Radoslav Atanasoski

Corrosion of aluminum at high voltages in non-aqueous electrolytes containing perfluoroalkylsulfonyl imides; new lithium salts for lithium-ion cells

New lithium bis-perfluoroalkylsulfonyl imide salts were recently synthesized in our laboratories whose properties make them promising candidates for lithium-ion batteries. The salts have good conductivity, cycling characteristics and excellent thermal and hydrolytic stability. In the focus of this paper are the corrosion properties of the aluminum current collector in electrolytes based on these salts. High protection potentials at aluminum in bisperfluoroethylsulfonyl imide electrolytes were observed. Based on the surface analysis of the protective film, a correlation of the molecular weight of the anions to their protective properties was established.

The surface film formed on a lithium metal electrode in a new imide electrolyte, lithium bis(perfluoroethylsulfonylimide) [LiN(C2F5SO2)2]

A newly developed imide electrolyte salt, LiN(C 2 F 5 SO 2 ) 2 (LiBETI) was found to give very uniform, thin, and stable surface films on a lithium metal electrode in the propylene carbonate (PC) solution. LiBETI/PC was studied and compared to determine its ability to form such a stable surface film, with conventional electrolyte systems such as LiCF 3 SO 3 /PC, LiPF 6 /PC, and LiN(CF 3 SO 2 ) 2 /PC (LiTFSI/PC). The surface film formed in LiBETI/PC system was a hemispherical, and the composition of the film consisted mainly of LiF, which is similar to that in a LiPF 6 /PC system. Quartz crystal microbalance (QCM) and cyclic voltammetry (after the tenth cycle) indicated that the surface film formed in LiBETI/PC (ca. 50 nm) was thinner than those in LiPF 6 /PC (ca. 90 nm), LiTFSI/PC (ca. 140 nm), or LiCF 3 SO 3 /PC (ca. 255 nm). The variation of the resonance resistance (AR) obtained from in situ CV/QCM measurement, which has been demonstrated to be a good measure of the surface roughness, also suggested that LiBETI/PC system gave a compact and smooth surface topology during lithium deposition-dissolution cycles. Impedance spectroscopy together with preliminary cycling tests showed that the LiBETI/PC system provides the highest cycling efficiency and improved cycleability among existing electrolyte salt systems in rechargeable battery systems employing lithium metal anodes.

Alternative Catalyst Supports Deposited on Nanostructured Thin Films for Proton Exchange Membrane Fuel Cells

A series of platinum-coated underlayer materials, alumina, gold, titanium carbide, and titanium disilicide, deposited by a high throughput magnetron sputtering method have been investigated as cathode catalyst supports in fuel cells. Orthogonal thickness gradients of the underlayer materials (0-100 nm planar equivalent) and the platinum top layer (0-75 nm planar equivalent) made up the 76 × 76 mm libraries. The resulting catalyst films were characterized by surface profilometry, X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. The electrochemical properties of the catalyst composition spreads were investigated simultaneously in 64-electrode proton exchange membrane fuel cells with emphasis placed on the determination of the electrochemical surface area (ECSA) as a function of underlayer thickness and chemistry. The present study shows that gold and titanium disilicide used as underlayers on 3Ms nanostructured thin film supports lead to a loss of ECSA during operation. Migration and surface accumulation were observed when gold was used as underlayer material. For titanium disilicide, alloying and the generation of platinum silicide phases occurred. Alumina and titanium carbide were found to be potentially acceptable underlayer materials as well as alternative support materials on the basis of their influence on the catalyst surface area.

Catalytic Activity of Pt1-xNix (0 < x < 0.8) Measured on High Surface Area NSTF-Coated GC Disks

Intermixed Pt1-xNix (0 < x < 0.8) and Pt were sputter-deposited onto high surface area, NSTF-coated GC disks and studied for oxygen reduction reaction (ORR) activity by rotating disk electrode (RDE). NSTF-coated GC disks used in RDE experiments is a viable alternative to the regular mirror-polished GC disks in screening catalyst activities because both the catalytic activities and the effects of high-surface area support can be examined in a single measurement. The sputtered Pt-Ni samples have a much higher Surface Enhancement Factor (SEF) and kinetic current density. The impact of potential cycling is also included in this work.

Toward standardizing the measurement of electrochemical properties of solid-state electrolytes in lithium batteries ☆

A brief discussion on transport measurements for lithium ion conducting polymer electrolytes is given. An engineering approach to obtain a complete set of transport and thermodynamic properties for a binary salt dissolved in a polymer electrolyte solvent is described. The technique is based on concentrated solution theory and requires a minimal amount of experimentation. Results from measurements on a representative polymer electrolyte system are given. The measured transport and thermodynamic properties of the polymer electrolyte are used to simulate the performance of symmetric Li/polymer/Li cells and compare to experimental data.

Efficient Oxygen Evolution Reaction Catalysts for Cell Reversal and Start/Stop Tolerance

Minute amounts of ruthenium and iridium on platinum nanostructured thin films have been evaluated in an effort to reduce carbon corrosion and Pt dissolution during transient conditions in proton exchange membrane fuel cells. Electrochemical tests showed the catalysts had a remarkable oxygen evolution reaction (OER) activity, even greater than that of bulk, metallic thin films. Stability tests within a fuel cell environment showed that rapid Ru dissolution could be managed with the addition of Ir. Membrane electrode assemblies containing a Ru to Ir atomic ratio of 1:9 were evaluated under start-up/shutdown and cell reversal conditions for OER catalyst loadings ranging from 1 to 10 μg/cm2. These tests affirmed that electrode potentials can be controlled through the addition of OER catalysts without impacting the oxygen reduction reaction on the cathode or the hydrogen oxidation reaction on the anode. The morphology and chemical structure of the thin OER layers were characterized by scanning transmission electron microscopy and X-ray photoelectron spectroscopy in an effort to establish a correlation between interfacial properties and electrochemical behavior.

Characterization of durable nanostructured thin film catalysts tested under transient conditions using analytical aberration-corrected electron microscopy

The stability of Ru0.1Ir0.9 oxidation evolution reaction (OER) catalysts deposited on Pt-coated nanostructured thin films (NSTFs) has been investigated by aberration-corrected electron microscopy. Accelerated stress tests showed that the OER catalysts significantly improved the durability of the Pt under cell reversal conditions. High-resolution images of the end-of-life NSTFs showed significant Ir loss from the whisker surfaces, while no Pt loss was observed, indicating that the OER catalysts had protected the catalyst coated whisker surfaces from degradation.

Development of Catalysts with Enhanced Tolerance to Fuel Cell Transient Conditions

A Ru1-xIrx binary over-layer was deposited on Pt-coated nano-structured thin film from 3M Company. XPS measurements were used to confirm the composition of the over-layer. Rotating disk electrode measurements were used to assess oxygen evolution activity of the catalysts. The results showed that a low Ir/high Ru over-layer had a higher activity but lower stability than high Ir/low Ru samples.