Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
1998-12-18
2001-05-29
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
C429S006000
Reexamination Certificate
active
06238814
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a fuel cell system and more particularly to a fuel cell system using a proton exchange membrane as an electrolyte.
2. Description of the Prior Art
A proton exchange membrane fuel cell comprises a proton exchange membrane (PEM) between two electrodes, that is a cathode to which an oxidizing gas is supplied and an anode to which fuel gas is supplied. PEM acts as an electrolyte and transports therethrough hydrogen ions obtained at the anode of the fuel cell toward the cathode, in the form of proton (H
+
). Each of the electrodes comprises a catalyst layer deposited on a porous base member through which the reactant gas is supplied. Mounted externally of each electrode is a separator or connector plate with grooves permitting the reactant gas to be introduced into the electrode at a constant flow rate. Excess gas which has not been consumed by the fuel cell reaction is exhausted to the open air through the grooved separator. The electricity generated by the energy conversion reaction at the anode is collected at the electrode porous base member and transported to the outside of the fuel cell system through the separator. In actual application, the system includes a plurality of fuel cells which are stacked in series with the separator being interposed between adjacent fuel cells.
Since the fuel cell generates heat in correspondence to the electric power generated, a fuel cell stack
2
usually includes cooling plates
803
between fuel cells
801
,
801
at predetermined intervals, as shown in FIG.
9
. Each cooling plate has a passage of a cooling medium such as air and water to prevent excessive overheat of fuel cells
801
in operation.
Proton is hydrated when being transferred through PEM electrolyte, so that PEM tends to be dehydrated as the fuel cell reaction proceeds. PEM must always be properly humidified to prevent decrease of ion-conductivity and energy conversion efficiency. In the conventional designs, hydrogen gas is humidified by suitable means which, in turn, humidifies the PEM when it is supplied to the anode.
Various attempts have also been proposed to humidify air to be supplied to the cathode. Since the cathode of the fuel cell in operation has been heated to 80° C., for example, the air of a normal temperature should be preheated by a humidifier so that its saturated vapor becomes consistent with the ambient vapor condition of the cathode. Such a humidifier that is required to have water supplying function and air preheating function can not be simple in construction.
In Japanese patent un-examined publication No. 7-14599, there is provided a water injection nozzle to inject a necessary quantity of water into an air introducing pipe through which an air is supplied to the cathode of the PEM fuel cell. Since the nozzle is located upstream of a compressor, liquid water injected from the nozzle is evaporated when subjected to heat generated by the compressor. Thus, the cathode is humidified by vapor, not by liquid water.
In the fuel cell system of Japanese patent un-examined publication No. 9-266004, a discharge gas from the anode containing hydrogen gas which has not been consumed during the anodic reaction is introduced into the cathode where the unconsumed hydrogen gas in the discharge gas is combusted with oxygen to generate water, which well humidifies PEM electrolyte. In this system, there is no need to install a humidifier for humidifying air to be supplied to the cathode.
During operation of the fuel cell system, a proton produced at the anode is moved to the cathode where it reacts with oxygen in the air or any other oxidizing gas supplied thereto to produce water. Accordingly, in accordance with the conventional recognition in the art, there is a greater need to humidify hydrogen gas to be supplied to the anode, than at the cathode where water can at least partially be self-sustaining.
As a result of the inventors' repeated tests and investigation, however, it has been found that water produced at the cathode permeates through PEM electrolyte toward the anode, which makes it unnecessary to humidify hydrogen gas to be supplied to the anode. On the other hand, a water content of the PEM electrolyte at the cathode side tends to decrease by contacting the air flow the cathode. Such finding is contradictory to the conventional knowledge and has been first recognized by the present inventors.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a fuel cell system, based on the above-described finding, which is capable of maintaining a proton exchange membrane to be in a suitable moist condition.
Another object of the present invention is to provide a fuel cell system which is simple in construction, small in size, easy to install and, therefore, particularly suitable to be mounted on a vehicle.
Still another object of the present invention is to smoothly and effectively supply liquid water to the surfaces of the cathodes in the respective fuel cells in a fuel cell stack.
According to an aspect of the present invention there is provided a fuel cell system in which water is supplied to the surface of the cathode not in a vapor state but in a liquid state. Thus, the fuel cell system of the present invention comprises a stack of a plurality of fuel cells each having an anode, a cathode and an electrolyte membrane interposed between the anode and the cathode; an air intake manifold mounted above the stack for supplying air to a plurality of longitudinally extending air flow passages of the fuel cells in the stack; one or more of nozzle means mounted to side walls of the air intake manifold for injecting water into the air intake manifold; and water supply means for supplying water to the nozzle means.
Liquid water supplied to the air intake manifold above the fuel cell stack will preferentially take latent heat from the air around the cathode to prevent water evaporation from the electrolyte membrane which, therefore, remains in a suitable and uniformly moist condition. This contributes to improvement of capacity and durability of the fuel cell system. Supply of the liquid water is also effective to cool the cathode which would otherwise be overheated to an excessive temperature, which means that the temperature of the fuel cell of the present invention may be controlled without need to use cooling plates. Mounting of the nozzle means to the side wall of the air intake manifold will prevent increase of the overall height of the fuel cell system, which is especially important when the system is mounted in a vehicle.
In a preferred embodiment of the present invention, the nozzle means are mounted respectively to a pair of opposite side walls of the air intake manifold at locations offset to each other. Alternatively, the nozzle means at opposite side walls of the air intake manifold have different angles of water injection. In either embodiment, the sprayed water is uniformly dispersed and distributed over the entire cross-section of the air intake manifold and, therefore, allowed to enter all of the air flow passages of the respective fuel cells mounted below a single air intake manifold.
In another preferred embodiment of the present invention, the air intake manifold has a double side wall structure having an outer side wall and an inner side wall to define therebetween a passage through which water is conveyed to the nozzle means which is mounted to the inner side wall of the air intake manifold for injecting water to a space within the inner side wall. There is a single water passage between outer and inner side walls of the air intake manifold, through which water is supplied to the respective nozzle means.
In another preferred embodiment of the present invention, each of the air flow passages has an enlarged top opening communicatable with the air intake manifold. This facilitates smooth entry of the sprayed water to the respective air flow passages. In a particular design, each of the fuel cells in the stack has a plurality of longitudinal
Horiguchi Munehisa
Takada Noriyuki
Ueno Masataka
Kabushikikaisha Equos Research
Kalafut Stephen
Lorusso & Loud
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