Tuning the Bifunctional Oxygen Electrocatalytic Properties of Core

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Tuning the Bifunctional Oxygen Electrocatalytic Properties of Core-Shell Co3O4@NiFe LDH Catalysts for Znair Batteries: Effects of Interfacial Cation Valences Xiaolong Guo, Xiaolin Hu, Dan Wu, Chuan Jing, Wei Liu, Zongling Ren, Qiannan Zhao, Xiaoping Jiang, Chaohe Xu, Yuxin Zhang, and Ning Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04217 • Publication Date (Web): 24 May 2019 Downloaded from http://pubs.acs.org on May 24, 2019

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ACS Applied Materials & Interfaces

Tuning the Bifunctional Oxygen Electrocatalytic Properties of Core-Shell Co3O4@NiFe LDH Catalysts for Zn-air Batteries: Effects of Interfacial Cation Valences

Xiaolong Guo†, Xiaolin Hu†, Dan Wu†, Chuan Jing‡, Wei Liu†, Zongling Ren†, Qiannan Zhao†, Xiaoping Jiang†, Chaohe Xu*,†,ζ, Yuxin Zhang*,‡, and Ning Hu*,§,# †College

of Aerospace Engineering and the State Key Laboratory of Mechanical Transmissions,

Chongqing University, Chongqing 400044, China ‡College

of Materials Science and Engineering, Chongqing University, Chongqing, 400044,

China §School ζKey

of Mechanical Engineering, Hebei University of Technology, Tianjin, 300401, China

Laboratory of Low-grade Energy Utilization Technologies and Systems, CQU-NUS

Renewable Energy Materials & Devices Joint Laboratory, Chongqing University, Chongqing 400044, China #The

State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University,

Chongqing, 400044, China Corresponding Authors: [email protected] (C. Xu); [email protected] (Y. Zhang); [email protected] & [email protected] (N. Hu)

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Abstract Rational design of excellent electrocatalysts is significant for triggering the slow kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in rechargeable metal-air batteries. Hereby, we report a bifunctional catalytic material with core-shell structure constructing by Co3O4 nanowire arrays as cores and ultrathin NiFe layered double hydroxides (NiFe LDHs) as shells (Co3O4@NiFe LDHs). The introduction of Co3O4 nanowires could provide abundant active sites for NiFe LDH nanosheets. Most importantly, the deposition of NiFe LDHs on the surface of Co3O4 could modulate the surface chemical valences of Co, Ni and Fe species via changing electron donor and/or electron absorption effects, finally achieving the balance and optimization of ORR and OER properties. By this core-shell design, the maximum ORR current densities of Co3O4@NiFe LDHs increase to 3~7 mA cm-2, almost an order of magnitude increases compared to pure NiFe LDH (0.45 mA cm-2). Significantly, OER overpotential as low as 226 mV (35 mA cm-2) is achieved in the designed core-shell catalyst, which is comparable to and/or even better than those of commercial Ir/C. Hence, the primary zinc-air battery employing Co3O4@NiFe LDH as air-electrode achieves a high specific capacity (667.5 mAh g-1) and first-class energy density (797.6 Wh kg−1); the rechargeable battery can show superior reversibility, excellent stability and voltage gaps of ~0.8 V (~60% of round-trip efficiency) in >1200 continuous cycles. Furthermore, the flexible quasi-solid-state zinc-air battery with bendable ability holds practical potential in portable and wearable electronic device.

Keywords: layered double hydroxide; Co3O4; oxygen evolution reaction; oxygen reduction reaction; Zn-air battery; electrochemical performance


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INTRODUCTION The enormous and increasing interest for clean, cheap, safe and high-energy-density battery technology has stimulated tremendous research efforts for developing advanced rechargeable Zn–air batteries in recent years.1-6 However, restricted by the slow efficiency and high overpotentials of the cathodic electrochemical ORR and OER 7-10, current Zn–air batteries in most reported works could be cycled less than 500 cycles or 100 h of continuous operations11-18, which can hardly satisfy the requirements of nowadays applications. The Ir-, Ru- and Pt-based noble metal electrocatalysts are the most efficient OER and ORR catalysts.19-22 Unfortunately, few reserves, high cost, poor stability and limited bifunctional electrocatalytic activity greatly impede their commercial applications. Therefore, quite a few works have concentrated on developing inexpensive, efficient and replaceable bifunctional electrocatalysts for rechargeable Zn–air battery. Layered double hydroxide (LDHs), especially NiFe LDH, has been recognized as the benchmark OER catalytic material owing to the good intrinsic electrocatalytic activity and stability over the past few years.23, 24 However, despite the great progress, low conductivity and self-aggregation are still current primary issues for NiFe LDHs, which apparently inhibit the optimization of electrocatalytic performances. Some strategies have been presented to overcome these problems, such as depositing of LDHs on nanocarbon materials (graphene, carbon nanotubes, carbon quantum dots),25-27 creating three-dimensional (3D) nanostructures (hollow nanoprism, core-shell),28,

29

introducing additional elements (Mn, Co, Cr, Se),30-33 adjusting the cation

valences (Fe3+ and Fe2+),34 anion exchange (OH- and S2-),35,36 in-situ transferring from Fe-Ni alloy,37 and so on. These NiFe LDH-based materials have shown improved OER activities, which



even outperformed commercial IrO2 and Ir/C catalysts under alkaline conditions. However,, the limitation of ORR performance is one of the key shortcomings of NiFe LDH-based electrocatalysts for using in rechargeable Zn–air batteries. Hence, to satisfy the requirements of long-life Zn–air batteries, the key scientific question is how to synchronously endow NiFe LDH-based electrocatalysts better ORR performance while keeping the first-class OER performance, and ultimately achieve the balance, optimization and compatibility between the two. Recently, spinel-type Co3O4, especially the Co3O4 nanowire arrays on conductive matrix has been investigated and demonstrated to be an efficient ORR electrocatalysts in alkaline media38-40, benefitting from their high conductivity, good structural stability, tunable electronic structure and large surface area.41-44 In view of the superior ORR performance of Co3O4 nanowire and the first-class OER activity of NiFe LDH, it is expected that combining these two together may give play to their advantages and achieve the effect of complementing each other, so as to build an excellent bifunctional electrocatalyst for application in Zn–air battery. However, to date, few systematic researches have been done in this aspect. Thus, we hereby develop an efficient bifunctional ORR and OER catalytic material with core-shell structures based on Co3O4 nanowire arrays as cores and ultrathin NiFe LDH nanosheets as shells. By this unique design, the Co3O4 nanowire arrays can offer more active sites and improve the electron conductivity of the ultrathin NiFe LDH nanosheets, achieving the balance between OER and ORR properties. The optimized Co3O4@NiFe LDH nanowire arrays demonstrated not only extraordinarily superior OER performance (an overpotential as low as 226 mV at 35 mA cm−2) but also greatly enhanced ORR performance. The SEM, XPS and LSV results confirmed that the morphologies and surface valence states of Co3O4@NiFe LDH can be adjusted


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via changing the growth time of NiFe LDH, finally determined and optimized the electrocatalytic performances. In nonstop rechargeable Zn–air battery tests, the discharge-charge voltages remained relatively stable over 1200 cycles (>200 h), outperforming most of previous results in literatures. Moreover, for the first time, the Co3O4@NiFe LDH nanowire arrays in-situ grown on flexible carbon cloths were directly utilized as an integrated binder-less/additive-free air-cathode and further fabricated a flexible quasi-solid-state Zn–air battery, which delivered a prominent open circuit potential, good cycling stability and excellent flexibility.

EXPERIMENTAL SECTION Reagents: The used chemicals were purchased from Aladdin and used without any pre-treatment. Preparation of Co3O4@NiFe LDH catalysts: Firstly, Co3O4 nanowires were synthesized through a chemical codeposition and thermal decomposition method, and a typical process was as follows. Ni foam (2 × 4.5 cm2) was successively cleaned in hydrochloric acid (5%), ethanol and deionized water for 10 min. The processed Ni foam was then placed into 35 mL of an uniform aqueous solution containing Co(NO3)2 · 6H2O (2.5 mmol), NH4F (5 mmol) and urea (12.5 mmol). The precursor solution with Ni foam was placed in a stainless Teflon-lined autoclave, which was heated at a 105 oC for 11 h. Afterward, the prepared Co3O4 precursor were calcined at 350 oC in air for 2 h to obtain Co3O4 nanowire arrays. Finally, for the growth of NiFe LDH, typically, the precursor solution was firstly prepared by mixing Ni(NO3)2·6H2O (0.8 mmol), FeSO4·7H2O (0.2 mmol), urea (4 mmol) and H2O (30 mL). Then, the precursor solution containing Co3O4 nanowire arrays was treated by hydrothermal method at 100 oC for 0.5, 1.0, 2.0, 4.0 and 7.0 h, respectively. The deposition time were used to control the amount of the NiFe LDH, which were labeled as



[email protected], Co3O4@NiFe-1, Co3O4@NiFe-2, Co3O4@NiFe-4 and Co3O4@NiFe-7, respectively. For comparison, the same method was employed to synthesize pure NiFe LDH on Ni foam. The Co3O4@NiFe LDH was also grown on the hydrophilic carbon cloth by the same synthetic conditions for using as flexible air cathode. Preparation of 20 wt.% Pt/C, 20 wt.% Ir/C electrodes on Ni foam: 30 mg catalyst, 150 μL Nafion (0.5 wt.%) and 850 μL ethanol/water (Vethanol/Vwater = 2:1) were sufficiently mixed via ultrasonication. Subsequently, the as-formed uniform suspension was added into the cleaned Ni foam, which was then naturally dried at atmosphere. The mass loading of each catalyst on Ni foam was ~ 2.0 mg cm-2. Preparation of Co3O4@NiFe LDH-2, 20 wt.% Pt/C, 20 wt.% Ir/C electrodes on glassy carbon electrode (GCE): For Co3O4@NiFe LDH-2, 3.5 mg catalyst, 0.5 mg Vulcan XC-72, 150 μL Nafion (0.5 wt.%) and 850 μL ethanol/water (Vethanol/Vwater =2:1) were sufficiently mixed to form a uniform suspension via ultrasonication. Afterward, 20 μL of the as-obtained suspension was dropped onto GCE with a mass loading of ~0.4 mg cm-2, which was then naturally dried at atmosphere. For Pt/C, and Ir/C, 4 mg catalyst, 150 μL Nafion (0.5 wt.%) and 850 μL ethanol/water (Vethanol/Vwater =2:1) were homogeneous dispersed for suspension by ultrasonic processing for 30 min. Then, 20 μL of the suspension was coated onto a GCE with the same mass loading, which was then dried in air at ambient environment. Materials characterization: The scanning electron microscopy (SEM, ZEISS AURIGA FIB/SEM) with energy dispersive X-ray spectroscopy (EDS) and transmission electron microscope (TEM, ZEISS LIBRA 200) were used for detecting the surface microstructure and crystal structure of the obtained catalysts. The phase compositions of the obtained catalysts were characterized by X-ray


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diffraction (Specification: D/max 2500; Target materials: Cu Kα). The surface electronic structure of the catalysts was analyzed by X-ray photoelectron spectroscope (XPS, Product model: Kratos Axis Ultra, Irradiation source: Al Kα source). The three-point-bend test was conducted on tensile strength testing with single column (EZTest, Shimadzu7) by applying a load at the center of the battery. The length was fixed as 40 mm. The mechanical loading speed is 10 mm min−1; the loading will be uninstalled immediately after achieving a deflection of 5 mm. Electrochemical measurements: A normal three electrode system working in an Electrochemical workstation (CHI 760E) was employed to test OER and ORR performances of the catalysts. The electrolyte of the electrode system was 1 M KOH solution. An Ag/AgCl electrode with 10% KNO3 filling solution and a Pt ring served as the reference and counter electrodes, respectively. The catalyst growing or coating on nickel foam (1.0×1.0 cm2), and glass carbon electrode (5 mm in diameter) were selected as work electrodes, respectively. The electrolyte always maintained oxygen saturated condition by injecting oxygen gas in the total experiments. All potentials in the work were converted to potential vs. reversible hydrogen electrode (RHE) via the following equation: ERHE = EAg/AgCl + 0.199 + 0.0591 × pH. Linear sweep voltammetry (LSV) potential range for OER was recorded from 0 to 0.65 V (vs. Ag/AgCl) with 90%-iR correction at 1 mV s-1. Linear sweep voltammetry (LSV) potential range for ORR was 0.1 to -0.8 V (vs. Ag/AgCl) at 5 mV s-1. Electrochemical impedance spectroscopy (EIS) tests were carried out on the working electrodes at a low anodic polarization overpotential section. The frequency range and amplitude of the EIS were 0.01-100000 Hz and 5.0 mV, respectively. The dependence relation of capacitive current and scan-rate in CV curves was used to determining the electrochemical surface area (ECSA). In this work, we chose the current density



at 1.26 V vs. RHE as calculative current density. For ORR, all LSV results on GCE were recorded at a scan rate of 5 mV s-1. The electron transfer number of one oxygen molecule during ORR was calculated via our previous works.16 The Tafel slopes of OER and ORR were calculated from the low overpotential section of LSV plots. Assembling of liquid Zn–air battery: A cleaned zinc plate (thickness: 0.1 mm) was used as cathode. The air electrodes were made up of carbon cloth with a gas diffusion layer and Co3O4@NiFe LDH catalyst on Ni foam. 6 M KOH and 6 M KOH + 0.2 M zinc acetate were employed as electrolytes for the primary Zn-air battery and the rechargeable Zn-air battery, respectively. The effective area of zinc plate and catalyst layer is 1×1 cm2. The current density in test was normalized to the geometry area (1×1 cm2) of catalyst layer. The testing environment was atmospheric environment (oxygen gas from air). Quasi-solid-state and flexible Zn–air batteries assembly: A cleaned zinc foil (thickness: 0.05 mm) was adopted for the battery anode. The detail processing of the quasi-solid-state electrolyte was: first, polyvinyl alcohol (PVA, 6 g) was fully dispersed in 42 ml H2O and heated to 90 °C under magnetic stirring, ultimately forming a clear solution; then, 18 mL of 6M KOH+0.2 M zinc acetate electrolyte was added into the former solution and kept stirring for another 30 min under 90 °C. Hence, the gel polymer electrolyte was obtained. Afterwards, the electrolyte was injected to a rectangle glass mold and the zinc foil was inset the mold. After freezing in a freezer at -18 oC over 2 h and naturally thawed at room temperature, the quasi-solid-state electrolyte with zinc foil was formed. The Co3O4@NiFe LDH on carbon cloth and zinc anode was putted onto the opposite sides of the electrolyte, respectively. Finally, the encapsulation of the batteries employed the punched heat contraction rubber cable (diameter: 1.5 cm; length: 5 cm) with shrinkable property.


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In order to promote the oxygen reaction for anode, a hole of 8 mm diameter was manufactured on the rubber cable via puncher.

RESULTS AND DISCUSSION The fabrication steps of the hierarchical core-shell Co3O4@NiFe LDH nanowire arrays are schematically shown in Figure 1a. In this work, we firstly employ Ni foam as substrate to support the growth of the Co3O4@NiFe LDH, and assess its electrocatalytic activities of and the energy storage performances of the corresponding liquid state Zn–air battery. Co3O4 nanowires arrays were firstly synthesized by a combination of hydrothermal and thermal decomposition process. Afterwards, NiFe LDH nanosheets were in-situ deposited onto the surface of Co3O4 nanowires via a chemical co-deposition approach, generating the free-standing 3D core-shell structural Co3O4@NiFe LDH hybrids (More details, please see the Experimental Section). Such synthetic strategy is easy, facile, can also be extended to other substrates, such as carbon cloths for using as air-electrode of flexible quasi-solid-state Zn–air battery. Figure S1a and b show the formation of Co3O4 nanowire arrays, with diameters ranging of 50-150 nm and lengths of around 5-10 μm. The Co3O4 nanowires are composed of numerous closely connected nanocrystallites with rich of mesopores (Figure S1c and d). After chemical co-deposition, the array structure of Co3O4 has been well maintained, while the surface of nanowires was covered by a layer of ultrathin NiFe LDH nanosheets, producing a typical core-shell structure (Figure 1b and c). TEM images (Figure 1d) distinctly exhibit that NiFe LDH nanosheets with diameter of 25-40 nm are vertically arranged with Co3O4 nanowires. Predictably, this as-obtained NiFe LDH shell can provide abundant exposed edges for Co3O4 nanowires, which will further improves the electrocatalytic performance



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Figure 1 (a) Schematic diagram of the synthetic process of the Co3O4@NiFe LDH hybrid nanowire arrays on Ni foam and flexible carbon cloth, respectively. The electronic images of Co3O4@NiFe LDH on Ni foam: (b, c) SEM images; (d, e) TEM images. (f) EDS mapping of Co3O4@NiFe LDH nanowire arrays to display the homogeneous dispersion of Ni, Fe, Co and O. Comparative the SEM images of the Co3O4@NiFe LDH nanowire arrays at various hydrothermal times: (g, h) 0.5 h, (i, j) 2 h, (k, l) 4 h, and (m, n) 7 h.

of Co3O4. A crystal lattice spacing of 0.264 nm (Figure 1e) is also identified, which is consistent


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with the spacing of (012) crystal face of NiFe LDH.45 The EDS results (Figure 1f) exhibit a uniform Ni, Fe, Co and O distribution, which clearly implies the homogeneous adhering of NiFe LDH nanosheets on the entire nanowire arrays. The X-ray diffraction are also proved the presence of NiFe LDH and Co3O4, where those diffraction peaks are identified with those of cobalt oxide (PDF#43-1003) and NiFe LDH (PDF#51-0463) (Figure S2). To further explore the relationship between growth time, morphologies, materials and electrocatalytic properties of the NiFe LDH shell, we use time-depended results to investigate the structure evolution process and the surface chemical valence states. As seen in Figure 1g and h, abundant nanoparticles or tiny “buds” on the smooth surface of the Co3O4 nanowires were observed after hydrothermally reacted for 0.5 h. By increasing reaction time to 2 h, and then 4 h, previous nanoparticles will transform into NiFe LDH nanosheets and gradually grow up to form a shell coating on Co3O4 nanowires (Figure 1i-l). Further prolonged to 7 h (Figure 1m and n), the dispersive and isolated core-shell structure begin connecting to each other, forming the “bamboo raft” structure, which leads to the reduction of inter-divided 3D network space. Obviously, the mass loading of NiFe LDH shell as well as the morphology of Co3O4@NiFe LDH nanowire arrays can be synchronously regulated by controlling the growth time of NiFe LDH. For disclosing the intrinsic relation between electronic structure and electrocatalytic properties, the surface chemistry of Co3O4@NiFe LDH is investigated by X-ray photoelectron spectra (XPS). As revealed by the high-resolution XPS spectra of Ni 2p3/2, Co 2p3/2 and Fe 2p3/2 (Figure 2), clearly, all peak values of the Co3O4@NiFe LDH that obtained in different reaction times are located at different binding energies. Taking Ni 2p3/2 as an example (Figure 2a), the peak centers first shifted negatively when the reaction time was less than 2 h, and then moved in the



positive direction if further extending the reaction time. Concretely, the broad Ni 2p3/2 spectrum of

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Figure 2 High-resolution Ni 2p (a), Fe 2p (b) and Co 2p spectra (c) in pure Co3O4 and Co3O4@NiFe LDHs obtained with different reaction times.

Co3O4@NiFe LDH-0.5 (the deposition time of NiFe LDH is 0.5 h) shows two distinguished nickel species including Ni3+ and Ni2+, and Ni3+/Ni2+ ratio is calculated to be 2.71. Increasing the reaction time to 1.0 h, Ni3+ peak at the Ni 2p3/2 location becomes sharper, while the corresponding Ni3+/Ni2+ ratio increases to 4.46. Remarkably, the Ni2+ species nearly disappear when the reaction time reaches or extends to 2 h. In the case of Fe 2p3/2, the peak values showed continuous negative migration as the reaction time increased (Figure 2b), thus implying the existence of electron donation effects of Fe species which can lead to the higher valence state. On the contrary, the binding energies of Co 2p3/2 in Co3O4@NiFe LDH shifted to higher values as time increases (Figure 2c), which indicate that the chemical valence value of the Co species in Co3O4 decreased after compositing with NiFe LDH. These above results demonstrate the occurrence of electron transfer from Ni and Fe ions to Co ions, which can be further confirmed by the left shift (relative to NiFe LDH) of M-O and M-OH (M is Ni or Fe) in O 1s spectrum (Figure S3a). Notably, despite of remarkable charge transfer between metal cations and changes of chemical valences, the crystal


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structures of Co3O4 and NiFe LDH are well persevered as proved by HRTEM and XRD results. XPS results suggest that the building of the Co3O4@NiFe LDH interface could tune the charge-transfer behavior of metal cations. Furthermore, the shifted amount of the binding energy is regulated by adjusting the growth time of NiFe LDH, thus finally affecting the electrocatalytic performances of Co3O4@NiFe LDH. Generally, additional OER active sites (i.e., Ni-Fe-Co) are likely to be created owe to the electron transfer from Ni, Fe species to Co species, and higher valence state of Ni and Fe ions will accelerate the OER reaction34,

46-48.

In term of ORR

performance, although Co3O4@NiFe LDHs with low valence state of Co ions are probably inferior to pure Co3O4 with higher valence of Co ions, the ORR activities of the Co3O4@NiFe LDH nanowire arrays should be obviously superior to the ORR performance of pure NiFe LDH. To minimize the capacitive current, we firstly recorded the IR-compensated LSV curves at 1 mV s-1 to evaluate OER activities (Figure 3a). The Ni2+/Ni3+ oxidation peaks are revealed at approximately 1.42-1.47 V vs. RHE34. Clearly, Co3O4@NiFe-2 delivers the lowest OER overpotential of 226 mV at 35 mA cm-2 .The overpotential value is signally lower than that of pure NiFe LDH (266 mV), single Co3O4 (371 mV), commercial Ir/C (334 mV) and Pt/C (388 mV), respectively, suggesting an enhanced and excellent OER performances. For more comprehensive comparison, the key indexed of other reported OER electrocatalysts are listed in Table S1, where the OER activity of the Co3O4@NiFe LDH electrocatalyst is comparable to OER activities of these advanced electrocatalysts. The smallest Tafel slope of 55.5 mV dec-1 for Co3O4@NiFe-2 further reveals the improved OER catalytic activities (Figure 3b). The Co3O4@NiFe-2 also has a smallest charge transfer resistance (0.64 Ω) than those of NiFe LDH (1.32 Ω) and Co3O4 (2.55 Ω) as displayed in Nyquist plots (Figure 3c), indicating an increasing OER kinetics.13 Besides,



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Co3O4@NiFe LDH catalysts show significantly increasing of the electrochemical active

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Figure 3 (a) OER polarization curves of different electrocatalysts at a sweep speed of 1 mV s−1. (b) OER Tafel curves of different electrocatalysts. (c) Nyquist plots of the low overpotential region. The inset is the electrical equivalent circuit. (d) Chronopotentiometry response of Co3O4@NiFe-2 at a constant current of 30 mA cm−2. The inset is a SEM image of Co3O4@NiFe-2 after 20 hours of chronopotentiometry test. (e) ORR polarization curves and (f) ORR Tafel curves of electrocatalysts.

surface area (ECSA) comparing with that of pure NiFe LDH (Figure S4 and S5). However, Co3O4@NiFe-2 delivers the lowest OER overpotential and Tafel slope in spite of the medium level of ECSA, as shown in Figure 3a, 3b and S5. We reasonably deduce that it is the balanced results of the modified surface chemical valence of interfacial cations and ECSA. Beyond that, the Co3O4@NiFe-2 catalyst also displays a stable current response as long as 20 h with almost no potential degradation, indicating a superior long-term stability (Figure 3d). No apparent morphology changes of Co3O4@NiFe-2 are observed after electrochemical test as shown in the inset SEM image, again proved the excellent structure stability. Generally, the catalysts first underwent the oxidation process, during which NiFe LDH and Co3O4 were oxidized to NiFeOOH


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and CoOOH23, 44, respectively, followed by OER reaction. After OER test, the 2p3/2 peaks of Ni, Fe and Co elements have shifted to different degrees (Ni 2p3/2: ~0.17 eV; Fe 2p3/2: ~0.34 eV; Co 2p3/2: ~0.19 eV) (Figure S6), indicating that all of the three elements probably acted as the OER catalytic active centers and participated in the OER process. For ORR performances (Figure 3e), as expected, the largest current densities of all Co3O4@NiFe LDH catalysts (3-7 mA cm-2) are an order of magnitude larger than that of NiFe LDH (~0.45 mA cm-2), confirming that introducing of Co3O4 to NiFe LDH can significantly improve the ORR catalytic activity of NiFe LDH. More important, Co tends to be a lower chemical valence with a higher binding energy, resulting in a relatively lower ORR current density. The reason is that the reduction of the Co3+ ions as ORR active site is unfavorable for ORR activity, thus causing the reduction of ORR current.39, 49 The Co3O4@NiFe-1 and Co3O4@NiFe-2 have lower Tafel plots, perhaps because of their smaller onset current (in Figure 3f). Besides, we also evaluated the durability of Co3O4@NiFe-2 as shown in Figure S7a. After over 2000 CV cycles, the maximum current density decreased by 22.7% compared with the initial LSV curve, but the core-shell morphology of Co3O4@NiFe LDH is well maintained (in Figure S7b). Remarkably, the peak center of Co 2p3/2 shifts ~0.44 eV to low binding energy, while that of the Ni 2p3/2 and Fe 2p3/2 have little changes (in Figure S8). The results indicate that Co ions should be the ORR active species. Summarily, we attribute the enhanced OER and ORR activities of Co3O4@NiFe LDH to the electronic structural modulation and synergistic effects of multi-component materials. For standard measuring the bifunctional electrocatalytic activity of Co3O4@NiFe LDH, we tested the potential gap (∆E) of Co3O4@NiFe-2 on the GCE (Figure S9). The Co3O4@NiFe-2 displays a ∆E



value of 0.78 V. This value is superior to those of Pt/C (0.81 V) and Ir/C (0.79 V), demonstrating good bifunctional performance of the Co3O4@NiFe-2. Figure S10a shows the ORR LSV measurements of the Co3O4@NiFe-2 under various revolving speeds and the corresponding Koutechy-Levich plots at various potentials vs RHE. The transferred electron number (n) is ~3.5-3.6 (Figure S10b), indicating a quasi-four electron reaction.

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Figure 4 (a) OER overpotential at 35 mA cm-2 versus Ni 2p3/2 binding energy of Co3O4@NiFe LDH with various NiFe LDH growth time, demonstrating the Ni binding energy-activity relationship. (b) The largest ORR current densities at ~0.2 V vs. RHE versus Co 2p3/2 binding energy of Co3O4@NiFe LDH with various NiFe LDH growth time, demonstrating the Co binding energy–activity relationship.

To clearer reveal the “material synthesis-electronic structure–electrocatalytic activity” correlation, the growth time of NiFe LDH, the Ni2p3/2/Co 2p3/2 peak center location and the OER overpotential (i.e. overpotential at 35 mA cm-2) and the largest ORR current density (at ~0.2 V vs. RHE) are synchronously shown in Figure 4. It can observe that a high positive correlation of the Ni oxidation states and OER activity is demonstrated (Figure 4a). To be specific, Co3O4@NiFe LDH with higher binding energy of Ni 2p3/2 possesses lower OER overpotentials, implying that the valence state of NiFe LDH can be regulated by changing the growth time, thus to tune the oxygen evolution electrocatalytic properties. In addition, The XPS spectrum of Co as an index


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expressly confirms that an obvious tendency correlation between ORR largest current densities and Co 2p3/2 binding energies (Figure 4b): Co3O4@NiFe LDH with lower Co 2p3/2 binding energies often have more ORR largest current densities. Interestingly, Co3O4@NiFe LDH catalysts with higher OER activity, such as Co3O4@NiFe-2 and Co3O4@NiFe-4, tend to exhibit lower ORR catalytic currents. In fact, achieving a scientific balance of both OER and ORR performance via the electron absorption/donor effects is essential toward bifunctional catalyst, which also avails Zn-air battery.

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Figure 5 Liquid phase rechargeable Zn−air battery with Co3O4@NiFe-2 as air-cathode in ambient air: (a) schematic illustration and (b) CV curves of Zn−air battery; (c) voltage curve of the initial discharge-charge tests; (d) the discharge−charge voltage profile of the Zn−air battery, the cycle numbers surpass 1200 cycles (at 15 mA cm−2, 10 min per cycle); (e) discharge-charge voltage curves of Zn−air battery before and after 1250th cycles.

To confirming the practical application of Co3O4@NiFe LDH electrocatalyst, we further assemble a liquid phase Zn−air battery, where employing Zn foil as the anode and Co3O4@NiFe-2 as the air-cathode. As a result, the primary battery delivers a power density of 127.4 mW cm-2 and



a specific capacity of 667.5 mAh g-1 at 10 mA cm-2 (Figure S11). The corresponding energy density is 797.6 Wh kg−1, indicating a high level. During a 26.5 h of galvanostatically discharge test at 10 mA cm−2, the working voltage maintains at ∼1.19 V without apparent drops (Figure S11b), indicating the excellent catalytic stability of Co3O4@NiFe LDH for ORR. The rechargeable Zn−air battery adopts the same configuration of the primary Zn−air battery (Figure 5a). As shown in Figure 5b, CV measurements firstly revealed distinct behaviors of Co3O4@NiFe LDH cathode-based Zn battery. A couple of redox peaks at ~1.70/~1.93 V are given, corresponding to redox reaction of M−O/M−O−OH (M = Ni 、 Co).50 The anodic peak and the high ratio of anodic to cathodic peak current (~ 2.05) indicate the strong OER performance from Ni, Co, or the presence of other anodic current source, such as possible OER current from Ni foam. The electrochemical energy storage of this battery probably not only contains ORR and OER courses in Zn−air battery, but also the reaction of M−O/M−O−OH in Zn−Ni batteries.51Therefore, the battery may be called as a hybrid battery of Zn−Ni/Co (Zn−Co3O4@NiFe LDH) and Zn−air. The initial discharge-charge-discharge voltage curves for the Zn−air battery are showed in Figure 5c. Notably, two flat plateaus at 1.175 V originated from the ORR and 1.964 V for OER in the voltage curves are typical characteristics of Zn−air battery. Besides the two plateaus, in the charging, the arc-shaped curve in the range of 1.4-1.9 V is due to the cation oxidation of M−O→ M−O−OH; and in the discharging the curve at 1.9-1.5 V is for cation reduction of M−O−OH→ M−O. This result highly matches with the CV results in Figure 5b. Impressively, the constructed Zn−air battery shows superior cycling stability (Figure 5d). The voltage gap of ~0.8 V and the round-trip efficiency of ~60% are well maintained after over 1200 continuous cycles (>200 h). The initial characteristic of charge-discharge profile is also retained in the 1250th cycling test


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(Figure 5e). The cyclic stability of the battery surpasses that of most reported rechargeable Zn−air batteries (Table S2), demonstrating the great suitability of our Co3O4@NiFe LDH electrocatalyst for rechargeable Zn−air battery.

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Figure 6 Performance of quasi-solid-state and flexible rechargeable Zn-air battery with Co3O4@NiFe-2 as air-cathode: (a) schematic representation and (b-d) optical images of the as-assembled flexible battery. (e) Open circuit plots of different battery packs (connected in series). The inset shows the corresponding photographs of the battery packs with an OCV. (f) Discharge platforms of different battery packs (connected in series) at 1.3 mA cm−2. (g) Photograph of a LED panel (3.0 V) powered by three batteries connected in series. (h) Cycling performance at 1.3 mA cm−2. (i) The discharge curves of the flexible sandwich-type Zn-air battery during simultaneous cyclic bending experiment and electrochemical testing.

Inspired by the growing attentions on portable electronic devices, a quasi-solid-state, flexible



and rechargeable sandwich-type Zn−air battery was further assembled. The microstructure and phase of Co3O4@NiFe LDH on carbon cloth were studied by SEM and XRD techniques (Figure S12). The air-cathode, Zn foil and gel-like PVA-KOH-Zn(AC)2 electrolyte are all highly flexible as shown in Figure S13a-c. The maximum applied strain of the gel electrolyte could reach up to ~50%, which could ensure the designed quasi-solid-state Zn−air battery a good flexibility and stretchablity.. Figure 6a, b and S13d show the schematic fabrication and photographs of the rechargeable sandwich-type Zn−air battery, respectively. The integral Zn-air battery can be neatly bended into different shapes (Figure 6c and 6d), and well finished the three-point bending experiment (Movie S1), which confirms its good flexibility. A battery pack in series with a different number of cells afforded a high open circuit voltage and flat open circuit plots (i.e., one electrode for 1.38 V, two electrodes for 2.67 V, three electrodes for 3.92 V) is shown as Figure 6e. The galvanostatic discharge curves of the battery packs in Figure 6f indicate that the Co3O4@NiFe LDH cathodes connected in series show multiple stable voltage plateaus. The steady open circuit voltage and discharge curves are the precondition for driving a LED panel (nominal voltage = 3.0 V). Clearly, a Zn-air battery group connected by three batteries is able to light a yellow LED panel (Figure 6g). More importantly, the continuous discharge-charge test (10 min per cycle) reveals its good long-term cycling performance; where relative stable charging/discharging potentials are delivered covering duration of 20 h (120 cycles) (Figure 6h). Amazedly, even in synchronously cyclic bending experiments and electrochemical testing, the discharge curve remained relative normal and stable (Figure 6i), showing the intactness of the battery under frequent bending and guaranteeing for its practical use.


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CONCLUSIONS In summary, we demonstrate that the design of Co3O4 @NiFe LDH nanowire arrays with core-shell structure can improve the limited ORR performance and reduce OER overpotential of NiFe LDH electrocatalysts, respectively. We find that the appropriate growth time of the NiFe LDH shells plays an important role for adjusting the electronic structure via the modulation of interfacial cation valences. Moreover, the XPS spectrum reveals that modulating of the electronic structure is an effective approach to balance the surface chemistry environment of the core and shell, thus achieving the optimization of ORR and OER performance. This strategy should be general and have implications for constructing advanced bifunctional electrocatalysts with appropriate valence state of both ORR and OER. Furthermore, Co3O4 @NiFe LDH electrode-based Zn−air battery shows prominent rechargeable performance. The flexible and quasi-solid-state Zn−air batteries with the Co3O4 @NiFe LDH air-cathode deliver good cycling stability with excellent mechanical flexibility. The flexible characteristics give it a huge potential for portable electronic devices.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM images, TEM images, XRD patterns, XPS spectra of electrocatalysts; Comparison of OER activity; Cyclic voltammograms, the curves of scan rate vs the current density; OER and ORR polarization curves; The comparison of Ej10, E1/2, and ΔE values; The K-L plots; Performance of Zn-air battery; Photographs of the flexible and stretchable electrolyte, Zn foils, flexible carbon cloth with Co3O4@NiFe-2, and the performance of flexible Zn-air battery (PDF). Three-point bending of the flexible Zn-air battery (MP4).

AUTHOR INFORMATION Corresponding Authors *E-mail addresses: [email protected] (C. Xu) *E-mail addresses: [email protected] (Y. Zhang)



*E-mail addresses: [email protected] & [email protected] (N. Hu) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (No. 21503025, 11632004, U1864208), Fundamental Research Funds for the Central University (No. 106112016CDJZR325520, 2018CDQYCL0014), Key Program for International Science and Technology Cooperation Projects of Ministry of Science and Technology of China (No. 2016YFE0125900), Venture & Innovation Support Program for Chongqing Overseas Returnees (cx2017060), Chongqing Research Program of Basic Research and Frontier Technology (NO. cstc2016jcyjA1059, cstc2017jcyjBX0063), and Hundred Talents Program of Chongqing University. REFERENCES (1) Fu, J.; Cano, Z.P.; Park, M.G.; Yu, A.; Fowler, M.; Chen, Z. Electrically Rechargeable Zinc–Air Batteries: Progress, Challenges, and Perspectives. Adv. Mater. 2017, 29, 1604685. (2) Li, Y.; Lu, J. Metal-Air Batteries: Will They Be Future Electrochemical Energy Storage of Choice? ACS Energy Lett. 2017, 2, 1370-1377. (3) Chen, Y.; Ji, S.; Zhao, S.; Chen, W.; Dong, J.; Cheong, W.-C.; Shen, R.; Wen, X.; Zheng, L.; Rykov, A. I.; Cai, S.; Tang, H.; Zhuang, Z.; Chen, C.; Peng, Q.; Wang, D.; Li, Y. Enhanced Oxygen Reduction with Single-atomic-site Iron Catalysts for A Zinc-air Battery and Hydrogen Air Fuel Cell. Nat. Commun., 2018, 9, 5422. (4) Ma, L.; Chen, S.; Pei, Z.; Li, H.; Wang, Z.; Liu, Z.; Tang, Z.; Zapien, J.A.; Zhi, C. Flexible Water Proof Rechargeable Hybrid Zinc Batteries Initiated by Multifunctional Oxygen Vacancies-rich Cobalt Oxide. ACS Nano, 2018, 12, 8597-8605. (5) Zhang, J.; Zhao, Z.; Xia, Z.; Dai, L. A Metal-free Bifunctional Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions. Nat. Nanotechnol. 2015, 10, 444-452. (6) Li, Y.; Fu, J.; Zhong, C.; Wu, T.; Chen, Z.; Hu, W.; Amine, K.; Lu, J. Recent Advances in Flexible Zinc-based Rechargeable Batteries. Adv. Energy Mater. 2019, 9, 1802605. (7) Cai, X.; Lai, L.; Lin, J.; Shen, Z. Recent Advances in Air Electrodes for Zn-air Batteries: Electrocatalysis and Structural Design. Mater. Horiz. 2017, 4, 945-976. (8) Wang Z.L.; Xu, D.; Xu, J.J.; Zhang, X.B.; Oxygen Electrocatalysts in Metal–Air Batteries: from Aqueous to Nonaqueous Electrolytes. Chem. Soc. Rev. 2014, 43, 7746-7786. (9) Lee, J.S.; Tai Kim, S.; Cao, R.; Choi, N.S.; Liu, M.; Lee, K.T.; Cho, J. Metal–air Batteries with High Energy Density: Li–air Versus Zn–air. Adv. Energy Mater. 2011, 1, 34-50. (10) Pang, H.; Gu, P.; Zheng, M.; Zhao, Q.; Xiao, X.; Xue, H. Rechargeable Zinc–air Batteries: A Promising Way to Green Energy. J. Mater. Chem. A 2017, 5, 7651–7666. (11) Chen, P.; Zhou, T.; Xing, L.; Xu, K.; Tong, Y.; Xie, H.; Zhang, L.; Yan, W.; Chu, W.;Wu, C. ; Xie, Y. Atomically Dispersed Iron–nitrogen Species as Electrocatalysts for Bifunctional


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Tuning the Bifunctional Oxygen Electrocatalytic Properties of Core

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