Catalytic materials, photoanodes, and photoelectrochemical cells for water electrolysis and other, electrochemical techniques

Abstract

Catalytic materials, photoanodes, and systems for electrolysis and / or formation of water are provided which can be used for energy storage, particularly in the area of solar energy conversion, and / or production of oxygen and / or hydrogen. Compositions and methods for forming photoanodes and other devices are also provided.

Description

RELATED APPLICATIONS[0001]This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61 / 103,905, filed Oct. 8, 2008, entitled “Catalyst Compositions and Photoanodes for Photosynthesis Replication and Other Photoelectrochemical Techniques,” by Nocera, et al., and U.S. Provisional Patent Application Ser. No. 61 / 187,995, filed Jun. 17, 2009, entitled “Catalytic Materials, Photoanodes, and Systems for Water Electrolysis and Other Electrochemical Techniques,” by Nocera, et al., each herein incorporated by reference.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT[0002]This invention was made with the support under the following government contract F32GM07782903 awarded by the National Institutes of Health and CHE-0533150 awarded by the National Science Foundation. The government has certain rights in the invention.FIELD OF THE INVENTION[0003]The present invention relates to photoanodes for electrolysis of water which can be used for energy storage....

AI Technical Summary

Benefits of technology

[0036]In all descriptions of the use of water (e.g., for the production of oxygen gas) for catalysis herein, it is to be understood that the water may be provided in a liquid and / or gaseous state. The water used may be relatively pure, but need not be, and it is one advantage of the invention that relatively impure water can be used. The water provided can contain, for example, at least one impurity (e.g., halide ions such as chloride ions). In some cases, the device may be used for desalination of water. It should be understood that while much of the application herein focuses on the catalytic formation of oxygen gas from water, this is by no means limiting, and the compositions, photoanode, methods, and / or systems described herein may be used for other catalytic purposes, as described herein.

[0037]In some embodiments, photoanodes are provided which may produce oxygen gas from water. As shown in Equation 1, water may be split to form oxygen gas, electrons, and hydrogen ions. Although it need not be, an electrode may be operated in benign conditions (e.g., neutral or near-neutral pH, ambient temperature, ambient pressure, etc.). In some cases, the electrodes described herein operate catalytically. That is, an electrode may be able to catalytically produce oxygen gas from water, but the electrode may not necessarily participate in the related chemical reactions such that it is consumed to any appreciable degree. Those of ordinary skill in the art will understand the meaning of “catalytically” in this context. An electrode may also be used for the catalytic production of other gases and / or materials.

[0038]As used herein, a photoanode is a photoactive electrode, in addition to any catalytic material adsorbed thereto. In some embodiments, the photoactive electrode may comprise a photoactive composition and a photosensitizing agent. The catalytic material may comprise metal ionic species and anionic species, wherein the metal ionic species and anionic species are associated with the photoactive electrode. The metal ionic species and anionic species may be selected such that, when exposed to an aqueous solution (e.g., an electrolyte or water source), the metal ionic species and anionic species are in dynamic equilibrium with the aqueous solution, as described herein.

[0039]In some embodiments, a photoanode of the present invention comprises a photoactive electrode and a catalytic material associated with the photoactive electrode. A “catalytic material” 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 may also be referred to as a catalyst and / or a catalyst composition. A catalytic material is not simply a bulk photoactive electrode material which provides and / or receives electrons from an electrolysis reaction, but a material which undergoes a change in chemical state of at least one ion during the catalytic process. For example, a catalytic material might involve a metal center which undergoes a change from one oxidation state to another during the catalytic process. In another example, the catalytic material might involve metal ionic species which bind to one or more oxygen atoms from water and release the oxygen atoms as dioxygen (i.e., O2). Thus, catalytic material is given its ordinary meaning in the field in connection with this invention. As will be understood from other descriptions herein, a catalytic material of the invention that may be consumed in slight quantities during some uses and may be, in many embodiments, regenerated to its original chemical state.

[0040]“Electrolysis,” as used herein, refers to the use of an electric current to drive an otherwise non-spontaneous chemical reaction. For example, in some cases, electrolysis may involve a change in redox state of at least one species and / or formation and / or breaking of at least one chemical bond, by the application of an electric current. Electrolysis of water, as provided by the invention, can involve splitting water into oxygen gas and hydrogen gas, or oxygen gas and another hydrogen-containing species, or hydrogen gas and another oxygen-containing species, or a combination.

[0041]In some embodiments, methods are provided for forming a photoanode comprising a photoactive electrode (e.g., an n-type semiconductor photoactive material), metal ionic species, and anionic species. The photoanode may be formed by exposing a photoactive electrode to a solution comprising metal ionic species and anionic species, followed by application of a voltage to the photoactive electrode. The term “application of a voltage,” as used herein, in some embodiments, is synonymous with the term formation of a photovoltage (e.g., formation of electron / hole pairs in a material by exposing the material to electromagnetic radiation). For example, the voltage may be applied to a photoactive electrode by an external power source (e.g., a battery) or by exposing a photoactive electrode to electromagnetic radiation (e.g., sunlight, to produce a photovoltage), as described herein. The metal ionic species and anionic species may associate with the photoactive electrode and form a composition (e.g., a catalytic material) associated with the photoactive electrode. In some cases, when associating with the photoactive electrode, the metal ionic species may be oxidized or reduced as compared to the metal ionic species in solution, as described herein.

Problems solved by technology

Voltage in addition to Eo that is required to attain a given catalytic activity, referred to as overpotential, limits the energy conversion efficiency.

It may be considered that oxygen gas production from water at low overpotential and under benign conditions using catalytic materials composed of earth-abundant materials presents the greatest challenge to water electrolysis.

While photoelectrochemical devices and photoanodes exist for the electrolysis of water, these devices are generally composed of expensive materials and / or operate with low energy conversion efficiencies.

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