Hydrogen Utilization: Fuel Cells - Hydrogen Generation, Storage, and Utilization - page 151

was first discovered by Christian Friedrich Schönbein [3]. Faced with major
environmental issues in the use of fossil fuels for applications such as elec-
tricity generation and automobiles, hydrogen fuel cells provide an attractive
alternative due to its high efficiency and clean byproduct (water) [7]. Sig-
nificantly increased efforts have been made recently to advance the fuel cell
technology and understanding of related fundamental issues.

9.1.2 Work Principles of Fuel Cells

A fuel cell produces electricity from electrochemical oxidation of the fuel.
An electrochemical cell usually consists of two electrodes that allow the
overall reaction described below to take place:

A + B → C + D ,

(9.1)

ox 1 red 1 red 2 ox 2

where A ox1 is the oxidant, usually O 2 in the fuel cells, B red1 is the reducer,
usually H 2 or other hydrocarbon fuels, C red2 is the reductive, usually H 2 O in
fuel cells, and D ox2 is usually CO 2 when the fuel is hydrocarbon. The Gibbs
free energy change of a chemical reaction is related to the cell voltage via:

∆ G =− nF U ∆

0 ,

(9.2)

where n is the number of electrons involved in the reaction, F is the Faraday
constant, and ΔU 0 is the voltage of the cell for thermodynamic equilibrium
in the absence of a current flow.
There are many types of fuel cells, but they all consist of an anode (nega-
tive side), a cathode (positive side), and an electrolyte that allows charges to
move between the two electrodes of the fuel cell [7]. Electrons are drawn
from the anode to the cathode through an external circuit, producing direct
current electricity. Fuel cells come in a variety of sizes. Individual fuel cells
produce relatively small electrical potentials, about 0.7 V, so cells are
“stacked” or placed in series to increase the voltage and meet specific appli-
cation requirements.
Figure 9.1 shows a schematic of a typical hydrogen/oxygen fuel cell and
its reactions based on the proton exchange membrane [4]. This fuel cell is
constructed using polymer electrolyte membranes (notably Nafion) as proton
conductor and platinum (Pt)-based materials as catalyst. Its noteworthy fea-
tures include low operating temperature, high power density, and easy
scale-up, making it a promising candidate as the next-generation power
sources for transportation, stationary, and portable applications. It is manu-
factured as a stack of identical repeating unit cells comprising a membrane