Effect of SiO x Nanocoating Deposition Route on Oxygen Evolution Reaction/Chlorine Evolution Reaction Selectivity of IrO x Anodes in Acidic Saline Water Electrolysis

Proton exchange membrane water electrolysis for green hydrogen production is typically designed for ultrapure water feeds. In offshore applications, for example, when coupled to desalination, residual chloride can still trigger the chlorine evolution reaction (CER), motivating the development of oxygen evolution reaction (OER)‐selective anodes. Silicon oxide (SiO x ) nanocoatings exhibit chloride‐blocking behavior and suppress Cl 2 formation. Previous studies used spin coating (SC) to fabricate SiO x coatings, but enhanced OER selectivity was limited to smooth catalyst surfaces, and the coatings showed poor mechanical stability. Here, we introduce atomic layer deposition (ALD) as a vapor‐phase alternative and benchmark it against SC to investigate how the SiO x fabrication method influences chlorine suppression on rough amorphous IrO x anodes. SiO x ‐coated anodes were evaluated in a low‐salinity acidic electrolyte using a rotating ring–disk electrode (RRDE) to quantify chlorine evolution and assess mechanical integrity. After 50 scans, plasma‐enhanced ALD and spray coating at a SiO x thickness of 0.5 nm both reduced CER selectivity by approximately 36% relative to uncoated IrO x , whereas thermal ALD showed no selective CER suppression. These findings show that the chloride‐blocking behavior of ultrathin SiO x nanocoatings depends strongly on the deposition route and likely arises from multiple coupled chemical, structural, and interfacial properties.