COMPOSITE PHOTOANODES
The provided method includes photoelectrodeposition of an electrocatalyst onto a semiconductor to form a photoanode. The method yields composite photoanodes showing enhancement of photocurrent (water splitting rate) when incorporated into a photoelectrochemical cell for water electrolysis.
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This application is a continuation of International Application No. PCT/US2011/027603, filed Mar. 8, 2011, which claims the benefit of U.S. Provisional Application No. 61/311,724, filed Mar. 8, 2010; the disclosure of each application is incorporated herein by reference in its entirety.
STATEMENT OF GOVERNMENT LICENSE RIGHTSThis invention was made with Government support under the Integrative Graduate Education and Research Traineeship (IGERT) awarded by the National Science Foundation (Award No. DGE-050-4573). The Government has certain rights in the invention.
BACKGROUNDThe photoelectrochemical (PEC) conversion of photon power into chemical fuels offers an attractive approach to storing solar energy, but it poses many fundamental chemical challenges. Hematite (α-Fe2O3) has emerged as a prototype photoanode material for testing strategies to overcome the challenging 4-electron oxidation of water, which under basic conditions is described by Equation 1:
4OH−→O2+4e−+2H2O (1)
Hematite meets many of the target photoanode requirements: It is inexpensive, oxidatively robust, environmentally benign, and it absorbs visible light (Eg˜2.1 eV). Although the α-Fe2O3 valence band edge potential is about 1 V or higher more positive than required for Equation 1 thermodynamically, water oxidation by photogenerated valence-band holes in α-Fe2O3 is kinetically inefficient, and additional anodic overpotentials are typically required before significant PEC water splitting is observed. A remaining fundamental limitation of α-Fe2O3 is that its conduction band edge potential resides ˜200 mV below that required to drive the cathodic half reaction (Equation 2).
2H2O+2e−→H2+2OH− (2)
Tandem PEC/photovoltaic (PV) configurations have been envisioned to provide the bias needed to meet these demands. Recent advances in controlled growth and doping of α-Fe2O3 nanostructures attempt to overcome many of the limitations associated with the short hole-diffusion length (˜2-4 nm), low electron mobility (˜10−1 cm2 V−1 s−1), and efficient charge carrier recombination characteristics of bulk α-Fe2O3 yielding promising PEC performance. For example, an overall solar-to-hydrogen power conversion efficiency of ˜2.1% has been estimated for one set of mesostructured α-Fe2O3 photoanodes when powered by a PV device providing 1.4 V in a tandem configuration. Unfortunately, many low-cost PV devices such as dye-sensitized solar cells or organic PVs typically provide about 1 V or lower, and two such PVs in series would thus be required to provide the necessary 1.4 V. The development of α-Fe2O3 photoanodes that require smaller overpotentials to oxidize water, such that they could be powered by single low-cost PV cells, would thus be attractive for reducing solar hydrogen production costs.
Recently, electrochemical water oxidation with low overpotentials was demonstrated over a range of pH values using an amorphous cobalt/phosphate catalyst (“Co-Pi”) electrodeposited onto ITO or FTO electrodes. Remaining uncertainties about the catalyst's precise microscopic identity do not diminish its attractiveness for water-splitting PECs. Co-Pi requires 0.41 V overpotential at pH 7 to oxidize water with a current density of 1 mA/cm2, whereas the α-Fe2O3 valence band edge potential provides about 1.2 V or higher. Photogenerated holes in α-Fe2O3 should thus be amply capable of driving water oxidation by this electrocatalyst.
What is desired, therefore, are new methods and materials for forming improved photoelectrodes for use in PECs.
SUMMARYThis summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
Composite photoanodes and methods for making the composite photoanodes are provided. The composite photoanodes comprise a semiconductor and an electrocatalyst.
In one aspect, a method of forming a composite photoanode by photoelectrodeposition is provided. In one embodiment, the method comprises photoelectrodepositing a solid conformal layer of an electrocatalyst from an electrolyte solution on a surface of a semiconductor submerged in the electrolyte solution by simultaneously:
-
- (1) impinging the surface of the semiconductor with electromagnetic radiation having a first wavelength and a first irradiance, to provide a first photoenergy that is sufficient to excite an electronic transition of the semiconductor; and
- (2) applying a first electric bias to the semiconductor, wherein the first electric bias is less than an electrochemical deposition bias, said electrochemical deposition bias being the minimum voltage required to electrodeposit the electrocatalyst onto the surface of the semiconductor without impinging the surface of the semiconductor with electromagnetic radiation having the first photoenergy.
In another aspect, a method for making an electrode is provided. In one embodiment, the electrode is formed by deposition of a competent electrocatalyst onto a photoanode from an electrolyte, wherein the deposition can be carried out by photodeposition, electrochemical deposition or a combination thereof. The electrolyte may comprise inorganic and organic ions, such as phosphate anion, acetate anion, sulfate anion, chloride, nitrate, sodium, potassium, or any combination thereof.
In another aspect, an electrode is provided comprising:
a photoanode having a first onset potential when incorporated into a photoelectrochemical cell for electrolysis of water into oxygen; and
a layer of an electrocatalyst conformally formed on a surface of the photoanode, wherein said electrocatalyst causes a cathodic shift in an onset potential of the electrode such that the electrode has a second onset potential when incorporated into a photoelectrochemical cell for electrolysis of water into oxygen, and wherein said second onset potential is less than said first onset potential.
The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:
The composite photoanodes provide enhanced performance compared to known photoanodes when incorporated into photoelectrochemical (PEC) systems (e.g., a PEC cell for splitting water into hydrogen). One particular benefit of the composite photoanodes is a cathodic shift in the onset potential of the photoanode when used in a PEC system. Such a cathodic shift allows for reduced electrical requirements to drive the PEC process, thereby increasing the efficiency of such systems.
The methods for forming composite photoanodes are light-enhanced deposition methods, referred to herein as photoelectrochemical deposition methods. Photoelectrochemical deposition of an electrocatalyst onto a semiconductor to form a composite photoanode provides enhanced photoanode performance in PEC systems, including increased cathodic shift, compared to composite photoanodes fabricated using traditional electrochemical methods. Such improvements are attributed to the conformal nature of electrocatalyst layers formed using photoelectrodeposition.
In one aspect, a composite photoanode is provided, said composite photoanode comprising a semiconductor having a solid conformal layer of an electrocatalyst formed on its surface. Herein, the electrocatalysts referred to are competent electrocatalyst that produce a cathodic shift in the onset potential of the photoanode when used within a PEC.
The semiconductor acts as a photoanode. The semiconductor is made of a photon-absorbing material. In one embodiment, the semiconductor is α-Fe2O3 (“hematite”). In another embodiment, the α-Fe2O3 semiconductor is mesostructured. In another embodiment, the semiconductor is a high-surface-area α-Fe2O3 photoanode. The use of hematite as a semiconductor is disclosed extensively herein, including in Examples 1-4 and 10.
While hematite is a preferred semiconductor, other semiconductors are contemplated. For example, Examples 5 and 6 disclose the use of titanium dioxide (particularly in nanowire form) as a semiconductor; Example 7 discloses the use of a cobalt-ion:zinc oxide semiconductor; and Example 8 discloses the use of a W:BiVO4 semiconductor. These listed examples are generally inorganic in character.
In one embodiment, the semiconductor comprises a group IV semiconductor having a formula selected from the group consisting of binary, ternary, and quaternary. In a further embodiment, the group IV semiconductor further comprises ions selected from the group consisting of cations and anions.
In one embodiment, the semiconductor is an n-type semiconductor.
Other photoanode materials useful in composite electrodes include a material selected from the group consisting of an iron oxide, a zinc oxide, a titanium oxide, a tungsten-bismuth-vanadium oxide, a tungsten oxide, a gallium-zinc-oxide-nitride, or these materials also containing additional cations or anions.
In another embodiment, the semiconductor comprises a sensitizer having a sensitizer absorbance wavelength, said sensitizer absorbance being different from a semiconductor absorbance wavelength. By incorporating a sensitizer with the semiconductor (e.g., embedded within, or coating the surface of the semiconductor), a broader spectrum of light can be usefully absorbed by the semiconductor and overall composite photoanode for use in the PEC process. Representative sensitizers include cadmium selenium, as described below in Example 6. Additional sensitizers include cationic or anionic impurities
In one embodiment, the semiconductor has a physical shape selected from the group consisting of dendrites, wires, belts, rods, mesostructures, nanotubes, and thin films. As described further in Example 2, a high semiconductor surface area and/or a high composite photoanode surface area produces improved results for PEC reactions. Certain physical shapes, such as dendrites, etc. are known to create a relatively high surface area. Such physical shapes are preferred in the embodiments provided herein. Nanoscopic high-surface area shapes are accordingly preferred. Therefore, in another embodiment, the physical shape has nanoscopic dimensions. As used herein, the term nanoscopic dimensions refers to a shape having at least one feature (e.g., dendrites) having a smallest size of 100 nm or smaller.
Semiconductors can be deposited on substrates, or otherwise formed, according to methods known to those of skill in the art, including those provided below in the Examples. For example, hematite can be grown using chemical vapor deposition (see Example 1).
The electrocatalyst is formed on a surface (e.g., a surface that will face a light source during PEC) of the semiconductor. In certain embodiments, the electrocatalyst produces a cathodic shift in the onset potential of a PEC process incorporating a composite electrode (semiconductor and electrocatalyst) when compared to the semiconductor alone. This comparison of electrodes can be found throughout the data provided herein so as to illustrate the efficacy of the disclosed materials, devices, and methods, in improving the PEC performance of semiconductors.
In one embodiment, the electrocatalyst is selected from the group consisting of a cobalt-containing catalyst, an iridium-containing catalyst, a manganese-containing catalyst, a ruthenium-containing catalyst, a nickel-containing catalyst (e.g., nickel borate, see Example 10), a cobalt-containing oxygen evolving catalyst, a cobalt oxide/hydroxide catalyst, and a cobalt oxide catalyst.
In one embodiment, the electrocatalyst is cobalt phosphate (Co-Pi). Co-Pi is used extensively in the examples provided herein. Examples 1-3 and 5-9 describe the use of Co-Pi to improve PEC performance.
In one embodiment, the layer of the electrocatalyst has a thickness of from 0.5 nm to 30 nm. As will be described further below in Example 2, a thick electrocatalyst layer will inhibit performance of composite photoanodes. Accordingly, nanoscale-thick electrocatalyst films are preferred. Electrochemical deposition does not allow for quality films of such a thickness to be deposited. Accordingly, photoelectrodeposition is preferred for forming nanoscale-thick films of electrocatalyst.
Examples of the competent electrocatalyst include, without limitation, a cobalt catalyst, iridium catalyst (e.g. IrO2), manganese catalyst (e.g. Mn-oxo complexes), ruthenium catalyst (e.g. [Ru(L)2(OH)2]2+ complexes, where L denotes ligand). In one embodiment, the cobalt catalyst was selected from the group consisting of cobalt based oxygen evolving catalyst and cobalt oxide catalyst (referred herein as “CoOx”, see Example 4).
The photoelectrochemical performance of composite the photoanodes provided herein is improved compared to “semiconductor-only” photoanodes. The EXAMPLES describe these improvements extensively.
For example, Co-Pi/α-Fe2O3 composite photoanodes for water oxidation are improved by optimization for front-side illumination in pH 8 electrolytes. Without being limited by theory, it is believed that a kinetic bottleneck appears to be related to the Co-Pi catalyst itself under these conditions. This kinetic bottleneck is overcome by more sparse deposition of Co-Pi onto α-Fe2O3. Following these improvements, sustained water oxidation by Co-Pi/α-Fe2O3 composite photoanodes was demonstrated in both photocurrent and O2 evolution measurements. Photoelectrochemical water oxidation by the Co-Pi/α-Fe2O3 composite photoanodes was enhanced relative to that of α-Fe2O3 alone: Under these conditions, a five-fold enhancement in the photocurrent density and water oxidation rate was observed at +1.0 V vs RHE. This enhancement is even more substantial at about 1.0 V or lower vs RHE, where α-Fe2O3 alone does not exhibit significant photocurrent at all.
It is also interesting to compare these results with those obtained for bulk electrolysis by Co-Pi without a photon-absorbing substrate. By itself, Co-Pi electrolysis current densities reached ˜1.2 mA/cm2 at an applied bias of +1.29 V vs NHE (pH 7), or ˜+1.7 V vs RHE. In conjunction with an inexpensive and robust photoanode such as α-Fe2O3 under 1 sun, AM 1.5 illumination, the applied bias necessary to achieve the same current density can be reduced by over 0.5 V in buffered salt water at pH 8, the average pH of sea water. The results described here thus demonstrate that sustained O2 evolution in mild salt water conditions can be achieved with significantly reduced external power demands relative to Co-Pi alone, particularly in the low current density regime, by integrating this catalyst with a light-harvesting semiconductor substrate. The overall process, in which photogenerated holes in α-Fe2O3 are converted to oxidizing equivalents in Co-Pi, yielding O2 evolution well below the Co-Pi bulk electrolysis threshold potential, is summarized schematically in
The combination of the first photoenergy and the first electric bias are sufficient to deposit catalyst components from the electrolyte to form the solid conformal layer of the electrocatalyst.
The electrocatalyst is formed from a buffer solution in which the semiconductor is submerged. The electrolyte solution can be any electrolyte solution known to those of skill in the art. Particularly, electrolyte solutions useful for traditional electrochemical deposition of an electrocatalyst onto a semiconductor are useful in the method. In a preferred embodiment, the electrolyte solution is a buffer solution of potassium phosphate (e.g., at pH 7) containing Co(NO3)2 if Co-Pi is to be the formed electrocatalyst.
In one embodiment, the pH of the electrolyte is about 7 or higher. In one embodiment, the pH of the electrolyte was about 13 or higher. In another embodiment, the pH of the electrolyte was about 8.
The surface of the semiconductor is irradiated with electromagnetic radiation (“light”) having a first wavelength and a first irradiance. The light can be a single wavelength or a broadband source. The only requirement is that the light provides a photoenergy sufficient to produce an electronic excited state in the semiconductor so as to provide a portion of the energy required to deposit the electrocatalyst from the electrolyte solution. In one embodiment, the electronic transition is a bandgap transition. By exciting the bandgap transition, photogenerated valence band holes can oxidize ions in the electrolyte to from an active catalyst at the surface.
Because the first photoenergy is not sufficient to drive the deposition of the electrocatalyst from the electrolyte solution, a first electric bias is simultaneously applied to the semiconductor. The first electric bias is significantly less than the bias required for electrochemical deposition. In one embodiment, the first electric bias is from 0.1 V to 0.4 V (e.g., versus Ag/AgCl). Therefore, the deposition (i.e., the photoelectrochemical deposition) of the electrocatalyst on the semiconductor is accomplished in the method by using energy from two sources (light and electricity) to facilitate the deposition reaction from the electrolyte. Neither of the two energy sources alone is sufficient to facilitate the deposition on their own.
The method according to this aspect utilizes light (e.g., sunlight or artificial sunlight) to assist in electrochemical deposition of an electrocatalyst onto a semiconductor. The method is useful, for example, to fabricate a composite photoanode according to the other aspects and Examples provided herein.
Examples 1 and 2 below disclose composite photoanodes fabricated with the generally known technique of electrochemical deposition. These examples are contrasted, by further Examples 3-10 utilize photoelectrochemical deposition.
Example 3, below, provides an in-depth development of the theory and results of photoelectrodeposition. While Example 3 primarily describes composite photoanodes of Co-Pi and α-Fe2O3, the method is not limited to these compounds. As illustrated in other Examples, photoelectrochemical deposition is compatible with any known semiconductors and electrocatalysts, particularly those used to make photoanodes using electrochemical deposition.
Without being bound by theory, in principle, photogenerated holes can be used to oxidize an ion from an electrolyte. For example, with reference to Co-Pi deposition on hematite, Co2+ can be deposited to form Co-Pi on the α-Fe2O3 photoanode and the electron can be removed by water reduction. Because photogenerated electrons in the conduction band of α-Fe2O3 are below the energy needed to reduce protons to hydrogen, a very low bias is applied to assist in photoelectrochemical deposition. Hence the addition of “electro” to photoelectrochemical deposition. The bias required for photoelectrochemical deposition is lower than that required for electrochemical deposition of similar compounds (i.e., deposition without the assistance of light).
Any light source with sufficient energy to excite the band gap of the semiconductor can be used in photoelectrodeposition. For example, sunlight (or artificial sunlight) can be used to drive photoelectrochemical deposition so as to test the possibility of a sunlight-driven reaction. In an exemplary embodiment (described further in Example 3) photoelectrodeposition on α-Fe2O3 was conducted in a three-electrode configuration from a solution of Co2+ in potassium phosphate (KPi) buffer under 1 sun AM 1.5 simulated solar irradiation. A Pt mesh was used as the counter electrode and saturated Ag/AgCl was used as the reference electrode. Typical current densities during deposition were ˜1-100 μA/cm2.
It will be appreciated that a broad-spectrum light source (e.g., sunlight) need not be used in the method, as any light source capable of exciting the bandgap of the semiconductor is compatible with the method. For example, a single wavelength light source can be sufficient to excite the bandgap as long as
One impetus for the development of photoelectrochemical deposition was to develop an electrocatalyst deposition method that would allow for nanoscale-thick, conformal, continuous layers (films) of electrocatalyst to be deposited on a semiconductor. Particularly if the semiconductor is nanostructured (e.g., dendritic). Traditional electrochemical deposition is insufficient in this regard. As demonstrated in the Examples (e.g., Example 3), thin, conformal electrocatalyst films are satisfactorily formed using photoelectrochemical deposition.
PEC reactions driven with photoanodes formed using photoelectrochemical deposition demonstrate improved absolute onset potential, cathodic shift of the onset potential, and maximum current density. All vital characteristics of, for example, PEC for water splitting, particularly in the context of solar-powered PEC devices.
In one embodiment, the electrocatalyst is selected from the group consisting of a cobalt-containing catalyst, an iridium-containing catalyst, a manganese-containing catalyst, a ruthenium-containing catalyst, a nickel-containing catalyst, a cobalt-containing oxygen evolving catalyst, a cobalt oxide/hydroxide catalyst, and a cobalt oxide catalyst.
In a preferred embodiment, the electrocatalyst is cobalt phosphate.
In one embodiment, the layer of the electrocatalyst has a thickness of from 0.5 nm to 30 nm.
In one embodiment, the cathodic shift is from 50 mV to 400 mV.
In one embodiment, the layer of the electrocatalyst has a thickness of from 0.5 nm to 30 nm. Such a thickness is indicative of the “thin” nature of the conformal electrocatalyst coating. As set forth herein, such a thin coating is essential, not only to allow light through the electrocatalyst to the semiconductor, but also due to the short charge diffusion lengths in the photoanode materials. Finally, as disclosed herein, thin films of electrocatalyst are less prone to defects (e.g., aggregates) than thicker electrocatalyst films are.
It will be appreciated that the range of 0.5 nm to 30 nm represents from about one molecular layer to about tens of molecular layers. Accordingly, it is preferred that a minimal number of molecular layers are used to conformally coat the semiconductor with electrocatalyst without pinhole defects (exposing the semiconductor) or aggregates (which diminish device performance).
In one embodiment, the electrocatalyst is deposited from an electrolyte by photodeposition, electrochemical deposition, or combination thereof.
In one embodiment, the electrode is formed by electrodepositing a conformal layer of an electrocatalyst from an electrolyte solution onto a surface of a photoanode.
In one embodiment, the semiconductor is an n-type semiconductor.
In one embodiment, the formed photoanode, when incorporated into a photoelectrochemical cell for electrolysis of water into oxygen, reduces a water electrolysis onset voltage compared to a second photoanode comprising the semiconductor without the electrocatalyst.
In one embodiment, the water electrolysis onset voltage is reduced by 50 mV to 400 mV.
In one embodiment, the first wavelength of the electromagnetic radiation is from 300 nm to 800 nm.
In one embodiment, the first irradiance of the electromagnetic radiation is from 0.1 W/m2 to 1100 W/m2, or the equivalent in pulsed irradiation.
In one embodiment, the electromagnetic radiation is selected from the group consisting of continuous radiation and pulsed radiation.
In one embodiment, the first electric bias is applied to the semiconductor as part of an electrochemical deposition system comprising a power source in electrical communication with the semiconductor and a counter electrode.
In one embodiment, the electrolyte solution comprises cations selected from the group consisting of cobalt, iridium, manganese, nickel, and ruthenium.
In one embodiment, the electrolyte solution comprises anions selected from the group consisting of phosphate, methyl phosphonate, borate, acetate, sulfate, and hydroxide.
In one embodiment, the semiconductor comprises a sensitizer having a sensitizer absorbance wavelength, said sensitizer absorbance being different from a semiconductor absorbance wavelength.
In one embodiment, the semiconductor comprises a group IV semiconductor having a formula selected from the group consisting of binary, ternary, and quaternary. In a further embodiment, the group IV semiconductor further comprises ions selected from the group consisting of cations and anions.
In one embodiment, the semiconductor comprises a material selected from the group consisting of an iron oxide, a zinc oxide, a titanium oxide, a tungsten-bismuth-vanadium oxide, a tungsten oxide, a gallium-zinc-oxide-nitride, or these materials also containing additional cations or anions.
In one embodiment, the combination of the first photoenergy and the first electric bias are sufficient to oxidize cations to deposit catalyst components from the electrolyte to form the solid conformal layer of the electrocatalyst.
In another embodiment, an electrode was made by deposition of cobalt catalyst onto mesostructured α-Fe2O3 from an electrolyte of Co2+. The deposition can be carried out by photodeposition or electrochemical deposition. Examples of the electrolyte include, without limitation, cobalt phosphate, cobalt borate, cobalt methyl phosphonate, cobalt nitrate, cobalt acetate, cobalt sulfate, and any combination thereof.
In one embodiment, the pH of the electrolyte is about 7 or higher. In one embodiment, the pH of the electrolyte was about 13 or higher. In another embodiment, the pH of the electrolyte was about 8.
In another embodiment, an electrode was made by electrochemical deposition of cobalt/phosphate catalyst (“Co-Pi”) onto mesostructured α-Fe2O3 and showed about 350 mV or higher cathodic shift of the onset potential for PEC water oxidation while retaining substantial photocurrent densities.
In another embodiment, Co-Pi was electrodeposited onto a mesostructured α-Fe2O3 photoanode. The photoelectrochemical properties of the resulting composite photoanodes were optimized for solar water oxidation under front-side illumination in pH 8 electrolytes. Relative to α-Fe2O3 photoanodes, more sparse deposition of Co-Pi onto the α-Fe2O3 resulted in a sustained five-fold enhancement in the photocurrent density and O2 evolution rate at +1.0 V vs RHE.
In one embodiment, the photoanode comprises a photoanode material selected from the group consisting of an iron oxide, a zinc oxide, a titanium oxide, a bismuth vanadium oxide.
In one embodiment, the photoanode material has a physical shape selected from the group consisting of dendrites, wires, and belts. In another embodiment, said physical shape has nanoscopic dimensions. As used herein, the term nanoscopic dimensions refers to a shape having at least one feature (e.g., dendrites) having a smallest size of 100 nm or smaller.
In one preferred embodiment, the photoanode comprises hematite iron oxide dendrites. In a further preferred embodiment, the photoanode consists of hematite iron oxide dendrites conformally covered with a layer of cobalt phosphate.
In one embodiment, the electrocatalyst is selected from the group consisting of a cobalt-containing catalyst, an iridium-containing catalyst, a manganese-containing catalyst, a ruthenium-containing catalyst, a cobalt-containing oxygen evolving catalyst and a cobalt oxide catalyst.
In one preferred embodiment, the electrocatalyst is cobalt phosphate.
In one embodiment, the cathodic shift is from 50 mV to 400 mV.
In one embodiment, the layer of the electrocatalyst has a thickness of from 0.5 nm to 30 nm. Such a thickness is indicative of the “thin” nature of the conformal electrocatalyst coating. As set forth herein, such a thin coating is essential, not only to allow light through the electrocatalyst to the semiconductor, but also due to exacerbated electron-hole recombination with thicker catalyst films.
It will be appreciated that the range of 0.5 nm to 30 nm represents from about one molecular layer to about tens of molecular layers. Accordingly, it is preferred that a minimal number of molecular layers are used to conformally coat the semiconductor with electrocatalyst without pinhole defects (exposing the semiconductor) or aggregates (which diminish device performance).
In one embodiment, the electrocatalyst is deposited from an electrolyte by photodeposition, electrochemical deposition, or combination thereof.
In one embodiment, the electrode is formed by electrodepositing a conformal layer of an electrocatalyst from an electrolyte solution onto a surface of a photoanode.
In one embodiment, the formed photoanode, when incorporated into a photoelectrochemical cell for electrolysis of water into oxygen, reduces a water electrolysis onset voltage compared to a second photoanode comprising the semiconductor without the electrocatalyst.
In another aspect, an electrode is provided, comprising:
a photoanode; and
a competent electrocatalyst that causes a cathodic shift in the onset potential of the electrode.
In another aspect, an electrode is provided, comprising:
a α-Fe2O3 photoanode;
a competent electrocatalyst selected from the group consisting of cobalt catalyst, iridium catalyst, manganese catalyst, ruthenium catalyst, cobalt based oxygen evolving catalyst and cobalt oxide catalyst.
In another aspect, an electrode is provided, comprising:
a α-Fe2O3 photoanode;
a competent electrocatalyst comprising a cobalt catalyst deposited onto the α-Fe2O3 photoanode from an electrolyte of Co2+ by photodeposition, electrochemical deposition, or combination thereof,
wherein the electrolyte comprises a composition selected from the group consisting of cobalt phosphate, cobalt nitrate, cobalt acetate, cobalt sulfate, and any combination thereof; and the electrode having about a several hundred millivolt cathodic shift of the onset potential for PEC water oxidation.
A system/device for converting water to hydrogen using only sunlight as an energy source is provided. The system includes a PEC comprising a photoanode formed using photoelectrochemical deposition and a photovoltaic cell. As described elsewhere herein, a water-splitting PEC typically requires over 1 V to produce hydrogen and oxygen from water, which is an electrical requirement that cannot be met by present PV technology. However, using the photoelectrochemical deposition method provided herein, the cathodic shift achieved in improving present photoanodes for PEC (e.g., Co-Pi/hematite), makes efficient sub-1 V water splitting in a PEC possible. Accordingly, by combining a PV cell with a PEC system having a photoanode formed using a photoelectrochemically deposited electrocatalyst on a semiconductor results in a system for converting water to hydrogen using only sunlight.
The following examples are provided to better illustrate the claimed invention and are not to be interpreted in any way as limiting the scope of the invention. All specific compositions, materials, and methods described below, in whole or in part, fall within the scope of the invention. These specific compositions, materials, and methods are not intended to limit the invention, but merely to illustrate specific embodiments falling within the scope of the invention. One skilled in the art may develop equivalent compositions, materials, and methods without the exercise of inventive capacity and without departing from the scope of the invention. It will be understood that many variations can be made in the procedures herein described while still remaining within the bounds of the invention. It is the intention of the inventors that such variations are included within the scope of the invention.
EXAMPLES Example 1 α-Fe2O3 Photoanode FabricationMesostructured Si-doped α-Fe2O3 photoanodes of 400-500 nm thickness were grown by atmospheric pressure chemical vapor deposition (APCVD) using Fe(CO)5 and tetraethoxysilane (TEOS) as precursors, delivered to an FTO substrate at 470° C. using Ar carrier gas. SEM images of a representative α-Fe2O3 photoanode are shown in
Mesostructured Si-doped α-Fe2O3 photoanodes were grown on F:SnO2 (FTO)-coated glass substrates (TEC15, 15 Ω/cm2 Hartford Glass Co.) by atmospheric pressure chemical vapor deposition (APCVD) following procedures known in the art. The precursors, Fe(CO)5 (Aldrich 99.999%) and TEOS (Aldrich 99.999%), were delivered by bubbling Ar gas (Praxair, 5.0 Ultra High Purity) at 11.3 and 19.4 mL/min, respectively, controlled by mass flow controllers. The gas was then mixed with air flowing at 2 L/min and directed by a glass tube onto the lower portion of a 50×13×2.3 mm3 FTO substrate kept at 470° C. The Co-Pi catalyst was electrodeposited onto the oxide anodes as known in the art. The anode was submerged in a buffer solution of 0.1 M potassium phosphate (pH 7) containing 0.5 mM Co(NO3)2 and a bias of 1.29 V (vs. NHE) was applied for 1 hr. For PEC measurements, the anode surface was masked during electrochemical deposition to yield a catalyst-covered area that matched the irradiated area (Ø=6 mm). Masking was achieved using electrical tape, which was then removed for PEC measurements. Composite Co-Pi/α-Fe2O3 anodes for which the mask was not used showed greater dark currents from the Co-Pi catalyst, but were otherwise very similar.
Electronic absorption spectra were measured using a Cary 500 UV/vis/NIR spectrophotometer (Varian). SEM images were collected using a FEI Sirion scanning electron microscope operating at 5 kV. Electrochemical measurements were performed in a 3-electrode configuration using an aqueous hydroxide electrolyte (1 M NaOH, pH 13.6), a Pt counter electrode, and an Ag/AgCl reference electrode. In a typical measurement, a titanium clasp was used to make contact with the upper 25% of the 5 cm long anode, where no α-Fe2O3 had been deposited. The bottom ˜50% of the anode was submerged in the electrolyte solution in a home-built optical cell. Cyclic voltammetry measurements were performed using a computer-controlled Eco Chemie μAutolab II potentiostat. Potentials are reported vs both Ag/AgCl and RHE, the latter obtained using the formula ERHE=EAgCl+0.059 pH+0.1976V. Photocurrent densities were measured as a function of applied voltage under simulated AM1.5 solar irradiation (1 sun), achieved using an Oriel 96000 solar simulator integrating a 150 W Xe arc lamp and Oriel 81094 filter, and delivered to the anode via fiber optic. Measurements were performed at a scan rate of 50 mV/s. IPCE measurements were performed using a Xe arc lamp with an Oriel Cornerstone 74000 monochromator with slits set to ˜10 nm spectral bandwidth at the designated bias voltage provided by the potentiostat. The wavelength was scanned at 1 nm/s. Photon power densities were determined using a calibrated Si photodiode. Dark current measurements probe the entire submerged FTO+α-Fe2O3 (or Co-Pi/α-Fe2O3) surface, whereas photocurrents represent the response achieved from just the irradiated area normalized to 1 cm2. This area was circular with a diameter of 6 mm. Typical monochromatic photon power densities in the IPCE measurements were ˜0.50 W/m2. For the data shown in
At 1.4 V (RHE), α-Fe2O3 photocurrent densities with front-side illumination were approximately 2× greater than with back-side illumination (
To test for the possibility that the cathodic photocurrent shift came from the action of solvated cobalt as a redox mediator, a set of control experiments involving deliberate addition of solvated Co2+ was performed by adding Co(OH)42− to the 1 M NaOH electrolyte of the PEC cell. Co(OH)42− was prepared by dissolving cobalt nitrate in a 50 wt % concentrated NaOH aqueous solution to make a ˜0.005M Co(OH)42− solution, which was then added to distilled water to reach pH ˜13. The final solution was added dropwise to the electrolyte of the PEC cell under operating conditions, where its influence on dark and photocurrent densities of various photoanodes could be monitored. Although sparingly soluble at pH 13.6, the precipitation of solid Co(OH)2 from the electrolyte solution was likely slow, as indicated by observation of the characteristic Co(OH)42-4T1(P) ligand field band in the absorption spectrum of the pH ˜13 stock solution even ˜30 min after preparation (
As the control experiments showed, the possibility of dissolved cobalt acting as redox mediator, or of an unidentified sacrificial reagent contributing to photocurrent, was eliminated by the following observations: (i) Addition of solvated Co2+ to the electrolyte had no noticeable effect on photocurrent densities; (ii) Replacement of the PEC electrolyte solution with new stock solution caused no change in photocurrent and did not lead to a photocurrent induction period; (iii) Continuous photocatalysis at 1 V vs RHE for about 10 hrs or longer showed no change in performance. Therefore, the cathodic shift in
Most α-Fe2O3 PEC cells operating under similar conditions show negligible photocurrent densities below 1 V vs RHE. Modification of the α-Fe2O3 surface by adsorption of Co2+ from aqueous 10 mM Co(NO3)2 was previously shown to cause an ˜17% increase in current density at 1.23 V vs RHE and an 80 mV cathodic shift of the onset potential. Similarly, growth of RuO2 onto α-Fe2O3 surfaces led to a 120 mV cathodic shift of the onset potential with about 80 μA/cm2 or lower at 1 V vs RHE. Interestingly, α-Fe2O3 nanorods have shown greater relative photocurrent densities at low bias than typical mesostructured α-Fe2O3 photoanodes, but with photocurrent densities of ˜2 μA/cm2. Without being bound to any theory, it is possible that the conformal catalyst deposition facilitates interfacial hole transfer from α-Fe2O3 to Co-Pi, allowing photon absorption and redox catalysis to be effectively decoupled while retaining photocurrent densities. Efficient hole transfer from α-Fe2O3 to Co-Pi should enhance the electron gradient in the α-Fe2O3 mesostructure under irradiation, also contributing to the driving force for electron diffusion to the FTO and reducing deleterious carrier recombination processes. Catalyst electrochemical deposition onto α-Fe2O3 may also passivate surface defects.
The experimental results for the Co-Pi/α-Fe2O3 composite photoanodes may be summarized in
Si doped α-Fe2O3 photoanodes were fabricated on fluorine doped tin oxide (FTO) glass (50×13×2.3 mm TEC 15 Hartford Glass Co.) at 470° C. for 5 min by atmospheric pressure chemical vapor deposition (APCVD) following procedures known in the art. The α-Fe2O3 films investigated here were typically ˜400-500 nm thick.
For Co-Pi deposition onto α-Fe2O3 photoanodes for the following data, electrical tape with an aperture that matched the irradiated area during photoelectrochemical (PEC) experiments (Ø=6 mm diameter) was applied onto the α-Fe2O3. As the working electrode, α-Fe2O3 was submerged into a solution of 0.5 mM cobalt nitrate in 0.1 M pH 7 potassium phosphate (KPi) buffer. A Pt mesh was used as the counter electrode and saturated Ag/AgCl was used as the reference electrode. Co-Pi was electrodeposited at +1.1 V vs Ag/AgCl for 15 (
Photoelectrochemical experiments. Current-voltage characteristics were measured using an Eco Chemie μ-Autolab II potentiostat in a home-built three-electrode optical cell using Ag/AgCl as the reference electrode and a Pt wire as the counter electrode. Contact to the photoanodes was made by a titanium clasp attached to the exposed FTO surface at the top of the anode, while the lower portion containing the sample was submerged in the electrolyte. Measurements were performed in 1 M NaOH(aq) at pH 13.6, 0.1 M KPi buffered at pH 8, and 0.1 M NaCl(aq) buffered at pH 8 with 0.1 M KPi. Potentials are reported vs Ag/AgCl (measured) or RHE (obtained using the relationship ERHE=EAg/AgCl+0.0591*pH+0.1976 V). Photocurrent densities were measured under 1 sun, AM 1.5 simulated sunlight using an Oriel 96000 solar simulator equipped with a 150 W Xe arc lamp and an Oriel 81094 filter. The photoanodes were masked to illuminate a circular area of 6 mm diameter. Power dependence measurements were performed using an Ag variable neutral density filter, Thorlabs NDC-50C-2M. Unless otherwise stated, all films in this example were illuminated from the front side of the photoanode. Unless otherwise specified, all experiments in this example were performed at room temperature in air atmosphere.
Oxygen detection. The detection of O2 was performed using a YSI 5000 dissolved oxygen meter equipped with a YSI 5010 self-stirring Clark-type probe in a three-neck flask with an optical window. Before use, the electrolyte (0.1 M KPi buffered at pH 8) was degassed and purged with argon gas. Measurements were conducted in argon in the same three-electrode configuration described for PEC experiments using the same light source. Again, the photoanodes were masked to illuminate a circular area of 6 mm in diameter. Consecutive measurements were taken at +1.0, 1.1, and 1.23 V vs RHE for two hours at each potential. While the light was off between voltages (˜160 seconds), there was no increase and sometimes even a decrease in the O2 level due to consumption by the Clark electrode.
Co-Pi/α-Fe2O3 photoanode performed under front-side illumination and mild pH conditions.
Optimization of the composite photoanodes for front-side illumination was carried out at mild pH conditions. To reduce photon absorption by the catalyst, Co-Pi deposition times were decreased from the original one-hour duration.
The basic electrolyte (pH 13.6) used previously herein is generally undesirable for practical applications. A gradual decrease in photocurrent density from α-Fe2O3 anodes alone was observed under continuous illumination in 0.1 M KPi electrolyte at pH 7 and +1.3 V vs RHE (
Kinetic bottleneck in Co-Pi/α-Fe2O3 composite photoanodes. In the course of efforts to optimize the Co-Pi/α-Fe2O3 composite photoanodes, it was recognized that improvements in efficiency were often accompanied by increasingly apparent symptoms of kinetic limitations. For example,
To detail this kinetic bottleneck, its symptoms were explored in various complementary measurements on a single Co-Pi/α-Fe2O3 composite photoanode in 0.1 M KPi electrolyte at pH 8. The resulting data are summarized in
Overall, four major symptoms of this kinetic bottleneck can be identified: (i) a scan rate dependence, (ii) a kinetic decay in the photocurrent density, (iii) photocurrent saturation upon increased illumination, and (iv) a sweep-rate-dependent maximum at the beginning of the J-V curve. Without being bound by any theory, the sweep-rate dependence of this maximum is a consequence of the superposition of an increasing current density from increasing bias with a current decay.
Parallel measurements were performed for α-Fe2O3 photoanodes alone under the same experimental conditions. Under the conditions represented in
To test the catalyst alone, Co-Pi was electrodeposited on FTO and electrochemical experiments were conducted in 0.1 M KPi electrolyte buffered to pH 7 with stirring.
Alleviating the kinetic problem in Co-Pi/α-Fe2O3. Without being bound to any theory, if there is a kinetic bottleneck in Co-Pi/α-Fe2O3 photoanodes, it is possible that even further reduction of the Co-Pi deposition time may remediate the problem. For example, thick layers of Co-Pi may inhibit rapid charge or proton transport from electrolyte through the catalyst, thus restricting current flow and allowing other non-productive recombination pathways to become competitive. To test this possibility, Co-Pi was electrodeposited on α-Fe2O3 photoanodes for 15 min.
PEC measurements were performed on these thinly covered Co-Pi/α-Fe2O3 photoanodes and the results were shown in
Decreased deposition of Co-Pi onto α-Fe2O3 largely overcame the kinetic limitations described in
Oxygen evolution. In addition to current density measurements, PEC O2 evolution by the Co-Pi/α-Fe2O3 composite photoanodes was also examined. Oxygen evolution was measured at various applied potentials before and after 15 min of Co-Pi electrochemical deposition onto an α-Fe2O3 photoanode. Measurements were performed in 0.1 M KPi electrolyte at pH 8.
Sustained photocurrent was observed for the Co-Pi/α-Fe2O3 composite photoanode over the course of this ˜6 hour experiment. This steady-state photocurrent was enhanced over that of the parent α-Fe2O3 film, even after several hours of illumination, and was accompanied by a correspondingly large enhancement in the O2 evolution rate. The photocurrent density and O2 evolution enhancement factors
respectively) measured at each applied potential were indicated in
Co-Pi catalyst was photoelectrochemical deposited onto α-Fe2O3 photoanodes by using light and an external applied bias to deposit Co-Pi. Without being bound by theory, in principle, photogenerated holes can be used to oxidize Co2+ from the electrolyte to form Co-Pi on the α-Fe2O3 photoanode and the electron can be removed by water reduction. Because photogenerated electrons in the conduction band of α-Fe2O3 are below the energy needed to reduce protons to hydrogen, a very low bias was applied to assist in photoelectrochemical deposition of Co-Pi. Any light source with sufficient energy to excite the band gap of α-Fe2O3 can be used in a photoelectrochemical deposition on α-Fe2O3. In this embodiment, photoelectrochemical deposition on α-Fe2O3 was conducted in a three-electrode configuration from a solution of Co2+ in potassium phosphate (KPi) buffer under 1 sun AM 1.5 simulated solar irradiation. A Pt mesh was used as the counter electrode and saturated Ag/AgCl was used as the reference electrode. Typical current densities during deposition were ˜1-100 μA/cm2.
In another exemplary embodiment, a photo-assisted electrochemical deposition approach (i.e., photoelectrodeposition) was used to deposit a cobalt-phosphate water oxidation catalyst (“Co-Pi”) onto dendritic mesostructures of α-Fe2O3. A comparison between this approach, electrochemical deposition of Co-Pi, and Co2+ wet impregnation showed that photo-assisted electrochemical deposition of Co-Pi yields superior α-Fe2O3 photoanodes for photoelectrochemical water oxidation. Stable photocurrent densities of 1.0 mA/cm2 at 1.0 V and 2.8 mA/cm2 at 1.23 V vs RHE measured under standard illumination and basic conditions were achieved. By allowing deposition only where visible light generates oxidizing equivalents, photo-assisted electrochemical deposition provides a more uniform distribution of Co-Pi onto α-Fe2O3 than obtained by electrochemical deposition. This approach of fabricating catalyst-modified metal-oxide photoelectrodes may be attractive for optimization in conjunction with tandem or hybrid photoelectrochemical cells.
By way of background, the maturation of photoelectrochemical (PEC) water splitting as a viable solar fuels technology has been hindered by the need to identify photoelectrode materials that are simultaneously efficient at solar energy conversion, stable under reaction conditions, and inexpensive. Whereas high solar-to-hydrogen conversion efficiencies of 12.4% have been demonstrated using semiconductor multilayer devices, these efficiencies are not sustainable even on the one-day timescale because of rapid electrode decomposition. Metal oxides have been widely studied as chemically robust alternatives, beginning with TiO2, but have been limited by various factors including low carrier mobilities, low absorption coefficients, or poor catalytic proficiencies. Hematite (α-Fe2O3) has emerged as a prototype photoanode for PEC water oxidation because of its balance of visible light absorption (bandgap of 2.1 eV), chemical stability, low cost, and large positive valence band edge potential. Low mobilities (10−2-10−1 cm2 V−1 s−1) and short hole diffusion lengths (2-4 nm or 20 nm) have generally led to low PEC water oxidation efficiencies in bulk α-Fe2O3, but doping and nanostructuring have been used to sidestep these shortcomings, by increasing carrier density, decreasing the distance minority carriers have to travel to reach the reactive surface, and increasing semiconductor-electrolyte interfaces. Doping with silicon has been suggested to increase photocurrent densities by several orders of magnitude in mesostructure α-Fe2O3 films. Nanowires and nanotubes of α-Fe2O3 have also shown increased photocurrent densities relative to bulk, although such structures have so far been limited to absolute one-sun current densities on the order of μA/cm2.
Interfacing such mesostructured metal-oxide photoanodes with competent water oxidation catalysts offers one approach to improving their performance. Similar to Nature's photosynthesis, the separation of photon absorption, charge separation, and water oxidation tasks in composite photoelectrodes allows components performing each task to be optimized independently and thereby enables a greater flexibility in the selection of component materials.
Electrochemical deposition of Co-Pi, as described herein (e.g., Example 2), forms an adequate junction between the catalyst and semiconductor for interfacial charge transfer, and the resulting Co-Pi/α-Fe2O3 composite photoanodes are stable under photolysis conditions. A kinetic bottleneck was observed with thick layers of Co-Pi that hindered the steady-state turnover of the composite photoanodes, especially at low applied potentials. This kinetic limitation was remediated by reducing the Co-Pi coverage, but at the expense of overpotential. With short electrochemical deposition times, however, Co-Pi was found to deposit preferentially at pinholes, scratches, or other imperfections in the α-Fe2O3 film, where more current can flow from the underlying conductive FTO substrate. This inhomogeneity affects the performance of Co-Pi/α-Fe2O3 photoanodes by creating areas where the catalyst layer is too thick (kinetic bottleneck), and it influences the reproducibility of the Co-Pi deposition itself. Ultimately, a stable and efficient water oxidation photoanode is desired, and methods to apply a uniform thin catalyst layer onto highly mesostructured metal-oxide photoanodes, such as α-Fe2O3 are therefore needed.
In this Example, we describe photo-assisted electrochemical deposition (“photoelectrodeposition”) of Co-Pi onto mesostructured α-Fe2O3 photoanodes, and present a comparison between this approach, electrochemical deposition of Co-Pi and Co2+ adsorption. These three approaches are summarized in
Mesostructured α-Fe2O3 photoanodes were fabricated on FTO glass by the APCVD method described in Example 2. Masks with apertures of 6 mm in diameter were applied to define the active surface areas. Co-Pi was electrodeposited onto α-Fe2O3 photoanodes by modification of published procedures. A three-electrode cell was used with α-Fe2O3 as the working electrode, Ag/AgCl as the reference electrode, and Pt mesh as the counter electrode. 0.9 V vs Ag/AgCl was applied in a solution of 0.5 mM cobalt nitrate in 0.1 M potassium phosphate buffer at pH 7. The amount of Co-Pi deposited was controlled by the deposition time, which ranged between 200-500 s. Current densities were typically ˜2-10 μA/cm2 during deposition.
Photo-assisted electrochemical deposition of Co-Pi onto mesostructured α-Fe2O3 was performed from the same electrolyte composition used for electrochemical deposition, 0.5 mM cobalt nitrate in 0.1 M potassium phosphate buffer at pH 7, but with 1 sun AM 1.5 simulated sunlight illumination. Because conduction-band electrons in α-Fe2O3 do not have sufficient potential to reduce water, an external bias (˜0.1-0.4 V) was applied. The amount of Co-Pi was again controlled by the deposition time, which ranged between 500-750 s. Current densities were typically ˜2-5 μA/cm2 during deposition.
Following Example 4, Co2+ adsorption onto mesostructured α-Fe2O3 photoanodes was achieved by dipping the photoanode in a solution of 0.1 M cobalt nitrate for 5 minutes. The amount of Co2+ adsorbed was optimized by repetition of this dipping process. Typically, PEC enhancement reached its maximum after about three cycles. Subsequent cycles resulted in either no change or a decrease in the PEC performance.
PEC measurements were conducted in 1M NaOH (pH 13.6) using a three-electrode configuration, with the photoanode as the working electrode, Ag/AgCl as the reference electrode, and Pt as the counter electrode. Photocurrent densities were measured with front-side illumination under 1 sun AM 1.5 simulated sunlight using an Oriel 96000 solar simulator equipped with a 150 W Xenon arc lamp and an Oriel AM 1.5 filter. Potentials vs. RHE are calculated using the Nernst equation ERHE=EAg/AgCl+0.0591(pH)+0.1976 V. Very similar α-Fe2O3 photoanodes were used for all PEC measurements. The amount of catalyst applied was optimized to give the largest sustainable cathodic shift and overall current density by controlling the amount of catalyst loading, either by adjusting the time of deposition for Co-Pi or the number of cobalt dipping cycles for Co2+ adsorption. Cathodic shifts were calculated as the average voltage shifts in the window where current densities range from 0.5-1.5 μA/cm2. For uniformity, reported photocurrent increases with catalyst deposition refer specifically to the difference in photocurrent at 1.1 V vs RHE. Photocurrent onset potentials were calculated by extrapolation to zero current from the linear portion of the J-V curve where current densities range from 0.5-1.5 mA/cm2.
Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses were performed using a FEI Sirion SEM equipped with an energy dispersive spectrometer. No conductive coating was deposited onto samples for these measurements.
SEM images of a representative mesostructured α-Fe2O3 photoanode are shown in
The amount of catalyst on α-Fe2O3 photoanodes that yields the largest sustainable PEC enhancement can be roughly estimated using the EDX results. As expected for surface deposition, increasing the probe depth by increasing the electron acceleration voltage from 10 to 15 keV results in a substantial decrease in the relative cobalt peak intensity. Approximating the probe depth of a 10 keV electron beam to be ˜200 nm the assumption of a uniform flat surface would yield a Co-Pi thickness of ˜30 nm, but this value represents an upper limit because of the very high surface roughness of the α-Fe2O3 mesostructure (roughness ˜20). The active Co-Pi cluster is believed to possess seven cobalt ions, with a volume of ˜700 Å, from which an upper limit of 34 clusters thickness is obtained. In all likelihood, the actual thickness is substantially smaller. For example, it is interesting to note that the amount of cobalt detected by EDX is about the same for optimized Co-Pi/α-Fe2O3 as for Co2+-impregnated α-Fe2O3. Co2+ adsorption has previously been suggested to yield only monolayer coverage, implying closer to one monolayer of Co-Pi cluster as well. Overall, these results clearly indicate that Co-Pi/α-Fe2O3 composite photoelectrodes optimized for steady-state photocurrents possess far thinner Co-Pi layers than the analogous Co-Pi-coated electrodes used in electrocatalysis. This difference relates to the kinetic bottleneck described previously, which likely reflects the important role of surface electron-hole recombination under PEC conditions.
The current densities for each of these films are stable and reproducible after multiple J-V scans and under illumination for over 72 hours, even after weeks of storage at room temperature in air.
Interesting variations in performance are observed from film to film, much of which derives from variations in the underlying α-Fe2O3 photoanode performance. Such differences are illustrated in
Incident-photon-to-current conversion efficiency (IPCE) measurements on a Co-Pi/α-Fe2O3 photoanode prepared by photo-assisted electrochemical deposition (
To put the above comparisons on a more quantitative footing,
Specifically,
Overall, these data clearly reveal the superiority of photo-assisted electrochemical deposition over simple electrochemical deposition for the preparation of Co-Pi/α-Fe2O3 composite photoanodes. They also illustrate the improvements in α-Fe2O3 PEC performance obtained using Co-Pi rather than surface-adsorbed Co2+ as the electrocatalyst.
In addition to their ease of preparation, Earth-abundant composition, and highly stable photocurrent densities, the absolute performances of Co-Pi/α-Fe2O3 photoanodes are comparable with those of IrO2/α-Fe2O3 photoanodes prepared by attachment of nanocrystals of the well-known water oxidation catalyst, IrO2, onto similar α-Fe2O3 photoanodes. Compared to the Co-Pi/α-Fe2O3 photoanode in
Despite the reduced onset potential, a positive voltage must still be applied in order to drive PEC water oxidation using α-Fe2O3. Ideally, this voltage would be supplied by a photovoltaic (PV) device in a tandem configuration. For the photoanode in
In summary, photo-assisted electrochemical deposition of Co-Pi onto mesostructured α-Fe2O3 yields better performing photoanodes than either electrochemical deposition of Co-Pi or simple Co2+ wet impregnation. A stable ˜170 mV cathodic shift was observed with photoelectrochemical deposition of Co-Pi, while the electrochemical deposition of Co-Pi gave cathodic shifts of ˜100 mV, and Co2+ impregnation gave ˜80 mV cathodic shifts. Photo-assisted electrochemical deposition provides a more uniform distribution of Co-Pi on α-Fe2O3 than obtained by electrochemical deposition by allowing deposition only where visible light generates oxidizing equivalents. Optimization of the photo-assisted electrochemical deposition conditions allowed elimination of all nodules and islands to yield thin uniform films of Co-Pi over the entire photoanode surface. The resulting catalyst-modified metal-oxide photoelectrodes are attractive for solar water oxidation in tandem or hybrid PEC cells.
Example 4 Deposition of Cobalt Oxide Catalysts on α-Fe2O3 by Deposition from an Aqueous Solution of Co2+, Such as from Cobalt Nitrate, Cobalt Acetate or Cobalt SulfateElectrochemical deposition and photoelectrochemical deposition of a cobalt oxide catalyst, referred to here as “CoOx,” on α-Fe2O3 were produced by deposition from an aqueous solution of Co2+, such as from cobalt nitrate, cobalt acetate or cobalt sulfate. X-ray diffraction experiments showed that CoOx did not match the typical diffraction patterns of known cobalt oxides, CoO, Co2O3, or Co3O4. In one embodiment, CoOx was electrodeposited from an aqueous solution of 10 mM cobalt nitrate (pH ˜4) at 0.7-1.4 V vs Ag/AgCl. A Pt mesh was used as the counter electrode and saturated Ag/AgCl was used as the reference electrode. In another embodiment, CoOx was photodeposited on α-Fe2O3 from the same Co2+ electrolyte under a light bias, 1 sun AM 1.5 simulated solar irradiation, and at 0.1-0.4 V vs Ag/AgCl.
A photo-assisted electrochemical deposition (photoelectrochemical) approach was employed to achieve selective deposition of Co-Pi onto TiO2 nanowires (NWs).
These data demonstrate that the cobalt-containing catalyst Co-Pi can be photoelectrochemically deposited onto other semiconductor materials of different shapes, such as TiO2 nanowires, as well as onto dendritic α-Fe2O3 photoanodes. Simple electrodeposition of Co-Pi onto the same TiO2 nanowire structures grown on conductive FTO substrates results in preferential catalyst deposition onto the exposed more-conductive FTO instead and does not improve PEC water oxidation performance. Direct photodeposition of Co-Pi did not result in successful application of the catalyst. By photo-assisted electrodeposition using a wavelength at which only the TiO2 absorbs, Co-Pi was successfully applied specifically to the TiO2 nanowires, yielding the significant cathodic shift in the PEC water oxidation potential. This result also shows that Co-Pi can be used to improve the PEC water oxidation of a semiconductor such as TiO2 with an already low onset potential towards PEC water oxidation. The successful Co-Pi modification of TiO2 nanowires demonstrates the versatility of this photoelectrochemical deposition method to apply cobalt-containing water oxidation catalysts onto semiconductor materials of various shapes and sizes.
Example 6 Composite Co-Pi/Amorphous TiO2/CdS/TiO2 NW PhotoanodesThese data demonstrate that the catalyst Co-Pi can be deposited by photoelectrochemical deposition onto complex electrodes involving visible-light-absorbing sensitizers, such as CdS, integrated with UV light absorbing wide-bandgap semiconductors, such as TiO2, via photoexcitation of the sensitizer and an applied potential. By modifying the TiO2 nanowire surfaces with CdS (bandgap 2.4 eV), the PEC water oxidation electrode is made more sensitive to visible light (i.e., sunlight), as seen by the large photocurrent enhancement. A cathodic shift is also observed after catalyst modification, demonstrating the compatibility of this catalyst deposition method with sensitizers such as CdS and with complex electrodes involving both sensitizers and wide-gap oxides.
Example 7 Composite Co-Pi/Co2+:ZnO PhotoanodesThese data demonstrate that photoelectrochemical deposition can also be applied to deposited Co-Pi onto wide-gap semiconductors doped with cationic impurities (Co2+) introduced to extend PEC water oxidation into the visible region and increase the solar photocurrent densities relative to undoped ZnO. Photoelectrochemical deposition of catalysts onto such doped semiconductors can also be achieved via excitation of mid-gap electronic transitions arising from the dopants, demonstrating that the photoelectrochemical deposition method is not limited to bandgap excitation of semiconductors. Regardless of the electronic transition used for photoelectrochemical deposition, the result is an increase in the overall PEC water oxidation efficiency.
Example 8 Composite Co-Pi/W:BiVO4 PhotoanodesThese data demonstrate that photoelectrochemical deposition of Co-Pi can also be used to deposition water-oxidation catalysts onto doped ternary semiconductors, such as W:BiVO4. The result is a substantial >300 mV cathodic shift in the onset potential for PEC water oxidation and a significantly lower onset potential (<400 mV vs RHE) than can be achieved with Co-Pi/α-Fe2O3 composite photoanodes. As with α-Fe2O3, a thin uniform layer of catalyst is desired for large stable photocurrent improvements. Increased deposition of Co-Pi onto W:BiVO4 results in decreased PEC performance associated with thick catalyst layers. Photoelectrochemical deposition is a useful approach for applying thin well-dispersed catalyst layers onto various types of semiconductor photoanodes.
Example 9 Composite Cobalt Methyl-Phosphonate/α-Fe2O3 PhotoanodesThese data demonstrated that other cobalt-containing water-oxidation electrocatalysts besides Co-Pi, such as Co-MePi, can be successfully photoelectrochemically deposited onto a semiconductor, such as α-Fe2O3 to improve electrode performance. The resulting cathodic shift in the onset potential for PEC water oxidation is very similar to that achieved by Co-Pi deposition, indicating that the photoelectrodeposition method can be expanded to include other oxygen evolving electrocatalysts as well.
Example 10 Composite Nickel Borate/α-Fe2O3 PhotoanodesThese data demonstrate that other electrocatalysts, such as the nickel-based Ni—Bi catalyst, can be applied onto semiconductors, such as α-Fe2O3, by photoelectrochemical deposition to yield a favorable cathodic shift of the onset potential for PEC water oxidation. This illustration indicates that photoelectrochemical deposition is a general approach for the application of various oxygen evolving electrocatalysts onto a variety of semiconductor photoanodes.
While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.
Claims
1. A method of forming a composite photoanode, comprising photoelectrodepositing a solid conformal layer of an electrocatalyst from an electrolyte solution on a surface of a semiconductor submerged in the electrolyte solution by simultaneously:
- (1) impinging the surface of the semiconductor with electromagnetic radiation having a first wavelength and a first irradiance, to provide a first photoenergy that is sufficient to excite an electronic transition of the semiconductor; and
- (2) applying a first electric bias to the semiconductor, wherein the first electric bias is less than an electrochemical deposition bias, said electrochemical deposition bias being the minimum voltage required to electrodeposit the electrocatalyst onto the surface of the semiconductor without impinging the surface of the semiconductor with electromagnetic radiation having the first photoenergy;
- wherein the combination of the first photoenergy and the first electric bias are sufficient to deposit catalyst components from the electrolyte to form the solid conformal layer of the electrocatalyst.
2. The method of claim 1, wherein the electronic transition is a bandgap transition.
3. The method of claim 1, wherein the semiconductor has a physical shape selected from the group consisting of dendrites, wires, belts, rods, mesostructures, nanotubes, and thin films.
4. The method of claim 3, wherein said physical shape has nanoscopic dimensions.
5. The method of claim 1, wherein the semiconductor comprises hematite iron oxide dendrites.
6. The method of claim 1, wherein the electrocatalyst is selected from the group consisting of a cobalt-containing catalyst, an iridium-containing catalyst, a manganese-containing catalyst, a ruthenium-containing catalyst, a nickel-containing catalyst, a cobalt-containing oxygen evolving catalyst, a cobalt oxide/hydroxide catalyst, and a cobalt oxide catalyst.
7. The method of claim 1, wherein the electrocatalyst is cobalt phosphate.
8. The method of claim 1, wherein the layer of the electrocatalyst has a thickness of from 0.5 nm to 30 nm.
9. The method of claim 1, wherein the semiconductor is an n-type semiconductor.
10. The method of claim 1, wherein the formed photoanode, when incorporated into a photoelectrochemical cell for electrolysis of water into oxygen, reduces a water electrolysis onset voltage compared to a second photoanode comprising the semiconductor without the electrocatalyst.
11. The method of claim 10, wherein the water electrolysis onset voltage is reduced by 50 mV to 400 mV.
12. The method of claim 1, wherein the first wavelength of the electromagnetic radiation is from 300 nm to 800 nm.
13. The method of claim 1, wherein the first irradiance of the electromagnetic radiation is from 0.1 W/m2 to 1100 W/m2, or the equivalent in pulsed irradiation.
14. The method of claim 1, wherein the electromagnetic radiation is selected from the group consisting of continuous radiation and pulsed radiation.
15. The method of claim 1, wherein the first electric bias is applied to the semiconductor as part of an electrochemical deposition system comprising a power source in electrical communication with the semiconductor and a counter electrode.
16. The method of claim 1, wherein the electrolyte solution comprises cations selected from the group consisting of cobalt, iridium, manganese, nickel, and ruthenium.
17. The method of claim 1, wherein the electrolyte solution comprises anions selected from the group consisting of phosphate, methyl phosphonate, borate, acetate, sulfate and hydroxide.
18. The method of claim 1, wherein the semiconductor comprises a sensitizer having a sensitizer absorbance wavelength, said sensitizer absorbance being different from a semiconductor absorbance wavelength.
19. The method of claim 1, wherein the semiconductor comprises a group IV semiconductor having a formula selected from the group consisting of binary, ternary, and quaternary.
20. The method of claim 19, wherein the group IV semiconductor further comprises ions selected from the group consisting of cations and anions.
21. The method of claim 1, wherein the semiconductor comprises a material selected from the group consisting of an iron oxide, a zinc oxide, a titanium oxide, a tungsten-bismuth-vanadium oxide, a tungsten oxide, a gallium-zinc-oxide-nitride, or these materials also containing additional cations or anions.
22. The method of claim 1, the wherein the combination of the first photoenergy and the first electric bias are sufficient to oxidize cations to deposit catalyst components from the electrolyte to form the solid conformal layer of the electrocatalyst.
Type: Application
Filed: Sep 7, 2012
Publication Date: Sep 19, 2013
Applicant: UNIVERSITY OF WASHINGTON (Seattle, WA)
Inventors: Daniel R. Gamelin (Seattle, WA), Diane K. Zhong (Woodside, NY)
Application Number: 13/606,439
International Classification: C25D 7/12 (20060101);