Iron-Salt Thermally Emitted Strategy to Prepare Graphene-like Carbon

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Energy, Environmental, and Catalysis Applications

Iron-Salt-Thermally-Emitted Strategy to Prepare Graphenelike Carbon Nanosheets with Trapped Fe Species for Efficient Electrocatalytic Oxygen Reduction Reaction in All pH Range Chen-Chen Weng, Jin-Tao Ren, Hui Zhao, Zhong-Pan Hu, and Zhong-Yong Yuan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07604 • Publication Date (Web): 15 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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ACS Applied Materials & Interfaces

Iron-Salt-Thermally-Emitted Strategy to Prepare Graphene-like Carbon Nanosheets with Trapped Fe Species for Efficient Electrocatalytic Oxygen Reduction Reaction in All pH Range Chen-Chen Weng,a,b Jin-Tao Ren,a,b Hui Zhao,a,b Zhong-Pan Hu,a,b and Zhong-Yong Yuan a,b,* a

National Institute for Advanced Materials, School of Materials Science and Engineering, Nankai University, Tianjin 300350, China. b Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China * E-mail: [email protected] ABSTRACT Earth-abundant, highly active and durable electrocatalysts towards oxygen reduction reaction (ORR) in all pH range are highly required for practical application of electrochemical energy conversion technologies. Here, non-noble-metal graphene-like carbon nanosheets with trapped Fe species (Fe-N/GPC) are developed by an iron-salt-thermally-emitted strategy, which integrates the modulation of electronic structure for boosted intrinsic activity with the engineering of hierarchical porosity for enriched active sites. The ORR electrocatalytic performance of Fe-N/GPC-800 achieves the half-wave potential of 0.86 and 0.77 V with limiting current density of 6.1 and 4.7 mA cm-2 in 0.1 M KOH and 0.1 M PBS solution, respectively, as well as respectable stability. Furthermore, Fe-N/GPC-800 also shows considerable ORR catalytic activity in acid media accompanied with superior stability to those of Pt/C catalysts. The as-prepared Fe-N/GPC-800, as a cathodic catalyst, is assessed in Zn-air battery test and delivers an open-circuit voltage of 1.44 V with power density of 134 mW cm-2 as well as the outstanding durability after 350 cycles at 10 mA cm-2, demonstrating appreciable promise in application of metal−air batteries. This work provides an enabling and versatile strategy for facile and scale-up preparation of high-performance non-noble-metal electroncatalysts. 1 ACS Paragon Plus Environment

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KEYWORDS: oxygen reduction reaction, carbon nanosheets, electrocatalyst, nonprecious metal, volatile iron salt

1. INTRODUCTION The fuel cells and metal-air batteries have been increasingly considered as green and efficient energy power technologies due to its high energy density and sustainable property. However, major challenge hindering the large-scale application resides the sluggish kinetics of cathodic oxygen electroreduction process.1-3 In spite of the efficient catalytic activity of Pt and its alloys, the prohibitive price and limited stability enormously impede widespread applications.4 Thus, a variety of non-precious-metal oxygen reduction reaction (ORR) electrocatalysts obtained by earth-abundant transition metal (e.g., iron, nickel and cobalt) and metal-free materials (e.g., graphene, graphitic carbon nitride and boron nitride) have gained widespread attention for the alternatives to the Pt-based catalysts.5-10 Especially, since Dai has reported that Ndoped carbon nanotubes show efficient oxygen electroreduction performance in alkaline medium in 2009,11 N-doped carbon materials with unique electronic properties, large surface area, high durability and cost-effectivity are increasingly considered as efficient catalysts to realize the high electrocatalytic activity. The inherent electronic structures of carbon materials can be tuned by incorporation of N into carbon network, which affords carbon materials with catalytic functionality, thus presenting great promise. Besides N dopant, some types of carbon defect sites, in both experimental and theoretical studies,12-20 have been proved to contribute to improving electrocatalytic activity, which results from the fact that these defect sites have an impact on the local electronic structure. For instance, the edge sites of graphite with delocalized charge distribution show a higher reactivity towards ORR as compared 2 ACS Paragon Plus Environment

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with basal plane.18 Defective carbon nanocages without dopant have been reported to deliver high ORR activity, indicating that certain kind of defect has a critical effect on oxygen reduction, which is also revealed by the theoretical calculation.12 Moreover, when transition metal species is introduced into N-doped carbon materials, especially in a trace amount, the ORR electroactivity can be remarkably improved. Thus, the transition metal–nitrogen–carbon (M–N–C) materials have been accepted as an important branch of nonprecious metal electrocatalysts toward ORR.21-28 The emerging iron-nitrogen-carbon (Fe-N-C) materials have gained ever-growing interest as a promising alternative electrocatalyst for ORR process in alkaline media.2933

In spite of extensive attention and efforts, further improvement of ORR

electrocatalytic performance for Fe-N-C materials, especially in all pH range, is required for practical application. When Fe-N-C catalysts are rationally designed, two design criteria which are of significant importance to the electrocatalytic ORR performance, include how to enrich the active sites and how to promote the transfer of ORR-relative species. Note that Fe-containing active centres are proposed to be accommodated in the slit of small pores (width ≤2 nm) in carbon material, and the larger pores are believed to be benefit for the high-flux mass transportation. Toward this end, hierarchically porous architectures with large surface areas are highly pursued due to the high capability of small pores to host abundant active sites and rapid mass transfer of reactants afforded by large pores.34-37 Nevertheless, further attempts regarding the facile strategy which can simultaneously tailor electronic structure and hierarchical porosity should be conducted to improve the intrinsic reaction kinetics, enrich the available active sites, and reduce the charge transfer resistance, thus synergistically contributing to an improved performance of the designed catalyst.

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In this work, we report an iron-salt-thermally-emitted method to fabricate the spatially scaffolded graphene-like carbon nanosheets with trapped Fe species via the pyrolysis of polyaniline and ferric trichloride (FeCl3). FeCl3, a highly volatile metal salt, is employed to realize hierarchical porosity design to endow the Fe-N/GPC with a large quantity of micropores for accommodating the active sites and plentiful macropores for facilitating the high-flux mass transportation. At the same time, electronic structure is favourably tuned for boosting ORR activity. The as-prepared catalyst Fe-N/GPC-800 exhibits efficient catalytic performance in 0.1 M KOH and 0.1 M PBS solution in terms of positive E1/2 of 0.86 and 0.77 V, respectively, even superior to that of Pt/C catalysts (0.83 and 0.68 V), as well as great stability. Also, considerable ORR catalytic activity of Fe-N/GPC-800 in 0.5M H2SO4 solution is confirmed, together with the prominent stability. Furthermore, the Fe-N/GPC-800, as a cathode catalyst in Zn-air battery, shows good reversibility and operation durability.

2. RESULTS AND DOSCUSSION 2.1. Precursor-to-Catalyst Transition via Iron-Salt-Thermally-Emitted Method. The synthetic route of Fe-N/GPC-T is depicted in Scheme 1. Briefly, the excessive FeCl3·6H2O is deliberately incorporated to the polymerization procedure to produce the mixed precursor (denoted as FePANI-precursor) containing polyaniline and iron salt. As shown in SEM images of FePANI-precursor (Figure 1a,b), irregular particles with a size of several micrometer can be observed in FePANI-precursor and some particles are covered by wrinkled thin-film-like shell. The XRD pattern of FePANI-precursor (Figure S1) shows the main crystalline structures of iron chloride hydrate (FeCl3, JCPDS No. 01-0277) and rokuhnite (FeCl2, JCPDS No. 25-1040). Then the FePANI-precursor is subject to an iron-salt-thermallyemitted process, where both morphology and hierarchical porosity can be tailored. The


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obtained sample, denoted as MFeNC-T, is subsequently leached in hydrochloric acid for the removal of iron oxides compounds, thus producing the final catalyst Fe-N/GPC-T. Here, excessive FeCl3·6H2O crystal acts as a key ingredient to tailor the nanoarchitecture of Fe-N/GPC-T through its accessible volatility to afford large volume of the gas during pyrolysis. Briefly, FeCl3·6H2O crystal is employed as a versatile pore-forming agent for both hierarchical pore construction and morphogenesis. The temperature of pyrolysis process is high enough for FeCl3 to volatize (boiling point, 316 oC). As FeCl3 crystal gradually volatizes, the released gas from volatilization of FeCl3 infiltrates into the carbon matrix derived from polyaniline and subsequently blows carbon matrix into numerous bubbles, which effectively detaches carbon matrix during thermal treatment, leading to the carbon sheets with multidirection channels rather than thick carbon bulks. The carbon matrix is gradually expanded and the walls becomes thinner and thinner due to the combined contribution of the released gaseous FeCl3 and surface tension of the carbon matrix, yielding loose structure consisting of graphene-like carbon nanosheets. At the same time, the carbon matrix is completely exposed to the FeCl3 gas, which gives rise to uniform and full deposition of Fe species on carbon matrix, thus enhancing the fraction of Fe species in Fe-N/GPC. As the temperature rises, FeCl2 crystal begins to melt (melting point, 677 oC). The restacking of carbon sheets could be prevented due to the flux of melted FeCl2, thus maintaining the loose structure. For the sample of MFeNC-800, the open frameworks with channels and pores in the range of several micrometres can be seen in Figure 1c. After leaching in HCl solution, as shown in Figure 1d Fe-N/GPC-800 has a two-dimensional loose morphology comprised by interconnected porous carbon nanosheets. A close observation of sheet-like morphology of Fe-N/GPC-800 by TEM in Figure 1e reveals that the characteristics of graphene-like crinkly and laminated carbon structure without aggregation or restack is achieved by taking 5 ACS Paragon Plus Environment


advantage of the blowing effect of released gaseous FeCl3, and no obvious metal-containing nanoparticles can be seen, indicating that Fe might be evenly distributed and incorporated into the carbon framework. The absence of lattice fringes in HRTEM image (Figure 1f) suggests the disordered and amorphous structure of Fe-N/GPC-800 with poor graphitization, and a high density of micropores in Fe-N/GPC-800 are also observed. Elemental mapping images in Figure 1g verify that N and Fe are evenly distributed over Fe-N/GPC-800. Both MFeNC-800 and Fe-N/GPC-800 exhibit the combined isotherms of type I and type II, suggesting a microporous and macroporous structure (Figure S2a). Compared with MFeNC-800, the sample of Fe-N/GPC-800, subject to the removal of Fe-containing compounds by HClleaching, possesses a high density of micropores as suggested by increased adsorption at low pressure in isotherms, which is in accordance with corresponding pore distribution (Figure S2b). Hysteresis loop in the isotherms of Fe-N/GPC-800 is found to be the H3 type, which is indicative of slit- or plate-like pores.22 The Fe-N/GPC-800 possesses a larger surface area of (1002 m2/g) than that of MFeNC-800 (282 m2/g), which is favour of the improvement of ORR performance.38 The active sites have been speculated to locate inside or around micropores of carbon matrix. Hence, abundant micropores are conductive to large number of active sites. While both FeNC and NC, which are prepared by carbonizing the polyaniline initiated by stoichiometric FeCl3 crystal and ammonium persulfate, respectively, exhibit a monolithic morphology with the absence of hierarchical pores (Figure S2c and Figure S3), suggesting that the iron-salt-thermally-emitted method is responsible for the formation of twodimensional loose sheet-like morphology with hierarchical pores. 2.2. Pyrolysis Temperature-Dependent Structure characterization. Fe-N/GPC-Ts with different crystalline structure and hierarchical porosity are synthesized by pyrolyzing at 700, 800, 900, 1000 and 1100 °C. The XRD patterns of Fe-N/GPC-Ts are recorded in Figure 2a, and it can be seen that the Fe-N/GPC-700, Fe-N/GPC-800 ， Fe-N/GPC-900 and Fe6 ACS Paragon Plus Environment

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N/GPC-1000 show broad diffraction peak at approximately 24o, which corresponds to the (002) plane of carbon, signifying a poor graphitization. As pyrolysis temperature further elevating to 1100 °C, an obvious incrase of the diffractions peaks at 2θ = 24o and 43o, which are ascribed to (002) and (101) plane of carbon respectively, are observed over the FeN/GPC-1100, indicating an increase of graphitization. Apart from diffraction peaks of carbon, the diffraction peaks of Fe2O3 (JCPDS No. 33-0664) in Fe-N/GPC-1100 are also observed (Figure S4). The iron oxide is not completely removed by HCl-leaching probably due to the protective graphited carbon layer formed in high-temperature pyrolysis process. Besides, the intensive scatter in low-angle scattering region for all the Fe-N/GPC-Ts are obviously noted, which is attributed to the presence of micropores in large quantities.39 Further insights into the carbon structure in Fe-N/GPC-Ts were investigated by Raman spectroscopy (Figure 2b and Figure S5). All the Fe-N/GPC-Ts show the D-band (1350 cm-1) and G-band (1580 cm-1). The deconvoluted Raman spectra of Fe-N/GPC-Ts show four bands including D1, D3, D4 and G band. The D1 band accounts for the small crystallite sizes, grains, or edge plane defects of graphite domains. The D3 band corresponds to amorphous carbon, and the D4 band represents the polyene-like structures or ionic impurities. The defect-free sp2 carbon networks are reflected by G band.40 The ID1/IG ratio of Fe-N/GPC-Ts varies with the annealing temperature: 1.81 for Fe-N/GPC-700, 2.40 for Fe-N/GPC-800, 2.31 for Fe-N/GPC900, 2.12 for Fe-N/GPC-1000, 1.69 for Fe-N/GPC-1100. That is, the sample of Fe-N/GPC800 exhibits the highest ID1/IG ratio, which indicates the largest defect domain size of graphite in Fe-N/GPC-800. From XPS survey spectra of Fe-N/GPC-Ts (Figure S6), one can see that the N content monotonically declines as the pyrolysis temperature rising from 700 to 1100 oC (Table S1), which may be due to the fact that some N atoms could be released at high temperature treatment. The carbon materials are considered to possess two main types of carbon atoms, 7 ACS Paragon Plus Environment


including trigonal bonded carbon atoms (sp2 carbon) and tetrahedral bonded carbon atoms (sp3 carbon). The sp3 carbon is considered to reflect defect level. The C1s peak of Fe-N/GPCTs (Figure 3a) are asymmetric and the deconvoluted spectra yields five peaks at 284.8, 285.9, 286.9, 287.7, and 289.5 eV, representing basal-plane carbon (sp2), defect carbon (sp3), C-O groups, C=O groups, and π–π* shake-up satellites, respectively. More trigonal bonded carbon centres with lower degree of defect are obtained after higher temperature pyrolysis, which is reflected by increased sp2/sp3 ratio from 3.45 (Fe-N/GPC-700) to 5.88 (Fe-N/GPC-1100). This is consistent with the observations from XRD and Raman results, which reveal that the high temperature pyrolysis can heal the carbon structure and promote graphitization, thus producing low degree of defect. The deconvoluted N 1s spectra (Figure 3b) shows four peaks including pyridinic N (398.5 eV), pyrrolic N (399.7 eV), graphitic N (401.3 eV) and oxidized N (403 eV), signifying that N atoms are indeed incorporated into carbonaceous skeletons. Moreover, the contents of the different nitrogen species are determined based on the integration of peak areas and shown in Table S1. The increasing pyrolysis temperature results in the decline of the total N content from 6.07% to 0.98%. In particular, both pyridinic N and graphitic N show declined relative content from 17% to 3% and from 66% to 43%, respectively. The pyridinic N and graphitic N species are believed to benefit the oxygen electroreduction process.36,38 The O 1s XPS spectrum shows that a peak centred at 531.6 eV represents carboxyls (COO−) and the other peak at around 532.6 eV is assigned to the hydroxyl (C-OH) and carbonyl (C=O) group (Figure 3c). There is no obvious O species associated with metal oxide centred at 529 - 530 eV, inferring the absence of Fe oxides in the prepared Fe-N/GPC-Ts. In addition, the Fe content of Fe-N/GPC-Ts (Table S1) are almost independent on the pyrolysis temperature, which is similar to previous reports.41 Note that for Fe-N/GPC-800, the Fe peak centers at 711.4 eV, which is upshifted by 0.6 eV as compared with standard spectrum of Fe2O3 sample (710.8 eV). This positive shift of binding energy 8 ACS Paragon Plus Environment

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could be assigned to the altered electronic structure of Fe atoms (Figure 3d). The similar upshifted binding energy can be seen in Fe-N/GPC-Ts, which are shown in Figure S7, indicating the altered electronic structure of Fe and the possibly boosted ORR electroactivities. The textual structure of Fe-N/GPC-Ts were assessed by the N2 adsorption−desorption measurement (Figure 3e). All the Fe-N/GPC-Ts exhibit combined type I and type II sorption isotherms with type H3 hysteresis loop, indicating that Fe-N/GPC-Ts possess disordered micro-/macroporous structure. The BET surface area increases with the elevated temperatures and the largest BET surface area (1904 m2/g) is achieved on the sample of Fe-N/GPC-1100. The corresponding pore size distributions of Fe-N/GPC-Ts (Figure S8) confirms high density of micropores. As summarized in Table S2, the micropore surface area and micropore volume increase with the increasing pyrolysis temperatures, suggesting that high temperature treatment is beneficial to promote the process in which the carbon matrix is activated to create micropores. It is observed that the number of mesopores raises and the pore size of micropore enlarges from Fe-N/GPC-700 to Fe-N/GPC-1100, which is due to the fact that higher temperature can smash the wall of micropores to create larger micropores and/or mesopores. The hierarchical porosity accompanied with surface area can be favourably tailored by pyrolysis temperature, which is beneficial for the further improvement of the electrocatalytic performance for the designed catalysts.42 2.3.

ORR

Electrocatalytic

performance

of

Fe-N/GPC-Ts.

The

oxygen

electroreduction activity of Fe-N/GPC-Ts in alkaline, neutral, acidic media were assessed by the rotating disk electrode (RDE) technique. The RDE voltammograms for Fe-N/GPC-Ts in 0.1 M KOH solution are shown in Figure 4a. For the Fe-N/GPC-800, when electrode potential scans positively and reaches 0.98 V (vs. RHE), nonzero cathodic current appears, which presents an onset potential as positive as 0.98 V. However, Fe-N/GPC-900 displays a little negative onset potential of 0.96 V, and Fe-N/GPC-700, Fe-N/GPC-1000 and Fe-N/GPC-1100 9 ACS Paragon Plus Environment


share a similar onset potential which is as negative as 0.93 V. Also, the Fe-N/GPC-Ts possess various limiting currents in diffusion-controlled region and the limiting current of Fe-N/GPC700, Fe-N/GPC-800, Fe-N/GPC-900, Fe-N/GPC-1000 and Fe-N/GPC-1100 at 0.3 V are 5.23, 6.14, 4.73, 5.59 and 3.37 mA cm-2, respectively. These observations signify that Fe-N/GPC800 stands out as the best electrocatalyst among Fe-N/GPC-Ts in alkaline medium. In addition, the onset potential (0.98 V) and half-wave potential (0.86 V) of Fe-N/GPC-800 reveal a more active ORR electrocatalytic activity than those of Pt/C (Sigma-Aldrich, 20 wt%, 0.96 and 0.83 V, respectively). To further assess the ORR electrocatalytic performance of FeN/GPC-800, CV polarization curves of Fe-N/GPC-800 and Pt/C were recorded in 0.1 M KOH solution (Figure 4b). No noticeable reduction peak can be seen in N2-saturated electrolyte. While the switch to O2-saturated electrolyte gives rise to a well-defined cathodic peak. The emergence of pronounced cathodic peak at 0.87 V, which is more positive than that of Pt/C (0.85 V), demonstrating the excellent oxygen electroreduction activity of Fe-N/GPC-800. The ORR kinetics of Fe-N/GPC-800 was evaluated by RDE experiments at various rotating rates. As displayed in Figure S9a, limiting diffusion current density of Fe-N/GPC-800 gradually rises with the increase of rotation speeds due to the reduced diffusion resistance under large rotating speed. The corresponding Koutecky-Levich (K-L) plots (Figure S9a inset) display well-defined linearity and parallelism, suggesting the first-order reaction kinetics toward the concentration of dissolved oxygen. It is calculated that the n ranges from 3.90 to 3.95 for Fe-N/GPC-800 from 0.3 to 0.6 V, suggesting a 4-electrons pathway during ORR process of Fe-N/GPC-800. Additionally, the formation of peroxide on the Fe-N/GPC800 and Pt/C were monitored by the rotating risk-ring electrode (RRDE) tests (Figure 4c). The H2O2 yield of Fe-N/GPC-800 is calculated to range from 4.8 % to 11.4 % from 0.1 to 0.8 V, accompanied with n ranging from 3.78 to 3.9, which surpasses to Pt/C (H2O2 yield: 8-13%, n: 3.78-3.83). 10 ACS Paragon Plus Environment

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Besides the activity, durability is also an important factor for ORR electrocatalysts. As depicted in Figure 4d, the Pt/C suffers a substantial current drop-off with about 77.8% retention after 20000 s of continuous operation, in contrast, the 90.2% of the original current is retained with Fe-N/GPC-800 under the same testing conditions, indicating an enhanced durability of Fe-N/GPC-800 electrocatalyst towards ORR process. The TEM images about Fe-N/GPC-800 after ORR durability test are supplied in Figure S10a,b. After ORR durability test, the used Fe-N/GPC-800 exhibits the folded lamellar structure which is similar to the fresh sample. Furthermore, the elemental mapping of iron images (Figure S10c,d) show that iron is uniformly distributed without aggregation after ORR durability test. The combination of both good structure stability and stable Fe species of Fe-N/GPC-800 could promote the long-time stability. Resistance to methanol crossover is another important parameter for electrocatalyst in fuel cells. As shown in Figure S11, the Pt/C exhibits an abrupt loss of current upon the injection of methanol, whereas the Fe-N/GPC-800 shows a slight change of current when methanol is added, suggesting strong methanol tolerance for Fe-N/GPC-800. In the aspect of acidic condition, Fe-N/GPC-800 is endowed with comparable ORR catalytic performance to that of Pt/C. It is illustrated from Figure 5a that the E1/2 of FeN/GPC-800 is 0.65 V, which is a bit negative when compared with of Pt/C (0.8 V). Note that the current density of Fe-N/GPC-800 is higher than that of Pt/C from 0.5 to 0.1 V, due to the fact that hierarchically porous open framework with large surface area is supposed to contribute to the large limiting current density in diffusion-control potential range.43-45 In Figure 5b, the Fe-N/GPC-800 displays a oxygen reduction peak at 0.70 V, approaching to Pt/C catalyst (0.76 V). The K-L plots of Fe-N/GPC-800 are shown in Figure S9b and the n is calculated to close to 4, signifying that Fe-N/GPC-800 follows a quasi-4-electron pathway during ORR process. Additionally, it is observed in Figure 5c that the yield of H2O2 and n of Fe-N/GPC-800 are approaching to those of Pt/C from 0.8 to 0 V, suggesting that the 11 ACS Paragon Plus Environment


electroactivity of Fe-N/GPC-800 towards ORR is comparable to Pt/C. Moreover, Fe-N/GPC800 exhibits a more enhanced durability than that of Pt/C, as demonstrated by a larger retention of the original current (89.3%) compared with Pt/C (70.7%) after 20000 s of continuous operation (Figure 5d). Besides, the highly-efficient ORR catalytic performance in neutral medium is also desired over the past decades due to the wide research and application in bioelectrochemical systems.46 In neutral medium (Figure 6a), Fe-N/GPC-800 exhibits an E1/2 of 0.77 V, superior to Pt/C catalyst (0.68 V). As displayed in CV curves, the oxygen reduction peak of FeN/GPC-800 locates at 0.74 V (Figure 6b). Furthermore, the Fe-N/GPC-800 shows a lower %HO2− yield as compared with Pt/C from 0 to 0.8 V, accompanied with the n of FeN/GPC-800 ranging from 3.98 to 3.85 (Figure 6c), confirming a prominent ORR activity in neutral medium. The stability test (Figure 6d) shows that the Pt/C catalyst suffers severe current drop-off of 47.3 % after 20000 s operation. While a slight loss of original current density with 88.9 % retention of Fe-N/GPC-800 is observed under the same condition, indicating a significantly better durability of Fe-N/GPC-800 as compared with Pt/C. The ORR performance of Fe-N/GPC-800 in all-pH range has also been compared with some previously reported ORR electrocatalysts (Tables S3–5), suggesting that it is highly efficient toward ORR in all-pH range. Also, the bifunctional electrocatalytic activity of FeN/GPC-800 towards ORR/OER is evaluated by the potential difference (ΔE), which is calculated by the E1/2 of ORR process and the potential at 10 mA cm-2 of OER process (Figure 7a). The Fe-N/GPC-800 achieves a ΔE of 0.95 V (Pt/C: 0.99 V), indicating that FeN/GPC-800 possesses good bifunctional activity towards ORR/OER. The rechargeable liquid zinc−air battery (ZAB) was constructed, where Fe-N/GPC-800 acted as a cathode (Figure 7b) and was compared with alongside Pt/C + RuO2. A ZAB with Fe-N/GPC-800 delivers an open-circuit voltage of 1.44 V (Figure 7c). Three ZAB assembled with Fe-N/GPC-800 in 12 ACS Paragon Plus Environment

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series could power the light-emitting diodes (LED) (Figure 7d). As shown in charge/discharge polarization profile (Figure 7e), the ZAB with Fe-N/GPC-800 can offer a current density of 50 mA cm−2 at 2.09 and 1.13V for charge and discharge process, respectively, 0.01 V lower and 0.08 V higher than those of Pt/C + RuO2 battery, together with a relatively low chargedischarge voltage gap in large current density region (50−150 mA cm-2), indicating better rechargeability of Fe-N/GPC-800 than mixture of Pt/C + RuO2. The Fe-N/GPC-800 ZAB exhibits a peak power density of 134 mW cm-2 at 210 mA cm-2, which outperforms the mixture of Pt/C + RuO2 (99 mW cm-2). Fe-N/GPC-800 displays outstanding cycling stability as indicated by negligible voltage variation for continuous 350 cycles (7000 min) in Figure 7f, whereas Pt/C + RuO2 ZAB shows an unacceptable rise of voltage gap between charge and discharge after 162 cycles, which results from a gradually inactivated Pt/C and RuO2 catalysts. The good performance and impressive operation durability of ZAB reveal the feasibility of Fe-N/GPC-800 in real energy-related devices. 2.4. Discussion The interrelation between ORR activity in alkaline medium and N doped species determined by XPS is summarized in Figure 8a. The elevated heating temperature (from 700 to 1100 oC) leads to the monotonic decline of N content, including corresponding pyridinic N and graphitic N content. Oppositely, as heating temperature increases from 700 to 800 oC, the E1/2 shifts from 0.81 to 0.86 V, suggesting a great ORR activity in pyrolysis temperature of 800 oC. Further increasing heat temperature from 800 to 1100 oC gradually downgrades the ORR activity. Both E1/2 and current densities at kinetic ranges (j at 0.85 V) indicates that 800 oC is the optimal pyrolysis temperature for Fe-N/GPC to deliver an enhanced ORR activity among the series. However, as for the Fe-N/GPC-700, the highest N content (including pyridinic and graphitic N species) cannot result in an excellent electrocatalytic activity, indicating that the electrocatalytic activity cannot be precisely controlled by the doped N content. Although the N content in Fe-N/GPC-700 is sufficient, 13 ACS Paragon Plus Environment


other nature of electrocatalysts still has an impact on its ORR activity. Subsequently, the electrochemical impedance spectroscopy (EIS) was used to elucidate the charge transfer resistance of Fe-N/GPC-Ts. It is found that Fe-N/GPC-700 exhibits the largest charge transfer resistance (Rct) among the resulting samples (Figure S12) and the Rct reduces in the order of Fe-N/GPC-1000 > Fe-N/GPC-800 > Fe-N/GPC-1100 > Fe-N/GPC-900 > Fe-N/GPC-700. The low Rct is believed to be an essential ingredient for improvement of activity. For FeN/GPC-700, poor charge transfer resistance impedes the efficiency of ORR process, thus resulting in less-efficient activity. The SCN− ion, which is employed to prove the important role of Fe-centred active centres, is added during oxygen electroreduction process. It is found in Figure S13 that the addition of SCN− can remarkably downgrade the activity of Fe-N/GPC-800. This result is consistent to the reports of Fe-centred active sites prisoned by SCN− ion, thus presenting a degradative the ORR activities.47-49 Thus, partial electrocatalytic activity towards ORR for FeN/GPC-Ts is demonstrated to attribute to the Fe-containing sites. Furthermore, it should be noted that Fe-N/GPC-1000 possesses Graphitic-N species dominated N dopant with low contents (1.02 %), yet delivering a high ORR electroactivity (Figure 8b). This result excludes the role of N-coordinated Fe moieties which account for such remarkable ORR activity. It has been suggested that transition-metal atoms can be trapped in the vacancy defects of graphene, wherein the electronic structures can be favourably tuned, thus boosting the electrocatalytic activity.50-52 Also, Wu group have reported that the carbon divacancy traps Pt atom, showing superior performance for HER.53 For Fe-N/GPC-Ts, it is proposed that the defective structure favours the accommodation of Fe atoms and thus the trapped Fe atoms could effectively tune electronic structures, which is responsible for the accelerated ORR reaction pathways. In the aspect of Fe-N/GPC-800, the robust defective structure reflected by the Raman results (Figure 2b) is deduced to host the Fe atoms, presenting distinctively different properties from bulk 14 ACS Paragon Plus Environment

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metal materials (Fe2O3) due to changed electronic structure of Fe species, which is confirmed by upshift of binding energy of 0.6 eV in Fe 2p XPS spectra (Figure 3d). Besides, iron-saltthermally-emitted strategy leads to the uniform and full deposition of Fe species onto carbon matrix, which endows the Fe- N/GPC-800 catalyst with enriched Fe-immobilized active site. With the further rise of pyrolysis temperature, on one hand, N atoms in carbon skeleton can be removed, which gives rise to the generation of carbon vacancy defects such as single vacancies and double vacancies. On the other hand, the high-temperature process can heal carbon structure and promote graphitization of carbon materials, resulting in the decline of defect sites. In other words, the generation of carbon defect is accompanied with the repair of defect. For Fe-N/GPC-1000 catalyst, carbonization process at 1000 oC promotes generation of defective structure left by the removal of N dopant, as demonstrated by low doped N content and relatively high ID1/IG ratio. When the carbonization temperature further rises to 1100 oC, such heat treatment repairs part of defect, presenting graphited structure as shown by XRD and Raman results. The correlation between ORR activity, pyrolysis temperature, and ID1/IG ratio is established (Figure 8c). The ORR activity is enhanced as pyrolysis temperature increasing from 700 to 800 oC. When pyrolysis temperature is further rises to 1100 oC, the ORR activity displays proportional deactivation. This is due to defect sites can be partially repaired, thus leading to the destruction of Fe-trapped-in-defect sites. A contrast experiment by annealing was performed to further identify the contribution of trapped Fe species to ORR activity. The iron-free sample NC1000 was obtained by the hightemperature carbonation at 1000 oC of the polyaniline initiated by stoichiometric ammonium persulfate. Then the mixture of NC1000, FeCl3 and FeCl2 was experienced a heat treatment at 800 °C in Ar atmosphere for 2 h for the sample NC1000-Fe. For NC1000, the carbonation process at 1000 oC removes the N-dopants and introduce vacancy defects. Subsequently, the heat treatment with Fe salt allows vacancy defects of NC1000 to interact with Fe, giving rise 15 ACS Paragon Plus Environment


to trapped Fe species in defect sites. Interestingly, the ORR activity of obtained sample NC1000-Fe is almost unchanged compared with Fe-N/GPC-1000 (Figure 8b). Therefore, this result combined with the dependence of ORR electroactivity on ID1/IG ratio (Figure 8c) imply that the ORR electroactivity of Fe-N/GPC-Ts is most likely related to the integrality of trapped Fe species and carbon defect sites such as single vacancy (I) and double vacancy (II) (Figure 8d), which can be tuned by pyrolysis temperature. The efficient ORR electrocatalytic performance in all pH range is realized on Fe-N/GPC800. Such excellent electrocatalytic performance could result from the following factors: (a) the unique spatially scaffolded graphene-like carbon nanosheets architecture with interconnected hierarchical pore and conductive carbonaceous skeleton is responsible to promote the accelerated mass and charge transport; (b) Fe species trapped in defective structure of carbonsheet can favorably tune electronic structures, which provides the accelerated reaction pathways, (c) large surface area and sufficient Fe species trapped by carbon vacancy defect are believed to contribute to the enhancement of ORR activity.

3. CONCLUSIONS The non-noble-metal and pH-universal ORR electrocatalyst Fe-N/GPC-800 has been synthesized via iron-salt-thermally-emitted strategy, in which the rich boosted active sites and unique spatially scaffolded graphene-like architecture with hierarchical pore have been achieved owning to the accessible volatility of FeCl3 salt. The prepared Fe-N/GPC-800 exhibits superior electrocatalytic behavior and high durability compared with commercial Pt/C in alkaline and neutral media. Also, an enhanced ORR electrocatalytic performance in acidic medium with prominent durability is obtained on Fe-N/GPC-800. When employed as air cathode in Zn-air batteries, the Fe-N/GPC-800 catalysts presents appreciable reversibility and operation durability, confirming its potential in energy-related devices. This work 16 ACS Paragon Plus Environment

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provides a valuable guidance for facile access of high-performance and non-noble-metal advanced energy material for diverse applications.

4. EXPERIMENT SECTION 4.1.1. Synthesis of Fe-N/GPC-Ts sample. Typically, excessive FeCl3·6H2O (7.5 g) was grinded into powder, which was dispersed into 1 M HCl solution under stirring at room temperature to produce claybank slurry. Then the aniline was added dropwise to the obtained claybank slurry. The color of slurry gradually changed from claybank to atrovirens. After continuous stirring at room temperature for 8 h, the mixture experienced a vacuum drying for 48 h, which was denoted as FePANI-precursor. Then, the FePANI-precursor was annealed in the Ar atmosphere for 2 h at controlled temperatures (700, 800, 900, 1000, or 1100 °C) with a ramping rate of 10 °C min−1. The obtained sample was labeled as MFeNC-T (T refers to the pyrolysis temperature). At last, MFeNC-T was transformed into the final product Fe-N/GPC-T by acid leach in 1 M HCl aqueous solution for 12 h. 4.1.2. Synthesis of FeNC and NC sample. The stoichiometric FeCl3·6H2O (2.9 g) or ammonium persulfate (1.59 g) was used to replace excessive FeCl3·6H2O to obtain samples FeNC and NC, respectively. 4.1.3. Synthesis of NC1000 and NC1000-Fe sample. The stoichiometric ammonium persulfate (1.59 g) was employed to replace excessive FeCl3·6H2O to initiate the polymerization of aniline in ice bath under continuous stirring. Then the obtained polyaniline was cleaned with copious ultrapure water, followed by centrifugation for collection. The obtained sample was dried in vacuum for 48 h. After dry the resultant powder was pyrolyzed for 2 h in 1000 oC with a ramping rate of 10 °C min−1 under the Ar flow to obtain NC1000. The mixture of NC1000 (0.1 g), FeCl3·6H2O (0.083 g) and FeCl2·6H2O (0.078 g) was



pyrolyzed at 800 oC. The resultant powder underwent acid leaching in 1 M HCl under continuous stirring for 12 h to give rise to NC1000-Fe. 4.2. Physicochemical characterization. The crystalline structure of Fe-N/GPC-Ts were determined by X-ray diffraction (XRD) spectra obtained from a Rigaku Mini Flex II diffractometer. The morphology of the Fe-N/GPC-800 was recorded on the Scanning electron microscopy (SEM) of Jeol JSF-7500L microscope. Transmission electron microscopy (TEM, Jeol JEM-2800 microscope) measurements was performed to obtain the nanostructures of asprepared samples. Raman spectroscopy was conducted on a RTS-HiR-AM spectrometer. Nitrogen sorption analysis was carried out by Quantachrome Autosorb-1 sorption analyzer. The chemical states of Fe-N/GPC-Ts were investigated by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi spectrometer). 4.3. Electrochemical measurements. A Pine WaveDriver 20 Bipotentiostat/Galvanostat workstation was utilized to examine electrocatalytic performance of as-synthesized FeN/GPC-Ts equipped with Ag/AgCl reference electrode and platinum wire counter electrode. The working electrode was produced by loading catalyst ink onto a polished glassy-carbon electrode. For preparation of the catalyst ink, 5 mg of as-prepared catalyst and 20 μL of Nafion solution (5 wt%, Sigma-Aldrich) were dispersed in 980 μL solution containing water and isopropanol at a volume ratio of 4:1. After 1 h sonication, the 10-µL aliquot of homogeneous suspension was evenly casted on the freshly polished surface of glassy carbon disk (5.6 mm in diameter), offering a catalyst loading amount of 0.25 mg cm−2. The alkaline, neutral and acid electrolyte were 0.1 M KOH aqueous solution (pH = 13), phosphate buffer solution (PBS, pH = 7, prepared by K2HPO4/KH2PO4) and 0.5 M of H2SO4 solution, respectively. The 5 mV s−1 and 20 mV s−1 were used as scan rate for recording Linear sweep voltammetry (LSV) and cyclic voltammograms (CVs) curves, respectively. The potentials calibrated to reversible hydrogen electrode (RHE) were obtained by the formula: E(vs RHE) = 18 ACS Paragon Plus Environment

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E(vs Ag/AgCl) + 0.197 + 0.059 pH. For the the number of electron transfer (n) and the HO2- yield on the Fe-N/GPC-Ts, the following two equations were used, according to the disk current (IDisk) and ring current (IRing) from rotating ring-disk electrode measuremnet. n = 4IDisk/(IDisk + IRing/N)

(1)

%HO2− = 200IRing/(IDiskN + IRing)

(2)

where N is the collection efficiency (37%). For Zn−air batteries measurements, the catalysts slurry was obtained in the same procedure to that of ORR test and the air cathode was obtained by loading catalysts slurry on hydrophobic carbon paper (1.0 mg cm-2). The homemade Zn-air batteries were constructed, in which the 6.0 M KOH + 0.02 M Zn(Ac)2 solution acted as electrolyte with polished Zn plate as anode. Galvanostatic discharge−charge cycling was performed on the LAND testing system.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XRD pattern of FePANI-precursor; SEM images of FeNC and NC; Raman spectra; XPS survey spectrum; LSV polarization curves operated at electrode rotating rate from 400 to 2025 rpm for the sample of Fe-N/GPC-800 in oxygen-saturated alkaline and acid electrolyte.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Z.-Y.Y.). ORCID 19 ACS Paragon Plus Environment


Zhong-Yong Yuan: 0000-0002-3790-8181 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21421001, 21573115, 21875118) and the 111 project (B12015). REFERENCES (1) Zhang, J.; Zhao, Z.; Xia, Z.; Dai, L. A Metal-Free Bifunctional Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions. Nat. Nanotechnol. 2015, 10, 444-452. (2) Wei, J.; Hu, Y.; Liang, Y.; Kong, B.; Zhang, J.; Song, J.; Bao, Q.; Simon, G. P.; Jiang, S. P.; Wang, H. Nitrogen-Doped Nanoporous Carbon/Graphene Nano-Sandwiches: Synthesis and Application for Efficient Oxygen Reduction. Adv. Funct. Mater. 2015, 25, 5768-5777. (3) Guo, D.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura1, J. Active Sites of Nitrogen-Doped Carbon Materials for Oxygen Reduction Reaction Clarified Using Model Catalysts. Science 2016, 351, 361-365. (4) Liang, J.; Zhou, R. F.; Chen, X. M.; Tang, Y. H.; Qiao, S. Z. Fe-N Decorated Hybrids of CNTs Grown on Hierarchically Porous Carbon for High-Performance Oxygen Reduction. Adv. Mater. 2014, 26, 6074-6079. (5) Wang, L.; Ambrosi, A.; Pumera, M. "Metal-Free" Catalytic Oxygen Reduction Reaction on Heteroatom - Doped Graphene Is Caused by Trace Metal Impurities. Angew. Chem. Int. Ed. 2013, 52, 13818-13821. (6) Zhang, H.; Osgood, H.; Xie, X.; Shao, Y.; Wu, G. Engineering Nanostructures of PGM-Free Oxygen-Reduction Catalysts Using Metal-Organic Frameworks. Nano Energy 2017, 31, 331-350. (7) Liu, L.; Zhu, Y. P.; Su, M.; Yuan, Z. Y. Metal-Free Carbonaceous Materials as Promising Heterogeneous Catalysts. ChemCatChem. 2015, 7, 2765-2787. (8) Wu, G.; Santandreu, A.; Kellogg, W.; Gupta, S.; Ogoke, O.; Zhang, H.; Wang, H.; Dai, L. Carbon Nanocomposite Catalysts for Oxygen Reduction and Evolution Reactions: From Nitrogen Doping to Transition-Metal Addition. Nano Energy 2016, 29, 83-110. (9) Gang, W.; Piotr, Z. Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction. Acc. Chem. Res. 2013, 46, 1878-1889. (10) Ren, J. T.; Yuan, G. G.; Weng, C. C.; Yuan, Z. Y. Rationally Designed Co3O4–C Nanowire Arrays on Ni Foam Derived from Metal Organic Framework as Reversible Oxygen Evolution Electrodes with Enhanced Performance for Zn–Air Batteries. ACS Sustain. Chem. Eng. 2017, 6, 707-718. (11) Gong, K. P.; Du, F.; Xia, Z. H.; Durstock, M.; Dai, L. M. Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science 2009, 323, 760-763.


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(43) Armel, V.; Hindocha, S.; Salles, F.; Bennett, S.; Jones, D.; Jaouen, F. Structural Descriptors of Zeolitic-Imidazolate Frameworks Are Keys to the Activity of Fe-N-C Catalysts. J. Am. Chem. Soc. 2017, 139, 453-464. (44) Guo, Z.; Zhang, Z.; Li, Z.; Dou, M.; Wang, F. Well-Defined Gradient Fe/Zn Bimetal Organic Framework Cylinders Derived Highly Efficient Iron- and Nitrogen- Codoped Hierarchically Porous Carbon Electrocatalysts towards Oxygen Reduction. Nano Energy 2019, 57, 108-117. (45) Zhao, H.; Weng, C.; Hu, Z.; Ge, L.; Yuan, Z. CdS-Polydopamine-Derived N,SCodoped Hierarchically Porous Carbons as Highly Active Electrocatalyst for Oxygen Reduction. ACS Sustainable Chem. Eng. 2017, 5, 9914-9922. (46) Sawant, S. Y.; Han, T. H.; Cho, M. H. Metal-Free Carbon-Based Materials: Promising Electrocatalysts for Oxygen Reduction Reaction in Microbial Fuel Cells. Int. J. Mol. Sci. 2016, 18, 25. (47) Jiang, W. J.; Gu, L.; Li, L.; Zhang, Y.; Zhang, X.; Zhang, L. J.; Wang, J. Q.; Hu, J. S.; Wei, Z.; Wan, L. J. Understanding the High Activity of Fe-N-C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe-Nx. J. Am. Chem. Soc. 2016, 138, 3570-3578. (48) Amiinu, I. S.; Liu, X.; Pu, Z.; Li, W.; Li, Q.; Zhang, J.; Tang, H.; Zhang, H.; Mu, S. From 3D ZIF Nanocrystals to Co-Nx/C Nanorod Array Electrocatalysts for ORR, OER, and Zn-Air Batteries. Adv. Funct. Mater. 2018, 28, 1704638. (49) Wang, Q.; Zhou, Z. Y.; Lai, Y. J.; You, Y.; Liu, J. G.; Wu, X. L.; Terefe, E.; Chen, C.; Song, L.; Rauf, M.; Tian, N.; Sun, S. G. Phenylenediamine-Based FeNx/C Catalyst with High Activity for Oxygen Reduction in Acid Medium and Its Active-Site Probing. J. Am. Chem. Soc. 2014, 136, 10882-10885. (50) Jiang, K.; Siahrostami, S.; Akey, A. J.; Li, Y.; Lu, Z.; Lattimer, J.; Hu, Y.; Stokes, C.; Gangishetty, M.; Chen, G.; Zhou, Y.; Hill, W.; Cai, W. B.; Bell, D.; Chan, K.; Nørskov, J. K.; Cui, Y.; Wang, H. Transition-Metal Single Atoms in a Graphene Shell as Active Centers for Highly Efficient Artificial Photosynthesis. Chem. 2017, 3, 950-960. (51) Jiang, K.; Wang, H. Electrocatalysis over Graphene-Defect-Coordinated TransitionMetal Single-Atom Catalysts. Chem. 2018, 4, 194-195. (52) Zhang, L.; Jia, Y.; Gao, G.; Yan, X.; Chen, N.; Chen, J.; Soo, M. T.; Wood, B.; Yang, D.; Du, A.; Yao, X. Graphene Defects Trap Atomic Ni Species for Hydrogen and Oxygen Evolution Reactions. Chem. 2018, 4, 285-297. (53) Qu, Y.; Chen, B.; Li, Z.; Duan, X.; Wang, L.; Lin, Y.; Yuan, T.; Zhou, F.; Hu, Y.; Yang, Z.; Zhao, C.; Wang, J.; Zhao, C.; Hu, Y.; Wu, G.; Zhang, Q.; Xu, Q.; Liu, B.; Gao, P.; You, R.; Huang, W.; Zheng, L.; Gu, L.; Wu, Y.; Li, Y. Thermal Emitting Strategy to Synthesize Atomically Dispersed Pt Metal Sites from Bulk Pt Metal. J. Am. Chem. Soc. 2019, 141, 4505-4509.



Scheme 1. Schematic illustration of the preparation process of Fe-N/GPC-T catalysts.


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Figure 1. SEM images of (a,b) FePANI-precursor, (c) MFeNC-800 and (d) Fe-N/GPC-800. (e) TEM image of Fe-N/GPC-800, (f) HRTEM image of Fe-N/GPC-800, (g) Elemental mapping images of FeN/GPC-800

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Figure 2. (a) XRD patterns of Fe-N/GPC-Ts. (b) Raman spectra of Fe-N/GPC-800.

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Figure 3. High-resolution XPS spectra for (a) C 1s, (b) N 1s, (c) O 1s and (d) Fe 2p of Fe-N/GPC-Ts samples. (e) N2 adsorption–desorption isotherm curves (inset is BET surface area) of Fe-N/GPC-Ts.

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Figure 4. (a) RDE voltammograms recorded for Fe-N/GPC-Ts and Pt/C in O2-saturated 0.1 M KOH solution. (b) CV voltammograms for Fe-N/GPC-800 and Pt/C in O2- and N2-saturated 0.1 M KOH solution. (c) Plots of H2O2 yield and n of Fe-N/GPC-800 and Pt/C catalysts in O2-saturated 0.1 M KOH. (d) Chronoamperometric curves of Fe-N/GPC-800 and Pt/C at +0.70 V vs RHE in O2-saturated 0.1 M KOH solution.


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Figure 5. (a) RDE voltammograms recorded for Fe-N/GPC-Ts and Pt/C in O2-saturated 0.5 M H2SO4 solution. (b) CV voltammograms for Fe-N/GPC-800 and Pt/C in O2- and N2-saturated 0.5 M H2SO4 solution. (c) Plots of H2O2 yield and n of Fe-N/GPC-800 and Pt/C catalysts in O2-saturated 0.5 M H2SO4 solution. (d) Chronoamperometric curves of Fe-N/GPC-800 and Pt/C at +0.70 V vs RHE in O2-saturated 0.5 M H2SO4 solution.

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Figure 6. (a) RDE voltammograms recorded for Fe-N/GPC-Ts and Pt/C in an O2-saturated 0.1 M PBS solution. (b) CV voltammograms for Fe-N/GPC-800 and Pt/C in O2- and N2-saturated 0.1 M PBS solution. (c) Plots of H2O2 yield and n of Fe-N/GPC-800 and Pt/C catalysts in O2-saturated 0.1 M PBS solution. (d) Chronoamperometric curves of Fe-N/GPC-800 and Pt/C at +0.70 V vs RHE in O2-saturated 0.1 M PBS solution.

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Figure 7. (a) LSV curves of Fe-N/GPC-800 and Pt/C in the full OER/ORR region in O2-saturated 0.1M KOH solution. (b) Scheme of ZAB with Fe-N/GPC-800 as air cathode. (c) Open-circuit voltage of ZAB with Fe-N/GPC-800. (d) Photograph of a LED illuminated by three ZABs in series. (e) Discharge-charge curves of ZABs and the corresponding power density plots. (f) Long-term galvanostatic charge–discharge plots at 10 mA cm-2; inset shows the corresponding galvanostatic charge–discharge plots for the last 200 min

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Figure 8. (a) The correlation between ORR activity in alkaline media and new bond formation associated with doped pyridinic (P) and graphitic (G) nitrogen. (b) LSV curves of Fe-N/GPC-1000, NC1000, NC1000-Fe and Pt/C in O2-saturated 0.1 m KOH solution. (c) The correlation between ORR activity, pyrolysis temperature and ID1/IG radio. (d) Schematic of Fe atoms trapped in single vacancy (I) and double vacancy (II).


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Iron-Salt Thermally Emitted Strategy to Prepare Graphene-like Carbon

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