Single-atom catalysis has become one of the most active new frontier in heterogeneous catalysis, as well as electrocatalysis. Single-atom catalyst (SAC) represents the ultimate in atomic effiency to minimise the consumption of expensive metals (Pt, Pd, Rh, etc.). Single-atom electrocatalyst (SAEC) also draw public attention to develop cleaner energy technique such as fuel cell to replace traditional fossil fuel. Aided by recent advances in synthetic methodologies, characterization techniques and computational modelling, amounts of SACs have been developed which exhibit distinctive performances for amounts of reactions. SACs are known to have specific active centres which are strongly affected by surrounding coordination structure, such that unique opportunities exist for the rational design of new catalysts with high activities, selectivities and stabilities. However, due to inevitable heterogeneities in SACs such as the nature of active sites and their distribution, understand of structure-performance relationship at atomic level and rational designs from fundamental insights are still challenging. Herein, we propose the use of the metal/supports SAC model system to understand the fundamental mechanisms, where we can precisely determine and even selectively modify the active site, and unravel the role of structure in catalytic activity. In this project, I describe how we will determine the sites that robustly anchor metal atoms on typical nitrogen doped carbon support in UHV, test their performance in newly-developing electrochemical cells, and further reveal reaction mechanism by in situ spectroscopy (IRAS, Raman) and theoretical calculation. The outcome will be promising to bridge the complexiy gap and recreate the optimal active sites on real SACs and lead the way into a new era where heterogeneous catalysts are designed based on fundemental insights.