One-Nanometer-Thick PtNiRh Trimetallic Nanowires with Enhanced

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One-Nanometer-Thick PtNiRh Trimetallic Nanowires with Enhanced Oxygen Reduction Electrocatalysis in Acid Media: Integrating Multiple Advantages in One Kan Li, Xingxing Li, Hongwen Huang, Laihao Luo, Xu Li, Xupeng Yan, Chao Ma, Rui Si, Jinlong Yang, and Jie Zeng J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08836 • Publication Date (Web): 02 Nov 2018 Downloaded from http://pubs.acs.org on November 2, 2018

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Journal of the American Chemical Society

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Revised ms#ja-2018-08836u.R2

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One-Nanometer-Thick

PtNiRh

Trimetallic

Nanowires

with

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Enhanced Oxygen Reduction Electrocatalysis in Acid Media:

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Integrating Multiple Advantages in One

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Kan Li,†,‖ Xingxing Li,†,‖ Hongwen Huang,*,†,‡ Laihao Luo,† Xu Li,† Xupeng Yan,† Chao Ma,†,‡

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Rui Si,¶ Jinlong Yang,† and Jie Zeng*,†

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†Hefei

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Strongly-Coupled Quantum Matter Physics of Chinese Academy of Sciences, Department of

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Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, P. R.

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China.

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‡College

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carbon materials and applied technology, Hunan University, Changsha, Hunan, 410082, P. R.

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China.

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¶Shanghai

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Academy of Sciences, Shanghai 201204, P. R. China.

National Laboratory for Physical Sciences at the Microscale, Key Laboratory of

of Materials Science and Engineering, Hunan province key laboratory for advanced

Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese

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‖These

authors contributed equally to this work.

22 23

Correspondence

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[email protected]), or to H.H. (email: [email protected]).

and

requests

for

materials

should

be

25

ACS Paragon Plus Environment

addressed

to

J.Z.

(email:

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Abstract

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Finding out an active and durable catalyst toward acidic oxygen reduction reaction (ORR), a

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key process for fuel cells, remains an open challenge due to the thermodynamically contradictory

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requirements for activity and durability. Here, we report that an active and durable ORR catalyst

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can be achieved by integrating multiple structural and compositional advantages into the one.

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The mass activity and specific activity of as-obtained one-nanometer-thick PtNiRh trimetallic

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nanowires/C catalyst were 15.2 and 9.7 times as high as that of commercial Pt/C catalyst,

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respectively. The compressive strain and ligand effects arising from the advantageous

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microstructure and optimal composition of the nanowires were revealed for the enhancement in

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activity. Besides, the PtNiRh trimetallic nanowires/C catalyst exhibited substantially improved

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durability relative to commercial Pt/C catalyst, due to the combination of one-dimensional

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structure and incorporated Rh atoms. This work provides a general guidance for the design of an

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impressive heterogeneous catalyst.

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2 ACS Paragon Plus Environment

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INTRODUCTION

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Proton exchange membrane fuel cells (PEMFCs) have been regarded as the most attractive

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clean power sources for automobiles due to the advantages in energy conversion efficiency,

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output power density and environmental sustanability.1,2 Two main challenges to an efficient

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PEMFC device lie in the kinetically sluggish oxygen reduction reaction (ORR) at cathode and

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the insufficient long-term operation durability because of the harsh chemical environment around

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the electrodes.3,4 Such problems at present are overcome by loading considerable amount of

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precious Pt nanocatalysts on the electrodes (e.g., about 0.4 mg cm-2 on the cathode) to ensure the

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performance of device over a long time.5 For this strategy, however, the prohibitive cost impedes

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the industrial-scale deployment of PEMFC devices. Accordingly, optimizing the mass activity

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and durability of Pt-based electrocatalysts toward the acidic ORR to minimize the usage of Pt is

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of paramount importance for the widespread application of technology given the unqualified

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durability of other catalysts under the acidic medium.6-9

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To maximize the mass activity of Pt-based electrocatalysts toward ORR, the research efforts

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have generally focused on increasing the utilization efficiency of Pt atoms via constructing

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favored microstructures and boosting the specific activity through optimizing the binding

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strength of oxygenated species on the surfaces of catalysts.10-26 Favorable structures include

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ultrathin low-dimensional nanostructures or hollow/open nanostructures which enable high

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utilization efficiency of Pt atoms.10-16 As for specific activity, engineering the surface facets,

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forming Pt-based alloy nanocatalysts and regulating the surface strain represent the powerful

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methods to achieve an enhanced specific activity via ligand or geometrical effects.17-26 The

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discovery of most active Pt3Ni with {111} surface facets was the well-known case that integrated

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the abovementioned factors.17 The great enhancement in specific activity of Pt3Ni {111} facets

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could be ascribed to a combination of ligand and geometrical effects17. In addition, the long-term

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durability of Pt-based electrocatalysts toward ORR, a key indicator to evaluate an electrocatalyst,

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has also received extensive studies.27-37 To prolong the durability toward ORR, typical

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approaches include the construction of one-dimensional (1D) nanowires (NWs),28-30

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incorporation of specific metals into Pt base31-33 and formation of ordered intermetallic

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electrocatalysts.34-37 The cardinal principle of these methods was to improve the dissolution

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potential of Pt atoms in the electrocatalysts.35 Despite the great progress has been made, it is still

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a standing challenge to simultaneously achieve a high mass activity and durability because of 3 ACS Paragon Plus Environment


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their contradictory requirements from perspective of thermodynamics.

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Very recently, we designed an active and durable catalyst toward ORR by integrating the

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advantages of ultrathin 1D NWs and doping of Rh atoms.38 Inspired by this design, we hereby

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upgrade the catalyst by alloying Ni into the NWs to form much more active one-nanometer-thick

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PtNiRh trimetallic NWs. Because of the multiple advantages of atomic-level thin diameter,

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optimal metal compositions and anisotropic 1D nanostructure, the one-nanometer-thick PtNiRh

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trimetallic NWs/C catalyst exhibit much greater mass activity and durability than commercial

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Pt/C catalyst toward ORR. The understanding to large enhancement in catalytic performance was

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revealed by a combination of the structural analysis and density functional theory (DFT)

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calculations.

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EXPERIMENTAL SECTION

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Synthesis of one-nanometer-thick PtNiRh trimetallic NWs. Typically, 4.0 mL of

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oleylamine solution containing Pt(acac)2 (20 mg, 0.051 mmol), Ni(acac)2 (10 mg, 0.017 mmol),

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Rh(acac)3 (2 mg, 0.005 mmol) and CTAB (50 mg, 0.137 mmol) in a 20.0 mL-vial was preheated

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at 170 oC for 5 min in an oil bath. Mo(CO)6 (20 mg, 0.076 mmol) was then quickly added into

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the preheated solution to start the reaction. The reaction was terminated after proceeding for 2 h.

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The product was then obtained by precipitating with ethanol and washing three times with

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hexane.

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Synthesis of PtNi bimetallic NWs. The similar procedure was used to produce PtNi bimetallic NWs without using Rh precursors. Synthesis of Pt NWs. The similar procedure was used to prepare Pt NWs besides the absence of Ni and Rh precursors.

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Electrochemical measurements. Before the electrochemical measurements, the as-prepared

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Pt-based NWs were loaded on carbon supports (Vulcan XC-72) with a loading content of 20

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wt%. Typically, a hexane solution containing 4.0 mg of NWs was dropwise added to another

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hexane solution containing 16.0 mg of carbon under violent stirring. The mixture solution was

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then stirred overnight to uniformly disperse the Pt-based NWs onto the carbon support. The

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loaded catalyst was subsequently collected by centrifugation and dispersed into acetic acid

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solution with a catalyst concentration of 0.5 mg/mL. Afterward, heating the mixture at 70 oC for 4 ACS Paragon Plus Environment

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2 h to remove the surfactant on the NWs. Finally, the acid-treated catalyst was collected via

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centrifugation and washed by ethanol for five times. To prepare the catalyst ink, 1.5 mg of the

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carbon-supported catalyst was dispersed into a mixture containing deionized water (0.745 mL),

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isopropyl alcohol (0.250 mL), and Nafion solution (0.005 mL). The mixture was further

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sonicated for 2 h to generate a homogeneous catalyst ink. Finally, 3.0 μg of the Pt-based NW

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was dropped onto a glassy carbon rotating disk electrode (RDE) with a diameter of 5 mm for the

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measurements. The suspension solution of commercial Pt/C catalyst (Johnson Matthey, 20 wt%

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loading) was prepared with a catalyst concentration of 1.5 mg/mL after 1h of sonication.

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Afterward, 3.0 μg of Pt nanoparticle was deposited onto a glassy carbon RDE for the

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measurements.

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An electrochemical workstation (Zahner IM6, Germany) was used for measuring all

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electrochemical tests using a three-electrode system. A leak-free Ag/AgCl (3M KCl) electrode

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was selected as a reference electrode and a Pt wire was selected as a counter electrode. For

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comparison, all potentials in this study were converted to the reversible hydrogen electrode

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reference. The CV measurement was conducted in a N2-saturated HClO4 (0.1 M) electrolyte at a

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sweep rate of 50 mV s-1. The ORR polarization curve was performed in an O2-saturated 0.1 M

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HClO4 electrolyte at a sweep rate of 10 mV s-1 under a rotation rate of 1600 rpm. The accelerated

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durability test (ADT) was conducted at room temperature by applying cyclic sweeps between 0.6

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and 1.1 V versus reversible hydrogen electrode (VRHE) in a 0.1 M HClO4 electrolyte at a sweep

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rate of 100 mV s-1 for 10000 cycles. The mass activity can be determined by dividing the kinetic

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current by the loading Pt mass. The specific activity can be obtained by dividing the kinetic

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current by the corresponding ECSA. Prior to the CO stripping, the working electrode was hold at

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the potential of 0.05 VRHE in a CO-saturated 0.1 M HClO4 electrolyte for 30 min. The CO

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stripping voltammograms were then recorded in a fresh 0.1 M HClO4 electrolyte (without CO) in

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the potential window of 0.05-1.26 VRHE at a sweep rate of 50 mV s-1.

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X-ray absorption fine structure (XAFS) characterizations. The Pt L3-edge XAFS spectra

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were conducted at BL14W1 beam line of Shanghai Synchrotron Radiation Facility (SSRF) using

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“top-up” mode (operated at 3.5 GeV with a constant current of 240 mA). The data were collected

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under transmission mode with the energy calibrated with reference to the absorption edge of pure

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Pt foil. The acquired data were fitted using the Athena and Artemis codes. For the X-ray

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absorption near-edge structure (XANES), the experimental absorption coefficients as function of 5 ACS Paragon Plus Environment


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energies μ(E) were processed by background subtraction and normalization procedures, and

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defined as “normalized absorption” with E0 = 11564.0 eV for all the samples and Pt foil/PtO2

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standard. For EXAFS analysis, the Fourier transformed (FT) EXAFS spectra in R space were

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obtained by applying the metallic Pt model for the Pt-Pt shell. The passive electron factors, S02,

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were calculated by fitting the pure Pt foil data and setting the coordination number (CN) of Pt-Pt

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bond as 12, which was then fixed for further analysis.

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DFT calculations. Considering the dominant exposed surface of (111) planes for NWs, a

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seven-layer 2  2 Pt (111) and 1  1 Pt3Ni (111) slabs were constructed to simulate the Pt

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NWs and PtNi bimetallic NWs, respectively. Besides, two Pt atoms of Pt3Ni (111) slab were

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substituted by two Rh atoms to form Pt19Ni7Rh2 in order to model the PtNiRh trimetallic NWs.

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Based on our energy calculations, the preferred positions of Rh atoms are one in the up

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subsurface and one in the down subsurface of Pt3Ni (111) slab. Furthermore, considering the

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dissolution of surface Ni atoms in acidic solution, the outmost layers of Pt3Ni (111) slab and Rh

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doped Pt3Ni (111) slab were all replaced by pure Pt monolayers. To study OH adsorption, only

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the adsorption on Pt top site was considered since previous work has shown that water could

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stabilize OH adsorption on the top site22 and the adsorption energy was calculated as

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EOH  Eslab-OH  Eslab  ( EH 2O  1 EH 2 )  EZPE , where Eslab-OH and Eslab are the total energies 2

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of slabs with and without OH adsorbed, and EZPE is the zero point energy correction,

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respectively. To understand the different durability of Pt NWs, PtNi bimetallic NWs and PtNiRh

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trimetallic NWs, the vacancy formation energy of outmost Pt atom was computed by

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EVPt  Eslab-VPt  Pt  Eslab , where Eslab-VPt and Pt are total energy of slab with one Pt vacancy

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and chemical potential of Pt atom (set to be the energy of face-centered cubic Pt crystal),

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respectively. The vacancy formation energy of outmost Ni atom (on the subsurface) was

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computed by EVNi  Eslab-VNi   Ni  Eslab , where Eslab-VNi and  Ni are total energy of slab with one

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Ni vacancy and chemical potential of Ni atom (set to be the energy of face-centered cubic Ni

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crystal), respectively. In the cases, the 4  4 Pt (111) and 2  2 Pt3Ni (111) slabs (pure and

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doped) were used.

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All calculations were conducted by Vienna ab initio simulation package (VASP) with

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GGA-PBE functional39 of D3 type correction of weak interaction.40,41 We used the projector

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augmented wave (PAW) potential42 and set the plane-wave cut-off energy to 400 eV. K-point 6 ACS Paragon Plus Environment

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meshes of 9  9 and 5  5 in the Monkhorst-Pack scheme were employed when studying OH

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adsorption and Pt vacancy formation, respectively. A vacuum space of 15 Å along the z-direction

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was applied to eliminate the image interactions. The lattice constants and positions of all atoms

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are fully optimized with a convergence threshold of 0.01 eV/Å. The total energy is converged

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within 1  10-6 eV. To confirm the method’s reliability, we optimized the lattice constants of Pt

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and Rh bulk crystals to be 3.925 Å and 3.788 Å, respectively, consistent well with the

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experimental values (the mismatch being smaller than 0.5%).

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Characterization techniques. The morphological structure was characterized by

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transmission electron microscopy (TEM, Hitachi HT-7700 operating at 120 kV) and High-angle

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annular dark-field scanning transmission electron microscopy (HAADF-STEM, JEOL

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ARM-200F microscope operating at an accelerating voltage of 200 kV). Energy-dispersive X-ray

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spectroscopy (EDX) technique was also performed on a JEOL ARM-200F microscope operating

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at an accelerating voltage of 200 kV. X-ray diffraction (XRD) patterns were recorded with a

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Philips X'Pert Pro X-ray diffractometer with a monochromatized Cu Kα radiation (λ = 1.542 Å).

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The quantitatively elemental analysis was probed by inductively coupled plasma atomic

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emission spectroscopy (ICP-AES, Atomscan Advantage, Thermo Jarrell Ash, USA). X-ray

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photoelectron spectroscopy (XPS) were performed on ESCALAB 250 XPS spectrometer with Al

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Kα as the excitation source. Fourier transform infrared (FT-IR) spectra were collected from

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3200-800 cm-1 with a Nicolet 380 FTIR spectrometer.

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RESULTS AND DISCUSSION

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Synthesis and structural characterizations. The microstructure of product obtained using

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the standard synthetic procedure was first analyzed with TEM and HAADF-STEM. Figure 1, a

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and b, shows the production of highly pure and uniform 1D NWs. The average diameter was

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estimated to be 1.0 ± 0.4 nm and length to be 80 ± 20 nm, displaying a high aspect ratio of 80

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(Figure 1c). To acquire more detailed structural information, the atomically resolved

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HAADF-STEM images were captured. Figure 1, d and e, demonstrates that 5~6 atomic layers

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make up the diameter of NW, in good agreement with the average diameter. The continuous

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lattice fringes suggested the single-crystalline nature of the NWs. The growth direction of

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direction was deduced from the distinguished lattice planes. Of note, the as-obtained 7 ACS Paragon Plus Environment


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one-nanometer-thick NWs may be broken into short NWs under the illumination of

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high-intensity electron beam due to their atomic-level thickness, as evidenced by the red dashed

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circle in Figure 1e. The compositional analysis was explored by the EDX. The elemental

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mapping (Figure 1f) and line-scanning profile (Figure 1g) demonstrated a homogeneous

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distribution of Pt, Ni and Rh throughout the NW. The ICP-AES result further quantitively

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determined the 3.05:1.00:0.26 for the atomic ratio of Pt/Ni/Rh of the NWs.

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The XPS survey spectrum also verified the coexistence of Pt, Ni and Rh in the as-prepared

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NWs (Figure S1), from which the surface atomic ratio of Pt/Ni/Rh was determined to be

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3.02:1.00:0.23. Notably, the signal of Br in the survey spectrum is probably related with the

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adsorbed CTAB on the NWs. Besides, we carefully examined the chemical states of Pt, Ni and

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Rh in the NWs by high-resolution XPS spectra, as illustrated in Figure 1 h-j. The majority of

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surface Pt and Rh atoms showed metallic state, with Pt 4f7/2 peak at 71.85 eV and Rh 3d5/2 peak

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at 308.2 eV, while the oxidized Nix+ was the dominant chemical state for surface Ni atoms. The

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XRD pattern (Figure S2) of the NWs shows that the positions of all characteristic diffraction

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peaks moved to higher diffraction angle relative to the corresponding diffraction peaks of Pt, in

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line with the reduced lattice spacing due to the formation of PtNiRh trimetallic NWs. All the

17

above analyses have confirmed the successful synthesis of one-nanometer-thick PtNiRh

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trimetallic NW. It is worth noting that such one-nanometer-thick PtNiRh trimetallic NW means a

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high Pt utilization efficiency, with a value more than 50% estimated from an approximate

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geometrical model (Figure S3).

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For comparison, we also prepared ultrathin PtNi bimetallic NWs and Pt NWs through

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slightly modifying the synthetic procedure. The structural and compositional characterizations

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(Figure S4 and S5) testified the production of PtNi bimetallic NWs with an average diameter of

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1.2 ± 0.4 nm and a length of 60 ± 20 nm, and Pt NWs with an average diameter of 2.1 ± 0.6 nm

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and a length of 50 ± 20 nm. The atomic ratio of Pt/Ni for PtNi bimetallic NWs was determined to

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be 2.99:1.00 by ICP-AES. The atomic-resolution HAADF-STEM images of the NWs confirm

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the same growth direction of direction (Figure S4b and S5c). The Pt 4f XPS spectra

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recorded from PtNi bimetallic NWs and Pt NWs (Figure S4e and S5d) indicate the Pt 4f7/2 peak

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of PtNi bimetallic NWs (71.8 eV) shifted to a higher binding energy compared to that of Pt NWs

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(71.65 eV), suggesting the electron transfer from Pt to Ni. To unearth the surface facet of the

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NWs, the electrochemical CO stripping voltammetry, a surface-sensitive technique, was 8 ACS Paragon Plus Environment

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employed.43-45 The Pt NWs exhibited a CO stripping potential of 0.87 VRHE (Figure S6), proving

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the dominant surface facets of {111} facets for the Pt NWs.43 Given the identical growth

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direction and morphology of the produced Pt-based NWs, the PtNiRh trimetallic and PtNi

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bimetallic NWs mainly exposed {111} surface facets.

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The electronic and local coordination structures of Pt atoms in Pt-based NWs were explored

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by XAFS analysis. Figure 2a reveals the normalized Pt L3-edge XANES profiles of Pt-based

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NWs, while Pt foil and PtO2 were used as references. The white line intensities of Pt-based NWs

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indicate the surface Pt in prepared Pt-based NWs are mainly in the metallic state, consistent with

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XPS results. The local coordination information of Pt atoms in Pt-based NWs was further

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obtained from the EXAFS spectra of the Pt L3-edge (Figure 2b). As shown in Table 1, the

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PtNiRh trimetallic NWs exhibited the lowest Pt-Pt coordination number (CN) of 4.4 ± 0.5,

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comparing to that of 5.4 ± 0.6 for PtNi bimetallic NWs and 9.5 ± 0.8 for Pt NWs. The lowest

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Pt-Pt CN of PtRhNi trimetallic NWs is related with the thinnest diameter and multi-metallic

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composition of the NWs. Importantly, PtNiRh trimetallic NWs presented the shortest Pt-Pt bond

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length (2.71 Å), which corresponds to a compressive strain of 1.8% with respect of commercial

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Pt/C.

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We investigated the formation mechanism of PtNiRh trimetallic NWs. The shape evolution

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process for PtNiRh trimetallic NWs was revealed by examining the morphology of

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time-dependent samples obtained at different reaction stages via TEM (Figure S7). Short

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nanorods were watched after the reaction proceeded for 30s. With increasing the reaction time,

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the nanorods evolved into long NWs and then remained unchanged after 2 h. The compositional

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evolution of PtNiRh trimetallic NWs was also detected by ICP-AES (Figure S8). In accordance

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with the morphology evolution process, the atomic ratio of Pt/Ni/Rh remains unchanged after the

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reaction proceeding for 2 h, which may be related with the depletion of reactants. Notably, the

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morphological evolution process of PtNiRh trimetallic NWs is nearly identical to the

26

observations on the previously reported 1D NWs, which can be generally attributed to the

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surfactant-directed growth mode where CTAB functions as a soft template to guide the growth of

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NWs.46

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ORR performance. We further evaluated the electrocatalytic ORR properties of PtNiRh

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trimetallic NWs by employing the rotating disk electrode (RDE) method. As a comparison, the

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PtNi bimetallic NWs, Pt NWs and commercial Pt/C were also studied. Before the 9 ACS Paragon Plus Environment


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electrochemical measurements, the carbon supported Pt-based NW catalyst was prepared via

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uniformly dispersing the NWs onto carbon supports with metal contents of 20 wt%. The

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surfactant on the surface of catalyst was further removed by heating with acetic solution and

4

washing for several times. The almost disappeared signals of CTAB in FT-IR spectra recorded

5

from different catalysts demonstrated the clean surface of the catalysts (Figure S9). The TEM

6

images of Pt-based NW/C catalysts prove the homogenous dispersions of Pt-based NWs on

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carbon supports (Figure S10a-d). The cyclic voltammograms (CVs) of the catalysts were

8

performed in a N2-saturated 0.1 M HClO4 solution at a sweep rate of 50 mV s-1 under room

9

temperature (Figure 3a). From the charge of underpotentially deposited hydrogen adsorption, the

10

ECSA of each catalyst was derived. Specifically, the PtNiRh trimetallic NWs/C catalyst showed

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the ECSA of 106.4 m2 g-1Pt, much larger than those of PtNi bimetallic NWs/C catalyst (89.8 m2

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g-1Pt), Pt NWs/C catalyst (74.7 m2 g-1Pt) and commercial Pt/C catalyst (67.9 m2 g-1Pt). The largest

13

ECSA of PtNiRh trimetallic NWs/C catalyst is in line with the thinnest diameter and lowest Pt-Pt

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CN. The positive-going ORR polarization curves of the catalysts were further recorded in an

15

O2-saturated 0.1 M HClO4 solution at room temperature, as shown in Figure 3b. From the

16

polarization curves, the kinetic currents of catalysts could be estimated based on the

17

Koutecky-Levich equation, which were then normalized against the ECSA and Pt mass to obtain

18

the specific and mass activity, respectively (Figure 3, c and d). At 0.9 VRHE, the PtNiRh

19

trimetallic NWs/C catalyst delivered the highest specific activity (2.71 mA cm-2), presenting

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1.2-fold enhancement relative to the PtNi bimetallic NWs/C catalyst (2.34 mA cm-2), 2.2-fold

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enhancement relative to the Pt NWs/C catalyst (1.23 mA cm-2), and 9.7-fold enhancement

22

relative to the commercial Pt/C (0.28 mA cm-2). The results reflected that the structure of NWs

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and alloying of Ni and Rh elements all contributed to the highest specific activity of PtNiRh

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trimetallic NWs/C catalyst. Because of a combination of the highest specific activity and Pt

25

utilization efficiency, the mass activity of PtNiRh trimetallic NWs/C catalyst reached 2.88 A

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mg-1Pt at 0.9 VRHE, which was 1.4 times higher than that of PtNi bimetallic NWs/C catalyst (2.10

27

A mg-1Pt), 3.3 times higher than that of Pt NWs/C catalyst (0.88 A mg-1Pt) and 15.2 times higher

28

than that of commercial Pt/C (0.19 A mg-1Pt).

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Besides, we assessed the durability of the catalysts through an accelerated durability test

30

(ADT) between 0.6 and 1.1 VRHE in 0.1 M HClO4 at a scan rate of 100 mV s-1. Figure 4a shows

31

the positive-going ORR polarization curves of PtNiRh trimetallic NWs/C catalyst before and 10 ACS Paragon Plus Environment

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after ADTs. The PtNiRh trimetallic NWs/C catalyst exhibited the mass activity of 2.51 A mg-1Pt

2

after 10000 cycles, showing a loss of 12.8% compared with the initial mass activity (Figure 4b).

3

By contrast, the decay in the mass activities of PtNi bimetallic NWs/C, Pt NWs/C and

4

commercial Pt/C catalysts reached 54.3%, 21.5% and 73.7% after 10000 cycles (Figure 4b and

5

Figure S11), respectively. On the basis of these results, 1D anisotropic structure and incorporated

6

Rh atoms were believed to be responsible for the highest catalytic durability of PtNiRh

7

trimetallic NWs/C catalyst. Importantly, the performance of such one-nanometer-thick PtNiRh

8

trimetallic NWs/C catalyst has excelled most of the lately reported Pt-based electrocatalysts in

9

terms of both mass activity and durability (Figure S12).

10

Understanding of the enhanced catalytic performance. To rationalize the greatest

11

enhancement in ORR activity for PtNiRh trimetallic NWs/C catalyst, the DFT calculations were

12

performed. The theoretical models were constructed for PtNiRh trimetallic NWs, PtNi bimetallic

13

NWs and Pt NWs based on the corresponding structural and compositional parameters (see DFT

14

calculations in Method Section and Figure 5a). It is found that the optimized PtNiRh (111) and

15

PtNi (111) slabs both show 1.3% compressed lattice constant relative to Pt (111) slab due to the

16

smaller radius of Ni atoms. Notably, the calculated value of compressive strain was consistent

17

with that obtained from EXAFS analysis. According to previous studies, compressive strain of Pt

18

would greatly facilitate the ORR catalysis by decreasing the adsorption energies of oxygenated

19

species.22-24,47 As the hydroxyl (OH) adsorption energy (EOH) can be regarded as an effective

20

descriptor of ORR activity for Pt-based catalysts,48,49 we compared the EOH on the surfaces of

21

PtNiRh (111), PtNi (111) and Pt (111) slabs. For convenience, we set the optimal EOH (∼0.1 eV

22

weaker than the value on Pt (111)48) to zero and use △ EOH to show the difference between a

23

calculated EOH and the optimal value. As shown in Figure 5b, the PtNiRh (111), PtNi (111) and

24

Pt (111) slabs presented △EOH values of 0.03 eV, 0.04 eV and -0.10 eV, respectively, supporting

25

the experimental results on the activity trend for catalysts. It is found that, in comparison with the

26

EOH on Pt (111) slab, alloying Ni into Pt caused a great decrease in EOH by 0.14 eV. Considering

27

the calculated 1.3% compressive strain may weaken EOH by ~0.04 eV according to the

28

established relationship,22 such a great decrease can be ascribed to a combination of compressive

29

strain and ligand effects. More intriguingly, the introduction of Rh into the PtNi (111) subsurface

30

increases the OH adsorption, which could be attributed to the ligand effect from the incorporated 11 ACS Paragon Plus Environment


1

Rh atoms. Overall, the substantially enhanced specific activity for PtNiRh trimetallic NWs/C

2

catalyst could be ascribed to a synergy of compressive strain and ligand effects arising from the

3

favored microstructure and optimal alloy composition.

4

We further provided an in-depth understanding of the enhanced catalytic durability for

5

PtNiRh trimetallic NWs/C catalyst. Since the operation durability is strongly related with the

6

structural and compositional stability of catalyst, the structures and compositions of catalysts

7

were examined after ADTs. It was found that the morphology of PtNiRh trimetallic NWs/C

8

catalyst was well preserved without obvious change, sharply contrasting with the severe

9

agglomeration/sintering for PtNi bimetallic NWs/C and commercial Pt/C catalysts after the

10

durability tests (Figure S10e-h). The observations verified that 1D structure and incorporated Rh

11

atoms promoted the structural stability of catalysts, in line with the previous reports.38 The

12

advantages of 1D structure can be generally ascribed to the ability to suppress the ripening

13

process and reinforce the contact between 1D catalyst and carbon support.28 The compositional

14

analysis after 10000 cycles of ADT conducted by ICP-AES suggested that the Pt/Ni/Rh atomic

15

ratio was 4.51:1.00:0.33, slightly deviating from the initial atomic ratio. By contrast, the atomic

16

ratio of Pt/Ni for PtNi bimetallic NWs/C catalyst was changed from 2.99:1.00 to 12.32:1.00.

17

Such a big change in composition for PtNi bimetallic NWs/C catalyst is believed to accelerate

18

the damage of their microstructure, causing the worst catalytic durability. Besides, the results

19

indicated that the incorporated Rh atoms could substantially retarded the leaching of Ni atoms

20

from NWs. The reasons for the functions of Rh in improving the structural and compositional

21

stability are probably related with the following two aspects. Specifically, the lower chemical

22

stability of Rh relative to Pt could prevent the Pt dissolution by preferentially dissolving Rh

23

atoms, resulting in the enhanced structural stability.38 Meanwhile, the anchoring effect of Rh

24

atoms in suppressing the diffusion of Pt atoms helps to maintain the Pt-skin structure during the

25

operation condition and thus protecting the Ni from the oxidation.32 To demonstrate the so-called

26

anchoring effect, the electrochemical analyses were performed to reveal the surface structure

27

variations of catalysts before and after ADTs. As shown in Figure S13, the results testified that

28

the PtNiRh trimetallic NWs preserved their Pt-skin structure well after ADTs, contrasting to case

29

of PtNi bimetallic NWs. The ability to maintain the Pt-skin structure for PtNiRh trimetallic NWs

30

is probably related with the suppressed diffusion of Pt atoms arising from the incorporation of

31

Rh atoms, supporting the anchoring effect. 12 ACS Paragon Plus Environment

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1

The role of alloyed Rh atoms in improving the catalytic durability of Pt-based catalyst were

2

also supported by DFT calculations. Since vacancy formation energies of outmost Pt and Ni

3

atoms represent the tendency of dissolution of Pt atoms and leaching of Ni atoms,35 respectively,

4

the effects of Rh atoms on vacancy formation energies of Pt and Ni atoms were investigated for

5

the catalysts. As shown in Figure 5c, the PtNiRh (111) slab has the highest vacancy formation

6

energy of Pt atom, while the PtNi (111) slab shows the lowest vacancy formation energy of Pt.

7

These results suggested that the incorporation of Rh atoms increased the vacancy formation

8

energy of Pt atoms, confirming the improved structural stability with the introduction of Rh

9

atoms. We noted that the incorporation of Ni would greatly decrease the vacancy formation

10

energy of Pt, which would lower the stability of Pt structure and in turn increase the possibility to

11

expose Ni atoms, facilitating the leaching of Ni atoms. Besides, it was found that the Rh element

12

could elevate the vacancy formation energy of Ni atom (Figure 5d), thus reducing the leaching

13

rate of Ni atoms from PtNiRh (111) slab. Obviously, the simulation results reproduced the

14

experimental trends in morphological and compositional changes for our catalysts. All these

15

analyses together suggested that the enhanced catalytic durability of PtNiRh trimetallic NWs/C

16

catalyst was due to the improved structural and compositional stability originated from the

17

integration of 1D structure and incorporated Rh atoms.

18 19

Conclusions

20

We have demonstrated a rational design of active and durable ORR catalyst by integrating the

21

multiply structural and compositional advantages. The multiply advantages in the catalyst can be

22

categorized on the basis of their effects on ORR performance. Specifically, the optimal

23

composition of incorporated Ni and Rh element, together with the active {111} surface facet,

24

afford the optimized electronic structure and thus boost the specific activity via a combination of

25

geometrical and ligand effects. Besides, the atomic-level thin diameter enhances the mass

26

activity via maximizing the utilization efficiency of Pt atoms. Moreover, the 1D anisotropic

27

structure and incorporated Rh atoms greatly improves the catalytic durability by elevating the

28

structural and compositional stability of the catalyst. Owing to the integration of these

29

advantages into the one, the one-nanometer-thick PtNiRh trimetallic NWs catalyst presented

30

enhancement factors of 15.2 for mass activity and 9.7 for specific activity relative to a

31

commercial Pt/C catalyst. Besides, the mass activity of PtNiRh/C trimetallic NWs/C catalyst 13 ACS Paragon Plus Environment


1

decreased by 12.8% after 10000 cycles of ADTs, comparing to 73.7% for commercial Pt/C

2

catalyst. This study not only achieves an active and durable acidic ORR catalyst, but also

3

demonstrates a general and effective approach to design remarkable heterogeneous catalysts.

4 5

ASSOCIATED CONTENTS

6

Supporting Information

7

This material is available free of charge via the Internet at http://pubs.acs.org.

8

XPS survey spectrum of PtNiRh trimetallic NWs, XRD patterns, approximate geometrical

9

models, structural characterizations for PtNi bimetallic NWs and Pt NWs, CO stripping

10

voltammogram, time-dependent morphological evolution and time-dependent compositional

11

evolution for PtNiRh trimetallic NWs, FT-IR spectra, TEM images of the catalysts for ADTs,

12

ORR positive-going polarization curves, ORR performance comparisons ， CVs and CO

13

stripping curves

14 15

AUTHOR INFORMATION

16

Corresponding Author

17

*Email:

[email protected] (J.Z.).

18

*Email:

[email protected] (H.H.).

19

Notes

20

The authors declare no competing financial interests.

21 22

ACKNOWLEDEGEMENTS

23

This work was supported by Collaborative Innovation Center of Suzhou Nano Science and

24

Technology, MOST of China (2014CB932700), NSFC under Grant Nos. 21603208 and

25

21573206, Key Research Program of Frontier Sciences of the CAS (QYZDB-SSW-SLH017),

26

Anhui Provincial Key Scientific and Technological Project (1704a0902013), Major Program of

27

Development Foundation of Hefei Center for Physical Science and Technology (2017FXZY002),

28

and Fundamental Research Funds for the Central Universities. This work was partially carried

29

out at the USTC Center for Micro and Nanoscale Research and Fabrication.

30


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23



1 2 3

Figure 1. Structural and compositional characterizations of PtNiRh trimetallic NWs. (a)

4

low-magnification TEM image. Scale bar, 50 nm. (b) HAADF-STEM image. Scale bar, 20 nm.

5

(c) Histogram of diameter and length distributions. (d, e) Atomic-resolution HAADF-STEM

6

images. Scale bars, 5 nm. (f) EDX elemental mapping images of Pt, Ni, and Rh. (g) EDX

7

line-scanning profile. (h-j) High-resolution XPS spectra of Pt 4f (h), Ni 2p (i) and Rh 3d (j).


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1

2 3 4

Figure 2. Local atomic and electronic structures of Pt-based NWs and commercial Pt/C. (a) Pt

5

L3-edge XANES profiles. (b) Pt L3-edge EXAFS spectra in R space.



Page 20 of 24

1 Table 1. Summary of Pt L3-edge EXAFS fitting results Sample

Pt-O

Pt-Pt

R (Å)

CN

R (Å)

CN

Pt foil





2.7640.002

12

Pt/C

2.000.01

2.10.3

2.760.01

5.10.5

Pt NWs

2.000.04

0.40.3

2.750.01

9.50.8

PtNi bimetallic NWs

1.960.02

0.80.3

2.730.01

5.40.6

PtNiRh trimetallic NWs

1.960.02

1.00.2

2.710.01

4.40.5

2 (Å2)

E0 (eV)

0.00490.0002

8.30.5 11.31.3

0.00460.0014(O) 0.00740.0004 (Pt)

7.11.1 5.71.3 5.71.3

R, distance between absorber and backscatter atoms; CN, coordination number; 2, Debye-Waller factor; E0, inner potential correction to account for the difference in the inner potential between the sample and the reference compound. 2


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1

2 3 4

Figure 3. Electrocatalytic performance of different catalysts. (a) CV curves recorded in

5

N2-saturated 0.1 M HClO4 solutions at room temperature with a sweep rate of 50 mV s-1. (b)

6

Positive-going polarization curves recorded in O2-saturated 0.1 M HClO4 solutions with a sweep

7

rate of 10 mV s-1 and a rotation rate of 1600 rpm. (c) Specific activities at 0.9 VRHE. (d) Mass

8

activities at 0.9 VRHE. The color scheme in panel (a) applies to all other panels.



1 2 3

Figure 4. The electrocatalytic durability of the catalysts toward ORR. (a) CV curves and the

4

corresponding mass activity Tafel plot (inset) for the PtNiRh trimetallic NWs/C catalyst before

5

and after ADTs of 10000 cycles. (b) The comparison of mass activities for the catalysts at 0.9

6

VRHE before and after ADTs of 10000 cycles.


Page 22 of 24

Page 23 of 24 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


1 2 3

Figure 5. DFT calculations. (a) Theoretical model structures. (b) The OH adsorption energies

4

relative to the optimal OH adsorption energy (△EOH) and (c) Pt vacancy formation energies ( EVPt )

5

for Pt NWs, PtNi bimetallic NWs and PtNiRh trimetallic NWs. The optimal OH adsorption

6

energy is set as zero. (d) Ni vacancy formation energies ( EVNi ) for PtNi bimetallic NWs and

7

PtNiRh trimetallic NWs.



1

Table of Contents

2

3 4


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One-Nanometer-Thick PtNiRh Trimetallic Nanowires with Enhanced

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