Facile fabrication of a 3D network composed of N-doped carbon-coated core–shell metal oxides/phosphides for highly efficient water splitting

Abstract

Development of robust, bifunctional, and non-precious catalysts for oxygen and hydrogen evolution reactions (OER and HER) is a prerequisite to realizing the overall splitting of water. This, however, remains a great challenge. In this context, we fabricated a novel three-dimensional (3D) network comprising N-doped carbon-coated core–shell NiFeO_x@NiFe–P (denoted as NC–NiFeO_x@NiFe–P) by two-pot high-temperature phosphorization and surface oxidation of a NiFe-Prussian blue analogue/polyvinylpyrrolidone (denoted as NiFe–PBAs/PVP) hybrid precursor. The as-synthesized NC–NiFeO_x@NiFe–P catalyst demonstrated exceptional performance for both OER and HER, offering a current density of 10 mA cm⁻² (a metric related to solar fuel) at small overpotentials of 285 mV for the OER and 237 mV for the HER in 1 M KOH, respectively. As expected, a NC–NiFeO_x@NiFe–P based alkaline electrolyzer with durability of 20 h was manufactured to achieve 10 mA cm⁻² at a voltage of 1.59 V, outperforming most non-precious metal-based electrolyzers. The exceptional performance could be attributed to the unique 3D network composed of core–shell NiFeO_x@NiFe–P and highly conductive N-doped carbon (NC), which provided a large amount of highly active sites for both OER and HER and favored fast electron transport during electrocatalytic processes.

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Introduction

As a clean and highly efficient energy, hydrogen has been widely recognized as a desirable candidate instead of fossil fuels to slow down the increasingly severe energy crisis and environmental pollution. The electrochemical splitting of water is an effective and easy way to produce hydrogen.¹ However, the anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER) in such water splitting often need a large overpotential to achieve preferable reaction rates.² To address these issues, excellent Pt- and Ru/Ir-based catalysts were developed for the HER and OER, respectively.³ Unfortunately, the scarcity and high-cost of those precious metals greatly hinder their actual application. Besides, because of the extraordinarily large overpotential required for the OER and vulnerability of most non-precious OER catalysts in acidic media, most of the water splitting reactions were performed in alkaline media.⁴ However, non-precious metal-based catalysts (i.e. MoS₂ and MoC₂) for the HER,⁵ which perform well in acidic media, often suffer from low-activity in alkali electrolytes. Therefore, development of non-precious metal-based electrocatalysts qualified for both OER and HER in alkali electrolytes is highly imperative for overall water splitting. This, however, still remains a challenging work.

Due to the advantages of high-activity and low-cost, non-precious transition metal phosphides (TMPs) have emerged as a new family of promising OER and/or HER catalysts. For example, nickel, cobalt and iron phosphides exhibited excellent performance for the HER with low overpotentials in acid or alkali electrolytes.⁶ Afterwards, surface oxidized NiP₂ was found to possess significantly improved OER activity due to the core–shell structure of NiP₂/NiO_x.⁷ Improved performance was also found for oxidized CoP₂ and CoMnP catalysts.⁸ Despite great achievements made on the use of these TMPs as catalysts for the OER, they still need a large overpotential (>300 mV) to reach a current density of 10 mA cm⁻². Moreover, previous reports indicated that coordination of different metal sites could improve the electroactivity by improving the adsorption of electrochemical intermediates.⁹ In this context, effective combination of different metal phosphides may be a promising way to further improve the electrocatalytic performance. Recently, a N, P-co-doped three-dimensional (3D) porous network carbon nanomaterial has shown great potential for both OER and HER.¹⁰ Therefore, in light of the possible electrocatalytic synergy, incorporation of surface oxidized binary TMPs and N-doped 3D network carbon may further enhance the activity for the OER and HER, and the new catalyst will be highly suitable for water splitting. Unfortunately, such special incorporation is still scarce.

Prussian blue analogues (PBAs), denoted as M₃^II[M^III(CN)₆]₂·nH₂O, are a family of well-organized metal–organic frameworks (MOFs), which contain transition metals (i.e., Co, Ni, Fe) as metallic nodes and CN⁻ as linkers.¹¹ By virtue of the variety in their chemical composition and microstructure, PBAs have been desirable precursors to construct non-precious metal-based electrocatalysts with elaborate architecture.¹² Inspired by the above results, herein, an easy yet valid strategy was adopted to fabricate a 3D network comprising N-doped carbon-coated core–shell NiFeO_x@NiFe–P (denoted as NC–NiFeO_x@NiFe–P) through two-step high-temperature phosphorization and surface oxidation of a NiFe–PBAs/PVP hybrid precursor. Intriguingly, we found that the outer PVP layer in the precursor could not only transform to N-doped carbon but also prevent the aggregation of the resulting mixed Ni/Fe phosphide (NiFe–P) nanoparticles (NPs) during high-temperature phosphorization, thus promoting generation of the well-ordered 3D network structure. Consequently, the as-synthesized NC–NiFeO_x@NiFe–P catalyst displayed high activity for both OER and HER in 1 M KOH. Based on electrocatalytic results, the excellent performance could be attributed to the following three reasons: (1) the well-organized 3D network with high conductivity giving rise to exposure of more active sites and facilitating the electron transfer during the OER and HER; (2) the highly active centers on both mixed Ni and Fe oxide (NiFeO_x) shell and N-doped carbon; (3) the NiFe–P core providing sites to adsorb the H atom during the HER^6b and facilitating the proton transfer during the OER.¹³ Notably, an alkaline electrolyzer that achieved 10 mA cm⁻² at a voltage of 1.59 V with durability of 20 h was successfully achieved by employing the NC–NiFeO_x@NiFe–P catalyst as both the cathodic and anodic electrocatalysts, providing a promising candidate for water splitting.

Experimental section

The synthesis of electrocatalysts

The NiFe–PBA/PVP hybrid composites were synthesized by a modified coprecipitation method. Solution A: 1.4928 g Ni(CH₃COOH)₂·4H₂O and 12 g PVP were dissolved in 100 ml alcohol to form a mixed solution. Solution B: 1.3270 g K₃[Fe(CN)₆] was dissolved in 20 ml deionized water to form another solution. Then solution B was added dropwise into solution A under intense stirring at room temperature. The resulting aqueous solution was aged for 2 h, and the precipitate was then collected by centrifugation without washing. The precipitate was dried at 80 °C for 24 h and is denoted as NiFe–PBA/PVP. After that, NaH₂PO₂·H₂O was selected as the phosphorus source and fully mixed with the above NiFe–PBA/PVP at the mass ratio of 1 : 8 by grinding. Then, the above mixture was calcined at 400 °C and 700 °C for 2 h, respectively, under an Ar atmosphere. Finally, the calcined product (0.2 g) was washed in 100 ml deionized water at 50 °C for 24 h under stirring to remove the sodium phosphate derived from NaH₂PO₂·H₂O. At the same time, mixed Ni/Fe layers were formed on the surface of NiFe–P NPs due to the oxidation by air, and the final product is denoted as NC–NiFeO_x@NiFe–P. Besides, to synthesize NC–NiFe–P without the NiFeO_x shell, the above calcined product (0.2 g) was also washed in 100 ml deoxygenized water at 50 °C under an Ar atmosphere for 24 h.

For comparison, 0.2 g of the NC–NiFeO_x@NiFe–P catalyst was dispersed in 80 ml deionized water. The resulting suspension was then transferred into a 100 ml Teflon-lined stainless-steel autoclave, which was heated at 100 °C for 12 h and then cooled to ambient temperature naturally. Finally, the product was collected by centrifugation, and is denoted as NC–NiFeO_x@NiFe–P-100. The NiFe–PBA precursor and resulting NiFeO_x@NiFe–P catalyst were synthesized through the same process as that for the NiFe–PBA/PVP precursor and NC–NiFeO_x@NiFe–P catalyst but without the addition of PVP, respectively. The NC–NiFe was synthesized in a similar manner to that for NiFeO_x@NiFe–P without addition of NaH₂PO₂·H₂O during calcination.

Characterization

The X-ray diffraction (XRD) data of various samples were recorded using a D8 ADVANCE diffractometer with a graphite-filtered Cu_Kα source (λ = 0.15418 nm) over a 2θ range of 5–70°. Field emission scanning electron microscopy (FESEM) was performed using a JEOL JSM-7800F. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were recorded using a JEOL 2100 electron microscope at an acceleration voltage of 200 kV. HAADF-STEM-EDS images were collected on a JEM-2100F instrument with an EDS system. Raman spectroscopy was performed using a microscopic confocal Raman spectrometer (Jobin Yvon Horiba HR800) at room temperature. Elemental contents were determined using a Vario EI elemental analyzer. X-ray photoelectron spectroscopy (XPS) data were collected on a Thermo VG ESCALAB 250 X-ray photoelectron spectrometer with Al_Kα X-ray radiation. Binding energies of different elements were corrected based on the graphitic C 1s peak at 284.6 eV.

Electrochemical characterization

The electrochemical performance of as-synthesized catalysts towards the OER and HER was studied on an electrochemical workstation (CHI 760 E) in 1 M KOH electrolyte. All electrochemical measurements were conducted on a three-electrode system with carbon electrodes as counter electrodes, saturated calomel as reference electrodes and catalyst modified glassy carbon electrodes (geometric surface area: 0.07 cm²) as working electrodes. The catalyst inks were prepared by dispersing as-synthesized catalysts and Nafion in absolute ethanol under 0.5 h sonication. Then the catalyst inks were loaded on glassy carbon electrodes with catalyst loading of 0.2 mg cm⁻². Electrochemical impedance spectroscopy (EIS) was carried out from 1 MHz to 1 Hz at a potential of 1.53 V for the OER and −0.2 V for the HER, respectively, in 1 M KOH solution. Cyclic voltammetry (CV) was performed on the NC–NiFeO_x@NiFe–P catalyst at a scan rate of 100 mV s⁻¹ for 2000 cycles in 1 M KOH to study the cycling durability for the OER. As for the two-electrode system in the overall splitting of water, the as-synthesized catalysts (i.e. NiFeO_x@NiFe–P, NC–NiFe, and NC–NiFeO_x@NiFe–P) were dispersed in the mixed solution of ethanol and Nafion, the resulting solution was dropped on nickel foam (1 × 1 cm) with catalyst loading of 1 mg cm⁻². The stability of the as-synthesized catalysts for overall splitting of water was investigated using the time-dependent current density curves in 1 M KOH.

Electrical double layer capacitance measurement

It is well known that the electrochemically active surface area (ECSA) can be measured by double layer capacitance (C_dl). C_dl was determined from the cyclic voltammograms (CVs) of various catalysts at different scan rates (0.04, 0.06, 0.08, 0.10, 0.12 and 0.14 V s⁻¹) in a non-faradaic potential range of 1.27 to 1.33 V vs. RHE. Afterwards, the corresponding capacitive current was plotted at 1.3 V at different scan rates of CV, and the slope of the fitted line was twice C_dl. Finally, the ECSA can be calculated as
here, the common specific capacitance C_s = 0.04 mF cm⁻² was applied in our calculation.

Results and discussion

The fabrication process for the 3D network comprising N-doped carbon-coated core–shell NiFeO_x@NiFe–P (denoted as NC–NiFeO_x@NiFe–P) involves three steps (Fig. 1). First, the NiFe–PBA/PVP hybrid precursors were synthesized through a facile coprecipitation method (upper right panel in Fig. 1). After high-temperature phosphorization, the metal ions (i.e. Ni²⁺ and Fe³⁺) transformed to correspondingly mixed metal phosphide (i.e. NiP, NiP₂ and FeP₄) NPs (denoted as NiFe–P). In this process, CN⁻ and PVP were employed as carbon and nitrogen sources to in situ form N-doped carbon, thus connecting NiFe–P NPs to create a 3D network structure (lower right panel in Fig. 1). Finally, the NiFe–P NPs were surface oxidized in air to form core–shell NiFeO_x@NiFe–P nanoparticles (lower left panel in Fig. 1). The XRD pattern of the NiFe–PBA/PVP precursor shows a series of characteristic diffractions related to (200), (220), (400), (422), (440), (660) and (620) planes for PBAs (JCPDS no. 20-0915) (Fig. S1†). After high-temperature phosphorization and surface oxidation, intense diffractions indexed to NiP (JCPDS no. 18-0882), NiP₂ (JCPDS no. 13-0213) and FeP₄ (JCPDS no. 32-0471) can be observed on the NC-NiFeO_x@NiFe–P sample, implying the formation of mixed Ni and Fe phosphides (NiFe–P) (Fig. S2†). Interestingly, no diffractions related to Ni/Fe oxides were observed, implying that the Ni/Fe oxides are in the amorphous form. Besides, as shown in Fig. S3,† the NC–NiFe catalyst (which served as the control sample; see the Experimental section) exhibits an intense (111) diffraction at 43.7° and a weak (200) one at 53.1°, suggesting the generation of cubic Ni_0.36Fe_0.64 alloys (JCPDS no. 47-1405).

Fig. 1 Synthetic process of NC–NiFeO_x@NiFe–P catalysts.

The morphology and microstructure of NiFe–PBA/PVP before high-temperature phosphorization as well as the resulting phosphorized and oxidized product (NC–NiFeO_x@NiFe–P) was studied by FESEM and TEM (Fig. 2). In the FESEM image of NiFe–PBA/PVP, a large number of NiFe–PBA NPs in the size of around 20 nm with PVP overlayers were found (Fig. 2a), further confirming the successful synthesis of NiFe–PBA/PVP hybrid composites. After high-temperature phosphorization and surface oxidation, a 3D network structure was observed, as shown in Fig. 2b. The corresponding TEM image in Fig. 2c exhibits numerous black NPs in the size of ∼25 nm embedded in the bright 3D network carbon. After increasing the magnification times, overlayers of carbon at a thickness of 1.7 nm can also be found on the surface of NPs (Fig. 2d). Moreover, the clear and continuous lattice fringes throughout the NPs in distances of 0.26, 0.22 and 0.34 nm are well in agreement with the d spacing of the (112) plane of NiP, (130) plane of FeP₄ and (200) plane of graphitic carbon (Fig. 2e), respectively, indicating the formation of mixed NiFe–P and graphitic carbon overlays. The situation is well consistent with the XRD result. The elemental mapping of NC–NiFeO_x@NiFe–P based on HADDF-STEM-EDS indicates the uniform distribution of Ni, Fe, P, O, C and N elements (Fig. 2g). It should be noted that the O element is more likely to be distributed around the NiFe–P (Ni, Fe and P elements) NPs to form a surface mixed Ni and Fe oxide (NiFeO_x) shell. Moreover, the distribution of O element in the EDS line spectrum is seemingly at the edge of the NiFe–P NPs (Ni, Fe elements) (Fig. 2h), rather than in the middle, further verifying the formation of the surface NiFeO_x shell. It is well known that the metal phosphides could be oxidized by air even at room temperature.¹⁴ Clearly, the surface NiFeO_x shell is in situ generated by the oxidation in air during the washing process at 50 °C. Besides, the Raman spectrum of the NC–NiFeO_x@NiFe–P catalyst indicates two Raman peaks at 1345 and 1580 cm⁻¹ (Fig. S4†), which can be assigned to the D and G bands, respectively. The relative high intensity ratio of the I_D/I_G band (∼1.71) (area ratio) indicates the formation of defects, which may be related to N atom doping into carbon.¹⁵

Fig. 2 SEM images of (a) NiFe–PBAs/PVP and (b) NC–NiFeO_x@NiFe–P; (c) TEM and (d, e) HRTEM images of NC–NiFeO_x@NiFe–P; (f) HADDF-STEM image of NC–NiFeO_x@NiFe–P with (g) EDX mapping; (h) EDX line spectra of Ni, Fe, O elements along the white line in (f), (i) Raman spectrum of NC–NiFeO_x@NiFe–P.

For comparison, FESEM was also conducted over the NiFe–PBA sample without addition of PVP and the corresponding high-temperature phosphorization and surface oxidation product (NiFeO_x@NiFe–P). The FESEM image of NiFe–PBA shows a uniformly distributed morphology similar to NiFe–PBA/PVP except for the PVP overlayers (Fig. S5a†). However, after high-temperature phosphorization and surface oxidation, the NiFeO_x@NiFe–P sample displays a poorly distributed morphology without the 3D network structure (Fig. S5b†), which is distinctly different from that of NC–NiFeO_x@NiFe–P. The huge NPs without the 3D network structure can also be observed in the TEM image of NiFeO_x@NiFe–P (Fig. S6†). On the other hand, it is reported that the PVP transformed to N-doped carbon through thermal decomposition.¹⁶ Moreover, as shown in Table S1,† the content of C and N elements in NC–NiFeO_x@NiFe–P is much higher than those in NiFeO_x@NiFe–P, indicating the presence of a much larger amount of N-doped carbon in NC–NiFeO_x@NiFe–P. The result further confirms that the pyrolysis of PVP in the NiFe–PBA/PVP precursors could form graphitic N-doped carbon. Thus, in the present system, it is evident that the PVP is transformed to a 3D N-doped carbon network during high-temperature phosphorization, thus prohibiting the agglomeration of NiFe–P NPs. Based on the above results, it can be speculated that the generation of the 3D network structure is related to the following three aspects. First, the Ni and Fe metal ions react with PH₃ gas derived from the pyrolysis of NaH₂PO₂·H₂O to form the mixture of NiFe–P. Second, the CN⁻ linkers on PBAs play the role of both carbon and nitrogen source to form N-doped graphene layers encapsulating metal phosphides. Finally, the large amount of PVP overlayers can transform to N-doped carbon, thus linking metal phosphides to generate the 3D network structure.

XPS was performed on NiFeO_x@NiFe–P and NC–NiFeO_x@NiFe–P catalysts to study their surface electronic states (Fig. 3). As shown in Fig. 3a, the survey XPS of NC–NiFeO_x@NiFe–P reveals the existence of Ni, Fe, P, O, C and N elements.

Fig. 3 XPS survey spectrum of (a) NC–NiFeO_x@NiFe–P; (b) N 1s for NiFeO_x@NiFe–P and NC–NiFeO_x@NiFe–P; (c) Ni 2p, (d) Fe 2p, (e) P 2p, (f) O 1s for NC–NiFeO_x@NiFe–P.

The high-resolution N 1s spectra of NiFeO_x@NiFe–P and NC–NiFeO_x@NiFe–P reveal the presence of three different N species, which are pyridinic-N (398.4 eV), pyrrolic-N (399.6 eV) and metallic-N (400.9 eV) (Fig. 3b).^10b,17,18 The percentage of pyridinic-N on the NC–NiFeO_x@NiFe–P catalyst is 53.32%, which is 1.1-fold of that on NiFeO_x@NiFe–P (47.36%). Notably, the pyridinic-N-doped carbon atoms show high activity for the OER and HER.¹⁹ The XPS spectrum of Ni 2p on NC–NiFeO_x@NiFe–P displays peaks at 855.5 and 873.6 eV for Ni²⁺ (Fig. 3c),²⁰ suggesting the presence of surface oxidized Ni species. The XPS spectrum of Fe 2p shows the peaks for Fe⁰ (707.3, 720.7 eV), Fe²⁺ (711.7, 724.0 eV) and Fe³⁺ (713.3, 726.2 eV) (Fig. 3d),²¹ indicating the generation of surface oxidized Fe species. The XPS region of P 2p reveals the presence of two different P species: phosphide (129.1, 130.1 eV) and phosphate (133.6 eV) (Fig. 3e).²² Notably, no obvious peak assigned to the P element (132.7 eV)²³ in P-doped carbon can be observed, signifying that only a small amount of P element is doped in carbon. Furthermore, the intense signal for the O 1s spectrum can be deconvoluted to two O species: lattice O (532.0 eV), adsorbed O (533.3 eV) (Fig. 3f). To further confirm the formation of core–shell NiFeO_x@NiFe–P, the XPS spectra of Ni, Fe, P and O elements for the NC–NiFeO_x@NiFe–P sample after Ar ion etching were also collected. As shown in Fig. S7,† after Ar ion etching of 30 or 60 s, the peaks for oxidized Ni²⁺, Fe²⁺, Fe³⁺ and phosphate species are significantly weakened, and, inversely, intense peaks for metallic Ni⁰, Fe⁰, and phosphide species are observed on NC–NiFeO_x@NiFe–P. These results indicate that these oxidized species only exist on the surface of NiFe–P NPs to form a NiFeO_x shell. The situation is well in agreement with the EDS mapping and line scan results.

The OER activities of the as-synthesized NC–NiFeO_x@NiFe–P and commercial RuO₂ (Alfa Aesar) catalysts with the same loading of 0.2 mg cm⁻² were evaluated in 1 M KOH and are shown in Fig. 4. It can be found that the NC–NiFeO_x@NiFe–P catalyst shows superior inherent OER activity with lower onset potential (1.46 V) and higher current density (j) at each overpotential compared with other catalysts (Fig. 4a). As expected, the overpotential (η) to achieve a j of 10 mA cm⁻² for the NC–NiFeO_x@NiFe–P catalyst is very small (285 mV), which is much smaller than that of NC–NiFe (360 mV) and commercial RuO₂ (370 mV) (Table 1). Notably, the η of NC–NiFeO_x@NiFe–P is also lower than the values of other previous reports for metal phosphides and in the range of leading non-precious metal-based OER catalysts (Table S2†). However, without the 3D N-doped network carbon, a very high η (477 mV) is needed to achieve a j of 10 mA cm⁻² for NiFeO_x@NiFe–P, indicating the important role of the 3D carbon in the OER. As shown in Fig. 4b, the Tafel slopes of catalysts increase in the order: NC–NiFeO_x@NiFe–P (48 mV dec⁻¹) < NiFeO_x@NiFe–P < NC–NiFe < RuO₂ (105 mV dec⁻¹), further verifying the high activity of NC–NiFeO_x@NiFe–P for the OER. In addition, electrochemical impedance spectroscopy (EIS) analysis was also conducted over different catalysts at a potential of 1.53 V (Fig. 4c). The Nyquist plots of all the three samples reveal two time-constants. As reported well in the literature, the small constant at high frequency belongs to the series resistance, and the other big one at low frequency is related to the charge-transfer resistance (R_ct) during electrocatalytic processes.²⁴ Clearly, the NC–NiFeO_x@NiFe–P catalyst displays the lowest R_ct value, which is in line with its highest OER activity. Moreover, in the absence of the 3D network carbon, a very high R_ct value is observed for the NiFeO_x@NiFe–P catalyst, suggesting that the highly conductive 3D N-doped network carbon can greatly promote the charge transfer during the OER. Besides, the NC–NiFeO_x@NiFe–P catalyst exhibits great stability for the OER with nearly negligible deactivation in 1 M KOH after 2000 cycles or 20 h continuous operation (Fig. 4d and S8†). The stability may be attributed to the 3D carbon stabilization on NiFe–P NPs. It can be concluded that the NC–NiFeO_x@NiFe–P catalyst displays robust OER performance with excellent activity, conductivity and stability.

Fig. 4 (a) Steady-state OER polarization curves, (b) Tafel plots and (c) EIS of different catalysts; (d) OER polarization curves of NC–NiFeO_x@NiFe–P before and after the cycling test for 2000 times.

Table 1 Summary information of different catalysts^a

Catalyst	OER		C dl (mF cm^−2)	ECSA (cm^2)	HER
	η (mV) at 10 mA cm^−2	Tafel slope (mV dec^−1)			η (mV) at 10 mA cm^−2	Tafel slope (mV dec^−1)
a η: overpotential, Cdl: double-layer capacitance, ECSA: electrochemically active surface area.
NiFeOx@NiFe–P	477	75	2.03	50.8	407	110.4
NC–NiFe	360	89	4.14	103.5	375	90.0
NC–NiFeOx@NiFe–P	285	48	6.01	150.0	237	65.0

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To further understand the exceptional performance of the NC–NiFeO_x@NiFe–P catalyst, the electrochemically active surface area (ECSA) of different catalysts was assessed in terms of double-layer capacitance (C_dl) (Fig. S9† and 5a). The ECSAs of different catalysts are listed in Table 1. One can see that the ECSA of NC–NiFeO_x@NiFe–P is markedly larger than that of NC–NiFe (control sample). This can be attributed to the core–shell structure of NiFeO_x@NiFe–P, which affords more active sites.²⁵ Moreover, a very small ECSA value is observed for the NiFeO_x@NiFe–P sample without the 3D network carbon, implying the presence of active sites on the 3D carbon. Usually, the catalytic activity of electrocatalysts is essentially associated with their ECSAs.²⁶ Correspondingly, the higher ECSA of the NC–NiFeO_x@NiFe–P catalyst compared with other catalysts results in higher OER activity. Furthermore, the ECSA of each catalyst is also used to calculate current density per ECSA (j_ECSA), and the corresponding polarization curves are shown in Fig. 5b. It can be found that the NC–NiFeO_x@NiFe–P catalyst exhibits higher j_ECSA than the other two catalysts across the whole potential window, indicating the higher activity per active area. On the basis of the above XPS results, there is a large amount of positively charged species including metal oxides (Ni²⁺, Fe²⁺ and Fe³⁺) and pyridinic-N-doped carbon atoms on the NC–NiFeO_x@NiFe–P catalyst. It is widely accepted that positively charged species can easily interact with OH⁻, thus facilitating the withdrawal of electrons from it and promoting the OER in the alkaline electrolyte.²⁷ Therefore, the exceptional activity of NC–NiFeO_x@NiFe–P results from the ample active sites with positive charge on both NiFeO_x shell and 3D N-doped network carbon.

Fig. 5 (a) Plots of the current density against scan rates and (b) OER polarization curves standardized by ECSA for NiFeO_x@NiFe–P, NC–NiFe and NC–NiFeO_x@NiFe–P.

The catalytic performance of the as-synthesized catalysts and commercial Pt/C (Alfa Aesar) catalysts towards the hydrogen evolution reaction (HER) was also evaluated in 1 M KOH. As shown in Fig. 6a, NC–NiFeO_x@NiFe–P displays a small overpotential (η) (237 mV) to achieve a j of 10 mA cm⁻², whereas NiFeO_x@NiFe–P and NC–NiFe require a large η of 407 and 375 mV to achieve a j of 10 mA cm⁻² (Table 1), indicating the higher activity of NC–NiFeO_x@NiFe–P for the HER. Notably, the excellent performance of NC–NiFeO_x@NiFe–P is also superior to that of other non-precious metal-based HER catalysts in alkaline electrolytes (Table S3†). Moreover, NC–NiFeO_x@NiFe–P exhibits a significantly lower Tafel slope of 65.0 mV dec⁻¹ than NiFeO_x@NiFe–P (110.4 mV dec⁻¹) and NC–NiFe (90.0 mV dec⁻¹) (Fig. 6b), further revealing the higher activity of NC–NiFeO_x@NiFe–P for the HER. Generally, the HER under alkaline conditions follows the Volmer–Heyrovsky or Volmer–Tafel process.²⁸ As shown in Fig. 7, both processes involve the adsorption of a H₂O molecule, the splitting of adsorbed H₂O to adsorbed OH⁻ and H atoms, desorption of OH⁻ to recover the surface, and the formation of H₂ from adsorbed H atoms. As reported previously, the TMPs have a strong affinity to H atoms,^6c,29 and the metal oxides and pyridinic-N-doped carbon with strong electrostatic affinity can attach to the OH⁻ derived from the decomposition of H₂O.³⁰ Correspondingly, in our composite catalysts, the OH⁻ produced by H₂O decomposition will preferentially be adsorbed on the surface of NiFeO_x and 3D pyridinic-N-doped carbon, and then the H atom will move to adjacent NiFe–P for recombination to produce H₂, thus accomplishing the Volmer process and synergic effect. On the basis of the greatly improved ECSA and HER performance caused by the NiFeO_x and pyridinic-N-doped network carbon, the exceptional performance of NC–NiFeO_x@NiFe–P can be attributed to the synergistic effect between the NiFeO_x shell, NiFe–P core and N-doped network carbon. Similar catalytic synergy for the HER was also found in Ni/NiO and Li⁺–Ni(OH)₂–Pt nanocomposites.^30a,31 To further confirm the catalytic synergy by experimental means, we further oxidized the NC–NiFeO_x@NiFe–P catalysts at 100 °C for 24 h to form a thicker Ni/Fe oxide (NiFeO_x) shell, and the new catalysts are denoted as NC–NiFeO_x@NiFe–P-100. Then the HER activity of NC–NiFeO_x@NiFe–P-100 was evaluated in 1 M KOH. As shown in Fig. S11a,† the NC–NiFeO_x@NiFe–P-100 catalyst displays an overpotential of 297 mV at a current density of 10 mA cm⁻², which is much larger than that of NC–NiFeO_x@NiFe–P (237 mV). This suggests that the thicker NiFeO_x shell could hinder the contact of active hydrogen atoms with the Ni/Fe phosphide (NiFe–P) core. In other words, the adsorption and recombination of active hydrogen atoms should happen on the NiFe–P core of NC–NiFeO_x@NiFe–P. Moreover, NC–NiFe–P without the NiFeO_x shell displays an overpotential of 315 mV at a current density of 10 mA cm⁻² for the HER (Fig. S12†), which is much larger than that of NC–NiFeO_x@NiFe–P (237 mV). This result implies that the water dissociation should happen on the NiFeO_x shell. On the basis of the above results, it is clear that the HER process in NC–NiFeO_x@NiFe–P catalysts should follow a catalytic synergy. Besides, the NC–NiFeO_x@NiFe–P catalyst exhibits excellent stability for the HER with nearly negligible deactivation after 20 h continuous operation (Fig. S13†), further confirming the 3D carbon stabilization on NiFe–P NPs.

Fig. 6 (a) Steady-state HER polarization curves and (b) Tafel slopes of different catalysts.

Fig. 7 The reported HER process in alkaline media.

Because of the excellent performance for both OER and HER, we constructed an electrolyzer with 1 M KOH as media through applying NC–NiFeO_x@NiFe–P as both a water oxidation and reduction catalyst. As shown in Fig. 8a, the voltage to reach a j of 10 mA cm⁻² is 1.59 V, which is lower than that of most non-precious metal-based electrolyzers (Table S4†). Meanwhile, evident bubbles for hydrogen and oxygen are shown on Ni foam at 1.59 V, indicative of the occurrence of overall water splitting (Fig. 8b). In sharp contrast, large voltages are required for NiFeO_x@NiFe–P (1.81 V) and NC–NiFe (1.75 V) catalysts to achieve a j of 10 mA cm⁻², further confirming the high activity of NC–NiFeO_x@NiFe–P for overall water splitting (Fig. S12†). As a key parameter to evaluate the performance of electrocatalysts, the stability of NC–NiFeO_x@NiFe–P was investigated at a j of 10 mA cm⁻² in 1 M KOH. As shown in the inset of Fig. 8a, the catalyst exhibits desirable stability without obvious deactivation in 20 h. Therefore, the bifunctional NC–NiFeO_x@NiFe–P with high activity and good durability may be a promising catalyst instead of precious catalysts for overall splitting of water.

Fig. 8 (a) LSV of water electrolysis based on the NC–NiFeO_x@NiFe–P catalyst in 1 M KOH, with the inset displaying the stability test of the electrolyzer at 10 mA cm⁻²; (b) optical photograph displaying the generation of hydrogen and oxygen bubbles on Ni foam.

Based on the above results, the exceptional activity of NC–NiFeO_x@NiFe–P for the OER and HER can be owing to the following aspects. First, the 3D N-doped network carbon derived from CN⁻ and PVP can not only significantly enhance the conductivity and charge transfer capacity but also afford ample active sites for reactions. Second, the well-organized 3D network structure without aggregation gives rise to exposure of more active sites and accelerating the transfer of reactants in the electrolyte. Third, the NiFeO_x shell is employed as highly active centers to further improve the activity of the OER and HER. Moreover, the NiFe–P core affords sites to adsorb H atom during the HER^6b,32 and facilitate the proton transfer during the OER.³³ Thus, the exceptional activity of NC–NiFeO_x@NiFe–P for both OER and HER can result from the synergistic effect between the above aspects.

Conclusions

In summary, we have developed a potentially scalable and cost-effective strategy to prepare a 3D network comprising N-doped carbon-coated core–shell NiFeO_x@NiFe–P by a two-step high-temperature phosphorization and surface oxidation of a NiFe–PBA/PVP hybrid precursor. The surface NiFeO_x shell and pyridinic-N-doped carbon atoms as new active sites could facilitate activation of OH⁻ for the OER and H₂O for the HER in alkaline solution, respectively. In 1 M KOH, the bifunctional NC–NiFeO_x@NiFe–P catalyst exhibited excellent performance with overpotentials of 285 and 237 mV for the OER and HER to achieve a current density of 10 mA cm⁻², respectively. The well-organized 3D network structure, highly active sites on both core–shell NiFeO_x@NiFe–P, and highly conductive N-doped carbon were responsible for the high activity for both OER and HER. Due to its advanced architecture and electrocatalytic performance, an alkaline electrolyzer based on NC–NiFeO_x@NiFe–P catalysts could be manufactured to achieve lasting 10 mA cm⁻² at a voltage of 1.59 V, which is lower than that of most non-precious metal-based electrolyzers. The above findings open a new door to construct bifunctional and non-precious electrocatalysts via PBA/polymer hybrid precursors, which may push forward the practical application of water splitting.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The financial support of the National Natural Science Foundation (NNSF) of China (21574084, 21571131 and 21374064), Postdoctoral Science Foundation of China (2017M612745), the Natural Science Foundation of Guangdong (2015A030313554 and 2017A040405066), and Shenzhen Government’s Plan of Science and Technology (JCYJ20160308104704791) is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7se00576h

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