67
6.3 Hydrogen Oxidation Reaction
The hydrogen oxidation reaction (HOR) and the hydrogen evolution reaction (HER)
are by far the most thoroughly investigated electrochemical reaction systems.
Driving the the HER with renewable sources of energy can lead to a sustainable
source of hydrogen fuel that can be stored, transported, and used in a zero-emission
fuel cell of combustion engine. In the reverse process, hydrogen oxidation can be
used as an efficient fuel in fuel cells. Achieving high energetic efficiency requires
the use of a catalyst to minimize the overpotential necessary to drive the HOR and
the HER. Platinum is the best-known catalyst for both processes as it requires very
small overpotentials even at high reaction rates in acidic solutions. However, the
scarcity and high cost of Pt limit its widespread technological use.
The HOR and the HER are a pair of highly reversible reactions:
H g
H aq
e
2
2
2
( ) ←→
( )+
+
−
(6.12)
with the equilibrium potential of that depends on the partial pressure of hydrogen
gas P H2 , absolute temperature T, and pH of the solution:
E
RT F
P
R T F p
=
( )
–
/
ln
– .
/
2
2303
2
H
H
(6.13)
At a Pt electrode, the reaction proceeds in steps, as proposed at the beginning of the
twentieth century [37–39]:
H
H
Tafel dissociative adsorption DA
ad
2
2
( )
(6.14)
H
H
H e Heyrovsky oxidative adsorption OA
ad
2
+
+
( )
+
−
(6.15)
H
H e Volmer oxidative desorption OD
ad
+
−
+
( )
(6.16)
The reactions are written for the HOR, while reverse reactions occur for the
HER. For the process of H 2 oxidation, two possible pathways can be distinguished:
Tafel-Volmer pathway, in which the Tafel dissociative adsorption of hydrogen molecules at Pt surface occurs without electron transfer, followed by two separate
Volmer one-electron oxidation processes of adsorbed hydrogen, and HeyrovskyVolmer pathway, in which Heyrovsky dissociative adsorption and one-electron oxidation occur simultaneously, followed by a Volmer one-electron oxidation of H ad . In
acidic solutions, it can be shown that the Tafel mechanism is responsible for high
activities of hydrogen oxidation at small overpotentials (<40 mV), whereas the
Heyrovsky mechanism becomes significant at higher overpotentials (>100 mV).
6.3 Hydrogen Oxidation Reaction
6.3 Hydrogen Oxidation Reaction
The hydrogen oxidation reaction (HOR) and the hydrogen evolution reaction (HER)
are by far the most thoroughly investigated electrochemical reaction systems.
Driving the the HER with renewable sources of energy can lead to a sustainable
source of hydrogen fuel that can be stored, transported, and used in a zero-emission
fuel cell of combustion engine. In the reverse process, hydrogen oxidation can be
used as an efficient fuel in fuel cells. Achieving high energetic efficiency requires
the use of a catalyst to minimize the overpotential necessary to drive the HOR and
the HER. Platinum is the best-known catalyst for both processes as it requires very
small overpotentials even at high reaction rates in acidic solutions. However, the
scarcity and high cost of Pt limit its widespread technological use.
The HOR and the HER are a pair of highly reversible reactions:
H g
H aq
e
2
2
2
( ) ←→
( )+
+
−
(6.12)
with the equilibrium potential of that depends on the partial pressure of hydrogen
gas P H2 , absolute temperature T, and pH of the solution:
E
RT F
P
R T F p
=
( )
–
/
ln
– .
/
2
2303
2
H
H
(6.13)
At a Pt electrode, the reaction proceeds in steps, as proposed at the beginning of the
twentieth century [37–39]:
H
H
Tafel dissociative adsorption DA
ad
2
2
( )
(6.14)
H
H
H e Heyrovsky oxidative adsorption OA
ad
2
+
+
( )
+
−
(6.15)
H
H e Volmer oxidative desorption OD
ad
+
−
+
( )
(6.16)
The reactions are written for the HOR, while reverse reactions occur for the
HER. For the process of H 2 oxidation, two possible pathways can be distinguished:
Tafel-Volmer pathway, in which the Tafel dissociative adsorption of hydrogen molecules at Pt surface occurs without electron transfer, followed by two separate
Volmer one-electron oxidation processes of adsorbed hydrogen, and HeyrovskyVolmer pathway, in which Heyrovsky dissociative adsorption and one-electron oxidation occur simultaneously, followed by a Volmer one-electron oxidation of H ad . In
acidic solutions, it can be shown that the Tafel mechanism is responsible for high
activities of hydrogen oxidation at small overpotentials (<40 mV), whereas the
Heyrovsky mechanism becomes significant at higher overpotentials (>100 mV).
6.3 Hydrogen Oxidation Reaction