EP0704555B1 - Aluminium electrode - Google Patents

Description

The invention relates to the use of an aluminum foil for the chemical reduction of liquid and / or gaseous components such as CO ₂ , and / or as a detector for electromagnetic radiation, for example in the UV range, using the photoemission process (photo effect).

The photo effect is the removal of electrons from the inside of a solid through its surface into the surrounding medium, air or vacuum, by irradiation with electromagnetic radiation such as light, X-rays or γ-rays. In this so-called external photo effect, an insulated suspended metal plate, which is irradiated with electromagnetic radiation, for example in the UV range, charges to an electrical potential if it is ensured that the released electrons are sucked off by an electric field. The number of photoelectrons or the current strength of the photocurrent formed by the photoelectrons is proportional to the frequency of the absorbed light intensity when exposed to monochromatic electromagnetic radiation. The kinetic energy of the released photoelectrons depends on the frequency of the incident electromagnetic radiation and on the so-called work function of the irradiated metal.

The invention proposes the use of an aluminum foil for the chemical reduction of liquid and / or gaseous Components such as CO ₂ and / or as a detector for electromagnetic radiation, for example in the UV range, using the photo effect. This goal is essentially achieved by subjecting the aluminum foil to a surface treatment to increase the surface roughness, exposing the roughened aluminum foil as a negative electrode to a potential voltage in an electrolyte bath which may contain the reducing liquid and / or gaseous components, and exposing the roughened to a potential voltage subjecting aluminum foil in the electrolytic bath to a photoemission process, for example using the electromagnetic radiation to be detected. An aluminum foil prepared in this way is particularly suitable for the chemical reduction of liquid and / or gaseous components and / or as a detector for electromagnetic radiation, since it has surprisingly been shown that a surprisingly high quantum yield is obtained even when relatively long-wave electromagnetic radiation acts on the aluminum foil is achievable. The quantum yield is defined as the ratio of the number of emitted electrons to the number of incident photons. Liquid components can be easily reduced by means of the strongly reducing emitted photoelectrons. There is also the possibility of using such an aluminum foil to reduce very stable gaseous substances such as CO ₂ or N ₂ by means of the photoelectrons emerging from the aluminum foil.

According to a preferred embodiment of the invention, the aluminum foil is roughened mechanically, for example by sandblasting, by electromechanical polishing and / or by electrochemical etching. These measures have a positive influence on the quantum yield.

According to a further embodiment of the invention, the surface of the aluminum foil is provided with a roughness corresponding to a roughness factor between 1.75 and 3.

An aluminum foil with a capacity between 0.5 and 2.0 µF cm ^-2 (at + 8 V (MSE)) is advantageously used.

According to another advantageous development of the invention, the surface of an untreated aluminum foil is enlarged by a surface treatment, in particular by electrochemical etching or the like, by a factor (surface enlargement factor (SEF) between approximately 10 and approximately 40.

It has proven to be advantageous that the surface of the aluminum foil is treated with perchloric acid and / or ethanol to enlarge the surface.

As an alternative or in combination with this surface treatment of the aluminum foil with perchloric acid and / or ethanol, the surface, in particular for blasting, can be treated with aluminum particles with a grain size or an average diameter between 1 μm and approximately 45 μm.

As an electrolyte bath, solutions with the exclusion of aggressive anions, e.g. Halogens used.

According to a further feature of the invention, the electrolyte bath advantageously has a pH between about 5 and about 10.

In the context of this invention, the term electrolytic bath also includes baths which have gaseous components, in particular CO ₂ and / or N ₂ , which can also be reduced by means of the aluminum foil using the photo effect.

The amount of potential voltage that can be applied to the aluminum foil is preferably set to values below approximately 2 volts. This measure advantageously allows the work function to be overcome by the photoelectrons when exiting the aluminum foil to be reduced. As a result, there is also the possibility of using long-wave electromagnetic radiation to release the photoelectrons from the aluminum foil, in which case a large number of suitable electromagnetic radiation sources can be used.

The use of electromagnetic radiation in the UV range has proven to be particularly advantageous.

In a special application, electromagnetic radiation with a wavelength λ of approximately 300 nm was advantageously used.

According to a special embodiment, a liquid electrolyte bath, a potential voltage of approximately 1.8 to 1.9 volts and an electromagnetic radiation with a wavelength λ of approximately 300 nm are used. Surprisingly, a high quantum yield of approximately 2% to approximately 4 was obtained under these conditions % reached.

Because of the high quantum yield, the aluminum foil is advantageously suitable for use as a detector for electromagnetic radiation, wherein the aluminum foil is exposed to electromagnetic radiation, in particular UV radiation, and the photocurrent is measured. Because of the very high quantum yield, a particularly sensitive measuring instrument or a sensitive detector for electromagnetic radiation is therefore made available.

Further objectives, features, advantages and possible uses of the present invention result from the following description of exemplary embodiments.

In order to use an aluminum foil for, for example, the chemical reduction of liquid or gaseous components or as a detector for electromagnetic radiation, the aluminum foil is subjected to a surface treatment in order to enlarge the effective surface or the surface roughness. The aluminum foil is then introduced as a negative electrode into an electrolyte bath and a potential voltage is applied. If the aluminum foil in the electrolyte bath is exposed to electromagnetic radiation, preferably with wavelengths in the UV range, an emission of photoelectrons from the aluminum foil can be observed directly into the electrolyte bath, provided the aluminum foil is exposed to a suitable surface treatment and placed on a suitable potential voltage and has been exposed to electromagnetic radiation of a suitable wavelength. This phenomenon of the emission of photoelectrons from the aluminum foil directly into the electrolytic bath has some similarities with the photo effect, as can be detected at the boundary layer between a metal and the vacuum when electromagnetic radiation hits the metal surface. However, the photon-induced emission of electrons from an aluminum foil immersed in an electrolyte bath also has the following different aspects:

wobei E_th (0) derjenige Energieschwellwert (entsprechend der sogenannten Austrittsarbeit) bei einer Potentialspannung von 0 bezogen auf die elektrochemische Skala ist und der Term eV die Potentialspannung der Aluminiumfolie in dem Elektrolytbad bezogen auf eine Referenzelektrode angibt. Ersichtlich variiert der Energieschwellwert E_th (eV) in Abhängigkeit von der angelegten Potentialspannung. Der wesentliche Unterschied zu dem Fotoeffekt an einer Metall/Vakuum-Grenzschicht ergibt sich aus der Tatsache, daß die angelegte Potentialspannung zu einer Polarisation der Metall/Lösung-Grenzfläche führt und die Funktion der Austrittsarbeit (W_Me/Sol) eines in eine Lösung eingebrachten Metalls im wesentlichen linear beeinflußt.At the boundary layer between the metal surface and the electrolytic solution, an electrical double layer builds up, at which the whole potential voltage with which the aluminum foil is applied, drops. From this it follows that a further variable influences the photoemission of the electrons from the aluminum layer into the electrolyte bath. Compared to the photo effect at a metal / vacuum interface, the energy threshold for photoelectron emission changes according to the equation $${\text{E}}_{\text{th}}{\text{(eV) = E}}_{\text{th}}\text{(0) - eV,}$$

where E _th (0) is that energy threshold (corresponding to the so-called work function) at a potential voltage of 0 based on the electrochemical scale and the term eV indicates the potential voltage of the aluminum foil in the electrolyte bath based on a reference electrode. As can be seen, the energy threshold E _th (eV) varies as a function of the applied potential voltage. The main difference to the photo effect on a metal / vacuum interface results from the fact that the applied potential voltage leads to polarization of the metal / solution interface and the function of the work function (W _{Me / Sol} ) of a metal introduced into a solution influenced essentially linearly.

While the emission of a photoelectron from a metal into the vacuum can be interpreted as a purely physical phenomenon without chemical reactions following the emission of the electrons, this is different in the case of a metal / solution interface. Then the electrons emitted due to the photo effect get into the solution or the electrolyte bath and initiate a series of chemical reactions. The end result is a chemical reduction of the liquid or gaseous components contained in the electrolyte bath.

It is relatively difficult to estimate the intensity of the photocurrent at a metal / solution interface. Under Disregarding surface enlargement effects, which are usually due to the surface roughness and / or electrons generated on the surface due to plasma vibrations of the metal, model calculations give quantum yields in the order of 10 ^-5 to 10 ^-4 for different metal / electrolyte surfaces.

In the special case of an aluminum / vacuum interface, quantum yields of about 4% could be measured with an energy of the incident electromagnetic radiation close to the plasma frequency (hγ = 10 eV). On the other hand, an emission limit value close to hγ = 2 eV could be determined for an aluminum / electrolyte interface, which is due to a reduction in the metal work function due to the applied potential voltage. The work function for the boundary layer / aluminum / electrolyte is approximately h γ = 4.15 eV. A further reduction in the work function at the aluminum / electrolyte bath interface could not be measured, since a strong hydrogen evolution at potential voltages more negative than -1.95 volts (based on MSE) starts in the electrolyte bath.

As can be seen from the table at the end of the description, an increase in the photocurrent can be achieved due to a suitable surface treatment of the metal. It is therefore appropriate to roughen the aluminum foil mechanically, for example by sandblasting, by electromechanical polishing or by electrochemical etching or a combination of these processes. In particular, electropolishing of the surface of the aluminum foil by means of perchloric acid and / or ethanol has proven itself, the surface of the aluminum foil being mechanically polished in a subsequent step with aluminum particles with a diameter between approximately 1 μm and approximately 45 μm. The surface of the aluminum foil has a roughness corresponding to a roughness factor between 1.75 and 3. These roughness factors are determined by measuring the capacity of the aluminum foil at 9 volts (MSE). With the surface treatment methods shown, the surface of the aluminum foil was increased by a factor (surface enlargement factor SEF) between approximately 10 and approximately 40.

The electrolytic bath consists of solutions that do not contain aggressive anions, e.g. Halogens. The pH of the electrolyte bath is in a range between about 5 and about 10. Because of the hydrogen evolution mentioned above in the electrolyte bath, the magnitude of the potential voltage is set to values below about 2 volts. The incident electromagnetic radiation is wavelengths in the UV range, in particular radiation with a wavelength of λ of approximately 300 nm was used. This corresponds to a photon energy of hγ = 4 eV. Under these conditions, a quantum yield of approximately 2% to approximately 4% could be achieved in the permanent state, i.e. in the steady state of the system.

If the extremely strong reducing effect of the emitted photoelectrons is taken into account, this system can also be used to reduce very stable gaseous substances, such as CO ₂ or N ₂ . Another type of use of the system described is the use of the aluminum / solution interface as a detector for electromagnetic radiation, in particular in the UV range, in which a high quantum yield can be achieved.

Claims (15)

1. Use of an aluminium foil for the chemical reduction of liquid and/or gaseous components such as CO₂, and/or as a detector for electromagnetic radiation, eg. in the UV range, where

   - the aluminium foil is exposed to surface treatment to increase the surface roughness,

   - the roughened aluminium foil, as a negative electrode in an electrolyte bath containing the liquid and/or gaseous components to be reduced, is exposed to a potential voltage, and

   - the roughened aluminium foil receiving a potential voltage in the electrolyte bath undergoes a photo-emission process, eg. under application of the electromagnetic radiation to be detected.
2. Use according to claim 1, characterised in that the aluminium foil is roughened mechanically eg. by sand blasting, by electromechanical polishing and/or by electrochemical etching.
3. Use according to claim 1 or 2, characterised in that the surface of the aluminium foil has a roughness corresponding to a roughness factor of between 1.75 and 3.
4. Use according to any of the previous claims, characterised in that an aluminium foil is used with a capacitance of between 0.5 and 2.0 µF cm^-2 (at + 8 V (MSE)).
5. Use according to any of the previous claims, characterised in that the surface of the aluminium foil is enlarged by surface treatment, in particular by electromechanical etching or similar, by a factor (surface enlargement factor SEF) of between 10 and approximately 40.
6. Use according to any of the previous claims, characterised in that the surface of the aluminium foil is treated with perchloric acid and/or ethanol.
7. Use according to any of the previous claims, characterised in that for surface treatment of the aluminium foil, in particular for polishing, aluminium particles are used of a grain size or an average diameter of between approximately 1 µm and approximately 45 µm.
8. Use according to any of the previous claims, characterised in that solutions, excluding aggressive anions eg. halogens, are used as an electrolyte bath.
9. Use according to any of the previous claims, characterised in that the electrolyte bath has a pH value of between approximately 5 and approximately 10.
10. Use according to any of claims 1 to 7, characterised in that the electrolyte bath has gaseous components, in particular CO₂ and/or N₂.
11. Use according to any of the previous claims, characterised in that the amount of the potential voltage is set to values under approximately 2 Volts.
12. Use according to any of the previous claims, characterised in that electromagnetic radiation in the UV range is used.
13. Use according to claim 12, characterised in that electromagnetic radiation is used of a wave length λ of approximately 300 nm.
14. Use according to any of the previous claims 1 to 9, 11 to 13, characterised in that with a liquid electrolyte bath, a potential voltage of approximately 1.8 to 1.9 V and an electromagnetic radiation with a wavelength λ of approximately 300 nm, a quantum yield (number of electrons emitted over number of photons received) of approximately 2% to approximately 4% is achieved.
15. Use according to any of the previous claims as a detector for electromagnetic radiation, characterised in that the aluminium foil is bombarded with electromagnetic radiation, in particular UV radiation, and the photoelectric current detected by measurement.