Controlled growth of porous oxygen-deficient NiCo₂O₄ nanobelts as high-efficiency electrocatalysts for oxygen evolution reaction

Abstract

Owing to their distinctive chemical properties and cost-effectiveness, transition metal oxides (TMOs) promise intriguing potential in electrocatalysis applications. Herein, porous NiCo₂O₄ nanobelts with controlled oxygen deficiencies were synthesized based on a facile strategy of hydrothermal growth followed by annealing under an inert atmosphere. By finely adjusting annealing temperature and time, concentrations of oxygen deficiencies within the nanobelts could be modulated. The oxygen-deficient NiCo₂O₄ nanobelts exhibit superior oxygen evolution reaction (OER) performance at a relatively low overpotential which is superior to the values of prepared pristine NiCo₂O₄ electrocatalysts. In particular, they show excellent stability for 10 h at 10 mA cm⁻². The enhanced OER activity and stability of the catalyst can be ascribed to the abundant oxygen deficiencies as well as porous architecture of the anisotropic nanobelts. This work paves a promising way in fabricating advanced electrocatalysts.

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1. Introduction

With the increase of fossil energy consumption and concerns relating to climate change and environmental problems, the demand for green and renewable energy is gradually increasing.^1–3 The oxygen evolution reaction (OER), as a vital half reaction of the water splitting reaction, is a promising way to solve the serious energy problem.^4–10 However, compared to hydrogen evolution reaction (HER), OER demands a much higher overpotential to complete a sluggish four-electron oxidation process, which suffers from sluggish kinetics.^11–14 Recently, a number of OER electrocatalysts have been applied, among which precious metal oxides are the most efficient like IrO₂ and RuO₂, but their high prices and exiguity hamper their extensive application commercially.^15–19 Therefore, numerous studies have been devoted to developing highly active, durable, and cheap electrocatalysts like transition metal oxides.^20–30

Among all transition metal oxides, NiCo₂O₄ is one of the most promising alkaline medium materials as electrocatalysts, because of its fast charge transfer and plentiful catalytically active sites. However, its intrinsic conductivity is still lower than those of precious metal oxides, as well as its active sites.^31–35 To overcome the limitation, Lee and co-workers synthesized a mesoporous NiCo₂O₄ nanoplatelet–graphene hybrid as a highly active bifunctional OER and ORR catalyst, owing to its improved electron transfer and more active sites facilitated by graphene as a support material.³⁶ Lim et al. successfully prepared oxygen-deficient NiCo₂O₄ materials with oxygen vacancies which promoted the electrochemical activity for ORR.³⁷ In light of the previous studies, introducing oxygen vacancies is a competitive strategy to improve both conductivity and the number of active sites.^38–41 First, moderate oxygen vacancies can help to diffuse charge, thus getting a high conductivity and capacitance.^42–44 Second, oxygen vacancies provide better adsorption of OH⁻.^45,46 Finally, oxygen vacancies can serve as active sites for redox reactions, which play an important role in accelerating oxidation reaction kinetics.^47–49

Presently, the common methods of creating oxygen vacancies can be classified into two ways: one is reducing metal oxides by strong reducing agents like NaBH₄,⁵⁰ and the other is annealing metal oxides in a reducing atmosphere like an NH₃ or H₂/Ar gas mixture.^51,52 Compared with chemical reduction method, the annealing method shows more convenience, less pollution and better safety. But to date, less research has been concentrated on the impact of an inert atmosphere and corresponding annealing temperature on oxygen vacancy performance and resultant electrocatalytic properties.

In this paper, we prepared an oxygen-deficient NiCo₂O₄ nanobelt composite by a facile and safe hydrothermal method and subsequent Ar-annealing process. The impact of annealing temperature on oxygen vacancy concentration has further been investigated. The NiCo₂O₄ nanobelts rich in oxygen vacancies thermally annealed at 400 °C under an argon flow reach a current density of 10 mA cm⁻² at 1.555 V versus the reversible hydrogen electrode (RHE) and show outstanding stability with no obvious current density loss at 10 mA cm⁻² after 10 h in 1 M KOH. It is very interesting to propose that the introduction of oxygen vacancies and the resulting higher conductivity and more active sites have been proved to be essential factors for the improvement of electrocatalytic performance.

2. Experimental

2.1 Materials

Cobalt nitrate (Co(NO₃)₂·6H₂O, 99.9%, Alfa Aesar), nickel nitrate (Ni(NO₃)₂·6H₂O, 99.9%, Alfa Aesar), ethanedioic acid dihydrate (H₂C₂O₄·2H₂O, 99.5%, Alfa Aesar), ethylene glycol (C₂H₆O₂, 99%, Alfa Aesar), potassium hydroxide (KOH, 99.9%, Alfa Aesar), ruthenium dioxide (RuO₂, 99.9%, Sigma-Aldrich), Nafion (5% wt, Sigma-Aldrich). All chemicals were used directly without any further purification.

2.2 Preparation of the NiCo₂O₄ nanobelts

The pristine NiCo₂O₄ nanobelts were synthesized by a hydrothermal treatment and subsequent annealing process. First, 1 mmol of Ni(NO₃)₂·6H₂O and 2 mmol of Co(NO₃)₂·6H₂O were dissolved in 15 mL deionized (DI) water and ultrasonicated for 5 min, named A. Simultaneously, 3 mmol of H₂C₂O₄·2H₂O was added into 45 mL C₂H₆O₂ and ultrasonicated for 5 min, named B. Then, the aqueous solution of A was poured into the solution of B to mix into a homogeneous solution by vigorous magnetic stirring for 5 min. Then, the mixed solution was transferred into a Teflon-lined stainless-steel autoclave at 120 °C for 24 h. After the autoclave was cooled naturally to room temperature, the precipitates were separated by centrifugation, washed with DI water and ethanol six times, and dried in a vacuum oven at 60 °C for 8 h. Finally, the prepared pink powder sample was then calcined in air at 350 °C for 0.5 h. The synthesized black powder sample is denoted as P-NiCo₂O₄.

2.3 Preparation of the oxygen-deficient NiCo₂O₄ nanobelts

To introduce oxygen vacancies into the NiCo₂O₄ nanobelts, the P-NiCo₂O₄ sample was annealed in a tube furnace under an argon flow for 2 h at different temperatures of 300 °C, 350 °C, 400 °C, 450 °C and 500 °C with a heating rate of 5 °C min⁻¹. The Ar-treated NiCo₂O₄ powder samples are denoted as NiCo₂O₄-OV-X, where X represents the annealing temperature.

2.4 Characterization

The surface morphologies of the powder samples were investigated by scanning electron microscopy (SEM, Hitachi SU8020) and transmission electron microscopy (TEM, JEM-2100F). The X-ray diffraction (XRD) patterns of all the samples were obtained at room temperature on an X'Pert Pro MPD (PANalytical B.V.) using a Cu target (λ = 0.154 nm) from 5 to 80 degrees. X-ray photoelectron spectroscopy (XPS) data were obtained with a Thermo ESCALAB250Xi using Al Kα radiation.

2.5 Electrochemical measurements

5 mg of the obtained catalyst were dispersed in a mixture solution of 990 μL of ethanol and 10 μL of 5 wt% Nafion. The resulting mixture was dispersed by ultrasonication for 0.5 h to form a uniform suspension. 10 μL of catalyst ink was deposited onto the surface of a glassy carbon electrode (GCE, 3 mm in diameter), and the electrode was naturally dried at room temperature.

The electrochemical tests were performed in a 1 M KOH aqueous electrolyte solution wherein the GC disk coated with the sample, Ag/AgCl electrode and graphite rod electrode were used as the working electrode (WE), reference electrode (RE), and counter electrode (CE), respectively. Each experiment was performed after the electrolyte solution was purged with O₂ for 0.5 h to form an oxygen-saturated solution. Linear-sweep voltammetry (LSV) was performed with a scan rate of 5 mV s⁻¹. The potentials herein were converted to RHE through the following equation:

E_RHE = E_Ag/AgO + 0.059 pH + 0.197 V (1)

η = E_RHE − 1.23 V (2)

where η stands for the overpotential.

Additionally, the Tafel slope was modelled by the Tafel equation:

η = b log j + a (3)

where b, j and a represent the Tafel slope, current density and a constant, respectively. Cyclic voltammograms (CVs) were performed at a series of scan rates in a narrow voltage range to obtain the double-layer capacitance (C_dl) values. The durability measurements of the OER properties were performed by chronoamperometry (CA) at 1.56 V (vs. RHE). All the above electrochemical measurements were conducted at room temperature.

3. Results and discussion

The oxygen-deficient NiCo₂O₄ nanobelts were fabricated by a three-step process as shown in Scheme 1. First, the nickel cobalt oxalate nanobelts were synthesized by a hydrothermal method; then, the obtained sample was annealed at 350 °C for 0.5 h in an air atmosphere in order to prepare the NiCo₂O₄ nanobelts; finally, the nanobelts were continuously annealed at 300, 350, 400, 450 and 500 °C for 2 h by switching the air to argon atmosphere to achieve oxygen-deficient NiCo₂O₄ nanobelts.

Scheme 1 Schematic illustration of the synthesis of the oxygen-deficient NiCo₂O₄ nanobelt catalyst.

The XRD patterns of the prepared NiCo₂O₄ nanobelts are shown in Fig. 1a. As can be seen, five major diffraction peaks located at 31.1°, 36.7°, 44.6°, 59.0°, and 64.9° can be indexed to the (220), (311), (400), (511), and (440) planes of NiCo₂O₄ (JCPDS card no. 20-0781), respectively. Compared to those of the pristine NiCo₂O₄, no obvious peak position changes were seen in the XRD patterns, which implied that no new phase exists. Interestingly, the peak intensity of the Ar-annealed NiCo₂O₄ is lower, demonstrating the weak crystallinity of the oxygen-deficient NiCo₂O₄ samples. Also, there is a negative shift of the peaks owing to the introduction of the crystal defects like oxygen vacancies.⁵³ As shown in Fig. 1b, the peaks at 184, 455, 505, and 648 cm⁻¹ match up with the F_2g, E_g, LO, and A_1g bands of NiCo₂O₄, respectively. Significantly, the Raman peaks of the oxygen-deficient NiCo₂O₄ samples shift towards the negative direction, which implies the successful introduction of oxygen vacancies.⁵⁴

Fig. 1 XRD patterns (a) and Raman spectra (b) of the as-prepared NiCo₂O₄ samples.

Fig. 2 shows the FESEM and TEM images of the P-NiCo₂O₄ and NiCo₂O₄-OV-400 samples. No obvious difference in the morphology between P-NiCo₂O₄ and NiCo₂O₄-OV-400 can be seen from the images. Both the NiCo₂O₄ samples are composed of well-defined nanobelts, indicating the favourable structural stability. After annealing at 400 °C, abundant NiCo₂O₄ nanoparticles grow in situ in the nanobelt, and even some particles grow to the edge of the nanobelt (Fig. 2e-inset), which formed mesoporous holes and therefore created a larger specific surface area. In addition, it can be seen from Fig. S1† that the annealing process makes the nanobelts curl and tend to form a tubular hollow structure, which is also consistent with the TEM results.⁵⁵ The clear lattice spacings (Fig. 2f) of about 0.202 nm, 0.237 nm and 0.247 nm can be assigned to the (400), (222) and (311) planes of NiCo₂O₄-OV-400, respectively. The inset image shows the corresponding SAED pattern of NiCo₂O₄-OV-400, which presents diffraction rings corresponding to the (111), (220), (311), (400) and (440) crystal planes. Fig. S1b† clearly shows the energy dispersive spectroscopy maps of NiCo₂O₄-OV-400, implying an even distribution of Ni, Co, and O elements.

Fig. 2 (a and d) FESEM images (inset: low-magnification images), (b and e) TEM images (inset: low-magnification images) and (c and f) HRTEM images (inset: the corresponding SAED patterns) of the P-NiCo₂O₄ nanobelt and the NiCo₂O₄-OV-400 nanobelt.

To verify the surface compositions and chemical states of P-NiCo₂O₄, NiCo₂O₄-OV-300, NiCo₂O₄-OV-350, NiCo₂O₄-OV-400, NiCo₂O₄-450 and NiCo₂O₄-OV-500, XPS test was also conducted. The survey spectrum of P-NiCo₂O₄ and NiCo₂O₄-OV-400 confirms the existence of Co, Ni, O, and C elements and no presence of any other impurity element (Fig. S2†). As shown in Fig. 3, the peaks at about 779.7, 781.2 and 787.5 eV can be indexed to Co³⁺, Co²⁺ and a satellite peak. As shown in Fig. 3, the Co²⁺/Co³⁺ ratios are 1.37 for P-NiCo₂O₄, 1.32 for NiCo₂O₄-OV-350, 1.78 for NiCo₂O₄-OV-350, 2.01 for NiCo₂O₄-OV-400, 1.88 for NiCo₂O₄-OV-450 and 1.57 for NiCo₂O₄-OV-500, respectively. Similar to Co 2p, as shown in Fig. S3,† Ni 2p3/2 can be carefully deconvoluted to Ni²⁺ at about 853.9 eV and Ni³⁺ at about 855.8 eV, as well as a satellite peak.^56,57 The Ni²⁺/Ni³⁺ ratios are 0.27 for P-NiCo₂O₄, 0.259 for NiCo₂O₄-OV-300, 0.279 for NiCo₂O₄-OV-350, 0.343 for NiCo₂O₄-OV-400, 0.33 for NiCo₂O₄-OV-450 and 0.323 for NiCo₂O₄-OV-500, respectively. Obviously, the proportion of Co²⁺ and Ni²⁺ is increased mostly after annealing at 400 °C.

Fig. 3 XPS spectra of Co 2p in the as-prepared NiCo₂O₄ nanobelts.

Fig. 4 shows the XPS spectra of O 1s and there exist three major peaks of O1, O2, and O3 located at ∼532.9, 531.5, and 529.4 eV, respectively, coinciding with the hydroxyl species of water molecules adsorbed on the surface, defect sites and the bonding of oxygen atoms and metals.⁵⁸ The O2 peak area ratio is 34.9 for P-NiCo₂O₄ and, similarly, increases with the annealing temperature and reaches a highest value of 38.9 for NiCo₂O₄-OV-400, then decreases at higher temperature. The largest area of the peak at 531.5 eV (O2) for the Ar-annealed NiCo₂O₄-OV-400 (Fig. 4) implies that the largest number of oxygen deficiencies existed in the as-prepared NiCo₂O₄ nanobelts. All the above results demonstrate that a mass of the Co²⁺ and a spot of Ni²⁺ ions is transformed by the reduction of Co³⁺ and Ni³⁺, respectively, owing to the formation of oxygen vacancies in the NiCo₂O₄ samples, which is consistent with the above XRD and Raman results. In addition, oxygen deficiencies coming from the annealing process under an inert atmosphere environment lead to the formation of a porous nanobelt structure.

Fig. 4 XPS spectra of O 1s in the as-prepared NiCo₂O₄ nanobelts.

As is well known, removing oxygen atoms in the crystal lattice sites (O_C) to the gas state is a practical way to form oxygen vacancies (OVs).

O_C ↔ OV + 1/2O₂(g) + e⁻ (4)

From the formula,⁵⁹ the reaction is favorable if the samples are annealed under an oxygen-deficient atmosphere, leading to the formation of oxygen vacancies. It is observed that the production of oxygen vacancies is essentially affected by the external partial pressure of oxygen. When the samples are annealed under low temperature conditions wherein the oxygen partial pressure is very low, the oxygen vacancy concentration increases as the temperature rises. Simultaneously, the proportion of M²⁺ (M = Ni/Co) increases as well. However, with the annealing temperature rising over 400 °C, the external O₂ pressure is higher. Therefore, the reaction will be shifted to the left side, leading to a decrease of oxygen vacancy concentration.⁶⁰ Meanwhile, the proportion of M²⁺ decreases. This explanation is supported and confirmed by the results shown in Fig. 5.

Fig. 5 Area ratio of the O2 peak components and peak ratio of M²⁺/M³⁺ (M = Ni/Co) of the as-prepared samples.

Electrocatalytic properties toward the OER of the samples were evaluated in 1 M KOH (Fig. 6a). Among these samples, NiCo₂O₄-OV-400 shows an outstanding activity at the lowest overpotential. The overpotential of NiCo₂O₄-OV-400 is about 325 mV (vs. RHE, reversible hydrogen electrode), which is much lower than that of P-NiCo₂O₄ (399 mV vs. RHE), NiCo₂O₄-OV-300 (404 mV vs. RHE), NiCo₂O₄-OV-350 (410 mV vs. RHE), NiCo₂O₄-OV-450 (387 mV vs. RHE), and NiCo₂O₄-OV-500 (400 mV vs. RHE), suggesting that the excellent electrocatalytic performance of NiCo₂O₄-OV-400 is apparently exhibited for the OER.

Fig. 6 (a) The polarization of OER and (b) the corresponding Tafel plots of the as-prepared NiCo₂O₄ samples in 1 M KOH. (c) Calculated C_dl values in the OER tests. (d) The chronoamperometry curve of NiCo₂O₄-OV-400 for OER at 1.56 V for 10 h.

Further activity evaluation could be seen from the Tafel plots of Fig. 6b. The Tafel slope of the as-prepared NiCo₂O₄-OV-400 samples was calculated to be approximately 71 mV dec⁻¹, which was the smallest of these obtained NiCo₂O₄ nanobelts, indicating that enhanced OER kinetics can be achieved by introducing oxygen vacancies. The CV method was tested to obtain the C_dl values, which can estimate the electrochemical surface area (Fig. 6c and S4†). The C_dl values are determined to be 12.79, 12.52, 3.04, 13.15, 8.55 and 2.66 mF cm⁻² for P-NiCo₂O₄, NiCo₂O₄-OV-300, NiCo₂O₄-OV-350, NiCo₂O₄-OV-400, NiCo₂O₄-OV-450 and NiCo₂O₄-OV-500, respectively, suggesting that NiCo₂O₄-OV-400 can provide more active surface area to the electrolyte, which is consistent with the as-mentioned analysis of FESEM and XPS.

The long-term stability of the NiCo₂O₄-OV-400 nanobelt electrode was also tested as shown in Fig. 6d, in which no significant difference in the current density was observed after long-term testing, illustrating the high stability of the as-prepared catalyst.¹⁰ To determine the stability of NiCo₂O₄-OV-400, the morphology, structure and composition of the sample were characterized by SEM, TEM and XPS after the long-time stability test process. The sample undergoing the stability test is denoted as NCO-ST whose morphology and structure are shown in Fig. S5,† which shows that the nanobelt structure is well-maintained. Furthermore, the XPS spectra of O 1s of NCO-ST are provided in Fig. S6,† which exhibit similar peaks to NiCo₂O₄-OV-400 before the stability test. The strong O2 peak corresponding to oxygen deficiencies can still be observed in NCO-ST, confirming the stable oxygen-deficient structure of the nanobelts even after long-term reaction. A new peak appearing at 532 eV likely comes from the KOH electrolyte residue on the nanobelt surface which can be evidenced by the presence of K 2s in the XPS survey spectrum.^61,62 The above results indicate the high activity and stability of the NiCo₂O₄-OV-400 catalyst.

For comparison, the OER performance of commercial RuO₂ and the obtained NiCo₂O₄-OV-400 have been listed and discussed in Fig. S7,† which indicates that NiCo₂O₄-OV-400 has a very close overpotential and a much higher electrochemical stability compared to commercial RuO₂. In addition, such OER catalytic activity of NiCo₂O₄-OV-400 is also superior to those of previously reported Ni or Co-based electrocatalysts. The detailed comparisons are shown in Table S1 (ESI†).

As observed in Fig. 7, there are two redox couples for the as-prepared NiCo₂O₄ nanobelts, respectively, which indicate that these electrocatalysts possess the same electrocatalysis mechanism. By comparing the CV curves of the different samples at the same scan rate of 10 mV s⁻¹, the NiCo₂O₄-OV-400 nanobelt electrode has more remarkable peaks than the other as-prepared NiCo₂O₄ nanobelt electrodes, implying that in comparison to the other electrodes, the NiCo₂O₄-OV-400 electrode has more rapid redox reactions.^63,64 Owing to the abundant surface oxygen vacancies of the material, which can promote the adsorption capacity of OH⁻, OER activity is promoted.

Fig. 7 CV curves at 10 mV s⁻¹ of the NiCo₂O₄ nanobelt electrodes.

To get further insight into the OER mechanism, the CV (Fig. S8†) tests of NiCo₂O₄-OV-400 were carried out at a scan rate of 10 mV s⁻¹ from 0.8 V to 1.5 V (vs. RHE). As shown in the magnified image in the inset, there were two oxidation peaks located at about 1.32 V and 1.38 V, which corresponded to the oxidation of Co²⁺ (Ni²⁺) to Co³⁺ (Ni³⁺) and further to Co⁴⁺ (Ni⁴⁺), respectively. Similarly, the corresponding reduction peaks located at 1.24 V and 1.36 V could be attributed to the reduction of Co⁴⁺ (Ni⁴⁺) to Co³⁺ (Ni³⁺) and further to Co²⁺ (Ni²⁺).^63–66

According to all the above analysis, the OER mechanism of the NiCo₂O₄ electrode material can be briefly explained by the following four steps:

M + OH⁻ ↔ M − OH + e⁻ (5)

M − OH + OH⁻ ↔ M − O + H₂O + e⁻ (6)

M − O + OH⁻ ↔ M − OOH + e⁻ (7)

M − OOH + OH⁻ ↔ M + O₂ + H₂O + e⁻ (8)

where M represents the active site of the catalyst.^67–69

According to these results, the superior OER activity and excellent stability of the oxygen-deficient NiCo₂O₄ catalysts synthesized by hydrothermal treatment and subsequent annealing process can be explained and proved by the several following advantages. Firstly, the as-prepared spinel-type NiCo₂O₄ synthesized at different annealing temperatures in an inert atmosphere can maintain the original nanobelt structure with no presence of a new phase and morphology. It provides a stable structure and large active surface area. Secondly, the successful generation of appropriate oxygen vacancies in the catalysts can help to adjust the electronic states and further provide effective electrocatalytically active sites, stimulating the adsorption capacity of OH⁻ and leading to a notably facilitated catalytic activity and electrochemical durability for the OER. And eventually, a series of NiCo₂O₄ samples with controllable oxygen vacancy concentration were prepared by adjusting the annealing temperature.

4. Conclusions

In summary, oxygen-deficient NiCo₂O₄ materials have successfully been synthesized by a facile and simple hydrothermal method and subsequent annealing process in an argon atmosphere. The important role of oxygen vacancies was clearly demonstrated during the OER process. The oxygen-deficient NiCo₂O₄ catalysts annealed at 400 °C showed superior catalytic activities with lower overpotential (only 325 mV to reach 10 mA cm⁻²), and accelerated OER kinetics with strong long-term stability was observed compared to the other obtained NiCo₂O₄ nanobelt catalysts. It can be inferred that the introduction of oxygen vacancies in the synthesized NiCo₂O₄ electrocatalysts plays an essential part in promoting efficient electrocatalytically active sites and enhancing the electrochemical properties. In particular, a series of NiCo₂O₄ samples with controllable oxygen vacancy concentration have been prepared by changing the annealing temperature; meanwhile, the relationship between oxygen vacancy concentration, M²⁺/M³⁺ (M = Ni/Co) ratio and annealing temperature was determined, and a reasonable explanation was shown as well. Thus, this oxygen-deficient NiCo₂O₄ structure shows high-performance for the OER at a large current density, which provides a promising pathway toward developing advanced electrocatalysts for developing sustainable energy materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This project is supported by the Key Project of Changfeng-HFUT Industrial Innovation Guidance Funds (JZ2020YDZJ0122), the National Natural Science Foundation of China (Grant No. 51772072, 51672065, and U1810204), and the Fundamental Research Funds for the Central Universities (PA2019GDQT002, PA2019GDZC0096). We also would like to thank the financial support from the 111 Project “New Materials and Technology for Clean Energy” (B18018).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0cy01669a

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