Yan, Jay Hon Cheung; (2025) Development and Characterisation of Pt-Based Catalysts for Proton Exchange Membrane Fuel Cells by X-Ray Spectroscopy. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The transition to a hydrogen economy is imperative for combatting climate change, with proton exchange membrane fuel cells (PEMFCs) emerging as a key technology due to the high efficiency and zero carbon emission. However, the widespread commercialisation of PEMFCs remains hindered by the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode, necessitating the use of expensive Pt based catalysts. This thesis explores the design, degradation, and performance enhancement of Pt based ORR electrocatalysts, leveraging advanced imaging, electrochemical techniques, and synchrotron based spectroscopy to uncover fundamental structure performance relationships. A correlative identical location X ray fluorescence and scanning transmission electron microscopy (IL XRF STEM) method was utilised to visualise the evolution of Pt nanoparticle catalyst over progressive degradation stages of accelerated durability test (ADT). The multi-modal and multi-length-scale nature of the technique enables statistical analyses of degradation pathways, allowing for the quantification of spatially resolved degradation distribution. IL XRF STEM provided insights into the site specific degradation mechanisms, including Pt dissolution, Ostwald ripening, particle agglomeration and detachment, and carbon corrosion. The integration of XRF and STEM facilitated a comprehensive understanding of the relationship between catalyst restructuring, and catalytic performance loss, bridging the gap between single particle behaviour and bulk electrochemical degradation. These findings establish IL XRF STEM as a powerful tool for guiding the rational design of durable PEMFC catalysts. In addition, the synthesis and characterisation of a novel high entropy intermetallic (HEI) catalysts for PEMFC cathode are reported. The carbon supported HEI catalyst (HEI/C) was synthesised via a seed mediated method using commercially available Pt/C and was shown to achieve rapid structural ordering at low temperature, as confirmed by in situ synchrotron X ray diffraction (XRD). HEI/C exhibited superior ORR activity and durability compared to conventional Pt based catalysts, in both rotating disk electrode (RDE) and practical membrane electrode assembly (MEA) setups. The electronic and structural properties of HEI/C were further elucidated using a suite of ex situ, in situ, and operando synchrotron based X ray characterisation techniques. Hard X ray photoelectron spectroscopy (HAXPES), X ray absorption fine structure (XAFS), and resonant inelastic X ray scattering (RIXS) analyses revealed that the Pt d band of HEI/C was lower and broadened compared to an intermetallic bimetallic PtCo/C catalyst, despite both sharing identical compressive strain effects. These findings indicate that the promoted electrochemical performance in HEI/C originated from a strengthened ligand effect arising from the unique multi-metallic local coordination environment, which optimally tuned the Pt d band electronic structure, reducing O binding energy and improving catalytic activity. Furthermore, X ray absorption near edge structure (XANES) difference spectra (Δµ) and time resolved quick XANES (QXANES) analyses demonstrated that HEI/C exhibited enhanced resistance and reversibility towards oxidation, along with more facile Pt redox kinetics for oxygenated species , supporting the superior stability and activity under fuel cell operating conditions. This thesis contributes to the ongoing effort to develop cost effective PEMFC cathode catalysts by providing a detailed mechanistic understanding of ORR electrocatalysis. By integrating cutting edge synchrotron based microscopy, spectroscopy, and computational analysis methods, this work advances the design principles for high performance catalysts, paving the way for more sustainable and economically viable fuel cell technologies.

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