Gas Sensor Device Containing Lithium Garnet

Abstract

An SO.sub.x sensor includes a lithium garnet electrolyte, a sensing electrode, a reference electrode, and a heating element. The sensing electrode includes Li.sub.2SO.sub.4 and at least one metal oxide or second metal sulfate. One surface of the sensing electrode is disposed on at least a portion of a surface of the lithium garnet electrolyte. A current collector is disposed on at least a portion another surface of the sensing electrode to electrically couple the sensing electrode to the reference electrode via a potentiometer. The reference electrode is disposed on the lithium garnet electrolyte. The heating element is capable of heating the sensing electrode and the lithium garnet electrolyte to a temperature sufficient to achieve a sensor response time of less than about 30 minutes.

Claims

1. An SO.sub.x sensor comprising: a lithium garnet electrolyte; a sensing electrode comprising Li.sub.2SO.sub.4 and at least one of a metal oxide or a second metal sulfate, wherein a first surface of the sensing electrode is disposed on at least a portion of a first surface of the lithium garnet electrolyte; a current collector disposed on at least a portion of a second surface of the sensing electrode; a reference electrode disposed on the lithium garnet electrolyte, wherein the reference electrode is electrically coupled to the current collector via a potentiometer; and a heating element capable of heating the sensing electrode and the lithium garnet electrolyte to a temperature sufficient to achieve a sensor response time of less than about 30 minutes.

2. The SO.sub.x sensor of claim 1, wherein the reference electrode is disposed on at least a portion of the first surface or a second surface of the lithium garnet electrolyte.

3. The SO.sub.x sensor of claim 1, wherein the sensing electrode comprises Li.sub.2SO.sub.4 and the second metal sulfate, and the Li.sub.2SO.sub.4 and the second metal sulfate are present in a mole ratio of about 1:4 to about 9.5:0.5.

4. The SO.sub.x sensor of claim 1, wherein: the second metal sulfate is at least one of CaSO.sub.4, K.sub.2SO.sub.4, Na.sub.2SO.sub.4, Bi.sub.2(SO.sub.4).sub.3, Ce(SO.sub.4).sub.2, ZnSO.sub.4, Zr(SO.sub.4).sub.2, Fe.sub.2(SO.sub.4).sub.3, Y.sub.2(SO.sub.4).sub.3, La.sub.2(SO.sub.4).sub.3, Ta.sub.2SO.sub.4, or BaSO.sub.4; and the metal oxide is at least one of FeO, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, VO, VO.sub.2, V.sub.2O.sub.3, V.sub.2O.sub.5, NiO, ZnO, CeO.sub.2, SnO.sub.2, Ga.sub.2O.sub.3, WO.sub.3, TiO.sub.2, Pt, In.sub.2O.sub.3, or La.sub.xSm.sub.1−xFeO.sub.3 where x is 0.2, 0.4, 0.5, 0.6, or 0.8.

5. The SO.sub.x sensor of claim 1, wherein a thickness of the lithium garnet electrolyte is about 100 nm to about 80 μm.

6. The SO.sub.x sensor of claim 1, wherein the sensing electrode additionally comprises at least one of lithium garnet phase or (La.sub.0.6Sr.sub.0.4).sub.0.99CoO.sub.3−δ phase.

7. The SO.sub.x sensor of claim 1, wherein the sensing electrode additionally comprises a catalyst to facilitate oxidation of SO.sub.x.

8. The SO.sub.x sensor of claim 1, wherein the lithium garnet has a cubic phase.

9. The SO.sub.x sensor of claim 8, wherein the lithium garnet electrolyte comprises Li.sub.xLa.sub.yZrO.sub.12 with at least one additional dopant element.

10. The SO.sub.x sensor of claim 9, wherein the lithium garnet electrolyte comprises Li.sub.7−xLa.sub.3Zr.sub.2−xTa.sub.xO.sub.12, where x is about 0.25 to about 0.6.

11. The SO.sub.x sensor of claim 1, wherein the reference electrode comprises at least one of (La.sub.0.6Sr.sub.0.4).sub.0.99CoO.sub.3−δ, gold, or platinum.

12. The SO.sub.x sensor of claim 1, additionally comprising a capping layer to substantially reduce exposure of the lithium garnet electrolyte to air.

13. The SO.sub.x sensor of claim 1, wherein the sensing electrode has a porous microstructure.

14. A method of sensing a SO.sub.x gas using an electrochemical sensor comprising: exposing the SO.sub.x gas to a sensing electrode comprising Li.sub.2SO.sub.4 and at least one of a metal oxide or a second metal sulfate, wherein a first surface of the sensing electrode is disposed on at least a portion of a first surface of a lithium garnet electrolyte; heating the sensing electrode and the lithium garnet electrolyte to a temperature sufficient to achieve a reaction between the SO.sub.x gas, Li.sup.+ and O.sub.2 to form Li.sub.2SO.sub.4, thereby changing a potential of the electrochemical sensor; and detecting a change in the potential of the electrochemical sensor.

15. The method of claim 14, wherein: the SO.sub.x comprises SO.sub.2; and the method additionally comprises oxidizing the SO.sub.2 to SO.sub.3.

16. The method of claim 14, additionally comprising determining a concentration of the SO.sub.x gas based at least in part on the change in the potential of the electrochemical sensor.

17. The method of claim 14, wherein the temperature is between about 200° C. and about 500° C.

18. A method of making an SO.sub.x sensing electrode comprising: creating a mixture of Li.sub.2SO.sub.4 and at least one of a metal oxide or a second metal sulfate; applying the mixture to at least a portion of a first surface of a lithium garnet electrolyte; and annealing the lithium garnet electrolyte and the mixture to form the SO.sub.x sensing electrode on the at least a portion of the first surface of the lithium garnet electrolyte.

19. The method of claim 18, wherein annealing comprises melting the mixture of Li.sub.2SO.sub.4 and the second metal sulfate.

20. The method of claim 18, wherein the step of annealing comprises heating the sensing electrode under a constant flow of oxygen gas to a temperature sufficient to adhere the sensing electrode to the at least a portion of the first surface of the lithium garnet electrolyte.

Description

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0016] The skilled artisan will understand that the drawings primarily are for illustrative purposes and are not intended to limit the scope of the inventive subject matter described herein. The drawings are not necessarily to scale; in some instances, various aspects of the inventive subject matter disclosed herein may be shown exaggerated or enlarged in the drawings to facilitate an understanding of different features. In the drawings, like reference characters generally refer to like features (e.g., functionally and/or structurally similar elements).

[0017] FIG. 1 is a cross-sectional schematic of an electrochemical potentiometric SO.sub.x sensor with a cross-plane configuration.

[0018] FIG. 2A is a planar schematic of an electrochemical potentiometric SO.sub.x sensor with an in-plane configuration.

[0019] FIG. 2B is a cross-sectional schematic of the SO.sub.x sensor in FIG. 2A.

[0020] FIG. 3A is a cross-sectional schematic of a dense sensing electrode and the triple phase boundary of the dense sensing electrode.

[0021] FIG. 3B is a planar SEM image of the sensing electrode in FIG. 3A.

[0022] FIG. 3C is a cross-sectional SEM image of the sensing electrode in FIG. 3A.

[0023] FIG. 3D is a planar SEM image of the dense sensing electrode in FIG. 3A.

[0024] FIG. 3E is a cross-sectional SEM image of the dense sensing electrode in FIG. 3A.

[0025] FIG. 3F shows a gold elemental analysis of the SEM image in FIG. 3F.

[0026] FIG. 4A is a cross-sectional schematic of a porous sensing electrode and the quadrupole phase boundary of the porous sensing electrode.

[0027] FIG. 4B is a planar SEM image of the porous sensing electrode in FIG. 4A.

[0028] FIG. 4C is a cross-sectional SEM image of the porous sensing electrode in FIG. 4A.

[0029] FIG. 4D shows a gold elemental analysis of the SEM image in FIG. 4C.

[0030] FIG. 5A is a cross-sectional schematic of a dense sensing electrode with a lithium garnet scaffold.

[0031] FIG. 5B is a planar SEM image of the dense sensing electrode of FIG. 5A.

[0032] FIG. 6A is a cross-sectional schematic of a dense sensing electrode with a lithium sulfate, calcium sulfate, and lithium garnet composite.

[0033] FIG. 6B is a planar SEM image of the dense sensing electrode of FIG. 6A.

[0034] FIG. 7 is a photograph of an electrochemical potentiometric SO.sub.x sensor with an in-plane configuration.

[0035] FIG. 8A shows emf response of an SO.sub.x sensor with the dense auxiliary sensing electrode of FIG. 3A to SO.sub.2 concentration step changes (0-10 ppm) at 480° C.

[0036] FIG. 8B shows average emf responses from the data in FIG. 8A.

[0037] FIG. 9A shows emf response of an SO.sub.x sensor with the porous auxiliary sensing electrode of FIG. 4A to SO.sub.2 concentration step changes (0-10 ppm) at 480° C.

[0038] FIG. 9B shows average emf responses from the data in FIG. 9A.

[0039] FIG. 10A shows emf response of an SO.sub.x sensor with the dense auxiliary sensing electrode with a lithium garnet scaffold of FIG. 5A to SO.sub.2 concentration step changes (0-10 ppm) at 480° C.

[0040] FIG. 10B shows average emf responses from the data in FIG. 10A.

[0041] FIG. 11A shows emf response of an SO.sub.x sensor with the dense auxiliary sensing electrode with a lithium sulfate, calcium sulfate, and lithium garnet composite of FIG. 6A to SO.sub.2 concentration step changes (0-10 ppm) at 480° C.

[0042] FIG. 11B shows average emf responses from the data in FIG. 11A.

[0043] FIG. 11C shows emf response of an SO.sub.x sensor with the dense auxiliary sensing electrode with a lithium sulfate, calcium sulfate, and lithium garnet composite of FIG. 6A to SO.sub.2 concentration step changes (0-10 ppm) at 240° C.

[0044] FIG. 12A shows cross-sectional SEM images of lithium garnet pellets after exposure to 4, 40, and 250 ppm SO.sub.2 at 240,320,400, and 480° C. for 24 hours.

[0045] FIG. 12B shows schematic illustrations of the images in FIG. 12A.

[0046] FIG. 13 shows X-ray diffraction and Raman characterization of the surface of the lithium garnet pellets in FIG. 12A after exposure to SO.sub.2.

[0047] FIG. 14 shows the melting temperatures of the sensing electrode as a function of its composition, as determined with differential scanning calorimetry and thermogravimetric analysis.

[0048] FIG. 15A shows the open circuit voltage response of an SO.sub.x sensor to four cycles of SO.sub.2 concentration step changes (0-10-0 ppm) at 480° C.

[0049] FIG. 15B shows sensitivity values from the data in FIG. 15A.

[0050] FIG. 16A shows the open circuit voltage response of an SO.sub.x sensor with an inorganic coating to SO.sub.2 concentration step changes (0-15 ppm) at 480° C.

[0051] FIG. 16B shows average sensitivity values from the data in FIG. 16A.

[0052] FIG. 17 shows planar SEM images of sensing electrode composites comprising Li.sub.2SO.sub.4, CaSO.sub.4 and (La.sub.0.6Sr.sub.0.4).sub.0.99CoO.sub.3−δ with varying amounts of (La.sub.0.6Sr.sub.0.4).sub.0.99CoO.sub.3−δ and different annealing temperatures.

[0053] FIG. 18A shows the open circuit voltage response of an SO.sub.x sensor with a sensing electrode with 10% (La.sub.0.6Sr.sub.0.4).sub.0.99CoO.sub.3−δ to SO.sub.2 concentration step changes (0-15 ppm) at 240° C.

[0054] FIG. 18B shows average sensitivity values of SO.sub.x sensors with sensing electrodes with 10%, 30%, and 50% (La.sub.0.6Sr.sub.0.4).sub.0.99CoO.sub.3−δ to SO.sub.2 concentration step changes (0-15 ppm) at 240° C.

[0055] FIG. 19A shows the open circuit voltage response of an SO.sub.x sensor with a sensing electrode with 50% (La.sub.0.6Sr.sub.0.4).sub.0.99CoO.sub.3−δ to SO.sub.2 concentration step changes (0-15 ppm) at 320° C.

[0056] FIG. 19B shows average sensitivity values of SO.sub.x sensors with sensing electrodes with 30% and 50% (La.sub.0.6Sr.sub.0.4).sub.0.99CoO.sub.3−δ to SO.sub.2 concentration step changes (0-15 ppm) at 320° C.

[0057] FIG. 20 shows average sensitivity values of SO.sub.x sensors as a function of the amount of (La.sub.0.6Sr.sub.0.4).sub.0.99CoO.sub.3−δ in the sensing electrode.

[0058] FIG. 21A shows the open circuit voltage response of an SO.sub.x sensor with a sensing electrode with 30% (La.sub.0.6Sr.sub.0.4).sub.0.99CoO.sub.3−δ to SO.sub.2 concentration step changes (0-10 ppm) at 400° C.

[0059] FIG. 21B shows average sensitivity values from the data in FIG. 21A.

[0060] FIG. 22 shows catalyst-assisted SO.sub.2 oxidation.

[0061] FIG. 23A shows an SEM image of a sensing electrode with 10% Fe.sub.2O.sub.3 as a catalyst.

[0062] FIG. 23B shows the open circuit voltage response of an SO.sub.x sensor with a sensing electrode with 10% Fe.sub.2O.sub.3 to SO.sub.2 concentration step changes (0-10 ppm) at 480° C.

[0063] FIG. 23C shows average sensitivity values from the data in FIG. 23B.

[0064] FIG. 24 is a photograph of a thin film electrochemical potentiometric SO.sub.x sensor.

[0065] FIG. 25A is a cross-sectional schematic of a thin film electrochemical potentiometric SO.sub.x sensor with an in-plane configuration.

[0066] FIG. 25B is a cross-sectional schematic of another thin film electrochemical potentiometric SO.sub.x sensor with an in-plane configuration.

[0067] FIG. 25C is a cross-sectional schematic of another thin film electrochemical potentiometric SO.sub.x sensor with an in-plane configuration.

[0068] FIG. 25D is a cross-sectional schematic of another thin film electrochemical potentiometric SO.sub.x sensor with an in-plane configuration.

[0069] FIG. 26 is a cross-sectional schematic of a thin film electrochemical potentiometric SO.sub.x sensor with a cross-plane configuration.

[0070] FIG. 27 is a cross-sectional SEM image of part of a thin film SO.sub.x sensor with an in-plane configuration.

[0071] FIG. 28A shows cross-sectional SEM images of sensing electrodes deposited on thin film lithium garnet using spray pyrolysis.

[0072] FIG. 28B shows a planar SEM image of a sensing electrode deposited on thin film lithium garnet using spray pyrolysis.

[0073] FIG. 28C shows a cross-sectional SEM image of a sensing electrode deposited on thin film lithium garnet using spray pyrolysis.

[0074] FIG. 29A shows the open circuit voltage response of a thin film SO.sub.x sensor to SO.sub.2 concentration step changes (0-10 ppm) at 320° C.

[0075] FIG. 29B shows average sensitivity values for the data in FIG. 29A.

[0076] FIG. 30A shows SEM images of thin film lithium garnet exposed to 10 ppm SO.sub.2 at different temperatures for 24 hours.

[0077] FIG. 30B shows SEM images of thin film lithium garnet exposed to 10 ppm SO.sub.2 at different temperatures for 24 hours.

[0078] FIG. 31 shows Raman characterization of thin film lithium garnet after SO.sub.2 exposure shown in FIGS. 30A and 30B.

[0079] FIG. 32 shows Raman characterization of thin film lithium garnet before and after post-annealing at 750° C.

DETAILED DESCRIPTION

[0080] A potentiometric solid-state electrochemical sensor uses a fast-conducting lithium-garnet (e.g., Li.sub.7La.sub.3Zr.sub.2O.sub.12 or Li.sub.6.5La.sub.3Zr.sub.1.5Ta.sub.0.5O.sub.12) solid electrolyte, a composite sensing electrode, and a reference electrode to detect sulfur dioxide (SO.sub.2) and sulfur trioxide (SO.sub.3) (cumulatively, SO.sub.x). The composite sensing electrode includes lithium sulfate (Li.sub.2SO.sub.4) and at least one other metal sulfate or metal oxide. The sensing electrode may be porous to define a high number of interfacial reaction sites. The Li-garnet-based electrochemical sensor successfully tracked SO.sub.2 gas at the dangerous levels of 0 ppm to 10 ppm with a high level of sensitivity and selectivity.

[0081] In comparison to resistive gas sensors, which have poor selectivity and high susceptibility to corrosion in acidic gases, potentiometric gas sensors have high selectivity and reduced susceptibility to corrosion. Resistive gas sensors detect changes in electrical resistance based on gas adsorption to the surface of the sensor. The potentiometric gas sensor operates under thermodynamic equilibrium and employs ion-conducting solid electrolyte, Li-garnet, where the conductivity stems from mobile Li.sup.+ ions rather than electrons, reducing susceptibility to corrosion and increasing selectivity, even at low operation temperatures. These potentiometric sensors also offer a promising alternative to the inconvenient UV and IR SO.sub.2 gas sensors, which have large power demands, high costs, and bulky sizes.

[0082] SO.sub.x gas reacts at the electrode/solid electrolyte interface in the potentiometric gas sensor, resulting in an electrical signal that is directly related to the concentration or partial pressure of the SO.sub.x gas species. The output is an electromotive force (emf). The emf of the cell is determined by the chemical potentials established at the sensing (μ.sub.Li.sup.SE) and reference electrodes (μ.sub.Li.sup.RE). At thermodynamic equilibrium, the measured voltage across the cell (E) is related to the partial pressure of the detected gas (p(SO.sub.x).sup.SE) according to the Nernst equation.

[0083] The solid-state potentiometric gas sensor is a type III sensor, where the ion species derived from the tracked gas do not coincide with either the mobile or the immobile ion but, rather, another ion species in the auxiliary sensing electrode. Compared to types I and II, type III potentiometric sensors provide detection of complex gas species using an auxiliary sensing electrode and a fast-ion-conducting solid electrolyte. Ionic conductivity in the solid electrolyte effects the sensor's response and recovery time. This mechanism obviates the need for separate gas environments for the sensing electrode (SE) and the referenced electrode (RE). Conventional type III potentiometric sensors use solid-state electrolytes based on O.sup.2− ion conductors (e.g., NASICON), which have low ion mobility so that they are typically operated at high temperatures above 500° C. to ensure sufficient ionic conductivity in the solid electrolyte.

[0084] These conventional devices typically have several drawbacks related to stability and transport, including relatively high power usage, sluggish sensor detection and regeneration due to slow diffusion processes and poor ion conductance (e.g., ˜10.sup.−8 S/cm at 300° C.), poor sensor detection and/or response times (typically >˜5 min), long recovery times, low electrolyte chemical stability, unstable voltage response, poor reproducibility and poor long-term stability.

[0085] Li-garnet has a high room-temperature ionic conductivity (e.g., ˜mS cm.sup.−1 for Li.sub.7La.sub.3Zr.sub.2O.sub.12 in the cubic phase), high chemical stability towards Li metal (e.g., reduction potential of 0.05 V vs. Lr/Li for Li.sub.7La.sub.3Zr.sub.2O.sub.12), and a wide electrochemical stability window. Li-garnet also has stable phase and transport properties during prolonged exposure to SO.sub.x gas. The Li-garnet-based potentiometric sensor can operate continuously for several days or weeks.

[0086] FIG. 1 shows one embodiment of the potentiometric gas sensor 100. The gas sensor 100 includes a Li-garnet solid electrolyte 110, a sensing electrode 120, and a reference electrode 130. The reference electrode 130 also acts as the counter electrode. In this cross-plane configuration, the sensing electrode 120 and the reference electrode 130 are disposed on opposite sides of the solid electrolyte 110. A current collector 140 is disposed on a surface of the sensing electrode 120 opposite the surface of the sensing electrode 120 disposed on the solid electrolyte 110. A potentiometer 150 is electrically coupled to the current collector 140 and the reference electrode 130 via conductive wires to measure emf across the device 100. SO.sub.2 gas 160 and O.sub.2 gas 170 interact with the surface of the sensing electrode 120, thereby changing the cell potential.

[0087] At the sensing electrode 120, SO.sub.2 gas 160 is oxidized with O.sub.2 gas 170 according to the following reactions:

[00001]$\begin{array}{cc}{\mathrm{SO}}_2+\frac{1}{2}{O}_2\stackrel{K}{.Math.}{\mathrm{SO}}_3;& \left[1\right]\end{array}$$\begin{array}{cc}K=\frac{p\left[{\mathrm{SO}}_3\right]}{p\left[{\mathrm{SO}}_2\right].Math.{p\left[O_2\right]}^{0.5}}& \left[2\right]\end{array}$$\Delta G_f^0=-97,780+92.78T\left[K\right],$

where K and ΔG.sub.f.sup.0 are the equilibrium constant and the standard Gibbs free energy change in both reactions, respectively. Thermodynamically, the oxidation of SO.sub.2 gas 160 is feasible at room temperature and up to 780° C. The overall reaction at the sensing electrode 120 is expressed by,

[00002]$\begin{array}{cc}2{\mathrm{Li}}^{+}+{\mathrm{SO}}_3+\frac{1}{2}{O}_2+2e^{-}.Math.L{i}_2{\mathrm{SO}}_4.& \left[2\right]\end{array}$

At the reference electrode 130, without being bound by any particular theory, Li.sup.+ ions may react primarily with oxygen and not SO.sub.2, according to the following equation:

[00003]$\begin{array}{cc}2{\mathrm{Li}}^{+}+\frac{1}{2}{O}_2+2e^{-}.Math.L{i}_{2}O.& \left[3\right]\end{array}$

Considering that i) the temperature and the partial pressure of O.sub.2 (p[O.sub.2]) are generally fixed in an atmospheric environment (0.21 atm) and ii) the activity of Li.sub.2SO.sub.4 and Li.sub.2O are kept constant and the concentration of Li.sup.+ is assumed to remain unchanged through the measurement, the cell potential (emf), E, is directly related to the partial pressure of SO.sub.2 (p[SO.sub.2]) according to the Nernst equation:

[00004]$\begin{array}{cc}E=E^0+\frac{RT}{2F}\ln \left(p\left[{\mathrm{SO}}_2\right]\right),& \left[4\right]\end{array}$

where E.sup.0 is a constant (standard potential), F is the Faraday constant, R the gas constant, and T is the absolute temperature.

[0088] The gas sensor 100 can operate continuously or intermittently. During operation, the gas sensor 100 is able to detect SO.sub.x concentrations of 0 ppm to 250 ppm. In one embodiment, the gas sensor 100 is able to detect SO.sub.x concentrations of 0 ppm to 10 ppm (e.g., 0.1 ppm. 0.5 ppm, 1 ppm, 2 ppm, 3 ppm, 4 ppm, 5 ppm, 6 ppm, 7 ppm, 8 ppm, 9ppm, or 10 ppm). In another embodiment, the gas sensor 100 is able to detect SO.sub.x gas concentrations of 4 ppm to 250 ppm (e.g., 4 ppm, 8 ppm, 12 ppm, 20 ppm, 50 ppm, 100 ppm, 200 ppm, or 250 ppm). In another embodiment, the gas sensor 100 is configured to detect low SO.sub.x concentrations of 0 ppm to 4 ppm. In another embodiment, the gas sensor 100 is configured to detect medium SO.sub.x concentrations of 4 ppm to 40 ppm. In another embodiment, the gas sensor 100 is configured to detect high SO.sub.x concentrations of 4 ppm to 250 ppm. At high and medium SO.sub.x concentrations, operating parameter (e.g., temperature) and materials are selected to accommodate the highly corrosive environment created by the presence of a substantial concentration of SO.sub.x gas.

[0089] Regardless of the SO.sub.x concentration, the gas sensor's components may begin to degrade once the gas sensor 100 is turned on and exposed to atmospheric gases. This degradation may determine a shelf-life of days, weeks, months, or years before the gas sensor 100 is either replaced or regenerated.

[0090] When the gas sensor 100 detects SO.sub.x gas in the environment, it has a response time of less than 30 minutes and a recovery time of 60 minutes. The response time is the amount of time between the introduction of the SO.sub.x gas and the sensor response, a change in emf, indicating the presence of the SO.sub.x gas. The recovery time is the amount of time between the removal of the SO.sub.2 gas from the environment and when the sensor stops indicating the presence of SO.sub.x gas. The sensor response time may be less than about 15 minutes. Preferably, the sensor response time may be less than 8 minutes. More preferably, the sensor response time may be less than 6 minutes. The sensor recovery time may be about 4 minutes to about 60 minutes. Preferably, the sensor recovery time is less than 24 minutes. More preferably, the sensor recovery time is less than 6 minutes.

[0091] The gas sensor 100 is heated by a heating element 132 thermally coupled to the sensing electrode 120. The reference electrode 130 is disposed on the heating element 132 in FIG. 1. In another embodiment, the heating element 132 is positioned along a side of the gas sensor 100 so that it is directly in contact with the solid electrolyte 110 and the sensing electrode 120. In another embodiment, the heating element 132 is positioned so that it is not in direct contact with any components of the gas sensor 100 to provide more even heating. The heating element 132 may have dimensions wider or smaller than the reference electrode 130. The heating element 132 may have a continuous surface or may have a grid shape. The heating element 132 may be a convection heater, an infrared heater, a resistive heater, or a combination thereof. Alternatively, the substrate may be heated via a light source (e.g., a laser) emitting light in a wavelength range (e.g., infrared light). Alternatively, the gas sensor 100 does not include a heating element and the sensing electrode 120 and the solid electrolyte 110 are heated by the environment in which the sensor is placed. For example, the gas sensor 100 may be placed in a combustion exhaust stream that heats the environment to 500° C. to 1000° C. or in an environmental monitoring area that is heated to 150° C. to 400° C.

[0092] The heat source (the heating element 132 or environmental heating) is thermally coupled to at least the solid electrolyte 110 and the sensing electrode 120. The solid electrolyte 110 and sensing electrode 120 are heated at a temperature selected based on the SO.sub.x concentration range which the sensor is configured to detect. The temperature selected is one that is hot enough to provide a reasonable sensor response time and not so hot as to accelerate degradation of the gas sensor components. In one embodiment, the gas sensor 100 operates in a temperature range of 200° C. to about 780° C. Preferably, the gas sensor 100 operates with the solid electrolyte and the sensing electrode in a temperature range of 240° C. and 500° C. (e.g., at 240° C., 280° C., 320° C., 360° C., 400° C., 440° C., 480° C., or 500° C.). In another embodiment, the gas sensor 100 is configured to detect low SO.sub.x concentrations (0 ppm to 4 ppm) and the solid electrolyte 110 and the sensing electrode 120 are heated at a temperature of 240° C. to 480° C. In another embodiment, the gas sensor 100 is configured to detect medium SO.sub.x concentrations (4 ppm to 40 ppm) and the solid electrolyte 110 and the sensing electrode 120 are heated to a temperature of 240° C. to 400° C. In another embodiment, the gas sensor 100 is configured to detect high SO.sub.x concentrations (40 ppm to 250 ppm) and the solid electrolyte 110 and the sensing electrode 120 are heated to a temperature of about 240° C.±10° C. At high and medium SO.sub.x concentrations, the gas sensor 100 is operated at lower temperatures to prevent degradation of the Li garnet in the highly corrosive environment created by the presence of a substantial concentration of SO.sub.x gas.

[0093] The solid electrolyte 110 is Li garnet. In one embodiment, the Li garnet is Li.sub.xLa.sub.3Zr.sub.2O.sub.12 (LLZO). In another embodiment, the Li garnet electrolyte includes Li.sub.xLa.sub.yZr.sub.2O.sub.12 with at least one additional dopant element. The additional dopant may be at least one of tantalum, niobium, aluminum, tungsten, bismuth, antimony, yttrium, samarium, barium, calcium, strontium, tin, hafnium, cerium, tellurium, gallium, or neodymium. The Li garnet may include Li.sub.7−xLa.sub.3Zr.sub.2−xD.sub.xO.sub.12, where D is the additional dopant and x is about 0.25 to about 0.6. For example, the Li garnet may include Li.sub.7−xLa.sub.3Zr.sub.2−xTa.sub.xO.sub.12, where x is about 0.25 to about 0.6. For example, the Li garnet may include Li.sub.6.5La.sub.3Zr.sub.1.5Ta.sub.0.5O.sub.12. The Li garnet may substantially maintain a cubic garnet structure during operation since this structure has a high ionic conductivity.

[0094] The solid electrolyte 110 may have a thickness of about 200 nm to about 1 cm. In one embodiment, the gas sensor has a thin film configuration and the solid electrolyte 110 is about 200 nm to about 1 μm. Preferably, in the thin film configuration, the solid electrolyte 110 is about 400 nm to about 600 nm (e.g., 400 nm, 500 nm, or 600 nm). In another embodiment, the gas sensor has a bulk configuration and the solid electrolyte 110 is about 1 mm to about 5 mm. Preferably, in the bulk configuration, the solid electrolyte 110 is about 2 mm to about 4 mm (e.g., 2 mm, 3 mm, or 4 mm).

[0095] The sensing electrode 120 includes lithium sulfate (Li.sub.2SO.sub.4) and at least one metal sulfate and/or metal oxide. The other metal sulfate may include at least one of CaSO.sub.4, K.sub.2SO.sub.4, Na.sub.2SO.sub.4, Bi.sub.2(SO.sub.4).sub.3, Ce(SO.sub.4).sub.2, ZnSO.sub.4, Zr(SO.sub.4).sub.2, Fe.sub.2(SO.sub.4).sub.3, Y.sub.2(SO.sub.4).sub.3, La.sub.2(SO.sub.4).sub.3, Ta.sub.2SO.sub.4, or BaSO.sub.4. The metal oxide may include at least one of FeO, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, VO, VO.sub.2, V.sub.2O.sub.3, V.sub.2O.sub.5, NiO, ZnO, CeO.sub.2, SnO.sub.2, Ga.sub.2O.sub.3, WO.sub.3, TiO.sub.2, Pt, In.sub.2O.sub.3, or La.sub.xSm.sub.1−xFeO.sub.3where x is 0.2-0.8 (e.g., 0.2, 0.4, 0.5, 0.6, or 0.8).

[0096] The other metal sulfate may act as a humidity absorbent (e.g., CaSO.sub.4). The metal sulfate may also lower the melting temperature of the composite sensing electrode, so that it has fewer defects and grain boundaries, and forms a strongly adhered interface with Li garnet. The metal oxide may act as a catalyst to facilitate the oxidation of SO.sub.2 to SO.sub.3. The metal oxide may be present in the sensing electrode 120 in a weight percent of about 0% to about 50% (e.g., 10%, 20%, 30%, 40%, or 50%). In one embodiment, the Li.sub.2SO.sub.4 and the other metal sulfate are in a mole ratio of about 1:4 to about 9.5:0.5. Preferably, the Li.sub.2SO.sub.4 and the other metal sulfate are in a mole ratio corresponding to or near the eutectic point on the phase diagram of the two materials. With a mole ratio close to or at the eutectic point, the sensing electrode has a lower melting temperature, and therefore a lower processing temperature and higher ionic conductivity. The Li.sub.2SO.sub.4 and the at least one metal sulfate and/or metal oxide are substantially homogenously mixed in the electrode. In some cases, the sensing electrode includes at least one of Li garnet or (La.sub.0.6Sr.sub.0.4).sub.0.99CoO.sub.3−δ to increase ionic conductivity. The Li garnet or (La.sub.0.6Sr.sub.0.4).sub.0.99CoO.sub.3−δ may be present in the sensing electrode in a weight percent of about 0% to about 50% (e.g., 10%, 20%, 30%, 40%, or 50%). In some cases, the sensing electrode includes an electronic conductor (e.g., gold, platinum, and/or (La.sub.0.6Sr.sub.0.4).sub.0.99CoO.sub.3−δ) to increase electronic conductivity in the electrode.

[0097] In some cases, the sensing electrode 120 has a dense (e.g., about >85%) morphology, which limits the reaction surface area and forms a triple phase boundary (as described below) where electrochemical sensing reactions occur. The lower surface area decreases the gas sensor's sensitivity but increases its lifetime when exposed to high temperatures and/or high concentrations of SO.sub.x gas. Therefore, the dense morphology is preferably for sensing SO.sub.x gas at medium and high concentrations of SO.sub.x or for operation at higher temperatures (>500° C.). Alternatively, the sensing electrode may have a microporous structure, which increases the surface area and creates a quadrupole phase boundary where electrochemical sensing reactions occur. The higher surface area increases the gas sensor's sensitivity but decreases its lifetime when exposed to high temperatures and/or high concentrations of SO.sub.x gas. Therefore, the microporous morphology is preferable for sensing SO.sub.x gas at low concentrations of SO.sub.x gas, at lower operating temperatures, and/or where fast sensing times are desired.

[0098] The reference electrode 130 includes at least one electrically conductive material. The reference electrode 130 includes at least one of (La.sub.0.6Sr.sub.0.4).sub.0.99CoO.sub.3−δ, gold, or platinum. The reference electrode 130 has a thickness of about 10 μm to about 500 μm (e.g., 10 μm, 20 μm, 50 μm, 100 μm, 200 μm, or 500 μm). The reference electrode 130 may be capped with an inorganic capping layer to prevent the reference electrode from interacting with atmospheric gases.

[0099] The current collector 140 includes at least one electrically conductive material. The current collector 140 includes at least one of (La.sub.0.6Sr.sub.0.4).sub.0.99CoO.sub.3−δ, gold, or platinum. The current collector 140 may include the same material as the reference electrode 130 for ease of sensor assembly. The current collector 140 may be applied to the sensing electrode 120 in the form of an electrically conductive paste. In one embodiment, wires electrically coupling the current collector 140 and reference electrode 130 to a potentiometer may be adhered to the current collector 140 and the reference electrode 130 with the same electrically conductive paste.

[0100] FIG. 2A shows another embodiment of the potentiometric gas sensor 200a. The gas sensor 200a includes a Li-garnet solid electrolyte 210a, a sensing electrode 220a, and a reference electrode 230a. The reference electrode 230a also acts as the counter electrode. This gas sensor 200a has an in-plane configuration, where the sensing electrode 220a and the reference electrode 230a are disposed on the same surface of the solid electrolyte 210a. A potentiometer 250a is electrically coupled to the sensing electrode 220a and the reference electrode 230a via conductive wires to measure emf across the device 200a, corresponding to a concentration of SO.sub.x. The gas sensor 200a uses the same materials as described for gas sensor 100. Any of the sensing electrode materials and configurations described below may be used with the in-plane sensor geometry. The in-plane geometry of the gas sensor 200a may be preferred for ease of manufacturing the sensor.

[0101] FIG. 2B shows another view of the in-plane configuration gas sensor 200b shown in FIG. 2A. The gas sensor 200b includes a Li-garnet solid electrolyte 210b, a sensing electrode 220b, and a reference electrode 230b. The reference electrode 230b also acts as the counter electrode. This gas sensor 200b has an in-plane configuration, where the sensing electrode 220b and the reference electrode 230b are disposed on the same surface of the solid electrolyte 210b. A potentiometer 250b is electrically coupled to the sensing electrode 220b and the reference electrode 230b via conductive wires to measure emf across the device 200b, corresponding to a concentration of SO.sub.x. SO.sub.2 gas 260 and O.sub.2 gas 270 interact with the surface of the sensing electrode 220b, thereby changing the cell potential of the gas sensor 200b. An inorganic adhesive coating 280 is disposed on the surfaces of the solid electrolyte 210b or the reference electrode 230b so that SO.sub.2 gas 260 and O.sub.2 gas 270 substantially do not interact with their surfaces, where the gases could degrade the materials in these components and cause measurement errors.

[0102] FIG. 3A shows a dense sensing electrode 320 disposed on a solid electrolyte 310 with a current collector 340 disposed on top of the sensing electrode 320. The dense sensing electrode 320 has a density of about 85%-95%.The dense sensing electrode forms a triple phase boundary (TPB) reaction zone between the gaseous species, the current collector 340, and the sensing electrode 320 at which the electrochemical sensing reactions occur. Gaseous SO.sub.2 and O.sub.2 react in the reaction zone. The current collector 340 conducts electrons and the sensing electrode 320 conducts Li.sup.+ ions from the sold electrolyte to the reaction zone where they participate in the electrochemical sensing reactions. FIGS. 3B-3F show different SEM images of the dense sensing electrode shown in FIG. 3A. The SEM images indicated that the sensing electrode is substantially homogeneous and dense, with little porosity. The current collector material 340 in the example shown in FIGS. 3B-3F includes gold particles. FIG. 3F shows an EDS SEM image with gold chemical mapping showing the location of gold in the gas sensor. FIG. 3F shows gold was located at the surface of the sensing electrode 320.

[0103] FIG. 4A shows a porous sensing electrode 420 disposed on a solid electrolyte 410. A current collector 440 is disposed on top of the sensing electrode 420 and is mixed into the sensing electrode matrix. The porous sensing electrode forms a quadrupole phase boundary (QPB) reaction zone between the gaseous species, the current collector 340, the sensing electrode 320, and the solid electrolyte 410 at which the electrochemical sensing reactions occur. The porous sensing electrode 420 provided a larger effective surface area for electrochemical reaction, favorable diffusion of SO.sub.2 into the porous sensing electrode structure, and greater SO.sub.x sensitivity. However, the porous structure may also provide the opportunity for additional degradation reactions between the solid electrolyte and the sensing electrode, which decrease the overall lifetime of the gas sensor. As described in greater detail below, the dense or porous sensing electrode structure can be chosen by changing processing temperatures during device fabrication. FIGS. 4B-4D show SEM images of the porous sensing electrode microstructure. As shown in the images, the sensing electrode has a large number of pores on the micrometer-scale. SEM EDS gold mapping in FIG. 4D shows that gold particles are dispersed throughout the electrode.

[0104] FIG. 5A shows a sensing electrode 520 that includes a porous Li garnet scaffold. The sensing electrode 520 is sandwiched between the solid electrolyte 510 disposed on one surface of the sensing electrode 520 and the current collector 540 disposed on an opposite surface of the sensing electrode 520. Adding Li garnet to the sensing electrode 520 provides improved ionic conductivity, and shorter sensor response and recovery times. The randomly connected Li garnet scaffold creates percolation pathways for improved Li.sup.+ ion conductivity through the sensing electrode 520. This configuration shortens the Li.sup.+ ion diffusion distance from the sensing electrode 520 to the Li garnet solid electrolyte 510. This configuration also provides a stable interface between the sensing electrode 520 and the solid electrolyte 510. FIG. 5B shows a top-down image of the surface of the sensing electrode 520.

[0105] FIG. 6A shows a sensing electrode 620 that includes a composite of Li.sub.2SO.sub.4 and Li garnet. The sensing electrode 620 is dense with Li garnet distributed more homogeneously in the sensing electrode. The sensing electrode 620 is sandwiched between a solid electrolyte 610 and a current collector 640. The composite sensing electrode 620 provided similar benefits to the Li garnet scaffold shown in FIG. 5A, with shorter Li.sup.+ ion diffusion lengths, improved ionic conductivity, and shorter sensor response and recovery times. FIG. 6B shows a top-down image of the surface of the sensing electrode 620.

[0106] FIG. 7 shows a photograph of a cross-plane configuration potentiometric gas sensor 700. FIG. 7 shows a gold current collector 740 disposed on top of a sensing electrode (not shown), all on top of a Li garnet solid electrolyte 710. Conductive wires (e.g., any solid conductive metal) electrically connect the current collector 740 and the reference electrode (not shown) to a potentiometer.

EXAMPLES

[0107] The SO.sub.2 sensor was composed of the following electrochemical cell, expressed as:

Au|Li.sub.6.54La.sub.3.00Zr.sub.1.36Ta.sub.0.50O.sub.11.73|Li.sub.2SO.sub.4—CaSO.sub.4|Au, SO.sub.2, O.sub.2

where gold (Au) is the reference electrode, Li.sub.6.54La.sub.3.00Zr.sub.1.36Ta.sub.0.50O.sub.11.73 is the solid electrolyte and Li.sub.2SO.sub.4—CaSO.sub.4 represents the binary compound of the auxiliary sensing electrode. When the cell is heated up to a stable thermal condition (a certain temperature), mobile Li.sup.+ ions in the sensing electrode and the solid electrolyte participate in the electrochemical reaction and the cell reaction may come into an equilibrium state. In this case, the emf of the sensor was measured and converted to SO.sub.2 concentrations.

[0108] Sensing Electrode Microstructure and the SO.sub.2 Sensor Voltage Response

[0109] The SO.sub.2 sensor operation was investigated in terms of its performance characteristics employing a dense sensing electrode microstructure, like that shown in FIG. 3A. The SO.sub.2 concentration was varied from 0 ppm to 10 ppm and back to 0 ppm with 2.5 ppm step changes in dry synthetic air. Electromotive force (emf) values were recorded and converted to SO.sub.2 concentrations to determine detection sensitivity, response time, and recovery time. Response and recovery times were defined as the time the sensor used to reach 90% of the steady final signal and the time the sensor takes to return to 10% of the steady signal, respectively.

[0110] FIGS. 8A-8B shows the emf response of the electrochemical cell to SO.sub.2 concentration steps at 480° C. During the concentration step change of SO.sub.2 from 0 to 10 ppm, the emf voltage response indicated a difference in the chemical potentials of Li.sup.+ ions in the Li garnet solid electrolyte at the interfaces with the sensing and reference electrode, respectively. Moreover, the initial negative polarity in the presence of synthetic air or low SO.sub.2 concentrations indicated an accumulation of Li.sup.+ ions at the reference electrode and the depletion of Li.sup.+ ions at the sensing electrode. The transfer of Li.sup.+ ions is accompanied by the occurrence of reaction [2] described above from right to left at the sensing electrode interface, and reaction [3] from left to right at the Au reference electrode. Increasing the SO.sub.2 concentration further, induces the reverse transfer of Li.sup.+ ions from the reference to the sensing electrode and the opposite occurrence of reactions [2] and [3]. The average emf values and the logarithm of the SO.sub.2 gas concentration was fitted by the Nernst equation to evaluate the sensitivity of the sensor. The average emf values were 144 (n=1.3 e.sup.−) mV/dec and 76 (n=1.96 e.sup.−) mV/dec for the sensor response step of 0 ppm to 10 ppm and a recovery step of 10 ppm to 0 ppm. This is similar or higher than the theoretical sensitivity for a two-electron reaction, which is 74.64 mV/dec. Without being bound by any theory, the differences between the theoretical and calculated sensitivity, especially in the response stage, may indicate more efficient desorption, compared to adsorption, of SO.sub.2 gas at the surface of the sensing electrode. It is evident that the recovery was faster, at ˜15-60 min depending on the SO.sub.2 concertation, whereas response time was typically longer than >60 min (except in the case of 0 ppm to 2.5 ppm where t.sub.90%=21 min). Although the emf in the recovery stage is stable, it did not return to the initial emf value, and it may use a longer period to fully recover. Response and recovery times may be decreased by using a catalyst to catalyze fast SO.sub.2 oxidation. Response and recovery times may be decreased by changing the operation temperature, so that adsorption and desorption of SO.sub.2 processes are shortened. The sensing electrode microstructure and composition can also be changed to change response and recovery times, as discussed below.

[0111] One promising strategy to shorten the response/recovery time is by introducing a porous auxiliary sensing electrode to facilitate faster adsorption and diffusion of sulfur dioxide through the porous sensing electrode structure, like that shown in FIG. 4A. A porous microstructure increases the effective surface area for reaction and the amount of sulfur dioxide molecules that participate in the electrochemical reaction. With a porous microstructure, the sensing reaction takes place at the quadruple-phase boundary (QPB) reactive sites, where the electrochemical equilibrium uses a quadruple contact between the auxiliary phase of Li.sub.2SO.sub.4—CaSO.sub.4, the Au current collector, the solid electrolyte, and the gaseous phase of SO.sub.2, as shown in FIG. 4A. To improve the distribution of Au particles and increase the density of QPB reaction sites throughout the sensing electrode, a sensing electrode with a porous microstructure with an approximate pore diameter ranging from 5 to 10 μm was used. This microstructure was made by lowering the processing temperature of the sensing electrode from 750 to 600° C.

[0112] FIGS. 9A-9B show the response time and recovery times of a gas sensor with a porous microstructure at 480° C. in the concentration ranges of 0 ppm to 10 ppm SO.sub.2 and 10 ppm to 0 ppm SO.sub.2, respectively, in 2.5 ppm steps. The graphs show a clear improvement in the response time of the porous sensing electrode compared to the dense electrode graphs shown in FIGS. 8A-8B. Here, the porous sensing electrodes revealed a shortened response time from >˜60 min for the dense electrode to ˜7-30 min for the porous electrode. The recovery time decreased from ˜15-60 min for the dense electrode to ˜5-24 min for the porous electrode, depending on the selected SO.sub.2 concentration step. Cross-sectional SEM images and EDS elemental mapping of the sensing electrode/solid electrolyte interface shown in FIG. 4D revealed that the porous sensing electrode structure has an average thickness of ˜20 μm and has a homogeneous distribution of the gold paste in the porous sensing electrode. This structure creates multiple quadruple contact points (QPB) for the cermet structure, which may account for the improvement in response time after SO.sub.2 exposure. Incorporating gold paste in the composite sensing electrode helped maintain better metallic conductivity and improved the quadruple contact areas. The porous sensing electrode structure supported a continuous ion and electron conduction paths. The porous sensing microstructure also provided pathways for the infiltration of sulfur dioxide into the porous structure, which effectively increased the available surface area for the electrochemical reaction. On the other hand, a porous microstructure with efficient quadrupole phase boundary may not only secure higher effective surface area for reaction and favorable diffusion of sulfur dioxide into the porous sensing electrode structure, but it may also instigate additional degradation reactions between the solid electrolyte and the sensing electrode. Indeed, once the average emf values and the logarithm of the SO.sub.2 gas concentration was fitted by the Nernst equation in order to evaluate sensitivity (mV/dec), it was determined to be 28.7 (n=5.2 e.sup.−) and 18.7 (n=7.9 e.sup.−) mV/dec for the sensor response step (0-10 ppm) and recovery step (10-0 ppm), respectively, significantly lower than the theoretical sensitivity of 74.64 mV/dec and the sensitivities determined in the case of a dense sensing electrode.

[0113] The investigation revealed that selecting the processing temperature of the Li.sub.2SO.sub.4—CaSO.sub.4 sensing electrode between 600° C. and 750° C. can be used to select either a dense or a porous microstructure sensing electrode. The porous microstructure provided increased surface area for the electrochemical sensing reaction to occur, which may facilitate faster gas adsorption/desorption processes and improve the response/recovery time. The porous microstructure also shortened the Li.sup.+ ion diffusion pathways in the low conducting Li.sub.2SO.sub.4—CaSO.sub.4 phase (˜10.sup.−6 S/cm at 300° C.) compared to the Li.sup.+ ion diffusion through a dense, several micron-thick sensing electrode. The low sensitivity values observed for the porous sensing electrode microstructure may indicate that the voltage does not approximately follow the Nernst law and that the system may not be in a thermodynamic equilibrium.

[0114] A potentiometric type III SO.sub.2 sensor based on the fast-conducting Li-garnet Li.sub.6.54La.sub.3.00Zr.sub.1.36Ta.sub.0.5O.sub.11.73 solid electrolyte was demonstrated by using different sensing electrode microstructures, namely dense and porous, and a porous gold layer as the reference electrode. To succeed here in defining the SO.sub.2 sensing electrode chemistry and phase, and its stability towards the Li garnet electrolyte, the phase compatibility was probed. Two configurations were presented, dense and porous structures of the Li.sub.2SO.sub.4—CaSO.sub.4 sensing electrode by changing the processing temperature. The sensor operated at 480° C. with sensitivities values ranging from 144 to 18 mV/dec and recovery/response time ranging from 5 to 60 min depending on the auxiliary sensing electrode configuration and SO.sub.2 concentrations. The QPB-based porous configuration showed up to 1-order of magnitude lower response time and up to a 75% decrease in the recovery time compared to the TPB-based dense configuration.

[0115] Sensing Electrode Incorporating Li Garnet and the SO.sub.2 Sensor Voltage Response

[0116] The interface between the solid electrolyte and the sensing electrode may be further stabilized by adding Li garnet to the auxiliary sensing electrode. Li garnet may further assist in both improving the ionic conductivity and response time of the sensor by i) creating percolation pathways, forming randomly connected clusters with improved Lition conductivity through the electrode; increasing the effective surface area for electrochemical sensing reactions by creating triple-phase boundary (TPB) reaction zones with shortened Lition diffusion distance from the sensing to the garnet LLZO solid electrolyte; and iii) stabilizing the solid electrolyte itself by playing an active role in forming an interfacial layer between the solid electrolyte and the sensing electrode, thus helping to achieve thermodynamic equilibrium. Two main processing strategies were undertaken in order to incorporate garnet LLZO in the auxiliary phase (Li.sub.2SO.sub.4:CaSO.sub.4): LLZO calcinated powder was directly added to the sensing electrode paste followed by heat treatment at 750° C., as shown in FIG. 6A, and a porous LLZO scaffold was created by preparing a porous LLZO layer on top of the LLZO sintered pellet, followed by infiltration of the sensing electrode paste and subsequent heat treatment at 750°, as shown in FIG. 5A.

[0117] SO.sub.2 sensor operation was investigated in terms of its performance characteristics employing a sensing electrode with an Li garnet scaffold, like that shown in FIG. 5A. The SO.sub.2 concentration was varied from 0 ppm to 10 ppm and back to 0 ppm with 2.5 ppm step changes in dry synthetic air. Electromotive force (emf) values were recorded and converted to SO.sub.2 concentrations to determine detection sensitivity, response time, and recovery time. Response and recovery times were defined as the time the sensor used to reach 90% of the steady final signal and the time the sensor takes to return to 10% of the steady signal, respectively.

[0118] FIGS. 10A-10B show the response time and recovery times of the gas sensor with a Li garnet scaffold at 480° C. in the concentration ranges of 0 ppm to10 ppm SO.sub.2 and 10 ppm to 0 ppm SO.sub.2, respectively, in 2.5 ppm steps. A consistent increase in the emf of the sensor was observed in the electrochemical experiments measuring the emf response upon increase of SO.sub.2 concentration steps, as shown in FIG. 10A. The response time was determined to be >˜30 min while the recovery time was achieved within ˜4-15 min. Nonetheless, the sensitivity of the sensor was determined to be 20.34 mV/dec (n=7.3 e.sup.−) and 8 mV/dec (n=18.6 e.sup.−) for the sensor response step (0-10 ppm) and recovery step (10-0 ppm), respectively, significantly lower than the theoretical sensitivity for a two-electron reaction of 74.64 mV/dec, as shown in FIG. 10B.

[0119] FIGS. 11A-11C show SO.sub.2 sensor operation in terms of its performance characteristics employing a sensing electrode with a dense Li garnet composite, like that shown in FIG. 6A. The Li garnet composite sensing electrode had a sensitivity of 48.4 mV/dec (n=3.1 e.sup.−) and 42.9 mV/dec (n=3.5 e.sup.−) for the sensor response step (0-10 ppm) and recovery step (10-0 ppm), respectively (FIGS. 11A-11C). The response time of ≥˜40 min and recovery time of ≥˜24 min was like the scaffold structure of Li garnet. The gas sensor with a Li garnet composite had a complete recovery to the initial sensor voltage with similar sensitivity values upon increase and decrease of SO.sub.2 concentration steps, as shown in FIG. 11B. In the composite structure where Li garnet was distributed more homogeneously between the sulfate components and chemically reacted with them, the sensing electrode may have a stable thermodynamic equilibrium through both the response and recovery steps. A significant improvement in the response ability of the sensor was demonstrated for the composite sensing electrode by the operation of the sensor at significantly lower temperatures, namely 240° C., as shown in FIG. 11C. At 240° C., the response and recovery times are ≥˜60 min and ≥˜4 min, respectively. These times may be improved by incorporating electronic conductors in the composite sensing electrode to meaningfully increase the active reaction zones. The sensitivity value was determined to be 47.7 mV/dec (n=2.1 e.sup.−) for the sensor response step (0 ppm to 10 ppm), a sensitivity value close to the theoretical value of 50.8 mV/dec (calculated for 240° C.). Without being bound by any theory, it is hypothesized that lower operating temperature inhibits decomposition reactions between Li garnet and Li.sub.2SO.sub.4—CaSO.sub.4, ensuring a sensitivity close to the theoretical value, and implying thermodynamic equilibrium when using a composite sensing electrode. However, the lower operating temperature evidently has a significant effect on the O.sub.2/SO.sub.2 desorption processes, decreasing the sensitivity to 8.2 mV/dec for the sensor recovery step (10 ppm to 0 ppm). A catalyst may be used to increase sensitivity for operation at lower temperatures.

[0120] Chemical and Thermal Stability of the Li-Garnet Li.sub.7La.sub.3Zr.sub.2O.sub.12 in SO.sub.2 Environments

[0121] The inventors have demonstrated SO.sub.2 electrochemical sensors with sufficient stability of the Li garnet electrolyte to maintain its phase and ensure sufficient Li conductivity to function for fast sensor response. To probe stability for the rather harsh environment, dense and cubic Li-garnet solid-state electrolyte pellets (Li.sub.6.5La.sub.3Zr.sub.1.5Ta.sub.0.5O.sub.12) were fabricated via classic solid-state synthesis and their phase stability and microstructural changes were investigated when exposed to low and high SO.sub.2 concentrations ranging between 4 ppm and 250 ppm at temperatures between 240° C. and 480° C. over a 24-hour period.

[0122] FIG. 12A shows morphological changes in Li garnet after exposure to SO.sub.2 observed using SEM. Without being bound by any theory, these morphological changes may indicate changes in near-order vibration of the Li garnet lattice resulting from reaction with SO.sub.2.

[0123] At low SO.sub.2 concentrations (4 ppm), the cubic garnet structure was maintained over the entire probed temperature range, 25-480° C. At temperatures of 320° C. and above, the XRD peak signature at 22.23° became more prominent, which corresponds to (111) diffraction of Li.sub.2SO.sub.4. The thickness of the Li.sub.2SO.sub.4 layer formed on LLZO increased from 0.5 to ˜2 μm upon increasing the temperature from 240° C. to 480° C., according to the SEM cross sectional images in FIG. 12A. For temperatures >320° C., a change in morphology from a continuous but porous layer of Li.sub.2SO.sub.4 to a non-continuous and dense island-like layer was observed. FIG. 12B schematically illustrates the Li garnet morphology changes upon exposure to SO.sub.2 at different concentrations and temperatures. Without being bound by any theory, the formation of Li.sub.2SO.sub.4 may be a product of leaching of Li.sup.+ ions from the surface of the Li garnet pellet followed by a reaction with SO.sub.2 gas. That said, the Li.sub.2SO.sub.4 layer did not substantially affect the Li.sup.+ ionic conductivity of Li garnet, with the Li garnet maintaining a conductivity of ˜0.7 mS cm.sup.−1 at ambient temperature. Electrochemical impedance spectroscopy was used to estimate the bulk ionic conductivity of the same LLZO pellet after 24-h exposure to 10 ppm of SO.sub.2 at 240 and 480° C. and indicated that the Li garnet maintained its cubic structure.

[0124] At medium SO.sub.2 concentrations (40 ppm), the appearance of Li.sub.2SO.sub.4 on the surface of the Li garnet was observed in the SEM, XRD, and Raman analyses at a temperature of 320° C. and above. The thickness of the Li.sub.2SO.sub.4 layer increased by one order of magnitude from ˜350 nm at 240° C. to ˜3.5 μm at 480° C. after exposure to 40 ppm SO.sub.2. The top-view SEM micrographs reveal more about the evolving growth of this layer starting from a porous morphology followed by an increase in grain size and densification of the Li.sub.2SO.sub.4 layer as a function of temperature, as shown in FIG. 12B. The complete coverage of the Li garnet pellet by the dense Li.sub.2SO.sub.4 layer aligns well with its principal observation in the Raman surface characterization and XRD bulk characterization. These results were accompanied by a color change of the pellet from yellowish to white at temperatures above 400° C.

[0125] At high SO.sub.2 concentrations (250 ppm), the Li garnet pellets were exposed to rather harsh conditions over a 24-hour period at temperatures of 240° C. to 480° C. Up to 40 ppm SO.sub.2 exposure, the phase of the bulk Li garnet did not change and remained cubic. At 250 ppm SO.sub.2, XRD analysis revealed that even though the cubic phase remained in Li garnet, there was pronounced Li loss and the formation of lanthanum zirconate (La.sub.2Zr.sub.2O.sub.7), as indicated by the increasing intensity of the (111) diffraction peak at 28.5° with temperature (at both 240 and 480° C.). The cross-section SEM micrographs indicate a continuous and dense Li.sub.2SO.sub.4 layer at 240° C. and 480° C. with thicknesses of ˜2.5 and 4 μm, respectively, exclusively covering the surface of the Li garnet pellet, as shown in FIG. 12A. Altogether, the cubic garnet structure was maintained as the majority phase up to 480° C. in the bulk at low and medium SO.sub.2 concentrations. In very harsh SO.sub.2 environments, there was a threshold of ˜240° C., above which Li leached out from the Li garnet structure significantly in the process of forming the Li.sub.2SO.sub.4 surface layer.

[0126] Exploring the spontaneous formation and morphological evolution of Li.sub.2SO.sub.4 under diverse SO.sub.2 concentrations and temperatures suggests that the major sensing electrode component may preferably be Li.sub.2SO.sub.4 to substantially avoid its in-situ formation during sensor operation and to better manipulate its morphology and thickness through the formation of an artificial sensing electrode layer. The was a tendency toward the formation of Li.sub.2SO.sub.4 on LLZO at the investigated temperatures and SO.sub.2 concentration ranges. The latter is of high importance, indicating the feasibility of SO.sub.2 oxidation under the explored conditions, without the use of catalysts (e.g., platinum (Pt) mesh) typically employed to catalyze the oxidation of SO.sub.2. The investigation clarified that the SO.sub.2 concentration may be kept at <40 ppm, where the gas sensor was operated at elevated temperatures (480° C.). Under these conditions, the cubic structure and high Li.sup.+ conductivity of Li garnet is maintained, indicating the potential long-term operation of the sensor.

[0127] Without being bound by any theory, once an artificial Li.sub.2SO.sub.4 layer is introduced on top of the Li garnet pellet as a sensing electrode, the formation of a new Li.sub.2SO.sub.4 layer, formed in situ on top of the LLZO layer during sensor operation, may be kinetically inhibited. Additional stability investigations of Li garnet with an artificially deposited Li.sub.2SO.sub.4 layer were executed to probe this theory. Qualitative analysis conducted using SEM micrographs and elemental mapping of a Li garnet pellet pre-deposited with Li.sub.2SO.sub.4 and exposed to 10 ppm SO.sub.2 at 480° C. for 24 hours revealed that the in-situ formation of the Li.sub.2SO.sub.4 layer was in fact inhibited. Thin (<500 nm) deposits (such as Li.sub.2CO3 and Li.sub.2SO.sub.4 deposits) were observed mainly at defected areas on the surface of the Li garnet pellet. Moreover, post-mortem SEM and elemental mapping analysis of the sensing electrode/Li garnet cross section revealed that the ˜15-20-μm-thick sensing electrode remained unchanged throughout the sensing experiment (˜24 h) without the appearance of a new deposit layer on top of the sensing electrode.

[0128] The phase and microstructure evolution indicate that the processing range to first establish the sensing electrode formation of Li.sub.2SO.sub.4 on LLZO may be separated from the operation conditions of the sensor. Manufacture of the sensing electrode on LLZO pellets may be fabricated artificially in a step prior to operation of the sensor. With this method, the electrode's microstructure and composition can be tuned efficiently. Operation of the SO.sub.2 sensor may proceed at a lower temperature and lower SO.sub.2 concentrations where long operation lifetimes are desired. The Li garnet pellet remained primarily in the cubic phase, with no occurrence of the Li-loss phase observed for a critical amount of ˜3 μm of Li.sub.2SO.sub.4 formed when operated between 240° C. and 480° C. for 4 ppm to 40 ppm SO.sub.2. Considering that the regulation values for SO.sub.2 detection in ambient atmosphere are 0.1-10 ppm, the gas sensor may be preferably operated in the regulated SO.sub.2 range at a temperature of 240-480° C.

[0129] Li Garnet/Li.sub.2SO.sub.4 Interface

[0130] The electrode/electrolyte interface plays a critical role in determining the performance characteristics of the gas sensor. The gas sensor responds to the difference in the chemical potential of Li.sup.+ at the interfaces of the electrodes. The Li garnet solid electrolyte acts as a Li.sup.+-ion source that facilitates Li.sup.+ diffusion to and from the sensing electrode so that Li.sup.+ can chemically react either during the processing of the sensing electrode (e.g., at 750° C.) or during the operation of the sensor (e.g., at 480° C.). The sensing electrode in the following experiments was a composite of Li.sub.2SO.sub.4 and CaSO.sub.4. This composite benefits from faster response time attributed to lower melting point and higher ionic conductivity than Li.sub.2SO.sub.4 alone. In the particular material composite case of Li.sub.2SO.sub.4—CaSO.sub.4LLZ0, both the sensing electrode and the solid electrolyte are Lition conductors. In other words, Li.sup.+ is the mobile ion. Thus, the electrochemical chain through the interface is achieved with Li.sup.+ ions. This contrasts with other type III potentiometric electrochemical sensors, where the sensing electrode and solid electrolyte are based on different mobile ions (e.g., Li.sup.+ conductor and O.sup.2− conductor for Li.sub.2SO.sub.4|MSZ), necessitating the formation of a mediating phase (ionic bridge) to provide a fast and stable electrochemical response by delivering a continuous path for ion conduction.

[0131] FIG. 13 shows XRD studies of possible chemical reaction between LLZO and the sensing electrode (Li.sub.2SO.sub.4—CaSO.sub.4). An interfacial layer may be formed at the sensing electrode/solid electrolyte interface during the sensing electrode coating process (750° C.) and/or operation of the sensor (480° C.). The interfacial layer may deteriorate the SO.sub.2 sensing ability and create a complex voltage response due to competitive electrochemical reaction. In one experiment, cross-sectional SEM micrographs and elemental mapping of a sensor after a prolonged sensing experiment revealed a ˜1-2-μm-thick Ca-rich interfacial layer sandwiched between the LLZO solid electrolyte and sensing electrode. The interface may be stabilized by adding Li garnet to the auxiliary sensing electrode, as described in more detail above.

[0132] Fabrication of the Gas Sensor

[0133] Synthesis of LLZO Solid Electrolyte

[0134] The solid electrolyte Li.sub.6.5La.sub.3Zr.sub.1.5Ta.sub.0.5O.sub.12 Li garnet was prepared via solid state reaction route using stoichiometric amounts of La(OH).sub.3, ZrO.sub.2, Ta.sub.2O.sub.5, and an excess amount of 50 wt. % LiOH to compensate for the lithium evaporation during the high temperature calcination of the powder. The precursors were homogenized by planetary milling in absolute isopropanol, and then dried at 90° C. The obtained powder was packed and first-calcinated at 750° C. for 10 h at a heating rate of 5° C. min.sup.−1 under the constant flow of synthetic air (50 sccm). The last two steps of ball-milling, drying and calcination were repeated once again. Finally, weighted amount of the Li garnet powder was pressed into a pellet using a die with a diameter of 12 mm and thickness of 1.5 mm in a uniaxial press (2.2 tons.Math.cm.sup.−2). The green pellets were sintered in a MgO crucible at 1100° C. for 5 hours at heating/cooling rates of 10° C. min.sup.−1 and under a constant flow of pure oxygen (50 sccm). The sintered Li garnet pellets were dry-polished to ensure consistency among all samples.

[0135] Fabrication of the Sensing Electrode and Sensor Device

[0136] The sensing electrode was prepared by mixing different mole ratios of lithium sulfate ((Li.sub.2SO.sub.4), anhydrous, 99.99% trace metal basis) and calcium sulfate ((CaSO.sub.4), anhydrous, 99.99% trace metal basis). A binder solution (mixture of α-terpineol and ethylene cellulose) was added in a weight ratio of 1:2 to the different Li.sub.2SO.sub.4:CaSO.sub.4 powder mixture, crushed and homogenized using a mortar until a smooth paste was produced. The paste was brushed on a quarter of the solid electrolyte and heated at 750° C. for 2 h at a heating rate of 10° C. min.sup.−1 under the constant flow of pure oxygen. After the sensing electrode preparation, a gold paste (Conductive Epoxy GOLD Paste, EMS) was brush-painted on the second quarter of the solid electrolyte surface, serving as the reference electrode (RE). A thin layer of the gold paste was brushed on the sensing electrode, serving as a current collector). Platinum wires (0.1 mm diameter, 99.995%) were connected to the sensing and reference electrodes using the gold paste. The complete sensor construction was annealed at 300° C. for 4 h in an oxygen atmosphere to cure the gold paste. An adhesive sealant was applied on the reference electrode (898FS, Cotronics corp.) and cured again at 300° C. for 2 h in an oxygen atmosphere.

[0137] In order to develop a porous sensing electrode two major processing approaches were taken: i) reducing the sensing electrode processing temperature to 600° C. instead of 750° C. while maintaining all other parameters, or ii) adding 10% of corn starch as a pore former to the auxiliary electrode paste followed by a heat treatment at 600° C. for 4 h at a heating rate of 10° C. min.sup.−1 under the constant flow of pure oxygen.

[0138] A composite sensing electrode including Li garnet was fabricated by adding 10 wt. % of the synthesized LLZO powder in the auxiliary electrode paste followed by the typical heating step at 750° C. for 2 hours at a heating rate of 10° C. min.sup.−1 under the constant flow of pure oxygen.

[0139] A porous LLZO scaffold layer was prepared by brushing a paste of the sintered LLZO powder and binder solution (mixture of α-terpineol and ethylene cellulose) in a 1:1 weight ratio on top of the LLZO pellet. Corn starch was added as a pore former. The sample was sintered at 1100° C. for 2 h under oxygen flow. Once a porous LLZO scaffold was formed, a sensing electrode paste was sprayed on top of the porous LLZO and further heated to 750° C. for 2 h at a heating rate of 10° C. min' under the constant flow (50 sccm) of oxygen.

[0140] When better control of the thickness and geometry of the sensing electrode was used (for instance, in the case of the porous LLZO scaffold layer), a spraying procedure was employed. Appropriate amounts of the Li.sub.2SO.sub.4 and CaSO.sub.4 powders were weighed and ball-milled in absolute isopropanol for 3 hours. The homogenized suspension was loaded into a spray gun and sprayed on the LLZO pellet using a shadow mask. The sprayed sensing electrode was then heated at 750° C. for 2 hours at a heating rate of 10° C. min.sup.−1 under the constant flow (50 sccm) of pure oxygen.

[0141] Chemical stability investigation of LLZO solid electrolyte and auxiliary sensing electrode components. Powder mixtures of LLZO and the sulfates, i.e., LLZO:Li.sub.2SO.sub.4:CaSO.sub.4, in different mole ratios (namely 1:0:1, 1:1:2, 0:1:1, 1:0:0, 0:1:0, and 1:1:0) were thoroughly mixed using a mortar and pestle, packed into a 12-mm diameter die, and heated at 750° C. for 2 h at a heating rate of 10° C. min' under the constant flow (50 sccm) of pure oxygen. The reaction products were identified using XRD.

[0142] Sensing System Setup and Electrochemical Sensing Measurements

[0143] The SO.sub.2 gas sensing experiments were conducted and measured in a Linkam stage (HFS600E) with an internal volume of ˜50 cm.sup.3 equipped with a heating element in the temperature range of 25-600° C. Two automated mass flow controllers were used to balance between 50 ppm SO.sub.2 balanced by dry synthetic air (21% O.sub.2 in N.sub.2) in order to achieve different concentrations of the analyzed gas, namely of 0-10 ppm of SO.sub.2 with 2.5 ppm steps, held for 1-2 h. The open-circuit voltage was measured using a Kiethley 2612B electrometer. The reference and sensing electrodes were contacted using the Pt wires, which were glued to the electrodes with gold paste. The sensitivity of the sensor was evaluated through sensing experiments where the open-circuit voltage (OCV) was measured as a function of SO.sub.2 concentration at a constant and calibrated temperature of either 240, 320, 400 and 480° C. with a heating/cooling rate of 10° C. min.sup.−1.

[0144] Surface and Bulk Characterization

[0145] Field Emission Scanning Electron Microscopes (FESEM Ultra Plus and FESEM Supra55VP equipped with an Energy Dispersive x-ray Spectrometer (EDS) for elemental analysis and mapping, Zeiss) and Scanning Electron Microscope (SEM JEOL 7900F) equipped with wide variety of detectors including EDS and a Soft X-ray Emission Spectrometer (SXES) allows efficient and parallel collection of very low-energy rays with chemical state analysis. The different scanning electron microscopes were used to characterize the microstructure of the sensing electrodes as a function of their processing procedures.

[0146] Melting temperature of the sensing electrode as a function of its composition was determined via Differential Scanning calorimetry/Thermogravimetric (DSC/TGA) analysis of different mole ratio of Li.sub.2SO.sub.4: 4, namely 1:0, 1:1, 4:1, 2:1, 1:2 and 4:1. The measurement was conducted under synthetic air from room temperature to 900° C., held for 5 min and cooled back to room temperature at a ramp rate of 10° C./min for both heating and cooling steps.

[0147] Phase Compatibility and Synthesis Protocol for The SO.sub.2 Sensing Electrode/Li-Garnet Electrolyte

[0148] A thorough electrochemical cell design and operational principles were explored for the Li.sub.7La.sub.3Zr.sub.2O.sub.12 garnet-based sensor tracking SO.sub.2. The sulfur dioxide sensor architecture had an in-plane geometry including a Ta-doped LLZO Li garnet pellet solid electrolyte with a sensing electrode and a gold reference electrode forming the gas sensor, as shown in FIG. 2B and FIG. 7. The gold reference electrode was brush-painted as a paste on top of the dense pellet of Li garnet and sealed with inorganic adhesive (on top of the pellet on all exposed surfaces except the sensing electrode). The inorganic adhesive served as a protective layer to substantially prevent water vapor and SO.sub.2 gas in the atmosphere reacting with the Li garnet or the reference electrode. The sensing electrode includes two or multiple phase components, which may help with sluggish recovery times. Examples of components that may be added to the sensing electrode include CaSO.sub.4 (with and without SiO.sub.2), BaSO.sub.4, V.sub.2O.sub.5, or La.sub.2O.sub.2SO.sub.4—K.sub.2SO.sub.4. In the current study, garnet solid electrolyte obviates the use of SiO.sub.2 to avoid decomposition of the electrolyte through the formation of lithium silicate (Li.sub.2Si.sub.xO.sub.2x+1) compounds.

[0149] In case of a two-component sensing electrode, i.e., Li.sub.2SO.sub.4—CaSO.sub.4, a less pronounced effect was observed in the average response time (˜20 sec) and average recovery time (˜12-18 min) as a function of the Li.sub.2SO.sub.4—CaSO.sub.4 mole ratio. Without being bound by any theory, the addition of other metal sulfates such as CaSO.sub.4, in eutectic proportions, i.e. ˜17 mol %, may suppress interference in the sensor performance attributed to the presence of humidity. Humidity can interfere with the voltage readout because water can affect the activity of SO.sub.2. If more than one electrochemical reaction takes place at the sensing electrodes, the sensor operates under a mixed-potential due to competitive reactions. This creates a more complex voltage response. Without being bound by any theory, the addition of other metal sulfates such as CaSO.sub.4, may also lower the melting temperature of the mixture. Materials with a lower melting temperature tend to have higher ionic conductivity due a higher defect concentration close to the melting point, which increases gas detection sensitivity at lower temperatures.

[0150] FIG. 14 shows the melting point of Li.sub.2SO.sub.4 and CaSO.sub.4 mixtures containing 0, 20, 33, 50, 67, 80 and 100 mol % CaSO.sub.4. The melting points indicate that the addition CaSO.sub.4 in 20 mol % to 80 mol % lowered the melting point of the mixture by ˜150° C., from 853° C. (100% Li.sub.2SO.sub.4) to ˜700° C. upon CaSO.sub.4 addition in 20-80 mol %. The results were also used to determine the firing temperature used to assure a complete melt of the sulphate's mixture and improve the adhesion between the sensing electrode layer and the solid electrolyte during fabrication of the gas sensor. The firing temperature was set to 750° C. so that the Li.sub.2SO.sub.4—CaSO.sub.4 sulfates mixture was completely melted. In-situ XRD investigation indicated that LLZO exhibited excellent stability properties and may support the processing temperature used, i.e., 750° C., to melt the solid electrolyte. Thus, the sensing electrode with a chemical composition of Li.sub.2SO.sub.4—CaSO.sub.4 in a mole ration of 1:1, was brushed directly on the surface of the LLZO solid electrolyte pellet, dried and heated up to 750° C. to provide adhesion between the dense sensing electrode and the solid electrolyte.

[0151] Further Mitigating LLZO Instability

[0152] FIG. 15A shows the open circuit voltage response of an SO.sub.x sensor to four cycles of SO.sub.2 concentration step changes (0-10-0 ppm) at 480° C. FIG. 15B shows sensitivity values from the data in FIG. 15A. The calculated number of electrons (slope of V vs. log[SO.sub.2] was in good agreement with the expected value of n=2 going from high to low [SO.sub.2] concentrations.

[0153] FIG. 16A shows the open circuit voltage response of an SO.sub.x sensor with an inorganic coating to SO.sub.2 concentration step changes (0-15 ppm) at 480° C. FIG. 16B shows average sensitivity values from the data in FIG. 16A. Using an inorganic adhesive (e.g., Al.sub.2O.sub.3) as a coating on top of the LLZO mitigated LLZO instability in the presence of an SO.sub.2 environment. The inorganic adhesive includes a ceramic material (e.g., Al.sub.2O.sub.3, ZrO.sub.2) that is substantially stable in a SO.sub.2 environment. The inorganic adhesive may be applied to the LLZO as a viscous paste and then heat treated. For example, a slurry including Al.sub.2O.sub.3 particles was applied as a coating on to the reference electrode using a brush. After, the slurry coating was applied, the slurry coating was thermally treated at 300° C. for two hours. In one implementation, the inorganic adhesive coated the reference electrode only. In another implementation, the inorganic adhesive coated the entire top surface of the sensor except for the sensing electrode. A sensor with the entire top surface of the sensor except for the sensing electrode coated with the inorganic adhesive demonstrated faster response time (˜20 min) and recovery time (˜10 min) but the sensor had a lower sensitivity.

[0154] Sensing Electrode Additives and Alternative Materials

[0155] The perovskite (La.sub.0.6Sr.sub.0.4).sub.0.99CoO.sub.3−δ (LSC) is an electrically conductive material that may be added to the sensing electrode composite and/or used as the current collector disposed on the sensing electrode. LSC may also act as a catalyst for the oxidation of SO.sub.2 to SO.sub.3 at the TPB or QPB in the sensing electrode. As a catalyst, LSC may support lower operation temperatures of the SO.sub.x sensor (e.g., 240° C.). LSC may be included in the sensing electrode composite in quantities of about 0% to about 60% by weight (e.g., 10%, 30%, or 50%). Preferably, the LSC is present in quantities of about 10% by weight. In place of LSC, other perovskite oxides may be used that have a high mixed ionic and electronic conductance and stability at temperatures greater than 350° C.

[0156] FIG. 17 shows planar SEM images of sensing electrode composites comprising Li.sub.2SO.sub.4, CaSO.sub.4 and LSC with varying amounts of LSC and different annealing temperatures. FIG. 17 shows 10%, 30%, and 50% by weight LSC in the composite of the sensing electrodes. Two different annealing temperatures were used for the preparation of the sensing electrode, 600° C. and 750° C. The 600° C. annealing temperature provided a porous sensing electrode morphology, while the 750° C. annealing temperature provided a dense sensing electrode morphology.

[0157] FIGS. 18A and 18B show the operation of gas sensors using sensing electrodes that include composites of Li.sub.2SO.sub.4, CaSO.sub.4 and LSC operated at 240° C. with SO.sub.2 concentrations ranging from 0 ppm to 15 ppm. The SO.sub.2 concentration was changed from 0 ppm to 15 ppm and back to 0 ppm, with 2.5 ppm step changes every 1.5 hours. FIG. 18A shows the open circuit voltage (OCV) of the gas sensor with a sensing electrode including 10% by weight LSC. FIG. 18B shows average OCV measurements vs. the log of the concentration of SO.sub.2 from gas sensors with sensing electrodes including 10%, 30%, and 50% LSC at 240° C. With 10% by weight LSC in the sensing electrode, the gas sensor operated at low temperature (240° C.) with sensitivity values close to theoretical (n=2).

[0158] FIGS. 19A and 19B show the operation of gas sensors using sensing electrodes that include composites of Li.sub.2SO.sub.4, CaSO.sub.4 and LSC operated at 320° C. with SO.sub.2 concentrations ranging from 0 ppm to 15 ppm. The SO.sub.2 concentration was changed from 0 ppm to 15 ppm and back to 0 ppm, with 2.5 ppm step changes every 1.5 hours. FIG. 19A shows OCV measurements of the gas sensor with a sensing electrode including 50% by weight LSC. FIG. 19B shows average OCV measurements vs. the log of the concentration of SO.sub.2 from gas sensors with sensing electrodes including 10%, 30%, and 50% LSC at 320° C. With 50% by weight LSC in the sensing electrode, the gas sensor had its highest sensitivity at a temperature of about 320° C.

[0159] FIG. 20 shows SO.sub.2 detection sensitivity as a function of the amount of LSC in the composite sensing electrode at three different operation temperatures, 240° C., 320° C., and 400° C. Three different amounts of LSC in the sensing electrode were probed, 10%, 30%, and 50% by weight. Sensitivity values are shown in mV per decade (mV/dec), where 1 decade is a 10-ppm change in SO.sub.2 concentration. Theoretical values are given based on the expected changes using the Nernst equation and the SO.sub.2 concentration. With 10% by weight LSC in the sensing electrode, the gas sensor had its highest sensitivity at 240° C. With 30% by weight LSC in the sensing electrode, the gas sensor had its highest sensitivity at 320° C. and 400° C.

[0160] FIG. 21A shows the open circuit voltage response of an SO.sub.x sensor with a sensing electrode with 30% (La.sub.0.6Sr.sub.0.4).sub.0.99CoO.sub.3−δ to SO.sub.2 concentration step changes (0-10 ppm) at 400° C. FIG. 21B shows average sensitivity values from the data in FIG. 21A. These sensing experiments indicated that 30% wt. of LSC addition to the sensing electrode showed good sensing performance at 400° C. with ˜10-minute response time. Sensing electrodes with LSC may be used for the detection of lower concentrations (e.g., less than 1 ppm) of SO.sub.2.

[0161] FIG. 22 shows the mechanism of catalyst-assisted SO.sub.2 oxidation. A catalyst may be added to the sensing electrode to facilitate oxidation of SO.sub.2. A catalyst may facilitate sensor operation at lower temperatures. The catalyst may be, for example, Fe.sub.2O.sub.3, NiO, Pt, LSC, or La.sub.xSm.sub.1−xFeO.sub.3 where x is 0.2-0.8 (e.g., 0.2, 0.4, 0.5, 0.6, or 0.8). The catalyst may be present in the sensing electrode in a weight percent of about 1% to about 50% (e.g., 1%, 10%, 30%, or 50%).

[0162] FIG. 23A shows an SEM image of a sensing electrode with 10% Fe.sub.2O.sub.3 as a catalyst. The SEM image shows particles of Fe.sub.2O.sub.3 2310 homogenously dispersed in the sensing electrode 2300. FIG. 23B shows the open circuit voltage (OCV) response of an SO.sub.x sensor with a sensing electrode with 10% Fe.sub.2O.sub.3 to SO.sub.2 concentration step changes (0-10 ppm) at 480° C. FIG. 23C shows average sensitivity values from the data in FIG. 23B. The addition of Fe.sub.2O.sub.3 catalyst assisted in recovering the sensor voltage response during decreasing SO.sub.2 concentration from 10 ppm to 0 ppm.

[0163] Thin Film SO.sub.x Sensors

[0164] FIG. 24 is a photograph of a thin film electrochemical potentiometric SO.sub.x sensor. FIGS. 25A-25D show several cross-sectional schematics of thin film electrochemical potentiometric SO.sub.x sensors with an in-plane configuration. These devices are deposited on a substrate (e.g., MgO) using one or more thin film deposition techniques (e.g., pulsed laser deposition, spray pyrolysis, sputtering, spin coating, tape casting). In one implementation, shadow masks are used to direct the fabrication of the sensor. In another implementation, photolithography is used to direct the fabrication of the sensor. In one embodiment of the in-plane configuration, a first PLD layer of crystalline LLZO was deposited on a MgO substrate and a second PLD layer of LSC was deposited on the LLZO. Then the sensing electrode was spray deposited onto part of the LSC layer and the LLZO and the sensor was post-annealed at a temperature between 600° C. and 750° C. Then a blocking layer (e.g., Al.sub.2O.sub.3, Gd.sub.2O.sub.3, HfO.sub.2, SrTiO.sub.3) was added to cover the LLZO but not the sensing electrode. In another embodiment of the in-plane configuration, a first PLD layer of crystalline LLZO was deposited on a MgO substrate. Then the sensing electrode was spray deposited onto part of the LLZO layer and then post-annealed at a temperature between 600° C. and 750° C. Then a second PLD layer of LSC was deposited and a blocking layer was added. In place of the LSC PLD step, a gold paste may be used instead.

[0165] FIG. 26 shows a cross-sectional schematic of a thin film electrochemical potentiometric SO.sub.x sensor with a cross-plane configuration. This sensor may be made by first depositing an LSC layer using PLD, then depositing an LLZO layer using PLD, then spray depositing the sensing electrode. The sensing electrode is then post-annealed before a blocking layer is applied. The in-plane configuration of the thin film sensor is preferred over the cross-plane configuration because the current collector (e.g., LSC, Au) may de-wet after post-annealing in the cross-plane configuration.

[0166] FIG. 27 is a cross-sectional SEM image of part of a thin film SO.sub.x sensor with an in-plane configuration. The sensor was post-annealed at 750° C. for 2 hours in synthetic air.

[0167] FIGS. 28A-28C shows SEM images of sensing electrodes deposited on thin film lithium garnet using spray pyrolysis. The sensing electrode (Li.sub.2SO.sub.4—CaSO.sub.4—Au) deposition morphology changed as a function of spraying time. For a dense and continuous morphology, about 10 μm of sensing electrode was deposited.

[0168] FIG. 29A shows the open circuit voltage response of a thin film SO.sub.x sensor to SO.sub.2 concentration step changes (0-10 ppm) at 320° C. FIG. 29B shows average sensitivity values for the data in FIG. 29A. The thin film SO.sub.x sensor operated at 320° C. with sensitivity values close to the theoretical sensitivity values in the response step.

[0169] FIGS. 30A-30B show SEM images of thin film lithium garnet exposed to 10 ppm SO.sub.2 at different temperatures (160° C. and 240° C.) for 24 hours. The lithium garnet substantially degraded when exposed to SO.sub.2 at 480° C. Therefore, the thin film SO.sub.x sensor is operated at temperatures lower than 400° C. to reduce or prevent substantial degradation of the solid electrolyte.

[0170] FIG. 31 shows Raman characterization of thin film lithium garnet after SO.sub.2 exposure, as shown in FIGS. 30A and 30B. The lithium garnet was a 1.5 μm thick layer of Ta-doped LLZO. Increasing the temperature from 240° C. to 480° C. during SO.sub.2 exposure led to a change in Li.sub.2SO.sub.4 layer morphology from a continuous and porous layer to a non-continuous and denser layer. These results indicate that the working temperature of the thin-film SO.sub.x sensor may be less than 320° C. to maintain cubic LLZO.

[0171] FIG. 32 shows Raman characterization of thin film lithium garnet before and after post-annealing at 750° C. Thin film Ta-doped LLZO was deposited by PLD on a MgO substrate using a multilayer deposition approach according to Pfenninger, Rupp et al., Nature Energy, 2019, 4(6), 475-483, which is attached as an Appendix. A cubic LLZO thin film was evident before and after depositing the sensing electrode on top of the LLZO electrolyte via spray pyrolysis and post-annealing at 750° C. La.sub.2Zr.sub.2O.sub.7 (LZO), a decomposition product, was also present in the LLZO film after post-annealing.

Conclusion

[0172] While various inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the inventive embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the inventive teachings is/are used. Those skilled in the art will recognize or be able to ascertain, using no more than routine experimentation, many equivalents to the specific inventive embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure.

[0173] Also, various inventive concepts may be embodied as one or more methods, of which an example has been provided. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments.

[0174] All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms.

[0175] The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.”

[0176] The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with “and/or” should be construed in the same fashion, i.e., “one or more” of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to “A and/or B”, when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

[0177] As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and/or” as defined above. For example, when separating items in a list, “or” or “and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of” or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e., “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law.

[0178] As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

[0179] In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” “composed of,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.