1 Department of Energy Conversion and Storage, Technical University of Denmark2 Fundamental Electrochemistry, Department of Energy Conversion and Storage, Technical University of Denmark
This thesis studied the electrochemical cells modified by NOx adsorbents for the NOx reduction under O2-rich conditions. The structure of a multilayer electrochemical cell with a NOx adsorption layer was optimized by removing a yttria-stabilized zirconia (YSZ) cover layer coated on a Pt/Ni/YSZ electrode. It was found that the NOx removal properties of the electrochemical cell were dramatically enhanced through this optimization, which was attributed to the extensive release of selective reaction sites for NOx species and a strong promotion for NOx reduction from the interaction of the directly connected adsorption layer with the electrode. Ag and (La0.85Sr0.15)0.99MnO3 (LSM) were investigated as electrode materials to substitute for Pt and Ni. Selective NOx reduction in the presence of excess O2 could be achieved for both Ag and LSM/Ce0.9Gd0.1O1.95 (CGO) electrodes by modifying the electrodes with NOx adsorbents. Performances of 82% NOx conversion with 7.7% current efficiency and 100% N2 selectivity for the Ag electrode, and of 85% conversion with 4% current efficiency and 74% N2 selectivity for the LSM/CGO electrode were achieved in 1000 ppm NO and 8-10% O2 at 500 °C with the addition of a K-Pt-Al2O3 adsorption layer. The effects of the NOx adsorbents on the electrode processes were characterized by electrochemical impedance spectroscopy (EIS). The impedance analysis revealed that the NOx adsorbents greatly enhanced the electrode activity, mainly contributed by the promotion of adsorption, surface diffusion, and transfer of NOx and O2 species at/near the triple phase boundary region, and the formation of intermediate NO2. Severe degradation was observed on both electrodes following long-term operation, caused by the corrosion of the Ag electrode covered by a nitrate melt, or associated with a profound change in the microstructure for the LSM/CGO electrode. Two different approaches to modify the electrochemical cell with NOx adsorbents, adding a Ba-Pt-Al2O3 adsorption layer on top of the electrode or impregnating of the BaO into the electrode, were studied on a LSM/CGO symmetric cell. A comprehensive comparison between the two approaches was provided based on systematic investigations, including conversion measurements, degradation tests, microstructure observations, and impedance characterization. It was found that both approaches significantly increased the activity and selectivity of NOx reduction on the LSM/CGO symmetric cell, by enhancing the adsorption and storage of NOx species, or by providing reaction sites for direct nitrate reduction. Cells with adsorption layers exhibited a superior performance at low temperatures (350 and 400 °C) and at low voltages (1.5 to 2 V) due to the NO oxidation ability of the Pt catalyst, although its performance was relatively poor at elevated temperatures and voltages due to the impedance of the diffusion of NOx to the reaction sites by the adsorption layer. The presence of a strong NO oxidation catalyst was important for lowering the operating temperature and minimizing the power consumption of the electrochemical cell. Square-wave (SV) polarization balanced the trapping and reduction rates of NOx species on the electrochemical cells, further improving the NOx reduction activity relative to that observed under direct current (DC) polarization.