(381b) Evaluating Bipolar Membrane Electrolyzers for Seawater Electrolysis and Undersea O2 Production

Water electrolysis (2 H₂O → 2 H₂ + O₂) using seawater and impure water feedstocks is of interest for enabling the large-scale production of renewable H₂ as well as for undersea O₂ production. Traditional electrolyzers rely on ultrapure water feeds to generate H₂ and O₂; direct electrolysis of seawater introduces distinct challenges with respect to ultrapure water. One critical challenge arises from the high concentrations of ionic species, and Cl^- in particular, in seawater. The chloride oxidation reaction (COR) to reactive ‘free chlorine’ species (i.e., Cl₂, HOCl, OCl^-) at the electrolyzer anode poses barriers to the safety, efficiency, and durability of electrolyzers during operation. When using seawater as the electrolyzer feedstock, where Cl^- concentrations are as high as 0.5 M, Cl^- oxidation, accumulation of corrosive/toxic free chlorine must be mitigated.

We hypothesized that a bipolar membrane (BPM) based electrolyzer would have inherent advantages with respect to traditional electrolyzer architectures when using ‘seawater-like’ water feeds. Thus, we evaluated the role that electrolyzer architecture plays dictating Cl^- crossover, COR selectivity, long-term electrolyzer stability, and energy efficiency; a proton exchange membrane (PEM) based electrolyzer served a reference of performance. Using an asymmetric feed of saline solution (0.5 M NaCl) or ‘real’ seawater to the cathode, and deionized water to the anode, we observed significantly reduced crossover of Cl^- from seawater and seawater-like catholytes to the anolyte for the BPM with respect to the PEM. Conversely, significant concentrations of free chlorine were observed in the PEM anolyte feed. Cl^- transport accounted for less than 1% of total current across the BPM device at 250 mA cm^-2 operation; cation transport across the device was also severely limited. These data suggest that inherently salt- and impurity-tolerant BPMs offer a promising route toward seawater electrolysis.