Dean R. Wheeler

Modeling of lithium-ion batteries

After reviewing the basic modeling framework for simulating battery behavior, three examples relating to mass-transfer effects are presented. Side reactions at the lithium electrode can change the surface concentration of lithium ions, introducing error into measurements of the cell potential as a function of bulk electrolyte concentration (concentration-cell measurements). This error introduced by a continuous side reaction is carried over into calculations of the transference number from the galvanostatic polarization method. Concentration gradients formed during passage of current are associated with a heat-of-mixing effect, which is the cause of heat generation during relaxation after cessation of the current. Finally, molecular dynamics simulations show that the decrease in conductivity with increasing salt concentration in liquid carbonate electrolytes is caused by ion association.

The Role of SPS, MPSA, and Chloride in Additive Systems for Copper Electrodeposition

This paper documents an experimental study to compare the behavior of bis-(3-sodiumsulfopropyl) disulfide (SPS)-Cl-polyethylene glycol (PEG) and 3-mercaptopropanesulfonic acid sodium salt (MPSA)-Cl-PEG additive systems during copper electrodeposition. These systems are analogous to those used industrially to plate copper lines onto integrated circuits. Galvanostatic experiments show that in the absence of chloride ion, either SPS or MPSA added to the plating solution inhibited copper deposition. Upon the addition of Cl - , a rapid transition from inhibition to acceleration was observed for both additives, illustrating the critical role played by the chloride ion in these systems. Potentiostatic experiments performed for the Cl-PEG-SPS and Cl-PEG-MPSA additive systems show that the potential dependency of the Cl-PEG-SPS system was much stronger than that of the MPSA-Cl-PEG system. Differences between the SPS and MPSA additive systems suggest that acceleration may occur through an MPSA pathway. Finally, results with chemical compounds similar to MPSA indicate that the thiol group is associated with inhibition and that a synergistic accelerating relationship exists between the sulfonate group and chloride, presumably due to complex formation.

Modeling of Particle-Particle Interactions in Porous Cathodes for Lithium-Ion Batteries

In this work, we present a mathematical model and associated experiments for describing the performance of porous electrodes under high rates of charge and discharge. By increasing the physical accuracy of porous battery modeling, we hope to enable improved design of cells for high-power applications, such as hybrid and plug-in-hybrid electric vehicles. The model includes an improved accounting of electron transfer between different-size particles or materials, including the conductive carbon additive, as well as a modified Bruggeman relation to handle liquid-phase ion transport through porous electrodes. Both types of resistance, electronic and liquid-phase ionic, are strongly coupled to particle properties, including size and volume-fraction distributions. The model is used to better understand the cause for decreased utilization of active material for relatively highly loaded lithium-ion cathodes at high discharge rates. It was found for Li x CoO 2 cathodes with loading around 1.6 mAh/cm 2 that voltage losses at 1C discharge rate are mostly governed by local interparticle resistances. At 5C discharge rate, diffusional resistance in the liquid electrolyte had the greatest influence on cell performance.

Non-equilibrium molecular dynamics simulation of the shear viscosity of liquid methanol: adaptation of the Ewald sum to Lees-Edwards boundary conditions

The Ewald sum method is commonly used in equilibrium simulations of polar fluids to enhance convergence of long-range Coulombic forces within modest-sized cubic simulation cells. In this work, we derive a form of the standard Ewald sum technique for use with non-equilibrium molecular dynamics (NEMD) simulations of viscosity that make use of the Lees–Edwards boundary conditions. This generalized Ewald sum can be used for any parallelepiped simulation cell. The method was tested by performing NEMD simulations at various temperatures and densities for simulated liquid methanol. The results were in excellent agreement with experimental data for methanol and Green–Kubo simulations of the viscosity using the standard cubic-cell Ewald sum. A simple truncation of the polar interactions at 10 A was found to produce errors of over 200% in the simulated viscosities. Values obtained with the polar interactions turned off (i.e. using only dispersion forces) were generally 40–60% below the experimental values. These re...

Shear viscosity of polar liquid mixtures via non-equilibrium molecular dynamics: water, methanol, and acetone

Non-equilibrium molecular dynamics (NEMD) with isobaric and isokinetic controls were used to simulate the shear viscosity for binary mixtures of water, methanol and acetone, and for ternary mixtures. In all, 22 different liquid composition points were simulated at 298.15 K and 0.1 MPa. A new set of acetone potential parameters was developed, while slight variants to existing water and methanol models were used. Long range Coulombic interactions were computed with the Ewald sum adapted to Lees-Edwards boundary conditions as formulated in Wheeler, D. R., Fuller, N. G., and Rowley, R. L., 1997, Molec. Phys., 92, 55. The attractive (dispersive) part of the Lennard-Jones (LJ) interactions also was handled by a lattice sum. A hybrid mixing rule was used for the LJ cross interactions. Viscosities extrapolated to zero shear compared well with experimental results, having a mean absolute error of 14% and no errors greater than 30%. Although the simulations successfully predicted viscosity maxima for mixtures high ...

Viologen Catalysts for a Direct Carbohydrate Fuel Cell

Deriving electrical energy from glucose and other carbohydrates under mild conditions is an important research objective because these biomolecules are abundant, renewable, have high energy density, and are convenient as fuels. This rich promise has not been realized because stable, inexpensive, and efficient catalysts are not available to oxidize carbohydrates and transfer all or nearly all of their electrons to fuel cell anodes. We report here that viologen catalysts meet these demanding criteria by catalytically oxidizing glucose and other carbohydrates in a mildly alkaline solution, making possible a direct carbohydrate fuel cell. Formate and carbonate are major products of carbohydrate oxidation, demonstrating that extensive carbon-carbon bond breaking has occurred. A rudimentary fuel cell utilizing viologen catalysts and glucose or dihydroxyacetone as fuels demonstrated electrical power production at up to 20 mA/cm 2 superficial current density. Improved catalyst function and cell design should significantly advance the efficiency and viability of direct carbohydrate fuel cell technology as a means of generating electrical energy from renewable biomass.

Correlation of chromatographic performance with morphological features of organic polymer monoliths.

Monoliths are considered to be a low pressure alternative to particle packed columns for liquid chromatography (LC). However, the chromatographic performance of organic monoliths, in particular, has still not reached the level of particle packed columns. Since chromatographic performance can be attributed to morphological features of the monoliths, in-situ characterization of the monolith structure in three dimensions would provide valuable information that could be used to help improve performance. In this work, serial sectioning and imaging were performed with a dual-beam scanning electron microscope for reconstruction and quantitative characterization of poly(ethylene glycol) diacrylate (PEGDA) monoliths inside a capillary column. Chord lengths, homogeneity factors, porosities and tortuosities were calculated from three-dimensional (3D) reconstructions of two PEGDA monoliths. Chromatographic efficiency was better for the monolith with smaller mean chord length (i.e., 5.23μm), porosity (i.e., 0.49) and tortuosity (i.e., 1.50) compared to values of 5.90μm, 0.59 and 2.34, respectively, for the other monolithic column. Computational prediction of tortuosity (2.32) was found to be in agreement with the experimentally measured value (2.34) for the same column. The monoliths were found to have significant radial heterogeneity since the homogeneity factor decreased from 5.39 to 4.89 (from center to edge) along the column radius.

A less expensive Ewald lattice sum

We present a treatment of the Ewald lattice sum which permits 25% or more decrease in program execution cost for the same level of accuracy. This is accomplished by optimizing on additional degrees of freedom introduced into the function that partitions the Coulombic potential between real- and reciprocal-space parts. The technique was tested in simulations of 1 M KCl in water. It is relatively simple to implement in existing codes, including those based on fast-Fourier-transform solutions to the lattice sum.

MPSA effects on copper electrodeposition investigated by molecular dynamics simulations

In superconformal filling of copper-chip interconnects, organic additives are used to fill high-aspect-ratio trenches or vias from the bottom up. In this study we report on the development of intermolecular potentials and use molecular dynamics simulations to provide insight into the molecular function of an organic additive (3-mercaptopropanesulfonic acid or MPSA) important in superconformal electrodeposition. We also investigate how the presence of sodium chloride affects the surface adsorption and surface action of MPSA as well as the charge distribution in the system. We find that NaCl addition decreases the adsorption strength of MPSA at a simulated copper surface and attenuates the copper-ion association with MPSA. The model also was used to simulate induced-charge effects and adsorption on a nonplanar electrode surface.

Intrinsic autotrophic biomass yield and productivity in algae: modeling spectral and mixing-rate dependence.

For non‐inhibitory irradiances, the rate of algal biomass synthesis was modeled as the product of the algal autotrophic yield ΦDW and the flux of photons absorbed by the culture, as described using Beer‐Lambert law. As a contrast to earlier attempts, the use of scatter‐corrected extinction coefficients enabled the validation of such approach, which bypasses determination of photosynthesis‐irradiance (PI) kinetic parameters. The broad misconception that PI curves, or the equivalent use of specific growth rate expressions independent of the biomass concentration, can be extended to adequately model biomass production under light‐limitation is addressed. For inhibitory irradiances, a proposed mechanistic model, based on the photosynthetic units (PSU) concept, allows one to estimate a target speed νT across the photic zone in order to limit the flux of photons per cell to levels averting significant reductions in ΦDW. These modeled target speeds, on the order of 5–20 m s(1 for high outdoor irradiances, call for fundamental changes in reactor design to optimize biomass productivity. The presented analysis enables a straightforward bioreactor parameterization, which, in‐turn, guides the establishment of conditions ensuring maximum productivity and complete nutrients consumption. Additionally, solar and fluorescent lighting spectra were used to calculate energy to photon‐counts conversion factors.