FUEL CELL BIPOLAR PLATE DESIGN FOR REDUCED CORROSION POTENTIAL

A fuel cell system includes: a first fuel cell; a second fuel cell; a cathode configured to receive a positive charge from the first fuel cell and the second fuel cell; an anode disposed apart from the cathode and configured to receive a negative charge from the first fuel cell and the second fuel cell; a manifold enclosing the anode and the cathode; coolant disposed within the manifold and surrounding the cathode and the anode; and a seal disposed between the cathode and the anode so as to prevent the coolant from leaking into the first fuel cell, wherein the cathode includes a seal portion disposed adjacent to the seal and a remaining portion separated from the seal by the seal portion, and wherein the remaining portion of the cathode is configured to be non-parallel with the anode so as to reduce shunt current at the seal portion.

TECHNICAL FIELD

One or more embodiments relate generally to fuel cell systems.

BACKGROUND

There is a shifting from fossil fuels to renewable energy as an alternative for industrial purposes, household purposes, vehicles, and small electronic products such as portable devices. Accordingly, there is active research and development on fuel cells. A fuel cell is a generator that converts chemical energy of a fuel into electrical energy through an electrochemical reaction.

A problem with a fuel cell is corrosion of electrodes that result gas leak in the fuel cell.

SUMMARY

An aspect of the present disclosure is drawn to a fuel cell system including: a first fuel cell; a second fuel cell electrically connected in series with the first fuel cell; a cathode configured to receive a positive charge from the first fuel cell and the second fuel cell; an anode disposed apart from the cathode and configured to receive a negative charge from the first fuel cell and the second fuel cell; a manifold enclosing the anode and the cathode; coolant disposed within the manifold and surrounding the cathode and the anode; and a seal disposed between the cathode and the anode so as to prevent the coolant from leaking into the first fuel cell, wherein the cathode includes a seal portion disposed adjacent to the seal and a remaining portion separated from the seal by the seal portion, and wherein the remaining portion of the cathode is configured to be non-parallel with the anode so as to reduce shunt current at the seal portion.

In some embodiments of this aspect, the remaining portion of the cathode includes a parallel portion that is parallel with the anode and a rising step portion that is not parallel with the anode. In some of these embodiments, the parallel portion is disposed a first distance from the anode and the rising step portion is disposed at a second distance from the anode, the first distance is greater than the second distance, and the rising step portion connects the parallel portion with the seal portion.

In some embodiments of this aspect, the remaining portion of the cathode is curved such that at least one portion of the remaining portion is disposed a first distance from the anode and the seal portion is disposed at a second distance from the anode, and the first distance is less than the second distance.

In some embodiments of this aspect, the anode includes an anode seal portion disposed adjacent to the seal and an anode remaining portion separated from the seal by the anode seal portion, and the remaining portion of the cathode is configured to be non-parallel with the anode remaining portion.

In some embodiments of this aspect, the remaining portion of the cathode includes an end that is bent toward the anode.

Another aspect of the present disclosure is drawn to a method of making a fuel cell system. The method includes: forming a first fuel cell; forming a second fuel cell electrically connected in series with the first fuel cell; forming a cathode configured to receive a positive charge from the first fuel cell and the second fuel cell; forming an anode disposed apart from the cathode and configured to receive a negative charge from the first fuel cell and the second fuel cell; forming a seal between the cathode and the anode; enclosing the anode and the cathode with a manifold; and disposing coolant within the manifold and so as to surround the cathode and the anode, wherein the seal prevents the coolant from leaking into the first fuel cell, wherein the forming the cathode includes forming the cathode so as to include a seal portion disposed adjacent to the seal and a remaining portion separated from the seal by the seal portion, and wherein the forming the cathode includes forming the remaining portion so as to be non-parallel with the anode so as to reduce shunt current at the seal portion.

In some embodiments of this aspect, the forming the cathode includes forming the remaining portion so as to include a parallel portion that is parallel with the anode and a rising step portion that is not parallel with the anode. In some of these embodiments, the forming the remaining portion includes forming the parallel portion so as to be disposed a first distance from the anode, the rising step portion is disposed at a second distance from the anode, the first distance is greater than the second distance, and the forming the remaining portion includes forming the rising step portion so as to connect the parallel portion with the seal portion.

In some embodiments of this aspect, the forming the cathode includes forming the remaining portion to be curved such that at least one portion of the remaining portion is disposed a first distance from the anode, the seal portion is disposed at a second distance from the anode, and the first distance is less than the second distance.

In some embodiments of this aspect, the forming the anode includes forming the anode so as to include an anode seal portion disposed adjacent to the seal and an anode remaining portion separated from the seal by the anode seal portion, and the forming the cathode includes forming remaining portion so as to be non-parallel with the anode remaining portion.

In some embodiments of this aspect, the forming the cathode includes forming the remaining portion so as to include an end that is bent toward the anode.

Another aspect of the present disclosure is drawn to a fuel cell system including a fuel cell, an anode, a cathode, a manifold surrounding the anode and the cathode, the improvement including: wherein the cathode includes a seal portion and a remaining portion, and wherein the remaining portion of the cathode is configured to be non-parallel with the anode so as to reduce shunt current at the seal portion.

In some embodiments of this aspect, the remaining portion of the cathode includes a parallel portion that is parallel with the anode and a rising step portion that is not parallel with the anode. In some of these embodiments, the parallel portion is disposed a first distance from the anode and the rising step portion is disposed at a second distance from the anode, the first distance is greater than the second distance, and the rising step portion connects the parallel portion with the seal portion.

In some embodiments of this aspect, the remaining portion of the cathode is curved such that at least one portion of the remaining portion is disposed a first distance from the anode and the seal portion is disposed at a second distance from the anode, and the first distance is less than the second distance.

In some embodiments of this aspect, the anode includes an anode seal portion disposed adjacent to the seal and an anode remaining portion separated from the seal by the anode seal portion, and the remaining portion of the cathode is configured to be non-parallel with the anode remaining portion.

In some embodiments of this aspect, the remaining portion of the cathode includes an end that is bent toward the anode.

DETAILED DESCRIPTION

Electrolytic corrosion is a major topic that affects the lifetime of fuel cells. Electrochemical oxidation of the surface of bipolar plates, particularly the cathode, leads to all kinds of functional degradations. In particular, the potential difference between the anode and cathode causes the corrosion reactions around the seal area. Eventually, the seal will lose its adhesive strength. This will be described in greater detail with reference toFIGS.1-5C.

FIG.1illustrates a related art fuel cell system100.

As shown in the figure, fuel cell system100includes a stack of an array of fuel cells102, a manifold104, a heat transfer system106, and a coolant108. Stack of the array of fuel cells102includes a plurality of fuel cells connected in series, a sample of which is indicated by plurality of fuel cells110. Each of the plurality of fuel cells connected in series includes individual fuel cells connected in series, a sample of which is indicated by fuel cell112and fuel cell114within plurality of fuel cells110.

Each fuel cell may be any known type of fuel cell.

Coolant108is contained within manifold104and may be any known type of coolant that is configured to absorb heat generated by the stack of the array of fuel cells102.

Heat transfer system106may be any known type of heat transfer system that is configured to extract heat from coolant108and expel the heat to outside of fuel cell system100.

In operation, each plurality of fuel cells will generate a voltage, which is added in series to the last cell in the series. For example, in plurality of fuel cells110, all the cells will add voltage until the final summed voltage is accumulated at the cathode at the end116. Channel120enables coolant108to circulate through the ends of each plurality of cells within the stack and absorb heat to prevent overheating of the fuel cells.

Heat transfer system106receives heated coolant108from manifold104, removes the heat from heated coolant108, and recycles cooled coolant back into manifold104. In this way, the stack of fuel cells does not overheat. The coolant path is provided to regulate the temperature of the array of fuel cells102. Coolant108flows through the array of fuel cells102and the manifold104of fuel cell system104.

Coolant108contains some electrolytes to enable transfer of the charge from end116to output cathode/anode pair118, and similarly for the rest of the cells in the stack. Unfortunately, the electrolytes in coolant108additionally react with the exposed cathodes within the stack. This will be described in greater detail with portion118and with reference toFIG.2A.

FIG.2Aillustrates an exploded view of portion118of related art fuel cell system100.FIG.2Billustrates a side view of an anode208in exploded view portion118.

As shown in the figure, portion118includes coolant108, fuel cell112, fuel cell114, membrane electrode gas-diffusion layer assembly (MEGA)202, a seal204, a cathode206, and anode208. Current flows from anodes to cathodes and through a hole214within coolant108as shown by arrows216. Each cathode and anode include a hole, similar to hole214, which together form channel120, as shown inFIG.1.

Each fuel cell includes a positively charged cathodic side, a sample of which indicated as cathodic side218, and a negatively charges anodic side, a sample of which is indicated as anodic side220. Each cathodic side is separated from a respective anodic side by MEGA202. MEGA202permits charge transfer from an anodic side to a respective cathodic side, without enabling the constituents of the cathodic side to physically contact the constituents of the anodic side, and vice versa. Positive charge flows from an anode to a cathode in MEGA202(or in side of the fuel cell), but positive charge flows from a cathode to an anode in the coolant path.

Coolant108contacts exposed cathode206and exposed anode208. As a result of this contact, coolant oxidizes (corrodes) these electrodes at different rates. In particular, as will be discussed in greater detail below, the rate of oxidation is related to the shunt current density at an electrode. Further, as will be described in greater detail below, the area of a cathode that is facing an anode and is closest to the seal has the greatest shunt current density and therefore has the greatest rate of oxidation.

Further, returning toFIG.1, the potential difference experienced throughout coolant108increases from right to left. As such, the area of a cathode, at area116, that is facing an anode and is closest to the seal has the greatest shunt current density and therefore has the greatest rate of oxidation.

FIG.3illustrates a graph300of shunt current as a function of the number of cells in related art fuel cell system100.

As shown in the figure, graph300includes an x-axis302representing the number of arrays of fuel cells, a y-axis304of shunt current measured in amps, and a plot306. Plot306includes an entry for each array of fuel cells, samples of which include entry308and entry310.

Let entry308correspond to the area of a cathode, at area116, that is facing an anode and is closest to the seal. Further, let entry310correspond to the area of the cathode, directly to the right of area116as shown inFIG.1, that is facing an anode and is closest to the seal.

As shown in graph300ofFIG.3, there is a dramatic increase in the shunt current at area116as compared to the next closest cathode.

The area of a cathode that is facing an anode and is closest to the seal, and which has the greatest shunt current density will now be further described in greater detail with reference toFIG.4.

FIG.4illustrates an exploded view of portion222of portion118ofFIG.2A.

As shown inFIG.4, portion222includes coolant108, MEGA202, seal204, cathode206, anode208. A portion302is outlined in a dotted line which includes the area of cathode206that is facing anode208and is closest to seal204. The effect of the oxidation of this area as a result of the increased shunt current will be described in greater detail with reference toFIGS.5A-C.

As shown inFIG.5A, cathode206includes a surface502in contact with coolant108and seal204. For purposes of discussion, presume that the cell including cathode206has not yet been used, such that there has been no reaction between coolant108and surface502.

As shown inFIG.5C, ultimately corroded area504has grown such that seal204might be removed at area506. The removal of seal204at area506may result in gases from the cell mixing with coolant108, which breaks the cell.

There are two conventional ways to protect bipolar plates against oxidation.

The first is to fabricate a layer of precious metal such as gold or platinum on the plate. Because they are anti-oxidant materials, only the electrolysis of water occurs and the surface metal condition remains unchanged. Carbon or resin are also be used as cheaper materials for similar purposes.

The second conventional way to protect bipolar plates against oxidation is to use the end cell as a sacrificial electrode. Because the currents from all the cells in the stack are balanced by Kirchhoff's current law, placing an easily oxidized plate at the end of the stack can reduce the amount of oxidation current flowing through the rest of the cells.

However, the above-mentioned techniques are expensive because they require the use of costly materials and equipment.

There is a lot of research trying to reduce the costs by tuning material composition, deposition method and so on. However, they are still too expensive to promote in the use of fuel cells.

What is needed is a system and method for protecting bipolar plates in a fuel cell system against oxidation without fabricating additional layers of precious metals on the plates and without sacrificing the electrodes in the end cell.

A system and method in accordance with the present disclosure protects bipolar plates in a fuel cell system against oxidation without fabricating additional layers of precious metals on the plates and without sacrificing the electrodes in the end cell.

The purpose of a fuel cell system in accordance with aspects of the present disclosure is to improve durability of the seal part which separate the coolant and gas domains.

A fuel cell system in accordance with aspects of the present disclosure focuses on corrosion in the coolant domain where very high voltage is potentially applied to hundreds of stacked cells and the large shunt current accelerates the chemical reactions on the bipolar plates.

The proposed technique does not require any additional layer to protect the surface of the bipolar plate from corrosion. As such a fuel cell in accordance with aspects of the present disclosure provides a more durable fuel cell in an economical manner

An example system and method for protecting bipolar plates in a fuel cell system against oxidation without fabricating additional layers of precious metals on the plates and without sacrificing the electrodes in the end cell in accordance with aspects of the present disclosure will now be described in greater detail with reference toFIGS.6-18.

FIG.6illustrates a fuel cell system600in accordance with aspects of the present disclosure.

As shown in the figure, fuel cell system600includes a stack of an array of fuel cells602, manifold104, heat transfer system106, and coolant108. Stack of the array of fuel cells602includes a plurality of fuel cells connected in series, a sample of which is indicated by plurality of fuel cells604. Each of the plurality of fuel cells connected in series includes individual fuel cells connected in series, a sample of which is indicated by fuel cell112and fuel cell114within plurality of fuel cells604.

Fuel cell system600operates in a manner similar to fuel cell system100discussed above with reference toFIG.1. However, as will be discussed in greater detail below, fuel cell system600includes modified cathodes which reduce shunt current near the seal and thus reduce the rate of corrosion.

Slowing down the reaction rate at a particular site can be achieved by changing the shape of each cell or the shape of the entire stack. This will be described in greater detail with reference toFIG.7.

FIG.7illustrates an exploded view of portion606of fuel cell system600.

As shown inFIG.7, portion606includes coolant108, MEGA202, seal204, a cathode702, and an anode704. Cathode702includes a seal portion706disposed adjacent to seal204and a remaining portion708separated from seal204by seal portion706.

As will be described in greater detail below, remaining portion708is configured to be non-parallel with anode704so as to reduce shunt current at seal portion706. In this non-limiting example embodiment, remaining portion708is configured to be non-parallel with anode704by including a parallel portion710and a square portion712. Parallel portion710is disposed to be parallel with anode704and at a first distance from anode704.

Square portion712includes a lowering step portion714, a flat portion716, and a rising step portion718. Flat portion716connects lowering step portion714with rising step portion718so as to form a “square” shape. Flat portion716is disposed at a second distance from anode704, wherein the first distance in which parallel portion710is located from anode704is greater than the second distance. In other words, flat portion716is closer to anode704than parallel portion710. Further square portion712is disposed between parallel portion710and seal portion706, so as to connect parallel portion710with seal portion706.

Cathode702has a shape to reduce the shunt current around the area next to seal204. Having square portion712right next to seal204and close to anode704will reduce the shunt current next to seal204. The current flow between cathode702and anode704is collected to the more at flat portion716and less at seal portion706.

It should be noted, that other shapes of cathodes may be implemented in accordance with exemplary embodiments. These will be described in greater detail with reference toFIGS.8A-D.

FIG.8Aillustrates a non-limiting example embodiment of a cathode in accordance with aspects of the present disclosure.

In this non-limiting example embodiment, remaining portion804is configured to be non-parallel with anode704so as to reduce shunt current at seal portion706. Remaining portion804includes a parallel portion805and a rising step portion806. Parallel portion805is disposed to be parallel with anode704and a first distance from anode704. However, rising step portion806is not parallel with anode704, and is disposed between parallel portion805and seal portion706, so as to connect parallel portion805with seal portion706.

FIG.8Billustrates another non-limiting example embodiment of a cathode in accordance with aspects of the present disclosure.

As shown in the figure, portion808includes coolant108, MEGA202, seal204, a cathode810, and anode704. Cathode810includes seal portion706disposed adjacent to seal204and a remaining portion812separated from seal204by seal portion706.

Remaining portion812is configured to be non-parallel with anode704so as to reduce shunt current at seal portion706. In this non-limiting example embodiment, remaining portion812is configured to be non-parallel with anode704by including a parallel portion813and a sawtooth portion814. Parallel portion810is disposed to be parallel with anode704and at a first distance from anode704.

Sawtooth portion814includes a lowering portion816and rising step portion806, which together form a “sawtooth” shape. Lowering portion816extends toward anode704so as to connect with rising step portion806at a point817, which is at a second distance from anode704. Accordingly, the first distance in which parallel portion810is located from anode704is greater than the second distance. In other words, point817of sawtooth portion814is closer to anode704than parallel portion710. Further sawtooth portion814is disposed between parallel portion810and seal portion706, so as to connect parallel portion810with seal portion706.

FIG.8Cillustrates another non-limiting example embodiment of a cathode in accordance with aspects of the present disclosure.

As shown in the figure, portion820includes coolant108, MEGA202, seal204, a cathode822, and anode704. Cathode822includes seal portion706disposed adjacent to seal204and a remaining portion824separated from seal204by seal portion706.

Remaining portion824is configured to be non-parallel with anode704so as to reduce shunt current at seal portion706. In this non-limiting example embodiment, remaining portion824is configured to be non-parallel with anode704by having a “wave” shape.

Remaining portion824starts at a highest point825, curves toward anode704to reach a lowest point827, and then curves up to connect to seal portion706. Highest point825is disposed at a first distance from anode704, whereas lowest point827is at a second, closer distance from anode704.

FIG.8Dillustrates another non-limiting example embodiment of a cathode and anode in accordance with aspects of the present disclosure.

As shown in the figure, portion826includes coolant108, MEGA202, seal204, a cathode828, and an anode830. Cathode828includes seal portion706disposed adjacent to seal204and a remaining portion832separated from seal204by seal portion706. Anode830includes seal portion834disposed adjacent to seal204and a remaining portion836separated from seal204by seal portion834.

Remaining portion832of cathode828is configured to be non-parallel with anode830so as to reduce shunt current at seal portion706. In this non-limiting example embodiment, remaining portion832of cathode828is configured to be non-parallel with anode830by having a first “wave” shape, whereas remaining portion836of anode830is configured to be non-parallel with cathode828by having a second “wave” shape. Accordingly, in this embodiments, the anode/cathode pair form a “double wave” shape.

Remaining portion824of cathode828starts at a highest point838, curves toward anode830to reach a lowest point840, and then curves up to connect to seal portion706. Remaining portion836of anode830starts at a highest point842, curves downward to reach a lowest point844, and then curves up to connect to seal portion834. Highest point828of cathode828is disposed at a first distance from highest point842of anode830, whereas lowest point840of cathode828is at a second, closer distance from lowest point844of anode830.

In this embodiment, bending anode830in the same direction as cathode828also reduces the current density. It has smaller effect but helps to avoid short circuits.

The stack durability is improved by utilizing the shapes discussed above with reference toFIGS.7-8Dfor cells throughout the stack. The durability of the stack may be further improved when there are more degrees of freedom for the shape design. In general, more currents flow on the cell placed on end of the stack, which determines the lifetime of fuel cell stack.

It is important to reduce the shunt current at the area of the cathode that is near the seal and that is facing the anode. It is particularly important for the highest cell in the stack, as the potential builds for each cell in the stack. This will be described in greater detail with reference toFIGS.9A-B.

FIG.9Aillustrates a portion900of a stack of cells in a related art fuel cell system.

As shown in the figure, portion900includes an array of individual cells, arranged as a stack of sets of serially connected individual cells, a sample of which is indicated as set of serially connected cells902, which includes a cell904that is serially connected to a cell906. Each cell includes a negative ion compartment, a sample of which is indicated as negative ion compartment908within cell906, and a positive ion compartment, a sample of which is indicated as positive ion compartment910within cell906. The positive and negative ion compartments are separated by a MEGA, a sample of which is indicated as MEGA911within set of serially connected cells902. A cathode is disposed on the side of each serially connected cells that has the negative ion compartments, a sample of which is indicated as cathode912. An anode is disposed on the side of each serially connected cells that has the positive ion compartments, a sample of which is indicated as anode914.

Electrons pass from each negative ion compartment, through a respective MEGA and into a respective positive ion compartment, as indicated by the set of arrows, a sample of which is indicated as arrows915. This transfer of ions through each set of serially connected cells is collected by a respective cathode, which creates a potential difference between the cathode and a respective anode. These potentials are added to provide a total amount of DC voltage for the entire fuel cell system, and can be represented by a circuit model. Gaskets, a sample of which in indicated as gasket916separates stacked fuel cells.

As shown inFIG.9B, equivalent circuit model918includes: three sets of anodes and cathodes, a sample of which is indicated as cathode920and anode922; and three DC voltage sources924,926and928. Cathode920corresponds to cathode912of set of serially connected cells902ofFIG.9A, whereas anode922corresponds to anode914of set of serially connected cells902ofFIG.9A. Similarly, DC voltage source924corresponds to the potential difference at cathode912ofFIG.9A. Accordingly, portion900ofFIG.9Ais able to provide a total DC voltage equal to the voltages of DC voltage sources924,926and928as shown inFIG.9B.

There is more chance of oxidation on the cathode surface that faces the anode because of the potential difference. While changing the shape of the cathode may reduce the rate of oxidation, there are additional mechanisms to further reduce the rate of oxidation.

Narrowing the manifold size (or placing the seal parts farther from the manifold end) is one of the solutions to reduce the current flow on the end cell. This will be described in greater detail with reference toFIG.10A.

FIG.10Aillustrates a non-limiting example embodiment of stack of cells1000having a modified cathode length in accordance with aspects of the present disclosure.

As shown in the figure, each electrode in stack of cells1000has a length extension, a sample of which is indicated as a cathode1004having an extension1006.

However, extending the electrode length as discussed above increases the pressure drop of the cooling system and more wider cells and/or higher pumping powers are required.

By bending the end of the cathode, the pressure drop of the cooling system is decreased as compared to that of the extended electrodes discussed above with reference toFIG.10A. This will be described in greater detail with reference toFIG.10B.

FIG.10Billustrates a non-limiting example embodiment of stack of cells1002having a modified cathode shape in accordance with aspects of the present disclosure.

As shown in the figure, each electrode in stack of cells1002has an end that is bent toward a corresponding anode, a sample of which is indicated as a cathode1008having an end1010that is bent toward an anode1012.

Further, more wider cells and/or higher pumping powers are not required, although the performance gain is somewhat reduced. This shape modification technique is more effective when the coolant conductivity is high.

The following discussion ofFIGS.11A-16describe how changing the cathode shape in accordance with aspects of the present disclosure decrease shunt current and therefore decrease the rate of oxidation of the cathode.

FIG.11Aillustrates electrolyte potential and current density vector results of a simulation of a portion1100of a related art fuel cell system.

As shown in the figure, portion1100includes a cathode1102, an anode1104and coolant1106. Current lines are shown as arrows, a sample of which is indicated as current line1108. A voltage key1110indicates the value of the electrolyte potential within coolant1006and is measured between 0.1-0.9V.

In this simulation, the electrodes materials are assumed as stainless steel (SUS 316L) and a coolant conductivity is 1 μS/m. The electrode distance is set to 1 mm and the potential difference is set to 1V. The average shunt current at the bottom of cathode1102near the seal (not shown) at area1112is evaluated in the 2D axisymmetric configuration. In the simulation, the average shunt current at area1112is 97 nA/cm2.

FIG.11Billustrates electrolyte potential and current density vector results of a simulation of a portion1101of a fuel cell system having a square-shaped cathode in accordance with aspects of the present disclosure.

As shown in the figure, portion1101includes a cathode1114, an anode1116and coolant1118. Current lines are shown as arrows, a sample of which is indicated as current line1120. A voltage key1122indicates the value of the electrolyte potential within coolant1118and is measured between 0.1-0.9V.

In this simulation, again, the electrodes materials are assumed as stainless steel (SUS 316L) and a coolant conductivity is 1 μS/m. The electrode distance is again set to 1 mm and the potential difference is set to 1V. The average shunt current at the bottom of cathode1114near the seal (not shown) at area1124is evaluated in the 2D axisymmetric configuration. In the simulation, the average shunt current at area1124is 25 nA/cm2, which is a 74% reduction as compared to the related art system discussed above with reference toFIG.11A.

FIG.11Cillustrates electrolyte potential and current density vector results of a simulation of a portion1126of a fuel cell system having a wave-shaped cathode in accordance with aspects of the present disclosure.

As shown in the figure, portion1126includes a cathode1128, an anode1130and coolant1132. Current lines are shown as arrows, a sample of which is indicated as current line1134. A voltage key1136indicates the value of the electrolyte potential within coolant1132and is measured between 0.1-0.9V.

In this simulation, again, the electrodes materials are assumed as stainless steel (SUS 316L) and a coolant conductivity is 1 μS/m. The electrode distance is again set to 1 mm and the potential difference is set to 1V. The average shunt current at the bottom of cathode1128near the seal (not shown) at area1138is evaluated in the 2D axisymmetric configuration. In the simulation, the average shunt current at area1138is 37 nA/cm2, which is a 62% reduction as compared to the related art system discussed above with reference toFIG.11A.

FIG.11Dillustrates electrolyte potential and current density vector results of a simulation of a portion1138of a fuel cell system having a double wave-shaped anode and cathode in accordance with aspects of the present disclosure.

As shown in the figure, portion1140includes a cathode1142, an anode1144and coolant1146. Current lines are shown as arrows, a sample of which is indicated as current line1148. A voltage key1150indicates the value of the electrolyte potential within coolant1146and is measured between 0.1-0.9V.

In this simulation, again, the electrodes materials are assumed as stainless steel (SUS 316L) and a coolant conductivity is 1 μS/m. The electrode distance is again set to 1 mm and the potential difference is set to 1V. The average shunt current at the bottom of cathode1142near the seal (not shown) at area1152is evaluated in the 2D axisymmetric configuration. In the simulation, the average shunt current at area1153is 36 nA/cm2, which is a 63% reduction as compared to the related art system discussed above with reference toFIG.11A.

As shown inFIGS.11A-D, neighboring plates may have a different physical structure to control the current density magnitude. In fact, the structures across the stack may be further tuned, as needed, although only end plate modifications in the stack may be needed. The wave structure inFIG.11Cand double wave structure inFIG.11Dare obtained by optimizing cathode1128in the wave structure ofFIG.11Cand cathode1142and anode1144in the double wave structure ofFIG.11D.

Returning toFIG.7, the durability of seal204is enhanced when flat portion716of square portion712of cathode702is closer to anode704. This will be described in greater detail with reference toFIGS.12A-B.

FIG.12Aillustrates a height relationship between a cathode and anode in a portion1200of a fuel cell system in accordance with aspects of the present disclosure.

As shown in the figure, portion1200includes a cathode1202and an anode1204. Cathode1202includes a seal portion1206and a remaining portion1208configured to be non-parallel with anode1204so as to reduce shunt current at seal portion1206. In this non-limiting example embodiment, remaining portion1208is configured to be non-parallel with anode1204by including a parallel portion1212and a square portion1210. Parallel portion1212is disposed to be parallel with anode1204and at a first distance,H, from anode1204.

Square portion1210includes a flat portion1214disposed at a second distance, h, from anode1204, wherein H is greater than h. In other words, flat portion1214is closer to anode1204than parallel portion1212.

FIG.12Billustrates a graph1216of current density change as a function of the ratio of distances between a cathode and anode as shown inFIG.12A.

As shown in theFIG.12B, graph1216includes a y-axis1218, an x-axis1220and a curve1222based on individual plots, a sample of which is indicated as plot1224. Y-axis1218indicates current density change measured in percentages at an underside of a cathode, near the seal. X-axis1220indicates a ratio of h/H measured in percentages.

It is clear from graph1216that the current density decreases as the ratio of h/H increases. However, the closer two electrodes become, the greater the possibility of an electrical short circuit. Accordingly, the appropriate step height should be decided from machining and assembly precision and size of the foreign particles in the coolant being used.

The length of the electrodes additionally affects a current density change in a cell. This will be described in greater detail with reference toFIGS.13A-B.

FIG.13Aillustrates a length relationship between portions of a cathode in a portion1300of fuel cell system in accordance with aspects of the present disclosure.

As shown in the figure, portion1300includes a cathode1302and an anode1304. Cathode1302includes a seal portion1306and a remaining portion1308. Cathode1302includes an extension1310such that a length, L, of seal portion1306and a portion1312of remaining portion1308is greater than a length, l, of extension1310.

FIG.13Billustrates a graph1314of current density change as a function of the ratio of lengths between portions of a cathode as shown inFIG.13A.

As shown in theFIG.13B, graph1314includes a y-axis1316, an x-axis1318and a curve1320based on individual plots, a sample of which is indicated as plot1322. Y-axis1316indicates current density change measured in percentages at an underside of a cathode, near the seal. X-axis1318indicates a ratio of l/L measured in percentages.

In this example, let the coolant conductivity be set to 1,000 μS/m, and let the voltage difference between the cathode1302and anode1304be 1 V. This value is too large comparing with the real situation and its impact becomes smaller when the conductivity is small. However, actual tests have shown that maintaining a distance between the manifold and the seal is effective at providing up to a 20% reduction in current density change at the end of the cell.

FIG.14illustrates a graph1400comparing normalized current density of the fuel cell system ofFIGS.11A-D.

As shown inFIG.14, graph1400includes a y-axis1402, a bar1404, a bar1406, a bar1408, and a bar1410. Y-axis1402indicates normalized current density at an underside of a cathode, near the seal. Bar1404corresponds to the normalized current density at an underside of a cathode, near the seal, of the related art cathode as shown inFIG.11A. Bar1406corresponds to the normalized current density at an underside of a cathode, near the seal, of the cathode as shown inFIG.11B. Bar1408corresponds to the normalized current density at an underside of a cathode, near the seal, of the cathode as shown inFIG.11C. Bar1410corresponds to the normalized current density at an underside of a cathode, near the seal, of the cathode as shown inFIG.11D.

As shown in graph1400, the reduction rate is slightly smaller in the square shape cathode as shown inFIG.11Bbecause of the filter radius constraint, which is a constraint on the shape optimization that is added to avoid deformation into a form that cannot be manufactured. It indicates distance between two plates is more important than the detail electrode shape.

FIG.15Aillustrates electrolyte potential and current density vector of a portion1500of a related art fuel cell system, wherein each cathode is parallel to its corresponding anode, for example as shown inFIG.11A.

As shown inFIG.15A, portion1500includes the end of a stack of 20 cells and a coolant1502. A key1504provides reference to the overall potential of the entire stack of 20 cells and ranges from 0.0-20.0 V, with 1.0 V per cell. Current lines are indicated, for example as indicated as current line1506. In this example, the current density at area1508, which is on the surface of the cathode that is near the seal (not shown) and is facing the anode, of the top most cell, is 0.04 A/m2.

FIG.15Billustrates electrolyte potential and current density vector of a portion1510a fuel cell system having square shaped cathodes in accordance with aspects of the present disclosure, for example as shown inFIG.11B.

As shown inFIG.15B, portion1510includes the end of a stack of 20 cells and a coolant1512. A key1514provides reference to the overall potential of the entire stack of 20 cells and ranges from 0.0-20.0 V, with 1.0 V per cell. Current lines are indicated, for example as indicated as current line1516. In this example, the current density at area1518, which is on the surface of the cathode that is near the seal (not shown) and is facing the anode, of the top most cell, is 0.035 A/m2. This is a 12.5% decrease over that of the fuel cell system having a cathode of the shape ofFIG.11Aas discussed above with reference toFIG.15A.

FIG.15Cillustrates electrolyte potential and current density vector of a portion1520of a fuel cell system having square shaped cathodes with an extended length in accordance with aspects of the present disclosure, for example as shown inFIG.11C.

As shown inFIG.15C, portion1520includes the end of a stack of 20 cells and a coolant1522. A key1524provides reference to the overall potential of the entire stack of 20 cells and ranges from 0.0-20.0 V, with 1.0 V per cell. Current lines are indicated, for example as indicated as current line1526. In this example, the current density at area1528, which is on the surface of the cathode that is near the seal (not shown) and is facing the anode, of the top most cell, is 0.033 A/m2. This is a 17.5% decrease over that of the fuel cell system having a cathode of the shape ofFIG.11Aas discussed above with reference toFIG.15A.

FIG.15Dillustrates electrolyte potential and current density vector of a portion1530of a fuel cell system having square shaped cathodes with bent ends in accordance with aspects of the present disclosure, for example as shown inFIG.11D.

As shown inFIG.15D, portion1530includes the end of a stack of 20 cells and a coolant1532. A key1534provides reference to the overall potential of the entire stack of 20 cells and ranges from 0.0-20.0 V, with 1.0 V per cell. Current lines are indicated, for example as indicated as current line1536. In this example, the current density at area1538, which is on the surface of the cathode that is near the seal (not shown) and is facing the anode, of the top most cell, is 0.034 A/m2. This is a 15% decrease over that of the fuel cell system having a cathode of the shape ofFIG.11Aas discussed above with reference toFIG.15A.

FIGS.15A-Dillustrate the result of the simulation with the stake of 20 cells. As the effect of extension or bending is very small when the coolant conductivity is kept low, it is set to 1,000 μS/m. The figures show the change of the manifold edge shape reduce the current density around the seal. The larger the distance between manifold and seal area becomes, the lower the current density near the seal becomes. Although the improvement of the bending the end of the cathode as shown inFIG.15Dis smaller than the extending the length of the cathode as shown inFIG.15C, the bending the end of the cathode as shown inFIG.15Dhas the merit to avoid increase of the pressure drop in the cooling system.

FIG.16illustrates a graph1600comparing normalized current density at the end of a cell in each of the fuel cell systems ofFIGS.15A-D.

As shown inFIG.16, graph1600includes a y-axis1602, a bar1604, a bar1606, a bar1608, and a bar1610. Y-axis1602indicates normalized current density on the surface of the cathode that is near the seal (not shown) and is facing the anode, of the top most cell. Bar1604corresponds to the normalized current density of the related art fuel cell system as shown inFIG.15A, bar1606corresponds to the normalized current density of the fuel cell system as shown inFIG.15B, bar1608corresponds to the normalized current density the end of a cell of the fuel cell system as shown inFIG.15C, and bar1610corresponds to the normalized current density the end of a cell of the fuel cell system as shown inFIG.15D.

FIG.17illustrates a method1700of making a fuel cell system in accordance with aspects of the present disclosure.

As shown in the figure, method1700starts (S1702) and an anode is formed (S1704), a cathode is formed (S1706) and the MEGA is formed (S1708). These parts may be formed by any known method. A non-limiting example method of forming an anode and cathode is by stamping, In accordance with aspects of the present disclosure, for example as discussed above with reference toFIGS.7-8D, the end of the anode that will be exposed to the coolant might be stamped to have a predetermined shape.

After the anode is formed (S1704), the cathode is formed (S1706), and the MEGA is formed (S1708), they are glued together with a seal to form a fuel cell (S1710). This may be performed by any known method to separate the last fuel cell from the area in which the end of the anode will be exposed to the coolant.

After the fuel cells are formed (S1710), multiple formed fuel cells are stacked to from a fuel cell stack (S1712). This may be performed by any known method.

After the fuel cell stack is formed (S1712), coolant is added (S1714). For example, coolant may be input into the manifold and connected to a heat transfer system as discussed above with reference toFIG.6.

After coolant is added (S1714), method1700stops (S1716).

It should be noted that fuel cell system in accordance with aspects of the present disclosure may be made using a different order of the processes discussed above with reference toFIG.17.

A benefit of a fuel cell system in accordance with aspects of the present disclosure is that it requires no additional investment to improve the durability of the fuel cell stack. Because electrolytic corrosion ratio is proportional to the shunt current, the shape design of the electrode can be used for tuning current distributions and the reaction speed at a specific area of the plate. The designed shape can be fabricated at the same time as gas flow channels are formed in the bipolar plates via stamping. Therefore, no additional process is required.