Microbial Fuel Cells in Power Generation and Extended Applications - Biotechnology in China: Biofuels and Bioenergy - page 172

precursor. The PVA-ACFs possess lotus-root-like axially meso- and macroporous
structures and a high specific surface area of 2128 m 2 /g, which greatly facilitates
mass transportation and bacterial adsorption. Using this ACF anode, Liu et al. [ 32 ]
successfully enriched a large amount of electrochemically active microorganisms
for MFC operation. A carbon mesh anode was recently reported [ 33 ] to also
significantly reduce cost while enhancing electricity production compared to
conventional carbon cloth. Granular activated carbon (GAC) was also used as the
anode, which as a three-dimension electrode material offer even more available
surface than flat-type carbon materials, and thus contribute to high bacterial
adhesion and low anode resistance [ 19 , 34 ].
Composite Electrodes
Despite their good surface properties, however, carbon-based materials mostly
have lower conductivity and mechanical durability compared to metal materials
like silver, copper, gold, and aluminum, which are also common choices for anode
materials. Notably, metal electrodes also have many shortcomings such as low
specific surface area and, most of all, relatively high cost. Therefore, one feasible
strategy is to combine metal and carbon materials to make a composite electrode.
Indeed, metals have been frequently incorporated into electrode design as a current
collector from the carbon-based electrode. One good example is the graphite fiber
brush anode that consists of a conductive non-corrosive metal rod surrounded by
graphite fibers [ 35 ]. This unique structure incorporates both the merits of graphite
fibers for bacterial adsorption and metal for electron transfer. As a consequence, a
high power density of 73 W/m 3 was achieved in an air-cathode MFC. However,
pretreatment of the fibers using high-temperature ammonia gas was usually needed
to create a more positively-charged surface, which could be costly and unfavorable
for large-scale application. To address this, Feng et al. [ 36 ] managed to develop a
less energy-intensive treatment method that combines the use of heat treatment
and acid soaking. The combined heat and acid treatment significantly improved the
power production to 1370 mW/m 2 , which is 25% higher than that using only acid
treatment and 7% higher than only heat treatment. Such a graphite filter brush
anode was also recently used by Zhang et al. [ 37 ] to enhance power generation in a
membrane-less air-cathode MFC, and a maximum power density of 154 W/m 3 was
obtained.
Surface Modification
Another approach to improve anode performance is surface modification by
coating metal, polymers or even immobilizing mediators onto the electrode sur-
face. Zhang et al. [ 38 ] constructed an air-cathode MFC using a mesoporous carbon
modified anode, which demonstrated distinctly better performance than a bare
carbon paper anode. The peak power density increased by 81%, the startup time
of MFC was 68% shorter, and the anode resistance decreased from 300 to 99 X.
A grapheme-modified stainless steel mesh anode was also recently developed and
evaluated for MFC operation for the first time [ 39 ]. This novel system delivers a
maximum power density of 2668 mW/m 2 , which is 18 times larger than that using
plain stainless steel mesh and is 17 times larger than that with a polytetrafluoro-
ethylene (PTFE) modified anode. In addition, attempts have also been made to