Johannes Fichtner

Sebastian Watzele

Batyr Garlyyev

Regina M Kluge

Felix Haimerl

Hany A El-Sayed

Wei-Jin Li

Frédéric M Maillard

Laetitia Dubau

Raphaël Chattot

Jan Michalička

Jan M Macak

Wu Wang

Di Wang

Thomas Gigl

Christoph Hugenschmidt

Aliaksandr S Bandarenka

Tailoring the Oxygen Reduction Activity of Pt Nanoparticles through Surface Defects: A Simple Top-Down Approach

Keywords: Electrocatalysis, Fuel Cell, Oxygen Reduction, Platinum Nanoparticles, Top-Down Synthesis

come   L'archive ouverte pluridisciplinaire

Introduction

With the increased awareness of preventing the climate change, efforts are made to decouple our society's energy demands from the so-called carbon cycle. One promising approach is to reduce emissions caused by automotive combustion engines, for instance by replacing this technology with electric motors that are powered by either batteries and/or hydrogen fuel cells. [START_REF] Thomas | Fuel Cell and Battery Electric Vehicles Compared[END_REF][START_REF] Pellow | Hydrogen or Batteries for Grid Storage? A Net Energy Analysis[END_REF] Even though the market launch of hydrogen fuel cell driven cars is on the rise, further cost reductions are required to make the technology commercially more competitive. Today, anode and cathode catalyst layer of proton exchange membrane fuel cells (PEMFCs) typically consist of Pt-based nanostructures supported on high surface area carbon (Pt/C). [START_REF] Yoshida | Toyota MIRAI Fuel Cell Vehicle and Progress Toward a Future Hydrogen Society[END_REF] While the hydrogen oxidation reaction taking place at the anode side of the cell is a rapid process, the oxygen reduction reaction (ORR) at the cathode side exhibits sluggish kinetics. In order to improve the activity of such catalysts, two approaches are typically pursued. One way is to alloy Pt nanoparticles with less precious transition metal elements (e.g., Co, Ni, Cu) or lanthanides (e.g., Pr), [START_REF] Chen | Origin of Oxygen Reduction Reaction Activity on "Pt 3 Co" Nanoparticles: Atomically Resolved Chemical Compositions and Structures[END_REF][START_REF] Oezaslan | PtCu 3 , PtCu and Pt 3 Cu Alloy Nanoparticle Electrocatalysts for Oxygen Reduction Reaction in Alkaline and Acidic Media[END_REF][START_REF] Garlyyev | High Oxygen Reduction Reaction Activity of Pt 5 Pr Electrodes in Acidic Media[END_REF][START_REF] Stephens | Understanding the Electrocatalysis of Oxygen Reduction on Platinum and its Alloys[END_REF] which alters the electronic properties of the catalyst surface by a combination of so-called "strain" and "ligand" effects. This weakens the strong surface-intermediate interactions and allows to optimize electron transfer. [START_REF] Kitchin | Role of Strain and Ligand Effects in the Modification of the Electronic and Chemical Properties of Bimetallic Surfaces[END_REF][START_REF] Bligaard | Ligand Effects in Heterogeneous Catalysis and Electrochemistry[END_REF] In particular, both Pt 3 Ni bulk and nanoscale alloy structures notably exceeded the activity of pure Pt. [START_REF] Stamenkovic | Improved Oxygen Reduction Activity on Pt 3 Ni(111) via Increased Surface Site Availability[END_REF][START_REF] Zhang | Synthesis and Oxygen Reduction Activity of Shape-Controlled Pt 3 Ni Nanopolyhedra[END_REF][START_REF] Mi | Porous Pt 3 Ni with Enhanced Activity and Durability Towards Oxygen Reduction Reaction[END_REF] However, due to the dissolution of the solute metal during PEMFC operation (commonly known as "dealloying"), the membrane electrode assembly can get poisoned, resulting in poor long term performance. [START_REF] Durst | Impact of Metal Cations on the Electrocatalytic Properties of Pt/C Nanoparticles at Multiple Phase Interfaces[END_REF] Another promising approach is to directly alter the catalytic properties of Pt nanoparticles by the modification of their surface structure. Exemplary, tailoring the size of the nanoparticles and the design of unusual catalyst shapes with enhanced surface area (e.g., hollow nanoparticles, aerogels) can be mentioned. [START_REF] Garlyyev | Optimizing the Size of Platinum Nanoparticles for Enhanced Mass Activity in the Electrochemical Oxygen Reduction Reaction[END_REF][START_REF] Dubau | Tuning the Performance and the Stability of Porous Hollow PtNi/C Nanostructures for the Oxygen Reduction Reaction[END_REF][START_REF] Henning | Pt-Ni Aerogels as Unsupported Electrocatalysts for the Oxygen Reduction Reaction[END_REF] Importantly, in such cases the increased activity towards the ORR can, amongst others, also be attributed to the generation of highly active defect-like surface sites. In particular, Calle-Vallejo et al. demonstrated that surface concavities on Pt significantly improve ORR kinetics. [START_REF] Calle-Vallejo | Why Conclusions from Platinum Model Surfaces Do Not Necessarily Lead to Enhanced Nanoparticle Catalysts for the Oxygen Reduction Reaction[END_REF][START_REF] Calle-Vallejo | Finding Optimal Surface Sites on Heterogeneous Catalysts by Counting Nearest Neighbors[END_REF] This is due to the optimized geometric structure at the concave sites, as quantified by the so-called generalized coordination number ( ) approach. Particularly 𝐶𝑁 high values have been predicted for e.g., hollow or coalescing nanoparticles at their 𝐶𝑁 boundary, confirming aforementioned experimental findings. Furthermore, the activity of concave-shaped nanocubes was experimentally demonstrated by Yu et al., [START_REF] Yu | Platinum Concave Nanocubes with High-Index Facets and Their Enhanced Activity for Oxygen Reduction Reaction[END_REF] but the degree of negative curvature was not very pronounced and the size of the concave nanocubes was larger than 20 nm, which results in low mass activity. Furthermore, development of a simple production method for such concave nanoparticles often is a non-trivial endeavor. Current research mainly focuses on the synthesis of such catalysts using conventional bottom-up techniques, which typically suffer from complex and timeconsuming multiple-step procedures. [START_REF] Chen | Platinum-Based Nanostructured Materials: Synthesis, Properties, and Applications[END_REF] Hence, the development of a facile synthetic method for Pt nanostructures, which contain these active sites, is a crucial step towards the commercial realization of PEMFC-driven cars on a large scale. Here, we take advantage of a novel top-down synthetic route, in order to prepare Pt/C catalysts with a high degree of surface defects. This top-down approach, which was initially discovered decades prior to the beginnings of nanotechnology and introduced as atomization of metal cathodes, [START_REF] Bredig | Ueber Zerstäubung von Metallkathoden bei der Elektrolyse mit Gleichstrom[END_REF] uses high alternating anodic and cathodic biases applied to metal wires immersed in an alkaline electrolyte, in order to produce nanoparticles at the electrode 6 surface and subsequently release them into the electrolyte. Fundamental experiments on the nanoparticle synthesis by this approach were provided by Koper's, Li's and Bandarenka's groups in recent years and extended to different types of noble and nonnoble metals as well as alloys and oxide materials. [START_REF] Yanson | Cathodic Corrosion: A Quick, Clean, and Versatile Method for the Synthesis of Metallic Nanoparticles[END_REF][START_REF] Feng | Cathodic Corrosion of a Bulk Wire to Nonaggregated Functional Nanocrystals and Nanoalloys[END_REF][START_REF] Huang | Facile Preparation of Pt Hydrosols by Dispersing Bulk Pt with Potential Perturbations[END_REF][START_REF] Chen | Facile Preparation of Bi Nanoparticles by Novel Cathodic Dispersion of Bulk Bismuth Electrodes[END_REF][START_REF] Fichtner | Top-Down Synthesis of Nanostructured Platinum-Lanthanide Alloy Oxygen Reduction Reaction Catalysts: Pt x Pr/C as an Example[END_REF] With the help of highresolution transmission electron microscopy (HR-TEM), high-resolution scanning transmission electron microscopy with a high-angle annular detector (HR-STEM-HAADF), STEM-HAADF tomography visualization, Rietvield refinement of wide-angle Xray scattering (WAXS), electrochemical techniques and positron annihilation spectroscopy (PAS), we provide evidence that the top-down synthesis leads to the formation of Pt nanoparticles with rough, disordered surfaces. In particular, this includes surface concavities. Furthermore, utilizing the rotating disk electrode (RDE) technique and single-cell PEMFC measurements, we show that Pt/C produced by this top-down approach (denoted as Pt/C TD in the following) is indeed highly active towards the ORR.

Experimental Section

Synthesis: In a typical synthesis of Pt/C TD , 20 mg of Vulcan XC72R active carbon (Cabot, USA) were dispersed in 25 ml of a 1 M KOH (85%, Grüssing, Germany) solution. To improve the wettability of the carbon support, ~100 µl of ethanol (puriss, >99.8%, Sigma Aldrich, Germany) were added to the suspension, followed by sonication for 15 min.

Subsequently, two Pt wires (99,999%, Goodfellow, Germany) with a diameter of ~0.2 mm were connected as electrodes and immersed in the suspension under vigorous stirring.

To produce Pt nanoparticles, a sinusoidal potential of ±10 V vs Pt pseudo-reference (frequency: 200 Hz) was applied, leading to the erosion of the Pt wires. After generation of the demanded Pt weight fraction, the suspension was stirred for 12h at room temperature, followed by filtering, washing with ultrapure water (Evoqua, Germany) and drying in the oven at 60°C. As a reference catalyst, TEC10V20E Pt/C (19.6 wt.%) from Tanaka Kikinzoku Kogyo (Pt/C TKK ) was used without further modification.

Pt on carbon weight loading:

The weight fraction of Pt TD nanoparticles on carbon was determined using TGA. The measurements were carried out with a METTLER TOLEDO TG-MS coupling instrument with an air flow of 50 ml/min and heating speed of 50 K/min to 1100°C. Prior to ramping the temperature, the sample was dried at ~135°C, followed by determination of its initial weight. Before cooling down, the sample was kept at 1100°C for 50 minutes until no further weight loss could be observed. The mass was compared after cooling down to 135°C.

Electron microscopy:

For TEM imaging of the Pt/C TD electrocatalyst, a Philips CM100 EM, operated at 100 kV, was used. The samples were prepared by sonicating the catalyst powder in isopropanol, followed by drop-casting the dispersion on formvar-supported carbon-coated Cu400 TEM grids (Science Services, Germany

Membrane electrode assembly measurements:

Single-cell PEMFC measurements were conducted in a home-made cell setup. Fuel cell performance was evaluated on a Greenlight Innovation G60 test station (Greenlight Innovation Corp., Canada), coupled with a Gamry reference 3000 potentiostat (Gamry Instruments, USA). 5 cm 2 catalyst layers were prepared using the decal transfer method, i.e., by rod coating on a PTFE decal, followed by drying at room temperature. As anode catalyst, commercial Pt/C TKK (TEC10V20E) with an anode loading of 0.08 mg Pt /cm 2 was used. As cathode catalyst, Pt/C TD with a cathode loading of 0.1 mg Pt /cm 2 was utilized. Synthesis of Pt/C TD electrocatalyst was conducted as described in the previous sections, however, the batch size was increased from ~20 mg to ~100 mg of Pt/C TD . For ink preparation, the catalyst was dispersed in a mixture of 1-propanol and water (~10 wt%) and an ionomer (Nafion, 5% wt in lower aliphatic alcohols and water, Sigma Aldrich, Germany ) to carbon (I/C) ratio of 0.65 was adjusted. The ink was mixed on a roller mill for 18 h at room temperature, using ZrO 2 beads. Anode, 15 μm membrane and cathode were hot-pressed for 3 min at 155°C and 4 kN. A Freudenberg H14C7 (Freudenberg & Co. KG, Germany) served as the gas diffusion layer. To record CO-stripping voltammograms, the cell was purged with 10% CO in N 2 for ~10 min at a constant potential of 0.1 V, followed by purging with pure N 2 to remove excess of CO . The voltammograms were recorded at a scan rate of 100 mV/s. Differential flow H 2 /O 2 (2000 nccm/5000 nccm) polarization curves were recorded at 80°C, 170 kPa abs and 100% relative humidity. The ohmic resistance was determined via electrochemical impedance spectroscopy. The H 2 crossover was determined in H 2 /N 2 atmosphere.

Positron annihilation spectroscopy:

For the present study, coincident Doppler broadening spectroscopy (CDBS) was performed at the CDB spectrometer using the mono-energetic positron beam at NEPOMUC located at the research neutron source Heinz Maier-Leibnitz (FRM II). [START_REF] Hugenschmidt | Unprecedented Intensity of a Low-Energy Positron Beam[END_REF] The positron implantation energy was varied between 0.10 and 30 keV. [START_REF] Gigl | Defect Imaging and Detection of Precipitates Using a New Scanning Positron Microbeam[END_REF][START_REF] Hugenschmidt | The Upgrade of the Neutron Induced Positron Source NEPOMUC[END_REF] In order to examine the near surface region, small energy steps of 0.10 and 0.20 keV were chosen up to an implantation energy of 8 keV. The energy of the emitted annihilation radiation was recorded by high-purity Ge detectors with a typical energy resolution of 1.30

keV at 511 keV. For the calculation of the S-parameter the central energy interval was set to (511± 0.85) keV. The CDB spectra were recorded at a positron implantation energy of 0.4 keV where the contribution of the Si substrate can be neglected according to the calculated Makhov profile. Preparation of the samples was performed by dispersing 10 mg of Pt/C TD and commercial Pt/C TKK catalyst powder in 1 ml of ethanol via sonication.

The as-prepared inks were drop-casted onto a silicon wafer, followed by drying of the sample at 60°C for 1 h.

WAXS measurements and

Results and Discussion

During the top-down synthesis process (see Figure 1A), Pt wires are exposed to strongly polarizing, alternating voltages. Thereby, nanoparticles can be rapidly eroded from the B and B´, as it is marked in Figure 1F. In addition, the complex shape of the nanoparticles can be seen from a variation in contrast within both crystals imaged by the HR-STEM-HAADF technique in Figure 1B andE. The resulting contrast at the used imaging conditions is predominantly dependent on atomic number Z and thickness of the sample. [START_REF] Nellist | The Principles and Interpretation of Annular Dark-Field Z-Contrast Imaging[END_REF] As the studied nanoparticles are pure Pt crystals, the contrast variations must originate from variations of the sample thickness. Therefore, it is also possible to demonstrate the topological complexity of both nanoparticles. Figure 1H and I show HR-TEM images of two Pt nanoparticles with "strawberry"-like shape, including up to three concave surface regions, which can be considered particularly interesting for electrocatalytic applications. To judge the complexity of the nanoparticle shapes in broader view, a state-of-the-art STEM-HAADF tomography technique, utilized for the visualization of complex 3D samples with high lateral resolution, was performed on unsupported Pt TD nanoparticles. The final 3D visualization of top-down synthesized single nanoparticles confirms the presence of surface concavities, as it is exemplarily illustrated in Figure 1J (left). Moreover, it is visible that surface concave regions appear on both, smaller particles, as well as on larger particles. Even though the unsupported nanoparticles tend to agglomerate, the used tomography technique revealed the irregular shape of many individual nanoparticles on surface of a bigger nanoparticle cluster, as it is illustrated in Figure 1J (right side). Compared to the typically spherically shaped commercial Pt nanoparticles such as the reference catalyst, [START_REF] Sneed | 3D Analysis of Fuel Cell Electrocatalyst Degradation on Alternate Carbon Supports[END_REF][START_REF] Mayrhofer | Fuel Cell Catalyst Degradation on the Nanoscale[END_REF][START_REF] Meier | Degradation Mechanisms of Pt/C Fuel Cell Catalysts Under Simulated Start-Stop Conditions[END_REF] the complex shapes of Pt TD nanoparticles prefigure an activity increase. However, a representative quantification of the particularly active sites is not possible due to the individual character of each single nanoparticle. In order to expand the study of single nanoparticles to the whole system, PAS was employed. It was chosen to examine the defective character of Pt/C TD in comparison to commercial Pt/C TKK . PAS is known as a powerful, non-destructive tool to detect open volume defects, i.e., vacancies or vacancy clusters in a crystal lattice. Due to the effective trapping of positrons in such crystal defects, the positron is applied as highly-mobile micro-probe with outstanding sensitivity to vacancy concentrations as low as 10 -7 in metals. [START_REF] Coleman | Positron Beams and Their Applications[END_REF] For the present study, a low-energy positron beam is required in order to investigate the near-surface region of the Pt/C samples. For better understanding, some elementary parts of PAS will be explained in the following text, whereas further details can be found in the literature. [START_REF] Schultz | Interaction of Positron Beams with Surfaces, Thin Films, and Interfaces[END_REF][START_REF] Hugenschmidt | Positrons in Surface Physics[END_REF] After being implanted in a solid, positrons thermalize within picoseconds and can then diffuse quasi-freely through the crystal lattice within a diffusion length typically in the order of 100 nm. They eventually annihilate with a core or valence electron either in the unperturbed crystal lattice or after being trapped at defect sites. The positron-electron annihilation leads predominantly to a back-to-back emission of two 511 keV γ-quanta in the center-of-mass-system. These γ-quanta are most important for gaining information about defects, as they will be detected during the experiment. In the laboratory system, the longitudinal component of the electron momentum (p L ) causes a Doppler-shift of the annihilation quanta. The resulting broadened annihilation photo peak, i.e., recorded counts as a function of γ-energy, is analyzed in DBS. For quantitative analysis of the line shape, the so-called S-parameter is commonly applied, which is defined as the fraction of counts in a fixed central region around the 511 keV annihilation peak. Since the central part of the annihilation peak is dominated by positrons annihilating preferably with low-momentum valence electrons, the S-parameter increases with an increasing number of vacancies or vacancy clusters. [START_REF] Krause-Rehberg | Positron Annihilation in Semiconductors: Defect Studies[END_REF] In CDBS, both γ-quanta of an annihilation event are detected simultaneously in a collinear detector setup. Due to the effective suppression of the background, the annihilation of high momentum (core) electrons becomes experimentally accessible, i.e., chemical information can be gained from the surrounding of the annihilation site. For evaluation, CDB spectra are normalized, mirrored at 511 keV and usually divided by a reference spectrum in order to obtain so-called ratio curves in which the elemental signatures can be clearly observed.

Figure 2A shows the S-parameter as a function of the positron implantation energy, which is a measure of the probed sample depth. [START_REF] Makhov | The Penetration of Electrons into Solids. 2. The Distribution of Electrons in Depth[END_REF] Towards high energies (> 25 keV), the commercial carbon reference material (Vulcan XC72R) in form of carbon nanoparticles with a particle size between 30-60 nm, [START_REF] Sanders | Carbon Black: Production, Properties, and Uses[END_REF] as well as both the commercial Pt/C TKK and Pt/C TD nanoparticles supported on Vulcan carbon approach the same S-parameter. This is expected since at higher energy the positrons are implanted with a higher implantation depth, i.e., most of the positrons annihilate in the Si substrate. At low energies (< 20 keV), the carbon reference possesses a higher S-parameter than both the commercial Pt/C TKK and Pt/C TD . This can be ascribed to the generally lower S-parameter of Pt compared to the carbon reference, as also observed in the normalized projection of the CDB spectra (Figure S1) and the low-energy region of the ratio curves (Figure 2B). More interesting, comparing both samples, Pt/C TD shows a significantly higher S-parameter than commercial Pt/C TKK . For the interpretation, it has to be mentioned that Pt/C TD and commercial Pt/C TKK slightly differ in Pt on carbon weight fraction (thermogravimetric analysis [TGA] reveals ~15 wt.% for Pt/C TD and ~20 wt.% for the commercial Pt/C TKK ), whereas the overall Pt loading on the substrate was adjusted. Therefore, due to its slightly smaller Pt content a slightly higher S-parameter of Pt/C TD compared to commercial Pt/C TKK was expected. However, the significantly higher S-parameter of Pt/C TD cannot be caused by only the comparably small difference in weight fractions of the samples with similar overall Pt loading. Instead, this effect is attributed to a higher concentration of (open-volume) defects in Pt/C TD than in commercial Pt/C TKK . In order to additionally access the element-specific high-momentum part of the annihilation line related to core electrons, CDBS was carried out. Figure 2B shows the ratio curves of Pt/C TD and commercial Pt/C TKK samples. These ratio curves are obtained by normalizing the individual CDBS raw spectra to same intensity and subsequent division by a reference spectrum, which is in this case a CDBS spectrum of a carbon reference. For comparison, a spectrum obtained for a bulk Pt reference is shown as well. For both, Pt/C TKK and Pt/C TD , the typical signature of Pt is visible (pL=10-80x10 -3 m 0 c). Assuming a simple model that positrons annihilate either in defect-free Pt or carbon, one would expect that the respective Pt signal relates to the measured Pt weight fractions (see TGA result above),

i.e., the Pt signatures in Pt/C TD would amount to 0.75 of that in Pt/C TKK . As determined by fitting the curves (see SI), however, the Pt signature in Pt/C TD (7.5%) is 0.59 less distinct than in the commercial Pt/C TKK (18.2%). Hence, the positron annihilation probability with Pt core electrons is significantly decreased in Pt/C TD . This effect is explained by the presence of a higher density of vacancy-like defects in Pt/C TD , since positrons are likely to be trapped at vacancies, where, in turn, the annihilation probability with core electrons is reduced. This conclusion is in accordance with the lower density of structural defects in the commercial sample, as observed in the DBS measurements.

Another approach to investigate the presence of structural defects in nanostructured catalysts is the use of high energy X-rays. Indeed, recent studies have used Rietveld refinement of WAXS patterns to extract the microstrain (i.e., the local strain) contribution from the experimental Bragg peak broadening. [START_REF] Dubau | Defects Do Catalysis: CO Monolayer Oxidation and Oxygen Reduction Reaction on Hollow PtNi/C Nanoparticles[END_REF][START_REF] Chattot | Beyond Strain and Ligand Effects: Microstrain-Induced Enhancement of the Oxygen Reduction Reaction Kinetics on Various PtNi/C Nanostructures[END_REF][START_REF] Chattot | Surface Distortion as a Unifying Concept and Descriptor in Oxygen Reduction Reaction Electrocatalysis[END_REF] Microstrain broadening in diffraction data originates in the local atom displacements from their ideal position, such as caused by atomic vacancies, stacking faults or grain boundaries. [START_REF] Qin | Origin of Lattice Strain in Nanocrystalline Materials[END_REF] Here, it should be noted that at the grain boundary of coalescing particles similarly active concave sites are formed.

Figure S2A displays the WAXS pattern measured at the ID31 beamline of the European synchrotron radiation facility (ESRF, Grenoble, France), including the calculated patterns for the Pt/C TD catalyst (details about the WAXS pattern of the commercial Pt/C TKK catalyst are available in Figure S2B). Results shown in Table S1 unambiguously support the conclusions obtained from DBS measurements: while commercial Pt/C TKK features zero value of microstrain, a value of 0.60% is found for Pt/C TD (being comparable or higher than some PtNi alloyed samples), [START_REF] Chattot | Surface Distortion as a Unifying Concept and Descriptor in Oxygen Reduction Reaction Electrocatalysis[END_REF] confirming its highly defective structure. The resulting surface distortion (SD) value was derived as 3.9 ± 0.2 % for the synthesized Pt/C TD (for further information see reference 45). It has to be noted that both techniques are not solely surface sensitive, but partially also include bulk information.

Electrochemical measurements further validate the presence of structural surface defects. Figure 2C RHE can be addressed to the H ads/des on Pt(110) facets, while the impact of ( 111) and

(100) facets cannot be clearly attributed. [START_REF] Vidal-Iglesias | Electrochemical Characterization of Shape-Controlled Pt Nanoparticles in Different Supporting Electrolytes[END_REF] Contrary, Pt/C TD shows an additional distinct peak shoulder at ~0.27 V vs RHE (orange dotted line), which can be assigned to the H ads/des on defective surface structures, as proposed by Chattot et al. [START_REF] Chattot | Beyond Strain and Ligand Effects: Microstrain-Induced Enhancement of the Oxygen Reduction Reaction Kinetics on Various PtNi/C Nanostructures[END_REF] Herein, we assume that the broad H des peak structure provides further evidence for the presence of structural defects on the Pt TD nanoparticle surface. nm can be observed. The corresponding particle size distribution (PSD) from the investigation of 125 isolated nanoparticles is given in Figure 3B, taking into account both edges of each individual particle and no particle agglomerates. PSD analysis was performed using the ImageJ software. Based on the broad size distribution, the numberaveraged (d N ) and surface-averaged (d S ) diameter of the isolated Pt nanoparticles was determined to be ~3.2 ± 1.0 nm and ~3.4 nm, respectively. In case of the commercial Pt/C TKK sample used, a number-averaged diameter of ~2.8 ± 0.8 nm can be found in the literature. [START_REF] Harzer | Cathode Loading Impact on Voltage Cycling Induced PEMFC Degradation: A Voltage Loss Analysis[END_REF] In contrast to the mean size determined from TEM, WAXS shows a global average size of 8.3 nm (Table S1), which indicates a bimodal size distribution consisting of smaller, isolated nanoparticles and larger particle aggregates. Similar observations have previously been reported by Chattot et al., where partially aggregated A-Pt/C electrocatalyst showed a TEM derived mean particle size of 3.4 nm, while WAXS revealed a global average size of 7.4 nm. [START_REF] Chattot | Surface Distortion as a Unifying Concept and Descriptor in Oxygen Reduction Reaction Electrocatalysis[END_REF] In order to evaluate the electrocatalytic activity of Pt/C TD towards the ORR, RDE measurements of freshly prepared catalyst inks were conducted. A typical polarization curve of Pt/C TD is depicted in Figure 4A. Evaluation of the kinetic current was performed using the Koutecký -Levich equation (Equation 1) [START_REF] Orfanidi | The Key to High Performance Low Pt Loaded Electrodes[END_REF] This is a significant improvement over typically used Pt/C electrocatalysts, especially as the activity is compared to one of the most active commercial Pt/C catalysts. Moreover, the activity is even comparable to some well-known PtNi and PtCo alloyed electrocatalysts. [START_REF] Mezzavilla | Structure-Activity-Stability Relationships for Space-Confined Pt x Ni y Nanoparticles in the Oxygen Reduction Reaction[END_REF][START_REF] Pizzutilo | The Space Confinement Approach Using Hollow Graphitic Spheres to Unveil Activity and Stability of Pt-Co Nanocatalysts for PEMFC[END_REF] Regarding the stability of Pt/C TD , preliminary accelerated durability tests of Pt/C TKK and Pt/C TD were conducted in RDE configuration. After 10,000 cycles between 0.6 and 1.0 V vs RHE, performed at 80°C, Pt/C TD still shows higher activity than Pt/C TKK (Figure S4A). As expected, post-mortem TEM analysis of Pt/C TD shows strong particle sintering at elevated temperature.

CO-stripping voltammograms of both commercial

Moreover, isolated particles appear to become more spherical (Figure S4B). To determine the specific activity of Pt/C TD , the kinetic current at 0.90 V vs RHE was normalized to the electrochemically active surface area (ECSA) of the catalyst. Integration of the H UPD charge of each CV was used to determine the ECSA. For the ad-and desorption of a monolayer of H on Pt, a charge density of 210 µC/cm 2 Pt was assumed.

Determination of the mass activity was conducted by normalizing the kinetic current to the Pt mass deposited on the working electrode.

The specific surface area (SSA) of Pt/C TD was determined to be ~46 ± 8 m 2 /g Pt , while commercial Pt/C TKK exhibits an SSA of ~68 ± 2 m 2 /g Pt (being comparable to literature data, considering the impact of the ionomer concentration). [START_REF] Orfanidi | The Key to High Performance Low Pt Loaded Electrodes[END_REF] In both cases, the SSA was normalized to the catalyst mass deposited on the working electrode. Comparing the average size of both catalysts, only a slightly reduced SSA compared to Pt/C TKK is expected. Hence, we assume that the actual difference in SSA is partially caused by agglomeration of the Pt nanoparticles, as can be seen in Figure 3A. Pt/C TKK (SSA: ~7 m 2 /g Pt ). For activity evaluation, the H 2 /O 2 data were corrected for the IR-drop and the H 2 crossover current. The reference data were taken from reference 56, however, the catalyst was examined in the same setup and under identical conditions.

Conclusions

Within this work, we show an up-scalable, top-down and one-pot synthetic approach towards highly active Pt/C electrocatalysts, circumventing typical demanding multi-step synthesis procedures. In RDE studies, synthesized Pt/C TD shows high activity towards the ORR with a specific and mass activity of 1.62 mA/cm 2 Pt and 712 mA/mg Pt , respectively. An ORR activity enhancement over commercial Pt/C was further confirmed in a single-cell PEMFC. Based on TEM, WAXS, CV and PAS experiments, the origin of the superior activity compared to commercial Pt/C can be correlated to the introduction of a high density of surface defects, such as surface concavities. Furthermore, considering the high complexity of classical nucleation driven bottom-up syntheses and costs of the precursor materials, the top-down approach does not only facilitate the synthesis, but should also save overall processing costs in large scale applications.

ASSOCIATED CONTENT

Supporting Information

Tailoring the Oxygen Reduction Activity of Pt Nanoparticles through Surface Defects: A Simple Top-Down Approach

  Rietveld refinement: WAXS patterns of Pt/C TD and the commercial Pt/C TKK were measured at the ID31 beamline of the European synchrotron radiation facility (ESRF, Grenoble, France). High energy X-ray radiations of 70 keV were focused on the catalyst powder contained either in a Kapton® tape (powder film removed from a rotating disk electrode) or Kapton® capillary for Pt/C TD and commercial Pt/C TKK , respectively. The scattered signal was collected using a Dectris Pilatus CdTe 2M detector within 5 s exposure time. The 2D signal was azimuthally integrated and reduced to a 1D curve using the pyFAI software package after the energy,[START_REF] Ashiotis | The Fast Azimuthal Integration Python Library: PyFAI[END_REF] detector distance and tilt were calibrated using a standard CeO 2 powder. The lattice parameter, crystallite size and microstrain values for the electrocatalysts were estimated by Rietveld refinement of the WAXS patterns using an Fm3m structure of metallic Pt and the Fullprof software.[START_REF] Rodríquez-Carvajal | Line Broadening Analysis Using FullProf*: Determination of Microstructural Properties[END_REF] Due to an external pollution by Ni, an additional Ni phase was added to refine Pt/C TD . The instrument resolution function was determined from the CeO 2 standard WAXS pattern, the peak shape and the background were described using the Thompson-Cox-Hastings (TCH) function and by a polynomial function, respectively. A possibility for uniaxial anisotropic description of nanoparticle size along the [111] direction axis was also implemented.

Page 14 of 51 ACS 5 .

 515 Paragon Plus Environment ACS Catalysis wire surface and subsequently be released into the electrolyte. Due to the rough conditions of our synthetic route, we assume to produce a wide variety of nanoparticle shapes, including surface concavities, combined with the introduction of local strain. In order to investigate the surface-defective character of Pt/C TD , five different approaches were exploited:1. HR-(S)TEM was used to visualize certain defective surface sites of unsupported Pt TD nanoparticles.2. STEM-HAADF tomography visualization was performed to obtain 3D information about the disordered surface regions of unsupported Pt TD nanoparticles.3. PAS was employed to compare the defect concentration of Pt/C TD to commercialPt/C TKK (detailed information about the reference catalyst is given in the experimental section).4. Rietveld refinement of WAXS measurements was performed to investigate and quantitatively compare the defect-induced degree of microstrain of Pt/C TD to commercial Pt/C TKK . Electroanalytical methods were used to monitor and compare structural defects and activities of both samples.

Figure 1 .

 1 Figure 1. (A) Schematic description of the synthetic procedure towards Pt/C TD

Figure 2 .

 2 Figure 2. (A) S-parameter as a function of the positron implantation energy obtained

  and D shows typical cyclic voltammograms (CVs) of Pt/C TD and commercial Pt/C TKK . In both cases, in the potential range of ~0.05-0.40 V vs the reversible hydrogen electrode (RHE), typical hydrogen adsorption and desorption (H ads/des ) features of polycrystalline Pt are visible. In detail, commercial Pt/C TKK depicts a sharp peak doublet with maxima at ~0.15 and ~0.22 V vs RHE (red dotted lines). According to Vidal-Iglesias et al., using HClO 4 as an electrolyte, features in the range between 0.09 V and 0.22 V vs

  Pt/C TKK and Pt/C TD are shown in Figure2E. While the commercial Pt/C only shows a single peak at a potential of ~0.78 V vs RHE, indicating majorly isolated, well-distributed nanoparticles, Pt/C TD shows a peak doublet with maxima located at ~0.67 V and ~0.69 V vs RHE and a singlet at a potential slightly lower than 0.78 V vs RHE. Based on the findings of Maillard et al., appearance of the peak doublet structure at lower potentials can be traced back to the presence of structural defects on complex isolated as well as agglomerated nanoparticles.[START_REF] Chattot | Beyond Strain and Ligand Effects: Microstrain-Induced Enhancement of the Oxygen Reduction Reaction Kinetics on Various PtNi/C Nanostructures[END_REF][START_REF] Maillard | Influence of Particle Agglomeration on the Catalytic Activity of Carbon-Supported Pt Nanoparticles in CO Monolayer Oxidation[END_REF] Recently, Chattot et al. compared the ratio between charge Q derived from CO ads and H ads/des peaks with the level of SD of different Pt-based ORR nanocatalysts, indicating a quasi-linear relationship.[START_REF] Chattot | Disclosing Pt-Bimetallic Alloy Nanoparticle Surface Lattice Distortion with Electrochemical Probes[END_REF] Taken the proposed evaluation protocols into account, Pt/C TD catalyst shows a Q CO /2Q H ratio of 0.85 ± 0.4, which indicates high density of surface defects and is comparable to e.g., hollow PtNi/C.

Figure 3 .

 3 Figure 3. (A) TEM image of Pt/C TD . The image reveals a homogeneous Pt nanoparticle

  𝐼 𝑘𝑖𝑛 = |(𝐼 • 𝐼 𝑙𝑖𝑚 )| |(𝐼 𝑙𝑖𝑚 -𝐼)| with I being the measured current, I kin being the kinetic current and I lim being the O 2 diffusion-limited current of the polarization curve. The kinetic current curve extracted from the polarization curve in a range of 0.85 V -1.0 V is shown in the inset of Figure 4A. The corresponding analysis of commercial Pt/C TKK is depicted in Figure S3. As described earlier, due to the high concentration of surface structural defects of Pt/C TD , we predict an activity increase compared to commercial Pt/C.

Figure 4 .

 4 Figure 4. (A) Typical IR-corrected polarization curve of Pt/C TD (anodic scan).

  The activity data and SSA values of Pt/C TD and commercial Pt/C are compared in Figure 4B-D. To further investigate the performance of Pt/C TD in an actual membrane electrode assembly (MEA), preliminary measurements in a single-cell PEMFC were conducted. To meet current industrial targets to produce PEMFCs with low Pt content, MEAs with a cathode loading of ~0.1 mg Pt /cm 2 were prepared and examined. The results are shown in Figure 5. Figure 5A depicts the CO-stripping voltammogram of the Pt/C TD catalyst layer.As observed in the RDE measurements, the voltammogram shows a characteristic double peak between 0.6 V and 0.9 V, associated with surface defects. Evaluation of the SSA gives a value of ~26.5 m 2 /g Pt , being ~40% lower than the values measured in the RDE tests (assumed standard charge of a CO monolayer on Pt: 420 μC/cm 2 ). This observation can be due to the increased catalyst batch size, leading to stronger agglomeration of the Pt nanoparticles, or due to non-uniform ionomer wetting of the of Pt/C TD through an improved synthesis procedure (e.g., improved mixing of the electrolyte during synthesis) and adjustment of the ionomer distribution could further improve the performance of the catalyst, especially at high current densities. Moreover, in future experimental studies, the stability of defect-rich Pt/C TD in the MEA setup has to be evaluated, as this task was out of the scope of this work. However, accelerated stress tests ofSchwämmlein et al. indicate that in the MEA, a lower SSA has beneficial effects on the stability of the catalyst, which should be considered prior to adjustment of the surface area.[START_REF] Schwämmlein | Activity and Stability of Carbon Supported Pt x Y Alloys for the ORR Determined by RDE and Single-Cell PEMFC Measurements[END_REF]

Figure 5 .

 5 Figure 5. (A) CO-stripping voltammogram of the Pt/C TD cathode catalyst layer, showing

  Indeed, at 0.90 V vs RHE, we observe a ~3.6-times enhanced mass activity (~712 ± 20 mA/mg Pt ) and a ~6.8-times enhanced specific activity (~1.62 ± 0.32 mA/cm 2 Pt ) of Pt/C TD compared to commercial Pt/C E-tek (activity values taken from literature),[START_REF] Kuttiyiel | Bimetallic IrNi Core Platinum Monolayer Shell Electrocatalysts for the Oxygen Reduction Reaction[END_REF][START_REF] Godínez-Salomón | Tuning the Oxygen Reduction Activity and Stability of Ni(OH) 2 @Pt/C Catalysts through Controlling Pt Surface Composition, Strain, and Electronic Structure[END_REF] and ~1.7-times mass, ~2.7-times specific activity enhancement compared to Pt/C TKK . The activities of Pt/C TKK are in good agreement with literature values of the same catalyst, considering the impact of the ionomer concentration.

ACS Paragon Plus Environment ACS Catalysis

We gratefully acknowledge the financial support from Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany´s excellence strategy -EXC 2089/1 -390776260, Germany's excellence cluster "e-conversion", DFG projects BA 5795/4-1 and BA 5795/3-1, and from the TUM International Graduate School of Science and Engineering, project 11.01. Support from CEITEC Nano Research Infrastructure and Karlsruhe Nano Micro Facility as well as financial support from Ministry of Youth, Education and Sports of the Czech Republic, projects LM2015041 and LQ1601, are gratefully acknowledged.

W.-J. Li gratefully acknowledges the financial support from the Alexander von Humboldt fellowship for postdoctoral researchers. We thank Prof. Hubert A. Gasteiger for providing reference electrocatalyst samples and for providing access to his fuel cell test equipment.

We are thankful to Jan N. Schwämmlein for conducting the fuel cell test measurements. We thank Prof. Hendrik Dietz for granting access to his transmission electron microscopy facilities.

TOC:

This information is available free of charge on the ACS Publications website. Coincident Doppler broadening spectrum, WAXS measurements, Rietveld refinement data, Reference ORR polarization curve, Accelerated durability test, TEM analysis

AUTHOR INFORMATION

Corresponding Author

*E-mail: bandarenka@ph.tum.de. Phone: +49 (0) 89 289 12531.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENT


Ricardo Sgarbi

Edson A Ticianelli

Frédéric Maillard
email: frederic.maillard@lepmi.grenoble-inp.fr

Frederic Jaouen

Marian Chatenet
email: marian.chatenet@grenoble-inp.fr

Frédéric Jaouen

Maillard

Oxygen Reduction Reaction on Metal and Nitrogen-Doped Carbon Electrocatalysts in the Presence of Sodium Borohydride

Keywords: metal-nitrogen doped carbons (M-N-C), catalyst, Fe-N-C, Co-N-C, oxygen reduction reaction, direct borohydride fuel cells

come   L'archive ouverte pluridisciplinaire

Introduction

Small mobile electric devices (e.g. either portable electronics or drones) require power systems of high energy density. They are usually powered by batteries, but these "closed systems" are inherently limited by their insufficient specific energy (e.g. < 250

Wh kg -1 for the most energetic Li-ion systems [START_REF] Linden | Handbook of Batteries[END_REF]). Fuel cells, being open systems, are intrinsically denser in energy, and could therefore advantageously be used in such devices. The present standard is the proton exchange membrane fuel cell (PEMFC), which is, in its state-of-the-art, fed by pure hydrogen [START_REF] Vielstich | Handbook of Fuel Cells: Fundamentals, Technology and Applications[END_REF]. While transporting, compressing and storing gaseous hydrogen may prove relevant for large quantities of energy stored, this solution is not always efficient, safe, economically-viable and therefore relevant in applications where smaller amount of energy is stored, i.e. the particular case for small mobile electric devices. In that situation, the direct oxidation of a liquid fuel may be more desirable, and direct methanol fuel cells (DMFC) have historically been studied as the pioneer direct liquid fuel cell (DLFC) [START_REF] Wasmus | [END_REF]. However, electrooxidizing methanol is a complex and slow process (not speaking from the toxicity of the fuel), which requires extensive amounts of platinum-group-metal (PGM) electrocatalyst, and this is a clear hindrance to the development of DMFCs, should they be acidic or alkaline [START_REF] Vielstich | Handbook of Fuel Cells: Fundamentals, Technology and Applications[END_REF][START_REF] Gasteiger | Hanbook of Fuel Cells[END_REF]. While other hydrocarbon fuels have been tried as well, none proved capable to provide both sufficient energy and power density (even in alkaline environments, known to facilitate the reactions and hence to render them faster [START_REF] Spendelow | [END_REF]), a clear drawback for the powering of small electronic devices. This triggered intense research to find more suitable liquid fuels for portable/small mobility application; from these works, sodium borohydride soon appeared as a promising candidate, owing to the properties of its fuel [6,7] and of its relevance for portable fuel cell system design [8][9][10][11].

Of course, the direct oxidation of the borohydride anion (BH4 -) in a direct borohydride fuel cell (DBFC) is not an easy process, and many works addressed the issue, so as to isolate the reaction mechanisms [12][13][14][15][16][17][18], the proper electrocatalysts for the anode [START_REF] Olu | Electrocatal. Low Temp. Fuel Cells Fundam. Recent Trends[END_REF] and the ideal fuel composition, electrode structure, operating parameters of the anode, etc. [START_REF] Merino-Jiménez | [END_REF][21][START_REF] Suda | Handb. Fuel Cells[END_REF][START_REF] De León | Direct and Indirect Borohydride Fuel Cells[END_REF]. While one shall not neglect research actions to improve our understanding of the borohydride oxidation reaction (BOR) and hence to find better materials to achieve it efficiently, one must also not neglect the membrane/separator that should separate the anode and cathode compartment. To date, cation-exchange membranes are more mature than their anion-exchange counterparts, and bipolar membranes are at an early stage of development. All types of ion-exchange membranes can be used in a DBFC (see for example [START_REF] Wang | [END_REF]25]), and, whatever the nature of the membrane/separator, there are drawbacks to their utilization. For a cation-exchange membrane, during DBFC operation, the Na + cation shall migrate to the cathode, where it will combine with the OH -species formed in the alkaline oxygen reduction reaction (ORR), thereby displacing the NaOH from the anolyte to the cathode side. This naturally has a clear detrimental effect on the operation of the cathode, as recently put forth by Ould-Amara et al. [26], owing to deleterious crystallization of NaOH in the cathode pores, leading to unavoidable mass-transport hindrances. Should an anion-exchange membrane be used (this is probably the desired situation, as the anolyte of a DBFC is necessarily alkaline [27,28]), there will be non-negligible crossover of borohydride anions from the fuel anolyte to the cathode compartment. In that latter case, of course, the cathode catalyst must be tolerant to BH4 -, otherwise its apparent ORR activity will be significantly affected, because either the net current will be the sum of the cathodic (desired) ORR and anodic (undesired) BOR contributions, or because borohydride species will adversely adsorb or react with the catalyst material and deactivate or chemically modify it. This shows that the cathode of a DBFC should also be tightly optimized, and in particular that BH4 --tolerant ORR catalysts must be selected for the application [10].

The literature regarding fuel-tolerant electrocatalysts for alkaline fuel cells is abundant, but mainly deals with tolerance to hydrocarbon molecules (methanol, ethanol, etc.) [29][30][31][32][START_REF] Ticianelli | Direct Alcohol Fuel Cells Mater. Performance[END_REF]. The case of borohydride, a very strong reducer, has been less under focus, even if some (rare) studies have explored the issue. Metal-oxides (e.g. perovskites, LaNi0.8Co0.2O3 [START_REF] Wei | [END_REF] and carbon-supported nanometric manganese oxides, MnOx/C) [35,36] have demonstrated some interesting properties for potential use in DBFC cathodes, but one could question the durability of such inherently-oxidized materials in strongly-reducing environments; as a matter of fact, long-term stability data was never reported in such studies, except for MnOx/C (but not in presence of BH4 -species) [37].

Cheng and Scott [38] explored PGM-free electrocatalysts (iron tetramethoxyphenyl porphyrin (FeTMPP), silver and nickel), regarding their ORR performances in direct borohydride fuel cell cathodes. FeTMPP cathodes outperformed silver and nickel ones, which led the authors to the conclusion that FeTMPP was somewhat BH4 --tolerant, unlike Ag and Ni (the latter is no surprise, as both Ag [39,40] and especially Ni [41][42][43] are active catalysts for the BOR). More recently Fe-aminoantipyrine (Fe-AAPyr) was successfully developed as a BH4 --tolerant ORR catalyst for a mixed-reactant DBFC [44].

These last two materials fall into the category of metal-nitrogen-carbon (M-N-C) catalysts, which are now under tremendous focus to replace Pt-based ones for PEMFC applications (see e.g. [45][46][47]). Because these classes of material are commonly-admitted to be tolerant to fuel species, and some of them have indeed shown tolerance to borohydride [38,44], the present contribution explores two families of M-N-C compounds, based on cobalt and iron metal centers. It notably aims to unveil whether any such catalysts are tolerant to BH4 -, or if they must have particular features to be tolerant; herein are compared the nature of the metal (iron, Fe, and cobalt, Co), the structure of the resulting electrocatalyst (atomically-dispersed vs. nanoparticles) and its mode of synthesis (flash vs. ramp pyrolysis in argon atmosphere).

Materials and methods

Catalysts syntheses

The electrocatalysts used in this study were synthesized using the method described by Ranjbar-Sahraie et al. [48]. In brief, a zinc(II) zeolitic imidazolate framework (ZIF-8, purchased from BASF, Basolite Z1200), Fe or Co acetate precursor and 1,10-phenanthroline were mixed via dry planetary ball milling. The masses of the precursors were 200 mg phenanthroline, 800 mg ZIF-8 and either 0.5 wt. % or 5.0 wt. % of metal (Fe or Co) compared to the overall mass of the three precursors. The dry mixed metal, carbon and nitrogen precursors were then pyrolyzed either in flash-(i.e. the powder was introduced at 1050°C in the oven) or ramp-(the powder was heated from room temperature to 1050°C at 5°C min -1 ) mode, and a temperature of 1050°C was maintained for 1 h. Then, the quartz tube and boat were opened and quenched to room temperature while still flowing Ar. As Ar was used, the weight loss during the pyrolysis was ca. 65-75 wt. % and it was independent on the pyrolysis mode.

Physicochemical characterizations

All physicochemical characterizations have been previously published, then, here only a brief overview is presented (for detailed experimental specifications, the readers could refer to References [48][49][50][51][52]). TEM measurements were performed on a JEM-2100HCKM (JEOL) microscope operating at 120 keV for the atomically-dispersed metal atoms electrocatalysts [48,50] and a JEOL 2010 TEM microscope operating at 200 kV for the metal nanoparticulated electrocatalysts [51]. To obtain 57 Fe Mössbauer spectra and confirm the absence of zerovalent iron crystalline phases in atomically-dispersed metal atom electrocatalysts, Fe-containing samples were measured with 57 Co-Rh source and analyzed doublet and sextet spectral components ( [51] for Fe5.0RP and Fe0.5RP; [52] for Fe0.5FP). The carbon nanocrystallites were characterized by Raman Spectroscopy using an argon LASER (514 nm) and X-ray Diffraction by an X'Pert PRO MPD PANalytical diffractometer operated at 45 kV [51]. Surface area and pore volume were estimated by the Brunauer-Emmett-Teller (BET) method using a Micromeritics ASAP 2020 equipment with N2 sorption at liquid nitrogen temperature (77 K) [49].

Electrochemical characterizations

Before the electrochemical measurements, the glassware, polytetrafluoroethylene (PTFE)-based materials, volumetric flasks, tips and electrodes were cleaned in 50% v/v solution of H2SO4 (Merck, Suprapur 96 wt. %) / H2O2 (Carl Roth, 30% v/v), rinsed in ultrapure water (MQ grade, 18.2 MΩ cm, 1-3 ppb TOC) and hot ultrapure water. The glassy carbon tips (working electrode substrates) were polished on diamond polishing paste (Presi, 3 and 1 µm). All fresh Ar-and O2-saturated electrolytes were prepared from NaOH (Alfa Aesar, 50% w/w aqueous solution) and ultrapure water to obtain a concentration of 0.1 M; similarly, NaBH4 fresh "mother solution" was prepared from previous 0.1 M NaOH solution and NaBH4 powder (Merck, ≥ 98.0%) to obtain a concentration total of 0.5 M.

All electrochemical measurements were performed using a three-electrode PTFEelectrochemical cell with temperature control at 25°C. The reference electrode was a commercial reversible hydrogen electrode (RHE, Gaskatel GmbH) connected to the cell by a Luggin capillary and the counter electrode was a carbon sheet. The working electrode, a homemade glassy carbon cylinder (glassy carbon Sigradur ® from Hochtemperatur-Werkstoffe GmbH) inserted in a PTFE cylinder, was connected to a commercial rotator (Origalys ® ). To investigate the electrocatalyst materials, a catalytic layer was made onto the glassy carbon disc substrate (0.196 cm 2 ) by dropping 20 µL of each prepared ink to obtain a total catalyst loading of 0.8 mgpowder cm -2 . The catalyst inks were prepared by dispersing 10 mg of catalytic powder, 50 µL of 5 wt. % Nafion solution (Sigma-Aldrich), 854 µL of isopropanol (Carl Roth) and 372 µL of ultrapure water, followed by ultrasonic homogenization [51]. Then, the electrochemical cell was coupled to an Autolab PGSTAT12 potentiostat to perform RDE (rotating disk electrode)

measurements.

An initial reproducible surface state was obtained by applying 50 cyclic voltammograms (CVs) between 0.0 and 1.0 V vs. RHE at 100 mV s -1 in Ar-purged 0.1 M NaOH. Next, the CVs of the catalytic layers were recorded in the same conditions at 10 and 5 mV s -1 , followed by polarization curves at 5 mV s -1 in Ar-and O2-saturated 0.1 M NaOH electrolyte at 400 rpm. All polarization curves were subtracted by capacitive currents obtained (CVs at 5 mV s -1 ) in Ar-purged electrolyte. This same procedure was repeated after each NaBH4 addition into the electrolyte (NaBH4 concentration of 0, 10, 20, 40 and 60 mM). All measurements were corrected from ohmic drop.

Mass activity (𝑗 MA =

𝑗 k 𝑚 at 0.85 V vs. RHE, where m is the catalyst mass onto the glassy carbon electrode) was calculated from the kinetic current density for ORR (jk) obatained by using the Koutecky Levich equation:

𝑗 𝑘 = - (𝑗 L .𝑗) (𝑗 L -𝑗) (1)
where jL is the O2-diffusion limited current density at 0.2 V vs. RHE and j is the Faradaic current corrected as described above.

Results and discussion

Physicochemical properties

Detailed physicochemical characterizations of the catalysts have been previously published by Zitolo et al. [48,53], Choi et al. [50] and Kumar et al. [51]. High-resolution transmission electron microscopy and 57 Fe Mössbauer spectroscopy showed that the catalysts with 0.5 wt. % metal content before pyrolysis (ca 1.5 wt% metal content after pyrolysis, due to the mass loss of ca 66 % experienced by the sacrificial metal-organic framework ZIF-8 during pyrolysis) feature only atomically-dispersed metal atoms coordinated by nitrogen atoms and embedded into a carbon matrix (FeNx sites). In contrast, the catalysts prepared with 5.0 wt. % metal content before pyrolysis (ca 15 wt% metal after pyrolysis) resulted in either metallic (Co) or metal carbide (Fe3C) nanoparticles surrounded by a carbon-nitrogen shell. No atomically-dispersed metal atoms coordinated by nitrogen atoms[f1] were detected from the spectroscopic characterization of these samples. The Mössbauer or EXAFS spectroscopic detection limit for FeNx sites is ca. 5% relative to the total amount of Fe; so the absence of detection of FeNx sites monitored by these techniques for the 5.0 wt. %-loaded Fe-N-C sample (ca 15 wt% Fe total content[f2]) means that, at maximum, 0.75 wt. % Fe might be present as FeNx sites in this catalyst. In comparison, the 0.5 wt.%-loaded Fe-N-C sample (1.5 wt% Fe bulk content) only contains FeNx sites, and this implies that the absolute number of FeNx sites in the latter is at least double that in the 5.0 wt. %-loaded Fe-N-C sample.

Raman spectroscopy and X-ray diffraction provided evidences that the content of carbon nanocrystallites (i.e. more graphitic carbon phase, with lower amount of surface defects and oxygen functional groups) increases with the increase of metal content. Because ramp pyrolysis results in slow but continuous escape of the volatile products from ZIF-8 and phenanthroline decomposition, ca. twice higher Brunauer-Emmett-Teller (BET) surface area was observed for the ramp pyrolyzed Fe catalyst with 0.5 wt. % Fe content compared to the flash pyrolyzed one [49]. However, this change mostly results in higher microporous surface area but little affects the meso-and macroporous surface areas [49].

In what follows, we refer to the catalysts as Mxmode, where M is Fe or Co, x is the wt. % metal in the powder composed of mixed precursors before pyrolysis, either 0.5 or 5.0 and "mode" is the pyrolysis mode (either ramp or flash pyrolysis, RP and FP, respectively).

Electrochemical properties

In agreement with our former findings [54], it is found that: (i) all the electrocatalyst materials present very interesting oxygen reduction reaction activity in alkaline electrolytes and (ii) Fe-N-C electrocatalysts perform better than Co-N-C ones, independently to their structure and BET surface area. To probe their tolerance to sodium borohydride, the electrocatalysts were tested subsequently in NaBH4-free and NaBH4containing Ar-saturated electrolytes. Fig. 1Fig. 1 shows the cyclic voltammograms obtained at 10 mV s -1 for all synthesized catalysts in 0.1 M NaOH electrolytes with different contents of NaBH4, in the absence of O2. Results reveal that none of the tested electrocatalysts are totally insensitive to the presence of BH4 -anions in the electrolyte: all of them show non-negligible oxidation current in the "high-potential range", the magnitude of which increases with the NaBH4 concentration, signing they present some activity towards the electrooxidation of the BH4 -anion. However, clear differences can be seen when results for Fe-N-C and Co-N-C electrocatalysts are compared.

Focusing firstly on the Fe-containing materials (Fig. 1a), one notices that the 5.0 wt. %-loaded Fe-N-C sample is much more sensitive to the presence of NaBH4 than its 0.5 wt. % counterparts (the oxidation current monitored is ca. 10-times larger in the former than in the latter cases). These results are ascribed to the presence of Fe3C nanoparticles in the Fe5.0RP catalyst (which may contain no FeNx sites at all, and in the most favorable hypothesis for minor fraction of co-existing FeNx sites, does not contain more FeNx sites than the Fe0.5RP catalyst-see section 3.1), whereas the two other samples (Fe0.5FP and Fe0.5RP) exclusively contain atomically-dispersed iron coordinated by nitrogen ligands (FeNx sites). It is believed that the lower reactivity of the Fe0.5-catalysts are related to the absence (or limited amounts) of neighbor sites required for the BH4 - anions adsorption and its subsequent oxidation. Comparing the Fe-N-C electrocatalysts prepared by the ramp pyrolysis (RP) or the flash pyrolysis (FP), results do not enable to differentiate the samples: Fe0.5RP and Fe0.5FP present similar reactivity in the presence of NaBH4 and are leading to very moderate BOR current (both contain atomicallydispersed iron).

Co-containing samples are, at given metal fraction, far more reactive for the BOR than their Fe-containing counterparts. Co0.5RP, although presenting atomically-dispersed Co coordinated by N-ligands, leads to significant BOR currents particularly for NaBH4 concentration higher than 20 mM, and the situation is even worse for the Co5.0RP sample, that is constituted of Co nanoparticles (Fig. 1b, see inserted image to better visualization).

In this last case, the electrode process must involve significant H2 release by BH4 - hydrolysis, as evidenced by the instability of the catalyst thin-film, which prevented measurements for NaBH4 concentrations above 20 mM. These results illustrate the high ability of Co-N-C electrocatalysts to promote reactions with NaBH4. This property is not surprising for the Co5.0RP sample, owing to the already-reported activity of Co electrocatalysts for the BOR [55,56] or for the hydrolysis of BH4 -followed by hydrogen release and subsequent oxidation [57][58][59]. The present results show that atomicallydispersed Co is also a good BOR electrocatalyst (Demirci et al. reported the activity of Co x+ (x = 2, 3) cations for BH4 -hydrolysis [60], but not for the BOR). the M-N-C electrocatalyst loading on the RDE tip was in all measurements 0.8 mgpowder cm -2 . For the Co5.0RP sample, the active layer was destroyed for [NaBH4] > 20 mM. The electrocatalysts were then tested for the ORR in absence and presence of NaBH4 at several concentrations in O2-saturated electrolytes (Fig. 2). In all cases, the "direct" measurement of the ORR in the presence of NaBH4 (represented by the curves (3), in symbols) are compared to the "reconstructed" ones, for which the ORR currents in absence of NaBH4 are added to the currents of BH4 -oxidation in Ar-saturated supporting electrolyte (represented by the curves (4 + 2), in dotted lines). For the majority of considered electrocatalyst considered, the "direct" and "reconstructed" curves of ORR in the presence of NaBH4 are near-superposed; this means that the processes at stake on these class of materials is indeed the sum of the ORR and of the BOR (or of BH4 - hydrolysis followed by the oxidation of the produced H2), meaning that the processes are independent from each other. Furthermore, one clearly sees that not all the electrocatalysts have the same reactivity for the ORR in the presence of NaBH4, in agreement to the conclusions obtained from Fig. 1. Firstly, Fe-N-C electrocatalysts presenting only atomically-dispersed iron (Fe0.5RP and Fe0.5FP) are somewhat tolerant to NaBH4: the ORR activity in presence of 10 mM NaBH4 is hardly changed compared to its value in absence of NaBH4. However, increasing the NaBH4 concentration progressively from 10 to 60 mM leads to an increase of BOR contribution to the total current, causing a slight negative shift of the apparent ORR onset potential (a sign of depreciated ORR kinetics and/or appearance of mixed electrode potentials, Fig. 2a). Surprisingly, increasing NaBH4 concentrations lead to practically unaffected absolute value of the ORR limiting currents (see curves (3), symbols), which one could associate to similar number of electrons involved in ORR electrocatalysis even in the presence of strong reducer. Another interesting aspect is the smaller "direct" ORR limiting currents compared to the sum of the ORR and the BOR (see curves (4 + 2), dotted lines); it can be related to the reduction of oxygen by BH4 - anions on the catalysts surface, which in fact reduces the local effective concentration of O2 in the "direct" measurements.

In contrast, when Fe3C nanoparticles are present (Fe5.0RP), the BH4 -tolerance is no longer maintained: the ORR activity of Fe5.0RP is severely affected by increasing amounts of NaBH4, and even at 10 mM NaBH4, the reaction onset potential is severely negatively-shifted (by more than 100 mV; the negative shift being > 400 mV for 60 mM NaBH4). Moreover, increasing NaBH4 concentration leads to decreased absolute values of ORR limiting currents, which indicates a lower number of electrons involved in the O2 electrocatalysis and/or a drastic reduction of O2 concentration in the catalyst surface (probably because of a reaction of O2 or ORR intermediates (i. e. HO2 -) with BH4 -or H2 [61]). Not surprisingly, Co-N-C samples, which showed non-negligible activity towards the BOR (Fig. 1), are not capable to maintain their good ORR activity in presence of NaBH4 in the electrolyte, even at low concentration (Fig. 2b). Of course, in presence of Co nanoparticles (Co5.0RP), the NaBH4 tolerance is even worse as compared to that of atomically-dispersed Co (Co0.5RP). tip was in all measurements 0.8 mgpowder cm -2 . (symbols) "direct" experimental curves of ORR in the presence of NaBH4; (dotted lines) sum of current of ORR in absence of NaBH4 with those for the BOR (similar as Fig. 1).

The ORR onset potential (Eonset), half-wave potential (Ehalf-wave) and mass activity (jMA at 0.85 V vs. RHE) of all the electrocatalysts were determined in the "direct" ORR experiments as a function of the NaBH4 concentration in the electrolyte, as shown Fig. 3.

The drastic reduction of the onset and half-wave potentials observed in Fig. 3a and Fig. 3b confirm that for Fe5.0RP or Co-N-C samples (Co0.5RP and the Co5.0RP), the tolerance to NaBH4 is insufficient: these electrocatalysts present highly-degraded ORR performances both in the kinetically-controlled and in the mixed kinetic-diffusion control regions as soon as some NaBH4 is present in the electrolyte.

In this way, the results evidence that only the atomically-dispersed Fe-N-C electrocatalysts present a sufficient tolerance to BH4 -to be used as ORR electrocatalysts in a DBFC cathode, even though their ORR performances would start to non-negligibly decrease if the crossover of NaBH4 is significant and the BH4 -concentration exceeds a few 10 mM. This is further highlighted by results in Fig. 3c, which demonstrate that the intrinsic mass activity of the Fe0.5RP and Fe0.5FP electrocatalysts are not drastically affected by concentrations of NaBH4 in the NaOH supporting electrolyte, at least for BH4 - concentrations of the order of 10 mM. These results also show that the electrocatalyst prepared by ramp pyrolysis (Fe0.5RP) is a little more tolerant than the one prepared by flash pyrolysis (Fe0.5FP). This behavior may be related to the above-mentioned difference in BET surface area, which facilitates the access of oxygen molecules. Note however that this effect progressively vanishes by increasing [BH4 -] concentrations in solution to values that far exceed the O2 solubility (1-2 mM).

Conclusions

The present study shows that by tuning the nature of the metal center, its structure (atomic dispersion vs. nanoparticles) and the BET surface area of the carbonaceous structure, the tolerance of the ORR to NaBH4 can be enhanced in the potential range of a direct borohydride fuel cell cathode. Evidences of a positive structural effect on the tolerance to BH4 -competition were obtained for catalysts containing atomically-dispersed Fe and Co sites, the latter being magnified for Fe-containing electrocatalysts because of combined poor activity towards the BOR and high activity towards the ORR. A small but non-negligible effect of the microporous surface area was observed; however, it is believed that it relates to higher density of active sites in catalysts after ramp-compared to flash-pyrolysis. These findings thus open the door to enhanced electrocatalysts for the DBFC both at atomic and macroscopic levels.

Acknowledgements

This study was financially supported by the French National Research Agency through the CAT2CAT and ANIMA projects. R.S. gratefully acknowledges the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Brazil (process number:

Fig. 1

 1 Fig. 1 Cyclic voltamperograms measured on a Fe-N-C and b Co-N-C electrocatalysts in

Fig. 2

 2 Fig. 2 Cyclic voltamperograms of oxygen reduction reaction measured on a Fe-N-C and

Fig. 3

 3 Fig. 3 Determination of representative oxygen reduction reaction kinetic parameters for

  1614344), CAPES/COFECUB program (process numbers: 88887-187755/2018-00 and Ph-C 914/18) for funding his research stay at LEPMI and the São Paulo State Research Foundation (FAPESPprocess number: 2013/16930-7) for financial supports. Some materials' analyses have been performed within "CEMAM", the Centre of Excellence of Multifunctional Architectured Materials (n° ANR-10-LABX-44-01).