Microbial Fuel Cells in Power Generation and Extended Applications - Biotechnology in China: Biofuels and Bioenergy - page 173

modify the carbon material with metal. For example, Sun et al. [ 40 ] prepared
a gold-modified carbon paper anode by homogeneously sputtering a layer of
gold onto a carbon paper matrix. This electrode exhibited considerably higher
conductivity, electrochemical capability, and biocompatibility, and contributed to
a rapid reactor startup and an elevated current output of the MFC.
In addition, nano-materials such as carbon nanotubes (CNTs), due to their
unique structure, high conductivity, and high surface-to-volume ratio, have also
recently received considerable research interest for potential application in
MFCs. However, because of their inherent cellular toxicity [ 41 ], CNTs are
usually more preferred for electrode modification than direct electrode fabri-
cation. It has been reported that the CNT-polyaniline (PANI) composite
material displayed significant enhancement in both mechanical strength and
conductivity [ 42 ] and showed good compatibility to neutral electrolyte [ 43 ].
Qiao et al. [ 44 ] successfully introduced such a PANI composite anode material
into an E. coli inoculated MFC to improve power generation. This enhanced
performance could be mainly attributed to the protective effects of PANI and
the larger available surfaces and high electron transfer properties of CNTs. Zou
et al. [ 45 ] constructed a novel polypyrrole (PPy)-coated CNT composite anode
by coating the synthesized PPy-CNT solution onto a plain carbon paper. This
PPy-CNT modified anode showed distinctly better performance than a plain
carbon paper in terms of internal resistance and biocompatibility. The MFC
exhibited a good performance even in the absence of any mediator. It is likely
that the PPy polymers might contain some molecular units similar to redox
mediators and form a redox active biocompatible layer that enhances electron
transfer, indicating a great promise of using such composite materials as MFC
anode. However, such composites usually have a relatively low CNT content,
and thus the surface-to-volume ratio and electron transfer capability may still
be limited. Moreover, a simple sprayed thin coat of CNTs are liable to loss
during long-term operations. To improve this, Sun et al. [ 46 ] fabricated a novel
multilayer CNT-modified anode by using a layer-by-layer self-assembly
method. This multilayer CNT-modified anode provided a free-standing three-
dimensional network structure of interwoven nanotubes, which enabled more
specific surface area and a more than four-fold decline in interfacial charge
transfer resistance. Operating in an MFC, the system demonstrated a 20%
enhancement in power production compared to that with a plain carbon paper
anode. In most studies, exoelectrogenic bacteria attach onto the CNT anode to
form a biofilm and transfer electrons to the anode. In fact, the electron transfer
can be accelerated simply by improving the contact between the bacteria and
electrode. This was recently demonstrated by Liang et al. [ 47 ], who incorpo-
rated CNTs directly into the anodic biofilm matrix to constitute a composite
biofilm.

Such a strategy enabled more close contact between bacteria and

electrode, and thus significantly enhanced the electron transfer rate and

improved the MFC performance.