H. Russell Kunz

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.

Nafion-Teflon- Zr ( HPO 4 ) 2 Composite Membranes for High-Temperature PEMFCs

Nafion membranes suffer high resistance due to dehydration when they are used at elevated temperatures (>100°C) and atmospheric pressures (Nafion 1135 > 0.6 Ω cm 2 at 120°C and 31% relative humidity). Thinner composite membranes containing Nafion and zirconium hydrogen phosphate, Zr(HPO 4 ) 2 , were prepared and showed lower resistance (<0.3 Ω cm 2 ) at these conditions. Thermogravimetric analysis showed that the composite membrane was stable below 200°C in oxygen. Hydrogen crossover of a 18 μm thick composite membrane was only 2 mA/cm 2 at 120°C. When the temperature of a cell using this membrane was raised from 80°C (100% relative humidity) to 120°C (31% relative humidity), the main performance loss at the elevated temperature was from low protonic conductivity and low oxygen permeability in the cathode catalyst layer and dehydration of the membrane.

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.

Study of blend membranes consisting of NafionR and vinylidene fluoride–hexafluoropropylene copolymer

An attempt to modify membranes for direct methanol fuel cells by blending NafionR with a (vinylidene fluoride)–hexafluoropropylene copolymer (VDF–HFP copolymer) from their solutions is reported. The purpose of this work was to reduce the methanol transport while still retaining the essential proton conductivity in a water-containing environment. The apparent conductivity, methanol barrier property, and equilibrium contact angle as a function of the membrane compositions are discussed. The blend membranes were also investigated using X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Compared with the pure NafionR membrane, the NafionR/VDF–HFP copolymer blend membrane with 62.5 vol % of the VDF–HFP copolymer shows a decrease in the apparent conductivity by about 2 orders of magnitude, and the methanol barrier properties increase substantially when only 25 vol % of the VDF–HFP-copolymer is incorporated. The equilibrium contact angles of water drops on the NafionR/VDF–HFP copolymer blend membranes as a function of the VDF–HFP copolymer content are rather similar to the plot of the advancing angle versus the percentage of the lower-surface-energy phase. X-ray diffraction studies indicate that these two polymers crystallize separately when blended and cast from their solutions, and the crystallization behavior is equivalent to that of the unblended state. DSC reveals that when the VDF–HFP copolymer is mixed with NafionR in their solution forms, an interdiffusion or other interaction takes place at the interfaces between their noncrystalline regions.