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Effect of ordering of PtCu3 nanoparticle structure on the activity and stability for the oxygen reduction reaction

Nejc Hodnik*, Chinnaya Jeyabharathi, Josef C. Meier, Alexander Kostka, Kanala L. N. Phani,

Aleksander Rečnik, Marjan Bele, Stanko Hočevar, Miran Gaberšček* and Karl J. J. Mayrhofer*

Supplementary Information

S1. Structural characterization

For transmission electron microscopy (TEM) observations ( Figure 1 in the main text)

the synthesized Cu3Pt/C samples were ground to a fine powder in an agate mortar with

alcohol. The suspension containing our sample was deposited onto the carbon-coated TEM

sample supports. TEM studies were performed using a 200 kV microscope (JEM-2010F, Jeol

Inc., Tokyo, Japan) equipped with an ultra-high resolution objective lens pole-piece having a

point-to-point resolution of 0.19 nm and a resolution limit at 0.11 nm (Cs = 0.48 mm, Δf = 8

nm), being sufficient to resolve the full lattice images of all major Cu3Pt low-index zone axes.

Fig 1a shows a 9 nm large PtCu3 particle carefully tilted into the [11-0] zone axis,

where we can observe two sets of Pm3-m symmetry specific reflections; {001} and {110}, as

well as the composition of {111} planes, which helps us to identify the actual mode of Cu-Pt

ordering in this compound. The outer shell of the particle shows interrupted sections with

clearly visible Pm3-m structure. It is easily identified by vertical arrays of strong and slightly

weaker white dots. This feature becomes diffuse and finally lost towards the crystal interior,

implying that both this composition and structure is not present at the crystal core. If we take

Fourier trans-form (FFT) from the shell and the particle core the main difference in the

reciprocal space image is the presence of Pm3-m symmetry specific reflections in the outer

shell (Fig. 1b), which away in the power spectrum from the core region (Fig. 1c). Some

residual intensity at the fade positions of superstructure reflections is due to overlapping with

the outer shell structure of the spherical particle in projection. A relatively high decrease of

these reflections implies that the core must have an Fm3-m symmetry, characteristic for

disordered (PtCu3)-alloy.

The procedure of X-ray powder diffraction measurement and qualitative and

quantitative analysis of the XRD pattern was described in ref. [1] . Shortly, data were

Electronic Supplementary Material (ESI) for Physical Chemistry Chemical Physics.This journal is © the Owner Societies 2014

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collected on a PANalytical X'Pert PRO diffractometer using CuKα1 radiation (1.54060 Å).

Data collection was carried out with the help of a flat disc-like sample holder in the 2Θ range

from 5 to 140º in steps of 0.008º 2Θ. The analyses were performed with Crystallographica

Search-Match programs [S-XRD1] and TOPAS V2.1 Rietveld refinement program [S-XRD2]

([S-XRD1] Crystallographica Search-Match, version 2,1,1,0, Oxford Cryosystems, UK, 2003.

[S-XRS2] TOPAS V2.1, Users Manual, Bruker AXS, Karlsruhe, Germany, 2000).

Figure 1 d) shows the powder XRD patterns of the annealed PtCu3/C with two

different structures, viz. a partially ordered and a fully disordered phase. The diffraction peaks

at the angles of 24° and 34° are characteristic for the ordered phase of PtCu3 (Pm3-m). All

other diffractions are due to solid solutions of Pt and Cu with random distribution of atoms

(Fm3-mphase). The phase composition of partially ordered catalyst is obtained from Rietveld

refinement and is 66.5% Fm3-m phase and 33.5% Pm3-m phase. In the case of fully disordered

catalyst the diffraction patterns reveals Fm3-m only. The position of the ordered phase is

theoretically predicted to be in the shell of the particles 4. This is indirectly confirmed in the

present study by the observation of a considerable difference in the surface properties like

activity and stability.

S2. Identical-Location Transmission Electron Microscopy measurements (IL-TEM)

The catalyst suspension for IL-TEM was prepared freshly by dispersing the catalyst

particles in Millipore water under bath-sonication. A drop of this dilute suspension was

pipetted on top of a gold finder grid coated with carbon film. After a few seconds the drop

was removed carefully from the grid using a tissue to avoid excessive loading and thus

overlapping of the catalysts particles. After drying, TEM investigation was done using a Jeol

JEM-2200FS instrument at the operating voltage of 200 kV. In the present work IL-TEM

imaging was done in a sequence of as-prepared - de-alloying - degradation of a particular

catalyst on identical locations. The gold finder grid with the catalyst particles was contacted to

a GC disk electrode and cycled in a similar way as mentioned for dealloying and degradation.

Bright field imaging of the catalyst was done at several locations with varying magnifications

and those locations of the catalyst portion were then marked in a way so that they could be re-

traced and imaged after different electrochemical treatments.

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S2.1. SEM images of the ordered and disordered catalyst that show similar particle size

distribution and inter-particle distances.

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S2.2: IL-TEM images of ordered catalyst of before (left) and after (right) 3600 cycles between 0 V and 1.2 V with 1 V/s in 0.1 M HClO4. Effect of shaping is nicely seen.

S2.3: TEM images showing good carbon embedment of the PtCu3 nanoparticles. A monolayer

of carbon (graphene) covers the surface of the nanoparticles.

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S2.4 IL-tomography of PtCu3 ordered nanoparticles as synthesized and after degradation is

attached a s movie.

S3. Electrochemical experiments

A three electrode cell made of Teflon was used for the electrochemical measurements.

A glassy carbon disk of a diameter of 5 mm mounted on a Teflon tip was polished using

alumina slurry in a micro-cloth before every experiment, cleaned with Millipore water in a

bath-sonication, modified with a catalyst film, dried and fixed on an RDE shaft (Radiometer

Analytical, France) and used as a working electrode. A graphite rod and an Ag/AgCl, 3M KCl

(Metrohm) were used as a counter electrode and reference electrode, respectively. Unless

otherwise mentioned, all the potentials were referred to RHE, which was determined before

each measurement by H2 redox reaction. To avoid Cl– ion contamination the counter and

reference electrodes were separated from the rest of the cell by a special cell construction

using a Nafion® membrane (Tschurl modification)2. A Gamry Reference 600

potentiostat/galvanostat was employed and controlled in combination with the rotator and the

gas system with an in-house developed LabVIEW software3. All the electrochemical

experiments were carried out using 0.1 M HClO4 solutions prepared from concentrated HClO4

(Suprapur®, Merck) and Millipore water (18.2 M.cm). Solution resistance was compensated

via positive feedback in such a way that the residual uncompensated resistance was less than 3

Ω in all experiments. All measurements were performed at room temperature.

The catalyst suspension was prepared with ultrapure water. RDE-GC tip was modified

by drop-casting 40 µL of the suspension and the solvent was evaporated in a desiccator. After

drying 5 µL of freshly prepared Nafion® (0.1%) in isopropanol was coated over the catalyst

layer and again dried. Final loading on the electrode was 26 µg.cm-2Pt. Dealloying of the

catalyst was performed in two different ways: (a) by sweeping the potential of electrode from

0.05 V to 1.2 V for 500 cycles at a scan rate of 0.2 V/s; (b) by holding the potential at 1.2 V

for 15minutes/2hours in de-aerated electrolyte. In both cases the electrode was kept at 0.05 V

before starting the dealloying process to avoid the leaching of copper at the equilibrium

potential (~0.34 V). This enabled us to record the initial dealloying processes. Accelerated

degradation tests were performed by cycling the dealloyed-PtCu/C catalysts in de-aerated

electrolyte from 0.6 V to 1.2 V for 7000 cycles at a scan rate of 1 V/s. For ORR studies using

hydrodynamic voltammetry, the working electrode was cycled from 0.05 V to 1.2 V at a scan

rate of 0.05 V/s in oxygen-saturated electrolyte by varying the rotation rate (400, 900, 1600 &

2500 rpm), while maintaining oxygen environment throughout the measurement. Finally the

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background current was subtracted from Ar-background curve to obtain the ORR current. A

CO stripping method was employed to measure the electrochemically active surface area

(ECSA) after dealloying and degradation. In this method CO molecules are chemisorbed on

the electrochemically accessible active Pt sites at the potential of 0.05 V in the electrolyte

containing CO molecules. Then all the CO molecules are removed from the electrolyte by

purging Ar, while maintaining the same low potential. The pre-adsorbed CO molecules are

stripped in CO-free electrolyte by cycling from 0.05 V to 1.2 V at a scan rate of 0.05 V/s for

three cycles and the charge corresponding to the CO oxidation peak is measured by

integrating the area under the peak, which is used to calculate the ECSA.

Figure 1. ORR measurement of the ordered PtCu/C – dealloying by 500 cycles.

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Figure 2. ORR measurement of the ordered PtCu/C – dealloying by hold @ 1.2 V.

Figure 3. Initial 500 cycles during dealloying of the ordered (on left) and the disordered (on

right) catalyst.

Activity of both catalysts towards ORR was studied after all electrochemical treatments by

performing Rotating Disk Electrode voltammetry. After accelerated degradation the same

electrode is used for RDE measurement of ORR. Electrochemical surface area (ECSA, m2/gPt)

of these electrodes are measured by integrating the charge of CO oxidative stripping. From

the oxygen reduction curves obtained at 1600 rpm, various parameters such as specific

activity (SA, mA/cm2Pt), mass activity (MA, A/mgPt) at 0.9 V were obtained (see table 1 and

2).

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Table S3.1:

Deg. = degradation; Bef. = before; Aft. = after

Table S3.2:

SA (mA/cm2Pt) MA (A/mgPt) ECSA (m2/gPt)

Hold at

1.2 V 15 min.

2 hrs

(bef. Deg.)

Aft.

Deg. 15 min.

2 hrs

(bef. Deg.)

Aft.

Deg.

15

min.

2 hrs (bef.

Deg.)

Aft.

Deg

PtCu/C

(Pm3m) 3.4 ± 0.3 2.4 ± 0.2 2 0.68 ± 0.07 0.77 ± 0.08 0.44 20±2 32 ± 3 22 ± 2

PtCu/C

(Fm3m) 2.6 ± 0.2 1.5 ± 0.04 - 0.6 ± 0.05 0.5 ± 0.1 - 24 30 ± 1 -

S4. Inductively Coupled Plasma Mass Spectrometry measurements

After the dealloying process, the electrolytes from the cell were carefully collected by

mixing the electrolyte via rotating the electrode for one minute to ensure the uniform

concentration of the ions, viz. Cu2+ ions produced due to oxidative leaching. Then the

electrolytes were used for ICP-MS analysis to quantify the amount of the materials leached

during the dealloying process. The concentration of dissolved Cu was determined by an ICP-

MS system (Nex-ION 300X, Perkin Elmer), which was equipped with a cyclonic spray

chamber and a Meinhard nebulizer operating at a pump rate of around 300 µL min−1. An

internal standard (115In, 5 ppb) was added to each sample and measured parallel to Cu to

compensate for instrument intensity fluctuations with time.

Catalytst SA (mA/cm2Pt) MA (A/mgPt) ECSA (m2/gPt)

CV Bef. deg. Aft. deg. Bef. deg. Aft. deg. Bef. deg. Aft. deg.

PtCu/C (Z) 2.5 ± 0.2 2.0 ± 0.2 0.58 ± 0.06 0.42 ± 0.05 23 ± 1 21 ± 2

PtCu/C (Q) 1.78 ± 0.2 1.56 ± 0.2 0.45 ± 0.1 0.3 ± 0.03 25 ± 1 20 ± 1

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Table S4:

ICPMS

(% of Cu in

electrolyte)

DA by

15 min.

DA by

Hold 2 hrs

(Bef. Deg.)

After

Deg.

DA by CV

(Bef. Deg.)

After

Deg.

PtCu/C (Pm3m) 10 ± 2 80 ± 1 9 ± 01 56 ± 1 15 ± 4

PtCu/C (Fm3m) 17, 4 90 8 73 ± 0,6 13 ± 0,9

1. Hodnik, N.; Bele, M.; Rečnik, A.; Logar, N. Z.; Gaberšček, M.; Hočevar, S. Energy Procedia 2012, 29, (0), 208-215.2. Mayrhofer, K.; Ashton, S. J.; Kreuzer, J.; Arenz, M. International Journal of Electrochemical Science 2009, 4, (1), 1-8.3. Topalov, A. A.; Katsounaros, I.; Meier, J. C.; Klemm, S. O.; Mayrhofer, K. J. J. Review of Scientific Instruments 2011, 82, (11), 114103-114103-5.4. Müller, M.; Albe, K. Beilstein Journal of Nanotechnology 2011, 2, 40-46.