Microfluidic-spinning construction of black-phosphorus-hybrid microfibres for non-woven fabrics toward a high energy density flexible supercapacitor

Abstract

Flexible supercapacitors have recently attracted intense interest. However, achieving high energy density via practical materials and synthetic techniques is a major challenge. Here, we develop a hetero-structured material made of black phosphorous that is chemically bridged with carbon nanotubes. Using a microfluidic-spinning technique, the hybrid black phosphorous-carbon nanotubes are further assembled into non-woven fibre fabrics that deliver high performance as supercapacitor electrodes. The flexible supercapacitor exhibits high energy density (96.5 mW h cm^-3), large volumetric capacitance (308.7 F cm^-3), long cycle stability and durability upon deformation. The key to performance lies in the open two-dimensional structure of the black phosphorous/carbon nanotubes, plentiful channels (pores <1 nm), enhanced conduction, and mechanical stability as well as fast ion transport and ion flooding. Benefiting from this design, high-energy flexible supercapacitors can power various electronics (e.g., light emitting diodes, smart watches and displays). Such designs may guide the development of next-generation wearable electronics.

Fig. 1

Schematic illustrations. a Synthesis of black phosphorous chemical-bridged carbon nanotubes (BP–CNTs) by heat treatment of black phosphorous (BP) and carbon nanotubes (CNTs), and chemical passivation of BP–CNTs using 4-nitrobenzene diazonium (4-NBD). Red phosphorous is abbreviated as RP. b Microfluidic-spinning-technique (MST) fabrication of microfibres via a triphase microfluidic device consisting of one core flow and two sheath flows and fibres assembled into non-woven fabrics, which can be cut into different shapes. Thermoplastic polyurethane is abbreviated as TPU. C₂H₅OH is ethanol. c Construction of flexible supercapacitor (SC) by hot pressing of two conductive fabric layers and one polymer-supported ionic liquid electrolyte layer. The supercapacitors have the potential to power various electronics for application. EMI⁺ is 1-ethyl-3-methylimidazolium, BF₄⁻ is tetrafluoroborate, EMIBF₄ is 1-ethyl-3-methylimidazolium tetrafluoroborate, and PVDF-HFP is poly(vinylidene fluoride-co-hexafluoropropylene). Guan Wu is the creator of the waterfall photo in the powered electronic device

Fig. 2

Structural characterization. a, b Transmission electron microscopy (TEM) images of black phosphorous chemical-bridged carbon nanotubes (BP–CNTs) at low and high magnifications, respectively. Scale bar: a 500 nm; b 100 nm. c High-resolution TEM (HRTEM) image of BP–CNTs, scale bar: 2 nm. d High magnification of the selected domain in a, scale bar: 100 nm. The corresponding energy dispersive X-ray spectroscopy (EDS) elemental mapping images of C (e) and P (f), scale bar: e 100 nm; f 100 nm. Surface scanning electron microscopy (SEM) images of BP–CNTs at low (g) and high magnifications (h), scale bar: g 2 µm; h 1 µm. i Structural illustration of BP–CNTs, scale bar: 400 nm. High-resolution C 1s X-ray photoelectron spectroscopy (XPS) spectra of black phosphorous (BP) (j) and BP–CNTs (k). l Micro-pore size distributions. Insets are typical nitrogen adsorption and desorption isotherms and meso-pore size distributions. 4-NBD is 4-nitrobenzene diazonium

Fig. 3

Electrochemical performance of flexible supercapacitors. a Microfluidic-spinning-technique (MST) fabrication of fibres-based non-woven fabrics. Left part: illustration of the device microchannel. Middle part: photograph of non-woven fabric. The inset: scanning electron microscopy (SEM) image of the fabric, scale bar: 500 µm. Right part: non-woven fabric that can be cut into various shapes and subjected to various deformations. All scale bars in photos are 2 cm. b–d Schematic illustration of the designed electrode structure. Carbon nanotubes is abbreviated as CNTs. BP is defined as black phosphorous. CNTs/BP refers to the physical mixture of carbon nanotubes and black phosphorous, CNTs/BP–CNTs refers to the physical mixture of carbon nanotubes and carbon nanotubes chemical-bridged black phosphorous. e Cyclic voltammetry (CV) curves of supercapacitors (SCs) at a scan rate of 10 mV s⁻¹. f Galvanostatic charge/discharge curves at a current density of 0.1 A cm⁻³. g Calculated specific capacitances under different current densities. h Electrochemical impedance spectroscopy (EIS) analysis of SCs. The inset figures show the depressed semicircle of the Nyquist plots and the equivalent circuit model. The symbols denote experimental data, while the green lines represent the fitted data. i Cyclic testing of SCs under a voltage of 3 V at a current density of 0.4 A cm⁻³; inset: galvanostatic charge/discharge curves after 10,000 cycles. j Energy density versus power density of SCs compared with other electrode-based energy storage systems. The BP and BP–CNTs are all modified by 4-nitrobenzene-diazonium (4-NBD). MoS₂-rGO/MWCNT is Molybdenum disulfide-reduced graphene oxide/multi walled carbon nanotubes. CNT is carbon nanotubes. rGO is reduced graphene oxide. Black phosphorous. CNTs/rGO is carbon nanotubes/reduced graphene oxide. MnO₂/carbon cloth is manganese dioxide/carbon cloth. Carbon nanosheet. Graphene/PANI is graphene/polyaniline. Co₃O₄/Co(OH)₂ is four oxidation of three cobalt/cobalt hydroxide. Ni@MnO₂ is nickel@manganese dioxide. MXene/Graphene is metal carbides/graphene. Li thin-film battery

Fig. 4

Flexibility, stability and application of supercapacitors. a Bending cyclic stability of a physical mixture of carbon nanotubes and hybrid carbon nanotubes chemical-bridged black phosphorous (CNTs/BP–CNTs) based supercapacitor (SC) at a current density of 0.4 A cm⁻³. Insets: galvanostatic charge/discharge curves and photographs under different bending angles. b Cyclic voltammetry (CV) curves of SCs under flatted, twisted, rotated and folded states at a scan rate of 10 mV s⁻¹. c Photographs of supercapacitor stably lighting up light-emitting-diodes (LEDs) under different bending angles. d Photographs of an SC powering watch. e Photographs of two capacitive pouches integrated in series to power a multi-colour display. Guan Wu is the creator of the waterfall photo in the powered electronic device

Fig. 5

Mechanism for flexible supercapacitors. a Schematic illustration of flexible supercapacitor. b Energy density of our supercapacitor compared with that of other electrode-based supercapacitors. c Charge distribution in three kinds of designed electrodes. d Fine distribution of ions in three kinds of designed electrodes. Carbon nanotubes is abbreviated as CNTs. BP is defined as black phosphorous. CNTs/BP refers to the physical mixture of carbon nanotubes and black phosphorous, CNTs/BP–CNTs refers to the physical mixture of carbon nanotubes and carbon nanotubes chemical-bridged black phosphorous