(609g) Dependence of Electrochemical Performance On Anode Surface Roughness in Microbial Fuel Cells

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, R_a =88.6 ± 13.4 nm and the root mean value R_q=116.3
± 8.4 nm. For the smoother surface
(Fig. 2-B), R_a =11 ± 0.5 nm and R_q =14.0 ± 1.0 nm. Fig. 3 shows the typical 3D topography
of the two different glassy carbon surfaces.

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

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

Figure
4. Complex
plane plot for (A) rough (R_a~100 nm) and (B) smooth glassy
carbon anode (R_a~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.

Figure
5. SEM
images for biofilms on (A) rough (R_a~100 nm) and (B) smooth
glassy carbon anode (R_a~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.