Binary Nitrogen Precursor-Derived Porous FeN-S / C Catalyst for Efficient Oxygen Reduction Reaction in a Zn-Air Battery

It is still a challenge to synthesize non-precious-metal catalysts with high activity and stability for the oxygen reduction reaction (ORR) to replace the state-of-the art Pt/C catalyst. Herein, a Fe, N, S co-doped porous carbon (Fe-NS/PC) is developed by using g-C3N4 and 2,4,6-tri(2-pyridyl)1,3,5-triazine (TPTZ) as binary nitrogen precursors. The interaction of binary nitrogen precursors not only leads to the formation of more micropores, but also increases the doping amount of both iron and nitrogen dispersed in the carbon matrix. After a second heat-treatment, the best Fe/NS/C-g-C3N4/TPTZ-1000 catalyst exhibits excellent ORR performance with an onset potential of 1.0 V vs. reversible hydrogen electrode (RHE) and a half-wave potential of 0.868 V (RHE) in alkaline medium. The long-term durability is even superior to the commercial Pt/C catalyst. In the meantime, an assembled Zn-air battery with Fe/NS/C-g-C3N4/TPTZ-1000 as the cathode shows a maximal power density of 225 mW·cm−2 and excellent durability, demonstrating the great potential of practical applications in energy conversion devices.


Introduction
The oxygen reduction reaction (ORR) plays an important role in the energy efficiency of polymer electrolyte membrane fuel cells (PEMFCs) and metal-air batteries (MABs).So far, platinum (Pt)-based materials are still the most effective catalysts for the ORR due to its sluggish kinetics.However, the high price and scarcity of Pt severely hinder the large-scale applications of PEMFCs and MABs.Therefore, extensive efforts have been devoted to develop low-cost and earth-abundant non-precious metal catalysts with efficient ORR performance.The transition metal (M=Fe, Co, etc.) and nitrogen co-doped carbon materials (M-N-C), such as graphene [1,2], nanotube [3,4], and porous carbon [5,6], have shown great progress in ORR electrocatalysis, especially for Fe-N-C materials, which have been considered as the most promising catalysts for substituting the expensive Pt catalysts [7][8][9].
The excellent ORR activity usually depends on two main factors, namely, the high intrinsic activity of single sites and high density of active sites.Therefore, the M-N-C electrocatalysts for efficient ORR require high heteroatom doping contents, high surface area, porous structure, and good conductivity [10][11][12].Heteroatom doping is an effective method to tailor the electronic structure of electroneutral carbon matrix, which would facilitate the adsorption of O 2 .High surface area and porous structure are beneficial to increase the number of accessible active sites and facilitate the mass transport of the ORR relevant species approaching the internal active sites of catalysts [13,14].Heat-treatment at high temperature is a vital process to form the ORR active centers, therefore, the precursors should be chosen carefully.Recently, the use of binary nitrogen precursors has been developed as an effective synthetic strategy to improve the porosity and heteroatoms doping contents, hence to improve the ORR activity.Wu et al. [15] synthesized a Fe-N-C catalyst derived from polyaniline (PANI) and dicyandiamide (DCDA) as binary nitrogen precursors, which possessed higher ORR activity than the individual PANI or DCDA-derived ones.The superior ORR activity can be ascribed to the increased content of pyridinic nitrogen doped into the carbon matrix.The combination of PANI and DCDA could enhance the porosity and increase the surface area, thanks to the different decomposition temperatures.Chen et al. [16] synthesized a class of Fe-N-C catalysts with 3D nanoporous structure using phenanthroline (Phen) and PANI as dual nitrogen sources.The Phen played the role of pore-creating agent due to its lower thermostability.A similar strategy was also reported by Zelenay et al. [17].Moreover, it has been proved that the additional doping of S atoms in the Fe-N-C catalyst will remarkably enhance the ORR activity [18].However, although the development of non-precious metal catalysts has achieved great progress, the application of these materials in practical devices, such as PEMFCs and MABs, is still far from satisfactory, especially for the long-term stability [19].
In this work, we have developed a facile method to synthesize Fe, N, S co-doped porous carbon materials (Fe-NS/PC) as efficient ORR catalysts with g-C 3 N 4 and 2,4,6-tri(2-pyridyl)-1,3,5-triazine (TPTZ) as binary nitrogen precursors.The TPTZ is able to coordinate with Fe 3+ [20], which could contribute to the uniformly-dispersed metal-containing species located at the N-doped carbon skeleton [21,22].The addition of g-C 3 N 4 sheets could inhibit the sintering of TPTZ during carbonization.The interaction of g-C 3 N 4 and TPTZ is beneficial to increase the doping amount of iron and nitrogen, and to facilitate the mass transfer of ORR relevant species.As a result, the binary nitrogen precursor-derived Fe-NS/PC catalyst exhibited better ORR performance than the single nitrogen precursor-derived ones.After second heat treatment, the Fe/NS/C-g-C 3 N 4 /TPTZ catalyst shows enhanced activity and long-term durability.The best ORR activity of Fe/NS/C-g-C 3 N 4 /TPTZ-1000 is even superior to that of the state-of-the art Pt/C catalyst.A Zn-air battery with the Fe/NS/C-g-C 3 N 4 /TPTZ-1000 cathode exhibits a maximal power density of 225 mW•cm −2 at room temperature and superior stability with only 4.03% loss of output voltage at a current density of 20 mA•cm −2 after 20,000 s.

Results and Discussion
Figure 1a-c show the scanning electron microscope (SEM) images of the as-synthesized Fe-NS/PC catalysts using g-C 3 N 4 and TPTZ separated as single nitrogen precursor and together as binary nitrogen precursors, denoted as Fe/NS/C-g-C 3 N 4 , Fe/NS/C-TPTZ, and Fe/NS/C-g-C 3 N 4 /TPTZ, respectively.The Fe/NS/C-g-C 3 N 4 has a fluffy morphology with high porosity (Figure 1a).By contrast, Fe/NS/C-TPTZ shows a denser morphology (Figure 1b), probably because of the collapse of the carbon skeleton during heat-treatment.Through the combination of both g-C 3 N 4 and TPTZ with various thermal stability (Figure S1), the Fe/NS/C-g-C 3 N 4 /TPTZ preserves the fluffy morphology (Figure 1c), probably due to the fact that the mixed g-C 3 N 4 sheets prevent the sintering of TPTZ.
The effects of N precursors on BET surface areas and pore structures were studied by N 2 adsorption-desorption isotherms.As can be seen in Figure 1d,f, the Fe/NS/C-g-C 3 N 4 exhibits a highest BET surface area of 928 m 2 /g, the most of which are external surface area.The micropore area is only 82 m 2 /g.The Fe/NS/C-TPTZ shows a slightly lower BET surface area of 849 m 2 /g, but a much larger micropore area of 317 m 2 /g.However, the amounts of mesopores and macropores are relatively few (Figure 1e), which are consistent with the dense structure displayed by SEM image (Figure 1b).After the combination of g-C 3 N 4 and TPTZ, although the BET surface area slightly decreases (759 m 2 /g), the Fe/NS/C-g-C 3 N 4 /TPTZ catalyst integrates the micropores of Fe/NS/C-TPTZ and the fluffy structure of Fe/NS/C-g-C 3 N 4 (Figure 1f), which might facilitate the mass transfer and the ORR catalytic activity [23,24].
Catalysts 2018, 8, 158 3 of 13 X-ray diffraction (XRD) was carried out to characterize the crystal structure of the Fe-NS/PC catalysts.According to the XRD patterns in Figure 1g, the Fe/NS/C-TPTZ exhibits two main diffraction peaks at around 25.5 • and 43 • , associated to the (002) and (100) planes of graphitic carbon, respectively.The (002) diffraction peak of the Fe/NS/C-g-C 3 N 4 shifts to a higher angle of 29.8 • , which can be associated with the carbon nitride (PDF-#78-1747).Noteworthy, the binary nitrogen precursor-derived Fe/NS/C-g-C 3 N 4 /TPTZ displays a broad peak at around 25.8 • corresponding to the (002) diffraction of graphitic carbon, and a weak swell at 29.8 • corresponding to the carbon nitride, which clearly indicates the interaction between two nitrogen precursors during pyrolysis.No other diffraction peaks can be observed, demonstrating the absence of any other Fe-containing crystalline phases.
Catalysts 2018, 8, x FOR PEER REVIEW 3 of 13 diffraction peaks at around 25.5° and 43°, associated to the (002) and (100) planes of graphitic carbon, respectively.The (002) diffraction peak of the Fe/NS/C-g-C3N4 shifts to a higher angle of 29.8°, which can be associated with the carbon nitride (PDF-#78-1747).Noteworthy, the binary nitrogen precursorderived Fe/NS/C-g-C3N4/TPTZ displays a broad peak at around 25.8° corresponding to the (002) diffraction of graphitic carbon, and a weak swell at 29.8° corresponding to the carbon nitride, which clearly indicates the interaction between two nitrogen precursors during pyrolysis.No other diffraction peaks can be observed, demonstrating the absence of any other Fe-containing crystalline phases.X-ray photoelectron spectroscopy (XPS) was implemented to investigate the states of each component within the Fe-NS/PC catalysts.The survey XPS spectra (Figure 2a) reveal that the main elements of Fe-NS/PC catalysts consist of Fe, N, C, O, and S. The elemental compositions are summarized in Table S1. Figure 2b-d display the high resolution N 1s spectra of the three Fe-NS/PC catalysts.All the spectra can be deconvoluted into four peaks corresponding to pyridinic N (N1, 398.1-398.7 eV), pyrrolic N (N2, 399.78-400.7 eV), graphitic N (N3, 400.99-401.3eV), and oxidized N (N4, 402-404.27eV) [25,26], respectively.Previous reports have demonstrated that both pyridinic N and graphitic N may participate the oxygen reduction reaction [27][28][29].These two N species (N1 + X-ray photoelectron spectroscopy (XPS) was implemented to investigate the states of each component within the Fe-NS/PC catalysts.The survey XPS spectra (Figure 2a) reveal that the main elements of Fe-NS/PC catalysts consist of Fe, N, C, O, and S. The elemental compositions are summarized in Table S1. Figure 2b-d  398.1-398.7 eV), pyrrolic N (N2, 399.78-400.7 eV), graphitic N (N3, 400.99-401.3eV), and oxidized N (N4, 402-404.27eV) [25,26], respectively.Previous reports have demonstrated that both pyridinic N and graphitic N may participate the oxygen reduction reaction [27][28][29].These two N species (N1 + N3) account for 4.03 at% of all the elements in Fe/NS/C-g-C 3 N 4 /TPTZ, remarkably higher than that of Fe/NS/C-g-C 3 N 4 (1.88%) and Fe/NS/C-TPTZ (3.46%), as shown in Table S1.Notably, the contents of Fe and N in Fe/NS/C-g-C 3 N 4 /TPTZ are 0.29 at% and 6.67 at%, respectively, both of which are the highest among the three catalysts.In addition, the co-doping of S element in Fe-N-C catalysts would further improve the ORR activity, probably due to the structural defects and electron distribution induced by S atoms [30,31].Based on the consideration of the high heteroatoms doping contents combined with the porous structure, the high ORR activity could be expected for the Fe/NS/C-g-C 3 N 4 /TPTZ catalyst.
Catalysts 2018, 8, x FOR PEER REVIEW 4 of 13 N3) account for 4.03 at% of all the elements in Fe/NS/C-g-C3N4/TPTZ, remarkably higher than that of Fe/NS/C-g-C3N4 (1.88%) and Fe/NS/C-TPTZ (3.46%), as shown in Table S1.Notably, the contents of Fe and N in Fe/NS/C-g-C3N4/TPTZ are 0.29 at% and 6.67 at%, respectively, both of which are the highest among the three catalysts.In addition, the co-doping of S element in Fe-N-C catalysts would further improve the ORR activity, probably due to the structural defects and electron distribution induced by S atoms [30,31].Based on the consideration of the high heteroatoms doping contents combined with the porous structure, the high ORR activity could be expected for the Fe/NS/C-g-C3N4/TPTZ catalyst.The ORR activity of the Fe-NS/PC catalysts were evaluated by rotating disk electrode (RDE) test in O2-saturated 0.1 M KOH solution.As displayed in Figure 3, the Fe/NS/C-g-C3N4/TPTZ catalyst exhibits the best ORR activity with an onset (E0) and half-wave (E1/2) potential of 0.95 V and 0.853 V (RHE), respectively, higher than that of Fe/NS/C-g-C3N4 catalyst (E0 = 0.946 V and E1/2 = 0.843 V).By sharp contrast, the Fe/NS/C-TPTZ shows inferior ORR activity with E0 = 0.917 V (RHE) and much smaller diffusion-limited current, probably due to the agglomerations of sintered carbon that are difficult to disperse obstruct the transfer of ORR-related species, in spite of the high BET surface area.The presence of g-C3N4 in dual nitrogen precursors might avoid the sintering of TPTZ, meanwhile maintain the fluffy structure and increase the Fe, N doping content, thus resulting in the full exposure of active sites, which could be responsible for the high ORR activity of the Fe/NS/C-g-C3N4/TPTZ catalyst.The ORR activity of the Fe-NS/PC catalysts were evaluated by rotating disk electrode (RDE) test in O 2 -saturated 0.1 M KOH solution.As displayed in Figure 3, the Fe/NS/C-g-C 3 N 4 /TPTZ catalyst exhibits the best ORR activity with an onset (E 0 ) and half-wave (E 1/2 ) potential of 0.95 V and 0.853 V (RHE), respectively, higher than that of Fe/NS/C-g-C 3 N 4 catalyst (E 0 = 0.946 V and E 1/2 = 0.843 V).By sharp contrast, the Fe/NS/C-TPTZ shows inferior ORR activity with E 0 = 0.917 V (RHE) and much smaller diffusion-limited current, probably due to the agglomerations of sintered carbon that are difficult to disperse obstruct the transfer of ORR-related species, in spite of the high BET surface area.The presence of g-C 3 N 4 in dual nitrogen precursors might avoid the sintering of TPTZ, meanwhile maintain the fluffy structure and increase the Fe, N doping content, thus resulting in the full exposure of active sites, which could be responsible for the high ORR activity of the Fe/NS/C-g-C 3 N 4 /TPTZ catalyst.To further improve the ORR performance, a secondary heat treatment was conducted to the best Fe/NS/C-g-C3N4/TPTZ catalyst at the range of 800-1000 °C, denoted as Fe/NS/C-g-C3N4/TPTZ-T (T = 800, 900, 1000).Figure 4a-h present the transmission electron microscope (TEM) images of Fe/NS/Cg-C3N4/TPTZ, and Fe/NS/C-g-C3N4/TPTZ-T (T = 800, 900, 1000) at diversed magnifications.All these samples show the morphological characteristics of agglomerations of amorphous carbon nanoparticles with the diameter of 20-50 nm.No crystalline iron-containing nanoparticles can be observed for all three catalysts, indicating no agglomerations of iron formed during the second heat treatment.The energy dispersive X-ray spectroscopy (EDX) mapping analysis of Fe/NS/C-g-C3N4/TPTZ-1000 was also carried out to observe the elemental distributions.As can be seen in Figure 4i, the TEM image and the corresponding elemental mapping reveal that all doping heteroatoms, which are regarded as the components of the ORR active sites, are uniformly distributed throughout the carbon matrix, leading to the full exposure of the active sites to the ORR related species.To further improve the ORR performance, a secondary heat treatment was conducted to the best Fe/NS/C-g-C 3 N 4 /TPTZ catalyst at the range of 800-1000 • C, denoted as Fe/NS/C-g-C 3 N 4 /TPTZ-T (T = 800, 900, 1000).Figure 4a-h present the transmission electron microscope (TEM) images of Fe/NS/C-g-C 3 N 4 /TPTZ, and Fe/NS/C-g-C 3 N 4 /TPTZ-T (T = 800, 900, 1000) at diversed magnifications.All these samples show the morphological characteristics of agglomerations of amorphous carbon nanoparticles with the diameter of 20-50 nm.No crystalline iron-containing nanoparticles can be observed for all three catalysts, indicating no agglomerations of iron formed during the second heat treatment.The energy dispersive X-ray spectroscopy (EDX) mapping analysis of Fe/NS/C-g-C 3 N 4 /TPTZ-1000 was also carried out to observe the elemental distributions.As can be seen in Figure 4i, the TEM image and the corresponding elemental mapping reveal that all doping heteroatoms, which are regarded as the components of the ORR active sites, are uniformly distributed throughout the carbon matrix, leading to the full exposure of the active sites to the ORR related species.To further improve the ORR performance, a secondary heat treatment was conducted to the best Fe/NS/C-g-C3N4/TPTZ catalyst at the range of 800-1000 °C, denoted as Fe/NS/C-g-C3N4/TPTZ-T (T = 800, 900, 1000).Figure 4a-h present the transmission electron microscope (TEM) images of Fe/NS/Cg-C3N4/TPTZ, and Fe/NS/C-g-C3N4/TPTZ-T (T = 800, 900, 1000) at diversed magnifications.All these samples show the morphological characteristics of agglomerations of amorphous carbon nanoparticles with the diameter of 20-50 nm.No crystalline iron-containing nanoparticles can be observed for all three catalysts, indicating no agglomerations of iron formed during the second heat treatment.The energy dispersive X-ray spectroscopy (EDX) mapping analysis of Fe/NS/C-g-C3N4/TPTZ-1000 was also carried out to observe the elemental distributions.As can be seen in Figure 4i, the TEM image and the corresponding elemental mapping reveal that all doping heteroatoms, which are regarded as the components of the ORR active sites, are uniformly distributed throughout the carbon matrix, leading to the full exposure of the active sites to the ORR related species.After a second heat treatment at different temperatures, all three Fe/NS/C-g-C 3 N 4 /TPTZ-T (T = 800, 900, 1000) catalysts show two main diffraction peaks at around 29.8 • and 43 • (Figure 5a), similar to Fe/NS/C-g-C 3 N 4 .There are no other diffraction peaks appear, further demonstrating the absence of crystalline iron-containing phases, which is consistent with the TEM results (Figure 4).After a second heat treatment at different temperatures, all three Fe/NS/C-g-C3N4/TPTZ-T (T = 800, 900, 1000) catalysts show two main diffraction peaks at around 29.8° and 43° (Figure 5a), similar to Fe/NS/C-g-C3N4.There are no other diffraction peaks appear, further demonstrating the absence of crystalline iron-containing phases, which is consistent with the TEM results (Figure 4). Figure 5b displays the N2 adsorption-desorption isotherms of the Fe/NS/C-g-C3N4/TPTZ-T (T = 800, 900, 1000) catalysts.The BET suface areas of Fe/NS/C-g-C3N4/TPTZ-800, Fe/NS/C-g-C3N4/TPTZ-900, and Fe/NS/C-g-C3N4/TPTZ-1000 are 876.8m 2 g −1 , 1026.4 m 2 g −1 , 1138.9 m 2 g −1 , respectively.The pore size distributions of the Fe/NS/C-g-C3N4/TPTZ-T (T = 800, 900, 1000) indicate that the three catalysts all possess a good porous structure (Figure 5c).The XPS survey spectra of Fe/NS/C-g-C3N4/TPTZ and Fe/NS/C-g-C3N4/TPTZ-T (T = 800, 900, 1000) were collected to measure the elemental compositions, as shown in Figure 6a and Table S2.The contents of doped Fe and N elements reduce along with the secondary heat treatment temperature elevates.The N doping contents of Fe/NS/C-g-C3N4/TPTZ-T (T = 800, 900, 1000) are 4.48%, 2.57%, and 1.40%, respectively, all lower than that of Fe/NS/C-g-C3N4/TPTZ, due to the formation of gaseous Ncontaining phases.The deconvoluted high-resolution N 1s spectra reveal that the pyridinic N and graphitic N predominate in all three catalysts, as shown in Figure 6b-d and summarized in Table S2.
The high-resolution S 2p spectrum of Fe/NS/C-g-C3N4/TPTZ-1000 can be deconvoluted into three peaks, as shown in Figure 6e.The two peaks at 164.0 and 165.2 eV can be described to S 2p3/2 and S 2p1/2 of thiophene-like C-S-C structure, respectively [32,33], while the third peak at 167.3 eV corresponds to sulfate species.The synergetic effects of N, S co-doping would significantly improve the ORR activity by reducing the electron localization around the Fe centers, and improve the interaction with oxygen, facilitating the four-electron pathway [34].The high-resolution Fe 2p spectrum of Fe/NS/C-g-C3N4/TPTZ-1000 presents two major peaks at around 711 and 724 eV, corresponding to Fe 2p1/2 and Fe 2p3/2, respectively (Figure 6f) [15,35].The dominant peak at 711 eV can be assigned to Fe 3+ or Fe 2+ coordinated with N, which are suggested to be the ORR active centers [15,36].The peak at 718.5 eV is a satellite peak indicating the co-existence of Fe 3+ and Fe 2+ in the Fe/NS/C-g-C3N4/TPTZ-1000 [35].The XPS survey spectra of Fe/NS/C-g-C 3 N 4 /TPTZ and Fe/NS/C-g-C 3 N 4 /TPTZ-T (T = 800, 900, 1000) were collected to measure the elemental compositions, as shown in Figure 6a and Table S2.The contents of doped Fe and N elements reduce along with the secondary heat treatment temperature elevates.The N doping contents of Fe/NS/C-g-C 3 N 4 /TPTZ-T (T = 800, 900, 1000) are 4.48%, 2.57%, and 1.40%, respectively, all lower than that of Fe/NS/C-g-C 3 N 4 /TPTZ, due to the formation of gaseous N-containing phases.The deconvoluted high-resolution N 1s spectra reveal that the pyridinic N and graphitic N predominate in all three catalysts, as shown in Figure 6b-d and summarized in Table S2.The high-resolution S 2p spectrum of Fe/NS/C-g-C 3 N 4 /TPTZ-1000 can be deconvoluted into three peaks, as shown in Figure 6e.The two peaks at 164.0 and 165.2 eV can be described to S 2p 3/2 and S 2p 1/2 of thiophene-like C-S-C structure, respectively [32,33], while the third peak at 167.3 eV corresponds to sulfate species.The synergetic effects of N, S co-doping would significantly improve the ORR activity by reducing the electron localization around the Fe centers, and improve the interaction with oxygen, facilitating the four-electron pathway [34].The high-resolution Fe 2p spectrum of Fe/NS/C-g-C 3 N 4 /TPTZ-1000 presents two major peaks at around 711 and 724 eV, corresponding to Fe 2p 1/2 and Fe 2p 3/2 , respectively (Figure 6f) [15,35].The dominant peak at 711 eV can be assigned to Fe 3+ or Fe 2+ coordinated with N, which are suggested to be the ORR active centers [15,36].The peak at 718.5 eV is a satellite peak indicating the co-existence of Fe 3+ and Fe 2+ in the Fe/NS/C-g-C 3 N 4 /TPTZ-1000 [35].The ORR activities of Fe/NS/C-g-C3N4/TPTZ-T (T = 800, 900, 1000) catalysts were measured in O2-saturated 0.1 M KOH solution.For a comparison, commercial Pt/C (20 wt%) catalyst was also evaluated.As displayed in Figure 7a, after secondary heat treatment, the ORR activity of the Fe/NS/Cg-C3N4/TPTZ is further improved.Fe/NS/C-g-C3N4/TPTZ-800 and Fe/NS/C-g-C3N4/TPTZ-900 exhibit similar activity with the half-wave potential (E1/2) of 0.863 V and 0.864 V (RHE), respectively.The best ORR activity is achieved at Fe/NS/C-g-C3N4/TPTZ-1000 with the onset and half-wave potential of 1.0 V and 0.868 V (RHE), respectively, higher than that of Pt/C catalyst (E0 = 0.97 V, E1/2 = 0.841 V).To evaluate the intrinsic catalytic activity, the mass activity (jm) was calculated based on the Koutecky-Levich equation (Table S3).The Fe/NS/C-g-C3N4/TPTZ-1000 catalyst shows the best mass activity of 5.73 A g −1 at 0.9 V, reaching up to 48.8% of that of Pt/C (11.73A g −1 ).The ORR activities of Fe/NS/C-g-C 3 N 4 /TPTZ-T (T = 800, 900, 1000) catalysts were measured in O 2 -saturated 0.1 M KOH solution.For a comparison, commercial Pt/C (20 wt %) catalyst was also evaluated.As displayed in Figure 7a, after secondary heat treatment, the ORR activity of the Fe/NS/C-g-C 3 N 4 /TPTZ is further improved.Fe/NS/C-g-C 3 N 4 /TPTZ-800 and Fe/NS/C-g-C 3 N 4 /TPTZ-900 exhibit similar activity with the half-wave potential (E 1/2 ) of 0.863 V and 0.864 V (RHE), respectively.The best ORR activity is achieved at Fe/NS/C-g-C 3 N 4 /TPTZ-1000 with the onset and half-wave potential of 1.0 V and 0.868 V (RHE), respectively, higher than that of Pt/C catalyst (E 0 = 0.97 V, E 1/2 = 0.841 V).To evaluate the intrinsic catalytic activity, the mass activity (j m ) was calculated based on the Koutecky-Levich equation (Table S3).The Fe/NS/C-g-C 3 N 4 /TPTZ-1000 catalyst shows the best mass activity of 5.73 A g −1 at 0.9 V, reaching up to 48.8% of that of Pt/C (11.73A g −1 ).
To explore the origin of the high ORR activity, we have probed the possible role of Fe and S within the Fe-NS/PC catalysts.As shown in Figure S2 To explore the origin of the high ORR activity, we have probed the possible role of Fe and S within the Fe-NS/PC catalysts.As shown in Figure S2, Fe/N/C-g-C3N4/TPTZ-1000 without sulfur doping exhibits lower onset and half-wave potentials for the ORR than Fe/NS/C-g-C3N4/TPTZ-1000, indicating the promoting effects of sulfur co-doping.By sharp comparison, the NS/C-g-C3N4/TPTZ-1000 without Fe doping presents inferior activity, with the half-wave potential 64 mV lower than that of Fe/NS/C-g-C3N4/TPTZ-1000.These results definitely reflect the indispensable roles of Fe and S doping in Fe-NS/PC for high ORR activity.It is well known that SCN − can strongly coordinate with Fe atoms.As shown in Figure S3, the Fe/NS/C-g-C3N4/TPTZ-1000 catalyst shows obvious ORR activity degradation after injecting 5 mM SCN − , the remaining ORR activity probably results from the N, S co-doped carbon materials.Interestingly, the ORR activity of the Fe/NS/C-g-C3N4/TPTZ-1000 can almost recover after rinsing and replacing with fresh electrolyte, indicating that Fe atoms are at least part of the ORR active sites.
Figure 7b shows the hydrogen peroxide yield (H2O2 %) and the electron transfer number (n) of Fe/NS/C-g-C3N4/TPTZ-1000 calculated from disk current (Id) and ring current (Ir) obtained by rotating ring-disk electrode (RRDE) test.The H2O2 yield is lower than 1.0% over the whole potential range.The corresponding electron transfer number is calculated to be larger than 3.975.Notably, the H2O2 starts to generate at the potential lower than 0.8 V, where the ORR polarization curve has reached the diffusion-limited current.These results indicate a direct four-electron pathway of ORR on the Fe/NS/C-g-C3N4/TPTZ-1000.The Tafel plots of Fe/NS/C-g-C3N4/TPTZ-1000 depicted in Figure 7c show the Tafel slope of 68 mV dec −1 , closed to that of Pt/C catalyst (73 mV dec −1 ).
An accelerated durability test (ADT) was carried out to assess the durability of Fe/NS/C-g-C3N4/TPTZ-1000 catalyst by potential-cycling between 0.6 and 1.1 V at a scan rate of 50 mV s −1 in O2- An accelerated durability test (ADT) was carried out to assess the durability of Fe/NS/ C-g-C 3 N 4 /TPTZ-1000 catalyst by potential-cycling between 0.6 and 1.1 V at a scan rate of 50 mV s −1 in O 2 -saturated 0.1 M KOH.As shown in Figure 7d, after 10,000 cycles, the E 1/2 of Fe/NS/C-g-C 3 N 4 /TPTZ-1000 slightly decreases by 12 mV, demonstrating the superior durability of the Fe/NS/C-g-C 3 N 4 /TPTZ-1000 catalyst, due to the robust structure of Fe and N anchored in carbon matrix and the low yield of corrosive H 2 O 2 .In sharp contrast, the E 1/2 of Pt/C remarkably decreases by 35 mV after only 5000 cycles.The insufficient durability of the Pt/C might suffer from the aggregation of Pt nanoparticles.
To further evaluate the potential for practical application, the Fe/NS/C-g-C 3 N 4 /TPTZ-1000 was assembled into a homemade primary Zn-air battery as cathode catalyst.Figure 8a presents the polarization curves of Zn-air batteries with Fe/NS/C-g-C 3 N 4 /TPTZ-1000 and Pt/C as cathodes, respectively.The Zn-air battery with Fe/NS/C-g-C 3 N 4 /TPTZ-1000 catalyst exhibits an open-circuit voltage (OCV) of 1.385 V and a maximal power density of 225 mW cm −2 at a temperature of ca. 25 • C, which is quite comparable to that of Pt/C with the OCV of 1.411 V and the maximal power density of 246 mW cm −2 .This performance outperforms the most of Zn-Air batteries utilizing analogous Fe-N-C cathode reported so far [37][38][39][40].In the meantime, the long-term stability of a Zn-air battery Catalysts 2018, 8, 158 9 of 13 with Fe/NS/C-g-C 3 N 4 /TPTZ-1000 was also tested by recording the galvanostatic discharge curves.As presented in Figure 8b, the Zn-Air battery with Fe/NS/C-g-C 3 N 4 /TPTZ-1000 catalyst only presents a voltage loss of 4.03% after 20,000 s at a current density of 20 mA cm −2 .For the Pt/C, the output voltage loss reaches 8.17% under the same conditions, demonstrating the improved durability of the Fe/NS/C-g-C 3 N 4 /TPTZ-1000.The slight fluctuation may due to the disturbance of testing circumstance such as humidity, which cannot be completely avoided.These results suggest that the as-prepared Fe/NS/C-g-C 3 N 4 /TPTZ-1000 catalyst has great potential to replace the precious metal catalysts in practical application of Zn-air battery.
polarization curves of Zn-air batteries with Fe/NS/C-g-C3N4/TPTZ-1000 and Pt/C as cathodes, respectively.The Zn-air battery with Fe/NS/C-g-C3N4/TPTZ-1000 catalyst exhibits an open-circuit voltage (OCV) of 1.385 V and a maximal power density of 225 mW cm −2 at a temperature of ca. 25 °C, which is quite comparable to that of Pt/C with the OCV of 1.411 V and the maximal power density of 246 mW cm −2 .This performance outperforms the most of Zn-Air batteries utilizing analogous Fe-N-C cathode reported so far [37][38][39][40].In the meantime, the long-term stability of a Zn-air battery with Fe/NS/C-g-C3N4/TPTZ-1000 was also tested by recording the galvanostatic discharge curves.As presented in Figure 8b, the Zn-Air battery with Fe/NS/C-g-C3N4/TPTZ-1000 catalyst only presents a voltage loss of 4.03% after 20,000 s at a current density of 20 mA cm −2 .For the Pt/C, the output voltage loss reaches 8.17% under the same conditions, demonstrating the improved durability of the Fe/NS/C-g-C3N4/TPTZ-1000.The slight fluctuation may due to the disturbance of testing circumstance such as humidity, which cannot be completely avoided.These results suggest that the as-prepared Fe/NS/C-g-C3N4/TPTZ-1000 catalyst has great potential to replace the precious metal catalysts in practical application of Zn-air battery.

Preparation of g-C3N4 Nanosheets
Bulk g-C3N4 powder was synthesized according to a procedure described in a previous paper [41].Typically, dicyandiamide (Aldrich, Milwaukee, WI, USA, 99%) powder was placed in an alumina crucible with cover and heated at 550 °C for 4 h in air with a ramp rate of 2.3 °C/min.The obtained yellow agglomerates were grinded into powders.The light yellow g-C3N4 nanosheets were prepared by thermal etching of bulk g-C3N4 in air at 500 °C for 2 h with a ramp rate of 5 °C/min.

Catalyst Synthesis
Commercial Ketjenblack EC 600J (KJ 600) carbon black was first pretreated in 6.0 M HCl solution for 12 h to remove metal impurities and collected by filtration.The obtained carbon black was then treated in concentrated HNO3 solution at 80 °C for 8 h to introduce carboxyl groups [42].

Preparation of g-C 3 N 4 Nanosheets
Bulk g-C 3 N 4 powder was synthesized according to a procedure described in a previous paper [41].Typically, dicyandiamide (Aldrich, Milwaukee, WI, USA, 99%) powder was placed in an alumina crucible with cover and heated at 550 • C for 4 h in air with a ramp rate of 2.3 • C/min.The obtained yellow agglomerates were grinded into powders.The light yellow g-C 3 N 4 nanosheets were prepared by thermal etching of bulk g-C 3 N 4 in air at 500 • C for 2 h with a ramp rate of 5 • C/min.

Catalyst Synthesis
Commercial Ketjenblack EC 600J (KJ 600) carbon black was first pretreated in 6.0 M HCl solution for 12 h to remove metal impurities and collected by filtration.The obtained carbon black was then treated in concentrated HNO 3 solution at 80 • C for 8 h to introduce carboxyl groups [42].
In a typical synthesis, 125 mg of 2,4,6-tri(2-pyridyl)-1,3,5-triazine (TPTZ, Adamas, Shanghai, China, 97%), 125 mg of g-C 3 N 4 nanosheets, and 50 mg of acid-treated KJ600 were dispersed in 50 mL of alcohol under vigorous stirring for 30 min, then Fe(SCN) 3 solution, prepared by mixing FeCl 3 (0.2 M, 1.2 mL) and potassium thiocyanate (KSCN, 0.2 M, 3.6 mL) in 50 mL of alcohol, was added into the suspension and stirred for another 30 min.The solvent was then removed by rotary evaporation and vacuum drying at 80 • C for 3 h.The resulting powder was pyrolyzed at 800 • C in a N 2 atmosphere for 1 h with a ramp rate of 10 • C/min.The pyrolyzed sample was subjected to acid leaching in 0.5 M H 2 SO 4 solution at 80 • C for 8 h to remove unstable and inactive species followed by filtration and thoroughly washed with ultrapure water.The sample was finally vacuum dried to obtain the Fe/NS/C-g-C 3 N 4 /TPTZ catalyst.As a comparison, Fe/NS/C-g-C 3 N 4 and Fe/NS/C-TPTZ catalysts were prepared by the same procedure except using 250 mg of g-C 3 N 4 or 250 mg of TPTZ as the single nitrogen precursor, respectively.

Characterizations
The morphology and elemental mapping of the samples were analyzed using field-emission transmission electron microscopy (FE-TEM, JEM-2100F, JEOL Ltd., Tokyo, Japan) with a working voltage of 200 kV.XRD analysis was performed using a D8 advance powder X-ray diffractometer (Bruker, Karlsruhe, Germany) with a Cu Ka (λ = 1.5418Å) at 0.2 • s −1 .The elemental composition and chemical states were measured by XPS (K-Alpha, Thermo Scientific, Waltham, MA, USA) with an Al Ka X-ray source.The surface areas and pore structures were characterized using a Micromeritics ASAP 2020 instrument (Micromeritics Instrument Corp., Norcross, GA, USA).

Electrochemical Measurement
All electrochemical measurements were conducted on a bipotentiostat (CHI-730E, Shanghai Chenhua, Shanghai, China) equipped with a rotating disk electrode (RDE) or rotating ring-disk electrode (RRDE) system (PINE Inc., Durham, NC, USA) at room temperature.An Ag/AgCl electrode and a graphite plate were used as the reference and counter electrode, respectively.The potential has been experimentally corrected to the range of RHE by the following equation: E(RHE) = E(Ag/AgCl) + 0.956 V.The electrolyte was O 2 -saturated 0.1 M KOH solution.
For all Fe-NS/PC catalysts, 12 mg of catalyst was dispersed in the mixture of 0.5 mL of de-ionized water, 0.45 mL of isopropanol and 0.05 mL of 5 wt % Nafion solution (Aldrich) by sonication for 1 h to produce the ink.A certain volume of the catalyst ink was pipetted onto the pre-polished glassy carbon disk (0.196 cm 2 for RDE and 0.2475 cm 2 for RRDE) resulting a loading of 0.4 mg cm −2 .For commercial Pt/C catalyst (20 wt %, ElectroChem, Inc., Woburn, MA, USA), the ink was prepared by dispersing 10 mg catalyst in 1.0 mL of de-ionized water, 0.95 mL of isopropanol, and 0.05 mL of 5 wt % Nafion solution.The loading of the Pt/C catalyst was 0.1 mg cm −2 .
In RDE tests, ORR polarization curves were measured at a scan rate of 10 mV s −1 .The electrode rotational speed was 1600 rpm.The background current was determined by recording the voltammogram in N 2 -saturated electrolyte.The accelerated durability tests (ADT) were carried out by cycling the potential in the range from 0.6 to 1.1 V in O 2 -saturated electrolyte with a scan rate of 50 mV s −1 .
Hydrogen peroxide yield and the electron transfer number (n) can be calculated by the following equations with the potential of ring electrode fixed at 1.4 V (RHE) in RRDE: where I d is the disk current, I r is the ring current, N = 0.37 is the collection efficiency of the Pt ring.

Primary Zn-Air Battery Test
The cathode of the Zn-air battery was prepared by loading the Fe/NS/C-g-C 3 N 4 /TPTZ-1000 or 20 wt % Pt/C catalyst onto carbon fiber paper (3.0 × 4.0 cm) with a catalyst loading of 1.0 mg cm −2 .Electrolytic zinc powder was used as the anode.The electrolyte was 6.0 M KOH solution.Polarization curves and galvanostatic discharge curves were measured on Arbin battery testing system.

Conclusions
In summary, a Fe, N, S co-doped porous carbon as ORR electrocatalyst was developed based on the interaction of binary nitrogen precursors during the pyrolysis process.After secondary heat treatment, the Fe/NS/C-g-C 3 N 4 /TPTZ-1000 catalyst displays superior ORR activity and durability in alkaline media, in comparison with the commercial Pt/C.Enhanced ORR activity and durability can be attributed to its good porous structure, high surface area, high contents of pyridinic N and graphitic N, and the synergy of N and S co-doping.Moreover, the Zn-air battery assembled with Fe/NS/C-g-C 3 N 4 /TPTZ-1000 as a cathode exhibits comparable power density and better stability than that of the Pt/C, demonstrating its potential for substituting precious metal catalysts in practical energy devices.S2: The element contents of Fe/NS/C-g-C 3 N 4 /TPTZ-T (T = 800, 900, 1000) obtained by XPS, Table S3: Comparison of ORR activity of Fe/NS/C-g-C 3 N 4 /TPTZ, Fe/NS/C-g-C 3 N 4 /TPTZ-T (T = 800, 900, 1000) and Pt/C catalysts.
display the high resolution N 1s spectra of the three Fe-NS/PC catalysts.All the spectra can be deconvoluted into four peaks corresponding to pyridinic N (N1,

Figure 3 .
Figure 3. Oxygen reduction reaction (ORR) polarization curves of Fe/NS/C-g-C 3 N 4 , Fe/NS/C-TPTZ, and Fe/NS/C-g-C 3 N 4 /TPTZ catalysts in O 2 -saturated 0.1 M KOH with a rotating speed of 1600 rpm and at a scan rate of 10 mV s −1 .RHE: reversible hydrogen electrode.