DE102004053938A1 - Electrochemical cell comprises used as a battery, fuel cell or superconductor an anode, a solid or liquid electrolyte and a cathode - Google Patents

Abstract

Electrochemical cell comprises an anode, a solid or liquid electrolyte and a cathode. A high frequency field of 0.3-300 GHz is spread in the cell.

Description

Technical field of the invention

The described development is in the field of electrochemistry and high-frequency engineering (microwave technology). She is for use on electrochemical cells of all kinds, i. galvanic elements (Primary- and secondary elements such as batteries, accumulators and fuel cells) and electrolysis cells suitable. The invention is concerned with the development of microwave technology for energy dissipation in such electrochemical cells.

Of the current The state of the art should be here first focused on fuel cell technology. the reason for this is that the inventors here the greatest application potential of their development see. Many of the statements made are also applicable to other electrochemicals Cells applicable.

Modern polymer electrolyte membrane fuel cell (PEMFC) aggregates consist of single cells with an active cell area between about 5-500 cm ² and are operated at temperatures between room temperature and 90 ° C. The anode consists of carbon black-supported platinum alloys (mostly Pt, Pt-Ru), the cathode of carbon black-supported platinum. The polymer electrolyte is a chemically and thermally stable polymer with proton-donating side groups (mostly sulfonates).

The state of the art in the automotive sector are PEMFC units which are supplied with pure hydrogen (H ₂ ) from metal hydride, pressurized and cryogenic liquid storages or with reformate gas (H ₂ , CO ₂ , CO). The reformate gas is recovered, for example, from natural gas. Natural gas storage systems, in contrast to hydrogen storage systems, are easy to build, cost-effective, light in weight and have a high energy density. A disadvantage of the technique is the conversion to climate damaging products (carbon dioxide, CO ₂ ) and the elaborate additional technology, since in addition to the reformer desulfurization, a shift converter and a carbon monoxide (CO) purification stage is needed.

An alternative to the hydrogen fuel cell could be the direct fuel cell, in which alcohols such as methanol and ethanol, dimethyl ether or other fuels are used. Due to the lower power density compared to hydrogen operation and the lower efficiency, the direct combustion of organic energy sources in the range of medium to high power (100W-100 kW) is currently considered uninteresting. However, if the shortcomings could be removed, this concept offers significant advantages over H ₂ technology: reformer and gas treatment would be completely eliminated and storage and refueling would be greatly simplified by existing solutions and infrastructures.

One previously unresolved Problem is the rapid commissioning of the vehicle at very much low temperatures. The ionic conductivity of the electrolyte is too low to have enough energy to start put. A backup battery with sufficient capacity needs this Phase over.

in the Low power range (0.1 W-100 W), plays the complexity of the system and the volume matters more, so here's the direct fuel cell, especially the direct methanol fuel cell, is advantageous. Main applications of such cell systems are mobile Electronics, such as mobile phones and laptops. In the market so far no series-ready systems available, However, some manufacturers will do so over the next few years provided in promising. The efficiencies of the systems are in the range by 20%. The German developer Smart Fuel Cell / Brunnthal already has a series of prototypes in the power range 25-50 W presented. Rechargeable opposite Secondary elements becomes a factor of 2-3 longer Operating time reached at the same volume. Regeneration through replacement The methanol tank can be done quickly and independently of the mains. Disadvantages of Systems are the low efficiency and high cost of the noble metal catalysts (Platinum, ruthenium). In the power range below 10 W, the peripheral components, such. As pumps, for a further volume and Cost reduction is a major obstacle. An improved activation of the catalyst could bring significant benefits.

Deficiencies of previously known designs

• – lange Inbetriebnahmezeit bei niedrigen Temperaturen
• – geringe Toleranz gegenüber Katalysatorgiften wie Kohlenmonoxid (CO), schlechte Regenerierungsfähigkeit (Entgiftung)
• – niedrige Leistungsdichten bei der Direktverbrennung organischer und anorganischer Brennstoffe, insbesondere von Methanol
• – hohe Verluste durch Durchtrittspolarisation bei niedrigen Katalysatorbelegungen an den Elektroden
• – hoher Gewicht- und Volumenverbrauch von Zusatzsystemen zur Reaktandenaufbereitung und Energiepufferung.

The most important shortcomings of known designs with respect to the invention are

• - long commissioning time at low temperatures
• - low tolerance to catalyst poisons such as carbon monoxide (CO), poor regeneration capacity (detoxification)
• Low power densities in the direct combustion of organic and inorganic fuels, especially methanol
• - High losses through penetration polarization with low catalyst occupancy at the electrodes
• - High weight and volume consumption of additional systems for reactant processing and Energy buffering.

The here presented concept could Progress is being made in several areas of application by: both increases the CO tolerance of a PEMFC aggregate and thus the demands on the quality of the reformate gas, as well as the power density of direct fuel cells improved. It also provides a solution for the launch of galvanic Elements at low temperatures. In electrolysis cells can reduces the Durchtrittspolarisation and the removal of reaction products be improved.

By the targeted thermal activation of the catalytic electrode from fuel cells Membrane and cell housing receives one both more efficient and faster heating. The effective working temperature of the cell can thereby over the previously possible Temperatures are. This achieves an increase in power density of cell stacks and increased CO tolerance. Since also the educts and products of the cell reaction, e.g. water and Alcohols have high dielectric losses in the microwave range, The microwave energy can also evaporate excess water of reaction on the electrode e.g. be used at very high current densities. Around To achieve this multiple benefit is an electronic stack monitoring, which switches the microwave source makes sense.

Technical task

development a cost effective, efficient coupling system from microwaves (MW) to electrochemical Cells for targeted activation of individual components, in particular the electrodes. The goal is to improve the overall balance of the This ancillary system extended energy source by adding power density elevated and / or the system complexity / cost and the commissioning time can be reduced. An important requirement particularly mobile energy sources is a volume and weight-saving Construction. For that reason is a good integration ability in electrochemical cells another important requirement.

Technical solution of task

In order to realize the method of microwave heating of an electrochemical cell under the requirements mentioned in the technical problem in operation, a concept for the supply of high frequency radiation has been developed. The basic idea for this is to use the electrodes of the electrochemical cell as antennas for radiating the high frequency. This bifunctional electrode will hereinafter be referred to as current collector antenna (SKA). The fact that such an SKA can achieve a uniform heating over the entire electrode surface and at the same time does not cause any additional volume and weight could be demonstrated in a simulation ( 5 . 6 ) already for a very simple SKA geometry ( 3 ) to be shown. The SKA consists of a material of high electrical conductivity and low attenuation quotient for the propagation of the radiation. Suitable materials are especially metallic conductors, eg gold-plated copper. The electrodes of the electrochemical cell must consist of a composite of materials with sufficiently high dielectric loss losses, so that heating is possible. This is true for many electrode composites. The heating behavior of a typical electrode for use in the PEMFC consisting of platinum catalyst on soot and solid electrolyte is shown below ( 1 ). Such electrodes are very good microwave absorbers and can be effectively heated. For the separation of MW radiation and direct current at the SKA passive electronic HF filters are sufficient.

For the coupling of microwave energy into the SKA coupling systems of waveguides, parallel conductors, coaxial conductors and microstrip conductors (micro stripes) can be used. Particularly efficient is the coupling of a waveguide into the cell by means of a Koaxialkopplungsstift ( 7 ). The waveguide can be filled with a dielectric. By changing the relative permittivity of the dielectric, the distance between two field maxima in the waveguide can be reduced in order to achieve a uniform coupling in closely adjacent coupling pins. This distance is at a frequency of 2.45 GHz in vacuum or air-filled at 6 cm. When using silicone as a dielectric, the distance between 2 maxima is reduced to 2 cm.

For the generation Electromagnetic radiation sources are from the field of Radio frequency generation, as described, for example, in the News, Communication, radar and microwave technology can be used. As a high-frequency source are in principle all microwave generator types can be used, as practical have mainly the magnetron and semiconductor oscillators Basis of high-frequency transistors or Gunn diodes proved they have high efficiency and simple construction and for one wide power range available stand.

The microwave energy can be delivered in pulses to the electrodes to produce short term temperature spikes. Such temperature peaks are useful, for example, catalyst gif Desorb CO, which strongly adsorbs on the Pt catalysts of PEMFC, without permanently raising the operating temperature beyond the maximum value permissible for the efficient functioning of the fuel cell. In the heating-up experiment it could be shown that at heating rates of more than 200 K / s temperature changes of at least 50 K return to their initial value within less than 10 s ( 2 ).

Heating behavior in the microwave field

The effect of microwaves (MW) at a frequency of 2.45 Ghz on a membrane-electrode assembly (MEA) of a polymer electrolyte membrane fuel cell (PEMFC) is shown in FIG 1 and 2 shown. The temperature of the catalytic electrode was measured at different MW field strength. Pulses with a repetition rate of 5 Hz and a pulse duration of 20 ms lead to the setting of a constant temperature level (magnetron 1) within the resolution (40 ms), pulses with a repetition rate of 0.2 Hz and a pulse duration of 225 ms cause strong temperature jumps with heating rates over 200 K / s (magnetron 2).

Simulation of the field strength distribution an MEA-SKA arrangement

For this purpose, a planar antenna shape, as in 3 can be seen in the supervision, elected. This construction consists of electrically conductive strips, for example of copper.

In 4 can be seen that this is integrated into the gas distribution layer and rests on the catalyst surface. This design allows the feeding of microwave radiation even in thin components. In the 3 shown antenna form is only one of several possible SKA geometries, which was chosen by way of example to show the technical feasibility of the method in the electromagnetic simulation.

For the simulation, a finite difference software for the calculation of electromagnetic fields was used. For reasons of clarity, only one MW coupling to the upper catalyst electrode was simulated, and the lower antenna, as used in US Pat 4 can be seen, left out. As gas distributors, transparent materials such as quartz wool fabric (SiO ₂ ) must be used for MW. The electrolyte membrane and the catalyst layer can absorb MW and thereby generate heat. The dielectric loss and thus the ability to heat in the MW field is orders of magnitude higher in the catalyst layer than in a water-swollen electrolyte membrane. The metallic conductor and the surrounding housing are made of a good electrical conductor (metal or graphite), reflect the MW radiation and are approximately lossless. The feed takes place via a strip-shaped coaxial conductor at a frequency of 2.45 GHz.

The 5 and 6 show field distributions and area-related absorbed MW power in different layers of the fuel cell.

The field distribution in the upper catalyst layer directly below the antenna shows that microwaves can be distributed with this arrangement over the entire electrode surface, but especially at the point of coupling and in the center of the antenna strong field peaks are present. The power density (heat dissipated power) shows the same image because the absorbed power is proportional to the square of the electric field strength E ² .

In the electrolyte membrane is only at the coupling point and in the center the antenna to detect an absorption of microwaves. reason this is the comparatively low dielectric loss of the used Electrolyte membrane. As a result, microwave radiation passes through the membrane on the lower catalyst layer and can also be absorbed here. The pattern of power density is very similar to the upper catalyst layer.

The Simulation shows that a plane Distribution of microwaves in a very thin arrangement is possible. For a more even field distribution To achieve an optimization of the antenna shape is necessary. in this connection may be the use of the second antenna at the lower catalyst layer help. If you turn this by 90 ° or If it arranges offset, then the sum of the two fields already exists to expect a much more homogeneous distribution.

The Requirement, a two-dimensional uniform irradiation of thin Systems with microwaves could already use a simple geometry be fulfilled. In the following simulation we want to show that the arrangement can also be efficiently transferred to a complete cell stack.

To generate microwaves of frequency 2.45 GHz, a magnetron can be used. The standard transmission line is usually an air-filled waveguide. Here, however, a minimum width of 86 mm is necessary to direct the radiation undamped and it propagates a wave pattern with a wavelength λ of about 12 cm. To decouple microwaves from such a system, coaxial pins can be inserted at intervals of λ / 2. For the operation of a fuel cell stack, in which the distances of the individual cells are in the mm range, however, a distance of 6 cm too large. Filling the waveguide with an MW transparent material of high dielectric constant reduces the wavelength and increases the number of decoupling pins per unit length.

7 shows the simulation of such a waveguide. About the length of the waveguide form about 4¾ maxima, compared to an unfilled waveguide with about 1¾ maxima at 2.45 GHz. Another advantage of a dielectric filled waveguide is that the conductor cross section can be reduced to 1/10 of the cross sectional area.

Innovation advantage, commercial Property rights

A Patent and literature research revealed that publications for reprocessing of fuel cell gases of the fuel cell to increase the Efficiency are available [1-3].

The Patent by Sharivker et al. [2,3] deals with the treatment of fuel cell gases outside and inside the fuel cell to reduce the penetration polarization. There will be no energy balance of the overall system, so that no indication of the effective improvement of the efficiency is possible. The activation The gases take place in a thermal way by formation energetically excited and ionized species such as hydrogen, oxygen and alkyl radicals as well as protons in plasmas and arcing discharges at strong microwave susceptors (High dielectric loss materials), especially soot.

in the In contrast to this, one's own development deals with conditions, where there is no training for plasmas and discharges and thus not to activate the same in the manner mentioned can. According to the authors, conditions exist where: The dielectric strength of the materials is exceeded in the long term to destruction the electrodes.

aim is a thermal activation of the cell components by radiation inside the cell. Furthermore, in contrast to the above Patent an integrated solution been developed, i. the microwave radiating unit is located in the cell and can be for reasons of weight and volume savings combined with the current collector (SKA). This is the result Arrangement especially for Systems from a larger Number of single cells (cell stacks, stacks) exist, suitable.

Moreover exist scientific publications that deal with the influence of microwaves on the kinetics of electrochemical reactions employ. Compton et al. For example, [4] investigated the influence of local overheating on microelectrodes in the microwave field.

The Writers are beyond no further publications known to the topic of their invention. In particular, the effect became from microwaves to the function of electrodes in galvanic Elements and electrolysis cells have not been investigated. Accordingly According to the author's knowledge, there are no further ones by own Touched invention Third party property rights.

Application example 1

The electrical performance of the described application example is in the range of fuel cell stacks, as used for example in the automotive industry (1-100 kW). 8th shows a fuel cell unit with integrated SKA electrodes.

One Magnetron with an output power of 10 to 1000 W generates the MW energy for the complete stack over the filled one Waveguide is transported. In the waveguide are at regular intervals strip-shaped coupling pins, which transmit the MW radiation into the fuel cell. Here are two Antennas per cell provided. These also act as current collectors for the reaction and are connected in series (anode (cell 1) - cathode (cell 2), etc.). At the two extreme Electrodes become the DC power for the electrical load tapped. For separation of MW radiation and DC passive electronic RF filters are sufficient. To periodically transmit MW radiation to the cells, is provided a field adaptation, which continuously the field maxima shifts. This method has the advantage that the magnetron does not need to be pulsed and achieves a higher efficiency. At the coupling points, where there is a field maximum, are for one defined time interval MW irradiated until the field adjustment a minimum over located the coupling pin. This achieves a pulse-like Coupling into the individual cells without having to pulse the source.

The Strength a single cell hitherto on the market stacks is about 4-5 mm; about 200 microns be for claimed the collector electrodes. The replacement of this conventional Electrodes caused by the new SKA do not cause any enlargement the cell strength.

To the energy balance of Arrangement:

The power delivered by the fuel cell assembly depends on the fuel. In the methanol / air mode, an output of 0.25 W / cm ^{2 is} achieved, in the hydrogen / air mode a quiet tion of 0.75 W / cm ² .

Continuous operation:

The required pulse energy is 10 mJ / cm ² . If an efficiency of 50% for the magnetron and an efficiency of 50% for the coupling is estimated, the average power required is 4 mW / cm ² at a pulse frequency of 100 mHz. The proportion of additional energy required compared to the energy generated is therefore only between 0.5-1.6%.

Start-Up:

It takes 50 mW / cm ² to bring the catalytic layer from ambient to 100 ° C in a fraction of a second. The influence on the total energy balance is already negligible with operating times of more than one minute.

Under Using a magnetron, the technology is inexpensive and efficient. The price for a commercially available 500 to 900 W units are less than 50 EUR. The efficiency such a magnetron is> 50%.

Further savings arise in the field of reformate gas treatment and purification by improving CO tolerance.

By The increase in efficiency can be achieved with smaller fuel cell aggregates to be worked.

By the faster start-up time is the driving readiness at low Temperatures reached much faster and an additional Onboard battery can be sized smaller.

Application Example 2

Also in the smallest power range (1 mW to 10 W) galvanic elements let yourself implement the arrangement. This power range is in the To find communication and computer technology. As generators of electromagnetic radiation find semiconductor devices application (semiconductor oscillators based on high-frequency transistors or Gunn diodes).

9 shows a concept for the activation of a methanol fuel cell unit in a mobile phone with radio frequency in the microwave range. In contrast to the example above, an existing RF source is used here. In addition, no waveguide is used for coupling, but a Koaxilkopplung or a microstrip line.

modern Mobile phones transmit in the microwave frequency range between 900 and 2400 MHz, which also for the activation method is suitable. The requirements of the microwave unit to the frequency stability the RF sources is much lower than that of the transmitting unit. As a rule, such Phones over 2 or 3 frequency generators with different output frequencies.

literature sources

• [1] O. Katsuya, N. Toru, H. Akira, Fuel Cell Power Generation System and its Starting Method, Publ. # 2001 2103 49 A, Japanese Patent Office, Date of Publ. 03.08.2001.
• [2] V. Sharivker, T. Honeycutt, S. Sharivker, Microwave Activation of Fuel Cell Gases, Publ. # US 2003/0027021 A1, United States Patent Application, Date of Publ. 06.02.2003.
• [3] V. Sharivker, T. Honeycutt, S. Sharivker, Microwave Activation of Fuel Cell Gases, Publ. # WO 02/091505 A2, International Application, Date of Publ. 14.11.2002.
• [4] R.G. Compton, B.A. Coles and F. Marken, Microwave activation of electrochemical processes at microelectrodes Chem. Commun., 1998, 2595-2596.

Claims (17)

Electrochemical cells consisting of an anode electrode, a solid or liquid electrolyte and a cathode electrode, characterized in that propagates in the galvanic cell, a high frequency field of frequency 0.3-300 GHz and that the galvanic cell is a battery, a fuel cell or a supercapacitor can be. Electrochemical cells according to claim 1, characterized characterized in that the propagation of the high frequency field means spatially suitable antennas is controlled Electrochemical cells according to claim 1, characterized characterized in that the high-frequency field in each case one electrode detected Electrochemical cells according to claim 1, characterized characterized in that the high frequency field detects only the electrolyte Electrochemical cells according to claim 1, characterized characterized in that the high frequency field is an electrode and the Electrolyte detected Electrochemical cells according to claim 1, characterized characterized in that the high-frequency field on a catalyst detected by the electrodes Electrochemical cells according to claim 1, characterized in that the high-frequency field detects a substance in the electrolyte Electrochemical cells according to claim 1, characterized characterized in that for feeding the high frequency field a or multiple antennas are used in microstrip technology Electrochemical cells according to claim 1, characterized characterized in that for feeding the high frequency field a or multiple patch antennas are used Electrochemical cells according to claim 1, characterized characterized in that for feeding the high frequency field a or more coaxial antennas Electrochemical cells according to claim 1, characterized characterized in that for feeding the high frequency field combinations different antennas are used Electrochemical cells according to claim 1, characterized characterized in that at least one of the antennas also simultaneously can serve as a current collector at least one of the two electrodes. In this case, a filter for the decoupling of low-frequency and high-frequency components be used. Electrochemical cells according to claim 1, characterized characterized in that locally high heating rates in the high frequency field can be achieved. Electrochemical cells according to claim 1, characterized characterized in that components of the cell desorbed by high-frequency heating, catalytically activated, mixed and dissolved. Electrochemical cells according to claim 1, characterized characterized in that locally high heating rates in the high frequency field can be achieved. Electrochemical cells according to claim 1, characterized characterized in that as antenna materials materials of high electrical conductivity and low high frequency attenuation be used. Electrochemical cells according to claim 1, characterized characterized in that electrode arrangements of medium electrical conductivity and average dielectric loss.