Structural and Electronic Stabilization of PtNi Concave Octahedral

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Energy, Environmental, and Catalysis Applications

Structural and Electronic Stabilization of PtNi Concave Octahedral Nanoparticles by P Doping for Oxygen Reduction Reaction in Alkaline Electrolyte Shan Wang, Laifei Xiong, Jinglei Bi, Xiaojing Zhang, Guang Yang, and Shengchun Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07742 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

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ACS Applied Materials & Interfaces

Structural and Electronic Stabilization of PtNi Concave Octahedral Nanoparticles by P Doping for Oxygen Reduction Reaction in Alkaline Electrolyte Shan Wang,a Laifei Xiong,a Jinglei Bi,a Xiaojing Zhang,a Guang Yangb* and Shengchun Yanga,c* a

School of Science, Key Laboratory of Shaanxi for Advanced Materials and

Mesoscopic Physics, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi’an, 710049, People's Republic of China b

Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education and International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China

c

Collaborative Innovation Center of Suzhou Nano Science and Technology, Suzhou

Academy of Xi’an Jiaotong University, 215000, Suzhou, People's Republic of China

Corresponding authors: [email protected] (G. Yang); [email protected] (S. Yang)

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KEYWORDS: P dopant; Concave octahedra; Pt frame; Ni core; Oxygen reduction reaction

ABSTRACT

The enhancement in catalytic activity of PtM (transition metals, TM) alloy NPs results from the electronic structure of Pt being modified by the TM. However, the oxidation of the TM would lead to the electronegativity difference between Pt and TM being much lowered, which induces a decrease in the number of electrons transferred from the TM to Pt, results in excessive oxygenated species accumulating on the surface of Pt, thus deteriorating their performance. In this work, the ORR performance of PtNi (Pt68Ni32) concave octahedral nanoparticles (CONPs) in alkaline electrolyte is much improved by doping small amounts of phosphorus. The P doped PtNi CONPs (P-PtNi) show about 2 and 10 times enhancement for ORR compared to PtNi and commercial Pt/C catalysts, respectively. The high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS) mapping characterizations reveal that P uniformly distributes throughout the concave octahedral, Pt mainly locates at the edges and corners, while Ni situates the center, forming a P doped Pt-frame@Ni quasi core-shell concave octahedral NPs. X-ray photoelectron spectroscopy (XPS) spectra indicates that the P dopant obviously increases the electron density of Pt compared with that of PtNi NPs, which contributes to the stabilization of electronic structure of PtNi CONPs, thus restrains the excessive 2 ACS Paragon Plus Environment

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HO2- species produced on the catalysts, which endow them with a high catalytic performance in ORR. In addition, the P attached to the Ni sites in the PtNi NPs partially prevents the Ni atoms being oxidized by external O species, which is conducive to the structural and electrochemical stability of the PtNi NPs during the ORR. The present results provide a new insight into the development of ORR catalysts with low utilization of Pt.



1. INTRODUCTION The development of clean energy technologies, such as proton exchange membrane fuel cell, alkaline anion exchange membrane fuel cells, and metal-air batteries, greatly depends on the oxygen reduction reaction (ORR). Platinum (Pt) has been regarded as the essential element in electrocatalysts for ORR, but the limited reserves inducing high cost and the sluggish ORR kinetics of Pt greatly limits the advancing and widespread adoption of above clean energy technologies. Therefore, the higher catalytic activity on a per-Pt-atom basis should be acquired to reduce the necessary Pt usage.1-3 In general, adjusting or optimizing the electronic structure (d-band center position) or/and surface structure of the Pt catalysts is believed to be an effective method to enhance their catalytic activity. The electronic structure of Pt can be well modulated by alloying with other transition metals (TM, such as Ni, Co, Cu, Pd, Mo et al.). The surface structure is largely dependent on the shape and architecture of catalysts.4-7 Stamenkovic et al. demonstrats that the Pt3Ni(111) has the highest electrocatalytic activity for ORR in cathode catalysts compared with the Pt3Ni(100), Pt3Ni(110) and Pt(111) surfaces. Such improvement was attributed to the weak interaction between the Pt atoms and nonreactive oxygenated species, thus increasing the surface active site availability for O2 adsorption.8 In recent years, combining the advantages of the “electronic effects” with “surface effects” to obtain the enhanced activity of PtM catalysts has obtained the comprehensive consensus in the field of ORR catalysts. PtM catalysts with different 4 ACS Paragon Plus Environment

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shapes including polyhedron9,10, jagged nanowire11, nanorods12, nanoframe13,14, hollow15, porous16 or dealloyed core-shell structure17 are known to be effective ORR catalyst due to their special surface structure increasing the number of active sites. However, the oxidation of the TM would lead to the electronegativity difference between Pt and TM being much lowered, which induces a decrease in the number of electrons transferred from the TM to Pt.18,19 Therefore, introducing the more weakly electronegative element as the electron donor into the PtM NPs should be conductive to the electron transformation from TM to Pt, thus increasing the electron density of Pt and improving their catalytic activities. In addition, due to the high density of low-coordinated atomic steps, kinks and edges on the facet of a concave structured NP, the PtM alloy NPs with such morphologies have exhibited an ORR-favorable electronic and geometric structure.20-23 For example, Iwasawa and coworkers synthesized concave octahedral Pt73Ni23/C, which showed remarkable ORR performance and long-time durability compared to commercial Pt/C.24 However, alloying PtM catalysts generally suffer from insufficient stability due to TM leaching and/or the aggregation of the NPs during the electrocatalytic reaction in acid electrolyte. Compared to the acid medium, an alkaline electrolyte provides a relatively small degree of degradation and corrosion for PtM catalysts, as well as an improved stability of such catalysts. However, a long time electrochemical corrosion induced compositional and structural degradation 5 ACS Paragon Plus Environment


was still a topic that should not be ignored. In addition, the study of ORR in alkaline electrolyte is also quite important due to the resurgence of interests on alkaline anion exchange membrane fuel cells, microbial fuel cells, and alkaline metal−air batteries. Nevertheless, the electrocatalytic activity of ORR on the Pt surfaces in alkaline electrolyte is lower than that in the acidic solution, mainly because of the excessive HO2- species being produced on the Pt surface in alkaline media.3, 25-27 The element P has a more weakly electronegative element which can efficiently modify the electronic structure and enhance their activity and stability by inhibiting the accumulation of HO2- species on their surfaces in ORR. Recently, Pd-Ni-P，P-PdAu, P-PtNi, Pt/P,N-C, P-carbon catalysts with much improved performances for MOR, EOR, HER, ORR have been successfully developed.28-35 Typically, Wang and coworkers found that P doping can shorten the distance between Pd and Ni active sites, thus being inductive to the chemical adsorption of OH- and desorption of *OH, and driving the formation of CH3COOH.29 Chen et al. revealed that the synergistic effect between the atomic Co and N-C active sites can be much improved with P doping, thus endowing the catalysts with an enhanced ORR performance in both alkaline and acidic solution.32 Although P-doped metal and nonmetal electrocatalysts for various reactions have been widely investigated, doping P into shaped PtM catalysts for ORR are still seldom reported. In this work, P doped PtNi concave ocatahedral NPs were prepared by a 6 ACS Paragon Plus Environment

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facile two step method. That is, PtNi concave octahedral NPs slurry were used as starting material prepared by modified one-pot method as our previous report,36 and then directly phosphorized the as-prepared PtNi NPs using NaH2PO2 in oil bath. EDS mapping results reveal that the P dopant uniformly distributed in PtNi concave octahedra. The P doping in PtNi CONPs obviously improved their electrocatalystic activity and durability for ORR in alkaline solution compared with the PtNi CONPs and commercial Pt/C. Therefore, our study suggests that the P doped PtNi CONP catalysts can be the potential alternatives for ORR catalysts with the low utilization of Pt. 2. EXPERIMENTAL 2.1. Reagents and Chemicals. Platinum (II) acetylacetonate [Pt(acac)2, AR.] was purchased from Kunming Institute of Precious Metals (KIPM). Nickel (II) acetylacetonate [Ni(acac)2, AR.], Ascorbic acid (AA, AR.), sodium hydroxide (NaOH, AR.) and potassium hydroxide (KOH, GR.) were purchased from Aladdin industrial corporation. Polyvinylpyrrolidone (PVP, MW of ~55000) was obtained from Sinopharm Chemical Reagent Co., Ltd. Sodium hypophosphite (NaH2PO2.H2O) was purchased from Tianjin Tian Li Chemical Reagent Co., Ltd. Ethylene glycol (EG, AR.), ethanol (AR.), acetone (AR.), Chloroform (CHCl3, AR.), diethylamine (DEA, AR.) and 2-propanol (AR.) were bought from Tianjin Fuyu Special Chemicals Co., Ltd. Deionized (DI) water (18.25 MΩ/cm) used in experiment was prepared by passing through an ultra-pure purification system. Before experiment, EG was added to an uncapped flask under stirring at 160 °C for 1h to remove water. Other chemicals 7 ACS Paragon Plus Environment


were used as received without further purification.

2.2. Synthesis of Octahedral PtNi NPs. PtNi octahedral nanoparticles were prepared through a modified one-pot method as reported in our group previously work.36 In detail, 0.2 g PVP was dissolved into 9 mL EG in a 50 mL round-bottomed flask and heated to 180 °C in oil-bath with magnetic stirring, then, 8 mL of 80 mM Ni(acac)2/EG solution was added and kept at 180 °C for 30 min. and then, 4 mL of 1.25 mM AA/EG solution was added into the flask and kept at 180 °C for 30 min. After that, 8 mL of 40 mM as-heated Pt(acac)2/EG solution (140°C) was added into above mixture, to make the molar ration of Pt(acac)2 and Ni(acac)2 1:2, and further kept at 180 °C for 2 h. Finally, the reaction was stopped by cooling the mixture down to room temperature and the black PtNi octahedral NPs slurry was obtained. The PtNi octahedral nanoparticles were isolated by centrifugation using acetone and ethanol (9000 rpm for 8 min) several times and re-dispersed in ethanol for further use.

2.3. Synthesis of P-PtNi octahedral NPs. 240 mg of NaOH was dissolved into 2 mL DI water to prepare NaOH/H2O solution. 70 µL of NaOH/H2O solution was mixed with 6 mL of above black PtNi octahedral NPs slurry in a 20 mL vial. Then, 100 mg of NaH2PO2.H2O was added to above vial and heated from room temperature to 200 °C and kept at such temperature for 12 h in oil-bath under magnetic stirring. The reaction was stopped by cooling the solution down to room temperature. The product was isolated by centrifugation using water and ethanol (6000 rpm for 5 min) several times. The obtained P doped octahedral Pt-Ni NPs were re-dispersed in


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ethanol for further use.

2.4. Synthesis of Carbon Supported P-PtNi (P-PtNi/C) catalysts. Firstly, 20 mg carbon black (Vulcan XC-72, Cabot) was dispersed in 20 mL CHCl3 and sonicated for 20 min to achieve a homogeneous dispersion of carbon. Secondly, 20 mL of P-PtNi NPs/ethanol solution was added to above mixture solution (to obtain a PtNi loading of ~20%) with stirring, and further sonicated for 30 min. Then, 1.5 mL of EDA was added to above mixture and stirred for 12 h at room temperature to remove the organic surfactants from NPs by ligand-exchange process.37 The P-PtNi/C catalysts were collected by centrifugation (10000 rpm for 10 min), washed with DI water and ethanol three times, and drying in air at 45 °C for 12 h. In the same manner, PtNi/C catalysts were also prepared.

2.5. Electrochemical Measurements. Typically, 5 mg of P-PtNi/C catalysts were added into a mixed solvent containing 40 µL Nafion (Aldrich Chemistry), 100 µL DI water and 1.06 mL 2-propanol followed by sonicated for 15 min to obtain the catalyst ink. The working electrode was prepared by pipetted 5 µL catalyst ink onto a pre-cleaned rotating disk electrode (RDE) and then dried at room temperature to prepare working electrode. The glassy carbon diameter of RDE is 5 mm and a geometric surface area is 0.196 cm2. An Ag/AgCl (in saturated KCl, aq) reference electrode and a 1×1 cm Pt mesh counter electrode used together with RDE working electrode to form a conventional three-electrode cell. The possibility of Pt ionic species redepositing on the working electrode in the ORR experiment was excluded



by the virtually identical LSV curves of P-PtNi (Figure S7). Electrochemical measurements containing cyclic voltammetry (CV) and linear sweep voltammety (LSV) were performed on an Electrochemical Analyzer Instrument (Pine AFCBP1). To avoid the silicates pollution induced by glass degradation from glass cell in KOH electrolyte for long time, the Teflon electrochemical cells was used in the accelerated stability test.

Before the measurement, the working electrode performed several cycles (~30 cycles) in nitrogen-saturated 0.1 M KOH ( PH ~13) solution from -0.918 to 0.262 V (vs. Ag/AgCl) at 150 mV/s until the stable CV curves were obtained. The CV curves were recorded in nitrogen-saturated 0.1 M KOH solution from -0.918 to 0.262 V (vs. Ag/AgCl) at a scan rate of 50 mV/s. The LSV curves were obtained in oxygen-saturated 0.1 M KOH solution from -0.918 to 0.262 V (vs. Ag/AgCl) at 10 mV/s and different RDE rotation speed (400-3600 rpm). ORR polarization curves were normalized to the geometric surface area (0.196 cm2). To evaluate the peroxide species yield in ORR process, rotating ring-disk electrode (RRDE; Pt ring, 0.189 cm2; GC disk, 0.248 cm2) experiments have been performed. The disk electrode was scanned from -0.768 to 0.232 V at 10 mV/s (vs. Ag/AgCl), and the ring potential was held at 0.5 V (vs Ag/AgCl). The measured potentials in the study were iR-corrected potentials and recorded vs. Ag/AgCl. The potentials reported herein vs. reversible hydrogen electrode (RHE) was according to eq1, in which EoAg/AgCl = 0.20 V versus SHE at 25 °C.38


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V vs RHE = V vs Ag⁄AgCl + 0.0591pH + ⁄ V vs SHE, 25 °C (1)

The kinetic current (!" ) can be calculated from ORR polarization curves using the Koutecky−Levich equation: # $

#

#

%

&

=$ +$

(2)

where ! is the current measured from experiment, !" refers to the kinetic current and !' is the diffusion-limiting current. From Levich equation, the jd can be described as: !' = 0.2()*+, -// 0 1#/2 3#/- 4+,

(3)

where ( is the number of electrons transferred of per O2, ) is the Faraday’s constant (96485 C/mol), *5, is the diffusion coefficient of O2 in electrolyte (1.9 × 10-5 cm2/s), 0 is the kinetic viscosity of 0.1 M KOH (0.01 cm2/s), 3 is the rotation speed in rpm when the constant expressed in 0.2, 45, is the concentration of O2 in 0.1 M KOH (1.2 × 10 6 mol/cm3).25, 39 −

In RRDE experiments, the peroxide yield (yperoxide/ %) with respect to the total products was evaluated according to the following equation: 67895:;'8 =

-

Structural and Electronic Stabilization of PtNi Concave Octahedral

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