Achieving high selectivity in electrochemical CO₂ reduction (CO₂RR) requires precisecontrol over the dynamic catalyst-electrolyte interface, a key challenge for developing scalable carbon conversion technologies. This dissertation confronts this challenge by employing high- resolution operando techniques to investigate the central role of the electrical double layer (EDL) and applied pulse frequency in governing reaction pathways. Using sub-second differential electrochemical mass spectrometry (DEMS), this work reveals how the EDL functions as a programmable nanoreactor rather than a passive capacitor. We demonstrate that pulse frequency is a primary control lever, dictating the balance between non- faradaic charging and faradaic reaction kinetics. This temporal control directly manipulates the competitive adsorption of key intermediates on the catalyst surface, providing a robust method for steering selectivity between C₁ and C₂ products on sub-second timescales. These fundamental, time-resolved insights are then extended from model systems to industrially relevant gas diffusion electrodes (GDEs) to explore how double layer interactions impact device performance at commercially relevant current densities. This framework is further validated in complex and heterogeneous electrolytes, including natural seawater, demonstrating the broad applicability of using dynamic potential modulation to stabilize CO₂RR in real-world environments. By explicitly connecting sub-second interfacial dynamics with applied device engineering, this research establishes a foundational understanding of how pulse frequency can be used to control electrocatalytic microenvironments. The findings offer a set of rational design principles for developing next-generation CO₂ conversion technologies with enhanced performance and stability.