Theoretical Anode ModelWe initially address the kinetic equations for the catalyst/ionomer interface that provides local current generation as a function of local hydrogen and CO concentrations and the local overpotential. Subsequently, we address the mass-transport equations that set the local conditions at the catalyst surface when a cell is operated with diluted

In this contribution, the effects of sodium chloride in the cathode side air stream of a polymer electrolyte membrane fuel cell were studied, since it is present in the atmosphere in many fuel cell operating environments, especially at coastal regions and on roads where NaCl is used as a deicer. The effect on PEMFC performance was investigated by injecting NaCl solution into the cathode side air stream of an operating fuel cell. The most significant effect of NaCl under test conditions is the replacement of H+ by Na+ in the ionomer, lowering performance by reducing proton conductivity. In addition, NaCl was found to absorb into the graphite components, making them a potential long‐term NaCl source after the initial exposure. In contrast to expectations, chloride ions did not appear to effect performance of the cell and the overall magnitude of the effects due to NaCl introduction were smaller than expected.

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The performance of polymer electrolyte membrane (PEM) fuel cells as a function of cathode inlet relative humidity (RH) and gas diffusion layer (GDL) properties has been characterized. The performance of 50 cm 2 fuel cells at high current densities was a strong function of the polytetrafluoroethylene (PTFE) content in the cathode GDL microporous layer (MPL). The voltage at a current density of 1.4 A cm -2 decreased at all inlet RHs as the PTFE content in the cathode MPL increased from 5 % by weight to 23 % by weight. This was associated with a corresponding increase in the mass transport resistance as measured by AC impedance. The low frequency resistance also increased with increasing cathode inlet RH. These results were validated by high-resolution neutron radiography on specially designed 2.25 cm 2 cells that showed increased water content in the GDLs at high inlet RHs and high microporous layer PTFE content. High-resolution neutron imaging also revealed higher water concentrations at the outlets, cathode GDL, anode flow channel, and MEA/GDL above the land when compared to the inlets, anode GDL, cathode flow channel, and MEA/GDL above the channel respectively.

The performance of single-PEM fuel cells at sub-freezing temperatures has been characterized. Fully humidified fuel cells that were subjected to multiple freeze/thaw cycles down to -40 oC showed degradation in performance associated with mass transport problems at the gas diffusion layer. The performance loss was negligible for cloth GDLs and slow cooling rates while fast cooling rates and paper GDLs showed significant degradation. When a single-fuel cell was operated at -10 oC at a constant voltage, the current first increased to a maximum value, then started decaying due to ice formation. This ice formation was monitored using neutron radiography and was found to be concentrated near the inlets, outlets and cell edges resulting in a significant increase in mass transport resistance as revealed by AC impedance spectroscopy. Cyclic voltammograms at -10 oC show no change in catalyst electrochemical surface area associated with this decreased performance. Finally the cell did not exhibit any degradation in performance at 80 oC even after 5 separate operations at -10 oC, where the cell was allowed to completely clog with ice each time until no further current was produced.

We have studied the effects of H2S and fuel impurity mixtures on hydrogen polymer fuel cell performance. The tests included a range of H2S concentrations, exposure times, and multiple impurity mixtures at various operating conditions such as cell voltage and times of exposure. Analytical methods included fuel cell polarization experiments, cyclic voltammetry, and effluent water and gas analysis using specific ion sensor probes. H2S concentrations as low as 10 ppb, produced negative effects on fuel cell performance. The extent of sulfur poisoning of the anodes was also shown to be a strong function of fuel cell operating voltage during H2S exposure.

Nanoscale graphenes were used as cathode catalyst supports in proton exchange membrane fuel cells (PEMFCs). Surface-initiated polymerization that covalently bonds polybenzimidazole (PBI) polymer on the surface of graphene supports enables the uniform distribution of the Pt nanoparticles, as well as allows the sealing of the unterminated carbon bonds usually present on the edge of graphene from the chemical reduction of graphene oxide. The nanographene effectively shortens the length of channels and pores for O 2 diffusion/water dissipation and significantly increases the primary pore volume. Further addition of p-phenyl sulfonic functional graphitic carbon particles as spacers, increases the specific volume of the secondary pores and greatly improves O 2 mass transport within the catalyst layers. The developed composite cathode catalyst of Pt/PBI-nanographene (50 wt%) + SO 3 H-graphitic carbon black demonstrates a higher beginning of life (BOL) PEMFC performance as compared to both Pt/PBI-nanographene (50 wt%) and Pt/PBI-graphene (50 wt%) + SO 3 H-graphitic carbon black (GCB). Accelerated stress tests show excellent support durability compared to that of traditional Pt/Vulcan XC72 catalysts, when subjected to 10,000 cycles from 1.0 V to 1.5 V. This study suggests the promise of using PBI-nanographene + SO 3 H-GCB hybrid supports in fuel cells to achieve the 2020 DOE targets for transportation applications. High stability is required for polymer electrolyte membrane fuel cells (PEMFCs) catalyst supports because they play a critical role in determining the overall durability of PEMFC systems.1 Carbon-based supports have been widely used to support Pt or Pt alloy catalysts in PEMFCs.2 High-surface-area carbon (HSAC) supports minimize the aggregation of the catalyst nanoparticles as HSAC provides more sites for catalyst to nucleate than low-surface-area-carbon (LSAC), thereby avoiding forming big aggregates. Thus, it increases the Pt utilization that leads to more active electro-catalysts with low platinum loadings (< 0.1 mg-Pt/cm 2 ), 2 yet HSAC is vulnerable for corrosion due to the increased surface area. Good electrical conductivity of the carbon materials provides good electron transport.3 In addition, the random aggregates of the primary carbon particles help the distribution of ionomer to access the active sites during the fabrication of membrane electrode assemblies (MEA), and also allow the construction of a highly porous catalyst layer that enables O 2 diffusion and the water dissipation in PEMFCs. The most commonly used carbon blacks in low temperature PEMFCs include Vulcan XC72 (Cabot Corp., USA), Black Pearls 2000 (Cabot Corp., USA) Ketjen Black EC300j (AkzoNobel Corp., the Netherlands), Ketjen Black EC600 (AkzoNobel Corp., the Netherlands), etc. These carbon blacks consist of inhomogeneous graphitic structures and amorphous carbons. 4 The small size of the graphitic domains cause a high density of edge sites, (particularly in the case of high BET (Brunauer-Emmett-Teller) surface area carbon supports), ...

H 2 S is a well known poison for polymer electrolyte fuel cells. In this work we have found that the severity of H 2 S poisoning depends on H 2 S concentration, dosage, cell temperature and anode humidification. We found catalyst poisoning by H 2 S was cumulative and highly irreversible. After poisoning, further cell operation on neat hydrogen resulted only in modest performance recovery. Full recovery could usually be attained after repetitive cyclic voltammetry between 0 and 1.4 V. We discuss improvements of cell tolerance to H 2 S and performance recovery using other strategies, such as holding the cell at open circuit and anode air bleed during operation. We discuss the possible implications of H 2 S crossover on cathode performance.