Investigating transport resistances in fuel-cell catalyst layers

Multiphase porous catalyst layers (CLs) find applications in a wide variety of electrochemical energy-conversion devices such as fuel cells, electrolysis, electrosynthesis, and batteries. The various phases enable transport of different species, while the porous structure exhibits a large interfacial area between the various phases in a small volume, thus enabling interfacial electrochemical reactions. In polymer-electrolyte fuel cells (PEFCs), the CL is composed of carbon agglomerates dispersed with Pt nanoparticles, which are the primary reaction sites. The carbon agglomerates are coated by an ion-conducting polymer (ionomer) that also acts as a binder. The three phases (solid carbon, ionomer, and pores between the agglomerates) allow the transport of electrons, ions, and reactant/product gases, respectively. A typical PEFC electrode is usually a few micrometers thick, while the carbon agglomerates are 50 to 300 nm in diameter and the ionomer thin film is only 5 to 15 nm thick. Overall, the structure is extremely heterogeneous with various length scales and multiple intercalating phases. Thus, it is extremely difficult to isolate and quantify the various transport processes./nGas transport is of particular interest since it determines the maximum power that can be derived from PEFC systems under typical operating conditions. Within the CL, the gas diffuses through the pores, followed by transport through the ionomer thin films to reach the Pt reaction sites. Several prior studies establish the presence of a large gas-transport resistance close to the Pt reaction site, commonly referred to as the “local” resistance. Further, the local resistance is known to increase with a decrease in Pt loading due to higher local fluxes. This finding necessitates the use of high-Pt mass loadings in PEFC CLs, thus driving up cost and posing a major barrier to the large-scale commercialization of PEFC systems. The primary aim of this dissertation is to understand the origin of the “local” resistance and propose mechanisms to ameliorate it./nTraditionally, CL transport resistance is quantified from O2-limiting-current measurements, where the mass-transport limit is achieved by using a low-concentration O2 feed and applying high overpotential. In chapter 2, we study the CL transport resistance as a function of CL Pt loading and ionomer content. By combining multi-scale continuum modeling and experiment, the various gas-transport sub-resistances are isolated and quantified. The local resistance close to the Pt particles is shown to dominate the overall gas-transport resistance in PEFC CLs. Our results further expose the ionomer thin films as the origin of the local resistance, which can be sub-divided into: (i) a transport component due to diffusion through the ionomer thin film, and (ii) an interfacial component due to sulfonate adsorption on Pt./nHaving identified the ionomer as the origin of the local resistance to gas transport in CLs, subsequent chapters (chapters 3, 4, and 5) explore methods to ameliorate this resistance. In chapter 3, the impact of ionomer content and chemistry on overall and component CL gas-transport resistance is investigated. CL transport resistance increases with ionomer content, potentially due to the formation of thicker ionomer films. Low-equivalent-weight ionomers allow better gas and proton transport due to greater water uptake and low crystallinity but also cause significant interfacial resistance due to the high density of sulfonic-acid groups. Conversely, high-equivalent-weight ionomers provide poor gas and ion transport, but result in low interfacial resistance. These observed effects of equivalent weight are validated via in-situ ionic conductivity and CO-displacement measurements. Of critical importance, the results are supported by ex-situ ellipsometry and scattering of model thin-film systems, thereby providing direct linkages and applicability of model studies to probe complex heterogeneous structures. Lastly, structural, and resultant performance changes in the electrode are observed above a threshold sulfonic-group concentration, thus highlighting the significance of ink-based interactions./nNext, we investigate the impact of carbon-support type. Carbon supports in CLs form the primary porous structure on which Pt particles are deposited and ionomer films are distributed. Unlike non-porous carbon supports, high-surface-area carbon supports possess interior micropores that are suitable for Pt nanoparticles deposition but prevent ionomer penetration. The micropores thus mitigate interfacial resistance by spatially segregating the ionomer and Pt particles, but they increase reactant transport resistance to the Pt reaction sites due to their small size and tortuosity. Additionally, the H+transport is forced through water pathways in the absence of ionomer, resulting in a humidity-dependent Pt utilization and power output. In chapter 4, we model the various mechanisms of water uptake by PEFC CLs, and the subsequent impact of water uptake on Pt utilization. The model results are in good agreement with literature data collected from various sources and highlight the role of wetted pores with surface adsorbed water. In addition to flooded pores that contain capillary condensed water, wetted pores significantly enhance Pt utilization at low to middle RHs. Additionally, Pt particles are exposed to very different local environments in wetted pores as compared to flooded pores. Until H+ transport is not limiting, wetted pores can significantly improve performance by enabling high gas transport through the pore gas phase as compared to flooded pores (which transport only gas dissolved in the liquid phase). Our results agree with recent studies, thereby suggesting that mesoporous carbons with highly hydrophilic surface are most suited to optimize both gas and H+ transport, and that should provide superior performance compared to traditional carbon-black supports./nLastly in Chapter 5, we examine the impact of ink properties on gas-transport resistance in terms of variations in the primary catalyst particle Pt loading (PPL). By investigating CL gas-transport resistance and zeta potential (ζ) of corresponding inks as a function of PPL, a direct correlation between CL high-current-density performance and ink zeta potential (ζ) is observed. We further explore ink behavior and find that ζ and pH are strong functions of PPL, indicating changes in ionomer-film distribution as a function of PPL. Resulting information is critical to unraveling ionomer-distribution heterogeneity in CLs to enable enhanced Pt utilization and improved device performance.In summary, this dissertation studies gas transport in PEFC CLs, the associated losses, and the impact of material properties on identified component gas-transport resistances. Most importantly, the results provide guidelines to improve existing material design to enhance gas transport, thus improving performance at high current densities.

Advisor: Radke, Clayton J
Weber, Adam Z