Hydrogen Storage Properties of N-Doped Microporous Carbon

Lifeng Wang and Ralph T. Yang*Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109-2136

ReceiVed: August 24, 2009; ReVised Manuscript ReceiVed: October 28, 2009

A N-doped microporous carbon was synthesized by using NaY as a hard template and acetonitrile as thecarbon and nitrogen precursor. The hydrogen storage measurements indicated that the N-doped microporouscarbon had an 18% higher storage capacity than the pure carbon with a similar surface area. Furthermore,hydrogen storage via spillover was studied on a sample comprising Pt supported on N-doped microporouscarbon, and a storage capacity of 1.26 wt % at 298 K and 10 MPa was obtained, showing an enhancementfactor of 2.4 by spillover. In addition, the Pt/N-doped microporous carbon exhibited 1.46 times the storagecapacity of Pt/microporous carbon. Significantly higher heats of adsorption were obtained on the N-dopedmicroporous carbon samples than that on undoped carbons for both H2 adsorption and adsorption by spillover.The experimental results were consistent with the theoretical calculations from the literature.

1. Introduction

With concerns with a potential energy crisis from the use offossil fuels and increasing demands for environmental protection,hydrogen has been proposed as one of the best alternative energysources for vehicles powered by fuel cells. Hydrogen storageplays a key role in the utilization of hydrogen as the energycarrier.1 Nanostructured and porous carbon materials, includingcarbon nanotubes (CNTs), graphite nanofibers, activated carbon,templated carbon, and graphene, are thought to be the promisingcandidates for hydrogen storage due to their high surface areas,light weight, and relative chemical stabilities.2-7 However, recentstudies showed that these carbon materials cannot store asufficient amount of H2 required for transportation applicationsmerely by physical adsorption at ambient temperature.8,9

A promising approach for solving this problem has been shownby which hydrogen storage in an adsorbent could be enhancedsignificantly by hydrogen spillover at room temperature.10-16

Hydrogen spillover is defined as the dissociative chemisorptionof hydrogen on metal nanoparticles and subsequent migrationof hydrogen atoms onto adjacent surfaces of a receptor viaspillover or surface diffusion.17-26 Enhancements in hydrogenstorage capacities on carbon materials by doping transitionmetals have been recently studied.27-33 In the system ofhydrogen storage in carbon via spillover, the hydrogen dis-sociation sources, the contact between the dissociation sourceand carbon receptor, and the nature of the carbon receptor areconsidered as main factors affecting the storage capacity.Different metals as hydrogen dissociation sources for hydrogenstorage have been intensively studied.27-33 Bridge-building andplasma-assisted doping techniques have been applied to improvethe contacts between the source and receptor and, hence,enhance the hydrogen storage capacity.12-16,34 Recently, devel-oping receptors has received much research interest becauseenhanced storage capacities could be achieved with high surfacearea receptors or chemically modified receptors. For examples,templated carbons and MOFs with high surface areas haveshown promising results.7,35 This can be understood because areceptor with a higher surface area would provide more

hydrogen adsorption sites than one with lower surface area. Asfor the chemically modified receptors, it was reported thathydrogen storage capacity could be enhanced on boron-dopedcarbon and nitrogen-doped carbon.36-45 Hydrogen uptakes onboron-doped microporous carbon and carbon nanotubes havebeen studied, respectively, by Chung et al.38 and Viswanathanet al.39 Our recent results showed an enhanced storage capacityin a boron- and nitrogen-codoped carbon.40 Badzian et al.synthesized a carbon with 1% nitrogen contents and observeda 0.7-0.8 wt % uptake in the doped carbon.41 Lee et al. obtaineda storage capacity of 0.28 wt % at 308 K by optimizing thenitrogen contents in carbon xerogel.42 Mokaya and co-workersinvestigated the hydrogen adsorption in nitrogen-doped carbonat 77 K.43 More recently, Zhu et al. theoretically investigatedthe interaction between hydrogen atoms and nitrogen-dopedcarbon materials and found that the doped nitrogen atomsincreased the adsorption energy of hydrogen atoms at theneighboring C-atom sites.44 An ab initio study of hydrogeninteraction with nitrogen-doped carbon nanotubes reported byZhang and Cho showed that doping the CNTs with nitrogenreduced the energy barrier for hydrogen dissociation.45 Fromthese theoretical calculations, one may expect, therefore, that anitrogen-doped carbon receptor exhibiting stronger interactionwith hydrogen or facilitating hydrogen dissociation would befavorable for hydrogen adsorption. However, hydrogen storagevia spillover has not been studied on N-doped carbon, and anunderstanding of the effect of doped nitrogen atoms on hydrogenstorage is needed. In this work, we prepared a N-dopedmicroporous carbon and the same sample that was doped withPt nanoparticles, and counterparts without N-doping, andinvestigated their hydrogen adsorption properties and the effectsof nitrogen on hydrogen storage.

2. Experimental Methods

2.1. Synthesis. N-Doped Microporous Carbon. N-dopedmicroporous carbon derived from zeolite NaY was preparedaccording to a procedure similar to that proposed by Mokaya.43

Typically, 2 g of NaY was degassed in a flask for 12 h at 473K, then placed in a vertical quartz tube and heated to 1023 Kunder a N2 flow. When the temperature reached 1023 K, theN2 flow was switched to acetonitrile (saturated in a N2 flow

* To whom correspondence should be addressed. Fax: (734) 764-7453.E-mail: yang@umich.edu.

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10.1021/jp908156v 2009 American Chemical SocietyPublished on Web 11/24/2009

rate of 150 cm3/min) to pass through the NaY for 4 h. After theCVD treatment, the composite was further heated at 1173 Kfor 2 h under a flow of N2. The obtained NaY/carbon compositewas treated in HF solution (40%) for 24 h and subsequentlyrefluxed by a concentrated HCl solution for 4 h to dissolve theNaY template. The resulting microporous carbon was collectedby filtration and washing with distilled water.

6 wt % Pt Supported on N-Doped Microporous Carbon.Typically, 200 mg of well-dried N-doped microporous carbonwas dispersed in 20 mL of acetone and was stirred for 0.5 h ina flask at room temperature. Five milliliters of acetone wasmixed with 26 mg of H2PtCl6, which was slowly added to theabove solution under vigorous stirring. The mixture wassubjected to ultrasonication (100 W, 42 kHz) for 1 h and thenmagnetically stirred at room temperature for 24 h. After beingdried in an oven at 333 K overnight, the impregnated samplewas transferred to a horizontal quartz tube and further dried ina He flow at 393 K for 2 h to remove the residual acetone inthe sample. The He flow was then switched to H2, and thetemperature was increased to 573 K and held for 3 h. Aftercooling to room temperature in H2, the sample was purged withflowing He and was stored under He atmosphere before furthermeasurement.

Plain Microporous Carbon. Microporous carbon derivedfrom zeolite NaY was prepared according to a procedure similarto that reported by Kyotani et al.46 NaY was degassed in a flaskfor 12 h at 473 K, then placed in a vertical quartz tube andheated to 1073 K under a N2 flow. When the temperaturereached 1073 K, propylene gas (2% in N2 by volume, flow rate) 150 cm3/min) was passed through the tube for 6 h. After theCVD treatment, the obtained NaY/carbon composite was treatedin HF solution (40%) for 24 h and subsequently refluxed byconcentrated HCl solution for 4 h to dissolve the NaY template.The resulting microporous carbon was collected by filtrationand washing with distilled water.

6 wt % Pt Supported on Microporous Carbon. Pt/mi-croporous carbon was prepared using the same procedure forpreparing Pt/N-doped microporous carbon except that the plainmicroporous carbon was used as the support instead of N-dopedmicroporous carbon.

2.2. Characterization. Powder X-ray diffraction (XRD) datawere recorded on a Rigaku Miniflex diffractometer at 30 kV,15 mA for Cu KR (λ ) 0.1543 nm) radiation, with a step sizeof 0.02° in 2θ. X-ray photoelectron spectroscopy was recordedon a Kratos Axis ultra XPS spectrometer. Nitrogen adsorptionand low-pressure H2 adsorption isotherms (0-1 atm) weremeasured with a standard static volumetric technique (Mi-cromeritics ASAP 2020). Hydrogen adsorption at 298 K andpressures greater than 0.1 MPa and up to 10 MPa were measuredusing a static volumetric technique with a specially designedSieverts-type apparatus. The apparatus was previously testedand proven to be leak-free and accurate through calibration byusing LaNi5, AX-21, zeolites, and MOFs at 298 K.47 Ap-proximately 200 mg of sample was used for each high-pressureisotherm measurement in this study.

3. Results and Discussion

N-Doped Microporous Carbon. Powder X-ray diffractionpatterns of NaY zeolite and N-doped microporous carbon areshown in Figure 1. NaY zeolite exhibited typical peaks assignedto FAU structure (Figure 1a). The N-doped microporous carbonsynthesized by using NaY as a hard template (Figure 1b) showed

a peak at 2θ ) 6.3°, indicating that the microstructure of thezeolite template had been replicated in the N-doped microporouscarbon.

Nitrogen adsorption at 77 K was employed to characterizethe porosity in the N-doped microporous carbon. As shown inFigure 2, the isotherm of N-doped microporous carbon exhibiteda sharp rise in the low relative pressure (P/P0 < 0.1) and agradual rise in the high relative pressure, indicating the presenceof microporosity and some mesoporosity. The presence ofmesopores is due to the incomplete infiltration of carbonprecursor into the channel of the NaY, which led to themesopores after removal of the NaY. The BET surface areaand pore volume of N-doped microporous carbon were 1663

Figure 1. X-ray diffraction patterns of NaY (a) and N-dopedmicroporous carbon (b).

Figure 2. Nitrogen isotherm on N-doped microporous carbon.

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m2/g and 1.43 cm3/g, respectively. The surface area of theN-doped microporous carbon was higher than previouslyreported high surface area N-doped carbon xerogel (1602 m2/g),42 N-doped mesoporous carbon (1271 m2/g),48 and N-dopedcarbon nanotubes (886 m2/g),49 indicating that the use ofmicroporous NaY as a hard template is helpful for synthesizinghigh surface area N-doped microporous carbon.

A high-resolution TEM image of the N-doped microporouscarbon further showed the detailed microstructure. As shownin Figure 3, microporous channels (marked by arrows) couldbe observed at the edges of the N-doped microporous carbonsample. This indicates the successful structural transfer fromthe zeolite template, in agreement with the XRD and N2 isothermobservations.

The X-ray photoelectron survey spectrum (Figure 4) ofN-doped microporous carbon showed three sharp signals forC, N, and O elements. The elemental mass ratio of the N-dopedmicroporous carbon was approximately 89% C, 7% N, and 4%O. This indicated that using the acetonitrile as both a carbonprecursor and a nitrogen precursor was efficient for the synthesisof N-doped microporous carbon. Our results are in agreementwith previous studies that showed acetonitrile can be also usedfor synthesis of N-doped carbon nanotubes,49-51 carbonnanofiber,52and mesoporous carbon.53 It was confirmed that thenitrogen doped in this manner was in the forms of pyridine-like nitrogen and quaternary nitrogen incorporated into graphenesheets.49-53

The high-pressure hydrogen isotherm at 298 K for theN-doped microporous carbon is presented in Figure 5, curve a.

As shown in Figure 5, curve a, the N-doped microporous carbonhad a hydrogen storage capacity of 0.51 wt % at 298 K and 10MPa. It has been suggested that doping of nitrogen into a carbonwas favorable for hydrogen adsorption.37 For comparison, a puremicroporous carbon was synthesized by using NaY as a hardtemplate and propylene as the carbon precursor according tothe literature.45 The obtained pure microporous carbon had aBET surface area of 1533 m2/g (Supporting Information,Figure 1). As shown in Figure 5, curve b, the pure microporouscarbon had a storage capacity of 0.43 wt % at 298 K and 10MPa. Therefore, the hydrogen uptake on the N-doped mi-croporous carbon was 18% higher than that of the plain carbonwith a similar surface area under the same conditions. DFTcalculation results reported by Viswanathan et al. also showedthat substitution of nitrogen in the carbon nanotube frameworkwas favorable for hydrogen molecular adsorption.37

The heats of adsorption of H2 on the N-doped microporouscarbon were calculated from the H2 adsorption isotherms at 273and 298 K by using the Clausius-Clapeyron equation, as shownin Figure 6. The isosteric heats of adsorption were determinedby evaluating the slope of the plot of ln(P) versus (1/T) at thesame adsorption amount. It can be seen that the H2 adsorptionamounts at all pressures up to 1 atm decreased with an increasein temperature. The inset in Figure 6 shows that the absolutevalues of the heats of adsorption decreased with adsorptionamount for the N-doped microporous carbon. The heats ofadsorption were ∼12 kJ/mol at low surface coverage and leveled

Figure 3. TEM image of N-doped microporous carbon.

Figure 4. XPS spectrum of N-doped microporous carbon.

Figure 5. High-pressure hydrogen isotherms at 298 K for N-dopedmicroporous carbon (∆) and pure microporous carbon (]).

Figure 6. Low-pressure H2 adsorption isotherm for N-doped mi-croporous carbon at 273 K (0), 298 K(]), and 323 K (∆). Inset:calculated isosteric heats of adsorption.

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off to ∼7.3 kJ/mol at relatively high surface coverage. The highvalues of heats of adsorption at low surface coverage wereattributed to the adsorption of H2 on the more energetic siteson carbon. Defect sites and edge sites are strong sites foradsorption. The heats of adsorption (∼7.3 kJ/mol) at a high H2

adsorption amount on the N-doped microporous carbon werestill significantly higher than that on pure carbon (∼5 kJ/mol).54,55

The relatively high heats of adsorption on N-doped microporouscarbon were in qualitative agreement with theoretical predictionsfrom the literature.39,40

Hydrogen Storage Properties of Pt/N-Doped MicroporousCarbon. Recent studies showed that the hydrogen storagecapacities at 298 K in nanostructured and porous materials,including carbon, zeolites, and metal-organic frameworks,could be enhanced by exploiting the hydrogen spilloverphenomenon.10-16 In addition, theoretical calculations indicatedthat the doped nitrogen atoms could increase the adsorptionenergy of hydrogen atoms at the neighboring C atoms andfacilitate hydrogen dissociation.43,44 Thus, we synthesized Pt/N-doped microporous carbon and investigated its hydrogenstorage properties.

The powder X-ray diffraction pattern of Pt/N-doped mi-croporous carbon is shown in Figure 7. In the low-angle XRDpattern, the Pt/N-doped microporous carbon sample exhibiteda peak at 2θ ) 6.3°, similar to that of N-doped microporouscarbon, indicating that the ordered microstructure of N-dopedmicroporous carbon was kept after doping Pt metals. In addition,the wide-angle XRD pattern of Pt/N-doped microporous carbonexhibited two peaks at 39.8° (111) and 46.3° (200), characteristicof the metallic platinum (ICDD-JCPDS card no. 4-802). Thesize of the Pt particle calculated from the Scherrer equation wasapproximately 7 nm. These results confirmed that nanosized Ptmetals had been successfully doped on the carbon supports byapplying our doping method.

Nitrogen isotherms at 77 K for Pt/N-doped microporouscarbon are shown in Figure 8. Pt/N-doped microporous carbonexhibited a similar isotherm to the undoped microporous carbon,revealing the presence of microporosity and some mesoporosity.Pt/N-doped microporous carbon had a BET surface area of 1388m2/g and a pore volume of 1.17 cm3/g, which were lower thanthose of N-doped microporous carbon (1663 m2/g and 1.43 cm3/g). This was due to the increased weight and micropore blockingcaused by the Pt metal particles. It is encouraging that thesurface area decreased only slightly after doping of the Ptparticles, indicating that most of the porosity of the carbonsupport remained open.

The high-resolution TEM image of the Pt/N-doped mi-croporous carbon is shown in Figure 9. The black spots of Pt

(3-9 nm) were well-dispersed on the surface of the Pt/N-dopedmicroporous carbon. The Pt size observed in TEM was inagreement with the calculated size from XRD data. This resultfurther confirmed that Pt metals have been successfully dopedon the N-doped microporous carbon support.

High-pressure hydrogen isotherms at 298 K for the N-dopedmicroporous carbon and the Pt/N-doped microporous carbonsamples are compared in Figure 10. The N-doped microporouscarbon had a hydrogen storage capacity of 0.51 wt % at 298 Kand 10 MPa. When 6.0 wt % Pt metal was doped on the

Figure 7. X-ray diffraction pattern of Pt/N-doped microporous carbon.Figure 8. Nitrogen isotherms on N-doped microporous carbon (O)and Pt/N-doped microporous carbon (0).

Figure 9. TEM image of Pt/N-doped microporous carbon.

Figure 10. High-pressure hydrogen isotherms at 298 K for N-dopedmicroporous carbon (∆) and Pt/N-doped microporous carbon (]).

21886 J. Phys. Chem. C, Vol. 113, No. 52, 2009 Wang and Yang

N-doped microporous carbon, the hydrogen uptake at 10 MPawas increased to 1.26 wt %, that is, by a factor of 2.4. Theenhanced hydrogen storage capacity cannot be attributed todifferences in surface area because the Pt/N-doped microporouscarbon had a lower surface area than that of the N-dopedmicroporous carbon. The enhancement in hydrogen storage wasdue to the spillover of atomic hydrogen from the Pt particles tothe N-doped microporous carbon. Pt metals are known ashydrogen dissociation sources, and the enhanced hydrogenstorage by metal doped on various carbon materials (CNTs,active carbon, carbon nanofiber, etc.) has been reported by manyauthors. In the present case, compared with the N-dopedmicroporous carbon, it is remarkable that the hydrogen uptakeon Pt/N-doped microporous carbon has been enhanced by afactor of 2.4. In addition, a hydrogen adsorption isotherm onPt/microporous carbon synthesized by doping Pt on puremicroporous carbon was also measured. As shown in theSupporting Information, Figure 2, Pt/microporous carbon witha surface area of 1308 m2/g had a storage capacity of 0.86 wt% at 298 K and 10 MPa. Thus, Pt/N-doped microporous carbonshowed 46% higher adsorption than Pt/microporous carbon.These results indicated nitrogen doping enhanced significantlyhydrogen adsorption by spillover.

Use of the Clausius-Clapeyron equation would yield theoverall heats of adsorption. The overall heats of adsorption ofH2 on Pt/N-doped microporous carbon were calculated fromthe H2 isotherms at 273 and 298 K by using the Clausius-Clapeyron equation. As shown in Figure 11, the H2 adsorptionamount at all pressures up to 1 atm decreased with an increasein temperature. The inset in Figure 11 shows that the absolutevalues of heat of adsorption decreased sharply with theadsorption amount for each sample. The heats of adsorptionwere >20 kJ/mol at low surface coverage and leveled off to∼11.8 kJ/mol at relatively high surface coverage. It is worthnoting that the heats of adsorption on the Pt/N-doped mi-croporous carbon were higher than that of hydrogen physisorp-tion on N-doped microporous carbon (∼7.3 kJ/mol), reflectingthe strong interactions between the spilt-over H and carbonsupports. The high values of heats of adsorption at low surfacecoverages can be attributed to the strong adsorption of H atomson the metal particles, as well as the H atoms on the strongestsites on carbon. As a first-order analysis, we take the heat ofadsorption at high H2 adsorption amount as an indicator of theadsorption strength of hydrogen atoms on the receptor surface.The heats of adsorption of H2 on Pt/microporous carbon werealso calculated from the H2 isotherms at 273 and 298 K. As

shown in the Supporting Information, Figure 3 inset, the heatsof adsorption at high H2 adsorption amounts on Pt/microporouscarbon were about 10 kJ/mol. The higher heats of adsorptionon Pt/N-doped microporous carbon relative to Pt/microporouscarbon suggest that more H atoms were favorably bonded toPt/N-doped microporous carbon, in agreement with high-pressure hydrogen adsorption results. It has been suggested thatdoping of nitrogen into carbon is favorable for hydrogenadsorption.

4. Conclusions

In this study, the hydrogen storage properties of N-dopedmicroporous carbon and Pt/N-doped microporous carbon wereinvestigated. It was found that nitrogen doping is favorable forhydrogen adsorption. N-doped microporous carbon had a storagecapacity of 0.51 wt % at 298 K and 100 atm, which was 18%higher than that of the pure microporous carbon with similarsurface area. Furthermore, hydrogen storage in Pt/N-dopedmicroporous carbon via spillover was studied, which showed astorage capacity of 1.26 wt %, an enhancement factor of 2.4compared with the N-doped microporous carbon. It is remark-able that Pt/N-doped microporous carbon showed 1.46 timesadsorption compared with Pt/microporous carbon (without Ndoping). These results were interpreted by the results onincreased heats of adsorption by N-doping, in agreement withtheoretical predictions.

Acknowledgment. The authors acknowledge NSF Grant No.CBET-0753008 and the funding provided by the U.S. Depart-ment of Energy’s Office of Energy Efficiency and RenewableEnergy within the Hydrogen Sorption Center of Excellence(HSCoE).

Supporting Information Available: Nitrogen isotherm onpure microporous carbon and high- and low-pressure hydrogenisotherms for Pt/microporous carbon. This material is availablefree of charge via the Internet at http://pubs.acs.org.

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Figure 11. Low-pressure H2 adsorption isotherms for Pt/N-dopedmicroporous carbon at 273 K (∆), 298 K (O), and 323 K (0). Inset:calculated isosteric heats of adsorption.

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