Electrostatic self-assembly of heterostructured black phosphorus–MXene nanocomposites for flexible microsupercapacitors with high rate performance

Quasi-solid-state microsupercapacitors (QMSCs) with neutral electrolytes are regarded as significant flexible microscale power sources for portable/wearable electronics with ensured user safety. Ti3C2Tx MXene nanosheets serve as the promising building block units for the fabrication of flexible QMSCs with in-plane electrode configuration. Yet, the compact restacking of MXene nanosheets unavoidably deteriorates the accessibility of charge carriers within multilayered electrodes and decreases the utilization rate of active sites, limiting the specific capacitance, rate performance, and cyclability of QMSCs with neutral electrolytes. Herein, positively-charged black phosphorus (BP) nanoflakes were first prepared by electrochemical exfoliation method and electrostatically assembled with negatively-charged MXene nanosheets, forming heterostructured BP–MXene nanocomposites. Afterwards, flexible BP–MXene thin films demonstrated lamellar structures with hierarchically porous networks and heterostructured interfaces. The formation of Ti–O–P bonds enhanced the atomic charge polarization at the heterostructured BP–MXene interfaces, resulting in efficient interfacial electron transfer. The incorporation of BP nanoflakes into MXene multilayers further facilitated reversible intercalation reactions of charge carriers that contributed fast charge storage. A flexible QMSC with symmetric BP–MXene electrodes and neutral gel electrolyte (BP–MXene QMSC) was finally fabricated, showing a high volumetric energy density of 28.17 mWh cm−3 at 0.043 W cm–3, outstanding rate performance (68.7% retention up to 20 V s–1), and high cyclability (90.9% after 40,000 cycles). The flexible BP–MXene QMSC further demonstrated high capacitance retention under repeated bending and provided favorable comparison with other MXene-based MSCs in neutral electrolytes.