Nejc Hodnik

New Insight into Platinum Dissolution from Nanoparticulate Platinum‐Based Electrocatalysts Using Highly Sensitive In Situ Concentration Measurements

Time‐ and potential‐resolved electrochemical Pt dissolution from commercial Pt and prepared PtCu alloy nanoparticulate catalysts have been studied under potentiodynamic conditions in 0.1 M HClO4 by using on‐line inductively coupled plasma mass spectrometry (ICP‐MS). For the first time the exact amount of dissolved Pt per cycle has been measured on real electrocatalysts. Results show clearly that Pt dissolution depends on the particle size: approximately seven times as much Pt is released into the solution from commercial 3 nm Pt particles as from a commercial 30 nm Pt sample. The stability of our prepared PtCu electrocatalyst is higher than that of a commercial 3 nm electrocatalyst, which is, however, still slightly lower than that of a commercial 30 nm Pt electrocatalyst.

Dissolution of Platinum in the Operational Range of Fuel Cells.

Abstract One of the most important practical issues in low‐temperature fuel‐cell catalyst degradation is platinum dissolution. According to the literature, it initiates at 0.6–0.9 VRHE, whereas previous time‐ and potential‐resolved inductively coupled plasma mass spectrometry (ICP–MS) experiments, however, revealed dissolution onset at only 1.05 VRHE. In this manuscript, the apparent discrepancy is addressed by investigating bulk and nanoparticulated catalysts. It is shown that, given enough time for accumulation, traces of platinum can be detected at potentials as low as 0.85 VRHE. At these low potentials, anodic dissolution is the dominant process, whereas, at more positive potentials, more platinum dissolves during the oxide reduction after accumulation. Interestingly, the potential and time dissolution dependence is similar for both types of electrode. Dissolution processes are discussed with relevance to fuel‐cell operation and plausible dissolution mechanisms are considered.

Importance and Challenges of Electrochemical in Situ Liquid Cell Electron Microscopy for Energy Conversion Research

The foreseeable worldwide energy and environmental challenges demand renewable alternative sources, energy conversion, and storage technologies. Therefore, electrochemical energy conversion devices like fuel cells, electrolyzes, and supercapacitors along with photoelectrochemical devices and batteries have high potential to become increasingly important in the near future. Catalytic performance in electrochemical energy conversion results from the tailored properties of complex nanometer-sized metal and metal oxide particles, as well as support nanostructures. Exposed facets, surface defects, and other structural and compositional features of the catalyst nanoparticles affect the electrocatalytic performance to varying degrees. The characterization of the nanometer-size and atomic regime of electrocatalysts and its evolution over time are therefore paramount for an improved understanding and significant optimization of such important technologies like electrolyzers or fuel cells. Transmission electron microscopy (TEM) and scanning transmission electron microscope (STEM) are to a great extent nondestructive characterization tools that provide structural, morphological, and compositional information with nanoscale or even atomic resolution. Due to recent marked advancement in electron microscopy equipment such as aberration corrections and monochromators, such insightful information is now accessible in many institutions around the world and provides huge benefit to everyone using electron microscopy characterization in general. Classical ex situ TEM characterization of random catalyst locations however suffers from two limitations regarding catalysis. First, the necessary low operation pressures in the range of 10(-6) to 10(-9) mbar for TEM are not in line with typical reaction conditions, especially considering electrocatalytic solid-liquid interfaces, so that the active state cannot be assessed. Second, and somewhat related, is the lack of time resolution for the evaluation of alterations of the usually highly heterogeneous nanomaterials. Two methods offer a solution to these shortcomings, namely, identical location TEM (IL-TEM) and electrochemical in situ liquid TEM. The former is already well established and has delivered novel insights particularly into degradation processes; however, characterization is still performed in vacuum. The latter circumvents this issue by using dedicated in situ TEM holders but introduces extremely demanding technical challenges. Although the introduction of revolutionizing thin SiN window cells, which elegantly confine the specimen from vacuum, has allowed demonstration of the potential of the in situ approach, the reproducibility and data interpretation is still limited predominately due to the strong interaction of the electron beam with the supporting electrolyte and electrode material. Because of the importance of understanding the nanoelectrochemical structure-function relationship, this Account aims to convey a timely perspective on the opportunities and particularly the challenges in electrochemical identical location TEM and in situ liquid cell TEM with a focus on electrochemical energy conversion.

Time Evolution of the Stability and Oxygen Reduction Reaction Activity of PtCu/C Nanoparticles

Crystalline Cu3Pt nanoparticles supported on graphitized carbon are synthesized by using a modified sol–gel method, and subsequent thermal annealing leads to alloying of Pt with Cu and formation of a partially ordered Pm

The Effect of the Voltage Scan Rate on the Determination of the Oxygen Reduction Activity of Pt/C Fuel Cell Catalyst

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SEM method for direct visual tracking of nanoscale morphological changes of platinum based electrocatalysts on fixed locations upon electrochemical or thermal treatments

m structure. Electrochemical dealloying under potentiodynamic conditions (potential cycling) induces not only changes from rather spherical high‐index faceted to more cuboctahedral low‐index faceted core–shell structures for particles in a size range of 10–20 nm but also percolation for some particles larger than 20 nm. In contrast, during dealloying under potentiostatic conditions (potential hold) the semispherical shape of small particles is completely retained and extensive porosity is formed on all particles larger than 20 nm. Other degradation processes are not observed on performing an additional accelerated aging test; hence, the high specific and mass activity of the catalyst decreases only slightly, mainly owing to continuing Cu leaching. The difference in dealloying protocols and their effect on the structure of the catalysts as well as their activities, considering the promising porosity formation, are discussed and indicate future directions for a rational design of active and stable oxygen reduction reaction catalysts.

Electrochemical Dissolution of Iridium and Iridium Oxide Particles in Acidic Media: Transmission Electron Microscopy, Electrochemical Flow Cell Coupled to Inductively Coupled Plasma Mass Spectrometry, and X-ray Absorption Spectroscopy Study

The oxygen reduction reaction (ORR) is one of the most important chemical reactions. Besides others, it plays a crucial role for the development of a sustainable energy scenario, as it is the decisive reaction in proton exchangemembrane fuel cell (PEMFC) [1, 2]. Improving the catalysis of the ORR can lead to a breakthrough in electrochemical energy conversion; therefore, the fundamental understanding of the reaction processes as well as the rapid assessment of the kinetic activity of different catalyst materials is of utmost important [3]. So far, it is well established that the surface coverage of electrosorbed oxygenated species like H2O, OH, and O determines platinum ORR activity, although the true nature of the potential dependent oxygenated species has yet to be resolved [4, 5]. Under the assumption that the Pt surface is almost fully covered at low overpotentials also referred as kinetic region, where Θad is the surface coverage of any adsorbate species, the ORR kinetic current becomes directly proportional to the pre-exponential factor (1-Θad) of the rate expression [6]. This know-how has helped to interpret effects introduced for instance by the particle size of Pt catalysts [7–9] or the development of catalysts with enhanced activities like Ptalloys with lower Θad compared to pure Pt [5, 6, 10–13]. The advancement in ORR fundamental understanding and performance evaluation has benefited to a great extent from informative half-cell kinetic survey studies of applied catalysts, as for instance performed with the thin-film rotating disk electrode technique (TF-RDE) [2, 14–16]. Such electrochemical model studies enable the determination of true kinetic current densities even of porous materials without complex interference with mass-transport or catalyst utilization effects, which are additional important factors for the final performance in electrochemical reactors [17]. In order to perform these ORR half-cell measurements accurately, certain practical guidelines have been shown to be essential. These include especially the cleanliness of the experimental setup [18], catalyst film preparation on the electrode [14], appropriate potentiostat sampling mode, evaluation of kinetic data [15], IR compensation [19], correction for true reversible hydrogen electrode (RHE) potential, and background currents, later especially at high scan rates [15]. Interestingly, although already reported in literature for polycrystalline and high surface area Pt catalysts [2, 16, 20], the effect of the voltage scan rate on the activity determination is often underestimated or not clearly separated from the effect of impurities. In the present work, the impact of the voltage scan rate on the activity determination of a high surface area Pt catalyst is systematically studied. We take special care in order to work extra clean and avoid the effect of impurities. We confirm that the relatively slow platinum surface oxidation process significantly influences the kinetic evaluation [20, 21], which is one reason behind varying ORR-specific activities in literature. Moreover, we describe the scan rate-dependent influence of chloride ions, as well as discuss the relationship to surface coverage and the implications of the results for practical applications. * Nejc Hodnik n.hodnik@mpie.de

Activation of carbon-supported catalysts by ozonized acidic solutions for the direct implementation in (electro-)chemical reactors

A general method for tracking morphological surface changes on a nanometer scale with scanning electron microscopy (SEM) is introduced. We exemplify the usefulness of the method by showing consecutive SEM images of an identical location before and after the electrochemical and thermal treatments of platinum-based nanoparticles deposited on a high surface area carbon. Observations reveal an insight into platinum based catalyst degradation occurring during potential cycling treatment. The presence of chloride clearly increases the rate of degradation. At these conditions the dominant degradation mechanism seems to be the platinum dissolution with some subsequent redeposition on the top of the catalyst film. By contrast, at the temperature of 60°C, under potentiostatic conditions some carbon corrosion and particle aggregation was observed. Temperature treatment simulating the annealing step of the synthesis reveals sintering of small platinum based composite aggregates into uniform spherical particles. The method provides a direct proof of induced surface phenomena occurring on a chosen location without the usual statistical uncertainty in usual, random SEM observations across relatively large surface areas.

In situ electrochemical dissolution of platinum and gold in organic-based solvent

Iridium-based particles, regarded as the most promising proton exchange membrane electrolyzer electrocatalysts, were investigated by transmission electron microscopy and by coupling of an electrochemical flow cell (EFC) with online inductively coupled plasma mass spectrometry. Additionally, studies using a thin-film rotating disc electrode, identical location transmission and scanning electron microscopy, as well as X-ray absorption spectroscopy have been performed. Extremely sensitive online time-and potential-resolved electrochemical dissolution profiles revealed that Ir particles dissolve well below oxygen evolution reaction (OER) potentials, presumably induced by Ir surface oxidation and reduction processes, also referred to as transient dissolution. Overall, thermally prepared rutile-type IrO2 particles are substantially more stable and less active in comparison to as-prepared metallic and electrochemically pretreated (E-Ir) analogues. Interestingly, under OER-relevant conditions, E-Ir particles exhibit superior stability and activity owing to the altered corrosion mechanism, where the formation of unstable Ir(>IV) species is hindered. Due to the enhanced and lasting OER performance, electrochemically pre-oxidized E-Ir particles may be considered as the electrocatalyst of choice for an improved low-temperature electrochemical hydrogen production device, namely a proton exchange membrane electrolyzer.

Insights into electrochemical dealloying of Cu out of Au-doped Pt-alloy nanoparticles at the sub-nano-scale

This work introduces a practical and scalable post-synthesis treatment for carbon-supported catalysts designed to achieve complete activation and, if necessary, simultaneously surface dealloying. The core concept behind the method is to control the potential without utilizing any electrochemical equipment, but rather by applying an appropriate gas mixture to a catalyst suspension.