Strain engineering of advanced Pt based electrocatalysts via atomic layer deposition
Abstract
Proton exchange membrane fuel cells are considered as the next-generation energy conversion system due to their high efficiency, simple structure and cleanness. However, the main impediment to the commercialization of proton exchange membrane fuel cells is the high cost of their noble metal Pt electrocatalysts in the electrodes. Thus, three challenges including (1) low atom utilization efficiency, (2) poor stability under long-term operation, and (3) low catalytic activity, especially during practical fuel cell applications need to be resolved before the large-scale commercialization of the Pt based electrocatalysts.
To address these challenges, several strategies were explored in this thesis:
Firstly, Ni-O-W dimer atoms decorated with Pt nanoparticles were prepared via atomic layer deposition. The surface stable compressive strain induced by the compact Ni-W dimer structure greatly enhanced the catalysts’ performance for oxygen reduction reaction both in rotating disk electrode and practical fuel cell applications.
Secondly, three additional elements, including Co, Ru, and W, were integrated and selectively deposited on the concave surface of PtNi octahedral nanoparticles to further improve their performance. The high entropy of the single atom alloy created a stable compressive strain that resulted in a tenfold increase in catalytic activity compared to commercial Pt/C catalysts. Notably, investigations of spatial effects revealed that randomly distributed high-entropy atoms did not improve catalytic activity.
Furthermore, to increase Pt atom utilization efficiency, Pt single atoms were successfully synthesized on an amorphous tantalum oxide support, featuring short Pt-O bonding paths. The compact Pt-O bonding from the amorphous structure led to a threefold increase in hydrogen evolution reaction activity compared to traditional Pt-N coordination.
Lastly, the site-specific strain of Pt nanoclusters of various sizes was analyzed using high-precision scanning transmission electron microscopy. A volcano-shaped relationship was discovered between active site characteristic strain, the d-band center, and catalytic activity. Computational studies further confirmed this relationship and proposed an optimized site-specific strain for improved hydrogen electrocatalysis.
In summary, these findings in this thesis contribute to overcoming the key challenges associated with Pt-based electrocatalysts and bring fuel cells closer to large-scale commercialization.