Monodisperse RuO2 nanoparticles for highly transparent and rapidly responsive supercapacitor electrodes

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 (Tvis) and areal capacitance (CA) simultaneously, and the capacitance of an electrode with Tvis 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 Tvis of 97.1% and CA 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.