16
Low temperatures used to run most of electrochemical reactions, and consequently low rates, are compensated by the possibility to increase the reaction rates
by applied potential. The rates of some reactions can be increased several orders of
magnitude by small change of potential. Such an increase in the rate of chemical
reaction would require very high temperatures. Important features of electrocatalytic reactions, facilitated by application of the electrode potential, include (i) high
reaction rates that can be achieved, (ii) high selectivity at defined potentials, and (iii)
the unique direct energy conversion in fuel cells that are likely to become one of the
major sources of clean energy. The main events in an electrocatalytic reaction are
adsorption/desorption, electron transfer, and bond breaking/formation.
Electrocatalytic reactions involve the strong interactions of reactants and/or
intermediates with the electrode surface. As a consequence, the rate of these reactions shows pronounced dependence on the nature of the electrode material. The
plots of the catalyst activity (reaction rate) against a descriptor of the adsorption
properties such as the adsorption energies or adsorption bond strength of the reactant or reaction intermediates pass through a maximum. These are volcano plots that
are generally based on the Sabatier principle [8], which states that the interactions
between the catalyst and the adsorbate should be neither too strong nor too weak. If
the interaction is too weak, the adsorbate will fail to bind to the catalyst, and no
reaction will take place. If the interaction is too strong, the catalyst gets blocked by
adsorbate or product that fails to desorb. Volcano correlations are important since
they help designing new catalysts. For the electrocatalytic reaction:
A
B
C
+ →
→
−
e
(2.1)
The rate expression can be written as:
I const C
RT
E RT
A
B
=
(
) (
)
1
1
0
–
exp –
/
–
/
θ
∆
∆G
G B
α
α
(2.2)
Assuming formation of B to be the rate-determining step, if B is adsorbed on the
electrode surface, it will form with a lower activation Gibbs energy than in the
absence of adsorption. For the limiting cases when θ B ~ 0 and θ B ~ 1, Eq. 2.2 at
constant potential, E, becomes ln i ~ − ΔG B or ln i ~ ΔG B , respectively.
As illustrated schematically in Fig. 2.1, the logarithm of the reaction rate varies
linearly with ΔG, increasing from weak adsorption (positive) to very strong adsorption (negative), which predicts a linear decrease of the reaction rate as result of the
blocking effect.
As an example, volcano plot for the activity of different metal catalysts for oxygen reduction reaction (ORR) versus the respective metal–oxygen bond strength is
shown in Fig. 2.2. Analogous three-dimensional plots can also be built against two
different descriptors, such as the adsorption bond strength of the two intermediates.
In that case, the plot is generally shown as a contour plot and is called a volcano
surface [9]. Figure 2.2 shows volcano surface for the activity of different metal catalysts for ORR versus both metal–O and metal–OH binding energy.
2 Electrocatalytic Reactions
Low temperatures used to run most of electrochemical reactions, and consequently low rates, are compensated by the possibility to increase the reaction rates
by applied potential. The rates of some reactions can be increased several orders of
magnitude by small change of potential. Such an increase in the rate of chemical
reaction would require very high temperatures. Important features of electrocatalytic reactions, facilitated by application of the electrode potential, include (i) high
reaction rates that can be achieved, (ii) high selectivity at defined potentials, and (iii)
the unique direct energy conversion in fuel cells that are likely to become one of the
major sources of clean energy. The main events in an electrocatalytic reaction are
adsorption/desorption, electron transfer, and bond breaking/formation.
Electrocatalytic reactions involve the strong interactions of reactants and/or
intermediates with the electrode surface. As a consequence, the rate of these reactions shows pronounced dependence on the nature of the electrode material. The
plots of the catalyst activity (reaction rate) against a descriptor of the adsorption
properties such as the adsorption energies or adsorption bond strength of the reactant or reaction intermediates pass through a maximum. These are volcano plots that
are generally based on the Sabatier principle [8], which states that the interactions
between the catalyst and the adsorbate should be neither too strong nor too weak. If
the interaction is too weak, the adsorbate will fail to bind to the catalyst, and no
reaction will take place. If the interaction is too strong, the catalyst gets blocked by
adsorbate or product that fails to desorb. Volcano correlations are important since
they help designing new catalysts. For the electrocatalytic reaction:
A
B
C
+ →
→
−
e
(2.1)
The rate expression can be written as:
I const C
RT
E RT
A
B
=
(
) (
)
1
1
0
–
exp –
/
–
/
θ
∆
∆G
G B
α
α
(2.2)
Assuming formation of B to be the rate-determining step, if B is adsorbed on the
electrode surface, it will form with a lower activation Gibbs energy than in the
absence of adsorption. For the limiting cases when θ B ~ 0 and θ B ~ 1, Eq. 2.2 at
constant potential, E, becomes ln i ~ − ΔG B or ln i ~ ΔG B , respectively.
As illustrated schematically in Fig. 2.1, the logarithm of the reaction rate varies
linearly with ΔG, increasing from weak adsorption (positive) to very strong adsorption (negative), which predicts a linear decrease of the reaction rate as result of the
blocking effect.
As an example, volcano plot for the activity of different metal catalysts for oxygen reduction reaction (ORR) versus the respective metal–oxygen bond strength is
shown in Fig. 2.2. Analogous three-dimensional plots can also be built against two
different descriptors, such as the adsorption bond strength of the two intermediates.
In that case, the plot is generally shown as a contour plot and is called a volcano
surface [9]. Figure 2.2 shows volcano surface for the activity of different metal catalysts for ORR versus both metal–O and metal–OH binding energy.
2 Electrocatalytic Reactions