Predicting in situ sediment fuel cell potential

Microbial fuel cells (MFCs) work by providing bacteria in anaerobic sediments with an electron acceptor (anode) that stimulates metabolism of organic matter. The buried anode is connected via control circuitry to a cathode exposed to oxygen in the overlying water. During metabolism, bacteria release hydrogen ions into the sediment and transfer electrons extra-cellularly to the anode, which eventually reduce dissolved oxygen at the cathode, forming water. The voltage between electrodes is operationally around 0.4 v and the current is chiefly limited by the rate of microbial metabolism at the anode. The Office of Naval Research has encouraged development of microbial fuel cells in the marine environment at a number of academic and naval institutions. Work at SPAWAR, a navy laboratory in San Diego, involves fuel cell design and testing, applications to low power, Navy sensors and studies of important environmental parameters that affect fuel cell performance. In San Diego Bay, we typically find that steady state power output is reached two to three weeks after deployment and power density ranges from 2 mW m^-2 to 15 mW m^-2 anode surface area. In order to determine which parameters are controlling power output, we developed small, self-recording fuel cells that could be deployed and retrieved from the surface - the `sea dart´. Typical sea dart deployments lasted two weeks. We combined data from the sea dart with oxygen, sediment and water temperature, and water depth data from simultaneously deployed instruments in many instances. We took additional data on sediment total organic carbon and grain size. For longer time series, we evaluated data from a much larger, self-recording fuel cell at one location for five months where the main environmental variant was seasonal water temperature. We found that the most important environmental parameters that control fuel cell power output in San Diego Bay were total organic carbon in the sediment and season- l water temperature. Parameters that we dismissed were dissolved oxygen levels, light level, and initial sediment bacterial populations. Parameters whose affect we could not separate were total organic carbon and grain size. Surprisingly, we also found that power density is inversely proportional to anode size, perhaps due to internal anode resistance or interference with local pore water diffusion of organics to the bacterial population. Derived relationships between power and organic carbon, temperature and anode size, along with extensive knowledge of sediment organic carbon and seasonal water temperatures in San Diego Bay, allowed us to develop seasonal contours of power output from a benthic microbial fuel cell with a nominal 1 m² anode. As an application, these contours were then translated into charging time for a satellite transmitter with a 10 minute transmission window.