WO2007047630A2

WO2007047630A2 - Carbon fiber-electrocatalysts for the oxidation of ammonia and ethanol in alkaline media and their application to hydrogen production, fuel cells, and purification processes

Info

Publication number: WO2007047630A2
Application number: PCT/US2006/040468
Authority: WO
Inventors: Gerardine G. Botte
Original assignee: Ohio University
Priority date: 2005-10-14
Filing date: 2006-10-16
Publication date: 2007-04-26
Also published as: US20100252422A1; WO2007047630A3; JP2009515036A

Abstract

Electrocatalysts useful for the oxidation of ammonia and ethanol in alkaline media. The electrocatalysts include a carbon support, a first plating layer having a strong affinity for OH, and a second plating layer having a strong affinity for oxidation of ammonia or ethanol. The carbon support may be selected from such materials as carbon fibers, carbon tubes, carbon microtubes, and carbon microspheres. The first plating layer is selected from rhodium, ruthenium, nickel and palladium and combinations thereof. The second plating layer is selected from platinum, indium, and combinations thereof. Also provided are electrolytic cells for the production of hydrogen including one or more electrocatalysts described herien, a basic electrolyte, and ammonia or ethanol. Also provided are ammonia fuel cells and ethanol fuel cells utilizing the electrocatalysts described herein.

Description

Carbon fϊber-electrocatalysts for the Oxidation of Ammonia and

Ethanol in Alkaline Media and their Application to Hydrogen Production, Fuel Cells, and Purification Processes

Cross-Reference to Related Applications

[0001] This application claims priority to U.S. Provisional Application No. 60/726,884, entitled "Carbon fiber-electrocatalysts for the Oxidation of Ammonia and Ethanol in Alkaline Media and their Application to Hydrogen Production, Fuel Cells, and Purification Processes," filed October 14, 2005, the entirety of which is incorporated herein by reference.

Background of the Invention

[0002] New electrocatalysts for the oxidation of ammonia and ethanol in alkaline media are necessary for several reasons: first, a continuous electrolytic cell for the electrolysis of ammonia and/or ethanol does not exist; second, none of the electrodes currently available are able to overcome the poisoning of the electrode due to surface blockage; and third no procedures that guarantee a hard rate performance of carbon fibers and/or carbon nanotubes exist. Accordingly, a need exists for new electrocatalysts that can be used in continuous electrolytic cells for the electrolysis of ammonia and/or ethanol. A need further exists for electrodes that are able to overcome the poisoning of the electrode due to surface blockage. And finally, a need exists for new procedures that guarantee a hard rate performance of carbon fibers and/or carbon nanotubes exist.

Summary of the Invention

[0003] Provided herein are electrocatalysts useful for the oxidation of ammonia and ethanol in alkaline media. The electrocatalysts described herein include a carbon support, a first plating layer having a strong affinity for OH, and a second plating layer having a strong affinity for oxidation of ammonia or ethanol. The carbon support may be selected from such materials as carbon fibers, carbon tubes, carbon microtubes, and carbon microspheres. The first plating layer is selected from rhodium, ruthenium, nickel and palladium and combinations thereof. The second plating layer is selected from platinum, indium, and combinations thereof. When combinations of platinum and iridium are used, the ratio of platinum:iridium may be in the range from about 99.99:0.01 to about 50:50. In some embodiments, the ratio of platinumdridium is from about 95:5 to about 70:30. In other embodiments the ratio of platinum:iridium is jrom about 80:20 to about 75:25. The first plating layer preferably has a coverage of at least about 2 mg/cm. In some embodiments, the total plating coverage is in the range from about 4 mg/cm to about 10 mg/cm.

[0004] Also provided are electrolytic cells for the production of hydrogen. Generally, the cell includes one or more electrocatalysts described herien, a basic electrolyte, and ammonia. In a second embodiment, the cell comprises one or more electrocatalysts described herien, a basic electrolyte, and ethanol. hi some embodiments, the basic electrolyte is added in excess of the stoichiometric proportions needed. In some embodiments, the basic electrolyte has a concentration in the range from about 3M to about 7M. The ammonia may be present in the cell at a concentration in the range from about 0.01 M to about 5M. In some embodiments, the ammonia is present in the cell at a concentration in the range from about IM to about 2M.

[0005] hi one embodiment, the ammonia electrolytic cell includes one electrode comprising an electrocatalyst described herein and a second electrode having an activity towards hydrogen evolution in alkaline media, hi this embodiment, the second electrode may be selected from platinum, rhenium, palladium and Raney Nickel or other electrodes known to those of ordinary skill in the art. hi another embodiment, the ammonia electrolytic cell includes two electrodes having the electrocatalyst described herein.

[0006] Also provided are fuel cells utilizing the electrocatalysts described herein as the anode. The cathode may be selected from nickel, platinum black, and other suitable electrodes, hi one embodiment, the fuel is ammonia, and the concentration of basic electrolyte is about two to about five times greater than the concentration of ammonia, hi another embodiment, the concentration of basic electrolyte is about three times greater than the concentration of ammonia.

Brief Description of the Drawings

[0007] Figure 1 shows cyclic voltammetry of ammonia on a Pt electrode on a Ti substrate. The decrease in the current density after a potential higher than -0.2 V is due to surface blockage.

[0008] Figure 2 shows yclic voltammetry of ethanol on a Pt-Rh (at different compositions) and Rh electrode on a Ti substrate. The decrease in the current density after a potential higher than 0 V vs SHE is due to surface blockage.

[0009] Figure 3 shows adsorption of OH on a Platinum cluster. [0010] Figure 4 shows experimental results on the electro-oxidation of ammonia on a Pt electrode. The experiments were performed on a rotating disk electrode. The comparison of the oxidation of ammonia with the baseline of KOH solution indicates that the first oxidation peak is due to the adsorption of OH^" at the surface of the electrode.

[0011] Figure 5 shows results of microscopic modeling of the electro-adsorption of OH. The results indicate that if the sites were available the adsorption of OH will continue producing higher oxidation currents

[0012] Figure 6 shows a representation of the electro-oxidation mechanism of ammonia on a Pt electrode. As NH₃ reaches the Pt surface it competes with the OH^" electro-adsorption. Since the Electro-adsorption of OH^" is faster on Pt the active sites of the electrode get saturated with the OH adsorbates causing deactivation of the electrode.

[0013] Figure 7 shows shows a schematic representation of the procedure used to increase the electronic conductivity of the carbon fibers during plating (and also during the operation of the electrode).

[0014] Figure 8 shows SEM photographs of the carbon fibers before plating and after plating.

[0015] Figure 9 shows cyclic voltammetry performance of Ammonia (IM Mϊ3) and IM KOH solution at 25⁰C. Comparison of the performance of the carbon fiber electrodes with different compositions.

[0016] Figure 10 shows cyclic voltammetry performance of Ammonia (IM NH3) and IM KOH solution at 25⁰C. Comparison of the loading of the electrode. Low loading 5 mg of total nobel metal /cm of fiber, high loading 10 mg of nobel metal/cm of carbon fiber.

[0017] Figure 11 shows cyclic voltammetry performance of Ammonia (IM NH3) and IM KOH solution at 25°C. Comparison of different electrode compositions at low loading: 5 mg of total nobel metal /cm of fiber. High Rh, Low Pt (80% Rh, 20% Pt), low Rh and high Pt (20% Rh, 80% Pt).

[0018] Figure 12 shows cyclic voltammetry performance of Ammonia (IM NH3) and IM KOH solution at 25°C. Effect of ammonia concentration. The concentration of NH3 does not affect the kinetics of the electrode. [0019] Figure 13 shows cyclic voltammetry performance of Ammonia (IM NH3) and IM KOH solution at 25⁰C. Effect of OH concentration. The higher the concentration of OH the faster the kinetics

[0020] Figure 14 shows cyclic voltammetry performance of ethanol (IM ethanol) and IM KOH solution at 25°C. The results indicate that the electrode is also active for the continuous electro- oxidation of ethanol.

[0021] Figure 15 shows energy (a) and Power balance (b) on the ammonia electrolytic cell. The energy consumption is low (much lower than a commercial water electrolyzer). This demonstrates the technical feasibility of the technology.

[0022] Figure 16 shows cyclic voltammetry of ammonia electro-oxidation on Pt-Rh electrode on a Ti substrate.

Detailed Description of the Invention

[0023] Provided herein is an electrocatalyst made of electrodeposited noble metals (by layers) on supported carbon fibers (nano-tubes or carbon fibers) has been developed for electrolysis of different chemicals in alkaline medium. The electrocatalysts provided herein eliminate the surface coverage effects leaving a clean active surface area for reaction.

[0024] In one embodiment, the electrocatalysts provided herein are used in an electrolytic cell for the production of hydrogen from Ammonia solution in alkaline media using the developed catalyst as anode or cathode. In another embodiment, the electrocatalysts provided herein are used in an electrolytic cell for the production of hydrogen from Ethanol solution in alkaline media using the developed catalyst as anode or cathode, hi another embodiment, the electrocatalysts provided herein are used in an Ammonia fuel cell using the developed catalyst as anode. In another embodiment, the electrocatalysts provided herein are used in an ethanol fuel cell using the developed catalyst as anode. In yet another embodiment, the electrocatalysts provided herein are used in an electrochemical treatment process where ammonia-contaminated effluents are purified by oxidation of ammonia using the developed catalyst. Those skilled in the art will recognized additional uses for the electrocatalysts provided herein.

[0025] Also provided herein are continuous electrolytic cells for the electrolysis of ethanol and ammonia in alkaline medium. The power generation system is integrated by ammonia and/or ethanol electrolytic cell, and proton exchange membrane fuel cell and/or alkaline fuel cell. [0026] Also provided herein are procedures for the preparation of the carbon fibers and nano- tubes electrodes. The procedures described herein increase the electronic conductivity of the fibers and nano-tubes to the levels of a metallic substrate for electrodes.

[0027] The electrodes comprising the electro-catalysts described herein are especially useful for ammonia oxidation and/or ethanol oxidation, particularly in alkaline media. These catalysts are particularly useful in following applications: electrolytic cells for the production of hydrogen from ammonia or ethanol solution in alkaline media using the catalyst as an anode or as a cathode; ammonia and/or ethanol fuel cells using the catalyst as an anode; sensors for ammonia concentration/activity measurements using the catalyst as part of the sensing element; electrochemical treatment processes where ammonia-contaminated effluents are purified by oxidation of ammonia using the catalyst.

[0028] In accordance with the methods described herein, the electrocatalysts are deposited in two layers. The first layer comprises a metal that has a strong affinity for OH. Examples include: Rh, Ru, Ni, and Pd. Rh is the prefer material. The first layer coverage should be of at least 2 mg/cm of fiber to guarantee a complete plating of the fiber. The second layer comprises one or more metals that have a strong affinity for the oxidation of ammonia and/or ethanol. Examples include: Pt and Ir. Monometallic deposition and/or bimetallic deposition of these materials may be performed. Ratios of Pt:Ir can range from 100% Pt-0% Ir to 80% Pt-20% Ir.

[0029] The electrodes comprising electro-catalysts described herein are useful in electrolytic cells for the production of hydrogen. The electrolytic cells comprise a) one or more electro- catalysts; and b) an alkaline, i.e., a basic electrolyte, solution; and c) ammonia. In other embodiments, the electrolytic cells comprise a) one or more electro-catalysts; and b) an alkaline, i.e., a basic electrolyte, solution; and c) ethanol. The basic electrolyte can be any alkaline electrolyte that is compatible with the electro-catalyst, does not react with ammonia or ethanol, and has a high conductivity. In most embodiments, the basic electrolyte is added in excess of the stoichiometric proportions needed. In some embodiments, the basic electrolyte is added at a level of about three times greater than the amount of ammonia added, stoichiometrically. In some embodiments, the electrolyte is potassium hydroxide, which may be used at a concentration of about 3M to about 7M. In some embodiments, the potassium hydroxide is added at a level of about 5M. Another example of a suitable basic electrolyte is sodium hydroxide at similar concentration levels. While potassium hydroxide and sodium hydroxide are illustrative of alkaline electrolytes that may be used, many other alkaline electrolytes known to those skilled in the art may also be used. [0030] The ammonia for the electrolytic cells described herein is an ammonia solution, wherein the source of the ammonia is not particularly limited. The ammonia for the electrolytic cells may be dissolved in water, i.e., ammonium hydroxide, which may be stored until ready for use, and then fed directly to the cell. The ammonia may be stored as a liquefied gas, at higher pressure, and then combined with water and the electrolyte when ready to use. The ammonia may also be obtained from suitable ammonium salts, such as but not limited to ammonium sulfate, dissolved in the electrolyte solution. The ammonia may be present at a concentration from about 0.01M to about 5M. In some embodiments, the ammonia concentration in the electrolytic cell will be from about IM to about 2M at ambient temperature. At higher temperatures, higher concentrations of ammonia may be desired.

[0031] The electrodes comprising the electro-catalysts described herein may be used as the anode of the electrolytic cell, or as both the anode and the cathode. In one embodiment, the electrolytic cell has a) at least one electrode comprising an electro-catalyst as described herein; b) potassium hydroxide at a concentration from about 3 M to about 7 M; and c) ammonia at a concentration from about 0.5 M to 2 M. hi another embodiment, the electrolytic has a) two electrodes comprising electro-catalysts described herein; 2) about 5 M potassium hydroxide; and 3) about 1 M ammonia. In other embodiments of the electrolytic cell, an electrode comprising the electro-catalyst described herein is used as only the anode or the cathode, the other may then be selected from any electrode having an activity toward hydrogen evolution in alkaline media. Some examples of such electrodes include, but are not limited to noble metals, such as platinum, rhenium, palladium, as well as Raney Nickel. Still other suitable electrodes can readily be determined by those skilled in the art.

[0032] The electrolytic cell can operate over a wide range of temperatures. Generally the electrolytic cell described herein may be operated from about 20°C to about 70°C. hi one embodiment, the electrolytic cell is operated at ambient temperature. In another embodiment, the electrolytic cell is operated in the temperature range from about 60⁰C to about 7O⁰C. hi another embodiment, the electrolytic cell is operated in the temperature range from about 2O⁰C to about 60⁰C. In another embodiment, the electrolytic cell is operated in the temperature range from about 3O⁰C to about 70°C. In another embodiment, the electrolytic cell is operated in the temperature range from about 30⁰C to about 60⁰C. In another embodiment, the electrolytic cell is operated in the temperature range from about 4O⁰C to about 50⁰C. In yet another embodiment, pressures higher than atmospheric pressure would be used, allowing temperatures higher than 70⁰C to be used. [0033] The current densities applied to the electrolytic cell to produce hydrogen may be in the range from about 25 niA/cm² to about 500 mA/cm². In some embodiments, the current densities are in the range from about 50 mA/cm² to about 100 mA/cm². hi some embodiments, the current densities are in the range from about 25 mA/cm² to about 50 mA/cm². In some embodiments, the current densities are in the range from about 50 mA/cm² to about 500 mA/cm². hi some embodiments, the current densities are in the range from about 100 mA/cm² to about 400 mA/cm². In some embodiments, the current densities are in the range from about 200 mA/cm² to about 300 mA/cm².

[0034] Also provided are methods of preparing hydrogen gas, wherein the method involves oxidizing ammonia in the above-described electrolytic cell. When preparing hydrogen gas using the electrolytic cell, a stoichiometric excess of basic electrolyte is used, hi some embodiments, the basic electrolyte is present at a level of at least three times greater than the amount of ammonia. In some embodiments, the electrolyte for the electrolytic cell is KOH. hi some embodiments, the concentrations for the alkaline electrolyte and ammonia in the electrolytic cell are 5M and IM, respectively, hi an electrolytic cell, the temperature of the electrolytic cell and the current densities affect the amount of hydrogen produced. Suitable temperatures and current densities are as described above.

[0035] When ammonia is used in the electrolytic cell, the electrolytic cell described herein produces both hydrogen and nitrogen gas. Accordingly, the same method for the production of hydrogen gas may be applied for the production of nitrogen gas by recovering the nitrogen gas produced by the oxidation of ammonia.

[0036] The electro-catalysts described herein also have application in ammonia fuel cells. An ammonium fuel cell has an anode, wherein the anode is an electrode comprising the catalyst described herein; a cathode, ammonia, and a basic {i.e., an alkaline) electrolyte. Nickel and platinum black are two non-limiting examples of suitable cathodes, though other cathodes known to those skilled in the art may be used. The fuel cell may be operated at any temperature from about ambient temperature, e.g., about 20°C to 25°C up to about 70°C. hi some embodiments, the fuel cells are operated at ambient temperature, hi some embodiments, the basic electrolyte used in the fuel cell may be any suitable basic electrolyte, for example, an inorganic hydroxide, such an alkali metal hydroxide or alkali earth metal hydroxide. Suitable basic electrolytes include alkaline media that do not adversely affect the catalysts described herein, do not react with ammonia, and have good conductivity, hi some embodiments, the alkaline electrolytes are potassium hydroxide or sodium hydroxide, hi one embodiment, the electrolyte is potassium hydroxide. The amount of the basic electrolyte used in the fuel cell is in stoichiometric excess of the amount of ammonia, hi some embodiments, the concentration of the alkaline electrolyte is at least about three times greater than the concentration of the ammonia. In one embodiment, the concentrations of basic electrolyte and ammonia are about 5M and about IM, respectively. The ammonia for the fuel cell can be from liquid or gaseous ammonia dissolved in water or in the basic medium, or it can be from a suitable ammonium salt dissolved in the basic medium.

[0037] The electro-catalysts of the present invention are also useful for in sensors for the detection of ammonia. The substantial current response collected at the anode during oxidation of ammonia (even at small concentrations) in the presence of the developed catalyst makes these catalyst ideal for use in ammonia electrochemical sensor. These electrochemical sensors may be used to determine even trace amounts of ammonia contamination in a sample.

[0038] The strong activity of ammonia on the present electro-catalysts, even with low ammonia concentrations are useful in processes for removing ammonia from contaminated effluents. Accordingly, also provided are methods for removing ammonia contaminants from contaminated effluents; the method comprising using the electro-catalysts described herein to oxidize the ammonia contamination in the contaminated effluent, hi this method, an electrolytic cell may be prepared which uses at least one electrode comprising the electro-catalysts described herein to oxidize ammonia contaminants in effluents. The effluent may be fed as a continuous stream, wherein the ammonia is electrochemically removed from the effluent, and the purified effluent is released or stored for other uses.

[0039] Significant current densities can be obtained from the oxidation of ammonia on noble metal but this electrode is far less reversible. Similar case occurs with the electro-oxidation of ethanol in alkaline medium. Furthermore, the activation of the electrode is limited by surface coverage. Figures 1 and 2 show the deactivation of the electrode for both reactions (ethanol and ammonia electro-oxidation in alkaline medium) during cyclic voltammetry experiments.

[0040] hi this application we propose a new design and recipe for an electrode active for the oxidation of ammonia and ethanol that overcomes both problems: reversibility as well as deactivation.

[0041] Development of a continuous in-situ hydrogen generator from ethanol and ammonia electrolysis Hydrogen is the main fuel source for power generation using fuel cells, but its effective storage and transportation still present technical challenges. Current hydrogen production costs make fuel cell technology for distributed power generation economically noncompetitive when compared to traditional oil-fueled power systems. Current technologies (for distributed hydrogen) are able to produce hydrogen at costs of $5 to $6 per kg of H₂. This high production cost is due in part to high product separation/purification costs and high operating temperatures and pressures. Using current technologies, hydrogen can be obtained by the partial oxidation, catalytic steam reforming, or thermal reforming of alcohols and hydrocarbons. However, all of these processes take place at high temperatures and they generate a large amount of CO_x as byproducts which must be removed from the hydrogen product. Most of these CO_x byproducts cause degeneration of fuel cell performance due to poisoning of the fuel cell catalysts. Their removal from the fuel stream makes these processes complicated, bulky and expensive. Currently, the cleanest way to obtain pure hydrogen is by the electrolysis of water. During the electrolysis of water electrical power (usually provided by solar cells) is used to break the water molecule producing both pure oxygen and hydrogen. The disadvantage of this process is that a large amount of electrical power is needed to produce hydrogen. The theoretical energy consumption for the oxidation of water is 66 W-h per mole of H₂ produced (at 25 ⁰C). Therefore, if solar energy is used (at a cost of $0.2138/kWh), the theoretical cost of hydrogen produced by the electrolysis of water is estimated at $7 per kg ofH_∑.

[0042] In co-pending US patent application 10/962,894 an electrolytic cell able to electro- oxidize ammonia in alkaline medium was disclosed. However, the cell describe was a batch cell. In this application a continuous electrolytic cell for the electro-oxidation of ammonia and ethanol is described.

[0043] Low electronic conductivity of carbon fibers and nanotubes Plating of carbon fibers and nano-tubes is a difficult job. Most of the problems have to do with the relatively low electronic conductivity of these materials, which may also cause a poor coating of the surface (for example, easy to remove). The electronic conductivity of the fibers decreases along the length from the electrical connection, therefore, the furthest point of contact to the electric connection transfers a low current when compare to the closest point to the electric contact.

[0044] Described herein is an electroplating procedure for carbon fibers and nano-tubes is described that overcomes these problems: it provides the electrode with uniform current distribution and also enhances the adherence of the coating and therefore the durability of the electrode.

[0045] Electrocatalyst for ammonia and ethanol oxidation. An electrocatalyst made of electrodeposited noble metals (by layers) on supported carbon fibers (nano-tubes or carbon fibers) has been developed for electrolysis of different chemicals in alkaline medium. The invention eliminates the surface coverage effects leaving a clean active surface area for reaction.

[0046] It was believed that the surface blockage caused during the ammonia electrolysis in alkaline medium was due to the presence of elemental Nitrogen, according to the mechanism proposed by Gerisher:

2| KB₃ +M<=^.±MNH₃ ^kl

2 MNH₃ +OH^" τ^→ MNH₂ +H₂O + e^'

2 f i MNH₂ ² + OH^" τ ^^r→^{^} MNH + H₂ ²O + e ^" ) / (^rds)

- (MNH + MNH <===± M₂N₂H₂

- 1 M₂N₂H₂ + 2OH^" <=± M₂N₂ + 2H₂O + 2e

M₂N₂ <==±N₂ +2M

Deactivation Reaction:

2 f MNH + OH- <=^→ MN + H₂O + e^"

I ^i ²

where M represents an active site on the electrode.

[0047] We have demonstrated by two independent methods that the proposed mechanism by Gerisher is not correct and that OH needs to be adsorbed on the electrode for the reactions to take place. Furthermore, the electrode gets deactivated by the OH adsorbed at the active sites.

[0048] Results from molecular modeling indicate that the adsorption of OH^" on an active Pt site is strong (chemisorption) and can be represented by the following reaction:

Pt₁₀ + OH- <» Pt₁₀ - 0H_(ad) + e^"

[0049] Figure 3 shows the bond between the UH ana the platinum cluster. The system was modeled using Density functional Methods. The computations were performed using the B3PW91 and LANL2DZ method and basis set, respectively. The binding energy for the Pt-OH cluster is high with a value of -133.24 Kcal/mol, which confirms the chemisorption of OH on a Pt cluster active site.

[0050] Additionally, results from microscopic modeling as well as experimental results on a rotating disk electrode (RDE) indicate that the adsorption of OH is strong and responsible for the deactivation of the catalyst.

[0051] Figure 4 compares the baseline of a KOH solution with the same solution in the presence of OH. The curves indicate that the first oxidation peaks that appear at about -0.7 V vs Hg/HgO electrode had to do with the electro-adsorption of OH^".

[0052] Figure 5 shows a comparison of the predicted results (by microscopic modeling) with the experimental results for the electro-adsorption of OH^". The results indicate that the model predict the experimental results fairly well. Furthermore, an expression for the surface blockage due to the adsorption of OH at the surface of the electrode was developed (notice that the active sites for reaction θ decay with the applied potential due to adsorbates). If the surface were clean (see results Model without coverage), the electro-adsorption of OH^" will continue even at higher potentials and faster).

[0053] Compiling the experimental results with the modeling results the author of this application proposes the following mechanism for the electro-oxidation of ammonia in alkaline medium: First the adsorption of OH takes place, as the ammonia molecule approaches the electrode, it gets also adsorbed on the surface. Through the oxidation of ammonia the some OH adsorbates are released from the surface in the form of water molecule. However, since the adsorption of OH is stronger and the OH ions move faster to the surface of the electrode this last get deactivated with increasing the potential. There will be a competition on the electrode between the adsorption of OH and MH₃. The results of the mechanism are summarized on the proposed reactions given below as well as Figure 6.

Pt₁₀ + OH^" o Pt₁₀ - OH^" ₍ad) (l)

2Pt₁₀ + 2NH₃ <z> 2Pt₁₀ - NH_{3(ad) (2})

Pt₁₀ - NH_3(ad) + Pt₁₀ - OH-(ad) <=> Pt₁₀ - NH_2(ad) + Pt₁₀ + H₂O + e^" (3)

Pt₁₀ - NH_2(ad) + Pt₁₀ - OH-(ad) <» Pt₁₀ - NH_(ad) + Pt₁₀ + H₂O + e^" _{(4> rds)}

Pt₁₀ - NH_(ad) + Pt₁₀ - OH-(ad) o Pt₁₀ - N_(ad) + Pt₁₀ + H₂O + e^" ₍₅₎

2Pt₁₀ - N_(ad) <=> Pt₁₀ - N_2(ad) + Pt_{10 (6)}

"ho ^ 2(ad) o Pt₁₀ +N_2(g)

(7)

[0054] This mechanism can be extended to the electro-oxidation of other chemicals in alkaline solution at low potentials (negative vs. SHE). For example, it has been extended to the electro- oxidation of ethanol. The propose mechanism clearly defines the expectations for the design of better electrodes: the materials used should enhance the adsorption of NH₃ and/or ethanol or other chemical of interest (it can also enhance the electrolysis of water in alkaline medium). It is necessary a combination of at least two materials: One of the materials should be more likely to be adsorbed by OH than the other; this will leave active sites available for the electro-oxidation of the interested chemicals (for example NH₃ and/or ethanol).

[0055] Continuous Electrolytic Cell A continuous electrolytic cell for the in-situ production of hydrogen from ethanol and ammonia is specified and tested in this application. Hydrogen is produced in both processes with a faradic efficiency of 100%. In both cases the reaction that takes place at the cathode is the reduction of water in alkaline medium:

2H₂O + 2e^~ → H₂ + 2OH- E⁰ = 0.82 V VS SHE

[0056] Design of carbon fiber and carbon nanotubes electrode The electronic conductivity of the carbon fibers and carbon nanotubes was enhanced by a deposition in different layers. The first layer of the catalyst is very active for the adsorption of OH while the second layer is made of materials that are very active for the adsorption of the chemicals of interest (for example, ammonia and/or ethanol).

[0057] Electrode preparation: Figure 7 shows a schematic representation of the procedure used to increase the electronic conductivity of the carbon fibers during plating (and also during the operation of the electrode). The fibers were wrapped on a titanium gauze, therefore, there were in electric contact with the metal at different points. This improvement allowed an easy and homogenous plating of the fibers at any point. The electronic conductivity at any point in the fiber was the same as the electronic conductivity of the Ti gauze (which is really high).

[0058] Figure 8 shows a Scanning Electron Microscope photograph of the electrode before plating and after plating. A first layer of Rh was deposited on the electrode to increase the electronic conductivity of the fibers and to serve as a free substrate for the adsorption of OH (OH was more affinity for Rh than for Pt). A second layer consisted of Pt was plated on the electrode. The Pt layer did not cover all the Rh sites, leaving Rh surface to act as a preferred OH adsorbent.

[0059] Performance of the electrode: Figure 9 shows the cyclic voltammetry performance for the electro-oxidation of ammonia on different electrode compositions. Notice that the carbon fibers plated with only Rh are not active for the reaction, while when they are plated with only Pt, the electrode is active but it is victim of poisoning. On the other hand, when the electrode is made by plating in layers: first Rh is deposited and then a second layer consisting of Pt is deposited the electrode keeps the activity. This is explained with the mechanism presented in the previous section. Figure 9 demonstrates that the new procedure and recipe for the preparation of the electrode takes care of the surface blockage.

[0060] Figure 10 shows the effect of different total loading on the electro-oxidation of ammonia. The results indicate that the catalyst with the lowest loading is more efficient for the electro- oxidation of ammonia. This is beneficial for the economics of the process (cheaper catalyst). Additional loading of the catalyst just causes the formation of layers over layers that are not really active for the reaction.

[0061] Figure 11 illustrates the effect of the catalyst composition of the electro-oxidation of ammonia in alkaline solution. There is not difference on the performance of the electrode with the composition of the electrode. This is basically because as long as a first layer of Rh is plated on the electrode the surface blockage will be avoided. Besides, additional plating of Pt will cause the growth or Pt island (see SEM picture, Figure 8) which are not completely active in the whole surface).

[0062] Figure 12 shows the effect of ammonia concentration on the performance of the electrode. The effect of ammonia concentration is negligible on the electrode performance. Because the active Pt sites had already adsorbed the NH3 needed for a continuous reaction. [0063] Figure 13 presents the effect of the concentration of OH on the electro-oxidation ot ammonia. The higher the concentration of OH the faster the reaction rate. Notice that the electrode keeps the continuous activity (no poisoning) independently of the OH concentration.

[0064] Figure 14 shows the evaluation of the electrode for the electro-oxidation of ethanol. Continuous electro-oxidation of ethanol in alkaline medium is achieved without surface blockage. This proves that the invention is also valid for this chemical.

[0065] Continuous Electrolytic Cell A prototype system for the continuous electrolysis of ammonia and/or ethanol in alkaline medium has been built. At the cathode, H₂ is produced continuously with a faradic efficiency of 100%. The design of the cell is small (4x4 cm) and allows a significant production of H₂ at a small energy and power consumption. A cloud of H₂ may be seen when generated at the cathode of the cell. The production of H₂ is massive, which demonstrates the use of the cell for in-situ H₂ production. Figure 15 shows the energy balance and the power balance on the ammonia electrolytic cell. The cell outperforms a commercial water electrolyzer. Both the energy and the power balance on the cell indicate that the cell could operate by stealing some energy of a PEM H₂ fuel cell and still the system (ammonia electrolytic cell/PEM fuel cell) will provide some net energy. This arrangement can be used to minimize hydrogen storage.

[0066] In one exemplary system produces of 480 kg of H₂ per day. A total capital investment of $1,000,000 is needed for the construction of the power system. A comparison of the economic analysis for the production of H₂ using the ammonia continuous electrolytic cell with current state of the art technologies (natural gas reforming and water electrolysis) for distributed power has been done. The continuous ammonia electrolyzer can produce hydrogen at less than $2 per Kg. Compared to other technologies for in situ hydrogen production, savings are substantial- using numbers provided by the National Academy of Science, the continuous ammonia electrolyzer produced H₂ about 20% cheaper than it can be produced using natural gas steam reforming and about 57% cheaper than using water electolysis.

[0067] Preparation of the Electrodes

[0068] The procedure is divided into two steps. The first step is the construction of the electrode. The schematic for the construction of the electrode is shown if Figure 7. The plating procedure consists into two steps: 1. First layer plating and 2. Second layer plating. [0069] First layer plating. This step consists on plating the carbon fibers or the carbon nanotubes with materials that show a strong affinity for OH. Examples include, but are not limited to Rh, Ru, Ni, and Pd. In one preferred embodiment, Rh is used. The first layer coverage should completely plate the fiber. In some embodiments, the first layer coverage is at least mg/cm of fiber to guarantee a complete plating of the fiber. In other embodiments, the first layer coverage may be 2.5, 3.0, 3.5 mg/cm of fiber, and so forth.

[0070] Second layer plating: This step consists on plating the electrode with materials that have a strong affinity for the oxidation of ammonia and/or ethanol. Examples include: Pt and Ir. Monometallic deposition and/or bimetallic deposition of these materials may be performed. Ratios of Pt:Ir can range from 100% Pt-0% Ir to 80% Pt-20% Ir.

[0071] Table I summarizes the plating conditions for the anode and the cathode of the electrolytic cell. After plating the Rhodium, the electrode is weighted. The weight corresponds to the Rhodium loading. Then, the Platinum is deposited on top of the Rhodium. After the procedure is completed, the electrode is measure again. The measurement will correspond to the total loading. The Platinum loading is obtained subtracting the total loading from the Rhodium previous measurement. The relation Platinum/Rhodium is then calculated so as the percentage of fixed loading. Because the loading depends on the length of the fiber, another measurement have to be calculated. It is known that 10 cm of fiber weights 39.1 mg, and because it is know the weight of fiber (calculated in step 1), then by proportionality it can be known the length of the total fiber that is being used in each electrode.

[0072] Table II summarizes the general conditions of the plating bath. During the whole plating procedure, the solution was mixed to enhance the transport of the species to the fibers and or nanotubes. Table III shows examples of some electrodes compositions, lengths, and loadings of noble metals.

[0073] Summarizing, the electrodes consist of a carbon fiber and/or carbon nanotubes substrates which were plated with a first layer of noble metal. This metal had a strong affinity for OH. Then after, the electrode was plated (single deposition and/or bimetallic deposition) with a noble metal that has a strong affinity for ammonia and/or ethanol. Table 1 Conditions for Electro-plating Technique in the Deposition of Different Metals on the Carbon Fibers and/or Carbon Nanotubes

Table 2 General Conditions of the Plating Bath

Table 3 Examples of some Electrode Compositions and Loadings

[0074] Electrolytic Cell

[0075] The anode of the electrolytic cell was constructed using the procedure described above. It consists of carbon fibers plated with two layers of materials. The first layer is made of a metal that has affinity for OH, while the second layer is made of a metal or metals that have affinity for ammonia and/or ethanol.

[0076] The cathode was made similar to the anode. It could be built exactly the same way or it can just consist of carbon fibers and or carbon nanotubes plated with Nickel.

[0077] The current fibers must rest (be wrapped) on a metal gauze. Any inert material for the acidic deposition bath as well as the alkaline medium of the solution could be used. In a preferred embodiment, the metal gauze is titanium.

[0078] The case of the cell can be made of any nonconductive polymer. Examples include: polypropylene, Teflon, acrylic, etc. In a preferred embodiments, the case is Teflon. The gaskets of the cell were made of Teflon.

[0079] The electrodes in the cell (anode and cathode) need to be separated by a membrane or separator that stands the strong alkaline conditions of the medium. Examples include: polypropylene, Teflon, and/or fuel cell grade asbestos. In a preferred embodiment, the separator is polypropylene.

[0080] The developed anode can be used in an ammonia and/or ethanol electrolytic cell for the production of hydrogen on site. It can also be used in an ammonia and/or ethanol fuel cell in alkaline medium. Surprisingly, previous tests done with a Pt-Rh electrode on a Titanium substrate did not show the activity reflected in this application. The electrode deposition described herein on carbon fibers and/or carbon nanotubes shows excellent activity towards ammonia and/or ethanol oxidation in alkaline medium.

[0081] The examples described herein are for illustrative purposes only and are not meant to limit the scope of this invention.

Claims

The invention claimed is:

1. An electrocatalyst comprising a carbon support, a first plating layer, and a second plating layer, wherein the first plating layer comprises a metal having a strong affinity for OH; and wherein the second plating layer comprises a metal having a strong affinity for oxidation of a fuel.

2. The electrocatalyst of claim 1 wherein the carbon support is selected from the group consisting of carbon fibers, carbon tubes, carbon microtubes, and carbon microspheres.

3. The electrocatalyst of either of claims 1 or 2 wherein the first plating layer is selected from the group consisting of rhodium, ruthenium, nickel and palladium and combinations thereof.

4. The electrocatalyst of claim 3 wherein the first plating layer is rhodium.

5. The electrocatalyst of any one of claims 1-4 wherein the second plating layer comprises platinum, iridium, or combinations thereof.

6. The electrocatalyst of any one of claims 1-5 wherein the fuel is selected from the group consisting of ammonia and ethanol.

7. An electrocatalyst for the oxidation of ammonia or ethanol in alkaline media, the electrocatalyst comprising a carbon support selected from the group consisting of carbon fibers, carbon tubes, carbon microtubes, and carbon microspheres; a first plating layer selected from the group consisting of rhodium, ruthenium, nickel, palladium and combinations thereof; and a second plating layer is selected from the group consisting of platinum, iridium, and combinations thereof.

8. The electrocatalyst of claim 7 wherein the first plating layer has a plating coverage of at least about 2 mg/cm.

9. The electrocatalyst of claim 7 wherein the second plating layer is a monometallic layer.

10. The electrocatalyst of claim 8 wherein the second plating layer is a bimetallic layer.

11. The electrocatalyst of claim 10 wherein the bimetallic layer comprises platinum and iridium; with a ratio of platinumdridium in the range from about 99.99:0.01 to about 50:50.

12. The electrocatalyst of claim 11 wherein the ratio of platinunriridium is from about 95:5 to about 70:30.

13. The electrocatalyst of claim 12 wherein the ratio of platmum:iridium is from about 80:20 to about 75:25.

14. The electrocatalyst of claim 7 having a total plating coverage in the range from about 4 mg/cm to about 10 mg/cm.

15. An electrolytic cell for the production of hydrogen, the cell comprising one or more electrocatalysts, a basic electrolyte, and ammonia; wherein the electrocatalyst comprises: a carbon support selected from the group consisting of carbon fibers, carbon tubes, carbon microtubes, and carbon microspheres; a first plating layer selected from the group consisting of rhodium, ruthenium, nickel, palladium and combinations thereof; and a second plating layer is selected from the group consisting of platinum, iridium, and combinations thereof.

16. The electrolytic cell of claim 15 wherein the basic electrolyte is selected from the group consisting of potassium hydroxide, sodium hydroxide, and combinations thereof.

17. The electrolytic cell of claim 15 wherein the basic electrolyte is added in excess of the stoichiometric proportions needed.

18. The electrolytic cell of claim 16 wherein the basic electrolyte has a concentration in the range from about 3M to about 7M.

19. The electrolytic cell of claim 15 wherein the ammonia is present in the cell at a concentration in the range from about 0.01 M to about 5M.

20. The electrolytic cell of claim 19 wherein the ammonia is present in the cell at a concentration in the range from about IM to about 2M.

21. The electrolytic cell of claim 15 comprising one electrode comprising the electrocatalyst and a second electrode having an activity towards hydrogen evolution in alkaline media; wherein the basic electrolyte comprises potassium hydroxide having a concentration in the range from about 3M to about 7M; and ammonia having a concentration in the range from about 0.5M to about 2M.

22. The electrolytic cell of claim 21 wherein the second electrode is selected from the group consisting of platinum, rhenium, palladium and Raney Nickel.

23. The electrolytic cell of claim 15 comprising two electrodes comprising the electrocatalyst; wherein the basic electrolyte comprises potassium hydroxide having a concentration of about 5M; and ammonia having a concentration of about IM.

24. An electrolytic cell for the production of hydrogen, the cell comprising one or more electrocatalysts, a basic electrolyte, and ethanol; wherein the electrocatalyst comprises: a carbon support selected from the group consisting of carbon fibers, carbon tubes, carbon microtubes, and carbon microspheres; a first plating layer selected from the group consisting of rhodium, ruthenium, nickel, palladium and combinations thereof; and a second plating layer is selected from the group consisting of platinum, iridium, and combinations thereof.

25. A fuel cell comprising an anode, a cathode, a fuel and a basic electrolyte; wherein

wherein the anode comprises: a carbon support selected from the group consisting of carbon fibers, carbon tubes, carbon microtubes, and carbon microspheres; a first plating layer selected from the group consisting of rhodium, ruthenium, nickel, palladium and combinations thereof; and a second plating layer is selected from the group consisting of platinum, iridium, and combinations thereof; wherein the fuel is selected from the group consisting of ammonia and ethanol; and wherein the basic electrolyte is selected from the group consisting of alkali metal hydroxides and alkali earth metal hydroxides.

26. The fuel cell of claim 25 wherein the fuel cell operates at temperature in the range from about 20°C to about 70°C.

27. The fuel cell of either of claims 25 or 26 wherein the cathode is selected from the group consisting of nickel and platinum black.

28. The fuel cell of claim any one of claims 25-27 wherein the fuel is ammonia, and the concentration of basic electrolyte is about two to about five times greater than the concentration of ammonia.

29. The fuel cell of claim 28 wherein the concentration of basic electrolyte is about three times greater than the concentration of ammonia.