Electrochemical reduction of NOx - DTU Orbit

1 Introduction
study by Gür et al. 23 , but since these studies were made in the absence of oxygen, they were of
little relevance for NOx removal in oxygen containing atmospheres, i.e. at conditions realistic for
exhaust gasses. In a review of the application of ceramic membranes Cicero et al. reported in 1990
the Helipump Corporation had developed an electrochemical membrane which could remove NOx
in the presence of up to 8% O2 24 . The electrochemical membrane/cell had an yttria stabilized
zirconia electrolyte and transition metal based electrodes, and was reported to remove up to 91%
NOx 24 , but unfortunately no further details were given on the experiments conducted and the
electrode materials used. In 1995 Reinhardt et al. performed studies of the reaction between O2,
NO and NO2 on LSM electrodes with yttria stabilized zirconia (YSZ) electrolyte and found the O2
exchange reaction on the LSM was very slow below 600 °C, while addition of 1000-3000 ppm NO
increased the current densities significantly in oxygen containing atmosphere due to reactions
with NO2 on the electrode surface 25 . Hibino et al. reported in 1994-1995 experiments on NOx
conversions in oxidizing atmosphere on YSZ based cells with Pd electrodes 26, 27 , and later the
studies were expanded to include Rh, Au, and Pt 28 . The results by Hibino et al. showed Pd was the
best electrode material at 800 °C, and in the presence of 1500 ppm NO and 3% O2 15% NO
conversion could be obtained with current efficiency ≈1% 28 . Furthermore it was shown by Hibino
et al. both NOx conversion and current efficiency decreased dramatically with increasing O2
content, and the use of AC voltage rather than DC voltage would decrease the degradation of the
electrochemical cell 28 . Walsh et al. studied the NO reduction at a slighter lower temperature range
compared to Hibino et al., namely at 500-600 °C rather than 700-800 °C, and studied Ir and Pt
electrodes on YSZ electrolytes 29, 30 . On the Ir electrodes Walsh et al. demonstrated the selectivity
towards NOx reduction compared to O2 reduction increased with decreasing temperature 29 , while
on the Pt electrodes it was demonstrated the NO conversion increased with NO concentration,
polarisation and temperature 30 . Bredikhin et al. also studied NOx conversion in an electrochemical
cell with YSZ electrolyte and Pt electrodes, but in the work by Bredikhin et al. the Pt cathode was
covered by a Ni-YSZ or LSCF(La1-xSrxFe1-yCoyO3)-YSZ electrode layer and a strong dependency of the
NOx conversion on the microstructure of this layer was found 31, 32 . While the studies described so
far all used a precious metal electrode, apart from the work briefly reported by Cicero et al. 24 , the
cylic voltammetry studies made by Hansen et al. in 2000 indicated that transition metal oxides
without addition of noble metals could be used for electrochemical NOx reduction, and especially
LSM15 appeared promising 33-35 . It was also demonstrated by Hansen et al. the redox properties of
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