Crab shells boost fuel cells

Remnants of crab and lobster shells could help prolong the life of microbial fuel cells that power sensors in the unlit depths of the sea, according to a team of Penn State University researchers.



Microbial fuel cells process organic material to produce energy, but deep sea sediments can be low in organic material because living things at a depth where light penetrates the water are used up before they reach the ocean floor. An absence of organics limits the lifetime of marine microbial fuel cells.



The researchers include chitin from processed crustacean shells into a cushioned anode made of carbon cloth. The anode is placed in the sediment or hung in the water where naturally occurring bacteria can eat the chitin.



‘This approach is good for deeper ocean areas or anywhere we want to increase the power of marine microbial fuel cells,’ said Bruce E. Logan, Professor of Environmental Engineering.



Microbial fuel cells work through the action of bacteria which can pass electrons to an anode. The electrons flow from the anode through a wire to the cathode, producing an electric current. In the process, the bacteria consume organic matter in the water or sediment. The Penn State approach uses the bacteria that naturally occur in the oceans and because so many sea creatures produce chitinous shells, many marine bacteria break down chitin.



Marine energy sources are often placed in remote areas to power sensors for such measurements as temperature, pressure, salinity, density, turbidity or particulate content. These sensors could be placed on buoys or used to monitor around offshore drilling platforms and to monitor for pollution or contamination, such as that caused by red tide, in both salt and fresh water. Other small devices can measure sound, light transmittance and conductivity. While the amounts of energy needed for these purposes are small, the locations often necessitate long-term remote operation.



While the team has not tested the marine microbial fuel cell in the ocean sediment, they did create a fuel cell in the laboratory consisting of a glass bottle with the anode embedded in the sediment on the bottom and the carbon paper and platinum cathode suspended in the water. In the ocean, no container is needed, but the anode and cathode must be close enough together so the protons or positive charge can pass through the water to the cathode.



The researchers tested two different sizes of chitin, one finer than the other and found that both increased power production over the same set up without the additional bacterial food supply. However, the finer particles produced almost twice the power as the larger particles, suggesting that the bacteria can more easily consume the smaller particles.



‘We can adjust the particle size to control the rate at which chitin is consumed and alter the power output and the fuel cell’s longevity,’ said Logan. ‘Technically, there is no reason why we cannot put a bigger bag of feed for the anode to supply more food.’