Patent Application
20190058204
In at least one aspect, the present invention relates to a high-stiffness interlayer to be incorporated into membrane-electrode assemblies for the purpose of reducing through-plane electrical shorts from fiber protrusion into electrolyte membranes.
Many approaches to improve the system cost and performance for polymer electrolyte fuel cells (PEFCs) also increase the risk for electrical shorting across the membrane separator layer of the membrane-electrode assembly (MEA). These include thinner membrane layers for lower proton resistance, thinner gas diffusion layers for lower cost, higher cell compression for lower electrical resistance from the flow field plate through the gas diffusion layer and thinner electrode layers for lower Pt loading. An electrical short across the membrane separator induces ohmic heating during operation which can lead to ionomer decomposition if the local temperature reaches 280° C. [1,2]. Sufficient membrane degradation requires a cell replacement in the fuel cell stack.
Two types of membrane shorts have been observed during the fuel cell testing: soft and hard shorts. A soft short is a sub-critical short that results when conductive carbon fibers protrude into the membrane layer. These shorts do not immediately lead to fuel cell failure; however, a significant accumulation can reduce the overall cell resistance and compromise fuel cell durability through cell voltage degradation. Soft shorts are categorized by an electrical resistance through the membrane layer that is below about 140Ω. The protrusion of carbon fiber into the membrane is often caused by a mechanical stress on a loose fiber at the membrane surface or on an oriented fiber emerging from the gas diffusion layer. The stress can force the fiber into the membrane layer during the MEA manufacturing process, fuel cell assembly process, or fuel cell operation.
A hard short is a critical short that causes significant reactant gas crossover of the membrane separator and cell failure. Hard shorts occur suddenly in an operating fuel cell stack when a high shorting current passes through an existing membrane soft short with sufficiently low electrical resistance that causes excessive ohmic heating and high local temperature. When the temperature of membrane surrounding the soft short reaches its decomposition temperature, the membrane loses its mechanical integrity and thereby allows direct contact of the conductive catalyst layers and gas diffusion layers from both sides of the membrane that gives a large shorting current and significant heat generation. The hard short typically originates from a soft short of less than 140 Ω when the cell potential exceeds than 2 V.
Accordingly, there is a need for membrane-electrode assemblies with reduced propensities for through-plane electrical shorts.
The present invention is directed to solving one or more problems of the prior art by providing, in at least one embodiment, a membrane-electrode assembly (MEA), for fuel cell applications, having a high-stiffness interlayer interposed between a gas diffusion layer and a conducting membrane layer to mitigate electrical shorting across the conducting membrane layer. The membrane-electrode assembly additionally includes a catalyst layer interposed between the gas diffusion layer and the conducting membrane layer. The stiffness of the interlayer may be controlled by varying, for example, ionomer content and thickness with minimal loss in gas or proton transport to the active catalyst site in the electrode layers.
In one embodiment, a membrane-electrode assembly with a high-stiffness interlayer interposed between the gas diffusion and electrode layers is incorporated within a polymer electrolyte fuel cell (PEFC), wherein the MEA is interposed between two electrically conductive flow field plates further comprising channels for reactive gases.
In another embodiment, a high-stiffness interlayer is interposed between the electrode and membrane layers within a PEFC cell. A method for fabricating the high-stiffness interlayer set forth is provided which includes steps for making a dispersion, for example of a carbon black and one or more ionomers, milling the dispersion, and applying the dispersion to a substrate layer of an MEA.
Reference will now be made in detail to presently preferred compositions, embodiments and methods of the present invention, which constitute the best modes of practicing the invention presently known to the inventors. The Figures are not necessarily to scale. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for any aspect of the invention and/or as a representative basis for teaching one skilled in the art to variously employ the present invention.
Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, “parts of,” and ratio values are by weight; the term “polymer” includes “oligomer,” “copolymer,” “terpolymer,” and the like; molecular weights provided for any polymers refers to weight average molecular weight unless otherwise indicated; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.
It is also to be understood that this invention is not limited to the specific embodiments and methods described below, as specific components and/or conditions may, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present invention and is not intended to be limiting in any way.
It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components.
Throughout this application, where publications are referenced, the disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains.
“BET” means surface area as determined using the Brunauer, Emmett, and Teller equation.
“DMA” means dynamic mechanical analysis.
“ePTFE” means expanded polytetrafluoroethylene.
“ETFE” means ethylene-tetrafluoroethylene co-polymer.
“GDL” means gas diffusion layer.
“I/C” means ionomer:carbon weight/weight ratio.
“MEA” means membrane-electrode assembly.
“MPL” means microporous layer.
“PEFC” means polymer electrolyte fuel cell.
“RH” means relative humidity.
“SEM” means scanning electron microscopy.
The following examples illustrate various embodiments of the present invention. Those skilled in the art will recognize many variations that are within the spirit of the present invention and scope of the claims.
Embodiments of the present invention provide a membrane-electrode assembly (MEA), for fuel cell applications, having a high-stiffness interlayer interposed between a gas diffusion layer and a conducting membrane layer to mitigate electrical shorting across the conducting membrane layer. The membrane-electrode assembly additionally includes a catalyst layer interposed between the gas diffusion layer and the conducting membrane layer. The catalyst may be, for example, platinum-, ruthenium-, platinum ruthenium-, palladium-, iridium-, silver-, gold-, cobalt-, copper-, iron-, nickel-, rhodium-, tin-, or carbon-based, or a combination thereof, among others. The stiffness of the interlayer may be controlled by varying, for example, ionomer content and thickness with minimal loss in gas or proton transport to the active catalyst site in the electrode layers.
In one embodiment, a membrane-electrode assembly with a high-stiffness interlayer interposed between the gas diffusion and electrode layers is incorporated within a polymer electrolyte fuel cell (PEFC), wherein the MEA is interposed between two electrically conductive flow field plates further comprising channels for reactive gases.
In another embodiment, a high-stiffness interlayer is interposed between the electrode and membrane layers within a PEFC cell. A method for fabricating the high-stiffness interlayer set forth is provided which includes steps for making a dispersion, for example of a carbon black and one or more ionomers, milling the dispersion, and applying the dispersion to a substrate layer of an MEA.
With reference to
The first and second electrically conductive flow field plates 240 and 260 further comprise gas channels 540 and 560. Within the polymer electrolyte fuel cell 600, the membrane-electrode assembly portion 700 comprises the polymeric ion conducting membrane 120, the anode catalyst layer 140, the cathode catalyst layer 160, the first gas diffusion layer 340, the second gas diffusion layer 360, the first high-stiffness interlayer 440, and the second high-stiffness interlayer 460. During operation of the fuel cell 600, a fuel such as hydrogen is fed to the flow field plate 240 on the anode side, and an oxidant such as oxygen is fed to the flow field plate 260 on the cathode side. The hydrogen gas passes through the first gas diffusion layer 340 and the first high-stiffness interlayer 440. The oxygen gas passes through the second gas diffusion layers 360 and the second high-stiffness interlayer 460. Hydrogen ions are generated by anode catalyst layer 140 and migrate through polymeric ion conducting membrane 120 where they react at cathode catalyst layer 160 with the oxygen to form water. This electrochemical process generates an electric current through the motion of electrons from the anode catalyst layer 140 to the cathode catalyst layer 160 through an external circuit. Preferably, the interlayer stiffness is typically from about 5 to about 19 N/mm, and the soft short density in a fuel cell incorporating the interlayer is at least about 2-fold lower than for an otherwise identical fuel cell operating under comparable conditions. The interlayer porosity may generally be from about 0% to about 80% v/v.
With reference to
The first and second electrically conductive flow field plates 240 and 260 further comprise gas channels 540 and 560. Within the polymer electrolyte fuel cell 620, the membrane-electrode assembly portion 720 comprises the polymeric ion conducting membrane 120, the anode catalyst layer 140, the cathode catalyst layer 160, the first gas diffusion layer 340, the second gas diffusion layer 360, the first high-stiffness interlayer 440, and the second high-stiffness interlayer 460. During operation of the fuel cell 620, a fuel such as hydrogen is fed to the flow field plate 240 on the anode side, and an oxidant such as oxygen is fed to the flow field plate 260 on the cathode side. The hydrogen and oxygen gas pass through the first and second gas diffusion layers 340 and 360, respectively. Hydrogen ions are generated by anode catalyst layer 140 and migrate through polymeric ion conducting membrane 120 where they react at cathode catalyst layer 160 with the oxygen to form water. This electrochemical process generates an electric current through the motion of electrons from the anode catalyst layer 140 to the cathode catalyst layer 160 through an external circuit. In a refinement, the first high-stiffness interlayer and the second high-stiffness interlayer each independently has a stiffness from about 5 N/mm to about 30 N/mm. In another refinement, the first high-stiffness interlayer and the second high-stiffness interlayer each independently have a stiffness from about 10 to about 30 N/mm, and the soft short density in a fuel cell incorporating the interlayer is at least about 2-fold lower than for an otherwise identical fuel cell operating under comparable conditions. In some variations, the first high-stiffness interlayer and the second high-stiffness interlayer each independently has a porosity from about 0% to about 80% v/v. In general, the first high-stiffness interlayer and the second high-stiffness interlayer have a porosity from about 0% to about 20% v/v.
In a refinement of both variations of the embodiment membrane-electrode assembly 700 or 720 described above and depicted in
In a refinement of both variations of the embodiment membrane-electrode assembly 700 or 720 described above and depicted in
In a refinement of both variations of the embodiment membrane-electrode assembly 700 or 720 described above and depicted in
In a refinement of both variations of the embodiment membrane-electrode assembly 700 or 720 described above and depicted in
In a refinement, the first or second high-stiffness interlayer has a sufficiently low ionomer loading such that an optimal balance between electrical shorting and reactant gas transport resistance to the electrode catalyst is achieved.
In another refinement, wherein the first or second high-stiffness interlayer has a sufficiently high ionomer loading such that an optimal balance between electrical shorting in the membrane separator and proton transport resistance to the electrode catalyst is achieved.
In a refinement of the variations 600 and 620 described above and depicted in
In another embodiment, a method for applying the high-stiffness interlayer set forth above to the membrane-electrode assembly is provided. A method for fabricating the high-stiffness interlayer set forth is provided which includes steps for making a dispersion, for example of a carbon black and one or more ionomers, milling the dispersion, and applying the dispersion to a substrate layer of an MEA.
With reference to
The GDL, which includes both the carbon fiber substrate and a microporous layer (MPL), has an approximate 100 to 300 μm thickness at 0.03 MPa compression. The MPL layer itself has an approximate 40 μm thickness and comprises a carbon black with a 50-200 nm pore diameter and a fluoropolymer for wetproofing. The fluoropolymer is added as a fine 200 nm diameter particle which flows above its melting temperature (270-330° C.) during a sintering step (340-400° C.) to remove hydrocarbon-based coating additives, but does not provide a sufficient mechanical modulus to resist fiber protrusion.
With reference to
In a variation, graphitized Vulcan® carbon black (33 nm solid carbon particle diameter, 90 m2/g BET) is dispersed in n-propanol-water solvent (nPrOH:H2O::3:1 w/w) with a blend of two ionomer equivalent weights (EW900:EW700::3:1 w/w). The carbon loading is formulated at 5.0% w/w ink, while the ionomer loading is stepped in separate inks from 0.80 to 1.20 to 1.60 w/w carbon. The carbon black dispersion is milled with ZrO2 beads and coated directly onto the GDL substrate with a set of Mayer rods with a normalized thickness of 21.0±0.5 μm/(mg C/cm2), which corresponds to a carbon porosity at 76.0% v/v. Table 1 lists the actual coated thickness measured by SEM cross-section for each interlayer ink at two laydowns.
Advantageously, the interlayer indentation modulus increases with ionomer loading. Table 2 lists the indentation modulus for reference interlayer coatings at 12 μm thickness on a solid ETFE (ethylene-tetrafluoroethylene co-polymer) substrate. The mechanical modulus (E′, GPa) is measured at five ionomer loadings with a Micromechanical Laboratory's MTS Nanoindenter XP device at 0.5 mN load equipped with a 5 μm radius 60° conical diamond tip stylus. Each coating is first conditioned at least 24 hours at 70 F/50% RH, and five replicate measurements are averaged. The modulus is extracted using Testworks software following the Oliver and Pharr method [3]. In a direct comparison, the nanoindentation and DMA methods do show good agreement in the measured modulus at room temperature for commercially available membrane films.
The mechanical modulus increases with ionomer fill volume of the available interlayer carbon porosity. Since the carbon and ionomer have nominally the same material density (2.00 g/cc), the carbon porosity is expected to saturate with ionomer at I/C (w/w)=3.17 as shown in Table 2.
With reference to
E′(GPa)=0.721*I/C(w/w)
This indicates that the interlayer modulus is proportional to the ionomer loading and ranges from 0-2.3 GPa for 0-3.2 I/C (w/w) or, equivalently, 0-100% ionomer saturation. Above 100% saturation, the carbon network is no longer tightly packed so the mechanical modulus drops significantly.
Electrical shorting density can be measured as follows, in characterizing MEAs containing high-stiffness interlayers.
A current distribution circuit board induces uniform compression over a 32 cm2 area of a sample that consists of a piece of proton exchange membrane or a membrane electrode assembly sandwiched between two opposing gas diffusion layers [4,5]. The shorting current distribution is measured in 0.5 cm2 individual elements by incrementing the compressive pressure a constant applied voltage of 0.6 V. The method determines the density of soft shorts (in shorting counts per unit area) and their ohmic resistance in 64 elements across a 32 cm2 sample area that may lead to hard shorts when an adverse fuel cell operating condition is met.
With respect to
With respect to
With respect to
Soft Short Mitigation with Interlayer
Advantageously, symmetrical insertion of a high-stiffness interlayer between the gas diffusion layers and a membrane layer improves soft short density in a symmetrical gas diffusion layer/membrane laminate.
With respect to
With respect to
The low mechanical modulus of the commercial MPL layer is implied by its low stiffness even though the layer (40 μm) is substantially thicker than the interlayer samples (9-16 μm). The commercial MPL in
A variation of an embodiment, for example provided in
The impact of this resistance is quantified by cell voltage performance in
An alternative variation of an embodiment, for example provided in
In this case, with respect to
While exemplary embodiments and variations are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.
