60
usually has value close to 1.0 V vs reversible hydrogen electrode (RHE), not exceeding 1.1 V. This potential is the starting point for the oxygen reduction as cathodic
reaction in electrochemical processes. For oxygen cathode in a fuel cell, the loss of
potential is 0.3–0.4 V. Reducing this loss in cathode potential and cell voltage is a
great challenge of electrocatalysis.
The open-circuit potential is due to a mixed potential as a result of simultaneous
occurrence of cathodic (O 2 reduction) and some side anodic reactions that compete
with it; the rest potential is going to be established in between the equilibrium
potentials of these competing reactions. It has been proposed that relevant anodic
reactions could be: either oxidation of impurities from the solution [8, 9] or chemisorbed oxygen-containing species, as OH or anodic dissolution of the electrode [1].
Platinum metal electrodes, silver, and some metal oxides can be placed in one
group that reduces oxygen to water directly or in parallel pathway. The second
group, which is represented by gold, reduces O 2 predominantly to hydrogen peroxide [1]. Based on the use of the rotating ring-disk methodology (See Sect. 7.4.3), it
was possible to follow reduction of O 2 on the disk and to monitor the H 2 O 2 production on the ring to distinguish weather oxygen reduction proceeds through hydrogen
peroxide as an intermediate or through direct four-electron multiple step reduction
without H 2 O 2 as an intermediate. There are several reaction schemes used to analyze
rotating ring- disk data.
Reaction pathways, rather than reaction mechanisms, were often analyzed, since
in many cases, particularly for a four-electron O 2 reduction, formulation of the latter
proved too complex. These analyses are based on data obtained by a rotating ringdisk technique that enables the quantitative determination of some of the reaction
intermediates. Since the first proposition by Damjanovic et al. [8], several reaction
schemes were proposed for analyzing the ring-disk data and calculating the rates of
various reaction steps. The formulation of the species entering these schemes was
partly based on the results of isotopic experiments. One of the difficulties in determining the correct scheme is that the number of experimentally obtained quantities
from the ring-disk measurements is usually not sufficient to determine all the
parameters of the model. Consequently, simple schemes were considered by several
authors. Wroblowa et al.’s [10] scheme, which has two fewer rate constants, is used
most often to analyze experimental ring-disk data. Oversimplification in some
schemes has had undesirable effects. Anastasijevic et al. [11] recently proposed a
general scheme for analyzing the ring-disk measurements, which include nearly all
the possible intermediates discussed in the literature.
The essential problem in determining the correct mechanism according to the
schemes is the mismatch in the number of experimentally obtained parameters from
the ring-disk measurements, and the number of possible pathways and intermediates. None of the authors were able to successfully solve the reaction mechanism
because the number of variables overpasses the number of calculated parameters
that are obtained by measuring.
Despite the problems in determining the exact values of all rate constants, the
careful analysis of the experimental data collected by frequently used electrochemical
techniques as: steady-state polarization, cyclic voltammetry, rotating disk electrode,
6 Important Electrocatalytic Reactions
usually has value close to 1.0 V vs reversible hydrogen electrode (RHE), not exceeding 1.1 V. This potential is the starting point for the oxygen reduction as cathodic
reaction in electrochemical processes. For oxygen cathode in a fuel cell, the loss of
potential is 0.3–0.4 V. Reducing this loss in cathode potential and cell voltage is a
great challenge of electrocatalysis.
The open-circuit potential is due to a mixed potential as a result of simultaneous
occurrence of cathodic (O 2 reduction) and some side anodic reactions that compete
with it; the rest potential is going to be established in between the equilibrium
potentials of these competing reactions. It has been proposed that relevant anodic
reactions could be: either oxidation of impurities from the solution [8, 9] or chemisorbed oxygen-containing species, as OH or anodic dissolution of the electrode [1].
Platinum metal electrodes, silver, and some metal oxides can be placed in one
group that reduces oxygen to water directly or in parallel pathway. The second
group, which is represented by gold, reduces O 2 predominantly to hydrogen peroxide [1]. Based on the use of the rotating ring-disk methodology (See Sect. 7.4.3), it
was possible to follow reduction of O 2 on the disk and to monitor the H 2 O 2 production on the ring to distinguish weather oxygen reduction proceeds through hydrogen
peroxide as an intermediate or through direct four-electron multiple step reduction
without H 2 O 2 as an intermediate. There are several reaction schemes used to analyze
rotating ring- disk data.
Reaction pathways, rather than reaction mechanisms, were often analyzed, since
in many cases, particularly for a four-electron O 2 reduction, formulation of the latter
proved too complex. These analyses are based on data obtained by a rotating ringdisk technique that enables the quantitative determination of some of the reaction
intermediates. Since the first proposition by Damjanovic et al. [8], several reaction
schemes were proposed for analyzing the ring-disk data and calculating the rates of
various reaction steps. The formulation of the species entering these schemes was
partly based on the results of isotopic experiments. One of the difficulties in determining the correct scheme is that the number of experimentally obtained quantities
from the ring-disk measurements is usually not sufficient to determine all the
parameters of the model. Consequently, simple schemes were considered by several
authors. Wroblowa et al.’s [10] scheme, which has two fewer rate constants, is used
most often to analyze experimental ring-disk data. Oversimplification in some
schemes has had undesirable effects. Anastasijevic et al. [11] recently proposed a
general scheme for analyzing the ring-disk measurements, which include nearly all
the possible intermediates discussed in the literature.
The essential problem in determining the correct mechanism according to the
schemes is the mismatch in the number of experimentally obtained parameters from
the ring-disk measurements, and the number of possible pathways and intermediates. None of the authors were able to successfully solve the reaction mechanism
because the number of variables overpasses the number of calculated parameters
that are obtained by measuring.
Despite the problems in determining the exact values of all rate constants, the
careful analysis of the experimental data collected by frequently used electrochemical
techniques as: steady-state polarization, cyclic voltammetry, rotating disk electrode,
6 Important Electrocatalytic Reactions