Binary Ni₂FeO_x anchored on modified graphite for efficient and durable oxygen evolution electrocatalysis

Abstract

Development of earth-abundant electrocatalysts for the oxygen-evolution reaction is highly desirable. Herein, a binary nickel-iron-oxide is supported on the surface of sulfonated-graphite (Ni₂FeO_x@G-Ph-SN), and it demonstrates superior electrochemical water oxidation activity (only 265 mV overpotential at 10 mA cm⁻²) and outstanding stability in 1.0 M KOH.

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Introduction

Development of renewable and clean energy is urgent for relieving the environmental and energy crisis.^1,2 Electrochemical water splitting is an efficient way to produce clean hydrogen energy.³ The oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are two pivotal reactions in the electrocatalytic water splitting process.⁴ However, the OER is kinetically sluggish, and needs to overcome a large overpotential to proceed.⁵ Although IrO₂ and RuO₂ exhibit conspicuous catalytic activity for the OER, the scarcity and high cost of the precious metals severely restrain the large-scale commercialization of clean electrochemical water splitting technologies.⁶

Recently, transition metal oxides have been paid tremendous attention due to their cost-effective and storage properties.^7–19 The intrinsic catalytic behavior of the metal oxides is significantly influenced by the surface geometric construction and electronic regulation.²⁰ Therefore, the electrocatalytic behavior can be appropriately enhanced by increasing the surface area, improving surface edge sites with low coordination, controlling the structures, and optimizing the composition of the catalysts.^21–24 Bimetallic or trimetallic nanocomposites are proven to be promising candidates due to their better OER activity than related monometallic ones.^25–29 Previous studies have indicated that binary Ni–Fe layered double hydroxides (LDHs) show high OER activity.³⁰ The increase in active site density and conductivity leads to the enhanced OER activity.³¹ For the Ni–Fe-based electrocatalysts, there is still debate over which element is the water oxidation center. Some in situ results indicate that Ni is the water oxidation active site since nanostructured alpha-nickel-hydroxide shows better electrochemical activity than ruthenium oxide.³² In addition, Stahl et al. inferred by Mössbauer spectroscopy that Fe⁴⁺ species are not kinetically competent to serve as the active site in water oxidation, but they considered that the presence of Fe has a positive role in enhancing the OER activity.³³ Moreover, Boettcher et al. used in situ electrical measurements to demonstrate that FeOOH is an insulator at high overpotentials <400 mV.³⁴ However, some results showed that Fe⁴⁺ should be the water oxidation active site.³⁵ Bell et al. employed operando X-ray absorption spectroscopy (XAS) and computational methods to demonstrate that the OER takes place at Fe sites with low overpotential, while Ni sites in Ni_1−xFe_xOOH are not active.³⁶

Since Fe and Ni are two of the most abundant elements on the Earth and are cost-efficient for the OER and less harmful to the environment, they are suitable for large-scale commercial applications. In this work, we used phenylsulfonic acid-functionalized graphite (denoted as G-Ph-SO₃H) as a host for NiFeO_x nanoparticles, aiming to combine the merits of these two materials to fabricate cost-effective non-precious metal based electrocatalysts. For comparison, single NiO_x and FeO_x loaded G-Ph-SO₃H samples were also synthesized, showing the influence of the metals on the OER. Meanwhile, G-Ph-SO₃H decorated with nanoparticles having different Ni/Fe atomic ratios was also fabricated to verify the effect of Fe ions on the OER. The experimental results show that the G-Ph-SO₃H anchored Ni₂FeO_x sample, Ni₂FeO_x@G-Ph-SN, demonstrates the highest OER activity among the synthesized catalysts. Remarkably, the OER performance of Ni₂FeO_x@G-Ph-SN is better than that of IrO₂ nanoparticles in alkaline electrolyte. Moreover, Ni₂FeO_x@G-Ph-SN is much more stable than IrO₂, indicating that it is a promising catalyst for use in renewable energy technologies.

Results and discussion

The synthesized samples were characterized by transmission electron microscopy (TEM). For the G-Ph-SO₃H support, a clean surface can be observed in the SEM (Fig. S1†) and TEM images (Fig. S2†). For the FeO_x@G-Ph-SN sample (Fig. 1a), the FeO_x nanoparticles are homogeneously dispersed on the phenylsulfonic acid-functionalized graphite. The histogram in Fig. S3† shows that the majority of the loaded FeO_x nanoparticles are around 1.8 nm in diameter. For the NiO_x@G-Ph-SN sample (Fig. 1b), the dispersion of NiO_x is worse than that of FeO_x in FeO_x@G-Ph-SN and the particle size increases to ca. 5.7 nm. However, for the Fe and Ni-codoped sample Ni₂FeO_x@G-Ph-SN (Fig. 1c and d), an amorphous dispersion is typically observed, implying that the FeO_x has been alloyed with NiO_x. The existence state of NiFeO_x in Ni₂FeO_x@G-Ph-SN was further investigated using the high-resolution TEM image and the SAED pattern. As shown in Fig. 2, no obvious lattice fringes can be observed for the NiFeO_x indicating its amorphous form. The ring-shaped diffraction pattern with dispersed bright spots in the SAED pattern can be attributed to the (002) plane of the graphite support. No ring-shaped diffraction pattern due to any crystalline oxides can be observed, further confirming the amorphous structure of NiFeO_x.

Fig. 1 TEM images of the prepared samples: (a) FeO_x@G-Ph-SN, (b) NiO_x@G-Ph-SN and (c and d) Ni₂FeO_x@G-Ph-SN.

Fig. 2 (a) High-resolution TEM image and (b) SAED pattern of Ni₂FeO_x@G-Ph-SN.

In addition, no obvious peaks due to FeO_x, NiO_x, or NiFeO_x can be observed in the X-ray powder diffraction (XRD) patterns (Fig. S4†) and Raman spectra (Fig. S5†). The Ni₂FeO_x@G-Ph-SN possesses a specific BET surface area of 5.8 cm² g⁻¹, slightly larger than that of graphite (2.3 cm² g⁻¹) (Fig. S6†). The amorphous NiFeO_x in Ni₂FeO_x@G-Ph-SN can be further evidenced from the SEM image (Fig. 3a). The corresponding energy-dispersive X-ray spectroscopy (EDS) elemental-mapping images of Ni₂FeO_x@G-Ph-SN in Fig. 3b–f show the distributions of C, Fe, Ni, O, and S, which further verify that Fe and Ni are present in the form of a NiFeO_x alloy in Ni₂FeO_x@G-Ph-SN, since the mapping images of Fe and Ni match very well in the relevant areas.

Fig. 3 SEM image of Ni₂FeO_x@G-Ph-SN used in the EDS mapping test (a), and the corresponding EDS mapping of C (b), Fe (c), Ni (d), O (e), and S (f).

XPS characterization was applied to confirm the chemical valence state of Fe and Ni in Ni₂FeO_x@G-Ph-SN (Fig. 4). The high-resolution Fe 2p spectrum (Fig. 4a) displays two obvious signals at 711.0 eV for Fe 2p_3/2 and 724.1 eV for Fe 2p_1/2 along with a satellite peak at 717.8 eV. The Fe 2p_3/2 and Fe 2p_1/2 spectrum can be fitted into two peaks, respectively. The fitted Fe 2p_3/2 at 710.8 eV and Fe 2p_1/2 at 724 eV can be attributed to Fe²⁺,^37,38 while the Fe 2p_3/2 at 711.6 eV and Fe 2p_1/2 at 724.8 eV can be ascribed to Fe³⁺.³⁹ For the Ni 2p XPS spectrum, the two main peaks observed at 855.85 eV and 873.35 eV with a spin–orbit splitting of ∼17.5 eV are ascribed to Ni²⁺ 2p_1/2 and Ni²⁺ 2p_3/2, respectively (Fig. 4b). Additionally, there are two shakeup satellite peaks at 861.5 and 880.4 eV, implying that Ni exists in the form of Ni²⁺.³⁷

Fig. 4 The high-resolution XPS spectra of Fe 2p (a) and Ni 2p (b) of Ni₂FeO_x@G-Ph-SN.

The OER performance of the prepared samples as well as that of IrO₂ nanoparticles (Fig. S7,† particle size: 20–30 nm) were evaluated in 1.0 M KOH solution. It can be seen from linear sweep voltammetry (LSV) curves that the potential required to drive a 10 mA cm⁻² OER current density on the FeO_x@G-Ph-SN catalyst is 1.577 V versus RHE (Fig. 5a), which is slightly higher than that of the IrO₂ nanoparticles (1.569 V) and much lower than that of the RuO₂ nanoparticles (1.61 V). It has been reported that the OER activity of FeOOH is limited by its low conductivity, and it does not conduct at overpotentials <400 mV.³⁴ However, we found that when the particle size of FeO_x is small enough (e.g. ∼1.8 nm), it shows good OER activity at overpotentials <400 mV. Interestingly, the OER activity significantly decreases when increasing the Fe loading content from 4% to 20% (Fig. S8†). The onset potential on 20% FeO_x@G-Ph-SN is ca. 1.63 V vs. RHE, which is similar to that observed by Burke et al.³⁴ Therefore, the OER can take place on the extrafine FeO_x (e.g. <2 nm), which should be more conductive than larger FeO_x particles or sheets.

Fig. 5 (a) Polarization curves of the prepared samples and IrO₂ and RuO₂ nanoparticles supported on a GCE in 1.0 M KOH solution without iR-compensation; (b) Tafel plots derived from (a); and (c) CVs measured in a non-faradaic region at scan rates of 20 mV s⁻¹, 40 mV s⁻¹, 60 mV s⁻¹, 80 mV s⁻¹, and 100 mV s⁻¹. (d) C_dl of Ni₂FeO_x@G-Ph-SN derived from current density differences vs. the scan rate. (e) Cyclic voltammograms (CVs) of FeO_x@G-Ph-SN, NiO_x@G-Ph-SN, and Ni₂FeO_x@G-Ph-SN in 1.0 M KOH at a scan rate of 50 mV s⁻¹. (f) Plot of current density vs. time for the Ni₂FeO_x@G-Ph-SN at a constant anode voltage of 1.5 V vs. RHE in 1.0 M KOH.

Meanwhile, the NiO_x@G-Ph-SN catalyst exhibits a smaller potential (1.534 V) at a current density of 10 mA cm⁻² than FeO_x@G-Ph-SN. It is interesting to note that the NiFeO_x@G-Ph-SN hybrid presents a lower applied potential (ca. 1.499 V) at an OER current density of 10 mA cm⁻² than the individual metal oxide composite. In addition, Ni₂FeO_x@G-Ph-SN shows the lowest potential at 10 mA cm⁻² (1.495 V), which is comparable to those of excellent Fe-/Co-/Ni-based OER electrocatalysts (Table S1†). Besides, it exhibits a lower Tafel slope (60.2 mV dec⁻¹) than that of FeO_x@G-Ph-SN (79.4 mV dec⁻¹), NiO_x@G-Ph-SN (79.2 mV dec⁻¹), NiFeO_x@G-Ph-SN (60.5 mV dec⁻¹), Ni₃FeO_x@G-Ph-SN (62.7 mV dec⁻¹), and the nanoscale IrO₂ catalyst (100 mV dec⁻¹), indicating the excellent OER kinetics of this novel catalyst (Fig. 5b). The Tafel slope of Ni₂FeO_x@G-Ph-SN is close to 60 mV dec⁻¹, which is associated with a rate-limiting chemical step following the first electron transfer.^34,40,41

The electrochemically active surface area (ECSA) of each electrode determined from cyclic voltammetry (CV) curves was compared. The linear slope was obtained by plotting the Δj (|j_charge − j_{off charge}|) in the faradaic silence potential range against the scan rates, and it has a positive correlation with the double-layer capacitance (C_dl), and can be used to represent the corresponding ECSA.⁴² The NiFe-series samples have higher ECSA compared with the individual metal component samples (Fig. 5c and d and S9†), demonstrating the collaborative contribution from Fe and Ni ions. In addition, among the test samples, Ni₂FeO_x@G-Ph-SN shows the highest ECSA, which can reach 246 cm².

The electrochemical impedance spectroscopy (EIS) spectra of FeO_x@G-Ph-SN, NiO_x@G-Ph-SN and Ni₂FeO_x@G-Ph-SN were investigated. As shown in Fig. S10,† a semicircle in the high-to-medium frequency region in the Nyquist plots indicates charge transport resistance (R_ct), while a straight line in the low frequency range reflects the diffusion resistance (e.g. oxygen desorption on the electrode).^13,43 It can be found that FeO_x@G-Ph-SN, NiO_x@G-Ph-SN and Ni₂FeO_x@G-Ph-SN have similar R_ct, which is due to the good electrical conductivity of the graphite support. However, the slope value of Ni₂FeO_x@G-Ph-SN in the low frequency range is obviously higher than that of FeO_x@G-Ph-SN, indicating faster oxygen desorption on the surface of Ni₂FeO_x.

The cyclic voltammogram (CV) of FeO_x@G-Ph-SN in 1.0 M KOH at a scan rate of 50 mV s⁻¹ (Fig. 5e) reveals one weak reversible reduction wave at E_1/2 = 1.387 versus RHE, ascribed to the one-electron redox reaction of the Fe^II/Fe^III couple.⁹ Moreover, NiO_x@G-Ph-SN shows one strong reversible reduction wave at E_1/2 = 1.316 versus RHE, due to the one-electron redox reaction of the Ni^II/Ni^III couple.^25,28,44 Interestingly, Ni₂FeO_x@G-Ph-SN shows one medium reversible reduction wave at E_1/2 = 1.377 versus RHE, which falls in between those of Fe^II/Fe^III and Ni^II/Ni^III couples. This indicates that there exists some interaction between Fe and Ni ions in Ni₂FeO_x@G-Ph-SN, which results in better oxygen evolution behaviour. Although both Ni and Fe are key for OER activity in NiFe-based electrocatalysts, to date no fully consistent view has been reached on which one is the water oxidation active site.³¹ Our CV results indicate that Ni and Fe interact and co-catalyze water oxidation. In addition to its good electrochemical water oxidation activity, the Ni₂FeO_x@G-Ph-SN catalyst exhibits high electrochemical stability in alkaline electrolyte (Fig. 5f). After 10 h of continuous operation at a constant anode voltage of 1.5 V vs. RHE in 1.0 M KOH, the current remained almost constant. In addition, the current density of Ni₂FeO_x@G-Ph-SN at 1.5 V vs. RHE after CV for 200 cycles and 2000 cycles is almost the same, further demonstrating its good electrochemical OER stability (Fig. S11†). The chemical valence state of Fe and Ni in Ni₂FeO_x@G-Ph-SN after the stability test was analysed by XPS. As shown in Fig. 6, the Fe 2p_3/2 at 711.6 eV and Fe 2p_1/2 at 724.8 eV may be due to Fe³⁺ in Ni₂FeO_x@G-Ph-SN after the stability test, while the Ni 2p XPS peak positions in Ni₂FeO_x@G-Ph-SN after the stability test are almost the same as those in fresh Ni₂FeO_x@G-Ph-SN, indicating the same valence state of Ni²⁺.

Fig. 6 High-resolution Fe 2p and Ni 2p XPS spectra of the fresh and used Ni₂FeO_x@G-Ph-SN.

Conclusions

In summary, we have demonstrated the fabrication of mixed Ni₂FeO_x on the surface of sulfonated-graphite by a simple hydrothermal process. The synergistic action of Ni and Fe not only modifies the morphology of the particles, but also enhances the OER activity. The practical application of Ni₂FeO_x@G-Ph-SN has been demonstrated by its use as a high-efficiency and stable electrocatalyst toward the OER. Our study demonstrates a simple hydrothermal approach for the development of multicomponent metal oxide OER catalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21303069) and Natural Science Foundation of Jilin Province (20150520013JH and 20180101291JC).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8se00307f

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