Durability of carbon-supported platinum nanoparticle is one of the limiting factors for PEM fuel cell commercial applications. In our research work, we applied both experimental and mathematical simulative tools to study the mechanisms of Pt/C catalyst degradation. An accelerated catalyst degradation protocol by cycling the cathode potential in a square-wave profile was applied to study the losses of cell performances, catalyst ORR activity, and Pt active surface areas. Post-mortem analyses of cathode Pt particle size by X-ray diffraction and platinum distributions in CCMs by SEM/EDS were also conducted. Increased cell temperature and relative humidity was found to accelerate the cathode catalyst degradation. High membrane water contents or abundant water/ionic channels within the polymer electrolyte were believed to accelerate Pt ion transport and cathode degradation. After degradation tests, significant amount of Pt loss into the membrane forming a Pt "band" was observed through cathode platinum dissolution and chemical reduction of soluble Pt ions by permeated hydrogen from the anode. Platinum deposition was confirmed at a location where the permeated hydrogen and oxygen had the complete catalytic combustion over the deposited Pt clusters/particles as the catalyst. A cathode degradation model was built including the processes of platinum oxidation, dissolution/replating, diffusion of Pt ions and Pt band formation in electrolyte. A simplified bi-modal particle size distribution was applied with equal numbers of small and large type particles uniformly distributed in the cathode initially. Processes of Pt mass exchange between two types of particles were demonstrated to cause the overall particle growth. This was due to the particle size effect that platinum dissolution from the small type particles and replating of Pt ions onto the large particles was favored. Parametric study found the increased upper cycling potential was the dominated factor to accelerate the catalyst degradation. Also high frequency of potential cycle and low surface oxide coverage accelerated the degradation rate. Pt dissolution and oxidation processes in perchloric acid were preliminary investigated, and both chemical and electrochemical processes of oxidation and dissolution were believed to be involved under closed-circuit fuel cell conditions with oxygen presence at cathode.

Fuel cells have the potential to change the energy paradigm by allowing more efficient use of energy. In particular, Polymer Electrolyte Membrane Fuel Cells (PEMFC) are interesting because they are low temperature devices. However, there are still numerous challenges limiting their widespread use including operating temperature, types of permissible fuels and optimal use of expensive catalysts. The first two problems are related mainly to the ionomer electrolyte, which largely determines the operating temperature and fuel type. While new ionomer membranes have been proposed to address some of these issues, there is still a lack of fundamental knowledge to guide ionomer design for PEMFC. This work is a computational study of the effect of temperature and water content on sulfonated poly(ether ether ketone) and the effect of acidity on sulfonated polystyrene to better understand how ionomer material properties differ. In particular we found that increased water content preferentially solvates the sulfonate groups and improves water and hydronium transport. However, we found that increasing an ionomer’s acid strength causes similar effects to increasing the water content. Finally, we used Density Functional Theory (DFT) to study platinum nano-clusters as used in PEMFCs. We developed a model using the atom’s coordination number to quickly compute the energy of a cluster and therefore predict which platinum atoms are most loosely held. Our model correctly predicted the energy of various clusters compared to DFT. Also, we studied the interaction between the various moieties of the electrolyte including the catalyst particle and developed a force field. The coordination model can be used in a molecular dynamics simulation of the three phase region of a PEMFC to generate unbiased initial clusters. The force field developed can be used to describe the interaction between this generated cluster and the electrolyte.