Thomas A. Zawodzinski

Characterization of Polymer Electrolyte Fuel Cells Using AC Impedance Spectroscopy

The ac impedance spectra of polymer electrolyte fuel cell (PEFC) cathodes measured under various experimental conditions are analyzed. The measurements were carried out in the presence of large dc currents. The impedance spectrum of the air cathode is shown to contain two features : a higher frequency loop or are determined by interfacial charge-transfer resistance and catalyst layer properties and a lower frequency loop determined by gas-phase transport limitations in the backing. The lower frequency loop is absent from the spectrum of cathodes operating on pure oxygen. Properties of measured impedance spectra are analyzed by a PEFC model to probe the effect of ac perturbation. Comparison of model predictions to observed data is made by simultaneous least squares fitting of a set of spectra measured for several cathode potentials. The spectra reveal various charge and mass-transfer effects in the cathode catalyst layer and in the hydrophobic cathode backing. Three different types of losses caused by insufficient cell hydration, having to do with interfacial kinetics, catalyst layer proton conductivity, and membrane conductivity, are clearly resolved in these impedance spectra. The data reveal that the effective tortuous path length for gas diffusion in the cathode backing is about 2.6 times the backing thickness.

Intrinsic thermodynamic and kinetic properties of Sb electrodes for Li-ion and Na-ion batteries: experiment and theory

A detailed comparative study between the electrochemical lithiation and sodiation of pure antimony (Sb), relating changes in structural, thermodynamic, kinetic and electrochemical properties has been carried out. For this purpose, a wide range of measurements using electrochemical (galvanostatic cycling, GITT, PITT), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) methods as well as density functional theory (DFT) based investigations have been undertaken. Assessment of the thermodynamics reveals that the reaction proceeds identically during the first and second cycles for Li whereas it differs between the first and subsequent cycles for Na as the reaction with Na proceeds through a different pathway associated with the formation of amorphous NaxSb phases. For the first time we rationalize the amorphization of NaxSb phases by the long ranged strain propagation due to Na-vacancy compared to Li–Sb. At full discharge, our XRD results show for the first time that a minor fraction of hexagonal Li3Sb forms concomitantly with cubic Li3Sb. The XRD results confirm that Sb crystallizes into hexagonal Na3Sb at full sodiation. The kinetics of the reaction is assessed by rate performance tests which highlight that both Li and Na can diffuse rapidly throughout micron thick films at room temperature. However, it is found that the (de)insertion of Li provides lower overpotentials and larger storage capacities compared to Na. The difference in rate performance is complemented by diffusion coefficient determinations near the 0 V region where both materials are crystallized into M3Sb (M = Li, Na). Interestingly, calculations show that the energy barrier for near-neighbor vacancy migration, predominant in these close-packed phases, is about twice for Na than for Li. Our analysis tries to relate the lower intrinsic diffusivity of Na compared to Li with the long-range strain propagation induced by the former, thereby leading to an intrinsic origin of differences in rates, mechanical properties and amorphization. Finally, the surface chemistry of Sb electrodes cycled in NaClO4 dissolved in pure PC with(out) the addition of 5 wt% EC or FEC shows presence of ethers and NaF for the EC- and FEC-based electrolytes, respectively, and SEI films rich in Na-based carbonates.

Two-Phase Transport in PEM Fuel Cell Cathodes

To date, multiphase computational fluid dynamics models for proton exchange membrane (PEM) fuel cells failed to provide even a qualitative depiction of the fuel cell water management. This was primarily due to the inability to capture two-phase phenomena in the cathode catalyst layer and the water saturation equilibrium at the interface between the fuel cell components. A model without the cathode catalyst layer cannot capture dominant mechanisms of water transfer and cannot explain correctly the fuel cell performance. We propose a multifluid, multiphase model consisting of separate transport equations for each phase. The model accounts for gas- and liquid-phase momentam and species transport in the cathode channel, gas diffusion layer (GDL), and catalyst layer and for the current density, ionomer-phase potential, and water content in the catalyst coated membrane. The model considers water produced at cathode by (I) electrochemical reaction, (II) change of phase, and (III) parallel, competing mechanisms of water transfer between the ionomer distributed in the catalyst layer and the catalyst layer pores. Liquid water is transported in the GDL and the catalyst layer due to liquid pressure gradient and in the channel due to gravity and two-phase drag. We have developed a transport equation for the water content. The source/sink terms of the transport equation represent the parallel, competing mechanisms of water transfer between the ionomer phase and the catalyst layer pores. They are (I) sorption/desorption at nonequilibrium and (II) electro-osmotic drag by the secondary current. Another distinguishing feature of this model is the capability to capture water saturation equilibrium at channel-GDL and GDL-catalyst layer interfaces. The computational results are used to study the dynamics of water transport within and between the fuel cell components and the impact of the GDL and catalyst layer properties on the amount of water retained in the fuel cell components during operation. A new dominant mechanism of water transfer between the ionomer distributed in the catalyst layer and the catalyst layer pores is identified. The amount of water retained in GDL is determined by GDL permeability and its pore size at the interface with the channel. The amount of water retained in the cathode catalyst layer is determined by the saturation equilibrium at the interface with the GDL. Models based on the two-phase mixture model are not applicable to PEM fuel cell electrodes.

Monitoring the State of Charge of Operating Vanadium Redox Flow Batteries

We describe methods for assessment of the state of charge (SOC) of vanadium redox flow batteries during operation using UV-vis spectrophotometry and full cell OCV measurements. The absorbance of the negative electrolyte solution at 433 nm, 600 nm and 750 nm is linear in SOC for SOC > 0.2. The absorbance of the positive electrolyte does not display a linear relationship with SOC, which we attribute to the formation of a strongly absorbing third species. An analysis of the excess absorbance of the positive electrolyte suggests that a complex with 1:1 stoichiometry is formed by VO 2+ and VO 2 + . Full-cell OCV measurements were found to be an inaccurate gauge of the state of charge of the battery.

A comparative ab initio study of the primary hydration and proton dissociation of various imide and sulfonic acid ionomers

We compare the role of neighboring group substitutions on proton dissociation of hydrated acidic moieties suitable for proton exchange membranes through electronic structure calculations. Three pairs of ionomers containing similar electron withdrawing groups within the pair were chosen for the study: two fully fluorinated sulfonyl imides (CF(3)SO(2)NHSO(2)CF(3) and CF(3)CF(2)SO(2)NHSO(2)CF(3)), two partially fluorinated sulfonyl imides (CH(3)SO(2)NHSO(2)CF(3) and C(6)H(5)SO(2)NHSO(2)CF(2)CF(3)), and two aromatic sulfonic acid based materials (CH(3)C(6)H(4)SO(3)H and CH(3)OC(6)H(3)OCH(3)C(6)H(4)SO(3)H). Fully optimized counterpoise (CP) corrected geometries were obtained for each ionomer fragment with the inclusion of water molecules at the B3LYP/6-311G** level of density functional theory. Spontaneous proton dissociation was observed upon addition of three water molecules in each system, and the transition to a solvent-separated ion pair occurred when four water molecules were introduced. No considerable quantitative or qualitative differences in proton dissociation, hydrogen bond networks formed, or water binding energies were found between systems containing similar electron withdrawing groups. Each of the sulfonyl imide ionomers exhibited qualitatively similar results regarding proton dissociation and separation. The fully fluorinated sulfonyl imides, however, showed a greater propensity to exist in dissociated and ion-pair separated states at low degrees of hydration than the partially fluorinated sulfonyl imides. This effect is due to the additional electron withdrawing groups providing charge stabilization as the dissociated proton migrates away from the imide anion.

Ionic conductivity and glass transition of phosphoric acids.

Here we report the low-temperature dielectric and viscoelastic properties of phosphoric acids in the range of H2O:P2O5 1.5-5. Both dielectric and viscosity measurements allow us to determine the glass-transition temperatures of phosphoric acids. The obtained glass-transition temperatures are in good agreement with previous differential scanning calorimetric measurements. Moreover, our analysis reveals moderate decoupling of ionic conductivity from structural relaxation in the vicinity of the glass transition.

A Fundamental Study of the Transport Properties of Aqueous Superacid Solutions

An extensive investigation of the transport properties of aqueous acid solutions was undertaken. The acids studied were trifluoromethanesulfonic (CF(3)SO(3)H), bis(trifluoromethanesulfonyl)imide [(CF(3)SO(2))(2)NH], and para-toluenesulfonic (CH(3)C(6)H(4)SO(3)H), of which the first two are considered superacids. NMR measurements of self-diffusion coefficients (D), spin-lattice relaxation times (T(1)), and chemical shifts, in addition to ionic conductivity (sigma), viscosity (eta), and density measurements, were performed at 30 degrees C over the concentration range of 2-112 water to acid molecules. Results showed broad maxima in sigma for all three acids in the concentration range of 12-20 water to acid molecules. This coincided with minima in anion Ds and is attributed to a local molecular ordering, reduced solution dielectric permittivity, and increased ionic interactions. The location of the maxima in sigma correlates with what is observed for hydrated sulfonated perfluoropolymers such as Nafion, which gives a maximum in ionic transport when the ratio of water to acid molecules is about 15-20. Of the three acids, bis(trifluoromethanesulfonyl)imide was found to be the least dependent on hydration level. The occurrence of the anticorrelation between the ionic conductivity maximum and the anion self-diffusion minimum supports excess proton mobility in this region and may offer additional information on the strength of hydrogen bonding in aqueous media as well as on the role of high acid concentration in the Grotthuss proton transport mechanism.

Nanometer-scale mapping of irreversible electrochemical nucleation processes on solid Li-ion electrolytes

Electrochemical processes associated with changes in structure, connectivity or composition typically proceed via new phase nucleation with subsequent growth of nuclei. Understanding and controlling reactions requires the elucidation and control of nucleation mechanisms. However, factors controlling nucleation kinetics, including the interplay between local mechanical conditions, microstructure and local ionic profile remain inaccessible. Furthermore, the tendency of current probing techniques to interfere with the original microstructure prevents a systematic evaluation of the correlation between the microstructure and local electrochemical reactivity. In this work, the spatial variability of irreversible nucleation processes of Li on a Li-ion conductive glass-ceramics surface is studied with ~30 nm resolution. An increased nucleation rate at the boundaries between the crystalline AlPO4 phase and amorphous matrix is observed and attributed to Li segregation. This study opens a pathway for probing mechanisms at the level of single structural defects and elucidation of electrochemical activities in nanoscale volumes.

Nanoporous polysulfone membranes via a degradable block copolymer precursor for redox flow batteries

Nanoporous polysulfone (PSU) membranes were fabricated via post-hydrolysis of polylactide (PLA) from PLA–PSU–PLA triblock copolymer membranes. The PSU scaffold was thermally crosslinked before sacrificing PLA blocks. The resulting nanopore surface was chemically modified with sulfonic acid moieties. The membranes were analyzed and evaluated as separators for vanadium redox flow batteries. Nanoporous PSU membranes prepared by this new method and further chemically modified to a slight degree exhibited unique behavior with respect to their ionic conductivity when exposed to solutions of increasing acid concentration.

Vapor synthesis and thermal modification of supportless platinum-ruthenium nanotubes and application as methanol electrooxidation catalysts.

Metallic, mixed-phase, and alloyed bimetallic Pt-Ru nanotubes were synthesized by a novel route based on the sublimation of metal acetylacetonate precursors and their subsequent vapor deposition within anodic alumina templates. Nanotube architectures were tuned by thermal annealing treatments. As-synthesized nanotubes are composed of nanoparticulate, metallic platinum and hydrous ruthenium oxide whose respective thicknesses depend on the sample chemical composition. The Pt-decorated, hydrous Ru oxide nanotubes may be thermally annealed to promote a series of chemical and physical changes to the nanotube structures, including alloy formation, crystallite growth, and morphological evolution. Annealed Pt-Ru alloy nanotubes and their as-synthesized analogs demonstrate relatively high specific activities for the oxidation of methanol. As-synthesized, mixed-phase Pt-Ru nanotubes (0.39 mA/cm(2)) and metallic alloyed Pt64Ru36NTs (0.33 mA/cm(2)) have considerably higher area-normalized activities than PtRu black (0.22 mA/cm(2)) at 0.65 V vs RHE.