Abstract:
A polymer electrolyte membrane fuel cell water management device is provided. The device includes a hydrophilic water transport element spanning from inside the fuel cell to outside the fuel cell and disposed between a gas diffusion layer and a current collector layer in the cell. The transport element includes an intermediate wick outside the fuel cell that is hydraulically coupled to the transport element, and has a transport element structure integrated with a flow field structure within the fuel cell. The device further includes an electroosmotic pump, where the pump is located outside the fuel cell and is hydraulically coupled to the intermediate wick. The hydraulically coupled pump actively removes excess water from the flow field structure and the gas diffusion layer through the transport element, where a key aspect of the invention is the decoupling of water removal from oxidant delivery and reduced parasitic loads.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates generally to fuel cells. More particularly, the invention relates to fuel cells with wicking elements spanning from inside to outside the fuel cell with an outside wick portion hydraulically coupled to an elecroosmotic pump for water management.  
       BACKGROUND  
       [0002]     Proton exchange membrane (PEM) fuel cells, also known as polymer electrolyte membrane fuel cells, require humidified gases to maintain proper membrane humidification. Water management is a persistent challenge for PEM fuel cells with perfluorosulfonic acid (PFSA) type membranes, such as Nafion®, which require high water activity for suitable ionic conductivity. Humidification of reactant gases ensures proper humidification of the membrane. Consequently, much of the water produced by the oxygen reduction reaction at the cathode is generated in liquid form. Several problems exists when the liquid water invades the pores of the catalyst layer and the gas diffusion layer (GDL) and restricts diffusion of oxygen to the catalyst. The primary problems occur when liquid water emerges from the GDL via capillary action. The water accumulates in gas channels, covers the GDL surface, thus increasing the pressure differentials along flow field channels, and creating flow maldistribution and instability. In-situ and ex-situ visualizations show that considerable flooding occurs in the GDL directly under the rib of the flow field, these effects occur in serpentine systems and in systems with multiple parallel channels.  
         [0003]     Currently, excessive air flow rates and serpentine channel designs are used to mitigate flooding at the cost of system efficiency. The air flow rates are large enough to force liquid water out of the system and the serpentine channels are for water accumulation at the cathode, where the serpentine channels minimize flow instabilities and are most commonly a small number of serpentine channels in parallel. These strategies act in concert as serpentine designs increase flow rate per channel, improving the advective removal of water droplets. Air is often supplied at a rate several times greater than that required by the reaction stoichiometry, increasing the oxygen partial pressure at the outlet. The larger applied pressure differentials required for these designs further reduce flooding since the pressure drop reduces local relative humidity, favoring increased evaporation rates near the cathode outlet. The use of high flow rate and high pressure contributes to air delivery being one of the largest parasitic loads on fuel cells. Miniaturization of forced air fuel cells exacerbates this parasitic load issue as the efficiency of miniaturized pumps and blowers is typically much lower than that of macroscale pumps. Parallel channels can reduce the pressure differential across the flow field by orders of magnitude compared to serpentine channels. A parallel channel design also simplifies flow field machining and can enable novel fabrication methods. However, truly parallel channel architectures are typically impractical as they are prone to unacceptable non-uniformity in air streams and catastrophic flooding. Typically, oxygen stoichiometries greater than four are necessary to prevent parallel channel flooding.  
         [0004]     To date, several passive water strategies employ additional components to mitigate flooding. One attempt fabricated a composite flow field plate featuring a thin water absorbing layer and waste channels for removing liquid water from the oxidant channels. This design, however, did not offer improved power density due to a significant increase in the Ohmic losses introduced by the new components.  
         [0005]     Another attempt used active water management strategies in which applied pressure differentials actively transport liquid water out of or into a fuel cell. A PEM fuel cell was made that actively managed the water content of the electrolyte by supplying pressurized water to wicks that were integrated into the membrane, where water was directly injected to the membrane. This approach had the undesirable effect of increased the parasitic loads and larger fuel cell size.  
         [0006]     In another design removes water through porous plates, where a bipolar plate that is porous and has internal water channels for cooling and water removal was used. An applied pressure differential between the gas and water streams drives liquid water from the air channels and into internal channels dedicated to water transport. This attempt requires completely porous plates dedicated to internal water channels, where the system is complex requiring thick porous plates for relatively low volumetric power density.  
         [0007]     Accordingly, there is a need to develop a water management device that reduces or eliminates the need for excessive air flow rates and large pressure differentials to reduce the largest parasitic loads on fuel cells, while providing an improved power density. There is an even greater need for such a device with miniaturized fuel cells, where the forced air exacerbates the parasitic load issue with the low-efficiency of miniaturized pumps and blowers. Further, there is a need for a water management device that enable use of parallel channels to reduce the pressure differential across the flow field, where flow field machining is simplified. It would be considered an advance in the field to provide a water management device that enables oxygen stoichiometries less than four without the onset of parallel channel flooding.  
       SUMMARY OF THE INVENTION  
       [0008]     The current invention provides a polymer electrolyte membrane fuel cell water management device. The device includes a hydrophilic water transport element spanning from inside the fuel cell to outside the fuel cell and disposed between a gas diffusion layer and a current collector layer in the cell. The transport element includes an intermediate wick outside the fuel cell that is hydraulically coupled to the transport element, and includes a transport element structure integrated with a flow field structure within the fuel cell. The device further includes an electroosmotic pump, where the pump is located outside the fuel cell and is hydraulically coupled to the intermediate wick. The hydraulically coupled pump actively removes excess water from the flow field structure and the gas diffusion layer through the transport element, where a key aspect of the invention is the decoupling of water removal from oxidant delivery.  
         [0009]     According to the current invention, the electroosmotic pump includes a secondary porous structure layer, a porous pumping element, at least two electrodes, and a housing, where the secondary porous structure layer and the intermediate wick are hydraulically coupled. The housing holds the secondary porous structure coupled to the porous pumping element, and holds the electrodes about the intermediate wick and porous structure, whereby the water is rejected from the cell.  
         [0010]     According to one aspect of the invention, the secondary porous structure layer is an electrical insulator between the pump and the fuel cell. The secondary porous structure layer is a particle filter to the pump, where the secondary porous structure layer can be polyvinyl alcohol sponge, glass microfiber, cotton paper, cotton cloth, wool felt, polyurethane foams, cellulose acetate, crosslinked polyvinyl pyrrolidone, or polyacrylamide. The porous pumping element can be glass-particle-packed fused silica capillaries, porous borosilicate glass, in situ polymerized porous monoliths, bulk-micromachined and anodically-etched porous silicon, aluminum oxide, porous silicon, or porous titanium oxide. In another aspect, the electroosmotic pump further includes an electric potential across the porous pumping element, where the electric potential is sufficient to induce a Columbic force on a mobile ion layer on the porous pumping element, whereas a viscous interaction between the mobile ions and the water generates a bulk flow. The electric potential across the porous pumping element can be a time varying potential, thus reducing parasitic loads to the fuel cell. The electric potential can be activated when flooding or dry-out is detected or imminent, whereby reducing parasitic loads to the fuel cell.  
         [0011]     According to another aspect of the invention, the fuel cell can be a fuel cell stack including at least two fuel cells. In one aspect, the fuel cell stack has a wicking bus disposed between the pump and multiple layers of the transport element, where the bus is operated by at least one EO pump. The bus can be a dielectric wick disposed outside the fuel cell, where when the dielectric wick saturates with water the dielectric wick hydraulically connects the transport elements with the pump and insulates an electric field of the cell from an electrical field of the pump. In a further aspect, the dielectric wick can be made from polyvinyl alcohol sponge, glass microfiber, cotton paper, cotton cloth, wool felt, polyurethane foams, cellulose acetate, crosslinked polyvinyl pyrrolidone, or polyacrylamide.  
         [0012]     According to one aspect of the invention, the transport element is an electrically conductive wick. The electrically conductive wick can be made from a material including carbon cloth, carbon paper, aluminum foam, stainless steel foam or nickel foam.  
         [0013]     In another aspect of the invention, the transport element is a porous hydrophilic water transport layer disposed between a bipolar plate and a gas diffusion layer in the fuel cell, where the water transport layer is hydraulically connected to the external electroosmotic pump.  
         [0014]     In a further aspect of the invention, the transport element is a porous hydrophilic water transport layer having a pattern of cut-outs or a pattern of hydrophobic regions a pattern of cut-outs and/or a pattern of hydrophobic regions arranged in a pattern, where the transport layer is hydraulically continuous, allowing for the fuel cell reactant gasses to flow freely through the transport layer in a direction perpendicular to the plane of the transport layer, where the transport layer is disposed between a gas diffusion layer and a current collector layer in the fuel cell. The transport layer is hydraulically connected to the external electroosmotic pump.  
         [0015]     According to another aspect of the invention, the electroosmotic pump is disposed to humidify a membrane electrode assembly when using dry gases and low humidity gases in the flow fields.  
         [0016]     In one aspect of the invention, the electroosmotic pump is disposed to humidify hydrogen in an anode current collector on the fuel cell.  
         [0017]     In another aspect, the electroosmotic pump actively distributes water in the cell between a cathode region and an anode region of the fuel cell.  
         [0018]     The proposed water management solution eliminates large fuel cell humidifier systems and reduces the size of air supply system by reducing the air flow requirements. This translates into reduction of power consumption, and complexity of auxiliary devices. Consequently, the proposed water management solution reduces the overall cost by reduction of system complexity and use of cost effective materials.  
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0019]     The objectives and advantages of the present invention will be understood by reading the following detailed description in conjunction with the drawing, in which:  
         [0020]      FIG. 1 ( a ) shows a planar cutaway schematic view of a fuel cell and EO pump assembly according to the present invention.  
         [0021]      FIG. 1 ( b ) shows a perspective exploded view of a fuel cell plate and EO pump assembly according to the present invention.  
         [0022]      FIGS. 2   a - 2   b  show planar schematic views of the current invention.  
         [0023]      FIG. 3  shows a partial cutaway perspective view of an integrated cathode/transport element embodiment according to the present invention.  
         [0024]      FIG. 4  shows a planar schematic view of transport phenomena related to water transport in PEM fuel cells.  
         [0025]      FIG. 5  shows a planar schematic view of cell water management according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     Although the following detailed description contains many specifics for the purposes of illustration, anyone of ordinary skill in the art will readily appreciate that many variations and alterations to the following exemplary details are within the scope of the invention. Accordingly, the following preferred embodiment of the invention is set forth without any loss of generality to, and without imposing limitations upon, the claimed invention.  
         [0027]     The current invention provides an active water management system utilizing electroosmtic (EO) pumps for redistributing and removing liquid water. Transient and polarization data demonstrate that the active removal of water with EO pumping according to the current invention eliminates flooding with a low parasitic load (˜10% of the fuel cell power). The EO pump uses an electric double layer (EDL) that forms between solid surfaces and liquids. By using porous glass EO pump structures, silanol groups on the surface of the glass spontaneously deprotonate, and create a negative surface charge and a net-positive layer of mobile ions with a generated potential of roughly −60 mV (a typical zeta potential for deionized water). Applying electric potential across a porous glass substrate induces a Columbic force on this mobile ion layer. The viscous interaction between ions and water generates a bulk flow. In the present invention, the working flow rate through an EO pump is a linear function of pressure load and the electric field imposed across the pump. The EO pump flow rates scale linearly with area, an appropriate scaling for fuel cells whose output power and water production rate also scale with area. According to the current invention, EO pumps present a negligible parasitic load. The EO pump is hydraulically coupled to an internal wick structure.  
         [0028]     Referring to the figures,  FIG. 1 ( a ) shows a planar cutaway schematic view of a fuel cell and EO pump assembly  100 . Shown is a fuel cell  102  with a hydrophilic water transport element  104  and an external EO pump  106  with water flow  108  in the assembly  100 . The hydrophilic transport element  104  absorbs water droplets  108  from the cathode channels  110 ( a ) (also known as flow field) of a cathode current collector  111  and gas diffusion layer  112 , including water  108  that normally accumulates under the rib  114  of the flow field  110 ( a ). Upon saturation with absorbed water  108 , the hydrophilic transport element  104  can no longer remove water without application of a pressure gradient to force water  108  across the hydrophilic transport element  104 . This forced transport action is accomplished by the external EO pump  106 . The EO pump  106  and the hydrophilic transport element  104  are hydraulically coupled through a secondary porous structure layer  116  which serves as both an easily-compressed coupler between the hydrophilic transport element  104  and a porous pumping element  118  that also keeps particles (e.g., carbon residue) from clogging the pump  106 . Furthermore, the non-conductive porous pumping element  118  helps to electrically isolate the pump  106  from the fuel cell  102 . According to the invention, the EO pump  106  is in close proximity to the air outlet (not shown) to exploit air pressure gradients within the cathode flow field  110 ( a ) in removing water  108  from the transport element  104 . The EO pump  106  further has at least two electrodes  120 , and a housing  124 , where the housing  124  holds the secondary porous structure  116 , the porous pumping element  118 , the electrodes  118  about an intermediate wick  126 , where the water is rejected from the cell. The intermediate wick  126  is hydraulically connected to the transport element  104 , where the intermediate wick  126  represents a portion of the transport element  104  that is outside the cell  102 . Further shown, is an anode current collector  130  having anode flow channels  110 ( b ), a membrane electrode assembly (MEA)  134  disposed between the gas diffusion layers  112 , and a seal  136  surrounding the gas diffusion layers  112  to seal the gases.  
         [0029]      FIG. 1 ( b ) shows a perspective exploded view of a fuel cell plate and EO pump assembly  126  that includes the transport element  104  and external EO pump  106 . Here, the transport element  104  is shown as a hydrophilic porous flow field plate having an integrated intermediate wick  126  that is hydraulically coupled to the external EO pump  106 . Further shown is a solid graphite base  128  for holding the transport element  104 .  
         [0030]     As shown, the secondary porous structure layer  116  has a horizontal tab that is disposed between the pump anode  120 ( a ) (pump inlet) and the porous pumping element  118 , where an opposite horizontal tab of the secondary porous structure layer  116  is disposed between the housing  124  and the intermediate wick  126  (or the portion of the transport element  104  that is outside the cell  102 ) of the hydrophilic transport element  104 . The secondary porous structure layer  116  is very hydrophilic and can have relatively large pores (as small as 10 μm) for low hydraulic resistance. The secondary porous structure layer  116  further can have an uncompressed porosity of 90%. The housing  124  consists of two plates which compress both the pump elements and the interface of the secondary porous structure layer  116  and porous pumping element  118 . The pump&#39;s anode housing plate  124 ( b ) has small openings (˜1 by 1 mm) to allow the oxygen generated by electrolysis to escape. The pump cathode housing plate  124 ( a ) has larger openings for the pump&#39;s water outlet.  
         [0031]     The secondary porous structure layer  116  can be an electrical insulator between the EO pump  106  and the fuel cell  102 . The secondary porous structure layer  116  provides a particle filter to the pump  104 , where the secondary porous structure layer  116  can be made from polyvinyl alcohol sponge, glass microfiber, cotton paper, cotton cloth, wool felt, polyurethane foams, cellulose acetate, crosslinked polyvinyl pyrrolidone, or polyacrylamide. Additionally, the porous pumping element  118  can be made from glass-particle-packed fused silica capillaries, porous borosilicate glass, in situ polymerized porous monoliths, bulk-micromachined and anodically-etched porous silicon, aluminum oxide, porous silicon, or porous titanium oxide.  
         [0032]     The EO pump  106  can further include an electric potential across the porous pumping element  118 , where the electric potential is sufficient to induce a Columbic force on a mobile ion layer on the porous pumping element  118 , whereas a viscous interaction between mobile ions and water generates a bulk flow (not shown). The electric potential across the porous pumping element  118  can be a time varying potential, thus reducing parasitic loads to the fuel cell  102 . The electric potential can be activated when flooding or dry-out is detected or imminent, whereby reducing parasitic loads to the fuel cell  102 .  
         [0033]      FIGS. 2   a - 2   b  show planar schematic views of the current invention having a PEM fuel cell  102  with active water removal through an integrated water transport element  104 , where the liquid flow  108  is driven by an external EO pump  106 . As shown, water  108  is removed from the channels  110 ( a ) and from the gas diffusion layer  112  underneath the ribs  114  (see  FIG. 1 ( a )) and transported to a wicking bus  200  that hydraulically connects the transport element  104  to the EO pump  106 . Current flow  206  is shown spanning across the fuel cell  102 . Shown in  FIG. 2 ( b ) is a fuel cell stack  204  having at least two fuel cells  102 . As shown, the fuel cell stack  204  has a wicking bus  200  disposed between the EO pump  106  and multiple layers of the transport element  104 , where the bus  200  is operated on by at least one EO pump  106 . When considering large area fuel cells  102 , such as for automobile usage, rapid response to saturation is important, where multiple EO pumps  106  (shown in dashed lines) can be connected to the bus  200  to minimize the distance required for the water to travel. The bus  200  can be a dielectric wick disposed outside the fuel cell  102 . When the dielectric wick  200  saturates with water it hydraulically connects the transport elements  104  with the pump  106 , while insulating the electric field of the fuel cell  104  from the electrical field of the pump  106 . According to the current invention, the dielectric wick  200  can be made from polyvinyl alcohol sponge, glass microfiber, cotton paper, cotton cloth, wool felt, polyurethane foams, cellulose acetate, crosslinked polyvinyl pyrrolidone, or polyacrylamide.  
         [0034]     In one embodiment of the current invention, the EO pump  106  is disposed to humidify the membrane electrode assembly (MEA)  134  when using dry gases and low humidity gases in the flow fields  110 . The EO pump  106  is further disposed to humidify hydrogen in the anode current collector  110 ( b ) on the fuel cell  102 , and/or disposed to actively distribute water  108  in the cell  102  between a cathode current collector  111  region and an anode current collector  130  region of the fuel cell  102  (not shown).  
         [0035]     According to the invention, the transport element  104  can be an electrically conductive wick. The electrically conductive wick  104  can be made from a material including carbon cloth, carbon paper, aluminum foam, stainless steel foam or nickel foam.  FIG. 3  shows a partial cutaway perspective view of an integrated cathode  111 /transport element  104  embodiment  300  of the invention, where the transport element  104  is a porous hydrophilic water transport layer disposed between a bipolar plate  302  and a gas diffusion layer  112  in the fuel cell  102 . The water transport layer  104  is hydraulically connected to the external electroosmotic pump  106  (not shown). The porous hydrophilic water transport layer  104  is shown having a pattern of gas permeable regions  304 , where the regions  304  are formed either as cut-outs or as locally hydrophobic zones of the hydrophilic transport layer  104 , where the transport layer remains hydraulically continuous. The gas permeable areas  304  enable rapid oxygen diffusion from the gas diffusion layer  112  into the channel  110 ( a ) even as the transport layer  104  is fully saturated with water. As the transport layer  104  conducts electricity  308  to the cathode  111  portion of the bipolar plate  302  it simultaneously transports water  108  from the cell  102  when hydraulically coupled to the EO pump  106 . The integrated embodiment  300  provides advantages of being thin, independent to the design of bipolar plate  302 , and low ohmic resistance.  
         [0036]      FIG. 4  shows a planar schematic view of transport phenomena  400  related to water transport in PEM fuel cells  102  having a MEA  134  disposed between two gas diffusion layers  112 , where according to the current invention, the MEA  134  a MEA is a membrane  402  with two catalyst layers consisting of a cathode catalyst layer  401  and a anode catalyst layer  403 . As shown, there are parallel and coupled mechanisms for transporting liquid and vapor within each distinct region of the fuel cell  102 ; each having its own characteristic transport physics. Water produced in the cathode  111  travels to the gas channels  110 ( a ) by vapor diffusion  404  and (liquid) capillary transport  406 . The vapor and liquid are coupled through phase change phenomena  408 . Once in the channels  110 ( a ), water  108  is removed from the fuel cell  102  by vapor advection  410  and droplet advection  412 . Additional water  108  may travel from the anode  130  to cathode  111  (see  FIG. 1 ), or vice versa, by a diffusion/hydraulic permeation combination  414 , and electroosmotic drag  416 . Flooding occurs and performance deteriorates when these mechanisms inadequately remove liquid water  108 , thus restricting oxygen from reaching the cathode catalyst layer  401 . By integrating EO pumps  106  and wicking structures  104  into PEM fuel cells  102  a comprehensive water management device is provided to address water removal limitations.  
         [0037]      FIG. 5  is a planar schematic view of cell water management  500  showing simultaneous MEA  134  hydration and mitigated flooding while operating the fuel cell  102  with negligible parasitic load. The electrically conductive transport element  104  rapidly absorbs water  108  by capillary action  404  (not shown) when the transport element  104  is unsaturated. The schematic shows an air hydration region  502 , a water redistribution region  504  and a water removal region  506  of the transport element  104 . If the air supply at the inlet  508  has a relative humidity below 100%, the water  108  absorbed by the transport element  104  near the outlet  510  is redistributed to dry regions up-stream by capillary forces  404 , during this time water  108  evaporates to humidify the air stream and improve membrane  134  conductivity. In the regions of evaporation, the latent heat of phase change removes heat produced by the fuel cell  102 . When water  108  completely saturates the system, including the dielectric wick  200  and EO pump  106 , the EO pumps circuit  132  (see  FIG. 1 ) is closed and the pump  106  is automatically activated. The pump  106  then generates a pressure gradient that removes excess water  108 . In the current embodiment, the EO pump  106  is disposed to humidify hydrogen in the anode current collector  130  on the fuel cell  102  such that the EO pump  106  actively distributes water  108  in the cell  102  between the cathode  111  region and the anode  130  region, where the water  108  removed by the EO pump  106  is diverted to the anode  130  for humidifying the hydrogen (not shown) at the hydrogen inlet  512 .  
         [0038]     The present invention has now been described in accordance with several exemplary embodiments, which are intended to be illustrative in all aspects, rather than restrictive. Thus, the present invention is capable of many variations in detailed implementation, which may be derived from the description contained herein by a person of ordinary skill in the art. For example, it can be extended to planar, air-breathing fuel cell designs. All such variations are considered to be within the scope and spirit of the present invention as defined by the following claims and their legal equivalents.