Vapor Synthesis and Thermal Evolution of Supportless, Metal Nanotubes and Application as Electrocatalysts

One of the major limitations of proton exchange membrane fuel cells (PEMFCs) is the high cost and poor durability of the currently preferred catalyst design, small Pt nanoparticles supported on high surface area carbon (Pt/C). Unsupported, high-aspect ratio nanostructured catalysts, or extended surface catalysts, are a promising paradigm as electrocatalysts for a number of electrochemical reactions. These extended surface catalysts generally exhibit higher specific activities compared to their carbon-supported nanoparticle counterparts that have been ascribed to their unique electronic, surface and structural properties. Extended surface catalysts frequently maintain enhanced durability over supported catalysts during fuel cell operation because they are not susceptible to the same modes of degradation inherent to small supported nanoparticles.

Considering the success of extended surfaces as catalysts, we have synthesized metallic, mixed-phase, and alloyed bimetallic nanotubes by a chemical vapor deposition (CVD) technique to catalyze a number of reactions relevant for fuel cells. In this CVD process, metalorganic precursors, namely metal-acetylacetonates, are decomposed by a mild thermal treatment and deposited as conformal nanoparticulate layers within a sacrificial anodic alumina template. Following vapor deposition, the nanotube samples may be annealed while still in the template to induce a series of changes with implications on electrocatalysis, including nano-porosity, alloying, and surface coordination. This synthesis technique and successive thermal modification is applicable for the deposition of a number of metals. The metallic nanotubes prepared by this method are highly active catalysts for a host of electrochemical reactions that are promising for fuel cell applications. The effects of composition, heat treatment temperature and gas environment on the activity and durability of these materials have been studied for oxygen reduction, methanol oxidation, formic acid oxidation, and hydrogen oxidation.