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Making Fuel from Leftovers

Researchers have designed a process to generate hydrogen from organic materials.
November 26, 2007

Unsure what to do with your Thanksgiving leftovers? According to Penn State University (PSU) researchers, feeding table scraps to bacteria may be a clean and efficient way to produce hydrogen that can be used as fuel. Bruce Logan, Kappe professor of environmental engineering, and his colleagues at PSU have designed a tabletop reactor that uses bacteria to break down biodegradable organic material. Adding a small jolt of energy to the system causes hydrogen gas to bubble up to the surface. Logan says that this biological process–compared with today’s existing techniques–may be a more sustainable and efficient alternative for generating hydrogen.

Waste not: A microbial electrolysis cell (left) uses bacteria to convert organic materials into hydrogen, with a boost from a small external power source (right).

The promise of hydrogen as a fuel source has led major automakers like BMW, Daimler Chrysler, Ford, and Toyota to develop test cars that run on hydrogen-powered fuel cells. These fuel cells convert hydrogen and oxygen into electricity, giving off water as a byproduct. It’s a zero-emissions model that could vastly reduce reliance on polluting fossil fuels. But there’s a catch: generating hydrogen itself can involve the burning of fossil fuels like natural gas. Cleaner methods of producing hydrogen include using geothermal, wind, and solar energy to separate water into hydrogen and oxygen, in a process called electrolysis. However, these processes are expensive and require large amounts of electricity. If scaled up, these methods could prove very inefficient.

Some scientists have concentrated on creating microbial fuel cells–reactors that use bacteria to catalyze reactions that produce electricity. Logan’s lab found a way to improve on existing microbial fuel cells by breaking down end products, such as acetic acid.

The researchers grew bacteria in a specially designed, oxygen-free reactor: a bioelectrochemically assisted microbial reactor, which they dubbed BEAMR. The reactor comprises two compartments. The first houses a negatively charged anode, composed of granulated graphite, which Logan sprayed with ammonia gas to help bacteria stick better. The second compartment contains a positively charged cathode of carbon, with a platinum catalyst. An ion-exchange membrane sits between the compartments. Logan used a small wire to connect both electrodes to a small external power source.

The researchers then fed the microbial reactor a varied diet of acetic acid and cellulose. They found that as bacteria fed, the reactor released protons and electrons. The electrons were immediately taken up by the anode, while the protons crossed the membrane to the cathode. The energy from the electrons (which amounted to 0.3 volts), coupled with a short jolt of external voltage (0.2 volts), passed into the cathode compartment, joining with the protons to produce hydrogen gas, which researchers captured and measured in a tube.

Penn State researchers have developed a microbial electrolysis cell, which they call BEAMR, to produce hydrogen. The process uses bacteria to break down organic material, such as acetic acid and cellulose. A small external burst of voltage aids in boosting hydrogen production.
Credit: Zina Deretsky, National Science Foundation

The entire process generated 288 percent more energy than the electricity required to produce the reaction. Logan and his colleagues estimate that, compared with conventional electrolysis, which has a 60 percent efficiency rate, BEAMR achieved an 82 percent efficiency rate.

Logan says that his recent experiment shows that acetic acid could be a rich source of hydrogen-generating material under specific conditions. This suggests that researchers may be able to get more hydrogen out of biomass than was previously thought. Another implication from the study: cellulose may turn out to be better suited for hydrogen production than ethanol fuel is because using cellulose for ethanol involves a more complicated process.

“If you think of cellulose as a starting material to make ethanol, people have to add enzymes to break it down to sugars, and then those can be fermented into ethanol,” says Logan. “But we can use cellulose directly to make hydrogen.”

He says that a potential first application for the technology may be in powering farms, wastewater treatment plants, and other facilities with large amounts of unused biomass. However, scaling BEAMR up to commercial applications may take some rejiggering. The materials used in the system, particularly the platinum cathode in the reactor, would be very expensive if manufactured at a large scale. In the future, Logan’s lab plans to reduce the cost of the reactor’s components, and it has already started looking for alternatives to platinum.

Lars Angenent, assistant professor in the Department of Energy, Environmental, and Chemical Engineering at Washington University, works to optimize fermentation processes to produce bioenergy. He says that while Logan’s technology successfully “circumvents biological limitations of hydrogen production,” bringing it to a commercial level may pose challenges.

“Scale-up will be the problem,” says Angenent. “This must be commercially viable while sustaining high efficiencies.”

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