GB2516866A

GB2516866A - Device for hydrogen and electricity production

Info

Publication number: GB2516866A
Application number: GB1313828.4A
Authority: GB
Inventors: Johan Martens; Jan Rong
Original assignee: Katholieke Universiteit Leuven
Current assignee: Katholieke Universiteit Leuven
Priority date: 2013-08-02
Filing date: 2013-08-02
Publication date: 2015-02-11
Also published as: GB201313828D0

Abstract

A device is disclosed that generates hydrogen from ambient air humidity under sunlight without any water oxidation co-catalyst (e.g. IrRuOx). The device comprises at least one semiconductor photoanode which performs the water oxidation without back reaction or deteriorated photoefficiency. The apparatus combines a photoelectrochemical process with a fuel cell for the generation of electricity. The photoelectrolysis of water to produce hydrogen and oxygen uses a semiconductor photoanode and a cathode, both comprising a macroscopic conductor, which is at least partially coated with a nanoscopic conductor. In particular embodiments this device uses an illumination controller, such a controller can be 1) an illumination intensity controller, 2) a filter for visible and infrared light to reduce dehydration or heating up and/or 3) a switch for intermittent periods of illumination of the semiconductor photoanode. In yet another embodiment the membrane electrode assembly (MEA) comprises incorporating hygroscopic silica. The preferred semiconductor material for the photoanode is TiO2 and the preferred nanoscopic conducting material is carbon nanotubes (CNT). Furthermore, plasmonic nanoparticles like silver-gold alloy nanoparticles are deposited on the layer of semiconductor at the photoanode.

Description

DEVICE FOR HYDROGEN AND ELECTRICITY PRODUCTION

Background and Summary

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates generally to energy device or fight driven apparatus for hydrogen production from water vapor and sunlight and transformation of the generated hydrogen into electricity production and, more particularly to a system and method for the production of hydrogen and electricity from ambient air humidity. This energy device can be lightweight.

Several documents are cited throughout the text of this specification. Each of the documents herein (including any manufacturer's specifications, instructions etc.) are hereby incorporated by reference; however, there is no admission that any document cited is indeed prior art of the present invention.

B. Description of the Related Art

Environmental pressure on carbon based fuels stimulates great interest in hydrogen as a secondary energy vector provided it is produced from alternative energy sourcesi. Sunlight driven photoelectrochemical water splitting is one of these appealing alternadves to produce hydrogen. Since the demonstration of photocatalytic water splitting by Fujishima and Honda in 19722 the prospect of a sustainable chemical energy source has spurred steady progress toward practical photoelectrochemicd cells (PECs).

The efficiency of the most advanced photoelectrodes has reached the 5% range3, and the durability of electrodes has been improved substantially4.

PECs bear the promise of affordable solar hydrogen generation devices made of earth abundant metals5. Several photoelectrochemica setups performing solar water splitting without sacrificial chemicals or electrical bias have been reported36 and small solar hydrogen production systems called "artificial leaves" for practical application have been demonstrated Most reported photoelectrochemical cells are wet systems. The need of liquid water for their operation in potential large scale application could be a hurdle in regions with limited water supp'y, and competition with the agricuhure sector where water scarcity has great relevance is a concern. In this respect the idea of using ambient air as a source of water is appealing, in appfications where weight is important the filling of a photoelectrochemical cell with air instead of water or liquid electrolyte will present a significant benefit.

At 20°C air at 100% relative humidity contains 1.5% water. Even a desert such as Sahara has an average relative humidity of 25% and rare are the places on earth where the relative humidity at the hottest time of the day drops below 5%.

Photocatalytic splitting of water vapor is a scientific challenge since reaction rates are critically depending on the presence of a condensed water film on the semiconductor surface for proton conductivity9. The use of air entails the additiond problem of the high concentration of molecular oxygen stimulating the back reaction. Oxygen molecules furthermore are mediators for recombination of electron-hole pairs generated by illumination of a semiconductor such as anatase Ti02, and deteriorate this way the photoefficiency.

Some concepts have been proposed to alleviate this oxygen problem. Application of a chromia shell has been shown to prevent oxygen photoreduction and back reaction on noble metal catalyst particles10. Surface carbonate species on anatase can assume similar function".

Application of an electrical or chemica' bias is another means to counter the negative effects of oxygen molecules, but it compficates the set up and reduces the practical use. In a two-compartment photoelectrochemical cell the back reaction can be prevented by the spatial separation of hydrogen and oxygen gases'2. but oxygen photoreduction and oxygen mediated charge recombination need a solution.

Thus, there is a need in the art for an operational device that under sunlight does generate hydrogen &om ambient air humidity. Present invention provides such solution, without any water oxidation co-catalyst (e.g. IrRuOx), by a hydrogen generating device comprising at least one semiconductor photoanode which surprisingly performs the water oxidation without back reaction and without deteriorated photoefficiency. In a particular embodiment this device to improve efficiency under ambient conditions of air and sunlight comprises an illumination controller, such controller can be 1) an illumination intensity controller, 2) a filter to filter out visible light to reduce dehydration or heating up and/or 3) a switch (illumination I non-illumination switch) to let follow in succession periods of illumination of

I

the at least one semiconductor photoanode with periods of non-illumination (darkness) of the at least one semiconductor photoanode. In yet another embodiment the membrane electrode assembly MEA) comprises incorporating hygroscopic material. By introducing dark periods switched with illuminations periods for instance of Ca. 200-l000s of illumination high photocurrents are maintained. The hydrogen generating device of present invention, with semiconductor photoanode without water oxidation co-catalyst, can combine any of the illumination controflers with the hygroscopic materia' comprised in the MEA. Such an operational device that does generate hydrogen in a cathode compartment and in which a semiconductor performs the oxidation of airborne water vapor in a separate anode compartment has not yet been demonstrated

SUMMARY OF THE INVENTION

In accordance with the purpose of the invention, as embodied and broadly described herein, the invention is broadly drawn to a membrane-electrode architecture in a photoelectrochemical device for production of hydrogen gas from water vapor contained in air for instance ambient air or another gas mixture using a light source such as sunhght. Such photoelectrochemical device can be constructed as a lightweight apparatus. The apparatus is particular suitable for ushg ambient air humidity as a supply of raw material for feeding the process of hydrogen production.

In one aspect of the invention, the hydrogen gas is produced at the cathode side of a two-compartment photoelectrochemical cell fed with ambient air and illuminated at the photoanode.

Another aspect is that the photoelectrochernical device of present invention does not need the two co-catalysts, water oxidation catalyst and proton reduction catalysts, for production of hydrogen gas from ambient air humidity.

The device has an electrode architecture, which can be robust. This electrode architecture comprises a proton exchange membrane and electric conducting macrostructured material on which electric conducting nanostructured material is mounted or which is coated with electric conducting nanostructured material.

At the photoanode the nanostructured conducting materid comprises a continuous nanocoating of semiconductor oxides which is photoactive and also protects the conducting substrate from being oxidized.

At the cathode the nanoconducting material comprises proton reduction hydrogen gas generating electrocatalyst nanoparticles.

Water adsorbing stubs composed of water adsorbing and water condensing inorganic nanoparticle aggregates are implanted in the proton exchange membrane for maintaining hydration.

Another aspect of the invention is the hydrogen production device composed of lightweight materials and requiring no other input but ambient air and sunlight. The operation of the device with air instead of for instance liquid water presents another weight gain. It is especially suitable for automotive applications and for aircrafts and aerospace application.

The cell can be run in reverse mode in the dark to produce electricity from stored hydrogen further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detafled description.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as Jaimed.

Some embodiments of the invention are set forth in claim format directly below: 1. An energy device that produces hydrogen gas from a water vapor containing gas mixture using sunlight.

2. An energy device that produces hydrogen gas from a water vapor containing gas mixture using solar energy and that converts hydrogen into electricity upon demand.

3. The energy device according to embodiment I and 2 in which the water vapor providing the Fl atoms for hydrogen gas production are contained in a gas mixture comprising molecular oxygen.

4. The energy device according to embodiment 1 in which the water vapor providing the H atoms for hydrogen gas production are contained iii ambient air.

5. The energy device according to embodiment 1 comprising a proton exchange membrane, macroscopic electric conducting electrode material and nanoscopic dectric conducting material characterized in that the nanoscopic electric conducting material of the photoanode is entirdy coated with semiconductor oxide nanolayer.

6. The energy device according to embodiment I comprising a proton exchange membrane, macroscopic electric conducting electrode material and nanoscopic electric conducting material characterized in that the nanoscopic electric conducting material of the photoanode is entirdy coated with a titanium dioxide layer.

7. The energy device according to embodiment I comprising a proton exchange membrane, macroscopic electric conducting electrode material and nanoscopic electric conducting material characterized in that the nanoscopic electric conducting material of the photoanode is entirely coated with semiconductor oxide layer with a thickness of more than 3 nm and less than 15 nm.

8. The energy device according to embodiment 1 comprising a proton exchange membrane, macroscopic electric conducting electrode material and nanoscopic electric conducting material characterized in that the nanoscopic electric conducting material of the cathode is supporting nanosize pktinum particles.

9. A multifunctional energy device according to embodiments 4 and 7 prepared using atomic layer deposition and plasma enhanced atomic layer deposition techniques.

Detailed Description

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equiva'ents thereof. c

The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents thereof.

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims.

The drawings described are only schematic and are non-limiting. In the drawings. the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes.

The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof Thus, the scope of the expression "a device comprising means A and B" shotfid not be limited to the devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B. Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various pbces throughout this specification are not necessarily all referring to the same embodiment, but may. Fur hermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details.

In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein.

It is intended that the specification and examples be considered as exemplary only.

Each and every claim is incorporated into the specification as an embodiment of the present invention. Thus, the claims are part of the description and are a further description and are in addition to the preferred embodiments of the present invention.

Each of the claims set out a particular embodiment of the invention.

The following terms are provided solely to aid in the understanding of the invention.

The object of the present invention is to provide a method for generating hydrogen from ambient air humidity and light as energy source. We discovered such an electrode architecture that can be operated using hydrated ambient air, and even ambient air as a source of water.

According to one embodiment this effective architecture of present invention comprises a proton exchange membrane, such as Nafion. flanked with macroscopic electric conducting material, such as carbon paper. On the macroscopic conducting material nanoscopic conduction material is attached, such as muhiwall carbon nanotubes grafted on carbon paper.

These conductive elements adjacent to the proton exchange membrane are either photoanode or cathode. The carbon nanotubes at the photoanode are coated with a continuous layer of semiconductor, such as oxides (e.g. Ti02, Fe203, ZnO, NaTaO. W03. BiVO. SrTiO. Cu,O).

sulfides (e.g. CdS, ZnS), nitrides (e.g. GaN, TaON) or their combinations. Optionally, plasmonic nanoparticles the continuous semiconductor layer of the photoanode can be as a method for increasing solar cell efficiency by forcing more light to be absorbed. Silver-gold alloy nanoparticles are a typical example. On the cathode the electric conducting multiwall carbon nanotubes are coated with platinum nanoparticles. The membrane-electrode assembly is mounted in a two-compartment photoelectrochemical cells. The cathode compartment can either be closed to accumulate the produced hydrogen, or left open, or purged with inert gas to transport and collect the produced hydrogen.

This embodiment of the invention advantageous'y comprises that when the anode is fed with a hydrated gas stream, such as hydrated air, or simply ambient air without any hydration operation and the photoanode is illuminated that hydrogen is produced.

This embodiment of the invention advantageously comprises that when electricity is needed, the illumination is interrupted and the cell converts the previously produced solar hydrogen into electricity like a ftc] cell.

Some of the techniques described above may be embodied as an apparatus according to the invention that when operationa' performs three tasks that otherwise would be performed by three distinguished devices: (i) a photovoltaic device to produce electncity from sunlight, (ii) an electrolyzer to electrolyze water to produce hydrogen, and a (iii) fuel cell to convert the stored solar hydrogen into electricity when needed. This embodiment of the invention advantageously comprises that the integration of functions minimizes space requirement which is advantageous for applications such as in the automotive sector and especially cars, trucks, traiis, busses, aircrafts, spacecrafts, and off-road vehicles and energy devices. An additiond advantage of the apparatus of present invention operation with air instead of liquid water according to the current systems represents an important weight saving. An additional advantage of the device according to the iiivention for small scale, household and automotive application is that the hydrogen can be produced at low concentration in an inert gas, such as below 4% in nitrogen. In and around the device it is unlikely that the lower explosion Umit of about 4 volume % hydrogen in air will be reached, which efiminates the explosion hazard.

Large scale systems according to the invention could be equipped with a hydrogen concentration and pressurization unit and be combined with CO2 mitigation systems. Mixtures of solar hydrogen and carbon dioxide could then be converted into gaseous or liquid ftes such as methane, carbon monoxide, formic acid, methanol, formaldehyde, or molecules with more than 1 C-atom and liquid mixtures suitable for substitution of fossil carbon sources.

Example I: Demonstration of the oxy2en problem usin2 standard electrodes The photoelectrochernical device consisted of a graphite flow-field plate at the cathode compartment and a fused silica plate at the anode side. Both the fused-silica plate and the graphite plate contain serpentine flows providing an optimal contact with the electrode surface'3. In this reference experiment, the flow of air through the anode compartment is 12.5 mI/mm. At the cathode side either there is no flow, or a nitrogen flow is applied at 25 mI/mm.

Air was hydrated by bubbling through a reservoir of water at room kemperature. The device was illuminated by means of a Xe lamp (66984, Oriel, Newport, 200 nrn < A < 1000 nm) adjusted such that the incident intensity equals 100 mW/cm2. The photocurrents were recorded using a potentiostat (Versastat 4)(Princeton Applied Research. Oak Ridge, TN) and Versastudio electrochemical analysis software. The outlet of the cathode compartment was analyzed with a Quantitative Gas Analyzer (QGA)(Hiden Analytical, Warrington, UK). The dectrodes consisted of conducting and chemically inert Toray paper. The cathode was prepared by depositing platinum on Toray paper via drop-casting of a suspension of Pt/CB and Nafion monomer in ethanol on Toray paper, followed by air drying at 60°C. The Pt loading was 0.03 mg/cm2 and the Nafion coating 0.01 mg/cm2. The anode was obtained by dectrophoretic deposition of commercial P25 Ti02 on Toray paper according to the method of Boccaccini et aL14. A home-made electrophoresis cdl was used consisting of Inox sheets (A1S1321) held 1.0 cm apart by a Teflon® holder. A circular hole in the Teflon® mask with a diameter of 1 cm ensured that an area of 0.78 cm2 of the Toray paper was coated. The potential was controlled by a potentiostat (Versastat 4, Princeton Research Systems). An electric field of 10 V/cm was applied for 60 s, After deposition, the electrodes were dried in air for 24 h and, subsequently, annealed in air for 2 h at 450°C, attahied at a ramping rate of 10°C/mm. The deposited amount of Ti02 was 0.5 mg/cm2.

Nafion® membranes were preconditioned according to an established procedure. Nafion® membrane was boiled in 5% hydrogen peroxide for I h and then treated in alternating baths of boiling 1 M H2S04 and boiling water for 2 and 3 h, respectively. After the last treatment in boiling water, the membrane was kept ii MilliQ water in the dark at room temperature. Prior to using a membrane, it was removed from the water. quickly blotted to remove bulk water and then kept in an Inox drying holder at room temperature for 24 h. Membrane-electrode assemblies were prepared by fixing the anode and cathode sheets on opposite sides of the pretreated Nafion® membrane, and hot-pressing between Teflon® liners at 5 MPa and 130°C for 2.5 mm. After hot-pressing, the assembly was carethily removed from between the liners and allowed to cool in an Inox holder.

Molecular oxygen from the air fed at the photoanode has a dramatic impact on the photocurrent obtained using the reference membrane-electrode assembly (Figure 1). The photocunent using air is systematically lower than using nitrogen calTier gas. Feeding the anode compartment with hydrated thert gas instead of air resulted in a photoculTent stabilizthg around 35 iA/cm2. This experiment confirming the dramatic impact of molecular oxygen contained in the air.

Example 2: Overcoming of the oxven problem using a membrane-electrode assembly according to the invention A membrane-electrode assembly according to the invention was prepared by grafting multiwafi carbon nanotube forests on the carbon paper and coating them with a continuous layer of a Ti02 with a thickness of at least 3 nm. and preferentially at least 5 nm and less than nm for the photoanode. Such continuous Ti02 layer can be achieved by Atomic Layer Deposition. Atomic layer deposition of Ti02 on the grafted carbon nanotubes was performed at 100°C. Tetrakis (dimethylamido) titanium TDMAT) (99.999% Sigma-Aldrich) and 03 generated by an ozone generator (Yanco Industries LTD) were alternatingly pulsed into the ALD chamber at pressures of 0.3 and 0.5 mbar, respectively. The concentration of ozone in the flux was 145 gImL. The pulse (20 s) and pump time (40 s) were optimized to allow for a uniform coating of Ti02 along the entire length of the MWCNTs and to prevent the occurrence of chemical vapor deposition type reactions. To achieve a 7 nm thick coating, 200 cycles were applied. The amorphous Ti02 was crystallized to anatase by heating at 450°C.

The photocathode was prepared by applying Plasma Enhanced Atomic Layer Deposition of platinum on supported CNT forests using (methylcyclopentadienyl)-trimethylplatinurn (MeCpPtMe3) precursor'5.

The photoanode and cathode were mounted on Nafion membrane as explained in Example I. SEM images were obtained with a Nova NanoSEM45O (EEl). The samples were mounted on aluminum stubs (Figure 2).

A photodectrochemical experiment was run using this membrane-electrode assemNy according to the invention (Figure 3). The use of air instead of inert nitrogen gas to introduce water vapor at the anode did not deteriorate the photocurrent demonstrating the achievement of an oxygen tolerant membrane-electrode assembly.

Example 3: Remediation of the membrane dehydration problem under illumination Maintaining the proton exchange membrane of the device hydrated is a critical issue. The relative air humidity determines membrane moisture content and proton conductivity from anode to cathode compartment. Under illumination, the temperature in the anode compartment is higher than in a reference dark condition (Figure 4) so relative humidity drops and water evaporates from the membrane. As a consequence, the photocurrent being representative of the hydrogen formation rate drops over time (Figure 5).

To prevent membrane dehydration, air flow in the anode compartment can be interrupted to counter dehydration (Figure 6).

In different embodiments of present invention four approaches to mitigate the membrane dehydration problem can be used: 1. Diminish the illumination intensity Under reduced illumination intensity, the membrane is less heated and dehydration is less of a problem.

2. Filter out ineffective wavelengths from the light source Visible light is only little absorbed by the Ti02 semiconductor and contributes little to hydrogen formation, but it produces heat. It can be filtered out to reduce heating up (Figures 4,7,8).

3. Modify the membrane electrode assembly by incorporating hygroscopic silica particles Hygroscopic particles dispersed in the proton exchange membrane capture moisture from the air and keep the membrane hydrated. Nafion with hygroscopic particles was added as a top layer on both sides of the Nafion membrane.

Many inorganic materials have water adsorbing properties, such as silica and alumina gels, zeolites, clays. Capillary condensation of water can be achieved in pores with nanometer dimensions, and especially in pores with diameters from 2 to iOO nm. Such pores can be present inside individual partides, or can be created by the interstitia] voids of aggregated nanoparticles.

Silicalite zeolite nanopowder introduced into the proton exchange membrane was found to be a means of adsorbing water from the air fed into the anode compartment. The introduced nanopowder was found concentrated at suiface stubs (Figure 9). The hydroxylated surfaces of the zeolite nanopowder favors water adsorption and capillary condensation in the interstitial voids and concentration of water in the membrane at reduced relative humidity. The operation of a cell with a proton exchange membrane equipped with water adsorbing stubs at 100% and 40% relative humidity is demonstrated in Figure 10.

4. Switch between illumination and dark regeneration In the dark, relative humidity increases in the anode compartment as temperature drops back to ambent temperature and moisture levels in the membrane are restored (Figures 8.10). By introducing short dark periods after ca. 200-l000s of illumination, an initial high photocurrent can be restored.

Any of these approaches can be combined in different embodiments of present invention.

The cell according to the present invention has high Faradaic efficiency. Within experimental error the Earadaic efficiency is close to 100%. Quantification of the hydrogen production using a membrane electrode assembly modified with hygroscopic nanopowder is shown in Figure 11.

Example 4: Operation of the cell in reverse mode for electricity production The two-compartment photoelectrochemical cell according to this invention can be operated in the dark in reverse mode to produce electricity from hydrogen with air as the oxidant. Open circuit potentials are relatively high for such system under ambient conditions, even at low hydrogen concentrations (Figure 12). Short circuit current is relatively low (Figure 13). This culTent can be increased by adding noble metal catalyst nanoparticles to the anode surface which in the hydrogen production mode of the cell can serve the purpose of plasmonic antennae.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

Drawing Description

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: Figure 1. is a graphic on the photocurrent in an experiment using conventional reference membrane-electrode assembly mounted in a two-compartment photoelectrochernical device illuminated with UV light from 5 to 125 s. The anode compartment was either fed with hydrated ambient air (Air anode trace) or hydrated N2 (N2 anode trace). The lower current using air demonstrates the detrimental influence of molecular oxygen on performance.

Figure 2 provides the following Scanning Electron Microscopy (SEM) photos: (a)carbon fiber mesh coated with MWCNTS; ), dense forests of the CNTs present on all sides of the carbon fiber; (b) magnified image of the multiwall carbon nanotubes on top of a carbon fiber; (c) magnified image of multiwall carbon nanotubes in the area between carbon fibers; (d, e) Pt particles visualized as white spots coated on the multiwall carbon nanotubes supported on the carbon fiber; (f) multiwall carbon nanotubeswith contirnious conformal Ti02 coating.

figure 3 is a graphic on the photocurrent in an experiment using membrane-electrode assembly according to the invention mounted in a two-compartment photoelectrochernical device illuminated with UV light from 5 to 65 s. The photoanode compartment was either fed with hydrated ambient air (Air anode trace) or hydrated N2 (N2 anode trace). Illumination intensity: 50 mW/cm2.

Figure 4 is a graphic on the internal temperature of the anode compartment of a cell under two different illumination intensities (50 mW/crn2 and 100 mW/cm2) and using filters for specific spectral ranges.

Figure 5: is a graphic on the current when the cell was illuminated with a broad spectrum Xe lamp at 50 mW/cm2. Current drops with time due to dehydration of the Nafion membrane.

Figure 6 is a graphic showing the culTent when the cell was illurniiiated with a broad spectrum Xe lamp at 50 rnW/cm2. Indicated are the anode (hurniditied) air flow rates, in mi/mm. When there is no flow in the anode compartment (0 mI/mm). the photocurrent is higher than in presence of flow. The higher the flow rate, the lower the photocurrent.

Figure 7: is a graphic that displays the current in a cell that was illuminated with a broad spectrum Xe bmp at 50 mW/cm2. When a UY band pass filter is added iii front of the light source, ineffective visible light is filtered and cell temperature drops (Figure 4). As a consequence, photocurrent is more stable over time.

Figure 8: is a graphic that displays the current in a cell that was illuminated with a broad spectrum Xe bmp at 100 mW/cm2, fitted with an AMI.5 solar simubtor filter. Dehydration of the proton exchange membrane is less pronounced under such reduced illumination, and dehydration can be further prevented by using additiona] filters or interrupting fight periods by dark periods. Dark periodes were present in the simulated solar experiment (100 mW/cm2) at 600 s, 1250 s and 1950 s. The intensity of the filtered light was only 60 rnW/crn2, which explains the lower current in this instance.

Figure 9 provides SEM pictures of zeolite nanopowder aggregate particles introduced as water adsorbing stubs in the proton conducting membrane (Nafion) of the membrane-electrode assembly.

Figure 10 demonstrates the current in a cell that was illuminated with a broad spectrum Xe lamp at 50 mW/cm2.equipped with water adsorbing stubs. Intermittent dark periods help to regenerate the membrane's moisture content and high photocurrents are maintained.. In the "untreated air" experiment the photoanode was fed with ambient air at 40% relative humidity at 27°C Even using air with such low relative humidity, high and stable photocurrent is obtained.

Figure 11 illustrates hydrogen production measured by mass spectrometry and estimated from the integrated photocurrent assuming 100% Faradaic efficiency. The cell was illuminated with a broad spectrum Xe lamp at 50 mW/cm2 for the indicated imes. During illumination, anode and cathode were sealed. The anode was filled with humidified air and the cathode was filled with dry nitrogen. After the illumination period, the light source was turned off and the cathode compartment was immediately sampled by a mass spectrometer.

Figure 12 illustrates the energy device run in electricity production mode. The cell was flushed with 12.5 mI/mm humidified air at the anode and 25 m/min dry nitrogen/hydrogen mixture (controlled with mass flow controllers) at the cathode. Hydrogen concentration was monitored with the mass spectrometry and open circuit potential was measured.

Figure 13 illustrates the energy device that runs in electricity production mode. The cell was short circuited with a 97 % hydrogen in nitrogen flow of 25 m/min at the cathode and 12.5 mllmin humidified air at the anode.

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Claims

Claims What is claimed is: 1. An apparatus for hydrogen production from ambient air humidity and light and for transformation of the generated hydrogen into electricity, comprising: at least one semiconductor photoanode comprising macroscopic electric conducting material coated at least in pail by nanoscopic conduction material, the photoanode being positioned to receive light and receiving ambient air, furthermore comprising at least one cathode comprising macroscopic electric conducting material coated at least in part by nanoscopic conduction material, wherein photoanode and cathode being conductivdy interconnected for electron flow and farther being separated by a proton exchange membrane or a membrane electrode assembly (MEA) comprising such proton exchange membrane for the proton flow there through to the cathode.
2. The apparatus according to claim 1, thrther comprising an element to switch the operation of hydrogen gas production from water vapor in the ambient air humidity to converting hydrogen into electricity.
3. The apparatus according to any one of the previous claims, whereby when operational the water vapor contained in a gas mixture comprising molecular oxygen provides the H atoms for hydrogen gas production and the photoanode is illuminated by sunlight.
4. The apparatus according to any one of the previous claims, characterized in that it is without any water oxidation co-catalyst, such frRuOx.
5. The apparatus according to any one of the previous claims, comprising furthermore an illumination controller.
6. The apparatus according to claim 5, whereby the illumination controller is an illumination intensity controller.
7. The apparatus according to claim 5, whereby the illumination controller is a filter to filter out visible and infrared light.
8. The apparatus according to claim 5, whereby the illumination controller is a switch (illumination I non-illumination switch) to let follow in succession periods of illumination of the at least one semiconductor photoanode with periods of non-illumination (darkness) of the at least one semiconductor photoanode.
9. The apparatus according to any one of the previous claims, whereby the membrane electrode assembly comprises hygroscopic nanoparticle aggregates.
10. The apparatus according to any one of the previous claims 1 to 9, characterized in that the nanoscopic electric conducting material of the photoanode is entirely coated with semiconductor oxide layer.
11. The apparatus according to claim 10, characterized in that the semiconductor oxide layer comprises titanium dioxide.
12. The apparatus according to claim 10, characterized in that the nanoscopic electric conducting material of the photoanode is entirely coated with a titanium dioxide nanolayer.
13. The apparatus according to any one of the claims 10 to 12, whereby the coating nano layer has a thickness of more than 3 nm and less than 15 nrn.
14. The apparatus according to any one of the previous claims I to 13, characterized in that the nanoscopic electric conducting material comprises carbon nanotubes.
15. The apparatus according to any one of the previous claims 1 to 13, characterized in that the nanoscopic electric conducting material consist essentially of multiwall carbon nanotubes.
16. The apparatus according to any one of the previous claims I to 13, characterized in that the nanoscopic electric conducting material is composed of carbon nanotubes.
17. The apparatus according to any one of the previous claims 14 to 16, characterized in that the carbon nanotubes at the photoanode are coated with a continuous layer of semiconductor of the group of the oxides consisting of Ti02. Fe,O, ZnO, NaTaO.W03, BiVO4, SrTiO3, Cu20 or of the group of sulfides consisting of CdS and ZnS) or of the group of nitrides consisting of GaNand TaON or of their combinations.
18. The apparatus according to any one of the previous claims 14 to 17, characterized in that plasmonic nanoparticles and in particular silver-gold alloy nanoparticles are deposited on the continuous layer of semiconductor at the photoanode.
19. The apparatus according to any one of the previous claims, characterized in that catalytic nanosize platinum particles are deposited on the nanoscopic electric conducting material of the cathode.
20. The apparatus according to any one of the previous claims, characterized in that membrane-electrode assembly is mounted in a two-compartment photoelectrochemical cell.
21. The apparatus according to any one of the previous claims I to 20, characterized in that the cathode compartment is closed to accumulate the produced hydrogen.
22. The apparatus according to any one of the previous claims 1 to 20, characterized in that the cathode compartment is left open or is foreseen from an inlet and outlet to purge with inert gas to transport and collect the produced hydrogen.
23. The apparatus according to any one of the previous claims, the apparatus being integrated in a vehicle of the group consisting of car. truck, train, buss, aircraft, spacecraft and off-road vehicles.
24. The apparatus according to any one of the previous claims, characterized in that when operational hydrogen is produced at low partial pressure below the lower explosion limit.
25. A method of operating the apparatus according to any one of the previous clams i to 24 by introducing non illuminating photoanode periods (dark periods) switched with illuminating photoanode periods for instance of ca. 200-l000s of illumination to obtain enhanced hydrogen production rates.
26. The method according to claim 25, whereby the apparatus produces hydrogen gas from a water vapor containing gas mixture using solar energy during the photoanode illuminations periods and that converts hydrogen into electricity during the darics periods upon demand.
27. Use of the illumination controller according to any one of the claims 5 to 7 to control or reduce dehydration of the membrane dectrode assembly or the proton exchange membrane and/or to control or reduce heating up of the semiconductor photoanode.