1：Experimental Design and Method Selection:

A facile one-step hydrothermal synthesis was used to prepare cobalt oxide ultrathin nanosheets (UTNS) and nanoplates (NPs) with controlled morphologies, phases, defects, and cobalt valences. Hypophosphorous acid and urea were employed to modulate the oxidation state and provide a basic environment.

2：Sample Selection and Data Sources:

Cobalt sulfate and cobalt nitrate were used as precursors for UTNS and NPs, respectively, grown on carbon cloth (CC) or carbon paper (CP) supports.

3：List of Experimental Equipment and Materials:

Equipment included SEM (JEOL JSM-7500F), TEM (JEOL JEM-2100), AFM (Bruker Dimension FastScan Bio), XRD (Rigaku MiniFlex 600), XPS (Thermo ESCALAB 250Xi), ICP-OES (Thermo Jarrell Ash ICAP-9000), and electrochemical workstation (CHI660D). Materials included cobalt sulfate, cobalt nitrate, urea, hypophosphorous acid, Nafion, ethanol, and deionized water.

4：Experimental Procedures and Operational Workflow:

For UTNS synthesis, precursors were dissolved in water, stirred, transferred to an autoclave with CC, heated at 120°C for 6h, washed, and dried. Electrochemical measurements were conducted in a three-electrode system with 1.0 M KOH electrolyte, using Ag/AgCl reference and Pt counter electrodes.

5：0 M KOH electrolyte, using Ag/AgCl reference and Pt counter electrodes. Data Analysis Methods:

5. Data Analysis Methods: Data were analyzed using techniques such as CV for redox behavior, LSV for OER activity, Tafel slopes for kinetics, and Cdl for electrochemical surface area. XPS and ICP-OES were used for composition analysis.