Room-temperature synthetic NiFe layered double hydroxide with different anions intercalation as an excellent oxygen evolution catalyst

Abstract

The Ni–Fe layered double hydroxide (LDH) is regarded one of the best catalysts for the oxygen evolution reaction (OER), yet bridging the relationship between the LDH nanostructure and OER performance still remains a big challenge. Instead of using other hydrothermal reactions to produce Ni–Fe layered double hydroxides, we adopted a method using a simple separate nucleation and aging steps to investigate the effect of crystallinity and the intercalated anions of LDH on OER performance. We found that improving the crystallinity and the size of NiFe-LDH by increasing the aging temperature led to a decrease of OER activity. Changing the interlayer spacing of LDH from 8.04 Å to 7.69 Å by introducing more CO₃²⁻ to replace NO₃⁻ causes the reduction of OER activity. These are probably attributed to the more exposed active sites, lower charger transferring resistance, and better exchange ability with OH⁻ in interlamination. Based on the abovementioned observations and the consequent optimizations, a very-low onset overpotential (∼240 mV) and Tafel slope value (33.6 mV dec⁻¹) (in 0.1 mol L⁻¹ KOH) for room-temperature synthetic NiFe LDH were achieved. This work proposes a strategy for the rational design of LDHs for the further enhancement of OER electrochemical activity, i.e. by decreasing the size and crystallinity of NiFe-LDH and by introducing more NO₃⁻ between layers.

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1. Introduction

High-efficiency, eco-friendly, low-cost energy storage applications are a critical element in the societal pursuit of sustainable and efficient energy conversion and storage solutions.¹ The oxygen evolution reaction (OER) in particular is a key factor of many renewable energy systems such as metal–air batteries and water splitting.^2,3 Thus, it is important to develop cost-effective, highly active catalysts for OER. Precious noble-metal oxides catalysts such as IrO₂ and RuO₂ show considerable catalytic activity for OER but they suffer from scarcity and high cost.^3–5 As a consequence, extensive efforts have been taken to discover and develop efficient OER catalysts based on earth-abundant metals, particularly nickel-based catalysts.^5–10

Layered double hydroxides (LDHs), which are a type of anion-intercalated material, have recently drawn great attention in the OER area due to their uniformly and densely distributed active sites.^10–14 Moreover, the abundance of constituent elements, the ease of fabrication, the capability of highly tunable compositions, and the superior OER performance make them rising stars, giving the possibility of practical utilization for electrochemically splitting water and in any applications involving OER such as metal–air batteries.^14,15 Among these LDH catalysts, NiFe-LDH is the most studied catalyst in recent years.^13–18 As the three important factors – metal elements; size and crystallinity; intercalated anions – which have strong effect on properties of LDH, when the metal elements are determined, the other two aspects may give great impact on the OER performance under given NiFe-LDH: one is the size and crystallinity of catalysts, the other is the intercalated anions between LDH's layers.¹⁹

In order to validate our assumptions, we finely tuned the crystallinity and sheet size by varying the aging temperature and altered the intercalation anions. It was noticed that the higher the aging temperature applied, the higher the crystallinity of LDH and consequently the more the active sites confined, as verified by OER data. Interlayer spacing was tuned by varying the ratio of OH⁻ with CO₃²⁻ from 24 : 0 to 24 : 2 and 24 : 4, while NO₃⁻ was used as the intercalated ion. Higher amount of CO₃²⁻ ions would cause a 4% decrease in the interlayer spacing, which leads to interlayer shrinking. Despite the small difference in interlayer spacing, we observed obvious OER current variance at the same applied voltages. Based on the abovementioned results, we safely hypothesized that the most active and efficient OER catalysts that were based on LDH could be generated based on small size, low crystallinity, and more intercalated NO₃⁻.

2. Experimental

2.1. Chemicals

Ni(NO₃)₂·6H₂O and Fe(NO₃)₃·6H₂O were obtained from Xilong Chemical Co., Ltd. and Tianjin Fu Chen Chemical Reagents, respectively. NaOH and Na₂CO₃ were of A.R. grade, were purchased from Beijing Chemical Reagent Co., Ltd., and were used as received without further purification. Deionized water was bubbled with N₂ for 1 h to remove CO₂.

2.2. Synthesis of NiFe-LDH

In a typical procedure, 10.0 mL of mixed salt solution containing Ni(NO₃)₂·6H₂O (2.4 mmol) and Fe(NO₃)₃·6H₂O (0.8 mmol) and 40.0 mL of mixed alkali solution containing NaOH and Na₂CO₃ (n_NaOH + 2n_Na2CO3 = 7.2 mmol, n_NaOH : n_Na2CO3 = 24 : 0, 24 : 2, or 24 : 4) were simultaneously poured out into an acolloidal mill and continuously stirred for 10 min. The resulting slurry was centrifuged at 12 000 rpm for 15 min and washed twice with deionized water to remove the excess free metal salts and alkali, and subsequently dispersed in 40.0 mL of deionized water. This aqueous suspension was transferred into a stainless steel autoclave with a teflon lining. The autoclave was then placed in a preheated oven, followed by hydrothermal treatment at room temperature (RT), 90, 120 and 150 °C for 12 h. The products were collected by centrifuge, repeatedly washed with CO₂-free deionized water and lyophilized.

2.3. Characterization

Transmission electron microscopy (TEM) was performed using a Hitachi-800 (accelerating voltage = 200 kV) system. X-ray diffraction (XRD) patterns were obtained with a Shimadzu XRD-6000 diffractometer using Cu K_α radiation (λ = 1.5418 Å) at 40 kV and 30 mA. Fourier transform infrared (FT-IR) spectra were recorded in the range of 4000–400 cm⁻¹ with 2 cm⁻¹ resolution with a Bruker Vector-22 Fourier transform spectrometer using the KBr pellet technique (1 mg of sample in 100 mg of KBr). XPS measurements were performed using an ESCALAB 250 instrument (Thermo Electron) with Al KR radiation. The CHN analysis was executed with a varioELcube elemental analyzer.

2.4. Electrochemical measurements

All the electrochemical measurements were performed using a standard three-electrode setup at RT. Electrochemical measurements (using a PARSTAT 2273 potentiostat from Princeton Applied Research) were conducted in an electrochemical cell using a saturated calomel electrode (SCE, 1.01 V vs. RHE in 0.1 M KOH) as the reference electrode, a 1 cm² Pt plate as the counter electrode and the sample-modified glassy carbon electrode (GCE) as the working electrode. For the rotating disk electrode (RDE) measurements, the working electrode was scanned cathodically at a rate of 5 mV s⁻¹ with a speed of 1600 rpm. Linear sweep voltammetry (LSV) was carried out at 5 mV s⁻¹ for the polarization curves after the catalyst was cycled 10 times by cyclic voltammetry (CV) in the oxygen saturated 0.1 M KOH aqueous solutions at a sweep rate of 50 mV s⁻¹ from 0.2 to 0.7 V. Prior to surface coating, the GCE was sequentially polished using 1.0 and 0.3 mm alumina slurry and then washed ultrasonically in water and ethanol for 5 min. The cleaned GCE was dried at RT for the next modification.

For the RDE measurements, 2.0 mg catalyst was dispersed in 490.0 μL of ethanol and 10.0 μL of 5% nafion solution and sonicated for 30 min to form a homogeneous ink. Then, 5.0 μL of the catalyst ink was loaded onto the cleaned GCE (5 mm in diameter, area of 0.196 cm²). All polarization curves were corrected with IR-compensation.

3. Results and discussion

The targeted NiFe-LDHs were prepared according to the separate nucleation and aging steps (SNAS) method in which the salt and the alkali solutions were quickly mixed and nucleated in a colloid mill and subsequently hydrothermally treated.^20,21 Fig. 1 and S1† show the typical TEM images and reveal that NiFe-LDHs display a plate-like morphology with growing size from RT to 150 °C (Fig. 1f). The corresponding XRD spectra (Fig. 1e and S2a and b†) showed that the (003) and the (006) peaks were significantly broadened and weaken with depressed aging temperature, suggesting lower crystallinity and more defects in the stacked structure.^22–28 The lateral sizes of NiFe-LDH nanosheets with no CO₃²⁻ were 16, 33, 61, and 143 nm on average for the RT, 90, 120 and 150 °C-aged samples, respectively (Fig. 1f). The typical size is labelled in the TEM image to highlight the difference (Fig. 1a–d). The other different anions-intercalated NiFe-LDHs also showed similar size growth at the same aging temperature (Fig. S1†).

Fig. 1 TEM images of NiFe-LDH plates under different resulting temperatures for 12 h with a 24 : 0 ratio of NaOH and Na₂CO₃: (a) RT, (b) 90 °C, (c) 120 °C, (d) 150 °C and (e) their XRD spectra, (f) size distribution.

To demonstrate the difference of intercalated anions (NO₃⁻ or CO₃²⁻), Fourier transformed infrared spectra of the RT-aged samples were collected and are shown in Fig. 2a, which were recorded in the range from 2000 to 1000 cm⁻¹. The infrared band at 1631 cm⁻¹ is attributed to the deformation mode of water (δ (H₂O)), while the band at 1384 cm⁻¹ stands for NO₃⁻.^21,24 As the CO₃²⁻ increased, two new broad bands at about 1470 cm⁻¹ and the one between 1000 and 1200 cm⁻¹ occurred, which were assigned to CO₃²⁻. Moreover, a typical vibration of the interlayer CO₃²⁻ band at 1357 cm⁻¹ appeared and grew strong; in addition, the ν₃ (asymmetric stretching) mode of NO₃⁻ at 1384 cm⁻¹ band turned weak, suggesting that NO₃⁻ had been exchanged by CO₃²⁻.^26,27 In addition, the other important modes, ν (OH⁻), δ (OH⁻) and ν₁ (CO₃²⁻), are labeled in Fig. S4.† Moreover, with a varioELcube elemental analyzer, we found that the nitrogen content of NiFe-LDH with 24 : 0-RT, 24 : 2-RT, and 24 : 4-RT was 0.676%, 0.413% and 0.22% in weight respectively, which agreed with the FT-IR results. As shown in Fig. 2b, with the more NO₃⁻ exchanged by CO₃²⁻ in the synthesis step (24 : 0 to 24 : 4), the (003) Bragg reflection gradually shifted from 8.04 Å to 7.82 Å and then to 7.69 Å. For the complete NO₃⁻ batch, the interlayer spacing is the same as that of NO₃⁻ intercalated MgFe-LDH (8.04 Å) prepared by the coprecipitation.²⁹

Fig. 2 (a) The FT-IR and XRD spectra (b) of NiFe-LDH OER catalyst with various ratios of NaOH and Na₂CO₃ at RT.

The products with the lowest applied aging temperature, i.e. RT, yielded the best electrocatalytic activity in 0.1 M KOH aqueous solution (Fig. 3a and S3a and b†). This is in accord with the fact that the defects are generally active sites for water splitting reactions.^30–32 In particular, those RT-catalysts exhibit superior OER activity to the commercial Ir/C in alkaline medium. Under high aging temperature, consequently elevated crystallinity and depressed disorder (Fig. 1e and S2†), the sheet size grew bigger (Fig. 1f) and those defects concentration decreased, which provided less activity towards OER reactions. This was consistent with the report that the amorphous phase shows superior OER activity to the crystalline ones.³⁰

Fig. 3 (a) The electrochemical performance of NiFe-LDH aged under different resulting temperatures with a 24 : 0 ratio of NaOH and Na₂CO₃. (b) The OER activity of NiFe-LDH with different ratios of NaOH and Na₂CO₃ at RT. (c) The potentials at 10 mA cm⁻² current for all samples. The loading of all samples was about 0.1 mg cm⁻².

In addition to the aging temperature, we found the ratios of different anion intercalation to be another vital parameter to enhance OER performance (Fig. 3b and c). Given the different ratio of intercalated anions, the interlayer spacing could be affected. XRD characterizations were performed to verify the varied interlayer spacing of the resulting LDHs. It was found that a larger interlayer distance induced by less CO₃²⁻ was relative to a smaller overpotential for OER, which means higher activity. However, considering the small change of the interlayer spacing, we safely believed that the intercalated anions play more important roles in OER catalytic process than the lamellar spacing of LDHs.¹⁹

In order to show that the intercalated anions and interlayer spacing are important factors that affect the overall OER performance for the RT-aged NiFe-LDHs, XPS analysis of the samples synthesized was performed at RT but with different ratios of OH⁻ and CO₃²⁻. The Ni 2p and Fe 2p XPS spectra are shown in Fig. S5a and b,† respectively. The oxidation states of Ni and Fe were found to be Ni²⁺ and Fe³⁺; the ratios of Ni and Fe remained almost the same for the three samples.¹³ Given the situation that no obvious binding energy (BE) shift occurred, we can safely conclude that the chemical states remain the same in these three samples obtained at RT.

The Tafel slopes were then measured to investigate the electrode kinetics. As shown in Fig. 4a, the corresponding Tafel slope values were 33.6, 36.9, and 44.5 mV dec⁻¹, meaning that the total NO₃⁻ intercalated NiFe-LDH was the most efficient OER electrocatalyst among these three samples. EIS plots (Fig. 4b) in the high frequency range indicated the intercalation by NO₃⁻ and agreed that a broader interlayer space possess a lower charger transferring resistance (R_ct) at the corresponding electrode/electrolyte interface, resulting in more favorable OER kinetics.^9,15,30 The chronoamperometric tests of NiFe-LDHs were performed at 0.55 V (vs. SCE, without iR correction) on the GCE. As seen in Fig. 4c, NiFe-LDHs without CO₃²⁻ showed the best stability in both low and high voltages, where 2000 s cycling caused only 4.8% activity decay at 0.55 V and 3% at 0.6 V. While for the other two samples synthesized at RT, the activity decays were 5.5% for 24 : 2 NiFe-LDH and 5% for 24 : 4 NiFe-LDH. However, their current densities were relatively low compared to that of 24 : 0 NiFe-LDH at the same applied voltage of 0.55 V.

Fig. 4 The current–time plots (a) of the different NiFe-LDH electrodes under the same alkaline conditions with various ratios of NaOH and Na₂CO₃ under RT and the corresponding Tafel plots (b), Nyquist plots (c) with a frequency range from 0.1 MHz to 1 Hz.

As a type of anion-intercalated material, LDHs possess a powerful ion exchange ability.^16,19,33 Inaddition, the accessibility to OH⁻ plays a very important role in OER because OH⁻ is the main reactant in alkaline solution.^1,13 Considering that NO₃⁻ is easier to be exchanged by OH⁻ than CO₃²⁻, it is obvious that the complete NO₃⁻ intercalated NiFe-LDH can gather more OH⁻.³³ Therefore, the reason why the sample with only NO₃⁻ intercalated showed the best electrocatalytic activity is because it has the lowest charger transferring resistance, as well as good anion-exchange ability with hydroxyl anion.

4. Conclusions

Size, crystallinity, and intercalated anions were all found to be important in determining the activity of OER. Samples with low crystallinity were achieved through lowering the aging temperature. The variations of intercalated ions can be obtained using different amount anions of NO₃⁻ and CO₃²⁻ in the synthesis procedures. Investigations of size, crystallinity and interlayer anions on OER demonstrated that efficient OER favored a small size and low crystallinity, which provided more unconfined active sites. More intercalated NO₃⁻ and a wider interlayer space could reduce the charger transferring resistance and improve the exchange ability with OH⁻. Our comprehensive understanding of the LDH-catalyzed OER process could be extended for synthesizing better layered materials for OER.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China, the 973 Program (2011CBA00503 and 2011CB932403), the Fundamental Research Funds for the Central Universities, and the Program for Changjiang Scholars and Innovative Research Team in University.

Notes and references

1. J. Suntivich, K. J. May, H. A. Gasteiger, H. A. Gasteiger, J. B. Goodenough and S. H. Yang, Science, 2011, 334, 1383–1385.
2. Y. Zhao, R. Nakamura, K. Kamiya, S. Nakanishi and K. Hashimoto, Nat. Commun., 2013, 4, 2390.
3. T. Grewe, X. Deng, C. Weidenthaler, F. Schüth and H. Tüysüz, Chem. Mater., 2013, 25, 4926–4935.
4. Z. Peng, D. S. Jia, A. M. Al-Enizi, A. A. Elzatahry and G. F. Zheng, Adv. Energy Mater., 2015, 5, 1402031; L. Trotochaud, J. K. Ranney, K. N. Williams and S. W. Boettcher, J. Am. Chem. Soc., 2014, 134, 17253–17261.
5. M. Zhang, M. T. Zhang, C. Hou, Z. F. Ke and T. B. Lu, Angew. Chem., Int. Ed., 2014, 53, 13042–13048.
6. M. J. Kenney, M. Gong, Y. Li, J. Z. Wu, J. Feng, M. Lanza and H. Dai, Science, 2013, 342, 836–840.
7. M. W. Louie and A. T. Bell, J. Am. Chem. Soc., 2013, 135, 12329–12337.
8. W. Zhou, X. J. Wu, X. Cao, X. Huang, C. Tan, J. Tian, H. Liu, J. Wang and H. Zhang, Energy Environ. Sci., 2013, 6, 2921–2924.
9. S. Chen, J. Duan, M. Jaroniec and S. Z. Qiao, Angew. Chem., Int. Ed., 2013, 52, 13567–13570.
10. Z. Lu, W. Xu, W. Zhu, Q. Yang, X. Lei, J. Liu, Y. Li, X. Sun and X. Duan, Chem. Commun., 2014, 50, 6479–6482.
11. C. G. Silva, Y. Bouizi, V. Fornés and H. Garcia, J. Am. Chem. Soc., 2009, 131, 13833–13839.
12. Y. Zhao, B. Li, Q. Wang, W. Gao, C. J. Wang, M. Wei, D. G. Evans, X. Duan and D. O’Hare, Chem. Sci., 2014, 5, 951–958.
13. M. Gong, Y. Li, H. Wang, Y. Liang, J. Z. Wu, J. Zhou, J. Wang, T. Regier, F. Wei and H. Dai, J. Am. Chem. Soc., 2013, 135, 8452–8455.
14. J. Jiang, A. L. Zhang, L. L. Li and L. H. Ai, J. Power Sources, 2015, 278, 445–451.
15. X. Long, J. Li, S. Xiao, K. Yan, Z. Wang, H. Chen and S. Yang, Angew. Chem., 2014, 126, 7714–7718.
16. F. Song and X. Hu, Nat. Commun., 2014, 5, 4477.
17. B. M. Hunter, J. D. Blakemore, M. Deimund, H. B. Gray, J. R. Winkler and A. M. Müller, J. Am. Chem. Soc., 2014, 136, 13118–13121.
18. D. Tang, J. Liu, X. Wu, R. Liu, X. Han, Y. Han, H. Huang, Y. Liu and Z. Kang, ACS Appl. Mater. Interfaces, 2014, 6, 7918–7925.
19. Q. Wang and D. O’Hare, Chem. Rev, 2012, 112, 4124–4155.
20. Z. Chang, C. Wu, S. Song, Y. Kuang, X. Lei, L. Wang and X. Sun, Inorg. Chem., 2013, 52, 8694–8698.
21. Q. Zhang, J. Xu, D. Yan, S. Li, J. Lu, X. Cao and B. Wang, Catal. Sci. Technol., 2013, 3, 2016–2024.
22. L. Li, R. Ma, Y. Ebina, K. Fukuda, K. Takada and T. Sasaki, J. Am. Chem. Soc., 2007, 129, 8000–8007.
23. N. T. Whilton, P. J. Vickers and S. Mann, J. Mater. Chem., 1997, 7, 1623–1629.
24. F. Millange, R. I. Walton and D. O'Hare, J. Mater. Chem., 2000, 10, 1713–1720.
25. C. Taviot-Guého, Y. Feng, A. Faour and F. Leroux, Dalton Trans., 2010, 39, 5994–6005.
26. Y. Lee, J. H. Choi, H. J. Jeon, K. M. Choi, J. W. Lee and J. K. Kang, Energy Environ. Sci., 2011, 4, 914–920.
27. F. Z. Zhang, L. L. Zhao, H. Y. Chen, S. L. Xu, D. G. Evans and X. Duan, Angew. Chem., Int. Ed., 2008, 47, 2466–2469.
28. Z. P. Liu, R. Z. Ma, M. Osada, N. Iyi, Y. Ebina, K. Takada and T. Sasaki, J. Am. Chem. Soc., 2006, 128, 4872–4880.
29. R. Ma, Z. Liu, K. Takada, N. Iyi, Y. Bando and T. Sasaki, J. Am. Chem. Soc., 2007, 129, 5257–5263.
30. R. D. L. Smith, M. S. Prévot, R. D. Fagan, Z. Zhang, P. A. Sedach, M. K. J. Siu, S. Trudel and C. P. Berlinguette, Science, 2013, 340, 60–63.
31. A. Bergmann, I. Zaharieva, H. Dau and P. Strasser, Energy Environ. Sci., 2013, 6, 2745–2755.
32. F. Y. Cheng, J. Shen, B. Peng, Y. D. Pan, Z. L. Zhan and J. Chen, Nat. Chem., 2011, 3(1), 79–84.
33. S. Miyata, Clays Clay Miner., 1983, 31, 305–311.

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Footnotes

† Electronic supplementary information (ESI) available: TEM images, XRD spectra, electrochemical characterization, Ni 2p XPS survey spectra and Fe 2p XPS survey spectra of samples and a table about OER activities of some benchmark catalysts in alkaline solution at 10 mA cm⁻². See DOI: 10.1039/c5ra05558j

‡ Contributed equally to this work.

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