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Enhancing Full Water Splitting Performance of Transition Metal Bifunctional Electrocatalysts in Alkaline Solutions by Tailoring CeO2-TMO-Ni Nano-interfaces Xia Long, He Lin, Dan Zhou, Yiming An, and Shihe Yang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01130 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 2, 2018
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ACS Energy Letters
Enhancing Full Water Splitting Performance of Transition Metal Bifunctional Electrocatalysts in Alkaline Solutions by Tailoring CeO2-TMO-Ni Nano-interfaces Xia Long,a,b He Lin,a Dan Zhou,a Yiming An,a Shihe Yanga,b*
[a] X. Long, H. Lin, D. Zhou, Y. An, S. Yang: Department of Chemistry, William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong [b] X. Long, S. Yang: Guangdong Key Lab of Nano-Micro Material Research, School of Chemical Biology and Biotechnology Shenzhen Graduate School, Peking University, Shenzhen, China
Corresponding Author E-mail:
[email protected] 1 / 12
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ABSTRACT Rational design of highly efficient bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is critical for sustainable energy conversion. Herein, motivated by the high activity of OER catalyst on water dissociation that is the rate-determining step of alkaline HER, a bifunctional catalyst of metallic nickel decorated transition metal oxides nanosheets vertically grown on ceria film (ceria/Ni-TMO) is synthesized by composition controlling and surface engineering. Due to the idealized electronic structure of the active centers and the abundance of such sites, as well as synergistic effect between the carbon cloth/ceria film and the in situ formed TMO/Ni nanoparticles, the as-synthesized ceria/Ni-TMO exhibited a long-time stability and a low cell voltage of 1.58 V at 10 mA/cm2 when applied as both the cathode and anode in alkaline solutions. Moreover, it is the first time that pH-independent four-proton-coupled-electron-transfer processes and multi adsorption/desorption processes were found to occur at the interfaces of ceria/TMO and Ni/TMO in a single catalyst for catalyzing OER and HER, respectively.
TOC Graphic
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ACS Energy Letters
Driven by the growing concerns on global warming
Herein, motivated by the superior water dissociation
and the increasing depletion of fossil fuels, people now
capability of OER catalysts that have been well
recognize the urgency to develop renewable energy
established, we designed and fabricated a novel catalyst of
sources and the associated energy conversion and storage
ceria film supported, metallic nickel nanoparticles
technologies. One of the most promising ways to tackle
decorated transition metal oxides (TMO) nanosheets
this fundamentally and practically important challenge is
(ceria/Ni-TMO) to fulfil the efficient full water splitting in
to produce hydrogen by water splitting, which has
alkaline electrolytes.
attracted more and more attention of both chemical and
As shown in Scheme S1, multi-transition metal
material scientists.1,2 To overcome the large barrier of the
(NiMnFe) hydroxide nanosheets were firstly vertically
strongly uphill reaction of water splitting, active
deposited on a ceria film, which is a good oxygenic
electrocatalysts for anodic oxygen evolution reaction
species conductor and thus suitable for hosting the OER
(OER) and cathodic hydrogen evolution reaction (HER)
catalyst. The hierarchical multi-level structure and the
are sorely needed. Though Ir-/Ru-and Pt- based materials
existence of multi-transition metal ions ideally combined
3
showed high activity on OER and HER respectively, the
two effective approaches to improving the electrocatalytic
prohibitive cost and scarcity of the noble metals greatly
activity, which have been well documented.24 These in
hinder their wide applications on a large scale. It is thus
isolation are 1) increasing the number of active sites,25,26
attractive to design efficient water splitting catalysts
and 2) enhancing the intrinsic activity of each active sites
comprising
earth-abundant
elements,2,4
especially
via tuning the electronic structure of transition metal
bifunctional catalysts that could greatly simplify the water
ions.27,28 Further, the ceria/TM-OH were annealed in
splitting system design and thus lower its cost by
reductive atmosphere (H2/Ar), resulting in the in situ
catalysing both OER and HER.5-10
formation of metallic nickel nanoparticles and transition
OER is kinetically sluggish and thus regarded as the
metal oxides (TMO) that is also an advanced OER catalyst.
critical half-reaction for water splitting. A number of
Therefore, the Volmer step of HER could occur at the
noble-metal-free catalysts have been well developed
boundary between metallic Ni and TMO/ceria, which
during past few years including transition metals based
absorb with H and OH species, respectively, greatly
hydroxides,
11,12
phosphates,16
oxides, etc.,
13-15
which
oxyhydroxides, showed
advanced
cobalt OER
enhancing the kinetics of water dissociation and hence the HER
performance
in
alkaline
as-synthesized
incorporated with multiple metal ions. Though many HER
bifunctional performance in accelerating both OER and
catalysts such as transition metal chalcogenides,17-19
HER in strong alkaline solutions with low Tafel slopes of
21-23
showed
The
performance in alkaline electrolytes especially when
20
ceria/Ni-TMO
solutions.29,30
advanced
etc. have also been synthesized
38 mV/dec and 69 mV/dec for OER and HER,
and showed good performance in acidic electrolytes, their
respectively, as well as a small cell voltage of 1.58 V at 10
performance in alkaline solution fell short, probably due to
mA/cm2 even when supported on carbon electrodes.
carbides, phosphides,
the low dissociation of adsorptive water (Volmer step).
3 / 12
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Figure.1 Morphology and structure characterizations of ceria/TM-OH and ceria/Ni-TMO on carbon cloth. (A-C) SEM images of (A) ceria/TM-OH, (B, C) ceria/Ni-TMO; (D) TEM image of ceria/Ni-TMO; (E) high resolution TEM (HRTEM) images of ceria/Ni-TMO showing the lattice fringes of metallic Ni particle (red rectangular) and TMO (blue rectangular) respectively; (F) XRD patterns of ceria film (blue curve), ceria/TM-OH (red curve), and ceria/Ni-TMO (black curve). The red arrows in D indicate Ni nanoparticles, the peaks indicated by # and * in F represent diffraction peaks of ceria and TMO, respectively.
Different from the relatively smooth surface of bare
annealing treatment at precisely controlled temperature
carbon cloth (Fig. S1A, B), a particulate film (Fig. S1C, D)
under H2/Ar atmosphere not only led to the in-situ
was formed on the surface of carbon fiber after the
formation of TMO and metallic Ni nanoparticles, but also
electrodeposition of ceria film. However, no obvious
enhanced the intimate contact between the underlying
diffraction peak could be detected from the X-ray
ceria film and the supported Ni-TMO compound. The
diffraction (XRD) pattern (Fig. 1F, blue curve), indicating
SEM (Fig. 1B&C) and transmission electron microscopy
the amorphous nature of the deposited ceria film. Then,
(TEM, Fig. 1D) images show that the nanosheet structure
NiMnFe hydroxides (TM-OH) were also deposited on the
of LDH was largely retained after the annealing treatment,
as-formed ceria film, forming a nanosheet structure as can
but the basal surface of the nanosheets became rough with
be seen from the scanning electron microscopy (SEM)
numerous pores. The high resolution TEM image
image (Fig. 1A). The EDX mapping shown in Fig. S2
(HRTEM, Fig. 1E) exhibits a lattice fringe spaced by
further indicates the uniform distribution of transition
0.201 nm, corresponding to the (111) plane of nickel metal,
metal ions. Besides the diffraction peaks at 26 º and 43 º
and this suggests that the spots indicated with red arrows
that could be ascribed to carbon cloth, an additional peak
in Fig. 1D, each having a size of 5~10 nm across, are
at ~10 º (Fig. 1F, red curve) for (003) plane of layered
actually Ni nanoparticles. On the other hand, the lattice
double hydroxides (LDH, a class of two-dimensional
fringes with a spacing of 0.299 nm are proposed to be
anionic clays made up of positively charged brucite-like
from the (220) plane of the transformed oxides. From the
host layers and exchangeable charge-balancing interlayer
XRD pattern (Fig. 1F, red curve), the diffraction peaks of
anions). Here the triple transition metals of Ni, Mn and Fe
carbon from carbon cloth could still be clearly observed,
composing the host layer could also be found, indicating
while the peak at ~10 º for LDH disappeared. Instead,
the successful formation of NiMnFe hydroxides with LDH
diffraction peaks at ~28.6 º for (111) plane of ceria
structure on the ceria thin film (ceria/TM-OH). The next
(JCPDF 02-1306), ~44.5 º for (111) plane of metallic 4 / 12
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ACS Energy Letters
nickel (black arrow indicated in Fig. 1F, black curve,
2A, blue curve). It is worth noting that this potential kept
JCPDF 04-0850), and ~ 30.3 º, ~ 35.7 º, ~57.6 º and ~62.7
unchanged for ceria/Ni-TMO (n) (Fig. S3, magenta curve)
º for (220), (311), (511), and (440) planes of spinel ferrite
but increased for ceria/TM-OH (n) (Fig. S3, red curve), as
with the formula of Ni(Fe, Mn)2O4 (JCPDF 74-2082)
can be seen from the LSV curves collected by sweeping
could be found, confirming the formation of metallic
polarization curves from high potential to low potential.
nickel nanoparticles and cubic phase of metal oxides
The OER performance of NiMn-OH on ceria film
spinel, respectively, in agreement with the high resolution
(ceria/NiMn-OH) was also tested (Fig. S4, olive curve),
TEM results. Therefore, the products transformed from the
which showed a relatively lower OER activity than
ceria/TM-OH are indeed the ceria film supported nickel
ceria/Ni-TMO, Ni-TMO and ceria/TM-OH, indicating that
nanoparticles
coexistence of the triple transition metal ions of Fe, Ni and
decorated
transition
metal
oxides
(ceria/Ni-TMO).
Mn in one compound was important for catalysing OER in
Given the fact that the transition metal oxides are 13,15,25,31
highly active OER catalysts
alkaline
solution.
The
superior
OER
activity
of
and the Ni/TMO
ceria/Ni-TMO is evidenced by the low overpotential at 10
29,30
composite had HER activity in alkaline electrolytes,
mA/cm2, which is much lower than those of the non-noble
the ceria/Ni-TMO that combines both functionalities in
metal OER catalysts and comparable to those of the
one unit, is expected to be a bifunctional catalyst and was
state-of-the-art OER catalysts of IrO2/RuO2 reported in the
then tested for full water splitting. Firstly, the OER
literature (Table S1).12,15,28,33-35 These results suggest a
performance of the catalysts were investigated in a typical
significant role that has been played by the underlying
three-electrode configuration in 1 M KOH. From the
ceria film in greatly enhancing the OER performance of
polarization curves shown in Fig. 2A, one can see that the
the catalysts. Moreover, the Tafel slope for ceria/Ni-TMO
ceria film showed little OER activity with negligible
was calculated to be 38 mV/dec (Fig. 2B, black curve,
current density even at the potential of 1.6 V (vs RHE,
fitting region was 1.44 ~ 1.48 V vs RHE), much smaller
olive curve). For ceria/TM-OH, an oxidation peak
than that of ceria/TM-OH (50 mV/dec, red curve, fitting
appeared at ~ 1.4 V (vs RHE), immediately followed by a
region was 1.435 ~ 1.475 V vs RHE), Ni-TMO (61
sharp current rise, indicating the onset of oxygen evolution
mV/dec, blue curve in Fig. 2B, fitting region was 1.50 ~
(Fig. 2A, red curve). Note that the pre-oxidation peak
1.54 V vs RHE) and ceria film (218 mV/dec, olive curve
stems from the abundant exposed catalytic active sites31 in
in Fig. 2B, fitting region was 1.56 ~ 1.62 V vs RHE),
LDH, and this peak is closely overlapped with the onset
implying a different rate determining step in the
current profile of OER (Fig. 2A, red curve). Interestingly,
water-splitting mechanism that favors more rapid OER for
the ceria/Ni-TMO catalyst exhibited an OER polarization
ceria/Ni-TMO electrode. It is worth noting that the small
curve with onset potential close to that of LDH, but
Tafel slope of 38 mV/dec of ceria/Ni-TMO is even smaller
without such a clear pre-oxidation peak. Moreover, a small
than the well-established noble metal OER catalysts of
potential of ~ 1.45 V (vs RHE, Fig. 2A, black curve) was
IrO2 (~49 mV/dec), Ir/C (~40 mV/dec), and most of the
2
sufficient to achieve the current density of 10 mA/cm ,
transition metals based OER catalysts supported on
similar to that of ceria/TM-OH but much smaller than that
conductive carbon nanomaterials (Table S1).11,14,36
of the control catalyst of Ni-TMO (~ 1.50 V vs RHE, Fig.
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Figure. 2 Electrochemical performance of catalysts on catalyzing oxygen evolution reaction in strong alkaline solutions. (A) polarization curves and (B) Tafel plots of as-synthesized catalysts on catalyzing oxygen evolution reaction, (C) electrochemical impendence spectroscopy (EIS) of the catalysts at the potential of 1.45 V (vs RHE), and (D) multi-current process of ceria/Ni-TMO, the current density started at 10 mA/cm2 and ended at 100 mA/cm2, with an increment of 15 mA/cm2 per 1000 s, the inset is the chronopotentiometric curve of ceria/Ni-TMO with a constant current density of 10 mA/cm2 for more than 30 hours. The electrolytes were 1 M KOH if not otherwise indicated.
structures, which would favorably influence the catalytic Two different mechanisms were invoked previously to
activity of the catalyst.28 On the other hand, cerium oxide
understand the OER processes on metal oxides. One
(ceria,
involves four proton-coupled-electron-transfer (PCET)
ion-storage capacity due to the flexible transition between
steps on metal-ion centers at the catalyst surfaces with the
the Ce(III) and Ce(IV) oxidation states, has been reported
37
O2 produced from electrolyte,
CeO2)
that
has
reversible
surface
oxygen
and the other involves
to be an excellent “co-catalyst” for improving the catalytic
non-concerted proton-electron transfer processes with the
performance of the catalysts dispersing on it.41,42 From
O2 produced from lattice oxygen.37,38 In our case, the
high resolution XPS spectra of ceria/Ni-TMO at the Ce 3d
current densities from polarization curves collected in
region (Fig. S6), the peaks located at 880-893 eV and
electrolytes with different pH values remained almost
895-925 eV could be ascribed to Ce 3d5/2 and Ce 3d3/2,
unchanged (Fig. S5), indicating a pH-independent activity
which showed the coexistence of Ce (IV) and Ce (III),
of the ceria/Ni-TMO on OER, signifying the operative
confirming the multivalence property of ceria. Further,
PCET process. Therefore, the intrinsic catalytic activity of
from the XPS peaks of Fe, Ni and Mn in ceria/Ni-TMO
the ceria/Ni-TMO be correlated with the oxidation state
(Fig. S7 red curves) were further positively shifted from
and electron configuration of transition metal ions and the
that of Ni-TMO (Fig. S7 black curves), suggesting the
surface
oxygen
binding
39,40
were
existence of ceria film could indeed offer the opportunity
investigated by XPS techniques. As shown in Fig. S6, the
energy,
which
to generate strong electron interactions with formed
peaks of Ni, Fe and Mn in the catalysts of both Ni-TMO
Ni-TMO.26 In addition, besides Ni ions (Fig. S8, blue and
and ceria/Ni-TMO were positively shifted, indicating
magenta fitted curves at 858.8 eV and 857.0 eV,
strong interactions involving the three cations of Ni, Fe,
respectively), metallic Ni spin-orbit split peak (Fig. S8,
and Mn that hence greatly altered their electronic
green fitted curve, peaked at 856.1 eV) could also be 6 / 12
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ACS Energy Letters
clearly found in ceria/Ni-TMO, confirming the formation
beginning from 10 mA/cm2 to 100 mA/cm2 (15 mA/cm2
of Ni nanoparticles after the annealing treatment under
per 1000 s). The potential immediately achieved 1.45 V
H2/Ar atmosphere. Further, from the deconvoluted
(vs RHE) at the start current density of 10 mA/cm2 and
spin-orbit split peaks of Mn 3d, ceria/Ni-TMO shows a
remains unchanged for the rest 1000s, and the other steps
high concentration of Mn (III) (Fig. S9), corresponding to
also show comparable results, implying the excellent mass
the electronic arrangement of t2g3eg1, which were at the
transportation, conductivity, and mechanical robustness of
31
the ceria/Ni-TMO on carbon cloth electrode. Moreover,
peak catalytic activity.
The good electronic/ionic conductivity and high
the long-term stability of this catalyst was also
oxygen-storage capacity of ceria are also beneficial to the
investigated (Fig. 2D, inset) and the potential of 1.447 V
To look into this
was stabilized to keep the current density of 10 mA/cm2
aspect, the charge transfer resistance of the catalysts was
for more than 30 h, further confirming the advanced
investigated by electrochemical impedance spectroscopy
durability
(EIS). From the Fig. 2C, ceria/Ni-TMO showed a much
Therefore, the superior OER activity of ceria/Ni-TMO was
smaller semicircle diameter than ceria/TM-OH and TMO,
reasonably proposed to result from the suitable electronic
confirming
of
structure of catalytically active transition metal ions, the
ceria/Ni-TMO arising from the better conductivity of
existence of underlying ceria film that facilitate the storage
TMO as well as the intimate contact of catalysts with the
and transferring of oxygenic species/intermediates, as well
underlying ceria film that has a unique character for
as the post-annealing treatment of the catalysts that
enhancement of reaction kinetics.
the
superior
26
charger
transfer
transferring oxygenic species/intermediates.
26,41
rate
Fig. 2D
exhibited a multi-step chronopotentiometric curve for
of
as-prepared
ceria/Ni-TMO
for
OER.
enhances the intimate contact between the TMO catalysts with the underlying ceria thin film.
ceria/Ni-TMO in 1 M KOH with the current density
Figure. 3 Electrochemical performance of catalysts on hydrogen evolution reaction in strong alkaline solutions. (A) polarization curves, (B) potentials for achieving the current densities of 10, 20, and 50 mA/cm2, (C) Tafel plots of the as-synthesized catalysts on HER, (D) the capacitive currents (∆j) at the potential of 1.05 (V vs RHE) as a function of scan rates for ceria/Ni-TMO, ceria/TM-OH and Ni-TMO (∆j=ja-jc). The electrolytes were 1 M KOH if not indicated otherwise.
Metallic nickel is normally considered to be HER inactive in alkaline solution,
43
but the complex of Ni
nanoparticles
formed
on
transition
metal
oxides/hydroxides were reported to be effective HER 7 / 12
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catalysts in alkaline solutions, probably due to the enhanced kinetics of water dissociation. the
hetero-interfaces
between
5,44
In this work,
mV/dec (fitting region was -0.15 ~ -0.25 V vs RHE), and 69 mV/dec (fitting region was -0.1 ~ -0.2 V vs RHE),
Ni
respectively, further confirming that the advanced HER
nanoparticles and TMO on ceria film provides more
activity of ceria/Ni-TMO was higher than most of the
opportunities in tuning the adsorption/desorption energies
reported HER catalysts (Table S2).30,45-49 From the charge
and facilitate the charge transfer, thereby making
transfer resistance of catalysts tested at the overpotential
ceria/Ni-TMO to be a good HER catalyst in alkaline
of 100 mV for HER (Fig. S10), ceria/Ni-TMO showed the
electrolytes.
smallest semicircle diameter among the three catalysts,
in-situ
formed
Page 8 of 12
As shown in Fig. 3A, the overpotentials for achieving 2
also confirming the fluent charge transfer at ceria/Ni-TMO
10 mA/cm for ceria/TM-OH, Ni-TMO and ceria/Ni-TMO
in the HER conditions. The electrochemical surface area
were 368 mV, 198 mV, and 93 mV, respectively, with
(ECSA), partly representing the number of exposed
ceria/Ni-TMO exhibited the best performance toward
catalytic active sites, was investigated by using s simple
HER
Ni
cyclic voltammetry (CV) test (Fig. S11) based on the
nanoparticles decorated on TMO played the critical role
linear relationship between ECSA and double layer
on the advanced HER performance of the catalysts.30 The
capacitance (Cdl). From Fig. 3D, the Cdl of ceria/TM-OH
much smaller onset potential and overpotentials for the
and ceria/Ni-TMO were calculated to be 16.6 µF and
in
the
alkaline
solution.
The
2
metallic
2
high current densities of 10 mA/cm , 20 mA/cm and 50
18.42 µF, respectively, two times larger than that of TMO
mA/cm2 of ceria/Ni-TMO than that of Ni-TMO indicates
(9.39 F), indicating the much more exposed catalytic
that the existence of underlying ceria film also facilitates
activity of ceria assisted transition metal based catalyst
the catalytic reaction. The calculated Tafel slopes (Fig. 3C)
and confirmed the effect of underlying ceria thin film on
for ceria/TM-OH, Ni-TMO and ceria/Ni-TMO were 159
dispersing and anchoring of the catalysts and hence
mV/dec (fitting region was -0.28 ~ -0.38 V vs RHE), 102
increasing their catalytic performance.25,26
Figure. 4 Performance of the full water splitting device using the ceria/Ni-TMO formed on carbon cloth as both anode and cathode. (A) digital image (left) showing vigorous H2 and O2 production on ceria/Ni-TMO electrodes at a voltage of 1.60 V, and schematic description (right) of OER and HER on the catalysts: water molecules/hydroxides were absorbed and activated at the interface of ceria and TMO, resulting in the oxygen evolution on TMO, or the electrons were rapidly transferred to the nearby Ni nanoparticles, triggering the hydrogen evolution, (B) LSV curves of full water splitting by using ceria/Ni-TMO (black curve and columns) and Pt nanowire (red curve and columns) as both anode and cathode in a two-electrode setup in 1 M KOH (without iR correction), (C) chronopotentiometry test of ceria/Ni-TMO for full water splitting in a two-electrode setup with a constant current density of 10 mA/cm2, SEM image (C inset, left) and LSV curve (C inset, right, red curve) of ceria/Ni-TMO after the water splitting reaction, (D) the amount
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ACS Energy Letters
of hydrogen experimentally produced (red spheres) and theoretically calculated (black line) as the function of reaction time for overall water splitting by ceria/Ni-TMO on carbon cloth as the electrodes.
Given that the as-synthesized ceria/Ni-TMO is an
both of the half reactions of water splitting, allowing to
active and stable catalysts toward both OER and HER in
assemble a high-performing whole cell device. The high
strongly basic solutions, the catalyst of ceria/Ni-TMO
bifunctional catalytic activity arises from both the
formed on carbon cloth was applied as both anode and
compositional tuning and surface engineering: 1) the
cathode in a two-electrode system to illustrate the real
idealized electronic structure of transition metal ions
application towards full water spitting (Fig. 4A). The cell
achieved by introducing Fe, Mn and Ni improved the
2
intrinsic catalytic activity of the catalysts; 2) the promoted
current density in 1.0 M KOH (Fig. 4B) with vigorous gas
electron interactions between the Ni-TMO catalysts and
evolution on both electrodes (Fig. 4A). This potential
underlying ceria thin film due to the intimate contact of
outperforms the behavior of Pt wire electrodes (1.68 V).
the hetero-layered structure, which also enhanced the
The long-term stability of ceria/Ni-TMO for full water
utilization of the catalysts because of the hierarchical
splitting was also investigated in 1.0 M KOH. As shown in
structure and large surface area; and 3) unique high
Fig. 4C, ~ 1.59 V was required and kept unchanged for
oxygen species storage/transferring property of ceria film,
more than 20 hours at 10 mA/cm2, which was even smaller
which is favorable to the adsorption/desorption of
than most of non-noble metal based bifunctional catalysts
intermediates during water splitting process. These
voltage as low as 1.58 V was enough to afford 10 mA/cm
for whole water splitting (Table S3).
5,7-9,23
Moreover, after
beneficial traits of the ceria/Ni-TMO catalyst, which
the long-time electrochemical tests, the catalysts showed
afforded low onset potentials on both OER and HER at the
same morphology as the original catalysts from SEM
same
image (Fig. 4C inset, left), and less than 1% positive shift
earth-abundance highlight the exciting promise for
was found between the original polarization curve (Fig.
commercial developments in full electrocatalytic water
4C inset, right, black curve) and the one after more than
splitting.
20 h’s chronopotentiometry test (Fig. 4C inset, right, red curve), further suggesting the good durability and structure robustness of the as-synthesized ceria/Ni-TMO for water splitting in strong alkaline solutions. Finally, the generated H2 and O2 were further measured quantitatively by using
time,
alongside
its
simple
synthesis
ASSOCIATED CONTENT Supporting Information Experimental section and additional characterizations and analysis of date for the catalysts.
gas chromatography (GC). The Faradic efficiency (FE) for
AUTHOR INFORMATION
water splitting was calculated by comparing amount of
Corresponding Author
experimentally quantified H2 (Fig. 4D, red spheres) with
Email:
[email protected] theoretically calculated one (Fig. 4D, black line). The
Notes
agreement between both values suggests that the FE is
The authors declare no competing financial interest.
nearly 100% for water splitting (Fig. 4D), with the atomic ratio of O2 and H2 being close to 1:2 (Fig. S12). In summary, we have developed a novel bi-junction nanostructured electrocatalyst directly on a current collector of carbon cloth. The catalyst is in the form of a ceria film supported transition metals oxides nanosheets array
that
decorated
(ceria/Ni-TMO),
which
with features
nickel
nanoparticels
enhanced
intrinsic
and
ACHNOWLEGEMENTS The authors acknowledge the financial support from the NSFC/Hong
Kong
RGC
Research
Scheme
(N_HKUST610/14), the RGC of Hong Kong (GRF No. 16312216 and 16300915) and Shenzhen Peacock Plan (2016).
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