(670e) Bipolar Membrane Electrolyzers Architectures for Ion Transport Control and Impure Water Electrolysis

Low temperature (LT) water electrolysis (2 H₂O → 2 H₂ + O₂) is a promising route to sustainable H₂ production. While contemporary LT membrane electrolyzers rely on ultrapure water, the direct electrolysis of impure water sources would offer an attractive set of advantages, including reduced complexity associated with water purification infrastructure as well as a larger pool of available feedstocks. However, impure water electrolysis introduces distinct challenges. Among them, ionic Cl^- impurities can lead to Cl^- oxidation to corrosive ‘free chlorine’ species (e.g., OCl^-, HOCl, Cl₂), and the transport of impurity ions can drive the formation of deleterious pH gradients.

Herein, we have evaluated a bipolar membrane (BPM)-based electrolyzer architecture for H₂ production from impure water streams. We aimed to leverage the BPM architecture to inhibit undesired ion transport and to promote an alkaline anode/electrolyte interface that would minimize the rate of Cl^- oxidation with respect to a proton exchange membrane (PEM) electrolyzer. We assessed the role that electrolyzer architecture plays in dictating four metrics: ion transport, Cl^- oxidation selectivity, long-term electrolyzer stability, and energy efficiency. Using an asymmetric saline solution (0.5 M NaCl) or ‘real’ seawater to the cathode, we observed nearly undetectable levels of free chlorine in the BPM; conversely, significant concentrations of ‘free chlorine’ species were observed in the PEM anolyte feed. Cl^- transport accounted for less than 1% of total current across both the BPM and PEM devices at 250 mA cm^-2, and cation transport across the devices was also limited. The limited Cl^- crossover from cathode to anode and low chlorine oxidation rates were correlated with stable operation over extended operation at 250 mA cm^-2, suggesting that inherently salt- and impurity-tolerant BPMs offer a promising route toward seawater electrolysis[1].

[1] Marin, Perryman, Boettcher*, Nielander*, Jaramillo*, et al., Joule, 2023