Imagine a submarine fleet of small machines at work in the ocean or a wastewater plant that can collect data while drawing their power from the “food” around them. 

Researchers from the U.S. Naval Research Laboratory (NRL) describe today on ES&T’s Research ASAP website (DOI: 10.1021/es052254w) the first step toward reaching that goal in the form of a miniature microbial fuel cell (mini-MFC) the size of a coin.

In a big step for little devices, this miniature microbial fuel cell produces remarkably high power levels. The spaghetti-like 3D graphite felt electrode visible inside the chamber is the key to its power.

MFCs harness the metabolic energy of bacteria to generate electrical power. Any organic material, including the wastes that go down the drain, can be a rich source of food for these bacteria and hence power for the fuel cell. However, generating enough power to do something useful has been a major hurdle.

In a notable advance, the NRL scientists upped power output by using a spaghetti-like 3D graphite electrode to capture electrons. “Three-dimensional electrodes . . . are a good approach,” says MFC researcher Uwe Schröder of the University of Greifswald (Germany). This electrode sits in the 1.2-milliliter (mL) anode chamber within a flow-through reactor, which is connected to a flask that holds bacteria and food. The mini-MFC produces 0.6 milliwatts of power, says corresponding author Bradley Ringeisen.

Moreover, the cell generates this power by using a pure bacterial culture of Shewanella oneidensis. Another MFC pioneer, Korneel Rabaey of Ghent University (Belgium), says that obtaining these power levels from a pure culture is noteworthy because MFCs that use a mix of bacteria usually produce more power than pure cultures. By implication, switching to a mixed culture may further increase power in a mini-MFC, Ringeisen indicates. Another advantage of S. oneidensis is that it can generate its own mediators—substances that act as mobile electron shuttles between bacteria and anode—improving the practicality of this new MFC. Many MFCs require the regular addition of chemicals that act as mediators, like the continual rewiring of a circuit. However, Rabaey cautions that “what needs to be determined is whether these systems can be operated on this small scale and in the ‘real’ environment.”

One of the stumbling blocks in the burgeoning MFC field is how to define an MFC’s power output. Power output is given in watts, but researchers compare different MFCs by determining their power density in terms of projected electrode surface area (watts per square meter [W/m2]) or power per reactor volume (watts per cubic meter [W/m3] or W/liter). However, the experts in the field sometimes disagree about how to calculate these values.

In this case, because the mini-MFC is a flow-through reactor with a flask supplying bacteria, food, and possibly mediators, exactly how to calculate power density is the subject of a debate—is the flask merely a feed chamber or an integral part of the reactor?

Ringeisen calculates the device’s power density as 3 W/m2 of projected anode surface area and 500 W/m3 on the basis of the 1.2-mL volume in the anode chamber. These values compare favorably with the top MFCs in the literature.

However, MFC authority Bruce Logan of Pennsylvania State University says that to calculate power density relative to volume, the volume of the entire apparatus—approximately 47 mL, which includes the 1.2-mL anode chamber, about 40 mL in the reservoir flask, and 6 mL in tubing—should be included. That approach would significantly lower the calculated power density of the mini-MFC. Logan suggests that the S. oneidensis growing in the reservoir could provide a high concentration of soluble mediators or nanowires that enhance electron transfer in the mini-MFC. “Without this separate reactor for growth, there is no evidence that the device would achieve a high power density,” he says.

Ringeisen disagrees. He insists that his team’s calculations and experiments prove that the anode chamber is where the electrons are generated. “Even though we have a larger volume of fluid in the reservoir . . . if we stop the flow . . . that big reservoir becomes nonexistent, basically. Then we are still able to sustain current generation [for several hours] just from substrate consumption in the anode chamber, that 1.2-mL chamber.”

Rabaey takes a middle ground, saying that he cannot offer “firm answers whether or not to calculate” the volume as as 47 or 1.2 mL, because not enough data exist for him to decide. “This type of discussion is constantly occurring between research groups,” he adds.

The U.S. Department of Defense, of which the NRL is part, hopes to develop MFCs for use in underwater sensing applications, according to an announcement for the Sustained Littoral Presence Program. Currently, Ringeisen and his collaborators are scaling up the mini-MFC and studying ways that a small MFC, or groups of MFCs, can be deployed in natural water environments. Likely modifications include switching to an oxygen reaction at the cathode and adapting the cell so that nutrients from the environment can flow into the chamber for use by the bacteria. —BARBARA BOOTH