the superfluid bulk, before annihilation. The prop-agation of the vortex through the superfluidbulk acts as a dissipative channel that gives riseto a resistive flow, which leads to an exponentialdecay of z(t). This mechanism can occur in ourcrossover superfluids: The three-dimensional char-acter of our junction, combined with the couplingto the transverse modes favored by the stronginterparticle interactions, may facilitate the leak-age of vortices from the barrier region (33).By performing a statistical study over several

time-of-flight images recorded after some timeevolution in the trap (22), we detected with non-zero probability the presence of topological de-fects, which appear as density depletions in theexpanded clouds (Fig. 4B, inset). By measuringtheir oscillation period in the trap after switch-ing off the barrier, we identified themas solitonicvortices (22, 34). The intimate connection be-tween the breakdown of the Josephson oscilla-tions and the appearance of vortices is furtherconfirmed by the data shown in Fig. 4B. Thisfigure shows the behavior of the Josephson fre-quency wJ at unitarity as a function of V0, to-gether with the occurrence of defects observedfor each V0 value over a statistical ensemble of40 images. Vortices appear only in the regimewhere coherent oscillations are absent (V0/EF >1.5). The interconnection between the quench ofthe coherent dynamics and the vortex nucleationis not peculiar to the unitary point; it extendsover the entire BEC-BCS crossover region. Thiscan be observed by comparing Fig. 4C and 4D,where the measured wJ is contrasted with thevortex occurrence probability, as a function ofV0/EF and 1/kFa. The trend of the first obser-vable is inversely correlated with the behaviorof the second one for all interaction regimes.Figure 4D highlights the robustness of thecrossover superfluid, which resists the forma-tion of topological defects while maintainingthe highest Josephson frequency. Our results dif-fer from those reported in a study of the limit ofvanishingly low barriers (V0 ≪ m), where pho-nonic excitations and pair-breaking effects, ratherthan vortices, respectively cause the breakdownof superfluidity in the BEC and BCS sides (30).Our work paves the way for studies of the

interplay between elementary and topologicalexcitations in the dissipative dynamics createdby varying the height and width of the inter-well barrier, and to the measurement of thesuperfluid gap, in close analogy with tunnelingexperiments in superconductors (3, 18). More-over, extending our studies of the tunnelingdynamics above the condensation temperatureTC may provide insight into the role of phasefluctuations in the regime where preformednoncondensed pairs appear in the system (17).

REFERENCES AND NOTES

1. B. D. Josephson, Phys. Lett. 1, 251–253 (1962).2. P. W. Anderson, Rev. Mod. Phys. 38, 298–310 (1966).3. A. Barone, G. Paternò, Physics and Applications of the

Josephson Effect (Wiley, New York, 1982).4. K. Sukhatme, Y. Mukharsky, T. Chui, D. Pearson, Nature 411,

280–283 (2001).

5. E. Hoskinson, Y. Sato, I. Hahn, R. E. Packard, Nat. Phys. 2,23–26 (2006).

6. F. S. Cataliotti et al., Science 293, 843–846 (2001).7. M. Albiez et al., Phys. Rev. Lett. 95, 010402 (2005).8. T. Schumm et al., Nat. Phys. 1, 57–62 (2005).9. S. Levy, E. Lahoud, I. Shomroni, J. Steinhauer, Nature 449,

579–583 (2007).10. L. J. LeBlanc et al., Phys. Rev. Lett. 106, 025302

(2011).11. M. Abbarchi et al., Nat. Phys. 9, 275–279 (2013).12. J. C. Davis, R. E. Packard, Rev. Mod. Phys. 74, 741–773

(2002).13. A. J. Leggett, Quantum Liquids: Bose Condensation and

Cooper Pairing in Condensed-Matter Systems (Oxford Univ.Press, Oxford, 2006).

14. W. Zwerger, Ed., The BCS-BEC Crossover and the Unitary FermiGas (Springer, Heidelberg, Germany, 2012).

15. M. Inguscio, W. Ketterle, C. Salomon, Eds., Proceedings of theInternational School of Physics “Enrico Fermi”, Course CLXIV,Varenna, Italy, 20 to 30 June 2006 (IOS Press, Amsterdam,2008).

16. M. W. Zwierlein, in Novel Superfluids, Volume 2, K. H. Bennemann,J. B. Ketterson, Eds. (International Series of Monographs onPhysics 157, Oxford Univ. Press, Oxford, 2014), pp. 269–422,and references therein.

17. Q. Chen, J. Stajic, S. Tan, K. Levin, Phys. Rep. 412, 1–88(2005).

18. S. Hüfner, M. A. Hossain, A. Damascelli, G. A. Sawatzky,Rep. Prog. Phys. 71, 062501 (2008).

19. E. Varoquaux, Rev. Mod. Phys. 87, 803–854 (2015), andreferences therein.

20. F. Piazza, L. A. Collins, A. Smerzi, New J. Phys. 13, 043008(2011).

21. A. Burchianti et al., Phys. Rev. A 90, 043408 (2014).22. Materials and methods are available as supplementary

materials on Science Online.

23. D. Stadler, S. Krinner, J. Meineke, J. P. Brantut, T. Esslinger,Nature 491, 736–739 (2012).

24. A. Smerzi, S. Fantoni, S. Giovanazzi, S. R. Shenoy, Phys. Rev.Lett. 79, 4950–4953 (1997).

25. I. Zapata, F. Sols, A. J. Leggett, Phys. Rev. A 57, R28–R31(1998).

26. J. K. Chin et al., Nature 443, 961–964 (2006).27. C. Kohstall et al., New J. Phys. 13, 065027 (2011).28. P. Zou, F. Dalfovo, J. Low Temp. Phys. 177, 240–256

(2014).29. A. Spuntarelli, P. Pieri, G. C. Strinati, Phys. Rev. Lett. 99,

040401 (2007).30. D. E. Miller et al., Phys. Rev. Lett. 99, 070402 (2007).31. F. Meier, W. Zwerger, Phys. Rev. A 64, 033610 (2001).32. G. E. Astrakharchik, J. Boronat, J. Casulleras, S. Giorgini,

Phys. Rev. Lett. 95, 230405 (2005).33. K. C. Wright, R. B. Blakestad, C. J. Lobb, W. D. Phillips,

G. K. Campbell, Phys. Rev. Lett. 110, 025302 (2013).34. M. J. Ku et al., Phys. Rev. Lett. 113, 065301 (2014).

ACKNOWLEDGMENTS

We acknowledge inspiring discussions with F. Dalfovo, A. Recati,and W. Zwerger. We thank C. Fort, A. Trenkwalder, A. Morales,and T. Macrì for collaboration at the initial stage of this work.We especially acknowledge the LENS Quantum Gases group.This work was supported under European Research Council grantno. 307032 QuFerm2D.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/350/6267/1505/suppl/DC1Materials and MethodsFigs. S1 to S7References (35–49)

7 July 2015; accepted 13 November 201510.1126/science.aac9725

ENERGY STORAGE

Nitrogen-doped mesoporous carbonof extraordinary capacitance forelectrochemical energy storageTianquan Lin,1,2 I-Wei Chen,3 Fengxin Liu,1 Chongyin Yang,1 Hui Bi,1

Fangfang Xu,1 Fuqiang Huang1,2*

Carbon-based supercapacitors can provide high electrical power, but they do not havesufficient energy density to directly compete with batteries. We found that anitrogen-doped ordered mesoporous few-layer carbon has a capacitance of 855 faradsper gram in aqueous electrolytes and can be bipolarly charged or discharged at afast, carbon-like speed. The improvement mostly stems from robust redox reactions atnitrogen-associated defects that transform inert graphene-like layered carbon into anelectrochemically active substance without affecting its electric conductivity. Thesebipolar aqueous-electrolyte electrochemical cells offer power densities and lifetimessimilar to those of carbon-based supercapacitors and can store a specific energy of41 watt-hours per kilogram (19.5 watt-hours per liter).

Carbon supercapacitors have outstandingattributes of low weight, very fast charging/discharging kinetics, and bipolar opera-tional flexibility. For carbon-based mate-rials, only electrical double-layer capacitance

(EDLC) is available; thus, surface area is the keyconcern. But even at a very large surface area(~2180 to 3100 m2 g−1), their specific capacitanceis still relatively low (~250 F g−1), which has lim-ited their appeal (1–4). Meanwhile, graphene has

a theoretical EDLC of ~550 F g−1 (5, 6) because ofits extraordinary conductivity and specific sur-face area (∼2630m2 g−1). In practice, however, itscapacitance has also been limited to ~300 F g−1,about the same as the best carbon-based EDLC(2, 5–7). Therefore, efforts have been made toenable redox reactions in ordered mesoporouscarbon (OMC) (8, 9) and conducting polymersby N doping, which via proton incorporation cantheoretically endow a capacitance of ∼2000 F g−1

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to conducting polymer polyaniline (10). Never-theless, such efforts have failed because con-ducting polymers are too unstable for practicalelectrochemical cells, while stable OMC is tooresistive to deliver a high capacitance or power.We demonstrate that N doping can turn inert

graphene-like layered carbon into an electro-chemically active substance. The preparationmethod is described in the supplementary ma-terials, starting with a sacrificial mesoporoussilica template, which contains self-assembled

tubes (11) later covered by few-layer carbon. Af-ter etching away silica, a self-supported orderedmesoporous few-layer carbon (OMFLC) super-structure in Fig. 1A remained. Various N-dopedOMFLC (OMFLC-N) having N incorporated atseveral OMFLC locations in Fig. 1B were alsoobtained, some further modified by a HNO3 oxi-dation treatment that partially converted N intoN−O. This set of OMFLC-N samples (table S1)includes 8.2 atomic percent (at%) N before andafter oxidation treatment (samples S1 and S2)and 11.9 at% N before and after treatment (sam-ples S3 and S4). For comparison, an orderedmesoporous (amorphous) carbon and a commer-cial activated carbon (YP-50, Kuraray Chemical)were also studied. To demonstrate the relevanceto practical applications, we further implementedthe idea using a simplified, template-free, scalablemethod producing essentially the same N-dopedmesoporous few-layer carbon materials with thesame overall performance.The ordered mesoporous nature of OMC

(Fig. 1C) and OMFLC (Fig. 1D) was confirmed

by electron microscopy. In both, OMFLC tubesappear as bright strips 4 to 6 nmwide. The tubesare porous, containing pores 1 to 2 nm in diam-eter (dark regions in strips), and are separated byaligned pore channels (dark regions betweenstrips) of about the same size or diameter. High-resolution imaging of OMFLC’s tube walls fur-ther revealed graphene-like sheets with ≤5 layers(Fig. 1E). These relatively homogeneous and uni-form mesoporous textures were largely preservedin OMFLC-N (fig. S1). The silica tubes in thetemplate are known to form a two-dimensional(2D) hexagonal “crystal”with space group p6mm(11). The same superstructure was confirmed inOMC, OMFLC, and OMFLC-N by their diffractionpatterns (Fig. 1F), which show decreasing peakintensities in the above order, indicating a pro-gressive distortion of the superstructure.Nitrogen adsorption-desorption suggests a bi-

modal pore size distribution (Fig. 1G) centeredaround 1.8 nm and 3.5 to 4.0 nm in all threemesoporous structures. They share similar N2

adsorption-desorption isothermswith a Langmuir

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Fig. 1. Structure of N-doped ordered mesoporous few-layer carbon andrelated materials. (A) Fabrication schematic of ordered mesoporous few-layer carbon (OMFLC). (B) Possible locations for N incorporation into a few-layer carbon network. (C and D) High-angle annular dark-field transmissionelectron microscopy (TEM) images of ordered mesoporous carbon (OMC)(C) and OMFLC (D); dark regions indicate connected pore channels. (E) High-

resolution TEM image of OMFLC; nanoporous walls consist of few-layered carbonsheets. (F) Low-angle x-ray diffraction patterns of OMC, OMFLC, and OMFLC-N(S1), showing characteristic (100), (110), and (200) peaks of hexagonal pack-ing. (G) Pore size distributions of OMC,OMFLC, and OMFLC-N (S1). (H) Wettingangles of 0.5 M H2SO4 droplet on OMFLC (85°) and OMFLC-N (S1) (21°)substrates.

1State Key Laboratory of High Performance Ceramics andSuperfine Microstructure and CAS Key Laboratory ofMaterials for Energy Conversion, Shanghai Institute ofCeramics, Chinese Academy of Sciences, Shanghai 200050,P.R. China. 2Beijing National Laboratory for MolecularSciences and State Key Laboratory of Rare Earth MaterialsChemistry and Applications, College of Chemistry andMolecular Engineering, Peking University, Beijing 100871,P.R. China. 3Department of Materials Science andEngineering, University of Pennsylvania, Philadelphia, PA19104, USA.*Corresponding author. E-mail: huangfq@mail.sic.ac.cn

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hysteresis (fig. S2A) typical of well-defined meso-pores. Among them,OMFLC-N (S1) has the largestsurface area (1580 m2 g−1), the largest total porevolume (2.20 cm3 g−1), the smallest average porewidth (2.25 nm), and the most prominent poressmaller than 2 nm (Fig. 1G). The characteristicRaman 2D band (fig. S2B) verified the formationof local graphene-like structure with ≤5 layers inOMFLC and OMFLC-N.Local graphene-like structure formation and

Ndoping profoundly altered other physical prop-erties as well. Whereas OMC is clearly an insu-lator, OMFLC andOMFLC-N displaymuch lowerroom-temperature resistance with much weakertemperature dependence (fig. S2C), indicating animprovement in the structural order of carbon(12).Meanwhile,whereas bothOMCandOMFLCare hydrophobic, OMFLC-N is hydrophilic, wettinga 0.5MH2SO4 droplet in Fig. 1H. This is consistentwith the zeta potential: Nearly neutral OMFLC(zeta potential =−6mV, almost the same asOMC’s−4 mV) becomes more nucleophilic OMFLC-N(−20mV) as a result of lone-pair N 2pz electrons;these improved physical properties of OMFLC-Nare generally conducive to the supercapacitiveperformance (see below).Spectroscopy studies identified C-bonding

and N-C locations in the carbon network. Thepresence of sp2 bonding expected for grapheneand local graphene-like structure was evidentfrom the high ratio of p* bonding to p* + s* bond-ing (fig. S3A), giving 98% (±2%) sp2 bonding inOMFLC versus 86% (±2%) in OMFLC-N, with

reference to graphite (100%). Evidence for N sub-stitution inOMFLC-Nwas also detected (fig. S3B),and the N/O content and bonding of OMFLC-Nquantified by x-ray photoelectron spectroscopy(XPS) (fig. S3, C to F, and table S1) provided thefollowing picture: (i) Deconvoluted N 1s XPS con-tains three characteristic peaks at 398, 400, and401 eV, corresponding to pyridinic (N-6), pyrrolic(N-5), and graphitic (N-Q) nitrogen, respectively,as shown in Fig. 1B (13, 14). (ii) As the N contentincreases from ~8.2 at% in sample S1 to 11.9 at%in S3, N substitution at “regular” graphitic C sites(N-Q) instead of defective sites (N-5 and N-6) be-comesmore abundant. (iii) OxidativeHNO3 treat-ment caused the least stable N-5 to substantiallyconvert to N−O (N associated with an O, shownin fig. S3D at 403.2 eV) (15) without affecting themost stable N-Q, as suggested by the correlationof N-O percentage (%N-O) to the decrement ofN-5 percentage (%N-5), denoted by D%N-5 intable S1. (iv) Non–N-Q fractions (i.e., %N-5 +%N-6 in table S1) decrease in the order ofsamples S3, S1, S2, and S4; their redox potentialsalso increase in the same order.These redox potentials in aqueous electrolytes

were determined in three-electrode electrochem-ical cells in 0.5 M H2SO4 (pH 0) electrolyte usingan Ag/AgCl reference electrode and a Pt counter-electrode. The working electrode was preparedby pressing together active-material powders (ata mass loading of 0.5 mg cm−2) and an inactive,highly compressible graphene foam (3D-graphene,specific capacitance = 30 F g−1) without any other

additive. In cyclic voltammetry (CV) at 2 mV s−1

(Fig. 2A), cells with both OMC and OMFLC work-ing electrodes have nearly rectangular CV curvesrepresentative of an ideal efficient EDLC. WithOMFLC-N electrodes, the curves may be decon-voluted into (i) a nearly rectangular EDLC-likecurve, albeit with a substantially higher charging/discharging current not seen with OMC andOMFLC, and (ii) a set of symmetric Faradaiccharging/discharging peaks. In (ii), the chargingpeaks are located at ~0.25 V to ~0.5 V, increasingin the order of S3, S1, S2, and S4 (S4 data omittedin Fig. 2A but listed in table S1), which is exactlythe same order that non–N-Q fractions decrease,thus strongly suggesting that the redox potentialis related to N-5 and/or N-6. The above shapeand symmetry features were maintained whenthe scan rate increased to 100 mV s−1, as shownfor S1 in fig. S4A. This indicates that both EDLC-like and redox reactions have fast charging/discharging kinetics.To proceed further, we note that pseudoca-

pacitive materials with a pronounced redox peakare usually inefficient electrodes in a symmetricelectrochemical cell, which renders the effortof incorporating faradaic capacitance ineffective.This is because a symmetric cell is electricallyequivalent to two serial capacitors, C1 and C2, soits total capacitance C1C2/(C1 + C2) is optimizedwhen C1 = C2. This condition is usually impos-sible to satisfy at all potentials unless the CVcurve is rectangular. We found that the followingsimplemethodcanovercome thisproblem,however.

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Fig. 2. Electrochemical evaluation. (A) Cyclic voltammetry (CV test, at2 mV s−1) from the first cycle for OMC, OMFLC, and OMFLC-N (S1 to S3)and for mixed OMFLC-N (SM). (B) Galvanostatic charge/discharge (CCtest at 1.0 A g−1) from the first cycle for OMC, OMFLC, and OMFLC-N (S1and SM). (C) Complex-plane plots of AC impedance. Inset shows phaseangle versus frequency. (D) Capacity versus square root of half-cycle time.Solid symbols, CV test data from 2 to 500 mV s−1; open symbols, CC testdata from 1 to 40 A g−1. Extrapolated intercept capacitance is rate-

independent capacitance k1, the remainder diffusion-controlled capaci-tance. (E) Tafel plots of electrode potential against pH at steady-statecurrent density of 10 mA cm−2. (F) Tafel plots of electrode potential againstcurrent i at pH 6.8 for OMFLC-N (S1 and SM). All potentials are relative toAg/AgCl reference electrode; all electrolytes except (E) and (F) are 0.5 MH2SO4 aqueous solution. In (E) and (F), theoretical slope (–59.2 mV/decade)is shown as a straight line to suggest reasonable agreement with the data(see text).

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By mixing three OMFLC-N powders at the ratioof S1:S2:S3 = 0.3:0.3:0.4 to form another OMFLC-N powder (SM), we obtain a new material that iscapable of supporting multiple faradaic peaks. Itexhibits a rectangular EDLC-like CV curve at avery large current (Fig. 2A)—a feature that shouldprove useful for constructing high-performancesymmetric electrochemical cells. Because this is aqualitatively different CV curve from those ofother OMFLC-N electrodes as well as OMC andOMFLC, we made further performance compar-isons between SM, S1, OMC, OMFLC, and YP-50.Consistent with the CV results, all galvano-

static charge/discharge tests (the CC test in Fig.2B) show symmetric features with a fairly lin-ear slope. A specific capacitance as high as855 Fg−1 at a currentdensityof 1Ag−1wasobtainedfor SM, versus 715 F g−1 for S1 (fig. S4, C and D)and 175 F g−1 for YP-50 (fig. S5). Over a widerange of current densities, SMcontinued toprovidea well-behaving CC curve and high capacitance(fig. S4, E and F), achieving 615 F g−1 at 40 A g−1,which is much higher than known EDLC andquite comparable to the capacitance of transitionmetal–oxide faradaic pseudocapacitors (16–18).Electrochemical impedance spectroscopy (Fig.

2C, enlarged in fig. S6A) found OMFLC-N (S1) tohave the lowest equivalent series resistance of~0.8 ohms, better than that of OMFLC andOMC.

This may be attributed to better wetting onOMFLC-N, which lowers the interface resistance,because OMFLC has at least comparable, if notlower, resistivity than OMFLC-N (fig. S2C). The>45° (negative) phase angle of both OMFLC-N(S1 and SM; inset of Fig. 2C) at relatively highfrequencies confirms their capacitive behavior atfast rates. Specifically, the frequency (of −45°)when the resistance and reactance have equalmagnitudes is 0.48 Hz for OMFLC-N, giving arelaxation time (t0 = 1/f0) of 2.1 s.The CV and CC tests are in broad agreement

with each other when compared at the samehalf-cycle time T as seen in Fig. 2D, which alsoprovides insight into the charging/dischargingkinetics. (In the CV test, T is the time to sweepover the voltage window. In the CC test, it is thetime to discharge.) In general, the capacitance Cmay contain a rate-independent component k1(classically attributed to EDLC) and a diffusion-limited component controlled by the scanningrate, n = T−1, taking the form (19, 20)

C ¼ k1 þ k2v−1=2 ð1Þ

In Fig. 2D, the k2v−1/2 term represents the long-T

data, which extrapolate to k1 at theT1/2 = v−1/2 = 0

intercept. (In the CV test, Fig. 2D reduces to thestandard C-v−1/2 plot in fig. S6B, from which

one can also obtain k1.) Apparently, k1 domi-nates inOMFLC-N, exceeding 700 F g−1 in SMand545 F g−1 in S1. Dominance of rate-independentcapacitances is common for EDLC, but it never-theless holds here in redox reactions of theabove materials because (i) OMFLC is a low-dimensional, fast-conducting, high-surface-area,few-layered material, and (ii) OMFLC-N is meso-porous (fig. S1) and hydrophilic (Fig. 1H). There-fore, they allow facile reactions both outside andinside the few-layer carbon tubes, as well asacross the tube thickness.The data from the slowest, near-equilibrium

tests allowed us to construct the Tafel plots inFig. 2, E and F, to compare the energetics offaradaic reactions and reveal a fundamental dif-ference betweenOMFLC-N andOMCorOMFLC.For both S1 and SM, the potential required tosustain a constant current density of 10 mA cm−2

from pH 4.0 to 7.0 (Fig. 2E) lies close to thetheoretical Tafel line with a slope of 2.3 × RT/F(−59.2 mV/pH) (21). Likewise, measuring thepotential required for different current densities(Fig. 2F) at pH 6.8 gives again a slope in closecoincidencewith 2.3 ×RT/F. Because both sets ofTafel lines imply a one-electron reaction, the pHdependence must arise from the concurrent in-corporation of one proton and one electron. Incontrast, for OMC and OMFLC, the slope is very

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Fig. 3. Electrochemical performance of symmetric cells. OMFLC-N SMcathode and anode were used in two aqueous electrolytes, 0.5 M H2SO4

(pH 0) and 2 M Li2SO4 (pH 1.8). (A) Cyclic voltammetry from the first cycleat 2 mV s−1 scan rate. (B) Galvanostatic charge/discharge curves from thefirst cycle at 1 A g−1. (C) Symmetric electrochemical cell devices retain>92% after 100 hours of sustained loading (blue symbols, upper scale) at1.2 V (in 0.5 M H2SO4) and 1.6 V (in 2 M Li2SO4), and retain >80% of theirinitial response after 50,000 cycles (black symbols, lower scale) from 0 tosame peak voltages in two electrolytes. (D) Gravimetric (left) and volu-metric (right) capacitance (at 1 A g−1) of symmetric electrochemical celldevice (counting electrode weight and volume only) versus areal massloading of OMFLC-N SM in two aqueous electrolytes. (E) Ragone plot of

specific energy versus specific power for OMFLC-N SM symmetric devices(counting all-device weight) using 0.5 M H2SO4 (solid squares) and 2 MLi2SO4 (solid circles) electrolytes, as well as several standard devices:electrochemical capacitors (EC) (2, 28), lead-acid batteries (1, 26), nickelmetal-hydride batteries (27), and lithium-ion batteries (28). Data countingelectrode mass only are shown as open symbols. (F) Ragone plots of en-ergy density versus power density for OMFLC-N SM packaged symmetricdevices (counting all-device volume) using 0.5 M H2SO4 (solid squares)and 2 M Li2SO4 (solid circles) electrolytes, as well as several standard devicesas in (E). Data counting electrode volume only are shown as open symbols.Dotted lines in (E) and (F) are current drain time, calculated by dividingspecific energy by specific power.

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flat, suggesting little redox activity. Recalling thatthe redox potential decreases with increasingconcentration of nongraphitic N (Fig. 2A andtable S1), we believe the redox reaction previouslyproposed for pseudocapacitance in N-containingpolypyrrole (22) and carbon nanotubes (8, 9)—that each pyrrolic (N-5) and pyridine (N-6) nitro-gen can incorporate an electron and a proton—isalso operational here: It fits all of the above de-scriptions. According to N 1s XPS that can “see”through tubes less than 2 nm thick, there is 8.2at% N in OMFLC-N (S1), which may store anadditional faradaic charge of 660 F g−1. This ismore than enough to account for the storagedifference (390 F g−1) between OMFLC-N (S1)and OMFLC.The proposed N-H mechanism dictates that

an acidic condition is more favorable for redoxreactions. This was verified for S1 in the three-electrode CC test at 1 A g−1: It has a larger ca-pacitance in 0.5MH2SO4 (715 F g−1, Fig. 2B) thanin 1 M KOH (pH 14) electrolyte (405 F g−1; fig. S7,A to D, also confirmed by the CV tests). In con-trast, OMFLC, which solely relies on EDLC, hasvery similar capacitances in the two electrolytes(fig. S7E). These results lend further support tothe proposed N-H redox mechanism that makesOMFLC-N a superior supercapacitor.Hoping to reduce these new mechanisms

into practice, we investigated whether the three-electrode performance of OMFLC-N can be trans-lated to electrochemical cells. Carbon-basedelectrodes are special in that they can be usedas both cathodes and anodes in symmetric elec-trochemical cells, with a per-electrode specificcapacitance nearly the same as that measured inthe three-electrode test. This was confirmed forYP-50, OMC, and OMFLC (table S2). Here, wemul-tiply the nominal specific capacitance of a sym-metric EC by 4 to obtain the per-electrode specificcapacitance (23). In contrast, asmentioned before,pseudocapacitive materials with a pronouncedredox peak are usually inefficient electrodes insymmetric electrochemical cells (23, 24) becausetheir two differential capacitances at the twoelectrodes, C1 and C2, are different. (Under thenormal circumstance when one electrode has asuitable potential for the major redox peak andthus a larger differential capacitance, the otherelectrode is at a potential away from the majorredox peak, hence having a smaller differentialcapacitance.) So their total capacitance C1C2/(C1 + C2) is lower than the maximum, which is½C1 = ½C2 when C1 = C2. In contrast, despitepredominant contributions of redox reactions,our SM electrode maintains a nearly rectangularCV curve (Fig. 2A)—that is, a constant differen-tial capacitance—in the three-electrode test. Sowe expect its symmetric electrochemical cell tosatisfy C1 = C2, thus to provide a per-electrodespecific capacitance identical to thatmeasured inthe three-electrode test. Indeed, its symmetric-cell CV curve (Fig. 3A) in 0.5 MH2SO4 electrolyteis rectangular and rather symmetric, and itssymmetric-cell CC test (Fig. 3B) gives a per-electrode capacitance of 840 F g−1 at 1 A g−1—within 2% of the three-electrode capacitance of

855 F g−1 (see table S2). In comparison, otherOMFLC-N electrodes (S1 to S3) each having adistinct redox peak in the CV curve all sufferedfrom capacitance losses of 10 to 15% when usedin a symmetric electrochemical cell (table S2). Allthe symmetric-cell electrochemical measurementswere conducted in 0.5 MH2SO4 electrolyte usingan operating voltage of 1.2 V, which did not causeany detectable H2 or O2 evolution (fig. S8A).The performance of OMFLC-N SM electrodes

in symmetric aqueous electrochemical cells wasfurther confirmed using another electrolyte,Li2SO4, which helps prevent carbon-electrodecorroding and allows a higher operating voltageup to 1.9 V (25). Indeed, in 2M Li2SO4 electrolyteat pH 1.8, a symmetric electrochemical cell withSM electrodes had a threshold water-splittingvoltage of 1.8 V; at 1.6 V there was no detectableH2 or O2 evolution after 24 hours (fig. S8B). Inacidic (pH 1.8) but not basic (pH 9.2) Li2SO4,pronounced redox was confirmed by the CV test(fig. S9). With this electrolyte, symmetric electro-chemical cells obtained a specific capacitanceof 740 F g−1 at 1 A g−1 from the CV and CC tests(Fig. 3, A andB), just 5%below the three-electrodecapacitance of 780 F g−1 (table S2).Having established the robust redox bipolar

activities of OMFLC-N SM as both cathode andanode, we further evaluated its suitability forpractical applications, starting with their sta-bility in sustained and cyclic loading (Fig. 3C).After 100 hours of sustained loading, the capac-itance retention was 93% at 1.2 V in 0.5 MH2SO4

electrolyte and 92% at 1.6 V in 2 M Li2SO4 elec-trolyte. The symmetric electrochemical cell with-stood 50,000 cycles between 0 and 1.2 V in 0.5 MH2SO4 electrolyte with 82% of the capacitanceremaining; a similarly cycled device between 0and 1.6 V in 2 M Li2SO4 (pH 1.8) electrolyte re-tained 80%.To pack more energy and power into the de-

vice, we increased the mass loading to the limitof not sacrificing full electrochemical efficien-cy. (To aid electrode formation at >2.0 mg cm−2

OMFLC-N SM loading, we added 5 wt% PVDFto the OMFLC-N powders.) Up to 6.0 mg cm−2

(∼0.69 g cm−3), the gravimetric specific capacitanceof the symmetric electrochemical cell changedminimally (Fig. 3D), indicating that OMFLC-Npowders had full access to the electrolyte with-out geometric or electric hindrance or diffusionlimitation. Such increased loading benefits thevolumetric capacitance, which is important forpractical applications. Peaking at 6.0 mg cm−2,the volumetric capacitance increases by morethan a factor of 8, so that OMFLC-N SM can reach560 F cm−3 and 810 F g−1 in 0.5 M H2SO4, and490 F cm−3 and 710 F g−1 in 2 M Li2SO4 (pH 1.8).The merit of our material relative to existing

battery and supercapacitor materials was eval-uated using Ragone plots (specific power versusspecific energy) for symmetric electrochemicalcells on both the device gravimetric basis (Fig.3E) and the device volumetric basis (Fig. 3F).In 0.5 M H2SO4 electrolyte, our device has aspecific energy E of 24.5 Wh kg−1 based onthe device weight (corresponding to 39.5 Wh

kgOMFLC-N−1 based on the active-material weight)

or 12.0 Wh liter−1 based on the device volume (or27.0 Wh literOMFLC-N

−1 based on the electrodevolume). The specific power P is 26.5 kW kg−1

(42.5 kWkgOMFLC-N−1) or 13.0 kW liter−1 (29.0 kW

literOMFLC-N−1), with a current-drain time (E/P) of

3.4 s. In 2 M Li2SO4 electrolyte, E increases to41.0 Wh kg−1 (63.0 Wh kgOMFLC-N

−1) and 19.5 Whliter−1 (43.5 Wh literOMFLC-N

−1) and P stays at26.0 kW kg−1 (44.0 kW kgOMFLC-N

−1) and 12.5 kWliter−1 (30.0 kW literOMFLC-N

−1), with a draintime of 5.7 s. For supercapacitor applications,these properties are notable in that high spe-cific power can be simultaneously achieved alongwith high specific energy, thus making carbon-based supercapacitors potentially competitiveagainst batteries, such as lead-acid batteries(3, 26), nickel metahydride batteries (27), andperhaps even lithium batteries (28) on a gravi-metric basis.Simplified fabrication of N-dopedmesoporous

few-layer carbon (omitting the sacrificial silicatemplate and post–carbon deposition etching asdescribed in supplementarymaterials) was final-ly implemented by combining chemical vapordeposition with a sol-gel process of inexpensive,environmentally friendly, Si-free precursors/catalysts. The material obtained is made of high-ly conductive (s = 360 S/cm) mesoscopically or-dered few-layer carbon with a large surface area(1900 m2 g−1). In 0.5 M H2SO4 electrolyte, itselectrode has a specific capacitance of 790 F g−1

at 1 A g−1, and its packaged device has a specificenergy of 23.0 Wh kg−1 and a specific power of18.5 kW kg−1 based on the device weight; in 2 MLi2SO4 (pH 1.8) electrolyte, the correspondingvalues are 720 F g−1, 38.5 Wh kg−1, and 22.5 kWkg−1. Indeed, in all important respects (figs. S10to S13), this material behaves within ~10% of thebest OMFLC-N SM described above, thus provid-ing an outstanding low-cost carbon-based mate-rial for electrochemical cells for electric powerapplications.

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(2011).23. M. D. Stoller, R. S. Ruoff, Energy Environ. Sci. 3, 1294–1301 (2010).24. V. Khomenko, E. Frackowiak, F. Béguin, Electrochim. Acta 50,

2499–2506 (2005).

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ACKNOWLEDGMENTS

Supported by National Natural Science Foundation of Chinagrants 51125006, 91122034, 61376056, and 51402336 and Scienceand Technology Commission of Shanghai grant 14YF1406500.

I.W.C. was supported by U.S. Department of Energy BES grantDE-FG02-11ER46814 and used the facilities (Laboratory forResearch on the Structure of Matter) supported by NSFgrant DMR-11-20901.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/350/6267/1508/suppl/DC1Materials and MethodsFigs. S1 to S14Tables S1 and S2References (29–32)

20 April 2015; accepted 13 November 201510.1126/science.aab3798

NANOMATERIALS

Synthesis of borophenes: Anisotropic,two-dimensional boron polymorphsAndrew J. Mannix,1,2 Xiang-Feng Zhou,3,4 Brian Kiraly,1,2 Joshua D. Wood,2

Diego Alducin,5 Benjamin D. Myers,2,6 Xiaolong Liu,7 Brandon L. Fisher,1

Ulises Santiago,5 Jeffrey R. Guest,1 Miguel Jose Yacaman,5 Arturo Ponce,5

Artem R. Oganov,8,9,3* Mark C. Hersam,2,7,10* Nathan P. Guisinger1*

At the atomic-cluster scale, pure boron is markedly similar to carbon, forming simple planarmolecules and cage-like fullerenes.Theoretical studies predict that two-dimensional (2D) boronsheets will adopt an atomic configuration similar to that of boron atomic clusters.Wesynthesized atomically thin, crystalline 2D boron sheets (i.e., borophene) on silver surfacesunder ultrahigh-vacuum conditions. Atomic-scale characterization, supported by theoreticalcalculations, revealed structures reminiscent of fused boron clusters with multiple scales ofanisotropic, out-of-plane buckling. Unlike bulk boron allotropes, borophene shows metalliccharacteristics that are consistent with predictions of a highly anisotropic, 2D metal.

Bonding between boron atoms is more com-plex than in carbon; for example, both two-and three-center B-B bonds can form (1).The interaction between these bondingconfigurations results in asmany as 16 bulk

allotropes of boron (1–3), composed of icosahedralB12 units, small interstitial clusters, and fusedsupericosahedra. In contrast, small (n < 15) boronclusters form simple covalent, quasiplanar mole-

cules with carbon-like aromatic or anti-aromaticelectronic structure (4–7). Recently, Zhai et al.have shown that B40 clusters form a cage-likefullerene (6), further extending the parallels be-tween boron and carbon cluster chemistry.To date, experimental investigations of nano-

structured boron allotropes are notably sparse,partly owing to the costly and toxic precursors(e.g., diborane) typically used. However, nu-merous theoretical studies have examined two-dimensional (2D) boron sheets [i.e., borophene(7)]. Although these studies propose various struc-tures, we refer to the general class of 2D boronsheets as borophene. Based upon the quasiplanarB7 cluster (Fig. 1A), Boustani proposed an Aufbauprinciple (8) to construct nanostructures, includ-ing puckered monolayer sheets (analogous to therelation between graphene and the aromatic ring).The stability of these sheets is enhancedby vacancysuperstructures (7, 9) or out-of-plane distortions(10, 11). Typically, borophene is predicted to bemetallic (7, 9–12) or semimetallic (10) and is ex-pected to exhibitweak binding (13) and anisotropicgrowth (14) when adsorbed on noble-metal sub-strates. Early reports of multiwall boron nano-tubes suggested a layered structure (15), but theiratomic-scale structure remains unresolved. Itis therefore unknown whether borophene is ex-perimentally stable and whether the borophene

structurewould reflect the simplicity of planar boronclusters or the complexity of bulk boron phases.We have grown atomically thin, borophene

sheets under ultrahigh-vacuum (UHV) conditions(Fig. 1B), using a solid boron atomic source(99.9999% purity) to avoid the difficulties posedby toxic precursors. An atomically clean Ag(111)substrate provided a well-defined and inert sur-face for borophene growth (13, 16). In situ scan-ning tunneling microscopy (STM) images showthe emergence of planar structures exhibitinganisotropic corrugation, which is consistent withfirst-principles structure prediction. We furtherverify the planar, chemically distinct, and atom-ically thin nature of these sheets via a suite ofcharacterization techniques. In situ electroniccharacterization supports theoretical predictionsthat borophene sheets are metallic with highlyanisotropic electronic properties. This anisot-ropy is predicted to result inmechanical stiffnesscomparable to that of graphene along one axis.Such properties are complementary to those ofexisting 2D materials and distinct from thoseof the metallic boron previously observable onlyat ultrahigh pressures (17).During growth, the substrate was maintained

between 450° and 700°C under a boron flux be-tween ~0.01 to ~0.1 monolayer (ML) per minute[see supplementary materials for details (18)].After deposition, in situ Auger electron spectros-copy (AES; Fig. 1C) revealed a boron KLL peak atthe standard position (180 eV) superimposed onthe clean Ag(111) spectrum.We observed no peaksdue to contaminants, and none of the distinctivepeak shifts or satellite features characteristic ofcompound or alloy formation (fig. S1).After boron deposition at a substrate temper-

ature of 550°C, STM topography images (Fig. 1D)revealed two distinct boron phases: a homoge-neous phase and a more corrugated “striped”phase (highlighted with red and white arrows,respectively). Simultaneously acquireddI/dVmaps(where I and V are the tunneling current andvoltage, respectively) of the electronic density ofstates (DOS), given in Fig. 1E, showed strongelectronic contrast between boron sheets and theAg(111) substrate and increased differentiationbetween homogeneous and striped islands. Therelative concentration of these phases dependsupon the deposition rate. Low deposition ratesfavored the striped phase and resulted in the

SCIENCE sciencemag.org 18 DECEMBER 2015 • VOL 350 ISSUE 6267 1513

1Center for Nanoscale Materials, Argonne NationalLaboratory, 9700 South Cass Avenue, Building 440,Argonne, IL 60439, USA. 2Department of Materials Scienceand Engineering, Northwestern University, 2220 CampusDrive, Evanston, IL 60208, USA. 3Department ofGeosciences, Center for Materials by Design, and Institutefor Advanced Computational Science, Stony BrookUniversity, Stony Brook, NY 11794, USA. 4School of Physics,Nankai University, Tianjin 300071, China. 5Department ofPhysics, University of Texas San Antonio, San Antonio, TX78249, USA. 6NUANCE Center, Northwestern University,2220 Campus Drive, Evanston, IL 60208, USA. 7AppliedPhysics Graduate Program, Northwestern University, 2220Campus Drive, Evanston, IL 60208, USA. 8Skolkovo Instituteof Science and Technology, Skolkovo Innovation Center, 5Nobel Street, Moscow 143026, Russia. 9Moscow Institute ofPhysics and Technology, 9 Institutskiy Lane, DolgoprudnyCity, Moscow Region, 141700, Russia. 10Department ofChemistry, Northwestern University, 2220 Campus Drive,Evanston, IL 60208, USA.*Corresponding author. E-mail: nguisinger@anl.gov (N.P.G.);m-hersam@northwestern.edu (M.C.H.); artem.oganov@stonybrook.edu (A.R.O.)

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storageNitrogen-doped mesoporous carbon of extraordinary capacitance for electrochemical energy

Tianquan Lin, I-Wei Chen, Fengxin Liu, Chongyin Yang, Hui Bi, Fangfang Xu and Fuqiang Huang

DOI: 10.1126/science.aab3798 (6267), 1508-1513.350Science 

, this issue p. 1508Scienceability to deliver power quickly.

an increase that was retained in full capacitors, without losing their−−energy density of the carbon more than threefold fabricated a porous carbon material that was then doped with nitrogen. This raised the et al.much more quickly. Lin

In contrast to batteries, capacitors typically can store less power, but they can capture and release that powerStore more energy with a touch of nitrogen

ARTICLE TOOLS http://science.sciencemag.org/content/350/6267/1508

MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2015/12/16/350.6267.1508.DC1

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