"This is the peer reviewed version of the following article: A Million Turnover
Catalytic Water Oxidation,which has been published in final form at

Molecular Anode for

http://onlinelibrary.wiley.com/doi/10.1002/anie.201609167/abstract
This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for SelfArchiving."

A Million Turnover Molecular Anode for Catalytic Water Oxidation
Jordi Creus,[a],[b],⊥ Roc Matheu,[b],[c],⊥ Itziar Peñafiel,[b] Dooshaye Moonshiram,[d] Pascal Blondeau, [b]
Jordi Benet-Buchholz, [c] Jordi García-Antón, [a] Xavier Sala,[a],* Cyril Godard,[b],* Antoni Llobet,[a],[c],*
Abstract:. Molecular Ru based water oxidation catalysts precursors
of general formula [Ru(tda)(Li)2] (tda2- is [2,2':6',2''-terpyridine]-6,6''dicarboxylato; L1 = 4-(pyren-1-yl)-N-(pyridin-4-ylmethyl)butanamide,
1b; L2 = 4-(pyren-1-yl)pyridine ), 1c), have been prepared and
thoroughly characterized. Both complexes contain a pyrene group
allowing ready and efficiently anchoring via π interactions on MultiWalled Carbon Nanotubes (MWCNT). These hybrid solid state
materials are exceptionally stable molecular water oxidation anodes
capable of carrying out more than a million Turnover Numbers (TNs)
at pH 7 with an Eapp=1.45 V vs. NHE without any sign of degradation.
XAS spectroscopy analysis before, during and after catalysis
together with electrochemical techniques allow to monitor and verify
their unprecedented oxidative ruggedness.

homogeneous phase occur via an “Interaction of 2 M-O units”
(I2M) might still be able to carry out the catalytic water oxidation
reaction at the surface of an electrode, but will need to proceed
through higher energy pathways that can lead to catalyst
degradation.6 Further, given the intrinsic high energy demands
for the water oxidation catalysis, it is essential that the anchoring
groups that act as an interface between the catalysts and
surface are oxidatively resistant.
Here on, we report new hybrid materials consisting of molecular
WOCs anchored onto Multi-Walled Carbon Nanotubes
(MWCNTs) via π-stacking interactions.9 The resulting materials
are extremely stable and allow the anchoring of a large amount
of catalyst giving Turnover Numbers (TNs) over a million without
apparent deactivation.

Visible light induced water splitting to produce hydrogen fuel is
one of the potential alternatives to fossil fuels.1 To achieve this
goal, powerful and rugged Water Oxidation Catalysts (WOCs)
that can be anchored onto solid-state devices to facilitate water
splitting cell assembling and engineering are needed.2,3,45 In the
molecular front, it is imperative to have water oxidation catalysts
that can work under restricted translational mobility conditions,6
and whose O-O bond formation step occurs via a “Water
Nucleophilic Attack” mechanism (WNA). 7 , 8 Molecular WOCs
whose low energy O-O bond formation pathways in

In a recent publication, 10 we have reported the synthesis of
complex {RuII(tda)(py)2}, 1a, (for a drawing of tda2- see Scheme
1) and have shown that in its high oxidation states (IV) acts as a
precursor for the formation of {RuV(O)(tda)(py)2}+. The latter is
the most powerful molecular water oxidation catalyst described
to date achieving Turnover Frequencies (TOF) in the range of
50.000 s-1. In addition, we showed that the rate determining step
for the water oxidation reaction is the O-O bond formation, which
in this case occurs via WNA, as evidenced by kinetics and
further supported by DFT calculations.

[a]

[b]

[c]

[d]

J. Creus, Dr. J. García-Antón, Dr. X Sala, Prof. A. Llobet
Departament de Química, Universitat Autònoma de Barcelona
Carrer dels Til—lers s/n,
08193 Bellaterra (Cerdanyola del Vallès) Barcelona, Spain.
E-mail: xavier.sala@uab.cat
J.Creus, R. Matheu, Dr. I. Peñafiel, Dr. P. Blondeau, Dr. C. Godard
Departament de Química Física i Inorgànica, Universitat Rovira i
Virgili
Carrer Marcel—lí Domingo s/n
43007 Tarragona, Spain
E-mail: cyril.godard@urv.cat
R. Matheu, Dr. J. Benet-Bucholz, Prof. A. Llobet
Institute of Chemical Research of Catalonia (ICIQ), Barcelona
Institute of Science and Technology
Avinguda Països Catalans 16
43007 Tarragona, Spain
E-mail: allobet@iciq.cat
Dr. D. Moonshiram
Chemical Science and Engineering Division, Argonne National
Laboratory
9700 S. Cass Avenue
Lemont IL 60439, U.S.A

Scheme 1. Drawing of the ligands discussed in the present work (top)

and complex labelling strategy (bottom).
Supporting information for this article is given via a link at the end of
the document.

"This is the peer reviewed version of the following article: A Million Turnover
Catalytic Water Oxidation,which has been published in final form at

Molecular Anode for

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This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for SelfArchiving."
Given the remarkable performance of {RuV(O)(tda)(py)2}+, we
proceeded to anchor it on conductive solid supports with the aim
of generating a powerful hybrid anode for the electrocatalytic
oxidation of water to dioxygen, that could be potentially
incorporated in water splitting devices. For this purpose, we
used MWCNTs as support, given their high stability, conductivity
and large electrochemically active surface area.9, 11 Further,
MWCNTs were selected because of their inertness as compared
to oxides that can potentially block labile Ru-aqua groups and
thus reduce or even suppress the activity of the catalyst. 12
Moreover, this anchoring approach avoids the use of
phosphonate or carboxylate moieties that have a limited stability
in water in the presence of a supporting electrolyte under
irradiation. 13 In order to anchor our catalyst on MWCNTs, we
prepared pyridyl type of ligands functionalized with the pyrenyl
group as shown in Scheme 1, so that they can be anchored on
MWCNTs via π-stacking interactions without significantly
modifying the intrinsic electronic and geometrical properties of
the parent complex.14 We synthesized ligand L1 that contains an
amide group as previously described,15 and a new ligand L2 that
contains a direct C-C bond between the pyridyl group and the
pyrene moiety, see Supporting Information (SI) for details. The
latter strategy avoids the use of easily oxidizing methylene
groups, which is fundamental for the long-term performance of
any molecular water oxidation catalysts.16

The synthesis of complexes {Ru(tda)(Li)2} (i = 1, 1b; i = 2, 1c), is
straightforward and similar to related complexes (see details of
the synthesis in the SI).10, 15 A single crystal X-ray structure has
been solved for 1b and its ORTEP plot is shown in Figure 1. The
latter complex shows a distorted octahedral coordination around
the Ru(II) metal ion with the tda2- ligand acting in a κ-N3O
fashion and leaving one of the carboxylate moieties
uncoordinated. The axial positions are occupied by two pyridyl
moieties from two L1 ligands. Overall, the structure of 1b shows
a very similar first coordination sphere for the reported Ru as
compared to 1a.10 In order to further electronically and
structurally characterize these complexes, X-ray Absorption
Spectroscopy (XAS) was carried out for powders of 1a,
{RuIII(tda)(py)2}(PF6) (1a(PF6)), {RuIV(tda)(py)2}(PF6)2 (1a(PF6)2),
1b and RuO2 and the results are shown in Figure 2A and the SI.
In all cases the half-edge energies obtained from X-ray
Absorption Near Edge Structure (XANES) were consistent with
the oxidation state assignment, and the metric parameters
obtained by Extended X-ray Absorption Fine Structure (EXAFS)
were very similar to those of related X-ray structures (Table
S3).10
Glassy Carbon Disks (GCd, S = 0.07 cm2) were used as working
electrodes (WE) for all the electrochemical work described here
except when larger surface areas were needed. In the latter
case, Glassy Carbon Plates (GCp, S = 1 cm2) were used.
Further, a Pt disk and a Hg/HgSO4 electrode were used as
auxiliary and reference electrode respectively. All the potentials
reported here are converted to NHE by adding 0.65 V.
Conductive electrode materials were prepared by depositing a
few L of a suspension of MWCNTs on the surface of glassy
carbon electrodes. The solvent was then allowed to evaporate
and the new materials were labeled as “MWCNT@GC”. They
were then soaked in a solution of the catalyst precursor 1b or 1c
affording
the
hybrid
anode
materials
“{RuII(tda)(Li)2}@MWCNT@GC” (i = 1, 2b; i = 2, 2c), that
contained the catalyst precursor attached to the MWCNTs and
were characterized by electrochemical techniques and XAS.

II

1

Figure 1. ORTEP plot of the catalyst precursor {Ru (tda)(L )2}, 1b, (ellipsoids
drawn at 50% probability). Color code: Ru, cyan; N, navy blue; O, red; C,
black; H, small cyan open circles.

"This is the peer reviewed version of the following article: A Million Turnover
Catalytic Water Oxidation,which has been published in final form at

Molecular Anode for

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This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for SelfArchiving."

number of 6 (5N, 1O) for Ru(II) and assuming the typical
pseudo-octahedral geometry expected for a Ru(II) d6 ion. On the
other hand, for Ru(III) a coordination number of “6.5” was
assumed (5N, 1O, 0.5O), with a distorted octahedral
coordination containing an additional oxygen contact (Ru-O
distance of 2.4 Å), in a similar manner as found in the X-ray
structure of 1a+ (Figure S32, Table S4, fit 12).10 The data fit
obtained for 1a+ is very similar to that obtained for 2b+ reflecting
their structural similarities.

To generate the active catalyst at the surface of the electrode
material, a potential of 1.25 V was applied for 500 s under
stirring at pH 12 to 2b or 2c. This process oxidizes the initial
Ru(II) complex to its oxidation state IV, where the coordination of
a hydroxide anion occurs readily,10 as indicated in the equations
1 and 2 for 2b.

+

Figure 2. A, Normalized Ru K-edge XANES for 1a (black), 1b (cyan), 1a (red),
2+
1a (blue) and RuO2 (dark green). Inset: Plot of half k-edge energy vs.
oxidation state for Ru0 metal (orange), 1a and 1b (black), 1a+ (red) and RuO2
+
0
(dark green). B, Normalized Ru K-edge XANES for 1a (red), 2b (magenta),
2b’ (light green) and 2b” (brown). Inset: Plot of half k-edge energy vs
0
oxidation state (same color code as in A) 2b (magenta), 2b’ and 2b” (light
+
+
green). C, Difference spectra for: 1b-1a (black), 1b-RuO2 (red), 1a -RuO2,
(cyan), 2b-RuO2 (magenta) and 2b”-RuO2 (blue). D, Fourier transforms of k3+
+
0
weighted Ru EXAFS for the Ru(III) complexes, 1a (red) and 2b : (2b -20%
1b) magenta, (2b’-10% 1b) (light green) and (2b’’-10% 1b) brown. D. Back
Fourier transformed experimental (solid lines) and fitted (dashed lines) k3χ(k).
Experimental spectra were calculated for k values of 1.941-10.9 Å-1.

The amount of molecular complex deposited on the surface of
the electrode turned out to be of ͸ = 6.35 nmol/cm2 for 2b and
2
͹ = 0.20 nmol/cm for 2c. Further, XAS was carried out for 2b
anchored on GCp in order to additionally characterize the nature
of these hybrid materials. Unfortunately, the lower catalyst
loading obtained for 2c, even supported in the GGp electrode,
prevented its XAS analysis. For 2b it was found that the nature
of the molecular species attached to the surface of the MWCNTs
was identical to those of the precursor complexes, except that
atmospheric oxygen had oxidized the initial Ru(II) complex to
Ru(III) by 80%, as revealed by XANES and EXAFS (Figure 2B,
Table S2). Additional evidence for this oxidation phenomenon
was obtained by measuring the Open Circuit Potential (OCP) as
a function of time for a sample of 2b in an open atmosphere
(see Figure S30). We labelled this partially oxidized material as
2b0, and showed that its Ru κ-edge at half peak neatly
correlates with oxidation state 2.8 and thus indicates that the
sample 2b0 contains 80% 2b+ and 20% 2b (Figure 2B, magenta).
In addition, the simulated EXAFS experiments for 2b+ (2b0-20%
1b) also gives very good fits and thus further supports this point
(see Figure 2D, Figure S33, Table S4 and Table S5 (fit 4)). The
EXAFS simulations were carried out assuming a coordination

Once generated, the active hybrid materials were removed from
the pH 12 solution, rinsed with water and introduced in another
solution at pH 7. Under these conditions, a mixture of 2b2+ and
3b is generated with an approximate ratio of 5:1 that remains in
equilibrium, as deduced from cyclic voltammetry (CV)
experiments at pH 7 (See Figure 3 left).
Figure 3 shows that for the precursor material, waves for the III/II
and IV/III couples are observed at 0.55 V and 1.10 V
respectively, together with a large current density that appears
at 1.3-1.4 V associated with the oxidation of the MWNCTs. On
the other hand, on the CV of the 2b2+:3b mixture, additional
small waves appear in the 0.6-0.9 V potential range associated
with the electroactivity of the anchored {RuIV(O)(tda)(L1)2}
catalyst, 3b, as we have earlier described for its homologue
{RuIV(O)(tda)(py)2} in homogeneous phase.10

Figure 3. Left, red line, CV for 2b at in pH 7 solution at a scan rate of 100
2
mV/s from 0.25 to 1.45 V, with a surface coverage of $< = 6.35 nmol/cm
using GCd as WE. Green line, CV of a mixture of 2b2+:3b ( ͸ $ = 2.66
nmol/cm2 and ͸ = 0.55 nmol/cm2) under the same conditions. In black a

"This is the peer reviewed version of the following article: A Million Turnover
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blank for MWCNT@GCd. Inset, enlargement of the 0.2-1.4 V potential zone.
2+
Right, linear sweep voltammetry at pH 7 for the 2b :3b mixture (grey solid
line). Inset, plot of i/QR vs. [1/(1+e((E0-E)*F/RT))]. The black dashed line in
both cases represents the experimental data used for the FOWA analysis, and
the black solid line shows the experimental data used for the extraction of
-1
TOFmax = 8935 s for 3b.

Finally, a very large electrocatalytic current due to the oxidation
of water to dioxygen associated with the Ru(V)/Ru(IV) couple
occurs at 1.2-1.3 V, manifesting the high activity of this catalytic
hybrid material. Interestingly, current densities above 10 mA/cm2
are achieved here that are assumed to be critical for a potential
construction of a water splitting device.17
We quantified the performance of this new solid state molecularanode for water oxidation, comparing its performances with its

homogeneous homologue by carrying out a Foot of the Wave
Analysis (FOWA)18 also at pH 7 (see Figure 3). A TOFmax = 8935
s-1 was obtained from the fitted data similar to that obtained for
{RuIV(O)(tda)(py)2}.10 This is extremely important because it
clearly shows that the activity of the catalyst anchored on a solid
support, under translationally restricted mobility conditions, is
maintained. It thus allows transferring the information obtained in
homogeneous phase to the desired solid-state anode material,
thanks to the WNA nature of the O-O bond formation step that
occurs both in homogeneous phase and anchored. This is in
sharp contrast with the related complex {RuIV(O)(bda)(4-Me(bda2is
6,6’-dicarboxylate-2,2’-bypyridine),
that
py)2}
mechanistically operates via a bimolecular I2M mechanism. 19
The latter, once anchored needs to change its mechanism to a
higher energy pathway that significantly decrease the TOFmax
values and leads to degradation.
The activity of “{RuIV(O)(tda)(L2)2}@MWCNT@GC”, 3c, at pH 7
was also evaluated in a similar manner as that of 3b, giving a
TOFmax = 8076 s-1. This is very similar to that obtained for 3b
(see Figure S24), further supporting the suitability of the chosen
heterogenization strategy. The long-term stability of these new
solid-state hybrid molecular anodes were evaluated at pH 7
based on repetitive CV, bulk electrolysis and XAS, shown in

Figures 2 and 4. The upper part of Figure 4 displays 1000
repetitive CV scans carried out at 100 mV/s for the anodes
containing mixtures of 2b2+:3b and 2c2+:3c, between 0.25 and
1.45 V. For the case of 2b2+:3b (Figure 4, top left), as the
repetitive cycles proceed, both the intensity of the
electrocatalytic current and the intensity of the waves due to the
catalyst precursor progressively decrease, until no electroactivity
is observed. Thus as the catalytic reaction proceeds, the catalyst
and catalyst precursor progressively disappear from the surface
of the electrode, most likely due to the oxidation of the linker. In
sharp contrast for the case of 2c2+:3c, the intensity of the
electrocatalytic current decreases by approx. 65% of its initial
value but the electroactivity of the precursor catalysts, 2c2+,
remains intact as shown in Figure 4, top right. The change in the
intensity of the electrocatalytic current at 1.45 V is mainly
attributed to a shift of the equilibrated species between
precursor 2c2+, and the active catalytic species 3c that occurs
during long-term catalysis (see figure S25 for an inset of the
molecular peaks). Also, a small decrease of the intensity can be
attributed to the partial detachment of the MWCNT due most
likely to a mechanical friction effect.
A similar trend is observed when bulk electrolysis experiments
using GCd electrodes are carried out at pH 7 with an applied
potential of 1.45 V as can be seen in Figure 4, bottom. For the
system 2b2+:3b (red line), the initial current density reaches a
value of 2 mA/cm2, but as time elapses the current density
progressively decreases to less than 0.25 mA/cm2 after 2.5 h.
On the other hand, for the 2c2+:3c system (blue line) the initial
current density is 1.5 mA/cm2, and it decreases to 0.7 mA/cm2 at
about 40 minutes and then remains constant. While the hybrid
anode 2c2+:3c is extremely stable generating roughly 0.18
million TNs without apparent deactivation, 2b2+:3b slowly
deactivates but still giving a remarkable final TNs of 0.67 million.
TNs in the range of 1.2 million can be obtained for 2c2+:3c under
similar conditions but by running the experiment for longer
periods of time (12h; see figure S27). The strikingly different
long-term performances of these two anode materials are
associated with the different oxidative stability of their linking
moieties as discussed above for the repetitive CV experiments.
These results manifest again the importance of ligand design for
long lasting anodes for water splitting applications that if properly
designed can parallel the performance of related oxide based
electroanodes. 20 Finally, a bulk electrolysis experiment was
performed in a GCp for 2b2+:3b under similar conditions, and the
amount of O2 generated was measured via a Clark electrode on
the gas phase giving a Faradaic efficiencies above 90%,
showing once more the ruggedness of the present system
(Figure S28).The remaining current is basically used for the
oxidation of the graphite electrode.

"This is the peer reviewed version of the following article: A Million Turnover
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The structure of the hybrid material 2b2+:3b was also analyzed
by XAS, using glassy carbon plates GCp and the results are
reported in Figure 2, S32, S33 and Tables S3 and S5. Two
samples of the hybrid material 2b2+:3b ( ͸ $ = 0.57 ± 0.16
nmol/cm2 and ͸ = 0.64 ± 0.24 nmol/cm2; ͸ $ : ͸ =0.89 ± 0.20
nmol/cm2) were exposed to a bulk electrolysis experiment at pH
7, with an applied potential of 1.45 V for 1000 s for the first
sample and for 1h for the second one.

contribution, and thus confirms the presence of 2b+. This is a
very important result since it shows that the nature of the
catalyst remains intact after catalysis. In addition, XAS
spectroscopy unambiguously shows the absence of any traces
of RuO2 after 1h catalysis. This can be monitored by the specific
peak at 22156 eV, nicely visualized through the difference
spectra in Figure 2C, that is highly characteristic of RuO2 as well
as by the absence of RuO2 in the EXAFS spectral features
shown in Figure 2D and S33. This is again very significant since
it clearly demonstrates the molecular nature of the catalysis in
heterogeneous phase, in sharp contrast with many instances
where the original molecular catalyst is transformed to the
corresponding metal oxide that ends up being the real active
catalyst.6
In conclusion, the present work reports a million turnover
molecular electroanode that consists of a molecular Ru catalyst
anchored on the surface of MWCNTs via a pyrenyl functionality.
The extraordinary unprecedented stability of the molecular
catalyst is a result of a bottom up approach that includes a
thorough mechanistic understanding of the water oxidation
catalyst steps involved in water nucleophilic attack events. XAS
spectroscopy has been shown to be a very valuable tool in the
solid state to monitor the long-term stability and molecular
nature of the anchored water oxidation catalysts.

Acknowledgements
Figure 4. Top, 1000 repetitive CV scans at pH 7 at a scan rate of 100 mV/s
2+
2
from 0.25 to 1.45 V for 2b :3b (left, ͸ $ = 2.07 nmol/cm and ͸ = 0.36
nmol/cm2) and 2c2+:3c (right, ͹ $ = 0.13 nmol/cm2 and ͹ = 0.03 nmol/cm2) at
GCd. Black solid line is the first cycle, black dashed line is the 1000th cycle. In
nd
th
2+
grey are 2 -999 cycles. Bottom, bulk electrolysis of 2b :3b (red, ͸ $ = 2.10
nmol/cm2 and ͸ = 0.40 nmol/cm2) and 2c2+:3c (blue, ͹ $ = 0.11 nmol/cm2
2
and ͹ = 0.03 nmol/cm ) in a phosphate buffered solution at pH 7 at Eapp =
1.45 V under stirring using GCd as WE. The experiments were stopped at
1000 s and 3000 s and then subsequently reinitialized. See Figure S29 for an
analogous experiment on bare MWCNT@GCd.

Subsequently a CV was carried out with a final potential of 0.2 V
that generated the catalyst precursor and the active catalyst at
oxidation state II, that are labeled as 2b’ and 2b’’ for the
samples exposed to 1000 s and 1h electrolysis, respectively,
and left in open air for a week. As was also the case for 2b0,
XANES spectra show half peak κ-edge energies that indicate
that initial Ru(II) complex is oxidized to Ru(III) by 90% in both
cases (see Table S2 and top inset in Figure 2B). In addition, the
EXAFS (see Figures 2D, S32, Table S3 and Table S5 (fits 4, 8
and 12)) point out that the samples before catalysis 2b0 and
after 1000 s and 1h catalysis 2b’ and 2b” respectively, are
practically identical to 1a+, after subtraction of their Ru(II)

A.L., X.S. and J.G-A thank MINECO (CTQ-2013-49075-R, SEV2013-0319; CTQ-2014-52974-REDC and CTQ2015-64261-R).
J.C. and R.M respectively thank UAB, “Euroregió Pirineus
Mediterrània” and “La Caixa” for PhD grants. D.M.
acknowledges support from the US DOE, Contract No. DEAC02-06-CH11357. J.G-A. thanks Serra Húnter Program.
Keywords: water oxidation catalysis, electrocatalysis, water
splitting, redox properties, transition metal complexes,

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"This is the peer reviewed version of the following article: A Million Turnover
Catalytic Water Oxidation,which has been published in final form at

Molecular Anode for

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This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for SelfArchiving."

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"This is the peer reviewed version of the following article: A Million Turnover
Catalytic Water Oxidation,which has been published in final form at

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COMMUNICATION
Oxidatively stable molecular water
oxidation anodes reaching more
than a million turnover numbers
with no molecular degradation

Jordi Creus, Roc Matheu, Itziar
Peñafiel, Dooshaye Moonshiram,
Pascal Blondeau, Jordi BenetBuchholz, Jordi García-Antón, Xavier
Sala,* Cyril Godard,* Antoni Llobet,*
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