Following Paths of Maximum Catalytic Activity in the Composition Space of High‐Entropy Alloys

The search for better and cheaper electrocatalysts is vital in the global transition to renewable energy resources. High-entropy alloys (HEAs) provide a near-infinite number of different alloys with approximately continuous properties such as catalytic activity. In this work, the catalytic activity for the electrochemical oxygen reduction reaction as a function of molar composition of Ag-Ir-Pd-Pt-Ru HEA is treated as a landscape wherein it is shown that the maxima are connected through ridges. By following the ridges, it is possible to navigate between the maxima using a modified nudged elastic band (NEB) model integrated in a machine learning NEB algorithm. These results provide a new understanding of the composition space being similar to an evolutionary landscape. This provides a possible new search and design strategy for new catalysts in which the composition of known catalysts can be optimized by following ridges rather than exploring the whole alloy composition space

Bayesian Optimization of High‐Entropy Alloy Compositions for Electrocatalytic Oxygen Reduction**

Active, selective and stable catalysts are imperative for sustainable energy conversion, and engineering materials with such properties are highly desired. High-entropy alloys (HEAs) offer a vast compositional space for tuning such properties. Too vast, however, to traverse without the proper tools. Here, we report the use of Bayesian optimization on a model based on density functional theory (DFT) to predict the most active compositions for the electrochemical oxygen reduction reaction (ORR) with the least possible number of sampled compositions for the two HEAs Ag-Ir-Pd-Pt-Ru and Ir-Pd-Pt-Rh-Ru. The discovered optima are then scrutinized with DFT and subjected to experimental validation where optimal catalytic activities are verified for Ag-Pd, Ir-Pt, and Pd-Ru binary systems. This study offers insight into the number of experiments needed for exploring the vast compositional space of multimetallic alloys which has been determined to be on the order of 50 for ORR on these HEAs

A New Approach to Probe the Degradation of Fuel Catalysts Under Realistic Conditions: Combining Tests in a Gas Diffusion Electrode Setup with Small Angle X-Ray Scattering

A new approach for efficiently investigating the degradation of fuel cell catalysts under realistic conditions is presented combining accelerated stress tests (ASTs) in a gas diffusion electrode (GDE) setup with small angle X-ray scattering (SAXS). GDE setups were recently introduced as a novel testing tool combining the advantages of classical electrochemical cells with a three-electrode setup and membrane electrode assemblies (MEAs). SAXS characterization of the catalyst layer enables an evaluation of the particle size distribution of the catalyst and its changes upon applying an AST. The straight-forward approach not only enables stability testing of fuel cell catalysts in a comparative and reproducible manner, it also allows mechanistic insights into the degradation mechanism. In contrast to standard rotating disk electrode measurements or identical location microscopy, typical metal loadings for proton exchange membrane fuel cells (PEMFCs), i.e. 0.2 mgPt cm-2geo, are applied in the GDE and the degradation of the overall (whole) catalyst layer is probed. For the first time, realistic degradation tests can be performed comparing a set of catalysts with several repeats within reasonable time. It is demonstrated that independent of the initial particle size in the pristine catalyst, for ASTs simulating load cycle conditions in a PEMFC, all catalysts degrade to a similar particle size distribution. <br /

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

[Image: see text] 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 IrO(2) 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