EP3268511A1

EP3268511A1 - Alkaline photoelectrochemical cell

Info

Publication number: EP3268511A1
Application number: EP16708672.7A
Authority: EP
Inventors: Sascha Hoch; Jan-Philipp BECKER; Patrick BOROWSKI; Matthias Blug; Friedhelm Finger; Jens Busse; Artjom Maljusch; Félix URBAIN; Vladimir Smirnov; Wolfram Jaegermann; Bernhard Kaiser; Jürgen ZIEGLER
Original assignee: Evonik Degussa GmbH; Forschungszentrum Juelich GmbH
Current assignee: Forschungszentrum Juelich GmbH; Evonik Operations GmbH
Priority date: 2015-03-10
Filing date: 2016-03-08
Publication date: 2018-01-17
Also published as: DE102015003003B4; WO2016142382A1; DE102015003003A1

Abstract

The invention relates to a photoelectrochemical cell (PEC) for the light-driven production of hydrogen and oxygen from an aqueous medium in a basic environment. The invention further relates to the production of the photoelectrode of the photoelectrochemical cell and to a method for the light-driven production of hydrogen and oxygen by using the photoelectrochemical cell. The problem addressed by the invention is that of specifying a PEC that operates under alkaline conditions and therefore can forgo noble-metal catalysts, that achieves high light efficiency, and that is durable despite the alkaline conditions. This problem is solved by means of a photoelectrochemical cell, the photoelectrode of which has a special multi-layer layer structure, namely at least d) having a transparent adhesion-promoting layer, which is applied directly to the mentioned solar cell or a further solar cell and which is composed of one of the following metals or of an alloy of one or more of these metals: nickel, chromium, tungsten, hafnium; e) having a mirror layer, which is applied directly to the adhesion-promoting layer and which is composed of one of the following metals or of an alloy of one or more of these metals: silver, copper, aluminum; f) and having an anti-corrosion layer, which is applied directly to the mirror layer and adjoins the reaction chamber and is composed of nickel or of an alloy containing nickel.

Description

Alkaline Photoelectrochemical Cell The invention relates to a photoelectrochemical cell for the light-driven production of

Hydrogen and oxygen from an aqueous medium in a basic medium. It also deals with the preparation of the photoelectrochemical cell photoelectrode and with a process for the light-driven production of hydrogen and oxygen using

photoelectrochemical cell.

A photo-electrochemical cell (engl .: photo electro chemical cell, short: PEC) is a device that allows, of water (H ₂ 0) to gain and molecular oxygen (0 ₂₎ molecular hydrogen (H _2), wherein the energy required for this from light, more precisely from sunlight. With the aid of a PEC, the high-quality hydrogen can be sustainably made available from water, which in turn can be burned or released without C0 ₂ emissions, but can also be further refined chemically into secondary products. The availability of efficient, low-cost photoelectrochemical cells is thus an essential prerequisite for a sustainable, hydrogen-based energy industry. A PEC essentially comprises two functional units, namely a photovoltaic element and an electrolyzer. The photovoltaic element (solar cell) converts light into electrical energy. With the help of electrical energy, the electrolyzer is operated, which splits water electrochemically into hydrogen and oxygen and separates the two gases. The splitting of the water is therefore not directly with light energy, but via the electrical detour. So that the electrical voltage generated by the solar cell can be transferred to the electrolyzer, photovoltaic element and electrolyzer are electrically interconnected.

In principle, photovoltaic element and electrolyzer can be provided as separate components and electrically connected. Both solar cells and electrolyzers are industrially available as separate components. In order to adapt the photovoltaic voltage tapped at the solar cell to the electrochemical voltage required for the water splitting in the electrolyzer, it is generally necessary to connect power electronics therebetween. This one described separate structure has the disadvantage that it is comparatively expansive and that occur through the current transport and power electronics losses.

Therefore, photoelectrochemical cells have been developed, which integrate the two functional units - photovoltaic element and electrolyzer - in one component. In such an integrated component, the individual functional units are realized in layers and interconnected electrically. The photovoltaic element and an electrode of the electrolyzer are combined in a so-called photoelectrode. A power electronics is not provided, but rather a solar cell is used, which is suitable for water splitting

Photovoltage supplies. The obvious advantage of an integrated PEC is that its space is smaller and that it can be produced cheaper in mass production. A further advantage of such a layered, integrated PEC can be seen in the fact that the surface area of the electrodes of the electrolyzer substantially corresponds to the surface area of the solar cell, precisely because the solar cell and an electrode of the electrolyzer are combined in the photoelectrode. Due to the large electrode area, the current density in the electrolyzer decreases, which increases its efficiency. These reasons mean that in the industrial implementation of a PEC usually a layered, integrated design is sought. The differentiated construction is more likely to be found in scientific experimental setups. Since the invention deals with problems that occur in the realization of layered, integrated PEC, an electrochemical cell is always to be understood below as an integrated device in which the two main functional elements (photovoltaic element and electrolyzer) are built up in layers , Such PEC always have a photoelectrode, ie the assembly which on the one hand comprises the complete photovoltaic element and on the other hand serves as an electrode of the electrolyzer, as a rule as a cathode. Furthermore, such PEC include a counter electrode and a separator. The separator electrically isolates the two electrodes from each other, but at the same time allows an ion exchange and causes the separation of the gases obtained. Inside the electrolyzer there is a reaction space separated by the separator into two parts. In the reaction space, the electrolysis of the water takes place. The two resulting gases collect on one side of the separator and are discharged separately. Most catalytically active substances are arranged in the reaction space, which accelerate the water splitting. Such an integrated PEC is known from WO2013 / 143885A1.

The problem with which the invention is concerned is due to the chemical aggressiveness of the aqueous medium used in the electrolyzer:

In the electrolyser, the water molecules are split into oxygen and hydrogen atoms, which in turn combine to form molecular hydrogen and oxygen. That's why the

Reaction space of the electrolyzer are filled with the water to be split. However, not only water can be used, since pure water has insufficient ionic conductivity for (H ⁺ or OH ^" ) .To increase the ionic conductivity of the water and thus lower the cell voltage necessary for the electrolysis, must in the reaction space In order to provide the water to be split on the one hand and also to realize an electrolyte on the other hand, in the simplest case an aqueous medium which is acidic or basic is filled into the reaction space Acid or a base, which acts as an electrolyte on the one hand due to their acidic or alkaline character and on the other hand also provides the to be split H ₂ 0 molecules due to their water content.

The acidic or basic character of the aqueous medium introduced into the reaction space of the electrolyzer first of all has an influence on the catalysts used in the electrolyzer. In order to carry out the water splitting with a low expenditure of energy, correspondingly catalytically active substances are provided in the electrolyzer. These may be noble metals such as platinum, ruthenium or iridium, but also base metals such as molybdenum, manganese, iron, nickel or cobalt, which act catalytically active in their oxidic or partially oxidic form. If an acid is used as an aqueous medium, a noble metal catalyst must be used, since only the precious metals of the acid lastingly withstand. The cheaper base catalysts based on nickel oxide or cobalt oxide, however, can only be used in an alkaline solution. So if you want to do without precious metal catalysts in the interest of material costs, you must inevitably create alkaline conditions in the electrolyzer. Since, in the case of a layered integrated PEC, the electrolyzer's reaction space filled with the acid or base directly adjoins the solar cell (because the photoelectrode comprises both the solar cell and an electrode of the electrolyzer), it must be resistant to the medium used.

In that regard, however, alkaline media prove to be problematic because they attack the semiconductor material contained in the solar cells. In particular, when the solar cell is based on inexpensive silicon, the contact with alkaline media should be avoided as much as possible because significant damage is to be feared - after all, it is not without reason that the semiconductor industry uses potassium hydroxide as an etchant for silicon components by default.

It is therefore obvious that a conflict of objectives between the material costs and the

Durability of an integrated PEC: In particular, if cost reasons do not apply to noble metal catalysts and inexpensive potassium hydroxide solution is to be used as an aqueous medium, precautions must be taken to protect the silicon-based solar cell or the entire photoelectrode from attacks from the alkali.

A use of a semiconductor material other than silicon does not lead further here: Although semiconductor materials for solar cells are known, which could be more resistant to alkalis than silicon. This may be the case with gallium arsenide solar cells. However, such solar cells are significantly more expensive than silicon-based solar cells because gallium is not as abundant on earth as silicon and little technology is available for its industrial processing. For cost and sustainability reasons, therefore, the use of solar cells, which do not consist of conventional silicon or its alloys excluded.

WO2013 / 143885A1 discloses a PEC which operates either in acidic or alkaline conditions. To protect the silicon-based solar cell from the electrolyte is the

Photo electrode provided on its side facing the reaction space with a corrosion-inhibiting coupling layer based on graphite, silver or stainless steel or other electrolyte-resistant metals. The corrosion-inhibiting coupling layer is considered a conductive

Passivierungsschicht carried out, which additionally has light-reflecting properties in order to increase the light absorption in the solar cell structure arranged above. Based on this prior art, the invention is based on the object to further optimize an integrated, working with the alkaline electrolyte PEC: It has namely shown that silver, which has the best reflector properties and thus increases the yield of solar cells, itself rapidly dissociates from the solar cell in the presence of alkaline media; probably because the alkali creeps behind the silver due to the poor bonding of silver to the silicon and etches the underlying silicon of the solar cell.

Specifically, it is an object of the invention to provide a cost PEC, which operates in alkaline conditions and therefore can dispense with noble metal catalysts, which achieves a high luminous efficacy, and which is durable despite the alkaline conditions and despite the use of a low-cost silicon solar cell.

This object is achieved by a photoelectrochemical cell whose photoelectrode has a special layer structure:

The basis is a transparent substrate, which is the simplest case of inorganic glass. The transparent substrate is disposed on the light side of the PEC and is reflected by the incident light. On the side facing away from the incident light of the transparent substrate, a transparent, electrically conductive layer is applied. This may consist of doped ZnO, In ₂ O ₃ or SnO ₂ , which can be deposited by sputter deposition, chemical vapor deposition (CVD) or liquid phase deposition (eg, spray pyrolysis), typically 50 nm in thickness has up to 3 μιη. The transparent, conductive layer makes the transparent substrate electrically conductive in order to serve as a current collector, because inorganic glass is known to be electrically insulating. In addition, the transparent, electrically conductive layer can be provided with a texture or be applied in textured form. More on that later.

At least one partially transparent solar cell based on silicon is applied to the transparent, electrically conductive layer. Such a solar cell, which is known per se, again comprises a plurality of functionalized layers of silicon which are characterized by a different doping (n- and p-type), intrinsic conductor behavior, crystallinity and alloying elements such as oxygen, Hydrogen, carbon or germanium can differ. As a rule, such a Si-based solar cell comprises three layers, namely an n-doped, a p-doped and in between an intrinsic layer. These three functional layers together form a so-called pin or nip junction. The functional layers of the solar cell do not have to by order

layered different materials, it can also be initially applied a single layer of a single material and this then partially functionalized material technology, such as by Randdotierung. However, these are all customary methods of producing solar cells; details are not relevant for the purposes of the invention. What is decisive is that a solar cell is provided which is based on inexpensive silicon and which is partially transparent. Such a solar cell is preferably deposited by way of thin-film technology from the gas phase, it is thus a so-called thin-film solar cell.

Optionally, further transparent solar cells may in turn be applied comprising a plurality of differently functionalized silicon layers in order to increase the luminous efficacy and to achieve a sufficiently high photovoltage (> 1.5 V) for direct water splitting. A single solar cell will usually not be able to deliver the necessary photovoltage. Specifically, the applied solar cells can have different band gaps to the

To expand absorption spectrum and thus to be able to convert different wavelengths of the incident light into electrical voltage. Not only visible light has to be adsorbed, it is also possible to use solar cells which adsorb in the UV or IR range. It is recommended to provide two or three solar cells (i.e., two or three pin junctions) on top of each other, that is, a tandem or triple solar cell. It is also possible to stack more than three solar cells one above the other. According to the invention, at least one solar cell, that is to say a pin junction, is necessary. The solar cells are to be designed as thin-film solar cells, so a piece of transparent, so that the unadsorbed light on the back of the solar cell exits again. The transparency of the thin-film solar cells is due to the low layer thicknesses of the silicon layers. Of course, the solar cell is not completely transparent, since it has to absorb light. That's why the term partial-transparent is used here. Since the solar cells used are hydrogenated silicon or an alloy of hydrogenated silicon with germanium,

Carbon or oxygen, these are inexpensive compared to solar cells from other semiconductors. The solar cell may also have other functional layers, such as ZnO: Al or μ ^ ίΟχ: Η. These materials can be applied in particular to the boundary layers of the solar cells in order to reduce plasmotic effects. Such layers based on ZnO or SiO are considered here as part of the solar cell.

It is essential to the invention that a transparent adhesion-promoting layer is applied directly to the last applied solar cell, namely from one of the following metals: nickel, chromium, tungsten or hafnium. The primer layer may also consist of an alloy of one or more of these metals.

It is also essential to the invention that directly on the adhesion-promoting layer a

Mirror layer is applied, which consists of silver, copper, aluminum or an alloy of one or more of these metals.

The mirror layer serves to reflect the light fallen through the solar cells in the direction of the transparent substrate, so that the hitherto unadsorbed light hits the solar cells again. This increases the luminous efficacy. It is crucial that between the solar cell and mirror layer, the adhesion-promoting layer is provided, which ensures that the mirror layer does not peel off. It has been found that the highly reflective metals silver, copper or aluminum with a thin layer of nickel, chromium, tungsten or hafnium can bind to the silicon of the solar cell and this compound is stable even in an alkaline medium. This makes it possible to use an inexpensive silicon solar cell in an alkaline PEC. Incidentally, a layer of nickel, chromium, tungsten or hafnium can be applied so thinly to the last solar cell that the adhesion-promoting layer is transparent and thus does not shade the mirror.

Another essential feature of the invention is that directly adjacent to the rear side of the mirror layer, ie toward the reaction space, one still adjacent to the reaction space

Corrosion protection layer of nickel or a nickel-containing alloy is applied. This anticorrosive layer need not be transparent, unlike the primer layer, since it is arranged on the shadow side of the mirror. However, it is directly exposed to the alkaline medium in the reaction space and must therefore be corrosion resistant. Here, a comparatively thick layer of nickel has proven itself, since it is stable in the alkaline medium and does not detach from the silver. In addition, the nickel layer acts catalytically in the water splitting, which is why it achieves a bonus effect over other anticorrosion layers of photoelectrodes.

A photoelectrode layered in this manner can be combined with a per se known separator and a counterelectrode to form a PEC, which achieves the stated objects.

The invention therefore relates to a photoelectrochemical cell for the light-driven production of hydrogen and oxygen from an aqueous medium in a basic medium, with a photoelectrode arranged on the light side, with a shadow side arranged

Counterelectrode, with a separator arranged between the photoelectrode and the counterelectrode, and with a reaction space which can be filled with the aqueous medium on both sides of the separator and in which basic conditions prevail when the aqueous medium is filled in, the photoelectrode comprising a transparent substrate arranged on the light side, which is coated in the direction of the separator with the following

Layer order: a) a transparent, electrically conductive layer;

b) at least one partially transparent, layer-wise applied solar cell based on silicon; c) a coupling layer adjacent to the reaction space; this is carried out in multiple layers, namely at least

d) with a coating applied directly to said or another solar cell,

transparent primer layer, which consists of one of the following metals or of an alloy of one or more of these metals: nickel, chromium, tungsten, hafnium; e) with a mirror layer applied directly to the primer layer and consisting of one of the following metals or of an alloy of one or more of these metals: silver, copper, aluminum;

f) and with a directly applied to the mirror layer, to the reaction space

bordering corrosion protection layer of nickel or of a nickel-containing alloy. An essential aspect of the invention is that the layer sequence described here and the mentioned coating materials are precisely adhered to, since the layers only fulfill their respective function and give a total of a photoelectrode, which achieves a good luminous efficacy, which is durable under alkaline conditions, therefore allows the use of an alkaline electrolyte and therefore does not necessarily rely on noble metal catalysts.

According to the invention, the photoelectrochemical cell operates in a basic medium. There are basically two possibilities in the electrolyzer to set alkaline conditions, namely via the aqueous medium and / or via the separator.

In the simplest case, the basicity is adjusted via the aqueous medium. Thus, a basic aqueous medium whose pH is greater than 7 is used. The aqueous medium must contain on the one hand the water to be split, and on the other hand at least one hydroxide or carbonate or phosphate or hydrogen carbonate or hydrogen phosphate or nitrate or sulphate compound of any of the following: lithium, sodium, potassium, magnesium, calcium,

Strontium, barium. These compounds make the aqueous medium alkaline. The aqueous medium is preferably potassium hydroxide solution or sodium hydroxide solution, ie a mixture of water and potassium hydroxide or sodium hydroxide. Potassium hydroxide and caustic soda are cheap mass chemicals. Of course, several alkaline compounds can be used mixed.

When an alkaline aqueous medium is used, it already serves as an electrolyte. The basicity of the separator then does not matter, and even a pH-neutral separator can be used. On the other hand, it is important for the selection of the material of the separator that it is electrically insulating, that it conducts hydroxide ions and is impermeable to gas. Electrically insulating means that the Durschlagspannung through the separator is greater than the electrochemical voltage that prevails in the electrolyzer. Thus, a short circuit between the photoelectrode and the counter electrode is avoided. The Durschlagspannung thus depends not only on the material of the separator, but also of its thickness and the electrode spacing. The

Breakdown voltage thus results from the overall system. Thus the appropriate

Breakdown voltage even in a compact design, ie at a low separator thickness and a small electrode spacing is achieved, the separator should be made of a material which does not conduct any electrons. Nevertheless, the separator must allow hydroxide ions to pass, so that a charge balance can take place and the hydroxide ions released by the electrolysis can diffuse through the separator from the cathode to the anode in order to be oxidized to oxygen there. Gas-impermeable must be the separator, so that the gas molecules collected on both sides of the separator do not mix again, otherwise it can lead to a back reaction to water up to a blast gas explosion. Gas impermeable means impermeable to 0 ₂ and H ₂ . Any permeability to other gases is not relevant. Suitable materials for the separator are anion-conducting ceramics, polymers functionalized with ion-conducting groups, such as polyolefins, polyethers, polyimides, polyamides and

Polysulfones, wherein said polymers are used in pure form, as a mixture or as co-polymers or as block polymers or as block copolymers. Especially preferred

Membrane material is a cross-linked, hydrocarbon-only polyolefin with quaternary ammonium groups.

The materials just mentioned show even alkaline behavior. A separator made of such basic materials is a solid state electrolyte. When the separator is a solid electrolyte, the aqueous medium itself need not have electrolytic properties. It is then possible to use pure water with neutral pH 7. The basicity of the system then comes out of the separator. Pure water within the meaning of this invention is water having an electrical conductivity of less than 1 * 10 ^"4 S / m. If the ionic conductivity of the separator is not sufficient (worse solid electrolyte), it may be useful, if necessary, still additionally a basic Liquid electrolyte (water or alkali with pH> 7) to use.

The particular embodiment of the photoelectrochemical cell with pure water and

Solid-state electrolyte is therefore characterized in that the aqueous medium is water having an electrical conductivity of less than 1 × 10 ^-4 S / m, and that the separator is a gas-impermeable, anion-conducting and electronically insulating

Polymer electrolyte membrane, wherein the separation-active material of the membrane is preferably selected from the following materials: ceramics, polyolefins, polyethers, polyimides, Polyamides, polysulfones, wherein said polymers are used in pure form, as a mixture or as copolymers or as block polymers or as block copolymers.

According to the invention, the photoelectrochemical cell has a reaction space which extends on both sides of the separator and in which the electrochemical water splitting takes place. The reaction space is not empty but at least filled with the aqueous medium. On the side of the photoelectrode, the reaction space at the corrosion protection layer ends, since it is not overcome by the aqueous medium. In the design of the reaction space again a conflict of objectives is to be solved: First, the two electrodes should be as close as possible to the separator to minimize the parasitic resistance contributions (caused by the ion transport within the electrolyte over a certain distance). In that regard, it would be ideal to place the electrodes directly on the separator. But this is not possible, because then the reaction space would be so small that on the one hand not enough water would be available for fission and on the other hand, the gases formed hydrogen and oxygen could not escape unhindered. The distance between the photoelectrode (more precisely, their

Anticorrosion layer) and the separator should therefore be as small as possible but as large as necessary.

This conflict of objectives can be solved by providing a porous structure made of an electrically conductive material between the anticorrosion layer and the separator, which is electrically contacted at least with the anticorrosion layer. This arrangement causes the reaction space to extend at least partially in the pores of the porous structure. The pores are therefore to be sized so that they can absorb the aqueous medium and release the resulting gases. Since the porous structure is electrically conductive and is electrically contacted with the anticorrosion layer, it becomes electrochemically a part of the electrode. As a result, the photoelectrode moves closer to the separator and thereby lowers the resistance contributions without consuming the reaction space. Preferably, the porous, electrically conductive structure is applied to the separator. A preferred embodiment provides that the porous structure consists of a catalytically active in the water splitting material and / or is provided therewith. Since the water splitting in the Pores of the porous material, it is promoted by the presence of the catalytically active material.

It is possible to use an electrically conductive, porous material, which is itself catalytically active. For this purpose, for example, nickel or an alloy of iron, carbon and nickel, more specifically stainless steel into consideration. Concretely, the porous structure may be foamed nickel or a textile fabric made of nickel fibers or stainless steel fibers. In addition, the porous structure may be impregnated with catalytically active substances, such as nickel oxide and / or cobalt oxide. In the interest of minimizing the resistance, the catalyst is preferably located at the interface between electrode and separator, and as close as possible to the separator. If the porous structure is impregnated with nickel oxide and / or cobalt oxide, the porous material itself need not be catalytically active, it is sufficient electrical conductivity. Thus, the porous material may be a fabric of carbon fibers impregnated with nickel oxide and / or cobalt oxide. Incidentally, the textile structure can also be integrated into the photoelectrode

Corrosion protection layer of nickel applied very thick and porous to the separator out. It is also possible to provide the nickel layer with corresponding grooves and depressions to make room for the reaction space. Likewise, the separator can be geometrically patterned to create reaction space in the separator.

In principle, a porous nickel structure can be provided on both sides of the separator. It is possible to provide different catalysts on both sides, each for the desired reaction, i. Water oxidation at the anode and water reduction at the cathode, are optimized.

Since the counterelectrode does not contain any silicon, the entire counterelectrode can be made porous, for example from a solid nickel foam. The catalytically active substances can also be applied to the separator in the form of an ionomer-containing paste: Under a lonomer-containing paste is a mixture of water and an organic solvent, a hydroxyl ion-conducting polymer as needed an electrically conductive additive and a in the Water splitting catalytically active substance to understand. Catalytically active substances are enriched in this paste in the water splitting, so that when the aqueous medium is filled in, it saturates the ionomer-containing paste. The reaction space then extends at least partially within the ionomer-containing paste. However, the paste may also be used in combination with a porous nickel-containing structure.

In the previously mentioned several times, in the water splitting catalytically active substances is preferably nickel oxide and / or cobalt oxide. Alternatively, palladium or platinum could be used, but these metals are significantly more expensive than nickel and cobalt. Since in the PEC invention, no acidic, but basic conditions prevail, no expensive precious metal catalyst needs to be used.

The solar cell can be designed as a tandem or triple solar cell:

With a tandem solar cell, the photoelectrode has two solar cells applied in layers, each of which in turn comprises several layers of differently doped silicon or its alloys, wherein the first solar cell is applied directly to the transparent, electrically conductive layer, wherein the second solar cell directly the first solar cell is applied, and wherein the coupling layer is applied directly to the second solar cell. With a triple solar cell, the photoelectrode has three solar cells applied in layers, each of which in turn comprises several layers of differently doped silicon or different crystallinity or its alloys, wherein the first solar cell is applied directly to the transparent, electrically conductive layer, wherein the second solar cell is applied directly to the first solar cell, wherein the third solar cell is applied directly to the second solar cell and wherein the coupling layer is applied directly to the third solar cell. In a tandem or triple solar cell, solar cells with the same or different band gap can be stacked. With different band gaps, the adsorption spectrum can be extended, so that the efficiency increases. The crystallinity and the degree of hydrogenation can be used to vary the band gap of the silicon, whereby tandem or triple cells can be produced purely from silicon whose sub-cells absorb in different spectral ranges. A

Alloying Si with Ge results in a smaller bandgap while alloying with C or O results in a larger bandgap. With regard to the choice of material of the solar cell, the invention does not differ from the prior art. It should be made clear at this point that the layer (s) applied

Solar cell (s) to so-called thin-film sola cell (s) acts. The layers of a thin-film solar cell are not cut out of a wafer, but separated from the gas phase. They are therefore significantly thinner than the layers of a wafer-based solar cell and are therefore partially transparent. A wafer solar cell is not transparent.

The thickness of the thin-film solar cell depends on the layer thickness of the individual layers. The intrinsic silicon layers (i-layer of the pin or nip structure) may well be more than 1 μm thick, in particular the με-5ϊ: Η-5εηϊεηΙθη. Amorphous intrinsic layers within the solar cell are typically 50-500 nm thick, microcrystalline intrinsic layers: 0.1-10 μιη. The doped layers on the backside (n-doped) can also be significantly thicker than 20 nm, since parasitic absorption plays no role there. As already mentioned, the solar cell can also comprise further functional layers, such as those based on SiO or ZnO, in order to reduce plasmotic effects. The transparent substrate is an organic or inorganic glass. An inorganic glass is usually made of Si0 ₂ . Organic glasses consist of one

transparent polymer such as polycarbonate or polymethyl methacrylate such as Plexiglas ^® from Evonik. The substrate is preferably provided on its coated side facing the coupling layer with a texture which allows incident light to pass in the direction of the coupling layer, but scatters incident light back from the direction of the coupling layer in the direction of the coupling layer. The texture scatters the light into larger angles, thus leading to a

Extension of the light path through the absorber layers. The texture thus increases the light output. The texture is a surface modification of the glass, which is etched wet-chemically. Alternatively, the texture can also be applied to the transparent, electrically conductive layer. This is done subtractively by etching the transparent, electrically conductive layer or additively by textured application of the transparent, electrically conductive layer.

Since the invention relates essentially to the layer structure of the photoelectrode, a method for producing this photoelectrode is likewise provided by the invention. The preparation is carried out by coating the transparent substrate successively as follows: a) with a transparent, electrically conductive layer;

b) with a partially transparent solar cell based on silicon, which in turn several

differentially functionalized layers of silicon and / or its alloys, which are applied successively;

c) optionally with a further partially transparent solar cell based on silicon, wherein the band gap of the further solar cell may differ from the previously applied solar cell but need not;

d) optionally with a further partially transparent solar cell based on silicon, wherein the band gap of the further solar cell may differ from the previously applied solar cell but need not;

e) with a directly applied to the last applied solar cell, transparent, primer layer, which consists of one of the following metals or of an alloy of one or more of these metals: nickel, chromium, tungsten, hafnium;

f) with a mirror layer applied directly to the primer layer, which layer consists of one of the following metals or of an alloy of one or more of these metals: silver, copper, aluminum;

g) and with a directly applied to the mirror layer, to the reaction space

bordering corrosion protection layer of nickel or of a nickel-containing alloy.

The deposition of the individual layers is carried out by well-known thin-film technology, in particular physical vapor deposition (PVD) such as sputter deposition,

Sputter coating or thermal evaporation, electrode beam evaporation or chemical vapor deposition (CVD). When assembled, the PEC according to the invention serves to produce molecular hydrogen and molecular oxygen from the aqueous medium introduced into the reaction space, the electrical energy necessary for the electrolytic water splitting being obtained from the visible and / or invisible sunlight with which the photoelectrode is illuminated ,

The invention therefore also provides a process for the light-driven production of hydrogen and oxygen from an aqueous medium, using a

The photoelectrochemical cell according to the invention, in which the aqueous medium is introduced into the reaction space of the photoelectrochemical cell and light is applied to the transparent substrate of the photoelectrode, so that incident and reflected light is adsorbed in the solar cell, resulting in electric current between the photoelectrode and the photoelectrochemical cell

Counter electrode is converted, which causes a cleavage of the water in the aqueous medium contained in oxygen and hydrogen and a separation of oxygen and hydrogen on both sides of the separator in the reaction space.

The structural design of PEC according to the invention will now be explained with reference to exemplary embodiments. 1: First embodiment I of a PEC with basic aqueous medium and nickel

Sponge;

Figure 2: as Figure 1, but in an exploded view;

FIG. 3: Second embodiment II of a PEC with neutral aqueous medium,

structured solid electrolyte and ionomer paste applied thereto; Figure 4: as Figure 3, but in an exploded view.

The first embodiment I of a photoelectrochemical cell according to the invention is shown in FIG. The three basic components of the PEC are a photoelectrode 1, a separator 2 and a counterelectrode 3. The photoelectrode here forms the cathode of the PEC, the counterelectrode forms the anode. But it is also the other way around. The photoelectrode faces the sun 0. The sun 0 is not part of the invention. The counter electrode 3 is located on the side of the shadow facing away from the sun 0. The separator 2 is arranged between the photoelectrode 1 and the counterelectrode 3. In embodiment I, the photoelectrode 1 and counterelectrode 3 are not directly adjacent to the separator 2, but are spaced therefrom. In this way, a comparatively large reaction space 4 is formed between the two electrodes 1, 3, which is divided symmetrically into two parts by the separator 2. An asymmetric division of the reaction space through the separator is also conceivable. The reaction space 4 is not empty, but filled with a, not shown in the drawing, aqueous medium having a pH greater than 7, such as potassium hydroxide, on the other hand, on both sides of the separator 2 each have a porous, nickel-containing structure 5K , 5A inserted. It is a nickel sponge, which is impregnated with catalytically active substances 6K, 6A. The porous, nickel-containing structure 5K on the side of the cathode is directly on the photoelectrode 1, so that there is an electrical contact with it. At its other end, the cathode-side, porous, nickel-containing structure 5K abuts the separator 2. On the anode side, another catalytically active substance 6A is introduced as the catalytically active substance 6K on the cathode side of the separator. Since the aqueous medium is in the pores of the nickel sponges 5K, 5A, it comes in contact with the respective catalytically active substance 6K, 6A everywhere.

The separator 2 is gas-impermeable and ionically conductive. He isolated photoelectrode 1 and

Counter electrode 3 electrically from each other, so there is no short circuit. The separator 2 therefore consists of a non-electrically conductive ceramic or of an organic polymer which has hydroxide ion-conducting groups. The counter electrode 3 consists of an alkaline

Solution stable metal such as nickel or titanium or of a conductive material which is provided with a protective layer of nickel or titanium or its alloys. The counterelectrode can also be porous itself. Decisive is the layer structure of the photoelectrode 1. This is based on a glassy substrate 7, on the side facing the separator 2 a texture 8 is introduced. On the textured side of the substrate 7, a transparent, electrically conductive layer 9 is applied, for example, doped ZnO, ln ₂ 0 ₃ or Sn0 _second As an alternative to the texture on the glassy substrate 7, the electrically conductive layer 9 can also be textured.

On the transparent, electrically conductive layer 9, a first solar cell 10 is applied and on this in turn a second solar cell 11. The two solar cells 10, 11 thus form a tandem Solar cell. Each of the two solar cells 10, 11 each again consists of three silicon-based layers, namely a p-doped silicon layer, an n-doped silicon layer and an intrinsically doped silicon layer therebetween. The three silicon layers form a pin junction and thus a solar cell. The layer sequence pin can also be reversed in nip, whereby the polarity of the solar cell changes and the photoelectrode becomes the anode. On the second solar cell 11 may still be a third solar cell to obtain a triple solar cell. However, this is not the case with embodiment I. Nevertheless, only one solar cell can be provided. In addition, the solar cell can contain several functional intermediate layers, for example of ZnO: Al or SiO _x : H, in order to minimize plasmotic effects.

However, on the last solar cell closest to the separator 2 (in this case the second 11), a coupling layer 12 is applied directly, which in turn is designed in multiple layers. It comprises an adhesion-promoting layer 13, a mirror layer 14 and a corrosion-protection layer 15.

The adhesion-promoting layer consists of nickel and is very thin, it preferably consists of a few atomic monolayers of nickel. This corresponds to a layer thickness between 0.5 nm and 5 nm. Due to its low layer thickness, the adhesion-promoting layer is transparent. Alternatively, the primer layer may consist of chromium, tungsten or hafnium. The mirror layer 14 is made of silver and therefore reflects light well. Alternatively, aluminum can be used, which is less reflective but less expensive than silver. Their layer thickness is between 10 nm and 500 nm. The corrosion protection layer 15 is made of nickel, but is thicker than the

Bonding layer (so between 100 nm and 1000 nm) and therefore not transparent. The adhesion-promoting layer 13 directly connects the last solar cell 11 with the mirror layer 14. The adhesion-promoting layer can be applied directly to an Si-based layer or to any functional ZnO or SiO-based functional layer. The corrosion protection layer 15 is applied directly to the mirror layer 14 and, with its other end of the layer, directly adjoins the reaction space 4. It therefore comes into contact with the alkaline aqueous medium contained therein. The nickel of the anticorrosive layer 15, however, is stable in an alkaline medium and protects the entire photoelectrode from attack by the base. Also the Primer layer is durable, so that the mirror layer is not peeled off by the solar cell if the caustic should creep behind the mirror.

The PEC is completed by an electrically conductive connection 16 between front contact (on the light side) of the photoelectrode 1 and counterelectrode 2. On the side of the photoelectrode, this is connected to the transparent, electrically conductive layer 9. In the case of the counter electrode, the place of connection is not so important because the counter electrode is completely metallic and therefore conducts the electric current everywhere. The electrically conductive connection 16 is led around the outside of the PEC. It does not have to

Power electronics between the photoelectrode 1 and the counter electrode 3 are switched because the solar cell provides a photovoltage, which is above that required for water splitting

electrochemical voltage is. Therefore, the electrically conductive connection 16 is, in the simplest case, a cable.

Not shown is a frame that encloses and seals the entire PEC. However, it must allow incident light to the transparent substrate 7, allow inlet and outlet of the aqueous medium and include two separate deductions for hydrogen and oxygen. In operation, the transparent substrate 7 is directed to the sun 0 or other light source. The light 17 enters the PEC through the transparent substrate 7. The light 17 may include several wavelengths 171, 172 also in the invisible area. Through the texture 8 and the transparent, electrically conductive layer 9, the light 17 strikes the first solar cell 10. In this case, the light with the band gap corresponding wavelength 171 and below is adsorbed as much as possible. Longer-wavelength light 172 passes through the first solar cell 10 and is largely adsorbed in the second solar cell 11 in accordance with a smaller band gap to be sensibly selected. Of course, the light 17 also includes even longer wavelength components that are not adsorbed, but the tandem solar cell 10, 11 is for the adsorption of two

Wavelength ranges 171, 172 optimized.

Light 173, which was not adsorbed by the two solar cells in the first pass, falls through the transparent adhesion-promoting layer 13 on the mirror layer 14 and is reflected there. It passes through the two solar cells again (this time in the opposite direction) and is further adsorbed. Light 174, which was still not adsorbed in the second pass, is reflected at the texture 8 of the substrate 7 and thereby prevented from leaving the photoelectrode. The incident light 17 is therefore repeatedly thrown back and forth between the mirror layer 17 and the texture 8, passing through both solar cells 10, 11 several times. This increases the luminous efficacy.

Due to the photovoltaic effect, a potential difference arises in the photoelectrode 1 between the electrically conductive layer 9 and the electrically conductive corrosion protection layer 15. Because the electrically conductive layer 9 is connected to the counterelectrode 3 via the electrically conductive connection 16, there is an identical potential difference also between

Corrosion protection layer 15 and counter electrode 3. This means that due to the structure of the photoelectrode 1, the electrically conductive corrosion protection 15 act as a cathode and the counter electrode 3 as an anode. This causes within the reaction space 4, the electrolysis of the water contained in the aqueous medium. The hydrogen H ₂ formed accumulates on the cathode side of the separator 2, during which the oxygen 0 ₂ formed is accumulated on the anode side of the separator. 2

Hydrogen H ₂ and oxygen 0 ₂ are drawn off separately on both sides of the separator. 2 Due to its gas impermeability, the separator prevents backmixing of hydrogen H ₂ and oxygen 0 ₂ and thus a blast gas explosion (2 H ₂ +0 ₂ -> 2 H ₂ 0).

During operation, the aqueous medium must be continuously pumped through the reaction space in the circulation process in order to ensure effective removal of the electrochemically formed gases (H ₂ and O ₂ ). The aqueous medium thus also assumes the function of a transport medium for O ₂ and H ₂ . The water splitting concentrates the basic

Component (e.g., potassium hydroxide or sodium hydroxide) in the aqueous medium as it passes through the cell. Therefore, the concentrated medium must always be rediluted with fresh water before it re-enters the PEC. The potassium hydroxide or the

However, sodium hydroxide itself is not consumed and therefore does not have to be topped up.

In the overall picture, the PEC absorbs light and water and releases hydrogen and oxygen. The layer structure of the first embodiment I is more apparent from the exploded view in Figure 2.

FIG. 3 shows a second embodiment II of the invention. This is particularly compact since photoelectrode 2 and counterelectrode 3 rest directly on the separator 2. The reaction space 4 is therefore minimal. The separator 2 is made of a material having anion conductive properties. Therefore, it can be called a solid electrolyte. As an alternative to a KOH or NaOH-containing electrolyte, it is also possible to use pure water (pH = 7, conductivity below 1 × 10 ^-4 S / m) as the aqueous medium 4 thus extends essentially in the depressions 18 of the separator 2. The catalytically active substances are present in an ionomer paste 19A, 19K, which is applied on both sides to the separator 2, on the anode and cathode side with the corresponding catalyst Thus, ionomer paste 19a has a different composition than the cathode-side ionomer paste 19k

Exploded view of the second embodiment II (Figure 4) recognizable.

Preparation example

A 800 nm thick, transparent, conductive aluminum-doped ZnO layer is applied to a transparent glass substrate by means of cathode ray evaporation (magnetron sputtering @ 13.56 MHz) of a ceramic ZnO: Al ₂ O ₃ target (1 at.%) At a temperature of 300 ° C, a power density of 2 W / cm ² and an argon pressure of 0.1 Pa deposited. The sample is heated for one hour prior to deposition and the base pressure in the chamber prior to deposition is 8x10 ^"5 Pa. The surface of the ZnO: Al layer is then wet-chemically roughened in dilute HCl (1%) to produce crater-like surface structures scattering visible light (texture).

On the thus prepared ZnO: Al layer are formed two pin junctions consisting of doped and intrinsic layers of amorphous hydrogenated silicon (a-Si: H) by plasma assisted vapor deposition (PECVD) at an excitation frequency of 13.56 MHz using the gases silane , Hydrogen, phosphine and trimethylborane deposited. The intrinsic layer of the first cell is at a substrate temperature of 180 ° C and a silane concentration in the

Gas phase of 4% deposited for 41 minutes so that it has a thickness of 110 nm. The intrinsic layer of the bottom cell becomes at a substrate temperature of 130 ° C at a Silane concentration in the gas phase of 10% for 120 minutes deposited so that it has a thickness of 400 nm. The p-doped layers are deposited at a temperature of 180 ° C with the addition of trimethylborane to the gas phase as a doping source for 90 seconds so that they have a thickness of about 15 nanometers. The same applies to the n-doped layers, but with the addition of phosphine to the gas phase.

By means of electron at a chamber pressure of 3xl0 ^"6 mbar, a 0.6 nm nickel layer is thick, optically almost completely transparent applied to the surface of the second n-doped silicon layer. This provides for improved adhesion of the subsequently also by means of electron applied 200 nm The layer stack is closed by a 100 nm thick, electron beam evaporated closed nickel layer, which protects the underlying layers from the electrolyte and is suitable as a catalyst for the hydrogen evolution in alkaline solution.

From a photoelectrode prepared in this way, a photoelectrochemical cell for light-driven water splitting can be built up as follows:

• The photoelectrode is inserted with its glass side to the outside in a holding frame / tray, whereby these z. B. can be sealed using a flat gasket to the frame.

On the nickel surface of the stack contact a 2 mm sponge electrode is placed, on one side of which (facing away from the stack contact) a catalytically active formulation was applied by means of spray coating and thermally cured / activated.

• An anion-directing membrane is placed on the catalytically active side of the sponge electrode (eg Fumatech FAA-3).

• Another sponge electrode is placed with its functionalized side on the membrane. This sponge electrode is fired by means of a cable to the front contact located at the edge of the solar cell.

• The entire stack is pressed together with a suitable further housing shell and with z. B. a flat gasket sealed against the front holder frame. This is constructively to ensure that there is no gas exchange between the two

Electron spaces can come on the two sides of the membrane. • The two sponge electrodes are perfused with IM KOH. The electrolyte flow simultaneously removes the gas produced during the splitting of water (H2 and 02).

• With this setup, the total efficiencies (solar to

Hydrogen) of> 8%. This means that 8% of the radiant energy introduced by the light is converted into chemical energy of hydrogen.

LIST OF REFERENCE NUMBERS

I first embodiment PEC

II second embodiment PEC

0 sun

1 photoelectrode

2 separator

3 counter electrode

4 reaction space

5K porous nickel-containing structure on the cathode side

5A porous nickel-containing structure on the anode side

6A catalytically active substance on the anode side

6K catalytically active substance on the cathode side

7 substrate

8 texture

9 transparent, electrically conductive layer (front contact)

10 first solar cell

11 second solar cell

12 coupling layer

13 adhesive layer

14 mirror layer

15 corrosion protection layer

16 electrically conductive connection

17 light

171 first wavelength of light

172 second wavelength of light

173 reflected light at the mirror layer

174 light reflected on the texture

18 wells

19K ionomer paste (cathode side)

19A ionomer paste (anode side)

H _{2 is} hydrogen

0 ₂ oxygen

Claims

claims

1. Photoelectrochemical cell for the light-driven production of hydrogen and oxygen from an aqueous medium in a basic medium, with a light side arranged

Photoelectrode, with a counterelectrode arranged on the shadow side, with a separator arranged between the photoelectrode and counterelectrode, and with a reaction space which can be filled with both sides of the separator and in which basic conditions prevail when the aqueous medium is filled in, the photoelectrode being a light side arranged, comprises, which is coated in the direction of the separator with the following layer order:

a) a transparent, electrically conductive layer;

b) at least one partially transparent, layered solar cell based on

silicon;

c) a coupling layer adjacent to the reaction space;

characterized,

that the coupling layer is designed in multiple layers, namely at least

d) with a transparent adhesion-promoting layer applied directly to said or another solar cell and consisting of one of the following metals or of an alloy of one or more of these metals: nickel, chromium, tungsten, hafnium;

e) with a mirror layer applied directly to the primer layer and consisting of one of the following metals or of an alloy of one or more of these metals: silver, copper, aluminum;

f) and with an applied directly on the mirror layer, adjacent to the reaction space corrosion protection layer of nickel or of a nickel-containing alloy.

Photoelectrochemical cell according to claim 1, characterized in that the aqueous medium is water having an electrical conductivity of less than 1 * 10 ^-4 S / m, and that the separator is a gas-impermeable, anion-conducting and electronically insulating polymer electrolyte membrane, wherein the separating active material of the membrane is preferably selected from the following materials: ceramics, polyolefins, polyethers, polyimides, polyamides, polysulfones, wherein said polymers in pure form, as Mixture or as co-polymers or as block polymers or as block co-polymers are used.

Photoelectrochemical cell according to claim 1, characterized in that the aqueous medium has a pH greater than 7, and that it contains water, and at least one hydroxide or carbonate or phosphate or hydrogen carbonate or hydrogen phosphate or nitrate or sulfate Compound contains one of the following elements: lithium, sodium, potassium, magnesium, calcium, strontium, barium; in particular, that the aqueous medium is potassium hydroxide solution or sodium hydroxide solution.

Photoelectrochemical cell according to Claim 3, characterized in that the separator is a gas-impermeable, anion-conducting and electronically insulating membrane, the release-active material of the membrane preferably being selected from the following materials: ceramics, polyolefins, polyethers, polyimides, polyamides , Polysulfone, wherein said polymers are used in pure form, as a mixture or as co-polymers or as a block of polymers or as block co-polymers.

Photoelectrochemical cell according to Claim 1 or according to one of Claims 2 to 4, characterized in that a porous structure made of an electrically conductive material is provided between the anticorrosion layer and the separator, which is electrically contacted at least with the anticorrosion layer.

Photoelectrochemical cell according to claim 5, characterized in that the porous structure consists of a catalytically active in the water splitting material and / or provided therewith.

Photoelectrochemical cell according to claim 6, characterized in that the porous structure is foamed nickel or a textile fabric whose fibers consist of nickel, of carbon or of an alloy with iron, carbon and nickel.

8. Photoelectrochemical cell according to claim 1 or one of claims 2 to 7, characterized in that at least on one side of the separator an ionomer-containing paste is applied, in which in the water splitting catalytically active substances are enriched, and that when filled aqueous medium this impregnates the ionomer-containing paste, such that the reaction space extends at least partially within the ionomer-containing paste.

9. A photoelectrochemical cell according to claim 6 or claim 7 or claim 8, characterized

in that the catalytically active substances are nickel oxide and / or cobalt oxide.

10. Photoelectrochemical cell according to claim 1 or according to one of claims 2 to 9, characterized

in that the photoelectrode has two solar cells applied in layers, each of which in turn comprises a plurality of layers of differently doped silicon and / or different crystallinity or its alloys, wherein the first solar cell is applied directly to the transparent, electrically conductive layer, the second solar cell being instantaneous is applied to the first solar cell, and wherein the coupling layer is applied directly to the second solar cell,

or

the photoelectrode has three solar cells applied in layers, each of which in turn comprises a plurality of layers of differently doped silicon and / or different crystallinity or its alloys, wherein the first solar cell is applied directly to the transparent, electrically conductive layer, wherein the second solar cell is direct is applied to the first solar cell, wherein the third solar cell is applied directly to the second solar cell and wherein the coupling layer is applied directly to the third solar cell.

11. Photoelectrochemical cell according to claim 10, characterized in that silicon-based solar cells are stacked with the same and / or different band gap. Photoelectrochemical cell according to Claim 1 or according to one of Claims 2 to 11, characterized in that at least one solar cell applied in layers is a thin-film solar cell.

Photoelectrochemical cell according to claim 1, characterized in that it is the transparent substrate is an organic or inorganic glass, which is provided on its coated side with a texture, or that the transparent, electrically conductive layer has the texture.

A process for producing a photoelectrochemical cell photoelectrode according to claim 1, wherein a transparent substrate is successively coated as follows: a) with a transparent electroconductive layer;

b) with a silicon-based, partially transparent solar cell, which in turn comprises a plurality of differently functionalized layers of silicon and / or its alloys, which are applied successively;

g) and with an applied directly on the mirror layer, adjacent to the reaction space corrosion protection layer of nickel or of a nickel-containing alloy.

A process for the light-driven production of hydrogen and oxygen from an aqueous medium, using a photo-electrochemical cell according to claim 1, in which the aqueous medium is introduced into the reaction space of the photoelectrochemical cell and light is applied to the transparent substrate of the photoelectrode, so that incident and reflected light in the solar cell is absorbed and converted into an electric current between the photoelectrode and the counterelectrode, which in the reaction space causes cleavage of the photoelectrochemical cell in aqueous medium containing water in oxygen and hydrogen and a separation of oxygen and hydrogen on both sides of the separator causes.