NO and NO2 (collectively referred to as NOx) are air pollutants, and the largest single contributor to NOx pollution is automotive exhaust. This study investigates electrochemical deNOx, a technology which aims to remove NOx from automotive diesel exhaust by electrochemical reduction of NOx to N2 and O2. The focus in this study is on improving the activity and selectivity of solid oxide electrodes for electrochemical deNOx by addition of NOx storage compounds to the electrodes. Two different composite electrodes, La0.85Sr0.15MnO3-δ-Ce0.9Gd0.1O1.95 (LSM15-CGO10) and La0.85Sr0.15FeO3-δ-Ce0.9Gd0.1O1.95 (LSF15-CGO10), have been investigated in combination with three different NOx storage compounds: BaO, K2O and MnOx. The main focus in the investigation has been on conversion measurements and electrochemical characterization, the latter by means of electrochemical impedance spectroscopy and cyclic voltammetry. In addition, infrared spectroscopy has been performed to study how NOx adsorption on the electrodes is affected by the presence of the aforementioned NOx storage compounds. Furthermore, non-tested and tested electrode microstructures have been thoroughly evaluated by scanning electron microscopy. The studies reveal addition of MnOx or K2O to the electrodes cause severe degradation problems, and addition of these compounds is thus unsuitable for electrode improvement. In contrast, addition of BaO to LSM15-CGO10 electrodes is shown to have a very positive impact on the NOx conversion. The increased NOx conversion, following the BaO addition, is attributed to a combination of 1) a decreased electrode polarisation resistance and 2) an altered NOx adsorption. The NOx conversion is observed to increase strongly with polarisation, and during 9 V polarisation of an 11-layer porous cell stack, 60% NOx conversion in a mixture of 1000 ppm NO and 10% O2 is achieved at 400 °C on entirely ceramic electrodes. This project thus demonstrates electrochemical deNOx is possible without the presence of noble metals at realistic operating conditions. However, several questions remain, among these how the BaO interacts with the solid oxide electrodes and how the electrochemical cell is optimally operated during electrochemical deNOx.
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Kammer Hansen, Kent
Department of Energy Conversion and Storage, Technical University of Denmark, 2012