Electrocatalytic activities of supported Platinum nanoparticles for low-temperature FC applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2010.Cataloged from PDF version of thesis.Includes bibliographical references.Low-temperature fuel cells (FCs) are highly efficient and environmentally friendly energy conversion devices that have been in the spotlight of many energy research efforts in the past few decades. However, FC commercialization is limited by several difficulties, one including the 60 % cell voltage loss caused by the sluggish oxygen reduction reaction (ORR) at the cathode despite the use of the active Pt nanoparticles (NPs) supported on high surface area carbon as the catalyst. In addition, the voltage loss due to the anode reaction kinetics in alkaline FCs (AFCs) remains unknown to the FC society, unlike for the proton exchange membrane fuel cell (PEMFC) case, where the loss due to the anode reaction has been well understood. Moreover, the high surface area carbon used for supporting nanoparticle catalysts is also known to corrode during the FC operation, degrading the cell performance over time. To offer a guideline to develop potential solutions to the above issues, this thesis seeks to explore and develop the fundamental understandings of both the cathode and the anode reaction kinetics for low temperature FCs of both PEMFCs and AFCs, and to demonstrate a new type of catalyst support that is resistant to corrosion. On the cathode side (the ORR), how the size of the Pt nanoparticle catalyst affect the performance is still under debate. By investigating the ORR on Pt NPs at different sizes and coupling the results to the spectroscopic information, we seek to explore the fundamentals behind the size effect on the ORR activities. We found that below 5 nm, particle size does not play a big role in the catalytic activity. However, the instability of Pt NPs in acidic environment, under simulated operation conditions of a PEMFC, is found to strongly depend on the particle sizes, which is proposed to be due to the Gibbs- Thomson effect. The findings of the particle size effect on the ORR activities and instability suggest that a trade off between smaller NP catalysts, which gives a benefit of a larger mass activity, can suffer from fast degradation. Therefore, a proper NP size that balances between high mass activity and stability would be the best for FC applications according to our study. On the anode side (the HOR), we found that the reaction kinetics in acid solution on Pt is solely limited by the diffusion of the reactant, and therefore, within the experimental uncertainty, the reaction rate of the HOR in acid is not measurable with conventional rotating disk electrode setup. However, once the same testing configuration is applied to the alkaline solution, the HOR kinetics on Pt electrode are found to be limited by the reaction kinetics, which is in contrast to the case in acid. From this finding, the anodic overpotential loss in AFCs/AMFCs is projected for the first time and is found to be 1/3 of the ORR loss. This thesis thus highlights the need for development of highly efficient HOR catalysts in alkaline in order to make AFCs/AMFCs more efficient. Carbon corrosion represents one of the biggest effects that contribute to the performance degradation in FCs. In this work, multi-walled carbon nanotubes (MWCNTs) supported Pt NPs as a novel corrosion-resistant electrocatalyst support for the ORR is proposed as a solution. The Pt/MWCNTs were synthesized through the electrostatic interaction between the Pt precursor and the functionalized MWCNTs, followed by chemical reduction in H2 at elevated temperatures. Our Pt/MWCNTs catalysts exhibit an enhanced durability after an anodic potential holding, which simulates a typical FC environment during the carbon corrosion. The ORR activity of Pt/MWCNTs is also in agreement with that of the commercial Pt/C. The results indicate that MWCNTs have great potential to serve as a novel carbon-support material with high stability without affecting the Pt catalytic activity.by Wenchao Sheng.Ph.D