Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor(s)

Xueliang (Andy) Sun

Abstract

Proton exchange membrane fuel cells (PEMFCs) are promising energy converting technologies to generate electricity by mainly using hydrogen as a fuel, producing water as the only exhaust. However, short life-time and high cost of Pt catalyst are the main obstacles for the commercialization of PEMFCs. In the conventional carbon black supported platinum nanoparticle (NP) commercial catalyst, carbon supports are prone to oxidation and corrosion over time that results in Pt NPs migration, coalescence, even detaching from the catalyst support. In addition, Ostwald ripening of the Pt NPs could also occur due to their high surface energy and zero dimensional structural features. All these contribute to the degradation of fuel cell performance. This research aims at fabricating various advanced nanomaterials, including (1) Pt-based highly efficient nanocatalysts and (2) alternative nanostructured durable catalyst supports, to address the above-mentioned challenges in PEMFCs.

It is well known that the catalytic activity and durability of Pt catalysts are highly dependent on their size and shape. In contrast to commercially-used Pt spherical nanoparticles, one-dimensional (1D) structures of Pt, such as nanowires (NWs), exhibit additional advantages associated with their anisotropy and unique structure.

We first reported a new approach to address both activity and durability challenges of PEM fuel cells by using 1D Pt nanowires (PtNWs) as electrocatalyst. Pt NWs were synthesized via a very simple environmentally-friendly aqueous solution route at room temperature, without the need of heating, surfactants or complicated experimental apparatus. This novel PtNW nanostructure showed much improved activity and durability than the state-of-the-art commercial Pt/C catalyst which is made of Pt nanoparticles. Further, Pt NWs were grown on Sn@CNT nanocable support to form a novel 3D fuel-cell electrode (PtNW/Sn@CNT). This approach allows us to combine the advantages of both PtNW catalyst and Sn@CNT 3D nanocable support for fuel cell applications. The PtNW/Sn@CNT 3D electrodes showed greatly enhanced electrocatalytic activities for ORR, MOR and improved CO tolerance than commercial Pt/C nanoparticle catalyst.

To save more platinum, ultrathin Pt NWs with even smaller diameters of 2.5 nm (vs. 4 nm reported in our previous work) have been successfully synthesized when using N-doped CNTs as support. Direct evidence for the formation of ultrathin Pt NWs was provided by systematically investigating their growth process under TEM. Nitrogen doping in CNTs played a key role in the formation of ultrathin Pt nanowires.

In terms of low durability of PEM fuel cell catalysts, the corrosion of current commonly-used carbon black support materials have been identified to be the major contributor to the catalyst failure. One of the major challenges lies in the development of inexpensive, efficient, and highly durable alternative catalyst supports that possess high corrosion resistance, high conductivity and high surface area. In this work, a series of promising alternative nanostructured catalyst supports, including 0D Nb-doped TiO2 hollow nanospheres, 1D TiSix-NCNT nanostructures, and 2D graphene nanosheets, have been synthesized by various methods and used as catalyst supports. Pt nanoparticles were then deposited on these novel supports, showing enhanced catalytic activities and durabilities. Most interestingly, a new technique, atomic layer deposition (ALD), was used to uniformly deposit Pt nanoparticles, subnanometer clusters and single atoms on graphene nanosheets. Downsizing Pt nanoparticles to clusters or even single atoms could significantly increase their catalytic activity and is therefore highly desirable to maximize the efficiency.

In summary, the discoveries in this thesis contribute to applying various novel nanostructured materials to design highly active and stable electrocatalyst and durable catalyst support to develop high performance and low cost PEM fuel cells.


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