Controllable Increase of Boron Content in B-Pd Interstitial Nanoalloy

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Controllable increase of B content in B-Pd interstitial nanoalloy to boost oxygen reduction activity of Pd jun li, Junxiang Chen, Qiang Wang, Wen-Bin Cai, and Shengli Chen Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b03732 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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Chemistry of Materials

Controllable increase of B content in B-Pd interstitial nanoalloy to boost oxygen reduction activity of Pd Jun Lia, Junxiang Chena, Qiang Wanga, Wen-Bin Cai*, b, Shengli Chen*, a a

Hubei Key Laboratory of Electrochemical Power Sources, Department of Chemistry, Wuhan

University, Wuhan 430072, China. b

Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative

Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Fudan University, Shanghai 200433, China. ABSTRACT: Boron doping can boost the catalytic activity of palladium for diverse reactions. Precise control of the doping content is crucial but remains difficult in current synthesis, which generally involves the use of instable and costly borane-organic compounds. Herein, by taking advantage of the relatively strong solvation of DMF to Na+ and the increased stability BH4- in DMF, we synthesize B-Pd interstitial nanocrystals in DMF with NaBH4 acting as reductant and B source. The B content, which can be readily tuned by changing the reaction time and NaBH4 concentration, can reach up to 20 at. %. Such a high B doping results in a great beneficial effect on the catalytic capability of Pd toward the oxygen reduction reaction (ORR). The synthesized B-Pd nanoalloy exhibits a mass and specific activity for ORR that is ca. 14 and 14.6 times higher than the state-of-the-art commercial Pt catalyst in alkaline solution. DFT calculations reveal three types of surface sites that are responsible for the enhanced activity, namely, Pd-BO2 assemblies, Pd atoms neighbored by the assemblies and the Pd atoms modified with sub-surface B atoms. The Pd-BO2 assembly has a Pt-like activity, while the Pd-BO2 assembly-neighbored and subsurface B-modified Pd atoms could catalyze ORR much more efficiently than Pt. The facile and controllable B doping in Pd should strengthen the power of Pd-based catalysts and thus provides a great prospect for their wide applications.

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Introduction Alloying is a general yet efficient route to enhance the catalytic performance of metals.1-5 Bimetallic nanoalloys have been the main focus in the past years. Recently, there is a growing interest in the use of non-metal elements to promote the catalytic capability of metal nanocrystals.6-13 Boron-doped palladium (B-Pd) has been shown to exhibit versatile catalytic capabilities for reactions such as hydrogenation of alkyne,14,

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furfural hydrogenation,16 H2

production,17, 18 electro-oxidation of ethanol and formic acid,19-21 and oxygen reduction,22, 23. In considering the structure-property relationship, one of the urgent challenges in extending the applications of B-Pd catalysts is to develop a simple and facile route that enables precise control of B-doping in Pd over a wide range. Historically, borane was used to deposit B atoms on metal surface based on the adsorption and decomposition of borane on metal surface.24-27. As for metallic Pd, the maximal B/Pd atom ratio up to 20% can be reached.28 Recently, researchers have reported the doping of B in metal nanocrystals in solution using borane-organic compounds (BOCs) as dopant. Tsang and co-workers15 prepared B-Pd catalyst with the high B content using borane tetrahydrofuran to deposit B onto as-synthesized Pd/C in tetrahydrofuran. Cai et al.19 reported the in-situ doping of B during the growth of Pd nanocrystals with Dimethylamine borane as reductant and dopant in aqueous solution, and a B content of ca. 7 at. % was achieved. It has been shown that BOCs can release borane in solutions. Given the cost, instability and safety issues of BOCs, the current solution synthesis has the limitations in precisely controllable doping of B content, as well as scale-up catalyst preparation and application. NaBH4 is a commonly used reductant for synthesizing metal nanocrystals in aqueous system. Although it has been used as a boronizing reagent in the synthesis of transition metal (e.g., Fe and Co) borides in aqueous solution29,

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or in high temperature condition31, NaBH4 is not


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effective for boronization of noble metals owing to the rapid hydrolysis of BH4-. As for Pd, the use of NaBH4 usually results in B uptake less than 2 at. %.19, 29 However, this problem can be overcome by using non-aqueous solvent. It is known that NaBH4 can be dissociated into Na+ and BH4- in N, N-dimethylformamide (DMF) due to the relatively strong solvation of Na+ by DMF molecules.32 On the other hand, BH4- is relatively stable in the polar aprotic solvent such as DMF.33 These advantages would be in favor of the tunable decomposition and deposition of B atoms on the Pd surface. In this work, we explore the use of NaBH4 as reductant and dopant for one-pot synthesis of BPd nanocrystals in DMF. It is found that B-Pd nanocrystals with different B content can be formed by altering the reaction time and concentration of NaBH4, with a B uptake up to ca. 20 at. % being reached. The as-prepared B-Pd nanocrystal shows a structure of B-Pd interstitial nanoalloy decorated with sub-oxidized B atoms on the surface. The catalytic performance of the B-Pd nanoalloy with 20 at. % B content for the oxygen reduction reaction (ORR) is tested in alkaline solution, and ca. 14 and 14.6 times enhancement in mass and specific activity is achieved comparing to the state-of-the-art commercial Pt catalyst, suggesting a great promise of B-Pd nanoalloys as cathode catalyst in alkaline fuel cells, which have attracted increasing interest due to the recent progress in developing alkaline polymer electrolyte membranes.34, 35 Furthermore, our DFT calculations show that both the sub-oxidized surface B and the subsurface B can shift the oxophilicity of surface Pd atoms towards the optimal value that corresponds to top of the activity volcano plot for ORR, while recent studies have attributed the ORR activity promotion only to the sub-surface B atoms. Experimental section Chemicals


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Palladium (II) acetylacetonate [Pd(acac)2, 98%], palladium chloride (PdCl2, 98%) and sodium chloropalladite (Na2PdCl4, 98%) was purchased form Kunming Sino-Platinum. XC-72R carbon was purchased form Vulcan. Ethanol (99.8%), N, N-dimethylformamide (DMF, 99.5%) and sodium borohydride (NaBH4, 96%) were purchased from Sinopharm Chemical Reagent (Beijing). Hydrogen (99.99%) stored in a tank was purchased from Tianke gases (Sichuan). All chemicals were used without further purification. Ultrapure water (Ulupure Chengdu) was used throughout the experiments. Synthesis of B-Pd/C catalysts 15 mg Pd(acac)2 and 16 mg XC-72R was added into 20 ml DMF in a 25 ml round-bottom flask. After an ultrasonic treatment of 1 hour, 50 mg NaBH4 (dissolved in 2 ml DMF) was injected into the solution in ice-cooled water with vigorous stirring for 2 hours. The product was then collected by centrifugation at 8000 rpm for 3 minutes and washed with water and ethanol for several times, and finally dried at room temperature overnight. The B-Pd/C catalyst thus prepared possessed a Pd content of 20 wt %. For comparison, similar synthesis using NaBH4 was also conducted in aqueous solution by replacing the DMF with 20 ml H2O and Pd(acac)2 with Na2PdCl4 while keeping the same content of Pd in solution. The resulted samples are denoted as B-Pd/C-H2O. The commercial pure Pd/C and Pt/C catalysts (metal mass contents: 20 wt %) from BASF and Johnson-Matthey (JM) were used as the reference catalysts. Catalyst characterization TEM images were obtained on JEM-2100 at an acceleration Voltage of 200 kV. X-ray powder diffraction (XRD) patterns were collected using Bruker D8-Advance X-ray diffractometer used Cu Ka radiation source (λ = 0.154178 nm) with a scan rate of 4° min-1. X-ray photoelectron spectroscopy (XPS) measurement was performed with a Thermo Fisher ESCALAB 250Xi


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spectrophotometer using Al Ka as excitation light source. XPS data was fitted using the software XPSPEAK41. The inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was taken using IRIS Intrepid II XSP (Thermo Fisher Scientific). Electrochemical measurements A three-electrode cell was used to perform electrochemical measurements, with a platinum foil as counter electrode and a Hg/HgO as reference electrode. The working electrode was prepared by dropping 6 µl aliquot of the catalyst ink onto a glassy-carbon electrode (5 mm, 0.196 cm2). This catalyst ink was prepared by dispersing 5 mg as-synthesized catalyst in 1ml isopropanol and 20µl Nafion solution (5 wt. %, Dupont), ultrasonicating for 40 minutes. The precious metal loading of all the catalysts on glassy carbon electrode was 30.6 µg·cm-2. Cyclic voltammetry was performed at 30 °C in Ar saturated 0.1 M KOH solution at a sweep rate of 50 mV·s-1. Oxygen reduction polarization curves were measured in O2 saturated 0.1 M KOH solution at a scan rate of 5 mV·s-1 with a rotating speed of 1600 rpm. The kinetic currents ik at a given potential was calculated from ORR polarization curves by using the Koutecky-Levich equation, i-1= ik-1+ id-1, Where i is the measured current, ik is the kinetic current and id is the diffusion-limiting current. The electrochemical workstation used was Chenhua CHI (Shanghai). Accelerated durability test was conducted between 0.6-1.0 V at 50 mV·s-1 in O2 saturated 0.1 M KOH solution at 30 °C. During the test, a carbon paper was used as counter electrode instead of platinum foil in case that platinic counter electrode would have dissolved and interfere the testing result. DFT calculations Spin-polarized DFT calculations were performed with periodic super-cells under the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) functional for exchange-correlation and the ultrasoft pseudopotentials for nuclei and core electrons. The Kohn-


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Sham orbitals were expanded in a plane-wave basis set with a kinetic energy cutoff of 30 Ry and the charge-density cutoff of 300 Ry. The Fermi-surface effects has been treated by the smearing technique of Methfessel and Paxton, using a smearing parameter of 0.02 Ry. Periodically repeated four-layer slabs of face centered cube (fcc) structure were used to model the Pd(111) electrodes, with the bottom two layers fixed to DFT-optimized equilibrium lattice constant of 4.00 Ǻ and the top two layers were allowed to relax with the adsorbates during the calculations until the Cartesian force components acting on each atom were below 10-3 Ry/Bohr and the total energy converged to within 10-5 Ry. A Slab of (2√3×2√3) is used. The Brillouin-zones were sampled with a 5×5×1 k-point mesh. The PWSCF codes contained in the Quantum ESPRESSO distribution36 were used to implement most of the calculations. The effect of B doping on the ORR activity of Pd is investigated by calculating the O adsorption energy at various surface sites relating to surface and sub-surface B atoms (details are given in RESULTS AND DISCUSSION), from which the ORR activity of each site is compared with that of pure Pd and Pt based on the theoretical volcano model proposed by Nørskov et al.37, 38

Figure 1. Illustrations of the synthesis procedure and formation mechanism of B-Pd/C.

Results and discussion Characterization and formation of B-Pd catalyst. The B-Pd/C interstitial nanoalloys (BPd/C) was synthesized by adding NaBH4 into carbon-dispersed DMF solution containing


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Pd(acac)2 (Figure 1). Monodispersed nanocrystals on carbon can be obtained because of the excellent dispersibility of carbon materials in DMF20, 39 without using surfactant. The TEM and HRTEM images of B-Pd/C are presented at Figure 2A and 2B. The size distribution of the B-Pd particles is determined to be 4.3 nm (Figure S1A). In contrast, B-Pd/C-H2O catalyst was synthesized in the same condition except using H2O instead of DMF, possessed large and agglomerative nanoparticles on carbon (ca. 8.7 nm), as shown by the TEM images in Figure S2B.. The TEM image of the B-Pd/C exhibited a lattice fringe of ca. 0.227 nm (inset of Figure 2B), which can be assigned to the (111) facet of Pd and was apparently larger than that of pure Pd (0.224 nm, Figure S2). The expansion of the lattice should be caused by the penetration of B atoms into Pd lattice, which was also indicated by the shift of the XRD diffraction peaks to the lower 2θ comparing to that of pure Pd/C (corresponding to the PDF# 05-0681). It is thus demonstrated that Pd and B form interstitial nanocrystals similar with other reports..15, 19, 22 Figure 2D displays the XPS spectra for Pd 3d in B-Pd/C. The peaks at 335.3 eV and 340.6 eV can be attributed to the 3d5/2 and 3d3/2 of Pd0 respectively; while those at 335.9 eV and 341.3 eV can be assigned to the 3d5/2 and 3d3/2 levels of Pd2+. In addition, the doublets at 337.9 eV and 343.1 eV correspond to the 3d5/2 and 3d3/2 states of Pd4+. Compared with that in pure Pd (3d5/2 335.1 eV, Figure S3), the 3d5/2 state of Pd in the B-Pd/C was elevated by 0.2 eV. The XPS spectra of B 1s (Figure 2E) and O 1s (Figure 2F) in the B-Pd/C were also monitored. The B 1s gave peaks at binding energies of 189.1 eV and 191.9 eV respectively, and the O 1s showed a peak at ca. 532.3 eV. Because the binding energy for B0 1s should be 186-187 eV24, 26 and B3+ 1s is generally around 193 eV,40, 41 the B 1s binding energy in Figure 2E suggest the surface B atoms on the prepared B-Pd/C are sub-oxidized. The B 1s binding energy of BxOy has been reported in the range of 188-190 eV

40

. To illuminate the oxidizability of surface B atoms, we


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estimated the reaction free energy for the successive addition of an O ad-atom through O2 dissociation (∆GO) on surface sites associated with a surface B (Figure S4) using DFT calculations. Negative ∆GO values were obtained for the formation of BOx surface groups for x=1-3 (Table S1), suggesting that the B atoms on Pd nanocrystals are easily oxidized. This should not mean that B can be oxidized to BO3, because the adsorbed O atoms are bonded by B and the neighboring Pd atoms. A possible structure like “Rh-BO2” has been reported previously42 when a B-contaminated Rh surface was exposed to oxygen. In fact, B atoms on metallic Pd surface are vulnerable to oxidation that turns into suboxides exposed to oxygen atmosphere.25, 28

Figure 2. (A), (B) The representative TEM and HRTEM images of B-Pd/C, (C) XRD patterns and (D) - (F) XPS spectra of Pd, B and O of the B-Pd/C catalyst.

It is noticed that the XPS peaks for B, O and Pd all shift to more positive binding energies. Similar phenomenon was also observed for a thin B2O2 film condensed on a clean Ag foil,43 indicating the electronic interaction between metal, B and O. Positive shift of O 1s in boron oxides has been generally observed,39,

44, 45

which should be due to the stronger binding of


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electrons in oxides. The positive shift of XPS peaks for Pd and B should be due to their electron donation to the co-adsorbed O. To gain deep insights into the charge transfer between elements in the B-Pd-O system, we performed Löwdin charge analysis using DFT calculations. It was shown that Pd would transfer electron to B in the absence of co-adsorbed oxygen (Figure S5A & S5B). Once B and O co-adsorb to form sub-oxides of B (Figure S5C), both B and Pd donated electrons to O; and Pd lost more electrons than that in the absence of O. The electron transfer from Pd to B was also observed in previous studies.15, 22 Similar charge transfer occurs in Pt-B system.46 It is worth mentioning that there were arguments that B would transfer electrons to Pd according to the larger Pauling’ electronegativity of Pd than B.21 Based on the present DFT calculations and the XPS results obtained in present study and that in the literatures,15, 22, 41 we tend to believe that Pd would transfer electrons to B in the B-Pd system.

Figure 3. B/Pd atom ratios (red) and Pd mass content (sky blue) of the B-Pd/C samples obtained with different reaction times.

To gain deep insight into the formation process of the B-Pd interstitial nanoalloy, the samples formed at different reaction times after adding NaBH4 were collected and the corresponding B/Pd atom ratios and Pd loadings on carbon were determined by ICP-AES. As shown in Figure 3, Pd were nearly completely reduced in less than 10 min upon adding NaBH4, while it took at least 2 hours for the B content to reach the maximal value. The XPS data shown in the Figure S6 gives


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similar implications. We think that boronizing process may include two boronizing steps. Step 1 (Figure 1) took place in the period of the growth stage of Pd nanocrystals with a B content of 4 at. %. The step 2 comes into effect after the completion of the Pd growth and an additional 15 at. % B can be doped. In step 2, the doping should proceed through BH4- decomposition on Pd nanocrystal surface, followed by diffusion of produced B atoms into the sublayers. The residual B atoms on surface would be oxidized when the nanocrystals are exposed in air, yielding a nanostructure of B-Pd interstitial alloy particles decorated with sub-oxidized B atoms. The possible reactions of NaBH4 involved in the formation of B-Pd nanocrystals in DMF are listed in Figure S7. The present synthesis has taken several advantages of NaBH4/DMF system. The strong solvation of Na+ by DMF (Figure S8) allows relatively high dissolution of NaBH4. The relatively mild decomposition of BH4- in DMF makes the boronizing process mainly takes place after Pd nanocrystal formation, which allows flexibly controllable B doping through reaction time. As well as the reaction time (Figure 3), the B content can also be tuned by adding different amount of NaBH4 (Figure S9). Thus, the present method shows an advantage to synthesize B-Pd catalysts with controllable B doping. Electrochemical properties of the B-Pd/C. The ORR activity of as-synthesized B-Pd/C was investigated by using a rotating disk electrode in 0.1 M KOH, along with the commercially available catalysts of Pt/C and Pd/C. As shown in Figure 4A, the B-Pd/C catalyst exhibited ORR polarization curve that was 70 and 57 mV more positive in half-wave potential than the commercial Pt/C and Pd/C respectively. In addition, it gave a Tafel slope of ca. 43 mV dec-1 in low overpotential region (Figure 4B), which was considerably lower than that seen for the Pt/C and Pd/C (~65 mV dec-1). The more positive half-wave potential and lower Tafel slope both indicated a facile kinetics of ORR on B-Pd/C. The recent theoretical calculations have suggested


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that the Tafel behavior of ORR in low overpotential region should be associated with the isotherm of the O ad-atoms.47, 48 The lower Tafel slope seemed to indicate the easier stripping of O ad-atoms from surface as potential goes negatively. Based on the polarization curves, the mass and specific activity at 0.9 V of each catalyst was estimated, which was 0.97 A·mg-1 and 1.27 mA·cm-2 for B-Pd/C, about 14 and 14.6 times higher than that for Pt/C, and 8.1 and 2.6 times higher than that Pd/C respectively (insert of Figure 4A). To gain the specific activities, the electrochemical active surface areas (ECSAs) of Pd/C and B-Pd have been estimated from the charges of the PdO reduction peak by assuming 425 µC/cm2 for a monolayer PdO. For Pt/C, both the charges involved in H desorption peak and PtO reduction peak have been used to estimate the ESCA and similar results were obtained. We didn’t use H charges for Pd-based samples because the absorption of H into Pd lattice may take place.

Figure 4. Electrochemical characterization: (A, B) ORR polarization curves and the corresponding Tafel plots for BPd/C (B/Pd atom ratio: 1:5), Pd/C and Pt/C, measured in the O2-saturated 0.1 M KOH at 30 °C with a sweep rate of 5 mV s-1 and an electrode rotating speed of 1600 r.p.m; the insert in (A) is the mass and specific activity at 0.9 V; (C) CV curves for B-Pd/C (B/Pd atom ratio: 1:5) and Pd/C recorded in O2-free 0.1 M KOH with a sweep rate of 50 mV s-1; (D) ORR polarization curves of B-Pd/C catalysts with different Pd/B atom ratios.


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The cyclic voltammograms (CVs) obtained in the O2-free solution for the B-Pd/C and Pd/C are compared in Figure 4C. The redox current below ~0.4 V should be associated with the formation and striping of adsorbed H atoms on Pd. For pure Pd catalyst, the oxidation current above 0.5 V can be assigned to the formation of oxygenated species, which gives a peak around 0.6V. For B-Pd alloy catalyst, this peak shifts to a more positive potential (~0.65 V), which suggested a weaker binding of oxygenated species on surface Pd sites. Another feature seen for B-Pd alloy is a shoulder peak appearing in potential region of 0.4-0.6V which separates the H striping and oxygenated species formation. We attribute it to the adsorption of oxygenated species on sites involving surface B atoms as shown in Figure S4. These CV features suggested that some surface Pd sites on B-Pd/C become less oxophilic than that on Pd/C, while the sites involving B atoms bind oxygenated species much stronger. Those with less oxophilicity should be responsible for the enhanced ORR activity according to the recent theoretical studies.22, 46 The ORR polarization curves of B-Pd/C with different B contents were tested and the results are displayed in Figure 4D. It clearly indicates that the ORR activity of B-Pd/C increases with the B content. According to the B 1s XPS responses of B-Pd/C with different B contents (Figure S6), little B is oxidized in the sample with very low B content, e.g., with a B/Pd atomic ratio of 1/25; while the oxidation of surface B increases with the increased B/Pd atomic ratio. Therefore, we believe the oxidized B could benefit the ORR kinetics. As will be indicated by the DFT calculation results shown later, the ORR activity of surface Pd atoms could be promoted by subsurface B atoms and the co-adsorbed surface B-O assemblies; and the latter is more effective. At very low B content, the promotion is only through the sub-surface B atoms. With increasing the B content, the amounts of surface B-O assemblies increase and the ORR activity would be increasingly promoted.


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The methanol tolerance of the ORR catalyst is very important for the methanol direct fuel cells. Therefore, the methanol tolerance of the B-Pd/C and the commercial Pt/C and Pd/C catalysts were surveyed and the results are shown in Figure S10. It is clearly seen that the B-Pd/C and Pd/C have a much better methanol tolerance than that of Pt/C. Especially, the B-Pd/C exhibits better ORR activity at the operating potentials of fuel cell cathode (0.8-1.0V) as compared with Pd/C. Implications from DFT calculations. For a deep understanding of the electrocatalytic properties of the B-Pd alloy, the adsorption energy of O atom on various possible surface sites was calculated and the corresponding activity trends are evaluated according to the Volcano model proposed by Norskov et al.38 (Figure 5) For better illustration, the O adsorption energy and ORR activity on various catalytic sites (EO*-M) are given with respected to that on Pt. Due to the strong affinity of B to O, the adsorption of the first two O atoms at B-involving surface sites is very strong, forming stable Pd-BO2 assemblies (Figure S4). The adsorption energy of O at the formed Pd-BO2 assembly sites (Structure A in Figure 5) is about 0.4 eV more positive than ∆EO*Pt,

which suggests that these sites should have Pt-like ORR activity according to activity-

adsorption energy volcano model.

Figure 5. ORR activity of different surface sites on the B-Pd alloy surface vs that on Pt according to the calculated O adsorption energies and the theoretical volcano model developed by Norskov et al.


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At surface sites nearby the Pd-BO2 or Pd-BO3 (Structures B and C), the O adsorption energy is about 0.1 eV more positive than ∆EO*-Pt. This O adsorption energy is very close to that on the surface of Pt segregated Pt3Ni alloy (Figure 5), which is among the best catalysts so far reported for ORR. Besides, the surface sites that are away from Pd-BO2 or Pd-BO3 but with subsurface B atom (Structure D) exhibit an oxygen energy of about 0.05 eV more positive than ∆EO*-Pt. Thus, the B-Pd alloy surface possesses two types of surface sites that are much more active for ORR than Pt, and one type of surface sites that are comparable with Pt to catalyze the ORR. This should be the reason why B-Pd alloy show greatly enhanced ORR activity than Pt. Especially, the formation of B sub-oxides should contribute more significantly. These DFT calculation results show very distinct promotion mechanism of non-metal elements to the ORR activity of noble metal. In a recent study by Zhu et al,49 it has been suggested that the oxidized states of Pd supported on a LaFeO3 perovskite oxide, especially those located in the B-site lattice of the perovskite structure, contributed considerably to the ORR activity due to the decreased eg filling induced by the interaction between the Pd ion and the perovskite. An interesting question is whether the oxidized states of Pd in present B-Pd/C system also contributed to the observed ORR activity enhancement. For carbon-supported catalysts, the oxidized states of noble metals were produced mainly through surface adsorption of oxygen, which has been generally recognized to be detrimental to the ORR activity due to the site-blocking effect.50-53 In the Pd/perovskite system, the perovskite oxide structure should have played an indispensable role in the improved activity of Pd oxidation states. We believe that the oxidized Pd states in present B-Pd/C catalysts should contribute little to the enhanced ORR activity as comparing to the effects of subsurface B and


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surface B sub-oxides. However, further detailed investigation may be conducted to explore the interaction between the oxidized Pd states and B oxides and its role in ORR.

Figure 6. Accelerated durability test. (A (B and (C is polarization curves, mass activity at 0.9 V vs RHE, and CVs of the catalysts before and after 3000 cycles, (D the partial CV profiles of B-Pd/C after 3000 cycles and Pd/C, (E the polarization curve and CVs of B-Pd/C after 3000 and 6000 cycles.

Durability of B-Pd/C Catalyst. To evaluate the stability of the synthesized catalysts, accelerated durability tests (ADT) were performed by cycling the catalyst-loaded electrodes between 0.6 V and 1.0 V for 3000 cycles in O2-saturated solution. Figure 6A shows the changes in the polarization curves. For Pt/C, Pd/C and B-Pd/C, the mass activity at 0.9 V reduced by 58%, 24% and 60% respectively (Figure 6B). As can be seen from the changes in the CV profiles (Figure 6C), the shoulder peak associated with the adsorption of oxygenated species at B-


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involving sites on B-Pd alloy disappeared after the ADT, which indicated that the surface B components dissolved into the solution during the ADT. Although the CV responses of the BPd/C after ADT became resembling that of the Pd/C, the OH adsorption peak on the CV of BPd/C after the ADT remained locating at more positive potentials (Figure 6D). This should be due to the existence of the subsurface B atoms, which implied that the bulk B atoms in the B-Pd alloy catalyst are fairly stable in the ORR process. As shown in Figure 6E, the ORR polarization curve and CV responses changed little when further ADT was performed after the dissolution of the surface B atoms. Thus, although the B-Pd/C showed a rapid activity loss at the initial stage of ADT, it remained a superior ORR catalyst for long-term use. As shown in Figure 6B, the Pd mass activity of B-Pd/C catalyst was ca. 4 and 5 times higher than the Pd/C and Pt/C after ADT, mainly due to the presence of bulk B. The ADT results also showed that Pt was much less stable than Pd in alkaline solution, which was in contrast with the stability trend in acidic solution. The slightly higher activity (Figure 4A) and much higher stability thus suggested that Pd is better than Pt as ORR catalyst in alkaline media. From the TEM images of the sample catalysts before and after ADT (Figures S11-S13), one can find that Pt/C was subjected to much more dissolution than Pd/C and B-Pd/C, which only underwent grain growth due to Ostwald ripening. In fact, Pt is much easier to be oxidized in alkaline solution than acidic solution54-56 while Pd is on the contrary trend57. What’s more, the coordination complexes of Pt with hydroxyl group are soluble in aqueous solution while the ones of Pd with hydroxyl group are not. It is therefore reasonable that the Pt/C catalyst is less stable than Pd/C in alkaline electrolyte.58 The similar activity (Figure 4) and much superior stability thus make Pd better ORR catalyst than Pt in alkaline media, and the ORR activity can be further boosted by doping B.


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Conclusion In summary, a facile yet effective method has been developed for synthesizing B-Pd interstitial nanocrystals with controllable B content using sodium borohydride as reductant and dopant in DMF. Especially, the present method enables a high B uptake up to 20 at. % in Pd, which is the highest values reported so far. Furthermore, this catalyst shows a remarkable ORR activity. DFT calculation reveals the improvement activity is due to the presence of sub-oxidized B on the outermost surface and B in the subsurface. In addition, ADT results suggest the B-Pd/C is an attractive ORR catalyst for long-term use. The present of boronizing method should be easily extended to other B-metal systems. The facile and controllable B doping should strengthen the power of B-metal catalysts and provides a great prospect for their wide applications. ASSOCIATED CONTENT The Supporting Information including additional TEM, XPS, DFT and electrochemical results is available free of charge on http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected], *[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT This work was supported by the National Natural Science Foundation of China (Grant No. 21633008 & 21673163). W.B. C acknowledges the financial supports from the 973 Program of MOST (No. 2015CB932303) and the NSFC (Nos. 21473039 and 21273046). REFERENCES


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Controllable Increase of Boron Content in B-Pd Interstitial Nanoalloy

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