Solvothermal Design of Metal Oxide Nanomaterials for Pseudocapacitors: From Synthesis Parameters to Intrinsic...

Metal oxide nanomaterials are among the most compelling electrode materials for supercapacitors. In contrast to carbon-based electrodes that store charge via electric double-layer mechanisms, pseudocapacitive materials operate through rapid, reversible surface or near-surface redox reactions, enabling higher densities. This review examines the non-aqueous solvothermal synthesis of nine major transition metal oxide nanomaterials (Fe₃O₄, Co₃O₄, NiO, RuO₂, V₂O₅, CuO, WO₃, Nb₂O₅, and MnO₂). Solvothermal synthesis is a powerful and indispensable strategy for tailoring the intrinsic physicochemical properties of these nanomaterials, that is, phase composition, crystallinity, morphology, and particle size, that directly govern their pseudocapacitive performance. We focus on a critical gap in literature: the limited reporting and understanding of key solvothermal parameters, particularly solvent identity and reactor fill ratio. These variables dictate autogenous pressure and determine whether the synthesis occurs under subcritical or supercritical conditions. Therefore, we propose a conceptual framework linking solvent choice and fill ratio to autogenous pressure, phase evolution, defect chemistry, morphology, and pseudocapacitive behavior. Because systematic studies remain scarce, existing evidence is critically evaluated to identify trends and knowledge gaps. The findings suggest that optimizing pristine metal oxide nanostructures through controlled solvothermal synthesis, rather than relying on conductive-carbon compositing, is essential for achieving meaningful improvements in next-generation supercapacitors.