Introduction

With the excessive exhaustion of traditional energy and the deterioration of ecological environment, finding clean and renewable energy with high combustion value has become the urgent demand of human beings [1, 2]. As a promising substitute for traditional energy, hydrogen with recyclability and high energy density has drawn increasing interest [3,4,5]. With the merits of high efficiency and no environment pollution, electrochemical water splitting is considered as a prospective and reliable technology for hydrogen production [6, 7]. In the process of water splitting, exploiting highly efficient and durable hydrogen evolution reaction (HER) catalysts are critical to reduce the overpotential that brings about excessive energy consumption [8,9,10]. Though Pt and Pt-based catalysts stick out from various prominent HER catalysts, the fatal drawbacks of exorbitant price and scarce reserves hinder their commercial applications [11]. Thus, the vigorous development of non-precious materials with high activity and stability as HER catalysts is highly imperative [12,13,14].

Recently, Mo-based catalysts, such as MoS2 [15], MoN [16], Mo2C [17], MoB [18] and MoP [19] etc., have been reported with excellent electrocatalytic HER performance. Among these catalysts, Mo2C catalyst has attracted extensive attention because of its impressive conductivity, strong corrosion resistance and analogous d-band electronic structure with Pt [20, 21]. Nevertheless, the catalytic performance of Mo2C is still limited by its inherent shortage of large unoccupied orbitals density and certain aggregation [22, 23]. For reducing the density of unoccupied orbitals of Mo2C, the design of bimetallic carbide as HER catalysts by adding electron-rich group VIII metal into Mo2C is an efficient strategy [24]. Hu et al. have reported that Ni-Mo2CCB/CFP electrocatalysts need overpotential of 121.4 mV at 10 mA cm−2 [25]. Lin et al. have prepared Fe3C-Mo2C/NC as HER electrocatalysts with overpotential of 116 mV at 10 mA cm−2 [26].

As well known, the intimate conjugation between Mo2C and carbonaceous materials promotes the electrons transport, stabilizes the overall structure, reduces hydrogen Gibbs adsorption free energy of Mo2C to optimize the absorption of H* and inhibits the aggregation of Mo2C nanoparticles to some extent, which is propitious to enhance electrocatalytic ability of catalysts [27,28,29]. Heteroatoms doping, especially nitrogen atoms, can further enhance the electrochemical performance by optimizing the electronic structure of carbon materials [30]. Among carbon materials, nitrogen-doped carbon fabricated by high temperature pyrolysis of melamine has sparked significant interest for its impressing conductivity, low cost and simple preparation [31]. To the best of our knowledge, there are few reports concerning the application of bimetallic Co-Mo carbide compounding with nitrogen-doped carbon as HER electrocatalysts. Taking these virtues into account, we anticipate that bimetallic Co-Mo carbide/nitrogen-doped carbon (Co-Mo2C/N-C) can serve as HER catalysts with low overpotential.

Herein, nanocoral-like Co-Mo2C/N–C electrocatalyst has been in-situ synthesized by high temperature pyrolysis of CoMoO4 and melamine. As a result, nanocoral-like CMCNC-3 catalyst only needs low overpotentials of 212 and 290 mV at the current density of 10 and 40 mA cm−2, respectively. In addition, CMCNC-3 catalyst shows favorable stability during durative hydrogen generation. The strategy to fabricate efficient and stable Co-Mo2C/-C catalyst offers a broad perspective for the exploitation of metal-carbide-based catalysts toward HER.

Experimental

Chemicals and materials

Na2MoO4 2H2O, Co(NO3)2 6H2O, NaOH, melamine, isopropanol and ethanol were obtained from Jiangsu Yatai Chemical Co., Ltd. (Jiangsu, China). Nafion solution (5 wt%) was provided by Sigma-Aldrich (St Louis, USA).

Fabrication of Co-Mo2C/N-C materials

Scheme 1 presented the fabrication procedure for Co-Mo2C/N-C materials. In the first step, as-obtained CoMoO4 precursor was fabricated via hydrothermal method. In detail, 2.0 mmol Na2MoO4 2H2O and 2.0 mmol Co(NO3)2 6H2O were added into 40 mL distilled water with stirring for 15 min. Next, the mixture was transferred into Teflon-lined autoclave for hydrothermal treatment at 160 °C for 6 h. The precipitate was washed several times with deionized water and ethanol, and finally dried at 60 °C overnight.

Scheme 1
scheme 1

Fabrication procedure for Co–Mo2C/N-C materials

Full size image

In the second step, Co-Mo2C/N-C materials were fabricated via high temperature pyrolysis of CoMoO4 and melamine. In detail, 100 mg as-fabricated CoMoO4 precursor and a certain mass of melamine were fully mixed by grinding in an agate mortar. Subsequently, the mixture above was transferred into a tube furnace for pyrolysis at 850 °C for 3 h with a heating rate of 5 °C min−1 in an argon atmosphere. After pyrolysis, collected product was ground and packaged to be used. The mass ratios between CoMoO4 and melamine were 1:5, 1:10, 1:15 and 1:20, and the corresponding samples were named as CMCNC-1, CMCNC-2, CMCNC-3 and CMCNC-4, respectively.

For comparison, CoMoO3 was fabricated following the steps above without melamine.

Materials characterizations

Scanning electron microscope (SEM, Sigma 300, Carl Zeiss SMT Pte Ltd., Germany) was operated to analyse the morphology. In order to investigate the structure of carbon components, Raman spectrometer (inVia, Renishaw Instrument Co., Britain) was employed. To study the crystal structures of as-fabricated materials, X-ray diffraction (XRD) tests were performed on X' Pert PRO diffractometer (PANalytical, Netherlands) using Cu Kα radiation (λ = 0.154060 nm). X-ray photoelectron spectroscope (XPS) tests were conducted on 5000 Versaprobe-II photoelectron spectroscope (ULVAC-PHI, Japan) with Al Kα (hυ = 1486.6 eV) to investigate the surface chemical states of materials.

Electrode fabrication and electrochemical measurements

Fabrication of working electrodes

Before use, glassy carbon electrode was polished on chamois leather using aluminum oxide powders and washed thoroughly with ethanol and distilled water and finally dried naturally.

As-fabricated active materials (5.0 mg) were ultrasonically dispersed in 1.0 mL of isopropanol/water (Visopropanol/Vwater = 3:7) containing 5.0 μL Nafion solution (5 wt%) to form homogeneous material ink. Next, the ink above (5.0 μL) was dripped onto GCE, and finally dried at ambient environment. The mass loading of material on GCE was about 0.35 mg cm−2.

Electrochemical measurements of as-fabricated materials

Polarization curves, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were operated on PARSTAT 2273 electrochemical workstation (Princeton Applied Research, USA), and chronoamperometry measurements were conducted on DH7000 electrochemical workstation (Jiangsu Donghua Analysis Instrument Co., Ltd., Jingjiang, China) in 1.0 M KOH solution at ambient environment with a standard three-electrode system. Reference electrode, working electrode and counter electrode were HgO/Hg electrode, as-fabricated electrode and graphite rod, respectively. In this work, all potentials measured were obtained after IR correction and converted to reversible hydrogen electrode (RHE) according to the following formula: ERHE = (EHgO/Hg + 0.098 V) + 0.059 pH. Before the electrochemical measurements, all 1.0 M KOH electrolytes were saturated with N2 for 1 h.

Results and discussion

Characterizations of structure, component and morphology

Shown in Fig. 1 is the Raman spectrum of CMCNC-3 catalyst. Two peaks at 1317.2 and 1590.4 cm−1 are ascribed to D and G bands, respectively, confirming the presence of disordered carbon and graphitic carbon in CMCNC-3 [32]. In addition, the calculated intensity ratio of D band and G band value for CMCNC-3 is 1.4, verifying that it possesses low graphitized degree and a large number of structural defects [33].

Figure 1
figure 1

Raman spectrum of CMCNC-3 catalyst

Full size image

XRD tests are carried out to study crystal structure of CoMoO4 (a), CoMoO3 (b) and CMCNC-3 (c) (Fig. 2). As for CoMoO4 (curve a), the peaks at 13.4°, 23.2°, 26.9°, 29.3°, 34.3° and 52.8° correspond to (001), (021), (002), (310), (022) and (440) crystal planes of CoMoO4 (JCPDS 21–0868) [34], respectively. As for CoMoO3 (curve b), the peaks at 18.0°, 25.4°, 32.7°, 36.1°, 37.2°, 40.5°, 45.5°, 49.2°, 52.1°, 56.2°, 59.8°, 62.5° and 64.6° are ascribed to (002), (102), (103), (200), (004), (104), (203), (114), (204), (006), (205), (303) and (220) crystal planes of CoMoO3 (JCPDS 21-0869) [35], respectively. As for CMCNC-3 (curve c), a peak at 26.5° is typical (002) crystal plane of graphitic carbon [36]. The peaks at 36.1°, 51.9°, 64.6°, 73.9° and 76.1° are ascribed to (100), (102), (110), (112) and (201) crystal planes of β-Mo2C (JCPDS 35–0787) [37]. The peaks at 32.1o, 44.5o and 49.1o are ascribed to (400), (511) and (442) crystal planes of Mo3Co3C (JCPDS 65-7128) [38]. The results prove the coexistence of β-Mo2C, Mo3Co3C and graphitic carbon in CMCNC-3.

Figure 2
figure 2

XRD patterns of CoMoO4 a, CoMoO3 b and CMCNC-3 c

Full size image

The surface electronic states of CoMoO4, CoMoO3 and CMCNC-3 catalysts are investigated by XPS analyses (Fig. 3). In Fig. 3a, Co 2p spectrum of CoMoO4 exhibits five peaks at 780.9, 782.1, 787.1, 797.5 and 803.2 eV. Two peaks at 780.9 and 782.1 eV belong to Co 2p3/2, and a peak at 797.5 eV belongs to Co 2p1/2 [39, 40]. In addition, two peaks at 787.1 and 803.2 eV belong to satellite peaks [41]. Mo 3d spectrum of CoMoO4 (Fig. 3b) shows two peaks at 231.9 and 235.1 eV, which correspond to Mo 2d5/2 and Mo 2d3/2, respectively [42, 43]. In O 1 s spectrum of CoMoO4 (Fig. 3c), a peak at 530.7 eV is related to lattice oxygen, while a peak at 533.2 eV is attributed to –OH on the surface of CoMoO4 [44].

Figure 3
figure 3

XPS spectra of CoMoO4 a–c, CoMoO3 d–f and CMCNC-3 g–j

Full size image

In Fig. 3d, Co 2p spectrum of CoMoO3 shows two peaks at 781.9 and 797.4 eV, which belong to Co 2p3/2 and Co 2p1/2, respectively. Additionally, two peaks at 786.4 and 804.3 eV correspond to shake-up satellite peaks [45]. In Mo 3d spectrum of CoMoO3 (Fig. 3e), two peaks at 229.9 and 233.0 eV belong to Mo4+, while two peaks at 231.7 and 234.7 eV are assigned to Mo6+ [46]. In Fig. 3f, O 1s spectrum of CoMoO3 displays two peaks at 530.6 and 532.3 eV, which belong to lattice oxygen and –OH on the surface of CoMoO3, respectively [47].

Co 2p spectrum of CMCNC-3 (Fig. 3g) exhibits the peaks at 781.7 and 797.9 eV, which are assigned to Co2+ [48]. In addition, two peaks at 783.9 and 799.4 eV are assigned to Co3+ [49]. The peaks at 787.6, 803.7 and 805.8 eV correspond to satellite peaks [50]. As displayed in Fig. 3h, Mo 3d spectrum of CMCNC-3 exhibits six peaks at 228.4, 229.2, 231.6, 232.3, 232.9 and 235.6 eV. Two peaks at 228.4 and 231.6 eV are attributed to Mo-C [51]. Two peaks at 229.2 and 232.9 eV belong to Mo4+, while two peaks at 232.3 and 235.6 eV belong to Mo6+ [52]. In C 1 s spectrum of CMCNC-3 (Fig. 3i), two peaks at 284.8 and 286.0 eV belong to C–C/C = C and C-N bond, respectively [53]. In Fig. 3j, N 1s spectrum of CMCNC-3 shows the peaks at 394.2, 397.7, 398.5 and 400.3 eV, which belong to Mo–N, pyridinic-N, pyrrolic-N and graphitic-N, respectively [54, 55].

SEM images of CoMoO4, CoMoO3 and CMCNC-3 are shown in Fig. 4a ~ c. CoMoO4 (Fig. 4a) displays uneven nanorods morphology with certain agglomeration. CoMoO3 (Fig. 4b) shows nanocoral-like morphology. However, its nonuniform and aggregation impede the exposure of active sites to a certain degree. After the addition of melamine in the precursor, CMCNC-3 catalyst (Fig. 4c) exhibits looser nanocoral-like morphology assembled by numerous more uniform nanoparticles in comparison with CoMoO3, exposing enough electrocatalytic active sites. EDS elemental mapping images of CMCNC-3 are exhibited in Fig. 4e~h, demonstrating that four elements (Co, Mo, C and N) are uniformly dispersed in CMCNC-3.

Figure 4
figure 4

SEM images of CoMoO4 a, CoMoO3 b and CMCNC-3 catalysts c; SEM image d and EDS elemental mappings e–h of CMCNC-3 catalyst

Full size image

Electrochemical characterizations

Polarization curves of as-fabricated electrocatalysts with a scan rate of 1 mV s−1 using a three-electrode system in 1.0 M KOH electrolyte are presented in Fig. 5a. Co-Mo2C/N–C catalyst displays improved electrocatalytic performance with lower overpotential toward HER in comparison with CoMoO3. Additionally, CMCNC-3 catalyst exhibits better electrocatalytic activity with onset potential as low as 106 mV, while 257 mV, 237 mV, 175 mV and 221 mV for CoMoO3, CMCNC-1, CMCNC-2 and CMCNC-4, respectively. Clearly, CMCNC-3 catalyst manifests lower overpotential (212 mV) than those of CoMoO3 (352 mV), CMCNC-1 (309 mV), CMCNC-2 (267 mV) and CMCNC-4 (288 mV) at 10 mA cm−2. CMCNC-3 catalyst displays an overpotential of 290 mV at 40 mA cm−2, which is lower than CoMoO3 (440 mV), CMCNC-1 (366 mV), CMCNC-2 (323 mV) and CMCNC-4 (350 mV).

Figure 5
figure 5

Polarization curves (a) and corresponding Tafel plots (b) of CoMoO3, CMCNC-1, CMCNC-2, CMCNC-3 and CMCNC-4 catalysts

Full size image

Co-Mo2C/N-C catalyst has looser structure than CoMoO3, which exposes more catalytic active sites, leading to excellent catalytic activity toward HER. Besides, owing to the synergistic effect between Mo2C, Mo3Co3C and N–C, CMCNC-3 catalyst shows commendable HER activity.

In Fig. 5b, Tafel plots are used to study the HER kinetics of as-fabricated catalysts. Tafel slope is calculated by Tafel equations as follows: η = a + b log j, where η is overpotential, a the constant, b the Tafel slope (mV dec−1) and j the current density (mA cm−2) [6]. As observed, CoMoO3, CMCNC-1, CMCNC-2, CMCNC-3 and CMCNC-4 catalysts show Tafel slope values of 99, 79, 68, 60 and 69 mV dec−1, respectively. Obviously, CMCNC-3 catalyst manifests smaller value of Tafel slope than other four catalysts, suggesting faster kinetics for electrocatalytic HER, which contributes to rapid reaction on its surfaces. In addition, Tafel slopes of 118, 40 and 30 mV dec−1 correspond to Volmer, Heyrovsky and Tafel reaction during HER process in alkaline solutions, respectively [56, 57]. The results indicate that rate-limiting mechanism of as-fabricated catalysts is Volmer-Heyrovsky mechanism.

To further investigate HER electrocatalytic performance of CoMoO3, CMCNC-1, CMCNC-2, CMCNC-3 and CMCNC-4, the electrochemically active surface areas (ECSA) are assessed by electrochemical double-layer capacitance (Cdl). In Fig. 6a ~ e, CV tests of as-fabricated catalysts are carried out in non-Faradaic potential region (0.290 V ~ 0.390 V vs. RHE) with diverse scan rates (40 mV s−1 ~ 200 mV s−1). As shown in Fig. 6f, current density Δj (janode—jcathode) at 0.34 V vs. RHE against scan rate shows the linear relationship. Meanwhile, it is clear that Cdl value of CMCNC-3 (9.67 mF cm−2) is higher than those of CoMoO3 (0.22 mF cm−2), CMCNC-1 (4.30 mF cm−2), CMCNC-2(6.51 mF cm−2), CMCNC-4 (5.33 mF cm−2), indicating higher ECSA and numerous exposed active sites of CMCNC-3. The high electrocatalytic activity of catalysts is positively related to the enhancement of ECSA and active sites [58]. Thus, nanocoral-like CMCNC-3 catalyst exhibits better HER electrocatalytic performance than all other as-fabricated catalysts.

Figure 6
figure 6

CV curves of CoMoO3 (a), CMCNC-1 (b), CMCNC-2 (c), CMCNC-3 (d) and CMCNC-4 catalysts (e) under different scan rates; plots of the capacitive currents as a function of scan rates (f)

Full size image

EIS tests are carried out in the range of 106 ~ 10–1 Hz at open circuit potential with a modulation amplitude of 5 mV, and Nyquist plots are displayed in Fig. 7. In equivalent circuit diagram (the insert in Fig. 7), Rs is the uncompensated solution resistance, Rct is the charge transfer resistance, Cdl is the double-layer capacitance, and Zw is the Warburg impedance. Furthermore, CoMoO3, CMCNC-1, CMCNC-2, CMCNC-3 and CMCNC-4 catalysts show Rct values (Table 1) of 9.96, 9.61, 7.37, 6.59 and 7.80 Ω, respectively. The lower Rct value of CMCNC-3 indicates faster electron transfer process.

Figure 7
figure 7

Nyquist plots of CoMoO3, CMCNC-1, CMCNC-2, CMCNC-3 and CMCNC-4 catalysts

Full size image
Table 1 Fitted values of equivalent circuit elements based on impedance spectra of as-fabricated catalysts
Full size table

As shown in Fig. 8, chronoamperometry tests of CoMoO3 and CMCNC-3 catalysts are performed at overpotential of 212 mV in 1.0 M KOH electrolyte. CMCNC-3 catalyst exhibits higher current density than that of CoMoO3 catalyst, indicating that CMCNC-3 catalyst has better HER catalytic activity. In addition, the current density of CMCNC-3 catalyst decreases at firstly and then maintains steady relatively during continuous hydrogen generation, demonstrating eminent long-term durability of CMCNC-3. The favorable HER electrocatalytic activity and stability of CMCNC-3 are ascribed to some reasons as follows: (1) the addition of N-C produced by pyrolysis of melamine, which increases the surface activity and conductivity in theory; (2) the loose nanocoral-like structure assembled by uniform nanoparticles, which provides larger ECSA and abundant ions transport channel; (3) the synergistic effect between Mo2C, Mo3Co3C and N-C.

Figure 8
figure 8

Chronoamperometry tests of CoMoO3 and CMCNC-3 catalysts at overpotential of 212 mV for 30 h

Full size image

Conclusions

In summary, Co-Mo2C/N-C catalyst has been in-situ synthesized by high temperature pyrolysis of CoMoO4 and melamine. The nanocoral-like CMCNC-3 needs overpotentials of only 212 and 290 mV at the current density of 10 and 40 mA cm−2, respectively. Besides, CMCNC-3 shows low charge transfer resistance and outstanding stability during continuous hydrogen generation. The high catalytic activity of CMCNC-3 originates from the efficient dispersion of Co-Mo2C nanoparticles by N-C and the synergistic effect between Mo2C, Mo3Co3C and N-C. The results suggest that nanocoral-like Co-Mo2C/N-C with excellent activity and long-term durability is promising in renewable energy conversion system to achieve massive hydrogen generation.