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Biosourced Nitrogen-Doped Microcellular Carbon Monoliths

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Biosourced Nitrogen-Doped Microcellular Carbon Monoliths
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  DOI: 10.1002/cssc.201301165 Biosourced Nitrogen-Doped Microcellular CarbonMonoliths Nicolas Brun,* [a] Petre Osiceanu, [b] and Magdalena M. Titirici [c] An srcinal approach based on the hydrothermal carbonizationof nitrogen-containing biomass derivatives within the continu-ous phase of a direct concentrated emulsion is reported forthe synthesis of nitrogen-doped microcellular carbon mono-liths. These biosourced foams show promising performancesas intrinsic electrocatalysts in the oxygen reduction reaction.Preliminary catalytic properties of powdered versus monolithicsamples are discussed and suggest interesting prospects fortheir introduction within electrochemical devices.The design of functional porous architectures depicting hier-archical and interconnected pore networks has emerged asa challenging field of research during the last decade. [1] Combining the structural advantages of macropores (i.e., porediameter Ø > 50 nm) and micro-/mesopores (i.e., Ø < 2 nm and2 < Ø < 50 nm respectively) leads to unique materials bearingenhanced properties. [2] The presence of such a hierarchical po-rosity within carbon architectures could offer many advantag-es: potential applications include electrode materials [3] (i.e.,electrocatalysts or electrocatalyst supports in energy storageand conversion devices), sorbents, [4,5] or even catalysts or cata-lyst supports for various important chemical reactions. Theopen macroporous structure can improve the mass-transportrate and the accessibility of the electrolyte and reagentsthrough the whole system. [6] The presence of micro- and meso-pores generates a high surface area, offering a large density of readily available active sites for adsorption and (electro)catalyt-ic reactivity. [7] In addition, continuous monolithic structures canprovide higher conductivity (avoiding the resistance associatedwith particles’ contact) and better mechanical integrity as com-pared with powders. From an engineering point of view, mon-olithic carbon-based architectures, with easy handling andshaping capabilities, clearly show a great potential for energydevice fabrication. However, the fabrication of such materialsremains most of the time laborious, unsustainable, and/or ex-pensive, often involving harsh conditions, [8] premade hard tem-plates [6,9] or supercritical post drying. [10–12] Although extensivelystudied for a wide range of applications, heteroatom-dopedcarbon architectures [5,12–17] have been rarely reported as hier-archical porous monoliths. [5,11,12,17] Particularly, a great deal of research effort has been recently devoted to the use of nitro-gen-doped (N-doped) carbons as platinum-free [16] or evenmetal-free electrocatalysts [12–14] for the oxygen reductionreaction.In this context, we recently developed emulsion-templatedcarbonaceous foams obtained through the hydrothermal car-bonization of biomass derivatives, that is, furfural. [18] This studyshowed one of the rare examples of biosourced polyHIPEs(HIPE for high internal phase emulsion) [19] leaving the beatenpath of the traditional resorcinol-formaldehyde and divinylben-zene-based systems. These monolithic materials were madeunder mild conditions and showed promising structural andmechanical properties, together with high electrical conductivi-ty up to 300 Sm  1 . Taking this “carboHIPE approach” one stepfurther, we propose herein the use of a N-containing biomassderivative precursor,  N  -acetylglucosamine, together with a de-hydration product of hexoses, 5-hydroxymethyl-2-furalde-hyde, [20] and the monomeric unit of phlorotannins, that is,phloroglucinol, for the design of functional N-doped carbon-based monoliths. After emulsification, hydrothermal carboniza-tion, soxhlet extraction, and further thermal treatment at950 8 C under inert atmosphere (see the Experimental Sectionfor details), monolithic N-doped carbon-based foams were ob-tained (Scheme 1).As shown in Figure 1a, these functional foams depict a typi-cal macrostructure of aggregated hollow spheres, named  cells ,resulting from the removal of the oil droplets initially dispersedwithin the high internal phase emulsion. For this reason, emul-sion-templated architectures are often referred to as cellularmaterials. In addition to these 10–20  m  m diameter cells, nar-rower connecting void spaces can be also observed, emphasiz-ing a high connectivity within the porous monolithic network.As shown in Figure 1b, the monolithic aspect of these carbon-based foams was conserved after pyrolysis.Interestingly, the N-doped carboHIPE samples further ther-mally treated at 950 8 C show a strong magnetic behavior andcan be attracted to and can stand on a magnet (Figure 1b).This is due to the use of FeCl 3 · 6H 2 O Lewis acid as a catalystduring the polymerization. Although the materials were soxh-let extracted with ethanol, the incorporated iron species could [a]  Dr. N. Brun + Department of Colloid Chemistry Max-Planck Institute of Colloids and Interfaces Am Mhlenberg 1, 14476 Golm/Potsdam (Germany) [b]  Dr. P. OsiceanuInstitute of Physical Chemistry Ilie Murgulescu, 060021 Bucharest (Romania) [c]  Dr. M. M. Titirici School of Materials Science and EngineeringQueen Mary University of LondonMile End Road, E1 4NS, London (United Kingdom) [   ]  Current address:Department of Chemistry Graduate School of Science, Kyoto University Kitashirakawa Sakyo-ku, 606-8502 Kyoto (Japan)E-mail: phd.nicobrun@googlemail.com Supporting Information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cssc.201301165.  2014 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  ChemSusChem  2014 , 7, 397–401  397 CHEM SUS CHEM COMMUNICATIONS  not be removed. This is also confirmed by XPS data (see theSupporting Information, Figure S2). Following the further ther-mal treatment under an inert atmosphere, the formation of iron-based nanoparticles was observed (Figure 1c). This featurewas also supported by the X-ray diffraction pattern of the N-doped carboHIPE (see Figure S3), showing an intense peak at2 q = 44.7 8  ascribed to the 110 reflection of   a -Fe. [21] The diffu-sion of such particles within the carbonaceous framework cata-lyzed the graphitization of amorphous carbons at rather lowtemperatures, leading to the formation of 30–50 nm diameterhollow graphitic rings (Figure 1c, see Figure S3 for XRD).As mentioned in our previous work, [18] this feature could ex-plain the enhanced electrical conductivity of up to 300 Sm  1 ,which is essential for integration as electrode within energydevices. As a reference, iron-free N-doped carboHIPEs werealso synthesized using  p -toluenesulfonic acid (TsOH) instead of FeCl 3 · 6H 2 O as a catalyst. XRD patterns support the influence of iron species on the partial graphitization of carbons (see Fig-ure S3). The iron-free foam, referred as to 950-AG_carboHIPE_TsOH, depicts two broad diffraction peaks centered at 2 q = 23 8 and 43.7 8 , typical of amorphous carbons. The iron-containingone, referred as to 950-AG_carboHIPE Fe, displays these twosimilar broad signals, together with a narrow peak growing at2 q = 26.5 8  (see Figure S3), associated with the graphite-like(002) line. Nevertheless, the iron-induced catalytic graphitiza-tion remains quite restricted as compared with previously re-ported N-free/iron-containing carboHIPEs, [18] limiting the elec-trical conductivity of 950-AG_carboHIPE_Fe foams to 10–20 Sm  1 . This lowered graphiti-zation degree could be ascribedto the use of   N  -acetylglucosa-mine and 5-hydroxymethyl-2-fur-aldehyde instead of furfural, pro-viding a larger heteroatom load-ing, which perturbs  in fine  thecrystallization of the carbonframework.Beyond a fully interconnectedmacrostructure, N-doped carbo-HIPE samples display significantBET surface area and porevolume as high as 568 m 2 g  1 (520 m 2 g  1 related to micro-pores) and 0.26 cm 3 g  1 (0.21 cm 3 g  1 related to micro-pores). As shown in Figure 1d,the nitrogen sorption curve de-picts a type I isotherm, typical of mainly microporous adsorb-ents. [22] The pore-size distribution(Figure 1e) underlines a mainmicropore contribution centeredat 0.8 nm, together with weak mesopore contributions be-tween 2 and 4 nm. A structuralcharacterization at the molecularlevel was carried out by FTIR spectroscopy (Figure S4 and S5),revealing the preservation of few carbon–oxygen and carbon–nitrogen vibration modes after further thermal treatment at950 8 C. This feature is supported by X-ray photoelectron spec-troscopy (XPS, Figure 2; Tables S2–4) and elemental analysis(EA; Table S2). As shown by EA, the N/C mass ratio of AG-car-boHIPE_Fe dropped from 3.7 to 2.4 wt% after pyrolysis. Thesevalues remain close to those determined from XPS, that is, 3.7and 2.9 wt%, before and after pyrolysis, respectively. This fea-ture supports a homogeneous introduction of nitrogen withinthe carbonaceous scaffold, from the surface to the wholevolume. Although EA provides data on the composition of thebulk of the samples, XPS gives the relative chemical composi-tion of the surfaces of the foams (depth 1–10 nm). Further in-formation was also obtained concerning the nature of thechemical bonds of nitrogen within the two N-doped carbonfoams (Figure 2b and the Supporting Information). The aminegroups (N 2  species; Figure 2a) are dominating just after the hy-drothermal carbonization. These were converted mainly toquaternary nitrogen (N 4 ; 22.3%), pyrrolic (N 3 ; 48.4%) and pyri-dinic species (N 1 ; 14.4%) after pyrolysis at 950 8 C (Figure 2band Table S4). A similar trend was also observed for HTC-AG_carboHIPE_TsOH (see Figure S1 and Table S4 in the SupportingInformation).As described in the introduction, the range of applicationsfor these monolithic hierarchically porous carbon materials isvery broad. We have selected herein their use as platinum-free Scheme 1.  Schematic representation of the N-doped carboHIPE synthetic pathway: (a) emulsification (oil-in-wateremulsion); (b) hydrothermal treatment; and (c) soxhlet extraction, drying and further thermal treatment at 950 8 Cunder inert atmosphere.  2014 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  ChemSusChem  2014 , 7, 397–401  398 CHEM SUS CHEM COMMUNICATIONS  www.chemsuschem.org  catalysts in the oxygen reduction reaction (ORR), the essentialreaction involved at the cathode of fuel cells. Recently, bio-mass-derived HTC monolithic aerogels have been proposed asmetal-free catalysts in the ORR, [12,14] appearing as sustainableand promising alternatives to conventional carbon-based cata-lysts. Nevertheless, their use has been limited to powderedmaterials mixed with Nafion, drastically lowering the interest indesigning monolithic porous architectures. None of these stud-ies proposed to compare powdered with monolithic samplesbecause of relatively narrow macropore diameters (i.e., lessthan 500 nm), drastically affecting the diffusion kinetics withinthe whole tridimensional porous network. Taking advantage of the microcellular structure of carboHIPE, we have proposedherein to discuss the potential of these monolithic N-dopedfoams as intrinsic electrocatalysts in the ORR.To compare with carbon-based electrocatalysts described inliterature, preliminary experiments were performed on milledsamples. As shown in Figure 3a, catalytic currents could be ob- Figure 1.  N-doped carbon foam (950-AG carboHIPE Fe): (a) Scanning elec-tron microscopy; (b) picture of a N-doped foam standing on a magnet;(c) transmission electron microscopy; (d) nitrogen sorption isotherm; and(e) pore-size distribution (DFT method, QSDFTequilibrium model). Figure 2.  N1s X-ray photoelectron spectroscopy band-like spectra (black solid lines) and deconvoluted curves (colored solid lines) obtained for(a) HTC-AG_carboHIPE_Fe and (b) 950-AG_carboHIPE_Fe. N 1 = pyridinic;N 2 = amine; N 3 = pyrrolic; N 4 = quaternary; and N 5 = pyridinic- N  -oxide. Figure 3.  Polarization curves measured in 0.1 m KOH solutions. Scanrate = 10 mVs  1 ; rotation rate = 1600 RPM. (a) Current density expressed inmA per mg of active sample. (b) Picture of a 950-AG_carboHIPE_Fe monolithpasted with carbon paint on a rotating electrode. (c) Current density ex-pressed in mA per cm 2 (geometrical area of the 5 mm diameter disc elec-trode, that is, 0.196 cm  2 ). The scatterplot ( * ) corresponds to the polariza-tion curve of the wet monolithic electrode measured within few secondsafter its introduction in the O 2 -saturated electrolyte.  2014 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  ChemSusChem  2014 , 7, 397–401  399 CHEM SUS CHEM COMMUNICATIONS  www.chemsuschem.org  served for both materials, 950-AG_carboHIPE_TsOH and _Fe,powdered and mixed with Nafion. Their intrinsic electrocatalyt-ic properties (i.e., overpotential and maximal current densityreached at low potentials) are competitive with sustainable N-doped carbon-based aerogels recently reported. [12,14] Consider-ing that no supercritical/freeze-drying post-treatments, whichare rather expensive and energy consuming methods, werenecessary to preserve their mechanical integrity, macromor-phology and porosity, one may see N-doped carboHIPEs aspromising “green” alternatives to the conventional Pt-free cata-lysts. The sample 950-AG_carboHIPE_Fe depicts better catalyticperformance as compared with 950-AG_carboHIPE_TsOH (Fig-ure 3a). This is related to higher porosity, graphitizationdegree, and loading of catalytically active nitrogen species (i.e.,N 3  and N 1 , see the Supporting Information for details) obtainedfor the iron-containing N-doped foam. Due to the graphiticcarbon rings coating the iron nanoparticles and keeping themaway from the readily accessible surface (Figure S2b), we donot believe that there is a direct and significant involvement of Fe species in the ORR. Another important aspect concerns thestrongest tendency of 950-AG_carboHIPE_Fe sample to pro-mote a 4-electron process versus a 2-electron one (Figure S8).This characteristic seems to be directly related to the pyrrolic/pyridinic nitrogen ratio (N 3 /N 1 ), as previously reported, [12] whichis about five times higher for the iron-containing N-dopedfoam (Table S4).As our materials are monolithic, we have looked into theirdirect use as electrodes for ORR without any additional prepa-ration (i.e., no crushing into powder and no mixing withNafion). Thus, one single piece of free standing 950-AG_carbo-HIPE_Fe foam was simply pasted with carbon paint on a rotat-ing electrode (Figure 3b). Comparing the polarization curves ata rotating disc electrode of both the powdered and monolithicN-doped foams (easily shaped in fine discs; Figure 3b) high-lighted some limitations most likely associated with the experi-mental setup. A comparison of specific currents per mass of active samples (Figure 3a) shows about tenfold larger maximalcatalytic current for the powdered foam than for the monolith.In contrast, a comparison of specific currents per geometricalarea (Figure 3c) shows an approximately twofold higher maxi-mal catalytic current for the electrode used as an intact mono-lith. It can be assumed that the O 2 -saturated electrolyte diffu-sion was not effective within the whole microcellular electrode,but was not limited to the readily accessible geometrical sur-face. Interestingly, the polarization curve of the wet monolithicelectrode measured within few seconds after its introductionin the O 2 -saturated electrolyte (Figure 3c; scatterplot/emptycircles) was compared with the one recorded at steady state,that is, after few minutes rotating before measurement (Fig-ure 3c; solid line/empty circles). Although at steady state, thecurrent reached a limiting catalytic current at high drivingforce (i.e., at low potential), at unsteady state, a peak shapecurve with a maximum catalytic current reached at   0.4 V vs.Ag/AgCl was observed. This former feature suggests a diffu-sion-limited behavior due to the depletion of O 2  near the sur-face of the pores within the bulk of the monolithic electrode.This phenomenon can be described as follow. Firstly, the O 2 molecules initially adsorbed onto the wet pores were rapidlyconsumed spreading a depletion layer towards the electrolyteand highlighting the high electrocatalytic activity of our mono-lith. Secondly, O 2  species diffused from the solution to the bulk of the monolithic electrode, finally reaching a steady state.Nevertheless, the convective movement of the electrolyte in-duced by the rotating system does not seem efficient enoughto provide sufficient amount of O 2  species for the microcellularelectrode. To fully expose the potential of our N-doped foamas monolithic electrode, further optimizations of the experi-mental setup, leading to a more efficient supply with electro-active species, will be necessary. [23] However, its first catalyticproperties are promising and its introduction within redoxflow battery devices as electrocatalyst or electrocatalysts sup-port could be envisaged. [23] In this way, the nitrogenatedgroups could be used as nucleation sites for the subsequentanchoring of metallic nanoparticles [24] or as coordination sitesfor the binding of transition metal complexes. [25] To the best of our knowledge, the materials presentedherein are the first microcellular N-doped carbons reported inthe open literature. The srcinal synthetic pathway involvingthe hydrothermal carbonization of biomass derivatives avoidedusing traditional resorcinol–formaldehyde precursors. Overall,these foams comply with all the requirements in terms of sus-tainability, functionally, easy design and shaping necessary formany environmentally friendly applications. In this way, we be-lieve that this study represents a significant step forward pro-moting the emergence in the near future of biosourced mono-lithic carbon-based foams for catalysis, electrocatalysis, and inenergy storage and conversion devices. Experimental Section In a typical synthesis, nonionic surfactant Tween 80 [polyoxyethy-lene (20) sorbitan monooleate (1 mL)] was added to a hydroalcohol-ic mixture of water (2.4 mL) and absolute ethanol (2.4 mL). Phloro-glucinol (0.286 g, 1 molar eq.) 5-hydroxymethyl-2-furaldehyde(0.286 g, 1 molar eq.) and  N  -acetylglucosamine (0.504 g, 1 molareq.) were mixed into the solution. After vigorous stirring overa few minutes, iron(III) chloride hexahydrate (0.123 g, 0.2 molar eq.)was added. The solution quickly turned black. The prepolymerizedhydroalcoholic solution was directly transferred into a 60 mL glassmortar, and dodecane (23.2 mL, corresponding to an oil volumefraction of 0.8) was added drop-by-drop while emulsification wasperformed manually with a glass pestle. The as-synthesized viscousemulsion was then added to a glass inlet (30 mL volume), sealed ina Teflon-lined autoclave (45 mL volume), placed in a laboratoryoven preheated to 180 8 C and left for 24 h. The hydrothermalcarbon-based monolith was then removed from the autoclave,washed by using soxhlet extraction with ethanol over 24 h, anddried at 80 8 C in an oven overnight. For the synthesis of HTC-AG_carboHIPE_TsOH, 188 mg of   p -toluenesulfonic acid monohydratepreviously dissolved in 500  m  L water/ethanol/Tween 80 mixturewas added to the premade emulsion while stirring vigorously.To prepare conductive N-doped carbon, the dried monolith wasplaced in a ceramic crucible within a carbonization oven andheated up to 950 8 C for 2 h under an inert atmosphere (i.e., N 2 ,flow: 10 mLmin  1 ). An initial ramp was used to reach 350 8 C ata speed of 5 8 C per minute, before a second ramp was applied to  2014 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  ChemSusChem  2014 , 7, 397–401  400 CHEM SUS CHEM COMMUNICATIONS  www.chemsuschem.org  reach 950 8 C at 2 8 C per minute. The N-doped carbonaceous foamwas then allowed to cool to ambient conditions and was removedfrom the oven prior to further analysis. 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Commun.  2013 ,  49 , 240–242.Received: October 30, 2013Published online on January 21, 2014  2014 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  ChemSusChem  2014 , 7, 397–401  401 CHEM SUS CHEM COMMUNICATIONS  www.chemsuschem.org
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