Low temperature (LT) water electrolysis (2 H₂O → 2 H₂ + O₂) is a promising route to sustainable H₂ production. While contemporary LT membrane electrolyzers (e.g, proton exchange membrane [PEM] electrolyzers) rely on ultrapure water as a feedstock, 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 also introduces distinct challenges that must be overcome. Among these, ionic Cl^- impurities can lead to Cl^- oxidation forming 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 platform for H₂ production from impure water streams. We hypothesized that the BPM-based electrolyzer would have inherent advantages with respect to a PEM electrolyzer under these conditions — specifically, 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. 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 feed (0.5 M NaCl) or ‘real’ seawater to the cathode, we observed nearly undetectable levels of free chlorine in the BPM anolyte and no oxidized chlorine detected in the O₂ stream at the anode; 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 severely 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].

Daniela Marin

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