ELECTRODE FOR PHOTOELECTRIC CATALYSIS, SOLAR CELL, AND METHOD FOR PRODUCING SAID ELECTRODE

Abstract

The invention relates to an electrode (10) for photoelectric catalysis, comprising a supporting layer (1) on which a catalytic layer (2) is arranged, which comprises particles (3) from a first semiconductor material, and a method for the production of said electrode and a solar cell with said electrode.

It is provided that the catalytic layer (2) further features a matrix (4) consisting of a second semiconductor material, which at least partially surrounds the particles.

Claims

1. An electrode (10) for photoelectric catalysis, comprising a supporting layer on which a catalytic layer is arranged, which comprises particles from a first semiconductor material, characterized in that the catalytic layer further features a matrix from an amorphous material which at least partially surrounds the particles.

2. The electrode for photoelectric catalysis according to claim 1, characterized in that at least 90% of the particles feature a deviation from an average particle diameter of maximum 20%, in particular of maximum 10%.

3. The electrode for photoelectric catalysis according to claim 1, characterized in that the semiconductor materials feature free charge carriers, wherein the type of charge carriers that represent a majority in the matrix material corresponds to the type of charge carrier that form in the particles as a result of the illumination.

4. The electrode for photoelectric catalysis according to claim 1, characterized in that the catalytic layer is formed as a multilayer system.

5. The electrode for photoelectric catalysis according to claim 1, characterized in that the electrode is a photoelectrochemical or a photovoltaic electrode.

6. The electrode for photoelectric catalysis according to claim 1, characterized in that an ion conductor, in other words, an electrolyte or an electron conductor, is arranged on the electrode.

7. A solar cell featuring an electrode for photoelectric catalysis according to claim 1 and an ion conductor, in other words an electrolyte or an electron conductor.

8. The solar cell according to claim 1, characterized in that the solar cell further comprises an in particular transparent counter-electrode, and the electrode for photoelectric catalysis is connected to the counter-electrode in an electrically conductive manner via the ion conductor.

9. A method for producing an electrode for photoelectric catalysis, comprising the following steps: a: Preparing a suspension of particles in a solvent and bringing the suspension into contact with a supporting layer, b: Applying a voltage pulse between the supporting layer and the suspension for depositing a layer of particles on the supporting layer, and c: Reducing the diameter of the suspended particles, and single repetition of step (b) or multiple repetition of steps (b) and (c).

10. The method according to claim 9, characterized in that the voltage pulse is applied for a time duration ranging from 10 seconds to 5 minutes.

11. The method according to claim 9, characterized in that the particle diameter is reduced using chemical etching.

12. The method according to claim 9, characterized in that during step (c) of the method, no voltage, or a lower voltage than in step (b), is applied between the supporting layer and the suspension.

13. The method according to claim 9, characterized in that a surface charge of the particles is determined through the addition of an oxidation or reduction agent to the suspension.

14. The method according to claim 9, characterized in that in step (c), the particles partially dissolve and in step (b) the dissolved particle material is deposited as a matrix simultaneously with the particles.

Description

[0057] The invention will now be explained below in exemplary embodiments with reference to the appended drawings, in which:

[0058] FIG. 1 shows a schematic profile view of an electrode for photoelectric catalysis according to the prior art (I) and in a preferred embodiment of the invention (II),

[0059] FIG. 2 shows a schematic view of the method according to the invention in a preferred embodiment,

[0060] FIG. 3 shows a diagram of the photocurrent densities of an electrode according to the prior art and of an electrode according to the invention as a comparison, and

[0061] FIG. 4 shows EDX images of an electrode produced using the method according to the invention in two resolutions.

[0062] FIG. 1 shows a schematic profile drawing of an electrode 10′ according to the prior art (partial view I) compared to an electrode 10 according to the invention (partial view II). Both electrodes, 10, 10′ feature a supporting layer 1 on which a catalytic layer 2 is deposited, which for its part comprises photoactive particles 3. The electrodes 10, 10′ can for example be represented using electrophoretic deposition (EPD).

[0063] The structure of the electrode 10′ as is typically maintained from the electrophoretic deposition of particles 3 with a larger diameter distribution, features in the catalytic layer 2 both smaller particles 3a and larger particles 3b. In accordance with the respective mobility of the particles 3 during the deposition in the solution, the smallest particles 3a are deposited fastest, while the larger particles 3b are deposited slowest. For this reason, smaller particles 3a are arranged closer to the supporting layer 1 than the larger particles 3b.

[0064] The partial depiction II of FIG. 1 shows the structure of the electrode 10 according to the invention, in which small particles 3a (diameter<diffusion length) are arranged monodispersed in several layers 5 on the supporting layer 1, while an embedded (amorphous) matrix 4, which comprises an amorphous electronic and electrochemically inert base material 4b and an amorphous semiconductor material 4a permits the removal of light-generated charge carriers through to the electrolyte interface. The inert base material 4b is particularly carbon-rich (carbon share higher than 30% weight, in particular higher than 50% weight), while the semiconductor material 4a, such as a metallic oxide, with a carbon share of less than 30% weight, in particular less than 20% weight, features a cation which corresponds to a cation of the photoactive particles 3a. Here, it is irrelevant whether the cations of the particles 3a and those of the semiconductor material 4a of the matrix feature the same oxidation stage.

[0065] With the electrode according to the invention, charge carriers are generated in the particles 3a which move from there through the matrix 4 starting within the semiconductor material 4a of the matrix 4 via what is known as a hopping mechanism. The inert base material 4b presents a high-impedance barrier for the charge carriers, which they can only cross via a tunnel effect. In order to realize good conductivity in spite of this, the expansion of the base material 4b and/or the distance between electrically conductive and semi-conductive particles 3a, 4a is as low as possible, preferably in a region of 1 to 5 nm.

[0066] A photoelectric electrode for photoelectric catalysis according to the invention uses coupled electron and ion transport for catalytic energy conversion. Here, the electrolyte takes on the role of the ion conductor. In this electrolyte, partially irreversible chemical processes take place such as the splitting of H.sub.2O into H.sub.2 and O.sub.2. The same electrode can now be operated in a comparable arrangement, in which the electrolyte, which is indeed not an object of the invention, is replaced by one that contains a so-called redox pair such as iodide/tri-iodide. The arrangement realized as a result converts light energy into a photovoltage and a photocurrent under reversible conditions (I.sup.−.fwdarw.I.sub.3.sup.− and I.sub.3.sup.−.fwdarw.I.sup.−), without consuming the electrolyte. This is then a photoelectrochemical solar cell. In a further embodiment, the electrolyte can be fully replaced by an electron conductor, which connects the electrode to the counter-electrode. This can occur through direct contacting of the surface with an electrically conductive, in particular metallic, material and/or through deposition of a transparent conductive oxide (TCO), which for its part is preferably connected via metallic contacts to the counter-electrode. Through this arrangement, a solid body solar cell is realized in which the required photovoltage and the photocurrent are generated in the structures known as catalyst particles.

[0067] FIG. 2 shows a schematic view of the method according to the invention in an n-step electrophoretic deposition of commercially available particles 3 with a large diameter distribution. Initially, the particles 3 which vary strongly in relation to the particle diameter, are suspended in a solvent 5 (not shown). The suspension is brought into contact with a supporting layer 1. This can occur both before and after the suspension. Subsequently, the deposition process begins, which generally occurs in two steps (b, c) which are repeated until the desired layer thickness of the catalytic layer 2 is achieved.

[0068] With the multi-stage deposition here according to the invention, the interim etching of the particles (c) and brief electrophoretic deposition phases (b) realize a homogeneous size distribution on the carrier material 1, while the reaction products of the etching process enable an embedding structure of the matrix material 4. In individual cases, with the addition of further additives, the chemical composition of the matrix material and its electronic properties can be suitably modified, so that favorable conductivity properties for the charge transport and interface deflections can be realized.

[0069] In a preferred embodiment, this specifically means the following:

[0070] 1) Pigment particles are suspended in a solvent which is suitable for an electrophoretic deposition (e.g. acetone or acetonitrile). The surface charge of the particles 3 is determined by the addition of a suitable reduction or oxidation agent (e.g. Iodine), so that a uniform transportation to the supporting layer 1 with the opposite charge is guaranteed. Via a brief voltage pulse, only the smallest particles 3a (with diameters smaller than the diffusion length, if already present in the suspension) are electrophoretically deposited on the carrier material. This effect is realized by the fact that larger particles (with a larger diameter) require longer transport times in the viscous solution due to the higher resistance (the resistance increases quadratically with the diameter).

[0071] 2) Through the addition of a chemical etching agent, the particle diameters are subsequently reduced, whereby areas of the particles 3a that are close to the surface dissolve (shown here with small arrows). At the same time, due to the etching (c), a particle surface is realized that features a lower density of surface states. In general, acids or lyes are suitable for this purpose. When iodine is used, however, the protic, i.e. proton-providing ethanol, can be added in order to achieve an acidic and thus slightly etching effect. During the course of this etching (c), material of the particles (3) is released into the suspension. A subsequent voltage pulse in turn transports the smallest particles 3a to the supporting layer 1. Simultaneously, the matrix 4 is formed electrochemically on the electrode, which during the course of etching is compiled from the chemical components of the reaction products that have dissolved. This matrix material 4 features e.g. an amorphous structure. If necessary, further chemical additives (e.g. Metal chlorides) can serve to modify the electronic properties of the matrix material 4.

[0072] 3) Steps 1 and 2 are repeated until the desired film thickness on the electrode has been realized. If an exhaustion of the iodine concentration occurs before the film thickness is achieved, iodine is preferably again added to the suspension. This exhaustion of the iodine concentration can alternatively or additionally serve as an indicator and/or regulator for the deposition. The entire light absorption behavior and thus the rate of incoming light to generated charge carriers is determined via the film thickness (incident photon to charge carrier efficiency. IPCE).

Exemplary Embodiment

[0073] Bismuth vanadate pigments (BiVO.sub.4) (from Bruchsaler-Farben GmbH) were used. The yellow pigments (35 mg) are according to step a of the method according to the invention suspended in an acetone (10 ml)/iodine (40 mg) solution in an ultrasound bath (37 kHz) for 10 minutes. Subsequently, the suspension for the electrophoretic deposition was electrophoretically deposited on a carrier (fluorinated tin oxide, FTO) with triple repetition for 1 minute and 40 seconds respectively. Between the individual depositions, the suspension was again set in the ultrasound bath. Since the added iodine forms the acetone solution together with residual water, a strong partial dissolution of the oxidic particles and thus a reduction in size is achieved. The resulting surface morphology was determined using EDX following the first and final deposition (FIG. 3) and corresponds to the electrode 10 shown schematically in FIG. 1 II.

[0074] In the example selected here, the matrix material 4 is primarily amorphous vanadium oxide (V.sub.xO.sub.y) which embeds the particles 3 consisting of BiVO.sub.4.

[0075] In FIG. 3, the behavior of the two electrodes 10, 10′ is shown in comparison with a constant potential (1.2 V) and a light switched on or off (approx. 50 mWcm−2). The points in time of the switching on or off of the light are shown with reference numerals 21 and 22. While the electrode 10′ according to the prior art (which can be produced for example through one-off electrophoretic deposition of a solution until the desired layer thickness is achieved) only features photocurrent densities of approximately 20 μAcm.sup.−2 during the light pulse, the electrode 10 according to the invention achieves photocurrent densities of up to 200 μAcm.sup.−2. The results were determined during the photoelectric generation of oxygen in a 0.1M NaOH electrolyte. The photocurrent response of the two compared electrodes 10, 10′ is shown in a 0.1M NaOH electrolyte. As a potential, 1.2 V was selected in comparison with an Ag/AgCl reference electrode.

[0076] FIG. 4 shows EDX images of the catalytic layer on the interface of the matrix material after step 1 (I) and at the same site in the matrix material following completion of the method according to the invention represented in FIG. 3 (II). In the overview (Ia) a (thinner) interface surface can be seen which is partially covered by larger particles or particle agglomerates. The amorphous phase in Ib shows smaller particles that are hardly embedded at all. By contrast, the overview IIa of the EDX image following completion of the method (II) shows a reduction in deposited larger particles or agglomerates. The detailed view IIB shows a nanoparticular heterolayer consisting of small individual constituents, surrounded by an amorphous phase. In IIb, a nanoparticular heterolayer can clearly be seen consisting of small (non-crystalline) individual constituents, which are surrounded by an amorphous phase. Here, the nanoparticular, non-crystalline constituents form the matrix material together with the amorphous phase.

LIST OF REFERENCE NUMERALS

[0077] 1 Supporting layer [0078] 2 Catalytic layer [0079] 3 Particles [0080] 3a Small particles [0081] 3b Larger particles [0082] 4 Matrix [0083] 4a Amorphous semiconductor material [0084] 4b Inert base material [0085] 5 Solvent [0086] 10 Electrode [0087] 10′ Electrode according to the prior art [0088] 20 Power density of an electrode according to the invention [0089] 30 Power density of an electrode according to the prior art [0090] 21/31 Light on [0091] 22/32 Light off [0092] 23/33 Photocurrent density