Low Temperature Electrochemical Reduction of NO and O2 on Pt Electrode Using YSZ, GDC and Lsgm Electrolyte

The pollutant NO mainly from diesel/gasoline engine exhausts without purification into environment can give rise to the acid rain and photochemical smog^[1] which causes great potential safety hazard to human health and global environment. Accompanied by NO, there exists excess O₂ in exhausts. Electrochemical reduction of NO and O₂ can save the huge reducing agents storage system with the low operation temperatures simultaneously^[2-3]. The exhaust temperature is low that high O^2- conductivity is needed to guarantee high conversions of NO and O₂ in the low temperature range.    

In this study, we fucused on the electrochemical reduction of NO and O₂ in solid oxide electrolyte cell(SOEC) on symmetric Pt electrode with the temperature varying from 250℃ to 500℃ using three different electrolytes including YSZ, Ce_0.9Gd_0.1O_1.95(GDC) and La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85(LSGM). The GDC and LSGM electrolyte were prepared by tape-casting method and characterized by SEM after electrochemical tests. The symmetric cells were characterized by electrochemical measurements including linear sweep voltammetry(IV) and electrochemical impedance spectroscopy(EIS) at open circuit voltage(OCV). The polarization range was set from 2V to 0V containing 800ppm NO and 8% O₂. The experiment testing temperatures and the concentrations of the NO and O₂ are choosen according to the exhausts of the diesel or gasoline engine. The GDC and LSGM electrolyte showed higher electrochemical performance than YSZ by polarization measurements especially at low temperature of 250℃ to 350℃. The EIS experiment results were fitted well with equivalent circuit model R(C(RW)) containing the serial resistance R_s, electron transfer resistance R_t, double layer capacitance C and Warburg component W. It can be observed that the activation energy of the R_s and R_t were close to each other and the activation energy of the R_s were 0.581eV, 0.702eV and 0.719eV in GDC, LSGM and YSZ electrolytes. The EIS results yielded typical semi-infinite diffusive character in three electrolytes. The activation energy of the electron transfer process for the YSZ, GDC, and LSGM electrolytes were 1.057eV, 0.933eV and 1.031eV in 800ppm NO with 8% O₂. And the YSZ electrolyte displayed the maxmium activation energy of electron transfer resistance R_t indicating the larger resistance of the charge transfer process. The gases in the Pt/YSZ/Pt cell showed the lowest diffusion coefficients obtained from the Warburg component in low frequency in the electrode resulted from largest charge transfer resistance that blocked the diffusion of the gases. This studies demonstrated that GDC and LSGM can be promising electrolytes for low temperature electrochemical removal of NO and provided theoretical guidance for the design of practical NOx elecrochemical application device at low temperatures.  

References

[1] M.T. Lerdau, J.W. Munger, J.D. Jacob, Science 289, 2291 (2000).

[2] T.J. Huang, C.Y. Wu, S.H. Hsu, C.C. Wu, Energy Environ. Sci., 4, 4061 (2011).

[3] R.M.L. Werchmeister, K.K. Hansen, Electrochim. Acta, 114, 474 (2013).