In situ growth of NiSe₂ nanocrystalline array on graphene for efficient hydrogen evolution reaction

Abstract

Nickel selenide electrocatalysts for hydrogen evolution reaction (HER) with a high efficiency and a low-cost have a significant potential in the development of water splitting. However, the inferiority of the high overpotential and poor stability restricts their practical applications. Herein, a composite nanostructure consists of ultrasmall NiSe₂ nanocrystals embedded on graphene by microwave reaction is reported. The prepared NiSe₂/reduced graphite oxide (rGO) electrocatalyst exhibited a high HER activity with an overpotential of 158 mV at a current density of 10 mA/cm² and a corresponding moderate Tafel slope of 56 mV/dec in alkaline electrolyte. In addition, a high retention of electrochemical properties (approximately 100%) was demonstrated with an unchangeable microstructure after 100 h of continuous operation.

Graphical abstract

Keywords

nickel selenide / carbon materials / nanoparticles / hydrogen evolution reaction (HER) / microwave reaction

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

(1)E_{(\text{RHE})}= E_{(\text{SCE})}+0.244+0.0591\times \text{pH}.

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 2p_3/2 and Ni 2p_1/2 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 3d_5/2 (54 eV), Se 3d_3/2 (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.

(2)\begin{array}{rl}& \mathrm{T}\mathrm{h}\mathrm{e}\mathrm{V}\mathrm{o}\mathrm{l}\mathrm{m}\mathrm{e}\mathrm{r}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}:\\ & {\text{H}}_{\text{3}}{\text{O}}^{\text{+}}\text{+ site +}{\text{e}}^{-}\to \text{site}-{\text{H}}_{\text{ads}}\text{+}{\text{H}}_{\text{2}}\text{O}\end{array}

(3)\begin{array}{rl}& \mathrm{T}\mathrm{h}\mathrm{e}\mathrm{H}\mathrm{e}\mathrm{y}\mathrm{r}\mathrm{o}\mathrm{v}\mathrm{s}\mathrm{k}\mathrm{y}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}:\\ & \text{site}-{\text{H}}_{\text{ads}}+{\text{H}}_{\text{3}}{\text{O}}^{\text{+}}\text{+}{\text{e}}^{-}\to \text{site}+{\text{H}}_{\text{2}}+{\text{H}}_{\text{2}}\text{O}\end{array}

(4)\begin{array}{rl}& \mathrm{T}\mathrm{h}\mathrm{e}\mathrm{T}\mathrm{a}\mathrm{f}\mathrm{e}\mathrm{l}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}:\\ & \text{site}-{\text{H}}_{\text{ads}}+\text{site}-{\text{H}}_{\text{ads}}\to 2\text{site +}{\text{H}}_{\text{2}}\end{array}

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 C_dl according to the CV curves (Fig.4(c) and S4). The values of C_dl 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.

References

[1] FangZ, PengL, QianY. . Dual tuning of Ni-Co-A (A = P, Se, O) nanosheets by anion substitution and holey engineering for efficient hydrogen evolution. Journal of the American Chemical Society, 2018, 140( 15): 5241– 5247

[2] SunY, Xu K, WeiZ. . Strong electronic interaction in dual-cation-incorporated NiSe₂ nanosheets with lattice distortion for highly efficient overall water splitting. Advanced Materials, 2018, 30( 35): 1802121

[3] LiuB, Wang Y, PengH. . Iron vacancies induced bifunctionality in ultrathin feroxyhyte nanosheets for overall water splitting. Advanced Materials, 2018, 30( 36): 1803144

[4] HuaW, Sun H, XuF. . A review and perspective on molybdenum-based electrocatalysts for hydrogen evolution reaction. Rare Metals, 2020, 39( 4): 335– 351

[5] GaoR, Zou J. Towards designing efficient catalyst for hydrogen oxidation reaction. Rare Metals, 2020, 39( 10): 1107– 1109

[6] PengX, YanY, Jin X. . Recent advance and prospectives of electrocatalysts based on transition metal selenides for efficient water splitting. Nano Energy, 2020, 78( 25): 105234

[7] ChenL, JangH, KimM. . Fe, Al-co-doped NiSe₂ nanoparticles on reduced graphene oxide as an efficient bifunctional electrocatalyst for overall water splitting. Nanoscale, 2020, 12( 25): 13680– 13687

[8] JiangH, WangZ, YangQ. . Ultrathin Ti₃C₂T_x (MXene) nanosheet-wrapped NiSe₂ octahedral crystal for enhanced supercapacitor performance and synergetic electrocatalytic water splitting. Nano-Micro Letters, 2019, 11( 1): 31

[9] FangX, RenL, Li F. . Modulating electronic structure of CoSe₂ by Ni doping for efficient electrocatalyst for hydrogen evolution reaction. Rare Metals, 2022, 41 : 901– 910

[10] MingF, LiangH, ShiH. . MOF-derived Co-doped nickel selenide/C electrocatalysts supported on Ni foam for overall water splitting. Journal of Materials Chemistry. A, Materials for Energy and Sustainability, 2016, 4( 39): 15148– 15155

[11] WangF, LiY, Shifa T. . Selenium-enriched nickel selenide nanosheets as a robust electrocatalyst for hydrogen generation. Angewandte Chemie International Edition, 2016, 55( 24): 6919– 6924

[12] SwesiA, MasudJ, NathM. Nickel selenide as a high-efficiency catalyst for oxygen evolution reaction. Energy Environmental Science, 2016, 9( 5): 1771– 1782

[13] KirubasankarB, MurugadossV, LinJ. . In situ grown nickel selenide on graphene nanohybrid electrodes for high energy density asymmetric supercapacitors. Nanoscale, 2018, 10( 43): 20414– 20425

[14] FangG, WangQ, ZhouJ. . Metal organic framework-templated synthesis of bimetallic selenides with rich phase boundaries for sodium-ion storage and oxygen evolution reaction. ACS Nano, 2019, 13( 5): 5635– 5645

[15] YanL, Jiang H, XingY. . Nickel metal-organic framework implanted on graphene and incubated to be ultrasmall nickel phosphide nanocrystals acts as a highly efficient water splitting electrocatalyst. Journal of Materials Chemistry A, Materials for Energy and Sustainability, 2018, 6( 4): 1682– 1691

[16] WangS, HeP, Jia L. . Nanocoral-like composite of nickel selenide nanoparticles anchored on two-dimensional multi-layered graphitic carbon nitride: a highly efficient electrocatalyst for oxygen evolution reaction. Applied Catalysis B: Environmental, 2019, 243 : 463– 469

[17] FuG, Yan X, ChenY. . Boosting bifunctional oxygen electrocatalysis with 3D graphene aerogel-supported Ni/MnO particles. Advanced Materials, 2018, 30( 5): 1704609

[18] DongQ, WangQ, DaiZ. . MOF-derived Zn doped CoSe₂ as an efficient and stable free-standing catalyst for oxygen evolution reaction. ACS Applied Materials & Interfaces, 2016, 8( 40): 26902– 26907

[19] WangZ, WuH, Li Q. . Reversing interfacial catalysis of ambipolar WSe₂ single crystal. Advancement of Science, 2020, 7( 3): 1901382

RIGHTS & PERMISSIONS

Higher Education Press