Three‐dimensional interconnected graphene network‐based high‐performance air electrode for rechargeable zinc‒air batteries

Although zinc‒air batteries (ZABs) are regarded as one of the most prospective energy storage devices, their practical application has been restricted by poor air electrode performance. Herein, we developed a free‐standing air electrode that is fabricated on the basis of a multifunctional three‐dimensional interconnected graphene network. Specifically, a three‐dimensional interconnected graphene network with fast mass and electron transport ability, prepared by catalyzing growth of graphene foam on nickel foam and then filling reduced graphene oxide into the pores of graphene foam, is used to anchor iron phthalocyanine molecules with atomic Fe‒N4 sites for boosting the oxygen reduction during discharging and nanosized FeNi hydroxides for accelerating the oxygen evolution during charging. As a result, the obtained air electrode exhibited an ultra‐small electrocatalytic overpotential of 0.603 V for oxygen reactions, a high peak power density of 220.2 mW cm‒2, and a small and stable charge‒discharge voltage gap of 0.70 V at 10 mA cm‒2 after 1136 cycles. Furthermore, in situ Raman spectroscopy together with theoretical calculations confirmed that phase transformation of FeNi hydroxides takes place from α‐Ni(OH)x to β‐Ni(OH)x to γ‐Ni(3+δ)+OOH for the oxygen evolution reaction and Ni is the active center while Fe enhances the activity of Ni active sites.


INTRODUCTION
5][6] The sluggish kinetics of ORR and OER result in unacceptable battery performance, such as low energy efficiency, inferior power density, and poor charge-discharge cycling stability.8][9] Noblemetal Pt and Ir/Ru-based materials are currently the best ORR and OER catalysts, respectively.0][21] Nevertheless, the preparation of high-performance TMCs/MeNC is still a big challenge since this process involves a hightemperature heat treatment, during which the presence of Ostwald ripening results in low-density active parts of either TMCs or Me-N 4 .6][27][28] For example, Zhang and coworkers developed a multiscale construction method to integrate FeNi hydroxides and cobalt porphyrin into one catalyst. 29Therefore, FeNi hydroxides accelerate the OER, while Co-N 4 sites in cobalt porphyrin boost the ORR, and as a result fast oxygen electrocatalytic reactions were demonstrated.Our group proposed a carbon nanotube-bridging way to anchor NiCo nanoparticles and Fe single atoms in a dual-phasic carbon. 30In situ Raman spectroscopy confirmed that NiCo hydroxides/oxyhydroxides (reconstituted from NiCo nanoparticles) and single-atom Fe species are independently stabilized on different carbon phases, reducing the interference during the OER-ORR (charge-discharge) cycling process, which greatly boosts the battery performance and charge-discharge cycling stability. 31Nevertheless, these reported catalysts are in the form of powders.When they are used to assemble an air electrode, additional polymer binders are required to realize a good integration between the catalyst and carbon paper/cloth.][34] Herein, we develop a free-standing air electrode based on a multifunctional three-dimensional interconnected graphene network.The intrinsic structure of threedimensional interconnected graphene network greatly shortens the transport paths of electrons, reactants, and products, which can decrease the current time resistance and mass transport losses. 35Furthermore, highly efficient active components of iron phthalocyanine (FePc) and FeNi hydroxides are anchored on the interconnected graphene network to boost the slow kinetics of oxygen reactions.Therefore, FePc containing atomic Fe-N 4 moiety and FeNi hydroxides are used to boost the catalytic kinetics of ORR and OER, respectively, reducing the kinetic loss of ZABs.The obtained multifunctional graphene network-based material (denoted as NF@GF/rGO-FePc||FeNi) is directly used as the air electrode of ZABs, showing outstanding battery output performance and charge-discharge cycling durability.Furthermore, in situ Raman spectroscopy experiments and density fuctional theory (DFT) calculations confirmed the electrocatalytic working mechanisms.

RESULTS AND DISCUSSION
The preparation process of NF@GF/rGO-FePc||FeNi is schematically shown in Figure 1A.First, graphene foam was grown on NF (NF@GF) by a chemical vapor deposition method, 35 which was revealed by scanning electron microscopy (SEM) imaging (Figure S1).Then, NF@GF was put into a well-dispersed graphene oxide (GO) solution to realize the GO filling in NF@GF (NF@GF/GO) by means of zinc foil stabilizing and freeze drying, showing a honeycomb-like structure (Figure S2).The NF@GF/GO precursor was annealed at high temperature to transform the GO into reduced GO and three-dimensional interconnected honeycomb-like graphene networks anchored in NF (NF@GF/rGO) were obtained (Figure S3).Subsequently, NF@GF/rGO was put into a N,N-dimethylformamide solution containing FePc to ensure that the FePc molecules were fully coupled with graphene.After washing and drying, a honeycomb-like FePc-modified NF@GF/rGO (NF@GF/rGO-FePc) was collected (Figure S4).Finally, NF@GF/rGO-FePc||FeNi was prepared through a coprecipitation process, in which FeNi hydroxides grew on NF@GF/rGO-FePc, showing a free-standing characteristic (Figure S5). Figure 1B shows a typical SEM image of NF@GF/rGO-FePc||FeNi.
Three-dimensional interconnected honeycomb-like graphene networks are demonstrated.Such a structure endows it with larger surface area compared to contrast sample prepared by directly using rGO to couple with FePc and then anchoring the FeNi layered double hydroxide (LDH) (Figure S6).Energydispersive X-ray spectroscopy (EDS) elemental mapping shows that Fe, Ni, N, and O are evenly distributed in the graphene substrate (Figure S7), illustrating that FePc and FeNi hydroxides are uniformly anchored on graphene.The structure of NF@GF/rGO-FePc||FeNi was further observed by transmission electron microscopy (TEM) imaging.Abundant small-sized metal species domains are attached to the surface of graphene layers (Figure S8).High-resolution TEM image shows that FeNi species have a distinct lattice fringe of 0.263 nm (Figure S9), which can be ascribed to the (012) plane of Ni 5.64 Fe 2.36 LDH.To further observe the Fe/Ni configurations in NF@GF/rGO-FePc||FeNi at the atomic level, aberration-corrected TEM imaging was conducted.Highangle annular dark-field images under a scanning TEM (HAADF-STEM) model show that metal single atoms and nanoparticles coexist in NF@GF/rGO-FePc||FeNi (Figure S10).High-magnification HAADF-STEM image in Figure 1C shows that atomically dispersed bright spots are densely anchored in the nanoparticle-free area of graphene support, which originates from Zcontrast imaging between isolated iron atoms from FePc molecules and carbon substrate. 36Figure 1D shows a high-magnification HAADF-STEM image of metal nanoparticle in NF@GF/rGO-FePc||FeNi.An ordered array of atoms indicates its good crystallization.By virtue of fast Fourier transformation analysis, the metal nanoparticle is identified to be Ni 5.64 Fe 2.36 LDH (Figure 1E).The FeNi hydroxides in NF@GF/rGO-FePc||FeNi are further demonstrated as Ni 5.64 Fe 2.36 LDHs by X-ray diffraction pattern analysis (Figure S11).The chemical composition was further examined by Xray photoelectron spectroscopy (XPS) analysis, showing the coexistence of Ni, Fe, C, O, and N in NF@GF/rGO-FePc||FeNi (Table S1), which is in good agreement with EDS elemental mapping results (Figure 1F).The Fe 2p spectrum (Figure 2A) consists of Fe 2p 3/2 and Fe 2p 1/2 regions.According to the binding energies of Fe configurations, six characteristic peaks are present in NF@GF/rGO-FePc||FeNi by fitting analysis.The peaks at 711.1 and 724.7 eV are indexed to Fe 2+ , while the peaks located at 713.5 and 726.3 eV are related to Fe 3+ .The other two peaks at 719.7 and 734.8 eV are satellite peaks. 37igure 2B shows the Ni 2p spectrum.Similar to Fe 2p spectrum, Ni 2+ and Ni 3+ coexist in NF@GF/rGO-FePc||FeNi, which is revealed by their binding energies of Ni 2p 1/2 at 856.0 eV (Ni 2+ 2p 3/2 ), 873.6 eV (Ni 2+ 2p 1/2 ), 857.5 eV (Ni 3+ 2p 3/2 ), and 875.8 eV (Ni 3+ 2p 1/2 ). 38N1s spectra demon-strate that the binding energy of NF@GF/rGO-FePc||FeNi has a shift compared to the FePc reference (Figure S12).Since N element in spectrum only comes from FePc, such an N binding energy shift should be caused by coupling interaction. 39Fourier-transform infrared spectroscopy was carried out to understand the coupling information, showing NF@GF/rGO-FePc||FeNi has three characteristic peaks at 723, 1083, and 1122 cm −1 , which are attributed to out-of-plane stretching vibration and in-plane deformation of FePc in NF@GF/rGO-FePc||FeNi (Figure S13). 40Compared to the FePc reference, the positions of these characteristic peaks have appreciable shifts, suggesting the presence of strong coupling interaction between FePc and graphene substrate.Compared to NF@GF/rGO, NF@GF/rGO-FePc||FeNi shows remarkably strong first desorption peak at ∼193.9 • C followed by multiple strong peak signals, suggesting the stronger adsorption capability of O 2 and therefore possibly better oxygen reaction performance (Figure S14). 41,42he chemical state and fine structure of iron in NF@GF/rGO-FePc||FeNi were investigated by X-ray absorption spectroscopy.Figure 2C shows the Fe Kedge X-ray absorption near-edge structure spectra of NF@GF/rGO-FePc||FeNi and reference samples.The absorption edge position of NF@GF/rGO-FePc||FeNi is found to be located halfway between phthalocyanine (FePc) and Fe 2 O 3 with a stronger inclination toward to Fe 2 O 3 but far from Fe foil.This confirms that the valence of iron in NF@GF/rGO-FePc||FeNi ranges between +2 and +3, approaching +3, which is consistent with the XPS results.Fourier-transform extended X-ray absorption fine structure (EXAFS) spectrum of NF@GF/rGO-FePc||FeNi is shown in Figure 2D.A primary peak at 1.51 Å is observed in the R space, which falls between the Fe-N (1.49Å) peak of FePc and the Fe-O (1.56 Å) peak of Fe 2 O 3 .This indicates the presence of both atomic Fe-N 4 sites and the hydroxide of Fe species in NF@GF/rGO-FePc||FeNi. Additionally, no Fe-Fe first-coordination shell scattering peak at around 2.26 Å is observed in NF@GF/rGO-FePc||FeNi.For the second-coordination shell scattering, a small peak near 2.56 Å is observed, which can be attributed to the Fe-Fe/Ni from FeNi hydroxides and/or Fe-C from FePc. Figure 2E shows the fitting curve of Fourier-transform EXAFS spectrum according to the back-scattering paths of Fe-N/O and Fe-Fe/Ni.The coordination number (CN) of Fe-N/O is around 3.9 (Table S2), smaller than those in FePc (CN = 4) and common FeNi hydroxides (CN = 6).This illustrates FeNi hydroxide species in NF@GF/rGO-FePc||FeNi are enriched with low-dimension structure, in good agreement with TEM results.Such a result is further supported by second-coordination shell fitting data.Besides, wavelet-transform diagrams of EXAFS oscillations are derived, showing that NF@GF/rGO-FePc||FeNi, F I G U R E 2 X-ray photoelectron spectroscopy (XPS) (A) Fe 2p and (B) Ni 2p spectra of NF@GF/rGO-FePc||FeNi. (C) Normalized Fe K-edge X-ray absorption near-edge structure spectra (XANES) spectra of NF@GF/rGO-FePc||FeNi, FeO, FePc, Fe 2 O 3 , and Fe-foil.(D) k 2 -Weighted Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectra of NF@GF/rGO-FePc||FeNi and reference samples.(E) Fourier-transformed EXAFS fitting curves of NF@GF/rGO-FePc||FeNi. (F) Wavelet-transform (WT) contour plots of Fe foil, Fe 2 O 3 , FePc, and NF@GF/rGO-FePc||FeNi. similar to Fe 2 O 3 and FePc, holds a maximum intensity in k-space at ∼3 Å −1 from Fe-N/O, while no Fe-Fe at ∼7.5 Å −1 is observed (Figure 2F).These above results clearly indicate the presence of atomic Fe-N 4 sites and FeNi hydroxides in NF@GF/rGO-FePc||FeNi, which is expected to show desirable catalytic performance in oxygen electrodes.
surpassing that of commercial Pt/C (0.823 V).This result demonstrates the excellent ORR activity of NF@GF/rGO-FePc||FeNi.The HO 2 -yield and the number of electrons transfered derived from common rotating ring-disk electrode measurements are shown in Figure 3B.With increasing overpotential, the HO 2 − yield of NF@GF/rGO-FePc||FeNi increases to a maximum peak value of 2.26% at 0.51 V and then decreases to 1.21% at 0.20 V.The average electron transfer number is 3.97, indicating that NF@GF/rGO-FePc||FeNi catalyzes the ORR maintaining an efficient four-electron transfer process.In addition to the ORR activity, the stability of NF@GF/rGO-FePc||FeNi was also evaluated by cyclic voltammetry (CV) accel-erated measurements.As shown in Figure 3C, the E j = −3 @ORR value of NF/GF-rGO@FePc/FeNi has only a slight negative shift of 7 mV after 5000 continuous CV cycles.HAADF-STEM imaging demonstrated that dense atomic sites were still present in NF/GF-rGO@FePc/FeNi (Figure S17).These results indicate good stability of NF/GF-rGO@FePc/FeNi. Figure 3D displays the OER polarization curves of NF@GF/rGO-FePc||FeNi and Ir/C.At 10 mA cm −2 , NF@GF/rGO-FePc||FeNi holds a potential (E j = −10 @OER) of 1.492 V (corresponding overpotential of 0.262 V), which is 110 mV more negative than that of Ir/C (1.602 V, corresponding overpotential of 0.372 V).This result indicates that NF@GF/ rGO-FePc||FeNi has much better electrocatalytic activity for OER.Accelerated measurement result demonstrates that NF@GF/rGO-FePc||FeNi has good OER stability, which is revealed by its slight decay of E j = −10 @OER value (13 mV) after 5000 CV potential cycles (Figure 3E).
To demonstrate the active sites for ORR and OER in NF@GF/rGO-FePc||FeNi, control experiments were performed to obtain the contrast samples of NF@GF/rGO-FePc without coprecipitating FeNi hydroxides and NF@GF/rGO-FeNi in the absence of FePc coupling.ORR performance evaluation result showed that the electrocatalytic activity of NF@GF/rGO-FePc was comparable to that of NF@GF/rGO-FePc||FeNi, but much higher than that of NF@GF/rGO-FeNi (Figure S18A).This clearly illustrates that the high ORR activity originates from FePc-coupled graphene in NF@GF/rGO-FePc||FeNi.For OER, high electrocatalytic activity should be attributed to FeNi hydroxides on graphene, which is uncovered by comparing the OER polarization curves (Figure S18B).Thus, NF@GF/rGO-FePc||FeNi holds high ORR and OER active parts, showing excellent bifunctional electrode performance with a small potential difference (ΔE = E j = 10 @OER -E j = −3 @ORR) of 0.603 V (Figure 3F), significantly outperforming the contrast samples, commercial Pt/C + Ir/C catalysts (0.779 V), and presenting one of the best bifunctional electrocatalysts in comparison with recently developed advanced catalysts (Table S3).The double-layer capacitance (Cdl), recognized as a key parameter to evaluate the electrochemically active surface area and electrocatalytic activity, demonstrates that NF@GF/rGO-FePc||FeNi holds a large Cdl of 2.73 mF cm −2 (Figure S19) and therefore good electrocatalytic oxygen performance.
0][31] To gain insights into the catalytic mechanisms of FeNi hydroxides in NF@GF/rGO-FePc||FeNi for OER, in situ Raman spectroscopy was conducted.As exhibited in Figure 3G, two weak peaks are observed in NF@GF/rGO-FePc||FeNi at 449 and 524 cm −1 at opencircuit potential and applied potentials below 1.37 V, which can be due to the Ni-O vibration in α-Ni(OH) x of FeNi hydroxides (Figure S20).The peak at 449 cm −1 shifts to 446 cm −1 , while the other peak at 524 cm −1 disappears when the applied potentials increase from 1.42 to 1.52 V.Meanwhile, a new peak at 493 cm −1 appears in NF@GF/rGO-FePc||FeNi.These results indicate the presence of phase conversion from α-Ni(OH) x to β-Ni(OH) x . 43 further increase in the applied potentials results in the transformation from Ni(OH) x species into γ-Ni (3+δ)+ OOH, which is uncovered by the characteristic peaks of γ-Ni (3+δ)+ OOH at 476 and 554 cm −1 .To further investigate the roles of Fe and Ni in FeNi hydroxides of NF@GF/rGO-FePc||FeNi, single-metal hydroxide-based reference samples of NF@GF/rGO-FePc||Ni and NF@GF/rGO-FePc||Fe were prepared under the same conditions, and in situ Raman tests were also performed.The Raman signals of raw Ni(OH) x species (α-Ni(OH) x ) in NF@GF/rGO-FePc||Ni are very weak and two new peaks from β-Ni(OH) x gradually appears (Figure 3H) when the applied potential increases from 1.42 to 1.57 V. Distinct γ-Ni (3+δ)+ OOH characteristic peaks are observed when the applied potential is further increased to 1.67 V. Completely different from NF@GF/rGO-FePc||FeNi and NF@GF/rGO-FePc||Ni, no phase conversion is observed for NF@GF/rGO-FePc||Fe during in situ Raman tests (Figure 3I).Six peaks at 220, 288, 399, 492, 606, and 656 cm −1 appear and their signals increase with increasing applied potentials, which may be due to the continuous adsorption of hydroxy species.Thus, it can be concluded that Ni in NF@GF/rGO-FePc||FeNi is the active center for OER.Furthermore, NF@GF/rGO-FePc||FeNi holds a lower conversion potential from β-Ni(OH) x to γ-Ni (3+δ)+ OOH than that of NF@GF/rGO-FePc||Ni (1.57V vs. 1.67 V), suggesting that the addition of Fe enhances the OER activity of Ni active center, which is in good agreement with OER performance comparison (Figure S21).To further reveal the active center for OER, we performed the DFT calculations.The limiting energy barrier on the Fe center is 0.49 eV in graphenesupported FeNi oxyhydroxide model (Figure S22), which is higher than that on the Ni center (0.44 eV).This illustrates that the OER tends to take place on the Ni center, which is in good agreement with in situ Raman result.
To validate the feasibility of practical applications, the NF@GF/rGO-FePc||FeNi electrode was assembled into liquid-state rechargeable ZAB.The battery displays an open-circuit voltage (OCV) of 1.411 V (Figure S23). Figure 4A shows the power density curves of ZABs derived from the corresponding discharge curves.A remarkable maximum power density of 220.2 mW cm -2 is demonstrated for NF@GF/rGO-FePc||FeNi-based ZAB, surpassing those of Pt/C + Ir/C one (171.9mW cm -2 ), rGO-FePc||FeNi (202.6 mW cm -2 ), and most recent reports (Table S4).By means of Zn mass loss during constant current discharge curves, the specific capacities and energy densities are demonstrated.Compared to Pt/C + Ir/C (specific capacity: 706.9 mAh g -1 , energy density: 830.6 mWh g -1 ) and rGO-FePc||FeNi (specific capacity: 710.8 mAh g -1 , energy density: 845.8 mWh g -1 ), NF@GF/rGO-FePc||FeNi has larger specific capacity of 739.3 mAh g -1 and energy density of 879.8 mWh g -1 (Figure 4B).Furthermore, the rate performance was studied through constant current discharging under different current densities.As displayed in Figure 4C, the NF@GF/rGO-FePc||FeNi cathode provides a higher discharge voltage than those of the Pt/C + Ir/C and rGO-FePc||FeNi ones at the same current density in the range of 1-20 mA cm −2 .When the current density is restored to 1 mA cm −2 , the average discharge voltage remains unchanged, demonstrating its excellent discharge reversibility.
In addition, galvanostatic charge-discharge cycling test was carried out to evaluate the rechargeable battery performance of NF@GF/rGO-FePc||FeNi. Figure 4D displays the long-term galvanostatic charge-discharge curve of NF@GF/rGO-FePc||FeNi at 10 mA cm −2 .Notably, we selected the 100th cycle as initial cycle to analyze the charge-discharge performance because test equilibrium is achieved after 100 cyclic measurements (Figure S24).[46] Subsequent charge-discharge cycling from 101st to 501st, and then 1136th cycle (Figure 4E-G) showed that its voltage gap (0.70 V) and energy efficiency (62.6%) remain almost unchanged.Furthermore, charge-discharge voltage platform is also stable even at large current densities (Figure S25).These results illustrate its outstanding charge-discharge cycling stability.Besides, we summarize the battery performances in Figure 4H, including charge-discharge voltage gap at 10 mA cm −2 and cycling time in comparison with recently reported FePc||CNTs||NiCo/CP, 30 Co@NiFe-LDH, 47 CoNi@CoCN, 48 Fe,Co/DSA-NSC, 49 NiCo 2 S 4 @g-C 3 N 4 -CNT, 50 Fe-N@Ni-HCFs, 51 and FeNi@NCNT-CP. 52We can see that the overall ZAB performance of our NF@GF/rGO-FePc||FeNi reaches a leading position, suggesting its great potential in real applications.In addition to liquid-state batteries, solid-state ZABs have also been assembled, mainly including zinc anodes, solid-state electrolytes, and oxygen catalyst-based cathodes (Figure 5A). Figure 5B displays the power density curves of solid-state ZABs.The peak power density of NF@GF/rGO-FePc||FeNi-based solid-state ZAB reached up to 77.6 mW cm −2 , which is superior to that of commercial Pt/C + Ir/C-based one (54.7 mW cm −2 ) and standing for one of the best among recently reported advanced batteries (Table S5).The rate performances of NF@GF/rGO-FePc||FeNi and Pt/C + Ir/C-based solidstate ZABs were evaluated through discharging under different current densities (Figure 5C), showing that NF@GF/rGO-FePc||FeNi has higher initial and reversible discharge voltages, which demonstrates the excellent discharge reversibility of NF@GF/rGO-FePc||FeNi-based solid-state ZAB. Figure 5D shows the OCV evolution of NF@GF/rGO-FePc||FeNi-based solid-state ZAB during bending at various angles.Its OCV value seems to have a slight increase after bending process.We believe that the bending process can improve the contact between electrolyte and catalyst, resulting in a slight increase in voltage output.Furthermore, good cycling stability of NF@GF/rGO-FePc||FeNi-based solid-state ZAB was demonstrated by galvanostatic charge-discharge cycling measurements (Figure 5E).To simulate the real application scenarios, NF@GF/rGO-FePc||FeNi-based solid-state ZABs were used to power electrical devices working.As shown in Figure 5F,G, four batteries in series can power a phone and three ZABs in series can drive a small electric fan.These results demonstrate excellent feasibility of NF@GF/rGO-FePc||FeNi-based solid-state ZABs for future practical applications.

CONCLUSION
A free-standing air electrode based on a multifunctional three-dimensional interconnected graphene network is fabricated to boost the output performance and charge-discharge cycling stability of ZABs.Thereinto, the intrinsic structure of three-dimensional interconnected graphene network shortens the transport paths of electrons, reactants, and products.Highly efficient active components of atomic Fe-N 4 species and FeNi hydroxides are anchored on the three-dimensional interconnected graphene network to boost the slow kinetics of oxygen reactions.Thus, the obtained air electrode exhibited an ultra-small electrocatalytic potential difference of 0.603 V for oxygen reactions, a large peak power density of 220.2 mW cm -2 and a small and stable charge-discharge voltage gap of 0.70 V at 10 mA cm -2 during 1136 cycles for its liquid-state ZAB, and a high peak power density of 77.6 mW cm -2 for its solid-state ZAB.Furthermore, in situ Raman spectroscopy and DFT calculations confirm that phase transformation of FeNi hydroxides takes place from α-Ni(OH) x to β-Ni(OH) x to γ-Ni (3+δ)+ OOH for OER and Ni is the active center while Fe enhances the activity of Ni active sites.This work proposes an innovative method for synthesizing high-performance air electrode, significant insights into the working mechanisms, and therefore pushes the development of ZABs.

A C K N O W L E D G M E N T S
This work was financially supported by the National Natural Science Foundation of China (52102046) and the Yunnan Fundamental Research Projects (202301AW070016).

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

F I G U R E 4
Liquid-state zinc-air batteries (ZABs): (A) power density curves; (B) discharge curves at 10 mA cm −2 ; (C) discharge curves at various current densities; (D-G) galvanostatic charge-discharge cycling curves; (H) battery performance comparison including galvanostatic charge-discharge cycling time and voltage difference at 10 mA cm −2 .

F I G U R E 5
Solid-state zinc-air batteries (ZABs): (A) a schemiatic showing its structure; (B) power density curves; (C) discharge curves at various current densities; (D) open-circuit voltage curve recorded under different bending conditions; (E) galvanostatic charge-discharge cycling curve; (F) four ZABs power a phone charging; (G) three ZABs power a small electric fan working.