sensors Article

Mixed-Potential Gas Sensors Using an Electrolyte Consisting of Zinc Phosphate Glass and Benzimidazole Takafumi Akamatsu *, Toshio Itoh and Woosuck Shin National Institute of Advanced Industrial Science and Technology (AIST), Inorganic Functional Materials Research Institute, 2266-98, Anagahora, Shimo-Shidami, Moriyama-ku, Nagoya-shi 463-8560, Japan; [email protected] (T.I.); [email protected] (W.S.) * Correspondence: [email protected]; Tel.: +81-52-736-7602 Academic Editor: W. Rudolf Seitz Received: 16 November 2016; Accepted: 3 January 2017; Published: 5 January 2017

Abstract: Mixed-potential gas sensors with a proton conductor consisting of zinc metaphosphate glass and benzimidazole were fabricated for the detection of hydrogen produced by intestinal bacteria in dry and humid air. The gas sensor consisting of an alumina substrate with platinum and gold electrodes showed good response to different hydrogen concentrations from 250 parts per million (ppm) to 25,000 ppm in dry and humid air at 100–130 ◦ C. The sensor response varied linearly with the hydrogen and carbon monoxide concentrations due to mass transport limitations. The sensor responses to hydrogen gas (e.g., −0.613 mV to 1000 ppm H2 ) was higher than those to carbon monoxide gas (e.g., −0.128 mV to 1000 ppm CO) at 120 ◦ C under atmosphere with the same level of humidity as expired air. Keywords: gas sensor; mixed-potential; zinc phosphate glass; hydrogen

1. Introduction Expired air contains over 100 different gases in a humid atmosphere at low concentrations, ranging from sub parts per billion (ppb) to parts per million (ppm), including hydrogen (H2 ), carbon monoxide (CO), and various volatile organic compounds (VOCs) [1–6]. Expired air can provide information that is useful for monitoring human health. The concentration of H2 in expired air has been used to monitor microbial metabolism in the colon [1–4], since H2 is produced by intestinal bacteria. High-performance gas sensors provide a simple and non-invasive method to monitor expired air, and are therefore one of the most promising approaches for the early detection of diseases. Mixed-potential gas sensors using yttria-stabilized zirconia (YSZ) electrolyte have been investigated for the detection of H2 , CO, nitric oxide, ethanol, and methane [7–11]. These gas sensors must be operated at temperatures above 500 ◦ C to provide both satisfactory response rates against target gases and good gas selectivity. These gas sensors require a heater to maintain the operating temperature above 500 ◦ C, which results in high power consumption. Mixed-potential gas sensors that use electrolytes with high proton conductivity—such as Nafion, zirconium phosphate, and antimonic acid—have been reported to function at reduced operating temperatures [12–15]. However, these electrolytes must be impregnated with water to achieve high proton conductivity, which necessitates the inclusion of a complicated humidity control system to allow these sensors to operate at room temperature and under high humidity. Moreover, these gas sensors show low selectivity for H2 against CO. One approach to overcoming this challenge is to use anhydrous proton conductors at an intermediate temperature between 100 and 300 ◦ C. However, to the best

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However, to the best of our knowledge, no mixed-potential gas sensors using a high-proton-conductivity electrolyte at intermediate temperatures have been reported (Table 1). 2 of 8 Sensors 2017, 17, 97 Oine et al. and Kato et al. have successfully prepared an anhydrous electrolyte that demonstrates high proton conductivity at temperatures between 100 and 200 °C by the reaction of of our knowledge, no mixed-potential gas sensors using a high-proton-conductivity electrolyte at zinc metaphosphate glass with benzimidazole [16,17]. This material has been investigated as an intermediate temperatures have been reported (Table 1). electrolyte in electrochemical devices that operate at intermediate temperatures, such as fuel cells. al. and Kato al. have successfully preparedhas an anhydrous electrolyte demonstrates Thus,Oine we etsuggest that etthis anhydrous electrolyte the potential to bethat applied as a ◦ C by the reaction of zinc high proton conductivity at temperatures between 100 and 200 mixed-potential gas sensor. metaphosphate glass benzimidazole [16,17]. This has been investigated as an electrolyte In this study, we with prepared a mixed-potential gas material sensor using an electrolyte consisting of zinc in electrochemical devices that operate at intermediate temperatures, such as fuel cells. metaphosphate glass and benzimidazole, and the sensor’s response to H2 gas in both humidThus, and we air suggest that this anhydrous has of thethe potential be applied as in a mixed-potential dry was investigated. An initialelectrolyte investigation sensor’storesponse to CO humid air was gas conducted sensor. also to determine its selectivity for H2 against other gases. In this study, we prepared a mixed-potential gas sensor using an electrolyte consisting of zinc metaphosphate glassexamples and benzimidazole, and the to H2 gas in both humid and dry Table 1. Typical of mixed-potential gassensor’s sensors response for H2 detection. YSZ: yttria-stabilized air was investigated. An initial investigation of the sensor’s response to CO in humid air was also zirconia. conducted to determine its selectivity for H2 against other gases. Sensor Structure Sensing Table 1. Typical examples of mixed-potential YSZ: yttria-stabilized zirconia. References H2 Concentration Sensing Reference gas sensors for H2 detection. Temperature Sensor Materials Electrode Electrode Sensor Structure

YSZ

Sensor Materials

ZnO

Sensing Electrode

Sb 2O5∙4H2O YSZ Sb2 O5 ·4H2 O Zr(HPO4)2∙nH2O Zr(HPO4 )2 ·nH2 O Sn0.9 In0.9 P20.1 O7P2O7 0.1In Sn

Pt ZnO Pt Pt Pt Pt/C Pt/C

Sensing H2 Concentration 400–600 °CTemperature50–500 ppm

Pt

Reference Electrode

Au Ag Pt

Pt Au Ag Pt

Room temperature ppm 400–600 ◦ C 200–10,000 50–500 ppm Room

Room temperature 200–10,000 ppm temperature 300–10,000 ppm Room temperature 300–10,000 ppm 100–30,000 30 °C 30 ◦ C 100–30,000 ppmppm

References [7]

[12] [7] [12] [14] [14]

[15] [15]

Materialsand andMethods Methods 2.2.Materials glasswith withaanominal nominalmolar molarratio ratioof of1:1 1:1ZnO:P ZnO:P22OO5 5was wasprepared preparedby bymelting meltingaabatch batchmixture mixtureof of AAglass ◦C the commercially available chemicals ZnO and and H3 PO in a4 platinum crucible at 1100 at the availablereagent-grade reagent-grade chemicals ZnO H43PO in a platinum crucible in air°C forin30 Themin. meltThe wasmelt poured an iron and plate quenched by iron pressing make glass 1100 airmin. for 30 wasonto poured ontoplate an iron and quenched by irontopressing to flakes.glass Theflakes. glass flakes wereflakes milledwere using an alumina mortar andmortar pestle to below 100 in diameter. make The glass milled using an alumina and pestle toµm below 100 μm ◦ C for 12 h to The glass powder waspowder mixed was withmixed benzimidazole, and the mixture was heatedwas at 170 in diameter. The glass with benzimidazole, and the mixture heated at 170 °C make an electrolyte. The weight ratio of the glass powder and benzimidazole was 1:3. Details the for 12 h to make an electrolyte. The weight ratio of the glass powder and benzimidazole wasof1:3. procedures electrolyte properties have been previously reported [17].reported [17]. Details of theand procedures and electrolyte properties have been previously Thesensor sensorelement elementwe weconstructed constructedisisshown shownschematically schematicallyand andasasaaphotograph photographininFigure Figure1.1. The Theelectrolyte electrolytewas wasdeposited depositedon onan analumina aluminasubstrate substratewith withplatinum platinum(Pt) (Pt)and andgold gold(Au) (Au)electrodes, electrodes, The withboth boththe theelectrode electrodegap gapand andelectrode electrodewidth widthbeing being11mm. mm.Finally, Finally,the theelectrolyte-coated electrolyte-coatedsubstrate substrate with was heated from behind by placing the assembly on a hotplate, resulting in the formation of an an was heated from behind by placing the assembly on a hotplate, resulting in the formation of electrolytemembrane membraneon onthe thesubstrate. substrate. electrolyte

Figure Figure1.1.The Thestructure structureofofthe thesensor: sensor:(a) (a)schematic schematicillustration; illustration;and and(b) (b)optical opticalimage. image.

The sensor element was placed in a test chamber and heated to 80–130 °C in an electrical tube The sensor element was placed in a test chamber and heated to 80–130 ◦ C in an electrical tube furnace. Synthetic dry air was introduced into the chamber for 20 min, and a gas mixture of H2 or furnace. Synthetic dry air was introduced into the chamber for 20 min, and a gas mixture of H2 or CO CO in synthetic dry air was then injected for 20 min. The gas mixture flow rate was 200 mL/min. in synthetic dry air was then injected for 20 min. The gas mixture flow rate was 200 mL/min. Synthetic Synthetic dry air and H2 or CO in synthetic air were prepared by mixing 99.99% N2 gas, dry air and H2 or CO in synthetic air were prepared by mixing 99.99% N2 gas, N2 -balanced 5% H2 gas, N2 -balanced 5% CO gas, and 99.5% O2 gas. The O2 content of synthetic dry air and that of H2 or CO in synthetic dry air were both 20 vol %. The H2 or CO gas concentrations in synthetic dry air were

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N2-balanced 5% H2 gas, N2-balanced 5% CO gas, and 99.5% O2 gas. The O2 content of synthetic dry or CO in synthetic dry air were both 20 vol %. The H2 or CO gas concentrations 3 ofin 8 synthetic dry air were controlled at values of 250, 500, 1000, 2500, 5000, 10,000, and 25,000 ppm. Synthetic humid air was prepared by passing air through ion-exchanged water at 25 °C before controlled values of 250, 500, 1000, 2500, 5000, 10,000, and 25,000 ppm. Synthetic humid air was flowing to at a test chamber. ◦ C before flowing to a test chamber. prepared by passing air through ion-exchanged water at 25 The potential difference (electromotive force, EMF) between Pt and Au electrodes of the sensor The was potential difference force,Logger EMF) between Pt and Au electrodes of the Japan). sensor element recorded using a(electromotive data logger (midi GL220; Graphtec Corp., Yokohama, element was recorded using a data logger (midi Logger GL220; Graphtec Corp., Yokohama, Japan). During the potential difference measurements, the Pt electrode was always connected to the positive During difference the Ptelement electrode always connected to are the denoted positive terminalthe of potential the data logger. Themeasurements, EMFs of the sensor in was the air and gas mixtures terminal data logger.The Thesensor EMFs response of the sensor in the are denoted as as Va andofVgthe , respectively. (ΔV)element is defined as air ΔVand = Vagas − Vgmixtures . Va and Vg , respectively. The sensor response (∆V) is defined as ∆V = Va − Vg . air and that Sensors 2017, 17,of 97H2

3. Results and Discussion 3. Results and Discussion Figure 2 shows that upon exposure to 25,000 ppm H2 in humid and dry air at 80 °C, 100 °C, Figure 2 shows that upon exposure to 25,000 ppm H2 in humid and dry air at 80 ◦ C, 100 ◦ C, 120 °C, and 130 °C, the sensor’s response increased with increasing operating temperature. Miura et 120 ◦ C, and 130 ◦ C, the sensor’s response increased with increasing operating temperature. Miura et al. al. described the potential response upon exposure to hydrogen in humid gas of a solid-state sensor described the potential response upon exposure to hydrogen in humid gas of a solid-state sensor that used a proton-conductor [12]. The EMF depends on the electrochemical oxidation and that used a proton-conductor [12]. The EMF depends on the electrochemical oxidation and reduction reduction reactions occurring at the Pt and Au electrodes: reactions occurring at the Pt and Au electrodes: H2 → 2H+ + 2e− (1) + − + 2e (1) H2 → 2H (1/2)O2 + 2H+ + 2e− → H2O (2) +

−

(1/2)O + 2H +transfer 2e → H (2) A mixed potential is established when the2electron rates 2 O of the hydrogen oxidation and the oxygen reduction are same. Since the kinetics of the sensor’s Pt and Au electrodes are different from A mixed potential is established when the electron transfer rates of the hydrogen oxidation and the those of the oxidation of H2 to water, the mixed-potential generated at each electrode varies, and oxygen reduction are same. Since the kinetics of the sensor’s Pt and Au electrodes are different from thus ΔV varies in accordance with the H2 gas concentration. We reported that the sensor response to those of the oxidation of H2 to water, the mixed-potential generated at each electrode varies, and thus H2 in synthetic dry air at 150 °C is higher than that at 130 °C [18]. However, in this paper, when the ∆V varies in accordance with the H2 gas concentration. We reported that the sensor response to H2 sensor response to H2 was◦investigated at 140 °C, the EMF was unstable and gradually decreased in synthetic dry air at 150 C is higher than that at 130 ◦ C [18]. However, in this paper, when the over the measuring time, and the sensor showed no response to H2 gas (not pictured). This behavior sensor response to H2 was investigated at 140 ◦ C, the EMF was unstable and gradually decreased may result from changes in the electrochemical properties of the electrolyte due to volatilization of over the measuring time, and the sensor showed no response to H2 gas (not pictured). This behavior the benzimidazole in the electrolyte, because the flash point of benzimidazole was 143 °C [19]. may result from changes in the electrochemical properties of the electrolyte due to volatilization of the Therefore, the optimum operating temperature of the sensor is below 140 °C. In general, the sensor benzimidazole in the electrolyte, because the flash point of benzimidazole was 143 ◦ C [19]. Therefore, responses are similar at each temperature. At 120 °C, the sensor response in dry air is higher, the optimum operating temperature of the sensor is below 140 ◦ C. In general, the sensor responses whereas for the other temperatures, the sensor response in humid air is higher. The electrolyte are similar at each temperature. At 120 ◦ C, the sensor response in dry air is higher, whereas for the derived from zinc metaphosphate glass and benzimidazole is an anhydrous proton-conducting other temperatures, the sensor response in humid air is higher. The electrolyte derived from zinc material, and has good water durability [16,17]. Therefore, the electrical properties of the electrolyte metaphosphate glass and benzimidazole is an anhydrous proton-conducting material, and has good were not changed by water vapor, and the sensor responses using the electrolyte was considered to water durability [16,17]. Therefore, the electrical properties of the electrolyte were not changed by be seen even in humid air. However, the difference between the sensor responses to H2 in dry air water vapor, and the sensor responses using the electrolyte was considered to be seen even in humid air. and humid air was significant. In future study, we will investigate the cycle characteristics and However, the difference between the sensor responses to H2 in dry air and humid air was significant. error evaluation of the sensor response. In future study, we will investigate the cycle characteristics and error evaluation of the sensor response.

Figure 2. Cont.

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◦ C; (b) 100 ◦ C; (c) 120 ◦ C; Figure Figure 2. 2. Sensor Sensor responses responses to to 25,000 25,000 ppm ppm H H22 in humid humid and and dry dry air air at at (a) (a) 80 80 °C; (b) 100 °C; (c) 120 °C; ◦ and C. and (d) (d) 130 130 °C.

Figure 33 shows shows the the sensor sensor responses responses to to H H22 concentrations concentrations ranging ranging from from 250 250 ppm ppm to to 25,000 25,000 ppm ppm Figure ◦ in humid humid air airFigure at 120 120 °C. These results show that the ∆V ΔV ofdry the sensor began to°C; decrease rapidly on on 2. Sensor responses to 25,000 ppm that H2 in the humid and airsensor at (a) 80 °C; (b) 100 (c) 120 °C;rapidly in at C. These results show of the began to decrease and (d) 130 °C. exposure to H 2 , and the response was stronger for higher concentrations of H 2 . Figure 2 shows that exposure to H2 , and the response was stronger for higher concentrations of H2 . Figure 2 shows that the the difference between the sensor responses to dry H2 in and air humid was largestage. at this difference between the sensor responses to H in airdry andair humid was air large at this In stage. order Figure 3 shows the sensor responses 2to H2 concentrations ranging from 250 ppm to 25,000 ppm In detect orderinH tohumid detect H 2 with high accuracy, error evaluation of the sensor response under different to with high accuracy, error evaluation of the sensor response under different humidity 2 air at 120 °C. These results show that the ΔV of the sensor began to decrease rapidly on humidity conditions investigated. Thefor model designed by of Miura et aal. suggests conditions must be The model designed by Miura et al. suggests linear relationship exposure to investigated. H2, must and thebe response was stronger higher concentrations H2. that Figure 2 shows that that a linear between relationship exists logarithm ofand the∆V, 2assuming concentration and atΔV, the difference betweenbetween the to H2 in dry airHand humid aira was large this assuming stage.for thea exists the logarithm of sensor the Hthe concentration Tafel-like equation 2responses In order to detect H 2 with high accuracy, error evaluation of the sensor response under different Tafel-like equation for the polarization behavior of the oxidation and reduction reactions the polarization behavior of the oxidation and reduction reactions at the electrode [12]. The ∆V canatthus humidity conditions must be be expressed investigated.inThe model designed by Miura et al. suggests that a electrode [12]. The ΔV can thus the form be expressed in the form: linear relationship exists between the logarithm of the H2 concentration and ΔV, assuming a ∆V ==aa++bblog CCH2 ΔV logthe H2 Tafel-like equation for the polarization behavior of oxidation and reduction reactions at the electrode [12]. The ΔV can thus be expressed in the form

(3) (3)

between the sensor response and the logarithm of the H2 concentration. The least squares regression ΔV= =13.1 13.1 −4.7 4.7log logCCH2 H2,, R analysis of the sensor can be expressed as−follows: ∆V R == 0.819 0.819

(4) (4)

where where aa and and bbare areconstants, constants,and andCCH2 H2 is is the the concentration concentration of of H H22.. Figure Figure 44 shows shows the the relationship relationship ΔV = a + b log C H2 (3) between the sensor response and the logarithm of the H concentration. The least squares 2 between the sensor response and the logarithm of the H2 concentration. The least squares regression regression analysis of can be and b are and Cas H2 follows: is the concentration of H2. Figure 4 shows the relationship analysis where of the theasensor sensor canconstants, be expressed expressed as follows:

CH2correlation , R = 0.819 coefficient. The sensitivity (4) of the where the CH2 is the concentration ofΔV H2=, 13.1 and− R4.7islog the where the CH2 is the concentration of H2 , and R is the correlation coefficient. The sensitivity of the sensor was −4.7 However, no good relationship between the logarithm where the mV/decade. CH2 is the concentration of H 2, and R islinear the correlation coefficient. The sensitivity of the of H2 sensor was −4.7 mV/decade. However, no good linear relationship between the logarithm of H2 concentration ΔV of the sensor was obviously sensor and was −4.7 mV/decade. However, no good found. linear relationship between the logarithm of H2 concentration and ∆Vand of ΔV theofsensor waswas obviously concentration the sensor obviouslyfound. found.

Figure 3. Sensor response to different concentrations of H2 in humid air at 120 °C. ◦ Figure 3. Sensor response to different concentrations of H2 in humid air at 120 C.

Figure 3. Sensor response to different concentrations of H2 in humid air at 120 °C.

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(mV) V V (mV)

0 0 -5 -5

-10 -10 100

o

120 C (humid) o

120 C (humid)

1,000

10,000

100,000

(ppm)100,000 100H2 concentration 1,000 10,000 H2 concentration (ppm)

Figure and thethe loglog of of thethe H2Hconcentration in humid air air at Figure4.4.The Therelationship relationshipbetween betweensensor sensorresponse response and 2 concentration in humid ◦ 120 C.°C. TheThe solid lineline shows the the least-squares linear fit. fit. at 120 shows least-squares linear Figure 4. The solid relationship between sensor response and the log of the H2 concentration in humid air at 120 °C. The solid line shows the least-squares linear fit.

According to toGarzon Garzon et etal., al.,ififthe theconcentration concentrationof ofthe thesensed sensedgas gas(H (H22)) isismuch muchlower lower than thanthe the According concentration ofO O22,Garzon , aa linear linearet relationship betweenthe theH H concentration and existsbecause because ofthe the concentration of relationship between of According to al., if the concentration of22 concentration the sensed gasand (H2∆V )ΔV is exists much lower than the diffusional mass transport limitation [8]. In this case, ΔV can be expressed as diffusional mass transport limitation [8]. In this case, ∆V can be expressed as concentration of O2, a linear relationship between the H2 concentration and ΔV exists because of the diffusional mass transport limitation [8]. InΔV this= case, a + K ΔV CH2 can be expressed as (5) ∆V = a + K CH2 (5) ΔV = a + K C H2 where a and K are constants. Figure 5 shows the relationship between sensor response and the (5) H2 where a and K are constants. Figure was 5 shows the relationship between sensor regression response and the Hof concentration. The sensor response adequately linear. The least squares analysis 2 where a and K are constants. Figure 5 shows the relationship between sensor response and the H 2 concentration. The sensor response was adequately linear. The least squares regression analysis of the the sensor can be expressed as follows: concentration. The sensor response was adequately linear. The least squares regression analysis of sensor can be expressed as follows: the sensor can be expressed as follows: ΔV = 0.021 − 0.00046 CH2, R = 0.996 (6) =H0.021 =2, 0.021 −0.00046 0.00046 CH2, ,RR==0.996 0.996 H2 where CH2 is the concentration∆V ofΔV and− R is theCcorrelation coefficient. The sensitivity of (6) the sensor −0.00046 mV/ppm. Rof for Equation (6) was higher than that for Equation (4). This whereC was CH2 isthe the concentration 2, and R is the correlation coefficient. The sensitivity of the where concentration of H2 , H and R2 may is thebe correlation coefficient. The sensitivity of the sensor H2 is suggested that the oxidation reaction of H mass transport-limited. sensor was −0.00046 R for (6) Equation (6) was higher than that(4). forThis Equation (4). that This was −0.00046 mV/ppm.mV/ppm. R for Equation was higher than that for Equation suggested suggested that the oxidation reaction of H 2 may be mass transport-limited. the oxidation reaction of H2 may be mass transport-limited.

(mV) V V (mV)

0 0 -5 -5

-10

o

● 120 C (Humid)

-10 0

o

● 120 C (Humid)

10,000

20,000

30,000

H2 concentration (ppm)30,000 10,000 20,000 H2 concentration (ppm) Figure 5. The relationship between the sensor response and the H2 concentration in humid air at 0

120 °C. The solid line showsbetween the least-squares linear fit. Figure The relationship thesensor sensor response andthe theHH 2 concentration in humid air at Figure 5.5.The relationship between the response and 2 concentration in humid air at ◦ 120 °C. The solid line shows the least-squares linear fit. 120 C. The solid line shows the least-squares linear fit.

Figure 6 shows the sensor’s responses to CO in humid air at 120 °C and at concentrations ranging from6 shows 250 to 25,000 ppm.responses When the sensor was exposed to these conditions, its ΔV Figure sensor’s toin CO in humid air ◦at 120 °Cconcentrations and at concentrations Figure 6 shows thethe sensor’s responses to CO humid air at 120 C and at ranging decreased rapidly, and the response decreased as thewas CO concentration increased. In CO ranging 250ppm. to 25,000 ppm. When sensor to its these conditions, its gas ΔV from 250 tofrom 25,000 When the sensor wasthe exposed to theseexposed conditions, ∆V decreased rapidly, sensing, the oxidation reaction of CO can take place simultaneously at both the Pt and decreased rapidly, and the response decreased as the CO concentration increased. In CO gas and the response decreased as the CO concentration increased. In CO gas sensing, the oxidation Au electrodes: sensing, oxidation reaction of CO can take reaction of the CO can take place simultaneously at both theplace Pt andsimultaneously Au electrodes: at both the Pt and Au electrodes: CO + H2O → CO2 + 2H+ + 2e− (7) + − COCO + H+2H O2→ COCO (7) + +2e 2 +2 2H O→ + 2H+ 2e− (7)

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◦ C. Figure6.6.Sensor Sensorresponses responsestotodifferent different concentrations CO humid 120 °C. Figure concentrations ofof CO inin humid airair atat 120 Figure 6. Sensor responses to different concentrations of CO in humid air at 120 °C.

Figure 7 shows the relationship between the sensor response and the logarithm of the CO Figure 7 shows the relationship between the sensor response and the logarithm of the CO Figure 7 shows the relationship relationship between between the the sensor sensor response of the CO concentration, and the response and and the the logarithm CO concentration in concentration, and the relationship between the sensor response and the CO concentration in humid concentration, and the relationship between the sensor response and the CO concentration in humid air at 120 °C. The equations of the relationship between the sensor response and CO air at 120 ◦ C. The equations of the relationship between the sensor response and CO concentration humid air at 120 The equations of the relationship between the sensor response and CO concentration were°C. determined from Equations (3) and (5), respectively: were determined from Equations (3) and (5), respectively: concentration were determined from Equations (3) and (5), respectively: ΔV = 0.376 − 0.168 log CCO, R = 0.939 (8) ∆VΔV = 0.376 −−0.168 = 0.376 0.168log logCC CO,,R R== 0.939 0.939 (8) CO ΔV = 0.101 − 1.015 × 10−6 CCO, R = 0.977 (9) ΔV = 0.101 − 1.015 × 10 (9) −−6 6 CCO, R = 0.977 ∆V =of0.101 − 1.015 10 correlation CCO , R = coefficient. 0.977 (9) where CCO is the concentration CO and R is×the Since R is slightly larger where CCO is the concentration of CO and R is the correlation Since R is slightly larger for Equation (9)concentration than for Equation oxidation reaction ofcoefficient. CO may be mass-transport-limited, where CCO is the of CO(8), andthe R is the correlation coefficient. Since R is slightly larger for for Equation (9) than for Equation (8), the oxidation reaction of CO may be mass-transport-limited, as seen for the Hfor 2 gas sensing. Equation (9) than Equation (8), the oxidation reaction of CO may be mass-transport-limited, as seen as seen for the H2 gas sensing. for the H2 gas sensing.

Figure 7. (a) The relationship between the sensor response and the logarithm of the CO Figure The relationship between the response sensor response and and theof logarithm of the CO Figure 7. 7. (a) (a) Theand relationship between the between sensor and response the logarithm the COconcentration concentration concentration (b) the relationship the sensor the CO in ◦ concentration and (b) the relationship between the sensor response and the CO concentration in and (b) the between the sensor and the linear CO concentration in humid air at 120 C. humid airrelationship at 120 °C. The solid lines show response the least-squares fit. humid at 120 °C.the Theleast-squares solid lines show The solidair lines show linearthe fit.least-squares linear fit.

The sensor responses to 1000 ppm H2 and 1000 ppm CO were determined to be −0.613 mV and ThemV, sensor responses in to 1000 ppm 1000 These ppm CO were determined be −0.613 mV and −0.128 respectively, humid airH at22and 120 are lowertothan seen for The sensor responses to 1000 ppm H and °C. 1000 ppm responses CO were determined to bethose −0.613 mV −0.128 mV, respectively, in humid air at 120 °C. These responses are lower than those seen for ◦ C. Theseoperating mixed-potential gas sensors in that used air proton conductor around and −0.128 mV, respectively, humid at 120 responsesatare lowerroom than temperature those seen mixed-potential gasppm sensors that−120 used proton conductor operating at around temperature (−190 mV for 1000 H 2 and mV for 1000 ppm CO) [12]. The value of H 2room selectivity against for mixed-potential gas sensors that used proton conductor operating at around room temperature (−190 mV for 1000determined ppm H2 andusing −120 mV for 1000 ppm CO) [12]. TheCOvalue of HΔV 2 selectivity against CO (S H2/CO ) was the equation S H2/CO = ΔV H2/ΔV , where H2 is the sensor’s (−190 mV for 1000 ppm H2 and −120 mV for 1000 ppm CO) [12]. The value of H2 selectivity against CO (SH2/COat ) was equation SH2/CO = at ΔV1000 H2/ΔV CO, where ΔVSH2 is of theour sensor’s response 1000 determined ppm H2, andusing ΔVCO the is the sensor response ppm CO. The H2/CO sensor response at 1000 ppm H2, and ΔVCO is the sensor response at 1000 ppm CO. The SH2/CO of our sensor

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CO (SH2/CO ) was determined using the equation SH2/CO = ∆V H2 /∆VCO , where ∆V H2 is the sensor’s response at 1000 ppm H2 , and ∆VCO is the sensor response at 1000 ppm CO. The SH2/CO of our sensor was 4.8, and that of the sensor using a proton conductor operating at around room temperature was 1.6 [12]. Semiconductor gas sensors using n-type SnO2 have been investigated for several decades for their high response to inflammable gases, such as H2 , CO, CH4 , and C2 H5 OH [20,21]. However, their low selectivity for H2 over CO was regarded as a problem to be solved. Moon et al. focused on the use of a CuO-doped SnO2 -ZnO composite material to improve the gas selectivity, and reported a selectivity of ~4 for H2 (200 ppm) over CO (200 ppm) at 350 ◦ C [21]. The sensor reported herein—which used an electrolyte consisting of zinc metaphosphate glass and benzimidazole—showed a higher selectivity for H2 against CO than the sensor reported by Moon et al. However, our sensor’s response to H2 is currently inferior, thus optimization of the sensor design is required to achieve a greater response. Further investigations of the sensor’s response to other interfering species (such as CH4 and VOCs) will be conducted in the future. 4. Conclusions In this study, we have investigated the responses of a mixed-potential gas sensor that used a proton conductor composed of zinc metaphosphate glass and benzimidazole for the detection of H2 (produced by intestinal bacteria) and CO gas (an interference in expired air). The gas sensor showed good sensor response to H2 in the concentration range 250–25,000 ppm under atmosphere having the same level of humidity as expired air at 120 ◦ C. The mixed-potential gas sensor without a heater can detect gases under the atmosphere at temperatures in the range 100–130 ◦ C. The sensor response varied linearly with the hydrogen and carbon monoxide concentrations, as expected under mass-transport-limited conditions. The gas sensor showed high gas selectivity for H2 against CO (SH2/CO = 4.8). Acknowledgments: This work was supported in part by Naito Research Grant. Author Contributions: T.A. performed the experiments and prepared the manuscript. T.I. and W.S. helped in designing the experiments and provided valuable guidance. All authors discussed the results of the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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