Biomedical Engineering Reference
In-Depth Information
2.2.2 Cathodes
Cathode Materials
The cathode is a highly challenging aspect of MFC design because a three-
phase interface of air, catholyte, and electrode is generally involved. In most cases,
the cathode is the limiting factor of power generation, thus a careful selection of
cathode material and configuration is of critical importance. An MFC cathode
should ideally have a high redox potential, high conductivity and strength, good
capability for proton capture, a sufficient interfacial area to enable electrochemical
reactions, and meanwhile an acceptable manufacturing cost. The bacterial
adsorption capability is of more critical importance when it comes to biocathode
MFCs. These stringent demands for electrode properties is driving an intensive
search for efficient and low-cost cathode materials.
At present, similar materials to the anode are frequently employed in the
cathode, such as various carbon-based materials and metals. A combined cathode
of graphite fiber brush and graphite granules was recently employed to favor the
development of biocathodes and to further improve MFC performance [
48
].
Another important material being widely used for oxygen reduction in MFC
cathodes is activated carbon, which provides even higher surface specific areas
than graphite. He et al. [
19
] built an MFC using a U-shaped cathode chamber,
which was filled with GAC and an inserted carbon fiber to constitute a three-
dimensional cathode. This configuration resulted in a significant decrease in the
cathodic charge-transfer resistance and enhanced power generation. Deng et al.
[
17
] developed an MFC using ACF felt as the cathode, which yielded markedly
elevated power density compared with the use of carbon felt or platinum-loaded
carbon paper. The ACF felt has an extremely high volumetric specific surface area
of 0.6 9 10
7
m
2
/m
3
. It is likely that the large surface area of activated carbon
compensates for its relatively poor oxygen reduction activity compared to a
platinum-catalyzed cathode. This ability for substantially increase power pro-
duction of the MFC without metal catalysts and relatively low manufacturing cost
indicates that activated carbon-based materials could be a promising cathode
material for large-scale application.
Cathode Catalysts
A major limitation to the MFC system is the reduction of molecular oxygen by
the cathode. Thus, since carbon-based materials mostly have poor catalytic
activity, an additional catalyst is required in most cases to enhance the oxygen
reduction rate at the cathode. One most commonly used catalyst is platinum (Pt),
which is highly reactive but costly and sensitive to poisoning. Several researches
suggested that some non-noble metals, such as pyrolysed iron(II) phthalocyanine
and cobalt tetramethoxyphenylporphyrin could be potential substitutes for
platinum [
49
]. However, most of these non-noble metals suffer from long-term
instability. In the search for novel catalysts, manganese dioxide (MnO
2
) was also
recently found to significantly improve the catalytic activity of the cathode [
50
].
To explore into the capability of manganese oxide (MnO
x
) as a cathodic catalyst,
Liu et al. [
51
] prepared nano-structured MnO
x
of various controlled sizes and
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