Hydrogen production via water electrolysis using desalinated seawater offers a sustainable energy solution and has attracted considerable attention in recent years. However, its efficiency depends heavily on the quality of water. Many studies have not explored the relationship between treated water quality and hydrogen generation efficiency at each stage of the seawater desalination process. This study examines a three-step seawater desalination process comprising softening with ballasted flocculation (SBF) as a pretreatment, reverse osmosis (RO) as the main desalination step, and ion exchange as a polishing step to provide high-quality water for electrolysis. Water from each purification stage was supplied to the electrolyzer to compare the impact on water quality and hydrogen generation efficiency. The SBF process removed magnesium (Mg) and calcium (Ca) from seawater, as well as turbidity and bacteria, but hydrogen production via water electrolysis continued for no more than 10 h. However, when feeding RO water and RO water processed by ion exchange after the SBF process, hydrogen was generated stably and continuously for 70 h, achieving high efficiency comparable to that of commercial pure water. High production of green hydrogen by water electrolysis is possible through RO seawater desalination combined with SBF pretreatment.

Hydrogen production via polymer electrolyte membrane (PEM) water electrolysis has attracted considerable attention as a promising technology to store renewable electricity and combat global warming. Although PEM water electrolyzers can produce high-purity hydrogen at high current densities, the use of low-purity water leads to device degradation because metal ions from the water are deposited on the membrane, thereby increasing its resistance and operating voltage. In this study, an in-situ recovery method was developed, in which degraded PEM water electrolysis cells were chemically treated without disassembly. Cells after degradation were subjected to a 24-h chemical treatment with either a strong acid (1.0 mol/L nitric acid) or a weak acid (12.9 and 1.0 mol/L phosphoric acid), followed by the supply of ultrapure water for 72 h. Recovery was evaluated using cell voltage measurements, while scanning electron microscopy (SEM)-dispersive X-ray spectroscopy (EDX) and inductively coupled plasma (ICP) analyses were performed to investigate membrane morphology, elemental distributions, and metal ion removal. Among the tested acids, 12.9 mol/L phosphoric acid showed the highest voltage recovery performance, achieving a 90% recovery ratio immediately after treatment (0 h). Moreover, a comparison of the voltage recovery ratios at 1 h post-immersion suggests that higher hydrogen ion concentrations are more effective for the recovery of degraded PEMs. These findings demonstrate that in-situ acid treatment can restore the performance of contaminated PEM water electrolyzers without disassembly and may provide a practical approach for extending cell lifetime and reducing maintenance requirements.