Eric L. Thompson

PEM Fuel Cell Operation at -20°C. I. Electrode and Membrane Water (Charge) Storage

An experimental procedure using isothermal galvanostatic operation was developed to quantify the charge (water) accumulation in proton exchange membrane (PEM) fuel cells at subfreezing conditions prior to voltage failure (i.e., zero cell voltage). The charge passed until voltage failure was compared to charge (water) storage estimates in the membrane phase and the cathode electrode void volume. Cryo-scanning electron microscope images of electrodes following voltage failure were used to assess ice filling of the cathode electrode void volume. At very low current densities, the membrane absorbs a maximum of ≈ 14 to ≈15 water molecules/per sulfonate group (λ max ≈ 14-15) and cathode electrode voids are completely ice filled. It is shown that the maximum charge storage of a membrane electrode assembly increases with electrode void volume and the difference between λ max and λ initial . With increasing current densities, decreasing fractions of the maximum charge storage can be utilized, which is shown to be related both to water transport resistances in the membrane phase and to reduced ice filling of the electrode void volume. Experimental results show that the charge storage utilization is mainly controlled by the current density and is less dependant on initial water content or electrode thickness.

Oxygen Reduction Reaction Kinetics in Subfreezing PEM Fuel Cells

An experimental procedure was developed to measure oxygen reduction reaction kinetics in subfreezing polymer electrolyte membrane (PEM) fuel cells. The procedure was also used to measure kinetics at temperatures above 0°C, and results compared to those collected with a traditional kinetic measurement technique. In general, because of brief time durations in which PEMFCs can be operated below freezing temperatures, short equilibration times were required and thus, enhanced catalyst activity was observed. At progressively lower subfreezing temperatures, suspected mass transport or uncompensated ohmic losses resulted in nonlinear Tafel plots, which at lower decades of current density become linear with a slope close to that predicted by Tafel kinetics, 2.303RT/α c F. Consistent with results of other researchers at nonfrozen conditions, low water (or ice) content in the fuel cell results in lower catalyst activity and performance at subfreezing temperatures. Cyclic voltammograms indicate that the rate of oxide formation decreases at subfreezing temperatures and low water contents, indicating proton activity as a likely reason for reduced catalytic activity. Arrhenius plots of current density at a constant overpotential are linear (constant activation energy) over the temperature range from 55 to - 40°C, indicating no fundamental change in reaction mechanism at subfreezing temperatures.

Performance of Nano Structured Thin Film (NSTF) Electrodes under Partially-Humidified Conditions

3M’s Nano Structured Thin Film (NSTF) electrode, being a core-shell catalyst, offers a novel mean to enhance the performance and lower Pt cost in a polymer electrolyte fuel cell (PEFC). In the present work, fuel cell performance of NSTF is reported and the underlying physics dictating NSTF behavior is probed. It was found that NSTF with 0.15 mgPt/cm 2 Pt loading shows comparable performance to that of a conventional Pt/C electrode with 0.4 mgPt/cm 2 loading in a highly humidified condition at 80 C. However, the NSTF performs poorly under dry conditions. A single-phase model was developed to elucidate the underlying phenomenon governing NSTF performance under partially-humidified conditions. NSTF proton conductivity as a function of relative humidity (RH) was determined and the model predictions were compared against a range of experimental data. Detailed results suggest that poor NSTF performance under dry operation is due to low proton conductivity over Pt surface, which reduces catalyst utilization. The importance of water management is highlighted to improve NSTF performance. VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3590748] All rights reserved.

Catalyst Development Needs and Pathways for Automotive PEM Fuel Cells

Mass production of fuel cell light-duty vehicles at competitive cost requires cathode (oxygen reduction reaction [ORR]) kinetic mass activities 4-fold higher than those of current state-of-the-art platinum / carbon black catalysts. 1 A catalyst-related cell voltage loss less than 50 mV over the entire current density range is sought over a >10 year automotive lifetime including ~300,000 large load cycles and ~30,000 start-stop cycles. While the materials set used in current demonstration vehicles falls short of these goals, pathways to these challenging targets are visible via increased attention to the structural details of Pt-containing multicomponent catalysts and through development of catalyst and support as a single system.

NMR studies of proton transport in fuel cell membranes at sub-freezing conditions

Water uptake activities and transport properties are critical for water management in fuel cell membranes. In this work, three perflourosulfonic acid (PFSA) fuel cell membranes, including Nafion®-117 and two Gore membranes, were evaluated at different relative humidity controlled conditions. These fuel cell membranes were studied using variable temperature 1H spin–lattice relaxation times (T1) and pulsed field gradient (PFG) NMR techniques in the temperature range of 298 to 239 K. Water self-diffusion coefficients and proton transport activation energies in the fuel cell membranes were obtained from the PFG-NMR experiments. The results show that the water self-diffusion coefficients increase with increasing hydration level, and decrease with decreasing temperature. The water molecular motion is significantly slowed at low temperatures; however, the water molecules in these membranes are not frozen, even at 239 K. The water uptake activity and diffusivity in these membranes were compared as a function of temperature and hydration level. At the same temperature and hydration level, the water self-diffusion coefficients of two Gore fuel cell membranes are higher than that of Nafion®-117. This is attributed to the lower EW of the Gore membranes. The presence of an expanded polytetrafluoroethylene (ePTFE) reinforcing layer in the membrane also has an impact on water diffusivity.

Catalyst Degradation Mechanisms in PEM and Direct Methanol Fuel Cells

While much attention has been given to optimizing initial fuel cell performance, only recent research has focused on the various materials degradation mechanisms observed over the life-time of fuel cells under real-life operating conditions. This presentation will focus on fuel cell durability constraints produced by platinum sintering/dissolution, carbon-support oxidation, and membrane chemical and mechanical degradations. Over the past 10 years, extensive R&D efforts were directed towards optimizing catalysts, membranes, and gas diffusion layers (GDL) as well as combining them into improved membrane electrode assemblies (MEAs), leading to significant improvements in initial performance of H2/air-fed proton exchange membrane fuel cells (PEMFCs) and methanol/air-fed direct methanol fuel cells (DMFCs). 3 While the required performance targets have not yet been met, current PEMFC and DMFC performance are close to meeting entry-level applications and many prototypes have been developed for field testing. This partially shifted the R&D focus from performance optimization to more closely examining materials degradation phenomena which limit fuel cell durability under real-life testing conditions. The predominant degradation mechanisms are sintering/dissolution of platinum-based cathode catalysts under highly dynamic operating conditions, dissolution of ruthenium from DMFC anode catalysts, the oxidation of carbon-supports of the cathode catalyst during fuel cell startup and shutdown, and the formation of pinholes in proton exchange membranes if