Saturday, May 1, 2010

Nanohair and Nanoholes from PACVD and Ion Etch Yield More Powerful Polymer Electrolyte Fuel Cell Membrane

Nanohairy and full of nanoholes best describes a new nanostructured polyperfluorosulfonate polymer electrolyte membrane with reduced platinum catalyst loading developed by Korean scientists for use in fuel cells. 

The effort to reduce the amount of platinum used in proton exchange membranes is the subject of billions of dollars in annual research expenditures by hundreds of corporate and public laboratories worldwide. Increasingly the tools and machinery used to manufacture microchips are being applied and adapted to fabricating nanostructured fuel cell membrane electrode assembly components in an effort to produce more durable and cheaper components. 
  
Hyundai Motor Company (Seoul, KR), Kia Motors Corporation (Seoul, KR) and the Korea Institute of Science and Technology (Seoul, KR) share U.S. Patent Application  20100102026  which discloses a method of forming a nanohair and nanohole nanostructured surface on a polymer electrolyte membrane for use in fuel cell electric vehicles. 

Kwang Ryeol Lee, Myoung Woon Moon, Sae Hoon Kim, and Byung Ki Ahn developed the method of forming a nanostructured surface (NSS) on a polymer electrolyte membrane (PEM) of a membrane electrode assembly (MEA) for a fuel cell. The nanostructured surface is suitably formed on a polymer electrolyte membrane by plasma-assisted chemical vapor deposition (PACVD), where catalyst particles or a catalyst layer are directly deposited on the surface of the polymer electrolyte membrane having the nanohairy surface.

The method preferably comprises: forming a plurality of nanostructures on a surface of a polymer electrolyte membrane by performing plasma treatment on the surface of the polymer electrolyte membrane placed in a plasma chamber, maintained at a pressure of 1.0.times.10-7 to 2.75.times.10-3 Pa, at a bias voltage of -100 to -50 kV for 1 second to 60 minutes by plasma-assisted chemical vapor deposition (PACVD); and supporting or depositing the catalyst on the surface of the polymer electrolyte membrane having the plurality of nanostructures formed by the plasma treatment.

The nanostructure is suitably formed into a nano-hair structure having a width of 1 to 1,000 nanometers and a length of 1 to 10,000 nanometers. According to the inventors, it is possible to suitably increase the surface area of the polymer electrolyte membrane and, at the same time, suitably increase the hydrophobic properties of the surface of the polymer electrolyte membrane by forming nano-sized structures (nanostructures) such as nanohair or nanohole structures on the polyperfluorosulfonate polymer electrolyte membrane by plasma-assisted chemical vapor deposition (PACVD). 

Moreover, it is possible to easily fabricate the membrane electrode assembly for a fuel cell by a simple process of directing coating platinum (Pt) catalyst on the nanostructured surface having increased surface area and hydrophobic properties by sputtering.

Since the surface of the polymer electrolyte membrane comprises the nanostructures such as nano-hair patterns, its surface area is suitably increased and, at the same time, the hydrophobic properties with respect to water are suitably increased. As a result, it is possible to considerably reduce the number of process of fabricating the membrane electrode assembly for a fuel cell and further reduce the amount of platinum catalyst.

FIG. 1A is a schematic diagram of a plasma treatment apparatus for forming a nanostructured surface on a polymer electrolyte membrane.
FIG. 2 is a scanning electron micrograph (SEM) image taken after performing surface treatment on a polymer electrolyte membrane using argon plasma at a bias voltage of -800 V for 3 minutes and sputtering Pt catalyst. As shown in the SEM image of FIG. 2,  it can be observed that bundles of nanostructures (also called nano-hair patterns, nanofibers, nanowools, etc.) having a width of 10 to 30 nanometers and a length of 0.2 to 2 micrometers are suitably distributed on the surface of the polymer electrolyte membrane, which has been preferably exposed to an argon gas plasma at a bias voltage of -800 V for 3 minutes
FIG. 3A is a SEM image taken after sputtering Pt catalyst on the surface of a polymer electrolyte membrane by argon plasma treatment at a bias voltage of -600 V for 3 minutes;

FIG. 4A is a SEM image of the surface structure of a polymer electrolyte membrane after oxygen plasma treatment;

FIG. 5 is a SEM image showing nanohole structures formed on the surface of a polymer electrolyte membrane by argon plasma treatment at a bias voltage of -800 V for 1 minute, taken after depositing Pt catalyst on the surface of nanohole structures by sputtering;

 FIG. 7A is a schematic diagram illustrating argon plasma treatment performed on the surface of a polymer electrolyte membrane at an oblique angle of 55.degree.;

FIG. 7B is a SEM image of an inclined nanostructured surface formed on a polymer electrolyte membrane; FIG. 7C is an enlarged image of a red rectangular box of FIG. 7B; and  FIG. 7D is an enlarged image of a portion of FIG. 7C.

A preferred manufacturing method includes forming a plurality of convex or concave nanostructures on the surface of the polymer electrolyte membrane by ion beam, reactive ion etching (RIE), and sputtering in addition to the PACVD. 

The method includes the step of changing an angle formed between the plasma flow direction and the surface of the polymer electrolyte membrane, thus forming a plurality of nanostructures suitably inclined at a specific angle in one direction on the surface of the polymer electrolyte membrane.

The step of supporting or depositing catalyst on the surface of the polymer electrolyte membrane may be performed by selected spray coating, in which platinum or platinum catalyst particles are suitably supported on carbon black and then formed on an ion exchange membrane, or by screen coating, tape casting, dual ion-beam assisted deposition or sputter deposition, in which a surface of a polymer electrolyte membrane is suitably modified and then coated with platinum catalyst nanoparticles.

A platinum alloy is directly deposited on the surface of the polymer electrolyte membrane by sputtering, electrodeposition, electrospray, supercritical deposition using carbon aerogel, platinum sol, and a method of supporting platinum or a platinum alloy using carbon nanotubes, amorphous supermicroporous carbons, or carbon aerogel supports.

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