A new catalyst
created by U of T Engineering researchers brings them one step closer to
artificial photosynthesis -- a system that, just like plants, would use
renewable energy to convert carbon dioxide (CO2) into stored chemical energy.
By both capturing carbon emissions and storing energy from solar or wind power,
the invention provides a one-two punch in the fight against climate change.
"Carbon
capture and renewable energy are two promising technologies, but there are
problems," says Phil De Luna, one of the lead authors of a paper published
today in Nature Chemistry. "Carbon capture technology is expensive,
and solar and wind power are intermittent. You can use batteries to store
energy, but a battery isn't going to power an airplane across the Atlantic or
heat a home all winter: for that you need fuels."
De Luna and his
co-lead authors Xueli Zheng and Bo Zhang -- who conducted their work under the
supervision of Professor Ted Sargent -- aim to address both challenges at once,
and they are looking to nature for inspiration. They are designing an
artificial system that mimics how plants and other photosynthetic organisms use
sunlight to convert CO2 and water into molecules that humans can later use
for fuel.
As in plants,
their system consists of two linked chemical reactions: one that splits H2O
into protons and oxygen gas, and another that converts CO2 into carbon
monoxide, or CO. (The CO can then be converted into hydrocarbon fuels through
an established industrial process called Fischer-Tropsch synthesis.)
"Over the
last couple of years, our team has developed very high-performing catalysts for
both the first and the second reactions," says Zhang, who contributed to
the work while a post-doctoral fellow at U of T and is now a professor at Fudan
University. "But while the second catalyst works under neutral conditions,
the first catalyst requires high pH levels in order to be most active."
That means that
when the two are combined, the overall process is not as efficient as it could
be, as energy is lost when moving charged particles between the two parts of
the system.
The team has now
overcome this problem by developing a new catalyst for the first reaction --
the one that splits water into protons and oxygen gas. Unlike the previous
catalyst, this one works at neutral pH, and under those conditions it performs
better than any other catalyst previously reported.
"It has a
low overpotential, which means less electrical energy is needed to drive the
reaction forward," says Zheng, who is now a postdoctoral scholar at
Stanford University. "On top of that, having a catalyst that can work at
the same neutral pH as the CO2conversion reaction reduces the overall potential
of the cell."
In the paper,
the team reports the overall electrical-to-chemical power conversion efficiency
of the system at 64 per cent. According to De Luna, this is the highest value
ever achieved for such a system, including their previous one, which only
reached 54 per cent.
The new catalyst
is made of nickel, iron, cobalt and phosphorus, all elements that are low-cost
and pose few safety hazards. It can be synthesized at room temperature using
relatively inexpensive equipment, and the team showed that it remained stable
as long as they tested it, a total of 100 hours.
Armed with their
improved catalyst, the Sargent lab is now working to build their artificial
photosynthesis system at pilot scale. The goal is to capture CO2 from flue
gas -- for example, from a natural gas-burning power plant -- and use the
catalytic system to efficiently convert it into liquid fuels.
"We have to
determine the right operating conditions: flow rate, concentration of electrolyte,
electrical potential," says De Luna. "From this point on, it's all
engineering."
The team and
their invention are semi-finalists in the NRG COSIA Carbon XPRIZE, a $20
million challenge to "develop breakthrough technologies that will convert
CO? emissions from power plants and industrial facilities into valuable
products."
The project was
the result of an international and multidisciplinary collaboration. The
Canadian Light Source in Saskatchewan provided the high-energy x-rays used to
probe the electronic properties of the catalyst. The Molecular Foundry at the
U.S. Department of Energy's Lawrence Berkeley National Laboratory did
theoretical modelling work. Financial and in-kind support were provided by the
Natural Sciences and Engineering Research Council, the Canada Foundation for
Innovation, Tianjin University, Fudan University and the Beijing Light Source.
As for what has
kept him motivated throughout the project, De Luna points to the opportunity to
make an impact on some of society's biggest environmental challenges.
"Seeing the
rapid advancement within the field has been extremely exciting," he says.
"At every weekly or monthly conference that we have within our lab, people
are smashing records left and right. There is still a lot of room to grow, but
I genuinely enjoy the research, and carbon emissions are such a big deal that
any improvement feels like a real accomplishment."
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