Peter S. Fedkiw

Improved corrosion behavior of nanocrystalline zinc produced by pulse-current electrodeposition

Pulse electrodeposition was used to produce nanocrystalline (nc) zinc from zinc chloride electrolyte with polyacrylamide and thiourea as additives. Field emission scanning electron microscopy (FESEM) was used to study the grain size and surface morphology of the deposits and X-ray diffraction was used to examine their preferred orientation. Corrosion behavior of the electrodeposited nc zinc in comparison with electrogalvanized (EG) steel in de-aerated 0.5 N NaOH solution was studied using potentiodynamic polarization and impedance measurements. A scanning electron microscope (SEM) was used to characterize the surface morphology of the EG steel before corrosion testing. Surface morphologies of nc zinc deposits and EG steel were also studied after potentiondynamic polarization by SEM. Nanocrystalline zinc (56 nm) with random orientation was produced. The estimated corrosion rate of nc zinc was found to be about 60% lower than that of EG steel, 90 and 229 lA/cm 2 , respectively. The surface morphology of corroded nc zinc was characterized by discrete etch pits, however, uniform corrosion was obtained after potentiodynamic polarization of EG steel. The passive film formed on the nc zinc surface seems to be a dominating factor for the corrosion behavior observed.

Fumed silica-based composite polymer electrolytes: synthesis, rheology, and electrochemistry

An overview of our research is presented on developing composite polymer electrolytes (CPEs) based on low-molecular weight polyethylene oxide (PEO) (namely, poly(ethylene glycol) dimethyl ether), lithium salts (e.g. lithium triflate, lithium imide, etc.), and fumed silica. These CPEs demonstrate high room-temperature conductivites (>10−3 S/cm), mechanical strength, and form stable interfaces with lithium metal as a result of the fumed silica. The surface groups on the fumed silica determine the mechanical properties of the CPE while the low-molecular weight PEO and lithium salt determine the ionic transport properties. These CPEs show promise as electrolytes for the next generation of rechargeable lithium batteries.

In Situ Electrode Formation on a Nafion Membrane by Chemical Platinization

Three chemical techniques to platinize the surface of a Nafion polymer electrolyte membrane (PEM) to form a Pt/PEM electrode have been studied. The resulting Pt/PEM composites were characterized visually by transmission electron microscopy and scanning electron microscopy and electrochemically by (i) hydrogen adsorption to determine surface area, and (ii) the polarization characteristics for the oxidation of hydrogen

Electrochemical impedance spectra of full cells: Relation to capacity and capacity-rate of rechargeable Li cells using LiCoO2, LiMn2O4, and LiNiO2 cathodes

Abstract Electrochemical impedance spectra (EIS) are reported for rechargeable lithium cells using cathodes LiM y O x (M=Co, Ni, and Mn) prepared from casting and high-pressure compacting. The composite cathodes were cast from a slurry mixture consisting of 30% NMP solvent and 70% solid (91% LiM y O x , 3% PVDF and 6% KS44 graphite) onto a 25-μm thick Al current collector. The compacted cathodes were made from the cast cathode using a laboratory press. The most compacted cathodes have the smallest impedance and the highest specific capacity and capacity-rate. Three equivalent circuits are proposed according to the effect of compaction pressure on our EIS results and work reported in the literature. The EIS results and relationship to the capacity and capacity-rate are discussed using these circuits. It appears that the ohmic resistance of the composite cathode is an important factor in the overall resistance of the cell and unfavorably affects the performance (capacity and capacity-rate) of the cathode.

Composite Electrolytes Prepared from Fumed Silica, Polyethylene Oxide Oligomers, and Lithium Salts

The conductivity of solution electrolytes containing lithium salts (imide and triflate anions), poly(ethylene glycol), and mono- and dimethyl-terminated poly(ethylene glycol) (Mw 200 to 400), and their corresponding composite electrolytes containing fumed-silica particulates (0 to 20 weight percent) are reported. At room temperature the ionic conductivity is as high as 1.5 {times} 10{sup {minus}3} S/cm for the composites studied, and they exhibit a gel-like consistency but flow under shear. The electrochemical stability of the composites and compatibility with lithium metal were also examined. A large potential window ({approximately}5.5 V) was found for Li imide-based electrolytes. The passive film formed on lithium in contact with the composite electrolyte is relatively more stable and less resistive than that formed in contact with the parent solution electrolyte. Considering the additional advantages of processability and low volatility, these composites should be good candidate electrolytes for lithium and lithium ion batteries.

Transport properties of lithium hectorite-based composite electrolytes

Conductivity and lithium-ion transference numbers are reported for physically gelled composite electrolytes using lithium hectorite clay as the charge carrier and carbonate solvents (ethylene carbonate, propylene carbonate, and dimethyl carbonate). Results are compared with those of typical lithium-ion battery electrolytes based on lithium hexafluorophosphate (LiPF 6 ) and carbonate solvents Room-temperature conductivities of the composite electrolytes as high as 2 × 10 4 S/cm were measured. Because of the nature of the anionic clay particulates creating the gel structure, near-unity lithium-ion transference numbers are expected and were observed as high as 0.98, as measured by the de polarization method using lithium-metal electrodes. Since the carbonates react with lithium and create mobile ionic species that significantly reduce the observed lithium-ion transference number care must be taken to minimize or eliminate the presence of the reaction-formed ionic species. These hectorite-based composite systems are possible electrolytes for rechargeable lithium-ion batteries requiring high discharge rates.

Composite polymer electrolytes using surface-modified fumed silicas: conductivity and rheology

Abstract We report results from our studies on composite polymer electrolytes based on novel surface-modified fumed silicas. The electrolytes were prepared by dispersing fumed silica in a matrix formed by methyl-capped polyethylene glycol and lithium salt. Silicas with widely different surface chemistries were synthesized in order to study the effects of surface modification, with the attached surface groups ranging from non-polar alkyl moieties (C 1 or C 8 ) to polar polyethylene oxide (PEO) oligomers (MW∼200). We find, rather surprisingly, that the conductivity is independent of the type of surface group present on the silica. Moreover, the conductivity decreases only slightly on addition of fumed silica, even at high weight fraction of solids. In contrast, the rheological properties of the composites are strongly affected by both the silica surface chemistry and weight fraction. Dynamic rheology measurements reveal that fumed silicas with silanol and octyl coverage both flocculate into gels (networks). The resulting materials are mechanically stable, with the elastic modulus of the gel being strongly dependent upon weight fraction of solids. The PEO-modified silica, in contrast, gives rise to a low-viscosity suspension where the silica units exist as distinct, non-interacting species. The findings of this study have significant implications for future work on composite electrolytes, in that we can tailor the mechanical properties of the system without affecting the electrochemical behavior.

Influence of Additives and Pulse Electrodeposition Parameters on Production of Nanocrystalline Zinc from Zinc Chloride Electrolytes

Pulse electrodeposition was used to produce nanocrystalline zinc from an aqueous zinc chloride electrolyte with polyacrylamide and thiourea as additives. The influence of additive concentration and pulse electrodeposition parameters, namely, current-on time, current-off time, and peak current density on the grain size, surface morphology, and preferred orientation was investigated. The grain size and surface morphology of zinc deposits were studied by scanning electron microscopy and field emission scanning electron microscopy. The preferred orientation of zinc deposits was studied by X-ray diffraction. The optimum concentrations of polyacrylamide and thiourea in the bath that give the finest grains were 0.7 and 0.05 g/L, respectively. At constant current-off time and peak current density, the grain size decreased asymptotically with increasing current-on time. An increase in the current-off time at constant current-on time and peak current density resulted in grain growth. A progressive decrease of the grain size was observed with increasing peak current density at constant current-on and -off time. Nanocrystalline zinc with an average grain size of 50 nm was obtained at a peak current density of 1000 mA/cm 2 . The crystal orientations developed were correlated to the variation in the cathode overpotential accompanied with changing the electrodeposition parameters. A (1013) preferred orientation was developed at low overpotential while higher overpotential developed a dual (1122) (1010) orientation.

Electrodeposition of high-surface area platinum in a well adherent Nafion film on glassy carbon

Abstract Platinum was electrochemically deposited within a Nafion film coated on glassy carbon (GC) to form a well adherent and high-platinum utilization electrode. Two potential-control procedures were evaluated to form a deposit: a cyclic potential scan and a constant potential, with Pt loadings ranging for each from 60–750 μg Pt cm −2 GC obtained by varying the coulombs discharged. The Pt/Nafion/GC electrodes were annealed at 170 °C before use. Transmission electron microscopy studies revealed that Pt grows as a dispersed, three-dimensional deposit within the film for both techniques. The average particle size for a loading of 60 μg cm −2 is 7.3 nm, which is in good agreement with that (7.1 nm) estimated from electrochemical formation of adsorbed hydrogen. With a Pt loading an order-of-magnitude greater, the particle number density increased, and the observed average particle is larger at 9.8 nm which is smaller than that (19.0 nm) evaluated from electrochemical hydrogen adsorption. The deposit thickness increased with loading and, for a given Pt loading, was less thick when using the steady-potential method; the latter technique, however, yielded a deposit which covered more of the GC-Nafion interface. The mass specific surface area of Pt decreased with loading and ranged from 32-8 m 2 g −1 as the loading varied from 60–640 μg cm −2 . The Pt/Nafion/GC structure is robust in that the electrode can be used to evolve hydrogen or oxygen without damaging the film. In contrast, a Nafion film on smooth Pt is lifted off the surface under similar gas-evolution conditions.

Inhibition of Lithium Dendrites by Fumed Silica-Based Composite Electrolytes

Lithium dendrite formation is investigated via in situ microscopy in a liquid electrolyte containing polyethylene glycol dimethyl ether 1 lithium bis~trifluoromethylsulfonyl !imide and composite gel-like electrolytes formed by dispersing nanometer-size fumed silica into the liquid. Fumed silicas with either hydrophilic silanol surface groups or hydrophobic octyl surface groups were employed. Dendrites with current density-dependent morphology are formed in liquid electrolyte but addition of fumed silica inhibits their formation, with hydrophilic fumed silica having a more pronounced effect than hydrophobic silica. The dendrite inhibition effect of fumed silica is attributed to its abilities to form a continuous network with elastic-like properties and scavenge impurities from the electrolyte.