Tommy Rockward

Model for Polymer Electrolyte Fuel Cell Operation on Reformate Feed: Effects of CO, H;2 Dilution, and High Fuel Utilization

We describe a polymer electrolyte fuel cell model emphasizing operation on hydrocarbon reformate, i.e., the anode feed stream consists of dry H 2 concentrations as low as 40%, inlet CO levels of 10-100 ppm, and hydrogen fuel utilization as high as 90%. Refinements of interfacial kinetics equations used in our previous work on CO effects in H 2 anodes have yielded a better quantitative fit to the measured dependence of voltage loss on inlet CO level [in Electrode Materials and Processes for Energy Conversion and Storage, J. McBreen, S. Mukerjee, and S. Srinivasan, Editors, PV 97-13, pp. 15-24, The Electrochemical Society Proceedings Series, Pennington, NJ (1997)]. We calculate anode potential losses by coupling such interfacial kinetic processes to reactant diffusion limitations and ionic resistance in the catalyst layer, and by accounting for the drop in local hydrogen concentration along the flow channel due to significant fuel utilization. As a result of internal readjustment of cell overpotentials when hydrogen concentration drops along the flow channel, we show that loss of current, or power, under the realistic condition of constant cell voltage is smaller than loss of current at constant anode potential. We show that voltage losses associated with CO poisoning are significantly amplified with diluted hydrogen feed streams and particularly so under high fuel utilization. We make projections on improvements required, qualitative and quantitative, in the physical parameters of the anode catalyst surface chemistry to significantly improve CO tolerance.

Imaging of Water Profiles in PEM Fuel Cells Using Neutron Radiography: Effect of Operating Conditions and GDL Composition

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 cm2 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 cm2 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 Impact of Impurities on Long Term PEMFC Performance

Electrochemical experimentation and modeling indicates that impurities degrade fuel cell performance by a variety of mechanisms. Electrokinetics may be inhibited by catalytic site poisoning from sulfur compounds and CO and by decreased local proton activity and mobility caused by the presence of foreign salt cations or ammonia. Cation impurity profiles vary with current density, valence and may change local conductivity and water concentrations in the ionomer. Nitrogen oxides and ammonia species may be electrochemically active under fuel cell operating conditions. The primary impurity removal mechanisms are electrooxidation and water fluxes through the fuel cell.

High Resolution Neutron Radiography Imaging of Operating PEM Fuel Cells: Effect of Flow Configuration and Gravity on Water Distribution

Neutron radiography, performed in situ within an operating PEMFC, is a powerful tool in developing greater understanding of water distribution profiles during cell operation. In particular, the ability to simultaneously correlate water distribution with cell operating conditions and cell performance provides a degree of understanding of water management that is not available with any other existing technique. High resolution imaging of through plane water profiles using a Micro-Channel Plate (MCP) detector with 25 m resolution produces water profile images in which one can readily distinguish between water in anode and cathode gas diffusion layers (GDLs), as well as between water in the GDLs above channels and above lands, and water in the channels themselves.

Measurement of H2S Crossover Rates in Polymer Fuel Cell Membranes Using an Ion-Probe Technique

A sulfide antioxidant buffer solution was used to trap and concentrate trace quantities of H 2 S that permeated through 50 cm 2 samples of fuel cell Nafion 117, 212, and 112 membranes using a H 2 S partial pressure difference up to 1000 ppm at room temperature. Experiments were conducted between 24 and 48 h to achieve sulfide ion concentrations high enough to be determined accurately by subsequent titration with Pb(NO 3 ) 2 . In contrast to previous work, a special H 2 S mixing system was employed to achieve reproducible delivery of H 2 S concentrations throughout each experiment. The calculated rates of H 2 S crossover, normalized for membrane area, thickness, and differential PH 2 S varied from as low as 7.58 X 10 ―10 to 4.65 × 10- 9 g/s atm cm depending on the type of Nafion sample and humidity conditions. Even low levels of humidification of the membrane and gases (saturated at 25°C) increased the crossover rate of H 2 S in the Nafion materials. H 2 S permeation through the electrolyte membrane negatively impacts the oxygen reduction rate occurring on the cathode and possibly causes an irreversible decrease in the overall fuel cell performance.

High Performance and Durable Low PGM Cathode Catalysts

There is a strong need to decrease the amount of Pt electrocatalyst used in fuel cells and increase its durability for transportation application. Conventional strategies include Pt nanocrystals and Pt alloy with well-controlled structures, durable carbon support, non-carbon support, etc. We have developed the so-called “metal-metal oxide-carbon” triple junction concept to stabilize Pt and protect carbon from corrosion. It also improved the activity of Pt. The good performance was not achieved in fuel cell test mainly because of the transport issue due to the use of 2D graphene. In this project, our main goal is to demonstrate the concept in fuel cell device test using 3D porous graphene as support so that the transport issue could be addressed.