The resistance for hydrogen gas to anode but

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The resistance for hydrogen gas to anode but

The microbial fuel
cells (MFC) can change  chemical energy  to electricity by anodic aspiration methods.The microbial
fuel cell (MFC) is a device that mimics the action of PEM fuel cells with microbes attached
on anode and/or cathode as the electrode catalysts. The power density level has role in energy generation unit.Logan and
Fan estimated the ultimate power generation by MFC to be 17e19 W/m.

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To lessen its
cost,in future the use of MFC to get energy from  wastewater is
recommended  .An estimation of the power generation from wastewaters can be made by taking an example of a US city of population of one million population
equivalent with producing 380L sewage per day.The wastewater stream has energy content of 7500 ± 100kJ/ m3,Then the total energy produced in the city’s wastewater is 2.8 109kJ/d. This energy
can power about 79,000 houses in the city.                                                          
 The market competitor for MFC is the PEM FC, whose power density can
easily reach 140 kW/m3.
The main differences between PEMFC and
MFC are the nature of anode and cathode  catalyst: (noble)
metal catalysts for the PEM FC and
biocatalyst for later. The anodic catalyst
in PEM FC is clusters of  metal clusters of size nm2  
but catalyst in MFC is  unit of biological cells, of sizes  mm2.The substrate used at anode for PEM FC is  H2and O2 gas, which produce water , for MFC it  is organic matters dissolved in
water. The  substrate from
anodic solution to be reacted on anode surface would experience 104 times
higher resistance for the MFC than for the PEM FC as there is no transport resistance for hydrogen gas to anode
but the transport resistance of MFC depends on the bulk mixing and the structure of
biofilm because diffusivities of the
dissolved organic compounds are greater in magnitude.On the cathode side, if both PEM FC and MFC use metal catalyst on cathode, the product of PEM FC is liquid water which is drained directly from cathode surface , while that of MFC is various oxidized compound that have to be transported across
liquid film in bulk.Since the microbes
in MFC need time for preadaption to an eco-environment while the metal
catalyst in PEM FC needs no pre-adaption. The microbes in MFC can be adapted to
various feeds with varying compositions while the
metal catalyst in PEM FC is very specific to the substrate and is  poisoned by any impurity added. The MFC handles liquid medium while PEM FC has to keep
the cathodic partially wet to allow the oxygen gas in, the optimal growth
temperature for microbes in MFC is  room
temperature while PEM FC needs higher temperature as it has to work on  metal catalyst.

If MFC and anaerobic biofilm
reactors are compared both utilize
microbes to degrade pollutants. They used 
high-strength wastewaters like both utilize
biological cells for biocatalyst; the bulk transports for pollutants to biofilm
are controlled by the same methods;
There are some differences between them,
including cell-cell electron transport
mechanism for bio?lm reactor and the cell-solid mechanism for the MFC.  
The MFCs were applied for handling of wastewaters or
wastes. Example Ma et al. used a
two-chambered, 1.1-L anion exchange membrane MFC using swine wastewaters and
potassium ferricyanide as anolyte and catholyte to generatemaximum power density of 13 mW/m2. The MFC was
also applied for utilizing numerous industrial
wastewaters, such as distillery wastewater, food, dairy and alcohol
wastewaters, petroleum refinery wastewater, bio refinery effluent,  Ping et al. 
used anion MFC to remove boron ions
from wastewaters and produce electricity.                                                                                                                        The MFC is designed in many different ways. A typical example of  the electrode fabrication is the
one who fabricated air cathode using  rolling-press method with  carbon powders. The novel designs proposed are
majorly to make MFC of practical sizes. For example, the MFC was made by sandwich form. Khaifbadam et al. used 9.64-L MFC
to treat municipal wastewaters with the anode carbon felt being used as a
filter to remove particulates in wastewaters.Efficiently designed
stacks can lead to practical MFC with minimum engineering efforts.
The membrane-less MFC’s are widely studied as they are economic. In MFC  for enhanced
cell performance chemicals are added.                                                                                                 
MFC should be used in applications of without the need of very high
electrical current ?uxes, such as in environmental pollution control units. In
particular, the integrating system with MFC should be the focus of further
research.

Conclusion:
The proposal of using MFC as an energy generation unit was
against by an order-of-magnitude argument based on power density level using
PEM fuel cells as the market competitor. The challenges and prospects for further research and development needs
on the MFC technologies are highlighted.

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