C Electrocatalysts: Carbon Support

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Porous Hollow PtNi/C Electrocatalysts: Carbon Support Considerations to Meet Performance and Stability Requirements. Tristan Asset, Nathalie Job, Yan Busby, Alexandre Crisci, Vincent Martin, Vaios Stergiopoulos, Celine Bonnaud, Alexey Serov, Plamen Atanassov, Raphael Chattot, Laetitia Dubau, and Frédéric Maillard ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03539 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 8, 2017

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Porous Hollow PtNi/C Electrocatalysts: Carbon Support Considerations to Meet Performance and Stability Requirements. Tristan Asset 1, 2, *, Nathalie Job 2, *, Yan Busby 3, Alexandre Crisci 4, Vincent Martin 1, Vaios Stergiopoulos 2, Céline Bonnaud 1, Alexey Serov 5, 6, Plamen Atanassov 6, Raphael Chattot 1, Laetitia Dubau 1, and Frédéric Maillard 1, *. 1. Univ. Grenoble Alpes, CNRS, Grenoble INP Ⱶ, Univ. Savoie Mont Blanc, LEPMI, 38000 Grenoble, France Ⱶ

Institute of Engineering Univ. Grenoble Alpes

2. University of Liège, Department of Chemical Engineering – Nanomaterials, Catalysis, Electrochemistry, B6a, Sart-Tilman, 4000 Liège, Belgium. 3. University of Namur ASBL, Department of Physics, Research Center in Physics of Matter and Radiation (PMR), LISE Laboratory, Rue de Bruxelles, 61, 5000 Namur, Belgium. 4. Univ. Grenoble Alpes, CNRS, Grenoble INP*, SIMAP, 38000 Grenoble, France * Institute of Engineering Univ. Grenoble Alpes 5. Pajarito Powder LLC, Albuquerque, NM 87109, USA 6. Center for Micro-Engineered Materials and Department of Chemical and Biological Engineering, University of New Mexico, Albuquerque, NM 87131, USA

*To whom correspondence should be addressed. E-mail: (A.T.) [email protected], [email protected] E-mail: (N.J.) [email protected] E-mail: (F.M.) [email protected]

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Graphical Abstract


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Abstract The influence of the texture, structure and chemistry of different carbon supports on the morphological properties, oxygen reduction reaction (ORR) activity and stability of porous hollow PtNi nanoparticles (NPs) was investigated. The carbon nanomaterials included carbon blacks, carbon nanotubes, graphene nanosheets and carbon xerogel, and featured different specific surface areas, degrees of graphitization and extent of surface functionalization. The external and inner diameters of the supported porous hollow PtNi/C NPs were found to decrease with the increase of the carbon mesopore surface area. Despite these differences, similar morphological properties and electrocatalytic activities for the ORR were reported. The stability of the synthesized electrocatalysts was assessed by simulating electrochemical potential variations occurring at a proton exchange membrane fuel cell (PEMFC) cathode during start-up/shutdown events. Identical location transmission electron microscopy (IL-TEM) and electrochemical methods revealed the occurrence of carbon-specific degradation mechanism: carbon corrosion into CO2 and particle detachment were noticed on carbon xerogels and graphene nanosheets while, on carbon blacks, surface oxidation prevailed (C → COsurf) and did not result in modified electrical resistance of the catalytic layers, rendering these carbon supports better suited to prepare highly active and stable ORR electrocatalyst.


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Introduction The development of renewable energy storage and conversion systems is pivotal to limit our dependence on fossil fuels and the pollution problems associated with their use. In this regard, Proton Exchange Membrane Fuel Cells (PEMFCs) have successfully passed initial demonstration stage and approach commercial viability 1,2. Pt nanoparticles (NPs) with a diameter 2 < d < 5 nm supported onto high surface area carbon are the reference electrocatalytic material both for the hydrogen oxidation reaction (HOR, anodic reaction) and the oxygen reduction reaction (ORR, cathodic reaction) in PEMFCs 3–9. However, the high Pt loading used to electrocatalyze the ORR along with the scarcity and the price of this element are hurdles to the widespread development of this technology. It was reported that a > 10-fold improvement of the ORR kinetics would open the way for PEMFC to enter the energy market 10–12. Strategies to achieve this goal include but are not limited to: (i) weakening the binding energy of ORR intermediates by strain (contraction of the lattice parameter) and ligand (modification of the Pt electronic structure) effects,

13–17

using alloys composed of Pt and a 3d-transition metal

18–22

or a rare-earth element 23–25, (ii) maximizing the exposure of (111) facets, this crystallographic orientation being the most active for the ORR (this holds also true for Pt-alloy surfaces 8,26,27), (iii) tuning the number and the intrinsic ORR activity of active sites at the atomic scale 28–35. Calle-Vallejo et al.

34,35

have recently shown that the generalized coordination number of the

optimal Pt catalytic site for the ORR is 8.3, i.e. slightly higher than the generalized coordination number of a (111) facet (7.5). Possible ways to form these favourable catalytic sites include increasing the density of highly coordinated atoms (such as missing atoms, concavities,

etc.) or controlling the structural disorder of the electrocatalyst Dubau et al.

3,37

6,36

. Using the last approach,

have recently reported 6-fold and 9-fold enhancement in mass and specific

activity for the ORR on hollow PtNi/C electrocatalysts compared to conventional solid Pt/C nanocrystallites of the same crystallite size. 4-fold and 3-fold enhancement in mass and specific activity over solid PtNi/C nanocrystallites with similar chemical composition, lattice contraction and crystallite size were observed, thereby providing experimental evidence that the disordered structure of these nanocrystallites is beneficial to the ORR kinetics. Interestingly, despite the fact that hollow porous PtNi/C electrocatalysts suffered the same Ni dissolution problems than solid PtNi/C NPs during simulated PEMFC operation, the beneficial effect of structural defects was maintained over time, thereby opening promising perspectives for the development of more sustainable ORR electrocatalysis 38,39. 4 ACS Paragon Plus Environment

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Now, many reports have shown that Pt or Pt-alloy NPs agglomerate or detach from the high surface area carbon support during PEMFC operation, leading to a decrease of the electrochemically active surface area and of the electrical performance of the device 40–43. This phenomenon is usually associated with the electrochemical corrosion of the carbon support (Carbon Oxidation Reaction, COR), especially under idle conditions or during abnormal events such as start-up/shutdown of the PEMFC. The COR kinetics depends on the degree of graphitisation and functionalization (i.e. the density of oxygen-containing surface groups) of the carbon support

44,45

. The COR kinetics is catalysed by the presence of Pt NPs onto the sup-

port, as the COsurf formed by the carbon corrosion might adsorb on the Pt surface (COsurf → COads) and be oxidized at lower potential than on the same Pt-free carbon support

46–48

. Obvi-

ously, high Pt mass fractions used at PEMFC cathodes accelerate the COR kinetics but this effect may be attenuated by playing with degree of graphitization of the carbon support 44,49 or partially encapsulating the metal nanoparticles 50,51. It is currently not clear whereas the COR kinetics is modified when Pt-alloys are used (instead of Pt/C). Based on identical location transmission electron microscopy measurements, Nikkuni et al.

52

suggested that the carbon

support is less subjected to corrosion when Pt3Co/C NPs are used as ORR electrocatalysts in the 0.1 - 0.9 V vs. RHE potential range: this was accounted for by considering that Co atoms may play the role of a sacrificial anode thereby diminishing the corrosion of the carbon support. Note that the chemical nature of the metal NPs is supposed to play a minor role at higher electrochemical potential (e.g. 1.0 – 1.5 V vs. RHE) where carbon supports are very strongly oxidized.

The structural and textural properties of the carbon nanomaterials (specific surface area, pore size and shape, etc.) are also pivotal for high PEMFC electrical performance as the reactants are fed under gaseous form and thus must diffuse through the pores of the carbon support to reach the catalytic sites. Nevertheless, the most important property of a carbon support remains its ability to maximize the distribution and avoid the agglomeration of the Pt-alloy NPs. Several types of high surface area carbon supports (see Figure 1) have been used as supports during the recent years, including: (i) graphene nanosheets (Figure 1A), composed of one or several stacked graphene layers (i.e. sp2 bonded carbon atoms 53,54), (ii) carbon blacks (Figure

1B), i.e. several graphene clusters combined as near-spherical particles (5 – 100 nm), connected by van der Waals bonds to form aggregates of diverse morphology and sizes 55–58, (iii) carbon nanotubes 59,60 (Figure 1C), i.e. graphene nanosheets rolled in a tubular form (single5 ACS Paragon Plus Environment

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walled carbon nanotubes) or several graphene nanosheets rolled and connected by van der Waals bonds (multi-walled carbon nanotubes) 61,62 and (iv) carbon xerogels (Figure 1D), i.e. amorphous carbon materials produced by drying and pyrolysis of phenol-formaldehyde polymers 63, and composed of covalently bonded near-spherical nodules with highly tuneable nodule and pore size 64–67.

These carbon supports can be classified according to their physical, chemical and morphological characteristics such as (i) their Brunauer-Emmett-Teller (BET) surface area (in m2 g-1), (ii) the surface area (in m² g-1) of their mesopores (2 nm < dpore < 50 nm

68

) and their mi-

68

cropores (dpore < 2 nm ), (iii) the average crystallite size in the graphene plane (La, in nm), (iv) or in the direction perpendicular to the graphene planes (Lc, in nm), and (v) the average interlayer spacing (d002, in nm). Items (iii - iv) are used to characterize the long-range order of partially graphitized carbons

69

: low La and Lc values are typical of amorphous or quasi-

amorphous carbon materials and hence feature poor resistance to COR. In contrast, high La and Lc values indicate large carbon crystallites, better organized and more robust carbon materials 70.

Figure 1. Scanning electron microscopy (SEM) micrographs of different carbon structures used in this study. (A) Graphene nanosheets (GNS), (B) carbon black (Vulcan XC72), (C) carbon nanotubes (CNT) and (D) carbon xerogel (CX).

In this study, we focus our efforts on the successful development of hollow PtNi/C nanocatalysts for PEMFC application, and selected seven carbon nanomaterials with different physicochemical properties: (i) carbon blacks, i.e. Vulcan XC72 (XC72 – Cabot), Ketjenblack 600 JD (KJB – Akzo Nobel) and YS (YS – Société du Noir d’Acétylène de l’Aubette), a graphitized carbon black, (ii) multi-walled carbon nanotubes (CNT) synthesized by the sacrificial support 6 ACS Paragon Plus Environment

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method following the procedure described by Andersen et al. 62, (iii) a carbon xerogel (CX), synthesized according to the procedure described by Job et al. 65 with a resorcinol / formaldehyde (R/F) molar ratio of 0.5, a R/Na2CO3 molar ratio of 1500 and a dilution ratio (water / reactants molar ratio) of 5.7, and (iv) 3D graphene nanosheets (GNS) synthesized by the sacrificial support method following the procedure described by Kabir et al. 71,72 and Serov et al. 73

. To investigate the effect of the density of oxygen-containing groups on the carbon surface,

the GNS surface was functionalized by an acid treatment for t = 8 h in 8 M HNO3. This carbon nanomaterial will be referred to as GNS – AL in what follows. The resulting changes in morphology, ORR activity and stability of the synthesized porous hollow PtNi/C electrocatalysts were then determined and benchmarked to those of hollow PtNi/C NPs supported on Vulcan XC72.


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Results & Discussion Physico-chemical properties of the carbon supports Figure 2 and Table S1 display the BET specific surface areas (SBET) of the different carbon supports used in this study. The SBET values ranged from 78 ± 2 m2 g-1 for the CNT to 1454 ± 5 m2 g-1 for the KJB. Such difference was related to different micropore (pores featuring a diameter dpores < 2 nm, according to the International Union of Pure and Applied Chemistry 68

) surface areas. Indeed, whereas CNT and KJB supports are not microporous, micropores

account for 20 to 80 % of the total SBET values for YS, GNS, GNS – AL, XC72 and CX. The SBET values excluding micropores (Smesopores), determined by the t-plot method, are reported in Figure 2 and Table S1. These values are of interest for the synthesis of porous hollow PtNi/C NPs featuring a large external diameter (dext ~ 10 – 15 nm) 3 because (i) these cannot be fully formed in pores with dpores < 2 nm and (ii) the precursors salts (Pt(NH3)42+ and Ni2+) hardly diffuse throughout the micropores within the time of the reaction (the Ni-cores are nucleated 2 min after the synthesis start 74).

Figure 2. Specific surface area values of the different carbon supports used in this study, calculated by the Brunauer, Emmett and Teller equation (SBET, solid circles), and specific surface area of the mesopores calculated by the t-plot method 75,76 ( Smesopores, open circles).

The nature and the concentration of oxygen-containing surface groups (carboxylic, carbonyl, alcoholic etc.) are also keys for the facile and successful synthesis of porous hollow PtNi/C NPs, as they act as nucleation sites for the metal salt precursors as well as anchoring sites for the metal NPs 77–79. These chemical properties also impact the resistance of the carbon materials to electrochemical corrosion (increased carbon coverage by oxygen leads to a decreased resistance to electrochemical corrosion 45). To gain insights into the chemistry of the different carbon supports, surface analysis was performed by X-ray photoelectron spectroscopy (XPS). 8 ACS Paragon Plus Environment

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The XPS measurements allowed (i) the identification of the nature of the chemical groups present on each carbon support and (ii) the determination of their elemental composition (see Figure S2 to Figure S4). The C, O, and N contents are reported in Figure S4 and Table 1. The oxygen content on the carbon supports was determined both from the O1s peak and from the peak-fitting of the C1s high-resolution spectrum. The latter also allowed the discrimination between C-O, C-C=O and O-C=O chemical components. The nitrogen content was determined from the N1s peak. Table 1 shows that CNTs displayed the lowest oxygen content (23 at. %) while the highest oxygen content was found in CX (42 at. %) and XC72 (38.5 at. %) supports. Preliminary treatments of the carbon nanomaterials strongly influenced the oxygen content, i.e. acid treatment in 8 M HNO3 during 8 h resulted in a 20 % increase in carboxylic acid groups (O–C=O), in agreement with literature 77,78,80. Table 1. Surface composition (at. %) of the different carbon substrates obtained by XPS analysis.

C-C-H

%C C-O, CC-C=O N

%O

%N

O-C=O

Other*

XC72

55

21.5

8

9

6.5

/

/

CNT

74

7

11.5

4.5

3

/

/

YS

66

14.5

12.5

4

3

/

/

GNS

64.5

11.5

14.5

3.5

3

2.5

0.5

GNS-AL

59

9.5

15.5

5

6.5

4

0.5

CX

49

24

9

9

9

/

/

KJB

65.5

14.5

12

6

2

/

/

* Surface contaminations

Insights into the structure of the different carbon nanomaterials, an important parameter to minimize the COR kinetics, were obtained by determining the average crystallite size in the graphene plane (La, in nm), perpendicular to the graphene planes (Lc), and the average interlayer distance (d002) (see Table S2 and Figure 3). The d002 values were determined from the analysis of XRD patterns using the position of the graphite peak at 26 < 2θ < 28° and the Scherrer and the Bragg laws. The La values were determined from the analysis of Raman spectra using the Knight and White formula 81 (Equation 1):


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= 4.4 ×

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. 1

where IG and ID1 stand for the integrated intensity of the G and the D1 bands (appearing at ca. 1585 and 1350 cm−1, respectively). Note that Equation 1 was established for λ = 514 nm (the position

82,83

and the intensity

84

of the D1-band depend on the laser wavelength). Further-

more, it is only valid for non-amorphous carbons ( ID1 / IG is proportional to La2 for amorphous carbons but no quantitative description of this relationship is provided in the literature 69

).

The position of the convoluted (G + D2) band (see Figure 3D and Table S2) was found to shift from v = 1597 ± 1 cm-1 for the GNS to v = 1584 ± 1 cm-1 for the CNT, respectively. The carbon supports with the highest degree of organization in the crystallite plane featured the smallest (G + D2) band wavenumber (i.e. the closest from the position of the G – band in an ideal graphitic lattice, see Figure 3B) and (G + D2) band was blue-shifted on the carbon supports with the lowest degree of organization. A graphite-like structure was observed for CNTs (ca. La = 7.0 nm and Lc = 6.3 nm), while a lower degree of organization in the graphene plane and perpendicular to the graphene planes was observed on carbon blacks (see Figure 3C and Table S2). Note that the position of the global (G + D2) band more accurately reflected the in-plane graphitization than the La value, which was determined after Raman bands fitting. As an example, one may note that, despite the YS support is more graphitic than the raw XC72, a higher La value was observed on the latter (Figure 3C). High Lc values (ca. 12.6 nm) were observed on GNS and GNS-AL, indicating that they are composed of several stacked graphene layers. However, the low La values (ca. 3.8 nm) determined on GNS and GNS-AL suggested that they are structurally disordered (i.e. contained a high density of defects) in the plane, most likely as a result of the sacrificial support method used for their synthesis (see Serov et al. 73).


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Figure 3. (A) XRD patterns measured on the porous hollow PtNi/C NPs synthesized in this work, (B) Raman patterns of the various carbon supports, (C) average carbon crystallite size in the plane (La) and perpendicular to the graphene planes (Lc) for the different supports used in this work and (D) position of the (G + D2)-band for the different non-amorphous carbons (because of its amorphous structure, no value is provided for the carbon xerogel (CX)).


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Morphology and electrocatalytic activity of porous hollow PtNi/C catalysts Porous hollow PtNi/C electrocatalysts were successfully synthesized on the different carbon supports, as shown by Figure 4 and Figure S5. Mild or no correlation was observed between the oxygen content on the carbon supports and the final diameter of the PtNi NPs. For example, despite their similar oxygen content (38.5 and 42 at. % for CX and XC 72, respectively, see Table 1), different diameters were observed for the NPs synthesized on XC72 and on CX (PtNi/CX, dext = 23.1 ± 7.5 nm; PtNi/XC72, dext = 12.5 ± 3.5 nm, see Table S3). This basically suggests that the relative density of O-containing surface groups present on the different carbon supports does not directly correlate with the density and the size of the porous hollow PtNi NPs. In contrast, the external and inner diameters of the hollow PtNi/C NPs were found to depend on the structural characteristics of the carbon supports (Figure 4B and Table S1); namely, the specific surface area of the mesopores (Smesopores) best correlated with the external and inner diameters of the NPs (see Figure 4B). This can be easily rationalized owing to the absolute number of anchoring sites (i.e. the total number of oxygen-containing surface groups, obtained from the product of the relative number of oxygen-containing surface groups by the specific surface of the carbon support, micropores excluded). Indeed, KJB featured a slightly lower oxygen coverage compared to the XC72 (oxygen groups represent 32.5 at. % and 38.5 at. % of the integrated intensity of the C1s peak in KJB and XC72, respectively see Table 1 and Figure S4); however, the specific surface of KJB is about 6 times larger than that of Vulcan XC72 (1454 ± 5 vs. 239 ± 2 m2 g-1, as reported in Figure 2 and Table S1). Therefore, the absolute number of available anchoring sites in KJB should roughly be 5 times higher than for XC72, resulting in an increased number of nucleation sites 77,78 and in the decrease of the external and inner diameters for the porous hollow PtNi/C NPs.


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Figure 4. (A) Transmission Electron Microscopy (TEM) micrographs of the porous hollow PtNi/C NPs synthesized on CNT, GNS, XC72 and KJB. (B) Variation of the external and inner diameters of the porous hollow PtNi/C NPs as a function of the mesoporous specific surface area of the different carbons supports.

To sum up: (i) both the inner and the outer diameters of the porous hollow PtNi NPs were found to decrease with an increase of the specific surface area (micropores excluded, i.e. Smesopores)

of the carbon support materials, owing to an increased absolute content in oxygen at the

carbon surface; (ii) under the chosen synthesis conditions (ratio between the Pt and Ni precursors equal to 1:3, synthesis at room temperature), the smallest dext (~10 nm) value was observed for the NPs synthesized on the KJB support. This carbon support featured a sensibly higher specific surface area (micropores excluded) than the other carbon supports and, thus, a larger absolute number of available oxygen groups; (iii) for low specific surface area carbon supports (micropores excluded, i.e. YS and CNT), the external diameter of the hollow PtNi NPs stabilized around 30 nm and the inner diameter around 20 nm (see Figure 4 and Table S3). On CNT, larger NPs of ca. 200 nm tend to form due to agglomeration (see Figure S6); these particles were excluded from the particle size distribution presented in Figure S5 and 13 ACS Paragon Plus Environment

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Figure 4 as their fraction within the material remains unknown. Their exclusion explains why the size of the porous hollow NPs supported on carbon was lower than expected according to the trend observed on Figure 4B.

The specific surface area of the carbon supports was also found to control the final Ni content of the NPs (see Table S3): the NPs synthesized on high specific surface area carbon supports (KJB and XC72) displayed the highest Ni content (ca. 27.9 at. % and 23.1 at. %, respectively vs. 16.5 at. % for the lowest loading, i.e. PtNi/CX). This results from the higher number of Ni nuclei, which acted as sacrificial templates for the deposition of Pt2+ ions via galvanic replacement during the synthesis of porous hollow PtNi/C NPs (a detailed mechanism is provided in Reference 74). Interestingly, porous hollow NPs synthesized on XC72 and KJB featured similar lattice parameter values (aPt-Pt = 0.384 ± 0.001 nm vs. aPt-Pt = 0.385 ± 0.001 nm), hence suggesting that a certain fraction of the Ni atoms present in the PtNi/KJB electrocatalyst were not fully alloyed to Pt. This may result from heterogeneous distribution of Ni atoms within the PtNi lattice and/or reduced Ni atoms trapped within the carbon porosity. The mean PtNi lattice parameter varied from aPt-Pt = 0.384 ± 0.001 nm (XC72) to aPt-Pt = 0.388 ± 0.001 nm (i.e. PtNi/CNT, PtNi/YS, PtNi/GNS-AL and PtNi/CX). The mean PtNi crystallite size calculated from the XRD patterns ranged from dXRD = 2.2 ± 0.1 nm (PtNi/KJB) to dXRD = 3.2 ± 0.1 nm (PtNi/CNT). The fact that dext >> dXRD was in line with our former conclusions and suggested that PtNi nanocrystallites surrounding a central cavity were formed 3.

A pronounced COads stripping peak at ~ 0.78 V vs. RHE was observed on the electrocatalysts displaying the smallest crystallites (PtNi/XC72 and PtNi/KJB, see Figure 5): this is large and small hollow PtNi/C crystallites feature different density of structural defects (see Ref. 85 for a detailed discussion). According to recent findings of Chattot et al. 6 the highest ORR activity should be observed for the electrocatalyst showing the lowest first moment of the potential weight of the COads stripping (µ1CO, mV vs. RHE 6).


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Figure 5. COads stripping voltammograms measured in Ar-saturated 0.1 M HClO4 on porous hollow PtNi/C NPs synthesized on different carbon supports, namely: Vulcan XC72 (XC72), carbon nanotubes (CNT), YS (YS), graphene nanosheets (GNS), acid leached graphene nanosheets (GNS – AL), carbon xerogels (CX) and Ketjenblack (KJB) – other conditions: v = 0.020 V s-1, T = 298 ± 1 K.

The specific activity (i.e. the activity per surface unit of Pt, SA) and the mass activity (i.e. the activity per mass unit of Pt, MA) for the ORR of the different electrocatalysts at E = 0.90 V and 0.95 V vs. RHE are shown in Figure 6 (see Table S4 for the electrochemical properties of a commercial Pt/C electrocatalyst). The ORR specific activities at E = 0.95 V vs. RHE (Figure 6A) ranged from 136 ± 18 µA cm-2Pt for PtNi/CX to 174 ± 15 µA cm-2Pt for PtNi/CNT, indicating a mild influence of the nature of the carbon support. In agreement with former findings of Chattot et al. 6, the electrocatalysts with the lowest µ1CO values (PtNi/CNT, PtNi/YS, PtNi/GNS–AL and PtNi/GNS) displayed the best performance for the ORR (see Table S3). The mass activities for the ORR at 0.95 V vs. RHE ranged from 40 ± 2 A g-1Pt for PtNi/CG to 68 ± 7 A g-1Pr PtNi/GNS-AL (see Figure 6B), in agreement with the different Pt specific surface areas (SPt,CO) obtained onto different carbon supports. Indeed, the SPt,CO values ranged from ~ 30 m2 g-1Pt for PtNi/CNT, PtNi/YS and PtNi/CX, 35 ≤ SPt,CO ≤ 41 m2 g-1Pt for PtNi/GNS and PtNi/GNS-AL and more than 42 m2 g-1Pt for PtNi/XC72 and PtNi/KJB (Figure 6C). Those differences were ascribed to the variation of the external and inner diameters of the NPs. By using the calculations described by Montejano-Carrizales et al. 86,87 and Dubau et al. 3, we were able to obtain confirmation (i) that the specific surface of the PtNi/C NPs decreased with an increase of their external diameter and (ii) that the porous hollow PtNi/C NPs were porous since, for all supports, the experimentally measured SPt,CO values were higher than the theoretical specific surface determined by considering that only the external surface was electrochemically active (see Figure S7). 15 ACS Paragon Plus Environment

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Figure 6. Specific activity (SA) and mass activity (MA) in O2-saturated 0.1 M HClO4 measured at (A) E = 0.95 V vs. RHE and (B) E = 0.9 V vs. RHE, corrected from oxygen transport resistance in solution and ohmic drop, for the porous hollow PtNi/C NPs synthesized on different carbon supports – other conditions: T = 298 ± 1 K, ω = 1600 rpm, v = 0.005 V s-1. (C) Pt specific surface area values determined from the COads stripping measurements.


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Stability of the carbon supports A procedure simulating the potential variations at a PEMFC cathode during start-up/shutdown events (i.e. square potential ramp between E = 1.0 V and E = 1.5 V vs. RHE with t = 3 s at each potential and T = 353 K see Figure S1) 44 was used to assess the stability of the different electrocatalysts. Fresh and 500-cycle aged electrocatalysts were characterized by electrochemical measurements and identical location transmission electron microscopy (IL-TEM), see Figure 7 to Figure 10 and Figure S8 to Figure S12. To assess the physico-chemical changes occurring during the accelerated stress test (AST), the variations of the electrical charge under the quinone/hydroquinone (Q/HQ) peak in the potential region 0.4 < E < 0.8 V vs. RHE (∆QHQ, see Figure S8A and Figure 7A) and the SPt,CO (see Figure S8B and Figure 7B) were used.

Figure 7. Variation of (A) the electrical charge under the Q/HQ peak in the potential region 0.4 < E < 0.8 V vs. RHE (∆QHQ) and (B) the Pt specific surface (SPt,CO) of the synthesized electrocatalysts during an accelerated stress test composed of 500 potential cycles between 1.0 V vs. RHE (t = 3 s) and 1.5 V vs. RHE (t = 3 s) with square ramp and T = 353 K.

The base voltammograms (Figure S9) featured an increase of the double layer capacitance (Cdl) and of the electrical charge under the Q/HQ peak

55

(see Figure 7) for the electrocata-

lysts supported on CNT and carbon blacks (i.e. XC72, YS and KJB). This suggested larger carbon support specific surface area and increased oxygen content after the AST

44,88

. On

PtNi/GNS, PtNi/GNS-AL and PtNi/CX, the COR was a very important degradation mechanism as illustrated by the decrease of (i) the double layer current (i.e. complete oxidation of 17 ACS Paragon Plus Environment

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the carbon into CO2 88), (ii) of the electrical charge in the Hupd region and (iii) of the Pt specific surface (see the decrease of the SPt,CO values, see Figure 9E), which are relevant markers of the detachment of the PtNi NPs. These results may be rationalized by (i) the amorphous structure of the CX support, which allows the COR initiating at a potential value as low as 0.6 V vs. RHE 44, (ii) the more disordered in plane structure of GNS and GNS-AL (G-band position = 1597 ± 1 cm-1 and La = 3.8 nm, see Figure 3 and Table S2), and (iii) the highly oxidized surface of the CX, GNS-AL and GNS supports 45. Since GNS and GNS-AL possess the largest crystallite size perpendicular to the graphene planes (Lc = 12.6 nm), the results also indicate that the in plane graphitization, and not the out of the plane crystallite size values, more closely control the stability of the various carbon supports under PEMFC operating conditions. A moderate detachment of NPs was observed on PtNi/CNT (see Figure 7B and Figure 8). Since the PtNi/CNT sample does not show carbon corrosion, i.e. ∆QHQ = 0.3 mC cm-2geo (see Figure 7A), the detachment of the PtNi NPs can be ascribed to (i) a weaker bonding of NPs due to the low surface functionalization (23% of oxygen groups, see Table 1) or (ii) the detachment of the Pt agglomerates observed in Figure S6. A decrease of the density of structural defects of the NPs (i.e. a higher charge under the peak at E = 0.78 V vs. RHE) was observed on the PtNi/KJB and PtNi/XC72 electrocatalysts, i.e. the electrocatalysts with the smallest Pt crystallite sizes 39.

Carbon corrosion and detachment of PtNi NPs were observed on the IL-TEM micrographs taken on Pt/GNS, PtNi/GNS-AL and, to a lower extent, on the PtNi/CX, the PtNi/CNT and the PtNi/KJB catalysts (Figure 8), therefore confirming the information derived from electrochemical measurements (Figure 7). However, less severe ageing was noticed by IL-TEM (i.e. fewer particle detachment events, see Figure 8) than what might have been inferred on the basis of the results of the ASTs. Several explanations may account for this observation: (i) the absence of intermediate electrochemical characterisations during the ageing (see Castanheira et al. 43 and Asset et al. 39) and (ii) the fact that IL-TEM provides only very local information, by opposition to electrochemical measurements. This led us to the conclusion that the ILTEM data must be considered in a qualitative way, i.e. to observe the trends in degradation (e.g. carbon corrosion for the GNS and GNS-AL support) but not to quantitatively assess the extent of degradation. Moreover, IL-TEM measurements were performed at T = 353 K between 1.0 and 1.5 V vs. RHE, i.e. under harsh conditions for gold TEM grids; as a result, gold re-deposition was sometimes observed, especially on PtNi/CG (Figure 8), in agreement with former findings of Schlögl et al. 89 18 ACS Paragon Plus Environment

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Figure 8. IL-TEM images recorded on the porous hollow PtNi/C NPs synthesized on different carbon supports [Vulcan XC72 (XC72), carbon nanotubes (CNT), YS (YS), graphene nanosheets (GNS), acid leached graphene nanosheets (GNS – AL), carbon xerogel (CX) and Ketjenblack (KJB)] before and after an accelerated stress test simulating the potential variations occurring at a PEMFC cathode during start-up/shutdown events (500 potential cycles between 1.0 V vs. RHE (t = 3 s) and 1.5 V vs. RHE (t = 3 s) with square ramp and T = 353 K).


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Figure 9. (A) Specific and (B) mass activity for the ORR in O2-saturated 0.1 M HClO4 measured at E = 0.95 V vs. RHE. (C) Specific and (D) mass activity in O2-saturated 0.1 M HClO4 measured at E = 0.90 V vs. RHE – after correction of O2 diffusion in solution and ohmic drop, T = 298 ± 1 K, ω = 1600 rpm, v = 0.005 V s-1. (E) Pt specific surface area determined from the COads stripping experiments for the porous hollow PtNi/C NPs synthesized on different carbon supports before and after an accelerated stress test simulating potential variations at a PEMFC cathode during start-up/shutdown events (500 potential cycles between 1.0 V vs. RHE (t = 3 s) and 1.5 V vs. RHE (t = 3 s) with square ramp and T = 353 K).


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The losses in Pt specific surface area (up to ca. 65 % for the PtNi/GNS) induced by the particle detachment and the corrosion of the carbon supports cannot solely explain the diminution of the ORR specific activity at E = 0.90 and 0.95 V vs. RHE. After ageing, the specific activity for the ORR at E = 0.95 V vs. RHE (SA0.95, Figure 9A) decreased by 62% and 19% for the PtNi/GNS and the PtNi/CNT electrocatalysts, respectively.

Figure S12 displays the ORR linear sweep voltammograms (LSV) after correction for the ohmic drop and the diffusion of oxygen in solution (the ORR LSV without correction of the diffusion of the oxygen in solution are provided in Figure S11). The potential difference (∆E) between the ORR kinetic current (i.e. the current corrected for the ohmic losses and the oxygen diffusion in solution, jk) of the fresh and of the aged electrocatalyst is presented in Figure S13 (for – 0.1 < jk < – 1 mA cm-2Pt). In this potential range, ∆E evolves linearly with jk, suggesting that the electrocatalytic losses partially result from ohmic losses due to the increase of the system resistance upon ageing. The electrochemical system (i.e. the glassy carbon (GC) electrode covered by the catalytic film immersed in the electrolyte) corresponds to a complex equivalent circuit that forbids the simultaneous determination of the resistance of the electrolyte, the resistance of the carbon support and the resistance of the glassy carbon 90–94, both in nature and value. The electrolyte properties were identical before and after ageing (the cell was washed with MilliQ water and fresh electrolyte was used for each characterization and the working electrode, Luggin capillary and counter-electrode were placed at identical positions before and after the AST; therefore the measured resistance was ca. 20 Ω) and the resistance of the GC disk was negligible, owing to its high electronic conductivity 94. Thus, the increase of the system resistance (related to the slope ∆Rk, the latter being an value determined according to Figure S8D) must be ascribed to (i) the decreased electrical conductivity of the catalytic layer following the electrochemical amorphization of the graphitic regions, especially in the interconnection regions between the elementary carbon particles 94, and (ii) to the increase of the Q/HQ surface content

57

(Figure 7A). The highest ∆Rk values (i.e. the

more degraded carbon supports in terms of electronic conductivity) were obtained for the GNS, GNS-AL and the CX supports, in agreement with the results of Figure 10A. The XC72, the CNT and the YS supports were more robust and no change of the ∆Rk value was observed for the KJB support during ageing (see Figure 10A). In an effort to confirm our findings, we also used the method recently introduced by Gribov et al.

94

to estimate the effective re-

sistance (Reff) of a carbon-supported electrocatalyst based on the difference in potential of the Q/HQ peak before/after ageing (see Figure S8C), according to Equation 2. 21 ACS Paragon Plus Environment

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=

, , 0,028 , !,

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. 2

where Ea,QHQ and Ec,QHQ are the potential of the anodic and the cathodic peak of the Q/HQ couple, and ia,QHQ and ic,QHQ are their respective intensity (Figure S8C), respectively. Reff corresponds to the connection in series of the resistance of the GC disk, of the electrolyte and of the carbon support. As such, a high Reff (see Table S5 and Figure 10B) indicates an increase of the carbon support resistance, thereby nicely confirming the results obtained using Rk (i.e. highest Reff for the GNS and GNS – AL, then CX and XC72). The initial Reff values were not calculated, as the Q/HQ peaks were not visible on the base voltammogram of the non-aged electrocatalysts (Figure S9).

Figure 10. (A) Slopes (∆Rk) determined from the fits of the curves presented in Figure S13 by a linear equation for the different carbon supports and (B) effective resistances (Reff) determined from cyclic voltammetry measurements using Equation 2. The cathodic Q/HQ peak was not observable after ageing on PtNi/YS and PtNi/CNT, resulting in no determinable Reff values for these two samples.

After the AST, 74 % and 36 % of the initial ORR specific activity measured at 0.90 V vs. RHE were lost for PtNi/GNS and PtNi/YS, respectively (Figure 9C). In agreement with the above discussion, these higher losses (compared to what was measured at 0.95 V vs. RHE) must be ascribed to: (i) the increased contribution of the ohmic drop to jk and (ii) the increased transport resistance of oxygen inside the catalytic layer. For each catalyst, the mass activity 22 ACS Paragon Plus Environment

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losses for the ORR (Figure 9B and Figure 9D) were more pronounced than the specific activity losses. This is due to hollow NPs collapsing and/or detaching during ageing (see Figure 9E) and the associated losses in Pt specific surface area, which were particularly marked for PtNi/CX ∼ PtNi/GNS > PtNi/GNS-AL (ca. 64%, 65% and 56%, respectively). The losses in mass activity for the ORR reached up to 90 % for PtNi/GNS. YS and KJB appeared to be the most promising carbon supports for the porous hollow PtNi/C NPs (slight increase of ∆Rk, minor decrease in ORR activity, etc.). Furthermore, the NPs supported on KJB had a dext = 10.6 ± 3.4 nm, resulting in a SPt,CO = 45 ± 5 m2 gPt-1 and, therefore, an almost optimal mass activity compared to PtNi/YS (MA0.95 = 56 ± 12 A gPt-1 vs. 40 ± 2 A gPt-1).


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Conclusion Porous hollow PtNi/C nanoparticles were successfully synthesized onto different carbon supports, i.e. three different carbon blacks, carbon nanotubes, graphene nanosheets and a carbon xerogel. The carbon supports were extensively characterized by nitrogen adsorptiondesorption, Raman spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The selected carbon materials featured various specific surface areas, degrees of graphitization and surface oxygen content. The external and inner diameters of the NPs decreased with an increase of the specific surface area of the carbon support, micropores excluded. The variation of the NPs size stems from an increased number of nucleation sites available for the Nibased sacrificial cores. Overall, the results showed that carbon supports provide a leverage to control the morphology, the size and the Ni content of porous hollow PtNi/C NPs. Besides, the structural properties of the porous hollow PtNi NPs supported on different carbon substrates were similar (mean lattice parameter between 0.384 ± 0.001 and 0.388 ± 0.001 nm and mean crystallite size between 2.2 ± 0.1 and 3.2 ± 0.1 nm), as well their electrocatalytic activity for the oxygen reduction reaction. The texture, the structure and the chemistry of the different carbon supports were however crucial during accelerated stress tests simulating the variations of the potential at a PEMFC cathode during start-stop events (500 potential cycles between 1.0 V vs. RHE (t = 3 s) and 1.5 V vs. RHE (t = 3 s) with square ramp and T = 353 K). Identical location transmission electron microscopy and electrochemical measurements showed that electrocatalysts supported on the graphene nanosheets and on the carbon xerogel suffered from pronounced nanoparticle detachment and carbon corrosion into CO2 while the electrocatalysts supported on carbon blacks underwent an incomplete oxidation of the surface (formation of COsurf). The degradation of the carbon support resulted in mass activity and specific activity losses, as the result of the increase of the resistance of the catalytic layers (∆Rk).


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Supporting Information The supporting information contains (i) the materials and methods, (ii) a table containing the BET, microporous and mesoporous specific surface area of the different carbon supports, (iii) the XPS spectra and illustrations of the position of the different C, O and N peaks, (iv) an histogram summarizing the degree of surface functionalization of the different carbon supports, (v) a table containing the physical properties (La, Lc, d002,) of the different carbon supports, (vi) the TEM micrographs and particle size distribution of the different electrocatalysts, (vii) the TEM micrographs of the PtNi/C agglomerates observed for the porous hollow PtNi NPs supported on CNT, (viii) the morphological properties of the porous hollow PtNi/C NPs and the electrochemical behaviours for the ORR of the commercial Pt/C, (ix) a description of the different parameters used to discuss the ageing of the carbon supported electrocatalysts, (x) the changes in the COads stripping voltammograms, the cyclic voltammetries in Arsaturated electrolyte and the linear sweep voltammetries in O2-saturated electrolyte measured after ageing and (xi) the figures explaining how the ΔRk values were determined.


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Acknowledgements This work was performed within the framework of the Centre of Excellence of Multifunctional Architectured Materials “CEMAM” n◦ AN-10-LABX-44-01. The French National Research Agency (HOLLOW project, grant number ANR-14-CE05-0003-01) and the University of Grenoble Alpes (AGIR SCALE UP, grant number LL1492017G and (AGIR PEPS IN FINE, grant number RECPEPINLEPG) financially supported this research. TA acknowledges IDS FunMat (Project 2012-04 LF) for funding his PhD thesis fellowship. NJ thanks the Walloon Region (project HYLIFE n°1410135) and the Fonds de Bay for funding.


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C Electrocatalysts: Carbon Support

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