Announcement Defense Shreyas Harsha - October 1, 2025 14:45hr Model PT Nanoparticle Electrodes by solid state dewetting for electrochemical hydrogen evolution reaction

Shreyas Harsha is a PhD student in the Photocatalytic Synthesis group. (Co)promotors are Dr. Marco Altomare and Prof.dr. Guido Mul - Department of Chemical Engineering - Faculty of Science and Technology, University of Twente.

Here is the summary of his thesis:

Context

Noble metal catalysts are typically used in the form of nanoparticles (NPs) where their high surface-to-volume ratio enables efficient catalyst utilization and allows for reduced catalyst loadings. In electrochemical conversion technologies, such as electrolyzers and fuels cells, catalyst NPs are synthesized by wet chemistry. Careful tuning of experimental conditions (temperature, metal precursor, ligands, solvents, and templating agents) allows for precise control of NP properties. Such NPs are deposited on electrically conductive supports such as carbon particles and mixed with binders (ionomers) to form catalyst inks. This results in complex, often undefined, NP properties in terms of morphology, composition, and mass transport, making it challenging to assess the intrinsic catalytic activity and isolate it from non-kinetic effects.

 Objective

This thesis investigates a chemical free route to produce model nanoparticle electrodes. The research demonstrates how solid-state dewetting (SSD) of thin metal films deposited by physical vapor deposition (PVD) can be used to produce binder-free NPs directly on electrode substrates. SSD refers to the controlled heat-induced agglomeration of thin metal films into metal NPs with defined loading, size, structure, and composition. The thesis shows how dewetted NP electrodes serve as model systems to investigate nanoscale effects in electrocatalysis, hence to draw criteria to design advanced catalyst with superior performance. The work focuses particularly on determining how factors such as catalyst ECSA, support interactions, exposed facets, and lattice strain, affect the electrocatalytic performance.

 Content 

The thesis is organized in 6 chapters. 

Chapter 1 provides an overview on climate change and mitigation strategies for the industry to reduce greenhouse gas emissions (GHGs), with emphasis on the role of green hydrogen in renewable energy and chemical processes. Electrochemical technologies powered by renewable electricity (from wind and solar energy) provide an attractive option to produce green hydrogen by water electrolysis, hence with no carbon footprint. Polymer electrolyte membrane water electrolysis (PEM-WE), one of the most soughtafter technologies for H₂ production, has higher energy efficiency and resilience under dynamic operation compared to alkaline water electrolysis. PEM-WE, however, uses electrodes containing expensive platinum-group metal (PGM) catalysts, making upscaling and commercialization of this technology a challenge due to high inherent costs. Research in this field aims to develop nanostructured PGM based electrodes with enhanced performance and stability featuring at the same time minimized catalyst loadings. To tackle this technological challenge, it is key to understand at the fundamental level what nanoscale factors govern the electrocatalyst activity and stability.

Chapter 2 introduces the design and fabrication by solid-state dewetting of Pt NPs on fluorine-doped tin oxide (FTO) as model electrodes for electrochemical hydrogen evolution reaction (HER). When comparing identical catalyst loadings, dewetted Pt NPs exhibit enhanced HER activity compared to thin films, despite the ca. 2-time higher electrochemical surface area (ECSA) of the latter. XPS analysis indicates that the enhanced HER activity is due to electronic metal-support interactions (EMSI), which affect the electronic structure of interfacial Pt sites (i.e., at the Pt/FTO interface) and improve the HER kinetics. This HER activity enhancement scales with the overall length of the Pt-support contact line.

Chapter 3 explores whether the enhanced HER activity observed for dewetted Pt NPs in stagnant electrolytes (Chapter 2) is purely intrinsic or also due to non-kinetic factors, such as mass transport effects. Pt NPs and thin films were tested under controlled hydrodynamic conditions using a rotating disk electrode (RDE) setup. For this, a custommade RDE adapter was designed to test non-standard-disk electrodes, i.e., dewetted Pt NPs and Pt thin films on FTO substrates. Under both hydrostatic and hydrodynamic conditions, dewetted Pt NPs showed significantly higher HER kinetics than thin films, indicating that the enhanced activity is intrinsic (likely linked to EMSI) and not a result of mass transport effects.

Chapter 4 deals with extending the SSD approach to carbon substrates and aims to investigate performance and stability of Pt thin films and dewetted Pt NPs on borondoped diamond (BDD) electrodes. Compared to FTO (the electrical conductivity of which degrades beyond 500°C), BDD substrates provide a broader temperature range for dewetting. As observed for Pt/FTO, when comparing identical catalyst loadings, dewetted Pt NPs on BDD exhibit enhanced HER activity compared to Pt thin films, despite the significantly higher ECSA of the latter. Increasing the dewetting temperature from 500 °C to 650 and 800 °C enhances EMSI effects, as supported by spectroscopic evidence, leading to a consistent shift in the Pt 4f signal towards lower binding energy. Consequently, the intrinsic HER activity of Pt NPs increases with increasing the dewetting temperature. NPs dewetted at moderate temperatures, e.g., 500 °C, show a ca. 30% HER activity enhancement after accelerated stress tests (AST), while NP electrodes dewetted at higher temperatures lose 80 to 90 % of the initial HER activity. The activity loss is linked to sp²-carbon corrosion, indicating that HER performance is influenced not only by catalyst/support interactions, but also by composition and stability of the carbon substrate.

Chapter 5 focuses on producing precisely faceted Pt NPs, with controlled high- or lowindex planes exposure, by SSD of Pt thin films on electrically conductive single-crystal (SC) supports (Nb-doped SrTiO₃, Nb:STO). This is achieved by tuning the dewetting temperature, atmosphere (gas composition), and crystallographic orientation of the SC substrate. Comparing identical catalyst loadings, dewetted Pt NPs on STO show higher intrinsic HER activity compared to thin films, despite the larger ECSA of the latter, which further confirms results in chapters 2-4. The higher HER activity of dewetted NPs compared to thin films is due to EMSI, as supported by XPS results. The activity of NPs increases with increasing the dewetting temperature, which is ascribed not only to stronger support interactions, but also to surface exposure of high-index facets with higher intrinsic HER activity, despite the gradual ECSA decreases. It is proposed that, in addition to factors like ECSA, SMSI effects, and exposed facets, lattice strain may also play a role in boosting the intrinsic performance of dewetted Pt NPs.

Chapter 6 summarizes the key findings of the thesis. It highlights how model electrodes produced by SSD make it possible to investigate nanoscale effects such as catalyst ECSA, EMSI, exposed facets, surface strain, and support properties (composition and electrochemical stability) and their influence on HER activity and electrode stability. Model NP electrodes are therefore key to draw criteria for rational catalyst design towards enhanced performance and reduced loadings and costs. The chapter briefly outlines also future research directions where sputter-dewetting methods can offer a new scalable chemical-free route to fabricate low-loading catalyst coated electrodes for polymer electrolyte membrane water electrolyzers (PEMWEs) and fuel cells (PEMFCs).