Electrocatalysis has been the cornerstone for enhancing energy efficiency, minimizing environmental impacts and carbon emissions, and enabling a more sustainable way of meeting global energy needs. Elucidating the structure and reaction mechanisms of electrocatalysts at electrode-electrolyte interfaces is fundamental for advancing renewable energy technologies, including fuel cells, and water electrolyzers, among others. One of the fundamental challenges in electrocatalysis is understanding how to activate and sustain electrocatalytic activity, under operating conditions, for extended time periods, which calls for the use of in situ/operando methods. This thesis first introduces the design and understanding of electrocatalysts for alkaline fuel cells since they enable the use of non-precious metals to catalyze the sluggish oxygen reduction reaction (ORR) at the cathode. Metal oxide-based ORR electrocatalysts, synthesized by a hydrothermal method, in particular, Mn-Co spinel nanoparticles, have demonstrated over 1 W/cm2 benchmark peak power density with a Pt-based anode and a record 200 mW/cm2 with a Ni-based anode for a completely non-precious metal-containing alkaline fuel cell, in membrane electrode assembly (MEA) measurements. Analytical scanning transmission electron microscopy (STEM) has been extensively employed to resolve the heterogeneous crystal structures and chemical environments at the atomic scale. Operando X-ray absorption spectroscopy (XAS) methods revealed that the superior performance of Mn-Co spinels in low humidity, relative to Pt, originates from synergistic effects in which the Mn sites bind O2 while the Co sites activate H2O to facilitate the proton-coupled electron transfer process. Moving beyond oxides, we have developed nitride-core oxide-shell Co4N/C and Pd-based alloys as ORR electrocatalysts for high-power alkaline fuel cells. The second part of this thesis focuses on operando studies of electrochemical interfaces. In situ heating STEM and heating X-ray diffraction were used to track the dynamic order–disorder phase transition of Pt3Co intermetallic ORR catalysts during annealing and quantify the degree of ordering as a key structural factor for long-term MEA durability. This thesis then presents the efforts to tackle a grand challenge in physical chemistry: understanding and spatially resolving the electrochemical double layer (EDL) at electrolyte/nanocrystal electrode interfaces. Preliminary studies, with heavy halide anion and/or alkali cations as chemical probes, while promising, have yet to provide compelling evidences of potential-dependent changes of ionic distributions. However, the pursuit of these EDL studies led to the unexpected observation of cathodic corrosion, an enigmatic electrochemical process in which noble metal electrodes corrode under sufficiently reducing potentials. I employed operando EC-STEM to reveal that cathodic corrosion at solid-liquid-gas interfaces yields significantly higher levels of structural degradation for nanocrystals than bulk electrodes. The dynamic evolution of morphology, composition, and structural information was retrieved by analytical and 4D-STEM. Such microscopic studies can provide unprecedented insights into the structural evolution of nanoscale electrocatalysts during electrochemical reactions under highly reducing potentials, such as CO2 and N2 reduction.