Since July 2023: Postdoc.Mobility Fellow of Swiss National Science Foundation (SNSF)
April 2022 - June 2023: Postdic Fellow of German National Academy of Sciences Leopoldina
- Mechanistic Insights into Aldehyde Production from Electrochemical CO2 Reduction on CuAg Alloy via Operando X-ray Measurements ACS CATALYSIS 2023: 9379-9391
- Tracking the Dynamics of a Ag-MnO x Oxygen Reduction Catalyst Using In Situ and Operando X-ray Absorption Near-Edge Spectroscopy ACS ENERGY LETTERS 2023
Influence of Temperature on the Performance of Carbon- and ATO-supported Oxygen Evolution Reaction Catalysts in a Gas Diffusion Electrode Setup.
2023; 13 (11): 7568-7577
State-of-the-art industrial electrocatalysts for the oxygen evolution reaction (OER) under acidic conditions are Ir-based. Considering the scarce supply of Ir, it is imperative to use the precious metal as efficiently as possible. In this work, we immobilized ultrasmall Ir and Ir0.4Ru0.6 nanoparticles on two different supports to maximize their dispersion. One high-surface-area carbon support serves as a reference but has limited technological relevance due to its lack of stability. The other support, antimony-doped tin oxide (ATO), has been proposed in the literature as a possible better support for OER catalysts. Temperature-dependent measurements performed in a recently developed gas diffusion electrode (GDE) setup reveal that surprisingly the catalysts immobilized on commercial ATO performed worse than their carbon-immobilized counterparts. The measurements suggest that the ATO support deteriorates particularly fast at elevated temperatures.
View details for DOI 10.1021/acscatal.3c01193
View details for PubMedID 37288094
Chemical Insights into the Formation of Colloidal Iridium Nanoparticles from In Situ X-ray Total Scattering: Influence of Precursors and Cations on the Reaction Pathway.
Journal of the American Chemical Society
Iridium nanoparticles are important catalysts for several chemical and energy conversion reactions. Studies of iridium nanoparticles have also been a key for the development of kinetic models of nanomaterial formation. However, compared to other metals such as gold or platinum, knowledge on the nature of prenucleation species and structural insights into the resultant nanoparticles are missing, especially for nanoparticles obtained from IrxCly precursors investigated here. We use in situ X-ray total scattering (TS) experiments with pair distribution function (PDF) analysis to study a simple, surfactant-free synthesis of colloidal iridium nanoparticles. The reaction is performed in methanol at 50 °C with only a base and an iridium salt as precursor. From different precursor salts─IrCl3, IrCl4, H2IrCl6, or Na2IrCl6─colloidal nanoparticles as small as Ir55 are obtained as the final product. The nanoparticles do not show the bulk iridium face-centered cubic (fcc) structure but show decahedral and icosahedral structures. The formation route is highly dependent on the precursor salt used. Using IrCl3 or IrCl4, metallic iridium nanoparticles form rapidly from IrxClyn- complexes, whereas using H2IrCl6 or Na2IrCl6, the iridium nanoparticle formation follows a sudden growth after an induction period and the brief appearance of a crystalline phase. With H2IrCl6, the formation of different Irn (n = 55, 55, 85, and 116) nanoparticles depends on the nature of the cation in the base (LiOH, NaOH, KOH, or CsOH, respectively) and larger particles are obtained with larger cations. As the particles grow, the nanoparticle structure changes from partly icosahedral to decahedral. The results show that the synthesis of iridium nanoparticles from IrxCly is a valuable iridium nanoparticle model system, which can provide new compositional and structural insights into iridium nanoparticle formation and growth.
View details for DOI 10.1021/jacs.2c10814
View details for PubMedID 36631996
- Investigating the Particle Growth in Bimodal Pt/C Catalysts by In-Situ Small-Angle X-ray Scattering: Challenges in the Evaluation of Stress Test Protocol-Dependent Degradation Mechanisms JOURNAL OF THE ELECTROCHEMICAL SOCIETY 2022; 169 (10)
Nanocomposite Concept for Electrochemical In Situ Preparation of Pt-Au Alloy Nanoparticles for Formic Acid Oxidation
2022; 2 (7): 1757-1768
Herein, we report a straightforward approach for the in situ preparation of Pt-Au alloy nanoparticles from Pt + xAu/C nanocomposites using monometallic colloidal nanoparticles as starting blocks. Four different compositions with fixed Pt content and varying Pt to Au mass ratios from 1:1 up to 1:7 were prepared as formic acid oxidation reaction (FAOR) catalysts. The study was carried out in a gas diffusion electrode (GDE) setup. It is shown that the presence of Au in the nanocomposites substantially improves the FAOR activity with respect to pure Pt/C, which serves as a reference. The nanocomposite with a mass ratio of 1:5 between Pt and Au displays the best performance during potentiodynamic tests, with the electro-oxidation rates, overpotential, and poisoning resistance being improved simultaneously. By comparison, too low or too high Au contributions in the nanocomposites lead to an unbalanced performance in the FAOR. The combination of operando small-angle X-ray scattering (SAXS), scanning transmission electron microscopy (STEM) elemental mapping, and wide-angle X-ray scattering (WAXS) reveals that for the nanocomposite with a 1:5 mass ratio, a conversion between Pt and Au from separate nanoparticles to alloy nanoparticles occurs during continuous potential cycling in formic acid. By comparison, the nanocomposites with lower Au contents, for example, 1:2, exhibit less in situ alloying, and the concomitant performance improvement is less pronounced. On applying identical location transmission electron microscopy (IL-TEM), it is revealed that the in situ alloying is due to Pt dissolution and re-deposition onto Au as well as Pt migration and coalescence with Au nanoparticles.
View details for DOI 10.1021/jacsau.2c00335
View details for Web of Science ID 000833992500001
View details for PubMedID 35911453
View details for PubMedCentralID PMC9327087
- Tracking the Catalyst Layer Depth-Dependent Electrochemical Degradation of a Bimodal Pt/C Fuel Cell Catalyst: A Combined Operando Small- and Wide-Angle X-ray Scattering Study ACS CATALYSIS 2022; 12 (3): 2077-2085
Anion Dependent Particle Size Control of Platinum Nanoparticles Synthesized in Ethylene Glycol
2021; 11 (8)
The polyol synthesis is a well-established method to form so-called "surfactant-free" nanoparticles (NPs). In the present study, the NP size resulting from the thermal reduction of the precursors H2PtCl6, H2Pt(OH)6, or Pt(acac)2 in presence of the bases NaOH or Na(acac) at different concentrations is studied. It is shown that the size control depends more strongly on the nature of the precursor (metal salt) than on the anion present in the base. The latter is surprising as the concentration of the base anion is often an important factor to achieve a size control. The reduction of H2PtCl6 or H2Pt(OH)6 in presence of NaOH and Na(acac) confirm the observation that the NP size is determined by the OH-/Pt molar ratio and expands it to the base anion/Pt molar ratio. In contrast, the reduction of Pt(acac)2 in presence of the bases NaOH (previous reports) or Na(acac) (shown in the present work) leads to larger NPs of ca. 3 nm, independent of the concentration of the base anions. Hence, the anion effect observed here seems to originate predominantly from the nature of the precursor (precursor anion dependence) and only for certain precursors as H2PtCl6 or H2Pt(OH)6 the size control depends on the base anion/Pt molar ratio.
View details for DOI 10.3390/nano11082092
View details for Web of Science ID 000690080400001
View details for PubMedID 34443923
View details for PubMedCentralID PMC8400561
- Surfactant-free colloidal strategies for highly dispersed and active supported IrO2 catalysts: Synthesis and performance evaluation for the oxygen evolution reaction JOURNAL OF CATALYSIS 2021; 401: 54-62
- Operando SAXS study of a Pt/C fuel cell catalyst with an X-ray laboratory source JOURNAL OF PHYSICS D-APPLIED PHYSICS 2021; 54 (29)
Insights from In Situ Studies on the Early Stages of Platinum Nanoparticle Formation
JOURNAL OF PHYSICAL CHEMISTRY LETTERS
2021; 12 (12): 3224-3231
Understanding the formation of nanomaterials down to the atomic level is key to rational design of advanced materials. Despite their widespread use and intensive study over the years, the detailed formation mechanism of platinum (Pt) nanoparticles remains challenging to explore and rationalize. Here, various in situ characterization techniques, and in particular X-ray total scattering with pair distribution function (PDF) analysis, are used to follow the structural and chemical changes taking place during a surfactant-free synthesis of Pt nanoparticles in alkaline methanol. Polynuclear structures form at the beginning of the synthesis, and Pt-Pt pair distances are identified before any nanoparticles are generated. The structural motifs best describing the species formed change with time, e.g., from [PtCl5-PtCl5] and [PtCl6-Pt2Cl6-PtCl6] to [Pt2Cl10-Pt3Cl8-Pt2Cl10]. The formation of these polynuclear structures with Pt-Pt coordination before the formation of the nanoparticles is suggested to account for the fast nucleation observed in the synthesis.
View details for DOI 10.1021/acs.jpclett.1c00241
View details for Web of Science ID 000636950500029
View details for PubMedID 33764071
The Gas Diffusion Electrode Setup as Straightforward Testing Device for Proton Exchange Membrane Water Electrolyzer Catalysts
2021; 1 (3): 247-251
Hydrogen production from renewable resources and its reconversion into electricity are two important pillars toward a more sustainable energy use. The efficiency and viability of these technologies heavily rely on active and stable electrocatalysts. Basic research to develop superior electrocatalysts is commonly performed in conventional electrochemical setups such as a rotating disk electrode (RDE) configuration or H-type electrochemical cells. These experiments are easy to set up; however, there is a large gap to real electrochemical conversion devices such as fuel cells or electrolyzers. To close this gap, gas diffusion electrode (GDE) setups were recently presented as a straightforward technique for testing fuel cell catalysts under more realistic conditions. Here, we demonstrate for the first time a GDE setup for measuring the oxygen evolution reaction (OER) of catalysts for proton exchange membrane water electrolyzers (PEMWEs). Using a commercially available benchmark IrO2 catalyst deposited on a carbon gas diffusion layer (GDL), it is shown that key parameters such as the OER mass activity, the activation energy, and even reasonable estimates of the exchange current density can be extracted in a realistic range of catalyst loadings for PEMWEs. It is furthermore shown that the carbon-based GDL is not only suitable for activity determination but also short-term stability testing. Alternatively, the GDL can be replaced by Ti-based porous transport layers (PTLs) typically used in commercial PEMWEs. Here a simple preparation is shown involving the hot-pressing of a Nafion membrane onto a drop-cast glycerol-based ink on a Ti-PTL.
View details for DOI 10.1021/jacsau.1c00015
View details for Web of Science ID 000651113200002
View details for PubMedID 34467289
View details for PubMedCentralID PMC8395656
- Carbon-Supported Platinum Electrocatalysts Probed in a Gas Diffusion Setup with Alkaline Environment: How Particle Size and Mesoscopic Environment Influence the Degradation Mechanism ACS CATALYSIS 2020; 10 (21): 13040-13049
- A New Approach to Probe the Degradation of Fuel Cell Catalysts under Realistic Conditions: Combining Tests in a Gas Diffusion Electrode Setup with Small Angle X-ray Scattering JOURNAL OF THE ELECTROCHEMICAL SOCIETY 2020; 167 (13)
Teaching old precursors new tricks: Fast room temperature synthesis of surfactant-free colloidal platinum nanoparticles
JOURNAL OF COLLOID AND INTERFACE SCIENCE
2020; 577: 319-328
A fast, simple, instrument-free room temperature synthesis of stable electroactive surfactant-free colloidal Pt nanoparticles in alkaline methanol and methanol-water mixtures is presented. Pair distribution function (PDF) analysis suggests that methoxy substitution of chloride ligands from H2PtCl6 occurs in methanol. X-ray absorption spectroscopy (XAS) studies and UV-vis measurements show that solutions of H2PtCl6 in methanol age and are reduced to Pt(II) species over time. These species are ideal precursors to significantly reduce the induction period typically observed in colloidal Pt nanoparticle syntheses as well as the temperature needed to form nanoparticles. The room temperature synthesis presented here allows designing simple in situ studies of the nanoparticle formation. In situ infra-red spectroscopy gives insight into the formation and stabilization mechanism of surfactant-free nanoparticles by CO surface groups. Finally, the surfactant-free nanoparticles ca. 2-3 nm in diameter obtained are shown to be readily active electrocatalysts e.g. for methanol oxidation. The synthesis approach presented bears several advantages to design new studies and new syntheses of surfactant-free colloidal nanomaterials.
View details for DOI 10.1016/j.jcis.2020.05.078
View details for Web of Science ID 000556588700032
View details for PubMedID 32497917
- Visible-Light-Induced Synthesis of " Surfactant-Free" Pt Nanoparticles in Ethylene Glycol as a Synthetic Approach for Mechanistic Studies on Nanoparticle Formation JOURNAL OF PHYSICAL CHEMISTRY C 2020; 124 (39): 21798-21809
- UV-induced syntheses of surfactant-free precious metal nanoparticles in alkaline methanol and ethanol NANOSCALE ADVANCES 2020; 2 (6): 2288-2292
- Testing fuel cell catalysts under more realistic reaction conditions: accelerated stress tests in a gas diffusion electrode setup JOURNAL OF PHYSICS-ENERGY 2020; 2 (2)
- Halide-Induced Leaching of Pt Nanoparticles - Manipulation of Particle Size by Controlled Ostwald Ripening CHEMNANOMAT 2019; 5 (4): 462-471
- Direct synthesis of H2O2 on PdZn nanoparticles: The impact of electronic modifications and heterogeneity of active sites JOURNAL OF CATALYSIS 2018; 368: 261-274