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.