Aluminum and Nitrogen Codoped Graphene: Highly Active and

Dec 7, 2018 - The development of a highly active and exceedingly durable ... Pt/C but also good durability in both three-electrode cell and Zn-air bat...
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Aluminum and Nitrogen Codoped Graphene: Highly Active and Durable Electrocatalyst for Oxygen Reduction Reaction Yong Qin, Hong-Hui Wu, Lei A Zhang, Xiao Zhou, Yunfei Bu, Wei Zhang, Fuqiang Chu, Yutong Li, Yong Kong, Qiaobao Zhang, Dong Ding, Yongxin Tao, Yongxi Li, Meilin Liu, and Xiao Cheng Zeng ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04117 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 7, 2018

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Aluminum and Nitrogen Codoped Graphene: Highly Active and Durable Electrocatalyst for Oxygen Reduction Reaction Yong Qin,†, ∇ Hong-Hui Wu, ‡, ∇ Lei A Zhang, §, ∇ Xiao Zhou,† Yunfei Bu,§ Wei Zhang,‡ Fuqiang Chu,† Yutong Li,† Yong Kong,† Qiaobao Zhang,*, ‖ Dong Ding,§ Yongxin Tao,† Yongxi Li,† Meilin Liu*, § and Xiao Cheng Zeng*, ‡ † the Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu, 213164, China ‡ Department of Chemistry, University of Nebraska, Lincoln, NE, 68588-0304, United States § School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, 30332-0245, United States ‖ Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen, Fujian 361005, China ABSTRACT: The development of highly active and exceedingly durable electrocatalyst at low cost for oxygen reduction reaction (ORR) is extremely desirable, but remains to be a grand challenge. Over the past decade, the transitional-metal (e.g., Fe, Co, Ni) and N codoped graphene materials have attracted most attention as the state-of-the-art non-preciousmetal-based effective electrocatalyst for ORR, but still entail unsatisfactory issues such as moderate activity and life. Herein, the main-group-metal Al and N codoped graphene (ANG) is successfully fabricated via thermal annealing treatment of N-doped graphene with aluminum tri-(8-hydroxyquinoline). As a highly effective electrocatalyst for ORR, the as-prepared ANG exhibits not only high electrocatalytic activity that even outperforms the commercial Pt/C, but also good durability in both three-electrode cell and Zn-air battery. Theory calculations show that the inhomogeneous charge density distribution and the interaction between Al and N are mainly responsible for the marked enhancement of ORR activity. The designed ANG electrocatalysts will provide a perspective application in energy storage and promote further exploration of main-group-element-based inexpensive, active and durable electrocatalysts. KEYWORDS：Al-doped graphene, N-doped graphene, oxygen reduction reaction, electrocatalyst, noble-metal free

Ever-growing demands for clean energy have stimulated extensive research effort in developing advanced energy storages and conversion systems with low-cost, high efficiency, and environmental benignity. Catalytic oxygen reduction reaction (ORR) plays the key role in both fuel cell and metal-air battery, two hallmark clean technologies with great potentials to power vehicles for long-distance transportation. Pt and Pt-based alloys are hitherto the best known ORR catalysts, but still exhibit some serious issues, such as high cost, scarcity, weak durability, and low poisoning-tolerance.1-3 Hence, it is imperative to develop highly active, highly durable, and inexpensive catalysts to meet practical applications in the future automobile industry. Carbon materials have shown great success in electrocatalysis because of their outstanding chemical stability and electric conductivity.4-6 Graphene, a popular two-dimensional sp2-hybridized material in carbon family, possesses abundant free-flowing π electrons, rendering graphene a promising catalyst for providing electrons needed in chemical reactions, such as ORR. However, the practical ORR activity of graphene is still fairly low due to the intrinsic

inertia of π electrons. Doping heteroatoms into graphene is an effective approach to activate the π electrons.7 The heteroatoms can be either electron-rich or electron-deficient. In the case of electron-rich N-doping, the carbon π electrons can conjugate with the lone-pair electrons from N atoms, resulting in the reduction of O2 molecule on the positively charged C atom, whereas in the case of electron-deficient Bdoping, the 2pz vacant orbital of B atoms can extract the carbon π electrons. Due to the low electronegativity of B atom, the electrons become very active so that O2 molecule can be reduced on the positively charged B atom.8-13 Consequently, either the electron-rich or electron-deficient dopant can break the electroneutrality of sp2 carbon and create positively charged sites favoring the O2 adsorption.14 However, the enhancement of ORR activity by this singletype heteroatom-doping strategy is still limited. In later studies, multi-heteroatom-codoped (i.e. B,N-,15, 16 S,N-,17-19 Fe,N-,20-22 Co,N-,23-25 and N,P,F-26 codoped) graphene was proposed. Wherein the nonmetal B,N- (BNG) and the transition-metal Fe and N-codoped graphene (FNG) received the most attention, as both being representatives in improving the ORR activity in at least two aspects: (i)

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increasing the number of active sites, and (ii) increasing the intrinsic activity of each active site.27, 28 For BNG, both B and N heteroatoms can break the symmetry of atomic charge density, resulting in more active sites (the positively charged B and C atoms),14, 29 while for FNG, the high spin density of FNG is responsible for the enhanced electrocatalytic activity of the -FeN4 active center.30 Notably, BNG features remarkable durability but moderate ORR activity,31 whereas FNG exhibits outstanding ORR activity but low durability due to the reduction of Fe active center at high overpotential during the ORR process.32-34 Currently, it is still a challenge to develop inexpensive electrocatalysts with both high activity and outstanding durability. Aluminum is the most abundant metallic element on earth and as such, it is inexpensive and widely used in our daily life. In the periodic table, Al locates in the same group as B, and has vacant 3p orbitals. In principle, the vacant 3pz orbital of Al can extract π electrons and form a positively charged Al active centre like B. It has been revealed that the O-O distance of O2 would be stretched from 1.21 Å to 1.53 Å after O2 is adsorbed on an Al site,35 implying possible good ORR activity of Al-doped graphene. Meanwhile, as an active metal, Al may bond with non-metallic N, generating the synergistic interaction favorable for ORR. Very recently, Lee et al. have predicted from density functional theory (DFT) calculation that Al-doped X-graphene (X=N, P) can catalyze ORR along a 4-electron pathway in acid media.35 Note that the redox potential of Al/Al3+ (-1.663 V) is more negative than that of Fe/Fe2+ (-0.409 V), hence the oxidation state of Al is harder to be reduced, suggesting that Al active center would be more stable than Fe. In these regards, Al and N codoped graphene (ANG) is a potentially active and durable electrocatalyst for ORR. However, the practical synthesis of ANG for ORR test has not been reported in the literature largely due to that Al is rather difficult to dope. Herein, ANG is firstly prepared through the thermal annealing treatment of Al precursors with N-doped graphene (NG). It is found that aluminum tri-(8-hydroxyquinoline) (AHQ) is optimal among various Al precursors, which can achieve the highest Al-doping level of 0.53 at.% in the graphene. On a rotation disk electrode (RDE) in a three-electrode tested system, the as-obtained ANG presents unprecedented ORR performance involving very positive onset potential (0.99 V) and half-wave potential (0.86 V), also substantially long durability (without loss of activity in 3,000 cycles) in alkaline media, which offer fascinating advantages over that of Pt/C. The Zn-air battery based on ANG delivers the high-power density of 103 mW cm-2 comparable to that of Pt/C (106 mW cm-2) and the super long cycling period of 263 hours. The results pave the way for the commercialization of inexpensive and highly efficient ORR electrocatalysts.

RESULTS AND DISCUSSION The fabrication process of ANG is schematically illustrated in Figure 1a. NG is firstly prepared through a method reported previously.36 Briefly, graphene oxide (GO), melamine, and formaldehyde are hydrothermally treated in a Teflon-lined autoclave at 180 ℃ for 12 hours to generate a melamine formaldehyde resin-bonded graphene aerogel. The graphene aerogel is calcined at 750 ℃ for 5 hours under Ar

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atmosphere to obtain the NG product. It has been shown that the NG possesses robust three-dimensional structure, hierarchical pore, and abundant nitrogen (~8 at.%).36 Al is further doped into graphene by a post-treatment of thermal annealing of AHQ with NG. Typically, AHQ is first adsorbed on NG, and then the AHQ/NG composite is annealed at 950 ℃ for 3 hours in Ar atmosphere, followed by acid- and alkalileaching successively to obtain the final ANG product. The morphologies of ANG are firstly captured by scan electron microscope (SEM) and transmission electron microscope (TEM), showing in Figure 1b-e, respectively. Clearly, SEM images show the interconnected 3D framework with random open pores constructed from graphene layers (Figure 1b and Figure 1c), similar to that of NG reported by us previously.36 TEM images display that the graphene sheets are very thin, transparent, and clean (Figure 1d and Figure 1e). Both SEM and TEM images show the high-quality surface almost free of any residuals, indicating that the carbonization products of Al precursor are fully removed. The high angle annular dark field (HAADF)-scanning transmission electron microscopy (STEM) image confirms the transparent and clean graphene as well (Figure 1f), and the corresponding elemental mapping analysis of ANG demonstrates the presence of C, N, O, and Al element (Figure 1g-j), implying that Al is successfully doped into the graphene. The structure of the ANG is further examined by X-ray diffraction (XRD), physical adsorption, and Raman spectroscopy, and compared with those of NG. As presented in Figure 2a, both the XRD patterns of ANG and NG exhibit two broad diffraction peaks at 26.2° and 43.3°, which correspond to the diffraction of C (002) and C (100),37 respectively. The XRD pattern of ANG is free of any Alrelated crystalline diffraction peaks, further confirming the clean surface of the graphene sheet. The nitrogen adsorption-desorption isotherm of ANG is a type II adsorption branch associated with an H3 hysteresis loop (Figure 2b), which is nearly overlapped with that of NG. The BET surface area of ANG (312 m2 g-1) is slightly lower than that of NG (357 m2 g-1) due most likely to that minor pores are probably collapsed during the process of Al doping at a high temperature. Besides, Raman spectra of ANG and NG show two peaks located at 1360 cm-1 and 1595 cm-1, which are the G band and D band peak of graphene,38 respectively (Figure 2c). Generally, the G band stems from the in-plane vibration of sp2-hybridized C atoms, while the D band is due to the defect or the edge of graphene.39 The ratio of the intensity at the D band to G band (ID/IG) can be used to evaluate the structural integrity of the graphene. As seen from Figure 2c, ANG shows the slightly lower ID/IG value than that of NG, suggesting that ANG presents the more integral structure, likely resulting from the loss of part of N during the higher heat-treating process. The chemical composition of ANG is lastly determined by X-ray photoelectron spectroscopy (XPS), as shown in Figure 2d. As compared with that of NG, XPS survey of ANG shows an additional weak Al peak except C, N, and O, also indicating the successful doping of Al. The amount of Al is 0.53 at.%, which is close to that of Fe

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Figure 1. (a) Schematic illustration of the fabrication process of ANG. (b, c) The SEM images and (d, e) the TEM images of the ANG. (f) The HAADF-STEM image of the ANG, and the corresponding elemental mapping of (g) C, (h) N, (i) O, and (j) Al in the selected area.

in FNG.40 The high-resolution N 1s XPS of ANG (inset in Figure 2d) exhibits an extra Al-N peak comparing with that of NG (Figure S1), suggesting the existence of bonded Al and N structure in ANG. The high-resolution Al 2p spectra confirmed the oxidized state of Al. The electrocatalytic performance of ANG is first evaluated in alkaline media in a three-electrode cell involving Ag/AgCl electrode as the reference electrode, graphite rod as the counter electrode, and the ANG-coated RDE as the working electrode. All the catalysts bear the mass loading of 0.15 mg cm-2 on the working electrode. For convenience of comparison, the potential was converted into reversible hydrogen electrode (RHE) potential. After Al-doping, the electric conductivity is significantly reduced (Figure S2), owing to that the Al-doping of graphene exhibits p-type behavior like B-doping.7 To improve ANG’s electric conductivity, 20 wt.% of conductive carbon black (CB) is mixed with ANG upon testing. The same amount of CB is added into NG. Cyclic voltammogram (CV) curves of ANG are firstly scanned in N2- and O2-saturated 0.1 M KOH

solution, shown in Figure 3a. Clearly, there is no redox peak in the N2-saturated solution. In stark contrast, a pronounced oxygen reduction peak centered at 0.8 V appear when the solution is saturated with O2, an indication of ORR activity of ANG. To explore the reaction kinetics and corresponding mechanisms, linear sweep voltammogram (LSV) curves of ANG at various rotating speed are subsequently measured on RDE in O2-saturated 0.1 M KOH, and compared with those of NG and the commercial Pt/C catalyst (20 wt.% Pt on Vulcan XC 72), shown in Figure S3. Their LSV curves at the rotation speed of 1600 rpm are collected in Figure 3b. It is found that ANG delivers the onset potential (Eonset) of 0.99 V and the half-wave potential (E1/2) of 0.85 V, respectively, more positive than those of NG (Eonset=0.85 V, E1/2=0.70 V), even the reference catalyst Pt/C (Eonset=0.98 V, E1/2=0.83). The Eonset and E1/2 values of the reference Pt/C is basically consistent with those reported in the literature,41 confirming the reliable values of the other catalysts. Besides, ANG outputs the higher current density (4.5 mA cm-2)

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Figure 2. (a) XRD, (b) N2 adsorption-desorption isotherms, (c) Raman spectra, and (d) XPS of ANG and NG. The insets in (d) are the high-resolution XPS of N 1s and Al 2p of ANG, respectively. at the potential of 0.8 V than that of Pt/C (3.5 mA cm-2) and NG (0.6 mA cm-2). In comparison with the ORR activity of the catalysts before and after adding CB, the contribution of CB to the intrinsically activity of the ANG can be ruled out (Figure S3). Certainly, CB can improve the ORR performance of the ANG, similar to the role of carbon in Pt/C catalyst. The optimal amount of the CB is 20 wt.%. The lower amount of CB cannot improve the electric conductivity of the ANG effectively, whereas the higher amount would decrease the content of active catalyst (Figure S4). Note that the single Aldoped graphene (AG) features poor ORR activity (Figure S3c), implying that the good performance of ANG results from the joint action of Al and N. The electron transfer numbers (n) of ANG calculated from the Koutecky-Levich (K-L) equation is 3.9 (Figure S5a), equal with that of Pt/C (Figure S5b), suggesting the same 4-electron reaction pathway as Pt/C. However, NG just exhibits the 2-electron pathway (Figure S5c). In these regards, ANG holds remarkable electrocatalytic activity toward ORR, clearly better than NG and most of the non-noble metal catalysts reported in literature (Table S1), and it even outperforms the state-of-the-art Pt/C. The considerably enhanced activity of

ANG compared to NG evidences the positive contribution of Al for ORR. To further confirm the conclusion above, the ANG annealed at various temperatures ranging from 750 ℃ to 1050 ℃ are obtained, and their components are analyzed by XPS (Table S2). As shown, Al is not found in ANG-750, suggesting that it cannot be doped below 750 ℃. The optimal doping temperature of Al is 950 ℃, much higher than that of the optimal temperature of Fe (750 ℃),40 confirming that Al is really harder to dope than Fe. The doping level of Al can be achieved to the maximum value of 0.53 at.% at 950 ℃. In comparison with the ORR activity of ANG fabricated under various temperatures, the ORR activity of ANG (Figure S6) is highly correlated with the Al content (Table S2). ANG-950 features the highest amount of Al, thus yields the best ORR activity, confirming that Al plays a critical role in promoting the ORR activity. Moreover, the content of nitrogen decreases gradually with the doping of Al below 950 ℃, signifying that the Al-doping is likely related with the decomposition of

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Figure 3. Electrochemical performance of catalysts for ORR. (a) CVs of ANG in N2-saturated (dash line) and O2saturated (solid line) 0.1 M KOH. (b) LSV curves of ANG, NG, and Pt/C at the rotation speed of 1600 rpm and the scan rate of 10 mV s-1. (c) Chronoamperometric curves of the ANG, and Pt/C at the constant potential of 0.8 V. (d) LSV curves of ANG before and after 3,000 cycles. Potential window: 0.2-1.1 V, scan rate: 5 mV s-1, catalyst loading: 0.15 mg cm-2. (e) V ~ i polarization and power density curves of ANG and Pt/C as the air electrode. (f) The durability of the Zn-air batteries based on ANG and Pt/C catalysts. Catalyst loading: 1.0 mg cm-2. The ANG and the NG are mixed with 20 wt.% CB before tests. C-N bond. In this point, the Al-doping may experience the two steps involving the cleavage of C-N and the formation of a more stable Al-N. To optimize the ORR performance of ANG, various available precursors involving inorganic (aluminum sulfate, aluminum nitrate, sodium aluminate) or organic (aluminum isopropoxide and AHQ) aluminum salt, are investigated, shown in Figure S7, AHQ is found to be the best precursor among those considered. As speculated, the condensed ring and conjugated aromatic structure of AHQ similar to that of graphene may be in favor of Al-doping. The accelerated durability test (ADT) of ANG is assessed through chronoamperometric measurements and compared with that of Pt/C. As shown in Figure 3c, ANG presents much better durability with the higher current retention (90%) after 20,000 s of continuous operation in comparison with those of Pt/C (74%) and NG (85%). Moreover, the cyclic stability is evaluated by the shift of the LSV curve at 1,600 rpm after 3, 000 cycles (Figure 3d). As expected, ANG gives negligible potential shift (including onset potential and halfwave potential) and slightly decay of limited current density, whereas Pt/C (Figure S8) and FNG40 exhibit very significant potential shift and current decay, indicating the super durability of ANG. Additionally, Pt/C catalyst usually suffers from the crossover effect from small molecular fuel (i.e. methanol, ethanol) in fuel cell.42, 43 In contrast, ANG shows little crossover effect from methanol, like most of the non-

noble metal catalysts (Figure S9). As a result, ANG achieves the overall excellent ORR performance, with the positive onset and half-wave potential, good anti-methanol toxicity, and long-term stability in alkaline media. The ORR performance of ANG in acidic media is also considered, but it fails to exhibit similar prominent performance as in alkaline condition (Figure S10). As a promising metal-air system to meet the growing demand of the global electric vehicle industry, the Zn-air battery has also attracted considerable attention for its high energy density and environmental safety.44, 45 To demonstrate the possibility of our catalyst for industrial applications, the Zn-air battery is constructed with using the ANG catalyst loaded on the air electrode as the cathode, Zn plate as the anode, and 6 M KOH as the electrolyte (Figure S11a). The photograph of the coin Zn-air batteries can be found in Figure S11b, and the V~i polarization and power density curves of the battery based on ANG and Pt/C air electrodes are shown in Figure 3e. Significantly, the ANG displays the power density of 103 mW cm−2, very close to that of Pt/C air electrode (106 mW cm−2). In addition, the discharge curves of each electrode at a current density of 5 mA cm−2 are recorded

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Figure 4. (a) The structure of ortho, meta, and para positions of the 5656-ring ANG, where gold, pink and purple balls represent C, Al, and N atoms, respectively. (b) Free-energy diagram for 4-electron ORR pathway at zero electrode potential, from DFT calculation. (c) Computed charge distribution of pristine graphene sheet. (d) Computed charge distribution of ortho-ANG through ELF analysis. in Figure 3f. Clearly, ANG features the approaching voltage (1.3 V) as Pt/C (1.38 V) as well, and the duration of the ANG (~12.6 h) is better than that of Pt/C (~11 h). With the replacement of the depleted aluminum foil, the long stability discharge test is shown in Figure S12. The durability of the ANG approached to ~263 hours, further confirming its stability and commercial potential. To gain deeper insights into the high ORR activity through Al and N codoping, we carry out theoretical analysis based on the DFT calculations. In the codoping, Al and N can exist in different local configurations, such as the isolated and bonded configurations. To identify the structure of the most active center, 18 possible configurations involving various types of rings (such as 5656 and 6666 rings, see Supplementary Information) and coordinates (Al, AlN1, AlN2, AlN3, AlN4) of ANG are considered, as shown in Figure S13. The free energies for each step and the overpotentials for each active site on ANG are summarized in Table S3. The computational results suggest that the 5656ring structure is the most active center because all the configurations with this structure exhibit relatively low overpotentials. Three scenarios involving ortho, meta, and para positions of Al and N in the 5656-ring ANG are

examined, and labeled as ortho-, meta, and para-ANG, respectively (Figure 4a). Clearly, the ortho-ANG exhibits the lowest overpotential among the three structures, indicating that the directly bonded Al and N structure provides the best performance with the lowest energy barrier. The result also suggests that the interaction between Al and N further facilitate the electrocatalytic ORR. It is worthy of noting that N 1s XPS evidences the existence of the bonded Al and N in as prepared ANG (inset in Figure 2d). The free-energy diagrams of ortho-, meta, and para-ANG are shown in Figure 4a. It can be seen that the transformation from OH* (* means adsorbed site) to OH- is the rate-determining step in all three cases. The free-energy diagram in the case of orthoANG is the closest to that of the ideal ORR state, further Supplementary that the orhto-ANG is the most active site. Note that the overpotential of the orhto-ANG from DFT calculation (~1.4 V) is much higher than that from our experiment (~0.4 V). The comprehensive ORR activity of a material is dependent on several factors, such as the charge,14 the spin density,30 the coordinate state,46 the edge,46 and the defect.47, 48 Especially, the GO derived from this modified Hummer's method has a large number of defects. To investigate the effect of the defect, the free-energy diagram

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and overpotential of the ortho-ANG with a defect (Figure 4a) are calculated, and compared with those of ortho-ANG without the defect, as shown in Figure 4b and Table S3. Indeed, the ortho-ANG with a defect type ANG-05 displays the significantly enhanced ORR activity than the ortho-ANG without the defect, suggesting the positive contribution of this defect. The electron localization function (ELF) analysis of pristine graphene sheet (Figure 4c), NG (Figure S14), and ortho-ANG (Figure 4d) are undertaken. Clearly, the Al-N codoping contributes to the most inhomogeneous distribution of the charge density. As described previously, O2 molecule is typically reduced on the positively charged atoms. The inhomogeneity of the charge density implies the more active sites from positively charged atoms. In combination with the DFT calculation result, it can conclude that the codoping of Al and N can enhance the ORR active sites in both quantity and quality. Additionally, the Al active sits is not reduced during oxygen reduced process due to its high redox potential, resulting in its good durability. Taken together, the ANG is a highly active and durable ORR electrocatalyst for ORR.

CONCLUSIONS In summary, Al and N codoped graphene is successfully fabricated via a facile thermal treatment of Al-containing precursors with NG. The highest level of 0.53 at.% Al doping can be achieved by using AHQ as the precursor. In a threeelectrode cell, ANG exhibits the unprecedented electrocatalytic performance for ORR in the alkaline electrolyte outperforming that of Pt/C, while involving the identical onset potential (~0.99 V), the more positive halfwave potential (0.85 V vs 0.83 V of Pt/C), and the higher current density at 0.8 V (4.5 mA cm-2 vs 3.5 mA cm-2 of Pt/C). As an air electrode for Zn-air battery, ANG affords the highpower density of 103 mW cm-2, approaching to that of Pt/C (106 mW cm-2), as well as longer cycle stability. Thus, ANG is a very promising non-precious-metal cathodic catalyst for fuel cell and metal-air battery. Theoretical calculations show that the bonded Al and N in the 5656-ring of ANG is likely the most active center. The inhomogeneous charge density induced by Al-N co-doping, together with the interaction between Al and N, is mainly responsible to the enhanced electrocatalytic activity of ANG. This work demonstrates, for the first time, that the main-group-metal element (e.g. Al) and N codoped graphene is capable of being a highly active and exceedingly durable ORR catalyst. The catalyst can be as good, or even better, compared to the state-of-the-art transition-metal (Fe or Co) and N codoped graphene catalyst, as both provide a novel perspective in the design and development of inexpensive, highly active and exceedingly durable electrocatalysts for ORR.

EXPERINMENTAL METHODS Fabrication of Materials. GO was prepared from natural graphite flakes by a modified Hummers’ method.49 Firstly, graphite flakes (1.0 g) and KMnO4 (6.0 g) were added into a mixture involving 120 mL concentrated H2SO4 and 13.3 mL H3PO4, which will generate a slight exotherm to 35 ℃. The mixture was kept stirring at 50 ℃ for 12 hours. After cooled to room temperature, it was poured into ice water (150 mL)

containing 10 mL H2O2 (30 wt.%). The mixture was then sifted through a polyester fiber. The filtrate was centrifuged (4000 rpm for 4 hours), and the obtained residue was washed in succession with 200 mL of water, 200 mL of 30 wt.% HCl, and 200 mL of ethanol. The solid was vacuum-dried overnight at room temperature, resulting 1.8 g GO. Secondly, 150 mg GO was dispersed in 15 mL water to produce a concentrated GO suspension, which was then mixed with 1 mL formaldehyde solution (37 wt.%) and 0.35 g melamine. The mixture was transferred into an autoclave and hydrothermally treated at 180 ℃ for 12 hours. The obtained composite hydrogel was dried at 120 °C for 24 h in an oven. The dry aerogel was subsequently calcined at 750 °C for 5 hours in Ar atmosphere to obtain the final NG product. Finally, The AHQ ethanol solution was mixed with NG, and dried in air. The aluminum precursor/AHQ composites were calcined at various temperatures in Ar atmosphere. The products were labeled as ANG-T, here, T represented the annealing temperature. The fabrication of AG is similar to that of ANG, expect that the hydrothermally treated mixture was freeze-dried and was free of melamine and formaldehyde, and the precursor is replaced by aluminum isopropoxide. Basic characterization. The morphology and structure of the samples were investigated by SEM (Gemini SUPRA55), STEM (FEI, Tencai G2 F30 S-TWIN) and X-ray diffraction (Rigaku Corporation, D/max 2500PC). Raman spectra were recorded with a Bruker RFS 100/S spectrometer (laser wavelength 532 nm). The XPS measurements were performed using ESCA 250Xi spectrometer (ThermoFisher Scientific) in an ultrahigh vacuum. Electrochemical measurements. The working electrodes were prepared as the following step: the ink (catalysts in water, 1.0 mg mL-1, 30 μL) of the as-obtained materials were dropped wise coated onto a glassy carbon disk with the diameter of 5 mm (mass loading ~0.15 mg cm-2) and dried at room temperature at ambient conditions for 6 hours. Then Nafion solution (5.0 wt.%) was cast on the electrode surface to adhere the catalysts on the electrode. Electrocatalytic performance measurements were conducted in a three-electrode cell using Ag/AgCl electrode (calibrated and converted to reverse hydrogen electrode, RHE) as the reference electrode, graphite rod electrode as the counter electrode, and the sample modified glassy carbon electrode as the working electrode. KOH solution (0.1 M) was used as the electrolyte solution, and saturated with O2 prior to the start of each experiment. For cyclic voltammetry (CV) test, an O2 flow was maintained over the electrolyte during the recording of CVs. Before data were recorded, the working electrode was cycled at least 5 times at a scan rate of 10 mV s−1. For comparison, CV measurements were also carried out in N2-saturated KOH solution. For RDE measurement, the working electrode was scanned at a rate of 10 mV s-1 with various rotating speed from 400 rpm to 2400 rpm. KouteckyLevich plots were analyzed at various electrode potentials. The electron transfer number (n) is calculated by the slopes of the linear fit lines on the basis of the Koutecky-Levich equation.50, 51 Assembly of the Zn-air battery. Zinc plate was used as the anode which separated by a nylon polymer membrane separator (Cell guard 3501 membrane) with the cathode and

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6 M KOH electrolyte was filled between the cathode and anode, and nickel mesh was used as the current collector. The air cathodes were prepared by coating a mixture of PTFE binder (60 wt.% PTFE emulsion in water, Sigma-Aldrich), activated charcoal (Darco G-60A, Sigma-Aldrich) and electrocatalyst (ANG) in a ratio of 65:8:27, respectively. The loading of the catalysts is 1.0 mg cm-2. An assembled full-cell was performed at several discharge and charge currents. Density Functional Theory (DFT) Calculations. A 6×6 of primitive cell of graphene with 72 C atoms was employed as the starting structure. Al and N were codoped into graphene by substituting one C atom each. Our spinpolarized DFT computations were performed using the projector-augmented plane-wave (PAW) method52 as implemented in the Vienna Ab Initio Simulation Package (VASP 5.4).53 The exchange-correlation function was described by the generalized-gradient approximation, and the kinetic energy cutoff of 520 eV was adopted. Conjugated gradient method was employed to the geometry optimization and all the atomic coordinates were fully relaxed till the Hellmann-Feynman force on each atom was less than 0.01 eV Å–1 eV, and the convergence criteria for total energy was 10-5 eV. The reciprocal space was sampled on the Gamma-centered meshes with a density larger than 10/Å. In the current work, the pathways on 18 types of Al and N codoping systems were calculated in detail, according to the electrochemical framework developed by Nørskov et al.54 Our experiments were operated under alkaline condition (pH = 13), since H2O/H3O+ may act as the proton donor, the overall reaction scheme of the ORR can be expressed as: O2+2H2O +4e-→4OH-. The ORR can proceed through the following elementary steps usually employed to investigate the electrocatalysis of the ORR on various materials: O2(g) + H2O(l) + e- + ∗ → OOH* + OH(1) OOH* + e- →O* + OH(2) O* + H2O (l) + e- →OH* + OH(3) OH* + e- →OH- +* (4) where * stands for an active site on the catalytic surface, (l) and (g) refer to liquid and gas phases, respectively. The chemical potential of each adsorbate is defined as: μ  Etotal  E ZPE  T  S (5) where

Etotal is the total energy obtained from DFT

calculations, E ZPE is the zero-point energy and S is the entropy at the temperature T=298 K. To get the reaction free energy of each elementary step of the ORR, we calculated the adsorption free energy of O*, OH*, and OOH*,





ΔGO *  ΔG H 2O ( g ) *  O*  H 2 ( g )  μO *  μH * (6)  μH 2O  μ*   1 ΔGOH *  ΔG  H 2O ( g ) *  OH *  H 2 ( g )   μOH * 2 (7)   1 *  μH 2  μH O  μ 2 2

  3 ΔGOOH *  ΔG  2H 2O ( g ) *  OOH *  H 2 ( g )   2 (8)   3 * μOOH *  μH 2  2μH 2O  μ 2

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For each elementary step, the Gibbs reaction free energy 𝛥𝐺 is calculated based on the standard hydrogen electrode (SHE) model, (9) ΔG  ΔE  ΔZPE  TΔΔ  ΔGU  ΔG pH where 𝛥𝐸 is the reaction energy of reactant and product molecules adsorbed on the catalyst surface, 𝛥𝑍𝑃𝐸 and 𝛥𝑆 are the difference in zero point energy and entropy during the reaction. The bias effect on the free energy of each initial, intermediate and final state involving electrons transfer in the electrode is also taken into account by shifting the energy of the state by ΔGU   neU , where U is the electrode applied potential, e is the transferred charge and n is the number of proton–electron transferred pairs. The change of free energy owing to the effect of pH value of the electrolyte is considered by the correction of H+ ions concentration ([H+]) dependence of the entropy,

ΔG pH  k BTln[H  ] = pH  k BTln10 where

(10)

k B is the Boltzmann constant and T is the

temperature. Given the fact that the high-spin ground of the oxygen molecule is poorly described in calculations, the free energy of the O2 molecule determined by GO 2 (g)  2GH 2 O (l)  2GH 2  4.92 (eV) .

state DFT was The

free energy for gas-phase water was calculated at 0.035 bars because this is the equilibrium pressure in contact with liquid water at 298 K. The free energy of gas phase water at these conditions is equal to the free energy of liquid water. The reaction free energy of (1)-(4) for the ORR can be determined by the following equations: ΔG1  μOOH *  μOH   μ H 2O  μ*  μO2  μe  ΔGOOH * (11)  4.92

ΔG2  μO *  μOH   μOOH *  μe   ΔGO*  ΔGOOH * (12) ΔG3  μOH *  μOH   μO *  μH 2O  μe   ΔGOH *  ΔGO * (13)

ΔG4  μOH   μ*  μOH *  μe    ΔGOH *

(14)

For the ORR, the onset potential is calculated by, Uonset= -max{ 𝛥𝐺1, 𝛥𝐺2, 𝛥𝐺3, 𝛥𝐺4} (15) Note that the equilibrium reduction potential U0 under the alkaline condition (pH = 13) is 0.46 V versus SHE, following the 4e- pathway. 55, 56

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal. the electrochemical impedance spectra and the LSV curves of NG and ANG at various rotation speeds, the effect of CB on the ORR activity of the catalysts, the LSV curves of ANG doped at various temperature and by different precursors at the rotation speed of 1600 rpm, the electrocatalytic ORR performance of ANG in acidic medium, the digital photograph of Zn-air batteries based on ANG catalyst, the optimized ANG configuration by DFT calculations (PDF)

AUTHOR INFORMATION

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Corresponding Author *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected]

ORCID Yong Qin: 0000-0003-4563-8828 Hong-Hui Wu: 0000-0002-1381-2281 Yong Kong: 0000-0003-1270-3232 Yunfei Bu: 0000-0002-1488-5925 Qiaobao Zhang: 0000-0002-3584-5201 Dong Ding: 0000-0002-6921-4504 Meilin Liu: 0000-0002-6188-2372 Xiao Cheng Zeng: 0000-0003-4672-8585

Author Contributions ∇ Y. Q., H. W., and L. Z. contributed equally to the work.

Y.Q. wrote the paper and X.C.Z. revised it, H.W. and L.Z. did the theory simulation. X.C.Z. and M.L led the project. All the other authors contributed to perform experiments (including sample fabrication, characterization, and measurement), data analysis, and extensively discussion.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is financially supported by the Natural Science Foundation of Jiangsu Province, China (BK20161191), the National Natural Science Foundation of China (21476031, 21673024, 21703185), the Project of Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Changzhou University, the Advanced Catalysis and Green Manufacturing Collaborative Innovation Center of Jiangsu Province, the National Key R&D Program of China (Grant No.2018YFB0905400). The computational work used Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number TG-DMR140083, and National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DEAC02-05CH11231. HW and XCZ acknowledge the computing support by UNL Holland Computing Center.

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(54) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108, 17886-17892. (55) Ge, X.; Sumboja, A.; Wu, D.; An, T.; Li, B.; Goh, F. T.; Hor, T. A., Zong, Y.; Liu, Z. Oxygen Reduction in Alkaline Media: from Mechanisms to Recent Advances of Catalysts. ACS Catal. 2015, 5, 4643-4667. (56) Pei, Z.; Gu, J.; Wang, Y.; Tang, Z.; Liu, Z.; Huang, Y.; Huang Y.; Zhao, J.; Chen, Z.; Zhi, C. Component Matters: Paving the Roadmap toward Enhanced Electrocatalytic Performance of Graphitic C3N4-Based Catalysts via Atomic Tuning. ACS Nano 2017, 11, 6004-6014.

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