William K. Epting

Gas Transport Resistance in Polymer Electrolyte Thin Films on Oxygen Reduction Reaction Catalysts.

Significant reductions in expensive platinum catalyst loading for the oxygen reduction reaction are needed for commercially viable fuel cell electric vehicles as well as other important applications. In reducing loading, a resistance at the Pt surface in the presence of thin perfluorosulfonic acid (PFSA) electrolyte film, on the order of 10 nm thick, becomes a significant barrier to adequate performance. However, the resistance mechanism is unresolved and could be due to gas dissolution kinetics, increased diffusion resistance in thin films, or electrolyte anion interactions. A common hypothesis for the origin of the resistance is a highly reduced oxygen permeability in the thin polymer electrolyte films that coat the catalyst relative to bulk permeability that is caused by nanoscale confinement effects. Unfortunately, the prior work has not separated the thin-film gas transport resistance from that associated with PFSA interactions with a polarized catalyst surface. Here, we present the first characterization of the thin-film O2 transport resistance in the absence of a polarized catalyst, using a nanoporous substrate that geometrically mimics the active catalyst particles. Through a parametric study of varying PFSA film thickness, as thin as 50 nm, we observe no enhanced gas transport resistance in thin films as a result of either interfacial effects or structural changes in the PFSA. Our results suggest that other effects, such as anion poisoning at the Pt catalyst, could be the source of the additional resistance observed at low Pt loading.

Spatially resolved, in situ potential measurements through porous electrodes as applied to fuel cells.

We report the development and use of a microstructured electrode scaffold (MES) to make spatially resolved, in situ, electrolyte potential measurements through the thickness of a polymer electrolyte fuel cell (PEFC) electrode. This new approach uses a microfabricated apparatus to analyze the coupled transport and electrochemical phenomena in porous electrodes at the microscale. In this study, the MES allows the fuel cell to run under near-standard operating conditions, while providing electrolyte potential measurements at discrete distances through the electrodes thickness. Here we use spatial distributions of electrolyte potential to evaluate the effects of Ohmic and mass transport resistances on the through-plane reaction distribution for various operating conditions. Additionally, we use the potential distributions to estimate the ionic conductivity of the electrode. Our results indicate the in situ conductivity is higher than typically estimated for PEFC electrodes based on bulk polymer electrolyte membrane (PEM) conductivity.

In Situ Measurement of Oxygen Partial Pressure in a Cathode Catalyst Layer

A micro-structured electrode scaffold (MES) was developed for performing in-situ measurements of through-plane oxygen partial pressure (OPP) distributions in the cathode of a polymer electrolyte membrane fuel cell (PEMFC). Alternating layers of insulating material and platinum are stacked and bonded, and surround a 100 micron diameter hole filled with catalyst layer material. The platinum layers serve as ultra-microelectrodes, allowing the measurement of local OPP at discrete intervals through the catalyst layer thickness. Following an investigation of ultra-microelectrode sensitivity and repeatability, pulsed amperometric detection was selected over potentiostatic operation as the most effective method for performing OPP measurements. Using calibration points at the full air and zero OPPs, the OPP distribution at a fuel cell current density of 450 mA cm-2 was measured.

Micron-Scale Diagnostics for Through-Plane Transport Phenomena in Porous Electrodes

This paper presents the development of a new method for characterizing the electrochemistry and transport phenomena in the porous electrodes of polymer electrolyte membrane fuel cells (PEMFCs). The new method uses a unique microstructured electrode scaffold (MES) that provide an architecture for obtaining measurements at discrete points through the thickness of an electrode. This paper reports on the design, fabrication and initial testing of an MES for measuring ionic potential across the thickness of a PEMFC’s cathode. The new fuel cell hardware and reference electrodes (REs), which gather electrolyte potential measurements through the thickness of the electrode via the MES, have been tested for accuracy and repeatability. The use of hydrogen oxidation reaction (HOR) REs versus oxygen reduction reaction (ORR) REs is analyzed and discussed. Polarization data was also gathered and the REs are used to separate the half-cell potentials. Finally, the preliminary fabrication of an MES and a micro-structural analysis are discussed.Copyright

In Situ, Through-Plane Measurements of Ionic Potential in a PEMFC Catalyst Layer

A novel apparatus for measuring in situ, through-plane, ionic potential distribution within the cathode catalyst layer of a polymer electrolyte membrane (PEM) fuel cell is presented here. This new diagnostic method makes use of a micro-structured electrode scaffold (MES), which is a stack of alternating insulating and sensing layers that surround a 100 µm diameter cylinder of catalyst layer material. Here we present measurements of ionic potential at four points through the thickness of a PEM fuel cell cathode. These preliminary results show significant variations of the ionic potential within the electrode at high currents. In addition, the local ionic potential provides an accurate measure of the local IR-free potential in the catalyst layer. Possible applications of the ionic potential data gathered from the MES include investigating mass and charge transport limitations, spatially characterizing electrode kinetics, and validating porous electrode models.