Judicious design photoelectrochemical seawater splitting for green hydrogen: nanostructured photoelectrodes, selectivity engineering, and durability and commerce

Abstract

Green hydrogen produced by solar-driven water splitting is a cornerstone of global decarbonization strategies, yet oceans hold about 96.5% of the planet’s water, and freshwater scarcity increasingly constrains where electrolytic and photoelectrochemical (PEC) hydrogen plants can be sited. Direct seawater splitting converting untreated or minimally treated seawater into hydrogen using sunlight has therefore emerged as one of the most intensively pursued frontiers in energy materials research. The central scientific obstacles are well defined but formidable: the chlorine evolution reaction (ClER) and hypochlorite formation compete with the oxygen evolution reaction (OER) at the (photo)anode within a selectivity window that we show, from first principles, is already close to its ~ 480 mV alkaline maximum at the pH of natural seawater; chloride and bromide ions aggressively corrode semiconductors, cocatalysts, and current collectors; and Mg²⁺/Ca²⁺ precipitation, biofouling, and particulate matter degrade interfaces over time. This review provides a critical, comparative synthesis of progress from roughly 2019 to 2026 in PEC and photocatalytic seawater splitting; monolithic PEC demonstrations in real or simulated seawater remain far less mature than the dark-electrolysis and PV-electrolysis (PV–E) benchmarks against which they are often implicitly compared, a distinction we maintain throughout via explicit device-type and electrolyte-tier classification. We first derive the thermodynamic and kinetic selectivity criteria that govern OER/ClER competition on a single, clearly defined potential scale, then critically assess rather than merely catalogue nanostructured photoanodes (TiO₂ nanotube arrays, BiVO₄, α-Fe₂O₃, WO₃, and protected Si and III–V architectures) and photocathodes operating in chloride media, weighing the activity gained from high surface area against the corrosion and scaling it amplifies. We analyze selectivity-engineering strategies alkaline operation, MnOx and chromate/phosphate permselective overlayers, NiFe-layered double hydroxides, sulfate-enriched anion shields, and hard-Lewis-acid local-environment modifiers alongside corrosion protection by atomic-layer-deposited TiO₂ and self-healing catalysts, and provide a dedicated, referenced degradation comparison across chloride-media photoanodes. System-level routes (membrane-based in situ desalination, forward osmosis, bipolar membranes, decoupled electrolysis, and the direct-versus-indirect seawater debate) are evaluated against techno-economic reality. Finally, we examine how density functional theory, high-throughput screening, and machine learning are accelerating the discovery of chlorine-tolerant, selective catalysts and photoelectrodes specifically for saline operation, and we articulate a roadmap of ten priority research directions each resolved into its core scientific bottleneck, the open scientific question it poses, and a concrete research proposal toward durable, scalable solar seawater-to-hydrogen technology.

Graphical Abstract