Thin Cation-Exchange Layers Enable High-Current-Density Bipolar Membrane Electrolyzers via Improved Water Transport

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.

Structures and properties of polymers in ion exchange membranes for hydrogen generation by water electrolysis

“…Amphoteric water electrolysers involve the use of bipolar membranes, that is, membranes containing both anionic and protonic characteristics 97 . Development so far in this regard has witnessed the use of mainly Nafion and other commercially available membranes, 98,99 along with noncommercial PBI‐ and poly(arylene ether ketone) (PAEK)‐based membranes.…”

“…Development so far in this regard has witnessed the use of mainly Nafion and other commercially available membranes,98,99 along with noncommercial PBI-and poly(arylene ether ketone) (PAEK)-based membranes. PBI, by virtue of possessing strong H-bonds and structural rigidity, forms dense membranes that show very low permeability toward gases.…”

Highly Efficient Bipolar Membrane CO2 Electrolysis

“…[14] The low current density in BPM configuration pointed towards the requirement of water transport to the bipolar interface through the membrane and water dissociation (WD) catalyst that split water into H + and OH À ions efficiently at the bipolar interface. [15]…”

Insights and Challenges for Applying Bipolar Membranes in Advanced Electrochemical Energy Systems

“… 54 Another option is operating with a pure water feed, avoiding the presence of ionic species in the membrane layer and circumventing the challenge to have strong relative affinity in multi-ionic systems. 49 , 50 …”