Material for sensing electrode of NO

1. A NO 2 gas sensing element, comprising:

an electrolyte;

a reference electrode in contact with the electrolyte; and

a sensing electrode selective to NO 2 in contact with the electrolyte spaced apart from the reference electrode, the NO 2 selective sensing electrode comprising an oxide material comprising:

(1) a mixed oxide according to the formula (Mn 2-u-v-w Co v Mg w SiO 4-u )+ξ(Mn 3 Al 2 Si 3 O 12 )+δ(SiO 2 ), wherein 0≤(u+v+w)≤2.00, 0≤ξ≤0.5, and 0≤δ≤0.1;

(2) a mixed oxide according to the formula (Mn 2-x-y-z Co y Mg z SiO 4-x )+ξ(ZnO)+δ(SiO 2 ), wherein 0≤(x+y+z)≤2.00, 0≤ξ≤0.5, and 0≤δ≤0.1; or

a combination comprising mixed oxides (1) and (2).

2. The sensing element of claim 1 , wherein the composition comprises the mixed oxide (1).

3. The sensing element of claim 2 , wherein 0.75≤(u+v+w)≤1.25, 0≤ξ≤0.025, and 0≤δ≤0.050.

4. The sensing element of claim 1 , wherein the composition comprises the mixed oxide (2).

5. The sensing element of claim 4 , wherein 0.75≤(x+y+z)≤1.25, 0≤ξ≤0.025, and 0≤δ≤0.050.

6. The sensing element of claim 1 , wherein the composition comprises the mixed oxide (1) and the mixed oxide (2).

7. The sensing element of claim 6 , wherein 0.75≤(u+v+w)≤1.25, 0≤ξ≤0.025, and 0≤δ≤0.050.

8. The sensing element of claim 6 , wherein 0.75≤(x+y+z)≤1.25, 0≤ξ≤0.025, and 0≤δ≤0.050.

9. The sensing element of claim 6 , wherein 0.75≤(u+v+w)≤1.25, 0≤ξ≤0.025, and 0≤δ≤0.050; and 0.75≤(x+y+z)≤1.25, 0≤ξ≤0.025, and 0≤δ≤0.050.

10. The sensing element of claim 1 , wherein the electrolyte comprises a solid oxide.

11. The sensing element of claim 10 , wherein the solid oxide comprises zirconia, ceria, lanthanum gallates, or mixtures comprising any of the foregoing.

12. The sensing element of claim 1 , further comprising a protective coating layer covering the sensing electrode.

13. The sensing element of claim 12 , wherein the protective coating comprises high surface area alumina-based oxide particles in an oxide glassy binder.

14. A method of detecting NO 2 , comprising contacting a test gas with the reference and sensing electrodes of a sensing element according to claim 1 , and measuring a voltage signal between the sensing electrode and the reference electrode.

15. A NO 2 sensor assembly comprising a heating cell, a temperature-sensing cell, and an electrochemical NO 2 sensing cell comprising the NO 2 sensing element of claim 1 and an electrical circuit connection between the reference electrode and the sensing electrode and configured to contact the reference and sensing electrodes with test gas.

16. The NO 2 sensor assembly of claim 15 , further comprising a protective layer, on the NO 2 -sensing electrode.

17. The NO 2 sensor assembly of claim 15 , further comprising an air-fuel sensing cell, an ammonia-sensing cell, or an air-fuel sensing cell and an ammonia-sensing cell.

18. The NO 2 sensor assembly of claim 17 , wherein the ammonia-sensing cell includes an ammonia sensing electrode and an ammonia reference electrode, and wherein sensing electrodes of the NO 2 -sensing cell and the ammonia-sensing cell are each covered by separate protective layers.

19. The NO 2 sensor assembly of claim 17 , further comprising an electronic memory chip containing stored calibration factors for any one or combination of the temperature-sensing cell, the NO 2 sensing cell, the air-fuel sensing cell, and the ammonia-sensing cell.

20. A combustion exhaust system, comprising the NO 2 sensor assembly of claim 17 disposed to cause the NO 2 sensing cell to provide contact of the reference and sensing electrodes with a source of combustion exhaust gas, and an electronic controller in contact with the heating cell, the temperature-sensing cell, and the electrochemical NO 2 sensing cell; the electronic controller configured to provide close-loop control of the heating cell based on temperature-sensing cell output, and/or to convert emf data from the NO 2 sensing cell electrical circuit to NO 2 concentration value.