| It is known that cell potential increases while anode resistance decreases during the start-up of microbial fuel cells (MFCs). Biological capacitance, defined as the apparent capacitance attributed to biological activity including biofilm production, plays a role in this phenomenon. In this research, electrochemical impedance spectroscopy was employed to study cell replication during the start-up period of MFCs so that the role of biological capacitance was revealed in electricity generation by MFCs. It was observed that the anode capacitance ranged from 3.29 mF to 120 mF which increased by 16.8% to 18～20 folds over 11～13 days. Notably, lowering the temperature and arresting biological activity via fixation by 4% para formaldehyde lead biological capacitance decreased by 16.9% and 62.6%, while anode resistance increased by 92.3% and 1.5 times, respectively, which indicated a negative correlation between anode capacitance and anode resistance of MFCs. Thus, biological capacitance of anode should play an important role in power generation by MFCs. We suggest that MFCs are not only biological reactors and/or electrochemical cells, but also biological capacitors, extending the vision on mechanism exploration of electron transfer, reactor structure design and electrode materials development of MFCs.The rotating disk electrode combined with chronoamperometry, electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV), were employed to investigate the effect of mass transfer on resistance, capacitance and electricity production of microbial fuel cells. The results showed that as the current increased, the anode resistance decreased, while the capacitance increased during the start-up period of MFCs. Biofilm capacitance exhibited strong linear correlation with increasing current that the Pearson product-moment correlation coefficient was 0.997. With 20 mmol L"1 acetate as electron donor and 50 mmol L-1 hexacyanoferrate as electron acceptor, a current output of～187 μAat-300 mVAg/AgcI was achieved after 9 d. With an initial current of 86μA, a highest current of 189 μA was observed at 480 rpm that was improved 1.2 folds. With an intial current of 68 μA, power output of the MFC increased with the increased rotation rate of the electrode, a maximum power output of 2852 mW m-2 was obtained, which increased by 85.04% at 480 rpm,568 mV. After the start-up period, the EIS results showed that the anode resistance decreased with the increased rotation rate of the working electrode, due to the enhanced mass transfer. However, the overhigh rotation rate (640 rpm) showed negative effect on power output of the MFC that it is undesirable for biofilm attachment resulting from the centrifugal and shear forces, thus increased the anode resistance.The effect of forced convection on mass transfer in the anolyte-biofilm-electrode system were analyzed by Koutecky-Levich equations combined with LSV. The thickness variation of hydrodynamic boundary layer and diffusion layer of the anode theoretically illustrated the accelerated mass transfer process by enhanced convection, thereby increased the power output. The results showed that the kinetic current that excluded impact of the diffusion process was 117±2μA at an initial current of 68 μA, while the maximum current obtained by LSV was 112 at 480 rpm, which indicated that the rotation rate effectively weakened the diffusion process. With the increased rotation rates of the electrode from 10 rpm to 480 rpm, the anodic hydrodynamic boundary layer reduced from 3.85 mm to 0.56 mm, and the diffusion layer reduced from 97 μm to 14 μm, both decreased by 85.6%, which effectively reduced the diffusion-controlled zone of slow reaction and expanded the convection zone. Therefore, the increased reaction rate of biofilm-electrode improved power output of the bioelectrochemical reactor. |
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