Multi-scale multi-technique characterization of PEM fuel cell and electrolyzer catalysts and catalyst layers
| dc.contributor.advisor | Pylypenko, Svitlana | |
| dc.contributor.author | Zaccarine, Sarah F. | |
| dc.contributor.committeemember | Pivovar, Bryan S. | |
| dc.contributor.committeemember | Steirer, K. Xerxes | |
| dc.contributor.committeemember | Alia, Shaun | |
| dc.contributor.committeemember | Richards, Ryan | |
| dc.contributor.committeemember | Trewyn, Brian | |
| dc.date.accessioned | 2022-10-13T21:47:30Z | |
| dc.date.available | 2022-10-13T21:47:30Z | |
| dc.date.issued | 2022 | |
| dc.date.updated | 2022-10-01T01:12:11Z | |
| dc.description | Includes bibliographical references. | |
| dc.description | 2022 Spring. | |
| dc.description.abstract | Polymer electrolyte membrane water electrolyzers (PEMWEs) and fuel cells (PEMFCs) are two important technologies that produce hydrogen from water and convert hydrogen into electricity, respectively. Slow kinetics of the oxygen-pertaining reactions in these devices require expensive noble metal catalysts. In fuel cells, the cathodic oxygen reduction reaction (ORR) uses Pt-based electrocatalysts; similarly, the oxygen evolution reaction (OER) at the electrolyzer anode utilizes an Ir-based catalyst. Once the catalyst layer (CL) is assembled, the catalyst, support, and ionomer must be considered holistically, including ratios in the ink, distribution and structure in the CL to increase interfaces, and how each component leads to overall CL degradation, to produce a high-performing, durable device. To meet cost targets at industrial-scale production, a shift towards efficient, low-loading CLs with good durability is needed. Advanced physicochemical characterization techniques are essential to interpret electrochemical data and understand how interrelated degradation mechanisms cause catalysts and CL components to degrade. In this thesis, various combinations of electron microscopy and X-ray based tomography and spectroscopy techniques were used to i) build a multiscale understanding of composition, morphology, and structure of catalysts and CLs, and ii) deconvolute the impact of individual constituents within the CL on overall electrode structure and stability. Physicochemical results were correlated with electrochemical measurements to advance fundamental understanding of these systems towards their optimization for long-term performance. Significant efforts have been dedicated to improve lower-Pt-loading PEMFC catalysts and elucidate degradation mechanisms of these catalysts in a device, but further developments are needed. Therefore, this thesis first investigated extended-surface PtNi nanowire (NW) catalysts produced via atomic layer deposition (ALD), aiming to improve their performance and durability. Chapter 2 discusses a series of post-synthesis modifications of the PtNi NWs, which were extensively characterized to understand their surface and bulk properties. This analysis allowed to achieve a combination of kinetically beneficial properties: increased PtNi alloy, Pt at the surface, and incorporation of defects. These properties resulted in high activity and surface area pre-and post-durability cycling compared to as-synthesized NWs, while also maintaining intact morphology, representing a significant advancement of extended-surface bimetallic ORR electrocatalysts. This success motivated further tuning of catalyst properties and optimization of the CLs made with PNi NWs, which is covered in Chapter 3, where integration with carbon and ionomer content were also carefully tuned. Guided by physicochemical findings, it was determined that the performance of the catalysts and electrodes was a result of both composition and defects in the skin of the PtNi NW “core-shell” catalyst. While the catalyst properties came to a fairly similar state after device testing, subtle performance differences were still observed based upon the original differences in the catalyst powders. While it is known that significant degradation issues will occur in PEMWEs as well, the extent and severity are not yet fully understood, especially at low loadings of Ir-based catalysts. The work in Chapter 4 builds upon characterization successes discussed in Chapter 2 and 3 and applies a thorough multi-technique approach to investigate degradation of CLs made with Ir and IrO2 catalysts, linking small-scale changes to individual CL constituents with macroscopic changes after testing. This work also featured development of novel approaches in data analysis and interpretation that were not previously applied to this system. The characterization approach identified numerous catalyst degradation changes, which further caused changes to the ionomer distribution and arrangement, leading to formation of voids and segregation of constituents within regions of the CL; and some conditions even lead to CL collapse and delamination. Motivated by the additional need to study lower-loading CLs, Chapter 5 focuses on low-loading IrO2-based electrodes and investigates the effect of additives on CL structure, porosity, catalyst and ionomer distribution, and their respective impact on performance and durability. Initial results show that several additives improve performance relative to the baseline, but impacts on kinetics, ohmic resistance, and mass transport are convoluted, and there is a complex interplay between the morphology, shape, size, and surface area of the additives. The experimental challenges and goals varied for each study, but in all cases, results demonstrate the necessity for a systematic, multi-scale, multi-technique characterization approach to enable a deeper understanding of catalyst properties as a powder, fresh electrode, and tested electrode. Collectively, it is shown how electrode fabrication and testing impact the properties of each CL component and their distribution and interfaces, and how observed degradation mechanisms are related, which are vital to understand in order to drive development of more efficient and stable PEM devices. | |
| dc.format.medium | born digital | |
| dc.format.medium | doctoral dissertations | |
| dc.identifier | Zaccarine_mines_0052E_12393.pdf | |
| dc.identifier | T 9339 | |
| dc.identifier.uri | https://hdl.handle.net/11124/15422 | |
| dc.language | English | |
| dc.language.iso | eng | |
| dc.publisher | Colorado School of Mines. Arthur Lakes Library | |
| dc.relation.ispartof | 2022 - Mines Theses & Dissertations | |
| dc.rights | Copyright of the original work is retained by the author. | |
| dc.rights.access | Embargo Expires: 09/30/2023 | |
| dc.subject | degradation | |
| dc.subject | electrolyzer | |
| dc.subject | fuel cell | |
| dc.subject | microscopy | |
| dc.subject | nanocatalyst | |
| dc.subject | spectroscopy | |
| dc.title | Multi-scale multi-technique characterization of PEM fuel cell and electrolyzer catalysts and catalyst layers | |
| dc.type | Text | |
| dcterms.embargo.expires | 2023-09-30 | |
| dspace.entity.type | Publication | |
| thesis.degree.discipline | Chemistry | |
| thesis.degree.grantor | Colorado School of Mines | |
| thesis.degree.level | Doctoral | |
| thesis.degree.name | Doctor of Philosophy (Ph.D.) |