287923 Dependence of Electrochemical Performance On Anode Surface Roughness in Microbial Fuel Cells

Thursday, November 1, 2012: 10:30 AM
322 (Convention Center )
Zhou Ye1, Junbo Hou2, Michael W. Ellis1 and Bahareh Behkam1, (1)Mechanical Engineering, Virginia Tech, Blacksburg, VA, (2)Institute for Critical Technology and Applied Science, Virginia Tech, Blacksburg, VA

Introduction

Microbial fuel cells (MFCs) utilize electrochemically active microorganisms such as Geobacter spp. and Shewanella spp. to oxidize organic fuels for electricity generation. MFCs are envisioned as clean and sustainable sources of energy for sensor systems in remote locations, miniature robots, and even wastewater treatment systems. However, the low power density prevents the wide application of MFCs in industry and other areas. This limitation is caused by multiple reasons, including little understanding of the mechanism of electron transfers between microorganisms and electrodes, low electrochemical activity of electrodes, low activity of microorganisms, and poor system designs. Previous studies have demonstrated that modification of the surface characteristics of the anode can have significant effects on the performance of MFCs; however, the underlying cause of this improvement has not been completely understood. Most surface modification methods not only changed the surface topography of the anode, but also changed the material or functional groups on the surface; therefore, the effect of change in surface topography alone has not yet been quantified.

In this work, we have developed model anodes that are chemically homogenous but each have a different surface roughness characteristic in order to systematically explore the effect of surface roughness on bacteria-anode interaction. We determine the relation between the anode surface topography and the biofilm growth, and correlate them to the electrochemical performance of the anodes. By quantifying the influence of anode surface topography, we plan to develop anodes with more ideal surface roughness to improve the power output of MFCs.

Materials and Methods

Cell Culture:

Shewanella oneidensis MR-1 (ATCC 700550) from -80 ⁰C glycerol stock was aerobically cultured on a Lysogeny Broth (LB) agar plate at 30 ⁰C overnight. A single colony was isolated from the plate and transferred to 10 ml of LB solution in a 125 ml flask. The culture was grown at 30 ⁰C and 150 rpm. When the optical density (OD600) of the culture reached 0.5, it was harvested and centrifuged at 3000g for 10 min. Subsequently, the bacteria were rinsed in defined medium (DM) twice and were finally suspended in 300 ml of DM with lactic acid as carbon source in a 1000 ml flask. The final culture was left at 30 ⁰C and 150 rpm to grow until OD600 reached 0.1, and then harvested as test medium.

Anode Preparation:

SIGRADUR® G glassy carbon plates (HTW Hochtemperatur-Werkstoffe GmbH Inc, Thierhaupten, Germany) were used as the anode material. A Buehler EcoMet® 3 Grinder-Polisher was used to create spatially uniform roughness of the orders of magnitude of 10s of nm and 100s of nm on two separate anodes. Scanning electron microscope (SEM) was used to show the surface topography of the two different anodes and atomic force microscopy (AFM) was used to quantify the roughness of the two anodes.

Electrochemical Performance Test:

A three-electrode system with graphite as the counter electrode and Ag/AgCl as the reference electrode was used (Fig. 1) and the electrochemical behavior of each of the glassy carbon anodes (i.e. working electrodes) was monitored in-situ. Pure nitrogen was bubbled into the medium during the test to replace the dissolved oxygen in the medium. A ModuLab system (Solartron Analytical, AMETEK Advanced Measurement Tehnology Inc, Oak Ridge, TN) was used as the potentiostat to measure the electrochemical performance of the anodes via the following measurement methods: potentiostatic, potentiodynamic, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV).

Figure 1. Three-electrode set up containing four glassy carbon electrodes with different roughness as working electrodes, Ag/AgCl electrode as reference electrode, graphite (or platinum) electrode as counter electrode and a nitrogen tube.

Biofilm Assay:

The anodes were gently removed from the three electrode system and were subsequently submerged into 2.5% glutaraldehyde at 4 ⁰C for 2 h. The samples were then gently rinsed in deionized (DI) water and  dehydrated in 20%, 40%, 60%, 80%, and 100% ethanol solution. The samples were air dried for 24 hours before SEM imaging.

Results and Discussion

The surface roughness of the textured anodes (shown in Fig. 2.) was measured using AFM. For the rougher surface (Fig. 2-A), the arithmetic mean of roughness, Ra =88.6 13.4 nm and the root mean value Rq=116.3 8.4 nm. For the smoother surface (Fig. 2-B), Ra =11 0.5 nm and Rq =14.0 1.0 nm. Fig. 3 shows the typical 3D topography of the two different glassy carbon surfaces.

             

(A)

(B)

Figure 2. SEM images for (A) rough (Ra~100 nm) and (B) smooth glassy carbon anode (Ra~10 nm) electrodes. Scale bars are 10 µm.

 

(A)

(B)

Figure 3. AFM 3D images for (A) rough (Ra~100 nm) and (B) smooth glassy carbon anode (Ra~10 nm) electrodes.

The EIS results (Fig. 4) show that the rough glassy carbon has much smaller polarization resistance than the smooth one. The polarization resistance of the rough glassy carbon after 11 h in test medium is around 850 Ω, whereas the polarization resistance of the smooth one after 11 h is 12,500 Ω, which indicates that the rough glassy carbon is much more electrochemically active than the smooth one. During the experiment period, the polarization resistance of the rough glassy carbon reduced significantly; however, the smooth one did not change much. This indicates that there is much more biological growth (biofilm) on the surface of the rough glassy carbon, which can improve the electrochemical performance as time progresses.

 

(A)

(B)

Figure 4. Complex plane plot for (A) rough (Ra~100 nm) and (B) smooth glassy carbon anode (Ra~10 nm) electrodes. Z' and Z" are respectively the real and imaginary parts of experimental impedances.

The SEM images of the biofilms taken after the electrochemical tests (Fig. 5) also demonstrate that the biofilms grew more on the rough glassy carbon's surface than on the smooth glassy carbon's surface. This may be due to better adhesion of bacteria to the rough surface, or better electron transfer between the bacteria and the rough surface.

                           

(A)

(B)

Figure 5. SEM images for biofilms on (A) rough (Ra~100 nm) and (B) smooth glassy carbon anode (Ra~10 nm) electrodes. Scale bars are 20 µm.

Conclusion

Our experimental findings indicate that anode surface roughness can significantly affect the electrochemical performance of MFCs. We have demonstrated that there is a substantially larger biomass growth on a rougher anode surface, which contributes to better performance of the anode. Results from this work can be used towards development of anodes with optimal surface topographical features for a better electron transfer rate and an improved power density.


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