Interconnected Fe, S, N-Codoped Hollow and Porous Carbon

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Interconnected Fe,S,N-Codoped Hollow and Porous Carbon Nanorods as Efficient Electrocatalysts for Oxygen Reduction Reaction Yinling Wang, Chengzhou Zhu, Shuo Feng, Qiurong Shi, Shaofang Fu, Dan Du, Qiang Zhang, and Yuehe Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13095 • Publication Date (Web): 03 Nov 2017 Downloaded from http://pubs.acs.org on November 3, 2017

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ACS Applied Materials & Interfaces

Interconnected Fe,S,N-Codoped Hollow and Porous Carbon Nanorods as Efficient Electrocatalysts for Oxygen Reduction Reaction Yinling Wang,†,‡ Chengzhou Zhu,† Shuo Feng,† Qiurong Shi,† Shaofang Fu,† Dan Du,† Qiang Zhang§ and Yuehe Lin*,† †School

of Mechanical and Materials Engineering, Washington State University, Pullman,

Washington 99164, United States ‡ College of Chemistry and Materials Science, Anhui Normal University, Wuhu, 241000, China §Department

of Chemistry, Washington State University, Pullman, WA 99164, United States

ABSTRACT As promising precious metal-free ORR electrocatalysts, Fe-N-C catalysts still face a great challenge to meet the requirement of practical applications. In this study, Fe, S, N co-doped hollow and porous carbon nanorods (Fe-S-N HPCNRs) were designed with the aim of improving the performance of Fe-N-C catalysts from the perspective of composition and structure. They were successfully prepared using cysteine, Fe2+ salt and polydopamine encapsulated ZnO nanorods (ZnO NRs@PDA) as precursors by a pyrolysis-acid etching strategy. The hollow and porous structure and composition of Fe, S, N and C were verified by TEM, XRD, BET and XPS tests. At the optimum ratio of ZnO NRs@PDA /cysteine and pyrolysis temperature, the Fe-S-N HPCNRs display higher ORR activities than the control samples which are lack of one of the precursors. Electrochemical tests show that the ORR follows a 4e pathway with the Fe-S-N HPCNRs. In addition, the long-term stability and methanol tolerance of Fe-S-N HPCNRs are good and superior to those of 20 wt % Pt/C. Keywords: Fe-S-N structures, porous nanomaterials, carbon nanorods, oxygen reduction reaction, electrocatalysts

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INTRODUCTION The oxygen reduction reaction (ORR) is an important cathodic reaction involved in fuel cells and other energy storage and conversion devices.1-4 To overcome its sluggish kinetics, Pt-based electrocatalysts for ORR are generally required, 5-7 which hampers the practical utilization of such devices due to the low abundance, high cost, and poor long-term stability of Pt. Therefore, various precious metal-free ORR electrocatalysts have been widely investigated as alternatives to the Pt-based catalysts.8-12 Currently, the M-N-C (M: Fe or Co) catalysts, especially Fe-N-C catalysts are the most promising candidates with efficient ORR activities comparable to Pt-based catalysts.13-17 However, M-N-C catalysts with higher catalytic activities and stabilities are still needed to meet the requirement of practical applications.18 To improve the ORR catalytic activities of M-N-C catalysts, many approaches have been attempted from the perspective of composition or structure. In terms of composition, it has been reported that the introduction of element sulfur into Fe-N-C system can improve the ORR activity. 19-22

For example, Sun’s group19 found that the doping of S increased the specific surface area and

changed the structure of carbon material, which led to the higher ORR activities. Ferrandon et al. 23

verified that the introduction of S with optimized amount could prevent the formation of Fe3C

and facilitate the formation of Fe-N active sites and consequently improve the ORR performance. On the other hand, the porous and hollow structure of catalysts are confirmed to enhance the ORR catalytic activities by facilitating the mass transport of ORR-related species,24,25 exposing more ORR active sites26 and generating confine effect.27 However, few ORR electrocatalysts were designed based on the combining strategies mentioned above. In this work, we prepared Fe, S, N co-doped hollow and porous carbon nanorods (Fe-S-N HPCNRs) by pyrolyzing the precursors containing cysteine, Fe2+ salt and polydopamine encapsulated ZnO nanorods (NRs). Among the precursors, polydopamine serves as the main carbon source and N source, Fe2+ salt serves as a Fe source, and ZnO NRs and cysteine play a role of template and S source, respectively. It should be noted that ZnO NRs were chosen as the template not only because of its special morphology but also the fact that ZnO NRs are expected to be reduced to Zn by carbon during the pyrolysis and the escape of Zn vapour may lead to the formation of micropores.28,29 Similarly, cysteine was selected as S source also due to its rich groups including amino, sulfhydryl and carboxyl, which not only can ensure the uniform


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dispersion of S in the catalysts for its good compatibility with polydopamine film,30 but also are helpful to disperse and stabilize Fe2+ and increase the active sites. In addition, two different N/C precursors including cysteine and polydopamine are involved in this study, which is also expected to enhance the ORR activities due to the possible synergistic effect.31 The as-prepared Fe, S, N co-doped hollow and porous carbon nanorods (Fe-S-N HPCNRs) demonstrated excellent ORR activities and high durability in basic medium. This work provides a new strategy to design Fe-N-C ORR electrocatalysts with high performance. RESULTS AND DISCUSSION The preparation process of Fe-S-N HPCNRs is illustrated in Scheme 1. First, the ZnO NRs templates were synthesized by a modified hydrothermal method.32,33 Then the polydopamine (PDA) film was generated on the surface of ZnO NRs via the self-polymerization of dopamine to replicate the structure of NRs. The corresponding products are denoted as ZnO NRs@PDA. To introduce Fe and S elements, ZnO NRs @PDA, Fe2+ salt and cysteine with the weight ratio of 10:1:10 were fully mixed and the mixture was named ZnO NRs@PDA/Fe/Cys. Subsequently, the mixture suffered a pyrolysis at 900 °C for 3 hours under N2 atmosphere and the products are denoted as Fe-S-N PCNRs. Finally, the Fe-S-N HPCNRs were obtained by etching the product of pyrolysis with 5% HCl.

Scheme 1. Schematic illustration of the preparation process of Fe-S-N HPCNRs. The products of key steps were first characterized by TEM. As shown in Figure 1a, the ZnO displays a morphology of typical NRs. The diameter of ZnO NRs ranges from 15 to 25 nm and the typical size of the length is 50-65 nm. Figure 1b shows that the ZnO NRs were successfully encapsulated by PDA with the thickness of 5-10 nm. As the final product, the Fe-S-N HPCNRs exhibit a well-defined hollow carbon nanorod structure (Figure 1c). The thickness of the carbon layer is consistent with that of PDA film. It is noteworthy that the Fe-S-N HPCNRs are not separated as expected but interconnected due to the poor dispersity of ZnO NRs @PDA. However, the interconnected Fe-S-N HPCNRs may be a better choice for ORR electrocatalysts because of



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their larger conductive network.

Figure 1. TEM images of (a) ZnO NRs, (b) ZnO NRs@PDA and (c) Fe-S-N HPCNRs. (d) XRD patterns of ZnO NRs, ZnO NRs @PDA/Fe/Cys, Fe-S-N PCNRs and Fe-S-N HPCNRs. For comparison, the TEM images of carbon materials prepared without ZnO NRs@PDA template (Fe-S-N CMs) and prepared without cysteine in the precursors (Fe-N HPCNRs) were shown in Figure S1 (Supporting Information). The Fe-S-N CMs are in an irregular block structure with nanoparticles ranging from 20-50 nm. For Fe-N HPCNRs, they display a similar hollow carbon nanorod structure to Fe-S-N HPCNRs except that there are some aggregations of nanoparticles in the structure. These results indicate that ZnO NRs as a template can effectively tune the morphology of the carbon materials. In addition, the existence of nanoparticles in both control samples but not in Fe-S-N HPCNRs will be explored later. The preparation process of Fe-S-N HPCNRs was also tracked by XRD. As shown in Figure 1d, the XRD pattern of the as-prepared ZnO NRs is consistent with the standard data of ZnO (JCPDS card no.36-1451). From the XRD pattern of ZnO NRs@PDA/Fe/Cys mixture, it can be seen that the crystalline structure of ZnO still remains after mixing fully with Fe2+ salt and cysteine. However, the XRD diffraction peaks of ZnO disappear and the peaks of ZnS (JCPDS card no.36-1450) and Fe4N (JCPDS card no.06-0627) appear after pyrolysis of the mixture, as can be seen from the XRD pattern of Fe-S-N PCNRs. Considering that ZnO was encapsulated by PDA,


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one possible reason for the existence of ZnS is that H2S gas was produced during the pyrolysis of cysteine which passed through PDA film and reacted with ZnO. Another possible reason is that Zn vapour derived from ZnO at high temperature permeated through the PDA film and reacted with the derivates of cysteine. No matter what the reason is, the presence of ZnS indicates the porous structure of the shell of hollow carbon nanorods. After acid etching, the peaks belonging to ZnS and Fe4N disappear and there are two broad and weak peaks appearing at 24 and 43°, which can be assigned to (002) and (100) reflections of graphitic carbon layers.34 However, no signal of Fe species is found in the XRD patterns of Fe-S-N HPCNRs. To understand the structure of nanoparticles appearing in Fe-S-N CMs and Fe-N HPCNRs as shown in Figure S1 (Supporting Information), their XRD patterns were displayed in Figure S2 (Supporting Information). The XRD data shows that the nanoparticles in Fe-S-N CMs are FeS (JCPDS card no. 37-0477) and those in Fe-N HPCNRs are Fe2O3 (JCPDS card no. 39-1346). The existence of nanoparticles in both control samples but not in Fe-S-N HPCNRs may be due to the fact that single PDA or cysteine has poorer ability to disperse Fe2+ than the composite of PDA and cysteine. This will lead to the high local concentration of Fe2+ and the consequent formation of Fe species with large size are difficult to be leached by acid in the same condition. The difference in the morphology and structure between the Fe-S-N HPCNRs and the control samples including Fe-S-N CMs and Fe-N HPCNRs may lead to different ORR activities. In order to make clear the composition of Fe-S-N HPCNRs, XPS tests were carried out and the results were shown in Figure 2 and Table S1. From Table S1, it can be seen that the content of Zn is only 0.06 at % for Fe-S-N HPCNRs while that of Fe-S-N PCNRs is 3.69 at %，indicating that the ZnO template has been removed by acid etching. The XPS survey spectrum of Fe-S-N HPCNRs in Figure 2a confirms the existence of S, N and C elements. However, the peak of Fe element has not been observed despite the addition of Fe2+ in the precursors. A possible reason is that most of the Fe element was removed during the acid etching process. Thus, the XPS survey spectrum of the sample before acid etching (Fe-S-N PCNRs) were also tested and displayed in Figure 2a. As expected, the peak of Fe 2p appears at 710.1 (Fe 2p 3/2) and 723.7 eV (Fe 2p 1/2) and the atomic percent of Fe in Fe-S-N PCNRs is 0.79%. The comparison of the composition between Fe-S-N HPCNRs and Fe-S-N PCNRs reveals that acid etching not only removes most Zn element



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but also decreases the content of Fe.

Figure 2. (a) XPS survey spectra of Fe-S-N HPCNRs, Fe-S-N PCNRs and Fe-N HPCNRs. (b) High-resolution XPS spectrum of N 1s of Fe-S-N HPCNRs. (c) The atomic percent of various N species in Fe-S-N HPCNRs and Fe-N HPCNRs. (d) High-resolution XPS spectrum of S 2p of Fe-S-N HPCNRs. However, the Fe-N species as real ORR active sites seem difficult to be leached off by acid since some Fe-N-C electrocatalysts show excellent stability in acid medium.22,35 In fact, the content of Fe measured by XPS is very low even in Fe-N-C catalysts with high ORR activities. 35-37

On the other hand, the N−Fe (N 1s) has a higher sensitivity than Fe-N (Fe 2p),23 therefore, the

existence of Fe element can be confirmed by the existence of N in the form of N-Fe. As shown in Figure 2b, the high-resolution N1s spectrum of Fe-S-N HPCNRs can be de-convoluted into four peaks with the high goodness of fit:23 pyridinic N (398.6 eV), N-Fe (399.6 eV), pyrrolic N (400.9 eV) and graphitic N (402.0 eV). The atomic percent of various N species were shown in Figure 2c. The atomic percent of N-Fe is 16.1 and that of total N in Fe-S-N HPCNRs is 3.54, therefore, the atomic percent of Fe coordinated to N is calculated to be 0.57%. In addition, the impact of cysteine as a S source on the composition of final carbon materials was investigated by comparing the composition of Fe-S-N HPCNRs and Fe-N HPCNRs. From Table S1 (Supporting Information) and Figure 2a, it can be seen that the addition of cysteine improves the content of N and S and decreases the content of total Fe. Moreover, Figure 2c shows


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that the N-Fe percent (16.1) in Fe-S-N HPCNRs is higher than that in Fe-N HPCNRs (13.6). Therefore, the atomic percent of Fe coordinated to N for Fe-N HPCNRs is only 0.24%, which is less than half of Fe-S-N HPCNRs in spite of its higher total Fe. This result agrees well with the report that the existence of S with the moderate amount of the precursors was beneficial to the formation of Fe-N active sites23. The final S species in Fe-S-N HPCNRs can be identified by the high-resolution S2p in Figure 2d. In Fe-S-N HPCNRs, the S element exists in the form of -C-S-C(163.9 eV), -C=S- (165.1 eV) and oxidized sulfur (168.7 eV) .22 The -C-S-C- species are generally considered as effective active sites for ORR.38 Doping S into the carbon lattice can change the atomic charge density and spin density of neighbor carbon, which is beneficial to promote the adsorption of oxygen, weak the O-O bonding and consequently improve the ORR catalytic effect.39-40

Figure 3. (a) N2 adsorption isotherm and (b) pore-size distribution of Fe-S-N HPCNRs and Fe-S-N PCNRs (c) Specific capacitance of Fe-S-N HPCNRs and the control samples

It has been inferred from the XRD data that the Fe-S-N HPCNRs have a porous structure due to the appearance of ZnS during pyrolyzing. This information can be further proved by BET tests. As shown in Figure 3a, both Fe-S-N HPCNRs and Fe-S-N PCNRs display a type IV adsorption isotherm, indicating typical mesoporous materials. However, the total pore volume of Fe-S-N HPCNRs is 0.62 cm³/g, larger than that of Fe-S-N PCNRs (0.38 cm³/g). Likewise, the specific surface area of Fe-S-N HPCNRs is 537 m²/g, which is almost 1.6 times that of Fe-S-N PCNRs (341 m²/g). These results show that the remove of the nanorod-like template by acid etching leads to the hollow structure as expected and improve the pore volume and surface area. The distribution of pore size in Figure 3b is consistent with the analysis of adsorption isotherm. Interestingly, the pores in the range of 15-25 nm increase obviously after acid etching, which is right the range of the diameter of ZnO NRs, meaning the removal of the template.



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Compared with BET surface area, the electrochemically accessible surface area (EASA) can truly reflect the performance of electrocatalysts. Therefore, the EASA of Fe-S-N HPCNRs was evaluated by testing the specific capacitance.41 To get the specific capacitance, the cyclic voltammetry (CV) curves ranging from 0.95 to 1.05 V at scan rates of 5, 10, 25, 50 and 100 mV/s were first recorded （Figure S3, Supporting Information）. At 1.0 V vs RHE, the current density is linear to scan rate and the specific capacitance can be obtained from the slope of the line. As shown in Figure 3c, the specific capacitance of Fe-S-N HPCNRs is far larger than that of Fe-S-N PCNRs, Fe-S-N CMs and Fe-N HPCNRs, implying the great roles of ZnO NRs templates and the combination of cysteine and PDA in improving the EASA. It should be noted that the specific capacitance of Fe-S-N HPCNRs is slightly smaller than that of S-N HPCNRs, which may be ascribed to the higher density of Fe-S-N HPCNRs because of the existence of Fe element. The electrocatalytic activities of Fe-S-N HPCNRs towards ORR were first evaluated by CV in 0.1 M KOH. As shown in Figure 4a, there is no redox peak in N2-saturated 0.1 M KOH, while there is a well-defined cathodic peak at 0.86 V in O2-saturated electrolytes, indicating good ORR catalytic activities of Fe-S-N HPCNRs. Likewise, the Fe-S-N CMs, Fe-N HPCNRs and S-N HPCNRs also display ORR activities through CV curves. However, the cathodic peaks of them are all more negative than that of Fe-S-N HPCNRs, which implies their inferior ORR activities to Fe-S-N HPCNRs. This result is further supported by the linear sweep voltammetry (LSV) curves obtained from rotation disc electrode (RDE) tests at 1600 rpm (Figure 4b). When the current density of ORR is 0.3 mA/cm2, the potential of Fe-S-N HPCNRs, S-N HPCNRs, Fe-N HPCNRs and Fe-S-N CMs is 0.93, 0.90, 0.88 and 0.75 V, respectively. At the potential of 0.5 V, the current density of Fe-S-N HPCNRs, S-N HPCNRs, Fe-N HPCNRs and Fe-S-N CMs is 4.2, 3.1, 3.0 and 1.7 mA/cm2. The most positive onset potential and the highest current density at the same potential indicate that Fe-S-N HPCNRs have superior ORR activities to the control samples. Moreover, for Fe-S-N HPCNRs, Fe-N HPCNRs and Fe-S-N CMs, there is a positive correlation between the ORR activities and ECSA. Notably, S-N HPCNRs has a lower ORR activity than Fe-S-N HPCNRs despite its larger ECSA, which proves the important role of Fe in the ORR active sites. It can be inferred that good ORR electrocatalysts should have high ECSA as well as effective ORR active sites.


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Figure 4. (a) CV curves of Fe-S-N HPCNRs and the control samples in N2 (dotted line) or O2 (solid line) saturated 0.1 M KOH electrolyte at 50 mV/s. (b) LSV curves of Fe-S-N HPCNRs and the control samples at the loading amount of 20 µg and (c) LSV curves of Fe-S-N HPCNRs at different loading amounts and 20 wt % Pt/C in O2-saturated 0.1 M KOH electrolyte at a rotation speed of 1600 rpm with a sweep rate of 10 mV/s. d) LSV curves of Fe-S-N HPCNRs at various rotation rates (inset: corresponding Koutecky–Levich plots). (e) Jk and (f) Tafel plots of the Fe-S-N HPCNRs and 20 wt % Pt/C. In addition, the effect of loading amount on the ORR activities of Fe-S-N HPCNRs was also studied. When the loading amount increases from 20 to 60 µg, the onset potential positively shifts and the steady-state current density improves (Figure 4c). However, when the loading amount is up to 80 µg, the ORR activity is very close to that of 60 µg, thus the optimum loading amount is 60 µg for Fe-S-N HPCNRs. We think that the active sites increase with the increasing loading amount of Fe-S-N HPCNRs which ensure the improvement of the oxygen reduction reaction rate



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and high diffusion limit current. However, when the loading amount is too high, the thick layer of catalysts may lead to a slow mass and electron transport which will hamper the ORR rate. Thus, the optimum loading amount is a result balanced between the increased active sites and the disadvantages such as slow mass transport due to the thicker layer of catalysts. At this optimum loading amount of 60 µg, the steady-state current density of Fe-S-N HPCNRs is larger and their onset potential at 0.3 mA/cm2 is only 20 mV negatively shifted compared with 20 wt % Pt/C (20 µg). Namely, the Fe-S-N HPCNRs (60 µg) show a comparable ORR activity to 20 wt % Pt/C (20 µg). The LSV curves of Fe-S-N HPCNRs (60 µg) at different rotating rates were recorded in Figure 4d, based on which the Koutecky-Levich plots at various potentials (Figure 4d inset) and the corresponding kinetic current density (Jk, Figure 4e) are obtained. As shown in Figure 4e, when the potential is 0.40 and 0.60 V, the Jk of Fe-S-N HPCNRs (60 µg) is lower than that of 20 wt % Pt/C (20 µg). While at high potentials such as 0.80 and 0.85 V, the Jk of Fe-S-N HPCNRs is higher than 20 wt % Pt/C (20 µg), indicating a faster kinetic process of ORR with Fe-S-N HPCNRs. At high potential, the ORR tends to be controlled by kinetics which can also be evaluated by Tafel slope. Figure 4f shows that the Tafel slope of Fe-S-N HPCNRs is 83.8 mV/dec, lower than that of 20 wt % Pt/C (91.1 mV/dec). The lower Tafel slope also implies that the kinetic process of ORR is faster with Fe-S-N HPCNRs than with 20 wt % Pt/C at high potentials, which is in accordance with the Jk data. RRDE tests were performed to investigate the mechanisms of ORR for Fe-S-N HPCNRs. As shown in Figure 5a, the electron transfer number of ORR for Fe-S-N HPCNRs is 3.82-3.91, indicating a 4e pathway of ORR. In comparison, Fe-N HPCNRs, S-N HPCNRs and Fe-S-N CMs show lower electron transfer number, implying a lower 4e selectivity. Similarly, the H2O2 yield of Fe-S-N HPCNRs is below 10% in the potential range of 0-0.8 V, while that of the control samples exceeds 10 % in the same potential range. This result further demonstrates that the Fe-S-N HPCNRs has a higher 4e-pathway selectivity than the control samples.


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Figure 5. Peroxide yield and electron transfer number from RRDE of (a) Fe-S-N HPCNRs and the control samples with the loading amount of 20 µg and (b) Fe-S-N HPCNRs with different loading amount and 20 wt % Pt/C (20 µg). Additionally, the RRDE results in Figure 5b reveal that the electron transfer number and the H2O2 yield of Fe-S-N HPCNRs almost keep the same with the increase of the loading amount. It is reasonable that the increase in loading amount of electrocatalysts can improve the ORR activities without changing the reaction mechanism. Notably, Fe-S-N HPCNRs (60 µg) have a better 4e-pathway selectivity than 20wt % Pt/C, which can be ascertained from both the electron number and the H2O2 yield. Excellent long-term stability is an important metric for ORR electrocatalysts. Therefore, the long-term stability of both Fe-S-N HPCNRs and 20 wt % Pt/C was measured by chronoamperometric technique at 0.7 V in O2-saturated 0.1M KOH. Compared with 74% for commercial Pt/C, the current density of Fe-S-N HPCNRs remains up to 92% after testing for 20000 seconds, indicating a better stability of Fe-S-N HPCNRs (Figure 6a). The methanol tolerance should be tested if the ORR electrocatalysts are expected to be used in direct methanol fuel cells (DMFC). Figure 6b and 6c depict the CV curves of Fe-S-N HPCNRs and 20 wt % Pt/C with and without methanol. There is almost no change in CV curves of Fe-S-N HPCNRs before and after adding 1.0 M methanol. However, for 20 wt % Pt/C, the cathodic peak of ORR disappears and the methanol oxidation peak appears upon adding methanol. These results indicate that Fe-S-N HPCNRs have a better tolerance toward methanol than commercial Pt/C.



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Figure 6. (a) Chronoamperometric response of Fe-S-N HPCNRs and 20 wt % Pt/C at 0.70 V in O2-saturated 0.1 M KOH at 200 rpm. CV curves of (b) Fe-S-N HPCNRs and (c) 20 wt % Pt/C with and without methanol in O2-saturated 0.1 M KOH at 50 mV/s. It should be noted that the effect of the weight ratio of ZnO@PDA/cysteine and the pyrolysis temperature on the ORR activities were also studied. The results show that the optimum ratio of ZnO@PDA / cysteine and pyrolysis temperature was 1:1 and 900 °C, respectively (Figure S4 and S5, Supporting Information). The Fe-S-N HPCNRs studied above were all prepared under the best conditions. In addition, the ORR catalytic activity of Fe-S-N HPCNRs in acidic medium was also studied and the corresponding results are shown in Figure S6. Compared with Fe-N HPCNRs without S, Fe-S-N HPCNRs also display a higher ORR catalytic activity in the acidic medium. Moreover, the ORR catalytic activity of Fe-S-N HPCNRs can also be improved by improving the loading amount. However, even with the high loading amount of 80 µg, the onset potential of Fe-S-N HPCNRs at 0.3 mA/cm2 is 0.77 V, almost 120 mV negatively shifted compared with 20% Pt/C. The corresponding current density at 0.5V for Fe-S-N HPCNRs is 2.9 mA/cm2, only 50% of that of 20 wt % Pt/C. It can be seen that the Fe-S-N HPCNRs show superior ORR activity in alkaline medium to that in acidic medium, which is similar to other S−Fe/N/C catalysts.21 CONCLUSION In summary, interconnected Fe, S, N co-doped hollow and porous carbon nanorods (Fe-S-N HPCNRs) have been successfully prepared with well-designed precursors by a simple pyrolysis-acid etching approach. The as-prepared Fe-S-N HPCNRs exhibit excellent ORR activities, long-term stability and methanol tolerance in basic medium. Their good performance can be well associated with their hollow and porous structure, large specific surface and ESCA, as well as effective ORR active sites such as Fe-N and -C-S-C-. Notably, the introduction of cysteine as a S source can increase the Fe-N active sites, but this result can’t be completely attributed to S


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element because N element is also contained in cysteine. This work provides a new perspective to design Fe-N-C electrocatalysts for ORR with high performance. EXPERIMENTAL SECTION Chemical and materials. Zinc acetate dihydrate (Zn(Ac)2.2H2O), dopamine hydrochloride (DA), iron (II) chloride hydrate (FeCl2.xH2O, 99%) and potassium hydroxide (KOH) were obtained from Alfa Aesar. L-cysteine, trizma hydrochloride, tris base, commercial 20 wt % Pt/C and Nafion perfluorinated resin solution were purchased from Sigma-Aldrich Co., Ltd. Methanol was bought from J.T.Baker. Ultrapure water (>18.2 MΩ.cm) was used throughout the whole experiment. Materials preparation. ZnO nanorods (ZnO NRs). ZnO NRs were prepared according to the literature with modifications.31,32 First, 2.70 g Zn(Ac)2.2H2O was dissolved in 125 mL methanol at 60 °C and 1.09 g KOH was dissolved in 65 mL methanol at room temperature, respectively. Then the methanol solution of KOH was added into the Zn(Ac)2 solution dropwise. The mixture was stirred to react and evaporate methanol at 60 °C until only 40 mL suspension remained. Next, the suspension was transferred into Teflon Vessel and heated at 120 °C for 24 hours. After centrifuging, washing and drying, the white ZnO NRs were obtained. ZnO nanorods encapsulated with polydopamine (ZnO NRs @ PDA). 0.2 g ZnO NRs were first dispersed into 100 mL 0.1 M Tris buffer solution (pH=8.5) by ultrasound and then 0.1 g dopamine hydrochloride was added into the suspension under stirring. After reacting for 7 hours, the mixture was centrifuged, washed and dried. Thus the black ZnO NRs@PDA composites were gained. Fe, S, N-codoped hollow and porous carbon nanorods (Fe-S-N HPCNRs). Fe-S-N HPCNRs were derived from three precursors: ZnO NRs@PDA, FeCl2.xH2O and cysteine. Generally, 0.3 g ZnO NRs @PDA, 0.03g FeCl2.xH2O and cysteine with different mass (0.18 g, 0.30 g and 0.60 g) was mixed in 30 mL DI water under stirring for 3 hours. After being freeze-dried, the mixture suffered a pyrolysis at different temperature (800, 900 and 1000 °C) with a temperature rate of 5 °C/min in N2 atmosphere for 3 hours. Next, the products of pyrolysis were dispersed in 5% HCl under ultrasound and reacted for 2 hours to wash off ZnO NR templates. After centrifuging, washing and freeze-drying, black Fe-S-N HPCNRs were finally harvested. For comparison, a series of control samples were also prepared according to the similar



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procedures. Fe,N co-doped hollow and porous carbon nanorods (Fe-N HPCNRs) were prepared without the addition of cysteine in the precursors. S, N co-doped hollow and porous carbon nanorods (S-N HPCNRs) was prepared in the absence of FeCl2.xH2O in the precursors. When the precursors were only FeCl2.xH2O and cysteine, the corresponding products were denoted as Fe-S-N CMs. Materials characterizations. The morphology, structure and composition of the as-prepared materials were tested by different instruments, including transmission electron microscopy (TEM, Philips CM200 UT), X-ray diffraction (XRD) instrument (Rigaku Miniflex 600), X-ray photoelectron spectroscopy (XPS, Kratos Axis-165 multitechnique electron spectrometer system) and nitrogen adsorption-desorption testing instruments (Micromeritics TriStar 3000, at 77 K). Electrochemical tests. Firstly, 2 mg/mL ink of ORR electrocatalysts was prepared by dispersing catalysts in a solution containing Nafion, 2-propanol and DI water (v/v/v=0.025/1/4) by ultrasound. Then, 10 µL ink of catalysts was dropped onto working electrode and dried in oven at 50 °C. For Fe-S-N HPCNRs, ink of catalysts with different volume (10, 20, 30 and 40 µL) were dropped onto the electrode surface to investigate the effect of loading amount on the ORR catalytic activities. Two different working electrodes were used in the electrochemical tests: rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE). RDE tests were performed on an electrochemical workstation (CHI 630E) coupled with a standard 3-electrode system, where a saturated calomel electrode (SCE), a Pt foil and a glassy carbon electrode (5 mm in diameter) were used as a reference electrode, counter electrode and working electrode, respectively. The current density was obtained by dividing the measured current by the geometric area (0.196 cm2 for RDE). The measured potentials vs SCE were converted to a reversible hydrogen electrode (RHE) scale according to the Nernst equation (ERHE = ESCE + 0.241 + 0.059pH). Cyclic voltammetry (CV) tests were carried out in N2 or O2 saturated 0.1 M KOH solution at a scan rate of 50 mV/s without rotating. Linear sweep voltammetry (LSV) tests were performed in O2 saturated 0.1 M KOH solution at a scan rate of 10 mV/s at 1600 rpm for the control samples and various rotating speeds (from 225 to 2500 rpm) for Fe-S-N HPCNRs and 20 wt %Pt/C. The long-term stability of Fe-S-N HPCNRs and 20 wt % Pt/C was tested by chronoamperometry in O2


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saturated 0.1 M KOH solution at 0.70 V vs RHE with a rotating speed of 200 rpm. The methanol-tolerance ability of Fe-S-N HPCNRs and 20 wt % Pt/C were evaluated by the CV curves before and after adding 1.0 M CH3OH into O2 saturated 0.1 M KOH solution. Rotating ring-disk electrode (RRDE) tests were carried out on an electrochemical workstation (CHI 1030 C) with a similar 3-electrode system to that of RDE tests except for the working electrode. For RRDE, the diameter of the glass carbon disk is 6.5 mm. The RRDE tests of different catalysts were performed in O2 saturated 0.1 M KOH solution with a scan rate of 10 mV/s at 1600 rpm and the potential of Pt ring was 1.5 V vs RHE. The yield of hydrogen peroxide and the electron transfer number (n) of different catalysts were obtained based on the following equation

݊ =4×

‫ܫ‬஽ ‫ܫ‬ோ ܰ + ‫ܫ‬஽

%‫ܪ‬ଶ ܱଶ = 200 ×

‫ܫ‬஽ ‫ܫ‬ோ + ‫ܫ‬஽ ܰ

where ID and IR are the disk current and the ring current, respectively, and N is the collection coefficient of the Pt ring (N = 0.37). ■ASSOCIATED CONTENT Supporting Information

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. TEM images of Fe-S-N CMs and Fe-N HPCNRs; XRD patterns of Fe-S-N CMs and Fe-N HPCNRs; CV curves of Fe-N HPCNRs, Fe-S-N PCNRs, Fe-S-N HPCNRs, S-N HPCNRs and Fe-S-N CMs at different scan rates; LSV curves of Fe-S-N HPCNRs with different weight ratio of ZnO@PDA/ cysteine in O2-saturated 0.1M KOH electrolyte; LSV curves of Fe-S-N HPCNRs with different pyrolysis temperature in O2-saturated 0.1 M KOH electrolyte; the composition of Fe-N HPCNRs, Fe-S-N PCNRs and Fe-S-N HPCNRs from XPS. ■AUTHOR INFORMATION Corresponding Author *

Email: [email protected] (Y. L.) Tel: +1 509 335 8523.

■Notes The authors declare no competing financial interest.

■ACKNOWLEDGMENT This work was supported by a start-up fund of Washington State University, USA. Dr. Y. Wang thanks the China Scholarship Council (CSC). The authors acknowledge Franceschi Microscopy



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