Yu-Min Tsou

Oxygen Reduction in a Proton Exchange Membrane Test Cell

Oxygen reduction in a gas-fed porous electrode attached to a proton exchange membrane is discussed. Experimental data and a mathematical model are presented for the test cell used. Various membrane and electrode assemblies were tested at different levels of platinum loading and Teflon , ,1 content. The model accounts for the diffusion and reaction of oxygen and the diffusion and reactions of hydrogen ions. Sulfuric acid was placed above the membrane in the test cell reservoir to provide a source of protons for the reduction of oxygen at the cathode. Based upon model predictions, it is shown that the transport of the protons in the active layer of the cathode is an important factor in the operation of the test cell

Hydrogen Diffusion, Solubility, and Water Uptake in Dow's Short‐Side‐Chain Perfluorocarbon Membranes

Hydrogen gas diffusion coefficients and solubilities as well as water uptake values are reported for Dows short-side-chain perfluoro-sulfonic and -carboxylic membranes of different equivalent weight (EW). The diffusion coefficients and solubilities were determined with an electrochemical test cell. Hydrogen solubility decreases with increasing EW in the lower EW range and tends to level off at higher EWs for both types of membranes. Both hydrogen solubility and diffusion coefficients of a sulfonic membrane with EW higher than 800 are higher than the corresponding values of a carboxylic membrane of similar EW

Study of Ionic Conductivity Profiles of the Air Cathode of a PEMFC by AC Impedance Spectroscopy

A characterization of the ionic conduction of the active layer of a polymer electrolyte membrane fuel cell ~PEMFC! cathode by ac impedance measurement at open-circuit potential conditions was conducted. Porous electrode theory was used to derive a compact equation, ] 2 F & 2 /]y 2 1 ] ln f(y)/]y 3 ]F & 2 /]y 2 R/ f ( y)(1 1 jV)F & 2 5 0, to solve for the impedance response of a cathode at open-circuit potential conditions. This equation includes a parameter R, the ratio of an ionic resistance ~evaluated at the active layer/membrane interface!, to the total charge-transfer resistance of the active layer. The influence of an assumed ionic conductivity distribution profile f ( y) on the error in the estimation of total double-layer capacitance of the active layer from the 21/(ZImv) vs. ZRe plot was also investigated in this work. The increase of ionic conductivity in the active layer of an air cathode with an increase in the ionomer loading was revealed from both impedance data and surface area measurements. A nonlinear parameter estimation method was used to extract the ionic resistance from the high-frequency region of the impedance data at open-circuit potential conditions. The assumed ionic conductivity distribution profile in the active layer was found to vary with ionomer loadings.

Estimation of the Diffusion Coefficient and Solubility for a Gas Diffusing Through a Membrane

Analysis of the data obtained by the electrochemical monitoring technique for diffusion of a gas through a membrane is considered. It is shown that combining a numerical method with a nonlinear parameter estimation technique provides a means to determine values for the diffusion coefficient and the solubility of the diffusing gas. It is shown that better accuracy can be obtained for the diffusion coefficient and solubility of this gas by using the method presented and all experimental data rather than only part of the data, as has often been done in the past

Crucial Role of Low Coordination Sites in Oxygen Reduction, CO Stripping, and Size Effect for Nano-sized Pt Particles

A new synthetic method for nano-sized Pt particles has been developed. It allows the manufacture of Pt particles with very small crystallite sizes even at very high loadings. A series of Pt catalysts were prepared with well defined crystallites. The weak hydrogen adsorption peak at -0.56 v is proposed to be associated to defect sites in addition to edge/corner sites. The ratio of strong/weak hydrogen adsorption peak is proposed to be an indication of “extent of disorder” of a Pt particle. A size effect theory consisting of two ORR (oxygen reduction reaction) size effects are proposed: the primary particle size effect that becomes important only when the percentage of corner/edge/defect sites is so high that they dominate the activity, i.e., as particle size falls below 18-20 A. The secondary size effect is associated with (111) terrace size. The decrease of surface specific activity from a crystallite size of 54 nm to 24 nm is estimated to be about 36 mv from CV peak shift. Experimental results are supplied to support the two-effect hypothesis. Several voltammetric peaks observed in H2SO4 and HClO4for nano-sized Pt/C are assigned to Pt-OH formation/reduction on low coordination sites. These peaks are separated form main Pt-OH peaks by 180-280 mv and they are hypothesized to be related to primary size effect. CO stripping on nano-sized Pt particles exhibits an onset potential close to the potential of hypothesized Pt-OH formation on low coordination sites. This assumed correlation is used to support the theory: adsorbed OH sites for CO stripping are related to low coordination Pt sites. Two distinct CO stripping peaks are observed for Pt particles with XRD sizes larger than 3.3 nm. The two peaks are assigned to different sites, terrace sites vs. other sites. The details of the CO stripping curves are discussed.

Integrated Manufacturing for Advanced MEAs

This program addressed a two-pronged goal for developing fuel cell components: lowering of precious metal content in membrane electrode assemblies (MEAs), thereby reducing the fuel cell cost, and creating MEAs that can operate at 120oC and 25% RH whereby the system efficiency and effectiveness is greatly improved. In completing this program, we have demonstrated a significant reduction in precious metal while at the same time increasing the power output (achieved 2005 goal of 0.6g/Kw). We have also identified a technology that allows for one step fabrication of MEAs and appears to be a feasible path toward achieving DOE’s 2010 targets for precious metal and power (approaches 0.2g/Kw). Our team partner Du Pont invented a new class of polymer electrolyte membrane that has sufficient stability and conductivity to demonstrate feasibility for operation at 120 oC and low relative humidity. Through the course of this project, the public has benefited greatly from numerous presentations and publications on the technical understanding necessary to achieve these goals.

Factors Affecting DMFC Fuel Cell Performances and Commercialization Consideration

By combining novel platinum and ruthenium chemistry, we have succeeded in developing a new process for making thoroughly mixed Pt:Ru alloy with minimum undesirable phases at all loadings, as evidenced by XRD analysis. XRD analysis indicated that most other commercial PtRu products have oxide impurities (RuO2 xH2O), or unalloyed Ru phase. Correlation of RDE (rotating disk electrode) results and XRD spectra indicates these phases are ineffective for catalysis. These phases reduce the electronic interaction for weakening Pt-CO bond and the bimolecular reaction rate for CO removal. A good PtRu catalyst should have thorough atomic mixing with large surface area. Oxygen cathodes are also crucial for DMFC performance. Newly developed high performance Pt black cathode catalysts possess high surface areas. 60%-80% Pt on carbon shows similar performance and is easier to process. DMFC anode structure needs to reach a compromise between catalyst access by methanol and cross-over; cathode structure needs to find the compromise between catalyst wetting and proton conduction.