High-Asymmetry Bipolar Membrane Electrode Assemblies Generate a Superconcentration of Cations and Hydroxide at a Catalyst Surface

Abstract

In electrochemical CO reduction reactions, a highly alkaline pH is typically desired to promote multicarbon liquid products and suppress hydrogen evolution, considerations that prioritize pH ≥ 14 (e.g. 1 M KOH). However, bulk electrolytes with pH exceeding 14 are prone to produce corrosion of catalyst and electrolyzer. Here we find that an engineered class of bipolar membrane assemblies (BPMEAs) achieves a superconcentration of local metal hydroxides, and generates a product slate consistent with local electrolyte pH = 15. We report that, in a cathode:anion exchange layer (AEL):cation exchange layer (CEL):anode architecture, a high thickness ratio of CEL:AEL generates a high local pH at the cathode, this achieved by blocking the transport of hydroxide ions, generated on the cathode, over to the anode side. This enables production of C2+ liquids at a total Faradaic efficiency of 93%, with an ethanol:ethylene productivity ratio of 70:1. Compared to anion-exchange membrane assemblies (AEMEAs) operating at the same 100 mA cm-2 current density for similar product selectivity, these BPMEA systems exhibit 28 hours stable operation (compared to <30 minutes in AEMEA), and a 12x lower rate of liquid product crossover, enabling us to report a liquid product concentration of 23 wt% on the cathode. Operando Raman spectroscopy shows that the optimal BPM enhances coverage, on the cathode catalyst, of surface-bound hydroxyl species, ~ 5x higher than AEM systems, simultaneous with maximizing the surface CO population. Mechanistic studies indicate that surface OH promotes hydroxylation of the CCH intermediate, steering the reaction pathway toward ethanol instead of ethylene, leading to the strong preference towards liquid production.