This invention relates to the design and development of a proton exchange membrane fuel cell. Fuel cell is a power conversion device that converts the chemical energy into electric energy through an electrochemical reaction of a fuel and an oxidant. For the fabrication of the fuel cell, acrylic sheets of 6 mm thickness were used and 10 individual cells with area 9 cm2 each were connected in series to make the stack assembly with the total area of the electrode being 90 cm2. The bipolar plates considered for the anode and cathode compartments were made of graphite. For the anode, the grooves for the flow of hydrogen were made in the serpentine pattern. For the cathode, forced air breathe was considered and the stacking method used was horizontal stacking. The fuel gas (Hydrogen) was supplied to the individual cells through manifolds and were then distributed inside single cells along passages or channels in flow field plates, so as to supply reactant gases to the entire area of the anode. At the anode hydrogen splits into constituent protons and electrons, while the protons pass through the electrolyte membrane and the electrons necessarily take up the path of an external circuit thus electric power is generated. At the cathode, protons and electrons react with atmospheric oxygen electrochemically forming water as a product. The electrolyte used was Nafion 117 and the membrane electrode assembly was prepared using Pt catalyst in Vulcan XC-72. The performance evaluation of the above mentioned MEA was carried out.
The present invention relates to the fabrication of a fuel cell stack and subsequent performance evaluation of the fabricated stack using a perfluorosulfonic acid membrane.
Background of the Invention:
During the last one to one and half decades, fuel cells are the subject of much attention due to its eco-friendly nature as there are no toxic emissions like COX, NOX etc. The fuel cells are considered to replace the conventional combustion engines in vehicles used for transportation.
Objective of the Invention:
The main objective of the current invention is to fabricate a stack containing ten fuel cells arranged in horizontal fashion with graphite plates for the flow of the fuel and oxidant gases and a serpentine flow pattern for the fuel gas and forced air breathe for the cathode.
Brief description of the prior art:
Yi et al (Fuel Cells 1, 111-117 (2010)) discusses PEMFC stack based on undulate MEAs with perforated bipolar plates. A hot-pressed 5-layer undulate MEAs with Nafion 112 was studied.
J. Scholta et al (Journal of Power Sources 127, 206-212 (2004)) reported stack concept using graphite composite bipolar plates. The tests reported show that the design goal of 10 kW could be significantly exceeded.
Song Tae-Won et al (US Patent Application 20090035611) reported a method of starting a polymer electrolyte membrane fuel cell (PEMFC) stack by rapidly increasing the temperature of the PEMFC stack. The PEMFC stack included a first flow line that is connected to upper parts of cooling plates installed in a plurality of unit cells of the PEMFC stack; a second flow line that is connected to lower parts of the cooling plates.
Kim Ji-Rae et al (US2002142205 (A1)) discusses a proton exchange membrane fuel cell (PEMFC) stack with at least two cell units with each cell unit includes: a catalyzed membrane formed by catalyzing both surfaces of an electrolyte layer; a fuel flow field portion and air flow field portion formed at opposite surfaces of the catalyzed membrane, at least one of the fuel and air flow field portions comprising a parallel flow field which induces gas flow in a direction parallel to the surface of the catalyzed membrane and an orthogonal flow field which induces gas flow in a direction orthogonal to the surface of the catalyzed membrane; a bipolar plate in contact with each outer surface of the fuel flow region portion and the air flow region portions.
Complete Specification:
The present invention relates to a stacking of individual cells in a fuel cell. Fuel cells are well defined as excellent electrochemical energy conversion device that directly convert the chemical energy of the fuel into electric energy without any intermediate mechanical energy. In recent years, considerable attention has been devoted to the development of polymer electrolyte membrane fuel cell (PEMFC) for transportation applications. Also, the fuel cells unlike the conventional power engines are interesting from the environmental point of view as they are nearly zero-pollution causing devices.
A typical fuel cell consists of a proton conducting membrane, electrodes (anode and cathode), graphite plates with passages for the fuel (hydrogen) and oxidant (generally oxygen and sometimes air), current collectors and the endplates. The two electrodes are sandwiched on either side of the proton conducting membrane and the resulting assembly is popularly known as the Membrane Electrode Assembly (MEA). The current collectors are utilized to draw the generated electric energy which may then be used for any purposes such as in vehicles, small electric devices etc. Also the current collectors provide mechanical support to the MEAs. All the aforementioned materials are compacted between the two endplates placed on either side.
The electrochemical reactions that take place in the fuel cell are (i) oxidation of fuel at the anode and (ii) reduction at the cathode. The two reactions along with the cell reaction may be represented as follows:
2H2 → 4H+ + 4e- (At Anode) ----------------------- (1)
O2 + 4H+ + 4e- → 2H2O (At Cathode) ----------------------- (2)
2H2 + O2 → 2H2O (Overall Cell Reaction) ----------------------- (3)
The half reaction that takes place in the anode as denoted in equation (1) is the oxidation of hydrogen and that in the cathode is the reduction reaction as denoted in equation (2) yielding water. The overall net reaction is as simple as hydrogen combines with oxygen to form water together with a little heat as the fuel cell reactions are exothermic in nature along with production of electricity.
In the current invention, a stack assembly was arranged in a horizontal fashion. A fuel cell stack is the arrangement of single cells connected electrically in series or in parallel. The total number of cells considered for the stack is ten and the dimension of the single cell is
30 mm x 30 mm with the total area of 900 sq.mm. The material taken for the passage of fuel is graphite and flow field in the graphite plates are of 4 mm thickness on the anodic compartment. The grooves for the flow of the fuel are of 2 mm width and a serpentine pattern is followed. The depth of the grooves is 0.7 mm and the total length is 270 mm/cell. The dimensions of the anode are schematically represented in Figure 1 and Figure 2. On the other hand, the cathodic compartment contains only a forced air breathe. Once again, the material chosen as the bipolar plate is graphite with 4 mm thickness. The width of the groove is 3 mm. There were 20 holes in each cell with a diameter of 2.5 mm. The dimensions of the cathode are schematically represented in Figure 3 and Figure 4. The endplate is made of acrylic sheet of thickness 6 mm that is carved in, to fix the single cells so that the displacement of cells at later stage is prevented. The pocket size for each cell is 30 mm x 30 mm and the depth of the pocket in the endplate is 4 mm. The overall dimension of the endplate is 270 mm x 120 mm x 6 mm and is schematically represented in Figure 5. The number of bolt holes made for tightening process is 14.
The membrane used for the fabrication of Membrane Electrode Assembly is Nafion 117. The fabrication of electrodes for the MEAs consists of three parts.
Teflonization and Coating of diffusion layer
Coating of the catalyst layer Stage I and
Coating of the catalyst layer Stage II.
Teflonization and Coating of diffusion layer:
Initially the carbon fabric of thickness 0.25 mm and of dimension 3.5 cm x 3.5 cm is dipped in a 12% Teflon dispersion. It is then sintered in a muffle furnace at 350 ËšC for 5 hours. It is then hot pressed at 100 ËšC for 5 minutes to enable the Teflon particles to accommodate the pores available in the carbon fabric. For the gas diffusion layer, Vulcan XC 72 (3 mg/cm2) was first mixed with Ammonium bicarbonate (50% with respect to Vulcan XC 72) and deionised water (150 ml for 20 g of Vulcan XC 72) and sonicated for 5 minutes. Here ammonium bicarbonate was added to facilitate the dispersion process. Then isopropyl alcohol (500 ml for 20 g of Vulcan XC 72) was added and again sonicated. Then 60% Teflon dispersion (40% with respect to Vulcan XC 72) was added and mixed gently with the help of a brush. The resulting mixture was coated immediately on both sides of the carbon fabric with the brush. It is then sintered at 350 ËšC for 5 hours.
Coating of catalyst layer Stage I:
The amount of catalyst for the two electrodes (anode and cathode) during the stage I is same. The catalyst used was 20% platinum in Vulcan XC 72. The catalyst powder
(0.125 mg/cm2) was mixed with ammonium bicarbonate (50% with respect to the amount of catalyst) and distilled water (0.6 ml/25 cm2 of the carbon fabric) and sonicated for 5 minutes. The sonication results in a fine dispersion of the catalyst. Then isopropyl alcohol (0.8 ml/ 25 cm2 of the carbon fabric) was added and once again sonicated for five minutes. To the resulting mixture, 60% Teflon dispersion (35% with respect to the amount of catalyst) was added, mixed and immediately coated on one side of the carbon fabric. It is then sintered in a muffle furnace at 350 ËšC for 5 hours.
Coating of the catalyst layer Stage II:
The amount of the catalyst for the anode and cathode during the stage II varies. Since the reduction process that occurs in the cathode is more difficult than the oxidation reaction of hydrogen at the anode, an excess of the platinum catalyst was taken at the cathode. The platinum catalyst (0.125 mg/cm2 for the anode and 0.375 mg/cm2 for the cathode) is mixed with distilled water (0.6 ml/25 cm2 of the carbon fabric) and sonicated for 5 minutes. Then a 5% Nafion solution (35% with respect to the amount of catalyst) was added and mixed. It is then coated on the same side as stage I.
On either side of the membrane, the electrodes were placed and pressed at 80 ËšC for
2 minutes. The resulting assembly is the membrane electrode assembly that is used in the fabricated fuel cell. After the electrodes were pressed on either sides of the membrane, Teflon gaskets were kept to ensure prevention of gas leakages during fuel cell operation. Then, the bipolar plates made of graphite were attached. The bipolar plates provide the path for the fuel and oxidant gases to flow inside the fuel cell. The current collectors are arranged over the bipolar plates in order to draw the generated electric energy from the fuel cell. Finally, the acrylic endplates were used to tighten the entire assembly by means of bolts and nuts. A schematic representation of the various components in the MEA is shown in Figure 6.
The Cell performance and the polarization curve are given in Table 1 and Figure 7 respectively. The amount of electric energy produced in each cell has also been determined by connecting the individual cells to a voltmeter. With the help of the electric energy generated, we were able to play a radio which consumed 1.5 Watts of power.
Brief Description of the Drawings
Figure (1) Schematic representation of Anodic Flowfield of a Single Cell in the Stack
Figure (2) Dimensions of Cells at Anode in the Stack
Figure (3) Schematic representation of Cathodic Flowfield of a Single Cell in the Stack
Figure (4) Dimensions of Cells at Cathode in the Stack
Figure (5) Dimensions of the Acrylic Plate
Figure (6) Various Components of the MEA
Figure (7) Polarization Curve of the Fuel Cell Stack
Detailed Description of the Drawings
Referring to figure 1, it shows the flowfield of the fuel gas (Hydrogen) in the anodic compartment. A serpentine pattern is followed for the flow of the fuel gas in the anode.
Referring to figure 2, it shows the dimensions of the cells in the anodic compartment, the interconnections between the cells for the fuel gas to flow through, the distance between the individual cells in the stack and also the holes made for the tightening purpose.
Referring to figure 3, it shows the flowfield of the oxidant gas (Air) in the cathodic compartment. A forced air breathe is followed for the flow of the oxidant gas in the cathode.
Referring to figure 4, it shows the dimensions of the cells in the cathodic compartment, the distance between the individual cells in the stack and also the holes made for the tightening purpose.
Referring to figure 5, it shows the dimensions of the acrylic plate used as the endplate and also the holes made for the tightening purpose. The acrylic plate gives a mechanical support to the cell.
Referring to figure 6, it shows the various components in the MEA. The order may be mentioned as anodic current collector - anodic graphite plate - anodic gasket - anodic electrode - membrane - cathodic electrode - cathodic gasket - cathodic graphite plate - cathodic current collector.
Referring to figure 7, it shows the power generated without load (OCV) and with load of the stack as well as the individual cells.
Examples:
We Claim:
A fuel cell hybrid electric energy source consisting of ten individual fuel cell connected horizontally wherein the dimension of each cell being 30 mm x 30 mm with the flowfield material made of graphite plates of thickness 4mm for both anode as well as cathode. For the anode, the grooves in the graphite plate are of 2 mm width with a depth of 0.7 mm and a serpentine pattern is followed.
The total length of the grooves as per claim 1 is 270 mm/cell.
The cathodic graphite plate as per claim 1 is made in such a way such that a forced air breathe is followed for the flow of oxygen in the cathode with the width of the groove for the air breathe being 3 mm. The number of holes for the air breathe is 10 in each cell with a diameter of each hole being 2.5 mm.
The endplate used for the tightening the MEA along with the graphite plates as per claim 1 and 3 on both sides of the cell are made of acrylic sheet of thickness 6 mm. The endplates are carved in to fix the individual single cells and the number of bolt holes made for tightening of the stack is 14.
A fuel cell hybrid electric energy source substantially as herein described with reference to and as illustrated in figures 1 to 6 of the accompanying drawings.
