Koichi Hamamoto (Post-Doctoral Research Scientist) of the Functional Assembly Technology Group (Leader: Masanobu Awano), the Advanced Manufacturing Research Institute (Director: Hideto Mitome) of the National Institute of Advanced Industrial Science and Technology (President: Hiroyuki Yoshikawa) (hereinafter referred to as AIST) has developed a high-sensitive NO_X sensor with a rapid response.

Conventional NO_X sensors do not have sufficient robustness or heat resistance in the harsh environment of engine exhaust gases. To solve this problem, a multi-chamber sensor that uses a solid oxide electrolyte (oxygen-ion conductor) has been developed. This, however, inherently does not give a high-speed response, because it has a rather complicated structure and it measures NO_X concentrations by combining multistep electrochemical reactions. This poses a problem in relation to the purification of exhaust gases and reduction of fuel consumption.

The newly developed sensor has extremely high selectivity toward NO_X molecules as a result of precise control of the nanostructure of the sensing electrode that detects NO_X. This structural improvement to the electrochemical cell gives the speed of response for the direct detection of NO_X molecules that is five times as fast as that of a conventional sensor and the detection sensitivity that is doubled.

The control of engine combustion with the aid of the high-performance sensor is expected to reduce NO_X emissions, particularly from diesel vehicles, thereby contributing to preserving the atmospheric environment and reducing carbon dioxide emissions.

The results of this research are scheduled to be presented at the 16th International Conference on Solid State Ionics, to be held in Shanghai, China, on 2nd–6th of July.

Fig. 1: The sensor array with the multilayer sensing electrode.

Strenuous efforts are being made to develop lean-burn technologies for gasoline-fueled vehicles to comply with societal demands for reducing CO₂ emissions and better fuel consumption. However, although lean-burn engines produce less CO₂ emissions, they produce more NO_X emissions than conventional engines. Existing three-way catalysts cannot be used to eliminate NO_X emissions under lean combustion because of the high concentration of oxygen in the exhaust gases. Instead of the three-way catalyst, a practical lean-burn engine uses a NO_X storage–reduction catalyst system.A NO_X trap material in this catalyst absorbs NO_X during lean-burn condition. When the catalyst becomes saturated with NO_X, a rich spike (excessive fuel supply) is generated in the engine, and this excessive amounts of fuel reduces and purifies the absorbed NO_X.

In the current system, the timing of when to add the rich fuel spike is based on the estimated amount of absorbed NO_X obtained by means of model studies on the production of emission gases under normal operating conditions. Because the system does not rely on direct detection of the NO_X concentration in the emissions, it is necessary to verify that it functions correctly under various operating conditions.

At the same time, there is a trade-off between emissions of NO_X and those of unburned carbon (particulate matter: PM). Because emissions of PM increase if those of NO_X are controlled. It is necessary to control NO_X emissions at the regulated limit by using a NO_X sensor, thereby minimizing emissions of PM. Therefore, the development of a small, high-performance, onboard NO_X sensor capable of rapid and precise detection is highly desired. Such a sensor should permit monitoring of NO_X emissions that are constantly changing, and would improve fuel economy by minimizing the number of rich fuel spikes required, thereby realizing highly efficient control of NO_X emissions and reduction of fuel consumption.

AIST successfully developed a high-performance electrochemical reactor with a solid oxide electrolyte and achieved drastic improvements of selective NO_X reduction even under a high oxygen partial pressure, where decomposition of NO_X had previously assumed to be impossible (AIST Today, June 2006 issue). In this electrochemical reactor, a self-assembled nanostructure was formed in a catalytic electrode to create a reaction site of highly selective adsorption and decomposition of NO_X molecules.

With the aim of utilizing the high selectivity of the electrochemical NO_X purification reactor in a sensor, we examined the optimal structure and production methods for the electrode; as a result, we developed a high-performance sensor for NO_X.

The results were obtained by controlling the nanostructure of the sensing electrode and by using a scandium-stabilized zirconia (ScSZ) ceramic with a high oxygen ionic conductivity as the electrolyte. The resulting electrode and electrolyte were shown to be applicable in a mixed-potential sensor that shows a high detection capability and a rapid response for nitric oxide (NO) molecules, even at 300 °C.

The sensor that we actually built was a single chamber electrochemical cell measuring about 1 cm x 2.5 mm x 300 nm. We formed the zirconia ceramic on the silica (SiO₂) substrate by the pulsed laser deposition method (Figure 2).

Figure 1 shows a sensor array containing four sensors. This sensor shows a change in electromotive force of about 100 mV for 1000 ppm of NO gas diluted by nitrogen gas containing 5% oxygen at 350 °C: this is the highest level of detection yet achieved for NO gas, a substance that is normally difficult to detect (Figure 3).

As shown in Figure 4, optimizing the nanostructure of the sensing electrode realized a response speed that is less than 5 seconds for a 90% response. (Judging from the measurement conditions, the actual detection speed is estimated to be faster than the measurement.) Detection by a device without optimized structure requires more than 1 minute at the temperature above 600 °C. The mixed-potential sensor has slow response at the low-temperature range, where the rate of adsorption and desorption of the gas is slow, because the response speed of the sensor depends on the gas adsorption (desorption) capability. This is why the mixed-potential sensor is rarely used for temperatures as low as 300 °C.

The dramatic enhancement in performance is the result of the following two improvements.

1) The perforation associated with formation of nano-nickel particles by the reduction of catalytic electrode is effective in providing a reaction space for a highly selective NO_X decomposition in the reactor, but it causes slow response of the sensor. We suppressed forming of the porous structure in the sensing layer by thinning the thickness (less than 100 nm).

2) We packaged most part of the reference electrode into the electrolyte to prevent the adsorption and desorption of gas on the open surface of reference electrode, which causes slower sensor response.

Multiple-chamber limiting current sensors, which are under active development, allow for a relatively high precision of detection, but the very low current value makes the system complicated and expensive. Furthermore, it does not allow for easy detection of NO_X levels of less than 100 ppm, as required for monitoring the performance of a catalyst. Our newly developed NO_X sensor technology, on the other hand, is capable of detecting small amounts of NO_X in a simple structure.

Fig. 2: Schematic illustration of the prepared electrochemical cell for NOX detection.

FIg. 3: Comparison of three different sensor responses for 1000 ppm NO.

Fig. 4: Transient response of the cell for 1000ppm NO in N2 (5%O2) at 350 °C. (total gas flow rate; 300 ml/min)

We will continue our efforts to develop the cell structure to allow for an even higher sensitivity and faster response in NO_X detection. We will also conduct further research by evaluating the effects of coexistent gases and the robustness of our cell, and we will make any improvements that are necessary for product realization of the sensor.