Electrocatalysis is the keystone to realize the transition from a fossil fuels-based energy landscape to one dominated by renewable energy sources. However, the sluggish reaction kinetics and low selectivity have severely limited their broad application. In this dissertation, I aim to advance electrocatalysis by combining atomistic insights to design high-performance electrocatalysts, and operando methods to reveal dynamic changes of electrocatalysts and reactants/products under real-time electrochemical conditions.As one of the most valuable operando techniques for electrocatalysis, differential electrochemical mass spectrometry (DEMS) was extensively employed to identify the potential dependent dynamic formation of gaseous or volatile products during the formic acid oxidation and alkene hydrogenation reactions. The successful employment of a dual-thin-layer flow cell enabled quantification of the current efficiencies of the main products and side products in the oxidation of formic acid, catalyzed by a series of Pt-Fe-Cu intermetallics. Lattice strain was proposed to account for the observed differences in current efficiencies based on a dual-pathway mechanism. DEMS was further used to evaluate alkene hydrogenation catalyzed by cobalt complexes, in which hydrogen signals were monitored as key products. The suppressed hydrogen evolution along with increased Faradaic current provided compelling evidence for alkene hydrogenation. Furthermore, the effects of electronic properties of catalysts and alkene structures on alkene hydrogenation efficiency were systematically investigated. Developing non-precious electrocatalysts for the sluggish kinetics of the oxygen reduction reaction (ORR) is regarded as one of the key bottlenecks for anion exchange membrane fuel cells (AEMFCs). I demonstrated that cobalt nitride electrocatalysts could overcome the conductivity challenge of oxide counterparts and enhance ORR performance by combining a conductive nitride core and an active oxide shell. Further, a systematic study of a family of transition metal nitrides identified cobalt nitride as the most active single metal nitride ORR electrocatalyst, which was in turn extensively evaluated by operando X-ray absorption spectroscopy during operating conditions. To unveil the underlying structure-property relationships in transition metal nitride electrocatalysts, we successfully synthesized well-defined manganese nitride nanocuboids and enriched an atomistic understanding of the interface between the surface oxide and the nitride core. A combination of theoretical calculations and microscopic study revealed that the ligand effects of the nitride core played a major role in determining the ORR performance of the surface oxide, rather than the effects of the applied tensile strain. Finally, I showed that a rigorous and systematic study of single-crystalline metal oxide/nitrides could benefit mechanistic understanding of ORR electrocatalysis for transition metal-based materials.