Water quality as an economic barrier to photocatalytic green hydrogen production.

Photocatalytic water splitting has emerged as a promising technology for sustainable green hydrogen production because it directly converts solar energy into chemical energy under mild operating conditions. Although extensive research has focused on developing highly efficient photocatalysts, cocatalysts, and charge-transfer strategies, the influence of water quality on photocatalytic hydrogen production remains comparatively underexplored. In practical applications, hydrogen generation is expected to utilize a wide range of water resources, including freshwater, groundwater, seawater, municipal wastewater, industrial effluents, and reclaimed water, all of which contain dissolved inorganic ions, natural organic matter, microorganisms, and suspended solids that can adversely affect photocatalytic performance. These contaminants may adsorb onto catalyst surfaces, block active sites, alter surface chemistry, promote charge-carrier recombination, induce photocorrosion, and reduce long-term catalyst stability, ultimately decreasing hydrogen production efficiency. Furthermore, maintaining the high water quality required for stable operation often necessitates energy-intensive and costly pretreatment processes, making water purification an important but frequently overlooked contributor to the overall cost of green hydrogen production. This review critically examines the relationship between water quality and photocatalytic hydrogen evolution by discussing the effects of major inorganic ions, heavy metals, natural organic matter, salinity, and real water matrices on catalyst activity and durability. Finally, current challenges, research gaps, and future strategies-including contaminant-tolerant photocatalysts, self-cleaning catalyst surfaces, integrated water treatment-hydrogen production systems, and standardized testing protocols using realistic water sources-are highlighted to facilitate the practical deployment of photocatalytic hydrogen production. By integrating photocatalysis, water chemistry, catalyst stability, and economic considerations, this review provides a comprehensive framework for advancing water-resilient and cost-effective solar hydrogen technologies.