Polymer Separators for High Power, High Efficiency Microbial Fuel Cells

   

Polymer Separators for High Power, High Efficiency Microbial Fuel Cells   Guang Chen, Bin Wei, Yong Luo, Bruce E. Logan*, Michael A. Hickner*

Supporting information EXPERIMENTAL Preparation of Poly(vinyl alcohol) (PVA) membranes Poly(vinyl alcohol) (PVA, 85000-124000 Daltons, 99+% hydrolyzed), tetra-nbutylammonium chloride, Nafion solution (Nafion 117 solution, 5 wt % in a mixture of lower aliphatic alcohols and water) and all other chemicals obtained from Sigma-Aldrich, Inc. were reagent grade and used as received. PVA membranes with different porosities were prepared as follows: PVA (2 g) was dissolved in 23 g H2O at 90 °C to prepare ~8 wt% transparent viscous polymer solution. To this solution, different amounts of porogen (tetrabutylammonium chloride) were added and dissolved. The prepared solution was cast onto a Teflon plate and then dried at 50 °C for 36 h. PVA membranes with different porosity were obtained after immersing the dried membrane into water during which the salt was dissolved to form pores in the water-swollen membranes. MFC reactor construction Anodes were composed of ammonia-treated carbon brushes.1 Control cathodes (area = 7 cm2) were constructed from carbon cloth (type B, 30% wet-proofed, BASF Fuel Cells, Inc.) with    

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    0.5 mg Pt per cm2 catalyst loading (10 wt% Pt on Vulcan XC-72,) on the MFC electrolyte side and four PTFE diffusion layers on the air side as previously described.2 Single-chamber, single air-cathode (SCa), cubic-shaped MFCs (2 cm long cylindrical chamber; 12 mL liquid volume) were employed for all performance tests in this work.3 The separators were placed against cathodes and secured with plastic mesh on the solution side of the cathode to hold the separator against the cathode surface.4 Characterization LSV was performed at 1 mV s-1 on the cathodes at 30 °C. The reactor with a working electrode consisting of plastic mesh, separator, and air-cathode was filled with 28 mL of 200 mM PBS (18.304 g L-1 Na2HPO4, 9.808 g L-1 NaH2PO4, 0.13 g L-1 KCl, 0.31 g L-1 NH4Cl, pH = 7) and equipped with 7 cm2 platinum foil square counter electrode and an Ag/AgCl reference electrode. Electrochemical impedance spectroscopy (EIS) was performed at 0.1 V (vs. NHE) over a frequency range of 105 to 0.006 Hz with a sinusoidal perturbation of 10 mV in the same reactor used for LSV tests. The charge transfer resistance (Rct) was obtained by fitting the charge transfer impedance to an RC circuit. The oxygen mass transfer was calculated by measuring the change in dissolved oxygen concentration (NeoFox, Ocean Optics Inc., FL) over time in a stirred abiotic reactor without an active anode as previously described.5 The produced voltage (Ecell) and electrode potentials were measured at a fixed eternal circuit resistance (1000 Ω) using a multimeter and Ag/AgCl reference electrode at 30 °C. After the reactors were operated on fresh substrate for about 2 h, the polarization and power density curves as a function of current density were measured using the single-cycle method4 after 20 min at each external resistance (1000-20 Ω). All the other parameters including current, current density, and power density, and coulombic efficiency (CE) were calculated from the ratio of the total    

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    electrical charge produced within every cycle at different external resistances (1000 Ω, 500 Ω, 200 Ω, 100 Ω, 75 Ω, 50 Ω) to the theoretical amount of electrons available from the oxidation of acetate to carbon dioxide.6,7

Table S1. Solution resistance (Rs) and charge transfer resistance (Rct) of reactors with different separators before and after cycling. Before operation

NS GF-1.0 PVA-0 PVA-3.2 PVA-5.6

After 17 cycles

Rs (Ω)

Rct (Ω)

Rs (Ω)

Rct (Ω)

5.0 6.3 8.0 7.1 7.1

7.0±0.4 10.0±0.7 10.0±0.8 9.0±0.5 8.0±0.5

9.9 9.2 7.7 7.5 7.8

27.6±0.5 31.8±0.8 27.3±0.3 23.2±0.7 20.5±0.5

Table S2. Maximum power density and CE (%) at 1000 Ω for reactors at 6 and 17 cycles. Pmax (mW m-2)

NS GF-1.0 PVA-0 PVA-3.2 PVA-5.6

CE (%)

Cycle 6

Cycle 17

Cycle 5

Cycle 18

1532±24 1017±30 1171±59 1267±75 1309±51

1112±91 969±30 1164±15 1181±41 1223±18

16.8±1.2 30.2±7 38.4±1.1 38.4±0.3 38.2±3.6

31.7±2.1 34.4±0.4 44.4±2.9 46.0 46.9±1.4

         

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      Table S3. Fabrication parameters and physical properties of separators. PVA solution (g)

Salt (g) (%)

Water uptake (%)

Thickness (cm)

KO (10-5) cm/s

DO (10-5) cm2/s

25.0 25.0 25.0

0 (0) 0.8 (3.2) 1.4 (5.6)

180±12 200±18 170±10

0.1 0.012±0.02 0.012±0.015 0.010±0.02

157.3 5.0 22.6 25.0

0.5 0.45 0.5

NS GF-1.0* PVA-0 PVA-3.2 PVA-5.6 * From Zhang et al. 19

Table S4. Coulombic efficiencies as a function of current density for MFCs with different separators. CE (%) at different external resistance (Ω)

NS GF-1.0 PVA-0 PVA-3.2 PVA-5.6

1000

500

200

100

75

50

23.9±1.5 31.3±1.2 42.2±0.5 43.5±0.9 43.7±2.2

37.8±0.4 48.4±1.6 66.8±6.2 76.1±2.5 72.2±1.4

41.3±3.0 52.1±2.8 68.7±2.9 76.0±5.6 81.3±0.5

55.1±0.9 61.1±2.5 76.9±3.2 82.3±5.2 87.6±2.7

60.1±0.7 79.2±2.7 83.6±4.4 90±2.2

78.8±2.1 63.3±0.2 81.0±2.8 84.6±7.3 94.6±3.3

Table S5. Coulombic efficiencies of MFCs with different separators at the current density of maximum power generation. CE (%) vs Pmax (mW m-2)

Pmax CE

   

NS

GF-1.0

PVA-0

PVA-3.2

PVA-5.6

1112±91 55.1±0.9

969±30 60.1±0.7

1164±15 79.2±2.7

1181±41 83.6±4.4

1223±18 94.6±3.3

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    ELECTROCHEMICAL CHARACTERIZATION Figure S1 shows electrochemical response of cathodes with different separators in LSV and EIS experiments. 0

(a)

-50

j (A/m2)

-100 NS

-150

GF-1.0 -200

PVA-0 PVA-3.2

-250

PVA-5.6 -300 -1.7

-1.2

-0.7

-0.2

0.3

0.8

E (V) vs. NHE 4

(b)

-Zim (Ω)

3 2 NS GF-1.0 PVA-0 PVA-3.2 PVA-5.6

1 0 3

5

7

9

11

13

15

17

19

Zre (Ω)

(c)

15

-Zim (Ω)

12 9 NS GF-1.0 PVA-0 PVA-3.2 PVA-5.6

6 3 0 5

10

15

20

25

30

35

40

Zre (Ω) Figure S1. (a) LSV curves and EIS plots for different separators in (b) initial cycles and (c) end cycles.    

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MFC PERFORMANCE Figure S2 shows the voltage output of MFCs with different separators over the 20 cycle operation period and the maximum voltage at each cycle. 0.8

(a)

0.7

NS

GF-1.0

PVA-0

PVA-3.2

20

25

PVA-5.6

0.6

Ecell (V)

0.5 0.4 0.3 0.2 0.1 0 0

5

10

15

30

35

Time (day) 0.8

(b) GF-1.0

PVA-0

PVA-3.2

PVA-5.6

Maximum Ecell (V)

NS 0.7

0.6

0.5

0.4 0

5

10

15

Cycle number (n)

20

Figure S2. (a) Voltage output over entire operation period during cycling; (b) Maximum cell voltage at each cycle. Figure S3 shows optical images of the cathodes after 17 MFC fed-batch cycles (32 d). The NS sample showed thicker biofilms on the surface of the cathode structure. Biofilm was ob   

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    served on cathodes with PVA separators, but the amount of biofilm formed was less than what was observed on the NS cathode. The biofilm impregnated within the GF-1.0 separator and mixed with stray glass fibers on the surface of cathode.

(a)

(b) NS

(c)

GF-1.0

PVA

Figure S3. Optical images of biofilms on cathodes after 20 cycles; (a) from top left to right: NS, GF-1.0, PVA-0, PVA-3.2, PVA-5.6; (b) biofilm on NS cathode; (c) biofilm on cathode with PVA5.6.

References

1. Logan, B. E.; Cheng, S. A.; Watson, V. J. Environ. Sci. Technol. 2007, 41, 3341. 2. Cheng, S. A.; Liu, H.; Logan, B. E. Electrochem. Commun. 2006, 8, 489. 3. Zhang, X. Y.; Cheng, S. A.; liang, P.; Huang, X.; Logan, B. E. Bioresource Technology 2011, 102, 372. 4. Zhang, X. Y.; Cheng, S. A.; Wang, X.; Huang, X.; Logan, B. E. Environ. Sci. Technol. 2009, 43, 8456. 5. Kim, J. R.; Cheng, S.; Sang-Eun, O.; Logan, B. E. Environ. Sci. Technol. 2007, 41, 1004. 6. Logan, B. E.; Hamelers, B.; Rozendal, R.; Schröder, U.; Keller, J.; Freguia, S.; Aelterman, P.; Verstraete, W.; Rabaey, K. Environ. Sci. Technol. 2006, 40, 5181. 7. Cheng, S.; Liu, H.; Logan, B. E. Environ. Sci. Technol. 2006, 40 (7), 2426.

   

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