James M. Fenton

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.

Membrane Degradation Mechanisms in PEMFCs

Nafion membrane degradation was studied in a polymer electrolyte membrane fuel cell (PEMFC) under accelerated decay conditions. Fuel cell effluent water was analyzed to determine the fluoride emission rate. Experimental findings show that formation of active oxygen species from H 2 O 2 decomposition or the direct formation of active oxygen species in the oxygen reduction reaction are not the dominating membrane degradation mechanisms in PEMFCs. Instead, membrane degradation occurs because molecular H 2 and O 2 react on the surface of the Pt catalyst to form the membrane-degrading species. The source of H 2 or O 2 is from reactant crossover through the membrane. The reaction mechanism is chemical in nature and depends upon the catalyst surface properties and the relative concentrations of H 2 and O 2 at the catalyst. The membrane degradation rate also depends on the residence time of active oxygen species in the membrane and volume of the membrane. The sulfonic acid groups in the Nafion side chain are key to the mechanism by which radical species attack the polymer.

Effect of Catalyst Properties on Membrane Degradation Rate and the Underlying Degradation Mechanism in PEMFCs

Nafion membrane degradation was studied in a polymer electrolyte membrane fuel cell (PEMFC) under accelerated decay conditions. Fluoride emission rate (FER) determined by fuel cell effluent water analysis was used to quantify the membrane degradation. Membrane degradation is most likely caused either directly or indirectly by the species formed as a result of the H 2 and O 2 reaction on the catalyst. To further understand the mechanism, the effects of the catalyst location, type, its interaction with O 2 and H 2 O, and cell current density on the FER were investigated and their implications on the underlying membrane degradation mechanism are discussed.

Is H2O2 Involved in the Membrane Degradation Mechanism in PEMFC

The involvement of H 2 O 2 in the membrane degradation mechanism in a polymer electrolyte membrane fuel cell (PEMFC) was investigated. Measurement of fluoride concentration in the effluent water was used as an indicator of the membrane degradation rate. It was found that H 2 O 2 is formed in the fuel cell in small concentrations but is not the main source of harmful species, which degrade the membrane. H 2 O 2 decomposition due to impurities or the catalyst leading to the possible formation of radical species would only account for a small fraction of the membrane degradation rate in a fuel cell.

Influence of Convection Through Gas-Diffusion Layers on Limiting Current in PEM FCs Using a Serpentine Flow Field

Three gas-diffusion layers (GDL) with distinctively different gas permeability were used to study the influence of convection through the GDL on the cathode limiting current in proton exchange membrane (PEM) fuel cells. Several flow rates between 50 and 1000 cm 3 /min were used in constant flow rate operation with oxidant gases being air, 4% oxygen/nitrogen, and 21% oxygen/helium. The study was conducted at three cell temperature/relative humidity conditions: 80°C/75% inlet cathode R.H., 100°C/70% inlet cathode R.H., and 120°C/35% inlet cathode R.H., with the objective to evaluate the influence of cell temperature, oxygen mole fraction, relative humidity, and cathode flow rate on the limiting current due to reactant gas transport under conditions where there is no significant flooding. A conventional single-serpentine graphite flow field was used. Cell relative humidity significantly affected the limiting current by reducing oxygen transport through the ionomer thin film of the cathode catalyst layer as the relative humidity decreased. At all three conditions, increasing the cathode dry flow rate increased the limiting current mainly due to more convection. A GDL with higher gas permeability in the microporous layer had a higher limiting current due to more enhanced convection. This accentuates the significance of high gas permeability as a criterion for optimization of GDL. Convection contributes to the limiting current of hydrogen/air PEM fuel cells even when using a conventional flow field pattern (i.e., not interdigitated).

Analysis of Polarization Curves to Evaluate Polarization Sources in Hydrogen/Air PEM Fuel Cells

A step-by-step technique to evaluate six sources of polarization, mainly associated with the cathode, in hydrogen/air proton exchange membrane fuel cells is demonstrated. The six sources of polarization were nonelectrode ohmic overpotential, electrode ohmic overpotential, nonelectrode concentration overpotential, electrode concentration overpotential, activation overpotential from the Tafel slope, and activation overpotential from catalyst activity. The technique is demonstrated as applied in the analysis of hydrogen/air polarization curves of an in-house membrane electrode assembly (MEA) using hydrogen/oxygen polarization curves as a diagnostic tool. The analysis results are discussed at three cell temperature/relative humidity (RH)/oxygen partial pressure (po 2 , atm) conditions at atmospheric pressure: 80°C/100% RH a n o d e /75% RH c a t h o d e /po 2 = 0.136, 100°C/70% RH/po 2 = 0.064, and 120°C/35% RH/po 2 = 0.064, which represent a near fully-humidified, a moderately humidified, and a low humidified condition, respectively. At the higher temperature operating conditions the RH and po 2 decrease resulting in higher electrode ohmic resistance (0.020, 0.020, and 0.035 Ω cm 2 , respectively), lower limiting current (2019, 1314, and 819 mA/cm 2 , respectively), and lower onset current density for significant electrode concentration overpotential (80, 60, and 40 mA/cm 2 , respectively). The technique is useful for diagnosing the main sources of loss in MEA development work, especially for high temperature/low relative humidity operation where several sources of loss are present simultaneously.

Effect of Elevated Temperature and Reduced Relative Humidity on ORR Kinetics for PEM Fuel Cells

As a measure of catalytic activity, i η = 0 . 3 v ,the current density at 0.3 V overpotential, was chosen to evaluate the oxygen reduction reaction (ORR) at elevated temperatures (> 100°C) and various relative humidities(RH) for polymer exchange membrane (PEM) fuel cells. The purely kinetic reaction order of the ORR with respect to oxygen partial pressure is less than 1.0 and changes with the RH. The activation energy is 49 kJ/mol at 100% RH and 55 kJ/mol at 50% RH. The active electrochemical surface area of platinum changes little with RH. RH has a strong effect on the catalytic activity under dry conditions (0-60% RH), but under wet conditions (>60% RH) its influence is unclear. The Tafel slope obtained in the 1-100 mA/cm 2 current density range changes significantly with RH: wet conditions produce low Tafel slopes ( 100 mV/dec). Dependence of the RH on the oxygen reduction reaction (ORR) may be explained by the changes of the rate-determining reaction, proton activity, and adsorbed -OH on the platinum surface. The ORR kinetic parameters obtained here are instructive for high-temperature fuel cell data analysis and performance improvement.

Investigation of Platinum Oxidation in PEM Fuel Cells at Various Relative Humidities

Surface oxidation of Pt cathode of proton exchange membrane (PEM) fuel cells at different relative humidities (RHs) was studied using cyclic voltammetry. The degree of platinum oxidation increased significantly with an increase in RH from 20% to 72%. Holding the cathode at high potentials and exposing it to air instead of N 2 resulted in the formation of more platinum oxides. At 20% RH, 0.85 V, and 100°C, both water and oxygen equally contribute to platinum oxidation; at 100% RH, 0.85 V, and 100°C, water produces more than 70% of the platinum oxidation while 30% forms from oxygen.

The rate of formation of P700+—A0- in photosystem I particles from spinach as measured by picosecond transient absorption spectroscopy

Photosystem I particles containing 30–40 chlorophyll a molecules per primary electron donor P700 were subjected to 1.5 ps low density laser flashes at 610 nm resulting in excitation of the antenna chlorophyll a molecules followed by energy transfer to P700 and subsequent oxidation of P700. Absorbance changes were monitored as a function of time with 1.5 ps time resolution. P700 bleaching (decrease in absorbance) occurred within the time resolution of the experiment. This is attributed to the formation of 1P700.* This observation was confirmed by monitoring the rise of a broad absorption band near 810 nm due to chlorophyll a excited singlet state formation. The appearance of the initial bleach at 700 nm was followed by a strong bleaching at 690 nm. The time constant for the appearance of the 690 nm bleach is 13.7±0.8 ps. In the near-infrared region of the spectrum, the 810 nm band (which formed upon the excitation of the photosystem I particles) diminished to about 60% of its original intensity with the same 13.7 ps time constant as the formation of the 690 nm band. The spectral changes are interpreted as due to the formation of the charge separated state P700+—A0-, where A0 is the primary electron acceptor chlorophyll a molecule.

Development of New CO Tolerant Ternary Anode Catalysts for Proton Exchange Membrane Fuel Cells

Four ternary catalysts, Pt-Ag-Ru, Pt-Au-Ru, Pt-Rh-Ru, and Pt-Ru-W 2 C, were investigated as anode electrocatalysts for oxidation of hydrogen-containing carbon monoxide. These catalysts were either alloys or intimate admixtures of the components. Membrane electrode assemblies were prepared for all the catalysts with anode Pt loading of 0.4 mg/cm 2 , and anode polarization was determined for oxidation of a H 2 stream containing 104 ppm CO. Potentials for CO oxidation were determined by stripping preadsorbed CO using cyclic voltammetry. The Pt-Ru-W 2 C (1:1:0.4 molar ratio) oxidizes CO at a lower potential, and the polarization for oxidation of hydrogen-containing CO was lower than the widely used Pt-Ru (1:1 molar ratio) catalyst. At low polarization, the Pt-Ru-W 2 C catalyst showed twice the activity of the Pt-Ru catalyst when the oxidation currents were normalized to the Pt area.