Team:SCAU-China/MFC,MDC-device

Physical Device

Theory

Microbial fuel cells (MFCs) use bacteria as the catalysts to oxidize organic and inorganic materials to generate electric current. Electrons produced by the bacteria are transferred to the anode and flow to the cathode through a conductive material containing a resistor (Fig.1). Meanwhile, a positive current flows from the positive to the negative terminal through a proton exchange membrane (PEM), with an opposite direction to that of electron flow.

Electrons produced by the bacteria may transfer from inside to outer environment by direct contact, shuttling via excreted mediators, a conductive biofilm or conductive pili. Additionally, the possibility exists that these methods are not mutually exclusive.¹ So we want to choose a better anode so the anode can be conductive and bacterial adhesive and electron transfer from bacteria to anode easily. When electrons reach to the cathode, they combine with the oxygen in the air and the protons in the solution resulting in water. So the material of cathode should benefit catalytic reaction and air diffusion inside the solution.²

Escherichia coli is not a friend bacterial organism for electric current generation because the cytoplasmic membrane has to be nonconductive to maintain the membrane potential absolutely required for free energy. Chemical mediators, such as neutral red, riboflavin or anthraquinone-2, 6-disulfonate (AQDS), allow electron transfer from a microbial electron carrier to an electrode.^3-5 It is known that ferricyanide is a good electron acceptor in the cathode chamber because it increases the power density at high concentrations.⁷

Device design

Many different configurations are possible for MFCs. A widely used and low-cost design is a two-chamber MFC in a traditional “H” shape, consisting of two bottles connected by a tube containing a PEM, such as Nafion, or by a plain salt bridge in the U-tube. The H-shape systems are acceptable for basic parameter research, but they typically produce low power densities. The limitation of power output may come from following factors, surface of the membrane, distance between two electrodes, high internal resistance and electrode-based losses. Compared with power produced by different types of bacteria, it makes the most sense to compare them on the basis of equal sized configurations and the solutions with the same composition.⁶

MFC type 1

The MFC (type1) contains two 100 ml bottles (Fig. 2a), each with three openings for the supplement of the solution, sampling and reference electrode insertion. A PEM is fixed between the bottles by a champ. We use common graphite plate as electrodes and they are connected to the resistor through two iron wires.

MFC type 2

The MFC (type 2) consists of two chambers. The sectional area of each chamber is 5 cm × 5 cm and the diameter of the chamber is 2.7 cm (Fig. 2b). We use PEM as proton passage. We use non-waterproof carbon cloth (FUEL CELLS ETC) for anode, whose roughened surface have been reported to produce a higher power density than flat graphite electrodes and has a high absorption capability. The air cathode contains a platinum coated carbon cloth (on the water side) with 5 diffusion layers (on the air side) as our cathode (FUEL CELLS ETC). The platinum can be used as a catalyst and the diffusion layers facilitate the air diffusion.

(a)	(b)
Figure 2. (a) Diagram of the “H” shape MFC (Version 1) used in the parallel experiment, (b) the Pt/C DL cathode and carbon cloth anode.

Electrical measurements

After the resistance measure, we found that the inner resistance of our fuel cell system was approximately 1600 Ω, so we connect our device to a 1 kΩ resistor as the external resistance. Voltage is measured with multimeter connected to the line between the anode and the cathode in the closed circuit configuration.
We design this simple data acquisition card (DAC) to monitoring the changes of the voltage during the long period of electrical generation.

References

[1] Lovley, D. R. The microbe electric: conversion of organic matter to electricity. Current Opinion in Biotechnology. 2008, 19: 564-574.
[2] Wei, J., Liang, P. & Huang, X. Recent progress in electrodes for microbial fuel cells. Bioresour Technol. 2011, 102: 9335-9344.
[3] Park, D. H. & Zeikus, J. G. Electricity generation in microbial fuel cells using neutral red as an electronophore. Appl Environ Microbiol. 2000, 66: 1292-1297.
[4] Jung, S. et al. Impedance and Thermodynamic Analysis of Bioanode, Abiotic Anode, and Riboflavin-Amended Anode in Microbial Fuel Cells. B Korean Chem Soc. 2012, 33: 3349-3354.
[5] Bond, D. R., Holmes, D. E., Tender, L. M. & Lovley, D. R. Electrode-reducing microorganisms that harvest energy from marine sediments. Science. 2002, 295: 483-485.
[6] Logan, B. E. et al. Microbial fuel cells: Methodology and technology. Environmental Science & Technology. 2006, 40: 5181-5192.
[7] Kim, H. J. et al. A mediator-less microbial fuel cell using a metal reducing bacterium, Shewanella putrefaciense. Enzyme Microb Tech. 2002, 30: 145-152.

MDC

Microbial Desalination Cell (MDC) is a new method of water desalination which is similar to water dialysis but without external energy input. This desalination process is driven by current and potential generated by bacteria in the Microbial Fuel Cell (MFC) (Kim and Logan 2013) (Fig.1). The energy in domestic wastewater ranges from 1.8 to 2.1 kWh/m3 and suffices most of energy demand in water desalination (3~4 kWh/m3) using reverse osmosis.
The MDC consists of anion exchange membranes (AEM) and cation exchange membranes (CEM), a desalination chamber, a two-chamber MFC and a connecting chamber. Because of the depletion of proton in the cathode chamber, cations move from the desalination to cathode chamber. The AEM prevents positive charged ions from leaving the anode chamber so charge is balanced by anions leaving the desalination chamber. As results, the ions in the desalination chamber are removed and the water is desalinated (Mehanna, Saito et al. 2010).

The ionic separation is repeated in every cell pair, the desalination effect is magnified by the number of cell pairs in the stack. For example, 100 pairs of sodium and chloride ions are separated for every electron transfer in a 100-cell paired electrodialysis stacks. The internal resistance was so large that the authors concluded that an MDC with more than 1.5 cell pairs (2 diluted and 1 concentrate chambers) would not be more efficient. A new developed electrodialysis stack make it possible to substantially increase the extent and efficiency of desalination, and allow high power densities through the minimization of Ohmic resistances in the stacks (Figs.2, 3). (Kim and Logan 2011)

(a)	(b)
(c) Fig.2 (a) Schematic design for the novel electrodialysis stack, (b) The configuration of the thin gasket, (c) A 4 cell-pair MDC in our project.

(a)	(b)
Fig3. Construction of the gaskets used to direct the flow.

MDC construction

◇The cathode (170 mL) and anode (170 mL) chambers were cylindrical with a cross-sectional area of 28 cm2.
◇The connecting chamber consists of a 90º turn for water influx and efflux and a same size lumen as electrode chamber.
◇Intermembrane distance (or cell width) was held constant by a piece of polyethylene mesh (10 × 10 cm2) 1.3 mm thick, with each cell having a rectangular cross section (4 × 2 cm2) (Figs. 2, 3). The dilute solution was introduced from the cathode side and flowed serially through every dilute cell (Hatzell and Logan 2013).
◇Anode is a non-waterproof carbon cloth with an active area of 28 cm2 (3 cm in diameter).
◇Cathode is a platinum (0.5 mg 60% Pt/C on the water side) coated carbon cloth with 5 diffusion layers (on the air side).
◇The dilute and concentrated water are introduced from the connecting chamber. The synthetic seawateand diluted water is controlled in fix flow rate (1 mL/min).

Result

In order to determine the conductivity standard curve, we weigh some quality NaCl according to a certain concentration gradient to make up NaCl solutions (0.0-4.0g/L), using the conductivity meter to determine their conductivities, and then, by manipulating the data we can obtain a standard curve.
We achieved a 5.8% desalination rate using a 3 V and 8 mA external power in 12 hours in a 4-cell MDC.

(a)	(b)
Fig 1. (a) Standard curve of conductivity against NaCl concentration omitting the low concentration segment, (b) Desalination efficiency of MDCs.

References

[1] Hatzell, M. C. and B. E. Logan. Evaluation of flow fields on bubble removal and system performance in an ammonium bicarbonate reverse electrodialysis stack. Journal of Membrane Science. 2013, 446(0): 449-455.
[2] Kim, Y. and B. E. Logan. Series Assembly of Microbial Desalination Cells Containing Stacked Electrodialysis Cells for Partial or Complete Seawater Desalination. Environmental Science & Technology. 2011, 45(13): 5840-5845.
[3] Kim, Y. and B. E. Logan. Microbial desalination cells for energy production and desalination. Desalination. 2013, 308 (0): 122-130.
[4] Mehanna, M., T. Saito, J. Yan, M. Hickner, X. Cao, X. Huang and B. E. Logan. Using microbial desalination cells to reduce water salinity prior to reverse osmosis. Energy & Environmental Science. 2010, 3(8): 1114-1120.