Overcoming the Trade‐Off between Optical Transmittance and Areal Capacitance of Transparent Supercapacitors for Practical Application

It is substantially challenging for transition metal oxide nanoparticle (NP)‐based electrodes for supercapacitors to achieve high transparency and large capacity simultaneously due to the inherent trade‐off between optical transmittance (T) and areal capacitance (CA). This study demonstrates how this trade‐off limitation can be overcome by replacing some electrode NPs with transparent tin oxide (SnO2) NPs. Although SnO2 NPs are non‐capacitive, they provide effective paths for charge transport, which simultaneously increase the CA and T550nm of the manganese oxide (Mn3O4) NP electrode from 11.7 to 13.4 mF cm−2 and 82.1% to 87.4%, respectively, when 25 wt% of Mn3O4 are replaced by SnO2. The obtained CA values at a given T are higher than those of the transparent electrodes previously reported. An energy storage window fabricated using the mixed‐NP electrodes exhibits the highest energy density among transparent supercapacitors previously reported. The improved energy density enables the window to operate various electronic devices for a considerable amount of time, demonstrating its applicability in constructing a reliable and space‐efficient building‐integrated power supply system. The inherent trade‐off limitation between transmittance and areal capacitance of the manganese oxide electrode for supercapacitors is overcome by incorporating an appropriate amount of tin oxide nanoparticles. The mixed oxide electrode‐based device shows the highest areal capacitance and energy density at a given transparency among transparent supercapacitors reported so far.