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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
In one aspect of the invention, the electroosmotic pump is disposed to humidify hydrogen in an anode current collector on the fuel cell.
In another aspect, the electroosmotic pump actively distributes water in the cell between a cathode region and an anode region of the fuel cell.
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.
The objectives and advantages of the present invention will be understood by reading the following detailed description in conjunction with the drawing, in which:
a-2b show planar schematic views of the current invention.
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.
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.
Referring to the figures,
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'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's water outlet.
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.
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.
a-2b 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
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).
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.
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.
Number | Date | Country | |
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60808492 | May 2006 | US |