DOI: 10.1126/science.1200832, 443 (2011);332 Science

, et al.Gang Wufrom Polyaniline, Iron, and CobaltHigh-Performance Electrocatalysts for Oxygen Reduction Derived

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reaction of an organometallic reagent at a specificposition, usually ortho to the directing group, followedby the reaction of the metallated aromatic compoundwith an electrophile. For application of these groups indirected ortho-lithiation see (31).

8. P. N. Rylander, Hydrogenation Methods (Academic Press,London, 1985), pp. 157–163.

9. M. K. Huuska, Polyhedron 5, 233 (1986).10. S. Utoh, T. Hirata, H. Oda, C. Yokokawa, Fuel Proc. Tech.

14, 221 (1986).11. E. M. Van Duzee, H. Adkins, J. Am. Chem. Soc. 57,

147 (1935).12. G. Stork, J. Org. Chem. 69, 576 (1947).13. A. Maercker, Angew. Chem. Int. Ed. Engl. 26, 972 (1987).14. P. Dabo, A. Cyr, J. Lessard, L. Brossard, H. Ménard,

Can. J. Chem. 77, 1225 (1999).15. Stoichiometric hydrogenolysis of aromatic C-O bond in

a palladium pincer complex was reported in (32).16. P. J. Dyson, Dalton Trans. 2964 (2003).17. E. Wenkert, E. L. Michelotti, C. S. Swindell, J. Am.

Chem. Soc. 101, 2246 (1979).18. E. Wenkert, E. L. Michelotti, C. S. Swindell, M. Tingoli,

J. Org. Chem. 49, 4894 (1984).19. J. W. Dankwardt, Angew. Chem. Int. Ed. 43, 2428

(2004).20. B. T. Guan et al., Chem. Commun. (Camb.) 1437 (2008).

21. M. Tobisu, T. Shimasaki, N. Chatani, Angew. Chem.Int. Ed. 47, 4866 (2008).

22. J. A. Widegren, R. G. Finke, J. Mol. Catal. A 198,317 (2003).

23. Materials and methods are available as supportingmaterial on Science Online.

24. We observed formation of cyclohexane and cyclohexenein the reaction of Ni(COD)2 and PCy3 with H2 inthe absence of diphenlyl ether under the sameconditions (23).

25. F. Glorius, Top. Organomet. Chem. 21, 1 (2007).26. S. Díez-González, N. Marion, S. P. Nolan, Chem. Rev.

109, 3612 (2009).27. Ni(COD)2 and PCy3 were reported to catalyze

silane-induced cleavage of polycyclic aromatic ethers(methoxynaphthalenes) and alkyl aryl ethers containingdirecting groups ortho to the C-O bond (33, 34).

28. No reaction was observed in the absence of Ni(COD)2,as shown through control experiments on the cleavage of4-tert-butylbenzyl methyl ether with hydrogen in thepresence of AlMe3 (1 equiv.), SIPr·HCl (0.4 equiv.),and NaOtBu (2.5 equiv.) in m-xylene at 120°C for32 hours (23).

29. For example, heterogeneous hydrogenolysis of alkylbenzyl ethers proceeds selectively in the presence ofdiaryl ethers over Pd(OH)2/C (35).

30. K. Poppius, Acta Chem. Scand. B 38, 611 (1984).31. V. Snieckus, Chem. Rev. 90, 879 (1990).32. M. E. van der Boom, S. Y. Liou, Y. Ben-David,

L. J. W. Shimon, D. Milstein, J. Am. Chem. Soc. 120,6531 (1998).

33. P. Álvarez-Bercedo, R. Martin, J. Am. Chem. Soc. 132,17352 (2010).

34. M. Tobisu, K. Yamakawa, T. Shimasaki, N. Chatani,Chem. Commun. Published online 24 January 2011(10.1039/c0cc05169a).

35. B. A. Ellsworth et al., Bioorg. Med. Chem. Lett. 18,4770 (2008).

36. We thank the BP for financial support of this projectthrough the Energy Biosciences Program andT. Rauchfuss for helpful discussions. A provisional patenthas been filed on the methods presented herein.

Supporting Online Materialwww.sciencemag.org/cgi/content/full/332/6028/439/DC1Materials and MethodsSOM TextFigs. S1 to S3Tables S1 to S6References

15 November 2010; accepted 11 March 201110.1126/science.1200437

High-Performance Electrocatalystsfor Oxygen Reduction Derivedfrom Polyaniline, Iron, and CobaltGang Wu,1 Karren L. More,2 Christina M. Johnston,1 Piotr Zelenay1*

The prohibitive cost of platinum for catalyzing the cathodic oxygen reduction reaction (ORR)has hampered the widespread use of polymer electrolyte fuel cells. We describe a family ofnon–precious metal catalysts that approach the performance of platinum-based systems at acost sustainable for high-power fuel cell applications, possibly including automotive power. Theapproach uses polyaniline as a precursor to a carbon-nitrogen template for high-temperaturesynthesis of catalysts incorporating iron and cobalt. The most active materials in the group catalyzethe ORR at potentials within ~60 millivolts of that delivered by state-of-the-art carbon-supportedplatinum, combining their high activity with remarkable performance stability for non–preciousmetal catalysts (700 hours at a fuel cell voltage of 0.4 volts) as well as excellent four-electronselectivity (hydrogen peroxide yield <1.0%).

Thanks to the high energy yield and lowenvironmental impact of hydrogen oxida-tion, the polymer electrolyte fuel cell (PEFC)

represents one of the most promising energyconversion technologies available today. Of themany possible applications, ranging from sub-wattremote sensors to residential power generators inexcess of 100 kW, automotive transportation isespecially attractive. PEFCs promise major im-provements over gasoline combustion, includingbetter overall fuel efficiency and reduction in emis-sions (including CO2). The spectacular progressin fuel cell technology notwithstanding, a large-

scale market introduction of fuel cell–poweredvehicles continues to face various challenges, suchas the lack of hydrogen infrastructure and thetechnical issues associated with PEFC perform-ance and durability under the operating condi-tions of an automotive power plant. The high costof producing PEFCs represents the most formi-dable challenge and has driven much of the ap-plied and fundamental fuel cell research in recentyears.

According to the latest cost analysis, the fuelcell—more precisely, the fuel cell stack—is re-sponsible for more than 50% of the PEFC pow-er system cost (1, 2). Although a state-of-the-artPEFC stack uses several high-priced components,the catalysts are by far the most expensive con-stituent, accounting for more than half of thestack cost. Because catalysts at both the fuel cellanode and cathode are based on platinum (Pt) orplatinum alloys, their cost is directly linked to the

price of Pt in the volatile and highly monopolizedprecious metal market. The precious metal cat-alyst is the only fuel cell stack component thatwill not benefit from economies of scale, and anincrease in the demand for fuel cell power sys-tems is bound to drive up the already high priceof Pt, about $1830 per troy ounce at present($2280 per troy ounce at its maximum in March2008) (3). Thus, PEFCs are in need of efficient,durable, and inexpensive alternatives to Pt andPt-based catalysts.

Ideally, Pt should be replaced at both fuel cellelectrodes; however, its substitution at the cath-ode with a non–precious metal catalyst wouldhave comparatively greater impact, because theslow oxygen reduction reaction (ORR) at thiselectrode requires much more Pt than the fasterhydrogen oxidation at the anode. As a conse-quence, the development of non–precious metalcatalysts with high ORR activity has recentlybecome a major focus of PEFC research (4–8).The Pt replacement candidates that have attractedthe most attention have been synthesized by heat-ing precursors comprising nitrogen, carbon, andgeologically abundant transition metals, iron andcobalt (M = Co and/or Fe) in particular (9–14).Although the nature of the active ORR catalyticsites in such N-M-C catalysts continues to be atthe center of an ongoing debate (6, 7, 10, 15),there is no doubt that the ORR performance ofN-M-C catalysts strongly depends on the type ofnitrogen and transition-metal precursors used, heattreatment temperature, carbon support morphol-ogy, and synthesis conditions.

We recently initiated a research effort to developnon–precious metal catalysts that combine highORR activity with good performance stability, orig-inally concentrating on materials obtained with-out heat treatment. The polypyrrole (PPy)-Co-Csystem prepared this way showed respectable per-formance durability for a non–precious metal

1Materials Physics and Applications Division, Los Alamos NationalLaboratory, Los Alamos, NM 87545, USA. 2Materials Science andTechnology Division, Oak Ridge National Laboratory, Oak Ridge,TN 37831, USA.

*To whom correspondence should be addressed. E-mail:zelenay@lanl.gov

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catalyst, but its oxygen reduction activity re-mained relatively low (5).We then shifted towardhigh-temperature systems synthesized using pre-dominantly iron, cobalt, and heteroatom polymerprecursors (polypyrrole and polyaniline) (16, 17).Such nitrogen-derived non–precious metal ORRcatalysts have been under development for sev-eral decades, starting with the early work byJasinski (18) and by Yeager and co-workers (19).Their research concentrated in particular onpyrolyzed transition metal–containing macro-cycles and yielded catalysts that offered goodORR activity but suffered from poor stability inan acidic environment (20). The expensive macro-cycles were later replaced in numerous studiesby various combinations of nitrogen-containingcompounds, transition-metal inorganic salts, andcarbons, which ultimately led to considerable im-provements in ORR activity but relatively littleprogress in stability. Polyaniline (PANI), whichrepresents a favorable combination of aromaticrings connected via nitrogen-containing groups,was selected for this study as a promising templatecompound for nitrogen and carbon. Because of thesimilarity between the structures of PANI and gra-phite, the heat treatment of PANI could facilitatethe incorporation of nitrogen-containing activesites into the partially graphitized carbon matrix.Furthermore, the use of such a polymer as a ni-trogen precursor promised a more uniform dis-tribution of nitrogen sites on the surface and anincrease in the active-site density. In our effort,although several catalysts have shown promisingoxygen reduction activity, only PANI-derived for-mulations appear to combine high ORR activitywith unique performance durability for heat-treatednon–precious metal catalysts. These catalysts arethe subject of this report.

A schematic diagram describing the catalystsynthesis is shown in Fig. 1. In the approachused, a short-chain aniline oligomer was firstmixed with high–surface area carbon material,pristine Ketjenblack EC-300J or modified Ketjen-black in the case of PANI-FeCo-C(2) (21), andtransition metal precursors [cobalt(II) nitrate and/or iron(III) chloride], followed by the addition of(NH4)2S2O8 (ammonium persulfate, APS) as anoxidant to fully polymerize the aniline. Afterpolymerization, water was evaporated from thesuspension and the remaining solid phase wassubjected to heat treatments in the range 400° to1000°C under a N2 atmosphere. The heat-treatedproduct was then preleached in 0.5 M H2SO4 at80° to 90°C for 8 hours to remove any unstableand ORR-nonreactive phases. The preleachedcatalyst then underwent a second heat treatmentunder N2 as the final step of the synthesis (21).

The ORR activity and four-electron selectiv-ity of various PANI-M-C catalysts and severalcontrol materials (as-received and heat-treatedcarbon, heat-treated metal-free PANI-C, and a20 weight percent Pt/C reference catalyst) areshown in Fig. 2A.All experiments involving non–preciousmetal catalysts were carried out in 0.5MH2SO4. The Pt/C reference catalyst was tested

in 0.1 M HClO4 to avoid performance loss causedby bisulfate adsorption, especially at higher elec-trode potentials (22). As established in previousresearch (23), at sufficiently high electrode over-potentials, carbon black alone can act as an oxy-gen reduction catalyst (mostly to H2O2 ratherthan H2O; see Fig. 2A, plot 1). Although the highORR overpotential on carbon can be lowered bya heat treatment alone (Fig. 2A, plot 2), a moresubstantial improvement in ORR activity, re-flected by a shift in the onset ORR potential from0.60 to 0.80 V (versus the reversible hydrogenelectrode, RHE), is achieved after the addition ofPANI (Fig. 2A, plot 3). Pyridinic, pyrrolic, andquaternary nitrogen centers are present in theheat-treated PANI-C sample (24). Nitrogen canbe viewed in this case as an n-type carbon dopantthat results in the formation of disordered car-bon nanostructures and/or donates electrons tothe carbon (24), thus facilitating the ORR (25).Iron or cobalt incorporation leads to a radicalenhancement in the ORR activity and four-electron selectivity of the catalysts (Fig. 2A, plots4 and 7). The ORR onset potential for the moreactive of the two PANI-FeCo-C catalysts andfor the PANI-Fe-C catalyst is ~0.93 V (bottompart of Fig. 2A, plots 6 and 7, respectively). Arotating ring-disk electrode (RRDE) study ad-ditionally reveals very high selectivity of the PANI-derived catalysts for the four-electron reductionof oxygen. The H2O2 yield with PANI-Fe-Cremains below 1% at all potentials, dropping to

as low as 0.6% at 0.40 V. Unlike other non–precious metal catalysts, which show a peroxideyield in excess of 50% at lower loadings (26),the H2O2 yield measured with PANI-derivedcatalysts shows little loading dependence. Evenat a low loading of 0.1 mg cm−2, the H2O2 yieldmeasured with the PANI-Fe-C catalyst is limitedto a range of 0.6 to 3.8%, depending on electrodepotential.

The disparity in the precious and non–precious metal catalyst loading notwithstanding,the performance gap between a state-of-the-artPt/C (E-TEK) and PANI-Fe-C, expressed as ahalf-wave potential difference (DE½) in rotatingdisk electrode (RDE) testing, has been substan-tially reduced in thiswork to 43mVrelative to Pt/Cat a “standard” loading of 20 mgPt cm

−2 (Fig. 2A)and 59 mV relative to Pt/C at a “high” loading of60 mgPt cm

−2.Whereas cyclic voltammograms (CVs) of

PANI-C and PANI-Co-C in N2-saturated H2SO4

solution are virtually featureless, theCVofPANI-Fe-Creveals a pair of well-developed redox peaks at~0.64 V (fig. S1). The full width at half maxi-mum (FWHM) of these peaks is ~100mV,whichis very close to the theoretical value of 96 mVexpected for a reversible one-electron process in-volving surface species (27). There are two sur-face processes that can possibly give rise to theobserved redox behavior in this case: (i) one-electron reduction/oxidation of the surface quinone/hydroquinone groups (28), and (ii) Fe3+/Fe2+

Fig. 1. Schematic diagram of the synthesis of PANI-M-C catalysts. (A) Mixing of high–surface area carbonwith aniline oligomers and transition-metal precursor (M: Fe and/or Co). (B) Oxidative polymerization ofaniline by addition of APS. (C) First heat treatment in N2 atmosphere. (D) Acid leaching. The second heattreatment after acid leach is not shown.

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reduction/oxidation. In support of the latter reac-tion, an in situ electrochemical x-ray absorptionstudy of the PANI-Fe-C system shows a cor-relation between the change in the oxidation stateof Fe species in the catalysts and the potential ofthe reversible CV feature in the PANI-Fe-C cat-alyst voltammetry (17).

Additional kinetic data (fig. S2 and table S1)reveal differences in the Tafel slope of oxygenreduction on the different catalysts studied in thiswork. ATafel slope of 67 mV decade−1 was mea-sured for PANI-Co-C—a much lower valuethan the Tafel slope of 87 mV decade−1 obtainedfor PANI-Fe-C. The rate-determining step of theORR for the latter catalyst is likely to simulta-neously involve the migration of reaction inter-mediates and charge transfer. The exchangecurrent density (i0) is nearly two orders of mag-nitude higher for the PANI-Fe-C catalyst (4 × 10−8

A cm−2) than for PANI-Co-C (5 × 10−10 A cm−2).Together with the differences in the onset potentialof oxygen reduction (21), the Tafel slope (masstransport–corrected), and four-electron selectivity(Fig. 2A) already described, the disparity of twoorders of magnitude in the i0 value for the twocatalysts implies that the ORR-active sites andreaction mechanisms are likely different in bothcases. Ex situ x-ray absorption analysis providesevidence for a different chemical environment ineach case. Cobalt coordination in the PANI-Co-Ccatalyst appears to closely resemble that in Co9S8,with the dominant x-ray absorption fine structure(XAFS) peak between 2 to 3 Å consistent with aknownCo-Co shell (29). XAFS of the PANI-Fe-Ccatalyst shows a peak at ~1.50 Å, which is indi-

cative of coordination to a lighter element (eitherN or O) at a much shorter distance than nearestneighbors in metallic Fe (XAFS peak at ~2.2 Å)or in FeN4-type structures in Fe macrocycles(~1.63 Å) (30).

In catalyst synthesis chemistry, the heattreatment temperature is a major factor in induc-ing catalytic activity of PANI-derived catalystsand assuring performance stability. We used anRDE to study the ORR activity of a PANI-Fe-Ccatalyst as a function of the heat treatment tem-perature in the range 400° to 1000°C. RDE studieswere conducted at room temperature and in 0.5M H2SO4 electrolyte (Fig. 2B). The performanceof the catalyst synthesized by heat treatment at400°C is very similar to that of Ketjenblack itself(bottom part of Fig. 2A, plot 1). The activity, asmeasured by the ORR onset and half-wave po-tentials (E½) in the RDE polarization plots, in-creases upon raising the heat treatment temperatureup to 900°C and then drops for catalysts synthe-sized at even higher temperatures. TheH2O2 yieldmeasured for the best-performing PANI-Fe-Ccatalyst, heat treated at 900°C, is below 1% overthe potential range from 0.1 to 0.8 V versus RHE,signaling virtually complete reduction of O2 toH2O in a four-electron process. This avoidance ofthe much less efficient, and therefore undesirable,two-electron reaction to peroxide matches, andpossibly exceeds, the four-electron selectivity ofPt-based catalysts (3 to 4%H2O2 yield at 0.4 Von14 mgPt cm

−2 Pt/C) (31).We next conducted extensive physical char-

acterization to obtain a structural explanation forthe correlation of the ORR activity of the cata-

lysts with the heat treatment temperature. Fouriertransform infrared (FTIR) spectra of PANI-Fe-C(fig. S3) show that between 400° and 600°Cthe benzene-type (1100 cm−1) and quinone-type(1420 cm−1) structures (32) on the main PANIchain break into smaller fragments, such as C=N(1300 cm−1), which may be precursor states forORR-active sites. The latter finding correspondswith scanning electronmicroscopy (SEM) results(fig. S4) indicating that in this range of heat treat-ment temperatures, PANI starts to lose its char-acteristic nanofibrous structure (fibers ~40 nmin diameter and ~200 nm in length) and grad-ually converts into more spherical particles. Thecarbon structure becomes more graphitic dur-ing the heat treatment at 900°C (HRTEM insetin fig. S4) (see below). After the treatment at aneven higher temperature of 1000°C, the particlemorphology becomes highly nonuniform, a changeaccompanied by a substantial surface area loss, asdetermined using Brunauer-Emmet-Teller (BET)technique.

The fuel cell polarization and stability plotsfor PANI-derived catalysts are shown in Fig. 3. Ingood agreement with electrochemical measure-ments, the addition of transition metals leads to aconsiderable activity enhancement of the cata-lysts relative to the metal-free PANI-C (Fig. 3A).Also in agreement with the RDE data in Fig. 2A,PANI-Fe-C exhibits higher ORR activity thanPANI-Co-C. The best-performing catalyst in fuelcell testing, with an excellent combination of highORR activity and long-term performance dura-bility, is the more active of the two FeCo mixed-metal materials, PANI-FeCo-C(2). This catalyst

Fig. 2. (A) Steady-state ORRpolarization plots (bottom)and H2O2 yield plots (top)measuredwith different PANI-derived catalysts and referencematerials: 1, as-received car-bon black (Ketjenblack EC-300J); 2, heat-treated carbonblack; 3, heat-treatedPANI-C;4,PANI-Co-C;5,PANI-FeCo-C(1);6: PANI-FeCo-C(2); 7, PANI-Fe-C; 8, E-TEK Pt/C (20 mgPtcm−2).Electrolyte:O2-saturated0.5 M H2SO4 [0.1 M HClO4 inexperiment involving Pt cat-alysts (dashed line)]; temper-ature, 25°C. RRDE experimentswere carried out at a constantring potential of 1.2 V versusRHE. RDE/RRDE rotating speed,900 rpm; non–precious metalcatalyst loading, 0.6 mg cm−2.(B) Steady-state ORR polariza-tion plots (bottom) and H2O2yield plots (top) measured witha PANI-Fe-C catalyst in 0.5 MH2SO4 electrolyte as a functionof the heat treatment temper-ature: 1, 400°C; 2, 600°C; 3, 850°C; 4, 900°C; 5, 950°C; 6, 1000°C.

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also shows the highest RDE activity, matchedonly by PANI-Fe-C (see low current-density rangein Fig. 2A, plots 6 and 7), as well as the highestmaximum power density: 0.55 W cm−2, reached

at 0.38 V. Unlike PANI-Fe-C, the mixed-metalcatalysts maintain their high ORR activity whencombined with Nafion ionomer in a fuel cell–type electrode under operation in the highly

acidic environment of the fuel cell cathode, thusreflecting better stability of the PANI-FeCo-Ccatalyst (see below). The open-cell voltage (OCV)of a hydrogen fuel cell operated with Fe-containing

Fig. 3. Fuel cell and performance durability testing. (A) H2-O2 fuel cell polarizationplots recorded with various PANI-derived cathode catalysts at a loading of ~4mg cm−2:1, PANI-C; 2, PANI-Co-C; 3, PANI-FeCo-C(1); 4, PANI-FeCo-C(2) (SDs from threeindependent measurements marked for all data points); 5, PANI-Fe-C. Performanceof an H2-air fuel cell with a Pt cathode (0.2 mgPt cm

−2) is shown for comparison(dashed line). All tests used a Pt/C catalyst at a loading of 0.25 mgPt cm−2 at theanode; anode and cathode gas pressure, 2.8 bar. (B) Long-term stability test of a

PANI-FeCo-C(1) catalyst at a constant fuel cell voltage of 0.40 V (2.8 bar H2/2.8 barair; 0.25 mgPt cm

−2 anode; cell temperature 80°C). (C) PANI-Fe-C catalyst RDEperformance at various potentials after potential cycling in nitrogen between 0.6and 1.0 V in 0.5 M H2SO4 (catalyst loading, 0.6 mg cm

−2). (D) PANI-Fe-C catalystH2-O2 fuel cell performance at various voltages after voltage cycling in nitrogenbetween 0.6 and 1.0 V (cathode catalyst loading, 2.0 mg cm−2, Pt/C anodecatalyst loading, 0.25 mgPt cm

−2; anode and cathode gas pressure, 1.0 bar).

Fig. 4. Micrographs of a PANI-FeCo-C(1) catalyst. (A) HRTEM image of a typicalnon–precious metal catalyst nanostructure involving carbon nanofibers andmetal-aggregates incorporated in graphitic nanoshells. (B) HRTEM image of hollownanoshells. (C) HRTEM image of onion-like nanoshells. (D to F) HRTEM, HAADF-

STEM, and SEM images of the same localized region in an area exhibiting agraphene sheet–like structure, where the green arrows designate the layeredgraphene sheet in each image and the red arrows designate the same FeCo-containing nanoparticle (technique used is noted in each micrograph).

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PANI-derived catalysts is ~0.90 V with an air-operated cathode and ~0.95 V with a cathode op-erated on pure oxygen. The OCV value remainsunchanged for more than 100 hours in the H2-airfuel cell (17).

A 700-hour fuel-cell performance test at a con-stant cell voltage of 0.4 V reveals very promisingperformance stability of the PANI-FeCo-C(1)catalyst at the fuel cell cathode. The cell cur-rent density in a lifetime test (Fig. 3B) remainsnearly constant at ~0.340 A cm−2. The currentdensity declines by only 3%, from the averagevalue of 0.347 A cm−2 in the first 24 hours to0.337 A cm−2 in the last 24 hours of the test (av-erage current-density loss, 18 mA hour−1). Fuelcell performance durability (Fig. 3B) repre-sents a substantial improvement over the dura-bility recently reported by the Dodelet group(6). The initially very active catalyst in the lat-ter work suffered from fast performance dete-rioration, losing ~38% of activity during 100hours of H2-air testing at 0.40 V. Although quitedurable by the standards of non–precious metalORR catalysts, both PANI-Fe-C and PANI-Co-Care less stable than PANI-FeCo-C(1), incurringperformance losses of ~90 and 130 mA hour−1,respectively. A stabilizing role of Co in the binarycatalyst is a distinct possibility.

The high stability of the PANI-derived cat-alysts does not apply solely to constant-potentialoperation of an RDE (nor to constant-voltageoperation of a fuel cell); it extends over potential-cycling conditions, and hence it is especiallyrelevant to practical fuel cell systems. High cycl-ing stability of a PANI-Fe-C catalyst at variousRDE potentials and fuel cell voltages is dem-onstrated in Fig. 3, C and D, respectively (seealso fig. S5). The cycling was carried out with-in a potential (RDE) and voltage (fuel cell) rangeof 0.6 to 1.0 V in nitrogen gas at a scan rate of50 mV s−1 (a protocol recommended by the U.S.automotive industry). The catalyst performanceloss calculated from linear regression was 10 to39% after 10,000 RDE cycles (Fig. 3C) and 3to 9% after 30,000 fuel cell cycles (Fig. 3D),further attesting to the high durability of PANI-derived catalysts, especially in the fuel cellcathode.

The morphology of the highly ORR-activeand durable PANI-FeCo-C(1) catalyst beforeand after the heat treatment at 900°C (followedby acid leaching) is depicted by the SEM im-ages in fig. S6. In this catalyst, as in PANI-Fe-C,the PANI nanofibers are replaced by a highlygraphitized carbon phase during heat treatment.High-resolution transmission electron micros-copy (HRTEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) imagesof the PANI-FeCo-C(1) catalyst heat-treated at900°C are shown in Fig. 4 and attest to the pres-ence of diverse carbon nanostructures, also high-lighted in HRTEM images in figs. S7 and S8.Metal-containing particles are to a large extentencapsulated in well-defined onion-like graphit-ic carbon nanoshells (Fig. 4, A and C). Such a

well-defined graphitized carbon shell surround-ing metal-rich particles was previously observedwith iron(III) tetramethoxyphenyl porphyrinchloride (FeTMPP-Cl) when the heat treat-ment temperature was raised to 1000°C (33).In the latter work, the graphite shell formationwas correlated to an increase in the catalystopen-circuit potential in oxygen-saturated solu-tion. Carbon nanosheets grown over the metalparticles are also observed in Fig. 4, A and B.In some cases, the metallic cobalt and/or ironsulfide phases (16) within the graphite-coatedparticles can be removed by acid leaching, leav-ing behind hollow and onion-like carbon nano-shells that are easily observable by HRTEM(Fig. 4B).

The carbon structure formed during the heattreatment, rather than being ideally graphitic, issomewhat disordered (e.g., turbostratic or meso-graphitic). This lattice distortion within the c-planesis reflected by a larger d-spacing of the (002)basal planes in the carbon relative to that in awell-ordered structure of graphite (34). The mea-sured (002) d-spacing, ranging from ~0.34 to0.36 nm, may facilitate incorporation of nitrogeninto the graphitic structure and thereby enhancethe number of active sites. The formation ofgraphene sheets appears to be closely associatedwith the improved durability of PANI-derivedcatalysts. A substantial fraction of multilayeredgraphene sheets in the catalyst are colocated withthe particles of Fe(Co)Sx, as shown by the greenand red arrows, respectively, in the complemen-tary TEM, HAADF-STEM, and SEM images ac-quired from the same location in Fig. 4, D to F,respectively. Apart from likely contributing toactive-site formation, the graphitization of theonion-like nanoshells and nanofibers as well asthe presence and formation of graphene sheetsthroughout the PANI-derived catalysts may alsoenhance the electronic conductivity and corrosionresistance of the carbon-based catalysts (35, 36).The presence of the graphitized carbon phase inan active catalyst deserves further study, as such aphase may play a role in hosting ORR-active sitesand enhancing stability of the PANI-derivedcatalysts.

Bridging the remaining performance gap inintrinsic activity and durability between non–precious metal catalysts and platinum in PEFCswill require determination of the active oxygenreduction site and a better understanding of thereaction mechanism.

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Phys. 12, 15251 (2010).37. We thank R. Adzic, Y. S. Kim, J. Chlistunoff, F. Garzon,

R. Mukundan, H. Chung, and S. Conradson for stimulatingdiscussions. Supported by the Energy Efficiency andRenewable Energy Office of the U.S. Department ofEnergy (DOE) through the Fuel Cell Technologies Program,and by Los Alamos National Laboratory through theLaboratory-Directed Research and Development Program.Microscopy research was supported by the Oak RidgeNational Laboratory’s SHaRE User Facility, sponsored by theDOE Office of Basic Energy Sciences. The authors have fileda patent through Los Alamos National Laboratory on thecatalysts described herein.

Supporting Online Materialwww.sciencemag.org/cgi/content/full/332/6028/443/DC1Materials and MethodsTable S1Figs. S1 to S8References

23 November 2010; accepted 24 March 201110.1126/science.1200832

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