Single-Site Active Iron-Based Bifunctional Oxygen Catalyst for a

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Single-Site Active Iron-Based Bifunctional Oxygen Catalyst for a Compressible and Rechargeable Zinc-Air Battery Longtao Ma, Shengmei Chen, Zengxia Pei, Yan Huang, Guojin Liang, Funian Mo, Qi Yang, Jun Su, Yihua Gao, Juan Antonio Zapien, and Chunyi Zhi ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b09064 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 13, 2018

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Single-Site Active Iron-Based Bifunctional Oxygen Catalyst for a Compressible and Rechargeable Zinc-Air Battery Longtao Ma1, Shengmei Chen1, Zengxia Pei 1*, Yan Huang2, Guojin Liang1, Funian Mo1, Qi Yang1, Jun Su3, Yihua Gao3, Juan Antonio Zapien1, Chunyi Zhi1, 4*. 1

Department of Materials Science and Engineering, City University of Hong Kong, 83

Tat Chee Avenue, Kowloon, Hong Kong 999077, PR China. 2

School of Materials Science and Engineering, Harbin Institute of Technology

(Shenzhen), Shenzhen 518055, PR China. 3

Center for Nanoscale Characterization and Devices, Wuhan National Laboratory for

Optoelectronics，School of Physics，School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, PR China. 4

Shenzhen Research Institute, City University of Hong Kong, Nanshan District, Shenzhen

518057, PR China. *Corresponding Authors: Prof. Chunyi Zhi E-mail: [email protected] Dr. Zengxia Pei E-mail: [email protected]

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ABSTRACT The exploitation of a high-efficient, low-cost and stable non-noble metal-based catalyst with oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) simultaneously, as air electrode material for rechargeable zinc-air battery is significantly crucial. Meanwhile, the compressible flexibility of a battery is the prerequisite of wearable or/and portable electronics.

Herein, we present a strategy via single-site

dispersing Fe-Nx species on a two dimensional (2D) highly-graphitic porous nitrogendoped carbon layer to implement superior catalytic activity toward ORR/OER (with a half-wave potential of 0.86 V for ORR and an over-potential of 390 mV at 10 mA·cm-2 for OER) in alkaline medium. Furthermore, an elastic polyacrylamide (PAM) hydrogel based electrolyte with the capability to remain great elasticity even under highly corrosive alkaline environment, is utilized to develop a solid-state compressible and rechargeable zinc-air battery. The creatively developed battery performs a low chargedischarge voltage gap (0.78 V at 5 mA·cm-2) and large power density (118 mW·cm-2). It could be compressed up to 54% strain and bended up to 90o without charge/discharge performance and output power degradation. Our results reveal that single-site dispersion of catalytic active sites on porous support for bi-functional oxygen catalyst as cathode integrating specially designed elastic electrolyte are feasible strategies for fabricating efficient compressible and rechargeable zinc-air batteries, which could enlighten the design and development of other functional electronic devices. KEYWORDS: single-site active sites; bi-functional oxygen catalyst; zinc-air battery; solid-state; rechargeable; compressible.


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With the ever-increasing energy demands for portable electronic equipment, numerous researches have been done to develop related functional (e.g., stretchable, compressible, bendable/flexible, self-healing) electrochemical energy storage and conversion devices.1-4 Zinc-air battery, which is typically comprised of air electrode containing a catalyst painted on gas diffusion layer, alkaline electrolyte and metallic zinc electrode to provide a high theoretical energy density (1370 Wh·Kg-1), is a promising energy storage appliance for next-generation portable electronics.5-7 Before zinc-air battery is utilized in daily life as effective power source, it is required to prepare all-solid-state battery to prevent the leakage of corrosive electrolyte (typically KOH or NaOH aqueous solution) under heavy and/or repetitive compression, and to integrate functional properties.8 Recently, great endeavors have been devoted to develop bendable, foldable and flexible zinc-air batteries.9, 10 However, few attentions are paid to enable compressible zinc-air battery that can retain good performance under various compression conditions, which is a critical factor for elastic electronics in daily life. Simultaneously, electrochemical OER and/or ORR play critical roles in zinc-air batteries.11, 12 In this context, exploring highly efficient and bi-functional catalysts for reversible air electrode has been the major pursuit to assemble rechargeable zinc-air battery during the past a few years.13,

14

Among various electro-catalysts, M-N-C

materials (M represents transition metal like Fe, Co, Ni etc.) are easily fabricated via pyrolyzing metal-macrocycles or calcining metal-nitrogen precursor with carbon materials, which is an auspicious candidate alternative to precious metal catalysts (such as Pt, Ir and their alloys) by exhibiting decent electro-catalytic activity towards ORR and OER in alkaline and/or acid medium.

10, 15-18

Fe-N-C is the most representative M-N-C


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material as electrocatalyst, in which different components including the Fe-N6,19 Fe-N420 and Fe-N2 coordination and the Fe@N-C21, 22 species have all been regarded as main active components for exerting ORR in alkaline and/or acid medium.23, 24 However, there is still the dearth of further improving their ORR performance to meet practical battery applications and to warrant a competent energy conversion efficiency. Moreover, the OER activity by Fe-N-C catalyzers are generally moderate, thus the simultaneous high ORR and OER activities of Fe-N-C catalyst (as efficient bi-functional electro-catalyst) for reversible oxygen electrode remains an impending challenge.25 Generally, improving the activity of electro-catalyst could be achieved either by increasing the number of active sites targeted electrode to expose more active sites, or by enhancing the intrinsic activity of each active site.26 For a given catalyst, single-site dispersing active sites is therefore an efficient strategy to increase the number of electro-catalytic sites, which is expected to boost the performance of oxygen electrode. Considering the above-mentioned facts, herein, we report a type of catalyst with singlesite active Fe-Nx species distributed on highly graphitic 2D porous nitrogen-doped carbon (PNC), which is utilized to develop high-performance oxygen electrode. Simultaneously, a compressible polyacrylamide (PAM) polymer is developed as electrolyte to construct a compressible and rechargeable zinc-air battery. Benefiting from the 2D PNC network, which accelerates charge transfer and possesses abundant single-site active Fe-Nx species, the as-synthesized FeNx-embedded PNC catalysts exhibited admirable electro-catalytic activity for both ORR and OER. The PAM hydrogel electrolyte, on the other hand, showed great compressible properties and superior ion transfer capability. Hence, the


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assembled zinc-air batteries showed prominent electrochemical performances and mechanical durability. RESULTS AND DISCUSSION The high-performance single-site bifunctional electro-catalyst is synthesized through pyrolysis of the coordinated iron ions with 2, 2-Bipy moieties dispersing on the surface of a 2D metal-organic framework. The schematic diagram of the synthetic process is illustrated in Figure 1(a). The ZIF-8 polyhedrons are immobilized on both sides of polypyrrole (PPy)-coated graphene, and the coordinated iron ions with 2, 2-Bipy species are dispersed on the surface of ZIF-8 polyhedrons. Specially, the surface of graphene is smooth, which makes it hard to immobilize the ZIF-8 polyhedrons on the both sides. The PPy is used to act as a binder to promote ZIF-8 polyhedrons to grow uniformly onto the graphene sheet. After pyrolysis and zinc sublimation27 under argon atmosphere at 1000 o

C, a homogeneous single-site active FeNx-embedded on porous nitrogen-doped carbon is

obtained. More detailed synthetic processes are given in Figure S1. The hybrid architecture is expected to exhibit several superiorities: Firstly, the single-dispersed Fe-Nx species expose more active sites, owing to the existence of pre-coordinated 2, 2Bipyrodine with iron ions, which can avoid the aggregation of FeCx nanoparticles. Secondly, the Fe species could render highly graphitized carbon domains,28 which could work cooperatively with the graphene sheets to boost the charge transfer capability. Thirdly, a high specific surface area is available due to the porous nitrogen-doped carbon structure originating from metal-framework of ZIF-8 precursor. Finally, abundant nitrogen functional groups resulting from the ligand of 2, 2-Bipye and 2-MeIM could contribute to additional catalytic sites within the porous carbon framework. The 5 ACS Paragon Plus Environment

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combination of these componential and structural properties offers a plenty of fully accessible electrochemical active sites and fast charge transfer kinetics, hopefully contribute to high electrochemical activities for ORR and OER. For the fabrication of solid-state and compressible zinc-air battery, a compressible polyacrylamide (PAM) hydrogel electrolyte with highly compressible flexibility is prepared, which could uniformly

dissipate

applied-pressure

through

changing

volume

induced

by

accommodating stress (Figure 1(b)). Owing to high-performance bifunctional oxygen catalyst and compressible electrolyte, it is expected to develop compressible and rechargeable zinc-air battery (Figure 1(c)).

Figure 1. (a) Schematically illustration for synthesizing FeNx-embedded in 2D porous nitrogen-doped carbon. The local amplification is the stricture of ZIF-8, coordination of (iron ion) -(2, 2-Bipy) and FeNxdispersing on the porous nitrogen-doped carbon surface. Illustration diagram of (b) compressible polyacrylamide hydrogel and (c) compressible, bendable and rechargeable zinc-air battery.

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are utilized to determine the micromorphology, microstructure and composition of the as6 ACS Paragon Plus Environment

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synthesized electocatalysts. The PPy-coated-graphene@ZIF-8 (Figure S2, Supporting Information) and FeNx-embedded porous nitrogen-doped carbon (PNC) (Figure 2a) inherits the two-dimensional (2D) structure of graphene with the sizes of several micrometers. The rough surface of the PPy-coated-graphene@ZIF-8 is different from graphene and PPy-coated-graphene, which validates the successful immobilization of ZIF-8 polyhedron on the surface of PPy-coated graphene. The morphology of porous polyhedron on 2D structure is also well retained during pyrolysis process (Figure 2a). It should be noted that without the assistance of PPy (acting as a binder), there are only isolated and inhomogeneous ZIF-8 polyhedrons immobilizing on the surface of graphene sheets (Figure S3, Supporting Information). This result obviously indicates that the PPy plays a crucial role in the formation of uniform ZIF-8 on graphene surface, as is stressed in our strategy in Figure 1(a). The TEM images of the PNC exhibit a typical MOF-derived porous nitrogen-doped carbon (Figure 2(b)). The overall morphology of PNC is maintained after the incorporation of 2, 2-Bipy-derived highly graphitic carbon layer in interface between graphene and porous carbon skeleton (area1 of Figure 2(b), the local amplification is shown in Figure 2(c)) and the embedment of Fe-Nx species (area 2 of Figure 2(b), the local amplification in Figure 2(e)). The 2D structure with porous nitrogen-doped carbon polyhedron could be observed, and the polyhedron is with size of about 30 nm. As shown in Figure S4 (Supporting Information), the thinness of porous carbon is around 60 nm. The highly graphitic carbon layer between the nitrogen-doped carbon and graphene is observed Figure 2(c) and is further confirmed by HRTEM as shown in Figure 2(d). The typical inter-planar distance of 0.35 nm, corresponding to the (002) planes of graphitic


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carbon layer, is observed. The result indicates that the overall porous carbon framework is packed with highly graphitic multiwall carbon, which is further confirmed by XRD pattern and Raman spectra. As displayed in Figure S5 (Supporting Information), XRD pattern shows two distinct characteristic peaks at around 26o and 43o, unambiguously assigning to the (002) and (101) planes of the graphitic carbon,29 respectively. The intensity of (002) planes increased after pyrolyzing iron-2, 2-Bipy on the surface of porous nitrogen-doped carbon, suggesting a better graphitization degree. Meanwhile, the Raman spectrum exhibits two main bands at 1351 cm-1 and 1592 cm-1, corresponding to the vibration of dispersive defective/disordered carbon moieties and sp2-bonded carbon atoms, respectively (Figure S6, Supporting Information). The relative ratio of ID/IG decreased significantly (by 27%) after the introduction of iron ions and bi-pyridine, revealing a highly-graphitic carbon layer exists in FeNx-embedded PNC catalyst, which could enhance ORR activities.30 Apparently, for the porous nitrogen-doped carbon with iron-nitrogen species dispersed, no nanoparticles are observed, evidencing that iron-nitrogen species are confined in porous carbon layer in a single-site dispersed form (Figure 2(e)). Intriguingly, from the distribution of pore size, it is observed that the diameter of pores (around 3 nm) within the carbon skeleton remains unchanged (Figure S7, Supporting Information). This result is contributed to dispersion of Fe-Nx species in single-site status. High angle annular dark field scanning transmission electron microscope (HAADF-STEM) image is then used to further validate the distribution of the Fe-Nx species in the porous nitrogen-doped carbon. As displayed in Figure 2(f), amounts of isolated bright dots (marked with red circle) are observed in nitrogen-doped carbon layer, which could be unambiguously attributed to


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individual iron atoms based the Z-contrast difference between the heavier iron and light elements (carbon, nitrogen). These results therefore confirm the existence of single-site Fe-Nx active species.31-34 Meriting by the homogeneously distributed single-site Fe-Nx, the hybrid catalyst still features a large specific surface area (1161.56 m2·g-1) as compared with that of the bare PNC (1251.13 m2·g-1, Figure S7, Supporting Information)), which provides abundant active sites. Moreover, a homogeneous distribution of C, N, O and Fe elements in the 2D porous nitrogen-doped carbon structure is verified by energydispersive X-ray spectroscopy (EDS) elemental mapping profiles (Figure 2(g)). Taken together, all these microstructure and morphology analyses provide solid evidence for the single-site dispersion of Fe-Nx species on surface and/or inside of 2D porous nitrogendoped carbon.


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Figure 2. FeNx-embedded PNC: (a) SEM image; (b) Low-magnification TEM image, the area 1 and area 2 is used to mark out the local amplification of porous nitrogen-doped carbon and the interface between nitrogen-doped carbon with graphene. (c) high-magnification TEM image for area-1, (d) HRTEM images, the d-spacing values of the carbon layers are also given; (e) high-magnification for area-2 and (e) STEMHAADF profile, the Fe-Nx single sites are marked with red circles. (g) HAADF-STEM EDS element mapping of O, Fe, C and N. Scale bar: (a) 500 nm, (b) 10 nm, (c) 5 nm, (d) 5 nm, (e) 5 nm and f) 2 nm.

To investigate the binding state of as-synthesized FeNx-embedded PNC and the referenced PNC catalyst, X-ray photoelectron spectroscopy (XPS) measurements are conducted. As displayed in Figure 3(a), the N 1s high-resolution spectrum of PNC obviously shows two peaks at 398.1 and 401.2 eV, attributing to four nitrogen atoms located in N-substituted methine environment, like a configuration to meso-aza nitrogen structure in porphyrin (named as N-C configuration), and graphitic-N, respectively.35-38 10 ACS Paragon Plus Environment

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Compared with PNC, there is a peak at 399.6 eV in the N 1s XPS spectrum of FeNx-PNC catalyst (Figure 3(b)). It could be safely assigned to six nitrogen atoms coordinate to iron atom as displayed in insert of Figure 3(b), corresponding to Fe-Nx moieties in FeNxPNC.39 Notably, the Fe-Nx species are considered to be the dominant electro-catalytic active sites responding for ORR reaction. The amount of nitrogen coordinating to iron atoms (Fe-Nx) is calculated to be 3.935 at. %, according to total nitrogen content and its fraction in FeNx-PNC catalyst. To investigate the Fe condition on the surface and inside of porous nitrogen-doped carbon, the Fe binding configurations are also determined by XPS depth profile (from 0 to 15 nm in depth through Ar+ etching) (Figure 3(c)). All Fe 2p XPS spectrum shows three kinds of peaks, where the two spin-orbit doublets at 710.2 eV and 724.5 eV, corresponding to the Fe 2p1/2, Fe 2p3/2 orbitals. While the peak at around 712.4 eV can be attributed to Fe-Nx configuration,40 demonstrating that the Fe-Nx active sites are dispersed on all surface area of FeNx-embedded PNC catalyst. Intriguingly, compared with binding situation of Fe-Nx species inside of porous nitrogendoped carbon, Fe peaks of Fe-Nx configuration on the surface shifts to higher binding energy (by 0.5 eV), implying less charge density on central Fe atoms. It is the clue of the interaction between Fe-Nx species and Fe-Cx components. The two signal inside porous nitrogen doped carbon at 708.1 eV and 720.5 eV could be safely indexed to zero-valence iron species (metallic iron or carbide).41 Hence, we successfully synthesized a catalyst dispersing abundant single-site active Fe-Nx species within FeNx-embedded porous nitrogen-doped carbon.


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Figure 3. XPS spectra of N 1s for (a) PNCand (b) FeNx-embedded PNC. (c) XPS depth profile of Fe 2p for FeNx-embedded PNCin different etching depth (0-15 nm).

To invesitigate whether FeNx-embedded PNC could serve as a highly-efficient ORR/OER bifunctional electrocatalyst, cyclic Voltammograms (CV), rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) tests of the as-fabricated catalysts are conducted in alkaline medium (0.1 M KOH), and the NC, PNC as well as comercial Pt/C (20 wt%) and FeNx-embedded PNC without a binder of PPy in precursor are also evaluated for comparison. Initially, CV curves of all catalysts are performed in nitrogen- and oxygen-saturated 0.1 M KOH aqueous solution. All catalyst exhibit an apparent oxygen reduction peak in oxygen-saturated condition (Figure 4(a), Figure S8 and Figure S9, Supporting Information), suggesting an evident oxyen reduction process. 12 ACS Paragon Plus Environment

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Among the catalysts treated at different temperatures, the FeNx-PNC pyrolyzed at 1000 o

C exhibits the best ORR activity (Figure S10, Supporting Information).Therefore, the

optimized catalyst is referred to the sample pyrolyzed at 1000 oC hereinafter. Specifically, the ORR peak of FeNx-embedded PNC reaches 0.85 V vs. RHE (Reversible hydrogen electrode), which is more positive than NC (0.76 V), PNC (0.79 V) and commercial Pt/C (20 wt%) (0.82 V). Moreover, the reduction current density of FeNx-embedded PNC (0.94 mA·cm-2) is even lager than that of Pt/C (0.82 mA·cm-2). The ORR activities of catalysts are also examined utilizing RDE in oxygen-saturated 0.1 M KOH aqueous solution. The FeNx-embedded PNC shows an onset potential (Eonset) of 0.997 V and a half-wave potential (E1/2) of around 0.86 V (Figure 4(b)). The half-wave potential is distinctly more positive than that of NC (0.77 V), PNC ( 0.79 V) and commercial Pt/C (0.84 V). Meanwhile, FeNx-embedded PNC provides higher limting current (jL) density at different rotating speeds, and a jL of 5.95 mA·cm-2 (at 0.3 V) is observed with 5 mV·s2

scan rate at 1600 rpm (Figure S11(a)). Furthermore, the Koutecky-Levich (K-L) plots

clues a first-order kinetic during ORR prodecess, according to good linearity and consistent slope of K-L plots42 (Figure S11(b)). The RRDE test for FeNx-embedded PNC further confirms that the electron transfer number (n) reaches to about 4.0 per oxygen molecule and the HO2- yield is below 6.9 % in the potential from 0 to 0.9 V , which is even better than commercial Pt/C (Figure S11(c) and Figure 4(c)). Based on the above results, the as-fabricated FeNx-embedded PNC catalyst could reduce oxygen with a direct four-electron pathway and pronounced superhigh ORR activity in alkline medium. The 3D architecture combinding 2D graphene and porous nitrogendoped carbon polyhedron with large numbers of pores could supply aboundant active


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sites and superior charge conductivity, which is verified utilizing electrochemical doublelayer capacitance10. As dispalyed in Figure S12 (Supporting Information), FeNxembedded PNC sample exhibits a capacitance of 17.25 mF·cm-2, which is larger than that of NC (4.35 mF·cm-2), PNC (5.85 mF·cm-2) and FeNx-embedded PNC without binder of PPy in precursor (12.15 mF·cm-2), suggesting the largest active surface area for FeNxembedded PNC catalyst. In addition, the semicircular diameter of electrochemcial impediance spectrum (EIS) for FeNx-embedded PNC is the smallest among all the control samples (Figure S13, Supporting Information), indicating that the FeNx-embedded PNC possesses superior ion and charge transport capability. Comparad to the FeNx-embedded PNC, the PPy-free sample shows a smaller electrochemical-active surface area and lager resistance, which is reasonably assigned to inhomogeneously imbolized of ZIF-8 ployhydens with a small quantity on surface of graphene. For the stability, there is only a negligible degration of jL and a small decline of E1/2 (20 mV after 10000 cycles), indicating the long-term durability of FeNx-embedded PNC catalyst in alkline medium (Figure S14, Supporting Information), which holds great potential for practical applications. The performance of OER activity plays another important role in bifunctional catalyst and rechargeable zinc-air batteries. To evaluate the OER performance, the as-fabricated catalysts is further performed in oxygen-saturated 0.1 M KOH aqueous solution. As displayed in Figure 4(d), FeNx-embedded PNC exhibits a low overpotential of 395 mV at a current density of 10 mA·cm-2, which is significantly better than that of NC(over 600 mV), PNC ( 530 mV) and Pt/C (over 600 mV), also very approaching to that of IrO2 (350 mV). Moreover, the Tafel slope of 80 mV·dec-1 for FeNx-embedded PNC is 14 ACS Paragon Plus Environment

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observed in Figure 4(e), validating a high OER kinetics comparable to the IrO2 catalyst. The bifunctional catalytic performace can be assessed by the potential difference (∆E=Ej=10 - E1/2) between the E1/2 for ORR and the current density at 10 mA·cm-2 (Ej=10) for OER, with a smaller ∆E for greater reversible oxygen electrode. Promisingly, FeNxembedded PNC exhibits the smallest ∆E value of 0.775 V (Figure 4(f)), which is better than most of bifunctional catalysts and the separate noble metal catalyst of even state-ofthe-art Pt/C, IrO2/C (Table S1, Supporting Information). The gratifying results are attributed to: a) a 3D mixture structure of porous polyhedron and 2D conductor to offer abundant acitive sites and improve charge transfer capability; b) a large number of highly reactive species as active sites (Fe-Nx species); c) positive charging carbon atoms induced by nitrogen atoms, stimulating the catalysts to adsorp oxygen species. The synergistic effect heightens the superior reversible oxygen eletrode nature of FeNxembedded PNC catalyst.


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Figure 4. (a) Cyclic Voltammograms (CV) curves of the FeNx-embedded PNC and referenced Pt/C catalyst in nitrogen and oxygen-saturated 0.1 M KOH aqueous solution. (b) ORR polarization curves of different catalysts recorded at 1600 rpm with 5 mV·s-1. (c) Electron transfer number (n) and HO2- yield derived from RRDE examination. (d) OER polarization curves of different catalysts for OER at 1600 rpm in oxygen-saturated 0.1 M aqueous KOH solution after IR-correction, (e) corresponding Tafel plots derived from (d). (f) The overall polarization curves of all catalysts in 0.1 M KOH solution, the bi-functional ORR/OER activities(∆E) are confirmed by half-wave potential for ORR and the metric potential at 10 mA·cm-2 for OER .

Building on the outstanding the bifunctional ORR/OER electrochemical activity of the FeNx-PNC catalyst, it is promising to assemble a high-performance zinc-air battery to simulate a real battery work condition. As shown in Figure S15 (Supporting Information), the zinc-air battery delivers an open-circuit voltage of 1.55 V and the voltage could maintain in 6 M KOH aqueous electrolyte over 12 hours. Encouragingly, it is observed that the battery delivers a power density of 278 mW·cm-2, much higher than that of Pt/Cbased battery (162 mW·cm-2) (Figure S16(a), Supporting Information). Meanwhile, a small charge-discharge (CD) voltage gap is observed (Figure S16(b), Supporting Information) and the voltage gap is only about 0.78 V at the CD current density of 10 mA·cm-2 (Figure S16(c), Supporting Information). After 300 continuous cycles (55h), no evident change is observed in both charging and discharging plateaus. The open circuit voltage of a single battery is also 1.55 V after several cycles and two batteries connected in series can power several LED lights (Figure S16(e), Supporting Information). Regarding to the requirement for accommodating stress, which is expected to avoid the volume expansion and contraction during practical operations of electronic devices. Here, we exploit a compressible PAM hydrogel electrolyte with high compressibility and 16 ACS Paragon Plus Environment

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superior ion conductivity to develop compressible solid-state zinc-air battery with superior compressible flexibility and dramatic electrochemical performance. The mechanism of compressibility for the hydrogel is illustrated in Figure 5(a). The PAM chains are polymerized by N, N'-methylenebisacrylamide as cross-linkers, and the hydrogen bonds exist between the PAM chains as another cross-linker. Both cross-linkers enable to hind the formation and propagation of cracks. When a large compression is imposed, the reversible intermolecular hydrogen bonds enable our PAM chains dynamically break and recombine to dissipate applied energy, resulting in a good mechanic capability for our compressible zinc-air battery. The high elasticity of the developed PAM can remain to be good even when highly corrosive KOH solution is incorporated into it to form an electrolyte. In addition, as displayed by the SEM image of freeze-dried PAM in Figure 5(b), the PAM hydrogel possesses a mass of interconnected macro-pores, which allows ions to transfer freely and adequately in the electrolyte. These results validate that the as-developed PAM hydrogel could be utilized to assemble compressible and highly efficient zinc-air battery. As displayed in Figure 5(c), FeNx-PNC catalyst modified carbon cloth as cathode integrating porous gas diffusion layer for oxygen taking into/passing out, electro-deposited Zn as anode (Figure S17, Supporting Information), and the PAM hydrogel as electrolyte, are used for assembling the compressible zinc-air battery. To investigate the flexibility of compression and bending, the device is reversely compressed up to 54% strain (Figure 5(c)) and bent to 90o (Figure S18, Supporting Information) respectively. After two hundred times compressing and bending, no obvious structural breakdown is observed. This is because that the compressible zinc-air battery works depending on dynamic process of PAM chains,


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which could endure stress rapidly and uniformly over the whole network.43 In contrast, for the traditional PVA hydrogel electrolyte, even in 275-fold device loading, there are negligible volume contraction observed (Figure S19, Supporting Information). The electrochemical performance of the developed compressible zinc-air battery under different compression is investigated. In detail, during bending and compression processes, the in-situ discharge polarization, corresponding power density curves (Figure 5(d)) and the charge-discharge polarization curves (Figure 5(e)) record from the zinc-air battery manifest only negligible changes. Specifically, as displayed in Figure S20 (Supporting Information), a 1.436 V of high open-circuit voltage (OCV) is achieved and it could maintain for 20 hours without any decay. The output power performance is further investigated under different compression strains (compression strain of 18%, 32% and 54%). The max power density is almost fixed at 98.93% when the Zn-air battery is subjected to different compression (Figure 5(f)). Furthermore, the Nyquist plots of A. C impedance of compressible zinc-air battery under different compression strains are recorded, which reveal that the charge-transfer resistances44 show no obvious change under different compression conditions (Figure S21, Supporting Information). Moreover, to evaluate the rechargeable characteristic of the as-assembled compressible zinc-air battery under different compression strains, five charge-discharge cycles are conducted at each strain at a constant current density of 5 mA·cm-2 (11 minute per cycle) (Figure 5(g)). It is observed that the charge and discharge voltage plateau are around 1.99 V and 1.21 V, respectively, and the plateaus reveal no vibration, indicating dramatic charge-discharge capability. Additionally, 220 charge-discharge cycles for about 40 hours are also performed to review the long-term durability of FeNx-PNC based 18 ACS Paragon Plus Environment

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compressible zinc-air battery. As shown in Figure 5(h), the functional Zn-air battery still exhibits stable charge-discharge profiles up to 220 cycles. Furthermore, after 500 times’ repetitive compression, the charge/discharge performance exhibit negligible decay, validating that our developed compressible and rechargeable zinc-air battery display good mechanical endurance (Figure S22, Supporting Information). The above analysis indicates superior compressible, excellent rechargeable performance and long-term durability for our FeNx-PNC based compressible zinc-air battery (Table S2, Supporting Information). It is envisioned to practical battery application even in volume changes condition of zinc-air battery.


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Figure 5. (a) Schematic diagram of the origin of high compressibility for PAM hydrogel electrolyte, the inset is the molecular formula of PAM. (b) SEM image of freeze-dried polyacrylamide (PAM) hydrogel. (c) The compression-release process of as-assembled compressible zinc-air battery, the inset is the schematic diagram of construction process for solid-state zinc-air battery. (d) Discharge polarization and corresponding power density curves indicating 0%, 18% and 54% compressive strains with PAM hydrogel as electrolyte. (e) Maximum power density retention obtained from power density curves as a function of the compressible strain. (f) Discharge-charge polarization curves and (g) Galvanostatic discharge-charge cycling curves at current density of 5 mA·cm-2, demonstrating at different compressible strains. (h) Cycling measurements for rechargeability at a current density of 5 mA·cm-2. Scale bar for (b) is 2 µm.

CONCLUSIONS In summary, we develop the compressible and rechargeable zinc-air battery with superior electrochemical performance and mechanical durability. The prominent functional Zn-air performance is bestowed by a catalyst with single-site active Fe-Nx species on 2D porous nitrogen-doped carbon, which is employed as bi-functional electrocatalyst for oxygen electrode. Meanwhile, an environmentally durable elastic polyacrylamide hydrogel based electrolyte is developed to endow the zinc-air battery with a compressible functionality. Thanks to the combination of 2D porous nitrogen-doped carbon with excellent conductivity and high specific surface area for integrating abundant active sites, and FeNx species with highly intrinsic active activity, the as-synthesized FeNx-embedded PNC exhibits exceptional OER/ORR bi-functional catalytic activity in alkaline medium. Utilizing the highly elastic polyacrylamide hydrogel based electrolyte and highperformance bi-functional catalyst, the FeNx-embedded PNC based solid-state compressible and rechargeable zinc-air battery shows admirable electrochemical performance and mechanic durability. The electrochemical performance sensationally maintains under mechanical deformations of compressibility (reach to 54% compression strain) and bending (up to 90o bend condition). This advanced compressible and


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rechargeable zinc-air battery provides an avenue for enhancing performance and mechanical durability of the future functional and wearable electronics. METHODS Synthesis of graphene@ppy@ZIF-8. For the graphene@ppy, 200 mg graphene was dispersed in 200 mL DI water and then, 200 µL pyrrole was injected into graphene solution under vigorously magnetic stirring. After that, 20 mL 1 M FeCl3 solution was poured into the above mixture solution in an ultrasonic bath and react under ice bath for 4 hours. Finally, the PPy-coated graphene was collected by centrifuged at 8000 rpm for 5 min and wash with DI water twice. The graphene@ppy@ZIF-8 was prepared according to the previous reported works. In detail, the obtained PPy-coated graphene was redispersed in 200 mL methanol by sonication for 10 min. In order to grow ZIF-8 on the surface of PPy-coated graphene, 100 mL 0.5 M Zinc nitrate methanol solution and 1 M 2Methylimidazole methanol solution were added in above solution under continuous magnetic stirring for 4 hours. The resulted products were collected by centrifuging and washed with methanol more than three times. Synthesis of Fe-Bipy embodied into ZIF-8 immobilized on PPy-coated graphene. The as-synthesized PPy-coated-graphene@ZIF-8 was dispersed in 100 mL methanol under ultrasonic bath for 20 min. 20 mL 0.2 M iron chloride methanol solution and 20 mL 0.6 M 2, 2-bipyrodine methanol solution were added to the dispersed solution which was kept under vigorous stirring for overnight. Finally, the synthesized products were obtained by centrifuging and washed with ethanol for more than three times.


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Synthesis of Fe-N species embodied in porous nitogen-doped carbon (Fe-embodied porous nitrogen-doped carbon (denoted as NC)). The Fe-embodied porous NC was obtained by pyrolyzing Fe-Bipy embodied into ZIF-8. In detail, the gray powder was heated at different temperature of 700 oC, 800 oC, 900 oC, 1000 oC, and 1100 oC for 5 hours at a heating rate of 5 oC·min-1 under Ar atmosphere. Then, the pyrolyzed products were treated using 0.5 M H2SO4 at 50 oC overnight to remove excess Zn and Fe nanoparticles. Synthesis of compressible polyacrylamide (PAM) hydrogel electrolyte. The PAM hydrogel was synthesized by in-situ polymerization method. Firstly, 2 g acrylamide was dissolved in 2 mL distilled water under vigorously magnetic stirring at 40 oC. Then 2 mg N, N'-methylenebisacrylamide as cross-linkers and 5 mg potassium persulfate as initiator were added into above solution and maintain at 40 oC for 2 hours. To remove dissolved oxygen, the mixture solution as degassed and sealed under nitrogen. After that, the freeradical polymerization was carried out at 70 oC for 2 hours. Finally, the as-synthesized PAM film was dried at room temperature and soaked in 20 mL mixture solution of 6 M KOH and 0.2 M Zn(CH2COO)2 for 24 hours. Fabrication of electro-deposited Zn electrode. The electro-deposited Zn electrode was fabricated utilizing two-electrode electrochemical deposition on carbon cloth. In detail, 100 mL 0.5 M Zn(SO4)2 was used as electrolyte, Zn Plate as negative electrode and carbon cloth as positive electrode. The electrochemical reaction was processed using chronoamperometry procedure (CHI 760E, Chenhua) at 1.0 V for 3000 s.


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Characterizations. The crystal structure of as-synthesized products was analyzed by using X-ray diffractometer (Bruker, D2 Phaser) with Cu Kα (λ=1.5418 Å) radiation. The surface morphology and microstructure were determined by field-emission scanning electron microscopy (FESEM, FEI Quanta 450 FEG) and transmission electron microscopy (TEM, JEOL-2001F). The N2 adsorption-desorption isotherms were collected using Miro-metrics analyzer (ASAP 2020) at liquid-nitrogen temperature. The Raman spectra were obtained using a micro Raman spectrometer (Renishaw inVia™ confocal Raman microscope) with 633 nm excitation laser. The surface properties of as-prepared products were characterized by using X-ray photoelectron spectroscopy (XPS, ESCALB 250) with Al Kα X-ray beam (E = 1486.6 eV). The Scanning Electron Microscopy (SEM, FEI Quanta 450 FEG) was used to evaluate the surface morphology of catalyst. Rotating disk experiment (RDE) and rotating ring-disk experiment (RRDE). All electrochemical measurements were carried out using CHI 760D workstation (Chenhua, China) integrating a rotating disk electrode (RRDE-3A) in a three-electrode system, in which a Ag/AgCl (3 M KCl) was used as reference electrode and a platinum foil as counter electrode. The recorded potentials refer to RHE, according to E(RHE) = E(Ag/AgCl) + 0.059 * pH + 0.210. All measurements were conducted at room temperature. In the experiment, a RDE with glassy carbon disk electrode (3 mm in diameter) and a RRDE with Pt ring (5 mm in inner diameter and 6.5 mm in outer diameter) and a glassy carbon disk electrode (4 mm in diameter). All electrochemical test was carried out under congruent condition with same loading mass. The catalyst ink was prepared by dispersing 2 mg catalyst in the mixture solution of 800 µL DI water, 200 µL isopropanol and 20 µL Nafion solution (Alfa Acesar, 5 wt%). All oxygen reduction reaction (ORR) 23 ACS Paragon Plus Environment

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measurements were conducted in 0.1 M KOH solution and the loading masses were 140 µg·cm-2. Pt/C (Alfa Acesar, 20 wt%) with a loading mass of 140 µg·cm-2 was used as reference. Before testing, the oxygen or nitrogen-saturated solution were obtained by accessing oxygen or nitrogen flow for 15 min. The cyclic voltammetry (CV) profiles were carried out in oxygen or nitrogen-saturated 0.1 M KOH solution at scanning rate of 20 mV·s-1 and linear scanning voltammetry (LSV) profiles were conducted in oxygensaturated 0.1 M KOH solution at a scanning rate of 5 mV·s-1. Long-term stability evaluation for ORR were carried out by measuring current changes at 0.7 V vs RHE with rotating speed of 1600 rpm in oxygen-saturated 0.1 M KOH solution. The oxygen evolution reaction (OER) performance was evaluated by LSV using RDE (1600 rpm) in 0.1 M KOH at scanning rate of 5 mV·s-1. To detect peroxide (H2O2) species formed in the process of ORR, the RRDE was conducted with Pt ring electrode potential set at 1.3 V. The electron transfer number (n) was calculated according to the Koutecky-Levich (K-L) equation as follows: 1 1 1 1 1 = + = + || | | √ = 0.2 ( )⁄ ⁄ Where J, JL and JK are the measured current, diffusion limiting current and the Kineticlimiting current density, respectively. ω is the rotating speed, F is the faraday constant (96485 C· mol-1), C0 is the bulk concentration of oxygen, D0 is the diffusion coefficient of oxygen in 0.1 M KOH (1.9*10-5 cm2·s-1) and ϑ is the kinetic viscosity (0.01 cm2·s-1).


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The n can also be evaluated using RRDE measurement profiles according the following equation: =

4

&

" ! #%$

Where the ID is the disk current, IR is the ring current and N is the collection efficiency of Pt ring. The HO2- yield was calculated from equation: H( =

200)* -+(), + )*% ) +

Where id is the disk current, ir is the ring current and N is the current collection efficiency of the Pt ring and in the experiment was determined as 0.44. The onset potential (Eonset) for oxygen reduction reaction is defined as the potential where the reduction current density gets to 1% of the limiting current density. Assemble of the compressible and rechargeable zinc-air battery and electrochemical test. The air electrode was fabricated using spraying FeNx-embedded PNC slurry on carbon cloth and dried at room temperature for 24 hours. The FeNx-embedded PNC slurry was prepared by dispersing 8 mg FeNx-embedded PNC catalyst into 1 mL mixture solution of isopropanol, distilled water and Nafion solution (5 wt %) (10:40:1). The air electrode as cathode and electro-deposited Zn electrode was directly coated on each side of the as-fabricated PAM film to assemble solid-state zinc-air battery without any separator. 25 ACS Paragon Plus Environment

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The charge-discharge polarization and electrochemical impedance spectroscopy (EIS) were determined utilizing an electrochemical work station (CHI 760e, Chenhua). The Galvanostatic test was performed using a Land 2001A battery test system at room temperature. The power density (P) of zinc-air battery was calculated by P=I·V where I is the discharge current density and V is the corresponding voltage. ASSOCIATED CONTENT Supporting Information Available: The supporting information including supplementary Figures S1-S22 and Table S1, S2 is available free of charge via the Internet at ACS Publications website or from the authors. ACKNOWLEDGEMENT This research was supported by NSFC/RGC Joint Research Scheme under Project N_CityU123/15 and City University of Hong Kong (PJ7004645). The work was also partially sponsored by the project 2017JY0088 supported by Science & Technology Department of Sichuan Province and was partially supported by the Chengdu Research Institute (2017JY0088), City University of Hong Kong (9610372).


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TOC


Single-Site Active Iron-Based Bifunctional Oxygen Catalyst for a

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