New applications for carbon dioxide could lead to new sustainable fuel source

New applications for carbon dioxide could lead to new sustainable fuel source

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News & Events - Engineering News

July 2, 2014

As the result of huge government initiatives and incentives for reducing emissions, carbon sequestration has been a hot topic for a few years now. Excessive greenhouse gases from power plants and automobiles are causing temperatures to rise on a global level, so efforts are being taken to collect carbon dioxide before it escapes into the atmosphere. As more sequestration projects prove successful, the question moves away from how to capture carbon dioxide and towards what should be done with it once it has been collected.

Carbon dioxide uses 
Some scientists suggest storing it in large geological structures such as shale bedrock or aquifers, but these ideas do not capitalize on the raw potential of carbon dioxide as a resource in and of itself. Back in 2012, a team of researchers from the University of California at Los Angeles developed a method by which formic acid can be used to turn carbon dioxide into isobutanol, an alcohol-based alternative fuel. Recently, a scientists from Princeton discovered a way to turn carbon dioxide into formic acid using solar energy, creating another outlet for CO2.

Solar power is a boundless source of clean energy, and while it is possible to capture sunlight and turn it into electricity with solar arrays, scientists have not yet designed a battery with a large enough capacity to balance generation and consumption. Basically, energy storage is the weak link in solar power. This got scientists thinking about other ways of storing energy. Plants, for instance, transform sunlight and carbon dioxide into energy through photosynthesis. However, instead of using that energy immediately, plants store it as sugar. Inspired by this, scientists at UCLA considered the possibility of storing energy from solar panels in a fuel instead of sugar.

Synthetic photosynthesis 
For their fuel, they chose to create isobutanol, which can be used as a liquid fuel in internal combustion engines. For the purposes of this experiment, the UCLA chemists genetically engineered a lithoautotrophic microorganism known as Ralstonia eutropha H16, which is able to create higher alcohol fuels from carbon and electricity.

In photosynthesis, a plant performs a light reaction and a dark reaction. The light reaction takes light energy and turns it into chemical energy, and during the dark reaction, that chemical energy is transformed into sugar with CO2. For the purposes of UCLA's synthetic photosynthesis, the solar panels played the role of chlorophyll, which is the biochemical in plants that allows them to transform sunlight into energy. That energy was then transferred through formic acid to help the Ralstonia eutropha H16 transform carbon dioxide into liquid fuel.

With this complex chain of events, the UCLA scientists were able to effectively transform sunlight and carbon dioxide into a combustible fuel, which takes the place of a battery for energy storage. Although liquid fuel is not as convenient as batteries, lithium-ion batteries must be recharged when they are depleted.

This means that a reliable source of electricity must be available to anyone using lithium-batteries, and this can be a logistical challenge when traveling to remote or less developed locations since electricity cannot be transported over very long distances without a degree of dispersal, eventually resulting in total power loss. Liquid fuel, on the other hand, can be used by existing technology, and it can be transported over long distances without losing any of its potency.

New sources of formic acid 
If carbon dioxide can be considered an unlimited resource, which it effectively is considering the rate at which it is produced by industry, then the only limited resource in UCLA's formulation is formic acid. Formic acid is a simple carboxylic acid that occurs naturally in organic compounds like wood fiber and ant venom. It is also created as a byproduct of other chemical reactions, such as the creation of acetic acid production. Recently, a team of scientists from Princeton has developed an even more eco-friendly method of formic acid production.

In collaboration with the start-up company Liquid Light Inc., Princeton chemists were able to harness the potential of solar energy to make formic acid out of carbon dioxide. Significantly, this process was accomplished with the use of a commercially available solar cell. Although more details are forthcoming, the researchers have indicated that they have been successful in this endeavor by feeding the output of the solar cell into an electrochemical cell, which was made up of metal plates sandwiching liquid-carrying channels, in order to turn CO2 into formic acid.

The major challenge to the project was in balancing the solar electricity output with the amount of power input that the electrochemical cell could handle. This optimization process, known as impedance matching, was accomplished by stacking several cells together to increase the amount of power input that the cells could process. The researchers report that by stacking 3 cells together, they were able to achieve 2 percent energy efficiency, which is actually twice that of photosynthesis.

Although the Princeton chemists who conducted this research are talking about using the formic acid to create formic salt, which is used as a deicer, the potential for coupling this work with that of the aforementioned UCLA research seems monumental.

Future applications
The fact that this process does not add any new components to the process of turning sunlight and carbon dioxide into liquid fuel means that these two processes be carried out at the same location. Facilities that create formic acid and isobutanol could conceivably be attached to the backend of fossil fuel burning power plants with carbon sequestration technology. If this technology were made ubiquitous, power plants would cut their emissions to next to zero.

Furthermore, isobutanol is not only a potential fuel source, it is also used in the manufacturing of many products including paints, lacquers and even, oddly, flavoring agents for food. It can even be dehydrated into butanol which is a building block for synthetic rubbers and lubricants.

In an age where carbon sequestration is becoming more common, the discovery that carbon dioxide can be turned into such useful chemicals as formic acid and isobutanol represents a significant advancement for environmentalists and industrialists alike.

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