This new microbial fuel cell is a redesign of a larger model that the researchers created a few years ago.
A quarter of the size of the original model, it can yet produce about three times the power, said Hamid Rismani-Yazdi, lead author of the study and a doctoral student in food, agricultural and biological engineering at Ohio State University.
In experiments, the researchers found that it took two of the new cells to produce enough electricity to recharge an AA-sized battery compared to four of the first-generation fuel cells that could recharge only one of those batteries.
According to Rismani-Yazdi, the source of power for these fuel cells comes from the breakdown of cellulose by a variety of bacteria in rumen fluid, the microbe-rich fluid found in a cow's rumen, the largest chamber of a cow's stomach.
As part of the experiment, the team first collected rumen fluid from a living cow, extracting the fluid through a cannula, a surgically implanted porthole that leads directly into its rumen.
Then they filled one compartment of a fuel cell with this microbe-rich fluid and cellulose.
The microbial fuel cell, which has two compartments, is about two inches wide and three inches in height and length. A thin membrane made of special material separates the two compartments. This material allows protons to move from the negative (anode) compartment into the positive (cathode) compartment, creating an electrical current.
A small piece of graphite placed inside each compartment serves as the fuel cell's electrodes.
The researchers filled the anode chamber with cellulose and with microbes derived from rumen fluid. Electrons are released as the microorganisms break down the cellulose.
"Energy is produced as the bacteria break down cellulose, which is one of the most abundant resources on our planet," said Rismani-Yazdi.
"The amount of electricity that we can get out of one of these cells is ultimately related to the resistance of the object that we want to power," he said.
The team is now working towards refining the microbial fuel cells, as well as trying to figure out how to grow mass amounts of rumen microbes in the laboratory for possible large-scale use in the future.
The findings were presented on August 21 at the American Chemical Society meeting in Boston.