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Sep 8, 2017 - which can be ascribed to partially filly bands formed as a result of d electron delocalization.24,25 Goodenough proposes a model. Figure...
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Metallic Transition Metal Selenide Holey Nanosheets for Efficient Oxygen Evolution Electrocatalysis Zhiwei Fang,†,§ Lele Peng,†,§ Haifeng Lv,‡ Yue Zhu,† Chunshuang Yan,† Shengqi Wang,‡ Pranav Kalyani,† Xiaojun Wu,‡ and Guihua Yu*,† †

Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States ‡ Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *

ABSTRACT: Catalysts for oxygen evolution reaction (OER) are pivotal to the scalable storage of sustainable energy by means of converting water to oxygen and hydrogen fuel. Designing efficient electrocatalysis combining the features of excellent electrical conductivity, abundant active surface, and structural stability remains a critical challenge. Here, we report the rational design and controlled synthesis of metallic transition metal selenide NiCo2Se4-based holey nanosheets as a highly efficient and robust OER electrocatalyst. Benefiting from synergistic effects of metallic nature, heteroatom doping, and holey nanoarchitecture, NiCo2Se4 holey nanosheets exhibit greatly enhanced kinetics and improved cycling stability for OER. When further employed as an alkaline electrolyzer, the NiCo2Se4 holey nanosheet electrocatalyst enables a high-performing overall water splitting with a low applied external potential of 1.68 V at 10 mA cm−2. This work not only represents a promising strategy to design the efficient and robust OER catalysts but also provides fundamental insights into the structure−property−performance relationship of transition metal selenide-based electrocatalytic materials. KEYWORDS: transition metal selenides, metallic, holey nanosheets, electrocatalyst, oxygen evolution

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configurations, intrinsic corrosion stability in alkali electrolyte, and earth-abundant nature.9−11 For instance, spinel Co3O4 nanocrystals grown on N-doped graphene exhibit high OER activities in alkaline solution, due to electrical coupling effects between wide-band-gap semiconducting Co3O4 and conductive graphene.12 However, low intrinsic electrical conductivity hinders the fast electron transport, leading to the poor O2 generation kinetics.13 To date, considerable efforts have been devoted to modulating the physical and chemical properties to improve the electrocatalytic characteristics. Strategies such as heterometal cation doping,14−17 anion substitution,18 nanostructure engineering, and surface treatment19 have been successfully applied to enhance the electrocatalytic activity. For instance, mixed-metal oxides have showed improved electrocatalytic properties due to the introduction of heteroatoms, offering enhanced charge transfer between different ions to lower the energy barrier.16,17 A recent study on nickel phosphide catalyst suggests that phosphorus substitution of

apid depletion of natural resources and increasing demand for renewable energy have led to a revolutionary era of exploring alternative energy storage and conversion systems with high efficiency, low cost, and environmental benignity. Some key renewable energy technologies, including fuel cells and water splitting, hinges on fundamental improvements in catalytic materials.1,2 Up to now, for the state-of-the-art catalysts, a large overpotential is required in water splitting to accelerate the multistep electron transfer process to produce H2, which is primarily due to the sluggish OER kinetics.3−6 Noble metal oxides such as RuO2 and IrO2 have demonstrated high OER activity. However, high cost and inferior stability at high anodic potential seriously restrict their widespread application.7,8 In this regard, designing efficient, durable, and low-cost non-noble electrocatalysts based on earth-abundant 3d metals is critically urgent. Highly efficient electrocatalysts for OER must possess abundant active sites for the adsorption/desorption process, desirable electrical conductivity for fast charge transport, and robust structure for long-term electrocatalysis. Recently, transition metal oxides have attracted considerable attention in electrocatalytic systems, thanks to their unique d electron © 2017 American Chemical Society

Received: August 1, 2017 Accepted: September 8, 2017 Published: September 8, 2017 9550

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Figure 1. (a) Rietveld refinement of the X-ray diffraction data of NiCo2Se4 (black line, data points; red line, calculation line; green vertical line, marker points; blue line, difference line). (b) Crystal structure of NiCo2Se4. (c) Calculated DOS and (d) charge density wave of NCS (isosurface is set to 0.07 e/Bohr3).

provides fundamental insights into the relationship of structure−property−performance of the electrocatalytic materials.

oxygen can lower charge transfer resistance and optimize hydrogen adsorption energy, thus improving the intrinsic electrocatalytic activity.20 Fabrication of single- and few-layered 2D nanosheets has also shown promise in improving the catalytic activities given that 2D structure significantly increases the electrochemically active area.21 However, irreversible restacking of 2D nanosheets during the processing and fabrication results in the decreased surface area and more difficult electrolyte diffusion pathway, which is still a significant challenge to date. In this regard, holey/porous nanomaterials, sparked by their interconnected open structures and structural stability, possess more active sites and continuous mass/charge transport pathway.22 Hence, it is highly desirable to design high-performance catalysts with the combination of aforementioned features for efficient oxygen evolution. Here, we report the rational design and controlled synthesis of metallic NiCo2Se4 (NCS) holey nanosheets with a monoclinic phase, as a highly efficient and robust OER electrocatalyst. Due to the metallic nature, synergistic interaction between Ni/Co atoms, and holey structure composed of interconnected nanoparticles, NCS holey nanosheets possess both superior catalytic activity and long-term durability for OER. Metallic NCS holey nanosheets show a low overpotential of 295 mV to motivate the oxygen evolution, a low Tafel slope of 53 mV/dec, and an improved cycling stability. The merits of NCS holey nanosheets are further supported by a systematic study on the relationship between composition and electrocatalytic properties among a series of NixCo3−xSe4 (x = 0−3.0). The experimental results, in combination with density functional theory (DFT) calculation, demonstrate that NiCo2Se4 shows the best electrocatalytic property among a series of NixCo3−xSe4 due to the largest hydroxyl ion absorption energy. Moreover, the NiCo2Se4 holey nanosheet catalyst enables a highly performed water splitting with a low applied external potential of 1.68 V at 10 mA cm−2. This work not only represents an advancement in rational design of highly efficient and durable electrocatalysts but also

RESULTS AND DISCUSSION Rational synthesis of NCS holey nanosheets is based on phase transformation from spinel NiCo2O4 (NCO) holey nanosheets (Figure S3) to monoclinic NCS holey nanosheets in a selenization process. The phase purity of a series of NixCo3−xSe4 is identified by the powder X-ray diffraction (XRD) patterns, which indicate the pure monoclinic selenide phases (except for Ni1−ySe with hexagonal NiAs structure) are formed (Figure S4). Figure 1a shows the XRD patterns and corresponding Rietveld analysis of the NCS sample. The low reliability factors in Table S1 indicate that the Rietveld-refined XRD pattern fits quite well with the experimental data points, giving calculated cell parameters of a = 12.0 Å, b = 3.59 Å, and c = 6.14 Å in monoclinic NiCo2Se4. Crystal structure of monoclinic NCS is shown in Figure 1b, which belongs to the Cr3S4-type structure.23 It is regarded as the monoclinic defect structure derived from NiAs-type layered structureafter removing one-fourth of the cations from alternate cation layers in an ordered manner, the vacancies are generated to every Ni layer in NiCo2Se4 (Figure S7).24 DFT calculation (Figure 1c,d and Figures S8 and S9) reveals that the density of states (DOS) of NCS across the Fermi level is more intense than that of NCO. This suggests the electrical property of NCS, in particular, the carrier concentration and electrical conductivity, can be further improved when transformed from oxides to selenides. The typical metallic characteristics of NCS can be further revealed by temperature-dependent electrical resistance test (Figure S10), where electrical resistance of NCS keeps increasing as temperature increases, displaying the metallic behavior. Previous studies have shown the metallic behavior for the AxB3−xX4 (A, B = Fe, Co, Ni...; X = S, Se, Te) compounds, which can be ascribed to partially filly bands formed as a result of d electron delocalization.24,25 Goodenough proposes a model 9551

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Figure 2. STEM images of NCO holey nanosheets (a), NCS holey nanosheets (b). Insets in (a,b) are the enlarged STEM images showing the holey structure. (c) HRTEM and SAED of a NCS holey nanosheet. (d) EDX images of NCS holey nanosheets. Scale bars: (a,b) 200 nm, inset of (a,b) 50 nm, (c) 20 nm, (d) 200 nm.

electron spectroscopy (XPS) spectra (Figure S13) are further conducted to confirm the composition of NCS holey NS. The binding energies at around 874.1 and 855.3 eV of Ni 2p are assigned to Ni3+, and those at 872.3 and 853.8 eV are attributed to Ni2+ with its shakeup satellite peak at 860.9 eV. Similarly, the Co 2p spectra, the peaks at 796.0 and 780.3 eV correspond to Co2+, whereas 794.4 and 779.3 eV corroborate with Co3+. The deconvoluted Ni 2p and Co 2p confirm the presence of mixed valence of metal ions (NiII,III, CoII,III) in NCS, which is expected to play a vital role in the electrocatalytic activity.14,15 Monoclinic NCS holey nanosheets offer the combined characteristics of metallic nature, 2D nanosheet structure, and interconnected holey architecture, making it a potential candidate for high-performance catalysts. To verify that metallic NCS holey nanosheets could serve as an excellent electrocatalyst for oxygen evolution, we compare the OER performance of as-prepared ternary selenide holey nanosheets with that of reference samples, including holey oxide nanosheets, selenide nanoparticles (NP), selenide nanosheets with no holes (NS) (Figure S14), and commercial RuO2 electrocatalyst. The electrodes are first activated in O2-saturated 1.0 M KOH aqueous electrolyte at a scan rate of 50 mV s−1 (Figure S15). Figure 3a,b shows 95% IR-corrected polarization curves and their corresponding overpotentials of NCS and reference samples. NCS holey nanosheets require an overpotential of only 300 mV, which is 60 mV lower than that of the NCO precursor and even better than that of the traditional commercial RuO2 electrocatalyst (Figures 3b and S6). This result suggests the phase transformation of metal oxides to metal selenides can significantly improve the catalytic activity.

to interpret the electrical properties of AB2X4, which includes both direct cation t2g interaction and indirect interactions between cation eg and anion s,pσ orbitals.26,27 In this regard, metallic NCS would bring efficient electron transfer between the surface of the catalyst and current collector, beneficial for improving electrocatalytic performance. Holey architecture in NCS is inherited from the NCO holey nanosheet precursor. NCO holey nanosheets are prepared in a template-directed process followed by a controlled thermal treatment according to the previous method reported by our group.28,29 Figure 2a shows the scanning transmission electron microscopy (STEM) image of the 2D oxide holey nanosheet precursor, consisting of interconnected nanoparticles (5−10 nm) with no obvious aggregation. Selenization process can be achieved through the reaction with Na2SeO3 in reductive environment (N2H4/ethylene glycol (EG) solution). Figure 2b shows the morphology of NCS holey nanosheets, and other selenides with different composition possess similar porous structures (displayed in Figure S11). High-resolution transmission electron microscopy (HRTEM) image in Figure 2c reveals that the clear lattice fringes of 0.27 and 0.53 nm (Figure 2c) correspond well to the (002) and (001) facets of the monoclinic NCS, respectively. The diffused concentric rings displayed in selected area electron diffraction (SAED) pattern (Figure 2c inset) indicate the polycrystalline structure. The diffraction rings can be indexed to monoclinic NCS, in agreement with the XRD analysis. Energy-dispersive X-ray spectroscopy (EDX) analysis (Figures 2d and S12) of NCS confirms the uniform distribution of Ni, Co, and Se in NCS and the complete formation of selenide compounds. X-ray photo9552

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Figure 3. (a) 95% IR-corrected polarization curves of NCS holey nanosheets (NS), NCO holey NS, NCS NP, NCS NS, and commercial RuO2 (j: current density). (b) Comparison of overpotentials requires j = 10 mA cm−2 of NCS and NCO with different morphology in 1.0 M KOH. (c) Tafel plots of NCS and NCO holey NS and NCS NP and NS in 1.0 M KOH solution. (d) TOFs with respect to all metal atoms of NCS and NCO holey NS at different overpotentials. (e) Chronopotentiometric measurement at current density of 10 mA cm−2 in 1.0 M KOH. Inset: Polarization curves of NCS holey NS after different cycles.

Moreover, stability is another key indicator to evaluate the performance of catalysts. Chronoamperometric response in Figure 3e verifies the high stability of NCS holey nanosheets (Ni/Co = 1:2), which can maintain a consistently low overpotential of 300 mV at 10 mA cm−2 for over 36 h, indicating the superior cycling stability of holey nanosheet structure. Interestingly, the overpotential decreases by 20 mV for NCS holey nanosheets after 5 h of galvanostatic conditioning at an anodic current density j = 10 mA cm−2. Anodic conditioning (AC) has been utilized to improve the activity of many Ni/Co-based oxygen-evolving catalysts, during the process which involves the formation of amorphous oxides on the surface of the electrocatalyst.31−34 This amorphous oxide layer not only protects the NCS phase from further oxidation but also provides necessary active sites for OER process.18 The composition and holey structure of NCS can be preserved after long-term cycling tests (Figures S20 and S21). After AC treatment, current density reaches 10 mA cm−2 at η = 290 mV and 27 mA cm−2 at η = 320 mV (Figure S22). For comparison, the overpotential for RuO2 increases by 70 mV after 3 h (Figure S23). Figure 3e inset shows the high catalytic activity can be maintained for more than 2000 CV cycles.

Although holey selenide nanosheets possess relatively smaller surface area than oxide precursor owing to the partial fusion of particles during the selenization (Figure S17), they provide larger electrochemically active surface area, which can be evaluated approximately by electrochemical double-layer capacitance (Cdl) test (Figure S18).30 In comparison, NCS NPs exhibit inferior electrocatalytic activity with an overpotential of 380 mV. In addition, the NCS nanosheet catalyst with no holey structure requires an additional overpotential of 85 mV to reach a same current density. To better understand the electrocatalytic activity of NCS holey nanosheets, Tafel slopes for all catalysts are investigated (Figure 3c). The Tafel slope of the NCS holey nanosheets is 53 mV/dec, smaller than that of NCO holey nanosheets (83 mV/dec), NCS NP (97 mV/dec), and NCS nanosheets (149 mV/dec). The intrinsic activity of holey selenide nanosheets is further confirmed by the Brunauer−Emmett−Teller (BET) surface normalized polarization curves (Figure S19) and turnover frequency (TOFs). As can be seen in Figure 3d, NCS holey nanosheet catalysts exhibit TOFs of 0.016 s−1 per total 3d metal atoms at an overpotential of 300 mV, which is much larger than that of NCO holey nanosheets. 9553

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Figure 4. (a) 95% IR-corrected polarization curves of a series of NixCo3−xSe4 (x = 0−3.0). (b) Comparison of overpotentials and Tafel plots of NixCo3−xSe4 in 1.0 M KOH solution. (c) Model of OH− on the (010) facet of NCS. (d) OH− adsorption energy of Co3Se4, NiCo2Se4, and NiSe.

Figure 5. (a) 95% IR-corrected polarization curves of NiCo2Se4 holey nanosheets for overall water splitting in a two-electrode configuration. (b) Chronoamperometric measurement at the constant current density of 10, 20, and 30 mA cm−2 in 1.0 M KOH.

over, DFT calculations are further applied to calculate the water adsorption energy of catalysts in alkaline medium (Figure 4c,d). It is generally accepted that hydroxyl ions (OH−) are initially adsorbed on the surface of catalysts during the OER process in basic media.19,35 That is, adsorption energy of OH− molecules plays an essential role in the OER activity. Large adsorption energy is in favor of the formation of an Oads intermediate (OHads → Oads + H+ + e−) and further reaction. To study the synergistic effect of Ni/Co on adsorption energy of OH−, the (010) facet of NCS holey nanosheets is chosen as the surface model (Figure 4c and Figures S24 and S25). In Figure 4d, it is clear that ternary NCS holey nanosheets possess adsorption energy (Eads, absolute value) of 3.71 eV, distinctly larger than that of Co3Se4 (3.56 eV) and NiSe (3.62 eV). The above calculated results indicate ternary selenide is more favorable for adsorbing OH− and thus promoting OER kinetics, which is in accord with the experimental results. Recently, transition metal selenides have shown desirable electrocatalytic activity for alkaline HER.11 The high OER activity of ternary NCS revealed here suggests that NCS holey nanosheets can serve as a bifunctional catalyst in overall water

To further investigate the effect of composition on electrocatalytic properties, OER activity of Ni xCo 3−xSe 4 catalysts with different Ni/Co ratio are further evaluated. The polarization curves of Ni/Co selenides with different composition in 1.0 M NaOH are shown in Figure 4a. Binary Co3Se4 and Ni1−ySe exhibit inferior electrocatalytic activity with the overpotentials of 349 and 345 mV, respectively. Remarkably, after the introduction of heteroatom, the catalytic activities of ternary metal oxides are improved. All Ni−Co mixed selenide catalysts deliver a desirable catalytic activity for oxygen evolution with the overpotential of 295−310 mV (Figure 4b). Among them, NiCo2Se4 yields the highest activities with a lowest operation overpotential of 300 mV after 20 CV cycling. The derived Tafel slopes of ternary selenides are between 50 and 60 mV/dec, smaller than that of Co3Se4 (65 mV/dec) and Ni1−ySe (76 mV/dec), indicating more rapid OER kinetics can be achieved in applications using mixed selenide holey nanosheets. The performance difference between ternary metal selenides and binary selenides primarily derives from the synergistic effect of Co and Ni, due to their modified electronic structure and mixed-valence state. More9554

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Hummers method. 0.4 mg/mL of GO suspension was prepared by adding 30 mg of GO into 75 mL of EG and was ultrasonicated for 2 h. After 0.5 mmol Ni(Ac)2·4H2O and 1.0 mmol Co(Ac)2·4H2O were added to 25 mL of EG, Ni2+/Co2+ mixed solution (Ni2+/Co2+ = 1:2) was added to GO suspension and refluxed at 170 °C for 2 h. After the reaction, the final products were centrifuged at 7800 rpm for 10 min and washed with water and ethanol. The hybrid intermediate was dried by freeze-drying method. All chemicals were used without further purification and purchased from commercial sources. Synthesis of NCO Holey Nanosheets. NiCo(OH)x/rGO hybrid intermediate is annealed at 400 in air for 2 h with a heating rate of 0.5 °C min−1 from room temperature. NCO nanoparticles were synthesized from hybrid intermediate without adding GO. Synthesis of NCS Nanosheets. NCS holey nanosheets were obtained by a solution-based phase transformation: 0.04 mmol oxide precursor (∼10 mg) was added into 25 mL of EG was ultrasonicated for 2 h, and 0.20 mmol Na2SeO3 (∼29 mg, a little excess) was dissolved into 15 mL of EG, and then NCO EG dispersion was added to Na2SeO3 EG solution under vigorous stirring. After dropping 1 mL of N2H4 into the solution, the mixture was refluxed at 180 °C for 4 h. Characterization. Powder XRD patterns were collected on a Philips vertical scanning diffractometer to identify the phase of the assynthesized samples. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, STEM (Hitachi S5500), and TEM (JEOL 2010F) were used to characterize the morphology of the samples. Nitrogen sorption isotherms and BET surface area were measured with an automated gas sorption analyzer (AutoSorb iQ2, Quantachrome Instruments). Electrical transport property measurement was conducted in a four-point probe method. The NCS powders were coldpressed (hydraulically) into pellets with diameter of 3 mm by a physical property measurement system (PPMS-9, Quantum Design) under high vacuum condition. Electrochemical Measurement. All of the OER measurements were performed under identical conditions with the same catalyst mass loading: 4 mg of catalyst and 40 μL of 5 wt % Nafion solution were dispersed in 1.0 mL of ethanol solvent by 30 min sonication to form a homogeneous ink. Five microliters of the catalyst dispersion (4.0 mg mL−1) was then transferred onto the glassy carbon rotating disk electrode (RDE, 0.07 cm2, Pine Research Instrumentation, USA) via a controlled drop-casting approach. All of the electrochemical measurements were conducted in a three-electrode electrochemical cell using saturated Ag/AgCl electrode as the reference electrode, a platinum wire as the counter electrode, and the sample modified glassy carbon electrode as the working electrode on a BioLogic Instrument (BioLogic VMP-3model). Electrocatalytic OER activity and kinetics were examined in O2-saturated 1.0 M aqueous KOH. Cyclic voltammograms (CVs) were performed at a scan rate of 50 mV s−1 from 0−0.8 V (vs saturated Ag/AgCl electrode). After CV activation for 20 cycles, linear sweep voltammogram polarization curves were obtained by sweeping the potential from 0 to 0.8 V at a sweep rate of 5 mV s−1 in 1.0 M KOH solution using a RDE (1600 rpm), corrected by 95% IR compensation. For a chronoamperometric test, a static overpotential was fixed for a certain time during continuous OER process to obtain the curve of time dependence of the current density. Chronopotentiometric measurement was performed at current density of 10 mA cm−2. For the preparation of the NCS/Ni foam electrode, the pretreated Ni foam with a fixed area of 0.5 cm × 1.0 cm coated with water-resistant silicone glue was drop-casted with the NiCo2Se4 catalyst ink (mass loading ∼ 1.0 mg cm−2). The cathode used was pure NiCo2Se4 catalyst, whereas the anode was activated by 20 cyclic voltammetric cycles (from 1.0 to 2.0 V). Electrochemical Calculation. The potentials in this work were converted to a reversible hydrogen electrode (RHE) scale according to the Nernst equation, ERHE = EAg/AgCl + 0.059 pH + 0.197). The overpotential (η) was calculated according to the following formula: η (V) = ERHE − 1.23 V. The values of TOF were calculated by assuming that every transition metal atom is involved in the catalysis (lower TOF limits were calculated):

splitting. To further investigate the overall water-splitting efficiency of NCS holey nanosheets, an alkaline electrolyzer based on the NiCo2Se4 holey nanosheets as both anode and cathode catalyst is assembled for both HER and OER applied on Ni foam substrates. Figure 5a shows the current−potential response of this electrolyzer in a two-electrode system. A current density of 10 mA cm−2 is obtained at about 1.68 V, which represents a combined overpotential of only about 450 mV for OER and HER. As a reference, water splitting using Ni foam required a combined overpotential of 640 mV to achieve the current density of 10 mA cm−2 (Figure S26). The potential is stable at 1.68 V without obvious degradation during a 24 h galvanostatic electrolysis when employing the electrolyzer of NCS/NCS holey nanosheets couple (Figure S27). Remarkably, as demonstrated in Figure 5b, this electrolyzer maintains the current density of 20 mA cm−2 at a cell voltage as low as 1.74 V and 30 mA cm−2 at 1.78 V over 6 h, respectively. Thus, NCS holey nanosheets enabled a highly performed overall water splitting with excellent bifunctional electrocatalytic activity and stability. The superior electrocatalytic performance of ternary selenide holey nanosheets can be ascribed to the synergistic effects of metallic characteristics, holey nanosheet structure, and synergistic interaction between Ni/Co atoms. First, the metallic characteristics favor the four-electron transfer kinetics between the catalyst and current collector, thus favoring the OER kinetics. Second, holey nanoarchitecture could enhance the electrolyte diffusion and augment the contact degree between reactants and active sites. This unique structure provides a direct pathway to facilitate electrolyte penetration and facile release of evolved O2 bubbles. Inside the holey structure, interconnected nanoparticles could not only facilitate electron transfer but also enhance the structural stability of electrocatalyst. Lastly, the synergistic effect of Ni/Co atoms can also improve electrocatalytic activity, due to their modified electronic structure and mixed-valence states. Thanks to these advantageous features, NCS holey nanosheets demonstrate one of the best electrocatalytic performance among the reported Ni/Co-based dichalcogenides (Table S3).

CONCLUSIONS In summary, we reported the rational design of metallic transition metal selenide NiCo2Se4-based holey nanosheets with a monoclinic phase, as a highly efficient and robust OER electrocatalyst. Monoclinic NCS nanosheets possess metallic behavior, holey nanoarchitecture, as well as abundant electrochemically active surface. Due to these synergistic characteristics, metallic NCS holey nanosheets show superior catalytic activity for OER, including low overpotential of 295 mV, low Tafel slope, and enhanced cycling stability. These features are supported by a systematic study combining experimental and theoretical aspects on a series of NixCo3−xSe4 holey nanosheets. Moreover, the NiCo2Se4 holey nanosheet catalyst enables a highly performed overall water splitting as an alkaline electrolyzer, which generates 10 mA cm−2 at 1.68 V over 24 h. This work not only represents a promising strategy to design the efficient and robust OER catalysts but also provides fundamental insight into the structure−property−performance relationship of the electrocatalytic materials. EXPERIMENTAL SECTION Synthesis of NiCo(OH)x/rGO Hybrid Intermediate. Graphene oxide (GO) is prepared from purified natural graphite by a modified 9555

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j×A 4×F×n

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where j (mA cm−2) is the measured current density at η = 275−375 mV, A (0.07 cm2) is the surface area of glassy carbon RDE, the number 4 means 4 electrons mol−1 of O2, F is Faraday’s constant (96485C mol−1), and n is the moles of coated metal atom on the electrode calculated from m and the molecular weight of the coated catalysts. Computational Details. Spin-polarized calculations were performed with the DFT method implemented in Vienna Ab initio Simulation Package. An effective U parameter of 3.7 eV was applied for Co 3d states and 6.6 for Ni 3d states under the approximation introduced by Dudarev et al. to describe well the strong correlated electronic states in Co2NiO4, Co2NiSe4, Co3Se4, and NiSe. The projector-augmented wave pseudopotential and the Perdew−Burke− Ernzerhof exchange-correlation functional were used in the calculations with a 450 eV plane-wave cutoff energy. The energy converge criteria was set to be 10−5 eV for self-consistent calculations and the lattice parameters were optimized until the convergence tolerance of force on each atom was smaller than 0.02 eV. All periodic slab models have a vacuum spacing of at least 15 Å. In this work, the adsorption energies were calculated via the following equation: EAds = Esub + EOH = Etotal, where Etotal is the total energy of the system in the equilibrium state, Esub and EH2O are the total energy of substrate and OH molecular, respectively. The structural models of Co3Se4, NiSe, and NiCo2Se4 all share the same geometry configuration, and the slab models consists of four layers of (010) facet. In calculations, the two bottom layers were kept fixed, whereas the rest of atoms were allowed to relax.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b05481. Experimental and computational details, additional XRD patterns, STEM images, EDX spectrum, XPS spectrum, electrical transport test, electrochemical characterizations, and calculation results (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Xiaojun Wu: 0000-0003-3606-1211 Guihua Yu: 0000-0002-3253-0749 Author Contributions §

Z.F. and L.P. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank Prof. J.B. Goodenough and Prof. J.S. Zhou at the University of Texas at Austin for valuable discussions and some instrumental support. G.Y. acknowledges the funding support from the Welch Foundation Award F-1861, ACS-PRF Young Investigator award (55884-DNI10), and the Camille Dreyfus Teacher-Scholar Award. REFERENCES (1) Chu, S.; Cui, Y.; Liu, N. The Path Towards Sustainable Energy. Nat. Mater. 2017, 16, 16−22. (2) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment in 9556

DOI: 10.1021/acsnano.7b05481 ACS Nano 2017, 11, 9550−9557

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DOI: 10.1021/acsnano.7b05481 ACS Nano 2017, 11, 9550−9557