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.

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.

The Gas Diffusion Electrode Setup as Straightforward Testing Device for Proton Exchange Membrane Water Electrolyzer Catalysts.

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.

Self-supported Pt-CoO networks combining high specific activity with high surface area for oxygen reduction.

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.

Catalyst Development for Water/CO2 Co-electrolysis.

Herein, we discuss recent research activities on the electrochemical water/CO2 co-electrolysis at the Department of Chemistry and Biochemistry of the University of Bern (Arenz and Broekmann research groups). For the electrochemical conversion of the greenhouse gas CO2 into products of higher value catalysts for two half-cell reactions need to be developed, i.e. catalysts for the reductive conversion of CO2 (CO2RR) as well as catalysts for the oxidative splitting of water (OER: Oxygen Evolution Reaction). In research, the catalysts are often investigated independently of each other as they can later easily be combined in a technical electrolysis cell. CO2RR catalysts consist of abundant materials such as copper and silver and thus mainly the product selectivity of the respective catalyst is in focus of the investigation. In contrast to that, OER catalysts (in acidic conditions) mainly consist of precious metals, e.g. Ir, and therefore the minimization of the catalytic current per gram Ir is of fundamental importance.

Accelerated Durability Test for High-Surface-Area Oxyhydroxide Nickel Supported on Raney Nickel as Catalyst for the Alkaline Oxygen Evolution Reaction.

We demonstrate a fit-for-purpose accelerated durability test (ADT) of a high-surface-area catalyst for the alkaline oxygen evolution reaction (OER). Using an automatized electrochemical setup enabled us to run a complex ADT protocol including online detection of the effective solution resistance as well as linear voltammetry, cyclic voltammetry, cyclic galvanograms, and electrochemical impedance spectroscopy (EIS) for 55 h in total. Using this protocol, we tested the service life stability of a nickel oxyhydroxide (NiOx) catalyst based on Raney Ni. The catalyst was prepared by growing nickel oxyhydroxide on high-surface-area Raney Ni and subsequent formation of the active phase. The successful synthesis of the active NiOx phase is supported by cyclic voltammetry and Raman spectroscopy. The as prepared and activated Raney NiOx exhibits an overpotential for the OER of 304 mV at 10 mA cm-2 with a Tafel slope of 53 mV dec-1 and roughness factors as high as 4515 determined by EIS during OER. By concentrating for the ADT protocol on current densities relevant for coupling water electrolysis to photovoltaics, it is demonstrated that Raney NiOx is a promising anode material candidate as it is earth abundant and its active phase exhibits high OER activity as well as stability.

Examining the Structure Sensitivity of the Oxygen Evolution Reaction on Pt Single-Crystal Electrodes: A Combined Experimental and Theoretical Study.

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.

Colloids for Catalysts: A Concept for the Preparation of Superior Catalysts of Industrial Relevance.

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.