Albert Epshteyn

Insights into PEMFC Performance Degradation from HCl in Air

The performance degradation of a proton exchange membrane fuel cell (PEMFC) is studied in the presence of HCl in the air stream. The cathode employing carbon-supported platinum nanoparticles (Pt/C) was exposed to 4 ppm HCl in air while the cell voltage was held at 0.6 V. The HCl poisoning results in generation of chloride and chloroplatinate ions on the surface of Pt/C catalyst as determined by a combination of electrochemical tests and ex-situ chlorine K-edge X-Ray absorption near-edge structure (XANES) spectroscopy. The chloride ions inhibit the oxygen reduction reaction (ORR) and likely affect the wetting properties of diffusion media/catalyst layer, while the chloroplatinate ions are responsible for enhanced platinum particle growth most likely due to platinum dissolution-redeposition. The chloride ions can cause corrosion of the Pt nanoparticles in the presence of aqueous HCl in air even if no potential is applied. Although the majority of chloride ions are desorbed from the Pt surface by hydrogen treatment of the cathode, they partially remain in the system and re-adsorb on platinum at cell voltages of 0.5-0.9 V. Chloride ions are removed from the system and fuel cell performance at 0.5-0.7 V is restored by multiple exposures to low potentials.

Pressure-induced polymerization of P(CN)3

Motivated to explore the formation of novel extended carbon-nitrogen solids via well-defined molecular precursor pathways, we studied the chemical reactivity of highly pure phosphorous tricyanide, P(CN)3, under conditions of high pressure at room temperature. Raman and infrared (IR) spectroscopic measurements reveal a series of phase transformations below 10 GPa, and several low-frequency vibrational modes are reported for the first time. Synchrotron powder X-ray diffraction measurements taken during compression show that molecular P(CN)3 is highly compressible, with a bulk modulus of 10.0 ± 0.3 GPa, and polymerizes into an amorphous solid above ∼10.0 GPa. Raman and IR spectra, together with first-principles molecular-dynamics simulations, show that the amorphization transition is associated with polymerization of the cyanide groups into CN bonds with predominantly sp(2) character, similar to known carbon nitrides, resulting in a novel phosphorous carbon nitride (PCN) polymeric phase, which is recoverable to ambient pressure.

Surface Passivated Air and Moisture Stable Mixed Zirconium Aluminum Metal-Hydride Nanoparticles

Synthesis of surface passivated Zr-Al mixed metal-hydride nanoparticles was accomplished via a multi-step process. The initial reaction to produce the zirconium aluminum hydride was via decomposition of zirconium tetrahydroaluminate (Zr(AlH4)4) while exposed to ultrasound produced by a bench-top ultrasonic cleaning bath. The particles were surface passivated using carbohydrates and were shown to be stable in air and partially stable in water. TEM imaging suggests the existence of smaller particles made of a Zr-Al alloy that range in size from 1.8 nm to 7.9 nm in diameter and are interspersed with larger particles that range from tens to hundreds of nanometers in diameter. It was also shown that the carbohydrate-derived coating of the nanoparticles is present as an aluminum alkoxide gel surrounding the core particles. INTRODUCTION Metal-hydride nanoparticles have been sought after, for one, due to their potential utility as hydrogen storage materials. There have been several patents issued, as well as several publications in recent years for metal-hydride nanoparticle materials. The authors of a recent review on nano-engineered hydrogen storage materials asserted that nanoparticle materials decrease the active particle size increasing surface area and grain boundaries, in turn improving the adsorption and desorption kinetics, decreasing the enthalpy of formation and thereby decreasing the release temperature of metal hydrides. A theoretical investigation by Clark et al. of various Zr-Al materials showed such materials as potentially having good hydrogen storage properties. It has recently been reported that NaAlH4 acquires significantly improved hydrogen adsorption and desorption properties when doped with Ti or Zr. Larger scale synthesis of air and moisture sensitive metal nanoparticles is a challenge, and poses an obstacle to investigating the physical properties of these materials via traditional techniques that would expose them to air. To obtain materials that do not oxidize when handled in air while still retaining certain useful properties, the surface of the nanoparticles can be protected by a passivating agent. In order to keep the intrinsic properties of the unpassivated material it is important to maximize the active metal content of the material while minimizing the amount of passivator present on the nanoparticle surface. We report a homogeneous solution-based method used to produce well-defined passivated air and moisture stable Zr-Al partial hydride nanoparticle materials on gram scale. Herein we report the initial characterization of these particles by Al magic angle spinning (MAS) NMR, SEM, TEM, oxygen bomb calorimetry, and microanalysis. EXPERIMENT General. All air and moisture sensitive manipulations were performed in a Vacuum Atmospheres glove box under an atmosphere of helium or via traditional Schlenk technique under an atmosphere of nitrogen. Dry diethyl ether (Et2O) was purchased from Aldrich packaged under Mater. Res. Soc. Symp. Proc. Vol. 1056

Tetracyanomethane under Pressure: Extended CN Polymers from Precursors with Built-in sp3 Centers

Tetracyanomethane, C(CN)4, is a tetrahedral molecule containing a central sp3 carbon that is coordinated by reactive nitrile groups that could potentially transform to an extended CN network with a significant fraction of sp3 carbon. High-purity C(CN)4 was synthesized, and its physiochemical behavior was studied using in situ synchrotron angle-dispersive powder X-ray diffraction (PXRD) and Raman and infrared (IR) spectroscopies in a diamond anvil cell (DAC) up to 21 GPa. The pressure dependence of the fundamental vibrational modes associated with the molecular solid was determined, and some low-frequency Raman modes are reported for the first time. Crystalline molecular C(CN)4 starts to polymerize above ∼7 GPa and transforms into an interconnected disordered network, which is recoverable to ambient conditions. The results demonstrate feasibility for the pressure-induced polymerization of molecules with premeditated functionality.

Surprising Stability of Cubane under Extreme Pressure

The chemical stability of solid cubane under high-pressure was examined with in situ Raman spectroscopy and synchrotron powder X-ray diffraction (PXRD) in a diamond anvil cell (DAC) up to 60 GPa. The Raman modes associated with solid cubane were assigned by comparing experimental data with calculations based on density functional perturbation theory, and low-frequency lattice modes are reported for the first time. The equation of state of solid cubane derived from the PXRD measurements taken during compression gives a bulk modulus of 14.5(2) GPa. In contrast with previous work and chemical intuition, PXRD and Raman data indicate that solid cubane exhibits anomalously large stability under extreme pressure, despite its immensely strained 90° C-C-C bond angles.

High-pressure phase transition of alkali metal–transition metal deuteride Li2PdD2

A combined theoretical and experimental study of lithium palladium deuteride (Li2PdD2) subjected to pressures up to 50 GPa reveals one structural phase transition near 10 GPa, detected by synchrotron powder x-ray diffraction, and metadynamics simulations. The ambient-pressure tetragonal phase of Li2PdD2 transforms into a monoclinic C2/m phase that is distinct from all known structures of alkali metal-transition metal hydrides/deuterides. The structure of the high-pressure phase was characterized using ab initio computational techniques and from refinement of the powder x-ray diffraction data. In the high-pressure phase, the PdD2 complexes lose molecular integrity and are fused to extended [PdD2]∞ chains. The discovered phase transition and new structure are relevant to the possible hydrogen storage application of Li2PdD2 and alkali metal-transition metal hydrides in general.

Pressure-induced polymerization of P(CN){sub 3}

Motivated to explore the formation of novel extended carbon-nitrogen solids via well-defined molecular precursor pathways, we studied the chemical reactivity of highly pure phosphorous tricyanide, P(CN)3, under conditions of high pressure at room temperature. Raman and infrared (IR) spectroscopic measurements reveal a series of phase transformations below 10 GPa, and several low-frequency vibrational modes are reported for the first time. Synchrotron powder X-ray diffraction measurements taken during compression show that molecular P(CN)3 is highly compressible, with a bulk modulus of 10.0 ± 0.3 GPa, and polymerizes into an amorphous solid above ∼10.0 GPa. Raman and IR spectra, together with first-principles molecular-dynamics simulations, show that the amorphization transition is associated with polymerization of the cyanide groups into CN bonds with predominantly sp(2) character, similar to known carbon nitrides, resulting in a novel phosphorous carbon nitride (PCN) polymeric phase, which is recoverable to ambient pressure.

Pressure-induced polymerization of P(CN)[subscript 3]

Motivated to explore the formation of novel extended carbon-nitrogen solids via well-defined molecular precursor pathways, we studied the chemical reactivity of highly pure phosphorous tricyanide, P(CN)3, under conditions of high pressure at room temperature. Raman and infrared (IR) spectroscopic measurements reveal a series of phase transformations below 10 GPa, and several low-frequency vibrational modes are reported for the first time. Synchrotron powder X-ray diffraction measurements taken during compression show that molecular P(CN)3 is highly compressible, with a bulk modulus of 10.0 ± 0.3 GPa, and polymerizes into an amorphous solid above ∼10.0 GPa. Raman and IR spectra, together with first-principles molecular-dynamics simulations, show that the amorphization transition is associated with polymerization of the cyanide groups into CN bonds with predominantly sp(2) character, similar to known carbon nitrides, resulting in a novel phosphorous carbon nitride (PCN) polymeric phase, which is recoverable to ambient pressure.