Decoupling and understanding the various mass, charge, and heat transport phenomena involved in the electrocatalytic transformation of small molecules (i.e., CO 2 , CO, H 2 , N 2 , NH 3 , O 2 , and CH 4 ) is challenging but it can be readily achieved using dimensionless quantities (i.e., Reynolds, Sherwood, Schmidt, Damköhler, Nusselt, Prandtl, and Peclet Numbers) to simplify the characterization of systems with multiple interacting physical phenomena.Herein we report the development of a gastight rotating cylinder electrode cell with welldefined mass transport characteristics that can be applied to experimentally decouple mass transfer effects from intrinsic kinetics in electrocatalytic systems. The gastight rotating cylinder electrode cell enables the dimensionless analysis of electrocatalytic systems and should enable the rigorous research and development of electrocatalytic technologies.

Decoupling and understanding the various mass, charge and heat transport phenomena involved in the electrocatalytic transformation of small molecules (i.e. CO2, CO, H2, N2, NH3, O2, CH4) is challenging but it can be readily achieved using dimensionless quantities (i.e. Reynolds, Sherwood, Schmidt, Damköhler, Nusselt, Prandtl, and Peclet Numbers) to simplify the characterization of systems with multiple interacting physical phenomena. Herein we report the development of a gastight rotating cylinder electrode cell with well-defined mass transport characteristics that can be applied to experimentally decouple mass transfer effects from intrinsic kinetics in electrocatalytic systems. The gastight rotating cylinder electrode cell enables the dimensionless analysis of electrocatalytic systems and should enable the rigorous research and development of electrocatalytic technologies.

Despite its increased relevance for the ongoing energy transition, the CO 2 -reduction reaction (CO 2 RR) continues to be poorly understood, particularly in the context of how its kinetics and product outcome are affected by surface-specific parameters such as the interfacial pH. The latter has received increased attention lately, with numerous studies reporting its impact on the product distribution of the CO 2 -electroreduction reaction but failing to quantify it reliably due to the poorly defined mass transport properties of the electrochemical cells in which most of those studies were performed. With this motivation, herein we investigate the CO 2 RR on polycrystalline Au and a Au aerogel in a rotating ring-disk electrode (RRDE) setup characterized by its well-defined hydrodynamic properties and quantify these surfaces' CO 2 -to-CO conversion performance and interfacial pH using a Au or an IrO x ring, respectively. The experimental ring pH was found to be in close agreement with the values computed from a mathematical model, from which we estimated disk surface pH maxima of ∼9.2 vs ∼9.0 for Au AG vs Au PC . The subsequent quantification of CO-specific kinetic currents made possible by the use of this RRDE approach unveiled that the polycrystalline surface features an ∼2-fold larger Au-surface-specific current than the aerogel, thus hinting at the existence of a roughness factor effect on this key performance metric.