A Self-Supported Porous Hierarchical CoreâShell Nanostructure of

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A self-supported porous hierarchical core shell nanostructure of cobalt oxide for efficient oxygen evolution reaction Han Xia, Zhipeng Huang, Cuncai Lv, and Chi Zhang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02320 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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A self-supported porous hierarchical core shell nanostructure of cobalt oxide for efficient oxygen evolution reaction

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Han Xia, †,‡ Zhipeng Huang,* † Cuncai Lv, †,‡ and Chi Zhang* †

†

School of Chemical Science and Engineering, and Advanced Research Institute, Tongji University, Shanghai, 200092, China

‡

Functional Molecular Materials Research Centre, Scientific Research Academy, ChinaAustralia Joint Research Center for Functional Materials, Jiangsu University, Zhenjiang, 212013, China.

*

Corresponding authors.

E-mail addresses: [email protected] (Z.P. Huang), [email protected] (C. Zhang)

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Abstract. Increasing the active surface area of an electrocatalyst is crucial for effective oxygen evolution reaction (OER). Here a sophisticated electrode taking simultaneously the advantages of porous/hollow nanostructure, hierarchical nanostructure, and self-supported structure is demonstrated for the first time. A self-supported porous hierarchical core-shell structure (PHCS) of cobalt oxide is synthesized by the combination of electrochemical deposition and electrochemical treatment. The treatment introduces numerous pores into the core of a core-shell structure, and decreases the particle size of cobalt oxide to smaller than 5 nm, markedly increasing the surface area of resultant structures. The electrochemical surface area of PHCS is 1.6 times of that of hierarchical core-shell cobalt oxide, and nearly 20 times of that of cobalt oxide nanowires. The PHCS is extremely active in the OER, with overpotential required for a current density of 100 mA cm-2 as small as 300 mV. The Tafel slope is 40.3 mV dec-1, and the PHCS can work stably for more than 40 h.

Keywords: Hierarchical, Core shell, Porous, Cobalt oxide, Oxygen evolution


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1. Introduction The application of molecular hydrogen (H2) as a clean energy has been stimulated by energy crisis, environment pollution, and greenhouse effect. Water electrolysis driven by clean and renewable energy (e.g. solar electricity and wind electricity) is a promising route to produce hydrogen.1 Water electrolysis includes hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Four electron process makes the kinetics of OER sluggish, and induces a significant efficiency loss in water splitting.2-3 Effective electrocatalysts are required to reduce the overpotential of OER. Noble metal compounds (e.g. IrO2, RuO2) are effective OER electrocatalysts, while they are too scare and expensive to be applied widely. The catalytic activity of low-cost nonprecious metal OER electrocatalysts remains far inferior to noble metal compounds. The exploitation of low-cost non-precious metal OER electrocatalysts remains highly desirable, and therefore stimulated.4-24 Effective electrocatalysts should possess simultaneously large electrochemical surface area (ECSA) and small electron transport resistance. Nanoparticles with small size can exhibit large surface area, whereas the electron transport is hampered by resistance correlated with binder and incompact inter-particles interface. Selfsupported nanostructures grown directly on the surface of current collector are promising electrode configuration, because such structures enable fast electron transport and therefore more effective catalyst loading. For example, metal oxide thin film,7 cobalt-phosphide nanorods,25 NixMy (M = P, S) nanofilms,26 NiCo2S4 nanowires,27 and cobalt phosphate nanowires28 grown on the surface of carbon fiber paper (CFP) or nickel foam have demonstrated enhanced catalytic activity in the OER. However, the performance of self-supported electrode remains unsatisfied. The increase of ECSA in self-supported OER electrode is a promising strategy and therefore highly desirable. Various approaches have been tried to increase ECSA of 3 ACS Paragon Plus Environment

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self-supported OER electrodes, and recent attempts were carried out in two different strategies. One strategy is the construct of porous or hollow nanostructures by removing unwanted interior material to expose more internal ECSA. The examples include amorphous nickel cobalt binary oxide nanoporous layers,29 Co3O4-carbon porous nanowire,30 Co4N porous nanowire,10 porous cobalt oxide nanosheets,31 NiO/CoN porous nanowires,32 and so on. The other strategy is the development of hierarchical core shell structures (e.g., three-dimensional NiCo2O4 core-shell nanowires,33 Ni3Se2 nanoforest,34 and three-dimensional CFP/carbon tubes/cobaltsulfide sheets,35), where additive nanoshell structure would bring more external ECSA. In principle, both internal and external ECSA increases can be integrated in a single nanostructure, and the resultant sophisticated electrode will take simultaneously the advantages of porous/hollow nanostructure, hierarchical nanostructure, and selfsupported structure, therefore certainly leading to remarkably enhanced catalytic activity in the OER. Nevertheless, such highly desirable advanced nanostructure has not yet been reported so far. Herein a novel self-supported CoOx porous hierarchical core-shell structure (PHCS), consisting of porous nanowires wrapped with ultrathin nanosheets, was synthesized by electrochemical deposition of ultrathin nanosheets on the surface of CoS2 nanowires and electrochemical treatment of solid nanowires. The CoOx PHCS produced a high current density of 100 mA cm-2 at small overpotential (300 mV) and exhibited a small Tafel slope of 40.3 mV dec-1. The performance is superior to commercial RuO2 electrocatalyst and other reported non-precious metal OER electrocatalysts. The excellent catalytic activity is ascribed to unique morphology and small particles size that markedly increase surface area while maintains effective electrons transport.


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2. Results and Discussion

Figure 1. Synthesis process of CoOx PHCS The synthesis process of CoOx PHCS is illustrated in Figure 1. CoS2 nanowires (NW) grown on the surface of CFP were adopted as precursor in a series of electrochemical treatment. The CoS2 NW was synthesized by the sulfurization of cobalt carbonate hydroxide (CoCH) NW hydrothermally grown on the surface of the CFP (Figure S1, SI). The resultant CoS2 NW were crystallized in cubic structure (JCPDS No. 41-1741, a = 5.5376 Å), as confirmed by the pattern of X ray diffraction (XRD) (Figure S2a, SI). The CoS2 NW was homogeneously grown on the surface of CFP (Figure S2b and Figure S2c, SI), and the CoS2 NW were assembled by nanoparticles with diameter of ca. 30 nm (Figure S2d and Figure S2e, SI). The CoS2 nanoparticles were well crystalized, as indicated by the image of high resolution transmission electron microscopy (HR-TEM) (Figure 2f, SI). In a series of electrochemical treatments, Co(OH)2 nanosheets (NS) were firstly grown on the surface of CoS2 NW by electrochemical deposition (Step 1 of Figure 1), resulting in a hierarchical core-shell structure (HCS) of CoS2-Co(OH)2 (Figure S3, SI). The CoS2-Co(OH)2 HCS consists of a solid CoS2 nanowire core surround by ultrathin Co(OH)2 NS. The morphology of CoS2 core nanowire (Figure S3f, SI) in CoS2-Co(OH)2 HCS is analogous to that of untreated CoS2 NW (Figure S2e, SI) , and 5 ACS Paragon Plus Environment

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solid nanoparticle in the core nanowire can be clearly found. Such a morphological feature was clearly revealed by a series of images of TEM images (panels b to h in Figure S3, SI) and scanning electron microscopy (SEM) (panels i to k in Figure S3, SI). Electrochemical treatment was then carried out to induce the morphology and composition variation of the CoS2-Co(OH)2 HCS (Step 2 of Figure 1). The treatment was implemented by repeatedly linear sweep voltammetry (LSV), and it was found that anode current decreased gradually with increasing LSV step. After 4 LSV scans a stable polarization curve was obtained (Figure S4, SI), indicating the completion of electrochemical treatment. The gradual decrease of polarization curve indicates the gradual oxidation of CoS2 NWs in the CoS2-Co(OH)2 HCS. It will be introduced in the following that the evolution of the CoS2-Co(OH)2 HCS includes the oxidation of CoS2 and Co(OH)2 to Co3O4 and the formation of CoOOH on the surface of Co3O4, resulting in CoOx PHCS. These reactions are in good accordance with that suggested by the Pourbaix diagram for cobalt in aqueous solution.36 Morphological and structural characterizations of CoOx PHCS were assessed by SEM and TEM (Figure 2). SEM images of the CoOx PHCS were shown in panels a to c of Figure 2. The CoOx PHCS grew homogeneously and densely on the surface of carbon fiber (Figure 2a), and the length of nanowire is ca. 8 µm, which was estimated from the difference between outer diameter of CoOx PHCS-loaded carbon fiber and a bare carbon fiber. The loading amount of CoOx PHCS is 1.4 mg cm-2 on the surface of CFP. It was further revealed by Figure 2b that the surface of nanowires was wrapped with a copious amount of ultrathin nanosheets. The width of nanosheet is several hundreds of nanometer and the thickness of nanosheets is typically smaller than 20 nm (Figure 2c).


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Figure 2. (a-c) SEM images of CoOx PHCS. (d,e) TEM images of CoOx PHCS. (f) SAED rings of CoOx PHCS. (g) Reciprocal distances derived from SAED. (h) HRTEM image of a nanosheet in CoOx PHCS. Inset of (d) shows the thickness distribution of nanosheets wrapping on the surface of nanowire. White arrows in (e) indicate typical pores on the core nanowire. Inset of (e) shows the diameter distribution of pores in nanowire. The morphology of CoOx PHCS was characterized by TEM in more details, as shown by Figure 2d. The semi-transparency of nanosheets wrapped on the surface of 7 ACS Paragon Plus Environment

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nanowires suggests that these nanosheets are relatively thin. An average thickness of 7.3 ± 1.1 nm (inset of Figure 2d) was estimated from TEM images. A large amount of pores can be found in the core, as indicated by the arrows in Figure 2e, and the average diameter of pores is 16.4 ± 1.5 nm (inset of Figure 2e). The morphology of CoS2 core nanowire (Figure S3c, SI) in CoS2-Co(OH)2 HCS is analogous to that of CoS2 NW (Figure S2e, SI), and solid nanoparticle in the core nanowire can be found. Figure 2e suggests that electrochemical treatment can introduce pores inside nanowires, and markedly reduce the grain sizes in nanowires. XRD experiment was performed to investigate the structure of CoOx PHCS, whereas the peaks were relative weak and cannot be used to identify the phase of activated sample (Figure S5, SI). Meanwhile, the peaks assigned to CoS2 in CoS2Co(OH)2 HCS vanished after LSV scans, confirming the structural and compositional variation induced by electrochemical treatment. Because the loading amount of CoOx PHCS is as large as 1.4 mg cm-2, the relative weak diffraction pattern implies ultrasmall grain size in the sample. The structure CoOx PHCS was derived from selected area electron diffraction (SAED) rings recorded in TEM (Figure 2f). The reciprocal distances-intensity relationship (Figure 2g) derived from the rotation average of patterns in Figure 2f corresponds well to cubic Co3O4 (JCPDS No. 42-1467, a=8.0837 Å), showing that the CoOx PHCS was mainly composed of Co3O4. HRTEM image of the ultrathin nanosheet is shown in Figure 2h. The distinct lattice fringes can be well indexed to (001) zone-axis of Co3O4 (d(220)=d(2-20)=2.8 Å), indicating that Co(OH)2 was also oxidized to Co3O4 by electrochemical treatment. Because the porous nanowires were wrapped by Co3O4 nanosheets, it is hard to access the HRTEM image of the core. Alternatively, a HRTEM image of CoOx NW was assessed to postulate the crystal structure of porous core. CoOx NW was produced by electrochemical treatment of CoS2 NW under the same condition to produce CoOx PHCS. The corresponding TEM 8 ACS Paragon Plus Environment

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characterization of CoOx NW can be found in panel a to d and panel f of Figure S6 (SI). No obvious pores can be found from the CoOx NW (Figure S6a and Figure S6b, SI), and the CoOx NW was assembled by tiny particles (Figure S6c, SI). Despite of their different morphologies, CoOx PHCS and CoOx NW exhibit identical SAED pattern (Figure S6d and Figure S6e, SI). Figure S6f (SI) is a HRTEM image of the CoOx NW, which shows that lattice fringes can be indexed to [-1 1 -1] zone axis of cubic phase Co3O4 (JCPDS No. 42-1467). The SAED and HRTEM images of CoOx NW suggest that continuous electrochemical treatment (LSV scans) in KOH solution (1 M) transforms CoS2 to Co3O4, and the grain size of resultant Co3O4 is typically smaller than 5 nm (Figure S6c and Figure S6f, SI). Because CoS2-Co(OH)2 HCS and CoS2 NW were subjected to the same electrochemical treatment, it is likely that similar composition variations occur during electrochemical treatment of the two samples. That is, CoS2 nanowires in the CoS2-Co(OH)2 HCS were transformed to Co3O4 in CoOx PHCS. This postulation is in accordance with the SAED of CoOx PHCS (Figure 2f), which shows patterns from Co3O4.

Figure 3. (a) HAADF image of CoOx PHCS. EDS elemental mapping of (b) Co and (c) O in CoOx PHCS The energy dispersive spectroscopy (EDS) spectrum of the CoOx PHCS shows distinct peak of Co and O, and a small peak corresponding to S (Figure S7a, SI). The quantitative analysis indicates that the atomic content of S is less than 0.4%, which is much smaller than the atomic content of S in CoS2-Co(OH)2 HCS (7.2%, Figure S7b, 9 ACS Paragon Plus Environment

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SI). To probe the spatial distribution of elements in CoOx PHCS, element mapping was carried out in scanning TEM (STEM) mode. Figure 3a shows a high-angle annular dark-field (HAADF) image of CoOx PHCS, which demonstrates a porous nanowire wrapped by ultra-thin nanosheets. The element mapping of a region enclosed by a square in Figure 3a was carried out, and the results were shown in Figure 3b and Figure 3c. The weak intensity of Co and O at the shell region is associated with the relatively small thickness of the nanosheet. Raman spectra was conducted to further analyze the structure of the CoOx PHCS (Figure S7c, SI). Two intensive peaks can be found in the spectrum, which locates at 463 and 570 cm1

. The most intense band at 463 cm-1 can be allocated to Eg mode of Co3O4,37-38 and band at

570 cm-1 can be assigned to CoOOH.38 The Raman spectra of CoS2-Co(OH)2 HCS was also measured (Figure S7d, SI), which shows peaks at 257, 457, and 521 cm-1. The dominated band at 521 cm-1 can be attributed to the CoO (Ag) symmetric stretching mode, the band at 457 cm-1 can be correlated to OCoO bending mode,38 and the peak at 257 cm-1 is associated to Co(OH)2.38-39 The difference between Raman spectra of treated sample and un-treated sample demonstrates that the surface Co(OH)2 was oxidized to Co3O4 and CoOOH after electrochemical treatment.

Figure 4. XPS spectra of (a) Co 2p window and (b) O 1s window collected from CoOx PHCS.


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To further confirm the chemical composition and the surface electronic structure of CoOx PHCS, X-ray photoelectron spectroscopy (XPS) measurements were carried out. Four peaks can be derived from the Co 2p window of the XPS spectrum (Figure 4a), and their binding energy (BE) is 779.7, 780.7, 781.3, and 790.1 eV, respectively. The peaks at 779.7 and 780.7 eV come from Co in Co3O4,38, 40-41 and those at 781.3 and 790.1 eV can be ascribed to Co in CoOOH.38, 41 Two peaks can be derived from the O 1s window of the XPS spectrum (Figure 4b), one at 529.3 eV corresponding to oxygen atom in oxide, and the other peak at 531.3 eV associated with oxygen in hydroxide.38, 41-42 The XPS analysis suggests that the presence of Co3O4 and CoOOH in CoOx PHCS, which is in good agreement with the results of TEM and Raman analysis. In addition, the S 2p window of the XPS spectra of CoOx PHCS and CoS2-Co(OH)2 HCS were also monitored. The spectrum of CoOx PHCS (Figure S7e, SI) showed two peaks locating at 168.4 and 169.5 eV, which can be allocated to oxidation state of S (SO42-).43 In contrast, a distinct peak at 164.2 eV can be found from CoS2-Co(OH)2 HCS (Figure S7f, SI), which corresponds to S2- in CoS2.44-45 Meanwhile, the atomic content of S varies from 5.45% for CoS2-Co(OH)2 HCS to 1.9% for CoOx PHCS. The variation of binding energy indicates that the S atoms on the surface of CoS2Co(OH)2 HCS were fully oxidized to SO42- after electrochemical treatment, and the decrease of S atomic content on the surface of samples indicates the loss of S after electrochemical treatment. Meanwhile, the surface S atomic content measured by XPS (1.9%) of CoOx PHCS is much larger than the bulk S atomic content derived from EDS (0.4%), suggesting that the SO42- resulted from electrochemical treatment is likely to adsorb on the surface of CoOx PHCS. CoOx NW shows no obvious porous structure while the core nanowire in CoOx PHCS is porous, suggesting that the formation of porous structure in CoOx PHCS is heavily correlated with Co(OH)2 nanosheets wrapping on the surface of CoS2 core nanowire. During the electrochemical treatment, the CoS2 was gradually oxidized, and Co2+ and SO42- ions were in11 ACS Paragon Plus Environment

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situ formed. Co2+ cations can be further oxidized to Co3O4 in basic solution via an electrochemical-chemical reaction between Co2+ and OH-.46 The mass transport occurrs in this reaction includes the diffusion of OH- anions from bulk electrolyte to the surface of CoS2 nanoparticles and the outward diffusion of Co2+ cations released from CoS2. In CoS2-Co(OH)2 HCS, the CoS2 nanowire was surrounded by Co(OH)2 nanosheets, so that the diffusion of OHto the surface of CoS2 is slower, in comparison with the release of Co2+. In chemical anion exchange the slower inward diffusion of secondary anions, in comparison with the outward diffusion of metal cations, was believed to be responsible for the convert of a solid nanoparticle to hollow shell.47-48 This process is well-known Kirkendall effect that generates hollow structures. Similar mechanism would work here and transfer each particle in the core CoS2 nanowire of CoS2-Co(OH)2 HCS to hollow Co3O4 shell. In contrast, the supply of OHis sufficient during electrochemical treatment of CoS2 NW. Once Co2+ anions were released they can react immediately with OH- to form Co3O4, producing rough Co3O4 nanowires composed of tiny nanoparticles.


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Figure 5. The electrocatalytic performance of the catalyst. (a) Polarization curves and (b) Tafel of different catalytic electrodes. (c) Current-time curve recorded in a potentiostatic electrolysis experiment. The applied potential is 1.52 V vs. RHE. Inset of (c) shows the LSV curves before and after potentiostatic electrolysis experiment. (d) Current efficiency for O2 produce under potentiostatic electrolysis experiment (applied potential is 1.6 V vs. RHE). All potentials are corrected with iR drop. The catalytic activity of the CoOx PHCS was evaluated by electrochemical measurements. The experiments were performed in oxygen saturated basic aqueous solution (KOH, 1M) using a three-electrode configuration. A graphite rod was adopted as a counter electrode. All measured potentials were corrected with iR drop, and referenced to a reversible hydrogen electrode (RHE) potential. The polarization curves measured from LSV was shown in Figure 5a. It can be shown that the CoOx PHCS exhibits excellent performance in the OER, whereas a bare CFP negligible activity in the OER. Namely, the prominent performance of the CoOx PHCS comes exclusively from the CoOx PHCS. RuO2 is one of the most active electrocatalysts for the OER. RuO2 nanoparticles were loaded onto the surface of CFP with 13 ACS Paragon Plus Environment

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the same loading amount as CoOx PHCS (1.4 mg cm-2), and the sample was measured for comparison. It is worth noting that the current density of the CoOx PHCS is comparable to that of RuO2 electrocatalysts load on the CFP in small overpotential, and larger than that of RuO2 loaded on the CFP in large overpotential, suggesting superb catalytic activity of the CoOx PHCS in the OER. To further elucidate the contribution of unique morphology of CoOx PHCS to its prominent catalytic activity, a series of comparative samples were synthesized and their catalytic activities were evaluated. All comparative samples were grown on the CFP to ensure similar capability of electrons transport. Comparative samples include Co(OH)2 NS, Co3O4 NW, CoOx NW, and CoOx HCS. The Co(OH)2 NS was deposited on the surface of the CFP by a method similar to that used to deposited Co(OH)2 on the surface of CoS2 NW (Figure S8, SI). The Co3O4 NW was synthesized by thermal annealing of the CoCH NW (Figure S9, SI). The CoOx HCS was produced by the electrochemical deposition of the Co(OH)2 NS on the surface of the Co3O4 NW and a electrochemical treatment similar to the treatment of the CoS2-Co(OH)2 CS (Figure S10, SI). It is shown that the hierarchical structures (CoOx PHCS and CoOx HCS) is more active than individual Co3O4 NW or Co(OH)2 NS, and the introduction of porosity into the core nanowire in CoOx PHCS further improves the catalytic activity in the OER. The catalytic activity of OER electrocatalysts were usually compared by the overpotential required for a current density of 20 mA cm-2 (η20), because solar-driven water splitting cell typically produces a current density of 10 - 20 mA cm-2 under AM 1.5G solar illumination. Here the η20 is as small as 270 mV for CoOx PHCS, which is close to that of RuO2 (265 mV). The η20 of CoOx PHCS is smaller than that of CoOx HCS (296 mV), Co3O4 NW (345 mV), Co(OH)2 NS (385 mV), and CoOx NW (318 mV). In water electrolysis the working current density of OER electrode is larger than that of photoanode, therefore the performance of electrode under large current density is also of importance for water electrolysis electrode. 14 ACS Paragon Plus Environment

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The overpotential required for a current density of 100 mA cm-2 (η100) is 300 mV for CoOx PHCS, and surprisingly this value is smaller than that of RuO2 (320 mV). The η100 of CoOx PHCS is also markedly smaller than that of CoOx HCS (351 mV), Co3O4 NW (402 mV), Co(OH)2 NS (494 mV), and CoOx NW (367 mV). Meanwhile, the η20 and η100 of CoOx PHCS is nearly smaller than corresponding values of all reported non-precious metal OER electrocatalysts, of which the performance was listed in details in Table S1

(SI). The

relatively small η20 and η100 rank CoOx PHCS one of the most efficient non-precious metal OER electrocatalyst reported so far. CoS2-Co(OH)2 HCS was subjected to continuous cyclic voltammetry (CV) scans, in order to monitor the influence of electrochmical treatment manner to the performance of resultant sample in the OER. The corresponding CV curves can be found in Figure S11a (SI). The first CV scan shows an obvious oxidation peak in positive scan. In the following CV scan no obvious redox peaks can be found, and different CV curves show little difference. This feature implies that the composition variation of CoS2-Co(OH)2 HCS occurs only in the first positive scan, and then the surface of sample was passivated, so that no further composition and/or morphology variation occur. The OER performance of resultant sample is shown in Figure S11b (SI), with η20 of 350 mV and η100 of 423 mV. The performance is inferior to sample treated by continuous LSV scans. To understand the kinetics of different samples, their Tafel slope was derived from corresponding polarization curves by data fitting. The Tafel slope of CoOx PHCS is 40.3 mV dec-1, which is smaller than that of CoOx HCS (69.8 mV dec-1), Co3O4 NW (62.0 mV dec-1), Co(OH)2 NS (97.8 mV dec-1), CoOx NW (67.8 mV dec-1), and RuO2 (70.9 mV dec-1). The Tafel slope of CoOx PHCS is also smaller than most of reported OER electrocatalyst (Table S1, SI) The long-term stability of OER electrocatalysts is of importance for practical application. The CoOx PHCS can work stably in long-term water electrolysis. The long-term stability of 15 ACS Paragon Plus Environment

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the CoOx PHCS was demonstrated by chronoamperometry (Figure 5c). The CoOx PHCS maintains large and stable current density around 50 mA cm-2 in 40 h. The fluctuation in current density (Figure 5c) is possibly associated to temperature variation during long-term measurement, the removal of big bubbles adhered on sample surface, or the replenishment of electrolyte. The long-term stability of sample was also confirmed by a comparison of polarization curves measured before and after the potentiostatic electrolysis. The overlap of two polarization curves demonstrates the invariably efficient surface OER kinetics of the CoOx PHCS (inset of Figure 5c). The stability of CoOx PHCS might be attributed to its selfsupported structure. Electrocatalysts grown directly on CFP would mitigate the peeling off of electrocatalysts associated with the evolution of a large amount of oxygen gas, which occurs usually in an electrocatalyst-post-coated electrode.30 We have carried out characterization of CoOx PHCS subjected to long-term OER measurement (denoted as CoOx-PHCS-it), including TEM, EDS mapping, XRD and XPS. TEM image (Figure S12a, SI) shows that CoOx-PHCSit remains hierarchical structure with ultrathin nanosheets wrapped on a core nanowire, while it is hard to identify porous structure in the core nanowire from a TEM image with larger magnification (Figure S12b, SI). Unnoticeable porous structure in the core nanowires might be attributed to the morphological variation of porous core, or the contrast reduction of image because the core nanowire was wrapped inside nanosheets. XRD pattern (Figure S12c, SI) recorded by a Co source XRD diffractometer shows only peaks from the CFP. The SAED pattern of CoOx-PHCS-it (Figure S12d, SI) matches well with that of CoOx PHCS (Figure 2f or Figure S12e, SI). XRD and SAED patterns suggest that CoOx-PHCS-it also consists of Co3O4 nanoparticles with tiny grain size, and that long-term electrolysis does not induce obvious variation of crystal structure. The HRTEM of ultrathin nanosheet wrapping on the surface of nanowire (Figure S12f, SI) shows clear lattice fringes, with indexed one corresponding to (400) plane of Co3O4. EDS Mapping (panels h to i of Figure S12, SI) indicates the homogeneous distribution of Co and O, a feature similar to that of CoOx PHCS 16 ACS Paragon Plus Environment

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(Figure 3). The Co 2p window of XPS spectrum of CoOx-PHCS-it (Figure S12j, SI) shows peaks with the binding energy of 779.7, 780.3, 781.2, 782.6, and 790.1 eV. CoOx-PHCS-it exhibits a new peak at 782.6 eV in comparison with CoOx PHCS. The peak at 782.6 eV can be assigned to CoOOH,49 and suggests more high valence Co species (Co3+) were formed because of long-term measurement under oxidative condition. The O 1s window of XPS spectrum of CoOx-PHCS-it (Figure S12k, SI) shows peaks at 529.4, 531.3, and 532.7 eV. A new peak at 532.7 eV can be assigned to H2O adsorbed on the surface of CoOx-PHCS-it.50-51 To detect the faradaic efficiency of the CoOx PHCS, a potentiostatic electrolysis was carried out in 1 M KOH for 4000 s. The volume of gathered oxygen bubbles was recorded and plotted in Figure 5d. Figure 5d also shows a curve corresponding to theoretical volume of oxygen calculated from charges transferring through circuit. A comparison between theoretical volume and experimental volume of generated oxygen shows a faradaic efficiency of 96% for the CoOx PHCS, confirming that most charges were converted to oxygen via the OER by the CoOx PHCS electrode. Electrochemistry impedance spectroscopy (EIS) of the CoOx PHCS and comparative samples were measured to go insight into their reaction kinetics during OER. The results are shown in Nyquist plots in Figure 6a, in which data were fit using an equivalent circuit shown in Figure S13 (SI). The semicircle in low frequencies range is associated with faradaic process (OER) on the surface of an electrocatalyst, and corresponding charge transfer resistance (Rct) is usually adopted as an indicator of the OER kinetics. A smaller Rct corresponds to a faster OER process. Figure 6a shows clearly that low-frequency semicircle corresponding to the CoOx PHCS has the smallest diameter among those of Co(OH)2 NS, Co3O4 NW, and CoOx HCS. Rct is 3.9 Ω for the CoOx PHCS, 5.3 Ω for CoOx HCS, 9.1 Ω for Co3O4 NW, 8.7 Ω for CoOx NW, and 54.6 Ω for Co(OH)2 NS. The much smaller Rct of the CoOx PHCS is in accordance with its superb performance in OER.


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The electron transport resistance (Ret) during the OER was estimated by the sum of Rs and R1 in the equivalent circuit. The Ret and Rct values of different samples were plotted in Figure 6b. The Rct of the CoOx PHCS is the smallest one among all samples (Figure 6b and Table S2, SI), whereas the Ret value of the CoOx PHCS is analogous to those of comparative samples. It is therefore suggested that the introduction of pores in the core nanowire of CoOx PHCS do not hinder the effective electron transport from current collector (CFP) to the surface of sample.

Figure 6. (a) EIS spectra of CoOx PHCS and comparative samples. (b) Rct, Ret, and Cdl of CoOx PHCS and comparative samples. To account for the superb catalytic activity of the CoOx PHCS in the OER, the ECSA of samples were probed. The ECSA was estimated from electrochemical double layer capacitance (Cdl) of corresponding samples. The Cdl values of different samples were calculated from corresponding CV scans (Figure S14 and Figure 18a, SI), and shown in Figure 6b. The Cdl is 709 mF cm-2 for CoOx PHCS, 441 mF cm-2 for CoOx HCS, 98.5 mF cm2

for CoOx NW, 41 mF cm-2 for Co3O4 NWs, and 19 mF cm-2 for Co(OH)2 NS. The Cdl values

of hierarchical structures, the CoOx PHCS and the CoOx HCS, are much larger than those of Co3O4 NW or Co(OH)2 NS, demonstrating that the construction of hierarchical structure can markedly increases the ECSA of samples. It is worth noting that the Cdl of the CoOx PHCS is 18 ACS Paragon Plus Environment

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1.6 times of that of the CoOx HCS. The extremely large Cdl of the CoOx PHCS is contributed by three factors, including ultrathin nanosheets wrapped on the surface of core nanowires, numerous pores inside core nanowires by electrochemical treatment, and ultra-small grain size (< 5 nm) of Co3O4 resulted from electrochemical treatment. The comparison of Rct, Ret, and Cdl in Figure 6b shows clearly that the CoOx PHCS maintains small electron transport resistance, which correlates with the morphology feature of self-support one-dimensional structure, and markedly increases the active areas for the OER, which is associated to the unique morphology and small particle size. These two features afford the superb performance of the CoOx PHCS in the OER. The influence of the loading amount of nanosheets on the performance of the CoOx PHCS was evaluated. The CoS2 NW were subjected to CV deposition of Co(OH)2 nanosheets for different CV cycles (i.e., 0, 1, 2, 3, and 4), and the samples subjected to electrochemical treatment was denoted as CoOx PHCS-x, where x is the CV deposition cycle of Co(OH)2. Here CoOx PHCS-0 is CoOx NW mentioned above. Figure S15 (SI) shows that the current density first increase with the cycle of CV deposition, with the largest current density from 2 cycles CV deposition. Further increase the cycle of CV deposition results in the decrease of electrode performance. The morphologies of different samples are shown in Figure S16 (SI). Only tiny amount of nanosheets can be found on the surface of CoOx PHCS-1, whereas the amount of nanosheets is so large that the nanowires were nearly buried under nanosheets in CoOx PHCS-3 and CoOx PHCS-4. Only the tip of nanowires can be found in the CoOx PHCS3, while no nanowires can be found in the CoOx PHCS-4. The EIS spectra (Figure S17, SI) of samples with different CV deposition cycles were also measured. The Cdl, Ret, and Rct derived from EIS spectra and CV measurements (Figure S18, SI) were shown in Figure S17b (SI). With little nanosheet (CoOx PHCS-1), the electron transport is fast, whereas the active sites is small. With amount of nanosheets increased, the Ret increases gradually, whereas the Cdl is firstly increased and then decreased. Increasing amount of nanosheets would contribute to 19 ACS Paragon Plus Environment

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more active sites, whereas excessive nanosheets would result in dense film on the surface of CFP. The dense film not only reduces active sites from nanosheets, also hinders the treatment of buried nanowires, resulting in smaller Cdl of samples. The variation of Ret is ca. 25% among all samples, whereas Cdl of CoOx PHCS-2 is 120% of that of CoOx PHCS-1 and 167% of that of CoOx PHCS-3. It is apparently shown that the performance difference is mainly associated with different numbers of active sites in different samples.

3. Conclusion CoOx PHCS have been synthesized by a facile method, and demonstrated efficient catalytic activity in the OER. The η20 is 270 mV, the η100 is 300 mV and the Tafel slope is 40.3 mV dec-1. The CoOx PHCS can work stably for long-term water electrolysis. The performances is superior to commercial noble metal electrocatalyst (RuO2) and reported non-precious metal electrocatalyst. The performance is correlated with the self-supported configuration and the integration of hierarchical core-shell structure and porous structure in the unique PHCS.

4. Experimental Section Reagents. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, AR), urea (CO(NH2)2, AR), and sulfur (CP) was purchased from Sinopharm Chemical Reagent CO., Ltd. Ammonium fluoride (NH4F, GR) was purchased from shanghai Aladdin Industrial Corporation. CFP was commercially available from Shanghai Hesen. All chemicals were used as received without further purification. Synthesis. CFP was cut into 10×60 mm2 pieces, and ultrasonically cleaned by ethanol, H2SO4 solution, and water in sequence, resulting in a hydrophilic surface. Then, Co(NO3)2·6H2O (0.87 g), CO(NH2)2 (0.90 g) and NH4F (0.22 g) were dissolved in 80 mL water, and the solution was transferred to a 100 mL Teflon-lined stainless steel autoclave. One piece of CFP 20 ACS Paragon Plus Environment

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was immersed in the solution. The autoclave was then heated in an electrical oven at 120 °C for 7 h, and naturally cooled down to room temperature. The products (CoCH NW) were rinsed ultrasonically several times with deionized water and ethanol, and dried at 80 oC in atmosphere. A CFP loaded with CoCH NWs and sulfur powder were put together in a porcelain crucible, and the crucible was covered by a quartz plate. The crucible was placed in the middle of a horizontal quartz tube inside a tube furnace. The air in the quartz tube was removed by purging the tube with nitrogen (99.99%), and the flow rate of nitrogen is 100 sccm during heat treatment. The tube furnace was heated to 450 oC with a ramping rate of 20 o

C min-1, and held at 450 oC for 1 h, resulting in CoS2 NW. Afterwards, the furnace was

naturally cooled to room temperature under nitrogen atmosphere. CoS2 NW loaded on CFP were used as the scaffold for the growth of ultrathin Co(OH)2 NS. The electrodeposition was carried out in a standard three-electrode glass cell at 25 °C, CoS2 NW loaded on CFP as a working electrode, saturated calomel electrode (SCE) as a reference electrode, and a graphite rod as a counter-electrode. The electrolyte is an aqueous solution of Co(NO3)2 (0.05mol l-1). Ultrathin Co(OH)2 NS was deposited onto CoS2 NW by CV .The CV deposition was conducted in a potential range of −0.5 to −1.1 V versus SCE with a sweep rate of 10 mVs-1 for different cycles (e.g., 1, 2, 3, and 4 cycles). The resultant sample (CoS2-Co(OH)2 HCS) was rinsed with deionized water. The CoS2-Co(OH)2 HCS was converted to CoOx PHCS by LSV in 1 M KOH solution at a scan rate of 5 mV s-1 for more than 4 LSV scan. A mercury/mercury oxide electrode (MOE) was used as a reference electrode, and a graphite rod was used as a counter electrode. Characterization. The morphology of the hierarchical structure is assessed by SEM (S4800, Hitachi) and TEM (Tecnai G2 F30 S-TWIN, FEI). XRD patterns were collected using a Bruker D8 Advance diffractometer with graphite-monochromated Cu Kα radiation (1.54178 Å). The XPS experiments were performed on an ESCALAB250Xi System (ThermoFisher) 21 ACS Paragon Plus Environment

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equipped with a monochromatic Al Kα (1486.6 eV) source and a concentric hemispherical energy analyzer. The Raman spectra were recorded by using a DXR Raman spectrometer (ThermoFisher, USA). The excitation source was a 532 nm laser. Electrochemistry measurement. Electrochemistry measurements were carried out by an electrochemistry workstation (CHI 614D) in a three-port glass chamber. A MOE was used as a reference electrode, and a graphite rod was used as a counter electrode. The counter electrode was separated from the chamber of working electrode by a porous glass frit. The electrolyte is an aqueous KOH solution (1 M). The RHE was determined by the open circuit potential of a clean Pt electrode in the solution of interest bubbled with H2 (99.999%). A scan rate of 5 mV S-1 is adopted in the measurement of polarization curve. The measured potential is corrected with the ohmic drop (iR), where i is the current corresponding to the experimental potential and R is the uncompensated cell resistance estimated by currentinterrupt method. The apparent Tafel slope was derived from the iR-corrected polarization curve by fitting experimental data to the equation η=a+blogj, where η is the iR-corrected potential, a is the Tafel constant, b is the Tafel slope, and j is the current density. Electrochemical impedance spectroscopy (EIS) measurements were carried out at 1.57 V vs RHE in the frequency range of 10-2 to 106 Hz with 10 mV sinusoidal perturbations and 12 steps per decade. The volume of H2 during the potentiostatic electrolysis experiment was monitored by the water displacement method, and the details has been introduced in our previous publication.52-53

Supporting Information XRD pattern, and TEM and SEM of CoCH NW, CoS2 NW, CoS2-Co(OH)2 HCS, CoOx NW, Co(OH)2 NS, Co3O4 NW, CoOx HCS, CoOx PHCS corresponding to different CV deposition cycles, and CoOx PHCS-it. EDS, Raman, and XPS spectra of CoOx PHCS and CoS2-Co(OH)2 HCS. Performance of typical reported OER electrocatalysts in alkaline media. CV curves of 22 ACS Paragon Plus Environment

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CoOx PHCS, CoOx HCS, Co3O4 NW, Co(OH)2 NS, and CoOx PHCS corresponding to different CV deposition cycles. Polarization curves of the CoOx PHCS corresponding to different CV deposition cycles

Acknowledgements This research was financially supported by the National Natural Science Foundation of China (51772214, and 51432006), the Ministry of Science and Technology of China (2011DFG52970), the Ministry of Education of China (IRT14R23), 111 Project (B13025), Jiangsu Province (2011-XCL-019 and 2013-479), Innovation Program of Shanghai Municipal Education Commission, and the Natural Science Foundation of Jiangsu (BK20131252).

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ToC figure


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Figure 1. Synthesis process of CoOx PHCS 409x186mm (150 x 150 DPI)

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Figure 2. (a-c) SEM images of CoOx PHCS. (d,e) TEM images of CoOx PHCS. (f) SAED rings of CoOx PHCS. (g) Reciprocal distances derived from SAED. (h) HRTEM image of a nanosheet in CoOx PHCS. Inset of (d) shows the thickness distribution of nanosheets wrapping on the surface of nanowire. White arrows in (e) indicate typical pores on the core nanowire. Inset of (e) shows the diameter distribution of pores in nanowire. 260x303mm (150 x 150 DPI)


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Figure 3. (a) HAADF image of CoOx PHCS. EDS elemental mapping of (b) Co and (c) O in CoOx PHCS 338x112mm (57 x 59 DPI)


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Figure 4. XPS spectra of (a) Co 2p window and (b) O 1s window collected from CoOx PHCS. 212x84mm (150 x 150 DPI)


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Figure 5. The electrocatalytic performance of the catalyst. (a) Polarization curves and (b) Tafel of different catalytic electrodes. (c) Current-time curve recorded in a potentiostatic electrolysis experiment. The applied potential is 1.52 V vs. RHE. Inset of (c) shows the LSV curves before and after potentiostatic electrolysis experiment. (d) Current efficiency for O2 produce under potentiostatic electrolysis experiment (applied potential is 1.6 V vs. RHE). All potentials are corrected with iR drop. 235x177mm (150 x 150 DPI)


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Figure 6. (a) EIS spectra of CoOx PHCS and comparative samples. (b) Rct, Ret, and Cdl of CoOx PHCS and comparative samples. 245x108mm (150 x 150 DPI)


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TOC Graphic 84x47mm (150 x 150 DPI)


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A Self-Supported Porous Hierarchical CoreâShell Nanostructure of

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