Metallic Transition Metal Selenide Holey Nanosheets for Efficient

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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 ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b05481 • Publication Date (Web): 08 Sep 2017 Downloaded from http://pubs.acs.org on September 8, 2017

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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, TX 78712, USA. §

Department of Materials Science and Engineering, University of Science and Technology of

China, Hefei, Anhui 230026, China.

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 nano-architecture, NiCo2Se4 holey

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nanosheets exhibit greatly enhanced kinetics and improved cycling stability for OER. When further employed as a 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 provides fundamental insights into the structure-property-performance relationship of transition metal selenides based electrocatalytic materials.

KEYWORDS: transition metal selenides, metallic, holey nanosheets, electrocatalyst, oxygen evolution

Rapid 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, IrO2, have demonstrated high OER activity. However, high cost and inferior stability at high anodic potential seriously restrict their widespread application.7,

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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 need possess abundant active sites for adsorption/desorption process, excellent electrical conductivity for fast charge transport, and


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robust structure for long-term electrocatalysis. Recently, transition metal oxides have attracted considerable attention in electrocatalytic systems, thanks to their unique d electron 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-bandgap 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 hetero-metal 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 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


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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 provide fundamental insights

into the relationship of structure-property-performance of the

electrocatalytic materials.

RESULTS AND DISCUSSION


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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 line: difference line; blue vertical line: marker points). (b) Crystal structure of NiCo2Se4. (c) Calculated DOS, and (d) charge density wave of NCS (isosurface is set to 0.07 e/Bohr3). 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 Xray 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 NCS sample. The low reliability factors in Table S1 indicate 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, c=6.14 Å in monoclinic NiCo2Se4. Crystal


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structure of monoclinic NCS was 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 Density functional theory (DFT) calculation (Figure 1c,d and Figure S8,S9) reveals 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 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 catalyst and current collector, beneficial for improving electrocatalytic performance.


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Figure 2. STEM images of NCO holey nanosheets (a), NCS holey nanosheets (b). Inset in (a, b) are the enlarged STEM images showing the holey structure. (c) HRTEM and SA-ED of NCS holey nanosheet. (d) EDX images NCS holey nanosheets. Scale bars in (a,b), the inset of (a,b), c and d represent 200 nm, 50 nm, 20 nm and 200 nm. Holey architecture in NCS is inherited from 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, consist of interconnected nanoparticles (5~10 nm) with no obvious aggregation. Selenization process can be achieved through the reaction with Na2SeO3 in reductive environment (N2H4/EG solution). Figure 2b shows the morphology of NCS holey nanosheets,


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and other selenides with different composition possess similar porous structures are displayed in Figure S11. High-resolution transmission electron microscopy (HR-TEM) image in Figure 2c reveals the clear lattice fringes of 0.27 nm 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 (Figure 2d, S12) of NCS confirms the uniform distribution of Ni, Co, Se in NCS, and the complete formation of selenide compounds. X-ray photoelectron 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 at 872.3 and 853.8eV is attributed to Ni2+ with its shake-up satellite peak at 860.9 eV. Similarly, the Co 2p spectra, the peaks at 796.0 and 780.3 eV correspond to Co2+ while 794.4 and 779.3 eV corroborates with Co3+. The deconvoluted Ni 2p and Co 2p confirm the presence of mixed valence of metal ions (NiⅡ, Ⅱ, CoⅡ, Ⅱ) 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 become 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 firstly 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%


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IR-corrected polarization curves, and their corresponding overpotentials of NCS and reference samples. NCS holey nanosheets requires an overpotential of only 300 mV, which is 60 mV lower than that of NCO precursor, and even better than the traditional commercial RuO2 electrocatalyst (Figure 3b,S6). This result suggests the phase transformation of metal oxides to metal selenides can significantly improve the catalytic activity. 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 NP exhibits inferior electrocatalytic activity with an overpotential of 380 mV. In addition, NCS nanosheets 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 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 BET surface normalized polarization curves (Figure S19) and turnover frequency (TOFs). As can be seen in Figure 3d, NCS holey nanosheets 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.


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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 to reach 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).


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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 hours, 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 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 (Figure S20,21). 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 2000th CV cycles.


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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 1.0M KOH solution. (c) The model of OH─ on (010) facet of NCS; (d) OH─ adsorption energy of Co3Se4, NiCo2Se4 and NiSe. To further investigate the effect of composition on electrocatalytic properties, OER activity of NixCo3-xSe4 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 overpotential of 349 mV 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


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4b). Among them, NiCo2Se4 yields the highest activities with a lowest operation overpotential of 300 mV after 20th CV cycling. The derived Tafel slopes of ternary selenides are between 50~60mV/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. Moreover, density functional theory (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─, (010) facet of NCS holey nanosheets is chosen as the surface model (Figure 4c, 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.


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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. 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 splitting. To further investigate the overall water-splitting efficiency of NCS holey nanosheets, an alkaline electrolyzer based on the NiCo2Se4 holy nanosheets as both anode and cathode catalyst is assembled for both HER and OER applied on Ni foam substrates. Fig. 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 1.68V, 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


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30 mA cm−2 at 1.78 V over 6 hours, 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. Firstly, the metallic characteristics favors the fourelectron transfer kinetics between the catalyst and current collector, thus favoring the OER kinetics. Secondly, holey nano-architecture 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 mixedvalence 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,


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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 24h. This work not only represents a promising strategy to design the efficient and robust OER catalysts, but also provide fundamental insight into the structure-propertyperformance relationship of the electrocatalytic materials.

EXPERIMENTAL SECTION Synthesis of NiCo(OH)x/rGO hybrid intermediate: GO is prepared from purified natural graphite by a modified Hummers method. 0.4 mg/mL GO suspension was prepared by adding 30 mg graphene oxidation into 75 mL EG, and was ultrasonicated for 2 h. After 0.5mmol Ni(Ac)2·4H2O and 1.0 mmol Co(Ac)2·4H2O was added to 25 mL EG, Ni2+ /Co2+ mixed solution (Ni2+ /Co2+ = 1:2) was added to GO suspension and refluxing at 170 Ⅱ for 2h. After the reaction, the final products were centrifuged at 7800rpm for 10min, 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 RT. NCO nanoparticles were synthesized from hybrid intermediate without adding GO. Synthesis of NCS nanosheets (Nickel cobalt selenide): NCS holey nanosheets was obtained by a solution-based phase transformation: 0.04 mmol oxide precursor(~10mg) was added into 25


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mL EG was ultrasonicated for 2h, and 0.20 mmol Na2SeO3 (~29mg, a little excessive) was dissolved into 15 mL EG. And then NCO EG dispersion was added to Na2SeO3 EG solution under vigorous stirring. After dropping 1 mL N2H4 into the solution, the mixture was refluxing at 180 °C for 4h. Characterization: Powder XRD patterns were collected on a Philips Vertical Scanning diffractometer to identify the phase of the as-synthesized 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 fourpoint probe method. The NCS powders were cold-pressed (hydraulically) into pellets with diameter of 3mm 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 ethanol solvent by 30 min sonication to form a homogeneous ink. 5 µL of the catalyst dispersion (4.0 mg mL─1) were 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.


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Cyclic voltammograms (CVs) were performed at a scan rate of 50 mV s─1 from 0~0.8V (vs. saturated Ag/AgCl electrode). After CV activation for 20 cycles, linear sweep voltammograms (LSV) polarization curves were obtained by sweeping the potential from 0 V to 0.8 V at a sweep rate of 5 mV s−1 in 1.0M KOH solution using a RDE (1600 rpm), corrected by 95% IRcompensation. For 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 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 was used pure NiCo2Se4 catalyst, while the anode was activated by 20 cyclic voltammetric cycling (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 (turnover frequency) were calculated by assuming that every transition metal atom is involved in the catalysis (lower TOF limits were calculated):

TOF =

j*A 4*F *n

where j (mA cm−2) is the measured current density at η = 275~375mV, 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 density functional theory (DFT) method implemented in the Vienna Ab-initio Simulation Package


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(VASP) 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 (PAW) pseudopotential and the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional were used in the calculations with a 450eV plane-wave cut-off 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: ‫ܧ‬஺ௗ௦ = ‫ܧ‬௦௨௕ + ‫ܧ‬ைு −‫ܧ‬௧௢௧௔௟ , where ‫ܧ‬௧௢௧௔௟ is the total energy of the system in the equilibrium state, ‫ܧ‬௦௨௕ and ‫ܧ‬ுଶை are the total energy of substrate and OH molecular, respectively. The structural model 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 Supporting Information. Experimental and computational details, additional XRD patterns, STEM images, EDX spectrum, XPS spectrum, electrical transport test, electrochemical characterizations and calculation results. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author


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*[email protected]. Author Contributions ‡These authors contributed equally to this work.

ACKNOWLEDGMENT 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 (55884DNI10), and the Camille Dreyfus Teacher-Scholar Award.

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TOC Graphic


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Metallic Transition Metal Selenide Holey Nanosheets for Efficient

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