Tomasz Kosmala

Nicolas Bibent

Moulay Tahar Sougrati

Goran Dražić

Stefano Agnoli
email: stefano.agnoli@unipd.it.

Frédéric Jaouen
email: frederic.jaouen@umontpellier.fr

Gaetano Granozzi

Stable, Active and Methanol Tolerant PGM-free Surface in Acidic Medium: Electron Tunneling at Play in Pt/FeNC Hybrid Catalysts for Direct Methanol Fuel Cell Cathode

Keywords: hybrid catalyst, core shell, electron tunneling, direct methanol fuel cell, methanol tolerance, model system, ORR, Fe 2 O

PGM-free catalysts have high initial activity for O 2 reduction reaction, but suffer from low stability in acid medium in PEMFC and DMFC. Here, we shed light on the atomic-scale structure of hybrid Pt/FeNC catalysts (1-2 wt% of Pt), revealing by STEM and EDXS the presence of Pt@FeO x particles.

The absence of exposed Pt on the surface is confirmed by the suppression of methanol oxidation reaction and CO stripping experiments. The promising application of such Pt/FeNC catalysts, comprising FeN x sites and Pt@FeO x particles, is demonstrated at the cathode of DMFC. To gain fundamental understanding on the stability in acid medium and on the intrinsic ORR activity of Pt@FeO x , we constructed model surfaces by depositing FeO x films with controlled thickness (from 1.0 to 6.4 nm), fully covering the Pt(111) surface, which resulted stable in acid medium in the potential range 0.45 -1.05 V vs. RHE. The specific ORR activity of Fe 2 O 3 /Pt(111) increases exponentially with decreasing overlayer thickness, which is explained by the tunneling of Pt electrons through Fe 2 O 3 . This special phenomenon sheds light onto recently reported excellent durability of Pt/FeNC composites in PEMFC and identify a promising core@shell strategy leading to stable PGM-free surfaces in acid medium, and tolerant to methanol.

INTRODUCTION

The electrochemical oxygen reduction reaction (ORR) to water is a key reaction for electrochemical energy conversion devices such as fuel cells and metal-air batteries. [1][2][3][4][5][6][7][8] A variety of fuel cell (FC) technologies are being developed with operating temperature and chemical environment being defined by the nature of the electrolyte, such as proton exchange membranes (PEM), 9 anion exchange membranes (AEM), 10,11 or solid oxides. Fuel cells based on PEMs (labelled PEMFC when fed with hydrogen, or direct methanol fuel cell, DMFC, when fed with methanol) are more mature than their AEM counterparts and do not suffer from complex issues related to airborne CO 2 , a challenge for alkaline liquid-electrolyte FCs and AEMFCs. 12 PEMFCs in particular are perceived today as the most promising fuel cells for automotive application, while DMFCs can target niche applications where easiness to handle and fuel storage are important. [13][14][15][16] The first commercial H 2 /air PEMFC cars were deployed in 2017 in Japan and USA and contain Pt-based catalysts both at the anode and cathode, with a higher amount of Pt in the latter. 17 Despite continuous progress in the ORR activity and durability of platinum-based catalysts for acidic medium, [18][19][20][21] the high cost and low resources of platinum and any platinum-group-metal remain an issue for reaching the final cost targets of fuel cell stacks and also for sustainability reasons. The mining of new platinum and its recycling significantly contribute to CO 2 emissions in the life-cycle analysis of PEMFC and DMFC devices. [22][23][24] The impact of platinum-groupmetals (PGMs) on the cost and life cycle analysis of DMFCs is stronger than for PEMFCs due to: i) Higher loadings of PGMs at both the anode and cathode of DMFCs compared to PEMFCs; ii) Lower power output per geometric area of membrane electrode assembly of DMFCs vs. PEMFCs.

The quest for PGM-free ORR catalysts for PEMFCs and DMFCs has resulted in the demonstration of highly active materials, with FeNC materials comprising atomically dispersed FeN x sites currently being the most promising subclass of PGM-free catalysts. [25][26][27][28] While progress is observed in the fundamental understanding of the nature of such active sites and the factors that control their initial activity, [29][30][31][32] slow progress is currently observed regarding the durability in PEMFC and DMFC of FeNC and other PGM-free materials. All FeNC and other Metal-NC catalysts suffer from rapid activity decrease in operating PEMFCs 2,33,34 and DMFCs. [35][36][37][38] The trend is also observed that the higher is their initial ORR activity, the worse is their durability. 2 Recent reports indicate that high initial ORR activity for FeNC materials corroborates with high microporous surface area and also with high Fe leaching rates in oxygenated acidic medium. [39][40][41] In particular, the simultaneous combination of low electrochemical potential and presence of oxygen seems to trigger Fe demetallation from FeN x sites, little or not observed in the absence of O 2 . [41][42][43] Such demetallation and degradation is likely related to the in situ production of H 2 O 2 and reactive oxygen species via Fenton reactions, catalyzed by Fe cations and other 3d transition metal cations. 44,45 Due to similar acidic environment at the cathode of DMFCs and PEMFCs, it is expected that the degradation rates and mechanisms are similar for FeNC catalysts in both devices. This is supported by recent observations of in situ formation of iron oxide nanoparticles at the cathode of both PEMFC and DMFC, even for FeNC materials that initially contain only FeN x sites. 37,41 Therefore, the identification of rational strategies for improving the durability of PGM-free ORR catalysts in acidic medium is acutely needed. In a recent study, we reported on the stabilization of FeNC by minute amounts of platinum (1-2 wt %). 46 The addition of 1-2 wt % Pt to a given FeNC catalyst resulted in Pt/FeNC composites with similar ORR activity, but with dramatically improved durability in PEMFC. While the ORR activity of FeNC at 0.8 V was divided by ca four after PEMFC operation for 50 h at 0.5 V, no measurable change in ORR activity was observed after functionalization of the same FeNC material with 1-2 wt % Pt. A longer durability test in PEMFC for 180 h at 0.5 V confirmed the improved durability. While Pt particles were observed on the SEM images of the Pt/FeNC composites, the lack of increase in activity towards both the ORR and the electroreduction of H 2 O 2 for Pt/FeNC relative to FeNC clearly indicated that no metallic Pt was present on the top surface. This was corroborated by the lack of CO-stripping signal of the Pt/FeNC cathode, both before and after the PEMFC durability test at 0.5 V.

In the present work, two Pt/FeNC catalysts were prepared as previously reported, 46 and further investigated with 57 Fe Mössbauer spectroscopy at 5 K and scanning transmission electron microscopy (STEM) coupled with energy-dispersive X-ray spectroscopy (EDXS). They were also electrochemically characterized for ORR catalysis with a rotating disk electrode (RDE) in the presence of methanol, as well as at the cathode of DMFC. The STEM-EDXS results show that both large and small Pt particles in these Pt/FeNC hybrid catalysts are covered with a thin shell rich in Fe and O, while 57 Fe Mössbauer spectroscopy at 5 K reveals the presence of ferric oxide, which is absent in the reference FeNC material.

The electrochemical results identify the complete tolerance to methanol of these Pt/FeNC powder catalysts. DMFC results with Pt/FeNC cathodes comprising only 40-80 µg Pt cm -2 (4 mg total mass of Pt/FeNC per cm 2 ) showed promising power performance compared to state-of-art Pt/C cathodes with 2000 µg Pt cm -2 , advantageously allowing the utilization of high methanol concentration at the anode without any drawback. In order to better understand the stability and possible activity of Pt@FeO x coreshell particles, electrochemical studies were then performed in acidic electrolyte on model surfaces prepared via the deposition of ultrathin films of Fe 2 O 3 on Pt(111). These studies revealed unexpected stability in acidic medium in the region 0.45 -1.05 V vs. RHE and moderate ORR activity when the Fe oxide layer is ≤ 3.2 nm. The ORR activity of Fe 2 O 3 layers on Pt(111) was negatively correlated with layer thickness, which can be explained by electron tunneling effect from the core through the shell. The insights gained from model surfaces of Fe 2 O 3 layers on Pt(111) and advanced characterization of the present Pt/FeNC hybrid materials explain well the methanol tolerance and lack of CO stripping signal of the latter. Overall, these insights can trigger novel approaches for the design of PGM-free or PGMbased catalysts for ORR and HOR by changing the nature of the metal, either in the core or in the shell, with tuned selectivity and tolerance to a range of fuels and chemicals.

EXPERIMENTAL SECTION

Pt/FeNC powder catalysts synthesis: Fe 1.0 d was synthesized from ZIF-8, a Zn(II) zeolitic imidazolate framework (Basolite Z1200 from BASF), Fe(II) acetate and 1,10-phenanthroline (phen). 800 mg of ZIF-8, 200 mg of phen and 31.5 mg of Fe(II) acetate were poured into a ZrO 2 crucible together with 100 ZrO 2 balls (5 mm diameter), defining a content of 1 wt% Fe in the catalyst precursor. After being sealed in ambient atmosphere, the crucible was subjected to 4 cycles of 30 min ballmilling at 400 rpm (Fritsch Pulverisette 7 premium). The resulting catalyst precursor was flash-pyrolyzed at 1050°C in Ar for 1 h.

Then 300 mg of Fe 1.0 d was impregnated with 550 µL (stepwise, 100 µL at a time with grinding for each addition) of an aqueous solution of Pt(NH 3 ) 4 Cl 2 •H 2 O. The concentration of the Pt salt solution was adjusted in order to reach 1.0 or 2.0 wt % Pt in the hybrid catalysts (9.9 and 17.9 mg/mL for 1 and 2 wt % Pt, respectively). The impregnated sample was then dried for 2 h in an oven at 80°C under air. The dry powder was loaded in a quartz boat, itself in a quartz tube and ramp-heated under N 2 flow from 300 to 560°C at a rate of 4°C per minute. The gas flow was then switched from N 2 to 5% H 2 in N 2 for 2 h at 560°C. After that, the split hinge oven was opened and the tube removed from it and let to cool down naturally under N 2 flow. The resulting catalysts are labelled Pt 1.0 Fe 1.0 d and Pt 2.0 Fe 1.0 d, for 1 and 2 wt% Pt added onto Fe 1.0 d, respectively. STEM/EDXS characterization of Pt 2.0 Fe 1.0 d: Probe Cs-corrected scanning transmission electron microscope Jeol ARM 200 F, equipped with a cold field emission electron source, was used for imaging Pt 2.0 Fe 1.0 d, and in particular for imaging the Pt nanoparticles. To minimize the beam damage, 80 keV and low beam current were used. High-angle annular dark-field (HAADF) images were obtained using 68-180 mrad collection half-angles at 24 mrad probe convergence semi-angle. Images were filtered with a non-linear filter, which is a combination of low-pass and Wiener filters.

57

Fe Mössbauer spectroscopy characterization of Pt 2.0 Fe 1.0 d: The 57 Fe Mössbauer spectrometer (Wissel, Germany) was operated in transmission mode with a 57 Co: Rh source. The velocity driver was operated in constant acceleration mode with a triangular velocity waveform. The velocity scale was calibrated with the magnetically split sextet of a high-purity α-Fe foil at room temperature. The spectra were fitted to appropriate combinations of Lorentzian profiles representing quadrupole doublets, sextets by least-squares methods. Isomer shift values are reported relative to α-Fe at room temperature. The powder catalyst was mounted in a 2 cm 2 holder. Mössbauer measurements at 5 K was performed in a helium flow cryostat (SHI-850 Series from Janis, USA). The ink was ultrasonicated for 30 min. Then, 7 µL was deposited onto a glassy carbon disk with a diameter of 5 mm leading to a total catalyst loading of 0.8 mg cm -2 . The electrodes were then immersed into 0.1 M HClO 4 in a glass cell with graphite as counter electrode, and a platinum wire in a separate compartment, saturated with H 2 , acting as a reversible hydrogen electrode (RHE). The rotation rate was 1600 rpm. For break-in, at least 5 cyclic voltammogramms (CVs) were applied between 0.0 and 1.0 V RHE at 10 mV s -1 in N 2 -saturated electrolyte. If the last two scans did not superimpose, more CVs were applied. Subsequently, CVs were recorded at 1 mV s -1 in O 2 saturated electrolyte between 0.1 and 1.0 V RHE , a scan rate sufficiently low to neglect capacitive currents.

Electrochemical characterization in RDE of

DMFC experiments:

The anode was a gas diffusion electrode already catalyzed with PtRu/C (2000 µg PtRu cm -2 , SIGRACET S10BC). The cathode was either a gas diffusion electrode already catalyzed with Pt/C (2000 µg Pt cm -2 , SIGRACET S10BC) or a Fe-N-C cathode obtained by depositing 4.0 mg cm - 2 of Pt/FeNC from an ink on a gas diffusion electrode (SIGRACET S29BC), resulting in 40 and 80 µg Pt cm -2 with Pt 1.0 Fe 1.0 d and Pt 2.0 Fe 1.0 d, respectively. The ink was prepared by sonicating for 1 hour 103.2 mg of catalyst, 3.364 mL of 5 wt. % Nafion solution (in lower aliphatic alcohols and 15 wt. % water), 1.682 mL 1-propanol, and 1.403 mL deionized water. The catalyst ink was drop cast in 3 aliquots onto the 25 cm 2 gas diffusion layer, giving a total (all elements) loading of 4 mg•cm -2 , then dried at 70 C for 1 hour. The cathode was hot-pressed onto Nafion 117 with the commercial anode using a pressure of 2 tons and 135 °C for 2 minutes. The cell assembly consisted of a Fuel Cell Technologies Inc. single cell with serpentine flow fields in graphite end plates. The methanol flow rate was 50 mL min -1 . The cell temperature, cathode flow rate and absolute pressure were i) 80°C, 1 bar, 400 mL min -1 or ii) 90°C, 1.5 bar, 600 mL min -1 or iii) 100°C, 2 bars, 600 mL min -1 . Polarisation curves were recorded point by point under galvanostatic control, from low to high current densities and then vice versa. For each current density, the cell voltage obtained when increasing the current and then decreasing it was averaged. This average is used to report the polarisation curves.

Preparation of α-Fe 2 O 3 /Pt(111) surfaces:

The α-Fe 2 O 3 on Pt(111) surface was prepared by adopting the procedures described in Refs. 47,48 . The surface of the Pt(111) single crystal (MaTeck) was cleaned by cycles of sputtering (1.5 keV, 10 -6 mbar of Ar, room temperature) and annealing at 1000 K. The residual carbon was removed by annealing in 2.0•10 -7 mbar of oxygen at 900 K and, subsequently, the adsorbed oxygen was removed by flash annealing at 1000 K. This procedure was repeated until the LEED pattern of a clean Pt(111) surface with sharp spots and low background was obtained. The Fe oxide films were grown by repeated cycles of reactive evaporation of Fe onto the clean Pt(111) substrate at 2 •10 -5 mbar O 2 partial pressure and keeping the substrate at 800 K. Then, the sample was annealed at 1050 K at 4•10 -3 mbar O 2 to fully convert the Fe oxide phases to α-Fe 2 O 3 . Oxygen was introduced in the preparation chamber by means of a capillary located at only ca 1 cm from the sample.

Structural characterization of α-Fe 2 O 3 /Pt(111) surfaces:

The composition of the samples and the chemical changes induced by the exposure to the EC environment were investigated by XPS using an EA 125 Omicron electron analyzer equipped with five channeltrons, working at a base pressure of 2•10 - 10 mbar. The XPS data were collected at RT with the Mg Kα line (h = 1253.6 eV) of a nonmonochromatic dual-anode DAR400 X-ray source using 0.1 eV energy step, 0.5 s collection time and 20 eV pass energy. The binding energy (BE) scale was calibrated using a gold sample (Au 4f at 84 eV).

The film thickness was determined before and after the electrochemical treatments by angle-resolved XPS (AR-XPS) measurements using the Fe 2p and Pt 4f photoemission peak intensity, according to the method reported in Ref. 49 . Low energy electron diffraction (LEED) patterns were acquired using an incident electron beam with energy between 30 and 80 eV. All LEED patterns were recorded at room temperature.

Electrochemical studies on α-Fe 2 O 3 /Pt(111) surfaces: Cyclic voltammetry (CV) and methanol

tolerance of α-Fe 2 O 3 /Pt(111) UT films were investigated in a conventional ex situ three-electrode cell. A glassy carbon rod was used as counter electrode and a saturated calomel electrode (SCE) as reference electrode. All potentials reported are however referred to the reversible hydrogen electrode (RHE) scale, and the conversion was done according to the equation: E(RHE) = E(SCE) + 0.258 + 0.059 × pH. A custom-designed sample holder with an area of 0.502 cm 2 was used.

In Situ Electrochemical Scanning Tunneling Microscopy (EC-STM) studies on α-Fe 2 O 3 /Pt(111) surfaces:

The in situ EC-STM measurements were carried out using a home-built EC-STM at constant current mode, as described by Wilms et al. 50 . The tunneling tips were electrochemically etched from a 0.25 mm tungsten wire in 2 M KOH solution and, subsequently, cleaned in high purity water, dried and coated by passing the tip through a drop of hot glue. Platinum wires were used as counter and reference electrodes. The Pt reference electrode was calibrated vs. SCE in Ar-saturated 0.1 M HClO 4 . The image analysis was carried out using the software WSxM 5.0.

RESULTS AND DISCUSSION

Two hybrid catalysts labelled Pt 1.0 Fe 1.0 d and Pt 2.0 Fe 1.0 d were synthesized as previously reported. 46 The synthesis involves first the preparation of an Fe-N-C catalyst, Fe 1.0 d, with 1.0 standing for 1 wt% Fe before pyrolysis and d for dry ball milling. Fe 1.0 d was then impregnated by given amounts of a Pt salt to result in 1.0 or 2.0 wt % Pt in the hybrid catalyst, then dried and annealed at 560°C in 5% H 2 in N 2 (see Methods). While Pt 1.0 Fe 1.0 d and Pt 2.0 Fe 1.0 d were demonstrated to be free of metallic platinum on their surface and they achieved at best only 1/50 th the ORR-activity of Pt/C, 46 the yet unknown structure and reactivity of Pt in such materials precludes any a priori statement on their tolerance to methanol while catalyzing the ORR. Figure 1a shows no significant difference in the ORR polarisation curves measured for Pt 1.0 Fe 1.0 d in acid in the absence of methanol and after addition of 0.1 or 2.0 M methanol. This clearly indicates that both Pt in Pt 1.0 Fe 1.0 d and the Fe-based actives sites in Pt 1.0 Fe 1.0 d are inactive towards methanol oxidation up to 1.0 V vs. a reversible hydrogen electrode (RHE). For Fe-based active sites, this comes to no surprise since all other FeNC catalysts previously studied to that end showed complete methanol tolerance. 51,52 In contrast, the methanol tolerance of Pt here is different from the well-known methanol electro-oxidation activity of state-of-art Pt/C, when metallic Pt is exposed to the surface. 53,54 The present hybrid Pt/FeNC catalysts are therefore promising for application at the cathode of DMFC, being tolerant to methanol crossover from the anode to the cathode and much more durable than FeNC alone, as previously shown in PEMFC. 46 ). For a), the rotation rate was 1600 rpm. For all DMFC measurements, the anode was PtRu/C (2 mg PtRu •cm -2 ), the membrane was Nafion 117. For b), the cell temperature was 80, 90 or 100°C and the methanol concentration was 2.0 M. For c) and d), the methanol concentration was 2.0 or 5.0 M and the cell temperature was 100°C.

The effect of cell temperature was first investigated, highlighting strong improvement from 80 to 100°C (Figure 1b). The cell performance obtained with Pt 1.0 Fe 1.0 d and Pt 2.0 Fe 1.0 d is comparable, the difference lying within the reproducibility error. In particular, the cell performance at high voltage (kinetically controlled region) is very similar for both cathode catalysts, as expected from our previous work. 46 Since the content of 1 wt % Pt was shown in PEMFC to be the lower-end limit for complete durability over at least 50 h in PEMFC, 46 we then focused the DMFC measurements on the Pt 2.0 Fe 1.0 d cathode.

The latter showed complete durability for all the 50 h PEMFC tests performed at our laboratory and for different batches of Pt 2.0 Fe 1.0 d. The effect of methanol concentration at the anode feed was then investigated, and the results confirm the methanol tolerance observed in RDE, with unnoticeable change from 2 to 5 M feed (Figure 1c). For comparison, the same measurements were performed with a commercial Pt/C cathode with high Pt loading, and the results show an important decrease of cell performance from 2 to 5 M methanol feed (Figure 1d). The effect would be even more significant for lower Pt loadings in Pt/C cathodes, due to enhanced CO poisoning on regular Pt particles with surfaceexposed Pt. This operando poisoning limits the methanol concentration that can be used in DMFC systems, with a strong impact on the footprint of the methanol reservoir and implies high cathode Pt loadings. The Pt 2.0 Fe 1.0 d cathode is therefore promising to overcome those limitations. The current density at 0.3 V and 100°C with 5 M methanol feed is about 110 mA cm -2 with the Pt 2.0 Fe 1.0 d cathode (80 µg Pt cm -2 ), ca 25 % of the current density reached in the same conditions with the commercial Pt/C cathode (2000 µg Pt cm -2 ) (Figure 1c-d, magenta curves). While the absolute performance is presently lower, due to lower ORR kinetics, a high margin exists to approach the same performance. This might be achieved for example by increasing the Pt content on FeNC, if such Pt structures have moderate ORR activity. Alternatively, it might be achieved by increasing the ORR activity of the FeNC substrate. In the former approach, the increased content of Pt should however be carried out by design, leading to the synthesis of methanol-tolerant Pt structures.

The structure of Pt 2.0 Fe 1.0 d was then investigated with STEM-EDXS in order to better understand the structure of the Pt particles and why they are inactive toward methanol oxidation. In our original paper on hybrid Pt/FeNC catalysts, 46 the distribution of Pt particles on the Fe-and N-doped carbon support of Pt 1.0 Fe 1.0 d was revealed by STEM (Figure 2 and Figure S1 in Ref. 46 ). Elemental mapping with EDXS on a catalyst area comprising both the FeNC background and particles expected to be Pt-rich confirmed the strong presence of Pt, but no information on the element distribution inside the particle could be obtained due to limits intrinsic to the microscope (Figure S1 in Ref. 46 ). Here, a STEM image of a large area of Pt 2.0 Fe 1.0 d shows again large and small bright particles (Figure S1a). These particles are identified by EDXS to be Pt-rich (Figure S1b). EDXS mapping on Fe identifies both a diffuse background signal related to Fe that is atomically dispersed in the support (Fe 1.0 d) but also a localized Fe signal matching with the position of the Pt-rich particles (Figure S1b-c). While this could be interpreted as the presence of PtFe alloy particles, EDXS mapping on oxygen reveals an even better match between Fe and O than between Fe and Pt (Figure S1c-d Pt 2.0 Fe 1.0 d was then characterized with 57 Fe Mössbauer spectroscopy at 5 K. The importance of performing 57 Fe Mössbauer spectroscopy at low temperature for characterizing FeNC materials was recently put forth in several reports. 31,32 Measurements above ca 60 K lead to an ambiguous quadrupole doublet signal with low quadrupole splitting value (ca 1 mm s -1 ), that may be assigned either to specific FeN 4 sites or to nanometric Fe oxide particles. At 5 K, the latter usually become paramagnetic, leading to a sextet spectral component while FeN 4 sites still result in a doublet component. 55 The spectrum of Pt 2.0 Fe 1.0 d (Figure S3 and Table S1) comprises a number of components that had previously been observed by us in the room-temperature spectrum of H 2 -Fe 1.0 d (Figure 3b in Ref 46 ). The latter is the reference material, obtained after subjecting Fe 1.0 d to the same annealing treatment in 5%H 2 /N 2 at 560°C as applied to prepare Pt/FeNC hybrids from Fe 1.0 d in the present work. The 57 Fe Mössbauer spectrum of Pt 2.0 Fe 1.0 d comprises two quadrupole doublets D1, D2 assigned to atomically dispersed FeN 4 moieties of different spin and oxidation states (from DFT-calculated quadrupole splitting values and experimental spectroscopic characterisations of a model Fe-N-C catalyst, D1 was identified to be mainly a Fe(III)N 4 C 12 single-metal-atom site in high-spin state and D2 a Fe(II)N 4 C 10 single-metal-atom site in low-or medium-spin state), 31 a singlet assigned to -Fe and a sextet assigned to α-Fe. However, one additional sextet component (accounting for 16% of the signal, Table S1) is observed in the 57 Fe Mössbauer spectrum of Pt 2.0 Fe 1.0 d at 5 K that is absent in the room-temperature spectrum of H 2 -Fe 1.0 d, and unambiguously assigned to high-spin ferric oxide, most probably Fe 2 O 3 . This component fits well with the observation by STEM-EDXS of a nanometric Fe-oxide shell around Pt nanoparticles in Pt 2.0 Fe 1.0 d. These high resolution STEM-EDXS and low-temperature 57 Fe Mössbauer spectroscopy characterizations of Pt 2.0 Fe 1.0 d therefore suggest that a fraction of Fe from H 2 -Fe 1.0 d migrated on the surface during the reductive annealing of the Pt salt impregnated on H 2 -Fe 1.0 d, leading to the Pt@FeO x structures supported on the Fe-and N-doped carbon substrate. These novel insights explain i) why the addition of 1-2 wt% Pt to H 2 -Fe 1.0 d followed by the present annealing treatment did not lead to significant increase in the initial ORR activity relative to H 2 -Fe 1.0 d, and ii) why no CO stripping signal was observed for Pt 1.0 Fe 1.0 d. 46 While the addition of 1-2 wt % Pt to H 2 -Fe 1.0 d did not lead to an ORRactivity increase, it resulted in a spectacular stabilization during operation in PEMFC with almost no performance loss at 0.5 V and almost no activity increase or decrease at 0.8 V, even after 180 h operation (Figure 10 in Ref. 46 ). This must imply that the Pt particles remained covered by the iron oxide shell in operando, in spite of the expected leaching of ferric oxide as ferrous cations at e.g. 0.5 V in the acidic medium of PEMFCs. We assign the stability of ferric oxide on metallic Pt nanoparticles to a strong interaction between ferric oxide and Pt, leading to a stabilization of the non-PGM overlayer. It is of interest to note that Pt@Fe 2 O 3 nanoparticles are well known in the field of materials science, with potential applications in magnetic storage, catalysis and biological labelling. [56][57][58][59] These novel structural insights into the nature of Pt in Pt 1.0 Fe 1.0 d and Pt 2.0 Fe 1.0 d also raise new questions on the true reason for the observed durability of these hybrid catalysts in PEMFC. Do Pt@Fe 2 O 3 nanoparticles act as scavengers for peroxide or reactive oxygen species, protecting the FeN x active sites from deactivation or degradation? Or do they have intrinsic ORR activity that is comparable to that of FeN x active sites, leading to apparent similar ORR activities for Fe The stoichiometry of the Fe 2 O 3 ultrathin film is confirmed by the Fe 2p core-level XPS data reported in Figure S5a. A clear satellite is observed at a binding energy of 719.3 eV, which is associated with Fe 3+ species 61 . Moreover, the O 1s photoemission line (Figure S5b) exhibits only one component at 529.5 eV, indicating a pure hematite phase 61 . Lastly, the Fe/O atomic ratio determined by XPS was 2/3, as expected for pure Fe 2 O 3 . To confirm the full coverage of the Pt(111) surface by the iron oxide layer, the C 1s energy region was checked after dosing hundreds of Langmuir of CO at room temperature. The lack of any C 1s signal confirmed the absence of adsorbed CO, implying the absence of surface-exposed Pt sites. By adopting the same synthesis protocol, but reducing the amount of the evaporated Fe, it was possible to grow Fe 2 O 3 with thickness of 1.0, 1.6 nm and 6.4 nm, with analogous structural and morphological properties. This gives us therefore four model surfaces with thickness of 1.0, 1.6, 3.2 and 6.4 nm of Fe 2 O 3 on Pt(111), labelled as x-Fe 2 O 3 /Pt(111) where x is the thickness in nm. For the thinnest film of 1 nm, we verified again with XPS of C 1s , after dosing hundreds of Langmuir of CO at room temperature, that no CO adsorbed on the 1.0-Fe 2 O 3 /Pt(111) film, verifying the absence of surfaceexposed Pt (Figure S6).

The electrochemical stability of the 3.2-Fe 2 O 3 /Pt(111) film was evaluated ex situ by cyclic voltammetry (CV) in Ar-or O 2 -saturated 0.1 M HClO 4 solution using a standard three-electrode cell. Figure 3a shows the CVs measured in Ar-saturated electrolyte between 1.05 V and 0.0 V vs. RHE. The negativeand positive-going scans of the second CV (CV2) show two redox peaks at 0.75 V and 0.83 V vs. RHE, respectively. Those peaks are absent from the CV of the clean Pt(111) surface (not shown) and can therefore be associated with the Fe 3+ /Fe 2+ redox couple. Their intensity is decreasing with the number of cycles (CV4 to CV60), suggesting the progressive dissolution of the α-Fe 2 O 3 film. This interpretation is supported by XPS measurements showing a constant decrease of the ratio Fe 2p /Pt 4f from the photoemission peaks (not shown). Interestingly, the redox couple is not observed when the 3.2-Fe 2 O 3 /Pt(111) surface is cycled between 0.55 and 1.05 V vs. RHE (see Figure S7). The appearance of the redox couple centered at 0.79 V vs. RHE seems therefore conditioned by the previous occurrence of the reduction event related with the reduction peak observed at ca. 0.12 V vs. RHE in the negative-going scans (CV1-4 in Figure 3a). This is especially obvious for CV1, where no reduction peak is observed at 0.75 V vs. RHE in the first negative-going scan while the oxidation peak at 0.83 V is clearly visible in the subsequent positive-going scan. This might imply that the Fe 3+ /Fe 2+ redox couple is not intrinsic to α-Fe 2 O 3 phase itself , but stems from a reduced Fe oxide top-surface layer formed on the surface of α-Fe 2 O 3 while the potential applied is in the range of 0.12-0.0 V vs. RHE.

A very similar behavior is observed when the acid medium is saturated with oxygen. When scanning the potential in the window 1.05 -0.0 V vs. RHE (Figure 3b), the iron oxide layer is progressively dissolved as deduced from the shift of the onset potential of the ORR toward more positive values, characteristic for a bare Pt(111) surface. Analogous conclusion of identical dissolution behavior in Arand O 2 -saturated acid electrolytes can also be deduced from the analysis of the Fe 3p /Pt 4f photoemission signal, which eventually strongly decreases after several cycles in the 0.0-1.05 V potential window, regardless of the Ar-or O 2 -saturation (Figure S8a).

In order to avoid dissolution of Fe 2 O 3 in the subsequent experiments, the lower-limit potential during the scans was set to 0.45 V vs. RHE, well above the onset of the reduction peak of the Fe 2 O 3 film located at ca 0.3 V vs. RHE. The CVs were recorded until a stable response was observed, then the sample was analyzed by XPS to determine possible chemical transformations as well as to measure the Fe-oxide film thickness. No chemical changes were induced in those cycling conditions ( In order to get an insight at the atomic level into the morphology and stability of the Fe 2 O 3 /Pt(111) system, the film with a nominal thickness of 1.6 nm was investigated by in situ electrochemical scanning tunneling microscopy (EC-STM). This technique directly demonstrates the stability of the Fe 2 O 3 /Pt(111) system even at the atomic level. Figure 4 shows low-and high-resolution STM images acquired in Ar-saturated 0.1 M HClO 4 solution at 1.05 V vs. RHE. The Fe 2 O 3 layer fully covers the Pt(111) surface, forming flat terraces characterized by an average surface roughness (R a ) and root mean square (RMS) roughness (see supporting information for details) for of ~ 0.9 and ~ 1.3 Å, respectively, where a primitive hexagonal unit cell, with the lattice parameter of can be clearly identified (Figure 4b).

Figure S9 reports a series of potentiodynamic EC-STM images starting from 1.05 V down to 0.45 V vs.

RHE. The red arrow on all panels indicates the same position on the surface. The electrode potential was scanned in the cathodic direction without observing any significant change on the surface (Figure S9b-c). Then, the potential was scanned back up to 1.05 V vs. RHE (Figure S9d-f). No changes in the morphology were observed in this potential window (0.45-1.05 V vs. RHE). Moreover, no differences were observed after 24 h in 0.1 M HClO 4 at 0.6 V vs. RHE and 100 cycles between 1.05 V and 0.45 V vs. RHE (Figure S10a) which is confirmed by the R a and RMS roughness analysis (Table S2). No Pt features were observed also in the CV in the region of 0.45 to 1.05 V vs. RHE, after the same protocol (Figure S10b), indicating that no Pt surface was exposed even after potential hold and potential cycling in this potential range. Overall, the experimental data suggest that in the potential window 0.45 -1.05 V vs. RHE, the ultrathin hematite film is highly stable in acid conditions. The origin of this high stability is

not clear yet, and several phenomena could be involved. Recent DFT calculations ascribed the increased stability in alkaline conditions of NiO x ultrathin films on Pt(111) surface with respect to the bulk counterpart to the strong adhesion energy with the underlying metal 62 . From calculated Pourbaix diagrams, a monolayer of NiOOH on Pt(111) was predicted to be stable down to a potential of ca 0.65 V vs. RHE while the bulk NiOOH phase was unstable already below 1.4 V vs. RHE. 62 Similar strong stabilization was also calculated for Mn-and Co-oxy(hydroxides) on Pt(111) vs. the bulk oxy(hydroxides). 62 The magnitude of stabilization predicted for these metal-oxy(hydroxides) films on Pt(111) are compatible with the stability down to 0.45 V vs. RHE for Fe 2 O 3 thin film on Pt(111) observed in this work. In addition, the peculiar electronic properties of ultrathin films and changes in the electronic structure at the interface between Fe 2 O 3 and Pt(111) can play a role. In particular, the Pt 5d band strongly hybridizes with Fe 3d and O 2p states at the interface and even in the inner layers 63,64 , which can influence the stability of the films. 111) is ca only 80 mV negative compared to that of clean Pt(111). In terms of ORR activity at 0.9 V vs. RHE, the 1.6 and 1.0 nm Fe-oxide layers on Pt(111) reach ca 1/70 th and 1/8 th the ORR activity of Pt(111), respectively. This is because although the metal surface is fully covered by α-Fe 2 O 3 , even for the 1.0 nm thick film, the electrons from the Pt metallic substrate can tunnel through the oxide layer and catalyze the oxygen reduction on the electrode surface [65][66][67] . As the oxide thickness increases, tunneling is progressively suppressed and the top surface then shows catalytic properties similar to the bulk phase of the overlayer, with little to no ORR activity expected for bulk Fe 2 O 3 in acidic medium. Figure 5b and S11 better show the correlation between Fe 2 O 3 layer thickness and ORR activity at 0.8 and 0.9 V vs. RHE, respectively. The lower potential of 0.8 V allows a more accurate reading of ORR activity for thick Fe 2 O 3 overlayer, while 0.9 V is more convenient to compare to the ORR activity measured for a bare Pt(111) surface. In both cases, the experimental data of ORR activity vs. Fe 2 O 3 thickness could be fitted with an exponentially-decaying law, supporting the electron tunneling hypothesis.

These results suggest that the observed ORR activity in the Pt/FeNC powder catalysts may arise in part from Pt@FeO x nanostructures and not only from FeN x centers. To estimate the activity that may be expected from Pt@Fe 2 O 3 nanostructures in the Pt 2.0 Fe S1, 16% Fe as Fe-oxide). The Pt core radius of 4 nm is also in line with the most frequent particle size of ca 8 nm reported by us previously for these Pt/FeNC materials (Figure 2 of Ref. 46 ). The expected contribution to the ORR activity at 0.8 V vs. RHE is then 0.71 mA•cm -2 (Table S3).

This is significantly lower than the total ORR activity of Pt 2.0 Fe 1.0 d in 0.1 M HClO 4 at 0.8 V vs. RHE (ca 6.8 mA cm -2 , Figure 6a in Ref. 46 ), and seems to support the stabilisation of FeN x ORR active sites by less ORR-active Pt@Fe 2 O 3 particles. The calculation is however highly sensitive to the assumed i s value, and the conclusion would be changed assuming the i s -value measured for 1 nm Fe 2 O 3 thickness (0.4 mA•cm -2 ). Future work with better-defined materials, in particular with more uniform Pt particle size in such Pt/FeNC composites, will thus be necessary to strengthen this conclusion, since the calculations also show that the assumed Pt core radius strongly affects the ORR activity contribution from Pt@Fe 2 O 3 particles for a fixed Pt content of 2 wt% (Table S3).

Finally, the methanol tolerance of the 1.0-Fe 2 O 3 /Pt(111) film was investigated using our model systems. This is also supported by the nearly superimposed negative-going scans in O 2 -saturated 0.1 M HClO 4 , with or without 50 mM methanol (orange and red curves in Figure 5c). Such a result is in line with the lack of methanol oxidation activity seen for Pt 1.0 Fe 1.0 d (Figure 1a). In contrast, the positive-going scan of Pt(111) in 50 mM methanol in O 2 -saturated 0.1 M HClO 4 shows methanol oxidation onset at ca 0.5

V vs. RHE and the sign of the current density even becomes positive ca 0.7 V vs. RHE indicating that the methanol oxidation current is larger than the oxygen reduction current. The methanol activity of Pt( 111) is also seen in the difference between the negative-going scans in O 2 -saturated 0.1 M HClO 4 , with or without 50 mM methanol (black and grey curves in Figure 5c). In summary, the 1.0-Fe 2 O 3 /Pt(111) surface is methanol tolerant, revealing that the 1 nm thick oxide surface on top pf Pt(111) imparts methanol tolerance, while reducing the ORR activity relative to a bare Pt(111) surface. A similar enhancement of the poisoning tolerance imparted by ultrathin films has previously been observed for the Co 3 O 4 /CoO/Pd(1oo) system for ORR in alkaline conditions 65 .

We note that the overall structural and electrochemical characterization results of the present study bear a strong analogy with a recent report from Gasteiger's group on a powder catalyst consisting of Pt nanoparticles deposited on TiO x , themselves supported on carbon black and finally subjected to a thermal treatment at 400°C in 5% H 2 /Ar (labelled Pt/TiO x /C 400°C, H2 in Ref. 68 ). The ORR activity normalized by the Pt mass was 50 times lower for Pt/TiO x /C 400°C, H2 compared to a reference Pt/C material with same Pt particle size. The authors identified Pt@TiO x core-shell particles with HR-TEM for particles of 6 nm and larger, while the detailed structure of smaller particles could not be accessed due to resolution limits. As is the case here and in our previous report, 46 no CO stripping and no H upd peaks could be observed with Pt/TiO x /C 400°C, H2 . While the tolerance to methanol was not verified in Ref. 68 , it can be expected that Pt@TiO x particles would also not oxidize methanol. While the authors reported much lower ORR activity for Pt/TiO x /C than for Pt/C, comparable activities were observed for the hydrogen oxidation reaction (HOR) in acidic and alkaline medium. 68 This behavior is, again, strongly analogous to that of Pt@FeO x core-shell particles, with high HOR activity in acidic medium (Figure 4 in Ref. 69 , HOR polarisation curves labelled D and E corresponding to the catalysts Pt 1.0 Fe 1.0 d and Pt 2.0 Fe 1.0 d, respectively). The authors in Ref. 68 proposed that the strongly suppressed ORR activity but retained HOR activity for their Pt@TiO x particles relative to Pt particles may be explained by the fact that only the Pt core is electrochemically active and O 2 (or H 2 ) must diffuse through the TiO x overlayer in order to react. The electrochemical results obtained in the present study on model Fe 2 0 3 /Pt(111) surfaces however challenge this hypothesis. First, if the Pt core had not modified the electrochemical properties of the Fe-oxide overlayer, the latter should have dissolved very quickly in acidic medium. Second, if the diffusion rate of O 2 and H 2 through the Fe-oxide layer controlled the ORR and HOR activity, respectively, the results should have shown linear increase of electrochemical activities with decreasing FeO x overlayer thickness. However, the present study reveals an exponential increase of ORR activity with decreasing FeO x overlayer thickness. This supports an electron tunneling effect from the Pt core through the Fe-oxide layer, the electron tunneling improving both the stability and electrochemical activity of the top surface Fe-oxide.

More theoretical and experimental work is needed however to better understand the relation between the ORR and/or HOR activity of Pt@MO x core-shell systems and the thickness of the metal-oxide shell (e.g. Pt@FeO x and Pt@TiO x core-shell systems) and why the methanol oxidation reaction is suppressed altogether. Last, while the model Fe 2 0 3 /Pt(111) surfaces reproduce well the general stability and reactivity trends of Pt@FeO x core-shell particles in the hybrid Pt/FeNC catalysts, a notable difference exists in the lower potential limit for stability. While it is 0.45 V vs. RHE for the model surfaces, it is downshifted to at least 0.05 V vs. RHE for Pt@FeO x particles in Pt/FeNC catalysts. The latter is deduced from the break-in procedure used in Ref. 46 for all RDE measurements, consisting of 20 CVs between 0.05 and 1.1 V vs. RHE. Since this break-in procedure did not remove the Fe-oxide layer (no H upd signal characteristic of surface-exposed Pt was observed after the break-in), it can be concluded that the FeO x overlayer in Pt/FeNC is stable down to this low potential. This different behaviour between model and real catalysts may be due to the presence of different Pt surfaces than Pt(111), or to strain effects improving the stability of FeO x overlayer in the powder catalysts. Support for a much stronger interaction between FeO x overlayer and Pt edge sites as well as Pt(100) surface relative to that

  Pt 2.0 Fe 1.0 d: We used a Pine MSR Electrode Rotator with a RDE/RRDE precision shaft and glassy carbon tip. The ink with 10 mg catalyst, 108.4 µL Nafion solution (5wt %, Sigma Aldrich), 300 µL ethanol and 36.5 µL ultrapure water (> 18 M Ω) was prepared.

  Pt 1.0 Fe 1.0 d and Pt 2.0 Fe 1.0 d were therefore evaluated at the cathode of DMFC.

Figure 1 .

 1 Figure 1. Electrochemical characterisation of hybrid Pt/FeNC catalysts and comparison to a commercial

  Figure 2dfurther identify the Fe and O shell, extending beyond the Pt core. This Pt@FeO x structure

Figure 2 .

 2 Figure 2. STEM-EDXS characterization of a single Pt-rich particle in Pt 2.0 Fe 1.0 d. a) HAADF and b)

  1.0 d, H 2 -Fe 1.0 d, Pt 1.0 Fe 1.0 d and Pt 2.0 Fe 1.0 d? To disentangle these different possibilities, we performed studies on model flat surfaces of Fe 2 O 3 layers of controlled thickness grown on Pt(111).

Fe 2 O 3 /

 23 Pt(111) model surfaces were prepared according to a modified version of the procedure described by Freund's group.48,60 Briefly, iron atoms were evaporated at constant rate on the clean Pt(111) surface in an atmosphere of 2•10 -5 mbar O 2 at 800 K. To improve the crystalline order and guarantee complete oxidation, a post annealing at 1050 K in 4•10 -3 mbar O 2 for 45 minutes was necessary. The atomic structure and chemical composition of the resulting Fe 2 O 3 ultrathin films were investigated by low energy electron diffraction (LEED) and X-ray photoemission spectroscopy (XPS).

Figure

  S4 reports the LEED patterns of the clean Pt(111) substrate and of Fe 2 O 3 /Pt(111) ultrathin film with an average thickness of 3.2 nm, as determined by angle-resolved(AR)-XPS measurements. The diffraction pattern corresponds to a α-Fe 2 O 3 (0001)-(1 x 1) surface with a hexagonal unit cell with a lattice constant of 5.0Å that is rotated by 30° with respect to the unit cell of the Pt(111) surface.47

Figure 3 .

 3 Figure 3. Cyclic voltammograms of the 3.2-Fe 2 O 3 /Pt(111) film in (a) Ar-saturated and (b-c) O 2saturated 0.1 M HClO 4 solution at a scan rate of 20 mV•s -1 between (a-b) 1.05 and 0.0 V vs. RHE and

13 cycles between 1 .

 1 05 and 0.45 V vs. RHE in O 2 -saturated 0.1 M HClO 4 , Figure S8b, curve 4) and the thickness of the iron oxide film after the cycling was estimated to be 92 ± 4% of the initial value. These results indicate that the Pt substrate stabilizes the α-Fe 2 O 3 thin film in acidic medium in the potential region 0.45 -1.05 V vs. RHE.

Figure 4 .

 4 Figure 4. Low-(a) and high-resolution EC-STM images (b) of 1.6-Fe 2 O 3 /Pt(111) in Ar-saturated 0.1 M

Figure 5 . 1 MFe 2 O 3 /

 5123 Figure 5. a) Comparison of CVs for the different thickness of Fe 2 O 3 /Pt(111) in O 2 -saturated 0.1 M

  CVs were recorded for Pt(111) and 1.0-Fe 2 O 3 /Pt(111) in O 2 -saturated 0.1 M HClO 4 electrolyte (Figure5c) before and after adding the amount of methanol necessary to reach a concentration of 50 mM. Due to the absence of convection of the electrolyte and semi-infinite diffusion mechanisms, the negativegoing scans identify best the ORR activity (low methanol concentration at the electrode surface, if the surface is methanol-active) while the subsequent positive-going scans identify best the tolerance to methanol (O 2 concentration at the electrode surface decreased during the negative going scan via ORR, while the methanol concentration was replenished at low potentials). The positive-going scan of the 1.0-Fe 2 O 3 /Pt(111) surface in 50 mM methanol in O 2 -saturated 0.1 M HClO 4 shows no features of methanol oxidation indicating complete tolerance to methanol (i.e. no activity for methanol oxidation reaction).

  1.0 d powder (J Fe2O3 ), one needs to estimate the surface specific activity of Fe 2 O 3 (i s ) and the surface area of Fe 2 O 3 in the Pt/FeNC layer per geometric area of glassy carbon (S). The value of i s may be estimated from the Faradaic current density observed at 0.8 V vs. RHE in Figure5a, red curve (0.065 mA•cm -2 ), corresponding to the 1.6 nm thick film. The latter is close to the Fe-oxide thicknesses observed in Pt 2.0 Fe 1.0 d. Then, the surface area of Fe 2 O 3 in the RDE active layer can be estimated assuming that the Pt cores have a single diameter, r (assumed between 2.5 and 4.0 nm), and the Fe-oxide shells have a fixed thickness of 1.6 nm (see supporting text).The calculations lead to S-values between ca 11 and 24 cm 2 Fe 2 O 3 per cm 2 geometric area for r = 4 and 2.5 nm, respectively. The former radius seems to better represent Pt 2.0 Fe 1.0 d, leading to ca 0.6 wt % Fe being present as Fe 2 O 3 (ca 20 % relative to all Fe), in line with the57 Fe Mössbauer spectroscopy analysis (Table

Funding Sources

The research leading to these results has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 779366 (CRESCENDO). This joint undertaking receives support from the European Union's Horizon 2020 research and innovation programme, Hydrogen Europe and Hydrogen Europe Research. F.J. also acknowledges financial support from AxLR. T.K., S.A. and G.G. also acknowledge financial support from the Italian MIUR (PRIN 2015: SMARTNESS, 2015K7FZLH; PRIN2017: Multi-e, 20179337R7). G. D. acknowledges the financial support from Slovenian Research Agency (P2-0393).

for Pt(111) was recently reported by Wen et al. 70 Upon formation of FeO x by atomic layer deposition (ALD) on Pt nanoparticles, it was observed that the CO adsorption signal characteristic for CO/Pt(edge) decreased most strongly and followed closely by CO/Pt(110), while the signal for CO/Pt(111) remained intense even after several ALD cycles. This suggests preferential deposition of FeO x on Pt edge sites and Pt(100) than on Pt(111). This in turn suggests stronger interaction between FeO x and such Pt sites compared to Pt(111). This is in line with the stability observed down to lower potential for the powder Pt/FeNC catalyst compared to Fe 2 O 3 model thin-film on Pt(111).

CONCLUSIONS

The electrochemical results on the Pt/FeNC powder catalysts with 1-2 wt % Pt identify complete tolerance to methanol while catalyzing ORR in acidic medium. DMFC polarisation curves with Pt/FeNC cathodes comprising only 40-80 µg Pt cm -2 showed promising power performance compared to state-of-art Pt/C cathodes with 2000 µg Pt cm -2 , allowing the utilization of high methanol concentration (5 M) at the anode without any drawback. STEM-EDXS characterization of the Pt/FeNC material with 2 wt % Pt identified the encapsulation of Pt nanoparticles by a thin Fe-oxide shell (on average, 1.3 nm), i.e. Pt@FeO x core-shell structures. 57 Fe Mössbauer spectroscopy at low temperature identified the coexistence of Pt@FeO x particles with FeN x sites. In order to understand the ORR activity and stability of Pt@FeO x particles, ultrathin films of Fe 2 O 3 on Pt(111) were prepared. It was shown that the Fe-oxide layer is stable in acidic medium in the potential range 0.45 -1.05 V vs. RHE, revealing a stabilization of the Fe-oxide by subsurface Pt(111). While Fe 2 O 3 overlayers showed similar stability for different Fe 2 O 3 thicknesses, the ORR activity increased exponentially with decreasing thickness. For example, the ORR activities at 0.9 V vs. RHE of the 1.6 and 1.0 nm Fe-oxide layers on Pt(111) were ca 1/70 th and 1/8 th that of Pt(111), respectively. Both the stability and ORR activity of Fe 2 O 3 can be explained by electron-tunneling effect from Pt(111) through the ultrathin Fe-oxide shell. In addition, it is demonstrated that the deposition of even only 1 nm Fe 2 O 3 on Pt(111) renders the surface fully tolerant to methanol.

From a practical viewpoint for DMFC or other direct alcohol fuel cells cathodes, the results show that Fe-oxide@Pt core particles are promising as i) methanol-tolerant, ii) stable and ORR-active particles in Pt/Fe-N-C hybrid catalysts and iii) as particles stabilizing neighboring FeN x sites. In addition, Feoxide@Pt particles could be promising HOR catalyst for PEMFC anodes, with improved tolerance to many species known to poison exposed Pt nanoparticles.

ASSOCIATED CONTENT

Supporting Information. Additional STEM-EDXS characterizations of Pt 2.0 Fe 1.0 d; 57

AUTHOR INFORMATION

Corresponding Authors *frederic.jaouen@umontpellier.fr, stefano.agnoli@unipd.it.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ABBREVIATIONS

PEMFC, proton exchange membrane fuel cell; DMFC, direct methanol fuel cell; AEM, anion exchange membrane; ORR, oxygen reduction reaction; PGM, platinum group metal; STEM, scanning transmission electron microscopy; EDXS, energy dispersive x-ray spectroscopy; EC-STM, electrochemical scanning tunneling microscopy; CV, cyclic voltammetry; LEED, low energy electron diffraction; XPS, X-ray photoemission spectroscopy; HAADF, high-angle annular dark-field; RHE, reversible hydrogen electrode; SCE, saturated calomel electrode; BE, binding energy.


Plenty of pitches

Fanny Gribenski, Max Planck Institute for the History of Science, fgribenski@mpiwgberlin.mpg.de

The note A tuned to 440 hertz only became the norm for musical performance in 1939 after decades of international and interdisciplinary disputes. Fanny Gribenski retraces the rocky path towards the adoption of this unified measure.

Though music is often referred to as "the universal language of mankind," in the words of American poet Henry Wadsworth Longfellow, for the longest time, the practice of this art was founded on local standards. Now regarded as "natural," the note A tuned to 440 hertz only became the norm for musical performance following an international conference held in London in 1939. For most of music history, pitches were fluctuating concepts: countries, cities, and individual musical institutions performed music according to their own tones. But with the development of railroads and the resulting integration of musical spaces, and in the context of a rise of experimental acoustics, many voices called for the adoption of a unified musical measure.

France was the first country to create such a standard. In 1859, a state-appointed commission gathering representatives of the worlds of music and science fixed a diapason normal at A 435. In the same way as a standard metre had been agreed upon after the revolutionary standardization of the metric system, a model of the musical standard -a tuning fork -was stored at the French Conservatory (pictured). The "French pitch" was subsequently chosen as a first international standard in Vienna in 1885 by several European countries and Russia. But in the interwar period the United States adopted A 440 as standard, which was agreed on by certain European nations in 1939, and became the first acoustic norm issued by the International Standards Organization in 1955 as ISO 16.

Although resembling that of other technical and scientific standards, the history of concert pitch is more complex. Crucially, whereas the metre was principally the product of industrial factories and physics laboratories, pitch was also the concern of opera houses and concert halls, churches and radio studios, instrument workshops and music schools. For other scientific and technical standards, it was physicists, mathematicians, engineers, and natural philosophers who determined at what measure these were set. In the case of pitch, such authorities were challenged by a diverse body of interested parties -whether composers, performers, piano makers or choir conductors -all with different ideas of what a standard pitch should be. Shaping these ideas were not only considerations about the physics of sound, but also conflicting notions of what sounded aesthetically pleasing, what was physiologically sustainable, and what was historically consistent with the works of celebrated composers.

Whereas in France, the diapason normal was fixed in reference to a historical canon of operas with the commission claiming that the choice of A 435 "would render the performance of old masterpieces easier," 1 these aesthetic considerations were challenged in other countries. In Britain, for instance, in the wake of the French decision, the Society of Arts started investigating "how far it would be practicable to do anything in this country in reference to it," 2 but with the aim of putting a greater emphasis on science. Drawing from the recommendations made by natural philosophers such as John Herschel the body created a competing standard in 1860, presented as a compromise between mathematical knowledge and contemporary musical practice: C 528. Following the advice of Giuseppe Verdi, the Italian association of musicians similarly adopted A 432 as standard in 1884, reconciling the demands of science and musical practice. Such a low pitch, it was argued, was more friendly to voices. The following year, however, this measure was defeated by the French pitch A 435 at the conference held in Vienna because of the commercial strength of the diapason normal, which had already been adopted by several acclaimed instrument makers. Eventually, the French pitch was superseded by the higher pitch A 440. This Concert pitch still remains a controversial standard. For several years, numerous blogs have been denouncing the use of A 440 and calling for its replacement by what they regard as the pitch of the "origins": A 432. Following in the footsteps of activist Lyndon LaRouche who, at the end of the 1980s, claimed that one should "revive Verdi's tuning [A 432] to bring back great music,"3 the Dutch political party Vrijzinnige Partij argues for A 432 as a standard on the unverified grounds that Joseph Goebbels was responsible for the adoption of A 440, and that it causes "disarray" in music and society. [START_REF] Korteweg | Vrijzinnige Partij: verlaag de grondtoon, deze wekt verdeeldheid en agressie op[END_REF] The history of our musical measure is actually one of many pitches, attesting to the great variety of sonic ideas and practices throughout the modern era.

Picture:

  standard, already chosen by a congress of natural scientists in Stuttgart in 1834, had been introduced in the United States in 1917, and enforced by means of a radio signal emitted by the Bureau of Standards in 1935. 1 Rapport et Arrêtés pour l'établissement en France d'un diapason musical uniforme (Paris: Imprimerie impériale, 1859), Archives nationales, F/21/768, 24-25. 2 Minutes of the Council, 122 (May 25 th , 1859), Royal Society of Arts, AD/MA/100/12/02/11.

Lyndon LaRouche, "Revive Verdi's Tuning to Bring Back Great Music," Executive Intelligence Review 15, no. 32 (1988): 24-34.

Ariejan Korteweg, "Vrijzinnige Partij: verlaag de grondtoon, deze wekt verdeeldheid en agressie op," Volkskrant, March 3, 2017, https://www.volkskrant.nl/nieuws-achtergrond/vrijzinnige-partij-verlaag-degrondtoon-deze-wekt-verdeeldheid-en-agressie-op~b718e98c/