Nanostructured hybrid NiFeOOH/CNT electrocatalysts for oxygen evolution reaction with low overpotential

Abstract

Oxygen evolution reaction (OER) has been recognized as a crucial half-reaction in water splitting for the production of hydrogen, one of the most important clean energies. In this article, we report the synthesis of a series of Ni-based NiMOOH layered double hydroxide (LDH, M = Cr, Fe, Co) nanosheets with sizes of about 20 nm with tetramethylammonium hydroxide (TMAOH) as a base source under mild reaction conditions. NiFeOOH shows much lower onset potential than NiCoOOH, NiCrOOH and Ni(OH)₂ in alkaline solution. To further improve the OER activity, NiMOOH/CNT hybrid composites was prepared by in situ addition of carbon nanotubes (CNT) during the synthesis process of NiMOOH. The hybrid composites afford much higher activity than NiMOOH alone, especially for NiFeOOH/CNT with the overpotential of 278 mV at 10 mA cm⁻² in alkaline solution. The significantly improved OER activity of NiMOOH/CNT hybrid composites is mainly attributed to the synergetic effect of CNT and nanostructured NiMOOH by improving the electric conductivity and increasing the exposure degree of active sites for OER. Moreover, the hybrid composites also possess high stability for a prolonged testing time.

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1. Introduction

The sustainable production of clean and carbon-free fuels constitutes one of the most important scientific challenges in the 21st century. The electrochemical water splitting to H₂ and O₂ is a desirable process for the large-scale storage of renewable and intermittent energy, such as solar, wind, or other renewable sources.^1–7 The primary challenge of such processes resides on the oxygen evolution reaction, which requires a proton coupled electron transfer process to oxidize two water molecules with a minimum potential of +1.23 V vs. RHE at standard temperature and pressure in thermodynamics. However, a high overpotential is usually required to carry out this reaction due to the slow reaction kinetics.^8–10 Thus the development of efficient OER catalyst to decrease the overpotential is one of the key issues for water oxidation.

Numerous catalysts have been developed to reduce the overpotential of OER, in which some noble metal oxides, like IrO₂ and RuO₂, are very active, but the high cost hindered their wide industrial application.^11–13 First-row transition metal oxides and hydroxides have attracted much research attention for water oxidation in view of the activity and stability in alkaline solution.^14–23 In recent studies, nickel-based materials have been viewed to be the most promising catalysts for OER,^18,24–27 especially nickel-based LDH.^28,29 Studies show that the Ni-based LDH possess a fast proton coupled electron transfer process,³⁰ which is a favourable factor to acquire high OER activity. In addition, doping other metals into LDH structure can modify local electronic structure to improve the OER activity, but the activities are usually restricted by their poor electric conductivity.³⁰ To further improve the catalytic performance, carbon materials with good conductivity, such as CNT, graphene and carbon quantum dot, have been blended with Ni-based LDH.^30–37

Most Ni-based LDH reported have particle size in the range of several hundred nanometers to micrometers.^30–32 Decreasing the particle size to nanoscale may help to improve the catalytic performance of Ni-based LDH due to the short electron transfer pathway and high amount of exposed active sites. Moreover, it will be easier for the uniform distribution of nanostructured Ni-based LDH on carbon materials to further improve their conductivity. At the same time, the synthesis of Ni-based LDH in previous reports generally requires high temperature treatment, in which a solvothermal procedure at over 100 °C was usually included. The synthesis of nanostructure Ni-based LDH under mild conditions is still a challenging issue.

Herein, Ni-based LDH doped with Cr, Fe and Co were synthesized and the catalytic activities of above nanostructured Ni-based LDH materials were tested with electrocatalytic OER in alkaline media, which verifies that the doped transition metal can effectively improve the catalytic activity. To enhance the electric conductivity of the electrocatalyst, the hybrid composites of NiMOOH/CNT were prepared subsequently, which show excellent electrocatalytic OER activities.

2. Experimental section

2.1 Chemicals and materials

All chemicals were used as received unless otherwise stated. Tetramethylammonium hydroxide solution (TMAOH, 25%), hydrogen peroxide (H₂O₂, 30%), nickel(II) chloride hexahydrate (NiCl₂·6H₂O), chromium(III) chloride hexahydrate (CrCl₃·6H₂O), cobalt(II) chloride hexahydrate (CoCl₂·6H₂O) and iron(III) chloride hexahydrate (FeCl₃·6H₂O) were purchased from Sinopharm Chemical Reagent Company. Potassium hydroxide (KOH) was purchased from Kermel Chemical Reagents. Deionized water was used throughout the experiments.

2.2 Synthesis of nickel-based layer double hydroxide

For NiMOOH, 8.8 mL of TMAOH, 4 mL of H₂O₂ (30 wt%) and 27.2 mL of H₂O were mixed together to obtain a 40 mL aqueous solution. Then, the above solution was added into a flask containing 20 mL of aqueous solution of NiCl₂·6H₂O and MCl_x (0.3 M, with Ni/M molar ratio of 5, M = Cr, Fe, Co, x = 2 or 3). The resulted suspension was stirred vigorously for 24 h at 30 °C. The precipitate was separated by filtration, washed with copious amounts of deionized water and ethanol, and then dried at 60 °C overnight. The products were sufficiently grinded in the mortar for further characterization and test. The materials are named as NiMOOH-n, where n refers to the molar ratio of Ni/M based on ICP results.

NiFeOOH with different Ni/Fe molar ratios (7 and 3), were prepared in a similar way to NiMOOH but with different Ni/Fe mole ratios in the initial mixture.

NiFeOOH/CNT was prepared in a similar way to NiFeOOH-5 except that the desired amount of CNT was added during initial mixing process. The products are named as NiFeOOH/CNT-x, where x refers to the mass content of CNT.

NiMOOH/CNT (M = Co or Cr) was prepared in a similar way to NiFeOOH/CNT-47%, the product is named as NiCoOOH/CNT-48% and NiCrOOH/CNT-48%, where 48% refers to the mass content of CNT calculated from the result of TG analysis.

2.3 Characterization

The transmission electron microscopy (TEM) was undertaken using a Hitachi HT-7700 at an acceleration voltage of 100 kV. The samples were placed onto an ultrathin carbon film supported on a copper grid. The high-resolution scanning electron microscopy (HR-SEM) was performed on Hitachi S-5500 scanning electron microscope operating at an acceleration voltage of 30 kV. The powder X-ray diffraction data (PXRD) were collected on a Rigaku D/Max2500PC diffractometer with Cu Kα radiation (λ = 1.5418 Å) over the 2θ range of 15–70° with a scan speed of 5° min⁻¹ at room temperature. The nitrogen sorption experiments were performed at 77 K on a Micromeritics ASAP 2020 system. Prior to the measurement, the samples were degassed at RT for 10 h. The metal content was determined by PLASAM-SPEC-II inductively coupled plasma atomic emission spectrometry (ICP). The thermogravimetric analysis (TGA) was performed using a NETZSCH STA 449F3 analyzer from 30 to 900 °C with a heating rate of 10 °C min⁻¹ under air atmosphere. XPS was recorded on a VG ESCALAB MK2 apparatus by using Al Kα (hl = 1486.6 eV) as the excitation light source.

2.4 Ink preparation for electrochemical measurements

To measure the electrocatalytic OER activities of as-synthesized catalysts, the preparation method of the working electrode is as following. In brief, 1 mg catalyst was dispersed in a solution consisting of 100 μL ethanol, 50 μL water and 0.75 μL Nafion solution (5 wt%, E. I. Du Pont Company). The mixture was then ultrasonicated for about 30 min to obtain a homogeneous ink. After that, 3 μL of the dispersion was transfered onto the glassy carbon electrode (3 mm in diameter) with the catalyst loading about 0.28 mg cm⁻². Finally, the as prepared catalyst film was dried at room temperature.

To test the stability and durability of NiFeOOH-5 and NiFeOOH/CNT-47%, the working electrode was prepared as follows: the ink was same with that for the measurement of electrocatalytic OER activity. Carbon paper was used to prepare the working electrode. Based on the catalyst loading about 0.28 mg cm⁻², a certain amount of ink was dropped onto carbon paper (∼0.2 cm²).

2.5 Electrochemical measurements

The electrochemical studies were carried on CH Instruments Model 760E electrochemical work station using 1 M KOH as an electrolyte. Catalyst powder casted on the glass carbon electrode was used as the working electrode. Platinum wire and saturated calomel electrode (SCE) were used as a counter electrode and the reference electrode, separately. The potential was converted to reversible hydrogen electrode (RHE) according to E (RHE) = E (SCE) + 0.059 pH + 0.241. Linear sweep voltammetry was measured at a scan rate of 10 mV s⁻¹. The catalyst was cycled ∼10 times by cyclic voltammetry (CV) before measuring the polarization curves. All polarization curves were corrected with 95% iR compensation.

3. Results and discussion

3.1 Synthesis and characterization of NiMOOH

A series of NiMOOH layered double hydroxides (M = Cr, Fe, Co) with different doping metals were synthesized by direct co-precipitation of Ni²⁺–M²⁺ (M²⁺ = Co²⁺) or Ni²⁺–M³⁺ (M³⁺ = Cr³⁺ and Fe³⁺) in the presence of TMAOH and H₂O₂ (Scheme 1).^17,38 Our previous study has demonstrated that TMAOH favors the formation of nanostructured metal oxide due to the slowly releasing of OH⁻,¹⁷ which is different from KOH and NaOH (Fig. S1 and S2†). H₂O₂ in the synthetic reaction was used to oxidize the metal ion with low valence state, which is necessary for the formation of LDH structure (Fig. S3†). Table 1 summarizes the Ni/M molar ratios in NiFeOOH-5, NiCrOOH-8 and NiCoOOH-5 synthesized in this work. Ni/M (M = Fe and Co) ratios in NiFeOOH-5 and NiCoOOH-5 are same with the initial Ni/M (M = Fe, Co) ratios. However, NiCrOOH-8 with Ni/Cr ratio of 8 is much higher than that in initial mixture of 5. This may be due to the larger difference in ion radius between Cr³⁺and Ni²⁺, which will make it difficult to dope Cr³⁺ into Ni-based LDH. The BET surface areas of NiFeOOH-5, NiCrOOH-8 and NiCoOOH-5 are almost same and vary in the range from 112 to 161 m² g⁻¹. Meanwhile, NiFeOOH with different Ni/Fe molar ratios were also synthesized to investigate the influence of metal doping amount. Ni/Fe ratios in all NiFeOOH are also same as the initial Ni/Fe ratios. The BET surface area of NiFeOOH-3 is only 19 m² g⁻¹, which is much lower than those of NiFeOOH-5 and NiFeOOH-7.

Scheme 1 Schematic representation of the preparation of NiMOOH (without CNT in the synthetic process) and NiMOOH/CNT.

Table 1 The chemical composition and BET surface areas of NiMOOH and NiMOOH/CNT composites

Catalysts	Ni/M molar ratio^a	CNT content^b (wt%)	BET surface area (m^2 g^−1)
a The Ni/M ratios of NiMOOH are the ratios used in the synthetic system. The data in parenthesis refers to Ni/M ratios based on ICP results.b The CNT contents are calculated based on TG analysis (Fig. 4). The observed weight loss in the temperature range from 320 °C to 600 °C under air in TG curves is identified as the weight loss of CNT.
NiCrOOH-8	Ni/Cr = 5 (8)	—	161
NiCoOOH-5	Ni/Co = 5 (5)	—	146
NiFeOOH-7	Ni/Fe = 7 (7)	—	83
NiFeOOH-5	Ni/Fe = 5 (5)	—	112
NiFeOOH-3	Ni/Fe = 3 (3)	—	19
NiFeOOH/CNT-24%	Ni/Fe = 5	24	76
NiFeOOH/CNT-47%	Ni/Fe = 5	47	175
NiFeOOH/CNT-57%	Ni/Fe = 5	57	150
NiCrOOH/CNT-48%	Ni/Cr = 5	48	—
NiCoOOH/CNT-48%	Ni/Co = 5	48	—

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The TEM images of NiMOOH (M = Cr, Fe, Co) are displayed in Fig. 1A. Ni(OH)₂ without transition metal doping has large particle size about 50–100 nm. NiCrOOH-8, NiFeOOH-5 and NiCoOOH-5 samples have smaller nanosheets morphology with size about 20 nm. The NiFeOOH-5 nanosheets are uniformly distributed; however, the nanosheets of NiCrOOH-8 and NiCoOOH-5 have tendencies to agglomerate together. TEM and AFM images clearly show that the nanosheets have the thickness about 1 nm (Fig. S4 and S5†). The morphology of NiFeOOH with Ni/Fe molar ratios of 7 maintains nanosheet. But it transfers from nanosheet to bulk particle for NiFeOOH-3 with Ni/Fe molar ratio further decreasing, which is in agreement with the result of the lowest BET surface area.

Fig. 1 (A) TEM images and (B) XRD patterns of (a) Ni(OH)₂, (b) NiCrOOH-8, (c) NiCoOOH-5, (d) NiFeOOH-7, (e) NiFeOOH-5, (f) NiFeOOH-3. The reflections labelled with ▲ in (B) are indexed to β-Ni(OH)₂. The reflections labelled with ★ in (B) can be assigned to LDH structure.

XRD patterns of NiMOOH are displayed in Fig. 1B. Ni(OH)₂ without transition metal doping is β-Ni(OH)₂ type structure.³⁹

The diffractions of (003), (006), (009), (015), (018), (110) and (113) assigned to typical LDH phase are clearly observed in the XRD patterns of NiFeOOH and NiCoOOH samples. But the diffractions are not obvious in the XRD patterns of NiCrOOH, which may be due to the difficulty of Cr³⁺ doping. The intensity of diffractions for NiFeOOH with different Ni/Fe molar ratios increases with the increment of Fe doping amount. This result shows that the incorporation of transition metals favors the structure transformation from β-Ni(OH)₂ to LDH structure.³⁰ Comparing with the XRD pattern of LDH structure in the references, the lower XRD diffraction intensities of NiMOOH samples suggest that the samples possess smaller particle size. This phenomenon is consistent with the TEM results.

Surface composition of materials was carried out using XPS technique (Fig. 2). All the samples clearly show the existence of Ni. The Ni 2p^3/2 and Ni 2p^1/2 spin–orbital splitting photo-electrons for all NiMOOH samples locate at 855.3 and 872.9 eV (Fig. 2A), indicating that the valence state of Ni in NiMOOH is about +2.⁴⁰ As shown in XPS spectra of NiCoOOH-5 (Fig. 2B), the binding energy of Co 2p locates at 780.6 and 796.0 eV, indicating that the oxidation of Co²⁺ is occurred in the synthetic reaction and the valence state of Co in NiCoOOH-5 is about +3.⁴¹ The binding energy of Cr 2p for NiCrOOH-8 locates at 577.0 and 586.5 eV,³ and the binding energy of Fe 2p for NiFeOOH-5 locates at 712.9 and 725.7 eV,^31,32 which demonstrates that the valence state of Cr and Fe in NiMOOH is +3.

Fig. 2 (A) XPS Ni 2p spectra of (a) NiCrOOH-8, (b) NiFeOOH-5 and (c) NiCoOOH-5; (B) XPS spectra of (a) Cr 2p in NiCrOOH-8, (b) Fe 2p in NiFeOOH-5, (c) Co 2p in NiCoOOH-5.

The formation process of ultrathin NiMOOH nanosheet was further studied using NiFeOOH-5 as a model sample. The samples taken out at different reaction time were characterized using TEM, nitrogen sorption isotherm and XRD techniques (Fig. S6 and S7†). A relative large block material was obtained after 25 min and 4 h, and the BET surface area of material obtained after 4 h is only 3.6 m² g⁻¹. With the reaction time prolonging, the block morphology gradually disappears and some nanosheets appear. Nanosheets with the size about 20 nm dominate after 24 h. The change of morphology can also be reflected from the result of BET surface area, which increases gradually with the prolongation of reaction time. The results of XRD show that diffractions of LDH structure appear after 1 h. With the reaction time prolonged, the diffractions of LDH structure become more obvious, suggesting that the degree of crystallinity increases. Above results suggest that the gradual transformation from amorphous bulk materials into crystalline nanostructured LDH samples is occurred during the synthetic reaction.

3.2 Synthesis and characterization of NiMOOH/CNT

To hybridize NiMOOH with carbon material, CNT was selected as carbon material. The NiMOOH/CNT hybrid composites were prepared by in situ addition of CNT during the synthesis of NiMOOH (Scheme 1). NiFeOOH-5 was selected as nanostructured LDH to investigate the influence of the contents of CNT in the hybrid materials. The CNT contents in NiFeOOH/CNT hybrid composites were calculated from thermogravimetry (TG) analysis (Fig. 3A). NiFeOOH-5 shows two weight loss steps. The first weight loss from 30 °C to 170 °C is corresponding to the loss of adsorbed water and the second weight loss from 170 °C to 320 °C is due to dihydroxylation. In addition to the above mentioned two weight loss steps, NiFeOOH/CNT composites give a new weight loss step ranging from 320 °C to 600 °C assigned to the weight loss of CNT.⁴² Based on this weight loss step, the CNT contents in NiFeOOH/CNT are calculated, which range from 24% to 57% (Table 1). It should be noted that the decomposition temperature of CNT in NiFeOOH/CNT is lower than those of pure CNT and physical mixture of NiFeOOH-5 and CNT, which is possibly due to the acceleration effect of NiFeOOH-5 for CNT decomposition. The results verified the close contact between NiFeOOH-5 and CNT. Similar with the synthesis of NiFeOOH/CNT-47%, the NiCoOOH/CNT and NiCrOOH/CNT were also synthesized by in situ addition of CNT during the synthesis of NiCoOOH and NiCrOOH. Based on the weight loss from 320 °C to 600 °C in TG analysis (Fig. 3B), the CNT contents in NiCoOOH/CNT and NiCrOOH/CNT are calculated, which are the same of 48% (Table 1). The decomposition temperature of CNT in NiCoOOH/CNT-48% and NiCrOOH/CNT-48% is also lower than that of pure CNT, which illustrates the close contact between NiMOOH (M = Co and Cr) and CNT.

Fig. 3 (A) TG curves of (a) NiFeOOH-5, NiFeOOH/CNT with different CNT contents (b) NiFeOOH/CNT-24%, (c) NiFeOOH/CNT-47%, (d) NiFeOOH/CNT-57%, (e) CNT, and (f) physical mixture of NiFeOOH-5 and CNT with mass ratio of about 1 : 1 for comparison; (B) TG curves of (a) NiCoOOH/CNT-48%, (b) NiCrOOH/CNT-48%.

The TEM images of all NiMOOH/CNT samples show the co-existence of CNT and NiMOOH (Fig. 4A and S8†), in which CNT and NiMOOH tangle together closely. Taken NiFeOOH/CNT-47% as an example, the HR-SEM image (Fig. 4A(d)) and element mapping of Ni and Fe (Fig. S9†) clearly show that NiFeOOH-5 maintains nanosheet morphology with particle size about 20 nm and contact closely with CNT. The XRD patterns of NiFeOOH/CNT with different CNT contents show the diffractions assigned to both CNT and NiFeOOH-5 (Fig. 4B). NiFeOOH-5 maintains the LDH structure after incorporation with CNT. As the CNT content increases, the intensity of (002) diffraction (belonging to CNT) increases. The results of XPS analyses show that the binding energies of Ni²⁺ and Fe³⁺ are consistent with those for individual NiFeOOH-5 (Fig. 5A). However, the binding energy of O 1s (Fig. 5B) for composites increases slightly in comparison with that for NiFeOOH-5, possibly due to the strong interaction between NiFeOOH-5 and CNT.

Fig. 4 (A) TEM images of (a) NiFeOOH/CNT-24%, (b) NiFeOOH/CNT-47%, (c) NiFeOOH/CNT-57% and (d) HR-SEM image of NiFeOOH/CNT-47%. (B) XRD patterns of NiFeOOH/CNT with different CNT contents (a) NiFeOOH/CNT-24%, (b) NiFeOOH/CNT-47%, (c) NiFeOOH/CNT-57%. The reflection labeled with ■ is indexed to CNT. And the reflections labeled with ★ are assigned to LDH, which is similar with individual NiFeOOH-5.

Fig. 5 (A) XPS survey spectra of (a) CNT, (b) NiFeOOH/CNT-24%, (c) NiFeOOH/CNT-47%, (d) NiFeOOH/CNT-57% and (B) XPS O 1s spectra of (b) NiFeOOH/CNT-24%, (c) NiFeOOH/CNT-47%, (d) NiFeOOH/CNT-57%, (e) NiFeOOH-5.

3.3 Electrocatalytic oxygen evolution reaction

The catalytic performance of NiMOOH for electrochemical oxygen evolution reaction was investigated in alkaline solutions (1 M KOH) using the standard three-electrode system with SCE as reference electrode and Pt wire as counter electrode. The mass-loading of all the materials on glass carbon (GC) electrode was kept consistent (0.28 mg cm⁻²) for comparison purpose. After reaching a relatively stable state by cyclic voltammetric scans, the OER activity was measured by linear sweep voltammetry. Fig. 6 shows the polarization curves of NiMOOH at a slow scan rate of 10 mV s⁻¹ to minimize the capacitive current.³⁰ The peak around 1.42 V vs. RHE is possibly ascribed to oxidation reaction of Ni²⁺/Ni³⁺ in LDH nanosheets (inset of Fig. 6).⁴³ Ni²⁺/Ni³⁺ oxidation potential has a negative and positive shift for NiCoOOH-5 and NiFeOOH-5 respectively, in comparison with that for Ni(OH)₂. NiCrOOH-8 has almost the same Ni²⁺/Ni³⁺ oxidation potential with Ni(OH)₂. The result indicates that Co and Fe doping facilitates and retards the oxidation from Ni²⁺ to Ni³⁺, respectively.⁴³ The current density at high potential for all NiMOOH is much higher than that for Ni(OH)₂. The results confirm that doping transition metal is beneficial to improve the electrocatalytic OER activity. Among these three electrocatalysts, the NiFeOOH-5 nanosheet displays the lowest onset potential, but for the other two materials, the onset potential are same with that of Ni(OH)₂. So we suggest that the active site for NiFeOOH-5 is Fe and the active site for NiCrOOH-8 or NiCoOOH-5 is Ni. And the current density at same overpotential for NiFeOOH-5 and NiCoOOH-5 is higher than that for NiCrOOH-8.

Fig. 6 Polarization curves of (a) Ni(OH)₂ and NiMOOH with different transition metal doped (b) NiCrOOH-8, (c) NiCoOOH-5, (d) NiFeOOH-7, (e) NiFeOOH-5, (f) NiFeOOH-3, and the inset is enlarged the part of Ni²⁺/Ni³⁺ oxidation peak of NiMOOH with different transition metal doped, (g) is the activity of blank GC electrode.

The results of linear sweep voltammetry of NiFeOOH with different Ni/Fe molar ratios are also shown in Fig. 6. The electrochemical OER activity increases with Fe content and reaches the maximum with Ni/Fe molar ratio of 5. The lower activity of NiFeOOH-3 may be related to its lower BET surface area and larger particle size. The above result also suggests that Fe is the possible active site in NiFeOOH LDH.^44–47

For further improving the activity of Ni-based LDH, CNT were selected to hybrid with NiMOOH LDH. Firstly, NiFeOOH/CNT hybrid composites with different CNT contents were prepared. The samples were coated on glassy carbon electrode with a catalyst loading of 0.28 mg cm⁻² and used as working electrode. The catalytic activity for OER in 1 M KOH aqueous solution was evaluated at a scan rate of 10 mV s⁻¹ in a standard three-electrode system. Fig. 7A displays the representative iR-corrected linear sweep voltammetry curve of NiFeOOH/CNT hybrid composites. In the polarization curves, the peak around 1.43 V vs. RHE is ascribed to oxidation reaction of Ni²⁺/Ni³⁺ in LDH nanosheets. It is apparent that NiFeOOH/CNT hybrid composites exhibit high OER activity in stark contrast with a weak activity response of NiFeOOH-5 and CNT. The anodic current recorded with the NiFeOOH/CNT hybrid composites shows a sharp OER current after onset potential. And the anodic current order at same potential of the hybrid composites is that NiFeOOH/CNT-47% > NiFeOOH/CNT-57% > NiFeOOH/CNT-24%. The results show that an optimized CNT content is existed in the hybrid composites. Compared with NiFeOOH-5, the current density is greatly improved but the onset potential remains unchanged for NiFeOOH/CNT hybrid composites. This result suggests that NiFeOOH-5 worked as active site and CNT could enhance the activity of NiFeOOH-5 by increasing the electric conductivity. At the same time, the exposure of active sites (NiFeOOH-5) for OER will be increased after hybridized with CNT. The synergetic effect of CNT and NiFeOOH-5 contributes to the enhanced activity of NiFeOOH/CNT hybrid composites. The Tafel plots of NiFeOOH-5 and NiFeOOH/CNT-47% are shown in Fig. 8A. After hybridized with CNT, the Tafel plot is significantly decreased from 198 mV dec⁻¹ to 58 mV dec⁻¹. And the EIS spectrum of NiFeOOH-5 and NiFeOOH/CNT-47% are shown in Fig. S10,† the charge transfer resistance is obviously decreased after hybridized with CNT, which further verifies the facilitating effect for the electric conductivity.

Fig. 7 (A) Polarization curves of (a) CNT, and NiFeOOH/CNT with different CNT contents (b) NiFeOOH/CNT-24%, (c) NiFeOOH/CNT-47%, (d) NiFeOOH/CNT-57%; (B) polarization curves of (a) NiCoOOH/CNT-48%, (b) NiCrOOH/CNT-48%.

Fig. 8 (A) Tafel plots of (a) NiCrOOH-8, (b) NiCoOOH-5, (c) NiFeOOH-5, (d) NiCrOOH/CNT-48%, (e) NiCoOOH/CNT-48%, (f) NiFeOOH/CNT-47%; (B) chronopotentiometry curves of (a) NiFeOOH-5 at the current density of 5 mA cm⁻² and (b) NiFeOOH/CNT-47% at the current density of 10 mA cm⁻².

Fig. 7B displays the representative iR-corrected linear sweep voltammetry curves of NiCoOOH/CNT-48% and NiCrOOH/CNT-48% hybrid composites. The Ni²⁺/Ni³⁺ oxidation potentials for NiCoOOH/CNT and NiCrOOH/CNT are same with those for NiCoOOH-5 and NiCrOOH-8, respectively. The OER activities of NiCoOOH/CNT-48% and NiCrOOH/CNT-48% hybrid composites are much higher than that of pure LDH, but the onset potential remains unchanged. And the Tafel plots of NiCrOOH-8, NiCoOOH-5, NiCrOOH/CNT-48% and NiCoOOH/CNT-48% are shown in Fig. 8A. It shows a little lower value after hybridized with CNT for NiCrOOH-8, but no obvious change is observed for NiCoOOH-5. And the EIS spectra of the samples are shown in Fig. S10,† which confirmed that the charge transfer resistance is significantly decreased after hybridized with CNT. We speculate that the improvement of the OER activities for NiCrOOH/CNT-48% and NiCoOOH/CNT-48% may be due to both the increase of the electric conductivity and the exposure of active site after hybridized with CNT. It is worth noting that the tendency of activities of NiMOOH/CNT is same with that of NiMOOH, NiFeOOH/CNT > NiCoOOH/CNT > NiCrOOH/CNT. The results suggest that during the oxygen evolution reaction with hybrid materials as catalysts, NiMOOH worked as active sites and CNT could enhance the activity by increasing the electric conductivity and the exposure of active sites.

The potential required to achieve a current density of 10 mA cm⁻² is an important parameter for evaluating the catalytic performance of an OER catalyst because it is approximately the current density for a 10% efficient solar-to-fuel conversion device.^4,48 For NiFeOOH/CNT hybrid materials, the corresponding overpotentials at 10 mA cm⁻² are 286, 278 and 285 mV for NiFeOOH/CNT-24%, NiFeOOH/CNT-47% and NiFeOOH/CNT-57%, respectively (Table 2). Compared with NiFeOOH/CNT-24%, NiFeOOH/CNT-47% shows a further enhancement in OER activity. However, NiFeOOH/CNT-57% shows slightly lower activity than NiFeOOH/CNT-47%. The results show that there is an optimized CNT content in NiFeOOH/CNT hybrid composites. For NiMOOH/CNT (M = Cr or Co) hybrid materials, the corresponding overpotentials at 10 mA cm⁻² are 364 and 378 mV for NiCoOOH/CNT-48%, and NiCrOOH/CNT-48%, which are much higher than that of NiFeOOH/CNT hybrid materials. Recently, conductive materials such as Ni foam, graphene, carbon nanotube and carbon quantum dot have been used to hybridize with NiFe-LDH. Compared with reported NiFe-LDH hybrid catalyst, NiFeOOH/CNT-47% is one of the most active OER catalysts (Table 2).

Table 2 Comparison of the activity of NiFeOOH/CNT hybrid composites with the previous reported OER catalysts

Catalysts	Electrolyte	Overpotential (mV)@10 mA cm^−2	Ref.
NiFeOOH/CNT-24%	1 M KOH	286	This work
NiFeOOH/CNT-47%	1 M KOH	278	This work
NiFeOOH/CNT-57%	1 M KOH	285	This work
CQDs/NiFe-LDH	1 M KOH	235	30
NiFe-LDH/CNT	1 M KOH	247	31
FeNi-rGO LDH	1 M KOH	210	32
NiFeLDH/G/Ni	0.1 M KOH	325	33
LDH/NGF	1 M KOH	337	36

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NiFeOOH-5 and NiFeOOH/CNT-47% hybrid composite were selected to investigate the stability and durability of materials (Fig. 8B). In the electrocatalytic system, carbon paper uniformly coated with the materials was used as the working electrode with mass loading of 0.28 mg cm⁻². The potentials were detected at the current density of 5 mA cm⁻² for NiFeOOH-5 and 10 mA cm⁻² for NiFeOOH/CNT-47%. As shown in Fig. 8B(a), chronopotentiometric curves measured at current density of 5 mA cm⁻² for NiFeOOH-5 indicate that the potential fluctuates slightly with no obvious change. The chronopotentiometric curves at 10 mA cm⁻² for NiFeOOH/CNT-47% also can be clearly seen in Fig. 8B(b). The working potential values for the hybrid material just show a little increase for a prolonged testing time (from 1.56 V to 1.59 V vs. RHE). The chronopotentiometric characterization results validate the good durability of the catalyst.

4. Conclusions

In summary, we report the synthesis of a series of nanostructured Ni-based LDH via a simple co-precipitation method. The doping of transition metal into Ni-based hydroxide can effectively improve the OER activity. An inexpensive, earth-abundant NiMOOH/CNT hybrid composite was constructed after hybridizing NiMOOH with CNT. This catalyst exhibits excellent electrochemical OER activity, where a small overpotential of 278 mV is obtained for NiFeOOH/CNT-47% hybrid material in 1 M KOH at current density of 10 mA cm⁻². Furthermore, the chronopotentiometry tests of the hybrid material reveal the good durability. Our studies provide a simple method to design and fabricate effective Ni-based hybrid OER electrocatalysts, and we believe that this strategy is applicable to prepare a wide range of hybrid functional materials for electrochemical applications.

Acknowledgements

This work was primarily funded by the NSFC Grant No. 21503217 and NSFC Grant No. 21273226. This work also was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences Grant No. XDB17020200.

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Footnote

† Electronic supplementary information (ESI) available: The additional figures mentioned in the text. See DOI: 10.1039/c6ra16450a

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