The NiFe2O4 - MgFe2O4 series as electrode materials for electrochemical reduction of NOx

Solid solutions of spinel-type oxides with the composition \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\rm Ni_{1-\mathit{x}}Mg_{\mathit{x}}Fe_2O_4$\end{document} (x = 0.0, 0.3, 0.5, 0.6, 1.0) were prepared with the glycine-nitrate combustion synthesis (x = 0.0, 0.3, 0.5, 0.6) and the citric-acid combustion synthesis (x = 1.0). The oxides were used as electrode materials in a pseudo-three-electrode setup in the temperature range of 400–600 °C. Cyclic voltammetry and electrochemical impedance spectroscopy were used to characterize the electrochemical behavior in 1% NO and 10% O2. Measurements show that NiFe2O4 has relatively high cathodic activity in both NO and O2, whereas MgFe2O4 shows much higher activity in NO compared to O2. MgFe2O4 was also measured with cyclic voltammetry in 1% NO2 and different gas mixtures of NO and O2 at 300 and 400 °C. Results show that the cathodic activities (−0.6 V) are relatively high with current ratios, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$ I_{\rm{NO}_{\mathit{x}}+\rm{O_2}}/I_{\rm{O_2}}$\end{document}, ranging from 10.1–167.7 and with a maximum at 400 °C. Dilatometry measurements were performed on the materials in air up to 1,000 °C, and they showed that the Curie temperature could be detected for all samples. Four-point DC resistivity measurements at elevated temperatures show that Ni0.4Mg0.6Fe2O4 has the highest conductivity, whereas Ni0.7Mg0.3Fe2O4 and NiFe2O4 have the highest conductivity at lower temperatures.