of magnitude! On the other hand, it has recently become clear that adding an aqueous or organic component to room-temperature ILs reduces their viscosity and can dramatically accelerate charging [7]. However, this comes at a cost of the decreased stored energy because solvent can narrow the electrochemical window of an electrolyte. A sound understanding of the fundamental mechanisms of electrode-electrolyte interactions is thus needed to enable a significant leap in the performance of supercapacitors. This is where the physics can help improve supercapacitors as is the goal of many works including this special issue. A brief read-map is provided below to navigate the readers through this special issue. We start from the works by Docampo-Álvarez et al [8] and Vatamanu et al [9] who approach the discussed problems by using atomistic molecular dynamics simulations. Docampo-Álvarez et al deals specifically with the behaviour of water-IL mixtures in charged nanopores, while Vatamanu et al focus on the effects due to polar organic solvents (viz. acetonitrile). Both studies conclude that the presence of solvent can have a profound and sometimes unexpected effect on charging. For instance, confinement increases the amount of water in the positively charged electrode, but a decrease is observed on neutral and negatively charge surfaces [8]. Such effects must be accounted for when designing supercapacitors and choosing an optimal voltage regime. The experimental study of Ohkubo et al [10] deals with the hydration structures of calcium ions in non-polarized nanopores. They find that calcium ion-assisted adsorption of water falls off significantly when the pores become narrow (0.63 nm). This finding therefore S Kondrat et al