# An Approach to Predicting Sediment Microbial Fuel Cell Performance in Shallow and Deep Water
## Materials and Methods  
A marine sediment microbial fuel cell is made by placing a non-metallic conductive surface (anode), usually graphite or carbon cloth, on or in sediment under anaerobic conditions. A wire (22 gauge insulated titanium, \(\sim\)0.1 ohm/m) conducts electrons to a conductive surface (cathode), again usually made of carbon cloth or carbon brushes, suspended in oxygenated seawater via a potentiostat circuit. The potentiostat controls the voltage between anode and cathode by varying the amount of current flowing between the anode and cathode, and may be teamed with a data recorder [15]. Non-metallic, non-corrosive materials are used to minimize corrosion-induced currents and extend the life of the electrodes. A reference electrode (usually Ag/AgCl) is often connected to the data recorder in order to assess the \"health\" of the electrodes relative to their expected voltage. It is important that the cathode is large enough, relative to the anode, so that it does not limit the MFC current. A cathode voltage \(\sim\)0.4 V above the reference electrode is indicative of a non-limiting cathode. Experimentally, we have found that open circuit voltage between and a stabilized MFC anode and cathode is \(\sim\)0.78 V. This voltage is typically reached in one to five days. The open circuit voltage, while smaller than the theoretical value when transferring electrons from glucose to oxygen, is reasonable, given various internal resistances [7]. The voltage of the cathode is typically \(\sim\)0.4 V above the reference when the open-circuit system is stable, and we operationally use this value to determine that the cathode is not limiting when current is allowed to flow from anode to cathode.  
Laboratory measurements of MFC performance were made using the following setup. Measurements at ambient pressures reported here used triplicate swatches of carbon cloth 2 cm on a side as anodes. One liter of sediment samples from different collection sites were nestled together in a temperature bath constructed from a covered 20 gallon cooler with aerated seawater and electrical chiller. Individual potentiostats outside of the bath controlled the current from each sediment sample and were connected to a large common carbon cloth cathode in the bath. Seawater was refreshed weekly. The cumulative anode surface area was approximately 36 cm\({}^{2}\), and the common cathode surface area was approximately 1200 cm\({}^{2}\). The cathode was suspended approximately 10 cm above the sediment samples in seawater to minimize ionic and internal resistance. The electrode wire lengths to the potentiostat were typically 30 cm or less. Laboratory measurements under pressure were conducted using a programmable custom system by Thar Designs. Approximately 0.5 L of sediment was placed in a 2 L cylinder filled with seawater. The anode and cathode were composed of carbon cloth swatches connected to a potentiostat outside of the bath. The anode surface area in the pressure housing was typically 12 cm\({}^{2}\) and cathode area was approximately 200 cm\({}^{2}\).  
In situ measurements of MFC performance were also made as follows. Shallow (<10 m) field measurements were made with a diver-deployed 8 m\({}^{2}\) graphite grid anode and a complex of carbon brushes acting as cathode. The potentiostat and data recorder were housed in a pressure vessel. In situ deep field measurements were made with triplicate hollow core graphite tubes driven into the sediment by weights on a remote vehicle [13]. The cathodes were large swatches of carbon cloth. The electronics were again housed in a pressure vessel. In the field deployments, the cathode was \(<\)1 m away from the buried anodes, the cathode surface area was approximately 20 times anode surface area, and wire lengths to the intervening potentiostat were approximately 0.5 m. An acoustic release was used to drop the weights (ballast) and allow the vehicle to return to the surface for retrieval.