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 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.

sales of light electric vehicles (battery electric vehicles and plug-in hybrid electric vehicles) are politically pushed and increasing exponentially. [1] Vehicles based on proton-exchange-membrane fuel cells (PEMFCs) have the potential to surpass the limitations of battery-based ones, especially, regarding the driving range. Unfortunately, the mass commercialization of this technology is hampered by the limited availability and high cost of Pt, which is required to speed up the anodic and cathodic reactions happening in a PEMFC. [2,3] Since ≈4 times more Pt is required on the cathode than at the anode side, the development of cathode materials containing low Pt amount is a promising way to reduce costs. Unfortunately, low-Pt cathode materials suffer from other limitations, such as losses due to mass transport; [4] also, when the Pt content is reduced below 100 µg cm −2 , the cost of other components rises. [3] For these reasons, completely replacing the Pt at the cathode side is a reasonable and promising way to go.In the last 10 years, platinum-group-metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) have drawn the attention of many research groups all over the world. Since it has been shown that bioinspired metal-nitrogen-doped-carbon (M-N-C, M = Fe, Co) catalysts could meet the requirements for Atomically dispersed Fe-N-C catalysts are considered the most promising precious-metal-free alternative to state-of-the-art Pt-based oxygen reduction electrocatalysts for proton-exchange membrane fuel cells. The exceptional progress in the field of research in the last ≈30 years is currently limited by the moderate active site density that can be obtained. Behind this stands the dilemma of metastability of the desired FeN 4 sites at the high temperatures that are believed to be a requirement for their formation. It is herein shown that Zn 2+ ions can be utilized in the novel concept of active-site imprinting based on a pyrolytic template ion reaction throughout the formation of nitrogen-doped carbons. As obtained atomically dispersed Zn-N-Cs comprising ZnN 4 sites as well as metal-free N 4 sites can be utilized for the coordination of Fe 2+ and Fe 3+ ions to form atomically dispersed Fe-N-C with Fe loadings as high as 3.12 wt%. The Fe-N-Cs are active electocatalysts for the oxygen reduction reaction in acidic media with an onset potential of E 0 = 0.85 V versus RHE in 0.1 m HClO 4 . Identical location atomic resolution transmission electron microscopy imaging, as well as in situ electrochemical flow cell coupled to inductively coupled plasma mass spectrometry measurements, is employed to directly prove the concept of the active-site imprinting approach.

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Electrochemical stability of a commercial Au/C catalyst in an acidic electrolyte has been investigated by an accelerated stress test (AST), which consisted of 10,000 voltammetric scans (1 V/s) in the potential range between 0.58 and 1.41 V RHE . Loss of Au electrochemical surface area (ESA) during the AST pointed out to the degradation of Au/C. Coupling of an electrochemical flow cell with ICP-MS showed that only a minor amount of gold is dissolved despite the substantial loss of gold ESA during the AST (∼35% of initial value remains at the end of the AST). According to the electrochemical mass spectrometry experiments, carbon corrosion occurs during the AST but to a minor extent. By using identical location scanning electron microscopy and identical location transmission electron microscopy, it was possible to discern that the dissolution of small Au particles (<5 nm) within the polydisperse Au/C sample is the main degradation mechanism. The mass of such particles gives only a minor contribution to the overall Au mass of the polydisperse sample while giving a major contribution to the overall ESA, which explains a significant loss of ESA and minor loss of mass during the AST. The addition of low amounts of chloride anions (10 −4 M) substantially promoted the degradation of gold nanoparticles. At an even higher concentration of chlorides (10 −2 M), the dissolution of gold was rather effective, which is useful from the recycling point of view when rapid leaching of gold is desirable.

A multifunctional electrocatalytic composite was synthesized for use as a generic catalytic material in the polymer electrolyte membrane (PEM) type reactors, working in either fuel cell, direct methanol fuel cell, or electrolyzer mode. It is constructed from graphene sheets evenly covered with TiON x on which PtCu, Ir, and Ru nanoparticles are immobilized. It enables high performances for oxygen reduction, hydrogen and methanol oxidation, and also hydrogen and oxygen evolution. Its activity is either higher or comparable to the monofunctional benchmark catalysts, namely, Pt, PtRu, and Ir nanoparticles. An important part of this study is the advanced electrochemical degradation investigation. With the use of an electrochemical flow cell coupled to ICP-MS and combining these results with identical location electron microscopy, we reveal not only potential shortcomings but also opportunities for our material. As a result, we also put forward general guidelines for the appropriate use of multifunctional electrocatalysts.

Nanoparticles of platinum-group metals (PGM) on carbon supports are widely used as catalysts for a number of chemical and electrochemical conversions on laboratory and industrial scale. The newly emerging field of single-atom catalysis focuses on the ultimate level of metal dispersion, i.e. atomically dispersed metal species anchored on the substrate surface. However, the presence of single atoms in traditional nanoparticle-based catalysts remains largely overlooked. In this work, we use aberration-corrected scanning transmission electron microscope to investigate four commercially available nanoparticle-based PGM/C catalysts (PGM = Ru, Rh, Pd, Pt). Annular dark-field (ADF) images at high magnifications reveal that in addition to nanoparticles, single atoms are also present on the surface of carbon substrates. Scanning electron microscopy, X-ray diffraction and size distribution analysis show that the materials vary in nanoparticle size and type of carbon support. These observations raise questions about the possible ubiquitous presence of single atoms in conventional nanoparticle PGM/C catalysts and the role they may play in their synthesis, activity, and stability. We critically discuss the observations with regard to the quickly developing field of single atom catalysis.