Microstructured-Black Photoanodes with Spatially Decoupled Light Absorption and Catalysis for Unassisted Solar Urea Oxidation

Photoelectrochemical (PEC) urea oxidation reaction (UOR) is a promising alternative anode reaction to the conventional oxygen evolution reaction (OER), which takes more than 90% energy input in PEC water splitting with oxygen byproducts lacking commercial value, enabling simultaneous energy-saving solar hydrogen production and urea-rich wastewater treatment. Current state-of-the-art photoanodes for PEC-UOR focus on large-bandgap semiconductors, including TiO₂, Fe₂O₃, and BiVO₄, reporting a highest saturated current density of 5.4 mA cm⁻². Further research efforts are highly desirable to enhance the current density value to above 10 mA cm⁻² for large-scale implementations of PEC-UOR. Recent research efforts start to explore Si and report a saturated current density of 39.5 mA cm⁻² with the monofacial architecture on Si photoanodes. However, the stability limits to less than 1 hour due to the compromise between catalytic activity and stability on the co-catalyst thickness in the traditional monofacial architecture with light absorption and reactive interface on the same side. In this work, we present microstructured-black Si photoanodes with spatially decoupled light harvesting and catalytic reaction sides for PEC-UOR. By employing ultrathin nickel-iron (Ni-Fe) alloy nanofilm that self-reconfigures the Ni-Fe (oxy)hydroxides-alloy structure as an efficient co-catalyst layer, the engineered bifacial microstructured-black Si photoanode shows a low onset potential of 0.807 V vs. RHE, a high saturated current density of 40.7 mA cm⁻², and long-term stability over 20 hours with 82% initial current retention. These performances represent the lowest onset potential among the reported Si-based photoanodes and an enhancement of 33% in stability compared to the most stable competitor. Moreover, the photoanodes show 94 mV lower onset potential in PEC UOR than OER, reducing around 10.8% energy input in urea-assisted solar hydrogen production. We employ aberration-corrected scanning transmission electron microscopy (STEM) to visualize the atomic-level structural evolution of the Ni-Fe alloy nanofilm during the PEC-UOR process and apply high-resolution X-ray photoelectron spectroscopy (HR-XPS) to analyze the corresponding chemical species changes. The tandem device with three microstructured-black Si connected in series reports a current density above 10 mA cm⁻² under unbiased status for stable running over 5 hours, validating the implementation of unassisted solar hydrogen production with urea-rich wastewater purification directly powered by sunlight. The engineered bifacial microstructured-black Si photoanodes with ultra-thin Ni-Fe alloy co-catalyst opens the pathway to the large-scale applications of simultaneous urea-rich wastewater treatment and solar fuels production beyond hydrogen, including CO₂-to-carbon fuels production and ammonia synthesis.