Study On Anodic Performance And Capacitive Characteristic With Modification By Iron(Ⅲ) Oxides In Microbial Fuel Cell

It is well recognized that microbial fuel cell (MFC) is specified as the green energy technology due to its wastewater treatment accompanying with energy recovery. This provides a new trial for resolving water pollution and energy crisis. However, considering the practical application for the future, high cost and low power are still the main problems. Especially, the electron transfer rate between anode and bacteria is the key factor.The exoelectrogenic bacteria in MFC mainly consists of dissimilatory metal-reducing bacteria (DIRB). It has been experimentally demonstrated that outer membrane c-type cytochromes (OMCs) have a high binding affinity to iron(Ⅲ) oxides. Furthermore, iron(Ⅲ) oxides can be recognized by DIRB and utilized as extracellular electron acceptor to be reduced. What’s more, stainless steel mesh (SSM) is superior to conventional carbon electrode with higher mechanism and conductivity, and already being the research tide in electrode materials.For the first time, a new type of anode (AcM) is composed of ultracapacitor activated carbon (UAC)/SSM and a polytetrafluoroethylene (PTFE) emulsion by a rolling press method. Sodium acetate is used as the synthetic substrate in single-chamber membrane-less air-cathode MFC. Comparing with carbon mesh (CM) as anode, it is lower cost and better mechanical strength with3.0%higher performance in power generation. Thus, there is bright prospect in favour of market promotion for MFC.For AcM modified by nanosemconductor goethite, based on the decrease of polar resistance and the increases of both exchange current density and anodic peak current, it is verified that α-FeOOH added AcM kinetically promotes the extracellular electron transfer. The increase of constant-phase element and decrease of Warburg element manifest that the capacitance and diffusion condition on the surface of anode are enhanced. So, the maximum power density (MPD) arrives to the biggest increase of36%from508±40mW/m2(the control) to637±51mW/m2(2.5%α-FeOOH added), and then to693±20mW/m2(5.0%α-FeOOH added). Whereas, with the excess increment of7.5%, the MPD inversely decreases to639±30mW/m2, probably due to the lower conductivity, diffusion and capacitance under the adsorption of biogenic iron(Ⅱ) on the anodic surface.For AcM modified by nano magnetite (AcFeM), Tafel tests indicate that AcFeM is also kinetically more advantageous, and the MPD of AcFeM (809±5mW/m2,5.0%Fe3O4added) is22%higher than that of AcM (664±17mW/m2) by polarization tests. It is the first time to point out that the dynamic activity of Fe3O4plays a leading role in accelerating the electron transfer, and beneficial to boost the transient charge storage of the anode. Furthermore, the net storage capacity initially increases followed by a decrease with the maximum capacitance of574.6C/m2for AcFeM under20min of open circuit interval. The cyclic voltammetry curves explain that the capacitance of AcFeM can be from three aspects:pore structure of carbon, capacitor-like behavior of anodic biofilm and the redox couple of Fe3O4/Fe(Ⅱ) as a solid-state electron shuttle.Power overshoot commonly makes the MFC performance evaluation inaccurate. Based on the capacitive characteristic of anodic biofilm, three types of carbon with different capacitance (UAC, plain activated carbon (PAC) and carbon black (CB1)) are rolled on SSM as anodes to investigate the relationship between overshoot and anodic capacitance. It is exciting that the overshoot is not observed for UAC-MFC from Cycle2to4, while the phenomenon is eliminated in PAC-MFC from Cycle3and in CB[-MFC from Cycle4. This is relevant with the Cmabiotic of the anode following the order of UAC (2.1F/cm2)> PAC (1.6F/cm2)> CB1(0.5F/cm2). It is inferred that the Cmabiotlc of anode stores charges and function as electron shuttle to overcome the phenomenon.In order to prove the credibility of the result above, another carbon black (CB2) with enhanced capacitance is produced via chemical activation with potassium hydroxide and nitric acid, and simultaneously made into composite anode with5.0%addition. It has been found that oxygen-containing functional groups are introduced and both the hydrophilicity and Cmabiotic of the electrode are also improved. But, before and after Fe3O4addition, there is a slight increase in anodic capacitance. It indicates that the overshoot phenomenon disappears only when Cm is larger than1.1F/cm2, in spite of through chemical activation or capacitance compensation from bacterial colonization and Fe3O4. This further strengthens that insufficient capacity of anode leads to the power overshoot.