Incorporation of Pt, Ru and Pt-Ru nanoparticles into ordered mesoporous carbons for efficient oxygen reduction reaction in alkaline media

Incorporation of Pt, Ru and Pt-Ru nanoparticles into ordered mesoporous carbons for efficient oxygen reduction reaction in alkaline media

Electrochimica Acta 153 (2015) 130–139 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/elect...

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Electrochimica Acta 153 (2015) 130–139

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Incorporation of Pt, Ru and Pt-Ru nanoparticles into ordered mesoporous carbons for efficient oxygen reduction reaction in alkaline media Marija Stojmenovi c a, *, Milan Mom9 cilovi c a , Nemanja Gavrilov b , Igor A. Pašti b , c d c , Biljana Babi ca Slavko Mentus , Bojan Joki a

Institute of Nuclear Sciences “Vin9ca”, University of Belgrade, P.O. Box 522, 11000 Belgrade, Serbia University of Belgrade, Faculty of Physical Chemistry, Studentski trg 12-16, 11158 Belgrade, Serbia c Serbian Academy of Sciences and Arts, Knez Mihajlova 35, 11000 Belgrade, Serbia d Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 5 September 2014 Received in revised form 28 October 2014 Accepted 12 November 2014 Available online 14 November 2014

Ordered mesoporous carbon, volume-doped up to 3 w.% with Pt, Ru and Pt-Ru nanoparticles was synthesized by evaporation-induced self-assembly method, under acidic conditions. The content of incorporated metal was determined by EDX analysis. The X-ray diffractometry confirmed the existence of highly dispersed metallic phases in doped samples. Specific surface area was determined by N2-physisorption measurements to range between 452 and 545 m2 g1. Raman spectroscopy of investigated materials indicated highly disordered carbon structure with crystallite sizes around 1.4 nm. In a form of thin-layer electrode on glassy carbon support, in 0.1 M KOH solution, the prepared materials displayed high activity toward oxygen reduction reaction (ORR) in alkaline media, with onset potentials more positive than 0.10 V vs. SCE. The kinetics of O2 reduction was found to be affected by both the specific surface area and the concentration of metal dopants. The ethanol tolerance of (Pt, Ru)-doped OMCs was found to be higher than that of common Pt/C ORR catalysts. Presented study provides a new route for the synthesis of active and selective ORR catalysts in alkaline media, being competitive with, or superior to, the existing ones in terms of performance and price. ã 2014 Published by Elsevier Ltd.

Keywords: ordered mesoporous carbon metal doping oxygen reduction reaction ethanol tolerance

1. Introduction Ever growing demand for portable/stationary devices that deliver electrical energy on demand presents a driving force for industrial advancement. Among various device types fuel cells present a clean way of producing electric energy through redox reactions using hydrogen and lower alcohols as fuel. A major drawback for their commercialization is the use of Pt-group metals, being the most active electrocatalysts for low temperature fuel cells, which significantly increases the cost of these devices. Hence the development of new electrocatalytic materials with high catalytic activity towards oxygen reduction reaction (ORR) is of significant interest. Alloying Pt with other metals in a form of

* Corresponding author. Institute of Nuclear Sciences “Vin9 ca”. Tel.: +381 11 3408 860; fax: +381 113408224. E-mail address: [email protected] (M. Stojmenovi c). http://dx.doi.org/10.1016/j.electacta.2014.11.080 0013-4686/ ã 2014 Published by Elsevier Ltd.

either bulk alloys [1–3] or Pt-monolayer over less precious metal nanoparticles [4,5] has resulted in superior ORR catalysts being developed. Alternative direction for the advancement is found through the use of carbon-based ORR catalysts which display high intrinsic ORR activity in alkaline media [5,6], being attractive in the field of alkaline fuel cells (AFC) and metal-air batteries. In addition, doping with different heteroatoms (nitrogen, boron, sulfur, etc.) their activity can be altered to reach Pt-based catalysts in terms of ORR activity [8–10]. However, these metal-free carbonaceous catalysts require appropriate pore structure for high ORR activity to arise, providing the access for O2 to large fraction surface area where it is reduced to HO2 or OH, depending on the applied electrode potential and the state of materials surface [11]. Ordered mesoporous carbons (OMCs) have drown great attention in recent years owing to their high surface area, large pore volume, chemical inertness, good thermal and mechanical stability, easy handling and low cost of manufacture [12]. Structure alterations, morphology and composition result many different

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applications particularly in catalysis as catalysts and catalyst supports [13,14], for the separation [15], as adsorbens [16], for the energy storage [17]. Related to electrochemical power source applications, OMC has been proven as a versatile material where single one material can be used in different fields without compromising the performances [18]. Attractive properties of OMCs define this class of materials as a basis for further modification and optimization for specific applications. As overviewed by Muylaert et al. [19] and Ma et al. [20], incorporation of metals in OMC can be done either after carbonization of polymeric resin (post-synthetic modification) or by addition of metal source into the resin before carbonization (in situ). Post synthetic modification of OMC with Pd [21] and Pt [22,23] was reported, while in the case of Ru both post-synthetic [24] and in situ approach were used [25,26]. According to Muylaert et al. [19], metal-modified OMCs were usually used in the field of heterogeneous catalysis, but also in some cases its capacitive properties were addressed [27]. Regarding the use of metalmodified OMC in ORR electrocatalysis, there are several examples found in the literature [22,28,29]. Typically, this functionalization used in situ Pt reduction with hard templates employed to synthesize OMC and rather high Pt loadings on OMC support, reaching up to 30% [29]. The necessity to remove the template after the synthesis complicates the route to obtaining the catalyst, while high Pt loading affects the price of such material. In this contribution we present the synthesis of ordered mesoporous carbons doped with Pt, Ru, and their mixture with the aim to demonstrate catalytic activity and selectivity of such doped carbonaceous material towards ORR in alkaline media. The proposed synthesis of OMC doped with Pt and Ru for highly efficient ORR catalysis has several advantages over previously reported ones: (i) due to the use of the evaporation-induced selfassembly (EISA) method there is no need for hard template during the preparation of polymeric precursor and its removal upon carbonization, (ii) modification of OMC by Pt and Ru is achieved in situ which reduces the number of step during the production of such ORR catalysts and (iii) the loading of the precious metals is much lower than typically reported. Materials were characterized by energy dispersive X-ray spectroscopy (EDX), N2-physisorption, X-ray powder diffraction and Raman spectroscopy. Assembled data on physico-chemical properties of prepared materials were used to rationalize high ORR activity displayed by these materials, which was investigated in alkaline media.

2. Experimental 2.1. Synthesis (Pt,Ru)-doped OMC materials were prepared using EISA method. Initially, 4.52 g of Pluronic F127 (EO106PO70EO106) (Sigma-Aldrich) was dissolved in the mixture of 18 cm3 of deionized water and 18 cm3 of ethanol (95 wt.%) and vigorously stirred for 15 min at room temperature. Then, 3 g of resorcinol was added and stirred for the next 30 min when the mixture was acidified with 0.35 cm3 of HCl (37 wt. %). In this step, H2PtCl64H2O (Sigma-Aldrich) and RuCl3H2O (Sigma-Aldrich) were introduced along with 0.1 g of NaBH4 (JT Baker) as reducing agent. Different amounts of dopants were used in order to obtain various Pt-to-Ru mass ratios in the final material. After 2 h, 4.5 cm3 of formaldehyde (37 wt. %) was slowly added dropwise, stirred for another hour, aged covered at room temperature for 3 days and dried at 85  C for two days. Obtained polymeric cakes were carbonized under nitrogen at 800  C for 3 h at a ramping rate of 5  C/min, cooled down in the same atmosphere and used for further examination.

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2.2. Characterization The morphology of investigated materials was investigated using scanning electron microscope (SEM) JEOL JSM-5800. The chemical composition of the samples was analyzed using an Energy Dispersive Spectrometer (EDS) Isis 3.2, with a SiLi X-ray detector (Oxford Instruments, UK) connected to the scanning electron microscope (SEM) JEOL JSM-5800 at 20 kV and a computer multi-channel analyzer. Porous structures of the samples were characterized by N2 adsorption/desorption using the gravimetric McBain method. From the obtained isotherms, the specific surface area (SBET), pore size distribution, mesopore including external surface area (Smeso) and micropore volume (Vmic) of the samples were calculated. Pore size distribution was estimated by applying BJH method [25] to the desorption branch of the isotherms. Mesopore surface and micropore volume were estimated using the high-resolution as–plot method [30–33]. Micropore surface (Smic) was calculated by subtracting Smeso from SBET. All powders of OMC-Pt-Ru were characterized at room temperature by XRPD using Ultima IV Rigaku diffractometer, equipped with Cu Ka1,2 radiation source, using a generator voltage of 40.0 kV and a generator current of 40.0 mA. The range of 10 - 90  2u was used for all powders in a continuous scan mode with a scanning step size of 0.02  and at a scan rate of 2  min1. Raman spectra excited with a diode pumped solid state highbrightness laser (532 nm) were collected on a DXR Raman microscope (Thermo Scientific, USA) equipped with an Olympus optical microscope and a CCD detector. The powdered sample was placed on X–Y motorized sample stage. The laser beam was focused on the sample using an objective magnification 10X. The scattered light was analyzed by the spectrograph with a grating 900 lines mm1. Laser power was kept at 2 mW. 2.3. Electrode preparation and electrochemical measurements Electrode material suspension was prepared by dispersing 5 mg of metal-doped OMC sample in 1 cm3 of ethanol/water mixture (40 v/v %), followed by 30 min homogenization in an ultrasonic bath. Glassy carbon (GC) disk electrode (geometrical cross section 0.196 cm2) was subsequently covered with a 10 mL drop of the prepared suspension and dried under N2 stream. Drying was followed by the addition of 10 mL of 0.05 wt.% Nafion in ethanol to insure the stability of the thin carbon layer. The solvent was removed by evaporation. Electrodes prepared in the described manner were further tested by cyclic voltammetry (CV) to investigate the capacitive and electrocatalytic properties of the samples in alkaline media. Conventional one-compartment threeelectrode electrochemical cell with wide Pt foil as a counter electrode and a saturated calomel electrode (SCE) as a reference electrode was used to conduct the CV measurements. Electrocatalytic activity towards ORR was investigated in O2-saturated 0.1 mol dm3 KOH aqueous solution using rotating disk electrode (RDE) voltammetry. High (5 N) purity N2 and O2 were used for these experiments. Measurements were done using Gamry PCI-4/750 potentiostat/glavanostat. 3. Results and Discussion 3.1. Morphology of (Pt-Ru)-OMC samples SEM analysis of prepared samples (Fig. 1) shows that metal doping of polymeric precursor does not affect the morphology of the final carbonaceous materials. Irregular morphology with no characteristic features has been observed while (Pt,Ru)-doped OMC particles have different sizes, ranging from nano to

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Fig. 1. SEM images of investigated OMC materials under the magnification of 10,000. For comparison, the SEM image of non-doped OMC-SAM-800/3 reported in ref. [18] is also included.

microscale. Moreover, the morphology of (Pt,Ru)-doped OMCs does not differ from the morphology of previously reported OMC-SAM-800/3 [18], which has been prepared using the same route but without the metal doping step. In addition, we not observed large metal agglomerates which points to finely dispersed metal within the OMC structure. 3.2. Energy dispersive X-ray spectroscopy (EDX) – Elemental composition EDX analysis confirmed incorporation of Pt, Ru or Pt/Ru nanoprticles in all samples (Fig. 2,Table 1). It is interesting to compare Pt-OMC and Ru-OMC samples for which the synthesis conditions were set to provide the same weight fraction of metal dopant relative to total carbon, amounting to 5 wt.% (evaluated by the ratio of metal and carbon sources used to synthesize polymer precursor). This is the upper limit of the metal weight fraction we set to have low noble metal loading ORR catalyst. When this targeted metal content is recalculated to atomic percents it yields 0.32 at.% of Pt for Pt-OMC or 0.62 at.% of Ru for Ru-OMC, which clearly indicates that there are certain loses of metal during the synthesis, and this loss is more pronounced for Pt (Table 1). As theoretical calculations suggest that Ru interacts much stronger than Pt with sp2 hybridized carbon network [34], while Ru, compared to Pt, has higher cohesive energy, more effective incorporation of Ru into OMC, can be attributed to (i) stronger Ru–C and (ii) stronger Ru–Ru interactions. It is possible that certain loses of metal dopant appeared on the vessel walls during synthesis or in decanted part reaction mixture during the synthesis of polymer precursor (two-phase system is obtained during the synthesis of the precursor and aqueous fraction was discarded upon polymerization). It is, however, important to note that described synthesis route is highly reproducible. Though it led to some losses of metallic component, these remain constant under controlled conditions. It is proposed that the identification of the specific step(s) of the synthesis at which metallic component is lost could further economize the synthesis as can enable the recycling of metallic component.

Fig. 2. Representative EDX spectra of selected Pt-OMC (top), PtxRuy-OMC-1 (middle) and Ru-OMC (bottom) samples.

M. Stojmenovic et al. / Electrochimica Acta 153 (2015) 130–139 Table 1 Ru and Pt content in at.% and wt.% of investigated samples obtained by energy dispersive X-ray spectroscopy. Sample

PtxRuy-OMC-1 PtxRuy-OMC-2 PtxRuy-OMC-3 Pt-OMC Ru-OMC

Metal fraction in at.%

Metal fraction in wt.%

Ru

Pt

Ru

Pt

0.09 0.18 0.12 – 0.24

0.03 0.03 0.15 0.08 –

0.67 1.36 0.88 – 1.78

0.43 0.44 2.13 1.16 –

3.3. N2-physisorption measurements (Pt,Ru)-doped OMC materials show similar isotherm profiles characteristic of type IV according to IUPAC classification [35] (Fig. 3) along with uniform mesopore size (Fig. 3, inset). The distribution is continual and shows that pore radius in all samples is about 2 nm. Specific surface areas calculated by BET equation (SBET) were found to be between 452–545 m2 g1 (Table 2). Mesoporous surface area (Smeso) including contribution of external surface and micropore volume (Vmic) were determined using as–plots, while micropore surface (Smic) was found by subtracting Smeso from SBET (Table 2). It was confirmed that samples are mesoporous but a large volumes adsorbed at low relative pressures suggest the presence of micropores in the samples. It is assumed that micropores were formed in the walls during the carbonization of the RF polymer. It is interesting to compare textural parameters of the OMC samples doped with Pt and Ru with the textural parameters of non-doped OMC sample reported previously (denoted as OMCSAM-800/3 [18]). For non-doped sample, which is synthesized in the same manner as the samples investigated here but without sources of metals and reducing agents, SBET was found to be 712 m2 g1 while Smic and Vmic was almost the same as for doped OMC samples presented here. This leaves Smeso to be affected by metal doping and, in fact, it was found that mesopore surface is reduced by a factor 2.5 upon OMC doping with Pt, Ru or the combination of these two metals. If one assumes that mesopores are formed between primary OMC particles and micropores in the particles one rationalize observed decrease of Smeso thorough sticking of primary OMC particles mediated by metal clusters/ particles present on the OMC surface. It is also possible that larger dopant particles might block pores and lower SBET.

Fig. 3. N2-physorption isotherms for PtxRuy-OMC-1 (SBET = 545 m2 g1) and PtxRuyOMC-3 (SBET = 452 m2 g1) samples. Inset gives corresponding pore size distribution curves.

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Table 2 Textural properties of investigated (Pt, Ru)-doped OMC samples. Sample

SBET/m2 g1

Smeso/m2 g1

Smic/m2 g1

Vmic/cm3 g1

PtxRuy-OMC-1 PtxRuy-OMC-2 PtxRuy-OMC-3 Pt-OMC Ru-OMC

545 497 452 486 532

147 169 120 130 127

398 328 332 356 405

0.185 0.195 0.215 0.174 0.186

3.4. XRPD analysis As a common feature of XRPD patters (Fig. 4) of investigated samples one finds wide and shallow graphite (002) (ICSD No. 617290) reflection at low angles characteristic for amorphous carbons with small regions of crystallinity. Characteristic reflections of hexagonal Ru (ICSD No. 43710) and fcc Pt (ICSD No. 64924) dopants are also clearly discernible, pointing that dopants are molecularly dispersed in the samples but form nanoparticles (observe diffuse and wide reflections of Pt and Ru). These reflections are superimposed onto diffuse (100) and (101) reflections of graphite. For PtxRuy-OMC-2 and PtxRuy-OMC3 samples we observed characteristic reflections of two crystalline modifications of graphite. The first one with reflections at 31.58 and 56.46 corresponds to a space group cma (67). This phase is clearly discernible in the case of PtxRuy-OMC-2 but its reflections were also found in PtxRuy-OMC-3 sample (denoted as C1, ICSC No. 88811). The second modification of graphite, having space group R-3 m (166) is found at lower extent in both PtxRuy-OMC-2 and PtxRuy-OMC-3 samples (denoted as C2, ICSD No. 53780), with a characteristic reflections at 28.44 and 45.26 . This points that in these two samples certain domains of high structural order exist. It is interesting to observe that the existence of crystalline domains in OMC samples correlates with high overall metal dopant content observed by EDX (Table 1). It is, however, unclear at this point why two mentioned graphite modifications have appeared in these two samples, but it is observed for (Pt, Ru)-co-doped samples with higher metal content. As investigated OMC materials have been produced at relatively low temperature of 800  C, high degree of

Fig. 4. The X-ray diffraction patterns of investigated metal-doped OMC samples. The identified diffraction lines were assigned to the corresponding elements observed in the samples.

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terminations, as well as other defects. Additional shoulder around 1200 cm1 has been found previously [38] and associated with double resonance process. Deconvolution of Raman spectra in these three components did not provided adequate fit of experimental data and additional band had to be included as done previously [39], which is associated with the presence of amorphous carbon, in line with the data provided by XRPD. This additional band is located around 1520 cm1 (Fig. 5, a-band). From the representative spectra given in Fig. 5 one can clearly observe that D-band dominates the signal, while small variations of the relative intensity can be observed. D-band is much wider than G-band (Full Width at Half Maximum, FWHM, 170 and 56 cm1, respectively) indicating the coexistence of well-defined graphitic domains and defect sites on the surface, the later ones being largely abundant. High level of structural disorder can be characterized thorough ID/IG ratio which was found to be around 3.1, indicating high relative concentration of amorphous carbon [40] on metaldoped OMC surface. Deconvoluted spectra also allow for the estimation of lateral size of crystallites (La), the empirical formula found by Tuinstra and Koenig [41]: IG La ¼ 4:35 ðnmÞ ID

Fig. 5. De-convoluted Raman spectra of selected Pt-OMC, Ru-OMC and PtxRuyOMC-1 samples in 1800–800 cm1 wavenumber window. Opened symbols are measured spectra, while the signals were decomposed into four components: Gband (graphitic component), a-band (amorphous component), D-band (disordered component) and a low wave number band found as a shoulder of the D band (patterned curve). Spectra were fitted with R2 > 0.99, while relative errors of the peaks are less than 0.1%.

graphitization is not expected, which agrees with the wide (002) reflection of graphite around 23 (Fig. 4). It should also be noted that the XRD patterns of the pure OMC do not indicate the existence of the two mentioned crystalline modifications of graphite [18]. Hence, there is a possibility that the presence of metal dopants during carbonization process has catalyzed structural ordering of carbon network. Such possibility seems plausible as previously Pt was found to catalyze thermally activated healing of defects produced in a graphene-ribbon network of H-irradiated glassy carbon [36]. Such healing action of Pt was observed at temperatures as low as 270  C which is much lower compared to carbonization temperature (Section 2.1). It is widely accepted that in-plane defects of graphite present the sites of increased reactivity and there is a possibility that Pt and Ru could interact with defects sites in carbon phase. However, incorporation of Pt into graphitic lattice of OMC is not very likely, as we expect that such sites would heal during the thermal treatment to recover the hexagonal graphene structure. This healing process is energetically much more favored than the metal atom interaction with the defects formed during carbonization of polymeric precursor [36]. 3.5. Raman spectroscopy Raman spectra of investigated materials (Fig. 5) display typical features of carbonaceous materials with two major bands commonly designated as G and D. The G-band, located around 1592 cm1, is due to vibration of sp2-bonded carbon atoms in a 2-D hexagonal lattice, i.e. the stretching modes of C¼C bonds in graphite) [37]. The D-band found around 1332 cm1 is associated with the presence of defects in the graphite layer, arising from sp3hybridized carbon. The disorder and defects are associated with vibrations of carbon atoms with dangling bonds in plane

(1)

For the investigated samples, we found La to be around 1.4 nm with very small variation of approx. 1% among different materials in investigated series. High level of structural disorder, presence of defects and incorporation of metallic component directly into OMC should be highly beneficial for the high catalytic activity of investigated materials, and the following section is addressed to this issue. 3.6. Electrocatalysis of oxygen reduction reaction on (Pt, Ru)-doped OMC Cyclic voltammetry of doped OMC material (Fig. 6) in de-aerated KOH solution revealed similar behavior with the main differences observable through the electrochemically active surface area, being proportional to the size of a cyclic voltammogram. Electrochemical response of investigated was found to scale approximately with the specific surface areas of the investigated materials as the largest capacitance was observed for PtxRuy-OMC1 (SBET = 545 m2 g1) and the smallest one for the PtxRuy-OMC-3 (SBET = 452 m2 g1) sample (Section 3.3). It is important to point out that for neither sample cyclic voltammetry has revealed characteristic electrochemical response of dopant metals. This means that Faradaic contribution to the overall capacitance is very small, allowing us to estimate electrochemical active surface areas of investigated materials using the capacitance evaluated from cyclic voltammetry measurements (Fig. 6). This was done assuming areal capacitance of 20 mF cm2, which is commonly accepted value. Such electrochemically determined specific surface areas range from 170 m2 g1 (PtxRuy-OMC-3) to 320 m2 g1 (PtxRuy-OMC-1). For PtxRuy-OMC-3, Pt-OMC and Ru-OMC these values are 217, 274 and 259 m2 g1. These values are lower than estimated BET surface areas (Table 2), clearly showing that pore network is not completely accessible to electrolyte, but also overrate the area of mesopores, showing that a fraction of micropores is also available to the electrolyte. This is due to the existence of suitable pore network which has a large impact on the ORR performance, as will be discussed later on. Carbon materials are rather promising electrocatalysts for O2 reduction in alkaline media [6,7] but metallic catalyst typically overate carbons in terms of ORR activity. Previously [18], we have showed that non-doped OMC-SAM-800/3 gives the ORR onset potential around –0.10 V vs. SCE, which was found to be comparable to or higher than the values found in literature for

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Fig. 6. Blank cyclic voltammetry of investigated materials in de-aerated 0.1 mol dm3 KOH at a common potential sweep rate of 50 mV s1 (left) and cyclic voltammograms of Ru-OMC in the same solution at different potential sweep rates.

various types of carbon materials [42]. For the OMC without metal dopants high ORR onset potential was explained by a relatively large number of surface defects and small crystallite size observed by XRPD and Raman spectroscopy [18]. For the materials studied in the present work, onset potentials were found to be even higher, above –0.10 V vs. SCE (Fig. 7), pointing that metallic dopants play important role in the kinetically controlled ORR region, enhancing charge transfer kinetics and possibly molecular O2 interaction with the electrocatalyst surface. Upon the comparison with the literature, one can observe that presented materials have ORR performance (in terms of onset potential) superior to that of carbon materials without metal dopant or heteroatoms [43]. When compared to some of N-containing carbon-based materials investigated recently [44–47] it can be seen that activities of (Pt, Ru)-doped OMCs are higher or, in some cases [48], comparable to that of N-containing nanocarbons. It should be noted that ORR activity of investigated (Pt, Ru)-doped OMCs is noticeable lower than the activity of advanced carbon-supported nanosized Pt catalyst where catalytically active Pt component is deposited on the external surface of carbon support [49–51]. The activity of investigated samples is also lower than the activity of

polycrystalline Pt (Pt-poly; for the detailed discussion of ORR activity of Pt-poly in alkaline media the reader is referred to ref. [52]). Differences in ORR onset potential make comparison rather difficult as there is no coincidence of kinetically controlled ORR regions. On the other hand, for such catalysts weight fraction of active metal is much higher than for the materials investigated here (usually around 20 wt.%), having large impact on its price. ORR polarization curves were subjected to Koutecky-Levich (K-L) analysis [53] which allows for the determination of the apparent number of electrons consumed per O2 molecule (n). This method of kinetic analysis enables an insight into the mechanism of ORR on particular material and presents an alternative to the rotating ring-disk technique. From the practical point of view, n is used to quantify the selectivity of O2 reduction to OH. Measured RDE current density at a given electrode potential (j(E)) can be expressed by Koutecky-Levich equation as: 1 1 1 1 1 ¼ þ ¼ þ jðEÞ jk ðEÞ jd ðvÞ jk ðEÞ B  v12

(2)

where jk(E) and jd(v) are kinetic current density and limiting diffusion current density. The constant B assembles the apparent

Fig. 7. Left: RDE polarization curves of oxygen reduction on Ru-OMC-modified GC disk electrode at different electrode rotation rates (given in rpm, revolutions per minute). Measurements were performed in O2-saturated 0.1 mol dm3 KOH and potential sweep rate was 20 mV s1. Right: Comparison of RDE polarization curves for selected material; the most active single metal doped OMC (Ru-OMC) and PtxRuy-OMC-1 as well as the least active PtxRuy-OMC-3 are presented (common electrode rotation rate of 300 rpm). Observed differences in diffusion limited region are due to different number of electrons consumed per O2.

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number of electrons transferred per O2, concentration of O2, its diffusivity, and the kinematic viscosity of the electrolyte. Hence, by determining the value of B one can determine also the apparent number of electrons consumed per O2 molecule if all the other constants involved in B are known. The sum of constants used for the analysis of ORR by means of the RDE polarization curves in alkaline media can be found in ref. [48]. Besides the selectivity of O2 reduction to OH, one can measure catalytic performance of a given material by mass activity (Imass), which is defined as a kinetic current (free of mass transport limitations) per unit mass of a material at a given potential. Imass is usually evaluated in the region of the kinetic control or in the region where diffusion limitation are low [54] and for this purpose we selected E = 0.2 V vs. SCE. Evaluated number of electrons consumed per O2 in the potential window available for K-L analysis (0.4 to 0.7 V vs. SCE) is above 2, reaching maximum values for PtxRuy-OMC-1 and Ru-OMC samples, where was found to be between 3.5 and 4 (Fig. 8, top). In this sense, doped OMCs overate non-doped counterpart analyzed previously [18]. This indicates that for these two materials ORR is being highly selective and O2 is almost completely reduced to OH. In this sense, PtxRuy-OMC-1 and Ru-OMC are comparable to some highly selective heteroatom-containing nanocarbons [8,48] and common Pt/C electrocatalysts [50]. It is important to mention that Ru is not typically considered as an electrocatalyst for ORR. However, we observe high selectivity of Ru-OMC catalyst which commensurate with the results of Mabena et al. [55] who found that for Ru-decorated nitrogen-doped carbon nanotubes prepared via thermal chemical vapor deposition and subsequent Ru

deposition using a microwave assisted reduction technique, 4epathway is favored for low Ru loading (2 and 5 wt.%) and not for high Ru loading. Mass activities of PtxRuy-OMC-1 and Ru-OMC are also among the highest, although the activity of PtxRuy-OMC-2 is the same as of Ru-OMC catalyst (Fig. 8, bottom left). By comparing the values of n and Imass of PtxRuy-OMC series, and having in mind elemental composition of these samples (Section 3.2) and specific surface areas (Section 3.3), it appears that high surface area of a catalyst dominates over the metal content for metal-doped OMC samples, being responsible for high ORR performance. As can be concluded from Fig. 8, PtxRuy-OMC-1 and Ru-OMC samples have the largest electroactive surface areas (as seen by the capacitive response under CV conditions in the absence of electroactive species) pointing to a suitable pore network which enable enhanced diffusion and transport of dissolved O2 and removal of the products formed in the pore network of cathode material. In this way, larger fraction of the catalyst surface is accessible to O2 where it gets reduced, in the first step, to HO2. Formed HO2 can undergo to further electroreduction or disproportiation to O2 and OH. Under suitable conditions, defined by appropriate pore structure, O2 can undergo to next charge transfer step, effectively increasing n above 2 [11]. According to an overview provided by Cao et al. [22], the main advantages of OMCs as a catalyst supports (Pt-based catalysts formed upon post-synthetic functionalization) are large surface area, large mesopores, uniform mesopore networks and high mesopore volume. However, it should be acknowledged that the applied in situ doping with low amount of Pt and Ru has rather large impact on ORR performance as activity

Fig. 8. Evaluated apparent number of electrons consumed per O2 molecule (n) as a function of the electrode potential (top). Bottom: mass activities of investigated materials (evaluated at 0.2 V vs. SCE) are provided and compared with the activity of non-doped OMC-SAM-800/3, ref. [18]. For the evaluation of mass activities, the ORR RDE polarization curves recorded at 2400 rpm were used (left); mass-transfer corrected ORR of investigated samples presented in Tafel coordinates (right).

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Fig. 9. The RDE polarization curves of ORR on Ru-OMC (left), PtxRuy-OMC-3 (middle) and PtxRuy-OMC-1 (right) in the absence of EtOH (dark line) and in the presence of EtOH (pale line); potential sweep rate 20 mV s1, electrode rotation rate 2400 rpm.

of investigated samples supersedes ORR activity of non doped OMC-SAM-800/3 (Fig. 8, bottom left, [18]). In the light of above discussion, metal component could enhance both charge transfer kinetics (presenting active sites for O2 reduction) and catalyze HO2 dissociation. The role of high degree of structural disorder, as observed by Raman spectroscopy (Section 3.5), and large number of defects also contributes ORR activity, as being well known from previously published studies [56]. ORR Tafel slopes for investigated materials are similar and found to be around 100 mV dec1 (Fig. 8, bottom right). At the end, we address ethanol tolerance of investigated materials, which is closely related to the applicability of investigated materials as cathode catalyst in direct alcohol fuel cells. This is an important question as during the operation of the cell alcohol (methanol, ethanol) can penetrate the membrane and cause severe poisoning of cathode catalyst due to parasitic reaction at the cathode catalyst. This effect is rather pronounced for common Pt/C catalyst, but purely carbon-based ORR catalyst show little or no poisoning when alcohol is present under ORR operating conditions [57,58]. ORR RDE polarization curves recorded in the presence of 0.1 mol dm3 of ethanol show certain reduction of the electrochemical response, indicating somewhat lower ORR activity in the presence of ethanol (see Fig. 9 for representative examples), yet being much more ethanol tolerant than common Pt/C catalyst, as it was clearly demonstrated that the presence of alcohols affects ORR activity of Pt/C to a great extent [58]. However, we have not observed faradaic current of EtOH oxidation superimposed over O2 reduction current, pointing to a specific origin of the hindered ORR performance. Though ethanol can poison metallic dopants in studied materials, it is more likely that reduced ORR activity is due to molecular adsorption of ethanol over carbonaceous surface [59,60], which reduces the number of active sites for O2 reduction. Surface defects, observed by Raman spectroscopy, being beneficial for the high ORR activity, might play important role in this case, too, as present the sites of increased reactivity and enable enhanced chemisorption properties.

4. Conclusions Series of Pt, Ru and Pt/Ru doped ordered mesoporous carbons has been synthesized using evaporation-induced self-assembly method, under acidic conditions, with resorcinol as the carbon precursor and Pluronic F127 triblock copolymer (EO106PO70EO106) as structure directing agent. Pt and Ru sources were added directly to the reaction mixture resulting in (Pt, Ru)-containing polymeric precursor. EDX analysis confirmed incorporation of metal dopants into carbonaceous matrix with total weight fractions below 3 wt.% in all cases. N2-physisorption measurements revealed specific surface areas of prepared materials between 452 and 545 m2 g1. In comparison to non-doped sample, it was concluded that the metal doping induced reduction of mesopore surface, while microporous domain remained almost unchanged. XRPD analysis revealed existence of highly dispersed Pt and Ru phases in doped samples. Based on the Raman spectra of investigated materials it was concluded that (Pt, Ru)-doped OMCs are highly disordered phases with crystallite sizes around 1.4 nm. Metal doping was found to be beneficial regarding the ORR performance of investigated materials in alkaline media. Measurements of ORR in alkaline media revealed the onset potentials of (Pt, Ru)-doped OMCs above 0.10 V vs. SCE, pointing to high intrinsic catalytic activity of investigated materials towards ORR. Selectivity of O2 reduction to OH was found to be affected by both specific surface area and the concentration of metal dopants. The most selective ORR catalysts in investigated series of materials had also the highest specific surface areas and displayed nearly 4e-pathway. Obtained results indicated that doping of OMC with small amounts of Ru, (without Pt), enhanced significantly both the ORR activity and the selectivity of doped carbon. ORR was found to be hindered in the presence of 0.1 mol dm3 of ethanol for all investigated materials, but the ethanol tolerance of (Pt, Ru)-doped OMCs was found to be higher in comparison to that of common Pt/C catalysts. The effect of reduction of ORR currents in the presence of ethanol was ascribed to the surface blockage caused by molecular adsorption of ethanol.

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