Patent Grant
10040050
Not applicable.
The invention relates to a process of forming an oxygen evolution catalyst and apparatus of using cobalt tungstate as a catalyst for the electrochemical and photochemical-electrolysis of water, and in particular to a process and apparatus using hydrated cobalt tungstate as a catalyst for the oxidation of water.
Hydrogen has long been considered an ideal fuel source, as it offers a clean, non-polluting alternative to fossil fuels. One source of hydrogen is the splitting of water into hydrogen (H2) and oxygen (O2), as depicted in equation (1).
2H2O→O2+2H2 (1)
In an electrochemical half-cell, the water-splitting reaction comprises two half-reactions:
2H2O→O2+4H++4e− (2)
2H++2e−→H2 (3)
and hydrogen made from water using sunlight prospectively offers an abundant, renewable, clean energy source. However, the oxygen evolution half reaction is much more kinetically limiting than the hydrogen evolution half reaction and therefore can limit the overall production of hydrogen. As such, efforts have been made to search for efficient oxygen evolution reaction (OER) catalysts that can increase the kinetics of OER and increase the production of hydrogen from water. In particular, oxides of ruthenium and iridium have previously been identified. However, as they are among the rarest elements on earth, it is not practical to use these catalysts on a large scale. Therefore, improved OER catalysts would be very useful in the development of hydrogen as an alternative fuel source.
In one aspect there is disclosed an oxygen evolution catalyst splitting water into oxygen and hydrogen ions that includes hydrated cobalt tungstate.
In another aspect there is disclosed a process for oxidizing water to produce oxygen. The process includes placing water in contact with hydrated cobalt tungstate, the hydrated cobalt tungstate catalyzing the oxidation of water and producing oxygen. The hydrated cobalt tungstate can be a plurality of hydrated cobalt tungstate nanoparticles which may or may not be attached to an electrode with an electrical potential applied between the electrode and the water to generate oxygen.
In a further aspect, there is disclosed a process of forming an oxygen evolution catalyst including the steps of: providing Co(NO3)4; providing Na2WO4; combining the Co(NO3)4 and Na2WO4 forming a solution; exposing the solution to a source of microwave energy and initiating a hydrothermal reaction forming hydrated CoWO4.
The present disclosure provides a method of forming, apparatus and/or catalyst composition for the oxidation of water to generate oxygen gases. The method includes providing a hydrated cobalt tungstate (CoWO4) catalyst material and adding the catalyst to water under a condition effective to generate oxygen. In one embodiment, the method further includes exposing the water, which contains the catalyst, to light radiation to generate oxygen gases.
A “catalyst” as used herein, means a material that is involved in and increases the rate of a chemical electrolysis reaction (or other electrochemical reaction) and which itself, undergoes reaction as part of the electrolysis, but is largely unconsumed by the reaction itself, and may participate in multiple chemical transformations. A catalytic material of the invention may be consumed in slight quantities during some uses and may be, in many embodiments, regenerated to its original chemical state. The reaction may include a water oxidation or oxygen evolution reaction.
In one aspect a water oxidation catalyst or an oxygen evolution catalyst includes hydrated cobalt tungstate that splits water into oxygen and hydrogen ions.
In a further aspect there is disclosed an electrode for electrochemical water oxidation splitting water into oxygen and hydrogen ions that includes a substrate and an active material in contact with the substrate. The active material includes hydrated cobalt tungstate.
In one aspect, the hydrated cobalt tungstate may be combined with conductive particles such as carbon black and may also include a binder such as NAFION®, a sulfonated tetrafluoroethylene based fluoropolymer copolymer sold by DuPont. The combined material may be attached to an electrode substrate using any method known to those in the art. Various electrode substrates may be utilized that are capable of conducting current such as for example, glassy carbon, carbon black or other materials.
The catalyst can include a plurality of hydrated cobalt tungstate nanoparticles. In some instances, the nanoparticles are uniform in size and can have an average particle size of 1 to 10 nm. In one aspect, the oxygen evolution catalyst including the plurality of hydrated cobalt tungstate nanoparticles may have a surface area of from 40-110 m2/g.
In one embodiment, the hydrated cobalt tungstate is attached to an electrode using any method known to those in the art. For example for illustrative purposes only, absorption techniques, adhesives, deposition techniques and the like can be used to attach the hydrated cobalt molybdenum to the electrode.
In some instances, the electrode can have channels and water can be brought into contact with the catalyst at a rate that allows the water to be incorporated into the electrode channels. In addition, the electrode can be in an aqueous solution and/or be part of an electrochemical cell and/or part of a photo-electrochemical cell, which may or may not include a container.
The container may be any receptacle, such as a carton, can or jar, in which components of an electrochemical device may be held or carried. A container may be fabricated using any known techniques or materials, as will be known to those of ordinary skill in the art. The container may have any shape or size, providing it can contain the components of the electrochemical device. Components of the electrochemical device may be mounted in the container. That is, a component, for example, an electrode, may be associated with the container such that it is immobilized with respect to the container, and in some cases, supported by the container.
In some instances, an electrochemical cell containing an embodiment of the present invention offers a highly efficient method of splitting water using solar illumination, without the need for an applied potential. Upon oxidation of water at a photo-anode, hydrogen protons are generated which are then reduced to form hydrogen gas at a counter electrode. In addition, the oxygen and hydrogen generated from the cell can be passed directly to a fuel cell to generate further power.
In a further embodiment, the electrochemical cell can be driven either by a photo-anode such as a dye sensitized semiconductor or an external potential. The dye sensitized semiconductor acts as a chemical/photo-electrical relay system. For example and for illustrative purposes only,
In another aspect, there is disclosed a process of forming an oxygen evolution catalyst including the steps of: providing Co(NO3)4; providing Na2WO4; combining the Co(NO3)4 and Na2WO4 forming a solution; exposing the solution to a source of microwave energy and initiating a hydrothermal reaction forming hydrated CoWO4. The exposing step may include exposure to microwave energy for various periods of time to elevate the temperature or heat the solution to a desired temperature range. Various time and temperature values are displayed in
The exposing step may include exposing the solution to microwave energy from less than one minute to 60 minutes. In one aspect, the exposing step may be from 1 to 10 minutes at a power of 800 Watts. The exposing step may raise the temperature of the solution to a temperature of from 200 to 300 degrees C. Following the exposing step the solution may be cooled and then washed and dried.
The process of forming an oxygen evolution catalyst produces a material that has a pseudo crystalline phase or a hydrated crystalline phase, that provides an increased stability and activity of the catalyst as will be discussed in more detail below. The hydrated phase may be formed at temperatures ranging from 100-240 and dwell times of from less than one minute to 60 minutes as shown in
The invention is further described by the following examples, which are illustrative of specific modes of practicing the invention and are not intended as limiting the scope of the invention defined in the claims.
Preparation of Hydrated CoWO4
Starting materials of Co(NO3)4 6H2O and Na2WO4.2H2O were purchased from Sigma-Aldrich and used directly without further purification. In a typical synthesis a (0.2M) Na2WO4 solution was combined with a (0.2M) Co(NO3)2 solution in a stoichiometric manner with strong agitation. The solution mixture was then placed into a glass microwave tube. A microwave assisted hydrothermal synthesis was conducted on a microwave reactor (Anton Paar Microwave 300). The microwave tube was heated to various temperatures at max power (800 W). The exposure to microwaves was maintained for various times as will be discussed in more detail below. Following the exposure to microwaves the tube was cooled by forced air flow. The resulting product was rinsed with DI water multiple times on a centrifuge followed by vacuum drying overnight at 60 degrees C.
A final powder product was examined by TEM as shown in
XRD data is shown in
Cyclic Voltammetry (CV) of CoWO4
To fabricate a working electrode, a catalyst ink was first prepared by sonicating a mixture of catalyst particles, acid-treated carbon black (CB), Na+-exchanged Nafion1 solution with tetrahydofuran, and then drop-casting (10 mL) onto pre-polished glassy carbon disk electrodes (5 mm in diameter). The catalyst film was allowed to dry at room temperature in a sealed container overnight and the final composition of the as expected to be 250, 50 and 50 mg cm22 for CoWO4, CB and Nafion1, respectively.
The electrochemical measurements were done in a three-electrode glass cell (125 ml) with the working electrode rotating at a rate of 1600 rpm, and Ag/AgCl (3M NaCl) as the reference. The counter electrode (Pt coil) was isolated from the main electrochemical cell using a fritted glass tube. The electrolyte utilized was of 0.4 M Na2HPO4 and 0.6 M Na2SO4, the pH of which was adjusted using NaOH or HNO3 solutions. During the electrochemical tests, the electrolyte was continuously purged and masked with ultra-high purity oxygen.
Cyclic voltammogram plots for the CoWO4 particles after one cycle, and after 20 cycles for amorphous CoWO4 and various hydrated CoWO4 samples are shown in
Tafel plot measurements, as shown in
Referring to
The invention is not restricted to the illustrative examples described above. Examples described are not intended to limit the scope of the invention. Changes therein, other combinations of elements, and other uses will occur to those skilled in the art. The scope of the invention is defined by the scope of the claims.
| Number | Name | Date | Kind |
|---|---|---|---|
| 20040242412 | Gulla | Dec 2004 | A1 |
| 20050136193 | Weidman | Jun 2005 | A1 |
| 20120267234 | Reece | Oct 2012 | A1 |
| 20130158322 | Nyce | Jun 2013 | A1 |
| 20130192999 | Jia et al. | Aug 2013 | A1 |
| 20130202889 | Ham | Aug 2013 | A1 |
| Entry |
|---|
| Electronic Supplementary Information, Different catalytic behavior of amorphous and crystalline cobalt tungstate for electrochemical water oxidation. Jia et al. The Royal Society of Chemistry. 2012. |
| Almeida, M. et al., Electronic and Magmetic Properties pf FeWO4 Nanostructures Synthesized by Microwave Assisted Hydrothermal Method, 2012, Brazil. |
| Kloprogge, J. Theo, et al., Microwave Assistaned Synthesis and Characterisation of Divalent Metal Tungstate Nanocrystalline Materials: Ferberite, Hübnerite, Sanmartinite Scheelite and Stolzite, Materials Chemestry and Physics, 2004, pp. 438-443, vol. 88, School of Physical and Chemichal Sciences, Brisbane, Australia. |
| Sczancoski, J.C., et al., SrMoO4 Powders Processed in Microwave-hydrothermal: Synthesis, Characterization and Optical Properties, Chemicha Engineering Journal, 2008, pp. 632-637, vol. 140, Brazil. |
| Number | Date | Country | |
|---|---|---|---|
| 20160121294 A1 | May 2016 | US |
