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Electrocatalytic Activities of Oxygen Reduction Reaction on Pd/C and Pd-B/C Catalysts Mengzhi Wang, Xueping Qin, Kun Jiang, Yu Dong, Minhua Shao, and Wen-Bin Cai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12026 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 25, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Electrocatalytic Activities of Oxygen Reduction Reaction on Pd/C and Pd-B/C Catalysts Mengzhi Wanga‡, Xueping Qinb‡, Kun Jianga, Yu Donga, Minhua Shao b*, Wen-Bin Cai a*‡

a. Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Centre of Chemistry for Energy Materials, Department of Chemistry, Fudan University, Shanghai 200433, China

b. Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

ACS Paragon Plus Environment

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ABSTRACT: The investigation of electrocatalysis of oxygen reduction reaction (ORR) on nonPt electrodes is of great interest to address the current technical bottleneck of using costly and rare metal Pt in the cathodes of low temperature fuel cells. The present work presents a comparative study of ORR on carbon supported Pd and B-doped Pd (with ca.7 at.% B doping) nanocatalysts with well controlled particle sizes, dispersions and loadings (both with 20 wt.% Pd). It is found that the Pd-B/C exhibits a modestly higher electrocatalytic activity towards ORR: the specific activity is enhanced by factors of ca. 2.0 and 2.7 on Pd-B/C as compared to that on Pd/C in acidic media at 0.85 V and 0.90 V, respectively. In contrast, the corresponding enhancement factors are ca. 1.3 and 1.6, respectively in alkaline media. In order to understand the promoted ORR activity by B-doping, density functional theory (DFT) calculations are applied, revealing weakened adsorption of the O-containing species on B-doped Pd surfaces, consistent with the XPS and CO stripping results. Despite the modest improvement at this moment, it raises the hope of further developing Pd- based ORR catalysts as well as the concern of reasonable comparison of two sets of non-Pt catalysts.


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INTRODUCTION Low temperature fuel cells have attracted extensive attention as a clean and efficient power source which could directly convert chemical energy into electric energy. Oxygen reduction reaction (ORR) is the common cathode reaction of fuel cells, and Pt-based nanoparticles are the most widely used catalysts for driving this kinetically sluggish reaction.1-3 The extremely limited availability and high cost of Pt4 trigger extensive efforts to search for suitable replacement for Ptbased catalysts.5 Despite that the ultimate solution is aimed at cost-effective non-precious catalytic materials,6-7 including but not limited to N8-9 and P10-11-doped C, M-C-N12-15, and Fe-16 and Co-17-18based catalysts, the catalytic activities towards ORR on these non-Pt catalysts are very low and unstable especially in the acidic media. For the time being, Pd may be considered as an alternative to Pt owing to sort of similarity in their electronic structures and chemical properties,19-20 as well as the lower cost1 of Pd. What`s more, Pd-based catalysts show better methanol tolerance in the cathode of direct methanol fuel cells than Pt-based catalysts.21 However, the ORR activity of the Pd/C catalyst is significantly lower than that of the Pt/C catalyst since Pd surfaces tend to be oxidized at less positive potentials,22 due to a higher d-band center of Pd, as compared to that of Pt.23 It is well recognized that the stabilities of Pd-based materials are much lower than Pt-based materials in a fuel cell since Pd has a larger tendency to dissolve especially in acidic media at ORR working potentials.24-27 According to the so-called volcano-plot of the reactivity versus d-band center proposed by Nørskov and co-workers,28 doping with appropriate heteroatoms could tune the electronic structure of Pd and then cause a downshift of its d-band center, leading to improved activity of Pd. So far, alloying Pd with another metal25, 29-30 to form an alloy or forming a coreshell structure31-34 has been a common way to enhance the activity of Pd.


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Incorporating a light metalloid element such as B to the interstices of the Pd lattice is a new strategy for that end. Similar to the conventional Pd-M alloys, B lodged into the Pd lattice would also downshift the d-band center of this metal because of the electron transfer between the metal d states and the p states of the metalloid element.35 The earliest application of this strategy may be dated back to 2009 when we first reported that Pd-B/C significantly promoted the dehydrogenation of formic acid,36 which triggers a successive application of this material in the hydrogen production and direct formic acid fuel cells.37-38 Pd-B/C

showed smaller and

broadened XRD patterns, suggesting an expanded and slightly disordered Pd lattice due to B doping.34 The downshift of the d-band center, which was indicated by a positively shifted Pd core-level binding energy in XPS measurement, is presumably beneficial to the ORR kinetics. Recently, further confirmation of d-band center shift was made by Vo et al.39 by using theoretical calculation. They also found that the ORR activity on Pd-B nanoparticles directly deposited on glassy carbon (instead of practical Pd-B/C) in alkaline media are significantly higher than that of commercial Pd/C. Nevertheless, the Pd/C and Pd-B catalysts for their measurement were markedly different in terms of nanoparticle sizes, dispersion and Pd loadings, and the ORR polarization curve for Pd/C appeared obviously distorted probably due to the poor preparation and dispersion of the catalyst ink. Unlike in alkaline media efficient non-Pt catalysts have been far less investigated in acidic media. In fact, the ORR activity on either carbon black supported or unsupported Pd-B in acidic media has not been reported. Therefore, it may be necessary to revisit the B-doping effect by obtaining more reliable data. To address the above concern, Pd-B/C (with an average of 7 at.% B) and Pd/C are carefully synthesized by facile aqueous-phase methods with similar nanoparticle sizes, dispersions and loadings on the same type of carbon black support, respectively. The electro-catalysis of ORR on


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the Pd/C and Pd-B/C catalysts is evaluated by using a rotating disk electrode system in order to clarify the actual effect of B-doping in Pd. Density functional theory (DFT) calculations are also adopted to understand the origin of the B-doping effect on the ORR activity, in particular to confirm the weakened adsorption of undesired ORR intermediate species Oad and OHad on Bdoped Pd surface.

EXPERIMENTAL SECTION Catalysts Synthesis. The Pd/C catalyst was prepared through a classic method using NaBH4 as the reducing agent. Specifically, 3.15 mL of 50 mM Na2PdCl4 and 40 mL of H2O was put into the round-bottom flask. 67 mg of Vulcan XC-72 carbon pretreated with HNO3 was also added to form a projected 20 wt.% metal loading. After being sonicated for 20 min and stirred for 4 h, a uniform carbon slurry was obtained. The pH of the carbon slurry was adjusted to 8.5 by adding NH3·H2O dropwise under vigorous stirring. Then 10 mL of freshly prepared 0.08 mol L-1 NaBH4 was added in dropwise through a constant-flow pump at a rate of 0.6 mL min-1. The mixture was further stirred at room temperature overnight. Finally, the Pd/C was repeatedly filtered and rinsed with copious amount of ultrapure Milli-Q water, and vacuum-dried at 70 °C for 12 h. The carbon black supported Pd-B (Pd/C) was also synthesized in an aqueous phase according to the previous work of our group.36 2 mL of 50 mM Na2PdCl4 was mixed in 20 mL of H2O. 35 mg of NH4F and 0.180 g of H3BO3 were also added into the reaction solution. Appropriate amount of NH3·H2O was added to adjust pH of solution to 8.5. Then keeping bubbling N2 until the color of the solution became transparent and colorless. According to the projected 20 wt.% Pd loading, 43 mg of carbon powder was added into the solution. By being


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sonicated for ca. 50 min and be stirred vigorously for 1 h, a carbon slurry was formed. After that, 10 mL of freshly prepared 0.1 mol L-1 dimethylamineborane (DMAB) aqueous solution was added dropwise into the slurry under strong stirring through a constant-flow pump at 0.6 mL min-1. Then, the reacting mixture was kept in an ice bath for 2 h and then in 30 °C for 2 h, followed by filtering and rinsing with ultrapure Milli-Q water repeatedly. The Pd-B/C was finally vacuum-dried at 70 °C for 12 h. Materials Characterizations. The metallic loading and the atomic ratio of Pd-based catalysts synthesized by facile aqueous phase reaction were analyzed by means of inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The structures of these Pd-based catalysts were examined by X-ray diffraction (XRD) with Cu Kα radiation from 20 to 80°. The morphology and size distribution of the catalysts were observed by transmission electron microscopy (TEM). The binding energies of Pd core level electrons were analyzed by X-ray photoelectron spectroscopy (XPS) and the C1s peak at 284.6 eV was used as the reference for calibration. Electrochemical Measurement. Cyclic voltammetry (CV) and CO stripping voltammetry experiments were carried out in a CHI 660B electrochemistry workstation. The working electrode was a glassy carbon rotating disk electrode (GC-RDE, 5 mm diameter, 0.196 cm2) with synthesized catalysts on it. The catalyst ink was ﬁrst prepared by mixing 800 µL of H2O and 200 µL of isopropanol and 2 mg of catalyst ultrasonically for 30 s. Then the ink was mixed with 20 µL of Naﬁon (5 wt.%, Aldrich) and sonicated for 5 min., 10.6 µL of this ink was transferred onto a freshly polished GC-RDE by a pipet. The Pd loading was controlled to be 21 µg cm-2 on each electrode. A platinum sheet was used as the counter electrode, and RHE electrode served as the reference electrode.


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The CV was carried out in 0.1 M HClO4 solution saturated with N2 at a scan rate of 50 mV s-1 within the potential range from 0.1 to 1.05V. The electrochemical surface area (ECSA) was calculated by CO stripping experiment. Firstly, bubbling CO at 0.2 V for 15 min, and then removing the dissolved CO by bubbling N2 for 40 min while keeping at 0.2 V. At last, the CO stripping was carried out by the potential range from 0.1 to 1.05 V at a scan rate of 10 mV s-1. The ORR polarization curve was measured in O2-saturated 0.1 M HClO4 solution at a scan rate of 10 mV s-1 from 0.15 V to 1.1 V. At the same time, the rotating rate of rotating disk electrode was set as 1600 rpm. All of these experiments were done at room temperature. And the experimental condition was largely same in alkaline media except that the electrolyte was 0.1 M KOH and the reference electrode was Hg/HgO (0.1 M KOH). Nevertheless, all potentials in this paper were given with respect to RHE. Theoretical Basis. The periodic electronic structure calculations in this study were performed using density functional theory (DFT) by the Vienna Ab-Initio Simulation Package (VASP) code.40 In all calculations, the generalized gradient approximation (GGA-PW91)41 was adopted to evaluate exchange-correlation energy. Projected augmented wave (PAW)42 pseudopotentials were implemented and plane-wave basis was set with 400 eV kinetic energy cutoff. Spin-polarization was considered in all DFT calculations. Brillouin zone integrations were performed using Monkhorst–Pack43 grids of 5 × 5 × 1 for all the slab calculations with a Methfessel-Paxton smearing σ= 0.2 eV. The convergence criteria for the electronic selfconsistent iteration and the ionic relaxation loop were set to 10-5 eV and 0.03 eV Å-1, respectively.


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To calculate and compare the binding energies of O and OH, both Pd (111) and Pd-B (111) slab models were adopted. The slab models contained four layers and each layer had 16 atoms (corresponding to a 4 × 4 unit cell), and a 16 Å thick vacuum layer was imposed to sufficiently avoid the periodic interactions. For Pd (111), derived from the optimized Pd bulk structure with a lattice constant of 3.96 Å, the bottom two layers were fixed at the lattice position and the top two layers were fully relaxed. For Pd-B (111), three boron atoms were introduced into the pure Pd (111) slab resulting in a 6 atom% B-doping level. Binding energies of O and OH on both Pd(111) and Pd-B(111) slabs were calculated. The binding energy, Eb is defined as the energy difference between the adsorbate−surface adsorption system (Etot) and the systems of isolated clean surface (Eslab) and adsorbates (O or OH, Ead), as shown in equation, Eb = Etot - Eslab - Ead.

RESULTS AND DUSCUSSION Characterizations of Pd/C and Pd-B/C. The morphology and size distribution of carbon supported Pd and Pd-B nanoparticles are shown in Figure 1. The TEM images indicate that Pd and Pd-B nanoparticles are uniformly dispersed on carbon black with the mean size of 4.2 nm (a and b) for Pd nanoparticles and that of 4.4 nm for Pd-B (c and d), respectively. The results of the ICP-AES suggested that ca. 7 at.% of B was doped in Pd-B nanoparticles, and the Pd loadings in Pd-B/C and Pd/C were ca. 18 wt.%, close to the value (20 wt.%) estimated from the Pd(II) mass in the precursor solution.


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Figure 1. TEM images and corresponding particle size distribution histograms for Pd/C (a, b and e) and Pd-B/C (c, d and f). The XRD pattern (Figure 2a) for the as-synthesized Pd/C shows the peaks featuring the fcc structure of Pd, matching well with PDF-#65-6174, indicating a negligible B doping if any. The corresponding XRD peaks of Pd-B/C are shifted negatively and broadened slightly as compared to those of Pd/C, confirming the effective doping of B in the interstice of Pd lattice, which is consistent with our previous results.38 Pd core level doublet peaks for as-synthesized Pd/C and Pd-B/C were shown in Figure 2b, indicating a positive shift of ca. 0.6 eV for the Pd0 3d5/2 and Pd0 3d3/2 peaks after B-doping. And the percentages of these species for Pd-B/C and Pd/C are listed in the Table S1. Usually, for a metal of different valences, the loss of electrons would cause an increase of its core level binding energy in XPS spectrum. However, this conclusion is open to discussion for alloys.44-45 Here, we make reference to the report proposed by M. Watanabe et al.,44 in which a positive shift of Pt core level binding energy is explained with partial electron to Pt from a less noble alloying metal, such as Ru or Co, resulting in a down-shift of the d-band center of Pt due to the downshift of the reference level (or the Fermi Level) and a positive shift of Pt 4f7/2 core level binding energy in the XPS measurements. It is well recognized that the direction of the core-level shift is


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consistent with that of the valence band shift.44-45 Along this line, a positive shift of Pd 3d5/2 core level detected on Pd-B/C in our XPS measurement could be attributed as well to the partial electron transfer from B to Pd atoms. The electron transfer direction is consistent with a larger electronegativity of B-element in Pauling scale, i.e., EN (Pd) = 2.20 eV, EN (B) = 2.04 eV, as well as a smaller work function of (WF) B, i.e., WF (Pd）=5.2 eV, WF (B) = 4.5 eV. What`s more, such an explanation is also in agreement with a previous review46 which suggests two directions of electron transfer depending upon the percentage of doped B, that is, from M to B in boron-rich MBx (x ≥ 2) and from B to M for metal-rich MBx (x ≤ 2). The following DFT calculation results also indicate that B is the electron donor. The downshift of d-band center may lead to the smaller adsorption energy of surface OH and O species. This argument is further supported by our anodic CO stripping and DFT calculation results that will be discussed later.

Figure 2. (a) X-ray diffraction patterns of Pd/C (red line) and Pd-B/C (black line).The peaks corresponding to Pd (111), (200), and (220) were characteristics of the Pd face-centered cubic


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(fcc) phase (2θ = 40.1°, 46.7°, and 68.2° respectively), and (b) Comparison of the XPS spectra over the Pd 3d region for Pd/C and Pd-B/C. Electrocatalytic Properties of Pd/C and Pd-B/C. Figure 3a shows typical cyclic voltammograms (CVs) for Pd/C and Pd-B/C on glassy carbon electrodes (hereafter simplified as Pd/C or Pd-B/C electrodes) in N2-saturated 0.1 M HClO4 solution between 0.1 and 1.05 V at a scan rate of 50 mV s-1. The upper limit was set not too high to avoid the dissolution of Pd. The nearly overlapped double layer region arises from very close metal loading, particle size and dispersion of the two catalysts on the same carbon black. In addition, similar voltammetric features are also observed in terms of hydrogen adsorption-desorption and oxide formationreduction. Nevertheless, small difference is noted, that is, the oxide reduction peak locates at relatively positive potential on Pd-B/C in addition to relatively large peak area. Further evaluation of the electrochemical surface area (ECSA) was made by recording the CO stripping voltammograms on these two catalysts (shown in Figure 3b). The onset and peak oxidation peak potentials are both negatively shifted on Pd-B/C as compared to Pd/C, probably due to the weakened adsorption of CO on the former, consistent with a lowered d-band centre of Pd with B-doping. In addition, the ECSAs calculated for Pd/C and Pd-B/C are 0.765 and 0.855 cm2 µg-1, respectively, by assuming 420 µC cm-2 for a monolayer of CO. For comparison, the ECSA values were also evaluated from the PdO reduction peak charges assuming 425 µC cm-2 for a monolayer of PdO, i.e., 0.675 and 0.726 cm2 µg-1 for Pd/C and Pd-B/C, respectively. Unless specified, the SA and MA values based on ECSAs determined by CO stripping curves are adopted in the following. It may be pointed out that similar ECSAs together with close metal


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loadings, sizes and dispersions ensure a reliable comparison of ORR performances on Pd catalysts with and without B-doping.

Figure 3. (a) CVs for Pd/C and Pd-B/C electrodes in 0.1 M HClO4 at a scan rate of 50 mV s-1, and (b) the anodic CO stripping voltammograms in N2-saturated 0.1 M HClO4 at a scan rate of 10 mV s-1.

ORR polarization curves for both catalysts are presented in Figure 4. The half-wave potential of Pd-B/C (0.852 V) is positively shifted by 15 mV as compared to that of Pd/C (0.837 V). According to the following DFT calculation and the above CO stripping results, doping with


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B could weaken the adsorption of ORR intermediate on catalyst, and that would be conducive to the oxygen reduction, yielding a positive-shifted polarization curve given a very close ECSA. The kinetic currents, specific activities (SA) and mass activities (MA) at 0.90 V and 0.85 V in acidic media evaluated according to the Koutecki-Levich’ equation (Equation 1) are listed in Table 1, together with the ECSA values determined from CO stripping curves.

Figure 4. ORR polarization curves for Pd/C and Pd–B/C measured in O2-saturated 0.1 M HClO4 solution at 10 mV s-1 with a RDE speed of 1600 rpm.

Table 1. Electrocatalytic parameters for ORR at Pd/C and Pd-B/C catalysts in 0.1 M HClO4 and 0.1 M KOH with ECSAs determined by anodic CO stripping charges, assuming a monolayer charge of 420 µC cm-2.

ik / mA

Half-wave potential/V

SA / mA cm-2


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@ 0.9 V

@ 0.85 V

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@0.9 V

@0.85 V

Pd/C(HClO4)

0.079

0.539

0.837

0.024

0.169

Pd-B/C(HClO4)

0.211

1.139

0.852

0.064

0.345

Pd/C(KOH)

0.369

1.930

0.866

0.130

0.606

Pd-B/C(KOH)

0.602

3.410

0.883

0.170

0.960

ORR polarization curves of Pd/C and Pd-B/C were also measured in 0.1 M KOH (Figure 5), a similar positive shift of the ORR polarization curve was observed on Pd-B/C, and the resulting parameters are listed in Table 1. It can be seen that both catalysts exhibit a better ORR activity in alkaline media than that in acidic media. This result is in agreement with previous reports which conclude that Pd-based nanocatalysts exhibit better ORR performances in alkaline solution.24, 26 Unlike the severely distorted ORR polarization curve for Pd/C in alkaline media reported,39 typical polarization curves for both Pd/C and Pd-B/C catalysts were obtained in both alkaline and acidic media, facilitating the comparison.


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Figure 5. (a) Cyclic voltammograms on two catalyst-coated electrodes measured in 0.1 M KOH at a scan rate of 50 mV s-1, and (b) their ORR polarization curves were measured in 0.1 M O2saturated 0.1 M KOH solution at a scan rate of 10 mV s-1. Rotational speed: 1600 rpm.

In an acidic media, the kinetic current density of the Pd-B/C is 0.211 mA, or ca 2.7 times of that the Pd/C ( 0.079 mA) at 0.9 V. The corresponding SAs for Pd-B/C and Pd/C are of 0.064 and 0.024 mA cm-2, respectively. The MA of Pd-B/C (0.055 A mg-1) is 2.9 times as high as that of Pd/C (0.019 A mg-1). At 0.85 V, the catalytic activities of Pd-B/C are also more than twice as high as those of Pd/C. In a basic solution, the SA for Pd-B/C is 1.6 (at 0.85 V) or 1.3 (at 0.9 V) times as high as that of Pd/C. Our results reveal that the beneficial B-doping effect on ORR kinetics at Pd surfaces is more significant in acidic media than in basic media. Besides, the MAs and SAs for the Pd catalysts and the commercial Pt/C in acidic and alkaline media at different potentials are shown in Figure S1 and Table S2 for comparison. As pointed out by a latest review on ORR,1 a large divergence in the SA and MA for ORR shows up in literature even with the same Pt/C catalyst determined by the RDE polarization measurement together with the Koutecky-Levich (K-L) equation. The different preparation of a catalyst layer on RDE is one of the main reasons. Our data obtained in basic media indicate that B-doping does improve the activity of ORR on Pd surfaces, but with SA and MA values as well as an enhancement factor much smaller than the previous report.39 Since no previous data of ORR on Pd-B or Pd-B/C can be seen from third independent party, we can only verify the validity of the ORR data obtained on Pd/C both in acidic and basic media. Very recently, Erikson


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et al.47 and Luesi et al.48 measured the MA and SA for a commercial Pd/C with a Pd loading of 20 µg cm-2. They reported that the SA of Pd/C was 0.13 mA in acidic media at 0.85 V, and 0.21 mA cm-2 in basic media at 0.09 V, comparable to our results, i.e., 0.19 mA cm-2 at 0.85 V in acidic media and 0.14 mA cm-2 at 0.90 V in basic media, based on the surface areas determined by PdO reduction charges.

Figure 6 Linear sweep voltammograms at various rotating speeds and the j-1 vs. ω-1/2 plots at different potentials for Pd/C (a, b) and Pd-B/C (c, d) electrodes in O2-saturated 0.1 M HClO4 solution at a scan rate of 10 mV s-1. Figure 6 shows the linear sweep voltammograms recorded at 10 mV s-1 from 0.15 to 1.05 V for rotating speeds 400, 900, 1600, 2500 and 3600 rpm and the j-1 vs. ω-1/2 plots at potentials 0.70, 0.75 and 0.80 V for the Pd/C (Figure 6a and b) and Pd-B/C (Figure 6c and d) catalysts


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toward the ORR in O2-saturated 0.1 M HClO4 aqueous solution. K-L equation (1) was used to calculate the kinetic current jk at 0.85 or 0.90 V and the number of electrons (n) transferred per O2 molecule in ORR, i.e.,

= + = − − . / / /

(1)

where F is the Faraday constant (96500 C mol-1), k is the rate constant for O2 reduction, Cb is the concentration of O2 in the bulk (1.22×10-6 mol mL-1), D is the diffusion coefficient of O2 (1.93×10-5 cm2 s-1), υ is the kinematic viscosity of solution (0.01 cm2 s-1), ω is the rotating speed (rad s-1). From the K-L equation, the reciprocal of square root of rotating speed (ω−1/2) and the reciprocal of measured current density (j−1) should exhibit a linear relationship. Indeed, the j-1 vs.

ω-1/2 plots (Figure 6b and d) for the two catalysts show good linear fitting, and in both cases the number of electron transferred in ORR is approximately 4, indicating that direct 4-electron reduction reaction on these electrodes, which means H2O is the main production during the oxygen reduction reaction. Calculated adsorption energies of O and OH on Pd (111) and B-doped Pd (111). In order to obtain further insights in the origin of the catalytic activity enhancement from B-doping, the binding energies of reaction intermediates O and OH on Pd (111) and Pd-B (111) were calculated. In an optimized Pd-B (111) slab model, three B atoms were evenly distributed and occupied the octahedral interstitial sites between the Pd layers, which was in agreement with experimental results.49 Unlike previous calculations conducted by Yoo et al.50 and Doan et al.39 where all the B atoms were distributed in one single layer between the first and second Pd layers, each B atom locates in its own layer in the current study. We believe that this configuration is more close to the real structure as it is unlikely that all B atoms segregate to the same layer


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during the synthesis of Pd-B at room temperature. Each boron atom was coordinated to six Pd atoms in the unit cell, forming an octahedral geometry. The cross-section and top view of slab models of Pd (111) and Pd-B(111) are shown in Figure S2 and Figure 7a, b, respectively. Upon B doping, the interlayer distance increases obviously by 2.3% from 2.282 Å to 2.335 Å, in good agreement with the XRD results38. The static DFT calculation6 revealed that boron atoms are the electron donators (Figure S3), in consistent with the XPS results in Figure 2b. The Bader charge analysis51 indicated that 0.4 electron transferred from each boron atom to surrounding Pd atoms. The detailed calculation procedure and charge density distribution could be found in SI.

Figure 7 The top view of Pd (111) (a) and Pd-B (111) (b) slab models without adsorbates. O adsorption at a fcc site of Pd(111) (c) and a hcp site of Pd-B(111) (e); OH adsorption at a fcc site of Pd(111) (d) and a bridge site of Pd-B(111) (f). Dark blue balls: Pd atoms; light pink balls: boron atoms; red balls: O atoms; white balls: H atoms. On a Pd(111) surface, among all the possible adsorption sites (fcc, hcp, top and bridge), the fcc site was found to have the strongest bindings for both O and OH, as shown in Figure 7c and d. The binding energies for O and OH at a fcc site was calculated to be -1.78 and -3.07 eV,


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respectively. On a Pd-B (111) surface, four possible adsorption sites including one fcc and three hcp sites, close to the B-doping site were considered (Figure S4). For the adsorption of atomic O, the binding at the hcp sites (-1.36 eV) shown in Figure 7e is much stronger than that at the fcc site (-0.97 eV) (Table S3). This value is 0.42 eV weaker than that on the Pd (111) surface. In the case of OH (Figure S5), one of the adsorption configuration at the hcp site turned into the bridge adsorption (Figure 7f) after optimization with the highest binding energy of -2.98 eV (Table S3). This value is slightly lower than that on a Pd (111) surface. In Yoo et al, the OH binding energy on Pd was higher after B doing.50 This discrepancy could come from the different models used in the current study and theirs. It has been well known that the oxygen binding energy on un-doped Pd surfaces are too strong, which makes the further reduction of O-containing species to form water or OH- difficult19,

32

The weaker oxygen binding energy caused by the B-doping is

desirable to balance the rates of electron transfer/ O-O bond breaking and removal of Ocontaining species from the Pd surfaces. We also calculated the oxygen binding energies at the sites defined by Pd atoms without direct coordinating with B atoms. The oxygen binding energies on two representative fcc sites (Figure S6) were calculated to be -1.79 and -1.81 eV, which are very close to the value on the Pd (111) surface (-1.78 eV). This result implies that the influence of B atoms on Pd atoms that are not directly coordinated with them is relatively small. The slightly stronger oxygen adsorption might be caused by the tensile strain caused by the interstitial B doping.

CONCLUSION


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In summary, ORR activities on Pd-B/C and Pd/C in acidic media have been comparatively studied for the first time with careful control of Pd loadings, nanoparticle sizes and dispersions on the same carbon black support in order to exclude the possible size, substrate and dispersion effects. It was found that the mass and specific activities on Pd-B/C were nearly 3 times as high as those on Pd/C at 0.9 V RHE in acidic media, and a somewhat smaller enhancement was found in basic media. The delicate control of measurement conditions as well as the reasonable comparison with previous relevant reports enables to justify the present ORR data for Pd-B/C. According to the experimental and theoretical calculation results, the enhanced ORR activity might be due the weakened adsorption of ORR reaction intermediates on Pd surfaces due to Bdoping. Further work is underway on varying B-doping concentration to further improve the ORR activity and stability.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (WBC); [email protected] (MHS). Author Contributions ‡: contribute equally to the present work Supporting Information


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Detemination of Pd loading on carbon black; XPS deconvulution result; ORR polarization curves as well as MAs and SAs of ORR on Pd/C, Pd-B/C and Pt/C; Cross-section view of Pd(111) and Pd-B(111) slab models; Static DFT calculations and Bader charge analysis on Pd-B(111). This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS We gratefully acknowledge the financial supports from the 973 Program of MOST (No. 2015CB932303) and the NSFC (Nos. 21473039 and 21273046). The work at the Hong Kong University of Science and Technology was supported by the Research Grant Council of the Hong Kong Special Administrative Region (26206115). And we also appreciate a technical support from Prof. Qiang Fu and Mr. Hao Wu of Dalian Institute of Chemical Physics, Chinese Academy of Sciences.


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REFERENCES 1. Shao, M.; Chang, Q.; Dodelet, J.-P.; Chenitz, R., Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116, 3594-3657. 2. Yang, H. Z.; Kumar, S.; Zou, S. Z., Electroreduction of O2 on Uniform Arrays of Pt Nanoparticles. J. Electroanal. Chem. 2013, 688, 180-188. 3. Adzic, R. R.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Shao, M.; Wang, J. X.; Nilekar, A. U.; Mavrikakis, M.; Valerio, J. A.; Uribe, F., Platinum Monolayer Fuel Cell Electrocatalysts. Top. Catal. 2007, 46, 249-262. 4. Antolini, E., Palladium in Fuel Cell Catalysis. Energ. Environ. Sci.2009, 2 (9), 915-931. 5. Nie, Y.; Li, L.; Wei, Z. D., Recent Advancements in Pt and Pt-free Catalysts for Oxygen Reduction Reaction. Chem. Soc. Rev. 2015, 44, 2168-2201. 6. Banham, D.; Ye, S.; Pei, K.; Ozaki, J.; Kishimoto, T.; Imashiro, Y., A Review of the Stability and Durability of Non-precious Metal Catalysts for the Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells. J. Power Sources 2015, 285, 334-348. 7. Liu, G.; Li, X. G.; Ganesan, P.; Popov, B. N., Development of Non-precious Metal Oxygen-reduction Catalysts for PEM Fuel Cells Based on N-doped Ordered Porous Carbon. Appl. Catal. B Environ. 2009, 93, 156-165. 8. Shui, J.; Wang, M.; Du, F.; Dai, L., N-doped Carbon nanomaterials are Durable Catalysts for Oxygen Reduction Reaction in Acidic Fuel Cells. Sci. Adv. 2015, 1, e1400129. 9. Gong, X.; Liu, S. S.; Ouyang, C. Y.; Strasser, P.; Yang, R. Z., Nitrogen- and PhosphorusDoped Biocarbon with Enhanced Electrocatalytic Activity for Oxygen Reduction. ACS Catal. 2015, 5, 920-927. 10. Wu, J.; Yang, Z. R.; Li, X. W.; Sun, Q. J.; Jin, C.; Strasser, P.; Yang, R. Z., PhosphorusDoped Porous Carbons as Efficient Electrocatalysts for Oxygen Reduction. J. Mater. Chem. A 2013, 1, 9889-9896. 11. Yang, D. S.; Bhattacharjya, D.; Song, M. Y.; Yu, J. S., Highly Efficient Metal-free Phosphorus-doped Platelet Ordered Mesoporous Carbon for Electrocatalytic Oxygen Reduction. Carbon 2014, 67, 736-743. 12. Zhao, Y.; Kamiya, K.; Hashimoto, K.; Nakanishi, S., Efficient Bifunctional Fe/C/N Electrocatalysts for Oxygen Reduction and Evolution Reaction. J. Phys. Chem. C 2015, 119, 2583-2588. 13. Wang, Y. C.; Lai, Y. J.; Song, L.; Zhou, Z. Y.; Liu, J. G.; Wang, Q.; Yang, X. D.; Chen, C.; Shi, W.; Zheng, Y. P.; et al. S-Doping of an Fe/N/C ORR Catalyst for Polymer Electrolyte Membrane Fuel Cells with High Power Density. Angew. Chem. Int. Ed. 2015, 54, 9907-9910. 14. Jiang, W. J.; Gu, L.; Li, L.; Zhang, Y.; Zhang, X.; Zhang, L. J.; Wang, J. Q.; Hu, J. S.; Wei, Z. D.; Wan, L. J., Understanding the High Activity of Fe-N-C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe-N-x. J. Am. Chem. Soc. 2016, 138, 3570-3578.


22

Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15. Liu, T.; Zhao, P. P.; Hua, X.; Luo, W.; Chen, S. L.; Cheng, G. Z., An Fe-N-C Hybrid Electrocatalyst Derived from a Bimetal-organic Framework for Efficient Oxygen Reduction. J. Mater. Chem. A 2016, 4, 11357-11364. 16. Zhao, P. P.; Xu, W.; Hua, X.; Luo, W.; Chen, S. L.; Cheng, G. Z., Facile Synthesis of a N-Doped Fe3C@CNT/Porous Carbon Hybrid for an Advanced Oxygen Reduction and Water Oxidation Electrocatalyst. J. Phys. Chem. C 2016, 120, 11006-11013. 17. Bezerra, C. W. B.; Zhang, L.; Lee, K. C.; Liu, H. S.; Marques, A. L. B.; Marques, E. P.; Wang, H. J.; Zhang, J. J., A Review of Fe-N/C and Co-N/C Catalysts for the Oxygen Reduction Reaction. Electrochim. Acta 2008, 53, 4937-4951. 18. Mamlouk, M.; Kumar, S. M. S.; Gouerec, P.; Scott, K., Electrochemical and Fuel Cell Evaluation of Co Based Catalyst for Oxygen Reduction in Anion Exchange Polymer Membrane Fuel Cells. J. Power Sources 2011, 196, 7594-7600. 19. Shao, M., Palladium-based Electrocatalysts for Hydrogen Oxidation and Oxygen Reduction Reactions. J. Power Sources 2011, 196, 2433-2444. 20. Konda, S. K.; Amiri, M.; Chen, A. C., Photoassisted Deposition of Palladium Nanoparticles on Carbon Nitride for Efficient Oxygen Reduction. J. Phys. Chem. C 2016, 120, 14467-14473. 21. Lo Vecchio, C.; Alegre, C.; Sebastian, D.; Stassi, A.; Arico, A. S.; Baglio, V., Investigation of Supported Pd-Based Electrocatalysts for the Oxygen Reduction Reaction: Performance, Durability and Methanol Tolerance. Materials 2015, 8, 7997-8008. 22. Norskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H., Origin of the Overpotential for Oxygen Reduction at a Fuel-cell Cathode. J. Phys. Chem. B 2004, 108, 17886-17892. 23. Greeley, J.; Norskov, J. K., A General Scheme for the Estimation of Oxygen Binding Energies on Binary Transition Metal Surface Alloys. Surf. Sci. 2005, 592, 104-111. 24. Cadle, S. H., Ring-Disk Electrode Study of Palladium Dissolution. J. Electrochem. Soc. 1974, 121, 645-648. 25. Pires, F. I.; Villullas, H. M., Pd-based catalysts: Influence of the Second Metal on Their Stability and Oxygen Reduction Activity. J. Hydrogen Energy 2012, 37, 17052-17059. 26. Alexeyeva, N.; Sarapuu, A.; Tammeveski, K.; Vidal-Iglesias, F. J.; Solla-Gullon, J.; Feliu, J. M., Electroreduction of Oxygen on Vulcan Carbon Supported Pd Nanoparticles and PdM Nanoalloys in Acid and Alkaline Solutions. Electrochim. Acta 2011, 56, 6702-6708. 27. Jiang, L.; Hsu, A.; Chu, D.; Chen, R., Oxygen Reduction Reaction on Carbon Supported Pt and Pd in Alkaline Solutions. J. Electrochem. Soc. 2009, 156, B370-B376. 28. Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Norskov, J. K., Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the SurfaceElectronic Structure. Angew. Chem. Int. Ed. 2006, 45, 28972901.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 26

29. Shao, M. H.; Sasaki, K.; Adzic, R. R., Pd-Fe Nanoparticles as Electrocatalysts for Oxygen Reduction. J. Am. Chem. Soc. 2006, 128, 3526-3527. 30. Savadogo, O.; Lee, K.; Oishi, K.; Mitsushima, S.; Kamiya, N.; Ota, K. I., New Palladium Alloys Catalyst for the Oxygen Reduction Reaction in an Acid Medium. Electrochem. Commun. 2004, 6, 105-109. 31. Javaheri, M., Investigating the Influence of Pd Situation (as Core or Shell) in Synthesized Ctalyst for ORR in PEMFC. Int. J. Hydrogen Energy 2015, 40, 6661-6671. 32. Shao, M. H.; Huang, T.; Liu, P.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Adzic, R. R., Palladium Monolayer and Palladium Alloy Electrocatalysts for Oxygen Reduction. Langmuir 2006, 22, 10409-10415. 33. Fu, G. T.; Liu, Z. Y.; Chen, Y.; Lin, J.; Tang, Y. W.; Lu, T. H., Synthesis and Electrocatalytic Activity of Au@Pd Core-Shell Nanothorns for the Oxygen Reduction Reaction. Nano Res. 2014, 7, 1205-1214. 34. Jiang, G. M.; Zhu, H. Y.; Zhang, X.; Shen, B.; Wu, L. H.; Zhang, S.; Lu, G.; Wu, Z. B.; Sun, S. H., Core/Shell Face-Centered Tetragonal FePd/Pd Nanoparticles as an Efficient Non-Pt Catalyst for the Oxygen Reduction Reaction. ACS Nano 2015, 9, 11014-11022. 35. Ge, Q. F.; Kose, R.; King, D. A., Adsorption Energetics and Bonding from Femtomole Calorimetry and from First Principles Theory. In Advances in Catalysis, Vol 45: Impact of Surf. Sci. on Catalysis, Gates, B. C.; Knozinger, H., Eds. 2000; Vol. 45, pp 207-259. 36. Wang, J.-Y.; Kang, Y.-Y.; Yang, H.; Cai, W.-B., Boron-Doped Palladium Nanoparticles on Carbon Black as a Superior Catalyst for Formic Acid Electro-oxidation. J. Phys. Chem. C 2009, 113, 8366-8372. 37. Jiang, K.; Xu, K.; Zou, S.; Cai, W.-B., B-Doped Pd Catalyst: Boosting RoomTemperature Hydrogen Production from Formic Acid-Formate Solutions. J. Am. Chem. Soc. 2014, 136, 4861-4864. 38. Jiang, K.; Chang, J.; Wang, H.; Brimaud, S.; Xing, W.; Behm, R. J.; Cai, W.-B., Small Addition of Boron in Palladium Catalyst, Big Improvement in Fuel Cell's Performance: What May Interfacial Spectroelectrochemistry Tell? ACS Appl. Mater. Interfaces 2016, 8, 7133-7138. 39. Tat Thang Vo, D.; Wang, J.; Poon, K. C.; Tan, D. C. L.; Khezri, B.; Webster, R. D.; Su, H.; Sato, H., Theoretical Modelling and Facile Synthesis of a Highly Active Boron-Doped Palladium Catalyst for the Oxygen Reduction Reaction. Angew. Chem. Int. Ed. 2016, 55, 68426847. 40. Kresse, G.; Furthmuller, J., Efficiency of Ab-initio Total Energy Calculations for Metals and Semiconductors Using a Plane-wave Basis Set. Comp. Mater. Sci. 1996, 6, 15-50. 41. Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C., Atoms, Molecules, Solids, and Surfaces – Applications of the Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B 1992, 46, 6671-6687. 42. Blochl, P. E.; Forst, C. J.; Schimpl, J., Projector Augmented Wave Method: Ab Initio Molecular Dynamics with Full Wave Functions. Bull. Mater. Sci. 2003, 26, 33-41.


24

Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


43. Monkhorst, H. J.; Pack, J. D., Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188-5192. 44. Wakisaka, M.; Mitsui, S.; Hirose, Y.; Kawashima, K.; Uchida, H.; Watanabe, M., Electronic Structures of Pt-Co and Pt-Ru Alloys for Co-tolerant Anode Catalysts in Polymer Electrolyte Fuel Cells Studied by EC-XPS. J. Phys. Chem. B 2006, 110, 23489-23496. 45. Hu, S.; Ha, S.; Scudiero, L., Temperature Dependence Study of Pd-Cu Supported Bimetallic Films by Photoelectron Spectroscopy and Cyclic Voltammetry. Electrochim. Acta 2013, 105, 362-370. 46. Carenco, S.; Portehault, D.; Boissiere, C.; Mezailles, N.; Sanchez, C., Nanoscaled Metal Borides and Phosphides: Recent Developments and Perspectives. Chem. Rev. 2013, 113, 79818065. 47. Erikson, H.; Luesi, M.; Sarapuu, A.; Tammeveski, K.; Solla-Gullon, J.; Feliu, J. M., Oxygen Electroreduction on Carbon-supported Pd Nanocubes in Acid Solutions. Electrochim. Acta 2016, 188, 301-308. 48. Luesi, M.; Erikson, H.; Sarapuu, A.; Tammeveski, K.; Solla-Gullon, J.; Feliu, J. M., Oxygen Reduction Reaction on Carbon-supported Palladium Nanocubes in Alkaline Media. Electrochem. Commun. 2016, 64, 9-13. 49. Tamaki, J.; Nakayama, S.; Yamamura, M.; Imanaka, T., Preparation and Catalytical Properties of Palladium (Boron, Phosphorus) Thin-films Using an RF Sputtering Method. J. Mater. Sci. 1989, 24, 1582-1588. 50. Yoo, J. S.; Zhao, Z. J.; Norskov, J. K.; Studt, F., Effect of Boron Modifications of Palladium Catalysts for the Production of Hydrogen from Formic Acid. ACS Catal. 2015, 5, 6579-6586. 51. Tang, W.; Sanville, E.; Henkelman, G., A Grid-based Bader Analysis Algorithm without Lattice Bias. J. Phys. Condens. Matter 2009, 21, 084204.


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TOC Graphic


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C and

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