Reduction-induced surface amorphization enhances the oxygen evolution activity in Co3O4

Article abstract of DOI:10.1039/c5ra00995b

Modifying crystalline Co₃O₄ by thermal H₂ annealing and air exposure produced an amorphous surface layer consisting of mixed hydrated cobalt hydroxide/carbonate species and remarkably enhanced the oxygen evolution activity

Full text of DOI:10.1039/c5ra00995b

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Reduction-induced surface amorphization
enhances the oxygen evolution activity in Co3O4†
Cite this: RSC Adv., 2015, 5, 27823
a
a
a
a
ab
*
Xue Leng, Qingcong Zeng, Kuang-Hsu Wu, Ian R. Gentle and Da-Wei Wang
Received 17th January 2015
Accepted 13th March 2015
DOI: 10.1039/c5ra00995b
www.rsc.org/advances
Modifying crystalline Co3O4 by thermal H2 annealing and air exposure
First-row transition metal oxides, such as spinel oxides and
produced an amorphous surface layer consisting of mixed hydrated perovskites, have received increasing interest for their great
cobalt hydroxide/carbonate species and remarkably enhanced the potentials as alternative low-cost OER electrocatalysts. Among
oxygen evolution activity.
them, spinel oxide Co3O4 has shown good activity and stability
in OER.9–11 By reducing the particle size or increasing the
porosity of Co3O4, more active sites can be generated. As a
result, porous Co3O4 nanoparticles exhibited superior OER
activity.12,13 Lattice substitution with Li, Ni, or Cu for Co also
enhanced the OER activity.11,14–17 Au-decorated Co3O4 showed
enhanced OER activity,18–20 which is attributed to the facilitated
oxidation of Co(2+) and Co(3+) in Co3O4. Gerken et al. suggested
that the surface of Co3O4 is reformed into a layered structure
during the OER process which is accompanied by the oxidation
of Co species.21 This is supported by the phase transformation
of Co3O4 to layered CoOOH during OER according to in situ
Raman spectroscopy and X-ray diffraction (XRD).19,22,23 Recently,
it has been reported that mesoporous Co3O4 nanowires treated
with NaBH4 exhibited excellent OER activity. And this superior
performance was attributed to the large surface area and the
oxygen vacancies which lead to higher conductivity and elec-
trocatalytic activity.24 Obviously the OER activity of Co3O4
depends on many factors such as morphology, phase structure,
crystallinity, composition, co-catalyst, and surface properties.
Amorphous OER electrocatalysts have shown great interest
recently. Amorphous Co3O4 lm occupied more active sites and
hence displayed superior OER activity compared to the crystal-
line lm at low overpotentials.25 An amorphous CoPi or CoBi
lm synthesized by electrodeposition has attracted intensive
study due to its application in mild environment (neutral pH,
room temperature).6,26–29 Its superior performance is attributed
to the small size of CoOx fragments. Besides cobalt, other metal
based amorphous materials also show promising performance
towards OER. Electrodeposited amorphous FeOOH lm
exhibits OER activity comparable to that of CoBi lm.30 Berlin-
guette et al. reported that the OER catalytic properties of
amorphous Fe2O3 is superior to those of crystalline Fe2O3 and
they synthesized an amorphous oxides Fe100ꢀyꢀzCoyNizOx with
OER activity comparable to those of noble metal oxides.31
The increasing public concern about global climate change and
the energy crisis demands more efficient sustainable technol-
ogies for renewable carbon-neutral energy. The oxygen evolu-
tion reaction (OER) plays a pivotal role in a number of energy
conversion and storage processes and devices, such as water-
splitting and rechargeable metal–air batteries.1–5 Taking the
anodic reaction in water splitting for instance, one dioxygen
molecule is generated from two water molecules through a four-
electron transfer reaction coupled with the formation of four
protons (eqn (1)).6,7
2H2O 4 O2[ + 4eꢀ + 4H+ E ¼ 1.23 V vs. RHE
(1)
In practical operation, overpotential (h) is required to over-
come the sluggish reaction kinetics. Overpotential requires the
input of extra energy which oen reduces the conversion effi-
ciency from electrical energy to chemical energy. How to reduce
the OER overpotential in order to improve the energy efficiency
remains a great challenge. Precious noble metal-based mate-
rials, such as IrO2 or RuO2, are the most active OER electro-
catalysts that have shown the best energy efficiency.8 However,
due to the scarcity and high cost of precious metals, their scale-
up application has been impeded to some extent. Therefore
searching for non-noble active OER electrocatalysts is of para-
mount importance.
aSchool of Chemistry and Molecular Biosciences, The University of Queensland,
Brisbane, QLD 4072, Australia
bSchool of Chemical Engineering, The University of New South Wales, Sydney, NSW
2052, Australia. E-mail: da-wei.wang@unsw.edu.au
† Electronic supplementary information (ESI) available: Experimental section,
XPS tting parameters, scheme of surface transformation, working principle of
RRDE, TOF calculation, and stability test. See DOI: 10.1039/c5ra00995b
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OER activity of carbon-supported amorphous nanoparticles
Ni0.69Fe0.31Ox outperformed the commercial noble Ir/C cata-
lyst.32 Even for the crystalline perovskites, surface amorphiza-
tion contributes to the improvement of OER catalytic activity.33
It is perceived that the number of active sites in amorphous
materials is higher than that in crystals due to the small clusters
in amorphous phase.
It has been reported that the thermal H2 treatment intro-
duced a disordered layer on the surface of TiO2 which facilitated
the photocatalytic activities substantially.34 It is speculated that
OER activity of Co3O4 may be improved by H2-reduction-
induced surface amorphization. In this work, we report the
surface amorphization of Co3O4 crystals caused by thermal H2
reduction and stabilization in air. We demonstrate the largely
improved OER activity which is facilitated by the amorphous
surfaces. The crystal structure and the surface properties of the
modied Co3O4 crystals were studied by using X-ray diffraction
(XRD), X-ray photoelectron spectroscopy (XPS) and trans-
mission electron microscopy (TEM). The results suggested that
the mixed hydrated cobalt hydroxide/carbonate species in the
surface amorphous layer are essential to the performance
improvement.
Fig. 1a shows the thermal gravimetric (TG) curve for the
thermal reduction of as-prepared Co3O4 (Co3O4-AP) under ow
of 5 vol% H2 + N2. Before 130 ꢁC, the 1.6% weight loss is
attributed to the removal of the absorbed species on the surface,
such as water. In the range of 130 ꢁC and 600 ꢁC, the total weight
loss is 26.6% which corresponds to the theoretical mass loss
(26.5%) for complete transformation of Co3O4 into Co metal.
The reduction proceeds in two steps: a 6.9% weight loss occurs
at the rst step ranging from 130 ꢁC to 320 ꢁC, while the second
step from 350 ꢁC to 600 ꢁC has a 19.7% weight loss. No obvious
weight loss is observed betweenꢁ320 ꢁC and 350 ꢁC. The Co3O4-
AP sample was reduced at 320 C for 1 h; otherwise the long-
term reduction at higher temperature could result in pure Co
metal. The reduced sample was stored in ambient air without
particular protection (denoted as Co3O4-R). We show later that
the surface of Co3O4-R became an amorphous layer containing
different Co(+2, +3) species aer the ambient storage. X-ray
diffraction (XRD) was adopted to study the phase evolution
from Co3O4-AP to Co3O4-R. Fig. 1b reveals that Co3O4-AP is pure
Co3O4 while additional reducing species (CoO and Co) exist in
Co3O4-R. Peaks related to CoO and Co are broad, indicating
their low crystallinity. Crystal sizes of Co3O4 are estimated to be
21 nm for Co3O4-AP and 17 nm for Co3O4-R by using the
Scherrer equation.
XPS was used to analyze the surface chemical species on the
original and reformed samples. The tted XPS proles are
shown in Fig. 2 and the tting parameters are listed in Table
S1.† As shown in Fig. 2a, Co3O4-AP exhibited typical Co 2p3/2
spectrum.35–37 The principle peak was tted by three sub-peaks:
one at 779.5 eV represents Co(3+); the other two at 781 eV and
782.5 eV are assigned to Co(2+). Co(2+) is characteristic of
higher binding energy and broad principle peak.37 Peaks S1 and
S2 are assigned to shake-up satellites. What is interesting is that
the Co 2p3/2 spectrum of Co3O4-R is almost the same with that of
Co3O4-AP, containing peaks related to Co(3+) and Co(2+) with
Fig. 1 (a) Thermal gravimetric curve of the reduction of Co3O4-AP
recorded at the heating rate of 2 ꢁC minꢀ1 under reducing environ-
ment of mixture 5 vol% H2 + N2. (b) XRD profiles of Co3O4-AP (blue)
and Co3O4-R (red).
similar ratios. This result is consistent with the result of
thermal-H2-treated TiO2 which showed similar Ti 2p spectrum
to that of pristine TiO2.34,38 But it is different from the XPS
results of NaBH4-reduced TiO2 and Co3O4 which exhibited
negative shi in the Ti 2p and stronger Co(2+) signal in Co 2p
respectively.24,39 This conrms the re-oxidation of the reduced
sample on exposure to air which is reasonable considering that
Co and CoO are thermodynamically unstable in atmospheric
condition. It has been known widely that CoO is easily oxidized
to Co3O4 under mild ambient condition.35,40 The surface of Co
metal is oxidized to hydrated cobalt oxide or cobalt (oxy)
hydroxide upon exposure to air at room temperature.41,42
Nonetheless the XRD results reveal Co and CoO phases in the
bulk of Co3O4-R sample, which could imply that the oxidation of
Co3O4-R is likely limited to the surface.
To elaborate the detailed surface chemical structure, the O
1s and C 1s spectra of Co3O4-R were recorded and compared
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Fig. 2 XPS high resolution spectra of Co3O4-AP (a–c) and Co3O4-R
(d–f): Co 2p3/2 (a and d), O 1s (b and e) and C 1s (c and f).
with those of Co3O4-AP. The tting parameters are listed in
Table S1.† O 1s spectrum of Co3O4-AP is tted by three
components which are assigned to stoichiometric O2ꢀ bonded
to Co, non-stoichiometric Oꢀ compensating for deciencies in
the subsurface and weakly absorbed species.35 The O 1s spec-
trum for Co3O4-R is quite different. Aside from the peak corre-
sponding to O2ꢀ, there are three peaks at higher binding energy
Fig. 3 TEM images of Co3O4-AP (a and b) and Co3O4-R (c and d).
Insets in (d) are FFT from ꢂ15 nm ꢃ ꢂ15 nm areas labelled by squares
A1 and A2, respectively.
hydroxide. Different types of lattice fringes were observed in a
single particle of Co3O4-R comparing to that of Co3O4-AP. The
fast Fourier transformation (FFT) of area A1 can be indexed
consistently to spinel Co3O4.43,44 The FFT of A2 could be
assigned to (311) planes of Co3O4 or (111) planes of CoO. This
morphological transformation occurred simultaneously with
the partial reduction of Co3O4 to CoO and Co, which may be
related to the higher atomic mobility because of the bond
breakage during phase transformation.45,46 Combining all the
above results, we propose Scheme S1† to describe the surface
reformation of the Co3O4 nanoparticles during thermal reduc-
tion and ambient stabilization. In summary, a novel structure
with crystalline Co3O4 inside and amorphous layer on the
surface was formed by thermal hydrogen reduction and the
following stabilization in air.
2ꢀ
that are assigned to oxygen in OHꢀ, CO3 and H2O, respec-
tively, which indicate the formation of hydrated/carbonated
Co(OH)2. TiO2 annealed under H2 also displayed additional
peak related to OHꢀ in the O 1s spectrum.34,38 C 1s spectrum of
2ꢀ
Co3O4-R also conrmed the presence of CO3 while all C 1s
signal of Co3O4-AP is only related to the carbon contamination.
2ꢀ
CO3 and H2O could come from air and may exist in the
interlayer space of Co(OH)2 so that a hydrotalcite-like layered
structure is formed. XPS results suggest that the surface of
Co3O4-R is chemically different from that of Co3O4-AP. The new
2ꢀ
surface contains Co(2+), Co(3+), OHꢀ, CO3 and H2O, which
form a material with a formula of Co(2+, 3+)(OH)x(CO3)y$H2O.
This surface layer should be very thin, or the crystallinity is low,
so that no corresponding peaks are observed in XRD.
To measure the OER activity, linear sweep voltammograms
(LSVs) of Co3O4-AP and Co3O4-R were recorded on a rotating
ring-disk electrode (RRDE). In Fig. 4a, the disk current of Co3O4-
R and Co3O4-AP starts to rise sharply at around 1.55 V. This
current rise is dominated by OER, which is conrmed by the
ring current in Fig. 4b since ring current is an indicator of
oxygen produced at the disk electrode according to the working
principle of RRDE (Scheme S2†). Apparently, Co3O4-R exhibited
higher OER activity relative to Co3O4-AP. An overpotential
h ¼ 580 mV is required to attain 10 mA cmꢀ2 on Co3O4-AP.
Co3O4-R showed a much lower overpotential of 490 mV to
obtain the same current density of 10 mA cmꢀ2. An important
parameter and benchmarking of the catalytic activity is the
onset potential. However, as shown in the inset of Fig. 4a, the
disk current around the onset potential was interfered by
another Faradaic process which is attributed to the oxidation of
Transmission electron microscopy (TEM) and high-
resolution TEM (HRTEM) were used to identify the morpho-
logical change and the atomic structures of the surfaces of
Co3O4-R and Co3O4-AP samples, as shown in Fig. 3. Co3O4-AP
consists of truncated octahedral single-crystal nanoparticles
with size from 15 to 25 nm (Fig. 3a). HRTEM image in Fig. 3b
shows the lattice fringes that are assigned to spinel Co3O4, in
consistence with the XRD results. Fig. 3c displays the irregular
particles of Co3O4-R. The larger particle size and the shape
change indicate the fusing of the nanoparticles during the
thermal reduction. The BET specic surface area of Co3O4-AP is
48 m2 gꢀ1 while that of Co3O4-R is reduced to 22 m2 gꢀ1, which
again suggests the particle agglomeration. In HRTEM image of
Co3O4-R (Fig. 3d), an amorphous layer with thickness of around
1–2 nm (indicated by dashed line) was observed. This amor-
phous layer should be the hydrated/carbonated cobalt
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Co in the catalysts (Fig. 4d). Since the current at ring is merely
related to the oxygen evolution, the onset potential of OER could
be decided unequivocally from the ring current. The onset
potential of Co3O4-R is 1.50 V (h ¼ 270 mV) according to ring
current, which is 50 mV lower than that of Co3O4-AP. The onset
potential and potential at current density of 10 mA cmꢀ2 for
Co3O4-R are 35 mV and 60 mV lower respectively than those for
mesoporous Co3O4 tested under similar conditions.13 The onset
potential is even 30 mV lower than that of mesoporous Co3O4
embedded with Au nanoparticles.20
Tafel plots of Co3O4-AP and Co3O4-R are shown in Fig. 4c.
The Tafel slope is related to the reaction mechanism and the
exchange current density is a measure of the catalytic activity at
the equilibrium potential. At low overpotential region, Co3O4-R
and Co3O4-AP showed Tafel slopes of 61 mV decꢀ1 and 66 mV
decꢀ1, respectively. These values are comparable to those of Co-
based spinels11 and perovskites,47,48 cobalt oxyhydroxides,49 and
amorphous CoPi lm,50 and are in the range of 60 mV decꢀ1
(2.3RT/aF), indicating the rate determining step on either
sample is the activation controlled process involving one elec-
tron transfer.11,51 The exchange current densities of Co3O4-R and
Co3O4-AP are 2 ꢃ 10ꢀ9 A cmꢀ2 and 4 ꢃ 10ꢀ9 A cmꢀ2, respec-
tively, one order of magnitude larger than the reported value of
electrodeposited cobalt oxides25 and CoPi lms,50 and in the
same order of magnitude with that of Co3O4.49 Although the
exchange current density of Co3O4-R is lower than that of Co3O4-
AP, Co3O4-R exhibited lower Tafel slope and thus still had
higher OER activity than Co3O4-AP at high overpotential. This is
consistent with the consideration that evaluation of electro-
catalytic activity should be based not only on the exchange
current density, but also on the Tafel slope.52 At higher over-
potential region, Tafel slope of Co3O4-R and Co3O4-AP increased
to 90 mV decꢀ1 and 117 mV decꢀ1, respectively. This Tafel slope
increase may result from the change in the coverage of OH
species,47 or the porous structure of the catalysts,53 or the
evolving bubbles blocking part of the active surface area.51
Turn-over frequency (TOF) was calculated (refer to ESI† for
TOF calculation) to evaluate the activity of OER. TOF was
calculated from the OER current density at an overpotential of
400 mV, assuming 100% faradaic efficiency and all Co sites are
involved in OER (a lower bound on activity). Co3O4-R shows
a TOF of 0.006 Sꢀ1, much larger than that of Co3O4-AP
(0.0023 Sꢀ1). TOF of Co3O4-R is slightly larger than that of
mesoporous Co3O4,13 but lower than that of Co3O4 nano-
particles reported by Esswein et al.12
Cyclic voltammograms (CVs) in Fig. 4d compare the redox
properties of Co3O4-AP and Co3O4-R. Two redox couples (A1, C1)
and (A2, C2) are observed in CV of Co3O4-AP. Redox couple (A1,
C1) centred at 1.49 V with peak separation of 75 mV is related to
the transformation between Co(2+) and Co(3+). Redox couple
(A2, C2) centred at 1.28 V with peak separation of 30 mV is
assigned to the oxidation of Co(3+) to Co(4+) and its reverse.54,55
In contrast to the dened peaks in Co3O4-AP, the redox peaks in
CV of Co3O4-R are too diffused to be distinguished. This peak
Fig. 4 Electrochemical performance of Co3O4-AP and Co3O4-R in 0.1 M
KOH. LSVs collected on a rotating ring-disk electrode with a rotating
speed of 1600 rpm: (a) disk current densities (inset is the enlarged view of
the disk current densities showing the sample oxidation prior to OER), (b)
ring current densities (inset is the enlarged view of the ring current broadness is connected with the heterogeneity of Co atoms in
densities around the onset potential region). (c) Tafel plots recorded at
the amorphous phase. The lattice disorder in amorphous phase
affects the energy distribution of the active sites, and thus
potential descending with steps of 20 mV. (d) CVs collected at 30 mV sꢀ1
.
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broadens the CV peaks. It has been suggested that the special the Centre for Microscopy and Microanalysis, The University of
energy status of the active sites in amorphous materials is Queensland.
promising for catalysis.56 In this study, the special energy status
of Co sites in the amorphous layer may be one of the factors that
facilitate OER. The anodic current in CV of Co3O4-R results from
Notes and references
the oxidation of hydrated Co(2+) species to Co(3+) and Co(4+)
species, while the cathodic current is caused by the reverse
processes.57,58 Therefore Co(4+) in Co3O4-R exists in a more
hydrous form which probably possesses a layered structure.
This structure benets the electron and proton transportation
and thus the OER process.21 Integration of the cathodic current
allows the estimation of cobalt ions that changed oxidation
states.28,59 The cathodic charge estimated by this method is
8 mC cmꢀ2 for Co3O4-R and 5.7 mC cmꢀ2 for Co3O4-AP.
Therefore, Co3O4-R has more oxidisable cobalt ions and thus
more Co(4+) ions were obtained during the cobalt oxidation
process. Since Co(4+) are considered as active sites for
OER,19,20,60 it is reasonable that Co3O4-R has higher OER activity
due to the larger number of Co(4+). In mesoporous Co3O4
nanowires reduced by NaBH4, authors attributed the enhanced
1 F. Cheng and J. Chen, Chem. Soc. Rev., 2012, 41, 2172–2192.
2 H. B. Gray, Nat. Chem., 2009, 1, 112.
3 A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253–278.
4 M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher,
Q. Mi, E. A. Santori and N. S. Lewis, Chem. Rev., 2010, 110,
6446–6473.
5 H. Wang and H. Dai, Chem. Soc. Rev., 2013, 42, 3088–3113.
6 M. W. Kanan and D. G. Nocera, Science, 2008, 321, 1072–
1075.
7 H. Dau, C. Limberg, T. Reier, M. Risch, S. Roggan and
P. Strasser, ChemCatChem, 2010, 2, 724–761.
8 Y. Lee, J. Suntivich, K. J. May, E. E. Perry and Y. Shao-Horn, J.
Phys. Chem. Lett., 2012, 3, 399–404.
9 R. N. Singh, J.-F. Koenig, G. Poillerat and P. Chartier, J.
Electrochem. Soc., 1990, 137, 1408–1413.
OER activity to the oxygen vacancies which enhance electro- 10 M. R. G. de Chialvo and A. C. Chialvo, Electrochim. Acta, 1990,
catalytic performance and improve the conductivity. The exis- 35, 437–443.
tence of the oxygen vacancies was conrmed by the stronger 11 M. Hamdani, R. N. Singh and P. Chartier, Int. J. Electrochem.
Co(2+) signal in XPS data than that in the pristine sample.24 But
Sci., 2010, 5, 556–577.
in our surface-engineered sample Co3O4-R, surface atoms 12 A. J. Esswein, M. J. McMurdo, P. N. Ross, A. T. Bell and
reduced by thermal hydrogen treatment were re-oxidized on
T. D. Tilley, J. Phys. Chem. C, 2009, 113, 15068–15072.
exposure to air, and no additional or stronger signal related to 13 H. Tuysuz, Y. Hwang, S. Khan, A. Asiri and P. Yang, Nano
Co metal or Co(2+) was observed in XPS in Fig. 2d. Therefore, we
Res., 2013, 6, 47–54.
correlate the enhanced OER activity with the layered surface 14 I. Nikolov, R. Darkaoui, E. Zhecheva, R. Stoyanova,
¨
¨
amorphous structure which provides more active sites. The
stability of Co3O4-R for OER was assessed by polarizing the
N. Dimitrov and T. Vitanov, J. Electroanal. Chem., 1997,
429, 157–168.
electrode at 1.63 V (overpotential of 400 mV) for different 15 X. Wu and K. Scott, Int. J. Hydrogen Energy, 2013, 38, 3123–
duration. In Fig. S1,† the OER current of Co3O4-R keeps almost
constant aer oxidation for 10 min, 20 min and 70 min, which 16 F. Svegl, B. Orel, I. Grabec-Svegl and V. Kaucic, Electrochim.
3129.
ˇ
ˇ
ˇ ˇ
indicates good stability of Co3O4-R.
Acta, 2000, 45, 4359–4371.
17 P. Rasiyah and A. C. C. Tseung, J. Electrochem. Soc., 1983,
130, 365–368.
18 B. Y. Kim, I.-B. Shim, Z. O. Araci, S. S. Saavedra,
O. L. A. Monti, N. R. Armstrong, R. Sahoo, D. N. Srivastava
and J. Pyun, J. Am. Chem. Soc., 2010, 132, 3234–3235.
Conclusions
We reported the improved OER activity of surface reformed
Co3O4. The amorphous surface contains hydrated/carbonated
cobalt hydroxide species and is more active compared to the 19 B. S. Yeo and A. T. Bell, J. Am. Chem. Soc., 2011, 133, 5587–5593.
highly crystalline surface of pristine Co3O4 nanoparticles. 20 X. Lu, Y. H. Ng and C. Zhao, ChemSusChem, 2014, 7, 82–86.
Through cyclic voltammetry, we show that the Co(4+) active 21 J. B. Gerken, J. G. McAlpin, J. Y. C. Chen, M. L. Rigsby,
sites in the amorphous layer exist in special environment and
agglomerate. The amorphous surface layer also exhibited good
W. H. Casey, R. D. Britt and S. S. Stahl, J. Am. Chem. Soc.,
2011, 133, 14431–14442.
stability aer prolonged anodic oxidation. This work demon- 22 W. Estrada, M. C. A. Fantini, S. C. de Castro, C. N. Polo da
strates a new avenue towards the development of more efficient
Fonseca and A. Gorenstein, J. Appl. Phys., 1993, 74, 5835–
OER catalyst through surface engineering.
5841.
ˇ
23 F. Svegl, B. Orel, M. G. Hutchins and K. Kalcher, J.
Electrochem. Soc., 1996, 143, 1532–1539.
24 Y. Wang, T. Zhou, K. Jiang, P. Da, Z. Peng, J. Tang, B. Kong,
W.-B. Cai, Z. Yang and G. Zheng, Adv. Energy Mater., 2014, 4,
1400696.
Acknowledgements
We acknowledge the nancial support from UNSW Australia,
The University of Queensland and Australian Research Council
Discovery Project (DP110100550). D.W.W. acknowledges the 25 J. A. Koza, Z. He, A. S. Miller and J. A. Switzer, Chem. Mater.,
support from UNSW FRG/ECR scheme. The authors acknowl- 2012, 24, 3567–3573.
edge the facilities, and the scientic and technical assistance, of 26 D. A. Lutterman, Y. Surendranath and D. G. Nocera, J. Am.
the Australian Microscopy & Microanalysis Research Facility at
Chem. Soc., 2009, 131, 3838–3839.
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˘
27 M. W. Kanan, J. Yano, Y. Surendranath, M. Dinca, 44 S. R. Walsh, I. Rusakova and K. H. Whitmire, CrystEngComm,
V. K. Yachandra and D. G. Nocera, J. Am. Chem. Soc., 2010,
132, 13692–13701.
28 M. Risch, K. Klingan, F. Ringleb, P. Chernev, I. Zaharieva,
A. Fischer and H. Dau, ChemSusChem, 2012, 5, 542–549.
2013, 15, 775–784.
45 K.-N. P. Kumar, K. Keizer, A. J. Burggraaf, T. Okubo,
H. Nagamoto and S. Morooka, Nature, 1992, 358, 48–51.
46 K.-N. P. Kumar, Scr. Metall. Mater., 1995, 32, 873–877.
˘
29 M. Dinca, Y. Surendranath and D. G. Nocera, Proc. Natl. 47 J. O. Bockris and T. Otagawa, J. Phys. Chem., 1983, 87, 2960–
Acad. Sci. U. S. A., 2010, 107, 10337–10341. 2971.
30 W. D. Chemelewski, H.-C. Lee, J.-F. Lin, A. J. Bard and 48 A. Grimaud, K. J. May, C. E. Carlton, Y.-L. Lee, M. Risch,
C. B. Mullins, J. Am. Chem. Soc., 2014, 136, 2843–2850.
W. T. Hong, J. Zhou and Y. Shao-Horn, Nat. Commun.,
2013, 4, 2439.
´
31 R. D. L. Smith, M. S. Prevot, R. D. Fagan, Z. Zhang,
P. A. Sedach, M. K. J. Siu, S. Trudel and C. P. Berlinguette, 49 Y.-C. Liu, J. A. Koza and J. A. Switzer, Electrochim. Acta, 2014,
Science, 2013, 340, 60–63.
140, 359–365.
32 Y. Qiu, L. Xin and W. Li, Langmuir, 2014, 30, 7893–7901.
33 K. J. May, C. E. Carlton, K. A. Stoerzinger, M. Risch,
50 Y. Surendranath, M. W. Kanan and D. G. Nocera, J. Am.
Chem. Soc., 2010, 132, 16501–16509.
J. Suntivich, Y.-L. Lee, A. Grimaud and Y. Shao-Horn, J. 51 R. N. Singh, M. Hamdani, J. F. Koenig, G. Poillerat,
Phys. Chem. Lett., 2012, 3, 3264–3270.
34 X. Chen, L. Liu, P. Y. Yu and S. S. Mao, Science, 2011, 331,
746–750.
35 T. J. Chuang, C. R. Brundle and D. W. Rice, Surf. Sci., 1976,
59, 413–429.
36 J.-C. Dupin, D. Gonbeau, P. Vinatier and A. Levasseur, Phys.
Chem. Chem. Phys., 2000, 2, 1319–1324.
37 N. S. McIntyre and M. G. Cook, Anal. Chem., 1975, 47, 2208–
2213.
J. L. Gautier and P. Chartier, J. Appl. Electrochem., 1990, 20,
442–446.
52 B. E. Conway, L. Bai and M. A. Sattar, Int. J. Hydrogen Energy,
1987, 12, 607–621.
53 J. N. Soderberg, A. C. Co, A. H. C. Sirk and V. I. Birss, J. Phys.
Chem. B, 2006, 110, 10401–10410.
54 R. Boggio, A. Carugati and S. Trasatti, J. Appl. Electrochem.,
1987, 17, 828–840.
55 M. Grzelczak, J. Zhang, J. Pfrommer, J. Hartmann, M. Driess,
M. Antonietti and X. Wang, ACS Catal., 2013, 383–388.
38 G. Wang, H. Wang, Y. Ling, Y. Tang, X. Yang,
R. C. Fitzmorris, C. Wang, J. Z. Zhang and Y. Li, Nano Lett., 56 C. Yoon and D. L. Cocke, J. Non-Cryst. Solids, 1986, 79, 217–
2011, 11, 3026–3033. 245.
39 Q. Kang, J. Cao, Y. Zhang, L. Liu, H. Xu and J. Ye, J. Mater. 57 L. D. Burke, M. E. Lyons and O. J. Murphy, J. Electroanal.
Chem. A, 2013, 1, 5766–5774. Chem. Interfacial Electrochem., 1982, 132, 247–261.
40 S. C. Petitto, E. M. Marsh, G. A. Carson and M. A. Langell, J. 58 M. E. G. Lyons and M. P. Brandon, Int. J. Electrochem. Sci.,
Mol. Catal. A: Chem., 2008, 281, 49–58. 2008, 3, 1325–1462.
41 H. G. Tompkins and J. A. Augis, Oxid. Met., 1981, 16, 355– 59 K. Klingan, F. Ringleb, I. Zaharieva, J. Heidkamp,
369.
P. Chernev, D. Gonzalez-Flores, M. Risch, A. Fischer and
H. Dau, ChemSusChem, 2014, 7, 1301–1310.
60 J. G. McAlpin, Y. Surendranath, M. Dincǎ, T. A. Stich,
S. A. Stoian, W. H. Casey, D. G. Nocera and R. D. Britt, J.
Am. Chem. Soc., 2010, 132, 6882–6883.
42 D. W. Rice, P. B. P. Phipps and R. Tremoureux, J. Electrochem.
Soc., 1979, 126, 1459–1466.
43 Z. Q. Wang, X. Y. Zhong, R. Yu, Z. Y. Cheng and J. Zhu, Nat.
Commun., 2013, 4, 1395.
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