Hierarchical CoNiSe nano-architecture as a high ... · Hierarchical CoNiSe 2 nano-architecture as a...

Hierarchical CoNiSe2 nano-architecture as a high- performance electrocatalyst for water splitting

Tao Chen and Yiwei Tan ()

State Key Laboratory of Materials-Oriented Chemical Engineering, School of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, China

Received: 10 May 2017

Revised: 11 June 2017

Accepted: 23 June 2017

© Tsinghua University Press

and Springer-Verlag GmbH

Germany 2017

KEYWORDS

bifunctional catalysts,

electrocatalysis,

nanostructures,

ternary selenide,

water splitting

ABSTRACT

Hierarchical nano-architectures comprised of ultrathin ternary selenide (CoNiSe2)

nanorods were directly grown on nickel foam (NF). The integrated CoNiSe2/NF

functions as a robust electrocatalyst with an extremely high activity and stability

for emerging renewable energy technologies, and electrochemical oxygen and

hydrogen evolution reactions (OER and HER, respectively). The overpotentials

required to deliver a current density of 100 mA·cm−2 are as low as 307 and 170 mV

for the OER and HER, respectively; therefore, the obtained CoNiSe2 is among

the most promising earth-abundant catalysts for water splitting. Furthermore,

our synthetic sample validates a two-electrode electrolyzer for reducing the cell

voltage in the full water splitting reaction to 1.591 V to achieve a current density

of 10 mA·cm−2, which offers a novel, inexpensive, integrated selenide/NF electrode

for electrocatalytic applications.

1 Introduction

Electrochemically converting water to H2 and O2 is a

promising strategy for developing clean, renewable

energy sources [1–3], and is greatly dependent on the

rational design of highly active, stable, and cost-effective

hydrogen- and oxygen-evolving catalysts. Water splitting

via an electrochemical route requires extremely large

overpotentials closely related to the coupled four-

electron and four-proton transfer processes with high

activation barriers [4], which is rather intractable.

Platinum-group metals and metal oxides are the most

efficient catalysts in this respect, but they are scarce

and expensive. Therefore, there has been intense

research into highly active, commercially viable

electrocatalysts based on earth-abundant elements

to overcome these challenges. Concomitantly, new

inspirations are expected to be acquired for developing

higher levels of catalytically active centers.

Recently, the design of inexpensive water splitting

electrocatalysts was achieved by optimizing the com-

position of nonprecious metal catalysts and fabricating

sophisticated nano-architectures, which provide a

large surface area, more active sites, and/or robust

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1332 Nano Res. 2018, 11(3): 1331–1344

structures for improving the activity and long-term

durability of the catalysts. In this respect, most

research has been targeted toward the synthesis and

modulation of nanostructured 3d transition metal

(3d-TM) chalcogenides [5–11], such as CoS [12], CoS2

[13], CoSe2 [14–21], Co(SxSe1−x)2 [22], CoSP [23], NiS2

[24], NiSe [25–27], Ni3S2 [28, 29], and Co(Fe)–Ni–S(Se)

[30–38], because they show higher activities and con-

ductivities than their oxide/hydroxide counterparts.

In contrast to binary 3d-TM chalcogenides and their

mixtures, ternary bimetallic chalcogenides, especially

bimetallic selenides [34–38], have rarely been inves-

tigated, while such nanostructured catalysts may be

optimized candidates because a greater number of

active sites and synergistic effects (typically, improved

d-band states of the bimetallic active centers) are

provided by different metal constituents. This gap

is largely due to a lack of synthetic methods for

engineering the nano-architecture of bimetal-based

chalcogenides.

To develop more active electrocatalysts with complex

compositions, such as the Co(Fe)–Ni–S(Se) systems,

stepwise synthetic procedures were generally adopted

by using nanostructured bimetallic oxides/hydroxides

as the templating precursors [29–36]. Compared to

direct synthesis of multi-component chalcogenides,

the stepwise synthetic strategy often lacks flexible

and versatile control over the diverse morphologies

and architectures of the resulting products because

of the topotactic conversion reactions. CoSe2 and NiSe

nanostructures exhibit high activities toward the

hydrogen and oxygen evolution reactions (HER and

OER, respectively) [15–21, 25–27]. Therefore, the Co–

Ni–Se system, which contains highly active Co and

Ni centers, could conceivably be a fascinating

electrocatalyst for the HER, OER, or both. However, a

pure compound of the ternary Co–Ni–Se system has

rarely been studied as a bifunctional electrocatalyst

for both the HER and OER, even though doped

systems and a two-phase mixture have been reported

in the literature [36–38]. Herein, we describe a simple

method for one-step growth of the hierarchical ternary

CoNiSe2 nano-architecture on Ni foam (CoNiSe2/Ni),

which serves as an excellent bifunctional electrocatalyst

for both the OER and HER, using Ni foam as both

the Ni source and catalyst support to fulfill the high

activity and long-term stability demands. We also

grew NiSe nanosheets (NSs) directly on the Ni foam

(NiSe/Ni) as a control. Furthermore, in a two-electrode

water electrolyzer comprised of CoNiSe2/Ni, a

10-mA·cm−2 water-splitting current is achieved at a cell

voltage of 1.591 V, which is superior to that provided

by most of the earth-abundant 3d-TM chalcogenide-

and phosphide-based electrolyzers reported to date.

2 Experimental

2.1 Chemicals

Selenic acid (Alfa Aesar, H2SeO4, 40% aqueous solution),

2-methoxy-5-nitroaniline (MNA, Alfa Aesar, > 98%),

cobalt(III) acetylacetonate (Co(acac)3, Sigma-Aldrich,

> 98%), nickel foam (1.5-mm thickness, Ailantian

Advanced Technology Materials Co. Ltd.), Pt/C

(Johnson Matthey, 20 wt.% of 3.2-nm Pt nanoparticles

(NPs) on a Vulcan XC-72 carbon support), pyridine

(Sinopharm Chemical Reagent Co., Ltd. (SCRL), 99%),

N-methyl-2-pyrrolidinone (Alfa Aesar, 99%), and

anhydrous ethanol (SCRL, 99.5%) were used without

further purification. All water used in this work was

purified using a Millipore Milli-Q system (resistivity

> 18.0 MΩ·cm−1).

2.2 Synthesis of the electrocatalysts

2.2.1 Synthesis of a hierarchical CoNiSe2 nano-architecture

on Ni foam

A piece of Ni foam was cut into equal parts (1 cm × 3 cm)

and sequentially pretreated with acetone, hydrochloric

acid (0.1 M), and ethanol under ultrasonication to

remove grease and the surface oxide layer. MNA

(121.1 mg) and Co(acac)3 (85.6 mg) were dissolved

in a mixed solvent of pyridine (20 mL) and water

(11.5 mL), and then mixed with 6 mL of an aqueous

solution of selenic acid (0.08 M) (Co/Se molar ratio of

1:2). Then, the reaction solution was transferred to a

40-mL Teflon-lined autoclave. The cleaned and dried

Ni foam was immersed into the autoclave. The autoclave

was sealed and maintained at 180 °C for 24 h and

then naturally cooled to ambient temperature. The

resulting Ni foam covered with CoNiSe2 (CoNiSe2/Ni)

was removed, washed 5 times with ethanol and

N-methyl-2-pyrrolidinone at 50 °C, and dried.

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1333 Nano Res. 2018, 11(3): 1331–1344

2.2.2 Synthesis of NiSe2 nanosheets on Ni foam

MNA (121.1 mg) was dissolved in 31.5 mL of pyridine;

then, it was mixed with 6 mL of an aqueous solution

of selenic acid (0.08 M) under vigorous ultrasonication.

Next, the reaction solution was transferred to a

40-mL Teflon-lined autoclave, and the cleaned Ni

foam (1 × 3 cm2) was immersed into the reaction

solution. The autoclave was sealed and maintained at

180 °C for 20 h and then naturally cooled to ambient

temperature. Finally, the resulting Ni foam with

the product (NiSe/Ni) was cleaned according to the

procedure described in section 2.2.1.

2.3 Electrochemical measurements

All electrochemical measurements were performed

using a CHI 660D electrochemical analyzer (CH

Instruments, Inc.) in a standard three-electrode con-

figuration consisting of a well-cleaned CoNiSe2/Ni or

NiSe2/Ni working electrode, Ag/AgCl (KCl saturated)

reference electrode, and platinum wire counter

electrode in one compartment. Note that the effects of

the platinum wire on the OER and HER are negligible

[39, 40]. For comparison, RuO2 NPs prepared according

to a recent report [41] and Pt/C (20 wt.%) benchmarks

were loaded onto Ni foam using a Nafion solution

(5 wt.%) with the same catalyst loading as that of the

CoNiSe2/Ni electrode. The Ag/AgCl reference electrode

was calibrated with respect to the reversible hydrogen

potential using a platinum wire as both the working

and counter electrodes after each measurement. Linear

scan voltammetry (LSV) and cyclic voltammetry (CV)

curves were recorded in a supporting electrolyte (1 M

KOH) at a scan rate of 2 mV·s−1. Electrochemical

impedance spectroscopy (EIS) measurements were

performed by sampling 130 points in the frequency

range from 100 kHz to 0.01 Hz centered at −0.20 or

1.45 VRHE (RHE = reversible hydrogen electrode) for

the HER or OER, respectively, with an AC perturbation

of 5 mV. All plots for evaluating the electrocatalytic

activity toward the HER or OER were recorded after

continuous purging of H2 or O2 and were corrected

by a background capacitive current and ohmic potential

drop (iR) based on the series resistances derived from

EIS. For the overall water splitting test, two symmetric

catalyst electrodes were used as the anode and

cathode.

2.4 Characterization of materials

Scanning electron microscopy (SEM) was performed

using a Hitachi S-4800 field-emission scanning electron

microscope operating at 5 kV to investigate the

morphology of the catalysts. Transmission electron

microscopy (TEM) and high resolution TEM (HRTEM)

micrographs were acquired using an FEI Tecnai G2

F20 S-Twin transmission electron microscope operating

at an accelerating voltage of 200 kV. Scanning TEM

(STEM) micrographs and energy-dispersive X-ray

spectroscopy (EDS) elemental maps were obtained in

high-angle annular dark field (HAADF) mode using

the same transmission electron microscope. The

specimens for TEM observations were scratched from

the Ni foam substrate and sonicated before dropping

them onto 300 mesh carbon-coated copper grids. X-ray

photoelectron spectroscopy (XPS) measurements were

carried out using a PHI5000 VersaProbe (ULVAC-PHI)

spectrometer with an energy analyzer, employing a

monochromatized microfocused Al Kα (hv = 1,486.58 eV)

X-ray source. Samples for XPS measurements were

scratched from the Ni foam substrate and then

pretreated by repeated cycles of Ar+ ion sputtering to

obtain clean sample surfaces. The binding energies

(BEs) of the core levels were calibrated by setting the

adventitious C 1s peak at 284.8 eV. Survey spectra of

the samples in the BE range of 0–1,000 eV, and the core

level spectra of the elemental signals were recorded

at resolutions of 1 and 0.125 eV, respectively. The

X-ray diffraction (XRD) patterns were recorded using

a Rigaku SmartLab diffractometer with Cu Kα radiation

(λ = 1.5406 Å) operating at 40 kV and 100 mA at a

scanning rate of 0.06°·s−1. The monolithic Ni foam

covered with an as-grown catalyst was used as the

specimen for XRD analysis after a cleaning treatment.

The chemical composition of each sample was

determined by inductively coupled plasma atomic

emission spectrometry (ICP-AES, Prodigy, Leeman

Labs Inc., λ = 165–800 nm, As = 200 nm) after dissolving

the sample in aqua regia. The amount of O2 and H2

evolved under illumination was quantified using

gas chromatography (GC, Shimadzu, GC-8A) with a

thermal conductivity detector and Ar as the carrier

gas. The applied potential for the gas evolution

measurements was set at 1.63 VRHE in an airtight

two-electrode cell.


1334 Nano Res. 2018, 11(3): 1331–1344

3 Results and discussion

3.1 Hierarchical CoNiSe2 nano-architecture

Figure 1 shows the morphological and structural

features of the hierarchical CoNiSe2 nano-architecture

that was solvothermally grown on a piece of Ni foam

(designated as CoNiSe2/Ni hereafter). Figures 1(a)–1(c)

display top-view SEM images of CoNiSe2/Ni at

different magnifications. In contrast to the Ni foam

(see Fig. S1(a) in the Electronic Supplementary Material

(ESM)), the overview SEM image in Fig. 1(a) reveals

that the skeletal surface of the Ni foam is evenly

covered with dense, multilayered CoNiSe2 nanorods

(NRs) that grow radially from the substrate to form

hierarchical sea urchin-like microstructures, as evidenced

by the image of these NRs at a high magnification

(Fig. 1(b)). Further, many NRs align in a certain

direction and closely pack into clustered needle-like

bunches (Fig. 1(c)). Evidently, the diverse orientations

of the alignment of CoNiSe2 NRs provide omnidirec-

tional contact with the electrolyte and sufficient

interspace for electrolyte percolation and transfer,

favoring mass-transport. In addition, such a high-

density packing of NRs substantially boosts the

Figure 1 (a) Low-, (b) moderate-, and (c) high-magnification SEM images; (d) TEM and (e) HRTEM images; and (f) XRD pattern of the CoNiSe2 NRs directly grown on the Ni foam. The peaks labelled with asterisks in (f) originate from the Ni foam substrate. Forcomparison, the intensities and positions for the pure CoNiSe2 reference are given according to the JCPDS database (orange lines at the bottom of (f). Insets: (a) schematic illustration of the clustered CoNiSe2 NRs grown on Ni foam; (f) HAADF-STEM image and the corresponding EDS elemental mapping images of Co, Ni, and Se for CoNiSe2 NRs. Elemental mapping was performed in the area enclosed by a yellow box in the STEM image.


1335 Nano Res. 2018, 11(3): 1331–1344

structural robustness for electrocatalytic applications.

To clearly depict the CoNiSe2/Ni morphology, a

schematic of the hierarchical arrays of CoNiSe2/Ni is

shown in the inset of Fig. 1(a). The cross-sectional

SEM image shows the CoNiSe2 NRs with lengths up

to 0.5 μm (Fig. S1(b) in the ESM).

Figure 1(d) presents a representative TEM image of

several CoNiSe2 NRs with very uniform, thin diameters

of approximately 7 nm, together with some broken

NRs caused by scratching while preparing the TEM

specimens. The corresponding HRTEM image of these

NRs shows their single crystalline nature, evidenced

by the well-resolved, perfectly ordered lattice fringes

with an interplanar spacing of 2.72 Å, in accordance

with the value (2.72 Å) of the (101) planes of CoNiSe2

(Fig. 1(e)). The HAADF-STEM image and the corres-

ponding EDS elemental mapping images demonstrate

that Co, Ni, and Se are homogeneously distributed

over the entire NRs (see the insets in Fig. 1(f)). The

stoichiometry of the NRs (determined by EDS and

ICP-AES quantitative analyses; Table 1 and Fig. S2 in

the ESM) suggests the chemical identity of the NRs as

CoNiSe2. The XRD pattern in Fig. 1(f) also confirms

that the product is CoNiSe2 because the Bragg peaks

can be perfectly indexed to the hexagonal CoNiSe2

phase with lattice constants a = 3.646 Å and c = 5.337 Å

(JCPDF No. 65-7038), except for three strong peaks

stemming from the Ni substrate.

XPS was employed to further refine the composition

and chemical state of the CoNiSe2 NRs, as shown in

Fig. 2. As expected, the XPS survey spectrum in Fig. 2(a)

shows the Co, Ni, and Se signals in addition to those

from oxides and adventitious C species. The Co 2p

core level spectrum reveals the presence of two spin-

orbit doublets at 781.6 and 796.1 eV, together with two

satellite peaks at 786.5 and 803.0 eV (Fig. 2(b)), which

are associated with the BEs of Co2+ and Co3+ at the

Table 1 Parameters for comparing the electrocatalysts

Electrocatalysts Bulk atomic ratio Co:Ni:Se

XPS atomic ratio Co:Ni:Se

Crystal structure

(hexagonal)

CoNiSe2 23.5:27.7:48.8a

24.1:25.6:50.3b 22.1:28.4:49.5 a = 3.646 Å

c = 5.337 Å

a Data determined by EDS quantitative analysis. b Data obtained from ICP-AES measurements.

surface of the CoNiSe2 NRs [14, 33]. Co3+ species form

because of the harsh solvothermal conditions. The

high-resolution spectrum in the Ni 2p region (Fig. 2(c))

exhibits two peaks at 855.5 and 873.3 eV assigned to

the BEs of Ni 2p3/2 and Ni 2p1/2, respectively [25, 33].

Concomitantly, the presence of two satellite peaks at

861.2 and 879.6 eV further confirms the formation of

Ni2+ ions at the surface of the CoNiSe2 NRs. The Se 3d

XPS spectrum in Fig. 2(d) shows the BE of the main

line that consists of a doublet at 54.2 (Se 3d5/2) and

55.1 eV (Se 3d3/2) and originates from the Se moiety of

CoNiSe2. Meanwhile, a peak centered at 60.0 eV and

a doublet obtained after deconvolution of the weak

hump can be attributed to selenium oxides from surface

oxidation and Co 3p peaks, respectively. Deconvolution

of the O 1s signal in Fig. 2(e) reveals that the peaks at

530.4, 531.2, 532.1, and 533.2 eV can be assigned to

the oxygen species that originate from metallic oxides,

hydroxyl groups, selenium oxides, and water, respec-

tively [36]. The XPS surface chemical composition

does not visibly differ from the bulk composition

(Table 1).

The above morphological, structural, and com-

positional analyses consistently suggest that hierarchical

CoNiSe2 NR arrays successfully grew on the Ni foam.

The growth mechanism for CoNiSe2/Ni is proposed

in the following. MNA plays an important role in

dictating the growth of the hierarchical CoNiSe2

nano-architecture. If the reaction had been performed

in the absence of MNA while keeping the other

experimental parameters the same, a monolithic

featureless film structure would have formed

(Fig. S3(a) in the ESM). MNA first functions as a

reducing agent that reduces selenic acid to elemental

Se; subsequently, ploy(MNA) forms after oxidative

polymerization and acts as a capping reagent for the

one-dimensional growth of the CoNiSe2 NRs in

virtue of the amine group (–NH–) ligand. Indeed,

HRTEM observations reveal that all the as-prepared

CoNiSe2 NRs that were not subjected to washing are

covered with a thin, compact polymer shell (Fig. S3(b)

in the ESM), which is indicative of a strong affinity

of ploy(MNA) for the surface of the CoNiSe2 NRs.

More importantly, the formation of such a CoNiSe2–

ploy(MNA) core–shell structure is extremely favorable


1336 Nano Res. 2018, 11(3): 1331–1344

for protecting CoNiSe2 NRs against oxidation by

water–O2 molecules during growth. The MNA-assisted

synthetic route for preparing the integrated CoNiSe2/

Ni electrode is schematically depicted in Fig. 3. To

demonstrate the generality of our synthetic method

and the effects of composition on the catalytic activity,

ultrathin NiSe NSs directly grown on Ni foam

(designated as NiSe/Ni thereafter) were also prepared

using a similar procedure (see Figs. S4 and S5, and the

related discussion in the ESM). Note that the NiSe

NSs are integrated into flower-like architectures. The

mass loadings of the CoNiSe2 NRs and NiSe NSs

are 1.84 and 2.93 mg·cm−2, respectively, based on the

ICP-AES quantitative analysis.

3.2 Electrocatalysis of water splitting

Figure 4(a) illustrates the polarization curves of the

CoNiSe2/Ni, NiSe2/Ni, and RuO2 NPs by plotting

the geometrical current density against the applied

potential to compare the OER activity. Note that all

potentials are referenced to an RHE in the text. As

shown in previous reports [25, 36, 38], the oxidation

Figure 2 XPS spectra collected from the CoNiSe2 NRs scratched from the CoNiSe2/Ni electrode. (a) XPS survey spectrum, and (b) Co 2p,(c) Ni 2p, (d) Se 3d, and (e) O 1s detail spectra.


1337 Nano Res. 2018, 11(3): 1331–1344

Figure 3 Schematic illustration of the procedure for fabricating the CoNiSe2/Ni catalytic electrode. The reaction processes are: (i) nH2SeO4 + 4nMNAs → nSe + poly(MNA) ((4MNAs)n) + 4nH2O; (ii) nCo(III)(acac)3 + 4nMNAs → nCo(II)(acac)2 + poly(MNA) ((4MNAs)n) + nHacac; (iii) 5Se + 2Co(II) + 2Ni + 3H2O → 2CoNiSe2 + H2SeO3 + 4H+. The bottom panel shows a digital photograph comparing the pristine Ni foam and the converted product, i.e., a piece of a CoNiSe2/Ni electrode. The color of the surface of the Ni foam changes from silver-white to black, demonstrating successful growth of the CoNiSe2 NRs on the surface.

Figure 4 (a) Polarization curves, (b) Tafel plots, and (c) Nyquist plots of different electrodes. (d) Chronopotentiometric curve ofCoNiSe2/Ni recorded at a constant current density of 100 mA·cm−2. Insets: a closeup of the Nyquist plot of CoNiSe2/Ni (left) and the equivalent circuit used to model all the electrode systems (right).


1338 Nano Res. 2018, 11(3): 1331–1344

peaks in the range of 1.40–1.45 VRHE are assigned to the

oxidation of Ni and/or Co species. The CoNiSe2/Ni

electrode shows the highest activity with the lowest

onset potential and the highest current density. A

steep rise in current density that accompanies the

vigorous release of O2 bubbles from the CoNiSe2/Ni

anode surface is observed while the potential is scanned

toward a positive value (see Movie S1 in the ESM). In

contrast, the bare Ni foam exhibits no current below

1.55 VRHE and the smallest anodic current above 1.55

VRHE. The CoNiSe2/Ni, NiSe/Ni, and RuO2 anodes

deliver a current density of 100 mA·cm−2 at overpo-

tentials (ηOER) of 307, 350, and 374 mV, respectively.

Notably, the activity of CoNiSe2/Ni is superior to

the NiSe/Ni and RuO2 benchmark and to most

reported chalcogenides, with only the exception of

the NixFe1−xSe2–DO hybrid nanoplates reported

recently (see Table S1 in the ESM) [33].

The Tafel slopes for the CoNiSe2/Ni, NiSe/Ni, and

RuO2 NPs obtained after iR correction are 79, 94, and

116 mV·dec−1, respectively (Fig. 4(b)), which verify

the fastest OER kinetics on the surface of CoNiSe2/Ni

because it shows the smallest Tafel slope value.

Figure 4(c) shows the Nyquist plots of EIS from

modeling with a Randles circuit comprising a series

resistance (Rs), charge transfer resistance (Rct), and

constant phase element (the right inset in Fig. 4(c)).

The small Rs values (< 3 Ω) for CoNiSe2/Ni and NiSe/

Ni suggest their outstanding electric conductivity

and integrated architecture. In particular, the Rct of

CoNiSe2/Ni (~ 0.27 Ω) is much smaller than those of

the NiSe/Ni (2.28 Ω) and RuO2 NPs (3.21 Ω), further

demonstrating an insignificant Rct and the fastest

reaction rate on the CoNiSe2/Ni surface. The time-

dependent potential curve recorded at 100 mA·cm−2

for 20 h is used to assess the stability of CoNiSe2/Ni

for the OER (Fig. 4(d)). The long-term stability of the

CoNiSe2/Ni electrode for OER is confirmed by the

maintained potential round 1.54 VRHE over a period of

20 h. Note that the potential required for delivering

100 mA·cm−2 increases slightly from an initial value

of 1.537 to 1.546 VRHE after 20 h of operation. Further-

more, the stability of CoNiSe2/Ni can also be validated

from the nearly identical current density after 3,000

potential cycles of CV measurements for the OER

scan (Figure S6 in the ESM). The elevated intensity of

the redox peaks after 3,000 cycles can be attributed to

the increased concentration of Ni(III) species, e.g.,

NiOOH [30], in the “skin” of CoNiSe2/Ni after the CV

measurements. However, the XRD pattern of this

tested sample shows no XRD peaks arising from other

matters, implying a stable bulk phase of CoNiSe2

during the continuous electrolysis process (Fig. S7 in

the ESM). The SEM images exhibit a perfect retention

of the CoNiSe2 nano-architecture, even after 20 h of

OER (Fig. S8 in the ESM), suggesting an excellent

structural robustness of the electrode stemming from

the bound NRs. As expected, the corresponding XPS

spectra exhibit that the surface of the CoNiSe2 NRs is

enriched by oxidized Ni and Co species after OER

(Fig. S9 in the ESM), where the appearance of the

essential Ni(III) and Co3O4 catalytic species is critical

for promoting OER [25, 42]. Similarly, Ni(III) species

are also formed on the surface of the NiSe NSs

(Fig. S10 in the ESM). Concomitantly, the surface

coverage of SeOx for both samples is increased because

the XPS peak of the metal–Se moiety disappears

(Figs. S9 and S10 in the ESM). Clearly, our CoNiSe2

NRs and NiSe NSs provide both catalytically active

surface species for the OER and a highly conductive

backbone for electron transport.

Figure 5(a) compares the capacitance- and

iR-corrected polarization curves of CoNiSe2/Ni, NiSe/Ni,

and Pt/C for testing the activity toward the HER. The

CoNiSe2/Ni and NiSe/Ni electrodes exhibit very high

electrocatalytic activities toward HER with onset

overpotentials of 42 and 56 mV, while their activities

are still slightly lower than that of the 20 wt.% Pt/C

that only needs a slight overpotential to induce an

onset current. Similar to the observations in a previous

study [25], the bare Ni shows a moderate HER

electroactivity and requires an overpotential of 262 mV

to deliver 10 mA·cm−2. In contrast, CoNiSe2/Ni requires

overpotentials of 87 and 146 mV to afford 10 and

100 mA·cm−2, respectively, and NiSe/Ni requires 171

and 239 mV to drive 10 and 100 mA·cm−2, respectively,

which clearly implies that the loaded CoNiSe2 and

NiSe nano-architectures offer the dominant electroac-

tivity to obtain a high throughput of H2 with a

relatively low overpotential. By comparing the onset

overpotential and the overpotential for marked H2

evolution as well as the catalyst loading with the


1339 Nano Res. 2018, 11(3): 1331–1344

overpotentials for the 3d-TM chalcogenides reported

previously, our CoNiSe2/Ni catalyst ranks among the

best non-precious metal HER catalysts (Table S2 in

the ESM).

Figure 5(b) shows Tafel slopes of 40, 72, and

29 mV·dec−1 for CoNiSe2/Ni, NiSe/Ni, and Pt/C, res-

pectively. The small Tafel slope within the low potential

region for CoNiSe2/Ni, from which the exchange

current density (J0, geometrical) is estimated to be

0.37 mA·cm−2 based on the intercept of the Tafel plot,

indicates that the HER follows the Volmer–Heyrovsky

mechanism [30, 31]. This J0, geometrical value is lower

than that of NiSe/Ni (0.69 mA·cm−2) but is two orders

of magnitude higher than those for binary CoS2 and

CoSe2 [14, 15]. Concomitantly, the Nyquist plots fitted

by the above Randles circuit reveal the faster reaction

rate for CoNiSe2/Ni, which is derived from its smaller

semicircle arc (Rct of 2.1 Ω) than that of NiSe/Ni (3.5 Ω)

(Fig. 5(c)). The performance stability of the CoNiSe2/Ni

electrode for HER is determined by the chronopoten-

tiometric curve, as shown in Fig. 5(d). The potential

required to deliver 100 mA·cm−2 varies from the initial

0.170 to 0.178 VRHE over 20 h of continuous electrolysis.

Such an insignificant increase in overpotential suggests

the excellent stability of the CoNiSe2/Ni for HER.

CV tests show a slight current degradation, further

verifying the excellent long-term durability (Fig. S11(a)

in the ESM). As for the OER, the morphology and

bulk crystalline structure of the CoNiSe2/Ni sample

that was subjected to continuous HER operation over

20 h remains unchanged (for brevity, the data are

not presented again). The corresponding Co 2p XPS

spectrum exhibits that the Co 2p3/2 and Co 2p1/2 lines

shift to lower BEs (778.6 and 793.9 eV, respectively),

suggesting the formation of Co2+ by cathodic reduction

[14, 16], while the Ni chemical state remains nearly

unchanged (Fig. 2 and Fig. S9 in the ESM). No per-

ceptible changes are observed in the Ni state for the

NiSe NSs after the HER (Fig. S10 in the ESM). The

XPS spectra reveal the increased content of SeOx after

the HER for both samples, while trace amounts of the

metal–Se moiety are still observed. The increase in

Figure 5 (a) Polarization curves, (b) Tafel plots, and (c) Nyquist plots of different electrodes. (d) Chronopotentiometric curve ofCoNiSe2/Ni recorded at a constant current density of 100 mA·cm−2.


1340 Nano Res. 2018, 11(3): 1331–1344

the coverage of SeOx species is probably attributed

to the adsorption and reaction of O2 produced and

diffused from the anode with Se anions in alkaline

solution. For both post-OER and post-HER, the con-

version of the most metal–Se moiety into SeOx might

be favorable for the adsorption of H2O, therefore

leading to an enhanced water splitting rate.

The double-layer capacitances (Cdl) of the CoNiSe2/Ni

and NiSe/Ni electrodes are 9.3 and 25.9 mF·cm−2,

respectively, based on the scan rate dependence of the

charging current density (Fig. 6), where the slope of

the ΔJ vs. scan rate plot is twice Cdl. Thus, CoNiSe2/Ni

has a smaller electrochemically active surface area

(ECSA) than NiSe/Ni because the NRs are bound

together in bundles and have a lower loading. In the

case of the HER, the normalized exchange current

(J0, normalized) of CoNiSe2/Ni is 1.49 times larger than

that of NiSe/Ni based on the relative ECSA values.

Concomitantly, the turnover frequencies are 0.082 and

0.0056 H2 s‒1 for CoNiSe2/Ni and NiSe/Ni, respectively,

at an overpotential of 0.1 V, according to the method

given in the literature (Fig. S12 and the related dis-

cussion in the ESM) [43]. These results suggest

that the enhanced activity of CoNiSe2/Ni relative to

NiSe/Ni for HER may be associated with its ternary

composition. The previous theoretical investigations

proposed that the variations in electronic structure

contribute to the enhanced water splitting performance

for the ternary compounds [23, 30]; however, until

now, no direct experimental data have been available to

support such a notion. We thus compare the valence

band (VB) structures of the CoNiSe2 NRs and NiSe

NSs to effectively reinforce the modification of the

electronic structure (Fig. 8). According to the d-band

center (d-BC) position of each catalyst, which is

derived from the weighted average energy of the VB

Figure 6 Cyclic voltammograms of (a) CoNiSe2/Ni and (b) NiSe/Ni electrodes, which are used to estimate the double layer capacitances(Cdl). Scan rates from 2, 5, 10, 20, 50, and 100 to 200 mV·s−1 were chosen. (c) The capacitance currents vs. scan rate for the CoNiSe2/Ni and NiSe/Ni electrodes. The current value Δj was obtained by adding the absolute values of the anodic and cathodic currents at thecorresponding intermediate applied potential (0.30 and 0.10 VRHE for CoNiSe2/Ni and NiSe/Ni, respectively). The potential window for CoNiSe2/Ni is chosen at the more positive region to avoid the larger cathodic current that could result from the reduction of the surfaceCo(III) species (i.e., E

oCo(OH)3/Co(OH)2 = 0.17 V vs. normal hydrogen electrode).


1341 Nano Res. 2018, 11(3): 1331–1344

Figure 7 XPS valence band spectra collected for the CoNiSe2 NRs (red curve), NiSe NSs (blue curve), CoSe2 nanoneedles (olive curve), and the mixture of CoSe2 and NiSe2 nanowires (black curve). A Shirley background is subtracted from each spectrum to eliminate the spectral background that originates from inelastically scattered electrons.

spectrum by subtracting the Shirley background and

integration, the d-BC energy (2.14 eV) of the CoNiSe2

NRs is lower than those of the NiSe NSs (3.24 eV),

CoSe2 nanoneedles (2.96 eV), and bi-phase mixture of

CoSe2 and NiSe2 nanowires (3.05 eV) (the latter two

controls were prepared by following the published

procedures as described in Refs. [19, 37], respectively).

Perceivably, charge transfer from Co to Ni (or vice

versa) is responsible for the change in the d-BC

energy. The diminution of the average energy of the

d electrons in CoNiSe2 weakens the chemisorption

energy of the intermediates (Had, OHad, and Oad) of

water splitting, which enables a low activation barrier

for the elementary reaction steps. Additionally, the

different morphologies for both samples could contribute

to the changes in the electronic structure and thus

provide another pronounced factor that influences

the water-splitting performance.

Considering that both CoNiSe2/Ni and NiSe/Ni

function as bifunctional electrocatalysts, a single

electrolyzer is constructed for overall water splitting

using a symmetric two-electrode setup consisting of

CoNiSe2/Ni or NiSe/Ni acting as both an anode and

cathode (designated as CoNiSe2/Ni||CoNiSe2/Ni or

NiSe/Ni||NiSe/Ni, respectively). A control RuO2||Pt/C

benchmark is also tested for comparison. Figure 8(a)

shows that a cell voltage of 1.59, 1.64, 1.56, or 1.91 V

is required to deliver a current density of 10 mA·cm−2

for the CoNiSe2/Ni||CoNiSe2/Ni, NiSe/Ni||NiSe/Ni,

RuO2||Pt/C, or blank Ni||Ni-based electrolyzer,

respectively. The CoNiSe2/Ni||CoNiSe2/Ni setup

outperforms or is comparable to other chalcogenide-

and phosphide-based electrolyzers (Table S3 in the

ESM), although this voltage is still lower than that for

the RuO2||Pt/C benchmark. The outstanding bifunc-

tionality endows CoNiSe2/Ni||CoNiSe2/Ni with a well-

improved overall water-splitting activity, albeit the

catalytic activity of single CoNiSe2/Ni toward HER or

OER is not supreme. The catalytic capability of the

CoNiSe2/Ni||CoNiSe2/Ni setup is evidenced by the

pronounced and vigorous H2 and O2 bubbles observed

on the electrodes at 10 and 180 mA·cm−2, respectively

(see Movies S2 and S3 in the ESM, respectively).

The long-term stability of CoNiSe2/Ni||CoNiSe2/Ni is

shown by the chronopotentiometric curve (Fig. 8(b)).

To preserve a current density of 10 mA·cm−2, a slightly

lower voltage at the initial stage and a nearly constant

voltage around 1.60 V is observed during the 20-h

operation. The faradic efficiencies (FEs) for the OER

and HER were obtained by comparing the H2 and O2

production quantified by GC with the calculated

amount according to the passed charge (Fig. 8(c)).

An FE of 100% for both the OER and HER is clearly

visible. Concomitantly, the ratio of O2 and H2 is close

to 1:2.

4 Conclusions

In conclusion, a simple and facile one-pot fabrication

process was developed to achieve a new, cost-effective

nanostructured ternary selenide (hierarchical CoNiSe2

NR arrays) supported on Ni foam, which serves as a

robust high-performance catalyst for electrochemical

water splitting. Compared to most 3d-TM chalcogenides

reported to date, our CoNiSe2/Ni electrode shows the

lowest overpotentials for delivering the same current

in the OER and HER, and a long-term durability. We

further demonstrate that an electrolyzer consisting of

CoNiSe2/Ni||CoNiSe2/Ni needs a remarkably low

cell voltage of 1.591 V to afford a current density of

10 mA·cm−2, which is among the best earth-abundant

3d-TM chalcogenide- and phosphide-based electrolyzers


1342 Nano Res. 2018, 11(3): 1331–1344

reported thus far. Additional experimental results

reveal that the electronic structure of the ternary

CoNiSe2 NRs changes with respect to the mixture of

monometallic CoSe2 and NiSe2, which is one of the

factors accounting for the improved activity toward

HRE caused by the bimetallic component. Our studies

pave new ways for designing and innovating nano-

architectures of multicomponent 3d-TM compounds

as inexpensive, highly efficient, and versatile catalysts

for clean fuel generation.

Acknowledgements

This work was financially supported by the National

Natural Science Foundation of China (NSFC) (No.

21371097) and the Key University Science Research

Project of Jiangsu Province (No. 16KJA150004).

Electronic Supplementary Material: Supplementary

material (additional characterization results and

electrochemical data for this article) is available in

the online version of this article at https://doi.org/

10.1007/s12274-017-1748-3.

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