Direct methanol fuel cell (dmfc)

0001The invention relates to a direct methanol fuel cell (DMFC), a plant consisting of several DMFC and a method for operating DMFC plants, with high voltage and Faraday efficiency.

0002Since 1922, the principle of DMFC is known, so far, the work focused on operating the DMFC with liquid fuel. Methanol is used as fuel in the DMFC. In earlier years, alternatives to methanol such as formic acid, formaldehyde or higher-chain alcohols were also tried out as fuel. The use of methanol has the greatest technical importance, which is why the name DMFC has become established. The operation of DMFC with liquid fuel takes place at relatively low temperatures and has the disadvantage that the reaction of the methanol takes place with relatively poor stress efficiency due to kinetic inhibition of the anode reaction.

0003From JP-22 34 359, the reaction of vaporous methanol is known. The water is used to humidify the membrane and to drain the reaction [CH₃OH + H₂O -> CO₂ + 6H⁺ + 6e^-] supplied separately on the back (ie on the cathode side). The cathode-side supply of water has the disadvantage that counteracts at higher current densities, the current proportional, electroosmotic water transport of water diffusion through the membrane. This leads to an increased water consumption because the membrane must be kept moist with additional water. In addition, the metered addition of water in this prior art is not load-dependent.

0004A general problem in the realization of the DMFC remains the diffusion of the fuel methanol through the electrolyte to the cathode, where this is also implemented. The consequence of this is in addition to the loss of fuel (lowering the Faraday efficiency), a reduction of the cell voltage (lowering the voltage efficiency).

0005The object of the present invention is therefore to provide a fuel cell and a fuel cell system and a method for operating the system in which high voltage and Faradaywirkungsgrade be realized at high current densities. In addition, an object of the present invention is that a fuel cell, a fuel cell system and a method for operating a fuel cell is provided, which works with low electroosmotic water loss in the cell and with the lowest possible water transport through the polymer electrolyte.

0006Faraday efficiency is the energy utilization rate, which is what percentage of the fuel actually reacted at the anode.

0007The voltage efficiency is the ratio between cell voltage under current load and thermodynamic open circuit voltage.

0008General knowledge of the invention is<ul id="ul0001" list-style="dash"><li>first, that an increase in the Faraday efficiency by minimizing the methanol diffusion within the cell is possible if the methanol is supplied depending on the load and consumed in accordance with the anode compartment. Then it is not in such a high concentration that a large diffusion pressure is produced towards the cathode.</li><li>Second, the invention is based on the finding that the voltage efficiency can be improved by increasing the operating temperature, because this leads to a minimization of the kinetic inhibition of the anode reaction. In addition, the Faraday efficiency is also increased by the load-dependent supply of the reactants at low current density.</li><li>thirdly, the problem of excessive water transport through the polymer electrolyte can be reduced by the addition of an inert gas such as carbon dioxide and / or nitrogen, thereby lowering the water content on the anode side of the DMFC and transporting less water to the cathode.</li></ul>

0009The subject of the present invention is therefore a DMFC, each comprising a supply and disposal channel for the fuel and the oxidant, a membrane-electrode unit and bipolar plates, wherein the verse orgungskanal for the fuel, an evaporator upstream so that the fuel in the implementation is present in gaseous form at the anode of the fuel cell. Furthermore, the subject matter of the present invention is a fuel cell system which comprises a cell stack of fuel cells according to the invention, an evaporator and, if appropriate, Up to three pumps (two metering pumps for the supply of methanol and water and a pump, the CO₂-Abgas, which is led in the circle, brings again on the necessary overpressure) in the supply line of the fuel as well as in the derivative of the fuel a CO₂Separator comprises, wherein in the, the fuel cell stack downstream CO₂ Separator the condensate of the gaseous fuel from the carbon dioxide is thermally or otherwise separable.

0010Furthermore, the subject of the present invention is a method for operating a DMFC system in which the fuel, at least consisting of methanol and water, is fed to the anode in gaseous form. Further advantageous embodiments of the invention are contained in the subclaims and in the description, the explanations to the figures and the figures themselves.

0011The fuel of the erfindunsgemäßen fuel cell can either consist only of methanol or any mixture of water and methanol. If the fuel consists of any mixture of water and methanol, then the concentration of both methanol or water can be load-dependent adjustable via a metering pump upstream of the evaporator. The fuel can be introduced with variable pressure in the fuel cell and it can be any amount of inert carrier gas such as CO₂, N₂, Argon, etc .... be mixed.

0012A preferred embodiment of the fuel cell provides that the methanol / water mixture, an inert gas such as carbon dioxide and / or nitrogen or the like is added. This reduces the water content on the anode side of the DMFC and less water is transported through the polymer electrolyte to the cathode side.

0013The degree of humidity x_f = V_nW / V_n [V_nW = Water vapor volume under normal conditions; V_n = Total volume under normal conditions] can be arbitrarily set by the inert gas. Moisture levels greater than 70%, preferably between 80 and 90% prove to be expedient, because then the polymer membrane does not dry out. The degree of humidity will be as high as possible so that the energy required for gas transport remains as low as possible. The degree of humidity also depends on the operating temperature of the DMFC. The higher this is, the higher must be the degree of moisture, because the water content in the membrane at temperatures above 100 ° C decreases rapidly. The degree of humidity x_f (by volume) is defined as follows:<maths id="math0001"><math display="block"><mrow><msub><mrow><mtext>x</mtext></mrow><mrow><mtext>f</mtext></mrow></msub><msub><mrow><mtext> = V</mtext></mrow><mrow><mtext>nW</mtext></mrow></msub><msub><mrow><mtext>/ V</mtext></mrow><mrow><mtext>n</mtext></mrow></msub><msub><mrow><mtext> = V</mtext></mrow><mrow><mtext>nW</mtext></mrow></msub><msub><mrow><mtext>/ (V</mtext></mrow><mrow><mtext>nW</mtext></mrow></msub><msub><mrow><mtext> + V</mtext></mrow><mrow><mtext>nL</mtext></mrow></msub><msub><mrow><mtext>) = p</mtext></mrow><mrow><mtext>W</mtext></mrow></msub><mtext>/ p</mtext></mrow></math><img file="EP0907979B1_D0001.tif" /></maths><ul id="ul0002" list-style="none" compact="compact"><li>V_nL Dry gas volume under normal conditions, ie the volume of gaseous methanol, with or without added inert gas;</li><li>p_W = Steam partial pressure</li><li>p = total pressure</li></ul>

0014The fuel cell system according to the invention preferably consists of a cell stack of fuel cells according to the invention, but it can also be built from different types of fuel cells combined. In this case, the evaporator and optionally one or two metering pumps, which supply the fuel or the water depending on the load, integrated in the supply line of the fuel to the cell stack.

0015In an anode cycle is in CO₂Separator the resulting CO₂ separated from the exhaust gas, which is rich in unused methanol. The exhaust gas is then present in condensed form and can be recycled, ie introduced into the evaporator. Furthermore, a portion of the separated carbon dioxide via a pressure pump, which also controls the addition amount of the inert gas, also be performed in a circle.

0016Anode circuit means that the fuel methanol or methanol / water mixture, each with or without inert gas additive, is passed past the anode in a closed-loop system, with additional fuel supplied to the system as needed and gaseous reaction product being separated out of the system.

0017The unconsumed fuel, which is contained in the fuel gas, is first condensed with the use of heat or cooled and then reintroduced into the supply line or into the evaporator. Of course, the load-dependent control of the metering pumps, which regulate the inflow of water / methanol in the evaporator, must be designed so that the concentration changes of the methanol / water mixture in the evaporator is taken into account by the supply of the exhaust gas.

0018The unconsumed fuel from the fuel exhaust gas is in the heat exchanger or CO₂Separator from the carbon dioxide contained physically or possibly chemically separated. Physical separation means that the separation takes place via the different physical properties of the substances (such as density, boiling point, etc.). The chemical separation is conceivable and means that the CO₂ is chemically bound, for example precipitated as carbonate (due to the high mass of the resulting carbonate energetically little useful but alternative chemical methods can be discussed).

0019As DMFC, the direct methanol fuel cell is referred to, which consists in analogy to the general principle of electrochemical energy converter of anode, cathode and a suitable electrolyte. The electrodes are generally backside ie contacted with the side facing away from the electrolyte by a current collector to which the task of gas or Reactant distribution has. Due to the nature of the electrolyte used, there are various possibilities for realizing a DMFC. In the context of the present invention, preferred acidic electrolytes, and in particular acidic solid electrolytes, are treated. In general, proton-conducting polymers (electrolyte membranes) are suitable which are stable under the corresponding operating conditions. As an example Nafion (registered trademark) is mentioned. As further electrolytes, apart from those mentioned, examples which are mentioned which are based on inorganic systems, such as tin phosphates or electrolytes based on siloxane skeletons, are mentioned.

0020As current collectors are usually carbon-based materials, eg Carbon fiber papers or fabrics. The catalysts used on the anode side are primarily platinum / ruthenium alloys, on the cathode side mostly pure platinum. In the realization of a fuel cell system, such as a battery, to achieve higher voltages, the individual cells are connected in bipolar series. The necessary bipolar plates can be made of graphite, metallic or other electrically conductive and corrosion-resistant materials. The bipolar plates simultaneously take over the task of Reaktandenzuführung. You are therefore possibly structured with appropriate channels.

0021The operation of the DMFC can be carried out at temperatures between 60 and 160 ° C, depending on the boiling point of the mixture. Preferably, the operating temperature will fall within a range of 100 to 150 ° C, typically between 120 and 130 ° C. Accordingly, methanol or corresponding methanol / water mixtures is heated above the boiling point and fed to the cell in gaseous form. The system pressure is adjusted so that it corresponds to the equilibrium pressure of the methanol / water mixture at the temperature of the fuel cell. In the anode compartment of the DMFC, therefore, the steam is in the state of saturation. By this vaporous feed of the reactant, the electroosmotic water transport is minimized because the amount of water at the anode is greatly reduced. The terms "fuel", "methanol" and "mixture of water and methanol" in the context of the present application always denote a vaporous fuel containing an indefinite amount of inert gas (ie from 0% to a moisture level of nearly 100). In the case of CO₂ as an inert gas may be a part of the anode exhaust gas, which is brought via a pump and a corresponding control valve back to the required pressure (see also Figure 2) and is driven in a circle.

0022As the fuel is, as I said, a methanol / water mixture or pure methanol, with or without inert gas additive used.

0023As stated, the fuel is circulated via a carbon dioxide separator connected to the exhaust line of the fuel cell, which at the same time has the function of separating the carbon dioxide formed from the remaining exhaust gas.

0024The oxidant is either pure oxygen or air or any mixtures of these components, wherein the oxidant of the cathode is preferably supplied in a superstoichiometric amount.

0025A particular problem of the DMFC is the search for suitable anode materials for the oxidation of the fuel. Therefore, depending on the state of the art, various anode materials and catalysts can be used on the anode in addition to the platinum / ruthenium alloys mentioned. By way of example, it should be mentioned that under certain circumstances by adding a third component, such as tin or nickel, the activity of the anode can again be slightly improved compared to the binary system platinum / ruthenium. The invention is not intended to noble metals as catalysts and anode materials or Limited cathode materials, but there are quite precious metal-free catalysts conceivable.

0026The concentration of methanol in the fuel mixture, based on the non-evaporated, liquid state may be between 0.05 and 5 mol / l. In this case, a concentration between 0.5 and 1.5 mol / l is particularly preferred.

0027As a further operating parameter is still the pressure mentioned, which can be between normal pressure and low pressure and vacuum. The above definitions apply to the description, the explanations to the figures and the claims.

0028In the following the invention will be explained in more detail with reference to two figures.

0029FIG. 1 shows a block diagram of a fuel cell system according to the invention.

0030Figure 2 also shows a block diagram of a fuel cell system according to the invention, but in which the fuel via a, connected to the fuel cell CO₂Separator is driven in a circle.

0031FIG. 1 shows a fuel cell system which can be operated both with a methanol / water mixture and with pure methanol as fuel. The system has metering pumps 3 and 3 ', which control the supply of methanol and / or water depending on the load via a control system. From left to right, FIG. 1 shows first the two containers 1 and 2, in which water and methanol are contained. From these reservoirs 1 and 2, the components of the liquid fuel, ie water and methanol, flow into the metering pumps 3 and 3 ', which respectively control the flow rate of the liquids. In the case of the storage container 1, which is for example the water tank, passes via the line 11, the load-dependent metered in the metering pump 3 amount of water in the evaporator. 4 Likewise, a certain amount of methanol from the reservoir 2 via the metering pump 3 'and the line 12 passes into the same evaporator. 4 In the evaporator 4, both liquids are heated above the boiling point and via the line 13, the vapor mixture formed in the evaporator is introduced into the fuel cell stack 5. There it is conducted via a supply channel into the respective anode chambers of the individual fuel cells. About the line 15 leaves the spent and with CO₂ Enriched fuel off the fuel cell stack again and enters the carbon dioxide separator or heat exchanger 6, where it is optionally condensed again using the heat energy. The resulting CO₂ can be separated there from the exhaust gas / condensate. Via the line 14 parallel to the line 13, the fuel cell is supplied on the cathode side with oxidant. The Oxidansabgas leaves the cell stack via line 16 again and is passed into the heat exchanger 7.

0032Figure 2 shows a similar block diagram of a fuel cell system according to the invention with the difference that the connected to the fuel exhaust pipe from the fuel cell stack heat exchanger or carbon dioxide separator 6 is connected via line 8 to the evaporator 4. Via the line 8 now passes in the heat exchanger or carbon dioxide separator 6 if necessary condensed or cooled fuel offgas back into the evaporator 4, where it is supplied via the line 13 again the fuel cell. Via a second line 18, the deposited CO₂ from the heat exchanger 6 also passed into the evaporator 4. In line 18 is a pump 19 through which the CO₂ is brought back to the required pressure.

0033The composition of the fuel mixture depends, as I said, according to the respective load of the fuel cell stack and the specification of the degree of moisture. By means of a regulating mechanism which compares the requirements of water / methanol concentrations given as a load with the actual value of the mixture given in line 13, the power of the metering pumps 3 and 3 'is adjusted. Similarly, the addition of inert gas via a control mechanism that compares the degree of moisture in the line 13 as an actual value with a predetermined degree of humidity as a setpoint, controlled. This embodiment of the invention thus enables optimization of the Faraday efficiency.

0034A fuel cell system, in particular a battery consisting of the fuel cells according to the invention, operates with a significantly increased voltage and Faraday efficiency compared to the prior art. In addition, due to the vaporous presence of the reactants, "flooding" means that methanol and also water enter the working layer of the cathode and lower the hydrophobicity of the gas transport pores, so that they are full of reaction water (= "flooded"). As a result, the supply of oxygen is hindered and the cell voltage collapses] prevents the cathode, at least largely suppressed.

0035The increase in the Faraday efficiency is achieved mainly by minimizing the methanol diffusion through the membrane. About the load-dependent controlled metering pump 3 evaporates only each amount of methanol in the evaporator 4, as required in the current operating state of the fuel cell stack. As "demand for fuel" is thereby a load-dependent setpoint, which is determined by the Faraday's equivalents of the reaction and an operational bandwidth, which preferably consists in an excess. The methanol concentration is variably adjustable at the anode load change following and can also be subjected to extreme operating parameters (such as standby and full load) in a state that is optimally close to the diffusion limit current (ie still at maximum power but close to the diffusion limit current along the characteristic curve in the voltage / current diagram). The change in the methanol concentration in the gas mixture need not be regulated by the water supply or the pressure, but it can of course be controlled by the addition of an inert carrier gas.

0036To adjust the metering pumps no extra measurement of the prevailing in the cell actual value of methanol or fuel concentration must be done because the metering pump is load-dependent adjustable, and the consumption of methanol and thus the amount that is still contained in the recirculated exhaust gas , over the current course is calculable.

0037An additional control statement of the actual value of the methanol concentration in the gaseous fuel mixture can, however, also take place, for example, in the feed line 13 from the evaporator to the fuel stack or in the evaporator itself. If the determination of the actual value takes place in the evaporator, the vessel of the evaporator must be dimensioned so that a complete evaporation is ensured under all conceivable operating conditions and thus no change in concentration due to condensation. In general, however, a control determination of the water-methanol mixture ratio, if any, then take place as possible in the direct supply line to the supply channel of the fuel cell stack.

0038According to the invention a simpler construction of the cell is realized in that the water on the anode side and not, as shown in the cited prior art, cathode side is supplied.

0039A battery consisting of fuel cells according to the invention is conceivable, inter alia, for use in the mobile power supply, such as in the automobile. But it is also conceivable for larger stationary energy supply systems, such as in power plants or for the supply of residential buildings or commercial buildings with electricity and heat.