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Enzymatic Biofuel Cells

 What would it be like if you 

could recharge your cell 
phone battery instantly by 
pouring your soft drink into it? 
Such applications may be a long way 
off, but the U.S. Air Force Office of 
Scientific Research is investing in 
such a future now. Under a Multi 
University Research Initiative, 
university professors from around the 
country are now focused on a fiveyear research program to look at the 
technical challenges surrounding a 
fuel cell that will run on such simple 
sugars as those found in our everyday 
foodstuffs.
The challenges are great. Most 
fuel cells in the world today run on 
hydrogen. However, as the fuel gets 
more complex, this oxidation process 
becomes vastly more complicated. 
Once carbon atoms are in the fuel, 
carbon monoxide poisoning of 
typical fuel cell catalysts becomes 
problematic. Researchers are turning 
to the natural world in an effort 
to see how sugars are oxidized by 
animals to produce power.
Using enzymes (nature’s catalysts) 
seems to be the answer, since they do 
not suffer from the contamination 
problems that more traditional 
metallic catalysts suffer from. They 
are also incredibly abundant and 
cheap to produce, something that the 
wine and detergent industries have 
known for decades. But applying 
enzymes to power electronics is very 
different from getting enzymes to 
clean our clothes. For one, enzymes 
do not like to give up electrons as 
easily as metal catalysts do, which 
means that generating an electric 
current from enzymes is much 
tougher. Enzymes can be made 
to give up their electrons with 
mediators, but using mediators can 
cause other problems in the fuel cell. 
Enzymes also are not used to staying 
put. In animals, enzymes are floating 
freely in the cells of the body, but 
to work in a fuel cell, they have to 
be put in a specific place and stay 
there, a process that scientists call 
immobilizing the enzymes. Finally, 
naturally occurring enzymes do not 
last that long. A typical enzyme in 
the human body lasts only a couple 
of days, but to be effective in a laptop 
or an automobile, an enzyme is going 
to have to last for months or years 
before needing replacement.
The benefits of the technology 
are as big as the risks, however. 
Enzymes, as we mentioned before, 
are cheap and plentiful. They are 
also green, and can be grown in 
quantity whenever they are needed, 
as opposed to the metal catalysts, 
which need to be mined and 
purified using expensive and less 
environmentally friendly processes. 
They are also “selective,” a word 
that scientists use to describe an 
enzyme’s ability to work with a very 
specific fuel, and only that fuel, so 
that the byproduct of one oxidation 
step could be the fuel for another 
enzyme. By doing this step, enzymes 
could conceivably reproduce what 
animals already know how to do: 
convert the sugars into just water 
and carbon dioxide. After all, there 
is as much energy in one jelly donut 
as you can find in 77 cell phone 
batteries. If you can get that energy 
out, it could have pretty, ahem, sweet 
consequences.
The basic enzymatic biofuel 
cell contains many of the same 
components as a hydrogen/oxygen 
fuel cell: an anode, a cathode, and 
a separator. However, rather than 
employing metallic electrocatalysts 
at the anode and the cathode, 
the electrocatalyst used are 
oxidoreductase enzymes. This is a 
class of enzymes that can catalyze 
oxidation–reduction reactions. 
Since these enzymes are selective 
electrocatalysts, the separator could 
be an electrolyte solution, gel, or 
polymer. Figure 1 shows a schematic 
of a generic biofuel cell oxidizing 
glucose as fuel at the bioanode and 
reducing oxygen to water at the 
biocathode.
Biofuel cells were first introduced 
in 1911 when Potter cultured yeast 
and E. Coli cells on platinum 
electrodes,
1
 but it was not until 1962 
that the enzymatic biofuel cell was 
invented employing the enzyme 
glucose oxidase to oxidize glucose at 
the anode.
2
 Over the last 45 years, 
many improvements have been 
made in enzymatic biofuel cells and 
those can be found in several review 
articles.
3-6
 However, there are still 
several main issues to consider with 
biofuel cells. These include short 
active lifetimes, low power densities, 
and low efficiency due to normally 
only incorporating a single enzyme


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