Grass-like alumina nanoelectrodes for hierarchical porous silicon supercapacitors

With the development of microscale and standalone electronic devices the demand for microscale energy storage is increasing. Micro-supercapacitors are emerging as the candidate for microscale energy storage, especially when combined with energy harvesters. In this work, we enhance the capacitance of porous silicon (PS) supercapacitor electrodes up to 4× by adding a new high-surfacearea nanoelectrode on the existing topography, thus forming a hierarchical 3D supercapacitor electrode that can be used in micro-supercapacitor applications. The nanoelectrode is based on grass-like alumina (GLA) - a recently reported conformal nanoporous coating - with two surrounding TiN films, all materials deposited by atomic layer deposition. The GLA nanoelectrode can be deposited conformally on complex topographies like here on PS, as the total thickness of the electrode structure does not exceed 200 nm allowing it to be fitted in existing electrodes. The GLA nanoelectrode increased the capacitance of the PS supercapacitors alone by up to fourfold or 4× and reduced the self-discharge to a mere 25% loss after 20 h compared to a TiN coated PS reference and state-of-the-art PS both with significantly higher losses. The GLA nanoelectrode showed remarkable stability for 10 000 galvanostatic cycles with a decrease in the capacitance only by 5% and no structural changes were identified from SEM images. Microelectronics compatible processing, the conformal deposition process and the nanoscale thickness of the GLA nanoelectrode allow it to enhance 3D electrodes commonly used in micro-supercapacitors.