Highly transparent energy storage devices have attracted attention owing to their applications in innovative electronics and displays. However, it is still challenging to fabricate devices with high optical transmittance (T_vis) and areal capacitance (C_A) simultaneously, and the capacitance of an electrode with T_vis exceeding 95% has not been reported yet. Here we demonstrate a simple process for fabricating ruthenium oxide (RuO₂) nanoparticle (NP)-based ultrathin film electrodes with a T_vis of 97.1% and C_A of 0.85 mF cm⁻². This high transparency and capacitance are achieved by the deposition of organic ligand-decorated, small (∼2 nm) RuO₂ NPs followed by subsequent removal of the ligands. The optical transparency of the electrode can easily be controlled by changing the concentration of the RuO₂ NP solution. Efficient contact with the electrolyte and metallic conductivity of the uniformly spread RuO₂ NPs result in a significantly high capacitance retention of 81.2% at a scan rate of 1000 mV s⁻¹, the highest reported value. The cycle stability is also considerably improved, and 93.5% of capacitance is retained after 5000 charge–discharge cycles. Furthermore, the monodisperse RuO₂ NPs can be uniformly and thinly deposited using a simple spray-coating technique, indicating their applicability to large-area transparent devices.

This study reports on an asymmetric supercapacitor consisting of two composite transition metal oxide (TiO₂–Fe₂O₃and TiO₂–MnO₂) nanoparticles-based electrodes working in different potential ranges.