Microbial Fuel Cells in Power Generation and Extended Applications - Biotechnology in China: Biofuels and Bioenergy - page 170

conventional up-down configurations. As a consequence, a maximum power
density of 29.2 W/m 3 was obtained in this system under a continuous operation
mode. While it is necessary to continuously supplement electron acceptors
(oxygen or hexacyanoferrate) into the cathode chamber in a two-chamber MFC, a
single-chamber air-cathode upflow MFC was found to be more cost-efficient and
yield a higher power density of 50.2 W/m 3 at tubular configuration [ 9 ].

2.1.3 Stacked Setup

Single MFC units can be assembled as a stack either in series or in parallel to
increase the overall output voltage or current [ 20 ]. However, stacking multiple
MFCs together in series can result in problems such as voltage reversal and contact
voltage losses [ 21 ]. Liu et al. [ 22 ] recently designed a novel stacked configuration
by physically bridging two parallel packed MFCs through an extra cation
exchange membrane (CEM). This extra CEM facilitated the proton transfer, as the
protons can move more conveniently to the cathode chambers that sandwich
the anode chamber. The total internal resistance of stacked MFCs was decreased
by connecting the circuits of individual reactors in parallel. As a consequence, the
maximum specific power density increased. In addition, only an air-saturated
buffer was used as the catholyte without addition of catalysts in this system; it thus
holds great promise for practical application. In another study, Zhuang and Zhou
[ 23 ] joined two single MFCs in series via a ball valve, which was employed to
control the conductive substrate flow between the twin cells. The individual MFC
units shared a feed passage and fuel continuously flowed into each cell through a
simple water distribution system. Because energy loss would occur due to parasitic
current flow under a substrate cross-conduction effect, the electrode spacing was
reduced to effectively lower the voltage loss. By optimizing the connection, a
minimal voltage loss of about 26.3% was achieved in such a system. To further
lower the voltage loss, Zhong et al. [ 24 ] developed a baffle stacking MFC, in
which a series of overflow plates and baffles were properly set between the units to
minimize the parasitic energy loss caused by the substrate cross-conduction effect.
A voltage loss of only 8.1% was obtained. Furthermore, the continuous flow mode
of high-strength molasses wastewater in this system is also helpful in avoiding cell
reversal caused by fuel starvation.

2.1.4 Mini-MFCs

While most researches have focused on scaling up of MFCs toward practical
applications, efforts are also underway to fabricate micro-sized MFCs (mini-
MFCs). Due to their small volume and low substrate consumption, mini-MFCs
could be especially useful for powering miniature devices and hold great promise
for potential application in medical and communication devices and in screening
of electricigens as well as electrode materials [ 25 ]. Ringeisen et al. [ 26 ] fabricated