Analysis of oxygen reduction reaction pathways on Co3O4, NiCo2O4, Co3O4–Li2O, NiO, NiO–Li2O, Pt,...
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Accepted Manuscript
Title: Analysis of oxygen reduction reaction pathways onCo3O4, NiCo2O4, Co3O4-Li2O, NiO, NiO-Li2O, Pt, and Auelectrodes in alkaline medium
Author: Trunov A. M.
PII: S0013-4686(13)00933-XDOI: http://dx.doi.org/doi:10.1016/j.electacta.2013.05.028Reference: EA 20516
To appear in: Electrochimica Acta
Received date: 4-10-2012Revised date: 9-5-2013Accepted date: 11-5-2013
Please cite this article as: T.A. M., Analysis of oxygen reductionreaction pathways on Co3O4, NiCo2O4, Co3O4-Li2O, NiO, NiO-Li2O,Pt, and Au electrodes in alkaline medium, Electrochimica Acta (2013),http://dx.doi.org/10.1016/j.electacta.2013.05.028
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Trunov A.M.
Analysis of oxygen reduction reaction pathways on Co3O4, NiCo2O4,
Co3O4-Li2O, NiO, NiO-Li2O, Pt, and Au electrodes in alkaline medium
Odessa National Maritime University, 34 Mechnikov St., Odessa, UA-65029, Ukraine
Abstract
The aim of present investigation is to theoretically identify parameters which are the root reason
for differentiation of oxygen reduction reaction (ORR) on two different pathways with formation on
oxide electrode materials either ОН─ or НО2─ ions. Theoretical analysis of the ORR data on cobalt
and nickel oxides was performed. New original ORR mechanism was proposed. The mechanism is
based on a concept of multistage electrochemical process with a slow chemical reaction stage.
Triad of requirements describing properties of electrode materials which fully determine pathways
of ORR was formulated. On a specific electrode material ORR proceeds with formation of ОН-
ions when three following requirements are simultaneously fulfilled. First, oxide or hydroxide with
atoms, which could change an effective positive charge as a result of electrochemical process, shall
be present on electrode material surface at ORR range of potentials. Second, the electrode material
shall have a surface crystal structure which allows formation of oxygen molecule “bridge” between
two surface atoms with effective positive charge (the “bridged’ chemical structure may be described
as a surface binuclear oxide nanocluster). Third, electrochemical potential of transfer of oxide
atoms with effective positive charge from oxidized to reduced state shall be more positive than the
potential of formation of НО2- ions.
Analysis of structure of Co3O4, NiCo2O4 and Co3O4-Li2O oxides indicated that on these oxides
all three requirements are fulfilled. Therefore on Co3O4, Co3O4-Li2O and NiCo2O4 electrodes ORR
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proceeds via pathway with formation of ОН─ ions. On NiO and NiO-Li2O oxides only first and
second requirements are fulfilled. Therefore ORR proceeds with formation of НО2─ ions.
The triad of requirements formulated in a course of analysis of ORR on oxide electrodes was
successfully used in cases with single-crystal Pt and Au electrodes. On Pt and Au(100) electrodes
all three requirements are fulfilled, therefore ORR proceeds via pathway with formation of ОН─
ions. On Au(111) and Au(110) electrodes the first condition is not fulfilled, therefore on such
electrodes НО2─ ions are formed.
Keywords: Oxygen Reduction Reaction Pathways, Surface Binuclear Oxide Nanoclusters, Cobalt
Oxide, Platinum, Gold
1. Introduction
Oxygen reduction reaction (ORR) is used in many practical electrochemical energy conversion
devices (for example, in low temperature fuel cells with alkaline electrolyte). Nevertheless
complete understanding of the ORR is still not achieved. There is no clear understanding which
characteristics of metals are responsible for proceeding of ORR either with rupture of two oxygen
molecule bonds (e.g., on single-crystal Pt [1 - 3], Au(100) [3], and Ag [4])
O=O + 2 H2O + 4 e─ ↔ 4 OH─, (1)
or with rupture of only one bond (e.g., on single-crystal Au(111) and Au(110) [3])
O=O + H2O + 2 e─ ↔ HO–O─ + OH─. (2)
Similarly, there is no understanding which characteristics of oxides are responsible for ORR
following reaction (1) on Co3O4 [5] and NiCo2O4 [5, 6] electrodes, and reaction (2) on NiO [5 - 7].
The aim of present investigation is to theoretically identify parameters which are the root reason
for described above differentiation of ORR pathways on oxide electrode materials. To achieve this
goal the author re-interprets his experimental studies of ORR on Co3O4, NiCo2O4, NiO, Co3O4–
Li2O, and NiO–Li2O electrodes [5, 7]. In author’s opinion, a thorough analytical comparison of
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characteristics of these oxides shall lead to identification of parameters determining ORR
directions.
Participation of oxide ions in ORR was described in multiple publications for variety of
electrode materials: for example, CoOOH and Co(OH)3 [8]; RuO2 [9] and IrO2 [10] oxide coverings
on Ti electrodes; MnO, Mn3O4, and MnO2 [11]; Fe3O4 and FeOOH [12].
Authors of study [9] hypothesized that oxides participate in ORR and proposed a description of
this participation with complex [Ru(OH)3] reaction scheme.
In study [10] ORR scheme with participation of various complex ions (e.g.,
[IrOx(OH)y·2H2O]2─, [IrOx−2(OH)y+2]─, and [IrOx−2(OH)y+2]) functioning on an electrode surface
was proposed. A core of the initial complex is positive Ir(IV) ion. In ORR this ion transfers
electrons to chemisorbed oxygen molecule. In author’s opinion, the described above complexes
could be interpreted as mononuclear clusters.
Other approaches based on mononuclear clusters (MNC) were also described in the literature
for description of ORR process. For example, redox cycle with mononuclear Co(OH)3(H2O)6 ↔
Co(OH)2(H2O)7 clusters functioning on an electrode surface were proposed in the study [8]. The
ORR process of CoOOH electrode proceeds mainly with formation of ОН─, but some limited
quantities of HO2─ were reported in the very same experiments. An ORR scheme of elementary
steps in a form of circled redox cycle with participation of surface Co(II) mononuclear hydroxide
clusters was proposed in the study [8]:
CoII(OH)2(H2O)7 ↓
+ O2 → CoII(OH)2(H2O)7O2 ↓
+ e─ → [CoII(OH)2(O2─) (H2O)7]
─
↓ [CoIII(OH)3(HO2) (H2O)6]
─
↓+ H2O → CoIII(OH)2(HO2) (H2O)7 + OH─
↓
+ e─→ CoII(OH)2(H2O)7 + HO2─
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The HO2─ ions formed in the cycle serve as intermediates in the further process of electrochemical
reduction up to OH─ ions. Details of the latter part of the process were not presented in the
publication [8].
At the same time, authors of the study [8] highlighted absence of any stable intermediate in the
ORR process. This absence is a principal differentiating feature of the proposed ORR scheme with
MNC comparatively with other published ORR schemes (e.g., OHad and Oad stable intermediates
are expected to be formed in the ORR schemes proposed in [1] and [3]). Participation of
mononuclear hydroxides clusters in ORR is the reason for this feature. Analysis of peculiarities of
the redox cycle from study [8] became a basis for investigation of possibility of utilization of
concept of mononuclear or binuclear hydroxide nanoclusters for description of ORR process on
Co3O4, NiCo2O4, Co3O4-Li2O, NiO, NiO-Li2O, Pt, and Au electrodes.
In study [11] a new ORR scheme was proposed. A reaction pathway was formed by a fast
redox process (Mn4+ + e─ ↔ Mn3+) coupled to a slow chemical redox step (Mn3+---O2,ads → Mn4+
---
O2─). The latter is a rate-determining process for the whole ORR. Let’s note that in the proposed
scheme there is no formation of nanoclusters – such formation was not discussed and even
considered in the study [11].
The author considers all discussed above facts of oxide participation in ORR as examples of
ORR with slow chemical reaction (SCR) stage. During such slow stage positive ions of surface
oxide clusters are oxidized by an oxygen molecule. This approach is an original one as authors of
the previously mentioned studies [9-12] did not use this interpretation of the experimental data. An
existence of the interval of potentials between onset of cathode process and onset of anode process
is a sign of presence of SCR stage in the electrochemical ORR. Such effect of anode-cathode
polarization was observed on NiCo2O4 oxide based air electrode [13]. The author considers this
experimentally observed effect as a proof of presence of SCR stage in ORR on NiCo2O4 electrodes.
In author’s opinion, high values of activation energy (60 - 70 kJ mol-1[13]) of ORR observed in
experiments with NiCo2O4 oxide based air electrodes also indicate that there is a chemical reaction
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stage in the ORR.
Transfer of first electron to an oxygen molecule is considered in many ORR related publications
as a rate limiting stage (e.g., ORR on single-crystal Pt [1-3], Au [3], and Ag [4]). There is also an
opinion that the rate-limiting stage of ORR is a transfer of the second electron (ORR on IrO2 [10],
MnO2 [14]). Nevertheless, authors of both points of view consider chemical oxidation of
electrochemically reduced oxide as a fast stage of ORR.
In this study for interpretation of experimental data the author introduces and uses a new model
of ORR with {(H2O)x(OH)2Co-O-Co(OH)2(H2O)x} and {(H2O)x(OH)Ni-O-Ni(OH)(H2O)x} surface
binuclear oxide (SBNO) nanoclusters. These SBNO nanoclusters may be considered as
modifications of Co(OH)2(H2O)7 and Co(OH)3(H2O)6 mononuclear clusters [8].
According to the proposed concept, ORR consists of two consecutive parts. Initial stage is an
activation of SBNO nanoclusters. Electrochemical reduction of positive ions, which happens on
this initial stage, is a result of transfer of electrons within electrode materials with low energetic
barriers (activation energy of electrical conductivity for NiCo2O4 oxide is ca. 0.06 eV). The final
stage is innercluster chemical oxidation of reduced positive ion by an oxygen molecule. This stage
proceeds with high activation energy (e.g., 60 - 70 kJ mol-1 on NiCo2O4 electrodes [13]) and,
therefore, determines a rate of the whole ORR.
2. Experimental
Comparison and correlation of various physical and chemical parameters of Co3O4, NiCo2O4,
and NiO oxides (electric conductivity, thermo electromotive force, type of crystal structure, Fermi
level, concentration and mobility of charge carriers, effective charge of positive ions, X-ray
radiography density, parameters of crystal lattice) with directions of ORR (either reaction (1) or (2))
was performed in studies [5] and [7]. It was concluded that features of positive oxide ions are the
major factors which affects ORR pathways. Therefore, in this study the data from [5, 7] which
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reflect influence of positive oxide ions on ORR pathways were used. Table 1 presents the compiled
data on structure dimensions, electric conductivity and relative ratios of electrical currents
recalculated into fractions of reaction (1) and (2) products.
The oxides experimentally studied in [5] and [7] were prepared by annealing of hydroxides
obtained by neutralization of nitrate solutions with NH4OH base. In study [5] the hydroxides were
annealed at 360 C. In study [7] the synthesized oxides were annealed at 800 C in a pure form and
in mixture with lithium carbonate. Amount of lithium in the mixtures was determined with flame
photometry.
In the same studies, X-ray radiography demonstrated that Co3O4, Co3O4-Li2O, and NiCo2O4
samples had cubic spinel structure with lattice parameter of about 805-808 pm. NiO and NiO-Li2O
samples had cubic lattice with lattice parameter of 416-417 pm. The studies of ORR were
performed with rotating ring-disk electrode method. The electrode surface of 0.192 cm2 was used
with efficiency Nd = 0.41. The oxide powders mixed with 1-2% of paraffin filler were pressed in
stainless steel cylinders with 5 mm inner diameter. Assembled ring-disk electrode was washed with
bidistilled water and with 0.1 M KOH working solution. Potentials of disk and ring electrodes were
measured in relation to RHE (reversible hydrogen electrode in the same electrolyte).
Current on platinized platinum ring electrodes was determined at constant potential of 1.2 V.
The disk electrode was not preliminary activated. Polarization of electrodes was performed with a
potentiostat. Polarization was done with 0.05-0.1 V stages with corresponding delays at each
voltage until stabilization of electrical current. All measurements were performed at 20 C in
oxygen saturated 0.1 M KOH solution. Fractions of reaction products, P1 and P2, were calculated as
P1 = I1/(I1 + I2) for reaction (1) and P2 = I2/(I1 + I2) for reaction (2), where I1 and I2 are currents of
formation of ions OH─ and HO2─, correspondingly.
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3. Result and discussion
3.1. Interpretation of experimental data
Analysis of data presented in Table 1 resulted in following observations.
Several orders of magnitude increase of materials electro conductivity was observed for
electrodes prepared with lithium oxides additives. Nevertheless, such increase did not affect
character of ORR (did not change the corresponding values of P1 and P2 parameters). On cobalt
oxides OH─ ions are the principal reaction product irresponsibly of electro conductivity value.
Similarly, on nickel oxides HO2─ ions are the principal product.
Introduction of Ni2+ ions into spinel structure (NiCo2O4 vs Co3O4) also did not change ORR
products. This indicates that in the discussed case the ORR proceeds with participation of only
Co3+ ions.
Comparison of cobalt spinel and nickel oxide electrodes indicates that potential of ORR start is
more negative for NiO oxide (1.00 V vs 0.65 V, see Table 1).
The change of positive ion (Co3O4 spinel vs NiO) is a principal factor which affects character of
ORR. While on cobalt oxides the OH─ ions are the principal reaction product (P1 is in 85-90%
range, see Table 1), on nickel oxides the principal product is HO2─ ions (P1= 0% and P2 = 100%).
The similar description of ORR products was also reported in study [6].
Values of potentials of ORR start, Estart, on oxides Co3O4 and NiCo2O4 (1.00 and 1.05 V,
correspondingly; see Table 1) are close to theoretically determined value of oxygen electrode
potential, E, for reaction (1) in 0.1 M KOH. This potential is equal to 1.23 V vs RHE [15]. Such
closeness of the potentials Estart and E confirms that ORR on Co3O4 and NiCo2O4 oxides proceeds
according to reaction (1).
In the study [5] increase of fraction of HO2─ ions in ORR products from 10 to 30% was
observed in response to a shift of potential from 0.9 to 0.5 V. An analogous effect was reported for
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Co/CoOOH electrodes: a shift of potential from -0.3 to -0.5 V vs SSCE results in increase of
fraction of HO2─ ions from 2 to about 30% [8].
Potential of ORR start, Estart , on NiO oxide (0.65 V, see Table 1) is close to theoretically
determined value (0.765 V vs RHE [15]) of oxygen electrode potential, E, for reaction (2) in 0.1 M
KOH. Such closeness of the potentials Estart and E confirms that ORR on NiO oxide proceeds
according to reaction (2). Accumulation of reaction (2) products inside utilized NiO oxide powder
electrode also contributes to such closeness of the potentials.
All the above observations may be explained by analyzing peculiarities of structure and
electrochemical properties of cobalt and nickel oxides.
3.2. Theoretical ORR schemes and redox cycles
Interpretation of experimental data presented in the previous chapter clearly demonstrates that
ORR on Co3O4 and NiO oxide electrodes proceeds via different reaction pathways. Therefore in the
following subchapters each electrode material is addressed separately.
3.2.1. Distributions of Co3+ ions in spinel’s octahedrons
In elementary cell of cobalt spinel positive ions with higher charge (Co3+ in the discussed case)
occupy 16 from the 32 octahedral holes formed by oxygen ions. Positive ions with relatively low
charge (Co2+ and Ni2+) occupy 8 from the 64 tetrahedral holes. Therefore, positive ions in
tetrahedral sites are isolated from each other and can not participate in ORR: the process proceeds
with participation of Co3+ ions which are located in spinel’s octahedrons.
A simple geometric calculations indicate that lattice parameter of 808 pm (see Co3O4 cases in
Table 1) corresponds to О2- ionic radius of 135 pm. In Co3O4 case each octahedron has square
cross section where Co3+ ion is surrounded by four O2─ ions located in the square’s corners. It
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means that minimal distance between Co3+ ions in two unadjacent octahedrons (there is an
octahedron with unfilled hole between them) is equal to double ionic O2─ diameter (2 x 270 = 540
pm). Such ordered distribution of Co3+ ions in a spinel (16 Co3+ ions in 32 octahedron holes) may
be symbolized by “○●○●” chain, where “○” – octahedron with a hole, “●” – octahedron filled with
Co3+ ion.
In disoriented spinel distribution of Co3+ ions may result in “○●●○” formation, where Co3+ ions
may fill holes in adjacent octahedrons. A distance between two Co3+ ions from the adjacent
octahedrons is equal to the ionic O2─ diameter of 270 pm.
On oxide/electrolyte interface spinel’s octahedrons with two different distributions of Co3+ ions
also form two different cobalt ions surface distributions/structures. These two structures create
conditions for formation of either surface mononuclear oxide (SMNO) nanoclusters or surface
binuclear oxide (SBNO) nanoclusters, subchapters 3.2.2 and 3.2.3 discuss each of these cases
separately.
3.2.2. ORR redox cycles on Co3O4 and NiCo2O4 oxide electrodes with SBNO
The distance of 270 pm between Co3+ ions in two adjacent octahedrons (“○●●○” chain) is
reasonably close to oxygen molecule size of ~240 pm (which may be estimated as double length of
121 pm bond in an oxygen molecule). Such similarity of distances provides conditions for
formation of SBNO nanoclusters {NC_Co} (realization of the “Bridge model” [15]) from two
SMNO nanoclusters [–O2─–CoIII(OH)2(H2O)x] and an oxygen molecule:
Figure 1 presents ORR pathways for Co3O4 and NiCo2O4 oxides with participation of SBNO
nanoclusters in the form of redox cycle. The {Ox} and {Red} states of SBNO nanoclusters in
{NC Co}
{(H2O)x(OH)2CoIII ---O=O---CoIII(OH)2(H2O)x} └───O2-───┘
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following below descriptions of presented in Figure 1 redox cycle correspond to CoIII and CoII ions
in reaction
CoIII (OH)3 + e─ ↔ CoII(OH)2 + OH─ (3)
For the Co(OH)3 /Co(OH)2 couple the standard potential E○ = 0.17 V vs NHE was reported in
study [8], the value may be recalculated as 1.00 V vs RHE. In 0.1 M КОН the corresponding
potential is also equal to E = 1.00 V.
Note that ORR on oxide electrodes is the most efficient when concentrations of CoIII and CoII
ions (representing concentrations of SBNO nanoclusters in {Ox} and {Red} states) are equal [20].
Such situation is realized in 0.1 M KOH at potential equal to the potential E of reaction (3).
Comparison of reported above E with Estart value of ORR start from the Table 1 (it is also equal to
1.00 V) leads to conclusion that start of ORR on oxides Co3O4 and NiCo2O4 also happens when
concentrations of CoIII and CoII ions are equal.
In author's opinion, an increase of fraction of HO2─ ions in products of ORR on Co3O4 and
NiCo2O4 electrodes (mentioned at the "Interpretation of experimental data" chapter) is a
consequence of action of electrochemical reaction (3). According to the Nernst equation change of
potentials during the cathode polarization should result in change of ratio of CoIII and CoII ions in
the {NC Co} nanocluster. There is statistically negligible quantity of SBNO nanoclusters with Co
III ions at electrode potentials less than 0.8 V vs RHE. Therefore ORR partially proceeds according
to redox cycle scheme with CoII ions (see "Introduction" chapter). Overall, fraction of HO2─ ions in
the ORR products gradually increases with increase of cathode polarization (see "Interpretation of
experimental data" chapter).
A core of the initial nanoclusters is always formed by a positive ion. This ion performs a
function of surface ADC. The main function of ADC is transfer of electrons during ORR from the
electrode material to oxygen molecules. In redox cycle of ORR on Co3O4 electrode SBNO
nanoclusters may function as two-electron ADC. To underline such prominent feature SBNO**
abbreviation is used below in the text.
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The ORR redox cycle with participation of SBNO** nanoclusters (see Figure 1) are described by
following reaction stages:
Stage I. Chemisorption of an oxygen molecule and formation of SBNO** nanoclusters with oxygen
molecule bridge. Oxygen chemisorption on stage I happens because of weak ion-dipole
interaction between positive oxide ions and induced dipole moment of an oxygen molecule.
This interaction is represented in Figure 1 by dashed lines between the oxygen and cobalt
atoms.
Stage II. First electrochemical activation of SBNO** nanoclusters — transfer from {Ox} into
{Red} state at E = 1.00 V (see reaction (3)), and formation of first transition state from
SBNO** nanocluster and an oxygen molecule.
Stage III. Innercluster chemical reduction of the chemisorbed oxygen molecule into first transition
state with breaking of first bond between the oxygen atoms, formation of peroxide group
(– O─ –– O─ –), transfer of SBNO** nanoclusters from {Red} into {Ox} state.
Stage IV. Second electrochemical reduction of SBNO** nanoclusters — transfer from {Ox} into
{Red} state at E = 1.00 V (see reaction (3)), hydration of the peroxide group, and formation of
second transitional state from SBNO** nanocluster and the peroxide group.
Stage V. Innercluster chemical reduction of the peroxide group in second transition state with
breaking of the second bond between oxygen atoms and transfer of SBNO** nanoclusters
into its initial {Ox} state.
Redox cycle of ORR includes two electrochemical reactions (Stages II and IV) with consequent
slow chemical reactions (Stages III and V). Such characteristic feature of the redox cycle
corresponds to a type of ORR on Co3O4 electrodes previously reported by the author in
experimental study [13].
In the described above redox cycle stages III and V are the most important. Formation of
bridging group Co(III)–(O22─)–Co(III) (see Stage III) was described in study [8]. Theoretical
calculations indicate that formation of the bridged peroxide is exothermic process with 65 kJ mol-1
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enthalpy change. In Co(III)–(O─ –– O─)–Co(III) bridging group oxygen atoms are pulled from each
other due to their interaction with Co(III) ions. This stretching and corresponding weakening of the
oxygen bond in peroxide group (– O─ –– O─ –) leads to its break-up in the final stage of ORR cycle.
The following formal simplified equations present innercluster chemical reactions which occur on
these stages:
2 Co2+ + O=O ↔ 2 Co3+ + (O – O)2─ (4)
2 Co2+ + (O – O)2─ + 2 H2O ↔ 2 Co3+ + 4 OH─ (5)
Chemical reactions on stages III and V have high activation energies. Therefore the rates of
these reactions fully determine an effective rate of ORR. One may conclude that innercluster
chemical reaction on stage III is slower than stage V reaction. The conclusion is based on
combination of following two factors. Formation of first transition state on stage III should proceed
slowly due to weak ion-dipole interaction between positive oxide ions of SBNO** cluster and
induced oxygen molecule’s dipole moment. On the other hand, formation of the second transition
state on stage V is a fast process governed by a powerful ion-ion interaction between the positive
oxide ions of SBNO** cluster and ions of the peroxide group.
3.2.3. ORR redox cycles on NiO oxide electrodes with SBNO
In elementary cell of NiO oxide all octahedral holes formed by oxygen atoms are occupied by
Ni2+ ions. A simple geometric calculations indicate that lattice parameter of 417 pm (see NiO cases
in Table 1) corresponds to Pauling ionic О2─ radius of 140 pm and Ni2+ radius of 69 pm. In NiO
case each octahedron has square cross section where Ni2+ ion is surrounded by four O2─ ions
located in the square’s corners. Therefore distance between two Ni2+ ions from the adjacent
octahedrons is equal to the ionic O2─ diameter of 280 pm. This distance is comparable with size of
oxygen molecule (~240 pm). In such case there are conditions for formation of SBNO nanoclusters
{NC_Ni 1} from two SMNO nanoclusters [–O2─–NiII(OH)(H2O)x] and an oxygen molecule:
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However in ORR redox cycle on NiO electrode SBNO nanoclusters functions only as one-electron
ADC. It happens because on NiO surface appears electrochemical equilibrium according to
reaction
NiII(OH)2 + e─ ↔ NiI(OH) + OH─ (6)
The standard potential E○ of reaction (6) is 0.61 V vs RHE [16]. In 0.1 M KOH the corresponding
potential E0.1 is also equal to 0.61 V.
For the range of reaction (1) oxygen potentials (around 1.20 V) the ratio of concentrations of
SBNO nanoclusters with NiI and NiII ions may be determined from the Nernst equation as 10-6 –
10-8 value. It means that there is a very small number of SBNO nanoclusters {NC Ni_2} with NiI
ions:
Such SBNO nanocluster functions in ORR as one-electron ADC. To reflect this SBNO*
abbreviation is used below in the text. The redox cycle presented in the study [8] may be used to
describe ORR on NiO electrodes with participation of SBNO* nanoclusters. The final product of
this redox cycle is HO2─ peroxide ion.
Significant quantity of SBNO* nanoclusters must form when ratio of concentration of NiI and
NiII ions is close to 1. This ratio corresponds, according to the Nernst equation, to potential E0.1 =
0.61 V. In practice, the reaction (2) starts on NiO electrode at about the same potential value (see
Table 1).
{NC Ni_1}
{(H2O)x (OH)NiII---O=O---NiII(OH) (H2O)x } └───O2-──┘
{NC Ni_2}
{(OH)NiII---O=O---NiI} └───O2-──┘
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3.2.4. Triad of requirements for determination of ORR pathways
A theoretical concept of multistage electrochemical process with SCR stage was used in the
previous chapters for re-interpretation of author’s experimental data on ORR on cobalt and nickel
oxide electrodes in alkaline medium. Utilization of this theoretical approach allowed to formulate
requirements to properties of electrode materials which fully determine pathways of ORR – either
with formation of ОН─ (e.g., on Co3O4, NiCo2O4 ) or НО2─ (e.g., on NiO) ions.
On a specific electrode material ORR proceeds with formation of ОН- ions when the following
triad of requirements to electrode material properties is simultaneously fulfilled.
First, oxide or hydroxide with atoms, which could change an effective positive charge as a
result of electrochemical process, shall be present on electrode material surface at ORR range of
potentials.
Second, the electrode material shall have a surface crystal structure which allows formation of
oxygen molecule “bridge” between two surface atoms with effective positive charge (the “bridged’
chemical structure may be described as a surface binuclear oxide nanocluster).
Third, electrochemical potential of transfer of oxide atoms with effective positive charge from
oxidized to reduced state shall be more positive than the potential of formation of НО2- ions.
On electrode materials on which at least one of the above requirements is not realized ORR
proceeds with formation of НО2─ ions.
While the concept of multistage electrochemical process with SCR stage was proposed in a
course of analysis of ORR on cobalt and nickel oxide electrodes, in author’s opinion the formulated
concept is universal. In other words, the concept could be used for predictions of ORR pathways
not only on cobalt and nickel oxide based electrodes but on wide range of other electrode materials.
Thorough simultaneous analysis of crystal structure and electrochemical potentials of electrode
materials determined reasons of proceeding of ORR via different pathways. Utilization of the
concept of triad requirements provides answers on following questions:
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- why on single-crystal Pt [1-3] and Ag [4], polycrystalline Pt, Ag, Ph, Pd [15], Ru [19], and Cu
[20], thermo oxidized Co [21, 22] and alloys Ni-Co [21, 23] are formed OH─ ions?
- why on polycrystalline Au [15], thermo oxidized Ni [21, 22] and alloys Ni-La [24] are formed
HO2─ ions?
- why on Au(100) products of ORR are OH─ ions, while on Au(111) and Au(110) the reaction
results in formation of HO2─ ions [3] ?
Examples of utilization of triad requirements concept for detailed analysis of ORR pathways on
Pt, Au, RuO2, and Fe3O4 electrodes are presented in the following chapters.
3.2.5. Analysis of ORR pathways on Pt electrodes
In author’s opinion, analysis of ORR pathways on Pt electrode with utilization of the triad
requirements concept is the most interesting one, as platinum is used in the most active
electrocatalysts of ORR in fuel cells. Efficiency and uniqueness of Pt electrodes were confirmed by
experimental [25 - 27] and theoretical [2] studies. In the monograph [27] Appleby had reviewed
and analyzed his own experimental data on ORR on Os, Ru, Rh, Ir, Pd, Rt, and Au noble metals in
phosphoric acid. Appleby had identified a functional dependence between ORR current and
potentials of oxide formation on noble metals. The dependence has a volcano-like shape with Pt
data located on curve’s maximum (top of the volcano). Theoretical calculations of oxygen
reduction activity and oxygen binding energy, E○, with atoms of d-metals (Ag, Au, Co, Cu, Fe, Ir,
Mo, Ni, Pd, Pt, Rh, Ru, and W) were performed in study [2]. The study’s results were presented in
graphical form as dependence of the oxygen reduction activity on energy E○. The dependence also
(similarly to results of experimental study [27]) has volcano-like shape with maximum
corresponding to Pt data.
Let’s analyze correspondence of Pt electrodes’ characteristics to triad’s requirements.
First triad’s requirement is fulfilled. According to study [1] data, in O2-purged electrolyte the
potentials higher than 0.9 V corresponds to onset of ORR (ca. 0.9 – 1.0 V vs RHE). On single-
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crystal Pt electrode in Ar-purged electrolyte the very same interval of potentials corresponds to
onset of formation of ‘irreversible oxide’ (covering about 15-20% of crystal surface) [1, 3].
Authors of study [1] indicated that a chemical state of such irreversible form of oxide is still
unknown. Nevertheless the mentioned above interval of potentials correlates with standard
potentials of electrochemical reactions
PtO + H2O + 2 e─ ↔ Pt + 2 OH─ (7)
Pt(OH)2 + 2 e─ ↔ Pt + 2 OH─ (8)
(0.90 V and 0.98 V vs RHE, correspondingly [18]).
The second triad’s requirement is also fulfilled. A platinum crystal has face-centered cubic (fcc)
lattice with lattice constant of 392 pm. Analysis of Pt atoms locations on [100], [110] and [111]
planes indicate that there are some Pt atoms (from 15 to 25 % of atoms pairs, depending on the
plane) with 278 pm distance between them. Such distance is comparable with oxygen molecule
size of 240 pm.
Existence of Pt atoms pairs with 278 pm distance between the atoms, especially considering
chemical reactions described by equations (7) and (8), is a beneficial condition for formation of
{NC_Pt} SBNO nanoclusters on Pt crystal’s surface:
Conventional superscript symbol II reflecting valence of Pt atoms is used as indicator of effective
positive charge of surface atoms, value of which is very difficult to interpret and quantify. Symbol
“─ O ─” in top part of the nanocluster structure demonstrates presence of the ‘irreversible oxides”
on the surface of platinum electrodes.
Finally, the third triad’s requirement is also fulfilled for Pt electrodes. Indeed, activation of
{NC_Pt} SBNO nanoclusters follows electrochemical reaction
[s,dPtII(OH)]+ + e─ ↔ [s,dPtI]+ + OH─ , (9)
{NC_Pt} ┌─── O ───┐ {(OH)PtII PtII (OH)} └ ─ ─ [xPt]─ ─ ┘
O – atom of ‘irreversible oxide’[xPt] - platinum electrode atoms
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where subscripts s and d indicate "surface" and "defect", correspondingly; therefore abbreviation
s,dPt indicates surface platinum atom which is at the same time a crystal defect.
Literature search for values of standard potentials for reaction (9) was unsuccessful. But, one
may assume that potential of reaction (9) is higher than the potential of reaction (7). Such
conclusion results from comparative analysis of potentials of copper oxide system, which is
analogous to the platinum electrode system. In copper oxide system
CuO + H2O + 2 e─ ↔ Cu + 2 OH─ (10)
CuIIO + 0.5 H2O + e─ ↔ 0.5 CuI2O + OH─ (11)
the potentials for reactions (10) and (11) are equal to 0.57 V and 0.67 V vs RHE, correspondingly
[18]. The analogy between copper oxide and platinum electrode systems leads to conclusion that
potential of reaction (9) would be a value of about 1.0 V (about 0.1 V higher than the 0.90 V
potential of reaction (7)).
The above analysis demonstrates that the Pt electrodes’ characteristics are in correspondence
with all triad’s requirements. Therefore ORR on Pt electrode, as already mentioned above,
proceeds up to formation of ОН─ ions.
Multiple schemes of ORR on Pt electrode were previously proposed and discussed in the
literature (e.g., [1-4, 15, 25]). But, SBNO nanoclusters were never used in such schemes. New
original scheme of ORR redox cycle on Pt electrode based on participation of {NC_Pt} SBNO
nanocluster is presented in Figure 2. A principal feature of the redox cycle presented in Fig 2. is a
combination of fast electrochemical reactions (stages II and IV) with consequent slow chemical
reactions (stages III and V). The chemical reactions on stages III and V may be formalized with
following equations, correspondingly:
s,dPt+ + O2 ↔ [s,dPt2+─(─O2)] (12)
[s,dPt+2 ─ (─O2)] + s,dPt+ ↔ [s,dPt2+ ─(─O─O─)─ s,dPt2+] (13)
An existence of chemical stage is supposed in some ORR schemes which were published in the
literature. For example, Yeager [25] proposed participation of ions of electrode’s metals,
represented in his ORR schemes with MZ symbol, where Z is valency of transitional element. After
chemical reaction with an oxygen molecule such ions are transferred into MZ+1 state [25]. In
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author’s opinion high values of activation energy (for example, for ORR on various planes of Pt
single-crystal experimental values of ca. 35-50 kJ mol-1 were reported in study [1]; similarly, in
study [17] theoretical values were determined to be ca. 40 kJ mol-1) also indicate an existence of
chemical stage in the ORR. Unfortunately, in the literature there are no discussions of relations
between rates of electrochemical and chemical stages. Typically, researchers limit themselves to a
priory assumption that rate of whole ORR is fully determined by the rate of electrochemical stage.
But ORR on Pt electrodes has very significant range of potentials from onset of oxygen molecule
ionization (ca. 0,9 V vs RHE [1, 15]) to onset of evolution of molecular oxygen (ca. 1,6 V vs RHE
[15]). In author’s study [13] was demonstrated that similarly positioned cathode and anode
branches of ORR polarization curve are a characteristic feature which indicates an existence of slow
chemical reaction. Therefore, character of polarization curves of ORR on Pt electrodes bear witness
to an existence of slow chemical reaction.
Stage III in redox cycle (see Fig 2) could be formally considered as a transfer of first electron
from electrode material through double layer to an oxygen molecule. Such interpretation of stage
III is in full agreement with typically proposed and used ORR schemes, where transfer of first
electron is considered to be the slowest ORR stage [1-3, 15, 25, 28]. But such interpretation of
stage III would require an explanation of reported in the literature variation of Tafel slope values on
platinum electrodes. First, there is an existence of two different Tafel slopes on platinum
polycrystalline electrodes in the low and high current density regions, 60 и 120 mV decade─1,
correspondingly [15, 28, 29]. Second, there is an experimentally observed dependence of Tafel
slope values on orientation of monocrystal planes [1, 3]: 112 and 86 mV decade─1 single slopes
were reported on Pt(100) and Pt(111), correspondingly; for Pt(110) plane two values of 90 и 190
mV decade─1 were reported for different voltage ranges. In the literature (e.g., in the review [29])
there are several alternative explanations of these experimental observations. In author’s opinion,
different Tafel slopes on different Pt monocrystal planes and on polycrystalline platinum electrodes
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could be explained by modifications of structure and composition of SBNO nanoclusters. But this
idea should be thoroughly examined in the separate independent study.
3.2.6. Analysis of ORR pathways on Au electrodes
Au crystal has fcc lattice with 407.8 pm lattice constant. Simple minded analogy with Pt
electrode case may lead to expectations that on Au electrode there are some conditions for
formation of SBNO nanoclusters (e.g. {NC_Pt} nanocluster) with an oxygen molecule “bridge” and
that ОН─ ions are formed in the result of ORR.
But experiments indicated that final products of ORR on polycrystalline Au electrode are НО2─
ions [15, 29, 30]. Contrary to Pt case, there are no oxide formations on Au electrode surface in the
range of potentials corresponding to proceeding of ORR (ca. 0.9 – 1.0 V vs RHE). According to
experimental cyclic voltammograms in Ar on polycrystalline Au [15, 31]oxide formations on Au
appear only at potentials higher than 1.3 V vs RHE. In Appleby’s anodic cyclic voltametry study
[27] start of formation of oxides on Au in phosphoric acid was reported at 1.40 V potential (for Pt
similar potential is equal to 0.89 V). Potential ca. 1.4 V correlates with standard potential 1.37 V vs
RHE [18] of electrochemical reaction
AuIIO + H2O + 2 e─ ↔ Au + 2 OH─ (14)
Absence of oxide formations on Au surface excludes a possibility of formation of SBNO
nanoclusters on Au electrode. Instead non-oxide nanoclusters may form. The ORR proceeds either
with participation of surface mononuclear non-oxide [xAu]─Au--O=O or surface binuclear non-
oxide [xAu]─Au--O=O--Au─[xAu] nanoclusters. The process follows either “Pauling model” or
“Griffiths model” scheme for mononuclear and “Bridge model” scheme for binuclear nanoclusters
[15]. A detail description of the redox cycles with participation of non-oxide nanoclusters should be
a subject of separate study. Let’s only comment here that neutral Au atoms perform a function of
nuclei in both types of the mentioned above nanoclusters. The final product of ORR is HO2─
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peroxide ion, as due to weak link in Au--(HO2─) complex there is a low energetic barrier for
separation of HO2─ ions from mononuclear or binuclear nanoclusters.
ORR on single-crystals Au is an exception from the above rule. Experimental data on pathways
of ORR on single-crystals Au in alkaline medium were presented in the studies [3, 32-35]. It was
reported that on Au(111) and Au(110) ORR proceeds, similarly to polycrystalline Au case,
following reaction’s (2) pathway. On Au(100) ORR proceeds according to the reaction’s (1)
pathway, i.e. with formation of OH─ ions. Analysis of ORR polarization curves, ring currents for
peroxide detection and base voltammetry [3], low energy electron diffraction (LEED) and Auger
electron spectroscopy (AES) [32-34], low energy ion scattering (LEIS) spectrum [3] data leads to
the following conclusions. Start of ORR on Au(111) and Au(110) happens at 0.9 V vs RHE with
formation of HO2─ ions; the reaction product does not change with further decrease of the potential.
On Au(100) ORR starts at 1.0 V with formation of OH─ ions. Decrease of the electrode potential
down to about 0.7 V value results in sharp change of ORR pathway: formation of HO2─ ions is
initiated. Further decrease of the potential results in the so-called "catalytic peak of current” [3].
After the peak the current decreases down to values comparable to corresponding currents on
Au(111) and Au(110) electrodes.
Base voltammetry of gold monocrystals performed in study [3] resulted in identification of
three potential regions: the double-layer region at about 0.6-0.7 V; OHad formation region at about
1.1 V; and "oxide" formation region at potentials higher than 1.1.V. The authors of study [3]
consider reversibly adsorbed OHad as a precursor state for the formation of the surface "oxide"
layer. Due to some unknown reasons, the study [3] authors did not accent peculiarities of cathode
branches of the base voltammetry on different surfaces of Au(hkl) monocrystals. The cathode
branches on Au(100) have two well defined peaks at 1.1 and 0.9 V; on Au(111) there is only one
peak at ca. 1.1 V; on Au(110) there is intense peak at 1.1 V and very small second peak at ca. 0.8 V.
LEED data confirm that Au(hkl) surface keep an order of the crystal structure. Therefore it
would be more correct to use in the discussion of surface atoms characteristics abbreviation
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s,dAu(hkl). By analyzing AES data authors of the studies [32-34] concluded that electrochemical
oxidation of gold followed by formation of surface AuOH structures. The authors described the
process by "electrosorption" term. The following reaction may formally represented the process:
s,dAu + OH─ - e─ ↔ s,dAuIOH, (15)
where superscript symbol I reflecting valence of Au atoms is used as indicator of effective positive
charge of surface atoms, value of which is very difficult to interpret and quantify.
In Au(100) case the cathode peak at 0.9 V , probably, corresponds to reduction of s,dAuIOH; the
peak at 1.0 V corresponds to more complex reaction
s,dAu2IO + H2O + 2e─ ↔ 2 s,dAu + 2 OH─ (16)
Energetic properties of s,dAu atoms, but not the properties of OH─ ions, should define
proceeding potentials of reactions (15) and (16). In a turn, the energetic properties of s,dAu atoms
are defined by configuration of atoms on monocrystal surface. A brief description of such
functional dependence was proposed in study [34], further detailed analysis should definitely be a
subject of separate research efforts.
Experimental data on pathways of ORR on single-crystals Au in alkaline medium were
presented in the study [3]. It was reported that on Au(111) and Au(110) ORR proceeds, similarly to
polycrystalline Au case, following reaction’s (2) pathway. On Au(100) ORR proceeds according to
reaction’s (1)
In author’s opinion, contrary to Au(111) and Au(110) cases, conditions for formation of SBNO
nanoclusters and simultaneous fulfillment of all triad’s requirements are realized on Au(100).
Therefore, according to the proposed concept of the triad requirements, ORR on Au(100) proceeds
following reaction (1) pathway. Unfortunately, literature search did not provide an electrochemical
reaction for Au with a potential which would correlate with ca. 0.9 vs. RHE potential of
experimentally observed voltammogram’s peak. Finally, let’s note the following experimental
phenomenon: introduction of small amount of Pd atoms [3] and presence of monolayer islands of
Ag atoms [36] on the Au(111) electrode surface changes ORR pathways. On pure Au(111) the
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ORR proceeds following reaction (2) pathway (formation of НО2─ ions); with Pd and Ag atoms
there is reaction (1) pathway (formation of OH─ ions). The concept of triad requirements explains
the phenomenon. On Au(111) the first triad’s requirement is not fulfilled, so ORR is following a
pathway to НО2─ ions. For cases with Pd and Ag atoms all three requirements from the triad are
fulfilled, so the SBNO nanoclusters are formed and ORR proceeds up to OH─ ions.
3.2.7. Analysis of ORR pathways on Fe3O4 and RuO2 electrodes
ORR with formation of HO2─ ion was reported on FeOOH hydroxide [12], steel [31], and Fe3O4
spinel [37]. All these materials have one common feature – presence of iron ions in various
valences in surface oxide structures.
Let’s take a closer look on Fe3O4 spinel – a material with the prominent oxide structure. Fe3O4
spinel is an analog of Co3O4 spinel, for which all three triad’s requirements for proceeding of ORR
reaction following reaction (1) pathway are fulfilled. But on Fe3O4 spinel electrodes the third
triad’s requirement is not fulfilled. Electrochemical potential of transfer of oxide positive ions from
oxidized to reduced state in a course of reaction
Fe(OH)3 + e─ ↔ Fe(OH)2 + OH─ (17)
is equal to 0.27 V vs RHE, this value is lower than the 0.765 V vs RHE standard potential of
reaction (2). Therefore, on Fe3O4 spinel electrodes the ORR proceeds following reaction (2)
pathway.
RuO2 oxide has a complex crystal structure. For interpretation of various RuO2 oxide properties
is used a simplified octahedral model formed by two RuIV atoms and four O2─ ions [9]. In the
octahedron the shortest distance between Ru atom and O2─ ion octahedron is ca. 192 pm; similarly,
shortest Ru-Ru distance is ca. 311 pm. Such excessively long distance between Ru atoms precludes
formation of SBNO nanoclusters with oxygen molecule “bridge”. Therefore, for RuO2 oxide the
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second triad’s requirement is not fulfilled, and the ORR proceeds following reaction (2) pathway, as
it was observed in study [9].
Conclusions
The study’s goal was to theoretically identify parameters responsible for differentiation of ORR
pathways (formation of either ОН─ or НО2─ ions) on oxide electrode materials.
Theoretical analysis of ORR pathways on Co3O4, NiCo2O4, and NiO electrodes was performed
simultaneously with analysis of structural and electrochemical characteristics of these oxides. The
structural surface characteristics were analyzed on the nanoclusters scale. The analysis of ORR was
based on a concept of multistage electrochemical process with a slow chemical reaction stage.
The analysis resulted in formulation of triad of requirements to properties of electrode materials.
On a specific electrode material ORR proceeds with formation of ОН─ ions when three following
requirements are simultaneously fulfilled. First, oxide or hydroxide with atoms, which could
change an effective positive charge as a result of electrochemical process, shall be present on
electrode material surface at ORR range of potentials. Second, the electrode material shall have a
surface crystal structure which allows formation of oxygen molecule “bridge” between two surface
atoms with effective positive charge (the “bridged’ chemical structure may be described as a surface
binuclear oxide nanocluster). Third, electrochemical potential of transfer of oxide atoms with
effective positive charge from oxidized to reduced state shall be more positive than the potential of
formation of НО2- ions (standard potential, E○, of reaction (2) is 0.765 V vs RHE).
The proposed ‘triad requirements’ rule was used for prediction and explanation of ORR
pathways on Pt, Au, Fe3O4, and RuO2 electrodes. For this purpose, literature review of ORR data
on these electrodes was complimented with analysis of structural and electrochemical parameters of
the same materials.
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On single platinum crystals all three triad’s requirement are fulfilled. Therefore, ORR proceeds
according to reaction (1) with formation of ОН─ ions.
On gold single-crystals there are different ORR pathways on different crystal surface planes.
On Au(111) and Au(110) crystal surfaces oxide structures did not form (the first triad requirements
is not fulfilled), therefore the ORR proceeds according to the reaction (2) pathway. On Au(100)
crystal surface all triads requirements are fulfilled, therefore ORR proceeds according to the
reaction (1) pathway.
On RuO2 and Fe3O4 spinel electrodes the second and third triad’s requirements are not fulfilled,
correspondingly. Therefore ORR on RuO2 and Fe3O4 proceeds according to the reaction (2)
pathway.
It was concluded that proposed “triad requirements” rule is universal one, and, as such, could be
used for prediction of ORR pathways in alkaline media on any electrode material. Therefore, the
author encourages utilization of the proposed theoretical methods for forecast of ORR pathways on
new electrode materials, for which there are no experimental data yet.
Acknowledgments
The author would like to thank Dr. M. A. Trunov for discussions of the manuscript’s draft.
References
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[3] T. Schmidt, V. Stamenkovic, M. Arenz et al, Electrochim. Acta 47 (2002) 3765
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Figure captions
Figure 1. Redox cycle of ORR on Co3O4 oxide electrodes with participation of SBNO**
nanoclusters and rupture of two oxygen molecule bonds
Figure 2. Redox cycle of ORR on Pt electrodes with participation of SBNO nanoclusters and
rupture of two oxygen molecule bonds
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Table 1
Characteristic parameters of cobalt and nickel oxides electrodes
OxidesParameter
Co3O4 Co3O4-Li2O NiCo2O4 NiO NiO-Li2OLithium content, at.% 0 1.5 0 0 9.1Lattice constant, pm 808 808 808 417 417
Electric conductivity, mS0.08[5];0.6[7] 200[7] 3600[5] 0.01[5];
0.04[7] 30[7]
Fraction of reaction (1) products, P1, %
85[5];90[7] 90[7] 90[5] 0[5, 7] 0[5,7]
Fraction of reaction (2) products, P2, %
15[5];10[7] 10[7] 10[5] 100[5,7] 100[5, 7]
Potential of ORR start on disk, Estart, V vs RHE* 1.00[5] N/A 1.05[5] 0.65[5] N/A
* RHE - reversible hydrogen electrode in the same electrolyte
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Figure 1
Initial State {NC Co}
{(OH)2CoIII CoIII(OH)2} └───O2- ──┘
O2- - oxygen ion of structure oxide
Stage I+ O2 → {(OH)2CoIII ---O=O---CoIII(OH)2} └───O2-──┘
Stage II+ 2e- - 2 OH- → {(OH)CoII ---O=O---CoII(OH)} └──O2- ──┘
Stage III.Innercluster transformation into{(OH)CoIII─O- ─O- ─CoIII(OH)}
└────O2-──┘Stage IV
HOH HOH ¦ ¦+ 2e- + 2 H2O
- 2 OH- → {CoII─O- ─O- ─CoII} └───O2- ──┘
Stage VInnercluster transformation into
{(OH)2CoIII CoIII(OH)2} └───O2- ──┘
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Figure 2
{NC Pt} ┌─── O ───┐ {(OH)PtII PtII (OH)} └ ─ ─ [xPt]─ ─ ┘
O – atom of ‘irreversible oxide’[xPt] - platinum electrode atoms
Stage I ┌─── O ───┐ + O2 → {(OH)PtII ---- O=O ----PtII (OH)} └ ─ ─ [xPt]─ ─ ┘
Stage II ┌─── O ───┐ + e─ → {PtI ---- O=O ---- PtII(OH)} + OH─
└ ─ ─ [xPt]─ ─ ┘ Stage III
Innercluster transformation into ┌─── O ───┐
{PtII( ─O2) ------- PtII(OH)} └ ─ ─ [xPt]─ ─ ┘
Stage IV ┌── O ──┐ + e─ → {PtII(─O2) ----- PtI} + OH─
└ ─ [xPt]─ ┘ Stage V
Innercluster transformation into ┌── O ──┐ {PtII (─O─O─) PtII} └ ─ [xPt]─ ┘
Stage VI ┌─── O ───┐ + 2 e─ + 2 H2O
→ {(OH)PtII PtII (OH)} + 2 OH─
└ ─ ─ [xPt]─ ─ ┘