Jean St-Pierre

Aging Studies of Pt/Glassy Carbon Model Electrocatalysts

A model Pt catalyst nanoarray on planar glassy carbon (GC) was synthesized using a diblock copolymer synthesis route and subjected to a potential cycle aging procedure. Preliminary experimental data show initial and degradation behavior similar to that of proton exchange membrane fuel cell gas diffusion electrodes. Commonalities include similar cyclic voltammetry and CO stripping voltammetry features, Pt active surface area temporal changes, and Pt particle size effect on CO oxidation peak potential. Furthermore, atomic force microscopy provides evidence for both Pt dissolution and Pt nanoparticle migration/coalescence as aging mechanisms. Experimental data obtained with Pt model catalysts also revealed that Pt dissolution may be the rate-determining step under certain operating conditions. Thus, the beneficial use of Pt/GC model electrocatalysts as surrogates for commercial gas diffusion electrodes is validated.

Nanoparticle Size Effects on the Electrochemical Dissolution Rate of Pt

Proton exchange membrane fuel cells are being extensively studied as power sources because of their technological advantages such as high energy efficiency and environmental friendliness. 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. One of the more significant causes of fuel cell performance degradation is the cathode catalyst deactivation. There are four mechanisms considered relevant to the loss of electrochemically active surface area of Pt in the fuel cell electrodes that contribute to cathode catalyst degradation, including: catalyst particle sintering such as Ostwald ripening, migration and coalescence, carbon corrosion and catalyst dissolution. The dissolution of the Pt nanoparticles is fundamental to a few of these mechanisms. Most approaches to study this role utilize membrane electrode assemblies (MEAs), which results in a complex system where it is difficult to deconvolute the effects of the metal nanoparticles. Our research addresses this need by taking a fundamental approach to study the electrocatalyst using a model support. We have recently shown this type of 2D model catalyst as a very useful tool for studying the sintering of Pd particles at high temperatures. We now apply this model system to study Pt nanoparticle electrochemical activity degradation. Electrochemical processes at the nanometer scale may dissolve Pt at potentials more negative than predicted based on the Nernst equation alone. An electrochemical equivalent of the Gibbs-Thomson equation can provide the effect of interfacial energy on the solubility in terms of particle radius. This curvature effect on EPt can be calculated by adding this effect to the Nernst equation.

PEMFC Reactant Mass Transfer Coefficient Measurement and Separation – Method Extension to the Mixed Kinetic and Mass Transfer Control Regime

A kinetic and mass transfer model for a proton exchange membrane fuel cell (PEMFC) current distribution was derived and validated with data obtained with a dilute oxygen stream and three diluents (5 % O2 + 95 % He, N2 or C3F8). The overall mass transfer coefficient k increased by a factor of ~2 to ~4 with a decrease in cell potential from 0.75-0.8 to 0.2 V. Mass transfer coefficients pertaining to gas phase molecular diffusion km, and, gas phase Knudsen and solid phase ionomer diffusion ke+K respectively increased by 20 % and a factor of ~2. Below 0.6 V, the km and ke+K increase is relatively smaller and is ascribed to the larger heat generation, local temperature and diffusion coefficients. The increase in km and ke+K between 0.75 and 0.6 V is relatively larger and is attributed to the development of steep oxygen and diluent concentration gradients affecting oxygen movement.