Eric L. Brosha

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.

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.

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.

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.

Mixed Potential Hydrocarbon Sensors based on a YSZ Electrolyte and Oxide Electrodes

The mixed potential response of perovskite-type oxide electrodes on a yttria stabilized zirconia (YSZ) electrolyte have been examined. The effects of varying the electrode and electrolyte composition and morphology on the sensor response have been studied in detail. La 0.8 Sr 0.2 CrO 3 was found to be an excellent electrode material for a highly selective nonmethane hydrocarbon sensor. Moreover, the sensor response was greatly improved by decreasing the heterogeneous catalysis on the YSZ electrolyte. Optimizing the electrode and electrolytes resulted in a sensor response of up to 100 mV for 500 ppm of propylene in a 1% O 2 stream at 550°C. The response times of these sensors were found to be strongly dependent on the porosity in the YSZ electrolyte and could be optimized to yield a sensor response time as low as one second.

A mixed-potential sensor based on a Ce{sub 0.8}Gd{sub 0.2}O{sub 1.9} electrolyte and platinum and gold electrodes

A Pt/Ce{sub 0.8}Gd{sub 0.2}O{sub 1.9}/Au mixed-potential sensor was constructed and its response to reducing gases in an oxygen containing stream was measured at T = 550 and 600 C. The sensor response was maximum for H{sub 2} and negligible for methane; other gases followed the trend methane < propane < CO, propylene < hydrogen. The mixed potentials developed at the Au and Pt electrodes were also independently monitored with respect to Pt air-reference electrodes. The mixed potential at the Au electrode was always higher (more negative) than that at the Pt electrode irrespective of the type of reducing gas used. The polarization curves of the two electrodes during oxygen reduction were also measured. These measurements revealed that the mixed potential at the electrodes was dependent on both the amount of electrochemical oxidation of reducing gas and the overpotential for oxygen reduction. While the response of the Pt electrode was found to be stable at both 600 and 550 C, the Au electrode had stability problems at 600 C due to the changing morphology of the gold film.

Dense diffusion barrier limiting current oxygen sensors

Abstract A new type of miniature amperometric oxygen sensor has been developed. The sensors are high temperature micro-electrochemical devices that use a dense diffusion barrier of a metal oxide that readily transports oxygen and conduct electrons. The diffusion barrier is deposited in thin film form on top of a zirconia-based electrochemical pump. When a voltage is applied to the pump, oxygen is depleted from one side of the diffusion barrier and the ionic current is proportional to the flux of oxygen across the thin film layer. If the pumping voltage reaches a high enough value, the transport of oxygen across the membrane and hence the device’s output current, will be limited by the external, oxygen concentration and the transport characteristics of the diffusion barrier. The sensors can be manufactured in a planar design that offers a faster time response, much simpler design and potentially lower cost than existing limiting current oxygen sensors.

Measurement of the local Jahn-Teller distortion in LaMnO{sub 3.006}

The atomic pair distribution function (PDF) of stoichiometric LaMnO{sub 3} has been measured. This has been fit with a structural model to extract the {ital local} Jahn-Teller distortion for an ideal Mn{sup 3+}O{sub 6} octahedron. These results are compared to Rietveld refinements of the same data which give the {ital average} structure. Since the {ital local} structure is being measured in the PDF there is no assumption of long-range orbital order and the real, local, Jahn-Teller distortion is measured directly. We find good agreement both with published crystallographic results and our own Rietveld refinements suggesting that in an accurately stoichiometric material there is long-range orbital order as expected. The local Jahn-Teller distortion has two short, two medium, and two long bonds. This implies that there is some mixing of the d{sub 3z{sup 2}{minus}r{sup 2}} and d{sub x{sup 2}{minus}y{sup 2}} states and the occupied state is not pure d{sub 3z{sup 2}{minus}r{sup 2}} symmetry. The Debye temperature of the Mn and O ions has also been calculated as {Theta}{sub D}(Mn)=1000{plus_minus}100hK, {Theta}{sub D}(O{sub apical})=980{plus_minus}30hK, and {Theta}{sub D}(O{sub basal})=601{plus_minus}8hK. {copyright} {ital 1999} {ital The American Physical Society}

Trace detection and discrimination of explosives using electrochemical potentiometric gas sensors

In this article, selective and sensitive detection of trace amounts of pentaerythritol tetranitrate (PETN), 2,4,6-trinitrotoluene (TNT) and cyclotrimethylenetrinitramine (RDX) is demonstrated. The screening system is based on a sampling/concentrator front end and electrochemical potentiometric gas sensors as the detector. Preferential hydrocarbon and nitrogen oxide(s) mixed potential sensors based on lanthanum strontium chromite and Pt electrodes with yttria stabilized zirconia (YSZ) solid electrolyte were used to capture the signature of the explosives. Quantitative measurements based on hydrocarbon and nitrogen oxide sensor responses indicated that the detector sensitivity scaled proportionally with the mass of the explosives (1-3 μg). Moreover, the results showed that PETN, TNT, and RDX samples could be discriminated from each other by calculating the ratio of nitrogen oxides to hydrocarbon integrated area under the peak. Further, the use of front-end technology to collect and concentrate the high explosive (HE) vapors make intrinsically low vapor pressure of the HE less of an obstacle for detection while ensuring higher sensitivity levels. In addition, the ability to use multiple sensors each tuned to basic chemical structures (e.g., nitro, amino, peroxide, and hydrocarbon groups) in HE materials will permit the construction of low-cost detector systems for screening a wide spectrum of explosives with lower false positives than present-day technologies.