From Hemoglobin to Porous N−S−Fe-Doped Carbon for EfficientOxygen ElectroreductionKoroush Sasan,† Aiguo Kong,†,§ Yuan Wang,‡ Mao Chengyu,‡ Quanguo Zhai,† and Pingyun Feng*,†,‡

†Department of Chemistry and ‡Materials Science and Engineering Program, University of California, Riverside, Riverside, California92521, United States§Department of Chemistry, East China Normal University, 500 Dongchuan Road, Shanghai 200241, P. R. China

*S Supporting Information

ABSTRACT: Nitrogen−sulfur−iron-doped porous carbonmaterial with high surface area (1026 m2 g−1) and large poresize is synthesized by the pyrolysis of hemoglobin, an abundantand inexpensive natural compound, with mesoporous silicafoam (MS) as a template and thiocarbamide (TCA) as anadditional sulfur source in an argon atmosphere. Our resultsindicated that as compared to the commercial 20% Pt/Ccatalyst, the synthesized catalyst exhibits not only highercurrent density and stability but also higher tolerance tocrossover chemicals. More importantly, the synthetic methodis simple and inexpensive.

■ INTRODUCTION

Renewable energy sources are attracting growing attention forfulfilling future energy requirements, since global warming isbecoming a major problem worldwide. The fuel cell, as a clean,efficient substitute for conventional energy systems, has quicklybecome a promising source of alternative energy.1,2 Decreasingthe cost per kilowatt of electric power output while improvingdurability and power density is a major challenge in thedevelopment of fuel cells. Currently Pt-based electrocatalystsare used for oxygen reduction reactions, but Pt is expensive. Inaddition, Pt-based electrodes can form stable Pt−O and Pt−OH species, which can limit the energy conversion efficiencyand slow the oxygen reduction reaction (ORR).3 Recent studieshave focused on a broad range of alternative catalysts based onnonprecious metals (Fe, Co, Ni, etc.) as well as nitrogen-coordinated metal species on nanostructured catalyst sup-ports.4−6 However, metal catalysts frequently suffer fromdissolution, sintering, and agglomeration during operation ofthe fuel cell, which can result in catalyst degradation. Toovercome this obstacle, extensive efforts have been devoted toreducing the amounts of metal species in the catalysts orreplacing them with nonmetal catalysts and substrates.3,7

Despite great progress in developing noble-metal-freecatalysts, the current ORR catalysts are still far from satisfactoryfor large-scale practical applications. Among these developedelectrocatalytic materials, carbon-based oxygen reductioncatalysts represent a family of promising candidates for fuelcell applications.8 Recently, Maruyama et al. reported theformation of noble-metal-free cathode catalysts by carbonizinghemoglobin and phthalocyanine (FePc).9−13 Additionally, theynoted that porous carbonaceous materials from hemoglobinshowed great practical potential as oxygen reduction catalysts

for polymer electrolyte membrane fuel cells (PEMFC), andabundant sources of natural hemoglobin can meet costrequirements for large-scale fuel cell application. About 2.5million tons of hemoglobin are discarded as meat waste aroundthe world every year. Despite this low cost, the carbonizedhemoglobin catalysts have low activity and durability, whichlimit its potential use as a replacement for conventional Pt-based catalysts.9,10 Recently, an efficient nonprecious-metaloxygen reduction reaction (ORR) catalyst from pyrolysis of aniron-coordinated complex with nitrogen-rich ligand, 11,11′-bis(dipyrido[3,2-a:2′,3′-c]phenazinyl) (bidppz), has been de-veloped. Two active sites in the M−N/C ORR catalysts havebeen proposed.5 One is the M−Nx species, and the other is thenonmetal−heteroatom doping in carbon matrix.5 Although theM−N/C catalyst shows good stability and catalytic activity, it isstill lower than that of the Pt catalyst. The low surface densityof catalytic sites might be one of the reasons for the lowercatalytic activity.1,3

Hence, we report the synthesis of nitrogen−sulfur−iron-doped porous carbon materials (Fe/N/S−C-4) made byannealing hemoglobin and thiocarbamide (TCA) in thepresence of mesoporous silica foam (MS) as a template in anargon atmosphere. The electrocatalytic tests show that thematerials exhibited better catalytic activity and long-termstability and higher current density and resistance than acommercial Pt/C in alkaline media for ORRs. Such excellentperformance may be attributed to more accessible active sites ofFe−N4 as well as high concentrations of dual-doping

Received: April 27, 2015Revised: June 1, 2015Published: June 2, 2015

Article

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heteroatoms (N and S) that could increase the catalytic activityperhaps due to induced charge distribution on the materials.1,5

■ EXPERIMENTAL DETAILSMaterials. Hemoglobin from bovine blood and thiocarba-

mide (Thiourea, TCA) were purchased from Sigma-Aldrich. Acommercially available catalyst of 20 wt % Pt/C catalysts fromAlfa Aesar was used as the conventional Pt-based catalyst. Thesilica oxide mesoporous (MS) also was purchased from AlfaAesar. The 5 wt % Nafion-isopropanol solution was purchasedfrom Ion Power.Sample Preparation. Mixtures of dry powder hemoglobin

(400 mg), TCA (1600 mg), and MS (1000 mg) were finelyground together and then heated up to 1000 °C for 2 h in aquartz tube furnace under a high purity argon atmosphere. Theresultant carbon−silica composite was treated with diluted HF(10 wt %, Aldrich) solution to remove the silica template. Thetemplate-free carbon was filtered, washed with water andmethanol several times, and dried in a vacuum at roomtemperature to give Fe/N/S−C-4. Then, by varying the TCA/hemoglobin ratios in the precursor mixtures with 0, 2:1, and6:1, and using the same procedure as described above, threesamples named Fe/N/S−C-0, Fe/N/S−C-2, and Fe/N/S−C-6were obtained, respectively (Table 1). The XRD data provedthat the formed materials are graphite phase carbon.5

Electrochemistry Experiment. The electrocatalytic activ-ities for ORR of the as-prepared catalysts were evaluated bycyclic voltammetry (CV) and rotating disk electrode (RDE)techniques on a CHI-650 electrochemical analyzer. A three-electrode cell was employed using a glass carbon RDE (5 mmin diameter, Pine). One served as a working electrode, oneserved as an Ag/AgCl, KCl (3 M) electrode as the referenceelectrode, and the final was a Pt wire electrode, serving as thecounter electrode. The ORR experiments were carried out in0.1 M KOH solution. The potential was scanned at the ambienttemperature between −1.2 and +0.1 V (vs Ag/AgCl) at a scanrate of 10 mV s−1 after purging O2 or Ar gas for 20 min. All theworking electrodes were prepared as following: 10 mg ofcatalysts was dispersed in a solution containing 1.2 mL ofethanol and 0.08 mL of 5 wt % Nafion−isopropanol solutionand then ultrasonicated for 30 min. The prepared catalyst inkwas pipetted onto a polished, glassy carbon electrode surface(0.196 cm2), and the electrode was dried at room temperature.The catalyst loading of active Pt-free catalysts on the workingelectrode is 0.2 mg cm−2 in 0.1 M KOH solution. All thepotentials were calibrated to the potentials vs RHE.

■ RESULTS AND DISCUSSIONThe surface area and pore size of the prepared materials weredetermined by the Brunauer−Emmett−Teller (BET) andBarrett−Joyner−Halenda (BJH) methods, respectively. Thesurface area of Fe/N/S−C-0 was 655 m2 g−1 (Figure 1d). In

the absence of MS, which has high surface area (over 800 m2

g−1), the prepared material showed a BET surface area of only60 m2 g−1. These results indicate that the MS plays a crucialrole in enhancing the surface area of the materials.The morphologies of Fe/N/S−C-4 are shown in Figures 1a

and 1b, which has sponge-like structural feature withcontinuous 3D networks. The N2 adsorption−desorptionisotherms indicated that the pore size increases with increasingmass of TCA (Supporting Information Figure 3). Furthermore,the surface area measurements show that the BET surface areaincreased from 655 to 737 to 1026 m2 g−1 for Fe/N/S−C-0,Fe/N/S−C-2, and Fe/N/S−C-4. When the mass ratio ofhemoglobin to TCA reached 1:6, as in the case of Fe/N/S−C-6, the surface area decreased to 850 m2 g−1 (SupportingInformation Figure 2), perhaps caused by the extensive TCApyrogenation.14 These observations show that the mass ratiosof hemoglobin to TCA play an important role in controlling thesurface area of the Fe/N/S−C products.Scanning electron microscopy (SEM) shows that the Fe/N/

S−C-4 possesses uniform pore distribution (Figure 1a,b). Thetransmission electron microscopy (TEM) image of Fe/N/S−C-4 exhibits both mesoporous and macroporous structuralfeatures (Figure 1c). The elemental analysis from the EDXconfirmed the presence of N, S, O, C, and a small amount of Fein the Fe/N/S−C-4. The uniform distribution of heteroatomsand Fe in the Fe/N/S−C-4 was verified by SEM-EDS and thecorresponding elemental mapping images (Supporting In-formation Figure 5). X-ray photoelectron spectroscopy (XPS)was used for further investigating the chemical status ofheteroatoms in the Fe/N/S−C-4. The XPS full spectrumshows a dominant C 1s peak (∼284 eV), an O 1s peak (∼532eV), an N 1s peak (∼400 eV), two S peaks (∼164 eV, 228 eV),and a weak Fe peak (Figure 2a). The data indicated that N, S,and Fe have been successfully doped into the carbonizedhemoglobin framework. The content of N, S, O, and C in theFe/N/S−C-4 was calculated to be 4.6, 2.3, 4.1, and 88.9 at. %,respectively, based on the XPS peaks. Also, trace amounts of Fe(∼0.1%) were found on the surface of Fe/N/S−C-4. The high-resolution spectrum of C 1s can be assigned into several singlepeaks, corresponding to C−C, C−N, C−O, C−S, CO, andCN bonds14 (Figure 2b). The high-resolution spectrum of N1s can be divided into three peaks at 398.1, 400.1, and 401.1eV, which can be assigned to pyridinic N, pyrrolic N, andgraphitic N, respectively. A peak was also observed at 404.6 eVwhich refers to pyridinic N+−O− groups14,15 (Figure 2c). Thepyridinic N and graphitic N are dominant N-types, which playsignificant roles in the ORR process by contributing to the π-conjugated carbonized hemoglobin with a pair of p-electrons.The XPS-S 2p spectrum also suggested the presence of −C−S−C− 2p2/3 (163.0 eV), −C−S−C−2p1/2 (164.9 eV), and C−SOx−C, x = 1, 2, 3 (168.0−169.0 eV), peaks (Figure 2d). Datafrom Figure 2d clearly showed the presence of two distinctforms of S: sulfide groups (−C−S−C−) and oxidized sulfurgroups (−C−SOx−C−, x = 1, 2, 3).16 The XPS data showed nochange in the S signal before and after sonication, supportingthat the S elements are covalently bonded in the bulk of thematerials and not on the surface.We performed rotating disk electrode (RDE) voltammo-

grams to investigate the electrocatalytic activity of the series ofFe/N/S−Cs for ORR in 0.1 M KOH electrolyte (Figure 3a).When the electrolyte solution was saturated with O2, theprominent cathodic, ORR peaks in the cyclic voltammetry(CV) curves were observed, demonstrating their effective ORR

Table 1. Mass Ratio of Hemoglobin/TCA/MS for Series ofFe/N/S−C’s Samples

samplesannealing temp

(°C)hemoglobin

(mg)TCA(mg)

MS(mg)

Fe/N/S−C-0 1000 400 0 200Fe/N/S−C-2 1000 400 800 600Fe/N/S−C-4 1000 400 1600 1000Fe/N/S−C-6 1000 400 2400 1400

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activities in the alkaline medium. The Fe/N/S−C-4 CV curves,the best sample, showed a single cathodic peak at 0.87 V (vsRHE). To gain further insight into the ORR performances ofthese samples, linear sweep voltammetry (LSV) measurementswere performed on a RDE at a scan rate of 10 mV s−1 and a

rotation rate of 1600 rpm. For comparison, a commercial Pt/C(20 wt %, Alfra Aesar) catalyst was also investigated at the sameconditions. The onset potential of 0.80 V (vs RHE) wasobserved for the Fe/N/S−C-0 electrode which does notcontain sulfur. After adding TCA, it can be seen in Figure 3b

Figure 1. (a, b) SEM images of Fe/N/S−C-4. (c) TEM image of Fe/N/S−C-4. (d) Adsorption−desorption isotherm of Fe/N/S−C-0, Fe/N/S−C-2, and Fe/N/S−C-4.

Figure 2. (a) XPS spectra survey scan, high-resolution (b) C 1s, (c) N 1s, and (d) S 2p for Fe/N/S−C-4.

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that all samples have more positive onset potential and highercurrent density than Fe/N/S−C-0. Additionally, Figure 3bshows that the onset potential for Fe/N/S−C-4 is similar tothat of the Pt/C and current density is much higher than that ofthe Pt/C. These results indicate that the Fe/N/S−C-4electrode can be a promising catalyst for the ORR in analkaline solution.As a control experiment, the influence of annealing

temperature on the ORR activity of carbonized hemoglobinmaterial was explored. Figure 3d shows LSV measurementcurves for carbonized materials obtained at 700−1100 °C. Ourobservations clearly indicate that the carbonized materialobtained at 1000 °C has the highest current density andmost positive onset potential of all the samples. On the basis ofour data, we speculate that the higher temperature, which iscapable of restoring defects, may be one of the factorscontributing to improved graphitic degree and catalytic activity.For comparison, the ORR activity of carbonized hemoglobinsamples with no MS as template was investigated. The LSVmonument showed the onset potential dramatically decreasedwhen carbonizing hemoglobin in the absence of MS. Theseresults indicated the MS plays a significant role in the ORRactivity of Fe/N/S−C-4 (Figure 4). Furthermore, the RDEmeasurement was performed at various rotating speeds todetermine the kinetic process of ORR over Fe/N/S−C-4(Figure 3c). The diffusion current densities depend on therotating rates, and the number of electron transfers (n)involved in the ORR can be calculated from the Koutecky−Levich equation (Supporting Information Figure 4). The n forFe/N/S−C-4 is calculated to be 3.80 at −0.40 V, suggesting afour-electron-transfer reaction to reduce oxygen directly toOH−, similar to ORR catalyzed by a commercial Pt/C catalystmeasured in the same alkaline media.17

Resistance to crossover effect and long-term stability of theORR catalyst are important considerations for practicalapplication. The chronoamperometric responses to methanolintroduced into an O2-saturated electrolyte were measured forthe Fe/N/S−C-4 and Pt/C catalysts. Figure 5a shows that afterthe addition of methanol to an electrolyte solution saturatedwith O2, no noticeable change was observed in the ORRcurrent for the Fe/N/S−C-4 electrode. In contrast, the ORRcurrent for the Pt/C electrode decreased drastically. Theseresults indicated high ORR selectivity and good resistance tothe crossover effect of Fe/N/S−C-4. The durabilities of the Fe/N/S−C-4 and Pt/C were also compared. The catalysts wereheld at −0.40 V for 50 000 s in O2-saturated 0.1 M KOHsolution. The result showed that the chronoamperometricresponse for Fe/N/S−C-4 exhibited very slow attenuation, and

Figure 3. (a) CVs for various catalysts obtained under different mass ratios of sucrose to TCA, (b) LSV curves for various catalysts, and a Pt/Ccatalyst at a rotation rate of 1600 rpm. (c) LSV curves for various rotating speeds for Fe/N/S−C-4. (d) LSV curves at a rotation rate of 1600 rpm forvarious catalysts obtained under different annealing temperature.

Figure 4. LSV curves for various carbon materials, Hemoglobin(black), hemoglobin + TCA (red), Fe/N/S−C-0 (blue), and Fe/N/S−C-4 (green) on a glass carbon rotating disk electrode in O2-saturated 0.1 M KOH, 1600 rpm.

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a high relative current of 92% persisted after 50 000 s. Incontrast, the Pt/C electrode exhibited a gradual decrease, withcurrent density drop to 80% after 50 000 s (Figure 5b).The presence of the Fe−N4 active sites could be one of the

important factors for high ORR performance for Fe/S/N−C-4.18 In addition, dual doping of heteroatoms (N and S),7−20

porous structure,21 and high surface area may all havecontributed positively to the ORR activities of the synthesizedmaterials.11 The XPS results of Fe/N/S−C-4 indicated anoxidation state of II for the Fe based on the peak positions ofthe Fe 2p3/2 binding energy.4,22 The presence of Fe2+ on thesurface of Fe/N/S−C-4 could play a significant role inenhancing ORR activity of Fe/N/S−C-4 since Fe2+ on thecatalyst surface is required as an O2 bonding site during the firststep of the multistep reaction.23 The ORR catalytic activity ofcarbonized material can be further improved by introducingheteroatoms. The XPS data indicate the successful codoping ofN and S on carbonized hemoglobin, resulting in N and Scontent of about 4.6 and 2.3, respectively. The codoping ofheteroatoms such as N and S in the carbonized hemoglobin canintroduce unpaired electrons and cause high spins and localcharge density, resulting in high catalytic performance of theORR.24 The excellent reactant transport caused by thehierarchical 3D porous structure of Fe/N/S−C-4 can beanother factor in enhancing the ORR activity.25 The highsurface area and large pore size of Fe/N/S−C-4 caused byusing MS as template and pyrogenation of TCA can also play asignificant role in improving ORR activity by introducing morecatalytic active sites on the surface of the materials. The porestructural feature plays a critical role in enhancing the catalyticactivity due to the increase in the availability of O2 moleculesfor individual active sites inside the pores of the catalyst.26

Watanabe et al. suggested the ORR activity would decreasewhen overlapping of the space occurs by a decrease in thedistance between the active sites.27 The increase in the specificsurface area could avoid this interference among the active sites,and thus increasing the pore volume of carbonized hemoglobincould expand the potential reaction space and current density.

■ CONCLUSIONIn summary, porous nitrogen−sulfur−iron-tridoped carbonizedhemoglobin (Fe/N/S−C-4) has been formed through aconvenient, economical, and scalable route. The experimentalresults have demonstrated that Fe/N/S−C-4 exhibits bettercatalytic activity, higher current density, longer-term stability,and higher methanol tolerance than the commercial 20% Pt/Ccatalyst. This study demonstrates that carbonized hemoglobinin the presence of MS and TCA is effective for the formation of

highly porous carbon materials that show enhanced activity asthe fuel cell catalyst.

■ ASSOCIATED CONTENT*S Supporting InformationMore experimental details, pore size distribution, XRD, SEMimage, SEM-EDS, and the corresponding elemental mappingimages of carbonized hemoglobin. The Supporting Informationis available free of charge on the ACS Publications website atDOI: 10.1021/acs.jpcc.5b04017.

■ AUTHOR INFORMATIONCorresponding Author*E-mail pingyun.feng@ucr.edu; Fax +1 951-827-4713; Tel +1951-827-2042 (P.F.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis research was supported by the U.S. Department of Energy,Office of Basic Energy Sciences, Division of Materials Sciencesand Engineering, under Award # DE-FG02-13ER46972 (Feng).

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Figure 5. Chronoamperometric responses of Fe/N/S−C-4 and Pt/C catalyst: (a) with 3 M methanol added at around 360 s and (b) at −0.40 V inan O2-saturated 0.1 M KOH solution.

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