necessary to account for the Donnan potential to establish AEM bulk values of the charged species (details are reported in the SI, section 1.3). When λ decreases, the presence of K + decreases drastically. This explains the slight increase in FE at low λ, suppressing the H 2 OR from K + microenvironments and leading to the highest contribution in CO current density from the TMA + microenvironments. Figure (c) illustrates that at less negative potentials the CO current density from TMA + microenvironments is higher than the current density from K + due to low K + surface coverages. However, at intermediate potentials (before the system switches to transport limited), the current density from K + microenvironments is prevalent as the [K + ] OHP increases. The K + -based current density switches to a transport-limited regime before the TMA + -based one and at more negative potentials the TMA + -based mechanism is prevalent again.