Photoanode for Photoelectrochemical Water-Splitting Device and Method of Fabricating Photoanode

The present application claims priority to Korean Patent Application No. 10-2024-0074182, filed Jun. 7, 2024, the entire contents of which is incorporated herein for all purposes by this reference.

The present disclosure relates to a system and a method for fabricating a photoanode. More specifically, the present disclosure relates to a technique for fabricating a photoanode used as an electrochemical water-splitting device by depositing a diethylenetriamine penta (methylene phosphonic acid) (DTPMP) cocatalyst cross-linked with cobalt (Co) on a surface of a BiVO(BVO) photoanode.

Bismuth vanadate (BiVO) has a small bandgap energy (up to 2.4 eV) and an appropriate band position. Bismuth vanadate is considered one of the most promising photocatalysts for water oxidation to generate oxygen.

However, the photocatalytic potential of BiVOis limited in photoanode applications due to low electron mobility of BiVO(up to 10cm/Vs), high charge recombination, and unsuitable water oxidation kinetics.

Although BiVOfor photoelectrochemical (PEC) water oxidation has a low starting potential, BiVOhas a photocurrent density much smaller than the theoretical expectation (7.5 mA cm). Due to that, research has been actively conducted to increase oxygen evolution reaction (OER) activity of a BiVOphotoanode through element doping, heterostructure construction, and structure control.

However, the research has mainly focused on charge separation of the BiVOphotoanode, but this approach has encountered a bottleneck, resulting in a slow OER reaction rate at the electrode interface.

Accordingly, the present inventors have proposed an approach for improving PEC performance and OER reaction rate, enhancing stability and durability, and preventing photocorrosion by forming a metal-organic complex Co-DTPMP layer. To achieve this, a surface of a fluorine-doped tin oxide (FTO) photoanode is subjected to an electrodeposition process to grow a Nano array structure of bismuth oxoiodide (BiOI), followed by drop-casting to fabricate a BiVOphotoanode. The surface of the BiVOphotoanode has a metal-organic complex Co-DTPMP layer formed thereon through a successive ionic layer adsorption and reaction (SILAR) process technique.

(Patent Document 1) Korean Patent No. 10-2055409 (Publication date: Dec. 12, 2019)

A technical solution of the present disclosure is to provide a photoanode used as an electrochemical water-splitting device and a method of fabricating the same photoanode, the water-splitting device having improved photoelectrochemical (PEC) water-splitting performance and an improved oxygen evolution reaction (OER) rate by coating a surface of the photoanode to have a metal-organic complex co-diethylenetriamine penta (methylene phosphonic acid) (Co-DTPMP) layer.

In addition, another technical solution of the present disclosure is to provide a photoanode used as an electrochemical water-splitting device and a method of fabricating the photoanode, the method enabling the photoanode to have improved stability and durability while preventing photocorrosion.

The purposes of the present disclosure are not limited to the purposes mentioned above. Other purposes and advantages of the present disclosure that are not mentioned may be understood from the following description and will be more clearly understood by the embodiments of the present disclosure. In addition, it will be readily apparent to those skilled in the art that the purposes and advantages of the present disclosure may be realized by means and combinations thereof as set forth in the claims.

According to one embodiment of the present disclosure, a photoanode used as an electrochemical water-splitting device is a photoanode that absorbs light and causes a water oxidation reaction to generate oxygen.

The photoanode is fabricated by coating the surface of a photoelectrode on which has a nanoporous material grown with a metal-organic complex cocatalyst through a successive ionic layer adsorption and reaction (SILAR) process.

Preferably, the photoelectrode may include a bismuth vanadate (BiVO).

Preferably, the cocatalyst may include diethylenetriamine penta (methylene phosphonic acid) (Co-DTPMP) bonded with cobalt (Co).

Preferably, the SILAR process may involve continuously performing a process of immersing the photoelectrode in a 15 mM DTPMP solution for 5 minutes, washing the photoelectrode with deionized water (DI water), and then immersing the photoelectrode in 15 mM Co (NO)6HO for 5 minutes a predetermined number of times.

According to another embodiment of the present disclosure, the method of fabricating a photoanode may include:

Preferably, the photoelectrode may include a bismuth vanadate (BiVO).

Preferably, the fabricating of a photoelectrode may include:

Preferably, the second step may involve:

The second step may further involve removing excess vanadium pentoxide (VO) by immersing the fabricated BiVOin 1 M sodium hydroxide (NaOH) solution, and then washing the BiVO4 with DI water and ethanol a predetermined number of times, followed by natural drying.

Preferably, the cocatalyst may include metal-organic diethylenetriamine penta (methylene phosphonic acid) (Co-DTPMP) bonded with cobalt (Co).

Preferably the coating of a photoelectrode may involve fabricating a BVO/Co-DTPMP through coating of the surface of the photoelectrode with a metal-organic complex (Co-DTPMP) cocatalyst by repeating the successive ionic layer adsorption and reaction (SILAR) process a predetermined number of times.

Preferably the SILAR process may involve continuously performing a process of immersing the photoelectrode in a 15 mM DTPMP solution for 5 minutes, washing the photoelectrode with deionized water (DI water), and then immersing the photoelectrode in 15 mM Co(NO)6HO for 5 minutes a predetermined number of times.

According to these embodiments, a BiOI photoactive material is electrodeposited on the surface of an FTO photoanode through an electrodeposition process. Then, 60 μL of dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)) is drop-cast on the FTO photoanode, and then the FTO photoanode is annealed at a temperature of 500° C. with a heating rate of 2° C./min., thus fabricating a BiVO. The surface of the fabricated BiVOis coated with a metal-organic complex (Co-DTPMP) cocatalyst through a SILAR process to fabricate a BVO/Co-DTPMP photoanode. Through this, it is possible to greatly improve photoelectrochemical performance and the oxygen evolution reaction (OER) rate.

In addition, on the basis of the present disclosure, long-term stability and durability of the BVO/Co-DTPMP photoanode are improved and photocorrosion is prevented.

Herein below, with reference to the attached drawings, embodiments of the present disclosure will be described in detail so that those skilled in the art can easily implement the present disclosure. However, the present disclosure may be implemented in many different forms and is not limited to the embodiments described herein. To clearly explain the present disclosure in the drawings, parts unrelated to the description are omitted, and similar parts are given similar reference numerals throughout the specification.

Herein below, one embodiment will describe a configuration of fabricating a BVO/Co-DTPMP photoanode in detail. The configuration is as follows: a BiOI photoactive material is electrodeposited on the surface of an FTO photoanode through an electrodeposition process.

Then, 60 μL of dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)) is drop-cast on the FTO photoanode, and then the FTO photoanode is annealed at a temperature of 500°° C. with a heating rate of 2° C./min., thus fabricating a BiVOphotoelectrode. The surface of the fabricated BiVOphotoelectrode is coated with a metal-organic complex (Co-DTPMP) cocatalyst through a SILAR process to fabricate a BVO/Co-DTPMP photoanode.

shows a flow chart showing a BVO/Co-DTPMP photoanode fabricating process according to one embodiment, andshows a diagram showing concept images for each step of the photoanode fabricating process of.

Referring to, the method of fabricating a BVO/Co-DTPMP photoanode in one embodiment is configured as follows: a BiOI photoactive material is electrodeposited on the surface of an FTO photoanode through an electrodeposition process. Then, 60 μL of dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)) is drop-cast on the BiOI photoanode, and then the BiOI photoanode is annealed at a temperature of 500° C. with a heating rate of 2° C./min., thus fabricating a BiVO4 photoelectrode. Finally, the surface of the fabricated BiVOphotoelectrode is coated with a metal-organic complex (Co-DTPMP) cocatalyst through a SILAR process to fabricate a BVO/Co-DTPMP photoanode. The method may include fabricating a BiVOand coating a BiVO.

Herein, in the fabricating of a BiVO, a BiOI photoactive material is electrodeposited on the surface of a fluorine-doped tin oxide (FTO) photoanode through an electrodeposition process, the FTO photoanode being a photoanode material for the photoelectrochemical water-splitting device. Then, 60 μL of dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)) is drop-cast on the BiOI photoanode, followed by annealing to fabricate the BiVO.

That is, referring to, the fabricating of a BiVOincludes the 11th to 14th stepsto. In the 11th step, 1.66 g of potassium iodide (KI) and 0.485 g of bismuth nitrate (Bi(NO)) were completely dissolved in 25 mL of de-ionized water (DI). Then, nitric acid (HNO) is added dropwise to the solution for the pH concentration to be adjusted to 1.7. 0.23 M benzoquinone in 10 mL ethanol solution is added to the pH adjusted solution.

In the 12th step, a BiOI photoactive layer is formed by electro-depositing the generated BiOI photoactive material on the surface of the FTO photoanode through an electrodeposition process. Herein, the electrodeposition process is a process of applying a constant potential of −0.1 V to the Ag/AgCl reference electrode for 400 seconds at room temperature. At this point, as shown in, the BiOI photoactive layeris a nanoarray grown with a nanoporous material.

Next, in the 13th step, 60 μL of dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)) is drop-cast on the surface of the FTO photoanode on which the BiOI photoactive material has been electro-deposited, followed by annealing to fabricate a BiVO.

Afterward, in the 14th step, the fabricated BiVOis immersed in a 1 M sodium hydroxide (NaOH) solution to remove excess vanadium pentoxide (VO) from the BiVOand washed several times with deionized water and ethanol, followed by natural drying in the air.

Meanwhile, the coating of the BiVOinvolves coating the surface of the BiVOwith a metal-organic complex diethylenetriamine penta (methylene phosphonic acid) (Co-DTPMP) cocatalyst bonded with cobalt (Co) by continuously performing a successive ionic layer adsorption and reaction (SILAR) process on the surface of the naturally dried BiVO(sample) a predetermined number of times to fabricate a BVO/Co-DTPMP (sample).

Herein, the SILAR process involves immersing the naturally dried BiVOin 15 mM DTPMP solution for 5 minutes, washing the BiVOwith deionized water (DI water), and then immersing the BiVOin 15 mM cobalt (II) nitrate hexahydrate (Co(NO)6HO) for 5 minutes.

The DTPMP solution is an organic derivative of phosphoric acid rich in N and P elements. Thus, the DTPMP solution may have multiple coordination sites capable of forming a stable complex by combining with the BiVOand cobalt (Co) cations.

That is, referring to, in the coating of the BiVO, the BiVOis immersed in 15 mM DTPMP solution for 5 minutes, washed with deionized water (DI water), and immersed in 15 mM Co(NO)6HO for 5 minutes. The SILAR process is performed a predetermined number of times in succession to coat the BiVOwith the metal-organic complex Co-DTPMP cocatalyst, thus fabricating a BVO/Co-DTPMP photoanode.

Due to this Co-DTPMP cocatalyst, tunneling of charge carriers is promoted, the OER reaction rate is improved, and photo-corrosion may be prevented.

shows a photograph showing crystallinity of the BVO/Co-DTPMP photoanode analyzed using scanning electron microscopy (SEM) in a further embodiment. Referring to, when looking at the BiOI photoanode analyzed using scanning electron microscopy (SEM), it may be confirmed that a nanoarray is well aligned on the surface of the FTO without surface aggregation.

show images and a graph showing crystallinity of the BVO/Co-DTPMP photoanode analyzed in scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) in a yet further embodiment. Referring to, the pure BiVOis made of soft nanoporous particles with an average size of 150 nm, so the BiVOhas a somewhat rough surface according to the SEM analysis results. From this, it may be confirmed that even after coating the surface of the BiVOwith the Co-DTPMP cocatalyst, the result does not change significantly.

show TEM analysis results for the BiVO, whileshow the TEM analysis results for the BVO/Co-DTPMP. Referring to BVO/Co-DTPMP shown in, it may be confirmed that the BVO photoanode coated with Co-DTPMP cocatalyst has a lattice spacing of 0.31 nm, consistent with planar d-spacing while exhibiting amorphous properties.

show images showing the results of HAADF-STEM analysis of the BVO/Co-DTPMP shown in. Referring to, the presence of Bi, V, O, C, Co, and P may be confirmed in the element mapping image and EDS spectrum results.

show photographs showing crystal structure and crystal phase of a pure BVO photoanode and a BVO/Co-DTPMP photoanode in a still yet further embodiment.shows a graph showing X-ray diffraction (XRD) patterns of the pure BVO photoanode and the BVO/Co-DTPMP photoanode.

Referring to, it may be confirmed that wettability of the BVO/Co-DTPMP photoanode is significantly improved after the deposition of the Co-DTPMP layer. For example, referring to, it may be confirmed that an contact angle of the BVO/Co-DTPMP photoanode decreases from 106.9°, which is the contact angle of the BVO, to 38.2°. From this, it may be confirmed that the coating layer of the BVO/Co-DTPMP photoanode has hydrophilicity.

Referring to, it may be confirmed that characteristic peaks of the BVO/Co-DTPMP photoanode were detected in the monoclinic phase corresponding to (), (), (), (), (), (), (), () and () planes in the results of X-ray diffraction (XRD) analysis.

are graphs showing the composition and surface chemical state of a pure BVO photoanode and a BVO/Co-DTPMP photoanode according to the analysis results of X-ray photoelectron spectroscopy (XPS) in a still yet further embodiment. The presence of Bi and V and Co, N, and P related to the BVO photoanode may be confirmed in the XPS results. Accordingly, it may be confirmed that the layer of metal-organic complex Co-DTPMP was successfully grown as an amorphous material on the surface of the BVO photoanode.

For example, referring to, Bi 4f of the BVO/Co-DTPMP photoanode appears in the form of two peaks, Bi 4f7/2 and Bi 4f5/2, at 158.5 eV and 163.9 eV, respectively. Referring to, the corresponding binding energy bands of V 2p are shown in the peaks of V 2pand V 2p, appearing at 516 eV and 523 eV, respectively. Referring to, it may be confirmed that the O 1s spectrum of the BVO sample shows only two peaks that correlate with lattice oxygen and adsorbed OH groups, respectively. Meanwhile, for the BVO/Co-DTPMP sample, the O 1s spectrum shows three main peaks attributed to the oxygen lattice, P—O species, and adsorbed OH groups, respectively.