WO2011014635A2

WO2011014635A2 - Electrochemical sulfur sensor

Info

Publication number: WO2011014635A2
Application number: PCT/US2010/043683
Authority: WO
Inventors: Douglas A. Rebinsky; Yong Tian; Xiaodong Liu; Svetlana Zemskova
Original assignee: Caterpillar Inc.
Priority date: 2009-07-29
Filing date: 2010-07-29
Publication date: 2011-02-03
Also published as: WO2011014635A3

Abstract

This disclosure relates to sulfur sensors that utilize sensing materials that can be used to detect ultra low concentrations of sulfur in liquids, such as below even 15 ppm. The sulfur sensors comprise a sensing electrode having a material that contributes an electrical element to the analysis and a material that contributes an ionic element to the analysis.

Description

Description ELECTROCHEMICAL SULFUR SENSOR

Technical Field

The present invention relates generally to sulfur sensors. More particularly, the present invention relates to sulfur sensors that utilize sensing materials that can be used to detect ultra low concentrations of sulfur in liquids, such as below even 15 ppm.

Background

It is important to be able to accurately and reliably measure the concentration of sulfur in liquids, as various chemical reactions may take place that would release sulfur compounds into the atmosphere or onto physical structures around the sulfur-containing liquid. For example, the combustion of diesel fuel typically generates sulfur oxides (SO₂, SO3) and sulfuric acid

(condensate H₂SO₄), both of which are components of acid rain. Further, these sulfur compounds have been linked to catalyst poisoning in diesel particulate filters (DPFs) and sulfuric acid condensation and corrosion of engine

components, such as the cooler and piston ring liner components. Such phenomena are found when using both high sulfur (>350 ppm) and low sulfur (15 -350 ppm) fuels.

For various reasons, including the sensitivity of aftertreatment components to sulfur compounds, many modern diesel engines are now being designed to use Ultra Low Sulfur Diesel fuel (<15 ppm S). Accordingly, the sulfur level of the fuel source is of utmost importance for optimum machine performance. While sulfur detection in liquids at levels below 15 ppm is attainable in a laboratory or other test setting, such detection is not feasible in the field with an accurate, portable, reliable, quick, and inexpensive sensor. Examples of known means of detecting sulfur at ultra-low levels include Flame Photometry Detection (FPD) and Inductively Coupled Plasma (ICP) devices, but both are more appropriately used in the laboratory setting because of their size and duration of test cycles.

United States Patent No. 6,716,336 B2 describes an electrochemical sulfur sensor based on an Ag(+) ion conductive ceramic. Such a sensor exhibits a change in electrical signal (measured as potential) because of the change in ionic conductivity of ceramic materials in contact with sulfur organics. Although such a sensor performs well with the "simulated diesel fuel" composed of mostly aliphatic sulfur organics and thiophene, evaluation by D.

Berglund shows that when the sensor disclosed in '336 is used with commercially available diesel fuel as being unsuccessful. Accordingly, a desire for a fast and inexpensive detection of sulfur level in diesel fuels, or possibly an on-board diagnostic tool for determining the same, persists. Summary of the Invention

In one aspect, the present disclosure is directed to an

electrochemical sulfur sensor for determining a sulfur concentration in a liquid. The electrochemical sulfur sensor comprises a reference electrode and a sensing material including an electrically conductive material and a metal cation.

Further, the sensing material is in association with a sensing electrode.

Moreover, the sensing material is both electrically and ionically conductive in the presence of the sulfur-comprising liquid.

In another aspect, the present disclosure is directed to various methods of making the electrochemical sulfur sensor of the present disclosure. Brief Description of the Drawings

Figure 1 is a schematic representation of one embodiment of an electrochemical sulfur sensor of the present disclosure. Figure 2 A is a graph showing the reaction of that has been exposed to diesel fuel having about 3600 ppm sulfur, about 350 ppm sulfur, and about 15 ppm sulfur.

Figure 2B is a graph showing the change in electrochemical potential of the electrochemical sulfur sensor over time, as well as the

electrochemical potential data relative to the varying levels of sulfur in the diesel fuel.

Figure 3 A is a graph showing the reaction of an Ag-doped graphite foam electrochemical sulfur sensor that has been exposed to diesel fuel having about 3600 ppm sulfur, about 350 ppm sulfur, and about 15 ppm sulfur.

Figure 3B is a graph showing the change in electrochemical potential of the electrochemical sulfur sensor over time and the electrochemical potential data relative to varying levels of sulfur.

Figure 4 is a graph showing electrochemical potential of an Fe- impregnated graphite foam electrochemical sulfur sensor that is exposed to diesel fuels having various sulfur concentrations.

Figure 5 is a schematic representation of an electrochemical sulfur sensor including a conductive metal tape and a sensing material.

Figure 6 is a graph showing the electric potential of an electrochemical sulfur sensor wherein the sulfur sensor comprises a graphite foam substrate impregnated with Cu cations to form a sensing electrode and an Ag wire as a reference electrode.

Figure 7 is a graph showing the electrical potential of an electrochemical sensor that is exposed to differing levels of sulfur in fuel, highlighted by an initial conditioning period.

Figure 8 is a schematic representation of an electrochemical sulfur sensor including an Ag wire reference electrode and a sensing electrode comprising ductile iron with graphite dispersed within the microstructure. Figure 9 is a photomicrograph showing the microstructure of a sensing electrode comprising ductile iron with graphite dispersed within the ferrite of the microstructure.

Figure 10 is a schematic representation of an electrochemical sulfur sensor according to the present disclosure in the final packaging.

Figures 1 IA and 1 IB are graphs showing successive trials of the same electrode varying concentrations of sulfur in fuel.

Figure 12A is a graph showing electrochemical sulfur sensors with a ductile iron sensing electrode that have been exposed to diesel fuels having various sulfur concentrations.

Figure 12B is a graph showing the relationship of electrical potential with respect to sulfur concentration for a ductile iron electrode.

Whenever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. Detailed Description

Figure 1 illustrates an electrochemical sulfur sensor 10. The sensor comprises three electrodes: a sensing electrode 13 and a reference electrode 12, as well as a sensor substrate 11 and a counter electrode 14. The sensing electrode 13 comprises a sensing material specifically chosen based on the material's ability to exhibiting both ionic and electronic conductivity. By doing so, the sensing material of sensing electrode 13 changes potential based on both electronic and ionic factors, yielding a more accurate, robust measurement of the sulfur concentration of the liquid to which the sulfur sensor is exposed. The electronic factors contributing to the overall change in conductivity, resistivity, or potential of sensing electrode 13 may include a change in potential of the portion of the sensing material that reacts to electrically neutral sulfur compounds in the liquid. This is accomplished by forming a partially charged sulfur-containing species on part of the surface of sensing electrode 13. The ionic factors contributing to the overall change in ionic conduction or potential of sensing electrode 13 may include a migration of metal ions within the material. Further, sulfur-containing species may form on sensing electrode 13 from the interaction between metal ions in the sensing material and sulfur compounds in the liquid.

The sensing material of sensing electrode 13 comprises an electronic conductor, such as a tape like a lead tape, a coupon like an aluminum or stainless steel coupon, conductive graphite, carbon fibers or sintered metal fibers, in any appropriate shape. In one exemplary embodiment, the electronic conductor is a metallic foam, which advantageously increases the total surface area in contact with the liquid. For example, the electrically conductive material may be a conductive graphite foam. In another exemplary embodiment, the electronic conductor is in powder form, such as conductive graphite powder or carbon powder in a dried slurry, paste, or thick film, The electronically conductive material is then impregnated with metal cations, such as, e.g., Fe²⁺, Cu²⁺, Ag⁺, Au³⁺, Ni²⁺, Zn²⁺, Pb²⁺, and/or Mo⁴⁺ cations. Electrochemical testing has shown that materials with Fe , Cu , Ag and Zn are most sensitive to S- organics in diesel fuel.

To form the electrochemical sulfur sensor 10 according to the disclosure, a compound comprising a metal cation is dissolved in an aqueous solution. Using Fe as an exemplary metal cation, a sufficient amount of Fe- compound, such as FeSO₄, is dissolved into an aqueous solution. In one example, between about 0.5 g and about 0.6 g, such as about 0.55 g, Of FeSO₄ may be dissolved in between about 18 - 22 mL, such as about 20 mL, OfH₂O with about 2 mL of APTS and about 8 mL of EtOH. Alternatively, other aminosiloxanes and alcohols can be used, as known by those skilled in the art.

To combine the metal cation with the conductive material, powdered conductive sensing material is added to the metal cation-comprising aqueous solution or, alternatively, the solution could be added to a foam piece of conductive material. When conductive material powder is added to the aqueous solution, for example, between about 1.7 g and about 2.0 g of graphite powder may be added to the aqueous solution. In the solution detailed above as an example, the graphite powder is added to the solution OfFeSO₄, H₂O, APTS, and EtOH. In such a process, the resulting slurry is allowed to age, such as for at least about 24 hrs or at least about 36 hrs, after which the conductive sensing material is filtered out and dried in air. Afterwards, the material may be washed and cured in a H₂OiEtOH (1 :1) mixture for at least about 10 hrs and dried.

Figure 4 shows another sample of a Fe-impregnated graphite powder electrochemical sulfur sensor that has been exposed to diesel fuels having various sulfur concentrations, specifically, about 15 ppm, about 350 ppm, and about 3600 ppm.

For the embodiment where the metal cation-comprising aqueous solution is added to a powder of conductive material, for example, a solution having a Cu-compound is introduced to a piece of graphite powder. The Cu- comprising solution may comprise, for example, between about 0.4 g and about 0.5 g, such as about 0.465 g, Of Cu(NOs)₂ is mixed with about 20 mL of H₂O, about 2 mL APTS and 8 mL EtOH. The Cu-comprising solution is then introduced to the graphite powder and dried for a time sufficient to facilitate drying, such as for at least about 12 hrs or at least about 24 hrs. Afterwards, the material may be washed and cured in H₂O:EtOH (1 :1) mixture for at least about 30 minutes, such as at least about 1 hr, and dried.

Figure 2A shows the reaction of a Cu-doped graphite electrochemical sulfur sensor that has been exposed to high sulfur diesel fuel having about 3600 ppm sulfur, low sulfur diesel fuel having about 350 ppm sulfur, and ultra-low sulfur diesel fuel having about 15 ppm sulfur. Figure 2B shows the change in electrochemical potential of the material sensor over time, as well as the electrochemical potential data relative to the varying levels of sulfur in the diesel fuel. Figures 3A, 3B, 1 IA, and 1 IB show other metal cation-doped graphite foam electrochemical sulfur sensors and the results of exposure of the sensors to diesel fuels having varying levels of sulfur. For example, Figure 3 A shows the reaction of an Ag-doped graphite electrochemical sulfur sensor that has been exposed to high sulfur diesel fuel having about 3600 ppm sulfur, low sulfur diesel fuel having about 350 ppm sulfur, and ultra- low sulfur diesel fuel having about 15 ppm sulfur. Figure 3B shows the change in electrochemical potential of the electrochemical sulfur sensor over time, as well as the electrochemical potential data relative to the varying levels of sulfur in the diesel fuel.

The sensor's ability to be repeatedly used is shown by further data in Figures 1 IA and 1 IB, which depicts two successive trials of the same electrode in each respective composition. That is, five different electrochemical sulfur sensors were prepared having Cu, Fe, Ag, and Zn cations joined to a graphite to form the electrochemical sulfur sensor. The electrochemical potential, which is also referred to as the open circuit potential (OCP) for these examples, was measured by immersing each sensor in diesel fuels in a sequence of fuels having about 15 ppm, about 350 ppm, and about 3600 ppm sulfur. The sensors were then rinsed with octane, dried in air, and re-tested to determine if the sensors had lost any sensitivity.

Figure 5 shows another embodiment of the disclosure, which is a two electrode design. Figure 5 shows that the construction of the electrochemical sulfur sensor 50 includes using a conductive metal tape, such as a Cu tape, connected to a sensing electrode 53 of the sensor 50. The sensor 50 further includes a reference electrode 51 , such as a Ag-based wire. Sensing electrode 53 and reference electrode 51 may be physically joined, so long as an insulating material 55, such as a PTFE insulating layer, is used to electrically isolate the two electrodes.

To form sensing electrode 50, a sol-gel composite is formed comprising a metal cation ionic component and an electronic component, such as graphite. The sol-gel composite is then applied to the conductive metal tape and cured for a sufficient time, such as at least about 12 hrs, at least about 24 hrs, or at least about 36 hrs. The electrochemical potential or OCP was measured for these sample electrodes as they were immersed in diesel fuels 57 having about 15 ppm, about 350 ppm, and about 3600 ppm sulfur. The sensors were rinsed with octane between exposures to each diesel fuel sample. Figure 6 shows the results of an electrochemical sulfur sensor wherein the sulfur sensor comprises graphite impregnated with Cu cations to form a sensing electrode and an Ag wire as a reference electrode.

In another embodiment the electrochemical sulfur sensor has a sensing electrode that comprises a metal-metal carbide-carbon matrix. For example, Figures 8 and 9 show a sulfur sensor having an Ag wire reference electrode and a sensing electrode comprising ductile iron. Figure 9 shows the microstructure of a sensing electrode comprising ductile iron with carbon inclusions 92 dispersed within the ferrite 91 of the microstructure, which is part of a sulfur sensor having a silver wire reference electrode and the sensing electrode. Figure 7 shows the results of a similar sulfur sensor, wherein the sensing electrode comprises a ductile iron, as it is exposed to fuels having varying sulfur concentrations. Figure 12A shows electrochemical sulfur sensors with a ductile iron sensing electrode that have been exposed to diesel fuels having various sulfur concentrations, specifically, about 5 ppm, about 17 ppm, about 38 ppm, about 386 ppm, and about 4940 ppm. It presents time-averaged results trending with fuel sulfur content for this embodiment. Figure 12B shows the relationship of electrical potential with respect to sulfur concentration for a ductile iron electrode in misfueling fuel. Industrial Applicability

Once an electrochemical sulfur sensor is assembled according to this disclosure, the sensor is exposed to a liquid, such as a fuel. After a sufficient time, such as e.g., at least about 1000 sec, at least about 2000 sec, at least about 3000 sec, at least about 4000 sec, or at least about 5000 sec, the sensor undergoes a change in potential relative to the reference electrode. This change in potential, which is based on both electronic and ionic factors, can then be correlated to a sulfur concentration in the liquid.

The electrochemical sulfur sensor may be used as an electrochemical electrode for detection of S-organics in diesel fuel as it is being introduced into a vehicle, at a fueling location before the fuel is introduced into the vehicle, or after the fuel is in the vehicle while diagnosing a vehicle in its environment. The sensor can also be used as a part of portable field test kits. Standard electrochemical cells and other commercial equipment may be used to measure the composite electrode potential in various fuel samples.

Figure 10 shows a finally packaged electrochemical sulfur sensor 100, including a CAN Bus connector 101 to relay the electrical signals from the sensing and reference electrodes to other analytical equipment, a stainless steel housing 102 to facilitate robust reliability and ease assembly or disassembly, a threaded head with an o-ring seal 103, and a perforated sheath 104 to protect the sensing electrode.

Although the present inventions have been described with reference to exemplary embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the sprit and scope of the invention. For example, although different exemplary embodiments may have been described as including one or more features providing one or more benefits, it is contemplated that the described features may be interchanged with one another or alternatively be combined with one another in the described exemplary embodiments or in other alternative embodiments. Because the technology of the present invention is relatively complex, not all changes in the technology are foreseeable. The present invention described with reference to the exemplary embodiments and set forth in the flowing claims is manifestly intended to be as broad as possible. For example, unless specifically otherwise noted, the claims reciting a single particular element also encompass a plurality of such particular elements.

Claims

1. An electrochemical sulfur sensor for determining a sulfur concentration in a liquid, the electrochemical sulfur sensor comprising:

a reference electrode; and

a sensing material including an electrically conductive material and a metal cation, the sensing material being in association with a sensing electrode;

wherein the sensing material is both electrically and ionically conductive in the presence of the sulfur-comprising liquid.

2. The electrochemical sulfur sensor of claim 1 wherein the electrically conductive material is selected from the group consisting of graphite foam, metal coupons, conductive metal tape, stainless steels, and ductile iron.

3. The electrochemical sulfur sensor of claim 1 wherein the electrically conductive material is a graphite foam.

4. The electrochemical sulfur sensor of claim 1 wherein the metal cation is selected from the group consisting OfFe²⁺, Cu²⁺, Ag⁺, Au³⁺, Ni²⁺, Zn²⁺, Pb²⁺, Mo⁴⁺, and combinations thereof.

5. The electrochemical sulfur sensor of claim 1 wherein the reference electrode is a wire comprising Ag.

6. The electrochemical sulfur sensor of claim 1 wherein the electrically conductive material is a copper-based metal tape.

7. The electrochemical sulfur sensor of claim 6 further including an insulating layer between the reference electrode and the copper- based metal tape.

8. The electrochemical sulfur sensor of claim 7 wherein the insulating layer includes PTFE.

9. The electrochemical sulfur sensor of claim 1 wherein the sensing electrode includes a metal matrix with graphite, wherein graphite particles are dispersed within ferrite particles of the metal matrix microstructure.

10. A method for forming an electrochemical sulfur sensor for determining a sulfur concentration in a liquid, the electrochemical sulfur sensor including a reference electrode and a sensing material including an electrically conductive material and a metal cation, wherein the sensing material is in association with a sensing electrode, the method comprising:

forming a solution including a metal cation; and

combining the solution with a conductive material.

11. The method of claim 10 wherein the metal cation is Fe²⁺.

12. The method of claim 10 wherein the conductive material is in powder form.

13. The method of claim 12 wherein the powder is a graphite powder.

14. The method of claim 12 wherein the solution is an aqueous solution, the method further including:

combining the aqueous solution with the powdered conductive material to form a composite slurry;

aging the composite slurry for at least about 24 hours;

curing the aged slurry for at least about 10 hours; and drying the cured slurry.

15. The method of claim 10 wherein the conductive material is a foamed material.

16. The method of claim 15 wherein the foamed material is graphite foam.

17. The method of claim 15 wherein the solution is an aqueous solution, the method further including:

combining the aqueous solution with the graphite foamed material;

aging the combined solution and foamed material for at least about 12 hours;

curing the aged foamed material for at least about 30 minutes; and drying the cured slurry.

18. The method of claim 17 wherein the metal cation is Cu²⁺.

19. The method of claim 17 wherein the metal cation is Ag⁺.

20. The method of claim 10 wherein the solution is a sol-gel solution; the method further including:

combining the sol-gel solution with a graphite foam; applying the combined sol-gel solution and graphite foam to a conductive metal tape.