any discrepancies are likely due to assumed Sherwood relationships for the calculation of the mass-transfer coefficient, which, in most cases, do not account for the effects of homogeneous buffer reactions and multi-ion transport. To fit the kinetics for CO formation, the reference-shifted experimental data for H2 partial current density versus voltage are imported into the CMA-ES algorithm and are held fixed. Doing so is critical for calculating the kinetic parameters for CO formation because H2 data enable the model to calculate changes in the local pH and CO2 concentration that result from HER, which, in turn, affect kinetics of CO formation. The COER current density is defined by Equation ( ). The CMA-ES algorithm then uses the transport and reaction model to obtain the best values of 𝛼 $,+, , 𝑖 ",+, , and 𝐿 () that give the best fit to the data (Figure ). Once the CO kinetic parameters and 𝐿 () are determined, they are fixed to their fitted values and the parameter for the HER partial current, 𝛼 . ! and 𝑖 ",. ! , described by Equation ( ) are fitted using the CMA-ES algorithm (Figure ). Post processing of the CMA-ES distributions provides the uncertainty. This process can be easily generalized to a system containing a greater number of competing reactions as described in Section S4.2 of the SI. While this overall process could be repeated to further reduce error, doing so results in only a minor change in the fitted parameters (< 0.1% change in 𝛼, < 0.2% change in 𝐿 () , and < 0.7% change in 𝑖 " ) and comes at greater computational expense. Thus, for this work, the fitting process was run once through each product.