Thin Cation-Exchange Layers Enable High-Current-Density Bipolar Membrane Electrolyzers via Improved Water Transport
Sebastian Z. Oener, Liam Twight, Grace Lindquist, Shannon W. Boettcher
Abstract
With suitable water dissociation (WD) catalysts, bipolar membranes (BPMs) can efficiently dissociate water into H+ and OH– at the junction between anion- and cation-exchange layers (AEL and CEL, respectively). First, however, water must be transported through the AEL or CEL and thus against the outward flow of hydrated H+ and OH–. This is a challenge intrinsic to the BPM architecture and limits operation to current densities typically less than ∼0.5 A·cm–2. Here we explore how water transport affects durability and performance in reference alkaline and acidic membrane electrolyzers, and we use the insight gained to design BPMs with improved water transport. We demonstrate a thin-CEL BPM (2-μm Nafion CEL|∼200 nm TiO2|∼200 nm NiO + ionomer|50 μm Sustainion AEL) which maintains a pH difference of ∼14 units between the anode and cathode for current densities of up to 3.4 A·cm–2 with a total water electrolysis voltage of ∼4 V and an estimated WD overpotential of ∼1.5 V. Such high-current-density operation is crucial for key emerging BPM applications, including in water and carbon-dioxide electrolyzers and in (regenerative) fuel cells.