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Article

Ni5P4-NiP2-Ni2P Nanocomposites Tangled with N-Doped Carbon for Enhanced Electrochemical Hydrogen Evolution in Acidic and Alkaline Solutions

by
Miaomiao Pei
1,
Xiaowei Song
1,2,*,
Haihong Zhong
2,3,*,
Luis Alberto Estudillo-Wong
4,
Yingchun Gao
1,
Tongmengyao Jin
1,
Ju Huang
1,
Yali Wang
1,
Jun Yang
5 and
Yongjun Feng
2,*
1
Key Laboratory of Functional Coordination Compounds of Anhui Higher Education Institutes, School of Chemistry and Chemical Engineering, Anqing Normal University, Anqing 246011, China
2
State Key Laboratory of Chemical Resource Engineering, Beijing Engineering Center for Hierarchical Catalysts, College of Chemistry, Beijing University of Chemical Technology, No. 15 Beisanhuan East Road, Beijing 100029, China
3
College of Science, Hainan University, Haikou 570228, China
4
Departamento de Biociencias e Ingeniería, CIIEMAD-IPN, Instituto Politécnico Nacional, Calle 30 de Junio de 1520, Alcaldía GAM, Ciudad de México 07340, Mexico
5
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(12), 1650; https://doi.org/10.3390/catal12121650
Submission received: 21 November 2022 / Revised: 12 December 2022 / Accepted: 13 December 2022 / Published: 15 December 2022
(This article belongs to the Special Issue Transition Metal Complexes as Catalysts)
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Abstract

:
Heterostructured non-precious metal phosphides have attracted increasing attention in the development of high-performance catalysts for hydrogen evolution reaction (HER), particularly in acidic media. Herein, a catalyst composed of ternary Ni5P4-NiP2-Ni2P nanocomposites and N-doped carbon nanotubes/carbon particulates (Ni5P4-NiP2-Ni2P/NC) was prepared from a Ni-containing hybrid precursor through approaches of a successive carbonization and phosphating reaction. Benefiting from the synergistic effect from three-component nickel phosphides and the support role of porous carbon network, the Ni5P4-NiP2-Ni2P/N-doped carbon catalyst presents the promising HER performance with overpotentials of 168 and 202 mV at the current density of 10 mA cm−2 and Tafel slopes of 69.0 and 74 mV dec−1 in both acidic and alkaline solutions, respectively, which surpasses the Ni2P/N-doped carbon counterpart. This work provides an effective strategy for the preparation and development of highly efficient HER non-precious metal electrocatalysts by creating heterostructure in acidic and alkaline media.

1. Introduction

Hydrogen is regarded as one of the most fascinating fuel carriers, with the ability to tackle the ever-growing energy demand and lessen the emissions of greenhouse gases [1,2]. Among those reported techniques, water electrolysis is one of the effective and sustainable approaches to produce high-purity hydrogen, which depends mainly on high-performance electrocatalysts for the cathodic hydrogen evolution reaction (HER) [3]. To date, Pt-based materials remain the state-of-the-art catalysts for the HER but suffer from high cost, low abundance, and difficult commercial applications. In order to supersede the Pt-based catalysts, it is desirable to explore non-precious metal electrocatalysts [4].
Recently, nickel (Ni) as the center element of [NiFe]-hydrogenases has gained increasing attention in developing Ni-based HER catalysts, including Ni-based sulfides, nitrides, selenides, and phosphides [4,5,6,7,8,9,10,11,12]. Among them, dinickel phosphide (Ni2P) has been widely investigated for HER, owing to its high performance and low cost [3,11,13,14,15,16,17,18]. Yet, improving the electrocatalytic performance of Ni2P for practical application is still desired [19,20,21,22,23,24]. Thus, in recent years, development of heterostructured nickel phosphides is considered to be a promising way to enhance HER intrinsic activity because of the potential synergistic effect among different components. For instance, biphasic nickel phosphides, such as Ni2P/NiP2 [25], Ni2P/Ni5P4 [26,27,28], and Ni2P-Ni12P5 [29,30], have emerged as superior HER electrocatalysts. Yet, the design of nickel phosphide catalysts with high conductivity and stability is still challenging.
One effective strategy is to grow nickel phosphide species on conductive substrates (e.g., nickel foam, carbon cloth, carbon sheets, graphene, and carbon nanotubes), inhibiting the agglomeration of catalysts and, thus, further enhancing catalytic activity and stability [31,32,33,34]. Among them, three-dimensional (3D) porous carbon materials favor the exposure of the active sites [35,36]. For instance, Mao and co-workers [37] employed the graphene as the supporting network to support the chainmail Ni2P nanoparticles encapsulated by ultrathin P-doped carbon shell for efficient hydrogen generation. Chen and co-workers [38] introduced reduced graphene oxide to fabricate carbon nanotubes/graphene-confined Ni-Ni12P5 nanoparticles towards sustainable hydrogen evolution. Lan and co-workers [39] reported commercial multi-walled carbon nanotubes supporting polynary nickel phosphide nanoflowers for HER with enhanced performance. Therefore, it might also be a good route to support nickel phosphides using in situ-formed carbon materials.
Herein, combining the synergy effect of multicomponents and the strategies of in situ-forming carbon substrates, a catalyst composed of ternary Ni5P4-NiP2-Ni2P nanocomposites supported on the N-doped carbon nanotubes/carbon particulates (Ni5P4-NiP2-Ni2P/NC) was prepared from a hybrid precursor Ni(bpy)(NO3)2/g-C3N4/Ni(dmgH)2 through carbonization and subsequent phosphating reaction. It is worth mentioning that the phosphating reaction proceeds via a sodium borohydride (NaBH4)-assisted sodium hypophosphite (NaH2PO2) method. The in situ-grown N-doped carbon nanotubes tangled with N-doped carbon particulates ensure the intimate contact with nickel phosphides and provide the interconnected 3D mesoporous structure, certainly improving the accessibility of active sites. The obtained Ni5P4-NiP2-Ni2P/N-doped carbon catalyst (Ni5P4-NiP2-Ni2P/NC) revealed good HER activity in both 0.5 M H2SO4 and 1 M KOH solutions, outperforming the Ni2P/N-doped carbon counterpart (Ni2P/NC) under the same conditions. Furthermore, the former catalyst maintained favorable stability in acid medium.

2. Results and Discussion

2.1. Characterizations of Morphology and Structure

Figure 1 illustrates that the synthetic process of the Ni5P4-NiP2-Ni2P/NC. Both 4,4′-bipyridine (bpy) and dimethylglyoxime (dmgH2) has strong coordination with Ni2+ to generate coordinate materials, which can be employed as precursors to prepare nitrogen-doped carbon materials via pyrolysis. In addition, g-C3N4 is a widely used self-sacrifice template in the preparation of porous materials [40]. Therefore, we prepared a hybrid precursor coupling g-C3N4 and nickel coordination materials through two-step coordination reactions. First, a simple solvothermal reaction led to the formation of Ni(bpy)(NO3)2/g-C3N4 hybrids, which were further etched by dmgH2 to form Ni(dmgH)2 microwires, ensuring the high dispersion of nickel source in the precursor matrix. Figure S1 presents the XRD pattern of Ni(bpy)(NO3)2/g-C3N4/Ni(dmgH)2 and the single crystal structure of Ni(bpy)(NO3)2. One can observe a broad peak at ~27.3°/2θ assigned to the (002) plane of the g-C3N4 phase [41]. The simulated XRD peaks from single crystal data of Ni(bpy)(NO3)2 and Ni(dmgH)2 match well with the powder XRD results, indicating the successful formation of Ni(bpy)(NO3)2/g-C3N4/Ni(dmgH)2. As displayed in Figure 2a, the needle-like Ni(dmgH)2 grows from the surface of bulk Ni(bpy)(NO3)2 and extends into the g-C3N4 matrix [42]. After pyrolysis, the precursor was carbonized into N-doped carbon-network-twined Ni nanoparticles (Ni/NC), and nickel sources were converted into metallic Ni nanoparticles, which were distributed in the in situ-generated N-doped carbon networks and subsequently transformed into nickel phosphide nanocomposites via the NaBH4 assisted phosphorization.
The SEM image of Ni/NC shows that carbon nanotubes and amorphous carbon particulates are intertwined into a 3D network structure, cf. Figure 2b. When the calcination temperature exceeds 700 °C, g-C3N4 is decomposed into carbon nitride fragments, such as C2N2+, C3N2+, and C3N3+, which can be further carbonized into N-doped carbon nanotubes under the catalysis of in situ-formed nickel nanoparticles [43]. Some bright points emerge on the surface of Ni/NC, which could be metallic nickel particles. Compared with Ni/NC, Ni2P/NC exhibits similar heterogeneous morphologies with slightly larger nanosheets, cf. Figure 2c. In addition, no obvious Ni particles can be observed on the surface of Ni2P/NC. After NaBH4-assisted phosphorization of Ni/NC, seen in Figure 2d, amorphous carbon structures are fused into flocculent-like carbon particulates, which aggregate on the surface of carbon nanotube skeletons to form a 3D network.
Figure 2e further shows the XRD patterns of three samples. One observes that Ni/NC presents three peaks located at 44.5°, 51.8°, and 76.4°/2θ, corresponding to the (111), (200), and (220) crystal planes of cubic Ni (PDF#87-0712). Additionally, the peak located at 25.9° can be assigned to the (002) facet of graphitic carbon [44]. For the sample Ni5P4-NiP2-Ni2P/NC, three diffraction peaks at 52.9°, 45.2°, and 36.1°/2θ are assigned to (303), (204) and (104) crystal planes of Ni5P4 (PDF#65-2075, hexagonal crystal system), respectively. The diffraction peaks at 55.8°, 36.8°, 32.7°, and 28.2°/2θ are individually assigned to (311), (210), (200), and (111) crystal planes of NiP2 phase (PDF#73-0436, cubic crystal system). Three additional diffraction peaks at 54.1°, 44.6°, and 40.5°/2θ are assigned to (300), (021), and (111) crystal planes of Ni2P (PDF#65-3544, hexagonal crystal system), respectively. These results indicate that Ni5P4-NiP2-Ni2P/NC is composed of Ni5P4, Ni2P, and NiP2 phases as well as graphitic carbon. In comparison, when the NaBH4 is absent during the phosphating reaction, the XRD peaks for Ni2P/NC could all be indexed to Ni2P (PDF#65-3544, cubic crystal system). To determine the mass percentage of nickel phosphide polymorphs in the sample Ni5P4-NiP2-Ni2P/NC, the Rietveld Refinement Method (RRM) was carried out using Crystallographic Open Database (COD) for Ni5P4 (COD#221-4342), Ni2P (COD#152-2619), and NiP2 (COD#901-2503) phases. Figure 2f and Table S1 further display the microstructural properties obtained by RRM analysis. Here, Ni5P4 (P63mc) and NiP2 (Pa-3) are the main phases in the sample, with crystallite average size of 90.4 ± 3.4 and 70.6 ± 1.3 nm, respectively. In addition, the Ni2P (P-62m) phase provides the lowest composition with a crystallite average size of 30.3 ± 1.3 nm.
Figure 3a–c demonstrate TEM images of Ni5P4-NiP2-Ni2P/NC. Here, nickel phosphide nanoparticles with sizes in the range of 10 to 100 nm are distributed in the carbon matrix composed of carbon nanotubes and carbon particulates. Furthermore, the SAED pattern in Figure 3d shows the polycrystalline diffraction rings and scattering dots, indicating the coexistence of Ni5P4, Ni2P, and NiP2. HRTEM analysis reveals that the lattice spacing of 0.32 nm is ascribed to the (111) plane of NiP2 phase, cf. Figure 3e, and the lattice spacings of 0.55 and 0.22 nm belong to the (002) plane of Ni5P4 and the (111) plane of Ni2P in Figure 3f, respectively. The Ni5P4 nanoparticles are wrapped by few-layer carbon shells with a lattice spacing of 0.34 nm, corresponding to the (002) plane of graphitic carbon. The elemental mapping images confirm that these nickel phosphide polymorphs appear as irregularly shaped particles in the nitrogen-doped carbon matrix, cf. Figure 3g. Combined with the XRD patterns, these TEM results clearly indicate that the sample Ni5P4-NiP2-Ni2P/NC consists of unique triphasic Ni5P4-NiP2-Ni2P nanocomposites.
The N2 adsorption–desorption isotherms of three samples in Figure 4a exhibit type IV with an H2-type hysteresis loop in the medium pressure region. The hysteresis feature suggests the capillary condensation in the interconnected mesoporous [45]. Figure 4b further shows the mesoporous size distribution in the range from 4 nm to 33 nm. The Brunauer–Emmett–Teller (BET) surface areas are individually calculated as 295.4, 110.3, and 207.6 m2 g−1 for Ni/NC, Ni2P/NC, and Ni5P4-NiP2-Ni2P/NC, respectively. The BET surface area difference among Ni/NC, Ni2P/NC, and Ni5P4-NiP2-Ni2P/NC could correlate with multiple factors, including the size effect, the structural features of carbon network, and the composition of nickel phosphides [46,47,48]. The mesoporous structure and large specific surface area for Ni5P4-NiP2-Ni2P/NC aid the contact between the active sites and the electrolyte and then improve the electrocatalytic activity [40].
Figure 4c displays Raman spectra of three samples with two typical bands centered at approximately 1350 and 1580 cm−1, which correspond to the structural defect carbon (D band) and graphitization carbon (G band), respectively. The intensity ratio of D and G-band (ID/IG) is approximately 1.00, 1.03, and 1.05 for Ni/NC, Ni2P/NC, and Ni5P4-NiP2-Ni2P/NC, respectively, suggesting that the phosphorization results in more defect carbon structures.
Figure S2 demonstrates the XPS survey spectra for all the samples, and Table S2 lists the relative atomic percentages of elements. The high-resolution C 1s spectra (Figure S3) for each sample show four fitted peaks centered at 283.9, 284.6, 287.6, and 290.9 eV, which are assigned to C-C/C=C, C-N/C-O, O=C, and O-C=O bonds, respectively [43]. Figure 4d presents the high-resolution N 1s spectra for all samples. The nitrogen chemical states correspond to four types of N configuration, including pyridinic N (398.2 eV), pyrrolic N (400.2 eV), graphitic N (401.3 eV), and oxidized N (405.3 eV) [49]. Furthermore, Figure 4e shows the comparative high-resolution Ni 2p spectra of three samples. The peak at 851.8 eV appears in Ni/NC, confirming the presence of zero-valent Ni. The sample Ni5P4-NiP2-Ni2P/NC exhibits two peaks located at 853.1 and 870.8 eV, which correspond to Ni 2p3/2 and Ni 2p1/2 spin-orbits of Niδ+ (0 < δ < 2) in nickel phosphides, respectively. The peaks at 855.6 and 874.2 eV are attributed to Ni2+ in nickel phosphides and surface oxidized Ni species, while the peaks at 861.2 and 880.1 eV are assigned to satellite peaks [18]. Moreover, the deconvolution peaks of the Ni 2p band for Ni2P/NC show a relatively low percentage of Niδ+ related to those of Ni5P4-NiP2-Ni2P/NC (35% in Ni5P4-NiP2-Ni2P/NC vs. 22.8% in Ni2P/NC). The peaks positioned at 128.7 and 129.6 eV in the high-resolution P 2p spectrum of Ni5P4-NiP2-Ni2P/NC are indexed to the P 2p3/2 and P 2p1/2 signals of the negatively charged Pδ− (0 < δ < 2), respectively, Figure 4f. It is noteworthy that the Pδ− peaks of the Ni2P/NC sample shifted to high binding energies, likely due to enhanced electron transfer from Pδ− to metal in metal-rich phosphides (Ni2P) [50]. In addition, for Ni5P4-NiP2-Ni2P/NC and Ni2P/NC, the main deconvolution peak at 132.9 eV is ascribed to the P-O species due to the surface oxidation of nickel phosphides [33,51]. In order to discover whether element B is doped into the carbon layer, the XPS high-resolution spectrum of Ni5P4-NiP2-Ni2P/NC was recorded in the binding energy range of 180–200 eV. It must be pointed out that the peaks of B 1s and P 2s overlap in this region. The P 2s peak for Ni5P4-NiP2-Ni2P/NC is obtained from P 2p and subtracted from the overlapping area, while the residual peak intensity is almost zero, cf. Figures S4 and S5. Additionally, both Ni/NC and Ni5P4-NiP2-Ni2P/NC have almost similar carbon and nitrogen states, as observed from their high-resolution C 1s and N 1s spectra. This result indicates that the element B is not doped into the carbon layer. Furthermore, no obvious B 1s-XPS signal is observed in Ni/NC-B, as shown in Figure S6. Therefore, the NaBH4 does not serve as a B-doping reagent in this system but does play a key role in producing polymorph nickel phosphide. In the phosphating process, the additional NaBH4 could act as an oxidant agent to promote the formation of zero-valent P, resulting in the formation of P-rich nickel phosphide.

2.2. Electrochemical Performance in Acidic Solution

For non-precious metal electrocatalysts, it is a great challenge to use them in the acidic electrolyte. Here, the HER performance of the Ni5P4-NiP2-Ni2P/NC catalyst was first evaluated in a 0.5 M H2SO4 solution. For comparison, Ni2P/NC, Ni/NC, and commercial Pt/C catalyst were also examined. In Figure 5a, the Pt/C electrode exhibits the best HER activity, with an overpotential of 40 mV at the current density of 10 mA cm−2 (referred to η10), while the Ni5P4-NiP2-Ni2P/NC electrode shows the overpotential (η10) of 168 mV, which is significantly better than Ni2P/NC (247 mV) and Ni/NC (312 mV). In Figure 5b, the Tafel slope of the Pt/C is as low as 30 mV dec−1, which agrees closely with the previous reports [26,35]. The Ni5P4-NiP2-Ni2P/NC exhibits a Tafel slope of 69 mV dec−1, smaller than that of Ni2P/NC (81 mV dec−1) and Ni/NC (103 mV dec−1) but larger than that of Pt/C. The Tafel slope of Ni5P4-NiP2-Ni2P/NC falls within the range of 40–120 mV dec−1, implying that the electrochemical desorption is the rate-limiting step, and the Volmer–Heyrovsky route is responsible for this HER [33]. The electrocatalytic HER performance of Ni5P4-NiP2-Ni2P/NC is still inferior in relation to the Pt/C catalyst but approaches to, and even outperforms, that of nickel phosphide/carbon systems in the literature, such as Ni2P/C [15], Ni2P/CNS [52], Ni2P@C-400 [53], Ni2P-MOF [54], and Ni-Ni12P5@CNTs/rGO-0.5 [38], in acidic electrolytes, cf. Table S3. In Figure 5c, the Nyquist plots of these catalysts at the overpotential of 170 mV are fitted by a facile equivalent circuit, in which the charge-transfer resistance Rct represents the HER kinetics, and a smaller Rct value corresponds to the faster HER rate. The Rct of Ni5P4-NiP2-Ni2P/NC (21.9 Ω) is lower than that of Ni2P/NC (131.1 Ω), suggesting a much faster electron transfer from active sites of Ni5P4-NiP2-Ni2P/NC to protons during HER. Of note, the catalyst Ni/NC shows Rct as high as 586.8 Ω, which may be due to the blockage of electron transfer at the electrode/solution interface caused by the corrosion of surface nickel particles in the acidic solution. These results further indicate that the HER activity of Ni5P4-NiP2-Ni2P/NC arises from nickel phosphide species.
In addition to electrocatalytic activity, the electrocatalytic stability is another key parameter, particularly for non-precious metal electrocatalysts in acidic electrolyte. Figure 5d shows chronopotentiometric results of Ni5P4-NiP2-Ni2P/NC at the current density of 10 mA·cm−2 in 0.5 M H2SO4. Here, after 20 h constant current electrolysis, the overpotential increases only 12 mV. Furthermore, from the insert graph, after 3000 cycles of CV scans at a scan rate of 100 mV s−1, the overpotential of Ni5P4-NiP2-Ni2P/NC at the current density of 10 mA·cm−2 displays a slight shift of 3 mV, suggesting good stability in the long-term electrocatalytic process. Additionally, the SEM image after chronopotentiometric testing further confirms that the morphology of the Ni5P4-NiP2-Ni2P/NC maintains the original features (Figure S7). In addition, the structural stability of Ni5P4-NiP2-Ni2P/NC is revealed by its XPS spectra. The high-resolution XPS spectra of Ni 2p (Figure S8) and P 2p (Figure S9) display deconvoluted peaks like those of the fresh sample, validating the preferable electrocatalytic stability of Ni5P4-NiP2-Ni2P/NC in acidic solutions.

2.3. Electrochemical Performance in Alkaline Solution

The HER performances of Pt/C and three catalysts were also examined in 1 M KOH solution. In Figure 6a, the Ni5P4-NiP2-Ni2P/NC electrode achieves a catalytic current density of 10 mA cm−2 at an overpotential (η10) of 202 mV, outperforming the electrodes Ni2P/NC (234 mV) and Ni/NC (250 mV), suggesting an enhanced catalytic activity. The η10 of Ni5P4-NiP2-Ni2P/NC in alkaline solution is comparable to that of other nickel phosphide-based HER catalysts (Table S3). Similar to the test results in acidic solution, the Pt/C manifests the highest electrocatalytic activity and the fastest HER kinetics, with a Tafel slope of 48 mV dec−1, cf. Figure 6b. The Tafel slopes of Ni2P/NC, Ni5P4-NiP2-Ni2P/NC and Ni/NC are 80, 74, and 81 mV dec−1, respectively, demonstrating that Ni5P4-NiP2-Ni2P/NC also possesses fast HER kinetics in alkaline solution. Moreover, the boosted HER kinetics of Ni5P4-NiP2-Ni2P/NC is further verified from its Nyquist plots, with a smaller semicircle diameter, as depicted in Figure 6c [55]. Additionally, Figure 6d shows the stability of the Ni5P4-NiP2-Ni2P/NC by chronopotentiometry and continuous CV scans. Although the potential at the current density of 10 mA cm−2 has a rapid negative shift of approximately 58 mV at first, it remains constant in the following 10 h. After accelerated degradation testing, the overpotential at 10 mA cm−2 increases by 50 mV. The XPS characterizations of Ni5P4-NiP2-Ni2P/NC after the 20 h chronopotentiometric experiment show that the intensity of the characteristic peaks of Niδ+ (Ni 2p3/2: 853.1 and Ni 2p1/2: 870.8 eV, Figure S10) and Pδ− (P 2p3/2: 129.0 and P 2p1/2: 129.9 eV, Figure S11) decreases obviously. The decrease in HER activity is attributed mainly to the decomposition of nickel phosphide species, owing to their exposure to air during the chronopotentiometry experiment. This result further indicates the nickel phosphide species plays a key role in electrocatalytic process towards HER.

2.4. Structure–Performance Analysis of Ni5P4-NiP2-Ni2P/NC

One can see that the catalyst Ni5P4-NiP2-Ni2P/NC exhibits enhanced catalytic activity related to Ni2P/NC in both acidic and alkaline solutions. Generally, larger electrochemical active surface area (ECSA) value indicates exposure of more electro-active sites, which positively correlate with the HER activity. The Cdl values of the catalysts were tested in 0.5 M H2SO4 and 1 M KOH through measuring the non-Faradaic capacitive current using CV, cf. Figures S12 and S13. As observed from the fitting results, the ECSA follows the order of Ni5P4-NiP2-Ni2P/NC > Ni2P/NC > Ni/NC in both acidic and alkaline solutions, cf. Figure 7a,b, and Table S4, which is in accordance with the activity trend. The polarization curves are normalized by the ECSA to compare the intrinsic HER activity of the catalysts. In Figure 7c,d, the Ni5P4-NiP2-Ni2P/NC catalyst still shows the highest HER activity among the three catalysts, with the smallest overpotential at the same current density, indicating its best intrinsic activity. These results demonstrate that the enhanced HER activity of Ni5P4-NiP2-Ni2P/NC arises from the enlarged ECSA and intrinsic activity that could be endowed by the synergistic effect of Ni5P4-NiP2-Ni2P and the interconnected 3D mesoporous structure.

3. Materials and Methods

3.1. Materials

All chemicals in this work were obtained from commercial sources and used without further purification. Urea (99%), nickel nitrate hexahydrate (Ni(NO3)2·6H2O, AR), 4,4′-bipyridine (bpy, 98%), dimethylglyoxime (dmgH2, 98%), sodium borohydride (NaBH4, 98%), sodium hypophosphite (NaH2PO2, 98%), and methanol (CH3OH, AR) were purchased from Shanghai Titan Co., Ltd., Shanghai, China. Commercial Pt/C (20 wt% Pt, Hispec 3000) and Nafion solution (5% w/w in water and 1-propanol, D-520) were supplied by Alfa Aesar Chemicals Co. Ltd., Shanghai, China. Ultrapure water was purified by a Milli-Q system and used throughout this work.

3.2. Synthesis of Ni(bpy)(NO3)2/g-C3N4/Ni(dmgH)2 Composites

Carbon nitride (g-C3N4) was prepared from urea following a reported method [56]. Typically, urea was placed in an alumina crucible with a cover and heated at 550 °C for 3 h in a muffle furnace. The resulting yellow powder (2.00 g) was dispersed to a mixed solution containing Ni(NO3)2·6H2O (2.90 g), 4,4′-Bipyridine (bpy, 1.56 g), and CH3OH (60 mL) through ultrasonication for 30 min. Then, the suspension was transferred to a 100 mL Teflon-lined autoclave and heated at 100 °C for 12 h. After being cooling to room temperature, the resultant hybrid Ni(bpy)(NO3)2/g-C3N4 was filtered, re-dispersed in 100 mL methanol, and then treated with dimethylglyoxime (dmgH2, 0.23 g) under vigorous stirring for 30 min. The product Ni(bpy)(NO3)2/g-C3N4/Ni(dmgH)2 was collected via centrifugation, washed with CH3OH for three times, and dried in a vacuum at 60 °C overnight.

3.3. Synthesis of Nitrogen-Doped Carbon-Twined Ni Nanoparticles

The hybrid precursor Ni(bpy)(NO3)2/g-C3N4/Ni(dmgH)2 was pyrolyzed at 900 °C for 1 h under flowing N2 atmosphere in a tube furnace. Before carbonization reaction, N2 gas was passed through the furnace tube for 1 h to remove the air. The heating rate was 5 °C/min. The resultant product was labeled as Ni/NC.

3.4. Synthesis of Nitrogen-Doped Carbon/Carbon Nanotube Network Entangled with Nickel Phosphides Nanoparticles

Ni5P4-NiP2-Ni2P/NC was prepared by sodium borohydride (NaBH4)-assisted phosphorization of Ni/NC. In detail, sodium hypophosphite (NaH2PO2, 2.00 g) and NaBH4 (0.20 g) were mixed uniformly via grinding. Subsequently, the mixture of NaBH4/NaH2PO2 and Ni/NC (0.20 g) was placed at the upstream and downstream of tube furnace, respectively. The furnace was heated to 600 °C for 1 h, with a heating rate of 5 °C/min under a N2 atmosphere. After phosphorization, the black product was rinsed with distilled water and methanol and finally dried under vacuum at 60 °C, marked as Ni5P4-NiP2-Ni2P/NC. For comparison, the counterparts Ni2P/NC and Ni/NC-B were prepared using the same procedure for Ni5P4-NiP2-Ni2P/NC but without NaBH4 for the former and without NaH2PO2 for the latter.

3.5. Characterization Techniques

The crystalline phases were characterized by X-ray diffraction (XRD) analysis. Diffraction data were collected on a Rigaku Ultima IV diffractometer (Rigaku Co., Tokyo, Japan) with a scanning rate of 0.4°(2θ) min−1 using Cu-Kα (λ= 0.15406 nm) radiation at 40 kV and 30 mA. Scanning electron microscope (SEM) images were obtained using Phenom LE scanning electron microscope (Phenom-World, Eindhoven, Netherlands). High-resolution transmission electron microscope (HRTEM) images, selected area electron diffraction (SAED), and elemental mapping images were recorded on a FEI Tecnai G2 F20 microscope (FEI Co., Hillsboro, OR, USA) operated at an accelerating voltage of 200 kV, with a line resolution of 0.102 nm, a point resolution of 0.24 nm, and an information resolution of 0.14 nm. X-ray photoelectron spectroscopy (XPS) was measured on a Thermo Scientific K-Alpha X-ray photoelectron spectrometer (Thermo Scientific, Waltham, MA, USA) equipped with Al anode. Low-temperature nitrogen adsorption–desorption experiments were performed with a Quantachrome 3QDS-MP-30 instrument (Quantachrome Instruments, Boynton Beach, FL, USA). The specific surface area was calculated according to the Brunauer–Emmett–Teller (BET) method on the basis of the adsorption isotherm. The pore-size distribution was calculated using the density functional theory (DFT) method. Raman spectra were acquired on a Thermo DXR micro-Raman spectrometer (Thermo Scientific, Waltham, MA, USA) with a visible laser beam of 532 nm.

3.6. Electrochemical Test

Typically, 5.0 mg of the electrocatalyst was dispersed in 500 μL N,N-Dimethylformamide and 500 μL Nafion aqueous solutions (0.5%) by ultrasonication for 2 h to form the catalyst ink. Then, 8 μL of ink was dropped onto the polished glassy carbon disk of rotating disk electrode (RDE, diameter: 5 mm). The loading amount was 0.204 mg cm−2 for each catalyst on the electrode surface. The commercial 20 wt% Pt/C catalyst was also measured under the same conditions for comparison.
The electrochemical measurements were conducted in a standard three-electrode cell on a CHI760E electrochemical workstation. The glassy carbon RDE coated with the catalyst was used as the working electrode. The graphite rod and saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. All the potentials were reported with respect to the reversible hydrogen electrode (RHE) based on the equation:
E (V vs. RHE) = E (V vs. SCE) + 0.059 pH + 0.24 V
Linear sweep voltammetry (LSV) was recorded in Ar-saturated 0.5 M H2SO4 or 1 M KOH solution at a scan rate of 5 mV s−1. The Tafel slope was calculated according to the equation:
η = b log (−j/mA cm−2) + a
where η, b, and j represent the overpotential (η = 0 V − ERHE), Tafel slope, and current density, respectively. The LSV polarization curves and corresponding Tafel plots were 95% internal resistance (iR)-corrected. Electrochemical impedance spectroscopy (EIS) measurements were performed with the frequency from 100 kHz to 0.1 Hz at the overpotentials of 170 mV and 220 mV in 0.5 M H2SO4 and 1 M KOH solutions, respectively. The EIS spectra were simulated using an equivalent circuit which consisted of a solution resistance (Rs) connected in series with two constant-phase element (CPE) that individually combined with the charge-transfer resistance (Rct) and polarization resistance (Rp) in parallel. The electrochemical double-layer capacitance (Cdl) of each sample was measured between 0.04 and 0.14 V vs. RHE at the scan rate from 10 to 100 mV s−1. The electrochemical active surface area (ECSA) was calculated on the basis of the equation:
ECSA = Cdl/Cs
where Cs is the specific capacitance, often assumed to be 40 μF·cm−2 [57]. The stability of Ni5P4-NiP2-Ni2P/NC was evaluated by accelerated degradation experiment and chronopotentiometry in both acidic and alkaline solutions. For accelerated degradation test, cyclic voltammetry (CV) measurement was performed with a scan rate of 100 mV s−1 between −0.32 V and 0.18 V vs. RHE for 3000 cycles, and the LSV curves before and after the cycling were recorded at 5 mV s−1. Chronopotentiometry measurement was conducted under a constant current density of 10 mA cm−2 for up to 70,000 s.

4. Conclusions

In summary, we have reported an NaBH4 assisted phosphorization strategy for the synthesis of ternary Ni5P4-NiP2-Ni2P nanocomposites dispersed in the N-doped carbon nanotubes and carbon particulates. The resultant product (Ni5P4-NiP2-Ni2P/NC) exhibits a better HER performance with a low overpotential and a small Tafel slope in 0.5 M H2SO4 and 1 M KOH electrolyte, related to the counterpart Ni2P/NC. Furthermore, the Ni5P4-NiP2-Ni2P/NC catalyst holds a good stability in acidic solution. The enhancement of activity is attributed to the synergistic effect among three-component nickel phosphides and the support of 3D mesoporous carbon network. This work opens a new avenue to develop heterostructured electrocatalysts towards HER with high-performance.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12121650/s1, Figure S1: (a) XRD pattern of the precursor Ni(bpy)(NO3)2/g-C3N4/Ni(dmgH)2 and simulated XRD patterns of Ni(bpy)(NO3)2 and Ni(dmgH)2, (b) and (c) the crystal structure of Ni(bpy)(NO3)2; Figure S2: X-ray photoelectron spectra of Ni/NC, Ni2P/NC, Ni5P4-NiP2-Ni2P/NC, and Ni/NC-B; Figure S3: High-resolution XPS spectra of C 1s for the samples Ni/NC, Ni2P/NC, and Ni5P4-NiP2-Ni2P/NC; Figure S4: High-resolution XPS spectrum of P 2s for the sample Ni2P/NC; Figure S5: High-resolution XPS spectra of P 2s + B 1s for the sample Ni5P4-NiP2-Ni2P/NC; Figure S6: High-resolution XPS spectrum of B 1s for the sample Ni/NC-B; Figure S7: SEM image for Ni5P4-NiP2-Ni2P/NC after the chronopotentiometric test in 0.5 M H2SO4 solution; Figure S8: XPS high-resolution spectrum of Ni 2p for Ni5P4-NiP2-Ni2P/NC after the chronopotentiometric test in 0.5 M H2SO4 solution; Figure S9: XPS high-resolution spectrum of P 2p for Ni5P4-NiP2-Ni2P/NC after the chronopotentiometric test in 0.5 M H2SO4 solution; Figure S10: XPS high-resolution spectrum of Ni 2p for Ni5P4-NiP2-Ni2P/NC after the chronopotentiometric test in 1 M KOH solution; Figure S11: XPS high-resolution spectrum of P 2p for Ni5P4-NiP2-Ni2P/NC after the chronopotentiometric test in 1 M KOH solution; Figure S12: Electrochemical double-layer capacitance (Cdl) measurements in 0.5 M H2SO4 for (a) Ni/NC, (b) Ni2P/NC, and (c) Ni5P4-NiP2-Ni2P/NC using CV scans at different scan rates from 10 to 100 mV·s−1; Figure S13: Electrochemical double-layer capacitance (Cdl) measurements in 1 M KOH for (a) Ni/NC, (b) Ni2P/NC, and (c) Ni5P4-NiP2-Ni2P/NC using CV scans at different scan rates from 10 to 100 mV·s−1; Table S1: Microstructural properties obtained by RRM for the sample Ni5P4-NiP2-Ni2P/NC; Table S2: Surface atomic compositions obtained from XPS spectra for the as-prepared samples; Table S3: Comparison of the HER Performance of the heterostructured catalysts comprising different nickel phosphides in the literatures and this work [3,15,25,26,27,28,29,30,32,33,34,35,37,38,52,53,54,58,59,60,61]; Table S4: Electrochemical active surface area (ECSA) of the as-prepared composites.

Author Contributions

M.P., methodology, validation, investigation, data curation, and writing—original draft preparation; X.S., methodology, resources, writing—review and editing, and funding acquisition; H.Z., investigation, funding acquisition, and writing—review and editing; L.A.E.-W., formal analysis, investigation; Y.G., data curation and investigation; T.J., J.H. and Y.W., methodology, validation, and data curation; J.Y. and Y.F., conceptualization, writing—original draft preparation, supervision, and funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the University Natural Science Research Project of Anhui Province (No. KJ2019A0549), Innovative Program of Anqing Normal University (202110732023 and XJ202110372019), Fundamental Research Funds for the Central Universities (ZY2117), the European Union (ERDF) ‘Région Nouvelle Aquitaine’, the Joint Funds of the National Natural Science Foundation of China (ZK20180055), Programs for Foreign Talent (G2021106012L) and the National Natural Science Foundation of China (22075290).

Data Availability Statement

Not applicable.

Acknowledgments

We are thankful for the help of the Analytical and Testing Centre of Anqing Research Institute, Beijing University of Chemical Technology, for the BET characterization.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 01650 g001 550
Figure 1. Illustration drawing of the synthesis process of Ni5P4-NiP2-Ni2P/NC.
Figure 1. Illustration drawing of the synthesis process of Ni5P4-NiP2-Ni2P/NC.
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Figure 2. SEM images of (a) Ni(bpy)(NO3)2/g-C3N4/Ni(dmgH)2, (b) Ni/NC, (c) Ni2P/NC, and (d) Ni5P4-NiP2-Ni2P/NC; (e) XRD patterns of Ni/NC, Ni2P/NC, and Ni5P4-NiP2-Ni2P/NC samples; (f) XRD experimental and fitting patterns for Ni5P4-NiP2-Ni2P/NC. G-of-F (Rwp%) parameter is 1.12 (4.1).
Figure 2. SEM images of (a) Ni(bpy)(NO3)2/g-C3N4/Ni(dmgH)2, (b) Ni/NC, (c) Ni2P/NC, and (d) Ni5P4-NiP2-Ni2P/NC; (e) XRD patterns of Ni/NC, Ni2P/NC, and Ni5P4-NiP2-Ni2P/NC samples; (f) XRD experimental and fitting patterns for Ni5P4-NiP2-Ni2P/NC. G-of-F (Rwp%) parameter is 1.12 (4.1).
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Figure 3. (a–c) TEM, (d) SAED pattern, (e,f) HRTEM, and (g) the corresponding elemental (C, N, Ni, and P) mappings of Ni5P4-NiP2-Ni2P/NC.
Figure 3. (a–c) TEM, (d) SAED pattern, (e,f) HRTEM, and (g) the corresponding elemental (C, N, Ni, and P) mappings of Ni5P4-NiP2-Ni2P/NC.
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Figure 4. (a) N2 adsorption–desorption isotherms and (b) pore-size distribution of the samples Ni/NC, Ni2P/NC, and Ni5P4-NiP2-Ni2P/NC; (c) Raman spectra of Ni/NC, Ni2P/NC, and Ni5P4-NiP2-Ni2P/NC; High-resolution XPS spectra of (d) N 1s, (e) Ni 2p, and (f) P 2p for all samples.
Figure 4. (a) N2 adsorption–desorption isotherms and (b) pore-size distribution of the samples Ni/NC, Ni2P/NC, and Ni5P4-NiP2-Ni2P/NC; (c) Raman spectra of Ni/NC, Ni2P/NC, and Ni5P4-NiP2-Ni2P/NC; High-resolution XPS spectra of (d) N 1s, (e) Ni 2p, and (f) P 2p for all samples.
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Figure 5. (a) Polarization curves of Ni/NC, Ni2P/NC, Ni5P4-NiP2-Ni2P/NC, and Pt/C in the 0.5 M H2SO4; (b) the Tafel plots derived from Figure 5a; (c) Nyquist plots of Ni/NC, Ni2P/NC, and Ni5P4-NiP2-Ni2P/NC at the overpotential of 170 mV; (d) Chronopotentiometric curves of the Ni5P4-NiP2-Ni2P/NC catalyst with a constant current density of 10 mA cm−2 for 70,000 s, insert is polarization curves of initial and after 3000 cycles of CV scans at 100 mV s−1.
Figure 5. (a) Polarization curves of Ni/NC, Ni2P/NC, Ni5P4-NiP2-Ni2P/NC, and Pt/C in the 0.5 M H2SO4; (b) the Tafel plots derived from Figure 5a; (c) Nyquist plots of Ni/NC, Ni2P/NC, and Ni5P4-NiP2-Ni2P/NC at the overpotential of 170 mV; (d) Chronopotentiometric curves of the Ni5P4-NiP2-Ni2P/NC catalyst with a constant current density of 10 mA cm−2 for 70,000 s, insert is polarization curves of initial and after 3000 cycles of CV scans at 100 mV s−1.
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Figure 6. (a) Polarization curves of Ni/NC, Ni2P/NC, Ni5P4-NiP2-Ni2P/NC, and Pt/C in the 1 M KOH; (b) the Tafel plots derived from Figure 6a; (c) Nyquist plots of Ni/NC, Ni2P/NC, and Ni5P4-NiP2-Ni2P/NC at the overpotential of 220 mV; (d) Chronopotentiometric curves of Ni5P4-NiP2-Ni2P/NC catalyst with a constant current density of 10 mA cm−2 for 70,000 s in 1 M KOH, insert is polarization curves of initial and after 3000 cycles of CV scans at 100 mV s−1.
Figure 6. (a) Polarization curves of Ni/NC, Ni2P/NC, Ni5P4-NiP2-Ni2P/NC, and Pt/C in the 1 M KOH; (b) the Tafel plots derived from Figure 6a; (c) Nyquist plots of Ni/NC, Ni2P/NC, and Ni5P4-NiP2-Ni2P/NC at the overpotential of 220 mV; (d) Chronopotentiometric curves of Ni5P4-NiP2-Ni2P/NC catalyst with a constant current density of 10 mA cm−2 for 70,000 s in 1 M KOH, insert is polarization curves of initial and after 3000 cycles of CV scans at 100 mV s−1.
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Figure 7. The capacitive currents of three samples were fitted as a function of scan rates from 10 mV s−1 to 100 mV s−1 in (a) 0.5 M H2SO4 and (b) 1 M KOH solutions; the polarization curves normalized by ECSA for the three catalysts in (c) 0.5 M H2SO4 and (d) 1 M KOH solutions.
Figure 7. The capacitive currents of three samples were fitted as a function of scan rates from 10 mV s−1 to 100 mV s−1 in (a) 0.5 M H2SO4 and (b) 1 M KOH solutions; the polarization curves normalized by ECSA for the three catalysts in (c) 0.5 M H2SO4 and (d) 1 M KOH solutions.
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MDPI and ACS Style

Pei, M.; Song, X.; Zhong, H.; Estudillo-Wong, L.A.; Gao, Y.; Jin, T.; Huang, J.; Wang, Y.; Yang, J.; Feng, Y. Ni5P4-NiP2-Ni2P Nanocomposites Tangled with N-Doped Carbon for Enhanced Electrochemical Hydrogen Evolution in Acidic and Alkaline Solutions. Catalysts 2022, 12, 1650. https://doi.org/10.3390/catal12121650

AMA Style

Pei M, Song X, Zhong H, Estudillo-Wong LA, Gao Y, Jin T, Huang J, Wang Y, Yang J, Feng Y. Ni5P4-NiP2-Ni2P Nanocomposites Tangled with N-Doped Carbon for Enhanced Electrochemical Hydrogen Evolution in Acidic and Alkaline Solutions. Catalysts. 2022; 12(12):1650. https://doi.org/10.3390/catal12121650

Chicago/Turabian Style

Pei, Miaomiao, Xiaowei Song, Haihong Zhong, Luis Alberto Estudillo-Wong, Yingchun Gao, Tongmengyao Jin, Ju Huang, Yali Wang, Jun Yang, and Yongjun Feng. 2022. "Ni5P4-NiP2-Ni2P Nanocomposites Tangled with N-Doped Carbon for Enhanced Electrochemical Hydrogen Evolution in Acidic and Alkaline Solutions" Catalysts 12, no. 12: 1650. https://doi.org/10.3390/catal12121650

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