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

Abstract

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 = Ni or Co) nanoparticles are synthesized and modified on nitrogen-doped hollow carbon sphere (NHCS). The NHCS loaded Ni_0.85Se (Ni_0.85Se-NHCS) with rich Ni³⁺ presents higher OER activity, whereas the NHCS-loaded Co_0.85Se (Co_0.85Se-NHCS) with abundant Co²⁺ displays better ORR activity, respectively. When Co_0.85Se-NHCS is mixed with Ni_0.85Se-NHCS in a mass ratio of 1 : 1, the resulting mixture (Ni_0.85Se/Co_0.85Se-NHCS-2) shows better ORR and OER dual catalytic functions than a single selenide. Moreover, zinc–air batteries equipped with Ni_0.85Se/Co_0.85Se-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.

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Introduction

Due to the exhaustion of fossil fuel energy and environmental deterioration, the development of efficient, clean, and sustainable energy storage technology has attracted worldwide attention.^1–3 Zinc–air batteries are considered an ideal candidate for renewable energy technologies due to their high power density, high output capacity, long-term stability, and environmental friendliness.^4–6 The performance of rechargeable zinc–air batteries depends mainly on the properties of the oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) that occur during discharging or charging on the cathode, respectively.^7–9 Usually, noble metal Pt-based and RuO₂ materials are considered as superior catalysts for ORR and OER, respectively, to accelerate the sluggish kinetics of the reaction.^10–13 However, due to the price, long-term stability, and limited reserves of noble metal catalysts, the practical application of rechargeable Zn–Air batteries will be severely limited when noble metals are used as cathode catalysts to achieve the dual function of ORR and OER.^14–17 Therefore, the development of low-cost, easy-to-prepare, durable, and dual-functional non-precious metal catalysts with high ORR and OER activities has become the key to achieve the practical application of Zn–Air batteries.

The transition metal selenide as the earth-abundant and low-cost materials have made great efforts to replace precious metal electrocatalysts to achieve durability and efficiency in electrochemical energy storage.^18–20 Compared with transition metal oxides and sulfides, transition metal selenides have greater intrinsic metallic conductivity, which facilitates electron transfer in the electrocatalytic process.^21–23 Especially, the non-stoichiometric transition metal selenides such as Co_0.85Se and Ni_0.85Se exhibit excellent electrocatalysis toward OER and ORR on account of the non-stoichiometric configuration, and corresponding varied metallic valence states and metal vacancies.^24,25 Besides, Ni³⁺ in Ni_0.85Se and Co³⁺ in Co_0.85Se are beneficial to the formation of NiOOH or CoOOH,^26,27 ascribing to the real active site during the OER process, whereas Ni²⁺ and Co²⁺are attributed to the active site for ORR.²⁸

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 electrocatalysis process.^29,30 Besides, to further improve the catalytic activity of carbon-based electrocatalysts, the electrocatalytic activity of carbon materials can be enhanced by doping with heteroatoms. For example, Gui et al.²⁸ reported the Co_0.85Se nanoparticle encapsulating the 1D nitrogen-doped carbon nanofibers (CNFs), which exhibited excellent bifunctional electrocatalysis activity toward OER and ORR and superior electrochemical stability because of the synergistic effect between Co_0.85Se and N-doped CNFs heterostructures and sufficient electrochemically active site. Because of the nitrogen-doped mesoporous carbon (NMC) modifying the CoSe, CoSe@NMC displays excellent performance for OER and ORR.³¹ However, the transition metal selenides practically gain the bifunctional electrocatalysis by many complicated strategies consisting of the construct of heterointerfaces,³² engineering of the defect,²⁵ and formation of electronic interaction^33,34 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 M_0.85Se (M = Ni or Co) nanoparticles modified on nitrogen-doped hollow carbon spheres (Ni_0.85Se-NHCS mixed with Co_0.85Se-NHCS) as OER/OER electrocatalytic materials. The simple physical mixing method simplified the preparation process and reduced the preparation cost. The nitrogen-doped hollow carbon sphere facilitated the mass/charge transfer and reduced aggregation of the nanoparticle during the electrocatalysis process. Due to the suitable ratio of Ni³⁺/Ni²⁺ and Co³⁺/Co²⁺, Ni_0.85Se-NHCS had better catalytic activity for OER, while Co_0.85Se-NHCS had higher performance for ORR. The mixture Ni_0.85Se/Co_0.85Se-NHCS-2 displayed the superior electrocatalytic performance for both ORR and OER than the Ni_0.85Se-NHCS or Co_0.85Se-NHCS alone because of the synergistic effect between Ni_0.85Se-NHCS and Co_0.85Se-NHCS. Compared with the zinc–air batteries using Pt/C and RuO₂ hybrid catalysts, the batteries using Ni_0.85Se/Co_0.85Se-NHCS-2 hybrid catalysts showed similar charge and discharge performance and better long-term stability. This work can provide a simple and low-cost method for the design and development of ORR/OER bifunctional electrocatalysts, which is beneficial for the practical application of zinc–air batteries.

Experimental section

Chemicals

NiCl₂·6H₂O, CoCl₂·6H₂O, formaldehyde, resorcinol, hexadecyl trimethyl ammonium bromide (CTAB) and tetraethoxysilane (TEOS) were purchased from Aladdin Reagent (Shanghai) Co., Ltd. Ammonia solution (25%) and ethanol were provided from Tianjin Baishi Chemical Co., Ltd. Hydrazine hydrate (N₂H₄·H₂O, 80%) were supplied from Tianjin Damao Chemical Reagent Factory.

Synthesis of NHCS

The synthesis of the nitrogen-doped hollow carbon sphere (NHCS) was performed following the hard template method.²⁷ 0.5 g of resorcinol was dissolved in 70 mL of ethanol and 10 mL of distilled water. Then 3 mL of ammonia solution added to the above solution under magnetic stirring. Subsequently, 0.72 mL of formaldehyde and 2.8 mL of TEOS were dropped in five minutes. For forming more mesoporous in the carbon sphere, 1.5 mL of TEOS was added to the above-mixed solution after 6 hours. The products were filtered and dried at 50 °C overnight before carbonation at 800 °C for 2 hours. Finally, NHCS was gained after performing 6 M of NaOH solution at 70 °C for 6 hours to etching silica spheres.

Synthesis of Ni_0.85Se-NHCS and Co_0.85Se-NHCS

50 mg of NHCS, 59.4 mg of NiCl₂·6H₂O, 59.4 mg of NaSeO₃, and 50 mg CTAB were dissolved in 60 mL of distilled water under ultrasonic operation for 1 hour. Subsequently, 8 mL of N₂H₄·H₂O was dropped in the above solution and transferred to a Teflon-lined autoclave and kept at 140 °C in an oven for 24 h. After filtered and washed with distilled water, the product was dried at 50 °C for 12 hours. Eventually, the product was performed further calcination in N₂ atmosphere at 600 °C for 2 hours. The loading masses of Ni_0.85Se and Co_0.85Se were calculated to be about 1.29 mg per gram of carbon sphere based on the addition of metal salts and carbon spheres. The synthesis of Co_0.85Se-NHCS was following the above process and the difference was that CoCl₂·6H₂O was used instead of NiCl₂·6H₂O. 50% Ni-Co_0.85Se-NHCS was prepared by a similar co-deposition method as a contrast. The mass of both CoCl₂·6H₂O and NiCl₂·6H₂O were 27 mg, other conditions remained the same.

Mixing of Ni_0.85Se-NHCS and Co_0.85Se-NHCS

The above-prepared Ni_0.85Se-NHCS and Co_0.85Se-NHCS were mixed with the mass radio of 1 : 2, 1 : 1 and 2 : 1, which was labeled as Ni_0.85Se/Co_0.85Se-NHCS-1, Ni_0.85Se/Co_0.85Se-NHCS-2 and Ni_0.85Se/Co_0.85Se-NHCS-3, respectively.

Characterization

The crystal structure's information of the samples was gained via X-ray diffraction (XRD, PANalytical, PW3040/60 diffractometer) with Cu Kα radiation (λ = 0.15418 nm). The surface chemical state of the above samples was tested by X-ray photoelectron spectroscopy (XPS, ESCALab 250) and performed by XPSPEAK software. The surface microstructure and morphology of the above-prepared samples were examined by field-emission scanning electron microscopy (FESEM, JEOL JSM-7001F) and transmission electron microscopy (TEM, JEM2010-HR). The BET specific surface area of the above samples was measured by nitrogen adsorption–desorption isotherm (Micromeritics ASAP 2460) and pore size distribution used the BJH desorption model.

Electrochemical measurements

The catalyst ink was contained with 3 mg of catalyst, 500 μL of isopropanol, 500 μL of distilled water, and 20 μL of Nafion®resin solutions (5.0 wt%). And then, 5 μL of catalyst ink was dropped on the glassy carbon rotating disk electrode (RDE, 0.126 cm²) or glassy carbon ring rotating disk electrode (RRDE, 0.196 cm²). Finally, the above catalyst was dried in the air.

The ORR and OER catalytic activity was examined on CHI 760e electrochemical workstation (Shanghai, Chenhua) using a three-electrode system, where the RDE (0.126 cm²) with Ni_0.85Se-NHCS or Co_0.85Se-NHCS film 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 activity were performed in O₂-saturated 0.1 M KOH electrolyte.

The electrochemical surface area (ECSA) was gained by CV measurement in the non-faraday current region by varying the scan rates from 2 mV s⁻¹ to 10 mV s⁻¹. The current is positively related to the scan rate:

I_c = C_dl × v

I_c represents the non-faraday current; C_dl represents the electrochemical double-layer capacitance; v means the scan rates. The ECSA is further gained by normalizing C_dl with the special capacitance of 40 μF cm⁻² for a flat surface, which would apply to the turnover frequency (TOF) calculations. According to the previous literature,³⁵ TOF for OER was calculated by the following equation:
Thereinto, N is the number of active sites

Since the real surface active site was unknown, we evaluated Co and Ni in the surface as the active site. The active sites per real surface area are shown in the following equation:

The number of active sites of Ni_0.85Se/Co_0.85Se-NHCS-2 is the average of the Ni_0.85Se-NHCS and Co_0.85Se-NHCS. Besides, the unit cell of Ni_0.85Se and Co_0.85Se are shown in Fig. S1,† according to the standard crystal card of Ni_0.85Se (JCPDS No. 18-0888) and Co_0.85Se (JCPDS No. 52-1008), respectively.

The electron transfer number (n) and hydrogen peroxide yield calculate the following equation:

I_D and I_R represent the disk current and the ring current of RRDE. N means the collection efficiency of the ring (34%).

The Zn–air battery measurement was carried out on CHI 760e electrochemical workstation (Shanghai, Chenhua) at room temperature. Zinc foil was worked as the anode and the catalyst coating in carbon paper was performed as the cathode with a geometric area of 1 cm². 5 mg of catalyst was dispersed into 20 μL of 5% Nafion solution, 300 μL water, and 700 μL of isopropanol solution. The electrolyte contained 6 M KOH and 0.2 M Zn(Ac)₂. As a reference, 2.5 mg of 20% Pt/C and 2.5 mg of RuO₂ were performed with the above-same process.

Results and discussion

The synthesis process of M_0.85Se-NHCS (M = Ni, Co) is shown in Fig. 1. Initially, the phenolic resin was coated on the SiO₂ spheres by polyreaction at room temperature. The yellow precursors collecting by filtration were carbonized at 800 °C for 2 hours in the nitrogen atmospheres. After the SiO₂ removal, the nitrogen-doped hollow spheres (NHCS) were decorated with M_0.85Se (M = Ni, Co) nanoparticles. The M_0.85Se-NHCS (M = Ni, Co) was successfully synthesized following the above process.

Fig. 1 The schematic of the preparation process of M_0.85Se-NHCS (M = Ni, Co).

The micromorphology and microstructure of the prepared catalysts Ni_0.85Se-NHCS and Co_0.85Se-NHCS were carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. S2a and b,† the NHCS with an average diameter of 400 nm size exhibited a roughly spherical structure. As shown in the results of SEM (Fig. S2c–f†) and TEM (Fig. 2a and d), the NHCS was uniformly immobilized with tiny nanoparticles (Ni_0.85Se or Co_0.85Se) abounding on the surface after hydrothermal reaction. For 50% Ni-Co_0.85Se, it can be seen the Ni_0.85Se and Co_0.85Se nanoparticles were co-deposited on the NHCS uniformly (Fig. S2j and k†). Furthermore, high-resolution transmission electron microscopy (HRTEM) was used to analyze the microstructure of the nanoparticles and to confirm the composition of the nanoparticles. As shown in Fig. 2b, the lattice fringes with interplanar width of 2.70 Å and 1.18 Å corresponded to the (101) and (110) planes of Ni_0.85Se, respectively. Besides, the crystal interplanar spacings of 2.69 Å corresponding to the (101) planes in Fig. 2e indicated the existence of the Co_0.85Se crystal. The EDX mappings of Ni_0.85Se-NHCS in Fig. 2c revealed the homogeneous distribution of Ni, Se, N, and C element. Similarly, the element of Co, Se, N, and C were uniformly distributed on the Co_0.85Se-NHCS in Fig. 2f. Furthermore, the EDX results implied the successful synthesis of non-stoichiometric Co_0.85Se and Ni_0.85Se (Fig. S3 and S4†). As we all know, the non-stoichiometric materials provide numerous active sites because of their specific stoichiometry of metal vacancies which makes them abundant unsaturated atoms and boost the ORR and OER activities.^24,28 Besides, the presence of the nitrogen component further demonstrated that the successful doping of nitrogen on the hollow carbon spheres, which changed the charge distribution of the carbon material and facilitated the absorption and desorption of oxygen in ORR or OER process.^36,37

Fig. 2 (a and d) The TEM images, (b and e) high-resolution transmission electron microscopy image, and (c and f) EDX mappings of Ni_0.85Se-NHCS and Co_0.85Se-NHCS, respectively.

X-ray diffraction (XRD) patterns revealed the crystal structure information of the as-prepared samples. In Fig. 3a, the XRD patterns of Co_0.85Se-NHCS and Ni_0.85Se-NHCS were in agreement well with the standard crystal card of Ni_0.85Se (JCPDS No. 18-0888) and Co_0.85Se (JCPDS No. 52-1008), respectively. The diffraction peak positions of 33.15°, 44.95°, and 50.49° were accountable with (101), (102), and (110) crystal planes of hexagonal crystalline Ni_0.85Se, correspondingly. Similarly, the diffraction peak positions at 33.26°, 44.74°, and 50.56° were related to (101), (102), and (110) crystal planes of hexagonal crystalline Co_0.85Se. The XRD patterns consistent with the standard card further demonstrated the successful synthesis of Co_0.85Se and Ni_0.85Se. Besides, as shown in Fig. S5a,† when the Co_0.85Se-NHCS and Ni_0.85Se-NHCS were mixed with the mass radio of 1 : 1, the XRD pattern of Co_0.85Se/Ni_0.85Se-NHCS-2 was unquestionably unchanged. As shown in Fig. 3b, the Raman peaks at 384 cm⁻¹ were ascribed to Se–Se librational and stretching vibrations of the NiSe.^38–40 The characteristic peaks at 188 cm⁻¹ and 678 cm⁻¹ were indexed to the A_g and A_1g of CoSe, respectively.^41,42 Additionally, there were distinctive peaks at 1335 cm⁻¹ (D band) and 1590 cm⁻¹ (G band) attributed to sp³ and sp² hybridized carbons.⁴³ The intensity ratios of D band and G band of Ni_0.85Se-NHCS, Co_0.85Se-NHCS, Co_0.85Se/Ni_0.85Se-NHCS-2 and NHCS (Fig. S5b†) were nearly 0.99, 0.98, 0.99, and 0.88, respectively. Raman results showed that the NHCS possessed a high degree of graphitization, resulting in high electrical conductivity.⁴⁴ After combined with Co_0.85Se or Ni_0.85Se, the Ni_0.85Se-NHCS, Co_0.85Se-NHCS, and Co_0.85Se/Ni_0.85Se-NHCS-2 had abundant defect because of the existence of the non-stoichiometric M_0.85Se (M = Ni or Co) nanoparticle, providing the more active site to boost the ORR and OER activities, which is consistent with previous literature.^45–47 The N₂ adsorption/desorption isotherms and corresponding pore size distribution of Ni_0.85Se-NHCS, Co_0.85Se-NHCS, and Co_0.85Se/Ni_0.85Se-NHCS-2 were revealed in Fig. 3c, d, and S5,† respectively. The specific surface area of the Ni_0.85Se-NHCS, Co_0.85Se-NHCS, and Co_0.85Se/Ni_0.85Se-NHCS-2 was 607.79, 546.49, and 554.67 m² g⁻¹, respectively. Also, the N₂ adsorption/desorption isotherms curves of the above samples had a hysteresis loop at a relative pressure between 0.5 and 0.9. These curves belonged to IV profiles and carried out numerous mesoporous structures in these catalysts. Besides, the pore size distribution curves demonstrated that these catalysts retained rich micropores and mesopores. The abundant porous structures of samples accelerated the mass and charge transfer and provided a large amount of specific surface area, which was conducive to the high performance of oxygen reduction and oxygen evolution reactions.⁴⁸

Fig. 3 (a) The XRD patterns and (b) Raman patterns of Ni_0.85Se-NHCS and Co_0.85Se-NHCS. (c) N₂ adsorption/desorption isotherms of Ni_0.85Se-NHCS (inset: pore size distribution) and (d) N₂ adsorption/desorption isotherms of Co_0.85Se-NHCS (inset: pore size distribution).

The surface chemical state and element composition of the catalyst was carried out by XPS analysis in Fig. 4. The survey spectrum of the Ni_0.85Se-NHCS presented the existence of C, N, O, Se and Ni elements, while Co_0.85Se-NHCS displayed the coexistence of C, N, O, Se and Co elements (Fig. S6a†). The O element was attributed to the exposure to air, which was consistent with the description in the literature.³² As displayed in Fig. 4a, the Ni 2p spectrum was deconvoluted into two main peaks along with two satellite peaks. The peak located at 853.6 eV of Ni 2p_3/2 and 870.9 eV of Ni 2p_1/2 were attributed to Ni²⁺ of Ni_0.85Se-NHCS, and the peak located at 856 eV of Ni 2p_3/2 and 873.9 eV of Ni 2p_1/2 were ascribed to Ni³⁺ of Ni_0.85Se-NHCS.^49,50 Similarly, the high-resolution spectrum of Co 3p was split into two main peaks and two satellite peaks in Fig. 4b. The peaks situated at 778.6 eV for Co 2p_3/2 and 793.78 eV for Co 2p_1/2 demonstrated the existence of Co³⁺ in Co_0.85Se-NHCS, and the peaks situated at 781.1 eV for Co 2p_3/2 and 797.1 eV for Co 2p_1/2 corresponded to Co²⁺ in Co_0.85Se-NHCS.^51–54 As the deconvolution of Ni 2p, the Ni³⁺/Ni²⁺ ratio of Ni_0.85Se-NHCS was 5.55, implying that the Ni³⁺ content was much higher than that of Ni²⁺. It has been reported that Ni³⁺ was considered the active site for OER by absorbing OH⁻ owing to the formation of NiOOH.^12,26 So it can be reasonably inferred that Ni_0.85Se-NHCS was favorable to OER. On the other hand, the Co 2p spectrum showed that the Co³⁺/Co²⁺ ratio of Co_0.85Se-NHCS was 0.06. It indicates a large amount of Co²⁺, which was considered to favor ORR.²⁸ In the spectrum of Fig. 4c, the N 1s peaks of Ni_0.85Se-NHCS and Co_0.85Se-NHCS were split into four main peaks ascribed to pyridinic N (398.3 eV), pyrrolic N (399.5 eV), graphitic N (400.9 eV), and oxidic N (402.8 eV), respectively.⁵⁵ As revealed in the Fig. 4d, the Se 3d spectrums of Ni_0.85Se-NHCS and Co_0.85Se-NHCS were split into Se 3d_5/2 (54.3 eV), Se 3d_3/2 (55.75 eV), and SeO_x (59.2 eV).⁵⁶ The existence of SeO_x was attributable to exposure to air. So the selenium of transition metal selenides was oxidized to selenites, then to selenates. The adsorption of selenate on the surface of the sample promoted the oxygen evolution reaction, strengthening the OER intermediates bond.⁵⁷ Interestingly, compared with Ni_0.85Se-NHCS, the extra peak located at 61.61 eV in the Se spectrum of Co_0.85Se-NHCS was ascribed to Co 3p, which was consistent with the previous literature.⁵⁵ Furthermore, the C 1 s spectrums of Ni_0.85Se-NHCS and Co_0.85Se-NHCS were deconvoluted into four main peaks corresponding to the C=C (248.8 eV), C-C (285.6 eV), C=N (286.9 eV), and C=O (289.4 eV) in Fig. S6b.†

Fig. 4 High-resolution XPS of (a) Ni 2p, (b) Co 2p, (c) N 1s and (d) Se 3d of Ni_0.85Se-NHCS and Co_0.85Se-NHCS.

The single NHCS without metal selenide only showed very weak ORR and OER activity (Fig. S7†). After M_0.85Se (M = Ni, Co) was decorated on NHCS, the ORR performance of catalysts had been greatly improved due to the intrinsic catalytic quality of Ni_0.85Se and Co_0.85Se (Fig. 5). As shown in Fig. 5a, the Ni_0.85Se/Co_0.85Se-NHCS-2 exhibited excellent ORR activities with the onset potential of 0.90 V, a half-wave potential of 0.77 V, and a limiting current density of 4.66 mA cm⁻², closing those of the Co_0.85Se-NHCS (E_onset = 0.89 V, E_1/2 = 0.76 V and J_L = 4.67 mA cm⁻²) and exceeding the Ni_0.85Se-NHCS (E_onset = 0.79 V, E_1/2 = 0.69 V and J_L = 3.71 mA cm⁻²). Obviously, the ORR performance of the Ni_0.85Se/Co_0.85Se-NHCS-2 mixture was mainly derived from the high content of Co²⁺ in Co_0.85Se-NHCS. Interestingly, the ORR activities of Ni_0.85Se/Co_0.85Se-NHCS-2 were better than that of 50% Ni-Co_0.85Se-NHCS (Fig. S9a†), which was prepared from CoCl₂·6H₂O and NiCl₂·6H₂O by co-precipitation. This result indicated that the catalyst prepared by the simple physical mixing method performed better than that prepared by the complex chemical method. As shown in Fig. 5b, the Tafel slope of Ni_0.85Se/Co_0.85Se-NHCS-2 was 58.2 mV dec⁻¹, lower than that of Co_0.85Se-NHCS (62.7 mV dec⁻¹), Ni_0.85Se-NHCS (71.7 mV dec⁻¹), 50% Ni-Co_0.85Se-NHCS (60.0 mV dec⁻¹) and 20% Pt/C (78.8 mV dec⁻¹), suggesting a faster kinetics rate of ORR process. Besides, electrochemical impedance spectroscopy (EIS) of these as-prepared catalysts was provided in Fig. S8.† According to the semicircle in the high-frequency region, the charge transfer resistance of Ni_0.85Se/Co_0.85Se-NHCS-2 was 3.68 Ω, between Co_0.85Se-NHCS (3.3 Ω) and Ni_0.85Se-NHCS (4.81 Ω), which was consistent with the fact that Ni_0.85Se/Co_0.85Se-NHCS-2 was a mixture of Ni_0.85Se-NHCS and Co_0.85Se-NHCS. Furthermore, the detailed data of LSV curves and Tafel slopes of these catalysts were shown in Fig. S9 and Table S1.† The Ni_0.85Se/Co_0.85Se-NHCS-2 had the highest ORR activity among the mixtures with different mass ratios. Moreover, its activity was even higher than that of some nickel-cobalt-selenides reported in the literature (Table S2†). Remarkably, Ni_0.85Se/Co_0.85Se-NHCS-2 achieved the same ORR performance as Co_0.85Se-NHCS with only half the cobalt content. And with the mass ratio changed, the ORR performance of the mixtures decreased significantly. Therefore, it can be reasonably inferred that there is a synergistic effect between Co_0.85Se-NHCS and Ni_0.85Se-NHCS, which can affect the ORR activity of the mixture.

Fig. 5 (a) Linear sweep voltammogram curves for ORR and corresponding (b) Tafel curves, (c) converted electron transfer number and hydrogen peroxide yield of Ni_0.85Se-NHCS, Ni_0.85Se-NHCS, Co_0.85Se/Ni_0.85Se-NHCS-2, and 20% Pt/C. (d) The ECSA of synthesized samples by CV measurement in different scan rates.

The electron transfer number (n) and H₂O₂ yields in the ORR process were carried out from the rotating ring-disk electrode (RRDE) measurement (Fig. 5c). The average n value evaluated for Ni_0.85Se/Co_0.85Se-NHCS-2 was 3.75, higher than that of Ni_0.85Se-NHCS (3.35), and closed to that of Co_0.85Se-NHCS (3.76) and noble metal catalyst 20% Pt/C (3.88), suggesting a four-electron reaction process. Moreover, the H₂O₂ yield of Ni_0.85Se/Co_0.85Se-NHCS-2 was less than 12% in the voltage range from 0.2 to 0.7 V, lower than that of the Co_0.85Se-NHCS (13%) and commercial catalyst 20% Pt/C (8%). This result of Ni_0.85Se/Co_0.85Se-NHCS-2 further confirmed its excellent selectivity for ORR activity. For measuring the electrochemical surface area (ECSA), the double-layer capacitance (C_dl) was conducted by cyclic voltammetry at a scan rate from 2 mV s⁻¹ to 10 mV s⁻¹. The CV curves of the as-prepared catalysts were shown in Fig. S10.† As shown in Fig. 5d, the C_dl of the Ni_0.85Se/Co_0.85Se-NHCS-2 was 5.69 mF cm⁻², which was second only to that of Co_0.85Se-NHCS and higher than that of other catalysts. This result demonstrates that the Ni_0.85Se/Co_0.85Se-NHCS-2 has a superior active surface area and active site for electrochemical activity. Besides, the super methanol tolerance was displayed in Fig. S11,† which was carried out the chronoamperometric test at 0.3 V in O₂-saturated 0.1 M KOH solution with 1600 rpm. The ORR activity of Ni_0.85Se/Co_0.85Se-NHCS-2 didn't change obviously after injecting 1 M methanol, while that of 20% Pt/C decreased evidently. It indicates that Ni_0.85Se/Co_0.85Se-NHCS-2 has outstanding methanol tolerance and a wide practical range. As shown in Fig. S12,† the limiting current density of Ni_0.85Se/Co_0.85Se-NHCS-2 only declined by 4.05% after 20 hours. By contrast, that of 20% Pt/C dropped off 13.14%. The above measurements result that Ni_0.85Se/Co_0.85Se-NHCS-2 has excellent ORR performance, good methanol resistance, and remarkable stability, which is derived from the synergistic effect of Ni_0.85Se, Co_0.85Se, and NHCS.

The excellent cathodic catalysts of the zinc–air battery should possess outstanding bifunctional electrocatalysis for both ORR and OER. The OER performance of the prepared catalysts was analyzed in a three-electrode cell in 0.1 M KOH electrolyte and compared with other catalysts including the noble metal catalyst RuO₂. The operating potential at a current density of 10 mA cm⁻² was used to evaluate the OER electrocatalytic activity of the material. As shown in Fig. 6a, the operating potential of Ni_0.85Se/Co_0.85Se-NHCS-2 was 1.63 V. It is a little worse than that of precious metal RuO₂ (1.59 V), close to that of Ni_0.85Se-NHCS (1.62 V), but better than that of Co_0.85Se-NHCS (1.65 V). Fig. 6b showed the Tafel slopes of these catalysts for OER. The Ni_0.85Se/Co_0.85Se-NHCS-2 exhibited a lower Tafel slope of 118.3 mV dec⁻¹ than that of Co_0.85Se-NHCS (141.6 mV dec⁻¹), Ni_0.85Se-NHCS (136.0 mV dec⁻¹), and only higher than that of RuO₂ (84.1 mV dec⁻¹). Besides, the LSV curves and Tafel slopes of other mixed catalysts were also shown in Fig. S9 and Table S1.† The OER performance of Ni_0.85Se/Co_0.85Se-NHCS-2 with less Ni content was similar to that of Ni_0.85Se-NHCS, and higher than that of the mixtures with other mass ratios. Moreover, to further evaluate the intrinsic performance for OER of the catalysts, the TOF of catalysts was evaluated. Ni_0.85Se/Co_0.85Se-NHCS-2 was calculated as 0.40 s⁻¹ at the potential of 1.58 V, lower than that of Ni_0.85Se-NHCS (0.73 s⁻¹) and higher than that of Co_0.85Se-NHCS (0.26 s⁻¹), which demonstrated the OER activity of the mixed catalyst Ni_0.85Se/Co_0.85Se-NHCS-2 was mainly derived from the Ni_0.85Se-NHCS. Like the ORR performance, the Ni_0.85Se/Co_0.85Se-NHCS-2 with the rich of Ni³⁺ displayed a better OER performance than that of the other metal selenide materials (Table S2†). Furthermore, the potential difference (ΔE) between half-wave potential (E_1/2) for ORR and OER potential at 10 mA cm⁻² (E_j=10) was applied to evaluate the performance of the bifunctional catalytic activity. The Ni_0.85Se/Co_0.85Se-NHCS-2 illustrated a ΔE value of 0.86 V, which was lower than that of Co_0.85Se-NHCS (0.89 V), Ni_0.85Se-NHCS (0.93 V), indicating that Ni_0.85Se/Co_0.85Se-NHCS-2 possesses the optimum bifunctional ORR and OER performance among the non-precious metal catalysts (Fig. 6c). From the above results, it is shown that the Ni_0.85Se-NHCS-2 as a mixture of Ni³⁺-rich Ni_0.85Se-NHCS and Co²⁺-rich Co_0.85Se-NHCS has good ORR and OER bifunctional electrochemical activity when the same catalyst mass is used.

Fig. 6 Linear sweep voltammogram curves for (a) OER and corresponding (b) Tafel slope curves, (c) the potential difference (ΔE) between E_1/2 for ORR and E_j=10 for OER.

Due to the superior bifunctional activity of Ni_0.85Se/Co_0.85Se-NHCS-2, the zinc–air battery was assembled with this mixture as cathode catalyst, zinc foil as an anode, and 6 M KOH and 0.2 M Zn(Ac)₂ as the electrolyte, respectively (Fig. 7a). The oxygen cathode in the zinc–air battery was made by coating the catalyst on carbon paper with a coating area of 1 cm². Moreover, the 20% Pt/C and RuO₂ noble metal mixture with a mass ratio of 1 : 1 was used as a comparison. As shown in Fig. 7b, the zinc–air battery equipped with Ni_0.85Se/Co_0.85Se-NHCS-2 exhibited the open potential of 1.40 V, coming near that of the battery with Pt/C&RuO₂ (1.46 V). Besides, the zinc–air battery based on Ni_0.85Se/Co_0.85Se-NHCS-2 worked and successfully drived a timer, testifying 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 Ni_0.85Se/Co_0.85Se-NHCS-2 based battery exhibited a potential gap in charge and discharge measure approaching that of 20% Pt/C&RuO₂, which pointed out the awesome charge and discharge ability of the Ni_0.85Se/Co_0.85Se-NHCS-2 based battery. Moreover, the Ni_0.85Se/Co_0.85Se-NHCS-2 based battery was manifested a power density of 118.34 mW cm⁻² at a current density of 185.1 mA cm⁻², while that of 20% Pt/C&RuO₂ achieved 154.13 mW cm⁻² at 210.10 mA cm⁻² (Fig. 7d). Moreover, the performance of zinc–air batteries with other non-precious catalysts was compared in Table S3.† To further appraise the practical stability application, the long-term charge and discharge activity of the Ni_0.85Se/Co_0.85Se-NHCS-2 and 20% Pt/C&RuO₂ was implemented at a current density of 25 mA cm⁻² with 10 min per cycle. As exhibited in Fig. 7e, the zinc–air battery based on Ni_0.85Se/Co_0.85Se-NHCS-2 displayed better long-term stability through long cycle charging and discharging than that with 20% Pt/C&RuO₂. The potential gap of the Ni_0.85Se/Co_0.85Se-NHCS-2 based battery manifested a subtle change from 1.27 to 1.25 V after 100 hours due to the balanced concentration of dissolved oxygen, which was consistent with previous literature.^9,10 In particular, the Ni_0.85Se/Co_0.85Se-NHCS-2 based battery showed a slight increase in charging potential from 2.18 V to 2.19 V, which confirmed the excellent long-term stability of the OER. For comparison, the Pt/C&RuO₂ based battery performed an initial voltage gap of 1.19 V. But after 100 operation hours, the voltage gap changed to 1.32 V, suggesting the performance of the zinc–air battery obviously deteriorated. It can be seen that although the charge and discharge performance of Ni_0.85Se/Co_0.85Se-NHCS-2 based battery is slightly worse than that of Pt/C&RuO₂ based battery, it has better long-term stability. Considering the low cost of the non-precious metal catalyst, the Ni_0.85Se/Co_0.85Se-NHCS-2 has the potential to replace precious metals for zinc–air batteries.

Fig. 7 (a) the schematic of the zinc–air battery. (b) The open-circuit curves, (c) charge and discharge polarization curves, (d) discharge polarization curves and corresponding the power density curves and the long-term stability by charge and discharge at 25 mA cm⁻² of zinc–air battery assemble with Ni_0.85Se/Co_0.85Se-NHCS-2 and 20% Pt/C&RuO₂.

Conclusions

In summary, we synthesized Ni_0.85Se-NHCS and Co_0.85Se-NHCS, respectively, by hydrothermal modification of Ni_0.85Se and Co_0.85Se nanoparticles on NHCS. The mixture Ni_0.85Se/Co_0.85Se-NHCS-2, which was prepared by simple mixing of Ni_0.85Se-NHCS and Co_0.85Se in a 1 : 1 mass ratio, exhibited better ORR and OER bifunctionality than the single Ni_0.85Se-NHCS and Co_0.85Se-NHCS. The zinc–air battery equipped with Ni_0.85Se/Co_0.85Se-NHCS-2 performed more durable than the battery equipped with the 20% Pt/C&RuO₂ catalysts. These results identify the rational mixture of nickel selenide and cobalt selenide as a simple, low-cost and efficient method for the preparation of oxygen electrode catalysts, further improving the performance and application prospect of zinc–air batteries.

Author contributions

Li-Juan Peng: conceptualization, investigation, writing – original draft. 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 draft, project administration.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by Natural Science Foundation of Guangdong Province (No. 2020A1515011551), Science and Technology Research Project of Guangzhou (No. 202102010484), and Innovative Training Program for University Students (No. 201911078017).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra02861h

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