Photoelectrochemical Redox Deposition of Rh₂P Nanoparticles on Cu₂O Photocathodes for Unassisted Solar Hydrogen Production Coupled with Glycerol Oxidation in Acidic Environments

Despite the theoretically validated approach of using photoelectrochemical (PEC) cells for solar-driven green hydrogen production, reported studies still fall short in achieving hydrogen production efficiencies suitable for practical applications. Among various studies on solar hydrogen production, the hydrogen evolution reaction (HER) under acidic conditions has been identified as a simple method to increase reaction rates and reduce overpotential losses. However, prolonged acidic exposure during PEC operation deactivates noble metal electrocatalysts (such as Pt and Rh), reducing device stability. To address catalyst degradation in acidic environments and enable stable/efficient PEC cell operation, we propose a process design based on the photo-illuminated redox deposition (PRoD) approach using an advanced Cu₂O-based photocathode. This approach allows for the growth of crystalline Rh₂P nanoparticles (NPs) with an average size of 20 nm on TiO₂/Al-ZnO/Cu₂O without annealing. Additionally, atomic-level precise reaction control was performed using several cyclic voltammetry coincident with light irradiation to create a system with optimal catalytic activity. The optimized photocathode, composed of Rh₂P/TiO₂/AZO/Cu₂O/Sb–Cu₂O/ITO, achieved an excellent photocurrent density of 8.2 mA cm⁻² at 0 V_RHE and demonstrated durable water-splitting performance in a strong acidic solution. Notably, the Rh₂P-loaded photocathode exhibited a 5.3-fold enhancement in mass activity compared to a catalyst utilizing Rh alone. Furthermore, <i>in situ</i> transmission electron microscopy (TEM) was performed to observe the real-time growth process of Rh₂P NPs in a liquid cell. Finally, the fabricated Rh₂P/Cu₂O-based photocathode was integrated with a BiVO₄ photoanode for unassisted solar hydrogen production in acidic environments. Here, the oxygen evolution reaction (OER) at the photoanode was replaced with the glycerol oxidation reaction (GOR). Driving GOR with glycerol, a low-value substance, significantly improved the photocurrent density and lowered the onset potential compared to OER, ultimately maximizing HER at the photocathode.