Mechanism Elucidation And Functionization Of Cathode Of Microbial Fuel Cells

Microbial fuel cell (MFC) is an electrochemical cell that directly captures the energy contained in bio-convertible substrates in the form of electricity. In an MFC, the microorganisms that completely oxidize organic compounds with an electrode as the sole electron acceptor are so-called exoelectrogens. The exoeleetrogens play a key role in the anode oxidation by facilitating electron transfer between the electron donor and the electrode. It has attracted increasing interests to employ MFCs to produce "green" energy from wastes. Just from the performance of producing electricity, MFC can not be compared with the traditional fuels in short term. So it can not be limited only to improve the electricity generation, but to deal with the development and utilization of waste in MFC. It is particularly necessary how to effectively use MFCs’cathode to enhance the overall functionality, so that the MFCs’ application range can been further to expand.In this thesis, two typical types of MFCs namely single-chamber and dual-chamber MFCs were builded, the reducing power of MFC cathode was applied separately in lithium cobalt oxide leaching and in situ remediation of groundwater contaminated by nitrates. Also, the cathode performance and factors of MFC were further extended, and reached the following conclusions:(1) Complete cobalt recovery from lithium cobalt oxide requires to firstly leach cobalt from particles LiCoO2 and then recover cobalt from aqueous Co(Ⅱ). A self-driven microbial fuel cell (MFC) can carry out the first step, in which Co(Ⅱ) is firstly released from particles LiCoO2 on the cathodes of MFCs and the outpower can be obtained continuously. The leaching effect on LiCoO2 by MFC is better than by acid leaching. The initial pH value of the catholyte has an influence on energy efficiency and cobalt recovery. pH, KCl concentration, gas/liquid ratio (S/L) and CuCl2 catalyst concentration all have significant influence on the maximum output voltage of MFC and Co(Ⅱ) concentrations of solution. The evidence of influence factors including pH, solution conductivity, gas liguid ratio and catalyst can contribute to improving understanding of and optimizing cobalt recovery with concomitant electricity generation in MFCs. The pH is the important thermodynamics and kinetics factor during the reduction of LiCoO2. So in a subsequent experiment pH is a key factor.(2)A rectangular and single bio-cathode MFC is constructed. The experiments that imitated groundwater flow and nitrate are the pollutants are tested. Firstly supply water, drainage and shower experiments are designed, then hydraulics characteristic coefficients such as 0.222 of porosity,0.148 of water retention and 5.13 m/d of permeability coefficient are obtained. All the results demonstrate that the feature of water flow in the rectangular MFC meets the characteristics of groundwater flow.(3) The single bio-cathode MFC starts successfully where microorganisms were inoculated and acclimated from anaerobic sludge, the maximum output voltage is about 500 mV stably after 4 cycles, the process needs 300 h. The carbon cloth surface of anode is different from cathode by scanning electron microscope morphology (SEM) observed, and the carbon cloth is covered by macroporous somethings in anode, however the carbon cloth is covered by lamellar somethings in cathode. But in the mixed bacterial system, SEM observation only is regarded as an adjunct to verify the macro results.(4) All evidences show that factors including C/N, NaHCO3 and HRT can influence on electricity generation of MFC and nitrate degradation, at the same time nitrite and ammonia accumulation effect appears. The experimental results showed:MFC output voltage and the degradation rate increases with C/N increasing due to more carbons. NaHCO3 and HRT all have significant influence on the maximum output voltage of MFC and NO3--N concentrations. More NaHCO3helps electricity generation and high nitrate degradation rates. MFCs gain higher output voltage and lower NO3--N concentrations under 2.0d of HRT than under 1.0d of HRT owing to more electron acceptors in the solution.(5) Experimental study shows that maximum output voltages increase and concentrations of NO3--N fall with hydraulic gradient increasing. These results show that electricity production performance increasing and total degradation of nitrate decreasing with hydraulic gradient ascending. Concentration in spatial and temporal distribution appears that high concentration of NO3-N is scattered in upper layer and low concentration is scattered in upper layer. In accordance with the horizontal flow, more NO3--N are discovered in the front and less NO3--N are found in the back of MFC.(6) The paper also found that there was an obvious reduction peak in the cyclic voltammetry curve of MFC cathode carbon cloth and an oxidation-reduction pair in the cathode solution of MFC. Blank anode and cathode is not linked, and the reaction solution is a solution medium, biofilm anode and cathode have CV curves. The results showed that biofilm anode is mainly oxidation catalytic oxidation occurring at the electrode surface microbes; biofilm cathode happen is a reduction reaction, the cathode catalytic electrode surface microbes occurring in the restore. There is a blank electrode CV curve reaction solution pair distinct redox pair, presumably the reaction solution by the presence of the microorganism metabolism secreted into the redox mediator extracellular, MFC system exists electron shuttle transfer mechanism, electron transfer is the main task of the producing microorganism electrode electrically completed.These results provide a broad application of MFC for recovery of cobalt of spent lithium ion batteries with no external energy consumption and nitrate situ remediation of groundwater.