1：Experimental Design and Method Selection:

A facile spin-coating and in-situ photoelectrochemical reduction method was developed to prepare dispersed Cu electrocatalysts on Si photocathodes. The key innovation is the use of thiourea as a dispersant in the precursor solution to achieve uniform Cu particle distribution.

2：Sample Selection and Data Sources:

Single crystalline n+p-Si wafers (LONGi Solar Technology, China) were used as photocathodes. The rear side was screen-printed with aluminium and silver as back contacts. Precursor solutions were prepared with CuCl and thiourea in 2-methoxyethanol.

3：List of Experimental Equipment and Materials:

Materials included CuCl (Aladdin), thiourea (Shanghai Lingfeng), 2-methoxyethanol (Sinopharm), NaH2PO4·2H2O, Na2HPO4·12H2O (Shanghai Lingfeng), HF (Shanghai Shenbo). Equipment included X-ray diffraction (XRD smartlab, 9 KW), Scanning electron microscope (SEM Nano Nova S230), energy-dispersive X-ray spectroscopy (EDX Genesis XM260S), X-ray photoelectron spectroscopy (Thermo K-Alpha), three-electrode cell (Shanghai Chenhua, CHI 760E), AM 1.5 G sunlight simulator, gas chromatograph (Shimadzu, GC-8A).

4：5 G sunlight simulator, gas chromatograph (Shimadzu, GC-8A). Experimental Procedures and Operational Workflow:

4. Experimental Procedures and Operational Workflow: Si photocathodes were etched in HF to remove native SiO2. Precursor solutions were spin-coated at 2500 rpm for 30 s, annealed in air at 600°C for 3 min, and then subjected to in-situ photoelectrochemical reduction in 0.2 M phosphate buffer solution (pH ~7) using cyclic voltammetry between -0.5 V and +0.3 V vs RHE for 30 cycles under illumination.

5：Precursor solutions were spin-coated at 2500 rpm for 30 s, annealed in air at 600°C for 3 min, and then subjected to in-situ photoelectrochemical reduction in 2 M phosphate buffer solution (pH ~7) using cyclic voltammetry between -5 V and +3 V vs RHE for 30 cycles under illumination. Data Analysis Methods:

5. Data Analysis Methods: XRD, SEM, EDX, and XPS were used for structural and morphological characterization. Photoelectrochemical properties were measured using linear sweep voltammetry and controlled potential electrolysis. Tafel slopes were calculated, and faradic efficiency was measured using gas chromatography.