Fernando H. Garzon

Surface Area Loss of Supported Platinum in Polymer Electrolyte Fuel Cells

Life tests of polymer-electrolyte fuel cells using supported Pt catalyst in thin film catalyst layers are run for up to 4,000 h at maximum power. Particle ripening is readily evident using these types of electrodes in which the high catalyst utilization efficiency apparently subjects in majority of the platinum to conditions that sustain particle growth. X-ray diffraction analyses indicate that the initial platinum specific surface areas of 100 m[sup 2]/g Pt eventually stabilize to about 40 to 50 m[sup 2]/g in the cathode and 60 to 70 m[sup 2]/g in the anode. Interestingly, this loss in surface area does not affect the apparent catalytic activity of these fuel cell electrodes. A crystallite migration particle growth mechanism is suggested by the shape of the particle size distribution curves. Since the presence of liquids is known to lower the activation energy for particle growth, the particle size difference between the two electrodes may possibly be attributed to the different hydration levels at the anode and the cathode in operating polymer electrolyte fuel cells.

Ruthenium Crossover in Direct Methanol Fuel Cell with Pt-Ru Black Anode

In this study, we provide electrochemical and X-ray fluorescence evidence of ruthenium crossover in direct methanol fuel cells using a state-of-the-art Pt-Ru alloy catalyst at the anode. We find ruthenium susceptible to leaching out from the highly active Pt-Ru black catalyst, crossing the proton-conducting Nafion membrane and redepositing at the Pt cathode on the opposite side of the fuel cell. After first detecting this phenomenon in a direct methanol fuel cell (DMFC) stack with a history of cell-voltage reversal, we have since observed ruthenium crossover under virtually all DMFC operating conditions, from single cell break-in (humidification) to stack life testing. The degree of cathode contamination by ruthenium species (of chemical form yet unknown) depends on, among other factors, the DMFC anode potential and the cell operating time. Once deposited at the cathode, ruthenium inhibits oxygen reduction kinetics and the catalysts ability to handle methanol crossover. Depending on the degree of cathode contamination, the overall effect of ruthenium crossover on cell performance may be from as little as ∼40mV up to 200 mV.

SOLID-STATE MIXED POTENTIAL GAS SENSORS: THEORY, EXPERIMENTS AND CHALLENGES

Abstract Solid-state mixed potential electrochemical sensors sense gases using differential electrocatalysis on dissimilar electrode materials. The response theory is typically expressed in terms of models invoking Butler–Volmer kinetics at high overpotentials (Tafel behavior). This model is not adequate for describing all types of mixed potential sensor responses. For low concentrations of analyte gas, mass transport limitations must also be considered. Experiments with sensors with air reference electrodes also demonstrate the importance of low overpotential oxygen reduction kinetics in establishing the device response. A sensor response model that predicts a linear relationship between response voltage and analyte gas concentration is derived. The development of oxide electrode based devices offers improved long-term response stability over metal electrode based devices.

Polyaniline-derived Non-Precious Catalyst for the Polymer Electrolyte Fuel Cell Cathode

A novel polyaniline (PANI)-derived non-precious cathode catalyst was developed in this work, exhibiting remarkable activity (onset potential: 0.9 V, half-wave potential: 0.77 V) and selectivity (0.4 % H20 2 at 0.4 V). As a result, the generated current densities at high voltages associated with electrochemically kinetic activity can be achieved to 0.04 Acm-2 for 0.80V and 0.21 Acm-2 for 0.6 V, when air was used in fuel cell tests. MEA life test at a constant voltage of 0.4 V demonstrated a promising stability up to 450 hours, without obvious degradation. The current density during the test was measured around 0.32 A cm-2, a respectable performance for a cell with non-precious cathode, operated on air rather than oxygen. The possible active sites, related to pyridine- and pyrrole-like metal species were discussed according to presented XPS and XRD analysis.

Sulfur Tolerant Anodes for SOFCs

Solid oxide fuel cells (SOFCs) using yttria-stabilized zirconia (YSZ) electrolytes, lanthanum strontium manganate cathodes, and La 1 - x Sr x BO 3 /YSZ anodes (where B = Mn, Cr, and Ti) were fabricated. The sulfur tolerance of the various perovskite-based anodes was examined at 1273 K in a H 2 /H 2 O fuel. The Sr 0 . 6 La 0 . 4 TiO 3 /YSZ (50/50 wt %) anode showed no degradation in the presence of up to 5000 ppm of H 2 S in a hydrogen fuel. This anode was also able to operate for 8 h with 1% H 2 S as a fuel and showed no degradation when the fuel was switched back to hydrogen.

Conducting polymer-colloidal silica composites

Abstract We describe the preparation of conducting polymer-colloidal silica composites by the in situ deposition of a thin coating of chemically synthesized polyaniline or polypyrrole onto monodisperse silica particles (∼1 μm diameter). These composite materials have been characterized by thermogravimetric analysis, scanning electron microscopy, four-point probe conductivity measurements, Fourier transform infra-red microscopy and Rutherford back-scattering spectrometry.

Relating Rates of Catalyst Sintering to the Disappearance of Individual Nanoparticles during Ostwald Ripening

Sintering of nanoparticles (NPs) of Ni supported on MgAl(2)O(4) was monitored in situ using transmission electron microscopy (TEM) during exposure to an equimolar mixture of H(2) and H(2)O at a pressure of 3.6 mbar at 750 °C, conditions relevant to methane steam reforming. The TEM images revealed an increase in the mean particle size due to disappearance of smaller, immobile NPs and the resultant growth of the larger NPs. A new approach for predicting the long-term sintering of NPs is presented wherein microscopic observations of the ripening of individual NPs (over a span of a few seconds) are used to extract energetic parameters that allow a description of the collective behavior of the entire population of NPs (over several tens of minutes).

CO/HC sensors based on thin films of LaCoO3 and La0.8Sr0.2CoO3−δ metal oxides

Abstract We have demonstrated a new type of mixed potential, zirconia-based sensor that utilizes dense, thin films of either La–Sr–Co–O or La–Co–O perovskite transition metal oxide vs. a Au counter electrode to generate an EMF that is proportional to the oxidizable gas species (carbon monoxide (CO), C3H6, and C3H8) concentration in a gas stream containing oxygen. The devices reported in this work were tested at 600°C and 700°C and in gas mixtures containing 0.1% to 20% O2 concentrations. The metal-oxide-based sensors exhibited an improvement in operating temperature and level stability at elevated temperatures compared to Au–zirconia–Pt mixed potential devices already reported in the literature. However, as with Au–zirconia–Pt devices previously reported, the response behavior reproducibility from device to device was dependent on the Au morphology, which could vary significantly between samples under identical thermal histories. The changing Au morphology on both the Au counter electrode and the Au current collector on the metal oxide electrode were responsible for sensor aging and changes in device response over time. No change in the crystal structure of the perovskite thin film could be seen from XRD. A significant hysteresis in sensor response was found as the background oxygen concentration was cycled through stoichiometry, and this may be attributed to a change in the oxidation state of the cobaltate-based metal oxide electrode. In an effort to mitigate device aging, we replaced the Au counter electrode with a second metal oxide thin film, doped LaMnO3, and demonstrated the operation of a mixed potential sensor based on dual metal oxide electrodes.

Titanium dioxide-supported non-precious metal oxygen reduction electrocatalyst

A new non-precious metal oxygen reduction catalyst was developed via heat treatment of in situ polymerized polyaniline onto TiO(2) particles in the presence of Fe species. The TiO(2) provides for improved performance relative to a carbon black-based catalyst and, at a high catalyst loading, allows for reducing the performance gap between non-precious-metal catalyst and Pt/C to ca. 20 mV in RDE testing.

Development of ceramic mixed potential sensors for automotive applications

Mixed potential sensors that utilize Gd{sub 0.2}Ce{sub 0.8}O{sub 2} electrolytes and patterned dense 1 {micro}m-thick LaMnO{sub 3} thin films were studied at 600 C and 1%O{sub 2}. The response to C{sub 3}H{sub 6} and CO of two different sensor configurations were studied continuously for 1000 hrs versus an air reference. Although two different current collection schemes and two different metal oxide electrode geometries were employed, the magnitude of the mixed potential generated by both sensors was remarkably similar. From previous work with Au-ceria-Pt mixed potential sensors, this behavior is attributed to precisely controlling the metal oxide electrode/solid electrolyte interface unlike the random interface produced when Au electrodes are used. Although doped ceria is not a suitable electrolyte for automotive exhaust gas applications, this work serves to illustrate design goals for zirconia-based sensors.