Physical and Chemical Considerations for Improving Catalytic Activity

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Physical and Chemical Considerations for Improving Catalytic Activity and Stability of Non-Precious Metal Oxygen Reduction Reaction Catalysts Kavita Kumar, Priyanka Gairola, Mathieu Lions, Nastaran Ranjbar-Sahraie, Michel Mermoux, Laetitia Dubau, Andrea Zitolo, Frederic Jaouen, and Frédéric Maillard ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02934 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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Physical and Chemical Considerations for Improving Catalytic Activity and Stability of NonPrecious Metal Oxygen Reduction Reaction Catalysts Kavita Kumar, 1, Pryanka Gairola, 1, Mathieu Lions, 1, Nastaran Ranjbar-Sahraie,2 Michel Mermoux, 1 Laetitia Dubau, 1 Andrea Zitolo, 3 Frédéric Jaouen, 2,* Frédéric Maillard 1,* 1

Univ. Grenoble Alpes, CNRS, Grenoble-INP Ⱶ, Université Savoie-Mont-Blanc, LEPMI, 38000

Grenoble, France 2

CNRS, Université de Montpellier, ENSCM, UMR 5253 Institut Charles Gerhardt Montpellier, 2

place Eugène Bataillon, F-34095 Montpellier, France 3 Synchrotron

ⱵInstitute

SOLEIL, L’orme des Merisiers, BP 48 Saint Aubin, 91192 Gif-sur-Yvette, France.

of Engineering Univ. Grenoble Alpes

 These authors contributed equally to this work.

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ABSTRACT Recent non-precious metal catalysts (NPMCs) show promise to replace in the future platinumbased catalysts currently needed for the electroreduction of oxygen (ORR) in proton-exchange membrane fuel cells (PEMFCs). Among NPMCs, the most mature sub-class of materials is prepared via the pyrolysis of metal (Fe and Co), nitrogen and carbon precursors (labelled as MetalN-C). Such materials often comprise different types of nitrogen groups and metal species, from atomically dispersed metal-ions coordinated to nitrogen, to metallic or metal-carbide particles, partially or completely embedded in graphene shells. While disentangling the different contributions of these species to the initial ORR activity of Metal-N-C catalysts with multidunous active sites is complex, following the fate of these different active sites during electrochemical ageing is even more difficult. To shed light onto this, herein, six Metal-N-C catalysts were synthesized and characterized before/after ageing with two different accelerated stress tests (AST) simulating PEMFC cathode operating conditions either in steady-state or transient conditions. The samples differed from each other by the nature of the metal (Fe or Co), the metal content and the heating mode applied during pyrolysis. Catalysts featuring either only atomically-dispersed metal-ion sites (Metal-NxCy) or only metal nanoparticles encapsulated in the carbon matrix (Metal@N-C) were obtained after pyrolysis of catalyst precursors containing 0.5 or 5.0 wt. % of metal, respectively. All six catalysts showed high beginning-of-life ORR mass activity but the ASTs revealed marked differences in their ORR activity at end-of-life. After the load-cycling AST (10,000 cycles), Metal-N-C catalysts with Metal-NxCy sites retained most of their initial activity at 0.8 V (60 to 100 %) while those with Metal@N-C particles retained only a small fraction of initial activity (10 to 20 %). Metal-N-C catalysts with Metal-NxCy sites lost only 25% of their initial ORR activity after 30,000 load cycles


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at 80°C, thereby reaching the 2020 stability target defined by US Department of Energy. After 10,000 start-up/shutdown cycles, no catalyst showed measurable ORR activity at 0.8 V. However, after 1,000 start-up/shutdown cycles, most of the Metal-N-C catalysts initially comprising MetalNxCy sites showed measurable ORR activity at 0.8 V, while those initially comprising Metal@NC particles did not. Energy-dispersive X-ray Spectroscopy and Raman spectroscopy measurements of the cycled rotating disk electrodes revealed that demetallation of the catalytic centres and corrosion of the carbon matrix are the main causes of ORR activity decay during load-cycling and start-up/shutdown cycling, respectively. In contrast to what could have been intuitively expected, the Metal-NxCy sites are more robust to both demetallation and carbon corrosion than Metal@NC sites.

KEYWORDS: Proton-Exchange Membrane Fuel Cells ; Oxygen Reduction Reaction ; NonPrecious Metal Catalysts ; Demetallation ; Carbon corrosion.


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INTRODUCTION Because of the world's growing population, the economic development of many countries and the resulting increase in gross income per capita, the world energy demand is rapidly increasing. Since the consumption of fossil fuels such as coal, gas, and petroleum accelerates global warming, pathways for energy production with zero net carbon dioxide (CO2) emissions are needed. In this context, renewable energies will play a predominant role. However, their naturally intermittent production calls for the development of efficient electrochemical energy conversion and storage systems. The combination of water electrolysers and fuel cells is envisioned as an energy efficient cycle to store electricity as hydrogen (H2) and re-produce electricity from it on demand later.

Despite the progress realized in recent years, Platinum-Group Metals (PGM) nanoparticles supported on high-surface-area carbon remain the state-of-the-art cathode electrocatalyst in proton-exchange membrane fuel cells (PEMFC). However, due to the slow oxygen reduction reaction (ORR) kinetics,

1

the mass of platinum required to meet power and efficiency

requirements of certain market segments (automotive application in particular) is considerable. A possible path to solve the economic and geological constraints associated with the use of platinum when deploying PEMFCs consists in developing catalysts based on non-precious metals. Many materials such as transition metal oxides, nitrides and/or carbides, chalcogenides, 3-5 nanostructured carbon materials, doped carbon materials,

7-8

6

2

transition metal

nitrogen-, sulphur-, boron-, phosphorous-

and transition metal/nitrogen/carbon (Metal-N-C) catalysts

9-12

have

been investigated for ORR electrocatalysis in acid and/or alkaline medium. Metal-N-C catalysts synthesized by pyrolysis of a transition metal (Fe, Co), nitrogen and carbon precursors have demonstrated initial ORR activity approaching that of Pt/C in rotating disk electrode (RDE) or


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PEMFC 9-11, 13-14. Their ORR activity strongly depends on their physical and chemical structure, namely the nature and coordination of the metal centres, 15 metal content, 9, 16 redox potential, 17 basicity of the N-containing groups 10-11, 16, 18 and pore size distribution in the carbon phase 17. For Metal-N-C catalysts, a bimodal pore size distribution composed of micropores (hosting catalytic sites

11)

and macropores (allowing Fickian diffusion of oxygen19) is often perceived as being

optimal for proper PEMFC cathode operation. Relatively high volumetric activity and efficient mass-transport at high current density, two pivotal criteria for automotive PEMFCs, can be obtained for Metal-N-C catalysts prepared by pyrolyzing a sacrificial Zn-based metal organic frameworks (MOF), phenanthroline and a transition metal salt precursor

10.

Moreover, the

possibility to synthesize model Metal-N-C catalysts comprising either i) only atomically-dispersed Metal atoms (referred hereafter as Metal-NxCy moieties) and free of metallic, metal-oxides or metal-carbide nanoparticles,

16, 18, 20

or ii) only metallic particles, most often, embedded in

graphene shells (referred to as Metal@N-C in what follows 21 ) and free of Metal-NxCy moieties has been recently evidenced, therefore opening the possibility to establish structure-activitystability relationships for this class of catalysts.

The ORR activity of Metal-N-C catalysts often decreases with electrochemical cycling in RDE setup and acidic medium, but the underlying degradation mechanisms remain under-investigated. Also, for Metal-N-C catalysts that showed little activity decay after electrochemical cycling in RDE setup, the reason for the stability of a particular catalyst is unclear. Stability of Metal-N-C catalysts in RDE conditions should be regarded as an important and necessary condition toward stable operation in PEMFC, even though it is certainly not a sufficient condition due to the


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importance of the stability of additional properties over time in fuel cell, such as hydrophobicity and resistance to H2O2 produced in-situ as a by-product of the ORR.

Demetallation is known to be a key factor of the ORR activity loss of Metal-N-C catalysts observed in RDE studies since the early studies of Alt et al.

22

and Gupta et al. 23. Choi et al.

24

have recently shown that Fe metallic nanoparticles (Fe@N-C sites) present in some Fe-N-C catalysts are prone to oxidize at potentials E > 0.77 V vs. the reversible hydrogen electrode (RHE), and then to dissolve into soluble ferrous (Fe2+) species under simulated PEMFC load-cycling conditions (cycling between 0.6 and 1.0 V vs. RHE). While this loss of metal did not lead to a significant ORR activity decrease in the specific Fe-N-C catalyst of that study (comprising mostly Fe-NxCy sites), this behaviour may however differ for other Fe-N-C catalysts whose activity stems mainly from metallic or metal-carbide particles encapsulated in a graphitic matrix. Investigating the stability and metal leaching upon load-cycling of Metal-N-C catalysts comprising only Metal@N-C sites (Fe or Co) thus remained to be investigated. Investigating the stability upon load-cycling of a broader set of Metal-N-C catalysts comprising only Metal-NxCy sites (different pyrolysis modes, different metals) is also important, and especially in experimental conditions closer to PEMFC conditions (higher temperature and extended cycling).

The second known degradation mechanism for Metal-N-C catalysts is the bulk electrochemical carbon oxidation reaction (COR), which leads to carbon monoxide (CO) and carbon dioxide (CO2) evolution 25. While thermodynamics predicts that COR may occur at potentials as low as ca 0.2 V vs. RHE, it is kinetically hindered up to ca. 0.9 V vs RHE in the operating conditions of a PEMFC. Former studies on Pt/C and Metal-N-C catalysts have shown that the COR rate increases


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exponentially at potentials > 1.0 V vs. RHE, with such high potentials being encountered during uncontrolled start-up/shutdown events of a PEMFC,25-31 COR can have cascading negative consequences for Metal-N-C-based catalytic layers, such as destruction or deactivation of the catalytic sites, increased hydrophilicity, loss of electron percolation through the electrode and, in the most severe cases, collapse of the porous electrode structure. 25, 28, 30, 32 Studies on the influence of the corrosion of the carbon matrix of Metal-N-C catalysts on the ORR activity and stability remain scarce. Possible synergies or antagonisms between the nature of the transition metal in Metal-N-C catalysts, the metal weight fraction and the COR kinetics have not been studied hitherto. They represent another important objective of the present study.

Herein, we focus on the effects of load-cycling (electrode potential stepped between 0.6 and 1.0 V vs. RHE) and start-up/shutdown cycling (electrode potential stepped between 1.0 and 1.5 V vs. RHE) on the degradation mechanisms of structurally- and chemically-different Metal-N-C catalysts in RDE setup and acidic medium. To this goal, six materials were synthesized by pyrolysis of ZIF-8, Metal-acetate (M = Fe or Co) and phenanthroline, and their ORR activity, metal content and carbon structure were assessed before and after the two types of accelerated stress tests (ASTs) by energy-dispersive X-ray spectroscopy (X-EDS) and Raman spectroscopy, measurements, respectively.


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RESULTS AND DISCUSSION Textural, structural and chemical properties of the Metal-N-C electrocatalysts investigated in this study The catalysts used in this study are referred to as MetalXMode, where X is the metal content before pyrolysis (0.5 or 5.0 wt. %) and “Mode” is the pyrolysis mode, either ramp pyrolysis (RP, catalyst precursor heated from room temperature to 1050°C at 5°C min-1, then 1 h dwell time at 1050°C under Ar) or flash pyrolysis (FP, catalyst precursor inserted directly in the oven preequilibrated at 1050°C, 1 h dwell time at 1050°C under Ar).

Figure 1. Representative transmission electron microscopy images of the six Metal-N-C catalysts investigated in this study.


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Figure 1 shows a transmission electron microscopy (TEM) survey of 0.5 wt. % and 5.0 wt. % FeN-C and Co-N-C catalysts pyrolysed under ramp or flash mode. Two different carbon microstructures were observed in the carbon matrix of each catalyst, stacked graphitic layers (mostly for Metal-N-C catalysts with high metal content) and sheet-like layers in which the carbon phase was poorly structured. For the Fe5.0 and Co5.0 catalysts, metal nanoparticles of 10 – 100 nm in size were observed and the carbon structure appeared more graphitic in their neighbourhood (third row in Figure 1).

X-ray absorption spectroscopy (XAS) spectra of Fe0.5RP and Fe0.5FP show that the Fe atoms were atomically dispersed on the carbon matrix (with superimposed spectra, Figure 2c), with the main peak of the Fourier transform (FT) of the extended X-ray absorption fine structure (EXAFS) signal at ca 1.4 Å assigned to the Fe-N backscattering in FeNxCy moieties, and the minor peak at ca 2.4 Å assigned to Fe-C backscattering from the second coordination sphere. The detailed EXAFS analysis of Fe0.5FP was reported in Ref. 16, and its result can be transposed to Fe0.5RP. In summary, the XAS data shows that the same Fe-based active sites are present in Fe0.5RP and Fe0.5FP, although the site density on the top surface (in contact with electrolyte) may be different due to a different ratio of bulk vs. surface location of such sites, as discussed later.

The X-ray absorption near edge spectroscopy (XANES) data, in contrast, show that Fe is mainly present under the form of Fe3C particles in Fe5.0RP (Figure 2a), the XANES spectra of Fe5.0RP superimposing on the reference spectrum for pure Fe3C. In this case, the FT of the EXAFS spectrum shows little to no signal at the Fe-N distance expected for FeNxCy moieties, but an intense peak at 2.1-2.2 Å, which can be assigned to Fe-Fe backscattering in Fe3C (Figure 2c). Fe


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Mössbauer spectroscopy of similar Fe-N-C catalysts prepared from Fe acetate and ZIF-8 has systematically revealed the appearance (when the Fe content is above the threshold content at which all Fe atoms can be present as Metal-NxCy moieties) of γ-Fe just above the Fe content threshold, and also of α-Fe and Fe3C at even higher Fe content 31, 33. The vast majority of Fe atoms in Fe5.0RP must thus be comprised in Fe3C particles, in order to explain the excellent match between the XANES spectra of a Fe3C reference compound and Fe5.0RP. If simultaneously present in that sample, γ-Fe, α-Fe and FeNxCy moieties may only represent a few relative % of the total Fe amount.

Figure 2. Characterization by X-ray absorption spectroscopy of the Metal-N-C catalysts investigated in this study. (a, b) XANES and (c, d) Fourier-transforms of the EXAFS spectra


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measured at Fe or Co K-edge. Distance is not corrected for phase-shift. The XANES spectrum for Fe3C reference was digitalized from data reported in Ref.

34

(EXAFS spectrum of Fe3C was not

reported in that paper and is therefore not shown in c).

To confirm this hypothesis, the Mössbauer spectra of Fe0.5RP and Fe5.0RP catalysts were acquired and analyzed with a number of spectral components, while the Mössbauer spectra of Fe0.5FP had already been reported by us in Zitolo et al. (catalyst labelled Fe0.5 in that work) 16. As can be seen from the comparison of the analysis of the spectra of Fe0.5RP and Fe0.5FP (Figure S1a of this work and Figure 1b of Zitolo et al.

16),

the lack of sextet and singlet components

demonstrates the absence of zero-valent iron crystalline phases in those catalysts. The two different doublet spectral components are assigned to FeNx moieties in different oxidation state and/or spin state. The quadrupole splitting parameters of the doublets D1 and D2 are also very similar in both catalysts (see Table S1). In contrast, the spectrum of Fe5.0RP can be fitted solely with a single sextet component, whose Mössbauer parameters (hyperfine field and isomer shift) are very similar to those of Fe3C 35. The fitting was also tried with the addition of minor signal of doublets, but this did not lead to improved fit quality, and we thus conclude that the catalyst Fe5.0RP does not contain any atomically-dispersed FeNx moieties, supporting the preliminary conclusion drawn from the similarity of the XANES spectra of Fe5.0RP and Fe3C (main text and Figure 2).

Similarly, the XANES and EXAFS spectra at the Co K-edge show that the Co atoms were atomically dispersed in Co0.5RP and Co0.5FP and mainly present under the form of nanoparticles in Co5.0RP (Figure 2b and Figure 2d). The detailed XAS analysis of Co0.5FP was reported in Ref. 36, and its result can thus be transposed to Co0.5RP, identifying CoN4C12 porphyrinic structures


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and defective porphyrinic structures (e.g. CoN3C10) as the active-site structures in such materials free of metallic particles. For Co5.0RP, the superimposition of the absorption-edge of its XANES spectrum with that of a metallic Co foil, as well as the match of all the peaks in the EXAFS Fourier transform with that of a cobalt foil, demonstrates that almost all Co atoms are integrated in metallic Co particles. The slightly less intense FT peak at 2.1-2.2 Å relative to a Co foil reflects the nanoparticle nature of a fraction of metallic cobalt clusters 37.

Complementary to the XAS analysis, X-ray diffraction (XRD) and Raman spectroscopy provided insights into the carbon structure of the Metal-N-C catalysts. The XRD peaks at 2Ɵ ≈ 26 ° and 2Ɵ ≈ 43 ° displayed in Figure 3a and Figure 3b correspond to the (002) and (10) reflections from the turbostratic graphitic carbon (PDF card# 00-041-1487), respectively. The large width of the XRD peaks for Fe0.5FP, Fe0.5RP, Co0.5FP and Co0.5RP suggests small graphitic crystallites and a high degree of structural disorder. The 2Ɵ ≈ 26° diffraction peak was more intense and narrower for Co5.0RP and Fe5.0RP relative to the Metal0.5 catalysts, thus confirming that the carbon phase was on average more crystalline for these two catalysts. This is explained by the well-known role of Fe and Co metallic and carbide particles in catalysing graphitization under the pyrolysis conditions employed in this study. Additional peaks detected at 2Ɵ ≈ 44.2° and 2Ɵ ≈ 51.7° for Co5.0RP, and between 37 and 60° for Fe5.0RP, are assigned to metallic cobalt nanoparticles and Fe carbides, respectively (PDF cards # 00-006-0696 and # 00-015-0806), in agreement with conclusions derived from the XAS characterisations.


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Figure 3. Structural properties of the Metal-N-C catalysts investigated in this study. (a, b) X-ray diffraction patterns and (c, d) Raman spectra. The Raman spectra were normalized to the intensity of the G-band at ca. 1580 cm-1.

The detailed investigation of both the graphitic and disordered carbon phases present in the metalN-C catalysts was carried out by Raman spectroscopy. As expected for disordered carbon sp2 materials, two bands dominate the Raman spectra (Figure 3c and Figure 3d): the Raman-allowed first-order G band at about 1580 cm-1 and the so-called disorder-induced D1 band peaking at about 1350 cm-1 for a laser excitation at about 2.41 eV 38-39. It is well-known that the intensity of the D1 band, a disorder-induced band, increases with the structural disorder of the carbon component 4041.

Another disorder-induced band (D2 band) is distinguishable at ca. 1610 cm-1. The origin of this


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band is physically identical to the origin of the D1 band (q ≈ 2k quasi selection rule) except that the corresponding phonon is located near the  point, not the K point. Finally, two other bands band namely the D3 band at  ~ 1495 cm-1 and the D4 band at  ~1190 cm-1 were considered for spectrum fitting purposes. A visual examination of the Raman spectra (Figure 3c and Figure 3d) confirmed the findings on the carbon structure derived from XRD and TEM measurements. The main bands observed for Fe5.0RP and Co5.0RP are narrower than those present in the other four catalysts with low metal content, clearly indicating a higher crystallinity of their carbon matrix. In contrast, the broad Raman peaks observed on the four samples with low metal content (Fe0.5FP, Fe0.5RP, Co0.5FP and Co0.5RP) are typical of smaller graphitic crystallites with less structural order within the hexagonal lattice plane.

The average in-plane graphitic crystallite size (La) was estimated from Raman spectra using the Knight and White equation 42. In addition, the Debye-Scherrer equation was used to estimate the average carbon crystallite size in the direction perpendicular to the graphene layers (Lc) and the average interlayer spacing between the graphitic layers (d002) was estimated by refinement of the XRD patterns. It is clear from Figure 4a and Figure 4b that Fe5.0RP and Co5.0RP feature larger graphitic crystallites (both in-plane and perpendicular to the graphene planes) as well as shorter interlayer spacing (Figure 4c) than the other catalysts, quantitatively confirming their more graphitic structure. The d002 values in the two Metal5.0 catalysts (0.342-0.343 nm) approach those of highly-oriented pyrolytic graphite (0.3355 nm)

43

whereas those determined for the four

Metal0.5 catalysts are situated between those reported for pure graphite and amorphous carbon (> 0.36 nm), thus suggesting a turbostratic stacking.


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Figure 4. Textural and structural properties of the Metal-N-C catalysts investigated in this study. (a and b) Crystallite size in the plane and perpendicular to the graphene planes (La and Lc, respectively), (c) interlayer spacing (d002), and (d) specific surface area. It is highlighted that no significant structural differences could be revealed by XRD and Raman spectroscopy between ramp- and flash-pyrolyzed catalysts at a metal content of 0.5 wt % (before pyrolysis). A key difference is however observed in the microscopic porosity as a function of the pyrolysis mode (ramp vs. flash), as revealed by N2 sorption isotherms. For a given metal element and metal content of 0.5 wt % before pyrolysis, the ramp pyrolysis mode resulted in circa twice higher Brunauer–Emmett–Teller (BET) surface area than flash pyrolysis (compare Fe0.5RP to


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Fe0.5FP and Co0.5RP to Co0.5FP in Figure 4d), as previously reported for similarly prepared Fe-NC catalysts.44 The different BET surface area with pyrolysis mode can be explained on the basis of slow and gradual transformation of ZIF-8 into a highly porous N-C structure during the ramp heating (with volatile products formed from ZIF-8 decomposition having time to be evacuated), while with flash pyrolysis, there is less time to evacuate the volatile products. The difference in BET surface area can mostly be assigned to a higher microporous area for ramp-pyrolyzed samples, while meso- and macropores are little affected by the pyrolysis mode. 44

In summary, the TEM images, XRD patterns, Raman spectra and XAS spectra allowed us classifying these six Me-N-C catalysts into i) structurally-disordered carbon nanomaterials with no diffraction peak from metal-based phases (all four Me-N-C catalysts with 0.5 wt. % metal before pyrolysis) and ii) into graphitic nanomaterials with intense diffraction peaks from metalbased phases (two catalysts with 5.0 wt. % metal before pyrolysis).

Electrocatalytic properties of the Metal-N-C electrocatalysts Insights into the electrocatalytic activity of the six Metal-N-C catalysts were obtained with a RDE set-up and recorded in O2-saturated 0.1 M H2SO4. A break-in of the electrodes was first carried out through cyclic voltammetry in Ar-saturated electrolyte (Figure 5a). Cyclic voltammetry shows smaller and potential-independent capacitive currents for Fe5.0RP and Co5.0RP. This may be explained by their more graphitic structure and consequently smaller pseudocapacitance (lower amount of surface defects and thus of oxygen functional groups). The ORR polarization curves obtained after subtraction of the background current measured in Arsaturated electrolyte are shown in Figure S2. For Fe0.5FP, Fe0.5RP, Co0.5FP and Co0.5RP, they


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featured an onset at ca. 0.90 V vs. RHE, a mixed kinetic-diffusion control for 0.65 < E < 0.90 V and a sometime ill-defined plateau at potential lower than 0.65 V vs. RHE. The plateau of diffusion-limited current density was particularly ill-defined for Fe5.0RP and Co5.0RP catalysts. TEM and XAS characterisations indicate that the most active sites in these catalysts are metal nanoparticles encapsulated in the N-C matrix (Figure 1, Figure 2, Figure 3). The ill-defined plateau for these two catalysts may thus be explained as a gradual switch from 2e to a direct 4e ORR pathway as a function of potential. Figure 5b shows the Tafel plots for the fresh Metal-N-C catalysts. At E > 0.75 V, the Fe-N-C catalysts were more active than the corresponding Co-N-C catalysts, in agreement with former literature findings 45. Note that despite the more positive ORR onset, the Tafel slopes were usually higher for Fe-N-C relative to Co-N-C catalysts, also in agreement with previous finding on a set of Fe-N-C and Co-N-C catalysts prepared in a complete different way 45. For example, the Tafel slope was in the range -90 to -100 mV decade-1 for Fe0.5RP, Fe0.5FP and Fe5.0RP, but only -60 to -70 mV decade-1 for Co0.5RP and Co0.5FP (catalysts based on atomically-dispersed cobalt). The catalyst Co5.0RP had however a Tafel slope comparable to that of Fe5.0RP, and this different behaviour is assigned to the nature of the active sites in those two catalysts, i.e. Metal@N-C.


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Figure 5. Electrocatalytic properties of the Metal-N-C nanocatalysts. (a) Cyclic voltammograms measured in Ar-saturated 0.1 M H2SO4 at 10 mV s-1 and no rotation, (b) Tafel plots and (c) mass activity for the ORR measured at E = 0.8 V vs. RHE (MA0.80) determined in O2-saturated 0.1 M H2SO4 at v = 5 mV s-1 and  = 1600 rpm. The error bars represent the standard deviation among at least four independent measurements. T = 25°C for all measurements.


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The mass activity for the ORR of the different catalysts was extracted by normalizing the kinetic current (i.e. after correction of the polarisation curve for Ohmic drop and O2 diffusion in solution) by the total mass of catalyst powder deposited on the glassy carbon tip. Figure 5c clearly identifies Fe0.5RP as the most active catalyst in the present series, seconded by the two other Fe-based catalysts, Fe0.5FP and Fe5.0RP. The higher ORR activity of Fe0.5RP vs. Fe0.5FP despite similar active sites as identified by XAS and similar bulk Fe content can be explained on the basis of the higher microporous surface area and BET area of Fe0.5RP. This probably resulted in a higher fraction of FeNxCy moieties located on the top surface in Fe0.5RP, and thus in a higher number of sites being electrochemically accessible. The relatively high ORR activity of Fe5.0RP might be due either to Fe@N-C sites and/or to a minor fraction of FeNxCy moieties coexisting with the Fe particles. While the EXAFS spectrum of Fe5.0RP does not reveal a significant signal corresponding to the Fe-N distance in FeNxCy moieties, it must be kept in mind that even if only 10% of all Fe atoms in Fe5.0RP are present as FeNxCy moieties (difficult to identify with XANES-EXAFS, whose signature is controlled by the majority species: metal nanoparticles), this would result in the exact same number of FeNxCy moieties per mass of catalyst than for Fe0.5FP. As shown later, the electrochemical stability of Fe5.0RP and Fe0.5FP are however totally different, supporting the hypothesis that the initial ORR activity of Fe5.0RP can mostly be assigned to Fe@N-C sites. The same general observations and conclusions can be derived for Co-N-C catalysts, with Co0.5RP the most active, followed by Co0.5FP and then Co5.0RP. The lower ORR activity of the latter combined with its higher Tafel slope and its XANES or EXAFS spectra nearly superimposed with those of a cobalt foil leaves little doubt that its initial ORR activity can be assigned to Co@N-C sites.


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We now discuss the changes in ORR mass activity during two ASTs: 10,000 square-wave cycles, either between 0.6 and 1.0 V vs. RHE (‘load-cycling’ protocol) or between 1.0 and 1.5 V vs. RHE (‘start-up/shutdown’ protocol) with a holding time of 3 s at each potential for each protocol. These protocols were designed by the Fuel Cell Commercialization of Japan (FCCJ) to simulate the potential range typically experienced by an automotive PEMFC cathode, and the excursions to high potentials experienced during unprotected start-up/shutdown or fuel starvation events, respectively 26-27, 29-30, 46-48. To accelerate the degradation of the Metal-N-C catalysts while better reproducing the operating conditions of a PEMFC, the ASTs were performed in liquid electrolyte at T = 80°C. As shown by Dubau et al.,

49

such conditions allowed, on Pt/C nanoparticles,

reproducing the morphological changes occurring in PEMFC cathodes. In contrast, the same AST but performed at only T = 25°C did not produce any detectable morphological change of the same Pt/C nanoparticles. Similar observations were also made by Goellner et al. 31 and Choi et al. 25 on Metal-N-C catalysts. While the electrolyte temperature has little influence on the ORR activity of Pt/C and Metal-N-C catalysts in the range 20-80°C, it greatly increases the COR, which is critical in the degradation induced by the start-up/shutdown protocol.45

Effect of load-cycling on the ORR activity and structure of Fe-N-C and Co-N-C catalysts For the load-cycling AST, the changes of ORR mass activity at 0.8 V vs. RHE as a function of the number of cycles are displayed in Figure 6a and Figure 6b. For all catalysts, 10,000 cycles resulted in a measurable decrease in ORR activity. The changes were pronounced during the first 1,000 potential cycles and less marked later on. For iron, the activity loss was milder for Fe0.5RP than for Fe0.5FP and the highest ORR activity loss was observed for Fe5.0RP.


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ACS Catalysis

Figure 6. ORR mass activity measured at E = 0.8 V vs. RHE after (a, b) load-cycling protocol (E stepped between 0.6 and 1.0 V vs. RHE, 3s at each potential at T = 80 °C) or (c, d) ‘startup/shutdown’ protocol (E stepped between 1.0 and 1.5 V vs. RHE, 3s at each potential at T = 80 °C). The ORR activities were determined in RDE setup in O2-saturated 0.1 M H2SO at T = 25°C and  = 1600 rpm. The scan rate was v = 5 mV s-1.

As shown in Figure S2, the ORR polarisation curve of Fe5.0RP was also more severely modified in the diffusion-limited region (E < 0.65 V vs. RHE) of the ORR polarization curves. These results may be interpreted either as an incomplete coverage of the geometric area of the working electrode after AST (through catalyst particle detachment), or as an average decrease of the number of electrons involved per O2 molecule reduced (decreased selectivity toward water formation). The


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latter effect might have been induced by the loss or modification of highly selective active sites, with less selective sites remaining after the AST. We discard catalyst detachment based on visual inspection of the thin-film electrodes and on the minor changes of the cyclic voltammograms (CVs, see Figure S3) and of the Raman spectra upon the load-cycling AST (Figure 7).

Figure 7. Raman spectra of the catalysts before (black) and after 10,000 cycles between 0.61.0 V vs. RHE (blue, load-cycling AST) or 10,000 cycles between 1.0-1.5 V vs. RHE (red, startup/shutdown AST) at T = 80 °C.

To check the validity of the second hypothesis (loss or modification of highly selective active sites), the thin-film electrodes were scratched and the metal content determined on several regions of the electrode with the help of X-EDS. The X-EDS analyses, performed on at least 10 different


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ACS Catalysis

zones of the thin-film electrodes, revealed a homogeneous distribution of the transition metal, both before and after ageing. These chemical analyses (Figure 8a and Figure 8b) showed a marked decrease in the amount of transition metal during the load-cycling AST, independently from the nature of the transition metal and the pyrolysis mode (flash or ramp). The relative decrease in metal content was however much more pronounced for the catalysts with higher initial metal content. For example, the Fe5.0RP and Co5.0RP catalysts lost nearly 80% of their initial mass of metal. This confirms the operando ICP-MS results of Choi et al.

24

who showed that Fe nanoparticles

imperfectly encapsulated in carbon dissolved fast under load-cycling conditions, while an Fe-N-C catalyst comprising only FeNxCy moieties (identically prepared as Fe0.5FP in the present study) showed negligible operando Fe leaching over several tens of cycles in the same conditions.


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Figure 8. Normalized metal content in the Fe- (a) and Co-based catalysts (b) before (plain), after 10,000 cycles of either load-cycling (grid pattern) or start-up/shutdown cycling (dashed pattern) at T = 80 °C, (c) correlation between normalized ORR activity after load-cycling and normalized metal content after load-cycling (d) lack of correlation between normalized ORR activity after start-up/shutdown cycling and normalized metal content after start-up/shutdown cycling. No cobalt could be detected in Co5.0RP after 10 k cycles of start-up/shutdown cycling. Since most of the thin-film electrodes detached from the glassy carbon tip or were visually destroyed during the start-up/shutdown protocol, X-EDS analyses and Raman were performed on remaining fragments thereby preventing any correlation between the normalized ORR activity after the AST and the normalized transition metal content.

In order to bridge electrocatalytic activity and chemical properties in this set of materials that initially comprise different types of sites, the normalized ORR mass activity (MAfinal/MAinitial) was plotted as a function of the normalized amount of transition metal (Metalat%, final/Metalat%,initial) estimated by X-EDS. It is striking from Figure 8c that a quasi-linear relationship holds between these two normalized quantities for the AST load-cycling protocol. This indicates that, regardless of the nature of the active sites initially present in a Metal-N-C catalyst (atomically dispersed moieties or Metal@N-C sites), the ORR activity fraction remaining after the load-cycling is proportional to the fraction of metal remaining in the electrode. It also indicates that the nature of the active sites after the load-cycling AST is likely the same as initially, the reduced ORR massactivity stemming only from a lower number of active sites per geometric area of the RDE. Also, the much higher relative loss of ORR activity of Fe5.0RP compared to Fe0.5FP in spite of similar absolute initial ORR activity, supports the hypothesis that the initial ORR activity of Fe5.0RP


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stemmed from Fe@N-C sites. With the present synthesis approach, the Metal@N-C sites are thus less stable than Metal-Nx-Cy moieties in the load-cycling protocol. This may be explained by the formation of imperfect graphene shells around the metallic particles, with access for the electrolyte and thus allowing metal leaching. Other syntheses approaches resulting in a uniform encapsulation by carbon may result however in Metal@N-C sites with better stability to the load-cycling. It is also promising that Metal-Nx-Cy moieties (Fe and Co) survive, for the majority, the load-cycling protocol after 10,000 cycles and at 80°C. The exact stability properties of such moieties is however tuned by synthetic conditions, such as the pyrolysis mode herein.

In conclusion, our combined Raman-electrochemistry-X-EDS measurements show that the ORR losses in load-cycling conditions in liquid electrolyte are mostly related to metal leaching of the active sites (Metal-Nx-Cy sites in 0.5 wt. % Metal-N-C catalysts and Metal@N-C sites in 5.0 wt. % Metal-N-C catalysts) during ageing in the 0.6-1.0 V vs. RHE potential range, while carbon corrosion does not play a significant role. The Fe0.5RP catalyst suffered only little ORR activity loss after extended cycling (10%) and little Fe loss (20%). It is of note that while stability to loadcycling protocol is a required condition for stable operation in PEMFC, it is not a sufficient condition. Other degradation mechanisms are specific to operation at high current density in PEMFC, such as retained hydrophobicity and resistance to minor amount of H2O2 produced during ORR, being produced in much larger absolute amount in PEMFC than RDE conditions due to several orders of magnitude higher current density.

Effect of start-up/shutdown cycling on the structure and ORR activity of Fe-N-C and CoN-C catalysts


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The scenario was completely different during the start-up/shutdown AST, leading to much higher loss of ORR activity (compare Figure S2 and Figure S4). The COR proved key to rationalize the changes in ORR mass activity during start-up/shutdown AST. The COR rate is so high at 1.5 V vs. RHE that it was directly visible from the gas evolution (CO2 and CO) on the electrodes, in agreement with former results

30, 50-52.

Actually, most of the thin-film electrodes

detached from the glassy carbon tip or were visually destroyed during the start-up/shutdown protocol, thereby preventing any straightforward correlation between the normalized ORR activity after the AST and the normalized transition metal content after the AST (Figure 8d). Fragments of the thin-film electrodes could however be recovered, and analysed with X-EDS and Raman spectroscopy. The Raman spectra monitored post-AST revealed a marked decrease of the D3 band intensity at 1495 cm-1 on all six catalysts, which is strong evidence that the disordered carbon domains of the Metal-N-C catalysts were preferentially corroded during the start-up/shutdown AST. Interestingly, the decrease of the D3 band was more marked for the Fe5.0RP and Co5.0RP catalysts and it was accompanied by narrower and more intense D1 and D2 bands, indicating increased concentration of surface and edge defects in the graphene planes on the aged Metal5.0 catalysts (Figure 7). These results agree with what was found by Castanheira et al.

30on

Pt

nanocatalysts supported on high surface area carbon blacks (Vulcan and Ketjenblack) after the same AST protocol conducted at 57°C. While the effect of the nature and quantity of the transition metal on the graphitization of the carbon phase is well-established 53, these results also suggest that the non-PGM transition metal might be able to catalyse the electrochemical corrosion of the carbon matrix of Metal-N-C catalysts in the start-up/shutdown protocol.


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In agreement with the massive structural changes caused by corrosion of the carbon phase, the ORR mass activity at 0.8 V vs. RHE decreased significantly already after 500 cycles (dashed patterns in Figure 8a and Figure 8b, see Figure S4 for the complete ORR polarisation curves) and all catalysts were much less ORR active after 10,000 cycles. To discriminate better the resistance of Metal-N-C catalysts relative to startup/shutdown AST (at high temperature), comparing the remaining activity after 500 or 1,000 cycles thus seems more accurate and meaningful than performing the same comparison after 10 k cycles (no measurable ORR activity at 0.80 VRHE). The electrochemical results are in agreement with former findings by Goellner et al. 31 who reported no residual ORR activity after 300 potential cycles between 0.9 and 1.4 V vs. RHE (square wave, holding time 3 seconds at each potential, T = 80°C) and by Martinaiou et al., 54,

who observed severe losses in both the kinetic and the diffusion-limited regions of the ORR

polarization curves (E = 0.75 or 0.50 V vs. RHE, respectively) after 5,000 potential cycles between 1.0 and 1.5 V vs. RHE at room temperature (the change in COR rate at a fixed potential is ca 2030 times lower at room temperature than at 80°C).

The CVs measured in Ar-saturated 0.1 M H2SO4 featured often a higher capacitive current density after 1 k or 5 k cycles of startup/shutdown AST relative to the initial CV of each catalytic layer (Figure S5). This can be assigned to both higher carbon area (formation of new pores due to COR, compensating for the loss of carbon as CO or CO2 gas) and formation of pseudocapacitive redox groups on the carbon surface (quinone/hydroquinone for example, as seen by broad redox peaks on the CVs after AST). Only for the extensive COR (after 10 k cycles) is a suppressed capacitive current response of catalytic layers observed (e.g. Fe0.5FP and Co5.0RP) in line with a complete loss of ORR activity for those layers after 10 k cycles in Figure S4. Overall, retained


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capacitive current or even increased capacitive current after AST is not synonym with retained activity.

Overall, the results derived from the start-up/shutdown AST protocol identify catalysts based on atomically-dispersed Metal-NxCy sites as more stable than those with Metal@NC sites. While this bears similarity with observations made with the load-cycle AST, the fundamental reasons seem different, carbon-corrosion for the former AST and demetallation for the latter.

Both the load-cycle and startup/shutdown ASTs identify Fe0.5RP as the most promising material in the present set of catalysts. In order to evaluate if it reaches the ex situ stability target for 2020 defined for ORR catalysts by the U.S. Department of Energy (US DoE), we extended the AST load-cycle to 30,000 cycles. The AST protocol employed here is in fact identical to the one defined by US DoE, except for the upper potential limit, 1.0 V vs. RHE here but 0.95 V vs. RHE as defined by the US DoE (55). Following 30,000 load cycles, the relative loss in mass activity at 0.8 V vs. RHE is only 25% for the Fe0.5RP catalyst (Figure S6). This catalyst therefore reaches the stability target for 2020 of 40% maximum loss in activity when subjected to this AST, and also shows promising initial ORR activity.

Overall, the main conclusion from this study is that Metal-NxCy centers are more stable to the load-cycling AST in acidic medium than Metal@NC centers, and the extent of activity loss is directly proportional to the demetallation for all catalysts. While somewhat counterintuitive, this general result is in line with previous stability reports that focused often on a single Metal-NC catalyst. In acidic medium, Serov et al observed a potential drop of ca. 40 mV at -1 mA cm-2 after


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only 5,000 triangular cycles between 0.2 and 1.1 V vs. RHE in 0.5 M H2SO4 for an Fe-N-C catalyst comprising 26 % as metallic iron or iron carbide and 74 % as FeN4 moieties 56. The relative high deactivation there may have been due to the high upper potential limit of 1.1 V, in between a loadcycle and a startup shutdown cycle. For Fe0.5FP, we previously showed that Fe is present only as atomically dispersed Fe ions in FeN4 type moieties and that no Fe is leached in the acidic electrolyte during several electrochemical cycling (on line mass spectroscopy) and that the ORR activity was unmodified after 5,000 cycles in acidic electrolyte 24-25. In alkaline medium, in Figure 4f in Ref. 57, the ORR activity of an Fe-N-C catalyst comprising Fe-based particles decreased after 30,000 cycles (ca. 25 mV negative shift) between 0.6 and 1.0 V vs. RHE in 0.1 M KOH. Remarkably, another Fe-N-C catalyst comprising only FeN4 moieties (according to TEM images) showed no measurable change in the kinetically-controlled region of the polarisation curve even after 60,000 cycles (Figure 4h in that same paper).

CONCLUSIONS In this study, we showed that Metal-N-C (Fe or Co) electrocatalysts prepared from precursors containing 0.5 wt. % metal comprise only atomically-dispersed Metal-NxCy centers, while those prepared from 5.0 wt. % metal comprise only metallic (Co) or metal-carbide (Fe) particles embedded in N-doped carbon shells (Metal@NC). High metal contents also graphitize the carbon phase of the Metal-N-C catalysts prepared via pyrolysis. The thin-film electrodes containing Metal-N-C catalysts with Metal-NxCy centers are initially the most active, but those with Metal@NC particles have however significant initial ORR activity as well. The physical, chemical and electrochemical characterizations of the thin-film electrodes before and after two different accelerated stress tests provided important correlations between structure, ORR activity and


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durability of Metal-N-C catalysts. Cycling between 0.6 and 1.0 V vs. RHE led to a small activity loss for catalysts comprising only Metal-NxCy centers but to a large activity loss for those comprising only Metal@NC sites. For all catalysts, a positive correlation is observed between residual metal content after AST and residual ORR activity. This shows that the metal in MetalNxCy centers is more stable in acidic medium than the metal in Metal@NC particles. Higher ORR activity losses were noticed after cycling between 1.0 and 1.5 V vs. RHE, due to a massive corrosion of the carbon phase for all those carbon-embedded catalysts. Overall, the Metal-N-C catalyst based on Fe, featuring only Metal-NxCy centers and pyrolysed in ramp mode revealed the most active and also the most robust in simulated PEMFC cathode environment. It showed only 25% loss in activity after 30,000 load cycles at 80°C, thereby reaching the 2020 stability target defined by US DoE.

MATERIALS AND METHODS Electrocatalysts Six different Metal-N-C catalysts derived from pyrolysis of ZIF-8 (Sigma-Aldrich, Fe impurities were detected with an amount of 50-70 ppm) have been synthesized using the procedure described in Ref. 10. These catalysts are composed of Fe or Co atoms embedded in a graphene structure doped with nitrogen. They differed from each other by the nature of the metal, the weight percent of metal (0.5 or 5.0 wt .%) and the method used to pyrolyse them (flash pyrolysis in which the sample was introduced at T = 1050°C or ramp pyrolysis in which the temperature was linearly increased from room temperature to T = 1050°C at 5°C min-1). Electrochemical Measurements Solutions


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All the glassware used in this study was cleaned by former immersion in a H2SO4: H2O2 (50 % v/v) solution overnight and thoroughly rinsed with Milli-Q-grade water (Millipore, 18.2 MΩ cm, total organic compounds

Physical and Chemical Considerations for Improving Catalytic Activity

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