Patent Application
20080199751
The present invention is directed to the field of bipolar plates for use in Fuel Cells or the like and, in particular, discloses a self air breathing bipolar plate design.
A fuel cell is an electrochemical device that converts chemical energy of a fuel (such as hydrogen or methanol) and oxidant (oxygen from air) into electrical energy and heat. The fuel cell has all the attributes of a battery, except that a fuel cell continues to produce electricity as long as fuel and oxidant are available, as opposed to a battery that stops producing power when the stored chemicals are exhausted. Several different types of fuel cells are under development. Amongst these, polymer electrolyte membrane (PEM) fuel cell is regarded as the most suitable technology for transport and small scale distributed power generation applications, because they operate at low temperatures (70-80° C.) and offer rapid start and shut down operation, unlimited thermal cycling capability and excellent load following characteristics. Around 50% of the power is available at cold start. A conventional polymer electrolyte membrane fuel cell stack consists of a number of cells called membrane electrode assemblies (MEAs) connected in series with the help of interconnect (bipolar middle and unipolar end ones) plates to produce the required stack voltage and power. Each cell (or MEA) consists of a proton conducting polymer membrane sandwiched between a hydrogen (anode) electrode and an oxygen (cathode) electrode. The interconnect plates serve dual purpose: to electrically connect one cell to the other (to conduct electrical current) and to distribute reactants (as well collect products) to (from) the respective electrodes of the MEAs. Hydrogen and air (source of oxygen) are supplied to the electrodes via flow field gas channels in the interconnect plates. On shorting the cell (or stack) through an external load hydrogen supplied to the anode gets oxidised to protons and electrons. Electrons travel through the external load and protons are transported through the membrane to the cathode, where they react with the oxygen supplied to cathode side and electrons from the external load to produce water as per following reactions.
At anode (Hydrogen electrode): H2=2H++2e
At cathode (Air electrode): 2H++½O2+2e=H2O
The oxygen depleted air along with the water formed on the air side of the MEA electrodes are collected by the gas flow channels. The air supplied to the oxygen electrode in addition to supplying oxygen, also helps in the removal of water formed at the electrode and thereby uncovering the reaction sites for more oxygen (air) access for the reaction. The voltage from a single cell under load conditions is in the range of 0.4 to 0.8V DC and current densities in the range 100 to 700 mA.cm−2.
In case of micro fuel cells for portable power applications, the fuel cell system is required to be smaller, simpler (without or less moving parts) and easily manufacturable at a mass scale. This is where the concept of self air breathing (no air compressors for oxygen supply to fuel cell), passive operation (no moving parts), miniaturisation of components (interconnects, micro fluid flow channels, overall system) and cheap fabrication methods have to be introduced to compete with batteries. There are two main configurations—stacking arrangement and planar or flat plate array design. In the planar configuration, the individual cells are laid flat side by side in a single plan, and oxygen (air) electrode side active area of each cell is exposed to atmospheric air for oxygen, and for water and heat exchange with the atmosphere. In a planar configuration series connections have to be established between individual cells with the negative of one cell connecting to the positive of the next cell on the other side of the array. In the stacking configuration, the cells are stacked one over the other with the help of bipolar interconnect plates. This simplifies connections between cells, however, it becomes difficult to provide atmospheric access to air side electrodes of the stack in a passive operation with no external air compressors. The stacking is generally used for bigger size fuel cell units (>10-20 We range). In a stacked configuration, the series connection between one cell to the next cell is in-built as the interconnect plate between any two cells acts as a bipolar plate and therefore, no special connections are required to be made between the cells. Secondly, the resistive losses due to connections between cells are expected to be very low as the contact area between cells is significantly higher (basically it's the resistance of the bipolar plate across its thickness).
Conventional fuel cells require the supply of compressed air to the oxygen electrode of the fuel cell to supply oxygen and to remove water produced by the electrochemical reaction. This increases the complexity of the system in portable power applications. However, if the oxygen electrode of each fuel cell in the assembled stack can be exposed to atmospheric air, the cells can self breath oxygen from the atmosphere. This requirement can be achieved by horizontal placement of cells in a planar configuration, whereby all the respective oxygen electrodes of cells are on one side and hydrogen electrodes are on the other side. However, planar array designs are limited to low overall power output due to limitations on the fuel cell area that needs to be exposed to air. Therefore, for higher power output (e.g. above 20-30 We), stacking configuration would be more appropriate. A stacked design offers substantial flexibility in terms of the electrode area and the number of cells that can be stacked (connected in series). The challenge, however, is how to expose oxygen electrode side of each of the cells in the stack to oxygen in atmospheric air without utilising a forced air supply thus combining the features of stacked configuration in a self air breathing compact design.
Examples of air breathing fuel cells exist in the prior art. For example, U.S. Pat. Nos. 4,407,904 to Uozum et al, 4,977,041 to Siozawa et al, 5,508,128 to Akagi, 6,218,035 to Fuglevand et al disclose air breathing fuel cell arrangements, the contents of which are hereby incorporated by cross reference.
It is an object of the present invention to provide an improved form of air breathing fuel cell arrangement.
In accordance with a first aspect of the present invention, there is provided a bipolar interconnect plate for a fuel cell, including: a first surface having a series of conductive interconnect posts for forming a conductive interconnect for conductively interconnecting, in use, with the cathode (air or oxygen) surface; the plate including a series of ridges surrounding the first surface having air access slots therein in fluid communication with the first surface.
The second surface of the plate preferably can include a series of fuel supply channels formed therein, the fuel supply channels mating with an anode surface in use to supply a fuel to the surface of the anode. Preferably, side ridges surround the first surface for, in use, forming a seal against a membrane surface.
The plate preferably can include a series of apertures for the transmission of fluid there through. The plate can be formed from fine grain graphite impregnated with a resin. The plate can be formed from a metal that has been processed by means of at least one of electoetching, electroplating, stamping or embossing.
Ideally, the plates are used in a mutltiplate fuel cell stack, each interposed and interconnected to a membrane electrode assembly. The fuel cell can be arranged in a stacked configuration. In one embodiment, the air can be fan fed to the fuel cell using power from the fuel cell.
In some embodiments, the plate can be formed from two sub plate joined together. The joining of the two sub plates are preferably joined together by one of spot welding, or using electrically conducting adhesives or glues. Pins or nails are preferably utilised to form a conductive interconnect between the subplates. Several portions of the plate are preferably fabricated separately and joined to form the interconnect plate.
Preferably, the plate can be formed from a metal that has corrosion protection coating. The plate can be utilised in a multi cell fuel cell array and preferably can include a multi cell interconnect where two or more cells are preferably interconnected in a planar arrangement and subsequently stacked with a number of such planar cell arrays. The conductive interconnect posts can have a cross section that can be one of rectangular, circular, hexagonal, elliptical, octagonal. The air access slots are preferably of different shapes. The interconnect plate also acts as a current collection plate.
Benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of exemplary embodiments and the appended claims, taken in conjunction with the accompanying drawings, in which:
The preferred embodiments provide the ease of series connection using bipolar interconnects (where two adjoining MEAs sandwich a bipolar interconnect plate) with a self air breathing concept of micro fuel cells to construct fuel cell stacks from a few watt to several hundred watts. The bipolar interconnect design of the preferred embodiment has been developed to provide oxygen access from atmospheric air to oxygen electrode of each cell in a fuel cell stack. It consists of an electrically conducting plate with hydrogen flow channels on one side and ‘pillar and land’ design on the other side. The pillars provide the electrical contact with the oxygen electrode of membrane electrode assembly (MEA), and lands (open areas) exchange oxygen and water with the atmospheric air through a series of slots on the periphery of the interconnect plate. By controlling the size and number of lands and pillars, fluid flow, current and heat distribution can be optimised in the stack. Compact packaging of a large number of cells (large effective surface area) is possible due to stacked configuration, avoiding the surface area limitations of the planar design.
The design permits the self management of excess heat (dissipation to atmosphere) and water as well as allowing effective membrane humidification. Manifolding and sealing is also very simple. In this design, in addition, a micro chip fan, driven by the fuel cell stack power, can be incorporated for forced air flow which further increases the power density available from the stack and allows even more effective heat and water management. This design extends air breathing concept to stacking of single cells thus allowing easy construction of 10-500 We small fuel cell systems while still keeping a compact overall size.
There are number of variations possible in design variables of this interconnect such as pillar dimensions, pillar-to-land ratio, cell active area-to-air access slot ratio etc., which can be optimised to cater for the particular application and power requirements of the device.
The design of the preferred embodiment of a bipolar interconnect plate allows oxygen (from atmospheric air) access to the oxygen electrode of each cell of the stack, making it possible to realise a self air breathing fuel cell device in a stacked configuration. The ‘lands and pillars’ design of the air side of the interconnect allows full control on the size and number of lands and pillars for optimisation of the fluid flow (air/oxygen circulation), current and heat distribution in the stack. The design of the preferred embodiment permits the self management of excess heat (dissipation to atmosphere) and water as well as allowing effective membrane humidification. Compact packaging of a large number of cells (large effective surface area) is therefore possible due to stacking configuaration, avoiding the surface area limitations of the planar design. In this design, in addition, a stack voltage driven micro chip fan can be incorporated for forced air flow which further increases the power density available from the stack and allows even more effective heat and water management. Manifolding and sealing is also very simple. The design extends air breathing concept to stacking of single cells thus allowing easy construction of 10-500 We, small fuel cell systems while still keeping the overall size fairly compact without forced air supply thus combining the good features of stacked configuration in a self air breathing compact design.
The preferred embodiment allows a series connection using bipolar interconnects (where two adjoining MEAs sandwich a bipolar interconnect plate) with the self air breathing concept of micro fuel cells to construct fuel cell stacks from a few watt to several hundred watts. The design has been demonstrated in the 10-50 We power range stacks with and without a stack voltage driven fan. However, the design can be scaled up for higher power outputs, in the 100-500 We range. This design can also be used in combination with a planar configuration, for example 4 cells assembled in a planar arrangement on a single multi cell interconnect, and stacked with a number of such 4-cell planar arrangements. This type of multi array, parallel cell design would have built-in redundancy in the case of a cell failure in the array.
The bipolar interconnect plate designed in accordance with the principles of the preferred embodiment was initially constructed in exemplary form by construction and assembly of a 6 cell self air breathing polymer electrolyte membrane (PEM) fuel cell stack.
As a variation of the above design, there can be any number of other MEAs in the stack, and there could be a different design of the gas manifolding for distribution (fuel) and collection of spent fuel, water and/or other products. The stack can also be used for any other fuel such as methanol, ethanol etc.
An interconnect plate of an assembled self air breathing fuel cell stack is designed in such a way that oxygen electrode of each cell has an access to the atmosphere for oxygen, and heat and water exchange. The bipolar plates 9,10 were constructed using fine grain graphite impregnated with a resin. However, as a variation of the above, the plate can be fabricated from a metal or an alloy that does not corrode or any metal (or alloy) with a corrosion resistant coating. The overall dimensions of the interconnect plates 9,10 for the six cell stack were 6 mm×60 mm×60 mm. As a variation, the dimensions (thickness and size) and shape (circular, square, hexagon, octagon, etc) of the interconnect plate can be different, as determined by the active area of each cell, gas manifolding design, heat distribution in the stack, and shape and size of the appliance (application). Interconnect bipolar plates for the six cell stack were fabricated by CNC machining.
Other methods for constructing the bipolar plates can be utilised. As a variation, the complete interconnect plate or some of the features can be fabricated using other technologies such as electroetching, electroplating, stamping, embossing etc. Instead of using a single block of material to fabricate both air and fuel side flow fields, there can be two separate plates fabricated—one with hydrogen flow field and the other with air flow field, and these plates are joined together with flow fields opposing each other. The joining of the two plates can be carried out by methods such as spot welding, or using electrically conducting adhesives or glues. Also where conducting adhesive are not used pins or nails may be used to make electrical contact between various components. In another variation several components fabricated separately may be joined as described above to form the interconnect plate. The end plates 11, 12 can be constructed in a similar manner but will only include one surface profile (Air or Hydrogen Profile) as required.
Hydrogen flow field: As illustrated in
Airflow field: The air flow field is illustrated in
Compact packaging of a large number of cells (large effective surface area) is possible due to stacking configuration, avoiding the surface area limitations of the planar design. This design permits the self management of excess heat (dissipation to atmosphere) and water as well as allowing effective membrane humidification. Manifolding and sealing is also very simple. In this design, in addition, a stack voltage driven micro chip fan can be incorporated for forced air flow which further increases the power density available from the stack and allows even more effective heat and water management. There are number of design variables of this interconnect such as pillar dimensions, pillar-to-land ratio, cell active area-to-air access slot ratio etc., which can be further optimised to cater for the application and power requirements of the device.
Returning initially to
As a variation interconnect plates at both ends of the stack can be used as interconnects as well as current collector plates. These plates would have extended tabs for electrical connection to the electrical load. This will avoid the use of additional current collector plates.
Titanium plates of thickness 4 mm have been used as stack assembling plates 16, 17.
In order to prevent any leakage of hydrogen gas from hydrogen compartment to atmosphere or air side of the MEAs, silicone rubber gaskets 25, 26 were used in the stack assembly.
The stack is assembled as schematically shown in
An example stack was tested in a test station on industrial grade hydrogen.
The six cell stack was operated for a period of about 2700 hours as shown in
In order to reduce the overall size, especially length of the stack, the thickness of the bipolar interconnect plate was substantially reduced in an alternative embodiment. The hydrogen flow field depth was reduced from 1 mm to 0.5 mm (i.e. channels are 1 mm wide and 0.5 mm deep, and ribs are 1 mm wide and 0.5 mm high). The pillars of the air flow field were reduced in height from 3 mm to 2 mm (i.e. the pillars of cross section 2 mm×2 mm are now 2 mm high with 2 mm space between each other). Now, there is a solid graphite thickness of 0.5 mm instead of 1 mm in the previous design, between the lands of air flow field and base of hydrogen flow field channels. This resulted in the final thickness of the plate as 3 mm.
In order to evaluate the new design of the interconnect plate, a 2-cell stack was assembled, with one MEA assembled with a thinner bipolar plate (new design with air breathing holes) and the other with a thicker interconnect plate (old design with air breathing slots).
Although the present invention has been described with particular reference to certain preferred embodiments thereof, variations and modifications of the present invention can be effected within the spirit and scope of the following claims.
