Title: Ion exchange system structure with a microtextured surface, method of manufacture, and method of use thereof
Abstract: A method for roughening a surface of an ion exchange system structure using laser interaction with a surface. The laser surface roughening process allows the use of a wide range of substrates such as metals, ceramics, silicates, polymers and the like, including varieties which can not be fabricated in a fine fibrous structure. The surface roughened ion exchange system structure may be used as an ion-exchange media in applications such as fuel cells, batteries, and other catalysis systems where a high surface exchange area is desirable.
Patent Number: 6,869,712 Issued on 03/22/2005 to Mittelstadt, et al.
| Inventors:
|
Mittelstadt; Laurie S. (Belmont, CA);
Smith; Joshua W. (Corvallis, OR)
|
| Assignee:
|
Hewlett-Packard Development Company, L.P. (Houston, TX)
|
| Appl. No.:
|
091485 |
| Filed:
|
March 7, 2002 |
| Current U.S. Class: |
429/30; 429/33; 429/40 |
| Intern'l Class: |
H01M 008//10 |
| Field of Search: |
429/30,33,40
|
References Cited [Referenced By]
U.S. Patent Documents
| 4673624 | Jun., 1987 | Hockaday.
| |
| 5473138 | Dec., 1995 | Singh et al.
| |
| 5733609 | Mar., 1998 | Wang.
| |
| 6051331 | Apr., 2000 | Spear et al. | 429/34.
|
| 6136412 | Oct., 2000 | Spiewak et al. | 428/143.
|
| 6326097 | Dec., 2001 | Hockaday | 429/34.
|
| 6361892 | Mar., 2002 | Ruhl et al. | 429/30.
|
| 6471993 | Oct., 2002 | Shastri et al. | 424/486.
|
| Foreign Patent Documents |
| 4119910 | Dec., 1992 | DE.
| |
| 4320408 | Dec., 1994 | DE.
| |
| WO8905707 | Jun., 1989 | WO.
| |
| WO9628574 | Sep., 1996 | WO.
| |
| WO0189018 | Nov., 2001 | WO.
| |
Other References
Kinsman G. et al: "Treatment of Metal Surface with Excimer Laser Radiation
for Radiative Applications" Applied Optics, Optical Society of America,
WA, US, vol. 32, No. 36, Dec 20, 1993, pp. 7462-7470, XP000425819.
Chun, William, et al., "Sputter Deposition of Catalysts for Fuel-Cell
Electrodes", Dec. 1999, pp i, 1-4, National Aeronautics and Space
Administration Contract No. NAS 7-918, NASA Tech Brief vol. 23, No. 12,
Item # from JPL New Technology Report NPO-20250.
Lee, Sang-Joon J., et al., "Miniature Fuel Cells With Non-Planar Interface
By Microfabrication", 198th meeting of the Electrochemical Society, Oct.
22-27, 2000, Phoenix, Arizona, 11pp.
|
Primary Examiner: Yuan; Dah-Wei
Claims
What is claimed is:
1. A substrate for an ion-exchange electrode structure, said substrate
comprising an exterior surface wherein at least a portion of the exterior
surface is irradiated by a laser radiation to enlarge a reactive surface
area on the exterior surface, wherein the reactive surface area has
two-scales of roughness, a first scale of roughness at least three orders
of magnitude different than a second scale of roughness.
2. The substrate of claim 1, wherein the portion of the surface is
irradiated by exposing the surface to the laser radiation near an ablation
threshold of the substrate.
3. The substrate of claim 1, wherein the portion of the surface is
irradiated by melting, boiling, or quenching part of the surface with
laser radiation.
4. The substrate of claim 1, wherein the laser irradiated surface is coated
with a layer of conductive material.
5. The substrate of claim 4, wherein the conductive material is a metal or
an alloy.
6. The substrate of claim 4, wherein the layer of conductive material is
further coated with a continuous or discontinuous layer of catalytic
material.
7. The substrate of claim 6, wherein the catalytic material is selected
from a group consisting of Pt, Pt alloys, V, V alloys, titanium dioxide,
iron, nickel, lithium and gold.
8. The substrate of claim 1, wherein the laser irradiated surface is coated
with a continuous or discontinuous layer of catalytic material.
9. The substrate of claim 8, wherein the catalytic material is selected
from a group consisting of Pt, Pt alloys, V, V alloys, titanium dioxide,
iron, nickel, lithium and gold.
10. The substrate of claim 8, further comprising micro openings wherein a
fuel flows through the micro openings to reach the catalytic material.
11. The substrate of claim 1, wherein the reactive surface area includes a
projecting surface feature.
12. The substrate of claim 11, wherein the projecting surface feature is
cone-shaped.
13. The substrate of claim 1, wherein the first scale of roughness is about
10.sup.-6 meters.
Description
TECHNICAL FIELD
The technical field relates to microtextured surfaces in an ion exchange
system structure and a method for making the same. The microtextured ion
exchange system structure may be used in electrochemical devices,
including fuel cells, batteries, sensors, electrolyzers and the like.
BACKGROUND
A desirable feature for ion-exchange media used in applications such as
fuel cells, batteries, sensors, electrolyzers and other catalysis systems
is the ability to deliver the highest exchange surface area while
minimizing the size and weight of the entire system. An important metric
used in comparing the performance of different system designs is the ratio
of the exchange area to the volume of the system. For example, in a fuel
cell, the increased contact area between the electrolyte, reactants and
the catalytic surface results in an increase in the number of reactions
per unit time. Therefore, the development of methods to increase surface
area is critical to the improvement of technologies dependant on ion
exchange. Common methods of increasing surface area fall into one of three
categories, namely, microfibers, porous materials and roughened or
microtextured surfaces.
With regard to the last category, a well known method for producing
roughened surface on a nano scale is the plasma process. The process,
however, requires high temperature and pressure that may damage certain
substrates. Other methods of roughening include the impingement of sand or
other particulates against a surface or the use of abrasives mounted on
substrates; grinding wheels and sandpaper are examples. These processes,
however, only provide limited surface area enhancement and are fraught
with problems associated with contamination.
In catalysis systems, such as fuel cells, batteries, sensors, and
electrolyzers, the ion exchange membrane is typically coated with a
continuous or discontinuous layer of catalyst to promote the rates of
chemical reactions. Commonly used catalysts include platinum (Pt) and Pt
alloys, vanadium (V) and V alloys, titanium dioxide, iron, nickel, lithium
and gold.
A fuel cell is an electrochemical apparatus wherein chemical energy
generated from a combination of a fuel with an oxidant is converted to
electric energy in the presence of a catalyst. The fuel is fed to an
anode, which has a negative polarity, and the oxidant is fed to a cathode,
which, conversely, has a positive polarity. The two electrodes are
connected within the fuel cell by an electrolyte to transmit protons from
the anode to the cathode. The electrolyte can be an acidic or an alkaline
solution, or a solid polymer ion-exchange membrane characterized by a high
ionic conductivity. The solid polymer electrolyte is often referred to as
a proton exchange membrane (PEM).
The simplest and most common type of fuel cell employs an acid electrolyte.
Hydrogen is ionized at an anode catalyst layer to produce protons. The
protons migrate through the electrolyte from the anode to the cathode. At
a cathode catalyst layer, oxygen reacts with the protons to form water.
The anode and cathode reactions in this type of fuel cell are shown in the
following equations:
Anode reaction (fuel side):2H.sub.2.fwdarw.4H.sup.+ +4e.sup.- (I)
Cathode reaction (air side): O.sub.2 +4H.sup.+ +4e.sup.-.fwdarw.2H.sub.2 O
(II)
Net reaction: 2H.sub.2 +O.sub.2.fwdarw.2H.sub.2 O (III)
The goal is complete hydrogen oxidation for maximum energy generation shown
in the equation. However, the oxidation and reduction reactions require
catalysts in order to proceed at useful rates. Catalysts are important
because the energy efficiency of any fuel cell is determined, in part, by
the overpotentials necessary at the fuel cell's anode and cathode. In the
absence of an catalyst, a typical electrode reaction occurs, if at all,
only at very high overpotentials.
One of the essential requirements of typical fuel cells, and indeed any ion
exchange system, is easy access to the electrode and a large surface area
for reaction. This requirement can be satisfied by using an electrode made
of an electrically conductive porous substrate that renders the electrode
permeable to fluid reactants and products in the fuel cell. To increase
the surface area for reaction, the catalyst can also be filled into or
deposited onto a porous substrate.
However, these modifications result in a fragile porous electrode that
needs additional mechanical support. An alternative is to sinter a porous
coating on a solid substrate and then fill or re-coat the porous coating
with a catalyst. The sintering process, however, is a multiple step
procedure that requires baking at high temperatures.
In U.S. Pat. No. 6,326,097 to Hockaday, a surface replica technique is used
to form an "egg-crate" texture on a membrane in a micro-fuel cell. The
catalyst and metal electrode are applied to the surface of the membrane,
and then the membrane is etched away so that the catalyst and electrode
surfaces replicate that texture. This procedure is complicated, requiring
blind etching and many separate operations.
Others have used silicon micro machining to increase the effective surface
area of an electrode (Lee, S. J. et al., Miniature Fuel Cells with
Non-Planar Interface by Microfabrication. In: Power Sources for the New
Millenium, Jain, M. et al. (eds.), Proceedings Volume 2000-22, The Ion
exchange Society Proceeding Series, Pennington, N.J., 2000). Etching of
silicon is a very time-consuming process.
SUMMARY
A process using laser interaction with a surface to enhance the production
of ions at a surface of an ion exchange system structure is disclosed. In
one embodiment, laser radiation is applied to a surface of an electrode
substrate near an ablation threshold of the substrate to create a variety
of shapes including cone-like and fibrous structures. In another
embodiment, the laser radiation is applied to the surface of an electrode
in an ion exchange membrane system to melt, boil or quench part of the
surface to create a rough and porous layer at the surface. In yet another
embodiment, an ion exchange membrane with a roughened surface is prepared
by solidifying a solution on a laser roughened surface or in a mold having
a laser roughened inner surface, or by stamping an ion exchange membrane
substrate with a laser roughened surface.
The laser radiation can be applied to a surface of an electrode after
fabrication of the electrode and, therefore, reduces the level of damage
and/or contamination of the surface. Since the roughness is formed only
where the laser beam strikes the surface, the surface roughening can be
patterned to fit a specific application with very tight positional
accuracy. In addition, the laser roughening operation can be performed
quickly in an ambient environment by a batch process or on a continuous
web, manufacturable process.
The laser surface roughening process allows the use of a wide range of
electrode substrates such as polymers, ceramics, silicates, and the like,
including varieties which can not be fabricated in a fine fibrous
structure. Using the laser roughening method, a solid film may be treated
to create enhanced surface areas in a single step as opposed to the
multiple-step processing required to fabricate a nonwoven solid composite.
The surface roughened electrode may be used in an ion exchange system in
applications such as fuel cells, batteries, and other catalysis systems
where a high surface exchange area is desirable.
BRIEF DESCRIPTION OF THE DRAWINGS
The detailed description will refer to the following drawings, in which
like numerals refer to like elements, and in which:
FIGS. 1A, 1B and 1C are schematics illustrating the equipment and process
of cone formation on a surface by laser radiation.
FIG. 2 is a schematic of a laser roughened surface where two scales of
roughness are produced.
FIG. 3 depicts an embodiment of cone formation using particles imbedded in
the substrate.
FIG. 4 shows an alkaline direct methanol fuel cell with a surface-roughened
electrode.
FIG. 5. shows a direct methanol fuel cell with a surface-roughened flex as
PEM.
FIGS. 6A, 6B and 6C depict the use of a roughened surface as a mold or
embossing tool for producing PEM.
DETAILED DESCRIPTION
The interaction of laser radiation with a material may result in
significant changes to the morphology of the surface and near surface of
the material. There are a number of mechanisms by which the surface change
may occur. Examples include: selective ablation by imaging the beam using
contact or projection mask, ablation-induced cone formation, preferential
ablation of the matrix of a multi-phase material, preferential etching of
grain boundaries, boiling and rapid solidification of the surface material
and other mechanisms. When properly controlled, the three-dimensional
surface topography produced by these treatments results in a surface area
that is many times greater than the original surface. The laser radiation
thus provides another method for producing ion exchange membranes with
enlarged exchange surfaces.
When light is applied to a light absorbing material, it is possible to
change the surface of the material significantly and to form a plethora of
different surface structures such as waves, ripples, pits, nodules, cones
and cracks. The character of features produced is based on the mechanisms
that create the features. The mechanisms themselves are varied and depend
on the characteristics of the light and the nature of the interaction of
the photons and the material.
In one embodiment, an excimer laser 101 is directed towards a substrate 105
as shown in FIGS. 1A and 1B. The wavelength, fluence and energy of a laser
beam 103 are chosen such that photons 107 remove material from surface 109
of the substrate 105 in a process known as ablation. During ablation, a
plume of ablation debris 111 is ejected above the surface 109 of the
substrate 105 (See FIG. 1C). By choosing the correct frequency and feed
rate and a fluence that is above the substrate ablation threshold and
below the debris ablation threshold, it is possible to encourage the
resettling of the debris 111 back onto the surface 109 of the substrate
105. The resettled debris 111 will shadow portions of the underlying
substrate material from the laser light and the substrate 105 will be
ablated non-uniformly, forming a cone structure 113 as shown in FIG. 1C.
The cone structure 113 with the debris 111 attached is a useful material
for an ion exchange membrane. The laser ablation process creates two size
orders of roughness on the surface of the substrate 105 (FIG. 2). A
large-scale roughness (i.e., the cone structure 113) having a size on the
order of 1-100 micron is created due to the shadowing provided by the
debris 111. A fine scale roughness in the size range of tens of nanometers
(indicated by the expanded portion of the diagram in FIG. 2) is created
due to the deposited ablation debris 111. This combination of large scale
and fine scale roughness significantly increases the surface area exposed
for ion exchange.
Redeposition of the debris 111 is just one of many methods capable of
providing shading of the substrate 105 to form structures on the surface
109. FIG. 3 shows another embodiment where particles 115 of higher
ablation threshold have been pre-deposited inside the substrate 105. The
substrate surface 109 is ablated down to expose the particles 115, which
then shadow the underlying material forming the cone structure 113. In
other embodiments not shown, various masks may be inserted between the
light source and the substrate 105 or deposited on the surface of the
substrate 105. Examples of masks include contact masks, projection masks,
films, particles and coatings deposited on the surface and the like.
Diffractive optics may be used to project an image on the surface 109.
There are other embodiments where the mechanisms are quite different. For
example, in metals and glasses, it is possible to melt and even boil the
surface of the substrate with a laser thereby forming a rough surface.
Membrane materials that may be surface treated by laser radiation include,
but are not limited to, metals, plastics, silicon, ceramics and composites
there of. Any material that can be manipulated with a laser is a potential
candidate. The types of light sources capable of inducing such changes on
the surface of a material are well known in the art. Examples include gas
lasers such as excimer and solid state lasers such as YAG lasers as well
as flash lamps, UV exposure tools and the like. What is important is to
match the material with a light source that will interact with the desired
material sufficiently to provide the roughening effect.
The membrane with laser roughened surface may be used in applications such
as fuel cells, batteries, and other catalysis systems where a high surface
area to volume ratio is desirable. FIG. 4 shows an embodiment where a
laser treated substrate is used in an electrode 131 of an alkaline direct
methanol fuel cell 200. In this embodiment, the alkaline direct methanol
fuel cell 200 contains an anode 131 (fuel electrode) and a cathode 141
(air electrode), separated by fuel/electrolyte mixture 133. The
fuel/electrolyte mixture 133 may be methanol (fuel) dissolved in a KOH
solution (electrolyte). The fuel/electrolyte mixture 133 is in full
contact with both the anode 131 and cathode 141. The application of
surface roughened material in the anode 131 would amplify the effective
surface reaction area and result in a higher reaction rate.
The anode 131 may include a plastic substrate 105, such as Kapton or any
other suitable polymer, with a laser textured surface 109 that is covered
with a conductive layer 135 and a catalyst layer 137. The conductive layer
135 may be formed by depositing onto the textured surface 109 a conductive
material by electroless plating, sputtering, atomic layer deposition, or
any other process that is capable of coating the surface of a
non-conductive material. The conductive material may be any material of
interest such as Ni, Cu, Al, Fe, Zn, In, Ti, Pb, V, Cr, Co, Sn, Au, Sb,
Ca, Mo, Rh, Mn, B, Si, Ge, Se, La, Ga, Ir, or an alloy. The catalyst layer
137 may be Pt or Pt alloys such as Pt--Ru and Pt--Ru-Osor, V, V alloys,
titanium dioxide, iron, nickel, lithium, gold, or any other material of
interest. The catalyst layer 137 may be deposited onto the conductive
layer 135 by electroplating, atomic layer deposition, chemical vapor
deposition, sputter deposition or any other process that is capable of
coating a conductive surface. The catalyst may be applied so that it forms
a discontinuous surface layer 137 over the conductor layer 135. The
formation of a discontinuous catalyst layer 137 is facilitated by the cone
structure, upon which catalytic material can be preferentially applied to
the tops of the cones. Alternatively, the non-conductive textured surface
109 may be directly coated with a continuous catalyst layer 137 (which
will serve both conductive and catalytic functions) by atomic layer
deposition, chemical vapor deposition, sputter deposition or any other
process that is capable of coating a non-conductive surface.
FIG. 5 depicts another embodiment utilizing the surface roughened electrode
membrane in a fuel cell with a solid polymer electrolyte membrane (PEM).
In this embodiment, a fuel cell 300 contains an anode 151 (fuel electrode)
and a cathode 153 (air electrode), separated by a PEM 155. The anode 131
is made from a surface roughened flex material 157 covered with a
conductor layer 135 and a catalyst layer 137. The surface roughened flex
material 157 is thinned and etched from the back side to form
micro-machined pores 139 so that fuel 143 on the anode side can reach the
active catalytic surfaces 137 through the openings 139. Here again, the
surface roughening of the flex material 157 provides higher reaction rates
and more efficient operation.
In another embodiment, a substrate with laser-roughened surface is used as
a mold or a stamp to produce a PEM with a roughened surface or surfaces.
As shown in FIG. 6A, an electrolyte material is melted or mixed with a
solvent to form a solution 161. The solution is cast onto a laser
roughened surface 109 and allowed to solidify into a membrane 163, which
is then separated from the surface 109. In this manner, a surface 165 of
the membrane 163 is a negative relief of the laser roughened surface 109
(FIG. 6B). The membrane 163 then may be covered with a conductor 135 and a
catalyst 137 and may be used as a PEM for a fuel cell.
The electrolyte material includes, but is not limited to, sulfonated,
phosphonated, or carboxylated ion-conducting aromatic polymer and
perfluorinated co-polymer. The solvent includes, but is not limited to,
lower aliphatic alcohols such as propanol, butanol, and methanol and water
or a mixture thereof FIG. 6C depicts another embodiment wherein an ion
exchange membrane 167 with a textured surface is produced by stamping the
membrane 163 and a laser roughened surface 109 with a roller 171.
In yet another embodiment, the solution 161 may be poured into a mold
having laser roughened inner surfaces to form an ion exchange membrane 163
with textured surfaces on both the up-side and lower side of the membrane.
The ion exchange membrane with textured surfaces on both sides may be used
as a PEM in a PEM-electrode structure, wherein both sides of the PEM are
covered by conductor layers and catalyst layers. Porous electrodes that
allow fuel delivery and oxygen exchange can then be pressed against the
catalyst layers of the PEM to form the PEM-electrode structure.
Although embodiments and their advantages have been described in detail,
various changes, substitutions and alterations can be made herein without
departing from the spirit and scope of the laser roughening process and
the use of surface roughened products as defined by the appended claims
and their equivalents.
*
