In Situ Coupling of Highly Dispersed Ni/Fe Metal‐NC Sites and N‐Doped 3D Carbon Fibers Toward Free‐Standing Bifunctional Cathode for Flexible Zinc‐Air Battery

Designing flexible free‐standing air‐electrode with efficient OER/ORR performance is of vital importance for the application of Zinc‐air batteries in flexible electronics. Herein, a flexible free‐standing electrode (Ni/Fe‐NC/NCF/CC) is synthesized by in‐situ coupling of binary Ni/Fe‐NC nanocubes and N‐doped carbon nanofibers (NCF) rooted on carbon cloth. The highly dispersed binary Ni/Fe‐NC sites ensure excellent ORR activity and create efficient OER active sites relative to Ni‐NC and Fe‐NC. The in‐situ coupling of Ni/Fe‐NC and NCF constructs a 3D interconnected network structure that not only provides abundant and stabilized reactive sites but also guarantees fast electron transfer and gas transportation, thus achieving efficient and fast operation of ORR/OER. Therefore, Ni/Fe‐NC/NCF/CC displays a much positive potential (0.952 V) at 4.0 mA cm−2 for ORR and a low OER overpotential (310 mV) at 50 mA cm−2. The Zinc‐air battery with Ni/Fe‐NC/NCF/CC air‐electrode exhibits excellent battery performance with outstanding discharge/charge durability for 2150 cycles. The flexible Zn‐air batteries with foldable mechanical properties display a high power density of 105.0 mW cm−2. This work widened the way to prepare flexible bifunctional air‐electrode by designing composition/structure and in‐situ coupling.

Designing flexible free-standing air-electrode with efficient OER/ORR performance is of vital importance for the application of Zinc-air batteries in flexible electronics.Herein, a flexible free-standing electrode (Ni/Fe-NC/ NCF/CC) is synthesized by in-situ coupling of binary Ni/Fe-NC nanocubes and N-doped carbon nanofibers (NCF) rooted on carbon cloth.The highly dispersed binary Ni/Fe-NC sites ensure excellent ORR activity and create efficient OER active sites relative to Ni-NC and Fe-NC.The in-situ coupling of Ni/Fe-NC and NCF constructs a 3D interconnected network structure that not only provides abundant and stabilized reactive sites but also guarantees fast electron transfer and gas transportation, thus achieving efficient and fast operation of ORR/OER.Therefore, Ni/Fe-NC/NCF/CC displays a much positive potential (0.952 V) at 4.0 mA cm À2 for ORR and a low OER overpotential (310 mV) at 50 mA cm À2 .The Zinc-air battery with Ni/Fe-NC/NCF/CC air-electrode exhibits excellent battery performance with outstanding discharge/charge durability for 2150 cycles.The flexible Zn-air batteries with foldable mechanical properties display a high power density of 105.0 mW cm À2 .This work widened the way to prepare flexible bifunctional air-electrode by designing composition/structure and in-situ coupling.
[41] Therefore, it is necessary to design an effective strategy to couple M-M-NC materials and the carbon substrate with a unique 3D structure toward a flexible binder-free electrode, thus promoting the performance of flexible Zn-air batteries.
Herein, a flexible binder-free electrode consisting of Ni/Fe-NC nanocubes, NCF network and carbon cloth (CC) is synthesized by pyrolyzation of the Ni/Fe-doped ZIF-8 absorbed on polypyrrole (PPy) nanofibers network (PFN).Owing to the synergistic catalytic effect of Fe-NC and Ni-NC, the binary Ni/Fe-NC structure provides efficient OER and ORR reactive sites.The in-situ coupling of Ni/Fe-NC and NCF not only provides abundant and stabilized active sites but also guarantees fast electron transfer and gas transportation.Therefore, the Ni/Fe-NC/ NCF/CC displays a much positive potential (0.952 V) at 4.0 mA cm À2 and a high current density of 13.5 mA cm À2 at 0.4 V, outperforming the vast majority of catalysts.Meanwhile, Ni/Fe-NC/NCF/CC electrode delivers an efficient catalytic activity for OER with a low overpotential (310 mV) at 50 mA cm À2 .The excellent performances of the rechargeable liquid Zinc-air batteries and corresponding flexible Zn-air batteries with Ni/Fe-NC/NCF/CC electrodes are demonstrated in this work.This work widened the way to prepare flexible bifunctional air-electrode by designing composition/structure and synergistic catalytic effect.

Morphological and Structural Characterization
The preparation process of Ni/Fe-NC/NCF/CC is illustrated in Scheme 1, which includes the electrodeposition of PFN on CC, the growth of Fe-ZIF-8 attached to PFN, the introduction of Ni in Fe-ZIF-8, and the final pyrolyzation under Ar atmosphere.Scanning electron microscopy (SEM) is conducted to analyze the morphology and structure evolution of catalysts.It is seen from Figure 1a and Figure S1, Supporting Information, that the crisscrossed PPy fibers are firmly rooted on carbon cloth, which constructs a 3D structure network.
Figure 1b and Figure S2, Supporting Information, show that a mass of Fe-ZIF-8 polyhedral nanoparticles enfold the PPy nanofibers looking like pearl necklaces, demonstrating that polypyrrole fibers are a satisfactory substrate.Compared with Fe-ZIF-8/PFN/CC, Fe-ZIF-8/CC shows that only sporadic Fe-ZIF-8 nanoparticles are absorbed on carbon cloth (Figure S3, Supporting Information).These results indicate that PPy nanofibers with large specific surfaces and electronegative surfaces are beneficial to adsorb the positively charged Fe ions, thus greatly increasing the reactive material. [42,43]When the Fe-ZIF-8/ PFN/CC is immersed in an n-hexane solution containing Ni(NO 3 ) 2 for 15 min, Ni(NO 3 ) 2 can be encapsulated in a small ZIF-8 cavity due to the two solvent routes. [28]As shown in Figure S4, Supporting Information, the morphology of pearl necklaces is still observed and the surface of Ni/Fe-ZIF-8 particles becomes relatively rough.The PPy nanofibers and Ni/Fe-ZIF-8 are transformed into N-doped carbon fibers and Ni/Fe-NC under the thermal treatment and pickling process, respectively.As shown in Figure 1c-e, although nanoparticles have large shrinkage and collapse phenomena, the microstructure of polyhedral shape of Ni/Fe-NC can still be observed, meantime, the 3D network structure is sustained well.The Ni/Fe-NC nanoparticles are absorbed on 3D connected network looking like pearl necklaces, which is intuitively observed in Figure 1f. Figure 1g-k show Ni, N, and Fe elements are evenly distributed in CC.These results indicate the successful introduction of Ni and the 3D spatial coupling of NCF and Ni/Fe-NC.However, when the pyrolysis temperature is increased to 1000 °C, too high a temperature leads to the fracture and pulverization of carbon nanofiber and the decomposition and disappearance of Ni/Fe-NC particles (Figure S5, Supporting Information).
Transmission electron microscopy (TEM) is conducted to further characterize the microstructure of Ni/Fe-NC/NCF/CC and the dispersion state of Ni/Fe atoms.As shown in Figure 2a-c and Figure S6, Supporting Information, the Ni/Fe-NC materials exhibit polyhedral nanostructures, which are adsorbed on carbon nanofiber tubes, and no obvious Ni/Fe metal clusters/particles were observed in the material.Furthermore, it can be obtained from the high angle annular dark fieldscanning TEM (HAADF-STEM) picture that the inexistence of Fe/Ni clusters/particles in Ni/Fe-NC/NCF/CC (Figure 2d,e).From Figure 2f, the white bright spots representing single atoms are observed, demonstrating the formation of highly dispersed Ni and Fe atoms.Figure 2h-j shows the Ni, Fe, and N elements are uniformly dispersed in nanoparticles, further indicating the existence of Ni/Fe elements.These results demonstrate the Ni/Fe atoms are highly dispersed in nanocubes instead of forming clusters or particles.
The crystal structure of catalysts is characterized by X-ray diffraction (XRD).The Ni/Fe-ZIF-8/PFN/CC exhibits the same crystal structure as Fe-ZIF-8/PFN/CC, which is in consistence with the standard pattern (Figure S7a, Supporting Information).After the pyrolysis at 900 °C for 1 h, the NCF/CC, Fe-NC/NCF/CC, Ni-NC/NCF/  3a).The surface valence states and composition of catalysts are further characterized by X-ray photoelectron spectroscopy (XPS) measurement.The fine N 1 s spectra of all the catalysts exhibit the same three peaks at 398.3, 400.9, and 404.4 eV, which are assigned to pyridinic-N, graphitic-N and N-O, respectively. [1](Figure 3b-e).Expressly, compared with NCF/CC, the other three catalysts display another peak at 399.4 eV, which is corresponding to the metalnitrogen (M-N) species. [32]The percentages of different N species of as-prepared samples are calculated and shown in the Table S1, Supporting Information, the Ni/Fe-NC/NCF/CC exhibits a content of peaks at 399.4 eV of 18.4%, higher than Fe-NC/NCF/CC (15.    ).This result demonstrates the fast OER reaction kinetics of Ni/Fe-NC/NCF/CC.Furthermore, the electrochemical active area of the gas electrode is evaluated by calculating the electrochemical double-layer capacitance (Cdl).The Cdl of Ni/Fe-NC/NCF/ CC is calculated to be 200.6 mF cm À2 , which is much higher than that of Ni/Fe-NC/CC (11.1 mF cm À2 ).The electrochemical area of Ni/Fe-NC/NCF/CC and Ni/Fe-NC/CC is further calculated to be 20.06 and 1.11 cm 2 , respectively, demonstrating the sufficient reactive sites of Ni/Fe-NC/NCF/CC (Figure S8a-c, Supporting Information).Therefore, the in-situ coupling of highly dispersed Ni/Fe-NC sites and 3D NCF interworking network not only can promote electron transfer and gas transportation but also provides abundant and stable reactive sites, thus realizing the efficient and fast operation of reaction sites and delivering high efficiency of bifunctional catalytic activity.
In addition, the OER/ORR durability of catalysts is evaluated in O 2saturated electrolytes (Figure 4e).The ORR polarization curves of Ni/ Fe-NC/NCF/CC show only a small decline after 5000 CV cycles, demonstrating excellent ORR stability.Furthermore, the Ni/Fe-NC/ NCF/CC exhibits a lower potential and smaller change of potential than Fe-NC/NCF/CC during 20 h OER testing in 20 mA cm À2 , indicating the outstanding catalytic stability for OER (Figure 4f).The XRD is conducted to analyze phase stability after 20 h OER testing.As shown in Figure S9, Supporting Information, the Ni/Fe-NC/NCF/CC electrode only exhibits the standard pattern of graphite (Ref.03-065-6212).This result shows that there is no agglomeration of metal atoms and excellent phase stability of Ni/Fe-NC/NCF/CC in the OER testing process.As shown in Figure S10a,b, Supporting Information, the Ni/Fe-NC nanoparticles are absorbed on 3D connected network still looking like pearl necklaces.Although the particles further shrink and collapse, the polyhedral shape structure of many nanoparticles and the 3D network structure of NCF is sustained well.Figure S10c-f  Energy Environ.Mater.2024, 7, e12541 much large electrochemical active area relative to p-Ni/Fe-NC/NCF/ CC, which is confirmed by the Cdl testing. [13]The Ni/Fe-NC/NCF/CC delivers a Cdl value of 200.6 mF cm À2 and an electrochemical area of 20.06 cm 2 , larger than p-Ni/Fe-NC/NCF/CC (162.8 mF cm À2 and 16.28 cm 2 ) (Figure S8d,e, Supporting Information).The smaller electrochemical active area of p-Ni/Fe-NC/NCF/CC is attributed to the fact that the surface of the material is covered with the binder (Nafion), thus lowering catalytic active sites.In addition, the Ni/Fe-NC/NCF/CC delivers a low polarization resistance (Rp) of 1.9 Ω, smaller than p-Ni/ Fe-NC/NCF/CC (4.1 Ω), demonstrating the outstanding electrical conductivity of Ni/Fe-NC/NCF/CC (Figure S8f, Supporting Information).These results demonstrate that the introduction of the binder causes the increment of internal resistance of p-Ni/Fe-NC/NCF/CC electrode and further impedes the electron/charge transfer.In conclusion, the Ni/Fe-NC/NCF/CC electrode delivers outstanding OER/ORR performance, owing to the following three advantages: 1) The construction of a highly dispersed binary Ni/Fe-NC structure ensures excellent ORR activity and creates efficient OER active sites; 2) The in-situ coupling of Ni/Fe-NC and NCF constructs the 3D interconnected network structure which not only provides abundant and stabilized active sites but also guarantees fast electron transfer and gas transportation, thus achieving the efficient and fast operation of ORR/OER.3) The flexible binderfree electrode displays excellent conductivity and avoids the side-effect of the binder, thus further decreasing the catalytic overpotential.As shown in Table S2, Supporting Information, the ORR half-wave potential and OER overpotential of Ni/Fe-NC/NCF/CC electrode are superior to many recent advanced M-M-NC materials, indicating the excellent ORR/OER catalytic activity of Ni/Fe-NC/NCF/CC.

Ni/Fe-NC/NCF/CC Based Zinc-Air Batteries Performance
A liquid Zn-air battery is assembled by Ni/Fe-NC/NCF/CC as an air electrode to evaluate the battery performance (Figure 5a).The Ir/C-Pt/ C/CC electrode is synthesized by dropping the Ir/C and Pt/C mixed ink onto carbon cloth, which is further used as contrast material.The power density of the battery with Ni/Fe-NC/NCF/CC is calculated to be 162.0mW cm À2 (Figure 5b), much higher than that of the batteries with Fe-NC/NCF/CC (122.5 mW cm À2 ) or Ir/C-Pt/C/CC (113.0 mW cm À2 ).The battery with Ni/Fe-NC/NCF/CC displays a lower discharge/charge overpotential of 1.02 V in 50 mA cm À2 compared to the batteries with Fe-NC/NCF/CC (1.45 V) or Ir/C-Pt/C/CC (1.46 V) (Figure 5c).In addition, the specific capacity of Zinc-air batteries are shown in Figure 5d  ).These results demonstrate that the Znair battery with Ni/Fe-NC/NCF/CC air-electrode delivers an outstanding bifunctional performance which benefits from the outstanding OER/ORR catalytic activity of the air-electrode.Another indispensable parameter of the durability of Zinc-air batteries is evaluated by longterm discharge/charge testing at 10 mA cm À2 (Figure 5e).The initial discharge/charge voltage gap of Ni/Fe-NC/NCF/CC is 0.78 V, which only increases to 1.11 V after 2150 discharge/charge cycles.However, the initial charge/discharge voltage gap of Ir/C-Pt/C/CC with Fe-NC/ NCF/CC is 0.89 V and 0.95 V and have successively lost their activity at 250th and 1120th, respectively.This result proves the excellent durability of the Zinc-air battery with Ni/Fe-NC/NCF/CC electrode.Besides, the Ni/Fe-NC/NCF/CC based Zn-air battery delivers higher    S3, Supporting Information).
The rechargeable flexible Zn-air batteries composed of solid-state electrolyte, zinc foil anode, and Ni/Fe-NC/NCF/CC air-electrode are assembled to evaluate the electrochemical performance of Ni/Fe-NC/ NCF/CC electrode in flexible battery devices (Figure 6a).It is seen from Figure 6b

The Water-Splitting Device Performance
A water-splitting device is assembled by 1 M KOH electrolyte, Pt/C/CC cathode, and Ni/Fe-NC/NCF/CC anode, driven by two liquid batteries with Ni/Fe-NC/NCF/CC electrode (Figure 7a).During the operation of the device, the H 2 O is constantly decomposed into O 2 and H 2 , which are observed in the cathode and anode (Figure 7b).As shown in Figure 7c, two Zn-air batteries exhibit an output voltage of about 2.05 V at a closed-circuit state and maintain stability during about 6 h measurement.Figure 7d shows a linear relationship of the volumes of H 2 /O 2 with time, and the slopes are 2.84 and 1.42 lL s À1 , respectively.The ratio of the production rates of H 2 /O 2 is 2:1, which is in accordance with the theoretical ratio.The excellent performance of the water-splitting unit driven by liquid Zinc-air batteries with Ni/Fe-NC/NCF/CC successfully proves the potential practicability.

Conclusions
In summary, we have demonstrated a flexible free-standing electrode is synthesized by in-situ coupling of highly dispersed binary Ni/Fe-NC sites and NCF network, for flexible Zinc-air batteries.Based on the synergistic catalytic effect of Ni-NC and Fe-NC, the binary Ni/Fe-NC provides high-efficiency ORR and OER reactive sites.Furthermore, the insitu coupling of Ni/Fe-NC and NCF not only provides abundant and stabilized reactive sites but also ensures fast electron transfer and gas transportation, thus achieving efficient, fast, and stable operation of ORR/OER.The flexible binder-free air-electrode avoids the side-effect of the binder, which benefits to improve the catalytic performance.Therefore, the Ni/Fe-NC/NCF/CC electrode exhibits excellent OER catalytic performance with a low overpotential (310 mV) at 50 mA cm À2 and outstanding ORR activity with a positive potential of 0.952 V at 4.0 mA cm À2 , which outperforms many bifunctional catalysts.Besides, the Ni/Fe-NC/NCF/CC driven liquid Zinc-air batteries have stability operated for 2150 charge/discharge cycles and the corresponding flexible Zn-air battery with Ni/Fe-NC/NCF/CC delivers superb mechanical property and high power densities of 105.0 mW cm À2 , demonstrating excellent battery performance.Moreover, the water-splitting device is steadily driven and operated for 6 h by two liquid Zn-air batteries with Ni/Fe-NC/NCF/CC electrodes.This work demonstrates a simple and feasible way to synthesize flexible free-standing air-electrode by designing composition/structure and synergistic catalytic effect, which is promising for flexible Zinc-air batteries.

Experimental Section
Synthesis of PNF/CC: The carbon cloth is soaked in concentrated nitric acid for 6 h to obtain the hydrophilic carbon cloth.The saturated calomel electrode, carbon rod, and hydrophilic carbon cloth were selected as the reference electrode, counter electrode and working electrode, respectively.The uniform electrolyte was formed by adding 0.532 g of LiClO 4 , 1.0 mL of pyrrole and 1.06 g of Na 2 CO 3 into 50 mL of H 2 O.The PNF/CC was prepared by constant voltage electrodeposition at 10 mA cm À2 last for 0.5 h.Synthesis of Fe-ZIF-8/PNF/CC and ZIF-8/PNF/CC: The 1.97 g of 2methylimidazole (MeIM) was added into 50 mL methanol to form solution A. Solution B was formed by adding 1.695 g of Zn(NO 3 ) 2 Á6H 2 O and 60 mg of Fe (NO 3 ) 3 Á9H 2 O into 50 mL methanol.The Fe-ZIF-8 grew on carbon cloth when PNF/CC is soaked in A and B mixed solution under stirred at 60 °C for 24 h.The as-prepared carbon cloth was washed with methanol several times and dried in a vacuum at 60 °C.Besides, under the same procedure, the ZIF-8/PNF/CC was prepared without adding Fe(NO 3 ) 3 Á9H 2 O, the Fe-ZIF-8/CC was prepared by replacing PNF/CC with CC.Synthesis of Ni/Fe-ZIF-8/PNF/CC: The 140 lL of 50 mg mL À1 Ni(NO 3 ) 2 methanol solution was added into 12 mL n-hexane to form solution C. The Fe-ZIF-8/ PNF/CC is soaked in solution C under ultrasound for 15 min at 25 °C, then washed with methanol and dried in a vacuum at 60 °C.Besides, under the same procedure, the Ni-ZIF-8/PNF/CC was prepared by replacing Fe-ZIF-8/PNF/CC with ZIF-8/PNF/CC, and the Ni/Fe-ZIF-8/CC was prepared by replacing Fe-ZIF-8/PNF/ CC with Fe-ZIF-8/CC.Synthesis of Ni/Fe-NC/NCF/CC: The Ni/Fe-ZIF-8/PNF/CC was pyrolyzed at 900 °C for 1 h under an inert atmosphere with the heating rate of 5 °C min À1 and subsequent acid leaching in 0.5 M H 2 SO 4 at 80 °C for 10 h.The Ni/Fe-NC/NCF/CC electrode was finally prepared.The Ni-NC/NCF/CC, Fe-NC/NCF/CC and Ni/Fe-NC/CC were prepared by the same procedure.The Ni/ Fe-ZIF-8/PNF/CC-1000 was prepared by the same procedure with 1000 °C pyrolysis temperature.
6%) and Ni-NC/ NCF/CC (6.7%), demonstrating that Fe and Ni are substantially coordinated with N instead of forming clusters or particles.As shown in Figure S7b,c, Supporting Information, the fine spectra of Fe 2p of Ni/Fe-NC/NCF/CC are fitted into two peaks of Fe (II) 2p 1/2 (711.4eV) and Fe (II) 2p 3/2 (724.2 eV), which is in accordance with the Fe-NC/ NCF/CC.To calculate the specific surface area of Ni/Fe-NC/NCF/CC, the Brunauer-Emmett-Teller (BET) measurement was performed.It is concluded from Figure 3f that Ni/Fe-NC/NCF/CC and NCF/CC display BET surface areas of 171.88 and 150.10 m 2 g À1 , respectively, which is much larger than Ni/Fe-NC/CC (10.37 m 2 g À1 ).The pores of Ni/Fe-NC/NCF/CC are mainly 1-2 nm micropores and macropores larger than 70 nm (Figure S7d, Supporting Information).This result confirms the large special surface area of Ni/Fe-NC/NCF/CC and NCF/ CC, which is attributed to the 3D interconnected network structure.

Figure 1 .
Figure 1.a) The SEM images of PFN/CC.b) Fe-ZIF-8/PFN/CC.b) and c-e, g) Ni/Fe-NC/NCF/CC.f) The SEM image of Ni/Fe-NC/NCF/CC is handled by Adobe Photoshop.i) The SEM-EDS elements mapping images of N. j) Fe, and k) Ni.

2. 2 .
Electrocatalytic Performance of Ni/Fe-NC/NCF/CC The electrocatalytic activity of Ni/Fe-NC/NCF/CC electrodes is demonstrated by an electrochemical workstation in a 1.0 M KOH electrolyte.The Ni/Fe-NC/CC is prepared and selected as the material for comparison.The Ni/Fe-NC/CC exhibits a current density of 4.69 mA cm À2 at the voltage of 0.7 V, much lower than Ni/Fe-NC/NCF/CC (8.35 mA cm À2 ), NCF/CC (7.74 mA cm À2 ), Ni-NC/NCF/CC (7.78 mA cm À2 ), and Fe-NC/NCF/CC (7.80 mA cm À2 ), indicating that the NCF with a 3D interconnected network structure guarantees fast electron transfer and gas transportation, thus improving the diffusion current density.And the Ni/Fe-NC/NCF/CC delivers an outstanding ORR performance with a more positive potential of 0.952 V at 4 mA cm À2 relative to NCF/CC (0.883 V), Ni-NC/NCF/CC (0.900 V), Fe-NC/NCF/CC (0.945 V), Ni/Fe-NC/CC (0.77 V), and Pt/C/CC (0.66 V) (Figure 4a).Besides, the Ni/Fe-NC/NCF/CC exhibits much more excellent ORR performance relative to Ni/Fe-NC/CC and NCF/CC, confirming the synergistic effect of highly dispersed Ni/ Fe-NC sites and NCF can promote the ORR catalytic activity.Besides, the OER polarization curves of as-prepared electrodes are shown in Figure 4b.The Ni/Fe-NC/NCF/CC delivers a lower potential to reach the same current density compared to NCF/CC, Fe-NC/NCF/CC, and Ni/ Fe-NC/CC, indicating the outstanding OER catalytic performance.The overpotential (DE OER = E j = 50 mA cm À2 -1.23 V) of Ni/Fe-NC/NCF/ CC is calculated to be 310 mV, much smaller than that of Fe-NC/NCF/ CC (398 mV), Ni-NC/NCF/CC (407 mV), and Ir/C/CC (365 mV), attributed to the efficient binary Ni/Fe-NC active sites.The DE OER of the Ni/Fe-NC/CC electrode is 383 mV, which is much larger than that of

Figure 2 .
Figure 2. a-c) The TEM images of Ni/Fe-NC/NCF/CC.d-g) The HAADF-STEM images of Ni/Fe-NC/NCF/CC.The elements mapping images of h) Fe, i) Ni, and j) N.
, Supporting Information, shows Ni, N, and Fe elements are evenly distributed in carbon cloth.These results demonstrate the excellent phase stability and morphology retention of Ni/Fe-NC/NCF/CC electrodes.To verify the advantage of the flexible binder-free electrode, the p-Ni/Fe-NC/NCF/CC as the contrast material is synthesized by using Nafion as a binder (synthetic procedure in experimental section).The ORR polarization curve of Ni/Fe-NC/NCF/CC shows a more positive potential at the same current density relative to that of p-Ni/Fe-NC/ NCF/CC (Figure S11a, Supporting Information), which strongly supports the more efficient ORR catalytic activity.Compared with p-Ni/Fe-NC/NCF/CC, the Ni/Fe-NC/NCF/CC manifests a more efficient OER activity with a lower overpotential at the same current density (Figure S11b, Supporting Information).The Ni/Fe-NC/NCF/CC exhibits a

Figure 5 .
Figure 5. a) Schematic of the Zn-air battery.b) The polarization curves of Zinc-air batteries with Ni/Fe-NC/NCF/CC, Fe-NC/NCF/CC, and Ir/C-Pt/C/CC.c) The discharge/charge polarization curves.The d) specific capacity curves and e) long-term discharge/charge cycle curves.

Figure 6 .
Figure 6.a) Schematic diagram of flexible Zn-air battery.b) The open-circuit voltage of the battery.c) The LEDs driven by batteries.d) The polarization curves and corresponding power density curves.e) The galvanostatic discharge/charge cycle curves.
and Figure S13, Supporting Information, that a high open-circuit voltage (1.4 V) of the flexible Zinc-air battery with Ni/Fe-NC/NCF/CC remains stable in different bending states.Figure 6c shows that a yellow/red light-emitting diode (LED) can be driven by two flexibles Ni/Fe-NC/NCF/CC based batteries and remains glowing when two batteries are at a bent state.Three batteries in series can lighten two LEDs (Figure S14, Supporting Information).Besides, owing to the excellent ORR activity of Ni/Fe-NC/NCF/CC (Figure 6d), the flexible Ni/Fe-NC/NCF/CC driven Zn-air battery exhibits a higher power density of 105.0 mW cm À2 relative to the battery with Ir/C-Pt/C/CC (58 mW cm À2 ).The durability of the batteries is evaluated by the galvanostatic charge/discharge cycle measurements in different bending states (Figure 6e).The flexible battery with Ni/Fe-NC/NCF/ CC displays a low charge/discharge voltage gap of 0.49 V and remains stable during 35 h testing at 1 mA cm À2 , which surpasses the Ir/C-Pt/ C/CC based battery (0.70 V and 17 h).A charge/discharge voltage gap of the battery with Ni/Fe-NC/NCF/CC is calculated to be 0.69 V and remains stable for 25 h at 2.5 mA cm À2 .This result demonstrates the benign durability of the Ni/Fe-NC/NCF/CC based flexible Zinc-air battery.It is concluded from Table S4, Supporting Information, that such excellent performance of flexible Zinc-air battery with Ni/Fe-NC/ NCF/CC electrode outperforms many reported flexible Zn-air batteries, indicating the promising potential of Ni/Fe-NC/NCF/CC in flexible Zn-air batteries.

Figure 7 .
Figure 7. a) The water-splitting unit powered by two Zn-air batteries in series.b) The images and the enlarged images (insert) of two electrodes.c) The voltage-time curves of unit.d) The ratio of the production rates of H 2 /O 2 .