Ellen Suhr

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

Li–Al–O materials are interesting for several applications. Herein, a Li–Al–O thin‐film materials library, deposited by inert magnetron sputtering and postdeposition annealing in O₂ atmosphere, is used to study the effects of different annealing temperatures (300–850 °C) and durations (1 min to 7 h) on crystallinity and composition of the films. X‐ray photoelectron spectroscopy depth profiling reveals inhomogeneous compositional depth profiles of conventionally annealed films (300–650 °C, 3–7 h) with increased Li contents toward the film surface and increased Al contents toward the film–substrate interface, as confirmed by a combination of Rutherford backscattering spectrometry and deuteron‐induced nuclear reaction analysis. The minimum temperature for the formation of crystalline LiAlO₂ in the otherwise amorphous films is 550 °C. The undesired inhomogeneous depth profiles are transformed into homogeneous ones by rapid thermal annealing (750–850 °C, 1–10 min), while simultaneously causing the formation of crystalline LiAlO₂. Therefore, rapid thermal annealing is shown to be more suitable than conventional annealing for the thermal processing of Li–Al–O thin films. Moreover, this conclusion is expected to have broader relevance beyond Li–Al–O materials, as it is likely to apply to other Li metal oxides, of which there are many technologically relevant ones.

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 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 perovskites provide the opportunity to develop stable and active catalysts for electrochemical applications. The challenge lies in the identification of single‐phase perovskites with optimized composition for high electrical conductivity. Leveraging a recently discovered effect of self‐organized thin film growth during reactive sputtering, La–Co–Mn–O and La–Co–Mn–Fe–O perovskite (ABO₃) thin‐film materials libraries are synthesized. These show phase‐pure La perovskites over a wide range of chemical composition variation for the B‐site elements for deposition temperatures ≥300 °C. It is demonstrated that this approach enables the discovery and tailoring of chemical compositions for desired optical bandgap and electrical conductivity and thereby opens the path for the targeted development of, for example, new high‐performance electrocatalysts.

La‐based perovskites are versatile materials that are of interest for solid oxide fuel cells and electrocatalytic water splitting. During fabrication of composition spread thin‐film libraries of La—Co‐based oxide systems, an unusual phase formation phenomenon is observed: instead of the expected continuous composition gradient, single‐phase regions with homogeneous composition form (La₂O₃ or stoichiometric La‐perovskite). This phenomenon, which occurs during reactive cosputtering, is independent of the used substrate. However, a dependency on the O₂‐portion in the process gas and the substrate temperature is observed. It can be described as a self‐organized growth, where excess transition metal cannot be incorporated into the lattices of the forming single‐phase regions, and therefore, not into the growing film. It is hypothesized that due to the high reactivity of La and the significantly low formation energies of La₂O₃ and La‐perovskites, the reactive sputter deposition of La‐based oxide films, which is a physical vapor deposition process, can turn partially—regarding film growth—into a chemical vapor deposition‐like process. The described single‐phase regions form and lead to a discontinuous composition spread, with preferred growth of the thermodynamically most stable phases. This phenomenon can be leveraged for the exploration of multinary perovskite thin‐film libraries, where the B‐site atoms of La‐perovskites are systematically substituted.