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The beginning of research related to the microbial fuel cell is based on the work of Potter in the year 1912[1]. At the beginning of the new century this research came back into focus because of its potential to generate renewable and ecologically friendly energy out of waste waters. In the first time, the focus of research was related to the species of the used bacteria, there growth conditions, metabolism at plate electrodes with a low yield of current.[2,3] Later, the electrode surface was more of interest because of the knowledge that a higher surface area on electrodes can yield a higher current caused by more bacteria. To proof this different porous carbon materials were tested. This led to the development of carbonized nanofibers as an electrode material which was made from nonwovens produced by blown assisted electrospinning of polyacrylonitrile (PAN). This material yields after colonization with bacteria for an microbial fuel cell the best current yields so far.[4] The disadvantages of such a carbon electrode are given by the relative bad mechanical and electrical properties of carbon related to other materials like metals. After carbonization these materials often crack or break even after low mechanical stress.[5] Compared to Copper and Silver in particular the carbon material has a lower conductivity in regions of magnitudes. This is a big disadvantage related to the use of this material as anode material in microbial fuel cells because of the low conductivity. Related to this the high resistance produces, (shown by Ohms Law (4)) a high loss of power in the electrode that rises with resistivity. In energy producing systems the usable power degreases with growing resistivity. This decreases the yield of energy produced by this system. In combination with the shown bad mechanical properties of carbon materials the high resistivity lowers the ability to produce such a fuel cell, causes high costs and makes the production of such a fuel cell in some cases even impossible. To overcome the disadvantages of carbon materials as use for anode materials in microbial fuel cells, the relative good mechanical properties of three-dimensional polymer backbones was combined with the relative good electrical conductivities of metals in this publication. At the beginning of this thesis different metals like Cu, Ag, Au and Ni were evaporated in Bayreuth on graphite plates in vacuum to find suitable metals for this approach. These plates were tested in Braunschweig in the working group of Uwe Schröder by André Baudler as electrodes in biological fuel cells. The current generated by these electrodes with a grown bacteria film out of wastewater was measured and compared to an uncoated graphite and stainless steel plate. Surprisingly the results showed that all used metals yielded nearly the same current like the uncoated graphite sample that was used as a reference. Related to this results Copper and Silver were chosen for metallization in Bayreuth because of their excellent electrical conductivity. Commercial melamine resin sponges were used as a substrate for metallization because of the pore structure of this material and its relative high inner surface. By this means metalized sponges were generated in Bayreuth which were successfully tested in Braunschweig as anode materials in biological fuel cells. The results show currents near to the best results achieved with electrospun PAN based carbon electrodes. But in comparison to the carbon based electrodes the metalized sponges show excellent mechanical and also electrical properties of magnitudes higher than carbon electrodes with similar density. The metal-polymer composite produced in this work also has some other interesting properties like a super hydrophobic surface, high current tolerances, lightweight, heating abilities at high currents, a high porosity at low density, and a good thermal insulating behavior. These multiple properties make the material very interesting for different other applications beside as an electrode for a biological fuel cell. It can be used as a conductive insulation martial or as a breathable heating material or sound insulation element. The successfully applied methods of this thesis can also be used to form metal layers on other porous materials which can lead to new fields of research for many other applications. In the last part of this work dispersions of nanofibers where created by the use of electrospinning and a suitable method to bring these in suspension. It was possible to generate membranes born by the wet laid method. These membranes show the same properties like membranes generated by direct electrospinning. Because of the nature of these suspensions these nanofibers can be combined with other kinds of conductive fibers or bacteria dispersed into a liquid. In the future this method could be a starting point to generate new conductive modified electrodes or membranes. |
