Researchers have developed a new type of low-temperature fuel cell that directly converts biomass to electricity with assistance from a catalyst activated by solar or thermal energy.
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| A new solar-induced direct biomass-to-electricity hybrid fuel cell can operate on fuels as varied as powdered wood. The fuel cell, shown on the right, relies on a polyoxometalate (POM) catalyst (shown in the vials) which changes color as it reacts with light. Credit: Georgia Tech, John Toon |
Although low
temperature fuel cells powered by methanol or hydrogen have been well studied,
existing low temperature fuel cell technologies cannot directly use biomass as
a fuel because of the lack of an effective catalyst system for polymeric
materials.
Now, researchers at the Georgia Institute of Technology have developed a new
type of low-temperature fuel cell that directly converts biomass to electricity
with assistance from a catalyst activated by solar or thermal energy. The
hybrid fuel cell can use a wide variety of biomass sources, including starch,
cellulose, lignin -- and even switchgrass, powdered wood, algae and waste from
poultry processing.
The device could be
used in small-scale units to provide electricity for developing nations, as
well as for larger facilities to provide power where significant quantities of
biomass are available.
The new solar-induced
direct biomass-to-electricity hybrid fuel cell was described February 7, 2014,
in the journal Nature
Communications.
The challenge for
biomass fuel cells is that the carbon-carbon bonds of the biomass -- a natural
polymer -- cannot be easily broken down by conventional catalysts, including
expensive precious metals, Deng noted. To overcome that challenge, scientists
have developed microbial fuel cells in which microbes or enzymes break down the
biomass. But that process has many drawbacks: power output from such cells is
limited, microbes or enzymes can only selectively break down certain types of
biomass, and the microbial system can be deactivated by many factors.
Deng and his research
team got around those challenges by altering the chemistry to allow an outside
energy source to activate the fuel cell's oxidation-reduction reaction.
In the new system, the
biomass is ground up and mixed with a polyoxometalate (POM) catalyst in
solution and then exposed to light from the sun -- or heat. A photochemical and
thermochemical catalyst, POM functions as both an oxidation agent and a charge
carrier. POM oxidizes the biomass under photo or thermal irradiation, and
delivers the charges from the biomass to the fuel cell's anode.
The electrons
are then transported to the cathode, where they are finally oxidized by oxygen
through an external circuit to produce electricity.
"If you mix the
biomass and catalyst at room temperature, they will not react," said Deng.
"But when you expose them to light or heat, the reaction begins. The POM
introduces an intermediate step because biomass cannot be directly accessed by
oxygen."
The system provides
major advantages, including combining the photochemical and solar-thermal
biomass degradation in a single chemical process, leading to high solar
conversion and effective biomass degradation. It also does not use expensive
noble metals as anode catalysts because the fuel oxidation reactions are
catalyzed by the POM in solution. Finally, because the POM is chemically
stable, the hybrid fuel cell can use unpurified polymeric biomass without
concern for poisoning noble metal anodes.
The system can use
soluble biomass, or organic materials suspended in a liquid. In experiments,
the fuel cell operated for as long as 20 hours, indicating that the POM
catalyst can be re-used without further treatment.
In their paper, the
researchers reported a maximum power density of 0.72 milliwatts per square
centimeter, which is nearly 100 times higher than cellulose-based microbial
fuel cells, and near that of the best microbial fuel cells. Deng believes the
output can be increased five to ten times when the process is optimized.
"I believe this
type of fuel cell could have an energy output similar to that of methanol fuel
cells in the future," he said. "To optimize the system, we need to
have a better understanding of the chemical processes involved and how to
improve them."
The researchers also
need to compare operation of the system with solar energy and other forms of
input energy, such as waste heat from other processes. Beyond the ability to
directly use biomass as a fuel, the new cell also offers advantages in
sustainability -- and potentially lower cost compared to other fuel cell types.
"We can use
sustainable materials without any chemical pollution," Deng said.
"Solar energy and biomass are two important sustainable energy sources
available to the world today. Our system would use them together to produce
electricity while reducing dependence on fossil fuels."
In addition to Deng,
the research team included Wei Liu, Wei Mu, Mengjie Liu, Xiaodan Zhang and Hongli
Cai, all from the School of Chemical and Biomolecular Engineering or the
Institute of Paper Science and Technology at Georgia Tech.
Story Source:
The above story is
based on materials provided
by Georgia Institute of Technology.
The original article was written by John Toon. Note: Materials may be edited for
content and length.
Journal
Reference:
1.
Wei Liu, Wei Mu, Mengjie Liu, Xiaodan Zhang, Hongli Cai, Yulin
Deng. Solar-induced
direct biomass-to-electricity hybrid fuel cell using polyoxometalates as
photocatalyst and charge carrier. Nature
Communications, 2014; 5 DOI: 10.1038/ncomms4208
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Georgia Institute of
Technology. "Solar-induced hybrid fuel cell produces electricity directly
from biomass." ScienceDaily. ScienceDaily, 18 February 2014.
<www.sciencedaily.com/releases/2014/02/140218100047.htm>.