Greg Birkett

A review of molecular modelling of electric double layer capacitors

Electric double-layer capacitors are a family of electrochemical energy storage devices that offer a number of advantages, such as high power density and long cyclability. In recent years, research and development of electric double-layer capacitor technology has been growing rapidly, in response to the increasing demand for energy storage devices from emerging industries, such as hybrid and electric vehicles, renewable energy, and smart grid management. The past few years have witnessed a number of significant research breakthroughs in terms of novel electrodes, new electrolytes, and fabrication of devices, thanks to the discovery of innovative materials (e.g. graphene, carbide-derived carbon, and templated carbon) and the availability of advanced experimental and computational tools. However, some experimental observations could not be clearly understood and interpreted due to limitations of traditional theories, some of which were developed more than one hundred years ago. This has led to significant research efforts in computational simulation and modelling, aimed at developing new theories, or improving the existing ones to help interpret experimental results. This review article provides a summary of research progress in molecular modelling of the physical phenomena taking place in electric double-layer capacitors. An introduction to electric double-layer capacitors and their applications, alongside a brief description of electric double layer theories, is presented first. Second, molecular modelling of ion behaviours of various electrolytes interacting with electrodes under different conditions is reviewed. Finally, key conclusions and outlooks are given. Simulations on comparing electric double-layer structure at planar and porous electrode surfaces under equilibrium conditions have revealed significant structural differences between the two electrode types, and porous electrodes have been shown to store charge more efficiently. Accurate electrolyte and electrode models which account for polarisation effects are critical for future simulations which will consider more complex electrode geometries, particularly for the study of dynamics of electrolyte transport, where the exclusion of electrode polarisation leads to significant artefacts.

Progress on the Surface Nanobubble Story: What is in the bubble? Why does it exist?

Interfaces between aqueous solutions and hydrophobic solid surfaces are important in various areas of science and technology. Many researchers have found that forces between hydrophobic surfaces in aqueous solution are significantly different from the classical DLVO theory. Long-range attractive forces (non-DLVO forces) are thought to be affected by nanoscopic gaseous domains at the interfaces. This is a review of the latest research on nanobubbles at hydrophobic surfaces from experimental and simulation studies. The review focusses on non-intrusive optical view of surface nanobubbles and gas enrichment on solid surfaces by imaging and force mapping. By use of these recent experimental data in conjunction with molecular simulation work, all major theories on surface nanobubble formation and stability are critically reviewed. Even though the current body of research cannot comprehensively explain all properties of surface nanobubbles observed, the fundamental understanding has been significantly improved. Line tension has been shown to be incapable of explaining the contact angle of nanobubbles. Dense gas layer theory provides a new explanation on both large contact angle and long-time stability. The high density of gas in these domains may significantly affect the gas-water interface which is in line with some observation made on bulk nanobubbles. Along this line of inquiry, experimental and simulation effort should be focussed on measuring the density within surface nanobubbles and the properties of the gas water interface which may be the key to explaining the stability of these nanobubbles.

Capacitance of Nanoporous Carbon-Based Supercapacitors Is a Trade-Off between the Concentration and the Separability of the Ions

Nanoporous carbon-based supercapacitors store electricity through adsorption of ions from the electrolyte at the surface of the electrodes. Room temperature ionic liquids, which show the largest ion concentrations among organic liquid electrolytes, should in principle yield larger capacitances. Here, we show by using electrochemical measurements that the capacitance is not significantly affected when switching from a pure ionic liquid to a conventional organic electrolyte using the same ionic species. By performing additional molecular dynamics simulations, we interpret this result as an increasing difficulty of separating ions of opposite charges when they are more concentrated, that is, in the absence of a solvent that screens the Coulombic interactions. The charging mechanism consistently changes with ion concentration, switching from counterion adsorption in the diluted organic electrolyte to ion exchange in the pure ionic liquid. Contrarily to the capacitance, in-pore diffusion coefficients largely depend on the composition, with a noticeable slowing of the dynamics in the pure ionic liquid.

Origin of Interfacial Nanoscopic Gaseous Domains and Formation of Dense Gas Layer at Hydrophobic Solid–Water Interface

Interfacial gas enrichment (IGE) covering the entire area of hydrophobic solid-water interface has recently been detected by atomic force microscopy (AFM) and hypothesized to be responsible for the unexpected stability and anomalous contact angle of gaseous nanobubbles and the significant change from DLVO to non-DLVO forces. In this paper, we provide further proof of the existence of IGE in the form of a dense gas layer (DGL) by molecular dynamic simulation. Nitrogen gas adsorption at the water-graphite interface is investigated using molecular dynamic simulation at 300 K and 1 atm normal pressure. The results show that a DGL with a density equivalent to a gas at pressure of 500 atm is formed and equilibrated with a normal pressure of 1 atm. By varying the number of gas molecules in the system, we observe several types of dense gas domains: aggregates, cylindrical caps, and DGLs. Spherical cap gas domains form during the simulation but are unstable and always revert to another type of gas domain. Furthermore, the calculated surface potential of the DGL-water interface, -17.5 mV, is significantly closer to 0 than the surface potential, -65 mV, of normal gas bubble-water interface. This result supports our previously stated hypothesis that the change in surface potential causes the switch from repulsion to attraction for an AFM tip when the graphite surface is covered by an IGE layer. The change in surface potential comes from the structure change of water molecules at the DGL-water interface as compared with the normal gas-water interface. In addition, the contact angle of the cylindrical cap high density nitrogen gas domains is 141°. This contact angle is far greater than 85° observed for water on graphite at ambient conditions and much closer to the 150° contact angle observed for nanobubbles in experiments.

Simulation study of methanol and ethanol adsorption on graphitized carbon black

GCMC simulations are applied to the adsorption of sub-critical methanol and ethanol on graphitized carbon black at 300 K. The carbon black was modelled both with and without carbonyl functional groups. Large differences are seen between the amounts adsorbed for different carbonyl configurations at low pressure prior to monolayer coverage. Once a monolayer has been formed on the carbon black, the adsorption behaviour is similar between the model surfaces with and without functional groups. Simulation isotherms for the case of low carbonyl concentrations or no carbonyls are qualitatively similar to the few experimental isotherms available in the literature for methanol and ethanol adsorption on highly graphitized carbon black. Isosteric heats and adsorbed phase heat capacities are shown to be very sensitive to carbonyl configurations. A maximum is observed in the adsorbed phase heat capacity of the alcohols for all simulations but is unrealistically high for the case of a plain graphite surface. The addition of carbonyls to the surface greatly reduces this maximum and approaches experimental data with carbonyl concentration as low as 0.09 carbonyls/nm2.

The adsorption of water in finite carbon pores

Grand canonical Monte Carlo simulations were applied to the adsorption of SPCE model water in finite graphitic pores with different configurations of carbonyl functional groups on only one surface and several pore sizes. It was found that almost all finite pores studied exhibit capillary condensation behaviour preceded by adsorption around the functional groups. Desorption showed the reverse transitions from a filled to a near empty pore resulting in a clear hysteresis loop in all pores except for some of the configurations of the 1.0 nm pore. Carbonyl configurations had a strong effect on the filling pressure of all pores except, in some cases, in 1.0 nm pores. A decrease in carbonyl neighbour density would result in a higher filling pressure. The emptying pressure was negligibly affected by the configuration of functional groups. Both the filling and emptying pressures increased with increasing pore size but the effect on the emptying pressure was much less. At pressures lower than the pore filling pressure, the adsorption of water was shown to have an extremely strong dependence on the neighbour density with adsorption changing from Type IV to Type III to linear as the neighbour density decreased. The isosteric heat was also calculated for these configurations to reveal its strong dependence on the neighbour density. These results were compared with literature experimental results for water and carbon black and found to qualitatively agree.

The impact of line tension on the contact angle of nanodroplets

The line tension for a Lennard–Jones (LJ) fluid on a (9, 3) solid of varying strength was calculated using Monte Carlo simulations. A new perturbation method was used to determine the interfacial tension between liquid–vapour, solid–liquid and solid–vapour phases for this system to determine the Youngs equation contact angle. Cylindrical and spherical nanodroplets were simulated for comparison. The contact angles from the cylindrical drops and Youngs equation agree very well over the range of surface strengths and cylindrical drop sizes, except on a very weak surface. Tolman length effects were not observable for cylindrical drops. This shows that quite small systems can reproduce macroscopic contact angles. For spherical droplets, a deviation between the contact angle of spherical droplets and Youngs equation was evident, but decreased with increasing interaction strengths to be negligible for contact angles less than 90°. Linear fitting of the contact angle data for varying droplet sizes showed no clear effect by line tension on contact angle. All calculated line tension values have a magnitude less than 4 × 10− 12 J/m with both negative and positive signs. The best estimate of line tension for this system of LJ droplets was 1 × 10− 13 J/m, which is smaller than the reported estimations in the literature, and is too small to be conclusively positive or negative in value.

Determination of contact angle by molecular simulation using number and atomic density contours

The contact angles of Lennard-Jones fluid droplets on a structureless solid surface, simulated using Monte Carlo simulation, are calculated by fitting isochoric surfaces and making a number of assumptions about the droplet. The results show that there are significant uncertainties in the calculated contact angles due to the choice of these assumptions, such as the grid size used in tracking the isochoric density profile, the omission of isochoric data points near the surface and the function used to fit the isochoric profile. In this study, we propose a new method of calculating density contours based on atomic density instead of number density. This method results in a much smaller variation in contact angle when applying different assumptions than using number density for isochoric contours. The most consistent results, across a range of assumptions about the droplet and the contact angle, come from averaging the contact angle from several isochoric density profiles. In addition, this gives a measurement of the variation due to the choice of isochoric density.

Mixed Matrix Carbon Molecular Sieve and Alumina (CMS-Al2O3) Membranes

This work shows mixed matrix inorganic membranes prepared by the vacuum-assisted impregnation method, where phenolic resin precursors filled the pore of α-alumina substrates. Upon carbonisation, the phenolic resin decomposed into several fragments derived from the backbone of the resin matrix. The final stages of decomposition (>650 °C) led to a formation of carbon molecular sieve (CMS) structures, reaching the lowest average pore sizes of ~5 Å at carbonisation temperatures of 700 °C. The combination of vacuum-assisted impregnation and carbonisation led to the formation of mixed matrix of CMS and α-alumina particles (CMS-Al2O3) in a single membrane. These membranes were tested for pervaporative desalination and gave very high water fluxes of up to 25 kg m−2 h−1 for seawater (NaCl 3.5 wt%) at 75 °C. Salt rejection was also very high varying between 93–99% depending on temperature and feed salt concentration. Interestingly, the water fluxes remained almost constant and were not affected as feed salt concentration increased from 0.3, 1 and 3.5 wt%.

Effects of surface mediation on the adsorption isotherm and heat of adsorption of argon on graphitized thermal carbon black

In this paper, the effects of surface mediation on the adsorption isotherm and isosteric heat of adsorption on a graphite surface were investigated, as the surface mediation is known to affect the intermolecular interaction of adsorbed molecules close to the surface. Kim and Steele (Phys. Rev. B 45 (11) (1992) 6226-6233) and others have assumed that the surface mediation is confined only to the first layer. This will be tested in this paper with a combined experimental and Grand Canonical Monte Carlo (GCMC) simulation of adsorption of argon on graphitized thermal carbon black (GTCB) over a range of temperatures (77-95.25K). By matching the simulation results against the experimental data, we have found that the surface mediation is extended up to the fourth layer, rather than only the first as suggested by Kim and Steele, and the extent of this mediation is reduced with distance from the surface. This reinforces the important role of surface on the intermolecular interaction. With regard to the heat of adsorption, we found that the isosteric heats obtained directly from the simulation agree fairly well with the heats calculated from the application of the Clausius-Clapeyron equation on experimental isotherms of 77 and 87.3K. The temperature dependence of the isosteric heat was investigated with the GCMC simulation results. One interesting observation is the existence of a heat spike at 77K and its absence at higher temperatures, a phenomenon which is common to both simulation results and experimental data. This lends good support to the molecular model with surface mediation as a proper one to describe adsorption of noble gases on GTCB.