Jaewoo Park, Narim Park, Doohwan Lee
High-solid-loading nanofluids are attractive for energy storage and related electrochemical applications but remain fundamentally limited by aggregation, sedimentation, and prohibitive viscosity growth. This work establishes that Brønsted acid site density, governed by the acidity, grafting density, and molecular structure of surface-grafted functional groups, serves as a unifying design parameter linking nanoscale surface chemistry to colloidal stability, macroscopic rheology, and electrochemical performance in metal oxide nanofluids. Using Fe2O3 nanoparticles as a model system, thiol (-SH), sulfonic acid (-SO3H), and phosphonic acid (-PO3H2) groups were systematically grafted at controlled densities, and their surface chemistry was quantitatively characterized. Systematic variation of these functional groups establishes a clear performance hierarchy of -PO3H2 ≈ -SO3H > -SH ≫ -OH across zeta potential, colloidal dispersion, nanofluid viscosity, and charge-transfer resistance. Phosphonate functionalization with etidronic acid reduces viscosity from ∼30 cP to near-water-like values (∼2-3 cP) even at a high solid loading of 40 wt %, maintains long-term dispersion stability, and improves discharge capacity from ∼130 to ∼245 mAh g-1. The identical anchoring chemistry of -SO3H and -SH confirms that functional group acidity governs surface charge development and ion accessibility, which in turn influence overall electrochemical kinetics. These results provide a transferable interfacial design framework for engineering high-loading, flowable metal oxide colloidal suspensions for semisolid flow batteries and related electrochemical technologies.