Urchin-like NiCo₂O₄ hollow microspheres and FeSe₂ micro-snowflakes for flexible solid-state asymmetric supercapacitors

Flexible solid-state asymmetric supercapacitors (FSASCs) have been considered to be excellent energy storage components for flexible and portable electronic devices. Pseudocapacitive materials involved in FSASCs may bring in high capacitance and energy density, which are crucial for their practical applications. In this work, a kind of NiCo₂O₄ pseudocapacitive cathode material with an urchin-like hollow micro-sphere structure (UHMS) was designed and synthesized. Due to the high proportion of active sites and fluent ionic channels created by the UHMS structure, the as-prepared NiCo₂O₄ UHMS sample shows an enhanced specific capacitance of about 942.2 F g⁻¹ at 0.5 A g⁻¹ and desirable rate capability. In addition, a pseudocapacitive anode material FeSe₂ with a hierarchical snowflake (SF) structure was also prepared by an anion exchange reaction through a unique morphological reconstruction process. The obtained material shows a much higher capacitance (304 F g⁻¹ at 0.5 A g⁻¹) and better rate capability than its parent material (micro-dendritic Fe₂O₃). A FSASC was fabricated using the NiCo₂O₄ cathode and FeSe₂ anode on a simple flexible current collector prepared from scotch tape and graphite paper, and it delivered a high operating voltage up to 1.5 V, an energy density of 10.4 W h kg⁻¹, a maximum power density of 1.2 kW kg⁻¹, and long term stability for 1000 cycles. Such an impressive FSASC is totally based on pseudocapacitance in both electrodes and would be a promising candidate for use in flexible and wearable electronic devices.