Highly conductive composites for fuel cell flow field plates and bipolar plates

Abstract

This invention provides a fuel cell flow field plate or bipolar plate having flow channels on faces of the plate, comprising an electrically conductive polymer composite. The composite is composed of (A) at least 50% by weight of a conductive filler, comprising at least 5% by weight reinforcement fibers, expanded graphite platelets, graphitic nano-fibers, and / or carbon nano-tubes; (B) polymer matrix material at 1 to 49.9% by weight; and (C) a polymer binder at 0.1 to 10% by weight; wherein the sum of the conductive filler weight %, polymer matrix weight % and polymer binder weight % equals 100% and the bulk electrical conductivity of the flow field or bipolar plate is at least 100 S / cm. The invention also provides a continuous process for cost-effective mass production of the conductive composite-based flow field or bipolar plate.

Description

[0001] The present invention is based on the research results of a project supported by the US Department of Energy SBIR-STTR Program. The US government has certain rights on this invention.FIELD OF THE INVENTION [0002] The present invention provides a highly electrically conductive composite material for use in a fuel cell bipolar plate or flow field plate. BACKGROUND OF THE INVENTION [0003] A proton exchange membrane (PEM) fuel cell is typically composed of a seven-layered structure, including (a) a central PEM electrolyte layer for proton transport; (b) two electro-catalyst layers on the two opposite primary surfaces of the electrolyte membrane; (c) two fuel or gas diffusion electrodes (GDEs, hereinafter also referred to as diffusers) or backing layers stacked on the corresponding electro-catalyst layers (each GDE comprising porous carbon paper or cloth through which reactants and reaction products diffuse in and out of the cell); and (d) two flow field plates (or a bi-polar plat...

AI Technical Summary

Benefits of technology

[0020] This invention provides a fuel cell flow field plate or bipolar plate having flow channels on faces of the plate, comprising an electrically conductive polymer composite. In one preferred embodiment, the composite is composed of (A) at least 50% by weight of a conductive filler, comprising at least 5% by weight reinforcement fibers, expanded graphite platelets, graphitic nano-fibers, and / or carbon nano-tubes (this at least 5% is based on the total weight f the composite); (B) thermoplastic at 1 to 49.9% by weight; and (C) thermoset binder at 0.1 to 10% by weight; wherein the sum of the conductive filler weight %, thermoplastic weight % and thermoset binder weight % equals 100% and the bulk electrical conductivity of the flow field or bipolar plate is at least 100 S / cm and, preferably, at least 200 S / cm. The thermoset binder resin has the advantage that it can be quickly cured so as to hold the reinforcement elements together, typically without a need to be heated to a high temperature and then cooled down slowly. The resulting preform is very easy to handle during subsequent molding operations. The thermoset resin is selected from the group consisting of unsaturated polyester resins, vinyl esters, epoxies, phenolic resins, polyimide resins, bismaleimide resins, polyurethane resins, and combinations thereof. A fast-curing or ultraviolet-curable resin is preferred.

[0021] The conductive filler comprises a conductive material selected from the group consisting of graphite powder, carbon / graphite fibers, metal fibers, carbon nano-tubes, graphitic nano-fibers, expanded graphite platelets, carbon blacks, metal particles, and combinations thereof. This filler may comprise some non-conductive fibers, such as glass fibers and polymer fibers, for the purpose of reinforcing or strengthening the composite without significantly reducing the electrical conductivity. Preferably, the thermoset binder is at 0.1 to 5% by weight and the thermoplastic is at 10 to 40% by weight. This composition is such that reinforcement fibers, carbon nano-tubes, graphitic nano-fibers, and / or expanded graphite platelets (those reinforcement elements having a high aspect ratio, such as a high length / thickness ratio or length / diameter ratio) form an overlapping, contiguous-strand backbone structure. Preferably, these high aspect-ratio elements are bonded together by the thermoset resin binder, or a combination of the thermoset binder and thermoplastic, to form a shape-retaining backbone. This shape-retaining backbone or “preform” makes it easily handleable for subsequent molding, embossing and / or stamping operations to form a flow field or bipolar plate.

[0022] In another preferred embodiment, the composite comprises an electrically conductive polymer composite having: (A) at least 50% by weight of a conductive filler, comprising at least 5% by weight reinforcement fibers, expanded graphite platelets, graphitic nano-fibers, and / or carbon nano-tubes; (B) a polymer matrix material at 1 to 49.9% by weight; and (C) a polymer binder at 0.1 to 10% by weight; wherein the sum of the conductive filler weight %, polymer matrix material weight % and polymer binder weight % equals 100% and the bulk electrical conductivity of the flow field plate or bipolar plate is at least 100 S / cm. In this case, the polymer matrix material is not a pure thermoplastic; instead, it may comprise a material selected from a thermoset resin, an interpenetrating network, a semi-interpenetrating network, an elastomer, or a combination thereof. The polymer binder can be advantageously selected from thermoset resins, but it does not have to be a thermoset resin. For instance, it can be a thermoplastic provided that heating and melting the thermoplastic to a high temperature (e.g., >200° C.) is not required. It is convenient to have a binder comprising a water soluble polymer. Vaporization of water allows the polymer to precipitate and bond to the reinforcement elements quickly. In one further preferred embodiment, the plate has a major surface having a skin layer less than 100 μm in thickness and having a polymer volume fraction less than 20%, preferably less than 10%. In other words, the skin layer is preferably composed of at least 80% conductive filler and more preferably at least 90% conductive filler. Such a skin layer prevents the formation of a resin-rich skin layer that otherwise has a high, dominating electrical resistance.

Problems solved by technology

The bipolar plate is known to significantly impact the performance, durability, and cost of a fuel cell system.

The bipolar plate, which is typically machined from graphite, is one of the most costly components in a PEM fuel cell.

These methods of fabrication place significant restrictions on the minimum achievable fuel cell thickness due to the machining process, plate permeability, and required mechanical properties.

Further, such plates are expensive due to high machining costs.

The machining of channels into the graphite plate surfaces causes significant tool wear and requires significant processing times.

Such laminated fluid flow field assemblies tend to have higher manufacturing costs than integrated plates, due to the number of manufacturing steps associated with forming and consolidating the separate layers.

They are also prone to delamination due to poor interfacial adhesion and vastly different coefficients of thermal expansion between a stencil layer (typically a metal) and a separator layer.

Because most polymers have extremely low electronic conductivity, excessive conductive fillers have to be incorporated, resulting in an extremely high viscosity of the filled polymer melt or liquid resin and, hence, making it very difficult to process.

It is well-known that CVI is a very time-consuming and energy-intensive process and the resulting carbon / carbon composite, although exhibiting a high electrical conductivity, is very expensive.

There are several drawbacks associated with this composite composition and method: (1) The fabrication process is tedious, consisting of many manual operations, and is not readily amenable to mass production.

Since engineering thermoplastics typically have a high melting point (e.g., >220° C. for polyester), it would take some time to heat up to that temperature and take some time to cool it down.

The cycle times are long and the process is energy-intensive.

(3) With this process, it appears difficult to achieve a graphite proportion above 50% (and, hence, conductivity above 100 S / cm) without interspersing additional graphite powder between layers of stacked preform sheets (an operation called “dry-lay”) prior to compression-molding.

Such labor-dependent operations make the whole process time-consuming and labor-intensive.

Dry-laid graphite powder between layers, although imparting high electrical conductivity to the composite, tend to form graphite-rich interfacial layers which are brittle and weak and tend to compromise the mechanical integrity of the resulting composite laminate.

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