Highly transparent supercapacitors based on ZnO/MnO₂ nanostructures

The recent rapid development of transparent electronics, notably displays and control circuits, requires the development of highly transparent energy storage devices, such as supercapacitors. The devices reported to date utilize carbon-based electrodes for high performance, however at the cost of their low transparency around 50%, insufficient for real transparent devices. To overcome this obstacle, in this communication highly transparent supercapacitors were fabricated based on ZnO/MnO₂ nanostructured electrodes. ZnO served as an intrinsically transparent skeleton for increasing the electrode surface, while MnO₂ nanoparticles were applied for high capacitance. Two MnO₂ synthesis routes were followed, based on the reaction of KMnO₄ with Mn(Ac)₂ and PAH, leading to the synthesis of β-MnO₂ with minority α-MnO₂ nanoparticles and amorphous MnO₂ with embedded β-MnO₂, respectively. The devices based on such electrodes showed high capacitances of 2.6 mF cm^-2 and 1.6 mF cm^-2, respectively, at a scan rate of 1 mV s^-1 and capacitances of 104 μF cm^-2 and 204 μF cm^-2 at a very high rate of 1 V s^-1, not studied for transparent supercapacitors previously. Additionally, the Mn(Ac)₂ devices exhibited very high transparencies of 86% vs. air, far superior to other transparent energy storage devices reported with similar charge storage properties. This high device performance was achieved with a non-acidic LiCl gel electrolyte, reducing corrosion and handling risks associated with conventional highly concentrated acidic electrolytes, enabling applications in safe, wearable, transparent devices.