Binary nickel–iron nitride nanoarrays as bifunctional electrocatalysts for overall water splitting

Abstract

Electrochemical water splitting provides a facile method for high-purity hydrogen production, but electro-catalysts with a stable bifunctional activity towards both oxygen and hydrogen evolution have been rarely developed. Herein we report a Fe₂Ni₂N material with a vertically aligned nanoplate array architecture as a bifunctional catalyst for overall water splitting in an alkaline environment. This advanced catalyst affords small onset overpotentials and fast current density increase, resulting in an excellent water splitting performance (requiring 1.65 V for achieving 10 mA cm⁻²), superior to the combination of benchmark noble metal catalysts.

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Xiaoming Sun

Prof. Xiaoming Sun was born in 1976. He received his B.S. degree and Ph.D. in the Department of Chemistry, Tsinghua University in 2000 and 2005, respectively. After postdoctoral work at Stanford University, he joined the State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology in 2008. He received the national science fund for distinguished young scholars from the Natural Science Foundation of China in 2011. His main research interests focus on the separation and assembly of inorganic nanostructures, synthesis and separation of carbon nanomaterials and their composites, and investigations of structure control and opto-/electro-properties of oxide nanoarrays.

The development of sustainable energy resources is of great importance to solve the increasing global demand for energy and the environmental pollution caused by burning of fossil fuels.^1–4 Water splitting driven by electricity or sunlight is widely considered to be critical for renewable hydrogen production.¹ The key to reduce energy supply and maximize the water splitting efficiency lies in the electro-catalyst screening for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER).^5–7 Currently, noble metal based electro-catalysts (e.g. IrO₂ and Pt) are identified to be the most active for these electro-catalyses, but the terrestrial scarcity and high cost of noble metals make them less attractive for commercial applications.^8–10 To address this issue, a variety of catalysts have been investigated with comparable activity as cheap alternatives, including metal oxides,^11,12 hydroxides,¹³ and selenides¹⁴ for OER, and metal alloys,¹⁵ carbides,¹⁶ sulfides,¹⁷ phosphides¹⁸ and nitrides¹⁹ for HER. However, the afore-mentioned electro-catalysts are usually active for a single half reaction because of the instability of the catalysts under the other half reaction conditions (e.g. positive or negative potential). Therefore, developing bifunctional electrocatalysts for water splitting is rarely achieved and still remains a big challenge.^20–22

In this study, we demonstrated mesoporous Fe₂Ni₂N nanoplate arrays (NPAs) vertically grown on Ni foam as bifunctional catalysts for overall water splitting. The porous nanoarray architecture not only provided a large electrochemical active surface area for ion diffusion, but also facilitated fast electron transportation due to a direct contact with the substrate. As a result, the NiFe-based nitride catalyst exhibited excellent activity toward both OER and HER, which allowed the construction of an efficient alkaline electrolyzer (requiring 1.65 V for achieving 10 mA cm⁻²) using this bifunctional catalyst, superior to the electrolyzer assembled by noble metal catalysts. Moreover, this electrolyzer showed prominent stability (retaining 94.5% of initial current for 10 h), possibly due to the stability of the Fe₂Ni₂N catalyst under both OER and HER conditions.

The synthesis of Fe₂Ni₂N NPAs involved two steps, as shown in Fig. 1A. A NiFe-based precursor (NiFe layered double hydroxides, denoted as NiFe-LDHs) with a nanoplate structure (Fig. 1B) was firstly synthesized by a hydrothermal method.¹³ Afterwards, the precursor was directly calcined at 380 °C under an NH₃ atmosphere to obtain Fe₂Ni₂N NPAs. As a comparison, NiFe-based oxide nanoplates were obtained by direct calcination of the NiFe-based precursor nanoplates in air (Fig. S1†). Scanning electron microscopy (SEM, Fig. 1C) demonstrated that the nanoplates with an average size of 0.5 μm and thickness of 20 nm were uniformly covered on the substrate, preserving the morphology of the NiFe-based precursor (Fig. 1B). However, the color of the final product changed from brown (Fig. 1B, inset) to black (Fig. 1C, inset), indicating a successful conversion. High resolution SEM showed that the final sample is porous with a rough surface (Fig. 1D), which arises from the gas release and dehydration of the precursor during annealing. Meanwhile, the transmission electron microscopy (TEM) image presented in the inset of Fig. 1D also confirmed the nanoporous nature of NiFe nitride.

Fig. 1 (A) Schematic illustration of the synthesis of Fe₂Ni₂N NPAs; (B) and (C), low-magnification SEM images of the NiFe-based precursor and Fe₂Ni₂N NPAs, the insets are the corresponding optical images; (D) high-magnification SEM image of Fe₂Ni₂N NPAs, the insert is the TEM image of an Fe₂Ni₂N NPA film, scale bar: 200 nm.

The structural information of the nitrided sample was investigated by X-ray diffraction (XRD, Fig. 2A), which showed a series of Bragg reflections matching well with Fe₂Ni₂N (JCPDF: 50-1350). The high-resolution TEM (HRTEM) image of the individual Fe₂Ni₂N nanoplate demonstrated a well-resolved lattice fringe of 0.217 nm, corresponding to the diffraction of a (111) plane (Fig. 2B). The energy dispersive X-ray spectroscopy (EDS) elemental mapping image proved the existence of N and indicated a homogeneous elemental distribution throughout the nanoplates, as shown in Fig. S2.†

Fig. 2 (A) XRD pattern and (B) HRTEM images of the Fe₂Ni₂N NPAs; (C), (D) and (E), XPS spectra of Ni 2p, Fe 2p, and N 1s peaks of Fe₂Ni₂N NPAs.

X-ray photoelectron spectroscopy (XPS) was used to investigate the oxidation states of elements in the nanoplate surface. For the NiFe-based oxide nanoplate, as shown in Fig. S3,† the Ni 2p spectrum showed two peaks at 855.45 and 872.55 eV corresponding to the +2 oxidation state of Ni.²³ The peaks at 712.4 and 724.9 eV of the Fe 2p spectrum demonstrated that the Fe species were mainly in their oxidation states of +3.¹² After thermal treatment under an NH₃ atmosphere, the Ni 2p XPS peaks of Fe₂Ni₂N can be deconvoluted into four peaks (Fig. 2C). Similar to the NiFe-based oxide, peaks at 854.95 (2p_3/2) and 872.3 (2p_1/2) eV in the Ni 2p spectra of Fe₂Ni₂N revealed the existence of Ni²⁺, while the other two peaks at 852.7 (2p_3/2) and 869.95 (2p_1/2) eV suggested a metallic state of nickel, indicating that Ni was partially reduced by the thermal treatment of NH₃.²⁴ To rule out the possible contribution from the substrate, we have synthesized Fe₂Ni₂N nanoplate arrays on carbon fiber paper and observed the XPS data (Fig. S4†), which showed similar XPS peaks as the sample grown in Ni foam, indicating that the metallic Ni peak cannot be attributed to the exposure of the substrate. For the other transition metal component of Fe species, thermal treatment showed no obvious reduction of Fe relative to Ni components (Fig. 2D); however, it did shift the Fe 2p_3/2 XPS spectrum to a lower binding energy of 0.5 eV which was also found for the Ni components. As nitrogen has a lower electronegativity than oxygen, the electron density outside the metal ions in Fe₂Ni₂N is higher than the NiFe-based oxide. Thus, the binding energies of 2p_3/2 peaks of both Ni and Fe were shifted to lower values (Fig. S3†).²⁵ The N 1s region showed a strong peak near 397.05 eV, which was consistent with the N environment in metal nitride (Fig. 2E).²⁶

The OER activity of Fe₂Ni₂N NPAs was firstly evaluated in O₂ saturated 1 M KOH solution. Fig. 3A demonstrates the polarization curve of Fe₂Ni₂N with a scan rate of 1 mV s⁻¹. For comparison, the NiFe-based oxide, Ni₃N NPAs (the synthesis method and characterization can be seen in ESI, Fig. S5†) and pristine nickel foam were also examined under the same conditions. Because the as-measured reaction currents did not directly reflect the intrinsic behavior of the catalysts due to the effect of ohmic resistance, an iR-correction was applied for further analysis.²⁷ It is obvious in Fig. 3A that the current density of Fe₂Ni₂N was indeed higher than that of the NiFe-based oxide, Ni₃N NPAs and the pristine Ni foam under a certain applied potential, and also showed a much earlier onset potential (∼1.43 V vs. RHE). Notably, the OER activity of Fe₂Ni₂N NPAs was even better than the 20 wt% Ir/C catalyst, which afforded an onset potential of ∼1.47 V. Furthermore, the Tafel slope using a Fe₂Ni₂N NPA electrode was measured to be 34 mV dec⁻¹ (Fig. 3C), which was smaller than the NiFe-based oxide (43 mV dec⁻¹), Ni₃N NPAs (56 mV dec⁻¹) and the Ir/C catalyst (69 mV dec⁻¹). The different performance may be caused by the difference in the intrinsic activity and resistance of materials and of charge transfer on the surface.²⁸ All the above results demonstrated that the Fe₂Ni₂N NPAs possess a higher intrinsic electrocatalytic activity for the OER than that of the NiFe-based oxide and commercial Ir/C. The better performance of nickel–iron nitride than nickel-nitride indicated the advantages of a binary metal compound in this case. The high activity towards the OER (low onset potential and the small Tafel slope) of Fe₂Ni₂N could be rationalized as follows: (1) the incorporation of nitrogen made the electrode structure more disordered, which could provide more active sites;¹⁹ (2) the porosity of the NPAs not only led to the exposure of more active sites but also facilitated sufficient transport of reactants and products.²⁹ To further prove the electrocatalytic OER performance of Fe₂Ni₂N, electrochemical impendence spectroscopy measurement was carried out to study the electrode kinetics under OER conditions. Nyquist plots (Fig. 3E) revealed an extremely decreased charge transfer resistance (R_ct in the equivalent circuit) for Fe₂Ni₂N in comparison with the NiFe-based oxide, Ni₃N NPAs and pristine Ni foam, indicating a much faster electron transfer process during the electrochemical reaction.³⁰ This may be associated with the porous structure and better electrical conductivity of metal nitride than oxide.³¹

Fig. 3 (A, B) OER and HER iR-corrected polarization curves and (C, D) corresponding Tafel plots of different materials; (E) Nyquist plots of the three electrodes in 1 M KOH; (F) time-dependent current densities of the Fe₂Ni₂N NPAs, the applied potentials are 1.5 V and −0.27 V for OER and HER, respectively after iR-correction.

In order to minimize the whole overpotential of water splitting, it is not sufficient to have a good OER performance for a bifunctional catalyst. Therefore, we assessed the catalytic activity of the Fe₂Ni₂N NPAs for the HER under the same basic conditions (Fig. 3B). For comparison, the NiFe-based oxide, Ni₃N NPAs, pristine Ni foam and Pt were also studied here. As expected, Pt showed superior HER activity with a negligible overpotential. Although the NiFe-based oxide, Ni₃N NPAs, and pristine Ni foam showed poor HER activities, Fe₂Ni₂N NPAs exhibited an onset overpotential as low as 110 mV, and a negative potential caused a rapid rise of the cathodic current, for example, only 180 mV of overpotential was required for achieving 10 mA cm⁻², suggesting that the Fe₂Ni₂N NPAs acted as a high-performance electrocatalyst for generating hydrogen from water. The smaller Tafel slopes for Fe₂Ni₂N NPAs than that for the NiFe-based oxide and Ni₃N NPAs further confirmed the better performance towards the HER of the binary metal nitride sample. All three samples exhibited classical Volmer–Heyrovsky behavior (ca. 120 mV dec⁻¹, Fig. 3D), which is similar to most Ni-based materials in alkaline water electrolysis.¹⁹

The stabilities of the Fe₂Ni₂N NPAs for both OER and HER were investigated under a constant voltage. When operating the OER and HER at certain applied potentials, stable corresponding current densities were observed with negligible degradation (8.3% for OER and 4.4% for HER, respectively) after 10 h of continuous testing (Fig. 3F), revealing the high stability as well as a high Faraday efficiency. After a durability test, the performance of the catalyst in both OER and HER was maintained with almost no noticeable difference (Fig. S6†). The high electrochemical stability was attributed to the outstanding stability of Fe₂Ni₂N under both positive and negative potentials, as revealed by the XRD patterns (Fig. S7†).

To illustrate practical relevance, the overall water splitting performance was investigated in a two-electrode setup using the same material as both cathode and anode in 1 M KOH solution. The linear sweep voltammetry showed that the electrolysis process proceeded at an applied potential of 1.6 V vs. RHE (Fig. 4A), and a current density of 10 mA cm⁻² was obtained at about 1.65 V, which illustrated a combined overpotential of only about 400 mV for the HER and OER, superior to the electrolyzer assembled by Ni₃N NPAs (Fig. S8†). Moreover, this electrolyzer also outperformed the electrolyzer assembled by Ir/C and Pt electrodes as an anode and cathode (requiring 1.78 V for achieving 10 mA cm⁻²), respectively. The stability of the catalyst is of importance because a practical water-splitting device should ideally last for a long time. The stability of the nitride catalyst electrode was operated under an applied potential of 1.8 V (Fig. 4B), and only a slight degradation over a 10 h testing was observed. The superb water splitting efficiency and good stability allowed the Fe₂Ni₂N NPAs as one of the most excellent bifunctional electrocatalytic candidates for overall water splitting.

Fig. 4 (A) Polarization curve of the two-electrode setup for the overall water-splitting reaction in 1 M KOH at 5 mV s⁻¹ (not iR corrected); (B) chronoamperometry of water electrolysis at a voltage of 1.8 V in 1 M KOH.

The above results clearly demonstrate the excellent bifunctional performance of Fe₂Ni₂N NPAs, which can be attributed to the following reasons. Firstly, the porous nanoarray architecture as well as the mesoporous nanostructure can offer a large surface area, enhancing ion diffusion and providing more active sites for electrocatalysis.³² The Brunauer–Emmett–Teller (BET) measurement results demonstrate that the surface area of Fe₂Ni₂N NPAs is ∼4 times higher than that of nickel foam (Fig. S9 and Table S1†), indicating the effectiveness of constructing a porous structure. Secondly, the direct growing nature of the transition metal nitride facilitates the electron transportation through the current collector to the active sites, thus accelerating the surface chemical reaction. Moreover, since the transition metal nitride has unique electronic structure,³¹ high electrocatalytic activity³³ and high stability in both positive and negative potentials, this kind of material demonstrates stable and outstanding bifunctional performance.

In summary, a nanoporous Fe₂Ni₂N NPA was prepared by converting NiFe-LDH under an NH₃ atmosphere, which demonstrated its high activity as a bifunctional catalyst for overall water splitting in alkaline medium. Due to the unique material and structural advantages, the Fe₂Ni₂N NPA electrode presented excellent catalytic activity with a small onset overpotential (ca. 200 mV for OER and 110 mV for HER), low Tafel slope (34 mV dec⁻¹ for OER and 101 mV dec⁻¹ for HER) and prominent electrochemical durability. This kind of catalyst can be directly utilized as a catalyst for both anode and cathode with superior efficiency and strong robustness. These findings can inspire a variety of opportunities for designing other binary or trinary transition metal nitrides towards the full electrocatalytic splitting reaction of water, as well as in other electrochemical devices.

This work was supported by the National Natural Science Foundation of China, the Program for Changjiang Scholars and Innovative Research Team in University. This work was also supported by the long-term subsidy mechanism from the Ministry of Finance and the Ministry of Education of PRC.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedure, optical and SEM images, high-resolution TEM image, EDX mapping, XPS spectra, polarization curves, XRD patterns and BET results. See DOI: 10.1039/c5qi00232j

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