1 Introduction
Developing highly efficient and stable electrocatalysts for hydrogen evolution reaction (HER) is crucial as a result of the low catalytic activity for electrolytic water splitting [1]. However, the noble-metal such as Pt/Pd-based catalysts still suffer from high price and insufficient supply, which hinders their large-scale application [2]. Recently, the transition metal-based phosphides, chalcogenides, and selenides have demonstrated to be promising alternatives to noble metal materials for HER [3–7]. Among these materials, nickel selenides have exhibited great potentials for full water splitting due to its high interface energy and intrinsic metallic activity [8,9], however, the poor stability is a thorny challenge to be solved [10].
Carbon materials (graphene, carbon nanotubes, etc.), showing excellent thermal stabilities and favorable electrical conductivities, are considered as ideal skeleton structures for the protection of electrocatalysts [11,12]. Based on the notion of prolonging the electrocatalytic life of nickel selenide materials, tremendous efforts have been made to develop nickel selenide/carbon compounds, among which, the graphene substrate with a faster charge transport and a high specific surface area can provide an outstanding electrocatalytic performance [13,14]. In this case, the simultaneous binding between graphene and nickel selenide nanocrystals is highly desirable, thus optimizing the HER electrocatalytic performance.
In this paper, a novel HER electrocatalyst composed of NiSe2 nanoparticles and 2D graphene nanosheet was successfully prepared via a facile and scalable microwave reaction and subsequent selenization treatment. Owing to its unique structural advantages, the uniform dispersion of NiSe2 nanoparticles in the reduced graphite oxide (rGO) has an additional synergistic effect on promoting the conductivity and stability. The obtained NiSe2/rGO composite displayed a remarkable electrocatalytic activity for HER (158 mV overpotential at 10 mA/cm2) with the synergistic effect of ultrafine NiSe2 nanocrystallines and graphene substrate. Meanwhile, this unique composite structure has an extremely stable performance in the 100 h H2 production test.
2 Experimental
2.1 Synthesis of the NiSe2/rGO composite
The graphene oxide (GO) solution was obtained via the traditional Hummers’ method [15]. 0.4 g nickel nitrate and 0.05 g urea were dissolved in 200 mL GO solution (0.1 mol/L) and stirred magnetically for 30 min. Transferred to the microwave reactor, the working steps were programmed as follow: 30 °C (for 10 min), 50 °C (for 10 min), 70 °C (for 10 min), and 90 °C (for 10 min), respectively. After the reaction was completed, the black precipitate was processed by centrifugation, washed with distilled water for several times, and then dried overnight in a cold trap. The precursor was placed downstream, while the selenium powder (80 mg) was placed upstream of the furnace. The powder was heated to 450 °C with a ramp rate of 5 °C/min and held for 2 h in N2. In the preparation of NiSe2 composite, the GO solution was replaced with distilled water during the microwave reaction.
2.2 Characterization
The crystal phases of the samples were identified using powder X-ray diffraction (XRD, Burker D8 Advance). The Fourier Transform InfraRed spectra (FT-IR) were collected on a Nicolet iS5 system (Thermo Scientific, Germany). The microstructure and morphology were observed using a field emission-scanning electron microscopy (FE-SEM, Hitachi S4800) and transmission electron microscope (TEM, FEI Tecnai G-20). The chemical composition and valence states of elements were probed using an X-ray photoelectron spectroscopy (XPS, Thermal Fisher, USA).
2.3 Electrochemical measurements
All the electrochemical data were collected using a standard three-electrode system in an electrochemical workstation (CHI760e, Chenhua). A standard saturated calomel electrode (SCE) and a graphite rod were used as the reference and counter electrode, respectively. 5.0 mg of sample and Pt/C were dispersed in a mixture solution (n(ethanol): n(H2O): n(Nafion) = 8: 1.8: 0.2, n is molar mass) to make uniform ink. Then, a glassy carbon electrode (d = 3 mm) coated with the catalyst loading of 0.85 mg/cm2 was used as working electrode. Linear sweep voltammogram (LSV) was performed with a scan rate of 5 mV/s. Electrochemical impedance spectroscopy (EIS) was collected at the frequency window between 100000 and 0.01 Hz. The cyclic voltammetry (CV) and chronopotentiometry (CP) methods were used to evaluate the long-term stability. All the potentials were transformed to the reversible hydrogen electrode (RHE) according to
3 Results and discussion
The NiSe2/rGO was synthesized via a facile two-step method of microwave and selenization reactions (Fig.1). A unique hybrid structure comprising NiSe2 nanocrystals grown in situ on the surface of graphene were obtained. The SEM image (Fig.2(a)) indicates that the pure rGO substrate possesses a smooth surface without any impurities. Meanwhile, the morphology of NiSe2 nanoparticles was shown in Fig. S1 in Electronic Supplementary Material (ESM), from which it can be seen that the size of the nanoparticles ranges from 40 to 100 nm. However, the NiSe2 nanocrystals with an ultra-fine size would be enriched on the rGO surface, forming a stable self-supporting structure following the selenization process and dispersing uniformly without any aggregation as displayed in Fig.2(b). The energy dispersive spectrometer (EDS) mapping images (Fig.2(c)) reveal that the distribution of the C, Ni, and Se elements are even throughout the view. In addition, the TEM image (Fig.2(d)) indicates that the graphene possesses a flawless two-dimensional structure, which is in agreement with the SEM images of rGO. In the meantime, the hybrid structure of NiSe2/rGO suggests that small nanocrystals are anchored on the entire surface of graphene (Fig.2(e)). Thousands of NiSe2 nanoparticles can supply abundant active sites to enhance the electrocatalytic activity. The HR-TEM image (Fig.2(f)) suggests that the well-crystallized NiSe2 nanoparticles have an interplanar spacing of approximately 0.36 nm, corresponding to the NiSe2 (210) lattice plane [7].
The XRD patterns in Fig.3(a) and S2 demonstrate that the characteristic peak around 26° belongs to the (002) plane of graphene (Joint Committee on Powder Diffraction Standards (JCPDS) #80-0017) while the other peaks at 29°, 33°, and 36° are assigned to the (200), (210), and (211) planes of NiSe2 (JCPDS #41-1495), respectively, indicating the successful bonding between graphene and NiSe2 nanocrystals [16]. The oxygen-containing functional groups are considered to be the pivotal driving force for the adsorption of metal ions to graphene. As exhibited in the FT-IR spectra (Fig.3(b)), the GO displays distinct peaks around 1059, 1366, 1599, and 3421 cm−1, which are attributed to the unique signals of epoxide, nitroxyl, carboxyl, and hydroxyl, respectively [17]. After this microwave process, the oxygen-containing functional groups of the NiSe2/rGO sample are converted into hydrogen bonds. In addition, the XPS analysis in Fig. S3 manifests that the elements of the composite sample are mainly C, O, Ni, and Se. The high-resolution Ni 2p spectrum of pristine NiSe2 (Fig.3(c)) shows two main peaks at 857.8 and 876.1 eV, corresponding to Ni 2p3/2 and Ni 2p1/2 of Ni2+, while two satellite peaks located at 863.7 and 882.4 eV belong to nickel oxide species because of the superficial oxidation. Another peak located at 855.8 eV can be attributed to the Ni 2p signal derived from Ni0. The high-resolution XPS spectrum of Se 3d in Fig.3(d) can be fitted to three peaks of Se 3d5/2 (54 eV), Se 3d3/2 (55 eV), and SeO2 (59 eV), demonstrating the formation of NiSe2 on the surface of graphene [18].
The HER activity of the prepared electrocatalysts were evaluated in 1.0 mol/L KOH electrolyte. Fig.4(a) exhibits the HER polarization curves for all electrocatalysts. The NiSe2/rGO shows a lower overpotential of 158 mV at the current density of 10 mA/cm2, which is lower than that of NiSe2 (344 mV), and close to that of Pt/C (35 mV). It is noteworthy that the HER kinetics are shown by Tafel plots (Fig.4(b)), where NiSe2/rGO records a low value of 56 mV/dec, indicating a fast absorption-desorption process of H+ ions at the electrode surface. The vital reaction in water electrolysis is the HER, which includes three reactions: the Volmer, the Heyrovsky, and the Tafel reaction.
The three reactions are the key steps in HER, which depend strongly on the electron transfer, the density of active sites, and the Gibbs free energy of adsorbed atomic hydrogen. The unique 2D graphene nanosheet with abundant defects and more accessible NiSe2 nanoparticles greatly facilitates the electroactivity and reduces the hydrogen adsorption energy. Meanwhile, the NiSe2/rGO catalyst apparently outperforms pure NiSe2 catalyst without rGO substrate, emphasizing the critical synergetic effect of highly active NiSe2 nanoparticles and the highly conductive rGO network to facilitate electron transfer [19]. The intrinsic characteristics of the electrochemically active surface area (ECSA) were evaluated from the Cdl according to the CV curves (Fig.4(c) and S4). The values of Cdl were 13.7 and 3.5 mF/cm2 for NiSe2/rGO and NiSe2, respectively, illustrating that the hybrid structure could create more active sites. The EIS curves in Fig.4(d) shows that the graphene substrate provides an excellent conducting medium that greatly improves the electron transfer. A negligible decay can be observed through the polarization curves of NiSe2/rGO and NiSe2 electrocatalysts after 1000 cycles (Fig.4(e)). The LSV curve of scanned HER barely shifts, implying the superior stability. The potential attenuation of the NiSe2/rGO catalyst is only 8 mV (compare with RHE) at a current density of 10 mA/cm2, which indicates its excellent long-term durability. The NiSe2/rGO also displays an excellent stability as measured by CP (Fig.4(f)), maintaining a long-term catalytic activity for at least 100 h. The magnified SEM images (Fig.2(b) and S5) reveal that the uniformly sized NiSe2 are homogenously dispersed on the surface of the rGO with no obvious aggregation. However, the morphology of NiSe2/rGO catalysts before and after the CP test has some differences compared with Fig.2(b) and S5, which may be attributed to the digestion of metal ions in the aqueous electrolyte. The above results, compared with the reported materials in Fig. S6, have validated that the composite structure of NiSe2/rGO has a splendid HER activity and a good stability, which may be attributed to the synergistic effect of NiSe2 nanocrystals and the graphene conducting network.
4 Conclusions
In summary, the hybrid structured NiSe2/rGO electrocatalyst has been successfully synthesized via a two-step process of microwave and selenization treatment. The unique microwave process not only facilitates the adsorption of metal ions on graphene, but also controls the nanocrystal size of NiSe2. In addition, it also has a long-term stability with no decay after 100 h of continuous testing. It is believed that the microwave process will provide a new highlight for the design and preparation of hybrid structured materials for electrocatalytic water splitting.