Electrochemical Gas Sensors Research Articles

Design and Characterization of a Microwave Transducer for Gas Sensing Applications

Gas sensors have wide applications in several fields, spanning diverse areas such as environmental monitoring, healthcare, defense, and the evaluation of personal and occupational exposure to hazardous chemicals. Different typologies of gas sensors have been proposed over the years, such as optical, electrochemical, and metal oxide gas sensors. In this paper, a relatively new typology of gas sensors is explored: the microwave gas sensor. It consists of a combination of a microwave transducer with a nanostructured sensing material deposited on an interdigitated capacitor (IDC). The device is designed and fabricated on a Rogers substrate (RO4003C) using microstrip technology, and investigated as a microwave transducer over the frequency range from 1 GHz to 6 GHz by measuring the scattering (S) parameters in response to gas adsorption and desorption. The sensing material is based on a nano-powder of barium titanate oxalate with a coating of urea (BaTiO(C2O4)2/CO(NH2)2). It is deposited on the IDC surface by drop coating, thus creating a sensing film. The developed prototype has been tested toward different oxygen (O2) concentrations and exhibits a sensitivity of 28 kHz/%O2. Special attention has been devoted to the measurement process. Besides the canonical short-open-load-thru (SOLT) calibration of the measured S-parameters, a thru-reflect-line (TRL) calibration has been performed in order to get rid of the parasitic electromagnetic (EM) contributions of the board connectors and the feedlines, thus moving the measurement reference planes to the edges of the IDC.

Assessment of ammonia sensors and photoacoustic measurement systems using a gas calibration unit

• Study shows measurement techniques for extensive tests of gas measuring devices. • We focused on suitability of devices for measuring ammonia at different humidities. • Photoacoustic gas analysers and electrochemical gas sensors were testet. • With a gas calibration unit, accuracy, response time and drift behavior can be tested. • Pre-testing at laboratory level is crucial to assess the suitability for field trials. The accurate measurement of environmentally-relevant gases such as ammonia (NH 3 ) is increasingly important, especially in agricultural science. Accurate and continuous measurement of gases is essential for evaluating NH 3 and other gases as resource-based indicators of air quality in forced ventilation barns and for determining emission rates. To better assess measuring devices under different barn conditions (e.g. relative humidity or concentration range) self-conducted tests should be implemented at the laboratory scale before, during and after (experimental) measurements in the barn, independent of the manufacturer's specifications. Therefore, in this study, a gas calibration unit was set up for measurements at the laboratory scale. Moreover, measurement protocols were developed to investigate the accuracy at different gas concentrations and relative humidity levels and the responsiveness to rapid gas concentration changes. Measurements were performed with photoacoustic gas analysers (INNOVA 1412) and transmitters with electrochemical gas sensors (Polytron 8100 and C300). In addition to the presentation of results of the devices used in this case, this study should above all offer suggestions for quality monitoring to test sensors more intensively at the laboratory scale in order to be able to use them in a more targeted manner according to their optimal suitability.

Electrochemical Gas Sensors as Microphones

Electrochemical Gas Sensors as Microphones

Errors in Ambient Gas Concentration Measurement Caused by Acoustic Response of Electrochemical Gas Sensors

Errors in Ambient Gas Concentration Measurement Caused by Acoustic Response of Electrochemical Gas Sensors

Optimized Triple-Phase Boundary in Ysz-Based Electrochemical Gas Sensor Enables to Lower the Operation Temperature for Sensing Ppb Level H2s

Optimized Triple-Phase Boundary in Ysz-Based Electrochemical Gas Sensor Enables to Lower the Operation Temperature for Sensing Ppb Level H2s

Fundamentals and Principles of Solid-State Electrochemical Sensors for High Temperature Gas Detection

The rapid development of science, technology, and engineering in the 21st century has offered a remarkable rise in our living standards. However, at the same time, serious environmental issues have emerged, such as acid rain and the greenhouse effect, which are associated with the ever-increasing need for energy consumption, 85% of which comes from fossil fuels combustion. From this combustion process, except for energy, the main greenhouse gases-carbon dioxide and steam-are produced. Moreover, during industrial processes, many hazardous gases are emitted. For this reason, gas-detecting devices, such as electrochemical gas sensors able to analyze the composition of a target atmosphere in real time, are important for further improving our living quality. Such devices can help address environmental issues and inform us about the presence of dangerous gases. Furthermore, as non-renewable energy sources run out, there is a need for energy saving. By analyzing the composition of combustion emissions of automobiles or industries, combustion processes can be optimized. This review deals with electrochemical gas sensors based on solid oxide electrolytes, which are employed for the detection of hazardous gasses at high temperatures and aggressive environments. The fundamentals, the principle of operation, and the configuration of potentiometric, amperometric, combined (amperometric-potentiometric), and mixed-potential gas sensors are presented. Moreover, the results of previous studies on carbon oxides (COx), nitrogen oxides (NOx), hydrogen (H2), oxygen (O2), ammonia (NH3), and humidity (steam) electrochemical sensors are reported and discussed. Emphasis is given to sensors based on oxygen ion and proton-conducting electrolytes.

Considerations of Thermodynamics and Kinetics for the Effects of Relative Humidity on the Electrolyte in Electrochemical Toxic Gas Sensors.

In this paper, the physical chemistry of the absorption and desorption of water vapor for electrochemical gas sensors with commonly used sulfuric acid as the electrolyte is investigated. Electrochemical gas sensors are being increasingly used for monitoring toxic gases in the environment, and they are, in principle, simple devices, but in practice, their operation is complex. In particular, changes in atmospheric humidity and temperature can have significant effects on the sensor output. A model has been developed for the calculation of sensor weight changes as humidity varies, which are in good agreement with the analysis of experimental results. This then allows for the calculation of the rather more important electrolyte volume variations. Changes in acid molarity and physical characteristics of the electrolyte have also been determined. The effects on working electrode (WE) electrocatalytic activity are discussed, and potential problems with sensors for environmental monitoring are highlighted. In particular, changes in the electroactive area of the WE and, consequently, of the sensor output, and flooding of the WE catalyst aggregates which can lead to problems with electrolyte leakage from sensors are considered.

Selective Recognition of Acetone in Air Against Hydrogen By Impedance Measurement of Two-Terminal Electrochemical Sensors Based on Ionic Liquids

Introduction Ionic Liquids (ILs) are molten salts at room temperature and have both solvent and electrolyte properties. ILs have been actively studied for use in electrochemical gas sensors, thanks to their gas absorption property, electrochemical stability, and low evaporation loss. For example, gas sensors based on the changes in electrical double layer capacitance or in bulk impedance induced by gas dissolution in ionic liquids have been reported [1, 2]. However, there are few reports on the selective recognition of volatile organic compounds such as metabolites in human exhaled air against interfering gases. In this study, we demonstrated selective recognitions of acetone, which is a typical low-molecular-weight metabolic gas in human exhaled air, against hydrogen, which is a metabolite of enterobacteria, by electrochemical impedance measurement using ionic liquids. Method The interdigitated electrodes made of Cr/Pt shown in Fig. 1 were fabricated on a glass substrate. The 3-nm-thick chromium film acting as the adhesion layer was deposited by electron beam evaporation technique, which was followed by the pattern definition and evaporation of 100-nm-thick platinum electrode acting as the main electrical conduction layer. The droplet of 3-μL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, a kind of ionic liquids, was casted onto the electrodes, which completed the fabrication of a two-terminal electrochemical sensor. The impedance measurements were performed under various atmospheres. The following shows the sequence of atmospheres under which the impedance responses of the fabricated sensors were measured. ⅰ) Dry air (3 min) ⅱ) (case Ⅰ) 25-ppm H2 in dry air (3 min)ⅱ) (case Ⅱ) 25-ppm CH3COCH3 in dry air (3 min) ⅲ) dry air (3 min)In the second process ii), case I or case II was chosen from two target gases; hydrogen and acetone. The gas flow rate was kept constant at 200 sccm. Impedance measurements were performed using an LCR meter (Keysight E4980A) with DC bias of 0.0 V and AC amplitude of 10 mV. The measurement frequency was varied from 300 kHz to 20 Hz. Results Fig. 2 shows the measured sensor response at (a) 200 kHz, (b) 30 Hz, and (c) 350 Hz. The sensor response is defined as impedance change ΔZ re due to target molecules normalized by the impedance in dry air Z re(Air), where Z re represents the real part of the impedance. The equation used to extract the sensor response is shown in Fig. 2. We noticed that the sensor response was strongly dependent on the measured frequency. Z re decreases at 200 kHz in both the CH3COCH3 and H2 atmospheres (Fig. 2(a)), whereas it is increased by the target molecules at 30 Hz (Fig. 2(b)). The directions of the impedance changes were the same for both the target molecules. On the other hand, at 350 Hz (Fig. 2(c)), Z re decreases in the CH3COCH3 atmosphere, although it stays constant in the H2 atmosphere. This clearly demonstrates that CH3COCH3 can be selectively detected against H2 by impedance measurement with an appropriate measurement frequency. Discussion and Conclusion We consider that the equivalent circuit of the present system includes an element related with the bulk IL resistance and one related with the electrical double layer at the electrode/IL interfaces. The measurement of the impedance phase angle reveals that the phase angle is close to 0 degrees at 200 kHz and close to -90 degrees at 30 Hz. Therefore, we consider that high-frequency (200 kHz) Z re is affected by the bulk IL resistance change induced by dissolved gaseous molecules and that low-frequency (30 Hz) Z re is influenced by the change in electrical double layer characteristics. On the other hand, at medium frequencies; namely 350 Hz in this study, the gas-induced effects both on bulk IL resistance and on the electric double layer characteristics causing Z re modulation may be compensated with each other, and no sensor response was observed under the H2 atmosphere as shown in Fig. 2(c). We will extract the equivalent circuit model for the system used in this experiment and examine how the parameters of circuit elements change in the H2 and CH3COCH3 atmospheres in comparison with the standard dry air atmosphere. By investigating the frequency dependence of the impact of each parameter change on Z re, we will clarify the mechanism of Z re constancy in the H2 atmosphere at medium frequencies.

Low-Cost Air Quality Measurement System Based on Electrochemical and PM Sensors with Cloud Connection.

This paper presents a portable device for outdoor air quality measurement that provides concentration values for the main pollutants: NO2, NO, CO, O3, PM2.5 and PM10, and other values such as temperature, humidity, location, and date. The device is based on the use of commercial electrochemical gas and optical particle matter sensors with a careful design of the electronics for reducing the electrical noise and increasing the accuracy of the measurements. The result is a low-cost system with IoT technology that connects to the Internet through a GSM module and sends all real-time data to a cloud platform with storage and computational potential. Two identical devices were fabricated and installed on a mobile reference measurement unit and deployed in Badajoz, Spain. The results of a two-month field campaign are presented and published. Data obtained from these measurements were calibrated using linear regression and neural network techniques. Good performance has been achieved for both gaseous pollutants (with a Pearson correlation coefficient of up to 0.97) and PM sensors.

MEMS Gas Sensors: A Review

Micro-electromechanical system (MEMS) gas sensors represent a system that is capable of detecting the existence and concentration of gasses in the atmosphere. These sensors have become incredibly popular and have been widely used in a variety of fields, including the biomedical field. Over the years, several gas sensors have been developed to detect and analyze a wide range of gases. As sensor applications are expected to be further extended in the near future, it is, therefore, essential to review crucial types of gas sensors, their features, challenges, and a range of important applications. This paper presents a comprehensive review of the state-of-the-art gas sensors, which have been developed for gas sensing applications using MEMS technology. The working principle, the structure, the sensitive material and the important characteristics of each of these sensors are discussed. The focus of this review is on applications surrounding the use of electrochemical, optical and acoustic gas sensors for biomedical applications.

Surface conductivity of binary carbonate as a performance-governing parameter of an electrochemical CO2 gas sensor

Surface conductivity of binary carbonate as a performance-governing parameter of an electrochemical CO2 gas sensor

Reducing the Influence of Environmental Factors on Performance of a Diffusion-Based Personal Exposure Kit.

Sensor technology has enabled the development of portable low-cost monitoring kits that might supplement many applications in conventional monitoring stations. Despite the sensitivity of electrochemical gas sensors to environmental change, they are increasingly important in monitoring polluted microenvironments. The performance of a compact diffusion-based Personal Exposure Kit (PEK) was assessed for real-time gaseous pollutant measurement (CO, O3, and NO2) under typical environmental conditions encountered in the subtropical city of Hong Kong. A dynamic baseline tracking method and a range of calibration protocols to address system performance were explored under practical scenarios to assess the performance of the PEK in reducing the impact of rapid changes in the ambient environment in personal exposure assessment applications. The results show that the accuracy and stability of the ppb level gas measurement is enhanced even in heterogeneous environments, thus avoiding the need for data post-processing with mathematical algorithms, such as multi-linear regression. This establishes the potential for use in personal exposure monitoring, which has been difficult in the past, and for reporting more accurate and reliable data in real-time to support personal exposure assessment and portable air quality monitoring applications.

Hierarchical Co3O4 decorated nitrogen-doped graphene oxide nanosheets for energy storage and gas sensing applications

Nano-sized cobalt oxide decorated nitrogen-doped graphene oxide (Co3O4@NGO) composite was produced by a feasible and cost-effective hydrothermal route for electrochemical supercapacitors and gas sensor applications. The composite materials formation was ascertained by Raman spectroscopy, X-ray diffraction, and X-ray photo electron spectroscopy analyses. Field emission scanning electron microscopy (FE-SEM) and field emission transmission electron microscopy (FE-TEM) results explored the controlled nanoscale-sized sheet-like morphology for the prepared composite materials. Electrochemical storage properties were studied by cyclic voltammetry (CV), galvanostatic charge–discharge process (GCD), and electrochemical impedance spectroscopy analyses using three-electrode configuration with 3 M KOH electrolyte. The observed results showed ~466 F/g specific capacitance at a current density of 1 A/g for Co3O4@NGO composite structure with the capacity retention of 96 % after 5000 cycles. Further, the synthesized Co3O4@NGO composite revealed improved detection response, cyclability, and linearity for dimethyl methyl phosphonate vapor gas sensing. The synthesized composite also demonstrated excellent selectivity, stability, sensitivity, and rapid response time.

High-sensitivity CO electrochemical gas sensor based on a superconductive C-loaded CuO-CeO2 nanocomposite sensing material

The surperconductive C-loaded CuO-CeO2 catalyst was fabricated by a high temperature solid phase reaction. The catalyst was annealed at 500 °C for 4 h then was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM), respectively. The results showed that the as-prepared catalyst was consisted of the particles with the size of 20–50 nm. The gas sensing properties of the superconductive C-loaded CuO-CeO2 to CO were investigated at room temperature. In addition, the as-prepared catalyst exhibited superior activity toward CO. The sensor shows characteristics of a practical-use level: range, 0.1–1000 ppm; sensitivity, 192 mV/ppm; response time, 9 s.

Using an RTIL-Based Electrochemical Sensor Platform for Trace Detection of Heptane As Biomarker for Lung Cancer

Lung cancer is one the most common diseases worldwide with more than 1.6 million dead since 2012 [1]. The survival rate for lung cancer is also very low as the disease is generally diagnosed only at an advanced stage when the treatment is not possible as significant damage has been done [2]. Hence, there is a need for an early and timely non-invasive method for the diagnosis of lung cancer. Previous studies have shown the use of endogenously produced volatile organic compounds (VOCs) as target analytes for the detection of lung cancer. Endogenous VOCs are produced in the bloodstream as result of cellular metabolic pathways involved in maintaining homeostasis[3]. Subsequently, the VOCs diffuse into the alveolar air in the blood-gas interface quickly in the lung because of their low solubility in the blood and they are exhaled out of the lung[4]. Therefore, detection of VOC levels in exhaled breath can be used to study the biochemical pathways that have been altered as a result of tumorigenesis. A breathomics device can be developed and used as a tool for non-invasive, sensitive, and specific detection of lung cancer. Moreover, the presence of lung cancer tumor is not masked by other diseases since every disease has its own volatiles fingerprint[5] , thereby making the detection mechanism specific in nature. Electrochemical sensors are gaining widespread interest as they are robust in nature, have faster response time, and specific and sensitive signal response. Electrochemical gas sensors usually involve a two or three electrode setup that causes change in signal as a function of the output current/potential. Electrochemical sensors require an electrode, a transducer, and an electrolyte. A good electrolyte should have high chemical and electrochemical stability, should be stable at elevated temperatures and have high charge transfer ability. Room temperature ionic liquids (RTIL) are being explored as suitable electrolytes for gas sensing applications as they follow all these parameters. There is an increasing interest in the field of non-invasive disease diagnosis, primarily using breath as a sample for monitoring the presence of various metabolites. Breath alkanes such as heptane can be used as a signature target analyte to differentiate the healthy subjects from lung cancer patients. Levels of heptane in breath for a healthy individual are within 50 ppb-200 ppb range. Concentrations greater than 400 ppb are found in breath as a result of altered metabolic pathways. We demonstrate the proof of concept of an electrochemical VOC sensor specific and sensitive for the detection of heptane up to 400 ppb toward developing a handheld breath analyzer for on-site applicability (Scheme 1). We use RTIL as a transducer and the inherent redox properties of these VOCs such as heptane make them a suitable candidate for electrochemical detection. Moreover, RTILs allow easy removal of VOCs by joule heating and prevent fouling of the system, thereby increasing the shelf life of the sensor.For this study, RTIL Triethylsulfonium Bis(trifluoromethanesulfonyl)imide with over 98 percent purity and used as received without any further purification. Trace Source™ disposable permeation tube for Heptane was purchased from KINTEK analytical. Chronoamperometry was used as transduction principle for detection and voltage applied was set within the electrochemical window of the RTIL. An electrochemical sensing platform is developed for the detection of heptane in vapor phase. The sensing platform can be used to monitor the presence of heptane in breath upto 400 ppb (LOD). The sensor showed dose dependent response for the concentrations ranging from 400 ppb to 5 ppm. Scheme 1- Schematic illustration of the sensing strategy used for the development of the electrochemical RTIL-based sensor Figure 1-Chronoamperometry scan was performed at −0.6 V for 60 s RTIL-modified interdigitated electrode (IDE) for the target analytes. Calibrated dose response chronoamperograms were observed for heptane, which varies with concentration. References J.-L. Tan, Z.-X. Yong, C.-K. Liam, J Thorac Dis, 8 (2016) 2772–2783.R.L. Siegel, K.D. Miller, A. Jemal, CA Cancer J Clin, 65 (2015) 5–29.W. Cao, Y. Duan, Clin Chem, 52 (2006) 800–811.A.G. Dent, T.G. Sutedja, P. V Zimmerman, J Thorac Dis, 5 Suppl 5 (2013) S540-50.G. Peng, M. Hakim, Y.Y. Broza, S. Billan, R. Abdah-Bortnyak, A. Kuten, U. Tisch, H. Haick, Br J Cancer, 103 (2010) 542–551. Figure 1

Design & Characterization of Pb Electrode for Sensing Trichloroethylene in Ionic Liquid/Acetonitrile Co-Solvents

Design & Characterization of Pb Electrode for Sensing Trichloroethylene in Ionic Liquid/Acetonitrile Co-Solvents

How to Make Ceramic Gas Sensor Films at Room Temperature - the Powder Aerosol Deposition Method

Introduction Typically, gas sensors are ceramic devices. They are manufactured in ceramic techniques like tape technology and/or conventional thick-film techniques (typically screen-printing with subsequent firing) [1]. During firing, interdiffusion processes occur between substrate and gas sensitive film. At least partly, they may deteriorate the gas sensing properties of the functional oxides. Some materials can even hardly be processed without decomposition. Therefore, room temperature deposition techniques are advantageous. The Powder Aerosol Deposition Method The Powder Aerosol Deposition Method (PAD) bases on the Room Temperature Impact Consolidation (RTIC) as the film densification mechanism. This allows producing dense ceramic films without any high-temperature processes directly from an initial ceramic powder on almost any substrate material. This contribution gives examples for PAD-applications in the field of gas sensing.Although the process is rather simple, the results are very promising. Driven by a pressure difference to a vacuum deposition chamber (evacuated only to rough vacuum), the aerosol is accelerated by a slit nozzle to several hundred m/s. This aerosol jet ejects particles into the deposition chamber. Here, the particles impact on a movable substrate and get fractured into nanometer-sized fragments that are compacted by subsequently impacting particles. Fig. 1 depicts the basic principle. Further process details can be found in the reviews [2], [3], or [4]. Results and Discussion Some examples for PAD-based sensors are surveyed here. Besides conventional conductometric gas sensors based on SnO2 and other metal oxides to detect limited components, applications for temperature independent oxygen sensors are reported, e.g. of SrTi1-x Fe x O3 [5], [6] or Alumina-doped BaFe1-x Ta x O3 (BFATx) [7]. Simultaneous powder aerosol co-deposition of inert and functional oxides to fine-tune the sensing properties may be promising [6].Figure 2 is a measurement protocol at 900 °C of the logarithm of the film conductivity of BFAT30 fabricated by PAD. The material is rapidly and reproducibly responding to stepwise changes of the oxygen partial pressure, pO2. In Figure 3, the log-log plot of the conductivity (log s vs. log pO2) of the sensor between 600 °C and 900 °C in the pO2 range from 0.01 to 1 bar is shown. Neither the sensitivity to oxygen (slope in the log-log-plot) nor the base line resistance changes with temperature. Log s depends linearly on log pO2 between 700 °C and 900 °C, showing a slope of 0.24. An improved formulation contains 1 % alumina (BFATx). Amongst the BFATx samples examined, particularly good properties were found with regard to temperature independency for BFAT25 (BaFe0.74Al0.01Ta0.25O3).Combined resistive and thermoelectric oxygen sensors with almost temperature-independent characteristics of both conductivity and Seebeck coefficient were manufactured using BFAT30 [8].Classical tin-oxide conductometric metal oxide gas sensors were deposited by PAD at room temperature on interdigital electrodes. As expected, they show a pronounced sensitivity to nitrogen oxides at around 300 °C and to hydrogen at around 450 °C [9].With PAD, solid electrolyte electrochemical gas sensors have also been successfully manufactured [10]. A nitrogen oxide sensor using the novel pulsed-polarization method, which attracted much attention recently with YSZ as solid electrolyte, can now be operated at lower temperatures due to the use of a Bismuth-based fast oxide ion conductors (Fig. 4).The powder aerosol deposition method is also suitable to produce thermistors with a negative temperature coefficient of resistance (NTCR) [11]. They may serve as sensitive detectors for pellistors. Conclusions It is possible to manufacture ceramic gas sensor films completely without any high-temperature process and directly from an initial ceramic powder on almost any substrate material. Even temperature sensors can be realized. Future research directions may focus on sensors on polymers or textiles by applying the powder aerosol deposition method.

Platinum–Nickel Bimetallic Nanosphere–Ionic Liquid Interface for Electrochemical Oxygen and Hydrogen Sensing

By exploiting the benefits of bimetallic platinum–nickel (Pt–Ni) alloy nanosphere and an ionic liquid (IL) (i.e., 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BmpyNTf2), an organic–inorganic hybrid interface of IL/Pt–Ni was designed and characterized for electrochemical sensing of oxygen and hydrogen gases for miniaturized electrochemical gas sensor development. The spherical Pt–Ni alloy nanoparticles (NPs) were synthesized through template-free, one-pot solvothermal method. The morphology, crystal structure, and chemical composition of Pt–Ni alloy NPs were thoroughly characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The obtained Pt–Ni alloy NPs were used to fabricate the planar sensor devices and tested for oxygen and hydrogen sensing. The oxygen-sensing performance of the resulting planar electrochemical sensor was investigated over a low concentration range of 500–5000 ppm of O2 at room temperature by using constant potential amperometry. The planar electrochemical sensor device exhibited a high sensitivity to O2 ((3.04 ± 0.18) × 10–5 mA cm–2 ppm–1) compared to commercial Pt/C-based sensor ((2.57 ± 0.22) × 10–5 mA cm–2 ppm–1). The planar electrochemical sensor device also showed good reproducibility and selectivity for oxygen detection during sensing tests. Moreover, the sensor device based on the obtained Pt–Ni alloy NPs was investigated for hydrogen detection with excellent analytical performance in hydrogen sensing. The outstanding gas sensing properties were attributed to unique interface properties and highly efficient catalytic reaction of gas species of oxygen and hydrogen at the interface of IL/Pt–Ni alloy NPs. This work demonstrated that the integration of Pt–Ni alloy NPs with ILs enabled beneficial electrode interface for O2 and H2 gases sensing with high sensitivity, rapid gas response, and superior reproducibility based on a novel planar electrochemical sensor platform.

(Invited) Effects of Noble-Metal Loading Onto Metal-Oxide Electrodes on CO-Sensing Properties and Mechanism of Potentiometric Gas Sensors Utilizing an Anion-Conducting Polymer Electrolyte

IntroductionSome kinds of electrochemical CO sensors using a cation-conducting polymer (e.g., Nafion®) as an electrolyte and carbon-based electrodes loaded with noble metal (mainly Pt) have been presently commercialized as a sensor operated at room temperature, and they practically show attractive CO-sensing properties. On the other hand, many kinds of metal oxides are basically limited to be used as the sensing-electrode materials of the sensors, due to the lack of chemical stability under the strong acidic environment. Recently, anion-conducting polymers (ACP) have been a focus of constant attention, in the field of various electrochemical applications, such as fuel cells and water hydrolysis, because of the large hydroxide-anion conductivity and the relatively long-term stability. We have so far developed various kinds of electrochemical gas sensors utilizing the ACP electrolyte, and we firstly demonstrated that Pt- or Pd-loaded carbon black (CB) was quite promising as a sensing-electrode material to detect H2 [1], CO [2], and CO2 [3]. However, their gas selectivity was relatively poor, and thus we have investigated various metal oxides loaded with noble-metal nanoparticles as a CO-sensing electrode material [4–7]. In this presentation, we will discuss the fundamental CO-sensing properties of their sensors and the CO-sensing mechanism. Experimental n wt% noble-metal (N) nanoparticles were loaded onto metal-oxide (MO) powder by precipitation-deposition (only for Au) or general impregnation (for other noble metals) technique, respectively, and they were heat-treated at elevated temperatures in air or H2 for 1 h. The obtained powder (nN/MO) was mixed with an appropriate amount of ACP solution (AS-4, Tokuyama Corp.) and then the paste obtained was applied on the surface of both sides of an ACP membrane (A201, Tokuyama Corp.) as sensing and counter electrodes by blade coating, and then EC(nN/MO(Tm)) sensors (T: annealing temperature (°C), m: annealing atmosphere (air or H2)) were obtained. The sensor was sandwiched with Au meshes (100 mesh) as a current collector and was set up in a gas-sensing measurement system with two electrode compartments. Electromotive force (E) of all sensors to 500 ppm CO or H2 balanced with synthetic air (O2: 20%, N2: 80%), which flowed over the sensing electrode, was measured at 30°C, while synthetic air was flowed over the counter electrode. Relative humidity (RH) of these flowing gases was controlled to be 0–80% (generally, 57%) at 30°C by bubbling the gases into distilled water. The magnitude of response was defined as a change in E value induced by a sample gas (ΔE SG, SG (sample gas): CO or H2). CO selectivity against H2 was defined as a ratio of CO response to H2 response (ΔE CO/ΔE H 2). Results and Discussion Different combinations of various N (Au, Ir, Ru, Pd, Pt, etc.) and MO (Bi2O3, CeO2, In2O3, TiO2, SnO2, V2O5, etc.) have been estimated as a CO-sensing electrode material. Among them, we confirmed that only three kinds of nN/MO, nAu/In2O3, nAu/SnO2, and nPt/SnO2, were promising candidates as the sensing-electrode material of highly sensitive CO sensors, together with relatively excellent operating stability. Response transients of representative EC(2Au/In2O3(Tm)), EC(2Au/SnO2(Tm)), and EC(2Pt/SnO2(Tm)) sensors to 500 ppm CO and H2 are shown in Fig. 1. Both the EC(2Au/In2O3(400air)) and EC(2Au/SnO2(400air)) sensors showed larger CO response than H2 response, but the CO selectivity against H2 (2.6 for EC(2Au/In2O3(400air)), 2.3 for EC(2Au/SnO2(400air))) was smaller than we expected. The annealing of the 2Au/In2O3 and 2Au/SnO2 at 250°C in H2 decreased the H2 response and thus enhanced the CO selectivity against H2 (28 for EC(2Au/In2O3(250H2)), 35 for EC(2Au/SnO2(250H2))). However, the small H2 response gradually increased and thus the CO selectivity against H2 decreased with their operating time, even though the CO response remained almost unchanged during the same period. On the other hand, the EC(2Pt/SnO2(500air)) sensor showed the largest CO selectivity against H2 (8.3) among all the EC(2N/MO(Tair)) sensors, together with the excellent long-term stability. On the other hand, the heat treatment of 2Pt/SnO2 powder at 250°C in H2 enhanced the responses to both CO and H2 and consequently reduced the CO selectivity against H2 (1.9 for EC(2Pt/SnO2(250H2))). The reasons for such variations in sensing property and the gas-sensing mechanism on the N/MO surface will be discussed in our presentation.

Effect of electrode pores on the performance of CO electrochemical gas sensor, experimental and modeling

The purpose of this paper is to provide a kinetic model for the CO gas electrochemical sensor in which the gas sensor behavior is simulated over time. This simulation was done by writing and solving sensor-equivalent artificial equations. The model showed that the sensitivity of a sensor, as well as its catalytic property, is a function of the input pores diameter, the gas passing membranes thickness, and the volume of the sensor. Different sensors were made with membranes with different porosity and thicknesses, to verify the accuracy of the proposed model, and it was observed that the model had a good match with the test results.