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The performance degradation and recovery of a polymer electrolyte fuel cell exposed to ammonia is described. The effect of ammonia contamination on each component of the MEA was studied using Neutron Imaging, lifetime tests, cell polarization measurements, potentiostatic measurements, and AC impedance spectroscopy. It is found that among the detrimental effects of the contaminant, mass transport and ORR kinectic are mostly affected. Reductions in electrolyte conductivity are not playing a major role in cell performance losses for the low levels of contamination studied here. Shifts in electrolyte proton activity reduce oxygen reduction reaction mixed equilibrium potential and so cell potential. In addition, cell potential might also be affected by overpotentials on the oxygen reduction reaction caused by oxidation of ammonia at cell potential above 0.7 V. It is demonstrated that cell initial performance can be recovered after contamination and ammonia-contaminated PEFC recovering mechanisms are discussed. [ABSTRACT FROM AUTHOR]

There are significant impacts to fuel cell performance when contaminant cations replace protons in the ionomer. The purpose of this paper is to directly correlate extent of cationic contamination with fuel cell performance and also to elucidate main mechanisms responsible for this performance degradation. A series of standard membrane electrode assemblies (MEAs) were contaminated by varying the ratio of protons to cesium cations in the ionomer phase. The amount of contamination was determined using X-ray Fluorescence (XRF) Spectroscopy. These MEAs were then run in fuel cell mode to determine fuel cell performance as a function of cationic contamination level. A strip cell configuration was also utilized to distinguish between ohmic, thermodynamic, and kinetic effects. Additionally, the distribution of contaminants was measured using XRF. The decrease in fuel cell limiting current, power density, and membrane conductivity as a function of contamination level were quantified. Performance decreased monotonically with increased contamination. The cationic contaminant was preferentially located near the cathode of the strip cell under load. This distribution led to increases in thermodynamic and kinetic polarizations that were more important than changes in membrane conductivity. [ABSTRACT FROM AUTHOR]

Sensors to detect mixtures of NO x /NH 3 are needed to monitor emissions of diesel automobiles where a selective catalytic reduction system uses an NH 3 mediated reaction to reduce NO x . We report on the application of a three electrode La 0.8 Sr 0.2 CrO 3 , Au 0.5 Pd 0.5 , Pt mixed potential sensor using yttria-stabilized-zirconia (YSZ) as a solid electrolyte to NO x /NH 3 sensing. Artificial neural networks were used to automatically decode the concentrations of NO x /NH 3 and errors of less than 15% are achieved. The optimal architecture for ANN decoding and the maximum density of training data points are also determined. The stability of the sensor was monitored by electrochemical impedance spectroscopy. The impedance associated with YSZ oxygen ion conduction and the electrochemical reactions at the three-phase interface are tracked over a period of over 100 days. [ABSTRACT FROM AUTHOR]

Abstract: In this article, the influence of perovskite electrode composition on an electrochemical based mixed potential sensor response is reported. Specifically, various stoichiometry of Strontium doped Lanthanum Chromite materials were used as the sensing electrode. Yttria-Stabilized Zirconia was used as the solid electrolyte and Pt as the counter electrode. The effect of varying doping levels of Strontium on the sensor response was studied. NO, NO2, C3H6, and C4H10 were used as test gases. The sensor response was studied under open-circuit and current biased conditions. Based on sensitivity and selectivity evaluations, the device configuration ‘La0.9Sr0.1CrO3/YSZ/Pt’ was found to behave as a selective total hydrocarbon sensor at 500°C when operated under open-circuit conditions. On the other hand, ‘La0.7Sr0.3CrO3/YSZ/Pt’ exhibited minimal cross-sensitivity to hydrocarbons facilitating its use as a selective NO x sensor when operated under a current bias. The variation in the sensor response and selectivity for different electrode materials indicate that it might be possible to develop multi-electrode sensor platform for identifying a suite of analytes with improved selectivity and sensitivity. [Copyright &y& Elsevier]

Abstract: The effect of trace quantities of ammonia on oxygen reduction reaction (ORR) on carbon-supported platinum catalysts in perchloric acid solutions is assessed using rotating ring disk electrode (RRDE) technique. The study demonstrates that ammonia has detrimental effects on ORR. The most significant effect takes place in the potential region above 0.7 V vs RHE. The effect is explained by the electrochemical oxidation of ammonia, which blocks Pt active sites and increases the formation of H2O2. This leads to losses in the disk currents and increments in the ring currents. The apparent losses in ORR currents may occur in two ways, namely, through the blocking of the active sites for ORR as well as by generating a small anodic current, which is believed to have a lower contribution. In addition, a detrimental effect of sodium cations in the potential range below 0.75 V vs RHE was demonstrated. This effect is most likely due to the co-adsorption of sodium cations and perchlorate anions on the Pt surface. [Copyright &y& Elsevier]