<p>This study investigates the influence of functionalized graphene nanoplatelets (F-GNP) on the thermo-physical performance of a water-based phase change material (PCM) in a spherical encapsulation system. The primary objective is to identify the optimal nanoparticle concentration that enhances solidification kinetics without the use of surfactants. Experiments were conducted at varying F-GNP concentrations (0.3, 0.6, 0.9, 1.2, and 1.5 wt%), and the phase change behavior was analyzed using radial temperature measurements, differential scanning calorimetry, and derived performance parameters. The results show a significant enhancement in thermal conductivity, increasing from 0.56 to 0.83 W/m K (48%) with increasing concentration. In addition, the solidification time is reduced by up to 38%, and the phase change acceleration factor increases notably up to 1.2 wt%. However, beyond this concentration, the improvement becomes marginal, indicating saturation due to particle interaction and clustering effects. The time lag parameter decreases by 2%, confirming improved radial heat transfer. However, the DSC-measured latent heat shows a moderate reduction of approximately 6–7%, while the latent heat retention ratio decreases to 0.88 at the highest nanoparticle concentration, indicating a limited compromise in energy storage capacity. The nanoparticle effectiveness factor decreases with concentration, highlighting diminishing returns at higher loadings. The results show that F-GNP can effectively enhance PCM performance without surfactants, with an optimal concentration of 1.2 wt % balancing heat transfer enhancement and energy storage efficiency.</p>