Corrosion resistance of a transition-metal-rich PtCu3/C oxygen reduction reaction (ORR) catalyst as a representative of Pt alloy-based materials has been significantly improved by doping with small amounts of gold (<1 at. %). Transmission electron microscopy imaging shows near-surface segregation of both platinum and gold with the underlying core consisting predominantly of intermetallic PtCu3. The resulting PtAu skin catalyst shows improved resistance against Cu dissolution, as well as against carbon corrosion if compared to its PtCu3 precursor. Also, it exhibits a much higher Pt and carbon stability than a widely used Pt/C standard. Most importantly, the Au doped sample shows a substantial improvement in stability at the elevated temperature (60 degrees C) degradation test (10 000 cycles; 0.4-1.2 Vim) simulating a real PEM fuel cell environment

Summary Achieving highly active and stable oxygen reduction reaction performance at low platinum-group-metal loadings remains one of the grand challenges in the proton-exchange membrane fuel cells community. Currently, state-of-the-art electrocatalysts are high-surface-area-carbon-supported nanoalloys of platinum with different transition metals (Cu, Ni, Fe, and Co). Despite years of focused research, the established structure-property relationships are not able to explain and predict the electrochemical performance and behavior of the real nanoparticulate systems. In the first part of this work, we reveal the complexity of commercially available platinum-based electrocatalysts and their electrochemical behavior. In the second part, we introduce a bottom-up approach where atomically resolved properties, structural changes, and strain analysis are recorded as well as analyzed on an individual nanoparticle before and after electrochemical conditions (e.g. high current density). Our methodology offers a new level of understanding of structure-stability relationships of practically viable nanoparticulate systems.

Understanding of less-noble-metal (M) dissolution from Pt-alloy-based oxygen reduction reaction (ORR) electrocatalysts, as well its interaction with Pt surface, is crucial for maximizing their performance. In pursuing this goal, two ORR electrocatalystsa benchmark Pt–Co/C and an in-house designed Pt–Cu/C materialsare investigated. Both are characterized with a range of standard techniques, such as X-ray diffraction (XRD), transmission electron microscopy (TEM) combined with energy dispersive spectroscopy (EDX) and thin film-rotating disc electrode (TF-RDE) measurements. A special focus is put on combining the latter with a highly sensitive electrochemical flow cell (EFC) online connected to inductively coupled plasma mass spectrometry (ICP-MS) measurements. A combination of standard and novel techniques provides unprecedented insights into the dissolution behavior and dynamics of metals, as well as their subsequent surface interactions and effects on the electrochemical performance. A special focus is devoted to the significance of electrocatalyst activation protocols and their effects on metal dissolution. Distinct differences in the behavior of Pt–Cu and Pt–Co alloys are revealed, which will help develop proper protocols for practical implementation of Pt-alloy electrocatalysts.

The present research provides a study of carbon-supported intermetallic Pt-alloy electrocatalysts and assesses their stability against metal dissolution in relation to the operating temperature and the potential window using two advanced electrochemical methodologies: (i) the in-house designed high-temperature disk electrode (HT-DE) methodology as well as (ii) a modification of the electrochemical flow cell coupled to an inductively coupled plasma mass spectrometer (EFC-ICP-MS) methodology, allowing for highly sensitive time- and potential-resolved measurements of metal dissolution. While the rate of carbon corrosion follows the Arrhenius law and increases exponentially with temperature, the findings of the present study contradict the generally accepted hypothesis that the kinetics of Pt and subsequently the less noble metal dissolution are supposed to be for the most part unaffected by temperature. On the contrary, clear evidence is presented that in addition to the importance of the voltage/potential window, the temperature is one of the most critical parameters governing the stability of Pt and thus, in the case of Pt-alloy electrocatalysts, also the ability of the nanoparticles (NPs) to retain the less noble metal. Lastly, but also very importantly, results indicate that the rate of Pt redeposition significantly increases with temperature, which has been the main reason why mechanistic interpretation of the temperature-dependent kinetics related to the stability of Pt remained highly speculative until now.

The dissolution of different platinum-based nanoparticles deposited on a commercial high-surface area carbon (HSAC) support in thin catalyst films is investigated using a highly sensitive electrochemical flow cell (EFC) coupled to an inductively coupled plasma mass spectrometer (ICP-MS). The previously reported particle-size-dependent dissolution of Pt is confirmed on selected industrial samples with a mean Pt particle size ranging from 1 to 4.8 nm. This trend is significantly altered when a catalyst is diluted by the addition of HSAC. This indicates that the intrinsic dissolution properties are masked by local oversaturation phenomena, the so-called confinement effect. Furthermore, by replacing the standard HSAC support with a support having an order of magnitude higher specific surface area (a micro-and mesoporous nitrogen-doped high surface area carbon, HSANDC), Pt dissolution is reduced even further. This is due to the so-called non-intrinsic confinement and entrapment effects of the (large amount of) micropores and small mesopores doped with N atoms. The observed more effective Pt re-deposition is presumably induced by local Pt oversaturation and the presence of nitrogen nucleation sites. Overall, our study demonstrates the high importance and beneficial effects of porosity, loading and N doping of the carbon support on the Pt stability in the catalyst layer.

The present study showcases the importance of temperature and potential window for evaluation of Pt-based supported electrocatalyst stability. A platinum based commercial material with an average size of Pt nanoparticles between 2-3 nm (Pt/C) and its thermally annealed analogue with an average particle size of ∼5 nm (Pt/C-HT) are considered. X-ray diffraction (XRD), ex situ transmission electron microscopy (TEM) imaging and thin film rotating disc electrode (TF-RDE) along with proprietary hightemperature disc electrode (HT-DE) are used for electrocatalysts inspection. The study shows a clear dependence between the electrochemical surface area (ECSA) loss and the temperature increase during the potentiodynamic accelerated degradation test (ADT). Additionally it is demonstrated that selection of the lower and upper potential limits in ADT protocol plays an important role in ECSA loss. Comparing various results obtained on Pt/C and Pt/C-HT, we show that varying ADT conditions of temperature and different potential windows is crucial for adequate evaluation and stability interpretation of potentially promising novel electrocatalysts and that relatively mild ADT conditions (i.e. 0.4-1.0 V RHE , RT) can be potentially misleading.

Single-atom catalysts (SACs) have quickly emerged as a new class of catalytic materials. When confronted with classical carbon-supported nanoparticulated catalysts (Pt/C), SACs are often claimed to have superior electrocatalytic properties, e.g., stability. In this study, we critically assess this statement by investigating S-doped carbon-supported Pt SACs as a representative example of noble-metal-based SACs. We use a set of complementary techniques, which includes online inductively coupled plasma mass spectrometry (online ICP-MS), identical location transmission electron microscopy (IL-TEM), and X-ray photoelectron spectroscopy (XPS). It is shown by online ICP-MS that the dissolution behavior of as-synthesized Pt SACs is significantly different from that of metallic Pt/C. Moreover, Pt SACs are, indeed, confirmed to be more stable toward Pt dissolution. When cycled to potentials of up to 1.5 VRHE, however, the dissolution profiles of Pt SACs and Pt/C become similar. IL-TEM and XPS show that this transition is due to morphological and chemical changes caused by cycling. The latter, in turn, is a consequence of the relatively poor stability of S ligands. As monitored by online ICP-MS and XPS, significant amounts of sulfur leave the catalyst during oxidation. Hence, in case catalysts with improved stability in the anodic potential region are desired, more robust supports and ligands must be developed.

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