Highly Active Fe Sites in Ultrathin Pyrrhotite Fe7S8 Nanosheets Realizing Efficient Electrocatalytic Oxygen Evolution.

Identification of active sites in an electrocatalyst is essential for understanding of the mechanism of electrocatalytic water splitting. To be one of the most active oxygen evolution reaction catalysts in alkaline media, Ni-Fe based compounds have attracted tremendous attention, while the role of Ni and Fe sites played has still come under debate. Herein, by taking the pyrrhotite Fe7S8 nanosheets with mixed-valence states and metallic conductivity for examples, we illustrate that Fe could be a highly active site for electrocatalytic oxygen evolution. It is shown that the delocalized electrons in the ultrathin Fe7S8 nanosheets could facilitate electron transfer processes of the system, where d orbitals of FeII and FeIII would be overlapped with each other during the catalytic reactions, rendering the ultrathin Fe7S8 nanosheets to be the most efficient Fe-based electrocatalyst for water oxidation. As expected, the ultrathin Fe7S8 nanosheets exhibit promising electrocatalytic oxygen evolution activities, with a low overpotential of 0.27 V and a large current density of 300 mA cm-2 at 0.5 V. This work provides solid evidence that Fe could be an efficient active site for electrocatalytic water splitting.

Introduction

Electrochemical water splitting was regarded as a promising method for clean and sustainable energy conversion and storage, where the oxygen evolution reaction (OER) is the bottleneck in water splitting owing to its complex four-electron redox process with slow kinetics.[1−5] Generally speaking, a catalyst must be applied during the OER processes to overcome the high overpotential and promote the reaction rate of water electrolysis. Currently, the most efficient OER electrocatalysts are noble metal based catalysts such as IrO2 and RuO2, while their scarcity and high cost greatly hamper large-scale applications.[6,7] For these reasons, extensive efforts have been undertaken to develop highly active, durable, and low-cost alternatives, such as the earth-abundant transition-metal-based (Ni, Co, Fe, Mn, etc.) compounds.[8−16]

A review of the pioneering study suggests Ni–Fe electrocatalysts to be the most active OER catalysts in alkaline media, which show significantly enhanced catalytic activity compared to that reported for either Ni- or Fe-based electrocatalyst alone, and even surpass some of the noble metal based catalysts.[11,17−21] For example, Lu et al. showed that the overpotential of electrodeposited Ni–Fe hydroxide nanosheets at 10 mA cm–2 could be as low as 215 mV,[22] while that for Ni- or Fe-based electrocatalysts is usually in the range of 350–450 mV and 500 mV, respectively.[19,23] In addition, the OER activity of Ni–Fe oxide thin film with 40% Fe atom is about 3 orders and 2 orders of magnitude higher than that of the deposited Fe film and Ni film, respectively.[11] Although great successes have been achieved in the design of highly active Ni–Fe electrocatalysts, the role of Ni and Fe sites played in the outstanding catalytic activity was under debate, and it is still uncertain which one is the actual catalytic active site.[24−28] Because the Ni-based compounds generally possess higher OER activity than Fe-based samples, Ni was usually assumed to be the reactive site for water oxidation in Ni–Fe electrocatalysts.[29−31] For instance, a series of studies implied that FeIII in Ni–Fe catalysts promotes the formation of NiIV during the electrocatalytic reactions, which was assumed to be the active site during the OER process.[18,32,33] Moreover, the work of Louie et al. showed that Ni sites in the electrodeposited Ni–Fe film contain a structural unit similar to NiOOH, conjecturing that Ni centers serve as active sites for OER.[11]

However, the latest spectroscopic and computational studies proposed that Fe is likely to be active site in Ni–Fe electrocatalysts too, especially for the detection of FeIV during OER processes.[34,35] Despite that numerous studies have shown that the incorporation of Fe into Ni-based compounds could dramatically enhance their catalytic activity, it is still hard to identify whether Fe would be the active site or not for the lack of efficient Fe-based OER electrocatalysts. So far, unary Fe-based electrocatalysts generally possess large overpotential and low current density in OER tests.[36,37] Thus, designing efficient electrocatalysts with highly active Fe sites is crucial for understanding the role of Fe played during the oxygen evolution processes, which may bring new vitality to the area of OER.

Inspired by the mixed-valence character of state-of-the-art NiII–FeIII catalysts, we focused on the Fe-based electrocatalysts with mixed-valence states of FeII and FeIII. Given that low electrical conductivity was usually accounted for the poor apparent OER activity of Fe-based catalysts, searching for Fe-based compound with high electrical conductivity would be better. Bearing these in mind, we paid our attention to the pyrrhotite Fe7S8 with NiAs-based hexagonal structure, which possesses both mixed-valence state and intrinsic metallic character.[38,39] From the point of view that the two-dimensional (2D) nanosheets with atomic thickness would fully expose the active sites at the surface, thus the ultrathin Fe7S8 nanosheets would be ideal platforms for studying the reactivity of Fe sites in OER processes. Herein, taking the ultrathin Fe7S8 nanosheets as an example, we studied the role of Fe played during the oxygen evolution reactions, where the highly active Fe sites enable the ultrathin Fe7S8 nanosheets to be efficient catalysts for oxygen evolution. The identification of highly active Fe sites in OER processes would give new understanding on the mechanism of state-of-the-art Ni–Fe-based catalysts.

Results and Discussion

Synthesis and Structural Characterizations

In this study, the ultrathin pyrrhotite Fe7S8 nanosheets with nonlayered structure were successfully synthesized via a modified liquid exfoliation for the first time (Figure a). The amines with various lengths led to different interlamellar spacings of the inorganic–organic hybrid intermediate (Figure S4), indicating that the amines are vertically arranged in the lamellar structure. The morphology of as-exfoliated nanosheets was studied by transmission electron microscopy (TEM) image (Figure b), which shows a free-standing sheet-like transparency character, revealing their ultrathin thickness. Furthermore, the atomic force microscopy (AFM) technique was carried out to measure the thickness of as-exfoliated nanosheets. As shown in the AFM image and corresponding height profile (Figure c,d), the nanosheets show average thickness of about 1.78 nm, close to the thickness of a single-unit-cell Fe7S8 slab along the c axis.

Figure 1

Synthesis and morphology study. (a) Schematic introduction for the formation of ultrathin Fe7S8 nanosheets. (b) TEM image of Fe7S8 nanosheets. (c, d) AFM image and corresponding height profiles of Fe7S8 nanosheets.

The X-ray diffraction (XRD) pattern of the recollected powder of nanosheets (Figure a) could be readily indexed to the hexagonal phase of Fe7S8 with lattice constants of a = 6.87 Å, c = 17.05 Å (JCPDS Card No. 71-0591), and no peaks of any other phases can be detected. Moreover, the structure of as-obtained ultrathin Fe7S8 nanosheets was further studied by Raman spectra (Figure b), where the sharp peak at 220 cm–1 was assigned to the Fe–S binding of the hexagonal Fe7S8, indicating that the exfoliated Fe7S8 nanosheets maintain the structure of corresponding bulk counterpart.[40] As shown in Figure c, the valence states of exfoliated nanosheets were studied by X-ray photoelectron spectroscopy (XPS), where the Fe 2p spectrum of the Fe7S8 nanosheets shows similar shifts to that of the bulk sample. In detail, the Fe 2p peaks were deconvoluted and divided into Fe2+ and Fe3+ states, where the peaks located at 706.7, 710.4, and 723.6 eV were attributed to the Fe2+ state, whereas peaks at 711.8 and 725.4 eV were ascribed to signals of the Fe3+ state.[41,42] It is obvious that the prepared nanosheets possess mixed-valence character of Fe2+ and Fe3+. The broadening and weakening XPS peaks for the Fe7S8 nanosheets compared with the bulk counterpart could be attributed to their ultrathin character.

Figure 2

Structural characterizations. (a) XRD pattern of the recollected powder of nanosheets. (b) Raman spectra of the Fe7S8 nanosheets and bulk sample. (c) XPS Fe 2p spectra of Fe7S8 nanosheets and bulk sample. (d) HRTEM image and SAED pattern of typical Fe7S8 nanosheets. (e) Atomic-resolution HAADF-STEM image of Fe7S8 nanosheets. (f) Crystal structure of Fe7S8.

The high-resolution TEM (HRTEM) image and corresponding selective area electron diffraction (SAED) pattern of the nanosheets show continuous and ordered lattice fringes with the lattice spacing of 0.297 nm, which correspond to the structure of hexagonal Fe7S8 (Figure d). To further evaluate the structure of the ultrathin Fe7S8 nanosheets, high-angle annular dark-field STEM image (HAADF-STEM) measurement was carried out on a typical nanosheet to see its atomic arrangements.[43] As presented in atomic-resolution HAADF-STEM image (Figure e), the bright and less bright spots are arranged in a hexagonal configuration, which correspond to the two distinct atoms, namely, Fe and S (blue and yellow dots representing Fe and S atoms, respectively), coinciding with the structure of hexagonal Fe7S8 as illustrated in Figure f. The above results clearly show that the ultrathin Fe7S8 nanosheets with mixed-valence states were successfully prepared.

Density-Functional Theory Calculations

Given the energetic degeneracy between the Fe2+ and Fe3+ configurations, extra electrons are easily delocalized between the two Fe sites of Fe7S8, which could be enhanced in the ultrathin 2D structure. Thus, it would be interesting for us to study the electronic structure of the ultrathin Fe7S8 nanosheets via density functional theory (DFT) calculations. As shown in Figure a,b, charge density of single-layered Fe7S8 shows enhanced orbital hybridization between Fe and S compared to the bulk material, implying the increased electrons in the single layer.[44] The DFT calculations in Figure c imply the metallic behavior of Fe7S8, in which the density of states (DOS) resides across the Fermi level and the single layer, showing an increase in DOS around the Fermi level compared to the bulk sample. The above results indicate that the single-layered Fe7S8 nanosheets would show enhanced conductivity with respect to their corresponding bulk counterparts. In order to confirm the electrical conductivity of the samples, temperature-dependent resistance tests were carried out (Figure d). As expected, resistivity of the Fe7S8 nanosheets takes on a slight increase with raised temperature, indicating an intrinsic metallic state of Fe7S8 nanosheets. Additionally, the value of resistivity for Fe7S8 nanosheets at room temperature (9.9 × 10–5 Ω·m) is about 8.8 times lower than that of the corresponding bulk counterpart, in agreement with the theoretical predictions. The intrinsic metallic character and superior electrical conductivity of the ultrathin Fe7S8 nanosheets render them to be efficient electrocatalysts for water oxidation.

Figure 3

DFT calculations. (a, b) The charge-density wave of the bulk and single-layered Fe7S8, respectively. (c) Density of state (DOS) for the bulk and single-layered Fe7S8. (d) Comparison of temperature-dependent electrical resistivity of Fe7S8 nanosheets and bulk sample.

It is well-known that the adsorption energy of H2O molecules onto the active sites of catalyst plays a vital role in OER activity. In order to explore the potential application of Fe7S8 nanosheets in water oxidation, the DFT calculations were performed to study their H2O absorption ability (Figure S8).[45,46] Specifically, the Fe7S8 nanosheets possess a more negative adsorption energy for H2O molecules of about −1.29 eV, compared with that of Fe7S8-bulk (−0.52 eV) and FeS-sheets (−0.28 eV), implying that the Fe7S8 atomic layers would be an efficient electrocatalyst for water oxidation with highly active Fe sites.

OER Activity Measurements

To evaluate the electrocatalytic performances of as-obtained samples, the steady-state electrochemical measurements have been carried out in 1 M KOH solution using a typical three electrode cell setup (see details in the Supporting Information). Linear sweep voltammetry (LSV) curves in Figure a show that the Fe7S8 nanosheets display large electrocatalytic current density of 300 mA cm–2 at 0.5 V, which is much larger than that of Fe7S8 bulk (56.1 mA cm–2, 5.38 times) and state-of-the-art RuO2 electrocatalyst (185.2 mA cm–2, 1.6 times), suggesting the Fe7S8 nanosheets to be excellent water oxidation catalysts. Moreover, the catalytic activity of the FeS sheets and FeOOH sheets with single-valence states were also carried out for comparison. Structural and morphology study of the FeS sheets and FeOOH sheets have been shown in Figure S2 and Figure S3. Detailedly, at 0.5 V, the current density of the FeOOH sheets and FeS sheets is only 1.54 mA cm–2 and 2.74 mA cm–2, respectively, which values are much lower than those of both the Fe7S8 nanosheets and Fe7S8 bulk, implying that the Fe-based compounds with single-valence state have little activity for OER. Besides, only 0.27 V is required for the Fe7S8 nanosheets to reach a current density of 10 mA cm–2, which is much smaller than that of all tested Fe-based samples and most previously reported OER catalysts (Tables S1 and S2), indicating their high electrocatalytic activity. Additionally, Tafel plots that reflect the OER increment rate of catalysts were carried out to examine their catalytic activity. A small Tafel slope is favorable for practical applications, because it will contribute to remarkably increased OER rate with an increase in overpotential. As seen from Figure b, the Fe7S8 nanosheet shows a smallest Tafel slope of 43 mV decade–1 among the tested samples, confirming the rapid kinetic information on Fe7S8 nanosheets. It is obvious that the turnover frequency (TOF) of the Fe7S8 nanosheets is 0.34 s–1 at a quite small overpotential of 0.35 V, which is 20 times higher than that of bulk Fe7S8.

Figure 4

Electrochemical behaviors. (a) LSV curves of the tested samples. (b) Corresponding Tafel plots. (c) A plot of TOFs with respect to Fe atoms of Fe-based catalysts at different overpotentials. (d) LSV curves of the Fe7S8 nanosheets after 5000 CV cycles; inset is the chronopotentiometric curve obtained in 1 M KOH solution.

In order to further estimate the exposure of active sites, the electrochemically active surface areas (ECSAs) for the samples were estimated through the tested electrochemical double-layer capacitance (Cdl).[47] The Cdl value is proportional to ECSA and, thus, can reflect the density of ion-accessible sites of an electrocatalyst in the electrolyte. As displayed in Figure S7a, Cdl of Fe7S8 nanosheets is 3.8 times larger than that of the bulk sample, indicating the exposure of larger number of ion-accessible active sites in the nanosheets. Apart from the catalytic activity, electrical impedance spectroscopy (EIS) (Figure S7b) unravels a small charge-transfer resistance of the Fe7S8 nanosheets.

The long-term activity and durability is also a significant factor to evaluate an advanced electrocatalyst. As seen in Figure d, the activity of Fe7S8 nanosheets was nearly unchanged after 5000 CV cycles, indicating their high stability during the electrocatalytic reaction. The durability of Fe7S8 nanosheets can be further evaluated by chronopotentiometric measurements at J = 10 mA cm–2 for a long time. It is worth noting that ultrathin and mixed-valence characters would dramatically hamper the tendency of oxidation of the Fe7S8 nanosheets during the OER process for their rich surface dangling bonds and enhanced electron transfers, which render the nanosheets with exceptional stability.[36,48] All of the results above indicate Fe7S8 nanosheets to be excellent catalysts for oxygen evolution with highly active Fe sites.

Synchrotron-Based XANES Studies

Given the generally low electrocatalytic activity of the Fe-based compounds, it would be interesting for us to gain insights into the mechanism for the presence of highly active Fe sites in the mixed-valence Fe7S8 nanosheets. Bearing this in mind, synchrotron radiation-based X-ray absorption near edge structure (XANES), an effective technique to provide the spin orbital information on transition metal, was carried out to explore the electronic structure evolution of the Fe7S8 nanosheets during the OER tests.[49] As is known, the XANES spectra of Fe L3 edge could be divided into two peaks that donated to be t2g and eg orbitals, respectively, whose intensity is a criterion to evaluate the total unoccupied states of the Fe 3d states.[50] As seen in Figure a, it is clear that the shoulder peak of t2g is higher than that of Fe7S8 bulk (the shadowed part), indicating that the nanosheets possess a high density of t2g unoccupied states, which may derive from their enhanced electron delocalization. Moreover, Fe L-edge XANES spectra of the Fe7S8 nanosheets in various OER stages, i.e., after electrochemical reaction at 1.52 V, 1.62 V, and 1.72 V (vs RHE) for 12 h, were further acquired to investigate the electronic structure of Fe atoms during the reactions. As seen from Figure b, the intensity of shoulder peak of t2g in 703.2 eV was gradually increased with the increasing in reaction voltage, indicating that the t2g unoccupied states are enhanced during the OER process. The phenomenon can be understood by the enhanced electron transfer in the ultrathin Fe7S8 nanosheets, where the d orbitals of Fe are overlapped with each other. As schematically illustrated in Figure c, owing to the delocalized electrons in the ultrathin nanosheets, the d orbitals of FeII and FeIII show greater tendency to overlap with each other under external voltages, resulting in the enhanced t2g unoccupied states of Fe. It is known that the enhanced unoccupied states indicate the loss of electrons in the system to some extent, where the metal atoms tend to be in high valence states. Although there is no obvious electron loss during the OER tests, the Fe sites in the ultrathin Fe7S8 nanosheets still represent the role of high valence played for the enhanced electron transfer between FeII and FeIII, which should be the reason for their high reactivity.

Figure 5

Spin states analyses. (a) XANES spectra of Fe7S8 nanosheets and bulk sample. (b) XANES spectra of Fe7S8 nanosheets collected at different bias voltages. (c) Schematic illustration for the formation of highly active Fe sites in the mixed-valence Fe7S8 nanosheets.

Conclusions

In conclusion, we have proposed, for the first time, that an Fe-based compound with mixed-valence and metallic character could be an efficient electrocatalyst for oxygen evolution reaction by using the ultrathin pyrrhotite Fe7S8 nanosheets as examples. Both theoretical calculations and experiments show that the ultrathin Fe7S8 nanosheets possess enhanced electric conductivity and electron delocalization in comparison with the corresponding bulk sample. As directly revealed by the XANES spectra, the d orbitals of FeII and FeIII in the ultrathin Fe7S8 nanosheets are intent to overlap with each other for the enhanced electron transfer process, resulting in the highly catalytic activity of Fe sites. Thus, the Fe7S8 nanosheets display improved OER catalytic activity compared with Fe7S8 bulk, FeS nanosheets, and FeOOH nanosheets, which manifest a small overpotential of 0.27 V and a large current density of 300 mA cm–2 at 0.5 V, to be the most reactive OER catalysts in Fe-based compounds. This work not only gives deep understanding on the role of Fe played in the OER process but also paves a practical way for the design of efficient electrocatalysts.