Oxygen evolution at functionalized carbon surfaces: A strategy for immobilization of molecular water oxidation catalysts

Article abstract of DOI:10.1039/c2cc35379b

A molecular Ru(ii) water oxidation catalyst was immobilized on a conductive carbon surface through a covalent bond, and its activity was maintained at the same time. The method can be applied to other materials and may inspire development of artificial photosynthesis devices. This journal is The Royal Society of Chemistry 2012.

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Oxygen evolution at functionalized carbon surfaces: a strategy for
immobilization of molecular water oxidation catalystsw
a
b
ac
¨
Lianpeng Tong, Mats Gothelid and Licheng Sun*
Received 25th July 2012, Accepted 22nd August 2012
DOI: 10.1039/c2cc35379b
A molecular Ru(II) water oxidation catalyst was immobilized on
a conductive carbon surface through a covalent bond, and its
activity was maintained at the same time. The method can be
applied to other materials and may inspire development of
artificial photosynthesis devices.
inevitably suffers from hydrolysis in aqueous surroundings,
applications of noncovalent binding have certain limitations with
respect to specific properties of electrode materials and catalysts.
Herein, we would like to report a novel and widely applicable
approach, by which molecular WOCs can be immobilized on
conductive carbon surfaces via a strong covalent bond.
Our strategy consists of modifying catalysts with terminal
acetylene groups on one hand, grafting azide-groups at the
electrode surface on the other, and subsequently coupling
the surface and WOCs via copper(I)-catalyzed azide–alkyne
cycloaddition (so-called CuAAC or ‘click’) reaction, as depicted in
Scheme 1a.17 Advantages of this strategy includes: (1) the linkage
between catalysts and surface is a robust triazole ring that is
resistant to high oxidative potential and O2-rich aqueous solutions;
(2) CuAAC is a conveniently fast reaction even regarding
heterogeneous coupling conditions. In the present work, a
readily obtained glassy carbon (GC) disk electrode (+ =
3 mm) was employed as a conductive platform, and a well-defined
mononuclear Ru WOC, [RuII(pdc)(pic)3] (pdc = 2,6-pyridine-
dicarboxylate, pic = 4-picoline), was chosen as a model complex
to be immobilized.5,18 It is important to note, however, that the
strategy described here can be applied not only on GC and
[RuII(pdc)(pic)3], but also on other carbon surfaces and various
kinds of molecular WOCs.
Promising progress has been made on the development of mole-
cular catalysts for water oxidation (2H2O - O2 + 4H+ + 4e–)
which is the critical and fundamental reaction of solar-energy
conversion in both natural and artificial photosynthetic systems.1,2
One of the latest breakthroughs in the field of water oxidation
catalysis is the discovery of ruthenium and first-row transition-
metal based complexes as highly efficient water oxidation
catalysts (WOCs).3–7 Typically, the catalytic activity of molecular
WOCs is measured in a homogeneous aqueous medium using
cerium(IV) (under acidic conditions) or tris(bipyridine)Ru(II) (under
neutral conditions) as sacrificial oxidants, and a remarkably high
turnover frequency (TOF) of 300 s–1 has been achieved very
recently.8 Moreover, accessible insight into mechanism and feasible
structure modification of molecular WOCs surely enable
further improvement in existing catalysts. However, for any
implementation in an electrocatalytic or photoelectrocatalytic
device toward water splitting, molecular WOCs have to be
immobilized on the semi-conductive or conductive surface of
an electrode.9,10
One of the major scientific challenges for WOCs immobilization
is the requirement of a stable linking manner under harsh oxidizing
conditions (the thermodynamic potential for water oxidation E =
1.23 ꢀ 0.059 ꢁ pH V vs. NHE). Reported approaches of tethering
WOCs to the electrode surface mainly lie in two categories: one is
covalent linkage through an ester type of bond formed between
hydroxyl groups of the oxide surface and carboxylic or phosphoric
groups of complexes;11–13 the other is noncovalent linkage through
electrostatic or p–p stacking interactions.14–16 While the ester bond
a Department of Chemistry, School of Chemical Science and
Engineering, KTH Royal Institute of Technology, 100 44 Stockholm,
Sweden. E-mail: lichengs@kth.se
b Department of Microelectronics and Applied Physics,
School of Information and Communication Technology,
KTH Royal Institute of Technology, 100 44 Stockholm, Sweden
c State Key Laboratory of Fine Chemicals, DUT-KTH Joint
Education and Research Center on Molecular Devices, Dalian
University of Technology (DUT), 116024 Dalian, China
w Electronic supplementary information (ESI) available. See DOI
10.1039/c2cc35379b
Scheme 1 Oxygen evolution at
a functionalized GC surface.
(a) Modification of the GC electrode with azide groups and coupling
of ruthenium complex 1 by ‘click’ reaction. (b) Schematic illustration
of a water-splitting electrochemical cell using functionalized GC as a
working electrode. (c) Molecular structure of ruthenium WOC 1.
c
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The acetylene–substituted complex (1 shown in Scheme 1c),
as a derivative of [RuII(pdc)(pic)3], was developed by firstly
substituting 4-H of pdc with trimethylsilylacetylene and
the functionalized GC, the peak current of this characteristic
redox potential did not show any discernible decay. Furthermore,
the current peak was found to depend linearly on the scan rate
from 20 to 500 mV sꢀ1 with zero intercept (Fig. S5, ESIw), which
is consistent with the behavior of surface-tethered redox couples.
In contrast, no redox waves were observed in the CV curve of a
control GC (black dash in Fig. 1), which is a pristine GC electrode
treated using the same immobilization procedure as that for
the azide-modified GC. These experiments clearly confirmed
that WOC [RuII(pdc)(pic)3] was immobilized on the GC
surface via a robust linkage, and a functionalized carbon
surface [RuII(pdc)(pic)3]@GC was yielded as we designed.
Analysis of [RuII(pdc)(pic)3]@GC by energy-dispersive
X-ray spectroscopy (EDS) revealed a 0.12% weight percentage
of ruthenium. By comparison, no weight percentage of ruthenium
obviously higher than the background noise was detected in the
EDS analysis of a pristine GC. The analyzed depth is on the order
of 0.5 mm and the weight of the Ru at the surface is therefore
small relative to the total mass.
The effective coverage of WOC (G0, mol cmꢀ2) on the
[RuII(pdc)(pic)3]@GC surface was estimated according to
the linear relationship between the peak current of RuII/RuIII
and the scan rate (see ESIw for details). An average concentration
of 1.2 ꢂ 0.4 (10–10 mol cm–2) was obtained from three independent
experiments. Notably, this value can be significantly enhanced
if a carbon electrode with a larger specific surface was used
instead of GC.20
The electrocatalytic activity of [RuII(pdc)(pic)3]@GC was
first investigated by CV in a standard three-electrode cell
consisted of a working electrode, a saturated calomel electrode
(SCE) reference electrode and a platinum wire counter electrode
(Scheme 1b). In a pH 7.0 phosphate buffer (IS = 0.1 M), the
CV curve of [RuII(pdc)(pic)3]@GC exhibits an onset of uprising
current at about 1.25 V vs. NHE compared with the CV curve
of a pristine GC (Fig. S7, ESIw), which is indicative of an
electrocatalytic water oxidation on the [RuII(pdc)(pic)3]@GC
surface. This catalytic onset potential agrees with the electro-
catalytic behavior of [RuII(pdc)(pic)3] in a homogeneous neutral
aqueous medium, indicating retention of the catalytic activity of
[RuII(pdc)(pic)3] after its immobilization.
The electrocatalytic activity of the [RuII(pdc)(pic)3]@GC surface
was further evaluated in a pH 7.0 phosphate buffer (IS 0.1 M) by
chronoamperometric experiments, in which the current density of
bulk electrolysis, either at a [RuII(pdc)(pic)3]@GC or a pristine
GC, was monitored continuously under a sequence of applied
potential steps in the range from 1.1 to 1.5 V vs. NHE (shown in
Fig. 2a and Fig. S9, ESIw). When the overpotential Z—referred
to an additional voltage to the thermodynamically required
oxidative potential of water at pH 7.0 (E = 0.82 vs.
NHE)—reaches 300 mV, an appreciable catalytic current
was observed at a [RuII(pdc)(pic)3]@GC in comparison with
the current at a pristine GC (as a background). And the
difference between current densities became quite significant
at Z = 500 mV, which implied a much more effective Faradaic
process on the [RuII(pdc)(pic)3]@GC surface, ascribed to an
efficient catalytic water oxidation. The catalytic current density
measured on [RuII(pdc)(pic)3]@GC as a function of Z followed
a Tafel behavior that is displayed in the inset of Fig. 2b. Under
the experimental conditions, the integral quantity of transferred
affording
4-(trimethylsilyl)ethynylpyridine-2,6-dicarboxylic
acid (H2Tepc), secondly coordinating ruthenium with Tepc
and pic ligands in one-pot reaction, and finally deprotecting
TMS with nBu4N+F–. The TMS group was used here in order
to prevent any coordination between the Ru core and the
terminal acetylene. Complex 1 was fully characterized by
1H NMR,13 C NMR, mass spectrometry and element analysis.
Synthetic and spectroscopic details were provided in the ESI.w
In principle, terminal acetylene can be installed to numerous
types of ligands and complexes via facile synthetic chemistry.19
Reactive azide groups were grafted to the GC by direct
electrochemical reduction of 4-azidobenzene diazonium tetra-
–
fluoroborate salt ([N3–C6H4–N2]+BF4 ) in 0.1
M HCl
(a literature procedure was followed).20 It is widely accepted
that during the electrochemical reduction, an aryl radical
generates from an aryl diazonium ion, attacks the carbon
surface and forms a covalent C–C bond with the surface.21
This process can be reflected from cyclic voltammogram (CV)
curves of [N3–C6H4–N2]+BF4 recorded in 0.1 M HCl on a
–
GC electrode (Fig. S3, ESIw). The first scan showed a broad
irreversible wave at around ꢀ0.4 V vs. NHE, corresponding to
the reductive potential of [N3–C6H4–N2]+BF4 . At the second
–
scan, however, the wave almost disappeared, indicating the
block of the surface by the organic layer grafted to the surface.
Besides GC, such an electrochemical grafting method has been
successfully applied to a variety of carbon materials, like
HOPG, carbon fibers and carbon nanotubes.21,22
Once the azide-modified GC and the acetylene-terminated
WOC 1 were readily prepared, immobilization of the catalyst
was carried out by simply immersing the modified electrode in
a nonaqueous solution of 1 containing [Cu(CH3CN)4]+BF4
–
and TBTA as a typical condition for click chemistry. After a
given incubation period, the catalyst-functionalized GC was
copiously washed with acetone and ethanol, and then sonicated
in acetone and ethanol, respectively, in order to remove any
physically adsorbed complexes. The CV curves of catalyst-
functionalized GC, recorded in the CH2Cl2 electrolyte, exhibit a
reversible wave at 0.5 V vs. NHE (red line in Fig. 1). This redox
potential is identical to the E1/2(RuII/RuIII) of [RuII(pdc)(pic)3]
in a homogenous CH2Cl2 solution.18 After multiple CV scans of
Fig. 1 CV curves of the [RuII(pdc)(pic)3] functionalized GC electrode
(red line) and control GC electrode (black dash); conditions: 0.1 M
nBt4N+PF6 in CH2Cl2, scan rate = 100 mV sꢀ1, T = room
–
temperature.
c
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In summary, our work herein provides a universal method,
by which molecular WOCs can be firmly immobilized on
conductive carbon surfaces, and meanwhile keep their catalytic
activity at the hybrid interface. In principle, this method should be
applicable to any molecular WOCs that can be modified with
terminal acetylene groups. An employment of an engineered
carbon electrode with large specific area, rather than GC, will
significantly improve the concentration of WOC on the surface as
well as promote catalytic current density in order of magnitude.
Moreover, our strategy reported here is not limited by the size or
specific properties of conductive carbon surfaces, which will
inspire fabrication of an efficient water-splitting device in the
context of exploitation of sustainable solar fuels.
We thank the Swedish Research Council, the K & A
Wallenberg Foundation, the Swedish Energy Agency, the
China Scholarship Council (CSC), the National Natural
Science Foundation of China (21120102036) and the National
Basic Research Program of China (2009CB220009) for financial
support of this work.
Notes and references
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Fig. 2 Electroactivity of the [RuII(pdc)(pic)3] functionalized GC surface.
(a) Chronoamperometric current density measured in potassium phos-
phate buffer (pH 7.0, IS = 0.1 M) at the [RuII(pdc)(pic)3]@GC electrode
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potential Z. The inset is a Tafel plot showing current density of the
[RuII(pdc)(pic)3]@GC surface vs. Z.
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´ ´
electrons, assigned to catalytic water oxidation by immobilized
[RuII(pdc)(pic)3], can be derived from the subtraction between the
integrated charge through a [RuII(pdc)(pic)3]@GC (Qc, C cmꢀ2
)
and that through a pristine GC (Qp, C cmꢀ2) during the integration
time (t = 360 s) of given overpotential. Thus, the TOF of
electroactive WOC on the [RuII(pdc)(pic)3]@GC surface can
be estimated by the following equation:
1
4
1
ðQc ꢀ QpÞ
FG0
TOF ¼
ꢁ
ꢁ
t
where F is Faraday’s constant. Generation of every O2 molecule
includes extraction of four electrons from two H2O molecules.
The logarithm of yielded TOFs varies linearly on the applied
overpotentials from 300 to 700 mV, as shown in Fig. 2b. This
Tafel-like plot implicates a rate-determining step of electron
transfer for O2 evolution from the [RuII(pdc)(pic)3]@GC surface.
An initial TOF of 0.073 s–1 was achieved at Z = 300 mV, which
reaches 1.6 s–1 at Z = 700 mV. Our previous study demonstrated
that water oxidation by [RuII(pdc)(pic)3] occurred in a typical
homogenous CeIV–CF3SO3H solution with a TOF of 0.23 s–1.5,18
Although the calculated TOF from [RuII(pdc)(pic)3]@GC
represented an upper bound of the realistic value, because
Faradaic efficiency was assumed to be 100% in the calculation,
the result emphasized a comparable, if not superior, catalytic
activity of [RuII(pdc)(pic)3] on conductive surfaces in relation
to homogeneous conditions.
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c
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Chem. Commun.