Shawn Litster

Electrification of heavy-duty transportation is motivated by a need to increase energy efficiency and resource flexibility (nuclear, renewables, etc.). In addition, local air quality is negatively impacted in areas of significant heavy-duty diesel vehicle use (ports, diesel train depots, etc.), with commensurate local respiratory health impacts and costs. Using hydrogen fuel cells as the electric power source for heavy-duty transportation is further motivated by the challenges of applying batteries to their electrification. Using Li-ion batteries for heavy-duty, long-range vehicles is challenged by the global transition to a market dominated by lower-cost but heavier lithium iron phosphate chemistry. Heavy-duty vehicles also present one of the most achievable costs for low-carbon hydrogen among the hard-to-abate sectors. Therefore, the rapid commercialization of fuel cells for heavy-duty vehicles is a crucial step in increasing low-carbon hydrogen production and driving down hydrogen costs through economies of scale and thus enabling low-carbon hydrogen in more challenging sectors, such as ammonia production. Establishing widespread use of PEMFCs in heavy-duty trucks requires significant improvements in efficiency and durability, including achieving a challenging lifetime target of over 25,000 hours while also meeting acceptable values for total cost of ownership. This talk will highlight our lab’s recent efforts in addressing these challenges and our innovations in materials, imaging, diagnostics, and simulations for PEMFCs. These highlights will include unique ionomer integration strategies and ionomer durability assessment, image-based modeling from catalyst to stack-scale, where we integrate micro- and nano-scale resolution X-ray computed tomography (micro-CT and nano-CT) along with plasma-focused ion beam scanning electron microscopy (pFIB-SEM) into our model geometries. Image-based modeling enables the exploration of physically accurate geometries and heterogeneity versus ideal geometries. The talk will further highlight new innovations being developed in our laboratory, including cathodes with high oxygen permeability ionomer, novel mesoporous supports, and ionomer-free electrode concepts.

This study investigates how various dispersion techniques influence the morphological and rheological properties of Vulcan XC-72R carbon black and a new nanostructured catalyst used in PEM fuel cells and electrolyzers. Each method was evaluated using rheology, dynamic light scattering (DLS), and scanning electron microscopy (SEM) to examine particle agglomeration, ink viscosity, and morphology. An optimized dispersion protocol was developed based on the Vulcan study and applied to the nanostructured catalyst system, followed by a catalyst loading analysis to explore structure–performance relationships. The optimized ink formulation showed improved dispersion stability, smaller particle sizes, and more uniform catalyst-ionomer distribution. Electrochemical performance was assessed with polarization curves, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The effect of the optimized dispersion techniques on catalyst utilization and mass transport resistance is also evaluated. These results emphasize the critical role of dispersion processing in shaping ink structure and producing high-performance membrane electrode assemblies (MEAs) for PEM applications.

Hydrogen proton exchange membrane fuel cells are a key clean energy technology suited to address hard-to-abate carbon emission sources in medium- and heavy-duty transportation sectors such as trucks, rail, ships, and aviation. Researchers continue to develop novel, functionalized/modified, high-surface-area, and mesoporous carbon supported catalysts to enhance electrochemical performance and durability. However, these novel carbon supports, as well as platinum group metal-free catalysts, often require unique ink formulation methods due to differences in initial catalyst aggregate size, surface functional groups, and surface wettability. These factors affect the homogeneity of the catalyst ink, film coatings, and catalyst layer structure. As a result, determining sufficient ink mixing protocols requires trial-and-error verification. This work examines how different ink mixing strategies, such as probe sonication and low-to-high speed ball milling, affect the structure of the catalyst layer and the cell performance for an emerging functionalized high-surface-area carbon-platinum catalyst (COR-Cat-C4™ by pH Matter). A high-throughput ink drop ‘witness sample’ coating method was applied to image the catalyst structure through the mixing process and to determine the necessary duration for homogeneity. Using this approach and other characterizations, our study further evaluates the relationship between mixing intensity and the resulting aggregate size distribution. In turn, we attempt to correlate the resulting size distribution with electrode coating quality, morphology, transport properties, and fuel cell performance. The inks were characterized systematically and specific metrics have been identified for optimal coating both in terms of quality and functionality. Overall, we observe that smaller aggregates reduce free ionomer agglomerates, improve electrode film uniformity, and decrease the catalyst protonic resistance, leading to enhanced cell performance. This work was supported by the DOE Office of Energy Efficiency and Renewable Energy (EERE) under award number DE-EE0011348-001.

Heavy-duty trucks account for most freight movement in the US and consequently contribute a large share of transportation emissions. Efforts to eliminate tailpipe emissions have focused on electrifying these vehicles through proton exchange membrane fuel cells (PEMFCs). While the scalability of PEMFCs makes them an attractive candidate for heavy-duty applications, further improvements in electrode durability are needed to achieve total cost of ownership targets to be commercially competitive. One challenge with understanding PEMFC lifetime is quantifying the extent of degradation to the the polymer electrolyte binder (commonly referred to as the ionomer) in the catalyst layers of the electrodes. The ionomer is needed for the mechanical structure and proton conductivity of the electrodes. However, the ionomer also negatively influences catalyst performance by restricting local oxygen transport at the catalyst and its catalyst adsorbtion that reduces activity. While the degradation of PEMFC membrane has been extensively studied, analysis of ionomer within the electrode is challenging due to the low total mass of material present. Typical methods for membrane degradation analysis, such as fluoride emission measurement, are frustrated by the need to distinguish degradation products of the catalyst layer ionomer from the membrane. In the present work, we assess a variety of postmortem chemical analysis techniques for their relevance to measuring catalyst layer ionomer degradation. We report methods of separating catalyst layer ionomer from the membrane in a membrane electrode assembly (MEA), and measurement conditions for a range of analytical techniques including nuclear magnetic resonance (NMR), Raman spectroscopy, and x-ray photoelectron spectroscopy (XPS). We also describe the results of fabricating a simulated degraded MEA through ex-situ treatment of the electrode on the decal backing layer prior to transfer to the membrane and compare the cell performance with MEAs which have been left untreated, and which have undergone open-circuit voltage degradation conditions. We evaluate the effectiveness and sensitivity of each characterization approach and identify potential directions for developing a postmortem MEA ionomer degradation assessment protocol.

Proton exchange membrane water electrolyzers (PEMWEs) offer a promising route to reduce the reliance on fossil fuels for hydrogen production. However, achieving high efficiency is critical for optimizing both cost and material usage. While the titanium fabrication and platinum coatings of porous transport layers (PTLs) contribute to their high cost, these components are essential for facilitating reactant transport, electronic conductivity, and mechanical integrity within PEMWEs. Hence, a thorough understanding and characterization of PTL properties is vital for advancing PEMWE technology. This study utilizes x-ray micro-computed tomography (micro-CT) to visualize selected PTLs and extract their morphological properties through image processing and computational simulation. Subsequently, electrochemical testing was performed to correlate these characterized morphological properties with PEMWE electrochemical performance and to analyze how performance variations are linked to PTL morphology. Preliminary tests highlight the importance of catalyst loading in PEMWEs and its relationship with PTLs. Optimized PTL morphology can potentially standardize PTL parameters, thereby facilitating wider PEMWE adoption.

In this work, a simplified vapor deposition method [1] is used to directly deposit iridium catalyst layers on a proton conducting membrane for proton exchange membrane water electrolyzers (PEMWEs). Currently, hydrogen production is dominated by carbon-heavy processes. PEMWEs generate hydrogen with a low carbon footprint and reach higher current densities compared to other electrolyzers, but the high cost of iridium-based anodes limits PEMWEs from having a larger role in hydrogen production. Depositing a thin film of iridium on high surface-area-to-volume proton conducting structures can increase iridium utilization. To accomplish this, a high surface area porous polymer support (PPS) layer was coated directly onto a perfluorosulfonic acid (PFSA) membrane. A thin film of iridium oxide was coated onto the PPS via chemical vapor deposition (CVD) and physical vapor deposition (PVD). CVD enables uniform coatings on complex surfaces, including surfaces outside of line-of-sight, but conventionally requires expensive equipment. Here, we present a low-cost CVD method to create iridium anodes directly on PFSA based substrates. Iridium acetylacetonate was selected as the precursor for low temperature vapor deposition. The precursor was heated in a vacuum oven, allowing the precursor to react with water in a nitrogen environment. First, this method was used for direct deposition onto a planar PFSA membrane. Physical characterization using scanning electron microscopy and energy dispersive X-ray spectroscopy confirmed a successful iridium oxide coating on the membrane. The resulting anode showed promising performance, reaching a current density of 2 A/cm 2 at less than 2.1 V. To the best of our knowledge, this is the highest performance recorded for a thin film iridium anode via a vapor-based process directly deposited onto a planar membrane. This low-temperature CVD process was then used to coat iridium oxide directly onto the PPS substrate. One challenge faced with CVD of iridium was the elevated temperature and the impact of heating on the PFSA. As an alternative to CVD, PVD was applied to coat the PPS. In contrast to CVD, PVD is limited to less complex thin-film surface coatings within the line-of-sight. However, it can be performed at lower temperatures with accessible, well-established methods with the capacity for parameter variation to enhance the coating. Iridium oxide sputtering was performed on both a planar and PPS substrate. Imaging then confirmed that the uniformity and thickness of the resulting coatings were sufficient. The sputtered samples were then electrochemically tested in a PEMWE, demonstrating a distinct improvement in performance from the enhanced surface area and achieving similar performance to the CVD samples. This work highlights the advantages and disadvantages of each deposition method while underscoring the benefits of increasing the electrode surface area for enhanced iridium utilization. This work was funded by the Office of Naval Research Undersea Power and Energy Program’s NEPTUNE project and additionally supported by the ATK-Nick G. Vlahakis Graduate Fellowship. References: [1] Papandrew, A. B., Atkinson, R. W., Goenaga, G. A., Kocha,S. S., Zack, J. W., Pivovar, B. S., and Zawodzinski, T. A., 2013. “Oxygen Reduction Activity of Vapor-Grown Platinum Nanotubes”. Journal of The Electrochemical Society,160(8), pp. F848–F852.

Polymer electrolyte membrane water electrolyzers (PEMWEs) are a critical technology for efficient hydrogen production to decarbonize fuels and industrial feedstocks. To make hydrogen cost-effective, the overpotentials across the cell need to be decreased and platinum-group metal loading reduced. One overpotential that needs to be better understood is due to mass transport limitations from bubble formation within the porous transport layer (PTL) and anode catalyst layer (ACL), which can lead to a reduction in performance at typical operating current densities. When operating at ultra-high current densities (UHCD), the rate of the OER may reach a critical point at which oxygen gas bubbles fill the pores of the ACL and PTL, completely blocking access of liquid water to the ACL. Because of this, there is a possibility that the cell will rely on water vapor diffusion through the evolving oxygen gas to deliver the water reactant to the OER catalyst. To assess the operational limitation of a PEMWE while relying on water vapor diffusion, a commercially manufactured membrane electrode assembly (MEA) was tested by flowing water vapor with an inert carrier gas into the anode as the reactant. To identify a limiting current density ( i lim ) of the electrolyzer under these conditions, potentiostatic polarization curves were obtained for a range of relative humidity (RH) and backpressures. The RH was varied to assess the impact of reactant concentration on the catalyst mass activity at low current and on the i lim , while the backpressure was varied to isolate the impact of the molecular gas diffusion coefficient on the i lim . Our findings highlight that water vapor diffusion through evolved oxygen is readily able to support the OER without notable mass transport overpotentials. However, our results show that water vapor feed inhibits high current density through reduced catalyst specific activity and polymer electrolyte membrane dry-out.