Ligand-assisted capping growth of self-supporting ultrathin FeNi-LDH nanosheet arrays with atomically dispersed chromium atoms for efficient electrocatalytic water oxidation†

Xiuyuan Xie ^ab, Changsheng Cao ^ac, Wenbo Wei ^ac, Shenghua Zhou ^a, Xin-Tao Wu ^a and Qi-Long Zhu *^a
aState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter (FJIRSM), Chinese Academy of Sciences (CAS), Fuzhou 350002, China. E-mail: qlzhu@fjirsm.ac.cn
^bFuzhou University, Fuzhou 350002, China
^cUniversity of Chinese Academy of Sciences, Beijing 100049, China

First published on 4th February 2020

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Electrochemical water splitting is rising as a promising technology to produce hydrogen as a clean and renewable fuel source, which holds the potential to mitigate energy crisis and environmental contamination.¹ This process involves two half reactions including the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).² However, the OER, as a vital half reaction at the anode, suffers from sluggish kinetics and complicated multistep proton-coupled electron transfer processes, causing the bottleneck to improve the whole efficiency of electrochemical water splitting.³ Therefore, it is urgent but challenging to develop high-efficiency and robust electrocatalysts to improve the OER performance. Although the state-of-the-art noble–metal oxides (e.g., RuO₂ and IrO₂) have been considered as the benchmark OER catalysts, their high cost, scarcity and poor long-term stability severely restrict their large-scale applications.⁴ It is thus critical to exploit new Earth-abundant, cost-effective, highly active and robust nonprecious metal based electrocatalysts for the OER.⁵

FeNi-based layered double hydroxides (LDH), as a new category of candidates as efficient OER catalysts, have recently attracted widespread attention because of their low cost, easy accessibility, and encouraging water oxidation activity.⁶ However, the intrinsic OER activity and conductivity of the pristine FeNi-LDH are far from satisfactory. Therefore, great efforts have been made to further improve their OER performance.⁷ Recently, the exfoliation or nano-engineering of FeNi-LDH to achieve thin nanosheet morphology can largely expose the unsaturated coordination active sites, which is favorable for improving the OER activity.⁸ Moreover, the introduction of a suitable foreign metal element into FeNi-LDH has been considered to improve the electrical conductivity and modulate the electronic structure, which would significantly enhance the OER performance.⁹ Sun and co-workers found that the incorporation of Co³⁺ to form ternary NiCoFe-LDH can tune the electron density of the active sites, much improving the OER activity.^9d Jin and co-workers reported that the ultrathin Ni₃FeAl_x trinary LDH nanosheets exhibited higher activity and stability for the OER than NiFe-LDH nanosheets, and the atomic ratio of Ni³⁺ and Ni species and the electrocatalytic activity of Ni₃FeAl_x-LDHs presented an identical volcano-type dependence on the contents of Al substitution.¹⁰ Taking inspiration from previous results, the Cr³⁺ ions, which possess the t_2g³e_g⁰ electron configuration and several oxidation states (from +1 to +6, commonly +3 and +4), could be a superior candidate as the third metal to effectively increase the intrinsic activity of the NiFe-LDH catalysts.¹¹ The multivalent property of Cr³⁺ is prone to generate strong electron interactions between different metals and thereby produce a favorable local coordination environment and electronic structure, which will presumably modulate the absorption/desorption of the reaction intermediate and finally boost the OER activity.¹² Furthermore, the incorporation of Cr³⁺ into NiFe-LDH could also be conducive to the promotion of conductivity, charge transfer and electron capture.¹³

In addition to the foreign metal doping, the integration of the electrocatalysts with conductive substrates also has a critical influence on the catalytic performance.¹⁴ To date, most of the reported FeNi-based LDHs are powder-type, making a binder essential for preparing the working electrodes, which is not only time-consuming, but also easily causes falling-off of the electrocatalysts during long-time electrolysis.¹⁵ Therefore, the direct growth of thin FeNi-based LDH nanosheets on conductive substrates with intimate contact and readily accessible active sites is considered to be an effective way to achieve better OER performance.¹⁶ In this regard, commercially available stainless steel (SS) mesh with low cost, high mechanical strength and flexibility and excellent electrical conductivity is considered to be a good choice as an electrode scaffold.¹⁷ More particularly, SS mesh contains several Earth-abundant elements like Fe, Ni, and Cr, and thus can be used as a superior semisacrificial template for in situ conversion to supported FeNi-LDH with desirable Cr³⁺ doping.

Herein, for the first time, we report a facile ligand-assisted capping growth approach (LACGA) to realize the cost-effective growth of self-supporting ultrathin atomic Cr-doped FeNi-LDH nanosheet arrays on SS (Cr₁/FeNi-LDH-SS) by directly using SS as both the metal source and substrate. Intriguingly, the organic multidentate ligand serves as an effective capping agent to govern the growth of Cr₁/FeNi-LDH in the synthetic procedure, achieving the uniform ultrathin nanosheet array morphology. Meanwhile, the in situ doping of atomically dispersed Cr³⁺ atoms into FeNi-LDH can be synchronously implemented to modulate the electronic structure owing to the sufficient content of the Cr element in SS. Owing to its unique nanostructure and atomic Cr³⁺ doping, the as-prepared Cr₁/FeNi-LDH-SS electrode exhibited exceptional OER performance with the overpotentials as low as 202 and 242 mV to achieve current densities of 10 and 100 mA cm⁻², respectively, and excellent stability, making it stand out against other LDH based catalysts reported to date for the OER. Density functional theory (DFT)+U computations indicate that the atomic doping of Cr could not only enhance the intrinsic electrochemical activity of the active sites but also narrow the bandgap to improve the conductivity of the catalyst, thus largely reducing the theoretical overpotential of Cr₁/FeNi-LDH compared with FeNi-LDH. This work shows great perspectives in designing cheap yet efficient self-supporting electrocatalysts and might provide atomic-level insights into the effect of element doping on water oxidation.

As schematically illustrated in Scheme 1, the ultrathin Cr₁/FeNi-LDH nanosheet arrays were directly grown on SS (304, 1000 mesh) with the assistance of organic multidentate ligand (2,5-dihydroxybenzenedicarboxylic acid, H₄DOBDC) under acidic conditions, where SS was used as both a metal source and substrate. Notably, H₄DOBDC has multi-coordination modes and strong coordination capability with metal ions and thus could serve as an effective capping agent to govern the growth. Briefly, 20 mg of H₄DOBDC was dissolved in a solution mixture of DMA and 2.0 M HCl (30 mL, v/v = 1:1), in which a piece of commercial SS pretreated with acid was placed. After aging at 150 °C for 18 h under solvothermal conditions, the Cr₁/FeNi-LDH-SS sample with dark green color was obtained, which can be directly used as the working electrode for subsequent electrochemical tests. During the growth process, H₄DOBDC as a capping agent played a critical role in mediating the formation of the Cr₁/FeNi-LDH nanosheet arrays on SS. For comparison, NiFe-LDH-SS was prepared with additional iron and nickel ions following similar procedures, where SS was merely used as a template for supporting NiFe-LDH. Without using SS as the metal source and substrate, CrFeNi-LDH was also synthesized from the corresponding metal salts.

Powder X-ray diffraction (PXRD) patterns of the as-prepared Cr₁/FeNi-LDH-SS and peeled Cr₁/FeNi-LDH are shown in Fig. 1a and S1.† All the diffraction peaks are consistent with the hydrotalcite-like layered double hydroxides (JCPDS#40-0215), indicating the successful generation of crystalline LDH-based phases, which is further evidenced by the color change of SS from the pristine argenteus with metal luster to dark green after synthesis (Fig. S2†). In contrast, the control sample FeNi-LDH-SS seems to be amorphous (Fig. S1†). The microstructure of Cr₁/FeNi-LDH-SS was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Remarkably, the SEM images show that the interconnected ultrathin nanosheets were vertically grown on the SS surface, forming a uniform and 3D honeycomb-like open porous nanostructure (Fig. 1b, S3c and d†), which is much different from the smooth surface of the pristine SS (Fig. S3a and b†). Since the Cr₁/FeNi-LDH nanosheets were in situ grown with the metal ions etched from SS, they can be tightly and chemically anchored to the surface of SS with coordination interactions. The average thickness of the peeled nanosheets measured by using an atomic force microscope (AFM) was found to be 1.69 nm (Fig. 1d), corresponding to about 2 LDH layers.¹⁸ Furthermore, the TEM image of the curly and semitransparent nanosheets confirms their ultrathin nanostructure without any metallic nanoparticles (Fig. 1e). The ultrathin nature of such a layered material indicates its great potential for electrocatalytic applications. The selected area electron diffraction (SAED) pattern reveals the crystalline nature of Cr₁/FeNi-LDH (inset in Fig. 1e), which is consistent with the PXRD result. Energy-dispersive X-ray (EDX) elemental mapping analysis demonstrates the uniform distributions of Fe, Ni and Cr elements in Cr₁/FeNi-LDH (Fig. 1f–i), implying the homogeneous self-doping of Cr in nanosheets.

Owing to the unique capping effect of the organic multidentate ligand, the effect of the amount of the capping agent H₄DOBDC added during the synthesis on the formation of the nanostructure was explored. As shown in Fig. S4,† the added amount of H₄DOBDC is of great importance for the thickness of the vertically grown Cr-doped FeNi-LDH nanosheets. Without the addition of the ligand, the nanosheets with the thickness in dozens of nanometers were obtained (Fig. 1c). With the increase in the amount of the added ligand, the thickness of the nanosheets was gradually decreased, while the 3D open porous nanostructure was still maintained. The excess addition of the ligand could further decrease the thickness of the nanosheets; however, the nanosheets may be too thin to support their vertical nanostructure and the height of the nanosheet array was much reduced, which is detrimental to promote the electrocatalytic OER performance (Fig. S5†). In stark contrast, only irregular nanoparticles without 3D open porous nanostructures were formed in FeNi-LDH-SS and CrFeNi-LDH synthesized from extra corresponding metal salts (Fig. S6†). Therefore, the semisacrificial conversion of SS and the capping control effect by the organic ligands are indispensable to obtain such a unique nanostructure of Cr₁/FeNi-LDH-SS. The 3D open porous nanostructure with ultrathin nanosheet arrays should be beneficial to expose more active sites and promote electron and mass transfer, which will lead to a better electrocatalytic performance.¹⁹

Moreover, the influences of Cr-doping and doping behavior on the surface chemical compositions and electronic states were investigated by X-ray absorption fine-structure spectroscopy (XAFS) and X-ray photoelectron spectroscopy (XPS). It is shown in Fig. 2a that the Cr K-edge X-ray absorption near-edge structure (XANES) spectrum of the Cr₁/FeNi-LDH nanosheets peeled from Cr₁/FeNi-LDH-SS is quite different from that of Cr-foil, suggesting the dominance of Cr³⁺ in Cr₁/FeNi-LDH,²⁰ which is also confirmed by the Cr 2p XPS spectrum (Fig. S7†). More structural information of Cr atoms can be further obtained from the extended X-ray absorption fine structure (EXAFS). As shown in Fig. 2b, the Fourier transforms (R space) display that both Cr₁/FeNi-LDH and Cr₂O₃ exhibit peaks at about 1.5 and 2.6 Å, which can be assigned to Cr–O coordination and the Cr–O scattering path in higher shells,²¹ respectively, corroborating the atomic dispersion of the Cr³⁺ atoms in Cr₁/FeNi-LDH. These results are aligned with the TEM observation. However, the Cr foil shows an obvious Cr–Cr coordination peak at 2.2 Å. In comparison with Cr₂O₃, the clear shifts of those two peaks were observed, demonstrating the slightly disturbed Cr–O environment, which may result from the interactions of the doped Cr with Ni and Fe in Cr₁/FeNi-LDH.

To further explore the influence of the atomically doped Cr atoms on the electronic structures of Ni and Fe in Cr₁/FeNi-LDH-SS, the high-resolution Fe 2p and Ni 2p XPS spectra were recorded. As shown in Fig. 2c, the two main peaks at around 712 and 724 eV, as well as the satellite peak at around 718 eV, can be observed in the Fe 2p spectra for Cr₁/FeNi-LDH-SS, implying the Fe³⁺ oxidation state in this electrode.²² In the Ni 2p spectra (Fig. 2d), the peaks at around 856 and 874 eV are attributed to the Ni²⁺ oxidation state, and besides, another pair of weak peaks at around 857 and 875.6 eV should be ascribed to the links between Ni atoms and hydroxyl groups.^18,23 Interestingly, as compared with FeNi-LDH-SS, both the binding energies of Fe 2p and Ni 2p spectra for Cr₁/FeNi-LDH-SS show positive shifts, revealing that the electron densities around the Fe and Ni sites are reduced. The dominant Cr³⁺ atoms in Cr₁/FeNi-LDH-SS possess a special electronic configuration of t_2g³e_g⁰, which is considered to be beneficial for electron capture and charge transfer. Therefore, the electron-withdrawing property of the Cr³⁺ atoms should be mainly responsible for the reduction of electron densities of the Fe and Ni sites in Cr₁/FeNi-LDH-SS. It has been widely accepted that the oxidation of Ni or Fe to high valent species in FeNi-based catalysts plays a crucial role in the catalytic OER performance,²⁴ as the more electrophilic Ni or Fe sites would promote the nucleophilic addition of hydroxyls and water molecules on the catalyst surface and improve the absorption of the reaction intermediate during the OER process.^12a As a result, such a modulated electronic environment of the Fe and Ni sites in Cr₁/FeNi-LDH-SS could be conducive to promote its electrocatalytic OER performance.

The electrocatalytic OER performances of the as-prepared Cr₁/FeNi-LDH-SS and FeNi-LDH-SS electrodes were evaluated in a typical three-electrode system, in which 1.0 M KOH, a saturated Ag/AgCl electrode and Pt mesh were used as the electrolyte, and reference and counter electrodes, respectively. The pre-synthesized CrFeNi-LDH and commercial RuO₂ powders loaded onto SS, namely CrFeNi-LDH/SS and RuO₂/SS, were also measured as the comparative samples. The potentials in all electrochemical tests were calibrated with respect to a reversible hydrogen electrode (RHE). Fig. 3a displays the linear sweep voltammetry (LSV) curves of these electrodes, where an oxidation peak in the region of 1.4–1.5 V, corresponding to the oxidation of Ni²⁺ to Ni^3+/4+, was obviously observed for Cr₁/FeNi-LDH-SS and FeNi-LDH-SS. Therefore, LSV curves of all electrodes were also scanned in the reverse sweep direction to exclude the influence of the trail of the oxidation peaks (Fig. S8†). As shown in Fig. 3a, b and S8,† Cr₁/FeNi-LDH-SS exhibits the best OER activity among all electrodes, only needing ultralow overpotentials of 202 and 242 mV to achieve current densities of 10 mA cm⁻² and 100 mA cm⁻², respectively, which are much superior to those of SS (330 and 380 mV), FeNi-LDH-SS (287 and 337 mV), CrFeNi-LDH/SS (286 and 333 mV) and RuO₂/SS (306 and 367 mV). To the best of our knowledge, the as-prepared Cr₁/FeNi-LDH-SS is among the best LDH-based and other OER electrodes reported to date (Table S1†).

Additionally, to understand the reaction kinetics, the corresponding Tafel plots for the electrodes were applied (Fig. 3c), among which Cr₁/FeNi-LDH-SS exhibits the lowest Tafel slope of 32.5 mV dec⁻¹ compared to CrFeNi-LDH/SS (42.8 mV dec⁻¹), FeNi-SS (42.0 mV dec⁻¹), SS (50.3 mV dec⁻¹) and RuO₂/SS (55.7 mV dec⁻¹), endorsing its much enhanced OER kinetics. To further shed light on the possible factors contributing to the enhanced OER activity of Cr₁/FeNi-LDH-SS, the electrochemically active surface areas (ECSA), generally proportional to the double layer capacitance (C_dl), of these electrodes were investigated based on the cyclic voltammetry (CV) curves at different scan rates (Fig. S9†). Apparently, Cr₁/FeNi-LDH-SS has an estimated C_dl value of 1.81 mF cm⁻², larger than those of CrFeNi-LDH/SS (1.21 mF cm⁻²), FeNi-LDH-SS (0.88 mF cm⁻²) and SS (0.78 mF cm⁻²) (Fig. 3d), suggesting that the ultrathin nanosheet arrays with a 3D open porous nanostructure are prone to expose more accessible active sites. Meanwhile, Cr₁/FeNi-LDH-SS presents much higher current densities normalized by the ECSA (Fig. S10†), manifesting that the intrinsic activity of the active sites in Cr₁/FeNi-LDH-SS for the OER is much enhanced with atomic Cr³⁺ doping. Moreover, the electrochemical impedance spectroscopy (EIS) plots were recorded to evaluate the charge transfer properties of the electrodes during the OER process. As shown in Fig. 3e, Cr₁/FeNi-LDH-SS presents the smallest semicircle diameter in the obtained Nyquist plots compared to other electrodes, implying that Cr₁/FeNi-LDH-SS has significantly improved charge transfer kinetics. Therefore, it can be concluded that the boosted OER activity of Cr₁/FeNi-LDH-SS could be highly correlated with its unique 3D open porous nanostructure featuring the synergistic effects of the abundant accessible active sites, favorable charge transfer kinetics, highly conductive substrate, and enhanced intrinsic activity of the active sites.

Besides its excellent OER activity, Cr₁/FeNi-LDH-SS exhibits prominent long-term durability. Chronopotentiometry curves without iR-compensation were obtained at the current densities between 10 and 20 mA cm⁻² in a galvanostatic mode for Cr₁/FeNi-LDH-SS and RuO₂/SS. It can be clearly seen in Fig. 3f that the potential to achieve a current density of 10 mA cm⁻² (ca. 1.50 V) had little change during continuous electrolysis for 15 hours, while it increased about 20 mV for RuO₂/SS, elucidating the satisfactory activity stability of Cr₁/FeNi-LDH-SS for the OER. Moreover, both SEM and XPS analyses for the recycled Cr₁/FeNi-LDH-SS confirmed that there was no significant change in morphology and electronic states after the stability test (Fig. S11†), further indicating its excellent structural stability.

According to the experimental results, the doping of atomically dispersed Cr atoms can greatly enhance the electrochemical performance during the OER process. To further study the relationship between electronic structures and the outstanding electrocatalytic activity of Cr₁/FeNi-LDH, density functional theory (DFT)+U computations were implemented to calculate the Gibbs free energy diagrams of the OER. The water oxidation 4e^– mechanism proposed by Norskov²⁵ is as follows:

* + H2O → *OH + (H+ + e−)	(I)

*OH → *O + (H+ + e−)	(II)

*O + H2O → *OOH + (H+ + e−)	(III)

*OOH → * + O2 + (H+ + e−)	(IV)

where * represents the reaction sites of the catalysts. And the overpotential η is defined as:

η = ΔGmax − 1.23 eV	(1)

where ΔG_max represents the highest Gibbs free energies among the 4 reaction steps of the OER process.

According to the experimental results, the computational structural configurations are depicted in Fig. S12† and the mechanism of the OER process on FeNi-LDH and Cr₁/FeNi-LDH is illustrated in Fig. 4a and b. Fig. 4c displays the Gibbs free energy diagram of the whole OER process, indicating that the formation of *O (ΔG_II = 3.25 eV) is the determining step in the OER process over FeNi-LDH. While for Cr₁/FeNi-LDH, the determining step is the formation of *OOH (ΔG_III = 1.80 eV), and the theoretical overpotential of Cr₁/FeNi-LDH is much lower than that of FeNi-LDH, which means that the atomic doping of Cr could facilitate the reaction process. To further understand the doping effects of the Cr atoms, the electronic structures of FeNi-LDH and Cr₁/FeNi-LDH were investigated. As shown in Fig. 4d, the narrower band gap was found after doping with the Cr atoms, which indicates the improvement of electric conductivity. Combined with the experimental results, the atomic Cr doping of Cr₁/FeNi-LDH can obviously enhance its electrocatalytic activity and lead to a better catalytic behavior as compared with the pristine FeNi-LDH.

In summary, with the assistance of the organic ligand as an effective capping agent, a self-supporting atomic Cr-doped ultrathin FeNi-LDH nanosheet array electrode (Cr₁/FeNi-LDH-SS) was cost-effectively fabricated by a facile LACGA concerning semisacrificial SS, which can offer a particular advantage in the control of the structure. Owing to its novel 3D open porous nanostructure, Cr₁/FeNi-LDH-SS is capable of exposing more accessible active sites and facilitating the electron transfer. Moreover, the in situ doping of atomically dispersed Cr atoms in Cr₁/FeNi-LDH-SS can modulate the electronic structure of the active sites to benefit charge transfer and electron capture, and thus greatly promote its intrinsic activity, as unraveled by the combination of the electrochemical analysis and DFT+U computations. The unique nanostructure and strong synergetic effect between the atomically dispersed Cr dopants and the active sites enable this electrode to afford an exceptional OER activity, as well as excellent long-term durability. This work opens a new path to fabricate more innovative and stable LDH-based electrocatalysts with outstanding electrochemical performances.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for the financial support from the One Thousand Young Talents Program under the Recruitment Program of Global Experts, the National Natural Science Foundation of China (NSFC) (21901246, 21905278 and 21771179), and the Natural Science Foundation of Fujian Province (2019J05158 and 2019J01133). We also appreciate the support from Shanghai Synchrotron Radiation Facility (SSRF) for EXAFS and XANES measurements.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nr10781a

This journal is © The Royal Society of Chemistry 2020