Olga A. Krysiak

High-throughput experimentation was used to generate a large dataset allowing derivation of composition–activity correlations across different compositionally complex electrocatalysts and reactions.

The discovery of high‐performance electrocatalysts for water electrolysis is highly important. We use a strategy for catalyst discovery based on high‐throughput screening of a broad range of materials compositions on a thin‐film materials library to identify noble‐metal‐lean multi‐metal compositions with high activity towards the hydrogen evolution reaction in alkaline electrolyte. We demonstrate this strategy using a quaternary materials library containing Mo, Ag, Ti, and Ru fabricated by combinatorial magnetron sputtering on a 10 cm diameter wafer providing 342 measurement areas. Surprisingly, binary Mo−Ru‐containing catalyst compositions with comparatively low Ru content exhibited the highest activity. Using the polymer/metal precursor‐based spray technique, Ni foam electrodes were modified with the Mo−Ru hit compositions and evaluated in a model electrolyzer in membrane electrode assembly (MEA) configuration. The electrodes showed a very low overpotential of only 132 mV at a comparatively high current density of −2 A cm⁻² and a 24 h electrolysis stability at −1 A cm⁻² with no observable degradation after the initial electrode conditioning.

Developing a hydrogen evolution reaction (HER) electrocatalyst that can steadily drive a large current density is of great significance for industrial electrochemical water‐splitting technology. Herein, the compositionally complex noble‐metal‐lean system Co–Cu–Mo–Pd–Re is investigated for alkaline HER by a scanning droplet cell‐assisted high‐throughput screening of co‐sputtered thin‐film material libraries. The identified hit compositions are used to prepare a catalyst‐coated Ni foam‐based layer that is hot‐pressed with a specifically designed PEEK‐covered carbon‐based gas diffusion layer to obtain a novel‐design gas diffusion electrode (GDE). H₂ bubbles that collect on the electrode surface are effectively reduced by controlling the hydrophobicity of the catalyst layer of the GDE, hence enabling a high current (−1 A cm⁻²) operation for HER in alkaline water electrolysis.

A thin‐film materials library (ML) of the La–Co–Mn–O system is fabricated by hot reactive combinatorial cosputter deposition and screened for its electrocatalytic activity for the oxygen evolution reaction. Within this ML, an area with superior catalytic activity is identified. In‐depth characterization of this region reveals a unique columnar‐grown microstructure showing a large catalytic surface and excellent stability during electrocatalytic measurements. A zoom‐in into these structures shows that the columns are compositionally and structurally not homogeneous but are composed of a mixture of the perovskite phase LaCoMnO₃ and Co–Mn–O oxide. Nanoelectrochemistry using the particle on a nanoelectrode approach confirms the high activity as well as stability of the single columns.

High‐entropy alloys (HEAs), especially in the form of compositional complex solid solutions (CCSS), have gained attention in the field of electrocatalysis. However, exploring their vast composition space concerning their electrocatalytic properties imposes significant challenges. Scanning electrochemical cell microscopy (SECCM) offers high‐speed electrochemical analysis on surface areas with a lateral resolution down to tens of nm. However, high‐precision piezo positioners often used for the motion of the tip limit the area of SECCM scans to the motion range of the piezo positioners which is typically a few tens of microns. To bridge this experimental gap, the study proposes a long‐range SECCM system with a rapid gas‐exchange environmental cell for high‐throughput electrochemical characterization of 100 mm diameter HEA thin‐film material libraries (ML) obtained by combinatorial co‐sputtering. Due to the gas–liquid interface at the positioned SECCM droplet on the sample, high‐throughput evaluation under industrial current density conditions becomes feasible. This allows the direct correlation between electrocatalytic activity and material composition with high statistical reliability. The multidimensional data obtained accelerates materials discovery, development, and optimization.

Alloying dissimilar elements presents an effective strategy for enhancing the electrocatalytic properties of multi‐metal materials. This enhancement can be attributed to the modification of electronic and geometric effects, which play a crucial role in determining the overall electrocatalytic performance. However, these effects are intricately intertwined and often interrelated due to their coexistence. As a result, the improved catalytic performance of multi‐metal systems is frequently attributed to synergistic or “cocktail” effects, without clear explanations of the role of alloying and the individual contribution of each element. A high‐throughput experimentation approach is employed to investigate 342 compositions within the quaternary thin film system Pd─Ag─Cu─Fe. The substitution of Cu with Fe (different number of valence electrons) or Ag (different atomic sizes) allows for selective manipulation of electronic or geometric effects, respectively. The substitution of Ag with Fe allows for the simultaneous variation of both effects. The number of valence electrons per unit cell volume is used as a descriptor for electrocatalytic activity, specifically with respect to the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), which can be optimized through independent or simultaneous alteration of electronic and geometric effects.

High‐entropy nitrides are largely unexplored materials with high potential to show good mechanical properties, high stability against chemicals, but also promising electrocatalytic properties. The latter is due to their good electrical conductivity compared to (high‐entropy) oxides. The high‐entropy nitride system (Ti–Co–Mo–Ta–W)N is chosen for investigation based on the idea to combine binary and ternary nitrides, which show good water‐splitting activities. Thin‐film materials libraries with continuous composition spreads are deposited using reactive cosputter deposition at 300 and 500 °C. X‐Ray diffraction results show that the films consist of a single‐phase solid solution in NaCl‐type structure. The surface morphology is examined using scanning electron and atomic force microscopy. (Ti–Co–Mo–Ta–W)N films show low resistivity values in the range from 1.72 to 5.2 μΩ cm. Their oxygen evolution reaction activity is measured using a scanning droplet cell, with a maximum current density of 1.78 mA cm⁻² at 1700 mV versus reversible hydrogen electrode. The results indicate that stability is a challenge for high‐entropy nitrides, at least for their use as oxygen‐related electrocatalytic reactions.

Compositionally complex materials such as high‐entropy alloys and oxides have the potential to be efficient platforms for catalyst discovery because of the vast chemical space spanned by these novel materials. Identifying the composition of the most active catalyst materials, however, requires unraveling the descriptor‐activity relationship, as experimentally screening the multitude of possible element ratios quickly becomes a daunting task. In this work, we show that inferred adsorption energy distributions of *OH and *O on complex solid solution surfaces within the space spanned by the system Ag−Pd−Pt−Ru are coupled to the experimentally observed electrocatalytic performance for the oxygen reduction reaction. In total, the catalytic activity of 1582 alloy compositions is predicted with a cross‐validated mean absolute error of 0.042 mA/cm² by applying a theory‐derived model with only two adjustable parameters. Trends in the discrepancies between predicted electrochemical performance values of the model and the measured values on thin film surfaces subsequently provide insight into the alloys’ surface compositions during reaction conditions. Bridging this gap between computationally modeled and experimentally observed catalytic activities, not only reveals insight into the underlying theory of catalysis but also takes a step closer to realizing exploration and exploitation of high‐entropy materials.

Compositionally complex materials such as high‐entropy alloys and oxides have the potential to be efficient platforms for catalyst discovery because of the vast chemical space spanned by these novel materials. Identifying the composition of the most active catalyst materials, however, requires unraveling the descriptor‐activity relationship, as experimentally screening the multitude of possible element ratios quickly becomes a daunting task. In this work, we show that inferred adsorption energy distributions of *OH and *O on complex solid solution surfaces within the space spanned by the system Ag−Pd−Pt−Ru are coupled to the experimentally observed electrocatalytic performance for the oxygen reduction reaction. In total, the catalytic activity of 1582 alloy compositions is predicted with a cross‐validated mean absolute error of 0.042 mA/cm² by applying a theory‐derived model with only two adjustable parameters. Trends in the discrepancies between predicted electrochemical performance values of the model and the measured values on thin film surfaces subsequently provide insight into the alloys’ surface compositions during reaction conditions. Bridging this gap between computationally modeled and experimentally observed catalytic activities, not only reveals insight into the underlying theory of catalysis but also takes a step closer to realizing exploration and exploitation of high‐entropy materials.

Multi‐metal electrocatalysts provide nearly unlimited catalytic possibilities arising from synergistic element interactions. We propose a polymer/metal precursor spraying technique that can easily be adapted to produce a large variety of compositional different multi‐metal catalyst materials. To demonstrate this, 11 catalysts were synthesized, characterized, and investigated for the oxygen evolution reaction (OER). Further investigation of the most active OER catalyst, namely CoNiFeMoCr, revealed a polycrystalline structure, and operando Raman measurements indicate that multiple active sites are participating in the reaction. Moreover, Ni foam‐supported CoNiFeMoCr electrodes were developed and applied for water splitting in flow‐through electrolysis cells with electrolyte gaps and in zero‐gap membrane electrode assembly (MEA) configurations. The proposed alkaline MEA‐type electrolyzers reached up to 3 A cm⁻², and 24 h measurements demonstrated no loss of current density of 1 A cm⁻².