Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2001-06-28
2004-03-09
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S006000, C429S047000
Reexamination Certificate
active
06703152
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of operating a phosphoric acid fuel cell having an assembly including an electrolyte impregnated with phosphoric acid and interposed between an anode electrode and a cathode electrode, in which fuel gas is supplied to the anode electrode by the aid of a fuel gas supply system, while oxygen-containing gas is supplied to the cathode electrode by the aid of an oxygen-containing gas supply system.
2. Description of the Related Art
A phosphoric acid fuel cell (PAFC) as a type of fuel cells is provided with a power-generating cell (fuel cell unit). The power-generating cell is constructed such that an electrolyte electrode assembly is interposed between separators (bipolar plates). The electrolyte electrode assembly comprises an anode electrode and a cathode electrode principally composed of carbon respectively and provided opposingly on both sides of an electrolyte matrix layer composed of a polymer membrane such as polybenzimidazole impregnated with phosphoric acid as liquid electrolyte. Usually, a predetermined number of the power-generating cells are stacked and used as a fuel cell stack.
In the fuel cell, a fuel gas such as a gas principally containing hydrogen (hydrogen-containing gas), which is supplied to the anode electrode, contains hydrogen which is ionized into ion on the catalyst electrode, and the ion is moved toward the cathode electrode via the electrolyte. The electron, which is generated during this process, is extracted to an external circuit, and the electron is utilized as DC electric energy. An oxygen-containing gas, for example, a gas principally containing oxygen or air (gas containing oxygen) is supplied to the cathode electrode. Therefore, the hydrogen ion, the electron, and the oxygen are reacted with each other on the cathode electrode and, thus, water is produced.
An operating temperature of the above phosphoric acid fuel cell is set to be relatively high (about 120° C. to 190° C.). In general, a heating means such as a heater is used to warm up the phosphoric acid fuel cell until it is steadily operated. The phosphoric acid fuel cell is operated after the temperature thereof is raised to be not less than about 100° C. Accordingly, the self-heat generation caused by generating the power is utilized to raise the temperature up to 120° C. to 190° C.
While the phosphoric acid fuel cell generates the power, reaction product water exists as liquid water at a high ratio at a relatively low temperature less than 100° C. as in starting the operation. The liquid water is hardly evaporated into steam. Therefore, concentration of the phosphoric acid is lowered by the liquid water while increasing an entire amount of the aqueous solution of phosphoric acid (hereinafter generally referred to “phosphoric acid” as well). Consequently, the phosphoric acid which is over a storable amount exists in the anode electrode and the cathode electrode.
If the phosphoric acid overflows from the anode electrode and the cathode electrode as described above, the phosphoric acid flows out which is retained in the electrolyte matrix layer. The flow-out phosphoric acid passes through the fuel gas flow passage and the oxygen-containing gas flow passage and is discharged out of a main body of the phosphoric acid fuel cell. Then, it is impossible for the phosphoric acid fuel cell to maintain an initial performance when it ceases operating and is restarted. As a result, there is a problem that the performance of the phosphoric acid fuel cell is worsened.
Further, if the phosphoric acid overflows in the anode electrode and the cathode electrode, an activity of the catalyst to facilitate the reaction necessary for generating the power is lowered in some cases. In addition, the fuel gas flow passage and the oxygen-containing gas flow passage are closed, thereby making it difficult to flow the reaction gas (fuel gas and/or oxygen-containing gas). As a result, there is a problem that the performance of generating the power is lowered.
If the phosphoric acid is flown out, it is assumed to use a separate apparatus for replenishing the phosphoric acid to prevent the performance from being worsened.
However, there is a problem that a size of the fuel cell system becomes inevitably large in its entirety.
It is also assumed to use a heater of a large size to quickly raise the temperature of the phosphoric acid fuel cell to be not less than 100° C. However, there is the same problem as described above that the size of the fuel cell system becomes inevitably large in its entirety. In addition, it is not economic to use the heater of a large size.
SUMMARY OF THE INVENTION
A principal object of the present invention is to provide a method of operating a phosphoric acid fuel cell, which makes it possible to reliably prevent reaction product water from worsening the performance of the phosphoric acid fuel cell, by using a simple arrangement and control without large equipments.
In the method of operating a phosphoric acid fuel cell according to the present invention, the operating condition is set so that a phosphoric acid concentration, at which an amount of reaction product water to lower a concentration of phosphoric acid and an amount of water evaporated from phosphoric acid are equilibrated, can be not less than a reference phosphoric acid concentration to successfully maintain desired performance, when the phosphoric acid fuel cell is operated under a condition in which the reaction product water exists as liquid water.
Usually, the phosphoric acid, which is used for the phosphoric acid fuel cell, is obtained by dissolving diphosphorus pentoxide in water. Especially, the high concentration phosphoric acid, which is used for the phosphoric acid fuel cell, is highly hygroscopic, and it tends to mix with water highly easily. In this case, the vapor component of the phosphoric acid, i.e., the vapor component of the aqueous phosphoric acid solution is diphosphorus pentoxide (exactly a dimer) and water. However, the vapor pressure of diphosphorus pentoxide is greatly low up to a temperature in the vicinity of 200° C. Therefore, although the vapor component of the phosphoric acid is substantially occupied by water, the vapor pressure of the phosphoric acid is different from that of water.
Specifically, the relationship between the saturated vapor pressure and the temperature of the phosphoric acid in the phosphoric acid concentration is shown in a diagram of FIG.
1
. The curve in
FIG. 1
, which resides in a phosphoric acid concentration of 0%, corresponds to the saturated vapor pressure curve of water. According to
FIG. 1
, the saturated vapor pressure of the phosphoric acid is greatly changed depending on the phosphoric acid concentration. The water in the phosphoric acid tends to be more evaporated as the phosphoric acid concentration is getting low. By contrast, the water in the phosphoric acid is more hardly evaporated as the phosphoric acid concentration is getting high.
When the temperature of the phosphoric acid fuel cell is lower than a usual operating temperature (about 120° C. to 190° C.) in a low output operation and at a low temperature as in starting the operation of the phosphoric acid fuel cell, the evaporating speed of the water in the phosphoric acid is slow. As a result, an amount of the product water is larger than that of the water to be evaporated. Accordingly, the phosphoric acid concentration in the electrolyte electrode assembly is lowered while the saturated vapor pressure of the phosphoric acid is increased. Thus, the water tends to evaporate.
Accordingly, the phosphoric acid concentration is lowered depending on the amount of the product water, and the amount of the product water to lower the phosphoric acid concentration and the amount of the water evaporated from the phosphoric acid arrive at the equilibrium at a certain point of time. In this state, the amount of the phosphoric acid is increased. If the amount of the phosphoric acid is larger than that of the phosphoric acid to be
Iguchi Masaru
Ise Masahiro
Komiya Teruaki
Lahive & Cockfield LLP
Laurentano Anthony A.
Parsons Thomas H.
Ryan Patrick
LandOfFree
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