Regents professor Meilin Liu holds a button fuel cell used to evaluate a new self-cleaning anode material based on barium oxide. The self-cleaning technique could allow fuel cells to be powered by coal gas. Georgia Tech Photo: Gary Meek. |
Using
barium oxide nanoparticles, researchers have developed a self-cleaning
technique that could allow solid oxide fuel cells to be powered directly
by coal gas at operating temperatures as low as 750 C.
The technique could provide a cleaner and more efficient alternative to
conventional power plants for generating electricity from the nation’s
vast coal reserves.
Solid
oxide fuel cells can operate on a wide variety of fuels, and use
hydrocarbons gases directly—without a separate reformer. The fuel
cells rely on anodes made from nickel and a ceramic material known as
yttria-stabilized zirconia. Until now, however, carbon-containing fuels
such as coal gas or propane could quickly deactivate these Ni-YSZ
anodes, clogging them with carbon deposits in a process known as
“coking”—especially at lower operating temperatures.
To
counter this problem, researchers have developed a technique for
growing barium oxide nanostructures on the anodes. The structures adsorb
moisture to initiate a water-based chemical reaction that oxidizes the
carbon as it forms, keeping the nickel electrode surfaces clean even
when carbon-containing fuels are used at low temperatures.
“This
could ultimately be the cleanest, most efficient and cost-effective way
of converting coal into electricity,” said Meilin Liu, a Regents
professor in the School of Materials Science and Engineering at the
Georgia Institute of Technology. “And by providing an exhaust stream of
pure carbon dioxide, this technique could also facilitate carbon
sequestration without the separation and purification steps now required
for conventional coal-burning power plants.”
The
water-mediated carbon removal technique was reported in Nature Communications. The research was supported by the U.S.
Department of Energy’s Office of Basic Energy Sciences, through the
HeteroFoaM Center, an Energy Frontier Research Center. The work also
involved researchers from Brookhaven National Laboratory, the New Jersey
Institute of Technology, and Oak Ridge National Laboratory.
Conventional
coal-fired electric generating facilities capture just a third of the
energy available in the fuel they burn. Fuel cells can convert
significantly more of the energy, approximately 50%. If gas
turbines and fuel cells could be combined into hybrid systems,
researchers believe they could capture as much as 80% of the
energy, reducing the amount of coal needed to produce a given amount of
energy, potentially cutting carbon emissions.
But
that would only be possible if the fuel cells could run for long
periods of time on coal gas, which now deactivates the anodes after as
little as 30 minutes of operation.
Liu and postdoctoral researcher Mingfei Liu examine a button fuel cell used to evaluate the new self-cleaning anode material. Georgia Tech Photo: Gary Meek. |
The
carbon removal system developed by the Georgia Tech-led team uses a
vapor deposition process to apply barium oxide nanoparticles to the
nickel-YSZ electrode. The particles, which range in size from 10 to 100
nm, form “islands” on the nickel that do not block the flow of
electrons across the electrode surface.
When
water vapor introduced into the coal gas stream contacts the barium
oxide, it is adsorbed and dissociates into protons and hydroxide (OH)
ions. The hydroxide ions move to the nickel surface, where they combine
with the carbon atoms being deposited there, forming the intermediate
COH. The COH then dissociates into carbon monoxide and hydrogen, which
are oxidized to power the fuel cell, ultimately producing carbon dioxide
and water. About half of the carbon dioxide is then recirculated back
to gasify the coal to coal gas to continue the process.
“We
can continuously operate the fuel cell without the problem of carbon
deposition,” said Liu, who is also co-director of Georgia Tech’s Center
for Innovative Fuel Cell and Battery Technologies.
The
researchers also evaluated the use of propane to power solid oxide fuel
cells using the new anode system. Because oxidation of the hydrogen in
the propane produces water, no additional water vapor had to be added,
and the system operated successfully for a period of time similar to the
coal gas system.
Solid
oxide fuel cells operate most efficiently at temperatures above 850 C, and much less carbon is deposited at higher
temperatures. However, those operating temperatures require fabrication
from special materials that are expensive—and prevent solid oxide fuel
cells from being cost-effective for many applications.
Reducing
the operating temperatures is a research goal, because dropping
temperatures to 700 or 750 C would allow the use of much
less expensive components for interconnects and other important
components. However, until development of the self-cleaning process,
reducing the operating temperature meant worsening the coking problem.
“Reducing
the operating temperature significantly by eliminating the problem of
carbon deposition could make these solid oxide fuel cells economically
competitive,” Liu said.
Fuel
cells powered by coal gas still produce carbon dioxide, but in a much
purer form than the stack gases leaving traditional coal-fired power
plants. That would make capturing the carbon dioxide for sequestration
less expensive by eliminating large-scale separation and purification
steps, Liu noted.
The
researchers have so far tested their process for a hundred hours, and
saw no evidence of carbon build-up. A major challenge ahead is to test
the long-term durability of the system for fuel cells that are designed
to operate for as long as five years. Researchers must also study the
potential impact of possible fuel contaminants on the new electrode.
Forming
the barium oxide structures can be done as part of conventional anode
fabrication processes, and would not require additional steps. The
anodes produced in the technique are compatible with standard solid
oxide fuel cell systems that are already being developed for commercial
electricity generation, home power generation, and automotive
applications.
“We
have started with state-of-the-art technology, and simply modified the
surface of the electrode,” said Mingfei Liu, a postdoctoral researcher
in the Center. “Because our electrode would be built on existing
technology, there is a lower barrier for implementing it in conventional
fuel cell systems.”
