Effect of Thermal Treatment on the Atomic Structure and

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Effect of Thermal Treatment on the Atomic Structure and Electrochemical Characteristics of Bimetallic PtCu Core-Shell Nanoparticles in PtCu/C Electrocatalysts Vasiliy V. Pryadchenko, Sergey V. Belenov, Darya B. Shemet, Vasiliy V. Srabionyan, Leon Aleksandrovich Avakyan, Vadim A. Volochaev, Alexey S. Mikheykin, Karina E. Bdoyan, Ivo Zizak, Vladimir E. Guterman, and Lusegen A. Bugaev J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03696 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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The Journal of Physical Chemistry

Effect of Thermal Treatment on the Atomic Structure and Electrochemical Characteristics of Bimetallic PtCu Core-Shell Nanoparticles in PtCu/C Electrocatalysts Vasiliy V. Pryadchenkoa*, Sergey V. Belenova, Darya B. Shemeta, Vasiliy V. Srabionyana, Leon A. Avakyana, Vadim A. Volochaeva, Alexey S. Mikheykina,b, Karina E. Bdoyana, Ivo Zizakc, Vladimir E. Gutermana, Lusegen A. Bugaeva. a

Physical Faculty and Chemical Faculty, Southern Federal University, 344006, B. Sadovaya str. 105/42, Rostov-on-Don, Russia b Southern Scientific Center of Russian Academy of Sciences, Rostov-on-Don 344090, 5 Zorge St., Russia c Institute for Nanometre Optics and Technology, Helmholtz-Zentrum Berlin, AlbertEinstein-Straße 15, 12489 Berlin, Germany * [email protected], +7 909 423 3020

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Abstract Supported PtCu/C electrocatalysts containing core-shell bimetallic PtCu nanoparticles were synthesized by sequential chemical reduction of Cu2+ and Pt (IV) in a carbon suspension, prepared on the basis of ethylene glycol–water solvent, and then treated at different temperatures in range from 250 to 350 °C. The structural characterization of “as prepared” and obtained after the thermal treatments PtCu nanoparticles was performed by TEM, XRD, Pt L3- and Cu K-edge extended X-ray absorption fine structures (EXAFS). Atomic cluster models of PtCu nanoparticles before and after thermal treatment, reflecting the character of components’ distribution, were generated. Electrochemical performance of the obtained PtCu/C electrocatalysts in oxygen reduction reaction (ORR) was studied by cycling and linear voltammetry.


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1. Introduction The efficiency of proton exchange membrane fuel cells (PEMFCs) 1 is determined by a number of factors, and one of the key factor is the rate of the oxygen reduction reaction (ORR). Currently, the catalytic layers used in the low-temperature hydrogen-oxygen fuel cells (FC) are usually based on metal-carbon nanocomposites, containing platinum nanoparticles 2–4. A great interest of researchers is also caused by “Pt-free” electrocatalysts, including in the development of alkaline fuel cells with anion-exchange membrane (AEMFCs) 5–10. Unfortunately, the efficiency non-platinum electrocatalysts in PEMFC is still far from the efficiency of Pt-containing materials. For recent years, many studies are devoted to the enhancing of the stability and catalytic activity of the platinum-carbon electrocatalysts, the decreasing of the mass fraction of the noble metal in such catalysts 11. Bimetallic PtM nanoparticles (where M is usually a transition metal such as Co, Ni, Fe, Cu, Ag, etc.) with a core–shell structure (with Pt atoms in the shell, and the M atoms in the core 12,13 ) deposited on highly developed surfaces of carbon substrates seem to be the most attractive among these materials 14–24. Using such bimetallic nanoparticles it is possible not only to reduce the mass fraction of platinum, but also to increase the specific activity of the catalyst 25,26. The thermodynamic stability of the metal M is typically much lower than that of platinum 27,28, that results in the selective dissolution of M during the operation and, as the consequence, in poisoning of the ion exchange membrane and decrease of the specific characteristics of the PEMFC. Therefore, the decisive factors determining the durability and catalytic activity of bimetallic core-shell nanoparticles, are the composition, thickness, and continuity of the shell, its atomic structure, shape, surface morphology, the stability of the core-shell structure during the operation. Another important factor, which affects the efficiency and stability of bimetallic PtM nanoparticles is the choice of metal M. Copper is one of the promising metals which can be used to form cores of bimetallic PtM nanoparticles, which generally provides high activity and morphological stability of PtCu/C electrocatalysts 24,29–31. The closeness of the crystal lattice parameters for copper and platinum is one of the main reasons that determines the efficiency to use copper as cores for the formation of PtCu nanoparticles with a desired continuous platinum shell. Identification of the structure of bimetallic nanoparticles is important for the studies aimed at optimizing the structure and enhancing the electrochemical performance of PtM/C electrocatalysts. Extended X-ray absorption fine structure (EXAFS) spectroscopy 32,33 is a powerful method for the study of nanoparticles because of its sensitivity to local atomic structure and composition in the absence of a long range order, high spatial resolution and applicability under in situ conditions of catalytic reactions 34–38. For bimetallic nanoparticles, EXAFS can be used as a source of structural information, which is complementary to particles characterization provided by such experimental methods as XRD, (S)TEM, XPS, etc. 34,39,40. Since the bimetallic nanoparticles form through self-organization, it may results in a certain deviation of the composition and architecture of separate nanoparticles 15,18. The architecture of bimetallic nanoparticles in PtM/C materials may be altered by the posttreatment 25–28. To improve the continuity of the shell and provide corrosive-morphological stability of such nanoparticles, the synthesized electrocatalysts may be treated in different ways: the acid treatment may lead to the dissolution of transition metal from shells of ACS Paragon Plus Environment

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defective core-shell nanoparticles 41, the thermal treatment may lead to the reconstruction of the shell. Depending on the composition and structure of the initial nanoparticles and the posttreatment parameters, the resulted nanoparticles’ architecture may vary from component segregation to a more uniform distribution of components over the particle volume. Recently, the effect of acid treatment on the structure of PtCu core-shell nanoparticles was studied 42. It was shown, that the “as prepared” PtCu/C electrocatalyst obtained using the suggested method of synthesis contains core-shell PtCu nanoparticles as well as large amount of amorphous copper oxide; acid treatment leads to dissolution of copper oxide and to relative thickening of Pt shell of “mean” nanoparticle. For the thermal treatment of PtCu core-shell nanoparticles it was found earlier 19 that thermal treatment at 300 °C leads to structural changes that are observed in the shift of diffraction maxima in X-ray powder diffractograms. It was shown, that these changes are connected with the alloying of the coreshell nanoparticles to partially ordered solid solutions. To establish the dynamic of evolution of structure and properties of the core-shell PtCu nanoparticles, the structural and electrochemical studies of PtCu/C electrocatalysts containing bimetallic PtCu nanoparticles synthesized by sequential chemical reduction of components and treated at different temperatures in range from 250 to 350 °C, were performed in the present work. 2. Experimental and Theoretical Methods 2.1. Synthesis of Materials Carbon supported PtCu nanoparticles were synthesized by wet-synthesis using NaBH4 as a reducing agent, this technique is described in detail in 31. Briefly: excess of a freshly prepared 0.5 M NaBH4 was added into the water - ethylene glycol suspension of a carbon powder (Vulcan XC72, Cabot) and metal precursors (H2PtCl6 and CuSO4) at pH = 10 (an excess of NH3). Temperature treatment was carried out using a PTC-1.2-40 furnace (NPP Teplopribor) in an argon atmosphere in the temperature range 250 - 350 °C according to the following scheme: rapid (about 10 minutes) heating to the set temperature, holding at the set temperature for 1 hour, slow spontaneous cooling after switching off the heating for 4 - 5 hours to room temperature. Materials after heat treatment were marked by adding a numerical value of the treatment temperature as an index to the name of the material (RT for “as prepared” material): PtCu_RT, PtCu_250, PtCu_280, PtCu_300, and PtCu_350. The composition of the synthesized PtCu/C nanocatalysts corresponds to mass fraction of the metallic component of ~30 % by weight. The chemical composition was determined by X-ray fluorescence analysis using the X-ray spectrometer ARL OPTIM’X and equal to Pt0.8Cu for all catalysts. 2.2. TEM, EXAFS, XRD and Electrochemical Measurements Pt L3- and Cu K-edge EXAFS spectra for the studied materials were measured at the µSpot beamline 43 of the BESSY-II Synchrotron Radiation Facility (Berlin, Germany). Mean electron current of a storage ring was maintained during experiment at 250 mA in Top-Up mode. The measurements were performed in the transmission mode utilizing a doublecrystal Si (111) monochromator and two ionization chambers and diode for the reference ACS Paragon Plus Environment

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channel. To avoid the intensity fluctuations during the EXAFS scan, the translation of the second crystal was stopped, simulating the channel-cut monochromator. The cut-off energy for the focusing mirror is 21 keV, resulting in a rather good suppression of the third harmonic for the energies higher than 8 keV. Additionally, the second monochromator crystal was tilted so the intensity of the first harmonic was reduced to about 70 %, resulting in the strong suppression of the third harmonic. Energy band width was about 2.3 and 2.9 eV for Cu K- and Pt L3-edge, respectively. The photon energy scanning steps were adjusted to δE = 1.0 eV and δk = 0.05 Å-1 in the XANES and EXAFS regions respectively, where E is the energy of the incident radiation and k is the corresponding photoelectron wavenumber. TEM analysis was performed using a JEM-2100 (JEOL, Japan) microscope and a FEI Tecnai G2 F20 S-TWIN TMP microscope with EDAX attachment operated at an accelerating voltage of 200 kV. Electrocatalyst powders (0.5 mg) were placed in 1 ml of heptane to prepare the samples for the TEM analysis. The suspension was then ultrasonically dispersed, and one drop of the suspension was deposited onto a copper grid sputter-coated with carbon. Histograms of the size distribution of nanoparticles in samples PtCu_RT, PtCu_280, and PtCu_300 were obtained by measuring sizes of at least 500 particles per sample. Synchrotron X-ray powder diffraction patterns were collected at the BM01 beamline 44 of the Swiss-Norwegian Beamlines at the European Synchrotron Radiation Facility (SNBL at ESRF, Grenoble, France). The monochromatic beam with the wavelength λ = 0.694(89) Å was used. The sample-to-detector distance, the tilt angles of the detector and the wavelength were calibrated using LaB6 NIST standard sample. Electrochemical characteristics were measured using a rotating disc electrode with standard three electrode cell setup 45. A saturated Ag/AgCl/KCl electrode was used as a reference electrode and as a counter electrode was used a platinum wire. The potentials values are given in the article had been recalculated to the reversible hydrogen electrode (RHE) values. A catalyst was placed at the electrode using “catalytic inks”, containing 0.006 g of the catalyst, 900 µl of isopropanol (purity: reagent grade, puriss.) and 100 µl 0.5 % of Nafion (DE-1020) 46. The exact amount (6 µl) of suspension was dropped on a glassy carbon electrode (diameter – 5 mm); after five minutes, when the droplet had been dried, a droplet (7 µl) of 0.05 % solution of Nafion in alcohol was placed on the top in order to fix the catalytic layer. To perform a standardization procedure, 100 cycles were recorded in the range from 0.03 to 1.26 V with a potential sweep rate of 200 mV/s at the beginning of each investigation for any droplet of “catalytic inks”. After the standardization, two cycling voltammetry curves (CVs) were recorded to determine the electrochemically active surface area (ECSA) of the catalyst in the same potential range, the sweep rate was 20 mV/s. The calculation of the ECSA value was based on the amount of electricity spent for adsorption/desorption of atomic hydrogen, taking into account the contribution of nonFaraday processes on the electrode, as described in 45. For comparison, the ECSA value was measured by CO-stripping, as described in 47. The measurements of activity of catalyst include four potentiodynamic curves recorded on the same setup in the potential range of 0.1 - 1.2 V with a potential sweep rate of 20 mV/s at the different electrode rotation speeds: 400, 900, 1600, 2500 rpm. The measured curves were normalized to the cell resistance (24


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Ω); the background curve measured in the electrolyte saturated with Ar was subtracted, as described in 48.

*

*

333fcc

224fcc

b 200fcc

Intensity, a.u.

*

400fcc 133fcc 331fcc 420fcc

220fcc

*

*

311fcc 222fcc

a 200fcc

002carbon

*

111fcc

111fcc

3. Results and Discussion 3.1. Results of XRD Analysis

Intensity, a.u.

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*

e d c b a

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30 40 2θ, deg.

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60

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Figure 1. (a) The powder diffraction data (λ = 0.694(89) Å) for PtCu_RT (dashed black line) and PtCu_350 (solid red line) sample, additional reflections that are not related to the space group Fm-3m (denoted as “fcc”) or to graphite are marked by *. (b) Selected area of the powder x-ray diffraction data of: a – PtCu_RT, b – PtCu_250, c – PtCu_280, d – PtCu_300, and e – PtCu_350. The vertical lines correspond to the maxima of 111 reflection in PtCu_RT (solid) and PtCu_350 (dashed). The diffractograms (Figure 1) show relatively strong metal platinum reflections (Pt, s.g. Fm3m), as well as quite visible, but rather wide (002) line of semi-amorphous carbon (Graphite, s.g. P63mc) at the around 12 degrees of 2θ. All additional reflections are weak, and any analysis of those peaks is sophisticated, as far as high broadening; nevertheless, the most of additional reflections would be classified as superstructure’s lines due to metal atoms ordering. It leads to the lower symmetry comparing to a pure metal or a disordered solid solution where any atom should be treated as a hypothetical “average particle” between two components proportionally to their molar ratio. Despite the mentioned in 25,49–53 a few possible structural models with two metal atoms ordering, there were no successful attempts to describe the superstructure yet with a certain good agreement regarding theoretical expectations (possible space groups are the following: R-3m, Pm-3m, I4/mmm) and the refinement as carried out for Fm-3m space group. Another plausible explanation is to find a good agreement with copper (II) oxide pattern, e.g. 2θ = 15.9° and 17.3° reflections would fit it rather well. But the very low intensity makes the identification difficult. The increasing of the annealing temperature for PtCu/C materials leads to expected decreasing of values both the unit cell parameter (a) and the (111) reflection’s FWHM, Table 1. Taking into the account complicated distribution of components, in this case FWHM depends not only from the average NPs’ size, but from their internal fine structure. Thus, there was no quite non-controversial solution to obtain Dav (average diameter of crystallites) through Scherrer’s equation. Nevertheless, a significant FWHM decreasing for


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temperature range higher than 250 °C shows the average crystallites’ size growing due to their possible intensive agglomeration during migration on the carrier’s surface. The sample PtCu_280 is the hardest case from the point of view structural details refinement due to probably multiple phases arising in the bulk as well as their presence as different, not quite large, clusters among the main volume. This fact explains well a strong reflections’ asymmetry could be seen at the powder X-ray pattern (Figure 1b, curve c).

Table 1. The Annealing Temperature Influence on FWHM and Unit Cell Parameter. sample FWHM, 2θ, ° lattice const., ° Å PtCu_RT 1.12 18.03 3.843±0.003 Å PtCu_250 1.30 18.01 3.847±0.003 Å PtCu_280 0.78 18.10 3.829±0.003 Å PtCu_300 0.58 18.24 3.798±0.003 Å PtCu_350 0.35 18.35 3.777±0.003 Å 3.2. Results of TEM TEM microphotographs for the samples PtCu_RT, PtCu_280, and PtCu_300 are shown on the Figure 2. TEM for the sample PtCu_RT demonstrates the presence of nanoparticles of different types: smaller nanoparticles with, probably, uniform structure, and larger nanoparticles with light inner region and dark outer region. According to TEM only, the former particles could be referred to core-shell nanoparticles with Cu inside (as lighter atoms) or to so-called hollow nanoparticles. In any case, thermal treatment leads to destruction of such nanoparticles: on Figure 2b one can see the presence of large particles comparative by size to non-uniform ones in PtCu_RT, but with uniform structure. At the same time there are no evidences of the presence of “core-shell” (or hollow) nanoparticles in the sample PtCu_300 (Fig. 2c). The observable qualitative changes in TEM for studied materials is the reproducible result obtained previously for PtCu/C electrocatalysts, synthesized and treated in similar conditions 31,42,54.


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Figure 2. TEM microphotographs (left side) of the samples PtCu_RT (a), PtCu_280 (b), and PtCu_300 (c) with corresponding nanoparticles’ size distribution histograms (right side) derived from TEM. Solid lines correspond to log-normal approximation of experimental data with parameters “mean” and “variance”, evaluated by fitting and presented in right-top corners. Size distribution histograms, shown on the right side of Figure 2, as well as corresponding approximations by log-normal density functions, show the increase of mean size of nanoparticles from 4.8 nm to 5.9 nm and its variance with the increase of treatment


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temperature. Line scanning of the composition of one isolated nanoparticle in sample PtCu_RT (Figure 3) confirmed its core-shell structure: the concentration of platinum is higher in the near-surface region (shell), while the concentration of copper higher in the internal region (core).

Figure 3. Result of EDX analysis (line scan) of composition of PtCu nanoparticle in PtCu_RT sample with non-uniform distribution of components over the volume. 3.3. Results of EXAFS Analysis Normalization of the absorption coefficients near Pt L3- and Cu K-edge and the subsequent extraction of their oscillatory parts χ(k) was performed using Athena – a part of the wide used Ifeffit software package for EXAFS data processing 55,56. Fourier analysis of the retrieved χ-functions was performed by two complementary approaches: 1) using Artemis, which is also a part of Ifeffit package and enables to perform fitting of the Fouriertransforms F(R) at certain ∆k-intervals and 2) using StepwiseExafsFiting software, developed by the authors, which implements the technique for reducing the effect of correlations among the fitting parameters on the obtained values of structural parameters. Fitting of F(R) of the experimental Pt L3- and Cu K-edge EXAFS was performed using the models of Pt and Cu local atomic structures in PtCu/C catalysts suggested earlier in 42 with several modifications described below. In the studied materials, which may contain bimetallic PtCu nanoparticles of different architectures or even separate


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monometallic Pt and Cu nanoparticles, each absorbing Pt or Cu atom can be surrounded either exclusively by Pt or Cu atoms (i.e. in “core” and “shell” or in separate nanoparticles) or by both of them (i.e. in intermediate region in core-shell or in alloy nanoparticles). Moreover, the nearest neighbors of Pt and Cu can be light atoms (oxygen and/or carbon). All possible cases are combined within the models for the Pt and Cu local structures in which the first neighbors of the absorbing atoms include platinum and copper atoms, as well as light ones. Denoting the contributions in χ(k), corresponding to photoelectron scattering processes on one nearest atom of a specific type, as χA-B(k), where the first label in subscript indicates the absorbing atom and the second label indicates the scattering one, we can write the oscillatory parts χPt(k) and χCu(k) of Pt L3- and Cu K-EXAFS, respectively, in the following form: Pt = Pt Pt-Pt ⋅ Pt-Pt + Pt-Cu ⋅ Pt-Cu + Pt-O,C ⋅ Pt-O,C + Pt (1)

Cu = Cu Cu-Cu ⋅ Cu-Cu + Cu-Pt ⋅ Cu-Pt + Cu-O,C ⋅ Cu-O,C + Cu

(2)

Using the Fourier-transform procedure in fitting enables to exclude the last terms Pt and Cu in expressions (1) and (2), which denote the contributions of the photoelectron scattering processes on distant atoms 42, as well as multiple scattering processes. Each term in the expressions (1) and (2) is determined by four varying parameters: amplitude factor – a product of spectroscope factor S02 and average coordination number, interatomic distance R, Debye-Waller parameter σ2 and energy shift ∆E. Using such a number of independently varying parameters in the fitting procedure leads to uncertainness of the resulting values of these parameters. This problem can be partially solved in frames of the technique for reducing the correlation effects mentioned above, which is based on varying ∆k-intervals and kn weight functions used for the Fourier-transformation and on the stepwise changing of some parameters to be determined. More detailed information one can find in 57. In frames of fitting procedure implemented in the used program Artemis, the problem of large number of fitting parameters can be solved by adding the restrains and dependencies between some parameters. First, we assumed that the energy shifts for contributions Pt-Pt and Pt-Cu and for contributions Cu-Cu and Cu-Pt should be pairwise equal and could be set to the corresponding values, obtained from spectra of Pt and Cu foils, respectively. Then, since the contributions of Pt-Cu and Cu-Pt correspond to the same bonds in the sample, their interatomic distances and the parameters of the DW should be equal. In our previous work 42 we used the coincidence of the values of these parameters as an additional indicator of the goodness of model. Here, to make the values of these parameters close, we applied restrains to their differences based on the permissible error in the determination of the parameters: |RPt-Cu - RCu-Pt| < 0.01 Å and |σ2Pt-Cu - σ2Cu-Pt| < 0.001 Å2. Finally, we assumed that the spectroscope-factors S02(Pt) and S02(Cu) do not depend on the type of scattering atom and can be retrieved from the fitting of EXAFS of corresponding bulks and then fixed during fitting of experimental spectra of the studied materials. The results of the fitting of χPt(k) of Pt L3-EXAFS performed using the suggested model show that the contribution of the term χPt-O,C(k) is neglectable, since the determined values of its amplitude factor are too small (less than 0.5 for the sample PtCu_RT and


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become even smaller for the rest of samples). Excluding this term from the fitting model almost does not change the parameters of Pt-metal terms. In the case of χCu(k), on the contrary, the term χCu-O,C(k) is rather significant. Furthermore, comparison of the results, obtained using phases and amplitudes of backscattering calculated for the atomic pairs Cu-O and Cu-C, allowed to conclude, that the main contribution of the backscattering on light atoms in χCu(k) correspond to Cu-O bonds in Cu(II) oxide. This conclusion is based on two factors: 1) the use of χCu-C(k) as a last term in (2) leads to physically unacceptable values of structural parameters, unlike χCu-O(k); 2) obtained value of Cu–O interatomic distance of ~1.93 Å corresponds to that in CuO. Further, to emphasize the conclusion drawn we replace the last term in (2) by χCu-O(k). Since the copper oxide as a separate phase is not observable in XRD, the presence of bonds Cu–O can be explained by too small size of the separate CuO nanoparticles and/or their amorphousness. According to made assumptions and restrains we can rewrite the expressions (1) and (2) in the following form: Pt = Pt Pt-Pt ⋅ Pt-Pt + Pt-Cu ⋅ Pt-Cu (3)

Cu = Cu ∙ Cu-Cu ⋅ Cu-Cu + Cu-Pt ⋅ Cu-Pt + 1 − ∙ 4 ⋅ Cu-O

(4)

Here, in (4) we also introduce the factor C, which enables to estimate the fraction of copper atoms in the bimetallic nanoparticles and in the oxide too. The factor of 4 in the last term of (4) correspond to the coordination number of CuO oxide. The values of structural parameters obtained by the fitting of Pt L3- and Cu KEXAFS spectra of samples PtCu_RT, PtCu_250, PtCu_280, PtCu_300, and PtCu_350 performed using the model (3), (4), are presented graphically on Figure 4. The estimation of the fraction of copper atoms in the oxide gives the slight decrease of its value from ~60 to ~50 %. The reported values of Debye-Waller parameters are higher than those typical for the inter-metallic bonds at a room-temperature 58, at which measurements were performed. This is due to the presence of structural distortions in the near-surface region of nanoparticles, which lead to an increase in the dispersion of interatomic distances, and is also typical of monometallic nanoparticles 59–61. At the same time, a decrease in the values of the DebyeWaller parameter for samples treated at temperatures of 300 and 350 °C indicates structural ordering, which may be due to the formation of an intermetallic alloy in the nanoparticle.


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0.000 PtCu_RT

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Figure 4. EXAFS-derived values of the structural parameters of local atomic structures of Pt (left side a-c) and Cu (right side d-f) atoms in PtCu/C electrocatalysts. From top to bottom: partial coordination numbers (NPt-Pt and NCu-Cu – blue circles, NPt-Cu and NCu-Pt – orange crosses); interatomic distances (RPt-Pt and RCu-Cu – blue circles, RPt-Cu and RCu-Pt – orange crosses); Debye-Waller parameters (σ2Pt-Pt and σ2Cu-Cu – blue circles, σ2Pt-Cu and σ2Cu-Pt – orange crosses). As one can see, the values of parameters change slightly from sample PtCu_RT to samples PtCu_250 and PtCu_280, then drastically change to sample PtCu_300 and again slightly change to sample PtCu_350. The presence of significant changes in structural parameters in the temperature region ~ 280–300 °C confirms the changes in lattice parameters which are observed in XRD patterns (Section 3.1). The increase in the mean partial coordination numbers NPt-Cu and NCu-Pt along with the changes in the values of interatomic distances RPt-Cu and RCu-Pt toward their mean value indicates the alloying of metals. Moreover, the increase in the total coordination number of platinum (NPt-Pt + NPt-Cu)


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indicates a decrease in the fraction of platinum atoms in the near surface region of nanoparticles. 3.4. Visualization of Atomic Structure of Bimetallic Nanoparticles PtCu and XRD Simulations EXAFS derived values of structural parameters presented on Figure 4 enable to make only indirect conclusions on the atomic structure of the bimetallic nanoparticles in the studied electrocatalysts. To determine the relationship between thermal treatment conditions – atomic structure – electrochemical performances of these materials, we used the method of atomic cluster simulations suggested in 62,63 in order to construct models of mean nanoparticle PtCu in materials PtCu/C. The comparison of |F(R)| of Pt L3- and Cu KEXAFS of the studied materials with the corresponding |F(R)| of Pt and Cu foils in the extended R-range (up to ~6.0 Å) enables to conclude, that the cluster models of PtCu nanoparticles structure can be generated using the fcc cluster, placing Cu and Pt atoms in its atomic positions. To be able to model nanoparticles with ordering of components like in intermetallic compounds, we imposed additional periodic conditions on the probability function for placing Pt or Cu atoms 62 along the directions of the lattice vectors, which are parametrized by the parameter of period l. Varying the value of l one can obtain the alloy structure from strong alternation of atomic planes (when the value of l coincide with the value of lattice parameter) to “checkerboard pattern” of atomic sub-clusters that stimulates aggregation of atoms of the same type inside the nanoparticle. The cluster models of mean PtCu nanoparticle generated by the suggested technique for each of the studied catalysts except of PtCu_280 are shown on Figure 5. As it was mentioned Section 3.1, the sample PtCu_280 seems to contain a mixture of bimetallic nanoparticles with different architectures resulting in a rather unstable results of structural refinement via the full profile powder XRD data. Similar conclusions could be made considering EXAFS data. F(R) of Cu K-edge EXAFS in PtCu_280 is significantly different from other samples. Despite the fitting gave adequate results for this material, the obtained values of structural parameters averaged over nanoparticles with drastically different structures cannot be used for constructing of atomic cluster model of “mean” nanoparticle, because in this case it will not be representative for entire ensemble of nanoparticles. It should be noticed, that the structural changes in the models of bimetallic nanoparticles do not reflect the changes occurring in entire samples due to the transformations of copper oxides that lead to different electrochemical performances for the samples PtCu_300 and PtCu_350 (Section 3.5).


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a

b

c

d

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Figure 5. Slices of the atomic cluster models, based on EXAFS-derived values of partial coordination numbers, which correspond to PtCu_RT (a), PtCu_250 (b), PtCu_300 (c), and PtCu_350 (d). Yellow circles – Cu atoms, grey circles – Pt atoms. The structures of atomic clusters obtained by the used approach were relaxed to produce reliable distortions of atomic structures induced by surface and differences of atomic species. The interatomic potential was accounted within effective medium theory EMT 64, which is one of the forms of embedded atom model (EAM), but with parameters calculated from first principles. The used set of parameters for platinum and copper was tested for bulk alloys, surfaces and nanoparticles 65–70. The structure relaxation was performed using modified velocity-Verlet molecular dynamics algorithm as it implemented in atomic simulation environment (ASE) 71,72. The obtained values of averaged interatomic distances, presented in Table 2, are in a reasonable agreement with the values of interatomic Pt-Pt, Cu-Cu, Pt-Cu distances obtained from EXAFS (Figure 4 b, e). Table 2. Average Interatomic Distances Obtained for the Cluster Models Corresponding to the Samples PtCu_RT and PtCu_350 after Structural Relaxation within EMT Potential. model

RPt-Pt, Å RPt-Cu, Å RCu-Cu, Å

PtCu_RT (core-shell) 2.73

2.67

2.65

PtCu_350 (alloy)

2.67

2.66

2.71

For the obtained clusters, the simulation of XRD patterns were performed on the atomic level using Debye formula 73:


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= $

sin# #

where q is a scattering vector (q = 4 · π · sin(θ) / λ), Rij is a distance between i-th and j-th atoms, and fi – atomic form factors which were analytically calculated 74 for each given q. The proportionality coefficient in this equation accounts for dynamics and instrumental effects and was estimated in the same way as in 73. The wavelength of simulation was set to λ = 1.54 Å, which is close to Cu Kα1 wavelength used in XRD experiment. Figure 6 presents the calculated XRD patterns obtained for considered models of core-shell and alloyed particles with account for structure relaxation. The simulated pattern of core-shell particle does not show the superstructural reflections, while they can be clearly noticed for alloyed particle (peaks are marked with asterisk (*) at Figure 6 b).

Figure 6. The comparison of simulated XRD patterns (solid red lines) of relaxed nanoparticles’ atomic models of size ~5 nm (containing 6699 atoms) of core-shell particle (a) and partially ordered alloyed particle (b) with experimental XRD patterns of corresponding samples PtCu_RT and PtCu_350 (dotted blue lines). 3.5. Results of the Electrochemical Studies An analysis of the results of electrochemical measurements take in account the processes and phenomena that caused by the standardization of electrodes (100 cycles of the


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potential sweep, Figure 7). In the process of standardization, an increase in the absolute values of currents during the first 20-25 cycles was observed for all samples, which is due to the cleaning of the surface of nanoparticles from the products of the previous interaction with the components of the medium and its development (reorganization) due to the selective dissolution of a part of copper atoms 75. The shape of CVs was stabilized for all studied electrodes approximately after 20-25 cycles. More or less pronounced anode peaks, associated with the dissolution of copper from a solid solution of Pt-Cu 49,76, were observed in the potential range 0.65-0.85 V (RHE) on the first CVs of PtCu/C materials (Figure 7). Anode peaks in the potential range 0.25 - 0.35 V (Figure 7c), which corresponds to dissolution of copper from its own phase 49, were observed for the sample PtCu_350 too. The content of copper in the catalysts upon completion of standardization was reduced significantly (Table 3). Thus, the electrochemical behavior was studied further for electrocatalysts, the composition, structure and, as a result, the properties of nanoparticles in which differed from those of the initial “as prepared” PtCu/C_RT-PtCu/C_350 materials 75. On the other hand, it is necessary to take into account that the features of the evolution of nanoparticles in the standardization process depend on their original architecture 75. The results presented in Sections 3.1 to 3.4 indicate that a number of differently directed processes and phenomena occur during the thermo-treatment of materials, which cause changes in the structure of the catalysts. It is logical to assume that these changes can influence the evolution of the composition and architecture of nanoparticles during the standardization of catalysts, thereby determining their electrochemical performance. An increase in temperature leads to an increase in the mobility of NPs, facilitates their movement along the surface of the carrier, and subsequent aggregation. It is obvious that the preservation of the core-shell architecture of a nanoparticle formed because of the aggregation of two nanoparticles is unlikely. At the same time, an increase in the mobility of platinum atoms in the surface layer of NP can lead to the healing of defects in the shell and to an increase in its protective capacity with respect to the atoms of the nucleus. The enhancement of the interdiffusion of copper and platinum atoms with temperature increasing, on the one hand, facilitates the transformation of core-shell NPs into solid solution particles, and on the other hand, causes ordering of the solid solution, i.e. its transformation into an intermetallic compound. As mentioned earlier, this phenomenon was registered by XRD and EXAFS methods for PtCu_350 material, and to a lesser extent for PtCu_300. Finally, the amorphized copper oxide II moieties present in the starting material can be reduced to metallic copper because of carbothermic reduction by carbon. The resulting copper atoms can form clusters of the intrinsic phase or be incorporated into existing nanoparticles, mainly into their surface layer. The multiple repetition of anodic and cathodic current during the standardization of catalysts not only leads to the cleaning of the nanoparticles surface, but also to the dissolution of phase inclusions of copper contacting with the electrolyte, as well as the copper atoms from the surface layers of the platinum-enriched solid solution. The value of the ECSA of catalysts after standardization is determined not so much by the growing size of the nanoparticles, caused by the thermo-treatment, as by the degree of development of the nanoparticle surface and the degree of platinum segregation in their surface layer.


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In our opinion, the influence of the above phenomena on the behavior of catalysts can be interpreted as follows. The original PtCu_RT sample contains a significant number of bimetallic NPs with a defective platinum shell. In the standardizing cycling process, selective copper dissolution and surface cleaning occur, accompanied by surface development to 55 m2/g(Pt) (Table 3). On the CVs of the PtCu_RT sample, there are pronounced anode peaks near 0.8 V due to the selective dissolution of copper from the solid solution (Figure 7a). In this case, the current intensity at the maximum decreases from cycle to cycle, as copper dissolves. During standardization, chemical dissolution of amorphous copper II oxide inclusions present on the surface of the carbon support and/or on the surface of the nanoparticles also occur. As a result, the amount of copper in the sample after standardization is reduced approximately 2 times compared to the initial one (Table 3). Heat treatment at 250 °C still does not lead to significant changes in the structure of nanoparticles, but a further increase in temperature activates the processes of carbothermic reduction of CuO to the metal and the diffusion of the formed copper atoms into the volume of nanoparticles. In combination with the reorganization of platinum atoms in the surface layer of nanoparticles, which increases the protective capacity of the shell, this leads to a decrease in the amount of copper dissolved during the standardization and causes an increase in the stability of the PtCu_280 and PtCu_300 materials (Table 3). The shape of CVs in the standardizing cycling process for PtCu_280 and PtCu_300 samples changes not the same as for PtCu_RT and PtCu_250: the anode current in the oxygen region of the CVs grows in the first 20-25 cycles, after which the CVs shape is stabilized (Figure 7b). This kind of change is connected, primarily with the development of a surface enriched with platinum atoms. Table 3. Some Characteristics of PtCu/C Electrocatalysts before and after Thermal Treatment at Different Temperature. sample Daver, nm ESA, Is, E1/2 composition PtСux 2 2 TEM m /g (Pt) А/m (Pt) at 1600 rpm after standardization (0.9 V) PtCu_RT 4.8 55±5 2.4±0.2 0.90±0.01 PtСu0.6±0.05 PtCu_250 PtCu_280 PtCu_300 PtCu_350

5.8 5.9 -

58±6 56±6 41±4 31±3

2.0±0.2 2.7±0.3 5.5±0.5 1.4±0.1

0.89±0.01 0.92±0.01 0.92±0.01 0.87±0.01

PtСu0.5±0.05 PtСu0.7±0.05 PtСu0.9±0.05 PtСu0.6±0.05

With a further increase in the processing temperature to 350 degrees, the main influence on structural changes is exerted by the interdiffusion of copper and platinum atoms, leading to a transformation of the core-shell nanoparticle architecture into a solid solution structure. That agrees well with EXAFS data and modeling of bimetallic clusters. Also, the structure of the solid solution is ordering (it transforms into an intermetallic compound, presumably Pt3Cu). These processes lead to an increase in the concentration of copper on the NPs surface, as a result of which the share of copper dissolved during standardization again increases significantly (Table 3). On CVs, peaks of anodic dissolution of copper from a solid solution based on platinum and even from the intrinsic phase are again observed (Figure 7c).


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Obviously, as the temperature increases, aggregation processes also contribute to the violation of the core-shell architecture of NPs and lead to a decrease in the ECSA. The obtained samples are arranged in a row with an increase in the ECSA values: PtCu_350 < PtCu_300 < PtCu_280 ~ PtCu_250 ~ PtCu_RT. The residual copper content in the catalysts after the completion of the standardization procedure increases in the row: PtCu_250 < PtCu_RT ~ PtCu_350 < PtCu_280 < PtCu_300 (Table 3).


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Figure 7. CV curves (100 cycles) of PtCu_RT (а), PtCu_300 (b) and PtCu_350 (c), electrolyte – Ar saturated 0.1 M HClO4, room temperature, sweep rate: 200 mV/s. Obtained results correlate with the published data, according to which thermotreatment of PtM/C catalysts can increase their catalytic activity due to formation of more ordered surfaces and additional metal co-alloying with bimetallic nanoparticles 25,77. A number of studies have demonstrated the optimum temperature (and duration) of treatment, the value of which depends on the composition and nature of bimetallic nanoparticles alloying component 77–79. For the investigated materials, an increase in catalytic activity with increase of treatment temperatures up to 300 °C (Figure 8) is observed. Then it sharply decreases due to a decrease in the ECSA caused by coalescence of nanoparticles, diffusion of platinum out of the surface in a volume of NPs at 350 °C and followed by dissolving the “weakly bound” copper (Table 3), the changing character of the surface reorganization during the selective dissolution of copper and reduction of its specific activity.

Figure 8. LSVs of PtCu/C electrocatalysts before and after thermos-treatment at different temperatures. Electrolyte – O2 saturated 0.1 M HClO4, room temperature, sweep rate: 20 mV/s, rotation speed: 1600 rpm. Insert: histogram of mass-activity dependence at 0.9 V on temperature of treatment. Comparison of the activity of the PtCu/C materials (Table 3) and commercial Pt/C catalysts with a close mass fraction of platinum showed a high quality of the catalysts


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obtained. In particular, the specific activity of the PtCu/C catalyst (225 A/g (Pt)) after heat treatment at 300 °C is significantly higher than the activity of the commercial analogue HiSPEC3000 (156 A/g (Pt)) 75 (see the sidebar in Figure 8). Conclusions Analysis of the composition, structure, and electrochemical behavior of the bimetallic PtCu nanoparticles deposited on carbon, obtained by chemical reduction of metals in suspension and subjected to subsequent thermo-treatment at different temperatures, allowed us to formulate the following conclusions. As a result of the synthesis based on the sequential reduction of Cu (2+) and Pt (IV) compounds by sodium borohydride in a carbon suspension, both bimetallic PtCu nanoparticles and nanoparticles of X-ray amorphous copper oxide II are formed on the carrier surface. About half the copper in the PtCu/C_RT sample is in the form of an oxide. The architecture of the "averaged" bimetallic nanoparticle, representative of the “as prepared” material, is a core-shell structure with a copper core and a platinum shell. On some NPs, the shell has defects, because of which it does not provide complete protection of the copper core from anodic dissolution. In the process of thermo-treatment of “as prepared” material, variously directed processes and phenomena occur, the result of which are changes in the structure of catalysts and the resulting peculiarities of their electrochemical behavior. According to the results of the EXAFS spectroscopy of materials, thermo-treatment at 250 °C leads to erosion of the core/shell boundary, and heating with the same duration at higher temperatures leads to a less or more pronounced destruction of this architecture (solid solution formation), nanoparticle aggregation and ordering of the alloy structure. Atomistic models of nanoparticle structures based on data on coordination numbers obtained from Cu K-edge and Pt L3-edge EXAFS spectra, and relaxed using EMT molecular-dynamic potential, allows to reproduce super-structural reflexes on experimental XRD patterns, simulated using Debye formula. Investigation of the electrochemical behavior of catalysts in combination with an analysis of the change in their composition after standardizing cycling shows that the thermo-treatment is accompanied by partial carbothermic reduction of CuO inclusions to copper atoms that are absorbed by bimetallic nanoparticles. At the same time, after treatment of the material in the 280-300 °C temperature range, the platinum atoms are still predominantly segregated in the surface layer, which determines the optimum character of nanoparticle surface development during electrochemical standardization of PtCu_280 and PtCu_300 samples, its enrichment with platinum and relatively high catalytic activity in ORR. For these samples, the minimum selective dissolution of copper is also characteristic. Increasing the treatment temperature to 350 °C leads to aggregation and an increase in the size of the PtCu particles, an increase in the surface concentration and selective anodic dissolution rate of copper, a decrease in the ECSA value, and a decrease in the mass activity of the catalyst in the ORR.


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Acknowledgements This work was supported by the Russian Science Foundation (grant no. 16-19-10115). Experimental EXAFS study was performed at the Synchrotron Radiation Facility BESSY II (Berlin, Germany). We are indebted to LLC “Systems for Microscopy and Analysis” (Skolkovo, Moscow) and Tabachkova N.Yu. (Materials Science and Metallurgy Joint Use Center, Moscow Institute of Steel and Alloys (National University of Science and Technology)) for the help in the electron microscopy studies. We thank SNBL for providing a beamtime. References (1) Sharaf, O. Z.; Orhan, M. F. An Overview of Fuel Cell Technology: Fundamentals and Applications. Renew. Sustain. Energy Rev. 2014, 32, 810–853. (2) Marković, N. M.; Schmidt, T. J.; Stamenković, V.; Ross, P. N. Oxygen Reduction Reaction on Pt and Pt Bimetallic Surfaces: A Selective Review. Fuel Cells 2001, 1 (2), 105–116. (3) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G.; Ross, P. N.; Markovic, N. M. Trends in Electrocatalysis on Extended and Nanoscale Pt-Bimetallic Alloy Surfaces. Nat. Mater. 2007, 6 (3), 241–247. (4) Wu, J.; Yang, H. Platinum-Based Oxygen Reduction Electrocatalysts. Acc. Chem. Res. 2013, 46 (8), 1848–1857. (5) Dekel, D. R. Review of Cell Performance in Anion Exchange Membrane Fuel Cells. J. Power Sources 2018, 375, 158–169. (6) Negro, E.; Delpeuch, A. B.; Vezzu’, K.; Nawn, G.; Bertasi, F.; Ansaldo, A.; Pellegrini, V.; Dembinska, B.; Zoladek, S.; Miecznikowski, K.; et al. Towards “PtFree” Anion-Exchange Membrane Fuel Cells: Fe-Sn Carbon Nitride-Graphene “Core-Shell” Electrocatalysts for the Oxygen Reduction Reaction. Chem. Mater., 2018, 30 (8), 2651–2659. (7) Vezzu’, K.; Delpeuch, A. B.; Negro, E.; Polizzi, S.; Nawn, G.; Bertasi, F.; Pagot, G.; Artyushkova, K.; Atanassov, P.; Di Noto, V. Fe-Carbon Nitride “Core-Shell” Electrocatalysts for the Oxygen Reduction Reaction. Electrochim. Acta, 2016, 222, 1778-1791. (8) Dai, L.; Xue, Y.; Qu, L.; Choi, H.-J.; Baek, J.-B. Metal-Free Catalysts for Oxygen Reduction Reaction. Chem. Rev. 2015, 115 (11), 4823–4892. (9) Qu, L.; Liu, Y.; Baek, J.-B.; Dai, L. Nitrogen-Doped Graphene as Efficient MetalFree Electrocatalyst for Oxygen Reduction in Fuel Cells. ACS Nano 2010, 4 (3), 1321–1326. (10) Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Du, A.; Jaroniec, M.; Qiao, S. Z. Hydrogen Evolution by a Metal-Free Electrocatalyst. Nat. Commun. 2014, 5, 3783. (11) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Marković, N. M. Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability. Science (80-. ). 2007, 315, 493–497. (12) Oezaslan, M.; Hasché, F.; Strasser, P. Pt-Based Core–Shell Catalyst Architectures for Oxygen Fuel Cell Electrodes. J. Phys. Chem. Lett. 2013, 4 (19), 3273–3291. (13) Gan, L.; Heggen, M.; Rudi, S.; Strasser, P. Core–Shell Compositional Fine Structures of Dealloyed Pt X Ni 1– X Nanoparticles and Their Impact on Oxygen Reduction Catalysis. Nano Lett. 2012, 12 (10), 5423–5430.


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TOC Graphic


27

111fcc

111fcc

*

*

333fcc

224fcc

Page 28 ofb35 200fcc

Intensity, a.u.

*

400fcc 133fcc 331fcc 420fcc

* *

311fcc 222fcc

220fcc

200fcc *

002carbon

Intensity, a.u.

1 2 3 4 5 6 7 8 9 0 10

a of Physical Chemistry The Journal

*

e d c b a


10

20

30 40 2, deg.

50

60

17

18

19

20 21 2, deg.

22

23

24

a

Page 29 of 35 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


b

c



254x190mm (300 x 300 DPI)


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Page 10 31 of 35

10 The Journal of Physical Chemistry

a

9

8

7

7

6

6

5

5

N

8

4

4

3

3

2

2

1

1

0

0 PtCu_RT

PtCu_250

PtCu_280 Sample

PtCu_300

2.76

PtCu_350

PtCu_RT

2.67

2.72

2.65 R (Å)

2.74

2.70

PtCu_250

PtCu_280 Sample

PtCu_300

2.69

b

2.61

2.66

2.59

2.64

2.57

2.62

PtCu_350

e

2.63

2.68

2.55 PtCu_RT

PtCu_250

PtCu_280 Sample

PtCu_300

PtCu_350

PtCu_RT

0.014

PtCu_250

PtCu_280 Sample

PtCu_300

PtCu_350

0.014

c

0.012 0.010

0.010

0.008

0.008

0.006

0.006

0.004

0.004

0.002

0.002

0.000

0.000 PtCu_RT

PtCu_250

PtCu_280 Sample

PtCu_300

f

0.012

σ2 (Å2)

σ2 (Å2)

R (Å)

N

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

d

9

ACS Paragon Plus Environment PtCu_RT PtCu_350

PtCu_250

PtCu_280 Sample

PtCu_300

PtCu_350


339x339mm (96 x 96 DPI)


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Scattering Intensity

PtCu_RT PtCu_RT Page The(a) 33 Journal of 35 of Physical Chemistry (111)

model

1 2 (200) 3 (220) 4 5 6 7 PtCu_350 8 (b) PtCu_350 model 9 (111) 10 11 12 (200) 13 (220) 14 * 15 * 16 ACS Paragon Plus Environment 17 15 20 25 30 18 2θ, degree

35


170x400mm (96 x 96 DPI)


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254x190mm (300 x 300 DPI)


Effect of Thermal Treatment on the Atomic Structure and

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