CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201400049

Seaweed-Derived Heteroatom-Doped Highly Porous Carbon as an Electrocatalyst for the Oxygen Reduction Reaction Min Young Song, Hyean Yeol Park, Dae-Soo Yang, Dhrubajyoti Bhattacharjya, and JongSung Yu*[a] We report the template-free pyrolysis of easily available natural seaweed, Undaria pinnatifida, as a single precursor, which results in “seaweed carbon” (SCup). Interestingly, thus-obtained SCup not only contains heteroatoms such as nitrogen and sulfur in its framework, but it also possesses a well-developed porous structure with high surface area. The heteroatoms in SCup originate from the nitrogen- and sulfur-containing ingredients in seaweed, whereas the porosity is created by removal of salts inherently present in the seaweed. These essential and fundamental properties make seaweed a prime choice as a precursor for heteroatom-containing highly porous carbon as a metal-free efficient electrocatalyst. As-synthesized SCup showed excellent electrocatalytic activity in the oxygen reduc-

tion reaction (ORR) in alkaline medium, which can be addressed in terms of the presence of the nitrogen and sulfur heteroatoms, the well-developed porosity, and the electrical conductivity in the carbon framework. The pyrolysis temperature was a key controlling parameter that determined the trade-off between heteroatom doping, surface properties, and electrical conductivity. In particular, SCup prepared at 1000 8C showed the best ORR performance. Additionally, SCup exhibited enhanced durability and methanol tolerance relative to the state of the art commercial Pt catalyst, which demonstrates that SCup is a promising alternative to costly Pt-based catalysts for the ORR.

Introduction In the last decade, fuel cells have drawn great attention as a clean alternative energy source, which can make up and counteract the depletion of expensive fossil fuels and the growing threat of environmental pollution. Fuel cells are recognized as an excellent power source because of their high efficiency, high power density, and negligible pollutant emission.[1] The oxygen reduction reaction (ORR) is one of the key reactions occurring in fuel cells, and to date mostly platinumbased catalysts or platinum alloys have shown the best electrocatalytic activity as cathode material for the ORR. However, wide-ranged applications of fuel cells are greatly hampered mainly as a result of the high cost and scarceness of platinum metal and its sluggish kinetics toward the ORR. Moreover, the gradual loss of the electrochemical surface area of platinumbased materials results in a decrease in the ORR activity and thus in an overall diminution in the fuel-cell performance.[2] To overcome these issues, much effort has been devoted to finding alternatives for platinum-based electrocatalysts by using nonprecious metals[3] and various heteroatom-doped carbonaceous materials.[4] Recently, carbonaceous materials doped [a] M. Y. Song, H. Y. Park, D.-S. Yang, D. Bhattacharjya, Prof. J.-S. Yu Advanced Materials Chemistry Korea University 2511 Sejong-ro, Sejong 339-700 (Republic of Korea) Fax: (+ 82)44-860-1331 E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201400049.

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with heteroatoms such as nitrogen, sulfur, phosphorus, and boron have been greatly investigated because of their excellent ORR electrocatalytic performances, relatively low cost, excellent methanol tolerance, and long-term durability. Furthermore, carbon materials doped with two or more heteroatoms have also been investigated.[5] Further improvement in the ORR activity was also suggested by the synergistic effect arising from the co-doping of heteroatoms.[5b, 6] These heteroatomdoped carbon materials have been also investigated as metal electrocatalyst supports, in which the presence of heteroatoms was found to reduce catalyst corrosion under oxidizing conditions and thus improve the catalyst durability along with a probable synergistic contribution towards the ORR.[7] The superior electrocatalytic performance and durability of heteroatom-doped carbon materials are mainly due to the covalent bonding of the catalytically active heteroatom to the carbon framework, unlike the physical bonding of platinum over carbon supports.[8] Various carbonaceous materials such as carbon nanotubes, graphene, mesoporous carbon, or amorphous carbon have been studied as hosts for heteroatom doping.[9] Among these, in particular, mesoporous carbon materials doped with nitrogen, phosphorus, or sulfur atoms have illustrated excellent electrocatalytic performance. This can be attributed not only to heteroatom doping in the framework, but also to their excellent physical properties such as their porous structure with a high surface area and large pore volume available for the mesoporous carbon.[4a, b] These carbon materials are traditionalChemSusChem 2014, 7, 1755 – 1763

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CHEMSUSCHEM FULL PAPERS ly prepared through nanocasting of mesoporous silica or a nanostructured assembly of inorganic particles by using heteroatom-containing organic molecules as the carbon precursor.[4a, 9e] However, most of these synthetic approaches are usually complex, time consuming, and costly and they often require the use of structure-directing agents or inorganic hosts. Additionally, not only heteroatom-containing carbon precursors but also hazardous substances (e.g., plenteous concentrated acid and base) as reagents or for the removal of the template have been employed. These negative aspects on the cost and environment seriously limit the development of nanostructured carbon materials on industrial scale. To diminish these aspects, researchers worldwide are still struggling to identify green and simple methods for the fabrication of heteroatomdoped highly porous carbonaceous materials in sufficient quantities at a relatively low cost to sustain commercial applications. Besides such conventional approaches, a variety of natural biomass-based materials, derived from plants and animals, have been transformed into these carbon materials through critical steps followed by post-activation.[10] In this work, we present a convenient, ultimately economical, and innovative route to synthesize highly porous carbon doped with heteroatoms such as nitrogen, sulfur, and phosphorus and with a high surface area from easily available natural seaweed as a single precursor. Seaweeds have been utilized as food for humans and in the pharmaceutical and cosmetics industries owing to their great nutritional value.[11] Seaweeds as marine algae are divided into three different groups since the mid-19th century based on their pigments and coloration: brown, red, and green algae. Brown algae represent the most common group with many different species. In general, the chemical composition of brown algae varies depending on several environmental factors of the natural habitats such as water temperature, salinity, light, and nutrients.[11, 12] Often inorganic ingredients are used as porogen for pore generation after dissolution.[13] These features make seaweeds useful as vital precursors for producing porous nanostructured carbons enriched with various heteroatoms.[13, 14] A literature search revealed that only one group has reported the use of seaweed carbon materials for electrochemical application. Beguin et al. reported the pyrolysis of a brown alga without any further activation process to obtain a porous carbon material for a supercapacitor.[14a, b] They used Lesonia Nigrescens as the species for the preparation of the porous supercapacitor carbon. Interestingly, although they tried to maximize the utilization of porosity in their carbon for improved supercapacitors, there was not much of an analysis of the heteroatoms (nitrogen, sulfur, and phosphorus) likely to be doped into the carbon apart from oxygen. We utilized Undaria pinnatifida (U. pinnatifida) as the alga species, which is also one of the brown algae and very popular as food worldwide, particularly in eastern Asian countries.[14c] Moreover, enormous quantities of raw U. pinnatifida are harvested worldwide (in 2013, Korea alone produced 500 000 tons of U. pinnatifida), which is beneficial in terms of cost and labor for commercial production. Thus, U. pinnatifida is abundant and of low cost, thus proving to be an excellent carbon pre 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org cursor. Herein, a simple inert atmosphere was used to obtain heteroatom-containing porous seaweed carbon (SCup) possessing a high surface area by pyrolysis of natural U. pinnatifida at different pyrolysis temperatures. The strategy was to use a cost-effective and easily available natural “green” source, that is, seaweed as a single precursor, to directly synthesize the heteroatom (nitrogen and sulfur)-rich porous carbon material without complex and time-consuming multistep processing of the commonly used templating methods. To the best of our knowledge, there is no report on heteroatom-containing porous carbon synthesized from natural seaweed U. pinnatifida and its use as a cathode in fuel cells for the ORR.

Results and Discussion A schematic illustration of the synthesis of SCup from U. pinnatifida as a single precursor is shown in Scheme 1. In a typical experiment, the SCup materials were obtained by pyrolysis of

Scheme 1. Schematic illustration for the preparation of nitrogen- and sulfurcontaining SCup from natural seaweed, U. pinnatifida.

beforehand-dried raw seaweed at pyrolysis temperatures of 800, 900, 1000, and 1100 8C under a N2 atmosphere for 5 h. The resulting pyrolyzed carbon materials were treated with 3.0 m HCl to remove the redundant inorganic impurities (mainly sodium, magnesium, and chlorine) and then further dried at 80 8C in air to obtain the final SCup materials. The as-prepared carbon materials before acid treatment are labelled as SCupB-X, whereas after acid treatment, the corresponding carbon materials are labelled as SCup-X, in which B and X represent before acid treatment and the pyrolysis temperature, respectively. In general, the dried raw seaweed was first pyrolyzed, followed by acid treatment to produce approximately 600–800 mg of carbon material through a simple template-free method by using 5.0 g of dried raw seaweed, a very abundant and cheap resource. To ensure the reproducibility of the work, we synthesized SCup materials in several batches and characterized their morphological and surface properties that mainly govern the ORR such as heteroatom content, surface area, and mesopore and micropore volume. The results showed excellent reproducibility with approximately less than  10 % variation, which is very common in the case of pyrolyzed carbon materials. The SEM images in Figure 1 show the surface morphology of dried raw seaweed, SCupB-1000, and SCup-1000. The surface of raw seaweed (Figure 1 a and b) was smooth, unchanged, and wrinkled. After pyrolysis at 1000 8C (i.e., SCupBChemSusChem 2014, 7, 1755 – 1763

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The morphological changes resulting from pyrolysis and acid treatment were also characterized by TEM. The bulk nature of the raw seaweed is shown in Figure 2 a and b, whereas the development of a mesh-like network having inorganic particles embedded in the carbon framework after pyrolysis is observed in the SCupB-1000 sample (Figure 2 c and d). Both the raw seaweed and SCupB samples illustrate a good dispersion of small Figure 1. Typical SEM images of a, b) dried raw seaweed, c, d) SCupB-1000, and e, f) SCup-1000. black spots, which can be attributed to the inorganic nanoparticles (see the XRD patterns of the samples in Figure 3). The TEM images clearly show that SCup1000 (Figure 2 e and f) possesses wrinkled and graphene-like features with a highly developed porous network after acid treatment, which was further confirmed by high-resolution (HR) TEM (Figure 2 g and h). The inorganic nanoparticles are not observed in the SCup-1000 sample after acid treatment. Similar morFigure 2. TEM images of a, b) dried raw seaweed, c, d) SCupB-1000, and e, f) SCup-1000; g, h) HRTEM images of phological transformations were SCup-1000. also observed in other SCup samples pyrolyzed at different pyrolysis temperatures. The details are provided in Figure S1 1000), the surface became rough, as shown in Figure 1 c and d. and S2 with the corresponding SEM and TEM images in the After subsequent 3.0 m HCl treatment, the degree of roughness Supporting Information. increased and the formation of various pore structures was The porous nature of the raw seaweed and SCup samples clearly visualized (see SCup-1000 in Figure 1 e and f). The inwas investigated by a N2 adsorption analyzer. Figure 3 a shows crease in roughness and the formation of a pore-like structure are attributed to the high porosity created after removal of the the N2 adsorption–desorption isotherms and the correspondinorganic impurities (mainly sodium, magnesium, and chlorine) ing pore-size distribution curves of SCup-1000. The N2 isotherms of the SCup-1000 sample show a typical type IV isoingrained in the raw seaweed, as will be shown in the XRD therm with a H3 hysteresis loop, which are indicative of the exand X-ray photoelectron spectroscopy (XPS) measurements istence of mesopores. Brunauer–Emmett–Teller (BET) analysis (see below).

Figure 3. a) Nitrogen sorption isotherms of SCup-1000 with corresponding pore-size distribution (inset), b) XRD patterns of raw seaweed and SCupB-1000 (* indicates other impurities), and c) XRD pattern of SCup-1000.

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CHEMSUSCHEM FULL PAPERS further confirmed the high surface area and the total pore volume of 1217.78 m2 g 1 and 1.45 cm3 g 1, respectively, with micropore and mesopore volumes of 0.53 and 0.92 cm3 g 1, respectively, for SCup-1000. The pore-size distribution was very narrow and centered at approximately 3.12 nm for SCup-1000 as determined by analysis of the adsorption branch using the Barrett–Joyner–Halenda (BJH) method. The textural parameters of raw seaweed and SCup-800, -900, and -1100 are shown in Figure S3 and are summarized in Table S1. The porous structures with a high surface area were generated by evaporation of the impurities present in raw seaweed during pyrolysis and by subsequent acid treatment to remove the residual impurities from the carbon structure. In general, the surface area increased gradually as the pyrolysis temperature was increased up to 1000 8C; however, upon a further increase in the pyrolysis temperature above 1000 8C, the BET surface area significantly decreased to 674.63 m2 g 1, as shown for SCup-1100 in Figure S3 d. The XRD patterns of raw seaweed, SCupB-1000, and SCup1000 are shown in Figure 3 b and c. The XRD pattern of raw seaweed shows peaks corresponding to halite (NaCl, JCPDS01-076-1688) and some unknown impurity marked with an asterisk (*, Figure 3 b). Interestingly, the XRD pattern of SCupB1000 shows new signals corresponding to MgO (JSPDS-01-0768598) as well as halite (JCPDS-01-076-1688) and other impurities. The presence of MgO signals in SCupB-1000 signifies that magnesium ions may react with oxygen-containing species such as H2O to form MgO during pyrolysis at high temperature. The SCupB sample was washed with 3.0 m HCl to remove halite rock salt and MgO. After acid treatment, SCup-1000 revealed two broad maxima peaks at approximately 2q = 238 (0 0 2) and 43.58 (1 0 0), which are typical of a turbostratic carbon structure, whereas characteristic signals corresponding to the salt, oxide, or other impurities disappeared (Figure 3 c); this is in agreement with the TEM images shown in Figure 2 e– h. The (100) reflection corresponds to the honeycomb structure, which is formed by sp2-hybridized carbon atoms, whereas the (002) reflection between 20 and 308 corresponds to coherent and parallel stacking of the graphene-like sheets. The (002) reflection of SCup at 23.38 was determined to give a lattice spacing of 3.75 , which is larger than that (3.3 ) for the corresponding reflection of graphite at 268. This indicates that the interlayer spacing of SCup-1000 is much larger than that of graphite, probably because of the presence of large sulfur heteroatoms in the framework. Again, for the SCup samples obtained after pyrolysis of seaweed at a high temperature followed by acid treatment, the relatively high surface areas along with the high porosity observed in Figure 3 a and Figure S3 are attributed to the removal of inherent inorganic salt (halite: NaCl), MgO, and other impurities present in the seaweed. As the temperature was increased, it is likely that the inorganic salts or Mg trapped inside the initial raw seaweed evaporated, and all the remaining impurities were removed again after acid treatment, which created a large amount of pores in the resulting SCup along with a high surface area. Figure S4 illustrates the XRD patterns of the SCupB and SCup materials prepared at different pyrolysis  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org temperatures. Interestingly, as the pyrolysis temperature was increased, the XRD signal intensity of halite (NaCl) gradually decreased, probably because of evaporation, whereas the signal intensity of MgO increased slightly or remained steady in the SCupB samples. XPS measurements were also conducted to elucidate the chemical structure of raw seaweed and the resulting SCup nanostructure (Figure 4, Figures S5 and S6, Table S2). The XPS

Figure 4. XPS spectra of a–c) raw seaweed and d–f) SCup-1000, for which a, d) is for the survey scan, b, e) is for the for N 1s binding energy region, and c, f) is for the S 2p binding energy region.

survey spectra revealed the presence of carbon, oxygen, nitrogen, sulfur, and phosphorus atoms as well as other elements such as sodium, magnesium, and chlorine for the raw seaweed and SCupB samples. In contrast, the XPS survey spectrum of SCup-1000 clearly showed the presence of carbon, oxygen, nitrogen, and sulfur signals, whereas magnesium, sodium, chlorine, and other elements almost disappeared, probably as a result of their removal by high-temperature pyrolysis and acid treatment, as also proven in Figures 2 e–h and 3 c. Elements such as nitrogen, sulfur, and phosphorus are widely believed to be responsible for the improvement in the activity of carbon materials towards various electrochemical reactions.[4a, d, 8] Nitrogen acts as an electron donor that increases the ntype conductivity of carbon, and nitrogen doping causes atomic-scale structural deformation. Moreover, owing to the strong electron affinity of nitrogen, the electron arrangement is altered, which in turn causes charge localization.[6a, 15] Recently, sulfur and phosphorus atoms were also studied as ideal heteroatoms because of their larger atomic size and different electronegativities, which can induce greater strain and defects in the carbon material relative to that induced by nitrogen, and ChemSusChem 2014, 7, 1755 – 1763

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this facilitates charge localization for favorable chemisorption ment with thiophenic sulfur atoms as a result of its spin–orbit of oxygen.[16] coupling, whereas the third peak C S (O)x C (x = 2–4,  168.5 eV) is oxidized sulfur.[19] As shown in Figure S6, the S 2p The deconvoluted high-resolution N 1s spectra of raw seaweed and SCup-1000 are shown in Figure 4 b, e. The obtained spectrum changed significantly with the pyrolysis temperature. binding energy values of 398.1, 400.5, and 401.5 eV can be atThe relative amount of oxidized sulfur species, C S (O)x C tributed to pyridinic, pyrrolic, and quaternary nitrogen atoms, decreased, whereas that of thiophenic sulfur atoms increased respectively, and the high energy peaks from 402 to 405 eV are with increasing temperatures. Notably, with an increase in the attributed to pyridinic-N-oxide or other oxidized nitrogen functemperature from 800 to 1000 8C, the highest energy peaks, tionalities.[4i, 17, 18] Pyrrolic nitrogen atoms are predominant in incorresponding to quaternary nitrogen atoms and thiophenic sulfur atoms, became dominant in the carbon network of itial dried seaweed (Figure 4 b). However, the amount of therSCup-1000 (Figure S7 b), and these species are known to be mally less stable pyrrolic nitrogen atoms decreased with inactive for the ORR. creasing pyrolysis temperature, and that of pyridinic and quaThe mechanism of electrochemical O2 reduction reaction internary nitrogen atoms became dominant in the case of SCup1000 (Figure 4 e). As the pyrolysis temperature was increased volves multielectron transfer and can proceed through two from 800 to 1100 8C, the overall nitrogen content decreased possible pathways: a direct four-electron pathway from O2 to from 5.21 % to 1.8 % (Figure S6 and Table S2). However, the relOH and an indirect two-electron pathway including the forative ratio of nitrogen species showed an interesting temperamation of intermediate HO2 , which needs to be further reture dependency as a function of pyrolysis temperature. The duced to OH in a subsequent two-electron step. To quantify pyrrolic nitrogen content largely dropped, possibly owing to the ORR electron-transfer pathway, we used the rotating ring its lower stability, whereas that of pyridinic nitrogen atoms indisk electrode (RRDE) technique, with which the amount of creased slowly with increasing pyrolysis temperature, reached HO2 generated at the disk electrode could be accurately dea maximum at nearly 900 8C, and then decreased at 1000 8C termined. It can be observed in Figure 5 a that all of the SCup and higher.[17] Interestingly, quaternary nitrogen atoms, known samples showed significant changes in ORR polarization curves to be the most stable, increased more significantly from an inibetween 0 and 0.3 V, among which the SCup-1000 sample tial low ratio and became predominant, particularly at 1000 8C showed an ORR onset potential of 0.01 V, which is very close and higher (Figure S7 a). It was proposed that both pyridinic to the 0.01 V for the commercial Pt/C catalyst; this demonand quaternary nitrogen atoms play an important role and strates the excellent ORR performance of seaweed-derived produce active sites for the ORR.[4i, 18a] The SCup-1000 sample SCup-1000. This excellent behavior can be attributed to the contains a high amount of quaternary and pyridinic nitrogen presence of catalytically active sites for O2 chemisorption and atoms, which could provide highly active catalytic sites for the reduction in the vicinity of the nitrogen and sulfur heteroORR. atoms and also to the high surface area, which results in Furthermore, it is very interesting to note that sulfur atoms remained in the SCup framework even at high pyrolysis temperatures of 1000 8C and higher unlike phosphorus atoms, which disappeared from the SCup samples although it was observed weakly in the initial raw seaweed. Although the absolute amount of sulfur in SCup decreased with temperature, its relative atomic percent remained steady or increased slightly, as shown in Table S2. The high-resolution S 2p spectrum of SCup1000 can be resolved into three different peaks at binding energies of approximately 163.5, 165.1, and 168.5 eV. Sulfur atoms can be doped into the carbon material in two distinct forms, that is, S 2p3/2 (  163.5 eV) and Figure 5. Steady-state RRDE experiments of the SCup and 20 wt % Pt/C (E-TEK) catalysts at 1600 rpm electrode roS 2p1/2 (  165.1 eV) for C S C tation rate and 10 mV s 1 potential scan rate. a) Disk and b) ring currents are shown separately for convenience. species with a higher S 2p3/2 c) Plot of percent peroxide formation, and d) number of electrons transferred at different potentials for the SCup peak intensity, which is in agree- and 20 wt % Pt/C (E-TEK) catalysts.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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a high volumetric density of these active sites. If Figure 5 a is examined more closely, it can be observed that the two important parameters for ORR activity, that is, low onset potential and high limiting current density, change in the order SCup1000 > SCup-900 > SCup-800 > SCup-1100, which is also the order of decreasing BET surface area (Table S1). The corresponding formation of HO2 was measured with a platinum ring electrode at the potential of 0.50 V. Figure 5 b shows the ring currents for the SCup samples in addition to that of 20 wt % Pt/C for comparison. The ring current corresponds to the amount of HO2 that reaches the ring electrode before it is reduced to HO at the disk electrode under the increase in negative potential. Figure 5 c and d shows the percentage of peroxide formation and the number of electrons transferred at different potentials for SCup and commercial Pt/ C. The electron number (n) and HO2 percentage were calculated using the equations shown in the Supporting Information.[18a, b] The SCup-1000 sample showed an n value higher than 3.7 and HO2 formation of 6 % or less over the whole potential range; this shows the excellent electrocatalytic activity. The result clearly emphasizes that the ORR proceeds mainly through a four-electron mechanism for SCup-1000. SCup-1000 showed not only the best ORR activity among all the SCup materials prepared at different pyrolysis temperatures (see Figure S8), but it also showed ORR activity that was similar to that of 20 wt % Pt/C (Figure 5 a). The enhanced ORR activity of SCup-1000 may be attributed to the high porosity with the high surface area and pore volume and also to the presence of a relatively large amount of active quaternary and pyridinic nitrogen sites as well as thiophenic sulfur sites. The electrocatalytic ORR activity of SCup-1000 was studied in a three-electrode geometry by using cyclic voltammetry (CV) in N2- and O2-saturated 0.1 m KOH solution, and it was compared to that of commercial Pt/C. Figure 6 a and b shows the CV curves of SCup-1000 and commercial 20 wt % Pt/C at a scan rate of 50 mV s 1. Featureless voltammetric currents within the potential range from 1.2 to + 0.3 V were observed in the N2-saturated solution for both materials. In contrast, if the electrolyte was saturated with O2, a well-defined cathodic peak current at approximately 0.12 V was observed, which is suggestive of pronounced electrocatalytic activity of SCup1000 for the ORR. A similar cathodic ORR signal was also observed for Pt/C. We also compared the performance of other

SCup materials prepared at different pyrolysis temperatures with an identical procedure. The other SCup materials also revealed similar cathodic peaks, which shifted to potentials more negative than that of SCup-1000 for the ORR, as shown in Figure S9. One of the disadvantages of platinum-based electrocatalysts in fuel cells, especially in direct methanol fuel cells, is methanol crossover from the anode to the cathode. In this case, methanol oxidation completely subdues the ORR, which reduces the current output. To check this effect in SCup-1000, we recorded CV curves in O2-saturated 0.1 m KOH mixed with 3.0 m methanol. For comparison, the same test was performed with a commercial Pt/C catalyst. It is evident from Figure 6 b that commercial Pt/C showed a pair of strong methanol oxidation peaks at 0.27 and 0.16 V (vs. Ag/AgCl), but the ORR signal completely vanished as a result of the preferred methanol oxidation of Pt within the potential range. However, SCup-1000 did not cause such methanol oxidation (Figure 6 a), and it maintained a high selectivity towards the ORR even in the presence of methanol, which indicates that SCup-1000 has an excellent tolerance towards the crossover effect of methanol. Recently, much attention centered on the stability of electrode materials. Thus, the long-term stability of catalysts for the ORR is also a major concern in fuel-cell technology. The durability of the SCup-1000 and commercial Pt/C catalysts was evaluated by chronoamperometric measurements at their respective ORR peak potential in O2-saturated 0.1 m KOH solution at 1600 rpm. The results of the durability test are shown in Figure 6 c, for which the commercial Pt/C catalyst showed a rapid current decay to 24.6 % after 30 000 s, which is indicative of its poor durability. However, the SCup-1000 material exhibited excellent durability performance with 73.2 % retention of the initial current after 30 000 s; this demonstrates its excellent durability relative to the state of the art Pt/C electrocatalyst. Electrical conductivity is an essential property of materials that affects the overall electrochemical performance. To study the variation of electrical conductivity with pressure, a cell with four probe configurations was used, as shown in Figure S10 a. The cell consisted of a hollow cylinder constructed with a nonconducting material (Teflon), in which two metallic pistons (brass) formed a pressure chamber. A current was applied to the sample through the metallic pistons, and the voltage was measured across the metal leads in the sample added in the hollow cylinder, as shown in Figure S10 a. The electrical resistivity of the SCup materials decreased as the pressure was increased for all samples. In general, as the pyrolysis temperature was increased, the overall resistivity of each sample decreased with respect to the pressure applied. This can be understood by the fact that the increase in temperature certainly improves graphi1 Figure 6. Cyclic voltammograms for the ORR recorded at 50 mV s in 0.1 m KOH solution for a) SCup-1000 and tization and, as a result, the b) commercial 20 wt % Pt/C at different conditions. c) Relative J–t chronoamperometric responses of the SCupoverall conductivity. On the basis 1000 and 20 wt % Pt/C electrodes at the ORR peak potential in O2-saturated 0.1 m KOH solution at 1600 rpm.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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CHEMSUSCHEM FULL PAPERS of the observations drawn from these results, the major issues of the ORR can be addressed in terms of heteroatom content, electrical conductivity, and surface properties, in particular mesopore surface area, which are the key factors governing the ORR activity. As the pyrolysis temperature was increased, graphitization and, as a result, the overall conductivity increased, and this is beneficial for the ORR; however, at the same time the doped heteroatom content decreased, which may be unfavorable for the ORR. The surface area increased up to 1000 8C and then decreased upon any further increase in the pyrolysis temperature. Hence, the trade-off should be evaluated and optimized for the best performance possible with the current system. The pyrolysis temperature is a key controlling parameter that determines the trade-off between heteroatom doping, surface properties, and electrical conductivity. There are several more factors that were not discussed in detail, but could contribute to the ORR activity, such as surface wetting, methanol adsorption, and impurities. The surface wetting parameter is expected to slightly change with an increase in the total heteroatom content; this variation is approximately 15–10 % on the basis of the total heteroatom content from SCup-800 to SCup-1100. Therefore, we believe that this change will not significantly affect the ORR performance. Given that methanol has almost no effect on the ORR for heteroatom-doped carbon materials, the adsorption resulting from the high surface area is also highly likely to have little effect on the ORR. The presence of trace amounts of impurities may have some effect on the ORR activity, but their amount is more or less the in all samples and very small. Thus, their effect on the ORR, if any, is likely almost the same for all samples and highly unlikely. The sulfur content is also more or less the same in all samples, and the effect is thus likely more or less similar in all samples. The impact of sulfur content is considered to be less than that of the nitrogen content in heteroatom-doped carbon materials. However, from the results of catalytic activity it can be concluded that surface area, conductivity, and nitrogen content remain as the most decisive factors influencing ORR activity. Figure 7 shows a comparison between the mesopore surface area, nitrogen content, and conductivity (at 8 MPa) along with an onset potential of the SCup materials as a function of pyrolysis temperature. It is interesting that SCup-1000, which has a much lower nitrogen content than SCup-800 and SCup-900, shows overall the best ORR performance. SCup-1000 showed much lower resistivity than the SCup-800 and SCup-900 samples, which revealed its better electrical conductivity. The higher electrocatalytic activity of SCup-1000 can be attributed to a better electrical conductivity and also to a higher mesopore surface area (1021.77 m2 g 1) relative to those of SCup800 and SCup-900, which facilitate the movement of O2 and the electrolyte toward the ORR active surface sites. Furthermore, SCup-1000 contains a higher amount of catalytically active quaternary and pyridinic nitrogen sites as well as thiophenic sulfur sites, and although SCup-1100 is highly conductive, it shows poor ORR activity, which may be attributed to other factors (such as a too low heteroatom content and a low surface area). Thus, on the basis of the trade-off between het 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 7. Correlative effects of mesopore surface area, electrical conductivity, and nitrogen content at the onset potential of the SCup materials as a function of pyrolysis temperature.

eroatom doping, surface properties, and electrical conductivity, SCup-1000 surpasses other SCup samples in terms of onset potential and current density, as shown in Figure 5 a, which proves that this sample has the best ORR performance close to the commercial Pt/C catalyst, and this is a tremendous achievement in heteroatom-doped catalysts synthesized from cheap biological resources.

Conclusions In this work, we demonstrated a simple, convenient, and ultimately economical innovative template-free green route to prepare porous carbon with a high surface area naturally doped with nitrogen and sulfur heteroatoms from easily available abundant seaweed, U. pinnatifida, as a single precursor for both carbon and the heteroatoms. U. pinnatifida is abundant and cheap and thus seems to be a prime choice as precursor for heteroatom-rich highly porous carbon with a high surface area to be used as a metal-free and efficient ORR electrocatalyst. As a first application, the resulting highly porous doped carbon electrocatalyst showed excellent electrocatalytic ORR activity through a four-electron pathway in alkaline media, and the activity was comparable to that of a commercial Pt/C catalyst, but it showed much better long-term stability and excellent resistance to alcohol crossover, which should be addressed in the cathode of low-temperature fuel cells; this clearly demonstrated that SCup is a cheap promising alternative to costly platinum-based catalysts for the ORR. The pyrolysis temperature was a key parameter that determined the trade-off between heteroatom doping, surface properties, and electrical conductivity. The trade-off was analyzed to obtain the best ORR performance. The best performance of the carbon electrodes obtained for SCup-1000 can be attributed to its highly porous nature with a high mesopore surface area and good electrical conductivity along with catalytically active nitrogen and sulfur sites, which significantly enhance the ORR activity. In addition, the current work provides a simple and scalable route to generate heteroatom-doped ChemSusChem 2014, 7, 1755 – 1763

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CHEMSUSCHEM FULL PAPERS porous carbon materials with excellent properties from renewable and abundant seaweed, which may have long-term significance in the field of energy conversion and storage.

Experimental Section Preparation of heteroatom-containing seaweed carbon Commercially available dried raw seaweed as a single carbon precursor was pyrolyzed at different temperatures ranging from 800 to 1100 8C under N2 flow for 5 h. After pyrolysis, the resultant product was treated with 3.0 m HCl solution for 24 h at room temperature to remove any inorganic impurities. The resulting black-colored powder was washed thoroughly with an excess amount of water until a neutral pH value was obtained, and was then finally dried at 80 8C overnight in an oven. Carbon materials prepared by using U. pinnatifida seaweed algae at different pyrolysis temperatures before HCl treatment were labeled as SCupB-X, whereas the corresponding carbon samples after HCl treatment were labeled as SCup-X, in which X represents the pyrolysis temperature.

www.chemsuschem.org voltammetry (LSV), and chronoamperometry. Electrochemical studies of the samples were performed at room temperature by using a rotating ring disk electrode (model ALS Co., RRDE-3A) in a threeelectrode geometry. A glassy carbon RRDE (4 mm) was used as the working electrode, and a platinum wire and Ag/AgCl saturated KCl were used as the counter and reference electrodes, respectively. The aqueous electrolyte (0.1 m KOH) used was degassed with nitrogen before the electrochemical measurements. The electrochemical studies were performed by using a Biologic VMP3 electrochemical workstation.

Acknowledgements This work was supported by a National Research Foundation (NRF) grant (NRF 2010-0029245) and the Ministry of Education, Science and Technology of Korea through the Global Frontier R&D Program on Center for Multiscale Energy System (NRF-20110031571). The authors would also like to thank Korea Basic Science Institutes (KBSIs) at Jeonju, Daejeon, and Pusan for SEM, TEM, and XPS measurements.

Instrumental analysis The morphology and microstructure of the obtained samples were investigated by scanning electron microscopy (SEM) by using a Hitachi (S-4700, Hitachi, Japan) microscope operated at an acceleration voltage of 10 kV. Transmission electron microscopy (TEM) was operated at 120 kV with an EM 912 Omega. High-resolution TEM (HRTEM) images were obtained by using a JEOL FE-2010 microscope operated at 200 kV. The purity of the samples was examined by X-ray diffraction (XRD) analysis with a Rigaku Smartlab X-ray diffractometer with CuKa radiation (l = 1.5406 ) operating at 40 kV and 30 mA. The average lattice spacing size (d) of the samples was estimated by applying the Scherrer formula: d = nl/2 sinq, in which l is the X-ray wavelength, n is the Scherrer constant taken as 0.89, and q is the Bragg diffraction angle. X-ray photoelectron spectroscopy (XPS) analyses were performed with an ESCALAB 250 XPS system by using a monochromated AlKa (150 W) source. Nitrogen adsorption–desorption isotherms were measured at 196 8C by using a Micromeritics ASAP 2020 accelerated surface area and porosimetry system. Specific surface areas of the samples were determined by nitrogen adsorption data in the relative pressure range from 0.05 to 0.2 by using the BET equation. Total pore volumes were determined from the amount of gas adsorbed at the relative pressure of 0.99. Pore-size distribution (PSD) was calculated from the adsorption branches by the BJH method.

Electrode preparation and electrochemical characterization The working electrode was polished with alumina slurry to obtain a mirror-like surface, was then washed with Mill-Q water and acetone and dried before use. The slurry was prepared by mixing SCup (1.0 mg) into a solvent mixture of Nafion (5 wt %) and water (1:9 v/v, 1.0 mL) for 20 min in an ultrasonicator. For comparison, a commercially available catalyst of 20 wt % Pt/C (E-TEK) was used, and a 1.0 mg mL 1 commercial Pt/C suspension was prepared according to the identical procedure described above. The slurry was placed on a precleaned working electrode, and the electrodes were allowed to dry at room temperature before measurements were taken. This led to a catalyst loading of 0.16 mg cm 2 for both SCup and commercial 20 wt % Pt/C. The electrochemical performance of the electrodes was characterized by CV, linear scan  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Received: January 13, 2014 Revised: March 15, 2014 Published online on May 8, 2014

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