Hydrogen Utilization: Fuel Cells - Hydrogen Generation, Storage, and Utilization - page 153

of hydrogen, the kinetics of this reaction is very fast on Pt catalysts, and in
a fuel cell, the oxidation of hydrogen at higher current densities is usually
controlled by mass-transfer limitations. The oxidation of hydrogen also
involves the adsorption of the gas onto the catalyst surface followed by dis-
sociation of the molecule and electrochemical reaction to two hydrogen ions
as follows:

2 Pt s ( )+ → − H Pt H + − Pt H

(9.6)

2 ads ads

Pt H − → + + H

+

e

−

Pt

( ) ,

(9.7)

ads s

where Pt (s) is a free surface site and Pt-H ads is an adsorbed H atom on the Pt
active site. The overall reaction of hydrogen oxidation is:

+ − 0

H → 2 H + 2 e U = 0 V .

(9.8)

2

The electrocatalytic oxygen reduction reaction (ORR) on catalyst surfaces
[RHE = reversible hydrogen electrode] is one of the most widely studied
reactions in electrochemistry. Its fundamental and technological importance
is based on the fact that the oxygen/water half-cell reaction is a strongly
oxidizing and ubiquitous redox couple [7]. Combined with an electron-
supplying redox process, a direct electrochemical conversion of hydrogen,
the overall Gibbs energy of reaction into electrical potentials is achieved.
This conversion is the scientific basis for electrochemical reactions in fuel
cells or metal-air batteries. The ORR is also used in oxygen depolarization
cathodes (ODC) in modern chlorine technologies, in which it replaces the
hydrogen evolution process to improve electrical efficiencies. The reverse
ORR process, that is, the evolution of oxygen from water, is crucial for
efficient water (photo)-electrolysis into hydrogen or in metal electrodeposi-
tion processes in the semiconductor industry [4, 5].
The efficiency of a fuel cell can be calculated from the Gibbs free energy
(ΔG) and the enthalpy change (ΔH) of the electrochemical reaction. Ideally,
the free energy of the reaction can be completely converted into electrical
energy and the efficiency is given by:

W nF Uo ∆

∆

∆
∆

G
H

T S
H

∆
∆

e
H

ε = (

) = ( ) =

= − 1 , (9.9)

∆ H

where W e is the electrical work performed, and ΔS is the isothermal entropy
change of the reaction. TΔS corresponds to the reversible heat exchanged
with the external environment. The change in entropy of the reaction (ΔS)
depends strongly on the reactants and products [4, 5].