Following a well-defined series of acid and heat treatments on a benchmark Pt3Co/C sample, three different nanostructures of interest for the electrocatalysis of the oxygen reduction reaction were ...tailored. These nanostructures could be sorted into the “Pt-skin” structure, made of one pure Pt overlayer, and the “Pt-skeleton” structure, made of 2–3 Pt overlayers surrounding the Pt–Co alloy core. Using a unique combination of high-resolution aberration-corrected STEM-EELS, XRD, EXAFS, and XANES measurements, we provide atomically resolved pictures of these different nanostructures, including measurement of the Pt-shell thickness forming in acidic media and the resulting changes of the bulk and core chemical composition. It is shown that the Pt-skin is reverted toward the Pt-skeleton upon contact with acid electrolyte. This change in structure causes strong variations of the chemical composition.
Despite the significant decrease in past decades, the cost of proton exchange membrane fuel cells, largely due to the rare and expensive electrocatalyst made of platinum, restrains their massive ...deployment. Therefore, reducing the platinum content in the electrode is the keystone of intense research efforts to increase the catalyst activity or utilization. The catalyst layer structure, especially the water and ionomer distributions, rules the active site availability for electrochemical reactions and thus catalyst utilization because of its influence on the transport of protons and oxygen. However, the rational design of more efficient electrodes faces the lack of accurate knowledge of their complex nanoporous structure. Specifically, ionomer and water distributions are very difficult to probe with conventional electron microscopy or X-ray techniques. This work provides quantitative information on the electrode structure, regarding ionomer and water distributions, thanks to an extensive analysis of small-angle neutron scattering profiles at different relative humidities and contrasts. A 2 to 3 nm-thin ionomer film spreads around the Pt/C catalyst particles, while a condensed water layer appears at the catalyst/ionomer interface depending on the type of carbon support.
The cathode is the most crucial component of the polymer electrolyte membrane fuel cell (PEMFC) because it is the most limiting in terms of performance, durability, and cost. Regarding the ...performance, the main losses are due to the cathode because of the negative coupling between a sluggish oxygen reduction reaction (ORR) and H+ and O2 transport loss issues. Therefore, many efforts have been conducted on the one hand to increase the kinetic of the ORR by developing a catalyst with improved activity and stability. On the other hand, attempts have been made to reduce mass transport losses in the cathode by tuning its nanostructure. This paper describes a multistep process to nanostructure the electrocatalyst in the view of simultaneously benefiting from enhanced activity toward the ORR and reduced O2 transport limitations. Thus, original carbon-free electrode’s architectures made of organized and well-ordered and oriented PtNi nanowires (PtNiNWs) and nanotubes (PtNiNTs) directly embedded onto a Nafion membrane were developed. Here, the nanotubes were templated from Ni nanowires grown on an anodic aluminum oxide (AAO) template. The fabrication process was optimized to improve the quality of the nanotubes and their integration into the membrane: the process includes thermal treatment in a H2/Ar environment and an acid leaching step as key steps to obtain the desired structure. After the electrodes were integrated in a complete membrane-electrode assembly (MEA), we tested the performance and durability of these nanostructures under real operating conditions. We compared the results to a Pt/C conventional electrode at low Pt loading (∼35 μgPt/cm2) exhibiting a roughness factor close to that of PtNiNWs and PtNiNTs electrodes. Results have shown a great improvement in the mass activity and stability of PtNiNTs electrodes in an accelerated stress test. Also, they have shown a significant sensitivity toward relative humidity variation.
Observations of microstructural evolution by scanning and transmission electron microscopy in different regions of aged membrane/electrode assemblies (MEA) have shown that degradation is not always ...uniform through the MEA surface; after load cycling operation, the degradation is more severe in the region located near the air inlet compared to air outlet. After constant load operation the degradation appears more uniform. Two types of microstructural evolution have been observed. The first one consists in the modification of the cathode leading to nanoparticles larger than initially but still well dispersed. The second type of evolution ends up with big agglomerates formed by large Pt particles in the cathode, with also a noticeable degradation of the carbon support, both phenomena being always coupled with the precipitation of Pt particles inside the membrane. The first type of evolution results from the electrochemical Ostwald ripening mechanism and appears when the cathode potential remains low. In contrast, the second one appears when the cathode is locally exposed to a high interfacial potential resulting from the reverse-current mechanism. Hydrogen starvation induced by the load cycles and oxygen crossover that increases with membrane damage, are the two main factors responsible for this severe degradation mechanism.
► Microstructure evolution of aged MEA under a constant load mode and a load cycling mode. ► Two MEA surface regions have been analyzed, near the air inlet and near the air outlet. ► Nanoparticle distributions have been studied by transmission electron microscopy (TEM). ► The two observed MEA microstructural evolutions are associated to two degradation mechanisms. ► The electrochemical Ostwald ripening and the reverse-current mechanisms are involved.
Long-term catalytic performance of electrode materials is a well-established research priority in electrochemical energy conversion and storage systems, such as proton-exchange membrane fuel cells. ...Despite extensive efforts in research and development, Pt-based nanoparticles remain the only – but an unstable – electrocatalyst able to accelerate efficiently the rate of the oxygen reduction reaction. This paper describes the synthesis and the atomic-scale characterization of hollow Pt-rich/C nanocrystallites, which achieve 4-fold and 5-fold enhancement in specific activity for the oxygen reduction reaction over standard solid Pt/C nanocrystallites of the same size in liquid electrolyte and during real proton-exchange membrane fuel cell (PEMFC) testing, respectively. More importantly, the hollow nanocrystallites can sustain this level of performance during accelerated stress tests, therefore opening new perspectives for the design of improved PEMFC cathode materials.
An electrochemical impedance model is developed to describe the impedance response of a volumetric electrode accounting for proton migration in the absence of oxygen transport limitation. It is used ...to fit experimental impedance spectra in H2/O2 configuration in order to estimate the protonic resistance of the electrode. Spatial resolution is achieved using a segmented cell, in dry, partially humidified and fully humidified conditions and at different cell currents. The local effective humidity in the cathode catalyst layer (CCL) and in the membrane are deduced from a correlation relating the protonic resistance measured in H2/N2 configuration to the relative humidity. Self-humidification of a cell operated in counter flow with dry oxygen is monitored using this technique, and the contributions of the membrane and of the CCL to the overall ohmic losses are eventually estimated.
Tuning the surface structure at the atomic level is of primary importance to simultaneously meet the electrocatalytic performance and stability criteria required for the development of ...low-temperature proton-exchange membrane fuel cells (PEMFCs). However, transposing the knowledge acquired on extended, model surfaces to practical nanomaterials remains highly challenging. Here, we propose 'surface distortion' as a novel structural descriptor, which is able to reconciliate and unify seemingly opposing notions and contradictory experimental observations in regards to the electrocatalytic oxygen reduction reaction (ORR) reactivity. Beyond its unifying character, we show that surface distortion is pivotal to rationalize the electrocatalytic properties of state-of-the-art of PtNi/C nanocatalysts with distinct atomic composition, size, shape and degree of surface defectiveness under a simulated PEMFC cathode environment. Our study brings fundamental and practical insights into the role of surface defects in electrocatalysis and highlights strategies to design more durable ORR nanocatalysts.
In this study, a carbon supported oxynitrides catalyst with ultra-low Pt concentration (2wt%, Pt-ON/C) is synthesized from Co(NO3)2⋅6H2O, (NH4)6Mo7O24⋅4H2O and PtCl4 precursors by using NH3 as ...reducing agent and nitrogen source. It is investigated as oxygen reduction reaction (ORR) catalysts for proton exchange membrane (PEM) fuel cells and compared to conventional 2wt% Pt/C (ETEK) catalyst. Electrochemical and physical properties of both materials were characterized in detail. Pt-ON/C shows competitive ORR activity similar to Pt/C (ETEK) while demonstrating an improved stability. By using post mortem analysis with transmission electron microscope/scanning transmission electron microscope (TEM/STEM), the degradation mechanisms of both catalysts are investigated. Two different dominant mechanisms were suggested to explain the decreased activity of Pt/C (ETEK) under different operation protocols: for an accelerated stress test (AST) with low maximum potentials, a loss of Pt surface area associated with carbon oxidation leads to the decreased activity; while for AST with high maximum potentials, Pt particle growth, detachment, dissolution/re-deposition and severe carbon corrosion dominate the performance loss. In addition, in the catalyst of Pt-ON/C, Mo dissolution occurs under the entire potential window which, however, leads to the enhanced activity after lifetime stability test protocol.
•A carbon supported oxynitrides with 2wt% Pt was developed, showing competitive activity and improved stability compared to Pt/C (ETEK);•Harmonized accelerated stress test (AST) protocols for PEMFCs ORR catalysts are implemented on both catalysts, demonstrating different dominant degradation mechanisms of Pt/C (ETEK) for different protocols;•Mo dissolution is observed in the sample of Pt-ON/C during the degradation test for all potentials;•Pt particle growth, detachment, dissolution/re-deposition under AST protocol are strongly potential dependent, and are only dominant for high potentials (up to 1.4V vs. RHE).
Carbon supported platinum–ruthenium (Pt–Ru/C) nanoparticles are used as anode catalysts in proton exchange membrane fuel cells (PEMFCs) operated under reformate owing to their good carbon monoxide ...tolerance. The stability of these catalysts during fuel cell operation is still not well known. In this work, we have studied by transmission electron microscopy (TEM) the microstructural evolution of a membrane/electrode assembly after a 1000 h ageing test under reformate (26 ppm CO). The analyses clearly show dissolution of Ru from the Pt–Ru/C anode catalysts, its diffusion and precipitation within the anode micro-porous layer and within the membrane. The structure and the chemistry of the membrane precipitates were accurately analysed. The high resolution TEM images and EDS (Energy Dispersive X-Ray Spectroscopy) Pt, Ru elemental maps show that the largest precipitates display a singular flower shape consisting of a Pt-rich face-centred cubic (fcc) crystallographic structure core and Ru-rich hexagonal close-packed (hcp) crystallographic structure shell. These results suggest that within the membrane the Ru reduction is catalysed by Pt. Moreover, the localization of the precipitation band near the cathode seems to indicate that the Pt in the precipitates comes from the dissolution of cathodic Pt/C and that both Pt and Ru ions are reduced by the hydrogen crossover.
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•Degradation of MEA with Pt–Ru/C anode catalysts after an ageing test with reformate.•Evidence of Ru dissolution and precipitation within the membrane or microporous layer.•Pt–Ru precipitate analysis using corrected HRTEM and EDS elemental mapping.•Pt–Ru membrane precipitate flower-shaped morphology with Pt rich core and Ru rich shell.•New precipitate formation mechanism: Pt coming from the cathode catalyses Ru ion reduction.
In this paper, we combined FTIR spectroscopy and COad stripping voltammetry to investigate COad adsorption and electrooxidation on Pt-Ru/C nanoparticles. The Pt:Ru elemental composition and the metal ...loading were determined by ICP-AES. The X-ray diffraction patterns of the Pt-Ru/C indicated formation of a Pt-Ru (fcc) alloy. HREM images revealed an increase in the fraction of agglomerated Pt-Ru/C particles with increasing the metal loading and showed that agglomerated Pt-Ru/C nanoparticles present structural defects such as twins or grain boundaries. In addition, isolated Pt-Ru/C nanoparticles have similar mean particle size (ca. 2.5nm) and particle size distributions whatever the metal loading. Therefore, we could determine precisely the effect of particle agglomeration on the COad vibrational properties and electrooxidation kinetics. FTIR measurements revealed a main COad stretching band at ca. , which we ascribed to a-top COad on Pt domains electronically modified by the presence of Ru. As the metal loading increased, the position of this band was blue shifted by ca. 5cm-1 and a shoulder around 2005cm-1 developed, which was ascribed to a-top COad on Ru domains. The reason for this was suggested to be the increasing size of Ru domains on agglomerated Pt-Ru/C particles, which lifts dipole-dipole coupling and allows two vibrational features to be observed (COad/Ru, COad/Pt). This is evidence that FTIR spectroscopy can be used to probe small chemical fluctuations of the Pt-Ru/C surface. Finally, we comment on the COad electrooxidation kinetics. We observed that COad was converted more easily into CO2 as the metal loading, i.e. the fraction of agglomerated Pt-Ru/C nanoparticles, increased.
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