FIELD OF THE INVENTION
The present invention relates to operation of Proton Exchange Membrane Fuel Cell (PEMFC) stacks for reactant gases of hydrogen and air. In genera), PEM fuel cells are operated between 50 - 70° C where physical existence of water is in liquid phase after the saturation of gases which in turn leads to flooding of anode or cathode. In particular, the present invention relates to a method of operation of a PEMFC to achieve flooding free operation using flow field designs of parallel interdigited for cathode side and serpentine for anode side gas distribution plates. The invention further relates to a device to drain the liquid water collected in the inlet manifold which is being carried by the inlet reactant gases in the form of liquid droplets/ misty condensate from the upstream is discharged to prevent the source of flooding by the inlet feed reactants. More particularly, the invention relates to an improved fuel cell stack assembly operably connected to an external device at inlet manifold to prevent the source of flooding due to entrained water droplets or condensate present in the reactant feed gas.
BACKGROUND OF THE INVENTION
Ionic conductivity of electrolyte membrane is known to be of critical importance to obtain better performance of PEM fuel cell. In order to obtain a higher ionic conductivity of electrolyte, the electrolyte has to be maintained under well hydrated condition which is defined by the activity coefficient (dimensionless parameter-A) of the electrolyte. If the value of A is 24 the membrane is said to be fully saturated, alternatively, the membrane is considered as unsaturated (value of A<24). Accordingly, operation of stack under such conditions may lead to drying of the membrane resulting in increase of electrolyte resistance. In order to avoid such drying conditions, the reactant gases of air or hydrogen or

both are humidified before they are fed at the stack inlet. In addition an excess amount of water (over saturated conditions) on either cathode or anode side may lead to flooding if the water is not evacuated properly. Under such conditions, the active sites are blocked from an access of the reactants which would result in drop of stack voltage.
To maintain the electrolyte under well hydrated condition, the reactant gases are humidified before they fed into the stack. In practice, the reactant gases of air, hydrogen both are humidified or any single reactant gas may be humidified which is governed by the characteristics of a membrane electrode assembly (MEA) being used in the stack. In practice, the gases are humidified either external to the stack using an additional subsystem or within the stack by means of internal humidification. As the conventional PEM fuel cell operates between 50-70°C, saturated water vapors carried by the reactant gases tend to form liquid water and lead to flooding which is undesirable phenomenon. Therefore, water management in the stack is of critical importance to avoid the situation of flooding.
In conventional PEM fuel cell stacks, excess liquid water of anode and cathode plates or either is being evacuated through a half bipolar plate (one side of the plate consists gas flow grooves and other side coolant grooves).
Grasso et al. (US2004/0258973A1, US2005/6916571B2) used a micro porous plate as an half bipolar plate, for the purpose of liquid water evacuation from the reactant flow field plates (anode or cathode) to the coolant circulation side provided between anode and cathode sides. According to this prior published patent, the water is evacuated from the reactant side to the coolant circulation side in vertical direction through the plate interconnected pores available

between the faces of reactant side and cooling side of half bipolar plate. In this invention, a passive water management mechanism is used to evacuate the excess amount of liquid water from the reactant gas distribution plate side by means of convection transport mechanism where the density of liquid water is less because of entrapped gas bubbles. The gas bubbles carried on the coolant side are separated in a separator unit prior to feed into the coolant circulation pump system.
WO 2005/031896 discloses a similar technology of water management i.e. transport of water from cathode to anode side, adopted by UTC Fuel Cells for better water management to avoid the drying of the membrane. In this cited invention, the Proton Exchange Membrane is modified by adding hydrophilic silicone carbide particles of size 1 to 2 size of length 5 microns using which the water formed at the cathode side is transferred to the anode side for assisting the internal humidification to the anode side reactant gas. Besides this, the size of vertical pores which are connected between the anode and cathode side plates through the water cooling plates are modified to ensure that the water is transferred from cathode to anode side by capillary action due to surface tension of water. In addition to that, the stack is operated such that the operating pressure of cathode side is higher than the anode side to pressure for having better water management by difference of pressure.
In case of external humidification where the reactant gases are being humidified prior to feed them into the fuel cell stack, Shailendra et al. (IP2015/264653) teaches improved water management. As per this cited invention, the reactant gases are fed through two inlets and exits through a single outlet {dual inlet-single exit). Multiple serpentine flow filed channels are used from both inlets, ends of each serpentine pattern are merged at one common internal manifold

(reservoir) which is connected to the exit gas manifold through straight channels. In this case, the excess water is evacuated by forced convection due to increase in the velocity of gases due to result of decrease in the effective area of exit gas flow path area.
In view of the above admitted prior art, there is a need to develop a technology to achieve a flooding free operation where interdigited and serpentine flow distribution paths for reactants of air (cathode side) and Hydrogen (anode side) respectively are used as reactant distribution plates, wherein a cathode side reactant gas alone is humidified externally, including the provision for draining out the liquid water or condensate accumulated in the cathode inlet manifold.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved fuel eel) stack assembly operably connected to an external device at inlet manifold to prevent the source of flooding due to entrained water droplets or condensate present in the reactant feed gas.
Another object of the invention is to propose an improved fuel cell stack assembly operably connected to an external device at inlet manifold to prevent the source of flooding due to entrained water droplets or condensate present in the reactant feed gas, in which a parallel interdigited flow pattern for cathode side reactant gas (Air) and serpentine reactant flow pattern for anode side reactant gas (Hydrogen) are operably connected to their individual manifolds of air and hydrogen respectively.

A still another object of the invention is to propose an improved fuel cell stack assembly operably connected to an external device at inlet manifold to prevent the source of flooding due to entrained water droplets or condensate present in the reactant feed gas, which is operably connected to a micro-porous tube at bottom of the anode exit manifold for operating the stack under back pressure mode and minimize differential pressure between cathode and anode side including an improvement in stack operation under low humidity conditions.
A further object of the invention is to propose an improved fuel cell stack assembly operably connected to an external device at inlet manifold to prevent the source of flooding due to entrained water droplets or condensate present in the reactant feed gas, in which feed inlet and outlet at fixed locations are provided to obtain flooding free operation including achievement of stack power and consistent cell voltage compared to various modes of operation available for the stack under the conditions of humidified reactant gas fed at cathode side assembly for wide range of inlet air dew points.
A still further object of the invention to propose, an improved fuel cell stack assembly operably connected to an external device at inlet manifold to prevent the source of flooding due to entrained water droplets or condensate present in the reactant feed gas, in which the reactant flow field distribution geometry patterns are parallel interdigited and serpentine respectively, and where only one humidified reactant gas preferably cathode (Air) gas passing over the cathode reactant distribution plate placed below the Membrane Electrode Assembly (MEA) while facing towards up and anode reactant distribution plate facing toward down above the MEA.

SUMMARY OF THE INVENTION
Ionic conductivity of Proton Exchange Membrane (PEM) is of critical importance to achieve better cell performance. The ionic conductivity of PEM depends more on the hydration level of the membrane, which is maintained by feeding the humidified reactants gases of Hydrogen (anode) and Air (cathode) or either of the gas may be humidified. In case of ionic transportation, the water molecules are being carried by the H+ ions during the process of electro-osmotic drag would result in loss water availability on anode side.
The water transport across the membrane is governed by means of i) back diffusion, ii) electro-osmotic drag and iii) differential water pressure between anode and cathode. The water transport by the differential water pressure is considered to be insignificant due to impermeability of the membrane, therefore it can be neglected which was studied by many research communities over the years. In case of air - hydrogen reactant gases, humidification of cathode side reactant (Air) is sufficient to avoid the drying of the membrane and both the reactants need to be humidified if the operating current density exceeds 1 A/cm2 because of water transport by electro-osmotic drag (anode to cathode) is more dominant than the water transport by back diffusion (cathode to anode). In case of low current densities, the membrane can be maintained under well hydrated condition by back diffusion of product water of cathode formed during reaction. Moreover, the flooding may arise due to result of designs used for cathode and anode side reactant gases distribution.
In present invention, a PEM fuel cell stack for example consisting of 25 cells, is operated, say at 60 °C using air and hydrogen as reactant feed gases for cathode and anode side respectively. In this case, air alone is humidified before the reactant gas is fed into the cathode side manifold. Parallel intedigted flow field geometry is used for distribution of cathode side reactant gas (air), whereas

serpentine geometry is used on anode side plate for distribution of hydrogen gas. Cathode side reactant gas is humidified externally before it is fed at the cathode inlet manifold (top side of stack), also the bottom side of the respective manifold is connected with a micro porous tube having a pore size of 2 -5 microns.
The assembled 25 cells stack is operated for a cumulative period of 500 hrs, each operation cycles varies from 2 hrs to 6-hrs. Through the testing of the cathode side, the reactant gas alone is humidified as the flooding can be observed when both the reactant gases are humidified and fed into the stack. The humidification level reactant air varied from dew point 0° C to 40° C and there were no flooding signatures found during the operation, during the testing of the stack was operated between 60 to 65 °C. The stack was operated for four combinations of hydrogen and air inlet positions, among these studied only two combinations resulted in flooding free operation for given designs of parallel interdigited and serpentine used on cathode and anode plates respectively.
The mechanism adopted for obtaining the flooding free operation is due to the fact that the reactants flow directions follows the pattern of counter and cross flow of reactants with each other over the MEA. As the water is transferred from cathode to anode is by back diffusion due to water concentration which can be intensified with the combination of counter and cross flow of the reactants distribution where the net mass transfer (cathode to anode or anode to cathode) is high in case of cross flow and counter flow distribution of reactants with reference to each other. The combined process of reactants distribution would enable near uniform distribution of water across the membrane. In view of the above, the stack was operated without any flooding until current densities of 0.4A/cm2 over wide range of air dew points ranges from 0- 40 °C.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 Conventional proton exchange membrane (PEM) fuel cell stack assembly according to prior art.
Figure 1.1 Serpentine flow field pattern for anode side reactant (hydrogen) gas
Figure 1.2 Parallel interdigited flow field pattern for cathode side reactant (air) gas
Figure 2 Proton Exchange Membrane (PEM) fuel cell stack assembly according to the present invention.
Figure 2.1 Section of Proton Exchange membrane (PEM) fuel cell stack for representation of one cell.
Figure 2.2 Shows reactants (air & hydrogen) flow distribution over the membrane electrode assembly for representation of counter and cross flow of reactants with each other.
DESCTIPTION OF THE PREFERRED EMBODIMENTS
A prior art PEM fuel cell stack is shown in figure 1, which comprises two end plates (1A, 1B) between which the balance components of the PEM fuel stack are tightened using at least two internal tie rods (2) and at least eight external tie rods (3). The components of the PEMFC stack includes a plurality of insulating plates (4A,4B) to protect against thermal and electrical loses, a multiple cooper plates (5A,5B) for collecting the electrons generated at the electrocatalyst sites

of the membrane electrode assemblies (6) through at least a pair of blank half bipolar plates (one side groove and other side blank) of anode (7) and cathode (8) respectively. An half bipolar plate (7) having a serpentine flow pattern and the other plate having parallel interdigited (8) flow pattern can be seen in Fig 1.1 and Fig 1.2 respectively. As the anode plate is facing towards down in the stack assembly, its flow pattern is invisible. In addition to this, a plurality number of bipolar plates (9) having gas flow grooves at both the sides is provided with cooling channels (10) in between the plates. The bipolar plate is formed by laminating the two half bipolar plates of anode (11) and cathode (12) using sealant (13). The bipolar plate has provision for reactant circulation on both the sides for air, hydrogen and coolant circulation through said cooling channels (10) in between the plates of anode and cathode half bipolar plates. Multiple number of such bipolar units (14) are stacked one over the other having MEA (6) in between the bipolar units (14). The total assembled plates form a common duct or manifold for air and hydrogen. The air inlet manifold (15A) is in fluid communication with the air out manifold (15B) which are in fluid communication with the air inlet (17) and out lets (18) provided for the stack end plates (1A, IB). Similarly, the hydrogen inlet manifold (16A) is in fluid communication with the hydrogen outlet manifold (16B) which are in fluid communication with the hydrogen inlet (19) and out lets (20) provided for the stack end plates (1A, 1B). In case of conventional PEM fuel cell stack, the other ends of each manifold of hydrogen and air are provided with dead end bolts (21).
Conventional PEM fuel cells operate below 70° C and require hydration for the membrane ionic conduction. One such method for maintaining the hydration of the membrane is by feeding the humidified reactants into the fuel cell stack. In practice, humidification of the reactants is achieved by external (outside the stack assembly) methods. In practice, there might be of vapor condensation

between the lines of humidification and reactant inlets of the stack, which may lead to local condensation of water vapor at the gas diffusion layer where electrocatalyst is coated. The condensed water vapor may lead to adverse effect of drop in cell voltage due to blockage of electrocatalyst (active reaction sites) by condensed water which would lead to non-availability of reactant gas at reaction sites. The process of water condensation at actives/ flow channels with progress of time leads to flooding of water which can occur due to multiple reasons i) over saturation of reactant gases ii) entrapment of water droplets with inlet reactants iii) design of reactants flow distribution plates and iii) inefficient water management within the cell. In present invention the source of flooding due to entrainment of water droplets with inlet reactant feed gases particularly air and inefficient design of reactants flow distribution plates were solved by modifying the conventional PEM fuel cell stack assembly as shown in Fig. 2. The modified assembly consists of two additional micro porous tubes (22, 23) fitted below the stack bottom plate. One micro porous tube (22) is fitted at opposite end of air inlet manifold and the other micro porous tube (23) is fitted at the hydrogen out manifold through which the hydrogen exits. The section of one cell in the stack is shown in Rg 2.1, consists of one MEA, anode half bipolar and cathode half bipolar plate. As shown in Rg 2.1, where cooling channels can be seen on top side of the plate and the other side gas flow channels of serpentine patterns as shown in Rg 1.1 are invisible (bottom side of plate). Also, reactant gas flow channels of cathode plate can be seen on the top side of the bottom plate as shown in Rg 2.1 where other side consists of cooling are invisible.
Present invention provides an assembly of complete stack which includes integrated micro-porous tubes of cathode side (22) and anode side (23) in addition to the assembly of conventional components used in PEM fuel cell stacks. The designs are made such that the flow direction of reactants maintains

counter and cross current flow with each other over the entire area of MEA as shown in Fig. 2.2. 1 kW stack is operated using hydrogen and air as reactant feed gases where air alone is humidified externally before it is fed through the air inlet (17) into the respective air manifold (15A) from which each cathode side plate receives the air. Also, dry hydrogen is fed through the hydrogen inlet (19) into the hydrogen manifold (16A) from which each anode plate receives hydrogen. As studied by many research communities that the humidification of cathode side (air) reactant gas is sufficient where operating current densities are below 1 A/cm2 to maintain the ionic conducting membrane under sufficient hydrated condition. In practice air carries entrained water droplets/condensate into the air manifold which may leads to flooding due accumulation of excess water on cathode electrode which is undesirable.
In the present invention the source of entrained/ condensate carried by reactants from upstream is removed with the use of microporous tube fitted below the end of air inlet manifold. The microporous tube is a hollow tube of certain length consists of fixed wall thickness to have outer diameter and inner diameter. Hollow micro porous tube consists of pores of size varies from 5 to 50 microns across the wall thickness. Further, the porosity of the micro porous tubes is reduced by treating them with hydrophilic solutions where the pores are closed to the maximum extent in this cases the micro porous tube acts like support structure for filling of hydrophilic material. After hydrophilic treatment of microporous tube, it is further heated to have firm bonding with the microporous substrate material. Finally, the microporous tube is made such that there would not be any reactant /gas leakages under the stack rated operating pressure (<0.2 bar), also the water collected at the bottom section of air inlet manifold (15A) would be allowed to train drain through the hydrophilic treated microporous tube (22) only. Similarly, untreated microporous tube (23) having

pore size of 50 to 100 microns is fitted at the exit section of anode (20) to ensure back pressure also to balance the differential pressure between the sections of cathode and anode sides during operation.
In the present invention the cathode side reactant gas (air) is alone humidified externally before it is fed into the cathode manifold (15A), dry hydrogen is fed into the anode manifold (16A) through their respective inlets of cathode (17) and anode (19). In present case, stack consists of 25 cells, each cell of cathode and anode plate receives air and hydrogen from the respective inlet manifolds of air/cathode (15A) and hydrogen/ anode (16A). Unreacted air and product water, unreacted hydrogen and water of each cell are drained through their bipolar plates (9, 14) which are in fluid communication with their outlet manifold of air (15B) and hydrogen (16B). As the reactant gases of air and hydrogen travel from inlet to the outlet through their gas distribution paths as shown in Fig 2.2, where the entry position of air (26) and hydrogen (28), also exit positions of air (27) and hydrogen (29) are at 180° as shown in Rg 2.2. The flow directions of inlet bulk air (24) and hydrogen (30) are counter current which are represented with directional arrows (24, 30). Similarly, air inlet flow path directions (24A) and hydrogen flow path directions (30, 31) are in cross flow directions where the angle between the arrows of hydrogen and air are 90°. As per the principles of mass transfer the rate of mass transfer by diffusion follows as the rate of mass transfer of increasing order is concurrent flow (0°)< counter current flow (180°) < cross current flow (90°). In present invention the flow of reactant gases over each MEA (6) in the stack follows the flow pattern of counter and cross flow at every point of location there ensures better water management within the stack because of overall uniform distribution of water concentration.

WE CLAIM:
1. An improved fuel cell stack assembly operably connected to an external device at inlet manifold to prevent the source of flooding due to entrained water droplets or condensate present in the reactant feed gas, each fuel cell in the stack having a membrane electrode assembly (MEA) consisting of at least one electrolyte and one each anode and cathode electrode between which the electrolyte is sandwiched to form the MEA, wherein the electrochemical reactions takes place at anode (H2 to H+) and cathode (oxygen and hydrogen gives water) sides, wherein one side anode electrode is in contact with a serpentine flow pattern, on the other side cathode electrode is contact with a parallel interdigited flow pattern using which the reactant of hydrogen and air supplied to the anode and the cathode sides of a fuel cell, wherein a plurality of fuel cells connected electrically in series and mechanically stacked one over the other to form fuel cell stack, the reactant of air is humidified external to the stack prior to allowing ingress into the electrochemical active area, wherein the reactants are fed into the stack such that flow directions of the reactants with each other are either in cross or counter flow distribution pattern over the entire area of the MEA to improve water management by preventing water flooding including fluctuation in stack power.
2. The improved fuel stack assembly as claimed in claim 1 wherein a tubular micro porous is placed at bottom of air inlet manifold to remove any liquid water/ entrained droplets/ condensate carried by the inlet reactant feed gas (air) to improve the water management in proton exchange membrane fuel cells by separation of liquid water from the reactant feed gas prior to ingress of the reactants at cathode side, and wherein a first

plurality of the microporous tubes are placed one at the bottom of cathode side inlet manifold through which only liquid water is drained, and wherein a second plurality of microporous tubes is placed at bottom of anode side manifold through which the gas the liquid water is drained, including balancing of the differential pressure between anode and cathode sides.
3. In addition to the above said claims of 1 & 2 the feed entry positions of hydrogen and air for the designs of anode (serpentine) and cathode (parallel interdigited) respectively are of critical importance to obtain flooding preventive operation over wide range of operating temperatures of 40 to 65 °C
4. For the above said designs of anode and cathode side, humidification of air alone is sufficient to obtain better performance of stack where the feed entry positions of hydrogen and air said to be adjacent to each other as closer as possible