Effect of La 2CuO 4 electrode area on potentiometric NO x sensor response and its implications on sensing mechanism

Abstract

The intent of this work is to look at the effects of varying the La 2CuO 4 electrode area and the asymmetry between the sensing and counter electrode in a solid state potentiometric sensor with respect to NO x sensitivity. NO 2 sensitivity was observed at 500–600 °C with a maximum sensitivity of ∼22 mV/decade [NO 2] observed at 500 °C for the sensor with a La 2CuO 4 electrode area of ∼30 mm 2. The relationship between NO 2 sensitivity and area is nearly parabolic at 500 °C, decreases linearly with increasing electrode area at 600 °C, and was a mixture of parabolic and linear behavior 550 °C. NO sensitivity varied non-linearly with electrode area with a minima (maximum sensitivity) of ∼−22 mV/decade [NO] at 450 °C for the sensor with a La 2CuO 4 electrode area of 16 mm 2. The behavior at 400 °C was similar to that of 450 °C, but with smaller sensitivities due to a saturation effect. At 500 °C, NO sensitivity decreases linearly with area. We also used electrochemical impedance spectroscopy (EIS) to investigate the electrochemical processes that are affected when the sensing electrode area is changed. Changes in impedance with exposure to NO x were attributed to either changes in La 2CuO 4 conductivity due to gas adsorption (high frequency impedance) or electrocatalysis occurring at the electrode/electrolyte interface (total electrode impedance). NO 2 caused a decrease in high frequency impedance while NO caused an increase. In contrast, NO 2 and NO both caused a decrease in the total electrode impedance. The effect of area on both the potentiometric and impedance responses show relationships that can be explained through the mechanistic contributions included in differential electrode equilibria.