1. Field of the Invention
Aspects of the present invention relate to a metallic separator for a fuel cell, and more particularly, to a metallic separator for a fuel cell having excellent anti-corrosive properties and a low contact resistance.
2. Description of the Related Art
In a fuel cell, a fuel such as hydrogen, natural gas, methanol, or the like is oxidized to produce electrons and hydrogen ions at an anode. The hydrogen ions produced in the anode pass through an electrolyte membrane to a cathode, and the electrons produced in the anode are supplied to an external circuit through a wire. The hydrogen ions that reach the cathode are combined with the electrons that reach the cathode through the external circuit and with oxygen supplied from the outside to form water.
Fuel cells have been promoted as next-generation energy conversion devices since they have a high electricity generation efficiency and are environmentally friendly. Fuel cells can be classified into polymer electrolyte membrane fuel cells (PEMFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), etc., according to the type of electrolyte used. The operating temperature, materials of constitutional elements and the like vary according to the type of fuel cell.
PEMFCs can be operated at relatively low operating temperatures, such as, for example, 80 to 120° C., and have a significantly high current density. Thus, PEMFCs can be used as power supplies for vehicles and homes.
The main constitutional elements of a PEMFC are a bipolar plate and a membrane electrode assembly (MEA). In order to make PEMFCs compact, light and economical, it would be desirable to improve particular aspects of PEMFCs such as the bipolar plate.
The MEA includes an anode in which the fuel is oxidized, a cathode in which an oxidizing agent is reduced, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane has ionic conductivity in order to deliver hydrogen ions generated in the anode to the cathode, and is an electric insulator in order to electronically insulate the anode from the cathode.
Typically, the bipolar plate has channels through which fuel and air flow and functions as an electron conductor for transporting electrons between MEAs. Thus, the bipolar plate should be non-porous such that the fuel and the oxygen can be kept separated, and should have excellent electrical conductivity and sufficient thermal conductivity to control the temperature of the fuel cell. Furthermore, the bipolar plate should have sufficient mechanical strength to bear a clamping force applied to the fuel cell and should be corrosion-resistant with respect to hydrogen ions.
Conventionally, graphite has been the most common material used to form bipolar plates in PEMFCs, and channels through which fuel and air flow have typically been formed using a milling process. A graphite plate has sufficient electrical conductivity and resistance to corrosion to meet the requirements of a PEMFC. However, graphite plate and milling processes for shaping the plates are expensive. Further, graphite plates are typically brittle and thus, it is difficult to process bipolar plates with a thickness of less than 2-3 mm. Due to the difficulty in decreasing the thickness of bipolar plates made of graphite, it is difficult to decrease the size of a fuel cell stack consisting of several tens to several hundreds of unit cells.
In order to reduce the production costs and the thickness of bipolar plates, an attempt has been made to use metals as bipolar plate materials. Metals have most of the physical properties required for bipolar plates, and the costs of metals and processing thereof are reasonable. If metal can be used as the bipolar plate material, the costs of the bipolar plate can be reduced by 99% or more.
However, a metallic bipolar plate may erode under the acidic conditions that are present inside a fuel cell, and thus, an oxidized film that imposes a high electrical resistance may be easily formed. As a result, serious problems such as membrane poisoning and increased contact resistance may occur. Corrosion of the metallic bipolar plate not only causes defects in the bipolar plate itself, but also poisons the catalyst and electrolyte due to the diffusion of metal ions into the electrolyte membrane. When the catalyst is poisoned, catalytic activity is decreased, and when the electrolyte is poisoned, the proton conductivity of the electrolyte is reduced, thereby resulting in deterioration of the performance of the fuel cell.
In addition, since the corroded metal dissolves, the contact between the separator and the MEA deteriorates and the electrical resistance is increased, resulting in deterioration of the performance of the fuel cell.
Thus, it has not yet been possible to use metallic bipolar plates in PEMFCs because of the likelihood of corrosion of the metal. For example, in a 1000-hour performance test, PEMFCs having bipolar plates formed of stainless steel, a Ti alloy, an Al alloy, and a Ni alloy, respectively, showed lower performance than a PEMFC having a graphite bipolar plate.
Therefore, research into ways to improve the anti-corrosive properties of a metallic bipolar plate, such as a method of coating a surface with a material having an anti-corrosive property, has been carried out.
For example, a method of coating the surface of a bipolar plate composed of Ti or stainless steel with a material such as TiN that has excellent anti-corrosive properties and is electrically conductive is disclosed in Korean Laid-Open Patent Publication No. 2003-0053406.
While an Al alloy or a Ti alloy is susceptible to forming an oxidized film, stainless steel, which is relatively less susceptible to forming an oxidized film and has high corrosion resistance, is an excellent alternative to graphite.
The above discussion with respect to the bipolar plate is also applicable to an end plate, a cooling plate, and a separator.
An end plate is an electrically conductive plate having channels for either a fuel or an oxidizing agent on only one side. An end plate having channels for a fuel is attached to an MEA disposed at one end of a fuel cell stack, and an end plate having channels for an oxidizing agent is attached to an MEA disposed at the other end of the fuel stack.
A cooling plate is an electrically conductive plate having channels for a fuel or an oxidizing agent on one side and channels for a cooling fluid on the other side.
A separator is commonly used when a flow field is formed in diffusion layers of an anode and a cathode and generally a bipolar plate is used when the flow field is not included. Advantageously, the separator may have low gas permeability, excellent electrical conductivity, and excellent anti-corrosive properties.
Herein, the term “separator” is used generally to refer to the bipolar plate, the end plate, the cooling plate, and the specific separator discussed above.
Problems relating to the bipolar plate or separator of a PEMFC have been described above, but such problems also occur in MCFCs, PAFCs, DMFCs, etc. Accordingly, the separator according to aspects of the present invention may be used in these types of fuel cells as well.
Based on the above description, the development of a separator having improved anti-corrosive properties and contact resistance is desired.