Back-diffusion of hydroxide ions governs failure mechanisms in asymmetric seawater electrolyzers

Seawater (SW) is an attractive alternative to freshwater for electrolysis due to its abundance, yet its direct use is hindered by competing chlorine evolution and cathodic precipitation failure. Half-seawater electrolyzers (half-SWE) have been proposed to partially mitigate these issues by coupling an alkaline anolyte with SW catholyte. In this work, we demonstrate how OH− spontaneously diffuses from the anolyte into the SW even in the absence of applied bias. This transport persists under operating conditions, progressively increasing catholyte pH and triggering precipitation of insoluble hydroxides. Through systematic experiments across different membranes, thicknesses, and electrode configurations, we show that OH− crossover is primarily governed by membrane properties rather than catalysts selection. While applied current partially counteracts back-diffusion, it does not suppress it, leading to delayed but unavoidable cathode failure. To quantitatively assess ion transport, we introduce an electron-transfer-based methodology, which decouples OH− migration from parasitic reactions. This approach enables direct evaluation of OH− transport efficiency, revealing intrinsic limitations below 80% due to concurrent back-diffusion and interfacial accumulation. These findings establish OH− crossover as a fundamental constraint in half-SWE systems and highlight the need for design strategies that minimize pH gradients, pointing toward full-SWE as a more viable pathway for stable operation.