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Topic Name: Research into Better Fuel Cell Materials and Designs Starts with Studying Failures Mechanismsin GTRI
Category: Fuel Technology
Research persons: Professor Fuller
Location: Center for Innovative Fuel Cell and Battery Technologies, Georgia Tech Research Institute, United States
Details
Researchers in the
Georgia Tech Research Institute’s (GTRI)
Center for Innovative Fuel
Cell and Battery Technologies believe that understanding how and why fuel
cells fail is the key to both reducing cost and improving durability.
Center director Tom Fuller has been trying to solve what he deems the top three
durability problems since he joined GTRI from United Technologies three years
ago.
“My philosophy is if we can really understand the fundamentals of these failure
mechanisms, then we can use that information to guide the development of new
materials or we can develop system approaches to mitigate these failures,” said
Fuller, who is also a professor in
Georgia Tech’s School of
Chemical and Biomolecular Engineering (ChBE).
The problems Fuller is addressing include chemical attack of the membrane,
carbon corrosion and platinum instability. Fuller described progress toward
solving these problems last month at the 212th Electrochemical Society Meeting.
In a typical fuel cell, hydrogen is delivered to the anode side of the cell that
contains a catalyst, such as platinum. The platinum splits the hydrogen
molecules (H2) into hydrogen ions and electrons. On the cathode side of the fuel
cell, an oxidant such as a stream of oxygen or air is delivered.
With a proton exchange membrane in the middle, only hydrogen ions can travel
through the membrane to the cathode. Electrons travel on a different path
through the electrical circuit to the cathode, creating an electrical current.
At the cathode, the hydrogen ions combine with oxygen and the electrons that
took the longer path to form water, which flows out of the cell.
Fuller’s research shows that the membrane, commonly made of a synthetic
polymer, is prone to attack by free radicals that create holes in the barrier.
The free radicals are formed by the decomposition of hydrogen peroxide (H2O2), a
strong oxidizing chemical that can form near the membrane.
Since a typical membrane is approximately 25-50 micrometers thick, or about the
thickness of a human hair, it’s impossible to see the degradation peroxide
causes with the naked eye.
In a paper published in March in the Journal of Power Sources, Fuller and
professor Dennis Hess, research scientist Galit Levitin and graduate student
Cheng Chen, all from ChBE, used X-ray photoelectron spectroscopy (XPS) to study
the membrane degradation. This work was funded by
GTRI, ChBE and the
Lawrence Berkeley National
Laboratory.
The researchers chose XPS because it is a quantitative technique that uses
X-rays to measure the presence and quantity of chemical elements and the
formation and breakage of chemical bonds within a material.
“We were able to see chemical differences in the membrane with XPS when it went
through the degradation process,” explained Fuller. “Now we’re trying to figure
out what really limits or controls the rate of degradation.”
The solution will be difficult because the formation of hydrogen peroxide
requires only hydrogen and oxygen to be present. Since these chemicals are
readily available in fuel cells, hydrogen peroxide can be produced many ways.
The problem is further complicated because free radicals are short lived and
difficult to detect.
Fuller will leave the actual engineering of new non-degrading membranes to the
materials scientists, but what he has learned can guide what properties new
membranes should have and how they can be tested for degradation.
Another challenge with low temperature fuel cells is that a blockage can occur
on the anode side of the fuel cell, possibly from a water drop formed in the
fuel channel. The blockage causes carbon (used to support the platinum) to
corrode, turn into carbon dioxide and leave the fuel cell as a gas. Frequently
starting and stopping the fuel cell also causes this mode of failure.
This can be catastrophic for the fuel cell because without carbon, the platinum
catalyst layer collapses and disappears. “If this happens, the fuel cell can be
destroyed in days rather than years,” noted Fuller. This problem is more common
in non-stationary fuel cell applications, such as cars that require the fuel
cell to start and stop when the vehicle is turned on and off.
“Researchers know this problem exists, but we’re trying to build physics-based
detailed models to evaluate different fuel cell designs that will reduce the
susceptibility to this type of corrosion,” said Fuller, who’s working on this
project with Norimitsu Takeuchi from Toyota’s material research department and
students Kevin Gallagher and David Wong with funding from Toyota.
The models can also be used to determine options for controlling and mitigating
this problem to find a more effective alternative material that is more
resistant to corrosion.
Another problem with fuel cells cycling on and off is that platinum has a small
but finite solubility in the acidic membrane given the high electrical potential
and oxidizing environment at the cathode.
“Platinum is one of the most expensive parts of the fuel cells, so researchers
study how to decrease the amount necessary to run a fuel cell,” explained
Fuller. “But if there is less platinum in the fuel cell to begin with, you can’t
afford to lose any by it dissolving.”
When the platinum layer dissolves, a band of platinum typically forms inside the
membrane. Fuller, GTRI senior research engineer Gary Gray and graduate student
Wu Bi, developed a model to predict where the platinum band would form to help
to understand why it was happening. This work was published in March in
Electrochemical and Solid-State Letters.
“We found that the platinum can also be deposited throughout the membrane and it
can move around to different places, but whenever it leaves where it’s supposed
to be, it’s no longer effective,” said Fuller.
Fuller aims to understand these very small platinum particles by modeling the
transport and thermodynamics of the particles in fuel cell systems. This work
was funded by Hyundai Motors Corporation.
A recent gift of $200,000 from the Hartley Foundation will allow Fuller to
purchase new research equipment and continue studying the degradation of fuel
cells and how to improve/extend the life cycle and technology of these energy
devices.
“Fuel cell failure can occur through many different mechanisms,” added Fuller.
“Results from these three projects show that new materials, new manufacturing
processes and new designs are required to improve the durability of fuel cells
and in turn lower costs.”
About Platinum
Platinum is a chemical element with the atomic symbol Pt and an atomic number of
78. It is in group 10 of the Periodic Table of Elements. A heavy, malleable,
ductile, precious, gray-white transition metal, platinum is resistant to
corrosion and occurs in some nickel and copper ores along with some native
deposits. Platinum is used in jewelry, laboratory equipment, electrical
contacts, dentistry, and automobile emissions control devices. Platinum bullion
has the ISO currency code of XPT. As of June 24, 2008, Platinum was worth $2,032
per troy ounce (approximately $65 per gram).
When pure, the metal appears greyish-white and firm. The metal is
corrosion-resistant. The catalytic properties of the six platinum family metals
are outstanding. For this catalytic property, platinum is used in catalytic
converters, incorporated in automobile exhaust systems, as well as tips of spark
plugs. Platinum has a cubic crystal structure.
Platinum's wear- and tarnish-resistance characteristics are well suited for
making fine jewelry. Platinum is more precious than gold. The price of platinum
changes along with its availability, but its price is normally more than twice
the price of gold. In the 18th century, platinum's rarity made King Louis XV of
France declare it the only metal fit for a king. Platinum possesses high
resistance to chemical attack, excellent high-temperature characteristics, and
stable electrical properties. All these properties have been exploited for
industrial applications. Platinum does not generally oxidize in air at any
temperature, but can be corroded by cyanides, halogens, sulfur, and caustic
alkalis. This metal is insoluble in hydrochloric and nitric acid, but does
dissolve in the mixture known as aqua regia (forming chloroplatinic acid). When
crude platinum is dissolved in aqua regia, gold is removed from the solution as
a precipitate by treatment with iron(II) chloride (FeCl2). The platinum is
precipitated out as impure (NH4)2PtCl6 on treatment with ammonium chloride
(NH4Cl), leaving H2PdCl4 in solution.
About X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique
that measures the elemental composition, empirical formula, chemical state and
electronic state of the elements that exist within a material. XPS spectra are
obtained by irradiating a material with a beam of aluminium or magnesium X-rays
while simultaneously measuring the kinetic energy (KE) and number of electrons
that escape from the top 1 to 10 nm of the material being analyzed. XPS requires
ultra-high vacuum (UHV) conditions.
XPS is a surface chemical analysis technique that can be used to analyze the
surface chemistry of a material in its "as received" state, or after some
treatment such as: fracturing, cutting or scraping in air or UHV to expose the
bulk chemistry, ion beam etching to clean off some of the surface contamination,
exposure to heat to study the changes due to heating, exposure to reactive gases
or solutions, exposure to ion beam implant, exposure to ultraviolet light, for
example.
XPS is also known as ESCA, an abbreviation for Electron Spectroscopy for
Chemical Analysis.
XPS detects all elements with an atomic number (Z) between those of lithium
(Z=3) and lawrencium (Z=103). This limitation means that it cannot detect
hydrogen (Z=1) or helium (Z=2).
Detection limits for most of the elements are in the parts per thousand range.
Detections limits of parts per million (ppm) are possible, but require special
conditions: concentration at top surface or very long collection time
(overnight).
XPS is routinely used to analyze inorganic compounds, metal alloys,
semiconductors, polymers, elements, catalysts, glasses, ceramics, paints,
papers, inks, woods, plant parts, make-up, teeth, bones, medical implants,
bio-materials, viscous oils, glues, ion modified materials and many others.
A typical XPS spectrum is a plot of the number of electrons detected (Y-axis,
ordinate) versus the binding energy of the electrons detected (X-axis,
abscissa). Each element produces a characteristic set of XPS peaks at
characteristic binding energy values that directly identify each element that
exist in or on the surface of the material being analyzed. These characteristic
peaks correspond to the electron configuration of the electrons within the
atoms, e.g., 1s, 2s, 2p, 3s, etc. The number of detected electrons in each of
the characteristic peaks is directly related to the amount of element within the
area (volume) irradiated. To generate atomic percentage values, each raw XPS
signal must be corrected by dividing its signal intensity (number of electrons
detected) by a "relative sensitivity factor" (RSF) and normalized over all of
the elements detected.
In figure 1, Tom Fuller's research shows that new materials, new
manufacturing processes and new designs are required to improve the durability
of fuel cells and in turn lower costs.
In figure 2, Since he joined GTRI three years ago, Tom Fuller -- director of the
Center for Innovative Fuel Cell and Battery Technologies -- has been trying to
solve what he deems the top three fuel cell durability problems.
In figure 3, Tom Fuller, a professor in Georgia Tech's School of Chemical and
Biomolecular Engineering, believes that understanding how and why fuel cells
fail is the key to both reducing cost and improving durability.
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