Photocatalytic hydrogen production presents a promising route for sustainable energy generation, yet the stability of photocatalysts remains a critical challenge. Long-term performance degradation due to photocorrosion, material leaching, and structural instability limits practical implementation. Addressing these issues requires fundamental understanding and targeted material design strategies that enhance durability without compromising catalytic activity.

Photocorrosion arises when photocatalysts undergo self-oxidation or reduction under illumination, particularly in aqueous environments. Metal sulfide semiconductors, such as CdS and ZnS, exhibit high activity for visible-light-driven hydrogen evolution but suffer from severe anodic corrosion. The oxidation of sulfide ions (S²⁻) to elemental sulfur (S⁰) or sulfate (SO₄²⁻) disrupts the crystal lattice, leading to activity loss. Similarly, oxide-based photocatalysts like TiO₂ and ZnO experience reduction-induced oxygen vacancy formation, altering electronic properties and surface reactivity. In non-oxide systems, elemental leaching—such as cadmium release from CdS—poses additional environmental and operational hazards.

Protective coatings offer a direct approach to isolate photocatalysts from corrosive environments. Atomic layer deposition (ALD) enables conformal coating of ultrathin barrier layers (e.g., Al₂O₃, TiO₂, or carbon) with precise thickness control. These coatings must balance impermeability to electrolytes with charge carrier transport. For instance, a 2 nm Al₂O₃ layer on CdS suppresses sulfide oxidation while allowing electron tunneling for proton reduction. Carbon shells, including graphene layers or amorphous carbon, provide chemical stability and enhance light absorption through plasmonic effects. However, excessive coating thickness (>5 nm) typically impedes charge transfer, reducing hydrogen evolution rates.

Defect engineering modifies bulk or surface properties to intrinsically resist degradation. Cation doping introduces metal ions (e.g., Co²⁺ in ZnS or Ni²⁺ in TiO₂) that stabilize the lattice against photocorrosion by redistributing charge carriers. Nitrogen doping in ZnO passivates oxygen vacancies, reducing photocorrosion while improving visible light absorption. Anion-rich surfaces, such as phosphorus-terminated InP, exhibit lower dissolution rates than stoichiometric surfaces due to stronger interfacial bonding. Controlled defect creation, like sulfur vacancies in MoS₂, can also promote catalytic sites while maintaining structural integrity under irradiation.

Heterostructure design leverages interfacial charge separation to mitigate degradation. Type-II band alignments or Z-scheme systems spatially separate oxidation and reduction sites, minimizing direct photocorrosion at vulnerable surfaces. For example, coupling CdS with ZnSe forms a staggered bandgap where holes accumulate in ZnSe, protecting CdS from oxidation. Core-shell architectures with wide-bandgap shells (e.g., SiO₂-coated Cu₂O) physically block corrosive species while facilitating charge extraction through energy level matching. The shell material must exhibit chemical inertness and lattice compatibility to prevent interfacial strain-induced cracking during prolonged operation.

Surface passivation techniques address reactive sites prone to dissolution. Molecular capping agents like thiolates or phosphonates bind to metal sites on nanoparticle surfaces, inhibiting ion leaching. Polymeric stabilizers such as polyvinylpyrrolidone (PVP) form steric barriers against agglomeration and electrolyte penetration. Inorganic ligands (e.g., sulfide or selenide ions) reconstruct surface coordination environments, reducing dangling bonds that initiate corrosion. Passivation must preserve active sites; excessive coverage diminishes catalytic turnover.

Charge carrier management reduces oxidative damage by minimizing hole accumulation. Cocatalysts like Pt or MoS₂ accelerate proton reduction kinetics, competing effectively with photocorrosion reactions. Electron mediators (e.g., reduced graphene oxide) extract charges from the photocatalyst bulk, lowering recombination-induced heating and lattice instability. Morphological control through nanostructuring (porous networks, ultrathin nanosheets) shortens carrier diffusion paths, decreasing the probability of destructive charge localization.

Operational parameter optimization extends photocatalyst lifespan without material modification. Neutral pH conditions generally minimize dissolution rates for oxide and sulfide photocatalysts. Lower light intensities reduce the generation of highly oxidizing holes, though this trades off against hydrogen production rates. Hole scavengers (e.g., methanol, sodium sulfite) sacrificially consume photogenerated holes, but their continuous replenishment increases system complexity. Temperature control below 50°C prevents thermally accelerated degradation pathways in organic-inorganic hybrids.

Stability assessment protocols must differentiate between reversible deactivation (e.g., surface poisoning) and irreversible degradation. Accelerated aging tests under intense illumination or elevated temperatures provide rapid screening but may introduce atypical failure modes. Long-duration testing (>1000 hours) under realistic conditions remains indispensable for evaluating mitigation strategies. Quantitative metrics include elemental leaching rates (measured by ICP-MS), phase purity (via XRD), and morphological consistency (from electron microscopy).

The interplay between activity and stability necessitates multi-parameter optimization. Highly stable photocatalysts often exhibit lower initial activity due to protective measures that partially inhibit charge transfer. Iterative refinement of coating thickness, defect density, and interfacial energetics can reconcile this trade-off. Emerging characterization techniques like in-situ X-ray absorption spectroscopy reveal real-time structural evolution during operation, guiding targeted stabilization approaches.

Material-specific strategies dominate current research, as no universal solution exists across different photocatalyst classes. Sulfide stabilization focuses on anodic protection, oxide stabilization targets cathodic protection, while hybrid materials require compatibility between organic and inorganic components. Future directions may exploit self-healing mechanisms inspired by biological systems, where embedded precursors autonomously repair corroded sites during operation.

Progress in photocatalyst stabilization directly impacts the economic viability of solar hydrogen systems. Extending operational lifetimes from hundreds to thousands of hours reduces replacement frequency and maintenance costs. Coupled with scalable synthesis methods for stabilized photocatalysts, these advances could transition hydrogen production from laboratory demonstrations to industrial implementation. The field increasingly recognizes that stability enhancements must be co-developed with activity improvements, rather than addressed as secondary considerations in material design.