Introduction:

NOx sensors are an enabling technology which will promote the advancement of diesel
engines by permitting improved engine control along with mandated on-board diagnostics.
In addition, NOx sensors are required for new NOx remediation technologies such as
selective catalytic reduction (SCR) utilizing hydrocarbon (HC) and urea injection. These sensors
should be operative at T ~600 oC (thus avoiding the prospect of the sensor being a "cold
finger" in the exhaust stream), able to measure [NOx] in the range ~1 ppmV to ~1500
ppmV, and be relatively insensitive to varying O2 concentrations in the exhaust stream.
"NOx" refers to mixtures of nitrogen monoxide (NO) and nitrogen dioxide (NO2). NO2
dominates at lower temperatures and higher O2 concentrations but NO is the dominant
species at higher temperatures and in O2-poor environments. Due to this equilibrium
between the mono- and dioxides of nitrogen, complete [NOx] characterization of exhausts
will require two of three concentrations to be accurately measured; [NO], [NO2], [NOx]
(=[NO] + [NO2]).

Objective:

This project seeks to develop NOx sensing elements able to operate at ~600 oC. The initial
focus has been on planar sensing elements that do not require a reference electrode. An
immediate goal is to develop sensing electrode materials sensitive to NO and/or "total NOx"
[NOx] with minimal [O2] sensitivity.

Approach:

Both "mixed-potential" and "biased" approaches are being evaluated. For both approaches,
the sensing elements consist of screen-printed co-planar electrodes on an oxygen-ion
conducting (YSZ) substrate.
Sensing element characterization is carried out with an automated test stand capable of
operation up to 800 oC. Gas concentrations are usually 7 vol% ≤ [O2] ≤ 20 vol% and 20
ppmV ≤ [NOx] ≤ 1500 ppmV, with the balance being N2. The ability to test cross-sensitivity
to interfering gas species (e.g., CO, CxHy) is available.

Results:

Evaluation of "mixed-potential" sensing elements typically indicates that the response to NO
is weaker and opposite in sign to that for NO2. Strong oxygen dependence of the NO2 response
is also often observed.

"Biasing" enables adjustment of the sign and magnitude of the NOx responses.

Minimal [O2] sensitivity at 600 oC achieved with both voltage (V) and current (I) biasing.

Summary:

Prototype NOx sensing elements have been fabricated and characterized. Investigations indicate
that "biasing" is a promising technique for both "NO selective" and "total NOx" sensing.
Future work will be targeted towards minimizing response and recovery times, decreasing the
O2 sensitivity, characterization of selectivity and cross-sensitivity, and determination of long-term
sensing element stability. Avenues for exploration include electrode composition and processing
as well as sensing element geometry.