Ultrathin Co3O4 Nanomeshes for the Oxygen Evolution Reaction

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Ultrathin Co3O4 Nanomeshes for the Oxygen Evolution Reaction Ying Li, Fu-min Li, Xin-Ying Meng, Shu-ni Li, Jing-Hui Zeng, and Yu Chen ACS Catal., Just Accepted Manuscript • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018

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Ultrathin Co3O4 Nanomeshes for the Oxygen Evolution Reaction Ying Li,†,⊥ Fu-Min Li,†,⊥ Xin-Ying Meng,‡ Shu-Ni Li,*,† Jing-Hui Zeng,‡ and Yu Chen,*,‡ † Key

Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, PR China.

‡ Key

Laboratory of Applied Surface and Colloid Chemistry (MOE), Shaanxi Key Laboratory for Advanced Energy Devices, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, PR China.

ABSTRACT: Ultrathin transition metal-based nanomeshes can perfectly combine the advantages of two-dimensionally (2D) ultrathin nanosheets and porous nanostructures, which have the wide applications in energy storage and conversion. In this work, we present an etch-free one-step approach to directly synthesize the ultrathin Co3O4 nanomeshes (Co-UNMs) by employing the CoCl2/K3Co(CN)6 cyanogel as reaction precursor. The 2D planar structural unit and solid property of the cyanogel result in the preferential assembly of generated crystal nucleus at solid-liquid interface (i.e., cyanogel-solution interface) in 2D direction, which plays a key role in the formation of nanomeshes. The as-prepared Co-UNMs with 1.5 nm thickness and abundant pores own the high surface area and numerous defected atoms, resulting in enhanced activity for the oxygen evolution reaction (OER) in the alkaline media, such as the low overpotential of 307 mV at 10 mA cm−2, small Tafel slope of 76 mV dec−1, and attractive durability in 1 M KOH electrolyte.

KEYWORDS: nanomeshes,

nanosheets, cyanogel, electrolysis, oxygen evolution reaction

The oxygen evolution reaction (OER) in electrolysis has been recognized as an important half reaction for the hydrogen production, carbon dioxide electroreduction, nitrogen electroreduction, and metal-air batteries because the kinetically sluggish OER significantly affects the overall reaction efficiency.1-3 Thus, the developement of highly efficient electrocatalysts for the OER will no doubt promote the industrialization of these industries. Currently, the Rubased and Ir-based nanomaterials show the best electrocatalytic performance for the OER, but thier practical application is hindered by the high price and rarity of these noble metals.4-5 More recently, the low-cost transition metals (such as Co, Ni, Fe, and Mn) based nanomaterials have emerged as highly promising Ru/Ir-alternative electrocatalysts for the OER due to their competitive activity in alkaline media.6-25 Triggered by the great success of the two-dimensional (2D) graphene, the ultrathin transition metals-based nanosheets have drawn extensive attention in recent years. In principle, the atomically thick ultrathin nanosheets not only supply the big specific surface area but also afford the numerous low-coordination atoms.26-29 As a result, the atomically thick ultrathin transition metals-based nanosheets (such as NiCo2O4 nanosheets, CoMoO4 nanosheets, Ni-Fe hydroxide nanosheets, Co-Fe hydroxide nanosheets, Co hierarchical nanosheets, Ni3S2 nanosheets, CoSe2 nanosheets, NiSe2 nanosheets, and Fe-Co oxide nanosheets) generally have the high electrocatalytic activity for the OER in alkaline media.30-41 Unfortunately, like

graphene nanosheets, the full utilization of ultrathin nanosheets may be limited by the low cross-plane substrate diffusion during the electrochemical reaction due to the big sheet aspect ratio. Compared to the ultrathin nanosheets, the ultrathin nanomeshes (UNMs, in-plane pores in ultrathin nanosheets) have the more remarkable advantages in the electrocatalysis.42-49 The unique porous structure of the UNM can provide more “space” for the mass transportation. Meanwhile, the existence of the holes inevitably generates the numerous defected atoms (i.e., the edge sites) with high reactivity, thus improving electrocatalytic activity. For example, Cui's group synthesized the porous LiCoO2 nanomeshes by acid etching method.48 The porous LiCoO2 nanomeshes revealed enhanced electrocatalytic activity for the OER compared to LiCoO2 nanosheets without pores, indicating the edge sites at nanosheets are more active than basal plane sites at nanosheets. Wang's group synthesized ultrathin CoFe nanomeshes with multiple vacancies by Ar plasma etching.49 The formation of multiple vacancies effectively enhanced the number of defected atoms and tuned simultaneously the electronic structure, which improved its intrinsic activity for the OER. Zhang's group synthesized the composition-tunable Ni1-xCoxSe2 mesoporous nanomeshes by selenization reaction and subsequent acid etching, which achieved a very small overpotential for the hydrogen evolution reaction in alkaline media.44 Xie's group synthesized the ultrathin βNi(OH)2 nanomeshes by an etching-intralayered Ostwald ripening route.45 Benefitting from the abundant nanopores,

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ultrathin β-Ni(OH)2 nanomeshes provided the rich defected atoms for the OER, resulting in the high OER activity. Although transition metals-based nanomeshes have wide and advanced application prospects in the catalysis/electrocatalysis and energy storage/conversion, their synthesis still remains elusive.45 As aforementioned, the present synthetic methods for the transition metalsbased nanomeshes generally suffer from the tedious and harsh etch procedures. Herein, we develop a facile surfactant-free cyanogel−NaBH4 method to directly synthesize the ultrathin Co3O4 nanomeshes (termed as CoUNMs). The 2D planar structural unit in the CoCl2/K3Co(CN)6 cyanogel and subsequent crystal nucleus assembly at the cyanogel-solution interface in 2D direction play the important roles for the generation of such intriguing architecture. Benefiting from the clean surface, big surface area, abundant pores, and numerous edge active sites, the Co-UNMs achieve a small overpotential (only 307 mV at 10 mA cm−2) for the OER in 1 M KOH electrolyte. EXPERIMENTAL SECTION Reagents and Chemicals. Sodium borohydride (NaBH4) and cobalt(II) chloride hexahydrate (CoCl2·6H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Potassium hexacyanocobaltate(III) (K3Co(CN)6) was purchased from Macklin Biochemical Co.,Ltd (Shanghai, China), and potassium hydroxide (KOH) were obtained from Alfa Aesar (Shanghai, China). RuO2 nanoparticles were purchased from Aladdin Industrial Corporation (Shanghai, China). Synthesis of the Co-UNMs. In a typical synthesis, 1 mL of 1 M CoCl2 and 1 mL of 0.5 M K3Co(CN)6 aqueous solutions were mixed at room temperature. After 8 h, the red jelly-like CoCl2/K3Co(CN)6 cyanogel generated. Then, 100 mL of 3.5 M NaBH4 aqueous solution was added into CoCl2/K3Co(CN)6 cyanogel under strong stirring. Finally, the black ultrathin cobalt oxide nanosheets (termed as the Co-UNMs) were separated by centrifugation, washed consecutively with water, and then dried at 80 °C in the air for 24 h. For comparison, the irregular and random aggregated Co nanostructures (termed as the Co-NS-ir) were synthesized by using CoCl2 as cobalt precursor under same experimental conditions. Physical characterization. Scanning electron microscopy (SEM) was measured by SU-8020. Transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy-dispersive X-ray (EDX) maps were conducted with TECNAI G2 F20 instrument. Powder X-ray diffraction (PXRD) patterns were obtained by a DX-2700 power X-ray diffractometer with Cu-Kα radiation source. Atomic force microscopy (AFM) was performed on Dimension Icon instrument. X-ray photoelectron spectroscopy (XPS) was carried out on a AXIS ULTRA spectrometer, and binding energy was calibrated with C1s peak 284.6 eV as standard value.

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Electrochemical measurements. Electrochemical tests were performed on an CHI 660 E electrochemical analyzer with a Gamry RDE710 rotating disk electrode in O2saturated 1 M KOH electrolyte at 30 ± 1 °C, using a threeelectrode system that consisted of the saturated calomel electrode as reference electrode, Pt plate as the auxiliary electrode, and a electrocatalyst modified glassy carbon electrode as the working electrode. All potentials are reported with respect to the reversible hydrogen electrode (RHE), where ERHE = ESCE +0.242 V + 0.0591 pH. The working electrode was prepared according to the previous work.50 The catalyst ink was prepared by dispersing 2 mg of the catalyst in 1.0 mL of water containing 5 μL of 5 wt % Nafion and 200 μL isopropanol under strong sonication conditions. Then, 12 μL of the catalyst ink was carefully loaded on the surface of glassy carbon electrode and dried at room temperature. The metal loading density of catalysts on working electrode is ca. ~0.34 mg cm–2. In all LSV curve, iR drop was compensated at 95% through the positive feedback model using the CHI 660E electrochemical analyzer. The turnover frequency (TOF) was calculated by the following equation TOF = jAn-1 F-1m-1. Where j was the current density at a given overpotential, A was the electrode area of glassy carbon electrode (0.071 cm2), n was the electron transfer number (herein, n=4 for the OER), F was the Faraday constant, and m was the Co moles on the surface of electrode.24 Electrochemical impedance spectroscopy was performed over a frequency range of 0.01 to 100000 Hz with an amplitude of 5 mV. RESULTS AND DISCUSSION Synthesis and characterization of the Co-UNMs. In a typical synthesis, the red jelly-like CoCl2/K3Co(CN)6 cyanogel was conveniently obtained by mixing the aqueous CoCl3 (1 mL, 1 M) and K3Co(CN)6 (1 mL, 0.5 M) solutions at room temperature for 8 h (Figure 1A). After adding aqueous NaBH4 (100 mL, 3.5 M) solution into the CoCl2/K3Co(CN)6 cyanogel for 24 h, the black Co-UNMs were obtained by the centrifugation (Figure 1B).

Figure 1. Schematic representation of the synthetic procedure of the Co-UNMs. (A) SEM image photograph of the CoCl2/K3Co(CN)6 cyanogel. (B) SEM image of the CoUNMs. The bulk chemical composition of the Co-UNMs was determined using the EDX spectroscopy. EDX spectrum shows the existence of Co and O elements (Figure 2A). The EDX mapping images show an atomically uniform

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distribution of the Co and O elements in the product (Figure 2B). After adding 1 M HCl solution in the product, no H2 bubbles generate on the product surface (Figure S1 in the Supporting Information), confirming the product is not metallic cobalt. XPS was carried out to analyze the composition and chemical state of the product. No B element is observed at XPS spectrum of the product (Figure S2 in the Supporting Information), which rules out the generation of the cobalt-borate. The fine-scanned Co 2p XPS spectrum shows a pair of sharp 2p1/2 and 2p3/2 doublet peaks with separation of 16.2 eV and a pair of weak satellite doublet peaks (Figure 2C), indicate the co-existence of Co2+ and Co3+ species on the Co-UNMs surface.51 The Co 2p XPS spectrum was further fitted to analyze the chemical state of Co element. The characteristic peaks at 797.2 and 781.7 eV binding energies are identified as Co2+ species, whereas the other two peaks at 796.1 and 780.1 eV correspond to Co3+ species (Figure 2C).41, 43 After adding 6 M HCl solution in the product, Cl2 gas is detected (Figure S3 in the Supporting Information). The generation of Cl2 confirms chemically the product contains the Co3+ species. Meanwhile, the appearance of the strong O 1s XPS peak also suggest the CoUNMs are cobalt oxide (Figure 2D). The fine-scanned O 1s XPS spectrum was deconvoluted into four characteristic OI, OII, OIII, and OIV peaks at 529.9, 530.5, 531.2, 532.1 eV, corresponding to oxygen atoms bound to basal plane, oxygen atoms bound to defect atoms, hydroxyl groups on metal, and adsorbed water molecules on metal, respectively.41, 43 The obvious OII signal indicates the CoUNMs contain the numerous defected atoms with lowcoordination numbers.41, 43

Figure 2. Chemical composition of the Co-UNMs. (A) EDX spectrum of the Co-UNMs. (B) High-angle annular dark field scanning transmission electron microscopy image and corresponding EDX mapping images of the Co-UNMs. (C) Co 2p and (D) O 1s XPS spectra of the Co-UNMs.

The primarily morphological and structural properties of the Co-UNMs were characterized by TEM, SAED, PXRD, and AFM. TEM images demonstrate that the high-quality cobalt oxide nanosheets have been synthesized successfully (Figure 3A,B). As observed, the yield of the cobalt oxide nanosheets almost reaches 100%. SAED pattern of the cobalt oxide nanosheets shows the clear and discrete reflection dots (Figure 3C), indicating the good crystallinity. The interplanar spacing are calculated to be 0.463, 0.284, 0.242, 0.200, 1.53 and 0.142 nm by diffraction spots, which perfectly match the PDF card of spinel Co3O4 (PDF#653103), indicating the cobalt oxide nanosheets are Co3O4. Although the Co-UNMs have high crystallinity, no welldefined diffraction peak is observed at PXRD pattern of the Co-UNMs (Figure 3D). In previous work, XRD diffraction peak couldn't be observed at ultrathin Rh nanosheets with 1.5 nm thickness.52 Since SEM and TEM image has displayed that the Co-UNMs have sheet structure, the present PXRD pattern hints that the 2D Co3O4 nanosheets are atomically thick. After the strong ultrasonic treatment, the few fragments are obtained. AFM image and the corresponding height profiles confirm the 2D structure of the Co-UNMs and show the ultrathin nanosheets have uniform thickness of ca. 1.5 nm (Figure 3E). This value corresponds to the thickness of 3 unit cells of spinel Co3O4 (a=8.056 Å, b=8.056 Å, c=8.056 Å, in PDF#65-3103).53

Figure 3. Morphology characterization of the Co-UNMs. (A, B) TEM images, (C) SAED pattern, (D) PXRD pattern and (E) AFM image of the Co-UNMs. The internal microstructure information of the CoUNMs was further investigated by high-resolution TEM (HRTEM) and N2 adsorption-desorption isotherm (SADI) tests. HRTEM images clearly show that the cobalt oxide nanosheets possess the obvious holes (Figure 4A,B). A numbers of pores with ca. 1.5 nm diameter uniformly distribute at the surface of the nanosheets (Figure 4B and insert). Considering that the ultrathin nature of the nanosheets, these pores should be through and open-style,54 which will effectively accelerate the mass transportation of the reactant. The magnified HRTEM image clearly shows well-defined

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lattice fringes in some regions (Figure 4C). The interplanar spacing values between adjacent lattice fringes are estimated to be about 0.286 and 0.244 nm, which correspond to (220) and (311) planes for the spinel Co3O4, respectively.55 The porous structure of the Co-UNMs was also confirmed by SADI analysis. The type-IV SADI curve with hysteresis loops is obtained for the Co-UNMs, which confirms the porous nature (Figure 4C). The pore-size distribution curve shows that the Co-UNMs contain numerous mesopores with ca. 1.45 nm diameter (insert in Figure 4D), in consistent with HRTEM observation. Using the Brunauer-EmmettTeller (BET) method, the specific surface area and total pore volume of the Co-UNMs is calculated to be 208.64 m2 g−1 and 0.299 cm3 g−1, respectively. Obviously, the ultrathin property and porous structure contribute to the high specific surface area and large pore volume. So far, all characterizations demonstrate that the Co-UNMs with a high surface and abundant defected atoms have been successfully synthesized through the convenient cyanogel−NaBH4 method. These structural characteristics are highly desired for the catalytic reaction due to the ultrathin thickness (i.e., high metal utilization and the full exposure the highly active sites) and abundant pores (i.e., numerous defected atoms at pore edge and favor mass transportation).

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the Co-UNMs. Cyanogel, a kind of insoluble cyano-bridged bi-metallic coordination polymer with high water affinity, can be facilely and massively achieved through the ligandsubstitution reaction between the chloride ligand at chlorometalates (K2PdCl4, K2PtCl4, RhCl3, SnCl4, NiCl2, CoCl2, and InCl3, etc) and nitrogen atom from cyano ligand at transition metal cyanometalates (K2Ni(CN)4, K3Co(CN)6, and K3Fe(CN)6, etc) in aqueous solutions (Figure 5A).56-62 According to structural formula of the cyanogel, 2D planar units exist in the CoCl2/K3Co(CN)6 cyanogel (Figure 5B). Due to the solid nature of the CoCl2/K3Co(CN)6 cyanogel, the new generated crystal nuclei preferentially assemble at solid liquid interface rather than in solution after adding NaBH4. Thus, the 2D planar units of the cyanogel can serve as a selftemplate for the growth of new-generated crystal nucleus in 2D direction, resulting in the formation of the 2D nanosheets. During the formation of the 2D nanosheets, the chloride and cyano ligands in the 2D planar units simultaneously enter into the solution due to their hydrophilic property, which may be responsible for the generation of the nanopores in the nanosheets.

Figure 5. The formation mechanism of the Co-UNMs. (A) Reaction equation for the cyanogel. (B) 2D geometrical unit in CoCl2/K3Co(CN)6 cyanogel.

Figure 4. Structural characterizations of the Co-UNMs. (A, B) HRTEM and (C) magnified HRTEM images of the CoUNMs. Insert in B: the pore-size distribution curve. (D) SADI curve of the Co-UNMs. Insert in D: the corresponding poresize distribution curve. The formation mechanism of the Co-UNMs. For comparison, Co nanostructures were synthesized by using CoCl2 as reaction precursor under same experimental conditions. Unfortunately, only irregular and random aggregated Co nanostructures (termed as Co-NS-ir) are obtained (Figure S4 in the Supporting Information). This fact indicates the application of CoCl2/K3Co(CN)6 cyanogel precursor plays a very important role for the generation of

The electrocatalytic performance for the OER. The cyclic voltammetry (CV) measurements were performed to investigate the electrochemical property of the Co-UNMs and Co-NS-ir in a standard three-electrode system. The electrochemically active surface areas (ECSA) of the CoUNMs and Co-NS-ir are measured by the double-layer capacitance (CDL) method in O2-saturated 1 M KOH electrolyte. In the potential range from 1.25 to 1.35 V, no Faradaic current is observed. Thus, the CDL values of the CoUNMs and Co-NS-ir are calculated by the scan-rate dependent CV curves (Figure 6A and 6B), using the followed equation: i = υCDL (where, i is current density and υ is scan rate). Then, the ECSA value is calculated using followed equation: ECSA=CDL/Cs (where Cs is the specific capacitance with 0.040 mF cm−2 in 1 M KOH electrolyte.41 At same scanrate, the charging current of the Co-UNMs (Figure 6A) is bigger than that of Co-NS-ir (Figure 6B), indicating the CoUNMs have bigger accessible area than the Co-NS-ir. According to linear relationship between i and υ (Figure 6C), the CDL values of the Co-UNMs and Co-NS-ir are calculated to be 0.7679 and 0.3395 mF, respectively. Consequently, ECSA

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values of the Co-UNMs and Co-NS-ir are calculated to be 240 and 106.09 m2 g−1, respectively. Obviously, the ultrathin and porous structures of the Co-UNMs play the crucial roles on the high ECSA. In the potential range from 0.85 to 1.56 V, both CV curves show two pairs of redox peaks around 1.13 V and 1.45 V, which relate to the Co2+/Co3+ redox couple and Co3+/Co4+ redox couple, respectively (Figure 6D).63 CV curves clearly show that the onset oxidation potential (ca. 1.31 V) of the Co3+/Co4+ redox couple at the Co-UNMs is lower than that (ca. 1.39 V) at the Co-NS-ir. Due to the interference of the big charging current, the differential pulse voltammetry (DPV) with the inherently high sensitivity was performed to further analyze the electrochemical response of the Co-UNMs and Co-NS-ir (insert in Figure 6D). DPV curves show the Co-UNMs have stronger Co2+/Co3+ and Co3+/Co4+ electrochemical response than the Co-NS-ir, indicating the Co-UNMs contain the more electroactive sites compared to the Co-NS-ir due to the abundant defected atoms at the edge of pores.

Figure 6. CV curves and Plots of the current density of the Co-UNMs and Co-NS-ir. CV curves of (A) the Co-UNMs and (B) Co-NS-ir in O2-saturated 1 M KOH electrolyte at different scan rates. (C) Plots of the current density at 1.30 V vs scan rate for the Co-UNMs and Co-NS-ir. (D) CV curves of the CoUNMs and Co-NS-ir in O2-saturated 1 M KOH electrolyte at the scan rate of 5 mV s−1. Insert: DPV curves of the Co-UNMs and Co-NS-ir in O2-saturated 1 M KOH electrolyte. Pulse amplitude, 50 mV; sample width, 16.7 ms; pulse width, 25 ms. Electrocatalytic performance of the Co-UNMs for the OER. Linear sweep voltammetry (LSV) measurements were performed to determine the electrocatalytic activity of the Co-UNMs and Co-NS-ir for the OER in O2-saturated 1 M KOH electrolyte (Figure 7A). The Co-UNMs show a lower OER onset potential (1.49 V) than the Co-NS-ir (1.53 V), indicating a higher electrocatalytic activity. The potential at the current density of 10 mA cm–2 is a vital experimental parameter for evaluating the OER activity of the electrocatalysts because this parameter is a metric related to solar-to-fuel conversion system.42, 64 The Co-UNMs

require a overpotential of 307 mV to obtain a current density of 10 mA cm–2, which is 46 mV lower than the CoNS-ir (Figure 7B). It is worth noting that this overpotential value (307 mV) is smaller than that of most monometallic Co-based OER electrocatalysts, and is even comparable to that of the most previously reported bimetallic Co-based OER electrocatalysts (Table 1).2, 12, 15-16, 31, 35, 38, 41, 43, 55, 65-83 To achieve a current density of 100 mA cm–2, the Co-UNMs require a overpotential of 407 mV while the Co-NS-ir require a overpotential of 469 mV (Figure 7B). In addition, TOF was calculated to prove the high activity of the CoUNMs. Specifically, the TOF of the Co-UNMs and the Co-NS-ir are 0.055 s-1 and 0.014 s-1 at a overpotential of 400 mV, respectively. The TOF of the Co-UNMs is higher than many reported catalysts, including Zn-Co layered double hydroxide nanosheets,79 Fe-Co oxide nanosheets,41 and hollow Co3O4 microspheres.55 Since the electrocatalytic performance of the electrocatalysts generally relate to their ECSA values, the high ECSA value of the Co-UNMs is responsible for it high OER performance. In general, the specific activity of electrocatalysts (i.e., the current is normalized to ECSA) can effectively reflect their intrinsic electrocatalytic activity. Thus, the electrocatalytic activities of the Co-UNMs and Co-NS-ir for the OER were further investigated by LSV, using the ECSA value. The ECSAnormalized LSV curves show the Co-UNMs still have a higher OER activity than the Co-NS-ir in O2-saturated 1 M KOH electrolyte (Figure S5 in the Supporting Information). For example, the specific OER activity of the Co-UNMs is 0.1 mA cm–2ECSA at 1.570 V, which is 23 mV lower than that (1.593 V) of the Co-NS-ir. Generally, the Co4+ species is regarded as the active site for the Co-catalysed OER in alkaline media.84 Consequently, the OER onset potential and OER current at Co-based electrocatalysts depend on the formation potential and amount of Co4+ species. CV (Figure 6D) and DPV (Insert in Figure 6D) measurements have demonstrated that the conversion potential of Co3+ to Co4+ at the Co-UNMs is lower than that at the Co-NS-ir and the CoUNMs can generate the more Co4+ species than the Co-NS-ir at same potential, which result in the OER activity enhancement. Table 1． ． OER activities of the Co-based electrocatalysts in KOH electrolyte. Catalysts

Electrolyte

Co-UNMs

1 M KOH 0.1 M KOH

η value at 10 mA cm-2

Co3O4 nanowires

1 M KOH

307 mV 335 mV (Figure S6 in the Supporting Information) 414 mV

CoCo layered double hydroxides

1 M KOH

Co(OH)2 nanosheets Co3O4 microplates Co nanosheet on Ngraphene Co hydroxide nanosheets Small-sized Co3O4 nanosheets

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Ref. (year)

This work

31

2015

393 mV

65

2014

1 M KOH

387 mV

66

2017

1 M KOH

385 mV

67

2017

1 M KOH

340 mV

35

2017

1 M KOH

326 mV

2

2017

1 M KOH

650 mV

68

2013

ACS Catalysis 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

Mesoporous Co3O4

0.1 M KOH

530 mV

69

2013

CoO/C nanocubes

0.1 M KOH

510 mV

12

2016

Pristine Co3O4 nanosheets

0.1 M KOH

500 mV

43

2016

CoxOy/nitrogen-doped carbon

0.1 M KOH

430 mV

70

2014

Hollow Co3O4 microspheres

0.1 M KOH

400 mV

55

2014

Au@Co3 O4 core-shell nanocrystals

0.1 M KOH

390 mV

16

2014

CoO nanosheet/graphene

0.1 M KOH

330 mV

71

2016

Plasma-engraved Co3O4 nanosheets

0.1 M KOH

300 mV

43

2016

Metallic Co4N porous nanowires

1 M KOH

370 mV

15

2015

CoS nanosheets

1 M KOH

361 mV

72

2015

CoP nanorods/C

1 M KOH

320 mV

73

2015

ZnxCo3-xO4

1 M KOH

320 mV

74

2014

CoMoO4 nanosheets Co–Fe hydroxide nanosheet Co-B nanosheet

1 M KOH

312 mV

31

2015

1 M KOH

ca. 310 mV

38

2017

0.1 M KOH

520 mV

75

2017

Co3O4–CoFe2O4 composite Co/Fe 32

0.1 M KOH 0.1 M KOH

500 mV 500 mV

76

2013 2014

Amorphous CoFe2O4

0.1 M KOH

490 mV

78

2014

Zn-Co double hydroxide nanosheets

0.1 M KOH

ca. 450 mV

79

2015

Amorphous Ni-Co-B nanocrystals

0.1 M KOH

430 mV

80

2017

Sea urchin-like Co–Fe phosphide

0.1 M KOH

370 mV

12

2016

NiCo2O4 nanocages

0.1 M KOH

340 mV

81

2015

O-NiCoFe-Layered double hydroxides

0.1 M KOH

340 mV

82

2015

Fe-Co oxide nanosheet

0.1 M KOH

308 mV

41

2017

0.05 M KOH

320 mV

83

2017

Fe-doped CoP nanosheet

77

The Tafel slope, which reveals the correlation between overpotential and steady-state current density, is an important parameter for evaluating OER kinetics.64 The lower value of the Tafel slope means a higher reactivity for the OER. OER kinetics at the Co-UNMs and Co-NS-ir were analyzed by Tafel plots (Figure 7C). The Tafel slope at the Co-UNMs is 76 mV dec–1, which is smaller than 95 mV dec–1 at the Co-NS-ir. The Tafel slope comparison suggests that the OER kinetics at the Co-UNMs is faster than that at the Co-NS-ir. Electrochemical impedance spectroscopy (EIS) tests were performed to investigate the charge transfer ability (Figure 7D). The charge transfer resistance (Rct) can be obtained from the Nyquist plots. A lower Rct value means a faster charge transfer rate. According to the diameter of the semicircle at the low-frequency region, the Rct value at the Co-UNMs is estimated to be 43.0 Ω, much smaller than that (124.6 Ω) at the Co-NS-ir, suggesting the Co-UNMs have relatively faster charge transfer rate for the OER compared to the Co-NS-ir. On the one hand, the abundant Co4+ species is favorable to the OER activity enhancement.34 On the other hand, the Co-UNMs have a high surface area and the porous structure, which can yield the easier mass transportation.31 What’s more, by investigating the O 1s XPS spectra (Figure 2D), a clear OIV peak can be easily observed, indicating the presence of oxygen vacancies on the surface of the CoUNMs.43 In particular, these oxygen vacancies can create more electrochemically active sites, and thus enhance the

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electrocatalytic activity for OER.43 These factors contribute to the improved OER kinetics of the Co-UNMs.85

Figure 7. Electrochemical measurement of the Co-UNMs and Co-NS-ir. (A) iR-corrected LSV curves of the Co-UNMs and Co-NS-ir in O2-saturated 1 M KOH electrolyte at the scan rate of 5 mV s−1. (B) Comparison of the overpotential at the current densities of 10 and 100 mA cm−2. (C) Tafel plots of the Co-UNMs and Co-NS-ir. (D) Nyquist plots of the CoUNMs and Co-NS-ir in O2-saturated 1 M KOH electrolyte at a overpotential of 310 mV. In general, RuO2 was used as a benchmark for the OER. Considering the potential application, we further compare the OER activity of the Co-UNMs and the state-of-the-art RuO2 electrocatalyst under the same experimental conditions (Figure 8A). The commercial RuO2 electrocatalyst display a overpotential of 408 mV at a current density of 10 mA cm–2, which is 101 mV higher than that at the Co-UNMs at the same condition. This fact indicates the Co-UNMs have the better electrocatalytic activity for the OER than the state-of-the-art RuO2 electrocatalyst. The durability of the electrocatalyst is a very critical factor for the practical application. The long-term stabilities of the Co-UNMs and RuO2 electrocatalyst were tested by chronopotentiometry at a current density of 10 mA cm–2 in 1 M KOH electrolyte (Figure 8B). The current oscillation originates from the accumulating and removal of O2 bubbles on the electrode surface. After 10 h running, the operation potential of RuO2 electrocatalyst considerably increases (108.9 mV) whereas the operation potential of Co-UNMs slightly increases (7.5 mV). After durability test, SEM images show the morphology of the Co-UNMs is well-preserved (Figure S7 in the Supporting Information). The long-term stabilities of the CoUNMs and RuO2 electrocatalyst were further tested by chronoamperometry at 1.54 V potential in 1 M KOH electrolyte (Figure S8 in the Supporting Information). In the whole running time, the OER current density at the CoUNMs is much bigger than that at RuO2 electrocatalyst. After 5 h, OER current density at the Co-UNMs only loss the 17.5% in initial current density whereas OER current density at

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RuO2 electrocatalyst loss the 86.4%. Combining the above electrochemical data, we conclude that the Co-UNMs can afford high electrocatalytic activity and durability outperforming the commercial benchmark RuO2 electrocatalyst.

characterization and electrochemical characterization of the ultrathin Co3O4 nanomeshes.

ACKNOWLEDGMENT This research was sponsored by the National Natural Science Foundation of China (21473111), the Fundamental Research Funds for the Central Universities (GK201602002 and 2016TS044), and the 111 Project (B14041).

REFERENCES

Figure 8. OER activity and durability of the Co-UNMs and RuO2. (A) iR-corrected LSV curves of the Co-UNMs and RuO2 in O2-saturated 1 M KOH electrolyte at the scan rate of 5 mV s−1. (B) Chronopotentiometry curves of the Co-UNMs and RuO2 in O2-saturated 1 M KOH electrolyte at a constant current density of 10 mA cm−2. CONCLUSIONS In summary, we have highlighted a particular solid-liquid interface synthetic strategy for the one-step synthesis of ultrathin Co3O4 nanomeshes by employing cyanogel with the solid property and 2D planar structural unit as precursor. Our approach is facial, efficient, and can be scaled up readily, which show a highly promising application in the synthesis of ultrathin transition metal-based nanomeshes. When used as an electrocatalyst for the OER, the Co-UNMs display the superior electrocatalytic activity compared to their counterpart Co-NS-ir due to the special advantages of ultrathin nanomeshes, including their atomic thickness, abundant pores and numerous defected atoms. Meanwhile, the obtained Co-UNMs can afford high electrocatalytic activity and durability outperforming the commercial benchmark RuO2 electrocatalyst, showing the promising application in water splitting.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Authors *Email for S.-N. L: [email protected] *Email for Y. C: [email protected] Author Contributions ⊥

Y. L. and F.-M. L. contributed equally to this work.

Notes The authors declare no competing financial interest. Supporting Information The Supporting Information is available free of charge on the Internet at http://pubs.acs.org. Material

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SYNOPSIS TOC Ultrathin Co3O4 nanomeshes with 1.5 nm thickness and abundant pores are directly obtained by a one-step surfactant-free cyanogel−NaBH4 method, which achieve a small overpotential (only 307 mV at 10 mA cm−2) for the oxygen evolution reaction in the alkaline media.

Table of Contents Artwork Ultrathin Co3O4 nanomeshes NaBH4 Cyanogel ∆E=101 mV at 10 mA cm–2 RuO2 20 nm Oxygen evolution reaction Title: Ultrathin Co3O4 nanomeshes for the oxygen evolution reaction

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