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

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.