J. Phys. Chem. C 2010, 114, 8661–8667
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Monodispersed Platinum Nanoparticle Supported Carbon Electrodes for Hydrogen Oxidation and Oxygen Reduction in Proton Exchange Membrane Fuel Cells Ch. Venkateswara Rao* and B. Viswanathan* National Centre for Catalysis Research, Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India ReceiVed: February 18, 2010; ReVised Manuscript ReceiVed: March 26, 2010
A series of Pt/C electrocatalysts with average Pt particle size of 1.8, 2.3, 3.4, 3.8, 4.7, and 5.8 nm are synthesized by reducing platinum(II) acetylacetonate with 1,2-hexadecanediol in the presence of long-chain carboxylic acid and alkylamine stabilizing agents. The prepared materials are characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and energy dispersive X-ray (EDX) spectrometry. The face-centered cubic structure of Pt in the materials is evident from XRD. Good spatial distribution of spherical shaped Pt nanoparticles is seen from the HRTEM images. The presence of Pt and C is observed from the EDX analysis. Linear sweep voltammetry (LSV), rotating disk electrode (RDE), and single-cell proton exchange membrane fuel cell (PEMFC) measurements are conducted to evaluate the electrocatalytic activity of Pt/C electrocatalysts. Electrochemical measurements indicate the good hydrogen oxidation and oxygen reduction activity for 1.8 and 3.4 nm size Pt particles, respectively. Kinetic analysis by RDE voltammetry reveals that the number of electrons transferred in hydrogen oxidation (HOR) and oxygen reduction (ORR) processes on 1.8 and 3.4 nm size Pt nanoparticles is 2 and 4, respectively. The high performance Pt/C anode and cathode are then used to fabricate a membrane-electrode assembly (MEA) and tested in a PEMFC. A maximum power density of 625, 760, and 1030 mW/cm2 is observed at 333, 343, and 353 K, respectively. 1. Introduction The salient features of proton exchange membrane fuel cells (PEMFCs) such as high electromotive force (∼1.23 V), good power density at ambient temperature, high energy density of hydrogen fuel (32 kW h/kg), and low fuel crossover flux make them one of the promising power sources for stationary, transportation, and portable device applications.1 In these fuel cells, hydrogen oxidation (HOR) and oxygen reduction (ORR) reactions take place at the anode and cathode, respectively, according to the following equations:
Anode: H2 f 2H+ + 2e-;
E° ) +0.0 V vs NHE
1 Cathode: O2 + 2H+ + 2e- f H2O; E° ) +1.23 V vs NHE 2
1 Overall: H2 + O2 f H2O; 2
OCV ) +1.23 V
These reactions are known to be taking place efficiently on Pt electrocatalysts.2 However, Pt is expensive and also less abundant in nature. To improve the catalytic activity and reduce Pt loading, various attempts have been made by researchers and most of the works have been focused on the dispersion of Pt nanoparticles on a carbon support.2,3 Electrochemical studies reveal that there is an optimum size of Pt nanoparticles for the maximum HOR and ORR activity.4 To the best of our knowledge, no one reported the investigation of PEMFC * To whom correspondence should be addressed. E-mail: vrao.chitturi@ ymail.com (C.V.R.);
[email protected] (B.V.). Telephone: +91 44 2257 4241. Fax: +91 44 2257 4202.
performance with the optimum sized Pt nanoparticle supported carbon electrodes for the corresponding anode and cathode reactions. In recent years, there has been great interest in the relationship between the activities of carbon supported Pt catalysts toward electrocatalytic reactions and the size of supported Pt particles. The investigation toward elucidating Pt particle size dependence for hydrogen oxidation was by Antoine et al.5 They examined the HOR activity using a GDE approach in the wide Pt particle size interval from 2.5 to 28 nm by increasing the Pt loading on a vulcan carbon support from 10 to 80 wt %. They concluded that intrinsic activity does indeed increase with decreasing particle size. If it is true, Pt nanoparticles of size less than 2.5 nm are expected to exhibit good HOR activity. The particle size dependence on ORR activity was studied well in literature.6–13 Although efforts were made, the conclusions drawn from the studies varied among the different researchers. Zeliger6 deposited Pt crystallites of size around 5-10 nm on asbestos and performed ORR measurements in acid electrolyte. They did not observe any change in the specific activity for ORR with Pt crystallite size. Further support to this conclusion was provided by Bett et al.7 and Vogel and Baris8 who showed a similar independence of particle size on ORR activity. On the other hand, Blurton et al.9 and Bregoli10 found a decrease in the specific activity for oxygen reduction with diminishing particle size from 12 to 3 nm and 2 nm, respectively. The studies reported by Blurton et al.9 involved highly dispersed Pt on carbon in concentrated H2SO4 at 343 K. They found a 20-fold decrease in activity with a particle size range of 2-12 nm and attributed the results to either a particle size effect or an effect of the support interaction or a combination of both these factors. The study by Bregoli10 involved highly dispersed catalysts in concentrated H3PO4 at 450 K. He found the specific activity for oxygen reduction to vary by a factor of 2 in the
10.1021/jp101481g 2010 American Chemical Society Published on Web 04/14/2010
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TABLE 1: Reactants and Conditions Employed for the Preparation of Carbon Supported Pt Catalysts catalyst Pt1C
a
Pt2C Pt3C Pt4Cb Pt5C Pt6C a
reactants
temperature and time
Pt(acac)2, 1,2-hexadecanediol, oleic acid, oleylamine, and octylether Pt(acac)2, 1,2-hexadecanediol, oleic acid, oleylamine, and octylether Pt(acac)2, 1,2-hexadecanediol, oleic acid, oleylamine, and octylether Pt(acac)2, 1,2-hexadecanediol, nonanoic acid, nonylamine, and diphenylether Pt(acac)2, 1,2-hexadecanediol, nonanoic acid, nonylamine, and diphenylether Pt(acac)2, 1,2-hexadecanediol, nonanoic acid, nonylamine, and diphenylether
573 K, 30 min 573 K, 45 min 573 K, 60 min 533 K, 30 min 533 K, 45 min 533 K, 60 min
Reference 23. b Reference 24.
range of particle size studied (12-2 nm). Later, Peuckert et al.11 in their investigations with highly dispersed 5, 10, 20, and 30 wt % Pt on carbon catalysts (particle sizes in the range 12-1 nm) in 0.5 M H2SO4 found the maximum ORR activity in the case of 4 nm sized Pt particles on carbon. However, a 20-fold decrease in ORR activity as the particle size decreased from 3 to 1 nm was observed, which was in contrast to expectations based on changes in surface area (or dispersion). Sattler and Ross12 also observed the same phenomenon in their investigations with dispersed Pt catalysts of crystallite size range 2.8-11.2 nm in 97% H3PO4. Later, ORR measurements were performed on carbon supported Pt catalysts with different Pt loading by several research groups also observed the decrease in specific activity (mA/cm2 of Pt) with a decrease in particle size (d < 3 nm) while specific activity for large (d g 5 nm) particles is approximately the same as that for bulk platinum.13–18 In all the reports, the authors prepared a series of catalysts with 5, 10, 15, 20, and 30 wt % Pt on carbon and studied the effect of particle size on oxygen reduction based on potentiodynamic rotating ring disk electrode (RRDE) measurements. Usually, supported catalysts contain a wide distribution of particle sizes, so experimental evidence for a particle size effect is often difficult to obtain. Hence, the inconsistent results and conclusions are most probably caused by the existence of Pt particles in different sizes and uncontrollable shapes by the procedures adopted for the preparation of catalysts. These studies also suggest that the kinetics of oxygen reduction on Pt is affected not only by the particle size but also by the particle shape which may modify the distribution of oxygen adsorption sites on the Pt surface. So if one could be able to prepare monodispersed Pt nanoparticles of different size on carbon and investigate the electrocatalytic HOR and ORR activity under potentiostatic measurements, it will provide information on the optimum size of Pt for maximum activity. By means of conventional preparation techniques, namely, wet impregnation method and chemical reduction of the metal precursors using reducing agents, achieving desired sizes of Pt nanoparticles (2-5 nm) with homogeneous dispersion on the carbon support appears to be difficult.19 Hence, in order to produce desired sizes of Pt nanoparticles with uniform dispersion on the carbon support, some kind of stabilizing agent, such as surfactant, ligand, or polymer, is usually employed during the preparation process.20,21 The polyol reduction method is one of the suitable methods to generate metal colloids and/or clusters on the nanoscale with greater uniformity.22–24 In the present study, monodispersed Pt nanoparticles of different sizes ∼2-6 nm are synthesized by employing the polyol reduction method and loaded onto a carbon support to achieve 20 wt % Pt. The prepared catalysts are characterized
by XRD, TEM, and energy dispersive X-ray (EDX) spectrometry. Steady-state linear sweep voltammetric measurements are performed to investigate the optimum size of Pt for maximum HOR and ORR activity with the hope that these studies will open an avenue for the rational design of PEMFC electrodes for practical application. Finally, the PEMFC performance is investigated with the fabricated electrodes consisting of optimum sized Pt nanoparticle supported carbon. 2. Experimental Section Materials. Platinum(II) acetylacetonate, 1,2-hexadecanediol, oleic acid, oleylamine, nonanoic acid, nonylamine, Nafion solution, and poly(tetrafluoroethylene) (PTFE) from SigmaAldrich and diphenyl ether, ethyl alcohol, isopropanol, and hexane from Merck were used as received. Nafion 212 membrane was purchased from DuPont Company. Water purified via a Milli-Q water purification system was used throughout the experimental work. Carbon black (CDX975, received from Columbian Chemicals Company) with the Brunauer-Emmett-Teller (BET) specific surface area of ∼300 m2/g was used as support for all the catalysts. 2.1. Preparation of Carbon Supported Pt Catalysts. Monodispersed Pt nanoparticles of different sizes are synthesized by reduction of platinum(II) acetylacetonate with 1,2-hexadecanediol in the presence of long chain carboxylic acid and long chain alkyl amine as protecting agents. To prepare different sized Pt nanoparticles, either the combination of oleic acid and oleylamine or nonanoic acid and nonylamine are used. The used reactants and conditions employed for the preparation of Pt nanoparticles are given in Table 1. The reaction is carried out using standard Schlenk line technique under dry nitrogen. In a typical synthesis, a mixture of 0.5 mmol of Pt(acac)2 and 2 mmol of 1,2-hexadecanediol are taken in a 100 mL three-necked round-bottom flask equipped with a N2 in/outlet, PTFE coated magnetic stir bar, septa rubber, and thermal probe. An amount of 25 mL of solvent is then transferred into the flask and the contents stirred while purging with N2 at room temperature. The flask is then heated to 373 K and held at 373 K for 15 min. During this hold, 0.5 mmol of protecting agent is injected into the flask while continuing the N2 gas purge. Then the required amount of carbon black (C) is added and the brown suspension is refluxed before cooling down to room temperature under the N2 atmosphere. Then it is filtered, washed copiously with ethanol, and dried at 323 K in a vacuum. The nominal metal content on the carbon black is 20 wt %. 2.2. Characterization Techniques. X-ray diffraction (XRD) and transmission electron microscopy (TEM) are used for the phase identification and for determining particle size, respectively. XRD measurements are performed on a Rigaku Miniflex
Pt/C Electrodes for HOR and ORR in PEMFCs X-ray diffractometer using a Cu KR source operated at 30 keV at a scan rate of 0.025°/s over the 2θ range of 10°-90°. TEM images are obtained by using a high-resolution JEOL 2010 TEM system operated with an accelerating voltage of 200 kV. The sample for TEM analysis is prepared by placing a drop of dispersed catalyst onto the carbon coated copper grid and dried at room temperature. An electron microscope with EDX (FEI, model Quanta 200) is used to observe the composition of the catalysts. 2.3. Electrochemical Measurements. Electrochemical HOR and ORR activity of the Pt/C catalysts is determined by cyclic voltammetry using a potentiostat (BAS 100 electrochemical analyzer). All experiments are performed at room temperature in a conventional one-compartment electrochemical glass cell assembled with a catalyst coated glassy carbon (GC) disk as the working electrode, Ag/AgCl, 3.5 M KCl (+0.205 V vs NHE) as the reference electrode, and Pt foil as the counter electrode. The working electrode is fabricated as described by Schmidt et al.25 In this process, 5 mg of Pt/C catalyst is dispersed in 5 mL of isopropanol by ultrasonication for 20 min. Subsequently, 20 µL of the catalyst suspension is pipetted on a mirror-finished glassy carbon electrode (3 mm diameter). It is dried at room temperature under argon stream to avoid the oxidation of Pt nanoparticles. Then the catalyst is covered with 10 µL of a diluted Nafion solution (5 wt % in 15-20% water/low aliphatic alcohols) and dried at room temperature. The glassy carbon disk is polished with slurry of 0.5 and 0.03 µm alumina successively and washed ultrasonically in distilled water prior to use. The apparent surface area of the glassy carbon disk is 0.07 cm2; thus, the Pt metal loading is 56 µg/cm2. For hydrogen oxidation measurements, linear sweep voltammograms are recorded between +0.0 and +0.65 V vs NHE at a scan rate of 10 mV/s in H2-saturated 0.5 M H2SO4. For oxygen reduction measurements, linear sweep voltammograms (LSVs) are recorded between +0.2 and +1.2 V vs NHE at a scan rate of 5 mV/s in both Ar- and O2-saturated 0.5 M H2SO4. Also, a thin-film rotating disk glassy carbon electrode (Pine Instruments Inc.) technique is employed to study the HOR and ORR pathway on the most active Pt/C catalysts. 2.4. Single-Cell PEMFC Tests. Single-cell PEMFC performance of the Pt/C catalysts is evaluated by preparing gas diffusion electrodes in a fuel cell test station. These electrodes are fabricated according to the method described in literature.26,27 Pt/C catalyst is sprayed onto the carbon fiber paper (Toray, Japan) in the form of a homogeneous suspension consisting of the required amount of the catalysts, 33 wt % Nafion, and isopropanol (approximately 0.5 mL for 20 mg of mass) followed by drying in a vacuum oven at 303 K for 2 h. The catalyst loading on the anode and cathode was about 0.2 and 0.4 mgPt/ cm2, respectively. The membrane-electrode assembly (MEA) is fabricated by sandwiching the Nafion 212 membrane between the anode (Pt1C) and cathode (Pt3C) by hot pressing at 398 K and 50 kg/cm2 for 40 s. The geometric area of the electrodes is 5 cm2. The fuel cell testing is carried out at 1 atmospheric pressure and operating temperature of 333, 343, and 353 K with pure hydrogen and oxygen gas reactants. Before the steadystate polarization curves are recorded, the cell is kept at a constant current density of 250 mA/cm2 for 6-8 h until the open circuit voltage (OCV) becomes steady and constant (MEA conditioning). Then the polarization curve is recorded by applying potential from 1.0 to 0.2 V. 3. Results and Discussion 3.1. XRD, TEM, and EDX Analysis. Figure 1 shows the powder X-ray diffraction patterns of the as-synthesized Pt/C
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Figure 1. Powder XRD patterns of the Pt/C catalysts: (a) Pt1C, (b) Pt2C, (c) Pt3C, (d) Pt4C, (e) Pt5C, and (f) Pt6C.
catalysts. All the catalysts exhibited five characteristic diffraction peaks at 2θ values around 40°, 46°, 68°, 81°, and 87° corresponding to the (111), (200), (220), (311), and (222) planes, respectively. These peaks indicated that Pt is present in the facecentered cubic (fcc) structure (JCPDS card 4-802). The broad diffraction peaks of Pt in the Pt/C catalysts are indicative of the nanosized Pt. The gradual increase of the full width at halfmaximum (FWHM) from (f) to (a) indicates the decrease of Pt crystallite size. The diffraction peak at a 2θ value of 25° corresponds to the (002) diffraction peak of the hexagonal structure of the carbon support. The average crystallite size of Pt supported on carbon is calculated from line broadening of the (220) diffraction peak (Gaussian-Lorentzian peak) according to Scherrer’s equation L ) (0.9λ)/(β1/2 cos θ), where λ is the wavelength of the X-ray (1.5406 Å), θ is the angle at the position of the peak maximum, and β1/2 is the width (in radians) of the diffraction peak at half height.28 The measured Pt crystallite size in the Pt1C, Pt2C, Pt3C, Pt4C, Pt5C, and Pt6C catalysts is 1.7, 2.3, 3.2, 3.7, 4.9, and 5.9 nm, respectively. The specific surface area of the catalysts was calculated using the equation Asp ) 6000/Fd, where F is the Pt density (21.45 g/cm3), d is the particle size (in nm),29 and values are 164, 122, 87, 76, 57, and 47 m2/g, respectively. Figure 2 shows the high-resolution TEM (HRTEM) images of the Pt nanoparticle supported carbon catalysts. Uniform sized and spherical shaped Pt nanoparticles on the carbon support can be seen in all catalysts. The particle sizes obtained by TEM are given in Table 2. Well-dispersed and well-separated Pt fringes of 1.8, 2.3, 3.4, 3.8, 4.7, and 5.8 nm in size are observed in the Pt1C, Pt2C, Pt3C, Pt4C, Pt5C, and Pt6C catalysts, respectively. HRTEM images also depict that the ratio of surface Pt atoms to the total atoms is approximately 90% in all the prepared electrocatalysts. The particle sizes measured from HRTEM images are almost comparable with those of the crystallite sizes determined from XRD. It indicates the existence of Pt particles as single crystallites on carbon support. EDX spectra of the Pt1C, Pt2C, Pt3C, Pt4C, Pt5C, and Pt6C catalysts confirmed the presence of Pt and C with a small amount of oxygen in the catalysts (Figure S1, Supporting Information). 3.2. Electrochemical Performances of Pt/C Catalysts toward HOR and ORR. Figure 3 shows the linear sweep voltammograms (LSVs) of Pt1C, Pt2C, Pt3C, Pt4C, Pt5C, and Pt6C catalysts in Ar-saturated 0.5 M H2SO4 between +0.0 and
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Figure 2. High-resolution TEM images of the Pt/C catalysts: (a) Pt1C, (b) Pt2C, (c) Pt3C, (d) Pt4C, (e) Pt5C, and (f) Pt6C.
TABLE 2: Particle Size, Onset Potential, and Electrochemical Activity for HOR and ORR on Carbon Supported Pt Catalysts catalyst
particle size (nm)
onset potential for HOR (V vs NHE)
HOR activity at +0.15 V vs NHE (mA/cm2)
onset potential for ORR (V vs NHE)
ORR activity at +0.7 V vs NHE (mA/cm2)
Pt1C Pt2C Pt3C Pt4C Pt5C Pt6C
1.8 ( 0.1 2.3 ( 0.1 3.4 ( 0.1 3.8 ( 0.2 4.7 ( 0.1 5.8 ( 0.2
+0.0 +0.0 +0.0 +0.0 +0.0 +0.0
1.15 1.0 0.86 0.75 0.67 0.52
+0.895 +0.905 +0.93 +0.915 +0.87 +0.855
1.9 2.5 4.3 3.4 2.8 2.2
+0.65 V vs NHE at a scan rate of 10 mV/s. The onset potential for hydrogen oxidation and HOR activity of the Pt/C catalysts are given in Table 2. The onset potential for hydrogen oxidation is +0.0 V on all the catalysts. The calculated HOR activity for Pt1C, Pt2C, Pt3C, Pt4C, Pt5C, and Pt6C catalysts at 0.15 V is 1.15, 1.0, 0.86, 0.75, 0.67, and 0.32 mA/cm2, respectively. It indicates the gradual increase of HOR activity with the decrease of Pt particle size. This is due to the increase of surface area with particle size. Among the Pt/C catalysts, Pt1C exhibited good HOR activity. Figure 4 shows the linear sweep voltammograms (LSVs) of Pt1C, Pt2C, Pt3C, Pt4C, Pt5C, and Pt6C catalysts in Ar- and
Figure 3. Linear sweep voltammograms for the carbon supported Pt catalysts in H2-saturated 0.5 M H2SO4; scan rate ) 10 mV/s.
O2-saturated 0.5 M H2SO4 between +0.2 and +1.2 V vs NHE at a scan rate of 5 mV/s. It shows that ORR on all the catalysts is diffusion-controlled when the potential is less than +0.65 V and is under mixed diffusion-kinetic control (or surface reaction control region) in the potential region between +0.65 and +0.95 V vs NHE. For all the Pt/C catalysts, when the potential was swept from +1.2 to +0.2 V, a single oxygen reduction peak was observed in the potential region of ca. 1.0-0.7 V. Moreover, the steep increase in peak current at +0.7 V indicates the facile kinetics of ORR. The activity obtained in O2 atmosphere is a
Figure 4. Linear sweep voltammograms for the carbon supported Pt catalysts in Ar- and O2-saturated 0.5 M H2SO4; scan rate ) 5 mV/s (empty and full symbols correspond to the LSVs in Ar- and O2-saturated 0.5 M H2SO4, respectively).
Pt/C Electrodes for HOR and ORR in PEMFCs sum of oxygen reduction current, current due to Pt oxide formation or reduction, and double layer charging current. So in order to eliminate the back ground currents, oxygen reduction activity was calculated by taking the difference in activity at +0.7 V vs NHE in Ar- and O2-saturated 0.5 M H2SO4. This method of determination of background current assumes that currents for Pt oxide formation and reduction are the same in Ar and O2 atmosphere.24 The calculated onset potential for oxygen reduction and ORR activity of the as-prepared catalysts are given in Table 2. It is evident that the Pt3C and Pt4C catalysts exhibit good oxygen reduction activity compared with other Pt1C, Pt2C, Pt5C, and Pt6C catalysts. The onset potential for ORR on Pt1C, Pt2C, Pt3C, Pt4C, Pt5C, and Pt6C is about 895, 905, 930, 915, 870, and 855 mV vs NHE, respectively. The differences in ORR onset potentials are caused by the differences in the surface activation of catalysts, which are related to the size and distribution of metal nanoparticles.12,30 These results indicate that the optimum size of Pt for maximum ORR activity is 3-4 nm. The relatively higher oxygen reduction activity of the Pt3C and Pt4C catalysts can be correlated with the homogeneous distribution of Pt particles and orientation of surface atoms in the crystal planes, alteration of redox energy levels as a function of Pt particle size, and anion (sulfate) effect. ORR on Pt surfaces has been widely studied, but the details of the mechanism still remain debatable. The overall ORR is a multistep process involving four-electron transfer during which bonds are broken and formed. Oxygen reduction on the platinum surface is a structure-sensitive reaction due to structure-sensitive adsorption of spectator species, such as O2,ads, OHads, and OOHads. The OH radical is one of the important intermediates of the ORR and is formed in large amounts during the processes of oxygen reduction and water oxidation.31 It is shown that oxygen reduction generally increases as the surface area decreases, and this effect is explained by variation of the different Pt crystal planes exposed to the electrolyte as a function of the particle size. The crystal planes of Pt(110), Pt(111), and Pt(100) can facilitate adsorption of oxygen molecules via the bridge model on the surface of the Pt particles and favor dissociation of adsorbed oxygen during the ORR.32,33 In general, oxygen reduction activity on different low-index Pt surfaces increases in the sequence (100) < (110) < (111) in the potential range (E > 0.75 V) where O2 reduction is under kinetic diffusion control. However, in aqueous H2SO4 solutions, the ORR rate increases in the sequence Pt(111) < Pt(100) < Pt(110).31 An exceptionally large deactivation is observed at the (111) surface probably due to competition of the strongly adsorbing (bi)sulfate anion.32,34 The decrease of the activity on the (100) surface is related to a high affinity of (100) sites for the adsorption of hydroxyl species, leading to a lack of active centers for O2 adsorption on Pt(100) as it is covered with OHads. It was proposed that the bridge mode of adsorption of oxygen molecules on the (110) surface is the most favorable one for the oxygen molecules among the three low-index planes of platinum, suggesting that it is the preferred adsorption site on a polycrystalline Pt substrate.33 In spherical shaped Pt particles, there should be the probable distribution of all planes. It means that all the planes have same probability to react with oxygen molecular species. Kinoshita30 has correlated the specific activity (mA/cm2 of Pt) of highly dispersed Pt electrocatalysts based on the change in the fraction of surface atoms on the crystal facets of Pt particles, which results from the change in surface coordination number with a change in the average particle size. Under the operating conditions, especially in the presence of electric potential, there should be a drastic change in the
J. Phys. Chem. C, Vol. 114, No. 18, 2010 8665 geometries, charges, and adsorption energies of the species on a catalyst surface and preferential exposure of Pt in a particular orientation. As a result, population of the antibonding π* orbitals in oxygen molecule increases and facilitates the reduction of oxygen to water. These reports support the higher performance of the Pt3C and Pt4C catalysts where there should be an optimum number of Pt atoms on various crystal facets and separated with optimum interatomic distance than the other catalysts. The higher ORR activity of 3-4 nm sized particles than that of other sized Pt particles can be viewed from the alteration of redox energy levels as a function of Pt particle size. Adsorption and dissociation of O2 occurs only when there exists a resonance (energetically and symmetrically) between the metal cluster’s highest occupied molecular orbital (HOMO) and the acceptor orbital of the oxygen molecule. In one of the reports, Heiz et al.35 investigated the chemical interaction of Pt clusters with oxygen in terms of the frontier orbitals. It is believed that the d-band position is the most decisive parameter, when describing molecule-surface bonding. The HOMOs (or d-band centers) of Pt clusters are found to be between the atomic limit (-9.00 eV, IP of the Pt) and the bulk limit (-5.32 eV), and the acceptor orbital of molecular oxygen is -6.16 eV. It was proposed that the energy of the highest occupied orbitals (HOMO) of the clusters, associated with the energy of the center of the d-band, can be tuned from -9.0 eV, the ionization potential (IP) of the atom, to -5.32 eV, the Fermi energy of bulk platinum, by changing the cluster size. Based on these observations, the high ORR catalytic activity of Pt3C and Pt4C catalysts can be assigned with the suitable energy of HOMOs (or position of the center of the d-band) and the concomitant enhanced resonance with the antibonding π* state of O2. As a result, there should be facile transfer of electrons from the HOMO to the antibonding π* state of O2 and this causes the dissociation of the O2 molecule. For the small particles (
