Catalysts play a critical role in photoelectrochemical (PEC) water splitting by lowering the kinetic barriers for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). These reactions are essential for efficient solar-to-hydrogen conversion, as they facilitate charge separation and surface redox processes on photoelectrodes. The choice of catalyst material—whether noble-metal or non-noble-metal—directly impacts the efficiency, stability, and cost-effectiveness of PEC systems. Recent advances in catalyst design have focused on improving activity, durability, and compatibility with semiconductor photoelectrodes.

The oxygen evolution reaction is particularly challenging due to its four-electron transfer process, which introduces high overpotentials and sluggish kinetics. Noble-metal catalysts such as iridium oxide (IrO₂) and ruthenium oxide (RuO₂) have demonstrated exceptional OER activity in acidic and neutral conditions. These materials exhibit high electrical conductivity and corrosion resistance, making them suitable for integration with photoanodes like bismuth vanadate (BiVO₄) or hematite (α-Fe₂O₃). However, their scarcity and high cost limit large-scale deployment. Recent studies have explored nanostructuring and alloying to reduce noble-metal loading while maintaining performance. For instance, IrO₂ nanoparticles dispersed on conductive substrates show enhanced mass activity compared to bulk counterparts.

Non-noble-metal catalysts offer a cost-effective alternative for OER, with transition-metal oxides, hydroxides, and phosphides gaining attention. Cobalt-based catalysts, such as cobalt phosphate (Co-Pi) and cobalt oxide (Co₃O₄), exhibit promising OER activity when deposited on photoanodes. Their mechanism involves the formation of high-valent metal-oxo intermediates that promote O-O bond formation. Nickel-iron (Ni-Fe) layered double hydroxides (LDHs) are another class of efficient OER catalysts, particularly in alkaline conditions, where they benefit from synergistic electronic interactions between Ni and Fe sites. Recent work has shown that doping these materials with elements like manganese or cerium can further enhance their catalytic activity and stability under illumination.

For the hydrogen evolution reaction, platinum (Pt) remains the benchmark catalyst due to its near-zero overpotential and high exchange current density. Pt nanoparticles are commonly integrated with photocathodes such as p-type silicon or copper indium gallium selenide (CIGS) to facilitate proton reduction. However, the high cost of Pt has driven research into non-precious alternatives. Molybdenum sulfide (MoS₂) is a prominent HER catalyst, with its edge sites providing active centers for hydrogen adsorption. Recent advances include phase engineering to increase the density of active sites and heteroatom doping to improve conductivity. For example, nitrogen-doped MoS₂ exhibits HER activity approaching that of Pt in acidic media.

Transition-metal phosphides, carbides, and nitrides have also emerged as effective HER catalysts. Nickel phosphide (Ni₂P) and cobalt phosphide (CoP) demonstrate excellent activity in both acidic and alkaline conditions, attributed to their favorable hydrogen-binding energies. Integration of these materials with photocathodes requires careful consideration of interfacial energetics to ensure efficient charge transfer. Recent studies have explored hybrid structures, such as CoP nanoparticles embedded in carbon matrices, to enhance electrical contact and prevent aggregation during operation.

The integration of catalysts with photoelectrodes is a critical aspect of PEC system design. Physical deposition methods, including sputtering and atomic layer deposition (ALD), enable precise control over catalyst loading and morphology. Chemical methods such as electrodeposition and sol-gel synthesis offer scalability but may require post-treatment to optimize performance. A key challenge is minimizing charge recombination at the catalyst-semiconductor interface. Strategies include the use of protective interlayers, such as titanium dioxide (TiO₂) or aluminum-doped zinc oxide (AZO), to passivate surface defects and improve charge collection.

Recent advances in catalyst design have focused on bifunctional and hybrid systems that simultaneously enhance OER and HER. For instance, cobalt borate (Co-Bi) catalysts deposited on bismuth vanadate photoanodes have shown dual functionality, acting as both OER catalysts and hole-storage layers. Similarly, nickel-molybdenum (Ni-Mo) alloys have been explored for their ability to catalyze HER while suppressing photocorrosion in III-V semiconductor photocathodes. Another innovative approach involves the use of single-atom catalysts, where isolated metal atoms on conductive supports maximize atomic efficiency and expose uniform active sites. For example, single-atom cobalt dispersed on nitrogen-doped graphene exhibits high OER activity due to its unique electronic structure.

Durability remains a significant challenge for PEC catalysts, particularly under prolonged illumination and harsh electrolyte conditions. Noble-metal catalysts generally exhibit superior stability but are prone to dissolution at high anodic potentials. Non-noble catalysts often suffer from phase transformations or surface oxidation over time. Recent efforts have addressed these issues through protective coatings, such as ultrathin carbon shells or conductive polymers, which shield active sites without impeding charge transfer. In-situ characterization techniques, including X-ray absorption spectroscopy and scanning electrochemical microscopy, have provided insights into catalyst degradation mechanisms, guiding the development of more robust materials.

The scalability of catalyst deposition techniques is another area of active research. Roll-to-roll printing and spray pyrolysis offer potential pathways for large-area photoelectrode fabrication, but they require optimization to ensure uniform catalyst distribution. Advances in ink formulation and substrate engineering have enabled the deposition of high-performance catalysts on flexible substrates, opening new possibilities for modular PEC systems.

In summary, the development of efficient and durable catalysts for PEC water splitting involves a careful balance of activity, stability, and cost. Noble-metal catalysts remain benchmarks for both OER and HER, but non-noble alternatives are rapidly closing the performance gap through innovative material design and engineering. Recent advances in bifunctional catalysts, single-atom systems, and protective coatings highlight the dynamic nature of this field. The integration of these catalysts with photoelectrodes will be crucial for achieving practical solar hydrogen production at scale. Future research should focus on understanding interfacial charge-transfer processes and developing scalable fabrication methods to accelerate commercialization.