Facile synthesis of nickel–copper hollow spheres as efficient bifunctional electrocatalysts for overall water splitting

Abstract

Sustainable energy conversion has motivated researchers to design and develop novel efficient bifunctional electrocatalysts for water splitting. In this work, we report the facile synthesis of Ni_xCu_y@C hollow spheres by using a template-free strategy. The activities of the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) are significantly enhanced by the hollow structure and optimal composition. The as-synthesized Ni₄Cu₂@C hollow spheres exhibit superior HER performance with overpotentials of 55 and 91 mV at 10 mA cm⁻² in alkaline and acidic media, and the low overpotential of 280 mV at 10 mA cm⁻² for the OER in alkaline media. When Ni₄Cu₂@C hollow spheres are employed as electrocatalysts for both the anode and cathode in an overall water splitting system, a cell voltage of only 1.49 V at 15 mA cm⁻² and long-term durability (50 h@40 mA cm⁻²) are achieved.

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Introduction

Hydrogen (H₂) possesses high gravimetric energy density and energy conversion efficiency, and is known as an ideal and promising sustainable energy candidate to replace fossil fuels.^1–4 Electrochemical water splitting is an efficient method of hydrogen production, which consists of two half reactions, including the hydrogen evolution reaction (HER) on the cathode and the oxygen evolution reaction (OER) on the anode. While platinum group metals (PGMs) such as Pt-, Ir-, and Ru-based materials are considered to be the state-of-the-art catalysts for the HER and the OER because of their high catalytic efficiency, their high cost and rareness severely hinder their wide application in water-splitting systems.^5–8

To address this issue, much effort has been made to develop earth-abundant, economic alternatives for efficient electrolytic water splitting,^9–12 and in particular the alloys and compounds containing first-row transition metals (e.g. Ni, Fe, and Co) have attracted tremendous interest owing to their earth abundance and remarkable HER and OER performances.^13–18 Nonetheless, the HER and OER performances of most Ni-based electrocatalysts are not comparable with that of the Pt/Ir or Pt/RuO₂ couple, thus limiting their practical applications in water electrolysis.^19–21

Recently, a number of hollow structure Ni-based nanomaterials, including nanochains,²² nanocages,²³ nanospheres,²⁴etc., have been synthesized to exhibit enhanced performance of energy conversion devices because the nanostructure can maximize the accessible active sites with a large surface area and a short diffusion path.^25–28 However, the synthetic processes based on hard-template and soft-template methods seem to be tedious and difficult to control.^29–33

Herein, we report a novel bimetallic Ni_xCu_y@C hollow sphere derived from the Ni_xCu_y-glycerate (Ni_xCu_y-G) hollow spheres fabricated via a facile template-free solvothermal method. Owing to the optimal composition and prominent structure, the as-obtained Ni₄Cu₂@C hollow spheres present outstanding HER electrocatalytic activity with overpotentials of 55 and 91 mV at 10 mA cm⁻² in alkaline and acidic media, and a low overpotential of 280 mV at 10 mA cm⁻² for the OER in alkaline media. When Ni₄Cu₂@C hollow spheres are employed as electrocatalysts for both the anode and the cathode in an overall water splitting system, a cell voltage of only 1.49 V at 15 mA cm⁻² and long-term durability (50 h@40 mA cm⁻²) can be achieved. These electrocatalytic properties are considerably better than most of the previously reported Ni-based electrocatalysts, making the Ni₄Cu₂@C hollow spheres versatile electrodes for highly efficient overall water splitting.

Experimental section

Chemicals and materials

Ni(NO₃)₂·6H₂O, Cu(NO₃)₂·3H₂O, glycerol, isopropanol, 5 wt% Nafion solution and carbon paper were purchased from Sinopharm Chemical Reagent Co., Ltd. 20 wt% Pt/C and Ir/C were purchased from Premetek Co.

Synthesis of Ni_xCu_y@C hollow spheres

Typically, 0.25 mmol of Ni(NO₃)₂·6H₂O, 0.125 mmol of Cu(NO₃)₂·3H₂O and 8 mL of glycerol were dissolved into 40 mL of isopropanol to form a transparent solution, which was then transferred to a Teflon-lined stainless steel autoclave and kept at 180 °C for 6 h. After being cooled to room temperature naturally, the precipitate was separated by centrifugation, washed several times with ethanol and dried in an oven at 60 °C. A series of Ni_xCu_y-G hollow spheres with different scales of cavities was obtained with different molar ratios of Ni(NO₃)₂·6H₂O and Cu(NO₃)₂·3H₂O. The as-prepared Ni_xCu_y-G precursors were placed in a tube furnace and heated at 500 °C for 1 h at a heating rate of 2 °C min⁻¹ under a H₂/Ar (5 : 95) atmosphere to obtain Ni_xCu_y@C hollow spheres.

Characterization

The structures and morphologies were obtained via transmission electron microscopy (TEM) (FEI, Tecnai G² 20 TWIN, 200 kV) and scanning electronic microscopy (SEM) (Zeiss, Ultra 55, 15 kV). The high-angle annular dark field-scanning transmission microscopy (HAADF-STEM) and energy dispersive spectrometry (EDS) were carried out on a JEOL, JEM-2100F. The crystallographic information was recorded by powder X-ray diffraction (XRD) (Bruker, D8 Advance diffractometer with Cu Kα radiation λ = 1.5406 Å). The parameters of XRD tests are listed: angular range = 10–80°, step size = 0.02°, and time per step = 0.2 s. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) (Thermo Scientific, iCAP 7400) was used to determine the Ni and Cu contents of the samples. X-ray photoelectron spectroscopy (XPS) was performed on a scanning X-ray microprobe (ULVAC-PHI, PHI 5000C & PHI 5300), using C 1s (284.8 eV) as a reference. The nitrogen adsorption–desorption experiments were operated on a Micromeritics ASAP 2020 system at 77 K. A laser of 532 nm wavelength was used to measure the Raman spectroscopy spectrum (Renishaw, RM 2000).

Electrochemical measurements

All electrochemical measurements were performed on a CHI 760E electrochemistry workstation (CH Instruments) at ambient temperature. The HER and OER catalytic activities were tested in a standard three-electrode system in 1 M KOH (pH 14) and 0.5 M H₂SO₄ (pH 0). A glassy carbon electrode (GCE) (area of 0.196 cm²), a saturated calomel electrode (SCE, in saturated KCl solution) and a graphite rod were used as the working electrode, reference and counter electrode, respectively. The catalyst ink was prepared by dispersing 3 mg of catalyst in 880 μL of ethanol and 120 μL of 5 wt% Nafion solution followed by ultrasonication for at least 30 min. Then, a certain volume of the catalyst ink was dropped on the glassy carbon electrode to reach the catalyst loading of 0.2 mg cm⁻². The HER performance was evaluated in H₂-saturated solution and the OER was in O₂-saturated solution. All potentials were converted versus a reversible hydrogen electrode (RHE): E(vs. RHE) = E(vs. SCE) + (0.242 + 0.059 pH) V. Prior to all the linear sweep voltammetry (LSV) experiments, a resistance test was applied to compensate the Ohmic potential drop. The level of iR compensation is 90%. LSV was performed at a scan rate of 5 mV s⁻¹ to obtain the polarization curves. Electrochemical impedance spectroscopy (EIS) measurements were performed with a frequency range from 0.1 to 100 000 Hz.

The values of turnover frequency (TOF) can be calculated assuming that all Ni and Cu ions in the catalysts are active and contributed to the catalytic reaction, since the exact number of active sites is not known (the lowest TOF values)

TOF = jS/(αFn)

here, j (A cm⁻²) is the measured current density at certain overpotential; S (GCE area, 0.196 cm²) is the surface area of the GCE; α is the number of electrons transferring in the HER or OER; F is the Faraday constant (96485.3 C mol⁻¹), and n is the metal ion molar number (both Ni and Cu) calculated from the ICP results of the as-prepared catalysts (Table S1, ESI†). Herein, the TOF was undervalued when all metal atoms were used to calculate it.³⁴

The faradaic efficiency was calculated by the equation

Faradic efficiency = nFm/Q

here, n = 2 and 4 for the HER and the OER respectively, F is the Faraday constant (96485.3 C mol⁻¹), m is moles of gas evolved, and Q (C) is the total amount of charge passed through the cell.

Overall water splitting tests were performed in a two-electrode system with the Ni₄Cu₂@C drop casted on pre-treated carbon paper as both the cathode and the anode in 1 M KOH. Before being used, carbon paper was cleaned by diluted HNO₃, deionized water, and acetone with ultrasonication for 5 min in each step. Then the carbon paper was immersed in concentrated H₂SO₄ for 1 h, followed by washing with deionized water and drying at 60 °C overnight. The mass loading was 1.5 mg cm⁻². For the benchmark noble metal-based catalysts, Ir/C/CP(+)‖Pt/C/CP(−) was adopted.

Results and discussion

Uniform Ni_xCu_y-G hollow spheres are synthesized via a facile one-pot solvothermal method. Fig. 1 shows the typical formation of Ni_xCu_y-G hollow spheres via transmission electron microscopy (TEM) and energy dispersive spectrometry (EDS). At stage I, uniform and spherical solid spheres are obtained. A compositional line profile shows that the amount of Cu in the internal domain is greater than Ni. The distributional difference of Ni and Cu can be ascribed to the faster nucleation rate of Cu with glycerol.^35–37 Therefore, slower nucleation of Ni with glycerol results in the higher content of Ni on the outer side. By extending the reaction time, the cavity can be observed at stage II. The formation of the cavity can be attributed to the diffusion rate difference (J_Cu − J_Ni) between Cu and Ni. Under solvothermal conditions, considering the difference of elemental distribution, the outward diffusion of Cu species is faster than the inward diffusion of Ni species in the nanospheres, thus generating internal voids.^38–40 On the completion of the reaction, the Ni_xCu_y-G spheres with enlarged hollow interiors are obtained at the end of stage III. Elemental mapping images distinctly show the homogeneous distribution of Ni, Cu, C and O on Ni₄Cu₂-G (Fig. S1, ESI†).

Fig. 1 Schematic illustration of the formation process of Ni_xCu_y-G hollow spheres.

Apart from the duration time at 180 °C, another crucial factor in the formation of hollow spheres is the Ni/Cu molar ratio. As shown in Fig. 2, solid spheres are obtained without the addition of Cu. At 4/1 for the Ni/Cu ratio, a small cavity can be seen. The hollow interior becomes bigger with more Cu content. The Ni₄Cu₃-G presents more obvious hollow structures. This is because higher Cu content has much faster diffusion rate, thus resulting in greater internal voids.

Fig. 2 TEM images of (a and e) Ni-G, (b and f) Ni₄Cu₁-G, (c and g) Ni₄Cu₂-G and (d and h) Ni₄Cu₃-G.

The Ni_xCu_y-G precursors could be converted into highly crystalline Ni_xCu_y alloys after annealing in a reducing atmosphere. Fig. 3 presents the evolution of the nanostructure along with the decreasing molar ratio of Ni/Cu. Compared to the Ni_xCu_y-G precursors, the Ni_xCu_y@C samples reveal spherical structures with more unambiguous cavities. Based on the results of N₂ sorption isotherms (Fig. S2, ESI†), the surface specific area of the Ni@C is smallest because of the solid structure. With the higher Cu content, the bigger cavity results in a larger specific area.

Fig. 3 SEM and TEM images of (a, e and i) Ni@C, (b, f and j) Ni₄Cu₁@C, (c, g and k) Ni₄Cu₂@C and (d, h and l) Ni₄Cu₃@C.

The XRD patterns demonstrate that the Ni_xCu_y@C can be indexed to Ni (JCPDS No. 65-0380) (Fig. 4a). No additional peaks can be observed after the incorporation of copper, indicating the pure phase of the crystal structure. However, the peaks (111) and (200) shift to the lower angle by increasing the amount of Cu in Ni_xCu_y@C, suggesting that Cu is indeed incorporated into Ni to form a Ni_xCu_y alloy.^41,42 In addition, XPS analysis (Fig. S3a, ESI†) indicates that the Ni 2p_3/2 region displays two dominant peaks of Ni⁰ and Ni–O in which the Ni–O species is ascribed to the inevitable surface oxidation exposed in the air.^43–45 The high-resolution Cu 2p_3/2 spectra can be deconvoluted into two major peaks centered at 932.6 and 934.5 eV, as shown in Fig. S3b (ESI†), corresponding to Cu⁰ and Cu–O species.^46,47 Taking Ni₄Cu₂@C hollow spheres as an example, the high-resolution TEM (HRTEM) presents the lattice fringes of 0.204 and 0.177 nm (Fig. 4c), which can be assigned to the interplane spacing of the (111) and (200) planes of nickel.^41,48,49 The diffusing concentric rings in the selected area electron diffraction (SAED) patterns indicate the polycrystalline structure (Fig. 4c inset). The diffraction rings can be indexed to Ni, in agreement with the XRD analysis. The HAADF-STEM image and the corresponding elemental mapping distinctly show the homogeneous distribution of Ni, Cu, and C (Fig. 4d), indicating uniform doping of Cu into Ni. Raman spectra reveal two characteristic peaks at about 1350 and 1590 cm⁻¹ (Fig. S4, ESI†), corresponding to D and G bands of graphitic carbon, indicating that the nanoparticles are covered with graphitic carbon.^50–52 Graphitic carbon encapsulating on the surfaces can impart the NiCu-alloys with fast electron transfer ability to enhance electrocatalytic activity and to protect from etching during the water splitting tests.^51,53,54

Fig. 4 (a) Typical XRD patterns of Ni_xCu_y@C with different Ni/Cu molar ratios of (I) Ni@C, (II) Ni₄Cu₁@C, (III) Ni₄Cu₂@C, and (IV) Ni₄Cu₃@C; (b) the corresponding zoomed-in regions showing evolution with Cu-incorporation; (c) high-resolution TEM image and SAED (inset) of Ni₄Cu₂@C and (d) HAADF-STEM and the corresponding elemental mapping images of Ni₄Cu₂@C.

To investigate the HER performance in basic electrolyte, the as-prepared Ni@C and Ni_xCu_y@C hollow spheres were loaded on the GCE in a standard three-electrode system. Fig. 5a displays the polarization curves with iR compensation obtained in 1 M KOH, with the commercial 20 wt% Pt/C catalyst for comparison. With the overpotential at the current density of 10 mA cm⁻² (η₁₀) as an indicator, the electrocatalytic performances vary with the molar ratios of Ni/Cu in Ni_xCu_y@C. The original Ni@C doesn’t display any catalytic activity for the HER in basic electrolyte. Incorporating a small amount of Cu can lead to improved performance such as Ni₄Cu₁@C. Furthermore, increasing the Cu addition reveals much better HER catalytic activity, e.g, for Ni₄Cu₂@C with as low as 55 mV overpotential to achieve a current density of 10 mA cm⁻², which is better than those of Ni₄Cu₁@C (152 mV) and Ni₄Cu₃@C catalysts (271 mV). Although 55 mV overpotential for Ni₄Cu₂@C is still not comparable to commercial Pt/C, it has outperformed most of the state-of-art HER electrocatalysts in 1 M KOH (Fig. 5g and Table S2, ESI†). The Tafel plot (η versus log j, η is the overpotential and j represents the current density) of Ni₄Cu₂@C is fitted to be 110 mV dec⁻¹, which is smaller than those of Ni@C (635 mV dec⁻¹), Ni₄Cu₁@C (137 mV dec⁻¹), and Ni₄Cu₃@C (161 mV dec⁻¹), further confirming its rapid HER reaction electrocatalytic kinetics. The turnover frequency (TOF) is another activity metric to gain insight into the intrinsic activity,^23,55,56 which can be calculated by assuming that all the metal sites (both Ni and Cu) in the catalysts are active. The Ni₄Cu₂@C has a TOF as high as 0.094 s⁻¹, at an overpotential of 100 mV (Fig. 5c), which is much higher than those of Ni@C (0.002 s⁻¹), Ni₄Cu₁@C (0.019 s⁻¹), and Ni₄Cu₃@C (0.006 s⁻¹). Remarkably, the TOF of Ni₄Cu₂@C can be boosted to 0.169 s⁻¹ at an overpotential of 150 mV, exhibiting the largest enhancement (0.075 s⁻¹) among other Ni_xCu_y@C catalysts. The Ni₄Cu₂@C presents remarkable activity towards the HER because of the following reasons: (i) the hollow structure provides a large specific surface area with more exposed active sites, simultaneously enhancing the ion transfer. (ii) Improved electrical conductivity is induced by the introduction of Cu, which can create more free electrons.^46,57 (iii) Moderate H adsorption free energy is another critical factor to determine the catalytic activity. After the optimal introduction of Cu, the binding strength of M–H_ad (M represents the surface active site) is weakened, facilitating the hydrogen adsorption and desorption processes.^29,41,57,58 However, the HER activity will decrease if the Ni/Cu ratio is further increased to 4/3, because Cu is less active than Ni in the water splitting system, and excessive Cu on the surface would prevent active sites with high activity from being exposed, resulting in less competitive performance.^24,59 To eliminate the contribution of the size, shape or morphology to electrocatalytic activity, the polarization curves have been normalized by ECSA (Fig. S5 and S6, ESI†).^60,61 As shown in Fig. S6b (ESI†), the Ni₄Cu₂@C still considerably outperforms other catalysts especially the original Ni, further confirming that the introduction of Cu plays a crucial role in improving the HER activity of Ni. Also, the EIS measurements reveal that the Ni_xCu_y@C electrocatalysts present small charge transfer resistance, suggesting fast electron transfer during the HER process (Fig. S7, ESI†). Especially after the introduction of Cu, R_ct decreases dramatically from 107.8 Ω (Ni@C) to 36.07 Ω (Ni₄Cu₂@C), indicating the promotion of charge transfer (Table S3, ESI†).

Fig. 5 HER polarization curves, the corresponding Tafel plots and TOFs obtained in (a–c) 1 M KOH, and (d–f) 0.5 M H₂SO₄. Comparison of the overpotential required at 10 mA cm⁻² with other recently reported HER electrocatalysts obtained in (g) 1 M KOH, (h) 0.5 M H₂SO₄ and (i) both acidic and alkaline electrolytes.

Moreover, the HER performance of the as-prepared samples in acidic medium was also investigated. Fig. 5d and e display the iR-corrected polarization curves of Ni_xCu_y@C samples and 20 wt% Pt/C in 0.5 M H₂SO₄, and the corresponding Tafel plots are employed to reveal the HER reaction mechanism. Obviously, the benchmark 20 wt% Pt/C still shows excellent performance toward the HER, which only needs the overpotential of 21 mV to achieve the current density of 10 mA cm⁻² and yields a small Tafel slope of 14 mV dec⁻¹. The Ni@C also shows relatively poor HER activity and a large Tafel slope. After the optimal amount of Cu incorporated into Ni, to drive the current density of 10 mA cm⁻², the applied overpotential for the Ni₄Cu₂@C is 91 mV, which is superior to other advanced HER electrocatalysts in 0.5 M H₂SO₄ (Fig. 5h and Table S4, ESI†). The Tafel slope value of Ni₄Cu₂@C is lower than that of the other Ni_xCu_y@C electrocatalysts, indicating much faster HER kinetics. The TOF values of Ni₄Cu₂@C are 0.057 and 0.137 s⁻¹ at the overpotential of 100 and 150 mV, which are the highest among other Ni_xCu_y@C samples, implying excellent intrinsic activity (Fig. 5f). Polarization curves normalized by ECSA further confirm the intrinsic activity of Ni_xCu_y@C (Fig. S8 and S9, ESI†), excluding the structural effect. R_ct decreases dramatically from 195 Ω (Ni@C) to 28.64 Ω (Ni₄Cu₂@C), indicating the promotion of charge transfer after the optimal introduction of Cu (Fig. S10 and Table S5, ESI†).

Based on the above results and analyses, Ni₄Cu₂@C as a promising electrocatalyst reveals outstanding HER performance both in alkaline and acidic electrolytes. Compared to the previously reported electrocatalysts (Fig. 5i and Table S6, ESI†), the Ni₄Cu₂@C is highly active to deliver 10 mA cm⁻² current at 55 and 91 mV in 1 M KOH and 0.5 M H₂SO₄. Ni₄Cu₂@C presents almost 100% faradaic efficiency for the HER in both alkaline and acidic media via the water–gas displacing method, indicating that negligible by-products are formed during the electrolysis reaction (Fig. S11, ESI†). In addition, the stability test of the Ni₄Cu₂@C electrocatalyst was evaluated for an advanced electrocatalyst under various pH values. By chronopotentiometry analysis (Fig. S12, ESI†), the electrode stabilizes around the given potential (−0.055 V in 1 M KOH and −0.091 V in 0.5 M H₂SO₄) for about 20 h.

The Ni_xCu_y@C electrocatalysts are further investigated as OER electrocatalysts in 1 M KOH electrolyte. In contrast to Ni@C (407 mV), Ni₄Cu₁@C (372 mV), Ni₄Cu₃@C (429 mV) and other previously reported catalysts (Fig. 6a, b and Table S7, ESI†), the Ni₄Cu₂@C (283 mV) has a great performance at 10 mA cm⁻², even outperforming the commercial Ir/C catalyst (405 mV). Similarly, the Tafel slope of Ni₄Cu₂@C is superior to other Ni_xCu_y@C samples, indicating the fastest kinetics of mass and electron transfer (Fig. 6c). The TOF value of the Ni₄Cu₂@C is 0.055 s⁻¹ at an overpotential of 300 mV (Fig. 6d), which is much higher than those of Ni@C (0.017 s⁻¹), Ni₄Cu₁@C (0.025 s⁻¹), and Ni₄Cu₃@C (0.016 s⁻¹). And the TOF value of Ni₄Cu₂@C can be further boosted to 0.079 s⁻¹ at an overpotential of 350 mV. These results further suggest that the Ni₄Cu₂@C possesses excellent catalytic performance toward the OER. The overall OER process includes four elementary steps as follows:

OH⁻ + M → M–OH + e⁻ (1)

M–OH + OH⁻ → M–O + H₂O + e⁻ (2)

M–O + OH⁻ → M–OOH + e⁻ (3)

M–OOH + OH⁻ → M + O₂+ H₂O + e⁻ (4)

Fig. 6 (a) OER polarization curves, (b) comparison of the overpotential required at 10 mA cm⁻² with other recently reported OER electrocatalysts, (c) the corresponding Tafel plots, (d) TOFs, (e) OER polarization curves normalized by ECSA and (f) chronopotentiometry response of Ni₄Cu₂@C hollow spheres at the current density of 10 mA cm⁻².

The formation from M–O to M–OOH intermediates (step 3) is regarded as the rate-limiting step. The optimal Ni/Cu ratio can endow Ni_xCu_y@C with better electrical conductivity and generation of more active M–O species during the OER.^62–64 Therefore, Ni₄Cu₂@C presents excellent catalytic performance toward the OER rather than other Ni_xCu_y@C catalysts. By eliminating the effect of specific nanostructures, the polarization curves normalized by ECSA (Fig. 6e and Fig. S13, S14, ESI†) also demonstrate the highest activity of Ni₄Cu₂@C. The measured EIS also confirms the effect of Cu introduction on the charge-transfer resistance decreasing from 21.04 Ω for Ni@C to 7.78 Ω for Ni₄Cu₂@C (Fig. S15 and Table S8, ESI†). The Faradaic efficiency of Ni₄Cu₂@C is measured to be about 99.0% for 60 min electrolysis, reflecting high OER efficiency (Fig. S16, ESI†). Besides the high activity, excellent stability of the electrocatalysts toward the OER is also critical. Under the given potential (1.513 V) in 1 M KOH, the Ni₄Cu₂@C can maintain the current density of 10 mA cm⁻² for 20 h without any decline (Fig. 6f).

Encouraged by the highly catalytic activity of the Ni₄Cu₂@C hollow spheres toward both the HER and OER in alkaline solution (Fig. 7a and Table S9, ESI†), we further utilized it as a bifunctional electrocatalyst for overall water electrolysis in a two-electrode system in 1 M KOH. Fig. 7b shows the polarization curves measured at a scan rate of 5 mV s⁻¹. The Ni₄Cu₂@C affords a current density of 15 mA cm⁻² at a cell voltage of as low as 1.49 V, comparable to commercial Ir/C-Pt/C (1.54 V). The high activity of Ni₄Cu₂@C for overall water splitting manifests the compatible integration of the HER and the OER.^65–67 The excellent performance also outperforms most of the recently reported bifunctional electrocatalysts (Table S10, ESI†). Moreover, the Ni₄Cu₂@C exhibits excellent stability upon long-term testing under 40 mA cm⁻² with only a very slight deactivation (∼0.6%) over 50 h (Fig. 7c). Besides, after the long-term test, the polarization curves show no obvious shift (Fig. 7d). In order to inspect if there is any change in the morphology or composition of the Ni₄Cu₂@C hollow spheres after the prolonged water splitting reaction, TEM, XRD, and XPS characterization studies were performed. On the cathode side, TEM images show that the morphology of Ni₄Cu₂@C is well maintained (Fig. S17a and b, ESI†). HRTEM and XRD analyses also reveal no change in material composition (Fig. S17c and d, ESI†). The surface species of Ni⁰ and Cu⁰ can still be recognized after a long-term test (Fig. S17e and f, ESI†), indicating that Ni₄Cu₂ is stable in basic solutions during the HER.^42,46,58 On the anode side, the nanostructure of Ni₄Cu₂@C also remains intact (Fig. S18a and b, ESI†). However, by HRTEM analysis (Fig. S18c, ESI†), an amorphous layer is found on the surface of the hollow spheres, which is supposed to be the hydroxide species produced from alloys during electrolysis.^62,68 Meanwhile, XRD patterns show no significant difference (Fig. S18d, ESI†), implying that the crystal structure is well-retained. XPS also displays that the surface metallic species of nickel and copper have disappeared and transformed into oxidized species (Fig. S16e and f, ESI†). Such in situ formed surface hydroxide species are considered to be the catalytic sites for the oxidation of OH⁻ to O₂.^69–71 Such excellent performance and durability render the Ni₄Cu₂@C electrocatalyst a promising alternative to noble-metal electrocatalysts for water splitting in practical applications.

Fig. 7 (a) Comparison of the overpotential required at 10 mA cm⁻² with other recently reported bifunctional electrocatalysts obtained in 1 M KOH, (b) polarization curves of overall water splitting in a two-electrode system, (c) chronopotentiometry response of Ni₄Cu₂@C hollow spheres at the current density of 40 mA cm⁻² (inset: the production of H₂ and O₂ bubbles on the surface of the electrode) and (d) polarization curves of Ni₄Cu₂@C recorded before and after the 50 h chronopotentiometry test.

Conclusion

In summary, we have designed and synthesized a novel nickel-copper alloy with a hollow structure and tunable composition via a facile template-free solvothermal method. Benefiting from the large surface area and the introduction of Cu, the Ni_xCu_y@C hollow spheres with optimized molar ratios of Ni/Cu = 4/2 exhibit superior HER and OER performances, which can afford a current density of 10 mA cm⁻² at small overpotentials of 55 and 283 mV in 1 M KOH, respectively. The Ni₄Cu₂@C bifunctional catalyst electrode requires an applied voltage of only 1.49 V at a current density of 15 mA cm⁻² to realize overall water splitting, outperforming most of the previously reported non-noble metal bifunctional electrocatalysts. This facile template-free strategy can hopefully be extended to other functional materials with different compositions and nanostructures for applications in energy storage and conversion.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We appreciate the financial support provided for this research by the National Key Research and Development Program of China (2017YFA0204600) and the National Natural Science Foundation of China (51721002 and 51673045).

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Footnote

† Electronic supplementary information (ESI) available: Characterization data (TEM, SEM, Raman spectra, XRD, XPS, electrochemical measurement tests, and so forth) and comparison of the catalytic activity of various bifunctional electrocatalysts. See DOI: 10.1039/c9qm00697d

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