Commercial NOx sensors for automotive applications are primarily YSZ electrochemical sensors of the amperometric type.
Figure 1 illustrates the basic operating principle. The sensor uses two or three electrochemical cells in adjacent chambers. The first cell electrochemically pumps O2 out of the sample so it does not interfere with the NOx measurement in the second cell. The need to remove O2 allows this type of NOx sensor to serve a dual purpose; it can also detect exhaust O2 level.
The O2 in the first cell is reduced and the resulting O ions are pumped through the zirconia electrolyte by applying a bias of approximately -200 mV to -400 mV. The pumping current is proportional to the O2 concentration. The remaining gases diffuse into the second cell where a reducing catalyst causes NOx to decompose into N2 and O2. As with the first cell, a bias of -400 mV applied to the electrode dissociates the resulting O2 which is then pumped out of the cell; the pumping current of the second cell is proportional to the amount of oxygen from the NOx decomposition. An additional electrochemical cell can be used as a Nernstian lambda sensor to help control the NOx sensing cell [Rheaume 2010].
All HC and CO in the exhaust gas should be oxidized before the NOx sensing cell to avoid interference. Also, any NO2 in the sample should be converted to NO prior to NOx sensing to ensure the sensor output is proportional to the amount of NOx.
If a dividing wall made of YSZ ceramics is placed between two chambers with different oxygen partial pressure, nothing will happen at room temperature. However, when the temperature of the ceramic wall is increased to approximately 600°C, oxygen ions can move through the gaps in the crystal lattice. An alignment takes place, where the chamber with the higher partial pressure pushes oxygen ions through the wall to the chamber with the lower pressure.
If both surfaces of the dividing wall are fitted with an electrode, it is possible to verify the movement of ions through voltage measurement. And this is precisely what happens in the binary (switching) lambda sensor. The reduction of oxygen to O2- that occurs in the chamber of a higher O2 pressure is described by Equation (1):
O2 + 4e- = 2O2-
and the sensor voltage is given by the Nernst equation:
Us = (RT/4F) ln(pref / pexh)
- Us – sensor signal, V
- T – temperature, K
- p – partial pressure of oxygen
- R – gas constant = 8.314 J/mol
- F – Faraday constant = 96,485 sA/mol
The diagram in Figure 2 presents the chamber with high oxygen partial pressure as the blue-colored area, and the chamber with low oxygen partial pressure as the gray area. If the brown-colored ceramic is heated to 600°C, the micro-porous platinum electrodes presented in yellow will generate approximately 1V.
Passive Cells. The chamber with the high partial pressure of oxygen is the reference air duct. Rich exhaust gas (λ ∠ 1) has a low oxygen content. If the zirconium oxide ceramics is heated using a heating element to approximately 600°C, oxygen ions move from the reference air duct through the ceramic wall onto the exhaust gas side and almost one volt signal voltage is generated. In the case of lean exhaust gas (λ> 1), the oxygen partial pressure difference relative to the reference air is low and a signal of only 0.1V or less is measured. At λ = 1, the signal voltage is approximately 0.4-0.5V, depending on the manufacturer and probe model. The voltage-lambda characteristic is almost stepwise, allowing the sensor to distinguish between two lambda values—rich and lean—hence the term “binary” lambda sensor. In such operation—representative of a binary lambda probe—the generated voltage correlates with the drop in oxygen partial pressure. The passive YSZ ceramics cell is also called the potentiometric or Nernst cell. Active Cells. It is also possible to actively operate the probes, as is the case in broadband (linear) oxygen sensors and in the amperometric cells in NOx sensors. In active operation, no voltage is picked up on the electrodes, but rather the electrodes are connected to a power source. In such active cells—referred to as “pump cells”—it is possible to “pump” oxygen ions from the oxygen-lean to the oxygen-rich side by reversing the polarity. The pumping current provides a measure of oxygen concentration. The current-lambda characteristic is linear, which makes it possible to measure O2 concentrations at various air-to-fuel ratios. NOx sensors include at least two oxygen pump cells (Figure 1)—one to remove excess oxygen from the exhaust gas, and another to measure the concentration of oxygen released from the decomposition of NOx.