1：Experimental Design and Method Selection:

A solvothermal method was used to synthesize hierarchical CoFe-LDH@g-C3N4 composites. The rationale was to combine the advantages of CoFe-LDH and g-C3N4 for enhanced catalytic activity.

2：Sample Selection and Data Sources:

g-C3N4 was prepared by thermal treatment of dicyandiamide, and CoFe-LDH was synthesized with Co/Fe molar ratio of 3:1. Composites with different weight loadings of g-C3N4 (3-15 wt%) were made.

3：Composites with different weight loadings of g-C3N4 (3-15 wt%) were made. List of Experimental Equipment and Materials:

3. List of Experimental Equipment and Materials: Chemicals included Co(NO3)2·6H2O, Fe(NO3)3·9H2O, KOH, DMF, C2H5OH, Al2O3 powder from Sinopharm Chemical Reagent. Equipment included XRD (Shimadzu XRD-6000), XPS (Thermo VG ESCALAB MK II), SEM (Zeiss SUPRA 55), TEM (JEOL JEM-2010F), PL spectrometer (Edinburgh FLS980), electrochemical workstation (CHI 660C).

4：Experimental Procedures and Operational Workflow:

g-C3N4 nanosheets were exfoliated and mixed with metal salts in DMF, stirred, and treated solvothermally at 120°C for 24 hours. Electrodes were prepared by drop-casting catalyst ink on glassy carbon. Electrochemical measurements were done in a three-electrode cell with Ag/AgCl reference, Pt counter, and working electrode in 1 M KOH.

5：Data Analysis Methods:

LSV curves were analyzed for overpotential and Tafel slopes. EIS was used to measure charge transfer resistance. ECSA was calculated from double-layer capacitance. PL spectra assessed charge transfer.