Semiconductor-based nanomaterials have been widely studied as promising electrocatalysts for energy conversion and storage. Understanding the reaction pathways and active sites on electrocatalysts are of critical importance for the development of highly active electrocatalysts made of earth abundant elements. In this talk, I will give a detailed summary about our approach using various in-situ and ex-situ techniques including X-ray absorption, X-ray diffraction, X-ray photoemission spectroscopy, electrochemical impedance spectroscopy, Rama spectroscopy, etc to study electrochemical reactions.