METHOD OF ELECTROCHEMICALLY DEPOSITING HIGH-ACTIVITY ELECTROCATALYSTS

Abstract

The method begins by forming a solution comprising catalyst precursors, electrolyte and a solvent. Electrodes are inserted into the solution comprising an anode electrode and a cathode electrode. Electrochemical deposition then occurs wherein a current is passed between the electrodes. In this method at least one additional step of: i) heating the solution prior to and during the electrochemical deposition; ii) increasing the concentration of the catalyst precursors in the solution to greater than 0.1 millimolar; iii) performing the electrochemical deposition by a pulsed current; and iv) adding chemical promoters to the solution.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application which claims the benefit of and priority to U.S. Provisional Application Ser. No. 61/764,708 filed Feb. 14, 2013, entitled “Method of Electrochemically Depositing High-Activity Electrocatalysts,” which is hereby incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None.

FIELD OF THE INVENTION

This invention relates to method of electrochemically depositing high-activity electrocatalysts.

BACKGROUND OF THE INVENTION

Hydrogen is currently produced by steam-reforming of natural gas. This process although economical, produces CO₂ as a by-product. With increasing demand for hydrogen in refineries, and the possibility of future carbon dioxide legislation, it is of interest to consider cost-effective alternative means of H₂ production. The electrochemical splitting of water into H₂ and O₂ gas is one known method. However, electrolysis requires electrical power, which either means significant CO₂ emission from fossil-based power, or reliance on currently expensive low-carbon power.

BRIEF SUMMARY OF THE DISCLOSURE

The method begins by forming a solution comprising catalyst precursors, electrolyte and a solvent, typically water. The electrolyte could be any chemical reagent, typically an acid or a base, that enables the solution to conduct electricity. Electrodes are inserted into the solution comprising an anode electrode and a cathode electrode. Electrochemical deposition then occurs wherein a current is passed between the electrodes. In this method at least one additional step of: i) heating the solution prior to and during the electrochemical deposition; ii) increasing the concentration of the catalyst precursors in the solution to greater than 0.1 millimolar; iii) performing the electrochemical deposition by a pulsed current; and iv) adding a chemical promoter to the solution.

In another embodiment the method begins by forming a solution comprising catalyst precursors, electrolyte and a solvent, typically water. Electrodes are inserted into the solution comprising an anode electrode and a cathode electrode. The solution is then heated to a temperature above room temperature. Electrochemical deposition then occurs when a current is passed between the electrodes.

In yet another embodiment the method begins by forming a solution comprising catalyst precursors, electrolyte and a solvent, typically water, wherein the quantity of the catalyst precursors in the solution is greater than 0.1 millimolar. The catalyst precursor could either be a metal, metal salt, metal alloy, oxometallate compound, an organometallic reagent or any combination of the above. Electrodes are inserted into the solution comprising an anode electrode and a cathode electrode. Electrochemical deposition then occurs wherein a current is passed between the electrodes.

In another embodiment the method begins by forming a solution comprising catalyst precursors, electrolyte and a solvent, typically water. Electrodes are inserted into the solution comprising an anode electrode and a cathode electrode. Electrochemical deposition then occurs when a pulsed current is passed between the electrodes.

In one embodiment the method begins by forming a solution comprising catalyst precursors, electrolyte, chemical promoter and a solvent, typically water. The chemical promoter could either act as a surfactant, chelating agent, reductant or any combination of the above. The promoter could belong to any of the following class of chemical compounds: polyol, polyacids, sulfonic acid, carboxylic acid, phosphonic acids, amines, paraquats and diquats. Electrodes are inserted into the solution comprising an anode electrode and a cathode electrode. Electrochemical deposition then occurs when a current is passed between the electrodes.

In yet another embodiment, the method begins by forming a solution comprising catalyst precursors, electrolyte, chemical promoter and a solvent, typically water, wherein the quantity of the catalyst precursors in the solution is greater than 0.1 millimolar. Electrodes are inserted into the solution comprising an anode electrode and a cathode electrode. The solution is then heated to a temperature above room temperature. Electrochemical deposition then occurs when a pulsed current is passed between the electrodes to produce enhanced-activity electrodes. Electrolysis is then performed on an oxygenate-containing solution using the enhanced anode and cathode electrodes.

In one embodiment, the method begins by forming a solution comprising catalyst precursors, electrolyte, chemical promoter and a solvent, typically water, wherein the quantity of the catalyst precursors in the solution is greater than 100 millimolar. Electrodes are inserted into the solution comprising an anode electrode and a cathode electrode. The solution is then heated to a temperature above 75° C. Electrochemical deposition then occurs when a pulsed current, with an on:off time ratio in the range of 1:0.001 to 1:1000, e.g. 1:99 would be on for 1 millisecond and off for 99 milliseconds, is passed between the electrodes to produce enhanced-activity electrodes. Electrolysis is then performed on an oxygenate-containing solution using the enhanced electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings in which:

FIG. 1 depicts the activity of the gold-platinum catalyst deposited under base case conditions (room temperature and 1 mM salt concentration) and tested as electrodes for the electrochemical hydrogen generation from oxygenate-containing water in alkaline conditions.

FIG. 2 depicts the scanning electron micrographs of catalysts deposited on nickel foam substrates under base case conditions.

FIG. 3 depicts voltammograms resulting from Au—Pt catalysts deposited at different temperatures, and tested as electrodes for the electrochemical hydrogen generation from oxygenate-containing water in alkaline conditions.

FIG. 4 depicts voltammograms resulting from Au—Pt catalysts deposited with different gold and platinum salt concentrations, and tested as electrodes for the electrochemical hydrogen generation from oxygenate-containing water in alkaline conditions.

FIG. 5 depicts voltammograms resulting from Au—Pt catalysts deposited with and without a chemical promoter, and tested as electrodes for the electrochemical hydrogen generation from oxygenate-containing water in alkaline conditions.

FIG. 6 depicts voltammograms resulting from Au—Pt catalysts deposited under different pulsing conditions (on:off times shown in the figure), and tested as electrodes for the electrochemical hydrogen generation from oxygenate-containing water in alkaline conditions

FIG. 7 depicts scanning electron micrographs of Au—Pt catalysts deposited on Ni foam substrate under optimized conditions.

FIG. 8 depicts a comparison of the activity of the catalysts deposited at the optimized parameters with the one deposited at the base case conditions

DETAILED DESCRIPTION

Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow.

The method begins by forming a solution comprising catalyst precursors, electrolyte and a solvent, typically water. Two electrodes, namely the anode and cathode, are immersed into the solution. Electrochemical deposition occurs wherein a current is passed between the electrodes. In this method at least one additional step of: i) heating the solution prior to the electrochemical deposition; ii) increasing the concentration of the catalyst precursors in the solution to greater than 0.1 millimolar; iii) performing the electrochemical deposition by applying a pulsed current; and iv) adding chemical promoters to the solution.

Catalyst precursor suitable for use in the solution can be selected from any metal, metal salt, metal alloy, oxometallate compound, an organometallic reagent or any combination of the above that is capable of operating as a catalyst. These metal precursors can be synthesized by any commonly known practice such as the combination of a metal with an acid or a metal with a non-metal. Examples of metals capable of being utilized as catalyst precursors include compounds, complexes, alloys and mixtures thereof, comprising at least one metal selected from the Group 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12 metals of the Periodic Table of the Elements.

The anode catalyst and the cathode catalyst selected for this process can be any metal, metal compound or a metal alloy. Non-limiting examples of anode or cathode materials that can be used include groups 4, 5, 6, 7, 8, 9, 10, 11 or 12 conductive metal and conductive metal alloys. Non-limiting specific examples of conductive metal or conductive metal alloys that can be used include, platinum and platinum alloy blends, palladium and palladium alloy blends, nickel and nickel alloy blends, iron and iron alloy blends, titanium and titanium alloy blends, gold and gold alloy blends. These anode and cathode materials are deposited on an electrically conductive substrate, preferably carbon-based materials.

Typically, during electrochemical deposition an anode and a cathode are both connected to an external power supply of direct current and residing in a solution called an electrolyte. The electrolyte typically contains one or more dissolved metal salts as well as other ions that permit the flow of electricity. Generally, the anode is connected to the positive terminal of the power supply and the cathode is connected to the negative terminal. When the external power supply is switched on, usually the metal at the anode is oxidized to form cations. These cations then are reduced at the cathode to form the electrochemical deposition. Although typical electrochemical deposition occurs with direct current, in some cases electrochemical deposition can occur with alternating current as well.

In one embodiment of the method an additional step of heating the solution prior to and during the electrochemical deposition occurring is added. This heating of the solution occurs prior to the external power supply being turned on. This method discloses that the solution can be at a temperature higher than that of room temperature. Examples of temperatures the solution can be include temperatures greater than 25° C., 50° C., 75° C., 80° C., 85° C., 90° C., 100° C. and even as high as 200° C. The heating of the solution can occur via any conventionally known heating step. One example of heating the solution can occur by using the latent heat from an electrochemical reforming system to elevate the temperature of the solution.

In another embodiment of the method an additional step of increasing the concentration of the catalyst precursors is added. In different embodiments the amount of catalyst precursors can be greater than 0.1 millimolar, 1 millimolar, 10 millimolar, 50 millimolar, 75 millimolar, 100 millimolar, 150 millimolar even greater than 200 millimolar.

In yet another embodiment of the method an additional step is added of having the electrochemical deposition occur by a pulsed current. The pulsing of the current occurs in an on:off mode. For example in one situation the pulse would be on for 1 millisecond and off for 4 milliseconds. Using the same 1:4 ratio, the pulse could be on for 2 milliseconds and off for 8 milliseconds, on for 3 milliseconds and off for 12 milliseconds and so forth. Other ratio's of the pulsed current would be 1:0.001, 1:0.01, 1:0.1, 1:1, 1:5, 1:10, 1:19, 1:20, 1:25, 1:50; 1:75, 1:99, 1:100, 1:150, 1:200, 1:500 even 1:1000.

In one embodiment of the method an additional step of adding a chemical promoter to the solution is incorporated. The chemical promoter could either act as a surfactant, chelating agent, reductant or any combination of the above. The promoter could belong to any of the following class of chemical compounds: polyol, polyacids, sulfonic acid, carboxylic acid, phosphonic acids, amines, paraquats and diquats. In one example 0.1 molar oxalic acid is added as a promoter. Different promoters that can be added include sodium dodecyl sulfate, cetyl trimethylammonium bromide, polyvinylpyrrollidone, and citric acid.

In different variations of the method at least two of the additional steps are incorporated into the method to have a cumulative effect. In yet another variation of the method at least three of the additional steps are incorporated into the method to have a cumulative effect.

The resultant anode and cathode, after electrochemical deposition, can be used for conventional electrochemical reforming. In conventional electrochemical reforming, electrolysis is performed on an oxygenate-containing mixture to produce hydrogen. Electrolysis occurs when an electrical power source is connected to two electrodes which are placed in water with supporting electrolyte present. The electrolysis for this process can occur as either a batch system or a continuous manner. Additionally, electrolysis in this process can occur with either an alternating current (AC) or a direct current (DC).

In order for the electrolysis to operate efficiently with a low CO₂ footprint it is desired that the electrochemical H₂ is produced at voltages less than 0.3 volts.

Although the electrolysis can occur at any range of temperatures the electrolysis of this process typically occurs at temperatures greater than 100° C. or even greater than 150° C. The pressure of the electrolysis typically occurs at pressures greater than atmospheric pressure or even greater than 200 psig. In one embodiment the electrolysis occurs at pressures from about 200 psig to about 400 psig.

Using the different electrolytes, electrodes and reaction conditions the current density of the electrolysis can range from about 15 mA/cm² to about 400 mA/cm² or even from about 100 mA/cm² to about 350 mA/cm². In one example, if one were to perform the electrolysis at temperatures below 100° C. the current density of the electrolysis is greater than 15 mA/cm².

In one embodiment the electrolysis does not form any significant amounts of oxygen. Significant amounts of oxygen can be defined as any amount more than 33% mol of oxygen. In other terms significant amounts of oxygen can be defined as any amount more than 100 ppm of oxygen or even 10 ppm of oxygen. In some embodiments no oxygen is formed by the electrolysis.

In another embodiment the electrolysis does not form any significant amounts of carbon dioxide or carbonates. Significant amounts of carbon dioxide or carbonates can be defined as any amount more than 500 ppm of carbon dioxide or carbonates.

The types of oxygenated mixtures that can be used for the electrolysis can be from any biomass or biomass derived stream. More specifically, the oxygenates can come from corn fiber/stover derived aqueous streams, lignin, lignocellulosic biomass, algae or fossil-fuels.

EXAMPLE 1

Gold-platinum (80:20) catalysts were deposited on Ni foils and Ni foams and used as electrodes for electrochemical hydrogen generation. In this example gold chloride (0.8 mM) and potassium tetrachlroplatinate (0.2mM) salts were added to 20 mL of 0.1 M hydrochloric acid (electrolyte) solution in water. A Ni foil was used as the cathode and platinum foil was used as the anode, and both were immersed in the salt solution. Both the electrodes were connected to a potentiostat. Electrochemical deposition was done by passing a current of 2A between the electrodes for a duration of 30 seconds. The deposition was carried out at room temperature without any external heating. FIG. 1 depicts the use of this catalyst both as an anode and cathode in corn fiber waste water (10-15 wt % organics) heated to 80° C. under alkaline conditions (30 wt % KOH). FIG. 2 shows scanning electron micrographs of the catalyst deposited under these conditions.

EXAMPLE 2

In this example, the steps were identical to Example 1 except for increasing the temperature of the solution for electrochemical deposition. FIG. 3 depicts voltammograms of this catalyst both as an anode and cathode in corn fiber waste water (10-15 wt % organics) heated to 80° C. under alkaline conditions (30 wt % KOH). FIG. 3 demonstrates the results from catalysts deposited at 5° C., 25° C., 50° C. and 75° C. The catalyst deposited at 75° C. showed the highest hydrogen generation activity (or current density).

EXAMPLE 3

In this example, the steps were identical to Example 1 except for increasing the concentration of the gold and platinum salts from 1 mM to 100 mM. FIG. 4 depicts voltammograms of this catalyst both as an anode and cathode in corn fiber waste water (10-15 wt % organics) heated to 80° C. under alkaline conditions (30 wt % KOH). As shown in FIG. 4, increasing the salt concentration improves the catalytic activity.

EXAMPLE 4

In this example, the steps were identical to Example 1 except for adding a chemical promoter to the solution. It is theorized that chemical promoters such as chelating agents bind strongly to certain crystal facets of the metal catalysts, allowing the growth of other crystal planes, hence affecting the catalyst size and activity. FIG. 5 depicts voltammograms of this catalyst both as an anode and cathode in corn fiber waste water (10-15 wt % organics) heated to 80° C. under alkaline conditions (30 wt % KOH). FIG. 5 shows the improved activity of AuPt catalyst due to the addition of 0.1 M oxalic acid.

EXAMPLE 5

In this example, the steps were identical to Example 1 except for applying a pulsed current (on for 1 millisecond and off for 99 milliseconds) instead of a continuous current. FIG. 6 depicts voltammograms of this catalyst both as an anode and cathode in corn fiber waste water (10-15 wt % organics) heated to 80° C. under alkaline conditions (30 wt % KOH). FIG. 6 shows the improved activity of the catalyst for hydrogen generation.

EXAMPLE 6

All the conditions of Examples 1-5 were incorporated in Example 6. In this example a solution temperature of 75° C., 100 mM Au—Pt salt concentration, 0.1M oxalic acid, and pulsed current signals (on:off 1:99) were used. Electron micrographs (FIG. 7) of the catalyst show that the catalyst completely covers the surface area on the Ni foam, most likely due to the increased nucleation and higher rates of deposition. The voltammogram for electrolysis of water in alkaline conditions is shown in FIG. 8 (green curve). The x-axis shows the voltage supplied for electrolysis of water, and the second x-axis shown the corresponding CO₂ footprint associated with the voltage applied. The y-axis shows the current density and the corresponding hydrogen generation rate (in standard cubic feet per day per m² area or SCFD/m²). The blue plot in FIG. 8 depicts a voltammogram of this catalyst used both as an anode and cathode in corn fiber waste water (10-15 wt % organics) heated to 80° C. under alkaline conditions (30 wt % KOH). As a comparison, a base case of an anode and a cathode in a similar corn fiber waste water is shown. As demonstrated in this figure the hydrogen rates for the optimized catalysts are a factor of 2 higher than that of the base case catalysts at a voltage of 1 V.

In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as an additional embodiment of the present invention.

Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents.

Claims

1. A method comprising:

forming a solution comprising catalyst precursors, electrolyte and a solvent;

inserting electrodes comprising an anode electrode and a cathode electrode in the solution; and

performing electrochemical deposition by passing a current between the electrodes,

wherein the method includes at least one additional step of:

i) heating the solution prior to and during the electrochemical deposition;

ii) increasing the concentration of catalyst precursor in the solution to greater than 0.1 millimolar;

iii) performing the electrochemical deposition by a pulsed current; and

iv) adding a chemical promoter to the solution.

2. The method of claim 1, wherein the catalyst precursor is selected from a metal, a metal salt, a metal alloy, an oxometallate compound, an organometallic reagent or any combination of the above.

3. The method of claim 1, wherein the electrolyte is selected from any chemical compound capable of conducting electricity.

4. The method of claim 1, wherein the solvent is selected from any chemical compound capable of dissolving the catalyst precursors and can enable the passage of electricity.

5. The method of claim 1, wherein the solution is heated to a temperature above room temperature.

6. The method of claim 1, wherein the solution is heated above 75° C.

7. The method of claim 1, wherein the quantity of the catalyst precursor in the solution is greater than 0.1 millimolar.

8. The method of claim 1, wherein the quantity of the catalyst precursor in the solution is greater than 100 millimolar.

9. The method of claim 1, wherein the current is pulsed with an on:off time ratio in the range of 1:0.001 to 1:1000.

10. The method of claim 1, wherein the current is pulsed with an on:off time ratio of 1:99.

11. The method of claim 1, wherein the chemical promoter contains one or more of the following functional groups: sulfate, sulfonate, phosphate, carboxylate, alcohols, polyol, polyacids, amines, quaternary ammonium, paraquats and diquats.

12. The method of claim 1, wherein at least two additional steps are performed.

13. The method of claim 1, wherein at least three additional steps are performed.

14. A method comprising:

forming a solution comprising catalyst precursors, electrolyte and a solvent;

inserting electrodes comprising an anode electrode and a cathode electrode in the solution;

heating the solution to a temperature above room temperature; and

performing electrochemical deposition by passing a current between the electrodes.

15. A method comprising:

forming a solution comprising catalyst precursors, electrolyte and a solvent, wherein the quantity of the catalyst precursors in the solution is greater than 0.1 millimolar;

inserting electrodes comprising an anode electrode and a cathode electrode in the solution; and

performing electrochemical deposition by passing a current between the electrodes.

16. A method comprising:

forming a solution comprising catalyst precursors, electrolyte and a solvent;

inserting electrodes comprising an anode electrode and a cathode electrode in the solution; and

performing electrochemical deposition by passing a pulsed current between the electrodes.

17. A method comprising:

forming a solution comprising catalyst precursors, electrolyte, solvent and a chemical promoter;

inserting electrodes comprising an anode electrode and a cathode electrode in the solution; and

performing electrochemical deposition by passing a current between the electrodes.

18. A method comprising:

forming a solution comprising catalyst precursors, electrolyte, solvent and a chemical promoter, wherein the quantity of the catalyst precursors in the solution is greater than 0.1 millimolar;

inserting electrodes comprising an anode electrode and a cathode electrode in the solution;

heating the solution to a temperature above room temperature;

performing electrochemical deposition by passing a pulsed current between the electrodes to produce an enhanced anode electrode; and

performing electrolysis on an oxygenate solution using the enhanced electrodes both as an anode and a cathode.

19. The method of claim 18, wherein the solution is heated above 75° C.

20. The method of claim 18, wherein the quantity of the catalyst precursor in the solution is greater than 100 millimolar.

21. The method of claim 18, wherein the current is pulsed with an on:off time ratio in the range of 1:0.001 to 1:1000.

22. The method of claim 18, wherein the current is pulsed with an on:off time ratio of 1:99.

23. The method of claim 18, wherein the chemical promoter contains one or more of the following functional groups: sulfate, sulfonate, phosphate, carboxylate, alcohols, polyol, polyacids, amines, quaternary ammonium, paraquats and diquats.

24. A method comprising:

forming a solution comprising catalyst precursors, electrolyte, solvent and a chemical promoter, wherein the quantity of the catalyst precursors in the solution is greater than 100 millimolar;

inserting electrodes comprising an anode electrode and a cathode electrode in the solution;

heating the solution to a temperature above 75° C.;

performing electrochemical deposition by passing a pulsed current between the electrodes, wherein the current is pulsed with an on:off time ratio of 1:99 to produce an enhanced anode and cathode; and

performing electrolysis on an oxygenate solution using the enhanced electrodes both as an anode and a cathode.