NiFe Layered Double-Hydroxide Nanosheets on a Cactuslike (Ni,Co

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NiFe Layered Double Hydroxide Nanosheets on a Cactus-Like (Ni,Co)Se2 Support for Water Oxidation Xin Li, Haijun Wu, Yue Wu, Zongkui Kou, Stephen J. Pennycook, and John Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01932 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 13, 2018

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ACS Applied Nano Materials

NiFe Layered Double Hydroxide Nanosheets on a Cactus-Like (Ni,Co)Se2 Support for Water Oxidation Xin Li, †, ‡ Haijun Wu, ‡ Yue Wu,‡ Zongkui Kou,‡ Stephen J. Pennycook,‡ and John Wang*,‡ †Centre

for Advanced 2D Materials, National University of Singapore, 117546 Singapore

‡Department

of Materials Science and Engineering, National University of Singapore, 117574

Singapore. E-mail (J. Wang*): [email protected]

ABSTRACT: Oxygen evolution reaction (OER) is a pivotal half-reaction for the nextgeneration energy storage and conversion technologies, for example, in metal-air batteries and water splitting. Herein, we report the preparation of a freestanding cactus-like structural (Ni,Co)Se2 support shelled with NiFe layered double hydroxides (LDHs), as a highly active and durable OER catalyst with low cost. This cactus-like selenide support provides an interconnected conductive and robust framework, which can ensure efficient electron transfer

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and prevent the aggregation of LDHs. The synergistic combination and coupling effects of active NiFe-LDH and conductive (Ni,Co)Se2 raise the intrinsic catalytic activity. As expected, when serving as an OER catalyst in 1 M KOH aqueous solution, the (Ni,Co)Se2/NiFe-LDH presents a high activity with overpotential of ≈205 mV at a current density of 10 mV cm-2, a small Tafel slope of ≈ 61 mV dec-1, as well as a high durability observed at 30 h chronoamperometric test. The present study shows the promise of the cactus-like structural (Ni,Co)Se2/NiFe-LDH as an effective OER catalyst, which provides a new thought for the fabrication of non-noble metal catalysts and desired nanostructures.

Introduction

The oxygen evolution reaction (OER), as the process of generating oxygen by a chemical reaction, has received growing attention recently due to its important role in energy storage and conversion technologies, for example, in metal-air batteries and water splitting.1 However, OER is a strictly kinetic sluggish process because of the four electron-proton coupled reaction involved.2 It is therefore of apparent value to develop effective catalysts that can boost the process and enable the reaction to proceed efficiently.3 Although the OER catalysts based on noble metal oxides, such as RuO2 or IrO2, exhibit excellent catalytic activity, their low abundance, high cost, and relatively poor stability make them not practical for wide-spread


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use and commercialization.4 Accordingly, tremendous efforts have been dedicated to developing highly active and durable OER non-noble metal catalysts with low cost.5. Among the various non-precious metal OER electro-catalysts reported, NiFe-LDH-based catalysts with low cost and low toxicity are shown to be highly active for OER in alkaline conditions.1,

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Nevertheless, the poor electronic conductivity of the NiFe-LDH limits their

applications toward OER, which is a common problem with almost all LDHs.7 Besides, the lamellar LDH flakes tend to stack or aggregate, leading to poor OER stability.8 On basis of the previous work, the exfoliation of bulk NiFe-LDHs appears to be one of the practical approaches, which can improve their electronic conductivity and create more active sites.9 Jia et al.10 showed that the exfoliated nanosheets exhibited enhanced oxygen evolution activity in comparison with the corresponding bulk LDHs. However, a polymer binder is needed to prepare the unsupported NiFe-LDH catalyst, which will add extra contact resistances and undesirable interfaces to the catalyst and reduce its OER activity.11 In addition, coupling of NiFe-LDH with carbonaceous materials can be another effective strategy for improving the conductivity and stability.12 For example, in the NiFe-LDH nanoplate/CNT complex, it was observed that the underlying CNT (carbon nanotube) network could facilitate the electron transport and improve the OER activity.1 Compared to pristine NiFe-LDH, the NiFe-LDH/rGO (reduced graphene oxide) displays better OER activity owing to the synergistic effect.13 However, carbonaceous materials generally exhibit limited OER activity although can as a conductive support, therefore, it would be desirable if the support material can both possess


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the structural integrity and contribute to the catalytic activity.3 The selection of an appropriate support material and the optimization of the overall structures are thus of great significance. Transition metal selenides are considered as a class of promising candidates for the support materials, because they possess comparable catalytic activity and distinct electronic configuration.14 Liu et al.15 confirmed the metallic property of CoSe2, which would allow an efficient charge transport during the electro-catalytic process. Also, a rational architecture design of the selenides, which will affect their surface area, stability, and conductivity, can further promote the catalytic activity.16 Herein, we present the successful synthesis of a freestanding cactus-like (Ni,Co)Se2 structural support shelled with NiFe-LDH, as an active and durable OER catalyst. Although there have been studies of developing cactus-like structural nanocomposite for water splitting applications, the overall progress is far from being satisfactory. For example, 1D Bi19Cl3S27 nanorods were used as the backbone for MoS2 being arranged like a stem supporting the trichome of a cactus.17 NixZn1-xOH nanosheets-derived sulfide ZnS/Ni3S2, consisting of interconnected nanosheets with nanoparticles, were presented as a cactus-like structure.18 Liu et al.19 presented a hierarchical ZnO support which showed a nanorod-nanosheet mixed dimensional structures for CdS coating layer, but the distribution of the nanorods and nanosheets was rather disordered. Different from these prior developments, our work has detailed a study on the freestanding (Ni,Co)Se2 support, which itself presents a unique cactus-like structure composed of the 1D-2D mixed dimensional structures. In our cactus-like (Ni,Co)Se2 support, both surfaces and edges of 2D nanoflake


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arrays scaffold the self-assembled 1D nanospines uniformly. The nanospines can provide a pathway for effective charge transport and promote rapid release of gas bubbles formed during the OER process.20 The nanoflakes give rise to the reasonable mechanical strength and stability of the electro-catalyst.21 Given the understanding above, the cactus-like (Ni,Co)Se2/NiFe-LDH would be a promising candidate for OER catalyst, which presents a better electro-catalytic activity and stability towards the OER compared to the single components (Ni,Co)Se2 or NiFe-LDH. Indeed, as demonstrated in the present work, in 1.0 M KOH, it exhibits a small overpotential of ≈205 mV at the current density of 10 mA cm−2, as well as shows the desired durability observed at 30 h chronoamperometric test. Such excellent performance suggest the promise of the cactus-like structural (Ni,Co)Se2/NiFe-LDH as an OER catalyst, which is attributed to the co-contributions of the novel cactus-like nanostructure and the synergistic combination of the hybrid components.

Results and Discussion

Morphology and Structure

As shown in Figure 1, the preparation procedure for (Ni,Co)Se2/NiFe-LDH is illustrated. The cactus-like NiCo-OH precursors were vertically aligned on carbon cloth (CC) substrate by hydrothermal reaction.22 Afterwards, the NiCo-OH precursors were transformed to (Ni,Co)Se2


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in situ by CVD selenization. Subsequently, the NiFe-LDH nanosheets were eletrodeposited onto the surface of the (Ni,Co)Se2 scaffold, to form a 3D (Ni,Co)Se2/NiFe-LDH nanohybrid. The carbon cloth was selected as a flexible and conductive substrate that has negligible OER activity. The freestanding (Ni,Co)Se2/NiFe-LDH directly grown on carbon cloth substrate provides a lower contact resistance, hence decreasing the ohmic losses in the system. We also prepared the sample (Ni,Co)Se2 cactus arrays and NiFe-LDH nanosheets on the substrate of carbon cloth for comparison experiments. As displayed in Figure S1 (Supporting Information), the molar ratio of Ni to Co during the synthesis shows a considerable influence on the morphology of the obtained NiCo-OH. We prepared different samples with starting Ni:Co molar ratios of 0:10, 3:7, 5:5, 7:3, 10:0, which are denoted as Ni/Co-0/10, Ni/Co-3/7, Ni/Co-5/5, Ni/Co-7/3, Ni/Co-10/0, respectively. The field-emission scanning electron microscopy (FESEM) images illustrate that the Ni/Co-10/0 is inclined to exhibit a flake-like morphology, while Ni/Co-0/10 presents a spine-like structure. Transitional morphologies can be observed for Ni/Co-3/7 and Ni/Co-7/3. For Ni/Co-5/5, the cactus-like morphology (1D-2D mixed dimensional structures) was obtained. The morphology of the as-synthesized samples was characterized by FESEM, which is presented in Figure 2. The NiCo-OH precursor presents a uniform growth with a cactus-like structure on carbon fibers (Figure S2). In this cactus-like structure, both surfaces and edges of 2D nanoflake arrays scaffold the self-assembled 1D nanospines uniformly (~2 µm in length) (Figure 2a-b). This cactus-like 3D structure integrates the 1D-2D mixed dimensional


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structures, which would be beneficial to efficient charge transport as well as the desired mechanical stability. After the selenization, it can be observed that the obtained selenide product (Ni,Co)Se2 (transformed from the NiCo-OH precursor) retains the cactus-like structure (Figure 2c). Detailed SEM studies of (Ni,Co)Se2 show that the “cactus” surface becomes rough, and consists of numerous nanocrystals (Figure 2d). The FESEM images of the sample (Ni,Co)Se2/NiFe-LDH demonstrate that densely packed and interconnected NiFe-LDH nanosheets are anchored on the surface of (Ni,Co)Se2 arrays (Figure 2e-f), and the interface contact between (Ni,Co)Se2 and NiFe-LDH will guarantee the fast electron transfer in between. The energy-dispersive X-ray (EDX) spectrum analysis and elemental mapping further support the successful surface electrodeposition of NiFe-LDH on the selenide support (Figure S3-4, Supporting Information). Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) studies were conducted to further observe the structure of the samples. The STEM HAADF (high angle annular dark field) image of a NiCo-OH precursor cactus with low magnification is shown in Figure 3a, which indicates a single crystalline nature of both the flake and the spines. Upon the selenization treatment, the (Ni,Co)Se2 cactus consists of numerous nanocrystals, the diameter of which is around 100 nm (Figure 3b). A polycrystalline structure helps increase the surface area and active sites of the catalyst.23 A properly controlled nanostructure would present open edges and numerous grain boundaries as the active centers and expose additional catalytic active sites, as well as pathways for the reactants, resulting in


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an improved performance and lower charge transfer resistance.24 After the hybridization with NiFe-LDH, a high density of rather uniform nanosheets are tightly anchored on the whole surface of the (Ni,Co)Se2 cactus, including on the nanospines and nanoflakes, forming a 3D hierarchical nanostructure, as shown in Figure 3c-d. The EDX spectrum images in Figure 3(e1e5), simultaneously acquired with the STEM HAADF image of Figure 3d, indicate the successful coating of NiFe-LDH on the selenide nanoparticles. Figures 4a and 4d are STEM ABF (annular bright field) images of nanoparticles from the spine part, indicating the difference of nanoparticles with and without NiFe-LDH nanosheets coating. The atomicallyresolved STEM ABF images of two particles along [112] and [001] are presented in Figures 4b and 4c, respectively. These structures are in accordance with that of NiSe2/CoSe2. After electrodeposition, the crystal structure of the core (Ni,Co)Se2 could be well maintained, as shown in Figure 4e. The STEM ABF image in Figure 4f focuses on the NiFe-LDH nanosheets, where the layered fringes can be seen. It is however difficult to get a clear image for the layered structure of hydroxides, due to its high sensitivity to the strong electron beam. The TEM results shown in Figure S5 is consistent with the results obtained from STEM images. According to the above characterization results, we have successfully constructed a 3D (Ni,Co)Se2/NiFe-LDH hybrids, where the cactus-like (Ni,Co)Se2 support is shelled with NiFeLDH nanosheets. The crystal structures of the as-synthesized samples were determined by X-ray diffraction (XRD). At first, the β-phase NiCo-OH precursor was formed, waiting for the further treatment


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(Figure S6, Supporting Information).25 After selenization, the XRD pattern of the as-obtained selenide is displayed in Figure 5a. Both NiSe2 and CoSe2 possess a pyrite structure in which the Ni/Co atoms are surrounded by an octahedral arrangement of adjacent Se atoms (Figure 5b). Therefore, the NiSe2 (PDF#41-1495) and CoSe2 (PDF#09-0234) deliver similar diffraction patterns. The peaks from the as-prepared nickel cobalt selenide lie between those of the NiSe2 and CoSe2, which can be well identified as NiSe2 or CoSe2 with a slight peak shift. Also, the weight and atomic ratio of Ni and Co are around 1:1 according to the ICP results (Table 1, Supporting Information). As described above, we successfully synthesized the compound of (Ni,Co)Se2, instead of the mixture of NiSe2 and CoSe2. From the XRD result of sample NiFeLDH (Figure 5c), the two broad diffraction peaks can be ascribed to α-phase NiFe-LDH. The XRD pattern of the sample (Ni,Co)Se2/NiCo-LDH exhibits no obvious change compared to that of the sample (Ni,Co)Se2. To gain further insight into the surface chemical composition of the as-prepared samples, X-ray photoelectron spectroscopy (XPS) analysis was carried out. For the sample NiFe-LDH, in the Ni 2p3/2 spectrum (Figure 5d), the binding energy at 855.5 eV and 856.0 eV can be ascribed to NiO and Ni(OH)2, respectively.26 The peak fitting analysis of Fe 2p3/2 (Figure 5e) shows that the chemical species of Fe can be identified as FeOOH.26 As for the sample (Ni,Co)Se2, the survey spectra of (Ni,Co)Se2 in Figure S7a (Supporting Information) reveals the presence of C, O, Ni, Co and Se elements. In the spectrum of sample (Ni,Co)Se2, the characteristic peaks of Ni shift toward lower binding energy direction compared with those of the sample NiFe-LDH, indicating that the electron-rich structure of Ni was induced by delocalized electrons near the Se atoms. In the Se 3d region (Figure 5f), the two peaks at 54.8


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eV (Se 3d5/2) and 55.2 eV (Se 3d5/2) show the metal–selenium bonds, which are due to the selenide phase. The peaks at higher binding energies (59.0 eV) could be ascribed to the SeO2, which means the surface of (Ni,Co)Se2 is oxidized to a certain degree.27 After assembling of the NiFe-LDH on the selenide scaffold, the selenide peaks almost disappear, since XPS is a surface technique. The C1s and O 1s spectra are shown in Figure S7b-c. Besides, in the Fe and Ni 2p3/2 spectrum, it is observed that there is a slight hypsochromic shift of binding energies in the spectrum of (Ni,Co)Se2/NiFe-LDH hybrid compared with that in pure NiFe-LDH sample (Figure 5f), suggesting the existence of electronic coupling between (Ni,Co)Se2 and NiFeLDH.28 The XPS analysis demonstrates the successful preparation of (Ni,Co)Se2/NiFe-LDH hybrid catalyst and provides important evidence for the interfacial coupling between (Ni,Co)Se2 and NiFe-LDH.

Electrochemical performance and OER catalytic activities

Compared with the monometallic catalysts, such as nickel or cobalt compounds, the bimetallic ones can possess a higher OER catalytic activity due to their improved conductivity and increased active surface sites, which has been demonstrated in other work.

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In this

work, the bimetallic cactus-like (Ni,Co)Se2 was chosen as the support. We prepared the sample NiSe2 nanoflakes and CoSe2 nanowires as well for comparison (Figure S8, Supporting Information). The OER performance of bimetallic (Ni,Co)Se2, along with NiSe2 and CoSe2, were examined in 1 M KOH. As expected, the overpotential and Tafel slope of (Ni,Co)Se2 are


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smaller than those of NiSe2 and CoSe2 (Figure S9a-b, Supporting Information). In Figure S9c (Supporting Information), Nyquist plots revealed a decreased resistance for the (Ni,Co)Se2. These results further demonstrate the higher intrinsic catalytic activity of the bimetallic catalyst (Ni,Co)Se2, possibly attributed to the atomic synergistic effect of Ni and Co.31 In addition, compared to the NiCo-OH precursor, the (Ni,Co)Se2 shows much improved electrocatalytic activity and conductivity, which can be seen in Figure S10 (Supporting Information). To evaluate the electrocatalytic performance of the sample (Ni,Co)Se2/NiFe-LDH for OER, the electrochemical measurements were conducted in a typical three-electrode configuration in 1 M KOH aqueous solution (pH ≈ 13.7) (Figure 6a). The as-fabricated catalyst (Ni,Co)Se2/NiFe-LDH was used as the working electrode. For comparison, sample (Ni,Co)Se2 and NiFe-LDH were also investigated under the same conditions. Linear sweep voltammetry (LSV) measurements for catalysts NiFe-LDH, (Ni,Co)Se2, and (Ni,Co)Se2/NiFe-LDH were conducted at a scan rate of 1 mV s-1 to investigate the activity of the catalysts for OER. During the electrochemical test, the rapid oxygen evolution from the surface of the catalyst was observed. The catalyst (Ni,Co)Se2/NiFe-LDH exhibits the smallest overpotential of 205 mV at the current density of 10 mA cm−1 (Figure 6b). The overpotential values are comparable to or smaller than those for recently reported NiFe-LDH-based OER electrocatalysts, such as NiFe NO3 (NiFe-LDH containing NO3- anions) (270 mV),32 NiFe-LDH/Co,N-CNF (Co,N-codoped carbon nanoframes) (212 mV),33 NiFe/NiFe:Pi (NiFe-LDH/NiFe Phosphate) (290 mV),34 NiFe-


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LDH HMS (hollow microsphere) (239 mV),35 NiFe LDH-NS@DG (nanosheet @ defective graphene) (210 mV),10 NiFe-LDH nanoparticles (260 mV),12 and other low-cost catalysts (Figure 6c). In addition, the current density of the (Ni,Co)Se2/NiFe-LDH hybrid at 1.53 V reaches ≈40 mA cm-2, which was much higher than that of (Ni,Co)Se2 (≈11 mA cm-2) and NiFeLDH (≈20 mA cm-2). Given the proportional relationship between the current density and the oxygen yield, (Ni,Co)Se2/NiFe-LDH appears to exhibit the highest OER activity, indicating the synergistic effect derived from the hybrid of NiFe-LDH and conductive (Ni,Co)Se2. Tafel slopes of the three catalysts were evaluated as well, which provide the insights into the kinetics of the oxygen evolution reaction. The (Ni,Co)Se2/NiFe-LDH exhibits favorable kinetic toward the OER with a lower Tafel slope of 61 mV dec-1 than that for the (Ni,Co)Se2 (102 mV dec-1), implying significantly accelerated OER kinetics owing to the electrodeposition of the NiFeLDH nanosheets shell (Figure 6d). Moreover, the durability of the prepared catalysts was evaluated at a constant current density of 10 mA cm−2 for 30 hours. As can be seen in Figure 6e, the catalyst (Ni,Co)Se2 possesses good stability, whose overpotential remained almost constant during the 30 h. The SEM image of (Ni,Co)Se2 collected after the chronoamperometric test showed that the cactuslike architecture was well preserved (Figure S11a, Supporting Information). However, by comparing the SEM figures of NiFe-LDH nanosheets taken before and after the chronoamperometric test (Figure 6f), one could see that the nanosheets had aggregated after the durability measurement. As expected, the overpotential of NiFe-LDH increases around 20


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mV after the test, indicating its relatively poor durability. The chronopotentiometric characterization curve verified a good stability of (Ni,Co)Se2/NiFe-LDH catalyst for OER (Figure 6f). It could well be attributed to the interaction between the cactus-like (Ni,Co)Se2 support and NiFe-LDH nanosheets (evidenced by XPS analysis), which would prevent the stacking and aggregation of NiFe-LDH nanosheets. From the XRD results of the electrode (Ni,Co)Se2/NiFe-LDH after the durability test (Figure S11b, shown above), the peak intensity of the selenides becomes fairly weak, which suggests the transformation of selenides to hydroxides during the test. However, the cactus-like morphology can be well retained after the test, as shown in Figure S11c. To obtain further insight into the enhanced catalytic activity as well as the excellent stability of (Ni,Co)Se2/NiFe-LDH catalyst in comparison with the catalyst of (Ni,Co)Se2 and NiFe-LDH, electrical impedance spectroscopy (EIS) and electrochemically active surface areas (ECSA) analysis were performed. As shown in Figure 7a, the (Ni,Co)Se2/NiFe-LDH hybrid catalyst shows a lower equivalent series resistance (RESR) than NiFe-LDH, for the (Ni,Co)Se2 support possessing the desired electrical conductivity. The hybrid catalyst presents a much lower charge transfer resistance (RCT) than that of the NiFe-LDH, supporting that the assembly of (Ni,Co)Se2 with NiFe-LDH facilitates the charge transfer. Furthermore, the layered structure of NiFe-LDH sheets show much higher RW due to the stacking problem (the upper layers of NiFe-LDH stack impede the electron transport and ion diffusion in the inner layers) (Figure S12, Supporting Information). By assembling a layer of NiFe-LDH nanosheets on the


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cactus-like (Ni,Co)Se2 support, the stacking problem of layers could be alleviated, and the diffusion resistance (RW) of the (Ni,Co)Se2/NiFe-LDH decreases a lot, compared to that of the NiFe-LDH sample as anticipated. The larger ECSA of (Ni,Co)Se2/NiFe-LDH is beneficial to the intimate contact with the electrolyte, along with rich active sites for catalytic reactions, leading to the enhanced catalytic activity. The linear slope of the capacitive current (idl) versus scan rate (υ), equivalent to the electrochemical double-layer capacitance (Cdl), is used to represent the ECSA. As presented in Figure 7b, from (Ni,Co)Se2 to (Ni,Co)Se2/NiFe-LDH, the ECSA increased by 2.8-fold, which can be attributed to the assembled NiFe-LDH nanosheets on the properly engineered cactuslike nanoarchitecture. From NiFe-LDH to (Ni,Co)Se2/NiFe-LDH, there is a 40% further increase in ECSA, which partially accounts for the smallest overpotential of (Ni,Co)Se2/NiFeLDH catalyst. Besides, the hypothetical relation between the edge length of NiFe-LDH nanosheets and catalytic activity can offer another explanation, which however requires further study. On basis of the observation, the nanosheets assembled on the (Ni,Co)Se2 support are smaller in dimensions than those on carbon cloth (Figure 7c), suggesting that the (Ni,Co)Se2 support can reduce the size of the NiFe-LDH nanosheets and thus increase the edge lengths. The edges are assumed to contain active sites for OER, leading to the intensified activity while maintaining a similar surface area. Such high catalytic activity as well as excellent stability indicate that (Ni,Co)Se2/NiFeLDH would be a promising candidate for OER catalyst (Figure 7d). The enhanced performance


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of the 3D (Ni,Co)Se2/NiFe-LDH can be ascribed to the following aspects: (1) The (Ni,Co)Se2 support plays an important role in the enhanced OER activity of (Ni,Co)Se2/NiFe-LDH with respect to the unsupported NiFe-LDH. According to the EIS results, the (Ni,Co)Se2 support can impart high conductivity to the hybrid. Also, the (Ni,Co)Se2 support provides improved surface area, and favors in achieving smaller-sized NiFe-LDH nanosheets in the (Ni,Co)Se2/NiFe-LDH catalyst, which are expected to increase the number of active sites, thereby showing enhanced OER activity. (2) The catalyst exhibits a cactus-like structure (1D-2D mixed dimensional structures), and such well-integrated hierarchical structure is more beneficial for OER catalysts. (3) The interfacial interaction and coupling between the (Ni,Co)Se2 support and the NiFe-LDH shell, help offer the effective pathway for charge transfer, and hinder the NiFeLDH nanosheets aggregation. The co-contributions of the novel cactus-like nanostructure and the synergistic combination of hybrid components, make (Ni,Co)Se2/NiFe-LDH a promising OER catalyst.

Conclusions

In conclusion, a freestanding cactus-like (Ni,Co)Se2 support shelled with NiFe-LDH nanosheets has been designed and developed for the first time, as a low-cost, high-efficiency catalyst for OER. The (Ni,Co)Se2 support plays a pivotal role in the enhanced OER activity of (Ni,Co)Se2/NiFe-LDH with respect to the unsupported NiFe-LDH, which provides an interconnected conductive framework and buffers the aggregation of active species. In this


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cactus-like (Ni,Co)Se2 support, both surfaces and edges of 2D nanoflake arrays scaffold the selfassembled 1D nanospines uniformly. The nanospines can provide a pathway for effective charge transport and promote rapid release of gas bubbles formed during the OER process, and the nanoflakes help achieve reasonable mechanical strength and stability. As a result, the (Ni,Co)Se2/NiFe-LDH exhibits good electrocatalytic performance for the OER in terms of a low overpotential of 205 mV, a small Tafel slope of 61 mV dec-1, and excellent stability demonstrated in 30 h chronoamperometric test. The present study shows the promise of (Ni,Co)Se2/NiFe-LDH as a low-cost efficient catalyst for electrochemical water splitting. The understanding of the cactus-like selenide support provides a new thought for the fabrication of non-noble metal catalysts and desired nanostructures.


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Figure 1. Schematic illustration of the fabrication strategy for the cactus-like (Ni,Co)Se2/NiFe-LDH 3D architecture on carbon cloth.


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Figure 2. (a) Low magnification SEM images of the NiCo-OH precursor directly grown on carbon cloth, with a cactus picture inset. (b) High magnification SEM images of the NiCoOH precursor. (c) Low and (d) high magnification SEM images of (Ni,Co)Se2. (e) Low and (f) high magnification SEM image of (Ni,Co)Se2/NiFe-LDH.


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Figure 3. (a-c) Low-magnification STEM HAADF images of the NiCo-OH precursor, (Ni,Co)Se2 and (Ni,Co)Se2/NiFe-LDH. (d) STEM HAADF image of (Ni,Co)Se2 spines with /NiFe-LDH coating. (e1-e5) STEM-EDX elemental mappings of Co, Ni, Fe, Se and O.


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Figure 4. (a) STEM ABF image of (Ni,Co)Se2 nanoparticles on the nanospines. (b) Atomicresolved STEM ABF image along [112], with an enlarged image inset. (c) Atomic-resolved STEM HAADF image along [001], with an enlarged image inset. (d) STEM ABF image of (Ni,Co)Se2/NiCo-LDH nanoparticles with LDH coating on the nanospines. (e) Atomicresolved STEM HAADF image along [001] from (d), with FFT image inset. (f) STEM ABF image of NiCo-LDH coating between nanoparticles on the nanospines.


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Figure 5. (a) XRD patterns of the sample (Ni,Co)Se2. (b) Crystal structure of NiSe2 and CoSe2. (c) XRD patterns of samples (Ni,Co)Se2, NiCo-LDH, and (Ni,Co)Se2/NiCo-LDH. XPS spectra of (d) Ni 2p, (e) Se 3d, and (f) Fe 2p and peak fitting analysis of these three different samples.


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Figure 6. (a) Schematic illustration of the OER catalyst based on as-prepared (Ni,Co)Se2/NiFe-LDH arrays grown on carbon cloth. OER performance of (Ni,Co)Se2/NiFeLDH conducted in 1 M KOH: (b) OER LSV curves at a scanning rate of 1 mV s-1. (c) Comparison of overpotential required at 10 mA cm-2 with other recently reported highperformance OER electrocatalysts. (d) Tafel plots derived from OER LSV curves. (e) Chronopotentiometry curves at a constant current density of 10 mA cm-2. (f) SEM images before (left) and after (right) the chronoamperometric test.


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Figure 7. OER performance of (Ni,Co)Se2/NiFe-LDH conducted in 1 M KOH: (a) EIS curves. (b) Capacitive currents as a function of scan rate. (c) SEM images of NiFe-LDH grown on the carbon cloth (left) and NiFe-LDH grown on (Ni,Co)Se2 support (right). (d) Comparison of overpotential required at 10 mA cm-2 and Tafel slope with NiFe-LDH and (Ni,Co)Se2/NiFeLDH. The results suggest that (Ni,Co)Se2/NiFe-LDH presents higher catalytic activity.


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ASSOCIATED CONTENT Supporting Information. Details on the experimental methods, SEM images, EDX spectrum and mappings, TEM images, ICP results, XPS measurements, and OER performance results. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the Ministry of Education, Grant number: MOE2016-T2-2-138, conducted at the National University of Singapore. Xin Li is supported by NUS CA2DM.

REFERENCES


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NiFe Layered Double-Hydroxide Nanosheets on a Cactuslike (Ni,Co

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