Fe- and Mg-modified binder-free Cu₂O thin films for solid-state symmetric supercapacitors: composition-dependent...

Binder-free cuprous oxide (Cu₂O) thin films were grown directly on copper foil by successive ionic layer adsorption and reaction (SILAR) using nominal Fe or Mg additions of 1, 3 and 5 at% relative to Cu. The films are denoted by modifier and nominal loading as Fe1, Fe3 and Fe5, and Mg1, Mg3 and Mg5, with the unmodified film written as pristine Cu₂O. Composition-matched electrodes were assembled into symmetric devices with a poly(vinylidene fluoride) (PVDF)-based LiCl/KOH gel electrolyte. X-ray diffraction shows a cuprite-dominant Cu₂O phase throughout both series, together with a composition-dependent shift of the (111) reflection and a corresponding change in coherent-domain size. Field-emission scanning electron microscopy shows compact granular Fe-modified films, whereas Mg modification produces a stronger evolution of particle size and surface architecture, culminating in an open agglomerated texture at Mg5. At 0.5 mA cm^-2, Fe1 and Mg1 deliver device areal capacitances of 129 and 80 mF cm^-2 against 12 mF cm^-2 for pristine Cu₂O, with fitted charge-transfer resistances of 759 and 18.5 Ω against (3.9 ± 0.7) × 10⁴ Ω for the unmodified film. The lowest charge-transfer resistances of the series belong to Mg1 and Fe5 rather than to the two highest-capacitance devices, and the Warburg coefficient falls by four orders of magnitude on modification. After 10 000 cycles all seven devices retain 95-99% of their capacitance, while coulombic efficiency remains strongly composition-dependent, from 61% for Mg1 to 97% for Fe5 and Mg3. Scan-rate current deconvolution assigns 59-94% of the stored charge to the surface-controlled term. Low-level Fe and Mg modification therefore tunes Cu₂O through distinct, non-monotonic combinations of interfacial kinetics, ion transport, redox accessibility and film morphology, and no single descriptor ranks the seven compositions in the same order.