Flexible Ammonia Detection with Voltammetric Microsensors

Ammonia (NH3) is a common reagent for and byproduct of numerous processes. Nearly 80% of all ammonia produced in the U.S. is used for agricultural applications including liquid fertilizer, protein for livestock feeds, and antifungal agents for fruit and corn. The petroleum industry uses ammonia to neutralize acids in crude oil; the mining industry, to help extract copper, nickel, and molybdenum from ore. Other applications include use in metal nitriding and annealing processes and as a reagent for manufacturing nitric acid, dyes, commercial household cleaners, detergents, pharmaceuticals, vitamins, and synthetic textiles.



Ammonia Detection

The average human nose can detect ammonia concentrations on the order of 50 ppm or less; 100 ppm significantly irritates human olfactory passages. Because the odor of ammonia is so potent, concentrations well below 300 ppm (the concentration immediately dangerous to life and health) can be detected before harm ensues.

Common ammonia detection instruments and technologies include UV absorption; NIR spectroscopy; color-changing cards capable of detecting concentrations in the 1–5 ppm range; and metal oxide, electrochemical, and polymer film sensors. UV sensors typically require high sample volumes and long warm-up periods for accurate measurements. NIR sensors are expensive; their cost limits the placement and coverage area of networks. Metal oxide sensors typically detect at 30 ppm thresholds and require extremely accurate temperature control for acceptable speciation. Electrochemical sensors require frequent electrolyte replacement, need stable oxygen concentrations for operation, and respond to interferent gases such as carbon monoxide and chlorine. Polymer-resistive devices are available but are still maturing and suffer from memory effects.

Continued demand for an inexpensive, rugged, sensitive ammonia detector capable of tolerating various gaseous interferents has encouraged the development of voltammetric cermet microsensors. These experimental devices are fabricated from thick films and extremely durable materials. The measurement technique is flexible for various gases and gas concentrations, and can measure both the target species and the interferents in a sample. An embedded microcontroller provides device intelligence including automated new sample learning, remote communication, and smart sensor standards such as the IEEE 1451.4 smart transducer interface.

Cermet Sensing Element

The sensing elements in the prototypes being developed at Argonne National Laboratory (ANL) consist of thick monolithic metal oxide and metallic films screen-printed on an alumina (Al2O3) substrate. The layered arrangement of the films forms an electrochemical cell, with a nickel oxide (Ni-NiO) film providing a source of oxygen ions for operation in extremely oxygen-deficient environments and a catalyst for ammonia reactions. Sensing electrode materials have included platinum, palladium, ruthenium, gold, and iron oxide films. Specificially selected film materials enhance some chemical reactions and inhibit others, tailoring the response of devices. Miniature arrays of various sensors further improve species identification.

Individual electrodes are 10–15 µm thick and sandwich a 25–30 µm solid electrolyte. Several electrolytes, designed to enhance specific chemical reaction, are used in fabricating the sensors. An yttria-stabilized zirconium oxide (YSZ) electrolyte has been ideal for small hydrocarbons, carbon monoxide, and ammonia. A YSZ electrolyte combined with tungsten bismuth oxide (WBO) suppresses many hydrocarbon reactions and provides reaction sites for carbon dioxide and chlorinated compounds. Both YSZ and YSZ/WBO (hereafter referred to as the WBO sensor) electrolytes limit chemical reactions by conducting specific ions. Alternative electrolytes, such as lanthanum fluoride, can be used to further partition the different species and aid in isolating compounds from mixtures. The thickness of the electrolyte influences the ideal sensor operating temperature and can be increased or decreased to respond best in a given environment (thicker electrolyte films for higher environment temperatures). A sample vertical cross-sectional arrangement of a Pt/Ni, NiO/YSZ/Pt sensor is shown in Figure 1).

 Figure 1. The vertical arrangement of the films dictates the cell s behavior. The horizontal geometry can be varied greatly to suit applications.
Figure 1. The vertical arrangement of the films dictates the cell s behavior. The horizontal geometry can be varied greatly to suit applications.

Several horizontal geometries, some as small as 2 x 4 mm and others as large as 20 x 20 mm (see Figure 2) have been developed for specific applications. Two thicknesses of Al2O3 substrates, 15 and 25 mil (375 and 625 µm), are routinely used during sensor manufacturing. Single inline package (SIP) sensing elements contain one sensor; dual inline package (DIP) miniature arrays contain multiple elements materially tailored for gas selectivity. The inexpensive SIP form incorporates silver-soldered leads to tolerate high temperatures; welded leads are useful in achieving even higher operating temperatures. The sensing element fabrication sequence has matured to the point where sensors can be produced in very low commercial batch production sizes (fewer than 1000 units) for less than $1.00 each.

Figure 2. The sensing element in (A) measures 10 3 10 mm (SIP). The four elements in (B) are 20 3 20 mm (DIP). The first commercially made voltammetric devices used a SIP configuration and soldered leads. An array was fabricated (for General Atomics Corp.) using different electrode materials and a DIP configuration.
Figure 2. The sensing element in (A) measures 10 3 10 mm (SIP). The four elements in (B) are 20 3 20 mm (DIP). The first commercially made voltammetric devices used a SIP configuration and soldered leads. An array was fabricated (for General Atomics Corp.) using different electrode materials and a DIP configuration.

To support very low power temperature control and extend the battery life span during operation, each device has a dual-element low-resistance/high-resistance heater/ thermometer circuit screened onto the substrate backside. Typical operating temperatures run between 200°C and 350°C. The sensors can operate in ambient environments throughout the U.S. Defense Department Military Specification environment ranges. Because they are made from cermet materials, the sensors can operate in elevated temperatures and extreme environments such as engine exhaust systems and industrial flue gases.

For normal atmospheric environments, the prototype sensors are housed in a PTFE (Teflon), acrylonitrile-butadiene-styrene resin, or glass enclosure (see Figure 3). Alternatively, application-specific ceramic housings can be substituted for extreme environments. Die-stamped stainless steel mesh cages are being evaluated as universal packages for common commercial applications. Cases are designed to protect the sensing element and to allow a controlled flow of air to be pumped across the sensor surface. If flow rates exceed 3.5 Lpm, internal baffles must be introduced to prevent convective cooling that can introduce measurement error.

Figure 3. The design of a protective enclosure is often application specific and contributes to the performance of the device. Both plastic and glass have been used to house sensors in the laboratory and in the field. Stamped stainless steel is the likely final commercial material.
Figure 3. The design of a protective enclosure is often application specific and contributes to the performance of the device. Both plastic and glass have been used to house sensors in the laboratory and in the field. Stamped stainless steel is the likely final commercial material.

Instrumentation

The thick-film electrochemical cell can be operated in different modes. In simple diffusion-driven mode, a potential is produced within the cell by the presence of atmospheric gaseous species. The potential produced can be described by the Nernst equation:



where:

C*O and C*R = concentrations of oxidized and reduced species, respectively

n= number of electrons transferred

F = Faraday constant

R = gas constant

T = absolute temperature

EO = electrode potential at a standard state

Briefly, the potential produced is proportional to the natural log of the ratio of the partial pressures of gas presented to the two surfaces of the device.

Figure 3. The design of a protective enclosure is often application specific and contributes to the performance of the device. Both plastic and glass have been used to house sensors in the laboratory and in the field. Stamped stainless steel is the likely final commercial material.
Figure 3. The design of a protective enclosure is often application specific and contributes to the performance of the device. Both plastic and glass have been used to house sensors in the laboratory and in the field. Stamped stainless steel is the likely final commercial material.

This is a very limited sensing reaction, but it does illustrate the influence of component concentrations on the cell's electrical behavior. This relationship also describes the principles of a common automotive lambda-type oxygen sensor. Varied concentrations of oxygen change the electrochemical potential of a cell; these changes can be used as feedback to electronically regulate engine operation.

In contrast, when the electrodes of the device are excited by an external controlled driving potential (as opposed to being a source of a potential), more complex and interesting chemical reactions are initiated. In a voltammetric microsensor, the analytes react (oxidize or reduce) at characteristic potentials. The working model for the NH3 reaction is shown in Equations (2–4). Oxygen is converted to oxygen ions (O2–) at the cathode, which subsequently migrate through the electrolyte (ionic current) to the anode. At the anode, direct oxidation of ammonia probably does not occur. Instead, ammonia likely decomposes into nitrogen and hydrogen according to Equation (3). The hydrogen from Equation (3) is oxidized into water by the incoming oxygen ions [1,2]. Nickel serves as a catalyst for the oxidation of hydrogen in Equation (4).

Completion of this reaction is measured as a change in electrical current through the electrode-electrolyte-electrode circuit and is the essence of the voltammetry.

The voltammogram produced describes the relationship between measured current vs. the applied potential. Voltammograms contain information usable for species identification and quantification. Initial gaseous chemical characterizations were performed by implementing a stair step triangular saw-tooth potential waveform applied over a 10–20 s period. Square wave sweep voltammetry was also used and was significantly faster (0.20 s period), but was not yet optimized for ammonia detection at the time of publication.

Figure 4. The typical full laboratory instrument is notebook CPU based. This provides a development environment for the voltammetry as well as the associated chemometrics.
Figure 4. The typical full laboratory instrument is notebook CPU based. This provides a development environment for the voltammetry as well as the associated chemometrics.

Measurement Electronics

At ANL, several programmable potentiostat instruments have been fabricated that implement gaseous voltammetry. These include palm-sized microcontroller versions as well as notebook CPU virtual instruments using commercial-off-the-shelf DA hardware. A version of the laboratory potentiostat is shown in Figure 4. The external box houses an Omega temperature controller, a circuit board containing the potentiostat interface (see Figure 5), and a battery for portable operation. The device functions with a single National Instruments 12-bit A/D PCMCIA DAQCard for data acquisition, driven by proprietary voltammetry software written using Mathworks MATLAB.

Figure 5. The potentiostat amplifier circuitry is small, with much of the control embedded in the software.
Figure 5. The potentiostat amplifier circuitry is small, with much of the control embedded in the software.

The laboratory instrument is designed to allow changes to be made and large amounts of data (voltammograms) to be stored for sample pattern recognition. The core of the instrument is available as a single microcontroller-based palm-sized device.

Temperature Control

Several different methods of controlling the sensing element operating temperature have been implemented, all using the dual-element heater/thermometer configuration. Microcontroller-based instruments use a pulse-width-modified or frequency-modified heater current. These circuits are simple and prevent exaggerated temperature swings commonly observed with low-mass devices. This is particularly important when longer measurement periods are initiated, as temperature changes during a voltage sweep can affect the signal. The prototype notebook systems have incorporated commercial Omega PID temperature controllers with an isolation amplifier built to trim and match impedance of the standardized controller to the experimental sensor heating/ measuring elements. This has been very successful and provides temperature control of ±0.2°C for the sensor, whose mass is <1 g.

Figure 6. The raw response of the YSZ microsensor displays a functional relationship between the reaction peak changes and the increased concentration of ammonia, and demonstrates good recovery after the ammonia is removed.
Figure 6. The raw response of the YSZ microsensor displays a functional relationship between the reaction peak changes and the increased concentration of ammonia, and demonstrates good recovery after the ammonia is removed.

Sensor Responses to Various Ammonia Concentrations in Air

Voltammetry can be used to analyze mixtures or can be tailored for single analyte detection. Figures 6 and 7) show voltammograms used to test a single analyte, ammonia, in air at several concentrations. Six voltammograms were taken at each concentration for statistical purposes. Each seemingly single line in the voltammograms at each concentration is actually six voltammograms superimposed to illustrate measurement reproducibility. The blue lines show the signature of ambient air prior to and following sensor exposure to ammonia; these lines also demonstrate sensor recovery. The light-green lines display an obvious functional relationship to changing concentrations of ammonia.

Figure 7. The raw response of the WBO microsensor to ammonia shows a distinct relationship between the reaction peak height and the concentration of ammonia, but at a different voltage region than exhibited by the YSZ microsensor. Varied sensor responses to gases are the key to enhancing gas recognition capability.
Figure 7. The raw response of the WBO microsensor to ammonia shows a distinct relationship between the reaction peak height and the concentration of ammonia, but at a different voltage region than exhibited by the YSZ microsensor. Varied sensor responses to gases are the key to enhancing gas recognition capability.

The WBO and YSZ sensors respond uniquely to ammonia. This is beneficial, as responses from sensors can be cross-validated before concentrations or compounds are assigned to responses. Neither of the sensors used in these tests displayed saturation effects, and similar well-behaved results were obtained across a wider range of concentrations from low ppm levels to percent levels.

For many individual gases, the dissociation potential range is narrow enough to allow single-current measurements at a specific potential to be gathered and translated directly into concentration values. In Figure 6, for instance, a single dissociation potential of 0.18 V could be used to evaluate concentrations of ammonia if no interferent gases are present. This can further simplify both the instrument and the measurement technique.

The voltammetry acquisition software uses several different chemometrics algorithms to extract chemical identification and quantification information from each voltammogram. Radial basis function neural networks performed well for voltammogram analysis. Measurement and/or identification error is a function of the number and value distribution of samples used for algorithm training purposes, and routinely is <1% of the training range. The execution time of any of the algorithms is <100 ms, delivering real-time results.

Future R&D

The ANL technology described here received a 2002 R&D100 Award. It serves as the core technology of several ongoing DOE and DoD research programs, and is being further improved by the application of nanotechnology. Research in the near future will be oriented toward refining detection and discrimination of multiple analytes in the presence of interferents.

References

1. J. Stanforth and R.M. Ormerod, Green Chemistry 5, 2003, pp. 606-609.

2. W.Z. Zhu and S.C. Deevi, Materials Science and Engineering A362, 2003, pp. 228-239.

For Further Reading

Bard, A.J., and L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, John Wiley & Sons, Inc., New York, NY, 1980, pp. 232-236.

Beebe, K.R., et al., Chemometrics: A Practical Guide, John Wiley & Sons, Inc., New York, NY, 1998, pp. 3-332.

Edmonds, T.E., Ed., Chemical Sensors, Blackie and Son Ltd., New York, NY, 1988.

Fraden, J., and M. Vogt, "Chemical Sensors," AIP Handbook of Modern Sensors: Physics, Designs, and Applications, 3rd ed., American Institute of Physics, New York, NY, 2003.

Neural Networks Toolbox Users Guide, V. 4, MathWorks, Inc., Natick, MA, 2003.

Radomski, R., et al., "Microcomputer-Controlled Electrochemical Universal Meter," Computer and Chemistry, 19 (3), 1995, p. 303.

Michael C. Vogt, Ph.D., a Computer Scientist, and Laura R. Skubal, Ph.D., an Environmental Engineer, are co-principal investigators for the Argonne Gas Microsensor Program, Energy Systems Division, Argonne National Laboratory, Argonne, IL; 630-252-7474, vogt@anl.gov (Vogt); 630-252-0931,

lskubal@anl.gov (Skubal).