The light, flexible and even fully transparent energy storage devices have wide applications in the future portable devices.

However, challenges still exist. The light transmittance and the energy density affect each other, and the improvement of a single performance often leads to a significant drop in the other performance. Also, it is necessary to increase the capacity of the energy storage device.

In a study published in Nature Communications, Prof. HUANG Fuqiang’s group at Shanghai Institute of Ceramics of the Chinese Academy of Sciences prepared a series of interstitial boron-doped mesoporous semiconductor oxides SnO_2-xB_y, ZnO_1-xB_y and In₂O_3-xB_y, through rational crystal doping design.

In this new type of transparent semiconductor oxide, the interstitial boron atoms can not only greatly increase the carrier concentration of the doped material, but also provide abundant binding sites for the embedding of OH^-. The pseudocapacitive reaction with OH^- converts the inert SnO₂, ZnO and In₂O₃ into highly electrochemically active supercapacitor electrode materials.

By controlling the interstitial boron doping concentration, the volumetric specific capacity of the mesoporous transparent semiconductor oxide of this kind can reach 1172 mF cm^-3 which is similar to the pseudocapacitive performance of other non-transparent metal oxides.

Such semiconductor materials can be uniformly blended with polyethylene dioxythiophene-poly (styrene sulfonate) conductive polymer and then coated on polyethylene terephthalate substrate by aerosol spray technology.

The transparent flexible supercapacitor assembled using this electrode has a capacitance retention rate close to 100% after 15,000 cycles, and its area energy density and light transmittance can reach 1.36 × 10^-3mWh cm^-2 and 85%, respectively, which are better than all other transparent energy storage devices reported.

This study provides a new strategy for the design and synthesis of transparent semiconductor oxides with excellent electrochemical activity.