Self-Templating Construction of Porous CoSe2 Nanosheet Arrays as

Subscriber access provided by University of Sunderland

Article

Self-Templating Construction of Porous CoSe2 Nanosheet Arrays as Efficient Bifunctional Electrocatalysts for Overall Water Splitting Shuai Wan, Weiyang Jin, Xiaoliang Guo, Jing Mao, Lekai Zheng, Jianling Zhao, Jun Zhang, Hui Liu, and Chengchun Tang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03804 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 18, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Self-Templating Construction of Porous CoSe2 Nanosheet Arrays as Efficient Bifunctional Electrocatalysts for Overall Water Splitting

Shuai Wan,†,‡ Weiyang Jin,†,‡ Xiaoliang Guo,†,‡ Jing Mao,§,⊥ Lekai Zheng,†,‡ Jianling Zhao,†,‡ Jun Zhang, ,†,‡ Hui Liu,,†Chengchun Tang†,‡

†School

of Material Science and Engineering, Hebei University of Technology, Dingzigu

Road 1, Tianjin 300130, P. R. China ‡Hebei

Key Laboratory of Boron Nitride Micro and Nano Materials, Guangrongdao Road

29, Tianjin 300130, P. R. China §School of Materials Science and Engineering, Tianjin University, Tianjin Haihe Education

Park, Tianjin 300072, PR China ⊥ Center

for functional materials, Brookhaven National Laboratory, Suffolk County, City

of New York 11973, USA

*Corresponding authors. Tel.: +86-22-60202660; fax: +86-22-60202660 E-mail address: [email protected] (J. Zhang), [email protected] (H. Liu)

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 36

ABSTRACT Developing

low-cost,

high

performance

and

stable

non-noble

bifunctional

electrocatalysts for overall water splitting is of great importance for future energy supplement. Despite recent advances in the synthesis of transition metal selenide nanostructures, the fabrication of porous nanosheet based binder-free electrode with more active sites remains a major challenge. Herein, the self-templating construction of a porous CoSe2 nanosheet array on carbon cloth (p-CoSe2/CC) has been reported by vapor selenizing the pre-prepared α-Co(OH)2 nanosheet array precursor. Arising from large active surface area, fast diffusion of generated gas and strong structural stability, the asobtained p-CoSe2/CC can serve as an efficient bifunctional electrocatalyst for both OER and HER in alkaline electrolyte, with a current density of 10 mA cm-2 at overpotential of 243 mV for OER and 138 mV for HER, respectively. Moreover, when p-CoSe2/CC is assembled as an alkaline electrolyzor, it only needs a cell voltage of 1.62 V at 10 mA cm2

and shows excellent long-term stability of 20 h. The versatile fabrication strategy with

self-templated porous structure proves a new way to construct other advanced metal selenide for energy conversion and storage. KEYWORDS: Cobalt selenide, Porous nanosheets, Self-templating, Bifunctional electrocatalyst, Overall water splitting

2


Page 3 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Electrochemical water splitting is an essential technology for the future hydrogen economy,1, 2 and has been considered as the cleanest way to produce H2 fuel when the required electricity comes from renewable energy sources. The overall water splitting consists of two half reactions: the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), occurring at the anode and cathode respectively. Although, noble metal-based materials, such as IrO2 or RuO2 for OER and Pt for HER, are usually efficient electrocatalysts, the exorbitant price and scarcity of these noble metal-based electrocatalysts severely restrict their widespread applications in electrolyzers.3-5 Consequently, it’s necessary to explore and optimize non-noble metals catalysts for water splitting. Over past few years, much effort has been devoted to develop earth-abundant transition-metal (Mn, Fe, Co, Ni and Cu)-based catalysts with excellent activities for HER/OER process as alternatives to noble metal-based electrocatalysts.6-12 Paradoxically, many HER catalysts preferably takes place in acidic conditions while OER in alkaline medium. As the OER is a sluggish reaction kinetics with several-fold higher overpotential than that of HER,

13, 14

and most OER catalysts in acid media suffers from using scarce

noble metal-based materials,15 conducting overall water splitting in alkaline media is more economically viable than in acidic electrolyte.16 In addition, it is inevitable to increase the cost using different HER and OER electrocatalysts in an integrated alkaline electrolyser.17 And, the excellent HER electrocatalytic candidates may not possess high efficiency for OER, and vice versa. Therefore, developing a bifunctional electrocatalyst with excellent 3



activity and stability for both HER and OER in alkaline electrolytes is of great important and challenging .18-20 Recently, as a type of highly active cobalt (Co)-based electrocatalysts, cobalt selenide (CoSe2) with a variety of crystal forms and features, has attracted considerable attention as an excellent electrocatalyst for water splitting owning to its earth abundance and low cost.21, 22

It was firstly found to be highly active for both HER in acidic solution and OER in basic

solution, though the real active materials are the in situ formed (oxy)hydroxides on the CoSe2 surfaces under OER conditions.23,

24

Lately, CoSe2 electrocatalyst has shown

considerable efficiency for HER under alkaline conditions by phase-transfromation engineering.25 As a result, CoSe2 is believed to be an ideal bifunctional electrocatalyst for overall water splitting in water-alkali electrolyte systems.26 In this aspect, many strategies have been applied to enhance the intrinsic activity of CoSe2-based catalysts for HER and OER in alkaline environments, for example, by cation or anion doping ( e. g. Ni-CoSe2,27 Zn-CoSe2,28

CoP2xSe2(1-x),29 and Co(SxSe1-x)230) for electron transfer and lattice distortion,

or by forming composite (e. g. CoSe2/Carbon,31 CoSe2/Co2B,32 and CoSe2/CoO33) to enhance the conductivity or utilize the synergistic effect. We notice that nanostructure engineering for CoSe2 is an effective strategy toward enhanced catalytic activity for both HER and OER half-reaction.34-37 Accordingly, it should take effects on CoSe2 bifunctional electrocatalysts for overall water splitting. However, few attention has been paid to nanostructure geometric modulation on CoSe2-based bifunction catalysts for water electrolysis. 4


Page 4 of 36

Page 5 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


It is believed that porous 2D structure is regarded as the most attractive elecrocatalyst structure for water splitting, due to its higher specific surface area with more active sites, faster channels for mass transfer and ion diffusion, and favoring for gas bubbles evolving and releasing from catalyst surface.38, 39 Moreover, porous nanosheet array electrocatalyst grown in situ on conductive substrate with binder-free configurations, especially on 3D foam, could provide highly desirable advantages, including good mechanical adhesion, excellent electrical transformation between catalyst and substrate, convenient diffusion of electrolytes and reaction products, and less blockage of active sites.40, 41 Therefore, it is of highly important to develop porous CoSe2 nanosheet arrays growth directly on conductive substrate, working as a bifunctional electrocatalyst to achieve optimized electrocatalytic performance toward overall water splitting, especially in alkaline medium. In this work, we report the self-templating fabrication of thin porous CoSe2 nanosheets (p-CoSe2/CC) arrays grown on carbon cloth as bifunctional electrocatalyst for alkaline overall water splitting, which were readily prepared by electrodepostion of α-Co(OH)2 nanosheets on carbon cloth followed by vapor selenization treatment. Featuring porous 2D nanosheet and binder-free structure, such 3D integrated p-CoSe2/CC electrode shows excellent activity and stability for both HER and OER in 1 M KOH: low overpotential of 243 and 138 mV at 10 mA cm-2 as well as small Tafel slope of 82 and 83 mV dec-1 for OER and HER, respectively, which were superior to most of the state-of-art CoSe2-based electrode materials. Most importantly, assembling an alkaline full water electrolyzer using p-CoSe2/CC as both cathode and anode, a low cell voltage of 1.62 V is needed to obtain 10 5



mA cm-2 for the overall water splitting with high stability.

EXPERIMENTAL SECTION Materials. Carbon cloth (CC) was provided by CeTech Co. Ltd. Pt/C (20 wt% Pt on Vulcan XC-72R), IrO2 and Nafion (5 wt%) were purchased from Sigma-Aldrich. Co(NO3)3∙6H2O, Se and KOH were supplied by Aladdin. All chemicals were used as received without any further purification. Synthesis of α-Co(OH)2 nanosheet precursor on carbon cloth (α-Co(OH)2/CC). Before electrodeposition, carbon cloths (CC) were activated by cleaning carefully with concentrated HNO3, and then washed in succession with deionized water and ethanol for several times. The deposition was performed in a two-electrode system by a Zahner IM6 electrochemical workstation (Global Analytical Testing Equipment Co., Ltd), using cleaned CC (1×1.6 cm2) as the cathode electrode and platinum wire as the anode electrode. 0.05 M Co(NO3)2·6H2O was used as the electrolyte. A constant cathode current of 8 mA was applied for 20 min. After the reaction, the as-prepared electrode was washed with deionized water, ethanol for several times to remove excess Co2+ and dried at 60 °C for 2 h. Synthesis of porous CoSe2 nanosheets on carbon cloth (p-CoSe2/CC). For the selenization reaction, two pieces of above obtained α-Co(OH)2/CC were put into the graphite box together with 0.4 g selenium powder. The slightly-closed graphite box was then moved to the center of a horizontal tube furnace, which was flushed with high purity 6


Page 6 of 36

Page 7 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ar gas for 10 min to remove air. The furnace was fast heated to 450 °C at a rate of 15 °C min-1 and held for 60 min under a steady flow of Ar at 100 sccm. Finally, the as-prepared p-CoSe2/CC was then annealed at 350 °C in Ar for 30 min to remove redundant Se. The loading amount of CoSe2 on carbon cloth was about 2.3 mg/cm2. Characterization. The powder X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker AXS diffractometer with Cu-Kα radiation. The surface morphologies of samples were studied by scanning electron microscope (SEM) (FEI Quanta-450 FEG). Transmission electron microscope (TEM, JEM-2010HR) and high resolution TEM (HRTEM, 200 kV) were also used to characterize the microstructures of the samples. The XPS spectrum was collected by X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi X-ray photoelectron spectroscopy). All of XPS spectra were corrected using the C 1s line at 284.6 eV. Electrochemical Measurements. All electrochemical tests were performed with a Zanher IM6 electrochemical workstation. The as-prepared p-CoSe2/CC was served directly as the working electrode, while a graphite rod and Hg/Hg2Cl2 (Saturated KCl) electrode were used as the counter electrode and the reference electrode, respectively. All measurements were carried out in 1 M KOH solution, and bubbled with O2 (OER) and N2 (HER) to reach saturation. Prion to recording the polarization curves, the working electrodes were electrochemically activated by using cyclic voltammetry (CV) test at a scan rate of 100 mV/s for some times. Both OER and HER polarization curves were obtained with iR compensation in all electrolyte solution and the resistance was 0.998 ohm 7



for p-CoSe2 NS/CC. CVs in Cdl determination were measured in a potential window nearly without Faradaic process at different scan rates of 20, 40, 60, 80 and 100 mV s-1. The frequency range of electrochemical impedance spectroscopy (EIS) is from 100 kHz to 0.1 Hz at the potential of -0.2 V vs. RHE. The CV cycling tests was performed at a wide potential range between -0.4 V and 0.1 V vs. RHE at a sweep rate of 100 mV s-1 in 1 M KOH solution. The long-term stability test was conducted using chronopotentiometric measurements without compensating iR drop. All the potentials in the text, if not specified, were calibrated to RHE using the following Equation: E (RHE) = E (Hg/HgCl) + 0.242 V + 0.059 pH, and the current density was normalized to the geometrical surface area. The faradaic efficiency was obtained by comparing the amount of gas theoretically calculated and experimentally measured. A home drainage device was set up to collect H2 and O2 were collected, and calculated the moles. The theoretical amount of H2 and O2 was calculated by applying the faraday law. For comparison, the electrocatalytic performance of commercially available IrO2 nanopowders and Pt/C catalysts supported on CC were also measured under the same conditions. The catalysts were dispersed in water/isopropanol solution mixed with Nafion (5 wt%, Sigma) and drop-cast onto the CC. The loading mass of either Pt/C or IrO2 catalysts was about 2.3 mg cm-2, similar to that of CoSe2 on the CC.

RESULTS AND DISCUSSION

8


Page 8 of 36

Page 9 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Schematic illustration of the synthesis of p-CoSe2 on carbon cloth. As shown in Scheme 1, the process could be divided into two steps: (i) growth of αCo(OH)2 nanosheets on CC by electrodepisition at 25 °C, (ii) conversion of α-Co(OH)2/CC to p-CoSe2/CC by reacting with selenium vapor at 450 °C. The crystallographic structure and phase information of the bare CC, α-Co(OH)2/CC and p-CoSe2/CC are confirmed by X-ray powder diffraction (XRD).

As Figure 1a presented, the observed broadened peak

at the angle region of ~25° in all samples is attributed to the diffraction characteristics of CCs, and the peaks at 2θ values of 10.46, 33.74 and 59.08 are respectively indexed to αCo(OH)2 (001), (100) and (110) lattice plane (PDF # 46-0605). Compared with the XRD pattern of α-Co(OH)2 nanosheet, there is no α-Co(OH)2 diffraction peaks in p-CoSe2/CC, suggesting the complete conversion of α-Co(OH)2. Notably, the synthesized p-CoSe2 possess cubic pyrite-structured phase (PDF # 09-0234), which is superior to orthorhombic phase in catalysis.25 In addition, the peaks at 2θ values of 30.48, 34.19, 37.62, 43.69, 51.75, 56.48, 58.85 and 63.44 are respectively indexed to p-CoSe2 (200), (210), (211), (220), 9



(311), (230), (321) and (400) lattice plane.

Figure 1. (a) The XRD patterns of CC, α-Co(OH)2/CC and p-CoSe2/CC. The SEM images of (b) α-Co(OH)2/CC and (c) p-CoSe2/CC, inset: corresponding high-resolution SEM images. (d) The TEM, HRTEM image and (e) corresponding elemental mapping of pCoSe2. In order to investigate the morphology, the SEM images of α-Co(OH)2/CC and p10


Page 10 of 36

Page 11 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CoSe2/CC are displayed in Figure 1b and 1c. As shown in Figure 1b, the entire surface of carbon cloth is fully covered by high-density α-Co(OH)2 nanosheets array. From the inset image, a single nanosheet is approximately 500 nm wide and only has a thickness of 15.8 nm. After selenization, it shows no significant change on their contour, but the thickness of nanosheet arrays becomes thicker (about 33.2 nm) and a large number of pore structures appear on the CoSe2 surface as a result of surface shrinkage (Figure 1c). As can be seen from the side, each p-CoSe2 sheet is assembled by many nanoparticles (Figure S1a). Such porous structure will expose more active sites, thus favoring electrocatalytic activity. Figure 1d displays the TEM image of a single p-CoSe2 nanosheet, which further confirms the porous structure. The high-resolution TEM image exhibits distinct lattice fringe of (210) and (211) plane with lattice spacing of ~0.260 nm and ~0.239 nm respectively, which belong to cubic CoSe2. The accurate value of lattice spacing is shown in Figure S1b. Moreover, the corresponding EDX mapping reveals that the Co and Se elements are uniformly distributed in the entire nanosheet (Figure 1e) and the atomics ratio of Co and Se is almost 1:2 (Figure S1c). All above observations confirm the successful selftemplating construction of porous CoSe2 nanosheet array from corresponding Co(OH)2 precursor. X-ray photoelectron spectroscopy (XPS) measurement was further employed to investigate the chemical composition of the as-prepared p-CoSe2. As shown in Figure S2, the XPS spectrum survey of CoSe2 indicates the presence of Co, Se, O and C without any other impurities, which is consistent with EDS results (Figure S1c). As shown in Figure 11



2a, the two peaks located at 778.4 and 793.3 eV can be assigned to Co 2p3/2 and Co 2p1/2 of CoSe2, respectively.29, 42, 43 Meanwhile, peaks at 780.7 and 796.7 eV are assigned to the Co 2p3/2 and Co 2p1/2 of the Co-O bonding structures at the surface for synthesized CoSe2.26, 44

The binding energies of Se 3d5/2 and Se 3d3/2 at 54.6 and 55.5 eV belong to Se22-, which

corresponds well with previous results of CoSe2 (Figure 2b).44 In addition, The higher binding energy at 59.4 eV and 61.5 eV are attributed to Co 3p and SeOx, respectively.45, 46 The characterizations above demonstrate the successful synthesis of CoSe2 nanosheets.

Figure 2. The XPS spectrum of (a) Co 2p and (b) Se 3d spectra for p-CoSe2 nanosheet. The electrochemical application of obtained porous CoSe2 nanosheet arrays as bifunctional electrocatalysts for overall water splitting is thoroughly characterized. Considering OER as a sluggish kinetics process and a major bottleneck for the water electrolysis,32, 47, 48 the electrocatalytic performance of as-obtained p-CoSe2/CC was first studied as OER electrocatalyst in O2-saturated 1.0 M KOH electrolyte under a threeelectrode glass cell. The electrode was initially conditioned by several linear sweep 12


Page 12 of 36

Page 13 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


voltammetric (LSV) scans (1.1-1.7 V vs. RHE) until a steady state was achieved. Figure 3a shows the iR-corrected LSV curve of the conditioned p-CoSe2/CC electrode recorded at 2 mV s-1 on the RHE scale. For comparison, LSV curves of a bare CC, α-Co(OH)2/CC and IrO2 catalyst supported on CC were measured under the same conditions. The bare CC does not show any obvious anodic current until the potential reaches ≈1.7 V. As a state-ofart OER catalyst, IrO2 exhibits a low overpotential of η = 306 mV at 10 mA cm-2, which is better than that of α-Co(OH)2/CC (316 mV). Even so, p-CoSe2/CC still show a lower overpotential of 243 mV at 10 mA cm-2, which is comparable or better than that of IrO2/CC and reported CoSe2-based catalysts (Table S1). The OER catalytic kinetic was evaluated by the Tafel plots (Figure 3b). The Tafel slope of p-CoSe2/CC is only 82 mV dec-1, slightly higher in comparison to that of IrO2 (67 mV dec-1) and smaller than that of α-Co(OH)2/CC (96 mV dec-1). The low Tafel slope of p-CoSe2/CC implies the favorable OER kinetic in alkaline solution.

13



Figure 3. (a) LSV curves of the CC, α-Co(OH)2/CC, p-CoSe2/CC, and IrO2 on CC with a scan rate of 2 mV s-1 for OER in 1.0 M KOH. (b) The corresponding Tafel plots derived from (a). (c) Chronopotentiometric curve of the p-CoSe2/CC with constant current density of 10 mA cm-2. Inset: the polarization curves of the p-CoSe2/CC before and after 1000 cycles. (d) Multistep chronopotentiometric curves of the conditioned p-CoSe2/CC at varying current densities without iR-correction.

Moreover, the self-supported porous CoSe2 nanosheets on CC also endow p-CoSe2/CC electrode with robust long-term electrochemical stability. As displayed Fig 3c, the chronopotentiometric (CP) curve of the obtained p-CoSe2/CC electrode was measured at a current density of 10 mA cm-2. Upon application of the anodic current, the potential rapidly 14


Page 14 of 36

Page 15 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


increases to 1.48 V (without iR-corrected) within 1 min. After 20 h later, the potential only goes up to 1.49 V. Moreover, as shown in the inset of the CP, the polarization curves of pCoSe2/CC almost overlaps with the initial one after the CV scanning for 1000 cycles (1.11.7 V vs. RHE). Figure 3d illustrates CP curves of the conditioned p-CoSe2/CC recorded under varying current densities ranging from 10 to 500 mA cm-2 without iR-correction. Upon increasing the current density, the potential of the conditioned p-CoSe2/CC electrode rises accordingly but gets stabilized quickly, indicating outstanding mass transfer property and mechanical robustness of the p-CoSe2/CC electrode.48, 49 Apart from excellent OER activity, the obtained p-CoSe2/CC electrode also exhibits outstanding HER performance under alkaline condition. The HER electrocatalytic performance of the p-CoSe2/CC electrode was evaluated in N2-saturated 1.0 M KOH. Commercial Pt/C, bare CC and α-Co(OH)2/CC were also used for comparison. Figure 4a shows the LSV polarization curves of different catalyst electrodes for HER at a scan rate of 2 mV s-1. Pt/C undoubtedly exhibits the highest catalytic activity towards HER with near 0 V vs. RHE onset potential and low overpotential of 37 mV at -10 mA cm-2. The pCoSe2/CC sample requires a much lower overpotential of 138mV to reach a current density of -10 mA cm-2 in comparison with α-Co(OH)2/CC precursor (η = 263 mV). Moreover, the HER overpotential of p-CoSe2/CC is superior or comparable with that of other CoSe2 catalysts ever reported, as shown in Table S1. Figure 4b presents the Tafel plots derived from the fitted polarization curves. p-CoSe2/CC possesses a favorable HER reaction kinetics with a small Tafel value of 83 mV dec-1, which is comparable to that of commercial 15



Pt/C (51 mV dec-1), but much smaller than those of α-Co(OH)2/CC (121 mV dec-1). Moreover, the Tafel slope of p-CoSe2/CC electrode is among the value of 38-116 mV dec-1, suggesting that the HER process may dominated by Volmer-Heyrovsky mechanism.50, 51

Figure 4. (a) LSV curves of the CC, α-Co(OH)2/CC, p-CoSe2/CC, and IrO2 on CC with a scan rate of 2 mV s-1 for HER in 1.0 M KOH. (b) The corresponding Tafel plots derived from (a). (c)

Chronopotentiometric curve of the p-CoSe2/CC with constant current

density of -10 mA cm-2. Inset: the polarization curves of the p-CoSe2/CC before and after 1000 cycles. (d) Multistep chronopotentiometric curves of the conditioned p-CoSe2/CC at varying current densities without iR-correction.

The long-term durability of p-CoSe2/CC electrode under HER measurement was also 16


Page 16 of 36

Page 17 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


evaluated at an current density of -10 mA cm-2, as shown in Figure 4c. The HER potential of the electrode remains nearly unchanged after 20 h of continuous test, indicating the electrocatalytic stability of the p-CoSe2/CC electrode. To further examine the stability, the continuous CV scan between -0.3 and −0.1 V (versus RHE) with a scan rate of 100 mV s−1 in 1.0 M KOH for 1000 cycles were investigated. Compared with the initial one, the polarization curve of the p-CoSe2/CC electrode after 1000 CV cycles (inset of Figure 4c) has the negligible difference, suggesting the outstanding stability of the electrodes under HER condition. Figure 4d shows a multi-step chronopotentiometric curve of p-CoSe2/CC with the current increasing from 10 to 500 mA cm-2 . The potential immediately levels off at -0.145V at the initial current value and remains stable for the remaining 500 s, and the other steps also show similar results, which is consistent with the OER results. To explore the HER kinetics, EIS investigations were conducted at an applied overpotential of 200 mV in 1.0 M KOH. The charge-transfer resistance (Rct) obtained from the semicircle in the high frequency zone is related to the electrocatalytic kinetics at the interface between electrocatalyst and electrolyte, and a lower value corresponds to a faster electron transfer rate.32,

52

The p-CoSe2 electrode showed an Rct value of 6.5 Ω at

overpotential of 200 mV (Figure 5a), which is smaller than that of α-Co(OH)2 electrode (24.5 Ω) and CC electrode (47.2 Ω), indicating that the p-CoSe2 has a better charge-transfer capacity. Similarly ， p-CoSe2/CC also possesses the lower charge transfer resistances under OER condition (Figure S3a). As shown in Figure 5b, the double-layer capacitances of p-CoSe2/CC were measured in the CV potential range, where no apparent Faradaic 17



process occurred (Figure S4). The specific capacitances obtained from the slope of linear fitting were 83.3 mF cm-2 (HER) and 78 mF cm-2 (OER) (Figure S3b), significantly larger than that of CC (0.12 mF cm-2 for HER, 2.4 mF cm-2 for OER) and α-Co(OH)2/CC (14.9 mF cm-2 for HER, 26.9 mF cm-2 for OER). The large electrochemical area is associated with more active sites at the solid-liquid interface and ultrahigh electrocatalytic activity.

Figure 5. (a) Nyquist plots of of the CC, α-Co(OH)2/CC and p-CoSe2/CC for HER tested at -0.2 V (vs. RHE) and (b) the double layer capacity Cdl for CC, α-Co(OH)2/CC and pCoSe2/CC.

While recently, increasingly studies have demonstrated that non-oxide based transition metal (TM) catalysts would undergo an in-situ electrochemical transformation under OER operational conditions converting to transition metal (oxy)hydroxide, which is proposed to be the true catalytically active species for the OER.36, 53, 54 Moreover, in most cases, the in situ transformed TM (oxy) hydroxide exhibits even better OER performance than the pristine TM (oxy)hydroxide obtained by direct synthesis,54-56 which may be due to surface 18


Page 18 of 36

Page 19 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reorganization that exposes more active sites.36 In order to further gain insights into the reaction mechanism, the p-CoSe2 electrode was characterized after CP in 10 mA cm-2 for 20 h. For the sample, after long-term HER measurement, the morphology and the crystalline remained nearly identical as before, which was evidenced by SEM images and XRD pattern (Figure S5a, b). As displays in Figure S5c, The atomic content of Co, Se and O is almost the same as the pristine sample. After long-term OER measurement, the surface of p-CoSe2 nanosheet was slightly damaged and still maintains its original morphology (Figure S6a). However, as shown in Figure S6b, the crystallinity of CoSe2 was greatly reduced, only weak (211) peak can be seen in the XRD pattern, no other peaks appeared, indicating that CoSe2 has been converted into amorphous phase. EDS spectra (Figure S6c) displays that the content of O increases obviously, while the Se content decreases, indicating the oxidation of catalyst surface after OER measurement.57 Comparing the XPS spectra before and after long-term OER measurement (Figure S6d, e), the binging energy position of Co 2p3/2 peak is shifted to a higher energy that can be assigned to Co 2p3/2 in Co-(oxy)hydroxide,36 meanwhile, the intensity of the Se peak decreases and virtually vanishes after 20 h OER. These results unambiguously demonstrate that CoSe2 was almost completely converted to amorphous Co-(oxy)hydroxide after the OER electrolysis at 10 mA cm-2 for 20 h, which is the true active species for catalyzing the OER during the longterm operation. On account of its robust electrocatalytic activity and stability for both HER and OER in strongly alkaline media, the p-CoSe2/CC electrode exhibits potential as an idea non19



noble bifunctional electrocatalysts for overall water splitting. Figure 6a shows the polarization curve of p-CoSe2/CC electrode measured in a two-electrode configuration without iR-correction (i.e., including real resistive loss). Two pieces of carbon cloth grown with p-CoSe2 act as both anode and cathode. As shown in the Figure 6b, obvious gas evolution was observed at both electrodes from the photographic image of the electrolysis cell for overall water splitting during electrolysis. Moreover, p-CoSe2/CC-based bifunctional electrocatalyst exhibits the significant performance with cell voltage of only 1.62 V at a current density of 10 mA cm-2, which is lower than most of the reported results (Table S1). Long-term electrocatalytic stability of the p-CoSe2/CC electrode was evaluated by galvanostatic water electrolysis at 10 mA cm-2 (Figure 6c). Impressively, the electrolyzer was able to operate at 10 mA cm-2 over 20 h with negligible degradation (from 1.62V to 1.64V). The superior operational stability suggests that the monolithic pCoSe2/CC electrode is a promising candidate for practical electrochemical water splitting. Furthermore, the faradaic efficiency of p-CoSe2/CC electrode are almost 100% for both HER and OER with the 2:1 molar ratio of H2 and O2 in alkaline media (Figure 6 d).

20


Page 20 of 36

Page 21 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Overall water splitting performance of the two-electrode electrolyzers. (a) Polarization curves of p-CoSe2/CC. (b) Optical photograph showing the generation of H2 and O2 bubbles on the p-CoSe2/CC electrode. (c) Chronopotentiometric curve of the pCoSe2/CC with constant current density of 10 mA cm-2. (d) The amount of gas theoretically calculated and experimentally measured versus time for overall water splitting of pCoSe2/CC.

CONCLUSIONS In summary，the 3D self-templated construction of porous CoSe2 nanosheet arrays have been reported via a facile vapor selenization treatment method. The p-CoSe2/CC electrode 21



exhibits superior activity and stability in basic media for both OER and HER. This excellent performance may be ascribed to two reasons: (i) the 3D porous nanosheets endow them with large active surface area, thus exposing abundant active sites;58, 59 (ii) directly growing on carbon cloth endows the p-CoSe2/CC electrode efficient electron conductivity and mass transport.60, 61 Moreover, this cost-effective 3D self-templated construction may point a new avenue to design and explore other novel hybrid materials as efficient catalysts for renewable energy applications.

ASSOCIATED CONTENT Supporting Information The SEM image, screenshot of ten times for lattice spacing, and EDS of p-CoSe2/CC (Figure S1); The XPS survey spectrum of p-CoSe2/CC (Figure S2); Nyquist plots of CC, α-Co(OH)2/CC and p-CoSe2/CC for OER, the double layer capacity Cdl for p-CoSe2/CC (Figure S3); Different scan rates of CV for OER and HER (FigureS4). The SEM image, XRD pattern, EDS and XPS spectra of p-CoSe2 after long term HER in -10 mA cm-2 for 20 h (Figure S5); The SEM image, XRD pattern, EDS and XPS spectra of p-CoSe2 after long term OER in 10 mA cm-2 for 20 h (Figure S6) ； Comparison of HER and OER performances of reported selenides in 1.0 M KOH for water splitting (Table S1).

AUTHOR INFORMATION Corresponding Authors 22


Page 22 of 36

Page 23 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


E-mail:

[email protected] (Jun Zhang)

E-mail:

[email protected] (Hui Liu)

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (No. 51402085), Nature Science Foundation of Hebei Province (No. E2015202295 and B2016202213) and Nature Science Foundation of Tianjin City (No. 16JCYBJC17500).

23



REFERENCES (1) Meyer, T. J. The art of splitting water. Nature. 2008, 451, 778-779. DOI: 10.1038/451778a. (2) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of electrocatalysts for oxygen-and hydrogeninvolving energy conversion reactions. Chem. Soc. Rev. 2015, 44, 2060-2086. DOI: 10.1039/C4CS00470A. (3) Danilovic, N.; Subbaraman, R.; Chang, K. C.; Chang, S. H.; Kang, Y.; Snyder, J.; Paulikas, A. P.; Strmcnik, D.; Kim, Y. T.; Myers, D. Using surface segregation to design stable Ru‐Ir oxides for the oxygen evolution reaction in acidic environments. Angew Chem. Int. Ed. Engl. 2014, 53, 14016-14021. DOI: 10.1002/anie.201406455. (4) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and activities of rutile IrO2 and RuO2 nanoparticles for oxygen evolution in acid and alkaline solutions. J. Phy. Chem. Lett. 2012, 3, 399404. DOI: 10.1021/jz2016507. (5) Leng, Y.; Chen, G.; Mendoza, A. J.; Tighe, T. B.; Hickner, M. A.; Wang, C.-Y. Solid-state Water electrolysis with an alkaline membrane. J. Am. Chem. Soc. 2012, 134, 9054-9057. DOI: 10.1021/ja302439z. (6) Wang, J.; Cui, W.; Liu, Q.; Xing, Z.; Asiri, A. M.; Sun, X. Recent progress in cobalt-based heterogeneous catalysts for electrochemical water splitting. Adv. Mater. 2016, 28, 215-230. DOI: 10.1002/adma.201502696. (7) Vij, V.; Sultan, S.; Harzandi, A. M.; Meena, A.; Tiwari, J. N.; Lee, W.-G.; Yoon, T.; Kim, K. S. Nickelbased electrocatalysts for energy-related applications: oxygen reduction, oxygen evolution, and hydrogen evolution reactions. ACS Catal. 2017, 7, 7196-7225. DOI: 10.1021/acscatal.7b01800. (8) Hulley, E. B.; Kumar, N.; Raugei, S.; Bullock, R. M. Manganese-based molecular electrocatalysts for oxidation of hydrogen. ACS Catal. 2015, 5, 6838-6847. DOI: 10.1021/acscatal.5b01751. (9) Anantharaj, S.; Ede, S. R.; Sakthikumar, K.; Karthick, K.; Mishra, S.; Kundu, S. Recent trends and perspectives in electrochemical water splitting with an emphasis on sulfide, selenide, and phosphide catalysts of Fe, Co, and Ni: A review. ACS Catal. 2016, 6, 8069-8097. DOI: 10.1021/acscatal.6b02479. (10) Faber, M. S.; Jin, S. Earth-abundant inorganic electrocatalysts and their nanostructures for energy conversion applications. Energy Environ. Sci. 2014, 7, 3519-3542. DOI: 10.1039/C4EE01760A. (11) Jiao, F.; Frei, H. Nanostructured cobalt and manganese oxide clusters as efficient water oxidation catalysts. Energy Environ. Sci. 2010, 3, 1018-1027. DOI: 10.1039/C002074E. (12) Zhou, M.; Weng, Q.; Popov, Z. I.; Yang, Y.; Antipina, L. Y.; Sorokin, P. B.; Wang, X.; Bando, Y.; Golberg, D. Construction of polarized carbon–nickel catalytic surfaces for potent, durable, and economic hydrogen evolution reactions. ACS Nano. 2018, 12, 4148-4155. DOI: 10.1021/acsnano.7b08724. (13) Kim, J. S.; Kim, B.; Kim, H.; Kang, K. Recent progress on multimetal oxide catalysts for the oxygen evolution reaction. Adv. Energy Mater. 2018, 8, 1702774-1702799. DOI: 10.1002/aenm.201702774. (14) Lu, F.; Zhou, M.; Zhou, Y.; Zeng, X. First-row transition metal based catalysts for the oxygen evolution reaction under alkaline conditions: basic principles and recent advances. Small. 2017, 13, 1701931-1701948. DOI: 10.1002/smll.201701931. (15) Han, N.; Yang, K. R.; Lu, Z.; Li, Y.; Xu, W.; Gao, T.; Cai, Z.; Zhang, Y.; Batista, V. S.; Liu, W.; Sun, X. Nitrogen-doped tungsten carbide nanoarray as an efficient bifunctional electrocatalyst for water splitting in acid. Nat. Commun. 2018, 9, 924-933. DOI: 10.1038/s41467-018-03429-z. (16) Zhang, J.; Wang, Y.; Zhang, C.; Gao, H.; Lv, L.; Han, L.; Zhang, Z. Self-supported porous NiSe2 nanowrinkles as efficient bifunctional electrocatalysts for overall water splitting. ACS Sustainable Chem. Eng. 2018, 6, 2231-2239. DOI: 10.1021/acssuschemeng.7b03657. 24


Page 24 of 36

Page 25 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17) Yan, Y.; Xia, B. Y.; Zhao, B.; Wang, X. A review on noble-metal-free bifunctional heterogeneous catalysts for overall electrochemical water splitting. J. Mater. Chem. A. 2016, 4, 17587-17603. DOI: 10.1039/C6TA08075H. (18) Wang, H.; Lee, H.-W.; Deng, Y.; Lu, Z.; Hsu, P.-C.; Liu, Y.; Lin, D.; Cui, Y. Bifunctional non-noble metal oxide nanoparticle electrocatalysts through lithium-induced conversion for overall water splitting. Nat. Commun. 2015, 6, 7261. DOI: 10.1038/ncomms8261. (19) Kim, H. W.; Oh, S. K.; Cho, E. A.; Kwon, H. S. A 3D porous cobalt-iron-phosphorous bifunctional electrocatalyst for the oxygen and hydrogen evolution reactions. ACS Sustainable Chem. Eng. 2018, 63056311. DOI: 10.1021/acssuschemeng.8b00118 (20) Ren, J. T.; Yuan, Z. Y. Hierarchical nickel sulfide nanosheets directly grown on Ni foam: A stable and efficient electrocatalyst for water reduction and oxidation in alkaline medium. ACS Sustainable Chem. Eng. 2017, 5, 7203-7210. DOI: 10.1021/acssuschemeng.7b01419. (21) Kong, D.; Cha, J. J.; Wang, H.; Lee, H. R.; Cui, Y. First-row transition metal dichalcogenide catalysts for hydrogen evolution reaction. Energy Environ. Sci. 2013, 6, 3553-3558. DOI: 10.1039/C3EE42413H. (22) Zheng, Y.-R.; Wu, P.; Gao, M.-R.; Zhang, X.-L.; Gao, F.-Y.; Ju, H.-X.; Wu, R.; Gao, Q.; You, R.; Huang, W.-X.; Liu, S.-J.; Hu, S.-W.; Zhu, J.; Li, Z.; Yu, S.-H. Doping-induced structural phase transition in cobalt diselenide enables enhanced hydrogen evolution catalysis. Nat. Commun. 2018, 9, 2533. DOI: 10.1038/s41467-018-04954-7. (23) Kong, D.; Wang, H.; Lu, Z.; Cui, Y. CoSe2 nanoparticles grown on carbon fiber paper: an efficient and stable electrocatalyst for hydrogen evolution reaction. J. Am. Chem. Soc. 2014, 136, 4897-4900. DOI: 10.1021/ja501497n. (24) Liu, Y.; Cheng, H.; Lyu, M.; Fan, S.; Liu, Q.; Zhang, W.; Zhi, Y.; Wang, C.; Xiao, C.; Wei, S.; Ye, B.; Xie, Y. Low overpotential in vacancy-rich ultrathin CoSe2 nanosheets for water oxidation. J. Am. Chem. Soc. 2014, 136, 15670-15675. DOI: 10.1021/ja5085157. (25) Chen, P.; Xu, K.; Tao, S.; Zhou, T.; Tong, Y.; Ding, H.; Zhang, L.; Chu, W.; Wu, C.; Xie, Y. Phasetransformation engineering in cobalt diselenide realizing enhanced catalytic activity for hydrogen evolution in an alkaline medium. Adv. Mater. 2016, 28, 7527-7532. DOI: 10.1002/adma.201601663. (26) Kwak, I. H.; Im, H. S.; Jang, D. M.; Kim, Y. W.; Park, K.; Lim, Y. R.; Cha, E. H.; Park, J. CoSe2 and NiSe2 nanocrystals as superior bifunctional catalysts for electrochemical and photoelectrochemical water splitting. ACS Appl. Mater. Interfaces. 2016, 8, 5327-5334. DOI: 10.1021/acsami.5b12093. (27) Xu, X.; Liang, H.; Ming, F.; Qi, Z.; Xie, Y.; Wang, Z. Prussian blue analogues derived penroseite (Ni,Co)Se2 nanocages anchored on 3D graphene aerogel for efficient water splitting. ACS Catal. 2017, 7, 6394-6399. DOI: 10.1021/acscatal.7b02079. (28) Wang, X.; Li, F.; Li, W.; Gao, W.; Tang, Y.; Li, R. Hollow bimetallic cobalt-based selenide polyhedrons derived from metal-organic framework: an efficient bifunctional electrocatalyst for overall water splitting. J. Mater. Chem. A. 2017, 5, 17982-17989. DOI: 10.1039/C7TA03167J. (29) Liu, K.; Wang, F.; Shifa, T. A.; Wang, Z.; Xu, K.; Zhang, Y.; Cheng, Z.; Zhan, X.; He, J. An efficient ternary CoP2xSe2(1-x) nanowire array for overall water splitting. Nanoscale. 2017, 9, 3995-4001. DOI: 10.1039/C7NR00460E. (30) Ling, F.; Wenxiang, L.; Yongxin, G.; Yangyang, F.; Huijuan, Z.; Shilong, W.; Yu, W. Tuning unique peapod-like Co(SxSe1-x)2 nanoparticles for efficient overall water splitting. Adv. Funct. Mater. 2017, 27, 25



1701008. DOI: 10.1002/adfm.201701008. (31) Sun, C.; Dong, Q.; Yang, J.; Dai, Z.; Lin, J.; Chen, P.; Huang, W.; Dong, X. Metal-organic framework derived CoSe2 nanoparticles anchored on carbon fibers as bifunctional electrocatalysts for efficient overall water splitting. Nano Res. 2016, 9, 2234-2243. DOI: 10.1007/s12274-016-1110-1. (32) Guo, Y.; Yao, Z.; Shang, C.; Wang, E. Amorphous Co2B grown on CoSe2 nanosheets as a hybrid catalyst for efficient overall water splitting in alkaline medium. ACS Appl. Mater. Interfaces. 2017, 9, 39312-39317. DOI: 10.1021/acsami.7b10605. (33) Kaidan, L.; Jingfang, Z.; Rui, W.; Yifu, Y.; Bin, Z. Anchoring CoO domains on CoSe2 nanobelts as bifunctional electrocatalysts for overall water splitting in neutral media. Adv. Sci. 2016, 3, 1500426. DOI: 10.1002/advs.201500426. (34) Lee, C.-P.; Chen, W.-F.; Billo, T.; Lin, Y.-G.; Fu, F.-Y.; Samireddi, S.; Lee, C.-H.; Hwang, J.-S.; Chen, K.-H.; Chen, L.-C. Beaded stream-like CoSe2 nanoneedle array for efficient hydrogen evolution electrocatalysis. J. Mater. Chem. A. 2016, 4, 4553-4561. DOI: 10.1039/C6TA00464D. (35) Zheng, Y.-R.; Gao, M.-R.; Yu, Z.-Y.; Gao, Q.; Gao, H.-L.; Yu, S.-H. Cobalt diselenide nanobelts grafted on carbon fiber felt: an efficient and robust 3D cathode for hydrogen production. Chem. Sci. 2015, 6, 45944598. DOI: 10.1039/C5SC01335F. (36) Dai, C.; Tian, X.; Nie, Y.; Tian, C.; Yang, C.; Zhou, Z.; Li, Y.; Gao, X. Successful synthesis of 3D CoSe2 hollow microspheres with high surface roughness and its excellent performance in catalytic hydrogen evolution reaction. Chem. Eng. J. 2017, 321, 105-112. DOI: 10.1016/j.cej.2017.03.068. (37) Chen, T.; Li, S.; Wen, J.; Gui, P.; Fang, G. Metal-organic framework template derived porous CoSe2 nanosheet arrays for energy conversion and storage. ACS Appl. Mater. Interfaces. 2017, 9, 35927-35935. DOI: 10.1021/acsami.7b12403. (38) Li, Z.; Niu, W.; Zhou, L.; Yang, Y. Phosphorus and aluminum codoped porous NiO nanosheets as highly efficient electrocatalysts for overall water splitting. ACS Energy Lett. 2018, 3, 892-898. DOI: 10.1021/acsenergylett.8b00174. (39) Yanshuo, J.; Haotian, W.; Junjie, L.; Xin, Y.; Yujie, H.; Kang, S. P.; Yi, C. Porous MoO2 nanosheets as non-noble bifunctional electrocatalysts for overall water splitting. Adv. Mater. 2016, 28, 3785-3790. DOI: 10.1002/adma.201506314. (40) Sheng, C.; Jingjing, D.; Pengju, B.; Youhong, T.; Rongkun, Z.; Shi-Zhang, Q. Three-dimensional smart catalyst electrode for oxygen evolution reaction. Adv. Energy Mater. 2015, 5, 1500936. DOI: 10.1002/aenm.201500936. (41) Wenxin, Z.; Rong, Z.; Fengli, Q.; M., A. A.; Xuping, S. Design and application of foams for electrocatalysis. ChemCatChem. 2017, 9, 1721-1743. DOI: 10.1002/cctc.201601607. (42) Zhao, X.; Zhang, H.; Yan, Y.; Cao, J.; Li, X.; Zhou, S.; Peng, Z.; Zeng, J. Engineering the electrical conductivity of lamellar silver-doped cobalt(II) selenide nanobelts for enhanced oxygen evolution. Angew. Chem. 2017, 56, 328. DOI: 10.1002/anie.201609080. (43) Fang, L.; Li, W.; Guan, Y.; Feng, Y.; Zhang, H.; Wang, S.; Wang, Y. Tuning unique peapod-like Co(SxSe1-x)2 nanoparticles for efficient overall water splitting. Adv. Funct. Mater. 2017, 27. DOI: 10.1002/adfm.201701008. (44) Chen, T.; Li, S.; Gui, P.; Wen, J.; Fu, X.; Fang, G. Bifunctional bamboo-like CoSe2 arrays for highperformance asymmetric supercapacitor and electrocatalytic oxygen evolution. Nanotechnology. 2018, 29. 26


Page 26 of 36

Page 27 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


DOI: 10.1088/1361-6528/aab19b. (45) Xia, C.; Jiang, Q.; Zhao, C.; Hedhili, M. N.; Alshareef, H. N. Selenide-based electrocatalysts and scaffolds for water oxidation applications. Adv. Mater. 2016, 28, 77-85. DOI: 10.1002/adma.201503906. (46) Liu, X.; Liu, Y.; Fan, L. Z. MOF-derived CoSe2 microspheres with hollow interiors as high-performance electrocatalysts for enhanced oxygen evolution reaction. J. Mater. Chem. A. 2017, 5, 15310-15314 DOI: 10.1039/C7TA04662F. (47) Han, L.; Dong, S.; Wang, E. Transition-metal (Co, Ni, and Fe)-based electrocatalysts for the water oxidation reaction. Adv. Mater. 2016, 28, 9266-9291. DOI: 10.1002/adma.201602270. (48) Li, W.; Gao, X.; Xiong, D.; Wei, F.; Song, W. G.; Xu, J.; Liu, L. Hydrothermal synthesis of monolithic Co3Se4 nanowire electrodes for oxygen evolution and overall water splitting with high efficiency and extraordinary catalytic stability. Adv. Energy Mater. 2017, 7, 1602579. DOI: 10.1002/aenm.201602579. (49) Tang, C.; Cheng, N.; Pu, Z.; Xing, W.; Sun, X. NiSe nanowire film supported on nickel foam: An efficient and stable 3D bifunctional electrode for full water splitting. Angew. Chem. Int. Ed. 2015, 54, 93519355. DOI: 10.1002/anie.201503407. (50) Zhou, H.; Wang, Y.; He, R.; Yu, F.; Sun, J.; Wang, F.; Lan, Y.; Ren, Z.; Chen, S. One-step synthesis of self-supported porous NiSe2/Ni hybrid foam: An efficient 3D electrode for hydrogen evolution reaction. Nano Energy. 2016, 20, 29-36. DOI: 10.1016/j.nanoen.2015.12.008. (51) Conway, B. E.; Tilak, B. V. Interfacial processes involving electrocatalytic evolution and oxidation of H2, and the role of chemisorbed H. Electrochim. Acta. 2002, 47, 3571-3594. DOI: 10.1016/S00134686(02)00329-8. (52) Li, K.; Zhang, J.; Wu, R.; Yu, Y.; Zhang, B. Anchoring CoO Domains on CoSe2 nanobelts as bifunctional electrocatalysts for overall water splitting in neutral media. Adv. Sci. 2016, 3. DOI: 10.1002/advs.201500426. (53) Wang, X.; Li, W.; Xiong, D.; Liu, L. Fast fabrication of self-supported porous nickel phosphide foam for efficient, durable oxygen evolution and overall water splitting. J. Mater. Chem. A. 2016, 4, 5639-5646. DOI: 10.1039/C5TA10317G. (54) Xu, X.; Song, F.; Hu, X. A nickel iron diselenide-derived efficient oxygen-evolution catalyst. Nat. Commun. 2016, 7, 12324. DOI: 10.1038/ncomms12324. (55) Feng, L.; Song, F.; Hu, X. Ni2P as a Janus catalyst for water splitting: the oxygen evolution activity of Ni2P nanoparticles. Energy Environ. Sci. 2015, 8, 2347-2351. DOI: 10.1039/C5EE01155H. (56) Chen, W.; Wang, H.; Li, Y.; Liu, Y.; Sun, J.; Lee, S.; Lee, J. S.; Cui, Y. In situ electrochemical oxidation tuning of transition metal disulfides to oxides for enhanced water oxidation. Acs Cent. Sci. 2015, 1, 244-251. DOI: 10.1021/acscentsci.5b00227. (57) Sun, Y.; Xu, K.; Wei, Z.; Li, H.; Zhang, T.; Li, X.; Cai, W.; Ma, J.; Fan, H. J.; Li, Y. Strong electronic interaction in dual-cation-incorporated NiSe2 nanosheets with lattice distortion for highly efficient overall water splitting. Adv. Mater. 2018, 30. DOI: 10.1002/adma.201802121. (58) Zhang, Q.; Ye, C.; Li, X. L.; Deng, Y. H.; Tao, B. X.; Xiao, W.; Li, L. J.; Li, N. B.; Luo, H. Q. Selfinterconnected porous networks of NiCo disulfide as efficient bifunctional electrocatalysts for overall water splitting. ACS Appl. Mater. Interfaces. 2018, 10, 27723-27733. DOI: 10.1021/acsami.8b04386. (59) Bae, S.-H.; Kim, J.-E.; Randriamahazaka, H.; Moon, S.-Y.; Park, J.-Y.; Oh, I.-K. Seamlessly conductive 3D nanoarchitecture of core-shell Ni-Co nanowire network for highly efficient oxygen evolution. Adv. 27



Energy Mater. 2017, 7. DOI: 10.1002/aenm.201601492. (60) Wang, A.-L.; Xu, H.; Li, G.-R. NiCoFe layered triple hydroxides with porous structures as highperformance electrocatalysts for overall water splitting. ACS Energy Lett. 2016, 1, 445-453. DOI: 10.1021/acsenergylett.6b00219. (61) Wang, Y.; Yan, D.; El Hankari, S.; Zou, Y.; Wang, S. Recent progress on layered double hydroxides and their derivatives for electrocatalytic water splitting. Adv. Sci. 2018, 5. DOI: 10.1002/advs.201800064.

28


Page 28 of 36

Page 29 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Use Only:

Porous CoSe2 Nanosheet Arrays on carbon cloth directly by vapor selenizing work as efficient bifunctional electrodes for full water splitting.

29



Scheme 1

1


Page 30 of 36

Page 31 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1

2



Figure 2

3


Page 32 of 36

Page 33 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3

4



Figure 4

5


Page 34 of 36

Page 35 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5

6



Figure 6

7


Page 36 of 36

Self-Templating Construction of Porous CoSe2 Nanosheet Arrays as

Recommend Documents