Silicon photovoltaic-coupled electrolysis represents a promising pathway for sustainable hydrogen production by integrating mature silicon solar cell technology with water electrolysis. This approach decouples light absorption and electrochemical reactions, enabling independent optimization of each component. The system typically consists of silicon solar cells connected to an electrolyzer, where the photocathode plays a critical role in determining overall efficiency and durability.
The photocathode in such systems must fulfill multiple requirements: efficient charge transfer, corrosion resistance, and catalytic activity for the hydrogen evolution reaction (HER). Silicon, while an excellent light absorber, is prone to corrosion in aqueous electrolytes, necessitating protective layers. These layers must be conductive, chemically stable, and thin enough to minimize resistive losses. Common protection strategies include the use of thin metal oxides such as TiO2 or Al2O2 deposited via atomic layer deposition. These oxides provide a barrier against corrosion while allowing electron transport to the catalyst layer.
Catalyst selection is equally critical for achieving high efficiency. Platinum remains the benchmark HER catalyst due to its low overpotential and high exchange current density, but its cost drives research into alternatives. Earth-abundant materials like molybdenum sulfide (MoS2), nickel-molybdenum alloys, and cobalt phosphides have shown promising catalytic activity. The integration of these catalysts with the protection layer requires careful interfacial engineering to minimize contact resistance and ensure long-term stability.
The efficiency of silicon PV-coupled electrolysis is governed by several factors. The solar-to-hydrogen (STH) conversion efficiency depends on the photovoltaic efficiency of the silicon cell, the overpotential losses at the electrocatalyst, and the ionic conductivity of the electrolyte. State-of-the-art silicon solar cells achieve efficiencies exceeding 26% under standard illumination, but the overall STH efficiency is lower due to electrolysis losses. Practical systems have demonstrated STH efficiencies between 10% and 15%, with the upper limit constrained by thermodynamic and kinetic losses in the electrolyzer.
A key challenge is minimizing voltage losses at the photocathode. The thermodynamic potential for water splitting is 1.23 V, but real-world systems require higher voltages due to overpotentials at both the anode and cathode. Silicon solar cells typically operate at voltages between 0.6 V and 0.7 V per cell, necessitating multiple cells connected in series to reach the required electrolysis voltage. Tandem configurations using perovskite-silicon cells have been explored to achieve higher voltages with fewer cells, potentially improving system-level efficiency.
Long-term stability is another critical consideration. Silicon photocathodes degrade in acidic or alkaline environments without proper protection. Accelerated lifetime testing has shown that ALD-deposited TiO2 layers can extend operational lifetimes to several thousand hours, but interfacial delamination and catalyst poisoning remain issues. Strategies such as conductive polymer interlayers and gradient doping of protection layers have been investigated to enhance adhesion and charge transport.
The scalability of silicon PV-coupled electrolysis depends on the cost and manufacturability of the photocathode components. Roll-to-roll deposition techniques for protective layers and catalysts could reduce production costs, while advances in catalyst ink formulations enable scalable printing methods. The balance between performance and cost is crucial for commercial viability, with systems needing to achieve a levelized cost of hydrogen below established fossil fuel-based benchmarks.
Future improvements may focus on optimizing the photocathode architecture at the nanoscale. Nanostructured silicon surfaces can enhance light trapping and increase the effective surface area for catalysis, while core-shell designs with tailored band alignments may improve charge separation. Additionally, the development of bifunctional protection layers that simultaneously act as HER catalysts could simplify the device structure and reduce interfacial losses.
In summary, silicon PV-coupled electrolysis leverages the high efficiency and maturity of silicon photovoltaics to drive hydrogen production. The photocathode design, incorporating corrosion-resistant protection layers and efficient catalysts, is central to achieving high STH efficiency and durability. While current systems operate below the theoretical efficiency limits, ongoing research into materials and interfaces holds promise for bridging this gap and enabling large-scale renewable hydrogen generation.