Potentiometric hydrogen sensors based on yttria-stabilized zirconia electrolyte (YSZ) and CdWO4 interface

Sensors and Actuators B 223 (2016) 365–371

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Potentiometric hydrogen sensors based on yttria-stabilized zirconia electrolyte (YSZ) and CdWO4 interface Yue Li, Xiaogan Li ∗ , Zhaoyun Tang, Jing Wang, Jun Yu, Zhenan Tang School of Electronic Science and Technology, Institute for Sensing Technologies, Key Laboratory of Liaoning for Integrated Circuits Technology, Dalian University of Technology, Dalian 116024, China

a r t i c l e

i n f o

Article history: Received 1 June 2015 Received in revised form 23 August 2015 Accepted 22 September 2015 Available online 26 September 2015 Keywords: Mixed-potential gas sensors YSZ CdWO4 electrode Hydrogen sensors

a b s t r a c t The yttria-stabilized cubic zirconia (YSZ) based potentiometric sensor coupled with CdWO4 as the sensing electrode was developed for hydrogen detection. The developed CdWO4 /YSZ/Pt sensor has found significant response to different concentration of hydrogen from 0.5 vol% to 3 vol% at 500 ◦ C. The sensor showed good selectivity to several possible interferents such as CO, NO2 and C3 H8 when the concentrations are less than 1500 ppm. The dc polarization curves and ac impedance spectra of the sensor were measured to study the sensing mechanism. The mixed-potential theory was confirmed to explain the sensing results. The reproducibility and signal repeatability of the sensor was examined to test the reliability of the sensor. The possible influence of oxygen variation and humidity in the background on the sensor response was also studied. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Hydrogen has been developed as a clean and renewable alternative to carbon-based fuels [1]. It has been explored for hydrogen fuel cells that have been used for stationary electricity generation or proposed for zero-emission combustion engines for automotive electric vehicles [2–4]. However, one critical aspect for the safe and efficient deployment of hydrogen is the ability of chemical sensors to meet the required performance specifications for the growing hydrogen infrastructure [1,5]. Leaked hydrogen will become dangerous once the concentration is above 4% when mixed with air [5–7]. Among several potential technologies developed for hydrogen detection, the yttria-stabilized zirconia based potentiometric sensor has found several advantages for hydrogen detection such as the excellent resistance to the harsh corrosive and high temperature environments [6,7]. This type of sensor has been proposed to work in accordance with a mixed-potential theory in which the sensor response is logarithmically correlated with the gas concentrations [8]. Three methods are usually investigated to improve the sensitivity and selectivity of the sensors including using new oxide sensing electrodes and electrolytes, optimization of the microstructure of the oxide sensing electrode and electrolytes (i.e. chemical

composition, thickness, particle size and morphology, etc.) and new process technologies for deposition of the sensing films [9–15]. Among these methods, finding out a better sensing electrode specifically for some targeted gas species still seems to be a more effective way. Several metal oxides and their composites have been investigated for hydrogen sensing as briefly summarized previously [16,17]. It has been recently demonstrated that the complex ZnWO4 oxide sensing electrode showed good sensitivity and selectivity to hydrogen from 0.5 vol% to 3 vol% at 600 ◦ C while MnWO4 electrode displayed some sensitivity and selectivity to hundreds of ppm-level hydrogen at 500 ◦ C [16,17]. Obviously, the substitution of metal M in the MWO4 compounds could have significant influence on the hydrogen sensing of MWO4 /YSZ/Pt sensors. In this work, CdWO4 has been studied in detail aiming at searching for a better oxide sensing electrode for detection of hydrogen leakage with a faster response and recovery. The results indicated that CdWO4 electrode showed a good sensitivity to different concentrations of hydrogen from 0.5 vol% to 3 vol% at 500 ◦ C.

2. Experimental 2.1. Materials preparation and characterization

∗ Corresponding author. E-mail address: [email protected] (X. Li). http://dx.doi.org/10.1016/j.snb.2015.09.110 0925-4005/© 2015 Elsevier B.V. All rights reserved.

The CdWO4 powders were prepared by the coprecipitation method. During the preparation, 3.29 g Na2 WO4 ·2H2 O (Sinopharm Chemical Reagent Co., Ltd, China) and 2.28 g CdCl2 ·2.5H2 O (Tianjin

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Kermiou Chemical Reagent Co., Ltd, China) were separately put into two beakers. Then, 70 ml deionized water was added into these two beakers respectively followed by being magnetically stirred to obtain the two corresponding homogeneous solutions. After that, the Na2 WO4 aqueous solution was added into the CdCl2 solution dorpwise with strong magnetic stirring for 2 h. The white precipitates were obtained and washed with deionized water and ethanol for several times, then dried at 80 ◦ C overnight. Finally, the as-prepared products were calcined at 750 ◦ C for 2 h and then grounded thoroughly to achieve the final CdWO4 powders. The crystal structure of the obtained oxide was determined by XRD (XRD-6000, Shimadzu Corp., Japan). The surface morphology of the sample was analyzed using the scanning electron microscope (SEM, FEI, USA). 2.2. Sensor fabrication CdWO4 paste painted on eight molar percentage yttria stabilized zirconia (8%YSZ) solid electrolyte was made by mixing as-synthesized CdWO4 powders with the commercial ␣-terpeneol (99%, Shanghai Baoman Biotechnology Co., Ltd, China). The amount of the ␣-terpeneol was carefully adjusted to obtain the printable paste. Pt paste was made by mixing the commercial Pt powders (mean particle size ∼100 nm, Kunming Youyan, China) with the ␣-terpeneol. The dense YSZ solid electrolytes with a dimension of 10 mm × 10 mm × 0.2 mm were provided by Shanghai Institute of Ceramics, China. The sensor consists of a planar-type electrochemical cell similar to that described in [18] and is illustrated in Fig. 1(a). The procedures for fabricating the sensor were involved with the following steps: The as-prepared Pt paste was painted on one face of the YSZ electrolyte plate. Then, the painted Pt was fired at 150 ◦ C for 2 h and then 1000 ◦ C for 2 h to sinter the Pt and increase the adhesion of the Pt to the electrolyte. This Pt thick film would work as the reference electrode (RE). Then, the other face of the YSZ plate was firstly painted with Pt paste serving as the current collectors. The painted Pt current collector was also sintered at 150 ◦ C for 2 h and then 1000 ◦ C for 2 h for the purposes mentioned earlier. The thickness of the Pt films was about 10 ␮m. Then, the CdWO4 oxide pastes were painted on the top of the Pt current collector and fired at 750 ◦ C for 2 h to burn away the organic binders and increase the adhesions of the sensing elements with the electrolyte. The oxide would work as the sensing electrode (SE) and the thickness is around 11 ␮m and this sensor was noted as Sensor1. The thickness

of both Pt and CdWO4 oxide films was estimated by a Micro figure Measuring Instrument (KOSAKA LAB ET 4000 Level Gauge, Japan). 2.3. Electrical measurements The sensing measurements were conducted in a conventional gas flow apparatus [18]. The electric potential difference (V) of the sensor was measured by a computer controlled Agilent high impedance digital electrometer (100 M, Agilent 34401A). During the electrical measurements, the oxide electrode was always connected to the positive terminal of the electrometer. Both electrodes of the sensor were exposed to the same sample gas. The commercially supplied certified gases containing various concentrations of H2 , CO, NO2 and C3 H8 were prepared by diluting a parent dry gas with the background gas (O2 and N2 balance), respectively, to obtain different concentrations of the test gas. The CO, NO2 and C3 H8 concentrations were changed from 60 to 1140 ppm and the background was 10% O2 by volume balanced with N2 all the time. The ppm refers to the volume concentration of the test gas in the gaseous mixture. The total gas flow rate was maintained at 200 sccm/min and the flow rate of different gases was controlled independently using computer controlled, pre-calibrated electronic mass flow controllers (D07-19B, Beijing Senvenstar Electronics, China). The response of the sensor (Rs , mV) was defined as: Rs = Va − Vb

(1)

where Va and Vb are the electric potential difference (EPD) of the sensor in the analytes and the background gas (10% O2 /N2 ), respectively. The ac impedance spectra of the sensor in different testing environments were measured by an Agilent LCR meter (20 Hz–20 MHz, Agilent E4980A) within a frequency range from 20 Hz to 2 MHz. Three-electrode configuration as shown in Fig. 1(b) with Pt as the counter and reference electrodes, respectively, is adopted for the dc polarization measurements. A variation of dc voltages from 0 to −400 mV at a step of 40 mV was applied across the oxide and counter electrodes and the current was measured simultaneously by a Keithley system source meter (1 pA–10 A, Keithley 2612). 3. Results and discussion 3.1. Materials synthesis and microstructure Fig. 2 shows the XRD pattern and SEM image of the synthesized CdWO4 powders. The XRD peaks as shown in Fig. 2(a) were indexed according to JCPDS No.: 14-0676. The crystal structure is a single monoclinic phase. Fig. 2(b) shows the surface morphology of assynthesized CdWO4 powders calcined at 750 ◦ C. The sample shows a typical Oswald ripening phenomena consisting of two different particle sizes. The larger particles indicate a rod-like shape with a length in the range of 100–500 nm and the smaller ones show a nearly circular shape with a diameter of 50–100 nm. Both types of particles are intimately aggregated together. Fig. 2(c) shows the SEM image of the CdWO4 sensing film after sintered at 750 ◦ C. The particles within the film grew larger (∼250 nm to 750 nm). The film shows porous microstructure which would be beneficial to the gas diffusion during the sensing process. 3.2. Sensing properties

Fig. 1. Illustrations of (a) sensor structure and (b) three-electrode configuration of CdWO4 /YSZ/Pt sensor and experimental setup for dc polarization measurement.

3.2.1. Response to hydrogen Fig. 3(a) shows the response of the potentiometric CdWO4 /YSZ/Pt sensor (Sensor1) to different concentrations of hydrogen from 0.5 vol% to 3 vol% at four operating temperatures

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Fig. 2. (a) XRD pattern, (b) SEM image of CdWO4 calcined at 750 ◦ C and (c) SEM image of CdWO4 sensing electrode after sintering at 750 ◦ C.

(a)

0

V(mV)

-100 -200 0.5

-300

1.0 o 1.5 450 C o 500 C o 2.0 550 C o 600 C vol%,10%O2

-400 -500 0

20

40

60

2.5

80

3.0

100

120

Time(min)

0

(b) Slope~41mV/Dec.

-100

Rs(mV)

-200 Slope~175mV/Dec.

-300 o

-400 -500

450 C o 500 C o 550 C o 600 C

Slope~182mV/Dec.

10000

20000

30000

H2 Conc.(ppm) Fig. 3. (a) Response curve and (b) calibration curves of the CdWO4 |YSZ|Pt sensor to different concentrations of hydrogen from 0.5 vol% to 3 vol% within the temperature range from 450 ◦ C to 600 ◦ C.

in a background of 10 vol% oxygen balanced with nitrogen. The response decreased as the operating temperature increased. The relationship between the response of the sensor and the logarithm of hydrogen concentrations from 5000 to 30,000 ppm could be fitted almost linearly as below equations: Rs (500

◦

C) = −182 lg(x) + 443

(2)

Rs (550

◦

C) = −175 lg(x) + 546

(3)

Rs (600

◦

C) = −41 lg(x) + 140

(4)

where x is the hydrogen concentrations in ppm. The sensitivity of the sensor, defined as the slope of the calibration curves shown in Fig. 3(b), shows the highest one (∼182) at 500 ◦ C. The response time taken for the sensor response to 90% of the saturated value and the recovery time taken to decrease down to 10% of the saturated response value is 35 s and 90 s, respectively. Further decrease in the operating temperature less than 500 ◦ C would lead to inappropriate response for a sensor as shown in Fig. 3(a) and (b) when exposed to higher concentration of hydrogen. Although the response is higher at lower concentrations (less than 2 vol% H2 ) than those at 500 ◦ C, it switches to a decrease tendency as the hydrogen concentrations increased above 2 vol%. The reasons for such unexpected performance are not clear at the moment. Therefore, the sensor could not work well for hydrogen detection at 450 ◦ C. This type of YSZ based potentiometric sensors could be thus explained by the mixed-potential mechanism as proposed previously. According to the mixed potential theory [19–21], the electrochemical redox reactions could occur at both electrodes of the electrochemical cell according to the below reactions: O2 (g) + 4e− → 2O2− (YSZ) 2−

2H2 (g) + 2O

(5) −

(YSZ) → 2H2 O + 4e

(6)

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80 1.0

0.5

1.5

2.0

2.5

0

3.0

-100 V(mV)

Vpt(mV)

60

40

-200 0.5

-300

20

1.0

VOL%

VOL%

(a)

0 20

40

60

80

100

0

120

1.5

sensor1 (~11) sensor2 (~102)

-400

o

T=500 C, 10%O2

20

60

80

100

120

Time(min)

Time(min)

65

40

2.0 2.5 3.0 o T=500 C, 10%O2

Fig. 5. Effect of thickness of CdWO4 sensing electrode on the response of the sensor to hydrogen at 500 ◦ C (Sensor1: ∼11 ␮m, Sensor2: ∼102 ␮m).

o

T=500 ,10%O2

60 100 Air 0.5%H2

80

50

O

40 35 10000

20000

1.0%H2

60

Slope 39.2mV/Dec.

45

Current(nA)

Rs-pt(mV)

55

30000

T=500 C,10%O2

40 20 0 -20

H2 Conc.(ppm)

-40 -400 -350 -300 -250 -200 -150 -100

Fig. 4. (a) Response curve and (b) calibration plot of the Pt/YSZ/Pt sensor (no CdWO4 ) to different hydrogen concentrations from 0.5 vol% to 3 vol% at 500 ◦ C.

-50

0

Potential(mV) Fig. 6. Polarization curves of CdWO4 |YSZ|Pt sensor at 500 ◦ C and 10% O2 .

The generated electric potential was different at each interface of the gas/electrode/electrolyte (also called as triple-phase boundary: TPB) thus leading to the observed response of the sensors according to: V = VCdWO4 − VPt

(7)

where V is the electrical potential difference of the sensor, and VCdWO4 and VPt are the mixed potential at oxide electrode and Pt counter electrode, respectively. As the operating temperature increased, more hydrogen would react with oxygen in the background during the diffusion within oxide layer according to: 2H2 + O2 → 2H2 O

(8)

This non-electrochemical catalytic conversion of hydrogen would decrease the hydrogen available to the TPB subsequently leading to the lower sensitivities as the temperature increased from 500 ◦ C to 600 ◦ C. The linear relationship between the open-circuit EPD and the logarithm of the hydrogen concentrations indicated that the polarizations were within Tafel region [21,22]. To confirm the role of the CdWO4 oxide electrode played in the sensing process, the sensor using only Pt as the two electrodes (i.e. no CdWO4 ) was also tested and the result is shown in Fig. 4. The sensor did show some response to different concentrations of hydrogen at 500 ◦ C. However, the response values within a range from 35 mV (to 0.5 vol% H2 ) to 65 mV (to 3%H2 ) are much lower compared with those of the sensor covered with CdWO4 oxide electrode. Moreover, the sensitivity (the slope of the calibration curve: 39.2 mV/Dec.) is also relatively smaller compared with those of the sensor with CdWO4 oxide electrode (182 mV/Dec.). These small responses and sensitivity should come from the slight variation of the microstructure such as thickness and porosity of Pt films on both sides of the YSZ electrolyte [23].

Further, the effect of the thickness of the CdWO4 sensing electrode on the response was thoroughly examined. One more sensor (Sensor2) was fabricated with the thickness of CdWO4 electrode around 102 ␮m. Fig. 5 shows the comparison of response of the two sensors with different thickness of CdWO4 electrodes. It could be observed that with thicker oxide layer, the response of the sensor was significantly reduced. This could be attributed to the nonelectrochemical catalytic conversion of hydrogen during hydrogen diffusion through the oxide layer according to reaction (8) which would decrease the hydrogen available to the TPB subsequently leading to the reduced sensitivities [11]. To testify the sensing mechanism, the polarization curves of the sensor (Sensor1) were measured as shown in Fig. 6. It has been proposed that the cross point at the polarization curves of sensors in the background and gas, respectively, could be used to estimate the EPD of the sensor [22]. Table 1 gives comparison between the measured sensor response and the estimated values from the polarization curves shown in Fig. 6. The measured ones are close to the estimated ones confirming the proposed mixed-potential theory for as-developed hydrogen sensors. The electrochemical process of the generation of the sensor response was studied by the impedance spectra as shown in Fig. 7. Table 1 Comparison between the measured sensor response and the estimated values attached with CdWO4 at 500 ◦ C. H2 conc. (ppm)

Measured sensor response (observed) (mV)

Mixed potentials (estimated) (mV)

5000 10,000

−230 −300

−240 −290

Y. Li et al. / Sensors and Actuators B 223 (2016) 365–371

-2500

(a)

Frenquency: 20Hz~20MHz o

T=500 C,10%O2/N2

-2000

Sensor#3 Sensor#4

-250

20Hz

10% O2/N2

o

T=500 C,10% O2

3.0 vol% H2

-1500

Rs(mV)

-Z"(Ohm)

369

-1000

-300

-350 -500 5KHz

0 0

1000

2000

3000

-400

4000

0.5

1.0

Z'(Ohm) Fig. 7. Impedance spectra in 10% O2 /N2 and 3 vol% hydrogen in the background of 10% O2 /N2 .

2.5

3.0

(b)

-100 -200 -300 -400

Sensor#3 Sensor#4

-500

20

40

o

T=500 C,2.0 vol% H2 60

80

100

120

Time(min) Fig. 8. Reproducibility of the three CdWO4 /YSZ/Pt sensors to different concentrations of H2 from 0.5 vol% to 3 vol% 500 ◦ C in the background of 10 vol% O2 /N2 (a) correlation plots and (b) response curve of the sensor with the hydrogen concentrations at 2 vol%.

participate in the electrochemical redox reactions at TPB according to reaction (6) that contributed to the sensor response. The sensor shows potential to work well under oxygen-rich environments. Fig. 11(a) shows the influence of the relative humidity (RH ∼98%) on the response of the sensor (Sensor1) to hydrogen at 500 ◦ C in 10%O2 background. It observed that the magnitude of the sensor response seems unaffected with the introduction of water vapor (RH ∼98%) as shown in the middle figure in Fig. 11(a). Impedance spectra of the sensor before and after the water vapor introduced are shown in Fig. 11(b) and the impedance within the testing frequency range from 20 Hz to 2 MHz remained almost unchanged which is consistent with those results shown in Fig. 11(a). However, the water vapor clearly has a great influence on the response and recovery speed of the sensor. The sensor showed poor response

5 3.05mv/Dec

0 -3.73mv/Dec

Rs(mV)

3.2.2. Interferences Fig. 9 shows the comparison of the pontentiometric CdWO4 /YSZ/Pt hydrogen sensor (Sensor1) to different possible interfering gases such as CO, NO2 and C3 H8 below 1500 ppm at 500 ◦ C in 10%O2 /N2 [26–34]. The sensor shows some sensitivity to these gases although the sensitivities are slightly lower compared to those to hydrogen. It indicates that the sensor will have a possible issue of cross sensitivities to these gases if the concentrations are below 1500 ppm. However, considering the possible practical applications of the developed sensor for monitoring the hydrogen leakage where the explosive hydrogen limit is high up to 4% mixed in air and these possible, coexisting interfering gases are usually below 1000 ppm, the developed sensor that shows much higher sensitivity to hydrogen from 0.5 vol% to 3 vol% could work well as a selective hydrogen sensor. Fig. 10 shows the influence of oxygen in the background on the response of CdWO4 /YSZ/Pt hydrogen sensor (Sensor1) at 500 ◦ C. When the oxygen concentrations in the background increased from 10% to 20%, the sensor shows a suppressed response. The decrease in response at lower hydrogen concentrations is more and the response varied slightly less at higher hydrogen concentrations above 2.5 vol%. Such decrease in the higher oxygen background could be attributed to the increased non-electrochemical catalytic conversion of hydrogen during it diffused through the oxide electrode. It subsequently resulted to a less hydrogen available to

2.0

0

V(mV)

The sensor (Sensor1) showed typical impedance spectra in both the 10% O2 /N2 background and 3 vol% hydrogen. Each spectrum includes a depressed semicircle at higher frequency range and a straight tail-line within the lower frequency range. The “depressed” semicircle is usually assigned to the bulk electrolyte impedance containing two electrical components such as resistance of the YSZ electrolyte and non-ideal capacitor (i.e. constant phase element (CPE)) while the “tail-line” corresponds to typical Warburge diffusion process within the electrodes. It observed that the part of the “depressed” semicircle of the impedance spectra of the sensor remained almost unchanged while a clearly reduced impedance of the “tail-line” part is observed when the sensor was exposed to 3 vol% hydrogen. It indicated that the gas diffusion process within the electrodes of the sensor would mainly contribute to the generation of the sensor responses [24,25]. Two sensors (Sensors#3and #4) with oxide thickness around 11 ␮m were fabricated and tested simultaneously to examine the sensor to sensor consistency and response repeatability. Fig. 8(a) shows the response curves of both sensors to different concentrations of hydrogen from 0.5 vol% to 3 vol% at 500 ◦ C. The response of the two sensors is very good consistent with each other. Fig. 8(b) indicates that both sensors displayed excellent signal repeatability.

1.5

H2 Conc.(vol%)

-5 o

-10

-15

T=500 C H2 CO C3H 8

-8.9mv/Dec

-10.5mv/Dec

NO2 100

1000 C3H8/NO2/CO/H2 conc. (ppm)

Fig. 9. Comparison of the response of the CdWO4 |YSZ|Pt sensor to the interferents C3 H8 /NO2 /CO at lower concentrations than 1500 ppm at 500 ◦ C.

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0

Table 2 Comparison of hydrogen sensing properties of MWO4 sensing electrodes (M = Zn, Cd, Mn) of MWO4 /YSZ/Pt sensors.

(a)

V(mV)

-100 -200

Opt. temp. (◦ C) Rs (to 960 ppm H2 ) Sensitivity (the slope: 60–960 ppm) Sensitivity (0.5–3% H2 ) Selectivity (60–960 ppm) Upper detection limita Water resistance

0.5

-300

1.0 vol% o T=500 C 10% O2 20% O2

-400 0

1.5

20

40

2.0

60

2.5

3.0

80

100

120

Time(min)

(b)

-100

a

-150

Rs(mV)

-300

o

T=500 C 10% O2 20% O2

-350 -400

10000

20000

30000

H2 Conc.(ppm) Fig. 10. Influence of oxygen to CdWO4 /YSZ/Pt hydrogen sensor at 500 ◦ C in the background of 20 vol% O2 /N2 and 10 vol% O2 /N2 (a) response curve and (b) correlation plots.

Bkgd: Dry

(a) Bkgd: Dry Again

Bkgd: RH~98%

V(mV)

-100 -200

0.5 0.5

0.5 0.5

0.5 0.5

1.0 1.5 2.02.5 3.0

-400

1.0 1.5 2.0 2.5 3.0 o vol%,T=500 C

o

vol%,T=500 C

0

40 80 120

0

40 80 120 Time(min)

1.0 1.5 2.0 2.5 3.0 o vol%,T=500 C

0

40 80 120

-Z"( Ohm)

-3500

(b)

-3000

Frequency:20Hz~2MHz 3.0vol%H2,10%O2

-2500

98%RH O T=500 C

20Hz

-2000 -1500 beforeRH RH afterRH

-1000 -500 2KHz

0 0

MnWO4 [17]

600 16 mV 12 mV/Dec.

500 13 mV 10 mV/Dec.

550 110 mV 75.2 mV/Dec.

0.22 mV/Dec.

182 mV/Dec.

Saturated

Good

Fair

Excellent

3 vol%

3 vol%

Good

Fair

960 ppm, then saturated Poor

The largest hydrogen concentration the sensor could detect.

and recovery speed to different hydrogen concentrations when the RH = 98% water vapor was introduced. After the water vapor was removed from the gas stream, the sensor would recover. Therefore, the water vapor has to be filtered away when the sensor was developed for the hydrogen detection in humid environments.

-181.06mv/Dec

-250

-300

CdWO4 [this work]

-346.46mv/Dec.

-200

0

ZnWO4 [16]

1000

3.2.3. Effect of M on sensing properties of MWO4 /YSZ/Pt sensors To discuss the effect of M in MWO4 on the sensing properties of MWO4 /YSZ/Pt mixed-potential sensors, the hydrogen sensing results including ZnWO4 and MnWO4 sensing electrodes reported previously are summarized in Table 2 [16,17]. Similar to ZnWO4 electrode, CdWO4 could detect higher concentrations of hydrogen up to 3 vol% but has a higher sensitivity than ZnWO4 electrode. In comparison, MnWO4 electrode showed very sensitive to lower hydrogen concentrations within 60–960 ppm but a worse waterresistant ability. Similar to Zn2+ , Cd2+ has the outermost shell of d orbital fully occupied by ten electrons. However, Mn2+ has the outermost d orbital with half empty states. It has proposed that the unoccupied d orbital could enhance the chemical adsorption of hydrogen containing species such as hydrogen, hydrocarbons and alcohols [35]. This might be a reason for the better sensitivity of MnWO4 to hydrogen even at lower concentrations since Mn2+ would help enhancing more adsorptions of hydrogen at TPB. Therefore, it might provide a new strategy in selecting M in MWO4 based oxide electrodes for highly sensitive and selective hydrogen sensors since there are plenty of transitional metals with partly-empty outermost d shell such as Ni, Co, and Cr, etc. More work is in progress to explore these interesting phenomena. 4. Conclusions The YSZ-based CdWO4 |YSZ|Pt sensor has excellent response to different hydrogen concentrations from 0.5 vol% to 3 vol% at 500 ◦ C in the 10%O2 /N2 background. The sensor also indicated excellent repeatability and reproducibility. Polarization curve and impedance spectra of the sensor testified the mixed-potential theory and sensing mechanism. The sensor also showed good selectivity to CO, NO2 and C3 H8 when the concentrations below 1500 ppm. The influence of oxygen and humidity to the sensor were all studied. Acknowledgements

2000 3000 Z'( Ohm)

4000

5000

Fig. 11. Influence of the humidity on the response of the CdWO4 /YSZ/Pt sensor at 500 ◦ C in the background of 10 vol% O2 /N2 (a) response curve and (b) impedance spectra of the sensor.

This study was supported by the National Natural Science Foundation of China (61474012, 61306091, 61176068, 61131004) and Hainan Key Projects in Science and Technology (ZDXM2014097). The financial support from the Fundamental Research Funds for the Central Universities is also acknowledged.

Y. Li et al. / Sensors and Actuators B 223 (2016) 365–371

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Biographies Yue Li is a Ph.D. candidate in the School of Electronic Science and Technology, Dalian University of Technology, China. She is currently working on exploration of compound oxide sensing electrodes for high performance mixed-potential hydrogen sensors for her Ph.D. thesis. Xiaogan Li is an associate professor in the School of Electronic Science and Technology in Dalian University of Technology, Dalian, Liaoning, P. R. China and serves as the Director of the Institute for Sensing Technologies. He received his Ph.D. in Materials Science and Engineering from University of Leeds, U.K. and conducted two-year postdoctoral research in chemical gas sensors at The Ohio State University in USA. His current research interests are in chemical gas sensors. Zhaoyun Tang is a master student in the School of Electronic Science and Technology, Dalian University of Technology, China. He is currently working on exploration of compound oxide sensing electrode for high performance mixed-potential hydrogen sensors for his master’s thesis. Jing Wang is a professor in the School of Electronic Science and Technology, Dalian University of Technology, China. She received her master’s degree from the Department of Electronic Engineering, Jilin University, China in 1981. Her current scientific interests are gas and humidity sensors. Jun Yu obtained her Ph.D. in Electro-Mechanical Engineering from Dalian University of Technology in 2006. She currently is an associate professor in the School of Electronic Science and Technology in Dalian University of Technology. Her research interests include micro-gas sensors and micro-scale thermal properties of thin films. Zhenan Tang received his Ph.D. in Mechanical Engineering from Dalian University of Technology in 1999. He is a professor and the head of School of Electronic Science and Technology in Dalian University of Technology. His current research interests include microsensors, integrated circuits, and microscale heat transfer in semiconductor devices.