*2.4. Characterization of the Coatings*

The surfaces of coated specimens were analyzed using XRD (model Rint-2500; Rigaku Corp., Tokyo, Japan) via a parallel-beam method using Cu–K*α* radiation (40 kV; tube current: 100 mA). Representative specimens were analyzed over the 2θ range from 10 to 60◦ using a step size of 0.02◦ and a scan speed of 0.25◦/min. The XRD patterns were obtained at 25 ◦C and analyzed qualitatively using PDXL2 software (Rigaku) based on the International Center for Diffraction Data (ICDD) database for phase identification and quantification.

To observe the coated layers and analyze their compositions on the cross-sectioned surface, specimens were encapsulated in an epoxy resin (Epofix; Struers, Copenhagen, Denmark) and cross-sectioned with a slow-speed, water-cooled diamond saw (Isomet; Buehler, Lake Bluff, IL, USA), then ground and polished using a series of silicon carbide abrasive papers and a final slurry of 0.05-μm alumina particles. All specimens were sputter-coated with pure gold for scanning electron microscopy (SEM) evaluation (model SSX-550; Shimadzu); the SEM was operated at 15 kV. The composition of a coated specimen was determined by energy-dispersive X-ray spectroscopy (EDS) at a working distance of 15 mm and a data acquisition time of 300 s.


**7. Controlling elements for plastic recycling**

Achieving a transition to a circular economy of plastics depends on the costeffective and efficient reuse and recycling of plastics at large scales. A combination of end-of-life management approaches for plastics can reduce use, reuse, and repair, leading to a decreased energy and materials [74]. Recycling is a vital part of industrial ecology that can reduce environmental impacts and resource depletion. PET and HDPE are widely recycled when used in specific categories such as plastic bottles. However, this is an exception and is not seen for other plastic categories (**Table 1**).

**Figure 9.**

*Schematic diagram showing process steps for pyrolysis and other thermo-catalytic processing of plastic waste.*

This could be due to the cost structure of recycled plastics, the challenges of sorting mixed plastics, and the variability of the cost of virgin plastics. Many post-consumer plastics may have to be converted to other products, and they are recycling a limited number of times. The controlling factors for recycling plastics include the effectiveness of collection and segregation of mixed materials, collection and storage of recyclables, geographical location of recycling facilities concerning the processing markets, and volume of the recyclable materials collected for the region.

The incompatible nature of most plastics due to their inherent immiscibility at a molecular level is one of the significant challenges for producing high-quality resins. For example, a small quantity of PVC in PET recycling degrades the quality of the recycled material. Hence, mixing recycled plastics with virgin plastics could reduce some of the attributes of the virgin materials, and hence, the recycled plastics are mainly used for non-critical applications. Hence, post-consumer recycling involves multiple steps: collection, sorting, washing, size reduction, and separation to minimize contamination by incompatible plastics [7]. Plastic additives such as plasticizers, color, and flame retardants used in manufacturing some plastics complicate the recycling process and present risks to human health and ecology.

#### **8. Infrastructure development and access**

The successful implementation of the plastic circular economy will require sufficient infrastructure to process-collected material. Due to the current lack of processing capacity, there is a need to modify or modernize existing recycling infrastructure [75].

#### **8.1 Availability of suitable feedstock**

The effective implementation of a CE for plastics is dependent upon the availability of feedstock, adequate recycling infrastructure, and technologies that can

#### *Are Reliable and Emerging Technologies Available for Plastic Recycling in a Circular… DOI: http://dx.doi.org/10.5772/intechopen.101350*

convert nonrecycled plastics to fuel or chemical feedstock. In 2016, only 16% of the inflow of polymers was collected for recycling, 40% were sent to landfills, and 25% incinerated. Recently, European countries have increased efforts to improve recycling rates. In 2018, 29.1 million tons of post-consumer plastic waste were collected in Europe. While less than a third of this was recycled, it represented a doubling of the quantity recycled and reduced plastic waste exports outside the European Union (EU) by 39% compared to what was recycled in 2006 [55].

The degree to which plastic recycling can be increased is limited by the availability of suitable feedstock. The ability to increase plastic recycling rates is inextricably linked to the collection rates, quality of the collected plastic, and the availability of recycling technologies to manage the post-use waste stream. Mechanical, or closed-loop processes are limited by the types of plastics that can be processed, resulting in sub-optimal yields. Plastic bottles are a highly desirable product for recyclers, but just about a third find their way into a recycling bin [76].

Chemical recycling enables the processing of mixed waste streams. However, just like mechanical recycling, advanced recycling facilities require a dependable flow of materials (feedstock) that fit the technology process [10]. Some chemical recyclers currently in operation rely on high-quality material, such as postconsumer bottles or recycled PET (rPET) polyethylene terephthalate flake as a feedstock to get a better quality of output. However, there is a concern in the recycling community that the new technologies are in danger of stripping the mechanical recycling industry of its much needed and increasingly scarce feedstock of high-quality post-consumer plastic waste. Because of the cost and challenge of sorting and separating, most chemical recyclers are not targeting low-quality waste but rather the same waste streams that would typically be used in mechanical recycling [10].


**Figure 5** exhibits SEM images of the WO3

results for sensor material of WO3

temperatures.

126 Electrochemical Sensors Technology

/WO3

WO3

SnO2

SEM images show that the particle size of the WO3

EDS data for sensor materials of 5 wt% SnO2

thick film at various calcination temperatures. These

only. **Figure 6(b)**, **(c)**, and **(d)** shows SEM photos and

, 1 wt% Ru/WO3

**Figure 6** represents SEM images and EDS results of each sensing material composed of

**Figure 5.** SEM image of WO3 powders calcined for 2 h at (a) 300°C, (b) 500°C, (c) 700°C and (d) 900°C.

**Figure 4.** X-ray diffraction patterns of the various sensor materials based on catalyst/SnO2

containing metal oxides and metal catalysts. **Figure 6a** shows SEM photos and EDS

/WO3

, respectively. The SEM photos obviously show the surface morphology of each

thick film increases with rising calcination

/WO3 .

, and 1 wt% Ru/5 wt%

This study produced sensor material for gas detection by varying the mixing ratios of SnO2 powder as main material, ZnO, transition metal catalyst and carbon black. The crystal property of each produced sensor material was analyzed by using XRD measurements. **Figure 9** shows each XRD pattern of various sensor materials such as SnO2 synthesized by the precipitation method, SnO2 /ZnO which is SnO2 with 20 wt.% ZnO, SnO2 /ZnO/Cu which is SnO2 /ZnO added of 1 wt.% Cu and SnO2 /ZnO/Cu/carbon black which is SnO2 /ZnO/Cu added of 5 wt.% carbon black. When the experimental data were compared to various values of SnO2 and ZnO in JCPDS, it was confirmed that the peaks were characteristic of the crystal structure held by SnO2 and ZnO. Meanwhile, it was not possible to confirm the peak of Cu and carbon black. The XRD

**Figure 6.** SEM images and EDS results of sensing materials. Upper: SEM (×50 K), middle: SEM (5 K) and lower: EDS (×5 K).

peak for Cu was not available because only small amount of Cu at 1 wt.% was added. The XRD peak for carbon black was not available because carbon black is a weak crystalline material.

**Figure 10** shows SEM images taken at 100,000 magnification showing ratios of SnO2 , SnO2 /20%ZnO, SnO2 /20%ZnO/1%Cu and SnO2 /20%ZnO/1%Cu/5% carbon black, respectively.

Fabrication and Characterization of Metal-Loaded Mixed Metal Oxides Gas Sensors for the Detection... http://dx.doi.org/10.5772/intechopen.68414 129

**Figure 7.** XRD patterns of catalyst/WO3 /SnO2 powder.

peak for Cu was not available because only small amount of Cu at 1 wt.% was added. The XRD peak for carbon black was not available because carbon black is a weak crystalline material.

**Figure 6.** SEM images and EDS results of sensing materials. Upper: SEM (×50 K), middle: SEM (5 K) and lower: EDS (×5 K).

**Figure 10** shows SEM images taken at 100,000 magnification showing ratios of SnO2

/20%ZnO/1%Cu and SnO2

SnO2

/20%ZnO, SnO2

128 Electrochemical Sensors Technology

,

/20%ZnO/1%Cu/5% carbon black, respectively.

**Table 1.** Surface area (m2 /g), pore diameter (nm), pore volume (cm3 /g) of catalysts/WO3 /SnO2 .

**Figure 8.** SEM images and EDS results of gas-sensing materials. (a) SnO2 , (b) 10 wt% WO3 /SnO2 and (c) 1 wt% Ru/10 wt% WO3 /SnO2.

**Figure 9.** XRD patterns of the various sensor materials.

**Figure 10.** SEM images of the various sensor materials.

From these SEM images, it could be seen that the sensor material made of SnO2 /20%ZnO/1%Cu/5% carbon black had uniform particle sizes and crystallinities compared to other sensor materials.

**Figure 11** shows the SEM images and EDS measurements for each sensor material composed of SnO2 , SnO2 /20%ZnO, SnO2 /20%ZnO/1%Cu and SnO2 /20%ZnO/1%Cu/5% carbon black, respectively. It was possible to observe the surface state of each sensor material through studying the SEM images shown in **Figure 11**. It was also possible to confirm the peaks of component elements existing in each sensor material from the EDS results. The existence of Sn and O was confirmed in **Figure 11(a)**; the existence of Sn, O and Zn was confirmed in **Figure 11(b)**; the existence of Sn, O, Zn and Cu was confirmed in **Figure 11(c)** and the existence of Sn, O, Zn, Cu and C was confirmed in **Figure 11(d)**.

#### **3.2. Response characteristics to acetaldehyde gas**

**Figure 12** shows the sensing characteristics of WO3 thick film sensor to acetaldehyde (100 ppm) at the various operating and calcination temperatures. In particular, WO3 calcined at 500°C is shown to give the best sensitivity at 350°C. The sensing characteristics of the sensor after addition of SnO2 and metal catalyst to acetaldehyde gas (100 ppm) at various operating temperatures were shown in **Figure 13**. While the highest sensitivity was reached at 350°C in the case of WO3 alone, this sensitivity peak occurred at 300°C in the presence of additional constituents, that is, SnO2 and Ru. This can be understood as follows: at a given temperature, the activation energy changes greatly depending on the amount of SnO2 and the kind of metal catalyst added. Because the sensitivity of a sensor degrades with the temperature higher than 300°C, the experiments were carried out at a fixed operation temperature of 300°C.

Metal catalysts were added to the sensor material in order to improve the sensitivity and selectivity of the gas sensor to acetaldehyde gas. Mixing a metal catalyst with a metal oxide

**Figure 11.** SEM images and EDS results of the various sensor materials.

**Figure 10.** SEM images of the various sensor materials.

**Figure 9.** XRD patterns of the various sensor materials.

130 Electrochemical Sensors Technology

induces an increase in the adsorption species caused by the activation of electrons. This affects the adsorption-desorption process, which results in an enhancement of electrical conductivity. To improve the effect of this catalytic reaction, the contacting area of metal catalysts with the sensor needs to be increased. Such a move has been found to greatly affect sensitivity, selectivity and operational temperature in terms of sensing the amount of gas [14].

**Figure 14** depicts the sensing characteristics of sensor materials to acetaldehyde gas with various metal catalysts. The metal catalysts were added to 5 wt% SnO2 /WO3 , which was used as main sensor material. As shown in **Figure 14**, the sensor material containing 1 wt% of Ru gives better sensitivity than those of other catalysts. This explains that 5 wt% SnO2 / WO3 with a metal catalyst of 1 wt% Ru shows better selectivity in oxidizing acetaldehyde gas

**Figure 12.** The sensitivity of WO3 thick films with various calcination temperatures for acetaldehyde gas (100 ppm) with different operating temperatures.

**Figure 13.** The sensitivity of the gas sensor for acetaldehyde gas (100 ppm) at various temperatures.

than any other sensing materials. The reason we used 1 wt% concentration is that it gave the best sensitivity, that is, in the range of 0–1 wt%, the sensitivity increases with elevated Ru concentrations. On the other hand, if the concentration exceeds 1 wt%, there is a decrease in sensitivity. In this case, the Ru particles were not able to spread homogenously on the surface of the thick film due to the coagulation.

**Figure 15** shows the results of the selectivity study of the sensing materials giving with the best selectivities toward acetaldehyde gas compared to various other volatile organic compounds. The sensitivity measurement of 1 wt% Ru/5 wt% SnO2 /WO3 gas sensor which showed the best sensitivity to acetaldehyde gas was performed within a concentration range of 100 ppm. The result showed that the sensitivity to acetaldehyde gas was much higher than other gases. It means that the sensing material of 1 wt% Ru/5 wt% SnO2 /WO3 works better for acetaldehyde gas in decomposing and activating reaction than for other VOC gases.

#### **3.3. Sensing characteristics to H2 S gas**

induces an increase in the adsorption species caused by the activation of electrons. This affects the adsorption-desorption process, which results in an enhancement of electrical conductivity. To improve the effect of this catalytic reaction, the contacting area of metal catalysts with the sensor needs to be increased. Such a move has been found to greatly affect sensitivity,

**Figure 14** depicts the sensing characteristics of sensor materials to acetaldehyde gas with

used as main sensor material. As shown in **Figure 14**, the sensor material containing 1 wt% of Ru gives better sensitivity than those of other catalysts. This explains that 5 wt% SnO2

with a metal catalyst of 1 wt% Ru shows better selectivity in oxidizing acetaldehyde gas

thick films with various calcination temperatures for acetaldehyde gas (100 ppm) with

/WO3

, which was

/

selectivity and operational temperature in terms of sensing the amount of gas [14].

various metal catalysts. The metal catalysts were added to 5 wt% SnO2

**Figure 13.** The sensitivity of the gas sensor for acetaldehyde gas (100 ppm) at various temperatures.

WO3

132 Electrochemical Sensors Technology

**Figure 12.** The sensitivity of WO3

different operating temperatures.

The sensing response of WO3 /SnO2 was investigated for different type of sensor materials where the amounts of metal catalyst added were varied. The kinds of catalyst were selected to give a good response and selectivity to H2 S gas. In general, metal catalyst mixed with metal oxide helps to transfer electrons smoothly and increases the number of absorbed species in the absorption process, which lead to the change of electric conductivity on the sensor material. It has also been known that increasing the contact area affects the catalyst to enhance the response, selectivity and operating temperature of the sensor [15].

**Figure 16** shows the response of hydrogen sulfide across different metal catalysts. Various metal catalysts were added to the basic sensor material composed of 10 wt% WO3 /SnO2 . As shown in **Figure 16**, the sensor material with added 1 wt% Ru compared to other metal catalysts, and the operating temperature was 200°C, which represented a reduction of 50°C. This meant that 1 wt% Ru/10 wt% WO3 /SnO2 sensor material oxidized hydrogen sulfide better than other sensor materials and also showed better selectivity.

**Figure 14.** Sensitivity of catalyst/5 wt% SnO2 /WO3 to various concentration of acetaldehyde gas at 300°C.

**Figure 15.** The sensitivity of 1 wt% Ru/5 wt% SnO2 /WO3 to various gases (100 ppm) at 300°C.

**Figure 17** shows the sensing response of H2 S gas for the samples of various amount of metal catalysts added to find the optimal condition. For the main sample of 10 wt% WO3 /SnO2 , the sensing response was examined by varying the amount of Ru catalyst added in the range of 0.5–5.0 wt%, and it was found to be the best for the case of 1 wt% Ru addition. When the quantity of catalyst was below 1 wt%, it is not sufficient to activate the reaction. While the quantity of catalyst was above 1 wt%, the Ru particles aggregated itself and changed the characteristics of sensor material which did not further improve responses.

**Figure 18** shows the response comparing the changes in different concentration levels of 100 ppm of hydrogen sulfide at 200°C when SnO2 was mixed with 10 wt% WO3 + 1% Ru. Finally, the sensor material of 1 wt% Ru/10 wt% WO3 /SnO2 showed the best response characteristics.

**Figure 19** presents the selectivity results of various gases based on the sensor material showing the best sensitivity for H2 S gas. The sensor prepared with the material of 1 wt% Ru/10 wt% WO3 /SnO2 showed the best sensitivity and was tested its sensitivities to various gases at an operating temperature of 200°C and the concentration range of 20–100 ppm. As a result, the sensor showed better response to H2 S gas than to other gases. Therefore, it is known that 1 wt% Ru/10 wt% WO3 /SnO2 adsorbs H2 S gas well and enhances the catalytic reaction to improve the selectivity.

#### **3.4. Response characteristics to toluene gas**

Carbon black is a material made by covalent bonding of carbon. It is known that carbon black has good electric conductivity and good thermal conductivity. There is existing literature

Fabrication and Characterization of Metal-Loaded Mixed Metal Oxides Gas Sensors for the Detection... http://dx.doi.org/10.5772/intechopen.68414 135

**Figure 16.** Response with catalysts/10 wt% WO3 /SnO2 to the hydrogen sulfide gas (100 ppm) at different operating temperatures.

**Figure 17** shows the sensing response of H2

**Figure 15.** The sensitivity of 1 wt% Ru/5 wt% SnO2

134 Electrochemical Sensors Technology

100 ppm of hydrogen sulfide at 200°C when SnO2

the sensor material of 1 wt% Ru/10 wt% WO3

the sensor showed better response to H2

**3.4. Response characteristics to toluene gas**

ing the best sensitivity for H2

/SnO2

that 1 wt% Ru/10 wt% WO3

improve the selectivity.

wt% WO3

of sensor material which did not further improve responses.

catalysts added to find the optimal condition. For the main sample of 10 wt% WO3

/WO3

sensing response was examined by varying the amount of Ru catalyst added in the range of 0.5–5.0 wt%, and it was found to be the best for the case of 1 wt% Ru addition. When the quantity of catalyst was below 1 wt%, it is not sufficient to activate the reaction. While the quantity of catalyst was above 1 wt%, the Ru particles aggregated itself and changed the characteristics

**Figure 18** shows the response comparing the changes in different concentration levels of

**Figure 19** presents the selectivity results of various gases based on the sensor material show-

an operating temperature of 200°C and the concentration range of 20–100 ppm. As a result,

Carbon black is a material made by covalent bonding of carbon. It is known that carbon black has good electric conductivity and good thermal conductivity. There is existing literature

/SnO2 adsorbs H2

/SnO2

S gas for the samples of various amount of metal

showed the best response characteristics.

S gas than to other gases. Therefore, it is known

S gas well and enhances the catalytic reaction to

was mixed with 10 wt% WO3

to various gases (100 ppm) at 300°C.

S gas. The sensor prepared with the material of 1 wt% Ru/10

showed the best sensitivity and was tested its sensitivities to various gases at

/SnO2

+ 1% Ru. Finally,

, the

**Figure 17.** Sensitivity with the catalyst/WO3 /SnO2 to the various concentrations of hydrogen sulfide gas at 200°C.

study that reported the addition of carbon black on sensor material to decrease sensor operation temperature [16–18]. In this study, carbon black is added to sensor material to decrease operation temperature and increase the sensitivity of the sensor. **Figure 20** is the sensitivity on 100 ppm toluene gas depends on carbon black added quantity. As seen in the figure, the

**Figure 18.** The sensitivity of 1 wt% Ru/10 wt% WO3 /SnO2 from 20 ppm to 100 ppm of hydrogen sulfide gas at 200°C.

**Figure 19.** Sensitivity of 1 wt% Ru/10 wt% WO3 /SnO2 to the various concentrations of various gases at 200°C.

Fabrication and Characterization of Metal-Loaded Mixed Metal Oxides Gas Sensors for the Detection... http://dx.doi.org/10.5772/intechopen.68414 137

**Figure 20.** Sensitivity of sensors with various contents of carbon black to the toluene gas (100 ppm) at various temperatures.

**Figure 18.** The sensitivity of 1 wt% Ru/10 wt% WO3

136 Electrochemical Sensors Technology

**Figure 19.** Sensitivity of 1 wt% Ru/10 wt% WO3

/SnO2

/SnO2

from 20 ppm to 100 ppm of hydrogen sulfide gas at 200°C.

to the various concentrations of various gases at 200°C.

sensitivity of sensor fabricated with material added of 5 wt.% carbon black of SnO2 has better sensitivity and 50°C lower operation temperature compared to sensors fabricated with material added of different wt% of carbon black. The reason is believed that the sensor sensitivity and operation temperature decreased by the increase in heat dispersion in sensor material when carbon black was added.

**Figure 21** is the sensitivity measurement on 100 ppm toluene gas using a sensor fabricated with different components such as SnO2 , SnO2 /20%ZnO, SnO2 /20%ZnO/1%Cu and SnO2 /20%ZnO/1%Cu/5%carbon black. As seen in the figure, the sensor fabricated with SnO2 /20%ZnO/1%Cu/5%carbon black has better sensitivity and lower operation temperature compared to sensors produced with other three materials.

**Figure 22** is the sensitivity measurement on 1–20 ppm low concentration toluene gas using a sensor made of SnO2 /20%ZnO/1%Cu/5%carbon black, which showed the best sensitivity in this study. This sensor showed a linear increase in sensitivity at a concentration of 20 ppm or less in general.

**Figure 23** is the selectivity on various volatile gases of the sensor fabricated with SnO2 / 20%ZnO/1%Cu/5%carbon black, which showed the best sensitivity on toluene gas in this study. Sensitivity was measured on various gases at 300°C operation temperature and 10–100 ppm concentration range. The result suggested that the sensor had the best sensitivity on toluene gas compared to the sensitivity to other gases. There was some interference effect

**Figure 21.** Sensitivity of the SnO2 /20%ZnO/1%Cu/5% carbon black sensor for toluene gas (100 ppm) to the various temperatures.

**Figure 22.** Sensitivity of SnO2 /20%ZnO/1%Cu/5%carbon black sensor from 1 to 20 ppm of toluene gas at 300°C.

Fabrication and Characterization of Metal-Loaded Mixed Metal Oxides Gas Sensors for the Detection... http://dx.doi.org/10.5772/intechopen.68414 139

**Figure 23.** Sensitivity of SnO2 /20%ZnO/1%Cu/5%carbon black to the various gases at 300°C.

from ethanol and trimethylamine gas; however, interference effect from other volatile gases was not so significant. A sensor fabricated with SnO2 /20%ZnO/1%Cu/5%carbon black has better selectivity on toluene gas, because it decomposes more toluene gas and activates more reaction compared to other volatile gas.

#### **4. Conclusion**

**Figure 21.** Sensitivity of the SnO2

138 Electrochemical Sensors Technology

**Figure 22.** Sensitivity of SnO2

temperatures.

/20%ZnO/1%Cu/5% carbon black sensor for toluene gas (100 ppm) to the various

/20%ZnO/1%Cu/5%carbon black sensor from 1 to 20 ppm of toluene gas at 300°C.

Semiconductor metal oxide gas sensors can be used for diverse applications, ranging from equipment to monitor environmental and occupational safety to facilitating quality assurance through novel measurement. The nature of the gas-sensitive material and the concentration of the target gas (usually a few ppb–ppm) will determine the measuring range and limitations of the device.

In this study, the metal oxide gas sensor was fabricated with SnO2 and WO3 as the basic substances added to the metal oxide (SnO2 , WO3 , and ZnO) and various metal catalysts to detect the toxic gas. By changing the added amount and types of metal oxide and catalysts, this study investigated the sensitivity, operating temperature and recovery properties for the toxic gas.

(1) The sensing characteristics of the fabricated gas sensors added with catalysts and metal oxides to SnO2 tend to rely on the operating temperature, calcination temperature and the kind and composition of the catalyst and metal oxide.

#### **Acknowledgements**

This research was financially supported by the Ministry of Education (MOE) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (no. 2015H1C1A1035858).

#### **Author details**

Chang-Seop Lee\* and Yong Jae Kim

\*Address all correspondence to<EMAIL_ADDRESS>
Department of Chemistry, Keimyung University, Daegu, South Korea

#### **References**

[4] Min BK, Choi SD. Development of methane gas sensor by various powder preparation methods. Journal of the Korean Ceramic Society. 1999;**5**(2):125–130

(2) The optimal conditions for the fabrication of the sensor material for detecting acetaldehyde gas were known to be 300°C as an operation temperature, 500°C as a calcination temperature, 20 μm as a film thickness and ethylene glycol as a binder. The best sensitivity, selectivity and response-recovery characteristics were achieved when 1 wt% Ru/5

 sensing material. (3) The sensing characteristics of hydrogen sulfide gas sensor made by adding catalyst and

added with 10 wt% WO3

(4) The optimum conditions for the toluene gas detection sensor are an operating temperature of 300°C with added metal catalyst Cu and carbon black. The best sensitivity and highest selectivity were achieved with 20% ZnO, 1% Cu and 5% carbon black, in terms of

This research was financially supported by the Ministry of Education (MOE) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for

[1] Wark K, Warner CF. Air Pollution: Its Origin and Control. New York: Haper Collins;

[2] Noordally E, Richmond JR, Tahir SF. Destruction of volatile organic compounds by cata-

[3] Hodgson AT, Faulkner D, Sullivan DP, DiBartolomeo DL, Russell ML, Fisk WJ. Effect of outside air ventilation rate on volatile organic compound concentrations in a call center.

by impregnation.

and kind and composition of catalyst and metal oxide. The ideal condition for the sensor

tend to rely on the operating temperature, calcination temperature

S gas is temperatures of 200°C for operation and 600°C for calcina-

and 1 wt% Ru.

wt% SnO2

140 Electrochemical Sensors Technology

wt% to SnO2

**Author details**

**References**

1981

**Acknowledgements**

metal oxide to SnO2

material to detect H2

tion, and the sample of SnO2

was added to the WO3

added to SnO2

Regional Innovation (no. 2015H1C1A1035858).

\*Address all correspondence to<EMAIL_ADDRESS>
Department of Chemistry, Keimyung University, Daegu, South Korea

lytic oxidation. Catalysis Today. 1993;**17**(1–2):359–366

Atmospheric Environment. 2003;**37**(39–40):5517–5527

Chang-Seop Lee\* and Yong Jae Kim

**State-of-the-Art Chemical Sensors**

## **Managing H2O Cross‐Sensitivity Using Composite Electrolyte NO***x* **Sensors**

Erica Perry Murray, Khawlah Kharashi and Kehinde Adedeji

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67827

#### Abstract

NOx sensors composed of partially stabilized zirconia (PSZ), fully stabilized zirconia (FSZ), and PSZ–FSZ composite electrolytes were investigated using impedance spectroscopy under dry and humidified gas conditions. The impedance data were used to interpret the electrochemical behavior of the various sensors as the water concentration in the gas stream varied. The sensors were operated in the presence of 0–100 ppm NO with 1–18% O2 and 3–10% H2O with N2 as the balance gas. The operating temperature of the sensors ranged from 600 to 700C. The impedance response for sensors containing ≥ 50 vol% PSZ slightly decreased under humidified gas conditions, in comparison to dry gas conditions; whereas, a significant increase in impedance occurred for sensor largely containing FSZ. This indicated water cross-sensitivity was substantial at FSZbased sensors. The microstructural properties, NOx sensitivity, oxygen partial pressure and temperature dependence, as well as the response time of the sensors composed of the various electrolytes were characterized in order to interpret the electrochemical response with respect to water cross-sensitivity. Analysis of the data indicated that sensors composed of a PSZ–FSZ composite electrolyte with 50 vol% PSZ were more suitable for detecting NOx while limiting water cross-sensitivity.

Keywords: NOx sensors, porous zirconia, impedancemetric gas sensing, partially stabilized zirconia, YSZ

#### 1. Introduction

Advancements in diesel engine technology and emissions regulations in various countries are driving the need for NOx sensors capable of greater sensitivity, selectivity, and accuracy. Zirconiabased NOx sensors are favored for their stability and electrochemical performance under stringent

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

exhaust conditions. Various research studies have achieved substantial NOx sensitivity at sensors utilizing novel porous electrolytes, as an alternative to the conventional dense electrolyte microstructure [1–3]. NOx sensors based on the conventional architecture commonly have porous precious metal electrodes to support exhaust gas transport to the electrode/electrolyte interface where NOx sensing reactions take place. Unfortunately, heterogeneous catalysis can occur within the porous electrodes, thereby reducing the amount of NOx gas arriving at the interface. This becomes a significant problem when it is necessary to detect low concentrations (i.e., <10 ppm) of NOx as sensor accuracy is compromised. Heterogeneous catalysis is negligible at NOx sensors using a porous zirconia-based electrolyte with dense electrodes. The porous electrolyte does not appear to interfere with the exhaust gas during transport to the electrode/electrolyte interface. Thus, the porous zirconia electrolyte enables greater NOx sensor sensitivity and accuracy. However, cross-sensitivity to interfering exhaust gases, particularly water vapor, adversely affects the selectivity of the device. Sensor operation at specific frequencies appears to be an effective measure for addressing cross-sensitivity to oxygen [1]. Yet, this approach has not been effective for limiting cross-sensitivity to water. Greater understanding of the behavior of water during sensor operation is necessary for managing water cross-sensitivity and improving the feasibility of porous zirconia-based NOx sensors.

Cross-sensitivity in NOx sensors can result from electrochemical reactions with exhaust gas species that proceed more readily and/or more rapidly than NOx reactions. Accumulation of various molecules at the electrode/electrolyte interface can also enable undesirable reactions to proceed leading to cross-sensitivity. The impact of water cross-sensitivity has been ambiguous as it can cause the NOx sensing response to increase or decrease [4–6]. Some studies suggest that the different behavior observed is related to the sensor materials, fabrication methods, and operating conditions. Few studies have reported the NOx sensing response for various water concentrations. Limited data with varying results has made it difficult to understand and control the impact of water on the sensing behavior of porous electrolyte NOx sensors.

Recently, the authors found composite electrolyte NOx sensors with a porous microstructure demonstrated reduced cross-sensitivity to water, in comparison to sensors with a single-phase electrolyte. To further study the potential of composite electrolytes for NOx sensing, various sensors based on electrolytes containing specific ratios of partially stabilized zirconia (PSZ) and fully stabilized zirconia (FSZ) were evaluated using the impedancemetric technique. The work presented in this chapter describes the electrochemical response, gas sensitivity, rate limiting mechanisms, and sensing response rate due to operation in humidified gas environments for composite electrolyte NOx sensors.

#### 2. Experimental

Standard ceramic processing techniques were used to fabricate NOx sensors containing partially-stabilized zirconia (PSZ, 4.7 mol% Y2O3–ZrO2) produced by advanced ceramics and fully stabilized zirconia (FSZ, 8 mol% Y2O3–ZrO2) manufactured by Tosoh as electrolyte

materials. There were five electrolyte slurries prepared using different ratios of PSZ and FSZ along with polyvinyl butyral (Butvar B-76) binder and ethanol for the solvent. The slurries were ball milled for 16 hours to form a uniform mixture. A portion of each electrolyte slurry was dried and the powders were pressed uniaxially at 200 MPa into pellets. The three types of PSZ–FSZ composite electrolytes studied contained 25, 50, and 75 vol% PSZ, and are identified as 25 PSZ–75 FSZ, 50 PSZ–50 FSZ, and 75 PSZ–25 FSZ, respectively. Electrolytes composed of single-phase FSZ or single-phase PSZ particles were fabricated for performance comparison purposes. Single-phase PSZ electrolytes containing coarse particles were also fabricated in order to study the impact of particle size on water cross-sensitivity. Gold (Au) electrodes were placed over the electrolytes and coated using the corresponding electrolyte slurry to complete the sensor configuration. The sensors were air dried and then fired at 1050C for a period of 1 hour. A schematic of the top view and cross section of the sensors is shown in Figure 1. The top view shows the position of the sensor electrodes without the electrolyte coating. The cross-sectional view includes the electrolyte coating over the electrodes. There was no distinction observed between the electrolyte coating and pellet as the materials were of the same composition.

exhaust conditions. Various research studies have achieved substantial NOx sensitivity at sensors utilizing novel porous electrolytes, as an alternative to the conventional dense electrolyte microstructure [1–3]. NOx sensors based on the conventional architecture commonly have porous precious metal electrodes to support exhaust gas transport to the electrode/electrolyte interface where NOx sensing reactions take place. Unfortunately, heterogeneous catalysis can occur within the porous electrodes, thereby reducing the amount of NOx gas arriving at the interface. This becomes a significant problem when it is necessary to detect low concentrations (i.e., <10 ppm) of NOx as sensor accuracy is compromised. Heterogeneous catalysis is negligible at NOx sensors using a porous zirconia-based electrolyte with dense electrodes. The porous electrolyte does not appear to interfere with the exhaust gas during transport to the electrode/electrolyte interface. Thus, the porous zirconia electrolyte enables greater NOx sensor sensitivity and accuracy. However, cross-sensitivity to interfering exhaust gases, particularly water vapor, adversely affects the selectivity of the device. Sensor operation at specific frequencies appears to be an effective measure for addressing cross-sensitivity to oxygen [1]. Yet, this approach has not been effective for limiting cross-sensitivity to water. Greater understanding of the behavior of water during sensor operation is necessary for managing water cross-sensitivity and improving the feasibility

Cross-sensitivity in NOx sensors can result from electrochemical reactions with exhaust gas species that proceed more readily and/or more rapidly than NOx reactions. Accumulation of various molecules at the electrode/electrolyte interface can also enable undesirable reactions to proceed leading to cross-sensitivity. The impact of water cross-sensitivity has been ambiguous as it can cause the NOx sensing response to increase or decrease [4–6]. Some studies suggest that the different behavior observed is related to the sensor materials, fabrication methods, and operating conditions. Few studies have reported the NOx sensing response for various water concentrations. Limited data with varying results has made it difficult to understand and

control the impact of water on the sensing behavior of porous electrolyte NOx sensors.

Recently, the authors found composite electrolyte NOx sensors with a porous microstructure demonstrated reduced cross-sensitivity to water, in comparison to sensors with a single-phase electrolyte. To further study the potential of composite electrolytes for NOx sensing, various sensors based on electrolytes containing specific ratios of partially stabilized zirconia (PSZ) and fully stabilized zirconia (FSZ) were evaluated using the impedancemetric technique. The work presented in this chapter describes the electrochemical response, gas sensitivity, rate limiting mechanisms, and sensing response rate due to operation in humidified gas environ-

Standard ceramic processing techniques were used to fabricate NOx sensors containing partially-stabilized zirconia (PSZ, 4.7 mol% Y2O3–ZrO2) produced by advanced ceramics and fully stabilized zirconia (FSZ, 8 mol% Y2O3–ZrO2) manufactured by Tosoh as electrolyte

of porous zirconia-based NOx sensors.

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ments for composite electrolyte NOx sensors.

2. Experimental

The microstructure and morphology of the electrolytes were analyzed using scanning electron microscopy (SEM) and Archimedes method. The electrochemical behavior of the sensors was characterized using a Gamry Reference 600 to perform impedance spectroscopy. Impedance measurements were performed for sensors operating over a temperature range of 600–700C where the concentration of NO and NO2 was varied from 0 to 100 ppm in O2 concentrations of 1–18% with N2 as the balance. Data were collected for dry and humidified (3–10% H2O) environments using a standard gas handling system with mass flow controllers that maintained a flow rate of 100 standard cubic centimeters per minute (sccm). The Gamry instrument was configured to apply a signal amplitude of 50 mV over an operating frequency range of 1 Hz–1 MHz. Measurements were collected in triplicate to insure the data were consistent and stable. Equivalent circuit modeling using Gamry EIS300 software was used to acquire a detailed understanding of the electrochemical behavior of the sensors.

Figure 1. NOx sensor diagram: (a) top view, and (b) cross-sectional view with embedded Au wire electrodes.

#### 3. Morphology and microstructure

The microstructure of the porous electrolyte can impact gas sensor reactions. Studies have found the porosity of the electrolyte effects gas transport to the triple phase boundary (TPB) [3, 7]. The TPB is the location where the electrolyte, electrode, and gas phase are in contact. The TPB is important as NOx sensing reactions take place along this boundary. Insufficient porosity within the electrolyte restricts gas transport to the TPB and limits sensor reactions. This usually results in a slow sensor response rate that is not practical for automotive applications. On the other hand, excess porosity allows gases to transport readily through the electrolyte, but also causes TPB reaction sites to become limited, which has the negative effect of decreasing NOx sensitivity. Figure 2 shows typical SEM surface images for PSZ, FSZ, and PSZ–FSZ composite electrolytes. The particles composing the PSZ electrolytes (Figure 2a) were irregular in shape and varied from 0.1 to 0.3 µm in size; whereas, the FSZ electrolytes consisted of finer particles that seemed to be more uniform in size (largest diameter ∼0.04 µm) and shape. The FSZ particleto-particle connectivity, shown in Figure 2b, appeared to be greater than the particle connectivity between PSZ particles. The PSZ–FSZ composite electrolytes contained a well-distributed mixture of PSZ and FSZ particles, as shown in Figure 2c. In addition, larger pores were observed throughout the composite microstructure, in comparison to the single-phase electrolytes. Figure 2d shows an image of the microstructure of electrolytes containing PSZ coarse particles where the particle size ranged from about 0.4–1 μm. Table 1 gives the porosity measurements for the various electrolytes that were determined via Archimedes method. The irregular shaped

Figure 2. SEM images of electrolytes composed of: (a) PSZ, (b) FSZ, (c) 50 PSZ–50 FSZ, and (d) PSZ coarse particles.

Table 1. Electrolyte porosity based on Archimedes method.

PSZ particles allowed for closer packing of the particles, thereby, lowering the porosity of PSZbased electrolytes.

#### 4. Electrochemical response

3. Morphology and microstructure

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The microstructure of the porous electrolyte can impact gas sensor reactions. Studies have found the porosity of the electrolyte effects gas transport to the triple phase boundary (TPB) [3, 7]. The TPB is the location where the electrolyte, electrode, and gas phase are in contact. The TPB is important as NOx sensing reactions take place along this boundary. Insufficient porosity within the electrolyte restricts gas transport to the TPB and limits sensor reactions. This usually results in a slow sensor response rate that is not practical for automotive applications. On the other hand, excess porosity allows gases to transport readily through the electrolyte, but also causes TPB reaction sites to become limited, which has the negative effect of decreasing NOx sensitivity. Figure 2 shows typical SEM surface images for PSZ, FSZ, and PSZ–FSZ composite electrolytes. The particles composing the PSZ electrolytes (Figure 2a) were irregular in shape and varied from 0.1 to 0.3 µm in size; whereas, the FSZ electrolytes consisted of finer particles that seemed to be more uniform in size (largest diameter ∼0.04 µm) and shape. The FSZ particleto-particle connectivity, shown in Figure 2b, appeared to be greater than the particle connectivity between PSZ particles. The PSZ–FSZ composite electrolytes contained a well-distributed mixture of PSZ and FSZ particles, as shown in Figure 2c. In addition, larger pores were observed throughout the composite microstructure, in comparison to the single-phase electrolytes. Figure 2d shows an image of the microstructure of electrolytes containing PSZ coarse particles where the particle size ranged from about 0.4–1 μm. Table 1 gives the porosity measurements for the various electrolytes that were determined via Archimedes method. The irregular shaped

Figure 2. SEM images of electrolytes composed of: (a) PSZ, (b) FSZ, (c) 50 PSZ–50 FSZ, and (d) PSZ coarse particles.

Impedance spectroscopy is a powerful and commonly used technique for interpreting the electrical response of electrochemical devices and systems [8]. The impedance is an AC measurement that describes the opposition to current flow due to resistance, inductance, and capacitance effects. The impedance, ZðωÞ, varies with frequency, f, and is generally represented by the following equation:

$$\mathbf{Z}(\omega) = \mathbf{Z}' + \mathbf{j}\mathbf{Z}'' \tag{1}$$

where the angular frequency, ω ¼ 2πf . In addition, the real, Z 0 , and imaginary, Z″, impedance typically produce a semicircular shaped response or arc that is presented in the complex plane. In practice, more than one arc is common due to various reactions occurring at different rates. The impedance measurements for the NOx sensors in this work consisted of a high and low frequency arc. The high frequency arc (HFA) described electrochemical reactions occurring at the porous electrolyte; and, the low frequency arc (LFA) reflected electrode and interfacial reactions. Figure 3 shows typical impedance data describing the electrochemical response of PSZ, FSZ, and 50 PSZ–50 FSZ-based sensors operating at 600�C with and without 100 ppm NO present under dry and humidified conditions. Data for sensors composed of PSZ coarse particles is included. For each type of sensor, the high frequency arc was independent of the gas concentration. In addition, the magnitude of the high frequency arc differed on account of the difference in ionic conductivity for PSZ (∼0.08 S/cm) versus FSZ (∼0.14 S/cm) [9]. The higher conductivity of FSZ contributed to the lower high frequency arc impedance. Thus, the magnitude of the high frequency arc decreased as the FSZ composition within the sensor increased. These observations were expected as the high frequency arc is known to depend upon the ionic conductivity and operating temperature of the electrolyte material [8]. In all cases here, the high frequency arc was incomplete on account of the high frequency limitation of the Gamry Reference 600 instrument. As the high frequency arc does not vary with gas concentration, it is generally not useful for interpreting the sensor response to NOx gas reactions. Therefore, acquiring a complete high frequency arc was not necessary for this study. For the low frequency arc, there was a decreased in magnitude when 100 ppm NO was added to the gas stream as shown in Figure 3 for each sensor type. This change indicated electrode and interfacial reactions proceeded more readily when NO was present. The impedance data for sensors operating with 100 ppm NO2 gas was

Figure 3. Impedance data collected at 600C where the sensor electrolyte was: (a) PSZ, (b) 50 PSZ–50 FSZ, (c) FSZ, and (d) PSZ coarse under dry and humidified conditions.

very similar to that for NO. Thermodynamic conversion of NO2 to NO takes place at elevated temperatures, such that at 600C approximately 90% of NO2 converts to NO. For this reason, data using NO gas is presented in this work.

The addition of water to the gas stream caused the low frequency impedance arc to slightly decrease for sensors with a PSZ, 75 PSZ–25 FSZ, and 50 PSZ–50 FSZ electrolyte. Quite the opposite behavior occurred for sensors containing a FSZ and 25 PSZ–75 FSZ electrolyte. The change in the impedance due to water cross-sensitivity is shown for sensors composed of PSZ, FSZ, and 50 PSZ–50 FSZ in Figure 3a–c. In other studies, analysis of water adsorption experiments at oxide surfaces have found that molecular water strongly adsorbs onto the surface of Y2O3 and surface reactions result in the formation of hydroxyl groups [10, 11]. Furthermore, computational studies indicate dissociation of water molecules is a mechanism for the formation of hydroxyl species at Y2O3–ZrO2 surfaces; and, interfacial reactions between the oxide and hydroxyl groups can enhance oxygen ion conductivity [12]. However, it is also possible for adsorbed water molecules and hydroxyl species to block oxygen adsorption sites along the electrolyte/electrode interface and subsequently hinder interfacial reactions with oxygen [5]. In the present study, the Y2O3 content of the sensor electrolyte was approximately 4.7 and 8 mol% for PSZ and FSZ supported sensors, respectively. It is possible that the higher Y2O3 content of FSZ allowed greater adsorption of molecular water and hydroxyl groups to take place at sensors containing an FSZ based electrolyte, in comparison to sensors with ≥50 vol% PSZ. This would decrease the available sites for NO and O2 adsorption. For such as case, triple-phase-boundary reactions requiring NO and O2 would be limited, thereby, causing the sensor impedance to increase in the presence of water, as shown in Figure 3b. The slight decrease in impedance for PSZ sensors likely occurred due to enhanced oxygen ion conductivity. The lower Y2O3 content of PSZ possibly resulted in less molecular water coverage such that sufficient adsorption of oxygen was able to take place. The adsorbed oxygen along with resulting hydroxyl species participated in reactions that produced oxygen ions, which enabled triple-phase-boundary reactions to more readily proceed at PSZ-based sensors.

As mentioned previously, the microstructure of the sensor electrolyte affects the number of particles and reaction sites along the triple-phase-boundary where NOx reactions take place. The SEM images in Figure 2 indicate the sensor electrolyte microstructure differed for each sensor type. To determine the impact of the electrolyte microstructure on water cross-sensitivity, impedance measurements of sensors composed of coarse PSZ particles were collected. The impedance data indicated the electrochemical response for the sensors containing the course PSZ electrolyte particles did not significantly change when humidified gases were present, as shown if Figure 3d. Since similar behavior was observed for sensors composed of smaller PSZ particles it is unlikely that the microstructure of the electrolyte influenced the NOx sensor response during humidified gas conditions.

#### 5. Equivalent circuit analysis

very similar to that for NO. Thermodynamic conversion of NO2 to NO takes place at elevated temperatures, such that at 600C approximately 90% of NO2 converts to NO. For this reason,

Figure 3. Impedance data collected at 600C where the sensor electrolyte was: (a) PSZ, (b) 50 PSZ–50 FSZ, (c) FSZ, and

The addition of water to the gas stream caused the low frequency impedance arc to slightly decrease for sensors with a PSZ, 75 PSZ–25 FSZ, and 50 PSZ–50 FSZ electrolyte. Quite the opposite behavior occurred for sensors containing a FSZ and 25 PSZ–75 FSZ electrolyte. The change in the impedance due to water cross-sensitivity is shown for sensors composed of PSZ, FSZ, and 50 PSZ–50 FSZ in Figure 3a–c. In other studies, analysis of water adsorption experiments at oxide surfaces have found that molecular water strongly adsorbs onto the surface of Y2O3 and surface reactions result in the formation of hydroxyl groups [10, 11]. Furthermore, computational studies indicate dissociation of water molecules is a mechanism for the formation of hydroxyl species at Y2O3–ZrO2 surfaces; and, interfacial reactions between the oxide and hydroxyl groups can enhance oxygen ion conductivity [12]. However, it is also possible for adsorbed water molecules and hydroxyl species to block oxygen adsorption sites along the electrolyte/electrode interface and subsequently hinder interfacial reactions with oxygen [5]. In the present study, the Y2O3 content of the sensor electrolyte was approximately 4.7 and 8 mol% for PSZ and FSZ supported sensors, respectively. It is possible that the higher Y2O3 content of FSZ allowed greater adsorption of molecular water and hydroxyl groups to take place at sensors containing an FSZ based electrolyte, in comparison to sensors with ≥50 vol% PSZ. This would decrease the available sites for NO and O2 adsorption. For such as case, triple-phase-boundary reactions requiring NO and O2 would be limited, thereby, causing the sensor impedance to increase in the presence of water, as shown in Figure 3b. The slight decrease in impedance for PSZ sensors likely occurred due to enhanced oxygen ion conductivity. The lower Y2O3 content of

data using NO gas is presented in this work.

(d) PSZ coarse under dry and humidified conditions.

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To further interpret the electrochemical response of the sensors equivalent circuit analysis was carried out. The impedance arcs in Figure 3 were simulated using an equivalent circuit model consisting of resistors, R1 and R2, and constant phase elements, CPE1 and CPE2. Figure 4 shows the equivalent circuit model that was used to analyze the impedance data for each of the sensors in greater depth. The components R1 and CPE1 described the resistance and nonideal capacitance behavior of the electrolyte. Components R2 and CPE2 corresponded to the interfacial resistance and nonideal capacitance behavior at the electrodes. The constants n1 and n2 describe the deviation from ideal capacitance behavior.

The behavior of R2 and CPE2 are useful for gaining insight about electrode reactions that influence sensing behavior. It was found that the interfacial resistance, R2, decreased as the operating temperature of the sensor increase. This was expected as interfacial reactions, such as charge transfer, are able to proceed more readily at higher temperatures. The relationship between the capacitance, C, and constant phase element, CPE2 is described by the following equation:

$$C = \frac{\left[ (R\_2)(CPE\_2) \right]^{1/n}}{R\_2} \tag{2}$$

where n ¼ 1 describes the behavior of an ideal capacitor. Studies have reported that oxygen coverage at the electrode/electrolyte interface is described by the low frequency arc capacitance [13]. Oxygen accumulation at the electrode/electrolyte interface can hinder NOx transport to the interface and subsequently limit NOx sensing reactions. Figure 5 shows the capacitance associated with sensors composed of PSZ, FSZ, and 50 PSZ–50 FSZ for dry, and humidified gas conditions. The capacitance for sensors with a PSZ electrolyte tended to be an order of magnitude greater than the capacitance of FSZ electrolyte sensors. It is possible that the difference in grain size between the PSZ and FSZ particles contributed to this difference in capacitance [14]. The capacitance of the 50

Figure 4. Equivalent circuit used to model the impedance data for PSZ, FSZ and composite PSZ–FSZ based sensors.

Figure 5. The capacitance of (a) PSZ, (b) FSZ and (c) 50 PSZ–50 FSZ based sensors with for dry and humidified gas conditions with respect to temperature.

PSZ–50 FSZ composite electrolyte sensors was about half of the capacitance calculated for the PSZbased sensors. Adding 3% water to the gas stream caused a slight increase in the capacitance for the PSZ-based sensors; whereas, a decrease in the capacitance occurred for FSZ electrolyte sensors and especially the 50 PSZ–50 FSZ-based sensors. Adding additional water did not cause further changes in the capacitance. The addition of NO did not significantly alter the capacitance, which suggests NO molecules did not significantly impact oxygen coverage at the interface. The data in Figure 5 also indicated a strong temperature dependence for capacitance of the FSZ-based sensors, whereas, the PSZ and 50 PSZ–50 FSZ-based sensors displayed a less significant decrease in capacitance as the sensor-operating temperature increased. Overall, it appears the FSZ content in the composite electrolytes was beneficial for by limiting oxygen coverage at the interface such that water and NO molecules had greater opportunity to participate in triple-phase-boundary reactions. The PSZ component of the composite electrolyte helped to limited temperature dependence.

#### 6. Sensor sensitivity

PSZ–50 FSZ composite electrolyte sensors was about half of the capacitance calculated for the PSZbased sensors. Adding 3% water to the gas stream caused a slight increase in the capacitance for the PSZ-based sensors; whereas, a decrease in the capacitance occurred for FSZ electrolyte sensors and especially the 50 PSZ–50 FSZ-based sensors. Adding additional water did not cause further changes in the capacitance. The addition of NO did not significantly alter the capacitance, which suggests NO molecules did not significantly impact oxygen coverage at the interface. The data in Figure 5 also indicated a strong temperature dependence for capacitance of the FSZ-based sensors,

Figure 5. The capacitance of (a) PSZ, (b) FSZ and (c) 50 PSZ–50 FSZ based sensors with for dry and humidified gas

conditions with respect to temperature.

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Figure 4. Equivalent circuit used to model the impedance data for PSZ, FSZ and composite PSZ–FSZ based sensors.

The angular phase component of the impedance, θ, is often more responsive the changes in NOx concentration, in comparison to other impedance parameters [1, 15]. For this reason, it is frequently used to evaluate NOx sensor sensitivity according to the following relationship:

Figure 6. NO sensitivity for sensors with: (a) PSZ, (b) FSZ, and (c) 50 PSZ–50 FSZ electrolyte under dry and humidified gas conditions at 650C.

$$\Theta = \arctan\left(\frac{Z''}{Z'}\right) \tag{3}$$

The change in the angular phase, Δθ, is given by:

$$
\Delta\Theta = \Theta\_{\bullet 2} - \Theta\_{\rm NO} \tag{4}
$$

where θO2 is the baseline response when 10.5% O2 is present with N2; and, θNO corresponds to the response when a specific amount of NO gas is added. Figure 6 shows Δθ with respect to NO and water concentration for the PSZ, FSZ, and 50 PSZ–50 FSZ composite-based sensors at an operating frequency of 40 Hz at 650�C. Data was collected at 40 Hz as the maximum sensitivity was achieved at this operating frequency. The slope of the data, Δθ/Δ[NO], is defined as the sensor sensitivity in units of degrees/ppm NO. The highest sensitivity was achieved during dry gas conditions for the various sensors. As the water concentration was increased in the gas

Figure 7. Change in angular phase response with respect to temperature for sensors composed of: (a) PSZ, (b) FSZ, and (c) 50 PSZ–50 FSZ electrolyte.

stream the sensitivity of the sensors decreased. However, the 50 PSZ–50 FSZ composite based sensors demonstrated the greatest tolerance to changes in the gas humidity as the sensitivity decreased very slightly. The sensors with an FSZ electrolyte had a higher sensitivity than sensors composed of PSZ, but the sensitivity of the FSZ based sensors decreased most significantly with increasing water in the gas stream. Although the FSZ electrolyte sensors provide higher sensitivity, the 50 PSZ–50 FSZ electrolyte contributes to a more reliable sensor.

In application the temperature of the sensor can fluctuate due to changes driving conditions. Thus, a stable NOx sensing response with respect to temperature is necessary in order to maintain accuracy. In the present study, the temperature dependence of the various sensors was observed by comparing Δθ with respect to the sensor-operating temperature, as shown in Figure 7. The sensing response of the FSZ-based sensors decreased with increasing operating temperature, as shown in Figure 7b. However, the PSZ and especially the 50 PSZ–50 FSZ based sensors were less dependent on temperature, as shown in Figure 7a and c. This data corresponded with the capacitance data presented in Figure 5 where the capacitance of the sensors with an FSZ electrolyte also had a strong temperature dependence, in comparison to sensors with a PSZ and PSZ–FSZ electrolyte. Based on the data shown in Figures 6 and 7, sensors with the 50 PSZ–50 FSZ electrolyte were less prone to water cross-sensitivity and more tolerant to changes in the operating temperature.

#### 7. Rate limiting mechanisms and activation energies

θ ¼ arctan

where θO2 is the baseline response when 10.5% O2 is present with N2; and, θNO corresponds to the response when a specific amount of NO gas is added. Figure 6 shows Δθ with respect to NO and water concentration for the PSZ, FSZ, and 50 PSZ–50 FSZ composite-based sensors at an operating frequency of 40 Hz at 650�C. Data was collected at 40 Hz as the maximum sensitivity was achieved at this operating frequency. The slope of the data, Δθ/Δ[NO], is defined as the sensor sensitivity in units of degrees/ppm NO. The highest sensitivity was achieved during dry gas conditions for the various sensors. As the water concentration was increased in the gas

Figure 7. Change in angular phase response with respect to temperature for sensors composed of: (a) PSZ, (b) FSZ, and

(c) 50 PSZ–50 FSZ electrolyte.

The change in the angular phase, Δθ, is given by:

154 Electrochemical Sensors Technology

Z″ Z0

Δθ ¼ θO2 � θNO ð4Þ

ð3Þ

The gases traveling through the porous electrolyte and reacting at the triple-phase-boundary undergo various reaction steps including adsorption, dissociation, diffusion, charge transfer, and oxygen ion transport (not necessarily in that order). The porous microstructure creates specific pathways for gas and ionic transport. The electrolyte material and microstructure can impact how readily gas transport and related reactions occur. A common approach to interpreting ratelimiting mechanisms that impact gas transport and associated reactions is to evaluate the oxygen partial pressure (PO2) dependence, which is described by the following power law relationship:

$$(\mathcal{P}\_{\mathcal{O}2})^{\mathcal{W}} \propto \mathcal{R}\_{\text{LFA}} \tag{5}$$

The power law exponent, m, is used to interpret specific rate-limiting mechanisms, and RLFA is the low frequency arc previously discussed. The PO2 dependence of the PSZ, FSZ, and 50 PSZ–50 FSZ based sensors was determined by measuring the impedance response at oxygen concentrations ranging from 1 to 18% for dry and humidified gas conditions with and without NO. A plot of log(RLFA) versus log(PO2) was generated for the various gas conditions in order to determine the exponent m. Table 2 provides the m values determined for the sensors with a PSZ, FSZ, and 50 PSZ–50 FSZ electrolyte. Dissociative adsorption is the rate-limiting mechanism commonly associated with at PO2 dependence where m ¼ 0.5 [11, 16]. The data in the table indicated under dry gas conditions m ≈ �0.5 for the PSZ-based sensors, which suggested dissociative adsorption was rate-limiting. Adding water to the gas stream without NO present decreased m to �0.43 suggesting an additional rate-limiting mechanism, possibly charge transfer, became involved.

Table 2. Power law exponent for sensors composed of the various electrolytes.

The charge transfer becomes a dominant rate-limiting step when (PO2) �0.25 ∝ RLFA. In the presence of NO and water, the PSZ-based sensors were apparently limited by dissociative adsorption as a (PO2) �0.51 dependence was observed. A slightly lower PO2 dependence was determined for the FSZ-based sensors. However, in the presence of both water and NO the dissociative adsorption seemed to be the dominant rate-limiting mechanism for FSZ-based sensors. Sensors with the composite 50 PSZ–50 FSZ electrolyte had a stronger PO2 dependence, in comparison to the other sensors evaluated. The exponent, m tended to be about �0.6, which suggested gas diffusion, may have participated as an additional rate-limiting mechanism. A (PO2) �1 dependence has been associated with gas diffusion limitations. The porosity of the 50 PSZ–50 FSZ sensor was about 47%, which is expected to provide sufficient gas diffusion. However, it is possible that the composite microstructure resulted in tortuous pathways that interfered with gas diffusion. As a result the dissociative adsorption and gas diffusion could serve as rate-limiting mechanisms for sensors with a composite electrolyte.

The activation energy associated with the various sensors also aids interpretation of how readily sensor reactions are able to proceed. The activation energy, Ea, of the sensors for operating temperatures ranging from 600 to 700�C was determined using the Arrhenius equation the given below:

$$
\ln\left(\frac{1}{R\_{LFA}}\right) = \left(\frac{E\_a}{R}\right)\left(\frac{1}{T}\right) + \ln(A\_o) \tag{6}
$$

where RLFA is the diameter of the low frequency impedance arc, R represents the ideal gas constant, T is temperature, and Ao is the y-intercept. Table 3 shows the activation energy

Table 3. Activation energy values for the various sensors.

calculated for each sensor type for dry and humidified conditions with and without NO present. The activation energy was the lowest for the single-phase PSZ-based sensors. The addition of water resulted in an increase in activation energy for the PSZ-based sensors and a decrease in the composite and FSZ-based sensors. Adding FSZ to the sensor electrolyte caused the activation energy to increase. This trend was observed for each of the various gas compositions measured and likely relates to the electrolyte microstructure. Other studies have reported that the activation energy in porous electrolyte NOx sensors tends to increase with increasing electrolyte porosity [3]. Table 1 shows the porosity of the electrolytes increased as the FSZ content in the sensor electrolyte increased. The increase in porosity causes particle contact along the electrode/electrolyte interface to decrease resulting in fewer sites for reactions to take place, thereby, resulting in an increase in the activation energy.

#### 8. Sensor response time and stability

The charge transfer becomes a dominant rate-limiting step when (PO2)

Power law exponent, m

Table 2. Power law exponent for sensors composed of the various electrolytes.

serve as rate-limiting mechanisms for sensors with a composite electrolyte.

ln 1 RLFA

Activation energy (eV)

Table 3. Activation energy values for the various sensors.

adsorption as a (PO2)

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(PO2)

below:

presence of NO and water, the PSZ-based sensors were apparently limited by dissociative

Dry 0 �0.494 �0.620 �0.412 Gas 100 �0.465 �0.591 �0.413 Humidified 0 �0.427 �0.626 �0.471 Gas 100 �0.512 �0.553 �0.472

determined for the FSZ-based sensors. However, in the presence of both water and NO the dissociative adsorption seemed to be the dominant rate-limiting mechanism for FSZ-based sensors. Sensors with the composite 50 PSZ–50 FSZ electrolyte had a stronger PO2 dependence, in comparison to the other sensors evaluated. The exponent, m tended to be about �0.6, which suggested gas diffusion, may have participated as an additional rate-limiting mechanism. A

�1 dependence has been associated with gas diffusion limitations. The porosity of the 50 PSZ–50 FSZ sensor was about 47%, which is expected to provide sufficient gas diffusion. However, it is possible that the composite microstructure resulted in tortuous pathways that interfered with gas diffusion. As a result the dissociative adsorption and gas diffusion could

The activation energy associated with the various sensors also aids interpretation of how readily sensor reactions are able to proceed. The activation energy, Ea, of the sensors for operating temperatures ranging from 600 to 700�C was determined using the Arrhenius equation the given

> ¼ Ea R 1

where RLFA is the diameter of the low frequency impedance arc, R represents the ideal gas constant, T is temperature, and Ao is the y-intercept. Table 3 shows the activation energy

Dry 0 1.067 1.119 1.148 1.150 1.230 Gas 100 1.017 1.101 1.122 1.125 1.205 Humidified 0 1.084 1.110 1.122 1.133 1.135 Gas 100 1.035 1.043 1.099 1.081 1.082

T

NO (PPM) PSZ 75 PSZ–25 FSZ 50 PSZ–50 FSZ 25 PSZ–75 FSZ FSZ

�0.51 dependence was observed. A slightly lower PO2 dependence was

NO (ppm) PSZ 50 PSZ–50 FSZ FSZ

�0.25 ∝ RLFA. In the

þ lnð Þ Ao ð6Þ

Time-based measurements for the angular phase response, θ, were collected for sensors with a PSZ, FSZ, and composite PSZ–FSZ electrolyte. Figure 8 shows the typical time-based response for sensors with a 50 PSZ–50 FSZ electrolyte as the NO composition was varied over 0–100 ppm. The dry and humidified data overlap as the 50 PSZ–50 FSZ electrolyte enables the sensor to be less prone to water cross-sensitivity. A baseline shift was observed for the time-based data collected for sensors composed the other electrolytes studied. The response time for each sensor was evaluated based on the τ90 response, which is the time required for the sensor to achieve 90%

Figure 8. Angular phase response with respect to time for sensors composed of a 50 PSZ–50 FSZ electrolyte.

of the steady state response after a step change in gas concentration. Table 4 shows the τ90 response for the various sensors. The sensors containing single-phase PSZ were the slowest as τ90 ¼ 16 s, which possibly relates to the lower ionic conductivity of the electrolyte, in comparison to single-phase FSZ-based sensors where τ90 ¼ 5 s. As seen in Table 4, the sensor response time decreased resulting in a faster sensor as the FSZ concentration increased.

The stability of a 50 PSZ–50 FSZ based sensors was evaluated for over 150 hours at 650�C and an operating frequency of 40 Hz with 3% water present. Figure 9 shows sensor drift was negligible for both the baseline response where θ remained about �17.25�, and the sensor response with humidified gas where θ remained nearly constant at about �16.25�. This data suggests that the 50 PSZ–50 FSZ electrolyte is capable of providing a very stable sensing response. Further evaluation is necessary to determine the extent of the 50 PSZ–50 FSZ based sensor stability over longer time intervals.

Table 4. The τ90 response of the sensors composed of the various electrolytes.

Figure 9. Stability of a 50 PSZ–50 FSZ sensor over several hours of operation.

#### 9. Conclusions

of the steady state response after a step change in gas concentration. Table 4 shows the τ90 response for the various sensors. The sensors containing single-phase PSZ were the slowest as τ90 ¼ 16 s, which possibly relates to the lower ionic conductivity of the electrolyte, in comparison to single-phase FSZ-based sensors where τ90 ¼ 5 s. As seen in Table 4, the sensor response time

The stability of a 50 PSZ–50 FSZ based sensors was evaluated for over 150 hours at 650�C and an operating frequency of 40 Hz with 3% water present. Figure 9 shows sensor drift was negligible for both the baseline response where θ remained about �17.25�, and the sensor response with humidified gas where θ remained nearly constant at about �16.25�. This data suggests that the 50 PSZ–50 FSZ electrolyte is capable of providing a very stable sensing response. Further evaluation is necessary to determine the extent of the 50 PSZ–50 FSZ based sensor stability over

Sensor electrolyte PSZ 75 PSZ–25 FSZ 50 PSZ–50 FSZ 25 PSZ–75 FSZ FSZ Dry gas 16 14 10 7 5 Humidified gas 14 12 11 5 4

decreased resulting in a faster sensor as the FSZ concentration increased.

Time constant – τ90 (s)

Table 4. The τ90 response of the sensors composed of the various electrolytes.

Figure 9. Stability of a 50 PSZ–50 FSZ sensor over several hours of operation.

longer time intervals.

158 Electrochemical Sensors Technology

Impedance spectroscopy was used to interpret the electrochemical response of NOx sensors composed of PSZ, FSZ, and PSZ–FSZ composite electrolytes during operation under dry and humidified gas conditions. Analysis of the electrochemical response of the 50 PSZ–50 FSZ based sensors indicated PSZ contributed to lower water cross-sensitivity, while FSZ promoted NOx sensitivity. It was also determined that the electrolyte microstructure influenced sensor sensitivity, but did not impact water cross-sensitivity. Dissociative adsorption was considered to be the dominant rate-limiting mechanism for each of the sensors. Adding water to the gas stream did not appear to significantly alter the rate-limiting mechanism. The response of the FSZ-based sensors was strongly depended upon the operating temperature, whereas the response of sensors containing PSZ, and particularly the 50 PSZ–50 FSZ, was less dependent on temperature. The response time of the sensors decreased as the concentration of FSZ increased within the sensor electrolyte indicating the FSZ contributed to a rapid sensor response rate. Overall, sensors composed of the 50 PSZ–50 FSZ composite electrolyte demonstrated significant sensitivity to NO and low cross-sensitivity to water with negligible temperature dependence.

#### Acknowledgements

The authors thank Mr. Robert Novak and Dr. Jaco Visser of Ford Motor Company for providing meaningful discussions that contributed to this work. Funding for this work was provided by the National Science Foundation under the Ceramics Program (DMR-1410670).

#### Author details

Erica Perry Murray\*, Khawlah Kharashi and Kehinde Adedeji

\*Address all correspondence to<EMAIL_ADDRESS>
Louisiana Tech University, Ruston, Louisiana, USA

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**4. Structure and activity of NK4 as HGF antagonist**

HGF is a stromal-derived paracrine factor that has stimulated cancer invasion at least *in vitro* (Matsumoto *et al*., 1994; Matsumoto *et al*., 1996a; Nakamura *et al*., 1997). Clinical studies suggest that the degree of serum HGF and Met expressions in cancer tissues appears to correlate with a given prognosis (Yoshinaga *et al*., 1993; Osada *et al*., 2008). Thus, it is hypothesized that *in vivo* inhibition of HGF-MET signaling may be a reasonable strategy to prohibit cancer metastasis. To test this hypothesis, we prepared NK4 as an intra-molecular fragment of HGF via a chemical digestive process (Date *et al*., 1997; Matsumoto *et al*., 1998). As expected, NK4 bounded to MET and inhibited HGF-MET coupling as a competitive inhibitor. An additional "unexpected" value was that NK4 inhibited tumor angiogenesis via a MET-independent pathway. This section focuses on the biological value of NK4 as an HGF-antagonist and as an angiogenesis inhibitor.