Our research concept is presented, which focuses on a design approach for supported electrocatalysts in order to optimize their properties independent of each other. In supported electrocatalysts, the active phase consists of metal or alloy nanoparticles distributed over a conducting support, mostly carbon based. Our aim is to elucidate the role of mesoscopic properties – such as the interparticle distance on the support – for their performance in a comprehensive and systematic manner by combining catalyst synthesis, electrochemical measurements with (ex-situ and in operando) spectroscopy and microscopy. Mesoscopic properties have so far received only little attention in academic electrocatalyst design, yet some few studies demonstrate that they bear significant potential for improving the electrocatalyst performance. A proof of concept has been our pioneering work on the particle proximity effect on size-selected Pt clusters where we could decouple the particle size from the particle loading (interparticle distance).In the talk focus on the catalyst synthesis via a so-called toolbox concept of surfactant-free colloidal nanoparticles, which plays an important role for the ability to investigate such mesoscopic effects. Different colloidal catalyst synthesis strategies are discussed which consist of two steps: the preparation of the active phase (nanoparticle) as well as the deposition of the nanoparticles onto a suitable substrate (support). To obtain well-defined catalysts, a stable colloidal nanoparticle suspension is key. In the presentation the classic use of surfactants in the colloidal synthesis to obtain stable suspensions is compared to so-called surfactant-free methods, in particular a recent approach from our group using low boiling point solvents [1].[1] Jonathan Quinson, Sara Neumann, Tanja Wannmacher, Laura Kacenauskaite, Masanori Inaba, Jan Bucher, Francesco Bizzotto, Søren Simonsen, Luise Theil Kuhn, Dajana Bujak, Alessandro Zana, Matthias Arenz, Sebastian Kunz; Angewandte Chemie International Edition 57 (2018) 12338-12341