Study On The Preparation Of Nano-Sized Iron Oxides Modified Carbon Paper Electrode And Its Application In Microbial Fuel Cell

In a microbial fuel cell(MFC), chemical energy in organic compounds could be converted into electricity directly through microbial metabolism by the way of extracellular electron transfer(EET). As a promising energy technology, the development of MFC is hindered by its low power output and relatively high cost. Anode as a key component of the device markedly affects power output because it provides surface active sites for microbial adhesion and interfacial electron transfer. Therefore, it is quite necessary to obtain a cheap anode with high bio-electrocatalytic prior to practical application of the MFC technology. Among employed electrodes, the carbon based materials such as carbon fiber, carbon paper(CP), graphite fiber brush are commonly used anode in MFCs due to not only good biocompatibility, high conductivity and chemical stability, but also low cost.Shewanella-a Gram-negative, dissimilatory metal-reducing bacteria(DMRB) found in soils, sediments, surface waters, and ground waters-is frequently used as the exoelectrogens in MFCs. It has the ability to recognize the surface of iron oxides and initiate EET to the attached iron oxides as a terminal process in its metabolism. Drawing inspiration from this fact, we employed nano-sized iron oxides to modify carbonaceous electrodes to increase the EET in order to improve the performance of MFC. The main results are summarized as the follows:(1) One dimensional(1D) α-FeOOH nanowires(NWs) were synthesized in situ on the surface of a CP electrode via hydrothermal reaction at 100℃ for 6h. The nanowires were 20-60nm in diameter and 650nm～1μ.m in length. The electrochemical activities of CP and NWs/CP electrode have been investigated by cyclic voltammetry(CV) and electrochemical impedance spectroscopy(EIS). The current density of 0.012mA/cm2 on α-FeOOH NWs/CP was 71% higher than that of 0.007mA/cm2 on bare CP after 41h cultivation of S. loihica PV-4, which was attributed to better growth of PV-4 and lower charge transfer resistance. CV was performed instantly after the termination of chronamperometry. A sharp cathodic peak was located at -0.285V and the peak current on NWs/CP(-0.170mA/cm2) was enhanced over a 20-fold than that on CP(-0.0082mA/cm2). Results above show a feasible way to obtain an inexpensive anode with high biocompatibility and bio-electrocatalytic activity.(2) Two dimensional(2D) α-Fe2O3 nanosheets(NSs) were synthesized in situ on the surface of a CP electrode via hydrothermal reaction at 260℃ for 24h. The nanosheets were 5～11μm in width and 450-800nm in thickness. The current density of 0.0036mA/cm2 on α-Fe2O3 NSs/CP was 1.9-fold higher than that of 0.0019mA/cm2 on bare CP after 19h cultivation of S. loihica PV-4, which was attributed to lower charge transfer resistance. CV was performed instantly after the termination of chronamperometry. PV-4 grown on NSs/CP electrode exhibited a clear redox wave; the anodic peak and cathodic peak were located at -0.130V and-0.336V respectively. Midpoint potential(Em=-0.233V) was calculated from anodic and cathodic peak positions and this value was in good accordance with the reported Em of outer membrane c-type cytochromes(c-Cyts) OmcA from Shewanella. These results indicated that the modified electrode markedly facilitated bacterial EET. What’s more, by controlling the amount of water added into the reactive solution, we successfully decreased the width of α-Fe2O3 NSs modified on the CP electrodes from micrometer to nanometer size. Electrochemical tests suggested that nano-sized α-Fe2O3 NSs could better improve the EET efficiency of PV-4 than micron-sized NSs.(3) Three-dimensional(3D) α-Fe2O3 mesoporous nanorods array(NRs-A) was synthesized on the surface of a CP electrode via hydrothermal reaction at 100℃ for 10h combined with calcination in N2 at 500℃ for 2h. A single nanorod was 70-150nm in diameter and 500-700nm in length. The current density of 0.0104mA/cm2 on α-Fe2O3 NRs-A/CP was 62.5% higher than that of 0.00640mA/cm2 on bare CP after 47h cultivation of S. loihica PV-4, which was attributed to lower charge transfer resistance. After EIS, the electrolyte was removed and replaced with fresh oxygen-free defined medium(DM) containing sodium lactate as the electron donor. CV tests indicated that medium replacement rarely affected the electron transfer between the biofilm and NRs-A/CP electrode, which could be ascribed to the much more compact contact between PV-4 and NRs-A/CP.