Microfluidic Fabrication Of Graphene-Based Fibers And Their Energy Storage Performance

With the rapid growth of flexible and wearable electronics,the high-performance integrated energy storage and power supply systems have already become significant research orientations in energy field.Fiber-based supercapacitors have shown great potential for development in the field of wearable electronic devices due to their high-energy density,excellent charge storage efficiency,and long cycle life.As an emerging carbon material,graphene is recognised as ideal electrode materials for supercapacitors,resulting from their special merits,such as large specific surface areas,good conductivity,structural designability,and light weight.However,the graphene fiber obtained through conventional spinning method exhibit bad mechanical property and poor electrochemical performance,resulting from disordered arrangement and low structural controllability of graphene.It is difficult to meet the demand of high-performance wearable electronics.In this paper,a series of graphene-based fiber and fabric electrodes are designed via a microfluidic spinning strategy.By the dual strategy of atomic doping and chemical deposition of metal oxide,the capacitance of electrode materials is significantly improved.The surfaces of fiber and fabric are coated with a biocompatible skin-friendly shell,improving the wearable comfort and circuit safety.This work can offer the application accumulation and theoretical support for graphene-based fibers and fabrics in wearable and flexible electronics.The main research contents and results are as follows:1.The large-layer graphene oxide with uniform size is prepared by modified Hummer’s method.The morphologies,sizes,and chemical structures of graphene oxide are also studied.The continuous graphene fibers（GFs）are successfully generated by using homogeneous graphene oxide via a microfluidic spinning strategy with the segmented microchannel.Highly ordered graphene nanosheets of fibers arranged along the main axis,which ensures excellent mechanical flexibility（stretchable fiber spring）and strength.After the heating treatment,the most of oxygen-containing functional groups on the graphene nanosheets are removed,resulting in outstanding electrical conductivity(91 S cm^-1).In the meantime,the space of calcined graphene nanosheets is expanded.It leads to larger specific surface areas(107.56 m²g^-1)of GFs,which is conducive for storing more electrolytes.The capacitance of all-solid-state supercapacitors（GFs supercapacitors,GSCs）assembled by two parallel GFs covered with acid electrolyte（H₃PO₄/PVA）reaches 76.6 F cm^-3 at 0.02 A cm^-3.2.The nitrogen-doped graphene fibers（NGFs）with a hierarchically inside-out porous network structure are successfully manufactured by using amino-modified graphene oxide via a microfluidic spinning strategy with the nitrogen source of ammonium bicarbonate.The appropriate nitrogen contents（3.6%）,small pore volumes(0.36 cm^-3g^-1),large specific surface areas(328.87m²g^-1),and uniform pore size distributions（2.1～158.2 nm）of NGFs are achieved by adjusting the fluid composition,fluid speed,and fluid content.The research result demonstrates that the hierarchically inside-out porous network structure of NGFs accelerates the infiltration of electrolyte and provides fast electron transport channel.The capacitance of all-solid-state supercapacitors（NGFs supercapacitors,NGSCs）assembled by two parallel NGFs covered with H₃PO₄/PVA reaches 236.1 F cm^-3 at 0.15 A cm^-3and only loss 5.1%of original capacitance after10000 cycles.The electrochemical performances are improved through adopting the organic ionic liquid electrolyte（DMF/EMIMBF₄/PVDF）and the operating voltage is expectantly extended to 3V.The capacitance of NGSCs is up to 183.6 F cm^-3 at 0.25 A cm^-3.The maximum volume energy density and power density are 10.6 m Wh cm^-3 and 1051 m W cm^-3,respectively.Considering above outstanding electrochemical properties,the fiber-shaped supercapacitors combined in series or parallel can power some electronics,such as light emission diode lamps,electronic timers,and electronic console.3.The core-shell manganese dioxide@nitrogen-doped graphene fibers（MNGFs）are synthesized by using various manganese sources with hydrothermal reaction and Y-shape microchannel via microfluidic spinning techniques.The growth of crystal type of Mn O₂（α,δ,andγ）is controlled by adjusting the reaction conditions,and the high-activity Mn O₂ is selected after optimization by comparing with their electrochemical performances.The hierarchically coaxial skin-friendly silk fibroin/γ-manganese dioxide/nitrogen-doped graphene fibers（SMNGFs）are firstly manufactured via a microfluidic spinning strategy.The all-solid-state supercapacitors（SF/γ-Mn O₂/NGFs supercapacitors,SMNGSCs）assembled by hierarchically coaxial SMNGFs as electrode materials preserve not only excellent electrochemical performances,but also meliorative wearable comfort and circuit safety.The capacitance of SMNGSCs is as high as 483.2 F cm^-3 at0.15 A cm^-3.The as-synthesized fibers exhibit hierarchically inside-out porous network structures and rich electrochemically active sites,which can smoothly transfer electrons and electrolyte ions,so it displays excellent rate performance and cycle stability.The capacitance of SMNGSCs can still maintain 62.8%of original capacitance when the current density increases up to 2.4 A cm^-3.The capacitance only loss 4.2%of original capacitance after 10000 cycles.The maximum volume energy density and power density are respectively 16.1 m Wh cm^-3 and 1863 m W cm^-3 in the DMF/EMIMBF₄/PVDF electrolyte.Considering those outstanding electrochemical properties,our fiber-shaped supercapacitors combined in series or parallel can light up some electronics,such as light emission diode phosphor screens,smartwatches,and hygrothermographs.4.Hierarchically sandwich-structural silk fibroin/γ-manganese dioxide@nitrogen-doped graphene fiber non-woven fabrics/silk fibroin（SMNGNFS）are exploited through hot-pressing method,roller-coating approach,and microfluidic spinning strategy.The all-solid-state fabric-shaped supercapacitors（SF/γ-MNGFs fabrics/SF supercapacitors,SMNGNFSSCs）assembled by two parallel SMNGNFS covered with H₃PO₄/PVA electrolyte demonstrate an areal specific capacitance of 989.3 m F cm^-2 at 1 m A cm^-2.The capacitance shows almost negligible deteriorations at different bending angles and during cyclic bending process.The capacitance can also reserve 95.7%of initial capacitance after 10000 cycles.Their maximum areal energy density and power density are respectively 51.3 u Wh cm^-2 and 839 m W cm^-2 in the DMF/EMIMBF₄/PVDF electrolyte.In conclusion,the high-performance graphene fibers and fabrics prepared by using graphene oxide via a microfluidic spinning strategy have a wide application prospect in the field of flexible and wearable intelligent textiles.The results of this work offer technical guidance and theoretical basis for not only the fabrication strategy of graphene-based fibers and fabrics,but also their assembly application of supercapacitors.It also provides an extended idea for the design and development of high-performance wearable graphene-based energy storage devices.