Hybridizing amorphous NiO_x nanoflakes and Mn-doped Ni₂P nanosheet arrays for enhanced overall water electrocatalysis

Abstract

Rational development and facile fabrication of efficient and low-cost bifunctional electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) play a vital role in hydrogen and oxygen generation by overall water electrocatalysis. Herein, a series of hierarchical core–shell hybrid nanostructured bifunctional electrocatalysts have been successfully synthesized by electrodepositing amorphous nickel oxide nanoflakes (A-NiO_x) on the surface of crystalline Mn-doped Ni₂P nanosheet arrays (Mn₅-Ni₂P). Benefitting from the synergistic effect generated from the outer amorphous NiO_x and inner highly crystalline Mn₅-Ni₂P nanosheets, the optimal A-NiO_x-20/Mn₅-Ni₂P sample exhibits quite low overpotentials for the HER (55 mV) and OER (255 mV) to afford a current density of 10 mA cm⁻² in alkaline electrolyte. Moreover, the two-electrode water electrolysis device only requires a small cell voltage of 1.54 V to deliver 10 mA cm⁻² and shows no obvious attenuation for 20 hours. Our work will provide a valuable method to design and synthesize efficient electrocatalysts for water splitting and other applications of energy conversion and storage.

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1. Introduction

The extensive use of fossil fuels has caused serious environmental problems and energy shortage which have promoted the re-emergence of research on renewable hydrogen generation.^1–4 Water electrolysis, which is driven by the electrocatalytic HER and OER, has long been regarded as one of the most promising methods to produce clean hydrogen energy.^1,5–7 However, the sluggish HER and OER processes still require efficient catalysts to enhance the low electrocatalytic efficiency and reduce the high overpotential.^4,8,9 For now, although noble-metal-based electrocatalysts (Pt, RuO₂ or IrO₂) exhibit excellent electrocatalytic activity, their scarcity and high price considerably impede their wide-spread utilization.^6,10–13 Therefore, the exploration of non-noble-metal based effective bifunctional electrocatalysts with high stability for water electrolysis devices is highly desirable.

Among all the non-noble-metal based bifunctional electrocatalysts, Ni-based compounds^14,15 have received wide research attention. Especially for NiO^16,17 and Ni₂P,^18,19 many prominent researchers are committed to enhance their catalytic performance through various methods, such as doping with heteroatoms,^20,21 constructing novel structures,^22–24 creating vacancy defects^25–27 and so on. However, their electrocatalytic activities still fail to compete with those of noble-metal-based catalysts.^28,29 Nowadays, fabrication of amorphous nanomaterials with abundant active sites has been considered as a promising method to enhance the activity of electrocatalysts due to their unique characteristics of short-range order structure.^30,31 But the short-range order structure also leads to an unsatisfactory conductivity, which is the fatal weakness of amorphous materials compared with crystal materials.^32,33 Therefore, it is of great interest to rationally design a novel structure which can combine the virtues of amorphous and crystalline materials to improve the activity of catalysts.

According to previous research,^34–37 construction of hybrid catalysts with heterostructures has been a promising way to fabricate effective electrocatalysts. For example, Wu et al.³⁸ revealed that the coupling interface between MoS₂ and Fe₅Ni₄S₈ could facilitate the adsorption of H⁺ and OH⁻, according to density functional theory calculations. Zhu et al.³⁹ devised a new approach of developing a Co–Ni₃N heterostructure array and found that the heterostructure could facilitate electron transfer between the two different domains. Kim et al.⁴⁰ found that MoS₂ basal planes were activated by decorating with Ni₂P nanoparticles, so the Ni₂P/MoS₂/NRGO catalyst displayed Pt-like HER performance. Han et al.⁴¹ found that the Mn–Co–P@MnCo₂O₄ nanowire array only required 269 mV overpotential to drive a current density of 10 mA cm⁻² in 1 M KOH during the OER, which was 93 mV smaller than that of the MnCo₂O₄ nanoarray. Based on the above research, it can be safely concluded that hybrid catalysts with heterostructures can effectively combine two single-component catalysts and fully exert a synergistic effect, leading to improved physical and chemical properties compared to monolithic catalysts.

Herein, a series of novel hybrid catalysts with 3D heterostructures on Ni foam (NF) have been synthesized by electrodepositing amorphous NiO_x nanoflakes (A-NiO_x) on the surface of Mn doped-Ni₂P nanosheet arrays and acted as bifunctional catalysts for water electrolysis. The inner crystal Mn doped-Ni₂P nanosheet arrays which straightly grew on the NF substrate can effectively accelerate the transfer rates of electrons and ions, while the amorphous ultrathin NiO_x nanoflakes are beneficial to expose more active surface and reduce the blockage of the active sites of Mn doped-Ni₂P. Consequently, the optimal A-NiO_x-20/Mn₅-Ni₂P catalyst exhibits superior bifunctional electrocatalytic performance in alkaline medium, and only needs small overpotentials of 55 mV and 255 mV to afford 10 mA cm⁻² for the HER and OER, respectively. More importantly, a home-made two-electrode water electrolysis device by employing the A-NiO_x-20/Mn₅-Ni₂P catalyst as the anode as well as cathode electrode can drive 10 mA cm⁻² current density only at a small voltage of 1.54 V, demonstrating its greatly promising prospects in the water electrolysis industry.

2. Experimental section

2.1. Chemicals and materials

50% manganese nitrate solution (Mn(NO₃)₂), ammonium fluoride (NH₄F), potassium hydroxide (KOH), urea (CO(NH₂)₂), nickel acetate (Ni(CH₃COO)₂), ammonium acetate (CH₃COONH₄) and sodium hypophosphite (NaH₂PO₂) were all of analytical grade. The Pt/C catalyst (20 wt%) was bought from Alfa Aesar and deionized water was used throughout. Before use, NFs (thickness: 1 mm) were first treated with acetone and 1 M HCl solution to remove impurities and oxides on the surface, and then they were washed with deionized water and ethanol in turn for each three times.

2.2. Synthesis of the Mn_x-Ni₂P nanosheet arrays

7.5 mmol CO(NH₂)₂, 4 mmol ammonium fluoride and different amounts of Mn(NO₃)₂ (3, 5 and 7 mmol) were first dissolved in 35 ml distilled water. Then the aqueous solution and treated NFs (2 cm × 2 cm) were transferred to a 50 ml Teflon-lined stainless steel autoclave, heated to 120 °C and maintained at this temperature for 5 h. After the hydrothermal reaction, the NFs coated with the precursor of Mn_x-Ni₂P were taken out, rinsed with deionized water and dried at 60 °C. Afterwards, the Mn_x-Ni₂P (x denotes the mole of Mn(NO₃)₂ used during the hydrothermal process) was obtained by direct phosphorization of the above precursor at 400 °C for 1 h with a ramp rate of 2 °C min⁻¹ in a N₂ atmosphere using NaH₂PO₂ as the phosphorus source. Typically, the precursor of Mn_x-Ni₂P was put into a porcelain boat with 20 mmol NaH₂PO₂ placed 4 cm away from the precursor of Mn_x-Ni₂P on the upstream side. For comparison, the pure Ni₂P sample was also prepared under similar conditions of the Mn_x-Ni₂P sample except that 5 mmol Ni(NO₃)₂ was used to substitute Mn(NO₃)₂ during the hydrothermal process. The mass loadings of Mn₃-Ni₂P, Mn₅-Ni₂P, Mn₇-Ni₂P and pure Ni₂P were about 2.2, 2.3, 2.3 and 2.8 mg cm⁻².

2.3. Synthesis of the A-NiO_x/Mn₅-Ni₂P catalysts

A series of A-NiO_x/Mn₅-Ni₂P catalysts were fabricated by the constant current electrodeposition process in 100 ml of 0.01 M nickel acetate and 0.002 M ammonium acetate mixed aqueous solution with the above obtained Mn₅-Ni₂P as the working electrode, graphite rod as the counter electrode and standard Ag/AgCl as the reference electrode. After being electrodeposited at a cathodic current of 7.5 mA cm⁻² for different times (10 min, 20 min, and 30 min, respectively), the resulting samples were dried at 80 °C, and accordingly named as A-NiO_x-10/Mn₅-Ni₂P, A-NiO_x-20/Mn₅-Ni₂P, and A-NiO_x-30/Mn₅-Ni₂P, respectively. Furthermore, single A-NiO_x was also synthesized by direct electrodeposition on the bare NF under the same conditions for 20 min. The mass loadings of A-NiO_x-10/Mn₅-Ni₂P, A-NiO_x-20/Mn₅-Ni₂P, A-NiO_x-30/Mn₅-Ni₂P, and A-NiO_x were about 2.8, 3.6, 4.5 and 1.9 mg cm⁻², respectively.

2.4. Characterization

The microstructure and morphology of the samples were investigated by scanning electron microscopy (SEM, JEOL JSM-6330F) and transmission electron microscopy (TEM, JEOL 2010, 200 kV). Energy dispersive X-ray (EDX) spectroscopy was conducted to analyze the elemental distribution using a JEOL JSM-6330F SEM. X-ray diffraction (XRD) data were recorded on a Rigaku/Max-2550 with Co Kα radiation (λ = 1.7890 Å). X-ray photoelectron spectroscopy (XPS) scans were obtained on a multifunctional imaging electron spectrometer (Thermo ESCALAB 250XI).

2.5. Electrochemical tests

The HER and OER measurements were conducted on a CHI660E workstation (Shanghai, Chenhua) with a three-electrode system, by using the prepared catalytic electrode as the working electrode, a graphite rod as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The HER measurements were performed in 1 M KOH and the OER measurements were performed in O₂-saturated 1 M KOH. Linear sweep voltammetry (LSV) was conducted at a scan rate of 2 mV s⁻¹ for both the HER and OER. Before the test, 50 cyclic voltammetric (CV) cycles were conducted between 0.1 and −0.4 V vs. RHE for the HER and 1.2–1.6 V vs. RHE for the OER with a scan rate of 50 mV s⁻¹. The measured potentials were converted to RHE using eqn (1):

E_RHE = E_measured + 0.059 × pH + 0.242 (1)

Electrochemical impedance spectroscopy (EIS) measurements were performed with the same configuration at a bias of −100 mV in a frequency range from 10⁵ to 0.01 Hz. All of the potentials and voltages were iR compensated unless noted. The electrochemical double-layer capacitance (C_dl) of the catalysts was estimated by CV at different scan rates (10, 20, 40, 60, 80, and 100 mV s⁻¹) from 0.1–0.2 V vs. RHE. The C_dl value could be estimated from the slope of the resulting (j_a − j_c)/2 vs. v plots (j_a and j_c represent the anodic and cathodic current densities at 0.15 V vs. RHE). The long-term durability tests for the HER and OER were all performed using chronopotentiometric (CP) measurements at a constant current density of 10 mA cm⁻² for 20 h.

The overall water electrolysis measurement tests were conducted in a two-electrode system employing the prepared catalytic electrodes both as the cathode and anode. The LSV curves were recorded at a scan rate of 2 mV s⁻¹ without iR correction and the CP measurement was also performed at a current density of 10 mA cm⁻² for 20 h. Faradaic efficiency was calculated according to the reported methods.^42,43 The amounts of H₂ and O₂ during the overall water electrolysis at 10 mA cm⁻² (Ni foam: 1 × 1 cm²) were measured on a gas chromatograph (Trace 1300). Turnover frequency (TOF) for the HER was calculated using eqn (2):

TOF = jS/2Fn (2)

where j is the current density (A cm⁻²) at a given overpotential for the HER, S is the area (cm²) of the working electrode, F is the Faraday constant (96 485 C mol⁻¹), 2 is the number of electrons involved during the reaction, and n is the number of active sites.

According to the research of Hu et al.,⁴⁴ the number of active sites was calculated by the CV measurement with a scan rate of 50 mV s⁻¹ in a potential range of −0.2 to 0.6 V (vs. RHE) in 1 M PBS (pH = 7). Assuming a one electron redox process, the upper limit of active sites could be obtained according to eqn (3):

n = Q/2F (3)

where Q is the whole charge of the CV curve.

3. Results and discussion

3.1. Synthetic protocol and materials characterization

The fabrication processes of A-NiO_x-T/Mn_x-Ni₂P catalysts are illustrated in Scheme 1, including the hydrothermal process, high-temperature phosphorization and electrodeposition. The pretreated NFs with a smooth surface were used as the precursor, substrate and current collector. Firstly, the precursor of Mn_x-Ni₂P nanosheet arrays was straightly grown on the NFs by a hydrothermal self-growth process. Mn(NO₃)₂ was purposely chosen as the reactant because it can be decomposed to produce HNO₃ during the hydrothermal process, so the surface of NFs would be partly etched by the acid and Ni²⁺ ions would be released into the reaction solution. With the hydrothermal reaction proceeding, the Mn-doped Ni-carbonate hydroxide precursor nanosheets successfully grew on NFs. After that, the above self-grown nanosheet precursor was then used as the starting material and converted into highly crystalline Mn_x-Ni₂P nanosheets through a high-temperature phosphorization process at 400 °C. Then, the amorphous NiO_x nanoflakes were coated on the surface of Mn_x-Ni₂P nanosheets by a simple electrodeposition method to form the final A-NiO_x/Mn_x-Ni₂P hierarchical core–shell electrodes.

Scheme 1 Schematic synthesis procedure of the A-NiO_x-T/Mn₅-Ni₂P catalytic electrodes.

The morphology and crystalline structure of Mn_x-Ni₂P were first characterized by SEM and XRD. As shown in Fig. 1a–c, all the Mn_x-Ni₂P (x = 3, 5, and 7) samples present a similar morphology with little difference and the original smooth Ni foams are fully covered with aligned nanosheet arrays. The EDS result presented in Fig. S1† shows that the Ni, P and Mn elements exist in all Mn_x-Ni₂P samples and the ratio of Mn to P is about 8.1%, 11.2% and 16.0% in Mn₃-Ni₂P, Mn₅-Ni₂P and Mn₇-Ni₂P, respectively. Moreover, the XRD patterns of Mn₃-Ni₂P, Mn₅-Ni₂P and Mn₇-Ni₂P shown in Fig. 1d indicate that except the strong diffraction peaks of the NF substrate, the apparent four peaks at about 47.5°, 52.2° (overlapping with the diffraction peak of Ni), 55.4° and 63.6° can be indexed to the (111), (021), (210) and (003) planes of Ni₂P (JCPDS no. 65-3544). It is noteworthy that there are not any XRD diffraction signals belonging to manganese phosphide species in all Mn_x-Ni₂P materials, demonstrating that Mn is doped into Ni₂P. For comparison, pure Ni₂P nanosheet arrays on NFs were also synthesized and characterized by SEM, XRD and EDS (Fig. S2†). The in situ self-supported Mn_x-Ni₂P nanosheet arrays have not only good electrical conductivity but also excellent mechanical properties, making them a suitable skeleton for further electrodeposition of other catalytically active materials. In order to determine which one among all the Mn_x-Ni₂P samples and pure Ni₂P nanosheet arrays is suitable as the skeleton for further electrodeposition of amorphous NiO_x, we first evaluated their electrocatalytic performance from LSV curves. As shown in Fig. S3,† the Mn₅-Ni₂P sample has the best electrocatalytic activity for both the HER and OER at the same time. So the Mn₅-Ni₂P sample was chosen as the target material for further investigation.

Fig. 1 SEM images of (a) Mn₃-Ni₂P, (b) Mn₅-Ni₂P and (c) Mn₇-Ni₂P. (d) XRD patterns of Mn_x-Ni₂P.

The low-magnification SEM image (Fig. 2a) reveals that after the electrodeposition process for 20 min, the resulting A-NiO_x-20/Mn₅-Ni₂P sample also presented the morphology of nanosheet arrays, which is similar to the Mn₅-Ni₂P precursor. However, the high-magnification SEM image (Fig. 2b) definitely indicates that the nanosheet arrays are actually coated with continuous chiffon-like nanoflakes. The ultrathin nanoflakes were interconnected with each other to form a higher specific surface structure and complete electron and ion transfer paths. The XRD pattern presented in Fig. 2c shows that no diffraction peaks of NiO_x species can be detected for the A-NiO_x-20/Mn₅-Ni₂P sample after the electrodeposition process, indicating that the electrodeposited NiO_x is amorphous. However, the EDS spectra and corresponding elemental mapping images (Fig. 2d and S4†) clearly reveal the existence of Ni, Mn, P and O elements in the A-NiO_x-20/Mn₅-Ni₂P sample and the elements are distributed uniformly. The hybrid structure of A-NiO_x-20/Mn₅-Ni₂P was further proved by the TEM image (Fig. 2e) and HRTEM image (Fig. 2f). The distinct lattice fringes with a d-spacing of 2.212 Å can be attributed to the (111) plane of Ni₂P. The amorphous nature of outer NiO_x nanoflakes was also confirmed by the HRTEM image and the fast Fourier transform (FFT) image, which is in agreement with the above XRD result. The morphologies of A-NiO_x-10/Mn₅-Ni₂P and A-NiO_x-30/Mn₅-Ni₂P were also studied by SEM. As shown in Fig. S5a,† the morphology of A-NiO_x-10/Mn₅-Ni₂P is very similar to that of A-NiO_x-20/Mn₅-Ni₂P but with little difference in which the quantity of amorphous NiO_x nanoflakes is less than that of the A-NiO_x-20/Mn₅-Ni₂P sample. However, along with the further increase of the electrodeposition time to 30 minutes, more NiO_x was deposited on the surface of Mn₅-Ni₂P nanosheet arrays to finally form a layered nanocomposite structure which looks very similar to a dry ground (Fig. S5b†).

Fig. 2 (a and b) SEM images, (c) XRD pattern, (d) EDS spectrum, (e) TEM image and (f) HRTEM image of A-NiO_x-20/Mn₅-Ni₂P (insert of (f) is the FFT for the red framed region).

XPS was used to analyze the chemical surface electronic states of A-NiO_x-20/Mn₅-Ni₂P and Mn₅-Ni₂P samples (Fig. 3). As shown in Fig. 3a, the Mn 2p spectrum could be fitted to two main peaks around ∼643 eV and ∼654 eV assigned to Mn 2p_3/2 and Mn 2p_1/2,⁴⁵ while the peaks around ∼643 eV can be divided into two peaks at ∼641 eV and ∼645 eV, which represent Mn²⁺ and Mn³⁺, respectively.^45,46 In the Ni 2p region (Fig. 3b), the peaks around ∼853 eV and ∼870 eV can be attributed to the Ni 2p_3/2 orbitals in Ni₂P.⁴⁷ Moreover, the peaks around ∼855 eV and ∼873 eV with two satellite peaks around ∼861 eV and ∼879 eV can be attributed to the NiO_x species in A-NiO_x-20/Mn₅-Ni₂P. The peaks at the same position are also observed for Mn₅-Ni₂P but much lower than that of A-NiO_x-20/Mn₅-Ni₂P, which is due to the surface oxidation of Mn₅-Ni₂P.^47,48 This superficial oxidation phenomenon can also be observed in the P 2p region (Fig. 3c). The two peaks at ∼129 and ∼134 eV corresponding to the phosphide⁴⁹ and phosphate/phosphite are owing to the surface oxidation state of P.⁵⁰ High-resolution O 1s spectra are shown in Fig. 3d. The peak at ∼531.5 eV can be observed for Mn₅-Ni₂P, which is assigned to phosphate species,^32,51 while the additional strong peak at ∼530 eV corresponds to the oxide species in A-NiO_x-20/Mn₅-Ni₂P, also indicating that the outside surface nanoflakes are NiO_x species.^51,52

Fig. 3 The high resolution XPS spectra of A-NiO_x-20/Mn₅-Ni₂P and Mn₅-Ni₂P. (a) Mn 2p, (b) Ni 2p, (c) P 2p and (d) O 1s.

3.2. Electrocatalytic performance of the prepared catalysts

The HER performances of the prepared catalytic electrodes were investigated in 1 M KOH electrolyte. For comparison, the bare Mn₅-Ni₂P and single A-NiO_x electrodes were also studied under the same conditions. As presented in Fig. 4a, the iR compensated LSV curves clearly show that the A-NiO_x-20/Mn₅-Ni₂P electrode only requires a small overpotential (η) of 55 mV to afford a current density (j) of 10 mA cm⁻², which is obviously more outstanding than that of A-NiO_x-10/Mn₅-Ni₂P (η = 76 mV), A-NiO_x-30/Mn₅-Ni₂P (η = 65 mV), Mn₅-Ni₂P (η = 96 mV) and A-NiO_x (η = 126 mV). In order to avoid the interference of scan rate, controlled-current electrolysis (CCE) at different current densities was conducted for 1 h to determine the accurate overpotential for the HER.^53,54 As shown in Fig. S6,† the A-NiO_x-20/Mn₅-Ni₂P electrode can achieve 10, 50, and 100 mA cm⁻² at small overpotentials of about 55, 132 and 166 mV respectively, and performs better than many reported catalysts as shown in Table S1.† Moreover, the A-NiO_x-20/Mn₅-Ni₂P sample also displays a superior exchange current density (j₀ = 0.62 mA cm⁻²) and mass activity (20 mA mg⁻¹ at an overpotential of 150 mV) as shown in Fig. S7 and S8.† All the above results indicate the excellent HER activity of the A-NiO_x-20/Mn₅-Ni₂P electrode and prove that the hybridization of amorphous NiO_x and crystallized Mn₅-Ni₂P in a suitable manner can have a great promoting effect on the HER performance. The intrinsic activity of active sites was evaluated by TOF based on the electrochemical method.^55,56 As shown in Fig. S9,† the TOF value of A-NiO_x-20/Mn₅-Ni₂P is 0.81 s⁻¹ at an overpotential of 200 mV, which is about 2 and 4.5 times that of Mn₅-Ni₂P (0.40 s⁻¹) and A-NiO_x (0.18 s⁻¹), revealing that A-NiO_x-20/Mn₅-Ni₂P delivers higher intrinsic activity than the single-components of Mn₅-Ni₂P and A-NiO_x. Furthermore, A-NiO_x-20/Mn₅-Ni₂P also possesses a better Tafel slope of 37 mV dec⁻¹ than that of Mn₅-Ni₂P (59 mV dec⁻¹) and A-NiO_x (76 mV dec⁻¹), suggesting admirable HER kinetics (Fig. 4b). The HER kinetics were further verified by the EIS measurement (Fig. 4c). Naturally, the crystalline Mn₅-Ni₂P shows superior conductivity (R_ct = 4.0 Ω) to A-NiO_x with amorphous structure (R_ct = 6.1 Ω). However, the heterostructured A-NiO_x-20/Mn₅-Ni₂P electrode unexpectedly shows the smallest R_ct of 2.3 Ω among all three electrodes, which is attributed to the surface interconnected nanoflakes. They can join the independent Mn₅-Ni₂P nanosheets to form a complete charge transport network. Moreover, the C_dl was also estimated by the CV cycles (Fig. S10a–c†) to represent the electrochemically active surface area.^30,55 As shown in Fig. S10d,† the A-NiO_x-20/Mn₅-Ni₂P sample possesses the highest C_dl value of 66.2 mF cm⁻¹ which is much higher than that of A-NiO_x (33.2 mF cm⁻²) and Mn₅-Ni₂P (22.3 mF cm⁻¹). The above measurement results clearly prove that the novel heterostructure of A-NiO_x-20/Mn₅-Ni₂P can fully exert the advantages of crystal and amorphous materials, leading to the admirable intrinsic activity, excellent conductivity and abundant active sites. In view of the favorable HER activity of A-NiO_x-20/Mn₅-Ni₂P catalytic electrodes, the stability and durability were evaluated by long-term electrolysis at a current density of 10 mA cm⁻² (Fig. 4d). The overpotential of the A-NiO_x-20/Mn₅-Ni₂P electrode shows a negligible increase after the CP determination for 20 h, demonstrating its excellent durability and stability for the HER. The excellent stability also can be verified from another side by SEM, XRD and HRTEM. The morphology and crystal structure of the A-NiO_x-20/Mn₅-Ni₂P catalytic electrode are perfectly maintained after the durability test (Fig. S11†).

Fig. 4 (a) LSV curves of A-NiO_x-T/Mn₅-Ni₂P, Mn₅-Ni₂P, A-NiO_x and Pt/C for the HER. (b) Corresponding Tafel plots of A-NiO_x-T/Mn₅-Ni₂P, Mn₅-Ni₂P, A-NiO_x and Pt/C. (c) Nyquist plots of A-NiO_x-20/Mn₅-Ni₂P, Mn₅-Ni₂P and A-NiO_x at −100 mV vs. RHE (insert is the equivalent circuit). (d) Long-term HER stability test of A-NiO_x-20/Mn₅-Ni₂P at 10 mA cm⁻² (without iR correction).

Moreover, the high resolution XPS spectra shown in Fig. S12† demonstrate that the peaks at ∼855, ∼873 eV in the Ni 2p region and ∼530 eV in the O 1s region corresponding to NiO_x do not change obviously, implying that the amorphous shell catalyst is still NiO_x. However, the intensity of peaks at ∼853 and ∼129 eV belonging to Ni₂P obviously decreases and a peak at ∼530.5 eV attributed to the O–H appears, indicating that the surface of the Ni₂P core catalyst is partially oxidized into amorphous Ni(OH)_x. As displayed in Fig. S13,† the XRD result shows that only the crystal Ni₂P phase exists in A-NiO_x-20/Mn₅-Ni₂P after the HER, implying that the produced Ni(OH)_x and the NiO_x shell are all amorphous. The above results definitely demonstrate that the surface of the core catalyst Ni₂P is oxidized to amorphous Ni(OH)_x after the HER stability test. According to previous reports,^48,54 the thickness of Ni(OH)_x on the surface of Ni₂P is usually very thin, while as for our A-NiO_x-20/Mn₅-Ni₂P catalyst, the generated amorphous Ni(OH)_x was mixed with the amorphous NiO_x shell, so it is very hard to determine the thickness of the Ni(OH)_x layer.

The electrocatalytic performance of the as-prepared electrodes towards the OER was evaluated using the same three-electrode system in O₂-saturated 1 M KOH electrolyte. The three electrodes all have a distinct oxidation peak between 1.3 V and 1.5 V vs. RHE due to the oxidation of Ni species (Fig. 5a).⁵⁷ The A-NiO_x-20/Mn₅-Ni₂P catalytic electrode possesses the best OER activity, and only needs a small potential of 1.485 V to afford 10 mA cm⁻², much better than that of A-NiO_x-10/Mn₅-Ni₂P (1.52 V), A-NiO_x-30/Mn₅-Ni₂P (1.505 V), Mn₅-Ni₂P (1.53 V) and A-NiO_x (1.567 V). The CCE was also used to determine the accurate overpotential for the OER at different current densities. As shown in Fig. S14,† the A-NiO_x-20/Mn₅-Ni₂P electrode only requires small overpotentials of about 255, 295 and 320 mV to achieve 10, 50 and 100 mA cm⁻² respectively, and is superior to many reported OER catalysts in alkaline medium (Table S2†). A-NiO_x-20/Mn₅-Ni₂P also has high mass activity (63 mA mg⁻¹ at a small overpotential of 350 mV), outstanding kinetics (Tafel slope is 55 mV dec⁻¹) and remarkable stability (the overpotential negligibly increased after the 20 h CP measurement) during the OER process (Fig. 5b–d). The morphological and structural changes of the A-NiO_x-20/Mn₅-Ni₂P catalyst after the OER process were also investigated by SEM, HRTEM, XRD, XPS and EDS. As shown in Fig. S15a,† although some agglomeration took place and the surface of the nanosheets became rougher compared to the HER process, the hybrid composite nanostructure was still basically preserved after the OER process. However, the HRTEM image (Fig. S15b†) clearly shows a typical triple layer structure after the OER stability test. The outer layer is the amorphous material, and the middle and the inner layer are the crystal material with lattice spacings of 3.908 and 2.212 Å, corresponding well to the (006) plane of Ni(OH)₂ and the (111) plane of Ni₂P, respectively. The XPS results of the A-NiO_x-20/Mn₅-Ni₂P catalyst after the OER process are shown in Fig. S16.† The peaks at ∼855, ∼873 and ∼530 eV can also be observed obviously in Ni 2p and O 1s regions, indicating that the outer layer amorphous material is NiO_x. However, peaks at ∼853 and ∼129 eV corresponding to Ni₂P disappear, while a new peak at ∼530.5 eV ascribed to the O–H species appears, implying that the middle layer crystal Ni(OH)₂ is transformed from the core catalyst Ni₂P. This change can also be verified by the XRD test, and the intensity corresponding to the Ni₂P peaks clearly decreased and the other two diffraction peaks belonging to Ni(OH)₂ (JCPDS no. 38-0751) appeared (Fig. S17a†). Moreover, the EDS result shown in Fig. S17b† demonstrates that the intensity of the P peak obviously decreased while the intensity of the O peak greatly increased. The above results indicate that after the OER stability test, the surface of the core catalyst Ni₂P was oxidized to Ni(OH)₂, which is very common in phosphide-based catalysts for the OER and consistent with previous literature reports.^58,59

Fig. 5 (a) LSV curves of A-NiO_x-T/Mn₅-Ni₂P, Mn₅-Ni₂P and A-NiO_x for the OER. (b) Mass activity of A-NiO_x-T/Mn₅-Ni₂P, Mn₅-Ni₂P and A-NiO_x for the OER. (c) Tafel plots for A-NiO_x-T/Mn₅-Ni₂P, Mn₅-Ni₂P, A-and NiO_x. (d) Long-term OER stability test of A-NiO_x-T/Mn₅-Ni₂P at 10 mA cm⁻² (without iR correction).

Considering the outstanding bifunctional catalytic performance of the A-NiO_x-20/Mn₅-Ni₂P electrode, we further used the prepared electrode as the anode as well as the cathode to prepare a two-electrode device for the overall water splitting in alkaline electrolyte (1 M KOH). As displayed in Fig. 6a, only a small voltage of 1.54 V is needed for A-NiO_x-20/Mn₅-Ni₂P||A-NiO_x-20/Mn₅-Ni₂P to achieve a current density of 10 mA cm⁻², which is much smaller than that of Mn₅-Ni₂P||Mn₅-Ni₂P (1.65 V), A-NiO_x||A-NiO_x (1.71 V), and even some reported Ir/C||Pt/C couples (1.62 V).⁶⁰

Fig. 6 (a) LSV curves of A-NiO_x-20/Mn₅-Ni₂P||A-NiO_x-20/Mn₅-Ni₂P, Mn₅-Ni₂P||Mn₅-Ni₂P and A-NiO_x||A-NiO_x for overall water splitting (without iR correction). (b) Long-term overall water splitting stability test of A-NiO_x-20/Mn₅-Ni₂P||A-NiO_x-20/Mn₅-Ni₂P at 10 mA cm⁻²(without iR correction).

Furthermore, the faradaic efficiency of the alkaline electrolyzer was measured at a current density of 10 mA cm⁻² as shown in Fig. S18.† The measured molecular ratio of H₂ to O₂ is about 2 : 1 and the faradaic efficiency of A-NiO_x-20/Mn₅-Ni₂P is determined to be 100% for the HER and 98% for the OER, indicating that any competitive side reaction can be ruled out. Furthermore, the A-NiO_x-20/Mn₅-Ni₂P electrode was very stable during overall water splitting. As shown in Fig. 6b, the voltage of the electrolytic device at 10 mA cm⁻² showed no fluctuation for 20 h. The above results indicate that the A-NiO_x-20/Mn₅-Ni₂P electrode is a promising bifunctional electro-catalyst towards overall water splitting.

4. Conclusions

In conclusion, a series of hierarchical core–shell hybrid nanostructured A-NiO_x-T/Mn₅-Ni₂P catalytic electrodes have been prepared by growing amorphous NiO_x on highly crystalline and conductive Mn-doped Ni₂P nanosheet arrays by a simple and scalable method. Benefitting from the large surface area provided by the outer amorphous NiO_x and good conductivity offered by the inner Mn₅-Ni₂P nanosheets, the optimal A-NiO_x-20/Mn₅-Ni₂P electrode shows excellent bifunctional catalytic performance in alkaline electrolyte, and is better than the single-component counterparts and some reported electrocatalysts. Furthermore, the alkaline electrolytic device fabricated by using A-NiO_x-20/Mn₅-Ni₂P as both the cathode and anode only needs a small voltage to deliver 10 mA cm⁻² for at least 20 hours, indicating that it is a promising and practical candidate for overall water splitting and other energy related applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful to the Fundamental Research Funds for the Central Universities of China (no. DUT17LK01) and the National Natural Science Foundation of China (no. 51575088).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9se00396g

‡ Wen-Zhuo Zhang and Guang-Yi Chen contributed equally to this work.

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