Chemical and Biological Engineering ETDs

Ronald Goeke

Publication Date

9-9-2010

Abstract

Proton exchange membrane fuel cells (PEMFCs) are being extensively studied as power sources because of their technological advantages such as high energy efficiency, high energy and power densities, environmental friendliness, low noise, rapid refueling, and fuel flexibility. The most effective catalyst in these systems consists of nanoparticles of Pt or Pt-based alloys on carbon supports. Understanding the role of the nanoparticle size and structure on the catalytic activity and degradation is needed to optimize the fuel cell performance and reduce the noble metal loading. Electrocatalyst degradation is believed to occur by four mechanisms; particle sintering (Ostwald ripening and migration and coalescence), carbon corrosion and catalyst dissolution. Increasing catalyst layer durability is becoming a major challenge and a growing focus of research attention in PEM fuel cell durability studies. Typical model systems utilized to study catalyst activity are single crystal materials or bulk polycrystalline materials which are not useful for studying catalyst degradation mechanisms as no particles exist. Powder catalysts on RDE electrodes can be very useful for catalyst evaluation and general degradation studies, but contain a distribution of particle sizes on a high surface area support that make them difficult to analyze. We addressed this gap in research needs with the development of model electrode structures, which consist of catalytic nanoparticles on planar model supports for easy analysis. We improve upon this basic concept by engineering the nanoparticle dispersion directly on the planar carbon support as ordered arrays. Ordered nanoparticle arrays of controlled size and spacing were fabricated by adapting a previously developed templating techniques using diblock copolymers. These well defined model electrode structures were applied to the study of electrocatalyst degradation and the dissolution reaction of Pt catalyst in acid medium. It is shown that these electrodes can be transferred between the electrochemical cell and SEM imaging without impact on the individual Pt nanoparticles. Control over the Pt particle size is demonstrated by changing the metal loading, annealing conditions and diblock copolymer templating. Pt nanoparticle sintering on many of these surfaces was observed to proceed through the formation of Pt nanowires. Degradation mechanisms of particle detachment were observed specifically under the harsh potential cycling conditions (0 — 1.4VRHE) used for the initial conditioning and when potential is cycled in oxygen saturated electrolyte. Ostwald ripening and dissolution were observed through the use of particle size distributions; particle migration and coalescence was observed upon close examination of repeat SEM images taken of the same area. For the first time ordered arrays of Pt nanoparticles on glassy carbon electrode have been fabricated and tested.

Keywords

electrocatalyst, platinum, model, degradation; Fuel cells--Electrodes., Proton exchange membrane fuel cells., Electrocatalysis., Nanoparticles., Catalyst supports., Platinum., Ostwald ripening.

Sponsors

National Science Foundation

Document Type

Dissertation

Language

English

Degree Name

Chemical Engineering

Level of Degree

Doctoral

Department Name

Chemical and Biological Engineering

First Committee Member (Chair)

Atanassov, Plamen

Second Committee Member

Han, Sang

Third Committee Member

Grey, John

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