Gu, Zhihui (2007-12). Dissolution of oxygen reduction electrocatalysts in acidic environment. Doctoral Dissertation. Thesis uri icon

abstract

  • Platinum (Pt) alloy nanoparticles are used as catalysts in electrochemical cells to reduce oxygen to water and to oxidize hydrogen; the overall reaction converts chemical energy into electrical energy. These nanocatalysts are deposited on a carbon substrate and their catalytic function takes place in acid medium. This harsh environment causes an undesired reaction, which is the dissolution of the metal atoms into the acid medium; thus affecting the catalyst life. This dissertation aims to investigate the dissolution mechanism of fuel cell cathode catalysts at the atomic level starting from the oxygen reaction intermediates on the cathode catalyst surface and propose guidelines to improve cathode catalysts durability based on our proposed mechanism. Density functional theory is employed to study various possible scenarios with the goals of understanding the mechanism of the metal atom dissolution process and establishing some guidelines that permit a rational design of catalysts with better stability against dissolution. A thermodynamic analysis of potential metal dissolution reactions in acid medium is presented first, using density functional theory calculations to explore the relative stabilities of transition metals in relation to that of Pt. The study is performed by comparing the change in reaction Gibbs free energies for different metals in a given dissolution reaction. Then, a series of density functional theory studies, tending to investigate the adsorbed atomic oxygen absorption process from cathode catalyst surface into its subsurface, includes: 1) the oxygen adsorption on various catalyst surfaces and oxygen absorption in subsurface sites to figure out the minimum energy pathway and energy barrier of on-surface oxygen migration and absorption into subsurface; 2) the oxygen coverage, the other oxygen reduction reaction intermediates, and water effects on the oxygen absorption process according to reaction pathways, energy barriers, and thermodynamic analysis; 3) the oxygen absorption process on several Pt-based alloys with various compositions and components to find out the best alloy to inhibit atomic oxygen absorption including both kinetic and thermodynamic analyses, and the effects of such alloyed species on the inhibition process.
  • Platinum (Pt) alloy nanoparticles are used as catalysts in electrochemical cells to
    reduce oxygen to water and to oxidize hydrogen; the overall reaction converts chemical
    energy into electrical energy. These nanocatalysts are deposited on a carbon substrate
    and their catalytic function takes place in acid medium. This harsh environment causes
    an undesired reaction, which is the dissolution of the metal atoms into the acid medium;
    thus affecting the catalyst life. This dissertation aims to investigate the dissolution
    mechanism of fuel cell cathode catalysts at the atomic level starting from the oxygen
    reaction intermediates on the cathode catalyst surface and propose guidelines to improve
    cathode catalysts durability based on our proposed mechanism. Density functional
    theory is employed to study various possible scenarios with the goals of understanding
    the mechanism of the metal atom dissolution process and establishing some guidelines
    that permit a rational design of catalysts with better stability against dissolution. A
    thermodynamic analysis of potential metal dissolution reactions in acid medium is
    presented first, using density functional theory calculations to explore the relative
    stabilities of transition metals in relation to that of Pt. The study is performed by
    comparing the change in reaction Gibbs free energies for different metals in a given
    dissolution reaction. Then, a series of density functional theory studies, tending to
    investigate the adsorbed atomic oxygen absorption process from cathode catalyst surface
    into its subsurface, includes: 1) the oxygen adsorption on various catalyst surfaces and
    oxygen absorption in subsurface sites to figure out the minimum energy pathway and
    energy barrier of on-surface oxygen migration and absorption into subsurface; 2) the oxygen coverage, the other oxygen reduction reaction intermediates, and water effects
    on the oxygen absorption process according to reaction pathways, energy barriers, and
    thermodynamic analysis; 3) the oxygen absorption process on several Pt-based alloys
    with various compositions and components to find out the best alloy to inhibit atomic
    oxygen absorption including both kinetic and thermodynamic analyses, and the effects
    of such alloyed species on the inhibition process.

publication date

  • December 2007