Page 137 - Entrophy Analysis in Thermal Engineering Systems

CHAPTER NINE

Entropy and fuel cells

9.1 Introduction
The power-generating systems that we have discussed so far are heat
engines, which produce useful work by converting a portion of heat sup-
plied through a high-temperature reservoir, fuel combustion, or a hot
stream. Fuel cell is a device that produces electrical energy by converting
the chemical energy of a fuel through a series of electrochemical reactions.
A fuel cell consists of the following main segments: anode, cathode, and
electrolyte (Fig. 9.1) The primary fuel of most fuel cells is hydrogen, which
is supplied to the anode. The cathode is fed by an oxidizer, i.e., oxygen or
air. The reactions taking place in the anode and cathode of a fuel cell include
¼
Anode : H 2 +O ! H 2 O+2e (9.1)
1

Cathode : O 2 +2e ! O ¼ (9.2)
2
The electrochemical reaction taking place in the anode between the hydro-
gen and oxygen ions leads to water production and a release of electrons. In
the cathode, oxygen reacts with the electrons, which are transferred from the
anode through an external circuit. On the other hand, the oxygen ions pro-
duced in the cathode travel across the electrolyte to the anode. The net cell
reaction can be expressed as

1
H 2 + O 2 ! H 2 O (9.3)
2
The byproducts of the cell reaction are water, electricity, and heat generated
due to various losses, including activation polarization, ohmic, and concen-
tration losses. Further details about the operation of various types of fuel cell
can be found in Refs. [1, 2].
Fuel cells have been regarded as highly efficient devices with applications
in transportation and power sectors. High-temperature fuel cells such as
solid oxide fuel cells (SOFC) are excellent candidate for stationary power

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Entropy Analysis in Thermal Engineering Systems 131
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