N Co-Doped Graphene-Like

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Facile synthesis of defect-rich and S/N co-doped graphene-like carbon nanosheets as an efficient electrocatalyst for primary and all-solid-state Zn–air batteries Jian Zhang, Huang Zhou, Jiawei Zhu, Pei Hu, Chao Hang, Jinlong Yang, Tao Peng, Shichun Mu, and Yunhui Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04665 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 5, 2017

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ACS Applied Materials & Interfaces

Facile Synthesis of Defect-rich and S/N Co-doped Graphene-like Carbon Nanosheets as an Efficient Electrocatalyst for Primary and All-solid-state Zn–air Batteries

Jian Zhang,† Huang Zhou,‡ Jiawei Zhu,‡ Pei Hu,† Chao Hang,† Jinlong Yang,§ Tao Peng,⊥ Shichun Mu*,‡ Yunhui Huang,*,†

†

State Key Laboratory of Material Processing and Die & Mould Technology, School

of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China ‡

State Key Laboratory of Advanced Technology for Materials Synthesis and

Processing, Wuhan University of Technology, Wuhan 430070, PR China §

Peking University, Shenzhen Graduate School University, Shenzhen 518055, PR

China ⊥

Department of Civil and Environmental Engineering, University of Windsor,

Windsor N9B 3P4, Canada

ABSTRACT: Developing a facile and low-cost of porous graphene-based catalysts for highly efficient oxygen reduction reaction (ORR) remains an important matter for fuel cells. Here, a defect enriched and dual heteroatoms (S and N) doped hierarchically porous graphene-like carbon nanomaterial (D-S/N-GLC) was prepared by a simple and scale strategy, and exhibits an outperformed ORR activity and stability as compared to 1

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commercial Pt/C catalyst in an alkaline condition (its half-wave potential is nearly 24 mV positive than Pt/C). The excellent ORR performance of the catalyst can be attributed to the synergistic effect, which integrates the novel graphene-like architectures, 3D hierarchically porous structure, superhigh surface area, high content of active dopants and abundant defective sites in D-S/N-GLC. As a result, the developed catalysts are used as the air electrode for primary and all-solid-state Zn–air batteries. The primary batteries demonstrate a higher peak power density of 252 mW cm−2 and high voltage of 1.32 and 1.24 V at the discharge current densities of 5 and 20 mA cm-2, respectively.. Remarkably, the all-solid-state battery also exhibits a high peak power density of 81 mW cm−2 with good discharge performance. Moreover, such catalyst possesses a comparable ORR activity and higher stability than Pt/C in acidic condition. The present work not only provides a facile but cost-efficient strategy towards preparation of graphene-based materials, but also inspires an idea for promoting the elctrocatalytic activity of carbon-based materials.

KEYWORDS: Fuel cell, Zn-air battery, Oxygen reduction, Electrocatalyst, Graphene-like

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INTRODUCTION The oxygen reduction reaction (ORR) in the cathode is recognized as a kinetically limited step in fuel cells because of its sluggish reaction mechanism.1-3 Platinum (Pt) and its alloys have long been considered as the state-of-the-art catalyst for ORR; however, the prohibitive cost, limited resources, susceptibility to fuel (e.g. methanol and carbon monoxide), and poor durability have greatly impeded their large-scale commercialization.3-5 Accordingly, the ongoing search for high efficiency, inexpensive and earth-abundant ORR catalyst to substitute Pt-based catalysts has aroused great interest. To date, heteroatom-doped graphene-based materials have been regarded as the most promising catalyst for ORR owing to their novel two-dimensional plane structure, high surface area, good conductivity and strength, durability, and fuel tolerance immunity2, 6-9. In real applications, however, such graphene nanosheets are pregnable to stacking and aggregating due to inherent π–π restacking behaviors,6, 10 which greatly decrease the specific surface area and porosity, and cause a considerable loss of effective active sites and inferior ionic accessibility, resulting in the decreasing overall properties of the electrocatalysts. To conquer this problem, many researchers have developed three-dimensional (3D) graphene with porous architectures, which can efficiently prevent stacking, increase the porosity, and expose more active sites, facilitating a faster transportation of reactant species in the triple-phase boundary for ORR.6-8, 11-12 In spite of the good ORR activity, they have normally suffered a high cost, time-consuming and complex synthesis procedures

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such as sacrificial template method, freeze-drying, as well as the long time post treatment, for the fabrication process of 3D porous graphene.6, 8, 13-15 which seriously limited the large scale preparation in practical production. Fortunately, recent studies have shown that graphene-like carbon materials can efficiently overcome the inherent π–π restacking, leading to high surface area with excellent porosity, and simultaneously keeping a similar property of graphene such as high electronic

conductivity and superior

chemical

stability.2,

16-18

Thus,

heteroatom-doped graphene-like materials have good ORR performance, which can be comparable to that of graphene-based material. Besides, the latest experimental and theoretical calculation results demonstrate that the defective site in carbon nanomaterial play a key role in ORR.19-22 Dai et al. has been proved that the defected graphite is much more active for ORR than pristine graphite due to the charge polarization of defective carbon atoms.23 Meanwhile, benefiting from its abundant defective sites, it is reported that even dopant-free nanocarbon materials can exhibit good ORR performance.19-20,

24

The theoretical

calculations are also suggested that some of defective sites (e.g., vacancies, voids, Stone–Wales defects) are more active than the heteroatom-doping for the ORR.24 Additionally, due to the synergistic reaction mechanism, the dual heteroatoms-doped carbon nanomaterials, such as N combined with B, S or P, can deliver higher ORR activity than single N doping.1, 19, 21, 25-28 Thus, in order to gain the optimum ORR catalyst, the design of dual heteroatoms-doped carbon nanomaterials with abundant defects becomes the promising strategy.

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In light of this, we develop a facile and cost-efficient strategy for construction of defect enriched and S/N dual heteroatom doped graphene-like nanocarbon electrocatalyst (D-S/N-GLC) using cystine as precursor via KOH activation and ammonia injection at high temperature. The as-prepared nanocarbon material not only owns a lamellar graphene-like morphology, with excellent hierarchically porous structure distribution from micro-, meso-, to macropore, and high specific surface area, but also possesses a high content of active dopant species as well as abundant defective sites. These properties and merits endow the material displays an excellent catalytic activity, stability and methanol immunity for ORR in both alkaline and acidic conditions. When used in Zn-air battery as cathode electrocatalyst, it displays a higher discharge performance than the benchmark Pt/C catalyst.

EXPERIMENTAL SECTION Electrocatalyst Synthesis. The sulfur and nitrogen-containing amino acid (cystine) was first pre-carbonized at 300 °C (0.5 h) in Ar2 atmosphere. The pre-carbonized sample were then permeated with 0.5 M KOH electrolytes (mass ratio of the pretreated sample: KOH= 1:2) under magnetic stirring for 2 h. After thoroughly drying, the mixture was heat treated at 800 °C for 30 min in Ar2 atmosphere. To remove the residual KOH and unstable species, the sample was washed extensively for 12 h with 0.5 M H2SO4 at 80 °C. After washing and drying, the product was subjected a further treatments including high temperature graphitization (2 h) and subsequent ammonia injection (15 min) at the same temperature to obtain D-S/N-GLC catalyst. For comparison, we also prepared S/N dual doped bulk carbon

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(S/N-BC) under the same conditions except for absence of KOH activation process. Characterization. The crystalline structure of the materials was analyzed by X-ray diffractometer (XRD) between 10°and 80°with Cu Kα radiation. The morphology of the samples was analyzed with field-emission scanning electron microscope (SEM) and high-resolution transmission electron microscope (TEM). The carbon structure and graphitization degree of the samples were performed by Raman measurements with the laser wavelength of 514.5 nm (LabRAM Aramis). The specific surface area and pore size distribution plot of the materials were calculated from the nitrogen adsorption isothems by using Brunauer−Emmett−Teller (BET) equation and Barrett−Joyner−Halenda model, respectively. The surface properties and composition of the materials were investigated by X-ray photoelectron spectroscopy (XPS, VG-Multi-lab2000). Electrochemical Measurements. All the catalysts were evaluated at room temperature on an electrochemical workstation (CHI660E) with a three-electrode system. A Pt foil, saturated calomel electrode (SCE) and glassy carbon disk (5 mm in diameter) were employed as counter, reference and working electrode, respectively. Experimentally, 5.0 mg of the prepared catalyst was mixed with ethanol (0.475 mL) and Nafion electrolyte (25 μL, 5 wt%, DuPont) to form a homogenous ink. The prepared ink (8 μL) was then coated on working electrode and dried in the air condition. As a benchmark, the state of the art of commercial Pt/C (Johnson Matthey, 20%) was coated on working electrode in a same way. For ORR measurements, cyclic voltammetry (CV) curves were tested in a nitrogen or oxygen saturated 0.1 M KOH

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electrolyte with a scan rate of 20 mV s-1. Linear sweep voltammetry (LSV) curves were measured in an oxygen saturated 0.1 M KOH electrolyte by using rotating disk electrode (RDE) technique with a sweep speed of 10 mV s-1. In acidic electrolyte, the ORR activity of D-S/N-GLC and Pt/C catalysts was tested in an oxygen saturated 0.1 M HClO4 from 0.4 to 1 V (vs. RHE) with a sweep speed of 10 mV s-1. The current versus time (i–t) chronoamperometric response was performed to test the stability of the catalyst. For the primary Zn–air battery, briefly, 10 mg catalyst and 40 μL 5 wt% Nafion solution were dispersed in 1 mL ethanol by sonication for 1 h. The obtained catalyst ink was sprayed onto carbon paper (Toray TGP-H-090) to form the air cathode (the exposed active area was 1 cm2 ), and then dried in an oven 80 °C for 2 h. The catalyst mass loading was 0.6 mg cm−2. A polished Zn plate was employed as the anode. These two electrodes and 6 M KOH aqueous electrolyte were assembled into the home-made Zn–air battery. For all-solid-state zinc–air battery, the battery is fabricated by laminating an Tokuyama anion-exchange membrane (A901) as the electrolyte between an air electrode (made of the above D-S/N-GLC catalyst loaded on carbon paper) and a polished zinc foil (0.03 mm thickness) as the anode. The assembled device was then pressed under a pressure of three MPa for one minute to enhance the integrity of the laminated structure.

RESULTS AND DISCUSSION The fabrication schematic of D-S/N-GLC is illustrated in Figure 1. The raw material of cystine contains abundant C, S and N atoms, which is most suitable as

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precursor to prepare S/N dual doped carbon material. At the pre-carbonized process, the cystine molecular will cross-link and gather together, then becomes a fluffy sponge-like biochar with many macroscopic pores (Figure S1). This porous property is beneficial to uniformly mix it with KOH solution and to facilitate KOH completely permeate the internal pore. At the second step, this porous biochar will fully react with KOH as required the overall reaction equation (1)29 6KOH + 2C ↔ 2K + 3H2↑ + 2K2CO3

(1)

As indicated in equation (1), the reaction will produce metallic potassium (K) and K compounds. The metallic K can be embedded into the carbon-based materials, cross and diffuse between the carbon layers, leading to the formation of ultra-thin lamellar carbon nanosheets.16, 30 On the other hand, the metallic K acts as the catalyst to induce the crystallization and recombination of these carbon nanosheets, leading to the production of similar graphene nanosheets.29 Meanwhile, with the elevated temperature, the fierce reaction and thermal decomposition of carbon-based materials constantly and repeatedly generate plenty of gases such as H2, H2O, CO, CO2, SOx, NOx. These gaseous phases are gathered together, and cross into the intermediate product, then overflow from the carbon matrix, resulting in the formation of 3D hierarchically porous graphene-like structure. In addition, at these violent reactions, a large number of edge and defective sites can be created in the product. After further high temperature treatments, the carbonaceous decomposition would further enlarge the pore size of macropores and mesopores, and simultaneously remove the most of oxygen-containing groups (no active for ORR). And then, the ammonia injection

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could create more micropores in the product.11,

29

Thus, the as-prepared material

displays a 3D hierarchically porous graphene-like structure with abundant defective sites. The morphology of this porous graphene-like nanomaterial were investigated by SEM and TEM. As revealed from SEM image of Figure 2a, D-S/N-GLC exhibits a typical lamellar nanosheet structure. These carbon nanosheets are irregularly inter-connected to form a series of porous strcture with the size ranging from dozens to hundreds of nanometers. TEM images (Figure 2b and 2c) also prove that numerous nanopores (including macropores and mesopores) present in D-S/N-GLC, which are cross-linked together to form a novel 3D hierarchically porous structure. Noteworthily, TEM image shows that these carbon nanosheets have sub-transparent and wrinkled lamellar properties, as similar as the morphology of graphene.8, 10, 14-15 As shown in Figure S2, the more such of hierarchically porous architectures and sub-transparent wrinkled graphene-like carbon nanosheets in D-S/N-GLC can be observed. Further, high-resolution TEM image (Figure 2d) displays that D-S/N-GLC is actually composed of few-layer graphene-like carbon nanosheets. Its lattice fringes are very short, and intricately stacked with each other. The selected-area electron diffraction (SAED) pattern of this graphene-like carbon reveals a typical ring-like mode (inset of Figure 2d). Meanwhile, the XRD pattern of D-S/N-GLC (Figure 3) also displays a broad and relatively low intensity of carbon diffraction peak (002). These characteristics indicate that a larger number of defective sites exist in the D-S/N-GLC,20, 22 which can offer abundant electrocatalytic active sites for ORR. It is

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worth noting that the carbon diffraction peak (002) of D-S/N-GLC is at ca. 21.8°, which shifts to smaller angles about 4.8°as compared with that of graphite at 26.6° (Figure S3). Thus, a larger lattice spacing (0.39-0.40 nm) can be observed in D-S/N-GLC (Figure S4), which is mainly attributed to intercalation impact of K ions. But for the sample without KOH activation (S/N-BC), both of SEM and TEM image (Figure 2e and 2f) display a typical bulk carbon morphology. This indicates that KOH not only serves as a pore-forming agent to form the 3D hierarchically porous structure, but also plays a key role in the creation of this unique graphene-like architectures. The specific surface area and pore size structure of D-S/N-GLC and S/N-BC were investigated by N2 adsorption-desorption isotherm measurements. As described in Figure 3a, D-S/N-GLC displays integrated characteristics of type I and IV sorption isotherms, indicating the existence of hierarchically porous structure spanning from micro- to macropores.13, 31 The pore size distribution (Figure 3b) further demonstrates the hierarchically porous architectures. For S/N-BC, it reveals a typical type I sorption isotherms, suggesting it only contains micropores, as conformed by pore size distribution (Figure 3b). According to calculation results, the BET surface area and pore volume of D-S/N-GLC are 1309 m2 g-1 and 0.86 cm3 g-1, respectively, more than twice as much as S/N-BC (BET surface area: 562 m2 g-1, pore volume: 0.29 cm3 g-1). Notably, owing to its inter-connected hierarchical porous graphene-like structure, the specific surface area is extremely higher than that of previously reported graphene-based materials and other carbon based aerogels, foams and frameworks.6, 8, 10, 13, 32

The high surface area can provide more active sites for oxygen reduction, and

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the hierarchical porous structure can more efficiently participate in the creation of triple-phase boundary and facilitate the movement of the ORR reaction species with the electrolyte, enhancing the interactions with catalytically active sites in ORR.1, 8, 11, 27, 33-34

The carbon structure of the catalysts was probed by Raman spectra. Generally, the relative intensity ratio of the D peak at ca. 1345 cm−1 to the G peak at ca. 1575 cm−1 (ID/IG) can be used to estimate the defect for carbon based material.35 As shown in Figure 3c, D-S/N-GLC displays a relative higher ID/IG value (around at 1.09) in comparison with S/N-BC (ID/IG= 1.03). Meanwhile, both of D and G peak features of D-S/N-GLC are broad and smooth, in contrast, the peak features of S/N-BC are narrow and sharp. These results indicate that D-S/N-GLC has more defective sites than S/N-BC.19-20, 36 As reported, the intrinsic defects can modulate the local band structure and tailor the electron charge distribution for carbon nanomaterial, resulting in the decrease of the reaction free energy and the promotion of electron transfer for ORR.5, 13, 19-21, 23 In addition, it is noteworthy that D-S/N-GLC displays a typical 2D peak at ca. 2700 cm-1 (the characteristic peak of graphene-based material),6, 10, 13, 16 further proving that D-S/N-GLC has a graphene-like morphology. For S/N-BC, it keeps a relative smooth spectrum line in the same area, suggesting that it has no graphene-like structure. This phenomenon is in good accordance with TEM results. The surface elemental composition and chemical status of D-S/N-GLC and S/N-BC were performed by XPS analyses (Figure 3d). The survey scan results show that they contain a nearly equal amount of N (3.70 at. % for D-S/N-GLC, 3.49 at. %

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for S/N-BC) and S (0.20 at. % for D-S/N-GLC, 0.21 at. % for S/N-BC) content. Nevertheless, according to the fitted results of N1s (Figure S5 and 3e), D-S/N-GLC contains a relative larger proportion of active edge N atoms (pyridinic-N and pyrrolic-N), in contrast, S/N-BC has a higher proportion of basal plane N atom (graphitic-N). It is reported that edge N atoms (especially pyridinic-N) are mainly responsible for ORR, while other N atoms have little impact on the electrocatalytic activity for carbon-based catalysts.1, 3-4, 7, 11, 33, 35 For S2p, as shown in Figure S6 and 3f, there is no significant difference about the type of S atoms between them, which is probably due to their extremely low content. The ORR activity of D-S/N-GLC and S/N-BC were investigated by LSV curves in an oxygen saturated 0.1 M KOH aqueous electrolyte using RDE measurements. As comparison, the Pt/C catalyst was also measured under the same conditions. As shown in Figure 4a, S/N-BC displays a lowest ORR activity in terms of E0= 0.835 V and E1/2= 0.733 V, respectively. Such low ORR activity of S/N-BC may be due to its less defective sites, low surface area and poor porosity. Turn around look at D-S/N-GLC, both of E0 (0.953 V) and E1/2 (0.849 V) shift more positively than those of S/N-BC, and even shift 5 and 24 mV more positively than those of Pt/C (E0= 0.948 V, E1/2= 0.825 V), respectively. CV curves (Figure 4b) also describe that the ORR peak potential of D-S/N-GLC (0.858 V) is 15 mV positive than Pt/C (0.843 V), further indicates its good electrocatalytic activity for ORR. To the best of our knowledge, our designed hierarchical porous graphene-like catalyst possesses the best ORR activity among the previously reported graphene-based and metal-free catalysts,1, 6-8, 10-11, 14,

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19-20, 22, 26-27, 35

as determined by the difference value (ΔE0) of onset potentials with

benchmark Pt/C (Figure 4c). Furthermore, to evaluate the S doping effect on electrocatalytic activity, the control experiment on the defect-enriched and single N-doped graphene-like carbon nanomaterial (denoted as D-N-GLC) was fabricated by using S-free amino acid (alanine) as the precursor via the same preparation process. As displayed in Figure S7a, D-N-GLC shows a morphology of ultrathin graphene-like carbon nanosheet with some irregular lattice fringes in the skeleton of carbon, which is similar to D-S/N-GLC. Raman spectrum (Figure S7b) shows that the ID/IG ratio is up to 1.10, suggesting abundant defective sites in D-N-GLC. Additionally, D-N-GLC has a high BET surface area (1257 m2 g-1, Figure S7c), abundant porosity (pore volume: 0.64 cm3 g-1, Figure S7d), and high proportion of active edged N species (Figure S7e-g). This phenomenon demonstrates that our strategy is a general way to fabricate graphene-like nanocarbon materials with high surface area and advanced porous structure. Figure S8 shows the LSV curves for D-N-GLC and D-S/N-GLC catalysts. It can be seen that the ORR activity of D-N-GLC is slightly lower than D-S/N-GLC catalyst, indicating that S doping can enhance the ORR activity for N-doped carbon materials. As reported, the introduction of S atoms can easily induce the polarization within the surrounding carbon atoms, which plays a synergic role with the doped N atoms, and thus boosts the ORR activity.7, 26, 27 In addition, we also prepared S and N co-doped graphene nanosheets sample (S/N-GNS) by annealing the graphene oxide/sulfur mixture and following ammonia

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injection. By comparison, the ORR activity of S/N-GNS is much inferior to D-S/N-GLC catalyst in terms of their LSV measurements (Figure S9). This can be attributed to the following two reasons. On one hand, as revealed in Figure S10a and S10b, S/N-GNS presents a relatively smooth surface structure (lack of defective sites),8, 13, 14 which cannot offer the efficient and accessible active sites for ORR. On the other hand, for S/N-GNS these graphene nanosheets suffer from a seriously irreversible stacking and aggregation, which leads to low BET surface area and poor porosity (Figure S10c and S10d), resulting in the compromise of the ORR activity. On the basic of the above results, the excellent electrocatalytic activity of D-S/N-GLC for ORR can be attributed to the synergistic promotion effect of the multiple factors. First, the lamellared graphene-like structure in D-S/N-GLC, owing to its similar properties to graphene, such as high electronic conductivity and superior chemical stability, is in favor of ORR. In addition, compared to traditional graphene nanosheets, such graphene-like structure can efficiently inhibit the stack and aggregation between the layers, resulting in high surface area and porosity. This could expose abundant extra catalytic sites to improve the mass activity density. Third, D-S/N-GLC possesses an open 3D inter-connected hierarchical porous network, thus improving the mass transfer to the active sites and significantly promoting the ORR diffusion kinetics. Additionally, the relative high content of edge N doped species (especially pyridinic-N) could result in the redistribution of charge density in the carbon framework, which is usually considered as the active sites for the ORR.19, 21 The S doping can further induce the polarization and bring high spin density into the

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carbon and thus improve the ORR activity.7, 26, 27 Most importantly, the abundant intrinsic defects in D-S/N-GLC can significantly modify the charge and spin distribution in the sp2-conjugated carbon matrix, lead to highly efficient adsorption of oxygen and intermediates, and facilitate the electron transfer, which is analogous to the heteroatoms doping.21 Moreover, the recent theoretical calculations and experiments demonstrate that the dangling bonds located at the edge of sp2 nano-carbon are regarded as high energy sites and catalytic centers for oxygen and evolution reactions.19-22, 24 Thus, it is concluded that both heteroatom dopants and defective sites are responsible for the ORR activity. Therefore, the highly efficient catalytic activity of D-S/N-GLC for ORR is contributed to the synthetic protocol of novel graphene-like architectures, superhigh surface area, 3D hierarchical porous structure, and the presence of high-content active heteroatom-doped species as well as abundant intrinsic defective sites. To investigate the electron transfer of D-S/N-GLC in the catalytic ORR, LSV at different rotation speeds were performed (Figure 4d). According to the data in Figure 4d, the corresponded five Koutecky–Levich (K–L, Figure 4e)10, 14 plots display a good characteristics of parallel lines at the potential range from 0.4 to 0.8 V. By calculating the slopes of the K-L plots (Equation S1), the average electron transfer number (n) of D-S/N-GLC is 3.96, corresponding to a 4e ORR pathway, which is similar to Pt/C catalyst (Figure S11). To further confirm the conclusion obtained from the above K-L plots, we carried out the measurement by rotating ring-disk electrode (RRDE) (Figure S12a and S12b) to accurately determine the yield of hydrogen

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peroxide (H2O2) and n values from the disk and ring currents during the ORR process. According to Equation S2, the measured H2O2 yields and n values for D-S/N-GLC are 1.37-8.26% and 3.79-3.96 over the potential range of 0.2-0.9 V (Figure S12c), respectively, which are very close to those of the Pt/C catalyst (0.36-5.12% and 3.82-3.98, Figure S12d). The low H2O2 production and four-electron pathway indicate the high efficiency of ORR on D-S/N-GLC, which agrees well with the result obtained from the K-L plots based on RDE measurements. Except for good ORR activity, stability of the electrocatalyst should be further regarded.2, 12, 37 Figure 4f shows the long term i–t chronoamperometric response of the D-S/N-GLC and Pt/C catslysts at 0.7 V. After long time test of 10,000 s, D-S/N-GLC only displays a slight loss (9%) in the current density, while Pt/C exhibits a significant current density loss (16%). This reveals the stability of D-S/N-GLC is superior to that of Pt/C. Moreover, in light of practical applications in fuel cells, fuel molecules (e.g., methanol) poisoning should be also considered.8, 27, 31-32, 38 The D-S/N-GLC catalyst was therefore subjected to further testing in the presence of methanol both in alkaline and acidic electrolytes by using i–t response measurements. As shown in Figure S13a, after adding methanol to the 0.1 M KOH electrolyte, D-S/N-GLC retains a stable current response (99%, after 150 s), whereas the current response for Pt/C catalyst is instantaneously declined (60%, after 150 s). The similar phenomenon can be observed in the acidic condition (Figure S13b). These results indicate our prepared catalyst has an excellent methanol immunity both in alkaline and acidic electrolytes, suggesting it

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is suitable for ORR catalyst in fuel cells. Considering the excellent ORR performance of D-S/N-GLC, a homemade Zn–air battery device was fabricated with a zinc plate as the anode and D-S/N-GLC loaded on carbon cloth as the air cathode (Figure 5a). For comparison, the commercial Pt/C catalyst is also tested under the same conditions. As shown in Figure 5b, the assembled Zn–air battery using D-S/N-GLC as the air cathode keeps a smoothly and high open-circuit voltage of ca. 1.50V, which is higher than that of Pt/C electrode (1.48 V). Figure 5c shows the discharge polarization and power density curves of the zinc–air batteries. It is reveal that D-S/N-GLC cathode has the higher peak power density of 252 mW cm−2 compared to Pt/C (198 mW cm−2) due to the excellent ORR activity of D-S/N-GLC. The galvanostatic discharge curves (Figure 5d) display that the D-S/N-GLC cathode has a high voltages of 1.32 and 1.24 V at the discharge current densities of 5 and 20 mA cm-2, respectively, which is slightly higher than Pt/C catalyst (voltages of 1.30 and 1.22 V at 5 and 20 mA cm-2, respectively) and other reported graphene-based cathode catalysts.1, 10-11, 25 When two Zn–air batteries are connected in series, as exhibited in Figure 5e, they can drive ten parallel high-power red light-emitting diode (LED, 1W) lamp beads and keep a high brightness for a long time. Additionally, it can also power high voltage LED lamp beads (green, blue and white, 3.0-3.2 V), as shown in Figure 5f-h. Furthermore, we also fabricated an all-solid-state zinc-air battery by using D-S/N-GLC (loaded on the carbon fiber paper) as the cathode, a polished Zn foil as the anode, and the anion-exchange membrane as the solid electrolyte. The fabricated

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battery shows a high peak power density of 81 mW cm−2 (Figure 6a). This is primarily attributed to its similar properties of graphene and open 3D hierarchically porous architectures, which supplies a fast electronic transport channel and accessible reaction mass transfer in reaction boundary for oxygen reduction. When the battery is discharged at a current density of 5 mA cm−2 for 10, 000 s, as shown in Figure 6b, only slightly voltage drop can be observed owing to the excellent stability of D-S/N-GLC for ORR. These results further demonstrate that such graphene-like catalyst has a better electrocatalytic performance as well as rapid mass and charge transfer, which can be act as good alternative to precious Pt/C catalysts for ORR in pratical battery applications. Additionally, we also investigated the ORR activities of D-S/N-GLC in acidic condition (oxygen saturated 0.1 M HClO4). As described in Figure S14a, the E0 and E1/2 of D-S/N-GLC are 0.809 and 0.609 V, respectively, which approach to Pt/C , and also are comparable to most of the graphene based ORR catalysts.8,

13-15, 31-32

To

determine the electron transfer of D-S/N-GLC in acidic condition, ORR polarization curves of D-S/N-GLC at diﬀerent rotating speeds from 400 to 2000 rpm were measured (Figure S14b). On the basic of the K-L equation (Equation S1), five J−1 versus ω−1/2 K-L plots were obtained (Figure S14c). According to the slope of each line, the average electron transfer number of D-S/N-GLC is 3.74, indicating that the D-S/N-GLC electrode also mainly favors a 4e ORR process in the acidic condition. Figure S14d shows the i–t response of the D-S/N-GLC and Pt/C catalysts in an oxygen saturated 0.1 M HClO4 electrolyte. After tests of 10, 000 s, D-S/N-GLC also

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displays a higher current density of retention (89%) than that of Pt/C (83%). This result also implies that our prepared catalyst can be applied in both alkaline and acidic condition.

CONCLUSION In this work, we developed defect-rich and S/N dual doped 3D hierarchical porous graphene-like carbon nanomaterials (D-S/N-GLC) by employing cystine as precursor along with KOH activation and ammonia injection. The KOH activation plays a multiple role in the formation of graphene-like architectures, hierarchically porous structure, high surface area, as well as abundant defective sites in the carbon material. Thus, the obtained D-S/N-GLC catalyst gets the superb ORR performance in alkaline media, which is higher than the state-of-the-art commercial Pt/C catalyst, as described by both RDE technique and practical Zn-air battery measurements. It also displays a comparable ORR property than Pt/C in the acidic electrolyte. We believe that our developed facile and efficient strategy towards the preparation of graphene-based material is useful for energy conversion and storage applications.

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ASSOCIATED CONTENT Supporting Information Supplimentary materials include: Optical photograph of the biochar, TEM images, XRD patterns, Raman and XPS spectra, the fitted N1s and S2p results, Koutecky-Levich equation, and other supplemental data.

AUTHOR INFORMATION Corresponding Authors *Shichun Mu, E-mail: [email protected] *Yunhui Huang, E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially sponsored by the National Natural Science Foundation of China (51602113), and the China Postdoctoral Science Foundition (2016M590692). The authors wish to thank Xiaoqing Liu and Tingting Luo (Materials Analysis Center of Wuhan University of Technology) for TEM (JEM-2100F) measurement support.

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Figure captions Figure 1 The formation schematic of D-S/N-GLC. Figure 2 SEM and TEM images (a, b, c and d) of D-S/N-GLC, the inset of (b) is the SAED pattern of D-S/N-GLC. SEM and TEM images (e, f) of S/N-BC. Figure 3 (a) Nitrogen adsorption-desorption isotherms, (b) pore size distribution, (c) Raman and (d) survey XPS spectra for D-S/N-GLC and S/N-BC; The fitted results of (e) N1s and (f) S2p for D-S/N-GLC and S/N-BC. Figure 4 (a) LSV curves for D-S/N-GLC, S/N-BC and commercial Pt/C catalyst in oxygen saturated 0.1 M KOH, (b) CV curces for D-S/N-GLC and Pt/C catalyst in nitrogen or oxygen saturated 0.1 M KOH, (c) LSV curves for D-S/N-GLC at different rotating rates from 400 to 2000 rpm in oxygen saturated 0.1 M KOH, (d) K-L plots for D-S/N-GLC in the range from 0.4 to 0.8 V, (e) Comparison of ΔE0 among D-S/N-GLC and other reported related catalysts, as derieved from Table S1, (f) The i-t chronoamperometric responses of D-S/N-GLC and Pt/C in O2-saturated 0.1 M KOH during a constant potential at 0.7 V with rotating speed of 1,600 rpm. Figure 5 (a) Scheme of the Zn−air battery. (b) Open circuit voltage measurements and (c) polarization curves and power density plots and (d) discharge curves of Zn–air batteries fabricated with D-S/N-GLC and Pt/C catalysts. Photograph of (e) ten parallel red and (f-h) high voltage (green, blue and white, 3.0-3.2 V) LED lamp beads driven by two Zn−air batteries with the D-S/N-GLC electrode connected in series. Figure 6 (a) Polarization curve and power density plot and (b) long-time discharge curve of the all-solid-state Zn-air battery with D-S/N-GLC catalyst. TOC 27



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N Co-Doped Graphene-Like

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