K.C. Neyerlin

Measurement of Oxygen Transport Resistance in PEM Fuel Cells by Limiting Current Methods

Limiting current measurements in a polymer electrolyte membrane (PEM) fuel cell are used to separate the oxygen-transport resistance into individual component parts. By varying the thicknesses of the diffusion medium (DM) and the microporous layer in different cell builds, the total transport resistance is separated into contributions from flow channels, DM, microporous layer, and all other sources. By varying the pressure, the transport resistance is separated into a pressure-dependent component (intermolecular gas diffusion) and a pressure-independent component (Knudsen diffusion or transport through ionomer/liquid water layers). In addition to oxygen diffusion in an anisotropic gas diffusion layer, the analysis accounts for coupled convective diffusion and reactant depletion in the flow channels. The present work is limited to conditions when no condensation occurs inside the cell. The analysis is applied to a large body of limiting current data collected on Toray diffusion media, both plain and treated with poly(tetrafluoroethylene) (PTFE), with and without a microporous layer. Effective diffusion coefficients obtained from these methods for plain Toray papers compare reasonably well with independent ex situ measurements of water vapor diffusion through the same materials.

Exceptional Oxygen Reduction Reaction Activity and Durability of Platinum–Nickel Nanowires through Synthesis and Post-Treatment Optimization

For the first time, extended nanostructured catalysts are demonstrated with both high specific activity (>6000 μA cmPt–2 at 0.9 V) and high surface areas (>90 m2 gPt–1). Platinum–nickel (Pt—Ni) nanowires, synthesized by galvanic displacement, have previously produced surface areas in excess of 90 m2 gPt–1, a significant breakthrough in and of itself for extended surface catalysts. Unfortunately, these materials were limited in terms of their specific activity and durability upon exposure to relevant electrochemical test conditions. Through a series of optimized postsynthesis steps, significant improvements were made to the activity (3-fold increase in specific activity), durability (21% mass activity loss reduced to 3%), and Ni leaching (reduced from 7 to 0.3%) of the Pt—Ni nanowires. These materials show more than a 10-fold improvement in mass activity compared to that of traditional carbon-supported Pt nanoparticle catalysts and offer significant promise as a new class of electrocatalysts in fuel cell applications.

The role of nitrogen doping on durability in the Pt-Ru/HOPG system.

Improving catalytic activity and durability are two major issues that must be addressed for fuel cells to become commercially viable. Surface modifications and doping of the catalyst support has been shown to effectively address both of these issues through significant improvements in the catalyst-support interactions [1, 2]. In this work we discuss the role of nitrogen doping via ion-implantation on the stability of a Pt-Ru nanoparticle catalyst phase supported on model highly-oriented pyrolytic graphite (HOPG) substrates.