ChEMS Seminar: Energy Conversion and Storage Explored with Synchrotron X-ray Tomography and Modeling

Abstract: Understanding transport processes in electrochemical devices (fuel cells, electrolyzers and batteries) is critical for effective energy conversion and storage. Interfacial phenomena of charge and mass transfer depends on local species distribution and probing these interfaces remains a challenge. Tools that are designed for characterization of porous media on a larger scale are not always applicable for thin (< 10 µm) electrodes. Furthermore, nano- and micro-scale transport processes need to be bridged, as these electrodes are hierarchically structured. Fine nano-structures of carbon and other materials are desirable for high surface area, and the features of a larger size are needed for higher mass and ion transport. Synchrotron X-ray techniques, such as transmission X-ray microscopy (TXM) and X-ray computed tomography (CT), are well-fit to characterize transport in porous electrodes due to the fast, non-intrusive measurements and relatively high resolution. Spatial resolution is inversely proportional to the field of view, i.e., higher resolution comes at the expense of a smaller field of view. Thus imaging at multiple scales is needed to visualize processes at micro and nano-scales.

For polymer-electrolyte fuel cells (PEFCs) and anion exchange membrane fuel cells (AEMFCs), effective water management is critical. By introducing the capabilities for operando synchrotron X-ray CT, mapping water distribution under various operating conditions became possible. Micro X-ray CT is a specifically useful tool for probing water distribution in platinum group metal-free (PGM-free) electrodes, as they are an order or two order of magnitude thicker (~200 µm) compared to conventional Pt/C electrodes. At 40 mA/cm², we observed water saturation of 0.4-0.5 at locations near the membrane for both 30^oC and 60^oC operating temperatures. These large saturation values are due to water pooling in the large voids (mean radius of 25 µm) near the membrane due to the fabrication method of depositing ink onto the gas diffuion layer (GDL), forming gas diffusion electrode (GDE)³. With nano X-ray CT, swelling of ionomer in nano-pores was observed when comparing the data at 50 percent and 100 percent RH, where mean radius increased from 273 to 325 nm.

Gas removal in PEM electrolyzers on the anode side is accomplished via bubble nucleation, growth and removal into the channel. This process is a function of materials selection and operating conditions. The subsecond nature of bubble growth dynamics requires imaging at higher scan rates that are difficult to achive with X-ray CT. We use X-ray radiography to capture subsecond dynamics, copled to X-ray CT to get three-dimensional layers microstructure for constant current density (50, 100 and 200 mA/cm²) operating conditions. Bubble detachment frequency was a strong function of current density.

For Li-ion batteries, evaluating morphological changes as a function of cycle numbers can help to assess degradation phenomena. Using X-ray CT and macro, micro and nano-scales, we compared the morhpology of pristine to cycled commercial battery. Micro-scale allowed observation of current collector pitting, whereas nano-CT allowed observation of tortuosity and porosity changes at the electrode-scale level.

Bio: Professor Iryna Zenyuk holds a bachelor's degree (2008) in mechanical engineering from the New York University Tandon School of Engineering. She continued her studies at Carnegie Mellon University, where she earned a master's degree (2011) and doctorate (2013). Her graduate work focused on fundamental understanding of meso-scale interfacial transport phenomena and electric double layers in electrochemical energy-conversion systems. After a postdoctoral fellowship at Lawrence Berkeley National Laboratory in Electrochemical Technologies Group with Adam Z. Weber, Zenyuk joined the faculty of the Mechanical Engineering Department at Tufts University in 2015.

With the recent technological advances in the transportation sector, robotics and implantable electronics, there is a growing need for reliable, lightweight and durable energy sources to power these technologies. At Tufts, Zenyuk’s group works on enabling energy solutions by researching high-power density low-temperature hydrogen fuel cells, Li-metal batteries and electrolyzers. Currently, fuel cell durability, low-temperature operation, cost and water flooding are still issues that need to be solved. Zenyuk works on addressing the problems of existing state-of-the-art fuel cells through a design strategy encompassing novel materials, chemistries, diagnostic tools and device-level testing. She is a recipient of the NSF CAREER award (2017), Interpore society Fraunhofer Award for Young Researchers (2017) and Research Corporation for Science Advancement, Scialog Fellow in Advanced Energy Storage (2017).

Host: Vasan Venugopalan