Thin film alloys and composites for application of hydrogen storage and oxygen reduction electrocatalysts

• Author / Creator

  Tan, XueHai

• This thesis encompasses aspects of alloy design, fabrication and characterization of thin films, which are employed as model systems to enhance hydrogen storage kinetics and thermodynamics in magnesium hydride and to enhance the catalytic efficacy and corrosion stability of platinum-based electrocatalysts towards oxygen reduction reaction. Chapter 2 describes the development of various bilayer Pd/X (X = Ti, Nb, Ta) surface catalysts that are used for all Mg-based alloys films in this thesis. The selected interlayer, which forms stable hydride both during absorption and during desorption, reduces the rate of intermetallic formation between Mg and Pd, allowing the Pd to remain catalytically active. The Pd coating is also effective to prevent the underlying alloy from oxidation during the transfer/storage before the hydrogen sorption measurement. Chapter 3 describes a new bimetallic Nb-V catalyst for enhancing the hydrogen storage kinetics of magnesium hydride. Mg combined with bimetallic Nb-V catalyst displays rapid and stable low temperature (200 °C) sorption kinetics even after 500 cycles. We employed JMA-type kinetic analysis and cryo-stage TEM analysis on fully and partially sorbed materials to provide insight into the rapid Mg to MgH2 phase transformation. Our results point to the surface catalyst distribution and stability against coarsening as being a key influence on the hydriding kinetics. The cycled Mg-V-Nb structure consists of a dense distribution of catalytic Nb-V nanocrystallites covering the surfaces of larger Mg and MgH2 particles. Chapter 4 describes the hydrogen storage properties of ternary Mg-Ni-Cu films using binary Mg-Ni and Mg-Cu as baselines, and aims to elucidate the influences of alloying elements on the hydrogen sorption kinetics, thermodynamics and cycleability. Mg-rich Mg-Ni-(Cu) alloys show two stages during absorption. The first stage due to the absorption of Mg is very quick, but the second one due to the absorption of intermetallic Mg2Ni and/or Mg2Cu is significantly slower. The rapid first stage absorption is catalyzed by the intermetallic phase Mg2Ni. Cu substitution improves the desorption kinetics, but decreases the kinetics of the second absorption stage. From pressure-composition isotherms, it is found that the Cu substitution has no influence on the plateau pressure of MgH2 from free-Mg phase, but slightly increases the plateau pressure of LT-Mg2NiH4. Chapter 5 describes a new metastable alloy hydride with promising hydrogen storage properties. Body centered cubic (bcc) (Mg0.75Nb0.25)H2 possesses 4.5 wt.% H capacity. The measured enthalpy of hydride formation of -53 kJ mol-1 H2, which indicates a significant thermo-destabilization relative to the baseline -77 kJ mol-1 H2 for rutile MgH2. The hydrogenation kinetics are remarkable: At room temperature and 1 bar hydrogen it takes 30 minutes to absorb a 1.5 μm thick film at cycle 1, and 1 minute at cycle 5. Using ab initio calculations we have examined the thermodynamic stability of metallic alloys with hexagonal close packed (hcp) versus bcc crystal structure. Moreover we have analyzed the formation energies of the alloy hydrides that are bcc, rutile or fluorite. Chapter 6 describes the support effects of a new highly conductive support that comprises of a 0.5 nm titanium oxynitride film coated by atomic layer deposition onto an array of carbon nanotubes for both pure platinum and platinum–nickel alloy electrocatalysts. Oxynitride induces a downshift in the d-band center for pure Pt and fundamentally changes the Pt particle size and spatial distribution. This results in major enhancements in oxygen reduction reaction electrocatalytic activity and corrosion stability relative to an identically synthesized catalyst without the interlayer. Conversely, oxynitride has a minimal effect on the electronic structure and microstructure, and therefore, on the catalytic performance of Pt-Ni alloy. Chapter 7 describes the oxygen reduction reaction electrocatalytic activity and the corrosion stability of several ternary Pt-Au-Co and Pt-Ir-Co alloys. The addition of Au fine-tunes the lattice parameter and the surface electronic structure to enable activity and cycling stability that is unachievable in the state-of-the-art binary Pt-Co baseline. Certain Pt-Au-Co ternary alloy shows an activity enhancement that is in fact the most optimum reported for skeleton-type Pt systems. The ternary catalysts exhibit improved corrosion stability with increasing levels of Au or Ir substitution. HRTEM and XPS show that Au alloying promotes the formation of an atomically thin Pt-Au-rich surface layer, which imparts kinetic stabilization against the dissolution of the less noble solute component.

• Subjects / Keywords

  • Catalysis
  • Magnesium
  • Thin films
  • Electrocatalyst
  • Hydrogen Storage
  • Oxygen Reduction Reaction
  • Alloy
  • Platinum

• Graduation date

  Spring 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R36Q1ST9Z

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.