Efficient Metal-Free Oxygen Reduction on High-Surface-Area Mesoporous Nitrogen-Doped Carbons

Ef?cient Metal-Free Oxygen Reduction in Alkaline Medium on

High-Surface-Area Mesoporous Nitrogen-Doped Carbons Made from Ionic

Liquids and Nucleobases

Wen Yang,*Tim-Patrick Fellinger,and Markus Antonietti

Colloid Chemistry,Max Planck Institute of Colloids and Interfaces,Am Mu¨hlenberg1,

14476Potsdam/Golm,Germany

Received September7,2010;E-mail:wen.yang@mpikg.mpg.de

Abstract:Mesoporous nitrogen-doped carbon materials with high surface areas up to1500m2g-1were conveniently made by the carbonization of nucleobases dissolved in an all-organic ionic liquid(1-ethyl-3-methylimidazolium dicyanamide).Using hard templating with silica nanoparticles,this process yields high-surface-area nitrogen-doped carbon materials with nitrogen contents as high as12wt%,narrow mesopore size distribution of ca.12nm diameter,and local graphitic carbon structure.It is demonstrated that the resulting nitrogen-doped carbons show very high catalytic activity,even in the metal-free case in the oxygen reduction reaction(ORR)for fuel cells.Speci?cally,the as-prepared materials exhibit a low onset voltage for ORR in alkaline medium and a high methanol tolerance,compared with those of commercial20wt%Pt/C catalyst.We regard this as a ?rst step toward an all-sustainable fuel cell,avoiding noble metals. Developing catalytic materials for oxygen reduction reaction (ORR)is the current technological bottleneck for the industrial development of fuel cells.1-4Up to now,the most ef?cient catalysts for ORR have been still platinum-based materials.3However,these catalysts are not really feasible.On the one hand,they are characterized by a quite sluggish oxygen reduction at any pH.3 Moreover,the high cost of platinum and its limited supply will hamper any the large-scale application of such fuel cells.1-3 Therefore,the search for non-precious-metal as well as metal-free catalysts for ORR is one of the most active and competitive?elds in chemistry.1-3

Nitrogen-doped carbons are already known to act as metal-free catalysts for oxygen reduction,1-3,5-16but still on a less competitive level.It is believed that one of the limitations for enhancing the catalytic activity of these systems is the low surface density of catalytic sites,3,10,15and turning to high-surface-area materials is a potential solution.10,15This is what we call“mesoscopic structure control”.In addition,there is much disagreement about the nature of the active site for oxygen reduction in nitrogen-doped carbons. Some believe that transition metals may be“accidentally”incor-porated during the fabricating process of the carbons materials and then play an important catalytic role during the ORR.2,3,5,10,16 Others propose that transition metals may not be necessary and that N-doped carbons possess inherent active sites for ORR,1,6-9,13 with a variety of N-C-N bonding patterns and local geometries being potentially active.This is called“microscopic structure control”.Showing ORR activity in metal-free nitrogen-doped carbon materials with high surface area is therefore at?rst a scienti?c issue, as this would prove the existence of non-metal-based ORR sites.1,7,13Beside this scienti?c question,also the application side is of larger importance,as a noble-metal-free catalyst based on simple carbon structures may enable fabrication of the next generation of cheap and sustainable fuel cells.

In a recent paper,17we described the(metal-free)synthesis of nitrogen-doped carbon materials by heating nonvolatile ionic liquids featuring dicyanamide anions to temperatures of1000°C.Carbons with nitrogen contents as high as10.4%,with an electronic conductivity and oxidation stability superior to those of pure graphitic carbon,were obtained.17,18This went well with the many other positive observations of improving electronic and mechanical properties of bulk carbon materials by N-doping.19-21Others showed that the density of states at the Fermi level(D(E F))and the work function will increase linearly with increasing nitrogen content,creating a metal-like conductivity.20,21As nitrogen also improves the HOMO position toward higher nobility,20,21one might also call N-doped carbons“noble carbons”(as it compares with the effect of,e.g.,chromium in noble steel).

It is the philosophy of the present contribution to gain higher nitrogen contents and microscopic structure control by incorporation of a set of natural compounds with a high atomic N/C ratio,high oxidation stability,and suitable atomic N-C-N bonding motifs that are easily available but have rarely been explored for functional carbon synthesis,namely the most familiar nucleobases,adenine

(A),guanine(G),cytosine(C),thymine(T),and uracil(U)(Scheme

1).This omission of study of such bases in carbon synthesis is surprising,as such nucleobases can be isolated from(bacterial) biomass in larger amounts and de?nitely represent a sustainable raw material base for such high-performance carbons.12As all these nucleabases readily dissolve in the ionic liquid1-ethyl-3-meth-ylimidazolium dicyanamide(Emim-dca),we simply use their solutions to synthesize nitrogen-doped carbon materials by car-bonization at1000°C for1h.As liquid and nonvolatile precursors, such educts also allow the fabrication of high-surface-area,narrowly Scheme1.Precursors and Heating Procedure for N-Doped Carbon

Materials

Published on Web12/14/2010

10.1021/ja108039j 2011American Chemical Society 2069J.AM.CHEM.SOC.2011,133,206–209

distributed mesoporous materials by using SiO 2nanoparticles as hard templates for mesostructure control.17,22

The resulting materials are tested in an standard ORR assay and are shown to exhibit excellent catalytic performance for the ORR and high resistance to the methanol crossover.1,13It will be shown that this metal-free,sustainable system can compare favorably with commercially available Pt/C catalysts.

The nitrogen-doped carbon materials with both narrowly dis-tributed mesoporosity and local N -C -N structural motifs were obtained from precursor mixtures of Emim-dca,nucleobase,and 12-nm SiO 2nanoparticles.Carbonization of these initial mixtures at 1000°C under nitrogen ?ow yielded carbon -silica composites.A preheating step at 300°C was performed in order to ensure removal of water,complete dissolution of the nucleobases,and homogeneity of the ?nal samples.After dissolution of the silica template,high-surface-area,mesoporous carbons were obtained.The materials derived from the pure ionic liquid (internal reference)and from the adenine,guanine,cytosine,thymine,and uracil solution were denoted as meso-Em,meso-EmA,meso-EmG,meso-EmC,meso-EmT,and meso-EmU,respectively.Typical transmis-sion electron microscopy (TEM)images are shown for meso-EmG in Figure 1.The mesopore structures nicely re?ect the previous

packing of silica spheres,con?ning the carbonization reaction.The textural properties of the resulting nitrogen-doped carbon materials were assessed by N 2sorption and are summarized in Table S1(Supporting Information).Gas sorption isotherms (Figure 1B)exhibit type IV curves,typical for mesoporous materials.13,17Meso-EmG exhibits the highest surface area (1553m 2g -1),with a total pore volume of 2.58cm 3g -1.These values rank these materials among the high surface carbons.23The mesopore radius distribution is centered at 7.4nm according to the Barrett -Joyer -Halenda (BJH)model (Figure 1C),in agreement with the size of the templating silica nanoparticles.

X-ray diffraction (XRD)patterns of the resulting mesoporous nitrogen-doped carbon materials (Figure S1,Supporting Informa-tion)show two broad XRD diffraction peaks for all samples,usually attributed to the inter-plane (002)(2θ)24.7°)and the inner-plane (110)(2θ)43.7°)re?ections of graphitic carbon.13,17After introduction of nucleobase additives,the dominating (002)XRD peak shifts to slightly higher angles (θ)25.1°),with a corre-sponding interplanar d -spacing (d 002)of 3.54?(as compared to d 002)3.60?of meso-Em).There are a number of reasons for such a compression,but at least it proves the introduction of the nucleobase additives into the graphitic body.The ?nal composition was analyzed by elemental analysis (Table S1).In agreement with expectations,the nitrogen content increases strongly from 8.8wt %(meso-Em)to around 12.0wt %with the various nucleobases.This value is,to our knowledge,the highest reported value for a one-step syntheses of N-doped carbon,although 13.6wt %could be obtained by postfunctionalization.24The molar composition of meso-Em is C 1N 0.12O 0.16H 0.18,while a typical composition for samples obtained with nucleobase additives is obtained for meso-EmG with C 1N 0.15O 0.13H 0.18.Nucleobases therefore act as ef?cient N-donors.

The electrocatalytic properties of the mesoporous nitrogen-doped carbon materials were evaluated for ORR using a three-electrode chemical station.1,13Figure 2depicts cyclic voltammograms (CV)

for O 2reduction on meso-EmG (A),as compared to a commercial Pt/C catalyst (B)as a reference (20wt %platinum on carbon black).The electrolyte was a 0.1M N 2-or O 2-saturated KOH solution.Meso-EmG (Figure 2A)does not show any signi?cant peak in the N 2-saturated solution but does show a quasi-rectangular voltam-mogram,typical of high-surface-area carbons and supercapacitor performance.25In the presence of oxygen,a well-de?ned cathodic peak appears at -0.192V (vs Ag/AgCl).This value is positively shifted by 68mV in comparison with previous reports.13This already indicates the excellent catalytic ability of our samples for ORR.Since typical commercial Pt/C catalysts are known to be prone to methanol poisoning,we speci?cally assessed the effect of methanol on ORR for both meso-EmG and commercial Pt/C.The CV scans for the commercial catalyst are shown Figure 2B.In an electrolyte saturated with O 2and containing methanol (3M),the cathodic peaks for oxygen reduction disappear,while one pair of peaks is observed at -0.174and -0.110V,which is characteristic for methanol reduction -oxidation.13When investigating the be-havior of meso-EmG in the same O 2-saturated methanol solution (Figure 2A),no activity speci?c to methanol is observed,while the characteristic peaks of ORR are maintained.These results indicate that the as-prepared mesoporous carbons can reduce oxygen but are tolerant to methanol.1,13This methanol inertness goes well with chemical expectations for the catalysis of such

systems.

Figure 1.TEM images of meso-EmG (A).N 2sorption isotherms (B)and pore size distribution (C)from BJH method of as-prepared high-surface-area nitrogen-doped carbon

materials.

Figure 2.Cyclic voltammograms of meso-EmG (A)and commercial 20

wt %Pt/C catalyst (B)on a glassy carbon electrode in N 2-saturated 0.1M KOH,O 2-saturated 0.1M KOH,and O 2-saturated 0.1M KOH and 3M CH 3OH solutions.

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For a potential in?uence of microstructure,the different nucleo-base materials were studied by linear voltamperometry on a rotating disk working electrode and compared to the 20wt %Pt/carbon sample.1,13Figure 3A shows the polarization curves of the different

materials in a 0.1M O 2-saturated KOH solution.The onset potentials of oxygen reduction on the carbon materials are ap-proximately 35mV (Ag/AgCl),thus corresponding to an overpo-tential ca.25mV higher than the commercial Pt catalyst.Interest-ingly,this is rather independent of the speci?c nucleobase which was used and also quite similar for the pure ionic liquid-based material;i.e.,the catalytically active functional site is simpler than the nucleobases as such.However,the plateau current is higher with carbons obtained from nucleobases than with the meso-Em sample,indicating a higher density of active surface sites.

Typical current -potential curves of N-doped carbons in an O 2-saturated 0.1M KOH electrolyte are shown for Meso-EmG (Figure 3B)over a range of electrode rotation rates.The current shows the typical increase with rotation rate due to the shortened diffusion layer.1,13Analysis of the plateau currents through Koutecky -Levich plots (Figure 3C)reveals the reduction of O 2with meso-EmU catalysts by a four-electron process (n )4.1),26as is the case for the Pt-based catalyst,thus yielding water as the main product.For the other nucleobase-based materials,as depicted for meso-EmG,we ?nd a slightly slower slope (n )3.2),which might indicate mixed processes.Detailed experiments are,however,underway to assess this speci?c point.

We also investigated the electrocatalytic properties of the mesoporous nitrogen-doped carbon materials for oxygen reduction in acid electrolyte.Figure S2(Supporting Information)shows the polarization curves for the ORR in O 2-saturated 0.1HClO 4on meso-EmG as an example and for commercial Pt/C catalysts.Meso-EmG exhibits high catalytic activity toward ORR,with an onset potential at 0.573V (vs Ag/AgCl)and a 197mV overpotential as compared with Pt/C.This higher overpotential in acidic medium supports the previous view that ORR activities of nitrogen-doped carbon

materials are higher in alkaline medium than in acid medium,while the ORR activity of low-index Pt(hkl )single-crystal electrodes in alkaline medium is lower than the value obtained in acidic medium.27,28

A methanol crossover test on meso-EmG was also performed in the chronoamperometric measurements.Figure 3D shows the corresponding response at a constant voltage of -0.26V for 3000s in an 0.1M KOH solution ?rst saturated with N 2(0-1000s),then gased with O 2(1000-2000s),and then after addition of methanol to yield an O 2-saturated 3M CH 3OH for the 2000-3000s period.The meso-EmG sample exhibits a strong and stable amperometric response after introduction of O 2(1000-2000s)into the solution,which does not suffer after introduction of methanol,suggesting a remarkably good tolerance to fuel crossover effects for nitrogen-doped carbon materials.For Pt/C,the ORR cathodic current vanishes,and an anodic current of methanol oxidation appears.To summarize,we were able to ?nd high ORR activity for micro-and mesoscopically structured high-surface-area nitrogen-doped carbons,with cathodic currents in the alkaline very similar to those of commercial Pt/C catalysts.In addition,the material based on a ?uid,nonvolatile mixture of an ionic liquid with nucleobases is rather sustainable and cheap and can be shaped,printed,or structured before calcination,as illustrated by successful hard templating here.As a carbon-based catalyst,it also avoids typical fuel spillover sensitivities of ordinary noble metals in alkaline medium,making the engineering and construction of fuel cells more simple.This suggests that such metal-free carbon catalysts could,in principle,indeed overcome the limitations of Pt-based systems and provide suitable,sustainable,and cheap solutions for the further technological development of fuel cells and oxygen electrodes in general.Work to improve oxygen reduction in acidic media is still ongoing,and we believe that adding simple metals (as Co and Fe)to the formulation might offer further potentialities.

Acknowledgment.We thank the Max Planck Society and the ERC (Senior Excellence Grant)for supporting this project.Dr.David Portehault and Prof.Xinchen Wang are thanked for coopera-tion and discussion.

Supporting Information Available:Experimental details and more characterization and results.This material is available free of charge via the Internet at https://www.wendangku.net/doc/d59287988.html,.References

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