Cells, an Alternative to Conventional Fuel Cells
Imagine running your
cellphone and mp3 player on alcohol, sounds surprising? Biofuel cells
will make it a reality in the near future. Here we explore the
tremendous possibilities of biofuel cells.
In October 2008, Japanese scientists from the Sony Corporation
developed a bio-battery that generates electricity from glucose using
enzymes as catalysts. When the researchers stacked four of the cells
together, they achieved a power output of 100 milliwatts, enough to run
an mp3 player or a remote controlled car.
Biological fuel cells (biofuel cells) use biocatalysts, which include
bio-molecules such as enzymes or even whole living organisms (microbes)
to catalyze oxidation of biomass-based materials for generating
electrical energy. A typical biofuel cell consists of an anode and a
cathode separated by a proton-conducting membrane. A renewable fuel,
such as a sugar, is oxidized by microorganisms at the anode, generating
electrons and protons. The protons migrate through a membrane to the
cathode while the electrons are transferred to the cathode by an
external circuit. The electrons and protons combine with oxygen at the
cathode to form water.
Biofuel cells are highly renewable and capable of using naturally
available biomass as fuel, as a result, they are an excellent
alternative to conventional fuel cells (and batteries) that are plagued
by non-renewability, non-implantability, size/weight, operating
conditions (high temperature, acidity and toxicity), waste issues, and
logistics. They are able to operate at a low substrate concentration
which can be even at the micromolar level.
Types of biofuel cells
There are a number of biologically-based fuel cell formats:
Cells which use a primary fuel (usually an organic waste such as corn
husks) and generate a material such as hydrogen, which is then used as
a secondary fuel within a conventional hydrogen/oxygen fuel cell. It is
to be noted that there is no direct generation of power by biological
Cells which generate electricity directly from an organic fuel such as
glucose, using either enzymes or complete microorganisms.
Cells which combine the utilization of photochemically active systems
and biological moieties to harvest the energy from sunlight and convert
it into electrical energy.
Biofuel cells which directly convert fuel to electricity utilize
biological moieties such as enzymes or living cells to directly
generate power from the chemical energy contained within various
organic or inorganic species. A typical fuel cell of this format has
two electrodes separated by a semi-permeable membrane placed in a
solution. A biological species such as a microbial cell (E.coli) or
enzyme (glucose oxidase) can either be in solution (or as a suspension)
within the anodic compartment of the cell—or alternatively be
immobilized at the electrode. Once a suitable fuel is introduced, it
becomes either partially or totally oxidized at the anode and the
electrons released by this process are used to reduce oxygen at the
cathode. The presence of mediator compounds such as methylene blue
increase the efficiency of the cell. Enzyme-based fuel cells remain as
a focus for research due to the high turnover rate and high
biocatalysis rate associated with enzymes.
The progress in the immobilization of enzymes at electrode surfaces has
facilitated efficient electron transfer between the enzyme and the
electrode, this indicates that the presence of oxygen in the anodic
environment would not affect the bioelectrocatalytic activity of the
enzyme and the enzyme-based electrodes could match the performance
attainable at platinum electrodes.
Photochemical biofuel cells use respiratory fuel as it makes use
microbes. For instance, a cell containing biological moieties in both
the anode (cyanobacteria) and cathode (bilirubin oxidase) compartments,
the cyanobacteria catalyze the photooxidation of water with the
production of electrons that are passed to the carbon felt anode and
are thereby made available for the reduction of oxygen at the cathode.
Although there is a huge potential for further improvement, the
development of biofuel cells for practical applications is still in its
infancy. However, based on their utility, here are some of the areas in
which they could find an application.
Transport and energy generation: At present fossil fuels are driving
the world’s power needs but they pose environmental hazards
and also cannot be used indefinitely. The utilization of biofuel cells
with carbohydrates as a power source would, if they could be developed,
help to mitigate at least some of these problems. It has been
calculated that a liter of concentrated carbohydrate solution
could power a car for 25–30 km. If a car running on
concentrated carbohydrate solution is fitted with a 50 liter tank, it
could travel over 1,000 km without refueling.
Implantable power sources: Biofuel cells can potentially be run in
living systems, since the oxygen and fuel required for their operation
can be taken from their immediate environment and this offers great
potential as power sources for implantable medical devices. For
example, a biosensor for glucose has been developed to give a
measurement of the glucose concentration in the range of 1–80
mM. A similar sensor for lactate has also been developed. Biofuel cells
can be used in pacemakers, catheters, defibrillators, active delivery
devices (insulin pumps etc.), drug delivery systems, middle ear hearing
devices, and more. Ideally an implanted biofuel cell would use a
biological metabolite fuel source such as glucose or lactate, both of
which are readily available in physiological fluids such as blood.
Numerous fuel cells have been shown to generate power by oxidation of
compounds found in wastewater streams. This could help in the removal
of organic compounds from the waste stream for the generation of
The most relevant application involves the use of a biofuel cell to
trickle-charge lithium-ion batteries used in consumer electronics such
as cell phones, laptops and PDAs. They could be used as power sources
for environmental sensors, pollution monitors, wildlife tracking
sensors, crop quality control sensors etc. Biofuel cell will also
enhances power supplies in remote areas as readily available fuel
sources such as plant saps with high levels of sugars could
be used as a fuel.
Although some specialist devices like short lifetime implantable power
devices could now be considered feasible. There is a need for a
continuous research effort to see large scale use of biofuel cells.