Compositionally complex solid solutions (CCSS) comprising five or more different elements mixed in a simple single-phase crystal structure provide conceptually unique, highly promising prospects in important scientific and technological areas, where the surface dominates properties such as – and ultimately not limited to –electrocatalysis and corrosion, crucial for future sustainable energy conversion systems.

The CRC aims to leverage the possibilities of CCSS as material design platforms by establishing a combined theoretical and experimental understanding of their atomic-scale surface features, as the unique properties of CCSS are caused by the large number of diverse poly-elemental active sites across their surface. Gaining control of and the ability to design these surface atom arrangements (SAA) has the potential to overcome limitations of conventional electrocatalysts and will pave the way to multifunctional materials, with unprecedented combinations of activity and stability as well as possibilities for cascade reactions. SAA are specific arrangements of (sub)surface atoms and their chemical identities. They form in statistical abundance the surface composition of CCSS. Knowledge of SAA is essential for designing composition-structure‑activity relations for CCSS surfaces.

The CRC aims to understand and control SAA – their formation, their variation for different systems and compositions, changes in SAA due to experimental conditions – to systematically tailor and manipulate them to design their properties. Development of new catalysts is not the primary goal; rather, advancement in fundamental understanding and exploration of new opportunities offered by SAA of CCSS. Initially, noble metal thin films will be used as model CCSS as their oxidation resistance supports SAA identification. Gaining detailed insights into the dependence of reaction mechanisms on the SAA and the understanding of CCSS surface metallurgy will then enable the design of multifunctional and sustainable electrocatalysts. This multi-dimensional challenge will be addressed by a fully interdisciplinary team (materials science, surface science, physics, chemistry, data science) using high-throughput methods in atomistic simulation, synthesis, characterisation, electrochemical probing, and in-depth experiments towards atomic-scale resolution. SAA determine electrochemical reactions on the atomic scale which is addressed by theory and atomic-scale characterisation. However, this atomistic view needs to be combined with the high-dimensional composition-property space of SAA and CCSS. Using materials informatics, we will integrate all data across scales and research areas, establish data-guided workflows, extract knowledge from data and organise it in knowledge graphs. Our holistic approach will be used to fulfil the vision of the CRC to control SAA on the atomic scale and across the surface and enable the design of ideal CCSS surfaces for specific applications.