Leonard J. Bonville

Characterization of Gas Diffusion Layers for PEMFC

In proton-exchange membrane fuel cells (PEMFC), gas diffusion layers serve as current collectors that allow ready access of fuel and oxidant to the anode and the cathode catalyst surfaces, respectively. Critical properties of five commercial and one in-house gas diffusion layers have been characterized and compared to determine factors limiting the oxygen transport in the cathode gas diffusion layer where there is no oxygen consumption. These properties are the limiting current, electronic resistivity, fraction of hydrophobic pores, gas permeability, pore size distribution, and surface morphology. Polarization curves using air and neat oxygen were collected to determine the air-limiting currents at three operating conditions: 80°C/75% relative humidity (RH) cathode inlet, 100°C/70% RH cathode inlet, and 120°C/35% RH cathode inlet, all at atmospheric pressure. Linear empirical relationships for permeability coefficient vs. limiting current were found at all three conditions. Characterization of the gas diffusion layers by porosimetry measurement provides the pore size distribution for the gas diffusion layers, which helps in understanding the correlation between the permeability coefficient and the limiting current at the temperatures and relative humidity tested.

High-Performance PEMFCs at Elevated Temperatures Using Nafion 112 Membranes

Operating proton exchange membrane fuel cells (PEMFCs) at elevated temperatures (>100°C) reduces the effect of CO poisoning, simplifies heat rejection, and results in more useful waste heat. Membrane electrode assemblies were developed to obtain high-performance PEMFCs at elevated temperatures and low relative humidity (RH) using the commercial Nafion 112 membrane. Cell polarization was obtained at three operating conditions (cell temperature °C/anode %RH/cathode %RH) of 80/100/75, 100/70/70, and 120/35/35, ambient pressure. Hydrogen/air cell performance at 400 mA/cm 2 was 0.72, 0.69, and 0.58 V at the three conditions, respectively. A cell voltage of 0.65 V was obtained at the 120°C condition when oxygen was used instead of air. Reproducible cell performance with the maximum voltage difference of 10 mV was obtained at all operating conditions. The hydrogen crossover rate through the Nafion 112 membrane was relatively low, between 0.6 and 1.3 mA/cm 2 at temperatures between 25 and 120°C. The electrochemical surface area of each cathode electrode, determined from cyclic voltammetry, was 97 m 2 /g Pt, with nearly 73% catalyst utilization. The performance optimization approach can be applied to more advanced high-temperature membranes that have higher conductivity at low RH when these membranes are available in the future.

Improving PEMFC Performance Using Low Equivalent Weight PFSA Ionomers and Pt-Co ∕ C Catalyst in the Cathode

The effects of lower equivalent weight (EW) perfluorosulfonic acid (PFSA) ionomers and Pt-Co/C catalyst on the cathode performance of proton exchange membrane fuel cells (PEMFCs) were investigated at two atmospheric pressure operating conditions: low temperature/high relative humidity (RH), 80°C/100% RH, and high temperature/low RH, 120°C/35% RH. Cell voltage at a current density of 400 mA/cm 2 was used for the performance comparison. The optimized content in the electrode changed with the ionomer EW, from 32% for 1100 EW Nafion, 28% for 920 EW Nafion to 25% for a developmental PFSA 800 EW ionomer. Compared to 1100 EW Nafion, 800 EW ionomer significantly improved the cell performance by 39 mV at 120°C/35% RH; however, at 80°C/100% RH, its effect was not apparent. The introduction of Pt-Co/C catalyst into the cathode increased the cell performance by 43 mV at 80°C/100% RH, which was much higher than a performance improvement at 120°C/35% RH. Compared to electrodes made of Pt/C and Nafion 1100 EW, the combination of 800 EW Ionomer and Pt-Co/C catalyst resulted in a 55 mV cell voltage increase at 80°C/100% RH and a 48 mV cell voltage increase at 120°C/35% RH.

Membrane Degradation Mechanisms and Accelerated Durability Testing of Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) have increasingly received worldwide attention as the technology that can lead to substantial energy savings and reductions in imported petroleum and carbon emissions. Cost, durability, performance, reliability, efficiency, and size, are some of the requirements that must be met before PEMFCs can be used commercially. The lifetime requirement for stationary applications is about 40,000 hours and for transportation applications 5,000 (cars) and 20,000 hours (buses) (1). Today, the typical operating temperature for both applications is between 60 – 80°C, but to meet the 2010 and 2015 Department of Energy targets, PEMFCs must operate at temperatures from below the freezing point to higher than 100°C (~120 °C maximum), humidity from ambient to saturated, and half-cell potentials from 0 to >1.5 V. Durability studies of proton exchange membrane fuel cells (PEMFC) show that, along with cost, the long-term stability of PEMFCs is a limiting factor in their commercialization (2-6). Degradation of PEM fuel cells is generally observed as slow, unrecoverable performance decay, followed by sudden failure. The gradual performance loss is typically associated with changes in the electrodes and the membrane. The degradation of electrodes is usually caused by catalyst degradation and carbon corrosion. Membrane chemical and mechanical degradation are related to reactant gas crossover, Pt dissolution and migration, transition metal ion contaminants, and hydroxyl radical formation, and cycling of relative humidity. The chemical decomposition of the side

Effects of Silicotungstic Acid Addition to the Electrodes of Polymer Electrolyte Membrane Fuel Cells

Operation of polymer electrolyte membrane fuel cells at elevated temperatures and low relative humidity is limited by the electrolytes dependence on water. The addition of a heteropolyacid, specifically silicotungstic acid (STA), is shown to increase the activity of platinum for the oxygen reduction reaction by nearly 73% at 118°C/40% relative humidity. However, STA-containing electrodes have a significantly higher diffusion resistance, countering any activity improvement at higher currents. By reducing the electrode ionomer weight fraction, the diffusion resistance decreases. Also, membrane durability is improved significantly by including STA at the cathode.

Evaluation of the Durability of Polymer Electrolyte Membranes in Fuel Cells Containing Pt/C and Pt-Co/C Catalysts under Accelerated Testing

One of the main sources of degradation in fuel cells is hydroxyl radical attack of the membrane. Radicals are formed where platinum, hydrogen, and oxygen are present. Radical formation occurs at the electrodes, where reactants diffusing through the membrane can react on the Pt catalyst. Additionally, Pt ions form at the cathode, and can then migrate into the membrane and form a Pt band where there is also significant radical generation. Radical attack of the membrane leads to pinhole and crack formation, resulting in significant hydrogen crossover, large performance losses, and shortened cell life.

Effect of Water Management Schemes on the Membrane Durability in PEMFCs

The authors report the degradation behavior of polymer-electrolyte membrane fuel cells tested under two types of water-management schemes: the solid plate (SP) cells that mange water via flow and dew point control of the reactant gases, and water transport plate (WTP) cells that actively remove liquid through micro-porous bipolar plates. Comparative experiments were conducted. Cell performance degradation was tracked by periodic diagnostic testing. After cell testing, the residual mechanical strength of the membrane electrode assemblies was characterized. It was found that both water management schemes led to cell performance reduction via the loss of the electrochemical active area. However, the SP cell led to rapid reduction of membrane toughness. The WTP cell tended to preserve the membrane mechanical toughness better. Post-analysis found localized membrane failure in SP cell test during which a step change in performance occurred. Abrupt performance degradation was not observed for either of the WTP cell tests.

Accelerated Durability Testing of Perfluorosulfonic Acid MEAs for PEMFCs Using Different Relative Humidities

Polymer electrolyte membrane fuel cells (PEMFCs) receive worldwide attention as the electricity-generating engine for the hydrogen economy. Cost, durability, performance, reliability, efficiency, and size, are some of the requirements that must be met before PEMFCs can be expanded commercially. The lifetime requirement for onsite, combined heat and power applications is about 40,000 hours and for transportation applications 5,000 (cars) and 20,000 hours (buses). Membrane durability is one of the most important factors limiting the lifetime of PEMFCs.

Accelerated Durability Testing of Perfluorosulfonic Acid MEAs for PEMFCs

There is a strong interest in durability studies of proton exchange membrane fuel cells (PEMFC) because, along with cost, the long-term stability of PEMFC is a limiting factor in their commercialization. Examining the characteristics of a membrane electrode assembly (MEA) over a prescribed amount of time under accelerated degradation conditions can give an indication of the degradation behavior of each MEA. Testing under low humidities and/or high temperatures or by humidity or temperature cycling are techniques that accelerate degradation.

Effect of Equivalent Weight of Phosphotungstic Acid-Incorporated Composite Membranes on the High Temperature Operation of PEM Fuel Cells

Fuel cells have shown great promise for future power sources and there has been substantial advancement in the technology of fuel cells over the past decades. For automobile application, however, there are still challenging issues related to its performance and durability. It is highly desirable to operate fuel cells at high temperature because of a number of benefits, e.g., improved reaction kinetics and carbon monoxide tolerance. Since the conventional polymer electrolytes such as Nafion are not stable at high temperatures, the development of novel membranes that are mechanically, thermally, and electrochemically stable at high temperatures while providing good conductivity under low relative humidity condition is one of the most challenging areas of research for automobile applications of fuel cells. In fact, extensive research efforts have been made to design new proton exchange materials that can overcome the limitations of conventional polymer electrolytes.