| Energy shortage and environment pollution is one of the critical challenges we are facing in 21th century. Renewable bioenergy without a net carbon dioxide emission is viewed as one of the ways to alleviate the current global energy and environment crisis. Microbial fuel cell (MFC) is a device that uses bacteria as the catalysts to oxidize organic and inorganic matter to generate current. It is a new method of producing electric power from wastewater and other waste, and it has great value for further research and development. The benthic sediment microbial fuel cell (BMFC) is a special type of MFC, which operates on the ocean floor. In the BMFC, the anode embedded in anoxic marine sediment is connected to the cathode positioned in overlying sea water by an electrical circuit. The organic matter in the marine sediment serves as the cell "fuel", and the oxygen dissolved in the sea water serves as the oxygenate. The BMFC has many advantages such as maintenance free, supply continuously, rich in substrate, low internal resistance, environmental friendly and low cost etc, so it is very promising to be used as energy installations to supply a wide range of low-power monitoring instruments in the remote marine environment. But the key problem for BMFC is its low output power density at the moment, which prohibits the widespread application of the BMFCs. In order to increase the output power density, on the basis of the BMFC construction in the laboratory, the effect of graphite anode chemical oxidation modification and electrode shape on the performance of the BMFC is studied. The research sketch map is attached in the appendix, and results are as follows:(1) Different chemical oxidation methods are conducted on graphite anodes and the surface properties are characterized by SEM, EDX and adsorption analysis. Results indicate that the specific surface area and surface wettability of the graphite anodes increase obviously after chemical oxidation modification. The true surface area increase by 49%,56% and 67%; the contact angle decrease from 132°to 63°,58°and 42°when the graphite anodes are modified with KMnO4, concentrated HNO3, and the mixed solution of H2SO4/HNO3 respectively.Electrochemical tests and other experiments demonstrate that the chemical oxidation modified graphite anodes have a clear active influence on the system performance and the kinetic activity increase compared with unmodified graphite anode in the BMFCs. The maximum power density (Pm) increase from 24.6 mW/m2 to 40.6 mW/m2,44.4 mW/m2 and 44.5 mW/m2 respectively; the apparent internal resistance of the BMFCs decrease from 732Ωto 443Ω,462Ωand 482Ωrespectively; the exchange current density increase from 1.965×10-3 A/m2 to 0.309 A/m2,2.586 A/m2 and 0.893 A/m2 respectively when the anodes are modified by acidic KMnO4, concentrated HNO3, and the mixed solution of H2SO4/HNO3.The quantitative analysis of bacteria by the method of plating indicates that:The bacterial density of anodes unmodified, modified with KMnO4, HNO3, and H2SO4/HNO3 are 5509/cm2,16526/cm2,51988/cm2, and 17559/cm2 respectively. The result is consistent to the fluorescence images.The long time discharge experiments indicate that modified anodes of BMFCs demonstrated a very good capability of anti-polarization in the start-up phase. The BMFCs with a modified anode output higher voltage than the BMFC with unmodified anode in the long time discharge.The increase of the cell performance may be due to the enhancement in anode surface wettability, increase in anode specific surface area, increase in active site for chemical reaction on the electrode surface, or by an increase in the biocompatibility by introducing the oxygen-containing functional groups on the anodes surface. The relationship between the anode surface wettability and the electrochemistry performance is studied in the BMFC.The materials and equipments used in the chemical oxidation modification of the graphite anodes are all low cost and common, and this method is suitable for the industrialization. Therefore, the chemical oxidation modification of the graphite anode is an ideal method to enhance the performance of the BMFCs.(2) As the electrode structure has a great effect on the performance of the BMFC, several graphite electrodes with different shapes (column, plane disk and tubular shape for example) are designed in this study. The maximum power density (Pm) of BMFC-column and BMFC-disk are 20.2 mW/m2 and 14.9 mW/m2 respectively, and the internal resistances are 333Ωand 598Ωrespectively. Three cells are composed of three different sizes of graphite tubes, and their internal diameters of these electrodes are 2.5 cm (called it BMFC-I for short); 1.0 cm (BMFC-II) and 0 cm (column shape for comparison, BMFC-III) respectively. Test results show that the Pm of BMFC-I, BMFC-Ⅱand BMFC-Ⅲare 13 mW/m2,11 mW/m2and 16 mW/m2 respectively, and their internal resistances are 435Ω,488Ω. and 419Ωrespectively. The Pm of BMFC-a (composed of porous electrode) and BMFC-b (composed of plane electrode) are 37.6 mW/m2 and 28.3 mW/m2 respectively, and the internal resistances are 203Ωand 265Ωrespectively. The Pm of BMFC-A (composed of plane cathode) and BMFC-B (composed of three-phase cathode) are 16.7 mW/m2 and 25.6 mW/m2 respectively, and the internal resistance are 357Ωand 268Ωrespectively.Results show that the column and porous structure electrode has a lower internal resistance and higher power density than the disk and tubular structure electrode. The three-phase in the cathode can enhance the performance of the BMFC. Results of the research can be utilized to design BMFC structure in practical application.So the performance of the BMFC can be further enhanced by the ways of anode chemical oxidation modification or optimization of electrode shape. Energy shortage and environment pollution is one of the critical challenges we are facing in 21th century. Renewable bioenergy without a net carbon dioxide emission is viewed as one of the ways to alleviate the current global energy and environment crisis. Microbial fuel cell (MFC) is a device that uses bacteria as the catalysts to oxidize organic and inorganic matter to generate current. It is a new method of producing electric power from wastewater and other waste, and it has great value for further research and development. The benthic sediment microbial fuel cell (BMFC) is a special type of MFC, which operates on the ocean floor. In the BMFC, the anode embedded in anoxic marine sediment is connected to the cathode positioned in overlying sea water by an electrical circuit. The organic matter in the marine sediment serves as the cell "fuel", and the oxygen dissolved in the sea water serves as the oxygenate. The BMFC has many advantages such as maintenance free, supply continuously, rich in substrate, low internal resistance, environmental friendly and low cost etc, so it is very promising to be used as energy installations to supply a wide range of low-power monitoring instruments in the remote marine environment. But the key problem for BMFC is its low output power density at the moment, which prohibits the widespread application of the BMFCs. In order to increase the output power density, on the basis of the BMFC construction in the laboratory, the effect of graphite anode chemical oxidation modification and electrode shape on the performance of the BMFC is studied. The research sketch map is attached in the appendix, and results are as follows:(1) Different chemical oxidation methods are conducted on graphite anodes and the surface properties are characterized by SEM, EDX and adsorption analysis. Results indicate that the specific surface area and surface wettability of the graphite anodes increase obviously after chemical oxidation modification. The true surface area increase by 49%,56% and 67%; the contact angle decrease from 132°to 63°,58°and 42°when the graphite anodes are modified with KMnO4, concentrated HNO3, and the mixed solution of H2SO4/HNO3 respectively.Electrochemical tests and other experiments demonstrate that the chemical oxidation modified graphite anodes have a clear active influence on the system performance and the kinetic activity increase compared with unmodified graphite anode in the BMFCs. The maximum power density (Pm) increase from 24.6 mW/m2 to 40.6 mW/m2,44.4 mW/m2 and 44.5 mW/m2 respectively; the apparent internal resistance of the BMFCs decrease from 732Ωto 443Ω,462Ωand 482Ωrespectively; the exchange current density increase from 1.965×10-3 A/m2 to 0.309 A/m2,2.586 A/m2 and 0.893 A/m2 respectively when the anodes are modified by acidic KMnO4, concentrated HNO3, and the mixed solution of H2SO4/HNO3.The quantitative analysis of bacteria by the method of plating indicates that:The bacterial density of anodes unmodified, modified with KMnO4, HNO3, and H2SO4/HNO3 are 5509/cm2,16526/cm2,51988/cm2, and 17559/cm2 respectively. The result is consistent to the fluorescence images.The long time discharge experiments indicate that modified anodes of BMFCs demonstrated a very good capability of anti-polarization in the start-up phase. The BMFCs with a modified anode output higher voltage than the BMFC with unmodified anode in the long time discharge.The increase of the cell performance may be due to the enhancement in anode surface wettability, increase in anode specific surface area, increase in active site for chemical reaction on the electrode surface, or by an increase in the biocompatibility by introducing the oxygen-containing functional groups on the anodes surface. The relationship between the anode surface wettability and the electrochemistry performance is studied in the BMFC.The materials and equipments used in the chemical oxidation modification of the graphite anodes are all low cost and common, and this method is suitable for the industrialization. Therefore, the chemical oxidation modification of the graphite anode is an ideal method to enhance the performance of the BMFCs.(2) As the electrode structure has a great effect on the performance of the BMFC, several graphite electrodes with different shapes (column, plane disk and tubular shape for example) are designed in this study. The maximum power density (Pm) of BMFC-column and BMFC-disk are 20.2 mW/m2 and 14.9 mW/m2 respectively, and the internal resistances are 333Ωand 598Ωrespectively. Three cells are composed of three different sizes of graphite tubes, and their internal diameters of these electrodes are 2.5 cm (called it BMFC-I for short); 1.0 cm (BMFC-II) and 0 cm (column shape for comparison, BMFC-III) respectively. Test results show that the Pm of BMFC-I, BMFC-Ⅱand BMFC-Ⅲare 13 mW/m2,11 mW/m2and 16 mW/m2 respectively, and their internal resistances are 435Ω,488Ω. and 419Ωrespectively. The Pm of BMFC-a (composed of porous electrode) and BMFC-b (composed of plane electrode) are 37.6 mW/m2 and 28.3 mW/m2 respectively, and the internal resistances are 203Ωand 265Ωrespectively. The Pm of BMFC-A (composed of plane cathode) and BMFC-B (composed of three-phase cathode) are 16.7 mW/m2 and 25.6 mW/m2 respectively, and the internal resistance are 357Ωand 268Ωrespectively.Results show that the column and porous structure electrode has a lower internal resistance and higher power density than the disk and tubular structure electrode. The three-phase in the cathode can enhance the performance of the BMFC. Results of the research can be utilized to design BMFC structure in practical application.So the performance of the BMFC can be further enhanced by the ways of anode chemical oxidation modification or optimization of electrode shape.
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