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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 Ir∼55 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.

Iridium nanoparticles are important catalysts for several chemical and energy conversion reactions. Studies of iridium nanoparticles have also been key for the development of kinetic models of nanomaterial formation. However, compared to other metals such as gold or platinum, there is very limited knowledge on the actual formation pathway of iridium nanoparticles on the atomic and molecular level. Here, we use in situ X-ray total scattering experiments with pair distribution function 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 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 IrxCly complexes, whereas using H2IrCl6 or Na2IrCl6, the iridium nanoparticle formation follows a sudden growth after an induction period and the brief ap-pearance of a crystalline phase. With H2IrCl6, the formation of different Irn (n= 55, 55, 85, 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 presented results introduce a new iridium nanoparticle synthesis model system and provide new chemical insights into nanoparticle formation and growth.

In the present study, different concepts for the development of bifunctional oxygen reduction reaction/oxygen evolution reaction (ORR/OER) electrocatalysts are explored and compared. Bifunctional O...

The development of highly active catalysts relies on the development of simple synthesis methods. Ir based materials are state of the art catalysts for the oxygen evolution reaction (OER), the anode reaction for water electrolysis, a key technology for more sustainable energy supply. Here, metallic Ir NPs ca. 2 nm in diameter are simply obtained by reducing IrCl3 in ethanol only, without any additives. The colloidal dispersion is used to obtain Ir NPs supported on a carbon material at different metal loadings which can be employed as OER catalysts.

Carbon supported Pt based nanoparticles are important electrocatalysts for energy conversion reactions such as the oxygen reduction reaction (ORR). Although this reaction has been extensively studied, the influence of factors such as the particle size and inter-particle distance of the nanoparticle-based or nano-sized electrocatalysts on the ORR activity and durability are not yet fully understood and often intertwined. This lack of understanding is mostly based on the limitation in the synthetic approaches of the electrocatalysts which usually do not allow an independent variation of particle size and inter-particle distance. In the presented work, we succeeded to disentangle both factors using a “colloidal toolbox” approach and have demonstrated an effect of the inter-particle distance on the electronic properties of the nanoparticle via operando electrochemical X-ray absorption spectroscopy (XAS).

Carbon supported Pt based nanoparticles are important electrocatalysts for energy conversion reactions such as the oxygen reduction reaction (ORR). Although this reaction has been extensively studied, the influence of factors such as the particle size and inter-particle distance of the nanoparticle-based or nano-sized electrocatalysts on the ORR activity and durability are not yet fully understood and often intertwined. This lack of understanding is mostly based on the limitation in the synthetic approaches of the electrocatalysts which usually do not allow an independent variation of particle size and inter-particle distance. In the presented work, we succeeded to disentangle both factors using a “colloidal toolbox” approach and have demonstrated an effect of the inter-particle distance on the electronic properties of the nanoparticle via operando electrochemical X-ray absorption spectroscopy (XAS).

Water electrolysis is a pivotal technology to drive the energy transition towards a system based on renewable resources. The scarce Ir is a crucial element for the synthesis of heterogeneous catalysts for the oxygen evolution reaction (OER). Carbon supported Ir oxide catalysts obtained from surfactant-free colloidal Ir nanoparticles (NPs) synthesized in alkaline methanol (MeOH), ethanol (EtOH), and ethylene glycol (EG) are investigated and compared. The comparison of independent techniques such as transition electron microscopy (TEM), small angle X-ray scattering (SAXS), and electrochemistry allows shedding light on the parameters that affect the dispersion of the active phase as well as the initial catalytic activity. The colloidal dispersions obtained are suitable to develop supported OER catalysts with little NP agglomeration on a carbon support. Due to the high dispersion of the active phase, initial catalytic activities of more than 400 A g−1Ir are reached at 1.5 VRHE when using carbon as a model support. While the more common surfactant-free alkaline EG synthesis requires flocculation and re-dispersion leading to Ir loss, the main difference between methanol and ethanol as solvent is related to the dispersibility of the support material. The choice of the suitable monoalcohol determines the maximum achieved Ir loading on the support without detrimental particle agglomeration. This simple consideration on catalyst design can readily assist the implementation of more relevant support materials for technical applications and significantly improved catalysts.

Several concepts for platinum-based catalysts for the oxygen reduction reaction (ORR) are presented that exceed the US Department of Energy targets for Pt-related ORR mass activity. Most concepts achieve their high ORR activity by increasing the Pt specific activity at the expense of a lower electrochemically active surface area (ECSA). In the potential region controlled by kinetics, such a lower ECSA is counterbalanced by the high specific activity. At higher overpotentials, however, which are often applied in real systems, a low ECSA leads to limitations in the reaction rate not by kinetics, but by mass transport. Here we report on self-supported platinum–cobalt oxide networks that combine a high specific activity with a high ECSA. The high ECSA is achieved by a platinum–cobalt oxide bone nanostructure that exhibits unprecedentedly high mass activity for self-supported ORR catalysts. This concept promises a stable fuel-cell operation at high temperature, high current density and low humidification. A high oxygen reduction reaction activity can usually be realized by increasing platinum specific activity at the expense of active surface area. Self-supported platinum–cobalt-oxide networks combining high activity and surface area now promise a stable fuel-cell operation.

In the present study different concepts for the development of bifunctional oxygen reduction reaction / oxygen evolution reaction (ORR / OER) electrocatalysts are explored and compared. Bifunctional ORR / OER catalysts are often suggested to improve the stability during startup and shutdown of fuel cells. Furthermore, they are proposed for so-called unitized regenerative fuel cells (URFCs) that would allow a closed loop system to use and produce hydrogen on demand. We compare the electrocatalytic performance of conventional PtxIry alloy nanoparticles (NPs) with Pt – IrO2 NP composites (nanocomposites), both immobilized onto a commercial carbon support. The Pt – IrO2 nanocomposites thereby consist of a mixture of Pt NPs and IrO2 NPs. By probing the electrocatalytic performance before and after exposing the electrocatalysts to accelerated degradation tests (ADTs) it is shown that the Pt – IrO2 nanocomposite concept offers advantages but also some disadvantages over the conventional alloy concept. In particular it is shown that while the nanocomposites are initially less active for the ORR due to an interparticle effect, their performance is less affected by the ADTs. However, all tested catalysts experience a decline of the Ir / Pt ratio upon the ADTs treatment, highlighting the challenging stability requirements for URFCs.

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.

Compared to conventional preparation methods for supported heterogeneous catalysts, the use of colloidal nanoparticles (NPs) allows for a precise control over size, size distribution, and distribution/location of the NPs on the support. However, common colloidal syntheses have restrictions that limit their applicability for industrial catalyst preparation. We present a simple, surfactant‐free, and scalable preparation method for colloidal NPs to overcome these restrictions. We demonstrate how precious‐metal NPs are prepared in alkaline methanol, how the particle size can be tuned, and how supported catalysts are obtained. The potential of these colloids in the preparation of improved catalysts is demonstrated by two examples from heterogeneous catalysis and electrocatalysis.

Cost and lifetime currently hinder widespread commercialization of polymer electrolytemembrane fuel cells (PEMFCs). Reduced electrode Pt loadings lower costs; however, the impactof metal loading (on the support) and its relation to degradation (lifetime) remain unclear. Thelimited research on these parameters stems from synthetic difficulties and lack of in situanalytics. This study addresses these challenges by synthesizing 2D and 3D Pt/C model catalystsystems via two precise routes and systematically varying the loading. Pt dissolution wasmonitored using on-line inductively coupled plasma mass spectrometry (on-line-ICP-MS), whileX-ray spectroscopy techniques were applied to establish the oxidation states of Pt in correlationwith metal loading. Dissolution trends emerge which can be explained by three particleproximity dependent mechanisms: (1) shifts in the Nernst dissolution potential, (2) redeposition,and (3) alteration of Pt oxidation states. These results identify engineering limitations, whichshould be considered by researchers in fuel cell development and related fields.

In the present work we investigate the structure sensitivity of the oxygen evolution reaction (OER) combining electrochemistry, in situ spectroscopy and density functional theory calculations. The intrinsic difficulty of such studies is the fact that at electrode potentials where the OER is observed, the electrode material is highly oxidized. As a consequence, the surface structure during the reaction is in general ill-defined and only scarce knowledge exists concerning the structure-activity relationship of this important reaction. To alleviate these challenging conditions, we chose as starting point well-defined Pt single-crystal electrodes, which we exposed to well-defined conditioning before studying their OER rate. Using this approach, a potential region is identified where the OER on Pt is indeed structure-sensitive with Pt(100) being significantly more active than Pt(111). This experimental finding is in contrast to a DFT analysis of the adsorption strength of the reaction intermediates O*, OH*, and OOH* often used to plot the activity in a volcano curve. It is proposed that as a consequence of the highly oxidizing conditions, the structure-sensitive charge-transfer resistance through the interface determines the observed reaction rate.

In this work, we present a facile and straightforward approach to synthesize, activate and benchmark small, i.e. 1.6 nm in diameter, Ir nanoparticles (NP) as oxygen evolution reaction (OER) catalysts. It is shown that the Ir NP, although oxidized after synthesis and drying, can be electrochemically reduced allowing determination of the electrochemically active surface area by CO stripping. Subsequently, an activation protocol is applied forming catalytically active Ir-oxide NP. This oxide formation is shown to be largely irreversible. It is then demonstrated that the activated Ir NP synthesized via our recently introduced colloidal method exhibit extremely high OER activities when normalized to their Ir mass. These high OER activities are related to superior dispersion as compared to state-of-the-art OER catalysts reported in the literature.

This work concerns a new technique to post-modify the catalytic material intended for use in H2O2 direct synthesis. The catalyst chosen was a commercially available 1 wt.% Pd/C. The catalyst was modified with the so-called Catalyst Wet Pretreatment Method (CWPM) that is used to post-modify prepared catalysts with aqueous solutions of NaBr, in different concentrations. The performance of pristine and the pretreated materials were then compared in the H2O2 direct synthesis and characterized before and after the catalytic tests in order to understand the role of the different concentrations of bromide in the CWPM procedure. The surface features of the different catalysts were analyzed with CO chemisorption (metal dispersion and mean particle size), Transmission Electron Microscopy (TEM, for Pd morphology and Pd particle size distributions), Inductive Coupled Plasma (ICP, for Pd content) and Ion Chromatography (IC, for bromide content). Various features of the materials prepared with the CWPM were correlated with the catalytic performance. It was found that the bromide has an active role in the reconstruction of metal phase and it does not only act as a poison for the most active catalytic sites as often reported in literature. By using this new protocol, the production H2O2 was almost doubled compared to the non-modified material when no direct promoters were added to the reaction environment.

We introduce a simple and scalable preparation method for colloidal precious metal nanoparticles (NPs) that overcomes the typical restrictions in catalyst synthesis, i.e. the independent control of individual parameters such as size, size-distribution, and particle density (metal loading). Our Co4Cat (Colloids for Catalysts) synthesis concept uses alkaline low-boiling point solvents without the use of additional surfactants. It can be applied to different mono- and multimetallic precious metal NPs as well as different support types. In the presented work, the Co4Cat concept is used to prepare Ir based NPs that are tested as oxygen evolution reaction (OER) catalysts for water electrolysis under acidic conditions. In electrochemical half-cell tests, it is shown that due to the very high surface area, exceptional mass activities can be achieved. The Co4Cat concept is an advancement of our “toolbox” synthesis approach that was previously applied to Pt based PEM fuel cell catalysts. In the presentation, the general idea as well as the individual synthesis steps are discussed in detail and the electrochemical testing is benchmarked against literature values as well as commercially available OER catalysts.