Electrocatalysts with high activity and stability for polymer electrolyte membrane fuel cells

In addressing the activity and durability challenges facing electrocatalysts in polymer electrolyte membrane fuel cells (PEMFCs), atomic layer deposition (ALD) is emerging as a powerful technique for deposition of noble metals and transition metal oxides due to its exclusive advantages over other methods. The primary advantages of ALD are derived from the sequential, self-saturating, gas-surface reactions, and angstrom level control that take place during the deposition process. Therefore, ALD possesses the advantage in precisely control the particle size and uniform distribution on the substrate. By forming chemical bonds between the initial layer of ALD precursor and support atoms during the first cycle of deposition, the strong interaction between the deposited material and support could be enabled, which is benefit for achieving the highly stable NPs.\nIn this thesis, an area-selective ALD of tantalum oxide (TaOx) on Pt/C catalyst to fabricate TaOx-anchored Pt NPs with triple-junction structure of Pt–TaOx–C was investigated in the first part of research work. By introducing a protective agent (oleylamine) to the Pt surface, TaOx NPs were selectively nucleated and grown around Pt NPs, thus forming the TaOx anchored-Pt NPs on the carbon surface. The electrochemical durability tests indicated that the 35ALD–TaOx–Pt/C catalyst exhibited superior durability compared to Pt/C. The enhanced stability of the 35ALD–TaOx–Pt/C catalyst is attributed to the anchoring effect of TaOx via the strong triple-junction of TaOx–Pt–C, which plays a significant role in stabilizing the Pt catalyst by preventing Pt NPs from migration/coalescence and detachment from the carbon support. Afterwards, to both improve the Pt/C catalyst activity and durability, a nitrogen-doped Ta2O5 was developed by ALD approach. It was found that the as-prepared Pt/N-ALDTa2O5/C catalyst showed enhanced catalytic activity and significantly improved electrochemical durability toward oxygen reduction reaction (ORR). X-ray absorption spectroscopy provided direct evidence of change of the electronic structure of Pt NPs on N-ALDTa2O5/C support compared to other supports, indicating the strong metal-support interactions formed between Pt NPs and the modified N-ALDTa2O5/C support. It was revealed that by tuning the metal support interface, the highly active and stable Pt catalyst could be enabled.\nTo achieve the extremely low Pt loading while maintaining the high catalytic activity and long-life stability, ALD technique was applied to deposit Pt NPs into the PEMFCs anode layer. By controlling the ALD cycle number, the Pt NPs with different size and loading amount were directly deposited on the carbon coating layers to form the anode catalyst layers. The PEMFCs composed with ALD catalyst showed much better performance and durability than that prepared by the commercial catalyst with a conventional method. The electron microscopy reveals that the application of ALD for Pt deposition directly on the electrode carbon layers could effectively reduce the Pt loading while enhance the Pt dispersion, utilization, and Pt-support interaction, which achieve the high PEMFCs performance and excellent durability under the ultra-low Pt loading. Furthermore, downsizing Pt NPs size to subnano-clusters or even single atoms is highly desirable to maximize Pt atom utilization efficiency. Here we report on a practical synthesis method to produce isolated Pt single atoms and subnano-clusters using ALD technique. The Pt single atoms/subnano-clusters catalysts supported on metal-organic framework-derived nanocarbon support are investigated for the ORR, where they exhibit significantly enhanced catalytic activity and superior stability in comparison with the Pt NPs catalysts. The X-ray absorption indicates that the partially unsaturated coordination environment of Pt single atoms/subnano-clusters on nanocarbon support is responsible for the excellent performance.\nThe last experimental investigation of this thesis is development of an alternative non-noble-metal electrocatalyst of nitrogen and sulfur-co-doped nanocarbon (N,S-co-doped nanocarbon) for ORR. The N,S-co-doped nanocarbon is synthesized using metal organic frameworks as a solid precursor, followed by carbonizing and pore size design, then further co-doping sulfur to generate more active sites. The resulting N,S-co-doped nanocarbon demonstrates a high catalytic activity toward ORR, remarkable long-term stability and strong methanol tolerance in alkaline media. First-principles calculations reveal that N,S-co-doped nanocarbons possess enhanced ORR activity compared to N-doped carbon. More importantly, this work for the first time report that the N,S-coupled dopants can create active sites with higher activity than the isolated N and S dopants. The approach and analysis adopted in this work offer a strategic consideration for designing the high performance nanocarbon electrocatalyst.

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