DFT and Experimental Evidences of Metal-Support Interaction in Pt

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DFT and Experimental Evidences of Metal-Support Interaction in Pt Nanoparticles supported on Nitrogen and Sulfur Doped Mesoporous Carbons: Synthesis, Activity and Stability Valentina Perazzolo, Riccardo Brandiele, Christian Durante, Mirco Zerbetto, Valerio Causin, Gian Andrea Rizzi, Isotta Cerri, Gaetano Granozzi, and Armando Gennaro ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03942 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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DFT and Experimental Evidences of MetalSupport Interaction in Pt Nanoparticles Supported on Nitrogen and Sulfur Doped Mesoporous Carbons: Synthesis, Activity and Stability Valentina Perazzoloa, Riccardo Brandielea, Christian Durantea,*, Mirco Zerbettoa, Valerio Causina, Gian Andrea Rizzia, Isotta Cerrib, Gaetano Granozzia, and Armando Gennaroa,*. a

Department of Chemical Sciences, University of Padua, Via Marzolo 1, 35131 Padova, Italy Toyota Motor Europe, Hoge Wei 33, 1930 Zaventem, Belgium

Abstract In this paper we report a comprehensive investigation of Pt nanoparticles (NPs) deposition on nitrogen and sulfur doped or co-doped mesoporous carbons (N-MC, S-MC and N,S-MC) to develop active and durable oxygen reduction catalysts for fuel cells. N-MC, S-MC and N,S-MC were prepared by employing mesoporous silica as hard template and suitable organic precursors. Pt NPs were deposited by solid state reduction of platinum acetylacetonate under N2/H2 flow on the three different supports. Pt NPs resulted to be well dispersed over the doped MC supports with size distributions (from 1.8 to 3.5 nm) that depend on the type of doping heteroatom (N or S and N,S). The influence of nitrogen and/or sulfur incorporated into the carbon matrix on Pt NPs nucleation and growth was rationalized also on the basis of Density Functional Theory (DFT) simulations. They highlighted that both nitrogen and sulfur increase the interactions between Pt and carbon support, but the interaction decreases as the nitrogen and sulfur functional groups become closer. The effect of sulfur content on Pt NPs size and activity was also evaluated. ACS Paragon Plus Environment

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Electrochemical measurements in 0.5 M H2SO4 electrolyte allow us to investigate the Pt NPs behavior and assess the relationship with electrochemical activity and stability. The Pt/S-MC showed mass activity and specific activity comparable with the state of the art commercial standard Pt/C Tanaka (Pt 46% on Vulcan XC72), and the highest catalytic activity with respect to Pt/N-MC and Pt/N,S-MC was associated to a stronger interaction between Pt NPs and thiophenic like group as proved by DFT calculations and XPS analysis. Pt/S-MC was incorporated in a membrane electrode assembly and tested as cathode material in a PEM fuel cell, while accelerated degradation tests up to 10000 voltammetric cycles were carried out in 0.5 M H2SO4: the influence of the doped support on the durability of the catalyst under harsh operational condition has been highlighted.

Keywords: sulfur, nitrogen, mesoporous carbon, doping, platinum, oxygen reduction reaction, density functional theory.

1. Introduction A large-scale polymer electrolyte membrane fuel cell (PEMFC) commercialization is currently bound to the cost reduction and to the durability of the materials used for electrodes. Both are technological challenges, which are still difficult to be met concurrently. In particular, the large amounts of Pt metal required to promote the sluggish kinetics of the oxygen reduction reaction (ORR) on the cathode side is strongly conditioning the actual cost of a PEMFC stack. Furthermore, aging phenomena of the carbon support are even more detrimental, since they lead to unacceptable performance losses, due to Pt NPs agglomeration, ripening and leaching.1 The United States Department of Energy (DOE) established a target of 20 $/kW per stack by 2020, for direct hydrogen fuel cell power systems employed in transportation. The Pt total loading on both electrodes must be reduced to 0.125 mg cm−2, with a mass activity of 0.44 A g−1 and a mass activity ACS Paragon Plus Environment

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loss lower than 40%.2 The only way to reach such and even more ambitious targets is to concomitantly increase the Pt catalyst reactivity and stability. Therefore, the development of suitable electrode support is of pivotal interest, and a lot of effort is focused to improve the morphological and chemical properties of the carbon materials. Several papers agree on the fact that the interactions occurring between the metal NPs and the support play a significant role in the final composite structure in terms of nanoparticle size and distribution, because the properties of the support directly affect the nucleation and growth processes during preparation.3–5 Furthermore, the catalyst/support chemical binding, would result in enhanced durability6–8 and intrinsic catalytic activity.9,10 The morphological aspects most heavily investigated are the high surface area and the pore dimension and interconnection. In fact, high surface area is necessary for optimizing the Pt active phase dispersion, whereas wide and interconnected pores are desirable for improving the reagents and products mass transport. The modification of the support chemical properties generally consists in the introduction of surface functional groups of heteroatoms such as nitrogen, boron, phosphorus and sulfur, a process that is commonly known as doping.11–13 Among many carbon materials, mesoporous carbons (MCs) are ideal materials for electrocatalysis, since they have large surface area, uniform and adjustable pore size, which allows a favorable mass transport and they can be easily doped. There are several examples in literature of single or doubly doped nitrogen and sulfur carbon supports including graphene,14–22 carbon nanotubes,23,24 carbon nanosheets25,26 micro and mesoporous carbon,11,27–31 etc. In this work, nitrogen- or sulfur-doped (N-MC and S-MC) and dual doped MC (N,S-MC) mesoporous carbons are modified with Pt nanoparticles (Pt/N-MC, Pt/S-MC and Pt/N,S-MC) in order to verify how heterofunctional groups affect the Pt NPs formation, activity and stability. The regular porous structure of the MC support is expected to confine the Pt NPs so that sintering is prevented as well as leaching effects, whereas the nitrogen and sulfur functional groups should trigger Pt NPs nucleation and growth. Actually, nitrogen and sulfur doped carbons

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are reported to be promising supporting material for metal deposition and activity towards ORR, even though data in the literature are somehow contradictory and often unclear.6,32–35In this paper we aim at making a step forward in understanding the effect of both nitrogen and sulfur on the Pt NPs activity and stability and whether N,S co-doping would synergically increase or hinder the Pt NPs performances. Actually, the dimension and shape of the employed MC particles are the same regardless of the dopant atoms, so that a fair comparison can be made on the basis of the sole chemical properties of the differently doped MCs. On this regard a Pt/MC catalyst was also prepared with sulfur present only in traces and used as reference to allow a better comparison and a clearer elucidation of dopant content on the metal-support interaction. Furthermore, density functional theory (DFT) calculations on doped carbon model surfaces contributed, with an adequate molecular basis, for better understanding the specific catalyst-support interactions, which occur during the nucleation and growth of Pt NPs. While DFT simulations investigating the adsorption and binding interactions between Pt and both nitrogen and sulfur doped graphene10,16 exist in the literature, there is an overall lack of fundamental understanding regarding the Pt – co-doped carbon interactions and the associated effect on ORR activity and durability. These findings were used to explain the activity and the stability of Pt on doped MCs, in comparison to the commercial standard Tanaka (Pt/C TKK).

Experimental 2.1

Chemicals

Mesoporous silica (Sigma-Aldrich, 200 nm particle size, 4 nm pore size), 1,10-phenanthroline (Sigma-Aldrich, 99.5%), phenothiazine (Sigma-Aldrich, 98%), dibenzothiophene (Sigma-Aldrich, 98%), sucrose (Sigma Aldrich >99%), Pt(acac)2 (Sigma Aldrich >97%) Nafion (Sigma Aldrich, 5 w% in EtOH), H2SO4 (Fluka, Traceselect® >95%) and acetone (Fluka, HPLC grade) were used as


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received without further purification. Alpha Gaz O2 and Ar were supplied by Air Liquid at the highest available purity (>99.99%).

2.2

Syntheses.

The synthesis and characterization of doped MCs are exhaustively reported in previous papers.11,31 The synthesis employs commercial mesoporous silica as templating agent and 1,10 phenanthroline, dibenzothiophene and phenothiazine, as organic precursors for the nitrogen doped, sulfur doped and nitrogen-sulfur co-doped mesoporous carbon, respectively. In the following, the resulting doped MC are named according to the heteroatoms content as N-MC, S-MC and N,S-MC. Sulfur doped mesoporous carbons at different percentages of sulfur were also prepared starting from a different dibenzothiophene/sucrose ratio i.e. 70/30, 50/50 and 30/70 and referred in the text as S-MC1, SMC2 and S-MC3, respectively. A further mesoporous carbon, labelled as MC, was prepared by employing only sucrose as carbon source and taken as reference undoped carbon. Pt NPs were deposited on the three differently doped MCs (hereafter indicated as Pt/N-MC, Pt/S-MC and Pt/N,S-MC) as well as on the S-MCx of different sulfur content (Pt/S-MC1, Pt/SMC2, and Pt/S-MC3) and on the undoped reference (Pt/MC), by a solid state deposition employing Pt(acac)2 as Pt precursor and H2 as reducing agent (Figure 1). Pt(acac)2 is more soluble than other Pt commercial salts in organic solvents, favoring the dispersion and the impregnation inside MC mesopores.35,36 The starting Pt precursor was weighed in order to obtain a catalyst powder with a final Pt loading of 25%. In a typical experiment 32.14 mg of Pt(acac)2, and 53.21 mg of doped MC were mixed together, dispersed in acetone and sonicated for 1 h. The resulting slurry was dried in an oven at 80 °C for 2 h, allowing the drying of the mixture and the obtaining of a fine powder. The mixture was then treated in a tubular furnace (Carbolite, UK) at high temperature under a H2/N2 flow of 2/23 standard cubic centimeter per minute (sccm). Before starting the heat treatment, the furnace was purged by fluxing N2 (30 sccm) for 1 h at room temperature (r.t.). Then, the temperature was allowed to increase to 300 °C and kept constant for 3 h; at the same time H2 was ACS Paragon Plus Environment

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gradually fluxed into the quartz tube to the desired ratio. Eventually, the furnace was allowed to cool down to r.t., while the H2 flow was switched off, allowing only N2 to flow inside the reactor. The obtained catalyst was then removed from the quartz boat, ground in an agate mortar and recovered in a glass vial.

Figure 1 Methodology scheme for the synthesis of Pt NPs on doped MC.

2.3

Electrochemical tests.

Electrochemical measurements were recorded using an Autolab Potentiostat/Galvanostat 101N in a three-electrode configuration cell, in 0.5 M H2SO4 with milliQ water at 25 °C. Glassy carbon electrodes (GC) as rotating disk electrode (RDE) (5 mm diameter, geometric surface area 0.196 cm2), were used as working electrode (WE). GC was preliminary polished using diamond pastes (3 µm, 1 µm, and 0.25 µm). The reference electrode was a reversible hydrogen electrode (RHE) or a saturated calomel electrode (SCE), whereas a Pt ring was used as counter electrode. Unless otherwise stated, all the potentials are reported versus RHE, which was freshly prepared before each experiment.36 The catalyst inks were prepared to obtain a Pt loading on the electrode of 15 µgPt/cm2; Nafion and isopropanol contents were optimized and ultrasonicated for 3-4 hours in order to obtain a uniform film thickness. Then 10 µL of suspension were carefully pipetted onto the clean GC electrode and allowed to dry under rotation at 700 rpm at room temperature.37 The electrolyte was ACS Paragon Plus Environment

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purged with Ar before each measurement, whereas for the ORR test, the electrolyte was bubbled with high-purity O2 gas for at least 30 min to ensure O2 saturation. The electrocatalysts were firstly activated by performing 300 voltammetric cycles at 50 mV s-1 between 0.05 to 1.2 V vs. RHE or until obtaining stable cyclic voltammograms (CVs). The ORR activity was evaluated by recording linear sweep voltammograms (LSVs) at a scan rate of 20 mV s-1 in the potential range 0.05–1.05 V vs. RHE in O2-saturated electrolyte using the RDE technique. The kinetic current density was calculated with the Koutecky-Levich (K-L) equation (Eq. 1):

ν

where j is the current density, jk the kinetic current density, jd the diffusion current density,

(1) the

concentration of molecular oxygen, ν the kinematic viscosity and ω the angular rate of RDE.5,38 The kinetic current density

was determined using a rotating electrode rotated at 1600 rpm. The current

density is taken at 0.9 V vs. RHE and corrected by mass-transfer according to Eq. 2 !

"

!

#

(2)

The electrochemical platinum surface area (EPSA) was calculated by the coulometry of the hydrogen under-potential deposition (Hupd), using a charge of 210 µC cm-2.39 The specific electrochemical surface area (ECSA), ORR specific activity (SA) and mass activity (MA) are defined according to Equations 3-5.

$%&' &' 3'

()*+ , -./ 0 1+

()*+ 2, + -./ 0

(3) (4) (5)

The SA of the ORR was calculated from the positive going RDE polarization curves recorded at a scan rate of 20 mV s−1. In order to exclusively analyze the ORR current, the RDE polarization curves were corrected by subtracting background surface oxidation and capacitive processes. This involves subtraction of the background CV recorded in Ar saturated electrolyte (obtained using the ACS Paragon Plus Environment

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same experimental parameters, i.e., scan speed, rotation rate, potential window) from the ORR polarization curves. Furthermore, the IR-drop was compensated by positive feedback. Catalyst durability was tested in accelerated stability tests by cycling the potentials in the range 0.6–1.0 V (vs. RHE) in O2 saturated 0.5 M H2SO4 solution at a scan rate v = 0.5 V s−1; ORR polarization curve and the EPSA change of the NP catalyst were recorded after each 2000 voltammetric cycles and results were compared to Pt/C TKK standard in the same conditions. The catalysts Pt/S-MC was incorporated in a MEA in order to be tested as cathode in a PEM fuel cell. The preparation of the MEAs was done by IonPower (New Castle, DE) according to inhouse protocols. The MEAs possess a Pt loading equal to 0.1 mg/cm2 at the anode and 0.3 mg cm−2 at the cathode. The different loading is due to the different kinetics for oxygen reduction (cathodic reaction) and hydrogen oxidation (anodic reaction). The MEA consists in a 50 µm thick Nafion membrane, supporting the commercial Pt/C catalyst on the anodic side and the Pt/S-MC on the cathodic side. The commercial Pt/C TKK was incorporated in a MEA, with the same properties, in order to realize a comparison between home-made catalysts and a commercial material.

2.4

Characterizations.

X-ray photoemission spectroscopy (XPS) measurements were performed in a UHV chamber (base pressure < 5×10−9 mbar), equipped with a double anode X-ray source (Omicron DAR-400), a hemispherical electron analyzer (Omicron EA-125) at r.t., using non-monochromatized Mg-Kα radiation (hν = 1253.6 eV) and a pass energy of 50 eV and 20 eV for the survey and the high resolution spectral windows, respectively. The calibration of the Binding Energy (B.E.) scale was carried out using Au 4f as reference (B.E. Au 4f = 84.0 eV). The XPS peaks of Pt, N and S were separated into single chemical shifted components (after Shirley background removal), using symmetrical Voigt functions; the χ2 was minimized by the use of nonlinear least squares routine. To perform XPS measurements, 2.5 mg of the Pt/MCs powders were dispersed in milliQ water and then gently sonicated (for 45 min) in order to efficiently disperse the powders; the solutions were ACS Paragon Plus Environment

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then drop-casted on electropolished polycrystalline copper or GC substrates (with a surface area of 1 cm2). Thus, the samples were first dried overnight under nitrogen flux to obtain homogeneous films; then they were vacuum-dried for 2 h at about 10-6 mbar. X-ray Diffraction (XRD) patterns were recorded in the diffraction angular range 5–85° 2θ by a Philips X’Pert PRO diffractometer, working in the reflection geometry and equipped with a graphite monochromator on the diffracted beam (Cu-Kα radiation). Thermogravimetric analysis (TGA) was performed using a Q5000IR (TA Waters) on 10 mg samples, at a heating rate of 10 °C/min from room temperature to 1000 °C in a N2 environment, to determine their thermal stability. Elemental analysis was carried out to determine the percentage of heteroatoms in the material, by using a Thermo Scientific Flash 2000 analyzer. Inductively coupled plasma mass spectrometry analysis (ICP-MS) for the determination of Pt content was carried out with an Agilent Technologies 7700x ICP-MS (Agilent Technologies International Japan, Ltd., Tokyo, Japan). A Microwave Digestion System (CEM EXPLORER SP-D PLUS) was used for the acid digestion.36 Transmission electron microscopy (TEM) images were obtained by using a FEI Tecnai G2 transmission electron microscope operating at 100 kV.

2.5

Quantum mechanical calculations.

Model doped surfaces have been prepared starting from a single, square graphene sheet with 8x7 carbon atoms per side, and saturated with hydrogen atoms at the borders (see Figure S3 and S4 in the Supporting Information). Single and double defects have been then introduced in the middle of the sheet. Three types of N defects have been considered: graphitic, pyridinic and pyrrolic, while the S-type defect was thiophenic. Co-doped surfaces were generated mixing pyridinic-pyrrolic, thiophenic-pyridinic, and thiophenic-pyrrolic defects. The N,S co-doped surfaces have been modeled with the two defects placed at three different distances in order to inspect the nitrogensulfur synergetic effect on Pt adsorption. A total of 12 surfaces (counting the pure graphene, also) have been simulated. ACS Paragon Plus Environment

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The adopted computational protocol was the following: first, each of the model surfaces has been energy minimized. Then, a single-point energy calculation has been carried out after placing a Pt atom 20 Å far away from the center of the surface, along the surface normal. The presence of the Pt atom is required in the adsorption energy calculation in order to avoid the basis set superposition error. Finally, the Pt atom has been moved at 2 Å from the middle of the surface (to simulate the approach of Pt to the surface close to the defect) followed by energy minimization. For those situations in which the defect caused an asymmetric deformation of the surface, two simulations have been performed placing the Pt atom “over” and “under” the surface, where with “over” is intended that the Pt is on the same side of the N atom (or S atom in thiophenic-only defect). The interaction energy has been evaluated as the difference between the energy of the surface+Pt system and the energy of the surface only, with the Pt atom far away. Since the model does not allow to distinguish a priori the two possible “over” and “under” configurations of the surface to which the Pt atom approaches, in order to (even if qualitatively) compare with experimental observations we calculated the adsorption energies as the average between the two configurations. All calculations have been performed at Hartree-Fock/DFT-D level (with D3 dispersion correction40,41) using the TeraChem 1.9 software package.42,43 The B3LYP functional has been employed for DFT calculations, while a mixed basis set has been used for the atomic orbitals. In particular, the 6-31G(d,p) basis set for H, C, N, and S atoms, while the LANL2TZ for Pt. Convergence of the self-consistent field cycle has been reached posing the two-electron integral threshold at 10-12, and employing the hybrid DIIS/A-DIIS scheme.42,43 Also, minimizations have been carried out in Cartesian coordinates.

3. Results and Discussion.

3.1

Morphological and chemical characterization of Pt on doped MCs.


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Pt NPs were deposited on the three differently doped MCs (N-MS, S-MC and N,S-MC), that have similar morphological aspect and properties, but different content of nitrogen and sulfur heteroatoms. In particular, the synthetized carbon support are nitrogen doped (N-MC, N = 7.32 w%), sulfur doped (S-MC, S = 13.79 w%), or nitrogen and sulfur co-doped (N,S-MC, N = 4.51 w% and S = 4.12 w%), respectively (Table 1).11,31 Doped MCs consist in spherical carbon particles characterized by a mean radius of 150 nm and with aligned pores of average radius of c.a. 4 nm. Two further peaks are centered in the mesoporous region at 8.4 nm and in the microporous range. The presence of micropores and mesopores with high pore volume confers a very high BET surface area to all three support: 881, 1183 and 855 m2/g for N-MC, S-MC and N,S-MC, respectively (Table S1).11

Table 1- Chemical and physical properties of Pt/doped-MC and Pt/C standard. Catalysts Pt/C TKKe Pt/N-MC Pt/N,S-MC Pt/S-MC

Na (%) -7.32 4.51 0

Sa (%) -1.02 4.12 13.79

β1b 4.08 1.83 1.57 2.23

dcc (nm) 2.4 5.5 6.4 4.3

Ptd (%) 46.2 24.1 35.7 29.4

a

Element content determined by elemental analysis, percentage are referred to the weight content. bHalf peak width determined from the XRD peak at 67.6 2θ. cCristallite size determined from Debye-Scherrer equation dDetermined by ICP analysis. eTanaka standard catalyst with a Pt loading of 46%.

Pt NPs were deposited according to a solid state reduction employing Pt(acac)2 as platinum precursor. The optimal deposition was carried out at 300 °C for 3 h under N2/H2 flow. The advantage in using Pt(acac)2 is the low sublimation temperature that at 760 mmHg corresponds to 187.6 °C; therefore the Pt precursor can easily diffuse inside MC pores and a homogeneous nucleation of NPs can be obtained inside and outside pores. Furthermore, the resulting catalyst does not need tedious and time consuming purification steps for removing chloride, sulfates or byproduct coming from the most common wet reduction with NaBH4. Figure 2 reports the TEM measurements at different magnification and the particle size distribution for the three catalysts Pt/N-MC, Pt/S-MC and Pt/N,S-MC. Pt/N-MC and Pt/S-MC show round shaped Pt NPs with high ACS Paragon Plus Environment

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loading and uniformly dispersed throughout the samples, without the formation of aggregates. The corresponding particle size distribution histograms (Figure 2c,f and i) were obtained by calculating the size of more than two hundred randomly selected particles in the magnified TEM images. For Pt/N-MC and Pt/N,S-MC samples, the mean Pt particle size diameter is centered at ca. 2.5 nm and 3.5 nm, respectively; in the case of Pt/S-MC even lower dimension (1.8 nm) and narrower size distribution were obtained. It is worth noting that in the case of Pt/N,S-MC some big clusters are also visible on the N,S-MC surface. ICP analysis revealed a different Pt loading among the three differently doped MC; this effect was attributed to the different thermal stability of the carbon support during thermal treatment in the presence or in the absence of Pt, which is known to catalyze the carbon combustion. In particular the degradation temperature decreases from 610 °C in the case of N-MC to 370 °C in the case of Pt/N-MC (Figure S1). Figure 3a shows the X-ray diffraction (XRD) patterns of Pt/C TKK and Pt NPs on differently doped MCs. All of the doped samples exhibit a wide diffraction peak at ca. 24.3°, which is related to graphitic materials and is indicative of the presence of amorphous carbon and small graphitic domains. The diffraction peaks at the Bragg angles of 39°, 46°, 67° and 81° correspond to the (111), (200), (220), and (311) facets of Pt, respectively, which suggests that Pt NPs on all the samples have the same face centered cubic (fcc) crystal structure. The diffraction peaks for Pt catalysts can be used to estimate the Pt particle size by measuring the full width half maximum of the (220), Pt XRD peak, using the Debye-Scherrer equation,

4

567 89: 2;< =>?@

(6)

where L is the mean size of Pt crystallites, ABC is the X-ray wavelength (1.5418 'D), E,FG is the angle of the peak, and H I is the half-peak width. Platinum crystallite sizes are 4.3, 5.5 and 6.4 nm for Pt/S-MC, Pt/N-MC and Pt/N,S-MC, respectively, which results higher to what observed from TEM results although the dimension trend is in agreement.


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The interactions occurring between a catalyst material (i.e. Pt) and the support play a significant role in terms of nanoparticle size and catalysts dispersion because the properties of the support directly affect the nucleation and growth processes during preparation. Several papers report that nitrogen-containing carbon can tremendously enhance the affinity of noble metal precursors onto carbon supports, via electrostatic interactions, and that the critical size of the clusters (nucleus with equal probability for growth and dissolution), and therefore the particle dimension, depends on the metal-support electronic interaction.3–5,15,32,44–47 Similar evidences are reported about the effect of sulfur on the nucleation and growth of Pt NPs.16,17,19 Conversely, much less information are available on the synergetic activity of nitrogen and sulfur when simultaneously present on the same materials.


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Figure 2. TEM and NPs size distribution of: (a-c) Pt/N-MC, (d-f) Pt/S,N-MC, (g-i) Pt/S-MC.

In Figure 3c the XPS data in the N 1s region of Pt/N-MC is reported along with the fitting of the N 1s peak. Four different components at different relative populations centered at 398.5 eV (pyridinic, 20%), 399.6 eV (pyrrolic, 24%), 400.7 eV (N-graphitic, 41%) and 401.5 eV consistent with oxidized nitrogen defects (15%), have been identified.11,48 If we compare the XPS N 1s components before (Figure 3b) and after the Pt deposition in a reducing environment (Figure 3c), we can observe the decrease in the amount of the oxidized nitrogen by 30% and a corresponding increase of the pyridinic nitrogen, the ratio between the other components remaining unchanged (Figure 3c). ACS Paragon Plus Environment

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Figure 3d provides a high resolution XPS scan of the S 2p signal that is composed of two peaks the 2p3/2 and the 2p1/2 components, and was fitted with a doublet considering a spin orbit separation of 1.20 eV and an intensity ratio of 2:1, as theoretically determined. The S 2p signal can be attributed to sulfur bonded directly to the carbon atoms in a thiophenic like heterocyclic configuration.11,31 The comparison of the S 2p region before and after Pt deposition does not reveal any particular change of the S 2p peak signal (Figure 3e). In Pt/N,S-MC both nitrogen and sulfur components were observed in the XPS spectra and the identified surface-species are the same as in Pt/N-MC (pyridinic 35%, pyrrolic, 28% and N-graphitic, 37%)) and Pt/S-MC (Figure S2). Therefore thiophenic, pyridinic, pyrrolic and graphitic nitrogen are the most abundant hetero-defects present in the three catalysts.


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Figure 3. a) X-Ray diffraction patterns of the Pt NPs loaded on differently doped carbon: (1) Pt/C TKK, (2) Pt/N-MC, (3) Pt/N,S-MC and (4) Pt/S-MC. XPS spectra and deconvolution into single chemical components of N 1s XPS region for Pt/N-MC before (b) and after (c) Pt deposition. XPS spectra and deconvolution of S 2p XPS region for Pt/S-MC before (d) and after (e) Pt deposition. XPS spectra and deconvolution of Pt 4f region for (f) Pt/C, (g) Pt/N-MC, (h) Pt/S-MC, (i) Pt@N,SMC.

To investigate and understand the effect of the support for differently doped carbon materials, a 4 × 4 supercell of graphene (Figure S3a) was used in the DFT calculations. Pyridinic, pyrrolic, graphitic nitrogen and thiophenic defects were modeled (Figure S3 and S4) and listed in


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ACS Catalysis

Table 2. The descriptor that is usually considered for representing the nucleation energy for platinum growth on the defect site is the adsorption energy between Pt and carbon (Eads).8,10,16,49,50 The interaction energy, defined as the energy difference between the two optimized structures (∆Eads,Pt), where the Pt atom is far and close, respectively to the doped graphitic surface are listed in Table 2. In Table 2 the mean values for the two-possible situations in which the defect caused an asymmetric deformation of the surface are also reported, so that Pt atom is placed “over” and “under” the surface, where with “over” is intended that the Pt is on the same side of the N or S atom. If a single defect is considered, the results of the calculations indicate that a Pt atom is stabilized according to the following interaction order : pyrrolic > N graphitic > thiophenic > pyridinic > graphene.10,16,19 Both nitrogen and sulfur involve a clear stabilization of the Pt atom with respect to a graphene layer by almost 100 kJ mol-1. The higher stabilization calculated when Pt is “over” the nitrogen and sulfur singular defects can support the experimental evidence that smaller Pt NPs are found on those surfaces, where smaller Pt critical nuclei are stabilized. In Pt/N,S-MC bigger Pt NPs were observed (3.5 nm) and this would be the result of a less stabilized Pt nucleus on nitrogen and sulfur co-doped surface with respect to nitrogen or sulfur singularly doped surface. For describing a N and S co-doped surface thiophenic and pyrrolic defects were concomitantly considered on the same graphene surface in three possible positions: near, halfway and far, with the idea of evaluating how the interaction between the two defects affects the Pt stabilization energy (Table 2). Furthermore, the Pt atom was considered in both “over” and “under” positions with respect to the thiophenic defect (Figure 4). Data reported in Table 2, highlight that ∆Eads,Pt for the co-doped model is lower than the surface with only the pyrrolic defects. Moreover, the Pt atom is more strongly interacting when the thiophenic group is farther from the pyrrolic group. Therefore, the close proximity of the two heterodefects would destabilize the formation of small Pt nuclei and this is in agreement with the experimental findings, where it is supposed that a high concentration of thiophenic and pyrrolic defects implies the vicinity of the twos. In the pyridinic-thiophenic co-doped model surface, instead, the “over” ∆Eads,Pt is nearly ACS Paragon Plus Environment

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similar in the three relative distances between the two groups. In this case the average ∆Eads,Pt value is higher with respect to the cases of singular thiophenic and pyridinic defects.

Table 2- Single atom Pt adsorption energy calculated via DFT as a function of the adsorption site and adsorption energies estimated using our simple classification methodology.

Graphene Graphitic Pyridinic Pyrrolic under Pyrrolic overc Thiophenic underd Thiophenic over Pyrinic-pyrrolic S-Npyrrolic V, over S-Npyrrolic V, under S-Npyrrolic HW, over

∆Eads,Pt(calc.) kJ mol-1 −156 (l) −335 (h) −176 (l) −394 (h) −394 (h) −215 (m) −264 (m) −156 (l) −174 (l) −322 (h) −283 (h)

S-Npyrrolic HW, under

−252 (m)

S-Npyrrolic F, over S-Npyrrolic F, under

−292 (h) −363 (h)

S-Npyridinic V, over

−244 (m)

Adsorption sites

∆JK ads,Pta kJ mol-1 −156 −335 −176

Changesb BED, 2×PtC, LSD 2×PtC, PtH, CH PtC, LSD 2×PtC, PtH, CH Formation of PtH2 PtC, PtH, CH, T PtS PtN, LSD, CR PtN, CR, SR 2×PtC, PtH, CH, T 2×PtC, PtH, CH, CR 2×PtC, BED, SR, LSD 2×PtC, PtH, CH, H 2×PtC, LSD 2×PtC, BED, SR, LSD 2×PtC, BED, SR, LSD 2×PtC, PtH, CH, CR 2×PtC, BED, SR, LSD 2×PtC, PtH, CH, LSD 2×PtC, PtH, CH, LSD, CR, T

−372 −240 −156 −248 −268 −328 −242

S-Npyridinic V, under

−239 (m)

S-Npyridinic HW, over

−299 (h)

S-Npyridinic HW, under

−239 (m)

S-Npyridinic F, over

−282 (h)

S-Npyridinic F, under a

−229 (m)

−269 −256 b

∆Eads,Pt(est.) kJ mol-1 −180 (l) −350 (h) −180 (l) −350 (h) / −130 (l) −300 (h) −130 (l) −180 (l) −330 (h) −300 (h)

|%Err|

−210 (m)

17

−270 (m) −380 (h)

8 5

−210 (m)

14

−210 (m)

12

−300 (h)

0.3

−210 (m)

12

−330 (h)

17

−260 (m)

14

15 4 2 11 / 40 14 17 3 2 6

Mean values calculated between under and over configuration, Contributions considered in the estimation of the energy: (PtX) Formation of a bond with S, C, N or H, (SR) strain/steric relaxation, (B) bridge adsorbed Pt, (LSD) local surface deformation, (T)atop adsorbed Pt, (CR) charge repulsion, (H) hollow Pt, (CH) breaking of a C-H bond, (BED) break of electron delocalization; cIn the pyrrolic (over) simulation it is observed the formation of PtH2, which is not adsorbed, and thus the model does not apply here. dThe thiophenic (under) configuration can be considered an outlier, showing an error twice higher than the highest error for the other surfaces. Inspecting the optimized geometry, the Pt atom is sitting atop a C atom that should be in sp3 hybridization, but a distorted geometry with a 90° H-C-Pt angle is observed. Also, this is the only case where the H atom is placed in “hollow” position over the 6C ring. This surface requires further study to rationalize what is happening upon adsorption.


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Figure 4. DFT computational simulation of mixed thiophenic–pyrrolic in the three surface position near (a,b) halfway (c,d) and far (e,f) and the two possible Pt atom configuration “over” (a,c,e) or “under” the surface (b,d,f). Answering to the question why the Pt atom is better adsorbed in the presence of certain defects of the graphene surface is complicated, since the difference of the chemical environment does not allow determining a common “reference” for all the defective surfaces. Moreover, the interplay of different electronic effects is not easily partitioned into the contributions of the single actors. However, two important details can be identified as the most relevant in distinguishing the force of the adsorption. Firstly, the availability of hydrogen atoms near the point of adsorption of the Pt atom. Such hydrogen atoms may be found around the defect to saturate the eventually dangling bonds of the C, N or S atoms formed during the creation of the defects. The incoming Pt atom is always observed to capture one or two H atoms if they are present in its surroundings. Secondly, bonding with Pt always occurs with more or less important deformations of the surface


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(e.g., as a result of double bonds breaking or change in the hybridization state of the defect). In some cases, such a deformation of the surface may be very important especially if the surface, in the absence of Pt, is highly stabilized by electrons delocalization. Quite relevant is the case of graphene, where the bonding of Pt with two C atoms implies the breaking of an aromatic double bond, followed by the corrugation of the planar surface due to the sp3 hybridization of the two C atoms. This makes the effective gain in energy, due to adsorption, the lowest with respect to the other (defective) surfaces, where the marked surface deformations (and thus breaking of the long range electronic delocalization) is the result of the presence of defects independently from the presence of the Pt atom. The results of the calculations were interpreted in terms of electron density difference maps and atomic partial charges distributions. The former quantity is obtained by subtracting from the electron density of the whole system (surface + Pt atom) the densities obtained by removing the surface or the Pt atom. These hypersurfaces highlight how the electron density is modulated due to the interaction of the Pt atom with the surface. Figures 5 and 6 show electron density difference maps plotted at ± 0.001 Bohr-3 (1 Bohr = 0.529 Å) isovalue for the series of single-defect and double-defect surfaces, respectively. Orange surfaces enclose regions in space where the electron density increases, while cyan regions are indicative of a reduction of the electron density upon adsorption. The orange regions in space can be interpreted as the regions where bonds are formed, while the cyan regions can be interpreted as those interested in the bonding by providing electrons. Figures S5 and S6 show as a color map the distribution of Mulliken partial charges for single-defect and double-defect surfaces, respectively. The color scale ranges from -0.8e (in red) to +0.8e (in blue), with e the elementary charge. Neutral atoms are colored in light gray in the figures.


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ACS Catalysis

Figure 5. Difference electron density isosurfaces plotted at ±0.001 Bohr-3 isovalue for the singledefect surfaces. Orange (cyan) surfaces enclose regions in space where the electron density in increased (decreased) upon adsorption. Adsorption energies are reported in the figure with a color code identifying high (red), medium (cyan), and low (green) surface-Pt interaction.

Figure 6 Difference electron density isosurfaces plotted at ±0.001 Bohr-3 isovalue for the doubledefect surfaces. Orange (cyan) surfaces enclose regions in space where the electron density increased (decreased) upon adsorption. Adsorption energies are reported in the figure with a color code identifying high (red), medium (cyan), and low (green) surface-Pt interaction.


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In order to discuss what is observed from the above described analysis in connection with the Pt adsorption capacity of the surfaces we have distinguished three levels of interaction, built around the average Pt-C bond energy of 245 kJ mol−1 obtained via calorimetric data.51 In particular, we have defined “high interaction” when the adsorption energy is < −280 kJ mol−1, “medium interaction” for energies in the −200 - −280 kJ mol−1 range, and “low interaction” if the energy is > −200 kJ mol−1. Figures 5, 6, S5 and S6 report the calculated adsorption energy with a color code for the high, medium, or low interactions, in particular: red, cyan, and green, respectively. In highly adsorbing surfaces, it can be observed that the Pt atom is bonded to one or two of the surrounding H atoms, situation that allows the Pt atom to increase its oxidation state. Within the “high interaction” series, the presence of S close to the Pt increases the adsorption capacity. This is due to the ability of S to transfer electrons to the Pt atom. As seen in Figures 5 and 6, large amount of electron density is transferred from S to Pt when they are close each other. Moreover, Figures S5 and S6 show that the S atom has always a partial positive charge. Based on these findings, it appears that a thiophenic defect serves as preferential anchoring site for Pt cluster nucleation and subsequent NP growth and this may justify the very small cluster observed in Pt/S-MC (1.8 nm). The double halfway or far S-N defective surfaces, labeled as highly adsorbing, show very similar adsorption energies, apart the thiophenic-pyrrolic defect in the far/under configuration where the Pt atom is bonded to 2 C atoms and no H and the close 1 S atom is able to provide electron density. Medium interaction situations have in common the fact that the Pt atom is in a lower oxidation state, with no H atoms to bond, while it is bonded to 2 C atoms, or 1 S atom. The S-N double-defect surfaces show very similar adsorption energies, with destabilization provided by the closeness of the N atom as clearly visible in the vicinal thiophenic-pyridinic surfaces. The “under” configuration shows a nitrogen atom close to the Pt, while in the “over” configuration the N atom is further away, and the S atom is closer to the Pt. This fact makes the adsorption energy higher (in


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ACS Catalysis

absolute value) than the “under” configuration. It is worth noting that the N atoms have always a partial negative charge (Figures S4 and S5). Finally, S-N double-defect models behaving as low interaction surfaces show again no H atoms and the Pt atom bonded only to 1 C atom, or to 1 N atom. Therefore, on one hand the trend of the stability with the oxidation state seems to be confirmed; on the other hand, the presence of more N atoms (such as in the mixed pyrrolic-pyridinic defective surface) reduces the adsorption capability of the surface. Observing Figures S5 and S6 it can be said that the larger electronegativity of N with respect to C reduces the tendency of the carbon atoms to bond the Pt atom, which remains in a lower oxidation state. The pure graphene surface has to be taken as a special case, as observed above; in fact, while the Pt atom bonds 2 C atoms, most of the energy gained by the bonding is lost in destroying the flat, highly delocalized graphene surface. In all the other cases, such a strong deformation contribution is not observed and can be estimated in contributing to only differences on the order of 10-20 kJ mol−1 (e.g., the deformation of the thiophenic ring). To summarize, we propose a tentative rationalization of the results of the simulations classifying a number of changes that occur upon adsorption and assigning to each of them an energy contribution, both stabilizing or penalizing. Energy contributions can be easily obtained examining the geometry of the isolated surface, the geometry of the surface with the cluster adsorbed and the distribution of charges; the classification is reported in detail in Table S2 of the SI. The results of the this analysis suggest that the adsorption energy is affected by the interplay of the following effects: i) the number of bonds with the surface atoms (C, N, S), ii) the formation of Pt-H bonds, iii) the eventual break of electronic delocalization, iv) the local deformations of the surface observed upon adsorption, v) the partial charges on the atoms, and vi) the top, bridge or hollow position of the Pt atoms closest to the surface (with the bridge as the most favorable).52 The first two effects are expected to give a stabilizing effect of the order of 102 kJ mol-1. It is worth noting that the adsorption energy increases in absolute value because of bonding in the order: PtS (−300 kJ mol-1) > PtN (−200 kJ mol-1)≈ PtC > PtH (−80 kJ mol-1). A further stabilizing contribution derives from the ACS Paragon Plus Environment

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release of a ring strain or a reduction of steric contributions. The remaining terms are all destabilizing. The breaking of the electron delocalization, and the breaking of a C-H bond are expected to be destabilizing with an increase of the energy of the order of 102 kJ mol-1. The remaining effects are all destabilizing, but with a smaller contribution of the order of 10 kJ mol-1. Electrostatic repulsion is estimated to be of the order of 50 kJ mol-1. These energy contributions have been estimated using knowledge of the orders of magnitude of the different events occurring upon adsorption, and then making some corrections to recover the trends from the calculations. Table 2 reports the estimation of the adsorption energies made using our empirical, rough, classification. It can be seen that the estimation catches the high, medium or low interaction classification that has been introduced above. The average error in the energy estimation is 10%, with maximum absolute relative error of 17%. Only the adsorption of Pt atom to the pyrrolic “over” surface is badly described. An extended exploration of Pt clusters – graphene surfaces by DFT calculations will allow refining this semi-empirical classification that we believe can be a useful tool as a rough, but practical, guide to rationalize and/or design the deposition of transition metal clusters over graphene surfaces. Clearly more complicated models are necessary for evaluating more quantitatively the favorable or unfavorable formation of adsorption site for Pt nuclei, however, it is clear that the PtS bond formation is the most stabilizing effect and this is in accordance with the formation of small Pt NPs. Furthermore, it seems that the Pt nuclei interact preferentially on the carbon surface, when the thiophenic and nitrogen defects are as far as possible from each other since the large electronegativity of N inhibits the tendency of the carbon atoms to bond the Pt atom. Therefore, it may be asserted that N and S co-doping favors the Pt atom adsorption and the formation of stable small nuclei when the two types of defects are almost non interacting as it is in Pt/N-MC and Pt/SMC. Figure 3 f-i show the photoemission from orbital 4f of Pt for Pt/MC, Pt/N-MC and Pt/S-MC. The three deconvoluted signals are associated to Pt(0), Pt(II) and Pt(IV), the most common ACS Paragon Plus Environment

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ACS Catalysis

oxidation states of Pt, characterized by increasing B.E. values. Each signal is split because of the spin-orbit interaction. Usually, metal support interaction results in a B.E. shift of both metal and support XPS signals due to a density charge transfer from the metal to the surface and vice versa. Kwon et al. observed a larger FWHM of the Pt 4f peak with a 0.3 eV shift towards higher B.E. for Pt NPs supported on sulfur containing ordered mesoporous carbon.28 The interaction of thiophene with Pt(111) surfaces was also studied by Stör et al. by NEXAFS.53 The authors verified that when thiophene is chemisorbed on Pt(111) a cleavage of the C-S bond occurs above 180 K so that the S atom is replaced by a Pt surface atom with the formation of a metallocycle of elemental S.53 Higgins et al. reported a negative shift of 0.11 eV in the S 2p doublet peak on sulfur doped graphene containing Pt NPs, in comparison to simply sulfur doped graphene.16 DFT analysis showed that upon Pt adsorption on nitrogen or sulfur defects the electron density of the Pt nucleus decreases (cyan color in Figures 5 and 6). Furthermore, S preferentially transfers electron density to the Pt atom, which in turn bond to one or two of the surrounding H atoms, resulting in the increase of the Pt oxidation state. Conversely N atoms have always a partial negative charge and the larger electronegativity of N reduces the tendency of other atoms to bond the Pt atom, which remains partially positively charged. In both cases a shift towards higher B.E. is expected. In Pt/S-MC it was observed a small shift of the Pt(0) photoemission peak by 0.2 eV towards higher B.E., as compared to Pt/C TKK, whereas Pt 4f signal for Pt/N-MC is almost identical to that recorded on Pt/C TKK.5,10. From the support point of view, small negative shifts were observed for the thiophenic functional groups (0.1 eV) as well as for pyridinic (0.2 eV), pyrrolic (0.3 eV) and graphitic (0.1 eV) defects. Pt/N,S-MC does not show any evidence of B.E. shift for nitrogen or sulfur, as well as for Pt. Therefore, the XPS experimental results are in accordance with the DFT calculations, and in line with previous results, so that a certain degree of interaction can be claimed between the Pt clusters and both sulfur and nitrogen functional groups. However, even though TEM and DFT data are in accordance with the XPS results, caution must be exercised because the small B.E. shift are of the same magnitude of the experimental error so that ACS Paragon Plus Environment

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high resolved experiments would be necessary to more strongly confirm the presence of the metal support interaction.

3.2

Electrochemical characterization of Pt/MCs.

Figure 7 reports the voltammetric curves for the Pt/N-MC, Pt/S-MC and Pt/S,N-MC samples along with Pt/C TKK standard, taken as reference, obtained in Ar purged 0.5 M H2SO4 after 300 activation cycles between 0.05 and 1.2 V vs. RHE. The voltammograms are characterized by three different voltammetric features; the left side corresponds to the H2 adsorption/desorption (Hupd) region and H2 evolution, the middle zone to the double layer charging current and the right side to the oxide-hydroxide formation and reduction. It is clear that catalysts supported on N and/or S doped MC show an increased double layer capacitance (0.3−0.8 V vs. RHE) with respect to standard Pt/C TKK, since doped carbon supports have large specific surface area and the presence of functional groups containing heteroatoms, gives carbon materials with acid/base character and thus enhances the capacitance by pseudocapacitive effects.11,31,54 In particular, the presence of peaks in the potential windows 0.3−0.8 V vs. RHE may be ascribed to redox hydroquinone/quinone couple or to the reversible reduction of nitro groups generated by the oxidation of nitrogen functional groups.55 The ECSA, which is an indicative parameter of the Pt-active site density, was obtained according to eq. 3, by the normalization of the EPSA, determined from the charge corresponding to the H2 desorption, with respect to the Pt loading determined by ICP analysis. These results are summarized in (Table 3). Pt/N-MC and Pt/S-MC showed ECSA values as high as 69.1 and 64.1 m2/gPt, which are slightly lower with respect to the 79.83 m2/gPt obtained for the Pt/C TKK standard. On the contrary Pt/S,N-MC appears to be less attractive in term of ECSA since a value lower by almost 50% was observed. The ECSA depends on both dispersion and size of the Pt NPs and on a good electrical contact of Pt with the underlying carbon support.56 The low ECSA value


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ACS Catalysis

for Pt/N,ScMC catalyst can be easily explained on the sole basis of different particle size since Pt/N,S-MC showed Pt NPs of mean diameter of 3.5 nm along with big clusters as visible in Figure 2d. By simple geometrical consideration it may be concluded that increasing the particle dimension from ∼2 nm (mean value between Pt/N-MC and Pt/S-MC) to 3.5 nm (Pt/N,S-MC)) the total surface almost halves. On the other hand, the slightly lower ECSA observed for Pt/N-MC and Pt/S-MC with respect to Pt/C is not easy rationalizable, but we retain that both: a lower conductivity of the doped carbon (σ ≈ 300 mS/m) with respect to the Vulcan XC72 (σ ≈ 3970 mS/m) and the possible confinement of Pt NPs inside the pores, which are hardly accessible by the electrolyte, can negatively affect the ECSA values. The LSV curves obtained at RDE in O2 saturated 0.5 M H2SO4 for all the investigated catalysts are shown in Figure 7b. Even though HClO4 should be preferred to H2SO4 for investigating the electrocatalytic properties of Pt NPs versus ORR, in this case H2SO4 was employed for comparing the activity and stability of the catalysts studied in the present work with the results of the sole carbon supports investigated in previous papers.11,31 It can be seen that Pt/NMC and Pt/S-MC show a remarkably high electrocatalytic activity towards ORR since the halfwave potential was 0.826 V and 0.824 V vs. RHE, respectively and very close to that obtained in the same experimental condition (0.838 V vs. RHE) for the Pt/C TKK standard (Table 3). The catalytic activity of Pt/X-MC, expressed as ∆E1/2= E1/2Pt/C − E1/2Catalyst, decreases as follows: Pt/C TKK > Pt/N-MC ∼ Pt/S-MC > Pt/N,S-MC. Table 3 reports also the values of the SA and MA determined in O2 saturated 0.5 M H2SO4 at 0.9 V vs. RHE. Further kinetic information may be extrapolated from Tafel plot after mass-transfer correction reported in Figure 7c. The kinetic currents at 0.9 V vs. RHE ranged in the order Pt/S-MC (jk = 0.081 mA cm-2), > Pt/C (jk = 0.076 mA cm-2),

>

Pt/N-MC

(jk

=

0.062

mA

cm-2)

>

Pt/N,S-MC

(jk = 0.052 mA cm-2). Furthermore, the similar Tafel slopes indicate that the reaction pathway and the rate determining step are similar for the newly synthetized catalysts and the TKK standard.


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Pt/S-MC showed the best performance in term of SA and MA (120.90 µA/cm2 and 77.44 A/gPt), followed by Pt/N-MC and Pt/N,S-MC (24.32 A/gPt). It is worth noting that Pt/S-MC has similar MA but decisively higher SA than Pt/C TKK. Both electrodes were tested with the same amount of Pt on the GC support (15 µg/cm2), but Pt/S-MC showed a higher kinetic current at 0.9 V with respect to TKK, that justify the slightly higher MA. Furthermore, both: the higher jk and the smaller Pt NPs observed in Pt/S-MC with respect to Pt/C TKK are also responsible for the higher SA in comparison to TKK. On the contrary, a different picture emerged for Pt/N-MC and to a different extent for Pt/N,S-MC, where the SA is comparable with TKK, but a decisively lower MA can be observed. In this case, the kinetic current jk recorded at 0.9 V is lower in comparison to both Pt/C TKK and Pt/S-MC, thus a lower MA is expected, being the Pt loading equal for each tested sample. We believe that the higher kinetic current observed for Pt/S-MC arises from a catalystsupport interaction, and precisely from the interaction between Pt NPs and thiophenic functional group as clearly pointed out by DFT calculations. Conversely, low kinetic current and, as a consequence, the lower catalytic activity in Pt/N,S-MC is in agreement with the weak metal support interaction observed for multi-defective surfaces (Table 2). Table 3- Electrochemical data obtained from liner sweep voltammetry at RDE in O2 saturated 0.5 M H2SO4.a Pt loading is 15 µg/cm2 for each sample. Catalyst

E1/2b

|jd| 2

∆E1/2c

Tsd

ECSAe

SA

MA

m2/g

mA/cm2

A/gPt

(V)

(mA/cm )

(mV)

(mV)

1 TKK Pt/C 2 Pt/N-MC 3 Pt/N,S-MC

0.838 0.826 0.761

4.71 4.42 3.91

-48 -77

96 102 114

79.83±1.9 69.1±2.1 28.7±3.5

98.81±7 86±5 86±11

76.4±3.7 59±4 24.3±0.2

4 Pt/S-MC

0.886

4.25

-14

84

64.1±1.6

120.9±0.4

77.4±1.7

a

At least 4 measurements were conducted for each catalyst: data are reported as mean values and the uncertainty as standard deviation. bAll potentials are referred to RHE. c∆E1/2= E1/2Catalyst - E1/2Pt/C. dTafel slope determined from diffusion corrected Tafel plot. e From cyclic voltammetry in Ar saturated 0.5 M H2SO4.


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Figure 7. Electrochemical responses for TKK standard and Pt NPs on doped carbons. a) CVs recorded at scan rate of 50 mV s-1 in 0.5 M H2SO4; b) LSVs recorded at scan rate of 20 mV s-1 in O2 saturated 0.5 M H2SO4 at 25 °C, rotation rate of 1600 rpm; c) mass-transfer corrected Tafel plots, derived from ORR polarization curves. To understand more in a deep the effects of sulfur doping on the Pt particle size and ORR catalytic activity, four new mesoporous carbons with the same morphological structure but different S contents were prepared i.e. S-MC1 (8.6%), S-MC2 (6.8%), S-MC3 (4.2%), and MC ( Pt/S-MC3 > Pt/S-MC2 > Pt/S-MC1 > Pt/S-MC (Table 4). Table 4. Chemical and electrochemical data of Pt/S-MC a different sulfur content.a

Sample

Sucb DTc

Pt/S-MC 0 Pt/S-MC1 30 Pt/S-MC2 50 Pt/S-MC3 70 Pt/MC 100

100 70 50 30 0

C

S

dcd

E1/2e

%

%

(nm)

(V)

68.1 74.2 73.8 75.3 71.0

13.8 8.6 6.8 4.2 Pt/S-MC1 ≈ Pt/SMC2 > Pt/S-MC3 > Pt/MC. In this case Pt/MC, can be taken as reference to gauge the effect of sulfur doping without any further effects arising from the carbon support morphology or structure. The ∆E1/2 is the difference in half wave potential between Pt/S-MCx and Pt/MC and it is indicative of the catalytic activity gain when the sulfur content increases. It is clear that passing from Pt/MC to Pt/S-MC1 and Pt/S-MC2 the catalytic activity increases of almost 100 mV, and an even higher value can be observed for Pt/S-MC (Table 4). Similarly, the mass transfer corrected Tafel plots reported in Figure S8c are indicative of higher kinetic currents for Pt NPs supported on those carbon that showed higher sulfur doping. Therefore, it is clear that sulfur functional groups play an active role in the nucleation and growth on Pt NPs and as a consequence also in their catalytic activity, so that a metal support interaction can be clearly claimed even though small B.E. shift were observed by XPS investigation on Pt 4f and S 2p peaks. Since Pt/S-MC showed to be the most promising material for ORR in H2SO4, it was incorporated in a membrane electrode assembly (MEA) in order to be tested as cathode material in a PEM fuel cell. The MEAs possess a Pt loading equal to 0.1 mg/cm2 at the anode and 0.3 mg/cm2 at the cathode. The MEA consists in a 50 µm thick Nafion membrane, supporting the commercial Pt/C catalyst on the anodic side and the Pt/S-MC on the cathodic side. A commercial material (Pt/C


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TKK) was incorporated in the cathode of a MEA with the same properties, in order to realize a comparison between home-made catalysts and a commercial material. The analysis of a MEA includes the measure of polarization curves, capacitance, impedance, ECSA and Tafel slope as reported in Table 5. Polarization curves, power densities and impedance were obtained at three different relative humidity (RH) percentage: 30%, 80% and 160%. ECSA and capacitance were obtained at 100% RH, while the Tafel slopes were calculated at 90% RH. Actually, a relative humidity of 160% indicates that the gas contains also liquid water. Humidity has a strong influence over the nafion membrane; in fact, this property determines its degree of hydration, and consequently, the performance of the catalyst. Low humidity value is deleterious for the performance, because membrane is not completely hydrated, and the migration of protons is hindered. The same effect is present also when the relative humidity is too high, because in this case the amount of water could determine clogging problems that prevent diffusion toward Pt catalyst or water removal. In Figures 8 a and b the polarization curves and the power density curves are reported for Pt/S-MC and the commercial catalyst Pt/MC at 80% RH, respectively. From the results reported in Table 5 and Figure 8 a and b it is clear that Pt/S-MC doesn’t show good performance as cathode material in a PEM fuel cells as expected from half cell analysis. In particular, the activity toward ORR is very low when compared to that of the commercial material, and this is not in agreement with the good results obtained from RDE analysis. It is reasonable that Pt/S-MC catalyst resulted not to be fully optimized for the MEA application, and this might be due to several factors, that can be explained considering the parameters in Table 5: ohmic resistance, oxygen transport resistance (LMN ) and ECSA. Table 5 Data obtained by testing the MEAs made of 1Pt@SMC and Pt/C as cathode, Pt loading 0.3 mg/cm2. In all cases anode is made of commercial Pt/C with 0.1 mgPt/cm2. RH (%)

I (0.6 V)a (mA/cm2)

I (0.4 V)a (mA/cm2)

LMN b (S/m)

Rc (mΩ)

160

199.8

403.0

368.0

215.5


Cd (mF)

ECSAe (m2/g)

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Pt/S-MC Pt/C TKK

80 30 80

195.0 61.2 856

a

427.0 146.3 1480

104.3 176.0 70

316.3 940.0 82

54.5

27.9

20

58

b

obtained from polarization curves. Air was fed to the cathode. obtained from CVs at low scan rate. Air was fed to the cathode; c obtained from impedance spectroscopy. Air was fed to the cathode. d obtained from CVs at four different scan rates. e obtained by CV at low scan rate. Relative humidity: 100%. N2 was fed to the cathode

S-MC has a multimodal distribution of pore size: two peaks are centered in the mesoporous region at 4 and 8.4 nm, while one peak is centered in the microporous range. Pore size distribution centered at around 8 nm is good for Nafion distribution and oxygen diffusion, but smaller pores prevent the Nafion from entering them.57 Pt particles that are not in contact with Nafion ionomer are not active during ORR, because oxygen cannot reach them by diffusion, thus a O2 limited diffusion computed as a mass transport resistance (LMN ) becomes the rate determining step affecting the kinetics of the whole reaction.58 This is consistent with the smaller ECSA obtained from MEA analysis compared to RDE data, which suggests that only a fraction of the whole Pt amount is active. Small pores are responsible as well for a bad oxygen diffusion, as highlighted in Table 5 for Pt/S-MC. Clearly, this negative effect is more evident in MEA measurements since it depends on the catalysts layer thickness that is few microns for microelectrode analysis whereas it can reach several hundred microns in MEA. Another parameter that adversely affects the performance toward ORR is the mesoporous carbon chemical composition, which affects the electrical conductivity, which in turn worsen the performance of the cell. To conclude it is possible to state that the low performance of Pt/S-MC toward oxygen reduction in a fuel cell is due to a non-optimized carbon support, so that the gain in catalytic activity due to Pt NPs/ support is lost due to ohmic drop and mass transfer resistance. In fact, while RDE method highlighted a purely kinetic favorable effect of sulfur doping on Pt activity toward the ORR, the more complex environment in MEA testing shows that the specific catalyst layer structure suffers from ohmic and mass transfer resistance. Carbon support improvements are mandatory, and


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will be focused on the realization of larger pores (ideally between 15-30 nm pore diameter) and higher electrical conductivity of the carbon support.

Figure 8. Cell polarization curves (a) and power density curves (b) for Pt/S-MC and Pt/C reference at 80% relative humidity. 3.2

Electrochemical stability of Pt/MCs.

To investigate the catalyst durability, accelerated durability testing (ADT) protocols are commonly employed to simulate the harsh conditions encountered at the cathode of PEMFCs during operation. Pt/N-MC, Pt/S-MC, Pt/N,S-MC and Pt/C TKK catalysts were subjected to 10000 cycles under oxygen saturated electrolyte with cyclic voltammograms (CVs) collected before and after ADT, in 0.5 M H2SO4 as shown in Figure 9 a-d, whereas Figure 9 e,f report the MA and SA before and after ADT for all the investigated materials. TKK Pt/C showed a slightly positive shift of E1/2 after 2000 cycles (Figure 9a). This effect can be explained by considering the increase of the mean Pt particle size to an optimal range 3-4 nm for ORR as evident from TEM analysis (Figure S9), and in agreement with recent findings.36 However, after 10000 cycles the mass activity decreased by 16% while the specific activity resulted almost unchanged (Figure 9 e,f). Pt/S-MC showed good ORR activity retention through ADT after 10000 cycles, showing only 14 mV loss in half-wave potential and retained almost 70% of the mass ACS Paragon Plus Environment

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activity at 0.9 V vs. RHE (Figure 9b,e). On the other hand, Pt/N-MC resulted a bit less performing after ADT since E1/2 is shifted by 30 mV towards more negative potential and the MA decreases by 36% (Figure 9c,e). It is interesting to observe that Pt/N,S-MC is the one that was less affected by ADT, since E1/2 resulted almost unchanged and MA changed only by a 10%. Concerning the specific activity, Pt/N-MC showed a slight increase, conversely to Pt/S-MC and Pt/N,S-MC, where SA decreases by 12 and 51%, respectively.


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Figure 9. Accelerated stability tests: first LSV (red line) and after 10000 cycles (black line), for: a) Pt/C TKK catalyst; b) Pt/S-MC; c) Pt/N-MC; d) Pt/N,S-MC. e) Variation of the mass activity. f) Variation of the specific activity.

Pt particle agglomeration and dissolution are among the primary causes of catalyst degradation and performance loss during PEMFC operation, therefore TEM analysis was employed for investigating the morphological changes occurring in catalyst materials during ADT. Figure S9, provides TEM images and corresponding particle size distributions at different ADT level for Pt/C TKK, whereas ADT TEM results for the newly prepared catalysts are reported in Figure 10 and Figure S10. In Pt/C TKK NPs dimension increases from 2.2 nm to 3.5 nm, and this point was already discussed to be responsible for the improved activity observed for Pt/C TKK after 2000 cycles; however, also big aggregates can be observed all over the sample, especially for longer ADT treatments, which explains the performance loss in term of catalytic activity (E1/2) and mass activity. The emerging picture is quite different among the three differently doped support: let’s first consider Pt/N-MC, which still shows small Pt NPs uniformly distributed inside pores along with some Pt NPs significantly grown after both 4000 and 10000 cycles (Figure 10a). The Pt average particle size increases from 2.5 to 4.5 nm after 10000 cycles, but Pt NPs in the range 6-18 nm are also present. It is worth noting that small Pt NPs confined inside pores are almost of the size of the MC inner pore diameter (∼ 4 nm), whereas the dimension increases going out to the MC surface. Therefore, it appears that ADT involves a general increase of the particle size because of agglomeration of the NPs that are not stably anchored over the MC surface, but the presence of mesopores prevents a further agglomeration of Pt NPs confined inside the carbon mesopores, where Pt NPs grow until the pore volume is completely filled. A similar picture emerged also from ADT experiment for Pt/N,S-MC even though the growing of big clusters is not clearly visible and only a slight increase of Pt mean particle size is present (Figure 10c,d). This observation is in line with the


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very superimposable LSVs recorded before and after ADT, confirming that even though Pt/N,S-MC is not as highly active as Pt/S-MC, it is less incline to performance loss. In the case of Pt/S-MC, the formation of big clusters in the range 5-35 nm is evident after 10000 cycles, but there is still a huge population of Pt NPs centered at 1.8 nm. This is in accordance with the electrochemical results that highlight a smaller performance loss in Pt/S-MC than for Pt/NMC and it provides further proof of the beneficial impact of using sulfur doped support materials. As a partial conclusion it is clear that the support plays a role in both preparation and stability of Pt NPs, in particular it appears that both nitrogen and sulfur heavily affect both the nucleation and growth of Pt NPs, but partially fail in stabilizing Pt NPs, since coarsening and agglomeration effects involve the increase of Pt NPs dimension up to sizes less favorable for ORR (> 6 nm), when subjected to harsh operational condition. On the other hand, the confinement effect of mesopores is effective in limiting the Pt NPs enlargement, but pore occlusion limits an effective electrolyte permeation till to the more remote Pt NPs. In our opinion carbon doping can help in reaching the DOE target, since a clear increase of kinetic current occurs when, for example, sulfur doping increases and this would allow to lower the Pt loading on the electrode. However, it is mandatory to optimize also the carbon support conductivity and mass transport as it was observed when the best performing catalysts was tested in a real MEA condition.


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Figure 10. TEM images after ADT at (a, c, e) 5000 and (b, d, f) 10000 cycles in 0.5 M H2SO4 between 0.6 and 1.0 V vs. RHE for: (a,b) Pt/N-MC, (c,d) Pt/N,S-MC and (e,f) Pt/S-MC.


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Conclusions Pt NPs were successfully deposited on novel nitrogen and/or sulfur doped mesoporous carbon by a solid state synthesis, which involves the reduction of Pt acetylacetonate in a H2/N2 flow at 300 °C. It was observed that both nitrogen and sulfur involve a clear stabilization of Pt NPs with respect to a non defective carbon layer and this resulted in the formation of Pt NPs in the range 1.82.5 nm. DFT simulation gave insights on the fact that when a nitrogen-sulfur co-doped support is employed, the close proximity of the two hetero-defects destabilizes the formation of small Pt nuclei in accordance with the fact that bigger ones (3.5 nm) were observed on Pt/N,S-MC. XPS analysis revealed a B.E. shift (0.1-0.3 eV) that according to other literature findings would be indicative of an interaction between Pt and sulfur or nitrogen defects, consisting in charge transfer from Pt orbitals to the doped support. These B.E. shift were observed in the singular doped catalysts but not in the co-doped Pt/N,S-MC. Pt/N-MC and Pt/S-MC showed a remarkably high electrocatalytic activity towards ORR in 0.5 M H2SO4 (E1/2: 0.826 V and 0.824 V vs. RHE, respectively), very close to that obtained in the same experimental condition for the Pt/C TKK standard. Furthermore, Pt/S-MC showed the best performance in terms of SA and MA (120.90

µA/cm2 and 77.44 A/gPt), followed by Pt/N-MC and Pt/N,S-MC. Four mesoporous carbons with the same morphological structure but different S contents were also prepared and a clear correlation between sulfur content and the shift of the half wave potential during ORR catalytic test was observed, i.e. the higher the sulfur doping, the higher the catalytic activity. The best performing Pt/S-MC was also tested toward oxygen reduction in a fuel cell, but the performances were lower with respect to the expectation since the gain in catalytic activity due to Pt NPs/ support is lost as ohmic drop and mass transfer resistance. Accelerated degradation tests revealed that nitrogen and sulfur functional groups partially fail in stabilizing Pt NPs since coarsening and agglomeration effects were observed after 10000 cycles between 0.6 to 1.0 V vs. RHE in 0.5 M H2SO4. However, it might be asserted that both TKK standard and the synthetized catalysts display a comparable stability. Furthermore, the confinement ACS Paragon Plus Environment

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effect of mesopores was effective in limiting the Pt NPs enlargement, even though Pt NPs in the core of the carbon sphere may be partially useless due to limitation of electrolyte permeation.

AUTHOR INFORMATION Corresponding Author *Corresponding author - Tel.: +39 049 8275112; fax: +39 049 8275829; e-mail: [email protected] e-mail: [email protected]

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: TGA analysis, XPS spectra and deconvolution into single chemical components for Pt/N,SMC before and after Pt deposition, DFT simulations and Mulliken atomic charges maps, X-Ray diffraction patterns of the Pt NPs loaded on differently sulfur doped carbon, TEM images of catalysts before and after accelerated degradation tests, electrochemical characterization of Pt NPs on sulfur doped carbons at different sulfur content, Pt NPs size distribution after accelerated degradation tests, table of BET data for doped support, table for classification of energy adsorption

Acknowledgment The research leading to these results has received funding from the University of Padova (PRAT CPDA139814/13) and the Hydrogen Initiative-Joint Undertaking (FCH-JU) within the CathCat project under contract No. 303492. We are in debt with Dr. Denis Badocco for ICP analysis.


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Methodology scheme for the synthesis of Pt NPs on doped MC 396x246mm (96 x 96 DPI)


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ACS Catalysis

TEM and NPs size distribution of: (a-c) Pt/N-MC, (d-f) Pt/S,N-MC, (g-i) Pt/S-MC. 204x192mm (150 x 150 DPI)


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a) X-Ray diffraction patterns of the Pt NPs loaded on differently doped carbon: (1) Pt/C TKK, (2) Pt/N-MC, (3) Pt/N,S-MC and (4) Pt/S-MC. XPS spectra and deconvolution into single chemical components of N 1s XPS region for Pt/N-MC before (b) and after (c) Pt deposition. XPS spectra and deconvolution of S 2p XPS region for Pt/S-MC before (d) and after (e) Pt deposition. XPS spectra and deconvolution of Pt 4f region for (f) Pt/C, (g) Pt/N-MC, (h) Pt/S-MC, (i) Pt@N,S-MC. 279x229mm (300 x 300 DPI)


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ACS Catalysis

Caption : DFT computational simulation of mixed thiophenic–pyrrolic in the three surface position near (a,b) halfway (c,d) and far (e,f) and the two possible Pt atom configuration “over” (a,c,e) or “under” the surface (b,d,f). 479x332mm (96 x 96 DPI)


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Figure 5. Difference electron density isosurfaces plotted at ±0.001 Bohr-3 isovalue for the single-defect surfaces. Orange (cyan) surfaces enclose regions in space where the electron density in increased (decreased) upon adsorption. Adsorption energies are reported in the figure with a color code identifying high (red), medium (cyan), and low (green) surface-Pt interaction. 525x205mm (96 x 96 DPI)


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ACS Catalysis

Figure 6 Difference electron density isosurfaces plotted at ±0.001 Bohr-3 isovalue for the double-defect surfaces. Orange (cyan) surfaces enclose regions in space where the electron density increased (decreased) upon adsorption. Adsorption energies are reported in the figure with a color code identifying high (red), medium (cyan), and low (green) surface-Pt interaction. 529x206mm (96 x 96 DPI)


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Figure 7. Electrochemical responses for TKK standard and Pt NPs on doped carbons. a) CVs recorded at scan rate of 50 mV s-1 in 0.5 M H2SO4; b) LSVs recorded at scan rate of 20 mV s-1 in O2 saturated 0.5 M H2SO4 at 25 °C, rotation rate of 1600 rpm; c) mass-transfer corrected Tafel plots, derived from ORR polarization curves. 160x134mm (300 x 300 DPI)


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ACS Catalysis

Figure 8. Cell polarization curves (a) and power density curves (b) for Pt/S-MC and Pt/C reference at 80% relative humidity 89x38mm (300 x 300 DPI)


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Figure 9. Accelerated stability tests: first LSV (red line) and after 10000 cycles (black line), for: a) Pt/C TKK catalyst; b) Pt/S-MC; c) Pt/N-MC; d) Pt/N,S-MC. e) Variation of the mass activity. f) Variation of the specific activity. 209x220mm (300 x 300 DPI)


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ACS Catalysis

127x203mm (150 x 150 DPI)


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465x268mm (96 x 96 DPI)


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DFT and Experimental Evidences of Metal-Support Interaction in Pt

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