Enhancing Full Water Splitting Performance of Transition Metal

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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]

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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.

5 / 12



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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



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).

REFERENCES

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(1)

increased number of active sites. Such a single

2004, 305, 972-974.

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Enhancing Full Water Splitting Performance of Transition Metal

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