Processing and functionalization of conductive substoichiometric TiO 2 catalyst supports for PEM fuel cell applications
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e development of substoichiometric TiO2-based nanostructured materials with high aspect ratios for future proton exchange membrane fuel cells is investigated. Nanostructures were manufactured using atomic layer deposition of TiO2 over both anodic aluminum oxide templates and silicon nanowires. It was observed in this work that nanostructures with aspect ratios of 100:1 can be fabricated using both methods. The conductivity of TiO2 films was enhanced following a postdeposition reducing anneal (at 450 °C in H2). Liquid phase-deposited Pt and plasma-enhanced atomic layer deposition of Pt were both found to be appropriate suited for metallization of TiO2 structures.
I. INTRODUCTION
The most common technology currently used for proton exchange membrane fuel cells (PEMFCs) utilizes Pt deposited on colloidal carbon black to function as the cathode material. This current electrode technology, while effective in its design, suffers from a number of well-known challenges. The first is carbon corrosion.1,2 Carbon corrosion is caused by the oxidization of carbon over time while functioning as a catalyst support material for Pt. Carbon corrosion can cause the supported Pt to be released from the support material and agglomerate with other Pt catalyst particles. This agglomeration limits the catalyst surface area that can be accessed by gases such as oxygen, which, in turn, leads to the degradation of overall fuel cell efficiency. The second issue is electrode flooding (a topic reviewed by Li et al.3). As a fuel cell cathode catalyzes the oxygen reduction reaction (ORR) O2 1 4H1 1 4e ! 2H2O, which combines hydrogen and oxygen, the resulting water must be transported out of the system. For this to take place efficiently, the catalyst support material must be structured to provide adequate gas transport pathways to prevent overaccumulation of water on the catalyst material surface. If too much accumulation occurs, the resulting water will begin to block the active Pt sites, which leads to increased local potentials at the carbon–catalyst interface and reduces the efficiency of the electrode. A strategy for overcoming these two issues involves changing both the catalyst support material and the structure of that material. In searching for a new material, there are several requirements that are important to cona)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.324 J. Mater. Res., Vol. 28, No. 3, Feb 14, 2013
sider. First, the material should be conductive enough to potentially allow for ultralow Pt loadings on the surface. Second, the material should bond well with Pt and be chemically stable in an oxidizing environment so as to maintain both adhesion and a conductive link to the Pt for the life of the fuel cell (i.e., no carbon corrosion). Third, the material must be structured to provide the needed gas pathways in the electrode, which will minimize water accumulation thereby preventing electrode flooding. It is anticipated that this will be accomplished by making structures that have high
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