Marine microbial fuel cell: use of stainless steel electrodes as anode and cathode materials

Global environmental concerns, the depletion of the earth's finite resources and economic considerations are all incentives to interest consumers in renewable energies. Although the latter cannot yet replace all existing energy sources, they can supplement power generated by utilities and increase the diversity of our energy supply. Since years an immerse reserve of energy is lying under seas, an inexhaustible fuel essentially composed of organic matter. The bacteria present either in sediments or in seawater could oxidise organic matter as well as reduce oxygen dissolved in subsurface seawater. These different reactions result in a voltage drop between sediments and water surface, thus generating current. In 2002, a US team1 has demonstrated that bacteria isolated from marine sediments, Geobacter sulfurreducens, could transfer electrons directly to electrode, without the contribution of soluble electron mediator2. This new way of electron transfer promises high yields of current production and enables facilities such as microbial fuel cells to be implemented easily. Concurrently, French researchers3 have demonstrated that a stainless steel cathode covered by a biofilm formed in seawater reveals extraordinarily high catalytic properties for the electrochemical reduction of dissolved oxygen. In this context, an installation was implemented in Genova harbour, simply composed of two stainless steel electrodes: one dipped in sea mud, and another immersed in surface seawater. First results were encouraging but indicated a current limitation. Various assumptions were emitted to explain this phenomenon: substrate diffusion, precipitation on electrode surface, low conductivity. Analyses of electrochemical kinetics and mass transfer revealed that the anode process was the rate limiting step. In order to better understand these phenomena, experiments on different anodic materials (graphite and stainless steel) were carried out in laboratory with Geobacter sulfurreducens. These tests showed on the one hand that stainless steel was not well suited for anode material, and on the other hand that graphite anode could sustain current densities up to 7A/m² which was seven times higher than figures previously reported in bibliography