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Article

Hydrothermal Synthesis of NiCo2O4 @NiCo2O4 Core-Shell Nanostructures Anchored on Ni Foam for Efficient Oxygen Evolution Reactions Catalysts

by
Lijuan Zhang
*,
Haichen Yuan
,
Xiang Li
and
Yan Wang
School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, China
*
Author to whom correspondence should be addressed.
Coatings 2022, 12(9), 1240; https://doi.org/10.3390/coatings12091240
Submission received: 18 July 2022 / Revised: 20 August 2022 / Accepted: 22 August 2022 / Published: 25 August 2022
(This article belongs to the Special Issue Research on Novel Energy Storage Materials and Devices)
Browse Figures
Versions Notes

Abstract

:
NiCo2O4@NiCo2O4 core-shell nanostructures anchored on Ni foam (NF) are synthesized by the hydrothermal and subsequent calcination process. In these structures, NiCo2O4 nanosheets are coated on NiCo2O4 nanocones, which are decorated on the surface of conductive NF. As an oxygen evolution reaction catalyst, the NiCo2O4@NiCo2O4 exhibits low overpotential of 440 mV at the current density of 100 mA cm−2. Furthermore, the composite shows outstanding long-term stability during 12 h continuous operation. Electrochemical impedance spectroscopy (EIS) reveals that the charge transfer resistance of NiCo2O4@NiCo2O4 is much smaller than other composites. The results reveal that the composite exhibits superior activity for OER in contrast to individual NiCo2O4 nanosheets or NiCo2O4 nanocones, which are attributed to the NiCo2O4@NiCo2O4 composite and integrate multiple advantages of nanostructures, abundant catalytic sites and outstanding stability.

Graphical Abstract

1. Introduction

Due to the increasing environmental issues and diminishing fossil fuels, a lot of efforts have been devoted to exploring renewable energy resources. Electrochemical water splitting has attracted great attention due to its low energy consumption, environmental benignity, and high purity [1,2,3]. Water electrolysis contains hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) [4,5]. However, OER involves a complex four electron transfer reaction, which is a thermodynamically and kinetically sluggish process. Thus, it causes a high overpotential, which is considered to be the main bottleneck of developing OER electrocatalysts [6,7,8,9]. In order to reduce the overpotential of OER, an electrocatalyst is introduced to accelerate the reaction. IrO2 and RuO2 have been proven to be two of the most efficient OER catalysts in the alkaline system. However, its application are greatly limited by low resource reserves and high price [10,11]. Therefore, developing a non-noble metal electrocatalyst with high OER activity and excellent cycling stability has become an urgent need in water splitting.
Transitional metal oxides with a spinel structure (AB2O4) has attracted tremendous attention due to its inexpensive, environment friendly and excellent stability. Recently, various NiCo2O4 nanostructures (nanowires [12], nanoflakes [13], nanosheets [14], nanowalls [15], et al.) have been synthesized as high performance electrodes. These nanostructures have a large specific surface area, which could increase the contact between the electrode materials and electrolyte and provide sufficient active sites. However, the NiCo2O4 powders exhibit low conductivity and weak connections, which limit the transfer of electrons and hinder the active materials taking part in reactions. These obstacles restrict their widespread applications. To solve this problem, NiCo2O4 nanostructures have been grown on some conducting materials is an effective strategy. The conductive materials could enhance electric conductivity and cycling stability. Among the various types of conductive materials for fabrication of high OER electrochemical performances and good stability of structures, Ni Foam (NF) has attracted more interest due to advantages of 3D porous structure, high specific surface area, and good electrical conductivity [16]. At present, various NiCo2O4 grown on NF were reported, which could keep stable for a long period of time [17,18]. Zhang and coworkers [19] synthesized NiCo2O4@rGO hybrid nanostructures on Ni foam via a hydrothermal and chemical reduction process, the hybrid electrode demonstrated high specific capacitance, good rate capability and superior cycling stability. Li et al. [20] synthesized 3D Ni-Fe layer double hydroxide (LDH) on NF via hydrothermal method as efficient OER electrode. Zhu et al. [21] reported on the electrodeposition route for the preparation of Ni1-xCoxSe nanostructures on nickel foam with outstanding OER electrocatalytic performances.
Herein, we designed a novel hybrid nanostructure of NiCo2O4@NiCo2O4 core-shell supported on Ni foam through a two-step hydrothermal and subsequent calcination process. Interestingly, the NiCo2O4 nanocones act as core and NiCo2O4 nanosheets as shell in the structure. In this catalyst, both the core and shell were NiCo2O4 with different morphologies, which is beneficial to expose more electrochemically active sites and accelerate the mass transport. As a result, the designed NiCo2O4@NiCo2O4 hybrid electrode presents lower overpotentia and excellent cycling stability as an efficient OER electrode.

2. Materials and Methods

2.1. Materials

NF was purchased from Shenzhen Kejing Star Technology Co. Ltd. (Shenzhen, China) All the reagents were of analytical grade and used without further purification. Before use, the NF (1 × 1 cm2) was washed with deionized water and ethanol in ultrasonic bath for several times to remove the impurities.

2.2. Synthesis of NiCo2O4 Nanocones/NF

In a typical procedure, 0.2 mmol Ni(NO3)2·6H2O, 0.4 mmol Co(NO3)2·6H2O and 0.8 mmol urea were successively added into 50 mL distilled water and 20 mL ethanol to form a homogeneous solution. The solution and pre-treated NF were placed in a 100 mL Teflon-lined autoclave and heated at 120 °C for 5 h. Finally, the prepared sample was annealed at 350 °C for 2 h with ramp rate of 2.5 °C min−1.The final product was NiCo2O4 nanosheets/NF.

2.3. Synthesis of Core-Shell NiCo2O4@NiCo2O4/NF

The core-shell NiCo2O4@NiCo2O4 /NF were prepared as following: 0.2 mmol Ni(NO3)2·6H2O, 0.4 mmol Co(NO3)2·6H2O, 0.6 mmol hexamethylene tetramine (HMTA) and 1.2 mmol urea were successively dissolved into 50 mL distilled water, and 20 mL ethanol to form a homogeneous solution. Then, the NiCo2O4 nanocones /NF were impregnated into this aqueous solution. Subsequently, the solution and the NiCo2O4 nanocones /NF were sealed in a 100 mL Teflon-lined autoclave and treated at 120 °C for 5 h. Finally, the product was heated in air at 350 °C for 2 h with a heating rate of 2.5 °C min−1, and the core-shell NiCo2O4@NiCo2O4/NF composites were fabricated after the consecutive washing and drying process. The average loading densities were about 0.5 mg cm−2. For comparison, the NiCo2O4 nanosheets/NF were prepared under similar conditions except adding NiCo2O4 nanocones /NF. The loading mass of NiCo2O4 nanosheets and NiCo2O4 nanocones was about 0.3 mg cm−2.

2.4. Synthesis of IrO2/NF

5 mg of IrO2 powder was dispersed in 540 mL of absolute ethanol, 400 mL of distilled water, and 60 mL of Nafion solution(5 wt%). Then, the mixture was ultrasonicated for 1 h to obtain a homogeneous ink. Then, 100 mL of as-prepared ink (containing 0.5 mg of IrO2) was dropped onto 1 × 1 cm2 NF, dried at room temperature. IrO2 was deposited onto NF with a loading of 0.5 mg cm−2.

2.5. Characterization

The crystal structure was investigated by a Rigaku (Tokyo, Japan) D/max-2400 with Cu Kα radiation. The morphology was analyzed by scanning electron microscopy (SEM) images using a JEOL JSM-6700 Field emission scanning electron microscope. Transmission electron microscopy (TEM) and High -resolution TEM was achieved by a JEOL JEM-3010. The surface area was determined by the Brunauer–Emmett–Teller (BET) method with a Micromeritics ASAP 2020. The surface properties of products were investigated by X-ray photoelectron spectroscopy (XPS) measurements on a Kratos AXILS ULTRA instrument using the monochromated Al K X-ray source.

2.6. Electrochemical Measurements

All electrochemical measurements were conducted on CHI 660 E electrochemical workstation (Chenghua, China) with a three-electrode system. 1.0 M KOH aqueous solution was used as the electrolyte in experimental. The as-prepared product served as the working electrode, graphite rod as the counter electrode and Ag/AgCl as the reference electrode. All the potentials were converted to the hydrogen electrode (RHE) scale as the following equation: E vs. RHE = E vs. Ag/AgCl + 0.098 + 0.059 pH. Linear sweep voltammograms (LSV) were recorded at a scan rate of 5 mV s−1. Electric impedance spectroscopy (EIS) was performed with scanning frequency from 105 to 0.01 Hz at the potential of 1.22 V (vs. RHE). The correction of ohmic losses was done by the following equation: Ecorrected = E − iR, Where Ecorrected was the potential corrected by iR, E was the potential measured in experiment, and R was the series resistance obtained from EIS. Tafel slopes were derived from OER polarization curves with 90% iR compensation. The double-layer capacitance (Cdl) of the working electrode was recorded by cyclic voltammetry curves in a non-Faradaic region in the potential range from 0.95 V to 1.05 V (vs. RHE) with 5 to 40 mV s−1. Stability tests were performed by Chronopotentiometry which was carried out at 100 mA cm−2 for 12 h.

3. Results

The core-shell NiCo2O4@NiCo2O4 composites were synthesized by two-step hydrothermal process (Figure 1). Firstly, NiCo2O4 nanocones precursors were loaded to the NF via hydrothermal method. Then, NiCo2O4 nanocones/NF were formed by calcination. Secondly, NiCo2O4 nanosheets precursors were coated on the as-prepared NiCo2O4 nanocones/NF via secondary hydrothermal process. Finally, the 3D core-shell NiCo2O4@NiCo2O4/NF nanostructures were synthesized after calcination. The hydrothermal process may contain the following chemical reactions [22]:
CO(NH2)2 + 3H2O→2NH4+ + 2OH− +CO2
CO2 + H2O→CO32− + 2H+
(Ni2+ +Co2+) + 0.5(2−x)CO32− + xOH−+ nH2O→(Co,Ni) (OH)x (CO3)0.5(2−x) ·nH2O
(Co,Ni) (OH)x (CO3)0.5(2−x)·nH2O +1.5O2→NiCo2O4 + (n + 0.5x)H2O + (1 − x)CO2
Generally, urea hydrolysis could provide OH−, which may possibly react with Ni2+ and Co2+ forming (Co,Ni) (OH)x (CO3)0.5(2−x)·nH2O intermediate. Then, after calcination at 350 °C, the intermediate was converted to NiCo2O4.
The crystal structures of NiCo2O4 nanocones, NiCo2O4 nanosheets and core-shell NiCo2O4@NiCo2O4 composite scratched from the NF were examined by X-ray diffraction (XRD), and the typical diffraction patterns are shown in Figure 2. It is found the three samples have the same diffraction peaks, which are at 2θ values of 18.9°, 31.1°, 36.7°, 44.6°, 59.1°, 64.9° and 77.0° assigning to the (111), (220), (311), (400), (511), (440) and (533) planes of NiCo2O4 (JCPDS No. 01-73-1702), respectively [23]. There are no other extra peaks in the samples, indicating that the NiCo2O4 have been successfully grown on Ni foam.
To study the morphology of the synthesized samples, SEM images are displayed in Figure 3a–d. Figure 3a shows the typical SEM of the pure NF. Clearly, the surface of NF is rough, and the scaffold of NF interconnected with each other forming a network 3D structure. Many NiCo2O4 nanocones we’re grown uniformly on the NF (Figure 3b). Moreover, a large number of nanocones are assembled and made up of urchin-like spheres with an average diameter of 8 μm. These randomly aligned nanocones are interwoven with each other and perpendicular to the spherical surface, forming 3D hierarchical structures with abundant open voids. Such urchin-like structures could expose more active sites and allow sufficient penetration of electrolyte, which is beneficial to the electrocatalytic activity [24]. Figure 3c shows the morphology of NiCo2O4 nanosheets. They have flower-like microstructures assembled by nanosheets. Such flower-like structure have a smooth surface area and can also provide effective with ion/electron transfer channels upon interaction with NiCo2O4 nanocones [25]. Figure 3d displays that the NiCo2O4 nanocones are homogeneously coated by NiCo2O4 nanosheets sites forming core-shell NiCo2O4@NiCo2O4/NF structure. These nanosheets are placed between the nanocones, which offer abundant activity surface area for minimize the ion diffusion length and faster charge ion transport [24].
Figure 4 shows the TEM images of NiCo2O4@NiCo2O4/NF. As shown in Figure 4a, the diameter of NiCo2O4 nanocones is about 20 nm. Moreover, both the nanocones and nanosheets consist of many nanoparticles, and its BET surface area is about 18.89 m2 mg−1 (Figure S1). The large surface could expose more active sites and increase the rate of electron transfer [18]. The high resolution TEM (HRTEM, Figure 4b,c) of nanocones and nanosheets reveal obvious lattice fringes. The spaces of 0.243 and 0.287 nm consist of the (311) and (220) planes of NiCo2O4 (PDF 73-1702), respectively [26]. This result demonstrates both nanocones and nanosheets are composed of NiCo2O4. This conclusion is consistent with the XRD result.
The effect of the reaction temperature on the morphologies of NiCo2O4@NiCo2O4/NF is also measured by SEM. As shown in Figure S2, when the temperature is lower than 120 °C, NiCo2O4 nanocones exhibit a microsphere-like structure with a diameter of ~3 μm. NiCo2O4 nanosheets are uniformly staggered on the NiCo2O4 nanocones formed NiCo2O4@NiCo2O4/NF structures. When the temperature is at 120 °C, the array structures can be enlarged the whole of Ni substrate. However, with the temperature higher than 120 °C, these structures are assembled and heavily agglomerated. All the samples illustrated only that NiCo2O4 are formed (Figure S3).
To investigate the surface properties of the core-shell NiCo2O4@NiCo2O4/NF sample, XPS measurements are performed. The signals of Ni, Co, and O can be easily observed in the survey spectra (Figure 5a), which is consistent with the result of XRD. Figure 5b shows the Co 2p spectra, and it can be split into two spin-orbit doublets of Co3+ (781.0 eV, 796.5 eV) and Co2+ (782.7 eV, 798.0 eV), along with two shake-up satellites at 786.0 eV and 802.8 eV, respectively [22,27]. At the same time (Figure 5c), the major peak of nickel species also presents similar spin orbit doublets of Ni3+ (855.3 eV, 874.6 eV) and Ni2+ (854.5 eV, 872.2 eV), and with a shake-up satellite in the higher binding energy region (879.6 eV, 861.5 eV) [28,29]. Figure 5d displays that the O1s spectrum, three oxygen peaks located at 528.6 eV, 530.2 eV, and 532.6 eV designated as O1, O2, and O3 are emerged, respectively [23]. The peak O1 can be attributed to M-O metallic bonds such as Ni-O and Co-O. The peak O2 is usually related to chemisorbed oxygen, hydroxyls, and under-coordinated lattice oxygen [30,31]. The peak O3 is associated with physically and chemisorbed water on the surface of samples. Overall, the XPS analysis demonstrates that NiCo2O4 nanosheets have been deposited on the surface of the NiCo2O4 nanocones.
The OER electrocatalytic performances of core-shell NiCo2O4@NiCo2O4/NF structure is examined by a three electrode system in an O2-saturated 1.0 M KOH solution. It is important to develop an OER electrocatalyst with smaller onset potential, lower overpotential and larger current density [4,21]. Figure 6 shows the linear sweep voltammetry (LSV) measurements for as-prepared catalysts at a scan rate of 5 mV s−1 after iR compensation (90%). Notably, there is a distinct oxidation peak between 1.3 ~1.4 V (vs. RHE), which could be ascribed to the electron-transfer from Ni2+/Ni3+ or Co2+/Co3+ in the samples [32]. The NiCo2O4@NiCo2O4/NF sample exhibits higher current density (at 2.1 V vs. RHE, and in all potential range) when compared with NiCo2O4 nanocones and NiCo2O4 nanosheets. This means the core-shell of NiCo2O4@NiCo2O4 structure greatly improved OER electrocatalytic activity. In Figure 6a, when the current density reaches 100 mA cm−2, the NiCo2O4@NiCo2O4/NF only needs 440 mV(vs. RHE) overpotential for oxygen evolution, which is lower than that of NiCo2O4 nanocones (543 mV) and NiCo2O4 nanosheets (540 mV), IrO2 (589 mV). The much-decreased overpotential means more efficient OER electrocatalyst for practical water electrolysis [33]. So, the NiCo2O4@NiCo2O4/NF with lower overpotential indicates much better electrocatalytic properties than NiCo2O4 nanocones and NiCo2O4 nanosheets. Also, its OER activity is superior to the previously reported electrocatalysts (see Table 1 for a detailed comparison). Figure 6b shows OER polarization curves of the as-prepared NiCo2O4@NiCo2O4/NF catalysts with different temperatures at a scan rate of 5 mV s−1 in 1 M KOH solution. The overpotential of NiCo2O4@NiCo2O4/NF is 398 mV for 120 °C, 406 mV for 100 °C and 420 mV for 140 °C obtained at the current density of 50 mA cm−2. It demonstrates that an appropriate hydrothermal time is beneficial to the OER activity, which is ascribed to the different structures of NiCo2O4@NiCo2O4/NF.
The double-layer capacitance (Cdl) is also an essential factor to evaluate the electrochemical active surface area (ECSA) [38]. The capacitances of the self-supported and coated electrode NiCo2O4 nanocones, NiCo2O4 nanosheets, and NiCo2O4@NiCo2O4/NF were 1.13 mF cm−2, 1.19 mF cm−2 and 2.38 mF cm−2, respectively (Figure S4). These results indicate the NiCo2O4@NiCo2O4/NF electrode contains a high density of active sites. The ECSA normalized LSV is also shown in Figure S5. The NiCo2O4@NiCo2O4/NF catalyst still shows lower overpotentials than NiCo2O4 nanocones and NiCo2O4 nanosheets. It suggests the NiCo2O4@NiCo2O4/NF catalyst exposes more active sites, which is beneficial to OER [39].
To confirm the OER kinetics, Tafel plots are investigated (Figure 7).Obviously, the Tafel slope of NiCo2O4@NiCo2O4/NF is 163 mV dec−1, which is slightly close to that of IrO2 (152 mV dec−1). However, it is much smaller than that of NiCo2O4 nanocones (234 mV dec−1) and nanosheets (229 mV dec−1), indicating the rapid OER kinetics and effective electron transfer of NiCo2O4@NiCo2O4/NF [36]. The OER mechanism can be assumed as follows [40]:
M-O + H2O → O-M-OH*abs + e− + H+
O-M-OH*abs+ OH− → MO2 + H2O + e–
MO2 →MO + O2
Firstly, water adsorption on metal oxide (M-O) and formation of adsorbed (ads) reactive intermediate-OH*as, at same time, releasing a proton and electron (5). Oxide intermediate is yielded in reaction of (6). Then, two oxide intermediates are recombined for a completed one (7), which is considered to be the rate-determining step for the OER [41].
To further study the electrode kinetics for OER, EIS measurement is used to investigate interface reactions at the frequency ranging from 0.01 Hz to 105 Hz. According to the previous works [42,43], the semicircle diameter at the low frequency region of the Nyquist plot stands for charge transfer (Rct), and a lower Rct value indicating the rapid charge transfer kinetics [44]. Figure 8 shows the Nyquist plot of NiCo2O4 nanocones, NiCo2O4 nanosheets and NiCo2O4@NiCo2O4/NF at the potential of 1.22 V (vs. RHE). It can be seen that the semicircular diameter of the NiCo2O4@NiCo2O4/NF is smaller than that of the NiCo2O4 nanocones and NiCo2O4 nanosheets, suggesting that NiCo2O4@NiCo2O4/NF electrode has a faster charge transfer and more favorable catalytic kinetics for OER. Therefore, higher ECSA and faster charge transfer of NiCo2O4@NiCo2O4/NF are beneficial to its superior electrocatalytic activity [32]. The Nyquist plots of NiCo2O4@NiCo2O4/NF at different potential are shown in Figure S6. It is more interesting that with increasing potential, the diameter of semicircle becomes smaller, which indicates a faster charge transfer for OER [44].
In addition, the long-term stability is also an important factor for the practical application of OER electrocatalysts. Herein, a long-term stability test is performed on the NiCo2O4@NiCo2O4/NF electrode at current density of 100 mA cm−2 for 12 h (Figure 9). The slight decrease of current density (2%) indicates it has favorable stability in long-term electrolysis process. Comparable to NiCo2O4 nanosheets/NF and NiCo2O4 nanocones/NF, the current density significantly decreased to 92% and 93% (Figure S7) of their initial value, respectively. Therefore, the NiCo2O4@NiCo2O4/NF as an electrocatalyst shows a superior stability for OER in an alkaline electrolyte.
Furthermore, after durability tests for 12 h, the XRD pattern of NiCo2O4@NiCo2O4/NF is employed. As shown in Figure S8, three of the Ni peaks are obtained, and other peaks are consisted with NiCo2O4. This implies the phase of NiCo2O4@NiCo2O4 after long-term catalyzing is maintained well. Thus, this NiCo2O4@NiCo2O4/NF electrode possesses good durability.
According to the above studies, the superior OER performance of the NiCo2O4@NiCo2O4/NF electrocatalysts may be attributed to the following reasons: Firstly, 3D core-shell structures and the synergistic effects between NiCo2O4 nanocones and nanosheets. The distribution of different NiCo2O4 nanostructures (nanocones or nanosheets) in the composite might accelerate charge transfer between the structure modulation and electrolyte, which are beneficial to the OER performance. Secondly, NiCo2O4@NiCo2O4/NF catalysts have good mechanical integrity, owing to the NiCo2O4 are directly grown on NF substrate, which not only guarantees good electronic transportation, but also may enhance the stability in continual testing. Thirdly, NF as a substrate provides large specific surface area and interconnected channels. Therefore, NiCo2O4@NiCo2O4/NF demonstrates better performance than NiCo2O4 nanosheets and NiCo2O4 nanocones.

4. Conclusions

This 3D of NiCo2O4@NiCo2O4/NF core-shell structure is successfully fabricated by a simple two-step hydrothermal and subsequent calcination process. The NiCo2O4@NiCo2O4/NF core-shell nanostructure exhibits superior OER performances with a lower overpotential and more electrochemical stability, compared with NiCo2O4 nanosheets, NiCo2O4 nanocones catalysts and even other catalysts. This work discloses a special structure that can be applied to other non-noble metal-based electrocatalysts for water splitting.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/coatings12091240/s1. Figure S1: BET curve of core-shell NiCo2O4@NiCo2O4. Figure S2: SEM of NiCo2O4@NiCo2O4/NF composites prepared at different temperatures (a) 100 °C and (b) 140 °C. Figure S3: XRD pattern of NiCo2O4@NiCo2O4/NF composites prepared at different temperatures. Figure S4: Cyclic voltammetry curves of (a) core-shell NiCo2O4@NiCo2O4 (b) NiCo2O4 nanosheets (c) NiCo2O4 nanoneedles at different scan rates from 5 to 40 mV/s in 1.0 M KOH (d) electrochemical surface areas of different samples. Figure S5: Polarization curves for different catalysts with current density normalized by ECSA. Figure S6: The Nyquist plots of NiCo2O4@NiCo2O4/NF at different potential. Figure S7: Long-time stability test of (a) NiCo2O4 nanosheets/NF and (b) NiCo2O4 nanocones/NF at current density of 100 mA cm−2 in 1.0 M KOH. Figure S8: XRD pattern of NiCo2O4@NiCo2O4/NF after the OER stability test.

Author Contributions

Conceptualization, L.Z. and H.Y.; methodology, H.Y.; software, L.Z.; validation, L.Z., X.L. and Y.W.; formal analysis, L.Z.; investigation, Y.W.; resources, H.Y.; data curation, L.Z.; writing-original draft preparation, L.Z.; writing-review and editing, L.Z.; visualization, X.L.; supervision, Y.W.; project administration, H.Y.; funding acquisition, H.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work has been supported by the National Natural Science Foundation of China, grant number 51521061 and U1435202, Natural Science Foundation of Shaanxi Province of China, grant number 2021JM433, and President’s Fund of Xi’an Technological University, grant number XAGDXJJ18008.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. A schematic illustration of the core-shell NiCo2O4@NiCo2O4 synthesis strategy.
Figure 1. A schematic illustration of the core-shell NiCo2O4@NiCo2O4 synthesis strategy.
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Figure 2. An XRD pattern of the NiCo2O4 nanocones, NiCo2O4 nanosheets and core-shell NiCo2O4@NiCo2O4 composite.
Figure 2. An XRD pattern of the NiCo2O4 nanocones, NiCo2O4 nanosheets and core-shell NiCo2O4@NiCo2O4 composite.
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Figure 3. An SEM image of different samples (a). pure NF, (b). NiCo2O4 nanocones on NF, (c). NiCo2O4 nanosheets on NF, (d). core-shell NiCo2O4@NiCo2O4/NF composites).
Figure 3. An SEM image of different samples (a). pure NF, (b). NiCo2O4 nanocones on NF, (c). NiCo2O4 nanosheets on NF, (d). core-shell NiCo2O4@NiCo2O4/NF composites).
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Figure 4. (a) TEM images and (b,c) HRTEM images of the core-shell NiCo2O4@NiCo2O4/NF composite.
Figure 4. (a) TEM images and (b,c) HRTEM images of the core-shell NiCo2O4@NiCo2O4/NF composite.
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Figure 5. The XPS spectrum of core-shell NiCo2O4@NiCo2O4/NF composites (a) signals of Ni, Co, and O, (b) Co 2p spectra, (c) the major peak of nickel species, (d) O 1s spectrum.
Figure 5. The XPS spectrum of core-shell NiCo2O4@NiCo2O4/NF composites (a) signals of Ni, Co, and O, (b) Co 2p spectra, (c) the major peak of nickel species, (d) O 1s spectrum.
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Figure 6. (a) LSV of core-shell NiCo2O4@NiCo2O4/NF composites, NiCo2O4 nanosheets/NF, NiCo2O4 naonocones/NF, pureNF, IrO2/NF; (b) core-shell NiCo2O4@NiCo2O4/NF composites prepared for different temperatures.
Figure 6. (a) LSV of core-shell NiCo2O4@NiCo2O4/NF composites, NiCo2O4 nanosheets/NF, NiCo2O4 naonocones/NF, pureNF, IrO2/NF; (b) core-shell NiCo2O4@NiCo2O4/NF composites prepared for different temperatures.
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Figure 7. The Tafel plots of core-shell NiCo2O4@NiCo2O4, NiCo2O4 nanosheets, NiCo2O4 nanocones and IrO2 in O2-saturated 1 M KOH solution.
Figure 7. The Tafel plots of core-shell NiCo2O4@NiCo2O4, NiCo2O4 nanosheets, NiCo2O4 nanocones and IrO2 in O2-saturated 1 M KOH solution.
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Figure 8. The electrochemical impedance spectra of core-shell NiCo2O4@NiCo2O4/NF composites, NiCo2O4 nanosheets, NiCo2O4 nanocones.
Figure 8. The electrochemical impedance spectra of core-shell NiCo2O4@NiCo2O4/NF composites, NiCo2O4 nanosheets, NiCo2O4 nanocones.
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Figure 9. The stability performance of a core-shell NiCo2O4@NiCo2O4/NF composite at 100 mA/cm2.
Figure 9. The stability performance of a core-shell NiCo2O4@NiCo2O4/NF composite at 100 mA/cm2.
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Table
Table 1. Comparisons of OER performance of the recently reported electrocatalyst [34,35,36,37].
Table 1. Comparisons of OER performance of the recently reported electrocatalyst [34,35,36,37].
CatalystsElectrolytesOverpotential@
100 mAcm−2 (mV)		Reference
NiCo2O4@CoMoO4/NF 1 M KOH480 [34]
FeCo2O4@Ni3S2/NF1 M KOH490 [35]
FeCo2O4@Ni3S2/NF1 M KOH445[36]
MoS2/ NiCo2O4/NF1 M KOH370 [37]
NiCo2O4@NiCo2O41 M KOH440 This work
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Zhang, L.; Yuan, H.; Li, X.; Wang, Y. Hydrothermal Synthesis of NiCo2O4 @NiCo2O4 Core-Shell Nanostructures Anchored on Ni Foam for Efficient Oxygen Evolution Reactions Catalysts. Coatings 2022, 12, 1240. https://doi.org/10.3390/coatings12091240

AMA Style

Zhang L, Yuan H, Li X, Wang Y. Hydrothermal Synthesis of NiCo2O4 @NiCo2O4 Core-Shell Nanostructures Anchored on Ni Foam for Efficient Oxygen Evolution Reactions Catalysts. Coatings. 2022; 12(9):1240. https://doi.org/10.3390/coatings12091240

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Zhang, Lijuan, Haichen Yuan, Xiang Li, and Yan Wang. 2022. "Hydrothermal Synthesis of NiCo2O4 @NiCo2O4 Core-Shell Nanostructures Anchored on Ni Foam for Efficient Oxygen Evolution Reactions Catalysts" Coatings 12, no. 9: 1240. https://doi.org/10.3390/coatings12091240

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