Oxygen reduction on nanocrystalline ruthenia-local structure effects

Article abstract of DOI:10.1039/c4ra10001h

Nanocrystalline ruthenium dioxide and doped ruthenia of the composition Ru_1-xM_xO₂ (M = Co, Ni, Zn) with 0 ≤ x ≤ 0.2 were prepared by the spray-freezing freeze-drying technique. The oxygen reduction activity and selectivity of the prepared materials were evaluated in alkaline media using the RRDE methodology. All ruthenium based oxides show a strong preference for a 2-electron oxygen reduction pathway at low overpotentials. The catalysts' selectivity shifts towards the 4-electron reduction pathway at high overpotentials (i.e. at potentials below 0.4 V vs. RHE). This trend is particularly noticeable on non-doped and Zn-doped catalysts; the materials containing Ni and Co produce a significant fraction of hydrogen peroxide even at high overpotentials. The suppression of the 4-electron reduction pathway on Ni and Co-doped catalysts can be accounted for by the presence of the Ni and Co cations in the cus binding sites as shown by the DFT-based analyses on non-doped and doped catalysts. This journal is

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PAPER
Oxygen reduction on nanocrystalline ruthenia –
local structure effects†
Cite this: RSC Adv., 2015, 5, 1235
Daniel F. Abbott,ab Sanjeev Mukerjee,b Valery Petrykin,a Zdenek Bastl,a
ˇ
c
c
a
*
Niels Bendtsen Halck, Jan Rossmeisl and Petr Krtil
Nanocrystalline ruthenium dioxide and doped ruthenia of the composition Ru1ꢀxMxO2 (M ¼ Co, Ni, Zn) with
0 # x # 0.2 were prepared by the spray-freezing freeze-drying technique. The oxygen reduction activity
and selectivity of the prepared materials were evaluated in alkaline media using the RRDE methodology.
All ruthenium based oxides show a strong preference for a 2-electron oxygen reduction pathway at low
overpotentials. The catalysts' selectivity shifts towards the 4-electron reduction pathway at high
overpotentials (i.e. at potentials below 0.4 V vs. RHE). This trend is particularly noticeable on non-doped
and Zn-doped catalysts; the materials containing Ni and Co produce a significant fraction of hydrogen
peroxide even at high overpotentials. The suppression of the 4-electron reduction pathway on Ni and
Co-doped catalysts can be accounted for by the presence of the Ni and Co cations in the cus binding
sites as shown by the DFT-based analyses on non-doped and doped catalysts.
Received 8th September 2014
Accepted 20th November 2014
DOI: 10.1039/c4ra10001h
www.rsc.org/advances
independent oxygen evolution (OER) or oxygen reduction
(ORR), the catalysts' application in regenerative fuel cells faces
Introduction
signicant restrictions in terms of minimizing the energetic
barriers of both kinetically irreversible processes. The fact that
the oxygen evolution reaction proceeds solely on oxide covered
surfaces disfavors the use of metal catalysts which are reported
to be of superior activity in oxygen reduction. The oxide activity
in the oxygen evolution was investigated in both acidic as well as
alkaline media on various systems based on oxides of ruthe-
nium,5–8 iridium,5,7–9 cobalt5,10 or manganese.5 Oxygen reduction
studies on oxides are less frequent and are generally restricted
to alkaline media. Oxygen reduction has been studied on
rutile,11,12 spinel,13,14 perovskite15 and pyrochlore16,17 structural
types based on ruthenium, manganese, nickel, cobalt and
iridium oxides. The investigated oxide catalysts were the subject
of electrochemical characterization which was phenomenolog-
ically analyzed in order to explain the possible reaction path-
ways leading to both 4-electron and 2-electron oxygen reduction
processes. In contrast to the studies of oxygen evolution, no
detailed investigations aiming at the role of the catalyst struc-
ture, including the local structure of the oxygen reduction active
site, have been reported so far.
The fuel cell related electrocatalytic processes based on
controlled hydrogen oxidation and oxygen reduction have
recently gained importance mainly in connection with the
increasing utilization of renewable energy sources. Despite
efforts devoted to the optimization of existing systems, the
performance of real fuel cells still lags behind the expectations
and the cathodic oxygen reduction is seen as the performance
limiting process. The electrochemical fuel cell reactions can
also be generally employed in the energy storage mode using
the excess electricity or solar energy to generate energetically
useful hydrogen (produced along with the oxygen), leading to
the introduction of the regenerative fuel cell concept.1,2 It also
needs to be stressed that the regenerative fuel cell applications
have sparked extensive catalyst development primarily for the
oxygen evolution/reduction processes.
Oxygen electrochemistry, including oxygen evolution as well
as reduction, represents the simplest example of multiple
electron charge transfer processes which have been extensively
studied both experimentally as well as theoretically.3,4 In
contrast to the development of suitable catalysts for
The theoretical approach allowing for the generalization of
oxygen electrochemistry on oxides based on DFT calculations
was recently reported.4,18,19 The DFT calculations identify the
active sites for oxygen activation and the charge transfer to so-
called coordination unsaturated sites (cus), the surface pop-
ulation of which is a function of the surface orientation. The cus
surface sites feature (n ꢀ 1) oxygen bonding partners, where n is
equal to number of metal–oxygen bonds present in the oxide
bulk. It is believed that only cus sites can form the atop reaction
a
´
Department of Electrocatalysis, J. Heyrovsky Institute of Physical Chemistry, Academy
ˇ
of Sciences of the Czech Republic, Dolejskova 3, 18223 Prague, Czech Republic. E-mail:
Petr.Krtil@jh-inst.cas.cz
bDepartment of Chemistry and Chemical Biology, Northeastern University, 360
Huntington Ave., Boston, MA 02115, USA
cCenter for Atomic-Scale Materials Design, Department of Physics, Technical University
of Denmark, Building 307, 2800 Kgs. Lyngby, Denmark
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra10001h
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intermediate(s), which are essential in the oxygen electro- prepared materials were measured using a modied ESCA 3
chemistry. In this respect one can easily predict that the cata- MkII multitechnique spectrometer equipped with a hemi-
lytic activity and selectivity of oxide catalysts may be altered if spherical electron analyzer operating in the xed transmission
one controls the population and stacking of the cus sites at the mode. Al Ka radiation was used for electron excitation. The
oxide surface. This trend has been shown for oxygen evolution binding energy scale was calibrated using the Au 4f7/2 (84.0 eV)
on heterostatically doped ruthenia when the incorporation of and Cu 2p3/2 (932.6 eV) photoemission lines. The spectra were
lower valency cations, such as Ni,20–23 Co,24–27 Fe28 or Zn29,30 into collected at a detection angle of 45ꢁ with respect to the macro-
ruthenia framework resulted in changes to both the activity and scopic surface normal. The studied materials were character-
selectivity of anodic processes including oxygen and chlorine ized using survey scan spectra and high resolution spectra of
evolution. Similar systematic studies focused on other oxide overlapping Ru 3d + C 1s photoelectrons, Ru 4s, Zn 2s and O 1s
systems are, so far, lacking.
photoelectrons. The spectra were curve tted aer subtraction
This paper focuses on the role of the local structure of the of Shirley background using the Gaussian–Lorentzian line
oxide catalysts in the oxygen reduction reaction. We report on shape and nonlinear least-squares algorithms. Quantication
the ORR activity of model nanocrystalline ruthenia based cata- of the elemental concentrations was accomplished by correct-
lysts with local structure controlled by doping with Ni, Co and ing the photoelectron peak intensities for their cross sections
Zn. The observed electrocatalytic activity and selectivity are and for the analyzer transmission function. The typical error of
related to the actual local structures and rationalized using quantitative analysis by XPS is ꢂ10%.
DFT-based thermodynamic analysis of the oxygen reduction
process.
Electrochemical measurements
The electrochemical oxygen reduction activity of the prepared
materials was assessed in a three-electrode single-compartment
cell with a rotating ring-disk electrode (RRDE) setup (Pine
Instruments, USA). The potential was controlled using an
Autolab PGSTAT30 (EcoChemie, The Netherlands). Catalyst ink
suspensions were prepared by sonicating 9.8 mg RuO2 or
Ru1ꢀxMxO2 (M ¼ Co, Ni, Zn) with 5.00 mL Millipore water, 4.95
mL isopropyl alcohol, and 50 mL of 5 wt% Naon® ionomer
solution until the suspension was well dispersed. A 10.0 mL
aliquot of the ink was drop cast on a 0.196 cm2 glassy carbon
disk electrode equipped with a platinum ring to yield a total
catalyst loading of approximately 50 mg cmꢀ2. All experiments
were conducted at room temperature in 0.1 M NaOH prepared
from sodium hydroxide pellets (semiconductor grade, 99.99%,
Sigma-Aldrich). A platinum wire served as the counter electrode
and a saturated calomel electrode (SCE) served as the reference
electrode. All potentials reported are quoted against RHE.
Electrolyte solutions were saturated with O2 for 30 minutes
prior to oxygen reduction measurements. The measured oxygen
reduction currents were corrected for the contribution of the
capacitive current by subtracting the cyclic voltammograms
obtained under identical conditions in Ar saturated solution.
Cyclic voltammograms were recorded at a scan rate of 20 mV sꢀ1
and the potential of the platinum ring electrode was held at 1.1
V vs. RHE during all measurements. The ring collection effi-
ciency was determined to be 0.275 according to the procedure
described in ref. 32.
Methods
Material preparation
Ruthenium dioxide and doped samples of the composition
Ru1ꢀxMxO2 (M ¼ Co, Ni, Zn) were synthesized using the spray-
freezing freeze-drying method as described in ref. 30 and 31.
Generally, an 8 mM solution was prepared by dissolving the
appropriate amount of Ru(NO)(NO3)3 (31.3% Ru, Alfa Aesar) in
100 mL of Millipore H2O. In the case of doped materials, a
stoichiometric amount of the appropriate transition metal salt
was added to the solution. Zinc-doped samples were prepared
from the acetate precursor, Zn(C2H3O2)2$2H2O (99.5% ACS
reagent grade, Fluka). Cobalt- and nickel-doped samples were
prepared from the nitrate salts, Co(NO3)2$6H2O and Ni(NO3)2-
$6H2O (99.999% trace metal basis, Sigma Aldrich), respectively.
The starting solution was then sprayed into liquid N2. The
resulting ice slurry was collected in an aluminum tray pre-
cooled with liquid N2 and quickly transferred to a freeze-dryer
(FreeZone Triad Freeze Dry System 7400030, Labconco) pre-
cooled to ꢀ30 ꢁC. The frozen solvent was sublimated at
reduced pressure (z1.0 Pa) while the temperature was ramped
according to the following program: ꢀ30 ꢁC (1 h), ꢀ25 ꢁC (5 h),
ꢀ20 ꢁC (4 h), ꢀ15 ꢁC (6 h), 30 ꢁC (4 h). Aer drying, the resulting
ꢁ
powder was annealed in air at 400 C for 1 hour.
XRD, XPS and SEM characterization
The crystallinity of sample powders was characterized using a
Rigaku Miniex 600 powder X-ray diffractometer with CuKa
DFT analysis of oxygen reduction
radiation operating at 40 kV and 15 mA. The average sample The thermodynamic analysis of the ORR on ruthenia based
compositions were evaluated with X-ray energy dispersive [110] surfaces was addressed using GPAW (grid-based projector-
spectroscopy using a Hitachi S4800 scanning electron micro- augmented wave) a DFT based code33 together with the ASE
scope (SEM) equipped with a Nanotrace EDX detector (Thermo (atomic simulation environment).34 For all surfaces the
Electron). Sample compositions did not deviate signicantly exchange correlation functional, revised Perdew Burke Ernzer-
from the projected ones. Particle size was evaluated by hof,35 was used. The grid spacing selected was 0.18 and the
analyzing SEM images and averaging the size of 300 randomly Brillouin zone was sampled using a 4 ꢃ 4 ꢃ 1 Monkhorst–Pack
chosen particles. The X-ray photoelectron spectra (XPS) of the grid. The two model systems, the non-doped and Ni-doped
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ruthenia [110], were approximated using a 2 ꢃ 1 and a 3 ꢃ 1
Surface composition of all doped samples reects the
supercell, respectively, with four atomic trilayers and with the metastable character of the materials and previous thermal
bottom two trilayers xed. The remaining layers and adsorbates treatment which result in a dopant enrichment of the surface
were relaxed until the residual forces in all directions were less layer.21,37 This effect is most pronounced in the case of the Zn
than 0.05 eV Aꢀ1. The positions of the Ni atoms were modeled doped materials when the actual surface compositions of
˚
using the approach described in ref. 33. The calculations con- both studied materials correspond to Ru0.73Zn0.27O2.43 and to
taining Ni were spin-polarized.
Ru0.63Zn0.37O2.23 for the materials with nominal Zn
contents of 0.1 and 0.2, respectively. It needs to be noted that
the apparent excess of the oxygen in the surface composition
can attributed to surface OH groups as well as to adsorbed
water.
Results and discussion
XRD and SEM characterization
X-ray diffraction patterns of all studied materials are shown in
Fig. 1. In all cases the recorded patterns conform to a single
phase tetragonal structure of the rutile type identical with that
of RuO2 (PDF le #431027). The average size of coherent crys-
tallite domains was evaluated using the Scherrer formula:
Electrochemical measurements
All prepared ruthenia materials are active ORR catalysts in
alkaline media. The ORR polarization curves for RuO2 and
Ru1ꢀxMxO2 (M ¼ Ni, Co, Zn) samples are shown in Fig. 3. The
disk current (iD) which reects the oxygen reduction shows a
pronounced peak at approximately 0.40 V to 0.55 V before
approaching a mass transport controlled region. The disk
current feature can be tentatively associated with a change in
the Ru oxidation state from Ru(IV) to Ru(III).11,36 This process is
usually connected with cation insertion into the oxide structure
to balance the charge in cationic and anionic sub-lattices.37 The
behavior giving rise to the peak in the disk current is also
manifested in the ring current, indicating a pronounced
formation of hydrogen peroxide in this potential region. The
hydrogen peroxide formation in the 0.40 V to 0.55 V interval
seems to be unaffected by the chemical composition of the
catalyst. The formation of hydrogen peroxide seems to be sup-
pressed with increasing rotation rate. The precise mechanism
of this reduction process is, however, not evident.
l
Di ¼
(1)
bi cos qi
where Di is the size of the crystallite domain, l is the wavelength
˚
of the incident radiation (CuKa ¼ 1.540598 A), bi is the width of
the diffraction peak at half maximum intensity measured in
radians, and qi is the angle of the hkl reection.
The average coherent domain size ranged between 4.3 and
5.7 nm (see Table 1). Representative SEM images of the doped
ruthenia are summarized in Fig. 2. The particle sizes evaluated
from SEM micrographs agree with the coherent domain size
values (see Table 1). Average sample compositions did not
deviate signicantly from the projected ones and are listed in
Table 1.
The overall ORR activity of the doped ruthenia catalysts is
lower than that of the non-doped ruthenia. The ORR activity as
reected in the disk currents (iD) generally decreases for the Co-
and Ni-doped samples. There is no apparent effect of the actual
dopant concentration on the oxygen reduction disc currents.
The corresponding ring currents (iR) are, however, higher than
that of the non-doped ruthenia, particularly at high over-
potentials (i.e. at potentials negative to 0.4 V vs. RHE). This
shows a pronounced tendency of Co and Ni-doped materials to
produce H2O2 namely at high overpotentials (n ranging between
3.0 and 3.4). In contrast, the Zn-doped materials show a pref-
erence for the 4-electron reduction pathway with n values
ranging between approximately 3.6 and 3.8 while the activity
remains comparable to that of the non-doped ruthenia. Also the
selectivity of the doped ruthenia in oxygen reduction is
controlled rather by the doping process itself than by the actual
dopant content.
The observed behavior reects the surface sensitivity of
oxygen reduction on oxide surfaces, which can be related to the
surface local structure. Quantitative visualization of this
behavior is shown in Fig. 4, which plots the potential depen-
dence of the average number of electrons transferred to an
oxygen molecule on different doped ruthenium dioxide mate-
Fig. 1 Powder X-ray diffraction patterns for nanocrystalline RuO2 and
Ru1ꢀxMxO2 (M ¼ Ni, Co, Zn) after annealing in air for 1 hour at 400 ꢁC. rials as calculated from the Koutecky–Levich equation:38
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Table 1 Results of the analysis of the powder diffraction data for RuO2 and doped RuO2 samples
Coherent domain
size [nm]
Strain
[%]
˚
a [A]
˚
c [A]
Actual composition
Particle size [nm]
RuO2
5.7
4.9
5.5
5.3
5.0
4.3
0.46
0.61
0.00
0.00
0.00
0.21
4.470
4.526
4.519
4.515
4.501
4.505
3.120
3.108
3.099
3.096
3.079
3.081
7.6 ꢄ 2.2
8.9 ꢄ 2.2
5.8 ꢄ 1.5
7.2 ꢄ 1.4
7.9 ꢄ 2.3
7.4 ꢄ 1.8
Ru0.9Zn0.1O2ꢀz
Ru0.82Zn0.18O2ꢀz
Ru0.95Ni0.05O2ꢀz
Ru0.91Ni0.09O2ꢀz
Ru0.9Co0.10O2ꢀz
reduction process. The measured disk current summarizes the
current contributions from the complete 4-electron reduction to
H2O and the 2-electron reduction to H2O2 while the recorded
ring current is proportional only to the amount of oxygen
reduced to H2O2. In this respect a ratio of iD/iR can be used as an
indicator of the actual mechanism which should yield a straight
line proportional to k1/k2 when plotted against uꢀ1/2 (see
Fig. 6).40
The actual iD/iR data deviate from linearity (see Fig. 6) as can
be expected since the formalism incorporated in the scheme
depicted in Fig. 5 disregards the nature of the individual reac-
tion steps composing both 2- and 4-electron reduction pathways
and their different dependence on the electrode potential.
The individual rate constants k1, k2 and k3 were evaluated
from ORR data assuming that all three processes proceed
simultaneously and that the values of kꢀ1, kꢀ2, and kꢀ3, corre-
sponding to reversed reactions, are negligible. The adsorption
of oxygen on the electrode surface is also assumed to proceed
sufficiently fast. The potential dependence of the rate constants
for all considered catalysts in the overall oxygen reduction
mechanism is shown in Fig. 7.
1
1
1
¼
ꢀ
(2)
2
3
1
6
1
2
i
ik
ꢀ
*
0:62nFADO
n
CO
u
2
2
where F is Faraday's constant, A is the geometric area of the
electrode, D is the diffusion coefficient (1.90 ꢃ 10ꢀ5 cm2 sꢀ1), n
is the kinematic viscosity (8.70 ꢃ 10ꢀ3 cm2 sꢀ1), and C is the
bulk concentration of O2 (1.22 ꢃ 10ꢀ6 mol cmꢀ3).39
It has to be stressed that in contrast to the behavior known
for metal electrocatalysts in acid media, the oxygen reduction
on ruthenia based catalysts apparently forms primarily
hydrogen peroxide, namely at low overpotentials. The observed
selectivity of ruthenia-based catalysts in ORR shows a complex
potential dependence which can be treated either by a
phenomenological or a local structure sensitive approach.
Assuming a general phenomenological model of the oxygen
reduction mechanism as proposed previously (see Fig. 5),40,41
oxygen can be reduced to water (4-electron pathway) either
directly or sequentially with H2O2 as the main adsorbed
intermediate.
In principle, H2O2 either desorbs and can be detected on the
ring or can be further reduced to water in the second 2-electron
Fig. 2 SEM images of nanocrystalline (a) RuO2, (b) Ru0.90Zn0.10O2, (c) Ru0.80Zn0.20O2, (d) Ru0.95Ni0.05O2, (e) Ru0.90Ni0.10O2, and (f) Ru0.90Co0.10O2
after annealing at 400 ꢁC in air for 1 hour.
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approximately one order of magnitude lower and seem to
correspond to decreased tendency of these materials to reduce
oxygen through the 4-electron pathway.
As follows from Fig. 7, the conversion of O2 to H2O2 appears
to be the dominant process on ruthenium based oxides at low
overpotentials. In this respect the reduction behavior of the
ruthenia differs signicantly from that of metals which prefer
the 4-electron reduction at low overpotentials. The role of the
chemical composition in selectivity of doped catalysts towards
2- and 4-electron reduction pathways can be visualized by the
potential at which the catalytic system shows the same prefer-
ence for the 4-electron and 2-electron reaction pathways, i.e.
potential at which k1/k2 ¼ 1 (see Fig. 8).
A fundamental description of the oxygen reduction on oxide
surfaces can be based on the thermodynamic analysis of the
observed trend, which highlights the enhanced tendency of the
Ni- and Co-doped materials to form hydrogen peroxide and
reects the local structure of the doped ruthenium oxides.
DFT analysis of oxygen reduction
A fundamental description of the oxygen reduction on oxide
surfaces can be based on the thermodynamic analysis of the
process utilizing the DFT modeling. Reverting to the formalism
used for the oxygen evolution reaction we can describe the
overall reduction process as a sequence of four consecutive
concerted electron/proton transfers – if one aims for the
complete 4-electron reduction – or of two consecutive electron/
proton transfers if hydrogen peroxide is considered as the
reaction product. The results of the DFT investigations of ORR
reduction on ruthenium dioxide based catalysts are summa-
rized in Fig. 9–12.
Fig. 3 ORR polarization curves and ring currents at 1600 rpm for RuO2
and Ru1ꢀxMxO2 (M ¼ Ni, Co, Zn) electrodes at 20 mV sꢀ1 in O2 satu-
rated 0.1 M NaOH. Ering ¼ 1.1 V vs. RHE.
A systematic description of the stable surface structures at
different potentials represents a prerequisite step in the theo-
retical investigation of oxygen reduction on an oxide surface,
which in this case is the [110] rutile surface of ruthenia. This
procedure results in computational Pourbaix diagrams where
the stable surface at any given potential features the highest
stabilization (i.e. the most negative surface energy) of the
system (see ESI† for details of the Pourbaix diagram
construction).
Bearing in mind that the [110] oriented surface of a rutile
type oxide features the transition metal cations in two local
environments, cus and bridge, one can visualize the surface of
non-doped ruthenium dioxide as changing from the surface
structure characterized by protonated oxygen on cus sites and
deprotonated oxygens in bridge sites (region C in Fig. 9) to the
surface featuring vacant cus and protonated oxygens in bridge
sites (region A in Fig. 9). Since the ORR was not observed at
potentials positive to 0.7 V (vs.RHE) one can restrict the DFT
investigations of the oxygen reduction on conventional RuO2 to
the surface stable in the region A. In the case of doped ruthenia
(as shown in the case of the Ni doped material presented in
Fig. 10) one needs to consider the complexity arising from the
chemical composition when both types of transition metal
cations enter the cus and bridge positions. This variability in the
chemical composition also increases the number of distinctive
Fig. 4 Potential dependence of the average number of electrons
transferred during oxygen reduction on RuO2 and Ru1ꢀxMxO2 (M ¼ Ni,
Co, Zn) electrodes. Presented data were calculated using Koutecky–
Levich equation.
It seems that the conversion of H2O2 to H2O through the
series pathway (k3) is negligible on all electrode surfaces at high
overpotentials. It has to be noted, that although the k3 values are
negligible with respect to k1 and k2 there is a signicant
difference between k3 of the Zn-doped and non-doped samples
and those obtained for the Ni- and Co-doped samples. The
values of k3 observed for Ni- and Co-doped samples are
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Fig. 5 Phenomenological mechanism of oxygen reduction according
to ref. 40.
Fig. 7 Potential dependence of the rate constants for the reduction of
O2 to H2O (k1), of O2 to H2O2 (k2), and H2O2 to O2 (k3) on nano-
crystalline ruthenia based catalysts. The presented data correspond to
experiments carried out in O2 saturated 0.1 M NaOH at 1600 rpm.
oxygen atoms available at the surface, the binding energy of
which depends on their nearest neighbors. Although the elec-
trode potential dependent variability of the surface structure is
more pronounced in this case, the stable structure predicted for
the potential range in which the ORR proceeds is qualitatively
the same and corresponds to vacant cus sites complemented by
protonated oxygen atoms connecting the bridge sites (see
structure D in Fig. 10).
The DFT models predict that the entire process begins with
oxygen adsorption at coordination unsaturated (cus) cationic
sites. The behavior of both the non-doped and the doped ruth-
enia catalysts is controlled by the local structure and depends on
the nature of the cation residing in the cus site as well as on the
electrode potential. In the case that the cus site is occupied by a
ruthenium cation (which are present on all investigated catalysts)
the rst electron reduction forms a rather strongly bound *OOH
intermediate, which is more stable than the hydrogen peroxide at
most reasonable electrode potentials (see Fig. 11). Consequently,
the further reduction of the *OOH intermediate located on Ru cus
site cannot form hydrogen peroxide unless one uses an rather
strong external electric eld to weaken the *OOH binding to the
surface. The actual potential(s) at which hydrogen peroxide
formation becomes thermodynamically allowed are indicated in
the legend of the Fig. 11.
Fig. 8 The potential of equal rate in 2- and 4-electron reduction for
different ruthenia based catalysts.
binding of the *OOH and *O compared with the Ru occupied
cus sites. This fact decreases the potential at which the reduc-
tion on Nicus starts to contribute to the overall reduction
In the case that the cus site is occupied by an heteroatom, e.g.
Ni or Co, (see Fig. 11) one observes a signicantly weaker
Fig. 6 |ID/IR| vs. uꢀ1/2 plots for (a) RuO2, (b) Ru0.80Zn0.20O2, and (c) Ru0.90Ni0.10O2 presented data were extracted from RRDE experiments carried
out in O2 saturated 0.1 M NaOH.
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Fig. 12 Volcano plot for the 2-electron (blue) and 4-electron (red)
reduction of O2 to H2O2 and H2O, respectively, using the binding
energy of OOH as a descriptor. The dotted lines represent the equi-
librium potentials for the reduction products. In the case of the Ni-
doped ruthenia the limiting over-potential for both possible reaction
sites (Rucus and Nicus) are shown along with that of conventional
ruthenia.
Fig. 9 Surface Pourbaix diagram for RuO2. Detailed description of the
diagram construction is given in the ESI.†
process. The weak interaction of the *OOH with the
heteroatom-containing cus site restricts the presence of such an
adsorbate in the potential region with low total surface
coverage, i.e. to relatively high over-potentials. It needs to be
noted though, that the formation of hydrogen peroxide from
*OOH conned on a heteroatom occurs at much more positive
potentials than in the case of *OOH conned to Ru-containing
cus sites and further reduction of the *OOH intermediate can
proceed via the 4-electron or 2-electron reduction pathway with
approximately the same probability.
Fig. 12 shows the dependence of the electrode potential
needed to drive the oxygen reduction on oxide based surfaces
either via the 4-electron (red) or 2-electron (blue) reaction
pathway as a function of the reaction descriptor – i.e. adsorption
energy of the *OOH intermediate. It needs to be noted that in a
similar manner one may describe the reaction with
the adsorption of *OH due to the interdependence of the
adsorption energies of the intermediate formed in the rst and
third charge transfer step.3,33
Fig. 10 Surface Pourbaix diagram for Ni-doped RuO2. Detailed
description of the diagram/s construction is given in the ESI.†
Fig. 11 Free energy diagrams for the reduction of O2 on three catalytic active sites, the Ru cus site on conventional ruthenia (green), the cus Ru
site on Ni doped RuO2 (magenta) and the cus Ni site on Ni doped RuO2 (blue). The dotted line represents the equilibrium potential of the
reduction O2 to H2O2. The key difference is the binding of O on the Ni cus site compared to the Ru cus sites.
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Such a dual volcano plot has been used with great success from meta-stable intermediates not reected in the DFT
in literature.42,43 The volcano curves presented in Fig. 12 calculations.
clearly show a quantitative prediction of the thermodynamic
preference of the 4-electron reduction pathway over the 2-
electron reduction on strongly adsorbing cus sites. As follows
Conclusions
from Fig. 11, oxygen reduction on Ni-doped ruthenia should
Nanocrystalline ruthenia based electrocatalysts offer a conve-
proceed at slightly more positive potentials compared with
nient model for investigating the role of the local structure in
the non-doped ruthenia as long as the cus sites are occupied
the oxygen reduction on oxide electrodes. The oxygen reduction
related activity of RuO2 is comparable with that of the doped
ruthenia. The selectivity of doped ruthenia catalysts differs from
that of the RuO2 in which the non-doped as well as Zn-doped
catalysts prefer 4-electron oxygen reduction while the Ni- and
Co-doped ruthenia produce signicant amount of hydrogen
with Ru cations. The reduction process on Ru occupied cus
sites should show a pronounced preference for 4-electron
reduction and the formation of hydrogen peroxide should be
excluded for potentials positive of 0.14 V or 0.43 V (vs. RHE)
for non-doped ruthenia and the Ni-doped material, respec-
tively (see Fig. 11). The easier formation of hydrogen peroxide
peroxide. The observed selectivity trends can be rationalized
on the Ni-doped material should be compensated by an
using a thermodynamic analysis of the oxygen reduction
earlier onset of the oxygen reduction process as predicted for
process based on DFT calculations.
Ni-doped material. In the case of weakly adsorbing sites, e.g.,
The DFT based analysis connes the oxygen reduction
in the case of Ni cus sites – there is no apparent thermody-
activity to cus sites the occupancy of which controls the selec-
namic preference for either the 4- or 2-electron reduction
tivity of the oxygen reduction process. Oxygen reduction on non-
pathway. The DFT model predicts the onset of the oxygen
doped ruthenium dioxide is controlled by the fourth electron
transfer. Doping the ruthenium dioxide shis the potential
control to the rst electron transfer. This trend can be attrib-
reduction process to occur at potentials comparable with the
ORR on non-doped ruthenia. The formation of hydrogen
peroxide is possible at signicantly more positive potentials
uted to decreasing occupancy of the cus sites with ruthenium.
(see Fig. 11).
The strong adsorption of the *OOH intermediate on the Ru cus
Analyzing the experimental behavior of the ruthenium
site steers the reaction mechanism towards 4-electron reduc-
dioxide based catalysts for the oxygen reduction process in
tion pathway. Incorporation of reactive transition metal cations
the light of the DFT results one can qualify the existence of
into bridge sites has negligible effect on the ORR activity. A
two classes of catalysts – one favoring the 4-electron reduction
connement of the reactive transition metal into cus sites
(non-doped RuO2 and Zn-doped RuO2) and another showing
weakens the adsorption of the reaction intermediates and
opens the 2-electron reaction pathway at relatively low
overpotentials.
signicant activity in hydrogen peroxide production (Ni- and
Co-doped ruthenia). Realizing that the Zn present in the Zn-
doped ruthenia is itself redox inactive one can assume that
the catalysts in the rst group have all active cus sites occu-
pied by Ru regardless of the actual chemical composition. The Acknowledgements
connement of the catalytic activity to Ru itself justies the
This work was supported by the Grant Agency of the Czech
selectivity towards 4-electron reduction pathway as it is shown
Republic (contract P108-12-1889) and European Commission
within the Initial Training Network ELCAT (Project no. 214936).
The support of the Danish Ministry of Science, Technology and
Innovation though the CASE is also gratefully acknowledged.
in Fig. 4 and 8. In the case of the Co- and Ni-doped ruthenia
the signicant amount of hydrogen peroxide formed in the
process can be attributed primarily to the Ni/Co cus sites
although the Ru cus sites also contribute to the hydrogen
peroxide formation at lower potentials. In contrast to the
complementary oxygen evolution process, the Ni (or Co) ions
located in the bridge sites, which play crucial role in the
Notes and references
complementary anodic process,33 apparently have no effect on
the oxygen reduction activity of these materials. A different
role of the catalysts local structure in oxygen reduction is not
entirely surprising given the irreversibility of oxygen evolu-
tion/reduction.
The DFT calculations, however, fail to explain pronounced
formation of the hydrogen peroxide on all ruthenium based
catalysts at low overpotentials (0.55–0.40 V) when the hydrogen
peroxide on Ru cus sites should be thermodynamically
excluded. Given the relatively short timescale of the RRDE
experiments one may therefore suggest that the system fails to
reach the thermodynamically stable surface structure on the
experimental timescale and the hydrogen peroxide is released
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