Fuel diffusion unit, fuel supply unit, and fuel cell system including the same

A fuel diffusion unit including: a fuel diffusion plate; a diffusion sheet disposed on fuel diffusion plate, to evenly distribute a fuel to the fuel diffusion plate; a primary transportation unit disposed on the diffusion sheet; secondary transportation units connected to the primary transportation unit, to distribute the fuel to the fuel from the primary transportation unit to the diffusion sheet. The diffusion sheet has a wetting direction that allows the fuel to flow in a predetermined direction. The fuel diffusion unit can be included in a fuel supply unit and a fuel cell system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Application No. 2007-140669, filed Dec. 28, 2007, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Aspects of the present invention relate to a fuel supply unit, and more particularly, to a fuel supply unit that can vaporize a fuel, and a fuel cell system that employs the fuel supply unit.

2. Description of the Related Art

A fuel cell is an electricity generator that chemically reacts an oxidant and a fuel, to produce electricity. A fuel cell can continuously generate electricity, as long as a fuel is supplied.

A direct methanol fuel cell (DMFC) is a fuel cell that generates electricity, through a reaction between methanol supplied to an anode, and oxygen supplied to a cathode. In a DMFC, the anode and the cathode face each other across an electrolyte membrane. An assembly of the anode, the cathode, and the electrolyte membrane is conventionally referred to as a membrane and electrode assembly (MEA).

The cathode is exposed to an oxidant, such as oxygen in air. The anode is surrounded by a spacer, and vaporized methanol is supplied to the anode, through a plurality of holes formed in the spacer. Electrons are generated at the anode, and the electrons move through an electrical circuit, to the cathode. At this point, when a load is applied to the circuit, work can done using the generated electricity.

A fuel cell is generally part of a fuel cell system. A fuel cell system can be classified as an active-type, or a passive-type, according to how a fuel is supplied to an anode. In the active-type, a low concentration liquid methanol is pumped to the anode.

Passive-type fuel cell systems can include a fuel supply unit. The fuel supply unit vaporizes liquid methanol stored in a cartridge, and then the vapor is supplied to the anode, at room temperature, by diffusion. Passive-type fuel cell systems are often used in mobile devices, because passive-type fuel cell systems generally are smaller in size.

It is generally beneficial for a fuel supply system to evenly supply fuel to an entire surface of an anode. In this way, a rapid initial start-up can be realized, by uniformly supplying vaporized fuel to an MEA.

If the fuel is supplied through a single supply path, it takes time to achieve a uniform concentration of fuel across the anode. Thus, an initial start-up may be delayed. Therefore, there is a need to develop a configuration of fuel supply system that can uniformly supply fuel across an anode of an MEA, in a short time.

SUMMARY OF THE INVENTION

To address the above and/or other problems, aspects of the present invention provide a fuel diffusion unit, and a fuel supply unit, which can realize a rapid initial start-up, by uniformly supplying fuel to a membrane electrode assembly (MEA), and a fuel cell system employing the same.

According to an aspect of the present invention, there is provided a fuel diffusion unit comprising: a fuel diffusion plate; a diffusion sheet disposed on the fuel diffusion plate, to evenly distribute fuel to the fuel diffusion plate; a primary transportation unit disposed on the diffusion sheet; and secondary transportation units connected to the primary transportation unit, to distribute the fuel from the primary transportation unit to the diffusion sheet. The fuel flows through the diffusion sheet in a wetting direction of the diffusion sheet.

According to an aspect of the present invention, there is provided a fuel supply unit for a fuel cell, comprising: a fuel storage unit; a fuel diffusion plate to evaporate fuel supplied from the fuel storage unit; a diffusion sheet disposed on the fuel diffusion plate, to uniformly distribute the fuel to the fuel diffusion plate, having a wetting direction along which the fuel is distributed; a primary transportation unit disposed on the diffusion sheet, connected to the fuel storage unit; an actuator to pump the fuel through the primary transportation unit, and secondary transportation units that are connected to the primary transportation unit, to distribute the fuel to the diffusion sheet.

According to an aspect of the present invention, there is provided a fuel cell system comprising: a unit cell to generate electricity using a fuel; a fuel storage unit to store the fuel; a fuel diffusion plate to vaporize the fuel, and to supply the vaporized fuel to the unit cell; a diffusion sheet to uniformly distribute the fuel to the fuel diffusion plate, having a wetting direction along which the fuel is distributed; a primary transportation unit disposed on the diffusion sheet, and connected to the fuel storage unit; secondary transportation units connected to the primary transportation unit, to distribute the fuel to the diffusion sheet; and an actuator to pump the fuel through the primary transportation unit.

DETAILED DESCRIPTION

FIG. 1is a plan view of a fuel supply unit100for a fuel cell, according to an exemplary embodiment of the present invention.FIG. 2is a cross-sectional view taken along line A-A ofFIG. 1, andFIG. 3is a drawing for explaining wet directions in diffusion sheets ofFIG. 2.

Referring toFIGS. 1 through 3, the fuel supply unit100includes: a fuel storage unit110that stores fuel; a fuel diffusion unit140; and an actuator120that is connected between the fuel storage unit110and the fuel diffusion unit140. The fuel is stored in the fuel storage unit110, as a liquid. The fuel may be, for example, methanol MeOH, ethanol EtOH, or sodium boromohydride NaBH4.

The fuel diffusion unit140includes a fuel diffusion plate130to vaporize the fuel, a primary transportation unit141, secondary transportation units142, and a diffusion sheet144. The primary transportation unit141can be a tube that extends in a lengthwise direction along the center of the fuel diffusion plate130. An end of the primary transportation unit141is connected to the actuator120, and the other end is blocked. The primary transportation unit141may have a diameter φ1, of 0.1 to 1 mm, in consideration of the portability and mobility of a mobile device in which a fuel cell is used. That is, the above range of diameters achieve a stable fuel supply to the fuel diffusion plate130, from the fuel storage unit110, without being significantly affected by the orientation of the primary transportation unit141. The primary transportation unit141may have a larger diameter than the secondary transportation units142.

The secondary transportation units142can be small tubes, such as capillaries, and can be disposed on opposing sides of the primary transportation unit141. The secondary transportation units142have attached ends that are connected to the primary transportation unit141, and open ends through which the fuel can exit the secondary transportation units142. The attached ends can be offset from one another, along the length of the primary transportation unit141, such that the attached ends do not face one another, across the primary transportation unit141.

The secondary transportation units142include first tubes142aand second tubes142b. The first tubes142acan be straight, such that the open ends of the first tubes142aface away from the primary transportation tube141. The second tubes142bcan be curved, such that the open ends of the second tubes142bgenerally face toward the primary transportation unit141. The first tubes142aand the second tubes142can be alternately disposed along each side of the transportation unit141. In other words, different types of the secondary transportation units142are adjacent to one other, on each side of the primary transportation unit141.

The different secondary transportation units142facilitate a rapid and uniform delivery of fuel, across the entire fuel diffusion plate130. The structures of the secondary transportation units142are not limited to the shapes depicted inFIG. 1. In other words, the secondary transportation units142can have various shapes, orientations, and/or lengths. For example, the second tubes142bcan be straight, and can be shorter than the first tubes142a.

The secondary transportation units142may have increasing diameters, the further each secondary transportation unit142is from the actuator120. The change in diameter facilitates a uniform supply of fuel to the entire fuel diffusion plate130, since a fuel supply pressure decreases as a distance from the actuator120increases. The diameter φ2of the secondary transportation units142may be in a range from 5 to 250 μm.

The secondary transportation units142can be connected to the primary transportation unit141, in various ways. For example, if the primary transportation unit141is formed from a flexible material, and the secondary transportation units142are formed from a harder material, the secondary transportation unit142can be stabbed into the primary transportation unit141. The secondary transportation units142can be welded or glued to the primary transportation unit141, for example.

The number of the secondary transportation units142can be determined, according to the area of the fuel diffusion plate130. For example, if the fuel diffusion plate130is relatively large, the number of the secondary transportation units142can be increased, to increase the amount of fuel flowing there through. If the number of the secondary transportation units142is increased in the same area, the amount of fuel supply per unit area can also be increased. Thus, the fuel diffusion plate130can be more rapidly supplied with fuel, as compared to a conventional fuel diffusion plate, in which fuel is supplied through a single path, since the fuel supply unit100supplies the fuel simultaneously through the secondary transportation units142.

The diffusion sheet144is formed to completely cover a surface of the fuel diffusion plate130. The diffusion sheet144can, in some exemplary embodiments, include a first diffusion sheet144aand a second diffusion sheet144b. The diffusion sheet144may have a wetting direction. Due to the wetting direction, the diffusion sheet144can rapidly transport fuel from the secondary transportation units142, to the entire surface of the fuel diffusion plate130, by absorbing and transporting the fuel. For example, the diffusion sheet144can transport fuel by capillary action, wicking, or the like.

The second diffusion sheet144bis positioned under the first diffusion sheet144a. As depicted inFIG. 3, the first diffusion sheet144aand the second diffusion sheet144bhave wetting directions that are different from each other. The different wetting directions, of the first diffusion sheet144aand the second diffusion sheet144b, facilitate the rapid distribution of fuel to the entire surface of the fuel diffusion plate130. The wetting direction of the first diffusion sheet144amay be generally perpendicular to that of the second diffusion sheet144b, or may be otherwise angled from the wetting direction of the second diffusion sheet144b. Accordingly, the diffusion sheet144can rapidly distribute fuel supplied from the secondary transportation units142, to the entire surface of the fuel diffusion plate130, by absorbing and transporting the fuel.

FIG. 4is a cross-sectional view of a fuel cell system200having the fuel supply unit100, according to an exemplary embodiment of the present invention.FIG. 5is a cross-sectional view of a configuration of a unit cell150, ofFIG. 4. Referring toFIGS. 4 and 5, the fuel cell system200includes the fuel supply unit100ofFIG. 1, and the unit cell150, which is connected to the fuel supply unit100, to generate a current.

The unit cell150includes an anode152, a cathode154, and an electrolyte membrane153interposed between the anode152and the cathode154. The cathode154is exposed to the outside, so that the cathode154can be supplied with an oxygen source (air). The anode152is surrounded by a spacer155. Fuel is evaporated from the fuel diffusion plate130, and is supplied to the anode152, through a plurality of supply holes155aformed in the spacer155.

As shown below, the Chemical Equation 1 occurs at the anode152, to generate electrons, and the electrons move to the cathode154, along a circuit156, to participate in the Chemical Equation 2. A load157can applied to the circuit156. An assembly of the anode152, the cathode154, and the electrolyte membrane153is conventionally referred to as a membrane and electrodes assembly (MEA).
CH3OH+H2CO2+6H++6e−[Chemical Equation 1]
3/2O2+6H++6e−3H2O  [Chemical Equation 2]

Generally, the current generated from a single unit cell150is not large enough to use, and thus, a plurality of the unit cells150are connected in a series, on the fuel diffusion plate130. Thus, a desired voltage/current can be obtained, by using multiple unit cells150.

FIG. 6is a graph showing unit cell voltages, according to time, and temperatures of the unit cells, when the unit cells are supplied with fuel using the fuel supply unit100. Referring toFIG. 6, the horizontal axis indicates time in minutes, the vertical axis on the left side indicates voltages V of the unit cells, and the vertical axis on the right side indicates cathode surface temperatures, in ° C.FIG. 6shows the voltage increase in each of the unit cells, when sixteen unit cells are installed on the fuel diffusion plate130of the fuel supply unit100.

Referring toFIG. 6, the voltages of the sixteen unit cells uniformly increase, without a large deviation. This indicates that fuel is uniformly supplied to each of the unit cells, since the voltages increase in nearly the same manner. Thus, the fuel supply unit100uniformly distributes fuel to the entire surface of the fuel diffusion plate130, in a short time.

FIG. 6also shows temperatures measured by three temperature detecting devices that are installed in three different positions. It can be said that temperature nearly uniformly rises, although there is a slight temperature deviation (approximately within 2° C.). This result denotes that the amount of fuel cross-over is similar in each of the cells, and fuel was uniformly supplied to all of the sixteen cells. Thus, it is seen that the fuel supply unit100uniformly distributes fuel to the entire surface of the fuel diffusion plate130, in a short time.