PEMFC MEA and System Design Considerations

Proton exchange membrane fuel cells (PEMFCs) are being developed and sold commercially for multiple near term markets. Ballard Power Systems is focused on the near term markets of backup power, distributed generation, materials handling, and buses. Significant advances have been made in cost and durability of fuel cell products. Improved tolerance to a wide range of system operation and environmental noises will enable increased viability across a broad range of applications. In order to apply the most effective membrane electrode assembly (MEA) design for each market, the system requirements and associated MEA failures must be well understood. The failure modes associated with the electrodes and membrane degradation are discussed with respect to associated system operation and mitigating approaches. A few key system considerations that influence MEA design include expected fuel quality, balance-of-plant materials, time under idle or open circuit operation, and start-up and shut-down conditions

Evaluation of alkali and alkaline earth doped samarium oxide catalysts for the oxidative coupling of methane

The catalytic oxidative coupling of methane involves the reaction of methane and oxygen at high temperatures (650°C to 900°C) in the presence of a solid metal oxide catalyst to produce the desirable products, ethane and ethylene, as well as the undesirable products, carbon monoxide and carbon dioxide. Although the homogeneous reactions are well understood, the heterogeneous reactions and their effect on the homogeneous reactions are still the subject of much research and discussion. The effect of the catalyst characteristics on the heterogeneous reactions is also an active area of research. The objective of this thesis was to characterize a series of catalysts, and to determine the effect of various catalyst properties on the oxidative coupling reactions. The experimental part of this thesis consisted of preparing and testing samarium oxide and alkali (Na and K) and alkaline earth (Mg and Ca) doped samarium oxide catalysts for the oxidative coupling of methane. The effects of the specific dopant used, varying dopant concentration (1:100 and 1:10 dopant:Sm mole ratio), and catalyst preparation were evaluated. The catalysts were tested in a bench scale packed bed reactor under conditions of varying temperature (650°C, 750°C, and 850°C) and methane to oxygen mole ratio (2 to 16). The catalysts were characterized by scanning electron microscopy, powder x-ray diffraction, surface area, estimated basicity, ability to form carbonates, and ionic radius of dopant. The addition of dopants to the samarium oxide catalyst resulted in changes in catalyst performance. No new phases were observed in the Sm203 crystal upon addition of the dopant cations, indicating that the cations were dispersed throughout the crystal, although probably not uniformly. The dopant concentration affected the catalyst performance; for example, at 750°C, the C2+ yield increased from 13.1% to 14.3% when the Ca:Sm mole ratio was increased from 1:100 to 10:100. A change in the catalyst preparation procedure resulted in an increase in the crystal dimensions and an improved combustion catalyst (e.g., the methane conversion increased from 9.8% for the standard catalyst to 17.4% for the revised catalyst) with, however, a decrease in the C2yield from 3.2% to 2.7%. The results of this study indicated that, over the range of surface areas tested (2.0 to 3.1 m2/g), surface area did not have a significant effect on the catalyst performance. The basicity of the catalyst appears to have a significant effect on the catalyst performance, with an increase in basicity resulting in an increase in C2+ selectivity (from 54.3% for the least basic catalyst, undoped samarium oxide, to62.8% for the most basic catalyst, 1:10 mole ratio Na:Sm oxide). The catalysts displayed temperature dependent behaviour, and there existed an optimum temperature for maximum C2+ yield, which is dependent on the amount and nature of the dopant, and is likely associated with the formation of carbonates on the catalyst surface. The ionic radius of the cation dopant must be similar to or smaller than the support cation to achieve effective inclusion in the crystal lattice.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat

Transient analysis of hydrogen sulfide contamination on the performance of a PEM fuel cell

A transient kinetic model for the anode catalyst layer of a proton exchange membrane (PEM) fuel cell is proposed to describe the performance loss introduced by the hydrogen sulfide contaminant. Reaction rates are considered as functions of cell current density and contamination level and are estimated based on the available experimental data. It is found that at a constant cell current density the surface coverage of Pt\ue2\u20ac\u201cS increases faster with time and the anode overpotential rises sharply when increasing the contamination level from 1 to 10 ppm, leading to a faster and more severe cell performance degradation. At a constant contamination level, the surface coverage of Pt\ue2\u20ac\u201cS also increases faster with time when increasing the cell current density from 0.1 to 1.0 A cm\ue2\u2c6\u20192, resulting in faster and more severe cell performance degradation. Simulation shows that the contaminant surface coverage at steady state is governed by current density. With the same contaminant concentration, to maintain a higher current density output, a significant decrease of steady-state contaminant coverage is required, while at a lower current density, steady-state contaminants coverage increases significantly.NRC publication: Ye

PEMFC MEA and System Design Considerations

Proton exchange membrane fuel cells (PEMFCs) are being developed and sold commercially for multiple near term markets. Ballard Power Systems is focused on the near term markets of backup power, distributed generation, materials handling, and buses. Significant advances have been made in cost and durability of fuel cell products. Improved tolerance to a wide range of system operation and environmental noises will enable increased viability across a broad range of applications. In order to apply the most effective membrane electrode assembly (MEA) design for each market, the system requirements and associated MEA failures must be well understood. The failure modes associated with the electrodes and membrane degradation are discussed with respect to associated system operation and mitigating approaches. A few key system considerations that influence MEA design include expected fuel quality, balance-of-plant materials, time under idle or open circuit operation, and start-up and shut-down conditions

Ti4O7 supported Ru@Pt core-shell catalyst for CO-tolerance in PEM fuel cell hydrogen oxidation reaction

A new method is developed for synthesizing Ti4O7 supported Ru@Pt core-shell catalyst (Ru@Pt/Ti4O7) through pyrolysis followed by microwave irradiation. The purpose is to improve the Ru durability of PtRu from core-shell structure and strong bonding to Ti4O7 oxide. In this method, the first step is to co-reduce the mixture of ruthenium precursor and TiO2 in a H2 reducing atmosphere under heat-treatment to obtain a Ru core on Ti4O7 support, and the second step is to create a shell of platinum via microwave irradiation. Energy dispersive X-ray spectrometry, X-ray Diffraction, High-resolution Scanning Transmission Electron Microscopy with the high-angle annular dark-field method and Electron Energy-Loss Spectroscopy are used to demonstrate that this catalyst with larger particles has a core-shell structure with a Ru core and a Pt shell. Electrochemical measurements show Ru@Pt/Ti4O7 catalyst has a higher CO-tolerance capability than that of PtRu/C alloy catalyst.Peer reviewed: YesNRC publication: Ye

3D Porous Fe/N/C Spherical Nanostructures As High-Performance Electrocatalysts for Oxygen Reduction in Both Alkaline and Acidic Media

Exploring inexpensive and high-performance nonprecious metal catalysts (NPMCs) to replace the rare and expensive Pt-based catalyst for the oxygen reduction reaction (ORR) is crucial for future low-temperature fuel cell devices. Herein, we developed a new type of highly efficient 3D porous Fe/N/C electrocatalyst through a simple pyrolysis approach. Our systematic study revealed that the pyrolysis temperature, the surface area, and the Fe content in the catalysts largely affect the ORR performance of the Fe/N/C catalysts, and the optimized parameters have been identified. The optimized Fe/N/C catalyst, with an interconnected hollow and open structure, exhibits one of the highest ORR activity, stability and selectivity in both alkaline and acidic conditions. In 0.1 M KOH, compared to the commercial Pt/C catalyst, the 3D porous Fe/N/C catalyst exhibits ∼6 times better activity (e.g., 1.91 mA cm^–2 for Fe/N/C vs 0.32 mA cm^–2 for Pt/C, at 0.9 V) and excellent stability (e.g., no any decay for Fe/N/C vs 35 mV negative half-wave potential shift for Pt/C, after 10000 cycles test). In 0.5 M H₂SO₄, this catalyst also exhibits comparable activity and better stability comparing to Pt/C catalyst. More importantly, in both alkaline and acidic media (RRDE environment), the as-synthesized Fe/N/C catalyst shows much better stability and methanol tolerance than those of the state-of-the-art commercial Pt/C catalyst. All these make the 3D porous Fe/N/C nanostructure an excellent candidate for non-precious-metal ORR catalyst in metal–air batteries and fuel cells

Web-like 3D Architecture of Pt Nanowires and Sulfur-Doped Carbon Nanotube with Superior Electrocatalytic Performance

Development of highly durable electrocatalysts for oxygen reduction reaction (ORR) is critical for proton exchange membrane fuel cells. Herein, we report the synthesis, characterization, and electrochemical performance of 1D sulfur-doped carbon nanotubes (S-CNT) supported 1D Pt nanowires (PtNW/S-CNT). PtNW/S-CNT synthesized by a modified solvothermal method possesses a unique web-like 3D architecture that is beneficial for oxygen reduction. We demonstrate that PtNW/S-CNT exhibits impressive activity retention under potential cycling between 0.05 and 1.5 V vs RHE over 3000 cycles. The reductions in electrochemically active surface area (ECSA, 7% loss) and mass activity (19% loss) of PtNW/S-CNT after accelerated durability testing (ADT) are found to be much lower than the dramatic losses observed with commercial Pt/C (>99% loss in ECSA and mass activity) under identical conditions. The PtNW/S-CNT catalyst also shows very high specific activity (1.61 mA cm^–2) in comparison to Pt/C (0.24 mA cm^–2)