A 100 mg sample of each pristine electrode material was immersed in 5 mL H2O and subjected to ultrasonic agitation for 15 minutes. Prior to the measurement, the dispersed electrode materials were mixed thoroughly using a vortex mixer, and 1 mL of the suspension was transferred into folded capillary zeta cells. Zeta-potential measurements were conducted at room temperature using a Zeta Sizer, Nano Series (Malvern Instruments).