Page 273 - Biofuels Refining and Performance

252 Chapter Nine

is involved in the hydrogen fuel cell process, no NO are generated. Since
x
sulfur is a poison to fuel cells, it has to be removed from fuel before feed-
ing it to a fuel cell; therefore, no SO is generated in the fuel cell.
2
The trend toward portability and miniaturization of computing and
communication devices has created a requirement for very small and
lightweight power sources that can operate for long periods of time with-
out any refill or replacement. Also, advances in the medical sciences are
leading to an increasing number of electrically operated implantable
devices like pacemakers, which need power supplies to operate for an
extremely long duration (years) without maintenance, as any mainte-
nance would necessitate surgery. Ideally, implanted devices would be
able to take advantage of the natural fuel substances found in the body
[7–8]. The idea of a biofuel cell that can generate electricity based on var-
ious metabolic processes occurring in our own cells is very appealing. A
biofuel cell converts chemical energy to electrical energy by the catalytic
reaction of microorganisms. Most microbial cells are electrochemically
inactive, and electron transfer from microbial cells to the electrode requires
mediators such as thionine, methyl viologen, methylene blue, humic acid,
and neutral red. In recent years, mediatorless microbial fuel cells have
also been developed; these cells use electrochemically active bacteria
(Shewanella putrefaciens, Aeromonas hydrophila, etc.) to transfer elec-
trons to the electrode. Amajor advantage of the biofuel cell over the hydro-
gen fuel cell is the replacement of expensive and precious platinum (Pt)
as a catalyst by much cheaper hydrogenase enzymes. A brief description
of the development and state of the art of hydrogen and biofuel cells is
presented in this chapter.

9.2 Fuel Cell Basics
Although fuel cells have been around for more than a century (William
Grove in 1839 first discovered the principle of the fuel cell), it was not
until the National Aeoronautics and Space Administration (NASA)
demonstrated its potential applications in providing power during space
flights in the 1960s that fuel cells became widely known and the indus-
try began to recognize the commercial potential of fuel cells. Initially,
fuel cells were not economically competitive with existing energy tech-
nologies; but with advancements in fuel cell technology, it is now becom-
ing competitive for some niche applications [6].
The main components of a fuel cell are anode, anodic catalyst layer,
electrolyte, cathodic catalyst layer, and cathode, as shown in Fig. 9.1.
The anode and cathode consist of porous gas diffusion layers, usually
made of high-electron-conductivity materials such as thin layers of
porous graphite. The most common catalyst is platinum for low-
temperature fuel cells. Nickel is preferred for high-temperature fuel