NiSe₂ pyramids deposited on N-doped graphene encapsulated Ni foam for high-performance water oxidation†

Jing Yu ^ab, Qianqian Li ^a, Cheng-Yan Xu *^b, Na Chen ^b, Yuan Li ^a, Heguang Liu ^a, Liang Zhen ^b, Vinayak P. Dravid ^a and Jinsong Wu *^a
aDepartment of Materials Science and Engineering, NUANCE Center, Northwestern University, Evanston, IL 60208, USA. E-mail: jinsong-wu@northwestern.edu
^bSchool of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China. E-mail: cy_xu@hit.edu.cn

First published on 18th January 2017

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Introduction

Electrochemical water splitting has been considered a promising strategy for renewable clean energy.^1–4 However, the sluggish water oxidation, namely the oxygen evolution reaction (OER), presents a bottleneck, restricting the efficient development of water splitting technology. This is because the OER process is kinetically challenging and must overcome a high activation barrier to perform the multistep proton-coupled electron procedure.^5,6 Although ruthenium and iridium oxides exhibit the most efficient water oxidation activity in pioneering studies, the exorbitant cost and scarcity of precious metals Ir and Ru severely hinder their widespread application.^7,8 To address this problem, extensive efforts have been made to synthesize earth-abundant and economical catalysts for enabling the OER efficiently, such as transition metal oxides, chalcogenides, nitrides and carbides. Yet their low electrical conductivity causes the formation of Schottky barriers,⁹ thus lowering catalytic performance. Metallic electrocatalysts, with unique electron states close to Fermi levels, are found to be beneficial for optimizing electron transmittability.^10–12 Earth-abundant Ni-based catalysts, especially nickel diselenide (NiSe₂), due to its intrinsic metallic properties, are used as an appealing alternative for water reduction.^13,14 Previous results reveal that Se-enriched materials possess remarkable hydrogen evolution reaction (HER) activity in acidic electrolytes and reveal impressive durability.¹³ The excess selenium on the surface could expedite the charge transfer and optimize their intrinsic catalytic ability. Additionally, first-principle calculations verify that the hydrogen adsorption free energy on Se sites is much lower than that on Ni sites. Nevertheless, because of the excessive overpotential required to overcome high energy barriers of the OER, which is usually associated with serious energy loss, there are few reports of NiSe₂ as the OER electrocatalyst showing desired performance.

Surface modification, such as elemental doping and decoration of other nanomaterials, becomes gradually a promising way of achieving electrocatalysts with enhanced performance. For instance, 3D porous NiSe₂/Ni hybrid catalysts have been constructed to promote robust electrochemical hydrogen evolution.¹⁴ Metal doping is an important approach to further promote the OER activity.^15–17 However, for Ni and Co based materials with a rich variable valence, the complicated requirements in stoichiometry for valence balance make it unviable via conventional chemical synthesis routes.^18,19 One could utilize highly conductive graphene, which has been proven to be a remarkable substrate for supporting foreign materials,^20–23 to modify initial nanocatalysts to gain higher conductivity and more exposed edge sites. In particular, heteroatom doped graphene can tune its geometric and electronic characteristics, producing more active sites and strengthening the interaction between graphene and foreign catalysts.^24–26 The optimized electronic structure, as well as the tight chemical and electrical coupling, can enhance the electrocatalytic performance and bring superior stability.²⁷

In this work, we have synthesized a hybrid catalyst of N-doped graphene and metallic NiSe₂ pyramids intrinsically deposited on a Ni foam matrix (denoted as NG/NiSe₂/NF). Compared with other nickel selenides with semiconducting nature, the metallic nature of NiSe₂ makes it more appropriate as an electroactive material for water oxidation. Furthermore, the synergistic effect between N-doped graphene and NiSe₂ is expected to enhance the catalytic performance of the initial material significantly. Most of the water oxidation properties of the NG/NiSe₂/NF hybrid were measured in 0.1 M KOH. The pH-dependent electrocatalytic activity of the hybrid catalyst has also been measured at different KOH concentrations.

Experimental section

Materials

All chemicals were purchased from Sigma-Aldrich and were used as received without any purification.

Synthesis of nitrogen doped graphene encapsulated Ni foam

Graphene was prepared via a simple sonication process. Briefly, 10 mg expanded graphite was added into a flask containing 8 mL DMF and 2 mL deionized (DI) water. Then the flask was sonicated for 3 h along with intermittent shake per 30 min for complete dispersion of graphite in organic solution. After that, 5 mL NH₃·H₂O was added into graphene suspension with sonication for another 5 min. The suspension was rapidly transferred into a 30 mL Teflon-lined autoclave. Two pieces of clean Ni foam (1 cm × 3 cm) were placed into the autoclave, which was then sealed and maintained at 180 °C for 12 h. The resulting product was washed with DI water and denoted as NG/NF. Graphene encapsulated Ni foam (G/NF) was prepared without the addition of NH₃·H₂O. Nitrogen doped graphene was obtained without the addition of Ni foam.

Synthesis of nitrogen doped graphene and NiSe₂ composite

NiSe₂ films were deposited on NG/NF substrates via a three-electrode electrodeposition technique with NG/NF, Pt wire and saturated calomel electrode (SCE) as the working, counter and reference electrodes, respectively. The electrolyte bath includes 25 mM NiCl₂, 25 mM SeO₂ and 100 mM LiCl. The potentiostatic electrodeposition process was carried out at −0.5 V versus SCE for 1 h using Gamry Instruments Reference 600 electrochemical station. After that, the working electrode was removed from the electrolyte and cleaned with DI water and ethanol, and then dried at room temperature. The obtained electrode was named as NG/NiSe₂/NF. The mass loading of NG/NiSe₂/NF was determined using a microbalance and calculated to be about 2.5 mg cm⁻². G/NiSe₂/NF and NiSe₂/NF films could be prepared using G/NF and clean Ni foam as work electrodes instead of NG/NF. Note that less deposition time might lead to the incomplete coverage of active materials on the surface of Ni foam.

Material characterization

X-ray powder diffraction (XRD) was performed on a Scintag XDS2000 X-ray diffractometer with Cu Kα radiation (λ = 1.54178 Å). The microstructure of the samples was characterized by using a Hitachi SU8030 scanning electron microscope (SEM) and a JEOL JEM-2100 transmission electron microscope (TEM). Energy-dispersive X-ray (EDX), high-resolution TEM (HRTEM) and selected-area electron diffraction (SAED) were performed by using a JEOL JEM-2100 TEM. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific ESCALAB 250Xi spectrometer. Raman spectroscopy was conducted on an Acton TriVista CRS Confocal Raman System with λ = 514 nm laser excitation.

Electrochemical measurements

The OER measurements were carried out in a typical three-electrode system using a Gamry Instruments Reference 600 electrochemical station, in which the as-prepared electrodes, Pt wire and SCE acted as the working, counter and reference electrodes, respectively. The potentials involved in this work referred to the reversible hydrogen electrode (RHE). The polarization curves were corrected against the ohmic potential drop. Linear sweep voltammetry (LSV) was performed in O₂ saturated 0.1 M KOH solution with a scan rate of 2 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) was conducted in the frequency range of 100 kHz to 100 mHz at a 5 mV AC voltage amplitude. pH-dependent electrocatalytic performance was evaluated in KOH solution with different concentrations of 0.1 M, 0.5 M, 1 M, 2 M and 3 M, respectively.

Results and discussion

The formation mechanism of N-doped graphene coupled metallic NiSe₂ pyramids is illustrated in Scheme 1. Firstly, expanded graphite was sonicated and exfoliated to become few-layer graphene in the mixed solvent of DMF and deionized water (Fig. S1†). Secondly, N-doped graphene was formed via a one-step hydrothermal process with NH₃·H₂O as the N source and coated onto the Ni foam (see Fig. S2†). To verify the doping of the N atom, X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray (EDX) element mapping were performed to characterize the N-doped graphene. As shown in Fig. S3,† the N 1s signal can be clearly detected in the survey spectrum of N-doped graphene. Additionally, the existence of the C–N bond and high resolution N 1s spectrum further confirms the moderate doping of the N element. From the EDX maps shown in Fig. S4,† we can observe obviously the uniform distribution of the N element in the graphene. Thirdly, NiSe₂ was deposited onto the N-graphene coated Ni foam to form an active catalyst. The coupling of N-doped graphene and NiSe₂ would lead to significant enhancement of electroconductibility and available active sites. Meanwhile, the intermediate graphene layer would serve as an interconnected conducting network to strengthen the close connection between the Ni substrate and foreign catalyst, thus enhancing the catalytic activity and cycle stability.

X-ray diffraction (XRD) was carried out to examine the phase and purity of the deposited samples. As described in Fig. 1a, aside from three obvious characteristic peaks of the Ni substrate, the rest of the peaks can be indexed to the cubic NiSe₂ phase (JCPDS no. 88-1711, a = 5.9629 Å). In the NiSe₂ phase, the Ni atoms are surrounded by adjacent Se atoms forming an octahedral arrangement. The weak peaks at 26.5° in the G/NiSe₂/NF and NG/NiSe₂/NF originated from the graphene layer. Scanning electron microscopy (SEM) images (Fig. S7†) show densely packed NiSe₂ films with large surface roughness. The enlarged SEM in Fig. 1b reveals that pyramid-like NiSe₂ with a smooth surface was vertically grown on the surface of Ni foam. NiSe₂/NF and G/NiSe₂/NF possess a similar morphology (Fig. S8†). The thickness of the film was estimated to be about 2.3 μm according to the cross-sectional SEM image (Fig. S9a†). Transmission electron microscopy (TEM) image in Fig. 1c shows quasi-triangle-shaped morphology, further confirming such pyramid-like configuration. The high-resolution TEM (HRTEM) image and the corresponding selected-area electron diffraction (SAED) pattern (Fig. 1d and e) clearly illustrate the high crystallinity of the as-deposited films with a distinct single crystalline structure. The well-resolved lattice fringes with interplanar spacings of 0.596 and 0.267 nm indexed readily to the (100) and (021) planes of cubic NiSe₂, respectively. The high-angle annular dark field (HAADF) image and the associated EDX elemental maps demonstrate the fairly homogeneous distribution of Ni and Se without discernible segregation throughout the cracked pyramid. The atom ratio of Ni:Se was calculated to be 1:2, in good agreement with stoichiometry of NiSe₂. The pyramid-like structure has considerable surface roughness and thus possesses highly specific surface area and active edge sites, facilitating charge/mass transport and oxygen diffusion during the OER process.

Raman scattering is highly sensitive to electronic structures and has been considered as an essential tool to characterize the structure and quality of graphene-based materials.²⁸ As shown in Fig. 1i, all the samples present a crystalline (G) band at 1581 cm⁻¹, associated with the E_2g vibration mode for sp² carbon domains.²⁹ However, the exfoliated graphene exhibits a weak disordered (D) band located around 1350 cm⁻¹, which is related to the sp³ defect sites. After N doping, the intensity ratio of D and G band (I_D/I_G) increased notably, implying the changed structure of graphene derived from the introduction of defects by heteroatom doping.³⁰

XPS was carried out to investigate the surface chemical composition and elemental bonding configuration of the as-grown NG/NiSe₂/NF. From the survey spectra in Fig. 2a, the NiSe₂/NF, G/NiSe₂/NF and NG/NiSe₂/NF exhibit similar XPS spectra except an obvious N 1s peak emerging at 399.0 eV for NG/NiSe₂/NF. The two peaks located at 855.9 and 873.7 eV in high-resolution Ni 2p can be assigned to Ni 2p_3/2 and 2p_1/2, respectively, corresponding to Ni^II. The satellite peaks centered at 861.7 eV and 879.6 eV can be attributed to the surface oxidized NiO due to air exposure.³¹ The shoulder peak of metallic Ni 2p at 853.6 eV arises from the Ni foam substrate. With respect to the Se 3d core level spectrum of NiSe₂/NF, the peak at 55.0 eV is consistent with the Se 3d binding energy, suggesting the presence of Se₂²⁻.³² Compared to the NiSe₂ deposited on pure Ni foam, the binding energies of Se 3d in G/NiSe₂/NF and NG/NiSe₂/NF present a negative shift of about 0.4 eV, which mainly originates from the variation of partial electron transfer due to bonding graphene. The negative shift indicates the enhanced electron occupation, thus improving the electron-donating ability. The broad peak at 58.8 eV reveals the surface oxidation of Se species.³³ The N 1s spectrum of the NG/NiSe₂/NF (Fig. 2d) can be split into four peaks. The peaks at 398.6, 399.5, 400.2 and 400.8 eV are ascribed to pyridinic N, amine groups or other sp³ hybridized C and nitrogen bonds (C–N), pyrrolic N, and graphitic N species, respectively.^34,35 The existence of graphitic N could promote the conductivity of the initial graphene, thus accelerating the electron transfer process.³⁶ These results clearly demonstrate the construction of N-doped graphene and NiSe₂ hybrid on the surface of Ni foam.

We evaluated the electrocatalytic OER performance of the deposited NG/NiSe₂/NF films in O₂-saturated 0.1 M KOH. Previously, Ni-based compounds have been proved to be good candidates for OER catalysts due to the unique 3d electron number and special e_g orbitals.^37,38Fig. 3a shows the iR-corrected polarization curves for representative selenide films with N-graphene encapsulated Ni foam as contrast. Limited by the intrinsic activity, the NG/NF exhibits inferior OER ability with a featureless LSV curve. The hybrid electrode of graphene and NiSe₂ exhibits better catalytic activity than pure NiSe₂/NF. Moreover, after N doping, the NG/NiSe₂/NF presents an earlier onset overpotential and much greater current density compared with the G/NiSe₂/NF electrode, indicating the synergistic effect between N-doped graphene and NiSe₂. An oxidation peak prior to the onset of water oxidation is clearly observed, which could be ascribed to the transformation of Ni^II to Ni^III.³³ In addition, compared to other two NiSe₂ electrodes, the increasing anodic peak area in the NG/NiSe₂/NF indicates the increase of accessible active sites.⁵ RuO₂ possesses a smaller onset potential than that of NG/NiSe₂/NF. However, its catalytic activity would be exceeded by NG/NiSe₂/NF at higher potential. The overpotential of NG/NiSe₂/NF to deliver 20 mA cm⁻² is 307 mV, which is lower than that of NG/NF (470 mV), NiSe₂/NF (358 mV) and G/NiSe₂/NF (337 mV). This value is also much smaller than that for previously reported OER catalysts, such as NiSe₂ (η₁₀ = 366 mV),³⁹ Ni₃Se₂ (η₁₀ = 310 mV in 0.3 M KOH),⁴⁰ CoSe₂/NG (η₁₀ = 366 mV),²⁶ N–Co₉S₈/G (η₁₀ = 409 mV)⁴¹ and CoSe₂ (η₁₀ = 320 mV).⁸ The reaction kinetics is assessed by the corresponding Tafel plots (inset of Fig. 3a). As shown in Fig. 3b, NG/NiSe₂/NF exhibits aTafel slope of 89 mV dec⁻¹, less than that of NG/NF (248 mV dec⁻¹), NiSe₂/NF (109 mV dec⁻¹) and G/NiSe₂/NF (95 mV dec⁻¹), suggesting a more rapid oxygen evolution rate. Another criterion of a good catalyst is the stability of the electrode for the ongoing water oxidation process. We first performed a continuous cyclic voltammogram (CV) scan between 1.0 and 1.65 V (vs. RHE) with an accelerated sweep rate of 100 mV s⁻¹. The overpotential required for 20 mA cm⁻² presents an increase of 11 mV after 2000 cycles (Fig. 3c). The long-term stability was further investigated at a constant potential of 1.55 V over 100 h. Notably, the NG/NiSe₂/NF electrode shows a slight current density variation of about 12%, while G/NiSe₂/NF exhibits a significant degeneration of about 52%. In contrast, the OER ability of the NiSe₂/NF electrode would almost disappear after 100 h of continuous operation. These results demonstrate the outstanding catalytic performance and stability of the NG/NiSe₂/NF electrode. The growth of metallic NiSe₂/NF on N-doped graphene networks could optimize the diffusion and transfer of electrons. Furthermore, the strong bonding along with high chemical and mechanical stability reduced the possibility of the active material being peeled off from the substrate during long-term operation. The adjacent N-graphene layer could effectively buffer their volume change during the OER process to afford high durability. These are the synergistic chemical coupling effects between N-doped graphene and NiSe₂.

To gain insight into the OER catalytic mechanism, we analyze the post-OER NG/NiSe₂/NF electrode using SEM, XPS and XRD. From the XPS spectra in Fig. S12,† we can observe that the satellite peaks of NiO and the peak of SeO_x become relatively stronger, which is attributed to the slight oxidization of the catalyst surface. Moreover, the appearance of nickel–oxygen bonds in the O 1s region at 530.6 eV further reveals the presence of nickel oxo/hydroxo species on the surface of NG/NiSe₂/NF. SEM images (Fig. S13†) reveal that the pyramid-like structure was well preserved, in spite of numerous crimped nanosheets covering its surface, which may denote emerged oxide layers. However, the nickel oxo/hydroxo species are difficult to detect by XRD (Fig. S14†), thereby demonstrating NG/NiSe₂/NF as the major phase during the OER. The thin oxide shell may function as an actual active site to promote water oxidation.³¹

Electrochemical impedance spectroscopy (EIS) was employed to examine the electrode kinetics and interface reactions. The Nyquist plots in Fig. 3e reveal a sharply decreased charge transfer resistance (R_ct) after the combination of NiSe₂ with N-doped graphene, enabling high-quality electrical integration. Furthermore, we measured the electrochemical double-layer capacitance (C_dl) to evaluate the relative active surface area of the electrodes. As shown in Fig. 3f, the C_dl value of NG/NiSe₂/NF was measured to be 3.68 mF cm⁻², much larger than that of NiSe₂/NF (0.89 mF cm⁻²) and G/NiSe₂/NF (2.54 mF cm⁻²). The increased active surface area leads to the proliferation of exposed edge sites, which may lead to a superior OER activity.

On the basis of these results, we believe that a unique and potential water oxidation catalyst has been successfully achieved. The synergistic catalytic effect between N-graphene and NiSe₂ film is proposed to enhance the OER performance. First, metallic NiSe₂ possesses intrinsically high activity and functions as the main active center during electrocatalytic reactions. The vertically grown pyramid-like NiSe₂ film with large surface roughness has more active sites and electron/proton transport pathways. Second, N-doped graphene with abundant chemical groups functions as a bond to strengthen the mechanical adhesion between the conductive substrate and active catalyst, lowering the electric resistance thus facilitating reaction kinetics. Simultaneously, the interconnected conducting N-graphene network serves as a moderate carrier for convenient and efficient electron transfer. Third, the interactions between N-graphene and NiSe₂ film significantly promote the water oxidation activity. The electron donation from the N-graphene layer to NiSe₂ could weaken the interaction of surface-oxygen to produce moderate bond strength, thus accelerating OER kinetics. Due to the low electronegativity of N atoms in N-graphene, strong N–Ni bonds may have been formed because of the interaction between N-doped graphene and NiSe₂, which could act as other active centers to accelerate water dissociation.⁴²

The catalytic performance of the NG/NiSe₂/NF electrode was further measured in different concentrations of KOH solution. As depicted in Fig. 4a, the concentrated basic solution can enhance the oxygen evolution ability acutely. The overpotentials to afford 100 mA cm⁻² exhibit a stepped decrease, from 376 mV in 0.1 M KOH to 346 mV (0.5 M KOH), 330 mV (1 M KOH), 317 mV (2 M KOH) and 300 mV (3 M KOH), respectively. The value of 330 mV in 1 M KOH is comparable to most state-of-the-art OER catalysts, such as NF–Ni₃Se₂/Ni (353 mV),⁴³ NiSe nanowire/Ni (η₃₅ = 400 mV),³¹ NiCo LDH (η₁₀ = 367 mV),⁴⁴ NiCo₂O₄ (∼470 mV)⁴⁵ and so on. Fig. 4b displays a multi-current curve for the NG/NiSe₂ electrode in 1 M KOH along with a continuously increasing current per 500 s. The potentials remain almost unchanged at each current step, even if up to 500 mA cm⁻² with a rapid O₂ evolution rate. Moreover, the nonsluggish potential response to current ramps indicates the effective mass diffusion, electrical conductivity, and mechanical robustness of the NG/NiSe₂/NF electrode.^33,46

Conclusions

In summary, we have successfully synthesized hybrid films with N-doped graphene and metallic NiSe₂ pyramids by using a combined technique of hydrothermal growth and electrodeposition. These NG/NiSe₂/NF films offer a feasible path for water oxidation with high performance and robust stability. An overpotential of only 307 mV is required to afford 20 mA cm⁻² in 0.1 M KOH. Furthermore, this advanced electrode preserved considerable OER performance over 100 h of continuous operation. Aside from enhanced electrical conductivity and abundantly accessible surface sites, the synergistic effect between N-graphene and NiSe₂ film significantly boosts oxygen evolution capacity. This work has implications to fabricate desired OER electrocatalysts by coupling suitably functional materials.

Acknowledgements

Part of this work was supported by the Initiative for Sustainability and Energy at Northwestern (ISEN). This work made use of the EPIC facility of the NUANCE Center at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN.

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Footnote

† Electronic supplementary information (ESI) available: Details of chemical structure characterization and electrocatalytic performance. See DOI: 10.1039/c6ta10303k

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