Intrinsic poorly-crystallized Fe₅O₇(OH)·4H₂O: a highly efficient oxygen evolution reaction electrocatalyst under alkaline conditions

Abstract

As the bottleneck of electrochemical overall water splitting, the oxygen evolution reaction (OER) needs efficient catalysts to lower the required overpotential. Electrocatalysts with an amorphous form are highly active but suffer with low structural stability. Poorly crystallized materials with activity like amorphous forms, while maintaining the mechanical robustness of crystalline forms, are expected to be ideal materials. Towards this direction, we, for the first time, developed low-crystalline Fe₅O₇(OH)·4H₂O as an excellent OER electrocatalyst with an overpotential of 269 mV, in order to drive a current density of 100 mA cm⁻² in a 1.0 M KOH environment, and this outperforms most of the reported Fe-based electrocatalysts. Notably, its activity can be maintained for at least 100 hours. A one-pot synthesis for the poorly-crystallized material using one of the most abundant metal elements to obtain effective OER catalysis will provide great convenience in practical applications.

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Introduction

The ever-increasing depletion of fossil fuels and worsening environmental contamination stimulate extensive research into clean and efficient energy generation systems.^1–3 Among these, electrochemical water splitting has attracted significant attention due to hydrogen production utilizing electricity from renewable but intermittent sources such as solar energy, wind, wave power, etc.^4,5 However, the associated energy conversion efficiency is greatly limited by the sluggish oxygen evolution reaction (OER), which is a complex four-electron redox process.^6–9 IrO₂ and RuO₂, as the most efficient electrocatalysts, exhibit high OER activity, yet their low-abundance and high cost dramatically hinder commercial utilization.¹⁰ Alternatives based on earth-abundant and inexpensive transition-metal based materials are actively being pursued.

As non-precious-metal compounds, Fe-based oxides can serve as active OER electrocatalysts, but they are restricted by their low conductivity and chemical stability.^11–16 Highly crystallized Fe nitrides,^17–20 phosphides,^21–23 sulfides,²⁴ etc. have better electronic conductive properties and are more thermodynamically/chemically stable, exhibiting progressive OER activity. However, their electrocatalytic performances are still far from the requirements of practical applications. A challenging route, the topological transformation strategy towards an amorphous structure, has been developed to improve OER dynamics by increasing the intrinsic activity and exposing more active sites. However, the resulting non-crystallized materials are generally susceptible to unavoidable collapse to some degree, exhibiting unsatisfactory robustness.^25–27 Designing poorly-crystallized materials with excellent catalytic activity similar to the amorphous form, and with structural stabilization of crystallization will provide a fresh and efficient perspective towards an ideal model.

On the basis of the above consideration, low-crystalline Fe₅O₇(OH)·4H₂O was proposed and was developed by a one-pot hydrothermal method in the presence of Ni foam (NF). Such material anchoring on NF (Fe₅O₇(OH)·4H₂O/NF) demonstrates excellent OER activity with an overpotential of 269 mV to drive 100 mA cm⁻² current density in a 1.0 M KOH environment, outperforming most of the reported Fe-based electrocatalysts. More remarkably, its activity can be maintained for at least 100 h. It is shown that the highly efficient electrocatalytic OER performance lies in the unrivaled structure superiority, stemming from the poor-crystallization state that is caused by the existence of massive oxygen-deficiencies.

Results and discussion

Fig. 1a shows the X-ray diffraction (XRD) pattern of the solvothermally obtained products. The diffraction peaks at 36.08°, 40.54°, 46.43°, 53.15°, 61.32°, and 63.02° are indexed to the (110), (200), (113), (114), (115), and (106) crystal planes of Fe₅O₇(OH)·4H₂O, respectively (JCPDS No. 29-0712).²⁸ Moreover, X-ray photoelectron spectroscopy (XPS) characterization was performed on the resulting Fe₅O₇(OH)·4H₂O. The corresponding XPS survey spectrum (Fig. 1b) provides evidence for the presence of Fe and O elements. The N element should be derived from the adsorbed ferric nitrate impurity.²⁹ In the Fe 2p region (Fig. 1c), the binding energy (BE) of 709.2 eV can be attributed to the ferric nitrate adsorbed on the Fe₅O₇(OH)·4H₂O nanocrystallites. The peak at 710.2 eV is assigned to the BE of Fe 2p_3/2 in Fe₅O₇(OH)·4H₂O.²⁹ Moreover, the peaks centered at 530.4 eV, 531.2 eV, and 533.1 eV in the O 1s region (Fig. 1d) are ascribed to the Fe–O bond, the oxygen vacancies and the hydroxyl group, respectively, in the Fe₅O₇(OH)·4H₂O.

Fig. 1 (a) XRD pattern of Fe₅O₇(OH)·4H₂O. (b) XPS survey spectrum of Fe₅O₇(OH)·4H₂O. XPS spectra of Fe₅O₇(OH)·4H₂O in the (c) Fe 2p and (d) O 1s regions.

Fig. 2a and b show the scanning electron microscopy (SEM) images of Fe₅O₇(OH)·4H₂O/NF. It can be seen that the Fe₅O₇(OH)·4H₂O nanosheets grow tightly on the NF and form arrays on the NF. The Brunauer–Emmett–Teller (BET) tests revealed that the surface area of Fe₅O₇(OH)·4H₂O/NF is 50.2 cm². The associating transmission electron microscopy (TEM) image of Fe₅O₇(OH)·4H₂O further confirms its nanosheet structure (Fig. 2c), and the high-resolution TEM (HRTEM) image demonstrates that the lattice is not well-defined (Fig. 2d), due to its intrinsic poorly-crystallized state. The electron spin resonance (ESR) spectroscopy determination unveils the presence of a surface oxygen vacancy. The obtained samples show two distinct symmetrical ESR signal peaks at g = 2.002 (Fig. S1†). The corresponding selected area electron diffraction (SAED) pattern shows rings indexed to the (115) and (110) planes of Fe₅O₇(OH)·4H₂O. Energy-dispersive X-ray (EDX) elemental mapping images of Fe₅O₇(OH)·4H₂O/NF (Fig. 2e and f) also show that Fe and O elements are uniformly distributed in the Fe₅O₇(OH)·4H₂O product.

Fig. 2 (a and b) SEM images for Fe₅O₇(OH)·4H₂O/NF. (c) TEM image of one single Fe₅O₇(OH)·4H₂O nanosheet. (d) HRTEM image and SADE pattern for the Fe₅O₇(OH)·4H₂O nanosheet. EDX elemental mapping images of (e) Fe and (f) O.

The electrochemical OER performance was examined in 1.0 M KOH using a typical three-electrode configuration. Fe₅O₇(OH)·4H₂O/NF (Fe₅O₇(OH)·4H₂O loading: 0.6 mg cm⁻²) was used as the working electrode, while Hg/HgO (filled with 6 M KOH solution) and a graphite plate were utilized as the reference and counter electrodes, respectively. For comparison, NF and RuO₂ deposited on NF (RuO₂/NF, RuO₂ loading: 0.6 mg cm⁻²) were also tested under the same conditions. Considering ohmic potential drop (iR) losses, experimental data were all corrected for further analysis, thus directly reflecting the intrinsic behavior of the catalysts. The overpotentials were converted and reported as values vs. the reversible hydrogen electrode (RHE). As observed from the linear sweep voltammetry (LSV) curves in Fig. 3a, Fe₅O₇(OH)·4H₂O/NF demonstrates an outstanding catalytic activity with an overpotential of 269 mV to drive 100 mA cm⁻² in 1.0 M KOH, outperforming most of the reported Fe-based electrocatalysts such as Ni–Fe LDH (269 mV – 5 mA cm⁻²),³⁰ Ni–Fe LDH/CNT (269 mV – 40 mA cm⁻²),³¹ FeOOH/Au (450 mV – 9 mA cm⁻²),³² etc. (Table S1†). More impressively, such an electrocatalytic performance even surpasses that of RuO₂/NF (279 mV, 100 mA cm⁻²). As for the Tafel plots shown in Fig. 3b, RuO₂/NF gives a Tafel slope of 81 mV dec⁻¹. The corresponding Tafel slope for Fe₅O₇(OH)·4H₂O/NF (118 mV dec⁻¹) is relatively larger than that for RuO₂/NF, implying the further engineering requirement towards more favorable catalytic kinetics. The high surface area of the array structure, affirmed by the BET measurements, and the existence of abundant oxygen-defects verified by the electron spin resonance spectrum, both contribute to the outstanding catalytic activity. The intrinsic oxygen-defects of Fe₅O₇(OH)·4H₂O result in its poorly-crystallized state. We deliberately treated the sample under high-temperature and the degraded catalytic activity was observed, related to the overpotentials of 280 mV and 480 mV to deliver 15 mA cm⁻² and 100 mA cm⁻², respectively (Fig. S2†). However, such a performance of the sample after high-temperature treatment is still very attractive for scientific researchers. To further gain insight into the excellent OER activity of Fe₅O₇(OH)·4H₂O, we examined the electrochemical double layer capacitances (C_dl), proportional to the electrochemical surface area (ECSA), for Fe₅O₇(OH)·4H₂O/NF. Fig. S3† presents the corresponding cyclic voltammograms. The slope for the linear plot of the non-faradaic capacitance currents as a function of scan rates is the value of C_dl. The C_dl for Fe₅O₇(OH)·4H₂O/NF is 18 mF cm⁻², and ECSA was determined to be 45 cm² on the basis of the equation: ECSA = C_dl/0.4. A series of cyclic voltammograms for Fe₅O₇(OH)·4H₂O/NF and Fe₅O₇(OH)·4H₂O/NF after heat treatment were collected, and are shown in Fig. S4.† A linear plot related to the oxidation currents for the redox species as a function of the scan rates can be obtained from the cyclic voltammograms, and the turnover frequency (TOF) was determined to be 0.017 mol of O₂ s⁻¹ at an overpotential of 269 mV for Fe₅O₇(OH)·4H₂O/NF, and this is superior to those of samples after heat treatment (0.00468 mol O₂ s⁻¹ for overpotential of 300 mV). The long-term stability of Fe₅O₇(OH)·4H₂O/NF was also evaluated by continuous cyclic voltammetry scanning at a scan rate of 100 mV s⁻¹. After 1000 cycles in 1.0 M KOH, the obtained LSV curve almost coincides with the initial one (Fig. 3c). The superb durability of Fe₅O₇(OH)·4H₂O/NF was also recorded by OER electrolysis at a fixed overpotential, preserving its catalytic activity for at least 100 h (Fig. 3d).

Fig. 3 (a) LSV curves of Fe₅O₇(OH)·4H₂O/NF, RuO₂/NF and bare NF for OER. (b) Corresponding Tafel plots for Fe₅O₇(OH)·4H₂O/NF and RuO₂/NF. (c) LSV curves for Fe₅O₇(OH)·4H₂O/NF before and after 1000 CV cycles. (d) Time-dependent current density curve of Fe₅O₇(OH)·4H₂O/NF. All experiments were tested in 1.0 M KOH.

The experimentally emerging gas species was qualitatively determined by gas chromatography analysis and was further qualified using a calibrated pressure sensor, monitoring the pressure change in the anodic compartment of the H-type electrolytic cell. The faradaic efficiency (FE) of Fe₅O₇(OH)·4H₂O/NF was then calculated to be 100% by comparing the molar number of the practically evolved oxygen with the theoretically calculated number (Fig. 4).

Fig. 4 The amount of oxygen theoretically calculated and experimentally measured versus time for Fe₅O₇(OH)·4H₂O/NF in 1.0 M KOH.

Conclusions

In summary, poorly-crystallized Fe₅O₇(OH)·4H₂O was designed and exploited by a one-pot hydrothermal method as a high-efficiency water-oxidation electrocatalyst. Profiting from its low-crystallization state, such a material exhibits a low overpotential of 269 mV at 100 mA cm⁻² current density, exceeding most of the reported Fe-based electrocatalysts. More encouragingly, the activity can be maintained for at least 100 h. This current work not only provides an outstanding OER electrocatalyst, but also demonstrates a new avenue to construct highly efficient poorly-crystallized catalysts, by a relatively quick method using one of the most abundant metal elements, for other electrocatalytic systems.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21575137).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra06374a

‡ Xiaoteng Ding and Wei Cui contributed equally to this work.

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