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Electrochimica Acta 125 (2014) 606–614

Contents lists available at ScienceDirect

Electrochimica Acta

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omplex electrochemical investigation of ordered mesoporous carbonynthesized by soft-templating method: charge storage andlectrocatalytical or Pt-electrocatalyst supporting behavior

ilan Momcilovic a,∗, Marija Stojmenovic a, Nemanja Gavrilovb,gor Pasti b, Slavko Mentusb,c, Biljana Babic a

University of Belgrade, Institute of Nuclear Sciences “Vinca”, P. O. Box 522, 11000 Belgrade, SerbiaUniversity of Belgrade, Faculty of Physical Chemistry, Studentski trg 12-16, 11158 Belgrade, SerbiaSerbian Academy of Sciences and Arts, Knez Mihajlova 35, 11000 Belgrade, Serbia

r t i c l e i n f o

rticle history:eceived 6 December 2013eceived in revised form 16 January 2014ccepted 26 January 2014vailable online 7 February 2014

eywords:rdered mesoporous carbonlectrochemical capacitorxygen reduction reactionydrogen oxidation reaction

a b s t r a c t

Ordered mesoporous carbon (OMC) was synthesized by an evaporation induced self-assembly method,under acidic conditions, with resorcinol as the carbon precursor and Pluronic F127 triblock copolymer(EO106PO70EO106) as a structure directing agent. The obtained OMC product was characterized by N2

sorptometry, X-ray diffractometry and Raman spectroscopy. The mean pore radius of 2 nm and specificsurface area of 712 m2 g−1 were found. The OMC sample was subjected to a complex electrochemicaltesting in order to check for its applicability in various energy conversion processes. For pure OMC,the charge storage properties and kinetics of oxygen reduction reaction (ORR) in alkaline solution weremeasured. The OMC sample delivered specific capacitance of 232 F g−1 at 5 mV s−1 with 83.6% capacitanceretained at 100 mV s−1. Effective ORR electrocatalysis by OMC in alkaline media was evidenced, with onsetpotential amounting to −0.10 V vs. saturated calomel electrode. A part of the OMC sample was used as

uel cell catalysis. a support of Pt nanoparticles, and examined as electrocatalyst for hydrogen evolution reaction (HOR)and ORR in acidic media. Reversible HOR kinetics was observed, while ORR performances were foundto be competitive to the ones on other carbon-supported Pt electrocatalysts reported so far. A superbelectrochemical behavior was correlated to physico-chemical properties of OMC. Described OMC standsout as a highly versatile material, which can be used to replace carbon materials developed for specific

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. Introduction

Ordered mesoporous carbons (OMCs) belong to a family oforous materials which have drown great attention in recent years,epresenting a huge challenge for material scientists. Research inhis area is based on the tuning of the features of OMCs for specificpplications and on elucidation of the mechanisms of their syn-hesis and physico-chemical interactions with other materials andhemical species.

OMCs are characteristic of hydrophobic surfaces, high surfacerea, large pore volume, chemical inertness, good thermal andechanical stability, easy handling and low cost of manufacture

1]. In addition, structure variances, morphology and composition

esult in a range of potential applications in: catalysis as cata-ysts and catalyst supports [2,3], separation [4], adsorption [5],nergy storage [6], as molecular sieves [7] etc. More interestingly,

∗ Corresponding author.E-mail address: milanmomcilovic@yahoo.com (M. Momcilovic).

013-4686/$ – see front matter © 2014 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2014.01.152

tion of carbon-based technologies aimed for energy conversion purposes.© 2014 Elsevier Ltd. All rights reserved.

nanostructured OMC composites which involve wide spectrumof chemical elements, oxides or sulphides exert novel electronic,optical, magnetic, and other properties [8].

Several strategies have been proposed for the synthesis ofporous carbons including carbonization of polymer blends [9],organic gels [10], colloidal imprinting [11] and catalytic activation[12]. In general, two approaches have been established. Nanocast-ing was first developed by Ryoo and Hyoen’s research groupsindependently, as an approach to fabricate ordered mesoporouscarbons using a mesoporous silicate as a hard template [13,14].The idea was based on preparation of adequate silica template,filling of the carbon precursor into the channels of the silica, car-bonization of the precursor, and final etching of the template withthe aid of acids [15,16]. Since this process is now regarded asbeing intricate, expensive and time-consuming, direct (soft) tem-plating imposed itself as a promising approach to obtain carbon

materials of preferred pore structure. This method includes theself-assembly of resol resin and block copolymer surfactant intoperiodical mesoscopic structure under controlled conditions fol-lowed by precursor cross-linking, subsequent removal of polymer

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ramework, and final carbonization of carbonaceous matrix whichesults in desired mesoporous carbon structure [15]. It may beonducted by employing evaporation-induced self-assembly (EISA)ethod, dilute aqueous route, macroscopic phase separation and

ydrothermal autoclaving [15]. The correlation between structural,echanical, electrochemical and catalytic properties of OMC will

elp better understanding of its structure-activity relationship.Sustainable energy development incites great challenges and

pportunities for industrial advancement in the near future. Fuelells are a clean way of producing electric energy through redoxeactions using hydrogen, methanol, ethanol etc. as fuels. Theost active electrocatalysts for proton exchange membrane fuel

ells (PEMFC) and electrolyzers are based on the Pt-group metals,hich have been used to catalyze both anodic reaction (hydrogen

xidation reaction, HOR) and cathodic reaction (oxygen reduc-ion reaction, ORR). Nanoscaling of noble metal particles to morective sites per mass unit is achieved through the use of properatalytic support. Good support has high surface area, good disper-ion of catalyst loadings, suitable pore formation for smooth massransfer of the fuel, high electrical conductivity for good electronransfer through the electrode, and good electrochemical corrosionesistance [17]. Current status in the field presents carbon-basedatalyst as most widely used ones [17]. Moreover, nanosized car-onaceous materials also have purposes in other fields of energyonversion applications. These materials are intensively investi-ated as ORR catalysts with an attempt to replace noble metallectrocatalysts with cheaper ones. If not doped with some het-roatom (N, P, B), these materials poses high intrinsic activityowards ORR in alkaline media, being attractive in the field oflkaline fuel cells (AFC) and metal-air batteries. In the case ofetal-free carbonaceous catalysts appropriate pore structure is

ecessary for high ORR activity allowing access of O2 to large frac-ion surface area where it is reduced to HO2

− or OH−, dependingn the applied electrode potential and the state of materials surface18]. Also, carbonaceous materials have been traditionally used aslectrode materials for electrochemical capacitors, where its largeurface area is utilized to store significant amounts of energy atarbon/solution interface, allowing fast delivery of stored energyhen necessary. Nevertheless, in a traditional approach, specific

arbon materials are used for different purposes related to energyonversion applications. For example, Vulcan XC-72 is often useds an electrocatalyst support but it has gravimetric capacitance ofnly 27 F g−1 [19] so it is not attractive as electrode material forlectrochemical capacitors.

According to the overview provided above, OMC has desirableroperties to be applied either as a catalyst support, ORR catalyst inlkaline media or electrode material in electrochemical capacitors.n fact, applicability of different OMCs in some particular fields ofnergy conversion and storage has been demonstrated so far, as itill be referred later on.

From practical aspect, it is interesting to develop the materialsoining a wide range of possible applications in themselves, which

ould economize synthesis and production costs, in a sense that material from a single batch can be used in different fieldsithout compromising performances, and OMCs seem to be a

atisfactory choice. Regarding energy conversion applicationshere is an interesting example from automotive applicationshere different power source systems are used complementary to

mprove engine performance. In specific, fuel cell powered engines augmented by electrochemical capacitors to achieve moreesponsive performance (we avoid to specify the manufacturer aso commercial interest exists in presented work). As presented

bove, carbon materials have significant role in fuel cell andlectrochemical capacitor technologies and an identification of aingle material which meets the standards required for both typef applications would have significant impact in the field. To prove

ca Acta 125 (2014) 606–614 607

the assumption that attractive physico-chemical properties ofOMCs provide its wide applicability, in this study we synthesizeda sample of OMC by a method known from the literature andsubjected it to a complex electrochemical investigation regardingits energy conversion applications. First, pure OMC was examinedas electrode material for electrochemical capacitors, and as ORRelectrocatalyst in alkaline media. In the second part, OMC wasinvestigated as inert catalyst support for Pt-based electrocatalystutilized to catalyze HOR and ORR in acidic media. It was demon-strated that the investigated OMC presents itself as a versatilematerial for a wide range of applications in the field of energyconversion, competitive in performances to the state-of-the-artmaterials derived for specific applications.

2. Experimental

2.1. Synthesis and physico-chemical charaterization of OMC

OMC was synthesized by EISA method under acidic conditionswith resorcinol as the carbon precursor, in a way almost identi-cal to that developed by Zhang group [20]. For this purpose, 1.65 g(0.015 mol) of resorcinol was dissolved in a solution composed of2.5 g of Pluronic F127 (EO106PO70EO106) (Sigma-Aldrich) and 20 g ofethanol/water (1/1 vol. %) under stirring at room temperature. After30 min, 0.2 g of HCl (37 wt. %) was added as a catalyst and stirredfor two hours. Afterwards, 2.5 g (0.030 mol, R/F = 1/2) of formalde-hyde (37 wt. %) was added dropwise and stirred for another hour.When the mixture turned cloudy and began separating into twolayers, it was further kept aging for 96 h. After this time, the upperlayer was discarded while the lower polymer-rich phase was stirredovernight until a sticky monolith was formed. Finally, the monolithwas cured at 85 ◦C for 48 h and carbonized under nitrogen atmo-sphere at 800 ◦C for 3 h at a ramping rate of 5 ◦C/min. After coolingdown to room temperature, the material was kept in sealed PVCbottle denoted as OMC-SAM-800/3.

Adsorption and desorption of N2 on OMC-SAM-800/3 was mea-sured at −196 ◦C using the gravimetric McBain method. From theobtained isotherms, the specific surface area (SBET) pore size dis-tribution, mesopore including external surface area (Smeso) andmicropore volume (Vmic) of the samples were calculated. Pore sizedistribution was estimated by applying BJH method [21] to the des-orption branch of the isotherms. Mesopore surface and microporevolume were estimated using the high-resolution ˛s–plot method[22–24]. Micropore surface (Smic) was calculated by subtractingSmeso from SBET.

Powders of pure OMC-SAM-800/3 and Pt-OMC-SAM-800/3were characterized at room temperature by XRPD using Ultima IVRigaku diffractometer, equipped with Cu K�1,2 radiation source,using a generator voltage of 40.0 kV and a generator current of40.0 mA. The range of 10 - 90◦2� was used for all powders in acontinuous scan mode with a scanning step size of 0.02◦ and at ascan rate of 2◦/min.

Raman spectra excited with a diode pumped solid statehigh-brightness laser (532 nm) were collected on a DXR Ramanmicroscope (Thermo Scientific, USA) equipped with an Olympusoptical microscope and a CCD detector. The powdered sample wasplaced on X–Y motorized sample stage. The laser beam was focusedon the sample using objective magnification 10X. The scatteredlight was analyzed by the spectrograph with a grating 900 linesmm−1. Laser power was kept at 1 mW.

2.2. Synthesis and physico-chemical characterization of Pt/OMC

catalyst

Platinum nanoparticles were deposited on the surface of theOMC-SAM-800/3 by a following procedure. The OMC-SAM-800/3

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ample was dispersed in water using the ultrasonic bath. Under agi-ation by a magnetic stirrer, the hexachloroplatinic acid was addedrop-wise, after which the pH value was adjusted to 10 by addi-ion of 1 mol dm−3 KOH solution. This was followed by adding anxcess of NaBH4 solution to ensure the deposition of PtNPs. Theomogeneous slurry was stirred for 2 h to allow sufficient time forhe deposition. Afterwards, the material was filtered, washed withopious amounts of de-ionized water, and dried at 60 ◦C for 12 ho obtain Pt-OMC-SAM-800/3. The amount of the added precursoras set to obtain 20 wt. % of Pt on carbon.

The mass fraction of Pt was determined by a TA SDT 2960 ther-obalance by combusting the OMC-SAM-800/3 support under an

ir stream, at a flow rate of 50 cm3 min−1 and a heating rate of0 ◦C min−1.

.3. Electrode preparation and electrochemical measurements

To use it as electrode material, 5 mg of OMC-SAM-800/3 sam-le was dispersed in 1 cm3 of ethanol/water mixture (40 v/v %),nd the suspension was homogenized in the ultrasonic bath for0 min. A drop of the suspension was transferred onto the glassyarbon (GC) disk electrode (geometrical cross section 0.196 cm2)nd dried under N2 stream. After the thin carbon layer was dried itas covered with 10 �L of 0.05 wt.% Nafion in ethanol. The solventas removed by evaporation. Total amount of 73.6 �g OMC-SAM-

00/3 was loaded onto GC electrode. The electrode thus obtainedas tested by cyclic voltammetry (CV) to investigate capacitive and

lectrocatalytic properties of the sample in alkaline media. This wasone in a conventional one-compartment three-electrode electro-hemical cell with wide Pt foil as a counter electrode and a saturatedalomel electrode (SCE) as a reference electrode. Capacitive prop-rties of OMC-SAM-800/3 were investigated using CV in deaerated

mol dm−3 KOH aqueous solution while electrocatalytic activityowards ORR was investigated in O2-saturated 0.1 mol dm−3 KOHqueous solution using rotating disk electrode (RDE) voltammetry.igh purity N2 and O2 were used for these experiments. Measure-ents were done using PAR potentiostat/galvanostat model 273A.Pt-OMC-SAM-800/3 catalyst was characterized electrochemi-

ally as follows. Desirable amount of synthesized catalyst wasispersed in the mixture of 980 �L ultrapure H2O and 12.5 �L of

wt.% Nafion solution in ethanol. The dispersion was homogenizedn the ultrasonic bath for at least 2 h. Then, desirable volume ofatalytic ink was transferred onto Au rotating disk electrode (pol-shed to a mirror finish, geometrical cross section area 0.328 cm2)nd dried naturally in air. Total amount of Pt loaded on Au disk

as 17.2 �g (88 nmol). RDE voltammetric measurements wereerformed in all-glass three-compartment three-electrode elec-rochemical cell. As a counter and a reference electrode Pt mashnd HydroflexTM Hydrogen Reference Electrode with internal H2

Fig. 1. a) Nitrogen adsorption isotherm of OMC (�- adsorption, �–d

ca Acta 125 (2014) 606–614

source were used, respectively. Measurements were performedusing Gamry PCI4/750 potentiostat/galvanostat. All measurementswere performed at (25 ± 1) ◦C. Electrolytic solution was 0.5 moldm−3 HClO4 (ACS grade). Catalytic activity towards HOR wasprobed using RDE technique coupled to linear sweep voltamme-try (quasi-stationary conditions, potential sweep rate 10 mV s−1) inH2-saturated 0.5 mol dm−3 HClO4. Potential was scanned between0 and 0.3 V vs. RHE (reversible hydrogen electrode). Measured cur-rents were corrected for Ohmic drop using uncompensated solutionresistance determined by AC impedance. This value was found tobe only 1.7 � due to high concentration of supporting electrolyteand appropriate cell geometry, but, later on, it was found to berather important. Catalytic activity towards ORR was probed usingRDE voltammetry (potential sweep rate 50 mV s−1) in O2-saturated0.5 mol dm−3 HClO4. Ohmic drop was compensated using posi-tive feed-back scheme as proposed by Arenz and Markovic [25].Background curve was obtained by means of cyclic voltammetry inquiescent N2-purged HClO4 solution after ORR measurements.

3. Results and discussion

3.1. Characterization of OMC-SAM-800/3 andPt-OMC-SAM-800/3

In the following section, the results regarding physico-chemicalproperties of OMC-SAM-800/3 and synthesized Pt-OMC-SAM-800/3 catalyst are described.

3.1.1. Characterization of OMC-SAM-800/3 - nitrogen sorptionanalysis, Raman spectra and XRPD

Nitrogen adsorption isotherm for OMC-SAM-800/3 sample isgiven as the amount of adsorbed N2 as a function of relative pres-sure at −196 ◦C (Fig. 1a). According to the IUPAC classification [26]isotherm is of type IV and with a hysteresis loop which correspondsto mesoporous materials. Specific surface area calculated by BETequation is 712 m2 g−1. The large volume adsorbed at low relativepressures suggests the presence of micropores in the sample.

Pore size distribution (PSD) of the sample is shown in the insetof Fig. 1. The PSD is very narrow with peak centered at 2 nm poreradius. �s-plot, obtained on the basis of the standard nitrogenadsorption isotherm is shown in Fig. 1b. The straight line in thehigh �s region gives a mesoporous surface area (Smeso = 320 m2/g)including the contribution of external surface, determined by itsslope, and micropore volume (Smicro = 392 m2/g, Vmic = 0.190 cm3/g)

is given by the intercept. It was confirmed that the sample is meso-porous with a certain amount of micropores. It is assumed that themicropores were formed in the walls during carbonization of theRF polymer.

esorption); inset - pore size distribution (PSD), b) �S - plots.

M. Momcilovic et al. / Electrochimica Acta 125 (2014) 606–614 609

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area OMCs obtained by hard templating and surpasses reportedOMCs obtained by EISA method. This points to a high performancematerial for electrochemical capacitors with significantly simpli-fied synthesis procedure. Capacity retention upon increasing the

Fig. 2. XRPD patterns of pure OMC-SA

Fig. 1 A Raman spectrum of prepared OMC-SAM-800/3 (Fig. 2a)isplays typical features of carbonaceous materials. The G-band,esignated around 1588 cm−1, is indicative of sp2-hybridised car-on and it corresponds to the graphitic lattice vibration modeith E2g symmetry (vibration of sp2-bonded carbon atoms in a

-D hexagonal lattice, i.e. the stretching modes of C = C bonds inraphite) [27]. As known, G-band position indicates perfections andmperfections in the graphite layers [27]. In this case, the position of-band points to a well-ordered mesoporous structure of the OMCample. The occurrence of an additional Raman line near 1337 cm−1

D-band) is associated with the presence of defects in the graphiteayer, arising from sp3-hybridized carbon. The disorder and defectsre associated with vibrations of carbon atoms with dangling bondsn plane terminations, as well as other defects. In addition, the shift- and D- bands towards lower energies is a result of the influencef considerably low crystallite dimensions (about 5 nm), which isevealed by XRPD analysis (Fig. 2b).

Fig. 2 Since absolute values of the intensities of the fitted peaksepend on parameters such as the smoothness of the sample sur-ace and the instrument that was used to obtain the spectrum, theatio of the intensities of the G- and D- bands is used to overcomehis problem. As known, it is possible to estimate the degree of crys-allinity (or degree of graphitization) from the peak position of the-band, the ID/IG intensity ratio and the Full Width at Half Maxi-um (FWHM) of the G-band. Well-ordered mesoporous structure

arbon material is confirmed, although the values ID/IG = 0.96 andWHM = 59 cm−1 are slightly higher that the values reported in theiterature. Higher values of ID/IG and FWHM indicate coexistencef well-defined graphitic domains and defect sites on the surfacef OMC and can be considered as consequences of the significantresence of micropores (Smic = 392 m2/g).

.1.2. Characterization of Pt-OMC-SAM-800/3 catalystCharacterization of prepared Pt-OMC-SAM-800/3 catalyst

nvolved thermogravimetric analysis (TGA) and XRPD analysis. TheGA curve of the sample indicated that Pt mass loading of 26.1 wt.

(Fig. 3, inset), after the combustion was completed at 600 ◦C.his result suggests that part of the carbon was lost during prepa-ation procedure resulting in higher amount of platinum in thenal sample (anticipated loading was 20 wt.% with respect to OMC-AM-800/3 equilibrated with air). The XRPD analysis revealed theverage size of deposited Pt nanoparticles to be 5.4 nm (Fig. 3). Par-icle size was obtained from the line broadening of XRPD peakocated at 2� = 39.22◦ (assigned to (111) reflection of Pt) by the

eans of Debye–Scherer equation.

.2. Electrochemistry of the materials for energy conversion

pplications.

In the following section, energy conversion applications of theamples are investigated. First we address capacitive properties

/3 (left) and Raman spectrum (right).

of OMC-SAM-800/3, followed by ORR electrocatalysis in alkalinemedia. Then, electrocatalysis by Pt-OMC-SAM-800/3, relevant forPEMFC applications, is considered.

3.2.1. Cyclic voltammetry of OMC-SAM-800/3 and investigationof capacitive properties

The cyclic voltammograms of the prepared OMC-SAM-800/3electrode, in 6 mol dm−3 KOH aqueous electrolyte, normalized bythe sweep rate and mass of the material, are shown in Fig. 4a.The nearly rectangular shape of CV curves at different potentialscan rates, ranging from 5 mV s−1 to 100 mV s−1, indicates goodelectrochemical double layer capacitive behavior. The CV curvesretain their identical basic shapes with increasing scan rates, whichis indicative of excellent quick charge propagation. The specificcapacitance values (Cspec) of OMC are calculated as:

Cspec = 12�Em

E2∫

E1

I

�dE (1)

where E1 and E2 are the vertex potentials, � is the potential sweeprate, m is the mass of active material on the electrode, I is the mea-sured current at given potential and �E is the potential window.Capacitance values, calculated as the average capacitance fromcharge/discharge sweeps values, are given in Fig. 4b. The highestcapacity of 232 F g−1 is measured at 5 mV s−1, and is comparable toor better than those found in literature (listed in Table 1) measuredin alkaline solutions. It should be noted that described OMC-SAM-800/3 by its capacitive behavior stands in line with high-surface

Fig. 3. XRPD patterns of Pt-OMC-SAM-800/3 catalyst; inset shows TGA analysis inair.

610 M. Momcilovic et al. / Electrochimica Acta 125 (2014) 606–614

Fig. 4. Capacitance currents of OMC in 6 mol dm−3 KOH at different scan rates (left) and measured specific capacitances of OMC as a function of potential sweep rate (right).

Table 1Textural properties of different ordered mesoporous carbons reported in the literature and corresponding specific capacitances, compared to OMC described in this work.

Marking in the respectivereference

Synthesis SBET/m2 g−1 Mean pore diameter/nm

Capacitance**

/F g−1Electrolyte Ref.

OMC-1 mesoporous silica MCM-48 hard template 1490 2.7 211.6 30 wt% KOH [28]OMC-M-6 mesoporous silica MSU-H hard template 868.5 8.5 205.3 30 wt% KOH [29]OMC-K-4 mesoporous silica KIT-6 hard template 955.8 5.7 190.4OMC-67-4.8 solvent EISA approach 690 4.5 130 6.0 mol dm−3 KOH [31]C–P carbonization of SBA-15/EO20PO70EO20

composite720 3.6 148 30 wt.% KOH [32]

OMC-SAM-800/3 712 4 232 6.0 mol dm−3 KOH this work

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transferred in a reaction, concentration of electroactive specie,its diffusivity, and the electrolyte kinematic viscosity. Hence,determining the value of B one can determine apparent numberof electrons consumed per O2 molecule if all the other constants

For the papers demonstrating capacitive performance of several OMCs the materia** measured in a potentiodynamic regime at 5 mV s−1 or 0.5 A g−1.

weep rate up to 100 mV s−1 has a value of 83.6%, indicating thathe solvated ions can diffuse fast enough through the mesopores ofhe carbon even at the voltage sweep rate of 100 mV s−1. Similarapacity retention values are found for different OMCs, present-ng its advantage over activated microporous carbons which looseapacitance rapidly upon increasing charging rate [28]. The arealapacitance has a value of 32.6 �F cm−2 at 5 mV s−1 (taking intoccount specific surface area of both micro and mesopores), beinglightly larger compared to the other OMCs reported in the litera-ure. This is due to the fact that part of the capacitance comes fromydrogen evolution in the potential range of–1.1 to–1.3 V vs. SCE. Ifhis region is excluded from the capacitance calculation the specificurface capacitance drops to 28 �F cm−2 which is in good agree-ent with those found in [29] for different OMCs. This speculationas indeed confirmed by investigating cyclic voltammograms ofMC-SAM-800/3 upon reducing cathode vertex potential to moreositive potentials (Fig. 5) [30]. While general shape of cyclicoltammogram is retained, a small bulge, present during anodicweep from deep cathodic potentials, gradually disappears. Thisharacteristic of cyclic voltammogram can be ascribed to the oxi-ation of cathodically evolved H2 during potential excursion below1.1 V vs. SCE, which is stored within the pore network of the carbon.his points to additional possibilities for exploitation of describedaterial as electrode material for electrochemical capacitors which

s rendered by suitable textural properties of OMC.

.2.2. ORR at OMC in alkaline solutionCarbon materials are relatively good electrocatalysts for O2

eduction in alkaline media [33]. For the studied OMC-SAM-800/3he onset potential for ORR is around–0.10 V vs. SCE (Fig. 6), whichs comparable to or higher than the values found in literature for

arious types of carbon materials [34]. High ORR onset potentialan be explained by a relatively large number of surface defects,s evidenced by Raman spectroscopy and quantified ID/IG rationd also small crystallite size observed by XRPD (Section 3.1.1.).

the highest performance is presented.

These defect sites can enhance O2 adsorption energetics and chargetransfer kinetics which commensurate with high ORR onset poten-tials observed here. ORR polarization curves were processed usingKoutecky-Levich (K-L) analysis [35]. Measured RDE current density(j) can be expressed by Koutecky-Levich equation as:

1j

= 1jk

+ 1jd (ω)

= 1jk

+ 1

B · ω12

(2)

where jk and jd(ω) are kinetic current density and limiting diffu-sion current density, while B assembles the number of electrons

Fig. 5. Cyclic voltammograms of OMC upon reducing cathode vertex potential tomore positive values (potential sweep rate 100 mV s−1).

M. Momcilovic et al. / Electrochimica Acta 125 (2014) 606–614 611

Fig. 6. RDE polarization curves of oxygen reduction on OMC-modified GC disk elec-trode and on high-surface area poly-oriented Pt (Pt-poly) disk electrode (roughnessfmpc

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actor around 30) at common electrode rotation rate of 600 rpm (revolutions perinute). Measurements were performed in O2-saturated 0.1 mol dm−3 KOH and

otential sweep rate was 20 mV s−1. Determined apparent number of electronsonsumed per O2 molecule (n) is given as a function of electrode potential.

ontained within B are known. Assembly of constants used forhe analysis of ORR RDE polarization curves in alkaline media cane found in [36]. At potentials E < –0.5 V vs. SCE a small currentlateau is discernible while the reduction current increases fur-her at more negative potentials indicating increase of apparentumber of electrons consumed per O2 molecule above 2 (Fig. 6).his feature is very similar to that found by Alexeyeva for carbonanotubes (CNTs) [37] and multi-walled CNTs [38] who attributedhis desirable behavior to the high number of active surface sites athich the reduction of oxygen proceeds on vertically aligned CNT.

his led us to conclude that OMC also provides a high number ofctive surface sites inside mesopores, due to large surface area andrdered mesoporous structure, which allows easy access for thelectrolyte and oxygen molecules. Recently, Yang et al. [39] demon-trated ORR performance of different platelet SBA-15-templatedMCs in alkaline media. If not doped with heteroatoms, such OMCisplayed high surface area of approx. 1900 m2 g−1 and ORR onsetotential (recalculated with respect to SCE for the sake of compari-on) of -0.24 V. At intermediate ORR overvoltages n was found to be.2, which is similar to the results presented here. However, highervervoltage of OMC described here and a simplified synthesisithout the use of silica template present obvious advantage with

espect to platelet OMC demonstrated by Yang et al. [39].The work of Tammeveski group [40] has shown that surface-

onfined quinones are active catalysts for ORR in alkaline solutionnd it has been proposed that the semiquinone radical anion isesponsible for the high oxygen reduction activity of carbon mate-ials in the solutions of high pH. In a recent study by Gavrilovt al. [18], it has been proposed that, if the material has apprecia-le contribution of mesopores (true in the case of OMC), HO2

–ionormed in the rate determining step, goes through chemical dis-roportiation step to OH–and O2. Furthermore, chemically formed2 can undergo the next catalytic cycle of the electrochemical

eduction, leading to an increase in the apparent number of elec-rons above 2, which can explain significant currents measuredor OMC-SAM-800/3 in this study. By its performance as ORRatalyst, OMC-SAM-800/3 also falls in line with some of the N-oped nanocarbons [41]. By comparing OMC with N-containingolyaniline-derived nanocarbon reported in [41] it is suggestedhat large mesopores surface area of OMC (320 m2 g−1) can com-

ensate the presence of N-dopant to provide efficient O2 reductionhrough the mechanism described above.

Fig. 6 As it is usual practice in electrochemical literature to com-are ORR performance of different carbonaceous materials with

Fig. 7. Stable cyclic voltammogram of Pt-OMC-SAM-800/3 electrode in de-aerated0.5 mol dm−3 HClO4 solution at a scan rate of 100 mV s−1.

platinum, considered as a benchmark ORR catalyst, it should benoted that ORR performance of OMC-SAM-800/3 is far from thatof Pt (Fig. 6). For such comparison, we have used high-surface areapoly-oriented Pt disk electrode in order to avoid complications aris-ing due to effects of Pt loading on the electrode which might leadto erroneous conclusions. ORR onset potential for OMC is almost150 mV lower than for Pt disk electrode but still high enough to setOMC-SAM-800/3 as a promising metal-free ORR catalyst in alka-line media for alkaline fuel cells and metal-air batteries. It is alsosuggested that described carbon is a good candidate for electro-chemical synthesis of hydrogen peroxide if operating potential islimited to low ORR overvoltages (electrode potentials below -0.5 Vvs. SCE).

3.2.3. Fuel cell electrocatalysis by Pt-OMC-SAM-800/33.2.3.1. Blank cyclic voltammetry of Pt-OMC-SAM-800/3 electrode.After being prepared, Pt-OMC-SAM-800/3 electrode was exten-sively cycled between 0.03 and 1.2 V vs. RHE at a scan rate of100 mV s−1 in de-aerated 0.5 mol dm−3 HClO4 in order to per-form electrochemical cleaning of the catalyst [25] and to obtainstable cyclic voltammogram of the investigated material. From theobtained cyclic voltammogram (Fig. 7), one can distinguish hydro-gen adsorption/desorption region (HUPD region, 0.03 V < E < 0.35 V),rather wide double layer region and Pt-oxide formation/reductionregion. As being commonly reported in the literature for carbon-supported nanodisperzed Pt, HUPD region does not display finestructure observed for poly-oriented Pt and single-crystal Pt elec-trodes [42]. Nevertheless, upon correction for double layer charging(Fig. 7), evaluation of total amount of charge associated withhydrogen adsorption/desorption peaks (QH) enabled estimation ofelectrochemically active surface area (ESA) of Pt-OMC-SAM-800/3catalyst as:

ESA = QH

210 �C cm−2(3)

where the value of 210 �C cm−2 is related to complete hydrogenmonolayer per 1 cm2 of platinum [43]. For the calculation above,average value of QH obtained for hydrogen adsorption and hydro-gen desorption was used, which lead to estimated value of ESA of1.11 cm2, which translates to roughness factor of 3.38. Width ofthe double-layer region indicates a significant capacitive contribu-

tion of OMC-SAM-800/3 support to recorded cyclic voltammogramof Pt-OMC-SAM-800/3 catalysts which sounds with high gravi-metric capacitances of OMC-SAM-800/3, discussed previously(Section 3.2.1.).

612 M. Momcilovic et al. / Electrochimica Acta 125 (2014) 606–614

Fig. 8. RDE polarization curves of hydrogen oxidation reaction on Pt-OMC-SAM-800/3 electrode in H2-saturated 0.5 mol dm−3 HClO4 solution at a scan rate of 10 mV s−1. Insets polaris tion ((

3HiOmirbos

utwpliIwilkska(ttf

wpSco

(ir

j

sm

and mass activities (jk-mass)

jk−mass = jk · A

mPt[A mg−1

Pt ] (7)

hows Koutecky-Levich plot for E = 0.3 V vs. RHE (left). Hydrogen oxidation reactionolution at a scan rate of 10 mV s−1 an 2500 rpm before (ERDE, �) and after IR correcright).

.2.3.2. HOR and ORR kinetics on Pt-OMC-SAM-800/3 electrode..ydrogen and oxygen electrode reactions have a significant impact

n the field of energy conversion applications. In specific HOR andRR represent anode and cathode reaction in PEMFC and AFC. Com-only used catalysts for both reactions are based on platinum as

t presents one of the most active catalytic materials for these twoeactions [44–46]. Nevertheless, the rates of HOR and ORR on Pt-ased catalysts differ by several orders of magnitude [42], formerne being exceptionally fast and the latter one being exceptionallylow.

Kinetics of HOR on Pt- OMC-SAM-800/3 was investigatedsing RDE voltammetry in H2-saturated 0.5 mol dm−3 HClO4 solu-ion. Recorded HOR polarization curves point to fast kinetics asell-defined diffusion limited currents (jd(�)) are reached for over-otentials above 75 mV (Fig. 8a). Diffusion limited currents depend

inearly on the square root of electrode rotation rate (ω, see insetn the Fig. 8a) as described by Koutecky-Levich equation (Eq. (2)).n our case, B was found to be 0.074 ± 0.001 mA cm−2

geom rpm−0.5,hich compares well with previous reports considering HOR kinet-

cs in acidic and alkaline solutions where it was found that diffusionimiting current are virtually the same [47]. It is known that HORinetics on Pt is fast in acidic media, with exchange current den-ities comparable to limiting diffusion current densities, so HORinetics should display high reversibility. In order to check thisssumption, we compared the measured HOR polarization curves2500 rpm) prior to and after compensation for IR drop, with Nerns-ian diffusion overpotential, d, which assumes ideal reversibility ofhe HOR and infinitely fast charge transfer kinetics. The expressionor d is given by:

d = −RT

2Fln

(1 − j

jd

)(4)

here R is universal gas constant (8.314 J mol−1 K−1), T is the tem-erature (298 K) and F is the Faraday constant (96485 C mol−1).uch comparison is presented if Fig. 8b. It can be seen that uponompensation for Ohmic drop, HOR polarization curve matches thene calculated on the basis of Eq. 4.

It should be noted that effect of IR compensation is rather smallseveral mV) but due to fast HOR kinetics even such fine effect playsmportant role. Kinetic current densities for HOR were further cor-ected for mass transport limitations using following equation:

k = jd (ω) · j(5)

jd (ω) − j

Polarization curves were extended to negative overpotentials toample hydrogen evolution reaction (HER) branch (Fig. 9). The sym-etry of HOR and HER branches around 0 V vs. RHE points to the fact

zation curve on Pt-OMC-SAM-800/3 electrode in H2-saturated 0.5 mol dm−3 HClO4

EIR-free, ©) are shown. Solid line is the calculated Nernstian diffusion overpotential

that HOR and HER proceeds through the same reaction mechanismand intermediates, as suggested by Sheng et al. [47]. The extractionof HOR/HER kinetic parameters related to charge transfer kinetics iscomplicated due to fast electrode reaction kinetics, but from resultspresented, one can estimate exchange current density to be roughly1 mA cm−2, which is in close agreement with previously reportedstudies of HOR/HER kinetics on Pt-based catalysts in acidic solution[48,49]. Although usability of RDE voltammetry for the evaluationof HOR kinetics on Pt-based catalysts has been reasonably ques-tioned [47], based on obtained results, it can be concluded that HORkinetics on investigated Pt- OMC-SAM-800/3 catalyst is compara-ble to the state-of-the-art carbon-supported nanosized Pt catalystsused to catalyze anode reaction in PEMFC.

After HOR, ORR kinetics was investigated on Pt-OMC-SAM-800/3 catalyst. Due to much slower kinetics, relevant kineticparameters can be extracted directly from recorded polarizationcurves after correction for mass transport limitation according toEq. 5. ORR polarization curves were transformed to specific activi-ties (jk-ESA) according to:

jk−ESA = jk · A

ESA[� A cm−2

Pt ] (6)

Fig. 9. The hydrogen evolution/oxidation kinetic current densities obtained fromIR-corrected polarization curves and corrected for hydrogen mass transport in thehydrogen oxidation branch using Kuotecky-Levich equation (electrode rotation ratewas 2500 rpm).

M. Momcilovic et al. / Electrochimica Acta 125 (2014) 606–614 613

F in O2

r , ©) a

i(atcd

fmOPP0sbowstbdcrP

4

dtcicctssosc

ig. 10. Tafel plots of oxygen reduction reaction on Pt- OMC-SAM-800/3 electrodeotation rate of 1500 rpm. Kinetic current densities were evaluated to specific (jk-ESA

n order to allow proper comparison with available literatureFig. 10). In Eqs. 6 and 7 A stands for geometrical cross section area of

rotating disk electrode and mPt is the mass of Pt loaded on the elec-rode. When specific and mass activities are represented in Tafelo-ordinates gradual change of Tafel slope from -60 to -120 mVec−1 is observable, being characteristic for ORR on platinum [50].

Fig. 10 Taking the electrode potential of 0.9 V vs. RHE as a pointor comparison, it is seen that jk-ESA amounts to 290 �A cm−2

Pt andass activity is 0.02Amg−1

Pt . In terms of ORR specific activity Pt-MC-SAM-800/3 catalyst stands side by side with state-of-the-artEMFC cathode catalysts and surpasses OMC-supported nanosizedt catalysts reported previously by Liu et al. [51] (450 �A cm−2

Pt @.85 V vs. RHE compared to 700�Acm−2

Pt found in this work at theame electrode potential). Besides high activity towards ORR, trueenefit of OMC support might be expected from enhanced stabilityf such Pt/OMC catalyst. During electrochemical characterizatione have not observed any catalyst activity deterioration which con-

olidates with previous reports available in the literature pointinghat the use of OMCs as catalyst supports enhances catalyst sta-ility [51,52]. It is also suggested that suitable pore network ofescribed OMC might augment mass transport in PEMFC appli-ations as OMC-supported nanosized Pt. Based on the obtainedesults, it has promising applications as both anode and cathodeEMFC catalyst.

. Conclusions

Carbon sample OMC-SAM-800/3 prepared through EISA methodisplayed high surface area (712 m2 g−1) and narrow pore size dis-ribution with pore radius centered around 2 nm. The obtainedarbon was tested for different energy conversion applicationsnvolving (i) capacitive (charge storage) properties (ii) ORR electro-atalysis in alkaline media (important in the field of alkaline fuelells and metal-air batteries) and (iii) as a support of Pt-based elec-rocatalyst (relevant for PEMFC applications). The sample deliveredpecific capacitance of 232 F g−1 in potentiodynamic regime at 5 mV

−1. Upon increasing the potential sweep rate to 100 mV s−1 83.6%f specific capacitance was retained. This was explained by largeurface area and appropriate pore structure. It was observed thatapacitance of OMC-SAM-800/3 can be increased to some extent

-saturated 0.5 mol dm−3 HClO4 solution at a scan rate of 50 mV s−1 and electrodend mass activities (jk-mass, �).

by the storage of hydrogen, evolved during cathodic potentialexcursions within the pore network. Effective ORR electrocatalysisby OMC-SAM-800/3 in alkaline media was evidenced, with onsetpotential amounting −0.10 V vs. saturated calomel electrode, whichwas explained by large mesopore surface area in combinationwith a significant number of surface defects (observed by Ramanspectroscopy) and small crystallite size (confirmed by XRPD analy-sis). Prepared Pt-OMC-SAM-800/3 catalyst was tested for HOR andORR in acidic media. Reversible HOR kinetics was observed, whileORR performance compares with previously reported carbon-supported Pt electrocatalysts, with specific activity of 700 �A cm−2

Pt@ 0.85 V vs. RHE. It is also expected that utilization of OMC as Ptsupport in PEMFC catalysts should enhance mass transport dueto developed pore network as well as catalyst stability. Owing toits attractive properties, described OMC-SAM-800/3 stands out asa highly versatile material, which can be used to replace differ-ent carbon materials developed specifically for various purposes.The presented results indicate a possible direction in rationaliza-tion of carbon-based technologies in the field of energy conversion,enabling replacement of a number of different carbons with a singleone without compromising targeted performances.

Acknowledgement

This work was supported by the Serbian Ministry of Education,Science, and Technological Development through the projects III45012, III 43009 and III 45014.

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