Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2000-07-20
2002-10-22
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000
Reexamination Certificate
active
06468683
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
This application is a national stage of PCT/DE99/00155 filed Jan. 19, 1999, and based upon German application 198 02 038.4, filed Jan. 21, 1998, under the International Convention.
FIELD OF THE INVENTION
The invention relates to a method and an apparatus for operating a direct methanol fuel cell with gaseous fuel.
BACKGROUND OF THE INVENTION
A fuel cell is an electrochemical converter in which, without recirculation of heat and mechanical energy, electrical energy can be recovered directly from a fuel and an oxidation agent. It is comprised of a cathode, an anode and an electrolyte lying therebetween. The oxidation agent (e.g. air) is fed to the cathode and the fuel (e.g. methanol) is fed to the anode. The cathode and anode in the fuel cell have continuous porosity as a rule so that the two operating agents (fuel and oxidation agent) can reach the electrolyte and the reaction products carried away. Carbon dioxide is a typical reaction product at the anode side. At the cathode side water is produced.
Fuel cells are known from the publication DE 195 31 852 C1 in which proton-conductive membranes are used as an electrolyte and which are operated at temperatures of 100° C. At the anode of such a fuel cell, protons are formed in the presence of the fuel by means of a catalyst. The protons traverse the electrolyte and bind at the cathode side with the oxygen stemming from the oxidation agent to form water. Electrons are thereby liberated and electrical energy produced.
Heat is generated in a fuel cell. This heat is, as a rule, carried off by a coolant flowing through the fuel cell. For this purpose a fuel cell is provided with coolant lines. The temperature of the coolant fed into the coolant lines lies below the working temperature of the fuel cell.
With a so-called “direct methanol fuel cell,” methanol together with water are converted at the anode to protons and carbon dioxide. The protons pass the electrolyte and are converted in the aforementioned form at the cathode to water. The carbon dioxide is carried away out of the anode compartment in gaseous reaction products. The direct methanol fuel cell is hereinafter referred to as DMFC.
It is basically possible to operate a DMFC with a liquid or vapor form mixture of methanol and water.
It is a disadvantage in the operation of a DMFC with liquid fuel that carbon dioxide forms as a second gaseous phase. The carbon dioxide formed immediately proximal to the electrolyte must be discharged through the pores of the anode at the region of the reaction. This restricts the uniform supply of the liquid fuel to the electrolyte. The power output of the fuel cell is as a consequence reduced.
Along the transport path of the fuel through the anode compartment the proportion of the gaseous phase continuously increases. There thus arises a very different distribution of the two phases (liquid and gaseous) in the anode compartment.
The dropping volume proportion of the liquid phase (formed by the fuel and the water) along the anode compartment gives rise to an increasing vaporization of the original liquid phase.
The anode compartment divides itself into two regions by the aforementioned dropping volume proportion and the thus associated increased vaporization. In a first region, the electrolyte is wetted with a fuel/water mixture. In the second or following region the electrolyte is not wetted with liquid.
Transport of liquid and of gaseous substances in a porous system run quite differently. If a liquid and a gaseous phase adjoin one another substantially only a gas transport through the pores which are not wetted with liquid occurs. The pores filled with liquid are generally not traversed by gas because of the high pressure loss in the gas. The different conditions in the anode compartment give rise to power losses.
At a suitably high pressure in the anode compartment, the aforedescribed distribution into two regions can be counteracted.
A higher pressure in the anode compartment has the disadvantage of increasing the permeation of fuel and water through the electrolyte. Power losses are the result. A higher pressure in the anode compartment in addition mechanically stresses the electrolyte membrane and gives rise, inter alia, to rupture.
The aforedescribed effects based upon a higher pressure at the anode side can be avoided by a suitably high pressure on the cathode side. An effort must be made to suitably increase the pressure on the cathode side. The efficiency of the fuel cell is correspondingly reduced.
An increased pressure, however, influences positively the transport and electrochemical processes in the porous cathode layer. Thus the efficiency is increased.
The pressure on the cathode side of a fuel cell of the type DMFC with a membrane serving as the electrolyte which is chosen based upon the aforedescribed counteracting effects is preferably 200 to 250 kPa. Typically the pressure at the anode side of a fuel cell is 30 kPa less than that on the cathode side.
If a vapor form methanol-water mixture is introduced into the anode compartment of a DMFC, there is only one gaseous phase available. Two phases are thus avoided in a simple manner along with the above-mentioned drawbacks.
OBJECT OF THE INVENTION
It is the object of the invention to provide a method for efficient operation of a DMFC and to provide an apparatus for carrying out the method.
SUMMARY OF THE INVENTION
The method of operating a direct methanol fuel cell of the invention uses the steps of:
evaporating a coolant in the fuel cell,
vaporizing a methanol-water mixture in an evaporator utilizing heat from the fuel cell, and
feeding the vaporized water methanol-water mixture to the anode compartment of the fuel cell.
Under the term “evaporator” (evaporator unit), a device is to be understood in which the liquid evaporates.
In the method according to the claims, coolant is evaporated in the fuel cell. The evaporated coolant is fed in a first alternative to a heat exchanger. The heat exchanger is a component of an evaporator unit. By condensation in the heat exchanger, the heat is released at the evaporator unit at a substantially constant temperature. The phase transformation between vapor or gas and liquid is thus used to transmit heat from the DMFC to the evaporator. At the same time an approximately constant temperature is maintained.
In the evaporator unit, a methanol-water mixture is evaporated. The heat exchanger thus transfers heat to the methanol-water mixture. The evaporation temperature lies, during this operation, below the condensation temperature of the coolant used.
Suitable coolants are basically all liquids which can boil in the foreseen temperature range. This can be at ambient pressure (standard pressure) although it can also be the case in superambient or subambient pressure ranges.
For example the following:
Boiling
Operating Temperature of
Liquid
Temperature/° C.
Pressure/kPa
the Cell/° C.
Water
90
70
100
Methanol
90
255
100
Ethanol
90
158
100
The evaporated methanol-water mixture is then fed into the anode compartment of the fuel cell.
In a second alternative, the methanol-water mixture and the liquid coolant which is evaporated in the fuel cell are identical. The external evaporator with the heat exchanger are omitted. The fuel cell functions simultaneously as the evaporator. The evaporated mixture is then fed to the anode compartment.
With the process according to the invention, as a consequence, in an especially simple manner, a predetermined temperature can be maintained in the evaporator unit. With the method it can be ensured that no liquid superheating arises in the evaporator. A superheating can operate to cause a coking of the methanol. Coked methanol can no longer be transformed to hydrogen. The electric current yield is correspondingly reduced.
With the evaporation according to the invention of the methanol-water mixture before admission into the anode compartment of the fuel cell it can be ensured in an energetically effective manner that a liquid phase will not arise disadvantageously in the anode
Hohlein Bernd
Menzer Reinhard
Dubno Herbert
Forschungszentrum Julich GmbH
Kalafut Stephen
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