The development of devices for the electrocatalytic transformation of carbon dioxide to fuels and chemicals at global scales is a promising alternative to store renewable electricity from intermittent sources and reduce carbon emissions. The realization of a technology that can produce C2+ liquid fuels at high rates requires a concerted effort to develop an efficient and selective electrocatalyst. However, transport phenomena inherent in aqueous electrocatalysis interfere with the observation of intrinsic reaction kinetics. The knowledge gap in the field to distinguish reaction kinetics from reactor kinetics has created discrepancies in interpreting experimental results, hindering the establishment of a fundamental design equation for the scale-up of electrolyzers.Decoupling and understanding multi-scale transport phenomena involved in the electrocatalytic transformation of small molecules is challenging, but it can be achieved using relationships between dimensionless quantities that simplify the characterization of transport properties. In the first part of this dissertation, we report the development of the gastight rotating cylinder electrode (RCE) cell and demonstrate how the mass transport characteristics of an electrochemical cell can be quantified. The gastight RCE cell enables kinetics studies under well-defined transport conditions, making it the closest experimental apparatus to differential reactors in thermal catalysis. The application of dimensional analyses on a lab-scale electrochemical reactor should enable rigorous research and development of electrocatalytic technologies. In the following sections of the dissertation, we present a large experimental dataset of electrochemical CO2 reduction on polycrystalline copper electrodes collected under a broad range of well-defined transport regimes in the gastight RCE cell. The control of relative timescales of electrode surface reactions and transport processes enables systematic parametrization of a multi-scale reaction-transport kinetics model. The model utilizes a continuous stirred-tank reactor (CSTR) volume approximation of the electrode reaction front that captures mesoscale stochastic processes of reactants and intermediates at the electrode/electrolyte interface, which determines product selectivity. Product distributions under different conditions of applied potential, mass transport, bulk electrolyte concentration, temperature, and electrode porosity are rationalized by introducing dimensionless numbers that reduce complexity and represent relative timescales of dynamic transport phenomena and electrocatalytic reactions on copper electrodes. The CSTR model demonstrates that one CO2 reduction mechanism can explain differences in selectivity reported for copper-based electrocatalysts using the residence time as a descriptor. With the insights gained from the RCE cell and the CSTR model, we propose mechanistic pathways for each CO2 reduction product. This combinative approach of experimental electrocatalysis under well-defined transport characteristics and a new framework of the reactor design equation that captures relative timescales of multi-scale processes will open up discussions from a new perspective to study kinetics and scale-up electrocatalytic processes.