Date of Award

Summer 8-2016

Embargo Period

1-25-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Advisor(s)

Shawn Litster

Abstract

Electrification of vehicles could enable the transportation sector to be more efficient with reduced emissions. Polymer electrolyte fuel cell (PEFC) vehicles powered by sustainably generated hydrogen fuel would be a viable replacement for internal combustion engines. PEFC powered vehicles are highly efficient and offer zero tailpipe emissions. State-of-the-art PEFCs rely on platinum (Pt) and platinum alloy catalyst nanoparticles supported on high surface area carbon black bound by ionomer. The high Pt loadings currently needed for the low-temperature acidic oxygen reduction reaction (ORR) incur significant costs due to the Pt raw material. Degradation of conventional carbon supported Pt electrodes remains a challenge for the commercialization of PEFCs. In the cathode, the ORR is sluggish and results in a large overpotential loss and hence reducing the Pt utilization. The strategy to make PEFC commercially viable is by reducing the cost through either reducing the amount of Pt in the electrode or replacing the Pt catalyst with alternative low-cost catalysts. Ionomer binder in conventional electrodes needed for proton conduction introduces undesirable high oxygen transport resistance that further reduces the Pt efficacy. However, novel ionomer-free electrodes, which have an advantage of no ionomer film resistance, relies on water for proton conduction and thus hindering the performance and stability at dry conditions. Alternative electrode designs can potentially alleviate some of the problems in these high power density devices. This work presents an alternative composite Nafion nanofiber catalyst support electrode, in which the oriented nanofibers provide robust internal proton transport to a conformal Pt catalyst coating without impeding oxygen transport. The high-surface-area electrodes are prepared by solution casting Nafion onto a sacrificial template, and thin Pt films are deposited on the nanofibers using either physical vapor deposition or chemical vapor deposition. The electrochemical characterization of the nanofiber electrodes demonstrates the high current density and specific activity of this nanofiber approach relative to prior electrodes fabricated by depositing Pt directly onto other Nafion surfaces. Even with the improved electrode architecture, the Pt raw material cost is still an obstacle. Hence, Pt group metal-free (PGM-free) PEFC cathodes are of significant interest for low-temperature ORR since they have the potential to reduce PEFC costs dramatically. The activity and durability of PGM-free catalyst have significantly improved in vii the last 10 years. However, several challenges remain before they can become commercially viable. The PGM-free catalysts have lower volumetric activity and hence the PGM-free cathodes are thicker than Pt-based electrodes. Thus, they suffer from significantly greater gas and proton transport resistances that reduce the observed performance and robustness of operation. To better understand the efficacy of the catalyst and improve electrode performance, a detailed understanding of the correlation between electrode fabrication, morphology, and performance is crucial. This dissertation reports the characterization of PEFC cathodes featuring a PGM-free catalyst using nano-scale resolution X-ray computed tomography (nano-CT) and morphological analysis. In this work, the pore/solid structure and the Nafion distribution was resolved in three dimensions (3D) using nano-CT for three PGM-free electrodes of varying Nafion loading. The particular PGM-free cathode being studied feature two distinct length scales of interest and was resolved using multi-resolution imaging in nano-CT. The associated transport properties were evaluated from pore/particle-scale simulations within the nano-CT imaged structure. These characterizations are then used to elucidate the microstructural origins of the dramatic changes in fuel cell performance with varying Nafion loading. The results show that this is primarily a result of distinct changes in Nafions spatial distribution. The significant impact of electrode morphology on performance highlights the importance of PGM-free electrode development in concert with efforts to improve catalyst activity and durability. To understand the potential distribution in the thick electrodes we utilize a novel experimental technique to measure the electrolyte potential directly at discrete points across the thickness of the catalyst layer and evaluate the ORR along the thickness of the catalyst layer. Using that technique, the electrolyte potential drop, the through-thickness reaction distribution, and the proton conductivity is measured and correlated with the corresponding Nafion morphology and cell performance. At this stage of PGM-free catalyst development, it is also necessary to optimize these thick electrodes along with the catalyst. To address the significant transport losses in thick PGM-free cathodes (ca. > 60 m), we developed a two-dimensional (2D) hierarchical electrode model that resolves the unique structure of the PGM-free electrode. The 2D computational model is employed to correlate the morphology and the electrochemical performance of the PGM-free electrodes. The model is a complete cell, continuum model that includes an agglomerate model representation of the cathode. A unique feature of the approach is the integration of the model with morphology and transport parameter statistics extracted from nano-CT imaging of the electrodes. The model was validated with experimental results of the PGM-free electrode with three levels of Nafion loading. We discuss the sensitivity of the PGM-free catalyst layer on the operating conditions and the morphological parameters to identify improved architectures for PGM-free cathodes. We employ the model to evaluate the targets for the volumetric activity of the catalyst. A notable finding is the impact of the liquid water accumulation in the electrode and the significant performance improvement possible if electrode flooding is mitigated.

Available for download on Thursday, January 25, 2018

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