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Article

In Situ Growth of Self-Supporting MOFs-Derived Ni2P on Hierarchical Doped Carbon for Efficient Overall Water Splitting

by
Neng Chen
†,
Sai Che
†,
Hongchen Liu
,
Na Ta
,
Guohua Li
,
Fengjiang Chen
,
Guang Ma
,
Fan Yang
and
Yongfeng Li
*
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2022, 12(11), 1319; https://doi.org/10.3390/catal12111319
Submission received: 6 September 2022 / Revised: 20 October 2022 / Accepted: 24 October 2022 / Published: 27 October 2022
(This article belongs to the Special Issue Metal-Organic Framework Based Catalysts for Energy Applications)
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Abstract

:
The in situ growth of metal organic framework (MOF) derivatives on the surface of nickel foam is a novel type of promising self-supporting electrode catalyst. In this paper, this work reports for the first time the strategy of in situ growth of Ni-MOF, where the metal source is purely provided by a nickel foam (NF) substrate without any external metal ions. MOF-derived Ni2P/NPC structure is achieved by the subsequent phosphidation to yield Ni2P on porous N, P-doped carbon (NPC) backbone. Such strategy provides the as-synthesized Ni2P/NPC/NF electrocatalyst an extremely low interfacial steric resistance. Moreover, a unique three-dimensional hierarchical structure is achieved in Ni2P/NPC/NF, providing massive active sites, short ion diffusion path, and high electrical conductivity. Directly applied as the electrode, Ni2P/NPC/NF demonstrates excellent electrocatalytic performance towards both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), with low overpotentials of only 58 mV and 208 mV to drive 10 mA cm−2, respectively, in 1 M KOH. Furthermore, Ni2P/NPC/NF acting as the overall water splitting electrodes can generate a current density of 10 mA cm−2 at an ultralow cell voltage of 1.53 V. This simple strategy paves the way for the construction of self-supporting transition metal-based electrocatalysts.

Graphical Abstract

1. Introduction

Hydrogen energy is considered to be a promising substitute for traditional fossil energy due to its advantages of high-energy density and carbon-free emissions [1,2]. The electrocatalytic overall water splitting (OWS) technique provides an eco-friendly and sustainable strategy for the large-scale production of hydrogen fuels [3,4]. At present, noble metal Pt and Ir/Ru oxides are considered to be state-of-the-art catalysts for the two half reactions of OWS [5,6], namely, the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER), respectively [7,8]. Unfortunately, its large-scale applications are hindered by their high cost and scarcity [9,10]. Therefore, there is an urgent need to explore low-cost and highly active electrocatalysts for OWS based on Earth-abundant elements [10].
Recently, transition metal phosphides (TMPs) have attracted extensive attention in the fields of energy conversion, such as electrocatalysis and energy storage [11,12,13]. In particular, nickel phosphide (Ni2P) with the Gibbs free energy change in hydrogen adsorption (∆G0) close to 0 is considered as one of the best candidates for OWS [14,15,16]. Numerous studies have shown that the phosphorus atom can easily attract electrons from metals as proton acceptors due to its powerful electronegativity [17]. The generation of negatively charged phosphorus sites and positively charged metal sites is beneficial to enhancing the activity of electrocatalytic OWS [18]. However, the electrochemical performance of Ni2P is severely hindered by its limited exposed active sites and poor electrical conductivity.
The following strategies have been adopted to overcome the above problems: On the one hand, the activity of catalysts depends on a well-organized micro/nanostructure, which can provide more active sites and augment the contact area between electrode and electrolyte. On the other hand, compositing Ni2P with highly conductive backbones not only facilitates the conductivity of the catalysts, but also boosts their electrocatalytic reaction kinetics. Therefore, it is of great significance to develop a general strategy to anchor active Ni2P nanostructure onto porous conductive backbones to realize enhanced catalytic activity.
Metal–organic frameworks (MOFs), a class of crystalline porous materials composed of isolated metal clusters and organic ligands [19,20], emerge as suitable precursors for the construction of transition metal/carbon composite materials thanks to their structural diversity, large specific surface area, and high density of ordered metal centers [21,22]. Specifically, metal ions are uniformly dispersed in MOF structures at the atomic level, while organic ligands in MOFs act as carbon sources for various carbon-based materials through pyrolysis [23,24]. The formation of Ni2P onto self-supporting substrate by pyrolyzing MOFs as precursors has been extensively explored [25]. However, the as-prepared Ni2P is usually attached to the substrate surface through loose adhesion, leading to not only easy detachment between the Ni2P and the substrate during long-term electrocatalysis, but also sluggish charge transfer and overwhelming interfacial impedance [26,27,28]. Moreover, the carbon network transformed from organic ligands can easily block the exposed active sites of Ni2P during high-temperature pyrolysis. Therefore, it is extremely challenging to develop MOFs-derived Ni2P with a tight contact to the substrate.
Based on the above considerations, we pioneered a simple “in situ transformation” strategy to fabricate a N, P-doped-carbon (NPC)-enwrapped Ni2P composite endogenously onto nickel foam (NF) substrate as self-supporting OWS electrocatalyst. In this method, Ni(OH)2 nanosheets were first grown on the surface of NF by the low-temperature “in situ etching growth” method. Subsequently, Ni2+ from Ni(OH)2 coordinated dynamically with 2-methylimidazole to transform into Ni-MOF. Finally, the Ni-MOF was transformed into Ni2P/NPC composite through simultaneous graphitization and phosphidation. With this novel strategy, no other exogenous were introduced in the entire synthetic process to prepare Ni-MOF-derived phosphide, maintaining the close contact of the interface to solve the problems of steric instability and large interfacial impedance. The synergistic effect between highly active Ni2P and highly porous NPC ensured fast charge transfer, high electrochemical activity, and long-term durability. As a result, the optimized Ni2P/NPC/NF catalyst showed excellent HER and OER performance in 1.0 M KOH solution, requiring overpotentials of 58 mV and 208 mV to drive 10 mA cm−2, respectively. Notably, it acted as a bifunctional electrocatalyst for overall water electrolysis, yielding a current density of 10 mA cm−2 at an ultra-low cell voltage of 1.53 V. This work emphasizes the importance of the three-dimensional network structure formed by in situ growth of MOFs on the surface of NF, which is conducive to the mass transfer of materials to the gas, and further phosphating is conducive to the adjustment of electronic structure. This “in situ transformation” strategy paves the way for the future design of self-supporting TMP/carbon composite electrocatalysts.

2. Results

The synthesis process of 3D network Ni2P/NPC coral-like shape on nickel foam involves three main steps, as shown in Figure 1. First, Ni(OH)2 nanosheets were grown in situ on the skeleton of nickel foam (Ni(OH)2/NF) by the low-temperature “in situ growth etching” method. Subsequently, Ni(OH)2 nanosheets was immersed in a 2-MeIM solution for various periods to naturally grow Ni-MOF 3D networks (Ni-MOF/NF). Finally, Ni-MOF/NF was phosphorylated to translate into Ni2P/NPC by an upstream and downstream strategy. During this preparation process, the metal source is only provided by the metal-based nickel foam substrate without external metal ions. The in situ grown MOF from the self-grown Ni(OH)2 and dimethylimidazole provides a solution to the problem of steric hindrance and boundary vacancies between the substrate and the MOF, exhibiting excellent electron transfer ability.
The Ni(OH)2 on the nickel foam substrate, the state between elemental nickel and nickel ions, can grow Ni-MOF in situ with dimethylimidazole. The whole process of Ni-MOF growth is shown in Figure 2a. First, a small number of Ni-MOF particles were grown on the surface of the nanosheets. Subsequently, the nanoparticles gradually kept growing to form a network structure with certain void channels. Finally, excessive Ni-MOF in situ growth covered all the Ni(OH)2 surfaces. To probe into the morphology evolution of Ni(OH)2/NF growing Ni-MOF in dimethylimidazole solution, scanning electron microscope (SEM) images of the samples obtained in the relevant reactions were provided. Figure 2b(i,ii) shows that neat and beautiful Ni(OH)2 nanosheets are uniformly grown on the nickel foam. Subsequently, the lamellae of the Ni(OH)2 nanosheets were thickened after an hour of transformation coordination with 2-methylimidazole (Figure 2c(i,ii)). With the extended growth time of 6 h, a coral network structure was formed on the surface of Ni(OH)2 nanosheets, (Figure 2d(i,ii)). After 9 h of reaction, its microscopic morphology gradually grew into a 3D granular stacked structure (Figure 2e(i,ii)). Finally, Ni-MOF grown on metal foam for 6 h was further phosphated to form a layered three-dimensional pore structure by gradually thickening the edges of the coral-like network (Figure 2f). The microstructures of Ni2P/NPC were characterized by transmission electron microscope (TEM). Figure 2g reveals that the Ni2P/NPC under ultrasound shows a folded and translucent lamellar structure. Figure 2h clearly illustrates the clear lattice spacing values of 0.221 nm and 0.203 nm corresponding to the (111) plane and (201) of Ni2P, respectively [29]. Figure 2i–m shows high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) and the corresponding energy-dispersive X-ray spectroscopy (EDS) elemental mapping images of Ni-MOF, indicating that C (red), N (brown), P (yellow) and Ni (green) are evenly dispersed in the selected region.
To further investigate the phase formation, X-ray diffraction (XRD) characterization was performed on the obtained samples (Figure 3a). The XRD pattern shows the characteristic peaks of the crystal form of Ni(OH)2, at 19.3°, 33.1°, 38.5°, 59.1°, 62.7°, 70.5° and 72.7°, fully demonstrating the successful growth of Ni(OH)2 (JPCDS No.14-0117) crystalline phase [30,31]. The exposed Ni2+ on Ni(OH)2 nanosheets can coordinate with dimethylimidazole to form Ni-MOF, and its XRD diffraction peaks are consistent with those previously reported in the literature. The characteristic peak of Ni-MOF did not appear after 1 h reaction of Ni-MOF grown on Ni(OH)2, but was more obvious at 6 h and 9 h. After the above sample phosphating, the diffraction peaks of the sample at 40.8°, 44.6°, 47.3°, 54.2°, 54.9°, 72.7° and 74.7° correspond to (111), (201), (210), (300), (211), (311) and (400) of Ni2P (JPCDS No.03-0953), respectively [32,33,34]. This indicates that phosphating calcination can successfully convert Ni(OH)2@Ni-MOF/NF to Ni2P/NPC/NF.
The prepared materials were analyzed by Raman spectroscopy (Figure 3b). Ni2P/NPC/NF exhibits two distinct peaks at 1341 and 1571 cm−1, corresponding to the D and G peaks, respectively [22]. The D peak may favor the consistent carbon structural defects for HER, while the G peak corresponds to electronically conductive graphitic carbon. In contrast, Ni2P/NF has no obvious characteristic peaks at the positions of D and G peaks. These results indicate that Ni-MOF grown in situ on nickel foam after phosphating can form a carbon layer with defects and a certain degree of graphitization at 500 °C, which is beneficial to improve the catalytic activity of the electrode material.
To deeply explore the chemical states of the prepared materials, X-ray photoelectron spectroscopy (XPS) was performed to analyze the changes in the coordination environment of Ni, P, N, and C. As shown in Figure 3c, the XPS spectrum of Ni 2p in Ni2P/NF can be decomposed into four peaks centered at 855.7 eV, 861.9 eV, which is ascribed to the binding energy of Ni 2p in the Ni2P phase, and the apparent satellite peaks of the latter are mainly attributed to the oxidation state of Ni, which is caused by inevitable surface oxidation in air [35]. The XPS spectrum of Ni 2p of Ni2P/NPC/NF is divided into these peaks: Ni0 (853.3 eV), Ni3+ (856.3 eV), Ni2+ (862.6 eV). Interestingly, Ni0 appears on the surface of the sample, mainly because the high-temperature phosphating was accompanied by carbonization, which is favorable for the reduction of Ni2+ to transform into Ni element.
Figure 3d demonstrates that the region of P 2p is decomposed into four peaks. The broad band of the peak on the left is fitted by two peaks at 133.8 and 134.5 eV, which are attributed to the characteristic peaks of P-C and P-O, respectively [36]. Oxidized phosphorus species (P-O) may arise from surface oxidation processes due to exposure to air. The peaks at 133.1 and 134.3 eV are assigned to the binding energies binding energy of P 2p3/2 and P 2p1/2 in the Ni2P phase, respectively. Compared with Ni2P obtained by phosphating Ni(OH)2 without N, C doping, the Ni 2p and P 2p binding energy of Ni2P/NPC/NF show a positive shift of 0.8 eV and a negative shift of 0.5 eV, respectively. This may be due to the transfer of electrons from P to adjacent C atoms. The high positive charge on the Ni atom can significantly increase the active sites for hydrogen ion acceptance, while the rich negative charge on the P atom is conducive to capturing protons due to its strong electrostatic affinity [37]. This tuning mechanism of the electronic structure of Ni2P is expected to improve the catalytic activity of HER. The C 1s XPS peaks of Ni2P/NPC/NF are deconvoluted into three characteristic peaks at 284.5, 285.3 and 286.3 eV (Figure 3f), corresponding to C-C, C-P/C-N and C-O/C-N bonds, respectively, fully demonstrating the formation of P-doped carbon. The three peaks at 399.3, 400.2 and 401.5 eV of the N 1s XPS spectrum correspond to the BE of pyridinic N, pyrrolic N and graphitic N (Figure 3e), respectively, which also confirms the successful formation of N-doped carbon [38].
Furthermore, the wettability of the catalyst is crucial for the water electrolysis reaction. A contact angle test was used to analyze the wetting properties of the prepared material in KOH solution. Compared with NF (80.5°), Ni(OH)2/NF (57.9°) and Ni2P/NF (28.6°), Ni2P/NPC/NF presents favorable hydrophilic properties (9.7°), which can enhance its full contact with the electrolyte and facilitate the desorption of air bubbles, thereby enhancing the performance of water splitting (Figure 4).
The electrocatalytic HER performance of Ni2P/NPC/NF was evaluated using a standard three-electrode system in 1 M KOH solution. In addition, nickel foam, Ni(OH)2/NF, Ni2P/NF, Ni2P/NPC/NF-1, Ni2P/NPC/NF-3, Ni2P/NPC/NF-6, Ni2P/NPC/NF-9, commercial Pt/C (20 wt.%) were evaluated and compared under the same conditions.
Ni2P/NPC/NF only possesses an overpotential of 58 mV to drive current densities of 10 mA cm−2 in KOH electrolyte, which the corresponding NF, Ni(OH)2/NF, Ni2P/NF requires 172, 139, 115 mV under the same conditions (Figure 5a,b). Meanwhile, it is worth emphasizing that the HER of Ni2P/NPC/NF activity is still slightly inferior to 20 wt.% Pt/C in alkaline electrolyte (η10 = 32 mV). These values of Ni2P/NPC/NF are superior to many other high-performance Ni2P-based catalysts in literature reports, as summarized in Table S1 in the Supporting Information. Therefore, the optimized Ni2P/NPC/NF catalyst shows a higher HER activity than Ni2P/NF, implying a synergistic effect of electrocatalytic activity between Ni2P and NPC that is conducive to the improvement of catalyst performance. Tafel slopes are often used to reveal reaction kinetics and rate-controlling steps in the HER process. Figure 5c reveals that Ni2P/NPC/NF has ultra-low Tafel slopes of 49.3 mV dec−1, which is much lower than those of Ni2P/NF (97.9 mV dec−1), Ni(OH)2/NF (107.0 mV dec−1) and NF (199.8 mV dec−1), directly reflecting the faster charge transfer kinetics.
To further comprehend the excellent HER performance, electrode transfer kinetics and interfacial reactions were evaluated by electrochemical impedance spectroscopy (EIS). In the Nyquist plot, Ni2P/NPC/NF exhibits smaller charge transfer resistance (Rct) than NF, Ni(OH)2/NF, and Ni2P/NF, which indicates that the synergistic effect of Ni2P and NPC in the Ni2P/NPC/NF exhibits excellent electron transport ability (Figure 5d). The time of in situ grown Ni-MOF possesses a non-negligible relationship to its HER performance. Apparently, Ni-MOF with the growth time of 6 h exhibits the best HER performance, mainly because the Ni-MOF array grown at 6 h shows sufficient nitrogen and carbon, and meanwhile it also avoids the dense state of excessive Ni-MOF generated for 9 h, which hinders the mass transfer and diffusion of a gas. Meanwhile, the impedance decreases with the increase in the growth time of Ni-MOF, which fully shows that the increased nitrogen and carbon in the Ni-MOF can be conducive to improving the electronic conductivity of the material.
The long-term stability of Ni2P/NPC/NF catalysts was evaluated by continuous CV cycling for 3000 cycles and chronoamperometry. Figure 5e shows that the shift of the LSV curve after 3000 CV cycles is negligible compared to the curve before the CV cycle test. Furthermore, the current density shows negligible degradation in the chronoamperometry test, which indicates that Ni2P/NPC/NF can work effectively for at least 48 h (Figure 5f). These results demonstrate that this in situ grown Ni-MOF-derived Ni2P exhibits excellent electrochemical stability.
The oxygen evolution reaction (OER) electrocatalytic performance of the product was also evaluated using a three-electrode battery system in 1.0 M KOH. Here, commercial IrO2 is evaluated as a standard with an overpotential of 191 mV to achieve a current density of 10 mA cm−2. Ni2P/NPC/NF exhibits the best OER performance with the lowest overpotential of 208 mV to achieve a current density of 10 mA cm−2, while NF, Ni(OH)2/NF and Ni2P/NF require larger overpotentials with 353, 288 and 235 mV, respectively (Figure 6a,b). Moreover, Ni2P/NPC/NF shows more superior performance compared with other previously reported electrocatalysts, such as NiCoP (290 mV), NiMoP (255 mV) NiFeP (271 mV) (Table S2), which is mostly ascribed to the dramatically minimized interfacial steric hindrance and the facilitated electron transport through this endogenous transformation strategy. At the same time, Figure 6c shows that Ni2P/NPC/NF exhibits a low Tafel slope of about 65.0 mV dec–1, smaller than that of NF (190.2 mV dec−1), Ni(OH)2/NF (105.5 mV dec−1) and Ni2P/NF (121.8 mV dec−1). It can be clearly seen that the introduction of N, P of Ni2P/NPC/NF enhances the kinetic advantage of water oxidation OER.
Furthermore, the charge transfer resistance in the OER was investigated by the EIS spectrum (Figure 6d). Apparently, the Ni2P/NPC/NF exhibits super-low Rct (7 ohm) compared with NF (>30 ohm), Ni(OH)2/NF (25 ohm) and Ni2P/NF (19 ohm), indicating its superior OER kinetics due to an appropriate amount of in situ ZIF-derived Ni2P, which plays an important role in promoting the OER process and lowering the energy barrier. The time of in situ grown Ni-MOF possesses an inconvenient relevance to its OER performance. Apparently, Ni-MOF with the growth time of 6 h exhibits the best OER performance, mainly because the Ni-MOF array grown at 6 h shows sufficient nitrogen and carbon, and it also avoids the dense state of excessive Ni-MOF generated for 9 h, which hinders the mass transfer and diffusion of gas. Meanwhile, the impedance decreases with the increase in the growth time of Ni-MOF, which fully shows that the increased doping of N and P can be conducive to improving the electronic conductivity of the material.
The OER stability test of Ni2P/NPC/NF adopts continuous CV scanning for 3000 cycles and chronoamperometry. Figure 6e shows the polarization curves before and after 3000 CV cycles of RHE at a scan rate of 100 mV s from 1.0 to 2.0 V in 1 M KOH. The OER polarization curve does not decay significantly before and after 3000 CV cycles. Meanwhile, the current density shows negligible degradation in the chronoamperometry test, which indicates that Ni2P/NPC/NF can work effectively for at least 48 h (Figure 6f). The reason for the excellent durability is the double protection of phosphate formed by surface oxidation and carbon layer formed by carbonization to improve stability and corrosion resistance.
Electrocatalytic-activity-specific surface area (ECSA) is an important parameter reflecting the intrinsic activity of catalysts. Under these circumstances, cyclic voltammetry (CV) measurements with RHE at 0.425~0.625 V at scan rates augmenting from 50 to 500 mV s−1 in the Faradaic range were employed to obtain the double layer capacitance (Cdl) (Figure S1). NF, Ni(OH)2/NF, Ni2P/NF and Ni2P/NPC/NF were calculated to be 0.9, 2.7, 5.6 and 22.1 mF cm−2, respectively, indicating that Ni2P/NPC/NF possesses the highest ECSA and exposes more active sites (Figure 7a). The larger Cdl of Ni2P/NPC/NF demonstrates that the dual synergy of Ni2P and NPC exposes more active sites, which is beneficial to the performance of water splitting.
To further explore the influence of mass loading on the electrocatalytic activity, the mass normalized LSV curves were plotted for OER and HER (Figure S3a,b). A similar trend was observed as the area normalized LSV curves, in which Ni2P/NPC/NF-6 h also demonstrated the most superior catalytic performance among all synthesized materials. In order to further estimate the intrinsic catalytic activity of Ni2P/NPC/NF, the turnover frequency (TOF) for HER and OER was investigated [39,40,41]. The relationships between the hydrogen and oxygen generation rate and the number of active sites through the CV test are shown in Figures S3a and S4a. Compared with Ni(OH)2 and Ni2P/NF, Ni2P/NPC/NF-6 h revealed a high TOF in HER (27.5 s−1) and OER (0.28 s−1) at an η of 350 mV, much higher than that of the samples with other growth times (1 h, 3 h and 9 h) (Figures S3b and S4b).
Furthermore, Ni2P/NPC/NF was assembled as a two-electrode electrolyzer with anode and cathode to evaluate the overall water splitting performance of electrocatalysis in 1.0 M KOH solution, while commercial Pt/C/NF and IrO2/NF (IrO2/NF||Pt/C/NF) of the same mass loading were also assembled and tested under the same conditions as a comparison. The tests were measured in 1.0 m KOH electrolyte with a scan rate of 5 mV/s. As shown in Figure 7b, Ni2P/NPC/NF||Ni2P/NPC/NF exhibits a decomposition voltage of 1.53 V at a current density of 10 mA cm–2, which is comparable to that of the reference electrode (IrO2/NF||Pt/C/NF) comparable to electrolyzed water. Meanwhile, compared with other reported work, such as doping other heteroatoms, introducing new metal ions, preparing Ni2P with different structures, etc., the in situ grown Ni2P material shows that it can be compared with competitive electrolyzed water splitting performance (Table S3). The steady chronoamperometric curve was performed at 1.53 V for 48 h, showing that the material possesses outstanding stability for long-time operation in the total water splitting system (Figure 7c).
It is the first report that the Ni-MOF is grown in situ only based on the metal source provided by the metal substrate without external metal ions. The possible mechanism for the growth of Ni-MOF is that Ni(OH)2 intervenes between the intermediate states of Ni and Ni2+, which releases Ni2+ to form coordination with 2-methylimidazole solution to crystallize Ni-MOF. The reasons for the outstanding performance of Ni2P/NPC are deduced, and the analysis is as follows: 1. The metals in the in situ-grown Ni(OH)2 and Ni-MOF derived from metallic nickel foams promote the tight bonding of the metal substrate and organic ligands, which exhibits extremely low intrinsic electrochemical impedance. 2. The synergy between Ni2P derived from Ni(OH)2 and NPC derived from Ni-MOF contributes to the electrode material to expose more electrocatalytically active sites. 3. Ni2P/NPC derived from Ni-MOF in situ growth at a suitable time presents a 3D network with a certain void channel structure (Figure 7d), which is beneficial to improve the gas diffusion and mass transfer processes generated by electrocatalysis, and further to promote the performance of water electrolysis.

3. Experimental Section

3.1. Chemicals

All the chemicals were of analytical grade. Hydrochloric acid (HCl), acetone (CH3COCH3), potassium hydroxide (KOH), methanol and ethanol were analytical grade, and were bought from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). 2-Dimethylimidazole (2-MeIM), Cetyltrimethylammonium Bromide (CTAB) and sodium hypophosphite (NaH2PO2) were purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China). Pt/C (20 wt.%) and RuO2 were purchased from Alfa Aesar (Heysham, Britain). Nickel foam (99.99% purity, 1.5 mm thickness, 0.45 μm pore size.) was purchased from Kunshan Shengshijing New Materials Co., Ltd. (Kunshan, China).

3.2. In Situ Growth of Ni(OH)2 Nanosheets on Nickel Foam

To remove the oxide layer of nickel foam, the nickel foam (1 cm × 4 cm) was ultrasonically cleaned by acetone and hydrochloric acid for 15 min, and then washed by deionized water and ethanol for several times. Subsequently, the nickel foam was immersed in deionized water at 80 °C for 3 h to grow Ni(OH)2 nanosheets on its surface. The obtained samples were repeatedly washed with ethanol and air-dried at room temperature. It was labelled as Ni(OH)2/NF nanosheets.

3.3. Synthesis of Ni-MOF/NF

The Ni(OH)2/NF nanosheets were placed in 270 mL aqueous solution containing 30 mg of CTAB and 27.24 g of 2-methylimidazole (MeIM) for different times (1, 3, 6, 9 h). The exposed Ni2+ of the nanosheets coordinated with dimethylimidazole to form Ni-MOF; thus, a hierarchically bi-layer nanosheet structure was formed. The sample after washing and drying with ethanol was labelled as Ni-MOF/NF.

3.4. Synthesis of Ni2P/NPC/NF Nanosheets

The above-prepared Ni-MOF/NF and NaH2PO2 were placed in two different positions in a ceramic crucible, with NaH2PO2 (1.0 g) and the sample located upstream and downstream, respectively. The sample was heated to 500 °C at a rate of 5 °C min−1 and kept corresponding temperature for 2 h, and the Ar gas flowed at a flow rate of 100 mL s−1 for phosphating to achieve Ni2P/NPC/NF.

3.5. Synthesis of Ni2P/NF

NaH2PO2 (1 g) was placed at the upstream boat and the above-prepared Ni(OH)2 was installed at the downstream boat in a tube furnace. The sample was heated to 350 °C and kept for 2 h at a ramp rate of 5 °C min−1 and an Ar flow rate of 100 mL min−1 to achieve Ni2P/NF nanosheets.

3.6. Characterization of Electrode Materials

X-ray diffraction (XRD) was applied by a Bruker D8 Advance X diffractometer to analyze the phase composition and crystal structure of the samples. Scanning electron spectroscopy (SEM, Quanta 200F, FEI company, Hillsboro, OR, USA) and transmission electron spectroscopy (TEM, Tecnai G2 F20, FEI company, Hillsboro, OR, USA) were employed to explore morphology and element distribution. X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA) was adopted to acquire the surface composition and valence state of the materials. The prepared self-supporting electrode materials were assembled into the equipment of a tablet press, which was pressed into a flat sheet. The contact angle of the material in 1.0 M KOH electrolyte was measured by the trapped droplet method using the DSA20 system (Kruss, Hamburg, Germany).

3.7. Electrochemical Measurements

The electrochemical measurement of the obtained electrode materials included linear sweep voltammetry (LSV), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) on a standard three-electrode system (CHI760E electrochemical workstation, CH Instruments, Inc., Shanghai, China) without iR-compensation. It was performed with Hg/HgO, graphite rod, and self-supporting material (1 × 1 cm2) as reference electrode, the counter electrode, and working electrode, respectively. Linear sweep voltammetry (LSV) was used to measure HER (0.1 V to −0.6 V (vs. reversible hydrogen electrode, RHE)) and OER (1.0 V to 2.0 V (vs. RHE)) for OER at a scan rate of 5 mV s−1. The stability of the catalyst was evaluated by chronoamperometry at the corresponding potential to reach an initial current density of 10 mA cm−2. Electrochemical impedance spectroscopy (EIS) measurements were performed in the range from 0.01 to 105 Hz to evaluate the charge transfer ability of the catalysts. The amplitude of the alternating voltage was 5 mV around a potential of −0.17 V versus RHE for HER and 1.58 V versus RHE for OER. The electrochemically active surface area (ECSA) was obtained by recording double-layer capacitance (Cdl) values by cyclic voltammetry in the Faradaic potential region from 0.425 to 0.625 V at different scan rates (50~500 mV s−1).

4. Conclusions

In summary, a self-supporting electrocatalyst of Ni-MOF-derived Ni2P/NPC was successfully prepared by a novel strategy of in situ growth and transformation without any external metal ions. The as-synthesized unique structure solved the problem of easy exfoliation during electrocatalysis as well as the large interfacial charge transfer between active sites and substrate. The 3D network structure formed by the in situ growth of MOF on NF surface benefited mass transfer and gas diffusion. As a result, extraordinary HER and OER performances were achieved for the self-supporting Ni2P/NPC/NF electrode with excellent reaction kinetics and considerable stability, which was attributed to the unique electrode bridging structure and facilitated electron transport. The as-prepared bifunctional catalyst, directly applied as cathode and anode electrodes in alkaline water electrolysis, demonstrated excellent performance in the OWS. This strategy opens up a new way to design and synthesize cost-effective and efficient multifunctional electrocatalysts for energy storage and conversion.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12111319/s1, Figure S1: CVs of (a) bare nickel foam, (b) Ni(OH)2/NF, (c) Ni2P/NF and (d) Ni2P/NPC/NF. The plots of the current density versus the scan rate of these samples; Figure S2: The mass normalized polarization curves of (a) HER LSV and (b) OER LSV; Figure S3: (a) CVs of different catalytic electrodes with a scan rate of 50 mV/s in 1.0 M KOH. (b) The calculated turnover frequency curves of different catalytic electrodes under HER conditions; Figure S4: (a) CVs of different catalytic electrodes with a scan rate of 50 mV/s in 1.0 M KOH. (b) The calculated turnover frequency curves of different catalytic electrodes under OER conditions; Table S1: Summary of some recently reported Ni2P/NF HER catalysts in alkaline electrolytes; Table S2: Comparison of OER performance in 1 M KOH solution for Ni2P/NF with other electrocatalysts; Table S3: Comparison of water splitting performance in 1 M KOH solution for Ni2P/NPC/NF with other electrocatalysts [42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68].

Author Contributions

N.C.: writing—review and editing, writing—original draft, formal analysis, investigation. S.C.: writing—review and editing, formal analysis, supervision. H.L., N.T., F.C. and G.L.: investigation. G.M. and F.Y.: formal analysis. Y.L.: writing review and editing, writing—original draft, supervision. N.C. and S.C. contributed equally to this work. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the National Natural Science Foundation of China, grant numbers No. 22108301, 22238012, and 22178384, and China Postdoctoral Science Foundation, grant number No. 2021M703577.

Data Availability Statement

All the relevant data used in this study are provided in the form of figures and tables in the published article, and all data provided in the present manuscript are available upon request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram of the preparation process of Ni2P/NPC/NF.
Figure 1. Schematic diagram of the preparation process of Ni2P/NPC/NF.
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Figure 2. (a) Schematic diagram of in situ growth of Ni-MOF on Ni(OH)2 nanosheets. SEM microscopic topography: (b) Ni(OH)2 nanosheets, (c) growth time of Ni(OH)2/Ni-MOF/NF for 1 h and (d) growth time of Ni(OH)2/Ni-MOF/NF for 6 h. (e) Growth time of Ni(OH)2/Ni-MOF/NF for 9 h, and (f) Ni2P/NPC/NF with the reticular structure. The sample of Ni2P/NPC/NF: (g) TEM morphology, (h) HRTEM image and (i–m) EDX image of Ni2P/NPC/NF.
Figure 2. (a) Schematic diagram of in situ growth of Ni-MOF on Ni(OH)2 nanosheets. SEM microscopic topography: (b) Ni(OH)2 nanosheets, (c) growth time of Ni(OH)2/Ni-MOF/NF for 1 h and (d) growth time of Ni(OH)2/Ni-MOF/NF for 6 h. (e) Growth time of Ni(OH)2/Ni-MOF/NF for 9 h, and (f) Ni2P/NPC/NF with the reticular structure. The sample of Ni2P/NPC/NF: (g) TEM morphology, (h) HRTEM image and (i–m) EDX image of Ni2P/NPC/NF.
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Figure 3. (a) XRD patterns of NF, Ni(OH)2/NF, Ni(OH)2/Ni-MOF/NF-1 h, 6 h, 9 h, Ni2P/NF and Ni2P/NPC/NF. (b) Raman spectra of Ni2P/NF and Ni2P/NPC/NF. XPS spectra of Ni2P/NPC/NF: (c) Ni 2p, (d) P 2p, (e) N 1s and (f) C 1s.
Figure 3. (a) XRD patterns of NF, Ni(OH)2/NF, Ni(OH)2/Ni-MOF/NF-1 h, 6 h, 9 h, Ni2P/NF and Ni2P/NPC/NF. (b) Raman spectra of Ni2P/NF and Ni2P/NPC/NF. XPS spectra of Ni2P/NPC/NF: (c) Ni 2p, (d) P 2p, (e) N 1s and (f) C 1s.
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Figure 4. Hydrophilic test in KOH solution of (a) bare nickel foam, (b) Ni(OH)2/NF, (c) Ni2P/NF and (d) Ni2P/NPC/NF.
Figure 4. Hydrophilic test in KOH solution of (a) bare nickel foam, (b) Ni(OH)2/NF, (c) Ni2P/NF and (d) Ni2P/NPC/NF.
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Figure 5. Diagrams of HER’s catalytic performance: (a) polarization curves, (b) corresponding overpotentials at 10 mA cm−2, (c) Tafel curves, (d) Nyquist plots of NF, Ni(OH)2/NF, Ni2P/NF, Ni2P/NPC/NF, Ni2P/NPC/NF-1, Ni2P/NPC/NF-3, Ni2P/NPC/NF-6, Ni2P/NPC/NF-9 and Pt/C of HER in 1 M KOH solution. (e) LSV curves of Ni2P/NPC/NF before and after 3000 cycles in 1 M KOH solution. (f) Timing current density curve for Ni2P/NPC/NF under a constant potential (overpotential = 58 mV vs. RHE) for 48 h.
Figure 5. Diagrams of HER’s catalytic performance: (a) polarization curves, (b) corresponding overpotentials at 10 mA cm−2, (c) Tafel curves, (d) Nyquist plots of NF, Ni(OH)2/NF, Ni2P/NF, Ni2P/NPC/NF, Ni2P/NPC/NF-1, Ni2P/NPC/NF-3, Ni2P/NPC/NF-6, Ni2P/NPC/NF-9 and Pt/C of HER in 1 M KOH solution. (e) LSV curves of Ni2P/NPC/NF before and after 3000 cycles in 1 M KOH solution. (f) Timing current density curve for Ni2P/NPC/NF under a constant potential (overpotential = 58 mV vs. RHE) for 48 h.
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Figure 6. Diagrams of OER’s catalytic performance: (a) polarization curves, (b) corresponding overpotentials at 10 mA cm−2, (c) Tafel curves, (d) Nyquist plots of Ni(OH)2/NF, Ni2P/NF, Ni2P/NPC/NF, Ni2P/NPC/NF-1, Ni2P/NPC/NF-3, Ni2P/NPC/NF-6, Ni2P/NPC/NF-9 and RuO2 of OER in 1 M KOH solution. (e) LSV curves of Ni2P/NPC/NF-6 before and after 3000 cycles in 1 M KOH solution. (f) Timing current density curve for Ni2P/NPC/NF-6 under a constant potential (Overpotential = 208 mV vs. RHE) for 48 h.
Figure 6. Diagrams of OER’s catalytic performance: (a) polarization curves, (b) corresponding overpotentials at 10 mA cm−2, (c) Tafel curves, (d) Nyquist plots of Ni(OH)2/NF, Ni2P/NF, Ni2P/NPC/NF, Ni2P/NPC/NF-1, Ni2P/NPC/NF-3, Ni2P/NPC/NF-6, Ni2P/NPC/NF-9 and RuO2 of OER in 1 M KOH solution. (e) LSV curves of Ni2P/NPC/NF-6 before and after 3000 cycles in 1 M KOH solution. (f) Timing current density curve for Ni2P/NPC/NF-6 under a constant potential (Overpotential = 208 mV vs. RHE) for 48 h.
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Figure 7. (a) Double-layer capacitance (Cdl) plots of nickel foam, Ni(OH)2/NF, Ni2P/NF Ni2P/NPC/NF samples. (b) Polarization curves of Ni2P/NPC/NF toward overall water splitting. (c) Chronoamperometric curve of Ni2P/NPC/NF at constant current densities of 10 mA cm−2 for water splitting in 1.0 M KOH. (d) Coupling synergistic effect of Ni2P and NPC of Ni-MOF on in situ growth of nickel foam and formation of gas mass transfer channel.
Figure 7. (a) Double-layer capacitance (Cdl) plots of nickel foam, Ni(OH)2/NF, Ni2P/NF Ni2P/NPC/NF samples. (b) Polarization curves of Ni2P/NPC/NF toward overall water splitting. (c) Chronoamperometric curve of Ni2P/NPC/NF at constant current densities of 10 mA cm−2 for water splitting in 1.0 M KOH. (d) Coupling synergistic effect of Ni2P and NPC of Ni-MOF on in situ growth of nickel foam and formation of gas mass transfer channel.
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Chen, N.; Che, S.; Liu, H.; Ta, N.; Li, G.; Chen, F.; Ma, G.; Yang, F.; Li, Y. In Situ Growth of Self-Supporting MOFs-Derived Ni2P on Hierarchical Doped Carbon for Efficient Overall Water Splitting. Catalysts 2022, 12, 1319. https://doi.org/10.3390/catal12111319

AMA Style

Chen N, Che S, Liu H, Ta N, Li G, Chen F, Ma G, Yang F, Li Y. In Situ Growth of Self-Supporting MOFs-Derived Ni2P on Hierarchical Doped Carbon for Efficient Overall Water Splitting. Catalysts. 2022; 12(11):1319. https://doi.org/10.3390/catal12111319

Chicago/Turabian Style

Chen, Neng, Sai Che, Hongchen Liu, Na Ta, Guohua Li, Fengjiang Chen, Guang Ma, Fan Yang, and Yongfeng Li. 2022. "In Situ Growth of Self-Supporting MOFs-Derived Ni2P on Hierarchical Doped Carbon for Efficient Overall Water Splitting" Catalysts 12, no. 11: 1319. https://doi.org/10.3390/catal12111319

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