Photoelectrochemical water splitting is a promising method for sustainable hydrogen production, utilizing sunlight to drive the electrochemical decomposition of water. However, the efficiency and longevity of these systems are significantly impacted by corrosion and degradation mechanisms. Understanding these challenges and developing effective mitigation strategies is critical for advancing the technology.

One of the primary degradation mechanisms in photoelectrochemical systems is photocorrosion. This occurs when photogenerated charge carriers participate in undesired redox reactions with the semiconductor electrode itself, rather than driving water splitting. For example, metal oxide photoanodes such as Fe₂O₃ and BiVO₄ can undergo self-oxidation, leading to the dissolution of metal ions into the electrolyte. Similarly, photocathodes like Si or GaAs are prone to reduction, forming unstable surface species that degrade performance. The extent of photocorrosion depends on the electrochemical stability window of the material relative to the water redox potentials. Materials with band edges that overlap with corrosion potentials are particularly vulnerable.

Chemical dissolution is another major degradation pathway. In aqueous electrolytes, electrode materials may dissolve due to thermodynamic instability or kinetic factors such as pH extremes. For instance, ZnO photoanodes dissolve rapidly in acidic conditions, while Mo-doped BiVO₄ suffers degradation in alkaline media. Even in neutral pH, certain materials exhibit gradual dissolution over time, leading to thinning of the active layer and loss of catalytic sites. The dissolution rates are influenced by electrolyte composition, temperature, and applied bias, making it essential to evaluate stability under operational conditions.

Passivation layers can form on electrode surfaces, either as a natural consequence of corrosion or due to deliberate surface treatments. While some passivation layers, like TiO₂ on Si, can protect against further degradation, others may impede charge transfer and increase interfacial resistance. The dynamic nature of these layers—often changing in composition and thickness during operation—complicates long-term stability. In some cases, passivation leads to irreversible performance loss, as seen with sulfide layers on Cu₂O photocathodes that block hole transport.

Mitigation strategies focus on preventing direct contact between the photoactive material and the corrosive electrolyte. Protective coatings are widely employed, with conductive oxides like TiO₂, Al-doped ZnO, and SnO₂ demonstrating effectiveness. These coatings must be thin enough to allow charge transport while providing a robust barrier against dissolution. Atomic layer deposition is particularly useful for achieving conformal, pinhole-free layers. Another approach involves using corrosion-resistant overlayers that also serve as electrocatalysts, such as NiFeOx on BiVO₄, which enhances both stability and reaction kinetics.

Material selection plays a crucial role in minimizing degradation. Compounds with higher inherent stability, such as Ta₃N₅ or SrTiO₃, are preferred despite their less optimal bandgaps. Doping strategies can further improve stability; for example, incorporating W into BiVO₄ reduces oxygen vacancy formation, slowing photocorrosion. Additionally, designing heterostructures where a stable material shields a more active but vulnerable component can extend operational lifetimes. An example is the use of GaN shells on InGaN nanowires to prevent oxidation while maintaining visible light absorption.

Electrolyte engineering offers another avenue for reducing degradation. Buffered solutions or non-aqueous electrolytes can suppress dissolution, though they may introduce other challenges like lower ionic conductivity. Additives such as hole scavengers or corrosion inhibitors can also be introduced, but their long-term impact on system efficiency must be carefully assessed. For instance, adding Na₂SO₃ to the electrolyte can protect sulfide-based photocathodes by consuming photogenerated holes before they attack the material.

Operational parameters must be optimized to balance efficiency and durability. Lowering the applied bias or reducing light intensity can decrease degradation rates, albeit at the cost of hydrogen production rates. Pulsed illumination schemes have shown promise in mitigating photocorrosion by allowing time for charge recombination, thus reducing cumulative damage. Temperature control is equally important, as elevated temperatures accelerate both desired reactions and degradation processes.

System-level design considerations include the use of membranes or separators to isolate sensitive components from harsh conditions. For example, proton exchange membranes can protect photocathodes in tandem cells by limiting exposure to oxygen. Flow-through architectures that continuously refresh the electrolyte near the electrode surface can also help by removing dissolved ions before they contribute to passivation.

Monitoring and diagnostics are essential for understanding degradation in real time. Techniques like in-situ spectroscopy and electrochemical impedance spectroscopy provide insights into corrosion mechanisms, enabling proactive adjustments. Long-term testing under realistic conditions is necessary to validate stability claims, as accelerated aging tests may not capture all failure modes.

The interplay between efficiency and stability remains a central challenge. Many high-performance materials are inherently unstable, while stable materials often suffer from poor activity. Future research must focus on optimizing this trade-off through advanced materials engineering, innovative protective strategies, and smart system designs. By addressing corrosion and degradation holistically, photoelectrochemical water splitting can move closer to commercial viability.

In summary, photoelectrochemical systems face multiple degradation pathways, including photocorrosion, chemical dissolution, and passivation. Protective coatings, stable material selection, electrolyte engineering, and operational optimizations are key strategies to enhance durability. Continued advancements in these areas will be critical for achieving efficient and long-lasting hydrogen production systems.