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Morphology-Controllable Synthesis of Zn-Co Mixed Sulfide Nanostructures on Carbon Fiber Paper Towards Efficient Rechargeable Zinc-Air Batteries and Water Electrolysis Xiaoyu Wu, Xiaopeng Han, Xiaoya Ma, Wei Zhang, Yida Deng, Cheng Zhong, and Wenbin Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16602 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 20, 2017

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ACS Applied Materials & Interfaces

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Morphology-Controllable Synthesis of Zn-Co Mixed

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Sulfide Nanostructures on Carbon Fiber Paper

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Towards Efficient Rechargeable Zinc-Air Batteries

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and Water Electrolysis

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† †§ † † † †‡ Xiaoyu Wu, Xiaopeng Han,*, , , Xiaoya Ma, Wei Zhang, Yida Deng, Cheng Zhong*, , and

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Wenbin Hu†,‡

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†

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and Engineering, ‡Key Laboratory of Advanced Ceramics and Machining Technology (Ministry

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of Education), Tianjin University, Tianjin 300072, China and §Key Laboratory of Advanced

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Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China.

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KEYWORDS: Zn-air battery, metal sulfide, morphology controllable, electrocatalysis, water

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splitting

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ABSTRACT: It remains an ongoing challenge to develop cheap, highly active and stable

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electrocatalysts to promote the sluggish electrocatalytic oxygen evolution, oxygen reduction, and

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hydrogen evolution reactions for rechargeable metal-air batteries and water-splitting systems. In

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this work, we report the morphology-controllable synthesis of zinc cobalt mixed sulfide (Zn-Co-

Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science

ACS Paragon Plus Environment

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S) nanoarchitectures, including nanosheets, nanoplates, and nanoneedles, grown on conductive

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carbon fiber paper (CFP) and the micro-nanostructure dependent electrochemical efficacy for

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catalyzing hydrogen and oxygen in zinc-air batteries and water electrolysis. The formation of

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different Zn-Co-S morphologies was attributed to the synergistic effect of decomposed urea

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products and the corrosion of NH4F. Among synthesized Zn-Co-S nanostructures, the

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nanoneedle arrays supported on CFP exhibit superior tri-functional activity for oxygen reduction,

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oxygen evolution, and hydrogen evolution reactions than its nanosheet and nanoplate

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counterparts through half reaction testing. It also exhibited better catalytic durability than Pt/C

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and RuO2. Furthermore, the Zn-Co-S nanoneedle/CFP electrode enables rechargeable Zn-air

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batteries with low overpotential (0.85 V), high efficiency (58.1 %), and long cycling lifetimes

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(200 cycles) at 10 mA cm-2 as well as considerable performance for water splitting. The superior

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performance is contributed to the integrated nanoneedle/CFP nanostructure, which not only

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provides enhanced electrochemical active area, but also facilitates ion and gas transfer between

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the catalyst surface and electrolyte, thus maintaining an effective solid-liquid-gas interface

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necessary for electrocatalysis. These results indicate that the Zn-Co-S nanoneedle/CFP system is

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a low cost, highly active, and durable electrode for highly efficient rechargeable zinc-air batteries

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and water electrolysis in alkaline solution.

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INTRODUCTION

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Developing high-performance metal-air batteries, fuel cells, and water-splitting energy

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technologies has been seriously precluded by the intrinsic sluggish kinetics and large

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overpotential of the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and

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hydrogen evolution reaction (HER).1-3 In particular, electrocatalysts with high ORR and OER

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activities are of significant importance to promote rechargeable metal-air batteries and


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regenerative fuel cells whereas efficient water splitting requires bifunctional catalysts for both

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HER and OER.4-7 Currently, Pt-based metals are recognized as the best ORR/HER catalysts and

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Ru/Ir-based oxides possess the best OER activities.8-10 However, considering their high cost and

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limited applicability, and more importantly, their non-bifunctional capability for ORR/OER or

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HER/OER, it remains a great challenge to develop high performance, inexpensive

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multifunctional alternatives based on earth-abundant elements for the wide-spread applications

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of the relevant renewable energy systems.

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Recently, transition metal sulfides were demonstrated to be one of the most promising catalysts

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for hydrogen and oxygen electrocatalysis due to their low cost, environmentally benign and

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significant catalytic properties.11-13 It is noteworthy that cobalt-based binary or ternary sulfides

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compounds have displayed bifunctional catalytic performance, such as CoS2, Fe0.5Co0.5S and

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NiCo2S4 for ORR/OER;14-16 and Co9S8, Co3S4, and Zn0.76Co0.24S for HER/OER.17-19 Furthermore,

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previous studies have demonstrated that the catalytic behaviors of these electrocatalysts could be

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dramatically enhanced using nanostructured strategies by tuning the surface electronic structure

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and increasing the number of exposed catalytic sites, as shown for ultrathin Co3S4 nanosheets,18

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ZnxCo1-xS nanowires arrays,19 CoS2 nanowires,20 and NiCo2S4 nanotubes.21 Understandably, the

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atomic arrangement and morphology of these electrocatalysts play a critical role in determining

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their electrocatalytic capabilities. Accordingly to the discussion above, development of cobalt

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metal-based sulfides with tailored active components may realize the multifunctionality of ORR,

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OER and HER. Moreover, use of the nanostructuring strategy could further enable the designed

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electrode materials to have high activity and durability through the optimization of surface

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configurations, electronic states and morphological architectures, resulting in the fabrication of


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rechargeable zinc-air batteries and water oxidation devices with high efficiencies and long

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operational life times.

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In this work, we demonstrate a novel two-step method of the controllable synthesis of mixed

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zinc cobalt sulfide (Zn-Co-S) nanostructures (nanosheet (NS), nanoplate (NP), and nanoneedle

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(NN)) supported on carbon fiber paper (CFP) substrate and a detailed investigation of their

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multifunctional electrocatalytic properties. The preparation of Zn-Co-S nanostructures was

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achieved via a facile hydrothermal method, followed by a thermal sulfidation technique. Three

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different Zn-Co-S morphologies (NS, NP and NN) were synthesized by tailoring the amount of

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urea and NH4F added before the hydrothermal process. The parallel formation of Zn-Co-S NS,

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NP and NN with identical phases and compositions allows us to elucidate the morphological

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influence on their ORR, OER and HER behaviors. Among these three different nanostructures,

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the one-dimensional (1D) Zn-Co-S NN in situ decorated CFP exhibits a high double-layer

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capacitance and a low charge transfer resistance, indicating a large electrochemical active

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surface area with numerous catalytic active sites and faster catalytic kinetics. Furthermore, the

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nanoneedle arrays, which are vertically grown on CFP, facilitate ion and gas transfer between the

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catalyst surface and the electrolyte, especially in the processes of OER and HER, thus

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maintaining the highly efficient solid-liquid interface for reagent/product displacement.

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Benefiting from these favorable factors, the Zn-Co-S NN/CFP electrode displays enhanced

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activity and remarkable long-term durability towards the ORR, OER and HER catalytic reactions

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under alkaline conditions as well as having a low overpotential and considerable cycle stability

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when employed in a rechargeable Zn-air battery and water splitting system.

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RESULTS AND DISCUSSION


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Figure 1. Schematic illustration of the preparation of Zn-Co-S nanosheet, nanoplate, and

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nanoneedle on CFP.

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Three different morphologies of Zn-Co-S nanostructures supported on CFP were synthesized

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using a facile hydrothermal preparation of the corresponding carbonate hydroxide precursors

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followed by thermal sulfidation, as depicted by the schematic in Figure 1. The surface

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differences in the intermediates are considered to be a result of the synergistic effect between the

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decomposed products of urea and the corrosion of NH4F during the hydrothermal process.22, 23

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The resulting Zn-Co-S samples maintain their original one or two-dimensional morphologies.

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The in situ growth of Zn-Co-S on conductive CFP ensures efficient electron transfer between the

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substrate and the supported active material. Powder X-ray diffraction (XRD) spectra of the three

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synthesized precursors and their final composites scraped down from CFP are presented in

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Figure S1. The diffraction peaks of Zn-Co-S precursors matched well with the standard peaks of

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Zn4CO3(OH)6•H2O (JCPDS No. 11-0287), but are slightly shifted to higher angles (Figure S1a,b),

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which may be ascribed to the substitution of smaller Co2+ ions (radius rCo2+ = 0.065 Å) for Zn2+


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(radius rZn2+ = 0.074 Å) in the metal sites. This hypothesis is supported by the energy dispersive

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spectroscopy (EDS) (Figure S2), which reveal that Zn and Co ions co-exist in the prepared Zn-

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Co-S precursors. Thus, the Zn-Co-S precursors can be regarded as Co-doped Zn4CO3(OH)6·H2O

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with a chemical formula of ZnxCo4-xCO3(OH)6•H2O. The broad peaks with low intensity indicate

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the poor crystallization of the precursors.24 As shown in Figure S1c, the diffraction peaks of final

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Zn-Co-S compounds could be readily assigned to the mixtures of Zn0.76Co0.24S (JCPDS No. 47-

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1656) and CoS2 (JCPDS No. 41-1471), demonstrating the successful phase conversion during the

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thermal sulfidation treatment. This is also evidenced by the colour change from pink to black of

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the CFP surface after heating (Figure S3).

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Scanning electron microscopy (SEM) images of the carbonate hydroxide precursors (a-c) and

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the resulting Zn-Co-S nanostructures (d-f) are displayed in Figure 2. Three typical morphologies,

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ultrathin nanosheet (a, d), nanoplate (b, e) and nanoneedle (c, f), can be clearly seen, which

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densely cover on the surface of the CFP. Although subtle morphological change can be observed

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before and after the sulfidation processes, for example, the distribution of the needles are

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different in Figure 2c vs. 2f, the nanostructured morphologies of the precursors are mostly

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sustained in the final Zn-Co-S with increased surface roughness. Moreover, EDS mapping

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revealed the homogenous distribution of Zn, Co, and S (Figure S4), suggesting the uniform

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deposition of metal sulfide on the CFP support. Transmission electron microscopy (TEM)

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images further show the detailed structure information of Zn-Co-S nanostructures. Compared to

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the ultrathin nanosheet in Figure 3a, the nanoplate (Figure 3b) and nanoneedle (Figure 3c) are

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composed of pore enriched structures interconnected by nanoparticles. The nanosheet is

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comprised of several single layers with a single thickness around 0.9 nm whereas the nanoplate

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is determined to be about 3.6 nm thick (Figure S5), as characterized by atomic force microscopy


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(AFM). Furthermore, in the high-resolution TEM (HRTEM) images of Zn-Co-S NS, NP, and

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NN (Figure 3d and Figure S6), the measured lattice spacings of 0.31 nm and 0.25 nm are indexed

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to the (111) plane of Zn0.76Co0.24S and (210) plane of CoS2, respectively, confirming the XRD

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analysis. The EDS elemental mapping further reveal the homogenous dispersion of Zn, Co, and

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S in a single nanoneedle (Figure 3e), suggesting that the two components of Zn0.76Co0.24S and

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CoS2 are homogeneously dispersed in the nanocomposite. In addition, the determined Brunauer–

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Emmett–Teller (BET) specific surface area is 25.6 m2 g-1 for Zn-Co-S NN and the corresponding

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pore distribution is about 60 nm in size calculated by Barrett-Joyner-Halenda (BJH) method

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(Figure S7). Moreover, the elemental ratio of Zn: Co: S in Zn-Co-S NN is determined to be

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about 1:1.7:4.5 by both the EDS and inductively coupled plasma (ICP) spectrometry analysis

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(Figure S8).

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Figure 2. SEM images of the as-synthesized precursors (a-c) and the corresponding sulfides on

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CFP (d-f). (a, d) Zn-Co-S NS/CFP, (b, e) Zn-Co-S NP/CFP, and (c, f) Zn-Co-S NN/CFP.

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Figure 3. TEM images of (a) Zn-Co-S NS, (b) NP and (c) NN in different magnification. (d)

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High-resolution TEM image of Zn-Co-S NN and (e) EDX elemental mapping of Zn, Co, and S

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in Zn-Co-S NN.

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The morphological evolution of Zn-Co-S precursors with various relative ratios of urea and

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NH4F was investigated in detail. We found that when the added urea was kept at 1.4 mmol, while

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the added amount of NH4F was increased from 0 mmol to 1.12 mmol, there was an obvious


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morphological evolution from the nanoplate to the interlinked ultrathin nanosheet (Figure 2a, b).

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The reason is speculated as follows: in the absence of NH4F, the Co2+ and Zn2+ ions in solution

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quickly reacted with CO32- and OH- by the decomposition of urea, leading to a faster reaction

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rate.23 However, with the presence of NH4F, metal ions would coordinated to form metal-Fx(x-2)-

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ions, leading to the decreased release rate of metal ions from the complex ions.22 The lower

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growth rate contributed to thinner lamellar, consequently forming the preferred ultrathin

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nanosheet morphology. Otherwise, when the added amount of urea was increased to 2.0 mmol,

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the morphology of the products changed from the nanosheet to the nanoneedle structures in the

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presence of NH4F due to the higher concentrations of CO32- and OH- by the hydrolysis-

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precipitation of urea (Figure 2a, c)25, while nanoplate structures were obtained in both cases with

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no NH4F added (Figure 2b and 4a). Meanwhile, when the added urea was kept at 2.0 mmol, upon

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increasing the amount of NH4F (0, 0.8, 1.2, 2.4 mmol), the final morphologies varied from

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nanoplate to narrow-nanoplate to nanoneedle and lastly to gathered agglomerations (Figure 4a-

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d), which may be ascribed to the gradually enhanced etching effect of NH4F. Too much NH4F

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led to the destruction of the needle structure and formed gathered products, approaching the

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“coalescence growth” mechanism reported previously.26 In summary, the decomposed species of

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urea and the corrosion of NH4F simultaneously play important roles in tailoring the

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morphologies of nanosheets, nanoplates, and nanoneedles in our experiments. Otherwise,

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without the support of CFP, the synthesized Zn-Co-S product trends to be agglomerated particles

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in sheet-like morphology (designated as bulk Zn-Co-S, Figure S9), which further reveals the

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critical role of CFP substrate in the formation of Zn-Co mixed sulfide nanostructures.


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Figure 4. Influence of NH4F on the morphology of Zn-Co-S precursors when the used urea was

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kept at 2.0 mmol. The added NH4F are (a) 0 mmol, (b) 0.8 mmol, (c) 1.2 mmol, (d) 2.4 mmol,

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respectively.

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The chemical valence states of Zn, Co, and S in Zn-Co-S NN/CFP were characterized by X-

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ray photoelectron spectroscopy (XPS, Figure 5). The XPS survey spectrum in Figure 5a shows

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the existence of Zn, Co, S, as well as C and O. The C might be introduced during the scraping of

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the sample from the CFP, and the existence of O is due to exposure to air.27 As revealed in

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Figure 5b, the Co 2p spectrum fit well with two spin-orbit doublets: the binding energies for Co

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2p3/2 at 779.3 eV and 782.5 eV can be ascribed to Co3+ and Co2+, respectively, while the two

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peaks at 795.1 eV and 798.9 eV, correspond to the spin-orbit characteristics of Co 2p1/2, also

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revealing the existence of Co3+ and Co2+.28 The peaks at 786.3 eV and 804.1 eV can be regarded

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as the shake-up satellites (larbelled as Sat. in Figure 5b), in accordance with the literature for

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Zn0.76Co0.24S.29 In the Zn 2p XPS spectrum (Figure 5c), the binding energies of 1022.7 eV for


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2p3/2 and 1045.9 eV for 2p1/2 are spin-orbits of Zn2+.30 For the high-resolution S 2p spectrum

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(Figure 5d), a weak doublet situated at 163.1 eV and 162.0 eV corresponds to S 2p1/2 and S 2p3/2,

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respectively.24 The peak at 163.9 eV indicates the presence of a metal-sulfur (M-S) bond in the

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Zn-Co-S compound.31 In summary, XPS results demonstrate that the chemical composition of

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Co2+, Co3+, Zn2+, and S2- coexist in the synthesized Zn-Co-S nanostructure, which are consistent

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with the reported results in the literatures.19, 29

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Figure 5. (a) XPS survey spectra, (b) Co 2p, (c) Zn 2p, and (d) S 2p high resolution spectra of

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Zn-Co-S NN, respectively.

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In order to evaluate their electrocatalytic properties toward catalyzing OER, the Zn-Co-S

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nanostructures along with their counterparts, bare CFP and RuO2, were tested in alkaline

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electrolyte (1.0 M KOH) using a three-electrode system. As shown in Figure 6a and Table 1, the


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Zn-Co-S NN/CFP achieves a current density of 10 mA cm-2 at an overpotential of 320 mV,

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which outperforms those of Zn-Co-S NP/CFP (330 mV) and Zn-Co-S NS/CFP (390 mV) and is

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similar to that of RuO2. Moreover, the evolved current density of RuO2 surpasses that of Zn-Co-

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S NP at 1.69 V, but is still lower than that of Zn-Co-S NN at more positive potentials (> 1.55 V

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vs. RHE). The fitted Tafel slope value of Zn-Co-S NN/CFP (55 mV dec-1, Figure 6b) is lower

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than other electrodes (Zn-Co-S NS/CFP: 136 mV dec-1; Zn-Co-S NP/CFP: 97 mV dec-1) and

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even that of RuO2 (69 mV dec-1), indicating more favorable OER kinetics over the nanoneedle

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surface. Therefore, the morphology of catalysts plays an important role in determining their

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electrocatalytic OER activities. In order to extract the double-layer capacitance (Cdl) of these

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three catalysts, we measured the capacitive currents as a function of scan rates within the voltage

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range from 1.19 to 1.29 V. The value of Cdl offers an estimate of electrochemical active surface

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area (ECSA) of the electrode-electrolyte interface.32 The corresponding Cdl of the Zn-Co-S NS,

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NP and NN nanostructures are determined to be 9.99, 32.23, and 35.47 mF cm-2, respectively

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(Figure S10), which demonstrate that the 1D nanoneedle structure could provide a larger

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effective active area as compared to the 2D nanosheet and nanoplate. Since ion and charge

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transports are crucial for efficient electrocatalysis, electrochemical impedance spectroscopy (EIS)

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was measured (Figure 6c). The fitted charge transfer resistance (Rct) values are 1.8, 3.3, and 7.2

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Ω for Zn-Co-S NN, NP, and NS samples (Table 1), respectively, which are consistent with the

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trend of corresponding OER Tafel slopes, further confirming that the superior activities might be

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ascribed to the faster catalytic kinetics.33 The OER performance of Zn-Co-S NN/CFP here is one

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of the most highly efficient electrocatalysts based on non-noble metals, as listed in Table S1.

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Moreover, no obvious change can be found in the polarization curves of Zn-Co-S NN/CFP

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catalyst after 1000 continuous potential cycles, demonstrating the remarkable long-term catalytic


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durability for OER electrocatalysis (Figure 6d). After 500 repeated CV cycles, the needle-like

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morphology of the Zn-Co-S NN electrode was still essentially preserved (Figure S11), further

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confirming the structural durability and electrochemical robustness in the range of oxidation

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potentials studied and mainly contributed to the intrinsic crystal stability and firm attachment

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between the in-situ grown nanostructures and the CFP support.

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Figure 6. (a) OER polarization curves of commercial RuO2, bare CFP and Zn-Co-S NS, NP,

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NN/CFP at a scan rate of 5 mV s-1 in 1.0 M KOH. (b) Corresponding Tafel curves. (c) EIS

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Nyquist plots. Inset shows the simplified Randles equivalent circuit used for fitting (Rs, CPE,

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and Rct are the electrolyte resistance, double layer capacity, and charge transfer resistance,

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respectively). (d) Polarization curves of Zn-Co-S NN/CFP before and after 1000 cycles. (e) ORR

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polarization curves of Zn-Co-S nanostructures and Pt/C in O2-saturated 1.0 M KOH. (f)

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Chronoamperometric response of Zn-Co-S NN/CFP and Pt/C at a constant potential of 0.7 V.

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In the context of developing bifunctional reversible oxygen electrode for rechargeable metal-

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air batteries and regenerative fuel cells, the Zn-Co-S/CFP samples were also evaluated as ORR

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electrodes. As shown in Figure 6e, the onset reduction potential of Zn-Co-S NN/CFP is located

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at about 0.81 V, which is more positive than that of Zn-Co-S NP/CFP (~0.78 V), Zn-Co-S

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NS/CFP (~0.76 V), and is only 70 mV lower than Pt/C (~0.88 V), respectively. Similar trends

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can also be observed in the case of oxygen reduction current densities. Moreover, rotation ring-

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disk electrode (RRDE) studies reveal that the ORR on Zn-Co-S NN scraped down from CFP

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proceeds via an apparent quasi-4e- pathway (Figure S12), further confirming the highly efficient

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ORR process on Zn-Co-S NN. Thus, the Zn-Co-S NN/CFP electrode exhibits considerable

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reversible oxygen electrocatalytic activity. Additionally, the chronoamperometric response in

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Figure 6f shows a current loss of 10.2 % after 36,000 s for Zn-Co-S NN/CFP, which is much

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lower than that of commercial Pt/C catalyst (45.9 % loss), suggesting a remarkable ORR

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catalytic durability of Zn-Co-S NN/CFP over a medium period of time. The morphological

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destruction and possible exfoliation of active catalyst from the CFP primarily account for the

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faster rate of current loss for Zn-Co-S NN after 32,000 s (Figure S13). Extensive understanding

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about the degradation mechanism during catalyzing ORR requires further investigations.


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In the context of developing bifunctional catalysts for overall water oxidation, the HER

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performance of synthesized samples were tested in the same electrolyte. All the linear sweeping

3

voltammetry (LSVs) measurements were taken at a scan rate of 5 mV s-1, while continuously

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purging with N2 during the experiment. Figure 7a compares the LSVs of the three different Zn-

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Co-S morphologies, commercial Pt/C, and bare CFP. It is clear that the bare CFP shows weak

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electrochemical activity. Among the synthesized Zn-Co-S nanostructures, the HER activities

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follows an order of NN>NP>NS. Specifically, the Zn-Co-S NN/CFP achieves a current density

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of 10 mA cm-2 at an overpotential of 234 mV, which is much lower than those of Zn-Co-S

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NP/CFP (304 mV) and Zn-Co-S NS/CFP (415 mV), respectively (Table 1). The corresponding

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Tafel slopes of Zn-Co-S NS, NP, NN, and Pt/C are 139, 131, 109 and 51 mV dec-1, respectively

11

(Figure 7b). In particular, Zn-Co-S NN shows the lowest Tafel slope of 109 mV dec-1 among Zn-

12

Co-S catalysts, which is comparable to those of transition metal sulfide catalysts reported

13

recently.19, 34 The disparity in Tafel slopes further confirms the advantage of the 1D nanoneedle

14

structure. Additionally, the slope values fall in the range from 100 to 140 mV dec-1, suggesting

15

the HER occurred on the surface of Zn-Co-S mainly proceeded via Volmer-Heyrovsky

16

mechanism.35 The presented HER performance catalyzed by Zn-Co-S NN/CFP is among the

17

most active catalysts based on transition metal elements (Table S2). In addition, in a continuous

18

polarization period of 36,000 s (Figure 7c), the HER current retention of Zn-Co-S NN (90.3 %)

19

is much higher than the benchmark Pt/C (58.9 %), verifying the remarkable catalytic durability.

20

After 500 potential cycles, the morphology of the composite still sustains the loosely porous

21

nanoneedle appearance (Figure 7d), indicating firm adhesion between the Zn-Co-S nanostructure

22

and the CFP support and good mechanical strength of the integrated electrode.


15


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Page 16 of 40

1 2

Figure 7. (a) HER polarization curves of commercial Pt/C, bare CFP, Zn-Co-S NS, NP, NN/CFP

3

at a scan rate of 5 mV s-1 in 1.0 M KOH. (b) Corresponding Tafel slopes. (c)

4

Chronoamperometric response of Zn-Co-S NN/CFP and Pt/C at overpotential to reach a current

5

density of 10 mA cm-2. (d) SEM image of Zn-Co-S NN/CFP electrode after 500 potential cycles.

6

It should be noted that the electrocatalytic activities of fabricated Zn-Co-S nanostructures on

7

CFP clearly outperformed those of bulk Zn-Co-S sample (Figure S14), further proving the

8

advantages of the in situ grown integrated Zn-Co-S/CFP catalytic electrode. Since the Zn-Co-S

9

nanoneedle decorated CFP achieved the best OER, ORR and HER properties, the Zn-Co-S

10

NN/CFP is anticipated as a promising efficient multifunctional and durable electrode applied in

11

realistic metal-air batteries and overall water splitting. Accordingly, a primary and rechargeable


16

Page 17 of 40

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1

zinc-air cell and a two-electrode water electrolyzer were constructed in alkaline electrolyte using

2

the Zn-Co-S NN/CFP as the air cathode in the zinc-air cell and as the cathode and anode in the

3

water electrolyzer. The state-of-art Pt/C and RuO2 electrodes were also prepared for comparison

4

with the same mass loading of 0.6 mg cm-2. The primary Zn-air battery with Zn-Co-S NN/CFP

5

as the cathode has a stable open circuit of about 1.36 V and could deliver a high discharge

6

capacity of 484.7 mA h g-1 based on the mass of consumed Zn plate, corresponding to a high

7

energy density of 565.5 Wh kg-1 at 10 mA cm-2 (Figure S15). The Zn-Co-S NN/CFP-based

8

rechargeable Zn-air battery displays comparable discharge/recharge behavior as the highest-

9

performing commercial Pt/C and RuO2 mixed catalyst (Pt/C+RuO2, Figure 8a). Specifically, the

10

initial charge and discharge potentials of Zn-Co-S NN/CFP electrode are ca. 2.03 and 1.18 V at

11

10 mA cm-2 with a low overpotential of 0.85 V and a high round-trip efficiency of 58.1 %,

12

similar to that of Pt/C+RuO2 cathode, confirming its excellent bifunctional catalytic capability

13

for ORR and the inverse OER. More interestingly, the cell catalyzed by Zn-Co-S NN/CFP shows

14

exceptional rechargeability for 200 cycles (Figure 8b), in sharp contrast to that of Pt/C+RuO2

15

(less than 55 cycles), indicating superior recycling durability. The reversibility of the

16

rechargeable Zn-air cells here is comparable to the reported results from recent publications

17

(Table S3). Meanwhile, Zn-Co-S NN/CFP was applied as both cathode and anode to electrolyze

18

water (Figure 8c), presenting a cell voltage of 1.71 V to achieve the water splitting current

19

density of 10 mA cm-2. The generated hydrogen and oxygen bubbles can be clearly observed on

20

the electrode surface (inset in Figure 8c). The long-term durability of the above water-splitting

21

system was evaluated with continuous polarization for 36,000 s at 1.90 V. The activity decay of

22

Zn-Co-S NN/CFP (27.1 %) is much slower than that of Pt/C+RuO2 (50.8 %, Figure 8d),


17


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Page 18 of 40

1

demonstrating the remarkable stability of the Zn-Co-S NN/CFP electrode in a full water-splitting

2

device.

3

The superior multifunctional catalytic performance of the Zn-Co-S NN/CFP can be attributed

4

to several synergistic factors. First, carbon fiber paper was employed as a low-cost, electrically

5

conductive support, which can ensure efficient electron transfer during the electrocatalytic

6

processes. Second, the nanoneedle nanostructure not only provides enhanced electrochemical

7

surface active area, but also facilitates gas and ion transfer between the catalyst surface and the

8

electrolyte, thus maintaining the effective solid-liquid interface necessary for electrocatalysis

9

(see Figure S16 for gas transfer mechanism in OER and HER processes).36 Third, the binder-free

10

integration feature of Zn-Co-S NN arrays grown in situ on CFP with firmed adhesion contributes

11

to the increased mechanical stability and electrochemical robustness, affording the respectable

12

long-term durability and cyclability.


18

Page 19 of 40

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1 2

Figure 8. (a) Voltage-current polarization curves of the Zn-Co-S NN/CFP and Pt/C+RuO2

3

catalyst. (b) Discharge/recharge curves of rechargeable Zn-air batteries catalyzed by Zn-Co-S

4

NN/CFP and Pt/C+RuO2 at 10 mA cm-2 with duration of 400 s per cycle. (c) Water-splitting

5

polarization curves of Pt/C+RuO2 and Zn-Co-S NN/CFP+Zn-Co-S NN/CFP at a scan rate of 5

6

mV s-1. Inset shows the gas evolution at 10 mA cm-2. (d) Chronoamperometric response of two

7

overall water splitting systems at 1.9 V.

8 9 10

Table 1. Summary of the electrochemical activities of Zn-Co-S/CFP electrodes with three different morphologies.


19


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

Catalysts

Loading (mA cm-2)

Zn-Co-S NS/CFP

~0.56

Zn-Co-S NP/CFP

~0.60

Zn-Co-S NN/CFP

~0.60

Reaction

Tafel slope Overpotential -2 (mA dec-1) @10 mA cm (mV)

HER

139

415

OER

136

390

HER

131

304

OER

97

330

HER

109

234

OER

55

320

Page 20 of 40

ORR onset potential

Rct Cdl (Ω) (mF cm-2)

0.76

7.2

9.99

0.78

3.3

32.23

0.81

1.8

35.47

1 2

CONCLUSION

3

In summary, we have successfully synthesized three morphologies of Zn-Co-S nanostructures

4

on CFP by tailoring the combined effect of urea decomposition products and the corrosion of

5

NH4F. Compared to Zn-Co-S NS/CFP and NP/CFP, the 1D Zn-Co-S NN/CFP exhibits superior

6

multifunctional catalytic performance in alkaline electrolyte with an ORR onset potential of 0.81

7

V, overpotentials of 234 and 320 mV to achieve HER and OER current density of 10 mA cm-2,

8

respectively, as well as considerable long-term durability. Remarkably, the Zn-Co-S NN/CFP

9

enables the assembled rechargeable Zn-air batteries with low overpotential (~0.85 V), high

10

efficiency (~58.1 %) and prolonged operational life (~200 cycles) at 10 mA cm-2 as well as

11

considerable performance for overall water splitting. The conductive CFP, increased effective

12

surface active area, favorable mechanism for ions transfer and gas diffusion, and firmly attached

13

in-situ integrated electrode synergistically contribute to the enhanced catalytic activity and

14

stability of Zn-Co-S NN/CFP. This work establishes the Zn-Co-S NN/CFP as a promising high-

15

performance functional electrode for rechargeable metal-air batteries and water electrolyzation

16

and demonstrates the micro- and nanostructuring as an effective way to fabricate earth-abundant

17

element-based catalysts with activities comparable or even superior to noble metal catalysts.


20

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1


EXPERIMENTAL SECTION

2

Materials Synthesis. The precursors were synthesized by a simple hydrothermal method.

3

Typically, 0.4 mmol Co(NO3)2·6H2O and 0.2 mmol Zn(NO3)2·6H2O were dissolved into 40 mL

4

of deionized water. After, 1.4 mmol urea and 1.12 mmol NH4F were simultaneously added to the

5

solution while vigorously stirring for 30 min at room temperature. Afterwards, a piece of carbon

6

fiber paper (1 cm*1 cm, Teflon-treated), which was cleaned by ultrasonication in acetone for five

7

minutes and dried at 60 ºC for several minutes, was immersed into the mixed solution. The

8

above aqueous solution was then transferred into a 50 mL Teflon-lined stainless autoclave and

9

then maintained at 100 ºC for 12 hrs. After cooling down to room temperature, the resulting

10

evenly distributed Zn-Co hydroxide carbonate nanosheet precursor on carbon paper were

11

removed and purified by washing with ethanol and dried at 60 ºC overnight. The nanoplate

12

precursor was obtained following a similar procedure except no NH4F was used. The nanoneedle

13

precursor was prepared by changing the amount of urea and NH4F added to 2 mmol and 1.6

14

mmol, respectively.

15

The as-prepared NS, NP, and NN precursors on carbon paper were thermally sulfidized to the

16

corresponding metal sulfide nanostructures under a sulphur atmosphere. Specifically, 500 mg

17

sulfur powder in an alumina boat was placed at the upstream position in a fused silica tube

18

equipped with gas flow and pressure controllers. The precursor-attached CFP substrate was put

19

in the center of the tube. Air was purged from the tube and Ar gas was flowed at 50 sccm. The

20

furnace temperature was raised to 500 ºC at a heating rate of 5 ºC min-1 and maintained for 1

21

hrs. After cooling to room temperature under Ar atmosphere, the samples were removed and the

22

three different morphologies of Zn-Co-S nanomaterials on carbon paper were obtained. Bulk Zn-


21


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Page 22 of 40

1

Co-S sample was synthesized following the similar procedures of Zn-Co-S NP except without

2

the CFP support.

3

Bulk Zn-Co-S and mixed commercial Pt/C and RuO2 supported on CFP electrodes were also

4

prepared for comparison. In a typical synthesis, 3 mg bulk Zn-Co-S powder and 7 mg Vulcan

5

carbon XC-72 or 5 mg Pt/C and 5 mg RuO2 was dispersed in a mixed solution consisting of 965

6

µL isopropanol and 35 µL Nafion. In order to form a homogenous ink, the mixture was

7

ultrasonically treated for at least 30 min. Then, 60.0 µL of the catalyst ink was dropped, coating

8

a piece of bare CFP (1 cm*1 cm) with a mass loading of 0.6 mg cm-2. After that, the catalysts-

9

covered CFP was dried at 60 ºC overnight at ambient atmosphere.

10

Materials Characterization. The phase structures of Zn-Co-S series and their precursors were

11

characterized using Bruker D8 Advanced X-ray diffractometer (XRD) with Cu Kα radiation at a

12

scanning rate of 2° min-1. The morphologies and micro-nanostructures were observed using

13

scanning electron microscopy (SEM, s4800, 30 kV), transmission electron microscopy (TEM,

14

jem-2100f, 200 kV) equipped with energy dispersive spectroscopy (EDS) and atomic force

15

microscopy (AFM-afm5500), respectively. The elemental compositions were determined using

16

inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7700x). The Brunauer-

17

Emmett-Teller (BET) specific surface area was analyzed by N2 adsorption/desorption isotherms

18

at 77 K using AutosorbiQ instrument (Quantachrome U.S.). X-ray photoelectron spectroscopy

19

(XPS) was performed on a Perkin Elmer PHI 1600 ESCA system.

20

Electrocatalytic Measurements. The electrocatalytic properties of the synthesized catalysts

21

were tested on an IviumStat workstation using a three-electrode configuration. All the

22

electrochemical data was performed in 1.0 M KOH electrolyte, which was saturated with high-

23

purity O2 (Air Product, purity 99.995 %) for ORR and N2 (Air Product, purity 99.995 %) for


22

Page 23 of 40

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1

OER and HER for at least 30 min before each test and maintained under the corresponding

2

atmosphere during the whole experiment. The catalysts modified CFPs, a saturated calomel

3

electrode (SCE), and a graphite rod were employed as the working electrode, reference electrode,

4

and the counter electrode, respectively. The OER and HER linear sweeping voltammetry (LSV)

5

data was corrected with iR-compensation. All the potentials were transferred with reference to

6

the reversible hydrogen electrode (RHE) according to following equation: Evs RHE = Evs SCE +

7

1.067 V in 1.0 M KOH.37 The electrodes were electrochemically scanned by repeated cyclic

8

voltammograms (CVs) to further improve the wettability and hydrophilicity of the surface before

9

collecting the final data. The OER polarization profiles were performed from 1.20 V to 1.72 V,

10

while the ORR the curves were measured from 1.0 V to 0.45 V at a scanning rate of 5 mV s-1.

11

RRDE data was collected on an electrochemical workstation with a rotating system

12

(PHYCHEMI). The catalyst-modified working RRDE was prepared by drop-casting 10.0 µL of

13

mixed solution of 3 mg Zn-Co-S and 7 mg Vulcan carbon XC-72 homogenously dispersed in

14

965 µL isopropanol and 35 µL Nafion. The ring potential was fixed at 1.5 V to detect the

15

generated peroxide species. Electrochemical impedance spectroscopy was carried out with the

16

frequencies from 10 kHz to 100 mHz. The electrochemical capacitance (Cdl) was obtained

17

through the CVs with the potential ranging from 1.19 V to 1.29 V at different scan rates (2, 5, 8,

18

10, 15 and 20 mV s-1, respectively). The overall water splitting performance was tested using a

19

two-electrode configuration in N2-saturated 1.0 M KOH media at 5 mV s-1.

20

Zn-air Battery Assembly. The zinc-air battery was constructed using a polished Zn plate

21

anode, 6.0 M KOH with 0.2 M ZnCl2 electrolyte and the synthesized Zn-Co-S NN/CFP or

22

Pt/C+RuO2 casted on CFP with a mass loading of 0.6 mg cm-2 as the air cathode (see optical

23

image in Figure S17). The voltage-current polarization curve was tested using IviumStat


23


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 24 of 40

1

workstation. The electrolyte was pre-saturated with high-purity oxygen before each test. The

2

battery discharge/recharge performance and cycling ability were analyzed by LAND-CT2001A

3

testing devices with oxygen fed to the cathode throughout the whole measurement at room

4

temperature.

5

ASSOCIATED CONTENT

6

Supporting Information.

7

Additional: XRD patterns, EDS, SEM, AFM, and HRTEM images, BET curve, calculation of

8

double-layer capacitance, RRDE data, characterization and activity of bulk Zn-Co-S,

9

performance of primary Zn-air battery, and illustration of a proposed gas transfer mechanism,

10

etc., are included in Supporting Information. This material is available free of charge via the

11

Internet at http://pubs.acs.org.

12

AUTHOR INFORMATION

13

Corresponding Author

14

*E-mail: [email protected];

15

*E-mail: [email protected]

16

Notes

17

The authors declare no competing financial interest.

18

ACKNOWLEDGMENT


24

Page 25 of 40

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1

This work was supported by the National Natural Science Foundation of China (51602216,

2

51472178, U1601216), National Key Research and Development Program (2016YFB0700205),

3

and Tianjin Natural Science Foundation (16JCYBJC17600).

4

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Catalyst for Hydrogen Generation and Oxygen Generation. Adv. Mater. 2015, 27, 3175-

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Figure 1. Schematic illustration of the preparation of Zn-Co-S nanosheet, nanoplate, and nanoneedle on CFP. 84x45mm (300 x 300 DPI)


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Figure 2. SEM images of the as-synthesized precursors (a-c) and the corresponding sulfides on CFP (d-f). (a, d) Zn-Co-S NS/CFP, (b, e) Zn-Co-S NP/CFP, and (c, f) Zn-Co-S NN/CFP. 160x107mm (300 x 300 DPI)



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Figure 3. TEM images of (a) Zn-Co-S NS, (b) NP and (c) NN in different magnification. (d) High-resolution TEM image of Zn-Co-S NN and (e) EDX elemental mapping of Zn, Co, and S in Zn-Co-S NN. 90x93mm (300 x 300 DPI)


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Figure 4. Influence of NH4F on the morphology of Zn-Co-S precursors when the used urea was kept at 2.0 mmol. The added NH4F are (a) 0 mmol, (b) 0.8 mmol, (c) 1.2 mmol, (d) 2.4 mmol, respectively. 140x100mm (300 x 300 DPI)



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Figure 5. (a) XPS survey spectra, (b) Co 2p, (c) Zn 2p, and (d) S 2p high resolution spectra of Zn-Co-S NN, respectively. 140x112mm (300 x 300 DPI)


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Figure 6. (a) OER polarization curves of commercial RuO2, bare CFP and Zn-Co-S NS, NP, NN/CFP at a scan rate of 5 mV s-1 in 1.0 M KOH. (b) Corresponding Tafel curves. (c) EIS Nyquist plots. Inset shows the simplified Randles equivalent circuit used for fitting (Rs, CPE, and Rct are the electrolyte resistance, double layer capacity, and charge transfer resistance, respectively). (d) Polarization curves of Zn-Co-S NN/CFP before and after 1000 cycles. (e) ORR polarization curves of Zn-Co-S nanostructures and Pt/C in O2saturated 1.0 M KOH. (f) Chronoamperometric response of Zn-Co-S NN/CFP and Pt/C at a constant potential of 0.7 V. 160x178mm (300 x 300 DPI)



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Figure 7. (a) HER polarization curves of commercial Pt/C, bare CFP, Zn-Co-S NS, NP, NN/CFP at a scan rate of 5 mV s-1 in 1.0 M KOH. (b) Corresponding Tafel slopes. (c) Chronoamperometric response of Zn-Co-S NN/CFP and Pt/C at overpotential to reach a current density of 10 mA cm-2. (d) SEM image of Zn-Co-S NN/CFP electrode after 500 potential cycles. 160x121mm (300 x 300 DPI)


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Figure 8. (a) Voltage-current polarization curves of the Zn-Co-S NN/CFP and Pt/C+RuO2 catalyst. (b) Discharge/recharge curves of rechargeable Zn-air batteries catalyzed by Zn-Co-S NN/CFP and Pt/C+RuO2 at 10 mA cm-2 with duration of 400 s per cycle. (c) Water-splitting polarization curves of Pt/C+RuO2 and ZnCo-S NN/CFP+Zn-Co-S NN/CFP at a scan rate of 5 mV s-1. Inset shows the gas evolution at 10 mA cm-2. (d) Chronoamperometric response of two overall water splitting systems at 1.9 V. 160x119mm (300 x 300 DPI)



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TOC 31x11mm (300 x 300 DPI)


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