Rodney L. Borup

Durability of PEFCs at high humidity conditions

This work addresses issues of long-term durability of hydrogen-air polymer electrolyte fuel cells (PEFCs). The chromium in a Pt 3 Cr binary alloy catalyst has been found to migrate from cathode to anode during the course of life testing when operating within the oversaturated, or high-humidity, gas feed regime (one or both inlet feeds with a dew point equal to or higher than cell operating temperature) above 1 A/cm 2 current density. Other major factors such as membrane degradation, dissolution of catalyst-layer recast ionomer, catalyst oxidation, and catalyst agglomeration/growth have been identified as simultaneous, gradual processes that can lead to long-term PEFC failure. In situ cyclic voltammetry measurement of electrochemically active catalyst surface area shows a continuous decrease, revealing that catalyst agglomeration and/or growth may be a major cause of membrane electrode assembly degradation during middle-term life tests (i.e., operation times up to about 2000 h) under high-humidity conditions. Membrane and/or recast ionomer degradation was confirmed by the presence of fluoride and sulfate anions in the cathode outlet water. Scanning and transmission electron microscopy observation of a tested MEA suggest the loss of carbon-supported catalyst clusters and possible dissolution of recast Nafion ionomer.

Microstructural Changes of Membrane Electrode Assemblies during PEFC Durability Testing at High Humidity Conditions

Morphological changes occurring in membrane electrode assemblies (MEAs) were monitored using transmission electron microscopy (TEM) during the course of life testing of H 2 /air polymer electrolyte fuel cells (PEFCs). In the fresh catalyst layers, anode Pt particles were found to have smaller particle sizes, better dispersion, and less agglomeration on the carbon-support surfaces than did the cathode Pt 3 Cr alloy particles. The operation-induced agglomeration of catalyst particles was evaluated for both the anode and cathode after defined life testing periods. Agglomeration of catalyst particles occurred primarily during the first 500 h of testing, which was confirmed by both TEM analysis and electrocatalytic surface area measurement. After 500 h, degradation of the recast Nafion ionomer network within the catalyst layers likely contributes more significantly to MEA performance degradation. Migration of metal catalyst particles toward the interface between the catalyst layer and membrane was observed at both electrodes. The Pt anode catalyst was less stable than the Pt 3 Cr cathode catalyst under high current density and high humidity conditions, which was confirmed by the higher extent of migration observed for the pure Pt than for the Pt 3 Cr. Some Pt particles (from both electrodes) were found to migrate into the membrane during the testing period.

Equilibrium products from autothermal processes for generating hydrogen-rich fuel-cell feeds

Abstract This work presents thermodynamic analyses of autothermal processes using five fuels—natural gas, methanol, ethanol, dimethyl ether, and gasoline. Autothermal processes combine exothermic and endothermic reactions. The processes considered here couple endothermic steam reforming with exothermic oxidation to create hydrogen-rich fuel-cell feeds. Of the fuels treated here, methanol, ethanol, and dimethyl ether are pure compounds. Methane simulates natural gas and a mixture of 7% neopentane, 56% 2,4 dimethyl pentane, 7% cyclohexane, 30% ethyl benzene simulates gasoline. In the computations, sufficient oxygen is fed so the energy generated by the oxidation exactly compensates the energy absorbed by the reforming reactions. The analyses calculate equilibrium product concentrations at temperatures from 300 to 1000 K , pressures from 1 to 5 atm , and water–fuel ratios from 1 to 9 times the stoichiometric value. The thermodynamic calculations in this work say that any of the five fuels, when processed autothermally, can give a product leading to a hydrogen-rich feed for fuel cells. The calculations also show that the oxygen-containing substances (methanol, ethanol, and dimethyl ether) require lower temperatures for effective processing than the non-oxygenated fuels (natural gas and gasoline). Lower reaction temperatures also promote products containing less carbon monoxide, a desirable effect. The presence of significant product CO mandates the choice of optimum conditions, not necessarily conditions that produce the maximum product hydrogen content. Using a simple optimum objective function shows that dimethyl ether has the greatest potential product content, followed by methanol, ethanol, gasoline, and natural gas. The calculations point the way toward rational choices of processes for producing fuel-cell feeds of the necessary quality.

Nafion Structural Phenomena at Platinum and Carbon Interfaces

Neutron reflectometry was used to examine the interactions of polymer electrolyte fuel cell (PEFC) materials that comprise the triple-phase interface. Smooth, idealized layers of Nafion on glassy carbon (GC) and Pt surfaces were used to experimentally model the PEFC electrode interfaces. Different multilayer structures of Nafion were found in contact with the Pt or GC surfaces. These structures showed separate hydrophobic and hydrophilic domains formed within the Nafion layer when equilibrated with saturated D(2)O vapor. A hydrophobic Nafion region was formed adjacent to a Pt film. However, when Nafion was in contact with a PtO surface, the Nafion at the Pt interface became hydrophilic. The adsorbed oxide layer caused a long-range restructuring of the perfluorosulfonic acid polymer chains that comprise Nafion. The thicknesses of the hydrophobic and hydrophilic domains changed to the same magnitude when the oxide layer was present compared to a thin hydrophobic domain in contact with Pt. A three-layer Nafion structure was formed when Nafion was in direct contact with GC. The findings in this research are direct experimental evidence that both the interfacial and long-range structural properties of Nafion are affected by the material with which it is in contact. Evidence of physical changes of aged Nafion films was obtained, and the results showed a permanent increase in the thickness of the Nafion film and a decrease in the scattering length density (SLD), which are attributed to irreversible swelling of the Nafion film. The aging also resulted in a decrease in the SLD of the GC substrate, which is likely due to either an increase in surface oxidation of the carbon or loss of carbon mass at the GC surface.

Accurate measurement of the through-plane water content of proton-exchange membranes using neutron radiography

The water sorption of proton-exchange membranes (PEMs) was measured in situ using high-resolution neutron imaging in small-scale fuel cell test sections. A detailed characterization of the measurement uncertainties and corrections associated with the technique is presented. An image-processing procedure resolved a previously reported discrepancy between the measured and predicted membrane water content. With high-resolution neutron-imaging detectors, the water distributions across N1140 and N117 Nafion membranes are resolved in vapor-sorption experiments and during fuel cell and hydrogen-pump operation. The measured in situ water content of a restricted membrane at 80 °C is shown to agree with ex situ gravimetric measurements of free-swelling membranes over a water activity range of 0.5 to 1.0 including at liquid equilibration. Schroeders paradox was verified by in situ water-content measurements which go from a high value at supersaturated or liquid conditions to a lower one with fully saturated vapor. ...

Surface Properties of PEMFC Gas Diffusion Layers

Understanding liquid-water transport phenomena inside operating proton exchange membrane fuel cells PEMFCs can only be achieved by a basic understanding of gas diffusion layer GDL wetting properties. This requires measuring and quantifying the combined properties of different GDL materials and layers. These properties include surface energy, liquid absorption, internal contact angle, equilibrium contact angle, and capillary constant. The stateof-the-art microporous layer MPL and GDL substrate configuration that is nearly ubiquitously used has been adopted without a basic understanding of these fundamental properties. This lack of understanding has created poor comprehension of the relationship between PEMFC operating variables most notably inlet relative humidity RH, temperature, and current density and GDL designs. Science applied to the GDL component design helps enable PEMFCs capable of running under wet and dry operating conditions. PEMFC GDLs have a complex wetting behavior due to the constituent materials from which they are made. 1-3 GDLs are typically composed of two distinct layers, a substrate of graphitized carbon fibers and an MPL coated onto one side. The macroporous substrate layer consists of a carbon-fiber matrix and a partial fluoropolymer coating of the fibers with a typical void volume of 75‐85%. The MPL consists of carbon black particles mixed with a fluoropolymer and has a smaller porosity 40 to 50% and a mean pore size than the GDL substrate. The MPL forms a penetrating intermediate layer with the fiber matrix, creating a trilayer structure in practice. GDLs participate in most functions of an operating PEMFC, including reactant distribution to the electrode layers, removal of cathode product water, conduction of electrons, and absorption of compressive loads. Quantification of surface wetting properties of GDLs is important for several reasons: i better understanding of the mechanisms of the two-phase transport of liquid water, ii improved understanding of fuel cell performance with different operating conditions, iii fundamental understanding of the nanoscale interactions of liquid water with the heterogeneous pore surface of GDLs, and iv more sophisticated mathematical model development by providing better capillary pressure and pore-size distribution PSD inputs. Recent research undertaken to enhance the understanding of liquid-phase transport includes capillary pressure Pc vs liquid saturation SL measurements 4-8 and pore-network modeling 9-12 of GDLs. Although these approaches are different from that taken in this work, they represent important steps in the understanding of the inter-related effects of surface chemistry and pore structure. Gostick et al. measured Pc vs SL curves for a spectrum of GDL materials for both the total and hydrophilic PSDs. These data were obtained using the method of standard porosimetry and correlated with Hg porosimetry for the total PSDs. Capillary-pressure data of the GDL substrates and MPLs were separately characterized using a Leverett J-function.

Parametric Study of the Morphological Proprieties of HT-PEMFC Components for Effective Membrane Hydration

A 1D non-isothermal model has been developed to study the optimum morphological properties of HT-PEMFC components that will help the catalyst layer retain water vapor generated by the electrochemical reaction and that delivered by the feed gases. The use of a microporous layer (MPL) helps retain water generated in the catalyst layer (CL), and the effectiveness of the MPL in retaining water vapor in the CL increases the MPL pore size and porosity are reduced. Reducing the GDL porosity is found to help retain more water in the CL but the pore sizes and porosity of the PEMFC components should not be too small, so as to avoid increased O 2 concentration overpotentials. The optimum values of MPL and GDL porosity depend on the operating conditions such as the cell voltage, operating pressure, and inlet relative humidity.

Estimation of Mass-Transport Overpotentials during Long-Term PEMFC Operation

A comprehensive method for separating proton exchange membrane fuel cell (PEMFC) cathode catalyst-layer and gas diffusion layer (GDL) mass-transport overpotentials was derived utilizing kinetic and ohmic analysis of high humidified H 2 /O 2 and low humidified H 2 /air polarization data. A hybridized method was applied accounting for electrochemical surface area, hydrogen crossover, and exchange current density. This method delineates separate cathode GDL and electrode mass-transport overpotentials as a function of current density and operating time. The methodology is systematic and generalized and can be applied to polarization data from any type of durability test. The derivation was applied to periodic polarization data from a steady-state 1050 h durability test and is shown to provide an accurate breakdown of the sources of performance losses. Increases in mass-transport overpotential for the cathode GDL and oxygen reduction reaction overpotential were predominantly offset by improvements in the mass-transport overpotential of the cathode catalyst layer and reductions in the high frequency resistance. Little increase in the GDL mass-transport overpotential was observed during the first ∼500 h period, but a substantial increase was seen during the second ∼500 h period. The mass-transport overpotential of the cathode catalyst layer was almost negligible at the end of ∼1000 h of operation, suggesting little O 2 diffusion resistance through the ionomer and adjacent void volume.

Accelerated Testing Validation

The DOE Fuel Cell technical team recommended ASTs were performed on 2 different MEAs (designated P5 and HD6) from Ballard Power Systems. These MEAs were also incorporated into stacks and operated in fuel cell bus modules that were either operated in the field (three P5 buses) in Hamburg, or on an Orange county transit authority drive cycle in the laboratory (HD6 bus module). Qualitative agreement was found in the degradation mechanisms and rates observed in the AST and in the field. The HD6 based MEAs exhibited lower voltage degradation rates (due to catalyst corrosion) and slower membrane degradation rates in the field as reflected by their superior performance in the high potential hold and open-circuit potential AST tests. The quantitative correlation of the degradation rates will have to take into account the various stressors in the field including temperature, relative humidity, start/stops and voltage cycles.

Measurement of Water Content in Polymer Electrolyte Membranes Using High Resolution Neutron Imaging

Sufficient water content within a polymer electrolyte membrane (PEM) is necessary for adequate ionic conductivity. Membrane hydration is therefore a fundamental requirement for fuel cell operation. The hydration state of the membrane affects the water transport within, as both the diffusion coefficient and electro-osmotic drag depend on the water content. Membranes water uptake is conventionally measured ex situ by weighing free-swelling samples equilibrated at controlled water activity. In the present study, water profiles in Nafion{reg_sign} membranes were measured using the high-resolution neutron imaging. The state-of-the-art, 10 {micro}m resolution neutron detector is capable of resolving water distributions across N1120, N1110 and N117 membranes. It provides a means to measure the water uptake and transport properties of fuel cell membranes in situ.