WO2013064726A1

WO2013064726A1 - A method for manufacturing a passive direct methanol fuel cell and a passive direct methanol fuel cell

Info

Publication number: WO2013064726A1
Application number: PCT/FI2012/051032
Authority: WO
Inventors: Aulis Tuominen; Arho SUOMINEN; Raimo Jokinen; Marjo FONSELL; Henrik Lagercrantz; Janne MÄKILÄ; Wukui ZHENG
Original assignee: Turun Yliopisto
Priority date: 2011-11-01
Filing date: 2012-10-26
Publication date: 2013-05-10
Also published as: CN103975471A; EP2774200A1; KR20140088884A

Abstract

The present invention relates to the field of generating electricity from chemical fuel and to fuel cells, and more particularly to a method for manufacturing a passive direct methanol fuel cell and to a passive direct methanol fuel cell. A passive direct methanol fuel cell (21, 22, 23) according to the present invention has a membrane electrode assembly structure (8, 12, 16, 27), which comprises a proton exchange membrane (9, 13, 17, 24, 26), current collector meshes (10, 14, 15, 18, 19) attached to both sides of said proton exchange membrane (9, 13, 7, 24, 26) by ultrasonic welding, and catalyst coating (11, 20) applied on and into said structure. The direct methanol fuel cell according to the present invention is very compact in size volume and weight, and produces substantial cost savings in manufacturing in comparison to direct methanol fuel cell according to prior art.

Description

A METHOD FOR MANUFACTURING A PASSIVE DIRECT METHANOL FUEL CELL AND A PASSIVE DIRECT METHANOL FUEL CELL

FIELD OF THE INVENTION

The present invention relates to the field of generating electricity from chemical fuel and to fuel cells, and more particularly to a method for manufacturing a passive direct methanol fuel cell and to a passive direct methanol fuel cell.

BACKGROUND OF THE INVENTION

A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. There are many types of fuel cells, but they all consist of an anode (negative side), a cathode (positive side) and an electrolyte that allows charges to move between the two sides of the fuel cell. In typical fuel cells there three segments (the anode, the electrolyte, and the cathode) are sandwiched to- gether. Take Hydrogen Fuel Cell as example, while fuel cell is working, Hydrogen is fed into the anode of the fuel cell. Oxygen (or air) enters the fuel cell through the cathode. Encouraged by a catalyst, the Hydrogen atom splits into a proton and an electron, which take different paths to the cathode. The proton passes through the electrolyte. The electrons create a separate current that can be utilized before they return to the cathode, to be reunited with the hydrogen and oxygen in a molecule of water. There are different types of fuels cells, such as alkaline fuel cells, proton exchange membrane fuel cells and high temperature fuel cells such as solid oxide fuel cells or molten carbonate fuel cells.

Proton exchange membrane fuel cells (PEMFC), also known as polymer electrolyte membrane (PEM) fuel cells, are a type of fuel cell being developed for transport applications as well as for stationary fuel cell applications and portable fuel cell applications. Proton exchange membrane fuel cells can be used in lower temperature/pressure ranges (0 to 100 °C) and they have a special proton exchange membrane, such as a polymer electrolyte membrane. To function, the membrane must conduct hydrogen ions (protons) but not electrons as this would in effect "short circuit" the fuel cell. The membrane must also not allow either fuel or gas to pass to the other side of the cell, a problem known as crossover. Finally, the membrane must be resistant to the reducing environment at the cathode as well as the harsh oxidative environment at the anode. The PEMFC is a prime candidate for vehicle and other mobile applications of all sizes down to mobile phones, because of its compactness.

Direct methanol fuel cells or DMFCs currently are using proton exchange membrane as the electrolyte and methanol as the fuel. DMFCs can be classified as a subcategory to proton exchange membrane fuel cells in which methanol is used as the fuel.

Direct methanol fuel cell is receiving increased interest in portable power applications, since DMFCs operate at ambient temperature with a good energy density. This significantly reduces the thermal management challenges of a small DMFC system that may extend the standby time of portable consumer electronics devices. Moreover, the fuel is far easier to refill, restore and transport compared to a hydrogen fuel cell. DMFC technologies are expected to account for a large portion of the energy sources for portable devices.

In the following, the prior art will be described with reference to the accompanying Figure 1 , which shows a basic structure of a direct methanol fuel cell according to the prior art.

Figure 1 presents a basic structure of a direct methanol fuel cell according to the prior art. The typical structure of a prior art direct methanol fuel cell (DMFC, Direct Methanol Fuel Cell) usually is a stack structure. The stack structure is sandwich-like, having the polymer electrolyte membrane layer 1 (PEM, Polymer Electrolyte Membrane) in the middle. On both sides of the PEM layer 1 there are catalyst layers 2, 3. In Figure 1 , an anode catalyst layer 2 and a cathode catalyst layer 3 are shown on both sides of the PEM layer 1 . On both sides of the catalyst layers 2, 3 there are gas diffusion layers 4, 5. Fur- thermore, there are current collectors 6, 7 on both sides of the gas diffusion layers 4, 5. In Figure 1 , an anode current collector 6 and a cathode current collector 7 are shown on both sides of the gas diffusion layers 4, 5. In the anode current collector 6 there are provided channels for guiding the flow of the methanol fuel. Respectively, in the cathode current collector 7 there are pro- vided channels for guiding the flow of the oxidant. In both sides of the current collectors 6, 7 there may be extra conductor plates to provide better conductivity for the electricity generated by the fuel cell.

In the anode side of the direct methanol fuel cell, methanol fuel is fed in the anode current collector 6. Methanol fuel and water diffuses to the anode catalyst layer 2 through the gas diffusion layer 4. The DMFC relies upon the oxidation of methanol on the anode catalyst layer 2 to form carbon dioxide. The half reaction at the anode side is:

CH₃OH + H₂0→ 6 H⁺ + 6 e^~ + C0₂

The half reaction shows that a catalyst, typically a platinum catalyst, encourage the methanol and water to split into positive hydrogen ions and negatively charged electrons, this forming carbon dioxide. Water is consumed at the anode side. Protons (H⁺) are transported across the proton exchange membrane layer 1 to the cathode side. Electrons (e^") are transported through an external circuit from anode side to cathode side, this providing power to connected devices. They all then reacted with oxygen to produce water which finishes the other half reaction.

In the cathode side of the direct methanol fuel cell, oxidant is fed in the cathode current collector 7. Oxygen diffuses by the cathode catalyst layer 3 through the gas diffusion layer 5. The half reaction at the cathode side is:

3

- 0₂ + 6 H⁺ + 6 e^~→ 3H₂0

The half reaction shows that at the cathode side, the electrons and positively charged hydrogen ions combine with oxygen to form water, which flows out of the direct methanol fuel cell. The overall reaction of the direct methanol fuel cell can thus be written as:

CH₃OH + ^ 0₂ → 2H₂0 + C0₂

The typical DMFC stack structure has advantages considering liquid/gas transport and methanol crossover. However, this structure also leads to additional impedance, weight, and volume. For example, the current collectors can account for 10-15% of the total fuel cell stack cost, more than 80% of the stack weight, and nearly all the stack volume. The structure used for a passive DMFC is as same as active DMFC structure. However, without additional power consumption, a passive system is usually operated at a low cur- rent density resulting in a reduced cooling load, less water management issues, less heat produced and a lower required fuel delivery rate. There are some researches in the DMFC field, discussing some minor adjustments to the prior art DMFC structure. But still there is still no discussion or indication on changing the foundation structure of passive DMFC.

As mentioned above, there are a lot of deficiencies in the current passive DMFC structure. There is a clear demand in the market for a new type of a passive direct methanol fuel cell and for a method for manufacturing a new type of a passive direct methanol fuel cell that would be better and more efficient than the current prior art passive direct methanol fuel cell solutions.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is thus to provide a method and an arrangement for implementing the method so as to overcome the above problems and to alleviate the above disadvantages.

The objects of the invention are achieved by a method for manufacturing a passive direct methanol fuel cell, which method comprises the steps of:

- fabricating a membrane electrode assembly structure by attaching current collector meshes to both sides of a proton exchange membrane by ultrasonic welding, and

- applying catalyst coating on and into said structure.

Preferably, in the method, first one current collector mesh is welded to one side of a proton exchange membrane, then another current collector mesh is welded to another side of a proton exchange membrane and then catalyst coating is applied on and into said structure.

Alternatively, in the method, first current collector meshes are simultaneously welded to both sides of a proton exchange membrane and then cat- alyst coating is applied on and into said structure.

Further alternatively, in the method, first catalyst coating is applied on and into a proton exchange membrane and then current collector meshes are welded to both sides of said proton exchange membrane.

Further alternatively, in the method, first catalyst coating is applied on and into a proton exchange membrane and on and into current collector meshes and then said current collector meshes are welded to both sides of said proton exchange membrane.

Further alternatively, in the method, first catalyst coating is applied on and into current collector meshes and then said current collector meshes are welded to both sides of a proton exchange membrane. Preferably, the current collector meshes are made of any metal, metal alloy or composite material having proper electric conductivity, with a thickness in the range of 1 -500 μιτι, typically in the range of 10-200 μιτι, and eye sizes in the range of 10-1000 μιτι, typically in the range of 20-300 μιτι. Preferably, the catalyst coating is applied by screen printing. Alternatively, the catalyst coating is applied by spraying, e.g. using air brush.

Preferably, the passive direct methanol fuel cell is tested with a special fuel cell measurement system suited for rapid fuel cell measurements, such as e.g. voltage/current measurements as well as impedance/frequency measurements of the fuel cells.

Furthermore, the objects of the invention are achieved by a passive direct methanol fuel cell, which passive direct methanol fuel cell has a membrane electrode assembly structure, which comprises a proton exchange membrane, current collector meshes attached to both sides of said proton ex- change membrane by ultrasonic welding, and catalyst coating applied on and into said structure.

Preferably, in said membrane electrode assembly structure, the catalyst coating has been applied on and into a proton exchange membrane and/or on and into current collector meshes before the ultrasonic welding.

Preferably, the current collector meshes are made of any metal, metal alloy or composite material having proper electric conductivity, with a thickness in the range of 1 -500 μιτι, typically in the range of 10-200 μιτι, and eye sizes in the range of 10-1000 μιτι, typically in the range of 20-300 μιτι. Preferably, the catalyst coating has been applied by screen printing. Alterna- tively, the catalyst coating has been applied by spraying, e.g. using air brush.

Preferably, in said membrane electrode assembly structure the fuel is transported to the membrane electrode assembly surface with the help of a wick made of linen, cotton, hemp or related wick material. Preferably, said membrane electrode assembly structure has a stripe structure. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a basic structure of a direct methanol fuel cell according to the prior art;

Figure 2 shows a partial membrane electrode assembly structure of a direct methanol fuel cell according to the present invention; Figure 3 shows another embodiment of a membrane electrode assembly structure of a direct methanol fuel cell according to the present invention;

Figure 4 shows a third embodiment of a membrane electrode as- sembly structure of a direct methanol fuel cell according to the present invention;

Figure 5 shows one embodiment a direct methanol fuel cell according to the present invention;

Figure 6 shows another embodiment direct methanol fuel cells ac- cording to the present invention;

Figure 7 shows a result of an experiment of a screen printing as a catalyst loading technique in the fabrication of a direct methanol fuel cell according to the present invention;

Figure 8 shows a cross section of a result of an experiment of a screen printing as a catalyst loading technique in the fabrication of a direct methanol fuel cell according to the present invention;

Figure 9 shows a microscopic figure of a result of an experiment of a screen printing as a catalyst loading technique in the fabrication of a direct methanol fuel cell according to the present invention;

Figure 10 shows an arrangement for testing of the membrane electrode assembly of a direct methanol fuel cell according to the present invention;

Figure 1 1 shows test results of voltage-current measurements for the membrane electrode assembly of a direct methanol fuel cell according to the present invention; and

Figure 12 shows test results of voltage-current measurements in terms of power density versus current density for the membrane electrode assembly of a direct methanol fuel cell according to the present invention.

The prior art drawing of Figure 1 has been presented earlier. In the following, the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings of Figures 2 to 12.

DETAILED DESCRIPTION OF THE INVENTION

Figure 2 shows a partial membrane electrode assembly structure of a direct methanol fuel cell according to the present invention. As shown in Fig- ure 2, the partial membrane electrode assembly structure 8 (MEA, Membrane Electrode Assembly) has polymer electrolyte membrane layer 9 (PEM, Polymer Electrolyte Membrane). In a direct methanol fuel cell according to the present invention the current collector layer gas diffusion layers, catalyst layers are embedded together. There are no clear layer boundaries between different layers. As shown in Figure 2, a mesh 10, typically a metal mesh 10, has been directly welded onto the PEM layer 9, and after this the catalyst ink 1 1 has been sprayed into the MEA structure 8. In the membrane electrode assembly structure of a direct methanol fuel cell according to the present invention the meshes 10 take the heaviest functional responsibility. The mesh 10 serves as the electrode which is holding the catalyst, the mesh 10 is also a part of the gas/water diffusion layer and a mechanical supporter.

Figure 3 shows another embodiment of a membrane electrode assembly structure of a direct methanol fuel cell according to the present inven- tion. As shown in Figure 3, another embodiment of a membrane electrode assembly structure 12 comprises a PEM layer (PEM, proton exchange membrane) 13 and mesh layers 14, 15, which mesh layers 14, 15 have been welded onto the PEM layer 13 using ultrasonic welding. The ultra-sonic welding is a welding type which uses high-frequency ultra-sonic vibrations as the energy source. By using ultra-sonic welding the PEM layer 13 and the mesh layers 14, 15 were welded together without damage and short cut. The cross-section picture of the welding result is shown in Figure 3.

In manufacturing a passive direct methanol fuel cell according to the present invention, the proton exchange membrane can be a typical standard membrane. For a mesh material any metal, metal alloy or composite material having proper electric conductivity may be used. Iron and gold are the metals commonly used in meshes when electric conductivity is a required feature. The meshes can have a thickness in the range of 1 -500 μιτι, typically in the range of 10-200 μιτι, and eye sizes in the range of 10-1000 μιτι, typically in the range of 20-300 pm.

Figure 4 shows a third embodiment of a membrane electrode assembly structure of a direct methanol fuel cell according to the present invention. As shown in Figure 4, a third embodiment of a membrane electrode assembly structure 16 comprises a PEM layer 17 and mesh layers 18, 19, which mesh layers 18, 19 have been welded onto the PEM layer 17. After welding the mesh layers 18, 19 onto the PEM layer 17, the catalyst ink 20 has been sprayed into the MEA structure 16. In the membrane electrode assembly structure of a direct methanol fuel cell according to the present invention the mesh layers 18, 19 serve as the electrodes which are holding the catalysts, and mesh layers 18, 19 also serve as a part of the gas/water diffusion layers and a mechanical supporters.

Compared with the conventional membrane electrode assembly structures, the MEA structure 16 according to the present invention can withstand curling to some extent. This means that this structure can be used in places where flow fields cannot be utilized, such as vaulted or camber structures. Moreover, the MEA structure 16 according to the present invention is ready product after welding and can be used directly. No mechanical pressure is needed to push the layers together in the structure.

In manufacturing a passive direct methanol fuel cell according to the present invention, the catalyst coating can be a typical catalyst coating. The catalyst coating can be applied on and into a proton exchange membrane and/or on and into current collector meshes prior welding. Alternatively the catalyst coating can be applied on and into the welded membrane electrode assembly.

Figure 5 shows one embodiment a direct methanol fuel cell according to the present invention. In the direct methanol fuel cell according to the present invention shown in Figure 5 the mesh layers of the DMFC have been welded into the PEM layer of the DMFC by ultrasonic welding. Thereafter the amount of 0.5 mg/cm² of catalyst were applied by air brush to the surfaces of the DMFC. The finished 9cm² direct methanol fuel cell 21 is shown in Figure 5.

Figure 6 shows another embodiment direct methanol fuel cells according to the present invention. Also in the direct methanol fuel cells according to the present invention shown in Figure 6 the mesh layers of the DMFCs have been welded into the PEM layer of the DMFCs by ultrasonic welding. Thereafter the amount of 0.5 mg/cm² of catalyst were applied by air brush to the surfaces of the DMFCs. The finished 1 cm² direct methanol fuel cells 22, 23 are shown in Figure 6.

In a direct methanol fuel cell arrangement according to the present invention the fuel may be fed using capillary action. The fuel may reside in the same space with the MEA structures or alternatively in a separate fuel compartment. The fuel is transported to the MEA surface with the help of a "wick" made of linen, cotton, hemp or related wick material. The wick material may for example be a thick woven fabric. As the wick material is in some contact with the fuel it transports the fuel to all parts of the said material due to the capillary action. When the wick material is in direct contact with the MEA surface it thus transports the fuel to the surface of the electrode. In one alternative solution the MEA wick is combined with the current collector mesh layer, i.e. the wick material comprises metal threads of a current collector. Furthermore, in another alternative solution the MEA wick is combined with the membrane electrode assembly structure, current collector meshes and catalyst coating applied on and into said structure.

In fabrication of a DMFC MEA the catalyst can be loaded either onto the gas diffusion layers, or onto the membrane. However, the primary challenge in the assembly of MEAs is to achieve good contact between the membrane, the gas diffusion layers, and the catalyst layers as a good contact max- imizes catalyst utilization during the cell's operation. It has been suggested that by using the catalyst coated membrane method makes a good contact between the catalyst layer and the electrolyte membrane, which can effectively reduce the catalyst loading without sacrificing the cell's performance.

Several methods have been developed for applying catalyst layers to DMFC, such as spreading, spraying, sputtering, painting, screen printing, decaling, electro-deposition, evaporative deposition, and impregnation reduction. Most commonly used methods to perform the loading of the catalyst are either spreading with a doctor blade or spraying using air brush technique.

Spraying usually refers to a process which uses air brush to load the catalyst ink. Spraying could be a manual or an automatic process. To the author's knowledge, there are no articles using an automatic spraying process, however machinery for this has been made available. In the process, a liquid ink is often used. The loading amount in a spraying process is usually in the range of 0.1 to 2.0 mg/cm².

The spraying method, can tolerate a variety of different catalyst contents. Metal powder or metal attached to the carbon black, or the catalyst mixed with PTFE and ionomer are all suitable. The method can also be applied to the automatic assembly line, although the loading process is time consuming. After each spraying, the ink should be remixed again or the ink should be mixed during the spraying process, as the liquid ink agglomerates easily. During the spraying process, parts of the catalyst will remain in the air and parts of the catalyst will be sprayed on the support material. The platinum lost in these two cases cannot be recovered.

Spreading refers to loading the catalyst layer by using a doctor blade or a similar instrument. During the process, the slurry/paste is dropped or squeezed onto the surface of the GDL or membrane. Then the surface is smoothened by using automated or hand tools to ensure a uniform catalyst layer. The loading amount obtained by the spreading method is usually more than I mg/cm².

Screen printing is another commonly used method for loading the catalyst in fabrication of a DMFC MEA. In screen printing a paste or also a liquid/suspension can be used as the deposition material. Commercial screen printers are designed to hold the screen parallel to and in proximity with the substrate. A squeegee can be used to provide the force necessary for forcing the paste through the openings onto the substrate. The screen printing pro- cess can handle a thin catalyst layer, starting from 0.2mg/cm2, which are controlled by the height or the weight of the catalyst layer. This process has a similar principle to that of the spreading method, but the screen printing process has been commonly used in electronics manufacturing thus is more controllable during the loading.

In spreading and screen printing, the catalyst loading process is fast and stable and the catalyst layer can be formed in a single process. The paste does not need to be remixed as often as with the spraying process but the solvent evaporation need to be concerned. Both of the methods are efficient; most of the catalyst can be used and recovered. These methods also have a high impurity tolerance. But the loading is restricted by the tools and equipment. Some kind of catalyst such as Pt black is seldom used in these methods, since the pure metal loading leads to a very thin layer which cannot handled by these methods.

Screen printing was experimented as a catalyst loading technique for the fabrication of a DMFC MEA. In this experiment the final goal of this loading process was to get a 0.5 mg/cm² catalyst layer on the novel structure.

The catalyst paste content used for cathode in this experiment was Pt 60% on Carbon black 16.7mg, Fumion 5 mg, Carbon black 173.3mg, Distilled water 5ml, and Isopropanol 10ml. Respectively, the catalyst paste content used for anode was Pt 30% Ru 30% on Carbon black 16.7mg, Fumion 5 mg, Carbon black 173.3mg, Distilled water 5ml, and Isopropanol 10ml. Having the paste ready for use, a screen printing machine was used (AP25, MPM Corporation) for loading the catalyst on the membrane. The welded membrane with mesh as the substrate was connected on the surface of a PCB. The catalyst ink was used as a paste, and stainless steel, with an opening of 1 cm by 1 cm was uses as a stencil.

The membrane used in this experiment is Fumapem F14100 (dry thickness: 100120 urn, Fumatech GmbH). This membrane was pretreated as follows: the membrane samples were put in an aqueous 10 wt% HNO3 solution for 12h at t=80°C, then treated for 1 h in distilled water at t=80°C, and last rinsed with distilled water. The catalyst ink was prepared by dispersing carbon supported Pt/PtRu (BASF) into carbon black, then added isopropanol : water = 5:1 solution, with 30 wt% Fumion (FLNA905, Fumatech GmbH).

Figure 7 shows a result of an experiment of a screen printing as a catalyst loading technique in the fabrication of a direct methanol fuel cell ac- cording to the present invention. The catalyst coated membrane is marked with a reference number 24. When physically scratching the catalyst layer of the membrane 24, it is shown that the catalyst has gone inside of the membrane meshes. The screen printing experiment result shows that, the screen printing method can be applied to the novel structure according to the present inven- tion. The membrane electrode assembly structure according to the present invention can for example have a stripe structure.

Figure 8 shows a cross section of a result of an experiment of a screen printing as a catalyst loading technique in the fabrication of a direct methanol fuel cell according to the present invention. The catalyst coated membrane is marked with a reference number 24. From the Figure 8 cross section it can be seen that the catalyst layer is a uniform layer and as thick as the membrane 24.

Figure 9 shows a microscopic figure of a result of an experiment of a screen printing as a catalyst loading technique in the fabrication of a direct methanol fuel cell according to the present invention. In the Figure 9 microscopic figure it is shown that the catalyst has gone inside of the membrane meshes.

In the novel structure according to the present invention there is no problem of Membrane swelling and wrinkling. The membrane can be used di- rectly. The other advantages of the method according to the present invention are a quick process, easy paste preparation, low waste, large catalyst loading range, impurity toluene, and the method is easy to scale up.

The membrane electrode assembly of a direct methanol fuel cell according to the present invention may be measured using a special fuel cell measurement system. The special fuel cell measurement system according to the present invention may have a control unit, a signal processing unit and a coupling unit. The control unit of the fuel cell measurement system may comprise a microcontroller unit or a microprocessor system, e.g. a laptop computer. The signal processing unit of the fuel cell measurement system may com- prise a signal processor unit. The coupling unit of the fuel cell measurement system may comprise A D-converters, D/A-converters and l/O-interfacing connectors. The measurement sequences are programmed into the signal processing unit and stored into the memory of the computer. Both voltage and current may be used as measured quantity. The special fuel cell measurement system according to the present invention is specially designed for rapid fuel cell measurements, such as e.g. voltage/current measurements as well as impedance/frequency measurements of the fuel cells. The measurement results are stored into the memory of the computer. The post-processing of the measurement results, e.g. Fourier-transformation, is carried out in the computer unit.

Figure 10 shows an arrangement for testing of the membrane electrode assembly of a direct methanol fuel cell according to the present invention. In the test arrangement, a normal fuel tank 25 was used for the testing. In the test arrangement the membrane 26 was positioned so that the solution 28 with fuel and water was supplied to the anode side of the membrane electrode assembly 27 and the cathode side of the membrane electrode assembly 27 was open to air.

As shown in Figure 10, the whole DMFC is a pump-free system and self-activated by electrochemical reactions. In this particular experiment the catalyst content of the fuel cell according to the present invention was 0.5mg/cm², the active area was 3x3cm, and the methanol concentration was 2M. All parts of the experiment were done in ambient room temperature (20°C).

Figure 1 1 shows test results of voltage-current measurements for the membrane electrode assembly of a direct methanol fuel cell according to the present invention. In the test arrangement the voltage-current measurements were started after the methanol solution had been in contact with the cell for 20 minutes. Each data point represents a typical steady state voltage that was measured after continuous operation for 5 minutes at the indicated current density. The comparative test results of voltage-current measurements for different meshes are shown in Figure 1 1 .

Figure 12 shows test results of voltage-current measurements in terms of power density versus current density for the membrane electrode assembly of a direct methanol fuel cell according to the present invention. The comparative test results of power density versus current density for different meshes are shown in Figure 12.

Figure 1 1 and Figure 12 demonstrate that the use of the gold plated meshes on both anode and cathode sides increases the passive DMFC performance. The maximum usable current density increases and the open cell voltage is increased to 120mV, subsequently there is a reduction in the voltage when the electronic load is started, and then the voltage stays at a level almost similar to the other tested cells. This indicates that the current collector character has great influence on the fuel cell performance before the load is connected. When the load connected, the current density is limited by electrochemical reactions, and the current collector does not have such a significant influence. In this case, if considering the cost and performance of DMFC, a thinner acid- proof steel mesh is the best choice, because there is no significant difference between the gold mesh and acid proof mesh in loading.

The direct methanol fuel cell according to the present invention produces substantial cost savings in manufacturing in comparison to direct methanol fuel cell according to prior art.

The direct methanol fuel cell according to the present invention is also very compact in size volume and weight in comparison to direct methanol fuel cell according to prior art. Furthermore, the direct methanol fuel cell according to the present invention is easily bondable or attachable to the electronic devices.

The direct methanol fuel cell according to the present invention has a simple structure and is therefore more reliable than prior art direct methanol fuel cell solutions. When implemented in electronic devices the direct methanol fuel cell according to the present invention savings, efficiency and reliability when compared to prior art solutions.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The in- vention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

1. A method for manufacturing a passive direct methanol fuel cell (21 , 22, 23), characterized by the method comprising the steps of:

- fabricating a membrane electrode assembly structure (8, 12, 16, 27) by attaching current collector meshes (10, 14, 15, 18, 19) to both sides of a proton exchange membrane (9, 13, 17, 24, 26) by ultrasonic welding, and

- applying catalyst coating (11 , 20) on and into said structure.

2. A method according to claim 1, characterized in that in the method, first one current collector mesh (10, 14, 18) is welded to one side of a proton exchange membrane (9, 13, 17, 24, 26), then another current collector mesh (15, 19) is welded to another side of a proton exchange membrane (9, 13, 17, 24, 26) and then catalyst coating (11, 20) is applied on and into said structure.

3. A method according to claim 1, characterized in that in the method, first current collector meshes (10, 14, 15, 18, 19) are simultaneously welded to both sides of a proton exchange membrane (9, 13, 17, 24, 26) and then catalyst coating (11 , 20) is applied on and into said structure.

4. A method according to claim 1, characterized in that in the method, first catalyst coating (11, 20) is applied on and into a proton ex- change membrane (9, 13, 17, 24, 26) and then current collector meshes (10,

14, 15, 18, 19) are welded to both sides of said proton exchange membrane (9, 13, 17, 24, 26).

5. A method according to claim 1, characterized in that in the method, first catalyst coating (11, 20) is applied on and into a proton ex- change membrane (9, 13, 17, 24, 26) and on and into current collector meshes (10, 14, 15, 18, 19) and then said current collector meshes (10, 14, 15, 18, 19) are welded to both sides of said proton exchange membrane (9, 13, 17, 24, 26).

6. A method according to claim 1, characterized in that in the method, first catalyst coating (11, 20) is applied on and into current collector meshes (10, 14, 15, 18, 19) and then said current collector meshes (10, 14,

15, 18, 19) are welded to both sides of a proton exchange membrane (9, 13, 17, 24, 26).

7. A method according to any one of claims 1 to 6, character- ized in that the current collector meshes (10, 14, 15, 18, 19) are made of any metal, metal alloy or composite material having proper electric conductivity, with a thickness in the range of 1-500 μιτι, typically in the range of 10-200 μιτι, and eye sizes in the range of 10-1000 μιτι, typically in the range of 20-300 μιτι.

8. A method according to any one of claims 1 to 7, characterize d in that the catalyst coating (11 , 20) is applied by screen printing.

9. A method according to any one of claims 1 to 7, characterize d in that the catalyst coating (11 , 20) is applied by spraying, e.g. using air brush.

10. A method according to any one of claims 1 to 9, characterized in that the passive direct methanol fuel cell is tested with a special fuel cell measurement system suited for rapid fuel cell measurements, such as e.g. voltage/current measurements as well as impedance/frequency measurements of the fuel cells.

11. A passive direct methanol fuel cell, characterized in that the passive direct methanol fuel cell (21, 22, 23) has a membrane electrode assembly structure (8, 12, 16, 27), which comprises a proton exchange membrane (9, 13, 17, 24, 26), current collector meshes (10, 14, 15, 18, 19) attached to both sides of said proton exchange membrane (9, 13, 17, 24, 26) by ultrasonic welding, and catalyst coating (11, 20) applied on and into said structure.

12. A passive direct methanol fuel cell according to claim 11, characterized in that in said membrane electrode assembly structure (8, 12, 16, 27), the catalyst coating (11, 20) has been applied on and into a proton exchange membrane (9, 13, 17, 24, 26) and/or on and into current collector meshes (10, 14, 15, 18, 19) before the ultrasonic welding.

13. A passive direct methanol fuel cell according to claim 11 or to claim 12, characterized in that the current collector meshes (10, 14, 15, 18, 19) are made of any metal, metal alloy or composite material having proper electric conductivity, with a thickness in the range of 1-500 μιτι, typically in the range of 10-200 μιτι, and eye sizes in the range of 10-1000 μιτι, typically in the range of 20-300 μιτι.

14. A passive direct methanol fuel cell according to any one of claims 11 to 13, characterized in that the catalyst coating (11 , 20) has been applied by screen printing.

15. A passive direct methanol fuel cell according to any one of claims 11 to 13, characterized in that the catalyst coating (11 , 20) has been applied by spraying, e.g. using air brush.

16. A passive direct methanol fuel cell according to any one of claims 11 to 15, characterized in that in said membrane electrode assembly structure (8, 12, 16, 27) the fuel is transported to the membrane electrode assembly surface with the help of a wick made of linen, cotton, hemp or related wick material.

17. A passive direct methanol fuel cell according to any one of claims 11 to 16, characterized in that said membrane electrode assembly structure (8, 12, 16, 27) has a stripe structure.