Patent Application
20110223517
This application relates to the field of PEM fuel cells, and more particularly, to avoidance of cathode flooding for improved performance in PEM fuel cells.
In some fuel cells, a polymer-electrolyte membrane (PEM) is disposed between an anode, where fuel is electrochemically oxidized, and a cathode, where oxygen is electrochemically reduced. The PEM enables ions evolved at the anode to travel to the cathode, resulting in a charge-balanced redox reaction between the fuel and the oxygen. Besides maintaining suitable ionic conductance, the PEM should be impervious to the fuel and the oxygen, to prevent unwanted mixing, and it should be dimensionally stable over the operating-temperature range of the fuel cell. In typical usage, a catalytic and/or reactant-retentive structure may be bonded to each side of the membrane—i.e., an anodic structure bonded to the anode side and a cathodic structure bonded to the cathode side. Such structures, together with the PEM in between, comprise the so-called membrane-electrode assembly (MEA) of the fuel cell.
One type of PEM, attractive for its extended operating-temperature range, is the PBI-PA membrane. This membrane comprises a polybenzimidazole (PBI) film in which a significant quantity of phosphoric acid (H3PO4, PA) may be sorbed. It is believed that protons (formally H+) are conducted through this membrane via the sorbed PA as well as the PBI polymer electrolyte. An MEA based on such a membrane may be used in a hydrogen-air fuel cell at temperatures approaching 180° C.
However, PA loss from a PBI-PA membrane in an operating fuel cell may degrade fuel-cell performance by reducing the ionic conductance of the membrane. Over an extended period of time, such loss may also affect the dimensional stability of the membrane, leading to sealing problems and reactant cross-over. PA loss may therefore limit the usable lifetime of a PBI-PA membrane in a fuel cell. Accordingly, a PBI-PA membrane engineered for use in a fuel cell may be intentionally doped with excess PA. Each of the catalytic and/or reactant-retentive structures bonded to the membrane may also be doped with excess PA. In this manner, the MEA may store a sufficient amount of PA to offer an acceptably long usable lifetime despite gradual PA loss.
One embodiment of this disclosure provides a method of making an MEA for a fuel cell. The method comprises arranging a cathodic structure on a first surface of a PEM, and arranging an anodic structure on a second surface of the PEM, opposite the first surface, the anodic structure containing more PA per unit volume than the cathodic structure. The method further comprises pressing the cathodic and anodic structures to the PEM to form the MEA.
Another embodiment provides an MEA as described above, wherein a cathodic bipolar plate is disposed in face-sharing contact with the cathodic structure of the MEA, and an anodic bipolar plate is disposed in face-sharing contact with the anodic structure of the MEA.
Another embodiment provides a method of assembling a fuel cell. This method comprises installing between two bipolar plates of the fuel cell an MEA as described above, and applying force to the bipolar plates to seal the bipolar plates to the MEA without first adding additional PA to the cathodic structure.
The summary above is provided to introduce a selected part of this disclosure in simplified form, not to identify key or essential features. The claimed subject matter, defined by the claims, is limited neither to the content of this summary nor to implementations that address problems or disadvantages noted herein.
Aspects of this disclosure will now be described by example and with reference to the illustrated embodiments listed above. Components, process steps, and other elements that may be substantially the same in one or more embodiments are identified coordinately and are described with minimal repetition. It will be noted, however, that elements identified coordinately may also differ to some degree. It will be further noted that the drawing figures included in this disclosure are schematic and generally not drawn to scale. Rather, the various drawing scales, aspect ratios, and numbers of components shown in the figures may be purposely distorted to make certain features or relationships easier to see.
As noted above, a PBI-PA membrane engineered for use in a fuel cell may be intentionally doped with excess PA. Each of the catalytic and/or reactant-retentive structures bonded to the membrane may also be doped with excess PA. In this manner, the MEA may store a sufficient amount of PA to offer an acceptably long usable lifetime despite gradual PA loss. However, the inventors herein have discovered a disadvantage in the approach noted above. In particular, excess PA present on the cathode side of the membrane may flood the reactant-retentive structure bonded to the membrane, restricting oxygen diffusion and occluding the catalytic sites where oxygen reduction takes place. Excess PA on the cathode side may also encourage dihydrogen phosphate (H2PO4−) to adsorb on the catalytic sites, causing contamination. These factors may significantly degrade the operating voltage of the fuel cell, which is normally limited by cathode kinetics.
To address these issues and to secure still other advantages, this disclosure describes an asymmetrically acidified MEA for a fuel cell, a method of making an asymmetrically acidified MEA, and a method of assembling a fuel cell using an asymmetrically acidified MEA.
At 14 the PEM is acidified by immersion in a bath containing aqueous PA at the desired temperature for certain period of time. In one example, the bath may contain 85% PA, although other concentrations may be used instead. After removal from the bath, the PEM may be wiped by dry paper towel to remove free PA and titrated to determine or estimate the PA concentration therein.
At 16 a cathodic gas-diffusion layer (GDL) and an anodic GDL are cut from a stock of carbon-fiber paper or carbon-fiber cloth to the desired shape and dimensions. In one embodiment, the carbon-fibers of the paper or cloth may be treated, before or after cutting, with a hydrophobizing agent, such as a polytetrafluoroethylene (PTFE) solution. At 18 an anodic microporous layer (MPL) is applied to the anodic GDL. The anodic MPL may be applied to the anodic GDL as a suspension of particles and dissolved solids in solvent vehicle. The suspension may be sprayed, painted, or screen-printed on the anodic GDL, for example. The particles in the suspension may include carbon—e.g., carbon black and/or graphite. In some embodiments, the suspension may also include silicon carbide particles to increase the amount of PA sorbable therein without sacrificing electrical conductivity.
The anodic MPL may also include PA. The PA may be initially present in the suspension of particles from which the anodic MPL is applied, or it may be added to the anodic MPL in a subsequent application.
At 20 an anodic catalyst layer is applied to the anodic MPL. The anodic catalyst layer may be applied to the anodic MPL as a suspension of particles and dissolved solids in solvent vehicle. It may comprise catalyzed carbon particles. Such particles may include any of the forms of carbon listed above and may support a suitable loading of a hydrogen-oxidation catalyst: finely divided platinum and/or ruthenium, as examples. The particles or vehicle may also include a hydrophobizing agent, such as PTFE. In one embodiment, the anodic catalyst layer may also include PA, whether delivered in the suspension of catalyzed carbon particles or in a subsequent application. In one embodiment, the combined amount of PA in the PEM and the anodic structure may be at least 15 milligrams per square centimeter (mg/cm2). Preferably, the combined amount of PA may be in the range of 20 to 28 mg/cm2.
The inventors herein have observed that partial occlusion of catalytic sites of the anodic catalyst layer is less problematic than occlusion on the cathode side, as the operating voltage of the fuel cell is typically limited by cathode kinetics.
At 22 a cathodic MPL is applied to the cathodic GDL, and at 24, a cathodic catalyst layer is applied to the cathodic MPL. In one embodiment, the cathodic MPL and the cathodic catalyst layer may be substantially the same as the anodic MPL and anodic catalyst layer described above, except with regard to PA content. More specifically, the cathodic MPL and cathodic catalyst layer may contain less PA per unit volume that the corresponding anodic layers. For example, the suspension from which the anodic structure is applied may include more phosphoric acid per unit volume than the suspension from which the cathodic structure is applied. In one embodiment, the cathodic MPL and the cathodic catalyst layer may contain substantially no PA, while in other embodiments, these layers may include a relatively small amount of PA, in order to shorten the MEA conditioning time (vide infra).
In other embodiments, the cathodic MPL and/or cathodic catalyst layer may differ structurally from the corresponding anodic layer. In one embodiment, the anodic structure may be engineered to be especially thick and/or porous, relative to the cathodic structure, to increase the relative amount of PA sorbable therein. In a more particular embodiment, the thicker anodic layer may be the anodic MPL, not the anodic catalyst layer. In this manner, the expensive catalyst need not be dispersed in areas too far from the PEM to catalyze oxidation of fuel. In another embodiment, the anodic structure may include more PA-absorbing silicon carbide per unit volume than the cathodic structure.
In still other embodiments, the cathodic catalyst layer may support a different catalyst, a higher catalyst loading, etc. Further, the cathodic MPL and/or cathodic catalyst layer may engineered in view of a reduced need for PA storage relative to the corresponding anodic layers. For example, the MPL may be thinner, or include less silicon carbide per unit volume than the anodic MPL.
At 26 the anodic and cathodic GDLs, which support their respective MPLs and catalyst layers, are arranged in registry on opposite surfaces of the PEM. In this manner, a cathodic structure including a cathodic MPL and cathodic catalyst layer is arranged on a first surface of the PEM, and an anodic structure including an anodic MPL and anodic catalyst layer is arranged on a second surface of the PEM, opposite the first surface. As described above, the anodic structure in this embodiment may contain more PA per unit volume than the cathodic structure.
At 28 the anodic GDL that supports the anodic MPL and anodic catalyst layer, the cathodic GDL that supports the cathodic MPL and cathodic catalyst layer, and the PEM are pressed together to form an asymmetrically acidified MEA. In one particular embodiment, the assembly may be subject to heat pressing for 30 seconds at 160° C. at a pressure of 5 to 20 kilograms per square centimeter.
As shown in
No aspect of
Returning now to
As shown in the graph, the symmetrically acidified MEA very quickly achieves a maximum voltage, which decays thereafter, presumably due to PA flooding at the cathode. By contrast, the operating voltage of the asymmetrically acidified MEA builds more slowly, but remains high for an extended period of operation. In method 54, accordingly, the actions taken at 70 may be continued during a conditioning phase of the assembled fuel cell, wherein the operating voltage of the fuel cell increases to a desired—e.g., normal operating—level.
Naturally, some of the process steps described and/or illustrated herein may, in some embodiments, be omitted without departing from the scope of this disclosure. Likewise, the indicated sequence of the process steps may not always be required to achieve the intended results, but is provided for ease of illustration and description. One or more of the illustrated actions, functions, or operations may be performed repeatedly, depending on the particular strategy being used.
Finally, it will be understood that the articles, systems, and methods described herein are embodiments of this disclosure—non-limiting examples for which numerous variations and extensions are contemplated as well. Accordingly, this disclosure includes all novel and non-obvious combinations and sub-combinations of the articles, systems, and methods disclosed herein, as well as any and all equivalents thereof.
