Facile Synthesis of Vertical Layered Double Hydroxides Nanosheets on Co@Carbon Nanoframes as Robust Bifunctional Oxygen Electrocatalysts for Rechargeable Zn–Air Batteries

The development of efficient and low‐cost bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is imperative but remains challenging in rechargeable zinc–air batteries. Herein, a metal oxidation‐assisted approach is developed for the facile synthesis of a highly efficient bifunctional catalyst (Co@NC@LDHs) which is composed of layered double hydroxides (LDHs) nanosheets in situ vertically grown on the surface of hollow carbon nanoframes encapsulating Co nanoparticles (Co@NC). Specifically, the vertical LDHs distributing on the carbon surface ensure the maximized number of accessible active sites and the interaction between LDHs with the carbon matrix. The hierarchically structured bifunctional Co@NC@LDHs display an overpotential of 0.33 V at 10 mA cm−2 for OER and a half‐wave potential of 0.88 V for ORR. The potential gap (ΔE) of Co@NC@LDHs is calculated to be 0.68 V, outperforming the mixed Pt/C + RuO2 catalysts (ΔE = 0.77 V), manifesting its superior bifunctional electrocatalysis performance. Moreover, the Zn–air batteries based on Co@NC@LDHs electrocatalyst exhibit a high peak power density (185.2 mW cm−2) and excellent durability (5.0 mA cm−2 over 500 h). This work may provide a facile strategy for the fabrication of LDHs on carbon substrate, acting as efficient catalysts for broad energy‐related applications.


Introduction
Rechargeable Zn-air batteries have attracted intensive attention for urgent demands on green and sustainable energy because of their high energy density, low cost, and environmental friendliness. [1,2]nfortunately, their widespread deployment was greatly inhibited due to sluggish kinetics process and high overpotential of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER).Considering the different reaction mechanisms of ORR and OER, a general strategy to ensure the efficient performance of monotonic electrode reaction (ORR/OER) is using two separate catalysts.Currently, the well-known state-of-the-art electrocatalysts for ORR and OER are Pt-and Ru/Ir oxide-based noble metal catalysts, respectively.5][6][7][8] Moreover, the unfavorable volumetric energy density causes poor compatibility and device complexities. [9,10]Thus, designing high-performance single nonprecious The development of efficient and low-cost bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is imperative but remains challenging in rechargeable zinc-air batteries.Herein, a metal oxidationassisted approach is developed for the facile synthesis of a highly efficient bifunctional catalyst (Co@NC@LDHs) which is composed of layered double hydroxides (LDHs) nanosheets in situ vertically grown on the surface of hollow carbon nanoframes encapsulating Co nanoparticles (Co@NC).Specifically, the vertical LDHs distributing on the carbon surface ensure the maximized number of accessible active sites and the interaction between LDHs with the carbon matrix.The hierarchically structured bifunctional Co@NC@LDHs display an overpotential of 0.33 V at 10 mA cm À2 for OER and a half-wave potential of 0.88 V for ORR.The potential gap (ΔE) of Co@NC@LDHs is calculated to be 0.68 V, outperforming the mixed Pt/C þ RuO 2 catalysts (ΔE = 0.77 V), manifesting its superior bifunctional electrocatalysis performance.Moreover, the Zn-air batteries based on Co@NC@LDHs electrocatalyst exhibit a high peak power density (185.2 mW cm À2 ) and excellent durability (5.0 mA cm À2 over 500 h).This work may provide a facile strategy for the fabrication of LDHs on carbon substrate, acting as efficient catalysts for broad energy-related applications.13] Layered double hydroxides (LDHs) with layered structures and adjustable multifunctionality, especially NiFe-LDHs, have been recognized as alternatives to Ru/Ir-based materials for OER. [14,15]Although a series of modification engineering processes like doping [16] and defect engineering [17] have been developed to enhance the intrinsic activity of NiFe-LDHs, the unsatisfying inherent electron conductivity and the selfaggregation of LDHs limit their catalytic activity in water oxidation. [18]In these respects, conductive carbon materials with high structural flexibility and unique surface properties were employed as preeminent substrates for LDHs deposition. [19]revious reports have demonstrated that the electron diffusion distance could be shortened by electron highways generated via the electrostatic attraction between the positively charged LDHs and negatively charged carbon materials. [20][23][24] For example, Dai et al. demonstrated that NiFe-LDHs/CNTs hybrids showed excellent OER activity and stability, outperforming Ir-based electrocatalysts under alkaline conditions. [25]In addition, the rational design of the catalyst structure is also crucial for the achievement of a high OER performance.Lou et al. presented that the hierarchically hollow nanoprisms composed of vertical NiFe-LDHs nanosheets with large surface areas and exposed active sites exhibited high OER activity. [26]Nevertheless, NiFe-LDHs electrocatalysts have not been extended so far to rechargeable zinc-air battery systems, due to their restricted ORR activity.
[29] Therefore, hybridizing LDHs on Co-NC materials has emerged as a superb strategy for the development of efficient bifunctional electrocatalysts for rechargeable zinc-air batteries. [12,30]onsiderable progress, in fact, has recently been made in pursuit of high-activity bifunctional electrocatalysts via the aforesaid hybrid strategy.However, extra alkaline reagents and hydrothermal treatment are necessary for previous reports. [31,32]In addition, complex steps are involved in the fabrication of hierarchical LDHs anchored on metal-NC substrates. [33]Thus, a facile method of synthesizing hierarchically structured LDHs nanosheets growing on the hollow carbon nanoframes remains severely challenging and is greatly essential.
In this article, we report a mild metal oxidationassisted method to obtain vertical LDHs nanosheets grown on Co-supported carbon nanoframes (Co@NC@LDHs).Unlike previous synthetic methods of LDHs, the current advantageous strategy opened a time-and cost-saving avenue that the target LDHs nanosheets could be obtained under room temperature conditions and neither any adscititious alkali sources nor oxidants for producing OH À ions and trivalent cations, respectively, are required.The LDHs nanosheets align vertically on the carbon surface which exposes more active sites, facilitating the diffusion of hydroxide ions and oxygen molecules.Benefiting from the above advantages, Co@NC@LDHs showed robust bifunctional activity compared to Pt/C and RuO 2 catalysts.More impressively, the assembled zinc-air batteries (ZABs) with use of the bifunctional catalyst Co@NC@LDHs displayed a low charge-discharge gap, high peak power density, and outstanding rechargeable lifetime.

Characterization of Electrocatalysts
The synthesis procedure for Co@NC@LDHs is illustrated in Figure 1a.As seen, the corresponding procedure starts from bimetal (Zn, Co) zeolitic imidazolate framework (BZIF).First, BZIF was coated with a resorcinol-formaldehyde (RF) layer to prepare BZIF@RF.Subsequently, BZIF@RF was pyrolyzed at 800 °C under N 2 atmosphere.In this process, Zn species were evaporated and some Co atoms were aggregated into Co nanoparticles.This carbon product exhibited a hollow polyhedral morphology, denoted as Co@NC.Finally, Co@NC powders were immersed in an aqueous solution containing Fe(NO 3 ) 3 and Ni(NO 3 ) 2 for 30 min under mild sonication to get Co@NC@LDHs.Detailed information of chemicals, synthesis steps, characterization, and electrochemical measurements of samples can be found in Supporting Information.
Structural characterizations were carried out to monitor the morphology transformation from BZIF to Co@NC and finally to Co@NC@LDHs.The transmission electron microscopy (TEM) image of BZIF displayed a typical rhombic dodecahedron with an average particle size of 300 nm (Figure S1a, Supporting Information).According to the sharp contrast, it is convinced that a thin RF shell was successfully coated on the surface of BZIF (Figure S1b, Supporting Information), and the RF shell displayed ignored effects on the crystal structure of BZIF according to their similar XRD patterns (Figure S2, Supporting Information).After pyrolyzing under N 2 atmosphere, Co@NC with a hollow structure was obtained (Figure S3, Supporting Information) and the scanning electron microscopy (SEM) image also confirmed the morphology-retaining transformation from BZIF@RF to Co@NC with a rough surface (Figure S4, Supporting Information).The RF coating enables carbon materials to have independent and hollow structures compared to Co/NC derived from direct carbonization of BZIF (Figure S5, Supporting Information).The carbon shells prevent the aggregation of Co nanoparticles and the hollow carbon structure will further favor fast mass transport. [34]s shown in Figure 1b, Co@NC@LDHs retained the basic size and framework structure of Co@NC.It can be clearly seen in Figure 1c that nanosheets were homogeneous and aligned vertically on the whole surface of Co@NC, meanwhile, the 3D hollow morphology was kept well.XRD technique was further employed to clarify the chemical components and crystals of as-prepared samples.As shown in Figure 1d, the peaks located at 44°, 51°, and 75°in the XRD patterns of Co@NC and Co@NC@LDHs are attributed to the Co nanoparticles formed during the carbonization of the BZIF. [1]The peak at around 25°is identified as the (002) crystal face of graphitic carbon formed under catalytic graphitization of Co nanoparticles. [35]he other diffraction peaks labeled at 11°, 23°, 34°, and 60°w are assigned to (001), (006), (012), and (110) crystal planes, indicative of hydrotalcite-like LDH phase in Co@NC@LDHs (JCPDS NO.51-0463). [20]These results manifest the successful in situ generation of LDHs nanosheets on the Co@NC support.High magnification TEM (HRTEM) and corresponding element mappings (Figure 1e) indicated that C, N, O, Fe, and Ni were homogeneously distributed in the carbon nanoframes while Co nanoparticles were confined in the interior of hollow carbon.It is worth noting that weak Co signal can be found in other places of the carbon nanoframes, which may be due to the Co ions dispersed in the internal of LDHs.As shown in Table S1, Supporting Information, the molar ratio of Ni:Fe in Co@NC@LDHs is about 1:1 from ICP-OES, which is lower than its feeding ratio (3:1).This should be attributed to higher solubility constant (k sp ) of Ni(OH) 2 (k sp = 2 Â 10 À15 ) than that of Fe(OH) 3 (k sp = 4 Â 10 À38 ). [36]-ray photoelectron spectroscopy (XPS) was used to investigate the electronic structure.In the XPS survey spectrum, the presence of C, O, Fe, Co, and Ni elements was confirmed (Figure S6, Supporting Information).In the Fe 2p XPS spectrum of Co@NC@LDHs (Figure 2a), two spin-orbit doublets at 712.1 and 725.3 eV can be identified as Fe 2p 3/2 and Fe 2p 1/2 signals of Fe 3þ .Interestingly, the binding energies of Fe 2p in Co@NC@LDHs were shifted %0.7 eV to high binding energies Figure 1.a) Schematic illustration for the synthesis of Co@NC@LDHs.b) TEM and c) SEM images of Co@NC@LDHs.d) XRD patterns of Co@NC and Co@NC@LDHs.e) HRTEM image of Co@NC@LDHs.f ) Corresponding element mappings of Co@NC@LDHs.compared with that of unsupported NiFe-LDHs, which can be ascribed to the interaction of LDHs with carbon supports.The Ni 2p XPS is shown in Figure 2b, and the two spin-orbit peaks were located at 873.8 and 856.2 eV with a binding energy gap of %17.6 eV, suggesting the presence of Ni 2þ species in these two samples.[37] As for the Co 2p XPS spectrum (Figure 2c), the binding energy peaks at 798.1 and 782.5 eV were ascribed to the Co 2þ 2p 1/2 and Co 2þ 2p 3/2 spin orbitals, and the binding energy at 796.5 and 780.6 eV were assigned to Co 3þ 2p 1/2 and Co 3þ 2p 3/2 , indicating the copresence of Co 2þ and Co 3þ in the Co@NC@LDHs.[36,38] According to the above-mentioned results, it can be concluded that the LDHs grown on Co@NC were trimetallic, including Fe, Co, and Ni elements.N 2 adsorption-desorption analysis was conducted to reveal the texture parameters.As seen in Figure S7, Supporting Information, the Co@NC and Co@NC@LDHs exhibited similar type-IV N 2 adsorption/desorption isotherms with obvious hysteresis loops, indicating their mesoporous characteristics.[39] The specific surface area based on the BET model was calculated with values of 322 m 2 g À1 for Co@NC and 249 m 2 g À1 for Co@NC@LDHs, respectively.The large specific surface area and abundant pores will be beneficial for O 2 transport, thus enhancing electrocatalytic performance.[40]

Formation Mechanism of Co@NC@LDHs
The fabrication mechanism of Co@NC@LDHs was clarified in detail, and a metal oxidation-assisted growth process is proposed in Figure 3. First, the highly hydrophilic Co@NC with a small contact angle (Figure S8, Supporting Information) holds well dispersibility in the mixed metal salt solution of Ni(NO 3 ) 2 and Fe(NO 3 ) 3 .The Co nanoparticles of Co@NC could be oxidized by H þ species produced via hydrolysis reaction, resulting in the accumulation of hydroxyl ions (OH À ) which further drives following formation of hydroxides.To support this viewpoint, the Co@NC was fully washed by HCl to clear the naked metal Co, and the resultant sample was named Co@NC-H.After the addition of Co@NC-H into the same mixture of nitrate solution, as expected, no LDHs were detected by the corresponding TEM image and XRD pattern (Figure S9, Supporting Information), demonstrating the vital role of metal Co nanoparticles.Subsequently, due to the small k sp , iron hydroxide preferentially precipitated as nuclei and quickly grew into the primary particles, in situ anchoring onto the surface of Co@NC.Finally, the mild basic condition enables the hydroxides to slowly crystallize and grow along the z-axis, gradually forming nanosheets via olation reactions.Given the LDH structure, Ni and Fe occupy divalent and trivalent cation sites, respectively, meanwhile a small amount of cobalt ions enter into the lattice according to the characterization results.To further understand the contribution of Co@NC, NaOH was employed as a basic source.As seen in Figure S10, Supporting Information, NiFe-LDHs were synthesized in the absence of Co@NC displaying bulk nanoparticle morphologies.A similar phenomenon was observed even Co@NC was added to the synthesis solution (Figure S11, Supporting Information).The obtained sample was named Co@NC@LDHs-OH.Moreover, the bulks are scattered randomly rather than on Co@NC support due to quick nuclear formation in the NaOH solution.In short, the advantages of Co@NC in the growth process are summarized as follows: 1) the 3D frame structure is beneficial for supporting and dispersion of LDHs nanosheets; 2) the slow oxidation of Co nanoparticles makes the formation of hydroxide slowly, in favor of the formation of nanosheets; 3) consumption of Co nanoparticles embedded in carbon structure drives the LDHs nanosheets growing on the carbon surface, without extra adhesive.Encouraged by the positive results, chloride (NiCl 2 and FeCl 3 ) solution was employed as the metal precursor of LDHs to prepare the controlled sample (Co@NC@LDHs-Cl).As shown in Figure S12, Supporting Information, similar hierarchical nanosheets on hollow Co@NC were observed which was further confirmed by XRD analysis as the resultant LDHs (JCPDS NO.51-0463, Figure S13, Supporting Information), indicating a certain universality of the novel metal-assisted method.

Electrocatalytic Activity
In this work, Co@NC for ORR and LDHs for OER were combined into the Co@NC@LDHs sample, making it a bifunctional catalyst.The OER electrocatalytic performance of Co@NC@LDHs was first investigated in O 2 -saturated 1 M KOH solution.Other controlled samples including unsupported NiFe-LDHs, sole Co@NC, Co@NC þ NiFe-LDHs (mixture of NiFe-LDHs and Co@NC), and noble metal-based catalysts (Pt/C þ RuO 2 ) were tested in identical conditions to evaluate their electrocatalysis OER activities.Figure 4a displayed no-iR corrected OER linear sweep voltammetry (LSV) curves on the synthesized electrocatalysts and commercial Pt/C þ RuO 2 at a scan rate of 10 mV s À1 .Among them, the current density of Co@NC@LDHs was dramatically superior to those of the other electrocatalysts over the entire potential region.Additionally, Co@NC@LDHs afforded a current density of 10 mA cm À2 at the smallest overpotential of 330 mV, which is lower than those of 440, 530, 380, and 460 mV for the Co@NC, NiFe-LDHs, Co@NC þ NiFe-LDHs, and Pt/C þ RuO 2 samples, respectively.As shown in Figure 4a, a significant peak ascribed to oxidized Ni 2þ is observed for Co@NC@LDHs but inconspicuous for other controlled samples due to advantageous support and Co dopant.To further demonstrate this term, NiFeCo-LDHs were prepared with Co salt added into the synthetic process of NiFe-LDHs, and the TEM image is depicted in Figure S14, Supporting Information.After electrocatalysis test under the same conditions, NiFeCo-LDHs possess a smaller overpotential compared to NiFe-LDHs (Figure S15, Supporting Information).It is worth noting that a weak oxidation peak shows up in NiFeCo-LDHs (Figure S15, Supporting Information), confirming the enhancement of Co species.Combined with the results of XPS (Figure 2), the obvious oxidation peak is ascribed to oxidized Ni 2þ and strongly enhanced by Co doping and carbon support, which could be regarded as a remarkable characteristic of OER performance.The Tafel slope is a key factor to evaluate the OER reaction kinetics. [41,42]As shown in Figure 4b, the Tafel slope for Co@NC@LDHs (76 mV dec À1 ) was lower than those for Co@NC (116 mV dec À1 ), NiFe-LDHs (124 mV dec À1 ), Co@NC þ NiFe-LDHs (125 mV dec À1 ), and Pt/C þ RuO 2 (134 mV dec À1 ).These results suggest that Co@NC@LDHs with a low overpotential can be perceived as a highly active OER catalyst.The excellent performance of Co@NC@LDHs might be owing to the supported NiFe-LDHs since the sole Co@NC had a weak OER ability with a high overpotential of 440 mV and a Tafel slope of 116 mV dec À1 .To further explore the advantage of LDHs grown on carbon nanoframe, electrochemical impedance spectroscopy (EIS) measurements were conducted.As demonstrated in Figure S16, Supporting Information, Co@NC@LDHs showed a lower electrochemical charge-transfer resistance (134.7 Ω) compared with Co@NC þ NiFe-LDHs (330.4Ω), NiFe-LDHs (20241.0Ω), and Co@NC (188.5 Ω), agreeing well with its exceptional OER activity.The above results revealed that growing LDHs on the surface of Co@NC had a positive effect on enhancing the OER performance compared to the direct mixed Co@NC þ NiFe-LDHs.First, Co@NC induced exceptional electroconductivity to the Co@NC@LDHs with accelerated electron transfer between Co@NC nanoframes and LDHs nanosheets as evidenced by XPS analysis.Second, the unique hollow features with high surface area and abundant pores were well maintained which favor mass diffusion, thereby enhancing OER activities. [32]Moreover, the Co@NC provided rich positions for the location of vertically aligned LDHs with more active sites exposed. [14]The current-time (i-t) measurement was carried out to study the stability of Co@NC@LDHs for OER.As shown in Figure S17, Supporting Information, the current density remained almost unchanged in the first 20 000 s, and gradually decreased and then stabilized, suggesting a certain stability of Co@NC@LDHs as an OER catalyst.
The ORR performance of Co@NC@LDHs was evaluated by cycle voltammetry (CV) tests in O 2 /N 2 -saturated alkaline solution (0.1 M KOH) using a standard three-electrode cell.As shown in Figure S18, Supporting Information, a notable oxygen reduction peak was observed in the O 2 -saturated solution, while no peak emerged in N 2 -saturated solution, indicating an effective ORR activity of Co@NC@LDHs.LSV measurements in O 2 -saturated alkaline solution (0.1 M KOH) were further conducted to study the ORR performance of Co@NC@LDHs and other comparative samples (Figure 4c).Apparently, Co@NC@LDHs, Co@NC, and Co@NC þ NiFe-LDHs possessed similar LSV curves and the same onset potentials (0.98 V).More importantly, the half-wave potential of 0.88 V for Co@NC@LDHs was slightly higher than that of Pt/C þ RuO 2 (0.85 V).The similar Tafel slopes for Co@NC@LDHs (83 mV dec À1 ), Co@NC (80 mV dec À1 ), and Co@NC þ NiFe-LDHs (80 mV dec À1 ) indicate the same ratedetermining step on these prepared catalysts (Figure 4d).In addition, the NiFe-LDHs sample possesses a low half-wave potential (0.60 V) and high Tafel slope (91 mV dec À1 ), which suggest that NiFe-LDHs are nonideal ORR catalysts.The above results further confirmed that the active site for ORR on Co@NC@LDHs is Co@NC.To clarify the advantages of micromorphologies on ORR activity, the controlled samples with destroyed morphologies were obtained via grounding Co@NC@LDHs (named as Co@NC@LDHs-g), and the TEM images (Figure S19, Supporting Information) display the collapsed structures.As expected, the conducted ORR test presents a dramatic decrease in half-wave potential and current density compared to the complete Co@NC and Co@NC@LDHs (Figure S20, Supporting Information).It could conclude that the unique hollow features with high surface area and abundant pores excel as positive promoters of electrocatalysis performance.The ORR activity of Co@NC@LDHs and Co@NC was also estimated under different rotating speeds (400-2025 rpm), and the electron transfer numbers according to the corresponding Koutecky-Levich (K-L) plots are 4.0 and 3.9 for Co@NC and Co@NC@LDHs, respectively (Figure S21, Supporting Information).The ORR performance of Co@NC@LDHs was further investigated by the rotating ring-disk electrode (RRDE).The electron transfer number is calculated with a value close to 4.0, and the corresponding H 2 O 2 selectivity is less than 10% at 0.2-0.8V, implying a desirable 4 e À reaction process (Figure S22, Supporting Information).More importantly, the Co@NC@LDHs almost remained at its initial ORR current after about 72 000 s measurement, manifesting its excellent stability as an ORR catalyst (Figure S23, Supporting Information).Additionally, the Co@NC@LDHs were subjected to the accelerated decay test (ADT) for 5000 CV cycles to evaluate their durability.As shown in Figure S24, Supporting Information, a negligible decrease was observed, implying good stability of Co@NC@LDHs.After ADT, SEM and XRD techniques were employed to determine the structural stability of Co@NC@LDHs.As exhibited in Figure S25 and S26, Supporting Information, Co@NC@LDHs remained its origin 3D structure and crystal phase.The peaks located at 11°, 34°, and 60°are identified as the (003), (012), and (110) lattice planes of the standard NiFe-LDHs phase (JCPDS NO.51-0463, Figure S26, Supporting Information), the rest of relative intense peak originated from carbon paper.Also, the double-layer capacitance (C dl ) method was used to measure the electrochemical active surface areas (ECSA) (Figure S27-S29, Supporting Information).Since C dl is linearly related to the ECSA, the Co@NC@LDHs seems to possess low ECSA but high intrinsic activity compared to the controlled catalysts (Co@NC and Co@NC þ NiFe-LDHs) (Figure S30, Supporting Information).The potential gap (ΔE) between the OER potential measured at 10 mA cm À2 (E J = 10 ) and the half-wave potential (E 1/2 ) of ORR (ΔE = E J = 10 -E 1/2 ) was the evaluation index of the catalytic activity of bifunctional catalysts.A lower ΔE value means a better catalytic bifunctional activity. [43,44]igure 4e,f displays that Co@NC@LDHs had a low ΔE of 0.68 V, which was lower than those of Co@NC (0.79 V), NiFe-LDHs (1.16 V), Co@NC þ NiFe-LDHs (0.73 V), Pt/C þ RuO 2 (0.76 V), and most other reported catalysts (Table S2, Supporting Information).Such results confirmed that Co@NC@LDHs can be employed as a promising alternative bifunctional catalyst for ORR and OER.

Zinc-Air Batteries
To explore the practical application potential of the as-prepared catalysts, the Co@NC@LDHs sample was applied as the cathode catalyst for the assembly of ZAB, as depicted in Figure 5a.For comparison, the cathode with Pt/C þ RuO 2 mixture was also fabricated.As shown in Figure 5b, the open-circuit voltage (OCV) of Co@NC@LDHs-assembled ZAB is 1.49 V, which is close to that of Pt/C þ RuO 2 -assembled ZAB.The charge and discharge polarization curves of Co@NC@LDHs and Pt/C þ RuO 2assembled ZABs are exhibited in Figure 5c.Obviously, the voltage gap between the charge and discharge potentials of Co@NC@LDHs-assembled ZAB is lower than that of Pt/C þ RuO 2 , indicating the remarkable rechargeability performance. [29]Meanwhile, it shows a higher peak power density of 185.2 mW cm À2 at a larger current density of 322.9 mA cm À2 than that of Pt/C þ RuO 2 -based ZAB (93.9 mW cm À2 at 202.1 mA cm À2 ).Furthermore, the Co@NC@LDH-assembled ZAB delivers a longer discharge time (41.6 h) than that of Pt/C þ RuO 2 -based ZAB (40.0 h) under a discharge current density of 10 mA cm À2 (Figure 5d).The outstanding performance of the Co@NC@LDH-assembled ZAB can be attributed to its hierarchical structures, which benefits the fast mass transportation, as well as the LDHs vertically grown on Co@NC nanoframes that expose a large amount of catalytic active sites.The cycling stability was evaluated by galvanostatic charge-discharge (20 min) cycling at a current density of 5.0 mA cm À2 , as shown in Figure 5e.The initial chargedischarge voltage gap of Co@NC@LDH-assembled ZAB is 0.80 V, which is lower than that of Pt/C þ RuO 2 -assembled ZAB (0.98 V).Impressively, the voltage gap of Co@NC@LDH-assembled ZAB (0.83 V) obtains negligible enlargement after 400 h service, rendering excellent cycling stability.Consequently, it can be concluded that the Co@NC@ LDHs holds great potential for practical ZAB technologies to meet the increasing energy demands.

Conclusion
In summary, a novel metal oxidation-assisted strategy was developed for the efficient construction of a superior bifunctional catalyst for OER/ORR and further rechargeable zinc-air batteries.Specifically, NiFe-LDHs nanosheets were successfully in situ vertically grown on the surface of Co@NC nanoframes without extra alkali sources and oxidants.The Co@NC@LDHs catalyst showed exceptional OER performance with E J = 10 of 1.56 V and outstanding ORR activity with E 1/2 of 0.88 V, outperforming Pt/C þ RuO 2 .Moreover, the Co@NC@LDHs-based ZAB exhibited high power density (185.2 mW cm À2 at 322.9 mA cm À2 ) and excellent stability (500 h).Benefiting from the vertical LDHs nanosheets, porous structure of Co@NC nanoframes, and the interface between LDHs and carbon supports, the hierarchical structured Co@NC@LDHs catalyst gained high utilization efficiency of active sites and fast mass transportation for high ORR/OER performance.These unique merits ensure Co@NC@LDHs a new type of efficient bifunctional catalyst for the paired reaction of OER/ORR in rechargeable ZAB.Moreover, our method presented renders considerable simplicity compared to the previously reported synthetic ones, and should be readily extended to the synthesis of other bimetallic or trimetallic LDHs nanosheets on carbon substrates.We believe that this work opens a new path for the design and construction of cost-effective electrocatalysts for urgent energy conversion and storage applications.

Figure 3 .
Figure 3.The mechanism illustration of LDHs grown vertically in situ on the carbon substrate.

Figure 4 .
Figure 4. a) OER LSV curves at a scanning rate of 10 mV s À1 without iR correction.b) Tafel plots derived from OER LSV curves.The electrolyte was 1 M KOH.c) ORR LSV curves at a scanning rate of 10 mV s À1 .d) Tafel plots derived from ORR curves.The electrolyte was 0.1 M KOH.e) LSV curves of the ORR and OER of prepared catalysts.f ) Potential gaps between the potential of OER current density at 10 mA cm À2 and the E 1/2 of ORR for prepared catalysts.

Figure 5 .
Figure 5. a) Schematic illustration of Zn-air battery configuration.b) Open-circuit potential plots of Zn-air batteries (inset: photograph of a battery with an OCP of 1.49 V). c) Charge-discharge polarization curves and power density plots of Zn-air batteries with Co@NC@LDHs and Pt/C þ RuO 2 catalysts.d) Discharge profiles at 10.0 mA cm À2 and e) Pt/C þ RuO 2 and Co@NC@LDHs catalysts at a current density of 5 mA cm À2 (the insets demonstrated the potential after the batteries run for 50 and 450 h).