Designing New Pt-Based Catalysts: Dual Manipulation of Highly-Branched Structures and Intermixed Compositions

This thesis investigates new design strategies for cost-efficiently fabricating Pt-based catalysts with exceptional oxygen reduction reaction (ORR) performance. Findings open new opportunities for developing next-generation, more cost-effective Pt-based catalysts, potentially advancing hydrogen fuel cell (HFC) technology by enhancing performance and addressing cost and energy security challenges through innovative catalyst design. The thesis is divided into four sections, with a particular emphasis on the control of two aspects of anisotropic nanoparticle (NP) growth; manipulating highly-branched morphologies and also composition. This dual control of properties promises to significantly reduce Pt reliance and synthesis cost and complexity while enhancing catalytic activity The first section explores the combination of Pt with Fe under various synthesis conditions, resulting in bimetallic NPs that provide insights into nanopod and nanodendrite formation. By finely controlling the reduction of metal precursors through the modulation of capping agents, reagents, and temperature, morphology control is then maintained while the composition is varied from Pt-rich to Pt-poor. Additionally, a methodology for promoting the collision-based branching of nanopod arms has been developed. This process is found to occur when the concentration of nanopods reaches a critical threshold, leading to diffusion-controlled collisions that induce nanodendrite formation. This allows the selective growth of compositionally controlled nanodendrites for the first time. The second section investigates the interplay between kinetic NP growth and thermodynamic reduction pathways, which opens opportunities for designing ultra-low-cost Pt-based catalysts. This part includes the synthesis of Pt/Fe nanopods that significantly reduce reliance on Pt through a simple one-pot synthesis protocol. This process is conducted over 30 minutes in an ambient pressure N₂ environment at 250°C. The method significantly lowers Pt dependence and increases yield by optimising the input ratio of Fe to Pt precursors. A reducing in the Pt precursor input from an initial ratio of 0.5 (relative to Fe) to 0.2 and eventually to 0.1 may improve the yield of Pt/Fe nanopods from 55% to 78% and potentially up to 94%. Nanopods, made with a Pt:Fe ratio of 0.1, exhibit exceptional ORR performance, reaching a mass activity of 1.27 A/mg_Pt at 0.9 V versus RHE, almost 32 times more mass-efficient than the commercial 10 wt% Pt NP/C standard, at an electrode loading as low as 0.56 μg_Pt/cm². Furthermore, the design strategy for these nanopod catalysts offers extensive chemical flexibility, allowing for effective manipulation of arm density and composition across different raw materials. Building on the successful dual control over Pt/Fe morphology and composition, the third section extends the developed methodology to a range of anisotropically grown Pt/Ni and Pt/Co bimetallic NPs. The Pt/Fe system is used to develop a method to promote collision-based branching of nanopod arms at critical nanopod concentrations, leading to diffusion-controlled collisions and subsequent nanodendrite formation. Leveraging this established protocol, the synthesis parameters for Pt/Ni and Pt/Co systems have been fine-tuned to replicate the controlled environment that facilitates similar branching behaviours. By ensuring the critical nanopod concentration is achieved in each case, the formation of nanodendrites is induced. This demonstrates the versatility and effectiveness of the original methodology across different bimetallic combinations, allowing a detailed understanding of how adjusting individual synthetic parameters affects the formation of Pt/M nanopods and nanodendrites with composition control. The obtained catalysts demonstrate superior ORR performance at an electrode loading as low as 2.2 μg_Pt/cm². Notably, nanodendritic Pt/Ni achieves a mass activity of 0.55 A/mg_Pt at 0.9 V versus RHE, making it 87 times more efficient in terms of Pt content than a commercial 10 wt% Pt/C NP standard. The final section extends the success in preparing anisotropic bimetallics to the synthesis of Pt/M1/M2 (M = Fe, Co, Ni) nanopods and nanodendrites with uniformly intermixed compositions. The resulting catalysts exhibit superior ORR performance. Particularly, Fe/Co/Pt nanopods achieve a mass activity of 1.01 A/mg_Pt at 0.9 V versus RHE at an electrode loading as low as 0.76 μg_Pt/cm², making them 28 times more efficient by mass than a commercial 10 wt% Pt/C NP standard. In contrast to previous reports, this study demonstrates the use of a non-empirical synthetic strategy to achieve a range of trimetallic compositions with different morphologies, addressing the critical issue of phase segregation in such systems and paving the way for new levels of catalyst design in this area.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2024

Author dc:creator

• Ming, Siyi

Advisor dc:contributor.advisor

• Wheatley, Andrew

• Nanodendrite
• Nanopod
• ORR
• Platinum
• Pt-based catalyst

Licence

• https://www.repository.cam.ac.uk/bitstreams/cc27cf9d-6bfe-4d83-aa56-f1dcf07dac40/download
• https://creativecommons.org/licenses/by/4.0/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.112791

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/374908