Lamas, Eduardo J. (2003-05). Theoretical studies of transition metal surfaces as electrocatalysts for oxygen electroreduction. Doctoral Dissertation. Thesis uri icon

abstract

  • In the last few years the quest towards a hydrogen based economy has intensified interest for effective and less expensive catalysts for fuel cell applications. Due to its slow kinetics, alternative catalysts for the oxygen electroreduction reaction are actively researched. Platinum alloys with different transition metals (for example: Ni, Co and Fe) have shown improved activity over pure Pt. The design of a Pt-free catalysts is also highly desirable, and different alternatives including metalloporphyrins and Pd-based catalysts are being researched. Pd-based catalysts constitute an attractive alternative to Pt alloys in some fuel cell applications, not only because of lower costs but also because of the lower reactivity of Pt alloys towards methanol, which is important for improved methanol crossover tolerance on direct methanol fuel cells. In this work we apply density functional theory (DFT) to the study of four catalysts for oxygen electroreduction. The electronic structure of these surfaces is characterized together with their surface reconstruction properties and their interactions with oxygen electroreduction intermediates both in presence and absence of water. The energetics obtained for the intermediates is combined with entropy data from thermodynamic tables to generate free energy profiles for two representative reaction mechanisms where the cell potential is included as a variable. The study of the barriers in these profiles points to the elementary steps in the reaction mechanisms that are likely to be rate-determining. The highest barrier in the series pathway is located at the first proton and charge transfer on all four catalytic surfaces. This is in good agreement with observed rate laws for this reaction. The instability of hydrogen peroxide on all surfaces, especially compared with the relatively higher stability of other intermediates, strongly points at this intermediate as the most likely point where the oxygen bond is broken during oxygen reduction. This adds to the argument that this path might be active during oxygen electroreduction. A better understanding behind the reaction mechanism and reactivities on these representative surfaces will help to find systematic ways of improvement of currently used catalysts in the oxygen electroreduction reaction.
  • In the last few years the quest towards a hydrogen based economy has intensified
    interest for effective and less expensive catalysts for fuel cell applications. Due to its
    slow kinetics, alternative catalysts for the oxygen electroreduction reaction are actively
    researched. Platinum alloys with different transition metals (for example: Ni, Co and Fe)
    have shown improved activity over pure Pt. The design of a Pt-free catalysts is also
    highly desirable, and different alternatives including metalloporphyrins and Pd-based
    catalysts are being researched. Pd-based catalysts constitute an attractive alternative to Pt
    alloys in some fuel cell applications, not only because of lower costs but also because of
    the lower reactivity of Pt alloys towards methanol, which is important for improved
    methanol crossover tolerance on direct methanol fuel cells.
    In this work we apply density functional theory (DFT) to the study of four catalysts
    for oxygen electroreduction. The electronic structure of these surfaces is characterized
    together with their surface reconstruction properties and their interactions with oxygen
    electroreduction intermediates both in presence and absence of water. The energetics
    obtained for the intermediates is combined with entropy data from thermodynamic tables
    to generate free energy profiles for two representative reaction mechanisms where the
    cell potential is included as a variable. The study of the barriers in these profiles points
    to the elementary steps in the reaction mechanisms that are likely to be rate-determining.
    The highest barrier in the series pathway is located at the first proton and charge transfer
    on all four catalytic surfaces. This is in good agreement with observed rate laws for this
    reaction. The instability of hydrogen peroxide on all surfaces, especially compared with
    the relatively higher stability of other intermediates, strongly points at this intermediate as the most likely point where the oxygen bond is broken during oxygen reduction. This
    adds to the argument that this path might be active during oxygen electroreduction.
    A better understanding behind the reaction mechanism and reactivities on these
    representative surfaces will help to find systematic ways of improvement of currently
    used catalysts in the oxygen electroreduction reaction.

publication date

  • May 2003