Page 287 - Biofuels Refining and Performance

266 Chapter Nine

average power of 7 kW with a peak power rating of 12 kW at 27.5 V.
A disadvantage of the AFC is that it is very sensitive to CO present in
2
the fuel or air. The alkaline electrolyte reacts with CO and severely
2
degrades the fuel cell performance, limiting their application to closed
environments, such as space and undersea vehicles, as these cells work
well only with pure hydrogen and oxygen as fuel.
Electrodes. A significant cost advantage of alkaline fuel cells is that
both anode and cathode reactions can be effectively catalyzed with non-
precious, relatively inexpensive metals. The most important character-
istics of the catalyst structure are high electronic conductivity and
stability (mechanical, chemical, and electrochemical). Both metallic (typ-
ically hydrophobic) and carbon-based (typically hydrophilic) electrode
structures with multilayers and optimized porosity characteristics for the
flow of liquid electrolytes and gases (H and O ) have been developed. The
2
2
kinetics of oxygen reduction in alkaline electrolytes is much faster than
in acid media; hence AFCs can use low-level Pt catalysts (about 20% Pt,
compared with PEMFCs) on a large surface carbon support [20].
Performance. The AFC development has gone through many changes
since 1960. To meet the requirements for space applications, the early
AFCs were operated at relatively high temperatures and pressures. Now
the focus of the technology is to develop low-cost components for AFCs
operating at near-ambient temperature and pressure, with air as the oxi-
dant for terrestrial applications. This has resulted in lower perform-
ance. The reversible cell potential for an H and O fuel cell decreases by
2
2
0.49 mV/ C under standard conditions. An increase in operating tem-
perature reduces activation polarization, mass transfer polarization, and
ohmic losses, thereby improving cell performance. Alkaline cells operated
at low temperatures (~70 C) show reasonable performance.
Pure hydrogen and oxygen are required in order to operate an AFC.
Reformed H or air containing even trace amounts of CO dramatically
2
2
affects its performance and lifetime. There is a drastic loss in performance
when using hydrogen-rich fuels containing even a small amount of CO 2
from reformed hydrocarbon fuels and also from the presence of CO in
2

the air (~350 ppm CO in ambient air). The CO reacts with OH (CO
2
2
2
2
2OH → CO 3 H O), thereby decreasing their concentration and thus
2
reducing the reaction kinetics. Other ill effects of the presence of CO are:
2
Increase in electrolyte viscosity, resulting in lower diffusion rate and
lower limiting currents.
Deposition of carbonate salts in the pores of the porous electrode.
Reduction in oxygen solubility.
Reduction in electrolyte conductivity.