Nanowrinkled Carbon Aerogels Embedded with FeNx Sites as Effective Oxygen Electrodes for Rechargeable Zinc-Air Battery

Rational design of single-metal atom sites in carbon substrates by a flexible strategy is highly desired for the preparation of high-performance catalysts for metal-air batteries. In this study, biomass hydrogel reactors are utilized as structural templates to prepare carbon aerogels embedded with single iron atoms by controlled pyrolysis. The tortuous and interlaced hydrogel chains lead to the formation of abundant nanowrinkles in the porous carbon aerogels, and single iron atoms are dispersed and stabilized within the defective carbon skeletons. X-ray absorption spectroscopy measurements indicate that the iron centers are mostly involved in the coordination structure of FeN4, with a minor fraction (ca. 1/5) in the form of FeN3C. First-principles calculations show that the FeNx sites in the Stone-Wales configurations induced by the nanowrinkles of the hierarchically porous carbon aerogels show a much lower free energy than the normal counterparts. The resulting iron and nitrogen-codoped carbon aerogels exhibit excellent and reversible oxygen electrocatalytic activity, and can be used as bifunctional cathode catalysts in rechargeable Zn-air batteries, with a performance even better than that based on commercial Pt/C and RuO2 catalysts. Results from this study highlight the significance of structural distortions of the metal sites in carbon matrices in the design and engineering of highly active single-atom catalysts.


Introduction
Climate change and environmental pollution have motivated the development of sustainable, clean energy technologies, of which rechargeable metal-air batteries have drawn tremendous attention owing to their high energy density and minimal impacts on the environment [1][2][3][4]. The overall efficiency of the charge-discharge process of metal-air batteries is determined by two major reactions, oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Although platinum group metal (PGM) materials, such as Pt/C, RuO 2 , and Ir/C, possess excellent catalytic activity for either ORR or OER, none of these noble metal catalysts displays a satisfactory performance for both [5], and their scarcity and high costs greatly hinder their practical applications [6,7]. Therefore, development of bifunctional catalysts with a low cost and high activity is of both fundamental and technological significance, but remains a great challenge.
Recent studies have demonstrated that PGM-free nanocomposites based on carbon materials, such as heteroatom-(including nonmetal and metal atoms) doped porous carbon, are promising bifunctional oxygen catalysts [8][9][10]. In fact, transition metal-doped carbon catalysts have been widely investigated due to the unique chemical properties caused by their adjustable 3D electronic orbitals. In particular, transition metal-based single-atom catalysts display overwhelming superiority as compared to their nanoparticle and nanocluster counterparts [11][12][13][14][15]. For instance, single-site dispersion of FeN x species in a two-dimensional nitrogendoped porous carbon layer has been found to exhibit a remarkable catalytic activity towards both ORR and OER in alkaline media [1], where a range of catalytic active sites has been proposed, such as CoN 2 C 2 , FeN 3 C, FeN 4 , and FeN 4 O [16][17][18]. However, the effects of structural distortion induced by the single-metal sites on the catalytic activity have long been ignored, although such structural defects are common in pyrolytic carbon.
Herein, biomass hydrogels (i.e., chitosan, gelatin, and agar), which have long been known for their diverse applications and economic advantages [19,20], were prepared and used as unique precursors, templates, and reactors to produce three-dimensional, nanowrinkled carbon aerogels embedded with FeN x single sites [21][22][23]. Due to the abundant functional groups on the hydrogel chains, defective single-metal sites were dispersed and stabilized within the nanowrinkled, porous carbon aerogels. First-principles calculations showed that the FeN x sites in the Stone-Wales configurations induced by the carbon nanowrinkles displayed a much lower free energy for oxygen electrocatalysis than the normal counterparts. Electrochemical measurements exhibited apparent and reversible oxygen electrocatalytic performance towards both ORR and OER. When the nanowrinkled carbon aerogels were used as the air-cathode of a zinc-air battery, the battery displayed a higher opencircuit voltage and higher energy density, as well as better cycling stability than that with commercial Pt/C-RuO 2 catalysts.

Results and Discussion
2.1. Synthesis and Characterization. In this study, flexible biomass hydrogels were synthesized in a facile process and employed as 3D templates to prepare carbon aerogels embedded with single-metal atoms (Figures 1(a) and S1). In order to achieve atomic dispersion, the hydrogel networks were modified by two strategies to minimize metal aggregation. The first is "headstream fixation," which means immobilization of metal atoms into the hydrogel reactor by complexation agents (e.g., phenanthroline (PM)); and the other, "roadblocks," is based on rigid templates, such as SiO 2 nanoparticles and Zn atoms. Experimentally, a variety of hydrogel/hydrosol networks, i.e., CS Si-Zn /FePM, CS Si /FePM, CS Si /Fe, and CS Si , were prepared by using chitosan (CS) as the structural scaffold, along with a select combination of other precursors, such as SiO 2 nanoparticles (Si), FePM, and Zn salt (details in Materials and Methods). The morphological details were first investigated by scanning electron microscopy (SEM) measurements. From Figure S2, freeze-dried CS Si /FePM, CS Si /Fe, and CS si hydrosols can be seen to consist of uneven microcavities. However, as shown in Figure 1(b), the microcavities of the CS Si-Zn /FePM hydrogel shows a more uniform size of ca. 50 μm, forming a 3D, continuous framework composed of intertwining CS-Zn chains. This suggests that Zn ions can induce the hydrogelation of CS hydrosol to form much more uniform 3D intertwining networks, which further facilitates the generation of nanowrinkles [24]. Circular dichroism (CD) and UV-vis absorption measurements were then carried out to monitor the structural evolution from CS Si sol to CS Si-Zn /FePM hydrogel. As depicted in Figure S3a, the incorporation of both FePM and Zn 2+ ions into chitosan led to marked conformational changes of the CS chains. In UVvis absorption measurements ( Figure S3b), two new peaks can be seen to emerge at ca. 226 nm and 268 nm, due to the strong complexation interaction between Fe and PM [25].
The freeze-dried CS Si-Zn /FePM hydrogel was then used as a 3D reactor to synthesize metal-doped carbon aerogels by controlled pyrolysis, which was then subject to HF etching to remove the SiO 2 templates (Figure 1(a)), producing NCA C-Zn /Fe (Figure 1(c) inset). Control samples of NCA C /Fe, CA C /Fe, and CA C were prepared in a similar fashion (details in Materials and Methods). From the transmission electron microscopy (TEM) images in Figure S4, one can see that the NCA C-Zn /Fe sample displays a highly porous, nanowrinkled structure with rich mesopores of ca. 10 nm in diameter. In both bright-field (Figure 1(c)) and dark-field (Figure 1(d)) scanning transmission electron microscopy (STEM) images, one can clearly see the formation of single-metal atoms (red circles) embedded into the porous carbon matrix. The corresponding elemental maps clearly show that the C, N, and Fe elements are homogeneously distributed across the aerogel (Figure 1(e)). These results indicate successful construction of N-doped carbon aerogels embedded with isolated Fe atoms by using biomass hydrogels as the reactors.
In order to examine the mechanical and electrical properties of the obtained porous carbon, Fast Force Mapping (FFM) measurements were then carried out. The data presented in Figures 1(f)-1(g) and S5 exhibit a ca. 10 nm variation in the mechanical and electrical properties of the porous carbon, confirming the formation of nanowrinkled carbon. Domains dictated by round features in topography ( Figure S5a) are outlined by prominent changes in max force ( Figure S5b) and an increase in the adhesion force (Figure 1(f)). Notably, the adhesion force, which represents the bulk modulus or stiffness of the sample, indicates that these round regions are stiffer in the center and softer around the edges. Typically, sp 2 -hybridized carbon exhibits hydrophobic characteristics, whereas defective carbons are more hydrophilic [26,27].
With an AFM tip that consists of a hydrophilic silicon oxide layer, a high adhesion force corresponds to a hydrophilic domain. This implies that the metal centers are most likely situated within the high adhesion force areas. Interestingly, from Figures 1(f) to 1(g), one can see that the soft nodes correspond to high electrical conductance. Taken together, these results suggest that the metal sites are mostly located in the high adhesion and high conductivity areas of the porous carbon aerogel [28,29]. Both features are conducive to oxygen electrocatalysis.
The porosity of the obtained samples was then quantitatively evaluated by N 2 adsorption-desorption measurements. The carbon aerogels obtained above all show a Type IV isotherm (Figures 2(a) and S6a), which suggests the formation of a complex porous network containing a myriad of mesopores with an average size of ca. 10 nm, in line with the diameter of the SiO 2 nanoparticle [30]. From the isotherms, the specific surface area of NCA C-Zn /Fe was estimated to be 609 m 2 g −1 , with a microporous surface area of 111 m 2 g −1 , which is the highest among the series, a condition favorable for the formation of abundant active sites ( Figure S6b and Table S1). The corresponding X-ray powder diffraction (XRD) patterns are shown in Figure S7, where only one broad peak at ca. 25°can be observed, due to the (002) diffraction of graphitic carbon [31]. This carbon diffraction became gradually sharpened from CS C to NCA C-Zn /Fe, indicating an increasing degree of graphitization. Importantly, the fact that no other diffraction features were observed suggests the absence of metal (oxide) nanoparticles.   Relative pressure (P/P o ) 1 1 0 In Raman measurements, the I D /I G ratio of NCA C-Zn /Fe was estimated to be ca. 1.15, much higher than those of the control samples (Figure 2(b)), signifying the generation of rich defects which may be conducive to the formation of metal active sites [31].
The elemental compositions of the obtained carbon samples were then quantitatively assessed by inductively coupled plasma atomic emission spectroscopy (ICP-OES) and energy-dispersive X-ray spectroscopy (EDS) measurements. Results from ICP-OES analysis showed that the Fe content in the carbon aerogel was about 0.22 wt% for CA C /Fe, 0.61 wt% for NCA C /Fe, and 0.72 wt% for NCA C-Zn /Fe, in good accordance with the EDS results ( Figure S8, Tables S2 and S3). The increased metal content suggests the important roles of PM (chelation) and Zn 2+ ions (porogen and gel initiator) into fixing Fe centers in the carbon matrix. Further analysis was conducted with X-ray photoelectron spectroscopy (XPS) measurements. First of all, no Fe−O peak can be resolved in the high-resolution scan of the O 1s electrons ( Figure S9a), suggesting that Fe atoms are most likely coordinated to other atomic sites such as N and C; and from the XPS spectra of the N 1s electrons of the series of samples (Figures 2(c), S9b, and S9c), one can see that the successive introduction of PM and Zn 2+ into the precursors increased the N doping from 2.25 at% to 3.60 at% in the carbon matrix (Tables S4 and  S5), and the pyridinic N fraction was the highest in the NCA C /Fe (0.52 at%) and NCA C-Zn /Fe (0.51 at%) samples. In addition, as compared to CS C /Fe, the much stronger Fe-N peak (0.44 at% vs. 0.04 at%) in the NCA C-Zn /Fe sample suggests the generation of more abundant FeN x moieties in the carbon aerogels.
The structural configuration of the FeN x functional moiety was then examined by X-ray absorption spectroscopy (XAS) measurements. From Figure 2(d), one can see that the Fe K-edge X-ray absorption near-edge spectrum (XANES) of NCA C-Zn /Fe is very similar to that of FePc but markedly different from that of an Fe foil, suggesting a comparable oxidation state (+2) of the Fe centers in NCA C-Zn /Fe and FePc. In the extended X-ray absorption fine structure (EXAFS) spectrum of the Fe foil, the Fe-Fe peak is welldefined at 2.21 Å (Figure 2(e)); however, this peak is absent in NCA C-Zn /Fe, consistent with the atomic dispersion of Fe in the NCA C-Zn /Fe sample ( Figure 1). In fact, both the NCA C-Zn /Fe and FePc samples display only a single major peak at 1.41 Å, which can be assigned to the Fe-N bond. Furthermore, the first shell of NCA C-Zn /Fe is well fitted with 3.8 N and 0.2 C with the same bond length of 1.94 Å (Figures 2(f) and S10). Taken together, these results suggest that the Fe centers in NCA C-Zn /Fe were mostly involved in the coordination structure of FeN 4 , with a minor fraction (ca. 1/5) in the form of FeN 3 C.  Figure 3(e)) is much closer to the Fermi level than that of normal FeN 4 , indicating a higher probability of donating electrons and reducing oxygen.
To evaluate the ORR activity of these Fe-N metal centers, the reaction free energy is calculated at the applied potential of +0.9 V vs. RHE and plotted in Figure 3(g). One can see that the first two electron-transfer steps are exothermic and the last two endothemic, with the rate determining step (RDS) most likely the fourth electron-transfer step of water formation and desorption. In comparison with normal FeN 4 and FeN 3 , both FeN 4 SW and FeN 3 SW show much lower endothermic energies (0.179 eV and 0.228 eV), implying a lower reaction overpotential. These results suggest that the nanowrinkles can enhance the electrocatalytic activity of Fe-N centers on the carbon matrices by forming SW defects, as manifested below in electrochemical tests.
2.3. Electrocatalytic Activity towards ORR. The electrocatalytic activity of the nanowrinkled carbon aerogels obtained above was then investigated in 0.1 M KOH. First, electrical impedance spectroscopy (EIS) analysis was carried out to investigate the electron-transfer kinetics. For the NCA C-Zn /Fe catalyst, the small diameter at high frequency and the steep tail at low frequency suggest excellent channels for both mass transfer and charge transfer. Such a low impedance is anticipated to facilitate ORR electrocatalysis ( Figure S13). Figures 4(a) and 4(b) show the ORR polarization curves and H 2 O 2 yields of the carbon aerogels, in comparison to commercial Pt/C (20 wt%). As a metal-free catalyst, the CA C sample shows a rather apparent electrocatalytic activity with an onset potential (E onset ) of +0.94 V vs. RHE and a half-wave potential (E 1/2 ) of +0.79 V, much more positive than those of other carbon catalysts reported in recent literature [15,32,33]. This suggests that biomass alone may be exploited as a carbon source to fabricate metal-free ORR electrocatalysts. Notably, doping of the FePM complex into the CS Si leads to a marked enhancement of the catalytic performance with E onset = + 1:10 V and E 1/2 = +0:90 V (NCA C-Zn /Fe), which is even better than those of commercial Pt/C (+0.99 V and+ 0.83 V) [34]. Likewise, the NCA C-Zn /Fe single-atom catalyst shows the lowest average H 2 O 2 yield (1.45%) within the potential range of +0.2 V to +0.9 V, signifying a high-efficiency 4e − reduction pathway (Figures S14-S16). From the Koutecky-Levich plots, the kinetic current density (J k ) at +0.85 V was estimated to be 9.12 mA cm −2 , about 3.3 times that of Pt/C (2.80 mA cm −2 , Figure 4(c)). Both NCA C-Zn /Fe and Pt/C show a low Tafel slope (85 vs. 87 mV E-E f (eV)  dec −1 ) in the high potential range, illustrating an efficient kinetic process of ORR on these two catalysts (Figure 4(d)). Besides, in contrast with Pt/C, the NCA C-Zn /Fe exhibits remarkable durability and methanol tolerance ( Figure S16). In addition, the E 1/2 and diffusion-limited current density of NCA C-Zn /Fe were comparable to those of Pt/C in acidic media (0.1 M HClO 4 ), suggesting the high ORR activity of the single iron atom catalysts even at low pH (Figure 4(e)).
To distinguish the contributions of the Fe center and adjacent nonmetal atoms to the electrocatalytic activity, electrochemical measurements were then carried out with the addition of SCNas the poisoning species. One can see that upon the addition of 10 mM SCNinto 0.1 M KOH, the E 1/2 of NCA C-Zn /Fe exhibited a negative shift of 20 mV ( Figure S17). The relatively mild performance deterioration suggests that in addition to the Fe sites, adjacent nonmetal atoms also play a critical role in driving the catalytic reaction. This is actually in good agreement with the formation of FeN x SW moieties in the carbon skeletons, where structural distortion leads to the activation of adjacent C atoms (Figures 3 and S12) [8].
Notably, other biomass hydrogels, such as gelatin and agar, can also be used as templates to fabricate nanowrinkled carbon aerogels embedded with single-metal atoms in a similar fashion. The resulting catalysts, NCA G-Zn /Fe and NCA A-Zn /Fe, both displayed excellent catalytic activities towards ORR in alkaline media, with an E 1/2 of +0.92 and +0.89 V and an E onset of +1.12 and 1.10 V, respectively (Figure 4(f) and inset). These results highlight the universality of the synthetic strategy in the preparation of high-performance ORR electrocatalysts ( Figure S1).

Electrocatalytic Activity towards OER and Zinc-Air
Battery Performance. The electrocatalytic activity of the NCA C-Zn /Fe aerogels towards OER was then examined and compared with commercial RuO 2 in 1 M KOH with iR correction. From Figure 5(a), one can see that for NCA C-Zn /Fe, an overpotential (η 10 ) of +370 mV was needed to achieve the current density of 10 mA cm −2 , a performance comparable to that of commercial RuO 2 (η 10 = +340 mV). The NCA C-Zn /Fe also exhibits a Tafel slope of 98 mV dec −2 , close to that of RuO 2 (71 mV dec −2 ), signifying a favorable OER kinetic ( Figure S18). Thanks to the excellent electrocatalytic performance towards both ORR and OER, the NCA C-Zn /Fe SACs show a low potential difference (ΔE) of only 0.71 V between the OER potential at 10 mA cm −2 (E OER,10 ) and the ORR potential at 3 mA cm −2 (E ORR,3 ), much smaller than those of bifunctional M−N−C catalysts reported recently in the literature [11,35,36].
With such a remarkable bifunctional performance, the NCA C-Zn /Fe aerogels were tested as the air-cathode for a Zn-air battery, in comparison with those using a commercial Pt/C-RuO 2 mixture (mass ratio 1 : 1), along with a Zn plate as the anode. From Figures 5(b) to 5(c), the NCA C-Zn /Fe-Zn-air battery can be seen to show an open-circuit voltage (OCV) of 1.50 V and a maximum power density of 231 mW cm −2 , about 6 mV and 20 mW cm −2 higher than those of the Pt/C-RuO 2 counterpart. Figure 5(d) shows the correspond-ing constant current discharge tests at various current densities (5,10,20, and 50 mA cm −2 ) of the two batteries. One can see that the NCA C-Zn /Fe-Zn battery exhibited a much higher discharge voltage within a wide range of current densities (5 to 50 mA cm -2 ). At the constant current density of 10 mA cm −2 , the NCA C-Zn /Fe-Zn battery displayed a stable and optimal potential of 1.36 V for 41 h. By normalizing the energy output to the weight of dissipated Zn, the calculated specific capacity and energy density were estimated to be 780 mAh g −1 and 956 Wh kg −1 , respectively, markedly higher than those of Pt/C-RuO 2 ( Figure S19). Also, the small charge-discharge voltage gap of the NCA C-Zn /Fe-Zn battery in Figure 5(e) indicates excellent rechargeability ( Figure S20). Impressively, the battery also delivers a stable potential plateau in the charge-discharge test at the constant current density of 10 mA cm −2 during prolonged operation. After 1100 continuous charge-discharge cycles (400 s for each cycle), the NCA C-Zn /Fe-Zn battery still afforded a high round-trip efficiency of 59% and a narrow discharge-recharge voltage gap of 0.79 V, much better than those of Pt/C-RuO 2 and other leading oxygen electrocatalysts reported in recent literature ( Figure 5(f)) [5,11,[37][38][39][40]. Taken together, these results demonstrate that the NCA C-Zn /Fe aerogels derived from biomass hydrogels can be used as high-performance bifunctional oxygen electrodes for Zn-air batteries, thanks to its high opencircuit voltage, large power density, and superb durability.

Conclusion
In this study, a facile, scalable strategy was developed for the preparation of nanowrinkled carbon aerogels embedded with FeN x active sites by utilizing biomass hydrogels as the precursors and reactors. The resulting nanowrinkled carbon aerogels (NCA C-Zn /Fe) showed an excellent and reversible ORR/OER electrocatalytic performance with a low voltage gap of only 0.71 V for oxygen electrocatalysis. With the obtained carbon aerogels as the (air) cathode catalysts of a Zn-air battery, the battery exhibited a higher open-circuit voltage, greater power density, and superior durability than that based on a mixture of commercial Pt/C-RuO 2 catalysts. First-principles calculation showed that FeN x sites in Stone-Wales defect formed by the carbon nanowrinkles were most likely responsible for the excellent electrocatalytic activity. Results from the present study suggest that creating structural distortion of metal sites in carbon matrices can be exploited as an effective strategy for the design and engineering of advanced electrocatalysts based on atomically dispersed metal centers. purchased from Sigma-Aldrich (USA). Commercial Pt/C (20 wt%) and high-purity zinc plate (99.999%) were obtained from Johnson Matthey. Polytetrafluoroethylene (PTFE, 60 wt%, D-210C) was purchased from Japan DaJin. All other reagents were analytical grade, and ultrapure water (Mill-Q, 18.3 MΩ cm) was used throughout this study.

4.2.
Instrumentation. TEM studies were carried out with a T20 FEI Tecnai G2 instrument. Scanning electron microscopy images were obtained with a Hitachi S-4800 fieldemission scanning electron microscope. STEM with EDS studies were performed on a JEOL JEM-ARM200CF with aberration-corrected STEM. Topography, maximum force, adhesion force, and current flow were measured using the Fast Force Mapping technique with an Oxford Instruments Asylum Cypher S AFM housed in an Ar-gas-filled glove box. Raman spectra were acquired with a Renishaw inVia Raman microscope. CD spectra were recorded on Jasco J-815 CD spectrometer (Japan). UV-vis spectra were acquired with a Shimadzu UV-2450 Spectrophotometer (Japan). ICP-OES studies were performed on a SPECTROBLUE SOP instrument. XRD and XPS measurements were carried out on a D/MAX2550 X-Ray Power Diffractometer and a Thermo Fisher-VG Scientific ESCALAB 250Xi X-Ray Photoelectron Spectrometer, respectively. N 2 adsorption-desorption isotherms were obtained with a Micromeritics ASAP 2020 Surface Area and Porosity Analyzer. Fe K-edge EXAFS measurements were performed at the Quick-EXAFS Beamline of the Taiwan Photon Source in transmission mode, and the results were analyzed by using the FeN x C y structural model with the Athena program. An RST 5200F electrochemical workstation (Zhengzhou, China) was used to perform the voltammetric measurements. Rotating Disk Electrode (RDE, Pine Research Instrument) tests were carried out at the rotation rates of 400 to 1600 rpm. , respectively). The CS Si-Zn /FePM hydrogel obtained above was freeze-dried and then heated to 900°C at the heating rate of 5°C min -1 in an Ar atmosphere (containing 3% H 2 ). After heating at 900°C for 3 h, the sample was cooled down to room temperature and subjected to HF etching to remove SiO 2 nanoparticles, affording an Fe-N-codoped carbon aerogel, which was referred to as NCA C-Zn /Fe.
Three control samples were prepared in the same fashion except that only one or two of the starting materials (SiO 2 nanoparticles, Zn 2+ ions, and Fe(PM) 3

2+
) were used to prepare the biomass hydrogel precursors (i.e., CS Si , CS Si /Fe, and CS Si /FePM). The corresponding aerogels were denoted as CA C , CA C /Fe, and NCA C /Fe, respectively.

Electrochemistry.
Electrochemical tests were carried out in a three-electrode electrochemical cell with a graphite rod as the counter electrode and an Ag/AgCl (saturated KCl) electrode as the reference electrode. The Ag/AgCl reference electrode was calibrated against a reversible hydrogen electrode (RHE) and all potentials in the present study were referenced to this RHE. To prepare a catalyst ink, 3 mg of the catalysts obtained above was dispersed in a 475 μL mixed solvent of H 2 O and ethanol (v : v = 1 : 1) and 25 μL of a Nafion solution (5%) under sonication for 1 h to form a homogeneous dispersion (6 mg mL −1 ). For ORR tests, the catalyst ink was loaded onto a cleaned glassy carbon electrode at the catalyst loading of 250 μg cm −2 for cyclic voltammetry and 400 μg cm −2 for RDE and RRDE measurements in 0.1 M KOH, respectively. For OER tests, the catalyst was loaded onto a carbon paper at the mass loading of 1.0 mg cm −2 in 1.0 M KOH.
In RDE measurements the disk current density (J) is defined by the Koutecky-Levich (K-L) equation: where J L is the limiting current density, J K is the kinetic current density, ω is the rotation rate, n is electron transfer number, F is the Faraday constant (96,485 C mol -1 ), C 0 is the O 2 concentration in the electrolyte solution (1:2 × 10 −6 mol cm -3 ), and ν is the kinematic viscosity of the electrolyte (0.01 cm 2 s -1 for 0.1 M KOH). J K can be determined from the intercept of the K-L plot (J −1 vs. ω −1/2 ). In RRDE measurements, the H 2 O 2 yield and electron transfer number (n) can be calculated by equation (2).
where I r and I d are the ring current and disk current, respectively, and N is the collection efficiency of the ring electrode (0.37).  [41]. A two-dimensional supercell was built based on an 8 × 8 unit cell (127-129 atoms in total). For avoiding the interactions between periodic images, the vacuum at the z-axis was set at 14 Å. The ultrasoft pseudopotential was adopted [42]. The kinetic and charge density cutoff were set at 40 and 200 Ry, respectively. The 2 × 2 × 1 Monkhorst-Pack K-point grids were sampled for the supercell. The total energy was converged to 10 -3 eV for geometric relaxation. The Marzari-Vanderbilt smearing was adopted with a smearing of 0.01 Ry [43]. The electronic energy and force were converged to 10 -8 Ry and 10 -4 a.u., respectively. The phonon contribution to zeropoint energy and entropy was calculated based on the density functional perturbation theory [44,45]. STM calculations were carried out based on the Tersoff and Hamann approximation [46] as implemented in the open-source Quantum ESPRESSO package [41] at a bias of -1.0 or +1.0 V, as described in the literature [47].

Conflicts of Interest
The authors declare no competing financial interests.  Table S1: BET surface area of CA C , CA C /Fe, NCA C /Fe, and NCA C-Zn /Fe. Table S2: EDS results of CA C , CA C /Fe, NCA C /Fe, and NCA C-Zn /Fe. Table S3: Fe contents in NCA C-Zn /Fe and NCA C /Fe determined by ICP-OES measurements. Table S4: elemental analysis by XPS measurements. Table S5: assignments of N species for different samples. Figure S1: schematic illustration of the preparation of NCA C-Zn /Fe carbon aerogels. Figure S2: SEM images of CS Si , CS Si /Fe, and CS Si /FePM. Figure S3: (a) CD and (b) UV-vis spectra of the series of biomass-derived hydrogels. Figure S4: TEM image of the NCA C-Zn /Fe aerogel. Figure  S5: AFM images of NCA C-Zn /Fe aerogels: (a) topography image, (b) max force image, (c) adhesion force image, and (d) current flow image. Figure S6: (a) N2 absorptiondesorption isotherm and (b) pore size distribution of CA C , CA C /Fe, NCA C /Fe, and NCA C-Zn /Fe. Figure S7: XRD patterns of CA C , CA C /Fe, NCA C /Fe, and NCA C-Zn /Fe. Figure  S8: EDS profiles of CA C , CA C /Fe, NCA C /Fe, and NCA C-Zn /Fe. The results are summarized in Table S2. Figure S9: (a) XPS of O 1s electrons of NCA C-Zn /Fe. XPS of the N 1s electrons of (b) NCA C /Fe and (c) CA C . Figure S10: EXAFS fitting curves for NCA C-Zn /Fe. Inset is the corresponding Kspace profiles. Figure Figure S13: EIS spectra of CA C , CA C /Fe, NCA C /Fe, and NCA C-Zn /Fe. Figure S14: RRDE polarization curves of CA C , CA C /Fe, NCA C /Fe, and NCA C-Zn /Fe, as well as Pt/C at 1600 rpm in 0.1 M KOH. Potential scan rate: 5 mV s -1 . Figure Figure S18: OER Tafel plots of (a) NCA C-Zn /Fe and (b) RuO 2 . Figure S19: specific capacity and energy density of NCA C-Zn /Fe and Pt/C-RuO 2 . Figure  S20: charge-discharge tests of NCA C-Zn /Fe and Pt/C-RuO 2 .

Supplementary Materials
(Supplementary Materials)