NOx Sensor Element Two-Layer Electrode to Reduce Sensitivity Loss
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Solution Overview
Problem
The reduction in sensitivity of NOx sensors due to the oxidation and vaporization of noble metals in the measurement electrode, caused by surplus oxygen not taken into the solid electrolyte, leads to a decrease in catalytic reactivity over time.
Innovation Solution
A sensor element with a measurement electrode configured in a two-layer structure, where the lower layer has a higher volume ratio of solid electrolyte and no pores, and the upper layer has a random distribution of noble metal, solid electrolyte, and pores, which suppresses the accumulation of surplus oxygen and maintains catalytic reactivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a single-layer porous measurement electrode is used, then mass transfer of O2 and NOx is facilitated, but surplus oxygen accumulates causing oxidation and vaporization of noble metal
Solution Approach 1:
The measurement electrode is divided into two distinct layers: an upper porous layer for mass transfer and a lower dense layer for preventing oxygen accumulation. This segmentation allows each layer to perform its specific function optimally without interfering with the other.
Solution Approach 2:
Different regions of the measurement electrode are given different properties: the upper layer has high porosity (30-70%) for efficient gas transport, while the lower layer has low porosity (0-10%) to prevent oxygen accumulation at the electrolyte interface, where it would cause harmful oxidation.
2Reliability
If the volume ratio of solid electrolyte in the lower layer is increased, then oxygen accumulation is suppressed, but catalytic reactivity may be reduced
Solution Approach 1:
The electrode is segmented into layers with different electrolyte volume ratios optimized for their specific functions. The lower layer has high electrolyte content (80-100% volume ratio) to prevent oxygen accumulation, while the upper layer has lower electrolyte content (20-70% volume ratio) to maintain catalytic activity and facilitate mass transfer.
Solution Approach 2:
The electrolyte distribution is made non-uniform: concentrated in the lower layer where oxygen removal is critical, and reduced in the upper layer where catalytic reactions occur. This local optimization allows simultaneous achievement of oxygen prevention and catalytic efficiency.
3Duration of action of stationary object
If the measurement electrode is exposed to oxygen for continuous operation, then NOx measurement function is maintained, but noble metal oxidizes and vaporizes reducing catalytic reactivity
Solution Approach 1:
The lower dense layer is pre-configured to intercept and remove oxygen before it can reach the noble metal in the upper layer during continuous operation. This preliminary oxygen removal prevents subsequent oxidation and vaporization of the catalytic noble metal, enabling long-term stable operation.
Solution Approach 2:
The presence of oxygen, which would normally cause harmful oxidation of noble metal, is converted into a benefit by using it to form oxygen ions in the lower layer that are then transported through the solid electrolyte away from the catalytic region, preventing metal degradation while maintaining measurement function.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The two-layer configuration effectively reduces the deterioration of the measurement electrode, maintaining sensor sensitivity and reducing the rate of sensitivity loss to 6% or less even with continuous use.
Implementation Method 1
oxygen-ion conductivity of zirconia used for the base
Implementation Method 2
NOx is then decomposed into O2 and N2 using a catalytic reaction of a measurement electrode
Implementation Method 3
O2 in a measurement gas introduced into a NOx sensor element is first removed (separated from NOx) by an electrochemical pump cell of the NOx sensor element
Data Source
AI summary
A sensor element includes: a base part containing an oxygen-ion conductive solid electrolyte as a constituent material; at least one internal space into which a measurement gas is introduced; and at least one pump cell including an internal electrode disposed to face the at least one internal space, an out-of-space pump electrode disposed at a location other than the at least one internal space, and a portion of the base part located between these electrodes, the internal electrode includes: an upper layer consisting of a noble metal, the solid electrolyte, and a pore; and a lower layer consisting of the noble metal and the solid electrolyte, and a volume ratio of the solid electrolyte in the lower layer is greater than a volume ratio of the solid electrolyte in the upper layer.