Crystal lattice distortion in ultrathin Co(OH)2 nanosheets inducing elongated Co–OOH bonds for highly efficient oxygen evolution reaction

Haidong Yang , Yu Long , Yan Zhu , Ziming Zhao , Ping Ma , Jun Jin * and Jiantai Ma *
State Key Laboratory of Applied Organic Chemistry (SKLAOC), The Key Laboratory of Catalytic Engineering of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000, P. R. China. E-mail: jinjun@lzu.edu.cn; majiantai@lzu.edu.cn; Fax: +86-931-8912582; Tel: +86-931-8912577

Received 20th August 2017 , Accepted 2nd November 2017

First published on 3rd November 2017


The exploration of highly efficient nonprecious metal-based electrocatalysts for the oxygen evolution reaction (OER) is of great importance for potential applications in sustainable energy conversion. Recently, the layered metal hydroxide (LMH) family is receiving extensive research attention owing to its unique structural properties. However, the gained electrocatalytic performance of LMH-based catalysts for the OER is still far from the state-of-the-art requirements because of its finite number and poor reactivity of exposed active sites. In response, we synthesized crystal lattice distorted ultrathin cobalt hydroxide (denoted as CLD-u-Co(OH)2) nanosheets with a great number of efficient catalytic active sites through the introduction of Ga into ultrathin Co(OH)2, followed by a selective removal of Ga, denoted as the “introduction @ removal” process. As a result, the crystal lattice distortion confined inside CLD-u-Co(OH)2 generates abundant elongated Co–OOH bonds on exposed (1[2 with combining macron]0) facets serving as efficient catalytic active sites for the OER. Besides, the optimized amount of “introduction @ removal” of Ga (4 at%) allows for an exquisite balance between distortion engineering and electrical conductivity, synergistically. The as-prepared CLD-u-Co(OH)2 achieves an overpotential of 265 mV at a current density of 10 mA cm−−2, an unexpectedly small Tafel slope of 47 mV dec−−1, and a long-term stability (beyond 20 h) in basic media. It is mainly attributed to abundant catalytic active sites, robust reactivity per site, and good electrical conductivity. Furthermore, the green and sustainable engineering of crystal lattice distortion to improve the intrinsic electrocatalytic activity of CLD-u-Co(OH)2 nanosheets presented in this work may provide a promising strategy to design and synthesize newly highly efficient LMH-based electrocatalysts for the OER.


Introduction

The electrolysis of water has been considered as a promising way for the production of renewable hydrogen (H2) to meeting the increasing global energy demand.1–3 The electrochemical production of hydrogen is impeded by the oxygen evolution reaction (OER), which involves complex four-electron redox processes.4,5 Therefore, exploring the highly efficient OER electrocatalysts is one of the most promising pathways to overcome this high kinetic barrier. Nevertheless, the low abundance and high cost of state-of-the-art RuO2 and IrO2 intensively limited their large-scale application.6 For this reason, in the past decade, considerable efforts have been directed toward the exploration of efficient and cost-effective OER electrocatalysts based on nonprecious metals, such as nonprecious transition metal sulfides,7 phosphides,8,9 oxides,10,11 and hydroxides.12,13 Among these catalysts, brucite-like layered metal hydroxides (LMHs) exhibit a good OER performance because of their multifarious chemical compositions and unique laminar crystal structures.14,15 In particular, Co-based LMHs are promising candidate electrocatalyst materials for the OER in alkaline electrolytes owing to their cost effectiveness, earth-abundance, and good catalytic activity.12,16 Although a great deal of progress has been made to design and fabricate inexpensive Co(OH)2 based OER electrocatalysts through tuning the morphology, composition, and exposed facets, an elaborate design and synthesis of Co-based LMHs as efficient and nonprecious OER electrocatalysts still presents as a great challenge.

Active sites play the vital role for the catalytic process.17,18 Therefore, increasing the number of surface exposed active sites is beneficial for enhancing the OER performance.19,20 It is well known that the surface chemical reaction will preferentially occur at defect sites, namely catalytic active sites; thus, the OER catalytic activity is closely correlated with the number of low-coordinated active sites, which can be generated through increasing the surface area when the total number of atoms is kept constant.17,21–23 In this case, ultrathin two-dimensional (2D) nanosheet materials represent a very promising structural motif to employ as catalysts for efficient OER, due to the fact that the 2D nanosheet structure can provide a great number of exposed low-coordinated active sites as the thickness decreased.14,22,24–26 Accordingly, in recent years, tremendous efforts have been devoted to preparing atomically thin OER electrocatalysts, such as ultrathin Co-based nanosheets,11,12,23,27 ultrathin transition metal dichalcogenide (TMD) nanosheets,28,29 and ultrathin layered metal hydroxide (LMH) nanosheets.30,31 Particularly, the laminar crystal nature of brucite-like metal-hydroxyl host layers are regarded as ideal candidates to prepare ultrathin or monolayer structured catalysts by in situ growth and exfoliation techniques,14 which can provide a larger number of catalytic active sites.

Additionally, the intrinsic reactivity of active sites in LMHs is another critical factor affecting the overall catalytic performance;17–20,29 thus, a properly engineered surface to improve the intrinsic reactivity is needed. It is well known that the crystal lattice distortion (e.g. the change in bond length or angle) of crystallized catalysts can effectively alter the reactivity of catalysts due to the multifarious electrons and orbital distributions, which is accompanied by significant changes in heterogeneous spin states of the active sites.8,27,32 Traditionally, introduction and variation of foreign atoms during the synthesis process for catalysts can induce crystal lattice distortion.8,27,32–36 Furthermore, for bimetallic compounds with the coexistence of both A and B cations, changing the relative concentration of cations (that is, the mole ratio of A/B) is proved to be an effective way to achieve crystal lattice distortion, especially when A and B cations have different ground state structures, coordination numbers, and local geometries.27,36 Due to these intrinsic differences between A and B cations, one cation should possess a relatively high catalytic activity, while another cation should display negligible catalytic activity. Notably, although some less active cations (especially noble-metal cations) in bimetallic compounds did not show any intrinsic catalytic activity during the reaction, the catalytic activity can be improved by synergistic effects.27,37,38 If these less active cations could be selectively removed by a suitable method with the other improved catalytic properties retained, the cost-effectiveness of these catalysts would be maximally elevated. Hence, we assumed that foreign cations could be firstly introduced into the catalysts to induce crystal lattice distortion and the corresponding change in bond length or angle, and then followed by a selective removal process to recycle these cations, especially those noble-metal cations. However, this assumptive green and sustainable strategy to design and fabricate advanced catalysts has multiple challenges.

Based on the above conceptions, ultrathin Co(OH)2 nanosheets should be an ideal platform to investigate the effect of “the introduction followed by a selective removal process of certain cations” on catalytic performance. In this work, the crystal lattice distorted ultrathin Co(OH)2 (denoted as CLD-u-Co(OH)2) nanosheets with a thickness of approximately 1.31 nm were successfully synthesized by a controllable method. Specifically, an optimized amount of Ga (4 at%) was firstly introduced into the ultrathin Co(OH)2 (denoted as u-Co96Ga4 LMHs, used as precursors), followed by a corrosion in 2 M KOH to selectively remove the introduced Ga (denoted as the “introduction @ removal” process). As a result, the crystal lattice distortion generates a great number of elongated Co–OOH bonds on the exposed (1[2 with combining macron]0) facets of CLD-u-Co(OH)2 nanosheets, which can serve as efficient catalytic active sites for the OER. Meanwhile, the optimized amount of “introduction @ removal” of Ga (4 at%) allows for an exquisite balance between distortion engineering and electrical conductivity, synergistically. As benefits to the synergistic modulation of abundant catalytic active sites, robust reactivity per site, and good electrical conductivity, the CLD-u-Co(OH)2 nanosheets possess an exceptionally high OER catalytic activity with a low overpotential of 265 mV at 10 mA cm−2 and a long-term stability beyond 20 h under basic media, which, as comprehensive parameters, are comparable to those of the state-of-the-art non-noble metal OER catalysts. Even more importantly, our successful strategy to synthesize CLD-u-Co(OH)2 nanosheets would open an exciting avenue for the green and sustainable synthesis of other brucite-like LMH based catalysts for the OER.

Experimental

Materials

All reagents were purchased from commercial sources and were of analytical grade. Cobalt(II) nitrate and sodium dodecylbenzenesulphonate were obtained from Sinopharm Chemical Reagent Co., Ltd. Gallium(III) nitrate was purchased from Aladdin Industrial Corporation. All chemicals were used as received. All aqueous solutions were prepared using deionized water (18.2 MΩ cm).

Synthesis of u-Co96Ga4 LMHs

u-Co96Ga4 LMHs were synthesized using a co-precipitation method. In a typical synthesis of u-Co96Ga4 LMHs, 2.79 g of Co(NO3)2·6H2O, 0.10 g of Ga(NO3)3·10H2O, and 10 mg of sodium dodecylbenzenesulphonate (the molar ratio of Co/Ga is 96[thin space (1/6-em)]:[thin space (1/6-em)]4) were firstly dissolved in 200 mL of deionized water and then mixed with a solution containing 0.5 g of KOH. Afterwards, the obtained mixture solution was ultrasonicated for 30 min. After being kept at 60 °C in a preheated oven for 3 hours, the mixed solution (300 mL) was cooled down naturally to room temperature. The obtained u-Co96Ga4 LMH precursors were washed with water (100 mL) followed by ethanol (100 mL) for five times and dried at 80 °C for 10 h, and the u-Co96Ga4 LMHs were finally harvested.

Synthesis of CLD-u-Co(OH)2

For the synthesis of CLD-u-Co(OH)2, 50 mg of u-Co96Ga4 LMHs were firstly dispersed in 100 mL of deionized water. Subsequently, 50 mL KOH (3 M or 6 M) aqueous solution was added dropwise (keeping a flowing rate of 5 mL min−1) to the u-Co96Ga4 LMH dispersion. The mixed dispersion with an actual concentration of 1 M or 2 M KOH was sonicated at 30 °C to selectively remove Ga atoms (see the ESI for a detailed description). Finally, the obtained CLD-u-Co(OH)2 was washed with water (200 mL) followed by ethanol (20 mL) for five times and dried at 80 °C for 10 h.

Electrochemical measurements

To prepare the working electrode, a homogeneous catalyst ink was prepared by dispersing 5 mg of an as-synthesized CLD-u-Co(OH)2 sample in 1 mL of a water–ethanol mixture (v = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) under a slight ultrasonic treatment. After a glassy carbon electrode (GCE, 3.0 mm in diameter) was polished, 3 μL of ink containing 15 μg of catalysts was drop-casted on it. The loading amount of catalyst on GCE was 0.21 mg cm−2. Then, an aqueous Nafion solution (2 μL, 0.5 wt%) was drop-casted onto the GCE and fully dried as a conductive binder. Electrochemical measurements were carried out on an electrochemical workstation (CHI 660E) in alkaline media, using a calibrated Hg/HgO electrode as a reference electrode, and a graphite electrode as the counter-electrode. In this paper, all potentials were firstly measured against a Hg/HgO reference electrode, and then were referenced to the relative hydrogen electrode (RHE) by adding a value of Evs Hg/HgO + 0.059 × pH. For LSV measurements, the scan rate was 5 mV s−1. Additional experimental methods details are in the ESI.

Results and discussion

The u-Co96Ga4 LMHs and CLD-u-Co(OH)2 were all synthesized via a simple aqueous phase reaction. Particularly, CLD-u-Co(OH)2 was obtained through an additional selective corrosive treatment in 2 M KOH using u-Co96Ga4 LMHs as precursors.39 The crystal structure was investigated on both the as-synthesized u-Co96Ga4 LMHs and CLD-u-Co(OH)2 samples by X-ray diffraction (XRD). As shown in Fig. 1a, the diffraction peaks of both u-Co96Ga4 LMHs and CLD-u-Co(OH)2 could be well indexed to Co(OH)2 (JCPDS card no. 45-0031, a = 3.191 Å and c = 4.664 Å). The diffraction peaks of these two samples at 2θ values of 18.8, 32.2, 37.7, 51.1, 57.6, and 61.3° can be assigned to the (001), (100), (101), (102), (110), and (111) crystal planes of hexagonal Co(OH)2, respectively. It is of note that after the introduction of Ga into Co(OH)2, the as-synthesized u-Co96Ga4 LMHs can still maintain the crystal structure of Co(OH)2. However, after removing Ga atoms from u-Co96Ga4 LMHs selectively, the peaks at 32.2, 37.7, 51.1, and 57.6° for CLD-u-Co(OH)2, which correspond to the (100), (101), (102) and (110) crystal planes of Co(OH)2, respectively, were slightly left shifted about 0.4–0.6°, indicating that the selective removal process of Ga can distort the original crystal structure and correspondingly enlarge the original interplanar distance along the [100] or [010] crystal orientation.40,41 According to previous studies, the increase in the interplanar distance that induced the elongation of the metal–oxygen bonds may lead to an improved OER performance because of the relatively low adsorption energy of H2O.8,42 The surface composition was then studied both on u-Co96Ga4 LMHs and CLD-u-Co(OH)2 samples by X-ray photoelectron spectra (XPS). In the Co 2p region (Fig. 1b) for these two samples, the peak for Co 2p 3/2 locates at the binding energy of 782.1 eV, and the spin–orbit splitting for Co 2p 1/2 and Co 2p 3/2 reaches a value at 15.9 eV, all corresponding to the Co2+ ions in Co(OH)2.43 Importantly, for u-Co96Ga4 LMHs in the Ga 2p region (Fig. 1c), the peak at 1119.0 eV can be ascribed to Ga3+ ions, which is consistent with previous reports.44 In striking contrast, for CLD-u-Co(OH)2, no single peak for Ga was observed, which can be explained by the fact that all the Ga atoms have been selectively removed from the surface of u-Co96Ga4 LMHs by a corrosion treatment, leaving only the crystal structure distorted ultrathin Co(OH)2 nanosheets (i.e. CLD-u-Co(OH)2). As for the O 1s XPS spectrum (Fig. 1d) of u-Co96Ga4 LMHs, the peak at 530.4 eV should be assigned to the Co–OOH bond, confirming the presence of Co(OH)2 in the subsurface layer.45 Because of the high degree of covalency of the M–O (M represents metal atom) bonds, these oxygen ions (O) can be well characterized by a low density of electrons in comparison with the classical “O2” ions. The typical peak at 531.1 eV is due to O atoms bound to Ga atoms in Ga(OH)3.46 The peak at 532.6 eV is associated with hydroxyl species of surface-adsorbed water molecules.11,47 Additionally, to clarify the elemental composition of CLD-u-Co(OH)2 and u-Co96Ga4 LMHs, ICP-OES measurements were conducted. As shown in Table S2, the mean ratio of Ga/Co in CLD-u-Co(OH)2 and u-Co96Ga4 LMHs matched perfectly with the XPS value (Ga/CoICP = 0 and Ga/CoXPS = 0 for CLD-u-Co(OH)2 and Ga/CoICP = 0.043 and Ga/CoXPS = 0.040 for u-Co96Ga4 LMHs, respectively), demonstrating that u-Co96Ga4 LMHs with 4 at% Ga and CLD-u-Co(OH)2 consisting of only cobalt were successfully synthesized.
image file: c7gc02543b-f1.tif
Fig. 1 (a) XRD patterns of CLD-u-Co(OH)2 and u-Co96Ga4 LMHs. (b) Co 2p, (c) Ga 2p and (d) O 1s XPS spectra of CLD-u-Co(OH)2 and u-Co96Ga4 LMHs.

The thickness was firstly studied on both u-Co96Ga4 LMHs and CLD-u-Co(OH)2 samples by atomic force microscopy (AFM) in Fig. 2a and d, respectively. The corresponding heights of the two samples are approximately 1.31 nm, confirming that the CLD-u-Co(OH)2 catalyst with ultrathin nanosheet morphology was successfully prepared using u-Co96Ga4 LMHs as precursors. The ultrathin nanosheet morphology of as-synthesized CLD-u-Co(OH)2 was then visually confirmed by light scattering; the colloidal suspension of CLD-u-Co(OH)2 was irradiated with a laser beam (shown in the right inset of Fig. 2d). As a result, a clear Tyndall light scattering effect was observed for the colloidal suspension, indicating the excellent dispersion of nanosheets. Even though the main crystal structure, surface composition, and thickness of these two samples are almost identical, they showed varied morphologies as revealed by transmission electron microscopy (TEM) images. As shown in Fig. 2b and c, the u-Co96Ga4 LMHs with smooth and tidy edges are exclusively high-quality flat nanosheets, which inherit the geometrics and dimensions of previously reported Co-based LMH nanosheets.48–50 In contrast, although CLD-u-Co(OH)2 with the similar tidy edges was observed, TEM images in Fig. 2e and f reveal the existence of a rough structure on the surface of CLD-u-Co(OH)2, which is caused by the crystal lattice distortion under a corrosion treatment.39 Additionally, the almost transparent u-Co96Ga4 LMHs and CLD-u-Co(OH)2 samples as shown in the TEM image further confirm the formation of ultrathin nanosheets, which can provide an abundance of exposed active sites during the catalytic process.


image file: c7gc02543b-f2.tif
Fig. 2 AFM images of (a) u-Co96Ga4 LMHs and (d) CLD-u-Co(OH)2. TEM images of (b, c) u-Co96Ga4 LMHs and (e, f) CLD-u-Co(OH)2. The inset of (d) shows a photograph of the as-synthesized CLD-u-Co(OH)2 colloidal suspension.

The high-resolution transmission electron microscopy (HRTEM) images of u-Co96Ga4 LMHs (Fig. 3a) reveal two sets of lattice fringes with interplanar distances of 4.70 and 2.71 Å corresponding to the (001) and (100) crystal planes of the hexagonal Co(OH)2 phase, respectively. The corresponding fast Fourier transform (FFT) image (shown in the inset of Fig. 3a) demonstrates that the selected area is single-crystal, and the zone axis is [010]. Thus, when the HRTEM image was obtained along the [010] orientation of the hexagonal Co(OH)2 phase, the observed angle between (001) and (100) facets is 89.7°, being in good agreement with the theoretical value of ∼90°. Consequently, these results indicate that the top exposed facet of u-Co96Ga4 LMHs is {1[2 with combining macron]0} (Fig. 3b). In contrast, as shown in Fig. 3c and d, CLD-u-Co(OH)2 exhibits two crystal planes with the spacing of 4.68 and 3.01 Å. The lattice fringe with an interplanar distance of 4.68 Å can be assigned to (001) crystal planes, which further confirms the fact that elongation of the interplanar distance occurs only along the [100] crystal orientation. Meanwhile, the facets with the interplanar distance of 3.01 Å should be ascribed the distorted (100) planes, reasonably (the detailed discussion is displayed in the ESI). The observed angle between the (001) and (100) crystal planes is 73.1°, lower than that of 89.7° for u-Co96Ga4 LMHs. These results confirm the existence of the lattice distortion of the Co(OH)2 crystal structure motif.27 Combined with FFT images, the CLD-u-Co(OH)2 samples also grow along the [001] and the interplanar distance enlarged [100] orientations with exposed {1[2 with combining macron]0} facets; correspondingly, the metal–oxygen bond length was elongated. Based on previous reports, the crystal lattice distortion and corresponding elongation of the bond can be explained by the occurrence of the self-adaptive atomic rearrangement of the original crystal structure, which is caused by the “introduction @ removal” process of certain cations (for example, Ga atoms in this experiment).8,27,32–36 To the best of our knowledge, this crystal lattice distortion engineering on the basal planes would be an efficient strategy to promote the catalytic activity of catalysts, especially the ultrathin nanosheet structured catalysts (for example, CLD-u-Co(OH)2 in this experiment), which is regarded as an ideal two-dimension structure motif with an abundance of active sites, therefore, displaying an outstanding OER performance.


image file: c7gc02543b-f3.tif
Fig. 3 HRTEM images of (a) u-Co96Ga4 LMHs and (c) CLD-u-Co(OH)2. Insets of (a) and (c) show the corresponding fast Fourier transform patterns. Schematic representations of the crystal structure of hexagonal Co(OH)2 before (b) and after (d) crystal lattice distortion.

The OER electrocatalytic activity of the CLD-u-Co(OH)2 catalyst was undertaken in a standard three-electrode system in O2-saturated 1 M KOH (pH = 14). At the same time, comparative studies were also performed for other samples (the as-synthesized u-Co96Ga4 LMHs, Co(OH)2, Ga(OH)3, and commercial IrO2; see the ESI for a detailed description). As shown in Fig. 4a and b, the OER polarization curve of u-Co96Ga4 LMHs achieves a current density of 10 mA cm−2 at a high overpotential of 410 mV. In contrast, CLD-u-Co(OH)2 shows a high catalytic activity for the OER with a lower overpotential of 265 mV at a current density of 10 mA cm−2, suggesting that the distorted crystal lattice induced by the “introduction @ removal” of Ga is the main active phase, which results in superior catalytic activity for the OER. Another very interesting observation is that the catalytic activity of CLD-u-Co(OH)2 is also obviously higher than that of Co(OH)2 and Ga(OH)3 (332 mV and 601 mV at 10 mA cm−2 in base, respectively), suggesting that the introduction of Ga may not directly correlate with improved OER performance, further confirming that CLD-u-Co(OH)2 possesses a superior catalytic activity as a result of the crystal lattice distortion and the corresponding elongated Co–OOH bonds. Of note, CLD-u-Co(OH)2 shows excellent OER catalytic performance compared to most previously published OER catalysts based on non-noble metals in basic media (Table S4). To further elucidate the OER kinetics of the catalysts, all the Tafel plots were recorded with the Tafel equation in linear regions (Fig. 4c). Obviously, CLD-u-Co(OH)2 with an abundance of new active sites exhibits the fastest kinetics for the OER, suggesting that the Volmer–Heyrovsky mechanism is the rate-determining step during the electrocatalytic process, which can be verified by its lower Tafel slope of 47 mV dec−1. The corresponding Tafel slopes of u-Co96Ga4 LMHs, Co(OH)2, and Ga(OH)3 in Fig. 4c are 70, 56, and 108 mV dec−1, respectively, suggesting a slower kinetics mechanism for the OER. The above experimental results reveal that the crystal lattice distortion and the corresponding elongated Co–OOH bonds play important roles in CLD-u-Co(OH)2 catalyzed OER. Moreover, turnover frequencies (TOFs) for each active site were further estimated via the following function,

TOF = (J × A)/(4 × F × n)
where J is the current density at a given overpotential, A is the surface area of the electrode, 4 represents 4 electrons per mol of O2, F is the Faraday constant, and n is the number of moles of metal in the electrode. We first assumed that all of the Co ions are catalytic active, and their TOF values were calculated. However, contributing to the existence of inaccessible metal sites during the OER, the calculated TOFs represent a lower limit.8,30 We found that CLD-u-Co(OH)2 exhibits a higher TOF under different overpotential values (Fig. 4d). The TOF of CLD-u-Co(OH)2 was 0.076 s−1 at an overpotential of 400 mV, which is about 8-times and 2-times that of u-Co96Ga4 LMHs and Co(OH)2, respectively.


image file: c7gc02543b-f4.tif
Fig. 4 (a) LSV curves and (c) corresponding Tafel plots of CLD-u-Co(OH)2, u-Co96Ga4 LMHs, Co(OH)2, Ga(OH)3, and commercial IrO2 for the OER in 1 M KOH. (b) A comparison of the overpotential values at different current densities of 10 and 20 mA cm−2. (d) TOFs with respect to corresponding metal atoms of CLD-u-Co(OH)2, u-Co96Ga4 LMHs, Co(OH)2, Ga(OH)3, and commercial IrO2 at different overpotential values.

The significantly improved catalytic performance for CLD-u-Co(OH)2 as compared to u-Co96Ga4 LMHs aroused our curiosity as to their chemical structures. Extended X-ray absorption fine structure spectroscopy (EXAFS) measurements were firstly performed to investigate the atomic arrangement around the photoabsorbers (Co atoms in this experiment). As shown in Fig. 5a, the Co K-edge EXAFS spectrum of CLD-u-Co(OH)2 shows some variations as compared to u-Co96Ga4 LMHs, indicating a change in the electronic structure of the Co species. The Co K-edge oscillation curves for CLD-u-Co(OH)2 exhibit a reduction in the oscillation amplitude relative to the u-Co96Ga4 LMHs, indicating a distinctly different atomic arrangement and coordination environment of CLD-u-Co(OH)2 (Fig. 5b).51 The Fourier transform (FT) curves present some peaks ranging from 1 to 4 Å, corresponding to Co–Co and Co–OOH coordination in both samples (Fig. 5c). Compared with u-Co96Ga4 LMHs, diffraction peaks in CLD-u-Co(OH)2 were shifted to a higher R value, which possibly resulted from the crystal lattice distortion caused by atomic rearrangement (Fig. 5d). Additionally, Co–OOH peak intensity significantly decreased in the presence of this crystal lattice distortion, because of the missing atoms in the Co coordination sphere. Estimated local structure parameters (Table S5) for CLD-u-Co(OH)2 show that the Co–OOH distance is 2.233 Å, which was larger than the distance of 2.112 Å in the u-Co96Ga4 LMHs. The coordination number (N = 4.53) of the Co–OOH octahedron in CLD-u-Co(OH)2 was almost the same as for that in u-Co96Ga4 LMHs (N = 4.62), suggesting that the missing atoms in the Ga sphere were compensated by the Co shells,52 whereby the Co distance increased by 0.12 and 0.28 Å.


image file: c7gc02543b-f5.tif
Fig. 5 (a) Co K-edge EXAFS spectra, (b) Co K-edge EXAFS oscillation functions k3χ(k), and (c) the corresponding Fourier transforms FT(k3χ(k)) of CLD-u-Co(OH)2 and u-Co96Ga4 LMHs. (d) Illustration of the crystal lattice distortion process (from u-Co96Ga4 LMHs to CLD-u-Co(OH)2).

Electrochemical stability is another important parameter that can influence the cost of hydrogen and oxygen production.53–57 Therefore, the long-time chronopotentiometry (CP) experiment of CLD-u-Co(OH)2 was carried out to estimate the lifetime of the electrode in 1 M KOH (Fig. 6a). In the CP measurement, the overpotential of ∼1.49 V was maintained even after 20 h of electrolysis, clearly showing its robust long-term electrocatalytic stability for OER. Besides, we further tested the polarization curves after 1000 and 2000 cycles, which are in agreement with the original ones (shown in the inset of Fig. 6a). Fig. 6b shows the curves for the multi-potential steps of CLD-u-Co(OH)2 in 1 M KOH with increasing potential from 1.47 to 1.55 V (10 mV per hour). The current density immediately levels off to 4.8 mA cm−2 at the initial potential value, and it is still maintained for the rest 1 h. The other steps have similar trends, undoubtedly confirming that as-synthesized CLD-u-Co(OH)2 possesses a robust electrochemical stability. Additionally, the ultrathin 2D material is more likely to suffer chemical corrosion in basic media and much easier to aggregate;58,59 thus, the stability of the as-synthesized CLD-u-Co(OH)2 dispersion was evaluated. Fig. 6c displays a clear Tyndall effect image of the homogeneous and stable CLD-u-Co(OH)2 colloidal suspension after dispersing in 1 M KOH for 24–96 h, and Fig. 6d shows the weight loss image for it, revealing that CLD-u-Co(OH)2 possesses a superior stability in a long-term chemical corrosion process. The above investigations, combined with the high specific activity and the high mass activity (Fig. S2), demonstrated that CLD-u-Co(OH)2 holds great potential as a highly efficient electrocatalyst for the OER in basic media.


image file: c7gc02543b-f6.tif
Fig. 6 (a) CP plot of CLD-u-Co(OH)2 recorded for over 20 h in 1 M KOH. The inset shows the LSVs of CLD-u-Co(OH)2 before and after 2000 cycles. (b) The multi-potential process of CLD-u-Co(OH)2. The potential started at 1.47 V and ended at 1.55 V, with an increment of 10 mV per hour. (c) An image of CLD-u-Co(OH)2 colloidal suspension after dispersing in 1 M KOH for 24, 48, 72, and 96 h. (d) The weight loss of 50 mg CLD-u-Co(OH)2 after dispersing in 1 M KOH for 24, 48, 72, and 96 h.

In addition, we have experimentally compared the OER activity for the four samples under different corrosion durations using u-Co96Ga4 LMHs as precursors (Fig. S3). Interestingly, we observed that the catalytic activity for the four samples typically increased with an increase in the corrosion durations until the whole Ga atoms were absolutely removed (i.e. CLD-u-Co(OH)2, ICP-OES results for samples are shown in Table S6). Based on the above experimental results, for CLD-u-Co(OH)2 in this work, the “introduction @ removal” process of Ga atoms is a reliable strategy to induce crystal lattice distortion and the corresponding elongation of Co–OOH bonds, thus displaying an outstanding OER catalytic performance.

Furthermore, to evaluate the OER catalytic performance, the CLD-u-Co(OH)2 samples with different amounts of “introduction @ removal” of Ga were synthesized. Specifically speaking, the CLD-u-Co(OH)2 samples using u-Co98Ga2 LMHs, u-Co96Ga4 LMHs, u-Co94Ga6 LMHs, and u-Co92Ga8 LMHs as precursors were denoted as s-2%, s-4%, s-6%, and s-8%, respectively. The XRD patterns of s-2%, s-4%, s-6%, and s-8% in Fig. S4 show that the diffraction peaks at the 2θ value of 32.2 and 37.7° apparently shifted to a lower 2θ value in the hydroxide, indicating that different amounts of “introduction @ removal” of the Ga can lead to the changes in the crystal lattice distortion degree and the corresponding interplanar distance of the catalyst, because the selective removal process of the introduction of Ga atoms can induce atomic rearrangement as previously observed. To further confirm this viewpoint, HRTEM images of s-2%, s-4%, s-6%, and s-8% (Fig. 7 and 3c) were obtained. What is noteworthy is that, the introduced Ga amount of 2% of CLD-u-Co(OH)2 was insufficient to introduce crystal lattice distortion of hexagonal Co(OH)2, as confirmed by the interplanar distances of 4.67 and 2.87 Å corresponding to the (001) and (100) crystal planes, respectively, and the angle between two crystal planes is ∼90°. A highly distorted structure was observed when the amount of “introduction @ removal” of the Ga increased beyond 6%. For s-6%, although the interplanar distance of 4.76 Å corresponding to (001) crystal planes is almost identical to s-2% and s-4%, the interplanar distance of (100) crystal planes shifted to 3.10 Å and the angle between two crystal planes is 63.8°, suggesting the existence of the highly distorted structure in s-6%. When the amount of “introduction @ removal” of the Ga increased to 8%, the disordered structure emerged in nanodomains, meaning that the as-synthesized s-8% nanosheets are nearly amorphous. The above results probably demonstrated that the amount of “introduction @ removal” of the Ga is a reliable parameter to obtain crystal lattice distortion and the corresponding elongation of Co–OOH bonds.


image file: c7gc02543b-f7.tif
Fig. 7 TEM images of (a, b) s-2%, (c, d) s-6%, and (e, f) s-8%. Insets of (b), (d), and (e) show the corresponding HRTEM images. Red and blue lines represent the (001) and (100) crystal planes, respectively.

Generally speaking, the relationship between distortion engineering and electrical conductivity is usually contradictory.22 The overall electrical conductivity in such a distorted nanostructure is usually low due to the poor electron transport, resulting in lower catalytic activity. Thus, adjusting the balance between the abundant active sites arising from crystal lattice distortion and good electrical conductivity to achieve a moderate distortion degree is highly desirable. Hence, the electrochemically active surface areas (Aechem) and the electrochemical impedance spectroscopy (EIS) analysis were performed on the above four samples. As can be seen in Fig. 8a, the double-layer charging is responsible for the current response within the potential window uniquely for the CV curves (1.1–1.2 V vs. RHE) at different scan rates (4–120 mV s−1). The double-layer capacitance (Cdl) is directly proportional to Aechem,60–62 which is extracted by plotting the Δj = jajc at a given potential (1.16 V vs. RHE) against the CV scan rates. As a result, the Cdl of s-4% (21.3 mF cm−2) was approximately 2 times higher than s-2% (11.2 mF cm−2), which is due to a significant increase in active sites caused by the excessive amount of “introduction @ removal” of the Ga. A relatively higher Aechem can offer more exposed active sites for the electrolyte, which consequently facilitates the electron/proton transfer or diffusion. Additionally, the electrical conductivity of s-2%, s-4%, s-6%, and s-8% was measured by EIS under OER operating conditions. As shown in Fig. 8b and S5, the impedance models for s-2%, s-4%, s-6%, and s-8% samples all consist of a series of resistances: the series resistance (Rs); the one at high frequency region relates to the charge transport resistance (Rp); and the one at low frequency reflects the charge transfer process during the electrochemical reaction (Rct).8,63–68 The calculated Rp values reveal a remarkable decrease from 51.7 Ω (s-8%) to 25.7 Ω (s-4%), indicating that the decreased distortion degree can give a facile charge transport capacity.34 It is noteworthy that the Rp values are almost constant between s-2% and s-4% samples and the Rct value of s-4% is higher than that of s-2%. This means that a suitable value of the “introduction @ removal” amount of the Ga for the s-4% sample could lead to the lowest charge transfer resistance and corresponding fast charge transfer kinetics. Besides, the high values of both Rct and Rp for s-6% and s-8% samples would induce the worst performance of those samples as compared to s-4%. In addition, the variability of Rs between 5 Ω and 12.8 Ω could be attributed to slight changes in the electrolyte conductivity or the distance between the reference and the working electrode among the different tests. To identify an optimized balance point between crystal lattice distortion degree and electrical conductivity of the samples, LSV measurements were conducted (Fig. 8c). As a result, due to its poor electrical conductivity, amorphous s-8% has a weaker catalytic activity as compared to other samples with a high overpotential of 563 mV at a specific current density of 10 mA cm−2. In contrast, due to a moderate amount of “introduction @ removal” of the Ga, s-4% has the highest catalytic activity among these samples with the smallest overpotential of 265 mV and the lowest Tafel slope (Fig. 8d), suggesting relatively fast OER kinetics. Remarkably, our results show that s-4% has an exquisite balance between crystal lattice distortion degree and electrical conductivity.


image file: c7gc02543b-f8.tif
Fig. 8 (a) Plots used for evaluating the Cdl of s-2%, s-4%, s-6%, and s-8%. (b) Nyquist plots of s-2%, s-4%, s-6%, and s-8%. (c) LSV curves and (d) corresponding Tafel plots of s-2%, s-4%, s-6%, and s-8%.

Conclusions

In conclusion, the crystal lattice distorted CLD-u-Co(OH)2 nanosheets using u-Co96Ga4 LMHs as precursors were synthesized by an “introduction @ removal” of the Ga. As visualized by HRTEM, XRD, and XAFS, the distortion generates a great number of elongated Co–OOH bonds on exposed (1[2 with combining macron]0) facets of these ultrathin nanosheets, which can serve as efficient catalytic active sites for the OER. Meanwhile, the optimized amount of “introduction @ removal” of the Ga (4 at%) allows for an exquisite balance between distortion engineering and electrical conductivity, synergistically. Benefiting from the abundant catalytic active sites, robust reactivity per site, and good electrical conductivity, the CLD-u-Co(OH)2 nanosheets exhibit a highly efficient OER performance. The green and sustainable engineering of crystal lattice distortion to improve the electrocatalytic activity of CLD-u-Co(OH)2 nanosheets presented in this work would open up a favorable route to synthesize other LMH-based catalysts with potential applications in heterogeneous and electrocatalytic water splitting.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the National Science Foundation of China (no. 21345003), the Fundamental Research Funds for the Central Universities (Grant No. lzujbky-2016-k08), the Natural Science Foundation of Gansu (145RJZA132), and the Key Laboratory of Catalytic Engineering of Gansu Province and Resources Utilization, Gansu Province.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7gc02543b

This journal is © The Royal Society of Chemistry 2017