A simple method for the preparation of a nickel selenide and cobalt selenide mixed catalyst to enhance bifunctional oxygen activity for Zn-air batteries

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A simple method for the preparation of a nickel

selenide and cobalt selenide mixed catalyst to

enhance bifunctional oxygen activity for Zn–air

batteries†

Cite this: RSC Adv., 2021, 11, 19406

ab

a

Li-Juan Peng, Jie-Ping Huang, Qiu-Ren Pan, Ying Liang, Na Yin,

a

Hang-Chang Xu and Nan Li

*

Developing a low-cost, simple, and eﬃcient method to prepare excellent bifunctional electrocatalysts

toward the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is critical in

rechargeable zinc–air batteries. Non-stoichiometric

M

0.85Se (M ¼ Ni or Co) nanoparticles are

synthesized and modiﬁed on nitrogen-doped hollow carbon sphere (NHCS). The NHCS loaded Ni0.85Se

3

+

(

Ni0.85Se-NHCS) with rich Ni

presents higher OER activity, whereas the NHCS-loaded Co0.85Se

2+

Co0.85Se-NHCS) with abundant Co displays better ORR activity, respectively. When Co0.85Se-NHCS is

mixed with Ni0.85Se-NHCS in a mass ratio of 1 : 1, the resulting mixture (Ni0.85Se/Co0.85Se-NHCS-2)

shows better ORR and OER dual catalytic functions than a single selenide. Moreover, zinc–air batteries

equipped with Ni0.85Se/Co0.85Se-NHCS-2 as the oxygen electrode catalyst exhibit excellent charge and

discharge performance as well as improved stability over precious metals. This work has developed

a simple and eﬀective method to prepare excellent bifunctional electrocatalysts for ORR and OER, which

is beneﬁcial for the practical large-scale application of zinc–air batteries.

Received 13th April 2021

Accepted 21st May 2021

DOI: 10.1039/d1ra02861h

rsc.li/rsc-advances

14–17

of ORR and OER.

Therefore, the development of low-cost,

Introduction

easy-to-prepare, durable, and dual-functional non-precious

Due to the exhaustion of fossil fuel energy and environmental metal catalysts with high ORR and OER activities has become

deterioration, the development of eﬃcient, clean, and sustain- the key to achieve the practical application of Zn–Air batteries.

able energy storage technology has attracted worldwide atten-

The transition metal selenide as the earth-abundant and

1–3

tion. Zinc–air batteries are considered an ideal candidate for low-cost materials have made great eﬀorts to replace precious

renewable energy technologies due to their high power density, metal electrocatalysts to achieve durability and eﬃciency in

1

8–20

high output capacity, long-term stability, and environmental electrochemical energy storage.

Compared with transition

The performance of rechargeable zinc–air metal oxides and suldes, transition metal selenides have

batteries depends mainly on the properties of the oxygen greater intrinsic metallic conductivity, which facilitates electron

4–6

friendliness.

21–23

reduction reactions (ORR) and oxygen evolution reactions (OER) transfer in the electrocatalytic process.

Especially, the non-

that occur during discharging or charging on the cathode, stoichiometric transition metal selenides such as Co0.85Se and

7

–9

respectively. Usually, noble metal Pt-based and RuO

are considered as superior catalysts for ORR and OER, respec- on account of the non-stoichiometric conguration, and cor-

2

materials Ni0.85Se exhibit excellent electrocatalysis toward OER and ORR

10–13

tively, to accelerate the sluggish kinetics of the reaction.

However, due to the price, long-term stability, and limited cies.

reserves of noble metal catalysts, the practical application of benecial to the formation of NiOOH or CoOOH,

rechargeable Zn–Air batteries will be severely limited when noble to the real active site during the OER process, whereas Ni and

responding varied metallic valence states and metal vacan-

24,25

3+

Besides, Ni in Ni0.85Se and Co in Co0.85Se are

26,27

ascribing

2

+

2

+

28

metals are used as cathode catalysts to achieve the dual function Co are attributed to the active site for ORR.

Owing to the poor electronic conduction, the transition metal

selenides usually anchor in the high electronic conduction such

as carbon-based materials, which eﬀectively are exposed to the

more active sites and achieve long-term stability in the electro-

a

School of Chemistry and Chemical Engineering, Guangzhou Key Laboratory for Clean

Energy and Materials, Guangzhou University, Guangzhou 510006, China. E-mail:

nanli@gzhu.edu.cn

b

29,30

College of Chemistry and Materials Science, Jinan University, Guangzhou 510632, catalysis process.

Besides, to further improve the catalytic

China

activity of carbon-based electrocatalysts, the electrocatalytic

activity of carbon materials can be enhanced by doping with

†

Electronic supplementary information (ESI) available. See DOI:

0.1039/d1ra02861h

1

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28

heteroatoms. For example, Gui et al. reported the Co0.85Se above solution under magnetic stirring. Subsequently, 0.72 mL

nanoparticle encapsulating the 1D nitrogen-doped carbon of formaldehyde and 2.8 mL of TEOS were dropped in ve

nanobers (CNFs), which exhibited excellent bifunctional elec- minutes. For forming more mesoporous in the carbon sphere,

trocatalysis activity toward OER and ORR and superior electro- 1.5 mL of TEOS was added to the above-mixed solution aer 6

ꢀ

chemical stability because of the synergistic eﬀect between hours. The products were ltered and dried at 50 C overnight

ꢀ

Co0.85Se and N-doped CNFs heterostructures and suﬃcient before carbonation at 800 C for 2 hours. Finally, NHCS was

ꢀ

electrochemically active site. Because of the nitrogen-doped gained aer performing 6 M of NaOH solution at 70 C for 6

mesoporous carbon (NMC) modifying the CoSe, CoSe@NMC hours to etching silica spheres.

31

displays excellent performance for OER and ORR. However, the

transition metal selenides practically gain the bifunctional elec-

trocatalysis by many complicated strategies consisting of the

Synthesis of Ni₀_.₈₅Se-NHCS and Co₀_.₈₅Se-NHCS

50 mg of NHCS, 59.4 mg of NiCl $6H O, 59.4 mg of NaSeO , and

32

25

construct of heterointerfaces, engineering of the defect, and

2

3

33,34

50 mg CTAB were dissolved in 60 mL of distilled water under

ultrasonic operation for 1 hour. Subsequently, 8 mL of

N H $H O was dropped in the above solution and transferred to

formation of electronic interaction

to optimize the

adsorption/desorption of reaction intermediates. Therefore, it is

important to develop an excellent, durable, and inexpensive

bifunctional electrocatalyst based on transition metal selenides

for ORR and OER by a simple and easy method, which really

contributes to the practical application of zinc–air batteries.

Because of the high electrocatalytic activity of nitrogen-doped

hollow carbon spheres, we synthesized mixed M₀_.₈₅Se (M ¼ Ni or

Co) nanoparticles modied on nitrogen-doped hollow carbon

spheres (Ni0.85Se-NHCS mixed with Co0.85Se-NHCS) as OER/OER

electrocatalytic materials. The simple physical mixing method

simplied the preparation process and reduced the preparation

cost. The nitrogen-doped hollow carbon sphere facilitated the

mass/charge transfer and reduced aggregation of the nano-

particle during the electrocatalysis process. Due to the suitable

2

4

2

ꢀ

a Teon-lined autoclave and kept at 140 C in an oven for 24 h.

Aer ltered and washed with distilled water, the product was

ꢀ

dried at 50 C for 12 hours. Eventually, the product was per-

ꢀ

formed further calcination in N

2

atmosphere at 600 C for 2

hours. The loading masses of Ni0.85Se and Co0.85Se were

calculated to be about 1.29 mg per gram of carbon sphere based

on the addition of metal salts and carbon spheres. The

^s^yⁿ^t^h^e^sⁱ^s^o^f^C^o0.85Se-NHCS was following the above process and

the diﬀerence was that CoCl

6H

deposition method as a contrast. The mass of both CoCl

6H O and NiCl $6H

the same.

2

$6H

2

O was used instead of NiCl

2

-

$

2

O. 50% Ni-Co0.85Se-NHCS was prepared by a similar co-

2

-

$

2

O were 27 mg, other conditions remained

3

+

2+

3+

2+

ratio of Ni /Ni and Co /Co , Ni₀_.₈₅Se-NHCS had better cata-

lytic activity for OER, while Co0.85Se-NHCS had higher perfor-

mance for ORR. The mixture Ni0.85Se/Co0.85Se-NHCS-2 displayed

the superior electrocatalytic performance for both ORR and OER

than the Ni0.85Se-NHCS or Co0.85Se-NHCS alone because of the

synergistic eﬀect between Ni0.85Se-NHCS and Co0.85Se-NHCS.

Compared with the zinc–air batteries using Pt/C and RuO₂

hybrid catalysts, the batteries using Ni₀_.₈₅Se/Co₀_.₈₅Se-NHCS-2

hybrid catalysts showed similar charge and discharge perfor-

Mixing of Ni₀_.₈₅Se-NHCS and Co₀_.₈₅Se-NHCS

The above-prepared Ni₀_.₈₅Se-NHCS and Co₀_.₈₅Se-NHCS were

mixed with the mass radio of 1 : 2, 1 : 1 and 2 : 1, which was

labeled as Ni₀_.₈₅Se/Co₀_.₈₅Se-NHCS-1, Ni₀_.₈₅Se/Co₀_.₈₅Se-NHCS-2

and Ni0.85Se/Co0.85Se-NHCS-3, respectively.

mance and better long-term stability. This work can provide Characterization

a simple and low-cost method for the design and development of

ORR/OER bifunctional electrocatalysts, which is benecial for the

practical application of zinc–air batteries.

The crystal structure's information of the samples was gained

via X-ray diﬀraction (XRD, PANalytical, PW3040/60 diﬀractom-

eter) with Cu Ka radiation (l ¼ 0.15418 nm). The surface

chemical state of the above samples was tested by X-ray

photoelectron spectroscopy (XPS, ESCALab 250) and per-

formed by XPSPEAK soware. The surface microstructure and

morphology of the above-prepared samples were examined by

Experimental section

Chemicals

NiCl

trimethyl ammonium bromide (CTAB) and tetraethoxysilane JSM-7001F) and transmission electron microscopy (TEM,

TEOS) were purchased from Aladdin Reagent (Shanghai) Co., Ltd. JEM2010-HR). The BET specic surface area of the above

2 2 2 2

$6H O, CoCl $6H

O, formaldehyde, resorcinol, hexadecyl eld-emission scanning electron microscopy (FESEM, JEOL

(

Ammonia solution (25%) and ethanol were provided from Tianjin samples was measured by nitrogen adsorption–desorption

Baishi Chemical Co., Ltd. Hydrazine hydrate (N H $H O, 80%) isotherm (Micromeritics ASAP 2460) and pore size distribution

2

4

2

were supplied from Tianjin Damao Chemical Reagent Factory.

used the BJH desorption model.

Synthesis of NHCS

Electrochemical measurements

The synthesis of the nitrogen-doped hollow carbon sphere The catalyst ink was contained with 3 mg of catalyst, 500 mL of

27

(NHCS) was performed following the hard template method.

isopropanol, 500 mL of distilled water, and 20 mL of

0.5 g of resorcinol was dissolved in 70 mL of ethanol and 10 mL Naon®resin solutions (5.0 wt%). And then, 5 mL of catalyst ink

of distilled water. Then 3 mL of ammonia solution added to the was dropped on the glassy carbon rotating disk electrode (RDE,

©

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2

0

.126 cm ) or glassy carbon ring rotating disk electrode (RRDE,

Since the real surface active site was unknown, we evaluated

Co and Ni in the surface as the active site. The active sites per

2

.196 cm ). Finally, the above catalyst was dried in the air.

The ORR and OER catalytic activity was examined on CHI real surface area are shown in the following equation:

60e electrochemical workstation (Shanghai, Chenhua) using

7

Number of active sitesðCo0:85Se-NHCSÞ

2

a three-electrode system, where the RDE (0.126 cm ) with

Ni0.85Se-NHCS or Co0.85Se-NHCS lm as the working electrode,

the Hg/HgO as the reference electrode and a carbon rod of

spectral purity as a counter electrode. But the electron transfer

numbers and hydrogen peroxide yield was used in an AMETEK

Princeton Applied Research PMC 2000A equipped with an

RRDE as a working electrode. All the ORR and OER catalytic

ꢀ

ꢁ

2

3

2 atom per unit cell

3

9:8 A per unit cell

¼

5

Number of active sitesðNi₀_:₈₅Se-NHCSÞ

ꢀ

ꢁ

2

3

2

atom per unit cell

activity were performed in O -saturated 0.1 M KOH electrolyte.

2

¼

3

60:1 A per unit cell

˚

The electrochemical surface area (ECSA) was gained by CV

measurement in the non-faraday current region by varying the

ꢁ

1

ꢁ1

scan rates from 2 mV s to 10 mV s . The current is positively

related to the scan rate:

The number of active sites of Ni0.85Se/Co0.85Se-NHCS-2 is the

average of the Ni₀_.₈₅Se-NHCS and Co₀_.₈₅Se-NHCS. Besides, the

unit cell of Ni₀_.₈₅Se and Co₀_.₈₅Se are shown in Fig. S1,†

according to the standard crystal card of Ni₀_.₈₅Se (JCPDS No. 18-

I_c¼ C_d_lꢂ v

0888) and Co0.85Se (JCPDS No. 52-1008), respectively.

c

I represents the non-faraday current; Cdl represents the elec-

The electron transfer number (n) and hydrogen peroxide

trochemical double-layer capacitance; v means the scan rates.

The ECSA is further gained by normalizing Cdl with the special

capacitance of 40 mF cm for a at surface, which would apply

yield calculate the following equation:

ꢂ I

þ I

ꢁ

2

4

D

ⁿ^¼I

to the turnover frequency (TOF) calculations. According to the

D

R

=N

35

previous literature, TOF for OER was calculated by the

following equation:

I

R

=N

turn over the number of O

2

H

2

O

2

% ¼ 200 ꢂ _I

R

TOF ¼

=N þ I

D

N ꢂ AECSA

D R

I and I represent the disk current and the ring current of

Thereinto, N is the number of active sites

RRDE. N means the collection eﬃciency of the ring (34%).

The Zn–air battery measurement was carried out on CHI

ꢁ

1

j ꢂ 1 C s

1 mol

96 485:3 C

Turn over the number of O

2

¼

ꢂ

1

000 mA

1

760e electrochemical workstation (Shanghai, Chenhua) at room

mol O

mol

2

ꢂ

temperature. Zinc foil was worked as the anode and the catalyst

coating in carbon paper was performed as the cathode with

4

2

3

6

:022 ꢂ 10

O

2

molecules

2

ꢂ

a geometric area of 1 cm . 5 mg of catalyst was dispersed into 20

1

mol O

2

mL of 5% Naon solution, 300 mL water, and 700 mL of iso-

propanol solution. The electrolyte contained 6 M KOH and

j ꢂ 1:56

1

5

ꢂ 10 molecules per mA per s0.2 M Zn(Ac)

2

. As a reference, 2.5 mg of 20% Pt/C and 2.5 mg of

RuO were performed with the above-same process.

2

Fig. 1 The schematic of the preparation process of M0.85Se-NHCS (M ¼ Ni, Co).

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provide numerous active sites because of their specic stoichi-

ometry of metal vacancies which makes them abundant

Results and discussion

2

4,28

The synthesis process of M0.85Se-NHCS (M ¼ Ni, Co) is shown in unsaturated atoms and boost the ORR and OER activities.

Fig. 1. Initially, the phenolic resin was coated on the SiO

2

Besides, the presence of the nitrogen component further

spheres by polyreaction at room temperature. The yellow demonstrated that the successful doping of nitrogen on the

ꢀ

precursors collecting by ltration were carbonized at 800 C for hollow carbon spheres, which changed the charge distribution

2

hours in the nitrogen atmospheres. Aer the SiO

2

removal, the of the carbon material and facilitated the absorption and

nitrogen-doped hollow spheres (NHCS) were decorated with desorption of oxygen in ORR or OER process.

36,37

M₀_.₈₅Se (M ¼ Ni, Co) nanoparticles. The M Se-NHCS (M ¼ Ni,

X-ray diﬀraction (XRD) patterns revealed the crystal structure

information of the as-prepared samples. In Fig. 3a, the XRD

0.85

Co) was successfully synthesized following the above process.

The micromorphology and microstructure of the prepared patterns of Co0.85Se-NHCS and Ni0.85Se-NHCS were in agree-

catalysts Ni0.85Se-NHCS and Co0.85Se-NHCS were carried out by ment well with the standard crystal card of Ni0.85Se (JCPDS No.

scanning electron microscopy (SEM) and transmission electron 18-0888) and Co0.85Se (JCPDS No. 52-1008), respectively. The

ꢀ

microscopy (TEM). As shown in Fig. S2a and b,† the NHCS with diﬀraction peak positions of 33.15 , 44.95 , and 50.49 were

an average diameter of 400 nm size exhibited a roughly spher- accountable with (101), (102), and (110) crystal planes of

ical structure. As shown in the results of SEM (Fig. S2c–f†) and hexagonal crystalline Ni0.85Se, correspondingly. Similarly, the

ꢀ

TEM (Fig. 2a and d), the NHCS was uniformly immobilized with diﬀraction peak positions at 33.26 , 44.74 , and 50.56 were

tiny nanoparticles (Ni₀_.₈₅Se or Co₀_.₈₅Se) abounding on the related to (101), (102), and (110) crystal planes of hexagonal

surface aer hydrothermal reaction. For 50% Ni-Co0.85Se, it can crystalline Co0.85Se. The XRD patterns consistent with the

be seen the Ni0.85Se and Co0.85Se nanoparticles were co- standard card further demonstrated the successful synthesis of

deposited on the NHCS uniformly (Fig. S2j and k†). Further- Co0.85Se and Ni0.85Se. Besides, as shown in Fig. S5a,† when the

more, high-resolution transmission electron microscopy Co0.85Se-NHCS and Ni0.85Se-NHCS were mixed with the mass

(

HRTEM) was used to analyze the microstructure of the nano- radio of 1 : 1, the XRD pattern of Co0.85Se/Ni0.85Se-NHCS-2 was

particles and to conrm the composition of the nanoparticles. unquestionably unchanged. As shown in Fig. 3b, the Raman

ꢁ

1

As shown in Fig. 2b, the lattice fringes with interplanar width of peaks at 384 cm

were ascribed to Se–Se librational and

38–40

˚

stretching vibrations of the NiSe.

The characteristic peaks at

Ni0.85Se, respectively. Besides, the crystal interplanar spacings 188 cm and 678 cm were indexed to the A and A1g of CoSe,

Additionally, there were distinctive peaks at

2

.70 A and 1.18 A corresponded to the (101) and (110) planes of

ꢁ

1

ꢁ1

g

41,42

˚

respectively.

of 2.69 A corresponding to the (101) planes in Fig. 2e indicated

the existence of the Co0.85Se crystal. The EDX mappings of 1335 cm (D band) and 1590 cm (G band) attributed to sp

ꢁ

1

ꢁ1

3

2

43

Ni0.85Se-NHCS in Fig. 2c revealed the homogeneous distribution and sp hybridized carbons. The intensity ratios of D band and

of Ni, Se, N, and C element. Similarly, the element of Co, Se, N, G band of Ni0.85Se-NHCS, Co0.85Se-NHCS, Co0.85Se/Ni0.85Se-

and C were uniformly distributed on the Co₀_.₈₅Se-NHCS in NHCS-2 and NHCS (Fig. S5b†) were nearly 0.99, 0.98, 0.99,

Fig. 2f. Furthermore, the EDX results implied the successful and 0.88, respectively. Raman results showed that the NHCS

synthesis of non-stoichiometric Co₀_.₈₅Se and Ni₀_.₈₅Se (Fig. S3 possessed a high degree of graphitization, resulting in high

44

and S4†). As we all know, the non-stoichiometric materials electrical conductivity. Aer combined with Co0.85Se or

Fig. 2 (a and d) The TEM images, (b and e) high-resolution transmission electron microscopy image, and (c and f) EDX mappings of Ni0.85Se-

NHCS and Co0.85Se-NHCS, respectively.

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Fig. 3 (a) The XRD patterns and (b) Raman patterns of Ni0.85Se-NHCS and Co0.85Se-NHCS. (c) N

2

adsorption/desorption isotherms of Ni0.85Se-

2

NHCS (inset: pore size distribution) and (d) N adsorption/desorption isotherms of Co0.85Se-NHCS (inset: pore size distribution).

Ni0.85Se, the Ni0.85Se-NHCS, Co0.85Se-NHCS, and Co0.85Se/ conducive to the high performance of oxygen reduction and

48

Ni₀_.₈₅Se-NHCS-2 had abundant defect because of the existence oxygen evolution reactions.

of the non-stoichiometric M₀_.₈₅Se (M ¼ Ni or Co) nanoparticle, The surface chemical state and element composition of the

providing the more active site to boost the ORR and OER catalyst was carried out by XPS analysis in Fig. 4. The survey

45–47

activities, which is consistent with previous literature.

The spectrum of the Ni0.85Se-NHCS presented the existence of C, N,

N

2

adsorption/desorption isotherms and corresponding pore O, Se and Ni elements, while Co0.85Se-NHCS displayed the

size distribution of Ni0.85Se-NHCS, Co0.85Se-NHCS, and coexistence of C, N, O, Se and Co elements (Fig. S6a†). The O

Co0.85Se/Ni0.85Se-NHCS-2 were revealed in Fig. 3c, d, and S5,† element was attributed to the exposure to air, which was

32

respectively. The specic surface area of the Ni0.85Se-NHCS, consistent with the description in the literature. As displayed

Co₀_.₈₅Se-NHCS, and Co₀_.₈₅Se/Ni₀_.₈₅Se-NHCS-2 was 607.79, in Fig. 4a, the Ni 2p spectrum was deconvoluted into two main

2

ꢁ1

5

46.49, and 554.67 m g , respectively. Also, the N adsorption/ peaks along with two satellite peaks. The peak located at

2

desorption isotherms curves of the above samples had 853.6 eV of Ni 2p and 870.9 eV of Ni 2p₁_/₂were attributed to

3

/2

2

+

a hysteresis loop at a relative pressure between 0.5 and 0.9. Ni of Ni0.85Se-NHCS, and the peak located at 856 eV of Ni 2p3/2

3

+

These curves belonged to IV proles and carried out numerous and 873.9 eV of Ni 2p1/2 were ascribed to Ni of Ni0.85Se-

49,50

mesoporous structures in these catalysts. Besides, the pore size NHCS.

Similarly, the high-resolution spectrum of Co 3p was

distribution curves demonstrated that these catalysts retained split into two main peaks and two satellite peaks in Fig. 4b. The

rich micropores and mesopores. The abundant porous struc- peaks situated at 778.6 eV for Co 2p3/2 and 793.78 eV for Co 2p1/2

3

+

tures of samples accelerated the mass and charge transfer and demonstrated the existence of Co in Co₀_.₈₅Se-NHCS, and the

provided a large amount of specic surface area, which was peaks situated at 781.1 eV for Co 2p and 797.1 eV for Co 2p

3

/2

1/2

2

+

51–54

corresponded to Co

in Co₀_.₈₅Se-NHCS.

As the

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Fig. 4 High-resolution XPS of (a) Ni 2p, (b) Co 2p, (c) N 1s and (d) Se 3d of Ni0.85Se-NHCS and Co0.85Se-NHCS.

3

+

2+

deconvolution of Ni 2p, the Ni /Ni ratio of Ni0.85Se-NHCS was the oxygen evolution reaction, strengthening the OER inter-

3

+

57

5

.55, implying that the Ni content was much higher than that mediates bond. Interestingly, compared with Ni0.85Se-NHCS,

2

+

3+

of Ni . It has been reported that Ni was considered the active the extra peak located at 61.61 eV in the Se spectrum of

site for OER by absorbing OH owing to the formation of Co₀_.₈₅Se-NHCS was ascribed to Co 3p, which was consistent

ꢁ

12,26

55

NiOOH.

So it can be reasonably inferred that Ni₀_.₈₅Se-NHCS with the previous literature. Furthermore, the C 1 s spectrums

was favorable to OER. On the other hand, the Co 2p spectrum of Ni0.85Se-NHCS and Co0.85Se-NHCS were deconvoluted into

3

+

2+

showed that the Co /Co ratio of Co0.85Se-NHCS was 0.06. It four main peaks corresponding to the C]C (248.8 eV), C-C

2

+

indicates a large amount of Co , which was considered to favor (285.6 eV), C]N (286.9 eV), and C]O (289.4 eV) in Fig. S6b.†

28

ORR. In the spectrum of Fig. 4c, the N 1s peaks of Ni0.85Se-

The single NHCS without metal selenide only showed very

NHCS and Co0.85Se-NHCS were split into four main peaks weak ORR and OER activity (Fig. S7†). Aer M0.85Se (M ¼ Ni, Co)

ascribed to pyridinic N (398.3 eV), pyrrolic N (399.5 eV), was decorated on NHCS, the ORR performance of catalysts had

55

graphitic N (400.9 eV), and oxidic N (402.8 eV), respectively. As been greatly improved due to the intrinsic catalytic quality of

revealed in the Fig. 4d, the Se 3d spectrums of Ni₀_.₈₅Se-NHCS Ni₀_.₈₅Se and Co₀_.₈₅Se (Fig. 5). As shown in Fig. 5a, the Ni₀_.₈₅Se/

and Co0.85Se-NHCS were split into Se 3d5/2 (54.3 eV), Se 3d3/2 Co0.85Se-NHCS-2 exhibited excellent ORR activities with the

5

6

(

55.75 eV), and SeO

attributable to exposure to air. So the selenium of transition a limiting current density of 4.66 mA cm , closing those of the

metal selenides was oxidized to selenites, then to selenates. The Co0.85Se-NHCS (Eonset ¼ 0.89 V, E1/2 ¼ 0.76 V and J ¼ 4.67 mA

adsorption of selenate on the surface of the sample promoted cm ) and exceeding the Ni0.85Se-NHCS (Eonset ¼ 0.79 V, E1/2

x x

(59.2 eV). The existence of SeO was onset potential of 0.90 V, a half-wave potential of 0.77 V, and

ꢁ2

L

ꢁ

2

¼

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Fig. 5 (a) Linear sweep voltammogram curves for ORR and corresponding (b) Tafel curves, (c) converted electron transfer number and hydrogen

peroxide yield of Ni0.85Se-NHCS, Ni0.85Se-NHCS, Co0.85Se/Ni0.85Se-NHCS-2, and 20% Pt/C. (d) The ECSA of synthesized samples by CV

measurement in diﬀerent scan rates.

ꢁ

2

0

.69 V and J ¼ 3.71 mA cm ). Obviously, the ORR performance 2 was a mixture of Ni₀_.₈₅Se-NHCS and Co₀_.₈₅Se-NHCS. Further-

L

of the Ni Se/Co Se-NHCS-2 mixture was mainly derived more, the detailed data of LSV curves and Tafel slopes of these

0

.85

0.85

2

+

from the high content of Co in Co₀_.₈₅Se-NHCS. Interestingly, catalysts were shown in Fig. S9 and Table S1.† The Ni₀_.₈₅Se/

the ORR activities of Ni0.85Se/Co0.85Se-NHCS-2 were better than Co0.85Se-NHCS-2 had the highest ORR activity among the

that of 50% Ni-Co0.85Se-NHCS (Fig. S9a†), which was prepared mixtures with diﬀerent mass ratios. Moreover, its activity was

2 2 2 2

from CoCl $6H O and NiCl $6H O by co-precipitation. This even higher than that of some nickel-cobalt-selenides reported

result indicated that the catalyst prepared by the simple phys- in the literature (Table S2†). Remarkably, Ni0.85Se/Co0.85Se-

ical mixing method performed better than that prepared by the NHCS-2 achieved the same ORR performance as Co0.85Se-

complex chemical method. As shown in Fig. 5b, the Tafel slope NHCS with only half the cobalt content. And with the mass

ꢁ

1

of Ni₀_.₈₅Se/Co₀_.₈₅Se-NHCS-2 was 58.2 mV dec , lower than that ratio changed, the ORR performance of the mixtures decreased

ꢁ1

of Co₀_.₈₅Se-NHCS (62.7 mV dec ), Ni₀_.₈₅Se-NHCS (71.7 mV signicantly. Therefore, it can be reasonably inferred that there

ꢁ

1

ꢁ1

dec ), 50% Ni-Co0.85Se-NHCS (60.0 mV dec ) and 20% Pt/C is a synergistic eﬀect between Co0.85Se-NHCS and Ni0.85Se-

(

ꢁ

1

78.8 mV dec ), suggesting a faster kinetics rate of ORR NHCS, which can aﬀect the ORR activity of the mixture.

process. Besides, electrochemical impedance spectroscopy (EIS) The electron transfer number (n) and H yields in the ORR

2 2

O

of these as-prepared catalysts was provided in Fig. S8.† process were carried out from the rotating ring-disk electrode

According to the semicircle in the high-frequency region, the (RRDE) measurement (Fig. 5c). The average n value evaluated

charge transfer resistance of Ni₀_.₈₅Se/Co₀_.₈₅Se-NHCS-2 was 3.68 for Ni₀_.₈₅Se/Co₀_.₈₅Se-NHCS-2 was 3.75, higher than that of

U, between Co₀_.₈₅Se-NHCS (3.3 U) and Ni₀_.₈₅Se-NHCS (4.81 U), Ni₀_.₈₅Se-NHCS (3.35), and closed to that of Co₀_.₈₅Se-NHCS (3.76)

which was consistent with the fact that Ni₀_.₈₅Se/Co₀_.₈₅Se-NHCS- and noble metal catalyst 20% Pt/C (3.88), suggesting a four-

19412 | RSC Adv., 2021, 11, 19406–19416

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electron reaction process. Moreover, the H

2

O

2

yield of Ni0.85Se/ excellent ORR performance, good methanol resistance, and

Co₀_.₈₅Se-NHCS-2 was less than 12% in the voltage range from remarkable stability, which is derived from the synergistic eﬀect

.2 to 0.7 V, lower than that of the Co₀_.₈₅Se-NHCS (13%) and of Ni₀_.₈₅Se, Co₀_.₈₅Se, and NHCS.

commercial catalyst 20% Pt/C (8%). This result of Ni₀_.₈₅Se/ The excellent cathodic catalysts of the zinc–air battery

0

Co0.85Se-NHCS-2 further conrmed its excellent selectivity for should possess outstanding bifunctional electrocatalysis for

ORR activity. For measuring the electrochemical surface area both ORR and OER. The OER performance of the prepared

(

ECSA), the double-layer capacitance (Cdl) was conducted by catalysts was analyzed in a three-electrode cell in 0.1 M KOH

ꢁ

1

ꢁ1

cyclic voltammetry at a scan rate from 2 mV s to 10 mV s

.

electrolyte and compared with other catalysts including the

. The operating potential at a current

Fig. S10.† As shown in Fig. 5d, the C of the Ni₀_.₈₅Se/Co₀_.₈₅Se- density of 10 mA cm was used to evaluate the OER electro-

The CV curves of the as-prepared catalysts were shown in noble metal catalyst RuO

2

ꢁ

2

dl

ꢁ2

NHCS-2 was 5.69 mF cm , which was second only to that of catalytic activity of the material. As shown in Fig. 6a, the oper-

Co₀_.₈₅Se-NHCS and higher than that of other catalysts. This ating potential of Ni₀_.₈₅Se/Co₀_.₈₅Se-NHCS-2 was 1.63 V. It is

2

result demonstrates that the Ni0.85Se/Co0.85Se-NHCS-2 has a little worse than that of precious metal RuO (1.59 V), close to

a superior active surface area and active site for electrochemical that of Ni0.85Se-NHCS (1.62 V), but better than that of Co0.85Se-

activity. Besides, the super methanol tolerance was displayed in NHCS (1.65 V). Fig. 6b showed the Tafel slopes of these catalysts

Fig. S11,† which was carried out the chronoamperometric test at for OER. The Ni0.85Se/Co0.85Se-NHCS-2 exhibited a lower Tafel

ꢁ

1

0.3 V in O

2

-saturated 0.1 M KOH solution with 1600 rpm. The slope of 118.3 mV dec than that of Co0.85Se-NHCS (141.6 mV

ꢁ

1

ꢁ1

ORR activity of Ni₀_.₈₅Se/Co₀_.₈₅Se-NHCS-2 didn't change obvi- dec ), Ni₀_.₈₅Se-NHCS (136.0 mV dec ), and only higher than

ously aer injecting 1 M methanol, while that of 20% Pt/C that of RuO (84.1 mV dec ). Besides, the LSV curves and Tafel

ꢁ1

2

decreased evidently. It indicates that Ni₀_.₈₅Se/Co₀_.₈₅Se-NHCS-2 slopes of other mixed catalysts were also shown in Fig. S9 and

has outstanding methanol tolerance and a wide practical Table S1.† The OER performance of Ni0.85Se/Co0.85Se-NHCS-2

range. As shown in Fig. S12,† the limiting current density of with less Ni content was similar to that of Ni0.85Se-NHCS, and

Ni0.85Se/Co0.85Se-NHCS-2 only declined by 4.05% aer 20 hours. higher than that of the mixtures with other mass ratios. More-

By contrast, that of 20% Pt/C dropped oﬀ 13.14%. The above over, to further evaluate the intrinsic performance for OER of

measurements result that Ni0.85Se/Co0.85Se-NHCS-2 has the catalysts, the TOF of catalysts was evaluated. Ni0.85Se/

Fig. 6 Linear sweep voltammogram curves for (a) OER and corresponding (b) Tafel slope curves, (c) the potential diﬀerence (DE) between E1/2 for

ORR and Ej¼10 for OER.

©

2021 The Author(s). Published by the Royal Society of Chemistry

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Paper

Fig. 7 (a) the schematic of the zinc–air battery. (b) The open-circuit curves, (c) charge and discharge polarization curves, (d) discharge

ꢁ2

polarization curves and corresponding the power density curves and the long-term stability by charge and discharge at 25 mA cm of zinc–air

battery assemble with Ni0.85Se/Co0.85Se-NHCS-2 and 20% Pt/C&RuO

2

.

ꢁ

1

Co₀_.₈₅Se-NHCS-2 was calculated as 0.40 s at the potential of Co₀_.₈₅Se-NHCS-2 illustrated a DE value of 0.86 V, which was

ꢁ

1

.58 V, lower than that of Ni₀_.₈₅Se-NHCS (0.73 s ) and higher lower than that of Co₀_.₈₅Se-NHCS (0.89 V), Ni₀_.₈₅Se-NHCS (0.93

ꢁ1

than that of Co0.85Se-NHCS (0.26 s ), which demonstrated the V), indicating that Ni0.85Se/Co0.85Se-NHCS-2 possesses the

OER activity of the mixed catalyst Ni0.85Se/Co0.85Se-NHCS-2 was optimum bifunctional ORR and OER performance among the

mainly derived from the Ni0.85Se-NHCS. Like the ORR perfor- non-precious metal catalysts (Fig. 6c). From the above results, it

3

+

3+

mance, the Ni0.85Se/Co0.85Se-NHCS-2 with the rich of Ni dis- is shown that the Ni0.85Se-NHCS-2 as a mixture of Ni -rich

2

+

played a better OER performance than that of the other metal Ni0.85Se-NHCS and Co -rich Co0.85Se-NHCS has good ORR and

selenide materials (Table S2†). Furthermore, the potential OER bifunctional electrochemical activity when the same cata-

diﬀerence (DE) between half-wave potential (E₁_/₂) for ORR and lyst mass is used.

ꢁ

2

OER potential at 10 mA cm (E

) was applied to evaluate the

Due to the superior bifunctional activity of Ni₀_.₈₅Se/Co₀_.₈₅Se-

j¼10

performance of the bifunctional catalytic activity. The Ni0.85Se/ NHCS-2, the zinc–air battery was assembled with this mixture as

19414 | RSC Adv., 2021, 11, 19406–19416

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cathode catalyst, zinc foil as an anode, and 6 M KOH and 0.2 M equipped with the 20% Pt/C&RuO

2

catalysts. These results

Zn(Ac)₂as the electrolyte, respectively (Fig. 7a). The oxygen identify the rational mixture of nickel selenide and cobalt

cathode in the zinc–air battery was made by coating the catalyst selenide as a simple, low-cost and eﬃcient method for the

2

on carbon paper with a coating area of 1 cm . Moreover, the preparation of oxygen electrode catalysts, further improving the

2

1

0% Pt/C and RuO

: 1 was used as a comparison. As shown in Fig. 7b, the zinc–air

2

noble metal mixture with a mass ratio of performance and application prospect of zinc–air batteries.

battery equipped with Ni0.85Se/Co0.85Se-NHCS-2 exhibited the

open potential of 1.40 V, coming near that of the battery with Pt/

Author contributions

2

C&RuO (1.46 V). Besides, the zinc–air battery based on Ni0.85Se/

Li-Juan Peng: conceptualization, investigation, writing – orig-

inal dra. Jie-Ping Huang: investigation, data curation. Qiu-Ren

Pan: validation, formal analysis. Ying Liang: investigation. Na

Yin: investigation. Hang-Chang Xu: investigation. Nan Li:

conceptualization, supervision, writing – original dra, project

administration.

^C^o⁰.85Se-NHCS-2 worked and successfully drived a timer, testi-

fying to the potential application of this catalyst in Fig. S13.† To

further assess the performance of zinc–air devices, the charge

and discharge curves were carried in Fig. 7c. The Ni0.85Se/

Co0.85Se-NHCS-2 based battery exhibited a potential gap in

charge and discharge measure approaching that of 20% Pt/

2

C&RuO , which pointed out the awesome charge and discharge

Conﬂicts of interest

ability of the Ni0.85Se/Co0.85Se-NHCS-2 based battery. Moreover,

the Ni₀_.₈₅Se/Co₀_.₈₅Se-NHCS-2 based battery was manifested

The authors declare that they have no known competing

ꢁ

2

a power density of 118.34 mW cm at a current density of 185.1



nancial interests or personal relationships that could have

ꢁ

2

mA cm , while that of 20% Pt/C&RuO achieved 154.13 mW

2

appeared to inuence the work reported in this paper.

ꢁ

2

ꢁ2

cm at 210.10 mA cm (Fig. 7d). Moreover, the performance

of zinc–air batteries with other non-precious catalysts was

compared in Table S3.† To further appraise the practical Acknowledgements

stability application, the long-term charge and discharge

This work was supported by Natural Science Foundation of

Guangdong Province (No. 2020A1515011551), Science and

activity of the Ni0.85Se/Co0.85Se-NHCS-2 and 20% Pt/C&RuO

2

ꢁ

2

was implemented at a current density of 25 mA cm with

0 min per cycle. As exhibited in Fig. 7e, the zinc–air battery

Technology

Research

Project

of

Guangzhou

(No.

1

202102010484), and Innovative Training Program for University

based on Ni₀_.₈₅Se/Co₀_.₈₅Se-NHCS-2 displayed better long-term

stability through long cycle charging and discharging than

Students (No. 201911078017).

that with 20% Pt/C&RuO

Co0.85Se-NHCS-2 based battery manifested a subtle change from

.27 to 1.25 V aer 100 hours due to the balanced concentration

of dissolved oxygen, which was consistent with previous litera-

2

. The potential gap of the Ni0.85Se/

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19416 | RSC Adv., 2021, 11, 19406–19416

Article Doi

A simple method for the preparation of a nickel selenide and cobalt selenide mixed catalyst to enhance bifunctional oxygen activity for Zn-air batteries

DOI: 10.1039/d1ra02861h

Source and publish data:

Authors:

Article abstract of DOI:10.1039/d1ra02861h

Full text of DOI:10.1039/d1ra02861h

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