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

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

Page 1 of 30

Accep

ted

Man

uscr

ipt

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

Page 2 of 30

Accep

ted

Man

uscr

ipt

2

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

Page 3 of 30

Accep

ted

Man

uscr

ipt

3

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─

Page 4 of 30

Accep

ted

Man

uscr

ipt

4

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

Page 5 of 30

Accep

ted

Man

uscr

ipt

5

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

Page 6 of 30

Accep

ted

Man

uscr

ipt

6

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.

Page 7 of 30

Accep

ted

Man

uscr

ipt

7

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

Page 8 of 30

Accep

ted

Man

uscr

ipt

8

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

Page 9 of 30

Accep

ted

Man

uscr

ipt

9

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-───┘

Page 10 of 30

Accep

ted

Man

uscr

ipt

10

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.

Page 11 of 30

Accep

ted

Man

uscr

ipt

11

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

Page 12 of 30

Accep

ted

Man

uscr

ipt

12

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:

Page 13 of 30

Accep

ted

Man

uscr

ipt

13

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-──┘

Page 14 of 30

Accep

ted

Man

uscr

ipt

14

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:

Page 15 of 30

Accep

ted

Man

uscr

ipt

15

- 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-

Page 16 of 30

Accep

ted

Man

uscr

ipt

16

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

Page 17 of 30

Accep

ted

Man

uscr

ipt

17

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

Page 18 of 30

Accep

ted

Man

uscr

ipt

18

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

Page 19 of 30

Accep

ted

Man

uscr

ipt

19

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─

Page 20 of 30

Accep

ted

Man

uscr

ipt

20

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

Page 21 of 30

Accep

ted

Man

uscr

ipt

21

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

Page 22 of 30

Accep

ted

Man

uscr

ipt

22

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

Page 23 of 30

Accep

ted

Man

uscr

ipt

23

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.

Page 24 of 30

Accep

ted

Man

uscr

ipt

24

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

[1] T. Schmidt, V. Stamenkovic, P. Ross Jr. et al, Phys. Chem. Chem. Phys. 5 (2003) 400

[2] J. Norskov, L. Rossmeis, A. Logadottir et al, J. Phys. Chem. B. 108 (2004) 17886

[3] T. Schmidt, V. Stamenkovic, M. Arenz et al, Electrochim. Acta 47 (2002) 3765

[4] B. Blizanac, P. Ross, N. Markovic, J. Phys. Chem. B. 110 (2006) 4735

[5] A. Trunov, A. Domnikov, G. Resnikov et al, Elecktrokhimiya 15 (1979) 783

[6] N. Heller-Ling, M. Prestat, J. Gautier et al, Electrochim. Acta 42 (1997) 197

[7] A. Trunov, Elektrokhimiya 17 (1981) 1051

Page 25 of 30

Accep

ted

Man

uscr

ipt

25

[8] T. Wass, I. Panas, J. Asbjornsson et al, J. Electroanalyt. Chem. 599 (2007) 295

[9] C. Chang, T. Wen, J. Appl. Electrochem. 27 (1997) 355

[10] C. Chang, T. Wen, C.Yang et al, Mater. Chem. Phys. 115 (2009) 93

[11] F. Lima, M. Calegaro, E. Ticianelli, Russian J. Electrochem. 42 (2006) 1283

[12] E. Vago, E. Calvo, M. Stratmann, Electrochim. Acta 39 (1994) 1655

[13] A. Trunov, Elektrokhimiya 22 (1986) 1093

[14] Y. Cao, H. Yang , X. Ai et al, J. Electroanalyt. Chem. 557 (2003) 127

[15] M. Tarasevich, A. Sadkowski, E. Yeager, in: B.E. Conway, J.M. Bockris, E. Yeager, S.U.M.

Kahn, R.E. White (Eds.), Comprehensive Treatise of Electrochemistry, Plenum Press, New

York, 1983, p. 301.

[16] A. Trunov, Kharkov University Bulletin (Ukraine, rus), 648 Chemical series (2005) 216

[17] T. Zhang, A. Anderson, Electrochim. Acta 53 (2007) 982

[18] W. Latimer. Oxidation Potentials, Prentice Hall, New York, 1938, chapter 4

[19] J. Prakash, H. Joachin, Electrochim. Acta 45 (2000) 2289

[20] F. King, M. Quinn, C. Litke, J. Electroanalyt. Chem. 385 (1995) 45

[21] V. Bagotzky, N. Shumilova, E. Khrushcheva, Electrochim. Acta 21 (1976) 919

[22] E. Krushcheva, O. Moravskay, V. Karonik at al, Elektrokhimiya 11 (1975) 620

[23] N. Ryasinzeva, N. Shumilova, A. Trunov at al, Elektrokhimiya 10 (1974) 822

[24] E. Krushcheva, O. Moravskay, N. Shumilova, Elektrokhimiya 10 (1974) 987

[25] E. Yeager, J. Mol. Catal. 38 (1986) 5

[26] S. Sugawara, K. Tsujita, S. Mitsushima at al, Electrocatalysis 2 (2011) 40

[27] A. Appleby, in: B.E. Conway, J.M. Bockris, E. Yeager, S.U.M. Kahn, R.E. White (Eds.),

Comprehensive Treatise of Electrochemistry, Plenum Press, New York, 1983, p.17.

[28] D. Sepa, M.Vojnovic, A. Damjanovic at al. Electrochim. Acta, 31 (1986) 97

[29] A. Appleby, J. Electroanal. Chem., 357 (1993) 117

[30] D. Schiffrin, The Electrochemistry of Oxygen. In: D.Pletcer (Ed). Electrochemistry. Royal

Page 26 of 30

Accep

ted

Man

uscr

ipt

26

Society of Chemistry. London. 8 (1983) 128

[31] D. Gervasio, I. Song, J. Payer, J. Appl. Electrochem. 28 (1998) 979

[32] R. Adzic, N. Markovic, V. Vesovic, J. Electroanal. Chem. 165 (1984) 105

[33] N. Markovic, R. R. Adzic, V. Vesovic, J. Electroanal. Chem. 165 (1984) 121

[34] S. Strbac, R. Adzic, J. Electroanal. Chem. 403 (1996) 169

[35] A. Prieto, J. Hernandez, E.Herrero et al, J. Solid State Electrochem. 7 (2003) 599–606

[36] A. Kongkanand, S. Kuwabata, Electrochem Commun 5 (2003) 133

[37] E Vago, E. Calvo, J Electroanalyt Chem. 339 (1992) 41

Page 27 of 30

Accep

ted

Man

uscr

ipt

27

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

Page 28 of 30

Accep

ted

Man

uscr

ipt

28

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

Page 29 of 30

Accep

ted

Man

uscr

ipt

29

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- ──┘

Page 30 of 30

Accep

ted

Man

uscr

ipt

30

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]─ ─ ┘