Synthesis of Pt-Mo/Carbon Nanocomposites from Single Source Molecular Precursors: A (1:1) PtMo/C PEMFC Anode Catalyst Exhibiting CO Tolerance
K. Kwiatkowski, S. B. Milne, S. Mukerjee, C. M. Lukehart J. Cluster Science, 16(2), 251 (2005)
Abstract
Reactive, thermal degradation of py2Pt[MoCp(CO)3]2, (Me)(cod)PtMoCp(CO)3, or {Pt3(dppm)3(CO)[Mo(Cp)]} [BPh4]/Vulcan carbon powder composites affords Pt–Mo/carbon nanocomposites containing metal nanoparticles of approximate compositions, PtMo2, PtMo, or Pt3Mo, widely dispersed on the carbon support. Total metal loadings range from 29–58 wt%. When tested as an anode electrocatalyst in a PEM fuel cell using either pure H2 or H2 containing 100 ppm CO as a fuel, the PtMo/carbon nanocomposite exhibits CO tolerance.
Dual Ion Beam Assisted Deposition as a Method to Obtain Low Loading-High Performance Electrodes for PEMFCs
Abstract
Ultralow loading noble metal (Pt) electrodes, for proton exchange membrane fuel cells (PEMFCs), were prepared via dual ion-beam assisted deposition of pure Pt metal particles directly onto the surface of a noncatalyzed E-TEK gas diffusion layer. Activity enhancement, based on normalization with electrochemical surface area and mass activity is reported relative to a commercial gas diffusion electrode containing carbon-supported Pt electrocatalysts. The enhanced performance was primarily dictated by the cathodic oxygen reduction reaction. Based on the morphological differences, which enable such enhanced activities and the mass manufacturability of this electrode system, we report a significant new development in terms of materials for PEMFC application.
Effect of Copolymer Composition on the Oxygen Transport Properties of Sulfonated Poly (arylene ether sulfone) Sulfonated (sulfide sulfone) Proton Exchange Membranes
Abstract
Mass transport properties were investigated by means of the chronoamperometry method for a series of sulfonated poly(arylene ether sulfone) (SPES) membranes and sulfonated polysulfide sulfone (SPSS) polymers of various ion exchange capacities (IEC) At a Pt (microelectrode)/proton exchange membrane (PEM) interface. The temperature and pressure dependence of oxygen transport parameters show similar trends for the SPES 30-60 (IEC = 1.2-2.2 meq g−1) and SPSS 20-50 (IEC = 0.7-1.8 meq g−1) membranes. The diffusion coefficient was found to increase with the IEC while solubility decreases. The results are discussed in the context of water/polymer interactions and morphology-facilitated mass transport characteristics.
Unexpected 5 V behavior for Zn doped Mn Spinel Cathode Material
Abstract
Zn-doped Mn spinel was investigated for its unique 5 V reversible Li intercalation. Only at a certain Zn doping, LiZn0.25Mn1.75O4, lithium ions can be extracted at 5 V, while at high Zn doping of LiZn0.5Mn1.5O4 no reversible capacity was observed. Electrochemical extraction of lithium ions during charge at 5 V is partially reversible. The two Zn doped compositions (x = 0.25 and 0.5 in LiZnxMn2–xO4) have a single phase cubic spinel structure, possessing a primitive cubic atomic arrangement, in contrast to the face centered cubic LiMn2O4 spinel. All materials synthesized have a tetradecahedra grain structure, bonded via the hexagon facets.
Activation Energies for Oxygen Reduction on Platinum Alloys: Theory and Experiment
Abstract
A combined theoretical and experimental analysis of the electrode potential dependencies of activation energies is presented for the first step in oxygen reduction over platinum and platinum alloy catalysts in both polycrystalline and carbon supported form. Tafel data for several of the catalysts are used to predict potentialdependent activation energies for oxygen reduction over the 0.6-0.9 V range in strong and weak acid. Comparisons with the theoretical curve show good agreement above 0.8 V, suggesting a fairly constant preexponential factor. Arrhenius determinations of activation energies over the 0.7-0.9 V range yield little trend for weak acid, possibly because of the larger uncertainties in the Arrhenius fits, but the strong acid results have smaller uncertainties and for them the measured activation energies trend up with potential.
Origin of 5 V Electrochemical Activity in Non Redox Reactive Divalent Cation Doped LiM0.5-xMn1.5+xO4 (0<x<0.5) Cathode Materials: In situ XRD and XANES Spectroscopy studies
Abstract
Divalent cation doped lithiated Mn spinel with Zn and Mg as cathode materials for a lithium battery are investigated and partial reversible behavior is observed at the 5 V region. The electrochemical charge and discharge potential profiles of the Zn-doped materials indicate a close relationship between the lattice energy and lattice parameters in the Zn-doped spinel system. Lithium ions extracted from octahedral sites at the 5 V plateau during the charge cycle are partially reinserted back into the tetrahedral sites during the discharge step, which contributes to the partial reversible 5 V behavior. The significant findings reported here are that the strong tetrahedral site preference of divalent nonreactive cations such as Zn and Mg force Li cations onto octahedral sites in these materials, thus resulting in electroactivity at 5 V. In situ X-ray absorption spectroscopy measurements show that the Mn K edge is shifted to higher energy at the 4 V plateau during charge cycle and remains unchanged at the 5 V plateau. In situ Zn K-edge X-ray absorption near-edge structure measurements reveal that the valence state of zinc ions is unchanged at the 5 V plateau region. In situ Mn K-edge extended X-ray absorption fine structure studies suggest that O2− ions in the Zn-spinel lattice are partially oxidized to O− at the 5 V plateau during the anodic process and O− ions are reduced back to O2− during the cathodic process at the 5 V plateau. The oscillations of the lattice parameters observed at the 5 V plateau region during the anodic charge step are attributed to chemical instability of O− ions.
Potential Shift for OH (ads) Formation on the Pt Skin on Pt3CO(111) Electrodes in Acid: Theory and Experiment
Abstract
A study combining theoretical predictions and experimental measurements was made to gain an understanding of the beneficial effect of alloying cobalt into platinum for electroreduction of oxygen. Carbon-supported Pt3Co catalyst particles were characterized by X-ray diffraction spectroscopy and X-ray absorption near-edge structure, which gave evidence for a surface layer composed of Pt, called the Pt skin. Electrochemical measurements were made in 1 M trifluoromethane sulfonic acid with a rotating ring disk setup. Cyclic voltammetry showed significantly less oxide formation in the .0.8 V range over the skin on the alloy compared to nonalloyed Pt. Tafel plots showed a 50-70 mV reduction in overpotential for O2 reduction over the Pt skin. The Vienna Ab Initio Simulation Program was used for calculating H2O and OH adsorption bond strengths on the Pt skin on Pt3Cos111d for comparison with prior work with the Pt~111! surface. The bond strength variations were used to estimate the shift in reversible potential for OHads formation from H2Oads oxidation. A shift of 80 mV was found, which indicates that an increase in the reversible potential for OHads formation correlates with the decrease in overpotential for O2 reduction over the Pt skin on Pt3Co nanoparticles.
Low Temperature Performance of Copper/Nickel Modified LiMn2O4 Spinels
Y. Ein-Eli, R. C. Urian, W. Wen and S. Mukerjee Electrochim. Acta50, 1931 (2005)
Abstract
This study presents an evaluation of structural changes resulting from cycling modified copper/nickel LiMn2O4 spinels at 263 K. In situ synchrotron XRD shows that cycling LiMn2O4 at 263 K resulted in the formation of mixed cubic and tetragonal phases with a consequent lower capacity. The differential capacitance profile normally exhibiting two peaks at 298 K is modified, showing only one oxidation peak at 263 K in the 4 V region. The changes observed are attributed to interactions between Jahn–Teller active Mn3+ species and Li+ ions. These changes are not observed once copper/nickel modified spinels are being evaluated, because of the decrease in Mn3+ population. All the observed changes are fully reversible once the material is cycled back at 298 K.