Biomedical Engineering Reference
In-Depth Information
Photocatalytic Cathodes
Another relatively new type of cathode is the photocatalytic cathode, which is
also attracting increasing attention in MFCs for energy generation and pollutant
degradation. Semiconductor photocatalysts have long been applied to catalyzing
electron transfer in photoelectrochemical cells, which convert solar energy into
electricity [
58
]. Lu et al. [
59
] incorporated a semiconductor mineral of natural
rutile as the cathodic catalyst into an MFC design. This novel design led to
additional photoelectrochemical reactions at the cathode, which significantly
enhanced electron transfer to the terminal electron acceptors in the MFC and
enabled an almost doubled power density (12.03 W/m
3
) under light irradiation
compared to that in the dark. Ding et al. [
60
] evaluated the performance of a rutile-
modified photocatalytic cathode in wastewater treatment. Rapid reduction of
methyl orange with concomitant electricity production was achieved when
exposing the cathode to visible light illumination. The internal resistance of the
rutile-cathode MFC decreased significantly from 1378 X in the dark to 443.4 X in
the light, demonstrating the practical feasibility of applying rutile as an efficient
photocatalyst to enhance the cathodic electron transfer process. By constructing a
TiO
2
-coated paper photocatalytic cathode, Yuan et al. [
61
] also significantly
lowered the cathodic electron-transfer resistance in an MFC and considerably
accelerated p-nitrophenol degradation.
2.3 Separators
Although the development and success of membrane-less air-cathode MFCs have
demonstrated that, at least in some situations, a highly efficient MFC without the
use of a proton exchange membrane (PEM) is possible [
2
], the need to prevent
substrate and oxygen crossover between the MFC chambers and to place the
electrodes closely together means that some kind of separator is ultimately in-
dispensible for efficient and sustainable operation of MFCs [
6
]. Conventional
PEMs have many drawbacks such as constrained proton diffusion, high internal
resistance, limited mechanical strength, and a high cost, which presents an
important bottleneck to MFC application. In the past few years, a variety of
separator materials have been investigated to substitute PEMs and to alleviate
these limitations. These materials include anion exchange membranes, bipolar
membranes, ultrafiltration/microfiltration membranes, micro-porous fabricates and
some composite separators [
6
,
62
]. Among the numerous options, the porous
fabricates like cloth and glass fibers seem to be more attractive for practical use
due to their excellent filterability, durability and, above all, low cost. Zhuang et al.
[
11
] developed a separator-cathode assembly (SCA) using canvas cloth instead
of membrane as separator material. A mixture of conductive nickel (Ni)-based
paint and MnO
2
catalyst was coated onto the separator to increase the electrical
conductivity and catalytic activity of SCA. In fed-batch mode, the tubular air-
chamber MFCs equipped with the Ni-SCA generated maximum power densities of
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