Patent Application
20090191351
The field to which the disclosure generally relates includes fuel cell bipolar plates.
A fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte between the anode and the cathode. The anode receives hydrogen-rich gas or pure hydrogen and the cathode receives an oxidant such as oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode, where the protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode are unable to pass through the electrolyte. Therefore, the electrons are directed through a load to perform work before they are sent to the cathode. The work may be used, for example but not limited to, operating a vehicle.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack includes a series of bipolar plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates and cathode gas flow channels are provided on the cathode side of the bipolar plates. The bipolar plates may also include flow channels for a cooling fluid.
The bipolar plates are typically made of a conductive material, such as a carbon-composite or metal, so that they conduct the electricity generated by the fuel cells from one cell to the next cell and out of the stack. The bipolar plates may be machined from relatively thin metal substrates or thin metal substrates that may be stamped to provide reactant gas flow fields and coolant fluid flow fields.
As is well understood in the art, most types of fuel cells need to have a certain relative humidity. During operation of the fuel cell, moisture may enter the anode and cathode flow channels due to the reactant gases being humidified or due to water produced at the cathode. As the size of the water droplets increases, the flow channel is closed off, and the reactant gas is diverted to other flow channels because the channels flow in a generally parallel direction between common inlet and outlet manifolds. Because the reactant gas may not flow through a channel that is blocked with water, the reactant gas cannot force the water out of the channel. As more and more flow channels are blocked by water, the electricity produced by the fuel cell decreases. Because the fuel cells are electrically coupled in series, if one of the fuel cells stops performing, the entire fuel cell stack may stop performing.
It is usually possible to purge the accumulated water in the flow channels by periodically forcing the reactant gas through the flow channels at a higher flow rate. However, on the cathode side, this increases the parasitic power applied to the air compressor, thereby reducing overall system efficiency. Moreover, there are many reasons not to use the hydrogen fuel as a purge gas, including reduced economy, reduced system efficiency, and increased system complexity for treating elevated concentrations of hydrogen in the exhaust gas stream.
Reducing accumulated water in the channels can also be accomplished by reducing inlet humidification. However, it is desirable to provide some relative humidity in the anode and cathode reactant gases so that the membrane in the fuel cells remains hydrated. A dry inlet gas has a drying effect on the membrane that could increase the cell's ionic resistance, and limit the membrane's long-term durability.
It is known in the art to coat the bipolar plate with a hydrophilic coating to reduce water accumulation.
One embodiment of the invention includes a first fuel cell component comprising a substrate comprising a first face, a first hydrophilic coating overlying at least a first portion of the first face, and a second less hydrophilic coating overlying at least a second portion of the first face.
Other exemplary embodiments of the invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the exemplary embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
Exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings.
The following description of the embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.
One embodiment of the invention includes a bipolar plate having variable surface properties to maximize the beneficial effects of low electrical resistance and low accumulated water mass, wherein the bipolar plate has a super-hydrophilic channel bottom and/or sidewalls, and less hydrophilic (or hydrophobic) lands that maximize product water transport from diffusion media to channels with no added electrical resistance. In another embodiment, a hydrophilic coating is applied so that the gas inlet areas of the fuel cell are more hydrophilic than the center of the cell.
A first coating 30 is formed over at least a portion of the bipolar plate 12. The first coating 30 may be formed over the entire surface of the bipolar plate including the lands 16 and channels 18, or the coating 30 may be selectively deposited over portions of the bipolar plate, for example, over only the channels 18. The first coating 30 may be a hydrophilic coating, for example a metal oxide coating including, but not limited to, silicon dioxide (SiO2), hafnium dioxide (HfO2), zirconium dioxide (ZrO2), aluminum oxide (Al2O3), stannic oxide (SnO2), tantalum pent-oxide (Ta2O5), niobium pentoxide (Nb2O5), molybdenum dioxide (MoO2), iridium dioxide (IrO2), ruthenium dioxide (RuO2), metastable oxynitrides, nonstoichiometric metal oxides, oxynitrides and mixtures thereof, as disclosed in U.S. Patent Application No. 2006/0216571A1. The first coating 30 may be a combination of a conductive material and a metal oxide as disclosed in U.S. Patent Application No. 2006/0194095A1. The first coating 30 may also be a SiOx coating. The first coating 30 may be formed by, for example, physical vapor deposition processes, chemical vapor deposition (CVD) processes, plasma enhanced CVD processes, thermal spraying processes, sol-gel, spraying, dipping, brushing, spinning on, or screen printing. The thickness, and consequently the hydrophilicity, of the first coating 30 may be increased by dipping multiple times. The thickness of the first coating 30 may be about 50 nanometers to about 1 micrometer.
Channel water accumulation in both anode and cathode flow field plates may significantly influence fuel cell performance at low load. In various embodiments, the coating 30 is a hydrophilic coating that may reduce or eliminate voltage instability at low load with fine-pitch flow fields, due to the spreading of product water into thin films that have little impact on plate flow resistance. Water transport out of the diffusion media and into the flow field channels may be enhanced, with no increase in electrical resistance. In other embodiments, the coating 30 may reduce the rate of carbon corrosion in the electrodes of a membrane electrode assembly by reducing the formation of full-channel water slugs in the anode channels and accumulation in anode diffusion media that can cause hydrogen starvation. In other embodiments, the coating 30 may reduce freeze damage and freeze start-up time by minimizing the accumulated water mass in the channels and diffusion media.
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In one embodiment of the invention, the hydrophilic character of the bipolar plate 12 is varied in the active area plane. For example, a hydrophilic coating may be applied such that the gas inlet areas are more hydrophilic than the center of the fuel cell. Referring now to
Neutron radiography experiments were performed to show the water distribution in various 50 cm2 fuel cells: a fuel cell where the bipolar plate has a SiOx hydrophilic coating over both the channels and the lands, a fuel cell where the bipolar plate has a SiOx hydrophilic coating over the channels but where the hydrophilic coating has been removed from the lands, and a fuel cell where the bipolar plate has a SiOx hydrophilic coating over the channels but where the hydrophilic coating has been removed from the lands near the inlet and the outlet of the flowfield.
Completely coating the bipolar plate (the lands and the channels) may increase the electrical resistance at the contact areas between the bipolar plates and diffusion media. For example, an SiOx coating with a mean thickness of 80 to 100 nanometers added an average resistance of 11.6 mΩ cm2, based on a sample of 160 plates and an average untreated plate resistance of 44.0 mΩ cm2. Placing the highly hydrophilic PTFE-coated diffusion media against the highly hydrophilic coated bipolar plate lands does not maximize the rejection of product water from the contact region, which may be beneficial for reduced mass transport resistance.
In various embodiments, minimal water accumulation and the best fuel cell performance can be realized with hydrophilic channels and less hydrophilic lands. The overall mass of accumulated water is less for the bipolar plate where the hydrophilic coating has been removed from the cathode lands. In a fuel cell where the channels of the bipolar plate are coated with a SiOx hydrophilic coating and the lands are less hydrophilic, the water may be more effectively ejected from the gas diffusion media layer in the lands. While the SiOx hydrophilic coating over both the lands and the channels may reduce the total accumulated water mass by 55% as compared to an untreated bipolar plate, the total accumulated water mass may decrease by an additional 13% when the SiOx hydrophilic coating is removed from the cathode lands.
When the terms “over”, “overlying”, “overlies”, or “under”, “underlying”, “underlies” are used with respect to the relative position of a first component or layer with respect to a second component or layer, such shall mean that the first component or layer is in direct contact with the second component or layer, or that additional layers or components are interposed between the first component or layer and the second component or layer.
The above description of embodiments of the invention is merely exemplary in nature and, thus, variations thereof are not to be regarded as a departure from the spirit and scope of the invention.
