Electrode comprising material to help stabilize oxide of catalyst for electrochemical sensor

An electrode comprises a catalyst and material to help stabilize an oxide of the catalyst. The electrode for one embodiment may be used for an electrochemical sensor or sensor cell.

TECHNICAL FIELD

One or more embodiments described in this patent application relate to the field of electrochemical sensors.

BACKGROUND ART

Electrochemical sensors may be used for a wide variety of purposes. Carbon monoxide (CO) sensors, for example, may be used to help detect unsafe levels of carbon monoxide (CO) in a home or garage, for example. Carbon monoxide (CO) sensors may also be used for flue gas analysis in an environment where both carbon monoxide (CO) and hydrogen (H2) may be present. Carbon monoxide (CO) sensors that use a measuring electrode of platinum black (Pt), however, also respond to hydrogen (H2) and therefore may falsely identify the presence of carbon monoxide (CO).

SUMMARY

One or more embodiments of an electrode comprise a catalyst and material to help stabilize an oxide of the catalyst.

One or more embodiments of a sensor cell comprise an electrolyte and an electrode comprising a catalyst and material to help stabilize an oxide of the catalyst.

One or more embodiments of an electrochemical sensor comprise a measuring electrode comprising a catalyst and material to help stabilize an oxide of the catalyst, a counter electrode, an electrolyte, and sensor operating circuitry coupled to the measuring electrode and counter electrode.

One or more embodiments of a method comprise forming two or more electrodes and positioning the two or more electrodes relative to a reservoir to couple the two or more electrodes to an electrolyte in the reservoir. At least one electrode comprises a catalyst and material to help stabilize an oxide of the catalyst.

One or more embodiments of another method comprise biasing one or more electrodes coupled to an electrolyte, measuring any current produced from one of the electrodes, and identifying whether a target particle is present based on the measured current. At least one electrode comprises a catalyst and material to help stabilize an oxide of the catalyst.

DETAILED DESCRIPTION

The following detailed description sets forth an embodiment or embodiments for an electrode comprising material to help stabilize an oxide of a catalyst for an electrochemical sensor.

Sensor

FIG. 1illustrates, for one embodiment, an electrochemical sensor100. Sensor100may be used to sense any suitable target particle in any suitable environment for any suitable purpose.

Sensor100for one embodiment comprises a measuring electrode120comprising a catalyst to sense one or more suitable target particles, such as carbon monoxide (CO) for example. Measuring electrode120for one embodiment also comprises material to help stabilize an oxide of the catalyst. The material for one embodiment may help stabilize an oxide of the catalyst formed on a surface of measuring electrode120. Stabilizing an oxide of the catalyst for one embodiment helps reduce or minimize sensitivity of measuring electrode120to a potentially interfering non-target particle, such as hydrogen (H2) for example, that measuring electrode120may otherwise sense as a target particle. Measuring electrode120for one embodiment comprises a catalyst comprising platinum (Pt) to sense carbon monoxide (CO), for example, and comprises a suitable metal oxide to help stabilize a platinum oxide (PtxOy) in measuring electrode120to help reduce or minimize the cross-sensitivity of measuring electrode120to hydrogen (H2).

As illustrated inFIG. 1, sensor100for one embodiment comprises a sensor cell102and sensor operating circuitry105conductively coupled to sensor cell102. Sensor operating circuitry105for one embodiment may also be coupled to or in wireless communication with an output device180. Output device180may be local to or remote from sensor operating circuitry105and may or may not be a component of sensor100.

Sensor cell102for one embodiment comprises a housing110, measuring electrode120, a reference electrode130, a counter electrode140, an electrolyte150, and electrical contacts122,132, and142. Although described as having three electrodes, sensor cell102for another embodiment may have only two electrodes or may have more than three electrodes. Sensor cell102for another embodiment may not have reference electrode130, for example.

Housing110for one embodiment defines an electrolyte reservoir of any suitable size and shape to hold electrolyte150and is configured to help support measuring electrode120, reference electrode130, and counter electrode140such that at least a portion of each of measuring electrode120, reference electrode130, and counter electrode140are coupled to electrolyte150. Housing110for one embodiment may help support measuring electrode120in any suitable position in the electrolyte reservoir such that at least a portion of measuring electrode120is immersed in electrolyte150. Housing110for one embodiment may help support both reference electrode130and counter electrode140in any suitable position in the electrolyte reservoir such that both reference electrode130and counter electrode140are immersed in electrolyte150.

Measuring electrode120, reference electrode130, and counter electrode140for one embodiment may each have any suitable size and shape for positioning in the electrolyte reservoir. Where the electrolyte reservoir is shaped as a hollowed cylinder, for example, measuring electrode120for one embodiment may be disc-shaped and reference electrode130and counter electrode140for one embodiment may be ring-shaped or disc-shaped.

Housing110for one embodiment defines an opening112of any suitable size and shape through which a target particle may pass from an environment external to housing110to measuring electrode120. Housing110may define opening112and help support measuring electrode120in any suitable position relative to one another. For one embodiment, housing110may help support a membrane124in any suitable position relative to opening112and to measuring electrode120to allow a target particle to diffuse through membrane124to measuring electrode120and to help prevent electrolyte150from passing through membrane124and out of the electrolyte reservoir. Membrane124for one embodiment, as illustrated inFIG. 1, may be coupled to measuring electrode120. Membrane124may be formed from any suitable material, such as polytetrafluoroethylene (PTFE) for example.

Housing110for one embodiment may also help support an optional chemical filter114in any suitable position relative to opening112to help prevent one or more poisons that may damage measuring electrode120and/or one or more potentially interfering non-target particles that may otherwise be sensed by measuring electrode120as target particles from reaching measuring electrode120. Housing110for one embodiment may also help support an optional dust filter116in any suitable position relative to opening112to help prevent dust, dirt, mites, etc. from interfering with sensor cell102.

Housing110for one embodiment may define an optional opening of any suitable size and shape through which oxygen (O2), for example, may pass from an environment external to housing110to counter electrode140. Housing110may define such an opening and help support counter electrode140in any suitable position relative to one another. For one embodiment, housing110may help support a suitable membrane in any suitable position relative to such an opening and to counter electrode140to allow oxygen (O2), for example, to diffuse through such a membrane to counter electrode140and to help prevent electrolyte150from passing out of the electrolyte reservoir.

Housing110for another embodiment may be configured to help support measuring electrode120, reference electrode130, and counter electrode140for coupling to electrolyte150in any other suitable manner. Housing110for one embodiment may be configured to help support measuring electrode120, reference electrode130, and/or counter electrode140external to an electrolyte reservoir for coupling to an electrolyte through wetting filters, for example, coupled to porous walls, for example, helping to define the reservoir.

Housing110for one embodiment helps support electrical contacts122,132, and142. Electrical contacts122,132, and142are conductively coupled to measuring electrode120, reference electrode130, and counter electrode140, respectively. Electrical contacts122,132, and142are to be conductively coupled to sensor operating circuitry105to conductively couple measuring electrode120, reference electrode130, and counter electrode140, respectively, to sensor operating circuitry105. Electrical contacts122,132, and142for one embodiment, as illustrated inFIG. 1, may be shaped as pins for insertion into corresponding socket openings of a connector for sensor operating circuitry105. Electrical contacts122,132, and142for another embodiment may be shaped in any other suitable manner.

For another embodiment, sensor operating circuitry105may be directly coupled to measuring electrode120, reference electrode130, and counter electrode140.

Housing110may be formed from any suitable material. Housing110for one embodiment may be formed from a suitable material that resists corrosion.

Sensor operating circuitry105is to operate sensor100to sense one or more target particles in an environment near sensor cell102. Sensor operating circuitry105may be conductively coupled to sensor cell102either locally in or near the same environment or remotely.

Sensor Use

FIG. 2illustrates, for one embodiment, a flow diagram200to use sensor100.

For block202ofFIG. 2, sensor cell102is exposed to an environment in which a target particle may appear. Sensor cell102may be exposed to any suitable environment in which any suitable one or more target particles may appear. The target particle(s) to be sensed may depend, for example, on the material used for measuring electrode120and counter electrode140and on the circuitry used for sensor operating circuitry105. Sensor cell102for one embodiment may be exposed to an environment in which carbon monoxide (CO) may appear. Sensor cell102for one embodiment may be exposed to an environment in which carbon monoxide (CO) appears in the presence of hydrogen (H2).

For blocks204,206,208,210, and212, sensor operating circuitry105operates sensor100to sense one or more target particles.

For block204, sensor operating circuitry105biases one or more electrodes to a suitable electrical potential. Sensor operating circuitry105for one embodiment may comprise any suitable circuitry to bias measuring electrode120, reference electrode130, and/or counter electrode140in any suitable manner relative to one another and/or relative to ground.

Reference electrode130for one embodiment helps provide a relatively stable electrochemical potential as reference for sensor operating circuitry105. Sensor operating circuitry105for one embodiment helps hold the electrochemical potential of reference electrode130relatively constant such that approximately no electrical current flows through reference electrode130to provide what is known as a quasi-reference electrode. For another embodiment, reference electrode130may be what is known as a true reference electrode by using, for example, Ag/AgCl, a standard calomel electrode, or Hg/Hg2SO4 for reference electrode130.

Sensor operating circuitry105for one embodiment may help bias measuring electrode120relative to reference electrode130at a relatively constant electrical potential. Sensor operating circuitry105for another embodiment may help maintain both measuring electrode120and reference electrode130generally at the same electrical potential. Sensor operating circuitry105for one embodiment may allow the electrical potential at counter electrode140to vary.

Measuring electrode120comprises a catalyst to help precipitate a reaction of target particles that pass through opening112and membrane124of sensor cell102and contact measuring electrode120. Measuring electrode120for one embodiment may catalyze the oxidation of a target particle to produce the oxidized target particle, ions (H+), and electrons (e−). Measuring electrode120may also be called a working electrode or a sensing electrode.

For one embodiment where a target particle is carbon monoxide (CO), measuring electrode120catalyzes the oxidation of carbon monoxide (CO) to carbon dioxide (CO2) as follows.
CO+H2O→CO2+2H++2e−
The resulting ions (H+) flow through electrolyte150to counter electrode140while the resulting electrons (e−) flow to sensor operating circuitry105, allowing sensor operating circuitry105to identify a presence of carbon monoxide (CO) in an environment near sensor cell102. The resulting carbon dioxide (CO2) passes back through membrane124, for example, into an environment external to sensor cell102.

Counter electrode140comprises a catalyst for a corresponding but converse counter reaction at counter electrode140. Counter electrode140for one embodiment may catalyze the reduction of oxygen (O2) to produce water (H2O).

For one embodiment where measuring electrode120catalyzes the oxidation of carbon monoxide (CO), counter electrode140catalyzes the reduction of oxygen (O2) as follows:
O2+4H++4e−→2H2O
and therefore helps avoid consumption of components of sensor cell102in completing the overall sensor cell reaction as follows.
2CO+O2→2CO2
Counter electrode140for one embodiment may receive oxygen (O2) saturated in electrolyte150and/or through an optional opening in the electrolyte reservoir near counter electrode140. Counter electrode140for one embodiment may receive ions (H+) flowing through electrolyte150from measuring electrode120and may receive electrons (e−) from sensor operating circuitry105.

The catalyst of measuring electrode120for one embodiment may also help precipitate a reaction of potentially interfering non-target particles that appear in an environment near sensor cell102and that pass through opening112and membrane124of sensor cell102and contact measuring electrode120.

As one example, the catalyst of measuring electrode120may catalyze the oxidation of hydrogen (H2) as follows:
H2→2H++2e−
while counter electrode140catalyzes the reduction of oxygen (O2) as follows:
O2+4H++4e−→2H2O
to complete the overall sensor cell reaction as follows.
2H2+O2→2H2O

Because the reaction of potentially interfering non-target particles at measuring electrode120may induce a current between measuring electrode120and sensor operating circuitry105, sensor operating circuitry105could falsely identify a presence of a target particle in an environment near sensor cell102.

To help reduce or minimize this cross-sensitivity to a non-target particle, measuring electrode120for one embodiment comprises material to help stabilize an oxide of the catalyst in measuring electrode120. The material for one embodiment may help stabilize an oxide of the catalyst formed on a surface of measuring electrode120. Measuring electrode120for one embodiment may comprise material that helps form an oxide of the catalyst in measuring electrode120.

For one embodiment where measuring electrode120comprises a catalyst comprising platinum (Pt) to sense carbon monoxide (CO) in an environment where both carbon monoxide (CO) and hydrogen (H2) may appear, stabilizing an oxide of platinum (PtxOy) of measuring electrode120helps reduce or minimize the cross-sensitivity of measuring electrode120to hydrogen (H2) because platinum oxide (PtxOy) favors carbon monoxide (CO) oxidation while metal platinum (Pt) favors hydrogen (H2) oxidation.

Measuring electrode120may comprise any suitable material to help form and/or to help stabilize an oxide of the catalyst in measuring electrode120.

For one embodiment, measuring electrode120may comprise any suitable material having an electrochemical potential more positive than that of the catalyst oxide formation for measuring electrode120and less positive than the oxygen evolution reaction at measuring electrode120. For one embodiment, the mixture of the catalyst with such material helps stabilize measuring electrode120at a more positive electrochemical potential relative to an electrode comprising only the catalyst. The higher electrochemical potential for measuring electrode120helps shift the equilibrium of the composition of measuring electrode120toward catalyst oxide formation, helping to stabilize the amount and composition of the catalyst oxide.

For another embodiment, measuring electrode120may comprise any suitable material having an electrochemical potential higher than the oxygen evolution reaction where the oxygen evolution reaction at measuring electrode120is kinetically too slow to happen to an observable extend.

The material to help form and/or to help stabilize an oxide of the catalyst of measuring electrode120for one embodiment may comprise a suitable reducible metal oxide. The material to help form and/or to help stabilize an oxide of the catalyst of measuring electrode120for one embodiment may comprise a suitable reducible transition metal oxide. The material to help form and/or to help stabilize an oxide of the catalyst of measuring electrode120for one embodiment may comprise a suitable reducible metal oxide and a suitable oxide of the catalyst material for measuring electrode120.

Material to help form and stabilize an oxide of the catalyst for one embodiment may comprise a manganese oxide (MnxOy), such as manganese dioxide (MnO2). For one embodiment where measuring electrode120comprises a catalyst comprising platinum (Pt), manganese dioxide (MnO2) is relatively stable in measuring electrode120because the electrochemical potential of platinum oxide (PtxOy) formation for PtO is Eo=0.98 volts (V) and for PtO2is Eo=1.045 V and because the electrochemical potential of the following reaction:
MnO2+4H++2e−→Mn2++2H2O (Eo=1.21V)
is almost equal to that of the following reaction.
O2+4H++4e−→2H2O(Eo=1.23V)
The MnO2/Mn2+reaction neither tends to oxidize oxygen of water (H2O) to elementary oxygen (O2), as higher potential systems do, nor reduces dissolved oxygen (O2) to water (H2O), as lower potential systems do.

For another embodiment where measuring electrode120comprises a catalyst comprising platinum (Pt), measuring electrode120may comprise a manganese oxide (MnxOy), such as manganese dioxide (MnO2), and a platinum oxide (PtxOy), such as platinum dioxide (PtO2), to help form and stabilize an oxide of the catalyst.

The material to help form and/or to help stabilize an oxide of the catalyst of measuring electrode120for another embodiment may comprise a ruthenium oxide (RuxOy), such as ruthenium dioxide (RuO2). For one embodiment where measuring electrode120comprises a catalyst comprising platinum (Pt), ruthenium dioxide (RuO2) is relatively stable in measuring electrode120because the electrochemical potential for the reaction RuO2/Ru2+is Eo=1.12 V.

The material to help form and/or to help stabilize an oxide of the catalyst of measuring electrode120for another embodiment may comprise an osmium oxide (OsxOy), such as osmic tetroxide (OsO4). For one embodiment where measuring electrode120comprises a catalyst comprising platinum (Pt), osmic tetroxide (OsO4) is relatively stable in measuring electrode120because the electrochemical potential for the reaction OsO4/OsO2is Eo=1.02 V.

Although described in connection with a catalyst comprising platinum (Pt) for measuring electrode120, the addition of material to a catalyst for measuring electrode120to help form and/or to help stabilize an oxide of the catalyst may possibly be extended to other suitable catalyst materials including, for example, a suitable platinum group metal or a suitable noble metal including silver (Ag) and gold (Au).

For block206, sensor operating circuitry105measures current flow between measuring electrode120and sensor operating circuitry105. Sensor operating circuitry105may comprise any suitable circuitry to measure such current in any suitable manner.

For block208, sensor operating circuitry105identifies whether a target particle is near sensor cell102based on the measured current. Sensor operating circuitry105may identify whether a target particle is near sensor cell102in any suitable manner based on the measured current.

Sensor operating circuitry105for one embodiment may compare the measured current to a predetermined value to identify whether a target particle is near sensor cell102based on the relationship between the measured current and the predetermined value. Sensor operating circuitry105for one embodiment may identify an amount or concentration of a target particle near sensor cell102based on the measured current, noting for one embodiment that the production of electrons resulting from an oxidation reaction at measuring electrode120is generally proportional to the amount or concentration of a target particle near sensor cell102.

Because stabilizing an oxide of the catalyst of measuring electrode120for one embodiment helps prevent the oxidation of potentially interfering non-target particles and therefore helps prevent inducing current between measuring electrode120and sensor operating circuitry105due to the presence of such non-target particles, sensor operating circuitry105may identify a presence and/or an amount or concentration of a target particle in an environment in which such non-target particles may appear with relatively more accuracy.

If sensor operating circuitry105identifies for block208that a target particle is near sensor cell102, sensor operating circuitry105for one embodiment for block210may output a signal indicating the presence of a target particle to output device180. Sensor operating circuitry105for one embodiment may output a signal indicating the amount or concentration of a target particle sensed with sensor cell102. If sensor operating circuitry105identifies for block208that a target particle is not near sensor cell102, sensor operating circuitry105for one embodiment for block212may output a signal indicating the absence of a target particle to output device180.

Output device180may comprise any suitable circuitry and/or equipment to respond to a signal output from sensor operating circuitry105in any suitable manner. Output device180for one embodiment may provide a suitable auditory output and/or a suitable visual output in response to a signal from sensor operating circuitry105. Output device180for one embodiment may provide a suitable auditory output and/or a suitable visual output to indicate the amount or concentration of a target particle sensed with sensor cell102. Output device180for one embodiment may provide a suitable tactile output, such as vibration for example, in response to a signal from sensor operating circuitry105. Output device180for one embodiment may actuate other circuitry and/or equipment in response to a signal from sensor operating circuitry105, for example, to help control a process involving a target particle or to help clear a target particle from an environment near sensor cell102.

Sensor operating circuitry105for one embodiment may repeat operations for blocks204,206,208,210, and/or212to continue to bias one or more electrodes of sensor100to a suitable electrical potential and monitor current flow between measuring electrode120and sensor operating circuitry105.

Sensor operating circuitry105may perform operations for blocks204-212in any suitable order and may or may not overlap in time the performance of any suitable operation with any other suitable operation. Sensor operating circuitry105for one embodiment may, for example, perform operations for blocks204,206,208,210, and/or212substantially continuously or discretely at a suitable rate.

Sensor operating circuitry105for another embodiment may output a signal to output device180for block210generally only when the absence of a target particle was identified based on a just prior current measurement and/or when an identified amount or concentration of a target particle near sensor cell102changes, or changes beyond a certain amount, from a prior sensing. Sensor operating circuitry105for another embodiment may output a signal to output device180for block212generally only when the presence of a target particle was identified based on a just prior current measurement.

Sensor Formation

FIG. 3illustrates, for one embodiment, a flow diagram300to form sensor100.

For block302ofFIG. 3, two or more electrodes are formed. Such electrodes may be formed in any suitable manner. For one embodiment, at least one electrode is formed to comprise a catalyst and material to help stabilize an oxide of the catalyst.

Measuring electrode120may be formed in any suitable manner from any suitable material.

Measuring electrode120for one embodiment may be formed by mixing a catalyst powder with powder for material to help form and/or to help stabilize an oxide of the catalyst. For one embodiment, the powder mixture may also be mixed with a powder or emulsion of a suitable binder material, such as polytetrafluoroethylene (PTFE) for example, to produce an emulsion that may be spread onto a suitable porous substrate for mechanical strength. The substrate for one embodiment may also serve as a diffusion barrier. The resulting emulsion for one embodiment may be spread onto membrane124. The substrate for one embodiment may then be subjected to a suitable heat treatment to help bond the emulsion to the substrate and remove solvents.

Measuring electrode120for one embodiment may comprise a catalyst comprising platinum (Pt) and comprise manganese dioxide (MnO2) to help form and stabilize a platinum oxide (PtxOy) in measuring electrode120. Measuring electrode120for one embodiment may be formed from any suitable powder mixture having any suitable amount of platinum black (Pt) and manganese dioxide (MnO2) powder. A higher manganese dioxide (MnO2) content may decrease the conductivity of measuring electrode120and decrease the total amount of platinum black (Pt) per square area, thereby decreasing sensitivity to a target particle and/or increasing the response time. A lower manganese dioxide (MnO2) content may not provide for sufficient contact of platinum black (Pt) particles with the oxidant particles, thereby increasing sensitivity to non-target particles. A lower manganese dioxide (MnO2) content may also potentially decrease the duration of low cross-sensitivity to non-target particles.

The mass mixing ratio of platinum black (Pt) to manganese dioxide (MnO2) for one embodiment is in the range of approximately 100:1 to approximately 3:1.

The mass mixing ratio of platinum black (Pt) to manganese dioxide (MnO2) for one embodiment is in the range of approximately 10:1 to approximately 5:1.

Measuring electrode120for one embodiment may comprise approximately 11 mg/cm2platinum black (Pt) and approximately 2 mg/cm2manganese dioxide (MnO2) for a mass mixing ratio of approximately 5.5:1.

Measuring electrode120for one embodiment may be formed from only platinum black (Pt) and manganese dioxide (MnO2).

Measuring electrode120for one embodiment may comprise a catalyst comprising platinum (Pt) and comprise both a platinum oxide (PtxOy), such as platinum dioxide (PtO2), and manganese dioxide (MnO2) to help form and stabilize a platinum oxide (PtxOy) in measuring electrode120. Measuring electrode120for one embodiment may be formed from any suitable powder mixture having any suitable amount of platinum black (Pt), a platinum oxide (PtxOy) powder, and manganese dioxide (MnO2) powder.

For one embodiment, the powder mixture for measuring electrode120may comprise less than approximately 20% of a platinum oxide (PtxOy) powder and less than approximately 30% of manganese dioxide (MnO2) powder of the total weight of the powder mixture.

For one embodiment, the powder mixture for measuring electrode120may comprise in the range of approximately 5% to approximately 15% of a platinum oxide (PtxOy) powder and in the range of approximately 5% to approximately 20% of manganese dioxide (MnO2) powder of the total weight of the powder mixture.

For one embodiment, the powder mixture for measuring electrode120may comprise approximately 10% of a platinum oxide (PtxOy) powder and approximately 10% manganese dioxide (MnO2) powder of the total weight of the powder mixture.

Measuring electrode120for one embodiment may be formed from only platinum black (Pt), a platinum oxide (PtxOy), and manganese dioxide (MnO2).

As measuring electrode120comprising either Pt/MnO2or Pt/PtO2/MnO2is produced in ambient air, surface oxides form on measuring electrode120in a manner that may depend, for example, on the pH of electrolyte150and on the electrochemical potential of measuring electrode120. Surface oxides may form independent from any added platinum dioxide (PtO2). The structure of the surface oxides is not known but is suspected to consist of stoichiometrically non-defined thin platinum oxide (PtxOy) clusters or films of different size and shape. Manganese dioxide (MnO2) and/or platinum dioxide (PtO2) are thought to help stabilize these surface oxides.

Reference electrode130may be formed in any suitable manner from any suitable material. Reference electrode130for one embodiment may be formed similarly as measuring electrode120to help prevent any non-target particles that leak to reference electrode130from shifting the electrical potential of reference electrode130to produce a negative zero current.

Reference electrode130for one embodiment may be formed by mixing a suitable powder or powder mixture for reference electrode130with a powder or emulsion of a suitable binder material, such as polytetrafluoroethylene (PTFE) for example, to produce an emulsion that may be spread onto a suitable porous substrate134for mechanical strength. Substrate134for one embodiment may also serve as a diffusion barrier. Substrate134for one embodiment may then be subjected to a suitable heat treatment to help bond the emulsion to the substrate and remove solvents. Substrate134may be formed from any suitable material, such as polytetrafluoroethylene (PTFE) for example.

Counter electrode140may be formed in any suitable manner from any suitable material that may depend, for example, on the reaction to be catalyzed by counter electrode140. Where counter electrode140is to reduce oxygen (O2), for example, counter electrode140may be formed to comprise gold (Au), silver (Ag), or platinum (Pt), for example.

Counter electrode140for one embodiment may be formed by mixing a suitable powder or powder mixture for counter electrode with a powder or emulsion of a suitable binder material, such as polytetrafluoroethylene (PTFE) for example, to produce an emulsion that may be spread onto a suitable porous substrate144for mechanical strength. Substrate144for one embodiment may also serve as a diffusion barrier. Substrate144for one embodiment may then be subjected to a suitable heat treatment to help bond the emulsion to the substrate and remove solvents. Substrate144may be formed from any suitable material, such as polytetrafluoroethylene (PTFE) for example.

For block304ofFIG. 3, each electrode is positioned relative to a reservoir to couple each electrode to electrolyte150. Each electrode may be positioned in any suitable manner relative to a reservoir to hold electrolyte150. For one embodiment, as illustrated inFIG. 1, housing110supports measuring electrode120, reference electrode130, and counter electrode140in the reservoir containing electrolyte150.

Electrolyte150may comprise any suitable solution. For one embodiment, electrolyte150may be a hydrophilic electrolyte, such as a solution of sulfuric acid in water for example. Hydrophilic separators or wetting filters may also be positioned relative to one or more electrodes and electrolyte150to aid ionic electrical contact between electrodes.

For block306ofFIG. 3, each electrode is conductively coupled to sensor operating circuitry105. Each electrode may be conductively coupled to sensor operating circuitry105in any suitable manner.

In the foregoing description, one or more embodiments of the present invention have been described. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit or scope of the present invention as defined in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.